Concrete is usually made of water, cement, admixtures and aggregates. Generally, the quantity of water added to the mix is greater with respect to the stoichiometric value which is essential to completely hydrate the cement. Mix water cannot be reduced to the theoretical quantity required even if superplasticisers are used. During concrete setting or hardening, the extra water that was used while making the concrete would eventually evaporate. The evaporation of the excess water isn’t a major problem in itself provided the whole process takes place in controlled conditions (at 20°C and 90% relative humidity).

The water evaporation speed primarily relies on factors like wind speed, relative humidity, ambient and concrete temperature. Usually, the condition of concrete casting is a lot different from the ideal condition that has been shown above. In that case, the water evaporation leads to the formation of micro fissures which collectively degenerates and forms cracks and macro fissures.

Cracking issues and joint failures usually bother the concrete floor and pavement owners due to the expensive repairs. A negative perception is usually associated with cracks regarding its longevity, quality and serviceability of pavements or concrete floors. Cracks are usually seen as aesthetic issues but it can create major disputes between consultants, clients and contractors resulting in cost overruns and delay in work process.

Conventional Steel Reinforcement For Concrete
Conventionally, standardized welded steel fabric in the forms of bars and meshes, has been used as reinforcement in various applications such as concrete floors, pavements, steel deck slabs, bridge decks, etc. for many years. Steel is primarily necessary to carry the loads after the concrete cracks and to hold together broken pieces of concrete. Steel in the forms of bars and meshes primarily functions as two-dimensional reinforcement. However, some structural concrete applications require a three-dimensional approach to reinforcement. Further, the use of steel in various forms, including steel fibers, has other problems related to either in-place performance or handling and placement including corrosion. Corrosion of reinforcing steel is a major concern as it affects the long-term durability and performance of any steel-reinforced concrete application. This corrosion concern is even more important in shotcrete applications that are constructed in a marine or water environment, or in an underground structure which are mostly wet. Equally important concern is ensuring the minimum necessary concrete cover for the steel mesh and rebar to protect it from corrosion and other chemical contaminants.

The reinforcement of mesh and steel rebar need to be spliced, bent, cut and attached to the project substrate, the entire process is laborious. In addition to this there are also chances of human injury when it comes to handling the steels. The availability issues and costing are also concerning factors regarding all forms of steels- mesh, bar and fibers.

Various fiber-reinforcing materials are available nowadays but structural applications of fiber-reinforced concrete are mainly made of steel fibers. But in recent years, new breeds of structural synthetic equivalents are proving their usefulness. Lighter weight, lower abrasion and better structural performance are making synthetic reinforcement an economic alternative.

How Do Fibers Work In Plastic Stage Of Concrete?
For architectural and aesthetic concrete products and for prevention of early age cracking Micro Synthetic fibers are generally used while Macro Synthetic Structural fibers or Steel fibers are mainly used to control properties of concrete in the hardened stage such as post-crack flexural strength, abrasion resistance, impact resistance and shatter resistance of concrete, etc.

Early age concrete shrinkage causes weak planes and results in the formation of cracks, because the stresses developed in the concrete exceeds its tensile strength at that specific time. The growth of these micro shrinkage cracks can be inhibited by the mechanical blocking action of both synthetic and steel fibers. The internal support system of the fibers inhibits the formation of plastic settlement cracks. The uniform distribution of fibers throughout the concrete discourages the development of large capillaries, caused by bleed water migration to the surface. Fibers thus lower the permeability of concrete through the combination of plastic crack reduction and reduced bleeding characteristics.

ECMAS EXF 54 is an easy-to-finish, blended fiber, made of 100% virgin polyolefin consisting of a twisted bundle non-fibrillating monofilament and a fibrillating network fiber, yielding a high-performance concrete reinforcement system. ECMAS EXF 54 disperses uniformly in the entire concrete mass and it helps to reduce plastic and hardened concrete shrinkage, improve impact strength, and increase fatigue resistance and concrete toughness. ECMAS EXF 54 is non-corrosive, non-magnetic, and 100% alkali-proof offers long-term durability, structural enhancements. They have a high tensile strength and a relatively high modulus of elasticity.

To maximize resistance to pull-out and post crack behavior, ECMAS EXF 54 Hybrid Macro Fiber involves a blend of two different fiber types & shapes:

1. A standard fibrillated polypropylene fiber to reduce and control shrinkage and temperature cracking.
2. A very heavy-duty twisted-bundle monofilament fiber made of a strong synthetic copolymer with embossed surface, to increase load-transfer and post-crack performance. This pre-blended fiber is typically used in long lengths (54 mm) and in high dosages to affect a higher replacement level of reinforcing steel than standard synthetic micro fibers.

ECMAS EXF 54 is mainly used for performance concrete applications such as industrial & warehouse floors, concrete pavements, steel deck slabs, bridge decks, shotcrete, loading docks, light precast products – anywhere when steel reinforcement reduction or replacement is the objective.

Concrete Floors, Pavements & Parking Lots
The concrete grade slab present in the industrial warehouse factories has two essential functions: firstly, it has to sustain the operational goods from the loaded racking system, they directly store the goods on the floor, fork-lift truck wheel loads and are responsible for transferring the same to the supporting soil. The job is done without any structural failures or disturbing the settlements. Secondly, it provides a good wearing surface on which the functions in the facility can be carried out with safety and efficiency. Both the functions achieved by the industrial floor slabs is the main reason behind the success of modern commercial advantages.

The primary purpose of industrial floors is to provide sufficient reinforcement to control the amount and size of cracks to achieve a consistent level with the appropriate use of the floor.

ECMAS EXF 54 Fiber Reinforced Concrete helps reduce the width of cracks, and permits the replacement of conventional steel reinforcement. This process is more prone to corrosion which will require a lot of maintenance of the floor in the near future. But when you use FRC, these cracks tighten up and prevent moisture and chlorides from entering the floor and penetrating down to the basic level of reinforcing. It also helps to bridge areas where there was no reinforcing present in the past. Hence, it reduces chipping, spalling and eliminates the formation of potholes and section losses.

By reducing the joint widths and frequency, and adding EXF 54 fiber reinforcement, it is anticipated that the concrete pavement will last much longer than a traditional concrete parking lot.

Composite Metal Deck Slabs
A typical metal deck consists of a corrugated steel sheet with a concrete topping, with the sheet serving as both a permanent form and as the principal reinforcement for the slab. Historically, welded wire mesh fabric has been used as a secondary (non- structural) reinforcement to control rather than prevent concrete cracks.

Synthetic Structural fibers have become a well-recognized, cost effective, and acceptable reinforcement for slab on metal deck applications. But beyond the cost-savings and long-term reduction or prevention of cracking is a more fundamental reason to choose synthetic fiber reinforcement:

The use of conventional steel reinforcement in slab on deck applications brings a host of ease of use and proper placement concerns. For the welded wire fabric to be effective in controlling this shrinkage/temperature cracking, it must be placed within the top-third of the concrete cross-section, a process that is quite challenging in execution. The simple transport of mesh rolls or sheets to upper-level deck projects is difficult and labor-intensive.

ECMAS EXF 54 fiber reinforcement system provides great time and labor savings as this is simply added to the concrete mix as an ingredient, in addition to the uniform three-dimensional reinforcement coverage it provides throughout the concrete deck matrix compared with single-plane steel.

ECMAS EXF 54 Synthetic fibers meet and exceed established measurement standards and building codes set forth by Underwriters Laboratories (UL), American Society of Testing and Materials (ASTM) and the American Concrete Institute (ACI). 

Shotcrete
Concrete is pumped through a hose and projected at high velocity onto the desired surface. Traditionally, welded wire fabric (WWF) is used as temperature-shrinkage reinforcement in shotcrete applications. Sometimes, configuration to the substrate by the steel does not happen because the steel is too stiff, and excess shotcrete material is used to cover the steel. Lastly, applying shotcrete through the steel “obstruction” makes the shotcrete system performance very dependent on the operator skill to reduce shadowing. These placement and performance deficiencies of steel reinforcement served as further incentive to develop a level of synthetic fiber reinforcement that could serve as a viable alternative

As with slabs-on-ground, synthetic structural fiber reinforcement is an advantageous alternative, providing several technical, economic, and safety benefits as compared to traditional secondary, steel reinforcement including, but not limited to, temperature shrinkage crack resistance, crack-width control, impact and abrasion resistance, and spalling resistance. The long-term durability benefits far outweigh the often-questionable performance of wire mesh at a very competitive cost. EXF 54 Fiber-reinforced concrete with greater ductility allows it to deform under tensile stress, as well as greater energy absorption capacity despite cracking.

Compared with steel mesh reinforcements, ECMAS EXF 54 fiber reinforced shotcrete also has many other benefits, such as:

• a greater homogeneity of the support structure
• a more efficient rock section profile, allows for a uniform thickness and uniform density following the contours of the receiving face
• offering simpler application logistics
• fibers help reduce rebound (cost advantage) and improve compaction

Fire: In the event of fire accidents in tunnels, synthetic structural fibers prevents the hazardous phenomenon of “spalling”, that is, violent explosion of the concrete structure. During a fire, once fibers reach their melting temperature, they decompose without producing any harmful gases and transform the volume they occupied in the cement into a series of interconnected empty channels. These provide escape routes for heat and steam generated in the fire due to sudden boiling of interstitial water.

Precast Structures
Macro synthetic fiber concrete reinforcement is premixed with the concrete and delivered straight to the precast mould, eliminating the steel installation process. It helps achieving an increase in production output and total cost savings. It Eliminate the need for storage and installation of standard steel reinforcement in precast elements. Project experience has shown that structural synthetic fiber concrete reinforcement can increase precast production speeds up to 50%. Macro synthetic structural fibers are mixed throughout the entire structure, eliminating concrete cover requirements. In many cases this will allow for a reduction in element thickness and a reduction in weight. Also, many precast items are exposed to the corrosive environments but macro synthetic structural fiber concrete reinforcement will never rust and will continue to perform for the full life of the concrete.

Attention to Application of Structural Synthetic Fibers
The main reason why synthetic structural fibers are not quickly adopted and used commonly as there are no proper guidelines on how exactly they should be utilized. There are no prior references for the correct usage of synthetic structural fibers. It is essential to know the proper application of the same, like how to add, mix, place and how finishing, compaction and curing is done and what are its effects on concrete entities. The ECMAS EXF 54 macro fibers are usually added to the concrete and dry mixture before water is added during the final mixing process. The rate of dosage of EXF 54 Fiber depends on the specific applications and the appropriate properties. It varies between 1.2 to 3.5kg/m3 for maximum applications. To achieve perfect results, you need to be careful about the proper mixture done while mixing the design and batching procedure for EXF 54 Fiber. The placing of structural synthetic fibers is precisely the same as regular concrete. It would help if you made sure that the concrete is sufficiently compacted, which would lead to take out the paste to the surface and allow its perfect finishing. After compaction, an easy float is usually passed over the concrete to close up the surface. The moment the fiber reinforced concrete has been levelled, compacted and floated; it can sustain with proper concerning practice. Surface friction is required to achieve Anchorage across a crack to acquire the best functioning of structural synthetic fiber. Cracking due to plastic shrinkage and drying shrinkage of the concrete is avoided by this process. It also improves the properties of concrete, such as the toughness of cracks, flexibility, impact and fatigue resistance.

Conclusion
Use of Synthetic Structural Fiber Reinforcement in Concrete has been rapidly growing throughout the construction industry since clients, contractors and consultants have started to recognize its benefits in terms of enhanced performance, durability, safety and convenience; reduced construction time and labor costs. Fiber Reinforced Concrete (FRC) has been proven to reduce cracking, reduce crack widths and in some cases, allow for replacement of conventional steel reinforcement which can be more prone to corrosion, accelerating the need for future maintenance.

With time, it has been proven that the use of fibre reinforced concrete will fortify reinforced concrete structures’ structural performance. Fibre-reinforced concrete improves the mechanical and structural properties of concrete. Various fibres and natural, synthetic, glass, steel, and others are used to enhance mechanical property concrete. The manufacture of steel items is increasing, and the accumulation of waste steel fibre is negatively affecting the environment. Therefore, effective and economical methods to convert this waste material to a building material is necessary. Different experimental and mathematical studies were conducted to study the feasibility of the use of waste in concrete.

In the modern construction world, the efficiency and the quality of the concrete cannot be compromised. Thus, the concrete should meet the structure’s demands. The concrete’s main problem is its weak tensile and ductile property. Effective use of fibre can overcome this issue and form a modified concrete. As mentioned earlier, there are many kinds of fibres; mild steel fibre from lathe waste can also be included in it. The addition of fibre can modify the crack propagation and failure crack under loading.

Mainly failure in fibre reinforced concrete will occur due to failure in bonding between the fibre and the concrete. The existing studies show that the changes in concrete properties are directly linked to the shape, length, aspect ratio, amount of addition, etc.^[1–5]. For the cases of mechanical properties, remarkable improvement in the tension, and flexural behaviour have been observed. Under the loading, after the formation of the crack, the fibre starts its function.

The objective of the fibre is to resist the propagation of the crack and prolong the failure load. It was observed that fibre-reinforced beam, even after the formation of the crack, the beam won’t fail, and the load will reach the peak value depending on fibre content^[8,12,13]. Beyond peak value, applied load decreased due to the failure of fibre and stress transformation from bottom to top of the section. The fibre reinforcement shows an improvement in stress-strain behaviour also ^[24-26]. Normally, the ultimate strain of reinforced concrete is 0.0035. But under the case of FRC, the strain value is increased to 0.012 and 0.018 at 1.6% and 3% of the fibre. This is an improvement in strain corresponding to peak stress with an increase in fibre content.

It is evident that the steel fibre uplifts the concrete’s structural integrity, and the usage of the steel fibre in concrete can be implemented in the construction. Due to the increase in steel usage, the accumulation of steel scrap increased and caused a lot of environmental issues. Various researches have been conducted to study the use of waste fibre as an alternative to the manufactured steel fibre. Some studies support the use of the lathe scraps, and from there, improvements in the mechanical properties were observed [19 – 21]. The effective usage of the lathe scraps paves the way to the production of economic and modified concrete.

The current research looks into the change in mild steel fibre reinforced concrete’s mechanical property and compares it with conventional concrete. Mild steel fibres from lathe waste are used as a substitute for the manufactured steel fibre. Lathe wastes are added at various percentages (0.0%, 0.5%, 1.0%, 1.5%, and 2.0%) by weight of concrete. For the experimental studies, concrete cubes, cylinders and beams are cast. Cubes and cylinders are employed to determine the compressive strength, split tensile strength, stress-strain behaviour, and elasticity modulus. Flexural strength is studied by testing the concrete beam.

Material Characteristics
The study’s aim is to understand the change in the properties of lathe waste added fibre in the concrete. Lathe waste fibre of the same material collected from a mechanical workshop was used for testing. Mild steel fibres with spiral cross-section were selected as the fibres. For the analysis, the fibre volume was kept as a variant, and the length was kept constant. The changes in mechanical and durable properties of concrete due to the change in the fibre volume were studied. The materials used for the casting works are:

• Cement: Ordinary Portland cement of grade 53 was used for the study. The initial and final setting time obtained was 30 minutes and 240 minutes. Cement has a specific gravity of 3.15 and of consistency of 33% was used for testing.
• Fine Aggregate: Dry sand, which passes through a 2.36mm sieve and retains 150 microns, was used. Specify gravity obtained was 2.67.
• Coarse Aggregate: Locally available coarse aggregate of size between 10-20mm was adopted. The specific gravity of coarse aggregate was 2.76.
• Lathe Waste: Mild steel fibre from the lathe waste was used as the fibre material. Spiral shaped fibres were selected for the casting. The fibre length was kept as a constant parameter and the volume of fibre addition as a variant. The length and width of the fibre will affect the concrete property, so careful selection of material should be done. The length of the fibre was maintained at 80mm, as shown in Fig 1.
• Water: Normal tap water was used for casting and curing. The water was free from salt content.

Fig.1: Mild Steel Fibre from Lathe Waste

Mix Design
A concrete mix design was done for M25 grade concrete and the Mix proportion is listed below in Table 1. The mix design was designed based on IS 10262, with a water-cement ratio of 0.5. Spiral shaped fibres are added at various percentages by weight of the concrete. Cubes, cylinders and beams of standard size are cast for the various percentages and kept curing for 7, 14 and 28 days.

Experimental Detail
The project’s main aim was to understand the change in the properties of lathe waste fibre reinforced concrete. By considering the fibre volume as a variant, parameter casting works for all the percentages were done. Concrete specimens of cubes, cylinders and beams were cast based on Indian standards.

Compression Test: Cubical specimen of size 100x100x100mm was cast for all the percentages, and they were tested in the universal testing machine at a loading rate of 140 kg/cm². Concrete mixes for all percentage of fibre were prepared, and it was compacted inside the mould. The samples were demoulded after 24 hours and kept in the curing tank for 7, 14 and 28 days. The test specimen was stored in a place free from vibration and cured in fresh water, the water was renewed every 7^th day, and the water was at a temperature of 27º ± 2º C. On the day of testing, the samples were taken out of the tank, and the surface moisture was wiped off before testing. The specimen was loaded in the universal testing machine, and the compressive strength was found using the formula

Split Tensile Strength: Standard size cylinders of diameter 100mm and height 150mm were cast for all percentage of fibre and kept under curing for 7, 14 and 28 days. On the day of testing, the samples were taken out of the tank, and the surface moisture was wiped off. The cylindrical specimens were loaded in the universal testing machine at 140kg/cm² per minute. The failure load was determined and the specimen’s split tensile strength was determined by using the formula.

Flexural Strength Test: Prismatic samples of size 10x10x50cm were cast for all the mixes and tested using a digital flexural testing machine with a loading rate of 180 kg/cm²/min after a curing period of 7,14 and 28 days. A similar method of casting and curing was adopted as done for compression testing. After the required curing, the samples were loaded in the digital flexural testing machine, and the flexure strength of the sample was displayed in the testing machine. The formula used for calculating the strength is

Where P is the load at failure and l is the length.

Modulus Of Elasticity
Cylindrical specimens of size 150mm diameter and 300mm height were cast to determine the modulus of elasticity, and the specimens were kept curing for 28 days. On the day of testing, the samples were taken out from the curing tank, and the surface moisture was wiped off. The strain measuring cage was attached to the cylindrical specimen after leaving a space of 50mm from bottom and 50mm from the top of the specimen. The specimen was loaded into the universal testing machine. From the test results, the load-deflection pattern and stress-strain behaviour of concrete can be studied. Load up to 200kN was considered for the study. A deflection dial gauge was attached to study the load-deflection pattern. Based on the load-deflection pattern stress-strain, values were determined. From the stress-strain curve, the change in the modulus of elasticity with the fibre content was determined by the formula

Water Absorption: For checking the concrete’s water absorption, concrete cubes of size 100x100x100mm were cast and cured for 28 days. After 28 days of curing the samples, air-dried and then placed inside the oven at 110°C for 24 hours. After 24 hours, the samples were weighed and recorded as dry weight. The samples were then kept submerged underwater for 24 hours, and they were weighed again and recorded as wet weight. Water absorption was expressed in percentage:

Sorptivity Test: The Sorptivity test was done to study the rate of water absorption of the concrete. For that concrete cylinder of diameter 100mm and height 150mm were cast, and it was cured for 28 days. After 28 days of curing, the cylinder was cut into a smaller size of the height of 50mm. The inner part of the cylinder was selected for the study, and the samples were kept inside the oven at 110°C for 24 hours. Using insulation tape, the top and sides of the cylinder were covered, and the bottom part was left open. After the sample preparation, the sample was immersed in water with 5mm height from the bottom. Weights of the samples were measured at 30, 45 and 60 minutes. This test measured the rate of water absorption through the capillary rise.

Where,
W₁ – Dry weight of the sample in kg
W₂ – Wet weight of the sample in kg
A – Surface area of the cylinder which is exposed to water
δ – Density of water in kg/m³
T – Square root of time during the measurement in minutes

Non- Destructive Test: Non -Destructive Test (NDT) consists of a wide range of testing to assess the quality of the concrete without causing any damage to the specimen. In this research, an ultra-sonic pulse velocity test and rebound hammer test was conducted.

Ultra-Sonic Pulse Velocity Method: The basic principle of ultra-sonic pulse velocity is to determine the time taken by an electronic wave to passing through the concrete specimen. Based on the received waves, the quality of the concrete was analysed. Electroacoustic transducers produce an ultrasonic pulse, and by transmitting this pulse through the concrete specimen, the quality of the concrete was studied. By a receiving transducer, the fastest waves was detected, and their velocity was measured. Pulse velocity only depends on the elastic property of the concrete, so it is the better method to understand the quality of the material. In this test, after selecting the line of propagation of the wave, the transducer which propagates the wave was placed in one end and the receiving transducer at the other end. As the wave passes the velocity of the wave was displayed in the UPV machine. Based on this value, the quality was determined. For the testing concert cubes of size 100x100x100mm of age 28 days was used.

Rebound Hammer Test: Hardness of the concrete was measured by the rebound hammer test. Based on the rebound hammer number obtained from the test the hardness of the concrete was determined. The test was conducted by the procedure based on IS 13311 (Part 2): 1992. Rebound hammer consists of a tubular housing having spring controlled mass that slides over the plunger within it. During the testing, the plunger was pressed against the test specimen and the spring controlled mass rebounds. The range of the rebound varies with the surface of the specimen. The rebound value number was then compared with the standard values. For testing, concrete cubes of size 100x100x100 were used and rebound hammer number of a surface were taken three times in different points and the average value was compared with the standard values.

Results And Discussion
Workability
The slump height of all the percentage concrete was calculated, and from the test results, it was observed that slump height was decreasing with an increasing percentage of fibre. The slump value is shown in Table 2. The addition of the fibre leads to an increase in the confining pressure between the fibre and the concrete mix, which causes a decrease in the value. The fibre will form a bond between the materials and arrest the falling of the concrete. Some other researches suggest the use of superplasticizers to overcome the decrease in workability. But for this study, no plasticizers were used since there was only a slight decrease in the value of slump height, and the resultant mix was sufficient.

Compression Test
From the compressive test results, it was observed that the fibre addition in the concrete would support the compressive strength properties of concrete. The fibre added in the concrete has helped to prolong the failure of concrete, and as a result, the value obtained for fibre reinforced concrete under compression was greater than conventional concrete. It was observed that the value of compressive strength was increasing with an increase in fibre content, and the maximum value was obtained at 2% of fibre addition. It was also observed that the fibre added concrete shows a different crack pattern in the concrete cube. The values of the compressive strength of the cube are shown in Table 3.

From Table 3, it was observed that there was only a slight increase in strength on the 7^th day compressive test, and after that, during the 14^th day and 28th day, there was a remarkable improvement in the strength. The result shows a gradual increase in the strength of the 14^th and 28^th days.

Fig. 2 shows the crack pattern in the cube after compression. The cube was not completely collapsed even after the failure, and the fibre added in concrete shows good resistance in the propagation of the crack. The failure pattern shows a small amount of collapse compared to conventional concrete.

Fig. 2: Cube after Compressive Test

Split Tensile Strength
Fibre-reinforced concrete is mainly adopted for improving the ductile behaviour of the concrete. From the test results, it was observed that the ductile properties of the concrete had been improved with an increase in fibre content ^[1-4]. The added fibre also takes part in load distribution in the specimen. Due to its ductile behaviour, the fibre can withstand load before its failure, and from the experimental results, the quantity of the fibre increases the load-carrying capacity of the concrete. Maximum tensile strength was obtained at 2% of fibre addition. Test results for all percentages of fibre addition at 7, 14 and 28 days are shown in Table 4.

Based on all the literature review and experimental data, the results show good agreement with the crack pattern and increased strength. Crack width of the cylinder was observed to be decreasing with the increase in the fibre content.

Fig. 3 shows the cylinder at failure. Only a slight crack is formed on the cylinder and it is also observed that the width of the crack decrease with increase in fibre content. For the case of conventional concrete, the cylindrical specimen was split into two pieces and for lathe waste reinforced concrete, the failure load of the cylinder was prolonged by the fibre.

Fig. 3: Cylinder under Split Tensile Strength

Modulus Of Elasticity
Based on the stress – strain values, the value of the modulus of the elasticity was determined. From the test data, it was observed that the value of the modulus of the elasticity was increasing with an increase in the fibre content. As per Indian code IS 456: 2000, Modulus of elasticity (Ec) is calculated by the 5000, where fck is 28^th day compressive strength of the concrete. Therefore, as per IS code, the value for modulus of elasticity for conventional concrete should be 25000 kN/mm². From the experiment, the value obtained for conventional concrete was 25,464 kN/mm², which shows the adaptability of the test results. The values of modulus of the elasticity are exhibited in Table 5.

From Fig. 4, it is evident that the strain value of the concrete specimen was decreasing with the increase in the fibre content. From various literature reviews and the experiment conducted, we can say that the fibre resists the deformation of the concrete under loading and prolong the stress value. Therefore, the concrete can yield heavy loading with low deformation, and due to a decrease in the strain value, the value of modulus of elasticity will increase.

Fig. 4: Stress-Strain Behaviour

Flexural Strength
Fibre-reinforced concrete was mainly adopted to improve the ductile and bending behaviour of the concrete. From the test results, it was observed that the addition of mild steel fibre in concrete shows a progressive behaviour in the bending behaviour of concrete. Improvement in the flexural strength of the concrete with an increase in the amount of mild steel fibre was observed. Mild steel fibre added in concrete will take part in load distribution in the concrete beam
and the fibre will support yielding under loading. During testing, all the concrete beam has undergone brittle, but for fibre reinforced concrete, the failure load was increasing with an increasing percentage of mild steel fibre. The value of the flexural test of fibre reinforced concrete is shown in Table 6 below.

From the results, it is evident that an increase in the amount of steel fibre has a positive impact on flexural behaviour. The change in strength is observed from the 7^th day onwards.

Water Absorption
The water absorption test was conducted under the study of the durability of concrete. The amount of water undergone absorption depends on the porosity of the concrete specimen. Thereby, an idea regarding the porosity in the concrete can be made. The addition of mild steel fibre helps to reduce the porosity in concrete by filling the voids and by providing strong bonding in concrete. From the test results, it was observed that the amount of water absorption was reduced with the increase in the amount of fibre. Table 7 shows the results, which reflect the decrease in the percentage of water absorption.

Sorptivity Test
It was observed that the rate of water absorption was decreased with an increase in fibre content. The value shows good agreement with the water absorption value because the value of water absorption was also decreasing with an increase in fibre content. The results from the sorptivity test are shown in Table 8.

It was observed that the rate of water absorption was increasing with time, but it was decreasing with increase in fibre content.
The maximum rate of absorption was obtained for conventional concrete (4.18X10⁴ mm/min^0.5 at 60 Min), and the minimum value was obtained for concrete with 2% fibre (2.93X10⁴ mm/min^0.5 at 60 Min).

Ultra-Sonic Pulse Velocity (UPV)
The UPV test was conducted, and the average values are recorded in Table 9. As per Indian standards, the values greater than 4 km/sec come under excellent quality concrete.

Both lathe waste reinforced concrete and conventional concrete come under good quality concrete, and the value of UPV was observed to be increasing with an increase in fibre content

Rebound Hammer Test
As per Indian standards, for very good and hard layer concrete, the value of the rebound hammer number should be greater than 40. Here, all the specimen have a value greater than 40, so all specimen comes are of good quality. The average value of all the specimens is shown in Table 10.

Conclusion
From the experimental study, the results indicate that:

From the slump test, it was observed that the addition the mild steel fibre increases the confining behaviour between the fibre and concrete materials, leading to a decrease in the value of slump height. For 2% fibre addition, the value of slump height was dropped to 54mm from 64mm at 0% addition.

The value of compressive strength was observed to be increasing with the increase in fibre content. For 2% fibre addition, the value of compressive strength obtained was 42 MPa, whereas, for conventional concrete, the compressive strength was 33.1MPa.

Remarkable improvement in the split tensile strength was observed in the fibre reinforced concrete. Due to the internal bonding provided by the fibre, the width of the failure crack was decreasing with an increase in the fibre content. The maximum value was obtained at 2% of fibre addition was 3.53 MPa.

Flexural strength was also improved with an increase in fibre content. The addition of the fibre resists the propagation of the crack and prolongs the failure load of the beam. The maximum value was obtained at 2% was 5.55 MPa.

The effect on modulus of elasticity of concrete was also observed to be increasing with the increase in the fibre content. From the test, it was observed that the fibre reinforced concrete could resist more stress with less strain value. At 2% fibre addition, the value obtained for modulus of elasticity was 42,441 MPa, and for convention concrete, the value was 25,464 MPa.

Due to the internal bonding provided by the fibre, the pore space inside the concrete was reduced. Therefore, from the test results, the value of water absorption was decreasing with the increase in fibre content. The rate of absorption measured from the sorptivity test shows a good correlation with this.

The sorptivity test results show that the rate of absorption was decreasing with the increase in fibre content.

From the NDT test, it was observed that the casted specimen was of good quality.

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Concrete mix proportions are selected for the desired workability and the target 28-day compressive strength of concrete. The mix proportions should be economical and at the same time, the concrete should satisfy the durability requirements, for the exposure condition, at which the structures shall provide the service. The Indian Standard Code of practice for plain and reinforced concrete provides the minimum cement content and maximum water-cement ratio in concrete, for different exposure conditions. The Code also recommends at least 25% fly ash or at least 50% Ground Granulated Blast furnace Slag (GGBS) as part replacement of low alkali ordinary Portland cement, to combat the deleterious alkali-silica reaction (if any), in concrete. In mass concrete foundation structures, (OPC + fly ash) or (OPC + GGBS) is required to be used in concrete, to avoid any temperature- related ill effects in concrete.

With the above-mentioned considerations, Indian Standard Guidelines are followed to select the concrete mix proportions and the quantities of cement, water, aggregates, admixture etc. per cubic metre of concrete are calculated. In this paper, details on different relationships as given in the guidelines, between water-cement ratio/water-cementitious materials ratio and 28-day compressive strength/flexural strength of concrete, those between workability of concrete and mixing water content, absolute volumes of coarse and fine aggregates for different Maximum Size of Aggregate (MSA), and finally their quantities per cubic metre of concrete, as required for different types of concrete have been covered. The types of concrete considered are: medium and high strength concrete, self-compacting concrete, mass concrete, roller-compacted concrete, fiber-reinforced concrete and pavement concrete.

Concrete Mix Proportions For Medium Strength And For High Strength Concrete
For estimating the target 28-day compressive strength of concrete, IS 10262¹ provides standard deviation values to be assumed, when sufficient test results are not available for a particular grade of concrete. As soon as 30 test results are available, actual standard deviation values shall be used for the mix proportions. The suggested standard deviation values are shown in Table 1.

Note 1: The above values correspond to the site control having proper storage of cement, weigh batching of all materials, controlled addition of water, regular checking of all materials, aggregate gradings and moisture content and regular checking of workability and strength. Where there is deviation from the above, the values given in the above table shall be increased by 1N/mm2.

Note 2: For grades M65 and above, the standard deviation may be established by actual trial, before finalizing the concrete mix.

The water-cement (w/c) ratio for the target 28 day strength is selected from an established relationship between the two. In the absence of such data, Figure 1 may be followed to start with.

Curves 1, 2 and 3 of Fig. 1 are for 28 day compressive strength of cement (33-grade, 43-grade and 53-grade level). While using Portland Pozzolana Cement (PPC) and Portland Slag Cement (PSC), the appropriate curve as per actual strength may be used. In the absence of test results of ppc and psc, curve 2 may be used to start with.

Fig.1: Relationship Between Water-Cement Ratio and 28-Day Compressive Strength of Concrete

For high strength concrete of M65 to M80, IS 10262 provides the following relationship, as given in Table 2.

Note: The W/Cem. ratios in Table 2 are for 28-day cement – strength of 53 MPa and above. For cement of other strengths, suitable adjustment may be made by reducing W/Cem. ratio. The mixing water contents of concrete for a slump of 50mm as given in IS10262 are shown in Table 3.

The entrapped air contents of concrete are also slightly different in the two types of concrete (Table 4). In high strength concrete mixes, the entrapped air content is expected to be less.

Note: It is suggested that the above values are approximate and the actual air content of concrete may be measured at the sites of construction and used in the mix design calculations.

Once the W/c ratio and mixing water content decided, the cement content of concrete is calculated. If mineral admixture e.g. fly ash, Ground Granulated Blast furnace Slag (GGBS) or silica fume is used, their percentage by weight of total cementitious materials can be based on project requirement. For example, minimum 25% fly ash or minimum 50% GGBS for hydro-electric project is required to combat the deleterious alkali-silica reaction in concrete. Silica fume, 5-10% can be used for abrasion resistance and to increase the strength of concrete. The silica fume is used as part replacement of cement and its proportion is 5-10% of the cement content of the mix, as per IS 456².

When fly ash is 20% or more and GGBS is 30% or more, to get the equal 28-day strength of concrete, the total cementitious material content is to be increased by 10%. Generally GGBS content is much higher these days, may be more than 50%. In coastal environment for concrete piles, 70% GGBS is required because of aggressive environment. In such case the increase in total cementitious materials content may have to be increased by more than 10%. This can be fixed based on experience and trials.

The cementitious material content so calculated and the w/c ratio or w/Cem ratio decided, shall be checked against the durability requirement as per IS 456, as shown in Table 5.

Note: The cement contents are irrespective of the grade and type of cement and is inclusive of additions i.e. mineral admixtures.

Adjustments
The following adjustments may be made to the minimum cement content for aggregates other than 20mm MSA.

The chemical admixture content may also be fixed by trials, to satisfy the workability requirement of concrete. The admixture must be compatible with the cement. If not, there will be segregation and bleeding of concrete. Next, the absolute volumes of these materials i.e. of cement, fly ash/GGBS/ silica fume, water and chemical admixture are calculated using their specific gravity values. The entrapped air content is % of volume of concrete as per Table 4. The remaining volume in a cubic metre of concrete is of (coarse aggregate + fine aggregate).

The absolute volume of coarse aggregate is decided based on MSA, and grading of fine aggregates (Table 6). These values are for the W/c or W/cem ratio of 0.50. For other values as decided based on target strength, the volumes need to be adjusted. The adjustments are as follows.

For every 0.05 decrease in w/c ratio or w/cem. ratio, the volume of coarse aggregate is to be increased by 0.01m³ and vice versa.

Notes:

1. Volumes of aggregates are based on aggregates in saturated surface dry condition.
2. Volumes are based on crushed rock angular aggregate.
3. Suitable adjustments are required for aggregates of other shapes e.g. rounded or sub-angular aggregates.
4. IS 383³ states that, fine aggregate conforming to grading zone IV shall not be used in reinforced concrete, unless tests have been made to ascertain the suitability of the proposed mix proportions.
5. Crushed stone fine aggregate or mixed sand (natural sand + crushed stone) may need lesser fine aggregate content and therefore in that case, volume of coarse aggregate shall be suitably increased.

The fine aggregate for medium strength concrete is generally in the range of 28-40% for different MSA and are based on the grading of fine aggregate. Typically, for 20mm MSA, zone II grading, the fine aggregate content is 33%. For 40mm MSA, it is 28% and for 10mm MSA, it is 40%. IS456 states, ‘the proportion of fine aggregate should be adjusted from upper limit to lower limit progressively, as the grading of fine aggregate becomes finer and the maximum size of coarse aggregate becomes larger. Graded coarse aggregates shall be used.’ For coarse fine aggregate of zone I grading, the quantity of fine aggregate is to be increased by about 1.5%, and for finer grading, say zone III grading, the quantity of fine aggregate is to be reduced by about 1.5%, and for zone IV grading, the quantity of fine aggregate is to be reduced by about 3%. For rounded coarse aggregate, the quantity of fine aggregate is to be reduced by about 7%. For high workability concrete or pumpable concrete, however, the quantity of fine aggregate shall be 40% and higher, irrespective of the grading of fine aggregate. IS 383 permits crushed stone fine aggregate and mixed sand i.e. blend of natural sand and crushed stone fine aggregate to be used in concrete. With the above considerations and using table 6, the absolute volumes of coarse aggregate and fine aggregate are decided, and their quantities per cubic metre of concrete are calculated, using their specific gravity values, and trial mixes can be conducted.

Trial Mixes
The concrete mix proportions thus fixed for a cubic metre of concrete, are for Saturated Surface Dry (SSD) aggregates. (i) For dry aggregates, the quantity of mixing water shall be increased by the amount required for their absorption. Coarse aggregate may absorb about 0.5% water, and fine aggregate may absorb about 1% water. (ii) In case of wet aggregates, the quantity of mixing water is to be reduced by the amount of extra water available in coarse and fine aggregates, other than their absorption. In both the cases i.e. (i) and (ii), the quantity of aggregates (both coarse and fine aggregates) are to be adjusted properly.

For the first trial mix of 0.05m³ concrete, the workability shall be measured. There should not be any segregation or bleeding. That may be because of incompatibility of super plasticizer with the cement. Sometimes, concrete mix may not be cohesive and may need higher fine aggregate content. If the workability is not achieved, the mix needs to be adjusted with more water or more chemical admixture. With the modifications in the mix, the mix proportions shall be recalculated, keeping w/c ratio or w/cem ratio at the preselected value, to obtain the desired 28-day compressive strength of concrete. This will comprise Trial mix no. 1. Two more mix proportions with + 10% of the w/c ratio or w/cem. ratio shall be worked out, and the three concrete mixes are to be made simultaneously, one after another, and workability measured. If the desired workability is achieved, concrete cubes can be cast for 7-day and 28-day compressive strength concrete. With the lower w/c ratio or lower w/cem ratio, if the workability of concrete is less, the chemical admixture content may have to be increased marginally. with the test results of 28-day compressive strength of concrete of the three trial mixes, a relationship between w/c ratio or w/cem ratio and 28-day compressive strength of concrete can be plotted and the right w/c ratio or w/cem. ratio can be estimated for the target 28-day compressive strength of concrete. The finalized concrete mix proportions can be recommended for field trials.

For obtaining the recommendation on concrete mix proportions early, the accelerated strength testing can be carried out by ‘boiling water method’ as per IS 9013⁴. In this method, 3 cubes from each trial mix i.e. total 9 cubes (along with their moulds) are to be normally cured in the laboratory for 23 hours under wet gunny bags and with cover plates fixed on them, to be placed in boiling water in a steel tank for 3½ hours. Next the cubes are removed from boiling water, cooled for 2 hours in the laboratory, and then de-moulded and tested for compressive strength. From the average accelerated strength of the three concrete mixes, the approximate 28-day compressive strength of concrete can be estimated from a relationship established beforehand. A typical correlation is shown in Fig. 2.

Fig. 2

Self-Compacting Concrete
The Self-Compacting Concrete (SCC) will have high workability, the workability measured by ‘slump flow test’, the slump flow⁵ being in the range of 550-850mm. the Maximum Size of Aggregate (MSA) can be 20mm. A smoother aggregate is preferred e.g. gravel aggregate or crushed gravel aggregate. The fine aggregate content will be higher in the range of 48-60%. The super plasticizer (polycarboxylate ether based) and a Viscosity Modifying Admixture (VMA) have to be used. The powder content (<0.25 mm size) of the mix will be in the range of 400-600 Kg/m³ of concrete. The water content is generally higher, 150-210kg/m³ of concrete. Fly ash content specified is 25-50% and GGBS content specified is 50-70%. The water-powder ratio will be 0.85 to 1.10 by volume. Typically for a M30 grade concrete for a high workability of 760-850mm slump flow, quantity of materials per cubic meter of concrete1 are : opc (43 grade) = 287kg, fly ash=155kg(35%), water = 190kg, super plasticizer = 0.6%, VMA=0.2%, water- powder ratio (by volume) = 0.99.

For low volume rural roads, IRC:SP:626 provides guidelines on self- compacting concrete, and suggests typical range of S.C.C. mix composition for M30 to M40 grades of concrete (Table 7). For village roads, the Guidelines suggests slump flow of 400mm and V-Funnel flow time⁵ of maximum 8 seconds.

Mass Concrete
Mass concrete generally refers to massive structures, e.g. foundations of bridge piers or high-rise building columns or concrete dams. In mass concrete, temperature of concrete due to heat of hydration of cement has to be lower. If the temperature difference within the mass of concrete is more than 20⁰C, cracks may develop in concrete.

It is essential therefore to use 33-grade opc or Portland Pozzolana Cement or Portland Slag Cement, or (opc + fly ash) or (opc + GGBS) in mass concrete. Generally, lower grade of concrete is used in mass concrete. In foundation structures, grades of concrete are: M20 to M40. In concrete dams, the concrete grade is generally M10. Quantity of mineral admixtures are not fixed, but generally fly ash content can be about 20-30% and GGBS content can be about 50%. For resisting the deleterious alkali-silica reaction in hydro-electric project structures, the minimum fly ash content specified is 25% and minimum GGBS specified is 50%. In foundation structures, 40mm MSA should be used, and in concrete dams, the MSA can be 75mm/80mm or 150mm. IS 10262 provides the mix design procedure, considering mixing water content for different MSA, water-cement ratio Vs 28-day compressive strength of concrete (Fig.1) and air content similar to those of normal mix design procedure.

But the target 28-day compressive strength calculated shall be increased by 20% for concrete with 75/80 mm MSA, and 25% for concrete with 150mm MSA. This is to account for higher strength achieved after wet-screening the fresh concrete through 40mm sieve, for casting 150mm size cubes. This increase in strength is because, after wet screening of concrete, the mix becomes richer in cement content. A special requirement of mortar content of mass concrete has been specified (Table 8) based on American practice.

Roller – Compacted Concrete
The mix proportioning method for zero-slump roller- compacted concrete differs from the conventional concrete in that the moisture content of the concrete should be dry enough to support the weight of the vibratory roller and yet a cohesive concrete mix. Instead of fixing the water-cement ratio, the water content of the mix is fixed in the range of 4-7% by weight of total dry material. The optimum moisture content which gives maximum density shall be determined. For pavement concrete, the MSA is 16.5mm, whereas for concrete dams, the MSA can be 50 or 75mm. IRC: SP: 687 suggests fly ash content up to 35% and GGBS content up to 50%. In roller- compacted concrete dams, the fly ash content of 65% has been used. For pavement, the characteristic strength is 35 MPa for State and National Highways, whereas for rural road, it is 30 MPa. The corresponding flexural strength shall be 4 MPa and 3.8 MPa respectively.

Since, roller-compacted concrete dams are of low grade concrete, say M10, the cement content of concrete is considerably lower, than that of pavement concrete of M30 or M35. With 75mm MSA, Willow Creek dam concrete contained cementitious material of 66.5 kg/m³. The mixture containing 47.5 kg/m³ of cement plus 19.0kg/m³ of fly ash, developed compressive strength of 18.2 MPa at 1 year. Generally, roller- compacted concrete dams containing cementitious material content between 104 and 178 kg/m³ produced an average compressive strength of 13.8 to 20.7 MPa at the age of 90 days to 1 year⁸.

Fiber Reinforced Concrete
The Fiber-Reinforced Concrete (FRC) is generally used for impact and abrasion resistance, especially for concrete pavement or for factory floors or in defence applications, where blast resistance is required for buildings, defence installations, air field pavements and bridges. Steel or polypropylene fibers can be used. For pavements with steel or polymeric fibers, usually fiber reinforced concrete having characteristic of flexural strengths of 5 to 8MPa may be used. The maximum size of aggregate is generally of 20mm. The fiber dose is percentage by volume of concrete. The polypropylene fiber of 32 µm size at about 1.5 kg/m³ of concrete can be used in high strength concrete. The steel fiber 0.5 to 1.0% by volume of concrete (about 40-80kg/m³), crimped, hooked ends or trough shaped, dia. 0.5-1.0mm, and aspect ratio 50-100. The polypropylene fibers 0.50 to 2.0% by volume of concrete (about 4.5-18 kg/m³ of concrete), can be used. The specific gravity of steel is taken as 7.85 and that of polypropylene is taken as 0.91. Normally fibers of 20mm length give good performance. Fiber-reinforced concrete controls shrinkage-cracking, and is resistant to drying shrinkage. For resisting cracks in early hours (1 to 8 hours), polymeric micro fibers of 0.1 to 0.2% by volume i.e. about 0.90 kg/m³ to 1.8 kg/m³ of concrete has been used with success9. Fiber-reinforced concrete has been used to provide durable concrete pavements and bridge decks, with improved crack-resistance and reduced slab thickness.

In 1981-’82, R&D work was carried out on steel fiber reinforced concrete, on the development of different shapes of steel fibers, their aspect ratio, concrete mix design and applications in air field pavements¹⁰. Out of different shapes (straight, crimped, hooked and trough), trough shaped fibers were found to be most efficient in the development of flexural strength and toughness of concrete. On the mix design front, it was observed that, about 15% more volume of paste (i.e. cement + water) was required than that of conventional plain concrete. This is because, the fibers need more paste to coat them properly. The fine aggregate content is about 50%, and the aspect ratio of fibers of 80 gave maximum increase in the flexural strength of concrete. The conventional concrete mix with 40mm MSA was designed for 3.8MPa flexural strength. The mix had a cement content of 330 kg/m³, with a water-cement ratio of 0.48. For fiber-reinforced concrete, MSA of 20mm chosen, the cement content was 410 kg/m³ and a water-cement ratio of 0.65. ‘Trough’ shaped steel fibers of 106 kg/m³ (which is about 1.4% by volume of concrete), with dia. of 0.45mm was used. The concrete which developed a flexural strength of 7.0-8.0 N/mm², was laid in the taxi tracks of the Indira Gandhi International airport, New Delhi. IRC: 1511 permits use of fiber- reinforced concrete to reduce shrinkage – cracking and to improve post-cracking residual strength of concrete pavements. For fibers and other details, the Code gives reference to IRC: SP: 469.

According to the guidelines of IRC: SP: 46, steel fibers should have an ultimate tensile strength of at least 800 MPa. Fibers can be straight or deformed. Fibers can be supplied loose or collated (i.e. glued with a water-soluble adhesive that dissolves during the mixing of concrete). Collated fibers have a lower tendency of balling. Sometimes, steel fibers are supplied with zinc coating. As a guide, for improved performance, steel fibers with hooked ends and having length of 50 to 60mm may be used. The polymeric fibers with low elastic modules are normally used to control plastic shrinkage cracking. Macro polymeric fibers of 30 to 60mm length of higher elastic modules, can increase the toughness and strength of FRC pavements. Macro fibers have diameter more than 0.2mm. Micro fibers are 12 to 40mm length and have diameter less than 0.2mm. Table 9 shows range of proportions of FRC for pavement applications, and the guidance is for an initial trial mix.

Pavement Concrete
Pavement concrete (for National and State highways) in generally of M40 grade. The corresponding flexural strength is 4.5 N/mm². Rural low-volume roads are of M30 grade concrete. For National and State highways, the workability of concrete shall be 20 to 30mm slump, for concrete laid with slip form paver and 40 to 60mm for concrete laid with fixed-form paver¹¹. For rural roads, a slump of 30 to 50mm has been suggested at the paving site for compaction by hand-operated machines⁶. The Guidelines for pavement with low-volume roads IRC: SP: 62 also suggests zero-slump roller compacted concrete, and high-workability self-compacting concrete, with super plasticizers and mineral admixtures e.g. fly ash, silica fume, rice husk ash, metakaoline and ground granulated blast furnace stag.

The pavement concrete shall contain both chemical admixtures and mineral admixtures. The maximum quantity of chemical admixture shall be 2% by weight of the cementitious materials (cement + fly ash/GGBS/silica fume). Fly ash up to 25% by weight of cementitious materials, and shall conform to IS 3812 (Part 1)¹². The GGBS shall conform to IS: 6714¹³ and up to 50% by weight of cementitious materials can be used with 43 grade or 53 grade OPC. The silica fume up to 10% by weight of OPC can be used, if specified in design for abrasion resistance. the Metakaoline of fineness of 700 to 900 m²/kg can be used up to 20%.

The MSA for pavement concrete shall be 31.5mm and the combined grading (of coarse aggregate + fine aggregate) for 31.5mm, 26.5mm and 19mm maximum sizes have been specified in IRC:1511, as well as in IRC:4414. IRC: 44 gives concrete mix design procedure based on 28-day compressive strength, as well as based on 28-day flexural strength. The suggested relationships (in the absence of an established relationship for the materials in hand) are shown in Table 10 and Table 11 respectively.

The approximate mixing water contents for different maximum sizes of aggregate are shown in Table 12.

The Volume of coarse aggregate per unit volume of total aggregate for different MSA and different grading zones of fine aggregate is shown in Table 13.

The quantities of different constituent materials per cubic metre of concrete are calculated by absolute volume method, considering the air content of concrete as shown in Table 14.

Concluding Remarks
The details on concrete mix proportions for different types of concrete have been highlighted. They are mostly based on Indian Standard recommended guidelines. The values of standard deviation suggested for high strength concrete of M65 and above are 6.0 N/mm². This is on the higher side, as lower values are expected in practice, with better quality control in high-strength concrete. The entrapped air in concrete, as suggested in IS 10262 and IRC 44, for different MSA are also approximate. The values should be determined at the sites of construction, and actual values should be used in mix design calculations. The relationships (given in figure and tables) between water-cement ratio/ water-cementitious materials ratio Vs 28-day compressive strength/28-day flexural strength of concrete (for pavement concrete) are tentative and approximate. Therefore, if established relationships are available, they should be used for mix design.

For self-compacting concrete, the procedure of mix selection is based on European guidelines. The fly ash content (25-50%) and GGBS content (50-70%) as suggested in IS 10262 are on the higher side, and may not be correct. Such high mineral admixture content will reduce the strength of concrete. The mass concrete mix proportioning method suggested is based on American practice, where mortar content for different MSA and two types of aggregate (crushed rock and rounded gravel) have also been suggested. The zero-slump roller-compacted concrete mix proportioning method as suggested in IRC: SP: 68 for pavement concrete is much different. In this case, the water content is based on percentage of total dry material, including fly ash (up to 25%) or GGBS (up to 50%). But in roller-compacted concrete dams, the fly ash content of 50-65% is generally used.

The fiber reinforced concrete is basically for impact and abrasion resistance and for crack resistance. The fiber content is expressed as % by volume of concrete. The compressive strength of concrete increases marginally but the flexural strength increases by about 80-100% than that of plain concrete without fibers. The R&D work carried out at the Cement Research Institute of India in 1982 provides the details on the mix proportions of steel fiber reinforced concrete and its pavement –applications. But those days, super plasticizers were not available. So, concrete mix proportions suggested will undergo some changes, for fiber reinforced concrete with super plasticizers.

The pavement concrete mix proportions are marginally different from the mix proportions for structural concrete. This is mainly because in pavement, flexural strength is the main consideration. Different MSA (31.5mm, 26.5mm, 19.0mm and 9.5mm) are used in concrete pavements. The 33-grade opc and fine aggregate of grading zone IV are excluded. Although IRC 44 provides mix design procedures based on 28-day flexural strength and also based on 28-day compressive strength of concrete, it is always preferable to follow the 28-day compressive strength, to choose the mix proportions and then we can have correlation made between 28-day compressive strength and 28-day flexural strength on 30 specimens, on the materials in hand. This is because, the test method for flexural strength is not consistent, and can vary considerably.

References

1. IS 10262 Indian Standard Concrete Mix Proportioning – Guidelines. Bureau of Indian Standards, New Delhi, 2019.
2. IS-456 Indian Standard Code of Practice for plain and Reinforced Concrete, 2000 (with Amendments, 1,2,3,4 and 5). Bureau of Indian Standards, New Delhi.
3. IS 383 Indian Standard Specification for Corse and Fine aggregate for Concrete. Bureau of Indian Standards, New Delhi, 2016.
4. IS 9013 Indian Standard Method of Making, Curing and Determining Compressive Strength of Accelerated Cured Concrete Test specimens. Bureau of Indian Standards, New Delhi, 1978.
5. IS 1199 (Part 6) Fresh Concrete – Methods of Sampling, Testing and Analysis. Part-6: Tests on Fresh Self Compacting Concrete. Bureau of Indian Standards, New Delhi, 2018.
6. IRC: SP: 62 Guidelines for Design and Construction of Cement Concrete pavements for Low Volume Roads. Indian Roads Congress, New Delhi, 2014.
7. IRC: SP: 68 Guidelines for Construction of Roller Compacted Concrete Pavements. Indian Roads Congress, New Delhi, 2020.
8. Sapre, Sunil, Shivgunde, Someshekar And Kapadia, Himanshu. Roller Compacted Concrete, In Handbook On Advanced Concrete Technology, Narosa Publishing House, New Delhi, 2012, Pp.27.1 To 27.14.
9. IRC: SP: 46 Guidelines for Design and Construction of Fiber Reinforced Concrete Pavements. Indian Roads Congress, New Delhi, 2013.
10. CRI. Development of Steel fiber reinforced concrete RB-21-82. Cement Research Institute of India, New Delhi, January 1982, 35p.
11. IRC: 15 Code of Practice for Construction of Jointed Plain Concrete Pavements. Indian Roads Congress, New Delhi, 2017.
12. IS 3812 (Part 1) Indian Standard Specification for pulverized fuel Ash, for use as Pozzolana in Cement, Cement Mortar and Concrete. Bureau of Indian Standards, New Delhi, 2013.
13. IS 16714 Indian Standard Specification for Ground Granulated Blast Furnace Slag for Use in Cement, Mortar and Concrete. Bureau of Indian Standards, New Delhi, 2018.
14. IRC: 44 Guidelines for Cement Concrete Mix Design for Pavements. Indian Roads Congress, New Delhi, 2017.

Concrete is not an elastic material, that is, it will not recover its original shape on unloading. In addition, the stress-strain curve of concrete is non-linear. Hence, modulus of elasticity and Poisson’s ratio, which are elastic constants, are not applicable. However, for the sake of simplicity, they are used in the analysis and design of concrete structures, assuming elastic behavior. The modulus of elasticity of concrete is required for the estimation of the deformation of buildings and members. In addition, it is used for determining the modular ratio,m. High-strength concrete (HSC) will have a higher modulus of elasticity and hence will result in reduced deflection and increased tensile strength.

The modulus of elasticity is dependent on the compressive strength of concrete, properties of the coarse aggregates, the proportion of the aggregates in the concrete, quality of cement paste and addition of mineral admixtures (Zhang and Gjvorv, 1991, Neville, 1996). Modulus of elasticity, however, is affected to a lesser extent by the chemical and mineral admixtures, curing conditions, age of the concrete and the type of cement (Russian Standard SP 52-101:2003). The fine and coarse aggregates generally occupy 60% to 75% of the volume of concrete (70% to 85% by mass) and are stiffer than the concrete paste (Neville, 1996). Hence, their E-value will have a significant effect on the E-value of concrete. The use of dense aggregates such as basalt than limestone, which in turn results in a higher modulus than lightweight aggregates. Specifying the largest practical maximum size of aggregate and a suitable grading may result in higher content of coarse aggregate in a concrete mixture. Such concretes tend to have a higher modulus of elasticity, provided the aggregates used have a high modulus of elasticity (Crouch et al., 2007). However, increasing the coarse aggregate size may result in reduced strength in high-strength concrete mixtures. Increasing the paste content may decrease the void content of concrete, and hence may increase the modulus of elasticity. Increasing the water-cement ratio will reduce the value of modulus of elasticity, similar to its effect on the compressive strength of concrete. A high modulus of elasticity is associated with a higher compressive strength of concrete, although the two are not directly proportional. For example, to increase the modulus of elasticity by 20% it may be necessary to increase the strength by 50%. Aïtcin, 2011 observes that the modulus of elasticity of concrete is as important as the water-cement ratio of the concrete mixture. Fig. 1 shows the various factors that may affect the modulus of elasticity of concrete.

Modulus of elasticity may be determined using an extensometer attached to the compression test specimen as described in IS 516:1959 or ASTM-C469M-14 (Subramanian, 2019). The test set-up for measuring the modulus of elasticity is shown in Fig. 2. 

Fig. 1: Factors Affecting Modulus of Concrete

Accurate prediction of modulus of elasticity is important in reinforced and pre-stressed concrete structures while calculating member deformations, elastic shortening of columns, shrinkage and creep loss as well as crack width. Note that restricting the crack width is directly related to the durability of concrete structures. The modulus of elasticity is also required in seismic analysis for rational calculation of drift and deformations.

Modulus Of Elasticity Of Concrete

The modulus of elasticity of concrete is defined as the ratio of normal stress to corresponding strain for tensile or compressive stresses below the proportional limit of the material. When the loading is of low intensity and short duration, the initial portion of the stress-strain curve of concrete in compression is linear, justifying the use of modulus of elasticity. When there is sustained load, however, the stress-strain curve will become nonlinear, even at relatively low stresses, due to inelastic creep. Moreover, the effects of creep and shrinkage will make the concrete behave in a non-linear manner. Hence, the initial tangent modulus is considered to be a measure of the dynamic modulus of elasticity (Neville and Brooks 2010). When the linear elastic analysis is used, one should use the static modulus of elasticity. Various definitions of modulus of elasticity are available: initial tangent modulus, tangent modulus (at a specified stress level), and secant modulus (at a specified stress level), as shown in Fig. 3. Among these, the secant modulus, which is the slope of a line drawn from the origin to the point on the stress-strain curve corresponding to 40% of the failure stress, is considered the average value of Ec under service load conditions (Neville and Brooks 2010).

Fig. 2: Test set-up for Measuring the Modulus of Elasticity of Concrete

Expressions Suggested By Different Codes

Different national codes suggest different expressions for the determination of the modulus of elasticity of concrete, to be used in the design. These expressions are given below. 

For normal-weight concrete, Clause 19.2.2.1 of ACI 318M-19 code allows it to be taken as

Where is the cylinder compressive strength of concrete in MPa.

Clause 8.6.2.3 of the Canadian code CSA A23.3-14 gives a similar expression, for normal density concrete with a compressive strength between 20 and 40 MPa as

Clause 6.2.3.1 of IS 456: 2000 suggests that the short-term static modulus of elasticity of concrete, Ec, may be taken as

Where is the cube compressive strength of concrete in MPa.

Both IS 456 and ACI 318 caution that the actual measured values may differ by about 20 % from the values obtained from Eq. (1). Moreover, the US code value is 16% less than the value specified by the Indian code. It has to be noted that the use of a lower value of E_c will result in a conservative (higher) estimate of the short-term elastic deflection.

For both normal-strength (NSC) and high-strength (HSC) concrete, the Comité Euro-International du Béton and the Fédération Internationale de la Précontrainte (CEB-FIP) Model Code and Euro code 2 suggest that the approximate value of secant modulus Ecm of concrete with quartzite aggregates can be obtained from the mean compressive strength as below

Where  =  + 8 MPa, and  is the cylinder compressive strength of concrete The coefficient α present only in the CEB-FIP Model code has a value of 1.2 for basalt and dense limestone, 1.0 for quartzite, 0.9 for limestone, and 0.7 for sandstone aggregates. When lightweight aggregates are used, the CEB-FIP equation was found to overestimates the modulus, and the calculated values decreased when coarse aggregate such as crushed quartzite, crushed limestone, and calcined bauxite was used (Vakhshouri and Nejadi, 2019).

Fig. 3: Various Definitions of Modulus of Elasticity of Concrete

Effect Of Unit Weight Of Concrete On The Modulus Of Elasticity

There is an increased awareness and use of lightweight concrete (LWC) in applications like elevated slab structures; bridge decks; wall, ceiling, and floor insulation; and insulation for fire protection. LWC normally has an in-place density of 800 to 2240 kg/m3. It is traditionally produced using lightweight aggregates such as expanded shale or clay, vermiculite, pumice, or scoria; however, it can be also produced using foaming technologies and polystyrene beads. Only a few codes provide formulae for the modulus of elasticity considering the density of concrete.

Clause 19.2.2.1 of ACI 318:2019 and also the AASHTO-LFRD-2006, provide the following formula for the modulus of elasticity, considering the density (unit weight) of concrete

where ρ_c is the unit weight of concrete (varies between 1440 kg/m³ and 2560 kg/m³).

As per clause 8.6.2.2 of the Canadian code, the modulus of elasticity, E_c for concrete with ρ_c between 1500 and 2500 kg/m³ may be taken as

where ρc is the unit weight of concrete.

The Australian code AS 3600:2018, clause 3.1.2 specifies that the modulus of elasticity be taken as below noting that this value may have a range of ± 20%

Where,  is the mean value of the in-situ compressive strength of concrete at the relevant age.

Table 3.1.2 of AS 3600:2018 gives the values of Ec calculated as per Eqn. (7) and is shown in Table 1.

The Architectural Institute of Japan specifies the following equation to estimate the modulus of elasticity of concrete

8.

As per BS 8110-2, 1985, the elastic modulus is related to its compressive strength as below

The equation for elastic modulus in the Russian SP 52-101-2003 has a different format as shown below:

High Strength Concrete

High-strength concrete (HSC) is often used in the columns of high-rise buildings, long-span bridges, parking garages, and offshore structures, where improved density, lower permeability, and increased resistance to freeze-thaw and corrosion are required. In these applications, designers can take full advantage of the increased compressive strength of HSC to reduce the amount of steel, reduce column size (to increase usable floor space in high-rise buildings), or allow additional stories. These benefits overshadow the higher cost of raw materials and increased quality control costs involved with HSC.

The ACI committee report on HSC (ACI 363R-92) provides the following equation for modulus of elasticity, which has also been adopted by NZS 3101- Part 1:2006.

The results of the above equation and also Eq. (1) of ACI 318-19, with the experimental values of several researchers, are compared in Fig. 4. It is seen that the ACI 318-19 expression overestimates the modulus of elasticity for concretes with compressive strengths over 41 MPa, and the Eqn. (11) provides better correlation, especially for high-strength concrete.

Fig.4: Modulus of Elasticity Versus Concrete Strength

Clause 3.1.2 of the Australian code AS 3600:2018, suggest the following equation, noting that this value may have a range of ± 20%

The Japanese code JSCE (2007) Clause 4.1.2 [equation C4.1.3] gives the following equation,

Where Ec(t) is the effective Young’s modulus at the age of t days; ф(t) is the compensating factor taking account of creep during concrete temperature increasing for up to 3 days ф =0.73, for after 5 days ф =1.0 (linear interpolation can be used from 3 to 5 days) and   (t) is the estimated compressive strength of concrete at t days.

The equations given in the various codes are simple to use because they require only the compressive strength and the concrete density to determine the value of the elastic modulus of concrete. Vakhshouri and Nejadi, 2019 have collated and presented the various other empirical models suggested by other researchers to predict the elastic modulus of normal strength concrete.

Swamy, 1985 showed that the elastic modulus for high strength concrete did not increase in proportion to its strength and the maximum value of modulus will be in the range of 45 to 50 GPa only.

Effect of Different Types of Aggregates of the Elastic Modulus

The equations discussed till now may not be accurate for concrete with all types of aggregates. It is because the elastic modulus and the compressive strength are influenced by several other factors, such as the type of aggregate, humidity, age of the concrete, and the type of binder used. As per Eurocode 2, the value of elastic modulus Ecm as computed by using equation (4), should be reduced for concrete with limestone and sandstone aggregates by 10% and 30% respectively. For basalt aggregates, the value should be increased by 20%. Though not mentioned in the Eurocode 2, equation (4) for concrete with quartzite aggregates is also valid for concrete with siliceous aggregates (Bamforth, et al., 2008). The value of the elastic modulus, for various values of cylinder compressive strength  and for different aggregates, as per Eurocode 2, is shown in Fig. 5.

Fig. 5: Modulus of Elasticity of Concrete with Different Aggregates related to the Compressive Strength of Concrete

Only when very high strength concrete is used, the type of aggregate will be known to the designer and hence can be used to predict the value of elastic modulus as per Fig.5. In the case of normal strength concrete, the designer will not know the type of aggregate used, until the concrete supplier is selected. Hence they should exercise caution while using the value of elastic modulus as per Fig.5.

Bamforth, et al., 2008 also recommend testing concrete specimens when the elastic modulus is critical for the performance of any structure. They also suggest adopting the following while designing structures:

• Use the mean value of Ecm for serviceability calculations
• Use a partial safety factor of γcE of 2, to get the design value of elastic modulus, Ecd = Ecm /γcE and use it in ultimate limit state calculations
• Use an effective modulus, Ec,eff =Ecm / (1+ф), where where ф is the creep coefficient, to take care of creep in long-term deflection calculations. The value of the creep coefficient, ф, may range between 1 and 3 (See Bamforth, et , 2008).

Noguchi and Tomosawa, 1995 and Noguchi, et al., 2009 proposed the following equation which applies to a wide range of aggregates and mineral admixtures used in concrete.

14. (a). 

where the correction factors k1 and k2 are given in Tables 2 and 3.

Substituting k1 = 1.0 and k2 = 1.0 and simplifying we get .

14(b). 

Fig. 6 shows the variation of strength and modulus of elasticity of concrete, made with different aggregates, calculated as per Eurocode 2 (Bamforth et al., 2008).

The dashed lines in Fig. 6 represent the values calculated by Bamforth et al., 1997 based on aggregate E-value and concrete strength for concrete used in nuclear Power Station Structures.

Fig. 6: Variation of Strength and Modulus of Elasticity of Concrete with Different Aggregates

A comparison of different formulae for the static elastic modulus of concrete is provided in Fig. 7. From Fig. 7 it is also seen that the values calculated as per Euro code 2 expression (Eqn. 4) provide the upper bound and those calculated using Noguchi, et al. (2009) (Eqn. 14) provide the lower bound values. As Eqn. 14 considers a wide range of aggregates and mineral admixtures used in concrete and also gives lower bound values, it is suggested to be included in the future versions of IS 456, instead of Eqn. 3, which considers only the compressive strength of concrete.

Fig. 7: Comparison of Different Formulae for the Static Elastic Modulus of Concrete

Variation Of Modulus Of Elasticity With Time

The value of modulus of elasticity varies with time (Singh et al., 2013). As per Clause 3.1.3(c) of Euro code 2, this time-dependent value of Ec(t) can be determined using the following expression

15(a).

Where Ec(t) and fc(t) are the modulus of elasticity and mean compressive strength ofconcrete at time t and Ec and fc aretherespective values determined at an age of 28 days. The value of fc(t) can be determined using the following expression given in the Euro code 2

15(b).

Where ‘s’ is a coefficient that depends on the type of cement and equals 0.20 for high early strength cement (Class R), 0.25 for normal early strength cement (Class N), and 0.38 for slow early strength cement (Class S).

As the cement class will not be known at the design stage, Bamforth, et al., 2008 recommend assuming Class R and suggest that Class N can be assumed when the quantity of ground-granulated blast-furnace slag (GGBS) exceeds 35% or fly ash exceeds 20% in the cement. Similarly, when GGBS exceeds 65% or fly ash exceeds 35% in the cement, Class S may be assumed. It is important to note the strength gain of cement after 28 days is more dependent on the cement type than the cement strength class. Euro code 2 also warns that Eqn. (15) should not be used retrospectively to justify nonconforming reference strength.

Dynamic Modulus Of Elasticity

The dynamic modulus of elasticity of concrete, Ecd can be determined by the non-destructive electro-dynamic method, by measuring the natural frequency of the fundamental mode of longitudinal vibration of concrete prisms, as described in IS 516:1959. The dynamic modulus of elasticity has to be used in structures subjected to dynamic loading (i.e., impact or earthquake). The value of Ecd is generally 20%, 30%, and 40% higher than the secant modulus for high, medium- and low-strength concretes, respectively (Mehta and Monteiro 2006). More details about the dynamic modulus of elasticity may be found from Popovics, 2008.

Summary And Conclusions

The modulus of elasticity of concrete is required for the estimation of the deformation of buildings and members. The modulus of elasticity is dependent on the compressive strength of concrete, properties of the coarse aggregates, the proportion of the aggregates in the concrete, quality of cement paste, and addition of mineral admixtures. Accurate prediction of modulus of elasticity is important in reinforced and prestressed concrete structures while calculating member deformations, elastic shortening of columns, shrinkage and creep loss as well as crack width. Under normal conditions, the static modulus is specified, which is usually the secant modulus (slope of a line drawn from the origin to the point on the stress-strain curve corresponding to 40 % of the failure stress). The formulae specified in different codes are reviewed. It is seen that several codes consider only the strength of concrete to evaluate the elastic modulus, though a few consider the density of concrete, as well. A few researchers have recommended a formula that considers the effect of a wide range of aggregates and mineral admixtures. It is important for the Indian code, IS 456, to specify a formula, which considers the effect of as many factors as possible, in the evaluation of the elastic modulus of concrete, so that the deformations of structures are predicted accurately.

Acknowledgment

The author wishes to thank Dr. Ahmad Fayeq Ghowsi, Post-Doctoral Fellow at IIT, Delhi, for his help in making the drawings.

References

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	Dr. Narayan V. Nayak Consultant		Siddhesh R. Kamat Mhamai DGM, QCP Alcon Construction (Goa) Pvt. Ltd.
	Vivekanand Laxman Harikantra Manager, QA/QC AFCONS Infrastructure Ltd.		Shridhar A. Behare Associate Professor & Head Department of Civil Engineering, Shreeyash College of Engineering Aurangabad

When considering durability of concrete, chloride attack is the most imminent enemy. In the presence of oxygen and water, chloride attack corrodes the steel reducing the strength of the structure drastically.

Chloride ion (Cl-) is formed when the element chlorine gains an electron or when a compound such as hydrogen chloride is dissolved in water. High concentrations of chloride ions in concrete, due to their electro-chemical nature, break down the passive layer of reinforcing steel, without the need to drop the pH levels. Corrosion takes place as the chloride ions react with steel and the surrounding passive material to produce a chemical process which forms hydrochloric acid. The hydrochloric acid reacts with the steel reinforcement, whereby the volume of steel is increased due to corrosion leading to concrete cracking, spalling and eventually failure of concrete.

There are two main sources of chloride ions, one is from the concrete making materials, and the other from the surrounding environment. The first could come from aggregates, admixtures, and from water used for making concrete and the second comes mainly from the concrete being exposed to marine environment such as sea salt spray, direct seawater wetting, concrete being in contact with soils that are rich with chlorides deposits or it can come from de-icing salts and use of chemicals. It is by the process of diffusion that chloride penetrates the concrete. The main problem involving the corrosion of the steel is the spalling of the concrete cover. The oxide resulting from the corrosion is very porous and takes up to 10 times the volume of the steel which causes the break-up of the concrete.

RCPT Test

The Rapid Chloride Permeability Test (RCPT) is a test that uses the total charge, driven by an applied electric field of 60 Volts, passing through a 2-inch thick and 4-inch diameter concrete sample over a period of 6 hours, as an index of the concrete permeability. This test has been standardized as ASTM C 1202/AASHTO T 277. The specimens are vacuum-saturated with de-aired water before testing. An electrical current is conducted from the power source by placing one end of the sample in a 0.3 N NaOH solution and the other end in a 3.0% NaCl solution. Although the test is intended as an indirect measure of the concrete pore system network, anything that changes the concrete resistivity will change the results. For example, certain admixtures such as calcium nitrite change the pore solution conductivity and change the results. Likewise, conductive fibers or aggregates that contain hematite can change the concrete electrical resistivity without changing diffusion properties. Changes in the vacuum pressure used during saturation or other specimen conditioning prior to testing can also change the sample resistivity due to the effect of the degree of saturation of the tested specimen on the measured result.

RCMT Test

The Rapid Chloride Migration Test (RCMT, NT Build 492) uses electrical voltage to accelerate chloride migration. The test specimen shall be 4 inches in diameter and 2 inches thick, and prior testing the specimen is vacuum-impregnated with saturated lime solution as described in NT Build 492. After the specimen is prepared, the concrete is exposed to a 10% NaCl solution on one side and a 0.3 N NaOH solution on the other. The test starts by measuring the initial current through the sample for an applied 30 volts.

Fig. 1: Schematic RCMT Set Up

The initial and final current through the specimen and specimen temperature are measured. After the test duration is completed, the concrete specimen is split open and a 0.1M silver nitrate reagent is applied to the sample. The chloride penetration depth, as evidenced by the precipitation on the specimen of silver chloride, is measured at least seven depths to an accuracy of 0.1mm (0.0039 inch). The surface chloride content can optionally be measured by cutting a 5mm (0.197 inch) concrete slice on the surface exposed to the chloride solution and measuring the acid-soluble chloride content in the slice. This chloride content can be used to get information on the concrete sample chloride binding capacity.

Use Of Secondary/Supplementary Cementitious Materials (SCMs)

The addition of SCMs to concrete can substantially increase the concrete’s resistance to chloride ingress. The use of SCMs can improve concrete’s durability, resistance to degradation due to multiple mechanisms, and strength gain behaviour. SCMs have two primary forms of reaction that influence the properties of the concrete. The hydration and chemical reactivity of SCMs are functions of their compositions, with many SCMs showing varying ranges of each type of reactivity.

Latent Hydraulic Reactivity

The material will react with water to form strength-bearing phases, with or without the presence of Portland cement. SCMs of this form typically will contain both calcium and reactive silicates.

Pozzolanic Reactivity

The SCMs will react chemically with water and the hydrated cement paste to form additional strength-bearing phases and cause a densification of the microstructure. Pozzolanic materials are typically siliceous in nature and need not contain lime-bearing phases. The reaction rates of SCMs impact their influence on the pore structure, and as a result reducing the rate of chloride diffusion through concrete. For the purpose of this study, the following materials were used in concrete and the test results evaluated to study the chloride migration characteristics of concrete:

• Ground Granulated Blast Furnace Slag (GGBS)
• Fly Ash
• Alccofine 1203

Ground Granulated Blast Furnace Slag (GGBS): Ground granulated blast-furnace slag, is a by-product of steel production. Slag is primarily composed of CaO, SiO2, aluminum oxide (Al2O3), and magnesium oxide (MgO). When used as part of a Portland cement concrete, slag reacts with both the water (latent hydraulic reaction) and the hydrated cement paste (Pozzolanic reaction), resulting in a more refined microstructure than that of a plain Portland cement.

Fly Ash: Fly ash is a by-product of coal combustion and composed primarily of silicon dioxide (SiO2) and calcium oxide (CaO). When added to concrete, fly ash reacts with the hydrated cement paste in a primarily Pozzolanic reaction; the result is a denser microstructure over time.

Alccofine 1203: Alccofine 1203 is a new generation, ultrafine, low calcium silicate product manufactured by Counto Microfine Products Pvt. Ltd. (CMPPL) – a joint venture between Ambuja Cement Limited & Goa based, Alcon group. The production facility is at Pissurlem Industrial Estate.

Concrete Mix Design

Concrete trials were conducted with the following proportions: a). OPC alone.

• OPC + Fly Ash, replacing 20%, 35% , 40% & 60% of Fly Ash in cementitious
• OPC + GGBS, with replacing 35%, 50%, 70% & 90% of GGBS in cementitious
• OPC + Alccofine 1203, with replacing of 5%, 15%, 25% & 35% of Alccofine 1203 incementitious
• OPC + Fly Ash + 5% Alccofine 1203, replacing 20%, 35%, 40% & 60% of Fly Ash in cementitious
• OPC + GGBS + 5% Alccofine 1203, with replacing 35%, 50%, 70%& 90% of GGBS in cementitious
• OPC + Fly Ash + 3% Alccofine 1203, replacing 20%, 35%, 40% & 60% of Fly Ash in cementitious
• OPC + GGBS + Alccofine 1203, with replacing 35%, 50%, 70% & 90% of GGBS in cementitious

The Test Method

The test method used to find the chloride ingress resistance of the concrete was the NT Build 492 test. The NT Build 492 test comes from the Nord Test (Based in Finland) family of test methods. This is an alternative to ASTMC1202, and the result is a chloride migration coefficient from non-steady state migration experiment, DNSSM x10¯12m2/sec, that can be used to access the quality of concrete. NT Build 492 is adopted widely in Europe and the USA. It is also fully adopted in China, as the National Standard GB/T50476. Apart from the above some experiments were also conducted by using the ASTM C1202 test method, with the mix proportion given in the Table 1 for Sr. No. 18 to Sr. No. 29, the results of the tests are presented in figure 5.

Difference Between ASTM C1202 Method And NT Build492

The NT Build 492“Chloride Migration Coefficient from Non- Steady State Migration Experiments” (RCMT) test method is different from the widely used ASTM C1202 test, the ASTM C1202 test is often criticized based on a report from a FHWA (Federal Highway Administration, U.S. Department of Transportation) study by Whiting[1].

The fundamental differences between the NT Build 492 “Chloride Migration Coefficient from Non- Steady Sate Migration Experiments” and the ASTM C1202 “Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration” (RCPT) test method are:

• RCPT test fundamentally is simply a conductivity test or inverse- resistivity test and in spite of it often being referred to as Rapid Chloride Permeability Test, it does not measure the “permeability” or “chloride diffusion” of concrete[2].
• High current flow in RCPT test results in heating of the sample and solution during the 6 hours test, raising the measured conductivity, thus a better quality concrete looks worse than it would
• The transport of ions in concrete depends on the pore structure of the concrete, while the electrical conductivity of concrete or RCPT results depend upon both pore structure and electrical conductivity of the pore solution, which in turn is determined by composition of the pore
• It is not correct to use electrical conductivity of concrete or RCPT results to rank rapid chloride permeability of concrete containing supplementary cementitious materials[4].
• ASTM C1202 states in clause 1, that this test method is applicable to types of concrete where correlations have been established between this test procedure and long term chloride ponding procedures such as those described in AASHTO T 259. In other words, establishing correlation with the ponding test is a prerequisite for using RCPT. How many laboratories actually conduct the classical 90-day soaking test and establish the correlation with RCPT, is not known.
• High water reducing admixtures work on the principle of electrical polarity to create electrical This may cause substantial changes to the overall electrical conductivity of concrete. It has been reported that different types of water reducing agents added to otherwise identical concrete mixes caused variance of up to 700 coulombs in total charge passed in RCPT.[9]
• On Precision and Bias, ASTM C1202 states that the single operator coefficient of variation of a single test result has been found to be 12.3%. The variability of results between two properly conducted tests by the same operator could be as much as 42%. The multi laboratory coefficient of variation of a single test has been found to be 18.0%. Therefore, results of two properly conducted tests in different laboratories could varyby as much as 51%.
• RCMT test is a non-steady state migration test, developed by Tang and Nelson[5]and adopted as NT Build 492 involves measurement of the depth of chloride ingress under applied DC potential (voltage and time of the test are determined from an initial current measurement).
• In RCMT for avoiding the influence of other carrying ions, the non-steady state migration coefficient is calculated from the actual chloride ion penetration measured from visible silver chloride precipitation that is developed after application of silver nitrate solution on the axial split specimen on completion of the test.
• The results of RCMT are not influenced by concrete pore fluid conductivity and depth of chloride penetration is measured
• The calculated diffusion coefficient can be used in service life prediction models[2].

Note: A detailed study on RCPT as per ASTM C 1202 can be found in the article – “Revisiting Rapid Chloride Permeability” by Milind Joshi and Siddika Mapari, POV, ICI 2010.

Brief Description Of Test (NT Build 492)

This test procedure and test conditions can be briefly summarized as follows:

• Samples of 100mm in diameter and 50mm in height are
• Vacuum-saturation of the samples with limewater performed prior to the
• 10% (wt.) NaCl solution used as the catholyte and 3M NaOH solution used as the anolyte.
• Applied voltage in the range of 10 – 60 V, decided upon the value of the initial current, measured at a voltage of 30
• Duration of the test of 6 – 96 h, decided upon the value of the initial current, measured at a voltage of 30
• Correction factor of 2 V used in the equation for calculating the DNSSM, accounting for the polarization of the
• Temperature during the test between 20 and 25 °C.

Use Of NT Build 492 Test In India

In India awareness about designing concrete structures for durability is slowly increasing, the practice of specifying durability test for qualification of concrete mixes is also increasing at a slow rate. Currently, RCPT tests as per ASTM C1202 is specified for most of the metro rail projects as well as in many roads and bridges in the country, RCPT NT Build has been specified in the Bandra-Worli sea link bridge with limiting factor for pre-qualification of concrete mixes to be <2×10-12m2/sec. Considering technical advantages of the NT Build 492 test method over the ASTM C1202 method and as per the current trends in the industry, NT Build 492 is expected to replace the ASTM C1202 test in the years to come.

Test Results

All the experiments for RCMT were carried out at the R&D Division of Alcon Construction (Goa) Pvt. Ltd., at Goa, where the test facilities of NT Build 492 is available, the test results of the experiments are tabulated in the Table 1 below and the graphical representation are given in Figure 2, 3 & 4.

Fig. 2: RCMT Results at 7, 28 and 56 Days by using Different Proportions of Fly Ash, Alccofine 1203 and GGBS with Different Proportions

Fig. 3: 28 Days and 56 Days RCMT Results of Concrete with 3% of Alccofine 1203 and Fly Ash/GGBS in Varying Proportion

Fig. 4: 28 Days and 56 Days RCMT Results of Concrete with 5% Alccofine 1203 and Fly Ash/ GGBS in Varying Proportions

Fig.5: 7 Days and 56 Days RCPT Results of Concrete with Varying Proportions of Fly Ash, GGBS and Alccofine 1203

Interpretation Of Results

The chloride migration coefficient under non-steady-state (DNSSM) is used for evaluating the concrete resistance to chloride penetration, according to the criteria presented in Table No. 2.

TPlease note that the results presented here are of single test for each different proportions of cementitious content (result is the average of three specimens), according to the results from Nordic round robin test between six laboratories, the coefficient of variation of repeatability of results is 9% and the coefficient of variation of reproducibility of results is 13% for Portland cement or for concrete mixed with silica fume and 24% for concrete mixed with slag [7]. The most effective SCM has been found as GGBS [10].

Conclusion

• RCMT value for fly ash concrete is very high at initial ages; the values are reduced at later ages of 28 and 56
• All the 56 days RCMT results of concrete with supplementary cementitious materials are less than 2 x 10-12m2/sec which relate to very good resistance to chloride
• RCMT values of concrete with only OPC are high and having least resistivity to chloride ingress as per the results of
• The results RCMT of OPC + GGBS/OPC + GGBS + Alccofine 1203 combination have the lowest results when compared to OPC + Fly Ash/OPC + Fly Ash + Alccofine 1203 combination, RCPT results in Figure 5 also show similar trends for OPC + GGBS combination.
• Addition of secondary/supplementary cementitious materials greatly improves the resistivity of concrete to chloride ingress, due to the improvement of pore structure of the concrete due to formation of additional calcium silicate hydrates in the hardened cement paste, which is able to resist the ingress of corrosion inducing
• RCMT values of concrete with 3-5% of Alccofine 1203 are the least of all the combination so far secondary/supplementary cementitious
• Highly durable concrete can be obtained by addition of 3-5% of Alccofine 1203 to OPC + GGBS & OPC + Fly Ash concrete.
• Concrete containing Alccofine 1203 which is a low calcium silicate based mineral additive is able to achieve the lowest values of chloride diffusion due to its unique particle size distribution resulting in enhanced hydration process, improved packing density of the paste component while lowering the water demand resulting in improved strength and durability of the concrete.

Acknowledgments

• RCPT tests were done at Goa Site of Gammon Engineers and Contractors Private Limited, Mumbai under the supervision of Mr Vivekanand
• Similarly, RCMT tests were done in Alcon Lab by Mr Siddhesh
• Results of both the tests were regularly monitored by Shridhar Behare.
• The whole investigations were done under the guidance of Dr V. Nayak.

References

1. Whiting, , “Rapid Determination of the Chloride Ion Permeability of Concrete,” Final Report No. FHWAJRD-371/1 19. US Federal Highway Administration (1981).
2. Development and Standardisation of Rapid Methods for Assessing the Fluid Penetration Resistance of Concrete,
3. Hooton, D., Charmchi, G. and Karkar, E., XII International Conference on Durability of Building Materials and Components (XII DBMC), RILEM Proceeding Pro 96, Sao Paulo, 2-5 September 2014.
4. Revisiting Rapid Chloride Permeability Test, Milind Joshi and Siddika Mapari, POV, ICJ, 2010
5. Shi Caijun, Another look at the Rapid Chloride Permeability Test (ASTM CI 202 or ASSHTO T277)
6. Tang, and Nilsson, L.O., “Rapid Determination of Chloride Diffusivity in Concrete by applying an Electrical Field,” ACI Matls. J., 89 (1) (1991)49-53.
7. Standard test method for electrical indication of concrete’s ability to resist chloride ion phenomenon, ASTM C1202-97, American Society for Testing and Materials,
8. NT Build 492 “Chloride Migration Coefficient from Non-Steady State Migration
9. State-of-the-Art Report (Draft), RILEM TC 230 – PSC, RILEM Technical Committee on Performance Specification for Concrete
10. Krieg, Willfried, Rapid Chloride Permeability Testing- a critical review,
11. Concrete Engineering International, Summer2007, pp-48-49.
12. Nayak, N. V., GGBS-The Most Effective SCM for Concrete-Civil Engineering and Construction Review, May 2020, pp-24-30.

Dr. M. R. Kalgal Technical Advisor UltraTech Cement Ltd.

Concrete As A Sustainable Material

In the search for materials and systems that will provide a durable foundation for sustainable communities, people are increasingly turning to concrete. When considered over its entire life cycle, concrete makes a significant contribution to the triple bottom line – environmental, social and economic – of sustainable development.

From runways to highways,  from subways to transit-ways, concrete helps develop and maintain a sustainable, environmentally-friendly transportation infrastructure. Regardless of the type of roadway or current pavement conditions, there is a concrete solution. Concrete can be used for new pavements, reconstruction, resurfacing, restoration or rehabilitation. Concrete pavements generally provide the longest life, least maintenance, and lowest life-cycle cost of all alternatives. Plus, due to higher bitumen prices, concrete has become the least expensive alternative for even new construction on a first-cost basis.

Making Concrete More Sustainable

Having realized the inevitability of using concrete for developmental works for quite some time to come, efforts are on throughout the world to try and make concrete more sustainable. The approaches can be broadly classified as follows:

1.      Make Cement More Sustainable

• Use less energy intensive kilns

• Use supplementary cementitious materials to reduce clinker component

• Use new types of clinker

• Use alternative fuels and renewable energy during production process

• Produce clinker free cement

2.      Use Alternative Materials In Concrete

• Alternative cementitious materials

• Alternative aggregates

• Wastes in Concrete (Converting Liabilities into Assets)

3.      Make Concrete More Durable

• sustainability through longer lasting structures

4.      Capture And Sequester CO2 In Concrete Or Store CO2 In bedrocks

5. Produce Clinker Free Concrete

An effort is made in this paper to look at the work being carried out across the world and the promise these hold towards achieving sustainability targets.

2050 Climate Ambition Of GCCA

Global Cement and Concrete Association (GCCA), is a CEO led industry initiative headquartered in London. It has amongst its members about 40 of the world’s leading cement and concrete companies as well as about 25 affiliates which are cement and concrete associations. GCCA announced the formation of a  strategic  partnership with the World Business Council for Sustainable Development (WBCSD) to facilitate sustainable development of the cement and concrete sectors and their value chains. The new partnership also created synergies between work program to benefit both the GCCA and WBCSD and their respective member companies. As part of the new agreement, the work carried out by the Cement Sustainability Initiative (CSI) was transferred from WBCSD to the GCCA on 1st January, 2019 with activities managed out of the GCCA’s London offices.

The vision of GCCA sees “a world where concrete supports global sustainable economic, social and environmental development priorities; and where it is valued as an essential material    to deliver a sustainable future for the built environment”. GCCA has unveiled the 2050 Climate Ambition on 1st September 2020 which demonstrates  the  commitment  of  the  industry across the globe to drive down the CO2 footprint of the world’s most used man-made product, with an aspiration to deliver society with carbon neutral concrete by 2050.

The statement identifies the essential levers that will be required to achieving carbon neutral concrete, including:

• reducing and eliminating energy related emissions,
• reducing process emissions through new technologies and
• deployment of carbon capture, more efficient use of concrete, reuse and recycling of concrete and buildings, and harnessing concrete’s ability to absorb and store carbon from the

5C Framework Of Cembureau

Cembureau the European Cement Association based in Brussels is the representative organisation of the cement industry in Europe. It is trying   to coordinate the efforts of the cement and concrete industry to play an essential role to help Europe achieve its strategic objectives on growth, innovation, social inclusion and climate and energy. They also have come up with a vision to achieve Carbon Neutral Europe by 2050. It is being attempted through the 5C Framework. The 5 Cs stand for clinker, cement, concrete, construction & built environment, and (re)carbonation.

Clinker

Cembureau recognizes that the circular economy goes hand in hand with carbon neutrality. Circularity is crucial to reduce emissions from clinker, which is the backbone of cement production. Efforts are on world-wide to use nonrecyclable waste to phase-out fossil fuels from cement production. It will become even more crucial tomorrow, as CO2 captured during clinker manufacturing will be used in other industrial applications.

The manufacturing process where raw materials are heated up and decarbonisation of the limestone is a chemical process which causes 60%-65% of cement manufacturing emissions (process emissions). The remainder of CO2 emissions comes from the fuels used to heat the kiln (combustion emissions). Since clinker production represents the lion share of emissions, this is obviously the area that offers most opportunities for deeper CO2 emission cuts.

The graphic given in Fig. 1 very nicely depicts the possible quantum of CO2 reductions by a multi-pronged approach.

Fig. 1: Opportunities to Achieve CO2 Reductions for Clinker (Courtesy Cembureau Website)

Cement

There are no further CO2 emissions at the stage of cement production. However, electricity is used for grinding and mixing, and incoming materials as well as final cement products are transported. Cembureau thus identifies that some cements can be made with less clinker, or even alternatives to clinker, to achieve significant emission savings. In addition, a reliable and affordable supply of renewable energy as well as zero carbon alternatives to diesel for industrial vehicles can further reduce emissions at the cement stage. Fig. 2 depicts the quantum of CO2 reduction at this stage.

Fig. 2: Opportunities to Achieve CO2 Reductions for Cement (Courtesy Cembureau Website)

Concrete

The direct CO2 emissions related to concrete largely come from cement production. The largest indirect CO2 emissions come from transportation of concrete to the end user. Fig. 3 indicates the possibility of reducing the CO2 impact of concrete.

Fig. 3: Opportunities to Achieve CO2 Reductions for Concrete (Courtesy Cembureau Website)

Construction

Concrete which is already ubiquitous offers a working life in excess of 100 years, provides fire resistance, and is able to reduce energy consumption for heating and cooling by 25%. This opens significant opportunities to reduce emissions not only for concrete itself, but for the overall construction sector. Cembureau’s Report lists the following possibilities to achieve reduction of CO2 emissions/impact in constructions:

• Energy efficiency in buildings: leverage thermal mass properties of concrete to cut energy used during the working life of buildings.
• Concrete used in buildings: reducing embodied carbon by using concrete more efficiently and using advanced techniques like 3D printing, leading to reduction in the quantum of concrete used.
• Design for adaptability and disassembly: using concrete structures’ adaptability for mixed use buildings and changing needs as well as exploring the “design for deconstruction” model where building is conceived at origin with the objective to disassemble at the end of life. This approach allows materials and components to be removed easily and to be re-used to construct new buildings.

Re-Carbonation

In addition to reducing emissions, carbon neutrality can also be reached through greenhouse gas emissions removal through carbon sinks, states Cembureau Report. Cement and concrete have here a key role to play through a process called re-carbonation, which effectively transforms the cities into carbon sinks.

Re-carbonation is the process whereby concrete re-absorbs some of the CO2 that was released during clinker production. It is a process that occurs naturally in all concrete structures, permanently trapping the CO2. Thanks to re-carbonation, cities effectively act as carbon sinks, allowing further reduction of emissions in the full cement and concrete value chain.

Cembureau identifies 3 major thrust areas here.

• Re-carbonation in the built environment: re-carbonation occurs naturally in all concrete infrastructure and it is said that 23% of process CO2 emissions of cement used, is being captured annually, which equates to about 8% saving of total CO2 emissions for the cement manufactured

Enhanced re-carbonation of recycled concrete: it is known that re-carbonation increases after demolition of a concrete building. This can be accelerated by using exhaust gases from a cement kiln (which have higher CO2 content and are also at a higher temperature) increasing the CO2 captured up to 50% of process CO2 emissions. Cembureau Report also suggests  separating the  aggregates from  recycled concrete and grinding the cement paste for higher capture of CO2 and the resulting material can be used as a clinker replacement in cement or as an additive in concrete Carbonation of natural mineral: it is stated that natural minerals such as basalt and olivine can be re-carbonated by exposing it to air and kiln exhaust gases. Such materials can be used as clinker substitutes.

Fig. 4: Carbon Neutrality Roadmap 2050 (Courtesy Cembureau Website)

In summary all the 5C initiatives combined with what is already achieved in terms of reducing CO2 reduction would result in Carbon neutrality in construction sector by 2050 says Cembrureau. This is shown graphically in the Fig. 4.

Other Major Initiatives Around The Globe

Worldwide, hundreds of companies and research groups are working to keep CO2 out of the atmosphere and store it someplace else, including in deep geologic formations, soils, soda bubbles, and concrete blocks. By making waste CO2 into something marketable, entrepreneurs can begin raising revenues needed to scale their technologies. Some of these are given below to indicate the approaches followed.

Fig. 5: Typical Components of LC3

Limestone Calcined Clay Cement (LC3) This new blend substitutes up to half of the usual Portland cement used to make concrete with highly abundant clay  and  limestone, promising to reduce cement-related CO2 emissions by up to 30%. When used together, the aluminates from the calcined clay interact with the calcium  carbonates from the limestone, leading to a less porous, and therefore stronger, cement paste. The efforts towards developing and testing this new blend was initiated by Ecole Polytechnique Federale De Lausanne (EPFL) with funding from Swiss Agency for Development and Cooperation (SDCC) with partners from the Indian Institutes of Technology and from universities in Cuba and Brazil.

The typical components of LC3 are indicated in Fig. 5. The major innovation in LC3 is to combine the use of abundantly available low-grade kaolinite clay with a further 15% of limestone, with no reduction in mechanical performance.

LC3 Vs. LC2
It is possible to produce LC3 by inter-grinding Limestone, Calcined Clay and Clinker or produce LC2 by grinding Limestone and Calcined Clay alone. This LC2 can be mixed with OPC anytime to obtain LC3. The processes are indicated in Fig. 6.

Fig. 6: Production of LC3 and LC2 (Courtesy EPFL)

Benefits of LC3

• Reduced Clinker factor reduces CO2
• Emissions of LC3are estimated to be 20-30% lower than Portland cement because:
  • Reduced clinker content leads to less process emissions from the decarbonation of limestone in clinker and less emissions from heating limestone to form
  • Grinding limestone takes less energy than heating
  • Calcination of clay takes place at 800°C and uses roughly 55% of the energy needed for clinkerisation at 1450°C.
• Kaolinite content needed for LC3is much lower and hence use of such ‘low grade’ clays would not compete with demand for resources by other Use of such resources would neither require opening of new quarries nor deplete agricultural soils.   In fact large stockpiles of low grade “waste clays” are seen stockpiled near kaolinite based industries as shown in Fig. 7.
• Limestone unsuitable for clinker production can be used. e.g. high dolomite content produces Periclase during clinker production, which causes expansion. Such materials can be utilised safely in interground applications, leading to more efficient use of limestone
• Depending on the exact scenario, the amount of cement that can be produced from the same identified limestone reserve could be increased two-fold.
• LC3 can be produced with existing manufacturing equipment, leading to only marginally increased investments for calcining equipment.

Fig. 7: Clay Suitable for LC3

CemZero
CemZero is a Swedish project by cement manufacturer Cementa and the energy company Vattenfall with the aim of reducing greenhouse gas emissions. The project includes studies of calcination and clinker mineral formation for carbonate based raw materials under electrical heating, carbonation under cooling in high CO2 concentrations, evaluations of technologies aiming at upscaling, and determination of gas composition in the CO2 rich process gases.

The suitability of the gases for capture, transport and geological storage, or other use, will be evaluated. There are 3 major thrust areas of research in this project which is expected to be completed by 2025. The thrust areas are:

• Heat transfer with plasma in rotary
• Direct separation of CO2 from calcination of carbonate based raw materials in the production of cement clinker and burnt
• CO2free products with electrified production-reactivity of cement clinker with secondary

LEILAC

Supported by the European Union, the LEILAC (Low Emissions Intensity Lime And Cement) projects are developing a breakthrough technology that aims toenable the cement andlime industries to capture those unavoidable CO2 emissions emitted from the raw limestone. The LEILAC technology is based on Calix’s Direct Separation technology, which aims to enable the efficient capture of the unavoidable process carbon emissions, derived from its original application in the magnesite industry. In addition to the main technology targets that will be demonstrated, the project scope includes a thorough analysis of the potential destination of the captured CO2, for use in processes, as well as for safe geological storage.

CIMENTALGUE

The CIMENTALGUE project is part of the development of a new ‘industrial symbiosis’combining the cement industry, which produces industrial effluent rich in CO2, NOx, trace elements and unavoidable energy waste, on the one hand and the emerging microalgae cultivation industry, which consumes CO2, nitrogen, trace elements and heat, on the other.

CIMENTALGUE is aimed at developing a process for exploiting CO2 and unavoidable heat waste from industrial sources by producing photosynthetic microalgae in natural light in photobioreactors under glass.

The project will install a 500m² demonstrator micro algae production unit inside a cement works. This installation will be operated for two and a half years to provide representative data for the entire value chain from capture and treatment of industrial waste gas to developing the economic potential of the microalgae biomass produced.

The project will perfect and optimise its process, prove its sustainability and ensure its economic and environmental validation in terms of norms, social acceptance and profitability on target markets (as additives for animal feed, dyes, materials, etc.)

CarbonCureTM Technologies

CarbonCure Technologies, from Canada, has demonstrated a technology which enables the production of concrete with a reduced water and carbon footprint without sacrifice to the material’s reliability. CarbonCure Technologies offers a technology to implement carbon dioxide (CO2) utilization in the ready mix concrete industry. Using this technology, waste CO2 can be put to a beneficial use as a feedstock in the production of concrete. The retrofit CarbonCure TM Ready Mix Technology adds CO2 to concrete during mixing. The CO2 reacts with the cement and is mineralized to produce nanoscale calcium carbonate. The carbonate formation can impart positive impacts on the concrete. The CO2 addition (hereafter, CarbonCure)can improve hydration and increase compressive strength without affecting the fresh concrete properties.

Utilizing CarbonCure Technologies’ system, a precise dosage of CO2 is injected into a concrete plant’s reclaimer system, which contains the water used to wash out concrete trucks and mixers. The CO2 is converted to a permanently embedded mineral with strength-enhancing properties which can then be incorporated into new concrete mixes. By reducing the amount of new freshwater, solid waste disposal and cement required, the team, which is backed by Bill Gates’ fund Breakthrough Energy Ventures, Amazon Climate Pledge Fund, BDC Capital and others, is able to reduce the material costs and increase profitability for concrete producers.

CarbonCure has a recommended dosage rate of 50 – 250g/100kg of cement (as distinct from total cementitious) for most applications. Dosages outside this range may be used if local testing shows acceptable performance. Pre-testing is required to determine the appropriate addition rate for desired performance. The optimum addition rate may be influenced by other concrete mixture components, cement types, ambient temperature, mineral additives, quality and gradations of aggregates, slump of concrete, mixing equipment, job conditions and desired performance characteristics. The optimum performance of the CarbonCure is said to be generally obtained with a delayed addition following the start of mixing. Packaging and handling CO2 is available in bulk and delivered by tanker truck to an on-site pressurized storage tank for dispensing by means of the CO2 metering equipment. CO2 must have a certified purity of 99% or above for use in this application. The dispensing control system is connected to the batching system and the CO2 addition is fully integrated into the batch sequencing of materials that are added to the mix.

CarbonBuilt™

CarbonBuilt’s core technology emerged out of the Institute for Carbon  Management  at  the  University  of  California,  Los Angeles (UCLA). The CO2 Concrete process developed by University of California directly converts carbon dioxide from power plants or other emitters into precast concrete and concrete masonry products (such as blocks and beams) that can be used for construction worldwide. This direct conversion bypasses any need for CO2 purification or enrichment that is endemic to nearly all other methods for absorbing CO2 smokestack emissions into concrete. The widespread adoption and use of conventional concrete by CO2 Concrete would dramatically cut CO2 emissions resulting from the production of cement. Due to its novel chemistry, CO2 Concrete achieves unprecedented levels of CO2 uptake, resulting in a carbon intensity (CI) that is up to 65% lower than that of conventional concrete.

CarbonBuilt’s Reversa™ technology is a low-cost solution for thermal energy, cement, steel or incineration operators seeking to beneficially utilize their waste CO₂, ash or slag. Their modular technology can be added to a site to permanently embed flue gas CO₂ into masonry or precast concrete produced nearby. CarbonBuilt’s Reversa™ process includes CO₂ emission-reducing innovations to both the concrete mix design and its curing process. On the formulation side, they introduce portlandite (also known as calcium hydroxide, a commodity chemical), reduce the usage of traditional cement and increase the use of waste materials like fly ash. The concrete is then formed using the same processes and equipment that are used today.

Fig. 8: CarbonBuilt’s Reversa™ Technology

After forming, we cure the concrete with waste CO₂ emissions using a process that does not require expensive capture, compression or purification of the CO₂.

The Reversa process reduces emissions through a combination of utilization (permanently embedding CO₂ into the concrete) and avoidance (reducing CO₂ emissions associated with the raw materials).

The process requires minimal CAPEX, since it features simple “stack-tap” integration with limited site utility tie-ins, does not require a carbon capture system, and readily integrates into existing construction supply chains and workflows. Their approach can also make greater and more flexible use of fly ash, slag or other supplementary cementitious materials than is possible with conventional concrete while providing engineering performance equivalent to typical concrete. The low cost of this technology is a significant advantage in the low margin concrete business.

Carbonated Calcium Silicate Concrete

It is a patented process from USA based Solidia Technologies. It is a low-lime calcium silicate (Ca2SiO4) cement (CSC) that cures by a reaction with gaseous carbon dioxide (CO2). The production of CSC requires less limestone and lower kiln temperatures than those used for ordinary Portland cement (OPC). This makes it possible to reduce the carbon dioxide emissions at the cement kiln from ~810 kg/t for OPC to ~565 kg/t for CSC. The carbon dioxide used in the curing process and captured within CSC-based concrete (CSC-C) is industrial- grade carbon dioxide sourced from waste flue gas streams. These translate to approximately 30% reduction in CO2 emissions.

ZERO Clinker Cement

Hoffmann Green Cement Technologies is a French company involved with production of Geopolymer (Zero Clinker) products. Majorly they have 3 products:

HP2A (High Performance Alkaline Activation) cement: It is a 100% mineral, non-flammable and VOC -free adhesive based on activated clay and silicate in the form of a two-component“paste & liquid”system. Activators and super-activators formulated are added to  flashed clay mixed with silicate to obtain H-P2A cement. It is said to have pull-out strength of more than 25 MPa.

H-UKR cement: This is a solution based on the use of blast furnace slag. Efficient activation system allows this co-product to be used without any addition of clinker in its formulation. It can be used       in various fields of construction, including ready-mix concrete and precast concrete.

H-EVA cement: It is an innovative binder, based on an alkaline ettringitic technology. Activators and super-activators formulated are added to flashed clay, mixed with gypsum/desulfogypsum to obtain H-EVA cement. It is said to have strength of up to 60 MPa at 28 days. It is said to be ideal as a road binder, but can also be used for mortars, plasters and construction concretes.

Fig. 9: Production of Zero Clinker Cement

The manufacture involves the following steps –

Step 1: Valorization (Conferring Value Upon Something)

Recover and enhance co-products from industry and construction, which are sent to production plant. Blast furnace slag comes from the metallurgical and steel industry, Flash clay is a co-product of clay sludge and Gypsum/Desulfogypsum are produced from construction site excavated material.

Step 2: Production

The manufacturing process is based on the systematic use of abundant co-products as a substitute for natural resources. The co-products, are mixed with activators and super-activators specifically formulated. This is followed by the packaging of cements (big bag, bulk or bags) and then shipment to the construction sites.

Concluding Remarks

Concrete is undoubtedly a sustainable material among other options. Efforts are on globally to make concrete more sustainable. In addition to the traditional approach of Replacing cement in concrete with larger amounts of supplementary cementitious materials (SCMs) than usual, using local, alternative and recycled aggregates, achieving higher strength and durability using chemical admixtures, efforts are on to bring in paradigm shift in producing cement and concrete. Many R&D projects are WIP (work in progress) but many are nearing fruition. The listed approaches, which are by no means exhaustive or complete are expected to trigger the imagination of budding concrete technologists to come forward and innovate.

Dr. S. C. Maiti Ex-Joint Director National Council for Cement and Building Materials

Concrete is the integral part of the structures. Being the part and in close contact with steel reinforcements, the reinforced concrete produces structural elements and structures, which carry load as per design and provide various kinds of services, in buildings and structures.

Cement discovered and developed is the binding material in concrete. Concrete has various other ingredients: Water, aggregates, chemical and mineral admixtures, and sometimes, fibers, steel or polypropylene. Some special types of concrete e.g. Polymer concrete, high-performance concrete, high-volume flyash concrete, self-compacting concrete and very high-strength concrete are also discussed briefly. 

Various stages of concrete e.g. elastic and plastic concrete- their significance, sustainable concrete construction and green concrete, the special property of concrete i.e. creep, the probable life of concrete structures, and the future with cement – less concrete are also discussed. 

Introduction

Versatility of concrete is widely accepted and well known. In good olden days, when Joseph Aspdin, a Leeds bricklayer, stone mason and builder used concrete for making slabs, there were apprehensions in using concrete in all types of structures as the construction of arches with masonry was common. Aspdin patented ‘Portland Cement’ in 1824.

The process of manufacture of cement consists essentially of grinding the raw materials (limestone and clay), mixing them intimately in certain proportions and burning in a rotary kiln at a temperature of upto about 14500 C, when the material sinters and partially fuses into balls known as clinker. The clinker is cooled and ground to a fine powder, with about 5% gypsum added, and the resulting product is the commercial Portland cement1 . Cement consists of four compounds: tricalcium silicate (C3 S), dicalcium silicate (C2 S), tricalcium aluminate (C3 A) and tetracalcium aluminoferrite (C4 AF). The two silicates (C3 S) and (C2 S) are primarily responsible for the strength development in concrete. 

Like all other disciplines, concrete production initially was an art, its science was developed later, when mechanics and structural engineering could be applied to understand the micro and macro properties of concrete. We apply a number of limit states to compensate the imperfections and intrinsic complexities like shrinkage, creep, fatigue etc. Prof. Neville, after elaborately discussing all the properties of concrete in detail, concluded that concrete making is an art, as much as it is also a science. 

Concrete is made of cement, water, sand, stone chips or river pebbles (we call coarse aggregates) and chemical and mineral admixtures (fly ash, ground granulated blast furnace slag, rice husk ask, silica fume etc.). The chemical admixtures change the physical characteristics of concrete e.g. setting time and workability. They can also reduce the water content of concrete for a fixed workability of concrete, thereby, reduce the water-cement ratio, and hence can increase the compressive strength of concrete. They can also produce high workability pumpable concrete for constructing high-rise buildings, roads and heavily congested reinforced concrete structures. These chemicals are mostly organic materials, their basis is either naphthalene, melamine or polycarboxylic ether. Some of the chemicals can entrain air inside the concrete, thereby making the mass concrete (with large size coarse aggregates) cohesive and other concrete, more resistant to freezing and thawing in cold climate.

The steel is used to reinforce the concrete structures, and thus we produce reinforced concrete structures. The steel is strong in both compression and tension, but concrete is strong in compression and weak in tension. The steel-concrete, combination makes the structures strong. As an integral part of the structure, steel is quite strong, except that it gets corroded in presence of oxygen and moisture. Further, chloride in moist concrete can also corrode the reinforcement. Once reinforcement is corroded, its volume increases (about 7 times the initial volume) and cracks are developed in the concrete structures. The cracks increase day by day and finally, the structure loses its integrity, and collapses. 

With its inherent weakness of cracking and corrosion of steel reinforcement, concrete has been a choicest material for making thin shell structures, starting from funicular shell to paraboloid, hyperboloid and natural shell, like Lotus temple in Delhi, Opera house in Sydney. There are number of such shells all over the world. Aesthetically and architecturally, shells are the most beautiful structures.

Different types of concrete are now produced suiting to the requirement of industry, and high rise buildings. Following are some of the special types of concrete.
1. Polymer concrete
2. High performance concrete
3. High-volume fly ash concrete
4. Self compacting concrete 

Polymer Concrete

Polymer concrete or resin concrete is a concrete containing polymer, a binder in place of Portland cement and inert aggregate as filler. This concrete has high strength, greater resistance to chemical and corrosive agents, lower absorption, higher freeze and thaw durability.

High Performance Concrete 

High performance concrete (HPC) is a concrete which is produced with some special properties like low permeability by adding micro filler like silica fume, flyash or ground granulated blast furnace slag (g.g.b.s.). The performance requirements can be high-strength, high early strength, high workability (including self-compacting concrete), low permeability and high durability for severe service environments. We call high performance concrete as a special concrete. But all concrete is supposed to provide high performance. The specially designed earthquake- resistant buildings and structures have to provide the required ductility to survive the earthquake forces. The fiber reinforced concrete, polymer concrete and epoxy concrete are all high performance concrete and have also to provide the required strength. Fly ash, a pozzolana and a mineral admixture, obtained as a by-product from thermal power stations, is being used in concrete to improve its properties. The Code of practice for plain and reinforced concrete IS4562 stipulates the use of at least 25% good quality fly ash or at least 50% g.g.b.s. as part replacement of low-alkali OPC, to prevent the durability risk associated with alkali-silica reaction in concrete structures, specially hydraulic structures. Some of the Himalayan aggregates may be reactive. Such aggregates react with the alkali of cement in concrete and alkali-silica gel is formed inside the concrete. This gel imbibes moisture and the volume increases causing expansion and cracking of concrete, over a period of many years. Two Indian dams (‘Hirakud’ and ‘Rihand’ dam) suffered this deleterious alkali-silica reaction. Although about 13,000 tons of fly ash was used in the structural concrete of the Rihand dam, yet the Power House structures cracked severely, because the OPC used had high alkali content, in the range of 1.2 to 1.8% as Na₂ O equivalent³ .

Silica fume is a very fine and highly reactive mineral admixture for concrete. It is a by-product of ferro-silicon industries. It’s BET fineness is more than 15,000m2 /kg and is being imported from Norway, Australia and China, in condensed form. For developing high-strength Silica fume is a very fine and highly reactive mineral admixture for concrete. It is a by-product of ferro-silicon industries. It’s BET fineness is more than 15,000m2 /kg and is being imported from Norway, Australia and China, in condensed form. For developing high-strength.

High-Volume Flyash Concrete

The high-volume (more than 50% replacement of OPC) fly ash concrete is being used in many places, specially in concrete roads, in mass concrete constructions, and in Roller Compacted Concrete Dams. The recently completed roller compaced concrete dam (Teesta IV Low dam) in Darjeeling district, 160 MW, 30m height, was constructed with 65% flyash in concrete.

Such concrete has benefit of reducing OPC content of concrete and reducing heat-development in mass concrete construction, and hence controls the thermal cracking in massive concrete structures. Prof. P.K. Mehta⁵ of the University of California, Berkley used high-volume fly ash concrete (106kg OPC and 142kg fly ash/m³ of concrete) in the construction of reinforcement-free and crack-free foundation structure of a Hindu temple, in Hawai island, which had 28-day compressive strength of 15.9MPa. The maximum temperature rise in concrete was only 13⁰ C.

Self-Compacting Concrete and Very High Strength Concrete

 The self-compacting concrete is a recent development. This concrete is like a thick liquid, a cohesive mass, and is being used in heavily reinforced concrete sections, where needle vibrator can not be inserted. Because, there is no vibration or compaction required, faster construction is possible. The essential ingredients of self – compacting concrete are the polycarboxylic ether – based superplastisizers and a viscosity modifying agent (VMA). Very high –strength concrete (of the order of 100 MPa) is only possible using this kind of superplasticizers, which is able to reduce more than 30% of the mixing water. The silica fume or the micro-silica (about 10% by weight of cement ) is also required to develop such high-strength concrete. In self-compacting concrete, superplasticizers provide the fluidity and VMA is used to reduce segregation. VMAs are hydrophilic, water – soluble polymers having high molecular weight. Such polymers can form a network of large molecules extending throughout the mass. The size of the polymers are in the colloidal range. Hence these are called ‘colloidal admixtures’.

The self-compacting concrete (M50 grade) has been used in the R.C.C. foundation raft (3.7 m thick) of the tallest man-made structure of the world, the Burj Khalifa6 . Creep of Concrete In the hardened states, we sometimes call concrete as ‘elastic’ material and we measure its modulus of elasticity. Creep and shrinkage are other hardened concrete properties. The creep is the time dependent deformation of concrete, and we calculate the ultimate creep strain using creep coefficient values given in our Code of practice2 . However, for long span structures, it is advisable to determine the actual creep strain likely to take place. To most of the civil engineers it is a neglected property of concrete, and many engineers do not understand its importance. Failure of structures due to creep of concrete is rare. However, creep-induced sagging was noticed in the midspan of a bridge in an American territory in the Pacific Ocean7. Elastic and Plastic Concrete We have thus used the term ‘elastic’ for the concrete. Similarly, we can use the term ‘plastic’. There can be plastic shrinkage cracks, when fast evaporation takes place in fresh concrete. ‘Plastic shrinkage’ by definition, occurs prior to setting of concrete. The drying shrinkage is preceded by elastic shrinkage, which occurs, when the water is lost from concrete, while it is still in a ‘plastic’ state. Concrete contracts on drying, and we call it as ‘drying shrinkage’. Withdrawal of water from concrete stored in unsaturated air causes drying shrinkage. Then there is “autogenous shrinkage” due to withdrawal of water from the capillary pores, by the hydration of the hitherto unhydrated cement in concrete.

In mass concrete, the thermal cracking can be avoided by controlling the placing temperature. Generally, lower the temperature of concrete, when it passes from a ‘plastic’ state to an ‘elastic’ state, the less will be the tendency towards cracking. We have just discussed the elastic and plastic states of concrete. We also sometimes estimate the ‘plastic rotation’ of steel-concrete composite beams, experimentally in the laboratory, and relate it to the bending moment8. This ‘plastic rotation’ on the loaded structures is in the inelastic state of concrete. 

We use ‘plastic concrete’ in cut-off wall or in diaphragm wall, in the dam project. The technique has been developed to make water-tight curtain wall, as the main element of foundation treatment for the embankment dams9. The diaphragm walls are generally excavated in panels, the excavated area being supported by bentonite slurry. The cut-off wall should behave in such a manner, that no crack is introduced as a result of imposed loading. A typical ‘plastic concrete’ mix includes gravel, sand, clay, cement and bentonite. They are mixed with water to produce a workable mass. The design of ‘plastic concrete’ wall involves section of a proper composition of the mix, to ensure the required permeability, deformability, workability, strength and durability. 

Sustainable Construction and Green Concrete 

The recent trend and technological innovation paved the way for new construction materials with the major focus on the sustainability in construction. The natural resources e.g. limestone, crushed stone and sand are reducing day by day, and we have to replace substantial proportion of cement and aggregates by industrial waste products such as flyash, bottom ash, blast furnace slag etc. The use of such waste products can partly solve the environmental pollution problem and simultaneously provide sustainable construction with less cement and aggregates. This can result in ‘green’ concrete using less of natural resources and use of waste materials. The coal ash consisting 80% fly ash and 20% bottom ash can replace substantial quantity of cement and fine aggregate in concrete. The resulting ‘green’ concrete will go a long way in providing sustainable construction and enhanced durability of concrete in structures.

Life of Concrete Structures 

High – strength concrete (grades M70 and M80) is now a days being used in lower portion of high-rise buildings, and also in bridge girders and in spillways of concrete dams. But the strength of concrete in structures decreases at higher ages. Like human beings, old concrete structures and buildings become weaker, may be after about 80 to 100 years’ of age. Delhi Metro structures have been designed for 120 years. So also the Euro tunnel (connecting England and France under the sea). The Burj Khalifa (in Dubai), the tallest tower of the world (828 m high) is also expected to provide the service life for at least 120 years. The Hindu temple foundation built in Hawai Island is made of precast concrete blocks, and without any reinforcement. This concrete structure is expected to provide service for 1000 years according to Prof. P.K Mehta.

Future with Cement-less Concrete 

So long the limestone is available, cement will be produced, and concrete structures will be built. But when the limestone reserve is exhausted, cement no longer available, cement-less concrete is the ray of hope for the construction industries. Such concrete is fly ash or g.g.b.s.-based geopolymer concrete, using sodium hydroxide and sodium silicate solutions as binders. Such geopolymer concrete using fly ash has been produced at a temperature of 650 C by Prof. Vijaya Rangan¹⁰ at Curtin University, Perth. The 7-day compressive strength of this concrete is 46.2N/mm2 . Rajamane an others11 produced geopolymer concrete using g.g.b.s. at ambient temperature of 30-350 , which had 28-day compressive strength of the order of 45MPa. 

The geopolymer concrete can be used in R.C.C. with grades of concrete upto M45. Such concrete can be produced with the normal equipment, similar to those used for conventional cement concrete, and is probably, the future of concrete in structures. 

Conclusions

Concrete, a versatile construction material is made of many ingredients with cement, a binding material. concrete is the integral part of the structures. With reinforcements, the reinforced concrete structures carry loads as designed and provide the desired service life, may be maximum upto about 100 years, unless they are exposed to severe environmental conditions or suffers some deleteratious reaction like alkali- aggregates reaction within the concrete. The chloride can corrode the reinforcements, if more quantity is inside or enters from outside and weaken the structures, reducing the life span. The chemical admixtures are a must in concrete, to develop the required property e.g. workability,cohesiveness, pumpability, self-compacting property and high strength. 

The mineral admixtures e.g. flyash (at least 25%) or ground granulated blast furnace slag (at least 50%) produce green concrete and help in combating the deleterious alkali- silica reaction inside the concrete. The other mineral admixture i.e. silica fume (upto 10%) help in producing high strength and abrasion resistance in concrete roads and spillways of concrete dams. The creep, shrinkage, elastic and plastic properties of concrete are developed in different stages and are termed in different contexts. The ‘plastic concrete’ is used to make a diaphragm wall in dam project. The future of cementbased concrete is uncertain, as the limestone deposits are getting exhausted. The geopolymer concrete with two chemicals i.e. sodium hydroxide and sodium silicate solutions, producing about 45MPa strength is the ray of hope, and may be the future of concrete construction. 

References:

1. Neville, A.M. properties of concrete, 4th edition, 2000, Pearson Education Pvt. Ltd. Singapore. 
2. BIS. code of practice for plain and reinforced concrete IS: 456-2000, Bureau of Indian Standards, New Delhi.
3. Rihand dam Expert Committee Report, Vol. 1, June 1986, published by Irrigation Department, Uttarpradesh.
4. IRC, Guidelines for use of silica fume in rigid pavement IRC:114-2013. The Indian Roads Congress, New Delhi. 
5. Mehta, P.K. and Wilbert S. Lngley. Monolith Foundation: Built to last a 1000 years. Concrete International, July 2000, pp. 27-32.
6. Baker, W.F. Burj Khalifa: A New Paradigm. The Indian Concrete Journal, Vol. 85, No.7, July 2011, pp, 8-12. 
7. Neville, A.M. Concrete Technology – an essential element of structural design. Concrete International, July 1988.
8. Maiti, Subhash Chandra. Plastic rotations in continuous encased beams, Journal of Structural Division, American Society of Civil Engineers, Vol. 101, NoST6, June 1975, pp. 1269-1281.
9. Mirghasemi A. Ali and Moshashai, H. Plastic concrete specification, a case study- Karkheh Dam project, in Advances in Concrete and Construction Technology, publication-3, 2002, Interline Publishing, Bangalore, p.p. 210-217
10. Rangan, B.V. Fly ash – based geopolymer concrete, Indian Concrete Journal, October, 2008.
11. Rajamane, N.P, Nataraja, M.C. Lakshmanan, N and Dattatreya. Rapid chloride permeability test on geopolymer and Portland cement concrete. The Indian Concrete Journal, Vol. 85, No. 10, October 2011, pp. 21-26.