Saturday, July 2nd, 2022
CECR

Concrete In Structures – A Typical State Of The Art

 

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 NaO 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.

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