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Rajeev Gupta Business Head ECMAS Construction Chemicals Pvt. Ltd. Hyderabad |
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.
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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.
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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:
- A standard fibrillated polypropylene fiber to reduce and control shrinkage and temperature cracking.
- 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.
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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.
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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.
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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.