The flexible concrete also known as bendable concrete is a unique construction material that can bring the revolution because of its some special quality such as flexibility, self-healing, lighter weight, etc. In India it is still a new material and requires proper research for its use in India.
The lack of bendability in concrete is a major cause of failure under strain and has been a pushing factor in the development of an elegant material. The recently developed smart building material namely Bendable concrete also known as Flexible concrete or Engineered Cementitious Composite (ECC), was developed by the Professor Victor Li at the University of Michigan in 2005. It is a class of ultra-ductile fibre reinforced cementitious composites characterised by high ductility and tight crack width control. This material is capable to exhibit considerably enhanced flexibility. A bendable concrete is reinforced with micro-mechanically designed polymer fibres.
ECC is made from the same basic ingredients as conventional concrete but with the addition of High-Range Water Reducing (HRWR) agent is required to impart good workability. However, coarse aggregates are not used in ECCs (hence it is a mortar rather than concrete). The powder content of ECC is relatively high. Cementitious materials, such as fly ash, silica fume, blast furnace slag, silica fume, etc., may be used in addition to cement to increase the paste content. Additionally, ECC uses low amounts, typically 2% by volume, of short, discontinuous fibres. ECC incorporates super fine silica sand and tiny Polyvinyl Alcohol-fibres covered with a very thin (nanometer thick), slick coating. This surface coating allows the fibre to begin slipping when they are over loaded so they are not fracturing.
For giving the concrete flexibility we have to alter the material of the conventional concrete. In the flexible concrete we eliminate the coarse aggregate. Instead of that we use the fibres that are used in the fibre reinforced concrete such as silica fibres, glass fibres, steel fibres, asbestos fibers, polyvinyl alcohol fibers, etc. The micro fibres provide the flexibility to the concrete. It also act as a reinforcement material in the concrete.
Additionally the slick coating (anti-frictional coating) is provided so that the fibers particles can slide over one another and does not have the friction which may result into the cracks in concrete. This tendency of slipping of fibers over one another helps the concrete to minimize the crack and provide flexibility to the concrete. The normal cement can be used for the concrete.
The fine silica sand is suitable for the flexible concrete which is used in the water treatment plant but if it is not available the normal sand can be used but it may affect the strength and flexibility of concrete. We can also add fly ash, silica fume, blast furnace slag can also be used in the concrete.
Polycarboxylated ether, lignosulfonates, lignin, naphthalene or melamine formaldehyde sulphonate (any one) is used in the concrete for increasing the workability of the concrete. Latex can also be used in the concrete for improving its flexibility.
Conventional concretes are almost unbendable and have a strain capacity of only 0.1 % and are highly brittle and rigid. An ECC has a strain capacity of more than 3 % and thus acts more like a ductile metal rather than like a brittle glass. It prevents the fibre from rupturing which would lead to large cracking. Thus an ECC deforms much more than a normal concrete but without fracturing. Figure 1 represents the behavior of ECC under flexural loading and it can be seen that the beam can deform well without direct failure. The different ingredients of ECC work together to share the applied load. ECC has proved to be 50 times more flexible than traditional concrete, and 40 times lighter, which could even influence design choices in skyscrapers. Additionally, the excellent energy absorbing properties of ECC make it especially suitable for critical elements in seismic zones. The strength of the bendable concrete is comparatively higher than the conventional cubes and slabs. This higher strength shows that the presence of PVA fiber in bendable concrete has increased its efficiency.
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