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Conventional concretes are almost un-bendable and have a strain capacity of only 0.1 percent making them highly brittle and rigid. This lack of bendability is a major cause of failure under strain and has been a pushing factor in the development of an elegant material namely, Engineered Cementitious Composites abbreviated as ECC. This material is capable to exhibit considerably enhanced flexibility. An ECC has a strain capacity of more than 3 percent and thus acts more like a ductile metal rather than like a brittle glass. A bendable concrete is composed of all the ingredients of a traditional concrete minus coarse aggregates or crushed stones and is reinforced with micromechanically designed polymer fibers.
Plain concrete possesses a very low tensile strength, low ductility and little resistance to cracking. Internal micro-cracks are inherently present in the concrete (due to drying shrinkage) and its poor tensile strength is due to the propagation of these cracks (under loading), eventually leading to brittle failure of the concrete.
It is a well-known fact that the addition of small, discrete and uniformly dispersed fibers in concrete acts as a barrier to crack propagation and improves its mechanical properties. Such type of concrete consisting of concrete mix (cement, sand, coarse aggregates, water and sometimes admixtures) containing uniformly dispersed discrete fibers is known as Fiber-reinforced concrete abbreviated as FRC. The conventional fiber-reinforced concrete is prepared by using different material fibers, the important ones being steel, polypropylene, asbestos, glass and carbon fibers.
The material ingredients which make up ECC are similar to FRCs that is it contains cement, sand, water, and admixtures. However, coarse aggregates are not used in ECCs (hence it is a mortar rather than concrete). Additionally, ECC uses low amounts, typically 2% by volume, of short, discontinuous fibers. ECC incorporates super fine (100 microns in diameter) silica sand and tiny Polyvinyl Alcohol-fibers covered with a very thin (nanometer thick), slick coating. This surface coating allows the fiber to begin slipping when they are over loaded so they are not fracturing. It prevents the fiber from rupturing which would lead to large cracking. Thus an ECC deforms much more than a normal concrete but without fracturing. The different ingredients of ECC work together to share the applied load.
According to geologists, sand particles range in diameter from 0.0625 mm to 2 mm. Silica sand is one of the most common varieties of sand found in the world. Silicon sand is defined as sand which contains 80-90% of silica (SiO2) particles. Polyvinyl alcohol (PVOH, or PVA) is a water-soluble synthetic polymer. Polyvinyl alcohol has high tensile strength and flexibility. Prior to use in ECC, these fibers are slick-coated. Although complete friction elimination is not possible by using any slick coating, the emphasis is to reduce the friction at its maximum. According to geologists, sand particles range in diameter from 0.0625 mm to 2 mm. Silica sand is one of the most common varieties of sand found in the world. Silicon sand is defined as sand which contains 80-90% of silica (SiO2) particles. Polyvinyl alcohol (PVOH, or PVA) is a water-soluble synthetic polymer. Polyvinyl alcohol has high tensile strength and flexibility and are well-suited to be used in making ECC.
Polyvinyl Alcohol Fibers
Figure Bendable concrete
ECC has proved to be 500 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.