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IntroductionIn the last several decades, concrete with increasingly high compressive strength have been used for structural applications. However, most of these materials remain brittle. In some cases, the brittleness as measured by the brittleness number (Hillerborg, 1983) actually increases as the compressive strength goes up. This poses potential danger and limitations of high strength concrete in structural applications. In certain locations, such as where steel and concrete come into contact (e.g. steel anchors in concrete at column base) or in connections of steel/concrete hybrid structures, the high stress concentration created can lead to fracture failure of the concrete. In seismic elements, high ductility in the concrete can make a significant difference in the seismic response of the overall structure. These and other examples discussed below point to the need to develop costeffective high ductility cementitious materials suitable for structural applications. In the last several years, the University of Michigan has been investigating a composite material known as Engineered Cementitious Composites, or ECC for short. In many respects, this material has characteristics similar to medium to high strength concrete. However, the 2 tensile strain capacity generally exceeds 1% with the most ductile composite in the 6-8% range. This article briefly reviews these emerging materials, and also reports on some ongoing developmental application studies. What is ECC?In terms of material constituents, ECC utilizes similar ingredients as fiber reinforced concrete (FRC). It contains water, cement, sand, fiber, and some common chemical additives. Coarse aggregates are not used as they tend to adversely affect the unique ductile behavior of the composite. A typical composition employs w/c ratio and sand/cement ratio of 0.5 or lower. Unlike some high performance FRC, ECC does not utilize large amounts of fiber. In general 2% or less by volume of discontinuous fiber is adequate, even though the composite is designed for structural applications. Because of the relatively small amount of fibers, and its chopped nature, the mixing process of ECC is similar to those employed in mixing normal concrete. Also by deliberately limiting the amount of fibers, a number of proprietary studies have concluded economic feasibility of ECC in specific structural applications. Various fiber types can be used in ECC, but the detail composition must obey certain rules imposed by micromechanics considerations (Li, 1998;Kanda and Li, 1998). This means that the fiber, cementitious matrix, and the interface (mechanical and geometric) properties must be of a correct combination in order to attain the unique behavior of ECCs. Thus ECC designs are guided by micromechanical principles. Most data so far has been collected on PVA-ECC (reinforced with PolyVinyl Alcohol fibers) and PE-ECC (reinforced with high modulus polyethylene fibers). 3The most fundamental mechanical property difference between ECC and FRC is that ECC strain-hardens rather than tens...

The micromechanics design theory has realized random short fiber-reinforced cement composites showing pseudostrain hardening (PSH) behavior with over 5% of strain capacity under tension. Nevertheless, this existing theory currently is limited to specific constituent properties, which does not account for chemical bond and fiber rupture. This article presents a new design theory that eliminates this restriction, achieving fiber rupture type PSH-random short fiber-reinforced cement composites with high-performance hydrophilic fibers like polyvinyl alcohol fibers. Uniaxial tensile tests are conducted employing polyvinyl alcohol fiber composites, the results of which support the validity of the proposed theory. Furthermore, parametric study employing the proposed theory quantitatively evaluates the effects of composite's micromechanics parameters, such as bond strength and fiber strength, on composite performance. This parametric study reveals that continuously increasing the degree of fiber rupture (fiber rupture intensity) enhances the strength performance of composites but not energy performance. However, an optimum rupture intensity exists for maximizing energy performance, which is critical for PSH behavior. The consistency between theoretical predictions and experimental results consequently demonstrates that the proposed theory can be utilized practically as a powerful and comprehensive tool for PSH composite design.

Engineered Cementitious Composites (ECCs) have recently demonstrated their high performance with pseudo strain hardening (PSH) behavior in civil engineering structures and buildings. These materials incorporate low cost fibers such as Polyvinyl Alcohol fibers, which often rupture in composites. Such fiber rupture type ECCs tend to have inferior and unsaturated PSH behavior compared with those incorporating properly designed pull out type fiber. The present study focuses on presenting practical design criteria to achieve saturated PSH behavior in fiber rupture type ECCs. These criteria are proposed based on two performance indices, which are measures of energy exchange during steady state flat crack propagation and stress level to initiate micro-cracks. The latter performance index necessitates a new cracking strength prediction theory, which is proposed in the current study. Finally the cracking strength theory is justified using tensile test data, and the criteria are proposed based on the data in terms of these two indices.

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