Experimental Research On Ultra High Toughness Cementitious Composites

Ultra High Toughness Cementitious Composite (UHTCC) is a new kind of high performance cementitious composite, which is reinforced with less than 2.5% short fibers by composite volume fraction and can be mixed with normal procedures, while the hardened composite possesses significant strain hardening properties with the ultimate tensile strain above 3% and ultimate crack width below 100μm. Some UHTCCs even can well control the ultimate crack width under 50μm, which is harmless to safety, durability and bearing capacity of structures, thus the corresponding composites can be called 'Crack Free Concrete'. It can be expected that the research and application of UHTCC will be great helpful to solve the widely existing problems caused by the lack of toughness, small deformability and easily cracking of concrete in engineering. Accordingly, a series of experiments and analyses regarding the basic mechanical properties and elementary applications of UHTCC are conducted:1. Uniaxial tensile test is the only test method to verify a cementitious composite is strain hardening or not. Appropriate specimen preparation and testing method for uniaxial tension are proposed for strain hardening cementitious composites. Uniaxial tensile tests are conducted to confirm which of the 23 composites, designed by the author, satisfy the basic properties requirement of UHTCC.2. The tensile strain hardening process and the whole cracking process of UHTCC are reasonably described according to the experimental results and the phenomena observed in uniaxial tensile tests, besides, UHTCC's ability of dispersing wide harmful crack into harmless ones is also evaluated accordingly.3. Reasonable simplifications of tensile stress versus strain relationship before ultimate tensile strength are conducted according to experimental curves, and the corresponding constitutive equations are also given accordingly.4. Property of notch insensitivity of UHTCC is verified with uniaxial tension tests on the double-notched plate specimens.5. Third-point bending tests are conducted to study the flexural properties of UHTCC, and great attention is paid to observe the ability of dispersing harm crack into harmlessly ones of UHTCC under flexural loading.6. Appropriate flexural toughness evaluating method is porposed for UHTCC. 7. Regarding how to measure the ultimate tensile strain of strain hardening material with third-point bending test, new reverse method named NRA method is proposed after analyzing the existing methods of JCI method and UM method.8. Flat-shape and U-shape small scale permanent formworks made of UHTCC are cast with different surface corrugation, and concrete is cast above to make UHTCC permanent formwork/concrete composite beams. Third-point bending tests on composite beams confirm that UHTCC is an ideal material for production of participating permanent formwork, UHTCC permanent formwork can promote structural toughness and structural bearing capacity, especially, it can effectively disperse the wide harmful crack into multiple fine harmless flat cracks. It's observed that the use of U-shape permanent formwork can effectively control the crack width under a very fine level along the whole depth of the beams, which may be greatly beneficial for improving structural durability.