The Basic Mechanical Performance And Strain Hardening Process Theoretical Analysis Of Ultra High Toughness Cementitious Composites

Ultra high toughness cementitious composites are a kind of high performance cementitious composites reinforced with medium fiber volume fraction random short fibers. This type of composites possesses strain hardening and multiple cracking under tensile and bending load with strain capacity more than 3% and maximum crack width below 0.1mm. Using this class of composites, it is potential to solve the structural problems inherent with today's typical concrete structures, such as security and long term durability. In this paper, the performance of UHTCC is researched by experimental studies and theoretical analysis.(1) The uniaxial tensile tests were carried out to characterize the tensile behavior of UHTCC. Test results show that UHTCC possess pseudo strain hardening and multiple cracking behaviors, as well as high ductility, toughness and crack width controlling capacity. Its tensile strain capacity is more than 3% and its maximum crack width is below 0.1mm. Moreover, the UHTCC specimens with double notches still exhibit pseudo strain hardening and multiple cracking under tensile load.(2) Four-point bending tests were carried out to characterize the flexural behavior of UHTCC. Test results show that UHTCC possess obvious deformation hardening, multiple cracking and high toughness properties under bending load. The ultimate deflection is more than 30mm which is comparable with the deformation capacity of aluminum and steel.(3) The uniaxial compression tests were carried out to characterize the compressive behavior of UHTCC. Test results show that UHTCC still possess multiple cracking behaviors. Those cracks coalesce gradually and inclined micro-cracks appear during the process of failure. The compressive strength is similar to that of ordinary concrete, and the elastic modulus is about 1/2-1/3 of that concrete, and the compressive strain at peak stress is about twice that of concrete, and the ductility and toughness have been improved obviously.(4) The relationships between tensile and flexural properties are established by the comparison of basic mechanical indexes under tensile and bending load. The comparison results show that the four-point bending test method is a simple and practical method to estimate the special mechanical performance of UHTCC substituting for the uniaxial tensile test method.(5) Based on the compression test results and referring to the existing stress-strain models of concrete, two analysis models were proposed for ultimate limit state design and non-linear analysis of UHTCC structures respectively. The study production provides necessary theoretical models for the practical engineering application of UHTCC.(6) The system for evaluating the compressive toughness properties is established by comparing different testing and evaluating standards of flexural toughness for steel fiber reinforced concrete used in various countries. Five toughness indexes are used from different points of view to quantitatively evaluate the compressive toughness of UHTCC in detail. The analysis results of all the indexes demonstrate that UHTCC possess high compressive toughness and plastic deformation capacity, and high post-crack load bearing capacity.(7) The strain hardening processes referring to multiple cracking and post multiple cracking behaviors are theoretically analyzed based on the micromechanics, fracture mechanics and mathematical statistics and the corresponding models are established. These models can be used to optimize material properties or to predict the macromechanical composite properties.