UCLA

This research is developed to investigate the influences of added phase change material (PCM) microcapsules on the freeze-thaw durability, the compressive behaviors, and the flexural behaviors of hybrid fiber-reinforced concrete. The concrete mix proportions are designed with the PCM volume fraction varying from 0 to 9%. The hybrid fiber, inclusive of steel fibers and polyvinyl alcohol microfibers, is added in a constant dosage of 1.1% by volume. Mechanical performance of the concrete is evaluated by performing uniaxial compression tests and three-point bending tests. Subsequently, the concrete specimens are weathered with a tailor-make freeze-thaw machine in a controlled environment. After the freeze-thaw cycles, the compressive strength, modulus of elasticity, fracture energy, equivalent flexural tensile strength, and residual flexural tensile strength are determined for each mixture design and compared with the values determined prior to the cycles for the evaluation of freeze-thaw damage. The internal damage extent of concrete is monitored by ultrasonic pulse velocity tests over the freeze-thaw cycles. The experimental results show that the mechanical performance is considerably improved with the added hybrid fiber. The improvement on the compressive strength sufficiently compensates for the strength loss due to the incorporated PCM microcapsules; the additions of the hybrid fiber and the PCM contribute to different aspects of freeze-thaw durability (i.e., internally or superficially), at least to a different degree. A novel damage model, considering the beneficial and detrimental effects of adding PCM, is proposed to quantitatively describe damage extents observed in the experimental results. From this model, the optimized dosage of PCM is provided for hybrid fiber-reinforced concrete subjected to severe freeze-thaw cycles.