Developing strain-hardening ultra-rapid-hardening mortar containing high-volume supplementary cementitious materials and polyethylene fibers

This study aims to develop a robust strain-hardening ultra-rapid-hardening mortar (URHM) with high-volume cementitious materials and polyethylene (PE) fibers. To achieve this, the combined effect of cement kiln dust (CKD) and silica fume (SF) on the initial hydration process of ultra-rapid-hardening cement and the tensile performance of URHM was analyzed. Optimum amounts of CKD and SF of 0.15 and 0.2, respectively, by weight ratios to cement, were determined to develop the strain-hardening URHM containing 2% PE fibers. As a result, the tensile strength of 7.3 MPa, strain capacity of 5.12%, and energy absorption capacity prior to tension softening of 297.5 kJ/m3, respectively, were achieved at a very early age (4 h) of air-drying curing. The tensile performance of URHM deteriorated when the CKD content was 0.4 or greater, regardless of the SF content. A lower SF content of 0.2 was effective in terms of the tensile performance enhancement compared with the higher content of 0.4 up to the CKD content of 0.2, but they became similarly lower at higher CKD contents due to insufficient initial hydration

Strain-hardening effect on the flexural behavior of ultra-high-performance fiber-reinforced concrete beams with steel rebars

This study evaluated the effects of volume fraction, aspect ratio, and shape of steel fibers on the mechanical properties of ultra-high-performance fiber-reinforced concrete (UHPFRC) and the structural behavior of reinforced (R-) UHPFRC beams. The tensile strength and energy absorption capacity of ultra-high-performance concrete (UHPC) are improved by adding steel fibers and increasing its volume contents by up to 3.0 %. Compared with short straight steel fiber, medium-length straight and twisted fibers at a volume fraction of 2.0 % result in twice higher energy absorption capacity and higher flexural strength of R–UHPFRC beams. The flexural strength of R–UHPC beams increases by increasing the fiber content up to 3.0 %. However, the strain-hardening characteristics of UHPFRC negatively influence the cracking behavior and stress redistribution in structural beams, causing 48.2–54.1 % lower ultimate ductility indices. The small amounts of steel fibers with volume fraction of ≤1.0 % that exhibit strain-softening behavior only improve the peak ductility