Mechanical properties and microstructure of ultra-high strength concrete with lightweight aggregate

In recent years, the development of concrete has been toward to the direction of lightweight and high strengthening. This study developed a kind of lightweight ultra-high strength concrete(L-UHSC) through the comprehensive design method, which had an apparent density rang of 1995 kg/m3-2114 kg/m3 and a compressive strength of 102.4–114.5 MPa. The specific strength of the designed L-UHSC is more than 50 MPa/(t/m3), which is higher than ordinary structural lightweight aggregate concrete (LAC). Additionally, the roles of high-performance paste, lightweight aggregate (LWA) and steel fiber in L-UHSC were revealed through scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP) and strength test. The findings revealed that SF content, rather than the binder-sand ratio and aggregate ratio, has a greater impact on density and strength, and the aggregate ratio having the least effect. As steel fiber content increases from 0.5 % to 2.0 %, compressive strength, flexural strength and splitting tensile strength of LAC increase by 44.3 %, 120 % and 151 %, respectively, which is higher than that of ultra-high performance (UHPC). The micromorphology and pore structure research suggested that the high-performance paste plays a keystone role in achieving the ultra-high strength, and LWAs mainly affected strength through improving pore structure. In addition, small size pre-wetted LWA is more conducive to concrete strength. Our preliminary results provide a reference for the research and development of lightweight ultra-high strength cement composites.