Mechanical degradation of ultra-high strength alkali-activated concrete subjected to repeated loading and elevated temperatures

In this work, a clinkerless alkali-activated slag-based ultra-high strength concrete (AAS-UHSC) with tailored mix proportions was developed at room temperature. To evaluate its practical serviceability, a systematic investigation was conducted on the fresh and mechanical properties (compressive, splitting tensile, and flexural strengths), with an emphasis on the uniaxial compressive behavior of AAS-UHSC subject to repeated loading and elevated temperatures. The results showed that despite the fast setting of AAS-UHSC, a significant improvement in flowability could be obtained with a slight increase in water-to-binder ratio. Regarding the strength development during the curing period, a higher early compressive strength was observed for AAS-UHSC when compared with ordinary Portland cement (OPC)-based UHSC, but a contrary behavior was found for the evolution of splitting tensile strength. Moreover, relative to the fiber-free AAS-UHSC, great improvements up to 31 times and 2.5/4.3 times in the flexural fracture energy and monotonic/cyclic compressive toughness were achieved for the specimens containing 1.5% steel fiber by volume, respectively. The superior high-temperature performance of AAS-UHSC free of explosive spalling could be attributed to its intensive shrinkage cracking upon dehydration, which likely leads to a significant enhancement of pore connectivity as the exposure temperature increases.