Cracking behavior of ultra-high strength mortar with CaO-based expansive agent and superabsorbent polymer

•The shrinkage cracking behavior of ultra-high strength mortar (UHSM) with CaO-based expansive agent (CaO-based EA) and superabsorbent polymer (SAP) was observed based on the temperature stress testing machine (TSTM) tests.•The coupling mechanism of EA and SAP was investigated by comparing the cracking process to hydration progress, hydration product and internal water distribution.•Although the addition of EA and SAP can significantly increase the expansive deformation of UHSM at early age, cracking still occurs due to the post-peak shrinkage.•“Time-zero” of shrinkage should be redefined in UHSM with EA and SAP due to the change in early age deformation rate. Employing CaO-based expansive agent (CaO-based EA) and superabsorbent polymer (SAP) in ultra-high performance concrete (UHPC) is a effective strategy to mitigate the shrinkage. The cracking identification can help to verify whether the cracking risk can be reduced. In this paper, the shrinkage cracking behavior of ultra-high strength mortar (UHSM, UHPC matrix) with different dosage of CaO-based EA and SAP was observed based on the temperature stress testing machine (TSTM) tests. The coupling mechanism of EA and SAP was investigated by comparing the cracking process to hydration progress, hydration product and internal water distribution at each stage. Furthermore, the creep and elastic modulus were also calculated to clarify the cracking risk development of UHSM with EA and SAP. Unlike ordinary concrete, “Time-zero” of shrinkage should also be redefined in UHSM with EA and SAP due to the change in early age deformation rate. Although the addition of EA and SAP can significantly increase the expansive deformation of UHSM at early age, cracking still occurs due to the post-peak shrinkage.