Improved TSTM for shrinkage cracking characterization of UHPC with expansive agent under various degrees of restraint

Determination of the shrinkage compensation “time window” of ultra-high-performance concrete (UHPC) is a multifaceted challenge influenced by the shrinkage and restraint degree. Due to the high requirements for measurement and control accuracy of constraint shrinkage test, it remained difficult to quantify the shrinkage compensation effect of expansive agent (EA) on the cracking behavior of UHPC under varying engineering structures. In this study, a non-destructive modification scheme was introduced to enhance the test precision of the commercial temperature stress testing machine (TSTM) in strain measurement, temperature regulation, and programming control. Moreover, the testing accuracy of different improvement strategies was compared and validated. Based on the improved TSTM, the cracking behavior, including shrinkage deformation, restrained stress, elastic modulus and creep, of UHPC containing EA under varying restraint conditions was conducted. Experimental results indicated that, under a high restraint degree, there is a limited shrinkage compensation time window after the final set and before cracking due to the very early cracking age of UHPC. Adequate expansive products need to be generated before cracking to ensure that the restrained stress is always lower than the tensile strength . Under a low restraint degree, EA can effectively reduce the cracking risk by compensating for shrinkage, increasing creep, and reducing elastic modulus . These results provide valuable insights into designing the compensation time of EA in UHPC under complex restraint conditions by using TSTM. • A non-destructive modification scheme was introduced to enhance the test precision of TSTM. • The shrinkage cracking behavior of UHPC with expansive agent was conducted. • The shrinkage compensation effect of EA on the cracking behavior of UHPC under varying restraint degree was analyzed.