Restrained Shrinkage Mechanism of Ultra High Performance Concrete

The understanding and controlling of the restrained shrinkage is critical for the application of ultra high performance concrete (UHPC). This study made an analysis of restrained shrinkage mechanism of four kinds of UHPCs based on the direct tensile test accompanied with acoustic emission (AE) source location, free shrinkage test and restrained shrinkage test. The effects of UHPC tensile properties (strain softening or strain hardening) and high performance calcium sulphoaluminate (HCSA, a new kind of expansion agent) dosages (0%, 3% and 6% by mass of total binder) were investigated. The restrained shrinkage cracking mechanism of strain hardening UHPC and strain softening UHPC was analyzed based on AE analysis method under direct tensile loading. The results indicates that strain hardening UHPC shows multiple micro-defects to relax the restrained tensile stress marginally while strain softening UHPC shows several hairline cracks to relax the restrained tensile stress evidently. In ASTM C1581-04 ring test, the restrained shrinkage of strain hardening UHPC with HCSA dosage of 0%, 3% and 6% at 80d is 141 μe, 96 μe and 16 μe, respectively. The HCSA expansion agent can effectively reduce the restrained shrinkage of UHPC and the influence on the structure.