Microstructural damage characterization of NC-UHPC composite under salt freeze-thaw cycles based on ex-situ X-ray computed tomography

In the harsh environment of extreme salt-freezing conditions, concrete structures face secondary durability issues after undergoing repairs, making it a critical yet challenging topic. This paper employs a novel ex-situ X-ray computed tomography approach to explore the mechanism of damage evolution in normal concrete (NC) to ultra-high performance concrete (UHPC) composites under salt-freezing conditions. For the first time, we consider evolution in both external and internal pores to quantitatively assess the damage. Additionally, slant shear tests, Mercury Intrusion Porosimetry (MIP) were also employed to validate the damage mechanism of NC-UHPC. Finally, empirical formulas summarizing the bond strength and pore changes in NC-UHPC were derived. The results reveal a crucial factor influencing the progression of damage in the NC-UHPC composite: the presence of external pores that directly interact with the salty solution. Damage initiation primarily occurs within the NC-UHPC composite due to these external pores located in the NC region, subsequently extending into the overlay transition zone (OTZ) and UHPC sections. Remarkably, the resistance of the OTZ, previously identified as the weakest zone in the NC-UHPC composite, surpasses that of the NC. This exceptional performance can be attributed to the higher porosity of the OTZ, offering additional space for the dissipation of pressure caused by freezing. What's even more important is that this highlights the consistent origin of damage in the NC-UHPC composite, emphasizing that it always begins within the NC. Considering this perspective prompts the question of whether the exceptionally high strength and durability of UHPC may lead to an excess of repair capabilities?