Interfacial debonding mechanism of magnesium phosphate cement onto old concrete substrates under freeze-thaw conditions

The interface created by the repair of damaged ordinary Portland concrete (OPC) structures using magnesium phosphate cement (MPC) is prone to cracking and debonding under freeze-thaw (F-T) conditions and may even cause engineering disasters. In this study, the entire debonding process, moisture migration and debonding mechanism of the MPC-OPC composite interface under F-T actions were investigated and a novel mixed mode fracture criterion was established. The primary findings are drawn as follows: (i) The interfacial crack initiates and propagates along the interface or kinks into the MPC side during F-T actions, depending on the position and thickness of the localized interface F-T damage zone (IFDZ) close to the interface on the MPC side; (ii) The net suction force causes capillary and film water in the mesopores and micropores to migrate to the freezing macropores inside the interface layer, whereas the increase in the liquid pressure caused by the water-ice phase transition causes crack initiation in the macropores and even the interface layer damage; (iii) The stiffness difference between MPC and OPC increases as the molar ratio of magnesia and phosphate (M/P) or the mass ratio of water to cement (W/B) of MPC increases, and the interface debonding mode shifts from propagating along the interface to kinking into the MPC side.