Shear behavior of the interface between magnesium phosphate cement and old Portland cement concrete under freeze-thaw action

Magnesium phosphate cement (MPC) is frequently used as a repair material for the damaged concrete engineering structures because of its strong adhesion to ordinary Portland cement concrete (PCC). However, studies on the fracture characteristics and mechanism of the MPC-PCC composite interface under freeze-thaw (F-T) action are scarce. In this study, the shear fracture behavior of the interface was investigated in-depth, and a novel F-T degradation model of the interfacial bond strength was established. Results show that F-T action results in serious strain softening of the interface, weakening its brittle damage characteristics. The shear failure modes of the interfaces under F-T action are divided into "cutting off along interfaces", "cutting off inside MPC", and "mixed shear-off between MPC and PCC". With increasing F-T cycles, the cohesive strength for MPC cutting gradually becomes the dominant factor to the interfacial shear strength. Based on the above results, the F-T degradation mechanism of MPC-PCC composite interface was further explained.