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.