Gradient microstructure and strain field at interfacial zone between cement-based repair and concrete substrate

For concrete repair, due to the discontinuity of the interface microstructure, stress concentration easily occurs at the interface between repair material and substrate, which leads to debonding failure. In this study, interface with gradient microstructure between cement-based repair and substrate was built up by using waterborne epoxy resin (WER) to modify the repair material. 1H low field nuclear magnetic resonance (1H LF-NMR) tests showed that WER in the cement-based repair permeated into the substrate and distributed in gradient along the permeating direction. The permeating depth of WER increased from 6 mm to 10.5 mm when the WER dosage in the repair increased from 5% to 20% (by weight of cement). The porosity at the zone below the interface in the substrate decreased by more than 40% and the micromechanical properties were thereby obviously improved, when the repair mortar was modified with 15% WER. As a result, the strain concentration at the interfacial zone was significantly mitigated by the WER and the bonding strength was enhanced by more than 80%.