Overlay transition zone in concrete repair: Insights into microstructural evolution and micromechanical properties

Abstract

Ultra-high toughness cementitious composites (UHTCC) are increasingly employed to repair and strengthen deteriorated concrete structures, yet the critical microstructural evolution and micromechanical properties of overlay transition zone (OTZ) remain underexplored. We report a comprehensive, curing time-dependent study of OTZ between cast and sprayed UHTCC and concrete substrates (CS). The findings reveal a dual-scale OTZ structure: (1) the narrow OTZ, impacted by the wall effect, and (2) the broad OTZ, comprising an air void-rich area, the narrow OTZ, and a reaction zone on the CS surface. The thickness of the broad OTZ, governed mainly by the air void-rich area, decreases over time to around 200 μm at 28 days in cast specimens. Spraying shows a dual effect on the broad OTZ, reducing interfacial gaps at 3 days by enhancing UHTCC-CS contact while generating a thicker (300 μm) air void-rich zone at 28 days. Ion migration from UHTCC to the CS surface increases the local mean elastic modulus through the formation of secondary hydrates, like calcium hydroxide and C-(A-)S-H gels. Ulteriorly, we discuss and summarize the evolution mechanisms driving the microstructure and micromechanical properties of OTZ. These insights lay the foundation for the bottom-up, cost-effective engineering regulation of OTZ in concrete repair.