Interfacial behaviour of bonding between ultra-high performance concrete and concrete substrate: Evolution of microstructure and micromechanical properties

Abstract

Ultra-high performance concrete (UHPC) is increasingly used to repair and strengthen deteriorated concrete structures. However, the crucial details of the microstructural evolution and micromechanical properties of overlay transition zone (OTZ) in composite structures are insufficiently understood. This study presents a systematic, curing-age-dependent investigation of OTZ between UHPC and concrete substrate (CS) across curing ages ranging from 1 to 28 days. A series of tests were performed to examine the hydration kinetics, grid elastic modulus, coefficient of friction, micromorphology, and 3D pore distribution of OTZ. Our findings suggest a dual-scale redefinition of OTZ: (a) a narrow OTZ affected by the wall effect and (b) a broad OTZ that encompasses the reaction zone on the CS surface, the narrow OTZ, and the air void-rich zone. The thickness of the broad OTZ is dominated by the air void-rich zone and decreases with curing age, measuring approx. 110 μm at 28 days. Ions from the fresh UHPC migrating towards the CS surface undergo mild, ongoing secondary reactions with the existing hydrates, generating additional Ca(OH)₂. After 1–3 days of curing, an easily identifiable blend band of Ca(OH)₂ and C–S–H gels and a tight bond between UHPC and CS can be seen simultaneously in the narrow OTZ. With prolonged curing (7 and 28 days), this band fades as most of Ca(OH)₂ is converted into C–S–H gels due to the pozzolanic activity of silica fume. This study concludes with an in-depth discussion of the evolution mechanisms driving the microstructure and micromechanical properties of OTZ.