Overlay transition zone in concrete repair under extreme marine conditions: Insights into microstructural deterioration and micromechanical properties

Today, the ongoing coastline hardening escalates the risk of corrosion-induced deterioration of coastal infrastructure, emphasising the necessity for sustainable, durable, and resilient concrete repair solutions. We present a systematic, exposure time-dependent study of overlay transition zone (OTZ) between cast/sprayed ultra-high toughness cementitious composites (UHTCC) and concrete substrates (CS) under simulated extreme marine conditions. Results validate the dual-scale OTZ structure model, differentiating among the air void-rich area, the narrow OTZ, and the reaction zone on the CS surface, collectively constituting the broad OTZ, and also reveal a deepening layer-by-layer zonation deterioration process of OTZ at the exposed surface. Three major zones are identified irrespective of casting methods or exposure durations: a superficial calcium carbonate layer, a calcium leaching and magnesium binding zone within bulk UHTCC, and a densifying zone within OTZ. Persistent coupled stresses are prone to release in the narrow OTZ, forming an inward-propagating major crack with a width below 50 μm. Spraying enhances OTZ durability by improving the initial interfacial packing and C-(A-)S-H gel distribution, which leads to reduced deterioration depth, fewer corrosion products, and limited crack propagation. Finally, we discuss and summarise the mechanisms driving microstructural deterioration and the evolution of micromechanical properties of OTZ.