In concrete repair systems, material properties in the repair material and interface are greatly influenced by the initial moisture content of the concrete (or mortar) substrate. In order to quantify moisture profiles inside the repair system, X-ray absorption was used. Preliminary studies are performed first to determine the absorption rate of a two-year-old mortar substrate. Cumulative water absorption and the penetration front of water are monitored as a function of time. Furthermore, absorption by the mortar substrate is investigated when a repair material is cast on top. Influence of water-to-cement ratio (w/c) of the repair material on water exchange, and moisture content change during drying in the repair system are studied. Ordinary Portland cement (OPC) paste was used as a repair material. It was shown that, initially, water from the repair material is absorbed by the substrate with the same speed as “free” water. Furthermore, not only can water be absorbed by the substrate, but it can also migrate back from the substrate to the repair material due to hydration of the repair material. Quantifying the dynamics of water exchange at an early stage between the repair material and the concrete substrate is a first step towards understanding the development of material and interface properties in repair systems. This can provide engineers with recommendations about substrate preconditioning in experiments and field practice.
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