Moisture exchange in concrete repair system captured by X-ray absorption

One of the main reasons for premature failure of concrete repair systems are stresses due to differential shrinkage between the old (concrete or mortar substrate) and new (repair) material. These are primarily caused by moisture exchange between the repair system and environment or moisture transport within the repair system itself. However, during the setting and hardening of the repair material, moisture exchange will also determine the development of material properties in the repair material and interface between repair material and substrate. In this paper, moisture profiles are quantified using X-ray absorption technique. The output of X-ray attenuation measurement is a spatial distribution of linear attenuation coefficients expressed as grey scale values. First, preliminary studies are performed to determine the absorption rate of the mortar substrate. Cumulative water absorption and penetration depth of the water front are monitored as a function of time. Furthermore, absorption of the substrate when water is absorbed from the top is investigated. Main advantages and drawbacks of the method are presented and compared with results from the gravimetric test. Following, moisture exchange in the repair system is investigated. In one specimen, a primer was used. When curing is stopped, drying profiles in the repair system are quantified. Knowledge about the water exchange between the repair material and the concrete substrate, which takes place at the early stage, is only the first step towards morejustified decisions about substrate preconditioning and possible use of primers and curing compounds in experiments and field practice.