Abstract
In concrete repair systems, material properties of the repair material and the interface are greatly influenced by the moisture exchange between the repair material and the substrate. If the substrate is dry, it can absorb water from the repair material and reduce its effective water-to-cement ratio (w/c). This further affects the hydration rate of cement based material. In addition to the change in hydration rate, void content at the interface between the two materials is also affected. In this research, the influence of moisture exchange on the void content in the repair system as a function of initial saturation level of the substrate is investigated. Repair systems with varying level of substrate saturation are made. Moisture exchange in these repair systems as a function of time is monitored by the X-ray absorption technique. After a specified curing age (3 d), the internal microstructure of the repair systems was captured by micro-computed X-ray tomography (CT-scanning). From reconstructed images, different phases in the repair system (repair material, substrate, voids) can be distinguished. In order to quantify the void content, voids were thresholded and their percentage was calculated. It was found that significantly more voids form when the substrate is dry prior to application of the repair material. Air, initially filling voids and pores of the dry substrate, is being released due to the moisture exchange. As a result, air voids remain entrapped in the repair material close to the interface. These voids are found to form as a continuation of pre-existing surface voids in the substrate. Knowledge about moisture exchange and its effects provides engineers with the basis for recommendations about substrate preconditioning in practice.
Highlights
Moisture transport between a cementitious repair material and a concrete substrate determines the microstructural development of the interface and repair material in concrete repair systems [1,2]
Tested repair systems were investigated by a CT scanner in order to study effects of moisture exchange on repair system microstructure
X-ray absorption technique was used to investigate the dynamics of moisture exchange in repair systems and its effects on the formed microstructure
Summary
Moisture transport between a cementitious repair material and a concrete (or mortar) substrate determines the microstructural development of the interface and repair material in concrete repair systems [1,2]. A few studies on the moisture exchange in multilayer systems when fresh, newly-cast material was placed on the matured substrate have been reported [3,4,5]. In all of these studies, nuclear magnetic resonance (NMR) technique was used. Some preliminary studies on layered “Lego blocks” specimens made of freshly cast cement pastes were performed by using X-ray absorption [6] Most of these studies only focused on the moisture exchange: they indicated the relative change of moisture content in the repair system.
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