Microstructure of the interface in concrete repairs

A key parameter of the performance of concrete repairs is the bond strength between the repair material and the concrete substrate. Sufficient bond strength is required to withstand the stresses at the interface induced by the environmental and/or mechanical loads. The bond strength is determined by the microstructure of the repair-substrate interface. The microstructure development of the repair-substrate interface is influenced by the moisture exchange between the repair material and the concrete substrate and the “wall effect”. To understand the fundamental bond mechanisms, the microstructure development in concrete repairs are experimentally investigated using the scanning electron microscopy. The experimental results reveal that when the concrete substrate has lower water content, it absorbs more water from the repair material and results in a lower porosity both in the interface and in the repair material. Due to the “wall effect”, the cement particles have a poor packing at the repair-substrate interface and the w/c ratio locally increases. The increased w/c ratio results in a porous interfacial zone and poor mechanical properties.