The influence of different bond primers on the bond strength of concrete overlays and the microstructure of the overlays transition zone

This study investigated the effect of different bond primers (BP) and the thickness of BP on bond performance. The mechanical and microstructural properties of BP were studied systematically using compressive strength, drying shrinkage, X-ray diffraction, and other methods; bond strength was evaluated using the slant shear test, and the failure modes were observed. Further, kernel density estimation was performed to qualify the crack width of the repair interface statistically characterised by analysing images obtained using the backscatteredelectron method. The results indicated that when the BP was modified using silica fume, nano-silica, and Portland cement after ball milling, the modified BP pores were refined because of the combined filler, packing, and pozzolanic effects. The drying shrinkage of BP modified by an expansive agent was reduced; the BP could narrow the repair interface gap or even disappear, and the modified BP made the interfacial zone denser and improved bond strength. The results of the microscopic characterisation method of the repair interface verify the multi-scale characterisation of the new and old concrete interface.