Factors affecting bond strength of RC column jackets

The demand for using concrete jackets to strengthen or repair reinforced concrete columns has been increasing in the past few decades. Previous research work studied the effect of substrate surface treatment on the bond strength between the substrate concrete and overlay concrete. This paper presents an experimental and theoretical study on the effect of dowels and jacket stirrups on generating shear friction and, therefore, enhancing the overall bond strength between reinforced concrete columns and added reinforced concrete jackets. Grinding and hand-chiseling were adopted as surface roughening techniques as well as using a bonding agent at the interface between the substrate concrete and overlay concrete. An experimental program consisting of seven reinforced concrete cube specimens having two-side jackets and four reinforced concrete cube specimens having four-side jackets was conducted. Direct shear tests were adopted for this study as they represent the state of stresses usually exists in actual situations. It was found that increasing the substrate concrete surface roughness by hand-chiseling considerably improved the bond strength compared with grinding. The confining effect of stirrups used in four-side reinforced concrete jackets proved to be effective in enhancing the overall bond strength and could outperform the effect of dowels.