Effect of pigments on bond strength between coloured concrete and steel reinforcement

The effect of pigments on mechanical properties of coloured concrete intended for structural applications, including the bond stress-slip behaviour to embedded steel bars, is not well understood. Series of concrete mixtures containing different types and  oncentrations of iron oxide (red and grey colour), carbon black, and titanium dioxide (TiO2) pigments are investigated in this study. Regardless of the colour, mixtures incorporating increased pigment additions exhibited higher compressive and splitting tensile strengths. This was attributed to the micro-filler effect that enhances the packing density of the cementitious matrix and leads to a denser microstructure. Also, the bond to steel bars increased with the pigment additions, revealing their beneficial role for improving the development of bond stresses in reinforced concrete members. The highest increase in bond strength was recorded for mixtures containing TiO2, which was ascribed to formation of nucleus sites that promote hydration reactions and strengthen the interfacial concrete-steel transition zone. The experimental data were compared to design bond strengths proposed by ACI 318-19, European Code EC2, and CEB-FIP Model Code.