Interfacial shear bond strength between old and new concrete

Abstract

The quality of the bonded concrete overlay depends on the bonding at the layer’s interface, which is affected by water to cement (w/c) ratio in new concrete, moisture condition of old concrete surface. The objective of this study is to quantify the interface shear bond strength affected by these variables. At the Phase I, twelve old concrete cubes with two different moisture conditions (air dry, saturated surface dry (SSD)) at the interface were chosen. Silica fume addition with two w/c ratios of 0.45 and 0.6 were used for new overlay concrete. It was found that silica fume significantly increased both compressive strength of new concrete and shear bond strength at the interface. New concrete with the w/c ratio of 0.45 resulted in higher compressive strength but lower shear bond strength It was found to be consistent that SSD resulted in higher bond strength. To better understand the bonding mechanism at the interface, overlaid cylinder specimens were fabricated with three moisture conditions (air dry, SSD, and wet) in Phase II. Seven different Supplementary Cementitious Material (SCM) addition with three w/c ratios (0.35, 0.45, and 0.55) were used for overlay concrete. The addition of fly ash developed lower bond strength compared to control group. This is considered due to slow strength gain of overlay concrete with fly ash at early age. The wet resulted in low bond strength and most of AD resulted in high. Similar to Phase I, SSD consistently developed high bond strength. In contrary to Phase I, new concrete with low w/c ratio resulted in high bond strength. Scanning Electron Microscope (SEM) and Energy-dispersive X-ray Spectroscopy (EDAX) were employed to characterize the material. C/S ratio at the surface was quantified from the EDAX. It was found that with the addition of G100 slag, C/S ratio decrease with increasing w/c ratio for all moisture conditions, i.e. the increase of shear bond strength. Due to the complexity of shear bond strength tests, an alternative method was seeked by estimating C/S ratio at the interface. However, the bond strength and C/S ratio at the interface did not have a strong relationship.