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Interfacial Properties between New and Old Concretes

To better understand bonding mechanism at the interface between new and old concrete surfaces, overlaid specimens were fabricated to measure shear bond strength. Old concrete cubes with two different moisture conditions (saturated surface dry (SSD) and air dry) were chosen. Three different materials with two w/c ratios and a silica fume addition were used for new overlay concrete. From the shear tests, it was found that silica fume in new concrete significantly increases not only the compressive strength of new concrete but also the shear bond strength at the interface. New concrete with low w/c ratio resulted in high compressive strength but lower shear bond strength for both surface conditions compared high w/c ratio. For all overlay concretes it was consistent that SSD surface condition results in much higher bond strength compared to air dry. SEM was employed to characterize material in micro-level at the interface and to determine interfacial zone.