Preliminary study on interfacial bond strength due to successive casting lifts of self-consolidating concrete - Effect of thixotropy

- Thixotropic SCC is sensitive to delays during casting of successive layers.
- Use of slant shear and pull-off methods to evaluate SCC interfacial bond.
- Determination of critical times to avoid drops in interfacial bond strengths.
- Correlating critical times during casting and interfacial bond strengths.

Vibration is prohibited during placement of freshly mixed self-consolidating concrete (SCC), making it particularly sensitive to delays or stoppages during multi-layers casting. This paper seeks to propose suitable testing methods that can be used to assess the effect of delays between successive lifts on interfacial bond strength of hardened material, including the proposal of time limits necessary to ensure proper mixing of cast layers. Two methods for testing bond by slant shear and pull-off are tested and compared. The SCC mixtures were proportioned to exhibit low to high thixotropic levels, while the relative delay between cast layers was set to 30, 60, and 120 min after initial mixing. Test results have shown that the lowest drop in interfacial bond strength occurred for moderately thixotropic mixtures, given the moderate bleeding rate that prevented dryness of the first cast layer while, at the same time, avoided excessive increase in water that reduces bond strength development along the interfaces.