Influence of time-dependent rheological properties on distinct-layer casting of self-compacting concrete

Abstract

The construction quality of self-compacting concrete (SCC) is possibly degraded by the distinct-layer casting in real structures due to the time-variable rheology of cement paste. This paper aims to investigate the influence of fly ash, water to binder ratio (w/b) and retarder dosage on distinct-layer casting of self-compacting concrete (SCC). Cubic specimens were cast in two layers with different delay durations of 20, 40 min and 60 min and the interface bond strength was tested using a self-designed method. On the other hand, yield stress and viscosity of cement mortars formulated from SCC mixtures were evaluated and the flocculation states of cement pastes were observed using an opto-digital microscope. Experimental results show that a weak interface is induced by the distinct-layer casting for SCC and the interface bond strength decreases with the longer delay for casting the second layer. The addition of fly ash, higher w/b ratio or higher dosage of retarder behave favorable effects on alleviating the distinct-layer casting induced degradation of interface bond strength for SCC specimens. This is attributed to the decreasing formation of floc particles in cement paste and lower time-dependent increase of static yield stress of cement mortar matrix. Relationships between the relative interface bond strength and static yield stress or static yield stress and average floc size can be expressed by exponential equations. Therefore, the negative effect of distinct-layer casting on SCC structures can be improved by controlling the flocculation process of cement paste.