Effect of structural buildup at rest of self-consolidating concrete on mechanical and transport properties of multilayer casting

Abstract

Multilayer casting of self-consolidating concrete (SCC) can be critical in situations involving delays between the placements of successive lifts. In the absence of mechanical consolidation, the increase in structural buildup at rest (SBR) of existing SCC lift prior to the placement of a successive lift can lead to lift lines. Furthermore, the bond strength deteriorates across the bonded interphases. In this study, eight SCC mixtures with different levels of SBR were considered. The thixotropy of concrete was determined using standard workability test methods and inclined plane test. Flexural strength and water permeability were determined using composite specimens cast in two SCC lifts. The second lift was placed after given periods of rest up to 60 min. Composite specimens developed residual flexural strengths and water permeability resistance with minimum values of 56% and 2%, respectively, compared to reference samples. To secure 90% residual flexural strength and impermeability, delays up to 25 and 10 min can be tolerated, respectively, depending on SBR level of the existing SCC layer. Statistical models were established to predict the residual flexural strength and water permeability resistance between successive lifts. SCC with low level of SBR, having static yield stress up to 250 Pa after 15 min of rest and rate of SBR up to 2.5 Pa/min evaluated using the inclined plane test, can secure relatively high level of bond strength and impermeability providing that the delay time before casting successive lifts is limited to 20 and 5 min, respectively. The interphases bond of successive SCC lifts was found to be affected by intermixing level of the layers. Both can be increased by the reduction in SBR of existing lift when concrete is cast with a given free fall height.