Title: Bond Strength in Multilayer Casting of Self-Consolidating Concrete
Author(s): Wael A. Megid and Kamal H. Khayat
Publication: Materials Journal
Appears on pages(s): 467-476
Keywords: bond strength; fold lines; lift lines; multilayer casting; rheology; self-consolidating concrete; structural buildup at rest; thixotropy
Multilayer casting of self-consolidating concrete (SCC) can be critical in situations involving casting of successive lifts. The increase in structural buildup at rest of freshly cast SCC material prior to the placement of a successive layer can result in lift lines and loss in interlayer bond strength. Delay in the casting of successive lifts without mechanical consolidation can further reduce bond. Eight SCC mixtures designed to develop different levels of structural buildup at rest were investigated. The structural buildup at rest was determined by multiplying the values of initial slump flow, T50, or J-ring flow by average rates of change in these properties with rest time. Bond between successive layers was determined using composite specimens cast with two lifts of SCC after rest periods of 17 to 52 minutes, which corresponds to 25 to 60 minutes of concrete age. Bond strength was determined using the slant shear and direct shear test setups. Compared to monolithically cast samples, composite specimens had residual bond strengths of 15 to 100%. The critical delay time to secure at least a 90% residual bond strength was found to vary between 5 and 55 minutes, depending on the structural buildup at rest of the concrete in the existing layer. Statistical models for predicting residual bond strength between successive lifts were established and account for the structural buildup at rest of the first lift and delay period between successive lifts. Based on the level of structural buildup at rest, three categories of SCC are proposed. Category III SCC with relatively low structural buildup at rest can develop high residual interlayer bond. Such concrete should have maximum slump flow filling ability index of 800 mm.mm/min (31.5 in.in./min), T50 viscosity index of 0.08 sec.sec/min, and J-ring passing ability index of 600 mm.mm/min (23.6 in.in./min).