Bonding agents to mitigate interfacial strength loss during multi-layer concrete casting

Self-consolidating concrete (SCC) is very sensitive to interruptions or delays during the placement process. Current literature converges that interfacial bond strength between successive casting lifts could drop by up to 60%, compared with monolithic casting. In this study, two series of SCC mixtures were tested to assess the suitability of bonding agents to mitigate the drop in interfacial bond strength resulting from successive casting lifts. The slabs were cast in two layers separated by 60 or 120 min rest intervals; different bonding agents, including styrene–butadiene rubber (SBR), waste latex paint and cement/water and cement/SBR slurries were compared. The results showed that the application of polymeric bonding agents on top of the first cast is efficient to improve tensile strength and restore the integrity of the hardened member. This is attributed to the polymer particles that coalesce and fuse to form continuous close-packed films between the first and second SCC layers. The cement/water slurry did not lead to considerable improvement in pull-off strength between cast layers. Series of charts enabling the prediction of the drop in bond for a given SCC composition and rest interval as a function of the polymer dosage to the area of the cast slab are presented.