Bond Strength between Successive Layers of SCC Incorporating Various Supplementary Cementitious Materials under Different Test Methods: Thixotropy Consequences

The multilayer casting of self-consolidating concrete (SCC) can lead to significant interlayer bond degradation, particularly when successive lifts are separated by extended time intervals. Despite its critical importance, systematic studies comparing different bond evaluation methods and their relation to the time-dependent structural buildup of SCC remain limited. This study investigates the effect of various supplementary cementitious materials (SCM), including Class F fly ash (FAF), ultrafine Class F fly ash (UFAF), Class C fly ash (FAC), and ground granulated blast furnace slag (GBFS), on the residual bond strength between SCC layers. To simulate real multilayer casting conditions, delay intervals ranging from 0 to 160 min were introduced. Interlayer bond performance was evaluated using slant shear, direct shear, and splitting tensile tests. Additionally, yield values at rest (YVR) were measured to assess the thixotropic behavior of the mixtures. The results revealed that both the type and dosage of SCM significantly influenced structural buildup and bonding efficiency. GBFS demonstrated the highest residual bond strength and yield stress at elevated replacement levels. UFAF and FAF improved bond performance at lower dosages but exhibited notable strength reductions beyond 30% replacement. FAC showed relatively stable behavior across all levels, with 30% replacement providing optimal results. The comparison of residual bond strength test methods showed that direct shear strength (DSS) and splitting tensile strength (STS) were most sensitive to weak interfaces, and slant shear strength (SSS) was less reliable due to compressive stress effects. Strong correlations were observed between YVR and residual bond strength, particularly in the control and FAF mixtures. Furthermore, direct shear and splitting tensile tests proved more sensitive to thixotropic variations than slant shear in SCM-modified SCC. The findings underline the importance of optimizing SCM type and dosage to maintain adequate interlayer bond performance in multilayer SCC applications.