Interfacial shear behavior between ultra-high performance mortar and concrete precast blocks

To improve the shear performance of concrete precast block (CPB) interfaces, and to promote the application of ultra-high performance mortar (UHPM) in masonry and precast block assembly structures, this study first carried out a suitability screening of UHPM mixtures. The effects of water-to-binder ratio, sand-to-binder ratio, quartz sand gradation, and steel fibers on the workability and mechanical properties of UHPM were analyzed. In addition, the suitability of a practically selected retarder dosage was evaluated in terms of setting time and specimen-preparation requirements. Based on this, 153 UHPM–CPB double-sided shear specimens were designed and fabricated. The main variables included interface treatment method, CPB strength grade, and steel fiber addition. The shear behavior, failure modes, and load-transfer mechanisms of different interfaces were systematically investigated. The results show that the screened UHPM has high strength and good stability. It can meet the requirements of interfacial bonding tests and engineering applications. The interface treatment method has a significant effect on the shear performance of UHPM–CPB interfaces. The grooved specimens show the highest shear strength. The reinforced specimens exhibit a more progressive failure process. Compared with the steel-brushed specimens, the average shear strength of the roughened, reinforced, and grooved specimens increased to 478.95%, 707.89%, and 813.16%, respectively. The CPB strength, number of shear keys, and shear key shape also affect the interfacial shear capacity. However, the improvement becomes less pronounced as the number of shear keys increases. Under the present test conditions, the S1 and S2 series exhibited differences in cracking resistance, shear resistance, and failure modes, which should be interpreted as the combined effects of steel fiber content, UHPM strength, and CPB strength grade. Based on the failure characteristics and interfacial load-transfer mechanism, a multi-factor model for calculating shear capacity was established. The model considers interfacial bonding, mechanical interlocking, friction, and dowel action. Within the calibrated parameter ranges, the calculated results showed good agreement with the experimental data used for model calibration. The proposed model can provide a reference for the design of UHPM–CPB interface connections and the application of concrete precast block masonry structures.