Strength compatibility and optimization of grout and coarse aggregate in two-stage ultra-high performance concrete

This study prepares two-stage ultra-high performance concrete (TS-UHPC) towards maximum volume coarse aggregates and low binder utilizations by combining two-stage casting process, high performance grout, and preplaced basalt aggregates. The strength compatibility and optimization of grout and coarse aggregates are discussed, as well as the effects of calcium formate content, coarse aggregate strength and size, casting and curing methods. The potential advantages such as binder efficiency, strength, volume stability, environmental benefits are explored and comparatively analyzed. The results show that TS-UHPC achieves compressive strength of 124.9 MPa by optimizing calcium formate content of 0.3 % in grout, crushing index of 4.9 % for coarse aggregates, and heat curing. The grout strength beyond 120 MPa and coarse aggregates with crushing index below 8 % are suggested to achieve a favorable strength compatibility in TSUHPC. The TS-UHPC exhibits excellent volume stability due to rigid skeleton and binder dilution effects of coarse aggregates, the total free shrinkage fluctuates around 316-398 mu epsilon and is reduced by 61.2 % and 28.5 % compared to the corresponding grout and the coarse aggregated ultra-high performance concrete (CA-UHPC). An appropriate particle size below 15 mm is suggested for coarse aggregates considering the adverse size effect on strength and chloride resistance. The TS-UHPC contains a low binder amount down to 525.1 kg/m3, a large coarse aggregate volume content of 51.6 %, a high binder efficiency of 0.19 MPa kg/m3, resulting in the CI and CP values around 3.9 kg/MPa center dot m3 and 1.9 $/MPa center dot m3, respectively, which demonstrate its superior cost-effectiveness and sustainability compared to normal UHPC and CA-UHPC.