Study on Mix-Proportion Optimization and Performance Test of Low‑Rebound Shotcrete for Tunnels

Abstract

To address the excessive rebound rate of shotcrete during tunnel construction, this study investigates the mix-proportion optimization and physico-mechanical properties of low-rebound shotcrete with an under-construction tunnel on the Cangtai Expressway as the engineering background. Three water-binder ratio levels (0.39, 0.42 and 0.45) were selected as the primary variable, while the sand ratio and unit water consumption were kept constant. Laboratory tests on slump and uniaxial compressive strength of shotcrete were carried out, and range analysis was utilized to quantify the effect of the water-binder ratio on the physico-mechanical performances of shotcrete. Field measurements were also performed to obtain the practical rebound rate under different water-binder ratio conditions. Experimental results reveal that the water-binder ratio is positively correlated with slump and negatively correlated with compressive strength. When the water-binder ratio increases from 0.39 to 0.45, the slump rises from 100 mm to 135 mm (an increment of 35%). The 7-day compressive strength decreases from 31.9 MPa to 22.3 MPa and the 28-day compressive strength declines from 38.2 MPa to 29.7 MPa, with reduction percentages of approximately 30.1% and 22.2%, respectively. the field-measured rebound rate first declines and then rises as the water-binder ratio increases. The lowest rebound-rate value is attained at a water-binder ratio of 0.42, where the slump equals 120 mm together with 7-day and 28-day compressive strengths of 27.1 MPa and 34.0 MPa. This mix balances construction pumpability, bearing capacity of the support and rebound suppression. This optimal mixture balances construction pumpability, bearing capacity of the support and rebound suppression. The research can furnish references for the mix-proportion design and construction of low-rebound shotcrete for tunnels.