Multi-scale comparative evaluation and property-optimized method of laboratory-prepared shotcrete

Abstract

On-site adjustments and variable environmental conditions during the tunnel construction potentially affect the quality of shotcrete, resulting in inconsistent properties and durability concerns. The article investigates the feasibility of preparing shotcrete under laboratory conditions with the identical raw materials and mix proportions employed on-site. And the effects of sand ratio on shotcrete properties are also studied. Results demonstrate that laboratory-prepared shotcrete can match or exceed the properties of site-prepared shotcrete. Improving the sand content within a specific range can optimize the shotcrete properties. For surface roughness evaluation, lab specimens exhibited a lower surface deviation (SD = 50.49 vs. 56.56) and a 3.25 % higher uneven area ratio (UAR), indicating more uniform paste coverage. Increasing the sand ratio can improve surface smoothness and reduce the SD value. Both the early-age and 28-day compressive strengths of the lab specimens reached 35 MPa, exhibiting a faster strength gain compared to the on-site samples. However, a 62 % sand ratio slightly reduced the 28-day strength due to its high specific surface area. Porosity of laboratory-prepared shotcrete was marginally higher than site‑prepared shotcrete (11.63 % vs. 9.69 % by MIP) under the same mix proportion, with NMR T₂ spectra confirming similar pore‑size distributions and porosity rising from 10.61 % to 11.14 % as sand ratio increased. SEM images revealed equivalent hydration products and C–S–H networks up to a 60 % sand ratio, with microcracking observed at a 62 % sand ratio. This article can serve as a reference for researchers to prepare shotcrete in the laboratory flexibly.