Effect of accelerators on the long-term performance of shotcrete and its improvement strategies: A review

Despite the wide application of shotcrete in underground constructions, its long-term performance is greatly affected by the challenging service conditions. Thus, improving the performance of this material is critical for the safety and stability of underground shotcrete structures. This paper reviews the long-term performance of shotcrete in varying service conditions. First, the microstructural characteristics of shotcrete were presented and analyzed. Then, the relevant long-term mechanical strengths, shrinkage, and durability were summarized and discussed. Results showed that shotcrete exhibited superior initial mechanical strengths, but gradually decreased over time. In terms of shrinkage performance, shotcrete had high shrinkage strain values resulting from high cement content and effect of accelerators. The durability of shotcrete deteriorates over time owing to increased porosity of the matrix caused by the effect of accelerating agents. Potential improvements in some of these properties can be achieved with the inclusion of supplementary cementitious materials (SCMs), fibrous reinforcements (FRs), chemical admixtures (CAs), and other additives (OAs). The review finally provides new research directions for further studies and development of innovative shotcrete mixes with improved long-term mechanical and durability characteristics.