Wet-mix shotcrete incorporating superabsorbent polymers for tunnel linings: From laboratory tests to large-scale validation

This study developed a wet-mix shotcrete mixture incorporating a superabsorbent polymer (SAP) to enhance water-leakage resistance and evaluate its field applicability. Accordingly, the type and dosage of SAPs were selected to maximize self-sealing while minimizing compressive strength loss. The developed SAP shotcrete mixture was evaluated through laboratory tests of its rheological properties, mechanical performance, and shrinkage behavior. Subsequently, large-scale shotcrete placement was performed on a tunnel mock-up structure (3.0 m in width, 3.2 m in height, and 2.5 m in length) to assess its field applicability in terms of mechanical properties and water-leakage resistance. The test results showed that the incorporation of 1 % acrylamide–acrylic acid copolymer SAP by cement weight led to a reduction in mechanical strengths; however, it markedly decreased water leakage by 68.3 %. In addition, the shrinkage was reduced, thereby improving the crack resistance. However, during the water absorption stage of the SAP, the static yield stress, dynamic yield stress, and plastic viscosity increased, resulting in decreased constructability. Conversely, when absorbed water was released from the SAP, it delayed the time-dependent increase in the dynamic yield stress and induced shear-thinning behavior, which partially improved the shotcrete placement performance. Therefore, in the application of SAP-incorporated shotcrete, time-dependent rheological changes should be considered when selecting the SAP type and dosage, as well as in determining the appropriate construction window.