We have a repair job for exterior concrete where we plan to use dry-mix shotcrete. The structure will be exposed to freezing and thawing. The engineer says air can't be entrained in dry-mix shotcrete and he questions whether the repairs will be durable under freeze-thaw conditions. Are there any research data that will ease his concerns?
Several studies have shown that dry-mix shotcrete is indeed resistant to cycles of freezing and thawing. In one study conducted by the Portland Cement Association, shotcrete test panels were obtained from contractors known for quality workmanship. The shotcrete contained only cement, water, and sand. The 28-day compressive strength of cubes from all panels tested ranged from 7340 to 11,400 psi. Specimens sawed from the panels were frozen and thawed in fresh and saltwater. Freeze-thaw durability after 300 cycles in fresh water was comparable to that of high-quality, air-entrained concrete. In saltwater tests, weight loss was high even though length change and relative dynamic moduli of elasticity were satisfactory. This led the researchers to suggest that for saltwater exposure a sealer or coating might be needed. They concluded, though, that dry-mix shotcrete can provide a high-strength durable shotcrete if made using sound materials, in the proper proportions, and applied by an experienced shotcrete crew. In another study, straight cement dry shotcrete mixes and mixes containing added silica fume were subjected to freezing and thawing. Compressive strengths varied from 6080 to 7510 psi. After 300 cycles of freezing and thawing, test specimens were still in excellent condition. There was no significant scaling or surface deterioration and sawcut edges were still sharp. Results of several other laboratory investigations are reviewed in Reference 3 below. References 1. Seegebrecht, Litvin, and Gebler, "Durability of Dry-mix Shotcrete," Concrete International, October 1989, page 47. Available from the American Concrete Institute, P.O. Box 19150, Detroit, MI 48219. 2. Morgan et al, "Freeze-thaw Durability of Wet-mix and Dry-mix Shotcretes with Silica Fume and Steel Fibres," Cement, Concrete, and Aggregates, Winter 1988, page 96. Available from ASTM, 1916 Race St., Philadelphia, PA 19103. 3. Morgan, "Freeze-thaw Durability of Shotcrete," Concrete International, August 1989, page 86. Available from the American Concrete Institute, P.O. Box 19150, Detroit, MI 48219.