My company is the structural engineering firm on a bridge repair project, and we have specified dry-mix shotcreting on several vertical and overhead surfaces. The project is running behind schedule and the contractor has recommended that we allow him to use a sodium carbonate accelerator in the shotcrete. He said that this will allow his nozzlemen to apply thicker, more frequent layers of shotcrete. We'd like to speed up the job, but we've heard that rapid-set accelerators reduce the compressive strength of shotcrete. Is this true?
The effect of a sodium carbonate accelerator on the hardened properties of dry-mix shotcrete was reported in "Durability of Dry-Mix Shotcrete Containing Rapid-Set Accelerators," ACI Materials Journal, May-June 1992, pp. 259-262. As part of the study, a shotcrete mix containing 4% sodium carbonate accelerator by weight of cement was compared to a control shotcrete mix containing no accelerator. Both mixes had a cement-sand ratio of 1:4. The 28-day compressive strength of the accelerated shotcrete was 54% lower than that of the control mix. Perhaps more importantly, the accelerated shotcrete had a reduced resistance to freezing and thawing (ASTM C 666). Concrete specimens are generally considered to have acceptable freeze-thaw resistance if they lose less than 5% of their weight after 300 freeze-thaw cycles. After 311 freeze-thaw cycles, the control mix had suffered a weight loss of only 0.9%. The accelerated shotcrete, however, lost 7.3% of its weight after 153 cycles and 17.1% after 210 cycles, at which time all three specimens had failed. The study concluded that the reduction in freeze-thaw durability was caused by not only the reduced strength of the shotcrete, but also an increase in the porosity of the accelerated shotcrete. Apparently, the quick or flash setting of the shotcrete inhibits compaction.