Q.: We are considering shotcrete for repair of a badly deteriorated dock face exposed to salt tidal waters in a cold climate where freeze-thaw is a major consideration. We are interested in shotcreting because it can be done much more easily and economically than alternative methods. We have, however, found it difficult to obtain any data about its suitability under such conditions, considering the difficulties of providing and testing for air entrainment in shotcrete.

A.: Your questions are answered in a report by Albert Litvin and Joseph J. Shideler, Laboratory Study of Shotcrete, Portland Cement Association Development Department Bulletin D 111. They found that wet-mix-process shotcrete seems to be more resistant to freezing and thawing than dry-mix-process shotcrete because it is capable of entraining air. Nevertheless, most dry-mix shotcrete was sufficiently durable in freeze-thaw tests because of its low water absorption. This was, however, not true of any dry-mix shotcrete that had a high water-cement ratio. They also found that durability of shotcrete is dependent upon good gunning practices; where gunning produced sand lenses the durability was generally lower than otherwise.

The tests by Litvin and Shideler did not include exposure to salt water. In general, however, good quality concrete does withstand salt water very well; the main consideration is that any reinforcing steel must have an adequate amount of concrete cover (ACI Committee 201, Durability of Concrete, recommends a minimum of 3 inches for marine construction). There should be no reason that shotcrete should not withstand salt water well, provided you have a thick enough cover over any steel in the shotcrete and the substrate.