Q.: The walls of some tunnels between several of our plant buildings were made by wet-mix shotcreting. The shotcrete material was brought to the job from a ready mixed concrete plant and fed into the hopper of the shotcreting equioment. In order to minimize delays the trucks were required to arrive well ahead of time, and much of the time there were two trucks at the job site. The walls are not satisfactory. In.some places they have a hollow sound when tapped, and the shotcrete appears to be of generally low quality. Since shotcrete is normally expected to be of high quality we are puzzled.
A.: It seems possible that your problem is the result of some of the shotcrete being too old when it was used. Forty-five minutes after mixing has generally been considered to be the maximum elapsed time for placing wet-mix shotcrete, whether it is mixed on the job or supplied by a ready mixed concrete producer.
It is interesting to note that in August 1979 the Gunite Contractors Association of 2837 Newell Street, Los Angeles, California 90039, sent out a news bulletin alerting the industry to this time limitation. They report that in a number of instances in the preceding months they had investigated reports of wet-mixed shotcrete of extremely low quality. In every case they found that the delivery tickets showed that "time spans varying from 1 hour 10 minutes to 4 hours 30 minutes had elapsed after the sand and cement had been introduced into the transit mix truck and until the material was placed."
Even 45 minutes may be too long, the Association points outs if 'there is excessive moisture in the aggregate, a very high ratio of cement to sand, high ambient temperatures and low humidity." On the other hand the reverse of all these conditions could make 1 hour 15 minutes acceptable. The aggregate grading, fineness modulus and capability of the mixing eqllipment might also affect the allowable time.
Unfortunately there is no good way to detect in the field whether the cement in the mix has begun to set, but the Association suggests that it is better to discard mixes that are too old than to suffer the penalties of an unsatisfactory job.