Q.: Does an air-entraining agent serve any purpose in wet-process shotcrete? I believe shotcrete is placed with such force that any air-entraining effect is negated. Should we use air-entraining agents in concrete that will be placed by wet-mix shotcreting?

A.: The American Concrete Institute Committee on Shotcrete recommends yes (for wet mixes, only). Section 2.7.2 of Guide to Shotcrete (ACI 506R-90) states, "Air-entraining admixtures do not entrain air in dry-mix shotcrete. Wet-mix concrete that will be exposed to freeze-thaw cycles should be air-entrained. Air entrainment tends to make some mixes more workable and may reduce rebound. Data is available to show that a significant quantity of the entrained air in wet mixtures is lost in gunning, but this may be offset by increasing the quantity of air-entraining agent."