We are concerned about the compressive strengths of shotcrete recently placed on one of our projects. The specification calls for 8000 psi (55 MPa). Test results indicate we are only at 5200 psi (36 MPa) at 28 days. Ambient temperatures are constant at about 45 °F (7 °C) at the point of placement. Should we be considering removal of the shotcrete?

Shotcrete, like any other concrete mixture, will continue to gain strength as long as there are unhydrated cement particles present along with sufficient temperature and moisture. Strength development will generally be quite slow at the ambient temperature reported. The inclusion of supplementary cementitious materials in this mixture is a benefit in this instance as strength will increase as long as calcium hydroxide is available from the hydration of the cement. The specified strength should eventually be attained as long as the ambient temperature does decrease further and some form of moisture is available to the shotcrete.