Q.: Some unusual job scheduling is going to require us to apply shotcrete in three layers applied several days apart. We know we'd better not cure the first two layers with curing compound since it would probably interfere with bond. Is covering with polyethylene satisfactory if we make all edges tight?
A.: Shotcrete usually has a quite low water-cement ratio, and it can benefit by having curing water added. This will ensure that water will find its way into all the pores that might otherwise become partially desiccated by the hydration of the cement. Some means should be provided to keep the shotcrete moist between times for example by fog spray or by water running down over it from soaker hoses. The final coat should be kept continuously wet for 7 days.