The specifications required 28 days curing time before applying a membrane. We applied a membrane to a deck that was more than 28 days old, and the membrane failed. We were told it was because the deck continued to crack. Is the cracking that will occur in a top deck completed in that time?
Not always. The 28-day curing time is a common requirement, but don't expect all cracking to be completed by 28 days. Drying shrinkage cracking can take place months after a concrete slab is placed. The rate of drying shrinkage depends on the size and shape of the slab, the surrounding relative humidity and temperature, and the mix constituents and proportions. As an example, PCA's Building Movements and Joints indicates that a 6-inch-thick slab exposed to 70 F and 50% relative humidity will undergo only 60% of its total shrinkage by the time it's 100 days old.