The actual drying rate of concrete slabs is still often misunderstood. Drying rates and the factors that affect them have been well-studied over the years, but again, the misconceptions in this area still exist. As a general rule-of-thumb, we can use 30 days of drying time for each inch thickness of concrete for a slab to dry to 85% RH. Again, this is just a rough guideline, and there are many factors that can negatively impact this drying rate.
Common denominators on low RH slabs are:
- Proper and functioning vapor retarder placed directly beneath the slab.
- Steel trowel/closed surface is not specified. The slab is finished with combo blades at most.
- No sealant applied to the slab. Water cure for three days then begins drying.
- Keep water off the slab. A day of rewetting can set the slab back a month of drying.
- Ambient conditions are conducive to drying. Low RH, air movement and warm temps all help dry the concrete.
- Proper mix design with a low W/C ratio, and low cement content. More cement means more water.