How often do you guys see projects resulting in 85% or less RH? Are there any common denominators that you see on jobs with 85% or less RH?

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: 

  1. Proper and functioning vapor retarder placed directly beneath the slab. 
  2. Steel trowel/closed surface is not specified. The slab is finished with combo blades at most.
  3. No sealant applied to the slab. Water cure for three days then begins drying. 
  4. Keep water off the slab. A day of rewetting can set the slab back a month of drying. 
  5. Ambient conditions are conducive to drying. Low RH, air movement and warm temps all help dry the concrete. 
  6. Proper mix design with a low W/C ratio, and low cement content. More cement means more water.