As a contractor, you understand the importance and challenge of completing a project on time and on budget. By now you have likely had one or more of your project schedules or budgets adversely impacted by the fact there is seldom time, or favorable enough conditions, for conventional concrete to dry naturally to the levels required by floor covering manufacturers.
When comparing concrete solutions, it’s important to understand when the average concrete will dry to acceptable levels (moisture mitigation methods notwithstanding). Inclement weather and regional climates are factors to take into consideration, but are ultimately secondary to the concrete product you use to meet your construction timeline goals.
Drying Time for Conventional Concrete
The drying rates for concrete slabs vary and are considerably longer than those reported from drying studies conducted in a laboratory. Unless proper on-site testing is done, you cannot accurately detect the level of moisture remaining in a given slab even if it seems dry on the surface. The most important factor to consider when determining moisture levels is relative humidity (RH) as measured within the slab itself.
According to a 1997 study conducted by Gorhan Hedenblad, a standard 4-inch thick concrete slab will take approximately 140 days on average to dry to an acceptable RH level for flooring materials (assuming a temperature of 70 degrees F and that the slab is protected from re-wetting.).
Because most construction projects do not have sufficient time or favorable enough drying conditions for slabs to dry naturally, moisture mitigation treatments are commonly needed to keep the project on schedule and meet the warranty requirements of the flooring materials. We’ve previously covered how these moisture mitigation solutions aren’t always effective, but have become a common means of dealing with conventional concrete that is not drying fast enough to meet the project schedule.
Other Construction Timeline-Related Factors
Inclement weather and temperature are both factors that can extend the amount of time required for a concrete slab to dry. The 140-day approximation by Hendblad could be extended by rewetting from rain, colder temperatures or high ambient humidity levels.
Furthermore, the amount of time required for moisture testing of the slabs should be considered. Multiple rounds of testing are often required to determine if a slab has dried to levels required for the installation of flooring.
Rapid-Drying Concrete and the Construction Timeline
Self-desiccating concrete, or rapid-drying concrete, will reach the 80% RH requirement for most flooring materials within a 30-40 day time period.
In a span of 40 days, rapid-drying concrete will dry close to a 75% RH level, which is a virtually impossible level to reach with conventional concrete.
The cost of rapid-drying concrete is comparable to the better topical moisture mitigation systems. There are advantages, however, to using rapid-drying concrete that translate to considerable cost and schedule savings to the project. Rapid-drying concrete eliminates the step of mitigating moisture before flooring is installed, instead giving construction crews the flexibility to complete other tasks in the interim. In addition, its very low shrinkage directly reduces the costs associated with floor leveling and joint treatment.