The mitigation of moisture in concrete floor slabs began as a late-stage reactionary measure when it was found the moisture levels were above that allowed by the floor-covering manufacturer at the time the flooring was scheduled to be installed.
As the need for mitigating slab moisture became commonplace around the country, design professionals, owners, project managers and general contractors began to utilize a proactive approach where moisture mitigation became an integral part of project specifications and budget. Given an increased demand for moisture mitigation, the number of moisture mitigation products and approaches introduced to the marketplace shot upwards.
Read our complete guide on moisture mitigation and how rapid-drying concrete can help keep your project on budget and on time.
Sadly, many of the approaches to moisture mitigation did not live up to their claims and it has taken decades to determine what works, what doesn’t, and why.
As the term “moisture mitigation” implies, the mitigation of moisture is a mitigation of risk, rather than an elimination of the root problem. Sealing the surface of a concrete slab may isolate the flooring system from the potentially damaging affects of moisture for a time, but the moisture remains present in the concrete – potentially leading to other damaging conditions. The worst of these is Alkali Silica Reaction (ASR) or Near Surface Alkali Reaction (NSAR).
Rapid-drying concrete, on the other hand, not only protects flooring materials from the potentially damaging affects of moisture and elevated pH levels, but by consuming free-water in the slab internally, the risk of an ASR or NSAR condition is eliminated. Tests on rapid-drying concrete show the relative humidity in the slab can reach acceptable levels for flooring within 45 days after placement. Conventional concrete, meanwhile, can take 3 to 6 months even after the building is closed in and favorable drying conditions are established.
The Evolution of Moisture Mitigation:
Fabric-Based Systems- The first moisture mitigation systems that achieved any measure of success involved adhering a fiber mat to the surface of the slab to provide a separation layer and lateral avenue for moisture to diffuse. While these systems did work on many projects, there were still several extreme cases where the fabric would break down. These systems are no longer being marketed.
Accelerated Drying- For elevated slabs or on-ground slabs placed directly over a vapor retarder, accelerated drying equipment has been used successfully. The approach is expensive and best utilized when the process can double as a temporary heating system during cold weather construction. To maximize slab drying, the slab surface must be opened to allow moisture to escape more readily. Accelerated drying can lead to the faster development of a slab curl if the slab design has not included measures to resist or offset the warping stresses that the slab will develop as it dries from the top down.
Reactive Chemistries- Chemically reactive treatments applied either topically or integrally have been tried as a means of mitigating slab moisture emissions and transmission. While some of these systems do have waterproofing and surface densification properties, there has never been a credible scientific study that could validate their effectiveness as a vaporproofing treatment for a concrete slab. In addition, the over-application of these types of products where an insufficient amount of calcium hydroxide is present can lead to unreacted material acting as a bond breaker on the surface region of the slab.
Moisture Mitigation Coatings- Acrylics, hybrid epoxies and moisture-cured urethanes are used as moisture mitigation treatments over the surface of a concrete slab. Even the best of these systems are prone to blistering if osmotic cells form at the interface of the concrete and the coating. Historically these have been late stage applications, which were not only costly but also disruptive to other construction activity.
Rapid-Drying Concrete as a Solution
Rapid-drying concrete effectively eliminates the need for other types of moisture mitigation, as the slabs reach the target moisture levels weeks after the slabs are placed. Compared to late stage approaches, rapid-drying concrete is an economical solution to mitigating slab moisture and can help shorten the overall construction schedule.