How to Rapidly Speed Up Concrete Drying Time
In flooring applications (either finished concrete or applied flooring), there is always a “hurry up and wait” element – wanting to move ahead but knowing that rushing too quickly may result in a flooring failure or flooring problem. Ideally, good curing conditions mean retaining that initial moisture content, but often a project schedule doesn’t allow the time necessary for optimal curing and drying. Meeting concrete and flooring specifications can seem to be a luxury that is not always available to the contractor on a tight schedule.
The rule of thumb is that you’ll need to allow 28 days of drying time for each inch of concrete thickness, if the slab is under ideal drying conditions (an enclosed area with the HVAC on, meaning there’s air circulation and a low ambient relative humidity).
Pre-Pour Steps to Speed Up the Drying Process
There are several steps you can do initially before you even make the concrete that can help to speed the drying process, but each also must strike a balance between initial mix water amounts and drying rates under job site conditions.
While a lower measure of water in the concrete mix sounds like an obvious solution, low water-cement ratio mixes typically have fewer capillaries – the natural pathways that allow water to move through the slab to the surface. Therefore, even with the reduced amounts of water, slower drying time can result.
For lightweight concrete, aggregate must be saturated before mixing the concrete, so a certain level of water must be present. However, the more water present in the initial mix, the more water must evaporate out of the slab.
Another option is to add “self-desiccation” agents to the concrete mixture. Adding a higher ratio of cementitious material to the mix does bind more water into the paste of the concrete, but not without its price: The potential risk for cracking or shrinking in the finished slab is significantly higher. Other chemical admixtures, like silica fume, hydrate rapidly enough to permanently capture a percentage of the initial mix moisture and negating the extra drying time that volume of water would normally require. These and other solutions also typically have a higher cost involved that may outweigh the benefits.
Synthetic particles have also been tried as a substitute for lightweight aggregate. These synthetic substitutes are thought to reduce drying time in slabs because they do not absorb significant amounts of water that then needs to be released.
Lastly, several mechanical factors also affect drying time. The first, of course, is how the slab is troweled. Hard troweling or slabs that are troweled to a burnished finish too quickly lose their ability to let moisture pass through. Those natural capillaries between the elements of the mix are, in effect, sealed off and drying time can be seriously impeded.
Vapor retarders are effective in reducing moisture from ground sources from entering the underside of the slab. But the initial effect on drying time can actually be slightly (although not always significantly) detrimental if it reduces another avenue for the moisture to escape the slab. This being said, for the long-term health of the building, strong vapor retarder, directly under the slab, is necessary.
Post-Pour Steps to Decrease Slab Drying Time
Keeping these preemptive steps in mind can do much to reduce the drying time for a newly-poured concrete slab. For a slab that has relative humidity (RH) levels that are too high for installation, the options are slim. A moisture mitigation product can be utilized on the surface of the slab to “encapsulate” the slab’s moisture, allowing for flooring installation. There are many products available that claim to accomplish this, so the specifier, installer, and/or contractor need to make sure they research each potential manufacturer and product.
The best bet is to encourage the slab’s natural drying process. Drying time ultimately depends on the balance of water and capillary structures in the concrete mix. Dehumidification tries to speed up the natural drying action.
Dehumidification is one of the processes often used to try to encourage a concrete slab to dry more quickly in order to reach the installation point. The basic concept is simple: Reduce the dew point of the air surrounding the slab so that more of the latent moisture within the slab can evaporate out through the surface. In the building industry, there are several common approaches to dehumidification: condensation dehumidification, heating (or drying) dehumidification, and desiccant drying.
Desiccant-based dehumidifiers use a chemical attraction to remove moisture from the air. Humid air is moved across a desiccant material that binds and holds the moisture. A hot air stream is then used to release the moisture from the desiccant and vent it away from the slab’s environment. The dried desiccant then absorbs more moisture and releases it again through heating in a continuous process. This process is not influenced by external weather, so it can be used year-round as long as the concrete slab can be isolated from weather conditions.
The condensation process uses cooling-based dehumidifiers which cool air, effectively dropping the dew point so that moisture can be collected and drawn away. By contrast, dehumidification uses heated air to raise the dew point of the surrounding air, allowing it to absorb more moisture from the slab surface. The saturated air is then circulated, either through a collection system or to the outside air where it releases the excess moisture as it cools. It should be noted here that most installed HVAC systems are not capable of removing the amount of moisture emitted through the drying process of a concrete slab. While it may be possible to raise or lower the ambient temperatures through the HVAC system, dehumidification must actively remove the excess moisture from the structure. If the moisture is simply moved around within the walls, excess moisture can cause mold growth or equipment deterioration over time.
The “Key” to Fast Drying Concrete
The real key to successful dehumidification is to have the concrete slab completely enclosed so that the moisture removed during the process is not re-introduced to the slab’s environment. Either the slab must be enclosed in service-ready indoor conditions, or a barrier must be erected around it for outdoor conditions.
Overall, the “natural” drying process of concrete takes time. Unfortunately, the necessary time is not usually allotted, so the need to have processes that help speed up drying or negate excess moisture, allowing for successful flooring installation, will continue to be necessary.
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