How to Rapidly Speed Up Concrete Drying Time

Clock in Dry Cracked Concrete

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 condition, 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

Mixing Concrete Admixtures

Admixtures or the right balance of water can greatly help decrease drying times

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.

While the Wagner [RH] probe takes only about a minute to read, the others take 2 hours to read. So if there 20 probes to be read, I can read the Wagner probes in 30 minutes, while it takes about 40 hours to read the other type of probe. I can find better things to do than stand around and wait 40 hours in order to read those probes.
John LowtherJKL Construction

 

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

wet concrete slab drying

After you pour, the strategy for drying changes

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.

HVAC system visualization

To really speed up concrete drying time, enclose your slab and ensure the HVAC system is on.

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.

Jason has 20+ years’ experience in sales and sales management in a spectrum of industries and has successfully launched a variety of products to the market, including the original Rapid RH® concrete moisture tests. He currently works with Wagner Meters as our Rapid RH® product sales manager.

22 Comments

  1. Cherian mathan says:

    There were various leaks in my tank wall external side. After repair, all the repair points are having moisture content over 6% due to which I am unable to start the coating. Will blow drying the specific point help in reducing the moisture content ?

    • Jason Spangler says:

      If the ambient conditions, temperature and relative humidity are reasonable levels where they can “accept” additional moisture, then moving heated air across the surface will help (how much is anyone’s guess). Left to its own though, the ambient air may become saturated to the point that it can’t accept any additional moisture.moisture, thus eliminating moisture movement from the surface of the concrete. Also, should you stop the heated air during this time, the likelihood of the moisture moving from the air back into the surface of the concrete is high. This is one of the reasons why some type of dehumidifier is critical in this process.

      Jason

  2. Kimberley Skinner says:

    We have built a new house….well still building….we took over after the contractor was done with the drywall. The drywall was completed about late October. We didnt do the flooring until after Christmas. We came in in may almost ready to move my mom into her new house and the flooring had buckled. We someone for moisture under the floor. We’ve had plumbers and everyone out. We can’t figure out why the concrete is now retaining moisture. It wasnt before. It doesn’t flood, it sits high. Its been over 100 degrees alot this summer. We live in West Texas so it’s not humid. We just can’t get it dry. You leave something on the floor, come back the next day and it’s damp underneath. We are at a stand still. Noone seems to be of any help around here. My dad passed away so this is my mom’s mother in law house. Right now we are all living out of boxes trying to this figured out! What do we do??

    • Jason Spangler says:

      Kimberly:

      It would be easier if you just give me a call. I am in the office Monday-Friday 7:30am-4:00pm PST. Our office number is (800) 634-9961 and you can ask for me directly.

  3. Joe Ebin says:

    Jason Spangler

    As the parking lot pavement is always cracking , we are considering pouring concrete onto a parking lot area at a business park which will be driven on by cars and occasional lighter delivery-type trucks. The tricky part is closing the parking lot so the concrete product can set up properly, all without driving the tenants into despair.

    Do you have any thoughts on this process?

    Thanks

    Joe Ebin

    • Jason Spangler says:

      Joe:

      Thanks for the comment. I think your best bet would be to get in contact with a local concrete finisher and get his/her input on this. I would imagine that with some concrete design modifications (faster curing, higher compressive strength) and timing (weekend pour, holidays) you could pull it off without a hitch.

      Jason

  4. kundan patel says:

    Joe,
    we had a concrete floor done in our backyard. It has been 3 months and it still looks like patches in it. Is it because it’s not dry yet?
    i called the company and they said it’s take time to dry. They just finished sealer coat yesterday. The cement pour was about 31/2″.
    please help. this company i am not happy with. all they care for is money.

    Thanks,
    kundan

    • Jason Spangler says:

      Kundan:

      Thanks for the question. I can’t say for sure, but my best guess, based on the information you provided, is that the concrete hasn’t had enough time to dry. I hope this helps.

      Thanks,

      Jason

  5. Barry Hyam says:

    We have had a water leak in a heater pipe in our bungalow, we have concrete floors and have industrial dehumidifiers and drying fans, but still struggling to dry the floor. The drying company say they need to skim the floor by about 2 mm. Do you think this will help to dry the floor. The bungalow was built in 1988.

    • Jason Spangler says:

      Barry:
      Thanks for the comment and sorry for all of the issues. I can’t say that I have ever heard a recommendation to skim because a floor was too wet, nor do I understand how it would work unless they theorize that the excess moisture on the surface can be utilized in the matrix of the skim coat. I understand drying, so for me, I would stay the course with drying. The other way may be effective, but I’m not sure.

      Thanks,

      Jason

  6. Carlotta Matherne says:

    I have a brick floor that was flooded with about 2″ of water when a washing machine hose failed. The flooded floor lasted about 7 hours before we got home. It’s about 600 sq. ft. of brick flooring. We had a professional crew come in and remove all the old sealer about 10 days ago. They had to re flood the floors to do their work. How long should we wait before applying the new water-based sealer? The mortar between the brick is still wet. There is a visquine (sp?) barrier under the slab.

    • Jason Spangler says:

      Carlotta:

      Thanks for the comment. I really can’t give you a timeframe on how long it will take the brick to dry. I would talk with the manufacturer of the sealer and tell them the situation to get their input. I can tell you though, air movement across the surface of the brick and good environmental conditions will help the process.

      Thank you,

      Jason

  7. Denise Supple says:

    We had a leak in the hot water pipe in the cement foundation. We had the leak fixed. We had engineered wood on top on the cement which was all pulled up. The problem now is that the flooring guy will not install the new wood flooring because the moisture in the cement is reading really high. Our contractor brought in the dehumidifiers and ran them for a week. The moisture in the cement is still very high. Do you have any suggestions on how to dry the concrete? We had a leak detection company come back out and there is not a new leak. We can’t seem to make any progress.

    • Jason Spangler says:

      Denise:

      Thanks for the comment. Consistent environmental conditions (temperature and humidity), air movement across the surface of the concrete, and dehumidification are key. The problem may take longer than a week to rectify. Another option to discuss may be a better adhesive or moisture mitigation product that will allow the floor to be installed in its current condition. It may be worth the discussion.

      Thanks,
      Jason

  8. Gill says:

    Hi , don’t know anything about pointing inbetween my patio slabs .The young man put this dry ciment i brushed it in how long does it take to dry

    • Jason Spangler says:

      Thanks for the comment. Dry is a relative term and is based off of the ambient conditions. I would have this discussion with the young man that poured the slab.

      Thanks,

      Jason

  9. lolos1 says:

    We are having a similar problem with Denise Supple, crazy as it may seem. We had a water leak 16 years ago in a hallway and the water pooled in the only room that had a wood floor under the existing Laminate flooring. The leak was fixed at the time as was the only place in the laminate flooring that had buckled, in the office. Fast forward to 2018, we are doing a little remodeling in our office and we pull up the laminate flooring and what do we find?? Water, moisture, mold, disinigrated wood. Guess the flooring contractor 16 years ago missed something. Everything was pulled up 3 weeks ago and we are down to the concrete slab, our moisture readings are so high that we as well have been unable to put a new laminate floor in the room. After reading your response to Denise, we felt a little better and we purchased a dehumidifier and have been running that for 2 days, the humidity in the room seems to be rising, but moisture is leaving the room so I’m a little unclear how this whole process is actually working. We live in sunny Arizona, where record temperatures are upon us. We also have a high powered carpet dryer running in the room. I’m assuming we just need to be patient, my small project turned into a huge project that in the end will be a good thing. We had a company come out to do a leak test (just in case) and no leaks were found thank goodness. Any other suggestions you could offer would be greatly appreciated.

    • Jason Spangler says:

      Lolos:

      Thanks for the question. Yes, patience is a big part, but based on my experience with older construction (in AZ, even newer construction at times), patience may not even be enough. This may be more of an issue than just the water leak from 2002. At times, slabs are/were constructed either without vapor retarders directly beneath the slab or if they had them, they have long since degraded due to the various materials used. Unfortunately, without this retarder, moisture vapor from the soil has direct access to the bottom of the slab which can lead to issues as you describe. I would continue the process you are doing, but I would also look to have someone (geotechnical engineer?) come out and do core samples of the slab to verify there is an intact vapor retarder. If there isn’t, all that you are doing is for not. Good luck.

      Jason

  10. J says:

    Hello Jason,
    I had a problem with moisture coming up through an old slab foundation in a bedroom, I had the slab removed and re- done, added a moisture barrier and repoured the slab. How long should I wait before adding flooring like laminate or anything else on top.

    • Jason Spangler says:

      J:

      Thanks for the question. It all depends on how long it takes for the slab to dry to an acceptable level for the laminate. Concrete dries based on ambient conditions, not on a clock, per se. Reasonable air temperatures and relative humidity in the environment are more compatible with concrete drying. This being said, measuring the moisture in the concrete is the only way to know whether the conditions in the concrete are acceptable for the finish installation. I would consult the laminate manufactures installation guidelines for this specific information on what those maximum moisture levels are and the methods of measuring they will accept. Good luck.

      Jason

  11. I do like it when you pointed out that the reason why concrete takes time to dry is because the water trapped in it needs to fully evaporate first. We really want to make it so that the concrete flooring will be dry a day or two after it has been poured, so I want to know what we can do. Perhaps, we can reduce the amount of water in the concrete but not in an exaggerated way so as to avoid adverse effects. I will talk to the contractor about what we can do. Thanks!

  12. I do like it when you said that dehumidification is one of the best ways to quicken the drying speed of the concrete because it will reduce the dew point so that the moisture in the concrete will evaporate. The only problem is that we do not know how to do this since none of us in the house knows anything about concrete works. Perhaps, we can ask the contractor that we will hire since we want the driveway to finish by the time dad returns home next week.

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