5 Tips to Help You Speed Up Concrete Drying Time
How Long Does Concrete Take to Dry?
Normally, concrete takes 28 days to fully cure at maximum strength. Concrete sets in 24 to 48 hours and will be hard enough to walk on. Concrete doesn’t “dry,” concrete cures. 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 is air circulation and a low ambient relative humidity).
Concrete drying time is a major factor in the schedule of most construction projects. Any way that time can be shortened can potentially save a lot of money.
When a flooring system will be installed over the concrete slab, drying is critical. If the slab isn’t sufficiently dry when the floor is installed, the floor might be seriously damaged by the excess moisture.
How to Speed Up Concrete Drying Time Tips:
- Use the correct amount of water in the mix. Too much can increase the drying time.
- Do not over-trowel or seal the surface. This can block the pores in the concrete and diminish moisture evaporation. This increases drying time.
- Once the space is enclosed, keep doors and windows closed, HVAC running, and fans circulating air.
- Use dehumidifiers to remove moisture from the air and speed up the overall drying process of the slab.
The Difference Between Concrete Curing and Drying
Concrete curing and concrete drying are different processes. Concrete curing is the process of hardening that begins immediately after the concrete is poured. A majority of the concrete curing process is completed after 28 days although concrete hardening continues for a significant amount of time after that.
In the drying process, excess water evaporates from the concrete. While concrete is considered sufficiently cured in only 28 days, concrete drying takes months to complete. The rule of thumb is that a slab takes one month of drying time for each inch of thickness and this is once the conditions in the space are conducive for drying (low ambient %RH and consistent temperature). Let’s look at how you can speed up the drying process in more detail.
How Concrete Cures
There are many types of concrete, but they all contain three basic components: cement, aggregate, and water.
When water and cement are mixed, a chemical reaction occurs that binds them together. This is what causes concrete to harden. In this process, the concrete becomes porous. A certain amount of water from the original mix becomes part of the concrete in this process. The water that’s left either evaporates or remains in the capillaries of the concrete as water.
How Concrete Dries
Concrete dries as the water evaporates from its surface. As water evaporates, water from deep in the concrete moves through the capillaries to the surface to replace it. As long as the surrounding air can hold more water vapor, evaporation continues. When the surrounding air can’t hold any more water vapor, evaporation or drying of the concrete stops.
How to Speed Up Drying: Before the Pour
These are some steps you can take before you pour the concrete that will speed the drying process.
Use the correct amount of water in the mix. If there’s too much water, there will be more water left after curing that will need to evaporate. That means a longer drying time.
Using a high cement content mix will reduce drying time, but at the risk of causing cracking due to shrinkage.
If you’re using lightweight concrete, the lightweight aggregates absorb a lot of water and slow drying time. You can reduce drying time by replacing these aggregates with synthetic aggregates that don’t absorb water.
If possible, don’t use curing, sealing, or bond-breaking agents. They can inhibit evaporation from the concrete surface and slow drying time.
How to Speed Up Drying: After the Pour
Once the slab is cured, enclose the space as soon as possible. The most obvious reason is to protect the slab from precipitation or water from any other source. The last thing you need is to add more water to the slab.
Other than protecting the slab from additional water, the other two factors that affect drying the most after the pour is ambient relative humidity (RH) and the temperature of the slab and air above the concrete.
RH determines whether or not the air can hold more water vapor. It’s defined as the amount of water vapor the air can hold at a given temperature and pressure. So it directly controls whether water can evaporate from the slab.
Enclosing the space allows you to use HVAC to control ambient conditions. And as it happens, normal service conditions provided by HVAC are also good conditions for concrete drying. In cooling mode, HVAC systems act like a refrigerating dehumidifier and will usually maintain RH around 50%, which is ideal for concrete drying. In heating mode, they lower RH by raising the air temperature, and tend to maintain even lower RH.
HVAC systems also provide good circulation. Circulation helps to speed evaporative drying by carrying moist air away from the slab, which is replaced by drier air that can absorb more moisture.
Some facilities have built-in central dehumidifiers which will speed the drying process. If necessary, you can rent portable dehumidifiers and fans.
There are many designs of dehumidifiers, but they all use one of two technologies.
The first technology is refrigeration. This works the way HVAC systems do in cooling mode. Fans draw the air over refrigerated coils. As the air cools to its dew point, water vapor in the air condenses on the coils. The water is collected and drained away. The refrigerant is pumped to a compressor where it’s compressed, and the heat is released through a heat exchanger and vented to the outside. The refrigerant is then recycled and expanded in the coils, which cools the coils. The cycle then repeats.
The second technology is desiccation. This works by blowing air over a moisture absorbing material (a desiccant). The desiccant absorbs moisture from the air and is then circulated to a heating area. The heat drives the moisture out of the desiccant as vapor. The vapor is vented to the outside and the desiccant is circulated back to absorb moisture again.
Both centrally installed and portable units are available that use one of these methods.
Testing the Concrete for Dryness
Of course, the only way to know if you’ve been successful in speeding up your concrete drying is to test the concrete for dryness. You can’t tell if it’s dry just by looking at the surface. As we’ve seen, moisture moves through the pores of the concrete to the surface and evaporates, so the surface is always drier than the center.
Concrete moisture testing has been studied since the 1960s, and researchers have developed a scientifically proven test for measuring moisture levels in a slab.
The test requires sensors for measuring the RH of the air trapped in the concrete. These sensors are inserted into the slab at specific depths. For slabs drying from one side, the depth is 40% of the thickness. For slabs drying from two sides, the depth is 20% of the thickness.
This test is called “the relative humidity test using in situ probes,” and it’s the basis for the ASTM F2170 standard. Wagner Meters provides an in situ RH testing system that conforms precisely to ASTM F2170.
The Wagner Meters Rapid RH® L6 system uses single-use sensors for speed, economy, and ease of use. The L6 sensors come calibrated and documented from the factory. Once the sensors are installed in the slab and equilibrated for the required 24 hours, repeat readings can be taken without additional equilibration time. And unlike reusable probes, the L6 sensors never need calibration.
How Do You Know When the Concrete Is Dry?
When you think the concrete should be dry, according to your best estimate, the only thing you can do is start testing. If you’re lucky, it’ll be dry, but many times it won’t be. So you give it more time and test again. How much more time is pretty much a best guess based on past experience. You continue this process until the test results show that the slab is sufficiently dry and ready to receive the floor.
The unpredictable nature of this process can cause schedules to slip and costs to mount up. Until recently, this was the best process for drying a concrete slab.
If drying cannot be completed when the floor covering must be installed, there is an option to use a moisture mitigation system. However, it is important to choose a high-quality product that can be trusted to do an adequate job of sealing the moisture into the slab.
New Technology for Data Logging and Trend Analysis
The good news is, there’s new technology that can make the art of predicting concrete drying more scientific by adding some hard data to your experience.
That technology comes in the form of data logging.
Data logging means you have automated devices installed in the concrete and around the site from the beginning of the drying process that continuously monitors conditions and store data. These devices measure concrete moisture and the temperature and humidity of the ambient air at regular intervals and store that data in their onboard memory. When it’s convenient, you can visit the site and quickly and easily collect the data with apps that run on your smart device.
Having complete and accurate data from the entire drying process can help you in some very important ways. First, if anything changes in the ambient conditions that might delay the drying process, you’ll be able to spot it immediately. That means you can correct it sooner, and you can more easily find out who’s responsible for it. So if there’s any delay in the schedule, you have the data to protect you from liability.
Having complete concrete moisture data allows you to perform trend analysis. Trend analysis is simply looking at the data on a chart, seeing a trend, and projecting where you think it will continue. This means you’ll be able to make an earlier estimate of when the drying process will be complete. As you gather data from more projects you’ll be able to make more accurate projections, helping you to schedule tighter and save more time and money.
The Tools for Data Logging and Trend Analysis
Data logging for concrete drying requires a system that tightly integrates concrete moisture testing with automated data logging.
The Rapid RH L6 system integrates the most advanced data logging technology available. It uses two small battery-powered devices, the DataGrabber® and the DataGrabber with Bluetooth®, that install in the L6 Smart Sensors to automatically log data.
The DataMaster™ L6 app runs on any iOS or Android device and can download the data via Bluetooth from either the DataGrabber with Bluetooth or the DataGrabber using the Total Reader®. The DataMaster L6 app can store, display, report and email the data in PDF format. Backups are stored in the cloud and in the sensors that are installed in the slab. This all-digital data path with backup ensures the highest data integrity, accuracy, and peace of mind.
To monitor ambient conditions, Wagner Meters also offers the Smart Logger™ that can record ambient temperature and RH for 300 days of replaceable battery life or up to 12,000 readings. The Smart Logger app can download the data via Bluetooth to store, report, or email it.
These two systems provide everything you need to set up a complete concrete moisture test plus an ambient condition logging system.
Concrete drying time is a major factor in the schedule of most construction projects. Follow the tips in this article to help speed your concrete drying time.
The only way to know that a concrete slab is sufficiently dry is to test it, and the scientifically proven most accurate test is the in situ RH test.
Data logging through the drying process enables you to detect problems earlier, make corrections and determine responsibility – protecting you from liability.
Trend analysis helps you to make more accurate projections, schedule tighter and save time and money.
The Rapid RH L6 system provides the fastest, easiest way to perform the in situ RH test, integrated with the most advanced data logging. Ambient logging systems can also add significant information to enable you to make more informed decisions.
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.