5 Tips to Help You Speed Up Concrete Drying Time
How Long Does Concrete Take to Dry?
According to the Portland Cement Association, once conditions are conducive for the concrete to dry — that is, consistent, reasonable heat in the air and low relative humidity — it typically takes about 30 days for every one inch of slab thickness to dry to 85-90% relative humidity. We say ‘once conditions are conducive’ because concrete that was put down a year ago might have been exposed to the elements for eleven months and only enclosed for the last month. If so, it has only been drying for one month.
Since concrete drying time is a major factor in the schedule of most construction projects, shortening that time can potentially save you 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.
Tips on How to Speed Up Concrete Drying Time:
- Use the correct amount of water in the mix. Too much water can increase the drying time.
- Do not over-trowel or seal the surface. This can block the pores in the concrete, diminish moisture evaporation, and increase the drying time.
- Keep doors and windows closed, the HVAC running, and fans circulating the air.
- You can also use dehumidifiers to remove moisture from the air. This will speed up the overall drying process of the slab.
The Difference Between Curing and Drying
Concrete curing and concrete drying are two different processes. Curing is the process of hardening that begins immediately after the concrete is poured. The concrete curing process is usually mostly complete after 28 days or so. However, the concrete will continue to harden further for a significant amount of time after that.
Even after the concrete is cured, excess water still must evaporate from the concrete. While it only takes around 28 days to cure concrete, drying can take months. The general rule of thumb is that concrete takes about 30 days to dry for every one inch of slab thickness. However, as we pointed out above, the conditions need to be right. That is, you’ll need low ambient relative humidity and a consistently warm temperature. You can achieve this by enclosing the space and then turning on the HVAC.
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 the process, the concrete becomes porous and a certain amount of this water becomes part of the concrete. The water that’s leftover either evaporates or remains in the capillaries of the concrete.
How Concrete Dries
Concrete dries as the water inside it evaporates through its surface. As this water evaporates through the surface, water from deep within the concrete moves through the capillaries and up 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
There are a few things you can do before you pour the concrete that will speed up the drying process:
- Use the correct amount of water in the mix. If there’s too much water, there will be more water left over after curing that will need to evaporate. That means a longer drying time.
- You can use a high cement content mix to reduce the drying time. However, there is the risk of cracking due to shrinkage.
- Are you using lightweight concrete? Lightweight aggregates absorb a lot of water and this increases the drying time. You can reduce the drying time by replacing these lightweight 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’s surface which increases the drying time.
How to Speed Up Drying: After the Pour
Once the slab has cured, enclose the space as soon as possible to protect the slab from absorbing any additional moisture.
While protecting the slab from additional moisture is important, there are a couple of other factors that affect drying after the pour:
- The ambient relative humidity and temperature of the air
- The temperature of the slab itself
The ambient relative humidity is important because it controls whether water can evaporate from the slab. If the ambient relative humidity is too high, your slab won’t be able to dry.
Enclosing the space allows you to use HVAC to control ambient conditions. In cooling mode, HVAC systems act like a refrigerating dehumidifier and will usually maintain a 50% relative humidity level, which is perfect for drying concrete. In heating mode, they lower the relative humidity by raising the air temperature.
HVAC systems are also a great way to get air circulating around the concrete and thereby reducing drying time.
Some facilities have central dehumidifiers that can be used to speed up the drying process. You can also rent portable dehumidifiers and even fans.
Both centrally installed and portable units are available that use one of these methods.
Testing the Concrete for Dryness
You can’t tell if concrete is dry just by looking at its surface because the surface is nearly always drier than the center of the slab. The only way to know if concrete is dry is to test it.
Concrete moisture testing has been going on since the 1960s and today there’s a scientifically proven way to easily test the moisture content of a concrete slab. The test is called “the relative humidity test using in situ probes” and is the basis for the ASTM F2170 standard.
The test uses sensors — inserted into the concrete at specific depths — to measure the relative humidity of the air trapped in the concrete. For slabs drying on one side only, the sensors are inserted to a depth that’s 40% of the slab’s thickness. For slabs drying on both sides, the sensors are inserted to a depth that’s 20% of the slab’s thickness.
The Wagner Meters Rapid RH® L6 system is an in situ relative humidity testing system that conforms precisely to the ASTM F2170 standard. The single-use L6 sensors are factory calibrated and easy to use. Once they’ve been installed in the slab and allowed to equilibrate for 24 hours, you can take repeated moisture readings whenever you want. Unlike reusable probes, the L6 sensors never need recalibration.
How Do You Know When the Concrete Is Dry?
You don’t unless you test it. If you’re lucky, it will be dry. However, many times it won’t be, so you’ll need to give it some more time to dry and then test it again. How much time you’ll give it to dry before testing again is pretty much a best guess based on past experience. At some point, the slab will be dry enough to receive the floor covering. Of course, this is an unpredictable process and because of that schedules can slip and costs can mount up.
When you absolutely must install the floor covering but the drying process isn’t complete, you can use a moisture mitigation system. If you go this route make sure you choose a high-quality product that will adequately seal the moisture into the slab.
New Technology for Data Logging and Trend Analysis
Technology is now available that adds some science to the art of predicting concrete drying time.
It’s called data logging.
Data logging involves installing small devices into the concrete. These devices have the ability to continuously monitor concrete moisture levels as well as the ambient temperature and relative humidity. The collected data is stored in their memory and whenever you visit the site you can easily download it via an app on your smart device.
This data can give you important insight into the drying process. For example, if a change in ambient conditions is likely to delay the drying process, you’ll be able to spot it and correct it sooner. Of course, you’ll also be able to easily see who’s responsible for it. (Hint: It won’t be the installer.) So, if the schedule gets delayed, you’ll have the data to protect you from liability.
Also, when you have complete concrete moisture data, you can perform trend analyses. This is where you look at data on a chart, see a trend, and then project how you think it will continue. This will allow you to more accurately estimate when the drying process will be complete. The more data you gather from all projects, the more accurate your drying estimates will become. This will help you schedule your next steps in the construction process, saving you both time and money.
The Tools for Data Logging and Trend Analysis
Data logging for concrete drying requires a system that integrates concrete moisture testing with automated data logging.
The Rapid RH L6 system has the most advanced data logging technology available. There are two battery-powered devices that automatically log data: the DataGrabber® and the DataGrabber with Bluetooth®.
The DataMaster™ L6 app runs on any iOS or Android device and can download the logged data from either the DataGrabber with Bluetooth or the DataGrabber using the Total Reader®.
The DataMaster L6 app stores, displays, reports, and emails the data in PDF format. For data integrity, accuracy, and peace of mind, backups of the logged data are stored in both the cloud and in the sensors installed in the slab.
For monitoring ambient conditions, the Wagner Meters Smart Logger™ records both the ambient temperature and the relative humidity for 300 days of replaceable battery life or up to 12,000 readings. The Smart Logger app can then download this logged data via Bluetooth for you to store, report, or share with others via email.
Used together, the DataMaster L6 app and the Wagner Meters Smart Logger are a complete concrete moisture testing and ambient condition logging system.
To sum up…
The only way to know if a concrete slab is dry is to test it. The most accurate test for doing this is the in situ relative humidity test.
Data logging throughout the drying process allows you to detect drying problems early, correct them, and will aid you to determine what is responsible for slow drying. This protects you from liability.
Trend analysis helps you make accurate projections and create tighter schedules. This saves you both time and money.
Concrete drying time is a major factor in the schedule of most construction projects and when you shorten that time, you save money. Following the tips in this article will help you do that.
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.