How Long Does Concrete Take to Dry
Concrete is usually dry enough after 24 to 48 hours to walk on. For concrete to dry and reach its full strength, it typically takes about 28 days per inch of slab thickness.
Once conditions are conducive for the concrete to cure at 85-90% relative humidity. We say ‘once conditions are conducive’ because concrete put down a year ago might have been exposed to the elements for eleven months and only enclosed for the last month.
Now that you know how long it takes concrete to dry, other things in this article you’ll learn are:
- The Difference Between Concrete Curing and Drying
- How to Speed Up Drying: Before the Pour
- How to Speed Up Drying: After the Pour
- How Do You Know When the Concrete Is Dry?
Since concrete drying time is a significant factor in the schedule of most construction projects, shortening that time can save you a lot of money.
Drying is critical when a flooring system is installed over the concrete slab. If the slab isn’t sufficiently dry when installed, the floor might be seriously damaged by the excess moisture.
The concrete curing time takes about 28 days to be fully cured. Differences in weather, mix, and other items can slightly change the timeframe of the curing period. The general rule of thumb for concrete drying is 28 days to dry for every 1 inch of slab thickness.
The Difference Between Concrete Curing and Drying
Concrete curing and concrete drying are two different processes.
How Long Does Concrete Take to Cure
The concrete curing process is usually mostly complete after 28 days. However, the concrete will continue to harden for a significant amount later.
Curing is the process of hardening that begins immediately after the concrete is poured.
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.
How Long Does it Take for Concrete to Dry
The general rule of thumb is that concrete takes about 28 days to dry for every inch of slab thickness. Within 24 to 48 hours, the concrete will be ready for foot traffic.
However, as we pointed out above, the conditions must be right. 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 essential components: cement, aggregate, and water.
When water and cement are mixed, a chemical reaction 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 leftover water 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, more water will be left over after proper 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, increasing 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, increasing 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 refrigerating dehumidifiers and usually maintain a 50% relative humidity level, perfect for drying concrete. They lower the relative humidity in heating mode by raising the air temperature.
HVAC systems are also a great way to circulate air through the concrete, reducing drying time.
[Tips] How to Speed Up Concrete Drying Time
- Use the correct amount of water in the concrete 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.
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 the concrete slab is dry just by looking at its surface because the surface is nearly always drier than the center of the slab. Testing the concrete is the only way to know if it is dry.
Concrete moisture testing has been going on since the 1960s, and today there’s a scientifically proven way to test the moisture content of a concrete slab easily. 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 of 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 installed in the slab and allowed to equilibrate for 24 hours, you can take repeated moisture readings whenever possible. Unlike reusable probes, the L6 sensors never need recalibration.
Free Download – 4 Reasons Why Your Concrete Is Taking Forever to Dry
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, it often won’t be, so you’ll need more time to dry and test it again. How much time you’ll give it to dry before testing again is pretty much the best guess based on experience.
The slab will be dry enough to receive the floor covering at some point. Of course, this is an unpredictable process; schedules can slip, and costs can mount up.
You can use a moisture mitigation system when installing the floor covering, but the drying process isn’t complete. If you go this route, choose a high-quality product that adequately seals the moisture into the slab.
Save time and money by using the most accurate concrete RH test.
Testing a concrete slab is the only way to know if it is dry. The most accurate test for this is the in situ relative humidity test.
Data logging throughout the drying process allows you to detect problems early, correct them, and will aid you in determining 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 significant factor in most construction projects’ schedules; when you shorten that time, you save money. Following the tips in this article will help you do that.
(Some of the information is according to the Portland Cement Association)
Are you looking for a concrete calculator to estimate how many cubic feet and cubic yards of concrete you need to fill your space? Try out our concrete calculator.
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.
Last updated on September 14th, 2023