Solved! This is How Long It Takes Concrete to “Dry”

Q: I will be pouring a concrete slab as part of a landscaping renovation project and I’d like to know how long it will take for the concrete to dry. Are there any guidelines or precautions I should follow?

A: Sounds like you have a fun project ahead of you! Concrete is one of the strongest and most durable construction materials around, but in order for it to reach its peak strength, it needs to “dry” gradually. This drying process, known as hydration, begins the moment you mix water with the dry concrete mix, giving you limited time to get the wet concrete into the forms before it hardens. Hydration continues over the next few weeks, progressing through a number of stages until the concrete is completely cured.

During the drying process, rock-hard crystals develop within the concrete that bind the sand and gravel components firmly together. If the concrete dries too quickly, the finished project won’t be as strong as it should be. Read on to find out how much time you have to pour and finish the concrete slab and what you can do during each stage to make your project a success.

Pour time is limited.

With most concrete mixes, such as Quikrete Ready-To-Use Concrete Mix (in the yellow bag) , you can expect to have about an hour of work time to get the concrete shoveled into the forms, spread out, vibrated, and leveled before the mix begins to harden. During this time, it’s vital that everyone who is working on the project knows what their job is and that they work somewhat quickly. You won’t have time to run out and buy an extra bag of concrete so make sure you have enough before you start the pour, as well as all the necessary tools (shovels, concrete rakes, screed board, floats, trowels, a broom, an edger, and a groover).

The weather affects work time.

In hot, dry weather, especially if you’re pouring under the blazing sun, the heat can draw water out of wet concrete quickly, reducing pour time to as little as 30 minutes, so it’s best to pour on a day when the temperature is between 60- and 80-degrees Fahrenheit. If you do have to pour in hot weather, the following tips will help extend pour time:

• Use cold mixing water.
• Wet down the sand in the forms before pouring. This will keep the dry ground from drawing water out of the concrete too quickly.
• Store the bags of concrete in the shade—the hot sun will increase the dry mix’s temperature.
• Schedule the pour during the early morning when the temps are cooler.
• Recruit plenty of workers because pouring concrete is labor intensive.

Don’t forget to factor in finish time.

When pouring concrete, plan to spend half of the work time on the pouring process, and the other half on the finishing process. If the concrete mix has an approximate one hour of work time (work time is listed on the bag), plan to spend no more than 30 minutes on the actual pouring and screeding process, so you have adequate time to finish the slab.

To start the finishing process, you must screed the surface. Screeding is the process of removing excess concrete by leveling the top level of the slab. A 2×4 is commonly used to do this, but whatever tool you use must be longer than the width of the area. To complete the process, move the straight edge in a sawing motion across the area to leave it flat. One pass is each direction is usually enough to achieve the desired result.

Finishing fresh concrete involves floating the surface with a concrete float to smooth it out and push large rocks down, hand-troweling around the edges where the float misses, using an edging tool to create rounded edges, adding a broom-finish texture to reduce slipperiness, and then using a grooving tool to create control joints. The larger the slab is, the longer it will take to finish, so take that into account and work quickly, and recruit extra help if needed. If the concrete begins hardening, skip the control joints and work on smoothing and brushing the surface and smoothing the edges. You can always cut the control joints the next day with a concrete saw.

Walk-on time depends on the specific type of concrete you’re using.

With basic concrete mix, you can usually walk on the slab about 24 hours after finishing the surface. If you need to be able to walk on it sooner, you may want to go with a fast-setting type of concrete, such as Quikrete Q•Max Pro Concrete Mix that offers a walk-on time of just three hours.

High-strength mixes can come with longer walk-on times, so you’ll need to read the particulars on the concrete bag. If you’re pouring an early high-strength mix, such as Quikrete 5000, while the work time is shorter, about 45 minutes, the concrete should not be walked on before 10 to 12 hours. Because different mixes differ in work time and walk-on time, it’s important to use only one type of mix in your project. Don’t blend different mixes in the same pour.

Curing will help the concrete reach its peak strength.

By keeping the new concrete from drying too quickly, you’ll have a stronger slab—and this is where curing comes in. You can slow the drying process by spraying the new slab frequently with water for the first seven days to keep it damp.  Known as “moist curing,” this will help ensure that the moisture deep within the slab is not drawn to the surface too quickly where it can evaporate.

Of course, not everyone has the opportunity to spray down new concrete multiple times during the first week to keep it damp. If this applies to you, no worries. You can apply a curing product, such as Quikrete Acrylic Concrete Cure & Seal, to the surface of just-finished concrete in order to keep the water within from evaporating and causing the concrete to dry too quickly. Acrylic Cure & Seal can be applied to the surface of the slab with a roller or garden sprayer just as soon as the concrete hardens and the surface sheen disappears.

After 7 days from pouring the concrete, you can drive on a new driveway.

By the time a week has passed the concrete will have reached approximately 90 percent of its final strength, and it’s usually safe to drive on it. If you have a large size vehicle, however, such as a 1-ton pickup truck, it’s a good idea to wait a few days or even a week longer.

At 28 days, concrete reaches its full strength and hydration is complete.

Although the bulk of the hydration process takes place in the hours and days immediately after the pour, concrete needs 28 days to fully dry. While the slab will harden and lighten in color before 28 days, don’t be fooled into thinking it’s done hydrating. If you plan to stain or paint the concrete, doing so before the process is complete can result in changes in the stain color or the paint peeling off.

This content has been brought to you by Quikrete. Its facts and opinions are those of BobVila.com.