Concrete is one of the most durable and strong construction materials available, but it doesn’t start out that way. In order to reach its full potential, concrete must dry from its liquid form into a completely solid material. This drying process is known as hydration and begins as soon as dry concrete mix is introduced to water. From this point, there is a limited time to get the wet concrete into a form before it hardens. Hydration then continues over the next few weeks, progressing through a series of stages until it is completely cured. Before you rush out to find concrete forms for sale, it is important to understand how this drying happens and how long it will take.
Pour Time Is Crucial
With most concrete mixes, you will have a window of about one hour to get the concrete into the forms, spread out, vibrated, and leveled before the mix starts to harden. In that period, everyone who is working on the project needs to understand their role and work quickly to ensure no time is wasted. There will not be time to go find or purchase additional bags of concrete, so having the right amount on hand is critical. You should also have any shovels, concrete rakes, screen boards, floats, trowels, brooms, edgers, and groovers that you need on hand.
Pay Attention to the Weather
The work time for any concrete will be heavily dictated by the weather on the job site. In hot and dry weather, especially when the sun is overhead, the heat can draw water out of wet concrete rapidly and reduce pour time to as little as 30 minutes. It is recommended, if you want to pour the perfect concrete slab, to on a day when the temperature is between 60 and 80 degrees Fahrenheit.
If you do have to pour in hot weather, some ways you can extend the pour time are to:
•Use cold water for mixing.
•Wet down the sand in the forms prior to pouring. This keeps the dry ground from drawing water out of the concrete at a faster pace.
Store bags of concrete in the shade to avoid the sun’s increasing temperature of the mix.
•Schedule the pout in the early morning when temperatures may be lower.
•Have additional workers to complete the pour faster.
Include Finish Time
When planning for a pour, you should expect to spend about half the time on the actual pouring process and the other half on the finishing process. So if your concrete mix has approximately one hour of work time listed, plan to spend no more than 30 minutes on pouring and screeding to allow adequate time for finishing the slab.
To start the finishing process, you will need to screen the surface or remove excess concrete by leveling the top of the slab. A 2×4 can be used to do this, but any tool works if it is longer than the width of the area. To complete this process, move the straight edge in a sawing motion across the slab to leave it flat. One pass in each direction is usually sufficient to achieve the desired result.
Finishing fresh concrete involves floating the surface with a concrete float in order to push large rocks down, hand-troweling around edges that the float missed, using an edging tool to create rounded edges, adding a broom finish texture to reduce slipperiness, and then using a groover to create control joints. The larger a slab is, the longer it will take to complete these steps, so that should be taken into account when planning for time and resources.
If the concrete begins to harden, you can skip the control joints and focus on smoothing and brushing the surface and smoothing the edges. You can cut the control joints the next day with a concrete saw if necessary.
With a basic concrete mix, you can usually walk on the slab about 24 hours after finishing the surface. However, if you need to walk on it sooner, you may be able to purchase a fast-setting concrete that can have a walk-on time as low as 3 hours.
Some high-strength mixes may have longer walk-on times, so it is important to read the particulars on each bag. Because different mixes can have varying work time and walk-in time, it is important to use only one type of mix for each project and avoid blending within a pour.
By keeping the concrete from drying too quickly, you will have a stronger slab as an end result. This is where curing comes into play. You can slow the drying process by spraying the new slab frequently with water in the first seven days to keep it damp in a process known as “moist curing.” This ensures the moisture deep in the slab is not drawn to the surface too quickly, where it will evaporate.
If you do not have the ability to spray your concrete over the first week, you can apply a curing product to the surface of freshly finished concrete. These products will keep the water from evaporating and can be applied with a roller or a garden sprayer as soon as the concrete hardens and the surface sheen disappears.
Seven Days After Pouring
Once a week has passed, your concrete should be at about 90% of its final strength. This is enough that you could safely drive on the concrete. However, if you have a heavy-duty vehicle like a 1-ton pickup truck, you may wait a few more days.
Full Strength Concrete
While the majority of the hydration process occurs in the hours and days after a pour, concrete takes 28 days to fully dry. The slab will harden and lighten in color before that time has elapsed, but this does not mean it’s done hydrating. If you plan to stain or paint the concrete, doing it early can lead to changes in the stain color or paint peeling off. This 28 day period is a good rule to follow before you consider any concrete dried.