The way concrete hardens is a function of the cement particles reacting with the water it is mixed with. As the cement bonds with the water molecules, the concrete gets harder.
The real question is how long does concrete take to set enough for whatever your purposes are for the concrete. For example, how long before you can walk on it without leaving footprints, drive or park on it without sinking into it, etc. The answer is that your concrete will be ready in a surprisingly short time.
Your concrete should be solid enough to walk on, without leaving footprints, after anything from 24 to 48 hours. 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. With basic concrete mix, you can usually walk on the slab about 24 hours after finishing the surface. You can slow the drying process by spraying the new slab frequently with water for the first seven days to keep it damp. 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. 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. 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.
So you use these methods to figure out how fast the bleed water is evaporating--if it's greater than 0. In the next section we'll discuss how to do initial curing. After initial set, the concrete surface still needs moisture and now there's no bleed water. This is when you really need to cure the concrete. You need to assume that your concrete needs to be cured-it does!
You don't want your perfect baby concrete to turn into a juvenile delinquent, do you? Now let's narrow this conversation down a bit. Let's talk only about horizontal concrete and only about the moisture part of curing. Let's also narrow things down to curing of colored concrete. We'll define that as any concrete with color, whether integral or dry-shake, whether it is going to be stamped or not.
First, and most importantly, colored concrete is not really different than any other concrete, it needs exactly the same treatment to end up with quality concrete. Some of the methods, though, need to be a bit different since appearance is so much more important than it is for an industrial slab. There are three ways to cure concrete: either we add water to the surface to replace the water that is evaporating or we seal the concrete to prevent the water from evaporating in the first place or we do both.
Note that adding water to the surface is NOT adding water that will be worked into the concrete mix--that would increase the water-cement ratio of the surface concrete and weaken it, ruining all our curing efforts.
You need to think about initial curing when the bleed water is evaporating too rapidly to keep the surface wet prior to initial set. Traditionally that has been specified at greater than 0. Many mixes today bleed at much lower rates than this, so if there is less bleed water then the evaporation limit needs to be set lower-more like 0. The best approach for decorative concrete is to try to alter conditions so you don't need to do initial curing: block the wind, keep the sun off the concrete, get cooler concrete.
If that's not possible, fogging just enough to keep the surface damp is possible, but the simplest approach is to use evaporation retardant. This chemical can be sprayed on to form a thin membrane on the surface that prevents the water from evaporating. It completely dissipates during finishing operations. Keep some of this around for dry windy conditions.
After concrete is placed, the concrete increases in strength very quickly for a period of days. Water curing can be done after the slab pour by building dams with soil around the house and flooding the slab.
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