Concrete slabs are one of two primary foundational sources for houses around the United States, especially in areas that are prone to having higher subterranean water levels that make a basement impossible.
When you’re building a house and you’re using a concrete slab, you should be aware of how long that slab needs to cure before you start framing.
You can start framing on a poured concrete slab about seven days after it’s poured. At this point, a concrete slab will have 70% of its fully cured strength. Poured concrete can take up to 28 days to fully cure.
Even though concrete can take four weeks or longer to fully cure, luckily you don’t need to wait that long to start framing.
Once you’re removed the forms you used to pour the concrete (usually a couple of days after pouring) and you’ve waited about a week, the concrete is usually strong enough for you to start framing.
That said, a quick call to a local concrete company can give you a more precise answer for your area since temperatures and moisture can affect curing (source).
How Long Until Concrete Is Strong Enough For Framing?
Concrete takes about seven days to reach 70% of its final required strength level (source). This corresponds to an adequate strength level to start supporting loads like the framing for a house.
There are a few factors that go into determining how long a concrete slab needs to cure before it can start supporting weight. The following table discusses a little bit about these factors.
|Amount of Water in Mixture||A higher water-to-cement ratio will cure slower but maybe stronger long-term|
|Ambient Temperature||Higher temperatures will cure faster but maybe less strong over time|
|Type of Mix Used||Rapid curing cement mixtures are available|
|Slab Size||Larger, thicker slabs will take longer to fully cure|
Keeping the cement moist during the curing process is also important to make sure it cures correctly. Spraying it down several times per day over the first week keeps it from losing moisture too fast, which can make it weaker when it’s fully cured (source).
Since you should be keeping long term strength in mind while curing your concrete slab, this is an important step to take.
What is Concrete?
Even though the two terms are often used interchangeably, concrete and cement are not the same things.
Cement is a component of concrete, and along with water and whatever aggregate is being used (usually sand), it combines and forms the hard, durable material known as concrete (source).
There are a lot of complexities that go into developing a good concrete mix, and the proper makeup of each mixture depends on the intended use for the concrete.
It also depends on environmental factors such as expected temperatures and the amount of movement in the ground, as well as how big of a load is going to be on the concrete.
Why Concrete Needs to Cure
Concrete needs to cure for a variety of reasons, but ultimately curing gives it long-term strength and durability. Essential during the curing process is keeping the right level of moisture in the concrete, which is done through either ponding, spraying, or covering the concrete.
Moisturizing concrete keeps it from losing moisture too fast, which can lead to cracking and loss of strength.
Concrete curing really is intended to accomplish three things:
- It retains moisture in the slab to help the concrete keep gaining strength
- It keeps concrete from cracking until it’s strong enough to resist cracking
- It improves strength, durability, water tightness, and wear resistance in the finished concrete product.
Drying is obviously necessary because wet cement isn’t strong enough to support any weight. However, curing ensures the concrete dries correctly to make sure it has the correct properties when it’s dried. Both processes, although slightly different, are essential to making sure concrete is able to do its intended job when it’s dry.
Speeding up Cure Times
There are a number of ways you can safely speed up concrete cure times without compromising the integrity of the finished product. Since time is money in a major construction project, any way to speed up the cure time for concrete should be considered a way to save money.
In general, these are the following ways that professional builders and DIY-ers can speed up the curing process:
- Do your best to reduce the humidity in the air. If you can enclose your concrete surface and run an HVAC system, that’s the best way to reduce dry times.
- Make sure you use the right amount of water. Using less water can speed up curing and drying times, but it may also make concrete weaker in the long run.
- Don’t apply a sealant to the surface. Sealing the concrete while it’s still curing can make it retain too much moisture and increase the dry time.
- Use a rapid-cure additive. These specially formulated additives can help your concrete cure quicker without weakening them over time.
It’s important to note that you don’t want to take too much moisture out of the concrete too quickly, as this can weaken the concrete and keep it from curing properly.
If you’re in doubt and you’re building your own house or doing your own concrete work for any project, you can consult with a professional in your area who might be able to give you a better idea of how to speed up your cure time without compromising the long-term strength of your concrete slab.
Other Ways to Strengthen Concrete
One of the most common ways to strengthen a concrete slab is to use metal rebar in the slab itself. These thick metal rods are laid at the bottom of the slab or within it while pouring the concrete into the form, and they can help improve concrete’s tensile strength, which is the strength it has to hold together while being pushed or pulled.
Another way to help deal with cracking is to put pre-staged joints in concrete slabs. These joints are placed at certain intervals, depending on the overall dimensions of the slab as well as its thickness, to help control the spread of cracks. Cracking in concrete is almost inevitable eventually, so putting these joints in will help keep cracks less conspicuous.
What to Do While Your Slab Is Curing
If you’re pouring a concrete slab, you should build the curing time into your schedule to make sure it doesn’t put you too far behind schedule. When you’re determining your schedule, however, you may wonder what else you can be doing while you’re waiting a week for your slab to cure so you can start framing.
Obviously, you should be applying moisture to your concrete for that entire time, but your building project doesn’t need to stop completely while it cures. You can use that time to accomplish some of the following other tasks:
- Gather materials
- Work on surrounding exterior work (yard grading, etc.)
- Start work on plumbing and electrical locations
When you’re developing your building plan and schedule, it’s important to take your concrete curing time into account. Doing so can help you avoid unforeseen delays and keep your project on schedule.
A concrete slab is a quick, effective way to build a strong foundation for a house, and it also can be good for patios, an outdoor shed, or even a driveway.
However, if you’re using it as the foundation for a house, you should make sure you let it cure the proper amount of time before you start to frame your house on it.
Applying weight to a slab before it’s properly cured can be disastrous for a house, as the concrete may crack or the framing materials could even sink into the weight.
Concrete is very strong but it depends on a good curing process to make sure it’s strong enough to hold something as heavy as a house.