While Curing Concrete, Why Do People Cover It With Plastic?

Rapid Moisture Loss Is Your Enemy

Image of a concrete driveway.
Concrete surfaces can be valued components of a landscape. Learn how to cure concrete properly. David Beaulieu

What causes cracking in new concrete? And how do you prevent these cracks? Curing concrete properly while it is fresh can save you a lot of trouble later. And the good news is that simply covering it with a sheet of plastic can help enormously. 

The Trick to Curing Concrete Properly

You have probably seen people cover freshly done concrete work with plastic while it is curing. Why do they bother with this?

So that dirt does not blow onto the concrete during the curing period? While it is true that covering the curing concrete with plastic will keep it cleaner, there is a more essential purpose behind this practice:

Remember, you mix water into concrete to activate the cement binding agent within. As the mix dries, it will harden. But the drying, or "curing" should be gradual, otherwise cracking may occur. To prevent cracking, plastic is placed over the curing concrete to trap the water inside, ensuring gradual curing. As an added step, you can also mist the concrete periodically with a spray bottle to prevent it from drying too quickly. For an example of a concrete project that you would need to cover with plastic, see How to Build a Concrete Patio.

Related Information on Masonry and Curing

If you are a DIY'er who is new to subjects such as curing concrete and mortar, there are many terms out there that can be confusing.

Let's take a look at a few, with an eye to clearing up any confusion before you undertake your hardscape project. It is better to take a little time now to become better acquainted with this subject than to make big mistakes -- perhaps even irreversible mistakes -- later on, when you are right in the middle of working with concrete, mortar, etc.

(these substances can be very unforgiving):

  1. "Masonry": Masonry is construction achieved through the use of units of various natural or artificial mineral products, such as stone, brick, mortar or concrete. The term, masonry can be applied to the craft itself or to the finished product.
  2. "Concrete": Concrete is a masonry product composed of cement, sand and gravel or other coarse aggregate. When water is mixed in with concrete, it activates the cement component, which is the element responsible for binding the mix together to form one solid object. Concrete is often used in hardscape construction. For instance, concrete may be used to form the flooring of a patio, or concrete footings may be used to support a wooden deck.
  3. "Mortar": Mortar is a masonry product composed of cement and sand. When water is mixed in with mortar, its binding element, the cement, is activated. You must distinguish mortar from "concrete," which acts in a similar way but which contains coarse aggregate which is bound together by the cement. Concrete can stand alone, while mortar more typically is used to hold brick, stone or similar materials together. For example, to construct a hardscape feature such as a brick patio or a mortared wall around an artificial pond, mortar would be used to hold the bricks or stones together.
  1. "Dry wall": In reference to stone walls, "dry wall" is construction wherein the stones are not held together by mortar. Dry-wall construction requires greater precision in the way stones are stacked than does mortared-wall construction, since proper placement alone keeps the wall intact. The equivalent term for installing stone pavers to build a patio or walkway is "dry-laid." Also notice that the term, "dry wall," used in the context of indoor construction, refers to something totally different: namely, the type of panel sometimes referred to as "gypsum board."

More on Curing Concrete -- and Mortar -- Properly

That's right: Even though mortar is a different masonry product from concrete, it, too should be cured properly. And, once again, improving its chances of curing properly is as simple as covering your just-completed project (such as a brick patio that is held together with mortar) with a sheet of plastic.

Optionally (to further improve your chances that your mortar cures properly), pick up the plastic occasionally and lightly dampen the mortar with water from a spray bottle. How much time does it take the mortar to fully cure? Generally it takes about two days, but this will depend on weather conditions.

But let's return to the issue of curing concrete properly. How long does it take concrete to fully cure? There is a technical answer and a practical answer. The technical answer is 28 days. But as Aurora Paving points out, "For practical purposes, we recommend that car traffic can generally enter onto concrete slabs after three days and truck traffic after seven days."

There is another step to take to help the concrete cure. Have you ever heard of a "control joint?"

Control joints are grooves inserted into a concrete surface to "control" cracking. Essentially, this groove is an intentional, controlled crack inserted to preclude the concrete's cracking on its own, in an uncontrolled manner. By placing control joints in it before it cures, any stress the concrete will be subjected to subsequently will not produce haphazard cracks that will be a landscaping eyesore.

Using a trowel or jointer, cut an even control joint that will be aesthetically pleasing. Control joints can also be cut into existing concrete surfaces using a saw with a masonry blade attached.