Concrete used on farms and in rural areas needs to be strong, durable, dense, watertight and able to withstand weathering, temperature changes and abrasion. Proper curing enhances all of these desirable properties.
If you've had something built with concrete and later had problems with it, odds are the curing wasn't done correctly. Making sure concrete cures properly costs little and is easy to do. And yet it is often overlooked, even by professional builders.
For concrete to cure properly, you must watch two factors carefully: moisture and temperature.
First, let's set something straight: Concrete does not dry. Instead, it cures or hardens due to a chemical reaction between cement and water. If concrete that is curing begins losing moisture to evaporation, it causes the concrete to shrink, creates stress and can lead to cracking on the concrete's surfaces.
To cure poured and finished concrete, cover the exposed surface and keep it moist, preferably for seven days but for a minimum of three days.
Common curing methods include covering concrete with plastic sheets, applying curing compounds to the surface, or wetting the surface with hoses or sprinklers.
Low-cost materials can also be used to cover and cure concrete if they are kept moist. These include sand or soil, burlap bags, hay or straw, old carpeting and even old newspapers.
For small slabs, you can build dikes around the perimeter and then flood the surface with water. Lawn sprinklers work well in keeping flat surfaces wet, and soaker hoses often are used on vertical surfaces.
Waterproof paper works well on concrete with flat surfaces and sharp edges. You don't need to add water with waterproof paper, which also can help protect concrete during later construction. Plastic sheets provide an effective moisture barrier and are often used on concrete walls. Wooden forms can be left in place to help protect against moisture loss if the surfaces are kept wet.
Curing compounds consist of waxes, resins and solvents. When spraying or brushing these compounds onto damp concrete surfaces, complete coverage is important because even small gaps can lead to excessive evaporation. To ensure a good cure, put on a second application the next day.
Temperature also plays an important role in curing. For a good cure, control the temperature and avoid extremes. On hot and dry days, water loss through evaporation can produce defective concrete. To help offset the heat, add water that is slightly cooler than the concrete to the finished surface.
If the temperature drops below 40° to the point of freezing, the chemical reaction that gives concrete its strength comes to a halt. It's best to plan concrete work for times when temperatures are likely to stay above 50°. As an alternative, steam can provide a good cure during extremely cold weather and will add to the early strengthening of concrete.
In short, curing is the one thing you can do to make concrete as solid as you want it to be.
For More Info
Portland Cement Association, 5420 Old Orchard Road, Skokie, IL 60077; www.cement.org