Curing Concrete - Critical!

Curing is the process of maintaining an optimum environment (temperature & humidity) around the fresh concrete for a specific period of time so that the concrete can achieve its designed strength and durability characteristics.

The chemical reaction that occurs when you mix water and cement is called hydration. This hydration is a two stage process. The initial stage happens quickly. This is the transformation from the liquid to the solid state. It can happen in less than four hours. After this, the reaction slows down. The reaction can continue for months and sometimes years! Crystals continue to grow and the concrete keeps getting stronger and stronger.

However, for this reaction to continue the concrete needs water! If the freshly poured concrete is left open to the atmosphere the relative humidity of the concrete will rapidly drop below 100 percent. The water is evaporating into the atmosphere. If the humidity level drops below 80 percent, the hydration reaction can actually stop. If this happens early in the life of the concrete, the concrete may only achieve half of its design strength!

Stopping the Evaporation

There are several ways to stop the water from leaving freshly poured concrete. You can cover it with burlap or other cloth and keep it wet; it can be covered tightly with plastic; or you can spray a liquid curing compound onto the concrete.

Liquid curing compounds are easy to work with. These products are basically liquid 'plastic'. They form a watertight barrier that prevents water from evaporating from the concrete. The use of water and burlap can waste huge volumes of water. Sheet plastic must be installed so that absolutely no air can get under the plastic. Wind can be a big problem. Curing compounds are the only way to go.

The key to using curing compounds is their immediate application once you have lifted your trowel from the concrete for the last time. Do not hesitate to apply these compounds! On sunny windy days, the water is leaving the concrete at an incredible pace. It is not uncommon to experience plastic shrinkage cracks in concrete under these conditions. The trick is to finish the concrete as quickly as possible and spray the curing compound right away. On large jobs, a separate person should be following behind the finisher and applying the curing compound as the finisher is working just ahead. Believe me, it is that critical.

Concrete Sealants

Earlier, we discussed the crystals that create concrete. If you could look through a powerful microscope at concrete, you would be amazed. It is far from a solid compound. It would look similar to a giant metal jungle gym that you often see in city parks, the steel bars being the needles of the crystals. Just like in the jungle gyms, there is free space between the needles of the crystals.

Water is easily absorbed into concrete that has a wood float or magnesium trowel finish. This is the type of finish that is most common on residential sidewalks, patios and driveways. These finishes are slightly rough for skid and slip resistance.

When water freezes, it expands. In fact, the volume increase is approximately 9 percent. Since water, in liquid form, is not compressible, this means that as it begins to freeze it pushes things out of its way. In concrete, the water and ice begin to push against the needles of the crystals. These needles break and fracture. Once enough needles are broken, the concrete begins to fall apart. The manifestation of this phenomena is spalling and pitting of the surface.

Deicing salts create the same problem. When the salts melt snow and ice, they create a saltwater brine which is absorbed into the concrete. Weird osmotic pressures build up inside the concrete as the salt brine solution becomes more concentrated. These pressures break the needles of the crystals as well.

What to do?

The best defense against freeze/thaw damage and salt brines is to minimize the absorption of these liquids into the concrete. There are wonderful sealants available that do just that. However, not all sealants can be used in all climates.

Certain sealants are comprised of a liquid in which millions of tiny solid particles are suspended.These sealants penetrate into the concrete surface. After the liquid evaporates, the tiny solid particles are left behind. These particles fill many, but not all, of the spaces between the needles of the crystals. Enough spaces are blocked so that water and other liquids can't squeeze into the concrete. However, water vapor that is in the concrete can escape from the remaining openings.

This is very important if your concrete is subjected to freezing temperatures. Water vapor which is trapped in concrete can build up, freeze, and break needles! Be sure to purchase a breathable sealant if you live in such a climate.

Certain common concrete sealants are not breathable! Beware!