Importance Of Concrete Curing

Ways To Cure Concrete For Maximum Strength

Mixing cement and then pouring the concrete mix does not complete the operation. The first 24 hours after concrete installation are critical to the ability of the mixture to gain strength and harden fully. This is stage one of the concrete curing process, which is one of the most important steps in concrete quality control. Proper curing greatly increases durability.

Cement requires a moist, controlled environment in order to perform properly. Hydration and hardening of concrete during the first three days is critical. About 3 weeks later, over 90% of the final strength is typically reached, though it may continue to strengthen for decades.

Concrete strength depends on the growth of crystals within the matrix of the material. These crystals grow from a reaction between cement and water, which is known as hydration. Without enough water, the crystals can't grow adequately and the concrete can't develop proper strength. If there is enough water, the crystals grow out like tiny rock-hard fingers wrapping around the sand and gravel in the mix and intertwining with one another.

Abnormally fast drying during the placement process can lead to increased tensile stresses. The early strength of the concrete can be increased by keeping it damp for a longer period during the curing process. High early-strength concrete is designed to hydrate faster, often by increased use of cement which increases shrinkage and cracking.

Temperature extremes make it difficult to properly cure concrete. On hot days, too much water is lost by evaporation from newly placed concrete. If the temperature drops too close to freezing, hydration slows to nearly a standstill. Under these conditions, concrete ceases to gain strength and other desirable properties.

During this period concrete needs to be in conditions with a controlled temperature and humid atmosphere. This can be achieved by spraying the surface with water, thereby providing protection from ambient conditions. Two common ways of achieving this would be to submerge the concrete in water or wrap it in plastic to keep water in the mix.

Other ways to keep new concrete wet is with soaking hoses, sprinklers or covered with wet burlap. It can also be coated with commercially available curing compounds, which seal in moisture.

Care must also be taken to avoid freezing, or overheating due to the exothermic setting of cement. By way of example, the Hoover Dam used pipes carrying coolant during setting to avoid damaging overheating. Improper curing can cause scaling, reduced strength, poor abrasion resistance and cracking.

The other important aspect of curing is temperature—the concrete can't be too cold or too hot. As fresh concrete gets cooler, the hydration reaction slows down. The temperature of the concrete is important, but not necessarily the air temperature. Below about 50 degrees Fahrenheit, hydration slows down significantly and below about 40 degrees Fahrenheit, it virtually stops.

A relatively new concrete curing power blanket is being used in some applications to keep a constant and consistent heat to help cure concrete. Concrete cannot be poured on frozen surfaces with ice and snow on them. The heating blanket acts as a wet cure, eliminating cracking and defects. These electric concrete curing power blankets are designed for use in both hot and cold weather to keep the concrete from drying too fast and freezing up.

Commercially manufactured concrete blocks are frequently hardened in steam rooms. These are called curing chambers and the process is known as steam curing. This type of curing keeps the products in a warm, vapor-laden air which not only hardens them more rapidly, but more uniformly, as a combination of warmth and moisture produces the most favorable conditions for uniform hardening of concrete.