Curing In-Place Concrete

External curing is a step during construction that involves maintaining newly placed concrete at adequate moisture and temperature conditions so that it can develop properties, such as strength and durability, the mixture was designed to achieve. Curing begins immediately after placement and finishing and should be continued for a sufficient period of time, typically 3 to 7 days.

Hydration is the chemical reaction of cementitious materials with water that produces concrete’s desired properties. The objective of curing is to prevent loss of moisture and maintain a favorable temperature to support continued hydration. Without an adequate supply of moisture, the cementitious materials in concrete cannot react to form a quality product.

Temperature is an important factor for curing. The rate of hydration, and therefore, strength development, is faster at higher temperatures. Temperature of placed concrete should be maintained above 50°F (10°C). Hydration generates heat that can be retained in cooler weather. In thicker sections, curing procedures should minimize the temperature differential between the core and surface to avoid thermal cracking. Curing can also regulate the cooling rate to prevent thermal shock.

Internal curing involves the use of absorptive materials, such as soaked lightweight sand and super-absorbent polymers, in the concrete mixture that release moisture with time. This is used in some applications. It does not negate the need for external curing.

Methods Using Application of Water

• Continuous fogging or sprinkling is an excellent curing method. Soaker hoses can be used on vertical surfaces. Air temperatures should be above freezing and water should be readily available. Absorbent materials can be used to retain water. Alternate wetting and drying is not an acceptable curing practice
• Ponding is the most thorough method of water curing but is rare. A dike is created along the edge of the slab to pond water on the slab surface. It is sometimes used on smaller slabs and bridge decks. Temperatures should be above freezing for the curing duration
• Use of absorbent materials like burlap or cotton mats can be used to hold water on horizontal or vertical surfaces applied by a soaker hose or sprinkler. The materials should be kept wet and weighted down to keep from blowing away. Materials should not stain the concrete surface
• Damp earth, sand, or sawdust can be used to cure flatwork, especially floors. Materials should be clean and free of organic or iron-staining contaminants
• Straw or hay sprinkled with water can be used on small areas. Straw can easily blow away and, if it dries is fire hazard. The layer of straw should be 6 inches thick and covered with a tarp

Methods Using Retention of Water

• Plastic sheets - either clear, white (reflective), or black. Plastic film should conform to ASTM C171, be at least 4 mils (0.10 mm) thick. Film reinforced with fibers are more durable and less likely to tear. Clear and dark sheets absorb solar radiation and are recommended in cool weather or on shaded areas. Reflective sheets minimize heat gain when exposed to sunlight and should be used in warm weather. Plastic should be laid in direct contact with the concrete surface as soon as possible without marring the surface. Edges should overlap, be taped, and weighted down. Sheets should extend beyond the edge of slabs at least twice the slab thickness.
• Liquid membrane-forming curing compounds must conform to ASTM C309 or C1315. These are wax or resin-based materials that form a surface film and minimize evaporation. Apply to the concrete surface, at the recommended rate, immediately after disappearance of water sheen on the surface after final finishing. Delayed application after surface has dried prevents the formation of the film.
• Waterproof paper - consists of two layers of kraft paper cemented together and reinforced with fiber. It is more resistant to tearing and can be reused. Paper is used like plastic sheeting and is less likely to mar the surface or cause mottling.