This invention relates generally to concrete and more specifically to apparatus particularly adapted to promote optimal curing or hydration of concrete during its early stages.
It is well known that in order to develop desirable properties of strength and durability concrete must be properly cured during its early stages, particularly the first few days, by maintaining a satisfactory moisture content at a suitable temperature. In order to ensure such moisture content it is conventional to either apply water and/or prevent excessive evaporation. With regard to the application of water the American Concrete Institute in their Standard Practice For Curing Concrete (ACI 308-92) lists a number of different systems which can be used to maintain satisfactory moisture including ponding, sprays, steam, or saturated cover materials such as burlap or cotton mats, rugs, earth, sand, sawdust, and straw or hay. It is stated that the system chosen must provide a complete and continuous cover of water. With regard to preventing evaporation it is noted that materials such as sheets of reinforced paper or plastic or membrane-forming curing compounds can be used.
The system chosen for a particular job is dependent on various factors such as availability and cost of water, labor, and curing materials, however, for proper development of desired properties it is important that moisture conditions be continuously maintained.
The above noted conventional systems have various limitations and disadvantages associated with them. For example, ponding can be very effective however it is impractical for many applications and subject to damage from premature or sudden release of the ponded water and, as a result, this system is not very widly used. Fog spraying can be effective at appropriate temperatures but requires a relatively costly set-up. Lawn sprinklers result in water run-off and can be objectionable in locations where the cost of water is high. Earth curing is generally limited to small jobs and requires labor intensive application and clean up. Straw and hay are subject to dislocation by wind unless held down by some means and also require intermittent wetting to prevent drying out. Absorbent materials will hold water on the surface of concrete but also must be wetted intermittently to prevent drying out.
All of the above methods can be effective if used properly. One of the most common methods currently in use is covering the concrete surface with burlap and wetting the surface on an intermittent basis to prevent the burlap from drying out. In recent times, as labor has become a more significant percentage of the total cost of construction, this method has become more and more expensive due to labor costs. For example, since the period for maintaining an acceptable level of moisture is generally considered to be seven days, a contractor must pay workers over time premiums to come to the construction site throughout the week end as well as nights to wet down the burlap every six or eight hours.