Concrete slabs on grade, and other concrete structures and objects, traditionally are made by building a form or a mold. The forms and molds are usually made from wood, plywood, metal and other structural members. Unhardened (plastic) concrete is placed into the space defined by opposed spaced form members. Once the concrete hardens sufficiently, although not completely, the forms are removed leaving a concrete wall or other concrete structure, structural member or concrete object exposed to ambient temperatures. For some applications, such as runways, roads and highways, slip forms are used to continuously place long lengths of concrete. For both conventional forms, molds and slip forms, the unprotected concrete slabs, structures or objects are exposed to the elements during the remainder of the concrete curing process after the forming process. The exposure of the concrete to the elements, especially temperature variations, often makes the curing of the concrete a slow process and the ultimate strength difficult to control or predict. To compensate for these losses, larger amounts of portland cement are used than otherwise would be necessary. Larger cement fractions, particularly for high surface area-to-volume ratio elements, such as these, can increase the likelihood of shrinkage, cracking, and curling.
The curing of plastic concrete requires two elements, water and heat, to fully hydrate the cementitious material. The curing of plastic concrete is an exothermic process. This heat is produced by the hydration of the portland cement, or other hydraulic cements, that make up the concrete. Initially, the hydration process produces a relatively large amount of heat. As the hydration process proceeds, less hydration occurs thereby resulting in the production of less heat. At the same time, moisture in the concrete is lost to the environment. If one monitors the temperature of concrete during the curing process, it produces a relatively large increase in temperature which then decreases over time. This chemical reaction is temperature dependent. That is, the hydration process, and consequently the strength gain, proceeds faster at higher temperature and slower at lower temperature. During conventional concrete curing, first, the heat is lost; then, moisture loss makes it difficult for the cementitious material to fully hydrate, and, therefore, the concrete may not achieve its maximum strength.
Concrete in conventional concrete forms or molds is typically exposed to the elements. Conventional forms or molds provide little insulation to the concrete contained therein. Concrete placed in forms for slabs on grade, roads, runways, and the like, is usually placed on a sheet of polyethylene placed on the ground first. Therefore, the bottom of the concrete slab is in contact with the ground, which absorbs the heat of hydration from the concrete. The top of the concrete slabs are sometimes covered with plastic sheathing to prevent some of the moisture loss to the environment. This also allows the concrete to lose the heat of hydration from the top of the slab surface to the air. Although during winter an insulated blanket may be placed on top of the concrete slab, road or runway to keep the concrete from freezing, heat produced within the concrete form or mold due to the hydration process usually is lost to the ground or to the air through a conventional concrete form or mold relatively quickly. Thus, the temperature of the plastic concrete may initially rise 20 to 40° C. or more above ambient temperature due to the initial hydration process and then fall relatively quickly to ambient temperature, such as within 12 to 36 hours. This initial relatively large temperature drop may result is concrete shrinkage, curling and/or concrete cracking. The remainder of the curing process is then conducted at approximately ambient temperatures, because the relatively small amount of additional heat produced by the remaining hydration process is relatively quickly lost to the ground or to the air through the uninsulated concrete form or mold. The concrete is therefore subjected to the hourly or daily fluctuations of ambient temperature from hour-to-hour, from day-to-night and from day-to-day. Curing concrete under ambient temperature conditions is not as significant a problem during summer temperature. It is cool or cold weather conditions that cause the most significant trouble for properly curing concrete.
Failure to cure concrete under ideal temperature and moisture conditions affects the ultimate strength and durability of the concrete. In colder weather, concrete work may even come to a halt since concrete will freeze, or not gain much strength at all, at relatively low temperatures. By definition (ACI 306), cold weather conditions exist when “ . . . for more than 3 consecutive days, the average daily temperature is less than 40 degrees Fahrenheit and the air temperature is not greater than 50 degrees Fahrenheit for more than one-half of any 24 hour period.” Therefore, in order for hydration to take place, the temperature of concrete must be above 40° F.; below 40° F., the hydration process slows and at some point may stop altogether. It is typically recommended that concrete by moisture cured for 28 days to fully hydrate the concrete. However, this is seldom possible to achieve in commercial practice.
Insulated concrete form systems are known in the prior art and typically are made from a plurality of modular form members. U.S. Pat. Nos. 5,497,592; 5,809,725; 6,668,503; 6,898,912 and 7,124,547 (the disclosures of which are all incorporated herein by reference) are exemplary of prior art modular insulated concrete form systems. Full-height insulated concrete forms are also known in the prior art. U.S. Patent Application Publication No. 2011/0239566 and applicant's co-pending patent application Ser. No. 13/247,133 filed Sep. 28, 2011 (the disclosure of which are both incorporated herein by reference in their entirety) discloses a full-height vertical insulated concrete form. Insulated concrete forms reduce the heat transmission to and from the concrete within such forms. However, vertical insulated concrete forms are not useful for forming slabs on grade, such as runways, road and highways and previously have not been proposed to be used in such applications.
Concrete insulating blankets are known in the art. Electrically heated insulating blankets are also known in the prior art, such as those disclosed in U.S. Pat. Nos. 7,183,524 and 7,230,213. Such concrete insulating blankets and electrically heated insulating blankets are known for use in northern climates for thawing frozen ground and/or preventing curing concrete from freezing. It is known that plastic concrete will not cure satisfactorily at temperature below 50° F. However, such electrically heated blankets are designed to provide a constant amount of heat to the plastic concrete and are used only for the purpose of preventing the concrete from freezing in cold weather.
Therefore, it would be desirable to produce a concrete forming or molding system for slabs on grade, such as runways, road and highways. It would also be desirable to provide a concrete curing system can be used specifically to cure slabs on grade, such as runways, roads and highways. It would also be desirable to provide a concrete curing system that accelerates concrete maturity or equivalent age to achieve improved concrete strength, particularly early concrete strength. Furthermore, it would be desirable to provide a concrete curing system for slabs on grade, such as runways, roads and highways, so that the concrete cures more quickly, is less permeable, more flexible, stronger, more durable and less prone to cracking and curling. It would also be desirable to produce concrete slabs on grade, such as runways, roads and highways, that are more environmentally friendly.