Concrete FAQ – Cold Weather Concrete FAQ – Frequently Asked Questions
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from Cement.org / Concrete FAQ – Curing is the process in which the concrete is protected from loss of moisture and kept within a reasonable temperature range. This process results in concrete with increased strength and decreased permeability. Curing is also a key player in mitigating cracks, which can severely affect durability.
The Link Between Concrete Sustainability and Curing
Sustainability, according to the Bruntland Report and adopted by many experts, is development that meets the needs of the present without compromising the ability of future generations to meet their own needs. This can be accomplished in one of two ways: either by using recyclable, reusable, or so little resources that future generations have the same access to them; or by producing development that meets our needs as well as the needs of future generations. We can use proper curing of concrete to advance towards the reduction of resource use.
A concrete element is expected to last a certain number of years. In order to meet this expected service life, it must be able to withstand structural loading, fatigue, weathering, abrasion, and chemical attack. The duration and type of curing plays a big role in determining the required materials necessary to achieve the high level of quality.
Curing is the process in which the concrete is protected from loss of moisture and kept within a reasonable temperature range. The result of this process is increased strength and decreased permeability. Curing is also a key player in mitigating cracks in the concrete, which severely impacts durability. Cracks allow open access for harmful materials to bypass the low permeability concrete near the surface. Good curing can help mitigate the appearance of unplanned cracking.
When smart, suitable, and practical curing is used, the amount of cement required to achieve a given strength and durability can be reduced by either omission or replacement with supplementary cementitious materials. Since the cement is the most expensive and energy intensive portion of a concrete mixture, this leads to a reduction in the cost as well as the absolute carbon footprint of the concrete mixture. Additionally, practical curing methods can enhance sustainability by reducing the need for resource intensive conditioning treatments, should the curing method be incompatible with the intended service environment.
Curing Pavements and Bridge Decks
While curing of concrete is an important issue with all concrete applications concrete pavements and bridge decks require careful consideration and have significantly different needs with regard to curing of the concrete of these structures. Both categories have basic requirements for the durability of the structures including strength, abrasion resistance, freezing and thawing and deicer resistance, and, in the case of bridges, low permeability for corrosion protection of the reinforcement of the structure.
Typical recommendations for curing of pavements allow the use of sheet curing, moist curing, or application of a film forming curing compound. Due to the large surface areas typical of concrete paving the application of curing compound to all exposed surfaces is the most common curing method. Moist curing and sheet curing of large surface areas may become cost prohibitive due to the large quantity of materials required to cover the full surface of concrete placed in any single day. In addition moist curing and sheet curing require maintenance to assure the curing method is properly completed for the full time duration chosen for paving (typically seven days). Moist coverings require rewetting and sheet goods are prone to being disturbed by wind, either of which would reduce the effectiveness of the curing method.
Curing compounds should be applied to pavements as soon as possible after bleed water has left the surface of the concrete at a rate of 200 ft2/gal for standard mixtures and application, 150 ft2/gal for fast track paving, and 100 ft2/gal for slabs thinner than 5.0 inches.
In contrast, concrete bridges require a higher standard of curing to achieve the low permeability required for protection of steel reinforcement. Standard recommendations for curing bridge decks is moist curing for a minimum of seven days for concrete mixtures containing only portland cement and as long as 14 days when supplementary cementing materials are included in the concrete mixture. Some states also require the application of curing compound upon removal of the moist curing methods.
Typical moist curing for bridge decks requires the application of adequate quality water saturated burlap or other approved absorptive material covered with minimum six mil plastic covering. The temperature of the saturated materials should be within 20 degrees Fahrenheit of the temperature of the in-place concrete. In most cases plastic will be specified to be white in color to reflect solar radiation, reducing the temperature rise beneath the plastic, while cold temperatures (less than 50 degrees Fahrenheit) may allow the use of black plastic to add heat to the system. Proper moist curing will also require uncovering and rewetting the absorptive material to assure that there is a constant supply of water available to satisfy the evaporation rate at the project site.
For more information, see Concrete FAQ Design and Control of Concrete Mixtures, 15th Edition, EB001.