Why Concrete Must Be Cured

In order for concrete to achieve its optimal strength and durability, it must be properly cured.  The American Concrete Institute (ACI) defines curing as the “action taken to maintain moisture and temperature conditions in a freshly placed cementitious mixture to allow hydraulic cement hydration and (if applicable) pozzolanic reactions to occur so that the potential properties of the mixture may develop.”  

Hydration is the process of concrete setting and hardening as a result of a chemical reaction between the cementitious materials and water.  Curing is the process of “maintaining satisfactory temperature and moisture conditions in concrete long enough for hydration to develop the desired concrete properties.”    It is done during the manufacturing of concrete products or after concrete is placed and finished. 

Curing concrete requires a number of days, and it can take weeks or even months for the concrete to reach its full strength.  The Portland Cement Association says that, “Properly cured concrete has an adequate amount of moisture for continued hydration and development of strength, volume stability, resistance to freezing and thawing, and abrasion and scaling resistance.”

Depending on the type of project, the concrete mix, and the outside temperature or humidity, different methods are used for curing concrete but all of them are designed to keep it moist and prevent the loss of moisture.  One way that concrete contractors prevent the loss of moisture during curing is to leave the formwork in place and/or to cover the concrete with an impervious paper, plastic sheeting or membrane-forming compounds.  Additional methods include ponding, spraying (or fogging) and wet coverings, which also work to cool the concrete as the water evaporates.

Variables in curing time depend on the type of weather the concrete will experience during its life-cycle, and the size, shape and strength required from the concrete.  As a general rule for most concrete structures, the ACI recommends a curing period of a minimum of 7 days or until 70% of the specified compressive strength is achieved in temperatures above 40º F.  In temperatures above 50º F, the curing period can be reduced to 3 days if an accelerant is used to speed up the hardening process. 

Sources: concrete.org/topicsinconcrete/topicdetail/curing, cement.org/for-concrete-books-learning/concrete-technology/concrete-construction/curing-in-construction

 

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