Poor curing practices adversely affect the desirable properties of high-performance concrete, just as they do any concrete. Proper curing of concrete is essential to obtain maximum durability, especially if the concrete is exposed to severe conditions where the surface will be subjected to excessive wear, aggressive solutions, or severe environmental conditions (such as cyclic freezing and thawing). Likewise, proper curing is necessary to assure that design strengths are attained.
Even when good quality concrete is placed on the job site, curing is necessary to ensure the concrete provides good service over the life of the structure. Good concrete can be ruined by the lack of proper curing practices. Curing is even more important today than ever before for at least three reasons (Neville 1996):
• Today’s cement gain strength earlier and allows contractors to remove formwork soon after concrete placement. This encourages discontinuing curing operations prematurely.
• The lower water-cement ratios being used with modern concretes (like HPC) tend to cause self-desiccation (Chapter 3). Ingress of water from proper curing is necessary to control this phenomenon.
• Many modern concrete mixtures contain mineral admixtures, such as fly ash and ground granulated blast furnace slag, that have slower reaction rates.
Curing over longer periods of time is needed for the proper development of the properties of these mixtures. Curing has a major impact on the permeability of a given concrete. The surface zone will be seriously weakened by increased permeability due to poor curing.
The importance of adequate curing is very evident in its effect on the permeability of the “skin” (surface) of the concrete. In the United States and other countries, contractors tend to either shortcut curing requirements in the field or ignore them almost completely. One survey conducted in the United States in 1979 estimated that 24 % of concrete used in nonresidential construction was not cured at all, and only 26 % was cured in accordance with project specifications (Senbetta and Malchow 1987). It is doubtful that the situation has improved very much since then. The concrete industry must do a better job of educating contractors, engineers, superintendents, and quality control personnel on the importance of good curing practices in the field.
This is especially true for high-performance concrete since it has been found to be even more sensitive to curing conditions than ordinary concrete, particularly at early ages. It has been suggested that one way to highlight the importance of curing is to make it a separately billed item in the schedule of prices for the project (Cather 1994). Contract specifications usually contain curing requirements; however, they are rarely adhered to in the field (Neville 1996). Similar to the batching and mixing operation for concrete, curing needs to be closely supervised and controlled.
As a construction project progresses, it is extremely difficult to prove whether proper curing has been applied. Although specifications may be adequate and complete, one of the biggest obstacles to ensuring proper curing in the field is the lack of standard methods to verify curing adequacy. Various penetrability methods have been proposed (Kropp and Hilsdorf 1995), but none has yet to be standardized for use. Without approved testing methods, it will continue to be difficult to verify desired levels of curing in the field. The curing of high-performance concrete has been identified as one of the critical areas in which more information and research are needed in order to realize the full potential of this class of concrete (Carino and Clifton 1990).
Current national curing specifications in the United States do not include specific requirements for high-performance concrete even though its use is becoming more widespread. Existing curing criteria are based on information from ordinary concrete, and may not be appropriate for the high-performance concrete mixtures being used today. Current standards are also deficient in that they do not address proper curing for durability. Historically, curing requirements have been based primarily on obtaining adequate strength. Some of the most recent research on high- 3 performance concrete has focused on how curing affects the surface layer and thus, the durability of the concrete. Finally, current curing requirements in the United States do not take into account the actual rate of hydration or strength development, both of which may be affected by in-place temperature and whether chemical and mineral admixtures are used.
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