1543Early Age Curing

Early age curing is a key step to achieve high performance for HPC. If early age curing is not carried out, even the best HPC that is made with excellent materials and proper compaction will not perform well. While this statement is true for all concretes, it is particularly important for HPC; ordinary concretes are rather more forgiving to the usual inadequate curing. This is because HPC does not usually exhibit much bleeding. Without protection from loss of surface moisture, plastic shrinkage cracks may form on the exposed HPC surface.

A great deal of research on the best curing techniques for HPC has been carried out at the Universite de Sherbrooke. This work is based on the premise that curing of HPC is carried out not only to maximize the amount of cement hydration, but also to minimize the amount of shrinkage. The basic conclusion of these studies is that, especially for HPC, adequate curing must begin as soon as the concrete is placed, and must be continued for as long as possible. In practice, this means that while the concrete is still in the plastic state (before initial set), when plastic shrinkage might occur, it should either be fog misted or covered with a curing membrane, to prevent water from evaporating. Once the concrete has set, when self-desiccation might occur (leading to cracking), the HPC should either be fog misted or water cured, to prevent the formation of menisci in the capillary pores. Finally, the concrete should then be coated with an impervious film, as it dries, to prevent further desiccation.

Unfortunately, the concrete construction industry is not accustomed to curing even ordinary concretes as well as they should. The steps outlined above will inevitably add some construction complexity and some costs to a project. However, there is little point in going through the trouble of designing and placing HPC mixes if their properties, in terms of both strength and durability, are ultimately going to be degraded by inadequate curing.

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