Curing Time Estimator for Portland and Portland/Fly Ash Concretes


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Title: Curing Time Estimator for Portland and Portland/Fly Ash Concretes

Author(s): L. J. Parrott

Publication: Special Publication

Volume: 192


Appears on pages(s): 35-52

Keywords: cover; curing; exposure; fly ash; porosity

Date: 4/1/2000

The development of a curing time estimator is described: it is based upon an existing model of the microstructure and porosity gradients in the cover concrete that correlates well with relevant hydration and pore structure measurements. The same model also yields capillary porosities that correlate with measurements of compressive strength and water absorption rate. The objective is to provide a single method to estimate curing times for CEM I and CEM II (portland and portland/fly ash) concretes in a wide range of climatic conditions and achieve a consistent, well-defined measure of cover concrete performance. New hydration and pore structure measurements are briefly reviewed in relation to the existing model of microstructure and porosity in cover concrete. Recent developments regarding European standards for curing and concrete durability are considered. A criterion of capillary porosity in the matrix of cover concrete is used to unify the durability-related effects of curing period, cement type, water/binder and climatic conditions. The initial input to the estimator is the cement type to be employed. The nest input is a maximum water/binder, as necessary to ensure durability under the expected exposure conditions; this automatically sets a target capillary porosity in the cover concrete, based upon recent curing period recommendations from European standards committees. Subsequent inputs define the climatic conditions in terms of exposure capillary porosities in the cover concrete for a wide range of curing periods so that a period can be chosen without exceeding the target porosity. Capillary porosities for reduced water/binders, 95and 90% of the input value, are also tabulated to illustrate the reductions in curing period that are possible with these higher concrete qualities. Examples are given to illustrate the effect of each of the eight inputs; water/binder, exposure relative humidity and cement type are the most influential. It is evident that in many cases control of cover concrete performance via curing options is limited relative to the control offered via small changes in the concrete mix proportions of alternative cements.