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In this paper, we have applied the SLABS model to investigate the influence of atmospheric conditions on the equivalent ages of concrete bridge decks at 24 h after the induction period with the following important findings:
As long as there is sufficient water available for hydration, the maximum equivalent ages coincide with the maximum concrete temperatures. Self-desiccation, especially deep within the slab, must be considered when determining concrete maturity of high performance mixes containing pozzolans under extreme environmental conditions. Note that self-desiccation is also a function of the water binder (w/b) ratio.
As expected, equivalent ages after 24 h of hydration averaged over the entire slab thickness are maximum under conditions of high air temperatures and relative humidities and low wind speeds.
Over the ranges of variables tested, equivalent ages can vary from less than 20 h to more than 50 h after 24 h of hydration.
The variations of equivalent ages with depth can approach 15 h with the maximum values generally occurring near the bottom of the slab where the maximum concrete temperatures developed.
The results from this investigation show clearly that many factors are involved in controlling NYSDOT Class HP concrete temperatures and maturity. At early ages, small differences in equivalent ages can represent large differences in concrete strength. Therefore, not only must the field engineer decide the time of placement and the curing regimen for a concrete structure, the engineer must also pay close attention to the atmospheric conditions that are expected to ensure that the structure attains the proper maturity at the time of critical construction operations.
Because other concrete mixtures may have different sensitivities to environmental conditions, the results presented here should be viewed as the first attempt to examine such sensitivities, but specific to Class HP concrete. Moreover, while a wide variety of conditions were simulated for this study, they are a subset of what can occur and therefore, care must be taken when applying the results presented here to other situations.
The engineer must consider the effect of depth variations of maturity in a concrete structure. Such variations become important when attempting to determine the overall maturity of a concrete element by a single point measurement. While some portions of the concrete may have reached the specified strength needed for continuing construction or opening a slab to traffic, other portions may have not reached the proper strength. Recognizing this depth variation may be necessary to maintain construction site and public safety. Because most heat transfer and the lowest temperatures occur near the top surface, sampling with a maturity meter near the top surface generally will indicate the lowest maturity attained by the slab, information that will provide a lower limit to the strength development.
The SLABS model indicates that self-desiccation of Class HP concrete may occur under some extreme conditions especially deep within the slab at early ages. Accounting for this self-desiccation when computing maturity indices will lower their value as the results show. Moist curing may provide water to the uppermost 50 mm of the slab, but this water probably does not influence hydration below this level (e.g., 2, 22). Current research by the author with an X-ray absorption system at the National Institute of Standards and Technology seeks to determine more rigorously this depth of influence as a function of w/b, curing regimen, and materials. There is a need to determine under what conditions and for what mixture proportions such desiccation may occur so that its effect on concrete maturity can be taken into account.
Other factors such as time of placement, initial concrete temperature, and spray water volume and temperature will also influence the concrete’s temperature and maturity (9, 23). These factors, then, could be used to offset any problems that could arise due to adverse atmospheric conditions. For example, the initial concrete temperature of a batch of concrete could be lowered to counter the effects of high ambient air temperatures or low wind speeds on concrete temperatures. A simplified field model, such as SLABS, that accounts for all of these factors in predicting concrete temperatures could be an important tool for determining the expected concrete maturity and temperatures on a case by case basis. Such a tool could aid in developing the best quality concrete and maintaining the safety of the construction site, perhaps preventing future disasters such as those at the Skyline Plaza Apartments in Fairfax County, VA and the Willows Island cooling tower.