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Ambient atmospheric conditions interact with the exothermic, temperature-dependent hydration reactions of concrete's binder components. This interaction may produce adverse high or low concrete temperatures and high temperature gradients, or insufficient concrete moisture content, perhaps resulting in cracking, reduced strength, or compromised long-term durability (e.g., 1,2). This interaction may also result in serious construction site safety problems. For example, the partial collapses of the Skyline Plaza Apartments in Fairfax County, VA in March 1973 (3) and a cooling tower in Willows Island, WV in 1978 (4), which resulted in the deaths of 14 and 51 workers, respectively, were attributed in part to low air temperatures that delayed concrete strength development.
Accurate predictions of concrete temperatures, temperature gradients, and moisture contents would provide engineers and field personnel with information they may need to make decisions about the proper time to place concrete or the proper time to continue the construction process. Such information would help to maintain safety of the construction site and ensure that high quality concrete is developed. Previous modeling studies of curing concrete exposed to the atmosphere did not use boundary conditions that could accurately account for a wide range of atmospheric conditions (e.g., 5).
In this study, the SLABS (SUNY Local Atmosphere Bridge Simulation) model is used to predict the temperatures and moisture content of curing concrete bridge decks containing New York State Department of Transportation's (NYSDOT) Class HP concrete, over a wide range of atmospheric conditions. The model boundary conditions include explicit formulations for sensible heat convective, evaporative, net radiative, runoff spray water, and conductive heat fluxes (Fig. 1) developed from field investigations of the energy balances of curing concrete bridge decks (6). The temperature predictions are used to compute values of the maturity index, the equivalent age, as a function of atmospheric conditions. Such information provides the first comprehensive view of the effects of a wide range of atmospheric conditions on the equivalent ages of early-age concrete. The equivalent age can be related to concrete strength using a previously established relationship for the concrete being used.
Fig. 1. Schematic cross-section of a bridge deck indicating the energy balance terms used as boundary conditions in the SLABS model. Not to scale. The arrows indicate the typical direction of energy flow when the peak concrete temperatures occur at night. "b" and "f" indicate grid points above a beam and above a form, respectively.
The maturity concept for estimating concrete strength is based on the idea that nominally identical specimens of concrete will acquire equal strength if they acquire equal values of a maturity index. For example, the equivalent age, te, is defined as the length of time at a specified temperature required to produce a maturity equal to the maturity achieved by a curing period at temperatures different from the specified temperature (7) and is given by:
where Ea (J mol−1) is the activation energy, R* (8.314 J K−1 mol−1) is the ideal gas constant, Tc (K) is the average temperature of the concrete during the time interval, Δ t (h), and Ts is the specified or reference temperature (K). To use te as an indicator of strength development, it is assumed that the initial rate of strength development obeys the Arrhenius equation and that there is sufficient water available for hydration. To implement Eq. (1), it is necessary to know the value of Ea for concrete and Ts must be selected. Traditionally, Ts = 293 K has been used (8) and this value is chosen for this study.