Curing Requirements for High-Early-Strength Concrete
I. Research Problem Statement
It is plausible that the limiting conditions under which deliberate curing activities are required are quite different for high-early-strength concrete compared to more conventional concrete. Conventional portland cement concrete typically requires several days to reach sufficient strength that curing can be stopped. If climactic conditions are unfavorable, enough of the original mixing water can evaporate that hydration either slows severely or ceases during this time. For this reason, deliberate curing activity is required to preserve the necessary fraction of the water, so that properties can develop normally. High-early-strength concrete can be proportioned that reaches the degree of hydration required for adequate physical property development within a few hours. This includes slab replacement during the rehabilitation of concrete pavements. During this shortened interval when evaporation of water can be detrimental, it is plausible that the amount of water lost would not reach critical levels. For example, based on the ASTM C 309 specification for curing compounds, it is commonly accepted that water losses of up to 0.55 kg/m2 after 72 hours are acceptable for preserving adequate water for hydration. Water evaporation from hardened concrete depends on climactic conditions, on water-cement ratio, and on maturity of the concrete, but rates of about 0.03 kg/m2/hr are plausible during the first few days under relatively severe drying conditions for w/cm around 0.4. Over a 72-hour period at this evaporation rate, 2.2 kg/m2 would be lost, which is severe enough to impact hydration rates. Over a 6 to 10 hour period used in the case of slab replacement, this drying rate would amount to 0.30 kg/m2, which probably would not be detrimental. The unknowns are the actual evaporation rates from high-early-strength concrete and the depth over which the evaporation occurs. Even a modest evaporation rate can be detrimental if all of the water loss is concentrated in a narrow near-surface zone, while a relatively larger amount of water loss can be tolerated if it is distributed over a wider zone.
II. Research Objective
The objective of the research would be to determine whether the curing requirements normally applied to conventional concrete are effective and required for high-early-strength concrete, such as commonly used in fast track paving and slab replacement. Also, if curing for high-early-strength concrete is not generally required, determine the criteria such as weather conditions, geometry and mixture proportions under which curing requirements must be applied to HES concrete.
III. Estimate of Problem Funding and Research Period
IV. Urgency, Payoff Potential and Implementation
If not actually needed, eliminating curing requirements would eliminate the cost of executing the work, and the cost of inspection and verification. This would also eliminate one potential source of contractor-owner disputes.