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Federal Highway Administration Research and Technology
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|This report is an archived publication and may contain dated technical, contact, and link information|
|Publication Number: FHWA-HRT-05-038 Date: August 2006|
Publication Number: FHWA-HRT-05-038
Date: August 2006
No major inconsistencies in the theoretical aspects of curing portland cement concrete were found as a result of this work, but the results did illuminate several important details in curing practice that need particular attention in the construction of portland cement concrete pavements that may not be adequately emphasized in current standard guidance These are described below.
Pavement engineers need to be more aware of the conditions that favor development of plastic shrinkage cracking and to be cognizant of when they become critical during a particular construction project. Most current guidance focuses on evaporation conditions as the only relevant variable. Bleeding behavior and time of setting of the concrete are also important variables in determining how and when evaporative water loss affects the development of early cracking. It is well known that the balance between bleeding and evaporation is the critical variable in the development of early-age cracking, but most guidance seems to assume the bleeding rate is approximately constant, so that evaporation condition is considered the only important operational condition. Slip form paving mixtures typically have low bleeding rates, so that conventional guidance on evaporation rates may not be correct in many instances.
Bleeding behavior and time of setting should be determined during mixture verification work. Also as part of preconstruction planning, engineers need to develop a forecast of probable drying conditions based on plausible conditions for a job site, then using direct measurement of evaporation rate, or calculations from on-site conditions, to make adjustments to the evaluation during construction. It is relatively common practice to use intuition rather than measurement to estimate drying conditions-hot, dry, windy conditions being associated with severe drying. Another extremely important variable is concrete temperature, which is often not considered at all. Given the relatively low bleeding rates of slip form pavement mixtures, seemingly modest drying conditions can be critical.
Guidance on the timing of the start of final curing needs to be revised. Most guidance directs that curing be applied after final finishing and disappearance of sheen. Applying this guidance to modern paving mixtures being used with slip form paving technology would often result in application of final curing within a few minutes of placing. This could result either in damage to the concrete surface or in poor curing. The optimal time for application of final curing is after time of initial setting and then when the sheen has disappeared.
Delaying application of final curing until after time of setting may require considerable attention to detail on managing evaporative losses in the time interval between placing and time of setting. Evaporation reducers show considerable promise for being effective in dealing with high evaporation rates from fresh concrete during this initial curing period. However, a serious impediment to their widespread acceptance is the absence of standard test methods and specifications with which users can evaluate materials and to give the user an accurate picture of the limits on performance that can be expected in field application.
Reducing concrete placing temperature below the maximum allowed by most guidance would have a major effect on reducing rate of evaporative water loss. However, it is unlikely that contractors will voluntarily do this for economical reasons. Revised prescriptive specifications on maximum placing temperature would help control incidents of surface cracking. Prescriptive specifications are declining in popularity in favor of performance specifications. A performance specification could plausibly limit development of surface cracking, however development of such a specification might be difficult. Accelerating the time of setting during particularly strong evaporative conditions should, in theory, be another effective way to reduce evaporative water losses and surface cracking, but the practical side of this has not been investigated.
Testing of curing compounds continues to be a contentious process, due principally to the large between-laboratory variation in the test method. As a partial result of this lack of precision, many agency specifications for water-retention may be more restrictive than theoretically needed. In this work, it was determined that time of application of the curing compound to the test specimen is probably at least one major source of between-laboratory error, but considerable additional work needs to be done on this method to make it suitable for acceptance testing.
Measuring application rates of white pigmented curing compound lends itself to optical methods. Visual verification of white pigmented curing compound application is probably a legitimate quality assurance technique for estimating uniformity. The human eye seems to be sensitive to relatively minor changes in shades of gray that result when insufficient application of curing compound occurs. But, to verify actual applications rates for specification compliance purposes, the observer needs to develop a visual concept as to what degree of whiteness constitutes an adequate application for a given curing compound. Using a portable paint reflectometer was shown to be a practical way to improve accuracy for curing compound application estimates.
The rebound hammer was found to be a simple and useful device for estimating effectiveness of curing. This instrument is often criticized as being overly sensitive to surface conditions, which makes it particularly suitable for evaluating curing.
Another research and development need for concrete paving technology is the development of a curing compound that can be applied to concrete relatively soon after placing concrete. The material would act as an evaporation reducer before time of setting, and then function as an effect curing membrane during the final curing period.
The maturity method has been shown to be effective in estimating early-age strength. Lack of experience by users appears to be the major obstacle to its more widespread use. Use of the maturity method should be pursued as an alternative to the common prescriptive curing time requirements. In many cases, this would probably shorten curing times. The method also gives a rational way to deal with the effects of cold weather on strength development.
Guidance developed for curing pavements as a result of this study is contained in Volume I entitled Guide for Curing of Portland Cement Concrete Pavements (FHWA RD 02-099).(5)