Curing Practices for Concrete Pavements (2023)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

1   Curing Practices for Concrete Pavements Curing is a major contributor to the short- and long-term performance of concrete pavements. Effective curing controls moisture loss and maintains adequate temperature of concrete pavements to allow for the continuation of cement hydration and formation of hydrated gel necessary to develop desired strength and durability. Moreover, adequate curing limits shrinkage and shrinkage cracking at early stages of concrete development and contributes to increased fatigue over the life of pavement under traffic. Despite recent advances in pavement design and construction quality control, challenges related to proper curing still remain. Understanding the need and process for curing based on state DOTs’ practices and related research reports in normal and adverse weather, as well as knowing the consequences for poor or inadequate curing, will assist DOT engineers and consultants to better specify and apply curing and assess its effectiveness. The objective of this synthesis is to document state DOTs’ practices and pertinent lit- erature on curing for concrete pavements. Information in the report was obtained from a literature review of reports and research publications, a survey of state DOTs on curing practices, and selected state DOT case examples of curing applications. The literature review included reports from industry organizations and associations such as ACI and American Concrete Pavement Association (ACPA). and research organizations such as CP Tech Center, as well as DOT specifications, FHWA reports, and other academic research reports. A survey questionnaire was prepared and transmitted electronically to members of AASHTO COMP (Committee on Materials and Pavements) of the 50 state DOTs and the District of Columbia. Thirty-eight DOTs responded to the survey question- naire, representing 76% of state DOTs. Furthermore, projects in three state DOTs—Texas, Florida, and California—were selected as case examples to obtain information about their use of sensors and testing to evaluate curing effectiveness. From the results of the literature review and of the survey responses, a number of key findings have been drawn: 1. DOTs responding to the survey have specifications for and require curing as an essential construction component for concrete pavements. 2. The standard practice among the surveyed states is to initiate curing measures on pave- ment surfaces and sides as soon as the finishing and texturing of the pavement have been completed. 3. The literature suggests that curing should remain effective until cement hydration has progressed to the degree that the desired concrete properties of strength and durability have been reached, or until it is clear that the properties will develop in the absence of additional curing measures. S U M M A R Y

2 Curing Practices for Concrete Pavements 4. Almost all surveyed agencies use AASHTO M148/ASTM C309 to specify curing for con- crete paving. There is general consistency among all state DOTs regarding reflectance requirements but less consistency in specification requirements regarding evaporation limits or the rate of application. 5. According to survey responses, state DOTs in dry, hot, and windy climates undertake initial curing measures during or after placement of the pavement section by applying fog spray to reduce rate of evaporation and risk of plastic shrinkage cracking. Fogging is used when the application of the standard curing compound is delayed to complete surface finish and texturing or withheld due to continuous bleeding action. 6. The responding agencies indicated that they use white-pigmented Type II curing com- pounds for concrete pavement construction across the full range of weather conditions. 7. The literature suggests that installing wind breakers in conjunction with other curing measures would help reduce the risk of rapid surface drying and shrinkage cracking in pavements; however, only one responding state DOT has used wind breakers. 8. The survey showed that 70% of responding state DOTs use insulation blankets in cold weather paving, and 54% of DOTs use them in accelerated paving projects such as slab replacements and lane replacement or widening. 9. The literature suggests that internal curing supplements external curing to achieve the desirable properties of strength and durability. The survey showed that only two state DOTs are currently using internal curing in concrete paving. Gaps in Information on Concrete Pavement Curing and Suggestions for Research Generally, there is a need to develop a more complete understanding of the implications of a specific curing regime on the performance of concrete pavement. Gaps in informa- tion related to curing for concrete pavements were identified based on the literature and responses from the survey of state DOTs. This information is provided for consideration in future research. Among the information gaps are the following: 1. Practices related to identifying timing of curing among state DOTs are likely based on contractors’ experiences, which vary across the country’s climatic regions. There have been some suggestions to use maturity concepts and/or short-term performance data to better define when a curing method should be applied. However, more information on the use of these or similar types of sensor technologies would assist state DOTs to better define the starting time for curing in construction. 2. The literature characterizes as almost non-existent the connection between the ASTM/ AASHTO standard rate of application for the laboratory test specimens and the specified application rate in the field. Presently, the state DOTs have little capability to distinguish between too little (under curing) versus too much (over curing) a pavement. Information on evaluation methods to assess the amount of curing could be better defined if there was a better connection between the test conditions in the laboratory prescribed in ASTM C156/309 and conditions in the field at the time of paving. 3. There is a need for protocols to evaluate new curing regimes that are relevant to actual field conditions. The Menzel/ACI 308 nomograph for estimating the maximum potential rate of evaporation from a water-covered surface is the only recognizable tool for evaluat- ing curing effectiveness. This nomograph was developed in 1954 with obvious shortcom- ings regarding both its applicability to a freshly cured concrete surface and its capability to yield any critical value pertaining to the performance of modern concrete paving. Improving the relevance of this nomograph or developing new and more appropriate techniques for evaluation of curing would better define the methods for managing the

Summary 3   curing process, including application rate, timing, and duration. Agencies today are will- ing to take measures to better monitor and manage curing if the right tools are in place. 4. The merits of any type of curing technology, whether external or internal, need to be better characterized to accurately specify and incorporate them under varying climatic conditions throughout a paving day or various construction seasons. 5. Curing techniques and technologies will continue to evolve, including the increasingly necessary tools to evaluate their effectiveness. 6. Information about specific contributions of any type of curing methodology on early and later concrete properties, as well as long-term performance of pavements, needs improved definition to encourage state DOTs to specify mixtures with ingredients that have the needed characteristics and properties in the finished product. 7. Sensor technology and related numerical tools have been developed sufficiently to allow for accurate monitoring of the curing quality and to identify deficiencies in concrete curing during construction. More work is needed to further verify their effectiveness and to qualify them as reliable tools for verification of quality and adequacy of pavement curing. This synthesis can benefit DOT engineers, consultants, and construction professionals, as well as university researchers and students. It provides state DOT practices and pertinent information on curing benefits, materials, and practices in normal and adverse weather. Furthermore, the synthesis raises awareness of the consequences of poor or inadequate curing on short- and long-term performance of concrete pavements. The references provide more detailed information on various aspects of concrete curing that may be of interest to engineers, practicing professionals, and the research community.