Guide for Curing Portland Cement Concrete Pavements, II

CHAPTER 1: INTRODUCTION

Background

"Curing of concrete is defined by the American Concrete Institute (ACI) as maintaining of a satisfactory moisture content and temperature in concrete during its early stages so that desired properties may develop."⁽¹⁾ The need to cure portland cement concrete, including pavements, in order to develop its physical properties sufficiently for strength and durability has been appreciated for many years. Jackson and Kellerman (1939) of the U.S. Bureau of Public Roads wrote, "Thorough and complete curing has always been recognized as one of the most important single factors involved in the construction of concrete pavement."⁽²⁾ Curing has been an item in the ACI Building Code (now ACI 318⁽³⁾) since 1904 (Meeks and Carino, 1999).⁽⁴⁾

There has been a considerable amount of research over approximately the last 70 years on the theoretical aspects of curing portland cement concrete. Early work focused principally on portland cement hydration chemistry and the physics of developing microstructure. Later work incorporated features of pozzolanic reactions and other supplemental cementing materials. Because of this work, there is sufficient understanding of the theoretical aspects of the curing process for concretes of conventional proportions using conventional materials to develop effective practical application rules. Features of the curing process of unusual concretes, such as extremely low water-cementitious concretes, have yet to be fully understood and still require significant research effort. Still, for the bulk of concrete placed, including portland cement concrete pavements, the existing theoretical knowledge is sufficient to support good practice. A substantial body of practice has been developed on curing that covers the critical steps that, if properly followed, result in properly cured concrete.

The major features of curing portland cement concrete pavements are not different from the major features of curing other portland cement concrete structures. The objective of all curing operations is to maintain a condition of moisture availability and temperature sufficient to allow development of physical properties before the structure is subjected to loading, abrasion, and excessive drying.

The problems related to curing portland cement concrete pavement may be due to changes in concrete technology that developed since the curing guidance was written or from some details of paving construction practice that differed from the types of concrete construction around which curing guidance was developed. In either case, the problems are in the details.

The most obvious changes in concrete practice since the development of standard curing practice are connected to the increased reactivity of cementitious materials, derived both from the more reactive chemical composition and from increased fineness. A practice that has changed is the common use of lower water-cementitious materials ratios.

Curing Problems Specific to Portland Cement Concrete Pavements

A major difference between concrete highway pavements and most other concrete structures is the large surface-area-to-volume ratio of the structure. This makes the pavement highly susceptible to environmental effects, such as drying or temperature extremes. Compounding this is the relatively large amount of such concrete that can be placed in a single workday and hence a large amount of surface area that must be managed without delay. For example, a kilometer (km) or more of concrete pavement can typically be placed in one shift using slip-form paving technology. This may represent as much as 10,000 square meters (m²) of concrete surface that is exposed to prevailing climatic conditions. As a matter of economics, this amount of surface area strongly affects choices in curing methods and materials.

Use of liquid membrane-forming curing compounds (henceforth called curing compounds) is the only economical approach for curing large placements of concrete pavement. The principal advantage in using curing compounds to cure concrete is the relatively low labor requirements involved with application and inspection during the curing period. The major disadvantage to using curing compounds is the amount of detail involved in specification-compliance issues and of application procedures required to insure proper performance. One of the major objectives of this work is to develop useful guidance on curing compound selection and usage practices as they apply to pavements.

Another important and somewhat unusual feature of pavement concrete is the relatively low bleeding or absence of bleeding of the fresh concrete, particularly for mixtures suitable for slip-form paving. Because of the large surface-volume ratio, such concrete is particularly susceptible to drying soon after placing if drying conditions are high, potentially leading to plastic shrinkage cracking. This phenomenon makes selection of early curing practices particularly critical.

Another important feature of concrete in pavements is the near-surface zone. This zone of concrete within about 50 millimeters (mm) of the surface is the most susceptible to the effects of poor curing. Uncontrolled cracks in the near-surface zone can be the initiation point for deterioration processes that shorten service life of the concrete. Poorly cured near-surface concrete is also more permeable to water and less abrasion resistant.

Because of these qualities of pavement concrete, details of generally accepted curing practice may be not be appropriate for use on pavements. Problems such as knowing when paving concrete is susceptible to plastic shrinkage cracking, understanding how to effectively control this condition in the field when it potentially exists, recognizing when to apply curing compounds, anticipating the effects of climatic conditions on drying of curing compound, and knowing the effects of less-than-optimal placing temperatures may require different practices than are used in more common structural applications.

For example, during a recent placement of a U.S. Army Corps of Engineers (USACE) airfield pavement, the climatic conditions were relatively hot and windy. The project specifications directed that curing compound be placed as soon as the sheen had disappeared from the surface of the concrete and finishing was complete, which is relatively common guidance. The water-cement ratio of the concrete was relatively low, so bleed water was not abundant. In such concretes, bleed water is sometimes slow to appear on the surface of the concrete, so a no-sheen condition developed within a few minutes after placing was completed. Curing compound was then applied. Bleed water subsequently developed, apparently after the curing compound had dried, and resulted in the delamination of 1 to 2 mm of the surface of the concrete. Guidance exists cautioning about this situation, but the pavement engineer onsite had no quantitative information on which to make judgments on details of bleeding and timing of curing compound application. Following the simplest guidance concerning the disappearance of surface sheen, curing compound was applied too soon.

Scope and Objectives

The scope of this guide is limited to pavements. It does not include curing of bridge decks or other structures associated with pavements. Curing for these types of structures depends on the same basic principles, but some details of execution differ significantly from pavements. References to work on the curing of bridge decks were found while researching this guide, and a summary of this literature is included in the literature review (appendix B).

The objective of this project is to develop practical, quantitative guidelines on curing of portland cement concrete pavements that will give the pavement engineer tools with which to anticipate the critical details of curing practice and to be able to plan for contingency conditions. Specific information includes recommendations on:

• Selection of curing procedures.
• Selection of curing materials.
• Accounting for climatic conditions.
• Duration of curing.
• Sequence of critical events.
• Verification of curing.
• Accounting for concrete materials and concrete mixture proportions.

Project Organization

The major tasks in the execution of the project are as follows.

• Formation of a technical advisory panel (TAP). A TAP was formed, comprised of individuals from State highway departments involved in concrete paving operations and familiar with the problems associated with curing practice. The purpose of the TAP was to advise on technical aspects of the project.
• Information search. Research literature and current guidance were reviewed to determine the existing understanding of technical matters involving curing and the state of current practice, at least as contained in guidance.
• Synthesis of information and draft guide. From the results of the information search, a description of current knowledge and practice was synthesized, and a draft guide for curing portland cement concrete pavements was developed. Deficiencies in the guide were identified.
• Resolution of deficiencies and development of completed guide. As a result of recommendations of the TAP, additional information was gathered as required to complete the guide.
• Additional investigations. Preparation of the draft guide suggested some areas that required laboratory work or other additional investigation to illuminate critical details of practice. Laboratory work was executed to evaluate these critical points of the guide.
• Final draft. A final draft of the guidelines was then developed and is printed as a separate report. ⁽⁵⁾