U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
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Publication Number: FHWA-HRT-06-117
Date: December 2006
In 2004, the value of concrete production for highway construction and maintenance has been estimated to be more than 9 billion dollars. Nevertheless, 34 percent of the United States’ major roads are still in poor or mediocre condition.(1) Although in cold climate regions, the most persistent problem is the concrete deterioration caused by freezing and thawing;(2) it is an issue not completely resolved.
Since the late 1930s, air-entraining cements and admixtures have been used to impart freeze-thaw resistance to concrete. Because air detracts from some other concrete properties (particularly strength), the goal of air entrainment is to provide sufficient air in the concrete to ensure freeze-thaw resistance, but no more than is required for that purpose. In non-freeze-thaw exposures some air is often used for economy or improved workability.
Research from the 1940s through 1960s by Gonnerman,(3) Powers,(4) Klieger,(5) Cordon and Merrill,(6) and others sought to establish air requirements for frost-resistant concrete. These initial research efforts concluded that at least 3 percent of air, by volume, in the fresh concrete was necessary to protect concrete from freezing and thawing (see figure 1, for example). Further research indicated that, since the air voids protect the paste, the required air content depended on the paste content, which is largely a function of aggregate size and gradation and of minimum cement content requirements. Therefore, 3 percent air per unit of concrete volume may be sufficient for a lean mix but not for a richer mix.
Figure 1. Graph. Freeze-thaw durability factor for different levels of total air contents.(6)
The air bubbles can be classified as entrapped or entrained. Entrapped air voids are relatively large, typically 1 to 10 millimeters (mm) or more in size. Air-entrained concrete contains much smaller voids that range from 0.01 mm to 1 mm in diameter(7) and that are stabilized in fresh cement paste through the action of the air-entraining admixture (AEA) (see chapter 2). The amount of entrapped air in concrete is also a function of aggregate size and gradation (especially fine aggregate gradation). Entrapped air usually comprises 1 to 2 percent of the concrete volume, but in some cases can comprise as much as 3 or 4 percent.(5) When air-entraining admixture or air-entraining cement is used to produce air-entrained concrete, the air void structure is usually smaller, with fewer larger air voids.
The American Concrete Institute (ACI) 211.1 Standard Practice for Selecting Proportions for Concrete(8) guidelines for air content reflect the factors discussed above, and over time certain recommendations (ACI 201.2R(9)), specifications (American Society for Testing and Materials (ASTM) C 94(10) and ACI 301(11)), and codes (ACI 318(12)) regarding air content and other air void system parameters have evolved. Most State departments of transportation (DOTs) where concrete is exposed to significant freezing and thawing specify target air contents of 5 to 7 percent in the fresh concrete for aggregate maximum sizes of 50 mm down to 12.5 mm (often with tolerances of ±2 percent).(13) Usually this specification is based on results of fresh concrete testing by either ASTM C231(14) and American Association of State Highway and Transportation Officials (AASHTO) T 152(15) (pressure method) or ASTM C173(16) and AASHTO T196(17) (volumetric method). Unfortunately, these methods provide only a measurement of the total air volume, not the size or distribution of the air voids. Furthermore, these tests are often performed before the completion of construction operations (such as placing, consolidating, and finishing) that can alter the air void system. Therefore, the actual in-place hardened air content and other air void system parameters may differ significantly from those in the fresh concrete.
Another commonly accepted hardened concrete parameter for freeze-thaw resistance is an air-void spacing factor (ASTM C 457(18)) of 0.200 mm or less (spacing factor is defined and discussed in chapter 2). A number of early research studies reported that a spacing factor of approximately 0.250 mm or less signified adequate freeze-thaw resistance. Although Powers first advocated void spacing as a means of specifying air entrained concrete in the 1950s,(19) few States have actually used a spacing factor specification. Until the recent advent of the Air Void Analyzer™ (AVA), the only means of determining the spacing factor was the labor-intensive ASTM C457,(18) which involves microscopical examination of a polished specimen of hardened concrete. The AVA method estimates the spacing factor from measurements on fresh concrete, which makes it a faster and more practical quality control test than ASTM C457.(18) Recently, some States have begun to specify spacing factor based on the AVA measurement. However, since the AVA and ASTM C457(18) methods are different, it is not clear whether a limit of 0.200 mm for the spacing factor determined by the AVA is appropriate for assuring freeze-thaw durability.
It is also very important to highlight that the current recommendations were established based mostly on data of concretes containing neutralized Vinsol® resin as an air-entraining admixture (AEA). On the other hand, the scarcity of Vinsol resin admixture is responsible for the increasing use of synthetic admixtures. Nevertheless, an extensive comparison of the freeze-thaw performances of Vinsol and synthetic air-entrained concretes with marginal air content has not yet been performed.
In 1994, the Strategic Highway Research Program (SHRP) published results from a research study on freezing and thawing of concrete, in which a number of concretes containing 2.5 to 3 percent total air performed adequately in freeze-thaw tests. These results seemed surprising in light of common minimum specification limits of 4 to 6 percent. The work reported here began as a followup study to the SHRP work, an attempt to corroborate the earlier results.
This report describes a laboratory investigation of the behavior of concrete with “marginal” air void systems, in which the air content and other air void system parameters do not meet commonly accepted thresholds for freeze-thaw durability.
The effect of deicing agents on concrete durability will not be covered in this document. Only evaluations using freezing and thawing in plain water were used in this study (AASHTO T 161(20) and ASTM C 666, Procedure A,(21) using freezing in water and thawing in water).
The objectives of this study are as follows:
The report contains five chapters. Chapter 1, the introduction, defines the objectives and scope of the study. Chapter 2 provides background information on freeze-thaw behavior of concrete, air entrainment, and freeze-thaw testing. Chapters 3 and 4 describe laboratory experiments performed as part of this research and discuss the experimental results. Chapter 5 provides a summary of findings, conclusions, and future research needs.
There are four appendices to the report. Appendix A contains the properties of the materials used in the project. Appendix B contains the complete test data for the experiments described in chapters 3 and 4 of the report. Appendix C presents the analyses of variance of the test results. Appendix D describes the equipment and method used to obtain time-domain data from ASTM C 215(22) (impact method) testing of freeze-thaw test specimens.