BACKGROUND. Laboratory testing and long-term field experience have shown that highway concrete must be properly air-entrained if it is to resist the action of freezing and thawing, particularly in wet climates. Experience has also shown that the effectiveness of air entrainment in providing resistance to freezing and thawing depends on the characteristics of the air void system. These characteristics are influenced by the materials used in producing concrete (e.g., cement type and composition, supplementary cementitious materials, air-entraining and other admixtures, and aggregate size); the practices for proportioning, mixing, and placing concrete; and field conditions. However in some situations, air entrainment has shown adverse effects on other concrete properties (e.g., strength).

Laboratory methods are currently available for characterizing the air void system in hardened concrete and for evaluating the freeze-thaw resistance of concrete. However, the parameters associated with this characterization and the results of these laboratory tests do not always reflect the observed field performance nor do they consider the possible effects on other concrete properties. There was a need to identify the characteristics of the air void system that relate to field performance and develop improved test methods for evaluating the freeze-thaw resistance of highway concrete. Ideally, these methods would evaluate the concrete mixture immediately before placement to allow adjustment of mixture if necessary. This information will help highway agencies prepare specifications for concrete procurement that will provide the air-void characteristics and freeze-thaw resistance needed for enhanced durability and thus ensure longevity of highway structures and pavements.

OBJECTIVES. The objectives of this research were to (1) identify the characteristics of the entrained air void system required for freeze-thaw durability of highway concrete, (2) identify/develop new or modified test methods for measuring these characteristics, and (3) identify/develop new or modified test methods for evaluating freeze-thaw durability.

STATUS: Research is complete. The final report has been published as NCHRP Research Report 961, Entrained Air Void Systems for Durable Highway Concrete, available at  http://www.trb.org/Main/Blurbs/181668.aspx.