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3Project Background Extensive laboratory testing and long-term field per- formance have demonstrated that concrete that is properly air-entrained can better resist the action of freezing and thawing. For more than 50 years, neutralized Vinsol resin has been used effectively for air entrainment. More recently, other air-entraining admixtures have been introduced and their use has increased primarily because of the higher cost and limited supply of Vinsol resin. AASHTO T 157, Air- Entraining Admixtures for Concrete, sets limits on the effects that any given air-entraining admixture under test may exert on certain properties of the fresh and hardened concrete mixture in comparison with a similar concrete mixture con- taining a standard reference air-entraining admixture such as neutralized Vinsol resin. However, many concrete mix- tures incorporating these newer admixtures meeting AASHTO T 157 limits have exhibited unacceptable proper- ties when used in bridge decks, pavements, and other high- way structures. There has been considerable debate recently about the effects of these admixtures on concrete properties and durability. Although a great deal of research has been performed to address various aspects of concrete resistance to freezing and thawing, there are no clear conclusions concerning the effect of newer air-entraining admixtures on concrete properties and durability. Further research was needed to address the issues associated with the use of these air-entraining admix- tures in highway concrete and to develop recommendations to help improve specifications for air-entraining admixtures. Objective and Scope of the Research The objective of this research was to develop procedures for evaluating and qualifying air-entraining admixtures for use in hydraulic cement concrete for highway applications. To accomplish this objective, the following tasks were performed: 1. Information relative to the use of air-entraining admix- tures in hydraulic cement concrete used in highway appli- cations was collected and reviewed. This information was obtained from domestic and foreign literature, contacts with public and private agencies and industry organiza- tions, and other sources. A summary of current use, field performance, test methods, test data, and other items pertaining to the use of air-entraining admixtures by state and other highway agencies was compiled. 2. Test procedures currently used in the United States and other countries for evaluating the effectiveness of air- entraining admixtures were identified based on the infor- mation gathered in Task 1. 3. A detailed experimental research plan, which encompasses laboratory tests, was developed for evaluating the relative importance of the various factors affecting air entrainment identified in Task 1, modifying the test procedures proposed in Task 2, and validating the modified procedures. 4. The plan developed in Task 3 was executed. The plan included testing of fresh and hardened concrete proper- ties. Also, a plan for putting the results of this research into practice was suggested. 5. A set of test procedures for evaluating air-entraining admixtures was developed based on the results of the entire research effort. Protocols for these test procedures were prepared in an AASHTO format. Research Approach The research performed in this project included review- ing the current procedures and a survey of field perform- ance of air-entraining admixtures (summarized in Chap- ter 2 and included in Appendix A), conducting laboratory tests to investigate factors affecting air-entrainment in concrete, and developing a procedure for evaluating air-entraining admixtures. C H A P T E R 1 Introduction
Factors Affecting Air-Entrainment A statistically designed experimental program was devel- oped to identify significant variables that influence concrete air-entrainment. Air-entraining admixture type, cement alkali, aggregate shape, mixing time, water-cement ratio, and mixing temperature were investigated. These factors were selected based on the literature review and survey of field performance of air-entraining admixtures. Forty-one com- mercially available air-entraining admixtures were analyzed and tested. Their physical properties and their compositions were identified and they were then subjected to preliminary (foam drainage) tests. Out of the 41, 6 admixtures were selected for further laboratory testing. The six chosen admix- tures had distinctive compositions with significantly different foam drainage data and were expected to exhibit significantly diverse performance when used in concrete. Statistical Analysis of Selected Factors An analysis of variance and stepwise least-squares linear regression analysis were used to determine whether differ- ences between fixed factors (experiment variables) and two- way interactions of fixed factors significantly affect the spacing factor. The spacing factor was chosen as an evaluation parameter because it is the most important air-void parame- ter affecting the freeze-thaw resistance of concrete. The effects of a change in significant variables on the air-void spacing factor were determined; the factors that cause increase in spacing factors were considered in developing the evaluation procedures. The results of the stepwise least-squares linear regression were used to identify statistically significant main factors and two-way interactions affecting strength. Factors that lead to strength reduction were also considered in the conditions of the procedures for evaluating air-entraining admixtures. Development and Validation of New Procedures The procedures currently available for evaluating air- entraining admixtures were reviewed and modified. Modified procedures that involve tests under simulated field conditions for comparing properties of air-entrained concrete with those of non-air-entrained concrete were proposed. Acceptance cri- teria were proposed for major highway concrete applications (pavement and structures); the admixture can be evaluated for use in either or both applications. The validation of the proposed procedures revealed that admixtures are sensitive to testing conditions. An admixture that satisfies the strength requirements when tested under laboratory conditions may not meet the same requirement when tested under simulated field conditions. A plan was developed for validation of the proposed proce- dures through a field performance study. This plan is discussed in Chapter 4 of this report. 4
