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S U M M A R Y The use of air entrainment has been an accepted practice in concrete technology for more than 60 years. Air is intentionally entrained in the concrete mixture to reduce the potential for dam- age from freezing and thawing. Until the early 1980s most air-entraining admixtures were based solely on the salts of wood resin (i.e., neutralized Vinsol resin). In recent years, a new generation of air-entraining admixtures was introduced and has been used to entrain air in concrete in many bridges and pavements. Air-entraining admixtures proposed for use in highway structures must conform to the requirements of AASHTO M 154 or ASTM C 260 test methods. The specifica- tions were developed in the late 1970s to assure users that new admixtures would not have significant adverse effects on concrete properties and to verify their effectiveness in reducing freeze-thaw damage. These methods compare the properties of concrete prepared with the tested admixtures in a controlled laboratory environment with concrete prepared with standard Vinsol resin admixture. Field performance of some AASHTO-certified admixtures has been questionable; many highway structures where these admixtures were used have exhibited poor performance or unacceptable properties. In this project, the current procedures available for evaluating air-entraining admixtures were reviewed and modified, and new procedures were developed to evaluate air-entraining admixtures. The main concept of the new procedures is to test the admixture under simulated field conditions and to compare the results with those of non-air-entrained concrete. Acceptance criteria were proposed for the two major highway applications (i.e., pavement and structural concrete) such that an admixture can be qualified for either or both applications. As part of the research program, a statistically designed program was developed to identify significant factors that influence concrete air-entrainment and select those variables that should be included in the test procedures. It was recognized that all concrete materials, production procedures, construction practices, and field conditions influence to a degree the concrete air void system. Based on a survey of field performance of air-entraining admixtures and review of the available literature, several potential key factors were chosen for evaluation in this project. Cement alkali level, air-entraining admixture type, mixing time, aggregate shape, water-cement ratio, and concrete temperature were key factors selected because of their reported effect on the air-void system and strength. Based on preliminary laboratory screening involving infrared analysis and foam drainage tests of 42 air-entraining admixtures, 6 commercially available air- entraining admixtures, each having different chemical compositions and expected to vary in performance, were selected for the evaluation. For each of the selected 6 admixtures, 16 different concrete mixes were prepared (96 mixes total). Fresh concrete properties were determined for each mix. Compressive, flexural strengths were determined at 28 days. In addition, beams prepared from each mix were used to measure the air-void parameters, including the spacing factor, specific surface, and total air content. Evaluating Air-Entraining Admixtures for Highway Concrete 1
2Findings of this study indicated that the type of air-entraining admixtures has a statistically significant effect on spacing factor. In addition, mixing duration, cement alkali level, and coarse aggregate shape were found to have a significant effect on the spacing factor. The spacing fac- tor decreases as the mixing duration increases and increases when a rounded aggregate instead of a crushed aggregate is used. The statistical analysis of the data obtained in the laboratory testing program indicated that compressive strength decreases with increasing temperature, water-cement ratio, and cement alkali, and by using round instead of crushed aggregate. The highest compressive strength (average of six air-entraining admixtures) was obtained for the mixes made with crushed coarse aggregate, having a low water-cement ratio, using low alkali cement, and mixed at stan- dard laboratory temperatures. The proposed evaluation procedures were validated by laboratory tests on mixtures containing five air-entraining admixtures, having different properties and performance records. Although these five admixtures were certified in accordance with AASHTO and ASTM standards, none of them conformed to all of the performance requirements of the proposed four testing protocols, indicating that changes in testing conditions (compared with standard laboratory conditions) better replicate the field performance of air-entrained concrete mixtures.
