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1 Introduction Problem Statement Laboratory testing and field experience have shown that highway concrete should be properly air entrained to resist the action of freezing and thawing, particularly in wet climates. The resis- tance to freezing and thawing greatly depends on the characteristics of the concrete air-void system, such as the air content and the size and distribution of the air voids. These air-void characteristics are influenced by every step of concrete production, from material selection and mixture proportioning to mixing, placing, and curing. Many have agreed that the spacing of air voids rather than the total amount of air voids plays the most critical role in improving concrete freezeâthaw (F-T) resistance (Wang et al. 2008). Much work has been performed to characterize the concrete air-void system, measuring not only the total air content but also the size and spacing factor of the air voids. However, much of the work has been performed on hardened concrete. ASTM C457, for example, determines the air content of hardened concrete. Although the measurement of air voids in hardened concrete provides valuable information, it does not provide timely information for concrete quality control on the jobsite. Recently, efforts have been made toward assessing the air-void size distribution or spacing factor of fresh concrete using an air-void analyzer (AVA) (AASHTO TP 75, Wang et al. 2008) and a super air meter (SAM) (Ley and Tabb 2014). However, the AVA captures only the air voids in a size range of 0.0003 in. to 0.12 in. (0.007 mm to 3.0 mm) (Magura 1996). Consequently, the test results are not always consistent with field concrete performance. Several gaps in the state of the knowledge still exist, as follows: ⢠Various admixtures are used for air entrainment in concrete. Although the effectiveness of these admixtures has been studied, their interactions with cementitious materials and other admixtures and their effects on the size and distribution of entrained air voids in concrete are not fully understood. ⢠Although widely used, the standard test method (AASHTO T 161) for resistance to rapid F-T conditions does not simulate field concrete saturation and F-T conditions and does not provide results that can be translated to actual concrete performance in the field. ⢠Correlation between the air-void system from laboratory testing and field experience is poor. ⢠Air-void clusters are frequently observed in concrete (Kozikowski et al. 2005) but have not been studied to any great extent. These gaps have made the establishment of specifications for field concrete F-T durability control challenging. While the industry is comfortable with bulk air measurements, technology and understanding of the issues have advanced, and an opportunity exists to provide improve- ments to the state of the practice. C H A P T E R 1
2 Entrained Air-Void Systems for Durable Highway Concrete This study addresses these gaps and, more specifically, investigates innovative test methods for effectively characterizing the air system in fresh concrete and identifying the charac- teristics of the air-void system that are related to the performance of field concrete under F-T conditions. Research Objectives The objectives of this research were to (1) identify the characteristics of the entrained air-void system required for F-T durability of highway concrete, (2) identify and develop new or modified test methods for measuring these characteristics, and (3) identify and develop new or modified test methods for evaluating F-T durability. Report Organization This report is presented in five chapters, including this introductory chapter. Chapter 2 presents a summary of existing literature on F-T durability and the air-void system of concrete mixtures as background information, and discusses the mechanisms of frost action, the effect of air voids on concrete properties, and test methods for measurement of air-void system properties in fresh and hardened states. Chapter 3 presents details of the experimental program undertaken in this project, including the scope and description of the laboratory work and an overview of field samples collected and evaluated. Chapter 4 presents guidelines for evaluating air-void systems and F-T abilities of concrete mixtures and for analyzing data. A summary of the research findings and recommendations for future research are presented in Chapter 5. This report also includes seven appendices: Appendix A provides details on the concrete mixture combinations investigated in the laboratory, mixing procedures, and field investiga- tion work. Appendix B contains the petrography report for the core specimens. Appendix C presents the data obtained from testing the air-void systems of the laboratory concrete mix- tures in the fresh state and comparisons with hardened data obtained from the fixed-focus optical microscope. Appendix D presents the flatbed scanner threshold optimization details and corresponding hardened air-void analysis results. Appendix E presents the data obtained from testing the concrete mixtures for clustering, and Appendix F presents the microcomputer control system for the AASHTO T 161 test cabinet. Finally, Appendix G details the observa- tions from testing hardened concrete samples exposed to various F-T testing scenarios.
