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5 Background Fly ashâa byproduct of coal combustionâis widely used as a cementitious and pozzolanic ingredient in hydraulic cement concrete. The use of coal fly ash (CFA) in concrete is increas- ing because it improves some properties of concrete and often results in lower cost of concrete. However, the chemical and physical compositions of CFA influence constructability, per- formance, and durability and may contribute to problems, such as cracking and alkali-silica reactivity (ASR) in concrete pave- ments, bridge decks, and other highway structures. Regulatory requirements have also contributed to changes in CFA proper- ties that may adversely affect concrete performance. In addi- tion, current specifications and test methods do not adequately characterize CFA properties, address the effects of CFA charac- teristics on fresh and hardened concrete properties, or consider the alkali content of the cement. For example, carbon content of CFA is not usually determined directly, but is often assumed to be approximately equal to the loss on ignition (LOI). Such in- adequate characterization may lead to unwarranted restrictions on the use of suitable materials. Although a great deal of research has been performed on the effects of CFA characteristics on concrete properties, the research has not dealt with the appli- cability of current specifications to the fly ashes that currently are produced. In addition, existing test methods for sampling and testing CFA used in concrete do not adequately address the characterization of CFA or the performance aspects of highway concrete. Further research is needed to develop recommenda- tions for improving CFA specifications and test protocols and thus help highway agencies better evaluate and use CFA that will provide acceptable structural performance and durability. NCHRP Project 18-13 was initiated to address this need. Objective The objective of this research was to recommend potential improvements to specifications and test protocols to deter- mine the acceptability of fly ash for use in highway concrete. Research Approach The research objective was accomplished through a num- ber of activities, including the following: ⢠A study of existing specifications and classification methods for CFA to recommend changes that would provide better criteria for selection of CFA for a given level of performance ⢠Identification of new test methods for characterizing the strength activity of CFA ⢠Development of new test methods for characterizing the prop - erties of residual carbon in CFA and approaches for estimat- ing air-entraining admixture or agent (AEA) dosage for CFA ⢠Evaluation of the use of CFA to mitigate alkali-silica reac- tion in concrete and to provide guidance on selection of CFA type and dosage for a specified level of field performance Report Organization The report consists of five chapters. Chapter 1 describes the background and research approach. Chapter 2 provides a sum- mary of the literature review pertaining to CFA specifications and test methods and the findings of a survey of state highway agencies. Chapter 3 describes the experimental investiga- tions performed in the project and the process for developing improved methods for characterizing fly ash. Chapter 4 sum- marizes the results of the investigations. A summary and rec- ommendations for further research are presented in Chapter 5. The report also includes a list of references and three attach- ments. Attachment A lists proposed changes to AASHTO M 295, Standard Specification for Coal Fly Ash and Raw or Calcined Natu- ral Pozzolan for Use in Concrete. Attachment B presents a proposed test method for determining the foam index of a CFA and port- land cement slurry and proposed changes to test methods dealing with (1) the determination of CFA iodine number and (2) the determination of AEA adsorption by CFA. Attachment C [avail- able by searching for NCHRP Report 749 on the TRB website (www.trb.org)] provides further details on the research performed in this project. C H A P T E R 1 Introduction
