Aggregates are the major constituents of concrete and typically occupy between 60% and 80% of the concrete volume. Properties of both fresh and hardened concrete are influenced by the quality of aggregates and yet their role is often overlooked. This chapter provides a brief review of the general requirements of concrete aggregates; the main focus concerns how aggregates can affect concrete durability. Although most aggregates are strong, hard, volumetrically stable, free of harmful impurities and chemically inert, some aggregates can behave in an undesirable manner and lead to premature failure of the concrete. Frost-susceptible aggregates can result in cracking (commonly called durability cracking, D- cracking or D-line cracking) and deterioration of concrete exposed to cycles of freezing and thawing in service; this phenomenon is discussed in the third section of the chapter (see also Chapter 9). The fourth section deals with harmful constituents of aggregates including organic matter, clay/silt content, coal/lignite materials, soft particles and salts. Sections 7.5 to 7.8 cover alkali-aggregate reactions (AAR) due to reaction between the alkali hydroxides from cement (and other sources) with either unstable siliceous (alkali-silica reaction, ASR) or carbonate (alkali-carbonate reaction, ACR) constituents of certain aggregates. AAR can result in expansion and cracking of concrete. In addition to a description of the reaction mechanisms, information is provided on testing to identify potentially reactive aggregates, measures for preventing deleterious expansion when reactive aggregates are used in new concrete and techniques for mitigating or suppressing expansion in existing structures affected by AAR. The final section discusses future trends in aggregate production including the use of manufactured fines and the use of recycled concrete and other waste materials as aggregates.
