For the most part, aggregates serve their intended purposes in concrete, both from an economic and technical perspective, allowing for a cost-effective end product with satisfactory long-term durability. However, there are certain cases where the inherent durability of aggregates is not adequate, or where the aggregate reacts chemically within the concrete.
This chapter summarizes some of the causes of aggregate-related distress or instability in concrete, with particular emphasis on frost resistance and alkali- aggregate reactivity. For the major causes of aggregate-related distress (AAR and frost attack), as well as the less common or damaging causes, substantial information exists to identify potentially non-durable aggregates. In many cases, measures can be taken to improve the durability of concrete containing these problematic aggregates, for instance, by using fly ash to suppress ASR. In other cases, however, adequate measures do not currently exist to eliminate concrete durability problems associated with particular aggregates (e.g., those prone to ACR and some frost-susceptible aggregates).
As natural resources become more depleted and high quality aggregates become less widely available, the need to evaluate new aggregate sources and types efficiently and accurately will become more critical. The increased use of marginal aggregates (ones that were historically avoided), manufactured sands (with low inherent resistance to polishing), and recycled products (e.g., recycled concrete as coarse aggregates) will all bring the potential issues of durability to the forefront. The key to making the most durable concrete from these aggregates is to understand the underlying causes of aggregate-related distress and to apply this knowledge to the design, construction, and maintenance of new concrete structures.