Durability refers to the ability of concrete to resist deterioration from the environment or service in which it is placed. The durability of aggregates can be conveniently divided into physical and chemical causes. The physical durability problems include aggregates that are susceptible to freezing and thawing or wetting and drying, as well as physical wear. Chemical durability problems are concerned with various forms of cement-aggregate reactions.

Physical Durability:

Aggregates are said to be unsound if volume changes that accompany environmental changes lead to the deterioration of concrete. Repeated freezing and thawing or wetting and drying cycles can cause volume changes. Rocks that undergo sizable volume change on wetting and drying are very rare; therefore, soundness is primarily a question of freeze-thaw resistance.

Freeze-thaw resistance of an aggregate depends on whether high internal stresses develop when the water inside the aggregate freezes and causes a volume increase. This stress is a function of the porosity of the aggregate, its permeability, the degree of saturation, and size.

Aggregates play an important role in determining the resistance of concrete to surface abrasion and wear. A good aggregate will be hard, dense, and strong, and free of soft, porous, or friable particles. The abrasion resistance of aggregates can be tested by the Los Angeles test (ASTM C 131).

Chemical Resistance:

Most chemical durability problems result from a reaction between alkalis in the cement and reactive silica contained in the aggregates, called alkali-silica reaction (ASR). Expansive alkali-carbonate reactivity is suspect only in extremely fine grained dolomitic limestones with large amounts of calcite, clay, silt, or dolomite chombs. Table (1) shows some potentially harmful reactive minerals, rocks, and synthetic materials that are susceptible to ASR and alkali-carbonate reactivity.

The factors that control the ASR expansion include, first, the nature of reactive silica, which was discussed above. Second, the amount of reactive silica causes maximum expansion when the content in aggregate is five percent, with decreasing expansion at higher percentages. Third, the particle size of reactive material shows the greatest expansion with intermediate-sized particles. Fourth, when the alkali content is below 0.6 percent deleterious expansions usually do not occur. Lastly, sufficient moisture must be available within the concrete for the ASR to proceed.

Table (1), Reactive Minerals, Rocks, and Synthetic Materials, PCA

Alkali-silica reactive substances		Alkali-carbonates  reactive substances
Andesites  Argillites  Certain siliceous limestones   and dolomites  Chalcedonic cherts  Chalcedony  Cristobalite  Dacites  Glassy or crptocrystalline  volcanics  Granite gneiss  Graywackes  Metagraywackes  Opal	Opaline shales  Phyllites   Quartzites  Quartzoses  Cherts  Rhyolites  Schists  Siliceous shales  Strained quartz and certain   other forms of quartz  Synthetic and natural silicious   glass  Tridymite	Calcitic dolomites  Dolomitic limestones  Fine-grained dolomites