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.
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).
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
Certain siliceous limestones
Glassy or crptocrystalline
Strained quartz and certain
other forms of quartz
Synthetic and natural silicious