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Alkali–aggregate reactions (AAR) occur between the alkali hydroxides in the pore solution of concrete and certain minerals found in some aggregates. Two types of AAR reaction are currently recognized depending on the nature of the reactive mineral; alkali–silica reaction (ASR) involves various types of reactive silica (SiO2) minerals and alkali-carbonate reaction (ACR) involves certain types of dolomitic rocks (CaMg(CO3)2). Both types of reaction can result in expansion and cracking of concrete elements, leading to a reduction in the service life of concrete structures.
This report describes approaches for identifying deleteriously reactive aggregates 1 and selecting appropriate preventive measures to minimize the risk of expansion when such aggregates are used in concrete. Preventive measures include avoiding the reactive aggregate, limiting the alkali content of the concrete, using supplementary cementing materials, using lithium-based admixtures, or a combination of these strategies.
The flow chart in Figure 1 shows the general sequence of testing and decisions that have to be made when evaluating a source of aggregate for potential AAR. Prior satisfactory field performance of the aggregate in concrete is considered in some cases to be sufficient for its acceptance in new concrete. However, the use of field performance in the absence of testing may not be sufficient to completely safeguard against damage due to AAR in new construction because of the difficulties in assuring that the materials used in existing structures built ten to twenty years ago (time frame needed to ensure that AAR has not occurred) are similar to those being proposed for use today. In most cases, it will be necessary to conduct laboratory tests to determine whether or not the aggregate is deleteriously reactive. There are many test methods available for evaluating aggregate reactivity, but only two expansion tests, together with petrographic examination are recommended in this report. If the aggregate is deemed to be non–deleteriously–reactive, it can be accepted for use in concrete with no further consideration of mitigation (assuming that the physical properties of the aggregate render it suitable for use). If the aggregate is found to be deleteriously reactive, it must then be determined whether the reaction is of the alkali–carbonate or alkali–silica type. There are no proven measures for effectively preventing damaging expansion with alkali–carbonate reactive rocks and such materials must be avoided by selective quarrying or beneficiation. If the aggregate is alkali–silica reactive, the aggregate may be either rejected for use or accepted with an appropriate preventive measure.
There are a number of options for minimizing the risk of expansion with alkali–silica reactive rocks. This report allows for preventive measures to be evaluated on the basis of performance testing or to be selected prescriptively from a list of options based on previous experience.
In the approach outlined by this report, the level of testing and the test limits vary depending on the level of risk that is acceptable to the owner. For example, in regions where occurrences of AAR are rare or where the aggregate sources in use have a long history of good field performance; it may be reasonable to continue to rely on the previous field history without subjecting the aggregates to laboratory tests. However, in regions where AAR problems are not infrequent and where the reactivity of aggregates are known to vary from source to source, it may be necessary to implement a rigorous testing regime to establish aggregate reactivity and evaluate preventive measures. In the report described here the level of prevention required is a function of the reactivity of the aggregate, the nature of the exposure conditions, and the availability of alkali in the system.
Figure 1. Summary of the Various Stages in the Process of Evaluation
1 The term deleteriously reactive is used to define aggregates that undergo chemical reactions in concrete which subsequently result in damage to the concrete. Some aggregates with minor amounts of reactive constituents may exhibit some small amount of reaction without producing any damage to the concrete; these are non-deleteriously reactive aggregates.
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