Durability of concrete and cement composites

Test methods for identifying aggregate reactivity

The first step in assessing the potential of an aggregate for AAR expansion and cracking is the performance of a petrographic analysis by a trained petrographer, the methodology being as recommended by a RILEM technical committee (Sims and Nixon, 2003). However, a petrographic analysis may not identify certain reactive materials (some may not be readily identified by optical microscopy), and the results of the analysis should not be used summarily to reject or accept an aggregate for use in concrete. Nevertheless, valuable insight into potential AAR reactivity can be gained through a petrographic analysis of a given aggregate, and information can also be obtained through petrography on physical, chemical, and mineralogical properties of aggregates that may affect other concrete properties. Although there are various rapid chemical test methods available for evaluating the potential for either the alkali-silica reactivity or the alkali- carbonate reactivity of an aggregate source, in the authors’ opinion, the most reliable means of assessment is to test the potential for expansion in concrete.

Concrete expansion tests for identifying aggregate reactivity have been standardized in many countries and, although the tests may differ in details, they typically involve the fabrication of concrete prisms using the aggregate under test and specific concrete mixture proportions. In most cases, either the cement content of the mixture or the alkali content of the cement, or both, are increased above normal values to increase the rate of reaction. The prisms are stored in conditions of high humidity (e.g. over water in sealed containers) and, usually, at elevated temperature (e.g. 38ëC/100ëF is widely used) to further accelerate the rate of reaction. The length change of the prisms is monitored during storage and the expansion is used to determine the reactivity of the aggregate. Figure 7.4 shows typical expansion curves for concrete prisms manufactured using a non- reactive, moderately-reactive and highly-reactive aggregate. Although expan- sion limits and test durations vary between different guidelines and specifica- tions, a widely-used limit for identifying reactive aggregates is 0.04% after 12 months storage at 38ëC. In other words, aggregates that produce concrete expansion less than this value are considered innocuous (non-reactive) and those that produce expansion above this value are considered to be potentially reactive. Concrete prism tests are generally considered to be suitable for evaluating both alkali-silica and alkali-carbonate reactivity. Potentially reactive aggregates should either be avoided or only used in concrete with appropriate preventive measures (see Section 7.7).

Various mortar bar tests have been developed to evaluate the potential for alkali-silica reaction (ASR). Such tests usually employ one or more methods for accelerating expansion including: augmentation of cement alkalis, addition of alkali to the mortar mix, immersion of mortar bars in alkaline solution, elevated temperature and even autoclaving. At this time the most commonly used mortar bar test, often referred to as the accelerated (in North America) or ultra accelerated (in Europe) mortar bar test, involves the immersion of mortar bars in a solution of 1M NaOH at 80ëC. Length change is monitored during immersion and the test is capable of identifying most reactive aggregates after just 14 days immersion in the hot alkaline solution. The test conditions are very aggressive and there are many aggregates with satisfactory performance in the field or in concrete prism tests that fail the accelerated mortar bar test. Consequently, this test should not be used to reject aggregates and the potential reactivity of aggregates that fail this test should be confirmed by more reliable concrete prism testing. The (ultra) accelerated mortar bar test is not suitable for evaluating some alkali-carbonate reactive rocks and an additional test method has been proposed recently by a RILEM technical committee for use in such cases (Sommer et al.,2005). Further consideration is beyond the scope of this chapter and readers interested in the ongoing international efforts aimed at developing improved screening procedures for AAR-susceptible aggregates are directed to the work of RILEM Technical Committee 191-ARP. A review of the work of this com- mittee’s activities and those of the earlier RILEM Technical Committee 106- AAR was presented at the 12th International Conference on Alkali-Aggregate Reaction in Concrete (Sims et al., 2004).


Related Articles