A measuring device known as a dilatometer — when combined with physical “rate theory” — has proven useful for quick measurement of alkali-aggregate reaction (AAR, also known as alkali-silica reactivity or ASR), according to “Development of a Rational Test Methodology for AAR,” by Chang-Seon Shon, Dan Zollinger, Ph.D, P.E., and Shondeep Sarkar, Ph.D, P.E., Texas A&M University.

The destruction of concrete due to the reaction between cement alkalis and certain types of reactive siliceous aggregates known as alkali-aggregate reaction (AAR) is a major problem across the U.S. AAR, the authors note, is largely a thermally activated process. “The use of rate theory to represent AAR relative to the susceptibility of an aggregate in terms of activation energy provides an entirely logical and unique material parameter to characterize AAR,” they contend.

Furthermore, this innovative method provides direct accountability for a variety of factors that affect AAR, such as temperature, moisture, concrete porosity, and concrete age, the authors affirm, adding that preliminary test results have been very promising and point to a new direction in AAR testing.

“The concept of applying a ‘rational’ approach using a dilatometer to assess expansion due to AAR is based on measuring the actual expansion produced by a siliceous gel,” the authors report. “This new method of using rate theory in terms of activation energy provides not only an entirely logical and unique material parameter to characterize AAR, but also direct accountability for a variety of factors that affect AAR in many instances less than 48 hours.”

Different tests have been used to measure AAR, including ASTM C 1260 (Accelerated Mortar Bar Test), which is one of the most commonly used methods because expansion results can be obtained within as little as 16 days, the authors note. But, they observe, aggregates with a good field track record and no history of AAR may be classified as reactive when tested according to that method.

“Another method, ASTM C 227, tests for potential alkali reactivity of cement-aggregate combinations,” they add. “However, unless very reactive materials are used, obtaining meaningful results from this method may require a year or more to obtain.” As with ASTM C 227, the amount of time necessary to carry out the ASTM C 1293 test method, which uses concrete prisms as test specimens, is a major drawback. Although ASTM C 289 provides a test method to detect potential reactivity of concrete aggregates, it fails to identify slowly reactive aggregates, the authors contend.

Finally, they assert, ASTM C 441 — a mortar bar test using Pyrex glass — is considered unsatisfactory because Pyrex is highly reactive and contains significant levels of alkali. Moreover, it is not adaptable for testing aggregates from different sources since it is mainly a test for cement AAR reactivity.

“The prospects of using the dilatometer to measure AAR expansion and to study the effect of AAR relative to aggregate components appear to be highly promising,” they affirm, citing several conclusions based on their analysis:

• Test results indicate that at higher temperature and higher reactivity of aggregate, AAR expansion is greater.
• The greater the reactivity of aggregate, the greater the activation energy.
• Different activation energies can be achieved for different levels of reactivity of aggregate.
• Different temperature ranges of the testing device were investigated to determine the optimal temperature range that can be used for measuring activation energy and AAR expansion.
• Using the fundamental modeling approach proposed by the study represents an entirely new concept for AAR testing in less than 48 hours.