Alkali-silica reactivity causes cracking in a concrete bridge column.
Specifications and guides provide the following list of test procedures which could be used for further investigation:
- ASTM C295, Standard Guide for Petrographic Examination of Aggregates for Concrete. This method consists of a geologist observing the aggregate for potentially reactive particles and grains. If you have a failing C1260 result, I would not suggest this method, as the geologist will see the grains in the sample which reacted during the C1260 test and call them out in the report as a potential issue.
- ASTM C289, Standard Test Method for Potential Alkali-Silica Reactivity of Aggregates (Chemical Method). This looks for the chemical compounds known to produce reactions in concrete. Again, if there are already failing results from ASTM C1260, it's evident the sample contains these compounds and this test wouldn't provide any new information.
- ASTM C1293, Standard Test Method for Determination of Length Change of Concrete Due to Alkali-Silica Reaction. This test method is the most reliable for determining the reactivity of aggregates, although it requires one year to complete.
- ASTM C1567, Standard Test Method for Determining the Potential Alkali-Silica Reactivity of Combinations of Cementitious Materials and Aggregates (Accelerated Mortar-Bar Method). This method is almost identical to ASTM C1260 although you are permitted to use combinations of cementitious products to prove mitigation of the aggregates. The validity of this test is still in flux as research continues.
Returning to our problem, all of these methods have flaws, and each has its own level of risk. Dealing with ASR is about limiting the risk involved in having a distressed concrete, and the level of risk which is acceptable is determined by the engineer of record. The question indicated there was no concern about ASR in your region, so providing field history of the aggregates and cementitious products may be sufficient to the engineer. These records should span 10 years, but this is up to the engineer.
If the level of risk around the aggregates is not acceptable, mitigation methods may be reviewed and implemented on the project. These may include limiting the alkali loading per cubic yard of concrete or using supplementary cementitious materials or a lithium admixture to mitigate the potential reactions.
ASR is a growing concern and specifications requiring data or methods to deal with it will become more frequent. As a concrete producer, your best defense is to better understand the raw materials you use. This begins with understanding whether the aggregates are reactive and, if so, to what degree. The best method we have so far is ASTM C1293, but the length of time required to complete the test demands foresight.
Contributed by Alfred Gardiner of Braun Intertec. Gardiner is chairman of ACI Committee 221 on Aggregates. Visit www.braunintertec.com.