Alkali-silica reactivity causes cracking in a concrete bridge column.
Alkali-silica reactivity causes cracking in a concrete bridge column.
  • Q: We received a request to submit proof that our aggregates are not alkali-silica reactive. We've never seen this type of request before in our area. They required ASTM C1260 testing, which we completed and failed. What do we do now?

    A: Issues surrounding alkali-silica reactivity (ASR) of aggregates are occurring in many areas where concrete producers were not historically required to produce such data. ASR is a reaction between the alkalies in the cement and silica in the aggregates. This reaction causes a gel which expands when water is available. The issues are related to more distressed concrete structures exhibiting ASR.

    In the past there were few areas where ASR was a concern. But now, almost every state has seen a concrete structure with ASR damage. Also, engineers work in multiple states, so if they encounter ASR issues in one state, they will use that experience in all of their future projects.

    Discussion surrounding ASR is increasing. The Federal Highway Administration and the American Association of State Highway and Transportation Officials have issued reports and guidelines to deal with reactive aggregates. ASTM and ACI are also working on documents to help the industry understand how to deal with ASR.

    If you are not aware of the requirements or test methods dealing with ASR, you should learn them. Tackling ASR in the concrete industry is key to producing sustainable concrete, and building structures and pavements with long life and low maintenance.

    Now back to the question at hand. The ASTM C1260 aggregate test does not have the best track record. It classifies about 50 percent to 60 percent of aggregates as potentially reactive when they are not. This test accelerates the reaction two ways: by increasing the alkali loading and increasing the temperature.

    The increased alkali loading is higher than the alkali loading found in any field conditions. This increase provides a continuous supply of alkali to the mortar sample and possibly causes aggregate particles to react that would not typically react.

    The increased temperature is perhaps a greater issue with the test. High temperature liberates more silica from reacting than what would be liberated at ambient temperatures. Often, the reactive elements of silica-based aggregate are strained from heat and pressure in geologic formations. Increasing the heat energy in the test, to a lesser degree, mimics these changes.

    For these reasons, the test method actually states: “When excessive expansions are observed, it is recommended that supplementary information be developed to confirm that the expansion is actually due to alkali-silica reaction.”

    Additionally, when describing ASTM C1260, the annex of ASTM C33, Standard Specifications for Concrete Aggregate, states: “Results of this test method should not be used for rejection of aggregates unless it has been established using the sources of supplementary information cited in the test method that the detected expansion is actually due to alkali-silica reaction.”

    Basically, when a result of potential reactivity is provided using ASTM C1260, we don't know if the aggregate is reactive, and we need to do more work to find out.

    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