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.