Q: We are being forced to look for a new source of coarse aggregate. The problem is our choices are limited. One of our options, and it appears our most economical one, will be from a quarry whose aggregate is known to be susceptible to alkali-silica reactivity.
What are the best test methods to help us determine whether these aggregates are safe from alkali-aggregate reaction?
A. There are two good sources to answer this question. Both contain current information on how to test the aggregate.
Probably the most complete review of the current information is the booklet titled, “Diagnosis and Control of Alkali-Aggregate Reactions in Concrete,” which was updated this year. Its authors provide the most current information on alkali-aggregate reactivity (AAR), including alkali-silica and alkali-carbonate forms. They report on the basic mechanism and conditions under which they occur.
They also included a guide to the visual symptoms of Alkali-Silica Reaction (ASR) and diagnosis. Important to your question, they report on the various standard test methods and discuss their limitations. Should you use aggregate that is susceptible to ASR, they advise on materials and evaluations to minimize the potential for AAR when designing concrete mixtures.
If you choose this source, it's important to make your customer aware of the potential problem. The Portland Cement Association's Durability subcommittee developed the “Guide Specification for Concrete Subject to Alkali-Silica Reactions.” Producers can provide the document to design professionals so they can incorporate it into their project specifications. Producers will also find information regarding the proper tests to qualify concrete mixtures.
Another interesting and informative document is a technical paper published in the March/April 2007 “ACI Materials Journal.” Dr. Charles Tremblay and a team of researchers reported the results of a two-year study on ASR mitigation efforts. The study's goal was to evaluate the effectiveness of lithium-based products in counteracting ASR. Euclid Admixtures Canada and the National Science and Engineering Research Council of Canada funded the work.
Researchers cast 87 concrete mixes that incorporated 12 reactive aggregates of various types and degrees of ASR. In the samples, the researchers used various dosages of LiNO3 and Li glass in combination or not with supplementary cementing materials. Samples were tested using the concrete prism test outlined in ASTM 1293.
The study yielded some important results producers should note.
They discovered that lithium glass was not effective in preventing ASR in any of the aggregates tested. They also reported that this was the case when the glass was finely-ground. They found that the glass did not release the lithium, even after 365 days.
Using a 0.04% expansion limit criterion at two years, the standard dose of LiNO3 was effective in preventing ASR with half of the aggregates. Three of the aggregates required substantially higher dosages to mitigate ASR. And with three of the aggregates, even at a dosage ratio of 1:11, the admixture was not effective at all.
They reported that the dosage rate of lithium is not related to an aggregate's degree of reactivity or its petrographic nature.
They also said that ASR was controlled in all the samples made with silica fume/slag blended cement, as well as samples made with slag and fly ash. Also interesting was the discovery that the addition of LiNO3 to mixes incorporating fly ash or slag had no effect on the mitigation process.
Tremblay's report, “Effectiveness of Lithium-Based Products in Concrete Made with Canadian Natural Aggregates Susceptible to Alkali-Silica Reactivity, Title No. 104-M23,” is available on the American Concrete Institute's Web site at www.concrete.org.