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Of the constituent materials in concrete, most suppliers regard aggregates to be the least prone to cause problems. Unfortunately, some aggregates react chemically when used in portland cement concrete and have the potential to destroy or damage a concrete structure. The problem occurs when alkalies in the cement react with siliceous material in the aggregate. This reaction forms a gel around the aggregate. The gel has an affinity for water and swells upon absorbing water. The swelling, in turn, creates pressures inside the concrete that, if high enough, will cause cracking. The best method to avoid ASR is examining structures that are at least 20 years old and made with the aggregate you wish to use. A careful visual examination should reveal if there is evidence of ASR. If you cannot examine structures in the field and lack service records for aggregates, several tests can determine if aggregates are potentially reactive. A quick chemical test takes about three days. Petrographic examination can identify aggregate types known to be reactive, and quantify the amount of materials present. The mortar bar test is probably the most accurate, and also the most time consuming test. There are several ways to minimize risk of ASR with reactive aggregates. Using a portland cement with an alkali content of 0.6% or less is one possibility. Incorporating pozzolanic admixtures and using blended cements can also help reduce reactivity. Research is under way on admixtures based on lithium salts that may be able to prevent or minimize ASR in concretes made with aggregates that are normally reactive.