Alkali Content Testing
In recent years, some deleterious reactions involving the aggregate and the surrounding cement paste have been observed. The most common reaction is that between certain forms of reactive silica in the aggregates and alkalis in the concrete, normally derived from the cement. The forms of reactive silica include opal, chalcedony and tridymite. They occur in some cherts, siliceous limestone, rhyolites and a number of other rock types. In order to reduce the risk of reaction, limits are now suggested for total alkali content of the concrete, where a potentially susceptible aggregate is involved. The suggested limit is either 3ikg/m3 total equivalent sodium oxide for the concrete or 0.6% total equivalent sodium oxide on the cement, where this is the only alkali source anticipated. ASR may also be controlled by additions of pfa or ggbfs. The alkali content of hardened concrete is determined by flame emission or atomic absorption spectrophotometric methods on an acid extract of the crushed concrete.