Sequential examination of slowly expanding alkali-reactive aggregates in accelerated mortar bar testing

The NBRI mortar bar test, now adopted with slight modification as ASTM C1260-94, has been used to study the sequential development of alkali-aggregate reactions jor certain slow/late expanding types of reactive rocks. The work presented here is concerned with the expansion produced by mylonites. meta-argillite, meta-greywacke, granite, limestone and silica glass. At different stages in the test, the situation at the reaction site and the formation of the gel reaction products have been investigated using both optical and scanning electron microscopes. The composition of the gel reaction products and the amount of mobilized silica have also been measured. The accelerated test method appears to echo the alkali-aggregate reactions occurring in, field concrete. The different rock types show different rates of expansion, and two different reaction styles are observed at the reaction sites. One of these two phenomena involves dissolution of quartz along grain boundaries and the other involves crack generation within the aggregate. The process of cracking and the amount of cracking appear to be the main, factors governing the amount of expansion produced by the reaction. No correlation was, found between the amount of alkali-silicate gel produced as the reaction product and the expansion of the mortar bar.