Friday, 22 September 2017
Austroads has published the results of tests to investigate whether the much faster accelerated mortar bar test could be employed instead of the slow concrete prism test to evaluate supplementary cementitious materials.
The tests identify the levels of supplementary cementitious materials, such as fly ash, slag or silica fume, required to suppress damaging expansion in concrete caused by alkali-aggregate reaction and avoid prohibitive repair costs.
Three potentially reactive aggregates were tested using the two methods.
The results indicate that:
- Both methods detected the reactivity of the three aggregates but the accelerated mortar bar test appears to be unsuitable for combinations of the more reactive rhyodacite aggregate with binders containing silica fume. A high dosage of silica fume (10%) was needed to suppress the reactivity in the accelerated mortar bar test but smaller amounts were found to be required when assessed by the concrete prism test
- The accelerated mortar bar test is suitable for assessing the combination of fly ash and slag with the aggregates, where more than 15% fly ash and more than 50% slag have been indicated to suppress the reactivity. However, based on previous experience with more reactive aggregates such as Ignimbrite from north Queensland, which required 30% fly ash and 65% slag, the above dosage cannot be used universally.
Overall, the comparison between the two methods indicated that the already-practised period of 21 days seems to be the shortest time that the effects of supplementary cementitious materials can be detected, and produces the best agreement between the results of the two methods. The results also showed that the dosage of supplementary cementitious materials for alkali-aggregate reaction suppression was highly dependent on the nature of the aggregate. As a result, a universal rule cannot be prescribed.
It is suggested that long-term field tests be undertaken for aggregates from various sources, so that a more reliable database can be generated for each area.