Role of PFA quality and conditioning in minimising alkali–silica reaction in concrete

Abstract

This paper describes the work of a study carried out to assess the effect of pulverised full ash (PFA) quality (including conditioned (moistened) PFA) on its ability to minimise the risk of alkali–silica reaction (ASR) in concrete and thereby, whether the wider range of materials covered in BS 3892-1, BS 3892-2 and BS EN 450 can be used in this role. A total of 11 PFAs (within and between sources) and three types of aggregate combination, covering materials used in the UK, were examined. Two series of tests were carried out, which used concrete mixes proportioned to (a) the BS 812-123 standard method (alkali content 7·0 kg/m3) and (b) the BRE method (alkali content from 3·6 to 6·0 kg/m3), using the BS 812-123 test procedure (38°C, above water). The results obtained over three years, established that the physical and chemical properties of PFA (fineness and alkali content (Na2Oeq)) had only a minor influence on its ability to minimise ASR risk. Conditioning of PFA and its loss on ignition appeared to have no effect. Overall, the results suggest that PFA to BS 3892-2 and to BS EN 450, may be considered to be equally as effective as BS 3892-1 PFA in minimising ASR in concrete, containing potentially reactive aggregates available in the UK. The outcomes of this and other work have been considered by the ASI concrete committee and this has led to a change in BS 8500, which now permits the use of BS EN 450 PFA.