The fineness of pfa over the BS EN 450 range will influence concrete strength, with reductions for each 5.0% increase in sieve retention of between approximately 0.5 N/mm2 and 1.5 N/mm2 over the practical binder range at 28 days. The permitted ±10.0% in the standard on the supplier’s declared mean can contribute to increased variability by up to 3.0 N/mm2 and may therefore add to the design margin of concrete producers. Pfa suppliers may, therefore, decide or be required to reduce the permitted variability. In contrast, the LOI limit of 7.0% is satisfactory, with little influence on compressive strength.
Tests using the range of Portland cements produced or available in the UK, combined with pfa to BS EN 450 indicate that the trends for pfa fineness, obtained with the reference cement, are maintained with the other sources of Portland cement. In addition, the variability in compressive strength of concrete associated with a range of pc/pfa combinations, using material from different sources, is no greater than that obtained for pc concrete.
A method of mix proportioning that takes account of the effect of pfa fineness on concrete strength has been devised. It is simple and can be integrated into existing concrete mix design procedures and production practice. Other aspects of concrete performance including engineering and durability properties are equivalent for pfa concretes over the BS EN 450 fineness range, providing the concretes are of the same design strength, achieved in the w/c ratio, via the mix water content.
the authors are currently undertaking a technology transfer programme for BS EN 450 pfa funded by DETR/industry. A technical guidance document will be produced and a number of seminars held during 1998 and 1999 in preparation for the new standard.