DISCUSSIONThe design of fly-ash concretes A. SMITH M r W. B. Knox, Transmission Project Group, Central Electricity Generating BoardIt is with considerable regret that I find that Mr Smith's Paper does not include details of his trial mixes and the results obtained, from which he deduced the values for his 'cementing efficiency' factor K of pulverized fuel ash (P.F.A.). I have in my possession mix details and cube strengths for some of his early work-for example that referred to in Table l-but I am not satisfied that his deductions are adequately supported, as it was only in one group that he prepared orthodox concrete control mixes and P.F.A. concrete mixes at the same aggregate/fines ratio and comparable workability as measured by the compaction factor test. In that group (aggregate/fines ratio 5.2, water/fines ratio 0.55, with P.F.A. replacing up to 15% of the cement weight) the strengths suggest a K value of zero. 54. The work to which I refer covered aggregate/fines ratios from 7.5 to 2.4, P.F.A. replacements from 0 to 55% of the cement weight, and the mixes gave compaction factor readings ranging from 0.99 to 0.60. 55.In 3 44 of his Paper MC Smith disposes easily of this variation in workability as measured by the compaction factor apparatus, by claiming that the P.F.A. cement concretes had 'placeabilities' approximately equal to those of their orthodox counterparts, but again there is no supporting test evidence.56. Persons familiar with concrete mix design will appreciate that Abrams law, as given in Road Note 4, should only be taken as a general indication of the relationship between water/cement ratio and the strength of orthodox concretes. Many other factors are involved and results of mixes at various aggregate/fines ratios, P.F.A./ cement ratios, waterlcement ratios and different workabilities are not directly comparable. To assess the effect of any particular variation in the mix, only one characteristic should be varied at a time. In MC Smith's test mixes it is noticeable that the P.F.A. concrete mixes which gave strengths significantly higher than might have been anticipated from the water/cement ratio, had each a high P.F.A. replacement, a very low compaction factor and a low aggregate/fines ratio, whereas the control concretes had relatively high aggregate/fines ratios and high workability.57. I consider that sufficient work has been done by others to establish that P.F.A. does not contribute significantly as a result of any chemical reaction to the strength of concrete mixes at early ages (e.g. 28 days). It is conceivable that by its presence in concrete mixes P.F.A. could contribute physically by increasing the specific surface of the mix, thereby reducing the thickness of water film around the cement particles and thus effectively reducing the waterlcement ratio. This phenomenon may also be true of other materials of particle size similar to cement. Even so it would have to be established that the omission of the P.F.A. and a reduction in the water content of the mix to provide similar workab...
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