Effect of Pulverised-Fuel Ash With a High Total Alkali Content on Alkali Silica Reaction in Concrete Containing Natural UK Aggregate

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Title: Effect of Pulverised-Fuel Ash With a High Total Alkali Content on Alkali Silica Reaction in Concrete Containing Natural UK Aggregate

Author(s): M. D. A. Thomas, P. J. Nixon, and K. Pettifer

Publication: Special Publication

Volume: 126

Issue:

Appears on pages(s): 919-940

Keywords: alkali-silica reactions; chert; electron probe; gels; petrography; pulverized fuel ash; Materials Research

Date: 8/1/1991

Abstract:
A number of seven-year-old, externally stored 500 x 100 x 100 mm concrete beams, some of which had suffered severe cracking due to alkali-silica reaction, have been examined. The concretes were produced using pulverized fuel ash (PFA) at a range of addition levels and contained a fixed proportion of a known reactive sand. Following 7 years of exposure, severe cracking was observed in the specimens without PFA or, with 5 percent PFA, surface crack widths were often in excess of 1 mm and examination of sawn surfaces indicated that the depth of visible cracks was up to 20 mm. Specimens containing 20 percent or more PFA did not exhibit any visible cracking. Expansion measurements, USPV, dynamic modulus of elasticity, and modulus of rupture tests were undertaken, and the results broadly confirm the visual condition of the specimens, with cracked specimens displaying significantly reduced engineering performance. Average carbonation depths were less than 3 mm for all the concrete specimens. However, depths of up to 20 mm were observed at the location of some of the wider cracks. Petrographic examination of thin sections showed evidence that alkali-silica reaction had occurred in all the concretes, but had only led to cracking in the concretes with no PFA or 5 percent PFA. In the concretes containing higher levels of PFA, the sites of gel were rare and there was no evidence of associated damage. Examination of polished sections by quantitative electron probe microanalysis showed differences between ordinary portland cement and PFA concrete in the composition of the alkali-silica gel and the cement hydrates. The gel in pores in the PFA concrete was lower in calcium than that in cracks in the ordinary portland cement concrete. In addition, hydrate rims around alite grains had lower Ca/Si ratios and higher K/Si ratios in PFA concrete. The lower quantity of available calcium in PFA concrete and the increased absorption of potassium by its contributions to the suppression of damaging alkali-silica reaction.