(Received 2 October 1998; accepted 3 December 1999)
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ASTM C 150 has optional limits on the equivalent alkalies in portland cement expressed as %Na2O + 0.658 × %K2O), often referred to as “Na2Oeq” where alkali silica reactivity is a potential problem. The prescribed limit of 0.6% by mass of the cement, which was developed over 40 years ago, has been questioned as not being sufficiently restrictive in some circumstances.
A 17-year-old bridge showing strong evidence of alkali-silica reactivity was studied as part of a larger investigation for the North Carolina Department of Transportation. The extent of noticeable map cracking was significantly different in different parts of the bridge. The differences could not be explained in terms of exposure to moisture or other environmental effects. Available information indicated that two different cements, with different alkali contents, were probably used at different times during construction.
Cores were removed from areas with obvious differences in cracking. All of the cores contained a reactive phyllite coarse aggregate. The alkali content was determined from selected cores. In addition, the area of material that fluoresced under ultraviolet light after treatment with uranyl acetate was determined on cores that had been split longitudinally. These factors were compared to the extent of cracking. No deleterious ASR was generated when the water-sol-uble alkali content was less than 1.5 kg/m3, or 0.55% by mass of the portland cement (Na2Oeq) with the reactive phyllite, at least for concrete in service for 17 years. A higher alkali content resulted cracking due to ASR.
Project engineer, McDevitt Street Bovis, Inc., Charlotte, NC
Assistant professor, North Carolina State University, Ralaigh, NC
Stock #: CCA10462J