Properties and Reactivity of Gabbro and Harzburgite Gravels Used in Concrete Work in ... Alkali-Aggregate Reaction Testing," Proceedings, 8th International.
Disc. 87·M461From the September-October 1991 ACI Materials Journal, p. 446
Properties and Reactivity of Gabbro and Harzburgite Gravels Used in Concrete Work in Kuwait. Paper by Osama E. K. Daoud Discussion by R. D. Hooton and Author
By R. D. HOOTON Fellow American Concrete Institute, Associate Professor, Department of Civil Engineering, University of Toronto, Toronto, Ontario
Dr. Daoud is to be commended for presenting a very exhaustive and complete study on testing of harzburgite and gabbro gravels for use in concrete. I only wish we had similar complete knowledge for the various aggregates we use in Ontario. More as points of information than any criticism, I would like to raise a few issues with respect to the alkali-aggregate reactivity testing reported. 1. The ASTM C 227 data reported in Table 4 only extends to 6 months. While the aggregates meet the suggested ASTM C 33 limits, in many cases, slowly reactive aggregates take 12 or even 18 months to exceed 0.10 percent expansion. Therefore, the ASTM limits can be deficient for some aggregates. While the accelerated mortar bar expansions are low (well below 0.10 percent), which is encouraging, I would be concerned with only testing C 227 mortar bar for 6 months. Also, as a word of caution, the type of C 227 storage container and wicking system greatly affect the expansion results. 29 •30 If the container and wick system shown in ASTM C 227-87 is used, alkalies may be leached out of the bars into the water reservoir and prematurely reduce or stop the expansion. As a preliminary measure,
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a cautionary note to this effect will be added to C 227. In light of this, I would be interested in knowing what kind of container was used. However, the main purpose of this discussion is not to criticize this very complete study but simply to make readers aware of the potential deficiencies in ASTM C 227. 2. Incidentally, a version of the accelerated mortar bar test (Oberholster and Davies31 ) now appears as proposal standard method P214 in the 1990 ASTM Book of StandardsY
REFERENCES 29. Rogers, C. A., and Hooton, R. D., "Leaching of Alkalies in Alkali-Aggregate Reaction Testing," Proceedings, 8th International Conference on Alkali-Aggregate Reaction, Kyoto, 1989, pp. 327-332. 30. Rogers, C. A., and Hooton, R. D., "Reduction in Mortar and Concrete Expansion with Reactive Aggregates Due to Alkali Leaching," Cement, Concrete and Aggregates, V. 13, No. I, Summer 1991. 31. Oberholster, R. E., and Davies, G., "Accelerated Method for Testing the Potential Alkali Reactivity of Siliceous Aggregates," Cement and Concrete Research, V. 16, No.2, 1986, pp. 181-189. 32. "Proposed Test Method for Accelerated Detection of Potentially Deleterious Expansion of Mortar Bars Due to Alkali-Silica Reaction (C-9 Proposal P-214)," 1990 Book of ASTM Standards, V. 04.02, ASTM, Philadelphia, pp. 737-742.
AUTHOR'S CLOSURE
The author would like to thank Professor Hooton for his comments. Regarding the two issues raised by him, the author has the following comments: ACI Materials Journal I July-August 1991
1. I agree totally that the 6-month limit may be deficient for some aggregates. However, in the case in point, ASTM C 227 is used to confirm the results of the accelerated mortar bar, the quick chemical test, and the petrographic examination. All of these indicated the aggregate's innocuous behavior. Had these tests indicated otherwise, longer durations would have been used. 2. The container used was made of plexiglass and was supplied with a shelf supporting the mortar bars vertically and separating them from water below. The inner sides of the box were covered by cotton cloth, with one end dipped in water. There was direct contact between water and the samples.
3. ASTM C 227 is prone to laboratory deficiencies such as variable temperatures and others resulting from its lengthy duration. It costs too much in terms of power consumed in operating the oven for 6 months. Both problems are solved when accelerated mortar bar is used. 4. One problem continues to exist in both tests - the unrealistic form in which aggregates are used in the test. Crushing the aggregate to powder form could produce misleading results. This is particularly true with nonhomogeneous aggregate where sampling can change the ratio of reactive minerals in the crushed powder. The use of concrete samples would certainly be a step in the right direction.
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