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Based on criticisms of the traditional approach to the control of concrete quality which often relies entirely on 28-day compressive strength assessment, a new approach is proposed which involves monitoring quality in various ways from the time of mixing. The alternatives examined are: recording batch weights to establish W-C ratio, monitoring consistency, analyzing freshly prepared mortar or concrete mixtures to determine water content and cement content, and assessing strength potential by means of autoclave or hot-water curing. On the basis of past case histories, it is concluded that assessment of strength potential must remain the primary assurance of concrete quality, and that the other alternatives can only complement rather than substitute for strength assessment.
The techniques proposed for mixture analysis and strength assessment involve testing specimens prepared from a sample of mortar extracted from the parent concrete, on the premise that just about anything that is detrimental to the quality of the concrete will manifest itself in the quality of the extracted mortar. Provided the relationship between mortar and concrete strength is established from trial mixtures or past plant records for the materials in question, it is shown that strength potential can be effectively and routinely assessed using autoclave or hot-water curing of 25 by 50 mm (1 by 2 in.) mortar cylinders, and that water content can likewise be determined from oven drying of small specimens of mortar. The problem of determining cement content with acceptable accuracy and precision is not completely solved either by the wet-sieving technique discussed herein or by any of the other available physical or chemical techniques.
The techniques proposed for strength assessment and mixture analysis offer considerable potential for cost savings in concrete quality control because of the small size of the facilities needed, their consequent portability, and their relatively low capital and operating costs compared with the equivalent costs of conventional concrete testing.
Professor of civil engineering, University of Calgary, Calgary, Alberta
Construction engineer, City of Edmonton, Edmonton, Alberta
Stock #: CCA10073J