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Fourteen concretes were cast, with and without air entrainment and fly ash, using various sizes of cylindrical mold made from different materials. Cylinders were cured in the standard manner and tested for compressive strength, primarily at an age of 28 days. The range of strengths measured was 30 to 50 MPa. Each cast consisted of 60 cylinders, with 12 replicates performed for each of the five types of mold: 150-mm diameter cardboard and plastic molds (notation, C150 and P150); 100-mm diameter plastic molds (P100); and 75-mm cardboard and plastic molds (C75 and P75). The length/diameter ratio of all cylinders was 2.0.
For 150-mm cylinders, the variability of strength measurement, as indicated by the average coefficient of variation (cv), did not depend upon the type of mold material used (cardboard or plastic). For 75-mm cylinders, the same observation was made; cv did not depend upon mold-material.
However, when plastic molds were used, there were significant differences in cv as cylinder size varied. The average cv for P75 cylinders was 4.9 MPa, while it was 3.2 MPa for P150 cylinders; the increase is highly significant. On the other hand, P100 cylinders gave a cv of 3.6 MPa, which is not significantly different than that for P150 cylinders.
For cardboard molds, where 150- and 75-mm cylinders were tested, the cv was not a function of cylinder size.
The effect of mold material and diameter on the magnitude of measured strength was also examined. No distinct trends were observed. Differences in observed strengths due to type and size of mold were influenced by the type of concrete tested and the procedure for initial storage during the first 24 h of curing. However, when all 28-day strengths were considered in paired-t analyses, the following differences were found to be significant: (1) the use of 75-mm molds (C75 and P75) resulted in a higher strength of about 1 MPa, compared to the larger molds (P100, P150, C150); (2) for 150-mm molds, the use of plastic instead of cardboard resulted in a lower strength, about 1 MPa; and (3) the strength obtained from concrete cast into plastic molds increased as the mold size decreased; the difference between P100 and P150 molds was 1 MPa, while the difference between P75 and P150 molds was, on average, 1.6 MPa.
Some of the differences in strength among the mold types was traced to differences in moisture loss during the first 24 h of storage. Cylinders stored under plastic sheet (in accordance with the American Society for Testing and Materials (ASTM) specifications), were about 3 MPa stronger than those stored in sealed plastic bags (in accordance with the Canadian Standards Association (CSA) requirements). Erratum to this paper appears in 17(1).
Professor, University of Calgary, Calgary, Alberta
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