When concrete sidewalks all over Montreal began scaling badly during the winters of 1948 and 1949, the public demanded to know why. Contractors blamed cement and aggregates. The cement producer blamed poor workmanship. Quarry operators blamed both inferior cement and inferior work. The results was confusion, assumptions and charges, but few facts. Early in 1950, W. S. Weaver and H. L. Isabelle of the Canada Cement Company Limited, Montreal, developed a program of field research designed to uncover the facts. The study, involving 48 test slabs in sidewalks selected at random from 15 sites in the Montreal area, was restricted to reporting what was observed. As many contractors as possible and a wide variety of aggregate materials, mix designs, placing techniques, curing methods, exposures and temperature ranges at the time of placement were included in the study. Specific findings of the study were highly significant and of value to any cement contractor. First, entrained air in concrete is highly beneficial. In the study, air entrapment did not insure good concrete performance if other factors were below par. However, all 12 test sections that rated above 7 were found to have low air content. Sixty-one percent of test slabs rated 6-10 had less than 4 percent air content. Second, sidewalks built in the late fall did not perform as well as those constructed during milder weather. Twelve of 16 sections constructed and cured at temperatures under 50 degrees Fahrenheit during the fall season rated 5 or higher, whether or not they contained a sufficient amount of entrained air. More than half, 52 percent, of all sections rated 6-10 were cured at temperatures below 50 degrees Fahrenheit. Third, the performance of concrete sidewalks was significantly influenced by exposure conditions. Fourth, concrete made with a water/cement ratio of less than .50, containing more than 525 pounds of cement per cubic yard, and testing approximately 4000 psi in compression after 28 days, performed best. Fifth, no correlation was found between slump and performance in the test sections. Sixth, results of the magnesium sulphate tests made on the coarse aggregate did not predict surface deterioration. Seventh, the manner in which concrete was batched, mixed, transported, placed, finished and cured helped explain the performance of concrete in certain test sections, but no definite patterns were established. Eighth, the performance of concrete sidewalks was predicted reasonably well by freezing and thawing tests carried out on 2 inch cubes molded from samples of screened concrete. Ninth, sidewalks constructed of a base course and a rich-mix surface course performed better than those of single course construction.
