569
CEMENT---SET AT DIFFERENT TEMPERATURES.
freezer with a day at 70° F., have their strength about doubled after six days additional time in the air at the higher temperature.
Comparing the strength of specimens which set in air at 70° only for a given period with those which set in air the same interval of time and at the same temperature plus a period in the freezer, it is found that the latter specimens show a deficiency in strength over the former. So far as these l-esults go, the influence of the zero-temperature interval appears to detract from the subsequent strength of the material. Thus, specimens 4 months old, three of which were passed in the freezer, at zero temperature and one at 70°, in strength fell below corresponding ones which were only 1 month old, but all of which time was passed in air at 70° F. This was noticeable in the Portlands examined. The mean of a number of natural cements showed substantially the same strength in each group.
The conditions found in the cooler at a temperature in the vicinity of 89° F., appeared more favorable for the ultimate attainment of high strength than setting wholly or in part in air at 70° F. Examining the l'esults of those cubes which set the longest intervals at each of these two temperatures, it is seen that the specimens from the cooler in the three months’ tests displayed higher results than those which set in the air at 70° F.
Making a further examination on material exposed for two or three months at 39° F., followed by intervals at 70° F., and comparing these results with specimens which were exposed to the higher temperature only, but over a period equal to the sum of the other two, it is seen that specimens which passed part of the time at 39° F. were the stronger, excepting in the case of one brand, where the difference in favor of the other temperature was small. The mean results showed a compressive strength of 2,737 pounds per square inch for those set in air in part at 39° F., against 1,929 pounds per square inch for those which set at 70° F. only, a difference sufficiently pronounced to justify the belief that the conditions attending the specimens set at a temperature in the vicinity of 39° F. were the more favorable for the material.
The results of the tests were arranged for another comparison, namely, to show the amount which was gained by setting six days additional in air at 70° F. over different periods of exposure at zero temperature. The activity of the cement in acquiring strength immediately after thawing is here a conspicuous feature, as shown in earlier comparisons. The gain in strength of the younger specimens was the greater, during the compared interval of time of six days, but the absolute strength was less in this period, hence the somewhat higher rate of gain presents no unusual feature in the general setting of cements at different ages. A corresponding comparison with specimens which set in air at 39° F. showed a substantial gain in strength both in the younger and older cubes, the greater gain pertaining to the younger specimens as in the last comparison. Among those which set in the air different intervals at 70° F. only there is a slight gain in the specimens 1 month old and a loss in those 3 months old, taking the mean results on cubes which are directly comparable. The actual gain, without regard to the initial strength, appears greatest in the specimens which were exposed to zero temperature as a group, and least in those which set in air at 70° F. only.
In the last table of comparisons the strengths of specimens of the