Factors affecting shrinkage Powers; Meininger; and Tremper and Spellman are the three references I have used most to explain the causes of drying shrinkage. Each of them point out that water demand of the separate materials used in concrete is the major determinant of the shrinkage of concrete. I maintain that variations in water demand caused by the separate concrete ingredients has a far greater effect on concrete shrinkage than does the common concern overthe variation in slump.

Powers, and Tremper and Spellman both emphasized the cumulative effect on shrinkage of making poor choices in the selection of material to be used. Powers’ shrinkage results, clarified by Mather in the form shown in Table 1, show the individual and cumulative effects of the most unfavorable versus the most favorable material choices with regard to six factors influencing the amount of shrinkage. Powers assumed a constant water-cement ratio, and concluded: “Wrong choices of alternatives (with respect to volume change) can result in about seven times as much shrinkage as would result from best choices.”

Table 1 below shows that concrete with ¾ in. (19 mm) maximum sized aggregate will shrink about 30 percent more than concrete with 1 ½ in. (38 mm) maximum sized aggregate. But, concrete placing costs may increase slightly when larger aggregate is used. Designers therefore should specify that the maximum sized coarse aggregate be slightly less than ? the slab thickness, with the understanding that the small increase in cost will be offset through lower shrinkage and a more productive floor slab.

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