The Effects of Sieving and Paste Content on the Setting Behavior of Mortars as Measured by ASTM C403

Abstract Setting of concrete is measured by ASTM C403 via penetration resistance of mortar extracted from concrete by removing coarse aggregates larger than the 4.75 mm (#4) sieve. However, C403 is sometimes performed on mortar prepared to represent the mortar fraction of the concrete in question; thus avoiding the time-and-labor intensive sieving operation. As noted in STM C403, “… it has been shown that the initial and final setting times may be increased when using the prepared mortar.” The authors previously reported this difference quantitatively, having observed or discovered in the literature prepared mortars with an initial setting time up to 8 %–14 % greater than that of sieved mortar, and final setting time 5 %–16 % greater than that of sieved mortar. These differences are 1 to 3 times greater than the single operator precision for acceptable range of test results, and are thus significant in the context of C403. The authors propose that this difference is associated with the reduction in paste content in the mortar that accompanies the sieving operation, having observed a 3 to 8 % paste volume reduction in sieved mortar. The authors suggest a general correlation between changes in setting time as a function of changes in paste content.