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Comparison of test methods on the compressive strength of slag and natural pozzolan cements

Tokgöz, Abdullah Usame
Among the two standard test methods of determining the compressive strength of cements which are described in EN 196-1 and ASTM C 109, the basic differences is in the amount of water used in preparing the mortars. According to EN 196-1 the former uses a constant water-cement ratio of 0.50 in the preparation of mortar specimens, for all types of cements whereas the latter uses a constant water-cement ratio of 0.485 and 0.460 for Portland and air-entrained Portland cements, respectively; and water-cement ratio that corresponds to a specified workability (110  5% flow) for blended cements. Presence of mineral additives in the blended cements would result in stiff mortar specimens when the same compactive effort is applied as described in EN 196-1. On the other hand, when the constant flow criterion of ASTM C 109 is applied, this would lead to higher water-cement ratios. Variability of the strength test results could be higher, in both cases either due to insufficient compaction or higher water-cement ratio, respectively. In this thesis, a third method of strength specimen preparation based on constant volumetric water-cement ratio was proposed. vi The aim of this thesis is to show the variabilities that can be encountered in the determination of compressive strength of blended cements. For this purpose, ground granulated blast-furnace slag (GGBFS) and natural pozzolan-incorporated blended cements were used. 20%, 35%, 55% GGBFS and 20%, 35%, 55% natural pozzolan-incorporated blended cements are selected and 7-day and 28-day compressive strength of mortars, which are obtained by the 3 different methods, constant water/cementitious by mass (TS EN 196-1), constant flow (ASTM) and constant water/cementitious by volume methods, are determined. Then, coefficients of variation of the results obtained by the three methods are compared. The variabilities involved in the two standard methods and the proposed method was comparatively studied by statistical analyses of the experimental results obtained. According to the results of the studies, deviations of the compressive strength results obtained from TS EN 196-1 mortars, is more than Constant Volumetric water/cementitious Mortars and ASTM C109 mortars. Therefore, it is proposed that using constant water/cementitious by mass is not suitable for blended cements.