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Comparison of ASTM C311 Strength Activity Index Testing vs. Testing Based on Constant Volumetric Proportions



Dale P. Bentz, Alejandro Duran-Herrera, Daniel Galvez-Moreno


Currently, the (pozzolanic) strength activity indices of fly ashes and natural pozzolans are typically evaluated using the procedures outlined in ASTM C311, “Standard Test Methods for Sampling and Testing Fly Ash or Natural Pozzolans for Use in Portland-Cement Concrete.” In this test, the 7 d and 28 d compressive strengths of mortar cubes with a 20 % mass replacement of cement by pozzolan are compared to those of a control without pozzolan, at constant flow conditions. In its current form, this procedure confounds two other properties of the pozzolan with its strength activity, namely its density and its water-reducing/increasing capabilities. In this study, the current C311 testing procedure is evaluated against an alternative in which the 20 % fly ash replacement for cement is performed on a volumetric basis and the volume fractions of water and sand are held constant, which should provide a true evaluation of the strength activity index of the pozzolan, free of these confounding influences. Class C and Class F fly ashes, a natural pozzolan, and a sugar cane ash are evaluated using both approaches, with some significant differences being noted. For a subset of the materials, the strength measurements are complemented by measurements of isothermal calorimetry on the mortars to an age of 7 d. For the constant volumetric proportions approach, a good correlation is exhibited between the cumulative heat release of the mortar at 7 d and the measured 7 d strength, suggesting the potential to evaluate 7 d pozzolanic activity via calorimetric measurements on much smaller specimens.
Journal of ASTM International


Compressive strength, density, flow, isothermal calorimetry, pozzolan, strength activity index, water reduction.


Bentz, D. , Duran-Herrera, A. and Galvez-Moreno, D. (2011), Comparison of ASTM C311 Strength Activity Index Testing vs. Testing Based on Constant Volumetric Proportions, Journal of ASTM International, [online], (Accessed June 20, 2024)


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Created September 12, 2011, Updated February 19, 2017