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This paper reports the loss of concrete stiffness due to the adverse effect of alkali-silica reactivity (ASR). The concrete cylinders prepared with fourteen aggregate groups were immersed in water at 20 °C (68°F), and in the 1.0 N NaOH at 80 °C (176°F). The percent loss in stiffness (PLIS) between water- and alkali-cured cylinders was determined at 4 and 26 weeks, and was correlated with the expansion of the mortar bars (immersed in the 1.0 N NaOH at 80 °C (176°F)) containing the companion aggregate source at the test durations of 2, 4 and 8 weeks. The results concluded that the loss in stiffness of the concrete cylinders occurred at 4 weeks, and it significantly increased at 26 weeks. The study also suggested that the ASR classification of the aggregates based on the expansion limits of mortar bars showed a good correlation with that produced from the proposed limit of stiffness loss of concrete.