affected by ASR Mario Berra*, Teresa Mangialardi**, Antonio Evangelista Paolini** 13h International Conference on Alkali-Aggregate Reaction in Concrete Trondheim (Norvegia) 16-19 giugno 2008 * CESI RICERCA ** UNIVERSITÀ DI ROMA "LA SAPIENZA" The primary objective of this study was to ascertain whether the Threshold Alkali Level (TAL) of the aggregates may be taken as a suitable reactivity parameter for the selection of aggregates susceptible of alkali-silica reaction (ASR), even when ASR expansion in concrete develops under restrained conditions. Concrete mixes made with different alkali contents and two natural siliceous aggregates with very different TAL were tested for their expansivity at 38°C and 100% RH under free and restrained conditions. Four stress levels over the range from 0.17 to 3.50 N/mm2 were applied by using a new appositely designed experimental equipment. The lowest stress (0.17 N/mm2) was selected in order to estimate the expansive pressure developed by the ASR gel under free expansion conditions. It was found that, even under restrained conditions, the threshold alkali level, TAL, proves to be a suitable reactivity parameter for designing concrete mixes that are not susceptible of deleterious ASR expansion. An empirical relationship between expansive pressure, concrete alkali content and aggregate TAL was developed in view of its possible use for ASR diagnosis and/or safety evaluation of concrete structures.