Abstract Knowledge of alkali concentration in concrete pore solution is key for long-term eval- uation of alkali-silica reaction (ASR) expansion. This study aimed at providing experimental data on the alkalinity evolution in concrete tested for ASR in order to determine critical alkalinity values that stop concrete expansion. The pore solution was systematically expressed (i.e. ex- tracted at high pressures) from concrete speci- mens tested at 38°C over water (Canadian standard). Alkali concentration showed a decreasing trend with time, which can be mainly explained by alkali leaching. After 52 weeks, the decrease in alkalinity was up to 25% of the ori- ginal Na 2 O eq content in the concrete, contributing to underestimate long-term expansion of reactive concrete. Comparisons were also made between pore solution expressed out of cement paste specimens and concrete specimens; differences (from 25% to 43% higher for cement paste) suggest that results obtained from cement paste are not suitable for concrete as they tend to overestimate the concrete pore solution alkalin- ity. The alkali concentration threshold varied from one mixture to another (205–335 mmol/l). Re ´ sume ´ La connaissance de la concentration en alkalis dans la solution interstitielle du be ´ton est un facteur cle ´ pour e ´valuer le potentiel d’expan- sion a ` long terme des structures touche ´es par l’alcali-re ´action. Cette e ´tude pre ´sente des donne ´es expe ´rimentales sur l’e ´volution de l’alcalinite ´ de be ´tons teste ´s pour leur re ´activite ´ afin de de ´ter- miner des valeurs critiques stoppant la re ´action. La solution interstitielle a e ´te ´ extraite sous pres- sion a ` partir d’e ´prouvettes maintenues au-dessus de l’eau a ` 38°C (norme canadienne). Les analyses montrent une de ´croissance de l’alcalinite ´ avec le temps, ce qui peut e ˆtre explique ´ en grande partie par le lessivage des alcalis; apre `s 52 semaines, la perte en alcalis atteint jusqu’a ` 25% de la teneur initiale en Na 2 O eq . Ceci contribue a ` sous-estimer l’expansion a ` long terme des be ´tons re ´actifs. Des comparaisons entre la solution extraite de pa ˆtes de ciment et de be ´ton ont e ´galement e ´te ´ re ´alise ´es. Des diffe ´rences importantes ont e ´te ´ note ´es (25% a ` 40% plus alcalins pour les pa ˆtes), ce qui indique que les donne ´es obtenues sur pa ˆte sont difficile- ment transfe ´rables aux be ´tons car elles tendent a ` surestimer l’alcalinite ´ de ces derniers. La con- centration seuil varie d’un me ´lange a ` l’autre (205 mmol/l a ` 335 mmol/l). P. Rivard (&) Æ G. Ballivy CRIB, De ´ partement de genie civil, Universite ´ de Sherbrooke, Sherbrooke J1K 2R1, Canada e-mail: patrice.rivard@Usherbrooke.ca M. A. Be ´ rube ´ CRIB, Universite ´ Laval, Quebec, Canada J. P. Ollivier LMDC, UPS - INSA Toulouse, Toulouse, France Materials and Structures (2007) 40:909–921 DOI 10.1617/s11527-006-9191-z ORIGINAL ARTICLE Decrease of pore solution alkalinity in concrete tested for alkali-silica reaction P. Rivard Æ M. A. Be ´rube ´ Æ J. P. Ollivier Æ G. Ballivy Received: 18 October 2005 / Accepted: 18 May 2006 / Published online: 3 October 2006 Ó RILEM 2006