Mitigation of alkali-silica reaction through carbonation of recycled concrete aggregates

Concrete waste management has become more and more challenging, especially in large cities, encouraging the use of recycled concrete aggregates (RCA). But their mechanical performances are generally lower, limiting their use to non-structural applications. Durability of recycled concrete can also be compromised, specifically if the original concrete is affected by a specific pathology, such as alkali-silica reaction (ASR) which could induce a secondary expansion. In order to prevent water-swelling gel formation, RCA are submitted to carbonation to reduce the quantity of alkali hydroxides and the mobility of the alkali ions in the cement paste. Concrete samples are produced from reactive natural aggregates in the laboratory and crushed at different expansion levels into coarse RCA. At the same time, coarse RCA were reclaimed from distinct members of an ASR affected bridge. Both types of RCA are submitted to accelerated carbonation. Their alkali reactivity is assessed through an autoclave mortar bar test and comparison of expansions with not carbonated RCA and original natural aggregates is performed. Results show that expansions of lab RCA are lower than the ones of original natural reactive aggregates, but higher than the 0.15% limit. In this case, expansions are significantly reduced by carbonation (20 to 50% reduction). On the contrary, expansions of RCA coming from in situ degraded concrete are below the acceptable limit and carbonation doesn't show any effect on the swelling potential.