Durability-related properties

Abstract

This chapter presents the work carried out to study the influence of recycled concrete aggregates on concrete durability properties. Numerous previous studies have highlighted that recycled concrete aggregates are more porous than natural aggregates (NAs) and can lead to a decrease in the durability properties. The durability properties investigated by the French national project RECYBETON and the ANR project ECOREB are the properties related to the corrosion risks of the reinforcement (carbonation, chloride migration, gas permeability, and porosity), the resistance to freeze/thaw cycles, the risks linked to alkali–silica reactions (ASRs), and the presence of sulfates (ettringite/thaumasite formation). The results obtained show that the porosity accessible to water is a durability indicator not relevant in itself to sufficiently predict the risks of corrosion. It is more rigorous to consider at least the chloride diffusion coefficient and the resistance of concrete against carbonation penetration. The recycled aggregates (RAs) predictably decrease the performances of the concrete relating to the transfer properties with an intensity which depends on the intrinsic characteristics of RAs (porosity), the substitution rate, and the compactness of the cement matrix of the new concrete. By optimizing concrete mix (reduction of the W/B ratio in particular), it is easy to produce concretes that are as resistant as concretes only constituted by NAs. The frost resistance of RA depends on the characteristics of the original concrete. RA concrete (RAC) can be resistant to freeze/thaw cycles with or without deicing the salts depending on the frost resistance of RA and the application of the rules of formulation (binder content, W/B ratio, entrained air content and so forth). Concerning the risks of ASRs, RA can release significant amount of water-soluble alkalis in particular for the fine aggregates that contain an important fraction of adherent cement paste (ACP). RA can also contain unstable silica phases provided by some specific NA and possible pollutants (broken tiles). The studies also show that the actual recommendations can be used. Meanwhile, the main tests currently used for NA have to be adapted (higher water absorption of RA), and nevertheless, some of them (microbar test) are not adapted to RA. The studies on ettringite and thaumasite formation due to the presence of sulfates lead to recommendations in order to avoid the risk of disorders: 0.3% and 0.2% for the maximum water-soluble sulfate content, in RA and total aggregate, respectively.