Effect of recycled coarse aggregate on the new interfacial transition zone concrete

Abstract

Due to its high water absorption, recycled aggregate (RA) disrupts the microstructure of concrete, mainly within the interfacial transition zone (ITZ). The objective of the present study is to examine the effect of Recycled Coarse Aggregate (RCA) on the microstructure of the ITZ of three different strength classes of concrete (C25, C35 and C45) with varying w/c ratios. The RA was used in SSD condition in order to check for interactions between it and the new paste, with the parameters for characterization and comparison being the porosity and anhydrous profiles which were evaluated by Scanning Electron Microscope (SEM) observation. The microstructure of the ITZ depends very much on the w/c ratio of the new paste, and despite the reduction in the w/c ratio the ITZ of the RA C25 and C35 concretes exhibit the same tendency, namely a higher porosity profile and lower anhydrous profile compared to the reference concretes (made with New Aggregate - NA). In addition to the “wall effect”, the ITZ of the recycled aggregate concrete is influenced by presaturation water which can be released into the new paste, disrupting its microstructure (the new ITZ and the bulk paste). C45 RA concrete with a w/c ratio of 0.41 has lower porosity and anhydrous profiles than the reference concrete (w/c of 0.43), the water released from the RA participates in the hydration of the cement, which generates less anhydrous and less porous.