Experimental investigation of concrete mixtures incorporating recycled concrete aggregates

Abstract

In the Netherlands, yearly 33 Mton new concrete (roughly 15 billion m3) is producewhile 12 Mton concrete waste becomes available. Recycling of the yearly 12 Mton concrete waste results in roughly 7 Mton coarse recycled concrete aggregate (CRCA) and 5 Mton fine recycled concrete aggregates (fRCA). Currently, fRCA is not used in structural concrete, but only for low grade applications. This research aims to investigate the effect of recycled concrete aggregates (RCA) incorporation on the performance of structural concrete mixtures. Three streams of fRCA were considered in this research and two deliveries of each source were examined. River sand was replaced by 25% fRCA, which was found to be the optimal amount that does not require cement increase. Both deliveries were used in order to identify potential differences. Additional concrete mixtures with simultaneous replacement of river sand by 25% fRCA and river gravel by 30% CRCA were evaluated. Parallel work was conducted during an 8months internship at TNO, where optimization of mortar mixtures with full replacement of river sand by fRCA was investigated. This was performed by considering the water demand of solid constituents (binder and fRCA) and packing of fRCA. Optical microscopy was used for closer inspection of the microstructure of fRCA. Fresh properties of studied mixtures were measured. Assessment of the mechanical performance of concrete mixtures was expressed by means of compressive strength and elastic modulus. Long term behavior was also investigated by shrinkage deformation measurements up to 95 days after casting. Results show that substantial rise in cement content is necessary for obtaining the desirable flow and high levels of compressive strength, when full replacement of fRCA is performed. Regarding concrete mixtures with 25% fRCA, increase in air content was measured. Entrapped air is strongly dependent on the porosity of adhered cement paste on RCA, content of 00.25 mm fraction as well as agglomeration and presence of impurities in as received fRCA. This affects negatively their mechanical performance. Additional porosity resulting from CRCA incorporation was responsible for further reduction in mechanical properties of concrete mixtures. Mixtures with RCA showed three times higher mass loss at the age of 95 days.