Experimental study on microstructure and structural behaviour of recycled aggregate concrete
Authors:   , Etxeberria Larrañaga Miren 
Published: July 2023

The use of recycled aggregates in concrete opens a whole new range of possibilities in the reuse of materials in the building industry. This could be an important breakthrough for our society in our endeavours towards sustainable development. <br/><br/>The trend of the utilisation of recycled aggregates is the solution to the problem of an excess of waste material, not forgetting the parallel trend of improvement of final product quality. The utilisation of waste construction materials has to be related to the application of quality guarantee systems in order to achieve suitable product properties. Therefore the complete understanding of the characteristics of new material becomes so important in order to point out its real possibilities. <br/><br/>The studies on the use of recycled aggregates have been going on for 50 years. In fact, none of the results showed that recycled aggregates are unsuitable for structural use. Only having inadequate number of studies in durability aspects, made recycled aggregates to be preferred just as stuffing material for road construction. <br/><br/>My thesis, aimed to focus on the possibility of the structural use of recycled aggregate concrete based on a better understanding of its microstructure.<br/><br/>To begin with the characteristics of the aggregates were established, to study their possible application in concrete production. After analysis, the dosage procedure was carried out in order to produce four concrete mixtures using different percentages of recycled coarse aggregates (0% (HC), 25% (HR25), 50% (HR50) and 100% (HR100)) with the same compression strength. Raw coarse aggregates (granite) and sand (crushed limestone) were used in the different concrete mixes.<br/><br/>Macroscopic and microscopic examination were carried out in HC, HR25, HR50 and HR100 concretes in order to observe the durability effects. The macroscopic examination determined the aggregates distribution, composition, the contaminants and aureoles around adhered mortar. Microscopic examination was carried out by Optical light transmitted microscope Leica Leitz DM-RXP, using Fluorescence Thin Sections, in order to analyse the cement paste, the new and old interfacial transition zones, secondary reactions as well as damage. Original aggregates and cement paste, interfacial transition zones and alkali silica gel produced due to reactive aggregates present in adhered mortar were analysed by SEM and EDX-maps. <br/><br/>Beside macro and micro observations, shear failure behaviour of recycled aggregate concretes was studied. Shear failure test was found more appropriate, as concrete properties had more influence in this type of failure behaviour compared to the flexural failure where the reinforcement plays the important role. Sixteen beam specimens were cast and the structural behaviour of these beams was analysed using four different transversal reinforcements for each kind of concrete. An analytical prediction of the experimental results are carried out using a numerical model (Modified Compression Field Theory), using the codes AASHTO LRFD, CSA, Eurocode-2 and expressions proposed in the Spanish code EHE-99.<br/><br/><br/>Organic and inorganic compounds were found to be released from waste materials through leaching and dispersed into the soil and surface water. The leaching of these compounds were measured employing different codes, the two Dutch codes (NEN 7341 and 7345) and the European Normative (EN 12457-2).<br/><br/>Some recommendations are given as to the aggregates characteristics to be used in concrete mixes, taking into account the European standards for recycled aggregates. Also suggestions are made for the production process of concrete using recycled aggregate. Mechanical properties of recycled aggregate concrete are studied and they are compared with that of conventional concrete. Based on the durability of the concrete, some suggestions are proposed with respect to possible alkali silica reaction between new cement and original fine aggregates. It is also determined that the effect of the use of recycled aggregate on the beams' shear strength depend on the percentage of coarse aggregate substituted. The applicability of concrete recycled aggregate with respect to its environmental behaviour is demonstrated. In conclusion, some suggestions for future studies are made which would help us in the evolution of our understanding in this field.