Improvement of the mechanical and durability performance of recycled aggregate concrete

Many countries around the globe are promoting the utilization of recycled concrete aggregates (RCA) in new concrete. However, the structural use of recycled aggregate concrete (RAC) is still limited owing to its inferior mechanical and durability performance. The focus of this research study is to improve the mechanical, durability and microstructural properties of RAC using different techniques. The main parameters considered in the study are the replacement ratios of RCA, the use of fibres (fibre types and dosages of fibre) and treatment techniques of RCA (acid immersion, acid immersion with rubbing, accelerated carbonation, acid immersion with accelerated carbonation and lime immersion with carbonation). The influence of the above parameters on the failure mode, compressive strength, stress-strain behaviour, tensile strength, flexural strength, elastic modulus, fracture energy, toughness and other durability properties like water absorption, carbonation, freeze-thaw and chloride penetration resistance of the RAC is studied during this research. Furthermore, microstructural characteristics of normal aggregate concrete (NAC) and RAC are also investigated using scanning electron microscopy (SEM). Results of post-peak performance and mechanical properties of plain and macro-synthetic fibre reinforced RAC show a reduction in mechanical properties of concrete with the rise in RCA content. However, an increase in mechanical properties particularly split tensile strength of NAC and RAC is observed with the increase in dosage of macro-synthetic fibres. Based on the experimental results and a test database from literature, a stress-strain model is developed in this study by amending the parameters of the top performing stress-strain model for NAC reinforced with steel fibres. The proposed model can effectively predict the stress-strain behaviour of both macro-synthetic and steel fibre-reinforced NAC and RAC. A positive effect of RCA treatment is also observed on the mechanical and stress-strain behaviour of RAC. The stress-strain behaviour of RAC having RCA treated through acetic acid immersion with mechanical rubbing and lime immersion with carbonation is observed very close to the stress-strain curves of NAC reflecting the positive impact of these RCA treatment techniques. Using the test results from this study and other data collected from the literature, empirical relations are developed between different mechanical properties of RAC incorporating treated and untreated RCA. Furthermore, a stress-strain model for NAC and RAC incorporating treated and untreated RCA is also developed in this study which may lead towards the development of design guidelines for the sustainable concrete structures. Improved resistance against water absorption, chloride penetration, carbonation, acid attack, freeze-thaw action and sulfate exposure is also observed for RAC having treated RCA than RAC having untreated RCA. Chloride migration coefficient values of RAC having RCA treated through immersion in acetic acid with mechanical rubbing and lime immersion with accelerated carbonation are observed very close to that for NAC. Moreover, SEM and mercury intrusion porosimetry results also show the enhanced microstructure, dense interfacial transition zone (ITZ) and improved porosity for RAC having treated RCA than RAC having untreated RCA, justifying the positive effect of RCA treatment on the durability of RAC. Based on the results, lime immersion with accelerated carbonation and immersion in acetic acid with rubbing techniques can be used to enhance the performance of RAC resulting in durable and cleaner construction. Regression analysis between properties of aggregates and concrete shows that the porosity of coarse aggregates can be used to predict the mechanical and durability performance of concrete effectively, which may be an optimistic step towards the durable and sustainable design of concrete structures.