Characterisation of shrinkage, carbonation and chloride penetration in multi-generation recycled aggregate concrete at equivalent compressive strengths

Abstract

Multi-generation recycled aggregate concrete is a rapidly developing material aiming to apply the principles of the circular economy to concrete by undertaking multiple cycles of use, crushing and reuse as aggregate for recycled aggregate concrete. A major area of concern within MGRAC application is the long-term durability which governs the design life of reinforced concrete structures. Current research on MGRAC durability is limited, and non-existent over extended periods of time. The objective of this study is to experimentally determine how common durability factors are affected by the use of MGRAC with equivalent compressive strengths utilising an extensive array of aggregate blends to link aggregate properties to concrete durability and service life. To do this, the total shrinkage, chloride penetration and carbonation of 45 MGRAC mixes over four recycling generations were investigated for one year. Shrinkage and carbonation were observed under natural exposure conditions and chloride ion penetration determined using the rapid chloride penetration test (RCPT). The experimental carbonation and chloride ion results were extended to predict the design life of structures with differing reinforcement cover depths. Analysis revealed that increasing generations of MGRAC have poorer durability, with concrete porosity governing shrinkage and chloride ion penetration confirming the trends observed in early day experimental results of current literature. Carbonation was found to be largely impacted by the concrete mixes effective water to cement ratio and the combination of inhomogeneous aggregate blends and high shrinkage strains producing micro cracks which increase the rate of transport of CO2 into the concrete matrix. This finding is new to MGRAC research, made possible by the multiple aggregate blend types explored.The predicted design life of MGRAC structures under chloride ion exposure decreased requiring all MGRAC mixes to have a reinforcement cover depth of 70 mm rather than the 45 mm required for VAC. The findings suggest that significant changes to reinforced concrete design cover requirements would need to occur to facilitate MGRAC use in marine environments. This work provides the durability performance for the largest set of mix designs over the longest period of ambient exposure explored in current MGRAC literature making it crucial to informing further research and industry design procedures. • The long-term durability was experimentally assessed over 45 MGRAC mix designs. • We assessed the carbonation, shrinkage and chloride penetration of MGRAC. • Multi-generational recycled aggregate concrete use negatively impacts durability. • Design life was negatively affected by MGRAC use. • This work outlines further research needed to improve material performance.

Topics & Concepts