Assessment of Field Durability of Recycled Concrete under Aggressive Environments

Thesis Abstract

This study addresses the growing need to understand the field performance of recycled concrete (RC) under aggressive environments, where exposure to chlorides, sulfates, freeze–thaw cycles, and alkaline environments can compromise durability and service life. The problem is the limited empirical evidence on how recycled concrete mixtures perform in real-world aggressive settings compared with conventional concrete, given the variability in recycled aggregate quality, cementitious binders, and mix designs. The aim is to quantify the field durability of RC structures subjected to multiple aggressive agents and to identify mix-design and exposure-condition interactions that govern long-term performance. Specific objectives include (i) developing a field monitoring framework for RC elements exposed to saline shoreline, industrial coastal, and de-icing salt environments; (ii) assessing the evolution of mass loss, surface corrosion indicators, and mechanical properties (compressive strength, modulus of elasticity) over a 36-month period; (iii) evaluating chloride and sulfate ingress using nondestructive and destructive tests, including surface resistivity, diffusion-based assays, and X-ray diffraction for pore-structure changes; (iv) determining the role of recycled aggregate content (0%, 25%, 50%), supplementary cementitious materials (fly ash, slag), and curing regimes on durability indicators; and (v) formulating predictive models for service life extension under combined aggressive exposures. The methodology adopts a comparative, longitudinal field study design anchored in durability-oriented performance evaluation. The population comprises RC structural elements in three representative environments coastal marine exposure, industrial inland with sulfate-rich soils, and urban roads subjected to de-icing salts. A stratified random sampling approach yields 60 RC specimens per environment across three mixture families, totaling 180 field-labricated panels, with two replicates per exposure condition for redundancy. Data collection instruments include automated embedded corrosion probes, embedded temperature and humidity loggers, periodic nondestructive evaluation (NDE) tools (half-cell potential mapping, infrared thermography, surface resistivity meters), and destructive sampling at 12, 24, and 36 months for core extraction, mass measurements, density, splitting-tension tests, and microstructural analyses. Laboratory analyses employ regression-based diffusion models to interpret chloride ingress, the Goldberg–Anderson approach for sulfate attack, and porosity characterization via mercury intrusion porosimetry. Statistical analyses will utilize mixed-effects ANOVA to assess the influence of recycled aggregate content, binder type, and curing regime on durability indicators, complemented by multivariate regression and survival analysis to predict service-life outcomes. The study will also apply the Theory of Planned Behavior to interpret contractor and maintenance decisions influencing field exposure conditions, integrating behavioral insights with material performance data. Expected findings include (i) quantifiable detrimental effects of higher recycled aggregate content on chloride diffusion coefficients and surface resistivity under marine exposure, mitigated by supplementary cementitious materials and optimized curing; (ii) evidence of accelerated sulfate ingress and ettringite formation in RC with lower-density aggregates, offset by fly ash partial replacement; (iii) a nonlinear relationship between exposure duration and strength retention, with RC 50% recycled aggregate maintaining comparable modulus of elasticity to conventional concrete when cured under controlled moisture and temperature; (iv) robust predictive models linking mix design variables, exposure type, and service life under combined aggressive environments, achieving R-squared values above 0.75 for durability indices. Contribution to knowledge includes clarifying the performance gap between laboratory-drafted durability predictions and field outcomes for RC in aggressive environments, delivering actionable design guidelines for mix proportions that balance sustainability with durability, and establishing a transferable empirical framework for field monitoring of RC structures. The main conclusion anticipates that recycled concrete can achieve acceptable durability in aggressive environments when optimized with judicious aggregate selection, appropriate supplementary cementitious materials, and rigorous curing practices, enabling lower embodied energy without compromising service life. Recommendations encompass standardized field-monitoring protocols, development of performance-based durability criteria for RC with recycled aggregates, and policy guidance promoting sustainable reuse of aggregates in infrastructure projects.