Comparative Performance of Recycled Concrete Aggregates in Coastal Regions

Thesis Abstract

Coastal regions face accelerated deterioration and higher life-cycle costs for concrete structures due to aggressive marine environments, high chloride ingress, and increased weathering, necessitating sustainable construction materials that combine performance with resource efficiency. The study addresses the comparative performance of recycled concrete aggregates (RCA) against natural aggregates (NA) in coastal applications, focusing on structural and durability domains to inform material selection and design practices. The aim is to evaluate whether RCA can meet or exceed the performance benchmarks of NA under coastal exposure, and to identify the conditions under which RCA delivers optimal life-cycle benefits. Specific objectives are (i) to compare mechanical properties (compressive strength, modulus of elasticity, splitting tensile strength) of concrete samples incorporating RCA and NA across 28-, 56-, and 90-day curing regimes; (ii) to assess durability performance under coastal-like chloride penetration, sulfate attack, and freeze-thaw cycling; (iii) to quantify workability and constructability implications for practical mixes; (iv) to develop predictive models linking RCA replacement ratios to key performance indicators; and (v) to evaluate life-cycle environmental and economic implications of RCA use in coastal structures. A cross-sectional experimental design is employed, incorporating both laboratory and field simulations. The population comprises concrete mixes prepared with RCA sourced from demolished coastal and inland structures and NA controls. A stratified sampling approach yields a total of 120 concrete specimens (60 with RCA varying replacement levels at 0%, 25%, 50%, 75%, and 100%, and 60 NA controls) to capture regional variability in RCA properties. Data collection instruments include standardized compressive and tensile test apparatus, rapid chloride permeability tests (RCPT), accelerated corrosion probes, and nondestructive testing equipment (ultrasonic pulse velocity, rebound hammer). Durability assessments span chloride diffusion coefficients, resistivity measurements, sulfate resistance tests per ASTM C1012/C1012M, and freeze-thaw cycles per ASTM C666. Analytical methods combine descriptive statistics with inferential techniques to establish significance and relationships. Regression analyses quantify the effect of RCA replacement on mechanical and durability outcomes, controlling for aggregate source, replacement level, and curing age. Analysis of Variance (ANOVA) tests detect differences between RCA groups and NA controls across performance metrics. Multivariate models integrate chloride diffusion, porosity, and capillary suction to predict service life under coastal exposure. The study also employs a life-cycle assessment (LCA) framework to compare environmental impacts (embodied energy, CO2 emissions, and waste diversion) and a simple cost-benefit analysis to evaluate economic viability. A theoretical basis integrates put forward expectations from the Theory of Planned Behavior for adoption by practitioners and the Fracture Mechanics perspective for durability phenomena, with the Concrete Durability Index developed to synthesize performance indicators into a single comparative metric. Expected findings indicate that moderate RCA replacement levels (25–50%) can achieve comparable 28- and 56-day compressive strengths to NA mixes, with marginal reductions at 75–100% replacement. Durability indicators are anticipated to show higher chloride diffusion coefficients for high-RCA concretes but remain within acceptable serviceability limits for coastal embankment and non-structural elements when coupled with appropriate supplementary cementitious materials (SCMs) and quality control. RCPT and resistivity results are expected to reveal diffusion patterns influenced by residual porosity and interfacial transition zone characteristics. Predictive models are projected to demonstrate robust relationships between RCA content, curing age, and durability outcomes, enabling design guidance for coastal applications. The LCA is expected to reveal notable environmental advantages for RCA use due to waste recovery and reduced natural aggregate extraction, albeit with a moderated energy footprint at higher RCA contents. Contribution to knowledge includes a systematic, regionally contextualized comparison of RCA and NA performance in coastal climates, development of integrated predictive models for mechanical-durability performance, and a pragmatic framework for decision-making that balances structural adequacy with sustainability. The study concludes that RCA can be recommended for specific coastal contexts—particularly for non-structural components and certain low to moderate replacement levels—when combined with SCMs and rigorous quality control. Recommendations emphasize standardized RCA characterization, mix design optimization, and adoption of performance-based specifications to encourage broader, safer, and more sustainable use of recycled aggregates in coastal infrastructure.