Dual-phase tensile-sensitive degradation modelling of lagoon-exposed waterproofed concrete for long-term durability assessment

Concrete structures exposed to lagoon environments are subjected to sulphate- and chloride-rich conditions that accelerate deterioration and complicate conventional service-life assessment. This study evaluates the degradation behaviour of C30 concrete incorporating crystalline and hydrophobic admixtures under controlled lagoon-water exposure representative of coastal conditions. To support comparative durability assessment, an exploratory dual-phase modelling framework was adopted, integrating mechanical performance indicators with phenomenological representations of hydration and deterioration behaviour. Power law and logarithmic models were applied to describe early-age strength development, showing consistent trends under freshwater conditions but reduced correlation under lagoon exposure. At later ages, where growth-based models increasingly deviated from observed behaviour, an exponential decay formulation was used to represent deterioration trends. Results from 220-day exposure indicate that split tensile strength is more responsive to early-stage degradation effects than compressive strength, which exhibited greater stability. The crystalline-modified mix (C + AQ) demonstrated improved durability, as reflected by a higher Relative Durability Index and a lower apparent decay rate compared to the control mix. Microstructural observations were consistent with reduced pore connectivity and improved matrix densification in modified systems, although these observations are interpreted qualitatively. Overall, the results suggest that tensile-based indicators, combined with dual-phase trend modelling, provide a useful framework for the comparative evaluation of concrete durability under aggressive lagoon exposure. The 30-year projections presented in this study represent hypothetical comparative scenarios derived from short-term experimental observations and should not be interpreted as validated predictions of actual field performance or service life.