Effect of varying w/c ratios and environmental conditions on interfacial transition zone evolution in roller compacted concrete pavement mixtures due to internal curing

Growing adoption of roller-compacted concrete (RCC) pavements has intensified the need to understand the interfacial transition zone (ITZ), which critically governs mechanical and durability performance in these low-paste, compaction-controlled systems. Unlike conventional concrete, RCC develops under restricted lubrication and limited early-age moisture, producing a hydration-limited ITZ highly sensitive to water–cement (w/c) ratio and environmental exposure. However, existing interpretations largely rely on conventional concrete concepts that overlook these constraints. This study examined the influence of internal curing using pre-soaked expanded clay aggregates (20% replacement of fine aggregates) on the microstructural, micromechanical, and transport characteristics of the ITZ in RCC. Mixtures with w/c ratios of 0.30, 0.35, and 0.40 were evaluated under tropical humid (RH 95%, 24 °C) and semi-arid (RH 50%, 36 °C) conditions. SEM–BSE, SEM–EDS, nanoindentation, and four-dimensional X-ray microscopy quantified ITZ porosity, thickness, hydration chemistry, elastic modulus, pore connectivity, and permeability. Higher w/c increased ITZ porosity (up to 19.46%) and thickness (up to 68.37%) due to wall-effect-dominated packing, while lower w/c produced dense but hydration-restricted ITZs. Internal curing reduced porosity and thickness, lowered Ca/Si ratios, enhanced high-density C–S–H, improved elastic modulus (9.4–13.2%), and increased compressive and flexural strengths by 24.6% and 18.6%, respectively.