Early-strength behaviour and microstructural evolution of calcium sulfoaluminate cement concrete under vehicle–bridge coupled vibration

To meet the demand for ultra-early-strength and vibration-resistant concrete in bridge widening projects driven by the rapid expansion of road networks in China, calcium sulfoaluminate (CSA) cement-based concrete has been proposed as a promising solution for early-age strength development. However, under uninterrupted traffic conditions, the early-age response of CSA-based concrete subjected to vehicle–bridge coupled vibration—particularly the coupled relationship among vibration intensity, pore structure evolution, and mechanical performance—has not yet been sufficiently understood. In this study, the effects of vehicle–bridge coupled vibration on the early-age mechanical behavior and microstructural evolution of CSA cement-based concrete were systematically investigated. The results show that a composite CSA–OPC binder system with a mass ratio of 7:3 was optimized based on early-age strength development and strength stability, achieving a compressive strength of 44.5 MPa within 4 h. In contrast, silica fume in this system primarily acted as a lubricating filler, which compromised early-age strength development. Mechanical test results indicate that low-intensity vibration (2 Hz, 2–6 mm) promotes pore refinement and interfacial densification, resulting in an approximately 5% increase in compressive strength compared with static conditions. Conversely, high-intensity vibration (6 Hz, 4–10 mm) induces pore coarsening and vertical aggregate segregation, leading to a significant reduction in flexural strength of up to 24.75%. Quantitative X-CT analysis further reveals that porosity, expressed as volume fraction, increases from 0.0358 to 0.0478 under high-intensity vibration. Multi-scale characterization using SEM, XRD, and X-CT demonstrates that although vibration does not alter the types of hydration products—with ettringite remaining the dominant crystalline phase—it significantly disrupts the spatial distribution and ordering of C–S–H and AH₃ gels. This disruption weakens their pore-filling capability and exacerbates interfacial defects. Overall, this study elucidates the coupled effects of vibration intensity, pore structure evolution, and mechanical performance, and provides quantitative mechanistic insight and an evidence-based framework for managing early-age vibration exposure of CSA concrete under dynamic traffic loading. • Ultra-early strength CSA concrete achieved a compressive strength of 44.5 MPa at 4 h. • Vibration intensity thresholds governing early-age strength degradation were identified. • X-CT revealed vibration-driven pore refinement and coarsening mechanisms.