Multi-Hazard Performance and Failure Mechanisms of Repair Techniques for Full-Diameter Damaged Agricultural Steel Pipelines

Although steel pipelines constitute the primary infrastructure of agricultural irrigation systems, they are highly susceptible to moisture-induced pitting corrosion and severe operational conditions, including internal pressure fluctuations and heavy overburden loads. This study evaluated the structural performance and durability of full-diameter steel pipe specimens (311.5 mm in diameter and 2.0 m long) repaired using CFRP (single-layer) and GFRP (single- and multi-layer) sheet wrapping as well as overlay welding. A 60-mm pinhole defect corresponding to a 6% circumferential damage ratio was introduced to simulate advanced localized corrosion. The repaired pipelines were experimentally assessed under four-point bending, internal hydrostatic pressure, and accelerated salt spray exposure. Under service-level flexural loading, all specimens exhibited similar global load–deflection responses regardless of defect or repair condition. However, localized strain measurements revealed that the unrepaired defect produced tensile strains up to 12 times greater than those of the intact pipe, whereas all repair techniques effectively suppressed the localized strain concentration. The effectiveness of the FRP systems improved with increasing reinforcement thickness. Overlay welding provided the highest structural performance, restoring localized strain behavior to a level comparable to that of the intact pipe. Under an internal pressure of 2.0 MPa, welding and CFRP maintained 100% pressure retention, whereas single-layer GFRP exhibited minor radial bulging, reducing its pressure retention ratio to 73.5%. Increasing the GFRP thickness restored the retention ratio to 93.0%. Accelerated salt spray exposure further demonstrated that GFRP provided effective barrier protection against corrosion by acting as an impermeable dielectric barrier under short-term exposure, whereas welded specimens still exhibited localized corrosion around the heat-affected zone despite epoxy coating. These findings demonstrate that overlay welding offers the greatest immediate structural restoration, whereas adequately dimensioned FRP systems can provide a more balanced solution for multi-hazard durability by simultaneously enhancing structural performance and mitigating electrochemical degradation in aging agricultural steel pipelines.