Prototype test study on external pressure-bearing capacity of large-diameter steel-reinforced concrete drainage pipe

The reinforced concrete drainage pipe with a steel cylinder features a novel structure, yet its load-bearing characteristics remain unclear. To promote its application and ensure safety during the construction and operational phases, this study investigates its performance.

To achieve these objectives, a full-scale experimental investigation was conceived and executed. The study was strategically located within the context of a major municipal drainage infrastructure project in the Xiong'an New Area, thereby ensuring the direct relevance of the research to real-world engineering applications. A meticulously designed external pressure prototype test was used as the core methodology. This test was engineered to simulate realistic burial conditions and loading scenarios. By applying incrementally increasing external pressure to a representative large-diameter pipe specimen and employing a comprehensive array of strain gauges and displacement sensors, the research aimed to capture and quantify the intricate structural strain response. The experimental setup allowed for detailed monitoring of stress redistribution, compatibility of deformation between the steel cylinder, reinforcement, and concrete, and the progression of damage from initial elastic behavior to ultimate failure.

The experimental findings yielded a detailed and multistage characterization of the structural performance of the pipe. First, a consistent positive correlation was observed between the applied external pressure and the induced strain across all monitored structural components throughout the testing sequence. Notably, the deformation patterns of the inner and outer layers of reinforcement exhibited substantial compatibility with the surrounding concrete in their respective layers during the initial loading phases, indicating effective composite action. The strain distribution under load revealed distinct stress states, with the steel cylinder demonstrating tensile stresses at its crown, invert, and springline. The inner concrete layer and its associated reinforcement were subjected to tension at the crown and invert but transitioned to compression at the mid-section (springline). Conversely, the outer concrete and reinforcement exhibited compression at the crown and invert and tension at the springline. The structural stiffness of the pipe was not constant but evolved with the applied load. Initially, under external pressures below 131 kN, the pipe operated within the elastic stage. Strain increased linearly across all sections, with the inner and outer concrete shells functioning as the primary load-bearing components and the steel and reinforcement elements serving a secondary role. As the load increased from 131 to 276 kN, the onset of visible microcracking on the concrete surfaces indicated the transition into the plastic stage. During this phase, the internal stress was considerably redistributed. The uncracked concrete, in conjunction with the reinforcement network that had become more actively engaged, became the principal elements resisting the applied load, thereby demonstrating the ductility of the structure. Upon exceeding 276 kN, the pipe progressed into the cracking and failure stage. The concrete in the tensile zones effectively ceased to contribute to load-bearing capacity owing to extensive cracking. At this final stage, the structural integrity predominantly relied on the integrated system comprising the steel cylinder, reinforcement cages, and confined concrete matrix between them. Together, these elements formed a final load-carrying mechanism until ultimate collapse.

This study successfully designed and implemented a definitive external pressure prototype testing method for a novel class of large-diameter reinforced concrete drainage pipes with an integrated steel cylinder. The study has systematically elucidated the complete spectrum of their external load-bearing characteristics from the initial elastic response through plastic deformation and culminating in failure. The results provide critical insights into the complex interaction among the constituent materials—concrete, steel, and cylinder—under load. By mapping the strain distributions, identifying key load thresholds for stage transitions (131 kN for the elastic limit and 276 kN for severe cracking) and clarifying the shifting roles of different components during each phase, this study delivers robust technical support for the large-scale and confidence-driven application of this pipe technology. Furthermore, the findings provide scientifically grounded reference guidelines and valuable empirical data for the future structural optimization design, performance-based specification, and safe engineering implementation of large-diameter reinforced steel-cased concrete drainage pipes in critical infrastructure projects.