Experimental Study on the Flexural Behaviour of Prefabricated Partially Encased Steel–Concrete Composite Beams

To address the issues of poor fire and corrosion resistance in traditional steel structure reinforcement methods, the construction constraints of cast-in-place partially encased steel–concrete composite beams (PEC beams), and their difficulty in application for the rapid retrofitting of existing structures, this paper proposes a novel fully dry-assembled prefabricated partially encased steel–concrete composite beam (PPEC beam). This structure reliably connects prefabricated concrete beams to an H-shaped steel beam using bolts, achieving an efficient, dry, and cast-in-place-free reinforcement method. To investigate its flexural mechanical behaviour and failure mechanism, one cast-in-place PEC control specimen and three PPEC specimens with different connection and assembly configurations were designed. Four-point static bending tests were conducted to systematically analyse the failure modes, load–deflection curves, cross-sectional strain distributions, and ultimate load-bearing capacity. The results indicate that the PPEC beams exhibit comparable flexural capacity to the cast-in-place PEC beam, demonstrating excellent overall composite action. The flange-bolted connection achieves stronger steel–concrete composite action, with elastic stiffness essentially identical to that of the cast-in-place specimen. The web connection provides weaker restraint against local buckling of the steel flange, resulting in slightly lower elastic stiffness. The fully prefabricated assembled specimen shows significantly better plastic deformation capacity than the segmentally assembled specimen. The ultimate loads of all specimens exceed the code-predicted values. The proposed PPEC beam, with its convenient construction and reliable mechanical performance, is suitable for the reinforcement, retrofitting and replacement of simply supported secondary steel beams, working platform beams, and walkway beams.