Ultimate capacity of acid-exposed steel–concrete–steel deep beams with UHPC considering interfacial slip: Experiments and modeling

Steel–concrete–steel (SCS) deep beams rely on effective load transfer between external steel plates and the concrete core to develop composite resistance. In acidic environments, deterioration of the concrete matrix and steel–concrete interface may weaken this interaction, resulting in reduced load-carrying capacity and altered failure behavior. This study investigates the structural behavior of acid-exposed SCS deep beams filled with ordinary concrete or ultra-high-performance concrete (UHPC). Static loading tests were conducted to examine load–displacement response, cracking behavior, failure mode, bottom steel plate strain, and interfacial slip. The results show that acid exposure markedly reduced the load-carrying capacity and ductility of ordinary concrete-filled beams and promoted interfacial slippage, whereas UHPC-filled beams maintained better interface integrity, exhibited higher ultimate load and ductility, and failed predominantly in flexure. Finite element models incorporating material degradation were established and validated against the test results, and were further used to reveal stress redistribution, plastic strain development, and damage evolution in the composite system. Based on the experimental and numerical results, simplified prediction models for shear and flexural capacities were developed by introducing a slip coefficient to quantify the degradation of interfacial interaction under acid exposure. The proposed approach extends conventional capacity prediction methods for SCS deep beams by considering acid-induced interface degradation and provides a practical basis for assessing UHPC-filled SCS members in chemically aggressive environments.