Structural performance of CFRP composites in UHPC sandwich walls and shock absorbing prosthetic pylons

Abstract

This thesis presents a comprehensive investigation into the performance of novel CFRP composite stirrups embedded in UHPC, with an extended structural application of CFRP composites in a novel shock-absorbing prosthetic pylon (SAP) assembly. The first phase examines the bond–slip and anchorage behaviour of unidirectional CFRP strips, fabricated with and without anchor heads using standard pullout test. A total of 48 specimens were tested to evaluate the influence of fibre type in UHPC (steel and POM fibre) and anchor geometry on bond performance. Anchor-headed specimens developed higher peak pullout loads than straight CFRP strip specimens. The ultimate tensile stress mobilized in anchor-headed CFRP strips ranged from 54–88% of the CFRP tensile strength in POM-fibre UHPC and 64–94% in steel-fibre UHPC, demonstrating superior anchorage efficiency in steel-fibre mixtures. The second phase investigated thermal bond degradation at elevated temperatures (70–200 °C) using steady-state, transient, and residual pullout protocols. Under steady-state exposure, bond strength decreased to 65–84% of ambient capacity, whereas residual tests showed only minor degradation. Modified push-off tests evaluated shear transfer and crack stabilization in UHPC reinforced with straight and oblong CFRP stirrups under concentric and eccentric loading. CFRP stirrups enhanced shear capacity and stabilized crack propagation relative to specimens without stirrups. Steel-fibre UHPC exhibited higher shear strength and reduced crack widths compared to POM-fibre UHPC, while concentric loading resulted in greater stiffness and lower slip. A 3-D finite element model was developed and validated against pullout tests of CFRP strips and shear transfer tests on sandwich walls, successfully reproducing experimental responses. Parametric studies examined UHPC and anchor-headed CFRP connectors, replacing steel connectors and conventional concrete. Based on these findings, a new anchor-headed CFRP shear connector was fabricated for UHPC sandwich panels with 25 mm wythes and a 150 mm XPS core. Compared to 6 mm GFRP grid connectors, CFRP connectors achieved 18% higher moment capacity under four-point bending and reverse cyclic loading. Finally, CFRP composites were used in SAPs, where finite element simulations demonstrated effective energy absorption and adjustable stiffness (56.4–140.2 N/mm), satisfying load demands and potentially enhancing comfort and gait stability for transfemoral amputees.