Orthotropic steel decks (OSDs) offer structural advantages but are highly sensitive to fatigue cracking, mainly initiated from trough-to-deck plate welds at the intersection between the trough and crossbeam. To mitigate existing fatigue cracks, a high strength concrete (HSC) overlay repair method was developed, which addresses two types of fatigue cracks but does not completely prevent root-weld crack propagation. To avoid damaging the HSC overlay by applying heating in traditional repair methods, the ‘Cold Repair’ method was developed, using a steel angle adhered to the trough-to-deck plate exterior. Recent studies suggest increased effectiveness with a carbon fiber-reinforced polymer (CFRP) angle, created by vacuum infusing carbon fiber fabrics with epoxy reason onto the OSD. However, limited knowledge exists on he use of fiber-reinforced polymer (FRP) to strengthen OSDs and control root crack growth in these weld details. This presents challenges in understanding its behaviour, material properties and design.

This study investigates CFRP-steel adhesive joints through experimental and numerical analyses. Thick-adherend shear tests (TASTs) were conducted to study the adhesive bond’s shear strength, and the failure mechanisms of these adhesive joints subjected to shear loading. Component-level three-point bending tests evaluated bond behaviour under bending loads and the structural performance of OSD strip components strengthened with a CFRP angle. Finite element (FE) models were developed to simulate the adhesive FRP-steel joint, employing both linear tied and non-linear tie-break interface conditions. Experimental and numerical results were compared to assess the FE models’ accuracy.

A comparative analysis was also performed between a full-bridge model, provided by supervising company Arup, and the component-level model developed in this thesis. Significant differences in boundary conditions, loading conditions, element formulation, and the scaling of global dimensions (apart from thicknesses) complicated precise comparisons. The component-level model used tie-break interface conditions to model adhesive interface failure, whereas the full-bridge model employed tied interface conditions, which limited its ability to predict failure.

TASTs results identified debonding between steel and primer -applied to enhance the adherend-adhesive bond- as the primary failure mode. In four samples, this debonding occurred alongside delamination of the first glass-fiber layer. The design value of the average shear bond strength was determined to be 5.42 kN. Component-level three-point bending tests consistently showed crack initiation at the outer edge of the horizontal adhesive bond at a load level of around 82 kN. Crack initiation was followed by linear behaviour up to the onset of yielding of the deck plate, after which full debonding of the horizontal leg of the Cold Repair occurred at an average load of 110 kN.

Numerical studies revealed that linear numerical modelling provides a sufficient approach to model the Cold Repair method up to the point of failure. Developed non-linear numerical models do not contribute to additional reliability of the Cold Repair method, as they were unable to accurately match observed failure behaviour and because adhesive bond failure occurs suddenly. Nonetheless, the additional capacity observed in component strips suggests that non-linear numerical modelling could extend the capacity of the Cold Repair method if its design allows for damage. To improve the accuracy of adhesive bond failure modelling, further experiments, including fracture mechanics tests, are recommended to determine essential adhesive properties, such as fracture toughness.