Steel-Reinforced Resin (SRR) is a particulate material originally developed as an injectant for anchoring applications. More recently, it has been proposed as a filler material for cavities embedding mechanical connectors in FRP–steel hybrid bridges. In this context, the compressive behaviour of SRR becomes critical due to the multiaxial stress states and fatigue demands at a joint scale. This paper presents a comprehensive experimental and numerical investigation of SRR under monotonic, incremental cyclic, and fatigue compressive loading in unconfined conditions. A custom triaxial setup is also used to evaluate pressure sensitivity and strength enhancement due to confinement under monotonic loading. In parallel, micromechanical finite element models are developed to simulate the interactions between the resin matrix and the steel balls at the microscale, incorporating interface damage, friction, and cohesive failure. The models reproduce the observed nonlinear behaviour and reveal distinct Poisson's ratio responses in tension and compression, offering deeper insight into the mechanisms governing stiffness degradation, strain softening, and plateau behaviour.

The need to renovate many European road bridges due to end-of-life conditions, fatigue, corrosion, and increasing traffic loads drives the demand for innovative solutions. The Dutch Ministry of Infrastructure and Water Management, RWS, estimates that 50 bridges in the Netherlands will require renovation annually over the next three decades. Lightweight, fatigue-resistant fiber-polymer composite panels have emerged as a promising solution for deck replacement in steel bridges. However, the adoption of this technology is hindered by the lack of a reliable connection method. This research, conducted at Delft University of Technology in collaboration with RWS, develops and evaluates the novel injected Steel Reinforced Resin (iSRR) connector to enable the use of glass fibre-polymer composite decks, often termed as GFRP. Initial tests demonstrate that the iSRR connector offers superior static, fatigue, and creep performance compared to existing \gls{gfrp}-steel connection technologies. The main objective of this PhD project is to understand the fatigue behavior of iSRR connectors and provide prediction methods for their performance that can be used for efficient design in engineering practice. The research pursues two specific goals: (1) characterizing the fatigue behavior of iSRR connectors under realistic GFRP panel configurations and load conditions, and (2) evaluating the effects of environmental conditions, such as moisture and temperature, on connector performance. The research involves extensive fatigue testing at both material and connector levels, together with finite element analysis. A specialized test set-up is developed in the Stevin 2 laboratory to apply cyclic shear loads, replicating realistic boundary conditions and load transfer. Temperature and moisture tests are also conducted to assess the impact of environmental conditions on the connectors. At the material level, coupon experiments are carried out to evaluate the SRR properties, and detailed finite element models are developed to better understand the mechanical behavior and failure mechanisms of SRR material and iSRR connectors. This dissertation identifies the fatigue mechanisms that lead to degradation in iSRR connectors for GFRP decks on steel girders. It establishes a fatigue detail that characterizes iSRR connectors, enabling the design of reliable connections between GFRP decks and steel girders. Furthermore, it provides a methodology to describe the fatigue degradation of iSRR connectors using a phenomenological model that builds upon existing Continuum Damage Mechanics (CDM) principles. The findings of this research fill a significant knowledge gap in understanding the behavior of iSRR connectors, offering a practical and sustainable solution for the upcoming bridge renovations in engineering practice. This project bridges theoretical understanding and practical application, aiming to enhance the performance and longevity of GFRP-steel bridge systems under growing traffic and environmental challenges.

The integration of Glass Fibre-Polymer composite (a.k.a. GFRP) deck panels in bridge infrastructure is hindered by lacking a robust connection technology. A promising bolted connection, utilising injected steel reinforced resin (iSRR) material, has demonstrated lower creep deformation and sustained significantly more shear load cycles than conventional bolts. Nonetheless, the production and testing conditions in all prior experimental campaigns followed idealized lab set-ups. This study bridges the gap between laboratory conditions and the challenges arising during connector's fabrication under representative conditions, coupled with cyclic load testing at room and elevated temperatures. The iSRR connectors design is modified and tested in actual composite sandwich web core panels, revealing excellent fatigue performance. The statistical analysis yielded F-N curves for shear performance of the connectors that can be used in the design. The slopes of the F-N curves of − 6.6 and − 5.8 were found at room and elevated temperatures, respectively. Finally, with post-cyclic static tests displaying significant connectors’ residual stiffness, resistance, and ductility, the research provides a step forward in enabling the integration of glass fibre composite deck in infrastructure.

Due to the low weight and excellent durability of composite materials, Fibre Reinforced Polymer (FRP) decks mounted on steel superstructures are becoming increasingly common in engineering practice. Bolted joints are generally used to facilitate connections between an FRP deck and steel girders in road bridges. The connections are subjected to both high magnitude static forces as well as fatigue loading due to overpassing vehicles. With ever increasing traffic on both road and railway bridges, fatigue performance is of critical concern. Bolted FRP joints have been extensively researched in the past under static loading, but less is known about the fatigue and creep behaviour of such joints. Furthermore, little research exists on non-pultruded FRP profiles connected using bolted connections. Therefore, the objective of this research is to investigate connectors’ feasibility by means of static, fatigue and creep experiments on four different types of bolted joints comprising mechanical connectors and injection techniques. The study focuses on application in vacuum infused GFRP panels with integrated webs made of multi-directional laminates) connected to steel bridge superstructures. In addition, experimental results are used to validate Finite Element Analyses (FEA). Based on the obtained results, the novel injected steel-reinforced resin (iSRR) connector shows promising potential in hybrid steel-FRP bridges where good fatigue endurance of the connection, are required.

Resins are essential structural materials predominantly used in load-bearing connections such as adhesively bonded joints and resin-injected bolted connections. In an effort to improve the latter, a new injection material, Steel Reinforced Resin (SRR), has been developed, consisting of spherical steel particles embedded in a resin. SRR's applicability has been explored in injection bolts used in composite steel-to-concrete floor systems and as an injection material in Fibre Reinforced Polymer (FRP) sandwich web-core panels of highway bridges. While the short-term and creep performance of SRR has been researched, its fatigue behaviour remains unexplored. From joint-level cyclic experiments with SRR, it was observed that joint endurance is dominated by SRR's fatigue performance. There is currently no research on the influence of temperature on SRR's mechanical properties and moisture uptake. This paper fills this gap, by investigating the static and fatigue performance of epoxy, unsaturated polyester polyurethane hybrid (UPE + PU), and vinyl ester based SRRs under ambient and elevated temperatures. Water absorption assessments were conducted on the three resins with and without steel particles to evaluate their durability and corrosion resistance. The results show that temperature significantly impacts tensile stiffness, tensile splitting strength, and fatigue life. Epoxy based SRR endured 20 times more tensile splitting stress cycles to failure compared to the UPE + PU resin based SRR at the same stress range. However, after 200 days of water exposure at maximum temperature, the epoxy resin (with and without steel shot) exhibited a weight increase of 4.0% and 0.50% respectively, suggesting that despite its superior fatigue performance, it may not be suitable for SRR applications in high moisture environments. In contrast, UPE + PU resin and its corresponding SRR displayed substantially lower water absorption.

In response to the sustainability requirements set in the EU Commission´s “Green Deal” towards reduction of the greenhouse gas emissions, it is estimated that the structural design for deconstruction is going to contribute considerably to the sustainable development of the built environment. The demountability of multi-material structural systems basically depends on the shear connectors used in the structural system. This paper focuses on a type of demountable injected shear connector with an injected steel-reinforced resin (iSRR) which consists of spherical steel particles embedded in a resin. Its application to steel-to-concrete and steel-to-Fiber Reinforced Polymer (FRP) decks is presented along with its benefits. In parallel, an overview of the experimental and numerical research on the evaluation of the mechanical properties of the demountable bolted connectors with iSRR is discussed. Last, detailed finite element (FE) models and a parametric study are performed to quantify the confinement level of the SRR material influenced by the oversized hole diameter.

Challenges of reliable prediction of resistance of bonded joints lie in the complexity of stress concentrations, multiple crack paths, mode-mixity and sudden crack propagation. The new Technical Specification for Design of fibre-polymer composite structures (TS) is bridging this gap by providing comprehensive sets of design recommendations and analyses to be used for the verification of adhesive joints. Two design approaches in the TS, based on stress analysis and fracture mechanics, are summarized and basic design assumptions on allowed failure conditions and partial factors in relation to execution and maintenance are highlighted. The application of the design recommendations is shown on the example of determining the design value and the ultimate resistance of a simple double lap joint. FE models built in Abaqus are used to analyse stress concentrations and crack initiation and propagation based on material level experiments. Variability of the material properties are taken into account through partial safety factors. Joint resistances obtained by the stress-based approach and fracture-mechanics approach are compared.