In Europe, including the Netherlands, a significant Replacement and Renovation (R&R) challenge is expected between 2040 and 2060 for concrete bridges and viaducts due to ageing infrastructure reaching their end of service-life. Various options emerge when existing structures fail to meet current requirements, including accepting additional risk without mitigation measures, making minimal adjustments for a short period, or implementing longer-term modifications. Existing frameworks for assessing R&R decisions, include technical and cost aspects, although lack of sustainability criteria and performance prediction.

The research objective is to develop a decision-making framework for evaluating different strengthening methods versus replacement, considering factors such as (environmental) cost, service-life, structural performance prediction and reliability. The research includes a comparison method on different strengthening methods, external prestressing (EP), memory-steel (MS), carbon fibre reinforced polymer (CFRP) and ultra-high performance concrete (UHPC), versus total replacement. The study mainly focuses on shear deficiencies and strengthening for shear in precast pre-stressed T-beam bridges.

The research starts with a literature review on decision frameworks, structural deterioration mechanisms, T-beam assessment and strengthening methods. The literature review defines and elaborates on key performance indicators relevant to the study. The First Order Reliability Method (FORM) is used to determine failure probabilities by comparing demand versus capacity, providing insights into the current condition and expected lifespan of the structure. A multi-objective optimisation process is introduced to determine the most effective strengthening method based on the desired service-life extension. The goal of this optimisation process is to minimise (environmental) costs while maximising strength, subject to fabrication and physical constraints. A multi-criteria decision-making approach is applied, using the Analytical Hierarchy Process (AHP) to support complex decision-making where multiple variables and criteria must be prioritised. A parametric study is conducted to explore how (geometric) parameters influence decision outcomes.

The parametric framework enables the decision for optimal strengthening to be run multiple times, allowing trends and patterns to appear. Within the analysis, when accounting for varying spans, cross-sections, different states of current reliability and distinct deterioration phenomena for each strengthening method, CFRP consistently proves to be the best-performing and most frequently chosen option. This is due to its high strength-to-weight ratio, which helps minimise material costs and environmental impact. External prestressing excels mainly for larger spans and applying memory-steel is very unfavourable in any case when compared to other strengthening methods. Replacement ranks high in many cases but requires careful consideration, as the design is not fully optimised to each case and impact assessment remains less developed.

For complex geometries, the decision-making framework become less reliable, because strengthening and reliability calculations grow significantly more complex, which is not accounted for. A more integrated approach, considering the interaction between bending and shear, would improve the strengthening designs and could be further refined. Ultimately, while the framework provides a structured approach to decision-making, it should be seen rather as a supporting tool than a stand-alone decision-maker.