The world is facing the challenge of climate change, a problem acknowledged by many governments in the Paris Agreement. All sectors, including the construction industry, must take responsibility for reducing their environmental impact. Using renewable bio-based materials, such as flax fibre composites, offers a promising solution towards reducing the construction sector's environmental footprint. Flax fibre-reinforced polymer composites, which consist of flax fibres embedded in a polymer resin, have gained attention for their potential application in structural elements. However, the lack of standardized long-term performance data, for example, its creep behaviour, limits its broader use in structural applications.

Creep is the gradual deformation of materials under a constant load, which can also affect their strength over extended periods of time. Conventional creep tests are often considered impractical as they are time-consuming and costly. Accelerated creep tests, such as the stepped isostress method (SSM), can offer a faster alternative for predicting long-term creep behaviour. The SSM predicts the long-term creep behaviour of a material by incrementally increasing the applied stress while maintaining constant environmental conditions. This method requires only a single test sample and approximately one day of testing to generate a prediction of the long-term creep master curve for a specific reference stress. However, the SSM is an empirical method that has not yet been standardized in any design codes. This research assesses the reliability of the SSM for flax fibre-reinforced polymer composites through an experimental study. Aiming to support the development of sustainable construction practices and encourage using renewable materials in the building industry.

To validate the SSM predictions, long-term creep tests are conducted using a custom-designed tensile creep test setup, supported by quasi-static tensile tests. Additionally, three different composite laminate configurations are compared, demonstrating the fibres' function as the main load-bearing component in the composite. One of the main findings of this research is the material's sensitivity to temperature. Although the SSM predictions produce creep master curves, these predictions are conservative compared to the long-term creep test results. Moreover, the data handling procedure of the SSM, particularly the horizontal shift, is extremely sensitive and partially subjective. Based on the findings, the current form of the SSM is deemed insufficiently reliable to replace conventional creep tests for structural material characterization. However, its potential as a highly time-efficient testing method should not be overlooked. With further refinement in both the experimental procedure and data processing procedure, SSM could become a more reliable approach, making further research on standardizing its methodology essential.

This research urges the need for further research to improve the reliability of the SSM and to standardize its data processing steps. An essential step in this development is understanding the relationship between the activation volume and the applied stress, which may be material-specific. Clarifying this relationship could help reduce the sensitivity and subjectivity of the horizontal shifting process in the SSM data analysis. Additionally, this research aims to inspire continued investigation into bio-based construction materials, contributing to a more sustainable building industry.

The last decades Fiber Reinforced Polymer (FRP) structures start to find their own place in the field of civil engineering applications and quite lately in bridge engineering. Composites are considered highly versatile materials that offer great tailoring on their design. Synthetic fibers and resins are already quite popular especially in applications where low weight, construction speed and high performance are the objectives. However, environmental concerns over the disposability of non-recycle composites is driving the engineering world towards the use of natural composites. Certain natural fibers appear to be advantageous not only in terms of mechanical properties but also in terms of sustainability compared to synthetic ones. However, environmental performance of bio materials is difficult to assess and predict due to their large natural variability. Durability assessment through literature review and optimization of the superstructure of an FRP bridge, accounting for failure modes mostly deriving from high concentrated vertical loading (wheel loading), are the main objectives of this report. In order to deal with the large number of variables and constraints for the design according to existing structural standards, advanced optimization algorithms are employed. In addition, due to lack of analytical formulas for local buckling resistance prediction, Finite Element (FE) analysis is used since it offers a better insight into buckling failure of composite laminates. Finally, an optimization workflow is developed that accounts for all variables and constraints and minimizes the objective, which in this case is the weight of the superstructure. The obtained results form recommendations for the optimum design of a flax and glass FRP bridge, regarding the thicknesses and orthotropy of the cross section’s laminates with L/300 and L/500 maximum deflection requirement, providing a deck slenderness L/16 and a spacing of the web sw = hSP /3. The assessment and the optimization reveals that flax or BioMid FRP could be promising candidates in replacing synthetic fibers for a bridge application as they can compete in performance and weight the synthetic glass FRP for the case of single span bridge and a span range of 10 - 30m.