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.

The construction industry faces significant challenges in managing the insulation material waste, expanded polystyrene (EPS) and extruded polystyrene (XPS) due to their low density, contamination
issues, and the presence of hazardous flame retardants. This study investigates the feasibility and environmental impact of different end of life methods for EPS and XPS waste contaminated with various construction materials, including glue and mortar as well as bitumen. A practical approach is taken, aiming to tailor existing recycling technologies to the specific needs and constraints of construction practices, thereby increasing the industry’s recycling rate and reducing its environmental footprint.
This work explores the entire lifecycle of EPS and XPS, from production and application to waste generation and recycling, and proposes solutions. By focusing on practical solutions and industry-specific needs, this work facilitates a shift towards a more circular and sustainable model for EPS and XPS waste management in the construction industry, offering actionable recommendations for construction companies and waste managing companies to adopt more environmentally responsible practices.

The construction industry is a major contributor to waste generation, with construction and demolition waste (CDW) constituting a significant portion of it. This research is centered on developing a technology to separate and recycle two types of plastics, EPS and PU, from cement matrices, along with recycling the cement itself. The goal is to recover high-quality materials for reuse within the construction sector, thereby promoting sustainable practices and enhancing the circularity of construction materials.
The research involved two classes of samples: samples containing cement and PU foam (Class
A) and samples containing cement and EPS foam beads (Class B). Different separation methodologies were employed for each class.
Class A samples underwent mechanical crushing and sieving, followed by electrostatic separation using a vertical triboelectrostatic separator.
Class B samples were successfully separated using mechanical milling and sieving.
The separated PU foam, due to a reduction in particle size during separation, could not be directly reused as insulation. Therefore, it was analyzed using Fourier-transform infrared spectroscopy (FTIR) to assess its quality for recycling into new polyurethane foam products. The FTIR results confirmed the effectiveness of the separation process, yielding clean polymer suitable for reintroduction into the market as a secondary raw material, particularly for particles smaller than 500 μm.
The separated EPS foam beads were reused to cast new insulating cement, and its thermal conductivity was tested. The results demonstrated that the separation process did not compromise the insulation properties of the EPS beads, allowing for their reuse in their original application.
The recycling potential of the separated cement powder was also analyzed. Chemical composition analysis, particle size distribution, and loss on ignition tests indicated its suitability for reuse as a partial cement replacement in new mortar samples. Mechanical tests on mortar samples with varying percentages of recycled cement powder (15% and 30%) and different reactivation temperatures (650°C and 750°C) showed a slight decrease in compressive and flexural strength compared to the reference mortar, particularly for the 650°C reactivation temperature. However, the recycled cement powder reactivated at 750°C and used at a 15% replacement ratio resulted in a 2.75% increase in compressive strength, highlighting the potential for optimizing the recycling process to achieve comparable or even enhanced performance in construction applications.

Through improved plastic waste separation EU recycling goals can be reached and environmental economic advantages can be unlocked. To help with this endeavour, this research explores dynamic separation efficiency determination and waste stream characterization through near infrared (NIR) separation unit and belt weigher data in a plastic waste sorting plant in Scandinavia. For the showcasing of these concepts, the goal was to predict the product quality of the high-quality (HQ) agglomeration line, using data of the first NIR-scanner in the agglomeration line as prediction input. In the agglomeration line two NIR-scanners are connected in series to ensure high-quality separation of the material. Through the NIR-scanners, material specific area flow data is available and through the belt weighers mass flow input to each NIR-scanner is provided. Quality criteria are weight shares of PO (target material) and PVC (main contaminant). Difficulties arose, as the material-specific mass flow is needed for quality determination but only the total mass flow is provided. This was addressed by modelling area densities using a linear regression model, with belt weigher and NIR-scanner data as input. Using the calculated area densities, the material-specific mass flow was determined. For validation, summed material flows were compared with belt weigher data, yielding a mean absolute error (MAE) of 141 [kg/h] and a mean relative error (MRE) of 3%. The separation efficiency was determined through an XGBoost model, to predict material-specific area flow of the second NIR-scanner. Results were a MAE of 50.02 [m²/h] and an MRE of 1.1% for the total area flow. The final separation step could not be validated, as no NIR-analyser is present behind the second NIR-scanner. Therefore, separation efficiencies from the previous separator were transferred. Joining all three concepts the weight share of PO and PVC could be predicted with a MAE of 0.36% and 0.007%. For the joint outcome, greater uncertainty contribution was ascertained for the area densities compared to the XGBoost application. Future research is recommended for separation efficiency determination of the last separation step and for improved modelling of the area densities.

Nowadays environmental issues like the depletion of the natural resources and the pollution of the environment have led many researchers to make new projects about the reuse of waste from construction and demolition again in concrete industry. Among available solutions are the partial replacement of cement with recycled ultrafine particles (RUP) and the reuse of recycled sand (RS) as substitute of natural sand in mortar/concrete mixtures. The aim of the present research is to investigate if the new mortar with recycled ultrafine particles can substitute the reference mortar made of virgin materials and to investigate the possible accompanying ecological and economic benefits. This will be demonstrated in terms of technical feasibility, ecology and economy of the new mortars. For the investigation of the main target of this MSc thesis, it was important three different experiments to be conducted. The 1st and 2nd experiment included the partial substitution of cement with RUP in 5%,10% and 15% respectively. The only difference was the size of RUP. In the 1st experiment were used only RUP below 125 microns and in the 2nd experiment only milled RUP below 45 microns. The 3rd experiment was more complicated with the simultaneous presence of RUP below 125 microns and the total replacement of natural sand with recycled sand. The w/b ratio was constant for the first two experiments at 0,5, but the w/c was increased as the replacement ratio of RUP also increased for all the experiments. The results were excellent about the fresh workability of the mortars in all the experiments with higher values of slump in contrast to the reference one from virgin materials. But there were some problems about the mechanical properties and especially the compressive strengths of the new mortars. The usage of smaller RUP below 45 microns improve the mechanical properties of the mortars with the highlight of the higher values for all the mortars in the test of flexural strength in the measurement after 14 days. Generally, it seems that as the replacement ratio of RUP was increased, the compressive strength of the mortars was decreased. For the 3rd experiment, the results regarding the mechanical properties were not very satisfactory despite the compensation with extra water. The quantity of this water has a crucial role for future researches and additionally lower replacement ratio than 100% is suggested for recycled sand.The next step was to reveal the possible ecological advantages from usage of this new mortar with recycled cement paste. For this investigation, an ecological analysis was conducted. This ecological analysis included in the first part a simulation in the lab experiments and in the second part a simulation in a conceptual recycling plant. The results are pleasant for the both cases regarding the GHG emissions and especially CO2. Initially, in the lab experiments, 5 % substitution with RUP instead of cement particles leads to 5,1% reduction of CO2 emissions, 10% substitution with RUP leads to 10,2% reduction of CO2 emissions and 15 % substitution with RUP leads to 14,8% reduction of CO2 emissions. Moreover, for the simulation of a conceptual recycling plant, the reuse of RUP in the mortar/concrete industry can lead to significant reduction of CO2 and less air pollution. Especially for the scenario in this MSc thesis, there is a potential environmental gain with total CO2 offset of 70,75 ton/day from this conceptual recycling plant. Up-to-day, the calcination of limestone to produce cement contributes to the emission of carbon dioxide. But the usage of recycled cement paste can reduce this bad consequence. The last step the investigation for the potential economic benefits from the reuse of the recycled materials and especially RUP in mortar/concrete industry. For this business case, an integration of Cost-Benefit Analysis with Cash-Flows was conducted for the previous conceptual recycling plant. According to this economic analysis, there are benefits from the creation of market for RUP, recycled sand and coarse aggregate like gravel and their demand as well as from the compensation by the European Union due to the reduction in CO2 emissions in this innovative conceptual recycling plant. The results revealed that the NPV was 5.955.631€ >0 for forecast of 18 year-operation. This can lead to a great payback period of the investment of about 33 months without the calculation of the taxation inside and with the conceptual discount rate at 10%. It has great perspectives for circular economy, with less waste, reuse of materials, less consumption of natural resources and less carbon emissions.In conclusion, although some technical drawbacks, the usage of RUP in new mortars instead of cement seems very satisfactory, and especially smaller RUP have beneficial effect to the mechanical properties of the mortar. Recycled sand is a material that needs more investigation about the appropriate parameters. The reuse of recycled materials and especially RUP contribute to some ecological advantages regarding the CO2 emissions. Also, there are possibilities for economic benefits for recycling plants with RUP as targeted material in products leading to the potential model of circular economy. Finally, RUP and other recycled materials can be the basis for sustainable and green development. Hence, recycled cement paste has as an impact the development of sustainable mortar in this industry.