Maintenance Challenges in Maritime Environments and the Impact on Urban Mobility: Machico Stayed Bridge

Abstract

This article investigates the challenges of maintaining the Machico Cable-Stayed Bridge in a marine environment, focusing on its implications for urban mobility. The primary problem addressed is the impact of harsh marine conditions on the structural integrity of the bridge, which poses significant challenges for ongoing maintenance and safety. The research highlights unique aspects such as the effects of saltwater exposure on materials and the interplay between infrastructure and urban transit dynamics. By emphasizing these critical issues, this study aims to provide insights into effective maintenance strategies and contribute to the broader discourse on urban mobility in coastal regions.

1. Introduction

Bridges are vital components of transportation infrastructure, especially in coastal areas, where they provide essential access, stimulate tourism, and drive economic growth as highlighted by Neuman [1]. However, maintaining bridges in marine environments presents significant challenges due to the corrosive effects of saltwater and high humidity. Previous studies, such as [2], have highlighted how material degradation accelerates under these conditions, requiring innovative solutions in terms of corrosion-resistant materials and monitoring systems.

Research has shown that the corrosive nature of marine environments and the impact of urban traffic on coastal infrastructures are key concerns. Studies like [3,4] emphasize the importance of preventive maintenance strategies and the use of durable materials. However, there remains a gap in the research concerning the combined effects of heavy urban traffic and marine exposure on the structural integrity of bridges, particularly in areas that face high tourist traffic, like Machico.

1.1. Research Gaps

While existing literature provides valuable insights into the degradation of marine infrastructure, there is limited exploration of how urban mobility pressures compound these effects. Specifically, there is a need for research that focuses on the long-term sustainable maintenance of bridges that must endure both heavy vehicular traffic and harsh environmental conditions, as is the case with the Machico Stayed Bridge (Figure 1).

1.2. Research Aim

This paper aims to investigate the maintenance challenges of the Machico Stayed Bridge, a crucial structure for urban transportation and tourism in Madeira, Portugal. The study will explore how the bridge’s marine environment and urban traffic contribute to its deterioration, and will propose sustainable urban planning and infrastructure management strategies to ensure its long-term preservation. By integrating sustainable planning principles and preventive maintenance strategies, this research contributes to the ongoing discourse on marine infrastructure management and sustainable development in coastal regions.

2. Machico Stayed Bridge

2.1. Importance of Bridges as Transport Infrastructure

Bridges are fundamental components of transportation infrastructure, particularly in coastal areas where they facilitate access, promote tourism, and contribute to economic development. The Machico Stayed Bridge, completed in 2009, plays a pivotal role in connecting both sides of the Ribeira de Machico, linking the city’s two main beaches—the Machico Beach and the São Roque Beach—and providing essential access for pedestrians, cyclists, and vehicles (Figure 2). This connectivity has significantly boosted local tourism and eased mobility within the city [5].

The construction of the Machico Stayed Bridge was a remarkable engineering achievement, employing the jet grouting technique to stabilize challenging soil conditions consisting of colluvial–alluvial gravel deposits reaching depths of up to 40 m [6]. This innovative approach ensured the foundation’s stability in a complex geological setting. Despite these efforts, inspection reports from 2019 [7] and 2024 [8] revealed progressive deterioration of the bridge, primarily due to corrosion affecting metallic components such as support bearings and cables (Figure 3). These findings underscore the ongoing challenges of maintaining coastal infrastructure in harsh marine environments [9], a well-documented issue in similar studies of marine-based structures [10].

The Machico Stayed Bridge exemplifies the need for integrating sustainable urban planning with infrastructure management. As highlighted by Gangopadhyay et al. [11], bridges in coastal regions require the use of durable materials, a rigorous maintenance schedule, and protective measures to ensure longevity. For the Machico Stayed Bridge, the implementation of advanced analytical techniques such as Empirical Mode Decomposition (EMD), as suggested by Zhu and Malekjafarian [12], could facilitate the early detection of structural issues and aid preventive maintenance strategies, helping to prolong the lifespan of the structure.

In addition to its engineering significance, the bridge plays a vital role in promoting soft mobility within the city. By providing safe passage for pedestrians and cyclists, the Machico Stayed Bridge supports sustainable urban mobility initiatives that reduce dependence on cars and enhance accessibility. This aligns with the city’s broader goals of improving urban mobility through the promotion of eco-friendly travel options. The bridge has also become a key tourist attraction due to its modern design and panoramic views of the city and coastline, contributing to both local commerce and the region’s appeal as a destination (Figure 4).

The Stayed Bridge plays a fundamental role in promoting soft mobility in Machico, especially for pedestrians and cyclists, connecting important areas of the city, such as the centre and the coastal zone, encouraging travel on foot or by bicycle, and reducing dependence on cars, while also becoming a popular tourist spot thanks to its contemporary design and ideal position, offering stunning views of the city and the ocean (Figure 5).

However, the harsh marine environment continues to pose significant challenges. The salt-laden air and high humidity accelerate the corrosion of metallic components, despite the use of protective measures during construction. The jet grouting technique, while effective at stabilizing the foundation, could not entirely mitigate the effects of the marine environment on the exposed metal parts. Components such as support bearings, cables, and guardrails remain vulnerable, highlighting the importance of regular inspections, rigorous maintenance, and the need for corrosion-resistant materials in future projects [9]. Recent studies have demonstrated that the use of corrosion-resistant alloys, advanced coatings, and cathodic protection can significantly extend the lifespan of such structures, providing insights into potential solutions for the Machico Stayed Bridge [13].

In recent years, the Machico City Council has implemented several measures to promote soft mobility, including the closure of streets in the city centre to reduce vehicular traffic. These changes have sparked debate among residents, with some expressing concerns about the potential impact on commerce and accessibility. Nevertheless, the Machico Stayed Bridge remains a crucial element in these urban transformations, serving as a vital link between key areas of the city while promoting a more sustainable approach to mobility.

2.2. Geological and Geotechnical Context

The construction of the Machico Stayed Bridge was challenging due to the complex geological conditions at the site. The heterogeneous mix of colluvial–alluvial deposits, including basalt pebbles and boulders up to 40 m deep, along with varying layers of tuffs, breccias, and weathered basalts, made it difficult to establish stable foundations.

To overcome the obstacles posed by the complex geological and geotechnical conditions at the site, innovative engineering solutions were required, leading to the adoption of the jet grouting technique for the bridge’s foundations (Figure 6), which illustrates the geological characteristics of the bridge location, showcasing the abutments (E1, E2) and pylons (P1, P2), along with the distinct soil and rock layers encountered during the geotechnical investigation. The geological investigation revealed several distinct layers:

• A superficial layer (ZG2) of colluvial–alluvial deposits with basalt pebbles and boulders, reaching up to 40 m deep [6].
• Deeper layers of compact to moderately compact tuffs and breccias (ZG1A), sometimes overlying weathered and fractured basalts [6].
• A zone of disaggregated tuffs (ZG1B) with clayey–sandy and sandy–clayey properties, characterized by NSPT values between 40 and 60 blows, indicating relatively dense material [6].

The decision to use jet grouting for the bridge’s foundations was a direct solution to the issues posed by the complex soil conditions in Machico, as it involves injecting cement slurry at high speeds to create soil–cement columns, improving the soil and enabling the construction of deep and sturdy foundations in soils with varying compositions and low bearing capacities (Figure 7).

2.3. Design and Structure

The Machico Stayed Bridge features a central span of 120 m and two adjacent spans of forty-five meters each, and it is composed of a deck made of reinforced concrete, held up by two concrete towers and a network of steel cables (Figure 8).

The decision to use a cable-stayed design influenced by the desire to cover a long distance while blending seamlessly with the surrounding environment.

The process of constructing the foundations with jet grouting required drilling 88 columns, each 1.5 m in diameter and ranging from 25 to 40 m in depth, followed by injecting a cement mixture under a pressure of 400 bar to create sturdy and long-lasting soil–cement columns, the effectiveness of which was verified through a comprehensive load test that demonstrated the columns’ capacity to bear the bridge’s operational weight and transmit it to the soil via lateral friction [6].

2.4. Challenges of Bridge Maintenance in Marine Environments

Madeira Island’s harsh marine conditions, including sea spray, high humidity, and temperature fluctuations, accelerate corrosion and cause common bridge problems like expansion joint wear and concrete cracking [14], with the 2024 inspection report highlighting worsening corrosion and current issues like water seepage and fractures (Figure 9).

2.5. Sustainable Urban Planning and Infrastructure Management

Sustainable urban planning, as discussed by Neuman [1], promotes urban growth that is economically viable, socially just, and environmentally responsible, and is crucial in infrastructure management, especially in challenging environments like the Machico Stayed Bridge (Figure 10), where proactive maintenance is essential for ensuring the long-term durability and safety of structures, as emphasized by Du and Greiving [15].

The Machico Stayed Bridge’s location in a harsh marine environment and challenging geological conditions necessitates tailored infrastructure management, exemplified using jet grouting for the foundation to address soil difficulties [6], and ongoing maintenance, including regular inspections, cleaning, protective coatings, and corrosion monitoring, and is essential for ensuring the bridge’s longevity and safety [15].

Sustainable urban planning should incorporate infrastructure maintenance, considering environmental problems and community needs, as exemplified by the Machico Stayed Bridge, which is crucial for urban mobility and regional economic growth, serving as a key entry point to the beach and a tourist attraction [5], and requires proper upkeep to ensure its safety, urban sustainability, and continued fulfilment of its social, economic, and environmental roles over time.

2.6. Materials and Construction Techniques

The construction of the stayed bridge utilized materials and methods focused on reducing the effects of corrosion and ensuring the longevity of the structure, and, for that, the towers and deck were constructed using concrete with a reduced water–cement ratio and mineral additives to enhance resistance to chloride penetration, a common cause of rebar corrosion. The steel cables were treated with an anti-corrosion paint system and covered with high-density polyethylene (HDPE) for extra defence against sea spray and saltwater.

However, the 2019 [7] and 2024 [8] inspection reports clearly show that the stayed bridge continues to face problems with corrosion and deterioration, emphasizing the importance of implementing a comprehensive maintenance strategy and incorporating corrosion-resistant materials in future projects; By selecting suitable materials and construction methods for marine environments, such as utilizing stainless steel or high-corrosion-resistant alloys, long-lasting coatings, and cathodic protection systems, it is possible to extend the bridge’s longevity and reduce maintenance expenses.

2.7. Objectives of the Study

This study aims to comprehensively analyse the maintenance challenges of the Machico Stayed Bridge, focusing on the impact of the marine environment, traffic effects, and potential shortcomings in design and construction. The research evaluates how these factors contribute to the bridge’s degradation, and proposes sustainable maintenance strategies that incorporate advanced material choices and proactive infrastructure management.

By examining the influence of the bridge’s deterioration on urban mobility, tourism, and the local economy, the study offers valuable insights into the significance of sustainable urban planning for managing marine infrastructure. The findings will provide recommendations for both proactive and reactive maintenance strategies, ensuring the bridge’s long-term preservation and contributing to the broader discourse on sustainable urban development in coastal regions.

3. Challenges in Maintenance and Monitoring of the Machico Bridge

Effective maintenance and monitoring of the Machico Stayed Bridge are critical to ensuring its longevity and safety. However, several challenges complicate these efforts:

Environmental Factors: The Machico Stayed Bridge faces significant challenges due to environmental factors. The coastal location exposes the bridge to harsh weather conditions, including high humidity and salt-laden winds, which accelerate the corrosion of the structural components. The impact of these environmental conditions necessitates more frequent inspections and advanced corrosion management strategies to prevent deterioration.

Technical Issues: Several technical challenges affect the bridge’s maintenance and monitoring. The design of the bridge includes complex structural elements that require specialized knowledge for accurate assessment. Additionally, the materials used in construction, while initially robust, have shown signs of wear and degradation over time. This has led to challenges in both detecting and addressing structural weaknesses.

Inspection and Maintenance Practices: Current inspection methods have proven to be limited in detecting all forms of damage or deterioration. The reliance on traditional inspection techniques sometimes results in the delayed identification of issues, which can exacerbate maintenance challenges. Moreover, there are constraints related to the allocation of resources and the frequency of inspections, which impact the effectiveness of maintenance efforts.

Summary of Challenges: In summary, the main challenges in the maintenance and monitoring of the Machico Stayed Bridge include:

• Corrosion Management: effective strategies are needed to combat the accelerated corrosion due to environmental conditions.
• Structural Integrity: ongoing issues with the bridge’s design and material degradation require careful monitoring and repair.
• Resource Allocation: limited resources and inspection constraints hinder timely detection and resolution of maintenance issues.

These challenges highlight the need for improved maintenance practices, enhanced inspection techniques, and better resource management to ensure the long-term durability and safety of the Machico Stayed Bridge.

3.1. Corrosion in Marine Environments

Located on the coastline of Madeira Island, in a marine environment, the Machico Stayed Bridge is exposed to severe weather conditions and faces significant difficulties due to the presence of chlorides in seawater, combined with high humidity and sea spray, creating a corrosive environment that accelerates the degradation of construction materials.

The Machico Stayed Bridge is exposed to intense winds, constant sea spray, and temperature fluctuations. This combination accelerates the corrosion process, particularly affecting metal components like steel cables and support bearings. Therefore, the bridge requires specific maintenance measures to combat corrosion (Figure 11).

The salt-laden sea spray deposits on the steel, initiating the corrosion process, which is accelerated by the moisture in the air and temperature fluctuations, which leads to the continuous decay of metal components and fastening elements, such as bolts and nuts, compromising the stability of the structure.

Although concrete is more resistant to corrosion than steel, it is still vulnerable in marine environments, leading to chloride infiltration into the concrete that can cause the reinforcements to corrode, compromising the bridge’s structural stability, and wave action and sand abrasion can further exacerbate the issue by wearing down the concrete surface, exposing the reinforcements and hastening the corrosion process.

The 2024 technical report [17], conducted by the LREC (Regional Laboratory of Civil Engineering), confirms the presence of chlorides at concerning levels in the bridge’s concrete, indicating a moderate risk of corrosion of the reinforcement, particularly in areas with higher exposure to the marine environment, underscoring the urgent need for proactive measures to mitigate corrosion and ensure the bridge’s long-term structural integrity.

The chloride content test showed elevated concentrations in some samples, especially near the surface. These levels exceed the European standard EN 206-1 [18], indicating a high risk of reinforcement corrosion. This corrosion can weaken the bridge’s load-bearing capacity and compromise safety. The corrosion potential test also revealed areas with a higher corrosion risk due to cracks and delamination in the concrete.

Therefore, implementing protective measures and continuous monitoring, as recommended by the report [17], is crucial to ensure the safety, durability, and economic viability of the Machico Stayed Bridge, as neglecting its maintenance can have direct consequences for safety, economics, local mobility, tourism, and the overall development of the region.

3.2. Additional Problems

Apart from corrosion, the Machico Stayed Bridge encounters other common challenges in marine structures, like the wear and tear of expansion joints due to exposure to sea spray, saltwater, and UV radiation, affecting their performance and potentially causing water infiltration into the structure (Figure 12); such joints are essential for accommodating the bridge’s thermal movements.

Concrete cracking is another common issue in marine bridges, due to temperature changes and concrete shrinkage, which can cause cracks to form, allowing chlorides to penetrate and accelerate the corrosion of reinforcements (Figure 13).

3.3. Traffic Impact

Traffic flow on the Machico Stayed Bridge, including automobiles, bicycles, and pedestrians, contributes to the structure’s wear and tear and exacerbates corrosion issues; the vehicle pressure, vibrations, and tire friction damage the surface and joints, generating stresses that can lead to concrete cracking.

The issue of traffic load is further compounded by instances of abusive parking on the bridge. In 2016, the Machico City Council took the initiative to address this problem by prohibiting parking on the cable-stayed bridge, recognizing its contribution to the bridge’s wear and tear, creating a bike lane delimited by flexible traffic posts (Figure 14).

The enforcement of these restrictions, with fines and towing for non-compliance, underscores the seriousness of the issue and the commitment to preserving the bridge’s structural integrity.

3.4. Importance of Preventive Maintenance

Preventive maintenance is crucial for mitigating the effects of corrosion and other issues in marine bridges, so it is recommended to implement key measures such as regular inspections, cleaning, application of protective coatings, and corrosion monitoring to extend the bridge’s lifespan and reduce long-term repair costs, all of which should be included in a comprehensive maintenance plan for the Machico Stayed Bridge.

• Routine Visual Inspections: early detection of corrosion, cracks, joint wear, and other problems [19].
• Structural Cleaning: removal of dirt, salts, and debris that can accelerate corrosion.
• Application of Protective Coatings: shielding metallic and concrete components from sea spray and seawater [20].
• Corrosion Monitoring: evaluation of corrosion levels and identification of areas needing attention [21,22].
• Repair of Cracks and Damage: preventing further damage and maintaining structural integrity [23,24].
• Replacement of Deteriorated Parts: when erosion becomes severe, replacing elements like support bearings and expansion joints may be necessary [25].

3.5. Specific Maintenance and Monitoring Techniques

Anti-Corrosion Measures—corrosion protection is achieved through a combination of strategies:

• Application of a multi-layered, zinc-rich epoxy primer system on steel components, followed by a polyurethane topcoat for enhanced durability.
• Implementation of cathodic protection, utilizing sacrificial anodes, on critical underwater elements to mitigate corrosion.
• Regular visual inspections conducted every six months to assess the condition of the protective coatings and identify any signs of corrosion.
• Localized repairs, including cleaning, surface preparation, and reapplication of coatings, are performed as needed.
• Annual electrochemical impedance spectroscopy (EIS) measurements at key locations to evaluate the effectiveness of the cathodic protection system and pinpoint areas requiring further attention.

Structural Health Monitoring (SHM) System:

• The bridge currently lacks a comprehensive, continuous SHM system. However, periodic inspections leverage technologies like the VSL Vibratest to assess the tension in stay cables. This method, based on vibration frequency analysis, revealed discrepancies between actual and design cable forces in the 2024 inspection, highlighting potential structural concerns
• To enable proactive maintenance and ensure long-term structural integrity, the implementation of a permanent SHM system is recommended. This system could incorporate technologies such as vibrating wire strain gauges and accelerometers to continuously monitor the bridge’s behaviour and provide early warnings of any developing issues.

3.6. Evidence of Deterioration and Its Causes

Extent and Speed of Deterioration—the inspection reports reveal a concerning acceleration in the bridge’s degradation between 2019 and 2024.

• Corrosion: The 2024 report indicates a significant worsening of corrosion on various metallic components, with many elements progressing from a poor to an extremely poor state of conservation. This suggests that the corrosive marine environment, rich in chlorides, is taking a toll on the bridge’s structural integrity at an alarming rate.
• Concrete Damage: The reports also highlight an increase in concrete cracking and spalling, particularly in areas exposed to the elements and those supporting critical components like the bearing supports. This suggests that the combined effects of environmental factors, such as sea spray and temperature fluctuations, and the dynamic loads from traffic, are contributing to the concrete’s degradation.
• Stay Cable Concerns: The VSL Vibratest results, as detailed in the 2024 report, reveal discrepancies between the actual and design forces in the stay cables. This raises concerns about potential prestress losses and their impact on the bridge’s structural behaviour, further emphasizing the need for close monitoring and timely interventions.

Connecting to Causes of Deterioration—these observations from the inspection reports directly correlate with the causes of deterioration discussed in this article:

• Marine Environment: The accelerated corrosion observed in the 2024 report aligns with the discussion on the aggressive marine environment and its impact on the bridge’s materials. The high chloride content in the concrete, as evidenced by the LREC report, further supports this connection.
• Inadequate Maintenance: The worsening condition of various components, despite the maintenance efforts mentioned in this article, suggests that the current maintenance strategies may not be sufficient to counteract the aggressive environmental conditions and the bridge’s heavy usage.
• Traffic Effects: The increased concrete cracking and potential prestress losses in the stay cables can be linked to the impact of traffic loads, especially considering the absence of a comprehensive traffic management plan in the bridge’s early years.
• Design and Construction: While the use of jet grouting was innovative, the inspection reports suggest that the initial design and construction may not have fully accounted for the long-term effects of the marine environment. The observed water infiltration and corrosion issues highlight potential vulnerabilities in the bridge’s design and material choices.

4. Inspection Reports Analysis

4.1. Comparison of 2019 and 2024 Reports

The comparison of the inspection reports from 2019 [7] and 2024 [8] reveals a significant deterioration in the Machico Stayed Bridge’s structural integrity over time, as highlighted in the 2019 report [7], showing that severe corrosion in critical components such as the bearing devices and stay cables, along with concrete pillar fractures and insufficient drainage visible on the Stayed Bridge (Figure 15).

The 2024 report presents a more alarming situation, with the corrosion in the bearing components and stay cables (water seepage in the stay cables and centralizers) escalating to critical levels, along with the presence of widespread concrete cracking and the significant erosion of the expansion joints, necessitating immediate and effective maintenance to counteract the combined effects of environmental conditions and vehicular traffic (Figure 16).

Table 1. Rapid deterioration of the bridge from 2019 to 2024.

Inspection Report	Corrosion in Bearing Devices	Corrosion in Stay Cables	Concrete Cracking	Water Infiltration	Expansion Joint Wear
2019	Advanced	Advanced	Pillars	Not Reported	Not Reported
2024	Extremely Poor	Very Poor	Various Areas	Present	Present

shows the rapid deterioration of critical bridge components between 2019 and 2024. The shift from ‘advanced’ to ‘extremely poor’ corrosion in the stay cables, and increased occurrences of water infiltration, emphasize the growing risk to the bridge’s structural integrity. These findings reinforce the thesis that immediate and proactive maintenance interventions, including the use of advanced materials and technologies like SHM, are essential to prevent further degradation and ensure the bridge’s long-term resilience.

Corrosion has worsened, new structural issues like water infiltration and concrete cracking have emerged, and the combined impact of the environment and traffic poses a risk to the bridge’s integrity and user safety. This also threatens the vital transportation link it provides for local mobility and tourism.

4.2. Analysis of Deterioration Causes

The decline of the Machico Stayed Bridge results from a combination of factors, including the harsh marine environment, inadequate maintenance, traffic impacts, and specific design and construction features.

The Machico Stayed Bridge, located on Madeira Island, is exposed to a subtropical climate with high humidity and sea spray, which accelerates the corrosion of the bridge’s metal components [7], further exacerbated by the chlorides in seawater causing damage to the steel cables and support structures, and penetrating the concrete, leading to reinforcement corrosion [22].

In addition to the marine environment, the bridge endures significant traffic volumes, including heavy vehicles, which contribute to structural wear and exacerbate corrosion issues, since the pressure and movements from vehicles can result in concrete fractures.

Design and construction characteristics of the bridge may have also played a role in its deterioration, since the selection of materials with lower corrosion resistance and the lack of an adequate waterproofing system for the cables and centralizers may have accelerated structural decline [8].

4.3. Evaluation of Force in the Tie Rods Using the VSL Vibratest

4.3.1. The VSL Vibratest System and Its Application

The evaluation of the structural condition of the Machico Bridge was enhanced using the VSL Vibratest system [26], an innovative technology that allows for the precise and non-destructive measurement of the force in the bridge’s tie rods (Figure 17).

The VSL Vibratest [26], like other Operational Modal Analysis (OMA) techniques, is based on the analysis of the natural vibrations of the cables, establishing a correlation between the frequencies of these vibrations and the tension present in the tie rods. The dependence between the natural and frequency ($f_n)$ and the tension ($T$) is expressed by the following formula:

$$f_n=f_0\left({\displaystyle \frac{1}{\pi }}\sqrt{{\epsilon }^2{k}_n^4+k_n^2}\right),$$

(1)

where:

• $f_0={\displaystyle \frac{1}{2L}}\sqrt{{\displaystyle \frac{T}{m}}}$ is the fundamental frequency of the string.
• $\epsilon ={\displaystyle \frac{1}{L}}\sqrt{{\displaystyle \frac{EI}{T}}}\mathrm{i}$s the parameter that models the effect of bending stiffness.
• $k_n(\epsilon ,{\mathsf{\lambda }}^2)$ is the wavenumber, a function of n, and the parameters λ² and $\epsilon$.
• λ² is the parameter modelling the bending effect.
• $T$ is the tension in the tie rod.
• L is the free length of the tie rod.
• m is the distributed mass of the tie rod.
• E is the modulus of elasticity of the tie rod material.
• I is the moment of inertia of the cross-section of the tie rod.

This approach eliminates the need for traditional measurement methods, such as the use of hydraulic jacks, which require heavy equipment and long testing periods [27].

The VSL Vibratest system [26] consists of a set of accelerometers, a data acquisition system, and analysis software, which together allow for the automatic collection and processing of cable vibrations, calculating the force present in each tie rod based on the collected data to provide accurate and reliable results, with a theoretical accuracy of the tension measurement estimated to be about 2.3%, comparable to the accuracy obtained with the hydraulic jack method, validating its effectiveness; the relative error committed during the tension measurement can be expressed as:

$${\displaystyle \frac{d{f}_n}{f_n}}=\left({\displaystyle \frac{EI}{f_n}}{\displaystyle \frac{\partial f_n}{\partial EI}}\right){\displaystyle \frac{dEI}{EI}}+\left({\displaystyle \frac{m}{f_n}}{\displaystyle \frac{\partial f_n}{\partial m}}\right){\displaystyle \frac{dm}{m}}+\left({\displaystyle \frac{L}{f_n}}{\displaystyle \frac{\partial f_n}{\partial L}}\right){\displaystyle \frac{dL}{L}}+\left({\displaystyle \frac{T}{f_n}}{\displaystyle \frac{\partial f_n}{\partial T}}\right){\displaystyle \frac{dT}{T}}$$

(2)

where: ${\displaystyle \frac{dT}{T}}$, ${\displaystyle \frac{dm}{m}}$, ${\displaystyle \frac{dL}{L}}$, ${\displaystyle \frac{dEI}{EI}}$, ${\displaystyle \frac{d{f}_n}{f_n}}$ are the relative errors made, respectively, in the tension, the distributed mass, the free length of the cable, the bending stiffness, and the frequency n.

4.3.2. Evaluation Results and Implications

The application of the VSL Vibratest [26] on the Machico Bridge revealed significant discrepancies between the measured forces in the tie rods and the original design forces (

Table 2. Comparison of measured forces using VSL Vibratest [26] and Original Design Forces in Machico Bridge tie rods.

Tie Rod	No. of Strands	Vibratest (kN)	Design Force (kN)	Fvib/Fpr Ratio
R1	12	879	804	109%
R2	7	672	391	172%
F1	12	765	669	114%
F2	7	661	293	226%
F3	12	1176	1183	99%
F4	12	848	703	121%
R3	12	1307	820	159%
R4	12	849	1339	63%

).

In some cases, the measured force was considerably higher than anticipated, indicating a possible excess of tensioning during construction or throughout the bridge’s lifespan, while in other cases, the measured force was lower, suggesting prestress losses due to factors such as steel relaxation, cable corrosion, or anchorage failures, which could be further contextualized by the research conducted by Barrios et al. [28] on the impact of corrosion on the structural stiffness and dynamic properties of steel structures.

Variations in tie rod force impact the bridge’s safety and durability. Excessive tensioning increases stress, potentially accelerating fatigue and compromising long-term integrity. Prestress losses decrease stiffness, making the bridge more prone to deformations and vibrations, especially under traffic loads. It is important to interpret these findings cautiously, as vibration-based techniques have limitations in detecting damage in prestressed elements [29].

4.3.3. Need for Intervention and Continuous Monitoring

The results of the evaluation with the VSL Vibratest [26] highlight the importance of the continuous monitoring of the Machico Bridge, especially in an aggressive marine environment where corrosion is a critical factor. Identifying tie rods with excessive tensioning or prestress losses allows for preventive and corrective measures, such as re-tensioning or cable replacement, ensuring the safety and longevity of the structure.

The use of the VSL Vibratest [26] proved to be a valuable tool in assessing the structural condition of the Machico Bridge, providing quantitative data that complement visual inspections and allow for a deeper analysis of the maintenance challenges of the bridge, demonstrating a commitment to efficient and sustainable infrastructure management through the adoption of innovative technologies, ensuring the safety and well-being of the community that depends on it, and opening up the potential for integrating the VSL Vibratest [26] with other monitoring technologies, such as the vision-based systems explored by Ngeljaratan et al. [30] for seismic structural health monitoring, to develop even more comprehensive and robust bridge health monitoring systems.

Tabiatnejad et al. [31] showed that combining the PTM method with vibration analysis is effective for damage detection in external tendons. This suggests the potential for future research to improve accuracy and efficiency in bridge tendon damage detection by integrating the VSL Vibratest with the PTM approach. This could lead to a powerful tool for proactive bridge maintenance and safer, more sustainable infrastructure.

Considerations on test results:

• Significant Discrepancies: The results from the VSL Vibratest [26] revealed substantial variations between the measured forces in the tie rods and the design values, with a tie rod showing both excessive tensioning (up to 172% above the design) and prestress losses (up to 37% below the design).
• Impact on Safety and Durability: These discrepancies can affect the safety and lifespan of the bridge, increasing the risk of fatigue, deformations, and excessive vibrations.
• Need for Action: The results from the VSL Vibratest [26] underscore the urgency for maintenance and rehabilitation interventions on the Machico Bridge, including the re-tensioning or replacement of tie rods, to ensure the safety and functionality of the structure.
• Continuous Monitoring: The adoption of a continuous monitoring program, utilizing technologies like the VSL Vibratest [26], is essential to track the structural condition of the bridge over time and identify issues at initial stages, allowing for timely preventive and corrective actions.

Recommendations:

• In-depth Analysis of Causes: Investigate the causes of the discrepancies between the measured forces and the design values, considering factors such as the quality of construction, the aggressiveness of the marine environment, and the maintenance history of the bridge.
• Intervention Plan: Develop a comprehensive intervention plan based on the VSL Vibratest [26] results and other structural assessments to address the identified issues and ensure the safety and durability of the Machico Bridge.
• Continuous Monitoring: Implement a continuous monitoring system for the bridge, using non-destructive technologies, to track the evolution of its structural condition and detect any signs of degradation early.

Combining detailed structural assessments, such as the VSL Vibratest [26], with a continuous monitoring program and initiative-taking maintenance interventions is essential to ensure the safety, functionality, and longevity of the Machico Bridge, preserving this important asset for the community and promoting the sustainable development of the region.

4.4. Assessment of Bridge Deterioration through Non-Destructive Testing (NDT)

4.4.1. Evaluation Techniques and Their Application

Non-destructive tests were strategically conducted at specific locations on the bridge to gain a comprehensive understanding of its condition. This approach ensured a representative assessment of the bridge’s overall health and identified areas needing immediate attention.

The two key non-destructive testing (NDT) techniques chosen to assess the bridge’s condition were [17]:

• Reinforcement corrosion potential was measured at strategic points on the pillars and deck (P2 to P4) using the ASTM C876-80 standard [32]. This method measures the electrical potential difference between steel and concrete, with more negative values indicating higher corrosion risk [33]. This technique is widely recognized for its accuracy in detecting corrosion in reinforced concrete structures [34,35].
• Chloride content analysis was performed on concrete samples from different bridge points (P1 to P5) following the NP EN 14629:2007 standard [36]. This test determines chloride content and penetration depth, crucial for evaluating concrete durability in marine environments. It helps estimate chloride penetration rates, predict the structure’s service life, and identify areas needing repair or protection [37,38].

4.4.2. Evaluation Results and Their Implications

The results of the corrosion potential measurement of the reinforcements, as depicted in

Table 3. Corrosion potential measurements at different points [17].

Statistics	Different Points
	P2	P3	P4
N° of Readings	48	50	47
Median (mV)	−47.5	−100	−83
Mean (mV)	−38.9	−60.5	−50.2
Standard Deviation (mV)	62.8	30.6	34.7
Lowest (mV)	−164	−142	−151
Highest (mV)	173	0	−6

, demonstrate significant variability in corrosion potential measurements across different points of the Machico Stayed Bridge, with notably negative values at P3 and P4, which signal a high likelihood of active corrosion in these areas. These data directly support the thesis by emphasizing the need for focused repair efforts in regions with the highest corrosion risk, as identified by the negative potential values. The differences in corrosion potential are critical for developing a targeted maintenance plan, reinforcing the importance of proactive monitoring and timely interventions to preserve the bridge’s structural integrity [39].

Figure 18 demonstrates the elevated chloride content in the bridge’s concrete samples, particularly at a depth of 30mm, where concentrations significantly exceed acceptable limits [36]. This finding is crucial to understanding the aggressive impact of the marine environment on the bridge. Elevated chloride levels lead to corrosion of the steel reinforcements, reducing the bridge’s load-bearing capacity [17]. This supports the thesis by emphasizing the importance of using corrosion-resistant materials and implementing ongoing protective measures to ensure the structure’s durability [39,40].

4.4.3. Urgent Interventions Required

The results of the non-destructive tests conducted on the Machico Bridge underscore the urgent need for conservation and repair interventions to ensure the safety and durability of the structure, prioritizing the areas with a higher risk of corrosion, identified through electrical potential measurement and chloride content analysis, in an intervention plan that includes the following measures:

• Repair of damaged areas by removing deteriorated concrete, cleaning and treating corroded reinforcements, and applying new concrete layers with adequate protection against corrosion as effective concrete repair techniques extending the service life of deteriorated structures, demonstrated by studies [41,42].
• Anti-corrosion protection by applying protective coatings on the exposed surfaces of the bridge to reduce chloride penetration and delay the corrosion process, as the use of protective coatings is a well-established strategy for enhancing the durability of concrete structures in aggressive environments [43,44].
• Continuous monitoring through the implementation of a corrosion monitoring system to track the evolution of deterioration and identify the need for future interventions enables initiative-taking maintenance and facilitates timely interventions, potentially preventing costly repairs and ensuring the long-term structural health of the bridge [45,46].

The use of non-destructive testing in the assessment of the Machico Bridge allowed for an accurate diagnosis of the structure’s condition, identifying critical areas and guiding the necessary maintenance and repair actions, highlighting the importance of adopting a proactive approach and implementing a comprehensive intervention plan to ensure the safety, durability, and extended service life of this vital infrastructure.

4.5. Evaluation of Proposed Solutions

The inspection reports from 2019 [7] and 2024 [8] suggest different strategies to address the issues identified with the Machico Stayed Bridge, which included replacing bearing devices and stay cables, repairing concrete, and installing a more effective drainage system [7], and nowadays these measures are insufficient, necessitating more extensive interventions such as replacing the expansion joints and applying a new waterproofing system [8].

A thorough analysis of the proposed solutions highlights the need for more rigorous maintenance, such as advanced monitoring techniques, as proposed by Erdogmus et al. [21], which utilize ultrasonic guided waves to detect faults in reinforced concrete bridges for early problem detection and targeted maintenance efforts.

Rehabilitating the Machico Stayed Bridge presents a complex challenge that requires a multifaceted approach, including replacing degraded components, applying protective coatings, installing a corrosion monitoring system, establishing a regular maintenance schedule, and considering the use of materials with higher corrosion resistance in future projects, such as stainless steel or high-performance concrete, to extend the bridge’s lifespan and reduce long-term maintenance costs.

5. Impact on Urban Mobility and Tourism

5.1. Traffic Data and Usage Patterns

The Machico Stayed Bridge serves as a vital artery for the town’s transportation network, accommodating a diverse range of users, including pedestrians, cyclists, and motorists, due to its strategic location as a direct link between the two main beaches and the town centre, making it a key facilitator of local mobility and a significant contributor to the region’s tourism industry [47].

The bridge’s significance is evident in the substantial volume of vehicular traffic it carries, as shown by data collected from sensors installed at Machico by CWay [48].

Table 4. Traffic data for the Machico Stayed Bridge.

Period	Accumulated Total Vehicles at Machico Stayed Bridge	Accumulated Total Vehicles at Machico Bridge
	North	North	South
Last day (July 30 of 2024)	279	1905	2047
Last 30 days (July of 2024)	4983	55,406	59,109
Last 2 months	8808	114,172	118,997
Last 6 months	17,067	117,734	113,780
Last 12 months	165,850	234,661	238,074

shows the heavy traffic on the Machico Stayed Bridge, with an accumulated total of 165,850 vehicles over the last 12 months. This volume of vehicular traffic, especially during peak tourist periods, exacerbates the structural wear, as evidenced by the advanced corrosion and concrete cracking described in earlier sections. The data support the thesis by illustrating the urgent need for sustainable infrastructure management strategies to accommodate ongoing traffic while mitigating the detrimental effects of heavy usage on the bridge’s long-term durability.

The Machico Stayed Bridge is unidirectional, while the Machico Bridge accommodates traffic in both directions, connecting various parts of Machico and facilitating the movement of residents, tourists, and goods (Figure 19).

The high traffic volume, especially during peak tourist seasons, can exacerbate the wear and tear on the bridge’s structure, particularly in conjunction with the corrosive effects of the marine environment.

5.2. Safety Concerns and Access Restrictions

The 2019 and 2024 inspection reports [7,8] show the Machico Stayed Bridge is deteriorating. Advanced corrosion, concrete cracking, and water infiltration raise safety concerns. This could lead to restrictions on the bridge’s use, like weight limits, speed reductions, or even closure. Such restrictions would disrupt mobility, hinder beach access, and create safety risks for pedestrians and cyclists forced to use alternative routes.

The 2024 LREC inspection report [17] further amplifies these concerns, emphasizing the risk of compromising the bridge’s long-term safety and durability due to the corrosion potential of the reinforcement, and the potential consequences of inaction are significant, with the possibility of restrictions or closures severely impacting local mobility and the tourism industry, which is crucial to the region’s economy.

5.3. Impact on Tourism

The Machico Stayed Bridge is not only a vital transportation link but also a tourist attraction, with its unique design and scenic location making it a popular spot for visitors to enjoy panoramic views of the city and the sea, but the bridge’s deteriorating condition and potential access restrictions could deter tourists from visiting Machico, impacting the local tourism industry.

The aesthetic appeal of the Machico Stayed Bridge, highlighted in online reviews [49], is a significant factor in attracting tourists, but the corrosion, cracks, and other signs of deterioration can diminish its visual appeal, making it less attractive to visitors, and any restrictions on access, such as closures or limitations on pedestrian and bicycle traffic, would further discourage tourists from visiting the bridge and the surrounding area.

The local tourism industry has both positive and negative impacts on the community. While it increases local accommodation and overnight stays, it also creates potential for overcrowding and strains resources. The high volume of vehicular traffic on the bridge, as shown by CWay data [48], further emphasizes this pressure.

5.4. Economic Implications for Machico

The economic consequences of the Machico Stayed Bridge’s damage extend beyond the tourism sector, playing a crucial role in the local economy by facilitating the movement of goods and services, and supporting various businesses that rely on tourism, such as hotels, restaurants, and shops.

The bridge’s disintegration and potential closure could disrupt economic activities, leading to a loss of revenue for local businesses, a decline in employment opportunities, and a strain on the local government’s budget by diverting resources from other essential services due to the cost of repairing or replacing the bridge.

The long-term economic impact of the bridge’s deterioration is a concern for the sustainable development of Machico, as a well-maintained and functional bridge is essential for attracting investment, promoting tourism, and supporting local businesses [49], and neglecting its maintenance needs could have far-reaching consequences for the town’s economic future.

The PAMUS RAM [50] highlights the low utilization rate of public transport in the region, with a modal share of only 9.4%, underscoring the urgent need to enhance its attractiveness and promote a shift towards more sustainable modes of transportation, especially in light of the challenges faced by the Machico Stayed Bridge and its impact on urban mobility due to decay, further emphasizing the importance of investing in sustainable transport solutions such as the creation of dedicated cycling infrastructure and the improvement of local public transport services in Machico, as proposed in the PAMUS RAM [51], aligning with the regional goals of reducing dependence on private vehicles and promoting a more balanced and environmentally friendly mobility.

6. Discussion

Building on the challenges and recommendations outlined in the previous sections, this chapter presents a vision for the sustainable future of the Machico Stayed Bridge and its integration into broader infrastructure and urban mobility strategies. By reflecting on the corrosion challenges, the impacts of tourism, and the bridge’s structural vulnerabilities, the following strategies are proposed:

• Proactive Maintenance and Monitoring Systems: The earlier analysis emphasized the deteriorating structural health of the bridge, largely due to the corrosive marine environment. As a direct response, future infrastructure management will integrate continuous Structural Health Monitoring (SHM), using technologies such as ultrasonic guided waves and corrosion sensors to monitor wear in real time. This allows for timely interventions and reduces the risk of unexpected failures.
• Material Resilience in Harsh Marine Environments: Reflecting on the environmental conditions of the Machico Stayed Bridge, future development projects should prioritize the use of corrosion-resistant materials such as stainless steel and high-durability concrete. These materials, combined with protective coatings (e.g., epoxy-based solutions), will better resist chloride penetration from the marine environment and extend the bridge’s lifespan.
• Integration of Sustainable Mobility Solutions: Given the earlier discussions on traffic congestion and its impact on the bridge’s deterioration, a shift towards soft mobility solutions is essential. Future urban planning will integrate pedestrian walkways and cycling paths along the bridge, encouraging non-vehicular traffic. This will relieve pressure on the bridge, contributing to both its structural longevity and a reduction in carbon emissions.
• Tourism and Economic Considerations: The bridge is a key connector for Machico’s tourism economy. Future planning should balance tourism growth with infrastructure resilience. The earlier issues of wear caused by high traffic highlight the need for a seasonal vehicle restriction policy during peak tourist times. This policy, aligned with sustainable tourism strategies, will ensure the bridge can accommodate increased pedestrian traffic without compromising its structural integrity.
• Community Engagement and Inclusive Urban Planning: Addressing earlier concerns about the lack of public consultation, future development must involve community dialogue to ensure that changes to the Machico Stayed Bridge, such as potential pedestrianization or vehicle restrictions, reflect the needs and preferences of residents. This participatory approach will help balance accessibility, mobility, and the economic interests of the local community.

6.1. A Critical Analysis

The case of the Machico Stayed Bridge serves as a valuable lesson in integrating sustainable urban planning with infrastructure management, especially in challenging maritime environments. The damage to the bridge, as evidenced by the inspection reports, underscores the consequences of neglecting long-term maintenance needs and the impact of environmental factors on infrastructure lifespan [7,8].

Revised Inspection Reports Analysis:

Alignment with Study Objectives:

To better reflect the study’s main objectives, the analysis of the inspection reports has been revised to emphasize how these findings contribute to understanding and improving maintenance strategies:

• Direct Relevance: The analysis now directly links specific issues identified in the inspection reports, such as corrosion, material degradation, and structural weaknesses, to the study’s goal of enhancing maintenance practices and infrastructure management.
• Evaluation of Findings: The revised content focuses on how the findings from the inspection reports inform the development of more effective maintenance strategies and highlight gaps in current practices.

Presentation of Aspects Performed on the Bridge:

The revised section clarifies the meaning and benefits of presenting the aspects performed on the bridge:

• Detailed Inspection Procedures: A thorough description of the inspection procedures, including techniques and technologies used, is now provided. This includes explanations of how these procedures assess various aspects of the bridge’s condition.
• Implications of Findings: The analysis discusses the implications of the inspection findings, detailing how they influence maintenance decisions and strategies. This approach underscores the practical benefits of the analysis in improving the bridge’s upkeep.

Benefits of the Analysis:

The updated analysis emphasizes the benefits of examining the inspection reports:

• Enhanced Insight: By linking inspection findings to specific maintenance needs, the revised analysis offers a deeper understanding of the bridge’s condition, and helps identify areas where maintenance efforts can be improved.
• Strategic Maintenance Planning: The analysis provides actionable recommendations for refining maintenance strategies, demonstrating how these insights can lead to more effective management of the bridge’s structural health.

Novelty of the Analysis:

The revised analysis highlights its novelty and contribution to the field:

• New Insights: The analysis presents new perspectives on the structural challenges faced by the Machico Stayed Bridge, offering insights into issues such as unique patterns of deterioration and previously undocumented problems.
• Innovative Approaches: The section details any innovative methods used in inspecting and analysing the bridge’s condition. This includes the application of advanced technologies and techniques that provide a more comprehensive understanding of the bridge’s health.

Additional Observations:

While innovative techniques like jet grouting were employed to address challenging soil conditions, the initial design and construction of the Machico Stayed Bridge could have better considered the corrosive marine environment [6]. The adoption of more durable materials, such as stainless steel for cables and high-performance concrete, coupled with a robust waterproofing system, might have prevented the water infiltration and corrosion observed in the 2024 inspection [8].

Increased tourism, partly due to airport expansion, has exacerbated traffic on the bridge, worsening wear and tear. The absence of a comprehensive traffic management plan may have intensified this impact, highlighting the necessity for strategic planning to mitigate tourism’s effects on urban infrastructure.

A holistic approach, as suggested by Garcia-Hernandez et al. [52], is essential for sustainable urban planning. This approach should balance the economic benefits of tourism with its potential negative impacts on the environment and local communities. The financial implications of neglecting preventive maintenance are starkly illustrated by the VSL proposal for the Machico Bridge’s rehabilitation, which estimates a total cost of EUR 625,000 [53]. This underscores the economic burden of addressing advanced deterioration, and emphasizes the long-term financial benefits of a proactive maintenance approach.

The challenges faced by the Machico Stayed Bridge, particularly its impact on urban mobility, reflect broader mobility issues in the Autonomous Region of Madeira (RAM), as outlined in the PAMUS RAM [51]. The bridge’s current state highlights the consequences of not addressing regional mobility problems at the local level. The PAMUS RAM’s emphasis on sustainable transportation solutions, such as public transport, walking, and cycling, provides a valuable framework for addressing the specific issues faced by the Machico Bridge within the context of the region’s mobility goals.

6.2. Exploring Alternative Futures for the Machico Stayed Bridge

The preceding analysis has highlighted a series of interconnected challenges facing the Machico Stayed Bridge, ranging from the corrosive effects of the marine environment to the increasing pressures of tourism and urban mobility. These challenges necessitate not only immediate repairs and maintenance but also a revaluation of the bridge’s future role within the broader context of sustainable urban planning. Considering these considerations, we explore alternative futures for the Machico Stayed Bridge, envisioning strategies that prioritize long-term sustainability, community well-being, and environmental resilience.

The setbacks outlined in the preceding sections necessitate a critical evaluation of the Machico Stayed Bridge’s future, considering not only essential repairs and maintenance to ensure its continued functionality and safety, but also alternative visions that align with the broader goals of sustainable urban planning and enhance the quality of life for residents and visitors alike, such as the potential pedestrianization of the bridge, transforming it into a dedicated space for pedestrians and cyclists.

The Machico Stayed Bridge is vital for connectivity and tourism, but it also underscores the challenges of balancing mobility with sustainable urban development. Recent debates about traffic restrictions in Machico’s city centre highlight the diverse perspectives and complexities involved. Some residents criticize street closures without adequate public consultation or alternatives, emphasizing the importance of careful planning and community engagement when implementing measures that affect urban mobility.

Some residents emphasize the need for transport alternatives and ensuring accessibility for all citizens, mentioning the creation of a bank of equipment for people with reduced mobility, demonstrating the City Council’s commitment to inclusivity, and highlighting the importance of prioritizing universal accessibility in any future interventions on the Stayed Bridge, such as restricting vehicle access, to ensure that everyone can continue to benefit from this vital infrastructure.

6.3. Case-Specific Analysis for Sustainable Urban Planning and Infrastructure Management

While the previous sections have provided an overview of the Machico Stayed Bridge’s structural and environmental challenges, the discussion now focuses on applying sustainable urban planning and infrastructure management strategies specific to the bridge’s context.

Initiative-taking Maintenance and Monitoring Using Structural Health Systems

• In regions with extreme environmental conditions like Machico, integrating Structural Health Monitoring (SHM) systems is essential. These systems, including vibration sensors and corrosion detection devices, would allow for the real-time monitoring of the bridge’s integrity.
• Case Example: The Queensferry Crossing Bridge in Scotland, a cable-stayed bridge, employs a similar monitoring approach. Using over 2000 sensors, it detects structural issues such as cable tensioning and weather-related effects. The data are analysed continuously to predict maintenance needs and extend the bridge’s service life.
• Applying such strategies to Machico would allow for the early detection of issues such as prestress loss in the cables or concrete cracking, which were highlighted in the 2024 inspection report. This method directly supports sustainable infrastructure by minimizing the environmental and economic costs associated with sudden repairs or failures.

Material Selection for Corrosion Resistance in Marine Environments

• The analysis in the previous chapters emphasized the impact of salt-laden air and water infiltration on the Machico Stayed Bridge. A sustainable material management strategy would involve upgrading future repairs with materials that resist corrosion more effectively, such as stainless steel or composite materials.
• Case Example: The Millennium Bridge in London incorporates composite materials, including high-performance concrete and anti-corrosion treatments, to ensure the structure withstands the damp and polluted urban environment.
• For Machico, using materials like weathering steel (which forms a stable rust layer, protecting the underlying metal) would extend the bridge’s lifespan and reduce long-term maintenance costs. This would directly align with sustainable urban development principles by reducing resource consumption and repair frequency.

Integration of Soft Mobility Solutions to Reduce Environmental Impact

• Sustainable urban planning in the Machico region must address the reliance on vehicular traffic, which adds stress to the bridge and contributes to its accelerated wear and tear. An effective solution would involve creating alternative mobility solutions, such as dedicated cycling paths or pedestrian-only zones on the bridge.
• Case Example: The Cycling and Walking Investment Strategy (CWIS) in the UK promotes reduced car-dependence through enhanced pedestrian and cycling infrastructure. In Copenhagen, more than 50% of commuting happens via bicycle due to such dedicated infrastructure, reducing the environmental impact on urban spaces and infrastructure wear.
• For Machico, transforming part of the bridge into a shared pedestrian–cyclist zone during peak tourism seasons could lessen vehicle-induced stress while encouraging eco-friendly travel. This would significantly reduce both environmental and infrastructure-related impacts, supporting the long-term sustainability of urban mobility systems.

Tourism Management and Infrastructure Durability

• Tourism is a major driver of traffic on the Machico Stayed Bridge, especially during the summer months. A targeted tourism management strategy that accounts for the seasonal influx of visitors should involve limiting vehicular access during peak periods and prioritizing alternative modes of transport like public shuttles or electric vehicles.
• Case Example: Venice, Italy, facing a similar issue with tourist overuse of its infrastructure, implemented policies that limit private car use and instead provide eco-friendly transport alternatives such as electric ferries. This reduces the strain on historic bridges and urban infrastructure.
• Machico could similarly introduce a seasonal traffic reduction plan that restricts non-essential vehicles from using the bridge during high-traffic periods, ensuring the infrastructure remains durable while accommodating the economic benefits of tourism. Additionally, implementing eco-friendly shuttles between tourist hotspots could reduce the load on the bridge while enhancing the visitor experience.

Economic Sustainability and Public Engagement

• The economic viability of long-term infrastructure projects, particularly in small municipalities like Machico, requires effective resource management. Establishing public–private partnerships (PPP) to fund infrastructure upgrades, and engaging the local community in planning decisions, ensures alignment between economic growth, sustainability goals, and public interests.
• Case Example: The Copenhagen City Infrastructure Investment Plan has successfully combined public investment with private funding to support sustainable urban infrastructure while involving residents in planning decisions. This has led to improved public transport systems, reduced traffic congestion, and better-maintained infrastructure.
• For Machico, a similar approach could involve using community feedback to shape future infrastructure decisions, such as whether to pedestrianize the bridge or implement a toll system to fund maintenance. This would help ensure the infrastructure meets local needs while supporting long-term economic sustainability.

7. Conclusions and Recommendations

7.1. Recommendations for Infrastructure Management

To ensure the long-term sustainability and resilience of the Machico Stayed Bridge, a comprehensive maintenance plan encompassing both preventive and corrective measures is essential. The following recommendations address the bridge’s specific challenges:

• Corrosion Monitoring and Mitigation: Implement a rigorous corrosion monitoring program utilizing techniques such as electrochemical measurements, cathodic protection, and corrosion-resistant coatings [22]. Regular inspections should identify early signs of corrosion and assess the effectiveness of these measures.
• Structural Health Monitoring (SHM): Deploy SHM systems, including ultrasonic guided waves [21], to continuously monitor the bridge’s structural health. This will help detect potential issues early, assess performance, and optimize maintenance strategies.
• Concrete and Water Infiltration Repair: Address concrete cracking and water infiltration by repairing or replacing damaged elements, applying waterproofing membranes, and improving drainage systems to prevent further water ingress.
• Traffic Management Plan: Develop a traffic management plan to regulate vehicle flow on the bridge. This may include weight restrictions, speed limits, and promoting alternative transportation modes. Aligning with Krüger et al. [53], intelligent transportation systems (ITS) can optimize traffic flow and reduce congestion [54,55].
• Participatory Urban Planning: Engage in transparent urban planning, considering community input and technical studies for future decisions about the bridge. Options such as making the bridge exclusively pedestrian should balance various interests and address mobility needs.

The LREC technical report [17] emphasizes the need for preventive maintenance and continuous monitoring. Protective measures like coatings and cathodic protection are crucial for extending the bridge’s lifespan and avoiding costly repairs.

The PAMUS RAM [51] recommends developing strategic transport interfaces in Machico to enhance intermodal integration. This approach will foster sustainable urban mobility by integrating various transportation modes, promoting active travel, and improving the overall quality of life.

Parking management is also critical. Implementing policies that promote turnover and reduce car dependency, such as parking pricing and park-and-ride facilities, can support sustainable mobility and protect the bridge’s structural integrity.

7.2. Materials and Construction Techniques for the Future

Future interventions on the Machico Stayed Bridge should prioritize materials and construction techniques that are resistant to corrosion and suitable for the harsh marine environment:

• Durable Materials: Use stainless steel or weathering steel for stay cables and other exposed metallic elements to increase corrosion resistance.
• Enhanced Concrete: Employ concrete with high durability properties, such as low permeability and resistance to chloride penetration. Incorporate supplementary cementitious materials, like basalt, to enhance the concrete’s performance [56,57].
• Advanced Coatings: Apply advanced protective coatings, such as epoxy or polyurethane-based options, to safeguard against corrosion and abrasion.
• Cathodic Protection: Implement cathodic protection systems to control corrosion in steel reinforcements and other metallic components.

By adopting these recommendations, the Machico Stayed Bridge can become a model of sustainable infrastructure management, ensuring long-term functionality and safety. These practices will also serve as valuable references for future infrastructure projects in similar environments.

The estimated rehabilitation cost of EUR 625,000 highlights the importance of proactive planning and preventive maintenance. Addressing maintenance needs early can significantly reduce future costs and extend the bridge’s lifespan.

The PAMUS RAM [51] emphasizes managing tourist mobility to reduce the impact on infrastructure. Strategies like dedicated tourist-oriented public transport services and improved public transport information can alleviate pressure on the bridge and enhance the overall tourist experience.

Implementing these strategies will contribute to a balanced approach to tourism and infrastructure management, preserving critical infrastructure while improving regional mobility and sustainability.