Transition Effects in Bridge Structures and Their Possible Reduction Using Recycled Materials

Abstract

This article serves as a review of the current challenges in bridge engineering, specifically addressing the transition effect and the utilization of recycled materials. It aims to identify research gaps and propose innovative approaches, paving the way for future experimental studies. As a review article, the authors critically analyze the existing literature on the transition effects in bridge construction, their causes, and their negative impacts. Integral bridges are discussed as a solution designed to work in conjunction with road or rail embankments to transfer loads, minimizing maintenance and construction costs while increasing durability. Particular attention is given to the potential use of modified plastic composites as an alternative material in integral bridge structures. This concept not only addresses the issue of plastic waste but also promotes the long-term use of recycled materials, a key consideration given recycling limitations. This article highlights the importance of the connection between the embankment and the abutment and provides examples of polymer applications in bridge engineering. By outlining the state of the art, this review identifies future development paths in this niche, but promising, field. Almost 240 literature items were analyzed in detail, and works containing 475 different key words contained in about 3500 individual works were used for scientometric analysis. The results of the analysis clearly indicate the novelty of the presented subject matter.

1. Introduction

The transition effect in bridge structures refers to the combination of adverse phenomena occurring at the junction of embankments and engineering structures. This complex issue, caused by differences in settlement, stiffness, and dynamic loads, leads to increased wear, damage, and maintenance costs. Addressing this challenge is critical for ensuring the long-term durability and safety of infrastructure.

In traditional bridge designs, expansion joints, bearings, and transition slabs are employed to manage these effects. However, these solutions often fail to provide a gradual transition, resulting in increased dynamic loads and accelerated wear. Integral bridges, which eliminate these elements, offer a promising alternative by enhancing durability and reducing maintenance needs. However, their application is limited due to challenges in managing differential settlements and stiffness transitions.

Simultaneously, the increasing accumulation of plastic waste has become a pressing global issue. Recycling polymer materials into construction applications, particularly in transition zones, presents an innovative approach to addressing both environmental and engineering challenges. Integrating recycled polymers into embankments could enhance their stiffness, mitigate differential settlements, and reduce the overall impact of the transition effect.

This article provides a comprehensive review of the transition effect in bridge structures and explores the potential for integrating recycled materials into embankments. By analyzing current solutions, identifying gaps in research, and proposing future directions, this study aims to pave the way for sustainable and efficient innovations in bridge engineering.

2. Methodology

The methodology used was appropriate for exact sciences. The dominant approach was the use of the quantitative counting method. Based on the collected finite set of biblio-graphic data, a graphical result of the research was prepared, and conclusions were drawn using specialist software. A graphical method was used for the development. The size of the cluster reflects the frequency of occurrence of the keywords. The thickness of the connections the frequency of co-occurrence of given words. The color distinction links the words with the date in which the texts containing them were published. This was carried out using simple mathematical formulas embedded in the software. On the set containing the elements necessary for the proper performance of systematic reviews and meta-analyses, relationships were assessed. Thanks to this analysis, other authors can obtain a transparent and complete state of knowledge from this research. Before delving into a detailed analysis of the available literature related to the proposed topics in this publication, a scientometric analysis was conducted based on publicly available library catalogs. This type of analysis aims to highlight the major scientific trends and areas requiring in-depth research. It facilitates the creation of visualizations for data analysis and establishes connections between sources, keywords, authors, and articles within a specific research area. Researchers from various fields of science utilize scientometric analysis [1,2,3,4,5].

Currently, scientists have access to a substantial collection of library catalogs promoted by various publishers [5,6]. The analyzed literature was selected based on the widely accessible and most reliable databases Scopus [7] and Web of Science [8]. Thanks to them, it was possible to prepare the search results in the form of Figure 1, which illustrates the frequency of occurrence of keywords, their mutual connections, and the novelty of their appearance. Due to the multifaceted nature of this article, the explored topics covered various fields, including civil engineering, material engineering, chemical engineering, chemical sciences, environmental engineering, and environmental sciences.

To correctly perform a bibliography analysis, it is necessary to determine the minimum number of keywords that must be included in a single work. Often, the overlap of two keywords is far from sufficient. Only with three keywords do single articles begin to share common parts. Data from Scopus and Web of Science databases for the selected searches (three and four keywords) were exported in comma-separated values (CSV) format for importing into the appropriate software tool. Mapping and visualizing the academic network were carried out using the VOSviewer software version 1.6.20 (developed by the Centre for Science and Technology Studies of Leiden University, Leiden, The Netherlands). VOSviewer is a software tool used to construct and visualize bibliometric networks based on citations, bibliographic couplings, co-citations, or co-authorship relations [9].

The visualization of co-occurrence networks of keywords, their relationships, and the density associated with the frequency of their correlations were examined and are presented in Figure 1. The size of the keyword node denotes its frequency, while its location represents its co-occurrence in the publications. The entirety of the connections is shown, where the material scope has been omitted. To identify clusters among over 4500 author keywords contained in nearly 3500 individual works, those appearing a minimum of three times were selected, reducing the number of analyzed words to 475. This demonstrates a significant diversity of topics and areas. Words that were entirely unrelated thematically were excluded from the visualization, leaving around 350. Many weakly connected words imply their infrequent occurrences, being referenced only a few times with other analyzed words from different texts. This indicates the vast fragmentation within the research area. The limitations of this study include the availability of the literature in the databases, the analyzed journals being in English, the limitation of the number of analyzed manuscripts and words, and the need to compensate for the analysis and review in this article.

This visualization can not only assist in finding trends but also aid future authors in selecting presented keywords to find published data on specific topics. The results presented in Figure 1 indicate that topics related to bridge engineering have been less frequently addressed in recent years. Popular areas in publications seem to revolve around recycling and the use of reinforced polymers. However, there is a lack of keyword connections between bridge-related topics and the use of reinforced polymer composites, not to mention the transition effect, for which there is a minimal number of keywords. The interesting range in this study is marked with a red square. It is clearly visible that it is not very developed and not very intensive. The analysis presented clearly indicates the necessity for further research in the highlighted subject matter.

3. General Purpose of Research

The aim of this review article is to showcase the current solutions in bridge engineering, focusing on reducing the transition effect and emphasizing the significance of the problem. Additionally, the issue of plastic recycling in the context of utilizing waste plastic as an advanced material base for construction is outlined.

First and foremost, our research introduces a novel approach to addressing the persistent challenge of transition zones in bridge structures. Integral bridges not only eliminate the need for expansion joints and bearings but also enhance durability and reduce maintenance costs—a departure from conventional bridge construction practices.

Furthermore, our study underscores the paramount importance of sustainable development in bridge engineering. We advocate for the utilization of recycled materials, particularly polymers reinforced with fibers, as viable alternatives to traditional construction materials. This emphasis on sustainability aligns with the evolving ethos of modern engineering practices, wherein environmental considerations play an increasingly pivotal role.

Additionally, our research explores the untapped potential of substitute materials processed and shaped to meet the rigorous demands of bridge engineering. By harnessing innovative solutions and materials, we envision a future where construction practices are not only more sustainable but also more cost-effective and ecologically sound.

Moreover, a comprehensive review was conducted on PET (polyethylene terephthalate) as a promising material for integral bridge construction, highlighting its superior strength properties and compatibility as fiber composites. This in-depth analysis lays the groundwork for the further exploration and utilization of PET and similar materials in bridge engineering applications, especially taking into account recycling restrictions. It should be emphasized that the construction of connecting structures forcing cooperation in integrated bridges between the facility itself and the embankment using the indicated polymer material will be a novelty in engineering, eliminating the transition effects.

4. Research Area

The research area focused on assessing transition effects using post-consumer polymer materials is multifaceted. In today’s world, solving individual problems in the short term is challenging. When advancing knowledge in one field, it is essential to consider whether materials available in other areas might be useful for solutions. Therefore, construction itself generates a large demand for materials and can effectively utilize materials that are considered waste in other sectors.

This study addresses the following aspects:

• Engineering Aspect: Current projects do not entirely solve the issue of stiffness discontinuity (flexibility) in the subgrade for linear structures approaching bridges. According to the authors, further progress supported by appropriately developed materials, and the spatial structure of the joint construction will undoubtedly enrich contemporary bridge engineering with new design solutions that could reduce the transition effect.
• Environmental Aspect: The new material and its application area offer opportunities to utilize large amounts of waste that accumulate in landfills or are incinerated. Due to their longevity anticipated in engineering constructions, the properties of processed waste will serve future generations. Pursuing the idea of sustainable development involves seeking applications for waste that is considered inefficient, difficult, or costly to process. This fact guarantees a constant demand for solutions enabling recycling.

5. Detailed Overview

5.1. Outline

Bridge construction history dates back several thousand years [10]. Bridges are an integral part of infrastructure and have played a pivotal role in civilization’s development. With technological advancements and heightened demands for durability and safety, engineers increasingly seek for superior solutions and construction materials. Bridges stand among the most significant engineering structures, serving as essential components of transportation infrastructure, facilitating access across natural obstacles like rivers, valleys, and artificial barriers, ensuring convenient and safe passage to human settlements. They form a fundamental factor in societal development by enabling the transportation of goods and people [10,11,12,13,14,15].

Bridge structures must have stability and the necessary load-bearing capacity to meet traffic intensity and load requirements. They must also be durable and safe for users [16,17]. Hence, the construction materials used must meet stringent strength requirements, including resistance to variable and dynamic loads, corrosion, and weather conditions [16,17].

Since the late twentieth century, experts have debated how to repair, renew, and modernize transportation infrastructure in Central Europe [18]. Over the past 25 years, there has been a focus on utilizing modern engineering technologies and decision-making processes to solve typical and regional environmental problems in land transport, especially roads and bridges.

The transition effect occurs at points where different types of permanent way or pavements are connected, laid on various substrates (e.g., soil subgrade, engineering structures) [19,20]. This effect is a complex phenomenon, as illustrated in Figure 2, showing the most significant causes of its occurrence, including permanent settlements at abutments δ_a (usually smaller) and embankments δ_e (usually larger), progressing over time with unequal values for individual elements and with momentary deformations from live loads δ_s (deflection of girders, rails, and substrate deformations), causing the rotation of the span φ_s. Among the surface-related causes, differences in stiffness between the ballast and ballastless tracks, as well as varying stiffness parameters across the transverse direction, can be listed. They also include the primary geometric irregularities of rails and the inaccurate alignment of the gradeline on and outside the facility. Structural causes include primary geometric irregularities, deformations, settlement, and increased stiffness. Causes related to the substrate involve poor sub-ballast compaction, excessive stiffness differences between sub-ballast and substrate, sub-ballast contamination, and improperly chosen geotextiles. Due to different settlement values at the abutment and embankment, a so-called “threshold” is formed, which leads, among other reasons, to additional dynamic loads on the structure and excessive permanent deformations of the surface, amplifying dynamic effects and causing increased wear and tear and damage to pavements, track superstructures, substrates, and structures.

In order to determine the impact of this effect, the so-called transition zone is distinguished. It is the area where the embankment meets the bridge structure, also referred to as the embankment–abutment interaction zone [21,22]. It is one of the most critical locations along a road or railway line. Due to the fact that the unit stresses acting on the ground beneath the embankment base are usually smaller than those under the abutment foundation slab of the bridge, the settlements of the abutment will be larger, causing unevenness in the grade line [23]. It is impractical to ensure the same settling for the entire embankment over many kilometers as for the rigid bridge abutment. Hence, a limited zone is defined, where a change in stiffness must occur.

5.2. Engineering Aspect

Designing and constructing bridges with their associated embankments require special attention. These structures are often placed in locations where there are poorly compacted, young alluvial sediments and sometimes clayey or peaty soils, which undergo significant settlement [24]. Crossing rivers and watercourses is always associated with the risk of riverbed erosion and changes in ground level directly in contact with the bridge support. The transmission of loads to the subsoil is significantly influenced by repeated, cyclic, and variable loads [25]. The practical application of new soil-strengthening technologies beneath embankments also introduces new risks to the overall stability of the structure. The transition zone at the embankment–abutment junction is particularly vulnerable. This issue is not new; it was previously addressed in early post-war standards [26,27]. In recent years, this problem has been extensively highlighted in the literature and at numerous conferences [28,29,30,31,32,33,34,35,36,37,38]. It is worth noting that the most significant issues in this area often occur during execution due to gaps in evaluating intermediate schemes during design phases [25]. The issue of ensuring uniform support on the road or railway at the junction of an engineering structure and a deforming earthwork concerns most existing and renovated structures of this kind and, to a lesser extent, new lines, where the issue of non-uniformity can be largely prevented through the use of appropriate designs [30,39]. So far, clear requirements and solutions ensuring a gradual change in the substrate’s stiffness for transition zones and reducing the effects of increased rolling stock actions in engineering structure areas have not been defined [30,39].

The embankment adjacent to engineering structures should be protected to equalize settlements at the junction with the support and prevent irregularities in the road pavement or rail track superstructure. Currently, a commonly used solution to mitigate substrate stiffness changes is a transitional slab. Its concept involves softening the stiffness by supporting it unilaterally on a rigid abutment on one side and through resilient embedding in the embankment on the other side The presented solution is demonstrated for integral abutments. The transition slab solution is depicted in Figure 3.

However, analyzing the most common damages during the “lifetime” of structures, such as road pavement cracks and track superstructure deformations, suggests that this solution might not be an entirely effective solution as it does not ensure a sufficiently gradual change in substrate stiffness (flexibility) [40]. The analysis of the most frequent damages, such as cracks and deformations, suggests that the transition slab is not entirely a suitable solution as it does not provide a sufficiently gradual change in the substrate’s stiffness (flexibility) [40]. This statement is vividly illustrated by photographs taken on numerous bridge structures, with selected examples shown in Figure 4 [40,41], and it is also supported by the content presented in publications by several authors [40,42,43,44,45,46,47,48,49,50,51]. By reviewing the available literature and observing existing bridge structures, it becomes evident that surface damages occur at the beginning of the transition zone, where the transition slab starts. Road pavement cracks mark the onset of gradual road degradation and can pose a danger to vehicles over time. However, for trains, there are even more risks involved. The track geometry always degrades faster in transition zones than in open tracks, causing significant irregularities (dips) [44]. Such irregularities in geometry can lead to substantial forces, potentially damaging track components, compromising passenger comfort and even causing derailment [31,32,52,53,54,55,56,57,58,59,60].

Very different design solutions for transition zones on railway lines are provided, but they do not include a comprehensive assessment of the transition effect [32,57]. In contrast to roads where reinforced concrete transition slabs are frequently used, they are not popular for railway lines. Instead, the reinforcement of the embankment with geopolymers, draining and vibration-isolating mats, synthetic resins, stabilized soil blocks, gabions, reinforcements using stone columns or micropiles, dogging, chemical stabilization of sub-ballast, extending and widening substructures, and using stiffening rails within track rails are employed. Known solutions also include the compensatory injection method and polyurethane mass-stiffening aggregate.

The use of substitute materials in the integral bridge engineering of embankments behind the abutment is often used to relieve this issue. By reducing the pressure on the abutment, its dimensions can be reduced, thus minimizing the difference in stiffness (compliance) between the surfaces. However, the resulting void between the back wall of the abutment and the embankment creates a discontinuity that can negatively impact operational qualities, increasing the transition effect. This type of solution has been studied, among others, by Horvath [61]. Engineers must therefore balance the selection of solutions to obtain a satisfactory effect.

Road and railway embankments, as well as bridge abutments, can be designed as reinforced soil structures [21,22]. Additional reinforcement at the bottom of the embankment, extending beyond the outline of the transition slab, is often used to minimize settlement differences [23]. The purpose of reinforcing the ground is to increase the soil shear strength and reduce the pressure on the abutment [62]. In such structures, active forces and external loads are partly transferred through friction by the soil and partly through the reinforcement anchored in the ground. Reinforced soil structures can particularly be applied as a soil block separated from the front and side walls of the abutment, bearing the embankment pressure, as a retaining structure forming the abutment walls and as a foundation support for piers. In the case of a significant increase in pressure on native soil, there is a risk of uneven settling or loss of stability. To reduce the differences between the embankment and abutment settlement in the transition zone, often, embankment soil reinforcement is necessary [21,22,23,45,63,64,65]. Using a reinforced platform allows for an even distribution of settling and can be controlled [66]. Reinforced soil structures, serving as retaining structures, can be executed using passive systems, where soil reinforcement comprises both structural elements (reinforcement wraps embedded within the reinforced soil block) and auxiliary components (anchoring inserts placed between the structural reinforcements), which anchor the protective elements of the wall. Alternatively, active systems incorporate reinforcing soil inserts anchored in the wall’s protective elements (reinforcing–anchoring inserts). Simultaneously, protective elements functioning as formwork for subsequent layers of fill material are placed alongside the formation of the reinforced earth block. The protective wall and reinforced soil block constitute separate structures, enabling their independent construction. For abutment elements, utilizing reinforced soil structures in a passive system is recommended due to the lesser impact of embankment settlements [63,64,65,67,68]. Flat steel strips, ribbed steel bars, polyethylene geogrids, polymer strips, or strips made of polyester fibers with a polyethylene coating serve as soil reinforcement. The reinforcement connects with the protective elements using systemic connectors.

The use of geosynthetics in constructing transition zones holds significant potential. Reinforcing the subsoil of the embankment and increasing its stiffness will reduce the transition effect.

Implementing engineering structures combined with embankments on weak ground is a complex task. Evaluating the load-bearing capacity and stability of road or railway embankments is the initial step [69,70,71]. Structures embedded within the embankment are usually supported on piles in challenging conditions [23,72,73,74].

Placing the structure on deep (pile) foundations results in minimal settlements [23], whereas allowable settlements for adjacent embankments can reach several dozen millimeters [69,71,72,74,75]. If the transition zone is not adequately designed or if the settlements are not uniform, unacceptable irregularities might emerge, which are also unacceptable to users. This aspect is more critical for railway lines [22,69,70].

Engineers often grapple with the durability issue of bridge structures. According to EC 0, bridges fall under category (class) S5, which implies an approximate design service life of at least 100 years. There are also structures over 200 years old that are still in use. Hence, the aspect of the durability and safety of the structure while limiting construction and operation costs is crucial [76].

The elements with the shortest lifespan in bridge structures are expansion joints and bearings. The most advanced ones can operate for only about 30 years [77]. While piers and girders determine a bridge’s load-bearing capacity, equipment elements significantly influence its durability and maintenance costs. The immediate cost of equipment element installation constitutes around 15–20% of the bridge’s construction costs. Their contribution to maintenance and repairs is often much higher, frequently exceeding 50% [17,62,76].

Ensuring the durability of bridge structures imposes specific requirements on the longevity of individual components. Despite advancements in bearing and expansion joint construction, production, assembly, and maintenance costs continue to rise. Additionally, there are increasing demands for the qualifications of personnel involved in these processes. The growing number of structures adds to maintenance challenges, highlighting the need for simpler solutions. Integral bridges present a viable option in this context.

Frame structures, particularly integral bridges, are not equipped with these expensive solutions [40,62,77,78,79,80,81,82,83,84]. Hence, they are less susceptible to natural and human-induced threats while requiring minimal maintenance throughout their service life. Most conventional bridges are equipped with components compensating for distortions and displacements. Water, salts, and other deicing agents can seep through expansion joints, leading to corrosion in bearings, load-bearing structures, sidewalk parapets, and other bridge elements. The absence of expansion joints in integral bridges reduces repair and maintenance costs throughout their service life [85,86], aligning with the urgent global need for low-maintenance transportation infrastructure [87].

Furthermore, integral bridges, when integrated into highways or railway lines, enhance riding comfort due to the absence of expansion joints and provide better horizontal stiffness perpendicular to the longitudinal axis of the bridge. This reduces the likelihood of rail misalignment [85]. Moreover, modern integral bridges exhibit resilience during earthquakes owing to their monolithic construction [85,86,87,88,89,90,91,92,93]. Thanks to these advantages, such structures can be appealing to entities managing transportation infrastructure.

These include single- or multi-span structures with a continuous deck connected to the abutment. The connection between the abutment and the deck can be rigid or semi-rigid, depending on the structural solution of the connection. The abutment can be articulated on the foundation or supported on piles. The fundamental structural solutions for these bridges are decks integrated with abutments based on shallow direct foundations, solid abutments, or partially integrated ones.

An integral bridge is a specific type of bridge structure designed to collaborate with an embankment in a way that minimizes the negative effects of the transition effect. Figure 5 shows an example of an integral bridge. In such cases, the embankment soil is often reinforced.

The longitudinal forces arising from the thermal expansion or contraction of the bridge deck are transmitted to the abutments and to the soil behind the abutment [94]. In the design assumption, the abutment does not necessarily have to absorb the entire horizontal forces due to thermal effects, as part of them will be transferred to the embankment, which will be integrated into the cooperation [94].

When calculating the ground reactions behind the abutment, the total length of the bridge is significant, determining the extent of its expansion or contraction due to temperature changes. However, the number of intermediate supports has no effect on the magnitude of the bridge’s expansion or contraction, and it does not influence the size of the ground reaction [62].

In integral bridges, forming a slip plane to stimulate movements in the upper layers of the embankment soil is essential to minimize vertical displacements that affect the magnitude of the transition effect [62]. Intermediate supports, typically reinforced concrete, can be articulated on the foundation or based on one or several rows of piles. Spans for structures with integrated supports are usually around 60 m, sometimes reaching 80–100 m. The number of piers can vary, and the lengths of the longest integral bridges can exceed 350 m (Happy Hollow Bridge, Hickman County, TN, USA).

The challenge in evaluating existing structures and applying them lies in the interaction between the bridge and embankment soil. In many cases, this interaction is misunderstood due to the inherent nonlinear behavior of soil during what is known as operational interaction, significantly altering stresses in the embankment soil due to abutment displacements [87]. A step toward a better understanding this phenomenon involves interpreting the support and embankment soil state at the beginning of dynamic excitation based on earlier interactions during operation and assessing the stiffness and strength of the existing cushioning properties of the supports under dynamic loads [87]. The typical geometry of an integral support and typical embankment soil are studied using fully constrained simulations, assuming a viscoelastic–plastic stress model for the soil (coupled approach) under static and dynamic loads [87]. Integral bridges are designed with the assumption that only minor settlements of the end supports of the structure and embankments are permissible for using this solution in a given location. It is essential to note that this assumption significantly restricts the application of integral bridges. The connection of the bridge structure with the embankment, depending on the length of the structure, the shape of the space under the structure, the type of obstacle, and the terrain conditions may be made through different solutions. For example, these may be: massive integral abutments (Figure 6a), integral wall abutments (Figure 6b,f), integral box frame walls abutment (Figure 6c) or pillars support (Figure 6d,e). A semi-integral solution can also be used with span overhang and placement in the embankment (Figure 6e,f) [21,48]. The stability of mutual interactions should be ensured, especially in bridge structures where elastic deformations of supports are used to accommodate the elongation of load-bearing structures [21,22].

When entering a bridge, a vehicle must cross a transition zone. During its design, engineers must consider that the road or rail and the bridge itself may have different mechanical properties, such as stiffness, density, and Young’s modulus. If the foundation at the entrance to the bridge is too stiff, it can lead to excessive stress in the bridge structure and increased risk of damage. Conversely, if the foundation is too soft, it may cause excessive settling of the bridge and a change in its geometry, which can result in hazardous situations for users. The mismatch usually occurs between a too compliant embankment and a too rigid bridge structure. Regulations seem to focus separately on the settling of individual elements, overly stiffening the structure (restrictively limiting deformations and settling) while overlooking the fact that soil structures will always settle more than those built from concrete and steel. Hence, it is reasonable to extend the bridge structure into the transition zone and connect it with a system that ensures efficient load transfer and smooth settling changes.

The problem of the transition effect, caused by the varying compliance (stiffness) of the pavement foundation in the transitional section onto the bridge, remains a current challenge for bridge engineers and continues to be researched and analyzed [29,30,31,33,34,61,80,81,83,84,94,95,96,97,98,99,100,101]. Intensity in addressing this issue has decreased in recent years, yet with increasing speed and tonnage, the problem continues to escalate. In the case of railways, any unevenness in the track profile becomes more important due to the possibility of jolting rolling stock, uneven wear on rail heads, or damage or dislodgement of track beds. For traditional roads, damage occurs to the surface, followed by subsequent structural elements. The issue of transition zones (and the transition effect within) in bridge engineering involves the use of numerous different solutions that are not standardized and have limited impact on reducing the set of adverse phenomena.

Several scientific publications describe the behavior and dynamic response of embankments, including transition zones [42,95,102,103,104]. Due to the recent increase in train speeds and transported tonnages, many researchers have undertaken studies into their dynamic effects and vibrations transmitted to the engineering object and embankment [105,106,107,108]. This aspect is crucial for the safety and durability of bridges, as dynamic forces generated by vehicle movement focus primarily in this area. The complex vehicle–soil–bridge system is where the bridge structure reacts with soil properties such as load-bearing capacity, shock resistance, and displacement [109]. Understanding the interaction between embankment and bridge is essential for the design, construction, and maintenance of bridges.

Presently, bridge design involves complex models encompassing the entire structure, considering mutual influences of parts with different stiffnesses and loads [37]. However, in the initial stages of such projects, simpler computational tools are used to analyze individual structural components. Paradoxically, conservative assumptions are adopted due to significant uncertainty. Consequently, actual settlements are much smaller than calculated, leading to uneven levels among intersecting parts and necessitating costly elimination of the resulting discrepancies. It is worth noting that requirements for limiting settlement differences [21,22,39] and the consequent design work do not consider time-variable settlements such as soil consolidation.

The analysis of deformations and mutual displacements of different parts of the same structure or neighboring structures is crucial. Both excessive and insufficient displacements are undesirable. Phasing of construction and the interaction between structures built at different times must be taken into account. Advancements in knowledge, research capabilities, and computational methods emphasize deformation analysis, displacements, and assessing serviceability limit states, pushing the issue of load-bearing capacity into the background [72,74]. Unlike previous practices, where in some cases only load-bearing capacity was analyzed, settlement matters were hidden in calculations through design assumptions.

The relevance of transition zone themes and the phenomena within them are confirmed by ongoing research [110,111]. The cited articles and their description of the interaction between the embankment soil and the bridge indirectly relate to the collaboration of these elements in response to the transition effect load. An important conclusion is the lack of examination of structures that facilitate and enhance the cooperation between the bridge structure and the embankment.

Despite numerous advantages, designing and constructing integral structures poses a challenge, with primary limitations in widespread application arising from the interaction between the abutment and embankment soil. This interaction results in vertical displacements of embankment soil and increases in passive soil pressure against the abutment [86]. Surprisingly, the literature has not reached a consensus on whether this is a beneficial or detrimental effect. Identified discrepancies point toward a conceptual gap in design and evaluation, requiring further research [86].

5.3. Environmental Aspect

In recent years, escalating environmental pollution and the depletion of natural resources have introduced new challenges to engineers. An effect of the increasing affluence of societies is the intensive growth in the quantity of waste generated. According to a World Bank report [112], currently, approximately 2 billion tons of solid waste are produced each year. Experts also predict that in about 30 years, we might expect nearly a doubling of garbage production. The enormous volume of waste poses a significant burden on the natural environment. Statistics indicate that an average resident of the European Union generates 530 kg of waste per year [113], of which approximately only 48% is processed. The share of polymer waste in recycling is only 25% [114,115].

According to the PlasticsEurope Foundation report [116] from 2022, in 2021, global plastic production increased by 4% to over 390 million tons, indicating a strong and continuous demand for plastics. However, Europe faces many challenges. The latest data show that China’s share in global plastic production continues to grow (reaching 32% in 2021), while Europe’s share—totaling 57.2 million tons in 2021—continues to decline (reaching 15%). In 2021, the recovery of plastic waste in the 27 EU27+3 countries exceeded 5.5 million tons, and post-consumer plastic waste used in new products and parts accounted for about 10% of plastic recycling and increased by about 20% compared to 2020 [116]. According to the cited report, global plastics production increased from 365.5 million tons in 2018 to 390.7 million tons in 2021. Plastic recyclates accounted for 30 million tons and 32.5 million tons, respectively. A slight increase in the share of recyclates from 8.2% to 8.3% can be observed [117]. In 2022, in the EU27+3 group of countries, out of 17.9 million tons of plastic packaging, 17% was landfilled, 46% was recycled, and 37% was recovered for energy [117]. In two countries, Belgium and the Netherlands, the goal of completely eliminating the landfilling of plastic waste was almost achieved [116]. In Central European countries, including Poland, the plastics industry is an important component of the economy, ranking third among industrial processing sectors in terms of gross added value generated. Employment in the entire industry is estimated at approx. 200,000 employees, and its development over the last dozen or so years has clearly outpaced other industries. The volume of local plastics production, estimated at 1.7 million tons, is insufficient to meet the needs of processors; therefore, a large part of the demand is covered with imported raw materials. Selective waste collection and the use of recycling can significantly improve the situation of the industry, where the collected fraction of PET is 28% [118].

Special attention needs to be paid to waste from high-molecular-weight plastics. Their largest source is packaging. According to data [119], only about 19.5% of plastic waste is sorted. This fact guarantees a constant demand for solutions allowing for recycling.

Growing environmental awareness and the need to reduce the construction industry’s impact on the environment encourage the search for alternative materials, such as polymers [120]. The diagram illustrating the global production structure of plastics in 2021 shown in Figure 7 was developed based on data from the PlasticEurope Foundation report [121]. By analyzing the structure of global plastics production, taking into account their frequency [121] and mechanical properties [122,123,124,125,126], we can identify several types of polymers that can be successfully applied in bridge engineering [120].

Polymers are materials with complex chemical structures, consisting of long chains of molecules called polymers, linked together through the process of polymerization [127,128,129,130,131,132]. The size and shape of monomers influence polymer properties [127,128,129]. Due to their exceptional properties, polymers are increasingly being used as structural materials in bridge construction [133,134].

Polymer materials are processed many times, which further reduces the need for limited resources, such as oil or gas, from which they are produced. However, it is important to note that with each subsequent recycling of polymer raw materials, their mechanical properties deteriorate. Essentially, after two cycles of plastic use (use–recycling–use), the material is no longer suitable for rational processing; its strength properties become too low [135]. It is desirable, therefore, to find a long-term use for waste material.

The main challenges hindering plastic recycling are the quality and price of recycled products compared to their virgin counterparts. This is due to the complex process and various difficulties to overcome [136]. This includes sorting technology, specific challenges related to mechanical recycling like thermo-mechanical degradation or degradation over the product’s lifespan, and the immiscibility of polymer mixtures. Plastic processors require large amounts of recycled plastics produced to tightly controlled specifications and at competitive prices [137]. Transitioning to a circular economy that retains plastic in its highest-value condition is essential to reducing the environmental impacts, promoting reduction, reuse, and recycling [138]. Mechanical recycling is an essential tool in an environmentally and economically sustainable economy of plastics, but current mechanical recycling processes are limited by cost, degradation of mechanical properties, and inconsistent-quality products [120]. Plastics can be easily customized to meet the functional or aesthetic needs of any manufacturer, complicating the recycling process and increasing its cost while affecting the quality of the final product [137].

A critical look at the recycling process can be found in the Center for Climate Integrity report [139]. Plastic pollution is one of the most serious environmental crises facing the world today. Between 1950 and 2015, over 90% of plastics were landfilled, incinerated, or leaked into the environment [139,140]. Plastic waste is ubiquitous—from our rivers, lakes, and oceans to roadways and coastlines. Underpinning this plastic waste crisis is a decades-long campaign of fraud and deception about the recyclability of plastics. Despite their long-standing knowledge that recycling plastic is neither technically nor economically viable [135,141], petrochemical companies and their trade associations have run misleading campaigns to promote plastic recycling. These efforts have expanded plastic markets and stalled effective regulation. By falsely promising the viability of plastic recycling, they have drastically increased virgin plastic production over the past six decades, worsening the global plastic waste crisis and imposing significant costs on communities [139]. Every product of plastic ever produced still exists today. Since 1950, 2 million tons of plastic have been created worldwide [116]. In studies [140], Roland Geyer et al. estimate that about 79% of all plastic waste ends up in landfills or directly in the natural environment. Ten million tons of plastic enter the oceans every year. The cost of pollution is estimated in billions of USD annually [142]. In 2019, an international research team published data in the Marine Pollution Bulletin stating that each ton of plastic waste in the oceans represents destroyed resources valued at up to USD 33,000. The scientists did not account for the indirect impact on tourism, transportation, and health. Considering these aspects, the social and economic costs of plastic waste in the oceans may be significantly underestimated [142].

Figure 8 shows the changes over time, between 2008 and 2020, in the way plastic waste is managed. We see that the share of landfilled waste has decreased by 47%. There has been an increase in the share of waste that is incinerated, and thanks to this, energy is recovered from it at 77%, and there has been a 117% increase in recycled waste. The charts are based on information contained in [116,117]. The following facts can be added regarding the above figure. In the EU27+3 group of countries, the percentage of waste going to landfills continues to decrease, but progress in this area is slow, especially in countries with a low level of plastic waste recovery. Half of the EU member states recover less than 30%. Every year, 25% of plastic waste in the EU27+3 still goes to landfills [121]. In Europe, 37% of plastic waste is incinerated in modern incinerators [116]. A study by Zero Waste Europe showed that even the most modern incinerators emit dioxins and other harmful pollutants [143,144]. Many of the countries accepting waste perform poorly in plastic waste management rankings. According to the World Bank, in developing countries, “over 90% of waste often ends up in illegal landfills or gets burned [145]”. This situation encourages intensified efforts to increase the amount of recycled materials [121].

Green energy still faces challenges, especially in wind energy, where durable, lightweight materials like carbon fibers are needed for producing turbine blades. However, their rapid consumption and the lack of comprehensive solutions for processing these materials pose societal problems [146,147,148,149,150,151,152,153,154,155,156,157]. From 80% to 90% of wind turbine installations can be recycled (concrete, steel, copper, and silica). The remaining 10% to 20% represents a critical issue in disposing of the materials used to build them [146,147]. This concerns the materials in the blades, known as fiber-reinforced polymers (FRPs), often made of glass, carbon, aramid, or basalt fibers. In the EU, demand for FRPs grew sharply from 5000 tons in 1991 to 346,000 tons in 2015. It is estimated that by 2030, 4 million tons of fiber-reinforced polymers will be used for zero-emission wind energy production in the European Union. The current proposed recycling methods do not solve the rapid increase in the amount of this type of waste [146]. New proposed actions in this area include prevention, involving services and repairs of turbine blades to enable their extended use; reusing them in other sectors of the economy, producing items like small architectural elements, playgrounds, furniture, and similar products; however, low processing capabilities and the dust generated during processing are issues with this; recovery through chemical or thermal treatment, which is currently unprofitable; and disposal through landfilling and incineration, which do not solve the problem and are banned in many countries; additionally, they are not in line with the principles of a circular economy [146]. A major problem is the disposal of used fiber-based components. Their large quantity poses a real challenge to societies [147,149,150,154,157]. Extracting fibers using pyrolysis and using them as dispersed reinforcement in polymer materials will certainly help manage large quantities of this specialized waste to some extent.

Currently, the simplest and most effective method of waste disposal appears to be the use of materials in civil engineering as construction materials. Construction accounts for the use of 4 million tons of recyclables in new products in the EU27+3 countries, which constitutes 46% of plastic recyclables [121]. Advances in science and technology allow for the utilization of polymers as structural materials in the construction of bridges and embankments, which may contribute to improved durability, corrosion resistance, and cost-effectiveness. Older structures, built using masonry technology, as well as those made of concrete and steel, require numerous analyses, such as acoustic emission to assess and monitor bridge integrity [93] or well-known non-destructive testing of bridges using acoustic and radar impulses [158,159,160]. Nowadays, most bridges are built using concrete and steel [16,93], employing well-established computational models and analyses. However, many new designs are being conceptualized as composite systems [161,162,163,164,165,166] or as concrete structures reinforced with polymer fibers from carbon, glass, aramid, or even basaltic materials [167]. These are structures still being explored, and their behavior in the future remains unknown.

5.4. Properties and Use of Polymers in Bridge Structures

In recent years, polymers have gained popularity due to their light weight, durability, corrosion resistance, flexibility, and ease of shaping [168,169,170]. The method that gives the greatest strength to polymers is the injection method, where the mass is pressed into a mold at high temperature and pressure [171]. These polymers are relatively easy adapt to the individual needs of a project, making them highly versatile construction materials. Moreover, polymers are readily available and widely used across various industries [116,117,119,121,133,134], making them more cost-effective than traditional construction materials. These are used as insulation material, adhesives, components, surfacing, reinforcement, and barriers [130,132,133,134]. There has been increasing attention given to thermosetting polymers such as epoxy, polyester, and epoxy resins [172,173,174], which are especially characterized by their high mechanical strength and corrosion resistance, making them attractive construction materials in bridge building [175,176]. Moreover, these polymers resist environmental factors like UV radiation, contributing to the durability of structures [177,178].

Polymer composites are used in construction [163,164,165]. They are useful for bridges near water bodies or in high-humidity areas and for underground structural elements due to their better corrosion resistance than steel. For this reason, soil reinforcements are realized using them. Despite numerous advantages, using polymers also has some drawbacks. One of the significant drawbacks is their lower resistance to high temperatures [167]. Polymers are less resistant to heat compared to traditional construction materials like steel or concrete. However, in certain applications, the risk of fire hazards to the structure might be minimal, or the polymer material might be adequately protected by non-combustible materials (e.g., soil or concrete). Another drawback is their reduced resistance to mechanical damage [167]. This aspect can be critical due to the cyclic and long-term dynamic loads that bridge structures experience. Here, in turn, the answer is composite materials, which, thanks to their high strength due to the reinforcement of polymers with fibers, are used as advanced construction materials. A serious threat to composite materials may be UV radiation; however, as engineers have shown, despite this, these materials are often used without additional protection. Protective coatings are also well developed and can effectively support polymer material. Ageing tests carried out on such materials with parameters selected for their use significantly reduce the risk of unexpected destruction. Taking these risks into account, a detailed analysis of the behavior of polymers and their performance in such specific environments should be required depending on the proposed solution. However, the scale at which polymers are used allows us to assume that the material can be appropriately and safely designed [168].

Polymers have been used in bridge construction for many years [133,161,162,163,166]. One of the initial uses of polymers in construction was the use of saturated polyesters to reinforce wooden bridges [179,180,181]. Polymers are used in various structural elements of bridges, such as girders, cables, beams, and spans. Presently, polymers are used in the form of laminates reinforced with fiberglass or carbon fiber [179,180,181,182,183,184,185,186,187,188,189,190]. Another application of polymers is their use as construction materials [161,163,164,166]. Polymers can also be used to increase bridges’ resistance to dynamic forces [191,192,193]. Fiber-reinforced polymer composites (GFRP), can enhance the flexibility and seismic resistance of bridge structures [194]. Another application involves using polymers as anti-slip materials [195,196]. Polymers like polyurethane or rubbers are employed to enhance traction on bridge surfacing, enhancing safety for pedestrians and vehicles. Finally, polymers can be used as fiber-reinforced materials to improve the strength and durability of bridges [180,181,182,184,185,186,187,188,189,190], for instance, epoxy resins reinforced with glass or carbon fibers.

The reinforcement of structures with polymer materials has long been popular [180,181,182,184,185,186,187,188,189,190]. In composite materials, glass, carbon, basalt, or aramid fibers are used as reinforcing fibers [77,164,167,197]. These materials possess excellent mechanical properties like high strength and rigidity, allowing for their usage in various applications [167]. Carbon fibers are among the most commonly used polymer-based materials for reinforcing structures [167,185,197,198]. They exhibit high tensile strength and a high modulus of elasticity. The fibers are highly resistant while remaining lightweight, making them an ideal material for bridge structures. The use of carbon fiber helps to reduce the mass of the structure, subsequently lowering production, transportation, and assembly costs. The process of reinforcing structures with carbon fibers usually involves placing a mat or mesh of carbon fiber on the structure’s surface and saturating it with epoxy resin, bonding the carbon fibers to the structure [185,197,198]. This process can be applied to reinforce concrete structural elements such as columns and beams, as well as wooden and metallic elements. Several concepts have been developed so far for creating composite materials using fiber-reinforced polymers [179,180,181,189,190]. Glass, carbon, aramid, or basalt fibers are used as dispersed fibrous reinforcement in these materials [133]. Research on such fiber-reinforced materials can be found, among others, in the studies [199,200,201,202,203,204,205]. When reinforcing structures with glass fibers, the process is similar. While glass fibers have slightly lower strength than carbon fibers, they are more cost-effective and flexible, allowing for their use in reinforcing more complex structural shapes [167,179,180]. Aramid fibers like Kevlar are also popular materials for structural reinforcement [167,206]. These fibers are highly durable and possess outstanding resistance to abrasion and perforation. Reinforcing structures with polymer-based fiber materials can improve their strength in extreme conditions like earthquakes, hurricanes, and tornadoes. Due to their strength and rigidity, these materials are excellent candidates for use in construction projects such as bridges.

Polymers have been extensively used in bridge and embankment construction [66,67,68,161,163,195,198,207,208,209,210,211,212,213]. Their application is known in hydro-insulation and surfacing [214,215,216]. For reinforcing embankments, geopolymers (geosynthetics) are particularly effective, especially in transition zones [66,67,68,207,208,209,211,212,213,217,218,219,220,221,222,223]. Initially, soil reinforcement was carried out using steel bars. The primary drawback of this solution, mainly in terms of durability, was the corrosion of the reinforcement. The use of reinforced concrete layers is also considered more historical. Currently, soil reinforcement primarily involves various forms of geotextiles [62] due to their advantageous properties and application aspects. Geosynthetics are synthetic materials used in geotechnical engineering to improve soil properties and geotechnical structures [62,210,224]. Geotextiles are one type of geosynthetic characterized by a low unit weight and high tensile strength [66].

Materials commonly termed geosynthetics are employed for soil reinforcement [66,67,68]. There are various types of geosynthetics used based on specific needs, including geogrids, geotextiles, geomembranes, and geocomposites [207,208,209,210,211,212,213,217,218,219,220,221,222,223]. The effectiveness of soil reinforcement can be increased by using high-strength geosynthetic geogrids, with a tensile strength of up to 1600 kN/m, which vertically absorb, distribute, or dissipate loads onto the soil [66]. Geotextiles for soil reinforcement constitute a kind of mat made from synthetic fibers, usually polypropylene, bonded together in the technological process of their production. It is essential to ensure suitable strength and deformation parameters (ductility) [62]. There are various solutions for constructing embankments using geotextiles. One popular approach involves using geogrids as reinforcements beneath the embankment. This solution increases the soil’s bearing capacity and reduces its deformation. Another method involves using geotextiles as a separation layer between different soil layers or as a filtration layer, preventing soil washout from the embankment by water, improving embankment stability and reducing erosion risk. Geotextiles used for embankment reinforcement at abutments cannot act as a diffusion barrier for air and water and must withstand low and high temperatures. They must also resist aggressive compounds (especially bases and acids), fungi, and decay [62]. A comprehensive analysis of geosynthetic reinforcement at the embankment base can be found in [66] and related studies [67,68,207,208,209,211,212,213,217,218,219,220,221,222,223]. It focuses on geosynthetic reinforcement in geotechnical structures, specifically the use of geogrids in embankment bases. The work presents results from laboratory tests and numerical analyses aimed at evaluating the effectiveness of using geogrids in reinforcing earth embankment bases. The research findings indicate that employing geogrids in reinforcing soil embankment bases enhances the structure’s load-bearing capacity and reduces deformation. The author also compares the costs of using geogrids against traditional reinforcement methods, which can be significant from an economic perspective. The study provides a new perspective on geogrid applications in geotechnics and presents research results that contribute to improving the efficiency and durability of soil structures. All these solutions use geotextiles to improve embankment properties and increase embankment durability. The use of geotextiles in embankment construction is extensively described in the scientific literature and engineering practice, and the choice of a specific solution depends on the soil conditions and design requirements.

An interesting application of polymers is their use in epoxy injection for repairing cracks in bridge structures [225]. One of the most intriguing applications can be found in a scientific work concerning an innovative method of reinforcing ground behind abutments using geopolymer injection [226]. This procedure enhances the load-bearing capacity and stiffness, improves embankment and structure cooperation, and enhances durability.

It is crucial to relate the above-mentioned aspect to the transition effect. Ensuring the safety of the structural design itself and appropriately designing the connection between the bridge structure and the embankment are significant aspects in the process of bridge construction. Excessive embankment settlements are one of the main causes of failures in this transition zone [227,228], often leading to numerous damages.

Commonly used embankment reinforcement methods mostly rely on soil replacement, subsoil consolidation, mechanical stabilization, vibratory and dynamic methods, deep-seated reinforcements, or the use of geosynthetics [229,230,231,232]. Using typical methods during renovations usually requires dismantling the pavement and conducting extensive earthworks, significantly increasing not only the costs but also hindering the operation of the structure during the works. Geopolymer material injection provides a non-intrusive alternative to traditional substructure work and piling.

For high abutments and abutments founded on piles, as well as ground behind the abutment with a low internal friction angle, horizontal forces play a significant role. These forces can be reduced by using polypropylene blocks resembling honeycomb patterns and expanded polystyrene (EPS) blocks—Styrofoam—for embankment construction [62,233,234,235]. In engineering, a solution for the embankment behind abutments involves using geosynthetic reinforcement and Styrofoam blocks to simultaneously reduce ground pressure on abutments and control settlement adjacent to the abutments [61]. The use of EPS geofoam as a filling material between viaducts and abutments has been described in several publications [61,62].

Studies on the application of dispersed reinforcement in concrete in the form of fibrous polymers have focused on mechanical properties and durability when used as concrete reinforcement [198,206,236,237,238]. The results indicate that fibrous polymers increase the strength and durability of concrete. Polymer reinforcement materials are typically provided in the form of mats or meshes, which can be easily cut and adapted to the dimensions of the surface to be reinforced. In the case of concrete, the fibrous polymer reinforcement material is usually placed inside the concrete mass before laying, allowing for better interaction between the materials.

5.4.1. Recycling Polymers

Polymers obtained from waste can be used in the construction of bridge structures [168,169,170]. One example of utilizing waste polymers is the production of plastics from secondary raw materials like PET bottles, which can be used to create unsaturated resins applied in bridge construction [239,240]. Another instance is the use of PET from recycling, in the form of sheets, to improve the strength parameters of sandy soil [241,242]. Ongoing efforts are also exploring realistic alternatives to classical materials used as reinforcements, such as steel or fiber-reinforced polymers, aiming to produce cheaper and more environmentally friendly structural elements with similar or superior performance during use. PET tapes and polyester yarn tapes could effectively compete with currently used materials. They are among the most commonly used materials in the packaging industry, boasting high tensile strength comparable to steel and being easily recyclable [243,244].

Currently, ongoing research is exploring the impact of reinforcing PET material with carbon fiber on its mechanical and thermal properties. Research has shown this to be the material with the best tensile strength properties among polymers commonly found in waste [116,119]. Due to its good recyclability and simplicity, it is one of the most commonly processed waste polymers [116,119]. A. Brzeski also focused on combining recycling polymers from waste with aggregate, creating a type of synthetic rock. Besides the obvious advantage of recycling, this solution allows for obtaining a material with controlled stiffness that is resistant to weather conditions, fire-resistant, elastic, durable, non-permanently deformable, and environmentally neutral [245]. Nowadays, there is a developing trend in using biopolymer fibers derived from organic organisms reinforced with polyester fibers (PES) in manufacturing construction materials that could be employed in bridge construction [236,237]. These examples clearly indicate that polymers derived from waste hold promise in construction, reaffirming the relevance of addressing this issue.

Numerous scientific articles exist on the recycling of polymer fibers. One such example involves recycling carbon fibers from wind turbine blades [146,147,151,152,156]. The use of post-recycled fibers to reinforce polymers, which are intended to become the building material, will be effective and environmentally justified.

5.4.2. Reducing the Transition Effect

The primary requirement in constructing a bridge abutment along with an embankment is to ensure smooth passage between these entities. Polymers that can be used to make ground reinforcements that reduce the pressure from the fill involve the occurrence of frictional forces between the ground and the reinforcement. Regardless of increasing the soil’s shear strength and reducing the pressure on the abutment, reinforced ground provides a more even distribution of deformations. This is particularly crucial in the case of integral bridges, where, due to temperature changes, displacements of the ground behind the abutments alter the direction multiple times [62].

Polymers are actively utilized to reduce the transition effect. These are applied in engineering structures to reinforce the subgrade, increasing its stiffness (while decreasing its susceptibility) and reducing the transition effect, primarily through injection masses.

An example is the use of polyurethane material as an adhesive that gradually stiffens the subgrade along the transition zone. Studies are being conducted on the use of polymer reinforcement in railway subgrades within transition zones at the approaches of engineering structures to mitigate the transition effect [246,247]. The technology involves injecting a polyurethane polymer mass in order to gradually bind the ballast of the track bed along its length. In terms of the vertical displacement of the rail, typical reductions of 25%, 16%, and 3% can be achieved. After static and dynamic testing, we can conclude that performance of polyurethane polymer in the transition zones can achieve the transition of the ballast bed from low stiffness to high stiffness [246]. The maximum reduction occurred with polyurethane alone, and when adding assistant rails and compared to having no settlement, the vertical rail acceleration was reduced by 16.5% and 25.6%, respectively [247]. The research results demonstrated increased subgrade stiffness and a reduced transition effect. In this scheme, the bonding surface of the aggregate with the polyurethane determined the variation in the subgrade stiffness. This solution was adapted in China for the transition zone of a railway line at the entrance to a tunnel, where the ground changed from flexible to the rigid concrete at the entrance.

Another example is the use of mats from used tires, which is a fully-fledged recycled polymer [248]. In this approach, after numerous analyses, engineers investigated a set of five-plate rubber mats placed under the rails in gradually increasing quantities. Based on the relationship between the rail deflection and rubber mat stiffness, the wheel–rail interaction force was significantly reduced from 120 kN to 85 kN. Rubber mats with different stiffnesses are designed to equalize rail deflection differences. With the proposed design, the smoothness of the transition zone can be significantly improved.

Another solution being studied is the use of dispersed polymer reinforcement for concrete, which can also be recycled [249]. The main difference from standard solutions is the use of a mixture with a continuously changing content and type of reinforcement along the length of the track. It should be noted that changing the rail support in the transition sections, in order to eliminate the transition effect, had a positive effect on the total stiffness of the track system in the transition zone, estimated at up to 15%.

Polymers are used as reinforcing materials in various construction projects, and there is substantial scientific research confirming their strength properties [161,162,198,250]. The aforementioned literature demonstrates that polymers can have numerous applications in the construction of bridge abutments and embankments, both as structural and auxiliary materials. It is worth noting that there is not an abundance of scientific studies linking recycled polymers with bridge construction, particularly with abutments, embankments, and transition zones. This area remains niche and requires interest from scientists. The examples cited clearly demonstrate that polymers possess applicable positive properties. The utilization of recycled polymers in bridge engineering has not yet been thoroughly investigated, creating opportunities for valuable research. Polymers are incredibly significant materials, and their application is crucial for enhancing the safety, durability, and load-bearing capacity of bridge structures.

6. Perspectives and Research Gaps

PET (polyethylene terephthalate or poly(ethylene terephthalate)) is a waste plastic material with excellent strength properties [136,251,252,253,254,255,256,257]. It is the most commonly processed material [135,136,137,138,244]. However, as mentioned earlier, it is not feasible to rely entirely on recycled raw material because with each processing, the long polymer chains fragmentize, causing a loss in strength properties. Therefore, adding virgin material in the recycling process is necessary to maintain the required strength properties [258]. Consequently, it is evident that there will always be a continuous influx of material for processing. Hence, finding a long-term use for the material to extend its service life is crucial. It appears that PET, as a matrix for fibers, has potential applications [199,200,201,202,203,204,205].

Its real strength (strength-to-weight ratio) in

Table 1. Comparison of the properties of reinforced polymers to steel (based on [199,200,201,202,203,204,205,258,259,260,261,262]).

Material [-]	Tensile Strength [MPa]	Density [g/cm3]	Strength-to-Weight Ratio [kNm/kg]
PET reinforced with glass fiber in a ratio of 55%	196	1.80	109
PET reinforced with carbon fiber in a ratio of 30%	220	1.45	152
PET reinforced with basalt fiber in a ratio of 30%	113	1.63	69
rPET reinforced with glass fiber in a ratio of 50%	140	1.75	80
rPET reinforced with glass fiber in a ratio of 40%	135	1.66	82
rPET reinforced with glass fiber in a ratio of 30%	122	1.58	77
Steel S235JR	360–510	7.85	46–65
Steel S355	470–630	7.85	60–80

shows that it could be a more efficient material than steel. The notations 55% and 30% mean the percentage fibers in relation to the weight of the composite. Other composite materials behave similarly. The “strength-to-weight ratio” value in the last column was determined by dividing the tensile strength by the density and converting the units to appropriate ones. It is worth noting that some of these are composite materials made of 100% recycled PET (rPET) with the addition of glass fiber in appropriate amounts. The results come from our research that is currently being conducted [258], research by other authors [199,200,201,202,203,204,205], and websites of polymer composite and steel manufacturers [259,260,261,262]. In most structures, it is the self-weight that constitutes the majority of the loads, and if this is significantly optimized thanks to innovative, light, and durable materials, engineers will be able to create more advanced structures.

Our experiments evaluated rPET composites reinforced with 20–50% glass fibers (GF) by weight, using standardized methods for mechanical, thermal, and structural analysis. Specimens were prepared via twin-screw extrusion and injection molding, with the process parameters optimized for each composition to ensure material quality. The mechanical tests, including tensile, flexural, impact, and abrasion resistance, followed ISO standards, while the DSC and SEM analyses examined the crystallinity, fiber distribution, and adhesion. Rheological properties were assessed using MFI tests, and the results were compared against theoretical models (Halpin–Tsai, Tsai–Pagano) to evaluate the reinforcement efficiency and predict the performance for construction applications.

In future research, a critical aspect will be the integration of the abutment with the ground using reinforcing profiles, along with selecting the composite’s composition and shaping the geometry of the structure to achieve the required stiffness. The authors are currently exploring the possibility of processing PET polymer (polyethylene terephthalate) with reinforcing fibers. In the recycling process, primarily used for new bottle production, issues arise from contaminated materials (adhesives, labels, chemicals) that cannot be used in products intended for food contact. Consequently, it is essential to find solutions where such limitations do not apply. The planned material’s utilization in the construction and modernization of roads, railways, and broadly understood bridge structures could be particularly efficient. Employing polymers as one of the building blocks of engineering structures, ensuring long-term usage, will undoubtedly aid the poorest countries in managing their waste.

Global material consumption in civil engineering is enormous and is still rising, especially in infrastructure [263,264]. The World Bank predicts that the share earmarked for infrastructure construction will be close to 90,000 tonnes per year in 2060 [263]. Using substitute materials in the form of recycled materials for even a small part of the demand will be a great benefit to societies. It is particularly important that these are minimally processed waste materials with a long-term use that will not be an ongoing burden on the recycling industry (also due to material limitations and a finite number of cycles). These materials that do not require expensive and energy-intensive processing and production processes can be defined as low-processed materials or minimally processed materials [265]. These, firstly, are effective, and secondly, they fit into the policy of sustainable development. Post-waste reinforced polymer composites can be successfully considered as such. Material in civil engineering can significantly manage large quantities of waste and ensure their utility for an extended period. The design period, which is often 100 years and is in fact much longer after renovations, also has a positive impact on the possible effectiveness of material recycling.

The construction of transportation infrastructure involves increasingly larger structures. This leads to increased vertical loads on the ground and, in the case of bridges and retaining structures, additional horizontal forces (pressure). The most commonly applied solutions to these problems are costly ground reinforcement techniques (such as piling, various types of columns) or enhancing slope stability and retaining structures (using ground anchors or nails). Moreover, the availability of high-quality natural materials (coarse sands, gravels, aggregates) is often limited, and their transportation over significant distances is uneconomical. An alternative, more cost-effective solution to these issues might involve the use of substitute materials.

In future research, a critical aspect will be the integration of the abutment with the ground using reinforcing bars, along with selecting the composite’s composition and shaping the geometry of the bar structure to achieve the required stiffness. It seems that there may be problems with the material’s fatigue, its brittle failure pattern, and low ductility. However, the authors provide for the possibility of modifying the composition of the composite, thus giving the material the desired properties.

There is a clear gap in the potential use of composites made from recycled materials in shaping transition zones in integral bridges. Future perspectives aim to propose a new design solution for bridge support structures based on the system of integral bridges, where the abutment will be closely linked to the embankment. In the future, a spatial structure will be designed and analyzed, ensuring cooperation between the abutment and the embankment. Through this, the change in the stiffness (flexibility) of the subgrade along the length of the structure and transition zones will be gradual, predicting a significant reduction in the transition effect.

Another probable outcome of the research will be an expanded scope for the use of integral bridges that will not be significantly restricted by the requirement for minimal settlements under the bridge supports and embankment. As mentioned, elements such as transition slabs, expansions, and bearings are not present in this type of structure. The absence of these elements will have a favorable impact on reducing the construction and maintenance costs of bridge structures.

It is anticipated that the designed structure will be composed of a composite made of polymer reinforced with dispersed fibers, with a direct and clearly defined correlation between the composition and the ‘stiffness’ of the resulting material. The ‘stiffness’ of the material and its geometric arrangement will directly influence the stiffness of the designed embankment structure.

The priority selection of components involves utilizing waste in the form of polymers in recyclate form and recycled fibers. The anticipated use of these raw materials will significantly reduce the carbon footprint and effectively utilize the materials (the expected durability of bridge structures is at least 100 years). The scope of the study will, in the near future, encompass a comprehensive exploration of this issue, starting from a theoretical analysis of computational models and concluding with laboratory research and a description of the proposed solution.

Despite significant advancements in the fields of recycling and bridge engineering, the intersection of these disciplines remains underexplored. Existing research on transition zones often lacks a focus on integrating recycled materials as a core component. Moreover, while polymers have been used in various construction contexts, their role in mitigating the transition effect has not been adequately quantified. Future studies should address the development of standardized methodologies for integrating recycled PET in geosynthetic applications; the evaluation of the long-term mechanical properties of recycled polymers under cyclic and dynamic loads; and the design and testing of composite materials specifically tailored for bridge embankments.

At this stage, concepts aligned with sustainable construction trends and the principles of the 6Rs are being formulated. The 6Rs typically refer to a set of principles aimed at promoting sustainability and environmental responsibility. These principles are often used in discussions about waste management and conservation. The 6Rs are rethink, refuse, reduce, reuse, recycle, and rot (or compost).

7. Conclusions

The presented state of the art in the field of bridge engineering and, more specifically, the issue of the transition effect and the field of polymers and their recycling provides several far-reaching facts:

• The relevance of the transition effect problem in bridge structures and the need for exploring new solutions is still current. The issue of transition zones and the transition effect in bridge engineering is associated with the use of various solutions that lack standardization and have a limited impact on reducing the discussed phenomenon. This topic has been known for quite some time. With the development of transportation, increasing cargo tonnages, and rising speeds, research in this area becomes necessary. The proposed solution related to integral bridges, allowing for the avoidance of expansion joints and bearings, which positively impacts durability and maintenance costs. Transition slabs often do not completely eliminate this and may cause damage.
• The ever-growing volume of waste poses challenges to humanity. Seeking multi-faceted solutions contributing to the common good is the essence of engineers’ and scientists’ efforts. There is the possibility of using substitute materials that are appropriately processed and shaped, which might find application in bridge engineering, representing a cheaper and more ecological alternative to traditional solutions.
• There is an opportunity to propose solutions involving the use of processed waste in the construction of transition zones connecting abutments to embankments, where materials made from waste can fulfill their function and contribute to reducing environmental pollution. The potential of recycling in construction and the use of composite materials based on recyclables reinforced with fibers in bridge engineering can contribute to waste reduction and the sustainable utilization of recycled materials. However, it is crucial to emphasize that polymers, including reinforced polymers, especially those made from recyclables, are not widely used in bridge construction, and their impact on reducing the transition effect has not been thoroughly explored.
• PET (polyethylene terephthalate) has the best strength properties among waste plastics and exhibits good opportunities for creating composites with fibers. Taking into account the fact that the percentage of recycling of this raw material is still not complete, this creates promising applications, including its use as a building material in bridge structures or even as an element of structures reducing the transition effect.

In conclusion, this prospective approach to bridge design, considering technical, durability, and environmental aspects through the utilization of innovative solutions and materials, could lead to more sustainable and efficient construction practices in the future. The integration of recycled polymers in transition zones offers a promising solution to addressing both engineering challenges and environmental concerns. By integrating recycled materials, such as PET composites, into the design of transition zones, it is possible to address both engineering challenges and environmental concerns. While current research provides a solid foundation, significant opportunities exist for future studies to validate these concepts through experimental and field investigations. The findings of this review aim to guide researchers and practitioners in exploring sustainable and cost-effective approaches to bridge construction. The next steps involve validating the proposed materials through computational models and experimental studies, which will be detailed in subsequent publications.