The combination of adverse phenomena such as excessive deformations, dynamic impact, wear and tear and damage at the junction between an embankment and a bridge structure is defined as the transition effect. It occurs at points where different types of permanent way or pavement are connected, laid on various substrates (e.g., soil subgrade, engineering structures) [
1,
2]. The transition effect is a complex phenomenon, as illustrated in
Figure 1, showing the most significant causes of its occurrence, including permanent settlements at abutments (usually smaller) and embankments (usually larger), progressing over time with unequal values for individual elements, and momentary deformations from live loads (deflection of girders, rails, and substrate deformations). 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. It includes also primary geometric irregularities of rails and inaccurate alignment of the gradeline on and outside the facility. Causes related to the structure include, but are not limited to, 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 pavement, track superstructure, substrate, and structure. 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 [
3]. 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, to a lesser extent new lines, where the issue of non-uniformity can be largely prevented through the use of appropriate designs [
4].
1.1. Engineering aspect
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 [
4]. Currently, a commonly used solution to mitigate substrate stiffness changes is the transitional slab. However, analyzing the most common damages during the "lifetime" of structures, such as road pavement cracks and track superstructure deformations, suggests that the transitional slab might not be an entirely effective solution as it doesn't ensure a sufficiently gradual change in substrate stiffness (flexibility) [
5]. Currently, very different design solutions for transition zones on railway lines are provided, but they do not include a comprehensive assessment of the transition effect.
In contrast to roads where transitional slabs are frequently used, they are not popular for railway lines. Instead, geosynthetics, draining and vibration-isolating mats, synthetic resins, stabilized soil blocks, reinforcement using stone columns or micropiles, dogging, chemical stabilization of sub-ballast, extending and widening substructures, and using stiffening rails within track rails are employed. Contemporary solutions sometimes also involve the use of reinforced concrete slabs.
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 still in use. Hence, the aspect of durability and safety of the structure while limiting construction and operation costs is crucial.
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 [
6]. While piers and girders determine the 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 construction cost. Their contribution to maintenance and repairs is often much higher, frequently exceeding 50% [
7].
The question arises whether there is a type of bridge structure without a transition slab, expansion joints and bearings? These are frame structures, particularly integral bridges [
5,
6,
7,
8,
9,
10,
11,
12,
13,
14]. These are 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. The interaction between the integral abutment and the embankment is crucial to ensuring comfort and safety for its users.
Figure 2 shows an example of an integral bridge. In such cases, the embankment soil is often reinforced. As mentioned earlier, this type of structure does not have expansion joints, bearings, or transitional slabs. The integral abutment is usually incorporated into the road or railway embankment, reducing the bridge's impact on the environment and ensuring smooth and safe road or railway traffic.
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 [
15]. 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 [
15]. Therefore, the crucial aspect is the connection between the abutment and the embankment.
In 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 hence, this number doesn't influence the size of the ground reaction [
7].
In integral bridges, forming a slip plane to stimulate movements in the upper layers of embankment soil is essential to minimize vertical displacements that affect the magnitude of the transition effect [
7]. 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 usually range around 60 meters, sometimes reaching 80-100 meters. The number of piers can vary, and the lengths of the longest integral bridges can exceed 350 meters (Happy Hollow Bridge, Tennessee, USA).
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.
Aware that the transitional slab does not entirely solve the transition effect issue and causes other negative effects, manifesting as pavement and track superstructure damages, designers of integral bridges opted for better cooperation between the embankment and the structure, abandoning the transitional slab [
7]. The assumption of minor settlements in integral bridges mitigated the transition effect issue but significantly limited their applicability.
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.
Reinforcing the ground behind the abutment is often used in bridge engineering to relieve the abutments. This is done by forming a block construction from reinforced soil, using geosynthetic materials, for instance. It can relieve the abutment bodies and wings of newly built bridges as well as existing ones. 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.
By reducing the pressure on the abutment, its dimensions can be reduced, thus minimizing the difference in stiffness (compliance) between the surfaces. Ensuring load transfer cooperation and standardized settling is crucial. Geosynthetic reinforcement itself is not used due to its low stiffness compared to the abutment. However, these two mediums can cooperate effectively. This type of solution has been studied, among others, by Horvath [
16].
The purpose of reinforcing the ground (or using layers) is to increase soil shear strength and reduce ground pressure on the abutment [
7]. By reinforcing the embankment soil, uneven settling can be controlled. In the case of a significant increase in pressure on native soil, there is a risk of uneven settling or loss of stability. Using a reinforced platform allows for even distribution of settling [
17]. The platform's effectiveness can be increased by using high-strength geosynthetic geogrids, with tensile strength up to 1600 kN/m, which vertically absorb, distribute, or dissipate loads onto the soil [
17].
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 [
10,
11,
13,
14,
15,
16,
18,
19,
20,
21,
22,
23,
24,
25,
26,
27,
28]. Intensity in addressing this issue has decreased in recent years, yet with increasing speed limits and tonnage, the problem is escalating. In the case of railways, any unevenness in the track profile gains importance 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 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. 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, such as excessive deformation, dynamic effects wear, and damage to surface elements, foundation, and the structure itself. 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.
Ensuring the durability of bridge structures places specific requirements on the durability of individual elements. Despite the increasingly modern solutions in bearing and expansion joint construction, reality verifies that production, assembly, and maintenance costs are significantly rising. There are also increasing demands for the qualification of personnel involved in assembly, maintenance, and replacement of individual elements during the operation of these structures. The growing number of structures poses increasing maintenance difficulties, necessitating the search for "simple" solutions. Integral bridges can be considered as such.
1.2. Environmental aspect
An effect of the increasing affluence of societies is the intensive growth in the quantity of waste generated. According to the World Bank report [
29], 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 [
30], of which approximately only 48% is processed. The share of polymer waste in recycling is only 25% [
31].
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 [
32]. It is desirable, therefore, to find a long-term use for waste material.
Comparing them to everyday items, the use of processed materials, especially in civil engineering like in bridge construction, ensures their long-term use, often exceeding 100 years. This fact can significantly reduce the carbon footprint of everyday products that eventually become waste. The issue of managing the increasing amount of waste from polymers has gained particular significance in recent decades, and if properly utilized, these can become valuable resources.
Special attention needs to be paid to waste from high-molecular-weight plastics. Their largest source is packaging. According to data [
33], only about 19.5% of plastic waste is sorted. This fact guarantees a constant demand for solutions allowing for recycling.
According to the PlasticsEurope Foundation report [
34] from 2022, in 2021, global 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 shows 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, 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 [
34]. 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 [
35]. 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 [
35]. In two countries, Belgium and the Netherlands, the intended goal of complete elimination of landfilling of plastic waste was almost achieved [
34].
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 [
36]. In Europe, 37% of plastic waste is incinerated in modern incinerators [
34]. The above data is presented in
Figure 3; the charts are based on information contained in [
34,
35]. A study by Zero Waste Europe showed that even the most modern incinerators emit dioxins and other harmful pollutants [
37,
38]. This situation encourages intensified efforts to increase the amount of recycled materials [
36].
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 [
39]." Every product of plastic ever produced still exists today. Since 1950, 2 million tons of plastic have been created worldwide [
34]. In studies [
40], 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 dollars annually [
41]. In 2019, an international research team published in the Marine Pollution Bulletin data stating that each ton of plastic waste in the oceans represents destroyed resources valued at up to
$33,000. 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 [
41].
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 [
42]. 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 [
43]. However, 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 [
43].
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 [
44,
45,
46,
47,
48,
49,
50,
51,
52,
53,
54,
55,
56]. 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 [
44,
45]. 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 5,000 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.
Current recycling methods proposed do not solve the rapid increase in the amount of this type of waste [
44].
Currently 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; recovery through chemical or thermal treatment, which is currently unprofitable; 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 [
44].
In Germany alone, where 30,000 wind turbines are currently operating, by 2024, 70,000 tons of materials from turbine blades are expected to be scrapped. Most of these devices will then be dismantled. The German Federal Environment Agency calls on the government and regional authorities to quickly develop regulations and procedures for recycling decommissioned turbines [
45]. Wind turbines installed in the United States in the late 1990s are gradually being decommissioned after 25 years of operation. A massive graveyard of buried turbine blades from decommissioned turbines is located in Casper, Wyoming. Until recently, a morally questionable practice occurred on a large scale, where dismantled wind farms were sold to markets in Eastern and Southeastern Europe, Russia, Africa, Asia, and Latin America. However, requirements in these countries are constantly increasing, and most are no longer interested in acquiring old technology that requires significant investment in maintaining such installations [
45,
50,
53,
54,
55]. 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 way 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 for EU27+3 countries, which constitutes 46% of plastic recyclables [
36]. The demand for materials for constructing embankments is very high. For standard embankment heights near bridge structures, this amounts to tens of cubic meters per meter of embankment using materials with a long period of use, as shown in
Table 1.