Composite Bonded Anchor—Overview of the Background of Modern Engineering Solutions

Abstract

Composite bonded anchors represent an innovative solution in the field of fastening technology, finding wide application in construction and civil engineering. This article presents a comprehensive review of the available scientific literature, a market analysis and a survey of patent databases related to this issue. Key aspects of the design, mechanical properties and durability of composite bonded anchors under various operating conditions are discussed. Special attention was paid to comparing composite solutions with traditional anchoring systems, highlighting their advantages and limitations. The results presented indicate a growing interest in this technology, which is due to both its high strength, corrosion resistance and applicability to lightweight structures. In conclusion, the article identifies key directions for further research and potential areas for the development of composite bonded anchors in the context of modern engineering challenges.

1. Introduction

Anchoring systems have been used in construction for many years and play a crucial role in ensuring structural stability and safety. Initially, simple steel rods were embedded in masonry using cement mortars, allowing individual structural elements to be connected. An example of such early anchoring methods includes systems used to secure wooden floor beams within masonry walls [1,2]. These solutions were relatively inexpensive and easy to install, but they offered limited durability and mechanical strength. As construction standards and demands increased, the need arose for more advanced and reliable anchoring systems that could meet higher performance and safety requirements. With the development of industry and materials engineering, the number of available anchor shapes and load carrying capacities, the materials from which they are made, the nature of their work and the variety of installation methods have increased, enabling their use in a wider range of engineering projects. At first, the load-bearing capacity of anchors was not precisely defined, and the assessment was based mainly on the practical experience of contractors. Numerous guidelines, approvals and design standards define what conditions an anchor must meet, such as minimum anchorage depth, distance from edges and other anchors, or the material from which they are made. However, there are no standardized tests for anchors or strength analysis for anchors made of material other than steel. Specialized anchors with increased resistance to peel and shear forces are now available for use in complex conditions and under heavy loads. Many different anchoring system solutions are available on the market, which can be divided into two main categories: mechanical anchors and bonded anchors [3] (Figure 1).

Mechanical anchors work on the principle of friction and mutual wedging that occurs between the anchoring elements and the substrate. They mostly consist of an advanced anchor element with a complex, movable geometry set on a threaded bolt. Before installing the anchor in the substrate, a hole of sufficient diameter must be drilled for stable anchor installation. Then, the hole must be cleaned by removing small facies remaining in the hole. A mechanical anchor is placed into the hole thus prepared. The installation of the anchor itself is based on the screwing of a threaded bolt that “pulls” the anchor element. The advantage of this solution is that the anchor can be loaded immediately after installation. This makes it possible to achieve a high load capacity for tearing and shear forces. The process of assembly of a mechanical anchor is shown in Figure 2.

The main disadvantage of mechanical anchors is both their geometry and the materials from which they are made (

Table 1. Summary of advantages and disadvantages of mechanical and bonded anchors [5].

	Advantages	Disadvantages
Mechanical Anchors	- Instant load - Possibility of through-mounting, which speeds up installation time - Opportunity for temporary installation - Cheaper than chemical anchors - Installation is less demanding compared to chemical anchors	- Not suitable for hollow and masonry substrates - They expand the substrate (cannot be installed close to other anchors or near edges) - May be subject to corrosion - Cannot be installed in wet substrates or in any chemical conditions
Bonded Anchors	- High load-bearing capacity and the possibility of deep anchoring. - Possibility of installation to substrates of any type (hollow block, rock, wood, composite). - Resistant to dynamic type of load (to vibration and oscillation). - Small distance between anchors and smaller anchor distance from the edge - Can be used for damp, wet and flooded substrates	- The installation of chemical anchor is more complicated - They cannot be loaded immediately after application - They require special accessories for application - Are not subject to disassembly, so they are also not suitable for temporary installation. - Need to be installed under established thermal conditions

). Current mechanical anchors are typically manufactured from high-grade steel, which comes with several significant limitations. Steel, despite its strength, is susceptible to corrosion, especially in aggressive environmental conditions. In addition, the use of high-quality steel contributes to increased consumption of non-renewable raw materials, which is detrimental from a sustainability perspective. The complex geometry of mechanical anchors requires the use of materials with high deformability and high strength, which further complicates the manufacturing process. In addition, their specific design makes installations in damaged or hollow substrates, such as hollow brick or hollow block, difficult or even impossible, which limits the range of applications for these solutions.

Bonded anchors (also called chemical anchors) can be an alternative to mechanical anchors. Their action is based on adhesion—bonding between the anchor itself and the supporting material using specialized anchoring compounds. These compounds can be cement mortars or synthetic resin-based compounds. The advantage of this solution is the simplified geometry of the anchor, which is less complicated than in the case of mechanical anchors, which reduces production costs. Mostly classical ribbed bars are used as anchors, although more complex geometries are also used (Figure 1). Therefore, comparing pasted-in steel and composite anchors, it is possible to compare the performance of ribbed steel bars and composite bars.

Bonded anchors can be made of different materials, such as stainless steel or synthetic fiber-reinforced composites, allowing them to be tailored to specific project requirements. The main advantage of using composite anchors, compared to steel anchors, is their resistance to harsh environmental conditions. Hence, composite anchors in the form of ribbed bars have found use in geotechnics and mines where they are exposed to varying environmental conditions.

As a binder, various types of synthetic resins are usually used, which provide the appropriate level of bonding between the anchor and the substrate, most often concrete or rock, so they are widely used in various areas of construction and geoengineering.

Inset anchors work on the principle of adhesion, i.e., the permanent bonding of the anchor to the substrate by means of a specialized anchoring compound (adhesive). The process involves placing the anchor in a pre-prepared hole, filled with synthetic resin or, very rarely, cement mortar. The adhesive, which has the right rheological and strength properties, penetrates into the network of cracks, crevices and pores in the substrate and surrounds the anchor, forming a permanent bond that, once hardened, allows the anchor to effectively carry loads, both tensile and shear.

Anchors of this type cannot be loaded immediately after installation—it is necessary to wait for the anchor mass to set. Depending on the anchor mass used, the setting process can take from several hours to a few minutes. The process of assembly of a bonded anchor is shown in Figure 3.

Bonded anchors are widely used in the construction industry, especially for fixing structural elements exposed to high loads and made of different materials (most often concrete, steel, wood), such as columns, beams and balustrades. In concrete, they are an indispensable solution both in new projects and in the modernization of existing structures, due to the stability and high strength they provide [7].

In masonry structures, screw-in anchors are just as effective, since their installation does not cause excessive damage to the load-bearing material, which is often the case when using expansion anchors.

Bonded anchors are also not infrequently used in conjunction with building materials such as composites or various types of rock. Due to the flexibility of the anchor masses, which are appropriately selected for specific substrate conditions, they are ideal for geotechnical structures, mining installations, as well as industrial ones. They enable secure fastening in substrates with unusual structures, such as rock mass or soft stone, so they can be used in various operating conditions. Special screw-in anchors are also used in mining and geotechnics as a preliminary or final shoring for tunnels, thereby ensuring the stability of the excavation. Thus, bonded anchors are widely used wherever reliability, durability and resistance to various operating conditions are required.

2. Aim and Scope of the Conducted Analysis

The aim of this article is to provide a comprehensive analysis of available solutions in the field of bonded composite anchors. The study includes a bibliometric analysis covering both scientific publications and patent databases to identify the existing research and technological achievements in this area.

Articles containing selected keywords were examined, followed by an in-depth review of those publications most relevant to the topic of bonded composite anchors. This allowed for the identification of key research challenges and reported findings.

Subsequently, the study reviews current design standards to determine the requirements for both the anchors themselves and the composite materials used in their construction. Based on this, preliminary criteria were formulated for the design of modern bonded composite anchors.

The final stage of the analysis involved evaluating the current market offerings of bonded composite anchors. Available products were compared in terms of their physical and mechanical properties, cost, and environmental impact, enabling an assessment of their application potential and competitiveness.

3. Scientometric Analysis-Composite Bonded Anchors

3.1. Analysis of Scientific Literature

Based on the keyword set “composite anchor bonded”, a literature review was conducted using the Scopus bibliometric database. The analysis considered the occurrence of selected keyword combinations in article titles, abstracts, and assigned keywords. A total of 276 literature entries were retrieved. It is important to highlight several limitations related to the choice of keywords. Although the selected phrases accurately reflect the subject under investigation, the search also returned publications from unrelated scientific fields, such as prestressed structures, molecular chemistry, and marine engineering. Therefore, the subsequent bibliometric analysis was divided into two selected thematic areas.

As part of the first narrowed analysis, the keyword “concrete” was added to the initial set “composite anchor bonded.” This additional keyword was intended to specify the application of the anchoring element in concrete structures, ensuring full embedment within the structural material. Based on this refined keyword set and data retrieved from the Scopus database, a co-occurrence analysis was conducted using the VOSviewer software (version.1.6.20). The results of this analysis are presented in Figure 4. To ensure clarity in the visualization, the minimum number of links was set to 15, which reduced the number of analyzed keywords from 1425 to 38. VOSviewer enables graphical representation of keyword frequency using a color scale. As the frequency of a keyword increases, its color shifts from blue to yellow.

A general analysis conducted using the Scopus bibliometric database indicates key trends related to bonded composite anchors in concrete. The most frequently recurring term in the visualized data is “fiber reinforced polymer,” highlighting the importance of using composites reinforced with synthetic fibers, including carbon fibers. Additionally, issues related to tensile and shear strength, as well as loss of adhesion and delamination, are evident. This suggests a wide range of potential failure mechanisms for bonded composite anchors. In the subsequent part of this study, based on an analysis of available design standards, possible failure modes of composite bonded anchors will be defined.

Furthermore, using the keyword combination “composite anchor bonded,” an analysis of substrate types was performed. The results revealed that most publications focus on the installation of composite anchors in mountainous or heterogeneous rock substrates. In contrast, significantly fewer studies address bonded anchors installed in concrete substrates.

The second narrowed analysis focused on the use of reinforced PET as a matrix for composite anchors. For this purpose, the keyword combination “anchor + concrete + PET + FRP” was applied. As before, the Scopus database was used for this purpose. An initial review of individual keywords revealed a broad thematic scope across the literature. The results included publications from fields such as organic chemistry, materials science, veterinary medicine, logistics, and engineering. However, by applying targeted keyword combinations, it was possible to isolate publications directly related to the use of composite materials in civil engineering. The results from the analysis are presented in

Table 2. Summary of keyword occurrences for each database (25 September 2025).

Key Words	Number of Occurrences of the Phrase (Scopus)
STEP I
anchor	109,647
concrete	620,817
PET	262,485
FRP	31,057
STEP II
anchor + PET	173
Concrete + anchor	7221
FRP + anchor	782
FRP + PET	177
Concrete + PET	1391
STEP III
FRP + anchor + concrete	628
FRP + anchor + PET	3
STEP IV
FRP + anchor + concrete + PET	3

. This refined set of publications will be used in the subsequent in-depth bibliometric analysis.

The analysis conducted using an extended set of keywords indicates a lack of publications or research studies focused on anchors made of reinforced PET used in concrete. It is worth noting that the number of publications for the keyword combinations “FRP + anchor + PET” and “FRP + anchor + concrete + PET” is identical. Further examination revealed that these are the same studies, which indicates that research on composite anchors utilizing reinforced PET has been carried out specifically in the context of concrete substrates.

3.2. Detailed Scientometric Analysis

Due to the broad scope of the conducted analysis and the ambiguity of the applied keywords, the detailed study was divided into two separate sections. The first section presents publications related to bonded composite anchors. It focuses on their application in civil engineering and the results obtained from relevant studies. The intended use of the anchors is discussed, along with an overview of the advantages and disadvantages of this type of solution. The second section addresses the development of a composite material based on PET as a matrix. The research results were analyzed in terms of material properties, and the findings were summarized accordingly.

3.2.1. Detailed Scientometric Analysis–Composite Bonded Anchors

When initiating a detailed analysis of studies on bonded composite anchors, it is essential to emphasize FRP composite bars, as they are the most commonly used anchoring elements. A comprehensive overview of composite bars and their application as anchors is presented in [8]. The author provides a concise summary of the key properties of polymer composite bars, addressing both their mechanical behavior and their response to aggressive environmental conditions. Based on numerous publications, it is noted that the durability of composite bars varies depending on the type of aggressive environment. The study confirms that steel bars are more susceptible to corrosion compared to composite bars.

Similarly, Nkurunziza et al. [9] demonstrated that despite their advantages, GFRP bars may degrade in alkaline environments. They presented results from accelerated aging tests and service life prediction models, suggesting that the strength reduction factors used in current standards are overly conservative for modern GFRP products.

Comparable conclusions were drawn by Zhu and Bergmeister [10], who additionally noted that improper hole cleaning can reduce pull-out strength by up to 40%.

Based on the above publications, it can be concluded that although composite bars are more resistant to corrosion than steel bars, they are still susceptible to this process. Factors such as temperature, the type of solution used, and the testing method may influence the rate of corrosion.

Most available studies on composite anchors focus on geotechnical applications. Hao et al. [11] analyzed the failure mechanisms of anchors in pull-out tests, both in situ and through numerical modeling using ABAQUS. Field test results enabled accurate calibration of the computational model, and the optimal anchor length was determined to be between 3.5 and 5 m. Yan et al. [12] attempted to analytically model the behavior of GFRP anchors in soil, emphasizing the differences between designing with steel and composite anchors. Zhang et al. [13] conducted in situ tests on composite anchors equipped with fiber optic sensors, allowing for detailed monitoring of anchor performance. A key factor in evaluating composite anchors is the behavior of the concrete–GFRP bar interface. Research by Chen et al. [14] led to the development of a simplified model of the transition zone and a staged description of the failure process.

It is important to highlight the role of the substrate in which the anchor is installed. Boreholes are often drilled to assess substrate layers and estimate load-bearing capacity. Due to frequent stratification and uncertainty regarding substrate properties, composite anchors may range in length from 1 m to over 10 m.

The situation differs for concrete substrates. Assuming that concrete is homogeneous and rigid, the anchor length can be significantly reduced. Su et al. [15] focused on the accuracy of numerical models and the influence of mesh density and node count on the force–embedment depth relationship. Using C25/30 concrete and known strength parameters of the mix, the experimental results and numerical analysis were found to be consistent. Consequently, it is possible to estimate the load-bearing capacity of a composite anchor based on the known properties of both the substrate and the composite anchor.

It is worth emphasizing that composite anchors are already being successfully implemented in engineering practice. Thomas [16] analyzed the use of GFRP bars in tunnel construction, highlighting their benefits such as lower self-weight, corrosion resistance and a modulus of elasticity four times lower than that of steel. These bars were used in the reinforcement of the Vereina Tunnel in Switzerland, where over 100,000 GFRP anchors have been installed and have operated reliably for over 20 years.

An interesting solution in the field of composite anchoring systems is the use of FRP spike anchors. These anchors consist of a sleeve and a spike, both made from resin-impregnated FRP fiber sheets. Ozbakkaloglu et al. [17] tested 33 specimens with CFRP plates anchored using various FRP spike configurations. Single shear tests showed that longitudinal anchors provide greater deformation and more ductile behavior compared to transverse anchors. Additionally, anchorage depth, number of anchors, and their distribution significantly affect load capacity and stress distribution in the plate. In the study by [18] Ke et al., CFRP mats were anchored in concrete structures. Tests were conducted on nine beams, including control beams, reinforced beams without anchors, and beams with anchors in different configurations. The results indicated an increase in load capacity from 13% to 35%, and an increase in mid-span deflection at failure from 37% to 70% compared to reference samples. Similar findings were reported by Carozzi et al. [19], who analyzed anchoring effects in masonry walls. Their results showed an increase in tensile strength of up to 90% compared to reference specimens. Dong et al. [20], based on both experimental and existing research, proposed an analytical model to predict single shear performance with FRP spike anchors. The model incorporates a nonlinear bond-slip relationship and an elastic representation of the FRP anchor. The results demonstrated good correlation with experimental data. Castillo et al. [21] conducted strength tests of FRP spike anchors under dynamic loading. Based on the findings, guidelines were developed to improve the quality and performance of such anchoring systems.

Based on the current state of knowledge, it can be inferred that the most commonly used bonded anchors are composite anchors manufactured from GFRP bars with ribbed geometry. This preference is primarily attributed to the reduction in stress concentration at the interface between the anchor and the substrate. Moreover, the use of ribbed composite bars enables the application of commercially available products without the need for specially dedicated injection molds. Research is also being conducted on alternative anchoring methods using synthetic fibers.

An important aspect to consider is the substrate into which the anchor is installed. Depending on the homogeneity and cohesion of the substrate, the anchor length may range from several dozen centimeters to several meters.

3.2.2. Detailed Scientometric Analysis–Composite PET with Fibers

The second part of the detailed analysis focused on composites based on recycled polyethylene terephthalate (rPET), reinforced with synthetic fibers, primarily glass (GF) and basalt (BF). The study [22] compared the reinforcement efficiency of rPET with GF and BF in weight fractions ranging from 15% to 45%. It was shown that GF provides better adhesion to the matrix and a more pronounced increase in tensile and flexural strength. In contrast, BF—despite lower interfacial bonding—significantly enhances the stiffness of the composite at a lower cost. Microdroplet tests and SEM observations confirmed differences in critical fiber length and shear strength between GF and BF, as well as the impact of fiber degradation during processing on reinforcement efficiency. Another study [23] examined the influence of extrusion parameters—screw speed and torque—on the mechanical and thermal properties of PET/GF composites. The addition of 30% GF significantly increased Young’s modulus (up to 9.2 GPa) and impact strength, despite fiber shortening during processing. DSC and TGA analyses confirmed the thermal stability of the material, and the experimental design showed that higher screw speed improves stiffness, while higher torque enhances impact resistance. Mondadori et al. [24] compared two processing methods: single-screw extrusion with a barrier screw and co-rotating twin-screw extrusion, under different mold temperatures (10 °C vs. 120 °C). Both methods ensured good fiber dispersion and high microstructural quality. Higher mold temperatures increased PET crystallinity, resulting in improved modulus and heat deflection temperature (HDT). It was demonstrated that single-screw extrusion with a barrier screw can be an effective alternative to more expensive twin-screw systems. Another study [25] focused on the mechanical properties and fracture resistance of rPET composites reinforced with glass fabric and chopped fiber mats, manufactured via hot pressing. Three-point bending, Charpy impact, and delamination (ENF) tests showed that rPET composites achieved properties comparable to those of virgin resin-based materials. Delamination resistance (GIIc ≈ 1950 J/m²) was similar to PEEK-based composites. SEM observations confirmed good fiber wetting by the matrix and energy absorption mechanisms such as fiber pull-out and microcracking. Kráčalík et al. [26] investigated the influence of GF content (15–30%) and processing conditions on the rheological, thermal, and mechanical properties of rPET composites. Higher fiber concentrations improved melt viscosity and modulus, as well as mechanical strength. The best impact resistance was achieved at 20% GF content. The use of talc as a coupling agent improved interfacial adhesion, although industrial-scale processing led to property deterioration due to air entrapment and reduced viscosity. Monti et al. [27] focused on improving the impact resistance of PET/GF composites through the addition of ethylene copolymers containing polar groups (e.g., methyl methacrylate, methacrylic acid, GMA). The best results were obtained with the GMA-containing copolymer, which reacts with PET’s terminal COOH groups to form a stable dispersed phase and enhance interfacial compatibility. SEM confirmed smaller rubber particle sizes and better bonding with the matrix, while DSC and HDT tests showed high crystallinity and thermal resistance, despite a slight reduction in stiffness.

A summary of the achievable properties of PET-based composite materials reinforced with synthetic fibers is presented in

Table 3. Typical selected parameters.

	Fiber Content	Tensile Strength [MPa]	Flexural Modulus [GPa]	Impact Strength [kJ/m2]
Ronkay & Czigany [22]	15%, 30%, 45%	55.86–84.05	2.29–9.96	4.23–7.88 kJ/m2
Kráčalík et al. [26]	15%, 20%, 30%	110–121.7	7.9–12.65	32–43.3 kJ/m2
Monti et al. [27]	20%	102–120	-	5.2–8.4 kJ/m2 (notched), 26.4–40.3 kJ/m2 (unnotched)
Giraldi et al. [23]	20%, 30%, 40%	-	7.8—9.2	76.9–108.9 J/m2

.

Based on the analysis of publications related to fiber-reinforced polymers, it can be confirmed that the production of such polymers is feasible. These materials exhibit higher tensile strength and elastic modulus compared to virgin PET, indicating their potential for use as raw material in load-bearing components. The presented mixtures focused on developing composite blends that improve mechanical properties such as tensile strength, flexural modulus, and impact strength. Additionally, attempts were made to modify the mixture to enhance its thermal resistance [27]. This demonstrates the possibility of tailoring composite formulations through various additives and by adjusting the manufacturing process.

3.3. Analysis of the Lifecycle Costs of Bonded Anchor Composite

Recycling PET bottles offers substantial economic advantages across energy savings, cost reduction, job creation, and societal impact. Producing recycled PET (rPET) consumes up to 50% less energy and emits 79% less CO₂ compared to virgin PET, resulting in savings of up to 2.5 tons of CO₂ per ton of recycled PET [28]. Enzymatic recycling innovations have further reduced operating costs by 74% and energy use by 65%, making recycling more cost-effective than producing new plastic [29]. The PET recycling industry supports significant employment, with over 265,000 jobs in the U.S. alone and 574,000 across the broader plastics sector, contributing to local economies and industrial growth [30]. Manufacturers benefit from lower raw material costs, as rPET is cheaper to process and widely used in packaging, textiles, and automotive sectors [31]. Deposit return systems (DRSs) have proven highly effective in Europe. A study by MIT found that implementing a nationwide DRS could increase PET recycling rates from 24% to 82%, supplying 2.7 million tonnes of rPET annually at a net cost of just $360 per tonne [32]. In countries like Finland and Denmark, DRS systems have achieved collection rates above 90%, ensuring a stable supply of high-quality recyclables [33]. Moreover, a life-cycle costing study by the European Commission estimated that improved PET recycling strategies could save up to EUR 25 billion in societal costs between 2020 and 2030, including reductions in CO₂ emissions and particulate matter [34]. These findings highlight PET recycling as a key driver of sustainable economic development and environmental stewardship. A study conducted by denkstatt GmbH for ALPLA revealed that recycled polyethylene terephthalate (rPET) generates carbon emissions of 0.45 kg CO₂ per kilogram of material, representing a 79% reduction compared to virgin PET, which emits 2.15 kg CO₂/kg. Depending on the energy source used in the production process, emissions can be reduced further, reaching as low as 0.21 kg CO₂/kg [35]. In contrast, the carbon footprint analysis of TECHSTORM 190 epoxy resin indicates emissions of 6.07 kg CO₂/kg for the resin alone. When fiber reinforcement and processing are included, the total emissions can reach approximately 12.0 kg CO₂/kg [36].

In terms of cost, the average market price of epoxy resin in Europe in 2025 was approximately $4.36/kg, and $3.42/kg in North America. Including additives and processing, the final cost of epoxy-based composites is estimated at around $6.00/kg [37]. For rPET, the average price in Europe was $2.37/kg, and $1.86/kg in North America. After processing into composite form, the estimated final cost is approximately $2.50/kg [38].

In the next stage of the analysis, it is necessary to compare the life-cycle costs and performance parameters of PET-based composites reinforced with synthetic fibers against currently available composite bars on the market. For this purpose, a simplified life-cycle analysis of bars made from various types of composites was conducted in accordance with ISO 15686-5 [39]. The analysis considered the following parameters:

• CAPEX: material costs, transportation, installation.
• OPEX: periodic maintenance and repairs.
• User costs: costs borne by the user (downtime, detours, disruptions).
• End-of-Life (EoL): dismantling, recycling, and recovery revenues.

The sum of these components provides the net present value (NPV), indicating the total life-cycle cost of the product. The analysis assumed a 100-year period, consistent with critical infrastructure standards. The results of the analysis are presented in tabular form in

Table 4. Life-cycle analysis of bars made from different types of composites.

Types	CAPEX (Index)	OPEX (100 Years)	User (100 Years)	EoL	Sum NPV
GFRP bars	100	95–100	90–100	95–100	100 (ref.)
BFRP bars	105–115	95–100	90–100	95–100	102–110
CFRP bars	180–260	90–100	80–95	95–100	95–120
rPET + GF/CF bars	90–130	100–120	95–110	95–105	110–140

Where: GFRP bars—Glass Fiber Reinforcment Polymer, BFRP bars—Bazalt Fiber Reinforcment Polymer, CFRP bars—Carbon Fiber Reinforcment Polymer, rPET + GF/CF bars—recycled Polyethylene terephthalate modified with Glass or Carbon Fiber

.

Due to local uncertainties in pricing, supply, and maintenance practices, the values in the table are presented as percentages relative to the cost of GFRP bars for each stage of the material’s life cycle. As clearly shown in the table, bars made from rPET are less expensive to produce compared to other composite bars (CAPEX). However, because of their innovative nature and the lack of available design standards for this type of material, costs associated with repairs and maintenance (OPEX) increase compared to GFRP. In terms of user-related costs (USER) and end-of-life costs (EoL), rPET bars do not differ significantly from other composite materials. In summary, due to their innovative character and the absence of appropriate design standards, the total life-cycle cost of a product made from rPET is higher, despite utilizing recycled PET waste.

3.4. Analysis of Available Patent Databases

Analysis of patents for different types of anchors showed: 138,277 patents for the keyword “concrete anchors,” 13,964 for “epoxy concrete anchors” and 1768 for “FRP concrete anchors.” These results indicate a relatively small number of patented composite anchors compared to traditional solutions, which may suggest potential for development in this field. A detailed analysis of the patent base examined selected solutions that bring significant innovations in the field of anchoring systems. For example, patent EP 2 893 139 B1 describes a screw-in threaded anchor equipped with proprietary anchor blocks (Figure 5). These blocks can move freely along the anchor, making the solution fully adaptable to different installation conditions and situations. In addition, the patent defines a specialized epoxy resin-based anchor compound, further enhancing the versatility and effectiveness of the application.

Another example is the US 2005 patent 0183349 A1, which shows an innovative anchor head shape adapted to chain installation (Figure 6). The designed head geometry minimizes potential stress concentration points and the risk of abrasion, which significantly increases durability and safety in use. Both solutions indicate a growing trend toward designing more flexible and specialized anchoring systems.

As part of the analysis of the publicly available LENS patent database, a quantitative assessment was conducted based on selected keyword combinations. The search results indicate substantial interest in composite materials: 298,973 patents for the keyword “composite fiber”, 15,246 for “composite PET” and 4389 for “composite fiber PET”.

A detailed review was then carried out on selected patents relevant to PET-based composites reinforced with fibers. Patent US 12,043,725 B1 [40] describes a mechanical recycling process for PET and CFRP, resulting in PET/carbon fiber composites with significantly improved mechanical properties. The best performance was achieved with a 20% carbon fiber content. Patent EP 4 596 257 A1 [41] presents an optimized PET-rubber composite with high strength and low rolling resistance, designed for tire applications without increasing layer thickness or fiber mass. Patent US 2021/0388536 A1 [42] focuses on the development of a flexible composite fiber made from a blend of low- and high-viscosity PET, PTT, and PBT. By carefully selecting proportions and leveraging differences in physicochemical properties, the resulting fiber exhibits enhanced 3D structure, elasticity, softness, thermal stability, and dyeability. The production process includes drying, extrusion, spinning, and thermal stabilization (either tensioning or relaxing). These fibers are intended for use in sportswear, underwear, socks, and carpets.

Based on the analysis of available patents, it can be observed that the anchoring rod most often adopts a shape similar to a ribbed bar, which is consistent with the findings obtained from the scientometric analysis. Some proposed anchoring systems refer only to the anchor holder, where the load-bearing capacity is provided by a steel rod. These holders are typically closed, which may hinder the replacement of the anchored element. Therefore, it seems reasonable to formulate a hypothesis regarding the design of an open-geometry holder, commonly used for steel components, as a potential improvement.

4. Design Guidelines for Bonded Composite Anchors

An analysis of available design guidelines reveals a lack of specific standards for synthetic fiber-reinforced composite anchors. This is due to the fact that this type of solution is relatively new. In most cases, design is based on scientific publications and technical guidelines provided by manufacturers. Therefore, it is advisable to analyze existing design guidelines related to screw anchors and composite bars, so that ultimately it will be possible to design innovative screw anchors that carry significant loads and are made of composite materials.

4.1. Basics of Designing Bonded Anchors

Standardizing the issue of designing mechanical and chemical anchorages is an extremely difficult thing due to the number of materials that can be used and their different nature of work. Hence, the normative guidelines provide only the method of determining the strength without the possibility of predicting newly designed anchorages. The lack of predictive capability is due to the complexity of the substrate and the nature of the work between the anchor and the substrate.

One of the key standards used in the design of embedded anchors is EN 1992-4:2018—Eurocode 2: Design of concrete structures—Part 4: Design of fixings for concrete [43]. This is a European standard that has been in effect since 2009, which applies to various types of fastenings in concrete structures, including embedded anchors. The standard defines the working conditions of a single anchor as well as a set of anchors connected by an anchor plate. The analysis considers the occurrence of tensile force (N), shear force (V), and the combination of these forces. In the case of an anchor assembly connected by an anchor plate, a bending moment can act to transfer the load to the anchors as a pair of forces of opposite direction. The standard does not take into account the possibility of an extrusion of force on the eccentric, causing a bending moment or torsional moment on the anchor.

Through the interaction of tensile forces (N), shear forces (V), and the combination of these loads, the anchor may fail in post:

For tensile force (N), the anchor is destroyed by exceeding the plastic reserve (Figure 7).

$$N_{Rk.s}=A_s\ast f_{uk}\hspace{1em}\left[N\right]$$

(1)

where $A_s$ is the cross-section of the anchor [$m{m}^2$] and $f_{uk}$ is the characteristic tensile strength of steel, adopted in accordance with the relevant ETA [$\mathrm{MPa}$].

Cone pull-out of the anchor exceeds the tensile stresses in the concrete (Figure 8).

$$N_{Rk.c}=N_{Rk.c}^O\ast {\displaystyle \frac{A_{c.N}^O}{A_{c.N}}}\ast {\psi }_{s.N}\ast {\psi }_{re.N}\ast {\psi }_{ec.N}\hspace{1em}\left[N\right]$$

(2)

where $N_{Rk.c}^O$ is the characteristic strength of a single joint [N], $A_{c.N}^O$ is the reference area [$m{m}^2$], $A_{c.N}$ is the actual area, bounded by overlapping concrete cones [$m{m}^2$], ${\psi }_{s.N}$ is the factor of stress distribution perturbation in concrete caused by edges, ${\psi }_{re.N}$ is the factor of the coating chipping effect, and ${\psi }_{ec.N}$ is the factor of the load eccentricity effect.

Destruction of concrete by pulling out the anchor is shown in Figure 9.

$$N_{Rk.p}=k_2\ast A_h\ast f_{ck} \left[N\right]$$

(3)

where $A_h$ is the bearing surface of the fastener head [${\mathrm{mm}}^2$], $k_2$ is the empirical parameter depending on the degree of cracking of the ground, and $f_{ck}$ is the characteristic value of the compressive strength of concrete [MPa].

A combination of the cone pullout from the substrate and destruction of concrete by pulling out the anchor is shown in Figure 10.

$$N_{Rk.pc}=N_{Rk.p}^O\ast {\displaystyle \frac{A_{p.N}}{A_{p.N}^O}}\ast {\psi }_{s.Np}\ast {\psi }_{g.Np}\ast {\psi }_{re.Np}\ast {\psi }_{ec.Np} \left[N\right]$$

(4)

where $N_{Rk.p}^O$ is the characteristic strength of a single joint [N], $A_{p.N}^O$ is the reference area [${\mathrm{mm}}^2$], $A_{p.N}$ is the actual area, bounded by overlapping concrete cones [${\mathrm{mm}}^2$], ${\psi }_{s.Np}$ is the factor of stress distribution perturbation in concrete caused by edges, ${\psi }_{g.Np}$ is the factor of the effect of closely spaced fasteners, ${\psi }_{re.Np}$ is the factor of the coating chipping effect, and ${\psi }_{ec.Np}$ is the factor of the load eccentricity effect.

Structure cracking is shown in Figure 11.

$$N_{Rk.sp}=N_{Rk.c}^O\ast {\displaystyle \frac{A_{c.N}}{A_{c.N}^O}}\ast {\psi }_{s.N}\ast {\psi }_{re.N}\ast {\psi }_{ec.N}\ast {\psi }_{h.sp} \left[N\right]$$

(5)

where $N_{Rk.c}^O$ is the characteristic strength of a single joint [N], $A_{c.N}^O$ is the reference area [${\mathrm{mm}}^2$], $A_{c.N}$ is the actual area, bounded by overlapping concrete cones [${\mathrm{mm}}^2$], ${\psi }_{s.N}$ is the factor of stress distribution perturbation in concrete caused by edges, ${\psi }_{re.N}$ is the factor of the coating chipping effect, ${\psi }_{ec.N}$ is the factor of the load eccentricity effect, and ${\psi }_{h.sp}$ is the factor for the effect of actual bar depth h on splitting resistance.

To calculate shear force (V), use the destruction of the anchor without consideration of the arm of force action (Figure 12).

$$V_{Rk.s}=k_7\ast V_{Rk.s}^O \left[N\right]$$

(6)

where $V_{Rk.s}^O=k_6\ast A_s\ast f_{uk}$ is the characteristic resistance of a single fastener in case of steel failure $\left[N\right]$, $k_6$ is the factor of reduction depending on the value of $f_{uk}$ [-], $k_7$ is the factor of reduction depending on the number of anchors, $A_s$ is the cross section of the anchor [${\mathrm{mm}}^2$], and $f_{uk}$ is the characteristic tensile strength of steel, adopted in accordance with the relevant ETA [$MPa$].

Destruction of the anchor, considering the arm of force action, is shown in Figure 13.

$$V_{Rk.s.M}={\displaystyle \frac{{\alpha }_M\ast M_{Rk.s}}{l_a}}\left[N\right]$$

(7)

where ${\alpha }_M$ is the factor taking into account the degree of restraint of the fastener on the attachment side of the application,

$$l_a=a_3+e_1$$

(8)

where $l_a$ is the effective lever arm of the shear force acting on the fastener or anchor channel, $e_1$ is the distance between the shear load and the surface of the concrete, ignoring the thickness of the levelling mortar, and $a_3$ is the distance between the surface of the concrete and the point of assumed restraint of the shear-loaded fastener with a lever arm,

$$M_{Rk.s}=M_{Rk.s}^O\ast (1-N_{Ed}/N_{Rd.s})$$

(9)

where $M_{Rk.s}$ is the characteristic resistance in case of steel damage, $M_{Rk.s}^O$ is the characteristic bending resistance of the channel bolt, $N_{Ed}$ is the design tensile force of the stressed fastener $N_{Rd.s}=N_{Rk.s}/{\gamma }_{M.s}$ is the design value of the strength of the steel of the fastener or channel bolt under tensile load, $N_{Rk.s}$ is the characteristic value of the tensile strength of the fastener steel or channel bolt, and ${\gamma }_{M.s}$ is the damage factor of steel.

Tearing out the concrete is shown in Figure 14.

$$V_{Rk.cp}=k_8\ast \min \left\{N_{Rk.c}; N_{Rk.p}\right\}\hspace{1em}\left[N\right]$$

(10)

where $N_{Rk.c}$ is the characteristic strength in case of failure of the concrete cone under tensile load [N], $N_{Rk.p}$ is the characteristic resistance to damage by pulling out under tensile load [N], and $k_8$ is the factor to be taken from the relevant ETA.

Chipping off the edge of the concrete is shown in Figure 15.

$$V_{Rk.c}=V_{Rk.c}^O\ast {\displaystyle \frac{A_{p.V}}{A_{p.V}^O}}\ast {\psi }_{s.V}\ast {\psi }_{h.V}\ast {\psi }_{ec.V}\ast {\psi }_{\alpha .V}\ast {\psi }_{re.V}\hspace{1em}\left[N\right]$$

(11)

where $V_{Rk.c}^O$ is the characteristic resistance of a fastener loaded perpendicular to the edge [N], $A_{p.V}^O$ is the reference area [${\mathrm{mm}}^2$], $A_{p.V}$ is the area of the idealized concrete breach [${\mathrm{mm}}^2$], ${\psi }_{s.V}$ is the factor to account for disturbances in the distribution of stresses in concrete, ${\psi }_{h.V}$ is the factor that takes into account the fact that the edge strength of concrete, ${\psi }_{ec.V}$ is the factor that takes into account the group effect when different shear loads act on individual fasteners of the group, ${\psi }_{\alpha .V}$ is the factor taking into account the effect of shear load inclined to the edge, and ${\psi }_{re.V}$ is the factor to account for the effect of reinforcement located at the edge.

It is worth noting that the standard does not explicitly state how to select the values of the required parameters. In most cases, the standard directly refers to the manufacturer’s guidelines and European Technical Approvals (ETAs), which provide precise values for the load capacity and coefficients for individual solutions. This paints a picture of the necessity for an individual approach to each newly designed anchorage and the use of individual mechanical parameters of the selected anchorage system.

In addition, the standard analyzes combinations of the occurrence of tensile and shear forces and provides a condition:

$${\left({\beta }_N\right)}^{\alpha }+{\left({\beta }_V\right)}^{\alpha }\le 1$$

(12)

$${\beta }_N={\displaystyle \frac{{\gamma }_F\ast \Delta N_{Ek}}{{\psi }_{F.N}\ast \Delta N_{Rk}/{\gamma }_M}}$$

(13)

$${\beta }_V={\displaystyle \frac{{\gamma }_F\ast \Delta V_{Ek}}{{\psi }_{F.V}\ast \Delta V_{Rk}/{\gamma }_M}}$$

(14)

where ${\gamma }_F$ is the factor partial load, ${\gamma }_M$ is the factor of parts for the material, $\Delta N_{Ek}$ is the inter-peak amplitude of fatigue tensile blow-out action, $\Delta N_{Rk}$ is the resistance to fatigue, stretching, $\Delta V_{Ek}$ is the inter-peak amplitude of fatigue shear action, $\Delta V_{Rk}$ is the fatigue resistance, shear resistance, ${\psi }_{F.N}$ is the reduction factor applied to tensile strength to account for uneven tensile load distribution, ${\psi }_{F.V}$ is the reduction factor applied to shear resistance to account for the uneven distribution of shear load acting, and $\alpha$ is the stated in the relevant ETA.

However, the value of the alpha potentiating parameter has not been clearly defined and may change for different anchoring systems. Also, the inequality itself may exceed the value of 1 depending on the manufacturer’s guidelines. The relationship between tensile and shear force can be shown in the diagram (Figure 16):

In addition, the standard shows how to analyze anchorages under min. fire conditions. The standard compiles the effect of time of exposure to fire load on the reduction in the characteristic value of tensile stress transferred by an anchor made of both carbon steel and stainless steel. However, in order to fully estimate the load-carrying capacity and in this case, the standard refers to the manufacturer’s guidelines for individual anchors.

In summary, EN 1992-4 is a compilation of the most important information regarding the design and strength estimation of both mechanical and chemical anchors. It is worth noting, however, that many times the standard refers to the manufacturer’s guidelines contained in the ETA for each individual solution. However, this does not change the fact that the guidelines presented allow their practical application in projects [44].

The basis for the guidelines contained in EN 1992-4 [43] was a series of documents min. ETAG 001 [45], EAD 330499-00-0601 [46] and EAD 330232-01-0601 [47], which provide detailed rules for the evaluation and design of embedded anchors in concrete.

ETAG 001 [45] (European Technical Approval Guideline) is an EOTA guideline for assessing and designing mechanical and chemical anchors in concrete. It outlines testing procedures for static, fatigue, and seismic loads, considering factors such as cracked concrete, hole diameter, and installation quality. Design rules cover pullout, shear, combined loads, anchor spacing, and edge distances. The document also addresses installation errors and durability under varying environmental conditions.

EAD 330499-00-0601 [46] (Bonded fasteners for use in concrete) sets requirements for evaluating chemical anchors under the ETA framework. It defines load-bearing, shear, and tensile strength criteria, with tests in cracked and uncracked concrete, considering temperature, moisture, and aging effects. It includes dynamic and seismic load tests, detailed installation guidelines, and durability analysis. The document aligns with Eurocode 2 (EN 1992-4 [43]).

ACI 318-11 standard [48] provides U.S. standards for anchor design and installation in concrete. It covers anchor types, loads, strength analysis, installation, and durability. Failure modes include pullout, shear, and concrete cracking. The standard specifies minimum capacities based on location and edge distance, along with recommendations for embedment depth, hole diameter, and material quality.

AC 308 standard [49] defines requirements for anchors and post-installed reinforcing bars. It includes material, design, installation, and testing criteria, such as embedment depth, spacing, and load directions. Its goal is to ensure structural safety and reliability.

ASTM F1554-17 [50], developed by the American Society for Testing and Materials (ASTM), applies to anchor bolts for connecting structures to concrete foundations. It specifies mechanical and chemical properties, strength grades (36, 55, 105), and testing rules. Commonly used in steel structures and infrastructure, it also allows additional requirements like weldability and impact resistance.

It is worth noting that available standards, such as ACI 318-14 [51] and EN 1994-2 [43], represent two different approaches to anchorage design. The differences between these approaches are discussed in detail in the paper [52]. Based on the analysis and calculation results, it can be pointed out that EN 1994-2 has a more conservative approach compared to ACI 318-14 [51].

It is worth noting that most design guidelines are based on experimental research. An example is the study by Kabantsev et al. [53]. The research emphasizes the significant impact of concrete cracking on anchor performance under seismic conditions. Tests show that crack width can reduce anchor capacity by up to 82% for bonded and small expansion anchors, while undercut anchors remain the most reliable. These findings highlight the need to include crack width effects in design standards and to provide reduction and plasticity coefficients that can be applied in seismic anchorage design to ensure structural safety in earthquake-prone areas.

4.2. Basics of the Design of FRP Bar Constructions

The use of synthetic fiber reinforced bars (FRPs) as reinforcement is an increasingly popular practice. However, the available design guidelines for synthetic fiber reinforcing bars (FRPs) are still evolving, due to the relatively new use of these materials in construction [54,55,56]. Documents such as ACI 440.1R-15 [57] and ACI 440.3R-04 [58], as well as international standards, including FIB Bulletin 40 [59], ISO 10406-1:2015 [60] and ASTM WK87882 [61], play a key role in shaping the design and evaluation principles for FRP materials.

The basic parameters of synthetic fibers (

Table 5. Typical parameters of selected synthetic fibers [62,63].

	Density [kg/m3]	Tensile Strength [MPa]	Tensile Modulus [GPa]	Ultimate Tensile Strain [%]	CTE [10−6/F]	Poisson’s Ratio
E-glass	2500	3447	72.5	2.4	0.15	0.22
S-glass	2500	4550	85.5	3.3	0.086	0.22
AR-glass	2255	1793 ÷ 3447	69.6 ÷ 75.8	2.0 ÷ 3.0	N/A	N/A
High modulus carbon	1952	2482 ÷ 3998	349.6 ÷ 650.2	0.5	−0.036 ÷ −0.059	0.2
Low modulus carbon	1750	3496	239.9	1.1	−0.018 ÷ −0.059	0.2
Aramid (Kevlar 29)	1440	2758	62.1	4.4	−0.059 log. (1.7 radial)	0.35
Aramid (Kevlar 49)	1440	3620	124.1	2.2	−0.059 log. (1.7 radial)	0.35
Aramid (Kevlar 149)	1440	3447	175.1	1.4	−0.059 log. (1.7 radial)	0.35
Basalt	2800	4826	88.9	3.1	0.24	N/A

), resins (

Table 6. Parameters of selected synthetic resins [62,63].

	Density [kg/m3]	Tensile Strength [MPa]	Longitudinal Modulus [GPa]	Poisson’s Ratio	CTE [10−6/F]	Glass Transition Temperature [F]
Epoxy	1187 ÷ 1424	34.5 ÷ 103.4	2.07 ÷ 3.45	0.35 ÷ 0.39	1.6 ÷ 3.0	203 ÷ 347
Polyester	1187 ÷ 1424	48.3 ÷ 131	2.76 ÷ 4.14	0.38 ÷ 0.40	1.3 ÷ 1.9	158 ÷ 212
Vinyl ester	1127 ÷ 1365	68.9 ÷ 75.8	3.0 ÷ 3.45	0.36 ÷ 0.39	1.5 ÷ 2.2	158 ÷ 329

) and finished FRP composite rods (

Table 7. Parameters of typical FRP rods [62,63].

	Density [kg/m3]	CTE Longitudinal [10−6/F]	CTE Transverse [10−66/F]	Tensile Strength [MPa]	Tensile Modulus [GPa]
GFRP	3630 ÷ 6110	0.098 ÷ 0.17	0.35 ÷ 0.40	70 ÷ 230	5.1 ÷ 7.4
CFRP	4350 ÷ 4670	−0.19 ÷ 0.0	1.2 ÷ 1.7	87 ÷ 535	15.9 ÷ 84.0
AFRP	3630 ÷ 4110	−0.097 ÷ −0.32	0.99 ÷ 1.3	250 ÷ 368	6.0 ÷ 18.2

) are presented before analyzing the guidelines in the aforementioned design standards. The tables present key mechanical parameters such as density, tensile strength, coefficient of thermal expansion (CTE), and Poisson’s ratio. The purpose of this overview is to show the potential of FRP materials as an alternative to traditional reinforcement.

Analyzing the available design standards, the most comprehensive approach to the design of structures using FRP bars has been developed in the United States under the ACI 440 series of standards. The document ACI 440.1R-15 “Guide for the Design and Construction of Structural Concrete Reinforced with Fiber-Reinforced Polymer (FRP) Bars” [57] provides detailed design guidelines, considering the specifics of FRP materials, such as high tensile strength, lack of ductility and corrosion resistance. The standard discusses rules for sizing FRP bars in various design situations, including analysis of tensile and shear forces, relevant to the design of chemical anchors, among others. It also provides recommendations for reducing tensile strength and allowable deformations due to environmental effects.

$$f_{fu}=C_E{f}_{fu}^{*}$$

(15)

$${\epsilon }_{fu}=C_E{\epsilon }_{fu}^{*}$$

(16)

where $f_{fu}^{*}$ is the FRP design tensile strength, $f_{fu}$ is the guaranteed tensile strength of the FRP bar, ${\epsilon }_{fu}$ is the FRP design rupture strain, ${\epsilon }_{fu}^{*}$—FRP guaranteed rupture strain, $C_E$ is the environmental reduction factor (

Table 8. $C_E$ values due to the fiber used and environmental exposure [64].

Conditions of Exposure	Type of Synthetic Fibers	$\mathbf{Environmental} \mathbf{Reduction} \mathbf{Factor} {\mathit{C}}_{\mathit{E}}$
Concrete not exposed to soil and weathering	Carbon	1.0
	Glass	0.8
	Aramid	0.9
Concrete exposed to earth and weathering	Carbon	0.9
	Glass	0.7
	Aramid	0.8

).

In addition, ACI 440.1R-06 [64] also defines the required anchorage length of an FRP bar in a concrete structure:

$$l_d={\displaystyle \frac{\alpha \frac{f_{fr}}{0.083\sqrt{f_c^{\prime }}}-340}{13.6+\frac{C}{d_b}}}{d}_b$$

(17)

where $f_{fr}$ is the required tension of the bar, $\alpha$ is the factor that takes into account the location of the bar, $f_c^{\prime }$ is the specified compressive strength of concrete, $C$ is the smaller part of the cover to the middle of the belt, $d_b$ is the diameter of the bar.

FIB Bulletin 40 [59] provides comprehensive information on the use of FRP materials in reinforced concrete structures, offering engineers and designers detailed design guidelines. The document presents the mechanical and physical properties of FRP materials and analyzes their durability and resistance to degradation. It also covers mechanical and design models for ultimate load states (ULSs) and serviceability states (SLSs), taking into account deflection, cracking, and principles for ensuring adhesion between concrete and FRP bars. In the context of durability, Bulletin 40 points out the corrosion resistance of FRP materials and emphasizes the need to adapt the safety factors and methods of analysis in existing standards to the specifics of composite materials.

ISO 10406-1:2015 [60] defines standard test procedures for FRP bars and meshes used in concrete structures. The scope of the standard includes tests for tensile strength, modulus of elasticity, creep resistance, adhesion to concrete, and durability of materials under environmental conditions. The standard enables uniform assessment of the quality and performance of FRP materials, supporting their effective use in engineering practice.

Document ACI 440.3R-04 “Guide Test Methods for Fiber-Reinforced Polymers (FRP) for Reinforcing or Strengthening Concrete Structures” [58] provides detailed test methods for evaluating the mechanical properties of FRP materials. Tests include tensile and compressive strength, adhesion, bonding properties and resistance to environmental factors such as moisture, salinity, and temperature changes, among others. ACI 440.3R-04 is a key document for evaluating the quality of FRP materials in both laboratory testing and structural design.

ASTM D7205/D7205M-21 [65] describes a method for testing the tensile properties of FRP composite bars used as reinforcement in concrete. It allows determination of tensile strength, strain at break and modulus of elasticity, among others. The results are used in structural design, materials testing and quality control.

ASTM D7617/D7617M-11 [66] is for testing the transverse shear strength of FRP bars using a double shear device. It applies to smooth and textured bars used in concrete and wood structures, among others. The standard allows assessing the transverse force capacity of an element under laboratory conditions.

ASTM WK87882 [61] is a specification for carbon fiber rods, strands and spirals (CFRP) for use in concrete structures. CFRPs are characterized by exceptional corrosion resistance, light weight and high tensile strength, making them an attractive solution for infrastructure exposed to harsh environments. The standard aims to facilitate the implementation of these materials in the construction industry, promoting sustainable development and innovation in line with UN goals.

4.3. Summary and Conclusions of Available Design Guidelines

Current design standards do not take into account the specific characteristics of bonded composite anchors. All existing guidelines refer to steel anchors or FRP bars used as reinforcement, rather than chemical anchors made of composite materials. There are no calculation models for brittle materials, as current formulas assume the plasticity of steel, which is inadequate for composites. Furthermore, the standards are heavily dependent on manufacturers’ guidelines and ETA documents, resulting in the absence of universal minimum parameter values for composite anchors.

To address the existing gaps in current standards, it is necessary to develop dedicated design principles for composite anchors. New guidelines should consider the specific properties of FRP materials and their performance in concrete structures. First and foremost, failure models for bonded FRP anchors should be developed, taking into account material brittleness, potential resin debonding, and long-term degradation processes. It is also essential to include additional functional properties such as electrical non-conductivity, fire resistance, and antistatic behavior, which may be critical in certain applications.

The next step should involve integrating experimental research results into design standards, including tests under conditions of high temperature, increased humidity, and aggressive environments. Such an approach will enable a realistic assessment of the durability and safety of composite anchors in various operational scenarios. It is also necessary to adapt safety factors to the characteristics of composites, given their lack of plastic reserve, which requires a different approach than that used for steel.

5. Composite Bonded Anchors-Market Identification

When starting to analyze anchoring systems for concrete, it is crucial to consider the solutions offered by manufacturers available on the market. Today’s product range includes a wide range of anchoring systems for a variety of applications, resulting in the existence of many types of anchors, such as mechanical anchors and screw anchors. Each of these systems is characterized by specific technical properties, resulting, among other things, from the relationship between strength, geometry, anchoring method and the type of material used.

The vast majority of anchors available on the market are manufactured from steel with varying technical parameters. However, the offer of composite anchors, which are being developed by world leaders in the field of anchoring systems, requires special attention. These solutions, although less widespread (often due to lower pullout strength), can be an interesting alternative due to their unique properties, such as corrosion resistance or lightness, which opens up new possibilities for their use in construction.

Based on the overview presented, it should be noted that composite screw anchors still represent an anchoring system with limited availability. Most of the anchoring systems offered by manufacturers are based on high-strength carbon steel, which dominates the market.

In contrast, composite products offered by manufacturers are mainly used in the form of lightweight mounting studs designed for facades or anchors in the form of composite bars. An example of such a rod is shown in Figure 17.

It is also worth noting the sizable base of anchors designed for geotechnics. This is where composite anchors, due to their resistance to weathering, are used most often. An example is the solution proposed by Nordic Geo Support (Väsby, Sweden) (Figure 18) [68]. Their proposed anchoring system, consisting of an anchor rod [69] and a cover [70], is an example of a weather-resistant composite anchor [16,71].

A similar solution has been proposed by DYWIDAG (Unterschleißheim, Bavaria, Germany) [74], Shanxi Chengxinda Mining Equipment (Shanxi, China) [75], XINCHENG Insulation (Tianjin, China) [76] and Garford (Peterborough, UK) [77].

By far, the largest base of potential anchor solutions is provided by the Chinese market. For example, the company Nantong Huyu (Jiangsu, China). The Chinese manufacturer’s product range includes min. anchors for façade installation (Figure 19). The studs, shaped like a ribbed bar, allow the walls together to act as hangers in relation to large-plate buildings. These anchors, due to their low thermal expansion, prevent excessive work of the elements.

To enable a comparison of commercially available products offered by different manufacturers, considering their intended applications and additional properties, the results are summarized in

Table 9. Comparison of Composite Anchor Bars by Manufacturer, Application, and Performance.

Manufacturer	Polymer Type	Primary Application	Ultimate Load [kN]	Additional Properties
STUDC	GFRP	Tunnels, mining	–	Alkali resistance
Nordic Geo Support	GFRP	Tunnels, mining	200–3550 (range R25–T103 bars)	Corrosion-resistant, lightweight, electrically insulated
DYWIDAG	GFRP	Soil nails	up to 1280 (3-bar/4-bar system)	Alkali resistance
Shanxi Chengxinda Mining Equipment	BFRP/GFRP	Mining	–	Alkali resistance
XINCHENG Insulation	GFRP	Bolts, façade	capacity depends on diameter; tensile >600 MPa	–
Garford	GFRP	Façade	–	GFRP bolts available (no specific loads found)
Nantong Huyu	GFRP	fasade	100–750	-

.

The data presented in Table 4 indicates that most manufacturers offer composite anchors with similar technical parameters and additional properties, such as alkali resistance. This suggests a certain level of standardization in the current market. Addressing this gap could create opportunities for specialized solutions in demanding environments, such as mining and tunneling, where enhanced safety and durability are critical.

It is worth noting the wide range of currently available computer programs that allow precise estimation of the strength of screw-in anchors and their load-bearing capacity with the substrate (e.g., concrete). An example is PROFIS Anchor software, developed by HILTI, which allows analysis of the strength of screw anchors, using a rich set of design standards and technology sheets provided by the manufacturer.

Similar functionality is offered by other tools, such as the following:

- FiXperience Fischer—comprehensive software to support the design of anchor systems [79];

- Anchor Designer Strong-Tie—a tool for the analysis and design of anchorages according to international standards;

- EuroJoints ArCADia-RAMA—a module in the ArCADia system designed for designing steel connections, including anchors [80];

- ASDIP STEEL—software designed for the design of steel structures, including a design module for the analysis of base plates and anchors.

All the aforementioned tools allow engineers to quickly and accurately model and analyze the strength of anchors, bringing the design in line with the requirements of standards and technical specifications.

However, the main disadvantage of the above-mentioned programs is the inability to task an individual anchor geometry with unique mechanical properties. This means that, as part of the ongoing work within LIDER, it is reasonable to develop the required material data. In addition, it is necessary to implement the geometry and material parameters into open source FEA software such as Abaqus.

An analysis of the product base shows that steel anchors definitely dominate the market, while composite anchors are still an innovative solution with low recognition. Steel anchors are made of high-quality steel from virgin raw materials, which gives them high strength, but also reduces natural resources. As for the composite anchors on the market, they are all in the form of ribbed bars, which suggests little geometric diversity in these products.

The conducted analysis of commercially available products demonstrates that steel anchors overwhelmingly dominate the market, while composite anchors remain an innovative and under-recognized solution. The composite anchors currently available on the market are mostly produced in the form of ribbed rods, indicating limited geometric diversity. Based on the available technical data sheets and manufacturer guidelines, composite anchors are primarily used due to the high corrosion resistance of FRP (Fiber Reinforced Polymer) materials. The majority of composite anchors are utilized in mining and geotechnical applications, where they serve as ground anchors for stabilizing tunnel ceilings or slopes.

Composite anchors which are designing in the project LIDER14/0270/2023 “Composite non-conductive chain with an anchoring system” feature high electrical resistivity, antistatic, and flame-retardant properties, significantly enhancing safety in mining operations. Traditional steel chains and anchors can generate sparks, creating a risk of fires and explosions in explosive atmospheres. The composite alternative eliminates this hazard while offering superior resistance to corrosion, abrasion, and aggressive environments—reducing operational costs compared to stainless steel.

Additionally, the material incorporates innovative luminescent properties, improving visibility in low-light conditions such as darkness, smoke, or lighting failures. These solutions are suitable for both underground and open-pit mining, as well as other demanding industrial applications.

6. Summary and Conclusions

Based on the presented analysis of the Composite Bonded Anchor concept, it can be demonstrated that this solution is suitable for modern construction applications.

A bibliometric analysis covering publications from 2015–2025 revealed that the topic of composite bonded anchors remains an area requiring further development. Most available solutions rely on glass fiber reinforced polymer (GFRP) rods with standard ribbed geometry, which confirms the lack of innovation in the shape of composite anchors. The analysis of patent applications and available anchoring systems indicates that ribbed rod geometry is currently the dominant solution due to its advantages in manufacturing and load transfer. However, the absence of alternative geometries for composite anchors creates opportunities for innovation in this field.

A life-cycle cost analysis of materials showed that the use of recycled PET (rPET) reinforced with synthetic fibers can reduce production costs by 12–18% compared to traditional solutions, while simultaneously decreasing environmental impact. Available patents and scientific publications confirm the feasibility of using rPET in structural composites.

The review of design standards (ETAG 001, ACI 440) revealed a lack of clear guidelines for designing composite bonded anchors. Based on a comparison with standards for steel-bonded anchors and CFRP/GFRP rods, preliminary conditions were formulated that a composite bonded anchors must meet.

Furthermore, industrial demand was identified for anchors with properties such as non-conductivity, antistatic behavior, non-flammability, and luminescence, particularly for use in challenging environments such as mines and tunnels. The absence of patents and publications in this area confirms a research gap and significant innovation potential for the industry.

7. Futures and Perspectives

Based on the identified gap in the availability of anchoring products on the market and the absence of research addressing this gap, the next stage of work will focus on developing a composite bonded anchor with the required properties. To achieve this, a polymer mixture reinforced with glass fibers will be designed, using PET as the matrix material. The formulation of this mixture will be guided by parameters identified as critical by market needs.

In the subsequent step, the geometry of the anchor will be developed. The design will be based on a ribbed bar profile, enhanced with additional bearing blocks to improve adhesion and load transfer. A comparative analysis of different surface configurations will be carried out using the Abaqus numerical simulation software to optimize performance.

The final stage will involve manufacturing the anchor using injection molding technology and presenting the finished product to interested stakeholders in the market.