Pultrusion of bendable rebars based on a glass fibre reinforced thermoplastic

Date：2025-04-14 Source：JEC Hits：133 Comment：0

Core tips：One of the key achievements of the COACH 2 project between Arkema and IRT M2P, which allowed the maturation of Elium®, an innovative liquid thermoplastic resin that is both post-formable and recyclable, is the development of a production process for benda

One of the key achievements of the COACH 2 project between Arkema and IRT M2P, which allowed the maturation of Elium®, an innovative liquid thermoplastic resin that is both post-formable and recyclable, is the development of a production process for bendable thermoplastic rebars. Boris Duchamp, expert in composite materials, IRT M2P, and Alexander Zoller, R&D Scientist in composite materials, Arkema, explain it to us.

Boris Duchamp, Expert in composite materials, IRT M2P, and Alexander Zoller, R&D Scientist in composite materials, Arkema

The first rebars appeared at the end of the 19th century and were designed to reinforce public works structures in the face of various climatic hazards, such as the earthquakes that struck the Californian region at the dawn of the 20th century. Concrete has weak tensile properties, which compensates for with very good compressive properties. When dimensioning structures, it is necessary to have reinforcement for stresses other than compression. This is the role of rebar in concrete reinforcement.
The rebars currently on the market are mainly made of steel, although some are available in composite form, mainly in thermosetting resin, and a few thermoplastic rebars are beginning to appear on the market.

Thermoplastic composite rebars

The main reason for transferring rebar technology from metal rebars to composites is the reaction to metal oxidation. When subjected to extreme climatic conditions such as humidity, rust can form. This causes the metal’s internal volume to swell, leading to cracking in the concrete and exposing the reinforcement structures. If not stopped in time, this phenomenon can lead to structural failure. In extreme corrosive environments, such as the seaside, or where temperature and/or chemical conditions are outside the norm, this phenomenon is amplified. Non-rusting composite rebar is the answer to the problem of metal rebar.
Epoxy resin-coated metal rebars are available to slow down corrosion, but corrosion still occurs. Stainless steel rebars also exist, but they are extremely expensive – around 6 to 7 times the cost of standard rebars. The composite rebars that have come onto the market offer excellent corrosion resistance, electrical and thermal insulation favourable to their use in construction and public works, high mechanical strength and satisfactory fatigue properties. They also are lighter than steel, with a density of around 1/4 that of steel. In terms of cost, several studies have shown that composite rebars are relatively more expensive than steel rebars [1]. However, if we consider the product’s entire life cycle, composite rebars have a much lower maintenance cost than metal rebars. That is why, in terms of the overall product life cycle, composite rebar is more economical than metal rebar.
Furthermore, traditional rebars available on the market are made of thermosetting resin and, as such, cannot be bent on-site. The advantage of developing a thermoplastic (TP) resin-based rebar is that, like metal rebars, it can be bent on-site, while retaining the advantages of composite rebars.
A composite rebar based on Elium® ThermoPlastic resin, produced by Arkema, is currently under development. With all the benefits of a composite rebar, it also has the added advantage of being bendable on site. As rebar is a constant-section component, the pultrusion process was the ideal solution for producing it economically (continuous production). Therefore, the work was focused on developing a specific grade of Elium® resin, studying the texturing and bending of these TP rebars.

Figure 1: Principle of a pultrusion line with knurling device

Process development

The pultrusion process (Figure 1) uses a conical die (not required to pultrude Elium® resin, but optimised for our process) developed to reduce the porosities inherent in composite production, while also enabling a layer of resin to be deposited on the outside of the rebar surface [2]. This thin layer of pure resin, fully bonded to the composite structure, is thermoplastic enough to allow texturing without degrading the product through heating or the application of mechanical stresses. The main idea here, of course, is to avoid degrading the fibrous structure, and therefore to play on the geometry or texturing of this thin resin layer. Knurling has been identified as the process best suited to this task. Standardised pull-out tests ensure that the solution developed is viable and passes the standard.
A specific resin grade was also developed by Arkema to achieve high mechanical properties and ensure certain guarantees regarding the environmental stresses to which rebars are subjected. Alkaline resistance was the focus here. Resin development work led to Arkema marketing their Elium® C599E resin grade specifically for rebar applications (Figure 2).

Figure 2: View of a pultrusion die and rebars

once these rebars have been produced, the next step is to develop the bending process needed to bend them to the desired shape.
The presence of Elium thermoplastic resin is highly advantageous here, as it can be heated to a sufficient high temperature, enabling the product to be formed into a new geometry. The idea here is to heat an area of the product in a controlled manner to allow it to be deformed. Several heating technologies have been studied, including microwave, Infrared Radiation (IR), conduction and convection heating. It turns out that the best compromise, both in terms of heating zone efficiency and surface heating, is IR heating. This allows a temperature gradient inside the rebar that is as controlled as possible, thus guaranteeing its integrity.
once the part of the rebar to be bent has reached the right temperature, it is time to apply a deformation that not only achieves the target geometry, but also guarantees the integrity of the fibre network. When bending a composite product of non-zero thickness, the main problem is fibre consumption (Figure 3), which is sometimes unequal between the intra- and extra-back.

Figure 3: Consumption of fibres in a bent rebar

Indeed, bending a rod requires controlling the consumption of the various fibres relative to the product’s neutral fibre (Figure 4). If not, creases and/or over-tensions appear in the induced product. This step can take several forms: traditionally, a twist that disorients the fibrous network and thus reduces the induced properties of the composite, or other methods that cannot yet be disclosed.

Figure 4: Steps of production of a bended rebar

Normalisation

In terms of characterisation procedures, 2 tests are standardised: the pull-out test, which involves pouring a straight, textured rebar into a concrete block and then exerting a tensile force on it. This test checks the adhesion between the rebar and the concrete. A second test is a pull-out on curved rebar, in which one part of the curved rebar is cast into a concrete block and the other is subjected to a tensile force. The test covers both the fracture zone and the maximum stress level obtained (Figure 5).

To ensure the viability of the solution developed, the Elium rebars resulting from the collaboration between Arkema and IRT M2P were compared not only to commercially available ThermoSet (TS) vinylester rebars, but also to the minimum required by the standards. According to the ASTM D8505 and ASTM D7957 standards, Elium-based rebars show properties relatively similar to those of a vinylester rebar and exceed the minimum required by the standards [3] [4].

Properties and commercial uses

The rebars developed using Elium® C599E fully comply with the requirements of the standards (Table 1), guaranteeing adhesion in concrete through an in-line texturing process developed by IRT M2P that does not remove any material and can be bent to those of metal rebars. It should be noted that the recycling of these rebars has been demonstrated both by mechanical recycling (by grinding and thermocompression by IRT M2P) and by depolymerisation (by Arkema). The issue of separating hard rebar from concrete remains unresolved however, just as with any other rebar on the market.
These Elium rebars are already being used commercially for repair and maintenance operations on civil engineering structures. Some of these rebars have already been installed on existing structures throughout Europe for repair purposes. Dextra and Sireg, 2 Elium rebar producers, have already positioned themselves as commercial suppliers of these TP rebars.

Table 1: Summary of TP Elium® rebar characteristics [3]

Acknowledgements
This research was supported by the French Technological Research Institute for Materials, metallurgy and Processes (IRT M2P) and the French National Research Agency (ANR) under the funding agreement No. ANR-10-AIRT-04. We also acknowledge the contributions of the project partner Arkema, whose support was instrumental in advancing this work.

Reference:
[1] Cost and environmental analyses of reinforcement alternatives for a concrete bridge – Structure and Infrastructure Engineering vol16, No4, 787-802, Thomas Cadenazzi et al., 2020
[2] WO2023233032A1, Procédé pour la production d’un composite thermoplastique façonné, composite thermoplastique mis en forme et système pour la production d’un composite thermoplastique façonné
[3] D’Antino, Tommaso, et al. “Tensile and interlaminar shear behavior of thermoset and thermoplastic GFRP bars exposed to alkaline environment.” Journal of Building Engineering 72 (2023): 106581
[4] Brahim Benmokrane et al., Physical, mechanical, and durability characteristics of newly developed thermoplastic GFRP bars for reinforcing concrete structures, Construction and Building Materials,
Volume 276, 2021, 122200

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