Textile-reinforced concrete with high-performance carbon fibre grids
(Published on October 2008 – JEC Magazine #44)
BERNHARD PLAGGENBORG, SALES ENGINEER, COMPOSITE MATERIALS, SGL TECHNOLOGIES GMBH
SILVIO WEILAND, CONSULTANT AND PROJECT MANAGER, DEUTSCHES ZENTRUM TEXTILBETON (GERMAN CENTRE FOR TEXTILE REINFORCED CONCRETE)
Reinforced concrete (RC) is the most successful building material of the past century, and most likely of the current century too. Combining steel bar reinforcement and concrete produces an almost ideal composite material. RC is extremely powerful, durable, and cost-effective. It fits nearly into every form, is extremely versatile and is therefore widely used as a construction material in buildings and bridges. There is just one problem: to achieve these features, RC structures need to have a minimum thickness in order to protect the metal reinforcement. A minimum concrete cover of 20–70 mm per layer is necessary to protect steel bar reinforcements from corrosion during a building's lifetime. However, bearing in mind that several layers are needed for most applications in major structures, this is no real limitation. But when such thick concrete sections are not required to resist loads, people usually start asking whether RC structures could be more lightweight, more elegant and more efficient. The answer is: Yes, they can – by using textile-reinforced concrete.
New opportunities with textile-reinforced concrete (TRC)
Textile reinforcement structures produced from carbon fibres represent an excellent alternative and complement existing reinforcement materials made from steel. Textile-reinforced concrete (TRC) has the same advantages as traditional RC, but also offers some new opportunities.
To produce textile reinforcements, continuous yarns or rovings are processed in a planar structure by a textile technique to produce an optimal alignment and arrangement of fibres in structural members (Figure 1). This in turn results in either a much higher load-bearing performance than fibre-reinforced concrete (FRC) with equivalent fibre content, or in a reduced amount of reinforcement fibres to reach an equal load capacity. TRC is the way to manufacture thin concrete parts with an extremely high load capacity. Even now, it is a very promising alternative for strengthening and repairing concrete structures, not least from a cost viewpoint.
TRCs, as described in this article, are made from carbon fibre grid structures which are embedded in a fine-grained concrete matrix (Figure 2). The result is a composite material with remarkable properties and advantages in use:
- No concrete cover is necessary to protect the reinforcement from corrosion since the reinforcing materials used do not corrode under normal environmental conditions. Considering that no concrete cover is needed, very thin strengthening layers can be produced.
- Carbon textile reinforcements have a much larger surface area than traditional steel-bar reinforcements. Thus, very high bond forces can be introduced into the concrete. This, in practice, is evidenced by both the ability to use short anchoring lengths and the presence of very dense crack patterns.
- TRCs produced from carbon possess distinctly higher strength than standard steel-bar reinforcements. The current generation of textile reinforcements has reached strengths well over 1,500 N/mm². In addition to increasing the ultimate loading capacity of reinforced concrete, TRC is also suitable for other repair applications.
- The use of additional TRC strengthening layers in RC structures has proved to have a positive influence on subsequent concrete cracking. The number of cracks increases while crack spacing decreases, and crack width is simultaneously reduced (Figure 3).
- Concrete elements become stiffer due to the application of additional material but also to the demonstrated fact that parts remain longer in an un-cracked state.
- More advanced repairs can be produced using a mineral-based matrix. In addition, pure re-profiling, re-passivation and the provision of a new or additional dense concrete cover are valuable options.
Building and infrastructure strengthening
In recent years, the need to strengthen and retrofit existing RC structures has increased. Over the course of normal building use, many situations may arise that require the strengthening of RC structures. Examples of such situations are structural damage, increased loads, design and construction faults, and system changes. A variety of methods can be used to increase the load-carrying capacity. The use of TRC can be an ingenious alternative, particularly since it is closely related to traditional, established strengthening procedures like shotcrete with steel bar reinforcement or externally bonded carbon fibre-reinforced polymers (CFRP).
Advantageous properties of externally bonded CFRP include: high tensile strength, low weight, easy handling, and high corrosion resistance. Disadvantages include high cost, sensitivity to moisture and UV radiation, low ductility, lack of fire resistance, cross-pressure sensitivity, and soaking. The repassivation of uncovered steel reinforcement – as a result of damage – requires additional measures.
The use of shotcrete, which is based on a cementitious matrix, is a suitable rehabilitation method that provides fresh protection against corrosion for existing reinforcement. Its primary advantage is the re-passivation of existing reinforcement, as influenced by the alkaline conditions of a new shotcrete layer while ruling out any sensitivity to soaking and UV radiation. However, the use of traditional steel reinforcement in combination with shotcrete has several drawbacks, including:
- the costly production and installation of strengthening layer(s),
- enlarged dimensions of the original total cross-section and required concrete covering, and
- high dead weight of the concrete added to the structure.
|Tab. 1: Performance comparison of shotcrete RC, CFRP lamellas and TRC for strengthening concrete structures|
|Ductile failure mode||++||-||+|
|Layer thickness/ weight||++||++|
The advantages of both externally bonded carbon fibrereinforced polymers and shotcrete with steel reinforcement are incorporated in the use of TRC strengthening (Table 1). The combination of textile reinforcement and concrete results in a composite material with prime qualities – high tensile strength, minimal layer thickness, low weight, high corrosion resistance, and additional corrosion protection for existing concrete reinforcement. TRC strengthening with carbon grids can improve the performance in different directions, such as bending or flexural strength, shear strength, torsion stiffness and axial forces. Moreover, the flexibility of textile reinforcement makes it possible to strengthen complex and curved geometries such as shell structures and columns.
Before the strengthening material can be applied, the surface of the RC member must be prepared, for example by sandblasting. The surface must be sufficiently rough to ensure that an adequate bond will form between the existing concrete and the strengthening layer. TRC strengthening is then accomplished by applying alternate layers of fine-grained concrete and textile reinforcement to existing RC members.
Application to lightweight concrete structures
Reinforcement with carbon fibre textile products has been used in hundreds of projects worldwide over the past few years. Some of them would certainly not have been possible without textile reinforcement.
The typical dimensions of textile reinforcement and resulting TRCs are one order of magnitude smaller than that of RC, providing totally new opportunities for the construction industry. Suitable production methods include laminating, casting, spraying, spinning and injection moulding, while possible applications may be used in a host of structures from facade panels to lost formwork and pedestrian bridges. To demonstrate the stage of development of TRC applications, a small footbridge for pedestrians and cyclists was constructed for a local horticultural show in Oschatz, Germany. The Döllnitz Creek Bridge is a single-span girder using a segmental construction method with internal unbonded pre-stressing. The span of the superstructure is approximately 8.60 m and the width between handrails is 2.50 m. The pre-cast members were assembled in a concrete plant. By optimizing the arrangement of the textile reinforcement to the flow of forces and using highstrength textile reinforcement, the amount of reinforcement can be considerably reduced and cost-effective structures are possible. The Döllnitz Creek Bridge weighs only a quarter of what a traditional RC bridge would have weighed. New construction elements can be easily produced. TRC façade elements are already in service and standing the test of time, while a second bridge with a span of more than 16 m has been coping well with the daily flow of traffic in Kempten, Germany, since the summer of 2007.
Due to the open grid structure, the textile reinforcement grid has a mechanical interlock in the concrete matrix. This results in excellent anchorage behaviour. The carbon fibre itself does not require substantial concrete cover due to its resistance to corrosion and alkalis, acids or chemicals in general. The surface design of the strengthening layer can vary within a wide range. A wide range of design options are available today for plasters that can also be utilized in TRC-applications, provided that suitable mixtures for matrices are used and the final layer is appropriately treated. These opportunities extend textilereinforced concrete applications into completely new fields.
Besides slender new concrete elements, strengthening of existing concrete structures by thin textile-reinforced concrete layers is already possible. In the near future, it is very plausible that we will find TRCs in concrete applications like roofs and balconies, noise barriers, precast elements and lost formwork for slabs, beams and columns, sandwich elements, tanks and lines. Glass fibre reinforcement materials are predominantly used for nonstructural applications, while carbon fibre materials are the preferred option for reinforcement of primary structures.
SGL carbon grid
SGL Group has developed a textile reinforcing grid for TRC using the SIGRAFIL® C PAN-based 50k carbon fibre (Figures 4 & 5). Used as continuous tow, SIGRAFIL® C is ideal for weaving, prepregging, filament winding, carbon fibre-reinforced plastic (CFRP) components, advanced composites, multi-axial production, unidirectional tapes, extrusion, pultrusion and other conversion processes.
For applications in textile-reinforced concrete, heavy tow carbon fibre improves cost efficiency in several ways. SIGRAFIL® C offers an excellent price-to-performance ratio by using a less expensive carbon fibre with high tensile strength and high modulus of elasticity. More importantly, it allows a greater fibre volume per layer of reinforcement. This improves cost efficiency dramatically because it is easier to place the reinforcement according to the stresses inside structures and, most importantly, it reduces the number of reinforcement layers needed. This leads to a shorter production time, reduced material costs and lower labour costs for the construction company.
600mm x 60mm x 10mm samples were used for the uniaxial tests with strain specimens. A hydraulic standard testing machine with a capacity of 100kN was used (Figure 6), Wedge clamping anchorage was used for the load introduction procedure. The load was applied under controlled deformation by using an external linear variable differential transducer (LVDT) at a rate of 1 mm/min. The strains were measured with a clip-on extensometer using two DD1 strain transducers from HBM (Darmstadt, Germany). The load, crosshead displacement and specimen’s deformation at front and back were measured during the test. From these data, stress-strain curves were generated for evaluation. The well-known multi-linear behaviour of reinforced concrete with high stiffness in the uncracked state I, very low stiffness during multiple cracking in state IIb, and increased stiffness with an almost linear section in state IIb is apparent for all specimens.
One of the main outcomes of the tests was that SGL’s carbon grid shows a 1,600 N/mm² tensile strength, which is more than 3 times the strength of traditional steel bar reinforcements (Figure 7). Compared to the performance of glass fibre grids in concrete reinforcements, the strength of the carbon material is still twice as high. Moreover, for applications under long-term permanent loading, glass has some significant shortcomings that let us conclude that carbon fibres are more favourable from an economic standpoint due to their perfect durability, even in alkaline environments such as cement-based matrices.
The investigation also shows a ductile material behaviour and better bond performance than steel bar reinforcements. This is important for all strengthening applications. The carbon grid reinforced material shows a ductile failure behaviour which leads to a pre-indication in the concrete construction before collapsing. This behaviour improves human safety, especially in extreme situations such as earthquake loading.
TRC combines the advantages of traditional steel reinforcements and external bonded CFRP plates. Using carbon fibre grids, the reinforcement material can be customized for specific performance demands in terms of strength, failure strain, stiffness and thickness, while guaranteeing durability for decades. The grid structure (grid width and spacing) can be customized for the application. Current knowledge has already enabled several practical applications for TRC, as shown in this paper. The ongoing development of high-performance carbon grids using heavy-tow carbon fibres will certainly lead to greater economic efficiency and thus push applications and new, innovative and lightweight TRC products.
This will certainly not entail the replacement of steel bar reinforcements but the first projects have demonstrated that TRC is a viable, cost-effective alternative to traditional steel reinforcements.
Further developments with the Technical University of Dresden, the German Centre of Concrete and SGL Group are in preparation to set European standards for the use of textilereinforced concrete in a broader range of applications. The goal is to make carbon fibre grids for TRC that will be available for various civil engineering applications, as well as to provide special TRC reinforcement solutions for the high-performance field. There are no limits for construction projects with this innovative material.