JEC Group have brought together the international community of composites leaders and executives in our Composites Circle as an unique networking opportunity to meet with both peers and future partners.
The need to strengthen and retrofit existing concrete structures will increase dramatically in the near future. Textile-reinforced concrete will open new opportunities by offering higher performance, better handling in use and lower overall cost.
(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:
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 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.