Concrete Reinforced with Fibre Reinforced Plastic

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Concrete is a very durable material. Fine examples of its first structural use by the Romans are still standing, and concrete is now probably the most widely used building material in the world. During Roman times and for many centuries after, its use was limited to compression structures, because of its poor tensile strength. But in the 19th century, the introduction of iron rods into the material led to the reinforced concrete of today, with its wide range of uses.

Iron and Steel Reinforcing in Concrete

Iron and steel rods cause potential corrosion and durability problems, however, embedded steel is generally very durable, as the alkaline environment of the concrete protects it from corrosion. But in highly aggressive environments, the protection given by the concrete is often insufficient. The protective layer is broken down and corrosion begins, the initial signs being the cracking and spalling of the concrete. Expensive remedial work is needed to repair this damage if the structure is to achieve its intended service life. Such repairs form a major part of the workload of the construction industry.

Tackling the Problem of Steel Corrosion in Reinforced Concrete

Tackling the problem of steel reinforcement corrosion has usually meant improving the quality of the concrete itself, but this has had only limited success. More recently, the construction industry has considered alternative steels for reinforcement, replacing carbon steel with stainless steel or using bars with an epoxy coating. In extreme cases cathodic protection is installed, although this is usually as part of a repair system and not for new structures.

Fibre Reinforced Plastic Reinforced Concrete

Now, the latest idea is to replace the steel with fiber-reinforced plastics (FRPs). These materials, which consist of glass, carbon, or aramid fibers set in a suitable resin to form a rod or grid, are well accepted in the aerospace and automotive industries and should provide highly durable concrete reinforcement. The durability is a function of both the resin and the fiber, while the amount and type of fiber determines the mechanical properties of FRPs. The strength of FRP reinforcement tends to be between that of high yield reinforcing steel and prestressing strand - about 1000 MNm-2 for glass fibers and 1500 MNm-2 for carbon fibers. However, the stiffness is generally much lower - about 45 GNm-2 for glass fibers and 150 GNm-2 for carbon fibers. All FRP materials have a straight-line response to failure with no plasticity.

Manufacturing and Limitations of FRP Reinforcing Elements

FRP reinforcing rods are normally made by pultrusion. However, thermoset resins are generally used and so once the material is fully cured, the rods cannot be bent into the range of shapes currently possible with steel. New manufacturing techniques are being developed to make such ‘specials’. Spiral reinforcement, both circular and rectangular, is being produced by several Japanese manufacturers, as are two- and three-dimensional grids. Other techniques are being developed where resin-impregnated fibers are wound onto mandrels to produce closed shapes, such as shear links. Alternatively, thermoplastic resins are being developed to allow the fully cured material to be warmed and bent to shape. However, this is likely to give weaker reinforcement where the bar is bent due to misalignment of the fibers.

History of FRP Concrete Structures

The potential of FRP concrete reinforcement has already been shown globally by the construction of many demonstration structures. Initially, concerns about the lower stiffness of FRPs compared to steel meant most structures were pre-stressed, with conventional steel being used as secondary reinforcement. Several footbridges and highway bridges have been built, mainly in Japan and North America.

Highway Bridge

The first major European structure was built in Dusseldorf in 1987 - a highway bridge with glass FRP pre-stressing cables. Later demonstration structures were important in the Eurocrete project, which was the first co-ordinated European program of development work on FRP reinforcement. Eurocrete was a collaborative research project between partners in the UK, France, the Netherlands, and Norway funded partly under the Eureka scheme. It was probably the first project of its kind in the world to bring together all the disciplines involved with FRPs, including materials suppliers, processors, research organizations, and designers.

Footbridges and Non-Magnetic Fencing

The first major European structure was built in Dusseldorf in 1987 - a highway bridge with glass FRP pre-stressing cables. Later demonstration structures were important in the Eurocrete project, which was the first co-ordinated European program of development work on FRP reinforcement. Eurocrete was a collaborative research project between partners in the UK, France, the Netherlands, and Norway funded partly under the Eureka scheme. It was probably the first project of its kind in the world to bring together all the disciplines involved with FRPs, including materials suppliers, processors, research organizations, and designers.

FRP Concrete Standards

drawn up globally. When introducing a new type of reinforcement with very different properties, there are two approaches - adapt the existing approach, or go back to square one and write new rules. The second is more technically correct but is costly and time-consuming. As real applications are the only way to get a good experience of the behavior of new material, modifying existing standards is the only feasible option.

The current standards for the design of reinforced concrete structures have developed over the last 100 years or so. They combine methods based on sound scientific principles and certain rules of thumb. For example, as reinforced concrete is a composite material, some aspects of its behavior, such as shear, are still not well understood and so empirical approaches are used. FRP-reinforced concrete follows similar rules to steel-reinforced concrete but differs in many ways.

Much experimental work has been carried out using FRP-reinforced concrete, mainly on simple beams and slabs, and basic design methods are being developed in several countries. The Japanese Ministry of Construction has published draft guidelines for design, the Canadian Bridge Code will have a chapter dealing with FRPs and the American Concrete Institute is preparing guidance. Proposed modifications to British Standards covering the design of reinforced concrete structures were developed under the Eurocrete project and are now being validated by the Institution of Structural Engineers. They will provide a document for use by design engineers in the absence of a formal code of practice. 

These design approaches will lead to safe structures but are unlikely to be the most economic use of the relatively expensive FRP materials. The cost of FRP rods is expected to be between that of epoxy-coated steel and stainless steel, two to eight times as expensive as a normal steel bar. Such a high initial cost can only be justified by looking at ‘whole life’ costs for structures in aggressive environments. Potential users need to consider the total costs for their structures, including repairs, and not just the material costs. In the future, such savings should become obvious as design approaches are developed accounting for the enhanced properties of FRP-reinforced materials.

Differences Between FRP and Steel Reinforced Concrete

  • Because of the high strength and relatively low stiffness of FRPs, failure is likely to occur by compression of the concrete and not rupture of the reinforcement.Crack widths in steel-reinforced concrete are controlled to prevent aggressive substances from reaching the steel, so improving durability. For FRP-reinforced concrete, aesthetics and possibly water-tightness will be critical for crack width control.
  • Deflections are likely to be higher than for equivalent steel-reinforced units.
  • FRP rods have low compressive strengths in comparison to their tensile capacities, so the traditional design approaches for columns are no longer valid. Studies looking at the effect of wrapping FRP around circular columns have found that the confinement increases the failure load and the failure strain.
  • Fire will be a design consideration for some types of structures. The main concern is to limit the temperature rise at the surface of the FRP bar so that it stays below the glass transition temperature of the resin. Above this temperature, the material stops acting as a composite and weakens.

Problems Associated with FRP Concrete

Durability

The major cause for concern in the use of FRPs as reinforcement is the durability of the material when embedded in concrete. The high alkaline environment degrades glass fibers and some resins, and manufacturers are reluctant to disclose the details of the materials used for commercial reasons. Work has concentrated on developing alkali-resistant glass and on using carbon and aramid fibers, but little attention has been paid to the resin. Ways of assessing the durability of the materials are urgently needed, but considerable work is still needed to develop an acceptance criteria.

A major assessment of durability was carried out in Eurocrete, including work on the materials and FRPs embedded in concrete. The latter samples were stored in laboratories under various environmental conditions and also on exposure sites in Europe and the Middle East. The results, which apply to the particular resin and fiber combinations studied, show that the composite rods resist the alkaline environment well, with no significant degradation during the test period

Industry Acceptance

Despite its excellent properties and durability, FRP reinforcement is unlikely to replace steel for most structures in the foreseeable future. Experiments and demonstration projects around the world have shown that FRP reinforcement is a viable and cost-effective alternative to steel in special circumstances, for example as an alternative to stainless steel. But the construction industry is extremely conservative, and so the most likely development route is the use of the new materials in non-structural applications or where the consequences of failure are not too severe. More highly loaded and critical applications will follow as confidence in the materials grows.

Summary

In summary, FRP reinforcement needs to move from low volume/high technology applications to high volume/relatively low technology applications. Before it becomes widely accepted for concrete structures, several significant aspects of the materials must be demonstrated, including the durability of FRPs embedded in concrete, the ability to produce suitable reinforcement shapes and the ability to produce large quantities of materials of consistent quality. All are essential if the true potential of FRP reinforcement is to be realized.

This article was updated on 12th February, 2020.

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