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 reinforced concrete as we know it today, with its incredibly 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 it is protected from corrosion by the alkaline environment of the concrete. 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 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 approach 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 fibre reinforced plastics (FRPs). These materials, which consist of glass, carbon or aramid fibres 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 fibre, while the amount and type of fibre are keys to determining 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 fibres and 1500 MNm-2 for carbon fibres. However, the stiffness is generally much lower - about 45 GNm-2 for glass fibres and 150 GNm-2 for carbon fibres. 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. One limitation of this method is that 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 in which resin-impregnated fibres are wound onto mandrels to produce closed shapes, such as shear links. As an alternative, thermoplastic resins are being developed that would 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 fibres.
History of FRP Concrete Structures
The potential of FRP concrete reinforcement has already been shown around the world by the construction of many demonstration structures. Initially, owing to concerns about the lower stiffness of FRPs compared to steel, most structures were pre-stressed, with conventional steel being used as secondary reinforcement. A number of footbridges and highway bridges have been built, mainly in Japan and North America.
The first major European structure was built in Dusseldorf in 1987 - a highway bridge with glass FRP pre-stressing cables. Later demonstration structures formed an important part of the Eurocrete project, which was the first co-ordinated European programme 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 organisations and designers.
Footbridges and Non-Magnetic Fencing
Two footbridges were built during the Eurocrete project, one at Chalgrove near Oxford, and the other in Oslo. Part of a berthing facility at docks in Qatar was also constructed using FRP reinforcement, and another application was as reinforcement for the concrete fencing around a test facility for sensitive electrical equipment where conventional steel bars would have caused magnetic interference, (figure 1). Many applications were tested in the laboratory and may move into practice shortly, including retaining wall units and cladding panels. Meanwhile, other programmes are developing larger structures fully reinforced with FRPs, such as an 80 metre-long footbridge in Denmark.
Figure 1. Non-magnetic concrete security fencing erected around a test facility for sensitive electrical equipment.
FRP Concrete Standards
As FRP-reinforced concrete is being developed, design standards for its use are also being drawn up around the world. 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 completely new rules. The second is obviously more technically correct, but is a costly and time consuming process. As real applications are the only way to get good experience of the behaviour of a 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 behaviour, such as shear, are still not well understood and so empirical approaches are used. FRP-reinforced concrete will follow similar rules to steel-reinforced concrete, but will differ in a number of 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 a number of countries. The Japanese Ministry of Construction has published draft guidelines for design, the Canadian Bridge Code will shortly 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 lead to 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 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 which take account of 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 reaching the steel, so improving durability. For FRP-reinforced concrete, aesthetics and possibly watertightness will be the only criteria 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 leads to increases in 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 so weakens.
Problems Associated with FRP Concrete
The major cause for concern in the use of FRPs as reinforcement is probably the durability of the material when embedded in concrete. The highly alkaline environment degrades glass fibres and some resins, and manufacturers are reluctant to disclose the details of the materials they use for commercial reasons. Work has concentrated on developing alkali-resistant glass and on using carbon and aramid fibres, but little attention has been paid to the resin. Ways of assessing the durability of the materials are urgently needed, but considerable work still needs to be done to develop acceptance criteria.
A major assessment of durability was carried out in Eurocrete, which included work on the materials themselves and on 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 fibre combinations studied, show that the composite rods resist the alkaline environment well, with no significant degradation during the test period.
Despite its excellent properties and durability, FRP reinforcement is unlikely to replace steel for the vast majority of 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 in ones where the consequences of failure are not too severe. More highly loaded and critical applications will follow later as confidence in the materials grows.
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 have to 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 a consistent quality. All are essential if the true potential of FRP reinforcement is to be realised.
Primary author: John Clarke
Source: Materials World vol. 6 no. 2 pp. 78-80 February 1998.
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