Non-ferrous reinforcement

Abstract

Traditionally, concrete structures have been reinforced with steel bars or prestressed with steel wires or strands. Generally, embedded steel is very durable, the concrete providing a suitable alkaline environment. However, for structures in highly aggressive environments, the protection afforded by the concrete is often insufficient to ensure the necessary service life for the structure. In addition to attempts to improve the quality of the concrete itself, there are a number of approaches for protecting the steel directly, such as by means of epoxy coating or the use of cathodic protection. A further alternative is to replace the steel completely by a material that will, hopefully, be more durable. One such option, which has great potential, is the use of fibre-reinforced plastics (FRPs), which consist of continuous fibres, generally of glass, carbon or aramid, set in a suitable resin to form a rod or grid. These materials are well accepted by the aerospace and automotive industries and are starting to find applications in the construction industry. The mechanical properties of FRPs are chiefly determined by the amount and type of fibre, while the durability will be a function of both the resin and the fibre. The strength of FRP reinforcement will tend to be between that of high-yield reinforcing steel and prestressing stand (say 1000 N/mm2 for glass fibres and 1500 N/mm2 for carbon fibres) but the stiffness will generally be significantly lower (say 45 kN/mm2 for glass fibres and 150 kN/mm2 for carbon fibres). All FRP materials have a straight line response to failure with no plasticity. The most common method for manufacturing FRP rods is pultrusion, in which the fibres are drawn off storage containers in a controlled pattern, are impregnated with resin and then drawn through a heated die which sets and cures the resin. One limitation at present is that thermoset resins are generally used and, hence, once fully cured, rods cannot be bent into the range of shapes currently used with steel. Thus ‘specials’ are required. Spiral reinforcement, either circular or rectangular in form, is produced by some Japanese manufacturers along with two- or three-dimensional grid. Other manufacturing techniques are being developed in which resin-impregnated fibres are wound on to suitable mandrels to producing closed shapes, such as shear links. As an alternative, thermoplastic resins are being developed, which would allow the fully cured material to be bent.