For many years concrete has been a popular construction material in view of its versatility and low cost. Concrete has a very low tensile strength, but higher compressive strength. To overcome this deficiency, structures are often formed using prestressed concrete, which is made by applying force to the concrete that induces compressive stresses in areas which will subsequently be subjected to tensile stresses caused by loads in use.

The two most common methods for prestressing the concrete are post-tensioned prestress and pre-tensioned prestress. In post-tensioned prestressed concrete, a tendon is stressed against the concrete after the concrete has been cast, either by placing the tendon in a duct, or by placing the tendon outside the concrete but connecting it to the concrete at anchor blocks and deflecting points. If a duct is used, the internal tendons are normally bonded to the concrete with a bonding agent, such as grout, to allow stress transfer between the concrete and the tendon. In some cases tendons remain unbonded. External tendons are normally free to slide at deflector points and only restrained against axial movement at their ends. In pre-tensioned prestressed concrete, the tendons are stressed against anchor blocks or reaction frames before the concrete is cast. The tendons must be straight, unless special deflector blocks are used. After the concrete has been cast and hardened, the tendons are released from the anchor blocks and contract. The resulting shortening of the tendon causes the concrete to be compressed. The tendons are normally bonded to the concrete throughout their length, although a few tendons are sometimes free at the ends of beams to prevent excessive prestress. When a section of the structure gets close to failure, large strains occur in the concrete and tendons. These are accompanied by large curvatures in the structure when the

strains on one surface of the structure differ from those on the other surface. If there are high curvatures, or the curvature is spread over a significant length of beam, there will be appreciable rotation of the adjacent parts of the beam relative to one another. A high rotation capacity is therefore desirable, since it allows internal forces to be displaced from one part of a structure to another or to give prior warning of impending failure.

Traditionally, prestressing tendons have been formed from steel, in the form of single wires, multi-wire strands, or, in the case of post-tensioned concrete, in the form of multi-strand tendons.

In recent years, new tendon materials in the form of continuous fibres, such as glass, carbon or aramid fibres, have become available. These can be formed into fibre reinforced plastic (FRP) rods which can be used to make pre-tensioning or post-tensioning tendons. These FRP rods have several advantages over their steel equivalent, in particular their lack of subceptability to corrosion and their non-magnetic properties.

A significant disadvantage of the tendons formed from these new materials, however, is that they do not have the ductile characteristics of metal, meaning that structures formed with bonded FRP tendons have low curvatures at failure, and that failure will occur when tendons snap, causing catastrophic failures without warning.

If FRP tendons are used in an unbonded manner, this problem can be overcome, but there is a significant lowering in moment capacity, making such an arrangement far more expensive to employ.

The present invention is directed toward solving the aforementioned problems.

According to the present invention there is provided a prestressed concrete structure comprising: at least one fibre reinforced plastic stressing tendon bonded in such a manner that the tendon can extend over a controlled length.

The required bonding may be provided by coating the tendon with a material having a predetermined shear strength. Alternatively, or in addition, the bonding may be provided by bonding the tendon at intervals along its length. For either arrangement, the tendon may be fully bonded at its ends.

The prestressed concrete structure may be of either the pre-tensioned or post-tensioned type. For post-tensioned prestressed concrete, a permanent anchorage system may also be employed at the ends of the tendon. The length of each of the intermittent bonded regions may be selected so that, when the force on the bond at a particular location exceeds a predetermined level under excessive load, the bond breaks down allowing the tendon to strain over a greater length. Alternatively, the unbonded length is sufficient to allow significant tendon extension before failure, whilst being short enough to ensure that the tendon force increases to an economic level. In this case, breakdown of the bond is not essential. A corresponding method for producing a concrete structure according to the invention is also provided.

One example of the present invention will now be described with reference to the accompanying drawings, in which:- Figure 1 is a schematic diagram showing the effect of excessive load on a prior art structure and a structure according to the present invention; and

Figure 2 is a schematic cross-sectional view of a structure according to the present invention. An example structure according to the invention is shown in figure 2. This structure 1 is formed from a standard concrete 2 which is placed around stressing tendons 3. The stressing tendons 3 are of the known fibre reinforced plastic (FRP) type. The type of concrete 2 used and the number of tendons 3 required, together with the relative spacing of the tendons 3, is dependent upon the size of the structure 1 to be formed and the application in

which the structure 1 is to be used. In this example, the structure 1 is of the pre-tensioned prestressed concrete type, meaning that tension is applied to the tendons 3 prior to them being surrounded by concrete 2. In this example, the bond between the tendon 3 and the concrete 2 is broken by covering the tendon 3 with a length of plastic tube (not shown) at intermittent regions along the tendons' length so that, when the structure has cured and set, each of the tendons 3 has a plurality of regions in which they are bonded to the concrete 2 and a plurality of regions in which they remain unbonded.

Figure 1 shows a prior art concrete structure 4 which comprises tendons formed from FRP, but which are bonded along their entire length. Also shown is a concrete structure 1 according to the invention. As can be seen from figure 1, when a load is applied to these structures 4, 1 there is a marked difference in the effects on each of the structures 4,1. With the prior art structure 4, there is little deflection, providing no indication of impending failure and allowing no re-distribution of the internal forces to avoid tendon failure. On the other hand, the concrete structure 1 according to the invention has increased rotation, providing a good indication of impending failure, and allowing for re-distribution of the forces across the whole structure 1. As mentioned above, an alternative example of the invention employs FRP tendons 3 that are coated with a material of predetermined shear strength. The shear strength should not be so excessive that shearing does not occur well before tendon failure. Such material can be of the epoxy resin type, and provides a similar effect to that shown in figure 1. Both examples may be used in combination to achieve a structure with the desired load carrying and rotation characteristics for a desired application.