GB2078814A - Method of prestressing a steel to the concrete member - Google Patents

Abstract

The elongate structure is formed of separately produced sections (A, B) in one (A) of which, during production, an end portion of a steel rod (3) is embedded and in the or another (B) of which the remote end portion of the rod is anchored; the intermediate portion of the rod being left unbonded to the concrete e.g. by means of a plastics sleeve (21). The other section or sections of the structure are formed and are assembled with the one section so that, during the formation or assembly, the unbonded portion of the rod can be constrained under tension and, on completion of formation of the structure, the constraint released to cause the rod to apply compression to the structure. <IMAGE>

Description

SPECIFICATION Method of prestressing a steel concrete member The present invention relates to a method of prestressing a steel concrete member such as a fence post.

Existing methods of prestressing concrete structures are not suitable for use with small elongate structures such as a fence post either on the ground or cost or of lack of technical feasibility.

This is especially true with a fence post as described in copending application No. 7937235.

This post is formed with a pointed end which would be at risk of damage from existing methods of prestressing.

It is an object of the present invention to provide an efficacious method of prestressing small elongate concrete structures.

In accordance with the present invention, the post is produced from separately formed sections providing an anchorage for respective end portions of a steel prestressing rod such that a portion of the rod between the end portions is left unbonded to the concrete and able to be extended to induce tension in the rod. In one embodiment, the post is formed of two sections. One of the sections is moulded round an end portion of the rod with the remaining portion of the rod extending from an end face of the section.The second section is then moulded round the exposed portion of the rod to form the next adjacent portion of the post, but the moulding is carried out to prevent bonding of the concrete to the rod except at a portion of the rod at the other end thereof, this being bonded to the concrete, and the two sections at their facing ends are formed with complementary ramp surfaces so that by twisting the two sections, they can be urged apart sufficiently to induce the required tension in the rod.

In a further embodiment, a first section is formed comprising an end portion embedding an end portion of the prestressing rod while the remainder of the section is prevented from bonding and is moulded at the remote end with a reinforcing band the axial length of which completes the full length the post is to have. In this instance, a rod is used of a length to leave a portion extending beyond the free end of the band, and this portion is used to apply tension to the rod.

With the rod under tension, the band is then filled with concrete to complete the post, the concrete bonding to the portion of the rod passing within the band. When this concrete has set, the extending portion of the rod is released to place the post under compression.

A single rod of a predetermined diameter may be used as the prestressing component, but preferably, rather than use one rod of some given diameter needed to impart the required strength to the post, a plurality of proportionally smaller diameter rods are employed. This enables the required strength of the structure to be maintained, but reduces the torque needed to effect the twisting of the prestressing component and also reduces the shear stress indiced therein.

In this case, the plurality of rods are loosely gathered together at the location at which a single rod would be used. However, purely for convenience of description hereinafter, the use of a single rod will be assumed. In a typical fence post, it is preferred to use seven of the rods.

In this embodiment, the first section may be formed by moulding said end portion thereof and the remaining portion thereof separately.

Conventional methods may be used to prevent bonding of the concrete and the prestressing rod, e.g. by using a sleeve of suitable material on the rod, or by application of a release agent, for example, grease.

Tensioning the rod in the second embodiment is preferably effected by means of a pair of wedges respectively bearing against a grip for the rod end and a plate supported on the free end of the band and bearing a hole to take the rod therethrough; the wedges having slots passing transversely thereof through which the rod passes, the arrangement being such that when the wedges are driven towards one another, the cable is tensioned. The degree of rod extension needed to set up the required tension is some 3 mm.

The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows in elevational section a concrete fence post of a first embodiment of the invention; Figure 2 is an elevation of a section of the post of Figure 1; Figure 3 is a detail of the section of Figure 2; Figure 4 is an elevational section of part of a concrete fence post of a second embodiment of the invention, and Figure 5 is an elevational section of a part completing the post of Figure 4.

Referring now to Figures 1 to 3, the post 1 comprises two separately formed sections A, B and contains a tensioned steel rod 3; the rod being embedded in the concrete of section A at one end portion of the rod and being embedded in the concrete of section B at a portion at the other end thereof. The section A, which forms a point to facilitate driving of the post into the ground, is moulded round the respective end portion of the rod so that the remaining portion of the rod extends from an end surface 5 of the section. That end surface is formed with a ramp surface 9 of spiral form passing through an angle of 3600. The ramp surface thus delimits a vertical face 11.

The section B is moulded around the exposed portion of the rod with part of the length of that portion bearing a sleeve 21 to prevent bonding of the concrete to the rod. The rod thus becomes embedded at its end portions, but is free intermediate those end portions to extend.

The end surface 7 of section B facing end surface 5 of section B is also formed with a ramp surface of complementary form to that of surface 5.

The extension of the rod, of some 3 mm, is effected by twisting the two sections relative to one another through an angle of some 300. This moves the vertical faces 11 of the two ramp surfaces apart to create a gap. While the two sections are constrained to maintain their relative position produced by the twisting, the gap is filled with a settable material, e.g. expoxide resin, and the material allowed to set following which the constraint is removed. This prevents the two sections from moving back to their original positions once the constraint is removed.

Referring now to Figures 4 and 5, the post comprises a first section A constituting a major portion of the length of the post and, at one end, is formed with a point to facilitate driving the post into the ground. The section is moulded round a prestressing rod 31 bearing a sleeve 33 over a portion at its length to prevent bonding of concrete to it; one end portion of the rod being embedded in the concrete of the part, indicated at A1, forming the point of the post, and the other end portion 35 extending from the remote end of the section. That end of the section is moulded so as to support a reinforcing band 37 made of sheet steel; and the end portion 35 is of sufficient length to extend beyond the free end of the band. The free end of the band supports a plate 41 bearing a hole through which the rod passes.

Before the plate is positioned the space within the band is filled with concrete 43.

Tension is applied to the rod before concrete 43 sets by two wedges 51, 53. Each wedge bears a slot 55 passing transversely therethrough and the rod is passed in succession through the two wedges by means of the slots therein. The end of rod portion 35 is gripped by a collet device comprising locking elements 61 urged against the rod by a sleeve 63. The wedges are arranged so that the end face 71 of wedge 51 acts against the sleeve 63 while the end face 73 of wedge 53 acts against plate 41 with the other end faces of the two wedges bearing on one another.

When the two wedges are urged towards one another, they cause the collet device to grip the rod and then force it in the direction away from the plate 41. This extends and tensions the rod; the extension being some 3 mm. The tension is maintained until the concrete sets. The collet device and the wedges are then removed from the rod, and the superfluous length thereof extending from the post is cut off.

The band is kept in place since it greatly increases the resistance of the concrete to resist impact forces it will be submitted to when being driven into the ground. It may be of any suitable material and steel need not be used. For instance, it may be made of plastics.

An advantage of a prestressed post is that it is less prone to cracking than one that is simply reinforced with steel rod, and is therefore less susceptible to corrosion of the rod employed, thus lengthening the life of the post. Further, it uses less steel than a reinforced post; and because the prestressing rod or rods may be centrally located, a post can be produced providing greater cover for the steel and therefore again a lesser susceptibility to corrosion.

Claims (10)

1. A method of forming an elongate concrete structure, wherein the structure is produced from separately formed sections providing an anchorage for respective end portions of a steel prestressing rod such that a portion of the rod between the end portions is left unbonded to the concrete and able to be extended to induce tension in the rod to impose a compressive force on the completed structure.

2. A method according to claim 1, wherein the post is formed of at least two sections, and one of the sections is moulded round an end portion of the rod with the remaining portion of the rod extending from an end face of the section, a second section is then moulded round the exposed portion of the rod to form the next adjacent portion of the post, the moulding being carried out to prevent bonding of the concrete to the rod over at least a portion thereof, and the two sections at their facing ends are formed with complementary ramp surfaces so that by twisting the two sections with the other end of the rod anchored in the concrete of the structure at the other end thereof, they can be urged apart sufficiently to induce the required tension in the rod.

3. A method according to claim 1 , wherein a first section of the structure, is formed comprising an end portion embedding an end portion of the prestressing rod while the remainder of the section is prevented from bonding and is moulded at the remote end with a reinforcing band the axial length of which completes the full length the post is to have, the rod being of a length to leave a portion extending beyond the free end of the band, tension is applied to the rod by applying tension to that end portion thereof while the band is filled with concrete to complete the full length of the post, the concrete bonding to the portion of the rod passing within the band, the concrete is allowed to set and the extending portion of the rod is then released.

4. A method according to claim 3, wherein said end portion and the remainder of the first section are separately formed.

5. A method according to claim 3 or 4, wherein tensioning the rod is effected by means of a pair of wedges respectively bearing against a grip for the rod end and a plate supported on the free end of the band and bearing a hole to take the rod therethrough; the wedges having slots passing transversely thereof through which the rod passes, the arrangement being such that when the wedges are driven towards one another, the cable is tensioned.

6. A method according to any of the preceding claims, wherein rod extension effected is some 3 mm.

7. A method according to any of the preceding claims, wherein a single rod of a predetermined diameter is used as the prestressing component.

8. A method according to any of the preceding claims, wherein a plurality of rods of predetermined diameters are employed.

9. A method of forming an elongate concrete structure substantially as hereinbefore described with reference to Figures 1 to 3 or to Figures 4 and 5 of the accompanying drawings.

10. An elongate concrete structure whenever produced by the method claimed in any of the preceding claims.