WO1998031891A1 - Improvements relating to reinforcing bars - Google Patents

Abstract

A bar is formed by drawing longitudinally extending resin-impregnated fibres (1) through forming eyes to remove excess resin, leaving an external surface which is non-smooth in appearance. After curing of the resin the bar can be used for reinforcing purposes and the roughened surface will tend to grip well within the material being reinforced. A secondary tow (2) can optionally be wound around the longitudinal fibres to give improved circumferential strength and to help prevent separation of the fibres.

Description

"Improvements relating to Reinforcing Bars"

The traditional method of reinforcing concrete is by using unprotected mild steel bars which tend to rust and expand causing spalling of the concrete and eventual failure. Several methods have been tried to protect the steel from corrosion but these tend to be expensive and have other disadvantages.

The formation of reinforcing bars by a process according to the invention comprises drawing bundles or 'tows' of continuous glassfibre strands and thermoplastic resin under controlled tension through forming eyes or the like to remove excess resin, leaving the resulting matrix with a high glass content (typically 60% - 70%) which is ideal for high strength reinforcing bars, the matrix subsequently being cured either by reaction in the resin or more quickly by heating.

Thus the invention overcomes the problem of steel corrosion by using continuous glassfibre strands impregnated with resin to form rods, bars or cages. The material, when cured, has a higher tensile strength than steel, is only a quarter of the weight and is highly corrosion resistant when incorporated into concrete.

Pultrusions which are formed in a heated die have similar properties but because of their generally smooth outer surface are unsuitable for use as reinforcing bars. The process is also slow and not currently an economic proposition for reinforcing bars.

The resulting bars produced by the process of the invention have a naturally rough outer surface giving them relatively good grip in the concrete. This can be further improved by twisting the tows of glassfibre while they are being drawn through the forming eyes to provide a spiral with a slightly varying cross section to give a better grip and resistance to 'pull through' (which can result from smooth reinforcing bars) . It also increases the circumfer- ential strength.

A further improvement in these properties can be obtained by drawing the tows of resin impregnated continuous glassfibres through two or more forming eyes and then twisting these together to form a rod with a pronounced spirally wound outer profile. This can be performed as these rods leave the eyes or some time after they have left the eyes, in either the uncured or semi -cured state.

A secondary operation can also be used to improve the circumferential strength and help prevent separation of the tows and this is performed by winding secondary tows circumferentially while the primary tows are being drawn longitudinally. The drawing process can be undertaken vertically or horizontally.

While glassfibre is the preferred reinforcing material, any continuous fibrous material can be used such as carbonfibre, aramid, ceramic, thermoplastic, metallic or natural fibres.

While polyester resin is the preferred binding resin it could alternatively be any thermosetting or thermoplastic resin.

If thermoplastic resin is used then a secondary benefit is that the rods can be post- formed after manufacture by heating them to a predetermined temperature to soften the thermoplastic to facilitate bending and forming to any desired shape. This will allow rods or even coils or reels of the bar material to be preformed into cages before delivery or the bars can be shaped on site before or during installation of the reinforcement by using locally applied heat to form the bars into any desired shape.

The use of heat can also be of advantage during the pre- or post- tensioning of thermoplastic bars where this is desirable. By applying controlled heat during the tensioning process the bars can be 'pulled' and tensioned to the optimum level and then allowed to cool to retain this tension. This helps to overcome the problem of 'relaxation' of pre- and post- tensioned reinforcing rods and cables.

The invention also extends to a reinforcing bar formed by a process of the invention as hereinbefore defined.

The invention may be performed in various ways and a preferred embodiment will now be described, by way of example, with reference to the accompanying drawings, in which: - Figure 1 comprises side and sectional views respectively on one form of reinforcement bar of this invention; and

Figures 2 to 4 are similar views of three alternative versions of a reinforcement bar of the invention.

Figure 1 illustrates a bar formed from longitudinally drawn, resin-impregnated fibres 1. These have been drawn through forming eyes to remove excess resin, leaving an external surface which is non- smooth in appearance. After curing of the resin the bar can be used for reinforcing purposes and the roughened surface will tend to grip well within the material being reinforced. As shown a secondary tow 2 can optionally be wound around the longitudinal fibres to give improved circumferential strength and to help prevent separation of the fibres.

In the arrangement shown in Figure 2 the fibres 1 have been twisted as they are pulled through the forming eye, thus increasing the grip characteristics of the external surface of the bar. As shown in Figure 3 two tows 3 and 4 have been twisted together to create a pronounced spirally wound outer profile. As can be seen from Figure 4 several tows 5 can be twisted together for further enhancement of the grip characteristic. Thermoplastic materials can be applied to the glassfibre in various ways, including the following.

The glassfibre can be drawn through a bath of molten thermoplastic.

The fibres can be drawn through a die whilst extruding a controlled quantity of thermoplastic onto the fibres using controlled pressure.

The fibres can be drawn through a controlled 'cloud' of electrostatically charged thermoplastic particles and heated afterwards to fuse them into and onto the fibres.

The glassfibres and pre-drawn thermoplastic fibres can be "co-drawn" simultaneously whilst heating the material so that the thermoplastic fuses around the glassfibre.

Claims

1. A method of forming reinforcing bars comprising drawing bundles or 'tows' of continuous fibre strands and thermoplastic resin under controlled tension through forming eyes or the like to remove excess resin, leaving the resulting matrix with a high fibre content, the matrix subsequently being cured either by reaction in the resin or by heating.

2. A method according to Claim 1, wherein the tows of fibre are twisted while they are being drawn through the forming eyes to provide a spiral with a slightly varying cross section to give a good grip and resistance to 'pull through' .

3. A method according to Claim 1 or Claim 2 , wherein the tows of resin impregnated continuous fibres are drawn through two or more forming eyes and are then twisted together to form a rod with a pronounced spirally wound outer profile.

4. A method according to Claim 3 , wherein the twisting is performed as the rods are leaving the eyes or some time after they have left the eyes, in either the uncured or semi-cured state.

5. A method according to any one of Claims 1 to 4, wherein secondary tows are wound circumferentially about the primary tows whilst the primary tows are being drawn longitudinally.

6. A method according to any one of Claims 1 to 5, wherein the fibres are of glassfibre or of any continuous fibrous material, such as carbonfibre, aramid, ceramic, thermoplastic, metallic or natural fibres.

7. A method according to any one of Claims 1 to 6 , wherein the binding resin is polyester resin or any other thermosetting or thermoplastic resin.

8. A method according to any one of Claims 1 to 6 , wherein the resin is a thermoplastic resin the rods are post- formed after manufacture by heating them to a predetermined temperature to soften the thermoplastic resin to facilitate bending and forming to any desired shape.

9. A method according to any one of Claims 1 to 8, wherein controlled heat is applied during the tensioning process and the bars are 'pulled' and tensioned to a predetermined optimum level and then allowed to cool to retain this tension.

10. A reinforcing bar formed by a process of the invention as defined in any one of Claims 1 to 9.