WO1990008877A1

WO1990008877A1 - Anchor bolt of oriented polymer

Info

Publication number: WO1990008877A1
Application number: PCT/AU1990/000037
Authority: WO
Inventors: Jan Havranek
Original assignee: The Broken Hill Proprietary Company Limited
Priority date: 1989-02-06
Filing date: 1990-02-05
Publication date: 1990-08-09
Also published as: ZA90869B

Abstract

An anchor bolt (21) of oriented polymer having an outer layer (26) of isotropic polymer in which a multiplicity of radial ribs or a helical thread (22) is formed, with the ribs/thread (22) having an angle of inclination with respect to the longitudinal axis of the bolt (21) of less than about 10°.

Description

ANCHOR BOLT OF ORIENTED POLYMER Field of the Invention

This invention relates to articles made from oriented polymers, such as anchor bolts used i mining and soil stabilisation . Background of the Invention

In our International Patent Application PCT/AU88/00248 , the contents of which are incorporated herein b cross- reference, we have described an oriented polymer rod having helical grooves formed therein for the purpose of keying the rod to the grouting material when the rod is used as an anchor bolt. Farther testing of rods made in accordance with the above patent application has shown that they perform relatively poorly in pull-out tests of the type which are conducted to test the suitability of anchor bolts, due primarily to the tendency of the rod to rotate under load, causing the rod to withdraw from the bonding material.

Using the method of formation described in the above patent application, grooves having a helix angle no steeper than about 60 - 70° are able to be formed in the surface of the rod, and this leads to the rod being relatively free to rotate in the grouting material when placed under load due to the absence of any positive bonding between the surface of the rod and the grouting material. Summary of the Invention and Object

While the previous research which resulted in the invention described in the above patent application was based on the premise that melting of the oriented polymer material was to be avoided to avoid the consequential weakening of the resultant anchor bolt or the like which was thought to occur, it has now been surprisingly discovered that melting of the surface material of an oriented polymer rod or other article may be achieved in a controlled manner without materially reducing the strength of the resultant anchor bolt or the like, and as a result, the rod may be ribbed or threaded at a much steeper angle than previously possible whereby the tendency of the rod to rotate in the grouting material under load may be avoided. The invention therefore provides an elongate article of oriented polymer having an outer isotropic layer having a multiplicity of generally radial ribs or a helical thread formed in said isotropic layer said ribs/thread having an angle of inclination with respect to the longitudinal axis of said article substantially falling in the range 0° to 45°, and preferably less than about 10°.

The profile of each rib/thread may take any suitable form, but is most preferably comprised of flat surfaces meeting at relatively sharp corners. In one presently preferred form, the ribbed profile is generally trapezoidal.

The ribs/thread may be melt formed in the surface of the oriented polymer article, which at the same time forms said isotropic layer, or the ribs/thread may be formed in a previously formed isotropic layer by means of a thread die or in lathe-type operation.

The invention also provides a method of forming an elongate article of oriented polymer with a multiplicity of radial ribs or a helical thread, comprising the steps of heating forming dies to a temperature exceeding the melting point of the polymer material, inserting the elongate article into the die and engaging the surface of the die with the surface of the elongate article for a period of time which is just sufficient to melt form said ribs or thread in the surface while not disturbing to a significant extent the orientation of the polymer material in the core of the elongate article.

The forming dies are preferably heated to a temperature between about 140°C and 250°C and most preferably around 160°C, and the dies engage the surface of the elongate article in a manner which melts the polymer material while not causing any crushing of that material. Several different methods of melt forming the ribs/thread will be described in greater detail below.

Alternatively, the invention provides a method of forming an elongate article of oriented polymer with a multiplicity of radial ribs or a helical thread, comprising the steps of heating a surface layer of said elongate article to a temperature exceeding the melting point of the polymer to form an isotropic layer, and cutting said ribs or said thread in said isotropic layer.

The invention also provides an elongate article of oriented polymer having an outer layer of isotropic polymer in which ribs or a thread may be formed.

While ribbed or threaded rods produced in the above way do not have the advantage of retaining the oriented fibrils in the ribs or threads, it has been surprisingly found that the reduction in the overall modulus of the rod due to the formation of the ribs/thread does not unacceptably reduce the pull-out shear stress at the bolt/grout interface of the rod, when used as an anchor bolt. In fact, the shear stress of a bolt formed in accordance with the invention is over twice that of the shear stress of a bolt formed by the method according to the abovementioned patent application, and exhibits an improvement of over ninety percent in the shear stress performance of a rod having a machined thread formed in its surface. These results may be further improved by optimising the helix angle and the profile of he ribs/thread and by gamma irradiating the ribbed/threaded rod in a (reducing) gas atmosphere. Brief Description of the Drawings

A presently preferred embodiment of the invention will now be described with reference to the accompanying drawings in which:

Figure 1 is a partly sectional elevation of a threaded rod formed in accordance with the invention, showing a nut attached to one end;

Figure 2 is a schematic perspective view of a heated die set for forming the thread shown in Figure 1 ;

Figure 3 is a sectional end view of the die set of Figure 2 showing a formed rod being released therefrom, and

Figures 4 and 5 are schematic end elevations of alternative die sets for forming ribs in accordance with the invention . Description of the Preferred Embodiments

Oriented polyethylene rods for use in the present invention are made by methods already known in the art. In one such method, a rod of isotropic polyethylene is drawn through a reducing die at a temperature of about 100°C, at a drawing speed of about 5 to 10 cm/min to produce a rod of about 20 mm in diameter from a 73mm isotropic rod.

The threaded rod shown in Figure 1 may be made using the heated die set shown in Figures 2 and 3 of the drawings, said die set comprising a top die 1, a bottom die 2 each carrying a pair of thermostatically controlled heating elements 3 and formed with thread forming grooves 4 and flash cutting edges 5.

Since the amount of flash formed using an oriented rod of circular section may be too great, the rod may be formed by the process shown in Fig. 1 but using a die which forms the rod with opposed flats. The width of each flat will depend on the diameter of the rod and the size and type of thread/ribs which are to be formed in the surface of the rod, and may be easily determined by a person skilled in the art.

The dies 1 and 2 are heated to a temperature between about 140°C and 250°C, the currently preferred temperature being of the order of 160°C. An oriented polyethylene rod (21) formed in accordance with the process described above, is placed between the dies 1 and 2 and the dies are closed gently so that the rod is not crushed by the dies. As soon as the dies 1 and 2 bottom when the flash cutting edges meet, the dies are immediately opened to minimise thermally induced structural changes in the oriented core 25 of the rod. Current experiments indicate that a die closing time of the order of 30 seconds produces satisfactory results.

The dies 1 and 2 may be formed with any suitable rib or thread profile, such as trapezoidal, square, rounded, or any other desired form, and the angle of inclination of each rib or thread element may be selected between 0° and 45°, and preferably less than 10°, say between 8° and 9°. The profile and angle of inclination will be selected according to the type of grouting material used, such as concrete or polyester. A threaded rod 21 formed using the dies sho n in Figures 2 and 3 is shown in Figure 1 with a helical thread 22 of isotropic polyethylene 26, formed by the heating process to a depth not substantially greater than the depth of the thread 22, on the the oriented polymer core 25. One of the advantages of forming a thread 22, rather than ribs, is that a nut such as 23 formed with a matching thread 24 may be attached to the rod for securement purposes.

The r i b b ed / thr ea d e d rod is subsequently gamma irradiated in a reducing gas such as acetylene in order to achieve the best mechanical properties. A radiation dose in the range 100 - 180 kGy should be sufficient to provide the required cross-linking without deterioration of the product, and for the ribbed rod described above, a dose of the order of 150 kGy has been, found to increase mechanical properties of the rod to the required extent.

Another example of the invention will now be further described. In this example, a rod of oriented polyethylene having a nominal diameter of 21.5 mm was formed with a series of parallel ribs using a die set similar to that shown in Figures 2 and 3of the drawings. Each rib was trapezoidal in shape having a base width of 4 mm, a top width of 1.5 mm and a depth of 2mm. The ribs had a pitch spacing of 10 mm and had an angle of inclination of 0°. The dies were heated to a temperature of 160°C and a die closing period of approximately 30 seconds was used, The overall modulus reduction due to the formation of the ribs was of the order of 10 percent, measured prior to gamma- irradiation. The melt forming of the ribs resulted in an isotropic rib followed by a transition region of the order of 0.25 to 0.5 mm in depth, with the core of the rod remaining oriented. The rod is subsequently gamma irradiated as described above.

Rods manufactured in accordance with the above example have been subjected to comparative pull-out testing in the following manner:

The efficiency of the mechanical interlock was evaluated by: i) Setti g the rod to be tested in a surrounding matrix eg: concrete/polyester grout; ii) Pulling the rod out of the matrix; iii) Evaluating the shear stress, where shear stress equals: Shear Stress = Peak Load to Pull Out Rod

Surface Area* of Embeded Rod

* surface area = surface area of a cylinder of same diameter as the rod at the roots of the ribs. COMPARISON OF MECHANICAL INTERLOCKS

Embedment length 150 mm in polyester grout. EXAMPLE 1

Trapezoidal melt ribbed rod as in above example.

Pull-out shear stress 6.7 MPa. EXAMPLE 2

Machined threaded rod (Standard metric thread - helix angle about 8°).

Pull-out shear stress 3.4 MPa. EXAMPLE 3

Ribbed and slotted rod

60°/18.7 mm root diameter

21.7 mm outside diameter

Produced as per above patent application.

Extracted pull-out shear stress 2.7 MPa (Extracted from shear stress for a non-irradiated rod).

The above results were quite surprising since it was initially felt that the melt forming of ribs or threads would significantly weaken the polyethylene material forming the ribs or threads, thereby resulting in unacceptable pull- out shear stresses. However, it will be noted that the pull- out shear stress of the rod produced according to the example was more than double the pull-out shear stress of the example according to International Patent Application PCT/AU88/00248 and provided an improvement of more than 90% over a rod having a machined thread formed in its surface. A further surprising discovery was that the rib/thread forming method according to the present invention results in a reduction in the modulus of the rod of the order 10 percent (prior to irradiation), whereas the method according to the patent application referred to above results in a reduction in the modulus of the rod exceeding 50% (also prior to irradiation) .

The ability to form a thread having a steep helix angle which improves the interlocking between the rod and the grouting material enables the use of a nut to attach a plate against a mine tunnel wall or the like. This represents a distinct advantage over the most commonly used fibre glass mining rib bolts in hich wooden wedges are used somewhat ineffectively to retain the plate. While fibreglass bolts are available with special threaded attachments, such products are very expensive and are therefore impractical for most projects.

Referring now to Figures 3 and 4, two alternative methods of melt-forming libs in an oriented polymer rod are shown schematically. In the case of Figure 3, a pair of driven rotary dies 6 and 7 are used, while in the case of Figure 4 of the drawings, three rotatably mounted dies 8, 9 and 10 are arranged as shown and the rod pulled through the dies to cause rotation thereof. In each case the dies may be formed with rib or thread forming grooves of any desired profile and pitch.

A further alternative method of forming a rod embodying the present invention comprises the steps of heating an oriented polyethylene rod to a temperature of about 160°C to 170°C for a period of about 6 minutes to form an isotropic polyethylene layer about 2 mm deep in an oriented rod about 20 mm in diameter. The rod may be heated in an oven or in a heated die similar to that shown in Figures 2 and 3 of the drawings but without the thread forming grooves. Following formation of the isotropic layer, a thread similar to the thread 22 may be formed in the isotropic layer using a thread forming die of the type shown in our international patent application No. PCT/AU88/00248 , or by means of a lathe-type operation well known in the art. Since the thread may be formed at any time following the formation of the isotropic layer, the invention also resides in the heat treated rod into which a thread may be subsequently formed. The depth of the isotropic layer will be dictated by the required depth of the thread or ribs to be formed in the rod, the pitch, depth and angle of which needs to be optimised for each particular type of grouting material. For example, the thread shown in Figure 1 of the drawings is optimum for a polyester grouting whereas a concrete or cement grouting may require a thread or ribs having different characteristics. The threaded or ribbed rod formed in the above way is preferably gamma irradiated in the manner described above.

Claims

CLAIMS:

1. An elongate article of oriented polymer, characterized by an outer layer of isotropic polymer in which ribs or a thread may be formed.

2. An elongate article of oriented polymer having an outer isotropic layer having a multiplicity of generally radial ribs or a helical thread formed in said isotropic layer said ribs/thread having an angle of inclination with respect to the longitudinal axis of said article substantially falling in the range 0° to 45°.

3. The article of claim 2, wherein said angle of inclination is less than about 10°.

4. A method of forming an elongate article of oriented polymer with a multiplicity of radial ribs or a helical thread, comprising the steps of heating forming dies to a temperature exceeding the melting point of the polymer material, inserting the elongate article into the die and engaging the surface of the die with the surface of the elongate article for a period of time which is just sufficient to melt form said ribs or thread in the surface while not disturbing to a significant extent the orientation of the polymer material in the core of the elongate article.

5. The method of claim 4, wherein said dies are heated to^" a temperature subs antially falling in the range 140°C to 250°C.

6. The method of claim 5, wherein said temperature is about 160°C to 170°C.

7. A method of formin an elongate article of oriented polymer with multiplicity of radial ribs or a helical thread, comprising the steps of heating the elongate article to a temperature exceeding the melting point of the polymer material to form an isotropic layer on said article, and forming said ribs or said thread in said isotropic layer.

8. The method of claim 7, wherein said ribs or thread have an angle of inclination with respect to the longitudinal axis of said article substantially falling in the range 0° to 45°.

9. The method of claim 8, wherein said angle of inclination is less than about 10° .

10. The article of claim 1, 2 or 3, when exposed to gamma radiation falling substantially in the range 100 to 180 K Gy

11. The method of any one of claims 4 to 9 comprising the further step of exposing the article to gamma radiation falling substantially in the range 100 to 180 K Gy.