Masonry/concrete anchors are widely used in the construction industry mostly for non-structural applications. These anchors can be broadly classified into expansion anchors, undercut anchors, bonded anchors, plastic inserts, powder actuated anchors and screw anchors.

To date the undercut anchor is the only type acceptable for structural application.

Almost all of the other type anchors have highly variable load carrying capacities, which are highly dependent on the particular installation. This variability does not allow them to be used as structural anchors.

A major limitation is that most types of anchors rely upon friction between the wall of a hole and the anchor within it. In order to increase this friction, the anchor is expanded in the hole. But the expansion forces induce significant tensile stresses in the concrete, which is undesirable.

Almost all the existing anchors are designed for standard strength concrete and masonry applications. As the use of high strength concrete is increasing, the need for an anchor installable after the concrete's setting (a post-cast installable anchor) especially designed for high strength concrete structural applications, is also increasing.

Most of the existing types of anchors induce substantial tensile stresses into concrete and this is undesirable because concrete has a significantly lower tensile strength than compressive strength. This is especially so if the concrete has cracked and, of the few anchor systems intended for installation into concrete after it is cast, only a very few can be installed into cracked concrete.

The following disadvantages and difficulties are identified in most of the existing product range of concrete anchors available up until now: (i) the strength of the connection is highly dependent on the care and skill used in the installation procedure (ii) even when installed with care, there can be a large variation in strength from fastener to fastener (iii) they generate significant tensile stresses on concrete (iv) they are unsuitable for cracked concrete (v) they are unsuitable for use in the bending or tensile zones of concrete (vi) pre-load is reduced due to concrete shrinkage and creep (vii) pre-load is reduced due to slip (viii) they need regular re-tightening (ix) they need relatively tight tolerances in hole size It is understood that at present the undercut anchor is the post-cast installation mechanical anchor mostly recognised as designed for structural applications because it does not depend on friction force and imparts no expansion force on the concrete.

An aim of the present invention is to provide a concrete anchor and a method of installing a concrete anchor which overcomes, or at least minimizes, these disadvantages and difficulties. A further aim of the invention is to provide a system for anchoring into concrete which is sufficiently reliable to be used in structural applications.

The parameter,"first slip load"of a concrete anchor is considered as equivalent to the yield load in a standard bolt. When the first slip occurs the concrete anchor loses its pretension partially or fully. The standard practice is to limit the permissible load of an expansion anchor to 0.65 of the first slip load. An increase in the first slip load is an important beneficial feature for a concrete anchor. An aim of the present invention is to provide this.

Accordingly, in one aspect the invention provides a thread-forming screw fastener for direct screwing into a pre-drilled hole in a concrete or masonry substrate, wherein:

(a) the fastener has: (i) at one end, a head for engagement with an installation tool for rotating the screw, (ii) at its other end a leading tip for insertion into the hole, and (iii) a shank comprising a core with a concrete-engaging or masonry-engaging helical thread ridge rising from the core and extending over at least part of the length of the shank; (b) on said threaded part of the shank, the thread ridge rises from the core over less than 30% of the core's surface; (c) commencing from the leading tip of the fastener, the outside diameter of the thread gradually increases over a first portion of the threaded part; (d) the outside diameter of the thread remains substantially constant over a second portion of the threaded part adjoining said first portion; and (e) over a third portion of the threaded part, adjacent the second portion and spaced from the first portion, the outside diameter of the thread is less than that of the second portion.

Preferably the thread ridge rises from the core over less than 15% of the core's surface. Preferably the pitch of the thread is from 0.5 to 1.5 times the diameter of the core. The increase in the outside diameter of the shank over said first portion of the threaded part may be accomplished by either an increase in the diameter of the core, or by an increase in the height of the thread ridge rising from the core or by both in combination.

In another aspect the invention provides a thread-forming screw fastener for direct screwing into a pre-drilled hole in concrete or masonry substrate, wherein the fastener has: (a) at one end, a head for engagement with an installation tool for rotating the screw, (b) at its other end a leading tip for insertion into the hole, (c) between the head and the tip, a shank comprising a core with a concrete-engaging or masonry-engaging helical thread ridge rising from the core over at least part of the length of the shank; and (d) on the leading tip, a twin-start thread wherein:

(i) one ridge of the twin-start thread gradually rises to the height of, and continues as, the thread ridge on the shank, and (ii) the height of the second ridge of the twin-start thread remains less than 50% of the height of the thread ridge on the shank Preferably the leading tip has a length of from 1 to 2 pitch lengths of the thread on the shank. Preferably the leading tip tapers from the core diameter of the shank to between 85% and 90% of the core diameter of the shank.

In a further aspect the invention provides a thread-forming fastener for direct screwing into a pre-drilled hole in concrete or masonry, said fastener having: (a) a shank comprising a core with a concrete-engaging or masonry-engaging helical thread ridge rising therefrom and extending over at least part of the length of the shank; (b) on said threaded part of the shank, the thread ridge covering less than 30% of the core's surface; and (c) over at least part of the threaded portion of the shank, grooves let into the core and spiraling along the shank at approximately right angles to the thread ridge to form a grooved portion of the shank.

Preferably the grooves pass through the thread ridge where the grooves intersect with the thread. Preferably the grooved portion of the core reduces in diameter at a rate which is substantially linear as it approaches the leading tip of the fastener.

In another aspect the invention provides a method of anchoring a fastener to concrete or masonry comprising: (i) selecting a fastener as defined above; (ii) drilling a hole in the concrete or masonry to a depth greater than the length of the shank of the fastener; and (iii) inserting the fastener into the hole and rotating the fastener so that said concrete engaging thread cuts a mating thread into the wall of the hole and is fastened thereby.

Examples of the invention will now be described with reference to the attached drawings where: Figure 1 is a side view of a first embodiment of a masonry screw according to the present invention; Figure 2 is a detail of the thread at the point marked A in Figure 1; Figure 3 is a side view of a second embodiment of a screw according to the invention; Figure 4 is a cross-section view through B-B shown on Figure 3; Figure 5 is a view of the screw portion where circled and labelled C in Figure 3; Figure 6 is a cross section detail view of the screw portion indicated in the direction and position of arrow D on Figure 3; Figure 7 is a partially cut away side view of a further embodiment of a masonry screw according to the present invention; Figure 8 is a view of an additional screw illustrating an alternative head shape; and Figure 9 is a cross section view of part of the screw shown in Figure 8.

Referring to Figure 1, the screw 2 has a hexagonal head 4 and a shank portion 6 extending to the leading tip 8. Immediately beneath the head is a shoulder portion 10 which acts in use as a bearing surface.

The shank 6 carries a single start helical thread 24 which rises from a core 22. The thread 24 varies in outside diameter as described later in this specification, but its pitch remains constant over its full length. The surface of the shank comprises mostly the surface of the core 22 and only about 12% of the core's surface is covered by the thread which rises from the core.

The shank 6 has six distinct zones along its length. Commencing from the leading tip 8 of the screw and working towards the head, these zones are respectively a tip zone 12, a lead-in zone 13, a main thread cutting zone 14, a mid tapering zone 15, a mid anchoring zone 16, and a plain shank zone 20. In order to describe the extent of these zones we will refer to"pitch lengths"or"pitches"as a unit of length. That unit is the pitch of the thread 24, which is equal to the axial distance between ridge tops for a single start thread.

The tip zone 12 extends lengthwise along the shank for about one pitch, the lead-in zone 13 for about one pitch, the main thread cutting zone 14 for about one pitch, the mid tapering zone 15 for about one pitch, and the mid anchoring zone for about 5.5 pitches. The length of the unthreaded plain shank zone 20 is selected to provide the required clearance of the shoulder 10 above the concrete surface and its selection depends on the height/thickness of whatever is to be fastened by the screw to the concrete.

Over the length of the mid anchoring zone 16, the core 22 is of uniform diameter and the thread 24 rising from it is of uniform height. The helix angle of the thread, indicated by the angle P in Figure 5 is 10.4°. This angle may be chosen from the range 8° to 26° and is more preferably in the range 9° to 12°.

Over the combined lengths of the tip zone 12 and lead-in zone 13, the core 22 of the shank is conically tapered at an angle shown as a on Fig. 1. The tapering provides a smaller diameter leading tip 8 for easier insertion of the tip into a hole in concrete.

The taper angle a used in this embodiment is 3° but this may be chosen from the range of 2° to 5°.

At the leading tip 8, the ridge 39 of thread 24 blends down to the core, again to facilitate easier insertion of the fastener tip and easy start of thread cutting. In the tip zone 12, starting from the tip end, the thread ridge 39 rises over about a quarter turn to a steady height of about 15% to 40% of the ridge height in the mid anchoring zone 16. In a slightly different embodiment, the thread ridge 39 gradually rises over a pitch length to a diameter substantially equal to that of the mid anchoring zone 16.

In the lead-in zone 13 the height of the ridge 39 increases gradually over a full turn until, at the head end of the lead-in zone 13, the thread 24 has reached its maximum diameter. That maximum diameter is maintained over the following turn (one pitch length) and this is the main thread cutting zone 14.

Over the mid tapering zone 15 (one pitch length) the thread 24 gradually reduces from its maximum outside diameter to the nominal diameter of the screw which is the diameter of the mid anchoring zone 16. As discussed earlier, in zone 16 the thread maintains its nominal outside diameter.

Extending over the tip zone 12 and continuing for about half the length of the lead-in zone 13 is an additional thread ridge 48. This is equally spaced between successive turns of ridge 39 so that, in effect, zone 12 and half of zone 13 carry a twin-start thread comprising ridge 39 and ridge 48. However ridge 48 remains relatively low and narrow, being only about 20% of the height of ridge 39 on the mid anchoring zone. In zone 12 both threads have similar thread heights. The twin-start thread, combined with the tapering of the zones 12 and 13 causes a greatly reduced tendency for the screw to tilt as it feeds into the hole and commences cutting the thread in the concrete. The small thread ridge 48 starts from close to the tip 8 and can conveniently extend for between one and two pitches, so that it may end by tapering to the core at any point along the lead-in zone 13.

The shape of the thread ridge 39 in the mid anchoring zone 16 is shown in Figure 2.

The ridge 39 rising from the cylindrical core 22 and has a head-side flank 40 (also called the leading flank) and a tip-side flank 42 (also called the trailing flank) on respective sides of a crest 44. The head-side flank is inclined at an angle y of 10° to a plane normal to the bolt axis and the tip side flank is inclined at an angle 8 of 45°.

These two angles may be varied a little (for example 0° to 15°, preferably 5° to 12° ; and 8=30° to 60°, preferably 40° to 50°) but it is important that the inclination of the head side flank is less than 15° in order to sufficiently minimize tensile stresses in the concrete.

The plain shank zone 20 carries no thread nor indentations and extends from where the end of the thread blends down to the core at 36 to the shoulder 10. The length chosen for this plain shank zone 20 corresponds to the thickness of the component to be held by the bolt to the concrete.

Along the full length of the thread 24, the core 22 is preferably circular but it may alternatively have a lobed cross section when viewed axially.

Along the main thread cutting zone 14 the height of the thread 24 is constant but is greater than the thread height in the mid anchoring zone 16. In the main thread cutting zone 14 the thread 24 continues to maintain a uniform height pattern above the core 22.

The term"uniform height pattern", when used above to describe the form of the thread in the tip zone 12 and the main thread cutting zone 14, is intended to encompass the situation where the thread maintains a uniform height above the core or the thread shows some cyclic variation in height above the core.

As the screw is driven into a hole in the concrete the low ridges on the tip zone 12 commence to locate the screw and feed it in at the desired rate of one thread pitch per rotation of the screw. What is effectively a two-start thread in this zone improves the stability of the location of the screw in the hole and the stability of the feed rate. As the lead-in zone 13 enters the hole, the ridge 39 which is increasing in height cuts more of the substrate material from the hole wall while the shorter ridge 48 diametrically opposite provides support against tilting or stripping without cutting further material from the walls.

When the main thread cutting zone 14 enters the hole, the ridge 39 has risen to its maximum height and cuts the full thread depth into the wall of the hole. The additional support of auxiliary ridge 48 is not required at this stage and ridge 48 tapers into the core 22 at the start of the main thread cutting zone 14. The thread is cut in the concrete by the ridge 39 where it is taller in the main thread cutting zone 14, whereas the slightly shorter ridge in the mid anchoring zone 16 would have a slight clearance from the cut thread. Nevertheless, the ridge in the mid anchoring zone 16 may perform a clearing operation as a follow up to the main cutting portion.

When the screw is tensioned, the load is transmitted to the concrete by the threads in the main thread cutting zone 14 and the mid anchoring zone 16. The thread ridges in

the tip zone 12 and lead-in zone 13 are of negligible use in supporting a tensile load applied to the screw.

The tip 8 is cupped, or concave, to a depth of about 30% of the screw diameter and this cup is useful for containing cuttings from the hole threading operation if the hole is not much deeper than the penetration required for the screw into the hole. The cupping is conveniently formed by metal displaced during a thread rolling operation forming the cup side wall.

The dimensions of the screw shown in Figures 1 and 2 are as follows: diameter of plain shank zone 20: 12.5 mm length from shoulder 10 to tip 8: 80 mm length of zones 12,13,14 and 16 combined: 67.5 mm pitch of thread 24 7.5 mm diameter of core 22 in zones 14 and 16: 11.7 mm OD of thread in anchoring zone 16: 14.0 mm OD of thread in cutting zone 14: 15.0 mm diameter of tip 8 : 10.9 mm length of tip zone 12 (ie. axial length of small auxiliary thread): 7.5 mm length of lead-in zone 13 (ie. where ridge 39 is same height as ridge 48) : 7.5 mm length of main cutting zone 14: 7.5 mm length of mid tapering zone 15 7.5 mm helical angle of thread 24: 10.4° conical profile angle at tip zone: 3° A preferred method of installing a screw of the present invention is to drill a hole in the concrete or masonry which is slightly greater in diameter than the diameter of the core 22. This significantly reduces the torque required for installation. A typical clearance would be 0. lmm all around so the hole would be 0.2mm greater in diameter than the diameter of the core 22.

The embodiment illustrated in Figure 3 is a screw 102 the same as that shown in Figure 1 except for the addition of helical grooves 126 let into the shank 106 of the fastener and notches 138 let into the thread 124. The grooves are let into the shank along the full length of the the main thread cutting zone 114. These grooves follow a left handed helix at an angle as shown on Figure 3, penetrating along their full length (but not very deeply) into the core 122 and passing fully through the right handed helical ridge of the thread 124 where they intersect it. Angle is preferably about 10° so the grooves 126 run substantially at right angles to the thread ridges 139 (not shown in Fig 3). Each groove 126 penetrates only a small distance into the core 122, typically less than 5% of the bolt diameter.

The preferred embodiment has six of these grooves evenly spaced around the circumference as best seen in Figure 4. The grooves 126 divide the thread 124 into segments 127 each extending for approximately 60° around the core 122. For each thread segment 127, the grooves provide a sharp edge to the end 128 closest to the leading tip 108 to facilitate better cutting of a thread into the concrete.

With the six thread segments 127 to each turn of the thread, there is a progressive cutting of the thread in small incremental steps. If less than four grooves were used, installation torque would be increased or the incremental steps would be much increased in size leading to a significantly lower thread cutting performance.

The grooves 126 also provide a space where cuttings, produced when threading the hole, can be accommodated and transported to the bottom of the hole in the concrete due to the action of gravity and vibration during installation. As the thread cutting proceeds in such small steps, the dust produced is fine and so is able to be transported along the grooves in the way described.

Each thread segment 127 tapers from a high point at its tip end 128 to a low point at its head end 130. This provides a small gap between most of the thread ridge in the main thread cutting zone 114 and the thread cut into the concrete and contributes to

reducing the friction torque acting against the rotation of the bolt during installation, so promoting efficient thread cutting.

In the mid anchoring zone 116 the thread ridge on the screw is of uniform height in order to maximize the load bearing area between the screw and the concrete, and there are no grooves let into the core.

Over the mid anchoring zone 116 the thread 124 carries a series of notches 138 on its crest. Some of these are shown in more detail in Figures 5 and 6. The notches 138 track helically around the shank at an angle 0 to the bolt axis as shown in Figure 5. In this embodiment the angle 0 is 20°, which is greater than the value of 10° for the corresponding angle + followed by the grooves 126. There are two such tracks of notches 138,180° apart on the shank.

As best shown in Figure 6 and on the right hand side of Figure 5, in the mid anchoring zone 116 each notch 138 penetrates to approximately 80% of the full depth of the thread ridge 139. The notches 138 serve to clean up the thread in the concrete as the bolt is rotated into position; they provide an additional space where minor debris may be accommodated so the debris does not hinder rotation of the bolt. This provides for a reduced installation torque.

A Bellville washer may be used, as a separate component, in conjunction with the screws described above. A key characteristic of a Bellville washer is that as its dished shape is compressed during installation, the reaction force it expresses first increases and then, after a certain further compression, decreases. Tightening a Bellville washer to this degree imparts a locking characteristic to the threaded fastening concerned as the forces involved mean that a greater turning force is required to uncompress the washer than to compress it further.

Use of a Bellville washer with a concrete screw as described above would provide a significant additional advantage. The negative gradient in the load displacement curve of the Bellville washer will increase the clamping force as the concrete creeps

and displace the fastener in the loosening direction. Accordingly the present invention envisages forming the head of a threaded fastener in such a way to incorporate into the head the above described displacement-load characteristic. In this way only a single component need be handled and, as incorrect washers cannot then be used by mistake, the correct load characteristics are ensured for any given anchor application. An example of a screw having such a head is shown in Figure 8 and a cross section detail of the head end of that screw is shown in Figure 9.

Extending from the underside of the screw head 304 is a flange extension 310. The flange extension 310 is dished towards the loading tip 308 in such a way that it flexes with similar displacement-load characteristics to a Bellville washer and therefore the screw 302 with its flexing flange extension has the same general performance characteristic of screw 2 with a separate Bellville washer in that it resists loosening of the screw. To provide the desired displacement-load characteristics, a circular ring- like channel 311 is formed in the underside of the head 304 deep within the dishing 309 and just inboard of the radially outer faces 305 of the head 304.

The screw 202 shown in Figure 7 is very similar to that shown in Figure 3. The difference is that the shank towards the tip end has a slightly different shape which may give improved performance in some situations. Whereas the screw in Figure 3 has a cylindrical core on the main thread cutting zone 114, and conical cores on the tip lead-in zones 112, the embodiment in Figure 7 has the core reducing over part of the main thread cutting zone 214 also.

The tip zone 212, lead-in zone 213 and main thread cutting zone 214 have a core with an ellipsoidally shaped longitudinal cross section. This means that the core of the entry and thread cutting zone 161, when viewed in cross-section as shown in Figure 7, has an elliptical form. The imaginary continuation of that ellipse is shown by dashed line 263. Therefore, over the entry and thread cutting zone 261, the diameter of the core gradually reduces at a rate which increases as it approaches the tip of the bolt. As an alternative, the cross-section may be made a close parabolic equivalent instead of elliptical without a substantial loss of performance.

An elliptical profile of this type provides a good reduction of the diameter of the screw leading tip 208 to provide ease of entry of the screw into the hole and a more even rate of thread cutting over the length of the tip zone 212, lead-in zone 213 and thread cutting zone 214 when compared with the configuration shown in Figure 1.

As the anchor screws into place, cutting its thread as it goes, initially the depth of thread increases relatively quickly with each 1/6 turn but then the increase in thread depth becomes less with each 1/6 turn. However as the thread increases in depth, a smaller increase in thread depth is required for the same volume of material to be removed because the thread is increasing in width as well as depth. The elliptical profile allows the rate of material removal to be constant, or at least more evenly spread, over the length of the tip, lead-in and thread cutting zones 212,213 and 214 respectively.

The above described anchors may be screwed into normal strength or high strength concrete or fibre reinforced concrete or suitable masonry materials.

The screws according to the above described embodiments of the present invention are preferably made from a steel which provides both a high degree of cold formability (to allow for the thread to be formed in a single rolling operation) and a high surface hardness after heat treatment.

The screws are preferably manufactured from headed blanks in a single thread rolling operation followed by a case-hardening operation. Such a thread rolling operation would preferably form all the grooves 126 and notches 138 in addition to the ridges.

Alternatively, but less preferably, sequential or multiple thread rolling operations may be used.

Throughout this specification, unless the context requires otherwise, the word "comprise", and variations such as"comprises"and"comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention. For example the hexagon head of the preferred embodiment described with reference to Figure 1 could be replaced by a conventionally threaded portion carrying a hexagonal socket on its end so the anchor would form a stud when screwed into concrete.