FIELD OF THE INVENTION This invention relates to a composite structure which includes a concrete slab or pavement which has a plurality of planned cracks formed therein during the manufacturing process which allow for concrete movements caused by temperature change and drying shrinkage. The composite structure of the invention has been found to reduce undesired cracking of the concrete slab due to curling of the joint edges of the slab and also to reduce linear shrinkage stresses which allows much greater loads to be applied to the concrete slab when compared to conventional slabs. BACKGROUND OF THE INVENTION

As mentioned in Walker and Holland Concrete International (1999) pages 47-53, curling occurs when the upper part of a floor tries to occupy a smaller volume than the lower part and this can happen with differences between the upper and lower parts with respect to shrinkage, temperature, moisture content and other variables. In other words curling is the distortion of a slab into a curved shape by upward or downward bending of the edges of the slab.

Significant curling is undesirable for a number of reasons. Substantial tensile stresses occur in the top of the slab from the edges curling up as gravity and any loads or vertical restraints try to pull the edges downwardly. This effect, in addition to linear shrinkage, can produce cracking. The great majority of cracks that are attributed to shrinkage are actually due to a combination of curling and linear shrinkage stresses, typically with the curling stresses far greater than the linear shrinkage stresses. Cracks can spall due to wheeled traffic and allowance of liquid penetration, and are also unsightly. Other problems are failure of joint filler or sealant and distress in floor materials covering a floor slab. It has also been found that excessive bleeding due to high water content in the concrete or water sprayed on the top surface, or a lack of surface moisture due to poor or inadequate curing can cause increased surface drying shrinkage relative to the bottom of the slab which can also cause curling.

It has also been found that the tensile stresses iri a top surface of a pavement which are caused by curling increase with panel size (i.e. the distance between joints) to a point around 6 - 7 metres when such tensile stresses plateaued. This has been described in the Walker and Holland reference referred to above. Therefore this reference has proposed that joint spacings of 4.6m should not be exceeded for unreinforced or lightly reinforced slabs.

Another problem with conventional concrete slabs is that it is often the case that overall design of such slabs has not taken into account both axial or longitudinal tensile stresses due to shrinkage as well as tensile stresses due to curling. It will be appreciated by the skilled addressee that both types of tensile stresses have to be considered in slab manufacture and design.

It is also commonplace in production of concrete slabs that spaced metal rods or dowels are placed across a planned crack in a concrete joint. For example in construction joints between panels it is proposed in

www.nrmca.orq/aboutconcrete/cips/06p.pdf that spacing between joints is usually between 3 - 4.5m but this reference refers to a slab using metal dowels in

combination with reinforcement mesh that is embedded in a top part of the slab above the construction joint to prevent curling of the panels.

It has also been previously proposed by Zollinger and Barenburg in an article entitled "Proposed Mechanistic Based Design Procedure for Jointed Concrete Pavements" in May 1989 by the University of Illinois on pages 6-17 that one or two hinge joints could be placed between contraction joints which were formed by dowels. This construction was proposed to eliminate or reduce an intermediate crack appearing between contraction joints if the joint spacing was greater, than 20-30 feet or between 6.3 and 10 metres. The hinge joints were proposed to be formed by using deformed bars and sawing the slab to about ½ - Α the slab thickness and making sure that the reinforcing steel was not cut in the process. The hinge joints were considered to relieve the curling stress and the concomitant cracking of the slabs. A similar proposal was made by an article published by the Illinois Department of Transport (IDOT) published at www.dot.il.gov/materials/research/pdf/ptad which stated that hinge joints using deformed bars were placed at intermediate locations between dowelled joints to control slab warping or curling. However it was also mentioned that IDOT no longer constructs hinge jointed pavements because of performance problems and patches could be used at failed existing hinge joints.

In US Patent 3437017 there is described use of a reinforcing steel mat comprised of spaced longitudinal rods extending in the lengthwise direction of the slab and spaced transverse rods intersecting and being affixed to the longitudinal rods. The spaced longitudinal rods extend transversely to joints located in the slab which include a sawcut located in a top surface of the slab. However this reference is directed to a different problem than the problem which is the subject of this present invention. In this reference there is provided a means to provide slidable engagement between intermediate longitudinal rods to thereby function as anchorage dowels for concrete bodies on either side of a sawcut or transverse groove located in the slab. This therefore means that the spacing between each longitudinal rod and each transverse rod within each joint region was not uniform and thus each reinforcing mat or sheet had to be specially manufactured which would raise production costs. DE 9210992 relates to a reinforcing structure in concrete which has a central longitudinal rod and a pair of end U shaped support stands and a central U shaped support stand. There is provided a plurality of transverse rods between adjoining support stands. The entire structure is embedded in a concrete slab which does not have planned cracks and thus this reference does not appear to be relevant in regard to solving the problem which is the subject of the present invention.

DE 4328831 refers to a joint structure in the form of an elongate frame which has a pair of spaced longitudinal rods at each side and a plurality of transverse rods which are welded to each pair of longitudinal rods wherein each transverse rod has a pair of L shaped ends. The elongate frame is located in a concrete slab directly underneath a sawcut with each longitudinal rod extending parallel to the longitudinal axis of the sawcut. The use of the elongate frame is intended to obtain a precise height location within the concrete slab and thus eliminates the need for bar chairs or other separate spacers which are required for use of dowels or reinforcement mesh. This reference is not relevant to the problem solved by the present invention.

OBJECT OF THE INVENTION

The problem which is relevant to the present invention and hot solved by the prior art discussed above is to substantially reduce tensile stresses in concrete slabs or pavements which are caused by shrinking whereby the concrete shortens in width and curls at the edges. The invention solves this problem by controlling both the location and width of cracks to joints and thus the residual tensile stresses are minimized to maximize the load carrying capacity of the slab or pavement.

SUMMARY OF THE INVENTION The invention provides a composite structure which (i) is a concrete slab having a plurality of joints each having a crack extending from a top surface of the slab to a bottom surface thereof wherein each crack defines a pair of slab sections on each side of said crack which have curled up joint edges wherein each joint edge curls uniformly, and (ii) a combination of (a) a plurality of dowels for facilitating lateral movement of each slab section which each extend across an associated crack and (b) a single joint tying device for inhibiting lateral movement of each slab section wherein the joint tying device has a multiplicity of transverse members extending

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across an associated crack characterised in that there is no reinforcement in the concrete slab above (a) and (b).

Preferably in the top surface of the slab at each joint there is provided a control groove or sawcut or alternatively a crack inducing insert. The bottom surface of the slab may also be provided with a crack inducing insert at each joint so that the crack extends between the control groove or sawcut or crack inducing insert at the top surface to the crack inducing insert at the bottom surface.

It is also desirable that each crack is separated by a spacing of 2.0 - 4.5 metres and more preferably 3.0 metres.

The plurality of dowels may be smooth metal bars which may be coated with a bituminous or paraffin based bond breaker to prevent concrete bonding to each dowel. The dowels may be bars with a round or circular cross section but dowels with a square cross section or flat bar dowels and plate dowels may also be used. To prevent misalignment of the dowels and ensure that they are accurately aligned with each other and installed in a single plane they are often supported by pre-fabricated cages or a plurality of bar chairs. . Usually dowels are from 400 - 600mm long and are around 16 - 50mm in diameter. Often a sawcut or control groove will be 3 - 6mm wide having a depth of ½ to ¼ of slab depth.

The joint tying device may be in the form of a continuous or elongate frame having at least one pair of longitudinal rods each parallel to a respective control groove or sawcut and which are joined by the multiplicity of transverse members referred to above which each interconnect the at least one pair of longitudinal rods. Such transverse members are suitably formed as channels such as being U shaped, M shaped, Y shaped or even W shaped that are each welded or otherwise attached to each longitudinal rod. Alternatively each transverse member is co-planar with the at least one pair of longitudinal rods and even more preferably a mesh sheet may be used with longitudinal rods of the mesh sheet being oriented parallel to an adjacent control groove. In the joint tying device described above it will be appreciated that each longitudinal rod provides the structure to resist lateral movement of each of the transverse members. The longitudinal rods also provide the opportunity to, with the use of pegs, accurately hold the joint tying devices in position during concrete placement. The composite structure also may have a plurality of crack promoting inserts which are preferably in the form of a grid or frame of crack promoting inserts which are described in detail in US Patent 7308892 which is reproduced in its entirety in this specification. Each crack promoting insert is suitably T shaped and the grid or frame is laid on the subgrade to promote cracking in a line between each sawcut and each crack promoting insert as described above which then causes or facilitates curling of the edges of the concrete slab.

Each of the joints may be applied transversely across the slab as well as

longitudinally within the slab to form a grid like pattern. Each joint or joint region may include a control groove or sawcut, crack and combination of dowels, and one or more joint tying devices as well as optionally a crack promoting insert. The dowels (D) and joint tying devices (JD) may be applied alternatively or in any other suitable arrangement, i.e:

(i) D-JD-JD-D-JD-JD-D; or

(ii) D-JD-D-JD-D-JD-D-JD; or

(iii) JD-D-D-JD-D-D-JD

The joint region in the concrete slab which includes the sawcut and the crack promoting insert may be cracked by mechanical means such as a crack inducing tool described in US Patent 7308892. Alternatively the cracks may be induced naturally.

It is also preferred that each dowel and joint tying insert has a height that is located at or below a mid-depth region of the concrete slab.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

Reference may now be made to a preferred embodiment of the invention as shown in the accompanying drawings wherein:

FIG 1 is a cross sectional view of a composite structure of the invention in relation to a joint region of the composite structure;

FIG 2 is a perspective view of a joint tying device for use with the composite structure; FIG 3 is a perspective view of another form of joint tying device for use in the composite structure of FIG 1;

FIG 4 is a perspective view of the composite structure of the invention showing use of dowels in combination with joint tying devices;

FIG 5 is a side view of the composite structure of the invention showing curling of the edges of each slab section;

FIG 6 is a side view of the composite structure of the invention;

FIG 7 is a side view of the composite structure of the invention showing spacing between joints; and

FIGS 7A, 7B and 7C are separate detailed views of parts of FIG 7 showing curling of inner edges of adjacent slab sections as well as curling at the outer edges.

In relation to FIGS 1-2 there is shown a composite structure 0 of the invention which has a concrete slab 11 which includes a joint region 12 which has a sawcut or control groove 13, a crack promoting insert 14, a crack 15 extending between sawcut 13 and insert 14, a joint tying device 16 which has a plurality of transversely oriented channel shaped members 17 which has a horizontal or transverse member part 8 extending across crack 15 as well as joint region 12, and a vertical part 19 which extends downwardly from a location 20 which is less than the mid-depth of concrete slab 11 to a base surface 21 of slab 11 and which is also provided with a bottom surface 22 which coincides with support surface 21. The joint tying device 16 also includes a pair of longitudinal members 23 which are oriented parallel to each sawcut 13. Each longitudinal member 23 is welded to vertical parts 19 so that joint tying device 16 is self-supporting prior to a pour of concrete 24. There are also provided a pair of slab portions 25 and 26 on each side of crack 15. The slab 11 is also provided with a top surface 9.

It will be noted that the two longitudinal members 23 are located at the same level coinciding with base surface 21 and bottom surface 22 terminates at support surface 21 which ensures that each joint tying device 16 is self-supporting and thus each joint tying device 16 is firmly supported on support surface 27 without the need for spacers which are shown in DE 4328831. The pair of longitudinal members 23 also serve to better anchor the joint tying device 16 in the slab 11 to help the channel shaped members 17 reach their full tensile capacity in preventing substantial lateral separation of slab portions 25 and 26. The pair of longitudinal members 23 also allow the channel shaped members 17 to be evenly spaced with regard to longitudinal members 23 without measuring on site.

It therefore will be appreciated that joint tying device 16 may take the form of a cage which can be manufactured to a desired length to facilitate quick arid efficient positioning on and attachment to support surface or subgrade 27 during the pour of concrete 24. Each longitudinal member 23 can be pegged to support surface 27 or otherwise fixed thereto to maintain the cage in a desired position and in particular to prevent movement during the concrete pour.

The joint tying device 16 may be formed from any suitable metal such as high grade tensile steel. In the embodiment of FIGS 1-2 the concrete slab 1 may be 175mm thick, the vertical parts 18 are 10mm in diameter and 300mm in length, and the vertical parts 19 are 10mm in diameter and 80mm long and connect to vertical parts 18 by an arcuate part 28 which has a 32mm diameter bend. Each longitudinal member 23 may be 6mm in diameter and extend 480mm between adjacent channel shaped members 7. These dimensions can be varied to suit the particular design of slab 11 having regard to thickness and concrete composition and crack line 15 spacing.

In one embodiment each channel shaped member 17 may be long enough to provide sufficient anchorage in concrete 24 but short enough and of sufficient diameter or ^ width that it does not experience significant deflection when stood on by a concreter during the concrete pour to form slab 1 .

In relation to the FIG 3 embodiment a variant of joint tying device 16A is shown.

Frame 16A has longitudinal members 23A and channel shaped members 17A welded to each longitudinal member 23A at 29. Each of channel shaped members 17A may be bent or otherwise formed to provide a transverse part 30 and a pair of angled parts 31 and 32. The junctions 33 between vertical members 19A and an adjacent angled part 31 or 32 may be of constant radius as in the FIG 2 embodiment. Thus it will be noted that each channel member 17A is approximately M shaped. Each longitudinal member 23A also serves to better anchor joint tying device 16A in the slab 11 to assist the channel shaped members 17A reach their full tensile capacity in preventing substantial lateral separation of slab portions 25 and 26.

Thus from FIG 3 it will be noted that vertical members 19A and a portion of angled parts 31 and 32 may project above the centerline of slab thickness and at least the transverse part 30 may be maintained at a location at or below the centreline of the slab thickness. Such an arrangement facilitates curling of slab 11. The location of horizontal part 18 of channel shaped members 17 shown in FIGS 1-2 being positioned closer to base surface 21 make the anchoring of frame 16 in slab 11 less effective because each vertical part 19 is shortened. However the embodiment of FIG 3 overcomes this problem by maintaining the position of transverse part 30 at a location at or below the centerline of the slab thickness without the need for shortening of vehicle members 19A.

Referring specifically to FIG 4 the tie joining device 16C takes the form of a supported mesh and has a plurality of laterally extending mesh members 45, each laterally extending mesh member 45 comprising two end portions 46 and an intermediate portion 47 between the end portions 45, and wherein the plurality of intermediate portions 47 extend across crack line 5.

The tie joining device 16C also has four longitudinally extending mesh members 48 comprising longitudinal mesh members 49 which extend through slab section 25 and two second slab section longitudinal mesh members 50 which extend through slab section 26. The laterally extending mesh members 45 are oriented orthogonally to the four longitudinally extending mesh members 48 and are welded to them. The longitudinal mesh members 48 serve to better anchor the laterally extending mesh members 45 in the concrete slab 11. The spacing between adjoining longitudinal members 48 and between adjoining transverse members 45 is uniform.

The intermediate portion 47 of each laterally extending mesh member 45 is that part of the laterally extending mesh member 45 that extends across crack 15 and extends into adjacent slab sections 25 and 26. This means that tie joining device 16C is very well anchored in slab 11 on either side of crack 15.

The tie joining device 16C also comprises a plurality of stands or bar chairs 39 for supporting tie joining device 6C on the subsurface 27. Each stand 39 has opposed recesses 41 to seat one of the four longitudinally extending mesh members 48 thereon. In use the stands 39 are positioned in a stable, spaced apart manner around the outer most of the four longitudinally extending mesh members 48. The stands 39 can take any other suitable form in other embodiments such that the mesh is maintained above the bottom surface 21 of the concrete slab 1 and, preferably, the intermediate portions 47 are located at or below a mid-depth region of the concrete slab 11. Alternatively a prefabricated cage may be used to support mesh sheet 16C. In this embodiment concrete slab 1 1 is 175mm thick, the grid spacing of the mesh is 200mm, and the height of the standi 39 is 80mm. If the mesh is well supported by a close array of stands 39 a relatively light grade mesh can be used. The dimensions of the grid and grade of the mesh can be varied to suit the particular concrete slab 11 design (e.g. thickness, concrete composition) and crack 15 spacing.

The tie joining device 16C may be made of any suitable engineering material such as high tensile strength steel.

In other embodiments the tie joining device may have more or less laterally and/or longitudinally extending mesh members.

In another embodiment the laterally extending mesh members 45 are made from larger or small bar than the longitudinally extending mesh members 48. Referring specifically to FIG 5 a concrete slab 1 is provided having a plurality of joint regions 12 to be cracked to form a plurality of crack lines 15 not shown in FIG 7. The plurality of joint regions 12 comprises four laterally extending joint regions 12A and four longitudinally extending joint regions 12B. The four lateral joint regions 12A extend orthogonally to the four longitudinal joint regions 12B.

5 The four longitudinal joint regions 12A are in spaced array and comprise two outer dowel joint regions 51 and two inner tie joint regions 52.

The four lateral joint regions 12B are in spaced array and comprise two outer dowel joint regions 53 and two inner tie joint regions 54.

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Each of the plurality of joint regions 12A and 12B comprises a crack promoting insert 4 and groove or sawcut 13 shown in FIGS 1 and 5 and omitted from FIG 5 for the sake of clarity. In other embodiments the crack promoting groove 3 may be replaced by another crack promoting insert 14. In yet other embodiments the 15 concrete slab 11. does not include any crack promoting inserts 4.

The two outer dowel joint regions 51 and the two outer dowel joint regions 53 each comprise a series of dowel joining devices placed across the respective joint region 12B. Each of the dowel joining devices comprises a substantially rigid dowel shown 0 in FIGS 6 and 7 thereby allowing controlled lateral separation of the slab sections 25 and 26 on either side of the crack line 15 also shown in FIGS T and 5 due to expansion or contraction of the concrete slab 1 .

The two inner tie joint regions 52 and the two inner tie joint regions 54 each comprise5 a single tie joining device 16 or 16A placed across the respective joint region 12A, the transverse portion 17 of the tie joining device 16 bridging the respective crack line 15.

Advantageously the concrete slab 11 , once cracked, has less residual tensile stress at an upper surface 9 reducing undesirable cracks and/or weakness in the slab since0 the portions of the concrete slab on either side of the plurality of joint regions 12 are allowed to curl and the cracks along the inner tie joint regions 52 and 54 are held • together by the tie joining devices 16 and 16A to reduce undesirable separation between slab sections 25 and 26. In this embodiment the two outer dowel joint regions 51 are spaced apart by 9 metres and the two inner tie joint regions 52 are spaced by 3 metres, each of the two inner tie joint regions 52 being spaced from the closer of the two outer dowel joint regions 51 by 3 metres. The two outer dowel joint regions 53 are spaced apart by 9 metres and the two inner tie joint regions 54 are spaced by 3 metres, each of the two inner tie joint regions 54 being spaced from the closer of the two outer dowel joint regions 53 by 3 metres. By allowing concrete curling at the inner tie joint regions 52 and 54 concrete curling at the outer dowel regions 51 and 53 is significantly reduced along with the surface tensile stress in the vicinity of the inner tie joints 52 and 54. The aforementioned spacing of joint regions can be varied to suit the particular

dimensions or composition of the concrete slab. Furthermore the number of longitudinal joint regions 12A and lateral joint regions 12B can be varied to suit the particular dimensions or composition of the concrete slab . A plurality of concrete slabs 1 may also be laid next to each other in a series or grid to form a mega-slab whilst retaining the benefits of the concrete slab 1 1.

If sawcut joints 3 are mechanically cracked at approximately 3 metre centres, not all joints will require dowels to accommodate longitudinal concrete shrinkage. Alternate dowelled joints mechanically cracked at approximately 6 metre centres will provide adequate capacity to accommodate longitudinal concrete shrinkage. This allows the intermediate joints to be tie joints which are significantly less expensive than dowelled joints. By mechanically cracking all sawcut joints 3 in a concrete slab 1 approximately 48 hours after concrete placement, not only do all dowelled sawcut joints 13 then open relatively uniformly, all sawcut joints 13 curl relatively uniformly.

In another embodiment the crack promoting insert 14 is located at a top region of the concrete slab 11.

In some embodiments the channel portion 17 may comprise a permanent bend ¹ deformation region to allow a greater degree of curling. In FIG 6 there is shown the use of dowels in combination with the joint tying devices in the composite structure of the invention as shown in FIG 5. The joint tying device 16 is shown along with the use of tapered dowel plates 55 with each dowel plate 55 having the taper 56 reversed alternatively as shown. Each of the dowel straps are supported by a pre-fabricated cradle or cage 57 which has a pair of longitudinal wires 58 and 59 on each side interconnected by vertical spacers 60. There also may be provided horizontal wires 61 interconnecting the top of wires 58. The use of cradle 57 avoids the use of bar chairs. The use of joints 12A and 12B shown in FIG 8 provides a grid like pattern as shown. The dowel cradles 57 are conventional and are available from PARCHEM Construction Supplies at www.parchem.com.au.

In FIG 7 there is shown the composite structure 10 of the invention which includes concrete slab and joint tying devices 16 and dowels 55. The spacing "X" between adjacent cracks 15 is suitably from 2.0 - 4.5 metres and more preferably 3.0 metres as described above and each of FIGS 7A, 7B and 7C show the formation of curled intermediate or inner joint edge portions 62 and 63 of adjacent slab sections 25 and 26 at both the top surface 9 and bottom surface 21 , and curling also of outer edge portions 64. It is also noted that dowel plates 55 are also bent at 65 and transverse members or channels 17 of joint tying devices 6 are bent at 66.

However it will be appreciated that curling of edges 62, 63 and 64 in FIG 7 is exaggerated for the sake of clarity but occurs uniformly wherein the angle to horizontal as indicated by support surface 27 of each curled edge 62 and 63 may be of the order of 0.2 degrees.

From the foregoing it will be appreciated that the composite structure of the invention 10 by having a combination of plurality of dowels 55 and joint tying devices 16 is able to produce a concrete slab 11 which allows far greater loads to be applied thereto than conventional slabs by reducing both axial or longitudinal tensile stresses due to shrinkage as well as tensile stresses due to curling. Thus in a more detailed scenario the dowels 55 are primarily responsible for reduction of tensile stresses due to axial shrinkage and both the dowels 55 and joint tying devices 6 reduce tensile stresses due to curling. Both the combination of dowels 55 and joint tying devices 6 inhibit differential vertical movement of each slab section 25 and 26 on either side of joints 2A and 12B. All of these factors enable greater loads to be applied to slab 11.