The present invention relates to reinforcing in concrete and, more particularly, to the support of slab ducts for post tensioning cable reinforcing at a required level within concrete structures.

BACKGROUND

Concrete "slab on grade" is the laying of a concrete slab on a plastic layer directly onto a prepared ground surface. Post tensioning of a grid of cables is employed, especially where large areas are involved and a minimum of jointing is, desired. These cables may be grouped in two or more cables which are run through slab ducts extending between opposite sides of the slab. The ducts may be plastic or are roll-formed from galvanized iron, to form a flattened sheath of sufficient width to accommodate the desired number of cables lying side by side and with sufficient clearance for free running of the cables in the duct.

Prior to pouring of the concrete the grid of slab ducts with their cables installed, is supported at a specified height above the ground surface on special "bar chairs". Once the concrete is poured and has sufficiently cured, hydraulic jacks apply tension to the cables. Once tensioned, a cement grout may be injected into the cable ducts to lock the cables in their tensioned state and are then in effect fully bonded with the surrounding concrete of the slab.

The same principles may be used for floor slabs of multi-storey buildings although here of course the duct and cable reinforcing is installed over a supporting ^' timber form work or steel decking, which may or may not be left in situ after curing of the slab. Post tensioning of concrete is typically used for multi-level car parks where high strength and reduction of slab thickness and beam cross section is an advantage.

The industry now relies on a wide range of so-called bar chairs, either formed of wire bent into a suitable shape or moulded of some polymer. In many cases the latter may be bulky to transport and wire bar chairs especially, may not be adequately stable when placed on plastic sheeting over soft fill, for example. Often also polymer bar chairs are moulded from the cheapest, inferior material so that such bars are - prone to collapse when significant weight is placed upon them, for example by personnel during preparation of the reinforcing.

It is an object of the present invention to address or at least ameliorate some of the above disadvantages. Notes

1. The term "comprising" (and grammatical variations thereof) is used in this specification in the inclusive sense of "having" or "including", and not in the exclusive sense of "consisting only of".

2. The above discussion of the prior art in the Background of the invention, is not an admission that any information discussed therein is citable prior art or part of "the common general knowledge of persons skilled in the art in any country.

BRIEF DESCRIPTION OF INVENTION

Accordingly, in a first broad form of the invention, there is provided a slab duct bar chair system for support of. concrete reinforcing slab ducts; said bar chair system including a plurality of bar chairs of substantially similar design, and a supporting base element; said bar chairs distinguished one from another dimensionally so as to provide for support and retention of standard sizes of said slab ducts; each said bar chair arranged for optional attachment to a said supporting base element.

Preferably, each said bar chair includes first and second substantially vertical, when in use, elongate strut elements; said strut elements depending from a bridging element extending between said strut elements; said bridging element comprising a web portion and a reinforcing rib element.

Preferably, said bridging element includes a slab duct retaining section; said retaining section comprising a generally rectangular, centralized cut-out portion extending along a portion of an upper edge of said bridging element.

Preferably, said cut-out portion is provided with rounded undercut portions in lower left and right corners of said cut-out portion; said undercut portions adapted to aid retention of a slab duct supported in said cut-out portion. Preferably, said reinforcing rib element extends continuously up a first of said strut elements, up along a first side of said web portion of said bridging element, along upper edges of said web portion, along edges of said cut-out portion, down an opposite side of said web portion and down second of said strut elements.

Preferably, said bridging element is further provided with a number of through apertures distributed symmetrically within said web portion.

Preferably, supporting feet elements are provided at lower ends of said elongate strut elements; said feet comprising horizontally extending elements, transverse to said bridging element. Preferably, said feet elements extend symmetrically both forwardly and rearwardly from said strut elements.

Preferably, said supporting strut elements, said web portion and said reinforcing rib element of said- bridging element, and said feet elements form an integrally moulded whole.

Preferably, said bar chair . and said base element are moulded in glass reinforced nylon.

Preferably, said elongate strut elements are further provided with integral clip elements; said clip elements adapted to clipping together two or , more of said bar chairs .

Preferably, the width of said bridging element lies in the range of 115mm to 135mm.

More preferably, the outside dimension of all size chairs of the bridging elements is 120mm.

Preferably, the height, of a horizontal section of said reinforcing rib in ,said cut-out portion above a supporting surface in use, is provided in a range of 25mm to 125mm. Preferably, said height increases incrementally by 10mm within said range of 25mm to 125mm.

Preferably, said generally rectangular centralized cut-out portion is provided in widths of 43mm, 70mm and 87mm.

Preferably, said base element provides a generally rectangular platform. Preferably, said base element is provided with securing elements; said securing elements adapted to receiving as a snap fit said feet elements of · said bar chair so as to maintain said bar chair in a secure upright position relative said base element.

In another broad form of the invention, there is provided a method of supporting slab ducts for post tensioned concrete at a level above a supporting surface; said method including the steps of:

(a) inserting feet elements of a bar chair into retaining elements of a base element, (b) placing an assembly of a said bar chair and a said base element onto a supporting surface, wherein undersides of said feet elements are substantially flush with the underside of said base element when said bar chair and said base element are assembled together for use.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described with reference to the accompanying drawings wherein:

Figure 1 is an unassembled perspective view of a bar chair and supporting base according to the invention,

Figure 2 is an assembled perspective view of the bar chair and supporting base of Figure 1, Figure 3 is a perspective view of a section of a grid of slab ducts and cable supported on the bar chairs and bases according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to figures 1 and 2, the bar chairs 10 of the present invention, comprises first and second supporting strut elements 12 and 14 depending from outer ends of a bridging element 16 The bridging element and the two strut elements form a web portion 18 and a reinforcing rib 20.

Bridging element 16 includes a generally rectangular cut-out portion 22 extending along a section of the upper edge of the bridging element 16 and of a depth sufficient to support and retain a slab duct placed therein. Preferably, cut-out portion 22 is provided with rounded undercut portions, 23 and 25, in the lower left and right corners to aid in retention of a slab duct 29 supported in the cut-out portion as shown in Figure 3.

The reinforcing rib 20 is continuous, extending upwardly along the outside of the first strut 12, around the web portion 18 of the bridging element 16, including edges of the cut-out portion 22, and downwardly along the outside edge of the second strut 14. The bridging element 16 is further provided with a number of through apertures distributed symmetrically within the web portion.

The pair of supporting strut portions 12 and 14 are provided at their lower ends with foot elements 24 and 26 aligned transversely to the bridging portion with portions 24A and 24B extending symmetrically forwardly and rearwardly from strut 12, and 26A and 26B similarly from strut 14 . The foot elements 24, 26, web portion 18 and the reinforcing rib 20 are all moulded as one integral unit, preferably from glass filled nylon.

The foot elements 24 and 26 are adapted to maintain the bar chair 10 upright when placed on a supporting surface. When placed on a suitable level surface, such as plywood or other timber based sheeting, the outwardly extending portions 24A, 24B and 16A, 26B allow the feet to be stapled to the surface.

In a preferred embodiment of the bar chairs of the invention, each chair 10 is provided with an integrally moulded clip (not shown) which allows bar chairs to be ^" clipped together to form a compact bundle of chairs for transport and convenience of use on site.

As part of the bar chair system of the present invention, a base element 40 is provided for extra stability when required, for . example when placing reinforcing on plastic laid over uneven fill. Base element 40 is of generally rectangular form and is provided with pairs of retention elements 42A, 42B and 44A, 44B adjacent respective opposing edges 46 and 48 of the base element 40. These pairs of retention elements are adapted to receive the feet elements 24 and 26 of the bar chair 10 as a snap fit.

r The struts are sufficiently flexible for the feet elements 2 and 26 to be pushed over the sloping upper portions of the retaining elements 42A, 42B and 44A, 44B. When fully inserted the undersides of the feet elements 24 and 26 are substantially flush with the underside of the base portion 40. As a result, the level of the horizontal section 54 of the cut-out section is at the same height when the bar chair is assembled to the base portion as when it is used in isolation. Projecting lugs 25 and 27 ensure the bar chair feet elements locate correctly in base element 40.

The system of the invention provides for a range of sizes of the bar chairs 10 of the invention. The variation in sizes relate to two principle dimensions. Firstly the height of the horizontal section 54 of the reinforcing rib in the cut-out section 22 above a supporting surface (that is to the underside of the feet elements 24 and 26, has a range of between 25mm and 125mm, and preferably includes intermediate heights in 10mm increments. The other variable dimension is that of the width of the cut-out portion 22 of the bridging element 16. This width ranges from 43mm to 87mm with intermediate width of 70mm to accommodate three standard sizes of slab duct.

In Use

The chairs of . the invention are considerably more compact then known chairs and, when a number are clipped together, may be carried conveniently over the forearm of a user (not shown) for placement. The fact that the chairs are moulded from high strength glass-filled nylon provides greatly improved resistance to deformation or collapse from vertically applied loading, for example when personnel tread on the slab duct adjacent to the chair.

The above describes only some embodiments of the present invention and modifications, obvious to those skilled in the art, can be made thereto without departing from the scope of the present invention.