NZ220693A - Load bearing structural member of cementitious laminate with tensioned reinforcing - Google Patents

Description

2 206 Priority Date(s): Complete Specification Filed: IS:.<P.'37 Class: ^rrP.✓..... . 6>^.6>.VS/O9?y.02-.-.

Publication Date: Z.7. RQV.

P.O. Journal, No: . ....taea.-.

NEW ZEALAND PATENTS ACT, 1953 No.: Date: COMPLETE SPECIFICATION STRUCTURAL MEMBERS 4-/We, CELLATE INDUSTRIES AUSTRALIA PTY LTD, a company incorporated in the State of Queensland, 'Australia, of 130 Cobalt.Street, Carole Park, Brisbane, Queensland 4300, Australia hereby declare the invention for which W we pray that a patent may be granted to me/us, and the method by which it is to be performed, to be particularly described in and by the following statement: - (.Followed by page la) - Mr ^ - V - 1A- * This invention is concerned with improvements in structural members and in particular to load bearing structural members such as wall, floor and roof panels.

Of recent years, there has been a trend towards the 5 construction of buildings from prefabricated components.

There are certain cost advantages in the prefabrication of structural panels at a remote site and then transporting these to a building site for rapid assembly and erection. In the handling and transportation of such prefabricating 10 components , both weight and dimensions are the most important and often limiting factors.

It is known to fabricate building panels from aerated foamed concrete in an endeavour to save weight. It is also known to manufacture foam cored panels for the same 15 reason. Foam cored panels may comprise a sheet of foamed plastics material such as polystyrene foam. The slab of foam is encapsulated within a concrete structural panel. If the panel has no structural requirements, the concrete "skins" on either side of the panel may be unreinforced or 20 include only a light reinforcing mesh. On the other hand, load bearing panels of this type may include one or more layers of a substantial reinforcing mesh or even internal stiffening ribs and pre-stressing tendons where a high flexural strength is required.

In general, the use of prefabricated structural panels of the type referred to above is limited to non-load bearing wall panels as there are serious difficulties associated with their use as roofing or flooring panels. For a panel comprising a sheet or slab core of polystyrene foam, the Youngs Modulus of the core is much greater than the concrete on the outside. Thus, under flexural load, the concrete skin will have passed its elastic limit and failed before any contribution is made by the foam core. From a design consideration, panels of this type must therefore include ribs to provide adequate strength. In effect, internally (or externally) ribbed panels would have to be individually designed for each construction application and all point load situations compensated for.

Panels of the above type are generally difficult to construct as it is almost impossible to accurately position and maintain a foam core insert when dealing with wet concrete slurries.

Other problems include 1. Water resistance.

Any imperfection in the outer skin of a roof panel will allow water to enter the cavity occupied by the foam insert. The trapped water can then find an imperfection in the inner skin and enter the interior of a building. 2. Fire resistance.

The fire rating of polystyrene foam cored panel with a thin concrete skin is quite poor. © m 220693 3. Visual effects.

When the panel is damp, the position of the foam cores between ribs, ends, etc. becomes clearly-visible like a patchwork effect. Differing o insulating properties over -the surface of the panel gives rise to condensation "patterns" under certain conditions. 4. Access.

In chasing a power or other service conduit through such prior art panels, obstructions such as ribs and other stiffening members will be encountered.

Other types of foam cored structural panels may be made with fibrous cement, plastics, metal or timber skins or combinations thereof but in the main these are used as 15 non-load bearing structural members due to their inherent weaknesses.

In all cases, solid plastics foam cores contribute little if any physical properties to such structural panels due to lack of inherent physical properties and the lack of 20 any significant bond with cementitious materials. In the main, the core is provided only as a physical means for separation of the skins during manufacture or for forming voids therebetween.

Various proposals for structural panels have been 25 made in respect of concrete skinned, low density cored panels. Those incorporating a slab or core of foamed O G '' V-?*'.- v. vw V n _ 4 - plastics materials have been found to be quite unsuitable for the reasons outlined above.

Other proposals have contemplated the use of a conventional concrete skin and a core comprising a 5 cementitious slurry incorporating foamed polystyrene beads. However none of the prior art proposals have really addressed the problems normally inherent in such panel structures intended for use as load bearing members.

In use of a conventional concrete skin comprising 10 cement, aggregate, sand and water, one could expect flexural strengths of around 2MPa with conventional concrete characteristics of high compressive strength combined with low tensile and flexural properties. In contrast, the present invention contemplates a steel fibre reinforced 15 cementitious skin having not only high compressive and tensile strengths but also flexural strength properties of the order of 8MPa in a non pre-stressed condition.

While the incorporation of foamed polystyrene beads in cement slurry mixes has been proposed as a core material, the practical problems of mixing a relatively low density material into a viscous medium of relatively high density have not really been addressed. At the present time, there are apparently no commercially available polystyrene foam/cement slurry cored structural panels in commercial usage and it is believed that the reason for this is that the practical problems of evenly mixing foam beads into a cement slurry have not been overcome on a commercial scale.

O O 220( If a cement (or concrete) slurry is too fluid (too "wet") , the foam, beads tend to remain near the surface during mixing or after mixing the beads rapidly migrate to the surface of the mix prior to removal from the mixer or ^uring the pouring or casting step. This leads to a laminar weakness in the resultant panel in the region of greater i bead concentration. When the mix is poured from a mixer such as a conventional concrete mixer, the initial portion of the pour has an excessively high bead concentration while the latter part comprises substantially entirely a cementitious -slurry. Having poured the initial polystyrene bead rich mix into a mould or the like, the latter portion, comprising a substantially bead free slurry, when poured over the top of the initial layer migrates to the bottom of the mould leaving the upper layer rich in polystyrene beads and consequently lacking in physical strength.

Alternatively, if the mix is too viscous (too "dry") better homogenization is achieved at the expense of the physical properties of the cementitious binder and lack of workability of the core mix.

French Patent specification No. 1 491 650 discloses a thin waterproof panel structure suitable as a lining for bathroom walls, shower cubicles and the like. The panel comprises a lightweight core of Portland cement admixed with perlite, vermiculite or crushed porous blast furnace slag in the preferred ratio of 1:2 to 1:4. The density of such core material lies in the range of 600 - 1800Kg/m .

The panel includes thin outer skins of cement reinforced with woven fibreglass cloth.

The main requirements of such a panel are that they are light in weight yet strong enough to enable handling by a single workman. Accordingly these panels are generally limited to a thickness of say 12mm to 25mm and measure 600mm x 900mm. The outer skins are necessarily very thin (i.e. of the order of 3-4mm) to facilitate fixing of the panels onto a support structure by nailing and further to permit the panels to be readily sawn.

The panels- described in French Patent No. 1 491 650 are inherently unsuitable as load bearing structures such as walls and suspended floors and roofs.

In Australian patent specifications 507249 and 520177 there are described load bearing wall panels comprising a non load bearing lightweight core materials sandwiched between layers of fibrous concrete reinforced with steel fibres.

The core material comprises slabs of cellular plastics material such as foamed rigid P.V.C., foamed rigid polyurethane, foamed rigid polystyrene or the slabs may comprise polystyrene bubbles set in a cement mortar or rigid plastics foam matrix.

Australian patent specification 507249 suggests that the panels may be stiffened by external webs or internal stiffeners such as timber strips. 2206 In contrast, Australian patent specification 520177 requires that the outer skin layers are interconnected by spaced webs to provide resistance to shear forces exerted between the layers when the panels are subjected to flexural 5 stress loads. The engineering principle underlying the wall panel structure of Australian patent specification 520177 is said to reside in the fact that the two skins share the compressive load end are separated to achieve a high modulus of section to resist buckling, the webs interconnecting the 10 skins serving to ensure that the two skins act as a single structural member.

Patent specification 520177 further suggests that if^required, post-tensioning cables may be located in conduits in the webs interconnecting the outer skins. In 15 the wall panels the tensioning cables extend between the roof plate and the foundations of the building incorporating the panels and presumably serve to tie the finished building structure together and thus only make a contribution to the strength of the panels after a building is constructed. 20 None of the prior art lightweight cored concrete panels have been proposed for use in suspended floor and roof constructions as conventional wisdom in this art actually teaches away from their use as load bearing structural members.

It is known to use as suspended floor and roof 220 panels pre-stressed concrete beams having a plurality of lightweight core members of expanded polystyrene. These floor members comprise thick concrete skins separated by thick closely spaced concrete webs, the webs including post-tensioning tendons. Foamed polystyrene blocks are placed between adjacent webs during manufacture simply as void formers to reduce the weight of the resultant panel. Such structural panels may therefore be regarded as a plurality of pre-stressed "I" beams placed side by side.

It is an aim of the present invention to provide a load bearing structural member of the type having a low density core material sandwiched between cementitious outer skins, which structural member embodies a principle of construction enabling its use in walls and suspended floors and ceilings.

It is a further aim of the invention to overcome or alleviate the problems of prior art low density cored panels and to provide a more economical panel having substantially improved load bearing characteristics when compared with prior art panels of similar mass and/or dimensions.

According to the present invention there is provided a structural member comprising A core of low density particulate material in a cementitious matrix sandwiched between thin outer skins of a steel fibre reinforced cementitious material on at least two opposing faces of said core, said structural member characterised in the provision of one or more post-tensioned tendons extending between opposed ends of said structural member, said post-tensioned tendons being positioned within the structural member between opposing faces thereof to distribute post-tensioning stresses substantially evenly throughout said outer skins.

Suitably the low density particulate material comprises an inorganic material such as perlite, vermiculite or cellular blast furnace slag or it may comprise an organic material such as cellular plastics.

Preferably the low density particulate material comprises foamed polystyrene beads and most preferably the foamed polystyrene beads are fragmented to form particles of uneven shape and size.

The cementitious matrix suitably comprises a sand/cement mortar and preferably the mortar includes entrained air to form a cellular matrix.

Suitably the density of the core is in the range of 3 3 from about 250 Kg/m to 800 Kg/m , preferably of from about 350 Kg/m3 to 450 Kg/m3 .

The thickness of the outer skin of the structural members may be in the range of from about 6mm to 30mm but preferably in the range of from about 10mm to 20mm. The post-tensioning tendons may be positioned directly within the core material and cast therewithin. Alternatively the post-tensioning tendons may be positioned in conduits cast within the interior of the structural members.

According to another aspect of the invention there is provided a method of constructing a structural member which method comprises:- placing on a shaping surface a first "layer of cementitious mortar containing steel reinforcing fibres; placing on said first layer before said first layer 1 has cured a second .layer of a cementitious mortar slurry containing a low density particulate material; and, placing on said second layer before said second layer has cured a further layer of a cementitious mortar containing steel reinforcing fibres, said further layer extending over the exposed surface of said second layer and allowing said first layer, said second layer and said further layer to cure to form a sandwich structure comprising an integral outer skin bonded to a low density core, said method including the further step of incorporating within the structural member between opposing faces thereof post-tensioning tendons extending between opposed ends of said structural member to distribute post-tensioning stresses substantially evenly throughout said outer skin.

As used hereinafter the expression "sand" means particulate silicious material such as an alumino-silicate e.g. river or beach sand. The sand comprises a mixture of particle sizes from a maximum of say 3 mesh (Tyler) down to a fine dust e.g. 200 mesh (Tyler) or less. When used in cement mortars the sand gives the effect of a miniature mixed aggregate similar to conventional concrete mixes but .. • T 230693 U | | on a substantially smaller scale.

In order that the invention may be more fully understood, reference may be made to the accompanying O drawings in which FIGS 1-3 illustrate a preferred method of constructing a structural panel in accordance with the * invention.

FIG 4 is a plan view of a typical multi purpose structural panel.

FIG 5 is a cross-sectional view along A-A in FIG 4.

FIG 6 is a plan view of an alternative form of- panel.

FIG 7 is a cross section along B-3 in FIG 6. FIG 8 is a cross section along A-A in FIG 6. 15 FIGS 9, 9a show alternate cross-sectional views along A-A in FIG 6.

FIGS 10 and 11 show alternative cross-sectional views of the structure of FIG 9.

FIGS 12-14 show alternative embodiments of the 20 invention.

FIGS 15 and 16 show respectively graphically O comparative stress analyses for a prior art cored panel and a panel according to the present invention. 220693 n IS O o FIGS 1-3 illustrate a preferred panel construction method with reference to the construction of a simple rectangular panel in a female mould although it will be clear to a skilled addressee that more complicated two or three dimensional shapes may be constructed in either male or female moulds.

I In FIG. lr the bottom or base layer 1 is formed on any suitable surface such as a sheet plastics, steel or concrete surface la or the like. This surface may be smooth or decoratively textured and, if required, coated with a mould release agent to facilitate release of the finished panel from the mould surface. The mould frame suitably comprises lengths of angle section steel, aluminium or plastics material 2 or the like movably secured to the mould surface la to define the perimeter of the mould and/or to define apertures such as window apertures in a structural wall. The inwardly directed faces of the angle sections 2 are subsequently lined with spacers 3 of timber, foam plastics or the like to form a predetermined space of width x around the perimeter of the mould., the purpose of which, spacers will be described later.

A cementitious skin mix is then prepared in a suitable mixer according to the ratio: cement water sand steel fibres - 800kg - 160 litres - 1500kg - 160kg •'-1 220693 The cement is preferably grey Portland cement and the steel fibres are preferably enlarged end rectangular end fibres made by A.W.I. Fibresteel having dimensions 14mm long x 0. 4ntm thick x 0.6mm wide. If required, a quantity of 5 conventional plasticizing agent may be employed to improve workability of the mix.

I The skin material 4 is poured into the mould to a required skin thickness and levelled, at least roughly by trowelling, screeding or the like and spacers 3 inserted in 10 appropriate positions against the mould frame 2.

A core mix is then prepared as follows according to the ratio: water • : - 200 litres cement : - 400kg foamed polystyrene beads : - In? Again the cement is preferably grey Portland cement and the foamed polystyrene beads have a diameter range of from 3—6mm with a density of approximately 15kg/ir? .

The water and cement are added to a mixer such as a 20 concrete mixer to form an homogenous' slurry. Once the slurry is formed, the beads are then added and mixed thoroughly to form an homogenous slurry/bead mix. The viscosity of the slurry is such that the tendency of the beads to migrate to the surface during mixing is effectively 25 negated.

Before pouring the core mix plastic conduits 6 are located within the confines of the mould by slots or O 2206,93 apertures in end blocks 3. The lateral spacing of the conduits and their distance from'the upper or lower skin is predetermined according to the required load bearing characteristics of the panels.

The core mix is then poured into the mould to form a second layer 5 on top of the first "wet" or uncured layer I l. The surface of the csment/bead mix 5 is levelled in a manner similar to the first layer 1. The thixotropic nature of the cementitious mix 5 is such that there is no tendency for the beads to migrate to the surface of the' second layer ' after pouring and levelling.

When the second layer has stiffened slightly or at least partially cured, spacers 3 are removed as shown in FIG. 3 and if required, exposed face 7 of first layer i is roughened by a suitable roughening tool. A final layer 8 substantially similar to that of layer l is then poured over the top of layer 5 and allowed to flow down the cavity between the edges of layers 1 and 5 and the mould wall formed by angle member 2. If required lifting eyes may be cast into the upper surface of the panel to facilitate later handling. In this manner, the resultant panel is formed with a peripheral edge skin 9 integral with and similar to upper skin 8 and lower skin"i respectively. The uppermost surface of skin 8 is screeded, trowelled or otherwise levelled and finished as desired.

In a typical structural panel, say a wall panel for a domestic dwelling, the panel may measure 2.5im x 4m x 100mm thick. The panel may comprise an 80mm core with 10mm skins on either side.

In a modified form of the invention, the cementitious slurry for the core may include a foaming or aerating agent to further reduce the density of the ' composite core material. Such a slurry composition may comprise the slurry mix as described above with the addition of 2 litres of FRO.B (Trade Mark) - an organic foaming agent manufactured by Sika. mixer with the FRO.B foaming agent and with the mixer rotating, the aqueous mix is aerated by the introduction of compressed air at the rate of about 65 litres/min. (lOcu.ft./min.) for about 3min. Two thirds of the cement is then added and during the mixing cycle, aeration is conducted for a further 3mins. The remaining portion of the cement is then added and when mixed thoroughly, the foamed polystyrene beads are added. The resultant mix has a "wet" 3 density of approximately 480Kg/m and when cured a density • 3 of approximately 450Kg/m . In practice, core densities of 3 3 between 250Kg/m and 800Kg/m will be found to be effective depending on the cost and engineering considerations of the structure. Foamed concrete slurries have the advantage of improved thixotropic properties which'further reduces bead migration.

The water is placed in a conventional concrete It will be noted that the viscosity of the c 22069 slurry is quite important to the effective practice of the invention. If the mix is too dry--then addition of further water will not give the required viscosity and equally if the mix is too wet, addition of further cement- results in the formation of cement "balls". Unlike conventional aggregate containing concrete mixes, the shear during mixing I of a foamed polystyrene containing cement slurry is insufficient to adequately mix water or cement added during the latter part of the mixing cycle.

As little as eight hours after the panel casting operation is completed, the mould edges may be removed and the panel stripped from the moulding surface la. The panels may then be stacked for complete curing prior to transportation and usage.

The structural panel 10 of FIGS 4 and 5 measures 5m x 3m and is 100mm thick. The outer skin 11 comprises a steel fibre reinforced cement mortar and the skin thickness is 12mm on all faces. The compositions of the cementitious skin and the core were the same as those employed in the embodiment illustrated in FIGS 1-3 and the panel was constructed substantially in accordance with the aforementioned method steps.

In the opposed ends 12 of the panel 10 are inwardly extending anchor supports 13 comprised of the same material as the outer skin 11. These anchor supports 13 locate and support the post-tensioning anchors 14 attached to opposed © o o o ends of post-tensioned cables 15 located within conduits 16 within the core 17 of the panel 10.

Before tensioning the cables 15, a pumpable cementitious grouting composition 18 is pumped into the 5 conduits 16.

The cables 15 are tensioned to a value of lOOKn and then are anchored by conventional post-tensioning anchors in the form of wedgable collets or the like The phantom outlines shown over the surface of the 10 panel 10 represent the zones of stress induced in the outer skin of the panel. As the zones overlap the tensile stress in the post-tensioning cables is dissipated as a compressive stress over substantially the entire skin surface of t'he panel 10.

This is in direct contrast to prior art cored panels having post-tensioning tendons located within large structural webs which have the purpose of tying the outer skins together and providing rigid stiffening ribs. In such a configuration the tensile stress in the post-tensioning 20 cable is dissipated as a compressive stress substantially only within the web and generally is intended only to stress the web to form a conventional pre-stressed concrete beam. The outer skins of such panels are substantially unstressed by the post-tensioning tendons and for this reason they may 25 be regarded as a plurality of adjoining pre-stressed concrete "I" beams having entirely predictable properties.

Under deflection loads the shear forces between the skins is greatest towards the ends of a cored panel and thus for prior art "adjoining I-beam" panels the interconnecting webs play a crucial role in conferring load carrying capability on the panels and, to a lesser extent, in binding the outer skins together as a unitary member. For this reason the webs are generally relatively thick when compared with say a steel "I" beam section.

FIGS 6-8 show an alternative form of structural panel suitable for use in suspended high load flooring applications and/or for long span flooring.

FIG 6 shows a plan view of a flooring panel having a length of 6 metres, a width of 2 metres and a thickness of 150inm. The upper skin is 20mm thick and the edge and bottom skins are 15mm thick. The skin and core compositions are as previously described.

For high load applications the post-tensioning cables 15* are anchored at opposed ends of panel 10* midway between the upper skin 30' and lower skin 31i.e. 75mm from both upper and lower skins 30' and 31' respectively. At the mid-span position 32' the post-tensioning cable 15' is located 50mm from lower skin 31' and lOOnun from the upper skin 30 '.

FIG 9 shows in cross sections an alternative embodiment of the structure of FIGS 6-8. Where a floor panel is likely to be subjected to very high point loads such as may be expected from a heavily laden trolley with small diameter wheels or upon impact by sharp heavy objects, it may be desirable to provide additional support for the upper skin. This additional support may be in the form of 220 thin webs having a thickness corresponding approximately to the skin thickness. As these thin webs do not contribute significantly to the ultimate mechanical properties of the floor panel, the webs 33 may extend parallel to the 5 post-tensioned cables 34 or they may extend transversely of the cables 34. The only meaningful contribution of such internal webs is to support point loads applied perpendicularly to the surface of upper skin 35. If required the cables 34a may be incorporated within the webs 10 33a as shown in FIG 9a.

For long span floor panels it is preferred to ."drape" the prestressing tendon within the floor panel as shown generally in FIG 7. In this manner the positioning of the central portion of the tendon adjacent the lower skin of 15 the panel substantially increases the load bearing capacity of the panel.

Due to the relative softness of the core material it is necessary to support the tendon in the draped position during and after the tensioning of the tendon to prevent 20 loss of tension due to crushing of the core material.

Accordingly where it is necessary to employ draped tendons which are subsequently tensioned a web of crush resistant material is formed around the draped tendon between the ends of the panel. Preferably the web extends 25 between the upper and lower skins of the panel to support the web against movement and subsequent loss of tension in the cable. Alternatively the webs may comprise broad flat members having a layer of core material located above and below the flat webs whereby the forces exerted on the webs by tensioning of the cable are dissipated in the upper and lower layers of core material without crushing.

The web of crush resistant material may comrise a cement mortar or a high aggregate containing concrete or for the sake of expedience the tendon supporting webs may be made.of the same material as the outer skin of the panel. Conveniently such tendon support webs may be formed integrally with the top skin of the panel.

FIG 10 shows a partial cross section of a panel wherein internal compression ribs 36 extend within inner core 37 between upper skin 38 and lower skin 39 transversely of post-tensioning cables 40.

FIG 11 shows yet another embodiment of the arrangement of FIG 10. To avoid the weight penalty imposed by the additional internal ribs 36 as shown in FIG 10, steel wire loops 41 may be arranged between upper skin 42 and lower skin 43. Loops 41 are held in place by upper and lower transverse arms 44 cast into the upper and lower skins 42 and 43 respectively and upright arms 45 extend through core 46 perpendicular to the upper and lower skins 42 and 43 respectively.

The loops 41 may comprise individual loops placed randomly or in parallel rows during formation of lower skin 43. Conveniently, the loops are arranged on the post-tensioning conduit to assist in alignment in rows and the individual loops may be manually spaced. Preferably however loops 41 are formed as a helical coil having a rectangular cross section. Again the helical coil is 5 conveniently located over the post-tensioning conduit 46 and during formation of lower layer 43, the coil is stretched into place and lower arms 44 embedded into lower layer 43.

The helical coil may be arranged parallel to the post-tensioning cables or the cables may be aligned 10 perpendicularly as shown in phantom.

Alhough the foregoing discussion is limited to flat panels for walls, floors and roofs, it will be clear to a skilled addressee that other structural members may be made in accordance with the invention.

FIG 12 shows a structural beam 50 having a thin outer skin 51 of steel fibre reinforced concrete mortar and a core 52 comprised of a particulate foam/mortar slurry as previously described. A post-tensioning cable 53 is grouted in a conduit 54 extending down the centre of the core 52. 20 For high load capacity beams a steel wire mesh 55 may be incorporated within the outer skin 51.

In further embodiments of the invention a structural panel 60 as shown in FIG 13 may include post-tensioning cables 61 arranged perpendicularly to each 25 other or as shown in FIG 14, a structural panel 70 may comprise a plurality of smaller panels 71 held together with post-tensioning cables 72 extending through the cores of the panels 71.

* I * V C •*''jtyip , , ■ ' ' .Vv—''' V-V.- r> O o - 2 2 - 220693 In order to appreciate the advantages of the present invention over traditional cementitious load bearing panels, comparisons may be made first with a steel reinforced concrete flooring slab and then with a webbed ~ 5 concrete flooring panel having polystyrene cores.

A. Steel Reinforced Solid Concrete Comparison may be made between a flooring panel as illustrated in FIGS 6-8 and a steel reinforced solid concrete flooring panel having the same dimensions i.e. 6 10 metres long, 2 metres wide and 15Oram thick.

For the solid concrete panel to possess the same mechanical properties steel reinforcing is required in the form of 12mm bars at 90mm centres extending longitudinally of the panel and 12mm bars at 300mm centres extending 15 transversely of the panel adjacent the lower face. Adjacent the upper face is a welded steel mesh comprising 6mm bars at 200mm centres in both directions.

The panel according to the invention weighs 0.14 3 tonnes and costs approximately A$50.00/m to produce. 20 The steel reinforced solid concrete panel weighs 3 0.42 tonnes and costs approximately A$66.00/m to produce.

While the panels may be considered as mechanically equivalent in terms of capability of handling and transportation and physical strength the panels according to 25 the invention are one third of the weight of solid concrete panels and three quarters of their cost. In addition to the prima facie cost advantage, there are other significant hidden cost advantages associated with the invention. The reduced mass of the panels according to the invention permit reduced handling and transportation costs and as well permit substantial cost savings in building constructions in reduced costs for support columns, footings and foundations normally associated with the heavier solid concrete panels. B. Foam Cored Structural Panels FIGS 15 and 16 show comparative theoretical analyses of the mechanical properties of foam cored panels and panels according to the invention respectively.

For the purpose of comparison a section of foam cor2d concrete panel is taken to represent the hypothetical "I" beam referred to hereinbefore.

In FIG 15 the "I" beam structure 70 comprises two flanges 71 and 72 connected by a thick central web 73. Centrally of the web 73 and flanges 71,72 is located a post-tensioned cable 74 stressed to lOOKn.

The flanges 71,72 measure 1000mm in width and 60mm in thickness. The central web 73 is 100mm wide and separates the flanges 71,72 by 80mm (this space normally being occupied by a foamed polystyrene block which does not contribute mechanically to the properties of the beam) .

The ratio of flange thickness t to unsupported flange width i.e. the distance from the edge 73a of web 73 to the outer edges 71a,72a of the flange sides is determined - 24 - 220693 by the Standards Association of Australia "Concrete Structures Code" No AS14 80 relating to load bearing concrete structural members which requires that the maximum outstand of a flange is 8t where t = thickness of flange.

FIG 16 illustrates a similarly dimensioned cross section of a panel 75 in accordance with the invention. The I surrounding skin thickness is 12mm and the skin and core compositions are as hereinbefore described.

Located centrally of panel 75 is a post-tensioned cable 76 stressed to lOOKn, the same as that in FIG-15.

As hitherto explained, the stress in panels according to the invention is substantially evenly distributed in the upper and lower skins whereas with webbed panels it is believed that the stress load distribution is confined substantially to the area of the web as shown in phantom in FIG 15. For the purposes of comparison however it is assumed that the stress in panel section 70 is distributed substantially evenly throughout the upper and lower flanges 71,72 respectively.

FIGS 15a and 16a represent graphically the respective stress values shown as compression in the flanges 71,72 and upper and lower skins 77,78. The stress values (represented as negative) are respectively 1.47MPa and 3.73MPa which illustrates the highly stressed nature of the thin skin surrounding the core of panel section 75.

FIGS 15b and 16b illustrate graphically the bending stresses under a 1.5KPa live load plus the dead load in both -2 5- 22069 cases. To further emphasize the advantages of the invention the bending stress values are calculated for a span of 3000mm in the case of the FIG 15 structure and for a span of 400 0mm for the FIG 16 structure according to the invention. The bending stress values show a compressive stress of 0.82MPa in the top flange of FIG 16 and a tensile stress of 0.82MPa in the lower flange. In FIG 16b the compressive stress is 2.66MPa while the tensile stress is 2.66MPa.

FIGS 15c and 16c show the resultant stresses in the structures under comparison. In FIG 15c the stress is compressive in both top and bottom flanges (consistent with conventional structural concrete design practice). The stress values are 2.29MPa for the top flange ;and 0.65MPa for the bottom flange.

FIG 16c shows compressive stresses of 6.39MPa and 1.07MPa in the top and bottom skins respectively.

Accordingly, despite having been subjected to a more severe bending stress over a larger span, the panel section according to the invention demonstrates a substantially greater bending stress capability than conventional foam cored panels of similar cross-sectional dimensions.

Conventional engineering wisdom dictates that the lower flanges 72 of "I" beam 70 should never under load stress achieve a compressive stress value of more than zero 1.e. the flanges should never go into tension due to the inherently poor tensile qualities of concrete. In practice however it could be expected that the lower flange could withstand a tensile stress of about lMPa before failure.

Comparison of FIGS 15c and 16c show that the highly-stressed nature of the skins 77,78 leaves a residual compression stress of 1.07MPa, substantially greater than the beam of FIG 15 notwithstanding the greater span over which the beam is stressed. In practice the beam according to the invention may be stressed to a point where the lower skin 78 can withstand a tensile stress of say 4MPa due to the tensile properties conferred by the steel fibres.

FIG 16 also shows that side or end flanges are not essential to the working of the invention due to the excellent bond strength between the sandwiched layers. In practice however side and edge skins are preferred to protect the edges of the highly stressed "working" skins i.e. the upper and lower skins.

While the substantially improved mechanical properties, of the present invention are considered to be derived from the highly stressed thin outer skin it is also considered that the nature of the core material and the excellent bond between the core material and the outer skin plays a large part in contributing to the mechanical properties of the structural members.

For very thin skins subjected to the stresses in the bending mode shown in FIG 16c, failure could readily occur if the skins were to undergo buckling. The core material has sufficient compressive strength to withstand 2206 inward buckling of a skin while outward buckling is prevented by the excellent bond between the core material and the skin. In addition, any tendency for shear to occur at the interface between the skins and the core material is 5 also resisted by the bond and the shear strength of the core material.

It will be clear to a skilled addressee that many modifications may be made to the invention. For example the core material may comprise any lightweight particulate 10 material such as wood chips and the like. Further the invention is not limited to planar panel constructions as in conventional laminating techniques utilizing pre-formed cores and skins. A particular advantage of the 'invention is that the structural members may be formed in a variety of 15 planar or three-dimensional shapes (in male or female moulds) due to the ability to form "integral" members in a plastic state.

The method of constructing panels or shaped members from concrete sandwich construction has been generally 20 described above. However, in construction of large unitary members such as large span roof panels or a complete roof structure or otherwise complex shaped objects not suitable for female moulding, the present invention permits a male mould to be employed. As very large structures are not 25 easily transportable, it is desirable where possible to manufacture the structure on or at least adjacent a site where the structure is to be used.

For a roof structure, a male mould is constructed of say a timber frame and a plywood or like sheet material covering. A plastics membrane may be placed over the mould surface to act as a mould release agent otherwise a conventional release agent may be employed.

In a manner similar to formation of the panels described above, the three layers of cementitious material are built up one upon the other until the desired thickness is achieved. The top or outer surface is then screeded and/or trowelled off to obtain a smooth finish.

During construction of the roof structure, post-tensioning cable conduits are placed at appropriate positions between the upper and lower skins and lifting bolts are located within the sandwich at desired position. If further reinforcing elements are required such as rods or mesh these may be incorporated in the sandwich structure by say placing the rods or mesh on top of the first layer and then spraying the successive second and third layers thereover.

If required, conduits for electrical cables and the like may also be incorporated in the panels or roof structure during construction.

For certain structural applications where extra strength or rigidity is required to resist application of point loads on the skins, "ribs" may be formed by building up the thickness of portions of the first layer of steel fibre reinforced concrete. 220693 It is envisaged that the present invention has application to virtually all presently employed structurally reinforced members such as floors, structural beam members including "I" beams, box beams and the like as well as piers, columns, foundations, etc. In a manner similar to that previously described, a first layer of steel fibre i reinforced cementitious material is formed against a mould surface, the stressing tendons are placed in position adjacent the surface of the first layer and additional steel fibre reinforced cementitious material is sprayed over the tendons to substantially encapsulate them. low density particulate plastics material is then formed over the first layer. If required, channel-like depressions may be formed in the second layer in the region where additional reinforcing tendons are to be placed. The channel-like depressions are then at least partially filled with a steel fibre reinforced cementitious material prior to positioning of the additional tendons. Alternatively the •stressing tendons may be incorporated into the core material directly or later placed in conduits cast into the core material. A further layer of steel fibre reinforced cementitious material is then formed over the exposed surface to encapsulate the tendons. stressed structural members according to the invention it is possible to place the tendons at considerably greater A second layer of cementitious material containing By using steel fibre reinforced skins in 2206 spacing than would otherwise be required. When point loads are applied to the "skin" the forces are dissipated for some considerable distance through the fibre reinforced skin.

Wall panels may be constructed as follows for most applications.

Skin 1 - 6-20mm thickness Core - 25-80mm thickness Skin 2 - 6-20mm thickness Wall panels constructed with two 10mm thick skins and a 50mm core have been found to have a significantly greater load bearing capacity than an equivalent 110mm thick clay brick wall.

It is believed that panels 'up to 5m x 10m with a 10mm/5Omm/10mm thickness and two lifting points can be easily handled. Such a panel has an estimated weight of 3 tonnes compared with an estimated weight of around 9 tonnes for an equivalent clay brick panel.

Similarly, roof panels may suitably be selected from the following range. Skin 1 - 8-20mm thickness Core - 50-100mm thickness Skin 2 - 8-20mm thickness Practical size limitations for handling a roof panel are considered to be of the order of a 10m x 10m member supported at three or four points. A panel of this size weighs approximately 7 tonnes. ■ n -3 1- 22069 Roof panels constructed in accordance with the above thickness ranges should be supported safely with span distances of around 6 metres.

O

Claims (13)

WHAT WE CLAIM IS: 32 220693

1. A method of constructing a load bearing structural member which method comprises placing on a shaping surface a first thin layer of a oememtitious mortar containing steel reinforcing fibres; placing on said first layer before said first layer has cured a second layer of a cememtitious mortar slurry containing a low density particulate material; and, placing on said second layer before said first layer has cured a further thin layer of a ceiremtitious mortar containing steel reinforcing fibres, said further layer extending over the exposed surface of said second layer and curing or allowing said first layer, said second layer and said further . layer to cure to form a sandwich structure comprising integral outer skins chemically bonded to a low density core, said method including the further steps of incorporating within said core between opposing outer faces of said structural ireiriber post tensioning tendons extending between opposed ends of said structural member and stressing said tendons to distribute post-tensioning stresses substantially evenly throughout said outer skins.

2. A method as claimed in claim 1 including the steps of locating each said post-tensioning tendon in a conduit positioned in the core and introducing a cementitious grout into said conduit before stressing said tendon.

3. A method as claimed in claim 1 or claim 2 including incorporator^ said low density particulate material which carprises foam polystyrene in said slurry.

4. A method as claimed in any one of claims 1 to 3 wherein said oeirentitious mortar comprises a foamed cementitious mortar. ' _•

5. A method as claimed in any preceding claim including stressing said post-tensioning tendons to between substantially 60 In and substantially 120Kn.

6. A load bearing structural member whenever made in accordance with the method of any preceding claim, said -33- 220693 load bearing structural member comprising a core containing low density particulate material in a cementitious mortar, sandwiched between thin outer skins of a steel fibre reinforced cementitious mortar on at least two opposing faces of said core, said structural member characterized in the provision of post-tensioned tendons extending between opposed ends of said structural member, said post-tensioned tendons being positioned within the core of the structural member between opposing faces thereof to distribute post-tensioning stresses substantially evenly throughout said outer skins.

7. A structural member as claimed in claim 6 wherein the density of said core is substantially within the range of frorn 250 Kq/m^ to 800 Kq/m^.

8. A structural member as claimed in claim 6 or claim 7 wherein the density of the core is substantially within the range of from 350 Kg/ir? to 450 Kg/m^-

9. A structural member as claimed in any one of claims 6 to 8 wherein the thickness of the outer skin is within the range of from 6mm to 30mm.

10. A structural member as claimed in any one of claims 6 to 9 wherein the thickness of the outer skin is within the range of from 10mm to 20mm.

11. A structural member as claimed in any one of claims 6 to 10 wherein said post-tensioning tendons are anchored in regions of increased skin thickness.

12. A structural member as claimed in any one of claims 6 to 11 substantially as hereinbefore described with reference to any of the accompanying drawings.

13. A method as claimed in any one of claims 1 to 5 when performed substantially as hereinbefore described with reference to any of the accompanying drawings.