CA1282611C - Structural members - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE:

A load bearing structural member such as a wall, floor or roof panel, 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 the core. One or more post-tensioned tendons extend bet-ween opposed ends of the structural member. These post-tensioned tendons are positioned within the structural member between the opposing faces thereof to distribute post-tensioning stresses substantially evenly throughout the outer skin.

Description

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This invention i~ eo~cerrled with improvemen~s in struc~ural members and in particular to load hearing s~ructural mem~ers such as wall, ~loor and roof panel~.
Of recent years, there has been a trend towards the cons~ruction of buildings from prefabricated components.
There are ~ertain cost advantages in the prefabric~tion of structur~l panels at a remote site ~nd then transporting these to a buil~in~ site for rapid asse~ly and erec~ion.
In the h~ndllng ~nd transportation of such prefabricating components, bo~h weight and dimensions ~re the most importan~ and ~ten li~iting ~ctor~.

It is known to f~hri~ate buil~ing panels from aerated fo~med con~rete in an endeavour ~o save weight. It is also known to manufacture foam cored panelY for the ~ame reason. Foam cored panels may comprise a sheet of foamed plastics materi~l such as polystyrene foam. The slab of foam is encapsulated within ~ concrete struc~ural pa~el. I~

the panel ha~ no struc~ural requirements, the con~rete "skins" on ei~her side oX the panel m~ he unreinforced or 2~ include only a li~ht reinforcing mesh. On the other hand, load bea~ panels of this type may include one or more layer~ of a substantial reinforcin~ mesh or even internal stiffening ribs and pre-stressin~ tendons where a high flexural s~ren~th is required~
~5 In general, the use of prefabricated structur~l panels of the type referred ~o a~o~e is limited to non-load bearing wall panel~ as there ~re ~erious diffi~ulties 1 ILLE~ H~lLFi-lF c:~ !ELL p~

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a~sociate~ with their use as roofing or El~vring pa~els.

For a panel compxisiny a sheet or slab core o poly~tyrene foam, the Young~ Modulus of the core i~ mu~h greater than the concrete on the outside.. Thus, under fleY.ural load, the concrete skin will have pas~ed its elastic limit and failed hefore any contribu~io~ is made by the foam core. From a design considexation, panels of this type must therefore include ribs to provide adequate streng~h. In effec~, internally (or externally) ribbed p~nels would have to ~e individually designed for each construction application ~nd all point load ~ituations compens~ted for.
Panels of the above type are gener~lly difflcult to construct a~ it i~ almost impossible to accurately position and main~ain a foam core insert when dealin~ with we-t concrete slurries.
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Other proble~s includ~:-1, Water resistanCe~
, O Any imperfection in the outer skin of a roof panel will ~llow ~ater to enter the c~vity occupied by the foam insert. The trapped w~ter can then find an imperfec~ion in the inner skin ~nd enter the interior of a building.
2. Fire resistance.
The fire rAting of polystyrene foam cored panel wi~h a thin concrete skin is quite poor.

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3.Visual effects.
When the panel is damp, the position of the foam cores between rihs, ends, etc~ becomes clearly visible like a patchwork effe~t. Pif~ering insulating properties over the surfa~e of the panel yi~S rise to ~onden~ation "p~tterns" ~nde~ certain çonditions .

4. Accesæ.
In chasing a power or other service conduit through such prior a~t 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, met~l or timber skins or combinations thereof but in the ~ain these ~r~ u~ed a~
lS non~load bearin~ st~uctural members due to thei~ inherent weaknesses .
In all caQes, solid plastlçs fo~m cores çontri~ute little if any physical properties to ~u~h stru~tu~al panel~
due to lack of inhe~ent physical properties and the lack of any significant bond with cementitio~s materials. In the main, the co~e is provided only as ~ physical mean~ for separation o~ the skins during m~nufacture o~ ~o~ forming voids therebetween.
Various proposals for structural panels have been made in respect of concrete skinned, low density çored panels. Those incorporating a slah or çore of foamed ~ . , L L t. 1 1 ~ H L r l_~ IT. L ' I ' I H, ' . ~ L L

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plastics materials have been found to ~e quite un~uita~le for the reasons ou~lined above.
other proposals have co~templated the use o conventional concrete ~kin and a core comprising a cementitious slurry incorporating foamed polystyrene ~eads.
However none of the prior art proposals have re~lly addressed the prohlems normally inherent in su~h panel structures intended for us~ as load ~earing members.
In u~e of a conventio~al concrete skin comprising 0 cament, aggregate, sand and wa~er, one could expect flexural streng~hs of around 2MPa with conventional concrete characteri~tics of high compressive strength combined with low tensile ~nd flexural properties. In contras~ the present invention contemplates a steel fibre reinforsed cementitio~s skin having not only hi~h compressive and ten~ile ~trengthc but also flexural streng~h properties of the order o~ ~MPa in ~ non pLe-stressed condition.
While the incorporation of .~oamed polystyrene beads in cemen~ sl~rry mixes ha~ been propos~d as a core material, 2d ~he practical problem~ of mixing a xelatively low density materi~l into a vi~ous medium of rela~ively high density have not really been addressed. A~ the present ti~e, there are apparently no commer~ially available polystyrene foam/cement slurry cored structur~l panels in commercial usa~e and it is believed that th~ ~eason for this is ~hat the practical proble~s of evenly mixing foam ~eads into a cement slurry have not been overco~e on a commercial scale~
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If a ce~ent ~or concre~e) slurry is too fluid (too "wet`'), the foam, be~ds tend to remain near the ~urfctce duri~g mixin~ or after mixing the head~ rapidly migrat~ to ~he surface of the mix prior to removal fro~ the mixer or durin~ the po~ring or Gasting step. This leads ~o a laminar w~kne~s in the r~sultant panel in the region of greater bead concentration. When the mix i~ poured rom a mixer : such as a conventional concrete mixer, the initial portion of the pour ha~ an excessively high bead concentration while the latteL part eomprise~ ~ubstantially entirely a cementitious slurry. Having poured the initial polystyrene bead ri~h mix into a mould or the like, the latter portion, comprisin~ a substantially bead free slurry, when poured over the top o the initial layer migra~es ~o ~he ~ot~om ~f the mould leaving the upper l~yer ri~h in polysty~ene beads and consequently lacking in phy~ical str~ngth.
Alternatlvely, if the mi~ is too visc~us ~too - ndry"~ better homo~enization is achieved at the expense of the physical properties o~ the cementitious binder and lac~
` ~0 of work~bility of the core mix. , : French Patent ~o. 1 491 650 di~closes a thin w~terproof panel str~c~r~ suitable as a lining for hathroom walls, shower cu~icle~ ~nd the like. The panel compr,~es a liyhtweight core of Portland cement ~dmixed with perlite, ; 25 vermiculite or crushed porous blast furnace slag in the ~:~ preferred ratio of 1:2 to 1: 4 . The density of such core materi~l lies in the range of ~00 - 1800Kg/m3.
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reinforced with woven fibreglass clo~h.
The main requirements of such a panel are th~t they are light in weight yet strong enough to enable handling by a single workman. Accordin~ly these panels are generally limited to a thickness of say 12mm to 25mm and measurP 600mm x 900mm. ~he ou~er skins are necessarily very thin (i.e. of the order of 3-4mm~ to fa~ilitate fixing of the panels onto a support structure by nailing and further to permit the panels to ~e re~dily sawn.
The panels described in Fre~çh Patent No. 1 491 6S0 are inherently unsui~able as load bearin~ structu~es such as walls and suspende~ ~loors and ~oos.
In Aust~lian patent specifications 507249 and 5~01~7 there are described load bearing ~all panels comprisin~ a n~n load bearin~ lightweight core materials sandwiched between layers of fibrous concre~e reinforced with s~eel fibres.
The core material comprises ~labs of cellular ~0 plastics materi~l such a~ foame~ rigid P.V,~., foamed riqid polyu~ethane, fo~med rigid polystyrene or the slabs may comprise polystyrene bu~bles set in a cement mort~r or ~igid plastics foam matrix.
Aust~alian patent specific~tion 50724g suggest~
tha~ the panels may be stiffened hy ex~ernal web~ or int~rnal stiffene~ such as ti~ber s~rips .

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In cvntra~t, Austr~liAn patent ~pecification $~177 requires tha~ the outer skin lAyers are interconnect~d ~y spaced we~s ~o provide resistance ~o shear forces exerted between ~he layers when the panels are subjected to flexural stress loads. The engineerin~ principle underlying the wall panel structure of Aus~ralian patent specific~tlon S20177 is said to reside in the ~act that the ~wo skins share the compressive load end are separ~ted ~o achieve ~ hi~h modulus of section to resist buckling, the we~s interconnecting the skins serving to ansure th~t the two skins act as a single st~uctural mem~ex.
Patent specification 5Z~177 further suggegts tha~
if re~uired, po~t-tensioning cables may b~ located in conduits in the webs interconnecti~g the outer skins. In the wall p~nels ~he ~en-~ionin~ cables extend between the roof plate and the foundations of the b~ilding incorporating the panels and presumably serve ~o tie the finished buildin~
~tructure together and thus only mAke ~ ~ontribution to the ~ st~eng~h of the pAnels after a building i5 constructed.
: 20 None of the prior art ligh~eight cored ooncrete panels have been propo~ed for use in ~uspended E]oor and roof constructions as conventional wisdom in this art : actually te~ches away from their use ~s lo~d hearing structural memhers, It is known to uæe as suspended floor and roof :`

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1 1. ~14 ~ C:ULLE~ HFlLF~ l!ELL F l - 8 - 1~8 panels pre-stressed concrete beams having a plurality of lightwei~ht eore members of exp~nded poly~tyrene. These ~loor members compri~e thi~k con~rete skins separated by thick closely spaced concrete webs, the we~s including post-tensioning tendons. FOamed polystyrene blocks are placedbetween adjacent webs during manufacture ~imply as void ~ormers to reduce the weigh~ of the res~ltant panel. Such structural panels may therefore be regarded as a plurality i of pre-stressed "I" be~ms pl~ced side by side.
It i~ an aim oi th~ present invention to provide a load bearing struc~ural member o~ the ~ype having a low density core mat~rial sandwiched between ceme~titious outer ~kins, which structural me~ber embodies a pxinciple of construction enabling it~ use in walls and suspended floors ! 1S ~nd ceilings.
j It is a ~urther ~i m of the invention to overc~me or alleviate the problems of prior ar~ low d~nsity cored p~nels and to provide a more economical panel having substantially improved load bearing charact~-ristics when compared with prior art panels of similar mas~ ~nd/or dimen~ions.
According to the present invention there is provided a structural member comprising:-A core of low density p~rticulate material in acementi~io~s m~trix sand~iched ~etween thin outer skins of a ~teel fibre reinforced cementi~-ious material on at least two opposing faces of said core, s~id structural member ; ~har~çterised in the provision of one or 1., '.

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~L2~2~1 more post-tensioned tendons extending between opposed ends of said structural member, s~id post-tensioned tendons being positioned within the structural mem~er hetween opposing faces thereof to di~tribute post-tensioning stresses 5 sub~tantially evenly throughout said outer skinS~

Suitably the low density particulate material comprises an inorganic mAterial such as perlite, vermiculit2 or cellular blast .~urnace slag or it may comprise an organic m~terial such as cellular plastic~.
Pre~erably the low density particul~te m~terial comprises foamed polystyrene be~ds and most preferably the ~oamed polystyrene beads ~re fragmented to form particles o~
uneven shape and slze.
The cementitio~s matrix ~ui~ably compri~es a sand/cement mortar and prefer~bly the mortar includes ~ntrained air ~o ~orm a cellular matrix.
Sui~ably the densi~y of th~ core is in the r~nge of from abo~t 250 Kg/m3 to 800 K~m3, preferably of from about 350 ~/m~ to 450 Kg/m3 .
The thick~e~s of the ou~er s~in of the structural members ~ay be in the r~e of from abbut 6mm to 30mm bu~
preferably in the range of f~om about lOmm to 20m~. The pos~-tensioning tendons may be positioned directly within the core material and c~st therewithin. Alternatively the post-tensioning tendons may be positioned in conduit~ cas~

within the interior of the str~ctural members, `:

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According to another aspeck of t~e invention there is provided a method of ~onstructing ~ structural member which method comprises:-placin~ on ~ shaping surface a first layer of cementitious mortar containing steel reinforcing fibres;
placing on sai~ first layer before said first l~yer has cured a second layar of a cementitious mortar slurry containing a low density par~iculate material; and, placing on said seco~d layer before said second layer has ~u~ed a further layer of ~ cementitio~s mortar containing st~el reinfo~cing fibres, said further layer extending over ~he exposed surface of said second layer and allowi~g said ~ir~t 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 incorporatin~ within ~he stru~ur~l ~em~er between opposin~
~aces thereof post-tensioned tendons ~xtendin~ between opposed ends of said structural member to distribute pos~-tensloning .s~resses suhstantially evenly throu~hout said outer skin.
~` As used h~reina~ter the expression "sand" means particulate siliciouæ mate~ial such as an alumino-silica~e e.g. river or beach sand. The sand comprises a mix~ure of 2S p~r~icle sizes ~rom a m~ximum of say 3 mesh (Tyler) down to a fine dust e~gb 200 mesh ~Tyler) or l~s~, When use~ in cement mortars the sand gives the effect of ~ miniature mixed aggregate similar to conventional concrete mix~s b~t .

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on a substantially smaller scale.

In order that the invention m~y be l~ore fully understood, re~erence may be made to the ~ccompanying drawinqs in which:-FIGS 1-3 illustrate a preferred method of constructing a st~uctural panel in accordance with the invention, FIG 4 is a plan view o a typical multi purpose ~tructural panel.
~ IG 5 i~ a cross-sectional view along A-A in FIG 4.
FIG 6 is a plan view of an alternativç form of pane 1 .
FIG 7 is a cross section along ~-~ in FIG 6.
FIG 8 is a c~o~s section along A-A in FIG 6.
FIG~ 9, 9a show alternate cross-sectional views along A-A in FIG b.
FIGS 10 and 11 show altern~tive cross~sec~ional vi~ws of the structure of FI~ 9.

FIGS 12-14 show alternative em~odiments of the inVention.

FIGS 15 and 1~ show respectively graphl~lly compara~ive stre~s analyses ~or a prior art cored panel and a panel accordin~ly.

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FIGS 1-3 illustrate a preferred panel construction me~hod wi~h referen~e to the constxuc~ion of a simple rectangular p~nel in a female mould ~lthough it will be Clear to ~ skilled addressee that more complicated two or three ~imensional ~hapes may be constr~cted in either male or ~emale moulds.
In ~IG. 1, the bottom or base l~yer 1 is formed on ~ny suitable surface such a~ a sheet plastics, s~eel or concrete surface la or the like. This surface m~y be smooth or decoratively textuLed ~nd, if required, coated with a mould release agent to facilitate rele~se of the finished panel from the mould surface. The mould frame suitably comprises length~ of angle se~tion s~eel, aluminium or plas~i~s material ~ or the like movably Qecured to the mould surface 1 to define the perimeter of the ~ould and/or to de~ine ap~rtures such ~s window apertures in a structural wall. The inwardly directed faces of the angle irons ~ a~e lined with spacer~ 3 of timber, fo~m plastics or the like to form a pxedetermined sp~e of width x around the perimeter of the ~ould, the purpose of whichl sp~cers will be ~escribed later~
A cementitiou~ skin mix is then prepared in a sui~a~le mixer according to the ratio;
` cement : - ~OOk~
water ~ 0 litres ~and : - 1500kg ~ ~eel fib~es : 160k~

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The cement is preferably grey Portland cement ~nd the ~teel fibres are preferably enlarged end rectangular end ~ fibres made by ~.W.I. Fik,resteel having dimen~ions 14mm long ; x 0.4mm thick x O.6mm wide. lf req~ired, a quantity of ~, conventional plastici2ing agent may be employed to improve workability o~ the mix.
The skin material 4 is pou~ed into the mould to a required skin thickness and levelled, at least roughly k,y trowelling, screeding or the like and spacers 3 inserted in appropriate positions against the mould frame 2.
A core mix is then prepared as follows accordin~ to the ratio:
water : - 200 litres . cemen~ : - 400kg - 15 foamed polystyrene beads : - lm3 Agai.n the cement is preferably grey Portland ~ement and the foamed polystyrene beads have a diamete~ range of ~` from 3-6mm with ~ density of approximately 15kg/m3 .
: The water and cement are ~dded to a mixe~ such as a ~; 20 ~oncrete mixer to form an homogenous slurry. Once ~he `~ ,slur~y is fo~med, the beads are then added and mixed thoroughly to form an homogenous slurryJ~ead mix. The viscosi~y of ~he slurry is ~uch that the tendency of th~
beads to migrate to the sur~ace during mixLn~ i~ e~fect-vely negated.
~ B~ore pouri~g the core mix plastiG conduits 4 are `` located within the confines of the mc,uld ~y slc,ts or ~` :
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~pertures in end blocks 3. ~he l~eral spacing of the condui~s ~nd their distance from ~he upper or lower skin is predetermined according to the required lo~d bearing characteri~tics of the panels.
~he core mix is tnen poured into the mould to form a second layer 5 o~ top of ~he first "wet" or uncured layer ;~ 4. The sur~ace of the cemen~/bead mix S is levelled in a m~nner similar to the first layer 4~ The thixotropic nature o~ the cementitious mix 5 is such that there is no t~ndency for the beads to migrate to the ~rface of ~he second layer after pouring and lev~lling.
When the second l~yer has stiffened slightly or at leas~ p~rtially cured, spacers 3 are remov~d a6 shown in FIG. 3 and if required, exposed face ~ of ~irst layer 4 is lS roughened by a suitabl~ roughening tool. A 1nal layer B
substanti~lly similar to that o~ layer 4 i5 ~hen poured over the top of layer S and allowed to ~low down the cavity between the edges of layers 4 and 5 and th~ mo~ld wall formed by angle me~ber 2. If required liftin~ eyes may be cast into the upper ~urface of th~ panel to f~ilitate latPr handling. In this manner, the resultant panel is formed wi~h a peripher~l edge skin 9 integral with ~nd similar to upper skin 8 and lower kin 4 respectively. The uppermost surface o~ skin 8 is ~creeded, trowelled or otherwi~e levell~d and ~inished as desired.
In a typical structural panel, say a wall paDel for ~`

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a domestic dwelling, the p~nel may measure 2.5m~.- Y. 4m Y.
lOOmm thi~k. The panel may compri~e an 80mm core wi-th lOmm skins on either ~ide.
In a modified ~orm o~ the invention, the cemen~itious slurry for the core may include ~ foaming or aerating agent to f~rther reduce the density of the composite core material. Such a slurry composition may comprise ~he slurry mix as descri~ed above with the addition of 2 litres of ~RO. B (Trade Mark) - an organic foaming agent manufactured by Sika.
The w~ter is placed in a conventional concrete mixer with the FRO.~ foaming agent and with the mixer rotating, ~he a~ueous mix is aerated by the introduction of compressed ~ir at the rate of ~bout lOcu.ft./min. for abo~t `;~ 15 3min. Two thirds o~ the cement is then added and durin~ the mixing cycle, aeration is conducted for ~ further 3mins.
The Lemaining portion of the cement is then added and when mix~d thorou~hly, the foamed polystyrene beads are added.
`~ Th~ resul~ant mix has a "wet" density of approximately 480K~/m3 and when cured a density of approximately 450Kg/m3.
In practice, ~ore densities of between ~50Kg/m and 800~g/m~
will be found to be e ~ective depen~ing on the cost ~nd engineering considerations of the structure~ Foamed concrete slurries have the advantag~ of improved thixotropi~
properties which further reduces bead mi~xation.
It will ~e noted that the viscosi~y of the core `

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slurxy is quite importank to the effsctive practice o~ ~he inv~ntion. If the mix is ~oo dry ~hen additio~ of further water will not give the re~uired viscosity and e~u~lly i~
the mix is too wet, addition of f~rther cement results in the formation of cement "balls". Unlike conventional a~regate containing concrete mixes, the shear durin~ mixing of a fo~me~ polystyrene containing cement slurry is insuff icient to adequately mix water or cemen~ added during : the latte~ part of the mixing cycle, As little as eight hours after the panel casting opera~ion is comple~ed, the mould edge~ mAy be removed and the panel ~tripped from the moulding surface 1. The p~nels may then be sta~ked for ~omplete curing prior to - transportation and usage.
The s~ructural pa~el 10 of FIGS 4 and 5 measures 5m x 3m and is lOOmm thic~. The outer skin 11 comprises a steel fibre rein~orced cement mortar and the skin thickness is 12mm on all aces. The compositions of the cementitious skin and the core were th~ same as those employed in the embodiment illustrated in FIÇS 1-3 and ~he panel was ' constructed su~stan~ially in ~ccordance with the `~ : aforemen~ione~ method steps~

In the oppos~d ends 12 of the panel 10 are inwardly extending an~hoL supports 13 ~omprised of the same ~aterial as the outer skin 11. These anchor supports 13 locate and ~uppo~t the po~t-tensioning anchor8 14 atta~hed to opposed . ~ ~
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` '1' ; ~ 1 1 ; l t~ I L L t. 11 ~I H L I~ L~ l l H ~ ~ L L _ ~ r 1 __ _ ___ ends of post-tensi~ned cables 15 located within condui~s 16 Within the core 17 of the panel 10.
Before ten ioning the cables 15, a pumpable cementi~io~s ~routing composikion 18 is pumped into the conduit~ 16.
The cables lS are tensioned ~o a value of lOOK~n and then are anchored by conventional p~st-ten~ioning anchor~ in ~he form of wedgable ~ollets or th~ like.
The phantom outlines shown over the surface of the panel 10 represent the zor~e3 of stress induced in the outer ~: skin of the panel. As the zones overlap the tensile stress in ~he po t-ten~ioning ~a~les is dis~ipated as a compressive ~` stre~ over su~stantialLy the enti~e skin ~urface of the panel 10~ .
This is in direct con~rast to cored panels having post-tensionin~ ~endon~ located within the structural webs tyin~ the outer skins together~ In ~uch a eonfiguration the ten~i}e stress in the p~st-tensioning cable is di~sipated as a compressive stress sub~tantially only within the weh. ~he ~0 outer skins o~ such panels are su~stantially unstressed ~y j ~he post-tensioning tendons and ~or this reason they may ~e regard~d as a plurality o~ adjoining pre,-stresssd concrete `. , "I" be~ms having entirely predict,able propertie~.
Under deflection lo~ds the shear forces ~etween the 2~ skins is grea~est towards the end~ 4f a ~ored panel and thu~
for prior art "adjoining I-beam" panels the interconnecting ~`

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; ~ we~s play a crucial role in binding the oUter skins ~og~ther as a unitary member. ~or this reason the webs are generally relatively thick ~hen compared ~i~h say a steel "I" ~eam sec~ionO In addition ~he concr~t~ web~ mus~ have a sufficient thi~kne~ to permit location of a post-tensioning cable in a conduit and to resist the compressi~ load indu~ed by the tensile ~tr~ss in the post-tensioning cahle.
FI~S 6-~ show an alternative form of structural panel suitabl~ for use in suspended high load flooring applica~ions and/or for lon~ span flooring FIG 6 shows a plan view of a ~loo~ing panel havin~ a length of 6 metr~s, a wi~th o~ 2 metres and a thick~ess of lSOmm. The upper skin is 20mm thick and the edge and ~otto~ skins are lSmm thick.
The skin and core co~positions are as pLeViOUSly described.
For high load applications the po&t-tensioniny - cables 15' are anchored at opposed ends of panel 10' midway between the upper skin 30' and lower ~kin 31', i~e. 75mm from both upper ~nd lower skins 30' and 31l respectively.
At the mid-span poYition 321 the post-tensioning cable 15' is located 50mm from lower skin ~l' and lOOmm from the upper skin ~0l.
FIG 9 shows in ~ross sections an altern~'ive embodiment of the s~ructure of FI~S G-~. Where a floor panel i~ likely to be suhje~ted tu very high point loads . 25 such a~ may be expected from a heavily laden trolley with i small diameter wheel~ or upon impact by sharp heavy objects, it may be desirable to provide additional support for the '` !
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~826 ~ upper ~kin. This additlonal support may be in th~ form of : thin webs having a thickness corre~ponding approximately t~
the skin thickness. As -these thin we~s do not contribute signifi~antly to ~he ul~imat,e mechanical propertieg of the floor panel, the webs 33 may extend parallel to the post-tensioned cables 34 or they may extend transversely of the ca~les 34. The only meaningful oont~ibution of such ¦ internal webs is to support point loads applied perpendicularly to the surface of upper skin 35. ~f re~uired the cables 34~ may be incorpora~ed within ~he webs ` ¦ 33a a~ ~hown in F~G ~a.
FI~ 10 ~hows a partial cross section of ~ pan~l wherein internal compression ribs 36 extend within inner . core 3? between upper skin 38 and lower skin 39 transversely of post-tensioning cable~ 40.
FIG 11 ~hows yet another embodiment of the arrangemen~ of FIG 10. To avoid the weight penalty imposed by ~he addi~i~nal in~ernal ribs 36 as shown in FIG 10~ st2el wire loops 41 m~y be arranged ~etween upper skin 42 ~nd lower s~in 43. Loops 41 a~e 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 ~ki~ 42 and 43 I respectively~
~` 2S The loops 41 may comprise individual loops placed randomly or in parallel row~ during formation of lower skin 43. Conv~niently, ~he loop~ are arrAnged on the -':`` I
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LLE~ HHLFIIFl[l~lH~ lllELL F':~4 - ~ ~ X6 post-ten~ioning conduit to assist in alignment in rows and the individual loops may be manually sp~ced. Preferably however loops 41 are ~ormed a~ a helical coil having a rectan~ular cross section. ~gain the helical CGil is 5 c~nveniently locAted over the post-tensioning conduit 46 an~
~uring formation of lower layer 43, the ~oil is ~tretched into place and lower arm~ 44 em~edded in~o lower layer ~3.
The helical coil may ~e arranged parallel to the post-tensionin~ cables or the ca~les ~ay ~e aligned perpendicularly as shown in phantom.
Alhough the foregoing discus~ion is limited to f lat panels for walls, floors and roofs, it will ~e clear to ~
skilled addressee that othe~ ~tructural members may be made in accordance with the invention~
FIG 12 shows ~ struct~ral beam 50 having a thin o~ter skin 51 of steel fibre reinforced concrete mortar and a core 5~ ~omprised of a particulate foam/mortar slurry as previously de~cribed. A post-tensioninq cable 53 is grouted in a conduit 54 extending down the centre of the core 52~
~o For hi~h load capacity ~eams a ~teel wire mesh 55 m~y be incorporated within the outer skin 51.
In further embodiments of the invention A
structural p~nel ~0 as shown in ~I~ 13 may include post-tensioning cahles 61 arranged perpendicula~ly to each other or as shown in FIG 14, a structural p~nel 70 may comprise a plurality of smaller panels 71 held ~o~ethe~ with post-tensioning cables 7~ extending through the cores o~ the panels 71.

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- 21 ~

In order to appreciate the advantages of the presen~ invention over traditional cementi~ious load bearing panels, comparisons may ~e made first wlth a steel reinforced concre~e flooring slab and then with a webbed S concrete floorin~ panel having polystyrene cores.
A~ Steel Reinfo~ed Solid Concrete ~ omp~rison may be made hetwe~n ~ flooring panel as illustrated in FIGS 6-8 and a steel reinforced solid concrete floorin~ panel having ~he s~me dimensi~ns i.e. 6 met~es long, 2 metres wide and 150~m thick.
~ or the solid concrete pan~l to possess the same mechanical properties steel reinforcing i5 required in the form of 12mm bars at 90mm centres extending longitudinally of the panel ~nd 12~m bars at 300mm centres ex~ending transve~sely of the panel adjacent the lower face. Adjacent the upper fa~e i~ a welded steel mesh comprisiny 6mm bars at 200mm cent~es in both di~ection~.
The panel according to the invention weighs 0.14 tonneQ and cost~ approxim~tely A$50.00/m~ to pxoduce.
~0 The steel reinforced solid concrete panel weighs 0.42 tonnes ~nd costs approximately A$66.00~m to produ~e.
While the panels m~y ~e ~onsidered as mechanically equivalent in terms of ~ap~bility of handling and transportation and physical strength the panels according to the i nvention ~re one thirc~ o~ the wei~ht of solid concrete p~nels and th~ee quarters of their cost. In addition to the ; .
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ILLE~ H~LFl-lFl[:~ lllELL F"~

Z~l prima acie cost advantage, there are other signi~ic~nt hidden cost advantdges associated with ~he invention, The xeduced mass o~ the panels according to the invention permit reduced handling and transpor~tion costs and as well permit ~ substantial ~ost savings in building constructions ~n i reduced costs for support columns, footings and fcundations normally a~so~iated with the heavier solid concrete panels.
! ~. Foam Co~ed Structur~l_Panels FI~S 15 and 16 show compar~tive theore~ical 1~ analyses of ~he m~chanical properties of foam co~ed panels and panels according to the in~entio~ respectively.
For the purpose of comp~rison a section of foam cored conc~ete panel is taken to represent the hypothe~ical "I" beam referred to hereinbefo~e.
In FIG 15 the "~" beam struc~u~e 70 comprises two flan~es 71 and 7Z connected by a thick central we~ 73.
Gentrally of the web 73 an~ flanges 71,72 is loG~ted a post-tensioned ~able 74 ~tressed to lOOKn.
The flanges 71,72 ~easure lOOOmm in wid~h and 60mm in thickness. The centxal web 73 i3 lOOmm wide and separ~tes the flanges 71,72 by 80mm ~this space normally ~eing occupied by a foa~ed poly~ty~ene ~14ck which does not contrib~te mechanically to the propertie~ of the beam).
The ratio of flange thickness t ~o uns~pported 2S flange width i.e. the dis~ance from the edge 73~ of web 73 to the outer edges 71a,7~a of the fl~n~e sides is de~ermined I

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L. ~ ILLE~ HFlLFl~lF~[~ JlJJELL

23 ~X~2 by the Stand~rds As~ociation of Australi~ "Concrete S~ructures Code" ~o AS1480 relating to load bearing concrete 5tructural membexs which requires that the maximum outstand of a flange is 8t where t ~ thickness of fl~nge.
FIG 16 illustrates a similarly dimensioned cross section o~ a panel 7~ in accord~nce with the inven~ion. The surrounding skin thicknes~ is 12mm and the skin and core ~ompositions a~e as herein~efore des~ribed.
Locatqd centrally of panel 75 is a post-tensioned cable 76 s~re5sed to lOOK~, the ame ~s th~t in FIG 15.

As hitherto explained, the p~e-stress in panels according to the invention is suhstantially evenly distributed in the upper and lower skins whereas with webbed p~nel~ i t is believed that ~he pre-s~ress load distribu~ion is confined substantially to the area of the weh as shown in phant~m in FIG 15. For the purposes of comparison ~owever it is assumed that the pre-~t~ess in p~nel sectio~ 70 is distributed subqtantially evenly ~hrou~hout the upper and lower fl~nges 71,72 respectively.
~0 Fl~ lSa and 16A represent graphically the re~pective pre-~tre~ v~ s shown as compression in the flan~es 71,72 ~nd uppe~ and lower skins 77,7~. The stress values (repre~ented as negativel are respectively 1. 47MPA
and 3.73MPa which illustrates the highly st~essed nature of the thin skin surrounding the ~ore of pAnel section 75.
`~ FIGS 15b and 16b illu~trate gr~phically the bending ~ stresses under ~ 1. 5KPa live load plus the dead load in b~th .

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4. ~ !LLEN H~lLFI-lF'~ !ELL F~

- 2 4 ~L~8~6 cases. To further e~phasiæe the advan~ages of the invention the bending s~ress values are calculated for a span of ~OOOmm in the ~ase of the ~IG 15 structure and for a span of 4000mm for the FIG 16 skructuxe acçording ~o the inven~ion S The ~ending s~ress 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.~Pa while the tensile stress is 2.~6~Pa.
FI~S 15c and 16c show ~he resul~ant stres~es in the str~ctures under comparison. In FIG l5c the Stress is ; compressive in both t~p and bottom flanges ~consistent with conventional s~ructural conorete d~sign practice). The stress value~ are 2.29MPa for the top Elange ~nd 0.65MPa ~or the bo ttom f lan~e.
~IG 16c shows compressive s~resses oi 6~39~Pa and 1.07MPa in ~h~ top and bottom skins re~pectively.
Accordingly, despite ha~ing been subjected to a more severe bending stress over a larger span, the panel section aecordin~ to the inYenti~n demonstrates a substantially g~eater bending ~tress c~p~ility than ~onventional foam cored panels of si~il~r cross-sectional dimensions.
Conventional en~ineering wisdom dictates that the lower flanges 72 of "I" be~m 70 should never under load stress ~hieve a compressive stress value of more than zero, i.e, ~he flange~ should never ~o into tension due to the inherently poor tensile qu~ ies of concrete. In practice ~; , , ~ . .

LILLEt~ HWLFI~lF~ lliELL F'~
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- 24a - ~ ~8X~l~

however it could ~e expected that the lower flange could withstand a -tensil~ stress of ~bout 1 MPa before failure.
Comparison of FI~S 15c and 16c show that the highly stressed nature of ~he skins 77,78 leaves a residual compreSsion stress of 1. 07MPa, cubstantially gre~t~r than the beam of FIG 15 notwithstanding ~he gre~ter span over which the beam is stressed. In practice the be~m ~ccording to the invention may be stressed ~o a point ~here the lower skin 78 can withstand a tensile stress of ~a~ 4.~Pa due to the tensile properties eonferred by the steel fibres.
~I~ 16 also ~hows that ~ide or end flanges are not essen~ial to the working of the invention d~e to the excellent bond ~tren~th between the ~andwiched layers. In prac~ice however side and edge ~kins are pt-eferred to protect the edges of the highly s~ressed "working" skins i,e, the upper and lower skins~

While the sub~tantially improved mechanical properties of the present invention are considered to be derived from the ~0 highly stressed thin outer skin it is also considered that the nature of ~he c~re material and the excellent bond between the core material and the outer skin -'.

.ULLE~ HI~ILFI-IF..[.~ .l.l!ELL F'~

- 2s - 1F~82 plays a large part ln con~ributi~g to the mechanlcal properties of the structural members.
~or very thin skins subjected to the stres~es in the bending mode shown in FIG 16~, failure could readily occur i~ the skins were to undergo bucklingO The core mat~rial has sufficient compressive strength ~o with~tand inw~ard buckling of a skin while outward buckling is pre~ented by the ex~ellent bond ~etwee~ the core material and the skin. In addition, any tendency for shear to occur at the interface between the skins and the core ma~eri~l is also re~isted by the bond and the shear strength of the core material It will be clear to a skilled ~ddressee that many modifi~ation~ may be made to the invention. For example the co~e material may comprise any li~htwei~ht particulat~
material such as wood chips and the li~e. Furthe~ the i~vention is not limited to planar panel constructions as i~
~onventional laminating techniques utilizing pre-formed cores and skins. A particular advantage of the invention is that the st~uctural members may be ~ormed in a ~riety of planar or three-dimensional shape~ ~in male or female mo~lds) due to the a~ility to fortn "integral" mem~ers i~ a plastic state.
~he method of constructin~ panels or shaped members ~rom conc~ete sandwich construction has been generally described above. However, in constructio~ of large unitary member~ such as la-ge span roof p-nels or a ~omplete roof `'~ .

~, :
, - 3 C:LILLE~ HliLFu~ lELL F ~, l . . _ . _ .. _ . . _, _ _ . _ . _ . ._ - ~6 --structure or o~.herwise complex shaped o~jects not suita~le for female moulding, the present invention permits a male mould to be employed. As very large structures are not easily transportable, it is desirable where possible to manu~ac~ure the s~ructure on or at least adjacent a site where the stru~ture is to be used.

For a roof structure, ~ male mould is construct~d of s~y a timber frame and a plywood or like sheet mat~rial covering. A pl~stic~ membrane may be placed over the mould ~urface t~ act as a mould rel~ase agent o~herwise a ~onventional release agent may be employed.
In a manner similar to form~ion of the panels descrihed ~bove, the three layerQ of cementitious mater~al ar~ built up one ~pon the other until the desired thick.ness is achieved. The top or outer surface is then screeded and/o~ troweIled off to obtain a smooth ~inish.

During conqtruction o~ the roof ~tructure, : post-tensioning ca~le conduits are placed at apprOpriate positions between the upper and lower skins and lif~in~

bolts are located within the sandwich at desired po~ition.
If further relnforcin~ elements are re~uired such as rods or mesh the~e may he incoxporated in the sandwich struc~ure by say pla~ing the rod~ or ~esh on top of the first layer and then spraying th~ successive second and third layers thereover.
If required, conduits for electrical cables an~ the like may also b~ incorpor~ted in the panels or roof- -~: .
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structure durin~ construction.~
For cert~in structur~1 ~pplications where extra strength or rigidity i~ required to resist app1ic~ation of point loads on ~he skins, "ribs" may be formed by ~uilding up the thickne~s o~ portion~ o~ the ~irg~ layer of steel fibre reinforced concrete~
It is envisaged that the present invention has application to vi~ually all presently employed structurally reinforced members ~uch a~ 100rg, structural ~eam members including "I" be~ms, box beam3 and the like a3 well a5 pier , columns, foundations, etc. In ~ maner simil~r to that previously described, a fi~st layer of st2el fibre reinforced cementitious material is formed against a mould suLf~ce, the pre ~tr~3ing tendons are placed in position adjacent ~he s~rf~e of the first layer and additional steel fibre reinforced ~ementitiou~ mat~ri~l is sprayed over the tendong to ~u~ ntially encapsulate them.
A second layer of cementitious ma~erial containing low density particulate plastics material is then formed ~0 o~er ~he ~irst l~yer. ~f required, channel-like depressions may be formed in the second layer in the reyion where additional rein~orcing tendon~ are to be pl~ced. The channel-like depressions are then at least pa~tia~ly ~illed with a steel fibre reinforced cementitious matexial prior to 2S po~itioniny o~ ~he additional tendons. Altern~tively the p~e-stre~sing tendon~ may be incorporated in~o the co~e material directly or la~er placed in conduits cast into the Simila~ly, roo~ panel~ may ~uitably be selected from the ~ollowing range.

1 r - ~ ~ ULL~ HHL~ L'l~lH;~ LL r ~_~'t _ __ core material. A further layer of steel fibre ~einfor~ed cementitious material i5 then formed over the exposed surface to en~apsulate th~ t~ndons.
By using steel fibre reinforced skins in pre-st~essed structural members according to the invention it is possi~le to place the tendons a~ considerably gre~ter spacin~ than would otherwise be required. When point loads are applied to the "skin" ~he for~es are dissip~ted for s~e consiaera~le distance throu~h the fi~re rei~forced skin.
Wall panels may be constructed as follows for most appli~a~ions~
Skin 1 - 6-20mm thickness Core - 25-~Om~ thickness Skin 2 - 6-20m~ thickne.ss Wall panels con~tructed with two 1 Omm thi~k skins and a SOmm Core have been found to have a significantly ; ~reater load bearin~ capacity than an equivalent llOmm thick ~lay brick wall.
It is believed that panels up to 5m X lOm wi~h a lOmmJ50mm/lOmm thickness and two lifting points can be easily handled~ Such a panel has an estimated weight of 3 tonnes compared with an estimated wei~ht of around 9 tonne~
for an equivalen~ clay brick panel~
Similarly, roof panels may suitably be selected irom the ~ollowing range.

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' r~ L L~ HHL ~ ' L;' l~lH;~ L L

2g ~ 3 - Skin 1 - 8-20mm thickness Core - 50-lOO~n thi~kness Skin 2 - ~20mm thickness Practical size limitation~ for handling a roof panel are considered ~o be of the order of a lOm x lOm member supported at three or four poillts. A panel of this si2e wei~hs approximately 7 tonnes.
Roof panels constructed in accordan~e with the above thick~es~ ranges should be supported safely with span distance~ of aro~nd 6 metres.

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Claims (14)

1. A method of constructing a load bearing structural member which method comprises:
placing on a shaping surface a first thin layer of a cementitous mortar containing steel reinforcing fibres;
placing on said first layer before said first layer has cured a second layer of a cementitous mortar slurry containing a low density particulate materia; and, placing on said second layer before said first layer has cured a further thin layer of a cementitous 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 chemically bonded to a low density core, said method including the further step of incorporating between opposing faces thereof post-tensioned tendons extending between opposed ends of said structural member to distribute post-tensioning stresses substantially evenly throughout said outer skins.

2. A method as claimed in claim 1, wherein said post-tensioning tendon is located in a conduit and before post-tensioning said tendon a cementitous grout is introduced into said conduit.

3. A method as claimed in claim 1, wherein said low density particulate material comprises foam polystyrene.

4. A method as claimed in claim 1 or claim 3, wherein said cementitous matrix comprises a foamed cementitous mortar.

5. A method as claimed in claim 1, wherein said post-tensioning tendons are tensioned to between about 60Kn and about 120Kn.

6. A load bearing member whenever made in accordance with the method of claim 1, said loading bearing structural member comprising a core of low density particulate material in a cementitous matrix, sandwiched between thin outer skins of a steel fibre reinforced cementitous material on at least two opposing faces of said core, said structural member characterized in the provision of one or more post-tensioned tendons extending between opposed ends of said structural member, said post-tensioned tendons being positioned with the structural member between opposing faces thereof to distribute post-tensioning stresses substantially evenly throughout said outer skin.

7. A structural member as claimed in claim 6, wherein the density of said core is in the range of from about 250 Kg/m3 to about 800 Kg/m3.

8. A structural member as claimed in claim 6, wherein the density of the core is in the range of from about 350 Kg/m3 to about 450 Kg/m3.

9. A structural member as claimed in claim 6, wherein the thickness of the outer skin is in the range of from about 6mm to about 30mm.

10. A structural member as claimed in claim 6, wherein the thickness of the outer skin is in the range of from about 10mm to about 20mm.

11. A structural member as claimed in claim 6, wherein said post-tensioned tendons are anchored in regions of increased skin thickness.

12. A structural member as claimed in claim 6, wherein said low density particulate material comprises foam polystyrene.

13. A structural member as claimed in claim 6, or claim 12, wherein said cementitous matrix comprises a foamed cementitous mortar.

14. A structural member as claimed in claim 6, wherein one or more support members is located within said core between opposed skin faces.