WO2008037001A1

WO2008037001A1 - Improvements in sandwich panels

Info

Publication number: WO2008037001A1
Application number: PCT/AU2007/001411
Authority: WO
Inventors: Jose Eli Hernandez
Original assignee: Building Technologies Australia Pty Ltd
Priority date: 2006-09-25
Filing date: 2007-09-24
Publication date: 2008-04-03
Also published as: PE20081075A1; AR062975A1; UY30609A1; CL2007002741A1

Abstract

A composite structural sandwich panel comprises spaced facing sheets of fibre reinforced cement or plywood and a lightweight cementitious core formed from a slurry of cement powder; water; a low density aggregate such as expanded plastics, exfoliated minerals such as perlite and vermiculite, ceramic or polymeric micro-spheres; and optional ingerdients including fly ash and lime. A modified carboxymethyl cellulose composition is added to the slurry to retain water and reduce localized migration of water into porous facing sheets at the interlace between the facing sheets and the core.

Description

IMPROVEMENTS IN SANDWICH PANELS

Field of the Invention

This invention is concerned with improvements in structural sandwich panels having a lightweight core. The invention is concerned particularly although not exclusively with structural sandwich panels having an improved bond strength between the core and the outer facing sheets and a process for the manufacture of such panels.

Background

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

The manufacture of structural sandwich panels with a core of lightweight material sandwiched between outer facing sheets is well known.

As early as 1920, United States Patent No 1333553 described a composite panel having a core of building plaster, saw dust, extract of cactus juice and heavy black molasses sandwiched between sheets of pasteboard. Reinforcement of the panel was achieved by incorporating into the core material longitudinally extending timber strips. British Patent No GB 1528816 describes a lightweight structural beam comprising a low density core, such as fibre-reinforced aerated concrete or bonded perlite or vermiculite, with corrugated metal reinforcing members extending longitudinally along opposed side edges of the core.

United States Patent No 4351670 describes a cast building panel having a low density core comprising a cement matrix incorporating fragments of cured cellular concrete. The low density core material is described as being non-shrinking and possessing both high strength and favourable insulation qualities.

United States Patents 4505449, 4303722, 4559263, 4617219, 4778718, 4894270, 4963408, 4050659, 4436274, 4876151 and 5209968 all describe composite structures having a low density core with fibre reinforced skins extending over opposite faces thereof. United States Patent 5387626 describes an additive combination for aqueous mineral building materials such as plaster, mortars, adhesive cements and fillers to improve the workability attributes for use with spray plastering machines and as cement based adhesives. Such attributes include structural stability, levelling, straightening, slurrying and smoothing. The compositions comprised mixtures of water-soluble cellulose ethers, polyacrylamides alkali-nietal or ammonium salts of cross-linked ungrafted or starch grafted polyacrylates, starch ethers and condensation products based on arylsulfonic acids and formaldehyde in the form of alkali metal or ammonium salts thereof. International Publication WO 02/42064 describes a reinforced lightweight dimensionally stable panel having outer skins of plywood or oriented strand board and a core of calcium sulfate alpha hemihydrate, hydraulic cement, an active pozzolan, lime and a reinforcing of glass fibres and ceramic or polymeric micro-spheres.

Australian Patent No 638567 describes a building panel having outer skins of fibre-reinforced cement, fibre-reinforced gypsum plaster or gypsum plaster sheets faced with paper or plywood and a lightweight inner core of foamed concrete, with or without a lightweight aggregate such as polystyrene beads, etc. The panels are reinforced by attaching metal strips to the inner faces of the outer skins by adhesive.

While generally suited for their respective intended purposes, a major problem associated with prior art sandwich or composite panels is the ability to obtain a high strength bond between the core material, particularly a relatively low strength lightweight core material, and the outer skins. When used as structural load bearing panels, the inner core effectively does no work other than to act as a spacer for the outer skins which have flexural, compressive and tensile strength properties appropriate for a chosen application. If, when a load is applied to such a structural panel, one or both of the outer skins begins to delaminate from the core, a fairly catastrophic failure of the panel under load is a common occurrence.

Australian Patent No 752467 sought to address the problem of limited bond strength between a lightweight cementitious core and fibre-reinforced cement sheets in a structural panel. In the process described, the panel is located in a mould or press while powdered aluminium reacted with a lime-containing cement slurry to produce fine bubbles of hydrogen gas. As the gas bubbles were generated, the internal pressure generated in the core within the closed mould forced cement particles into intimate contact with the porous fibre reinforced cement facing sheets to improve the bond strength between the facing sheets and the ultimately cured core.

Hitherto, the use of porous or water absorbent facing panels such as fibre- reinforced cement or plywood has imposed a limitation on the ultimate strength of the panel due to the tendency of water in the cementitious mix to migrate into the water permeable facing sheets whereby at the core/facing sheet interlace, cement particles are deprived of water otherwise required for hydration. The poorly hydrated cement particles were quickly immobilized in the layer of slurry adjacent the surface of the facing sheet and in the absence of complete and timely hydration of the cement particles, the physical/chemical bond between the core and the facing sheets was compromised, thus diminishing the strength of the panel.

Australian Patent No 638567 sought to address this problem by effectively increasing the surface area of the facing sheets by forming grooves therein, while Australian Patent No 752467 sought to create a more intimate contact of cement particles with the facing sheet interface by internally pressurizing the cementitious slurry with a chemically generated gas.

Another shortcoming associated with prior art structural panels having lightweight cores arises from the use of process additives in the cement slurry. Air entrainment agents, typically fatty acid sulphonate surfactants, are added to the slurry during the mixing step at a rate of 50-150 ml per 100 Kg of cement powder in a special mixer designed to fold air into the slurry. Entrained air bubbles, apart from reducing the density of the cementitious matrix, improves the workability of the liquid slurry.

In seeking to maximize the strength of the cement matrix of the lightweight core to compensate for entrained air and low density aggregate fillers, it is important to use a water : cement ratio as close to that theoretically required to hydrate the cement particles fully, but without surplus water which can reduce the strength of the cementitious mix. The ideal water : cement ratio is about 36 parts by weight of water to 100 parts by weight of cement powder. At this ratio however,, the slurry is very viscous, difficult to mix thoroughly and difficult to pour. To improve workability, a plasticizer is added to the cementitious mix at a concentration of about 2% by weight of the mix. Where pozzolanic fly ash and lime are incorporated in the mix, a super-plasticizer is often used. Lignosulphonate plasticizers and sulphonated naphthalene formaldehyde or sulfonated melamine formaldehyde function by dispersing flocculated cement particles by absorption on to the particle surfaces and thereby form a highly negative charge thereon giving rise to a colloidal-like electrostatic repulsion between adjacent particles. Polycarboxylate ethers, a more recent form of super-plasticizer function by steric stabilization rather than electrostatic repulsion but these compounds are generally more expensive. The typical dosage of a plasticizer is from 200-450 ml per 100 Kg of cement powder while for a super-plasticizer, the dosage is from 750 ml to 2500 ml per 100 Kg of cement powder.

While aerating agents and plasticizers or super-plasticizers are incorporated routinely into cementitious mixtures to improve workability, there are quite serious disadvantages in the manufacture of structural sandwich panels having a low density cementitious core.

Aerating agents and many of the plasticizers or super-plasticizers exhibit strong surfactant properties which are used to advantage in dispersion of cement powders and water distribution through a cement slurry. Surfactant compounds are relatively stable and remain dispersed throughout a panel core such that in the presence of any moisture, water is readily drawn into the porous cementitious core material.

Where structural panels are used in internal wet areas of a structure or on external walls, special care needs to be taken to ensure that exposed surfaces are thoroughly waterproofed as damage to decorative surfaces can occur due to early peeling of paint or mould. When plasticizers or super-plasticizers are utilized in a cementitious slurry, they permit a reduction in the amount of water required to fully hydrate the cement particles and still permit flowability.

In all of the known processes for the manufacture of structural sandwich panels having a low density cementitious core sandwiched between porous facing sheets such as fibre cement, plywood and paper laminated gypsum, water is removed from the cement slurry at the interface of the core material and the inner surface of the facing sheets by absorption into the porous facing sheet material. This localized removal of water from the slurry at the interface deprives the cement particles of hydration water thus leading to an impaired bond between the core and the facing sheets. This phenomenon is exacerbated when plasticizers and super-plasticizers are used in the core slurry mix as a reduced quantity of hydration water is required for flowability and workability.

Similarly, where any surfactant composition is present whether as an aerating agent or a plasticizer, the rate and extent of absorption of water into the facing sheets is increased, further diminishing the ultimate bond strength. It is therefore an aim of the present invention to overcome or alleviate at least some of the problems associated with the manufacture of structural building panels having a lightweight cementitious core and facing sheets on opposite faces thereof, or otherwise to provide users with a convenient choice.

Summary of the Invention

According to a first aspect of the invention there is provided a method for the manufacture of a structural building panel, the method comprising the steps of: locating in a mould, spaced facing sheets to form a cavity therebetween; introducing into the cavity a flowable cementitious core material to form, when cured, a lightweight panel core, the method characterized in that the flowable cementitious core material includes at least a partially water-soluble cellulosic polymer adapted to retain moisture in the cementitious core material in a region adjacent inner surfaces of the facing sheets to improve a bond between the core and the facing sheets by controlling the rate of hydration of cement particles in the region. The cementitious core material may comprise cement powder, for example

Portland cement powder.

The cementitious core material may further comprise a low density particulate filler. The low density particulate filler may have a density of between about 0.01 g/cm³ and 4.0 g/cm³, or between about 0.1 g/cm³ and 3.5 g/cm³, or between about 0.1 g/cm³ and 3.0 g/cm³, or between about 0.1 g/cm³ and 2.5 g/cm³, or between about 0.1 g/cm³ and 2.0 g/cm³, or between about 0.1 g/cm³ and 1.5 g/cm³, or between about 0.1 g/cm³ and 1 g/cm³, or between about 0.2 g/cm³ and 0.9 g/cm³, or below 1.5 g/cm³, or below 1.4 g/cm³, or below 1.3 g/cm³, or below 1.2 g/cm³, or below 1.1 g/cm³, or below 1.0 g/cm³, or below 0.95 g/cm³, or below 0.9 g/cm³, or below 0.8 g/cm³, or below 0.7 g/cm³, or below 0.6 g/cm³, or below 0.5 g/cm³, or below 0.4 g/cm^'3, or below 0.3 g/cm³.

The low density particulate filler may be, but is not limited to, expanded plastic material, expanded polystyrene beads, chopped expanded polystyrene, exfoliated vermiculite, perlite of micaceous aggregates, organically derived fillers such as rice and wheat husks, cellulose fibres, shredded paper, ceramic, vitreous or polymeric micro- spheres, pumice, expanded clay, expanded shale, polyvinyl chloride, polystyrene vinyl acetate, polyurethane, blast furnace slag, ceramic beads, glass beads, silicate beads and crushed cured porous cement mortar, or combinations thereof. Those skilled in the art will realise that other known materials having suitable densities may also be used as an alternative to, or in addition to, those recited above.

The cementitious core material may consist essentially of cement powder, a low density particulate filler and a partially water-soluble cellulosic polymer. 5 The cementitious core material may be other than a gypsum-based material.

The cementitious core material may not contain primarily gypsum.

The cementitious core material may comprise gypsum in an amount of no more than 50%, 45%, 40%, 35%, 30%, 25%, 20%, 15%, 10%, 8%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1% or 0.01% by weight of the cementitious core material. o The cementitious core material may not include gypsum.

The cementitious core material may riot include a retardant or a dispersant.

The cementitious core material may not include a plasticizer or super-plasticizer.

The cementitious core material may not include a surfactant or an air-entraining or aerating agent. 5 The cementitious core material may have a density of less then 0.45 g/cm³.

The method of the first aspect may not include the step of aerating the flowable cementitious core material.

The cellulosic polymer may comprise a non-ionic modified cellulose ether selected from the group consisting of: methyl cellulose, methylhydroxyethyl cellulose,o methylhydroxypropyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, ethylhydroxyethyl cellulose, methylethylhydroxyethyl cellulose, butylglycidylether- hydroxyethyl cellulose, laurylglycidylether-hydroxyethyl cellulose, carboxymethylated- methylhydroxyethyl cellulose, sodium carboxymethyl cellulose or mixtures thereof.

The cellulosic polymer may be of the following formula:

Where R = H, CH₃, CH₃-CH₂, CH₃-CH₂-CH₂ or CH₂CHOHCH₃ and n = 75-500

The cellulosic polymer may be included in the cementitious core material in a ratio of from about 0.01 % to 5%, or about 0.01% to 4%, or about 0.01% to 3%, or about 0.01 to 2%, or about 0.01% to 1.5%, or about 0.01% to 1.25%, or about 0.01% to 1%, or 5 between about 0.02% to 1%, or about 0.02% to 0.9%, or about 0.04% to 0.8%, or about 0.05% to 0.7%, or about 0.05% to 0.6%, or about 0.05% to 0.5%, or about 0.06% to about 0.4%, or about 0.08% to 0.25% w/w of the cement powder.

The cellulosic polymer may be included in the cementitious core material in the ratio of from about 0.05% to 0.4%, or about 0.05% to 0.3%, or about 0.05% to 0.25%, or0 about 0.05% to 0.2%, or between about 0.1 % to 0.2% w/w of the cement powder.

The facing sheets may be selected from fibre reinforced cement sheets, plywood, PVA coated metal, plywood/metal laminates, fibre reinforced gypsum, paper faced gypsum panels or the like.

If required, a normally inner surface of the facing sheets, in use, bonded to the5 cementitious core, is textured to enhance a bond between the facing sheets and the cementitious core.

The inner surface of the facing sheets may include a plurality of ribs separated by channels.

Whilst not necessary in the method of the first aspect, the cementitious materialΌ may be aerated by air entrainment during mixing of the cementitious slurry and/or by chemical gas generation during curing of the cementitious slurry between the facing sheets. Conveniently however, this step can be omitted in the method of the first aspect.

A pozzolan such as fly ash may also be incorporated into the cementitious slurry. According to second aspect of the invention, there is provided a structural building panel, the panel comprising: spaced facing sheets having located therebetween and bonded thereto a lightweight cementitious core including a low density particulate filler and a bond 5 enhancing agent in the form of at least a partially water soluble cellulosic polymer, the at least partially water soluble cellulosic polymer being adapted, in use, to retain moisture in the cementitious core material in a region adjacent inner surfaces of the facing sheets to improve a bond between the core and the facing sheets by controlling the rate of hydration of cement particles in the region. o The cementitious core may comprise cement powder, for example Portland cement powder.

The cementitious core may further comprise a low density particulate filler having a density of between about 0.01 g/cm³ and 4:0 g/cm³, or between about 0.1 g/cm³ and 3.5 g/cm³, or between about 0.1 g/cm³ and 3.0 g/cm³, or between about 0.1 g/cm³ and 2.5s g/cm³, or between about 0.1 g/cm³ and 2.0 g/cm³, or between about 0.1 g/cm³ and 1.5 g/cm³, or between about 0.1 g/cm³ and 1 g/cm³, or between about 0.2 g/cm³ and 0.9 g/cm³, or below 1.5 g/cm³, or below 1.4 g/cm³, or below 1.3 g/cm³, or below 1.2 g/cm³, or below 1.1 g/cm³, or below 1.0 g/cm³, or below 0.95 g/cm³, or below 0.9 g/cm³, or below

0.8 g/cm³, or below 0.7 g/cm³, or below 0.6 g/cm³, or below 0.5 g/cm³, or below 0.40 g/cm³, or below 0.3 g/cm³.

The low density particulate filler may be, but is not limited to, expanded plastic material, expanded polystyrene beads, chopped expanded polystyrene, exfoliated vermiculite, perlite of micaceous aggregates, organically derived fillers such as rice and wheat husks, cellulose fibres, shredded paper, ceramic, vitreous or polymeric micro-5 spheres, pumice, expanded clay, expanded shale, polyvinyl chloride, polystyrene vinyl acetate, polyurethane, blast furnace slag, ceramic beads, glass beads, silicate beads and crushed cured porous cement mortar, or combinations thereof. Those skilled in the art will realise that other known materials having suitable densities may also be used as an alternative to, or in addition to, those recited above. o The cementitious core may consist essentially of cement powder, a low density particulate filler and a partially water-soluble cellulosic polymer.

The cementitious core may be other than a gypsum-based material.

The cementitious core may not include calcium sulfate.

The cementitious core may not include a retardant or a dispersant. The cementitious core may not include a plasticizer or super-plasticizer.

The cementitious core may not include a surfactant or an air-entraining or aerating agent.

The cellulosic polymer may comprise a non-ionic modified cellulose ether selected from methyl cellulose, methylhydroxyethyl cellulose, methylhydroxypropyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, ethylhydroxyethyl cellulose, methylethylhydroxyethyl cellulose, butylglycidylether-hydroxyethyl cellulose, laurylglycidylether-hydroxyethyl cellulose, carboxymethylated-methylhydroxyethyl cellulose, sodium carboxymethyl cellulose or mixtures thereof.

The cellulosic polymer may be of the following formula:

Where R = H, CH₃, CH₃-CH₂, CH₃-CH₂-CH₂ or CH₂CHOHCH₃ and n - 75-500

The cellulosic polymer may be included in the cementitious core in a ratio of from about 0.01 % to 5%, or about 0.01% to 4%, or about 0.01% to 3%, or about 0.01 to 2%, or about 0.01% to 1.5%, or about 0.01% to 1.25%, or about 0.01% to 1%, or between about 0.02% to 1%, or about 0.02% to 0.9%, or about 0.04% to 0.8%, or about 0.05% to 0.7%, or about 0.05% to 0.6%, or about 0.05% to 0.5%, or about 0.06% to about 0.4%, or about 0.08% to 0.25% w/w of the cement powder.

The cellulosic polymer may be included in the cementitious core in the ratio of from about 0.05% to 0.4%, or about 0.05% to 0.3%, or about 0.05% to 0.25%, or about 0.05% to 0.2%, or between about 0.1 % to 0.2% w/w of the cement powder.

The facing sheets may be selected from fibre reinforced cement sheets, plywood, PVA coated metal, plywood/metal laminates, fibre reinforced gypsum, paper faced gypsum panels or the like. The inner surface of the facing sheets may include a plurality of ribs separated by channels.

If required, the cementitious material may be aerated by air entrainment during mixing of the cementitious slurry and/or by chemical gas generation during curing of the cementitious slurry between the facing sheets.

A pozzolan such as fly ash may also be incorporated into the cementitious slurry.

Brief Description of the Figures

In order that the invention may be fully understood and put into practical effect, reference will now be made to the accompany drawings in which: FIG. 1 illustrates a moulding press for manufacture of building panels according to the invention;

FIG. 2 shows graphically axial compression tests performed on panels made in accordance with the invention;

FIG. 3 shows graphically shortening of specimens against compressive load; FIGS. 4 and 5 show graphically the results of bending tests on specimens; and

FIG. 6 shows graphically the results of a racking test.

Definitions

In the context of this specification, the term "comprising" means "including principally, but not necessarily solely". Furthermore, variations of the word "comprising", such as "comprise" and "comprises", have correspondingly varied meanings.

The term "improve" as used herein in relation to the bond between the cementitious core material and the facing sheets may mean that where a water-soluble cellulosic polymer is included in the cementitious core material, as compared to when such a polymer is absent, a superior bonding between the cementitious core material and the facing sheets is obtained.

The term "structural building panel" may mean a panel adapted to be used as a load-bearing structure in a building, for example a house.

Detailed Description of an Embodiment of the Invention As illustrated in FIG. 1 , the press 1 may comprise a series of modules 2 supported on a concrete base 3. The upper portions of respective modules are supported by a head frame 5 of which portion is shown. Head' frame 5 supports an elevated walkway 5a. Press module 2 comprises a robust frame with spaced fixed upright members 6 reinforced by a bracing structure 4 and spaced movable upright members 6a slidably mounted on base rails 6b. Located within module 2 are spaced metal mould frames 9 defining panel mould cavities 10 with contoured closure plates (not shown) supported on mould frames 9 to define side and base closures to the mould cavities. The contoured side plates are paired such that one has a longitudinally extending tongue projection and the other having a longitudinally extending groove to form respective groove and tongue formations on the opposite sides of a panel, these formations enabling interconnection of adjacent panels, in use. The side plates also include locating slots to locate the edges of the facing sheets to form sealed edges to the mould 9. The base^' plates are similarly contoured to form either a tongue like projection extending along the base of a resultant panel or alternatively a groove extending along the base. Like the side plates, the base plates also include locating slots to locate the edges of facing panels to form a sealed interior cavity between the facing sheets located in respective slots of side and base plates within the mould cavity 10.

Extending between fixed frame member 6 and movable member 6a along the opposite sides of module 2 are a plurality of threaded shafts 12 with one end secured to fixed frame member 6 and the other slidably located in an apertured locating bracket 13. When the module is assembled with spaced facing panels coupled to side and base plates in each mould cavity 10, threaded nuts 14 on the ends of shafts 12 are tightened to place the entire module under compression.

If required, reinforced mould top closures 11 , each spanning five mould cavities 10 are frictionally engaged between the respective top edges of mould frames 9 and facing sheets (not shown) and the undersides of spaced longitudinal beams 7 to form a sealed top closure to each mould cavity 10.

After location of pairs of facing sheets between mould frames 9 and positioning of the mould side and base plates (not shown), a flowable slurry of cementitious core material is introduced into each mould cavity via respective top openings and the core material is allowed to cure at least to a "green" state providing sufficient structural integrity to allow removal of the semi-cured panels from the press. Because of the closely packed nature of the panels being moulded a considerable amount of heat is built up in the module as the cementitious slurry undergoes an exothermic hydration reaction. This build up of heat in the panel cores is considered to cause at least a partial thermal expansion of the core material which is resisted by the side, base and top plates surrounding the cores and under compression from tensioned shafts 12. This is a possible explanation as to why there is no noticeable shrinkage of the core materials or the resultant panel thickness when the green panels are removed from the press 1. The panels are then allowed to cure at ambient temperature for several days or they may be more rapidly cured in an autoclave.

If the core material is to be internally pressurized by a chemical aeration agent, top closures 11 are engaged against the tops of mould cavities 10 to prevent expansion of the core material therefrom.

Building panels according to the invention typically will have a thickness of from 50 mm to 100 mm with facing sheets having a thickness of from 3 mm to 6 mm preferably 4.5 mm, depending upon the specific panel use. It should be understood however that these dimensions may be varied to suit particular applications. Where one or more of the facing sheets of the building panel comprises an impervious material such as sheet steel, the inner face of the steel sheet may be coated with a water soluble PVA adhesive compound and/or the slurry mix may include PVA adhesive to aid in the bonding of the facing sheet to the core.

In the manufacture of panels according to the invention, it has been found that a wide variety of low density particulate fillers may be used including expanded polystyrene beads, chopped expanded polystyrene, exfoliated vermiculite, perlite of micaceous aggregates, organically derived fillers such as rice and wheat husks, cellulose fibres, shredded paper, ceramic or polymeric micro-spheres, or crushed aerated cement slurries. Whilst not necessary, the cementitious slurry may be aerated with a conventional aerating agent to produce a dispersion of fine air bubbles to reduce core density. The cementitious slurry may also include a pozzolan such as fly ash together with lime and, when lime is present, a quantity of aluminium powder may also be incorporated into the slurry to generate gas bubbles in the slurry during mixing and otherwise to create an internal pressure in the mould to enhance bond strength even further.

The addition of the modified cellulosic ether composition to the slurry imparts a number of distinct unexpected benefits.

As the cellulosic composition functions almost as an internal lubricant in the slurry, little if any aerating agent is required if the slurry is to be aerated. Moreover, the internal lubrication effect obviates the need for plasticizers or super-plasticizers such that the incorporation of traditional surfactant compositions into the slurry is also avoided. This overcomes the problem of moisture "wicking" within the panel core due to the dispersed chemically stable surfactant. It is considered that once the panel core has cured and dried, the residue of cellulosic material initially acts as a sealant to prevent moisture ingress but over a relatively short period of time, the cellulosic residue disappears due to degradation by micro-organisms and cellulose enzymes associated with naturally occurring bacteria and yeasts.

Examples Example 1 - Slurry preparation

A cementitious slurry was prepared as follows:

• Cement powder: 300 Kg

• Water: 152 L

• Lightweight aggregate (polystyrene beads): 540 litres • Additive - non-ionic modified cellulose ether 300-800 gm (depending upon aggregate type and ambient temperature)

The slurry of Example 1 was then introduced into a mould frame as described generally with reference to the apparatus of FIG. 1. The mould frame was adapted to produce a 75 mm thick panel with facing sheets of 4.5 mm fibro-cement spaced within the mould frame.

Specimens of a lightweight concrete sandwich panel manufacture in accordance with the invention under the trade mark "Styrocon" were tested for structural behaviour under a variety of actions as described in the American Standard Test Method covering panels for building construction, ASTM E72 2002. There are no relevant Australian

Standards which describe test methods for light building sandwich panels.

Example 2 - Specimens

The wall panels are a sandwich consisting of outer layers of 4.5 mm fibre-cement sheet each side of a layer light concrete in which the coarsest aggregate consists of small styrene spheres. The panels tested in this programme of testing had an overall thickness of 75 mm, were 600 mm wide and of varying heights to three metres. A shallow tongue on one vertical edge engages a matching groove in the vertical edge of the abutting panel 1.4

to form a vertical joint. Three beads of construction adhesive are run into the joint. Generally the panels are to be supported along their bottom edge on a concrete slab or floor and restrained against translation in plan. A suspended floor or roof is intended to be supported on the wall line formed by the top edges of the panels.

Example 3 - Test Methods

3.1 Compression Tests

Three specimens each consisting of a pair of 75 mm x 600 mm x 3 m high panels assembled side by side to form a 1.2 m wide x 3 m high section of wall were tested in vertical axial compression as required by Section 9 of ASTM E72 - 1980. Each specimen was set up vertically and loaded by a downward force at the midpoint of a steel beam placed centrally on the specimen's 75 mm wide top face. The central tongue and grooved joint was glued according to the manufacturer's instructions. Vertical axial force was applied using a hydraulic ram reacting against the reinforced concrete roof of a strong room. The force was quantified by an electronic load cell. Deflection transducers on rods were mounted over a gauge length of 2.5 metres on the front and back faces to assess axial strain. Lateral deflection (and buckling tendency) was assessed by a scale and telescope. As the load was gradually increased, readings of axial force and deflection were logged to a data acquisition system. While it was the intention to test to destruction, no clear failure of any panel was encountered at the load limit of the apparatus.

3.2 Bending Test

The bending resistance of the panels was examined by applying bending action to a series of single-panel (600 wide x 75 mm thick x 3 m long) specimen arranged horizontally and supported over a span of 2.5 metres. A screw actuator and load cell were used to apply and quantify load to the outer quarter-points of the span as described in section 11 of ASTM E72. Net central deflection was obtained by subtracting any downward support deflection from that of. the mid-span of the loaded panel. Three replicates were tested. The load was increased to failure in bending.

3.3 Bending and Axial Load Test:

In addition to the standard ASTM horizontal bending test, three similar replicate panels were tested in bending as described above and with a simultaneous axial load of approximately 10 kN while the bending load_.was increased to failure. A block and tackle with lever and dead weight were used to apply the axial load.

3.4 Racking Test: The capacity of a group of panels to resist overall shear deformation in the plane of the wall was assessed by a procedure after that of the racking test described in Section 14 of ASTM E72. A group of five panels each 75 mm thick x 600 mm wide x 3 m tall were assembled to form a section of wall 3 m long, 3 m high and 75 mm thick. The tongue and grooved vertical joints were flued as directed by the client and the wall's bottom edge restrained by screwing to a 90 x 95 xx radiata pine bottom strip in turn fixed to the floor of the test chamber. Two 150 x 4 mm hex head screws were driven through each panel (at outer quarter points) into the bottom strip. A second 90 x 45 radiata pine strip was attached as a ledger beside the top edge of the specimen using 100 x 4 mm hex- head screws at similar spacings to the bottom edge. In addition to the screws, the top ledger was also glued to the inner face of the panel assembly using the same construction adhesive as used in the joints. As indicated by Section 14 of ASTM E72, a bottom corner was restrained and the diagonally opposite top corner of the wall specimen felt a racking load applied by a hydraulic jack and measured by a load cell. To prevent the panel rotating about the bottom corner (and force it to deflect into a parallelogram in shear), the top surface adjacent to the load point was restrained against vertical movement by the use of a block acting against the top of the steel test chamber. As for previous tests, deflection transducer signals were logged with that of the load cell as the load was increased to failure. One 3 m x 3 x wall specimen only was tested for racking.

3.5 Fixing Tests:

Several fixings used to attach to or restrain the panels were examined:

• Connection between the top ledger and the panel face of the racking specimen

• Pullout on 10 gauge x 65 mm screws driven 55 mm into green star plugs in a 10.5 mm hole

• Shear on 10 gauge x 65 mm screws driven 55 mm into green star plugs in a 10.5 mm hole

• Opening force on corner connection, 450 long specimen fixed with 2, 10 mm dowels and adhesive • Ultimate shear on 600 mm length of kerf 35 mm deep x 4.5 mm wide over aluminium angle.

The resistance of the ledger was assessed by applying a perpendicular point load to the mid span of ledger orientated in the plane of the panel. A hydraulic ram was used for the force and a load cell measured the peak load only.

All other fixing tests were conducted by applying force to the fixing in a small section of 75 mm thick panel restrained in a 100 kN Universal testing machine.

Example 4 - Results and Observations 4.1 Compression Tests

The first two panel specimens tested remained unbroken under a maximum axial compression of 100 kN, the load limit of the apparatus. That apparatus was modified for a further test but no clear failure was observed at the new limit of 160 kN. While the third replicate specimen remained intact under a maximum load of 164 kN, FIG. 2 shows that the central horizontal deflection had increased to 8 mm - it is probably that a buckling failure would have been close. It is unlikely that the specimen would have endured axial forces significantly greater than the maximum of 164 kN applied during the test.

FIG. 3 plots the vertical shortening of the wall specimens against axial compression load. These results suggest that (after some initial anomalies) there is an approximately linear relation between axial load and axial shortening of the wall. All three 3 m high double-panel specimens showed a shortening close to 1 mm over 2.4 m at an axial compression load of 100 kN. Although likely to be close to buckling, the more highly loaded Panel 3 continued to show a linear relation between axial compression and axial shortening up to its maximum of 164 kN.

4.2 Bending Tests

Table 4.1 below shows the ultimate loads and calculated ultimate bending moments for the bending and bending plus axial load tests of 600 wide x 75 mm thick panels loaded at outer quarter points and spanning 2.5 metres. The results suggest that the approximately 10 kN of axial load increased the bending capacity by approximately 10%. This may be attributed to a mild pre-stress effect applied by the axial load. The lowest bending moment sustained was 1.49 kN m. Table 4.1: Ultimate loads and calculated ultimate bending moments for the bending and bending plus axial load tests of 600 wide x 75 mm thick panels loaded at outer quarter points and spanning 2.5 metres

FIGS. 4 and 5 show load deflection curves for the bending and bending plus axial tests. While there is a transducer problem evident in the first specimen, the curves show a linear response up to central deflections of about 12 mm with a loss of stiffness preceding failure at central deflections between 20 and 28 mm. No cracking was observed prior to failure.

4.3 Racking Tests

The greatest load applied to the top corner during the racking test was 22.7 kN after which there was a local crushing failure of the panel material at the point of application of the load. At 22.7 kN, the 3 m x 3 m specimen's net out of square deflection was 11.5 mm. FIG. 6 shows a plot of racking load vs net deflection - while some loss of stiffness at peak load may be read from the curve, the effect is not conclusive and it is conceivable that a more uniformly loaded wall could resist greater racking loads.

4.4 Fastener Tests

4.4.1 Point load on Ledger: The ledger separated from the face of the panels adjacent to the point of loading at an applied force of 25.5 kN. The test is no more than indicative and only this one result is reported.

4.4.2 Screw Withdrawal from Star Plugs: Table 4.2 below shows the withdrawal loads perpendicular to the panel surface for a series of six tests. Table 4.2: Pullout on 10 gauge x 65 mm screws into green star plug in 10.5 mm hole Rate: 500N/min

4.4.3 Shear Capacity of Screws in Star Plugs: The ultimate loads of the screws in shear (in plane of surface) are presented in Table 4.3 below. The variability of the results for screws in shear is better than that of the tension results.

Table 4.3: Shear on 10 gauge x 65 mm screws into green star plug in 10.5 mm

I O hole Rate: 1 kN/min

4.4.4 Corner Dowel Connection: The maximum loads required to break away 450 mm lengths of wall from the corner joint are presented in Table 4.4 below.

I ₅ Table 4.4: Opening force on corner connection, 450 mm long specimen Ωxed with 2, 10 mm dowels and adhesive Rate: 2.5 nK/min

4.4.5 Lateral Capacity of Base Edge Ken: Table 4.5 shows the ultimate lateral loads required to cause failure of the bottom edge kerf. For two specimens, the material on each side of the kerf was tested (sides A and B). It can be seen in Specimen 2 that a strong result on one side is matched by a weaker result on the other, reflecting a slight deviation of the ken from the midline of the bottom edge.

Table 4.5: Ultimate shearon 600 mm length of kerf 135 mm deep x 4.5 mm wide Rate 2.5 kN/min

In addition to the ASTM structural test procedures described, other tests were performed as follows:

Bond strength is normally tested by determining peel strength between the core and the facing sheet in Kg/cm². Typical results for prior art panels exhibit a peel strength of between 1.6 - 3.0 Kg/cm² with a clean separation between the facing sheet and the core material. In the present invention the bond strength or "peel" tests were not able to be measured as the core material failed before the facing sheet/core bond. The ultimate strength of the panels according to the invention were thus a function of core strength rather than bond strength as with prior art panels, A simple "rule of thumb" test to assess bond strength consisted of laying a 100 mm x 50 mm timber stud on a concrete surface and then dropping a panel, face down, across the timber stud. Where the panel snapped cleanly across the width along the line of impact with the edge of the timber stud, that was indicative of a bond strength superior to the core strength. Where the panel did not snap through cleanly, a jagged diagonal break in the facing sheets and clear evidence of delamination between the core and the facing sheets demonstrated an inferior bond strength. For panels according to the invention, a clean transverse break was obtained in all of these "ad hoc" bond strength tests.

It readily will be apparent to a person skilled in the art that many modifications and variations may be made to the invention without department from the spirit and scope thereof.

Claims

The claims defining the invention are as follows:

1. A method for the manufacture of a structural building panel, the method comprising the steps of: locating in a mould, spaced facing sheets to form a cavity therebetween; introducing into the cavity a flowable cementitious core material to form, when cured, a lightweight panel core, the method characterized in that the flowable cementitious core material includes at least a partially water-soluble cellulosic polymer adapted to retain moisture in the cementitious core material in a region adjacent inner surfaces of the facing sheets to improve a bond between the core and the facing sheets by controlling the rate of hydration of cement particles in the region.

2. The method of claim 1, wherein the cementitious core material does not include a surfactant or an air-entraining or aerating agent.

3. The method of claim 1 or claim 2, wherein the cellulosic polymer comprises a non-ionic modified cellulose ether selected from the group consisting of: methyl cellulose, methylhydroxyethyl cellulose, methylhydroxypropyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, ethylhydroxyethyl cellulose, methylethylhydroxyethyl cellulose, butylglycidylether-hydroxyethyl cellulose, laurylglycidylether-hydroxyethyl cellulose, carboxymethylated-methylhydroxyethyl cellulose, sodium carboxymethyl cellulose or mixtures thereof.

4. The method of any one of claims 1 to 3, wherein the cellulosic polymer is of the following formula:

5. The method of any one of claims 1 to 4, wherein the cementitious core material comprises cement powder.

6. The method of claim 5, wherein the cellulosic polymer is included in the cementitious core material in the ratio of from 0.08% to 0.25% w/w of the cement powder.

7. The method of claim 6, wherein the cellulosic polymer is included in the cementitious core material in the ratio of from 0.1 % to 0.2% w/w of the cement powder.

8. The method of any one of claims 1 to 7, wherein the facing sheets are selected from the group consisting of: fibre reinforced cement sheets, plywood, PVA coated metal, plywood/metal laminates, fibre reinforced gypsum and paper faced gypsum panels.

9. The method of any one of claims 1 to 8, wherein a normally inner surface of the facing sheets, in use, bonded to the cementitious core, is textured to enhance a bond between the facing sheets and the cementitious core.

10. The method of claim 9, wherein the inner surface of the facing sheets include a plurality of ribs separated by channels.

11. The method of any one of claims 1 to 10, wherein the cementitious core material includes a low density particulate filler.

12. The method of claim 11, wherein the low density particulate filler is selected from the group consisting of: expanded plastic material, expanded polystyrene beads, chopped expanded polystyrene, exfoliated vermiculite, perlite of micaceous aggregates, organically derived fillers such as rice and wheat husks, cellulose fibres, shredded paper, ceramic, vitreous or polymeric micro-spheres, pumice, expanded clay, expanded shale, polyvinyl chloride, polystyrene vinyl acetate, polyurethane, blast furnace slag, ceramic beads, glass beads, silicate beads and crushed cured porous cement mortar, or combinations thereof.

13. The method of any one of claims 1 to 12, wherein the cementitious core material further comprises a pozzolan.

14. The method of any one of claims 1 to 13, wherein the cementitious core material is other than a gypsum-based material.

15. A structural building panel, the panel comprising: spaced facing sheets having located therebetween and bonded thereto a lightweight cementitious core including a low density particulate filler and a bond enhancing agent in the form of a substantially water soluble cellulosic polymer, the at least partially water soluble cellulosic polymer being adapted, in use, to retain moisture in the cementitious core material in a region adjacent inner surfaces of the facing sheets to improve a bond between the core and the facing sheets by controlling the rate of hydration of cement particles in the region.

16. The panel of claim 15, wherein the cementitious core does not include a surfactant or an air-entraining or aerating agent.

17. The panel of claim 15 or claim 16, wherein the cellulosic polymer comprises a non-ionic modified cellulose ether selected from the group consisting of: methyl cellulose, methylhydroxyethyl cellulose, methylhydroxypropyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, ethylhydroxyethyl cellulose, methylethylhydroxyethyl cellulose, butylglycidylether-hydroxyethyl cellulose, laurylglycidylether-hydroxyethyl cellulose, carboxymethylated-methylhydroxyethyl cellulose, sodium carboxymethyl cellulose or mixtures thereof.

18. The panel of any one of claims 15 to 17, wherein the cellulosic polymer is of the following formula:

19. The panel of any one of claims 15 to 18, wherein the cementitious core comprises cement powder.

20. The panel of claim 20, wherein the cellulosic polymer is included in the cementitious core in the ratio of from 0.08% to 0.25% w/w of the cement powder.

21. The panel of claim 21, wherein the cellulosic polymer is included in the cementitious core in the ratio of from 0.1 % to 0.2% w/w of the cement powder.

22. The panel of any one of claims 15 to 21, wherein the facing sheets are selected from: fibre reinforced cement sheets, plywood, PVA coated metal, plywood/metal laminates, fibre reinforced gypsum or paper faced gypsum panels.

23. The panel of any one of claims 15 to 22, wherein a normally inner surface of the facing sheets, in use, bonded to the cementitious core, is textured to enhance a bond between the facing sheets and the cementitious core.

24. The panel of claim 23, wherein the inner surface of the facing sheets include a plurality of ribs separated by channels.

25. The panel of any one of claims 15 to 24, wherein the low density particulate filler is selected from the group consisting of: expanded plastic material, expanded polystyrene beads, chopped expanded polystyrene, exfoliated vermiculite, perlite of micaceous aggregates, organically derived fillers such as rice and wheat husks, cellulose fibres, shredded paper, ceramic, vitreous or polymeric micro-spheres, pumice, expanded clay, expanded shale, polyvinyl chloride, polystyrene vinyl acetate, polyurethane, blast furnace slag, ceramic beads, glass beads, silicate beads and crushed cured porous cement mortar, or combinations thereof.

26. The panel of any one of claims 15 to 25, wherein the cementitious core further comprises a pozzolan.

27. The panel of any one of claims 15 to 26, wherein the cementitious core is other than a gypsum-based material.