WO1981001979A1

WO1981001979A1 - Moulding of articles

Info

Publication number: WO1981001979A1
Application number: PCT/GB1981/000002
Authority: WO
Inventors: C Bevan
Original assignee: Bevan Ass C G; C Bevan
Priority date: 1980-01-07
Filing date: 1981-01-05
Publication date: 1981-07-23
Also published as: JPS56501843A; BR8108680A; US4522772A; AU546692B2; JPH0213882B2; AU6643481A; DE3164784D1; EP0050114B1; EP0050114A1

Abstract

Construction products are moulded by mixing the dry constituents, including a proportion of fine particulate material, feeding (1, 2) the mixture into a mould (5), compacting the mixture, removing part (6) of the mould (5), lightly spraying (9) an exposed upstanding surface of the product with setting liquid, removing the product from the mould (5) and allowing it to set. Sufficient fine particulate material to surround the coarse particles and compaction, using vibration (7) of the mould (5) and compression of the mixture, to cause the fine particles to fill the interstices between the coarse particles, provide sufficient support of the exposed surface to prevent collapse or erosion thereof during wetting even though no fibre reinforcement is included in the mixture. Sufficient liquid to wet the product but not to saturate it is applied by the spray (8, 9).

Description

MOULDING OF ARTICLES TECHNICAL FIELD OF THE INVENTION

This invention relates to the moulding of articles and in particular to the moulding of construction 5. products, such as partition panels, roof decking and pipes, from liquid setting particulate materials. STATE OF THE ART

- It has been customary hitherto to mould such articles as aforesaid by mixing the constituent

10. materials, applying a sufficient quantity of setting liquid to the mix, introducing the moistened mix to the mould and allowing the mix to set before removal of the set article from the mould. This process is time-consuming and for quantity production of such

15. articles, since the setting of the article occurs in the mould, a large number of moulds is required.

It has been proposed for example in British Patents Nos. 528,657, 1,067,671, 1,346,767, 1,417,001 and 1,466,663 that a dry mixture of constituent materials

20. be introduced into the mould and compacted therein. The mould is then immersed in a setting liquid or the liquid is allowed to permeate the mix by capillary action. Of these Patents, only in the case of 1,346,767 is the liquid applied to a vertical surface which is

25. unsupported by a part of the mould apparatus, and in that case the mould is immersed in the water so that the buoyancy effect thus created offsets the tendency of such unsupported walls to collapse due to the increase in weight of the mix.

It has also been proposed in U.S. Patent 1,427,103 that for producing very small moulded articles, for 5. example buttons, the dry constituent materials be pressed into the mould, removed therefrom and then sprayed with setting liquid. However, this process is restricted to use for the production of very small articles and has not been used for the production of 10. relatively very large articles, such as construction products, since such articles would be expected to collapse under their own weight on demoulding and may also shrink and crack during the spraying operation. In consequence it has been considered that if there 15. is to be any vertical surface of mix which is unsupp¬ orted by a part of the mould apparatus during the wetting process by seepage rather than by total immersion then it is essential to incorporate into the mixture of constituent materials some reinforcing 20. means from which the moulded article can derive support during the spraying and setting stages of the process. The reinforcing means may be fibres, and examples of processes incorporating the use of such fibrous reinforcing material for the supporting of the moulded 25. article whilst unsupported at. least in part by the mould are described in German Patent 1,683,829, British Patent 1,346,767 and our co-pending. Application No. 8006777.(2045150). DISCLOSURE OF THE INVENTION 30. The invention provides a method for producing moulded construction products from a liquid setting mixture of fine and coarse particulate materials comprising the steps of mixing the dry constituenet materials, said materials including a proportion of 35. fine particles sufficient to substantially surround all coarse particles but not including fibrous rein¬ forcing materials, introducing said mixture into a mould, compacting said mixture in said mould to an extent that said fine particles substantially fill the 5. interstices between said coarse particles, removing at least a part of the mould from contact with the thus moulded product, spraying the product at a surface unsupported by said mould with a predetermined quantity of a setting liquid, being a quantity sufficient to wet

10. all of the compacted constituents but insufficient completely to saturate the same, and allowing said product to set.

Surprisingly, it has now been found that provided that there is sufficient compaction and a sufficient

15. proportion of fine particles in the mixture of con¬ stituents no fibres or other reinforcement are required and a satisfactory moulded article may be obtained which, without collapse, can be demoulded before the onset of chemical curing and which does not shrink or crack

20. during the spraying and setting process. Because of this the method of the invention can be used for the manufacture of high quality precast concrete products' having no fibrous reinforcing therein, and in respect of which removal of the article from the mould after

25. compaction and prior to spraying can be used to economic advantage by reducing the number of moulds needed for quantity production of such articles.

In addition to immediate demoulding, concrete products produced by the new method have an unusually

30. high quality finish, high immediate demoulding strength and can be moulded to intricate shapes, without the application of high pressure or heavy ramming or tamping. This combination of features is unique in concrete making.

35. In conventional concrete practice, immediate de- - Λ -

moulding can be achieved by vibrating or ramming so- called "earth damp" mixes into moulds but the products are generally characterised by a granular surface finish as in "breeze" blocks. At present, smooth finishes 5. for immediately demoulding products can only beObtained by using extremely high compacting forces, such as the centrifugal forces, used in the "Packer-head" process for pipe manufacture. Such processes, however, are only suitable for simple shapes, compared to the intricate 10. section, which can be produced by the new method. Alter¬ natively, relatively smooth finishes can be obtained by conventional wet casting but here the wet concrete sticks to the moulds and can only be removed once the material has set. Furthermore, although these surfaces 15. tend to be smoother than those made from "earth damp" mixes, they are characterised by "pin holes" and other blemishes, arising from bubbles within the liquid which do not occur with the new method.

Another departure from the core spray method of . 20. our co-pending Patent Application No. 8006777 (2045150) is the discovery that with adequate compaction and suitable powder formulation, it is possible for com- . pletely dry mixes to stand intact with one or more of the mould sides removed. If rigid steel bar or mesh 25. reinforcement is incorporated, it is sometimes possible to remove all the sides of the mould (other than the base) without collapse of the dry compacted mix. This means that, whereas previously access for spraying could only be via internal core holes, it is now possible to 30. spray onto free-standing external vertical surfaces. This widens the range of shapes which can be handled. Also, water penetration can be speeded (particularly for thick sections) by, for example stabilizing the core zones by an initial internal spray and then 35. removing both main sides of the mould for further spraying via the outer surface.

These developments are surprising when viewed in relation to normal preconceptions in the industry or in relation to the published prior art. So far, 5. it has been considered essential that some form of support be provided to the dry vertical surfaces to prevent collapse either prior to or during the appli¬ cation of liquid, typical means of support being either some form of external support (such as per-

10. forated plates or membranes) or more recently internal fibres. We have now found that if the correct pro¬ cedures are followed, no support of the dry surfaces being sprayed is needed at all. (Such support as may be required for the mass as a whole can be provided

15. at the surfaces which do not need to be sprayed as described later) . Furthermore, it was previously thought that at least some fibres were needed to prevent erosion of the free-standing material in, for example, the core holes, and it has now been found that

20. with sufficient compaction and fines content and a sufficiently fine spray, remarkably smooth bores can be obtained with no fibrous support.

Another very surprising feature is the unexpect¬ edly high strength of the dampened, compacted material

25. immediately after demoulding. In slow setting Portland cement-based formulations, this so-called "green" strength occurs well before any strength can develop from the chemical reaction with the water. Hence the unusual stiffness and cohesiveness of the moulding at

30. this stage can only be due to physical properties, such as mechanical particle interlock and surface tension effects.

It is possible for example to demould some pro¬ ducts made by the new method by hand, without requiring

35. vacuum lifting or other special equipment designed to minimise demoulding stresses.

In common with fibrous panels made by the afore¬ mentioned core spray method, it is worth noting the lack of any adhesion to the mould sides after spraying, 5. despite the very strong adhesion between the particles themselves. Provided the amount ^~of water sprayed is such as not to saturate the mass, mould sides come away remarkably cleanly and sufficiently dry to be ready for the next filling. 10. Broadly, the range of products and manufacturing sequence for the present method follows the method of fibrous core spraying, except that fibres are omitted and spraying can be other than via the core holes. Spraying is largely on vertical (or approximately 15. vertical) surfaces, which generally comprise at least half of the total vertical surfaces of the products. In the case of spraying via the cores in panel products, the spray area is significantly more than half the total vertical area. Spraye'd surfaces can 20. be ribbed or textured, particularly in the case of exterior sprayed surfaces, where the moulds do not have to be withdrawn by sliding parallel to the surface, as is usually the case with core hole sur¬ faces. Generally, in the case of rectangular products, 25. the dry compacted material needs at least two mould sides to remain in place during spraying, so the dry material can support itself by arch action between the remaining two mould surfaces. In the case of annular shapes, generally at least the outer or inner 30. mould surface should remain in place during spraying to provide support to the dry compacted mass.

The remaining distinctions between the present method and the method of Application No. 8006777 (2045150) largely relate to the degree of dry ccrα- 35. paction applied and the provision of adequate fines

OM in the mix.

For example, in the science of soil mechanics, particles are broadly categorised as clays, silts, or sands. The particle sizes of clays are extremely 5. cohesive when in a damp, compressed state. Sands, on the other hand, are not cohesive under any circumstances and silts occupy an intermediate position. It is not necessary with the present process to do down to -clay- like particle sizes and the process will not work

10. solely with sands (unless the sand is combined with finer material) .

Common commercially available liquid setting powders such as Portland cement or gypsum would probably be classified (in terms of particle, size) as

15. silts. It has been found that such powders work well with the present process. Finer powders would give more stable mouldings, but these are more difficult to compact properly (unless the mix contains a pro¬ portion of coarse particles or compacting means other

20. than vibration alone are used) . Broadly, it has been found that to achieve adequate compaction, powder feed rates have to be slower, e.g. up to half the . speed that has been used heretofore. If filling rates are too fast (and/or vibration insufficient) , some of

25. the interstices may not be completely filled before subsequent layers of material compact into an effective bridge above. If this happens no further downward percolation is generally possible and the voids remain only partly filled, even if subjected to prolonged

30. or even increased vibration.

Optimtαm filling .rates depend very much on mix proportions, particle size, etc. Generally for mixes with near to the optimum economic proportions of coarse aggregate, filling rates are generally slow - i.e. less

35. 10mm per sec. Compacting vibration must be more intense and of a higher frequency than has been usual hereto¬ fore e.g. preferably at least 12,000 cycles per minute. The more effective the compaction, the less critical is the quantity of fines present, provided 5. at least sufficient fines are present to surround the coarse particles. Mixes need to be as dry as possible to obtain optimum compaction as even a small degree of dampness can inhibit full compaction.

"Coarse" in this context means everything above 10. the "silt" fraction discussed earlier i.e. it includes the proportion of sand which is generally added to concrete mixes. The ideal mix is one in which the cement (for example) compacts into all the interstices between the sand and the sand/cement mix in turn com- 15. pacts into all the interstices between the coarse aggregate.

From the processing point of view, there appears to be no particular upper limit to the size of coarse aggregate, provided that they fit readily into the 20. mould and are completely surrounded by compacted sand/ cement. Provided the aggregate component in the mix is not too coarse, in some cases the proportion of cement powder in the mix- needed to generate adequate final cured strength provides all the fines needed for 25. dry stability during manufacture. Where this is not sufficient, additional fines are added, usually in the form of pulverised fuel ash or some other suitable cheap extender. Aggregates usually consist of a range of larger particle sizes and include sand and light- 30. weight aggregates such as those manufactured from expanded clay or sintered pulverised fuel ash. For small sectioned products, such as sewerage pipes' or hollow concrete blocks, the maximum aggregate size is generally around 5mm. 35. Although readily processible by the present method

OMPI Ji'-m V/IPO thin sectioned, large area panels are generally not suitable as fibre reinforcement is usually required in the end product for structural reasons. However, the present method can be used for making products 5. containing non-fibrous reinforcement, for example, such rigid reinforcement steel rods or bars as used in conventional reinforced concrete. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a diagrammatic elevation of one form of 10. apparatus suitable for use in practising the invention; Fig. 2 . is a plan view of the apparatus of Fig. 1 with the core removed; and Figs. 3 and 4 are cross-sectional elevations of typical 15. construction products manufactured in accordance with the present invention. BEST MODE OF CARRYING OUT THE INVENTION

One of the simplest types of equipment using the new method is shown in Fig. 1. A vibrating tray 1 20. distributes the dry powder mix into a laterally oscill¬ ating chute 2 so that two equal streams of material pass either side of a bore former support 3 and are guided by a hopper 4 into a mould 5, containing at least one bore former 6 which is fitted at its base with a 25. vibrator 7. While filling the mould, the bore former 6 and/or the hopper and bore former support, are vibrated to settle and thoroughly compact the mixture. After filling the mould, the upper parts of the mixture which are not compacted by a head of material above them, may 30. be further consolidated by pressing the bore former support 3 (preferably together with the bore former 6) onto the powder mix surface until the whole mass is uniformly compacted. Vibration then ceases and the bore former 6 and bore former support are withdrawn 35. from the mould, which then^' oves laterally to locate over one or more spray tubes 8. Each tube 8 is fitted at its end with a fine spray nozzle 9, which is oscillated vertically in a bore until sufficient liquid has been delivered to the bore surface(s) to just wet 5. the mixture throughout.

The spray needs to be fine and of modest velocity to avoid surface pitting and should generally deliver liquid at an average rate which does not exceed the rate at which the liquid can be absorbed into the powder 10. by capillary action. This prevents the surface from becoming saturated and causing drip marks or local collapse. Spraying is usually terminated before full wetting occurs, so that wetting of the still dry thicker parts of the moulding is completed by capillary action, 15. drawing liquid from the adjacent wet parts. This allows the minimum quantity of liquid to be applied for full wetting, thus avoiding the risk of over-wetting which can cause the mixture to stick to the mould sides and reduce demoulding strengths. When the damp areas have 20. spread throughout the mass, the mould is opened and the uncured product is removed therefrom (by vacuum lifting methods, for example) and allowed to cure.

Fig. 2 illustrates the method described above as applied to -the manufacture of paving flags or the like, 25. two such flags 10 being formed simultaneously in mould 12. The process is described in greater detail in Example 1 below.

Figs. 3 and 4 illustrate other construction products which may be manufactured by the present process as 30. described in Examples 2 and 3 below. Example 1

Simple paving flags and the like can be produced, without core holes, as shown in Fig. 2. In this case the "core former" 6 in Fig. 1 is two complete mould sides, 35. which on withdrawal, expose the compacted particulate

OMFI ° material for spraying (items 10 in Fig. 2)^'. The dry material is held up by arch action between mould sides 11. Sides 12 restrain buckling in one direction but not the other, .so they can also be removed before 5. spraying. This allows both faces of material 10 to be sprayed, which is an advantage with relatively thick products like paving flags (typically measuring 50mm thick X *600mm X 600mm.) .

It the material is correctly formulated and

10. compacted, it can freely span the 600mm without any support other than at the base and at the sides 11. Product thickness for this span can be as little as 15mm, which is surprisingly. slender bearing in mind there is no binding material at all between the particles.

15. To be competitive, paving flags require a high coarse aggregate fraction of sufficient size to minimise the surface area^"and hence the amount of relatively expensive cement needed to bind the aggregate together. A typical mix which gives a satisfactory product strength

20. for this application and can be processed satisfactorily consists of 1:0.3:1.2:4 parts by weight of ordinary Portland cement, pulverised fuel ash (as commonly used for concrete manufacture) standard fine grade "sharp" concreting sand and granite aggregate chippings passing

25. a 12mm mesh and retained on a 6mm mesh.

The dry mixture is poured evenly into the vibrating mould, so that the level rises at approximately 500mm per minute, while vibration frequency iε maintained at 12,000 cycles per minute. Amplitude is adjusted so

30. that the coarse aggregate on the surface is just mobile but the layers below are locked into position with the fines flowing and .compacting around them. After filling, the top layer can be compacted by plunger 3 Fig. 1 but generally with the specified mix this is not very effective

35. (due to the almost point-to-point contact of the coarse aggregate preventing movement) . On removal of the core former/mould sides 6, the free surfaces are lightly sprayed until the material is just dampened throughout and the mouldings then removed by vacuum 5. lifter to the curing zone. Example 2

Pulverised fuel ash (PFA) is a silicious waste material from coal fired power stations and is one of the cheapest fillers available. If the mix is auto- 10. claved after dampening, the silica reacts with the free lime in the cement, resulting in a strong chemical bond between filler and binder. In these respects therefore it is advantageous to increase the PFA content and adjust the production procedures and mix proportions

15o to overcome the fine powder compaction problems mentioned in Example 1.

With high PFA concentrations it has been found almost impossible to achieve the required compaction by vibration alone and a preferred method is to rely

20. largely on direct externally applied pressure. For compression compaction to be effective, the proportion of coarse aggregate in any case has to be limited, as point-to-point contact of the latter tends to cause a series of "bridges" which shield the loose powder in

25. the interstices from externally applied pressure. It is also preferable to limit the size of coarse aggregate to sand rather than gravel, as the former is generally easier to compact by direct pressure.

A typical application for such mixes is the manu-

30. facture of sewerage and drainage pipes of approximately 100mm internal diameter and 15 to 20mm wall thickness and a suitable mix would be 1:1:3 of ordinary Portland cement, PFA and sand. This is poured fairly rapidly into a moulding plant similar to that shown in Fig. 1

35. except that core former 6 is vibrating rather than the mould. On filling, core former 6, together with top plunger 3, move downwards to compress the powder/sand mix, while still vibrating. After full compaction vibration ceases, core former 6 is completely 5. withdrawn downwards and plunger 3 withdrawn upwards, before the mould moves to the spray station.

In this method, filling and top compression rates are not critical, provided there is provision for the escape of air (e.g. between the mould side and top

10. plunger 3) . Vibration is also not critical, provided it is sufficient to disrupt dry resistance to compaction by arch action in the material immediately below the top plunger 3. With the apparatus shown in Fig. 1, the core former acts as a poker vibrator, dislodging any

15. potential arching, so that the top pressure can be fully effective -throughout the product. Also, core former 6 is one of the abutments against which the material arches, so moving the core former relative to the mould side 5 (forming the other abutment) also has

20. a powerful arch breaking effect during compaction. Example 3

Insulating lightweight aggregate concrete blocks can be manufactured by the new method, particularly multi-slotted, thin-walled sections as shown in Fig. 4.

25. Although it has been known that such sections have considerably greater thermal insulation than conventional concrete blocks, the wet manufacturing methods for the latter are not suitable for such extreme shapes. By using dry methods and a specifically designed spray

30. system, it is possible to reduce slot ^"dimensions to lOmm and leaf thicknesses to under 5mm (using 4mm max aggregate size) . This is a surprisingly delicate structure, considering that prior to spraying there is no adhesion between the particles.

35. Manufacturing conditions and mix properties for this product are intermediate between Examples 1 and 2. A typical mix is .1:0.5:3 parts by weight of cement, PFA and "Lytag" lightweight, aggregate from 4mm down to dust. The latter is made from sintered pulverised fuel 5. ash and is about half the density of the aggregates in the previous Examples. This aggregate also contains fines, so the mix properties are therefore not directly comparable to those in earlier Examples.

The process described in our co-pending Application 10. No. 8006777 (2045150) relies on the fibres contained in the constituent mix acting as tensile reinforcement, preventing the dry compacted particles from cracking - or, if cracks do form, by preventing these from spreading to complete rupture. .This is achieved by 15. fibres penetrating across a crack or potential crack and holding the^" sections or clumps of compacted material together. Fibre pull-out is prevented by the frictional resistance of the particles bearing on the length of fibre embedded on either side of the 20. crack.

In addition to these effects, the interlocking network of fibres acts as a barrier or screen, resisting the flow of particles between them. With such small apertures between fibre barriers, relatively modest 25. compaction enables the particles to arch between the fibre restraints and so prevent flow. Even modest amounts of fibre have very marked affects on both dry and wet stability. For example, the green strength of the formulations in Examples 2 and 3 can be more than 30. doubled by adding under 1% of lOOmm glass fibre strands to the constituent mix.

In^"the process of the present invention there are no such arch restraints, screen effects or tensile reinforcement to stabilise the material. The dry 35. particulate mass has to be rendered stable enough for subsequent processing by the frictional resistance between particles and some slight mechanical inter¬ locking with angular particles. This is why the fines content and compaction requirements are so much more 5. critical with this method than with the aforementioned mixes containing fibre reinforcement. In the present process, the fine particles promote interlocking by packing into all available spaces, while the applied vibration and/or pressure ensures that the particles

10. penetrate between the coarse aggregate and pack firm enough to generate the required frictional resistance. The tensile strength generated by such frictional effects is generally too small for the dry material to stand entirely on its own and the structure stands

15. by arching between at least one pair of opposite mould sides ^' (or by ring compression, in the case of annular structures like pipes) . If suitable non-fibre reinforce¬ ment is included in the product, it is possible to remove all vertical support provided by the mould.

20. Stability of the mix is much enhanced by capillary cohesion effects, when only just enough liquid is added. In consequence local overwetting during liquid application should be avoided, since this can cause collapse of the upstanding surfaces. However, by means

25. of the process of the present invention i.e. providing sufficient fines are present in the mix which is then adequately compacted, the mix can possess adequate dry and wet stability and a high enough green strength to enable the mould to be removed completely after wetting

30. and before curing.

Claims

Claims:

1. A method of producing moulded construction products from a liquid setting mixture of fine and coarse particulate materials comprising the steps of mixing 5. the dry constituent materials, said materials including a proportion of fine particles sufficient to substantially surround all coarse particles but not including fibrous reinforcing materials,^' introducing said mixture into a mould, compacting said mixture in said mould to an

10. extent that said fine particles substantially fill the interstices between said coarse particles, removing at least a part of the mould from contact with the thus moulded product, spraying the product at a surface unsupported by said mould with a predetermined quantity

15. of a setting liquid, being a quantity sufficient to wet all of the compacted constituents but insufficient completely to saturate the same, and^' allowing said product to set.

2. A method according to claim 1, comprising removing 20. the wetted product from the mould before the onset of chemical curing.

3. A method according to claim 1 or 2, wherein said part of the mould removed before spraying comprises an inner part or former of the mould.

25. 4. A. method according to claim 1 or 2, wherein said part of the mould removed before spraying comprises an outer wall part.

5. A method according to any one of claims 1 to 4, comprising spraying an exposed upstanding surface of

30. said product.

6. A method according to any one of claims 1 to 5, comprising vibrating at least a part of the mould to compact the dry constituent materials.

7. A method according to claim 6, comprising applying 35. pressure to an upper surface of said product to

O -* ^* compact said dry constituent materials. 8. A method according to claim 6 or 7, wherein the frequency of said vibration is at least 12,000 cycles per minute. 5. 9. A method according to any one of claims 1 to 8, comprising introducing said mixture into said mould at a feed rate not greater than 10mm per second.

10. A method according to claim 9, comprising oscillating the feed of said mixture into said mould to distribute

10. said mixture in said mould.

11. A method according to any one of claims 1 to 10, comprising moving a spray nozzle relative to and adjacent an exposed surface of said compacted product to wet the same.

15. 12. A method according to any one of claims 1 to 11 wherein the proportion of fine particles in said mixture is in the range 15% to 22% by weight.