GB2229132A - Pressing concrete bodies and expressing water therefrom - Google Patents

Abstract

Concrete slabs are manufactured by pressing wet concrete between a ram and a mould to compact the concrete and express water from it. A layer of filter material is provided between the ram and the wet concrete or between the concrete and the base of the mould (it is known to make this layer from a non-woven fibre sheet supported on a scrim, the scrim being manufactured from flat tape). This invention utilises a filter material which is made of non-woven fibre and which includes a scrim, in which the filaments of the scrim are such that the thickness in one direction is a significant proportion of the thickness in an approximately perpendicular direction. The filaments of the scrim are preferably of circular cross- section. The filter material may be secured to the ram and/or the base of the mould by a discontinuous coat of adhesive.

Description

Method and Apparatus for Pressing Concrete Bodies This invention relates to a method and apparatus for pressing concrete bodies and, more particularly, to a method and apparatus for pressing concrete paving stones.

Concrete paving stones are usually produced by inserting a wet concrete mix into a mould and compressing the concrete mix in the mould to compact the mix. The concrete mix In the mould was conventionally compressed by the Introduction of a ram member into the upper portion of the mould, the concrete mix being compressed between the internal surfaces of the mould and the ram member. During the compression, the concrete mix is compacted and formed Into a paving stone the shape of which corresponds to the shape of the mould and in addition at least part of the water content of the wet concrete mix is extracted therefrom.

In order to assist the release of the water content from the concrete mix during compression, it is usual to place a sheet of filter material between the base of the mould and the concrete mix and between the ram member and the concrete mix. When the concrete mix has been compressed and the paving stone removed from the mould, the sheet of filter material is peeled back from the paving stone. In the past it was usual for the filter material to consist of a paper sheet and It was found that the sheets of paper had to be discarded after they had been used to produce only one paving stone.

In the specificatlon of U.K. Patent No. 1,578,000 there Is described apparatus for producing concrete bodies in which the filter material Is secured to the ram or the base of the mould or both as the case may be, so that it remains In the mould when the compacted concrete body is removed. That specification also describes the use of filter material made of thermoplastic fibres such as polypropylene.

The use of filter material secured to the ram and/or the base of the mould, particularly If it is made according to the preferred features described in specification No. 1,575,000 enables the filter material to be used repeatedly in the manufacture of the number of concrete bodies, and In some Instances, such a filter material has been used to produce some hundreds of concrete bodies before it was necessary to change it.

However, wth certain types of concrete mix, good results have not been achieved and in extreme cases, the filter material has become Ineffective after only a small number of concrete bodies has been produced.

According to a first aspect of the present invention a method of producing concrete bodies comprises pressing wet concrete between a ram and a mould to compact the concrete and express water from it, there being a layer of filter material attached to at least one of the ram and the base of the mould, the filter material comprising a non-woven fibre sheet having a smooth water porous surface, the fibre sheet including a scrim the warp and/or weft of which Is formed from filaments, those filaments being formed so that the thickness in one direction Is a significant proportion of the thickness In an approximately perpendicular direction. Preferably the significant proportion is at least one third. In the preferred construction, the filaments of the scrim are of circular cross-section.

The filter material may be secured to at least one of the ram and/or the base of the mould. It may be secured by a discontinuous coat of adhesive applied over a sufficient area of the filter material to ensure the adhesion of the filter material to the ram and/or the base of the mould under normal working conditions but to allow water expressed from the mix to pass through it without allowing the fines to escape.

The use of a scrim, the filaments of which are of a thickness in one direction which is a significant proportion of the thickness in an approximately perpendicular direction apparently plays a significant part in determining the performance of the filter material, especially with regard to the number of times that the filter material can be used before It has to be changed. It is believed that the effect of the scrim on the flow of water through the filter material when water is being expressed from the concrete is of considerable importance. Similarly, the preferred method of adhering the filter material to the supporting structure appears to play a significant part In the performance of' the filter materlal.

References throughout this specification to the filter material being adhered to the ram or base of the mould, are to be taken to Include adhesion of the filter material to a foraminous element such as a perforated plate which is Itself secured to the underside of the ram or lying on the base of the mould. However, by use of the invention, it has been found possible to adhere the filter material directly to the ram (i.e. not directly to the perforated plate as hitherto). This is clearly an advantage in that the perforated plate may not be required for all applications.

According to a second aspect of the present invention, apparatus for pressing concrete bodies comprises a ram member and a mould into which the ram member is insertable to compress a wet concrete mix inserted in the mould to form the concrete body, and a layer of filter material disposed so that it Is either between the wet concrete mix, when the mix is in the mould, and the ram member and attached to the ram member, or disposed so that it Is between the wet concrete mix when the mix is in the mould, and the base of the mould and attached to the base of the mould, the filter material comprising a non-woven fibrous sheet having a smooth water porous surface, the non-woven fibrous sheet including a scrim the warp and/or weft of which is formed from filaments, those filaments being formed so that the maximum thickness In one direction is a significant proportion of the maximum thickness in an approximately perpendicular direction, the surface of the filter permitting the passage of water therethrough, whilst retaining the concrete mix on the surface so that the filter can be used repeatedly. Preferably the significant proportion is at least one third.

Preferably, the or each layer of filter material is formed, at least partially, from nylon. Preferably the scrim Is formed from a nylon monofilament. It Is further preferred that the monof i I lament is of substantially circular cross-section.

The surface of each layer of the filter material which is arranged to contact the wet concrete mix is preferably subjected to a hot calendering process to produce the smooth porous surface on the layer of filter material. The hot calendering process may comprise passing the layer of filter material over a heated roller. Preferably, also, the filter material is subjected to a process to "burst" the material after calendering, thereby releasing any fibres In the core of the material which had adhered together as a result of the calendering process. The "bursting" may be achieved by needling the fabric on a needle loom using barbless needles. By using a suitable thickness of needles and/or holes per unit area (I.e. needle density) any desired level of porosity can be accurately achieved.The porosity determines the efficiency with which the filter material releases the slab. The filter material may be subjected to a waterproofing process and the side edges of each layer of filter material may be sealed (by heat, for example) to reduce the fibrous characteristic thereof.

The or each layer of filter material is preferably secured to the ram member and/or the mould by a discontinuous adhesive (as defined herein and also In U.K. Patent Application No. 8730127). Suitablyu the adhesive is pressure-sensitive.

The filter material may be secured to a perforated metal plate which is mounted on the mould or on the ram member. Alternatively, the layer of # i filter material may be secured to a layer of porous metal foam and/or a metal grid which Is mounted on the mould or on the ram member.

Whilst the exact theory of operation is not yet understood, It appears that the use of a scrim fabric in which the warp and/or weft of which Is formed from filaments, those filaments being formed so that the maximum thickness in one direction is a significant proportion of the maximum thickness in another approximately perpendicular direction ensures that when the concrete mix In the mould is compacted (possibly under the order of 1 ton per square inch of pressure) there are significant areas of the filter material through which water can flow substantially unimpeded by the scrim.Prior to the present Invent ion, the scrim was formed from a polypropylene woven slit film (i.e. a wide thin film or tape), which had the disadvantage that when under pressure, with certain concrete mixes, the film would be forced flat against the non-woven fabric and/or the smooth porous surface of the filter material so that It acted as a partial seal, preventing significant amounts of water from being expelled from the mould. In the present case, the use of a "three dimensional circular cross-section filament rather than a "two dimensional" flat film section filament allows much more efficient operation for certain concrete mixes than hitherto thought possible.In addition, the use of a combination of circular cross-section nylon monofilament for the scrim and polypropylene for the needled staple allows the filter material to be used many more times (with certain aggregate mixes) than has been possible using a filter material in which the scrim is formed of flat-tape. The use of a nylon monofilament means that the porosity of the filter material may be controlled more readily. In some Instances a polypropylene based scrim has exhibited shrinkage when processed and/or variations in porosity over a particular time period. A nylon based scrim may not have these disadvantages.

One embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic sectional side elevation of an apparatus for forming concrete bodies; Figure 2 shows part of the reverse face of a filter medium for use in the apparatus shown in Figure 1, illustrating only one form of discontinuous adhesive coating; Figure 3 is a cross-section through a prior art flat-tape filter material in its natural uncompressed state; Figure 4 Is a cross-section through the prior art flat-tape scrim filter material as shown in Figure 3 when compressed; Figure 5 is a cross-section through a filter material having a scrim made of circular section nylon monofilament yarn in Its natural uncompressed state; and, Figure 6 is a cross-section through the filter material of Figure 5 when compressed.

The apparatus which 5 illustrated In Figure 1, is Intended to be used for forming concrete paving stones, and It generally comprises a mould 10 having sidewalls 12 which define a substantially rectangular cavity open at the top and bottom. The bottom surface of the mould 10 is closed by a lower press plate 14 which is adapted to move upwardly through the mould cavity by an injection mechanism (not shown). The lower press plate 14 is also provided with a series of drain holes and connecting channels 16 to allow liquid to drain therethrough from the mould cavity.

A ram mechanism indicated generally at 18 which includes a die head Is mounted above the mould 10 and is adapted to be moved downwardly towards the mould 10, and a compression plate 20 is mounted on the die head of the ram mechanism 18. When the ram mechanism 18 moves downwardly towards the mould 10, the compression plate 20 (sometimes referred to as a former plate) slides into the open top of the mould cavity. The compression plate 20 is provided with a series of drain holes and connecting channels 21 through which liquid can be drawn from the mould cavity.

A perforated metal plate 22, which is arranged to be a close fit in the mould cavity is mounted on the lower surface of the compression plate 20 and is connected thereto by countersunk set screws 24. The perforated metal plate 22 is arranged to conform closely In shape to the shape of the mould cavity and the edges thereof are ground to give a sharp edge to the concrete body.

A similar perforated metal plate 26 is carried on the upper surface of the lower press plate 14. The perforated metal plate 16 provides an effective foraminous base for the mould, and the perforated metal plate 22 provides a foraminous bottom surface on the compression plate 20 which can be regarded as a ram.

Two layers of filter material 30 and 32 are provided which both closely conform to the shape of the mould cavity, and each layer of filter material 30, 32 is formed mainly of thermoplastic fibres such as polypropylene fibres. In this particular arrangement, the upper and lower surfaces of the layers of filter material 30 and 32 are subjected to a hot calendering process to produce a surface of smooth non-fibrous nature which is porous to water. The hot calendering process consists of, for example, passing the layer of filter material over a heated roller. The layers of filter material are cut to size by die stamping on a cutting press.

The layers of filter material 30 and 32 are secured to their associated metal plates and 26 by means of a discontinuous hot melt adhesive. Such a hot melt adhesive is described in detail in U.K. Patent Application No. 8730127. However, for the sake of completeness, it will be described, In general terms, hereinafter. In many embodiments lower securing screws 28 will not be used since the heads thereof may leave an impression upon the finished slab.

The method of using the apparatus illustrated in Figure 1 will now be described: The lower press plate is disposed in the bottom of the mould cavity as illustrated in Figure 1, with a layer of filter material 32 secured thereto. A quantity of wet concrete mix 34 is placed in the mould 10 to fill the mould cavity to the required depth.

The ram mechanism 18 is operated to move the plate 20 downwardly Into the mould to the position illustrated in Figure 1. During the movement the concrete mix 34 is compressed between the layers of filter material and the compaction of the concrete mix causes water to be expressed from the concrete mix. This water passes through the porous surfaces of the layers of filter material 30 and 32 which will allow the passage of water, but retain the cement and the fines in the mould.

The water flows through the filter material, then through the perforations of the metal plates 22 and 26 and then out of the mould cavity via the series of holes and channels 16 and 21 formed in the lower press plate 14 and the compression plate 20. The concrete mix 34 takes up the shape of the concrete paving stone defined by the mould cavity.

After compression, the compression plate 20 is withdrawn from the mould by actuation of the ram mechanism 18 and the concrete paving stone Is ejected from the mould cavity by actuating the ejection mechanism (not shown) which moves the lower press plate 14 upwardly through the mould 10.

The concrete paving stone thus formed is removed from the layer of filter material 32 and is then allowed to cure. The layers of filter material 30 and 32 are retained on the compression plate 20 and the lower press plate 14 respectively and the apparatus can then be used to repeat the operation to produce further paving stones.

It will be appreciated that the expressloh of water from the concrete mix during the compression thereof Is an important part of the method of manufacture described herein.

The adhesive described In UK Patent Application No. 8730127 is provided in the form of a discontinuous coating on one face of a substrate. The substrate Is In the form of a roll, but its two faces have a different release effect so that as the substrate Is unrolled, ali the adhesive is on one face. The discontinuous adhesive coating is appi led to the substrate In the form of discrete particles 50. It will be observed from Figure 2, that the particles 50 are applied to the substrate in a pattern of ranks and files, which cover the entire surface of the substrate. When the filter material 30 or 32 has been prepared it is placed in face-to-face contact with the substrate which is then cut to the same size as the filter material .The filter material and the substrate are then pressed together using a hot rotary ironer. In effect, the discontinuous adhesive coated substrate is a transfer, but the adhesive particles have sufficient tackiness to enable them to secure the substrate to the face of the filter material I particularly when subjected to the hot ironing process.

The adhesive particles are pressure-sensitive.

On the first operation of the apparatus to form a paving stone after application of the adhesive particles, they are able to flow under the compressive load, remaining discontinuous but improving the adhesion substantial ly.

When a particular piece of filter material is to be used, the substrate is peeled off, leaving the pattern of adhesive particles 50 transferred on to the reverse face of the filter material. The filter materiai is pressed with its reverse face against the top face of the perforated plate 20 or the underside of the perforated plate 22 to cause the filter material to adhere to its respective perforated plate.

The construction of the filter material will now be described. In Figure 3 a prior art filter material 100 comprises generally a woven scrim 101 (having warp and weft components 102 and 103 respectively made from slit film (flat-tape) yarn) and an arrangement of needle punched stapled fibres 104. The stapled fibres are needle punched through the scrim so that the scrim provides a reinforcing for the filter material but it is embedded In the centre of the thickness of the filter material. The scrim 101 may be made more stable by weaving it by means of leno weaving, and the leno construction where the warp yarns are in crossed pairs is illustrated. The thickness of the warp and weft yarns has been exaggerated in Figures 3 to 6 for clarity.

Whilst some of the stapled fibres 104 are located at least partially in a vertical direction perpendicular to the faces of the filter material, such fibres have been omitted for illustrative simplicity. The remainder of the fibres 105 (I.e. those having a predominantly horizontal direction) are indicated diagrammatically in Figures 3 and 4.

In Figure 3 It will be observed that the uncompressed fibres lie virtually parallel to the warp components 102 of the scrim. Even when uncompressed a large number of fibres are In close contact with the surface of the scrim 101.

When the filter material 100 Is compressed (under the significant pressures previously mentioned) the fibres 105 are pressed closer still to the surface of the scrim 101. Since the scrim itself is so thin in the vertical direction and so wide in the horizontal direction this means that very little space 107 remains between the fibres near to the edges of the warp/weft components or near to the region where those components cross over. In essence then It seems that this prior art arrangement of filter material will act as a limited form of seal, with the water being unable to travel easily In a horizontal plane, the result of which is that water Is partially prevented from escaping from the moulds.

In use, this will not be of great concern in the central area of the filter material since the water will be able to escape vertically through the spaces between the fibres and into the drainage channels of the apparatus. However, towards the edges and corners of the filter materlal, the wet concrete mix will be forced sideways from the centre. The effect of this is two-fold: firstly, the filter material itself will become worn towards its edges and corners for mixes having a high abrasive property; and, secondly, at least a proportion of the water in the mix will be forced through the filter In a direction having an at least part horizontal component.

The fact that the water is restricted from draining horizontally through the filter material and the additional wear on the edges/corners produces a deterioration In the filtering properties of the filter material with the result that with certain mixes the filter material has to be replaced after the production of a relatively short run.

As an example, tests have shown that for one specific mix comprising granite chippings, granite dust, sand, and (possibly) fly ash and cement each filter constructed In accordance with the prior art needed replacing after approximately 15 pressings. Tests have also shown that filter materials manufactured in accordance with the present invent ion may last as long as 400 pressings with the same type mix.

Turning now to Figures 5 and 6 a filter material 110 (in accordance with the Inventlon), comprises a nylon scrim 111 (having warp and weft components 112 and 113 respectively made of generally circular cross-section yarn) and an arrangement of non-woven stapled fibres 114 needle punched thereto. In certain embodiments the scrim 111 may be woven and/or heat sealed in a manner to fix the grid pattern.

As with Figures 3 and 4, some of the non-woven fibres 114 are located at least partially in a vertical direction these fibres have been omitted for illustrative simplicity. The remainder of the fibres (I.e. those having a predominantly horizontal direction) are indicated diagrammatically in Figures 5 and 6.

in Figure 5 it will be observed that, especially in the region of the cross-over points of respective warp and weft fibres, the uncompressed fibres lie much less parallel to the warp components 112 than In the Figure 3 construction. When compressed It is believed that the said fibres tend to follow a generally undulating path, the peaks and troughs of which are associated with the presence or absence of warp components (as shown In Figure 6).

When the filter material 110 Is compressed (under the significant pressures mentioned) the fibres 115 are pressed close to the scrim 111. Whilst, it is believed, the concentration of fibres around the curved surfaces such as 116 of Figure 6 Is relatively acute, the arrangement of fibres under pressure is such that spaces 117 are maintained near to the cross-over of warp and weft components. Comparing the size of the spaces 117 of Figure 6 to the spaces 107 of Figure 4 (which are almost non-existant) It will be appreciated that the difference is very significant.

Whilst in the present invention, limited regions of the filter material 110 may act as a seal (due to the acute concentration of fibres as at 116) horizontal drainage of water will be permitted by the existence of the spaces 117. In any case, water will be able to drain through the filter material in a vertical direction more easily since there will be less of an obstruction due to narrower fibres being used to form the scrim.

Also, since the scrim is a thicker construction than the prior art scrim, any wear upon the filter material in the region of Its corners and/or edges will be minimised.

Needle punching with barbiess needles helps to create a track for the water to pass through the filter (vertically). It is stressed, however that when the needle Is withdrawn the fibres close up again, but a needle hole is left In the calendered surface, and the number of needle penetratlons affects the porosity of the filter. This assists water flow vertically through the calendered surfaces as opposed to the horizontal drainage previously referred to.

This invention is not restricted to the details of the foregoing embodiment. The invent ion extends to any novel one, or any novel combination, of the features disclosed in this specification and/or drawings, or to any novel one, or any novel combination, of the steps of any method or process disclosed herein.

Claims (20)

CLAIMS:

1. A method of producing concrete bodies comprising pressing wet concrete between a ram and a mould to compact the concrete and express water from it there being a layer of filter material attached to at least one of the ram and the base of the mould, the filter material comprising a non-woven fibre sheet having a smooth water porous surface, the fibre sheet including a scrim the warp and/or weft of which is formed from filaments, those filaments being formed so that the thickness in one direction is a significant proportion of the thickness in an approximately perpendicular direction.

2. A method of producing concrete bodies as claimed in Claim 1, in which the thickness of the filaments forming the scrim in one direction is at least one third the thickness in an approximately perpendicular direction.

3. A method of producing concrete bodies as claimed in Claim 1 or Claim 2, in which the filaments of the scrim are of circular cross-section.

4. A method of producing concrete bodies as claimed in any one of Claims 1 to 3, in which the filter material is secured to at least one of the ram and/or base of the mould.

5. A method of producing concrete bodies as claimed in Claim 4, in which the filter material is secured by a discontinuous coat of adhesive applied over a sufficient area of the filter material to ensure the adhesion of the filter material to the ram and/or the base of the mould under normal working conditions but to allow water expressed from the mix to pass through it without allowing the fines to escape.

6. Apparatus for pressing concrete bodies comprising a ram member and a mould into which the ram member is insertable to compress a wet concrete mix inserted in the mould to form the concrete body, and a layer of filter material disposed so that it is either between the wet concrete mix, when the mix is in the mould, and the ram member and attached to the ram member, or disposed so that it is between the wet concrete mix when the mix is in the mould and the base of the mould and attached to the base of the mould; the filter material comprising a non-woven fibrous sheet having a smooth water porous surface, the non-woven fibrous sheet including a scrim the warp and/or weft of which is formed from filaments, those filaments being formed so that the maximum thickness in one direction is a significant proportion of the maximum thickness in an approximately perpendicular direction, the surface of the filter permitting the passage of water therethrough, whilst retaining the concrete mix on the surface so that the filter can be used repeatedly.

7. Apparatus for pressing concrete bodies as claimed in Claim 6, in which the maximum thickness of the filaments forming the scrim in one direction is at least one third of the maximum thickness in an approximately perpendicular direction.

8. Apparatus for pressing concrete bodies as claimed in Claim 6 or Claim 7, in which the or each layer of filter material is formed at least partially from nylon.

9. Apparatus for pressing concrete bodies as claimed in Claim 8, in which the scrim is formed from a nylon monofilament.

10. Apparatus as claimed in any one of Claims 6 to 9, in which the filament forming the scrim is of substantially circular cross-section.

11. Apparatus for pressing concrete bodies as claimed in any one of Claims 6 to 10, in which the surface of each layer of the filter material which is arranged to contact the wet concrete mix is subjected to a hot calendering process to produce the smooth porous surface on the layer of filter material.

12. Apparatus for pressing concrete bodies as claimed in Claim 11, in which the hot calendering process comprises passing the layer of filter material over a heated roller.

13. Apparatus for pressing concrete bodies as claimed in Claim 11 or Claim 12, in which the filter material is subjected to a process to "burst" the material after calendering, thereby releasing any fibres in the core of the material which adhered together as a result of the calendering process.

14. Apparatus for pressing concrete bodies as claimed in Claim 13, in which the "bursting" is achieved by needling the fabric on a needle loom using barbless needles.

15. Apparatus for pressing concrete bodies as claimed in any one of Claims 6 to 14, in which the filter material is subjected to a waterproofing process and the side edges of each layer of filter material are sealed to reduce the fibrous characteristic thereof.

16. Apparatus for pressing concrete bodies as claimed in any one of Claims 6 to 16, in which the or each layer of filter material is secured to the ram member and/or the mould by a discontinuous adhesive.

17. Apparatus for pressing concrete bodies as claimed in any one of Claims 6 to 16, in which the filter material is secured to a perforated metal plate which is mounted on the mould or on the ram member.

18. Apparatus for pressing concreted bodies as claimed in any one of Claims 6 to 16 in which the layer of filter material is secured to a layer of porous metal foam and/or a metal grid which is mounted ~ on the mould or on the ram.

19. A method of producing concrete bodies substantially as herein described with reference to the accompanying drawings.

20. Apparatus for pressing concrete bodies constructed and arranged substantially as herein described with reference to the accompanying drawings.