GB2127343A

GB2127343A - Improvements in or relating to methods of casting concrete articles

Info

Publication number: GB2127343A
Application number: GB08323740A
Authority: GB
Inventors: Rupert John Sidney Mcbean; Thomas Eric Horncy; Michael Johnstone
Original assignee: MYRAYARN Ltd
Current assignee: MYRAYARN Ltd
Priority date: 1982-09-03
Filing date: 1983-09-05
Publication date: 1984-04-11
Also published as: GB8323740D0; GB2127343B

Abstract

The disclosure relates to methods of casting concrete articles in which a mould (11,12,13,14,15) is filled with a wet concrete mix from a hopper (17) and the mix is consolidated by pressure applied by a ram (22) and vibration applied by a vibrator (23). In order to allow immediate stripping of the mould from the cast article a concrete mix containing only sufficient water to activate the cement in the mix is used and the mix is subjected whilst under said pressure to a high amplitude/low frequency vibrator, and then, whilst subjected also to vacuum, to a low amplitude/ high frequency vibration whilst the pressure is first increased and then relieved. <IMAGE>

Description

SPECIFICATION Improvements in or relating to methods of casting concrete articles This invention relates to methods of casting concrete articles and is particularly although not exclusively applicable to the casting of arcuate concrete lining segments.

The invention provides a method of casting a concrete article comprising the steps of: i) preparing a concrete mix with a water to cement ratio substantially equal to a ratio sufficient only to activate all the cement present; ii) filling a mould for the article with the mix; iii) applying a relatively high amplitude/lowfrequency vibration to the mix in the mould whilst maintaining the vacuum; iv) subjecting the mix to vacuum; v) applying a relatively low amplitude/high frequency vibration to the mix and progressively increasing the pressure on the mix to a maximum and maintaining the vacuum;; vi) applying a relatively low amplitude/high frequency vibration to the mix whilst maintaining the pressure on the mix and also the vacuum vii) and then decreasing the pressure on the mix progressively whilst maintaining the relatively low amplitude/high frequency vibration and also the vacuum.

It has been found that the combination of the use of a minimum amount of water in the concrete mix sufficient only to activate the cement in combination with the vibration steps employed to consolidate the mix enables an adequate cast and to have sufficient strength shortly after casting to be removed from the mould rather than cured in the mould for a period of up to twenty-four hours as in conventional concrete casting. Thus very shortly after the completion of one casting operation, the mould is available for a further casting operation and so the number of moulds which would be required for continuous production of the concrete articles is very considerably reduced.In practice it has been found that using the method of the invention, a concrete article has achieved approximately 6 per cent of its 28 day strength within 8 hours and 30% of the 28 day strength within 24 hours of the consolidation/vibration stage without any form of accelerated curing.

The mix may be subjected to vacuum before the high amplitude/low frequency vibration or after the high amplitude/low frequency vibration.

The mould for the article to be produced may initially be overfilled with concrete to between forty and fifty per cent of the mould volume, the mixture being consolidated by the following treatment to the required volume of the mould.

Alternatively the mould can be filled with a weight of mix to provide the requisite volume of concrete.

The following is a description of a specific embodi mentofthe invention reference being made to the accomanying drawings in which: Figure 1 is a perspective view of an apparatus of casting arcuate concrete tunnel lining segments; Figure 2 is a detailed view of a mould filling and mould compacting station of the apparatus; Figure 3 is a graph showing part of the pressure cycle applied to the mould in the compacting station, Figure 4 is a perspective view of an alternative form of apparatus for performing the method; and Figures 5 and 6 illustrate a further method in diagrammatic form.

Referring firstly to Figure 1 of the drawings, there is shown apparatus for filling and compacting moulds for arcuate concrete tunnel lining segments.

The moulds each comprise an open-topped opensided base 11 having an upwardly facing convex moulding surface 12 to define the inner surface of the arcuate concrete lining to be moulded and integral upstanding end walls 13 to define the ends of the moulding. Pairs of side walls 14 are provided for the moulds which are detachably connected to the moulds 11 by releasable pins (not shown). The side walls 14 have convexly curved upper edges 15 to extend between the end walls 13 of the moulds to define the sides of the lining to be moulded.

The apparatus comprises a first, input conveyor 16 to which mould bases 11 are supplied cleaned, oiled and fitted with reinforcement for the segment to be moulded at an entry station A. The mould bases are orientated on the conveyor with the ends 13 of each mould base disposed along the conveyor.

The conveyor is advanced step by step to move the moulds from station to station.

The moulds pass from station A to a second station indicated at B at which pairs of side walls 14 are supplied to the mould sides. The next station along the conveyor is indicated at Cat which there is a feed hopper 17 for concrete for filling the moulds disposed over the conveyor. The feed hopper has a controlled outlet for discharging a predetermined quantity of a concrete mix into the mould disposed below the hopper to provide a required overfill of the mould. A screed 18 is mounted on swing arms 19 on the downstream side of the feed hopper in the direction of movement of the conveyor to ride over the upper edges 15 of the side walls 14 of the filled mould to scrape off any excess material as the mould passes below the screed leaving the mould overfilled by the required level.

At the end of the conveyor 16 there is a station D which comprises a box-shaped enclosure 20 having a base 21 to receive filled moulds one at a time. The enclosure 20 is opened and closed to receive and discharge moulds by a mechanism not shown. A downwardly acting ram 22 is mounted in the top of the enclosure 20 and there is at its lower end a curved plate 22a to complete the mould 10 on the conveyor and define the outer surface of the arcuate concrete segment to be cast in the mould. The plate 10 is a close sliding fit between the side walls 14 and the ram 22 is employed to apply pressure to the plate to consolidate the concrete mix in the mould as described later. In addition a vibrating mechanism 23 is provided in the enclosure for applying vibration to the mould at one or more required frequencies during the consolidation of the concrete in the mould.Finally a vacuum pump is provided connected to the enclosure 20 by a conduit 24to evacuate the enclosure 20 during the consolidation and vibration of the segment described later.

Afurther intermittently moving conveyor 26 extends away from the enclosure D to receive moulds 10 in the enclosure following the compaction/vibration of the concrete in the mould. The moulds pass through a receiving station E to a station F at which the pins holding the side walls 14 on the mould are released from the mould and are raised vertically from the mould and so wiping the vertical faces of the segment and avoiding "plucking" out of the surface of the "green" segment. The side walls 14 are then returned through a cleaning cycle to the entry station A of the first conveyor. The moulds then pass to a delivery station G from where they are passed to a storing area for curing before the moulded segments are finally removed from the mould.

In operation, the moulds 11 are over-filled (in volumetric terms) with a very dry concrete mix, that is a mix with just sufficient water to activate all the cement present. The required overkill for the mould depends on the shape of the mould and the particular concrete mix but for a product of uniform thickness, an over-fill of about 45 or 46% of the volume of the required finished segment has been found to be appropriate. Any reinforcement incorporated in the segment is fitted into the mould before it is filled with concrete.

When the mould is full and is located in the enclosure 20, the pressure plate 22a is brought down on to the top of the mould by the ram 22. The enclosure 20 is then evacuated and the mould vibrator 23 is activated.

Initially the full mould is subjected to a relatively low frequency, high amplitude vibration and the concrete is pressurised slightly by the weight only of the plate 22a. The purpose of the low frequency vibration is to rid the concrete of air voids and to remove any excess water vapour in the concrete.

The vacuum prevents the formation of laminar air voids at the top of the concrete and the vacuum pump removes excess water vapour shaken out of the concrete from the chamber 10.

The amplitude of the low frequency vibration during this stage is in the range of 15 to 100 millimetres and is preferably of the order of 50 millimetres. The frequency of the vibration is in the range 0.4 to 2.5 Hertz, and is preferably about 1 Hertz. The duration of the low frequency vibration is 10 to 25 seconds and preferably about 15 seconds although this depends to some extent on the size of the article being formed.

The moulding is then subjected to a high frequency low amplitude vibration and at the same time is pressurised by the ram 22 through the plate 22a and the vacuum in the enclosure is maintained. The second vibration stage is intended to consolidate the concrete and drive out further excess water. The amplitude of the vibration during the second vibration stage is in the range of 0.5 to 3 millimetres and is preferably of the order of 1 millimetre. Thefrequen- cy of the vibration in the second stage is in the range 5 to 100 Hertz and is preferably about 70 Hertz. The duration of the high frequency vibration is between 30 and 60 seconds and preferably about 45 seconds.

It will be appreciated of course that the time any of the above operations is dependent on the mass of the cast article. During the second vibration stage, the pressure applied to the concrete by the ram 22 is progressively increased from the commencement of the high frequency vibration and Figure 3 shows a graph of the pressure cycle during this vibration stage. The pressure is maintained at its maximum value while the vibration and vacuum are also maintained and the pressure is then progressively decreased. The pressure is decreased progressively while the vibration continues to prevent the concrete from adhering to the plate, the gradual reduction of the pressure allowing the plate to be released from the concrete without damaging the surface of the concrete formed by the plate.

During the second vibration stage the hood pressure is progressively increased at a rate between 0.3 and 10 kilo pascals per second and preferably at a rate of 6.5 kilo pascals per second. The pressure is subsequently decreased at a rate between 0.3 and 20 kilo pascals per second and preferably at the rate of 6.5 kilo pascals per second. The maximum pressure to which the mixture is subjected in the second vibration stage is in the range 5 to 110 kilo pascals and preferably of the order of 65 kilo pascals. The vacuum to which the chamber is evacuated in the range 50 to 100 kilo pascals and preferably of the order of 85 kilo pascals.

Reference is now made to Figure 4 of the drawings which shows an alternative form of apparatus for moulding arcuatetunnel lining segments but embodying the basic principals of the method outlined above.

The apparatus comprises a carousel or turntable 30 mounted on a circular track 31 and having spaced work stations 32 to receive segment moulds within which arcuate lining segments are to be cast.

Pairs of segment bases 33 are delivered to the carousel by a conveyor 34. Each segment base comprises an arcuate plate 35 corresponding in shape and dimensions to the circumferentially extending edge of a segment to be cast with upstanding end members 36 corresponding in dimensions to the axially extending ends of the segment to be cast.

The mould bases are delivered by the conveyor clean, oiled and fitted with such reinforcement as is required for the segments to be cast. The two mould bases are located side-by-side with their concave sides adjacent but spaced apart from one another for the moulding of two segments simultaneously sideby-side one another.

The pair of mould bases are transferred to a work station 32 on the carousel by means of a hoist 37 and a central mould section 38 is then lowered between the two mould bases. The central section 38 is formed with concave mould surfaces 39 on either side thereof to form the outer circumferential faces of the segments to be moulded. Two further mould walls 40 are then located in place against the mould bases to form the inner circumferential faces of the segment. The assembly of mould members are securely fixed together to create two side-by-side oppositely facing moulds for two arcuate concrete lining segments. The Carousel then moves to a filling station indicated generally at 41. The dry concrete mix as described above is delivered to the filling station by means of an overhead conveyor 42 along which skips 43 carrying loads of concrete move.At the filling station a hopper 44 is provided to receive a load of concrete from the skip and to deliver the concrete to a feed and weigh hopper. The hopper 44 has an outlet controlled in accordance with the weight of concrete received by the feed hopper 45 so that just sufficient weight of concrete is delivered to the feed hopper 45 for the casting of two segments. The feed hopper has an outlet end 46 extending over the carousel to deliver the concrete into the two mould cavities provided by the mould assembly. The mould assembly then moves on to the compacting/vibrating enclosure 46 which is substantially the same as the enclosure 20 of the previously described embodiment. In this case however the downwardly acting presser member 48 carries a pair of arcuate plates similar to the plates 35 of the mould bases to complete the moulds.A vibrator plate (not shown) is raised in the enclosure 20 as an air bag is inflated, pushing the mould and segments on the support 32 up the vacuum chamber when the air is evacuated. This automatically operates the compacting ram and vibrating plate simultaneously. When compaction has been completed, the moulds are released from the enclosure 20 and then passed to the stripping station 49 where the segments and segment bases 33 are removed from the side walls 40 and central section 38 by a further hoist 50 and transferred to a delivery conveyor 51 whereby they are taken to a storage section for curing. The lifting of the segments from the side walls 40 and central section 38 serves to wipe the intrados and extrados of each segment to improve the surface finish.The side walls 40 central portion 40 are transferred by the carousel to the assembly station to receive a further pair of segments bases 33, the side walls and central portion being cleaned if necessary.

Reference is now made to Figures 5 and 6 of the drawings which show yet a further form of apparatus for moulding arcuate tunnel lining segments utilising the basic method including frequencies of vibration, pressure and vacuum for the methods outlined above. Like parts have been allotted the same reference numerals used in the previously described embodiment.

The vacuum chamber 20 is disposed at the centre of the apparatus with the hopper 44 for concrete disposed above and to one side of the chamber and having feed hoppers 45 for delivering measured quantities of wet concrete mixture to the mould. Two series of moulds indicated at 60 are disposed on either side of the chamber 20 and the chamber has opening doors 20a on either side thereof to receive filled moulds from one or other of the sets of moulds on either side of the chamber.

At each station in the two series of stations there is a base 61 to receive a mould for an arcuate concrete tunnel lining segment. The mould comprises inner and outer arcuate walls 62,63 joined by end walls (not shown) to form an arcuate tunnel lining segment of the required radius. The lower edges of the wall 62, 63 are formed without turned flanges 60,65 which bear on the base 61 and are engaged by releasable clamps 66 mounted on the base to lock the mould to the base for a moulding operation. One circumferentially extending edge of the cast segment is formed by the upper surface of the base 61 and a separate top plate 67 is insertable between the mould walls 62, 63 to define the other circumferential edge of the segment.

Five stations are provided in each series of stations to either side of the vacuum/vibration chamber as best seen Figure 5 and the sequence of operations carried out in each of the series of stations will now be described. At station one there is a mould which, on a previous station as described below, is filled with concrete, subjected to high amplitude/low frequency vibration cycle and then fitted with a top plate. The mould is moved into the chamber 20, air is evacuated from the chamber and the mould is then subjected to the high amplitude/ low frequency vibration described above whilst the vacuum is maintained in the chamber. The mould is then returned to station one. The base plate clamps are then released. The mould is then moved to station two. At station two the mould is lifted from the segment leaving the latter on the plate.The mould is then moved to station three from where the moulded segment is removed to a pallet at station five. The mould after cleaning and preparation for moulding is then reinstated on the base and reclamped to the base. The mould then moves to station four and is filled with the required weight of concrete from the feed hopper. The mould is then subjected to the high amplitude/low frequency vibration as described in the earlier embodiments except that vacuum is not applied at this stage and then the top plate for the mould is fitted and the mould is transferred to the station one and the sequence of operations continued.

At station five the moulded segment with the bottom plate and fittings are transferred by a "tippler" where the bottom plate and fittings are removed and the segment is loaded on to a pallet.

When the pallet is fully loaded with segments, it is transferred to a curing area for curing of the segments.

Claims

1. A method of casting a concrete article comprising the steps of: i) preparing a concrete mix with a water to cement ratio substantially equal to a ratio sufficient only to activate all the cement present; ii) filling a mould for the article with the mix; iii) applying a relatively high amplitude/lowfrequency vibration to the mix in the mould; iv) subjecting the mix to vacuum; v) applying a relatively low amplitude/high frequency vibration to the mix and a progressively increasing pressure on the mix to a maximum and maintaining the vacuum; vi) applying a relatively low amplitude/high frequency vibration to the mix whilst maintaining the pressure on the mix and also the vacuum; vii) and then decreasing the pressure on the mix progressively whilst maintaining the relatively low amplitude/high frequency vibration and also the vacuum.

2. A method as claimed in claim 1 wherein the mixture is subjected to vacuum prior to application of the high amplitude/low frequency vibration to the mix.

3. A method as claimed in claim 1 wherein the mixture is subjected to vacuum after the application of the high amplitude/low frequency vibration.

4. A method as claimed in any of claims 1 to 3 wherein the mould is initially overfilled by between forty and fifty percent of its volume.

5. A method as claimed in any of claims 1 to 3 wherein the mould is initially overfilled by an amount in the region of forty-five per cent of its volume.

6. A method as claimed in any of claims 1 to 3 wherein the mould is filled with a mixture equivalent in weight to the weight of the volume of concrete of the article to be moulded.

7. A method as claimed in any of the preceding claims wherein the first vibration of the mix is carried out with an amplitude in the range 15 to 100 millimetres and frequency in the range 0.4 to 2.5 Hertz for a duration in the range 10 to 25 seconds.

8. A method as claimed in claim 7 wherein the first vibration is carried out with an amplitude of the order of 50 millimetres and a frequency of the order of 1 Hertz for a duration of the order of 15 seconds.

9. A method as claimed in any of the preceding claims wherein the second vibration is carried out with an amplitude in the range 0.5 to 3 millimetres and a frequency in the range 50 to 100 Hertz for a duration in the range 30 to 60 seconds.

10. A method as claimed in claim 9 wherein the second vibration is carried out with an amplitude in the region of 1 millimetre and a frequency in the region of 70 Hertz for a duration of 45 seconds.

11. A method as claimed in any of the preceding claims wherein the vacuum to which the mix is subjected throughout the process is in the range 50 to 100.

12. A method as claimed in claim 11 wherein the vacuum to which the mixture is subjected is in the region of 85 kilopascals.

13. A method as claimed in any of the preceding claims wherein the maximum pressure to which the mixture is subjected is in the range 15 kilopascalsto 110 kilopascals.

14. A method as claimed in any one of the preceding claims wherein the maximum pressure to which the mixture is subjected is in the region of 65 kilopascals.

15. A method as claimed in any of the preceding claims wherein the pressure to which the mixture is subjected is increased at a rate in the range 0.3 kPa/sec to 10 kPa/sec.

16. A method as claimed in any of the preceding claims wherein the rate at which the pressure to which the mixture is subjected is increased is in the region of 6.5 kilopascals per second.

17. A method as claimed in any of the preceding claims wherein the pressure to which the mixture is subjected is decreased at a rate between 0.3 to 29 kilopascals per second.

18. A method as claimed in any of the preceding claims wherein the pressure to which the mixture is subjected is decreased at a rate of the order of 6.5 kilopascals per second.

19. A method as claimed in any of the preceding claims wherein pressure is applied to the mixture in the mould by means of one wall of the mould which is movable into the mould.

20. A method as claimed in claim 19 and in the case where the article to be cast is an arcuate concrete tunnel lining segment, wherein the wall to which pressure is applied is the wall which bounds the outer circumferential surface of the segment.

21. A method as claimed in claim 19 and in the case where the article to be cast is an arcuate concrete tunnel lining segment, wherein the wall to which pressure is applied is a wall which bounds one of the circumferential edges of the segment.

22. A method of casting a concrete article substantially as hereinbefore described with reference to Figures 1 to 3 or Figure 4 of the accompanying drawings.

23. A method of casting an article substantially as hereinbefore described with reference to Figures 5 and 6 of the accompanying drawings.

24. A concrete article cast according to the method as claimed in any of the preceding claims.