EP0478604A1 - Formwork for forming structural beams - Google Patents

Abstract

A method and apparatus for forming a structural beam for a building or the foundations of a building comprising, forming and placing a formwork which comprises at least two opposing prefabricated and freestanding elements (25) arranged to define a mold for the beam. Each element (25) has a side wall (26) integrally formed with a foot (27) having an outwardly projecting portion (28) and, preferably, an inwardly projecting portion (29). In use, the two elements (25) are arranged in opposition to each other so that they define a mold which has two side walls formed by the side walls (26) of the elements (25) as well an impermeable base formed by the inward projecting portions (29) of the legs as well as outward projecting stabilizing portions formed by the outward projecting portions (28) of the legs.

Description

Formwork for Forming Structural Beams

The invention relates to a method and apparatus for forming concrete structural beams used in a building or building foundation such as, for example, ground beams, aerial beams or like structures. In forming the foundations of a building, concrete ground beams in conjunction with driven concrete piles are a well known way of providing good foundation support for later structural work. In preparing the foundation, concrete piles or pillars formed of concrete and having embedded within thick metal reinforcement bars, are pile-driven into the ground as deep as possible. Where the ground is not particularly firm, the piles are driven in until the end of the pile strikes a layer of more solid material. After a pile is fully driven in, the top end of the pile is capped and cracked and the ends exposed. Concrete ground beams are then formed between piles to link up piles. The width and depth of the beams are determined by what is to be built on top of the foundations. Conventionally, a trench is dug, the bottom of which is lined with drylene concrete. This is necessary to provide a good surface to mould the beam and to protect the beam when formed. Wooden shuttering and bracing is then erected by a carpenter on the base of the trench between the two piles to provide a mould or formwork for the ground beam. The mould comprises the drylene-coated base of the trench and two walls provided by the shuttering. The shuttering is then covered with releasing oil, to enable subseguent removal of the shuttering, and a reinforcement cage is placed on loose spacer blocks on the floor of the trench. The reinforcement cage comprises a metallic meshed cage, substantially rectangular in cross-section, running the length of the trench inside the two walls of the shuttering. The cage needs to be raised above the floor of the trench by the spacer blocks to ensure that cage will not be exposed to damp when subsequently in place in the ground beam. The ends of the cage are meshed with the reinforcement bar ends protruding from the concrete piles. Then, the concrete is poured into the mould formed. After the concrete has set, the shuttering is removed to be used again and the volume of the trench left by the removal of the shuttering is backfilled with an inert material.

This method of forming ground beams suffers from a number of disadvantages. Principally, there are many problems associated with the amount of waste that has to be excavated. In order to provide sufficient space in the trench for adequate bracing and support to be provided to enable the shuttering to hold still while the concrete is poured, a trench of at least three times the width of ground beam has to be excavated. This space is also needed to allow the carpenter to erect the shuttering and dismantle the shuttering after the beam is set. Excavation of unnecessary amounts of material increases both time and labour costs and provides a problem in stockpiling and disposing of the unwanted earth. This problem of 'muck away' is proving increasingly important due to the difficulty in finding sites to dump the excess material, the cost of transporting the material etc and is exacerbated by the fact that excavated soil expands in volume. Furthermore, after the beam is set, the remaining space in the trench must be filled in again with inert material and the material compacted on each side of trench.

Also, the conventional method of forming ground beams by constructing wooden shuttering is extremely time-consuming. Due to the expense in providing large amounts of wooden shuttering (including possibly plastic or metal faced wood for the internal walls of the mould) it is frequently the case that only part longitudinal sections of the ground beam can be constructed at a time. After the concrete has been poured in there then ensues a wait of at least 24 hours while the concrete sets, after which the shuttering is dismantled and re¬ assembled to form the next portion of the ground beam. Not only does this greatly increase the time needed to construct the foundations, it also results in a ground beam formed with at least one 'cold joint', rendering the beam less structurally sound. Furthermore, the job of shuttering is a skilled job and generally requires the services of a skilled formwork carpenter to construct. These problems of slowness and complexity are also associated with conventional methods of forming aerial beams which are formed by setting concrete around the metal girders of a building, the concrete being set in a mould defined by a wooden shuttering similar to that used in forming ground beams. Furthermore, a relatively wide platform may have to be provided to support the shuttering around the beam.

The problems associated with formation of ground beams are to some degree addressed in EP 0141465 which discloses a method for forming a foundation in which a shuttering unit is used comprising a polystyrene box formed from four walls and a bottom and having a reinforcement cage within, the box remaining in place around the ground beam formed after the concrete has set.

By using a disposable formwork that remains in place after the concrete has set, the trench can then be backfilled immediately prior to pouring in the concrete, the backfill then providing the main support for the walls of the formwork against outward displacement as the concrete is poured in. Using the backfill to provide the support also means that the trench can be excavated to a width not much greater than that of the ground beam, reducing the amount of material to be excavated, and resulting in a saving of time and money in excavation, storage and muck away. The amount of material to be backfilled is also less than in the usual method which results in further cost saving if special inert material is used.

Backfilling at the earlier stage also enables easy access to fill the formwork with concrete and removes the need for providing any further working platforms or the like.

The shuttering unit of EP 0141465 suffers from a number of disadvantages, particularly with regard to stability and convenience of use. In order to enable the unit of EP 0141465 to be sufficiently supported to both stand upright and to hold its shape during backfill the reinforcement cage is provided on spacers inside the box and the walls and floor of the box bound against the cage by bracing wire. Even using this bracing it seems likely that the unit is not particularly stable particularly during backfill and binding the unit may prove an awkward and time consuming process. Furthermore, using a box of set dimensions clearly reduces the adaptability of the structure in forming ground beams of different lengths and widths. To produce a ground beam of double the length of the box, two boxes must be placed adjacent to each other, resulting in the formation ground beam of broken length. According to one aspect of the invention there is provided a method of forming a structural beam for a building or building foundation comprising providing a formwork having two side walls arranged to provide a mould for the beam, characterized in that the formwork comprises at least two pre-fabricated self-standing opposing parts, each part comprising a side wall formed integral with a supporting foot extending outwardly from the side wall, stabilising the supporting feet to assist in maintaining the position of said parts, filling the mould with poured concrete and allowing the concrete to set.

Where used in the formation of a ground beam, the method comprises excavating a trench, positioning the formwork parts in the trench so that the supporting foot of each extends outwardly from the respective side wall to the adjacent wall of the trench, backfilling with material onto the supporting feet at least part of the volume between the side walls of the formwork and the adjacent walls of the trench and, after backfilling, filling the mould with concrete and allowing the concrete to be set, the formwork permanently remaining in place around the set concrete. The use of a formwork having two pre-fabricated self-standing parts shaped to define the mould provides a number of significant practical advantages over prior art methods regarding installation time and cost, manufacturing cost, and adaptability. Location of the mould parts is simple to achieve, and stability may be provided e.g. by depositing material onto the feet to weigh each part down, fixing the feet by using a road pin or the like extending through the feet or by merely stopping movement of the parts by using a block at the end of the feet. In each case the use of complex bracing is avoided.

Using integral feet to provide self-standing parts means that each part can be quickly and easily positioned and, by eliminating the need for further bracing, can provide a mould free of obstruction such that a person can work inside the mould, if desired.

As compared with the prior art box structure disclosed above, the formation of a mould by separate opposed self-standing parts also leads to improvements concerning ease of manufacture, transportation and storage, and the lack of end walls means that a reinforcement cage may conveniently extend continuously from the central region between the parts to around a pile for intermeshing with the reinforcement wires extending from the pile.

When used in the formation of a ground beam, the feet will be weighted down and stabilised by the initial backfill of material onto the feet and will provide good support or anchorage to the side walls during any further backfill and during the subsequent pouring in of concrete. After this first backfill minor adjustments of position of the formwork may then be conveniently made prior to further backfill and concrete pouring. Whilst it will be usual that the remainder of the volume between the side walls and the trench walls is backfilled prior to the pouring of the concrete the stability of a preferred formwork is such that this may not be necessary, particularly if additional supports such as road stakes (see below) are provided.

By using a pre-fabricated formwork that remains in place, a complete section of the ground beam can more readily be formed at once, without the need to dismantle and reassemble a support assembly. Thus, this method is particularly applicable to form a single piece ground beam linking two or more piles, avoiding problems such as cold joints and reducing the time needed to form the ground beam. Furthermore, by virtue of the simple construction of the invention, two lines of opposing parts can be arranged along the length of the trench to provide a mould of desired and unbroken length. Advantageously, each part may be formed with a tongue and groove arrangement in each end such that a line of parts can be matingly engaged.

The method is also applicable to the forming of ground beams or like foundation structures above ground and outside of a trench where full backfilling is not desired but rather only a small amount of material placed onto the feet may be used to assist in stabilising the formwork. The method may also be used in the formation of aerial beams around girders used in the structure of a building above ground. In such an application the formwork may be positioned around the girder on a platform some distance above ground. Stability may be best provided by use of road stakes or by use of a block or rail fixed to the platform at the end of the feet to stop lateral movement of the parts.

In these above ground situations the formwork may be left in place or removed, as desired.

In a particularly preferred embodiment, each formwork part has a foot also extending inwardly of its respective side wall such that in use the formwork as a whole defined by the two parts has an internal U-shaped cross section and has two side walls and an impermeable base. Provision of a base as part of the formwork means that the surface of floor of the trench does not have to be covered with drylene concrete. Instead, the method can include lining the floor of the trench with a layer of sand reducing the cost considerably. Furthermore, the substantial inverted T-shape cross-section of this embodiment is especially stable when put in position by the very nature of its shape. Advantageously, each or one of the parts can be designed to be of an asymmetric T-shape, such that different internal formwork widths can be formed by re-arrangement of the two parts.

Alternatively or in addition to having a foot extending inwardly to form a base, a separate base may be placed between opposing parts to vary the width of the formwork.

The method may further include the step of placing one or more restraint straps or top braces between the top of the side walls to provide extra initial support to the walls, in particular where backfill material is used to stabilise the parts. These can be removed after the concrete has set and need not form part of the beam. Stakes may also be driven through the foot of each part to provide additional initial support to the side walls.

Viewed from a further aspect the invention provides a formwork having two side walls which can be arranged in use to provide a mould for forming a structural beam for a building or building foundation characterized in that the formwork comprises at least two pre-fabricated separate self-standing opposing parts, each part comprising a side wall formed integral with a supporting foot extending outwardly from the respective side wall. For the reasons explained earlier, the formwork of the invention when used to form a ground beam or the like is particularly stable in use during backfill, as the backfill weighs down and anchors each part in position. Advantageously therefore, an outwardly extending part of each foot may be provided with a recessed portion or portions to catch the backfill.

Preferably, each foot also extends inwardly of its respective side wall, such that each part has a substantially T-shape cross-section. In use, as explained before, opposing parts may then be arranged to form a formwork having an impermeable base. Alternatively, each part may have a simple L-shaped cross-section. Advantageously, each or one of the parts can be designed to be of an asymmetric T-shape cross-section. Preferably, each part includes supporting webbing integrally formed with and between the side wall and foot. This webbing lends extra support and rigidity to the side wall.

Preferably, an inwardly extending part of each foot is pre-fabricated with integral spacers upon which a reinforcement cage may be placed. This removes the need for having loose spacers and the problems associated with keeping them secure during the formation of a ground beam. These integral spacers may be formed as protruding ridges or as blocked areas separated by grooves.

The formwork parts in either aspect of the invention may be formed from concrete as this proves an inexpensive, easily manufactured and reliable construction. Alternatively, other materials could be used, such as metal or plastic. The material chosen should be resistant to the effects of moisture to which the formwork may be exposed after many years in situ in a building or in a buildings foundations. In a particularly advantageous embodiment, however, the formwork parts are formed from a lightweight foamed plastic material, such as polystyrene, by, for example, a conventional moulding process. In addition to ease of manufacture and cost benefits, such a formwork is easily transportable and positionable, leading to a reduced time in setting up the formwork. Also such formworks may be cut and shaped on site and in use to provide for trenches of different widths, to arrange two adjacent formworks in a mitred joint to vary the width of a particular trench etc. Furthermore, formworks formed from foamed plastics such as polystyrene have the additional advantage of being flexible and less rigid than those made from concrete so that, after the formwork is in place in the foundation, ground movements can be accommodated by the formwork, which will bend or flex. This reduces the chances of cracks appearing in the formwork and results in better resistance to damp etc. and better weathering. When left in place surrounding a ground beam, a foamed plastic formwork also provides an advantageous insulation effect. Viewed from a further aspect the invention comprises a formwork part moulded from a foamed plastic, such as polystyrene, for use in formation of a structural beam for a building or building foundation, characterized by a side wall formed integral with a supporting foot extending at least outwardly in one direction perpendicular to the side wall, said supporting foot having a recessed portion or portions adapted to receive material so as to anchor the formwork part in use. Such part may also include a foot portion extending in the opposite direction for defining a base part for a mould in use.

Embodiments of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which:

Figs. 1 to 3 illustrate the conventional method of forming a ground beam, showing a cross-sectional views of the trench, shuttering and ground beam.

Figs. 4 to 7 illustrate a method and apparatus according to an embodiment of the invention for forming a ground beam, showing cross-sectional views of the trench, formwork and ground beam so formed,

Fig. 8 is a side or cross-sectional view of the formwork of Figs. 4 to 7,

Fig. 9 is a perspective view of the formwork of Figs. 4 to 7 and Figs. 10 and 11 are a side view and a perspective view respectively of a further formwork part according to the invention.

Conventionally, and with reference to Figs. 1 to 3, first a trench 1 is dug, of approximately the same height as the ground beam to be formed, but of approximately three times the width of the ground beam. The floor of the trench is then covered with drylene concrete mix and compacted. Typically, for a ground beam of cross-sectional dimension 6 m by 6 m, a trench of approximately 6.5 m by 20 m would be dug, and the bottom of the trench lined with 0.05 m of drylene. Then, plywood shuttering and timber bracing is erected by a carpenter on the floor of the trench 1, as shown in Fig. 2. The inside of the shuttering 3, forming the mould 4, is then coated with releasing agent in preparation for the concrete. Spacer bars 5 are placed on the floor of the trench 1, and the reinforcement cage 6 is placed loosely on top of the spacers. Then, the concrete 7 is poured into the mould 4 and allowed to set accordingly. After a wait of 24 hours, the shuttering 3 is dismantled and moved to the next part of the ground beam 8 to be formed. Once a complete length of ground beam between two piles has been formed, the space between the walls of the trench and the ground beam 8 is backfilled with a large quantity of expensive inert material. As mentioned, the conventional method suffers from a number of disadvantages, particularly relating to the size of the trench to be dug, the amount of backfill required, and the time required to construct the ground beam. Figs. 4 to 9 show a method and apparatus for preparing a ground beam according to embodiments of the invention. Firstly, a trench 10 of approximately the same cross-sectional dimensions of the ground beam to be formed is dug. Typically, to form a ground beam of cross-sectional dimension 6 m by 6 m, a trench of height 6.5 m and width 9 m is dug. The bottom of the trench is then lined with approximately 0.05 m of sand 11. A pre¬ fabricated formwork 12 is then placed in the trench. The formwork 12, which is seen in better detail in Figures 8 and 9, comprises two parts 14,15 of an inverted asymmetric T-shape arranged in use opposing each other. The formwork so formed defines an internal U-shape cross-section and two outwardly protruding feet or ledges 16,17. During backfill, material will fall on these feet and anchor the assembly, giving good support to the integral side walls 18,19. Inwardly protruding feet portions 16' ,17• are provided to define a base for the mould provided by the parts 14,15. By reversing one or both of the parts 14,15 so that either one or both feet 16,17 face inwardly and the corresponding portions 16* ,17* face outwardly, a formwork of reduced internal width can be formed. This is advantageous in that a number of different ground beam widths can be formed from the same shape components. Any number of parts may be used to cover the whole length of the trench between the piles, and may be conveniently assembled in situ in the trench. In order to provide part of the support for the next stages, a number of stakes or road pins (not shown) may be driven through the base into the ground, extending above the ground to support and engage with the side panels 18 and 19. Also, a restraint bar 21 may be placed over the top of the formwork, holding the tops of the side walls 18 and 19 at a constant spacing. Where a number of side panels are used to cover the length of a trench, the restraint bar may additionally be arranged to bridge adjacent side panels along the length to provide additional support. However, these additional supports are not essential.

Next, a reinforcement cage 22 is placed between the parts 14,15, optionally on integrally formed spacer blocks (not shown) , and the space between the walls of the trench 10 and the side walls 18 and 19 is backfilled with material 24. The support provided by the anchoring of the feet 16,17 at the first load backfilled together with the support provided by the restraint bar 21 and stakes (if used) ensures that the formwork retains its shape during backfill.

After backfill, concrete 23 is poured into the mould formed by the formwork 12 and allowed to set. During this time, the backfill material provides support for the side walls and the additional support provided by the restraint bar 21 (if used) is no longer required and this may be removed, to be used again. The presence of the backfill 24 also enables easy access to the formwork 12 during this stage. The formwork 12 is allowed to remain in place, and the finished ground beam and formwork arrangement is shown in Fig. 7.

Although the formwork of Fig. 8 could be made of any material, such as concrete, solid plastic or metal, advantageously it is formed from a foamed plastic such as polystyrene.

In forming a structural beam in an above ground situation the formwork parts are arranged to define the mould for the beam, the formwork stabilised by use of material placed on the feet or by any other means, after which concrete is poured into the mould and left to set. After the beam has been formed the formwork may be left in place or removed as desired. Figs. 10 and 11 show side and perspective views of an asymmetric T-shape formwork part according to a preferred, especially stable, embodiment of the present invention, usable in the same manner as the embodiment described above. Part 25 comprises side wall 26 integrally formed with a foot 27 having outwardly and inwardly extending portions 28,29 respectively. Part 25 also includes recessed portions 42,43 adapted to receive and contain backfill in use to enhance stability. Further hollowed portions 30-33 may also be formed to receive backfill and to provide a weakened portion through which a road stake may be driven. Webbing supports 34-36 may be formed integral with the part 25 to provide additional support to the side wall 26 and spacers 37-39 separated by grooves may also be formed integral with the part 25 to support the reinforcement cage of the ground beam. Due to the inherent rigidity and stability of this design restraint bars are not needed to provide additional support.

In order to facilitate interlocking of adjacent parts in a line so as to strengthen adjacent parts, each part may formed with a respective tongue 40 and groove 41.

Whilst the part 25 may be formed from any suitable material, advantageously it is formed by moulding a suitable foamed plastic such as polystyrene. In order to speed up the moulding process, the part may include scalloping 42 on the side not used in forming the ground beam. Similarly, the central webbing support 35 is split into two parallel supports 45,46 to reduce the moulding time and a further recessed portion 47 introduced between them. A further hollowed portion (not shown) may be formed at the base between the two supports 45, 46 to receive backfill and to provide a weakened portion through which a road stake may be driven.

Claims

Claims :

1. A method of forming a structural beam for a building or building foundation comprising providing a formwork having two side walls arranged to provide a mould for the beam, characterized in that the formwork comprises at least two pre-fabricated self-standing opposing parts, each part comprising a side wall formed integral with a supporting foot extending outwardly from the side wall, stabilising the supporting feet to assist in maintaining the position of said parts, and filling the mould with poured concrete and allowing the concrete to set.

2. A method as claimed in claim 1 for forming a ground beam for a building foundation and further comprising excavating a trench, positioning the formwork parts in the trench so that the supporting foot of each extends outwardly from the respective side wall to the adjacent wall of the trench, backfilling with material onto the supporting feet at least part of the volume between the side walls of the formwork and the adjacent walls of the trench and, after backfilling, filling the mould with concrete and allowing the concrete to be set, the formwork permanently remaining in place around the set concrete.

3. A method as claimed in claim 1 or 2 wherein each opposing part has a foot also extending inwardly in use of its respective side wall such that the formwork as a whole defines an internal U-shaped cross section and has two side walls and an impermeable base.

4. A method as claimed in any preceding claim comprising providing a separate base portion placed in use between the opposing parts.

5. A method as claimed in any preceding claim further comprising providing each part with a tongue and groove arrangement in each longitudinal end such that a line of parts can be matingly engaged.

6. A method as claimed in any preceding claim further comprising providing pre-fabricated formwork parts formed from a lightweight foamed plastic material such as as polystyrene.

7. A formwork for use in a method as set out in any preceding claim, comprising two side walls to provide a mould for forming a structural beam for a building or building foundation, characterized in that the formwork comprises at least two pre-fabricated separate self- standing opposing parts, each part comprising a side wall formed integral with a supporting foot extending outwardly from the respective side wall.

8. A formwork as claimed in claim 7 wherein each foot also extends inwardly of its respective side wall, such that each part has a substantial T-shape cross-section and such that, opposing parts may be arranged to form a formwork having an impermeable base.

9. A formwork as claimed in claim 7 or 8 wherein each part includes support webbing integrally formed with and between the side wall and foot.

10. A formwork as claimed in any of claims 7 to 9 wherein an outwardly extending part of each foot is provided with a recessed portion to receive backfill material.

11. A formwork as claimed in any of claims 7 to 10 wherein an inwardly extending part of each foot is pre¬ fabricated with integral spacers upon which a reinforcement cage may be placed.

12. A formwork as claimed in any of claims 7 to 11 wherein said formwork parts are formed from a lightweight foamed plastic material such as polystyrene.

13. A formwork part moulded from a foamed plastic, such as polystyrene, for use in formation of a structural beam in a building or building foundation, characterized by a side wall formed integral with a supporting foot extending at least outwardly in one direction perpendicular to the side wall, said supporting foot having a recessed portion adapted to receive material so as to anchor the formwork part in use.

14. A formwork part as claimed in claim 13 further comprising a second foot portion extending in the opposite direction from said side wall.