Abstract:
A parallelepiped structure ( 10 ) comprises interleaved panels ( 12 ), each panel being moulded from concrete and being rectangular in plan and comprising a plurality of slots ( 22 ) along their top edges ( 16 ). The panels can be interleaved in order to provide the structure ( 10 ). Tongues ( 13 ) between adjacent slots ( 22 ) are supported on both sides against movement perpendicular to the plane of the panel ( 12 ) by ridges ( 24   a,b ) on the supporting panel, and so that the weakness inevitably constituted by them is mitigated. The structure ( 10 ) is suitable to accept vehicular loading, while the panels that make it up are still individually light enough to be manipulated by hand. The panels are moulded in moulds assembled from blow-or vacuum-moulded sheets placed together in a sandwich.

Description:
[0001]    This invention relates to a matrix structure used in underground systems where substantial voids are required. Such structures are required in transportation and storage of fluids, and as passageways for the reception of cabling. Other applications are also feasible. 
       BACKGROUND 
       [0002]    It is well known to use plastic matrices of different construction in storm water drain-off tanks, where storm water can collect before being released slowly to the normal drainage run-off into streams and rivers. Such collection tanks counter-balance the increasing areas of concrete and tarmac in built-up areas that, with heavy rain-falls, precipitate flooding. They mimic the effect of absorbent earth that releases rain water to rivers more slowly and progressively. Nevertheless, by definition, such tanks are in built-up areas and indeed form part of such built up areas. Frequently they are under parking garages or car parks so that they are subject to loading from vehicles. Consequently, the matrices need to be strong. In the case of plastics, which is not a strong material, this means complex arrangements to maximise the structural rigidity. Nevertheless, it involves the use of considerable amounts of plastics material. For example, WTB Geotechnics manufacture a high void ratio matrix with high crushing strength, possibly as described in GB-A-2417733. Hoofmark manufacture a polypropylene matrix structure under the brand Raintank®, as do Polypipe Civils under the brand Polystorm, and Wavin Osma under the brand AquaCell. 
         [0003]    Geoplast Elevetor® (www.geoproducts.ca) is a different arrangement assembled on site employing special caps seated on lengths of plastic drain pipe that form formwork for subsequently poured concrete. 
         [0004]    In one respect, using, for example, recycled plastics material for this application would be a good means of disposing of waste plastics in an environmentally sound and in a useful way. However, given the structural requirement virgin plastics is generally always employed. Even as recycled waste, the use of such material for the present application is a waste of a valuable resource, and one which merely postpones the disposal problem for an, albeit lengthy, period of time. This is especially the case, however, with virgin plastics material. Even waste plastics material represents substantial energy, firstly (in what it represents in a waste format) as a source of fuel or hydrocarbons, but secondly in what it takes to make it in the first place. Thus to simply incorporate it into underground tanks, if alternative material could be employed, is, in fact, wasteful. 
         [0005]    Concrete, on the other hand, would be much more suitable, because it takes relatively low energy to produce and, once formed, represents neither a valuable asset tied up in a low value application, nor a disposal problem waiting to happen. At the end of the lifetime of the tank, a concrete structure can simply be integrated with the rest of the rubble of a building site. Moreover, there would be no leaching out of chemicals into the ground water, which is a risk with plastics based matrices, particularly from recycled plastics. 
         [0006]    However, while there is no fundamental problem in constructing concrete matrices, there are several difficulties therewith. The first is that concrete is much denser than plastics. Consequently, the costs of transportation of large cubic structures made of concrete, despite the fact that they will consist largely of void, will therefore be high—they are both large and heavy. Because they are heavy, their handling on site will need to be effected by machinery—cubic structures made of concrete will be too heavy for man-handling on-site. Secondly, moulding complex open structures from concrete is not especially easy because formwork has to have sufficient strength to resist the pressure of dense flowable concrete, and yet be removable from the final cast product. 
         [0007]    U.S. Pat. No. 6,581,352 discloses a concrete panel employing an isogrid web. JP-A-10-68162 discloses I-section concrete elements that are connected together using special corner pieces to form the walls of multiple small cells for storm drainage. 
         [0008]    It is an object of the present invention to provide a structure that does not have these aforementioned disadvantages, or at least mitigates their effects. 
       BRIEF SUMMARY OF THE DISCLOSURE 
       [0009]    In accordance with the present invention, there is provided a parallelepiped structure comprising interleaved panels, each panel being made from moulded concrete and being rectangular in plan and comprising front and back opposing sides, each with side edges, a base edge and a top edge,
       wherein in each panel at least two slots are formed in the base edge parallel the side edges through from the front to back sides, which slots comprise two opposing side walls that extend from an open end of the slot in the base edge to a closed end of the slot, the length of the slots being at least half the distance between the top and bottom edges, and the slots being of a width at least as much as the thickness of the panel between the closed end and the top edge on a line parallel said side edges; and   wherein a ridge is formed between the closed end of each slot and the top edge on each side of said line, positioned so as to enclose the side walls of the slot of another identical panel when one of its slots is interleaved with a slot of a first panel, and so as to limit movement of the tongue between adjacent slots in one panel in a direction perpendicular to the sides of said one panel.       
 
         [0012]    Preferably, there are N panels interleaved with M panels to form said structure, and wherein there are M slots in each of said N panels and N slots in each of said M panels. N and M may be the same and greater than 2. 
         [0013]    The interleaved panels are preferably perpendicular to one another, the structure being cuboid. 
         [0014]    Thus a structure according to the present invention, when assembled, comprises at least four panels, each with at least two slots, the four bottom edges being interleaved so that a parallelepiped structure is formed having a top and bottom surfaces formed by interleaved top and bottom edges of the four panels. The fundamental structure is familiar in the packaging industry where cardboard boxes are compartmentalised by cardboard panels of the basic structure defined above. Here, however, the same structure is modified by the ridges that give strength to each panel in the area thereof being “tongues” between adjacent slots in a direction transverse to the panel. It is here, in a direction perpendicular to the sides, that the panels are weakest and would be susceptible to failure if of the form as employed in such cardboard box dividers. In the present invention, the interleaving panels provide support for one another, the tongues of one panel being supported in the direction that the supporting panel is strongest (ie in its plane). 
         [0015]    At least, this is the case if the panels are perpendicular to one another, which is their natural orientation. However, the slot walls could be inclined if desired, and the ridges shaped accordingly, so that the orientation between interleaved panels would not be perpendicular—the structure would be rhomboid. In that event, the support for the tongues would be better in one direction, ie towards the supporting panel, than in the other direction. If this orientation was employed, then possibly one ridge could be dispensed with (that one on the supporting side of the line), while the one on the other side might require modification to provide a re-entrant to support the tongue in the direction away from the supporting side. However, this depends on the extent of the inclination and the nature of the ridges. 
         [0016]    The ridges could conceivably not comprise a discernable localised thickening of the panel on either side of said line. Instead, they could be in the form simply of a groove along said line. However, this is the same as a generalised thickening of the panel, which would be contrary to the general desire that the panels want to be strong enough for the purpose required, but as light as possible. Given that each panel has a through slot across at least half its height, a groove formed in each side surface for the panel, for the remaining height of the panel extending from the end of the slot to the top surface, is clearly a fundamental source of weakness of the panel. The remaining thickness between the opposing bases of the grooves must be sufficient to provide adequate strength to the panel, which means that a general thickening of the panel elsewhere is largely redundant. Nevertheless, the term “ridge” as used herein needs to include the possibility of being merely the sides of a groove formed along said line. 
         [0017]    Preferably, however, said panel is as thin as it can be without losing structural integrity for its intended application. Given the fact that the panels are moulded, particularly if employing a method as described further below, it is feasible not only to mould the ridges mentioned above, but also to mould thickened edges of the panel around its entire periphery, thereby providing extra strength and permitting the panel to be correspondingly thinned. 
         [0018]    Preferably, the panels have through holes that permit water and other things to communicate between the cells formed by the interleaving panels. Indeed, while the invention has so far been described in the context of the panels forming a storm water retention tank, it is also suitable for other purposes, such as for providing trunking routes for cables or drains or water or gas services. 
         [0019]    A great advantage of the present invention is that the panels can be transported from their place of manufacture “flat-packed” so that their volume is efficiently small. On site, the structures can be assembled by hand, with each panel individually being easily manipulated by hand. 
         [0020]    Preferably the slots are of a width w and are equally spaced from one another with a centre to centre separation of slots d so that the width D of the panel between its side edges is N*d or M*d, as the case may be. This provides half a width (d/2) of tongue and slot at the end of each panel, and that enables two panels to be butted end to end so that the size of the structure can be doubled, or more. This applies in both directions, of course. Preferably, if an enlarged structure is provided, the breaks between two adjacent pairs of panels are staggered so that the integrity of the overall structure is maintained and the inherent weakness caused by breaks between panels are mitigated. 
         [0021]    Moreover, it is possible to enlarge such structure vertically, as well as horizontally, by stacking one structure on top of the other. Preferably, the panels are aligned one above the other so that the weight of structures above is taken by the entire length of each panel, both above and below, so that concentrated stresses are not imposed at points of contact between bridging panels suspended above supporting panels. 
         [0022]    Preferably, the thickness t of the panel is between 7 and 15 mm thick, or preferably between 9 and 11 mm thick, or about 10 mm thick. The width w is preferably between 11 and 19 mm, more preferably between 13 and 15 mm, or about 14. Preferably there is between 2 and 6 mm difference between the thickness t of the panel and width w of slot  22 , or about 4 mm. This same clearance also preferably exists between the abutting ridges of two interleaving panels. 
         [0023]    In accordance with a different aspect of the present invention there is provided a method of constructing a panel for a structure as defined above. It is apparent from the foregoing description that the accuracy of manufacture of the panels is important. Great tolerance in the dimensions of the slots or ridges cannot be provided, otherwise the integrity of the overall structure will be compromised. However, if no tolerance is provided, and yet the manufacturing provides inaccuracies, then it is likely to be impossible to assemble the structure. Moulds formed from rigid metal, machined to great accuracy would provide the requisite assurance that the panels can be moulded with the accuracy needed. However, such moulds would be expensive for the purpose of forming the concrete panels themselves. 
         [0024]    Accordingly, the present invention also provides a formwork comprising a plurality of form-sheets disposed in side by side relationship, each form-sheet having a form-side and a back-side and alternating in the direction they face, between facing form-sides of the form-sheets a form being defined in which to cast a panel as defined above, facing back-sides of adjacent form-sheets abutting one another in first and second regions, said first region being where the thickest parts of the panels to be cast are formed, and said second region being where an aperture in the panel to be cast is formed, said aperture being defined by rims having abutting form-sides and central bosses, extending from the base of said rims, whose back-sides abut the backsides of corresponding bosses on adjacent form-sheets. 
         [0025]    Preferably, said first and second regions lie in a single plane. 
         [0026]    Preferably, two form sheets are joined back-side to back-side in a pair, adjacent pairs defining a form between facing form-sides from each pair. 
         [0027]    Preferably, said form-sheets are inserted in a close fitting box having ends and sides, the ends pressing said first regions towards one another. 
         [0028]    Preferably, bolts extend through said formwork through a line of said bosses to clamp the sheets together, as well as to press said second regions towards one another. 
         [0029]    In this way effective seals between adjacent sheets can be assured. Moreover, each sheet may be blow moulded, or vacuum moulded in a very inexpensive way and yet with sufficient accuracy on one face, which face is that which lies adjacent the mould of the blow or vacuum mould, and which preferably forms said front face of the sheet. 
         [0030]    Preferably, the face of the form-sheet that lies adjacent the mould of the blow mould forms said form-side of the sheet. 
         [0031]    Preferably, said sheets are all identical, being placed front to front and back to back with respect to one another, adjacent fronts forming the mould cavity of the form. 
         [0032]    Preferably, a core is inserted after assembly of said formwork to mould said slot. The form-sheets may comprise longitudinal beads to locate the core when inserted, the core having corresponding grooves to engage with the beads. The core is preferably tapered to facilitate removal thereof from the slot when cured, said formwork providing a corresponding taper to the walls of the panel above the slot so that said wall is a snug fit in the slot when interleaved therewith. 
         [0033]    In accordance with a different aspect of the present invention, a method of constructing a panel for a structure as defined above is provided, the method comprising the step of casting concrete in a formwork as defined above. Preferably, said concrete is first mixed with reinforcing fibres. 
         [0034]    In accordance with another aspect of the present invention, a stormwater retention tank is provided comprising a structure as defined above or made in a formwork as defined above or made by a method as defined. 
         [0035]    Indeed, in its broadest aspect, the present invention provides panels, each being one of the panels to construct a structure as defined above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    Embodiments of the invention are further described hereinafter, by way of example, with reference to the accompanying drawings, in which: 
           [0037]      FIGS. 1   a, b  and  c  are, respectively, a side view of a panel forming a structure according to the present invention; a detail as indicated in  FIG. 1   a;  and a section along the line A-A in  FIG. 1   a;    
           [0038]      FIG. 2   a  and  b  are, respectively, a side view; and a section along the line A-A in  FIG. 2   a  of a side sheet of a structure according to the present invention; 
           [0039]      FIGS. 3   a  and  b  are, respectively, an exploded view of panels; and an assembled view of those panels, forming a different embodiment of a structure according to the present invention; 
           [0040]      FIG. 4  is a side section through two interleaved panels according to the present invention; 
           [0041]      FIGS. 5   a  to  f  are sections through the interleaving between panels according to different embodiments of the present invention; 
           [0042]      FIGS. 6   a, b  and  c  are sections through form work according to the present invention, forming panels for making structures according to the present invention; and 
           [0043]      FIG. 7  shows an enlarged structure according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0044]    Referring first to  FIG. 7 , a structure  10  according to the present invention is made from a plurality of panels  12  interleaved together. An individual panel  12  is shown in side view in  FIG. 1   a,  and comprises a thin flat panel sheet  14  having two sides  14   a,    14   b,  a top edge  16 , a bottom edge  18  and two side edges  20 . The panel has a plurality of through slots  22  that extend from the top edge  16  to an end wall  22   a  disposed halfway between the top and bottom edges  16 , 18 . The slot  22  is defined between side walls  22   b,c  of the slot and has a mouth  22   d.    
         [0045]    A ridge  24   a,b  is formed on either side of a line  23 , the line being an extension of the centre line between the walls  22   b,c  of the slot  22 . The ridges  24   a,b  extend for the entire height of the panel  12  between walls  16  and  18 , although, depending on its geometry, may extend only from the slot end  22   a  to the bottom edge  18 , as described further below. 
         [0046]    Two panels  12   a,b  (see  FIG. 4 ) can be interleaved when the mouths  22   d  of two panels are engaged with one another and moved towards one another in the direction of the line  23 , mutually engaging the slots  22  so that, when fully engaged, the panels form a cross, lying substantially in the same plane. In  FIG. 4 , it can be seen that ridges  24   a   1  and  24   b   1  of one panel  12   b  lie against the corresponding ridges  24   a   2  and  24   b   2  of the other panel  12   a.    
         [0047]    The problem overcome by the panel construction  12  is that, between adjacent slots  22 , the tongues  13  formed thereby have very little strength in the direction perpendicular to the plane of the panel  13 . Thus, in  FIG. 4 , tongue  13  is weak in the direction perpendicular to the plane of the panel  12   a,  indicated by the arrow F. However, engagement of the ridges  24   a   1   ,a   2  and  24   b   1   ,b   2  means that panel  12   b  acts as a supporting panel for the tongue  13  and prevents deflection of it perpendicular to the plane of the supported panel  12   a.    
         [0048]    From the foregoing, it is apparent that the shape of the ridges  24   a,b  can be varied without departing from this essential function. However, it should be appreciated that, in the case of the mitred arrangement shown in  FIG. 4 , the ridges  24   a,b  must extend the full height of the panel  12 , since it is not just the ridges on the supporting panel  12   b  that result in support for the tongue  13  on the supported panel  12   a,  but that the ridges adjacent the slot are also needed. 
         [0049]    However, in  FIG. 5   a , instead of being mitred, and part of the ridge being also adjacent the slot  22 , the ridges  24   a ′,  24   b ′ are rectangular, so that the tongue  13  is still supported in a direction perpendicular to its plane, despite the absence of any ridges adjacent the slot. That is, the ridges  24   a ′, 24   b ′ only extend in the region beyond the slot, between the slot end  22   a  and the bottom edge  18 . Indeed, they serve only to strengthen the panel where it least needs it (beyond, of course, its support for the tongue of an interleaving panel). This is why the arrangement of  FIG. 4  is preferred, because the ridge extending from edge to edge increases the strength of the panel and enables the remainder of sides  14  to be relatively thin. Indeed, they can, and are, provided with large apertures  42 , without undermining the strength of the panel. 
         [0050]    Nevertheless, the arrangement shown in  FIG. 5   a  is feasible, as indeed is the arrangement in  FIG. 5   b . This arrangement is preferable to that shown in  FIG. 5   a , since there is reinforcement of the panel sides from top  16  to bottom  18 , with a flange  24   a   2 ″,  24   b   2 ″ being provided down the side of the slot  22 ″ to cooperate with the ridges  24   a   1 ″,  24   b   1 ″. However, there is still a disadvantage with this arrangement in that there is a transition between the ridge forms  24   a   1 ″,  24   b   1 ″ and the flanges  24   a   2 ″,  24   b   2 ″ at the slot end  22   a.  This inevitably has an adverse effect on the strength of the panel. 
         [0051]    In  FIG. 5   c  is shown an arrangement in which the ridges  24   a , 24   b  are not evident at all. However, this is only because the panel sides  14  have been given the same thickness as the ridges. There is still support for the tongue  13  of the panel  12   a  in the direction perpendicular to the panel  12   a  since the slot walls extend into the grooves  44 , 46  formed in the panel  12   b ′. Nevertheless, there would, in reality, be little purpose in the arrangement shown in  FIG. 5   c . The thickness t between grooves  44 ,  46  in the panel  12   b  determines the strength of the panel. Accordingly, the excessive thickness of the sides  14  away from the line  23  is entirely superfluous. This merely renders the panel heavier, and using of more material, than is necessary. 
         [0052]      FIG. 5   d  is a development of the preferred arrangement shown in  FIG. 4 , where the ridges have been reduced to their minimum, but while otherwise maintaining the functionality shown in  FIG. 4 . 
         [0053]    In  FIG. 5   e , the arrangement is similar to that shown in  FIG. 5   c , but here with a square cut to the grooves  44 ′, 46 ′. This arrangement is slightly advantageous over that shown in  FIG. 5   c  in that tendency for the tongues  13  to move in a direction perpendicular to the panel  12   a  does not result in any spreading of the slot  22 , which inevitably occurs with the mitred arrangement of  FIG. 5   c.    
         [0054]    Finally,  FIG. 5   f  shows an arrangement in which the panels  12   a ″′, b ′″ are not perpendicular to one another, as in the embodiments described above, but are at an angle α, which may be as little as 45 degrees. In this arrangement, the supporting panel  12   b ″′ has a supporting side  12   c  on one side of the normal  120  to the panel  12   b ″′, and an unsupporting side  12   d  on the other side of line  120 . Only on the unsupporting side  12   d  is a ridge  24 ′ required, the ridge on the other side being integrated into the supporting side  12   c  itself. That is, tendency of the tongue  13  to move perpendicularly to its plane is restrained by the supporting side  12   c  itself. However, on the unsupporting side, the ridge  24  needs to provide a re-entrant  25  under which the wall of the slot  22  can “hook” to prevent its movement in the direction of the normal to the panel  12   a″′.    
         [0055]    Turning to  FIG. 1   a  again, it can be seen that the ridges  24   a,b  extend between the top edge  16  and bottom edge  18 . However, it also extends around the top edge  16  and bottom edge  18  to complete a frame that gives substantial rigidity to the panel and helps support the tongue  13 . The weakness of the system is the joining wall  48 , which can only be as thick as the slot  22  is wide (less clearance). However, at its base  50 , the ridge extends between adjoining sections of the panel  12  again, substantially improving the overall strength of the arrangement. This means that the mouth  22   d  (see  FIG. 1   b ) is stepped inwardly at  52  in order to accommodate the base  50 . Thus, the wall  58  and accompanying ridges  24   a, b  and base  50  fill the gap provided by slot  22  and its mouth  52 . Accordingly, there is little space for movement when two panels are joined in a cross and their combined strength and rigidity is substantial. 
         [0056]    The overall width D of the panel  12  may be 900 mm. In the embodiment shown in  FIG. 1   a  the number N of slots is 4. The separation n between each slot is equal to D/n, which in the case of  FIG. 1   a,  is 225 mm. In the embodiment shown in  FIGS. 1   a  and  4 , the thickness t of the panel is 10 mm and the width w of the slot is 14 mm, providing 2 mm clearance c on either side. Indeed, the clearance c is extended to between the ridges  24   a   1 , 24   a   2  and  24   b   1 , 24   b   2    
         [0057]    The distance between the last slot and the edge  20  is d/2. Accordingly, when two panels  12  are placed end to end the separation between two slots separated by a break between two ends  20  is the same as the separation between slots of a panel  12 . Thus, turning to  FIG. 7 , panels  12  can be butted end to end, forming part of a larger structure  10  than merely as many panels as there are slots. Indeed, if the breaks  58  between adjacent panels are staggered with respect to one another (substantially as shown in  FIG. 7 ) then the weakness in the overall structure, constituted by such breaks, can be minimised and spread over the entire structure. 
         [0058]    It will be appreciated that panels running in one direction have their top edges  16  uppermost while those running in the other direction will have their bottom edges  18  uppermost. Accordingly, the top and bottom edges are substantially identical so that a planar surface is formed by the top and bottom edges, which is flat and contiguous. Thus, the structure  10  will stand evenly on a flat floor. Similarly, concrete lids or flagstones can be placed directly on the structure  10 . 
         [0059]    The structure  10  so formed provides a useful storage tank for rain water drainage (eg a soakaway). The sides of the tank may be formed by panels  60  as shown in  FIG. 2   a  and  b.  The panels  60  comprise a plurality of tapered apertures  62  through which water filters on entering or leaving the structure  10 , thereby preventing the structure  10  from becoming clogged by larger debris. The holes  62  prevent ingression of large stones. A fabric sheet can be attached to stop fines from entering. Alternatively, the tank may be fed from drains and roof gutters by one or more pipes, and water collected in the tank  10  is able to seep out gradually into the surrounding ground through the holes  62 . 
         [0060]    Turning to  FIGS. 3   a  and  b,  a structure  10 ′ is illustrated which employs the same principles as the present invention, but wherein the panels in different directions are different. Two side panels  72 , 74  are provided, and several intervening cross panels  76 . Again, if the length of one side panel  72 , 74  is insufficient for the application under consideration, then numerous panels  72 , 74  can be butted end to end and their ends  78  can be staggered with respect to each other. The side panels  72 , 74  are provided with slots  82   a  and the cross panels  76  are likewise provided with corresponding slots  82   b.  Accordingly, side panels  72 , 74  and cross panels  76  can be interleaved as shown in  FIG. 3   a  to form an overall structure that is suitable as a conduit chamber for services such as water, gas, electricity and telecommunications conduits. 
         [0061]    Manufacture of the panels  72 , 74  and  76  of the  FIG. 3  arrangement, and panels  12  of the  FIGS. 1 and 7  arrangement, is by casting concrete, for example glass reinforced concrete, into appropriately shaped moulds. 
         [0062]    It is a further aspect of the invention to provide a mould constructed from a plurality of blow-moulded plastics sheets  102 , 104 , which are advantageously identical with one another and which act together, in pairs, to form mould forms  103 . Each form  103  comprises the form-sides  102   a,   104   a  of the form sheets  102 , 104 . Forms  103  are positioned next to each other in a “box”  114  with their back-sides  102   b,   104   b  (opposite to those of the form-sides  102   a , 104   a ) facing and abutting one another. 
         [0063]    The void formed by the form  103  into which concrete is cast is indicated by hashed lines in  FIG. 6   a  and  c,  which are respectively horizontal and vertical sections through the mould where it is forming the panel  12  along the line VIa/d-VIb/d and VIf-VIf in  FIG. 1   a.  That is to say, in the top of  FIG. 6   a , in the direction of the arrows VIa and VIc, the section is along the line VIa-VIc in  FIG. 1   a.  In the top right and bottom left of  FIG. 6   a , in the direction of the arrows VIa and VIb, the section is along the line VIa-VIb in  FIG. 1   a.  In the bottom right and top left of  FIG. 6   a , in the direction of the arrows VId and VIc, the section is along the line VId-VIc in  FIG. 1   a.  In the bottom of  FIG. 6   a , in the direction of the arrows VId and VIb, the section is along the line VId-VIb in  FIG. 1   a.    FIG. 6   c  is a section along the line VIf-VIf in  FIG. 1   a.    
         [0064]    The sheets  102 , 104  have the following regions:
       regions A, that forms the skin or web thickness of the tongue  13  and side generally of the panel  12 ;   regions B, that form the ridges  24   a,b;      a lip C, that is pressed against the corresponding lip of the other sheet so that the gap between the ridges  24   a,b  is closed, thereby forming the slot  22 . However, also shown in  FIG. 6   a  is the wall  48  formed lower down the panel (see line VIe in  FIG. 1   a ) that corresponds with the view direction VIe in  FIG. 6   a );   a rim D, that is provided to create the apertures  42 . However, the rims D do not extend right across the aperture  52 , but, instead, they have:   central bosses E, that extend out of the plane of the aperture  42 . Holes  110  extend through the centre of the bosses E, through which a bolt  112  can be passed; and finally,   rims F that form the scallops  21  in the side edge  20 .       
 
         [0071]    Thus a plurality of sheets  102 , 104  are stacked together and clamped by the bolts  112 . The bolts  112  also pass through end walls  113  of the box  114  and, when nuts  109  are tightened, the walls  113  press all the sheets  102 , 104  against one another, ensuring that the gaps between facing form-sides where they are intended to close the form to prevent ingress of the concrete (at regions C and D) to form the slot  22  and apertures  42 . Not all the apertures  42  require the bolts. 
         [0072]    Preferably, the back-sides of  102   b , 104   b  of two for sheets  102 , 104  are physically connected together around their periphery  154  to form single units  105  each forming one side of two adjacent forms  103 , except for the unit that lies against the end walls  113 , which is not, therefore employed other than to provide reaction surfaces X in a single plane. 
         [0073]    Once the form-work is assembled and bolts  1112 , 109  tightened, concrete is poured into the open tops of the mould forms  103 . The concrete percolates down and sits on base  150  on which the form-work is assembled, forming there the bottom edges  18  of the panel  12 . It also percolates sideways and is retained by side walls  117 , forming there the side edges  20  of the panels  12 . Once filled, the top surface of the concrete form the top edge  16  of the panel  12 . 
         [0074]    Once the concrete has set, the bolts  112  are released, the sides  113  and  117  removed, and the mould forms separated to expose the cast panels  12 . 
         [0075]    A preferred alternative to the lip C shown in  FIG. 6   a  is shown at C′ in  FIG. 6   b . Here, the wall- 48 -forming profile of the sheets  102 , 104  is maintained into the region where the slot  22  is required, except that a thin, longitudinal bead  17  is provided down the length of the slot form. When the sheets are stacked together as described above, and before pouring the concrete, a core  19  is inserted between sheet  102 ″, 104 ″. The core  19  has grooves  21  that correspond with and locate on the bead  17 . This facilitates separation of the mould since this would otherwise be hindered by the lip C being caught in the formed slot  22 . With the arrangement of  FIG. 6   b , there is no problem of mould separation, and the cores can be extracted easily from the slots  22 , especially if the core has a slight taper from top edge  16  to slot end  22   a.  The wall  48  can, of course, be given a corresponding taper so that it fits snugly in the tapered slot  22 . 
         [0076]    As already mentioned, sheets  102 , 104  are preferably identical. Moreover, the form-face  102   a , 104   a  of each sheet  102 , 104  (that defines the form for each panel  12 ) is that face of the blow moulded sheet  102 , 104  that lies against the blow mould when the sheet is formed. Therefore, this surface has the precision of the mould, so that the surface details of the panel  12 , when formed, are precise. While the form sheets  102 , 104  are clearly capable of deflection, and therefore of producing misshapen panels, the use of the bolts  112 , and fitting an appropriate number in a retaining box  114 , means that the dimensions and overall shape of the panel is accurate and enabling the panels to be joined as described above and to form strong and rigid structures. 
         [0077]    In the event that the mould form sheets  102 , 104  are identical, the bosses E are of equal height, and meet one another in the same plane as the ridges  24   a,b.  However, it is not essential that they be identical; the bosses E could be of different height, but there seems little point in deliberately making sheets  102 , 104  different. Certainly, doing that does not affect their operation. 
         [0078]    Although blow-moulded plastics sheets are convenient, the invention is not limited to their use. For example, pressed steel sheet would be perfectly acceptable. Also, the combined reverse pair  105  of sheets could be a solid injection moulding or two castings of resin. 
         [0079]    Thus, by virtue of the present invention the panels  12  can conveniently be made. Once made, they can be transported easily and efficiently by being flat-packed and carried to site. Individual panels are easily manipulated by operators at the site, assembling the structure  10  as desired. The strength of the assembled structure is adequate to take a vehicle loading. Indeed, using glass reinforced concrete and the dimensions mentioned above, a loading of 150 tonnes per square meter can be accommodated. 
         [0080]    Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, means “including but not limited to”, and is not intended to (and does not) exclude other moieties, additives, components, integers or steps. 
         [0081]    Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
         [0082]    Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. 
         [0083]    The reader&#39;s attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. 
         [0084]    All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. 
         [0085]    Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
         [0086]    The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.