Abstract:
This invention relates to a shear dowel assembly for distributing loads between adjacent concrete slabs, separated by a joint. The shear dowel assembly includes a dowel sheath having a front face and a rear face. The rear face of the dowel sheath is imbedded within the first concrete slab, whereas the front face is left substantially flush with the side of said first concrete slab. The front face also has a portion defining an opening therein. There is also provided a shear dowel adapted to be inserted into the opening in the dowel sheath, with a first end of the shear dowel abutting the rear face of the dowel sheath, and a second end of the shear dowel extending out from the opening in the dowel sheath, whereby the shear dowel may be embedded within a second adjacent concrete slab to be poured. The shear dowel has a width of constant cross section, and is preferably trapezoidal (or in the shape of a trapezium) in plan view. The shear dowel assembly allows for increased differential movement of the adjacent concrete slabs, as the joint gap between the concrete slabs increases, whilst retaining the load carrying capacity of the shear dowel at all times.

Description:
FIELD 
       [0001]    This invention relates to an improved shear dowel assembly, for use in relation to construction using concrete. The invention may be particularly suitable for use in distributing loads between adjacent concrete slabs, separated by a joint, and especially during the curing, shrinking and/or thermal expansion of the adjacent concrete slabs. 
       BACKGROUND 
       [0002]    A majority of the floors in industrial buildings at ground level are constructed in concrete. The use of concrete introduces a widespread problem in the performance of these floors, since concrete is a material that shrinks slightly during the curing process. 
         [0003]    Concrete shrinkage is typically about 8 one thousandths of an inch in a one foot length, which, in a section of concrete 100 feet long, will give total shrinkage of about 0.8 inches 
         [0004]    The shrinkage in the concrete requires designers and constructors to install joints or breaks in the concrete, to establish locations at which the concrete shrinkage can be taken up. If joints are not included in floors on a planned basis, then random and unsightly cracking will take place. In many basic floor designs, joints are spaced at about 20 feet in each direction throughout the floor. 
         [0005]    The need for joints creates a problem for designers because the design for the thickness of the concrete assumes that the concrete is continuous, and any location at which the concrete terminates becomes a location of reduced strength in the floor. 
         [0006]    To maintain the strength of the slab at a joint, short steel bars or plates may be installed to span across the joint, penetrating into the concrete on each side of the joint. These are called shear dowels, and help distribute applied loads from the concrete on one side of a joint to the concrete on the other side of the joint. These bars or plates are needed if the width of the joint rises above about two thousandths of an inch, which is a very low gap width for a large floor. The concrete in slabs is generally not thickened at the edges or joints to give increased strength, as this would restrain the shrinkage movement and would therefore further increase the stresses in the concrete. 
         [0007]    The design of shear dowels is a complex process, since several design factors must be considered, and the governing factors may change depending on how wide the concrete joint becomes. The design must allow for shear stresses in the steel; bending stresses in the steel; deflection in the steel; bearing stresses between steel and concrete, and punching of the concrete. 
         [0008]    Traditionally, round steel bars were used, however, these have a disadvantage in practical use. While much of the movement at a joint occurs in a direction perpendicular to the line of the joint, some relative movement also occurs between adjacent sections of concrete, along the direction of the joint. This is commonly known as differential movement. Round dowels restrain this movement since they are tightly encased in concrete, and this restraint can contribute to cracking in the concrete, which is undesirable. 
         [0009]    A development in the design of shear dowels was the use of square steel dowel bars encased in sheaths, with the sheaths being made of plastic or sheet steel. The sheaths were designed to have small voids between their sides and those of the steel dowel bars, which allowed the steel dowel bars to move slightly sideways within the sheath. This therefore allowed for some differential movement between the sections of concrete on each side of the joint without any significant restraint from the dowel bars. However, use of these steel dowel bars only allowed for very small amounts of differential movement of adjacent concrete slabs, and so their use is limited. 
         [0010]    A further development in shear dowel design was the introduction of plate steel dowel bars. Research showed that, in narrow joint widths, a governing factor in shear dowel capacity was the bearing stress between the steel of the dowel and the concrete. This occurs over a relatively short length of steel, so, if the dowel bar is also narrow, then a low effective contact area results, and stresses become high, which may govern the dowel capacity. 
         [0011]    The use of plate steel dowel bars enabled the cross section dimensions of the dowel to give more width than thickness, and to increase the bearing area and reduce the bearing stresses. 
         [0012]    A common early design of plate steel dowel bars uses sections of square steel plate oriented so that one of the longest or diagonal dimensions is located along the initial join line The steel plate is inserted into a triangular shaped plastic sleeve on one side of the joint. Since this gave an effective diamond shape in plan view, the common and trade names became DIAMOND DOWEL, and these dowels are well known in the industry. 
         [0013]    The diamond shaped dowels have an advantage in some installations. That is, as concrete shrinkage opens the joint and pulls the steel plate out of the triangular sleeve, a gap develops between the steel plates and the edges of the triangular sleeves. This gap allows for some differential movement to occur between the concrete sections on each side of the joint, without restraining forces between the steel and the concrete. The steel plate can therefore slide sideways inside the plastic sheath. 
         [0014]    However, the use of diamond dowels has its limitations, as they are only efficient where joint widths remain relatively narrow, that is, up to a maximum joint width of about ⅜ inch, or possibly a little more. 
         [0015]    However, with many common design methods for concrete floors, the widths to which joints will develop cannot be accurately determined. In addition, many concrete floors are now designed so that there are fewer joints. Such floor designs may use high dosages of steel fibre reinforcing, or may use pre-stressing steel reinforcing. A reduction in the number of joints in floors increases the distance between the joints. Since approximately the same total shrinkage occurs in the concrete, the lower number of joints must each develop to a greater width. In some floors, joints may develop as wide as 2¾ inches or even wider, and joint widths are commonly in the ⅝ to ¾ inch range in some floor designs. 
         [0016]    Furthermore, in some designs, there may be many joints in the basic floor design (typically at 20 foot centers), but only a few of these joints may be free to open. That is, the other joints in these floors may be held closed by a sufficient quantity of reinforcing steel which passes through the joint, so that movement is prevented at such joints, and increased movement therefore appears in the few joints that are free to open. 
         [0017]    Diamond shaped dowels cannot provide effective load transfer when relatively wide joint widths develop (above about ⅜ th  of an inch), such as in those situations as described above. This is because the 45 degree taper of the steel plate reduces the width of the steel plate that is still embedded in concrete on at least one side of the joint. The reduced width rapidly reduces the load carrying capacity of the dowel as the joint width increases, and hence the dowels become ineffective as relatively wide joint widths develop. 
         [0018]    Other designs of plate dowels use steel plates which have a constant width of steel, in the axis perpendicular to the joint. These plates are inserted into sheaths that also have a constant width in relation to the axis perpendicular to the concrete joint. Such designs must allow a sufficient gap (between the sides of the steel plate and the sleeve) for all the expected differential movement of the sections of concrete at the joint, since this gap cannot increase with increased differential movement. However, the amount of differential movement that occurs between two slabs of concrete generally increases in proportion to the increasing gap at the joint in the concrete. Hence in situations (as described previously) where there is significant movement of the slabs both at the joint and differentially, there is no increase in the allowance for differential movement as the joint gap increases. Hence, these constant-width plate dowels have similar limitations as the diamond dowels. 
       OBJECT 
       [0019]    It is an object of the present invention to provide an improved shear dowel assembly which goes some way towards addressing the aforementioned problems or difficulties, or which at the very least provides the public with a useful choice. 
       DEFINITIONS 
       [0020]    Throughout this specification unless the text requires otherwise, the word ‘comprise’ and variations such as ‘comprising’ or ‘comprises’ will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. 
         [0021]    Throughout this specification, the term ‘trapezoid’ or ‘trapezoidal’ is defined as being a quadrilateral having two parallel and two non-parallel sides. 
         [0022]    Throughout this specification, the term ‘trapezium’ is defined as being a quadrilateral having no two sides parallel. 
         [0023]    Throughout this specification, the term ‘joint gap(s)’ is defined as being the gap formed between adjacent concrete slabs. A joint gap will increase (widen) as the concrete slabs cure and/or shrink after pouring and setting. 
         [0024]    Throughout this specification, the term ‘differential movement’ as used in relation to adjacent concrete slabs, is defined as meaning movement between the concrete slabs in an axis parallel with the axis of the joint gap between the concrete slabs. 
         [0025]    Throughout this specification, the term ‘effective load carrying capacity’, as used in relation to the effectiveness of shear dowels, is defined as being the minimum amount of load carrying capability required of the shear dowels, which retains the visual and structural integrity of adjacent concrete slabs during movement of the slabs (for example during curing or shrinkage) both across the joint gap between the slabs and in relation to differential movement between the slabs. 
         [0026]    Throughout this specification, the term ‘significant movement’, as used in relation to the joint gap between two adjacent concrete slabs and/or the amount of differential movement between two adjacent concrete slabs, is defined as meaning more than ½ inch movement. 
       STATEMENTS OF INVENTION 
       [0027]    According to one aspect of the present invention, there is provided a shear dowel assembly for distributing loads between adjacent concrete slabs, separated by a joint, said shear dowel assembly including:
       a) a dowel sheath adapted to be embedded into the side of a first concrete slab, said dowel sheath having a front face and a rear face, with the rear face being imbedded within said first concrete slab, and the front face being substantially flush with the side of said first concrete slab, and said front face having a portion defining an opening in the side of said first concrete slab,
           said dowel sheath having tapering sides which are not parallel in relation to the axis perpendicular to the joint between the adjacent concrete slabs, and whereby the width of the dowel sheath at the rear face is less than the width of the dowel sheath at the front face,   
           b) a shear dowel adapted to be inserted into the opening in the dowel sheath, with a first end of the shear dowel abutting the rear face of the dowel sheath, and a second end of the shear dowel extending out from the opening in the dowel sheath, whereby the shear dowel may be embedded within a second adjacent concrete slab to be poured, and said shear dowel having a width of constant cross section,
           the arrangement and construction being such that the shear dowel assembly allows for increased differential movement of the adjacent concrete slabs, as the joint gap between the concrete slabs increases.   
               
 
         [0032]    According to another aspect of the present invention, there is provided a shear dowel assembly, substantially as described above, wherein the effective load carrying capacity of the shear dowel is retained during significant movement of the adjacent concrete slabs across the joint gap and/or during significant movement differentially between the adjacent concrete slabs. 
         [0033]    According to another aspect of the present invention, there is provided a shear dowel assembly, substantially as described above, wherein there is a gap of at least ¼ inch between the sides of the shear dowel and the tapered sides of the dowel sheath at the region where the shear dowel abuts the rear face of the dowel sheath, once the shear dowel has been inserted in the dowel sheath. 
         [0034]    According to another aspect of the present invention, there is provided a shear dowel assembly, substantially as described above, there is a gap of at least 1 inch between the sides of the shear dowel, and the tapered sides of the dowel sheath at the region where the sides of the dowel sheath meet the front face of the dowel sheath, once the shear dowel has been inserted in the dowel sheath. 
         [0035]    According to another aspect of the present invention, there is provided a shear dowel assembly, substantially as described above, wherein the dowel sheath is substantially trapezoidal in plan view. 
         [0036]    According to another aspect of the present invention, there is provided a shear dowel assembly, substantially as described above, wherein the dowel sheath is in substantially the shape of a trapezium, in plan view. 
         [0037]    The shear dowel assembly may be suitable for distributing loads between adjacent concrete slabs, separated by a joint. The concrete slabs may comprise part of any concrete structure, for example, flooring (particularly large concrete flooring for industrial buildings), pavements or roading. 
         [0038]    Most joints between adjacent concrete slabs are formed along armour joints, and the shear dowel assembly may be particularly for use with such armour joints. Armour joints generally have at least two main components to them, namely:
       1. Steel sections anchored into the concrete slabs, to armour the edges of the concrete joint or joint gaps.   2. Shear dowels assemblies for distributing loads between the adjacent concrete slabs, particularly during movement of the concrete slabs, for example during curing and/or shrinking of the concrete.       
 
         [0041]    Many armour joints also include a metal form to create the joint between adjacent concrete slabs, and to separate same. Alternatively, some armour joints utilise timber forms to create the joint, with the timber forms being removed after the pouring of the first concrete slab, and before the pouring of the adjacent second concrete slab. 
         [0042]    Armour joints are well known in the industry, and it is not intended therefore to describe them in any great detail herein, other than as their use relates to the incorporation of the present invention. Moreover, it is also envisaged that the present invention could be used for distributing loads between adjacent concrete slabs that do not utilise an armour joint. 
         [0043]    The dowel sheath of the present invention may preferably be adapted to be embedded into the side of a first concrete slab. 
         [0044]    If an armour joint utilising a metal form is to be used, then the dowel sheath may be fixed to the metal form, for example of the use of screws. The metal form may be placed along the line of where the concrete joint is to be located, and a first concrete slab may be poured on the side of the metal form which houses the dowel sheath. The shear dowel may then be placed in the opening in the dowel sheath, and the second concrete slab may then be poured, thus encasing the shear dowel within same. 
         [0045]    If an armour joint utilising a timber form is to be used, then the dowel sheath may be nailed to the side of the timber form where the first concrete slab may be poured (with the opening in the dowel sheath abutting the timber form and thus preventing any concrete from entering the opening). A first side of the armour joint (with anchoring arm) may also be nailed to this same side of the timber form. Once the first concrete slab has been poured and cured sufficiently, the timber form may be removed and the shear dowel may be placed in the opening in the dowel sheath, and the second side of the armour joint (with anchoring arm) may be fixed to the first side of the armour joint, for example by the use of tack welding. The second concrete slab may then be poured, encasing both the shear dowel and the second side of the armour joint. 
         [0046]    In the situation where an armour joint is not utilised, but a timber form is, then the procedure would be the same as stated in the paragraph above, but without the steps involving the inclusion of the first and second sides of the armour joint (and anchoring arms). 
         [0047]    The dowel sheath may have a front face and a rear face, with the rear face being embedded within the first concrete slab, and the front face lying substantially flush with the side or edge of the first concrete slab. The front face preferably has a portion defining an opening therein, with this opening being adapted to receive the shear dowel, as described above. 
         [0048]    Preferably, the dowel sheath may have tapering sides which are not parallel in relation to the axis perpendicular to the joint between the adjacent concrete slabs. Preferably, the width of the dowel sheath at the rear face is less than the width of the dowel sheath at the front face. 
         [0049]    Preferably the dowel sheath may be substantially trapezoidal when viewed in plan view, with the two parallel sides of the trapezoid preferably being the front and rear faces. 
         [0050]    Alternatively, the dowel sheath may be substantially in the shape of a trapezium, when viewed in plan view. In such an embodiment, it may be appreciated that the shear dowel may be housed closer to one side of the dowel sheath than the other, and this may be desirable, for example, in a situation where differential movement of the concrete slabs is expected to be greater on one side of the shear dowel than the other side (for example, if one concrete slab is significantly wider or longer than the adjacent concrete slab). 
         [0051]    The shear dowel to be inserted into the dowel sheath may preferably have a width of constant cross section. Preferably, a first end of the shear dowel may abut the rear face of the dowel sheath, with the second end of the shear dowel extending out from the opening in the dowel sheath. 
         [0052]    The proportion of the length of the shear dowel housed within the dowel sheath and the proportion extending from the dowel sheath opening may be of any suitable proportion, whereby the effective load carrying capacity of the shear dowel is maintained for any two adjacent concrete slabs. Preferably however, the larger proportion of the length of the shear dowel may be housed within the dowel sheath, when the dowel is placed within same. 
         [0053]    Once the shear dowel has been placed within the opening in the dowel sheath, the second concrete slab may be poured, thus embedding the shear dowel within same. 
         [0054]    The shear dowel may preferably be made of a strong metal material, and a mild steel plate may be particularly suitable. Once embedded in the second concrete slab, the shear dowel will adhere to the concrete (or vice versa) and the shear dowel will then move in concert with any movement of the second concrete slab, for example as it cures or shrinks (or possibly even expands due to thermal expansion). For example, the shear dowel will move slightly out of opening in the dowel sheath as the joint gap increases, and it will move sideways in the dowel sheath as differential movement occurs. 
         [0055]    It may be seen that the arrangement and construction of the shear dowel assembly allows for increased differential movement of the adjacent concrete slabs as the joint gap between the adjacent concrete slabs increases, and furthermore the load carrying capacity of the shear dowel is maintained due to it being of a constant cross section. 
         [0056]    That is, as the joint gap develops its width, the shear dowel is pulled out of the dowel sheath, and the further the shear dowel is pulled out, the further the distance between the side edges of the shear dowel and the tapered sides of the dowel sheath, thus allowing for increased differential movement of the adjacent concrete slabs as the joint gap increases. 
         [0057]    This is a major advantage as compared to the presently available diamond dowel assemblies because the load carrying capacity of the diamond dowel rapidly decreases as the diamond-shaped dowels are pulled out from the sheaths during significant movement across the joint gap. Namely, diamond dowels tend to lose their load carrying capacity when the joint gap exceeds approximately ⅜ th  of an inch. This is because the tapered nature of the diamond dowel means that the width of the dowel that remains in the sheath reduces as the joint gap increases. The reduced width rapidly reduces the load carrying capacity of the dowel as the joint width increases, and hence the dowels become ineffective as significant movement across the joint gap develops. 
         [0058]    Furthermore, presently available dowel assemblies that have sheaths and dowels of a constant width, have the limitation in that they are not able to provide for increased differential movement of the adjacent concrete slabs as the joint gap increases. Hence, for concrete slabs of significant length or width (say 20 yards or more as is reasonably common these days), these dowel assemblies are not able to offer an effective load carrying capacity for the amount of differential movement (sometimes more than two inches) that will occur in concrete slabs of this size. And what will in fact happen if the dowel assembly has reached its maximum allowable differential movement limit is that the adjacent concrete slabs may buckle or crack, which is clearly undesirable. 
         [0059]    Preferably, the inside of the dowel sheath may be provided with one or more positioning means to guide and/or retain the shear dowel in position within the dowel sheath. The positioning means may be in the form of small pins or pegs formed within the dowel sheath. Alternatively, the positioning means may be in the form of flanges extending from the sides of the dowel sheath. 
         [0060]    The positioning means may preferably be located so as to guide the shear dowel into a substantially central position within the dowel sheath, for example the positioning means may be located at substantially the same location on each side of the dowel sheath. Alternatively, the positioning means may be located so as to guide the shear dowel closer to one side of the dowel sheath than the other, for example in a situation where differential movement of the concrete slabs is expected to be greater on one side of the shear dowel than the other side (for example, if one concrete slab is significantly wider or longer than the adjacent concrete slab). 
         [0061]    Preferably, the positioning means may preferably allow for the shear dowel to fit substantially snugly within the dowel sheath, with the sides of the shear dowel abutting each positioning means. 
         [0062]    Preferably, the positioning means may adapted to break off if differential movement occurs between the adjacent concrete slabs, thus allowing for the differential movement of the shear dowel. 
         [0063]    Preferably, there may be provided a gap of at least ¼ inch between the sides of the shear dowel and the tapered sides of the dowel sheath at the region where the shear dowel abuts the rear face of the dowel sheath, once the shear dowel has been inserted in the dowel sheath. In such an embodiment, it may be appreciated that the shear dowel may be able to move differentially a total of ½ inch (that is ¼ inch in either direction) even if there is no movement of the adjacent concrete slabs across the joint gap (however, it would be very uncommon for the concrete slabs to only move differentially without any movement of the slabs across the joint gap). 
         [0064]    Preferably, there may be a gap of at least I inch between the sides of the shear dowel, and the tapered sides of the dowel sheath at the region where the sides of the dowel sheath meet the front face of the dowel sheath, once the shear dowel has been inserted in the dowel sheath. More preferably, this gap may be approximately 1 and a half inches. 
         [0065]    Preferably, the shear dowel may be substantially rectangular in plan view (including being substantially square in plan view). 
         [0066]    Preferably the dowel sheath may be made of a plastics material, and may preferably be made by injection moulding techniques. In such an embodiment, the dowel sheath may be made as one piece or possibly as two identical pieces which may be clipped together. 
         [0067]    According to a further aspect of the present invention, there is provided a shear dowel assembly for distributing loads between adjacent concrete slabs, said shear dowel assembly including:
       a) a dowel sheath, said dowel sheath having a front face and a rear face, with the front face defining an opening in the dowel sheath,
           said dowel sheath having tapering sides whereby the width of the dowel sheath at the rear face is less than the width of the dowel sheath at the front face,   
           b) a shear dowel adapted to be inserted into the opening in the dowel sheath, with a first end of the shear dowel abutting the rear face of the dowel sheath, and a second end of the shear dowel extending out from the opening in the dowel sheath, and said shear dowel having a width of constant cross section.       
 
         [0071]    According to another aspect of the present invention, there is provided a method of allowing for effective load transfer between adjacent concrete slabs separated by a joint, said method utilising the shear dowel assembly substantially as described above, and said method including the steps of:
       a) embedding a dowel sheath, substantially as described above, into a first concrete slab to be poured,   b) inserting a shear dowel, substantially as described above, into the opening in the dowel sheath,   c) pouring the second concrete slab adjacent the first concrete slab, whereby the shear dowel is encased in said second concrete slab,
           the arrangement and construction being such that the shear dowel assembly allows for increased differential movement of the adjacent concrete slabs, as the joint gap between the concrete slabs increases.   
               
 
       PREFERRED EMBODIMENTS 
       [0076]    The description of a preferred form of the invention to be provided herein, with reference to the accompanying drawings, is given purely by way of example and is not to be taken in any way as limiting the scope or extent of the invention. 
     
    
     
       DRAWINGS 
         [0077]      FIG. 1 : is a cutaway plan view of one possible embodiment of a shear dowel assembly of the present invention, 
           [0078]      FIG. 2 : is a perspective, exploded view, of the dowel sheath illustrated in  FIG. 1 . 
           [0079]      FIG. 3   a:  is a cross-sectional view of a joint to be formed between two adjacent concrete slabs, utilising a timber form and the embodiment of the invention as illustrated in  FIG. 1 , 
           [0080]      FIG. 3   b:  is a cross-sectional view of the embodiment illustrated in  FIG. 3   a,  after the pour of the second concrete slab, and 
           [0081]      FIG. 4 : is a cross-sectional view of a joint formed between two adjacent concrete slabs, utilising a metal form and the embodiment of the invention as illustrated in  FIG. 1 . 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0082]    With reference to  FIG. 1 , there is shown a shear dowel assembly generally indicated by arrow  1 . The shear dowel assembly  1  is used for distributing loads between adjacent concrete slabs, separated by a joint. 
         [0083]    The shear dowel assembly  1  includes a dowel sheath generally indicated by arrow  2  The dowel sheath  2  consists of two equal halves  2   a,b  which are then clipped together. For convenience, only one half of the dowel sheath  2  is illustrated in  FIG. 1  so that the inside of the dowel sheath  2  may be shown. 
         [0084]    A perspective, exploded view of the two halves  2   a,    2   b  comprising the dowel sheath  2  is illustrated in  FIG. 2 . 
         [0085]    The dowel sheath  2  has a front face  5  and a rear face  6 , and there is an opening  7  formed in the front face  5  between the two positioning flanges  8   a,b.    
         [0086]    In use, the dowel sheath  2  is adapted to be embedded into the side of a first concrete slab, with the rear face  6  being embedded well within the concrete slab, and the front face  5  adapted to sit flush with (and run along) the side of the concrete slab. The joint or joint gap between two adjacent concrete slabs that utilise the shear dowel assembly  1  would run in a line parallel with the longitudinal axis of the front face  5 . The direction of this longitudinal axis is shown by arrow  9 . 
         [0087]    The dowel sheath  2  is trapezoidal in plan view and has tapering sides  10 , 11  which are not parallel in relation to the axis perpendicular to the joint between the two adjacent concrete slabs (the direction of this perpendicular axis is shown by arrow  12 ). 
         [0088]    Furthermore, the width  17  of the dowel sheath  2  at the rear face  6  is less than the width  18  of the dowel sheath  2  at the front face  5 . 
         [0089]    The dowel sheath assembly  1  also includes a mild steel shear dowel  13 , which is rectangular in plan view, and which has a constant cross section. The shear dowel  13  is adapted to be inserted into the opening  7 . When inserted, a first end  14  of the shear dowel  13  abuts the rear face  6  of the dowel sheath  2 . The second end  15  of the shear dowel  13  extends out from the opening  7  of the dowel sheath  2 . 
         [0090]    The front positioning flanges  8   a,b  serve to guide the shear dowel  13  into place when it is being inserted into the dowel sheath  2 . A second set of positioning flanges  16   a,b  also perform the same function, and it can be seen that these positioning flanges  8   a,b  and  16   a,b  together serve to position the shear dowel  13  in a roughly central position within the dowel sheath  2 . 
         [0091]    The dowel sheath  2  is made from a plastics material and the two halves are formed using known injection moulding techniques. 
         [0092]    The positioning flanges  8   a,b  and  16   a,b  are relatively weak and they are therefore adapted to snap off to allow the shear dowel  13  to move sideways in either direction (of axis  9 ) during differential movement between two adjacent concrete slabs. 
         [0093]    Each side of the dowel sheath  2  is provided with a sleeve  25   a,b.  Each sleeve  25   a,b  may be used to receive a nail, for example when nailing the dowel sheath  2  to one side of a timber form. 
         [0094]    The dimensions of the various features of the shear dowel assembly  1  are as follows:
       1. The width  17  of the dowel sheath  2  at the rear face  6  is 4¾ inches.   2. The width  18  of the dowel sheath  2  at the front face is 8 inches.   3. The depth  19  of the dowel sheath is four inches.   4. The length of the shear dowel  13  is six inches (hence, in use, approximately four inches of the shear dowel  13  remains inside the dowel sheath  2 , and two inches extends from the dowel sheath  2 ).   5. The width of the shear dowel  13  is four inches.   6. The thickness of the shear dowel  13  is ⅜ of an inch.   7. The width of the opening  7  is only slightly more than the thickness of the shear dowel  13 , say about 4⅛ th  inch.       
 
         [0102]    The trapezoidal arrangement and construction of the shear dowel assembly  1  allows for increased differential movement (along axis  9 ) of adjacent concrete slabs as the joint gap between the adjacent concrete slabs increases (along axis  12 ). Furthermore, the load carrying capacity of the shear dowel  13  is maintained, especially during significant movement (along axis  9  or axis  12 ) of the two adjacent concrete slabs, due to the shear dowel  13  being of a constant cross section. 
         [0103]    Hence the present invention has clear advantages as compared to diamond dowel assemblies, which rapidly lose their load carrying capacity as the joint gap increases, due to the tapered shape of the diamond dowels. The present invention also has clear advantages over dowel assemblies of constant width which do not allow for significant and/or increased movement differentially between adjacent concrete slabs, especially as the joint gap increases. 
         [0104]    As can be appreciated from  FIG. 1 , as the joint gap between two adjacent concrete slabs (not shown) develops its width (along axis  12 ), the shear dowel  13  will be pulled out of the dowel sheath  2 , and the further the shear dowel  13  is pulled out, the further the distance between the sides  22 ,  23  of the shear dowel  13 , and the tapered sides  10 ,  11  of the dowel sheath  2 , thus allowing for increased differential movement (along axis  9 ) of the adjacent concrete slabs as the joint gap increases. 
         [0105]    There is provided a gap of approximately ¼ inch between the sides  22 ,  23  of the shear dowel  13  and the tapered sides  10 ,  11  of the dowel sheath  2  at the region where the shear dowel  13  abuts the rear face  6  of the dowel sheath  2  (once the shear dowel  13  has been inserted in the dowel sheath  2  as shown). This allows the shear dowel  13  to be able to move differentially a total of ½ inch (that is ¼ inch in either direction) even if there is no movement of the adjacent concrete slabs across the joint gap. 
         [0106]    If the shear dowel  13  is pulled out of the dowel sheath  2  a total of one inch (ie, during the development of the joint between the adjacent concrete slabs), then the minimum distance between the sides  22 ,  23  of the shear dowel  13  and the tapered sides  10 ,  11  of the dowel sheath  2  increases to approximately ⅞ of an inch on each side, thus allowing for a significantly larger amount of differential movement between the concrete slabs (which will usually occur in proportion to any increase in the width of the joint gap). Furthermore, in cases of extreme movement of, say, two inches across the joint gap between adjacent concrete slabs, the minimum distance between the sides  22 ,  23  of the shear dowel  13  and the tapered sides  10 ,  11  of the dowel sheath  2  increases to approximately 1¼ inch each side. 
         [0107]    There is a gap of at approximately 1½ inch between the sides of the shear dowel  13 , and the tapered sides  10 ,  11  of the dowel sheath  2  at the region where the sides  10 ,  11  of the dowel sheath  2  meet the front face  5  of the dowel sheath  2 . This distance is indicated by arrow  24 . 
         [0108]    Several possible methods for use of the shear dowel assembly  1  will now be described in relation to  FIG. 3   a  and  3   b,  and  FIG. 4 . For convenience, the same numbering of the main components of the shear dowel assembly  1 , as described in relation to  FIG. 1 , will be used in relation to  FIGS. 3   a  and  3   b,  and  FIG. 4 . 
         [0109]    Turning first to  FIG. 3   a,  there is shown a cross sectional view of a joint  35  to be formed between two adjacent concrete slabs  27 ,  34  utilising a timber form  26 . 
         [0110]    The dowel sheath  2  is first nailed to the side of the timber form  26  where the first concrete slab  27  is to be poured (with the opening  7  in the dowel sheath  2  abutting the timber form  26  and thus preventing any concrete from entering the opening  7 ). The dowel sheath  2  is nailed to the timber form  26  by the use of nails  28 , which pass through the sleeves  25   a,b  (see  FIG. 1 ). 
         [0111]    The dowel sheaths  2  may preferably be spaced at approximately 1 yard intervals along the timber form  26 . 
         [0112]    A first side of an armour joint  29  (with anchoring arm  30 ) is also nailed to this same side of the timber form  26 , by the use of nails  31  (only one nail shown). Once the first concrete slab  27  has been poured and cured sufficiently, the timber form  26  may be removed and the shear dowel  13  may be placed in the opening  7  in the dowel sheath  2  (see  FIG. 3   b ). The nails  28 ,  31  may be cut off at this point or alternatively removed from the dowel sheath  2  and first side of the armour joint  29  respectively. The second side of the armour joint  32  (with anchoring arm  33 ) may then be fixed to the first side of the armour joint  29 , for example by the use of tack welding. The second concrete slab  34  may then be poured, encasing both the shear dowel  13  and the second side of the armour joint  32 . 
         [0113]    Over time (perhaps up to  12  months or more), the adjacent concrete slabs  27 ,  34  will cure and/or shrink and this will result in movement between the concrete slabs  27 ,  34 . Usually (but not always) the more pronounced movement will occur across the joint gap  35  between the adjacent concrete slabs  27 ,  34  as they move away from each other (in the direction of arrow  36 ). This will result in the shear dowel  13  being pulled out of the dowel sheath  2 , by virtue of the shear dowel  13  being retained by, and within, the second concrete slab  34 . However, because the shear dowel  13  is of a constant cross section, the load carrying capacity of the shear dowel  13  (and therefore the shear dowel assembly  1  as a whole) does not noticeably reduce as the joint gap  35  increases. 
         [0114]    Furthermore, and as described previously, as the shear dowel  13  is pulled out of the dowel sheath  2 , the tapered sides  10 ,  11  of the dowel sheath  2  allow for increased differential movement of the shear dowel  13  (and therefore increased differential movement of the concrete slabs  27 ,  34 ). This is advantageous because differential movement of the concrete slabs  27 ,  34  will usually increase in proportion with increased movement of the slabs  27 ,  34  across the joint gap  35 . Hence, the shear dowel assembly  1  will allow for significant movement of the adjacent concrete slabs  27 ,  34  both across the joint gap  35 , and also differentially, and without the load carrying capacity of the shear dowel  13  being compromised or noticeably reduced. 
         [0115]    The tack welding between the two sides  29 ,  32  of the armour joint is designed to break as soon as movement between the concrete slabs  27 ,  34  begins to occur. 
         [0116]    Turning now to  FIG. 4 , there is shown a cross-sectional view of a joint  37  to be formed between two adjacent concrete slabs  38 ,  39 , utilising a metal form  40 . 
         [0117]    The metal form  40  includes a thin stainless steel metal strip  41  which runs the length of the joint  37  to be formed between the concrete slabs  38 ,  39 . The metal strip  41  is joined at approximately 1 yard intervals to metal legs  42   a,b  which serve to support the metal strip  41 , and the metal form  40  as a whole. The metal form  40  also includes an armour joint  43 , with the requisite anchoring arms  44   a,b.  There is also shown a reinforcing rod  45  which runs the length of the metal form  40 . 
         [0118]    In use, the metal form  40  is placed where a joint  37  is to be formed between two adjacent concrete slabs  38 ,  39 . The dowel sheath  2  may then be fixed to the metal strip  41  as shown, for example by the use of rivets. The dowel sheaths  2  may preferably be spaced at approximately 1 yard intervals along the metal form  40 . Shear dowels  13  may then be placed in each dowel sheath  2 . 
         [0119]    The adjacent concrete slabs may then be poured, either together or separately, thus encasing the dowel sheath  2  within the concrete slab  38 , and the shear dowel  2  in the concrete slab  39 . 
         [0120]    As the two adjacent concrete slabs  38 ,  39  shrink and/or cure the shear dowel assembly  1  will function substantially the same as described previously in relation to the timber form arrangement of  FIGS. 3   a  and  3   b.    
       VARIATIONS 
       [0121]    While the embodiments described above are currently preferred, it will be appreciated that a wide range of other variations might also be made within the general spirit and scope of the invention and/or as defined by the appended claims.