CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of my application Ser. No. 330,551, filed Feb. 8, 1973, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to expansion joints of the kind that are used between the edge surfaces of adjacent slabs of thermally expansive material (such as steel or concrete) used in the construction of roadways, walkways, bridges, and the like. 
     The invention also relates to a method of forming an expansion joint between a pair of such slabs. 
     PREVIEW OF THE PRIOR ART 
     In accordance with established practice in the construction of roadways, walkways, bridges and the like, concrete slabs of selected length are cast in situ along and upon a previously prepared roadbed, formed for example of compacted crushed gravel to other suitable aggregrate material. To allow for expansion of the concrete during hot weather, a gap of predetermined width is left between the parallel edge surfaces of immediately adjacent slabs. It is the practice to bridge or close this gap with an expansion joint, which usually must be liquid impermeable to thereby keep water from running between and under the slabs where it could freeze and cause heaving of the slabs during cold weather. The joint may also be required to serve as a supporting surface for vehicular and predestrian traffic. Where required, a layer of asphalt may, of course, be applied onto the upper surface of each concrete slab by hot-rolling or the like. 
     One prior art expansion joint utilizes a pair of stainless steel extrusions and an accordian-section web with bead-shaped edges, the web being formed from a flexible polymeric material such as neoprene. The steel extrusions serve as anchor elements for the web and are bolted, cast in place or otherwise secured to the respective edge surfaces of a pair of adjacent concrete slabs. The web is then forced between the extrusions until its bead-shaped edges slip into cooperating channels provided in the extrusions. A suitable high strength adhesive is usually preloaded in the channels to bond each edge of the web to its respective steel extrusion. 
     Such an expansion joint entails a number of disadvantages. In particular, the stainless steel material of which the extrusions are formed is not only very expensive but is heavy and therefore difficult to handle. Moreover, because of the dissimilarity of materials, a poor bond is sometimes formed between the polymeric web and the steel extrusion. Furthermore, the aforesaid method of assembling the prior art expansion joint is both laborious and time-consuming, in that is requires individual handling of each of the three expansion joint components; the pair of steel extrusions and the flexible polymeric web. 
     DEFINITION OF THE INVENTION 
     An object of the invention is to provide an expansion joint that utilizes anchor elements which are cheaper, lighter, and bond better to the polymeric web, while retaining the corrosion-resistant property of stainless steel. 
     Another object of the invention is to provide a fast and simple method for forming an expansion joint between adjacent slabs which entails handling of fewer separate components. 
     In accordance with the present invention there is provided an expansion joint for joining a pair of substantially parallel edge surfaces of adjacent slabs, said expansion joint comprising: 
     a. a pair of elongated anchor elements of a substantially rigid polymeric material, each adapted to extend along the edge surface of a respective one of said slabs; 
     b. securing means on each anchor element for securing the respective element to the respective slab edge; 
     c. a laterally compressible and extensible elongate web of normally folded configuration, of more flexible polymeric material than the said anchor elements and having two parallel longitudinal edges which in the completed joint are permanently secured to respective ones of the said anchor elements; and 
     d. spacing means interposed between the anchor elements for maintaining them at a preselected spacing to one another until said elements are secured to the respective slabs, said spacing means comprising an extensible membrane joined to the folds of the web to resist spreading thereof, said membrane being adapted to rupture during normal movement of said slab surfaces away from each other in operation. 
     Also according to the invention there is provided an expansion joint web element for use in an expansion joint for joining a pair of substantially parallel edge surfaces of adjacent slabs, said joint comprising a pair of elongated anchor elements of a substantially rigid polymeric material, each adapted to extend along the edge surface of a respective one of said slabs, securing means on each anchor element for securing the respective element to the respective slab edge, and the web element which in the completed joint is permanently secured to and extends between the two anchor elements, wherein the said web element comprises an elongate member of normally longitudinally folded configuration to be compressible and extensible transversely of the length of the anchor elements, the member having two longitudinal edges which in the completed joint are permanently secured to respective ones of the said anchor elements, and the member having an extensible membrane joined to the folds of the web to resist spreading thereof, thereby constituting spacing means for the said anchor elements for maintaining them at a predetermined spacing from one another, said membrane being adapted to rupture during normal movement of said slab surfaces away from each other in operation. 
     Further in accordance with the invention there is provided a method of forming an expansion joint between a pair of substantially parallel edge surfaces of adjacent slabs of settable material, said expansion joint comprising first and second elongate anchor elements of substantially rigid polymeric material and an elongate, laterally compressible and extensible web of substantially more flexible polymeric material than that of the said anchor elements with the longitudinal edges thereof secured permanently to the respective anchor elements, wherein at least one of the two longitudinal edges of the web has a pronounced bead and a corresponding first one of the anchor elements has a channel adapted to receive and retain the said bead said method comprising the steps of: 
     a. casting a first slab with the said first anchor element embedded therein to be thereby secured thereto, 
     b. with the second anchor element permanently joined to the respective longitudinal edge of the web inserting the said bead in the channel of the first anchor element and permanently securing it therein by means of an adhesive, the anchor elements being positioned at a preselected spacing from one another, and 
     c. casting a second slab with the second anchor element embedded therein to be thereby secured thereto. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention and its advantages, particular preferred embodiments thereof will now be described, by way of example, with reference to the accompanying diagrammatic drawings wherein: 
     FIG. 1 is a perspective view, partly in section, of a roadbed utilizing expansion joints formed in accordance with the invention; 
     FIGS. 2a, 2b and 2c are sequentive cross-sectional enlarged views in elevation taken along line 11 -- 11 of FIG. 1, wherein FIGS. 2a and 2b illustrate one constructive embodiment, FIG. 3a illustrates another constructive embodiment, and the three figures together illustrate a method of the invention of forming an expansion joint, and 
     FIGS. 3a and 3b are sequentive cross-sectional enlarged views in elevation taken along the said line 11 -- 11 to illustrate another constructive embodiment and method of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Throughout the various figures of the drawings, like reference numbers are used to denote like elements. 
     FIG. 1 shows several slabs 1a, 1b, 1c of a settable material such as concrete supported upon a roadbed 2 of compacted crushed gravel or the like. Adjacent slabs are separated by a gap of a size preselected to allow for thermal expansion and contraction of the slabs during changes in the ambient temperature. An elongated block 4 of resistingly compressible material, such as styrofoam, may be used as a spacing gauge to determine the width of the gap between immediately adjacent concrete slabs 1a, 1b and 1b, 1c. An expansion joint (generally designated by the reference E) bridges each gap by joining each pair of substantially parallel edge surfaces (for example the surfaces 3a, 3b) of adjacent slabs. Such expansion joints constitute liquid impermeable seals to keep water from running between and under the slabs where it could freeze and cause heaving of the slabs, or in the case of a roof construction leak through into the structure below. 
     As illustrated in greater detail in FIGS. 2a,  2b, 2c, a novel expansion joint of this invention comprises a pair of elongated anchor elements 5a, 5b of a substantially rigid polymeric material, and an elongated, laterally compressible and extensible web 6 of a polymeric material substantially more flexible than that used for the anchor elements. Each of the anchor elements 5a, 5b is shaped to form an edge for the respective slab and also to create a respective recess formation 8a, 8b extending along the corresponding edge surface 3a or 3b. Securing means 7a, 7b are also provided extending from the respective anchor elements for securing each element 5a or 5b to a respective one of the slabs 1a, 1b. At least one longitudinal edge 6b of the web 6 is permanently secured to anchor element 5b  before securing of either anchor element 5a, 5b to its respective slab 1a, 1b. The other longitudinal edge 6a of web 6 is permanently secured to anchor element 5a at least before both anchor elements 5a, 5b are secured to their respective slabs 1a, 1b by casting of the said slabs with the anchor elements in place. 
     Preferably, the expansion joint also comprises spacing means which in this embodiment are constituted by a breakable clamp 9, a compressible mass 10, and rupturable membrane 11, these spacing means maintaining anchor elements 5a , 5b at a preselected spacing to each other until they are secured to slabs 1a, 1b. As illustrated in FIGS. 2a, 2b and 2c, the breakable clamps 9 are C-shaped and of plastic material or the like, being spaced along the expansion joint with their free ends engaged in cooperating slots in the anchor elements 5a, 5b, thereby preventing separation thereof. The clamps are adapted to break under the normal thermal expansion and compression of slabs 1a, 1b. Movement of the anchor elements 5a, 5b, toward one another is prevented by a compressible mass 10, e.g., asphalt, disposed between the anchor elements. A resistingly extensible membrane 11 is adapted to rupture during movement of the edges of the slabs 1a, 1b, away from one another and may be used instead or in addition to prevent separation of anchor elements 5a, 5b during installation of the expansion joint. 
     Depending upon local circumstances, preferences and like special considerations, any of a variety of securing means 7a, 7b may be employed to secure anchor elements 5a, 5b to slabs 1a, 1b. For example, as described above, the securing means of FIGS. 2a, 2b, 3a, and 3b comprises an integral anchor portion for each anchor element 5a and 5b, the concrete slabs 1a and 1b being cast in place around them. 
     In the embodiments particularly described both web 6 and anchor elements 5a, 5b are composed of polymeric material, which is less expensive and less cumbersome to handle during installation than the steel anchor elements commonly used hitherto. By way of example only, the web 6 may be composed of neoprene, whereas the anchor elements may be composed of rigid polystyrene. 
     Because of their similar material composition, the web 6 and at least one of anchor elements 5a, 5b may be formed integrally by co-extrusion, resulting in a highly effective liquid impermeable junction therebetween. 
     In a construction in which both anchor elements 5a, 5b are co-extruded with web 6, then membrane 11 may also be co-extruded in joined relation to the folds of web 6 to resist spreading thereof, thereby providing in a minimum number of manufacturing operations a substantially complete expansion joint permitting simple installation. 
     In a construction in which only one anchor element 5b is co-extruded with, and integrally joined to one longitudinal edge 6b of web 6, the other longitudinal edge 6a may be formed with a pronounced bead 16 which may be subsequently bonded to anchor element 5a. For this purpose, in the embodiment illustrated anchor element 5a is provided with a channel 17 adapted to mate with bead 16 so as to receive it and mechanically retain it therein. The bead 16 subsequently is permanently secured in this channel by means of an adhesive such as an epoxy resin, which is particularly well suited for bonding between polymeric materials. 
     FIGS. 2a, 2b and 2c also illustrate a preferred sequence of steps in carrying out one method of installing an expansion joint in accordance with the invention. 
     With web 6 secured permanently to both anchor elements 5a , 5b (as by co-extrusion or adhesive bonding), the anchor elements are positioned at a preselected spacing, the means 9, 10, 11 being applied to maintain the anchor elements 5a, 5b at the preselected spacing, as described above. As shown in FIG. 2a, slab 1a is then cast to mold securely to anchor element 5a. As shown in FIG. 2b, a block 4 of resistingly compressible material such as styrofoam is then positioned contiguous to edge surface 3a, whereupon concrete slab 1b is cast to mold securely to the anchor element 5b, the block 4 serving as a form for spacing the formation of edge surface 3b. 
     FIG. 2c also illustrates an embodiment in which the web 6 is provided along both of its longitudinal edges with respective pronounced beads 16 each of which may be bonded at the appropriate time to the respective anchor element. Thus both edges may be bonded prior to installation of the joint, or one may be bonded prior to installation and the other after installation, or both may be inserted into their respective channels and bonded to their respective anchor elements after the installation of the elements. 
     FIGS. 3a and 3b also illustrate part of the sequence of steps in carrying out another method embodying the invention. As shown in FIG. 3a, concrete slab 1a is cast to mold securely to anchor element 5a, which is provided with the above-described channel 17, bead 16 is then permanently secured in channel 17 by means of an adhesive. With web 6 secured permanently to anchor elements 5a, 5b, they are positioned at a preselected spacing, spacing means being applied to maintain the anchor elements at the preselected spacing. For example, as described in relation to FIGS. 2b and 2c, a block 4 may be positioned beneath the joint and concrete slab 1b cast to mold to ahcnor element 5b.
An elongated expansion joint of the kind used between adjacent slabs in a road or building construction comprises, a pair of elongated anchor elements or rigid polymeric material adapted to be secured to the respective slab edges, and a laterally compressible web of more flexible material securable between the two anchor elements. At least one of the anchor elements may be secured to the web by means of a bead which snaps into a channel. The web may be of folded configuration with a rupturable membrane to hold the web to a predetermined spacing. The invention also comprises a method of forming an expansion joint between a pair of surfaces using a structure as specified.