BACKGROUND OF THE INVENTION 
     This invention relates generally to joints in concrete slabs, and more particularly to an improved joint and method of installation to prevent concrete surface deterioration caused by spalling at edges of the joint spaces. 
     Concrete floor slabs having exposed surfaces subjected to repeated impact loads, such as those produced by hard wheel tires on industrial lift trucks, are susceptible to localized failure at unprotected edges of cracks and joint spaces because of the inherent brittleness and weakness of concrete in both tension and shear. The breakage and crushing type failure at the unprotected edges is generally referred to in the art as &#34;spalling&#34;. To reduce the likelihood of edge spalling, joint spaces and cracks are routinely filled with sealant materials in an effort to avoid edge exposure. In today&#39;s market, various liquid plastics including epoxies, urethanes and polysulfides are available as joint fillers. Nevertheless, floor joints and cracks in concrete surfaces subjected to hard-wheeled traffic continue to eventually break down because of spalling, regardless of the joint or crack filler material utilized. 
     Concrete slab shrinkage is a well known ongoing process because of hydration and drying within the concrete mass, and is manifested by steady growth in the width of joint spaces and cracks. The filler material selected must therefore accommodate such long-term slab shrinkage by virtue of its elastic and adhesive bonding properties. While the stresses induced by slab shrinkage are resisted both in the body of certain rigid types of filler materials and at their bonding interfaces with the concrete, eventually the tensile strength of adjacent layers of concrete is exceeded to cause adjacent layer fracture or &#34;re-cracking&#34;. Such re-cracking phenomenon creates the very same condition the filler was intended to prevent or repair, i.e., concrete edge exposure. In an attempt to avoid re-cracking failure resulting from induced stresses, a semi-rigid, low-adhesive type of filler material has been formulated, wherein the concrete bonding interfaces of the filler are adhesively weaker than the tensile strength of the filler or the concrete alone, so as to preclude re-cracking of the concrete in spaced adjacency to the filler, as aforementioned. However, filler separation or fracture at the concrete bonding interfaces then occurs in response to shrinkage induced stress resulting in edge exposure and spalling under repeated impact loading. 
     Various joint filler modifications other than changes in material formulation have been proposed in an effort to deal with the foregoing spalling problem, including the use of plastic divider strips in an enlarged spalling repair patch, or insert elements embedded in the filler during joint installation. For example, a filler body is held compressed by an insert element during joint installation, for subsequent expansion within the joint space according to U.S. Pat. Nos. 3,276,334 and 3,255,680 to Rhodes and Cooper et al, respectively. According to U.S. Pat. No. 4,699,540 to Gibbon, a preformed cylindrical insert is utilized to relieve any strain at the concrete bonding interfaces of the filler caused by concrete expansion. However, none of the foregoing joint filler modifications provides a completely reliable solution to the problem of eventual failure by spalling at filled joint spaces and cracks, related to the aforementioned re-cracking phenomenon caused by long term slab shrinkage. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, the filler within a concrete crack or joint space has an insert embedded therein with means on one side thereof to enhance bonding to the filler material so as to establish a path along the other low adhesive side of the insert for separation from the filler in response to stress induced in the concrete by long-term shrinkage, for example. The adhesive strength of the bond between the insert and the filler is accordingly arranged to be less than that of the concrete bonding interfaces. 
     When installed, the insert has means for maintaining at least its low adhesive side spaced throughout from the side wall surfaces of the crack or joint space to be filled by the filler, in order to avoid any fracture or separation capable of weakening the concrete bonding interfaces of the filler and to ensure the maintenance of concrete edge protection by the filler against spalling. According to one embodiment, insert spacing from the concrete bonding interfaces is established by lateral projections from the insert contacting the concrete side walls of the joint space. In another embodiment, a narrow retention slot is initially cut to receive and hold the insert in position while the slot is partially widened to the joint space dimension. 
     Pursuant to the present invention, the aforementioned insert is embedded within the filler during establishment of the joint to prevent spalling of the concrete edges at such joint, as distinguished from repair treatment of spalling damage at an existing joint, involving enlargement of the joint space to remove the damaged surface portions of the concrete. As a result of the treatment provided by the present invention, the only separation or re-cracking occurring because of induced stress is located within the joint itself and in spaced relation to the concrete edges so that the filler edge protection remains intact. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout. 
     FIG. 1 is a side section view through a concrete slab and expansion joint in accordance with a prior art arrangement, showing spalling damage under loading and stress-induced cracking conditions. 
     FIG. 2 is a side section view through a concrete slab showing an expansion joint in accordance with the present invention, under loading and stress-induced cracking conditions, similar to those shown in FIG. 1, but without spalling damage. 
     FIGS. 3A-3D are section views of a concrete slab showing different stages in the formation of the joint shown in FIG. 2. 
     FIG. 4 is a partial perspective view of the insert to be embedded in the filler of the joint shown in FIGS. 2 and 3D, in accordance with one embodiment of the invention. 
     FIG. 5 is an enlarged partial section view taken substantially through a plane indicated by section line 5--5 in FIG. 4. 
     FIG. 6 is a side section view of the same joint shown in FIGS. 2 and 3D, installed between two abutting slabs. 
     FIGS. 7A, 7B and 7C are side section views showing different stages in the formation of an expansion joint in a concrete slab, in accordance with another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates by way of example a horizontal, concrete floor slab, generally referred to by reference numeral 10, having an upper exposed surface 12 to which moving impact loads are applied through a hard wheel 14 rolling over the surface. In an effort to pre-establish the location of all shrinkage induced fractures, such as the crack 16 shown in FIG. 1, narrow expansion joints were heretofore provided in the slab either during installation or by subsequent repair treatment, such as the expansion joint generally referred to by reference numeral 18. The expansion joint 18 is formed by a slot or joint space 20 in the concrete slab, cut to a predetermined depth and width and filled with a semi-rigid epoxy sealant material or laminant 22 in accordance with standard practice. The laminant or filler material 22 when fully cured exhibits a relatively high impact-resistant strength because of its resiliency, and completely fills the joint space so that its rigidity protects the surface edges 24 of the concrete at the intersections of the surface 12 with the side walls of the joint spaces. Low adhesive bonding interfaces 26 are formed between the filler 22 and the concrete side walls of the joint space so that re-cracking of adjacent layers of concrete is avoided. Such type of joint space filler is marketed as &#34;MM-80 Semi-Rigid Epoxy Joint Filler&#34; by the Metzger/McGuire Company of Concord, N.H. 
     The foregoing known type of expansion joint 18, while preventing stress-induced surface fracture between joints, is susceptible to adhesive rupture of the bonding interface at one side of the filler 22. Therefore, under impact loading of hard wheel traffic by wheels 14, for example, the concrete edge exposed at the surface 12 by separation or fracture 28 along one bonding interface, will rupture as shown by the spalled zone 30 in FIG. 1. If the filler material were made more adhesive and elastic to avoid fracture and separation at the bonding interface, it will not be sufficiently rigid to protect the concrete edges 24 from impact loads and spalling will also eventually occur. 
     In order to avoid such spalling failure, the stress-induced surface fracture is relocated within the epoxy filler itself despite its high tensile strength, in accordance with the present invention. Thus, fracture 28&#39; as an extension of the underlying crack 16 is spaced from both of the concrete bonding interfaces 26, as shown in FIG. 2 with respect to a modified form of expansion joint 18&#39;. The expansion joint 18&#39; is modified in accordance with the present invention by the provision of a plastic separation strip or insert 32 extending between the lower end surface and the upper exposed end surface of a filler 22&#39; which may be made of the same material as described for filler 22 shown in FIG. 1, or may alternatively be made of a more rigid and more adhesive material. When installed, one side 34 of the insert is roughened to enhance bonding to the filler 22&#39;  leaving the other side 36 with an adhesive bond to the filler that is less than that of the concrete bonding interfaces 26, aforementioned. Fracture 28&#39; along such lesser adhesive side 36 of the insert 32 thereby ensures that the concrete edges 24 remain protected by the filler 22&#39; of joint 18&#39;, to prevent spalling. 
     The joint 18&#39; is formed during concrete slab installation, in accordance with the present invention, rather than as a repair treatment. As shown in FIG. 3A, the slab 10 has the joint space 20 cut therein, after which the insert 32 is positioned therein as shown in FIG. 3B. The filler 22&#39; is then poured into the joint space and cured to its final state with the insert embedded therein, as shown in FIG. 3C. The insert 32 and filler 22&#39; when installed project above the surface 12 as shown, and are subsequently cut flush with the surface 12 as shown in FIG. 3D. In actual practice, it may be convenient to reverse the order of insert and filler installation. That is, the filler 22&#39; may first be poured into the joint space, with the insert 32 being pushed down into the joint while filler 22 is still liquid. 
     It is essential that the side surface 36 of the insert 32 be spaced throughout from the bonding interfaces 26 when the filler is installed. Toward that end, spacing projections or dimples 38 are formed on the insert and extend laterally therefrom for contact with the side walls of the joint slot 20 as more clearly seen in FIG. 3B, pursuant to one embodiment of the invention. The projections are spaced from each other and are non-aligned on opposite sides of the insert as shown in FIGS. 4 and 5 so as to accommodate free flow of the filler material in a fluent state when poured into the joint slot 20 during installation. In the particular embodiment of insert 32 shown in FIGS. 4 and 5, the insert body is made of polypropylene, with the dimple projections 38 struck out therefrom. The side surface 34 of the insert is roughened to enhance bonding by the formation of dovetail striations 42 therein. 
     FIGS. 7A and 7B show another method of maintaining an insert 32&#39; spaced throughout from the concrete bonding interfaces, without any lateral projections from the insert body. Initially, a narrow retention slot 40 is cut into the slab 10 to a depth 42 as shown in FIG. 7A, dimensioned to receive the insert 32&#39;. The slot 40 is widened to a depth 44 above 42 to form the joint space 20&#39;, as shown in FIG. 7B. The filler is then installed within joint space 20&#39; bonding to the concrete and the insert to complete the joint 18&#34;, as shown in FIG. 7C, having the properties hereinbefore described with respect to FIGS. 2-5. 
     The same joint 18&#39; as hereinbefore described with respect to FIGS. 2-5, is shown installed between abutting concrete slabs 10&#39; and 10&#34; in FIG. 6. The joint 18&#39; will accordingly accommodate expansion or strain of the abutting slabs along gap 16&#39;, while protecting the concrete edges 24 against spalling by restricting formation of any fracture separation to the weaker adhesive side 34 of insert 32 as hereinbefore described. 
     The foregoing is considered as illustrative only of the principles of the invention. Further since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and, accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.
Joint spaces within structural concrete bodies are filled with semi-rigid fillers to avoid adjacent concrete layer re-cracking and protect the concrete surface edges of the joint spaces against spalling by repeated impact loading. Inserts embedded in the fillers locationally restrict stress-induced fracture to the joint spaces and in spaced relation to the concrete bonding interfaces of the fillers so as to maintain filler protection for the concrete edges against spalling damage.