Patent Document

FIELD OF THE INVENTION 
     The invention relates to athletic playing surfaces and more particularly to an athletic track having a post-tensioned concrete foundation. 
     BACKGROUND OF THE INVENTION 
     Track and field sport competitions are frequently held on athletic tracks. Such tracks usually have a standard shape with opposed straight sides connected by semi-circular ends. A foundation usually supports an athletic surface. The track should be durable yet resilient to aid athletic performance and minimize injury. Originally, cinder tracks and other loose aggregate surfaces were used. More recently composite surfaces have been employed. Composite surfaces have a foundation, often poured-in-place concrete, and an athletic surface, often a resilient composite such as asphalt, a polymeric substance, or a mixture of materials. In any case, it is important that the track not develop cracks which could cause injuries. The track should also have a configuration which sheds water easily, particularly if the track is outdoors. 
     The development of cracks in concrete foundations is a well-known phenomenon. Ground-supported, reinforced concrete slabs, commonly referred to as “floating slabs”, began to be used in residential and commercial construction after about 1945. Although floating slabs worked very well in many cases, they developed unacceptable cracks when used on “expansive” soils. Expansive soils absorb water and swell during rainy conditions, but shrink and dry during hot, dry weather. The combination of absorbent soils and a climate characterized by extended dry periods interspersed with significant rain can be found in many parts of the United States and particularly in California, Texas and Louisiana. During the mid 1960&#39;s, post-tensioned concrete slab foundations began to be used to support residential buildings in parts of Louisiana, Texas and California. Such slabs are called “post-tensioned” because they are stressed in tension after the concrete has set. A post-tensioned concrete slab comprises a poured-in-place concrete slab with metal strands or cables imbedded in the slab. The cables protrude from opposed parallel edges of the slab. Generally, there are two sets of parallel cables at right angles to each other, allowing the slab to be stressed in orthogonal directions. After the concrete has been poured and has set, the cables are pulled to a preselected tension, putting the slab under a compressive load. Concrete is very strong in compression, but weak in tension. A permanent compressive load on the concrete helps the slab resist cracking as the supporting ground swells and shrinks with changing water conditions. 
     Cracking occurs as the moisture content of the soil beneath a foundation changes differentially across the foundation. For example, the slab may be distorted in either a center lift mode (also termed “edge drying,” “center heave”, or “doming”) or an edge lift mode (also, “edge swell”, “edge heave” or “dishing”). Center lift mode is a long-term condition which occurs when the moisture content of the soil around the slab edges decreases and the soil sinks relative to the soil beneath the center of the slab. Edge lift mode is usually a seasonal, short-term condition which occurs when the ground at the edge of the slab becomes wetter than the ground beneath the center of the slab. The distance measured inward from the edge of a slab over which the moisture content varies is known as the “edge moisture variation distance” or “edge penetration distance”. The edge penetration distance is generally a function of climate. Wetter climates create larger edge penetration distances, and potentially greater bending in either center lift or edge lift modes. 
     Athletic tracks with concrete foundations are subject to the same kinds of changes experienced by residential or commercial foundations. Moreover, the extended shape of the track may present additional problems because a track is relatively long and narrow. The curved ends also present peculiar problems for stressing a concrete slab. 
     There remains a need, therefore, for an athletic track having a post-tensioned concrete slab foundation. An object of the present invention is to provide such an athletic track. 
     SUMMARY OF THE INVENTION 
     The present invention comprises an athletic track having a post-tensioned concrete slab supporting a resilient surface. The athletic track has opposed, parallel rectangular side areas and opposed semi-circular end areas connecting the side areas. The concrete slab is cast in place without requiring expansion joints between segments. Cables are cast into the slab for providing a tension load in the concrete slab. The cables are not perpendicular to exposed sides of the rectangular side areas. A first set of cables is tilted away from the perpendicular in a first direction while a second set of cables is parallel to the edges of the rectangular areas such that members of the second set of cables cross members of the first set of cables. In another aspect of the invention, a third set of circumferential cables is provided in the semi-circular end areas. A fourth set of cables intersects the circumferential cables. The fourth set of cables is preferably non-radially disposed in the semi-circular end area. In another aspect of the invention, the slabs forming the track have a relatively flat lower surface, without downwardly extending footings. In a further aspect of the invention, pour strips are provided between adjacent quadrants of the end areas. In another aspect, the pour strips are situated along a line of symmetry. 
     It is an object of the invention, therefore, to provide an athletic track with a substrate of post-tensioned concrete. 
     It is a further object of the invention to provide an athletic track with a concrete substrate without features extending from a bottom surface thereof for use on expansive soils. 
     It is another object of the invention to provide an athletic track with a concrete substrate with increased longitudinal compression. 
     It is also an object of the invention to provide pour strips for a post-tensioned athletic track wherein the stresses on the pour strips are reduced. 
     Another important object is to provide a stable substrate for an athletic track to increase the longevity of a resilient surface on the substrate. 
     Yet another object of the invention is to provide a method of constructing an athletic track having a post-tensioned concrete substrate. 
     These and other features and objects of the invention will be apparent from the following detailed description, in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view of an athletic track, with embedded cables shown in solid lines. 
     FIG. 2 is a top plan view of one half of the athletic track of FIG.  1 . 
     FIG. 3 is a partial sectional view of an edge of the athletic track. 
     FIG. 4 is a partial sectional view of a pour strip from the athletic track. 
    
    
     DETAILED DESCRIPTION 
     A preferred embodiment of the invention will now be described with reference to the accompanying drawings. Like numerals will be used to identify like parts in all the drawings. FIG. 1 illustrates an athletic track  10  comprising two opposed generally rectangular side areas  12 ,  14  and two opposed generally semi-circular end areas  16 ,  18 . Generally semi-circular should be understood to mean any smooth curve or arch or combination of arches and straight segments which would permit a runner or other athlete using the athletic track to make a circuit around the track. A true semi-circular configuration is preferred, but a segment of an ellipse or other such curve could also be used. Each rectangular area has an inner edge  20 ,  22  and an outer edge  24 ,  26 . Each semi-circular end area has a curved inner edge  28 ,  30  and a curved outer edge  32 ,  34 . The curved inner and outer edges conform to the shape chosen for the end areas. If the end areas are constructed in the shape of a smooth curve or arch or a combination of segments, the curved edges would usually conform thereto. Preferably, the rectangular areas  12 ,  14  and the semi-circular end areas are symmetrical about a line  36  bifurcating each of the end areas. 
     Extensions  38 ,  40  may be provided, co-linear with one of the rectangular side areas  12 , for sprints and similar races and activities. 
     It is desirable that the athletic track  10  comprise as few separate concrete segments as possible. In the preferred embodiment, the rectangular areas are poured first, with encapsulated cables and reinforcing structures to be described below. The semi-circular end areas are then poured, each end area comprising two symmetrical quadrants  42 ,  44 ,  46 ,  48 . Each quadrant joins its adjacent rectangular area at a cold joint  50 ,  52 ,  54 ,  56 . A key way may be cast in the end of a rectangular area so that a mating key will be formed at an adjacent end of a quadrant as the concrete for the quadrant is poured. Adjacent quadrants  42 ,  44  and  46 ,  48  forming end areas  16 ,  18  are separated by a pour strip  58 ,  60 . The pour strips  58 ,  60  are filled after tensioning of cables in the quadrants. Preferably, the pour strips lie on the line of symmetry  36 . Because the athletic track is generally symmetrical around the line  36 , strains caused by thermal expansion or contraction of the track or by expansion or contraction of the soil are minimized at the line of symmetry  36 . Since the pour strips are relatively weaker structures, placement of the pour strips at the line of symmetry  36  reduces the probability of failure of these structures. The extensions  38 ,  40  may be poured at the same time that the quadrant  44 ,  48  to which they are attached is poured. The concrete may be vibrated as the segments of the athletic track are poured which reduces voids in the concrete and thereby increases the strength of the concrete when set. This process requires pouring in a fixed width equal to the operating width of the concrete pouring machine. In such circumstances, a quadrant should be poured and the extension poured thereafter. 
     Referring now to FIG. 2, a rectangular area, such as rectangular area  12 , is prepared by laying a set of longitudinal cables  62  parallel to the inner and outer sides  20 ,  24  of the rectangular area  12 . The longitudinal cables  62  extend from an end  64  of the rectangular area  12  where the cold joint  52  will be formed, past a midline  66 , to selected locations  68  (see FIG. 1) in the rectangular area. Longitudinal cables  62  also extend from a second end  70  where the cold joint  56  will be formed, past the midline  66  to selected locations  72  in the rectangular area. Ends of the cables at the selected locations  72  are staggered such that adjacent cables do not end on a common line perpendicular to the edges  20 ,  24 . Conventional anchors (not shown) secure ends of the longitudinal cables at the selected locations  68 ,  74  within the concrete of the rectangular area when it is poured and set. The longitudinal cables, as well as other tensioning cables described hereafter, are sheathed to allow the cables to be pulled into tension after the concrete has set. 
     A set of transverse cables  76  extends from the inner edge  20  to the outer edge  24 . The transverse cables  76  are preferably parallel to each other and form an acute angle (and complimentary obtuse angle) at a cable&#39;s intersection with an edge. The acute angle may be 45°, but any suitable angle may be selected. This angled orientation of the transverse cables makes the transverse cables longer than they would be if they were only perpendicular to the edges  20 ,  24 . Longer cables can stretch further in response to a load. Moreover, angled transverse cables resist crack formation perpendicular to the edges  20 ,  24  where cracking is most likely and which would otherwise be resisted only by the very long and relatively few longitudinal cables  62 . The transverse cables  76  should cross the longitudinal cables  62  such that no path can be found from inner edge  20  to outer edge  24  or from end  64  to end  70  which does not cross a cable. 
     In addition, near the ends  64 ,  70 , sets of perpendicular cables  78 ,  80  extend from edge to edge in an area where angled transverse cables cannot extend fully from edge to edge. 
     To further strengthen the outer edge  24 , tendons  82  may be provided at regular intervals along the edge. For clarity, only a few representative tendons  82  are illustrated in FIG.  2 . Tendons may be provided along the entire outside edge  24 . Tendons comprise a length of re-enforcing rod (rebar), bent into a “J” shape, and imbedded in the concrete such that the long part is parallel to and adjacent the upper surface of the slab. 
     At the ends  64 ,  70  of the rectangular area  12 , ends  84  of a set of circumferential cables  86  for a quadrant of a semi-circular end area are anchored with conventional anchors (not shown). The ends  84  of the circumferential cables  86  are also off set to distribute the load experienced by the concrete slab when the cables are tensioned. 
     With cables  62 ,  76 ,  78 , and  86  and tendons  82  positioned, a concrete slab  88  is poured in the rectangular area  12 . Preferably, the slab  88  is of uniform thickness. As illustrated in FIG. 3, the slab  88  may have a thickened area  90  along the outer edge  24  and the inner edge  22 . This additional thickness spreads the stresses caused by tensioning a transverse cable  62  or perpendicular cable  78  with a conventional tensioning block  92  available from Suncoast Post-Tension of Houston, Texas, for example. Similar thickened areas may be provided at the ends  64 ,  70  for tensioning of the longitudinal cables  62 . There are no footings as such extending transversely down from the slab. The athletic track  10  supports only the nominal loads of athletes using the surface. The track  10  essentially floats on the top of the soil. Particularly where that soil is expansive, footings would only provide surfaces against which expanding or contracting soil would push in horizontal directions, which would tend to crack the slab  88 . Such features should be minimized or avoided. 
     Both rectangular areas  12 ,  14  may be constructed and poured at the same time. The rectangular areas  12 ,  14  should then be allowed to cure until the cables can be tensioned. In particular, tensioning blocks for the longitudinal cables  62  are concealed by the semi-circular end areas and are inaccessible after those areas have been poured. 
     After the rectangular areas have been poured, the quadrants of the semi-circular end areas are prepared for pouring. As the quadrants are similar, a description of one quadrant will be given. As mentioned above, circumferential cables  86  already have ends  84  imbedded in the rectangular areas. The circumferential cables are positioned concentrically from the end of the rectangular area  12  to the location where the pour strip  58  will be constructed. A set of fanned cables  94  extend from the curved inner edge  28  to the curved outer edge  32 . The cables in the set of fanned cables do not lie on radii, but form an acute angle (and complementary oblique angle) with a tangent to either the curved inner edge or the curved outer edge at the intersection of a cable with an edge. Adjacent cables are slightly closer together at the curved inner edge than at the curved outer edge, but two intersections on the curved inner edge subtend the same degrees of arc as the two intersections of the same cables at the curved outer edge. As with the transverse cables  62 , slanting the fanned cables  94  allows the cables to be longer and imparts a component of tension in a circumferential direction, which would otherwise have to be provided solely by the circumferential cables  86 . The fanned cables  94  should cross the circumferential cables  86  such that no path can be found from the curved inner edge  28  to the curved outer edge  32  or from a first end  96  to a second end  98  which does not cross a cable. In addition, near the ends  96 ,  98 , sets of radial cables  100 ,  102  extend from curved edge to curved edge in an area where fanned transverse cables cannot extend fully from edge to edge. 
     If an extension  38  is attached to a quadrant, the fanned cables  94  are extended into the extension. Some  104  of the fanned cables are curved until they lie parallel to the outer edge  24 . This outer edge  24  is common to both the extension  38  and the adjacent rectangular area  12 . The curved, fanned cables  104  terminate at conventional tension blocks at an end of  106  of the extension  38 . Perpendicular cables  108  cross the extension  38  perpendicular to the outer edge  24 . Transverse cables  110  cross the extension  38  in the area where area where the curved, fanned cables  104  bend and form acute (and complimentary oblique) angles at their intersection with the outer edge  24 . 
     Where all the aforementioned cables are exposed at edges or ends of rectangular areas, quadrants of semi-circular end areas, or extensions, conventional tension blocks are provided so that the cables may be placed under tension when a surrounding slab of concrete has been applied and has set to a sufficient hardness. 
     The two quadrants of a semi-circular end area meet at the pour joint  58 , illustrated in cross section in FIG.  4 . Lower reinforcing rods  110 ,  112  are placed across a gap  114  and extending into areas which will be the ends of concrete slabs forming the quadrants. The lower rods are fastened together with wire ties  116  such that a rod extending from one quadrant is fastened to a rod extending from an adjacent quadrant. A second series of upper reinforcing rods  118 ,  120  is placed above the tension blocks attached to ends of the circular cables  94 . Free ends of these upper rods  118 ,  120  are not fastened together at once but are bent upward, making it easier to reach the cables  94 . Slabs of concrete for the quadrants and extensions  38 , if any, are poured, encasing cables and ends of the rods  110 ,  112 ,  118 ,  120 . The same considerations for the shape of the slabs apply to the quadrants and extensions as explained above in connection with the rectangular areas. When the slabs have set, the cables in the quadrants and extensions are tensioned. Free ends of the upper rods can then be bent down and rods from adjacent quadrants can be fastened together with wire ties. Transverse rods  126  perpendicular to the upper or lower rods may also be tied to the upper and lower rods. The gap between the ends of the adjacent quadrants is then filled with concrete, forming the pour strip  58 . 
     After all the concrete has been poured, set and tensioned, a resilient athletic surface  128  is placed on the structure. Because the concrete slabs are tensioned, cracks sufficient to damage the athletic surface  128  are less likely to develop. Also, the relatively flat bottom of the track  10  enables the track to float on expansive soils so that forces which would otherwise contribute to cracking are reduced or eliminated. The resistance to cracking is enhanced by placing the pour strips on a line of symmetry, where the weaker pour strips are subjected to weaker forces. 
     These and other features and advantages of our invention, as well as modifications, will be apparent to the skilled artisan. The foregoing description is intended, therefore, to be illustrative only and the scope of the invention is to be defined by the following claims. All variations and changes which are within the scope of equivalency of the claims are intended to be encompassed therein.

Technology Category: 0