Track and field sporting events are commonly held on running tracks. As would be understood by those in the art, running tracks come in various sizes and configurations that can vary depending on the standards implemented by different athletic organizations. Typically the main portion of the running track has an oval shape defined by two opposing semi-circular end sections or “turns” that are connected by two parallel and opposing rectangular side areas.
In some instances the base and the surface of the track (i.e., the useable surface that competitors run on) are defined by a single structure. However, in some configurations, the track is a composite structure defined by multiple substrate layers and multiple different materials. Typically, in such composite configurations, a concrete or bituminous concrete (i.e., asphalt) base/foundation is poured or placed on top of a prepared sub-base and the base acts as the primary structural layer that supports the top-most running surface. The running surface is often a resilient composite such as a polymer based material, asphalt, or a mixture of materials.
Irrespective of the track construction, it is preferable that the base provide a structurally sound and uniform base. It is also preferable that the track not develop cracks and maintain a consistent and generally level surface (e.g., without undulations or variations) through changing environmental conditions and usual wear and tear, as irregularities can cause injuries.
Cracking or irregularities in the level of the base often occurs in concrete slab surfaces and asphalt bases due to the expansion or contraction of the ground that supports the slab and the areas surrounding the base. More specifically, the moisture content of the soil that supports a floating slab can vary across the area of the slab and in the immediate vicinity and, accordingly, the ground expands or contracts differentially across the underside of the slab causing the slab to bend accordingly. For example, moisture trapped underneath a section of the base can freeze in the winter-time thereby lifting that section of the base relative to the surrounding portions of the base. These stresses can result in deformation of the base and cracking if the stresses exceed the tensile strength of the base. Swelling, or shrinking of the earth under or around the base can result from seasonal, short-term environmental conditions as well as more long-term environmental conditions also relating to climate and environment. By way of further example, moisture trapped within the base can also cause stresses and cracking of the base, for instance due to freezing and expansion of the moisture. In addition to experiencing significant flexing due to changes in the earthen base underneath the slab and the weight of any loads on the slab as described above running tracks can also expand and contract with changes in temperature. These conditions can result in cracking, which allows water and other environmental elements to further deteriorate the base and any metal structural reinforcements within the slab.
The foregoing problems are common to various floating slab constructions. One factor bearing upon the use of concrete is the fact that it is strong in compression, but relatively weak in tension. One method that is used to overcome the disadvantages of concrete's low tensile strength is through reinforcement. This can be accomplished by use of high strength steel wires, cables, or rods embedded within the concrete. One method for reinforcing concrete is post-tensioning in which un-bonded cables are suspended within the slab. After the slab of concrete has been poured and has set, the cables are pulled to a preselected tension using tensioning equipment, thereby putting the slab under a compressive load acting in the direction of the cables. Accordingly, when the slab receives a load, the compression is relieved on that portion, which would otherwise be in tension if not pre-compressed. Thus, for example, a slab is pre-stressed so that when the concrete is under load, the side of the slab that would otherwise be in tension if not pre-stressed, has less net tensile forces acting upon it. A permanent compressive load on the concrete helps the slab resist cracking as the supporting ground swells and shrinks with changing environmental conditions.
As noted above, a typical layout of a running track primarily consists of a flat, continuous oval shaped slab that surrounds an interior area. The generally narrow width of the track surface and the overall end to end length of the track presents special concerns in the installation and construction of a post-tensioned floating slab base that has sufficient structural integrity. One such concern is to minimize the thickness of the slab, which results in a lower total volume of concrete required for the slab and associated cost, while still providing a running track base that can withstand the stresses brought by environmental conditions and use and have a long useful life.
There remains a need, therefore, for an athletic track having a post-tensioned concrete slab foundation that has improved resistance to cracking than existing post-tensioned athletic track designs. It is with respect to these and other considerations that the disclosure made herein is presented.