Permeable Aggregate Systems for Playing Fields
Posted by Graniterock on Mar 18, 2015
It’s no doubt that synthetic athletic fields are becoming more popular. Organizations that decide to install synthetic fields have realized lower maintenance costs and better playability. Using a sophisticated system of drainage channels, filter fabrics and specialized aggregate, a well-designed field draws water quickly from the turf layer allowing year-round play and reliable playability in wet weather.
The aggregate blends underlying the turf are extremely important components of the athletic field system and must be designed correctly to ensure stability, permeability and smoothness. Specifications for permeable aggregate typically require a non-recycled crushed stone that is durable and abrasion resistant. Soft materials such as sandstone, limestone or shale are often prohibited. Minimum durability and sulfate-soundness requirements are often specified, as well as abrasion resistance. All blend components should be washed, since clay and silt-sized fines impede permeability and are usually limited in the specifications to no more than 3 percent of the blend. Blends must also be permeable enough to allow water to pass through at a rate of at least 30 inches per hour.
Most projects use a dual-blend aggregate system to meet performance requirements: a base-stone blend for stability and permeability and a top-stone blend that provides a level and permeable surface for the artificial turf. Although the dual-blend system is most often used, the allowance of an alternative single-stone blend is also common.
The purpose of the top-stone layer is to promote rapid drainage of water and to provide a level surface. Top-stone blends typically consist of a durable gravel of uniform size, with a small addition of clean sand to promote smoothness, planarity and improve particle interlock.
The purpose of the base-stone layer is to provide stable support while permitting sufficient drainage. A good base stone blend balances the need for stability with the need for drainage and does so by balancing the blend’s opposing qualities of compactability and permeability. A well-graded blend with angular or cubical particles may provide great stability but may restrict permeability if over-compacted, so specifications will usually include a target or range of acceptable compaction rates. The producer should ensure that the blends are tested for permeability at the same rate of compaction that will be used during product placement.
The success of this new technology has implications on the development of other permeable technologies such as permeable pavements for storm-water management. Green Building systems such as LEED® , created by the U.S. Green Building Council, recognize permeable stone systems as an effective way to manage storm water and alleviate waterway pollution. Permeable-stone systems designed for athletic fields may also be effective in meeting these emerging needs.
Abrasion resistance is usually required to be no greater than 40% loss at 500 revolutions using an L.A. Rattler device. A typical minimum durability index value of 40 and maximum average sodium sulfate loss of 12% may also be specified.
Specifications typically require a minimum of 30 inches per hour using ASTM Method D2434. Compaction should be determined using a vibratory table to avoid particle fracture; ASTM Method D4253 may be used to determine maximum unit weight. The use of a mechanical rammer as used in ASTM D1557 is not representative of actual product installation and should be avoided.
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