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Playgrounds and Surface Materials

By Arthur Mittelstaedt Jr., EdD, executive director, Recreation Safety Institute

Since the late 1970s, studies by the Consumer Product Safety Commission (CPSC) revealed a great disparity in analyses of various surfacing materials.

Chart A reflects this disparity. Chart B reflects more definitive information on the material. Instead of accentuating the G forces, it emphasizes the depth of materials.

The traditional types of surface might be termed unitary (tiles, mats, carpets) and loose fill (sand, pea gravel and coffee bean husks). Unitary materials more recently include poured-in-place (loose manufactured particles bonded together by a binder) and multiple layering of different compositions (sheets bounded together by a binder). These products, unlike the monolithic tiles or mats, can have lab testing done of a specified nature or field-testing after installation. However, such products require greater quality control measures in their installation than the monolithic tiles or mats. Loose fill materials now include engineered wood fiber, shredded tires and palletized compositions.

Since the application and properties of these products vary it is highly urged that after the material is installed it be field tested to ascertain the critical fall height that the installed material is suited for. Adjustments can then be made to place more material or to provide equipment with shorter fall heights. Loose fill materials are often placed on a synthetic geothermal material to eliminate fines from mixing with the particles. Unitary materials are required to be placed on a solid case so that adhesive or other method of binding the surface to the under surface is practical.

Appendix C of CPSC Publication 325 lists many of the fall absorbing characteristics, the installation/maintenance requirements, and advantages and disadvantages of loose materials, now divided into organic and in organic materials: sand and gravel or wood chips, bark mulch, engineered wood fibers, etc.

Though the CPSC describes some of sieve sizes in Appendix D of sands and gravels used in its testing no further detailed information was specified for wood chips, mulch or engineered wood fibers, shredded tires or other particles. Field tests should be conducted by a reputable laboratory with equipment that meets the standards of ASTM F 1292 and not by unqualified individuals with equipment not calibrated or otherwise uniformly contracted to meet the instrumentation requirements.

To be safe when using products that can be changed in application field tests are recommended to reduce the risk of injury.

Laboratory and field tests are critical to establish if the installation has the acceptable shock-absorbing properties for impact attenuation established for the critical fall height of the equipment. Though CPSC has conducted tests, as illustrated in the charts, to determine the relative values in an uncompressed and compressed state there are many other factors that can affect the instrument readings that alter the consistency of the readings of the drop tests. Therefore, to be safe when using products that can be changed in application field tests are recommended to reduce the risk of injury.

The 1990 Report to CPSC by COMSIS Inc. stated under Section 5.1.1 Review of Fall Injury Data. "...the effectiveness of resilient surfaces in reducing injuries has received very little systematic studies." When a head impacts a surface, it is subjected to an impulsive force, whose magnitude, direction, and duration depend primarily upon impact velocity, and on mechanical properties of the head and the surface. The impulsive force can cause a deformation of the skull, a linear acceleration of the head, a rotation of the head with respect to the neck and torso, or some combination of these. A deformation of the skull can result in a skull fracture and concussion. Deformation is usually accompanied by head acceleration. When the head strikes a resilient surface or a surface that consists of loose materials, head acceleration can occur without significant skull deformation.

A linear acceleration of the head and a head rotation may cause relative motion between the skull and brain and changes in intracranial pressure; both of these effects can lead to concussion. (NBS, 1979a; Rutherford, 1979).

Due to the flatness of surfaces under playground equipment, linear skull fracture and/or concussion are more likely consequences of head impact than depressed skull fracture. Linear skull fractures involve failure of the overall skull, whereas depressed fractures involve localized failure of the skull due to the concentration of forces on a small area of the skull. In addition, most of the concussion tolerance data for humans were derived from linear skull fracture data. Therefore, the NBS used linear skull fracture data as the basis for their impact performance criterion. Peak acceleration was chosen as the criterion measure "because this greatly simplifies the testing procedure." Two studies were cited as justification for the recommended 200 g peak-acceleration criterion. Headfirst drops of adult cadavers onto a flat surface showed that when impact load was sufficient to cause skull fracture, peak accelerations were between 190 and 370 g's (Hodgson, Thomas, and Prasad, 1970). Head injury tolerance data for the headfirst falls of children indicated that a conservative tolerance limit for head injury is 150-200 g average acceleration for 3 msec, or 200-250 g peak acceleration (Mohan et al., 1978). Based on these data, the National Building Specification (NBS) concluded that "the risk of serious head injury due to headfirst fall is minimal when the peak acceleration imparted to the head is 200 g's or less". (NBS, 1979a).

The use of impact attenuating surfaces under and around play equipment does not mean that an injury cannot occur. All testing is done to reduce the risks of injuries to the head. However, a misplaced arm, finger, leg or angle when falling such an unusual or even abnormal positioning when impacting any surface could cause an injury. The use of "safety surfacing" minimizes the chance of injuries occurring and their severity. Design and specify products that are safe, will enlighten the playground for fun and be cost effective.

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December 14, 2019, 7:54 am PDT

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