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g-max Testing - An Overview

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When a player falls, the impact is absorbed either by the playing surface or the player's body. The "harder" the surface, the greater the amount of the impact absorbed by the player's body; and, not surprisingly, the greaterConcussions are a common injury the likelihood the fall will result in injury. This is especially true with respect to concussions - traumatic injuries to the brain - which can occur when the player's head hits the playing surface. Concussions are an insidious form of injury. The effects of individual concussions are most often fairly mild. In the case of multiple concussions, however, the effects can be cumulative and the consequences can be long lasting -  even permanent. Additionally, if a player has already experienced one concussion, they are more likely to experience another; and subsequent concussions carry a much higher risk of serious injury - up to, and including, death. If for no other reason than to lessen players' exposure to this type of injury, it is important to monitor impact-related characteristics of sports surfaces.

Impact testing (commonly referred to as g-max testing) measures the shock-attenuation performance of sports surfaces - including synthetic (artificial) turf and natural turf athletic fields. Test results (g-max values) are a ratio: the ratio of the maximum acceleration (deceleration) experienced during an impact, to the normal rate of acceleration due to gravity. The higher the g-max value, the poorer the shock-attenuation performance of the surface. Measuring impact-attenuation is a fundamental tool of athletic field safety testing. It is also useful in assessing the playability of a field.

Testing g-max values involves measuring the shock-attenuation performance of a playing surface in situ, and comparing the results against a standard. The most commonly used standard is the one established by ASTM International (ASTM). For synthetic surfaces, ASTM requires that the reported g-max value for every test point be less than 200g's (as measured in accordance with ASTM Standard F355-A and ASTM Specification F1936). If the requirement isn't met, the field is considered unsafe and remediation is needed.

While the maximum threshold value of 200g's is the de facto standard for evaluating the safety of an artificial field, many architects set different standards for the fields they design. These alternate standards typically involve a range of acceptable g-max values. The range sets an upper limit that addresses safety, and a lower limit that focuses on playability; the operative assumption being that fields that are too "hard" are dangerous, while fields that are too "soft" contribute to excessive fatigue and poor player performance. The range of acceptable values will vary in accordance with the materials and methods used to build a field, and also as a matter of the architect's personal preferences.

How often should you test?

It is important to note that g-max measurements are not static. On synthetic turf surfaces, g-max values will generally increase as the field ages. The amount of the increase, and the rate at which it occurs, are tied to construction methods, materials used, levels of play, environmental conditions, and the frequency and types of maintenance. On a natural turf field these same factors are at work, along with the moisture content of the soil, the type of turfgrass, and the density of coverage. This suggests that regular g-max testing is important - but how often is often enough? Everyone agrees that new synthetic fields should be tested before they are accepted from the builder. Beyond that, opinions differ: some advocate testing at least twice a year; some suggest an annual test; others say that testing can be less frequent. Among researchers and other industry experts, there is a general consensus that an annual test is most desireable:
  • Annual testing demonstrates a serious and proactive commitment to safety
  • Fields rarely become unsafe overnight, so testing once a year should provide adequate warning of emerging safety issues
  • Significant changes in g-max readings can be an early sign that problems are developing within the turf system - an annual test can alert you to these problems before they become critical
  • Annual testing generates a meaningful historical record - a record that can be important if warranty or liability issues arise
  • Testing once a year isn't a "budget buster"

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Do it right!

If you make the commitment to have your synthetic field(s) tested, you should also commit to having the tests done properly:
  • You should insist on using the services of an independent test provider. While many manufacturers will offer to do the testing, there is an inherent conflict of interest if they do it themselves.
  • Your test provider should be knowledgeable of, and in compliance with, applicable ASTM standards. Your provider should be able to show you copies of current standards and discuss them with you.
    • Equipment used to test the field should conform to ASTM Standard F355-Procedure A (F355-A) which requires a cylindrical missile with a circular, flat, metal impacting surface. The weight of the missile must be 20lbs (+/- 0.11 lb).
    • The accelerometer used in the test equipment should have a current calibration certificate, and the calibration should be NIST traceable.
    • Procedures used to test an artificial field, plus the number and location of individual test points, should conform to ASTM Specification F1936.
  • Do not accept values determined with a "Clegg" tester - except on a natural turf field tested in accordance with ASTM Standard F1702. Both a Clegg tester and a "F355" tester can be used to calculate g-max values, but the numbers are not comparable. (Be wary of anyone who tells you they are!)

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But what does this mean to you?

  • As the owner or manager of a field, you have an obligation to provide safe facilities. Where standards exist, you should insure that your facilities are in compliance and that you and your organization aren't exposed to unnecessary liability if an injury occurs.
  • As an athletic director or coach, you have a duty of care to the athletes under your supervision. Your duty of care encompasses the obligation to make sure the facilities you use are inherently safe and properly maintained.
  • As a concerned parent or player, you owe it to your child and/or yourself to know that a field has been built in accordance with established standards. You also want to know that it is regularly tested, by an independent test provider, to insure continuing compliance with those standards.
  • As an architect or engineer, you should insure that g-max standards for safety and playability are clearly defined in the specifications you write. You should require that all tests of synthetic surfaces be conducted by an independent test provider, in accordance with ASTM standards F355-A and F1936. And, you should specify that fields be evaluated prior to acceptance and at least annually throughout the warranty period. Click here to view a sample specification in pdf format. (You will need Adobe Reader to view this file.)

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