G-max (also known as impact) testing measures the surface hardness of an activity field. The measurement is taken by dropping a 20 lb. weight from a 24-inch height onto the playing surface. The weight contains an accelerometer that measures how fast the missile stops once it hits the surface. The maximum acceleration magnitude recorded is the G-max. Three drops are performed at each of a minimum of 10 test locations per field. The G-max for the location is the average of the second and third drops.
The State of Minnesota does not require G-max testing. However, following construction and renovation of numerous synthetic activity fields in 2017, Edina Public Schools established a protocol to G-max test all synthetic turf fields each spring. Testing is done by the Institute for Environmental Assessment (IEA), an independent, third-party organization based in Minnesota, using trained staff and certified equipment to provide EPS with annual updates on the condition of the fields. IEA also measures the infill level at each of the drop sites in order to further enhance field safety and consistency.
ASTM International has set a maximum threshold of 200 G-max for every test point, however EPS subscribes to taking corrective actions at a threshold of 150-165 G-max. A G-max test score that exceeds 150 results in field corrections that may include dragging the field, adding infill or other work. Regardless of G-max score, all EPS fields receive routine field maintenance to sustain the safety and playability of the fields over time and reduce the need for corrective actions.
