Surface Hardness (Gmax) Method
Surface hardness was measured using a CIST equipped with a 2.25 kg (5 lb) missile and a drop height of 455 mm (American Society for Testing and Materials, 2000b) and the F355 method equipped with a 9.1 kg (20 lb) missile and a drop height of 610 mm (American Society for Testing and Materials, 2000a) (Fig 15). Impact attenuation, as measured by an accelerometer mounted on the missiles, was used to indicate surface hardness and is reported as Gmax, which is the ratio of maximum negative acceleration on impact in units of gravities to the acceleration due to gravity. The average of six CIST and three F355 measurements taken in different locations on each subplot was used to represent the surface hardness of that subplot. A single F355 measurement consists of dropping the missile three times in the same location with a three minute interval between each drop. The value reported as Gmax is the average of the second and third drop in the same location. When using the CIST we report the Gmax value we obtained with the first drop on the surface. Measurements were taken when the surface was free of moisture from dew or precipitation.
The experimental design was a completely random split-plot statistical design with three replications. The split was wear and no wear. The means of the six CIST and three F355 measurements were analyzed using analysis of variance and Fisher's least significant difference test at the 0.05 level. A LSD was not calculated when the F ratio was not significant at the 0.05 level.
Results and Discussion
For all results in this section it should be noted that this data is from the first two years of a long term study and represent only that time frame. These results will be updated as more data is collected during subsequent years.
The Gmax, Severity Index, and Head Injury Criteria (HIC) for data collected during 2003 and 2004 are shown in Table 1 and 1a. We found that Gmax had a high correlation with the severity index (0.97) and head injury criteria (0.96). For this reason, we are currently suggesting that Gmax should be the main focus when comparing surface hardness values.
Table 1. Surface hardness (Gmax), Head Injury Criterion (HIC), Severity Index (SI), subsurface temperature, and infull depth of ten synthetic turf products.
Table 1a. Surface harness (Gmax), Head Injury Criterion (HIC), Severity Index (SI), and pad temperature of ten synthetic turf products
The question remains: how hard is too hard? The following information is taken from ASTM 1936 (American Society for Testing and Materials, 2000d).
The need for a systematic means of evaluating the impact attenuation of an installed North American football playing system has been amply demonstrated by the current difficulty in establishing the shock absorbing properties of new and aging systems. The aim of this specification is to provide a uniform means and relatively transportable method of establishing this characteristic in the field based on historical data. According to historical data, the value of 200 G is considered to be a maximum threshold to provide an acceptable level of protection to users.
The test method used in this specification (Procedure A of Test Method F 355), has been documented, through "unofficial" use for testing impact in fields for over 20 years. The development of this 2 ft fall height method can be traced back to the Ford and General Motors crash dummy tests of the 1960's, medical research papers from the 1960's and 1970's, and a Northwestern University study in which an accelerometer was fixed to the helmet of a middle linebacker to measure the impact received during actual play. This study found the impact to be 40 ft/lb that translates to the 20 lb at a height of 2 ft used in Procedure A of Test Method F 355. The maximum impact level of 200 average Gmax, as accepted by the U.S. Consumer Product Safety Commission, was adopted for use here.
All of the surfaces measured were well below the maximum level of 200 Gmax even after the equivalent of 296 games of simulated traffic over two years. While open to debate, I suggest the upper limit should be set to 175 Gmax using the F355 method A. After two years of simulated wear, all synthetic surfaces in this study measured well below the suggested upper limit for surface hardness.
Clegg Impact Soil Tester (CIST)
The CIST is the standard method to measure the surface hardness of natural turfgrass playing surfaces (American Society for Testing and Materials, 2000b). The device is similar to the F355 method. Both systems have a weighted missile that is dropped through a guide tube and impacts the playing surface. Each missile contains an accelerometer that measures how quickly the missile stops upon impact. This impact attenuation is representative of surface hardness. The two devices use different weights. The F355 method uses a 20 lb weight and the CIST uses a 5 lb weight but has a smaller impact surface area. The impact energy of both devices is very similar; however, the CIST results in a lower Gmax reading compared to the F355. The F355 method also has a velocimeter that measures the velocity of the missile just prior to impact. This gives the F355 method an added measure of accuracy since the velocity of the missile is actually measured during each drop. When using the CIST, the velocity of the missile is assumed. The velocity of the CIST missile has been measured, but for any individual drop, the user must assume that the missile is traveling at the calculated velocity and nothing has interfered with that velocity.
In a previous study, McNitt and Landschoot (2004) reported that under the conditions of their study the relationship between the Gmax values generated by the first drop of the F355 method can be compared to the values generated by the CIST using the regression equation (F355 x 0.66) - 9.3 = CIST. The regression coefficient for this equation was 0.95. Although this study was limited to the Sofsport infill system, the high regression coefficient would indicate that the CIST would be a suitable device to measure the surface hardness of Sofsport installations. After two years of data on varying infill systems at varying levels of wear we have generated the following regression equations:
For the first drop of both devices:
(F355 x 0.76) - 27.5 = CIST with an R squared value of 0.87.
For the first drop of the CIST and the average of the second and third drop in the same location using the F355:
(F355 x 0.81) - 27.1 = CIST with an R squared value of 0.81.
Using the above regression equation a Gmax of 200 measured with the F355 would be equivalent to a Gmax of 135 measured with the CIST and a 2.25 kg missile. None of the treatments in this study exceeded the 200 Gmax limit measured with the F355 or the 135 Gmax measured with the CIST (Table 2).
Table 2. Surface hardness (Gmax) of infill systems in 2003 determined with the Clegg Impact Tester prior to and after grooming1.
We connected the CIST to a laptop computer and generated both the HIC and SI. The HIC and SI generated with the CIST were highly correlated to the Gmax value suggesting that Gmax is a sufficient measure of surface hardness (Table 3, 4 and 5).
Table 3. Severity Index (SI) and HEad Injury Criterion (HIC) of infill systems determined in 2003 with the Clegg Impact Tester prior to and after grooming1.
Table 4. Severity Index (SI) of infill systems on 2004 determined with the Clegg Impact Tester prior to and after grooming¹.
Table 5. Head Injury Criterion (HIC) of infill systems determined in 2004 with the Clegg Impact Tester prior to and after grooming¹.
The lower half of the first two columns of data in Tables 2 and 3 is after simulated traffic equaling 44 and 84 games, respectively. The top half of the table is data collected from the half of the plot not receiving traffic. The third column (20 Nov) is data collected immediately after grooming. It is apparent from this data that grooming significantly reduced the surface hardness of all treatments in 2003. This was not the case in 2004 as grooming seemed to have little consistent affect on surface hardness (Table 6). This may be due to the aging of the systems or due to the unseasonably cold wet summer we experienced in 2004. Weekly Gmax measurements were collected using the CIST. The results are shown in Table 6. This was done in an attempt to monitor the duration of the effect of grooming on Gmax values. Our results indicate that the Gmax values of these surfaces remained relatively consistent from fall of 2003 through October 2004. It is unlikely that the effects of grooming last this long. In fact, the F355 data indicates that the surfaces trended higher in Gmax values in 2004 compared to 2003.
Table 6. Surface hardness (Gmax) of infill systems determined in 2004 with the Clegg Impact Tester prior to and after grooming¹ but were not consistent.
These data may be a result of the cool wet conditions that prevailed in 2004. Another possibility is that as these systems age and become less resilient, the heavier missile of the F355 method, which takes the average of the second and third successive drop in the same location, is experiencing a higher Gmax due to the heavier load at impact. After maximum impact attenuation of the infill and fiber system is reached, the accelerometer in these devices will begin to be affected by the impact attenuation of the surface below the backing. Henderson (1986) found that a rock can be sensed by the F355 method when it is four inches below the surface of a natural turfgrass playing field. The reason for this difference will likely become clearer as successive years of data are collected. Currently, we are suggesting the CIST as a tool for grounds managers to monitor their field throughout the season and recommending the F355 device be employed at least annually.
The following data is provided to give some reference points for the Gmax values generated using the CIST.
Table 7. Impact values for high school fields vs. impact values for other surfaces
During 2004, we applied simulated foot traffic to a very well established Kentucky bluegrass (Poa pratensis, L.) turfgrass (33% Liberator, 33% Washington and 33% Touchdown) at the same rate and intensity as we applied it to the synthetic turf systems.
A significant amount of turfgrass cover was lost. On the 9 Jul 205 rating date, the average turf cover was 65% (Fig. 16). In order to coincide with grooming of the synthetic turf, the natural turf area was aerated in August using 3/4 inch tines on 3 x 2 in. center and rested for about one month. The plots recovered to about 90% turf cover (Fig. 17). Wear continued and by 8 Nov 2004 the plot area averaged 40 - 45% turf cover (Fig. 18). This turf area was well maintained. During the 2004 growing season nitrogen fertilization on this plot area was 3.5 lbs N per thousand feet squared and irrigation was applied to prevent drought stress. The soil was a Hagerstown silt loam. Prior to the beginning of simulated wear, the thatch thickness of the plot area averaged 3/4 in. The data listed in Table 8 indicate that the surface hardness of the natural turf area was the same or higher than most of the infill systems' Gmax values listed in Table 2. On a native soil turfgrass surface, Gmax is greatly affected by soil moisture. Soil moisture values for some rating dates are listed and were high for 2004 as it was a very wet summer and fall. During dry conditions, we would expect these values to increase.
Table 8. Surface hardness (Gmax) and soil moisture content in 2004 of a natural turfgrass test area