Topdressing with Crumb Rubber from Used Tires on Athletic Fields and Other High Traffic Turf Areas
Dr. J.N. Rogers, III, and J.T. Vanini, Department of Crop and Soil Sciences, Michigan State University

Introduction
Over the last few years the focus on athletic field maintenance has intensified. Primary reasons for this occurring include an increase in recreational sport activity, the gradual shift from artificial turf to natural grass, and an increase in concern for player safety and liability. Most of this maintenance responsibility falls on the shoulders of athletic department administrators and turf managers who desperately need more information specific to their turf problems. The athletic field research program at Michigan State University has feverishly attempted to keep up with these shift in attitudes through various turfgrass studies. Most of the studies have been aimed toward increasing turfgrass wear tolerance through variety testing or physically protecting the soil from compaction and the plant from damage.

In 1991, 234 million tires were discarded in the United States, many of them in landfills. However, 25 out of 47 states have prohibited these tires from landfills, and 46 out of the 47 have legislated government funding to recycle the tires (1). This usually means the tire must be broken down into very small pieces and subsequent uses for these parts sought out. While the metal/steel in the tire are easily sold, finding a market for crumb rubber particles (1/4" and less) has been more challenging. One idea researched at Michigan State University since 1990, is the use of crumb rubber as a soil amendment in different turfgrass situations. The theory is that the crumb rubber particles introduced to the turf/soil system will increase turfgrass wear tolerance, reduce soil compaction, and subsequently reduce turf system inputs.

Original studies of incorporating crumb rubber into the soil profile have proven it to be an ideal soil amendment for high trafficked areas. However, it required taking an area out of play for three or four months, an often impractical task for an athletic field manager. Thus, the objective of this research was to provide an incorporation method which is less disruptive and easier than tilling crumb rubber into the soil profile. One method investigated was topdressing the crumb rubber into the turf/soil surface. Topdressing plays many roles in enhancing the turfgrass environment. Among these benefits include thatch modification, a smooth playing surface, modification of the soil surface, and winter protection (2). Putting greens and sportsfields profit from this maintenance practice, primarily because they are intense traffic areas with high quality standards, and the games played rely on the importance of a smooth and uniform surface. Specifically, soccer and football fields are subject to more abrasive action due to the nature of the games played on them. In most cases, a sand/organic matter mix or 100% sand is used to promote the forementioned qualities. However, the most intensively worn out areas, usually by mid-season, are past the point of repair in terms of turfgrass cover, and topdressing will not alleviate the problem. This can be detrimental for the playing field due to the intense traffic areas on the field becoming the most sparse areas or the least dense turf stand (Soccer and football fields are most vulnerable to wear in between the hashmarks and the goal mouth and mid-field portions, respectively). This effect is magnified on low to medium maintenance sports fields. With the absence of turf, the aesthetics and playing field quality are dramatically reduced which can ultimately lead to player injuries. Additionally, sand has more abrasive edges, leading to scarification of the crown tissue area particularly those areas under stress and poor environmental conditions for recovery. The abrasive action of the sand can also be detrimental to any high traffic turf area as well as those under reduced light, (e.g. shade) growing, and recuperative conditions (ie. cooler weather). The crown tissue area of the turfgrass plant is the source of regeneration of leaves, stems, and roots. While the importance of keeping it alive is obvious, just as important is to provide a favorable environment for this regeneration to take place. Once this area is thrashed and mangled, either by the sand particles or the play on the field, the plant could very easily die thus resulting in bare soil. Consequently, aesthetics and the playability are dramatically reduced, and the potential of surface-related injuries increases exponentially.

Thus the hypothesis was that topdressing crumb rubber, applied in the same manner as any other topdressing, can dramatically reduce the abrasive action caused by the nature of the athletic activity. With an increase in surface area and rounder edges, (in comparison to sand), the crumb rubber is able to cushion the crown tissue area while still providing a smooth and uniform surface, improve overall turf quality and reduce compaction. Inevitably, this improves the playability and aesthetics of the playing surface. Further, if areas that are reseeded or established on an annual or frequent basis are not worn substantially because of topdressing with crumb rubber, one can logically assume a reduction in inputs for turfgrass management and subsequent dollar savings.

Materials and Methods
A trial plot was established on an 80% sand : 20 % peat mix (Table 1) at the Hancock Turfgrass Research Center (HTRC) at Michigan State University, East Lansing, Michigan on 29 July 1993 to determine optimum topdressing rates for high trafficked areas, specifically targeting high school and collegiate athletic fields and playgrounds. To determine optimal particle sizes and application rates, crumb rubber was topdressed in a 2x5 randomized complete block design with three replications. There were two sizes of crumb rubber (10/20 mesh and 1/4" size) (Table 1) and five treatment amounts (0.0, 0.05, 0.10, 0.125, and 0.25 inch) added to the surface in equal applications on 29 July, 11 September, and 5 October 1993, and reached final levels at 0.0, 0.15, 0.30, 0.38, and 0.75 inch. Crumb rubber was not applied in 1994. Treatment areas were 10ft x 12ft. Crumb rubber was topdressed with a rotary spreader and then dragged in for as even distribution as possible on a perennial ryegrass (Lolium perenne var. Dandy, Target, and Delray) and Kentucky bluegrass (Poa pratensis var. Argyle, Rugby, and Midnight) turfgrass stand. (Because of the relatively small plot sizes, the rotary spreader was a suitable means for application. On larger areas, a belt-type topdresser works excellent.). On 16 May 1994 trafficked lanes were slit-seeded with Lolium perenne var. Dandy at 1.1 lbs./1000ft². The rubber particles eventually settle down to the soil surface. However, crumb rubber will not transgress through the soil profile because of being lighter or having a lower particle density: (rubber's particle density is 1.2 g/cc versus soil particle density at 2.65 g/cc). During the study, measurements were made as to the crumb rubber's effectiveness in reducing impact absorption (surface hardness measured with the Clegg Impact Tester) (3), reducing compaction (thereby providing a favorable environment for growth and recovery), improving turfgrass color, and sustaining turfgrass density.

In 1994, impact absorption was collected by the Clegg Impact Soil Tester (2.25kg hammer). Impact absorption values were recorded with the Br‘l and Kjaer 2515 Vibration Analyzer, replacing the read-out box. This analyzer allowed for further evaluation of surface hardness characteristics as described by Rogers and Waddington at The Pennsylvania State University (4). The values recorded was an average of four measurements. Shear resistance was measured with the Eijkelkamp Shearvane (5). The value recorded was an average of three measurements. Surface temperature was read by the Barnant 115 Thermocoupler Thermometer. Soil moisture recordings were provided by the gravimetric method (6). Three soil samples (7.6 cm) per treatment were used for this method. Density and color ratings were observed on 27 October and 4 December.

In 1993, wear treatments were initiated on 26 August and ended 14 November and in 1994, wear treatments were initiated on 6 September and ended 15 November, for a total of 48 games simulated each year. Wear treatments were applied by the Brinkman Traffic Simulator (BTS) (7). Two passes by the BTS is equivalent to the traffic experienced in one football game between the hashmarks between the forty yardlines (8). The field tests conducted were to originally assess athletic field conditions. However, we quickly realized the results observed translated to a wide range of highly trafficked and compacted areas commonly seen on campuses, from intramural fields, game and practice fields to areas where a Saturday afternoon tailgate party takes place.

Results and Discussion
While our data were collected throughout the 1993 and 1994 seasons, due to spatial costraints, our purpose is to only highlight the trends we observed in 1994, and results from one data collection date will be presented. Whenever a test regarding turfgrass wear and soil compaction is conducted, chief among the components to be evaluated following the wear is turfgrass density and color. These are important to the field manager as they are often indicators of good playing conditions. Turfgrass color and density ratings provided substantial evidence that turfgrass conditions have been maintained despite intense traffic (Table 2). These findings are consistent throughout the 1994 season and were attributed to the crumb rubber particles protecting the crown tissue area of the plant (Values shown are after 40 games simulated with the BTS). During the 1993 season the density ratings were dependent upon the amount of rubber used as well as the size of the rubber (data not included). We had higher turfgrass densities where we used the smaller rubber size (10/20 mesh) and high rates. We believe this was because the smaller particles were able to work to the surface faster, in 1993 during the fall, thus protecting the plant. When this relationship was not evident in 1994 it was logical to conclude the larger particles had also worked to the surface during the winter and were also now providing the protection to the turfgrass plant. This is significant because the smaller particle sizes are more expensive, an important factor in all decision making processes.

While there were no turfgrass color responses in 1994 regardless of crumb rubber size or rate, we saw a increase in color immediately after putting the rubber down in 1993. This response was positively correlated to crumb rubber rate. We still do not have the exact reason for this response as we saw no increases in color in 1994.

The amount of crumb rubber used as a topdressing played an important role in affecting surface characteristics. Impact absorption values (Gmax) were significantly lower at high crumb rubber rates in 1993. While this phenomenon did not continue in 1994, the surface characteristics, duration of impact (Tt), time to peak (Tp), and rebound ratio (rr%) showed the effectiveness of crumb rubber (0.75") in providing a softer, more resilient surface (Table 3). Tp, Tt, and rr% values increased at the high rates of crumb rubber. These characteristics are important parameters as they further define critical elements of surface hardness, such as, duration and severity of impact as described by Rogers and Waddington at the Pennsylvania State University (3). When an object is in contact with a surface the longer the time of impact the more resilient that surface and more likely the surface will resist compaction. Crumb rubber particle size was not significant in regards to these surface hardness characteristics.

In 1993, shear values decreased significantly as crumb rubber levels increased. In 1994, as crumb rubber levels increased, shear values increased significantly and stabilized (Table 2). However, differences between particle sizes were not significant. To help explain this scenario, crumb rubber was topdressed in 1993 but not 1994. In 1993, the crumb rubber had not settled down to the crown tissue area, when the shear vane apparatus was applied to take a measurement, the teeth or fins could not grip the surface as well. One possible correlation to this is when a player digs his/her cleat into the surface, and it slips out from underneath. However in 1994, after a growing season and the crumb rubber settled to the soil surface and stabilized, shear values increased significantly as crumb rubber levels increased. This settling process, in part, also explains the lack of significant differences in impact absorption values in 1994 as compared to 1993.

In 1993, surface temperatures were significantly higher as crumb rubber levels increased. Although data was not significant on 10 November, the effect of crumb rubber on surface temperatures was significant due to the relationship between turfgrass growth and soil temperatures. As surface temperatures drop below 50 F the growth and recovery of turfgrass slows. Keeping temperatures higher can lead to increased playing quality conditions. This also holds true in the spring time as well, a factor very important to golf courses in the northern United States. For instance, on 7 April (data unpublished), there was a 7.5oF from the check treatment to the highest crumb rubber treatment. The exposure of crumb rubber at the surface heats the turf surface and revitalizes dormant turfgrass. This translates to a quicker spring green-up, an important factor for any field used in early spring. One concern we had was the effect of the crumb rubber on turfgrass during the summer. As the density of the turf stand increases during the growing season, the effect of crumb rubber on surface temperatures moderates due to the shading effect of the turfgrass, an effect measured and confirmed during 1993 and 1994.

Although crumb rubber is an excellent tool, it is not a "cure-all". Therefore the use of crumb rubber cannot be an exclusive means for maintaining turf in any high traffic turfgrass area, and must be used as a tool integrated into the management program. It should also be noted that we recommend the field manager must have a 100% turfgrass stand, or as close to this as possible, before making any crumb rubber applications. Therefore, our research does conclude topdressing between 0.375 and 0.75 inch (not more than 0.25 inch at any given application) would be a good level to achieve for high traffic areas. Thus, the "Take Home Message" is that crumb rubber will not resurrect the turfgrass, but it will protect the crown tissue area of the plant which becomes vital in improving the longevity and quality of a high traffic turfgrass stand. This is shown in the magnified photographs of sand and crumb rubber particles (Figures 1-4). The sand particles have sharp edges and will tear turfgrass tissue. Crumb rubber has more rounded edges and subsequently there is less abrasive action to the turf plant.

While the research to date has been extremely promising, we have not covered every scenario in the turfgrass industry. First, we do have confidence that the crumb rubber topdressed at 0.50 to 0.75 inch levels (1200 to 1800 lbs/1000 ft²) will increase turfgrass wear tolerance and prevent soil compaction in turfgrass maintained above 0.63 inch, specifically at 0.25 inch (putting green collar) height. Our results have been consistent with earlier findings. Application rates are obviously smaller (rate were equal to 0.25 inch of rubber) and care should be exercised to not smother the plant. We still do not have any data for cool season grass putting greens. We have done little testing at cutting heights below 0.63 inch, and while we remain optimistic that crumb rubber would provide similar findings, obviously there will be some limits. Second, except for early in 1993, we have seen little differences in response from different crumb rubber sizes in our studies. However, we have noted the smaller sizes are easier to work with in terms of working into the turf area. It comes as no surprise this is a more expensive product, and we also caution against using a too finely granulated product as this could cause a detrimental effect in the soil profile relationship. A cost breakdown of 1000ft² for re-establishing an area versus topdressing crumb rubber can provide an idea of the necessary steps involved to continually re-establish an area. (Table 4). This is only an estimation of the costs however it does provide an idea of the dollar savings that can be attained over an extended period of time.

Another point not researched to date is the effect of crumb rubber on warm season grasses. We have no reservations about it protecting the turf plant but there is some question as to overseeding cool season grasses in the fall. Informal research in California showed little adverse effects to seedlings and actually may have aided establishment in some instances. Other reports with warm season grasses have shown the rubber may help prevent winter damage. The final area is the question of contamination from crumb rubber particles in terms of soil and water quality. We have had crumb rubber tilled in the ground at MSU since 1990 and monitor soil samples annually. The major constituents of rubber are iron, sulfur, and zinc. While iron and zinc levels have increased in our tests, none have approached levels of concern nor do these elements pose concerns to the water quality. At no time have we seen any toxicity to the turfgrass plant during our studies.

We are confident, we have found another use for a difficult to reuse product that poses environmental hazards and takes up landfill space. When topdressed, crumb rubber can extend turfgrass wear tolerance and reduce soil compaction in high traffic areas. These high traffic areas exist on every athletic field as well as walk paths, golf courses, and main event venues on campus. The more this product is researched and tested the more uses will be found. Michigan State University was issued a patent (#5,622,002, "Method for reducing abrasion of turfgrass on activity fields") on this use of crumb rubber in April 19997, and has sold the use rights to Jai-Tire Industries, Denver, CO (800-795-TIRE). Royalties paid to Michigan State University will go toward turfgrass research. A deep appreciation to the Michigan Turfgrass Foundation goes out for funding the assistantship to research this project.

**Note** All particle size figures are averaged over three samples.

1 The sieve analysis of the sand used for the modified rootzone for the Crumb Rubber Topdressing Study at the Hancock Turfgrass Research Center.
 

Table 2. Effects of crumb rubber size and topdressing rates on color and density ratings on a Kentucky bluegrass/perennial ryegrass stand under trafficked conditions at the Hancock Turfgrass Research Center, Michigan State University, East Lansing, MI. 1994.
	Color Ratings		Density Ratings
Crumb Rubber Particle Size	27 Oct	4 Dec	27 Oct	4 Dec
1/4"	5.9	4.6	67.7	60.0
10/20 mesh	5.9	4.4	72.1	64.0
Significance+	-NS-	-NS-	-NS-	-NS-
Crumb Rubber Topdressing Depth
0.00"	5.7	4.7	53.3	41.7
0.15"	6.0	4.7	61.7	50.8
0.30"	5.9	4.6	71.7	63.3
0.38"	5.7	4.6	73.7	65.8
0.75"	6.2	4.1	89.5	88.3
Lsd (0.05)	-NS-	-NS-	11.0	14.3

**Note** Scale for Color Ratings: 1-9; 1-Brown, 9-Best, 6-Acceptable

+ indicates a significant difference at the 0.05 level.
 

Table 3. Effects of crumb rubber size and topdressing rates on a variety of field measurement values measured on a Kentucky bluegrass/perennial ryegrass stand after 46 football games simulated at the Hancock Turfgrass Research Center, Michigan State University, East Lansing, MI, on 10 November 1994.
Crumb Rubber Particle Size	Impact Absorpt. (Gmax)	Time of Duration (Tt)	Time to Peak (Tp)	Rebound Ratio	Shear Resist. (Nm)	Soil Moisture (%)	Surface Temp. (F)
1/4"	60	10.3	5.7	0.216	14.2	16.3	47.5
10/20 mesh	62	10.2	5.8	0.236	14.7	16.6	47.8
Significance+	-NS-	-NS-	-NS-	-NS-	-NS-	-NS-	-NS-
Crumb Rubber Topdressing Depth
0.00"	58	10.1	5.6	0.168	11.9	16.2	47.6
0.15"	60	9.7	6.1	0.181	15.3	16.5	47.6
0.30"	62	9.9	5.5	0.210	13.7	16.3	47.6
0.38"	61	10.5	5.7	0.257	16.0	16.8	47.7
0.75"	72	11.1	5.8	0.314	15.4	16.4	47.8
Lcd (0.05)	-NS-	1.0	0.4	0.03	2.1	-NS-	-NS-

Table 4. Estimated costs to re-establish a 1000 ft2 area annually per year versus topdressing with crumb rubber at 0.75 in. For a five year period.
Items	Re-establishing	Topdressing Crumb Rubber
Seed, Fertilizer, Labor, Aeration, Watering, and Seedbed preparation for one year	$400	$400
Topdressing crumb rubber at 0.75" (1800 lb. X $0.15/lb.)	$0	$270
Re-establishing annually for four years at $400/year	$1600	$0
Miscellaneous costs	$200	$200
Total	$2200*	$870*

* Prices are an estimation of costs and do not include equipment or inflation.

References

1. Riggle, D. 1994. Finding markets for scrap tires. Biocycle. March p.41.
2. Beard, J.B. 1973. Turfgrass: science and culture. Prentice-Hall, Englewood, NJ.
3. Rogers, John N. III, and D.V. Waddington. 1988. Portable apparatus for assessing impact characteristics of athletic field surfaces. Journal Paper NO. 8017, Pennsylvania State University, College of Agriculture, Agricultural Experiment Station, University Park, PA, p. 96-110.
4. Rogers, John N. III, and D.V. Waddington. 1992. Impact absorption characteristics on turf and soil surfaces. Agronomy Journal. 84:203-209.
5. van Wijk, A.L.M. 1980. A soil technological study on effectuating and maintaining adequate playing conditions of grass sports fields. Agric. Res. Rep. 903. Centre for Agri. Publishing and Documentation. Wageningen, Netherlands 124pp.
6. Gardner, W.H. 1965. Water Content: Gravimetry with oven drying. Methods of Soil Analysis. American Society of Agronomy. 1:92-96.
7. Cockerham, Stephen T., and D.J. Brinkman. 1989. A simulator for cleated shoe sports traffic on turfgrass research plots. California Turfgrass Culture. 39:(3&4) p.9-10.
8. Cockerham, Stephen T. 1989. Cleated-shoe traffic concentrated on a football field. California Turfgrass Culture. 39:(3&4) p.11-12.

For more information about the Crumb Rubber project at Michigan State University, contact Dr. J.N. Rogers.

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