Pros and cons of cork as an infill for artificial turf pitches

published in sb 2/2017

Granulated cork piloted in Hamburg, Germany

 

Authors: Torge Hauschild, landscape architect for district sports facility construction at Hamburg-Mitte district authority and Oliver Schneider, Labor Lehmacher | Schneider GmbH & Co. KG
Photos: Torge Hauschild (Figures 1 and 8 to 11) and Labor Lehmacher | Schneider (Figures 2 to 7)

The surfacing of sports facilities and sports grounds in Hamburg with artificial turf has been underway on a relatively large scale since 2006. Defined as the standard by the district authority, the “2nd-generation” (or 2G) turf construction method has been the subject of heated debate among users and manufacturers. Infills of granulated plastic are no longer currently in use; instead, the granulated cork on the market and under discussion since about 2007 is now undergoing exhaustive testing. The author explains the background to this trend and reports on initial experience with granulated cork.

The story so far

Some of the construction projects implemented from 2007 to 2009 utilised “3rd-generation” (or 3G) artificial turf surfaces, i.e. partially infilled with rubber and sand. These involved artificial turf surfaces from different manufacturers and of different systems in terms of gauge, fibre count, fibre length and fibre properties.

The infill used on these grounds also varied, with both EPDM and TPE being employed. As a recycled product with the associated stressing and the familiar drawbacks in terms of colouration and odour, SBR granulate was the only one to be rejected from the outset. All the same, a number of clubs used the material in their own projects.

Maintenance- and material-related damage, the scope for maintenance and the basic economic framework called for a review of construction methods in 2011. This resulted in the definition of a standard valid for Hamburg which is based on an artificial turf with textured fibres and an infill consisting purely of quartz sand, i.e. the 2G construction method.

Although choosing an established and proven product may seem “uninspiring” to clubs, users and particularly manufacturers and meet with criticism from them, this decision by the district authority in Hamburg has been vindicated. The district authority set itself the goal of implementing construction projects cost-consciously and efficiently, providing a reliable sports offering all-year round and maintaining it in the long term with the available means of maintenance and care.
At present, this construction method is still the method of choice, although new products are being tested, as regards not only infills, but also fibre systems such as combined fibres from different manufacturers. The approach of employing fibres and granulates of identical materials (PE) is also being pursued with interest.

By 2016, the City of Hamburg had equipped some 70 large pitches with artificial turf. The first turf renewal proj­ects are now in preparation.

Relevant factors

In its decision, Hamburg-Mitte district authority complies with its commitment to treat all users and clubs equally and offer them if possible identical usage and operating conditions. For the specification of an appropriate construction standard, a large variety of factors were investigated to prevent wasted investment and unwanted consequences:

Functional suitability: A long, smooth and soft fibre with an infill of quartz sand combined with a suitable granulate infill still represents the best replication of natural turf and thus offers optimum playing comfort.

Maintenance needs: Systematic maintenance is an indispensable precondition for operating an artificial turf pitch and keeping it functional. It is therefore a point of criticism that the finance provided in precisely this sensitive area only permits the minimum amount of maintenance. This trend stretching back several years is unlikely to be reversed. The choice of construction method is capable of diminishing the risk of a significant reduction in the useful life of the artificial turf system.

Cost-effectiveness: By its very nature, a high-end artificial turf calls for higher investment than a 2G artificial turf. The infills (EPDM, TPE or cork) are more cost-intensive in terms of initial investment and refills, and maintenance is more elaborate (if it can even be afforded) and hence more expensive. In addition, the green wastes of the adjoining surfaces, enriched with migrated granulated plastic, are more expensive to dispose of.

Environmental compatibility: Synthetic artificial turf obviously has its drawbacks in terms of environmental compatibility. The recycling of artificial turf is basically uneconomic at present, as it consists of a mix of different plastics. As far as the infill is concerned, it can be said that quartz sand as a natural product is safe and is available without a “use-by date” date. This also applies particularly with regard to the migration of material into sewers, onto adjoining surfaces and into containers of green wastes. The situation for granulated plastics as synthetic infills is entirely different.

Material durability: In terms of durability and susceptibility of the various artificial turf systems, Hamburg first had to gather experience. Fibres faded, became detached or broke; granulated plastic tended to clump. The most obvious choice must be in favour of an artificial turf system whose maintenance is affordable and that has proven itself and established itself on the basis of experience.

Multifunctionality: In a comparatively densely populated city state, artificial turf has to be versatile. It should always be compatible with football, hockey and other sports and must also withstand leisure activities and school playground usage with as little damage as possible.

Is cork infill the alternative? (Figure 2)

n the efforts to regularly update and optimise the stan­dard, systems and products are tested that embody promising improvements in terms of the factors listed above. This applies to artificial turf as such as well as to the infill. On the last-mentioned count, granulated cork may well even be ushering in a change of policy, as the material displays a number of fundamental advantages.
As a product of nature, granulated cork is a renewable raw material derived from the bark of the cork oak (Quercus suber) from sustainably managed sources. In addition to its classical use for bottle corks, cork’s special properties make it suitable for use in many areas – even in space travel (heat shields). It is also known as an insulating material, which is attributable to the raw material’s low density.

Since about 2007, granulated cork has been put to use to a growing extent as an infill in artificial turf systems. For infill production, leftover material from bottle cork production is granulated and usually heat-treated.

Cork’s use as an infill is favoured by its many positive properties:

  • Low density: insulating properties with low heat absorption when exposed to sunlight
  • High elasticity because of its large share of air-filled cell structure
  • High strength and low wear
  • High fire resistance; slow to react to a large number of acids and other chemical substances
  • Sustainable product with an attractive life-cycle assessment: sustainable production and straightforward processing mitigate the impact of relatively long transportation distances
  • No environmental toxicity: cork contains and binds pollutants owing to its natural protective function for the cork oak. In terms of current limit values, the pollutant content is admissible and can be considered safe.
  • Susceptibility to mould so far not demonstrated
  • Largely odour-neutral

At the same time, there are still a number of unresolved issues concerning granulated cork as the material has not yet been standardised or certified and, in addition, it has not yet proven itself definitively on the market and in practice. Taking into account the matter of cost-effectiveness, these issues still have to be critically assessed.

Cork pilot project

Having firmly established itself in the technical discussion, cork was taken up by the specialist authority as the subject of a pilot project. The associated theoretical appraisal and discussion set out to investigate whether using granulated cork would call for a type of artificial turf deviating from the defined standard. It was suspected that the dense structure of the favoured artificial turf system coupled with the low effective pile height of the curly fibres could make it much more difficult to introduce the coarser cork granulate infill into the artificial turf. This suspicion was also initially echoed by most artificial turf manufacturers.

Finally, a field test on a small pitch in spring 2014 decided whether the idea could be pursued further at all. The test showed that the introduction of the granulated cork was more time-consuming but certainly possible. As a consequence, four more large pitches in need of modernisation were selected in 2014 for a farther-­reaching test of granulated cork.

Because of the lack of standards and test procedures and owing to the obligation to observe product neutrality in invitations for tenders, the granulated cork was defined primarily by means of its grain size and weight per cubic metre. To put the findings from the pilot project on a broader footing, greater variation was admitted for the standard of the adopted artificial turf systems.
The following turf systems were adopted:

  • Two relatively dense systems (3/8” gauge, textured fibres, 35-38 mm pile height, approx. 220,000 fibres/m²) (Figure 3)

  • A relatively open system (3/8” gauge, textured fibres, 38 mm pile height, approx. 130,000 fibres/m²)  (Figure 4)

  • A relatively open system of combined fibres (curly fibres with monotapes, 45 mm pile height, approx. 120,000 fibres/m²) (Figure 5)

To ensure objective results, the pitches have been monitored by experts Lehmacher-Schneider since going into operation in 2014/15 who have surveyed the maintenance and operating data.

A sensible scale of testing was based on the technical standard DIN EN 15330-1 and the FIFA Handbook of Test Methods. The tests can be subdivided into three basic areas.

The protective function of the artificial turf system, i.e. the effects of the system on athletes (on the joints and ligaments, for example), is investigated by measuring force reduction, energy return and vertical deformation (Figure 6), and rotational resistance (Figure 7).

 

The sports function of the artificial turf system, i.e. the special requirements to be met by the system to ensure its suitability for sports activities, is investigated with tests of ball roll and bounce.

Wear, material durability and thus cost-effectiveness are investigated by measuring infill height and by establishing the infill grading curve.

Other factors arise additionally during ongoing pitch use, above all due to the weather. The unusually large number of heavy rainfall events in Hamburg in summer 2016 revealed the “floating” phenomenon.

When more rain falls on artificial turf than can run off or seep away, water accumulates. If the accumulating water exceeds the pile height of the artificial turf fibres, the relatively light granulated cork is lifted by its own buoyancy out of the composite fibres (even textured fibres) to the surface where water currents can cause it to migrate significantly. Even if it does not migrate, the granulate remains after water drainage on top of the artificial turf and has to be worked back in again. Loss of material and extra work are the consequences, and the economic impact of this must not be underestimated
(Figures 8 to 11).

Provisional conclusion

On the basis of the findings, it can be concluded at present that granulated cork represents a genuine alternative to granulated plastics in terms of protective and sports functions.

Granulated cork is expected to be show higher wear than of granulated plastics, although the difference after initial field testing is not as high as in the laboratory.

In addition, the quality of granulated cork can vary. It is extremely important for quality standards and requirements to be set and test methods defined. The lack of standards and certification will always be an argument against choosing granulated cork. Its susceptibility to the weather is a further drawback and the economic effects need to be investigated further.

On the face of it, dense and high-pile artificial turf systems are less likely to lose material, be it through usage or the weather. In the third year of pitch operation with these systems, the need for refill is low, while the two grounds with more open systems are already showing acute refill needs.