Which is better: brushing or raking?

Opinions about whether brushing or raking third-generation synthetic turf fields is better for the field are divided. A grant from the Norwegian FA, the Dutch industry body BSNC, as well as the Dutch Ministry of Health, Welfare and Sport enabled the Municipality of Amsterdam to contract research institute ERCAT of the University of Ghent to investigate this in more detail.

Brushing or raking

Opinions about whether brushing or raking third-generation synthetic turf fields is better for the field are divided. A grant from the Norwegian FA, the Dutch industry body BSNC, as well as the Dutch Ministry of Health, Welfare and Sport enabled the Municipality of Amsterdam to contract research institute ERCAT of the University of Ghent to investigate this in more detail.

Despite the thousands of third-generation synthetic turf fields that have been installed around the world, there is still no consensus about what corrective action should be taken to keep infill loose and evenly spread over the field. Most maintenance manuals include the brushing of a field, but many municipalities with a significant number of these fields have opted to rake fields. “I regularly get students, who actually do it dutifully but don’t know what they’re doing and why they’re doing it,” says Gerrit de Koe of Amsterdam Municipality, referring to brushing. De Koe has followed the developments in the synthetic turf market from the very beginning and now offers courses to bring new people in the industry up to speed. “That is why I will ask them to work two or three lengths of a field in a specific way and judge the difference afterwards. Raking a compacted field once is often not enough. Especially with a 60 mm pile-height, you’ll have to do it twice or even three times to make a difference. However, once finished, the field will feel like it was installed yesterday.”

John van Gennip of installing company J&E Sports shares the experience but favours brushing. “I always put a lot of effort into brushing and I always emphasise that the person who will perform the maintenance should really focus on what is happening on the ground instead of looking up into the air.” Nevertheless, van Gennip is often asked to send out his crew again to do the maintenance. “It looks like a simple and undemanding task but there are a lot of details one has to pay attention to. Failing to do so will result in problems only getting worse.” Wijnand Hendriks of Henko Adhesives & Tools shares that opinion. “You have to become one with the field so that you understand all its characteristics,” says the man who could fill an entire afternoon just talking about the choice of brushes.

Important in view of ECHA

Back to the research. With the dossier on microplastic pollution from polymeric infill almost concluded, the European Chemical Agency (ECHA) has now shifted its focus to microplastic contamination from fibre wear. This triggered de Koe to investigate what brushing or raking does in terms of wear of the fibre. “We asked ERCAT to develop a maintenance device that drove autonomously over a field in a laboratory environment and where the pressure on the brushes and rakes was a constant 38 g/cm2.” The maintenance device was fitted with three different brushes, with the fibre thickness varying from 1.5 to 3 mm, while the rakes had teeth of 3 mm. The study focussed on a third-generation synthetic turf system with SBR infill as well as a system with cork infill. A so-called non-filled surface was also studied. “All test sections were first subjected to 100 cycles in the Lisport XL without abrasive plates before the maintenance robot was sent out to conduct 110 cycles of maintenance.” The surface as well as the thickness and width of the fibres were examined afterwards. “We got a very diverse picture, but light damage to the fibre was noticed.” De Koe attributes this to the brushes that rubbed over the fibres. “Fibres with notches or grooves actually became hairier under the influence of brushes. That’s because the thinner parts of the filaments are more susceptible to breakage or deformation. I believe that coarse brushes are too hard for these thin filaments and therefore advise against using a brush with fibres of 3 mm or more.” As for the rake, it is important that there are no burrs on the various pins. “Apart from those sharp edges, the rake had no effect on the fibre,” de Koe concluded.

In addition to the impact of the brush or rake, ERCAT also drew a conclusion about the infill. “Softer types of infill materials seem to cause slightly higher fibre wear compared to SBR, but the difference in this study was not significant,” he points out.

De Koe: “The research was done on brand new synthetic turf surfaces. I expect that if aging of the fibres due to, for example, the influence of UV, usage of the field or maintenance are also taken into account, the wear due to maintenance with mainly brushes will increase exponentially.”

Importance of brushing exaggerated?

Despite being recommended by virtually every installing company, de Koe believes that brushing is pointless in 80% of cases. “Brushing does not decompact the infill and is only effective when you replenish the infill. By raking a field, you actually pull the infill loose. In addition, raking offers the advantage that you can work in any desired direction, while, for brushing, you actually have to drive against the nap. The latter rarely happens because it is a more time-consuming procedure, as you have to drive over the width of a field. That will take more time than when you drive in the length of the field.” Working speed therefore still exceeds the importance of quality. “That is an additional reason why the City of Amsterdam has purchased extra-large rakes,” de Koe notes.

Van Gennip understands the logic but does not fully share de Koe’s opinion. “First of all, these are two different activities that you do for different reasons. If a field is brushed regularly enough, decompaction should not actually be necessary.” Van Gennip also questions the necessity of raking the so-called 45 mm systems. “The layer of performance infill in these systems is so thin that it can hardly compact.” He warns that, if one decides to still rake this type of field, to do so with care. “More is not always better. Particularly with a larger, heavier rake where you stand a chance that the pins are flush on the backing, you’ll run the risk of hitting a seam or line while raking, which can result in you pulling the entire carpet loose. That is why it is important that you always use a rake that can be adjusted in height and that you set the rake to the correct height.” According to van Gennip, the rake must be high enough to only touch the layer of performance infill and not pull through the layer with stabilisation sand.

More is not always better

More is not always better is also the reason why Wijnand Hendriks gets a stomach ache when he sees pictures of brushing units that consist of multiple brushes. “Working with a triangular brush is like pulling a ship through the water. As long as you use one, you’ll be fine. If you have to pull several through the field simultaneously, this requires more strength. Fibres don’t like that because you start forcing things,” says the director of Henko Adhesives & Tools. “The best maintenance activities are those that require as little energy as possible.” Hendriks does not dispute the results of de Koe’s research, but does point out that one should first ask oneself what exactly the goal is. “Do you want to keep the top layer free of leaves, mosses and branches or do you want to decompact and straighten the fibres? In what kind of environment is the field situated? Once you have that information, you must determine what kind of synthetic turf system you want to maintain: is it a field with SBR, or, for example, with cork infill? Only then can you decide how you want to approach things. In terms of brush design, you don’t want a brush whose fibres drag. Such a brush won’t collect anything. You want a brush with the fibres positioned in such a way that you can work between the stitches of the synthetic turf.” Before he designed his brushes, Hendriks carefully examined all aspects. “Using a high-speed camera, we looked at how a brush behaves. Only then did we start drawing.”

Van Gennip points out that the downside may be that if you use a brush with a setting that is too dense, the brush will actually fill up with granulate. “In that case, you have to stop every few metres to knock that granulate out of the brush.” The choice for the right fibres for the brush, therefore, is important. “It is because of the vibrations of those fibres that the dirt works its way up into the brush. But the number of tufts and the total weight of the brush also play a role,” says Hendriks. Apart from the specs of the brush, the result also depends very much on when the field is brushed. “Brushing a field with dew in the morning or brushing it when the impact of the sun is at its peak, also makes a big difference.” According to him, it is precisely the major maintenance with a rotating brush that kills the field and leads to microplastic pollution from fibre fragments. “We all know that, in the summer, the temperature on an artificial turf field can rise to around 70 degrees. To then go over a field with a rotating brush at high speed is asking for trouble.” That it does happen is mainly a matter of ignorance. “For most brush manufacturers, the artificial grass market is a small market. As a result, their material is not really specifically adjusted for use on artificial turf pitches.” Therefore, Hendriks claims that one should just accept that a brush for a synthetic turf pitch wears out within two years. “You want the brush to wear out, not the surface.”

Three experts with experience, three opinions on the question of which is better: brushing or raking. However, they all agreed on one thing: both the brushing and raking activities are not valued enough. That will have to change before the result will automatically improve.

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