Playground safety mat – 90 cm FS "Grass green"

Apparent density is the ratio of mass to total volume, including all pores and voids. It differs from true density, which refers only to the solid substance itself and excludes voids. For comparison, water at 4 °C has a density of 1,000 kg/m³. Recycled tyre rubber granules (ELT), consisting mainly of SBR and natural rubber, have a true density of around 1,200 kg/m³. EPDM rubber granules have a true density of around 1,600 kg/m³.

For a moulded component made from rubber granules bonded with polyurethane, apparent density is calculated from its mass and external dimensions: length, width and overall height. This volume includes two types of void. The first consists of pores between the granules that remain after pressing during manufacture. Their proportion depends on moulding pressure, granule size, granule shape and the proportion of binder. As a simplified example, a uniformly compacted tile with a flat underside and an apparent density of 800 kg/m³ has a pore volume of around one third, assuming a granule true density of 1,200 kg/m³ and disregarding the binder proportion.

The second type consists of moulded voids created by a structured underside. Drainage channels, studs, drainage feet or pedestal feet leave open spaces beneath the wearing layer. These spaces form part of the overall height but contain no rubber granules. A moulded component with a pronounced underside profile can therefore have a low apparent density even where its wearing layer is densely pressed. Apparent density describes the complete moulded component, not the constituent substance alone. At WARCO, apparent density ranges from around 600 to 1,250 kg/m³.

The compaction of the wearing layer shapes the properties of the floor covering.

• Surface: the higher the density, the less open and more resistant the surface becomes.

• Water permeability: lower density means more open pores and, in most cases, greater water permeability.

• Elasticity: lower density makes the floor covering more yielding and elastic, while higher density makes it harder and stiffer.

• Weight: together with overall height, apparent density determines dry weight per square metre. When wet, water held in the pores adds to this weight. This matters for transport, installation and the load-bearing capacity of balconies, terraces and roof areas.

The scale runs from Rating 1 to Rating 5. Each rating represents a density range, for example Rating 2 for 780 to 840 kg/m³. For moulded components with a structured underside, the rating says less about the wearing layer than it does for a tile with a flat underside.

Rubber is an elastomer. Under load, it deforms and, once the load is removed, its cross-linked molecular chains draw it back largely to its original shape. In floor coverings made from rubber granules bonded with polyurethane, the porous structure adds a further mechanism. Pores remain between the rubber granules, while the elastic binder holds them together. Under load, it is mainly these pores that close, while the granules and binder deform at the same time. As deformation increases, the floor covering generally becomes stiffer and its resistance to further deformation increases. Elasticity and internal friction determine how it absorbs impacts, vibrations and footstep sound.

Shock absorption: when a body strikes the floor covering, its kinetic energy is absorbed over the deformation distance. One part is stored elastically and returned as rebound, while another is converted into heat through internal friction. The longer the deceleration distance, the lower the forces acting on the body and substrate. Under light loads, the floor covering yields only slightly. Only severe impacts make substantial use of the available deflection. A greater thickness of floor covering extends this distance. For safety tiles, the effect is assessed as critical fall height in accordance with BS EN 1177.

Vibration damping: rubber is viscoelastic, combining spring-like and viscous behaviour. Under cyclic loading from weights being set down, equipment in operation or machinery, deformation follows the applied force with a delay. The work done in this process is dissipated as heat through internal friction. The floor covering therefore acts as both a spring and a damper. Above the natural frequency of the system formed by the mass and floor covering, it decouples the vibration source from the substrate and reduces transmitted vibrations and shocks. The amount transmitted depends on the excitation frequency, the moving mass and the dynamic stiffness of the floor covering.

Footstep sound reduction: footstep sound is structure-borne sound. Walking, jumping or setting down objects introduces an impact into the substrate. It travels through the floor or ceiling structure and walls and may be heard as noise in other rooms. An elastic floor covering introduces a compliant layer into this transmission path. The peak force is spread over a longer period and is therefore lower, while part of the energy is converted into heat within the floor covering. The amount of structure-borne sound entering the building depends on the dynamic stiffness and thickness of the floor covering, as well as on the substrate.

The damping provided by a rubber granule tile depends on its density and pore content, the granule size and shape, the proportion of polyurethane binder and its thickness. As a rule, a dense, fine-grained tile deforms less and provides lower damping, whereas a porous, coarser or thicker tile provides greater damping. The scale ranges from Rating 1, noticeable damping, to Rating 5, very high damping, and is based on comparative measurements of WARCO products. Whether a product is suitable for a particular application is best assessed on site using a sample tile.

Whether a shoe gains traction on a floor covering depends on friction between the sole and the surface. Static friction acts when starting to move and when standing, while dynamic friction acts during sliding. The dynamic coefficient of friction is the force parallel to the surface needed to keep a body moving, relative to the force pressing it onto that surface. With rubber, friction has two components. The adhesion component results from close contact between the elastic surface of the floor covering and the sole. The deformation component arises because the rubber deforms beneath the sliding body and absorbs energy through internal friction. The contribution of each component depends on the roughness of both surfaces, rubber stiffness, applied load, sliding speed and temperature. An elastic surface generally conforms to the sole more closely than a rigid one, creating a larger actual contact area.

BS EN 14041 defines class DS for resilient floor coverings. Under dry, clean conditions, the dynamic coefficient of friction must be at least 0.30. It is determined in accordance with BS EN 13893 using a standardised slider drawn across the floor covering at a specified load and speed. The force required is recorded. The result is a comparative value obtained under consistent conditions, rather than a measurement for a particular shoe sole, and it describes the dry state. Moisture, dust or a film of dirt can form a separating layer between sole and surface, reducing the adhesion component in particular.

The scale runs from 1 to 5. Rating 1 corresponds to a dynamic coefficient of friction of 0.30, which is the minimum requirement for class DS. Rating 5 corresponds to 0.60, twice that value. The ratings are based on tests of representative tiles, with intermediate results interpolated.

For rubber granules, density, granule size and granule type determine the surface texture. In WARCO products, higher pressure during pressing produces a denser, smoother surface and therefore lower dynamic friction. Coarse or fibrous granules and a more porous structure create a rougher surface, and the slider then passes over the individual granules. As the rating increases, the individual rubber granules are more readily felt.

New floor coverings often require a running-in period before reaching their final dynamic friction level. Residues of mould-release agents from manufacture are removed through use and cleaning. Dynamic friction also depends on how the covering is maintained. Dirt and residues reduce it, while regular cleaning helps maintain it.

Abrasion resistance describes how effectively a floor covering resists mass loss caused by rubbing and abrasive action. Typical sources include shoe soles, wheels, and sand or dirt moving across the surface under load. Rubber wears differently from a rigid solid. As the surface yields elastically beneath a sliding body, it is locally stretched and sheared before being relieved again. These repeatedly changing stresses fatigue the rubber over time. Fine cracks may develop and small particles can detach from the surface. In floor coverings made from rubber granules bonded with polyurethane, the bond between the granules is also relevant. The binder must retain each granule so that frictional loading does not pull whole granules from the surface. Abrasion resistance is therefore a property of the complete system, including the rubber, granule size and shape, binder content and curing, adhesion between granules and binder, and compaction, rather than of the rubber alone.

Testing to BS 7188 uses a Taber Abraser. Two standardised abrasive wheels pass over the test specimen at a defined contact force and rotational speed for a specified number of revolutions. The specimen is weighed before and after each test interval, and the resulting mass loss provides the measure of abrasion. The abrasion index and abrasion ratio are calculated from the test intervals specified in the standard. This indicates not only how much mass is lost during the first interval, but also whether abrasion decreases, remains constant or increases as loading continues.

The scale runs from Rating 1 to Rating 5. Rating 1 means that the requirements of BS 7188 are met. Ratings 2 to 4 denote good, high and very high abrasion resistance. Rating 5 is the highest rating on the scale. Classification is based on tests of representative products, with results interpolated between the tested values. In use, a high rating is reflected in a surface that retains its granule texture and visual appearance for a long period under foot traffic, rolling loads and the ingress of sand.

Water permeability describes the quantity of water that enters a porous floor covering and passes through it. In floor coverings made from rubber granules bonded with polyurethane, water does not pass through the rubber itself, which is impermeable. It moves through the open, interconnected pores between the granules under gravity. The flow rate depends on the proportion of open pores, their size and whether they form continuous paths. Coarse granules and lower moulding pressure during manufacture create larger open channels. Fine granules and higher moulding pressure reduce the pore spaces and can result in a practically impermeable tile.

Water permeability is measured in accordance with BS EN 12616 as an infiltration rate. This is the quantity of water passing through the floor covering within one hour under defined test conditions, expressed in mm/h. A rate of 1 mm/h corresponds to 1 litre per square metre per hour. Even a heavy rain shower supplies only a fraction of the water volume that a floor covering with a high rating can pass through over the same period.

The scale ranges from Rating 1 to Rating 5. Rating 1 represents an infiltration rate of approximately 0 mm/h, so the floor covering is practically impermeable to water. Rating 5 represents approximately 1,000 mm/h, equivalent to around 1,000 litres per square metre per hour. Classification is based on tests of representative tiles, with the results interpolated for the relevant product.

The tile is only one part of the construction. At Ratings 3 to 5, rainwater can pass through the tiled covering provided that the laying course and sub-base can also receive and drain it. On an impermeable substrate, water accumulates beneath the tiles. The area then requires a slope leading the water towards drainage, for example a drainage channel or outlet. Floor coverings with Ratings 1 and 2 are intended only for indoor and covered areas.

Water permeability can decrease over time if pores become blocked by tracked-in sand, leaves and fine dust. In stable mats, inadequate cleaning can also block the pores. Cleaning with water helps keep the pore structure open.

Slip resistance describes how reliably a floor covering can be walked on in footwear, including when wet or soiled. It is governed by the friction between the sole and the surface. During each step, the sole contacts the surface at an angle and then pushes off. The foot remains in position only when friction can absorb the resulting shear force. A film of liquid between sole and surface reduces friction because it interrupts close contact between them. A rough, porous surface counteracts this effect. Surface roughness displaces the liquid film at the contact points, while liquid can move into open pores.

BS EN 16165 specifies a harmonised European method for determining the slip resistance of floor coverings under shod and barefoot conditions. In Germany, it replaced DIN 51130, while the familiar R9 to R13 classifications remain in use. The ramp method uses a floor covering wetted with oil as the standard test liquid. A trained test operator walks across the surface while the inclination is increased in stages. The angle at which the operator still just accepts the surface as walkable is the acceptance angle. It indicates friction on the wetted surface. On an incline, static friction must at least balance the downslope force, which increases with the angle. The R10 classification covers mean acceptance angles above 10° and up to and including 19°.

WARCO floor coverings are generally assigned the R10 classification. Within this classification, a scale from 1 to 5 describes the mean acceptance angle. Rating 1 corresponds to approximately 12°, while Rating 5 corresponds to approximately 17°. The classification is based on tests carried out on representative sample tiles under DIN 51130 and interpolated for the relevant product. Slip resistance complements the dynamic friction of class DS under BS EN 14041, but the two results are not directly comparable. DS is measured dry with a standard slider, whereas the R classification is determined with oil and a walking person.

The density, granule size and structure of the rubber granules determine the surface. Greater moulding pressure during manufacture produces a smoother surface, while coarser granules create a rougher and more porous structure. New floor coverings often require a running-in period because residues of release agent from manufacture are removed only through use and cleaning. Regular cleaning maintains slip resistance. Dirt and deposits reduce it.

Heat is transferred by three mechanisms: conduction through a material, convection in air or water, and radiation. For a floor covering, conduction is the principal mechanism. It is described by thermal conductivity, expressed in W/(m·K). This value defines the quantity of heat passing each second through one square metre of a material one metre thick when the temperature difference between its two sides is one kelvin. The lower the value, the less readily the material conducts heat and the greater its insulating effect.

Solid rubber has a thermal conductivity of approximately 0.13 to 0.18 W/(m·K). Still air, at approximately 0.026 W/(m·K), conducts heat considerably less readily. Tiles made from rubber granules bonded with polyurethane contain pores that retain air and therefore conduct less heat than solid rubber. In general, lower density and a greater pore volume result in lower thermal conductivity. Moisture in the pores increases it again. Depending on density and structure, WARCO values range from approximately 0.07 to 0.14 W/(m·K).

Overall insulation depends not only on thermal conductivity but also on thickness. Thermal resistance equals thickness divided by thermal conductivity, so a tile with twice the thickness provides twice the thermal resistance. A structured underside with drainage channels or studs can also retain a layer of air beneath the tile, which may further reduce heat flow towards the substrate. This applies while the air remains still and dry. The effect is lower at contact points and in channels through which air can circulate.

The scale ranges from 1 to 5. Rating 1 corresponds to approximately 0.14 W/(m·K), while Rating 5 corresponds to approximately 0.07 W/(m·K). The values are based on measurements of representative samples and interpolated for the relevant product. Insulating materials such as extruded polystyrene (XPS) or mineral wool achieve values of approximately 0.03 W/(m·K).

Tolerates frost and freezing pore water without bursting, cracking or breaking.