CHAPTER 8
8.1
Introduction:
The road user desires a road surface where he can drive safe and comfortable. This requires a pavement structure with enough stiffness, a fast run-off from the rainwater, an even road surface, a good reflection of (artificial) light at the road surface and a limited production of noise in the contact area between the vehicle tyre and the road surface. These properties should preferably be present during a long period of time, e.g. during the life of the pavement structure. In the next paragraphs the desired road surface characteristics are discussed in somewhat more detail.
8.2
Skidding resistance:
A vehicle is able to drive on a road because of the friction in the contact area between the tyres and the road surface. The skidding resistance is defined as the friction coefficient measured according to a standardised method.
The friction force between the rubber tyre and the road surface may be composed of 3 components, which are:
1. The adhesion or stick component that occurs through molecular attraction between the tread of the tyre and the road surface. On a dry road surface this component is the most important one. This component decreases with increasing texture of the road surface.
2. The hysteresis or deformation component that occurs through deformations of the tyre. This component increases with the texture of the road surface and it is the most important component on a wet road surface.
3. The cohesion or wear component that occurs through the resistance of the rubber against breaking of the internal coherence.
8.2.1 Influencing factors:
The most important factors that affect the friction coefficient are: 1. road surface,
2. tyre,
3. weather conditions, 4. vehicle speed,
5. wheel slip and drift angle. Re 1. Road surface
With respect to the effect of the road surface on the friction coefficient the following parameters can be distinguished:
a. The texture of the road surface. The texture is the geometry of the road surface. Two texture scales are distinguished, i.e. a macro scale and a micro scale (figure 8.1). The texture at macro scale is required to remove on a wet road surface, especially at higher vehicle speeds, the water from the contact area between the tyre and the road surface. The macro texture is determined by the size of the aggregate particles at the road surface.
of the last traces of water from those locations where high contact pressures between the aggregate and the tyre are present.
Figure 8.1: Different road surface textures.
b. The aggregate at the road surface. The following properties of the aggregate are relevant with respect to the skidding resistance:
- the shape and the size
- the resistance against polishing - the resistance against crushing
- the bond with the binding agent (bitumen or cement).
In general crushed aggregates have sharp and rough surfaces that favours the texture and the skidding resistance. However, polishing of the aggregates occurs under the repeated traffic loadings. The rate of polishing is among other things dependent on the type of aggregate and the traffic intensity. The skidding resistance is also negatively influenced by crushing and loss of aggregate at the road surface.
c. Condition of the road surface. On a wet road surface the water may partially or totally interrupt the contact between the tyre and the road surface and that leads to a decrease of the friction coefficient µ. Therefore the rainwater has to be removed fast from the road surface and one should prevent puddles on the road surface (figure 8.2).
In this respect the aim of the crossfall (transversal slope of the road surface) is relevant: the fast transversal removal of the rainwater. The applied percentage of crossfall depends on the type of road pavement; e.g. it has been internationally decided to apply on asphalt pavements a crossfall of 2.5% to prevent as much as possible puddles in ruts.
Also the presence of dirt may have a negative effect on the friction coefficient. Dust and water may form an emulsion that temporary acts as a lubricant (figure 8.3). Continuous rainfall causes cleaning of the road surface and then the friction coefficient increases again.
Figure 8.2: Risk for puddles near road markings and in ruts.
Figure 8.3: The friction coefficient as a function of time during a rain shower after a long dry period of time.
Re 2. Tyre
The tread of luxury car tyres is generally made of synthetic rubber. Compared to natural rubber the synthetic rubber yields a greater friction coefficient due to a greater adhesion and hysteresis. The disadvantage of the synthetic rubber is more development of heat.
For economical reasons (less wear) truck tyres are usually made of natural rubber.
The tread of the tyre should have such a profile that the water in the contact area between the tyre and the road surface can disappear quickly. Both the shape and the depth of the tyre profile are relevant.
Re 3. Weather condition
At skidding resistance measurements it is assumed that due to winter maintenance the road surface is free of snow and ice. The influence of rainfall manifests itself as a decrease of the skidding resistance and the risk for aquaplaning. Aquaplaning is the phenomenon that a driving vehicle on tyres looses contact with the road surface (as there remains a thin layer of water in
high vehicle speed, the thick water layer on the road surface and the insufficient tyre profile.
Figure 8.4 schematically shows how a rubber tyre rolls or slides over a wet road surface. Three zones are distinguished: zone 1 (no contact), zone 2 (local contact) and zone 3 (dry contact). When zone 1 enlarges the friction coefficient decreases, finally resulting in aquaplaning. This effect is intensified if a standing wheel is brought to rotation very quickly; this occurs at a landing aircraft where the phenomenon of aquaplaning was observed first.
Figure 8.4: Schematic representation of a rolling tyre on a wet road surface. Re 4. Vehicle speed
On a dry road surface the influence of the speed of the wheel (vehicle) on the friction coefficient in general is limited.
However, on a wet road surface the friction coefficient strongly decreases with increasing vehicle speed and increasing thickness of the water layer (figure 8.5). The friction coefficient only becomes greater if the driver slows down or if the thickness of the water layer decreases (e.g. through a greater crossfall).
Figure 8.5: The relationship between the friction coefficient and the vehicle speed as a function of the thickness of the water layer (100% wheel slip, profiled radial tyre, concrete road with a fine texture).
Re. 5 Wheel slip and drift angle
A wheel can be braked off in such a way that solely forces in the longitudinal direction occur in the contact area between the tyre and the road surface. If ω1 is the rotary speed of the braked wheel and ω0 is the rotary speed of a purely rolling wheel, then the percentage of wheel slip is (ω ω0ω 1)
0 −
x 100%.
The magnitude of the longitudinal force varies with the percentage of wheel slip.
A braking wheel can also have a so-called drift angle with the direction of travel (e.g. braking in a curve). The occurring transversal braking forces are dependent on the magnitude of the drift angle.
8.2.2 Standards for skidding resistance:
For reason of traffic safety there exist minimum requirements for the skidding resistance in the longitudinal direction.
The standards used in The Netherlands are based on research of S.W.O.V. (Foundation for Scientific Research into Traffic Safety) that has found relationships between the skidding resistance of a road surface and the chance for traffic accidents.
Characteristic values for the skidding resistance of roads outside built-up areas in The Netherlands are given in table 8.1. The skidding resistance has to be measured according to the standardised method of the Road and Hydraulic Engineering Division (‘DWW’) of the Ministry for Transport, Public Works and Water Management. The standards are based on skidding resistance measurements with a ‘retarded wheel’ with 86% wheel slip, a 0.5 mm thick water layer and a vehicle speed of 50 km/h. The measurement tyre does not have any profile and it is internationally standardised.
Minimum value
Guideline value = reject value Deduct value
0.38 0.45 0.52
Table 8.1: Characteristic values for the skidding resistance of roads outside built-up areas.
If at the delivery stage of the road the skidding resistance appears to be below the deduct value, the contractor gets a penalty. In the case that at the delivery the skidding resistance is below the guideline value, the contractor has to take such measures that after that the guideline value is exceeded everywhere.
8.3 Ride-ability and evenness:
amplitude, the speed and the acceleration of the vibrations that passengers experience due to unevenness (roughness) of the road. The movements of the passengers can be divided into horizontal and vertical movements. Research has learned that especially the vertical component of the movement is experienced as being unpleasant.
On the other hand ride-ability can be expressed as traffic safety; this is a measure for the ‘risk’ of driving the vehicle. This traffic safety mainly concerns the relation between the characteristics of the (laden) vehicle, the vehicle speed, the road surface properties and the driver’s capabilities. In the case of a unbalance between these factors a dangerous situation arises, not only for the own vehicle but also fellow road users.
Evenness can be defined as a measure for the magnitude and nature of relative deviations in a road surface compared to the absolute height level of the road surface desired beforehand (e.g. a straight line or a vertical curve). With respect to evenness a further distinction can be made in macro and micro evenness. Macro evenness determines the ride-ability, i.e. the driving comfort and traffic safety, at higher vehicle speeds while the micro evenness (= road surface texture with ‘wavelengths’ up to 3 mm) does effect the skidding resistance, the road illumination and the traffic noise production (tyre noise), so the traffic safety. The micro evenness is hardly ever taken into account when the road evenness is reviewed.
Nowadays road pavements are to a great extent constructed in a mechanical way and that results in very good initial road evenness (at the time of delivery). The decrease of the road evenness and ride-ability in time may be considered as a relation between the total pavement structure and the traffic. The changes of the macro evenness mainly arise from (unequal) settlements in the subsoil, permanent deformations in the pavement materials as well as cracking and ongoing shear failures. The micro evenness is influenced by ravelling (loss of aggregate) and disintegration of the road surface.
8.3.1 Deformations in longitudinal direction:
The deformations of the road surface in the longitudinal direction are usually schematised by means of waves. In general the deformations consist of combinations of wavelengths, which to some degree is an advantage as a regular pattern of impact loads negatively affects the ride-ability of the road (e.g. the regular occurring impact that one experiences when driving over old concrete roads with faulted transverse joints).
It is known from measurements, and also from experience, that on most type of pavements the deformations with greater wavelengths always have a greater amplitude than those with smaller wavelengths (except very local potholes and bumps). At higher vehicle speeds the vibrations of the vehicle are determined by the greater wavelengths. Increasing the vehicle speed on the same road surface thus leads to greater accelerations of the vehicle (and its passengers) and a lower steer-ability of the vehicle.
The other way around it can be stated that the road surface must exhibit a high level of (macro) evenness to limit the accelerations of the vehicle and its
passengers and the related forces acting on the vehicle.
8.3.2 Deformations in transversal direction:
Possible deformations in transversal direction are rutting on asphalt and small element pavements, unevenness around a longitudinal joint in concrete pavements and cracking at the ‘seam’ between two strips of asphalt wearing course laid next to each other. Rutting is the most important type of transversal unevenness, both with respect to nature and magnitude.
Rutting may be described as a change in the cross profile of a road surface due to the repeated traffic loadings. In time a wave pattern grows that is characterised by wave-troughs in both wheel tracks (figure 8.6). As already indicated in figure 8.2, rutting implies the risk of aquaplaning under wet weather conditions.
Figure 8.6: Scheme of the cross section of a road exhibiting rutting.
Also under dry weather conditions rutting hampers the steer-ability of a vehicle, especially at higher vehicle speeds or for heavy loaded trucks.
8.4 Realisation of the desired road surface properties:
When discussing the realisation of the desired road surface properties a distinction has to be made between the available types of pavement because of the different (natural) properties of the various pavement materials.
8.4.1 Small element pavements:
Concrete paving blocks and burnt clay bricks are mainly applied in built-up areas on roads where the vehicle speed is limited (residential streets, district roads). For such applications there are no strong requirements with respect to the evenness while the skidding resistance (considering the vehicle speed) and the reflection of (artificial) light are sufficient. The discharge and storage of water are good because a small element pavement to some extent always remains open (permeable).
In historical city centres sometimes so-called natural stone setts are applied that are rather susceptible for polishing. Under wet weather conditions the skidding resistance of such pavements is insufficient.
8.4.2 Concrete pavements:
The desired texture on concrete pavements is obtained by treating (brooming) the surface of the fresh concrete. A properly constructed concrete pavement usually does not exhibit any unevenness. The diffuse reflection of (artificial) light is sufficient because of the natural light colour of the concrete.
8.4.3 Bituminous pavements:
The skidding resistance originates from the angular aggregates in the wearing course at the road surface. The film of bitumen around the aggregates wears off under the repeated traffic loadings or is not present at all, which is the case when so-called chippings (small-sized stone aggregates) are strewed during compaction of the wearing course. Chippings are therefore (nearly) always applied on new dense asphalt wearing courses.
The reflection of light is improved by the application of light-coloured aggregates in the wearing course.
Under wet weather conditions the skidding resistance of a porous asphalt wearing course (in Dutch: ZOAB) generally is not any problem because the rainwater is removed through this asphalt layer. However, under dry weather conditions a newly constructed porous asphalt wearing course may exhibit a somewhat lower skidding resistance during a certain period of time, because the aggregates are then covered with bitumen. The micro texture of the aggregates only can contribute to the skidding resistance when the bitumen has worn off. Strewing chippings for a better skidding resistance during the first months is impossible as they would disappear into the porous asphalt.
8.5 Noise production of road surfaces:
Traffic noise is not only caused by the vehicles’ engines and exhausts but also by the road surface together with the vehicle tyres driving over it. Therefore a distinction is made between operating noise and rolling noise, and this latter noise is generated by the vehicle tyres and the road surface. With increasing vehicle speed the rolling noise increases more strongly than the operating noise. The rolling noise is dominant over the operating noise for vehicle speeds of more than about 30 km/h (for luxury cars) and 60 km/h (for trucks) respectively.
To solve the rolling noise problem solutions have to be found in the tyres and in the road surface. In this paragraph only the effects of the road surface on the production of rolling noise are very briefly discussed.
It appears from acoustic considerations that both the depth and the wavelength of the surface texture have a substantial effect on the noise production. The diameter of the applied aggregate influences both factors: the smaller the diameter of the aggregate, the smaller the texture depth and the smaller the texture wavelength. A study for VBW-Asfalt (1) learned that, compared to dense asphalt that serves as a reference material, the various pavement types yield the difference in noise production as given in table 8.2. This table shows that porous asphalt results in a substantial reduction of the rolling noise.
Small element pavement Broomed concrete pavement Dense asphalt wearing course
Porous asphalt wearing course (ZOAB)
+2 till +5 dB(A) +2 dB(A)
0 dB(A) -3 dB(A)
Table 8.2: Equivalent noise pressure levels on various types of road surface compared to a dense asphalt wearing course.
8.6 References:
1. Asphalt and traffic noise (in Dutch)
