Surface roughness - Wikipedia
They represented the relation of surface depth and density with pavement friction by developing a model. The pavement surface texture according to its size is. Smooth contact pads that evolved in insects, amphibians and mammals to enhance the attachment abilities of the animals' feet are often dressed with surface. Surface roughness often shortened to roughness, is a component of surface texture. .. a or Ra and Sm) to find the strongest correlation. Common Surface structure is often closely related to the friction and wear properties of a surface.
The value a is a function of speed and increases with speed. Thus, FR increases with speed. In the constant-braked mode figure 10an additional force called the braking slip force FB is required to counter the added moment MB created by braking. The force is proportional to the level of braking and the resulting slip ratio. The total frictional force is the sum of the free-rolling resistance force FR and the braking slip force FB.
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Rolling resistance force with a free-rolling tire at a constant speed on a bare, dry paved surface Andresen and Wambold, Forces and moments of a constant-braked wheel on a bare, dry paved surface Andresen and Wambold, The coefficient of friction increases rapidly with increasing slip to a peak value that usually occurs between 10 and 20 percent slip critical slip. The friction then decreases to a value known as the coefficient of sliding friction, which occurs at percent slip.
The difference between the peak and sliding coefficients of friction may equal up to 50 percent of the sliding value, and is much greater on wet pavements than on dry pavements. Vehicles with ABS are designed to apply the brakes on and off i. The braking is turned off before the peak is reached and turned on at a set time or percent slip below the peak. The actual timing is a proprietary design of the manufacturer.
Pavement friction versus tire slip. The relationship between the forces acting on the vehicle tire and the pavement surface as the vehicle steers around a curve, changes lanes, or compensates for lateral forces is as follows: It shows how the side-force friction factor acts as a counterbalance to the centripetal force developed as a vehicle performs a lateral movement. Dynamics of a vehicle traveling around a constant radius curve at a constant speed, and the forces acting on the rotating wheel.
When operating at the limits of tire grip, the interaction of the longitudinal and lateral forces is such that as one force increases, the other must decrease by a proportional amount. The application of longitudinal braking reduces the lateral force significantly. Similarly, the application of high lateral force reduces the longitudinal braking. Figure 13 shows these effects Gillespie, Commonly referred to as the friction circle or friction ellipse Radt and Milliken,the vector sum of the two combined forces remains constant circle or near constant ellipse see figure When operating within the limits of tire grip, the amount of braking and turning friction components can vary independently as long as the vector sum of these components does not exceed the limits of tire grip as defined by the friction circle or friction ellipse.
The degree of ellipse depends on the tire and pavement properties. Brake Fx and lateral Fy forces as a function of longitudinal slip Gillespie, Lateral force versus longitudinal force at constant slip angles Gillespie, It is a function of the interface shear strength and contact area.
The hysteresis component of frictional forces results from the energy loss due 25 Figure The deformation is commonly referred to as enveloping of the tire around the texture. When a tire compresses against the pavement surface, the stress distribution causes the deformation energy to be stored within the rubber.
As the tire relaxes, part of the stored energy is recovered, while the other part is lost in the form of heat hysteresiswhich is irreversible. That loss leaves a net frictional force to help stop the forward motion. Although there are other components of pavement friction e. Thus, friction can be viewed as the sum of the adhesion and hysteresis frictional forces.
Also, because tire rubber is a visco-elastic material, temperature and sliding speed affect both components. As a result of this phenomenon, adhesion governs the overall friction on smooth-textured and dry pavements, while hysteresis is the dominant component on wet and rough-textured pavements.
Table 2 lists the various factors comprising each category. To prevent hydroplaning occurrence, the surface needs to have adequate macrotexture for quick dispersion of water accumulated on the surface of the pavement. Additionally, adequate macrotexture helps the development of the hysteresis component of friction which is related to energy loss due to tire deformation because Figure 2.
Micro-texture and macro-texture . Pavement wavelength and surface characteristics . Simulation of the percentage of contact points within the contact area of a tire and pavement surface could give an estimation of the macrotexture of the pavement .
Moreover, they reported that according to HMA mix design properties, prediction of some of the frictional properties of the wearing surface mixes is possible . Bloem  indicated that in order to assure enough depletion of water from under the tire, an average texture depth of about 0.
Balmer  showed that about 0. Microtexture plays a considerable role in the road-tire contact in wet surfaces. The size of microasperities has a significant trace in overcoming the thin water film. Squeezing and overcoming the thin water film present in the pavement-tire contact area and generating friction forces requires the existence of microtexture . Moreover, to maintain a confident contact between tire and pavement, the microtexture has a great role to penetrate into thin water film present on the surface of the pavement .
The shape of micro asperities controls the drainage process   . Savkoor  also showed that the amplitude and number of microasperities on the surface affects the drainage of the water film between the tire and pavement . Forster  presented a parameter in relation to microtexture based on a combination of average height and average spacing between microasperities. In this research, it was shown that hydroplaning in pavements that contain coarse aggregates with high microtexture in the range of 0.
Horne  also stated that it is possible to delay hydroplaning in pavements with a good microtexture . A study by Pelloli  on five different types of surfaces showed that the relationship between friction coefficient and the water depth accumulated on the pavement surface is a function of the amount of microtexture .
Leu and Henry  explained the differences of skid resistance tests taken from different pavement surfaces based on their specific microtexture and macrotexture. However, Horne and Buhlmann  demonstrated the poor relation of surface friction measurements and pavement texture measurements  . Hogervorst  reported that both microtexture and macrotexture of the pavement surface affects the changes of skid resistance with vehicle speed.
The magnitude of skid resistance is defined by microtexture, and controlling the slope of reduction of skid resistance as speed increases is related to macrotexture. Moreover, macrotexture by reducing the friction-speed gradient and helping the drainage of water affects the skid resistance of pavements at high speed, but it has little effect at low speed.
On the other hand, at low speeds, the dominant factor which defines the level of friction is microtexture    Figure 5. Researchers declared that the characteristics of the coarse aggregates exposed at the wearing course affect the macrotexture and microtexture of the pavement surface  ; Figure 5. According to microtexture and macrotexture of a pavement surface, they could be classified into four categories : In some researches, surface irregularities are divided into four categories: Megatexture includes major surface irregularities like roughness, cracks and potholes.
It is measured in a centimeter scale and influences tire-pavement contact and decreases tire-pavement adhesion .
Ride comfort is highly related to megatexture . Unevenness affects the safety and comfort of riding on a road due to its effect on dynamic parts of the vehicle . A summary of the effect of different factors on the microtexture and macrotexture of the pavement surface is presented in Table 2.
This information can be used to obtain required characteristics for pavement surface design . Wetting of the Pavement Surface The standard way of skid resistance measurement is on wet pavements. One of the most important parameters influencing skid resistance is the wet or dry condition of the pavement surface. Many researchers demonstrated that there is a relationship between accidents in wet weather conditions and pavement surface friction   - .
In places where there are long intervals between precipitations, after a dry period, the number of accidents increases during the first precipitation . When the pavement becomes wet, the layer of water covering the pavement acts like a lubricant and reduces the contact between the tires and the pavement surface  .
Therefore, the friction Figure 6. Types of pavement surface texture with the scope of wavelength and amplitude . Factors affecting pavement micro-texture and macro-texture . In addition to this lubricating effect of water at high speeds, lack of drainage facility in the presence of certain depths of water film may result in hydroplaning hydroplaning occurs when a water film builds between the tires of the vehicle and the pavement surface, leading to the loss of traction and thus disabling the vehicle from responding to actions like steering, braking or accelerating which is considered the main cause of accidents in wet weather conditions  .
Implementing corrective actions in hazardous areas can reduce the rate of these accidents. It is necessary for future safety improvements to evaluate the safety of roads and analyze the different factors affecting pavement friction.
The occurrence of dynamic hydroplaning is influenced by pavement macrotexture and tires tread depth in two ways. First, the critical hydroplaning speed directly depends on these factors because they provide a pathway for water to digress from the pavement?
Second, the critical hydroplaning speed indirectly depends on these factors because the larger macro-texture requires more water presence to cause hydroplaning.
However, the pavement surface must also have an adequate microtexture to develop good friction. Research studies have shown that a 63 percent decrease in wet-pavement crashes can be obtained by an increase in average pavement friction from 0.
Research by Kamel and Gartshore also showed 71 percent reduction in wet weather crashes in intersections and 54 percent on freeways by improving the skid resistance  . The Organization for Economic Cooperation and Development OECDfound a linear relationship between the slipperiness of the road surface and the accidents. Moreover, with an increase in slipperiness of the road surface, the rate of crashes increased  .
The effect of wet weather conditions on road safety was also demonstrated by a study conducted in Germany, where the proportion of wet crashes was compared to pavement surface friction, as shown in Figure 7. This figure clearly shows a significant decrease in wet pavement accidents as the pavement friction increases . The relationship between pavement skid resistance and crash rates and the effect of pavement friction improvement on these crash rates is also demonstrated by several researchers  - .
Material Characteristics While most researches are focused on increasing the life span of pavement materials, there is no clear criterion for the selection and use of aggregate and mixture design to assure desirable frictional performance. In addition, available methods of evaluating aggregates for use in asphalt mixtures are mainly based on the old viewpoint of aggregate performance  . Relationship between crash rates in wet weather conditions and pavement surface friction .
The high correlation between pavement skid resistance and rate of accidents shows that it is required for developing a system for comprehensive material selection and mixture design. Mix designers always deal with the challenge of selection of proper aggregates.
Furthermore, it is necessary to pay attention to frictional properties of aggregates and their ability to resist the polishing action of the passing traffic. Considering these important factors prevents probable additional costs for surface treatment. The cost of maintenance and rehabilitation of pavements can reduced by developing a comprehensive system for selecting aggregates based on a quantitative evaluation of the physical properties of aggregate related to pavement skid resistance.
This system would consider the effects of pavement microtexture and macrotexture to propose the optimal pavement skid resistance and facilitate the selection of aggregate type and mixture design to satisfy safety requirements. To achieve this optimization, it is required to find a proper accelerated polishing method and systematic test method for measuring the frictional properties of the pavement.
The ability of aggregates to keep their texture against polishing action of traffic has long been known as a highly important requirement for its use in pavement construction   - . Coarse aggregate characteristics like angularity and resistance to wear, have a substantial role in providing sufficient skid resistance in pavements.
Using hard and irregularly shaped coarse aggregates is a key to attaining and retaining the desired texture. To avoid reducing skid resistance of the asphalt surface, it is essential to implement a hard and polish-resistant coarse aggregate . Fine aggregates show their significant role only when used in relatively large quantities . The adhesion component of pavement friction could be highly enhanced by using Sharp, hard sand particles . Aggregates, based upon their mineralogy, are polished differently in the asphalt mixture.
Aggregates polish or become smoother at different rates because of their different ability to resist the polishing action of traffic  . Texture and Friction Measurement There are a number of test procedures and devices available to measure the skid resistance of the pavement surface.
Every single of them has specific features and it is vital to know that these devices all measure slightly different parameters and hence their results cannot be compared directly but in some aspects we can compare these measurement tests. In a general classification we can divide the measurement of skid resistance into two categories: These measurements are also divided into other categories. Figure 8 shows the classification of measurement methods: Field Measurement Field skid resistance is generally measured by the force generated when a locked tire slides on a pavement surface .
These measurement methods should be precise, and Figure 8. Classification of skid resistance measurement method. Friction testing is comprised of applying a standard test tire to the pavement surface with a controlled wheel slip 0 to percent slip and measuring friction between the test tire and pavement surface American society for testing and materials ASTM E, E, E, E, E, E .
In the field of direct measurements, there are four main types of skid resistance measuring approaches  - : The friction coefficient is measured while a percent slip condition is produced. The friction is measured for wheels that are constantly slipping. Measures the friction at any desired slip. In all methods related to locked wheel and variable slip of tires, the friction coefficient is measured on wet pavement surfaces .
Henry  found that the locked wheel method ASTM E is the most common method for pavement friction measurement in the U. This method aims to measure the frictional properties of the pavement surface under sudden braking conditions for a vehicle without anti-lock brakes. Unlike the side-force and fixed-slip methods, in the locked-wheel method the slip speed is equal to the vehicle speed, which means that the test wheel is locked and unable to rotate .
The ribbed tire is insensitive to the thickness of the pavement surface water film and is insensitive to the pavement macro-texture, but the smooth tire is sensitive to the macro-texture of the pavement surface  . Other measurement methods comprise the fixed slip, variable slip, and the sideway force or cornering method.Physics - Factors affecting Friction
In the fixed and variable slip method, the coefficient of friction is a function of the slip of the test wheel while rolling over the pavement surface . Fixed-slip devices use vehicles with anti-lock brakes to measure the friction. They maintain a constant slip, typically between 10 and 20 percent, as a vertical load is applied to the test tire .
These devices are more sensitive to micro-texture at low slip speeds . Variable-slip devices ASTM E use a predetermined set of slip values for measuring the frictional force.
Pavement Friction and Skid Resistance Measurement Methods: A Literature Review
The side-force method ASTM E output is a sign of the ability of vehicles to maintain control in curves. In this test the test wheel must maintain a constant angle yaw angle to the direction of motion . The side-force method test wheel moves at an angle to the direction of vehicle motion because the critical situation for skid resistance occurs in cornering .
Side-force testers are sensitive to changes in the pavement microtexture but they are generally insensitive to pavement macro-texture. The ability of continuous friction measurement throughout a test section is the primary advantage of side-force measurement devices, because this ability ensures that areas with low friction are not skipped due to a sampling procedure .
Portable and Laboratory Testers Methods explained before are categorized as field methods. Other testing methods are portable and laboratory testers.
The British pendulum tester  is a simple and cheap instrument used in the measurement of friction characteristics of pavement surfaces. This device can be used to measure pavement friction characteristics in the laboratory or at low speeds in the field. The BPT is easy to handle, can be used in the laboratory and in the field, but its result is only a measure of frictional property at a low speed . Experience has shown that although this tester measurement is largely influenced by the microtexture of the pavement surface, the macrotexture can also affect the measurements .
BPT results from coarse-textured surfaces friction measuring can be misleading . Other researchers also showed that the British pendulum tester exhibited unreliable behavior in coarse-textured surface measurements  - . The DFT is a tester device that measures the friction force between the surface and three rubber pads attached to a rotating disc.
The rubber pads can touch the pavement surface at different speeds so the DFT can measure the skid resistance at various speeds . In a study, Saito et al. Both British pendulum and DFT device procedures are based on determining the loss in kinetic energy of a sliding pendulum or rotating disc when they are in contact with the pavement surface.
Measuring friction at various speeds is an advantage for the DFT device because it is able to measure the speed dependency of the pavement friction .
Pavement researchers have been long concerned about measuring the pavement microtexture and macrotexture and relating these measurements to pavement skid resistance.
The practice of pavement macrotexture measurement has been a common practice in recent years  . It is favorable to use a computer model considering that test methods are not easily repeatable and prediction methods are fast and low cost . Pavement macrotexture is generally measured by a volumetric method. Essentially, in this method a known volume of a homogenous material sand, glass beads, or grease is spread on the pavement surface and the resulting area is measured.
The volumetric methods are burdensome for use in routine testing . The outflow meter can be used for measuring the relative drainage ability of the pavement surface and detecting surface wear and predicting correction measures .
In the Outflow Meter test, a transparent vertical cylinder with a rubber ring under it, placed on the pavement surface. Then, water is allowed to flow into the pavement, and the required time for passing a determined volume of water in the transparent vertical cylinder is recorded.
This time indicates the ability of the pavement surface to drain water and shows how fast water depletes from the surface. This time is reported as the outflow time and can be related to pavement macrotexture .
This test output is an indication of the hydroplaning potential of a surface by relating to the escape time of water beneath a moving tire. The measurement parameter, outflow time OFTdefines the macro-texture of the pavement surface. Pavement surfaces with smooth macrotexture have high OFTs and pavement surfaces with rough macrotexture have low OFTs . In the past decade, with significant advances in laser technology and in the computational power and speed and creation of small and high-speed computers, several systems are now available to measure macrotexture at traffic speeds.
Various profile statistics such as the Mean Profile Depth MPDthe overall Root Mean Square RMS of the profile height and other parameters that reduce the profile to a single parameter can be computed by these systems . The Mini-Texture-Meter developed by British Transport and Road Research Laboratory the Selcom Laser System developed by researchers at the University of Texas at Arlington  and the noncontact high speed optical scanning technique developed by the researchers at Pennsylvania State University   are examples of these systems.
The Mini Texture Meter and the Selcom Laser System use a laser beam to scan the pavement surface and estimate pavement texture depth. The noncontact high-speed optical scanning technique uses a strobe band of light with high infrared content to generate shadowgraphs. This technique can use a vehicle moving at normal highway speeds to collect information from the pavement surface .
This device can be used in the laboratory or in the field. It measures the profile of a circle The profile is divided into eight segments of 4. The MPD represents the average of the highest profile peaks in eight individual segments comprising the circle of measurement. The RMS is a statistical value, which presents a measure of deviation of the actual data from the measured profile and a best-fit of the data from the modeled profile .
There are several methods for measurement of the microtexture . A research accomplished at Pennsylvania State University showed that there is a high correlation between the zero speed intercept of the friction-speed curve of the Penn State model and the RMS of the microtexture profile height. In addition, researchers found that the BPN values have a high correlation with this parameter.
Therefore, the BPN values could be considered as the substitute for microtexture measurements . Observations of pictures of road stones taken by means of the Scanning Electron Microscope SEM showed how microtexture of the aggregates is affected by the polishing actions, as simulated in the laboratory by the British Accelerated Polishing Test  - .
It should be noted that the test results are highly sensitive and have a large variability. For the test results to be purely indicative of aggregate textures, other factors need to be controlled. Coupon curvature, the arrangement of aggregate particles in a coupon for heterogeneous materials such as gravel, the length of the contact path, and slider load have significant effects on the results, and any change in these parameters would yield misleading results  .
The degree of polishing of various aggregate types is different because the aggregates are further polished or conditioned during the slider swing . For the laser scanning method, a 3d laser scanner including an enhanced sensor was utilized to inspect a full range of colors and depths on the selected asphalt pavement surface. The laser equipment was mounted on a portable vehicle attached to a computer.
The device measures texture by means of laser light. This study was conducted on sections which were exposed to the same environment but different traffic loading conditions. This study also demonstrated the feasibility of assessing the surface texture of the pavement by means of laser scanned image analysis.
The main advantage of the utilized 3d laser scanning system is acquisition time and accuracy as compared with the sand patch test and it would give an accurate and detailed assessment of pavement texture.
The application of optic or laser devices in direct measurements are gaining popularity because of their simplicity and being easy to use. Forster  used cameras to digitize and measure road profile images obtained from a projection device. He combined measurements of the average height and average spacing of the microtexture asperities and developed a parameter according to it. Yandell and Sawyer  proposed a device using almost the same measurement procedure for in-situ use.
Samuels  used a laser sensor to directly record profiles. The laser system, with a measuring range of 6 mm and a spot size of around 0.