INTRODUCTION TO PAVEMENT
A highway pavement is a structure consisting of superimposed layers of processed materials above the natural soil sub-grade, whose primary function is to distribute the applied vehicle loads to the sub-grade. The pavement structure should be able to provide a surface of acceptable riding quality, adequate skid resistance, favourable light reflecting characteristics, and low noise pollution. The ultimate aim is to ensure that the transmitted stresses due to wheel load are sufficiently reduced, so that they will not exceed bearing capacity of the sub-grade (Mathew 2007).
TYPES OF PAVEMENTS
The pavements can be classified based on the structural performance into two, flexible pavements and rigid pavements.
FLEXIBLE PAVEMENT
Flexible pavements will transmit wheel load stresses to the lower layers by grain-to-grain transfer through the points of contact in the granular structure. The wheel load acting on the pavement will be distributed to a wider area, and the stress decreases with the depth.
Taking advantage of this stress distribution characteristic, flexible pavement normally has many layers. Hence, the design of flexible pavement uses the concept of layered system. Based on this, flexible pavement may be constructed in a number of layers and the top layer has to be of best quality to sustain maximum compressive stress, in addition to wear and tear.
The lower layers will experience lesser magnitude of stress and low quality material can be used. Flexible pavements are constructed using bituminous materials. These can be either in the form of surface treatments (such as bituminous surface treatments generally found on low volume roads) or, asphalt concrete surface courses (generally used on high volume roads such as national highways). Flexible pavement layers reflect the deformation of the lower layers on to the surface layer (e.g., if there is any undulation in sub-grade then it will be transferred to the surface layer). In the case of flexible pavement, the design is based on overall performance of flexible pavement, and the stresses produced should be kept well below the allowable stresses of each pavement layer (Mathew 2007).
TYPICAL LAYERS OF A FLEXIBLE PAVEMENT
Typical layers of a conventional flexible pavement includes seal coat, surface course, tack coat, binder course, prime coat, base course, sub-base course, compacted sub grade, and natural sub-grade.
Binder course
This layer provides the bulk of the asphalt concrete structure. Its chief purpose is to distribute load to the base course the binder course generally consists of aggregates having less asphalt and doesn't require quality as high as the surface course, so replacing a part of the surface course by the binder course results in more economical design.
Base course
The base course is the layer of material immediately beneath the surface of binder course and it provides additional load distribution and contributes to the sub-surface drainage it may be composed of crushed stone, crushed slag, and other untreated or stabilized materials.
Sub-Base course
The sub-base course is the layer of material beneath the base course and the primary functions are to provide structural support, improve drainage, and reduce the intrusion of fines from the sub-grade in the pavement structure If the base course is open graded, then the sub-base course with more fines can serve as a filler between sub-grade and the base course A sub-base course is not always needed or used. For example, a pavement constructed over a high quality, stiff sub-grade may not need the additional features offered by a sub-base course. In such situations, sub-base course may not be provided. Transportation Research Board (2001).
RIGID PAVEMENT
Rigid pavements have sufficient flexural strength to transmit the wheel load stresses to a wider area below. A typical cross section of the rigid pavement: Compared to flexible pavement, rigid pavements are placed either directly on the prepared sub-grade or on a single layer of granular or stabilized material. Since there is only one layer of material between the concrete and the sub-grade, this layer can be called as base or sub-base course.
In rigid pavement, load is distributed by the slab action, and the pavement behaves like an elastic plate resting on a viscous medium (Figure 2:4). Rigid pavements are constructed by Portland cement concrete (PCC) and should be analyzed by plate theory instead of layer theory, assuming an elastic plate resting on viscous foundation. Plate theory is a simplified version of layer theory that assumes the concrete slab as a medium thick plate which is plane before loading and to remain plane after loading. Bending of the slab due to wheel load and temperature variation results in tensile and flexural stress.
Types of Rigid Pavements
Rigid pavements can be classified into four types:
Jointed Plain Concrete Pavement: These are plain cement concrete pavements constructed with closely spaced contraction joints. Dowel bars or aggregate interlocks are normally used for load transfer across joints. They normally have a joint spacing of 5 to 10m.
Jointed Reinforced Concrete Pavement: Although reinforcements do not improve the structural capacity significantly, they can drastically increase the joint spacing to 10 to 30m. Dowel bars are required for load transfer. Reinforcement's help to keep the slab together even after cracks.
Continuous Reinforced Concrete Pavement: Continuous Reinforced Concrete Pavement is a Portland cement concrete (PCC) pavement that has continuous longitudinal steel reinforcement and no intermediate transverse expansion or contraction joints. The pavement is allowed to crack in a random transverse cracking pattern and the cracks are held tightly together by the continuous steel reinforcement.
PAVEMENT FRICTION AND SURFACE TEXTURE
Definition
Pavement friction is the force that resists the relative motion between a vehicle tire and a pavement surface. This resistive force is generated as the tire rolls or slides over the pavement surface.
The resistive force, characterized using the non-dimensional friction coefficient, µ, is the ratio of the tangential friction force (F) between the tire tread rubber and the horizontal travelled surface to the perpendicular force or vertical load (FW) and is computed using equation 1.
µ = FFw Eq. 1
Pavement friction plays a vital role in keeping vehicles on the road, as it gives drivers the ability to control/manoeuvre their vehicles in a safe manner, in both the longitudinal and lateral directions. It is a key input for highway geometric design, as it is used in determining the adequacy of the minimum stopping sight distance, minimum horizontal radius, minimum radius of crest vertical curves, and maximum super-elevation in horizontal curves. Generally speaking, the higher the friction available at the pavement - tire interface, the more control the driver has over the vehicle.
Longitudinal Frictional Forces
Longitudinal frictional forces occur between a rolling pneumatic tire (in the longitudinal direction) and the road surface when operating in the free rolling or constant-braked mode.
In the free-rolling mode (no braking), the relative speed between the tire circumference and the pavement - referred to as the slip speed - is zero. In the constant-braked mode, the slip speed increases from zero to a potential maximum of the speed of the vehicle. The following mathematical relationship explains slip speed (Meyer, 1982):
S = V - Vp = V - (0.68 x ω x r) Eq. 2
Where:
S = Slip speed, mi/hr.
V = Vehicle speed, mi/hr.
VP = Average peripheral speed of the tire, mi/hr.
ω = Angular velocity of the tire, radians/sec.
r = Average radius of the tire, ft.
Again, during the free-rolling state of the tire, VP is equal to the vehicle speed; thus, S is zero. For a locked or fully braked wheel, VP is zero, so the sliding speed or slip speed is equal to the vehicle speed (V). A locked-wheel state is often referred to as a 100 percent slip ratio, and the free-rolling state is a zero percent slip ratio. The following mathematical relationships give the calculation formula for slip ratio (Meyer, 1982):
SR = V-VpV X 100 = SV X 100 Eq. 3
where;
SR = Slip ratio, percent.
V = Vehicle speed, mi/hr.
VP = Average peripheral speed of the tire, mi/hr.
S = Slip speed, mi/hr.
Similar to the previous explanation, during the free-rolling state of the tire, VP is equal to the vehicle speed and S is zero, thus the slip ratio (SR) is zero percent. For a locked wheel, VP is zero, S equals the vehicle speed (V), and so the slip ratio (SR) is 100 percent.
The ground force acting on a free rolling tire. In this mode, the ground force is at the centre of pressure of the tire contact area and is off centre by the amount a. This offset causes a moment that must be overcome to rotate the tire. The force required to counter this moment is called the rolling resistance force (FR). The value 'a' is a function of speed and increases with speed. Thus, FR increases with speed.
In the constant-braked mode, an 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).
The coefficient of friction between a tire and the pavement changes with varying slip as shown in figure 11 (Henry, 2000). 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 100 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.
The relationship is the basis for the anti-locking brake system (ABS), which takes advantage of the front side of peak friction and minimizes the loss of side/steering friction due to sliding action. Vehicles with ABS are designed to apply the brakes on and off (i.e., pump the brakes) repeatedly, such that the slip is held near the peak. 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.
Lateral Frictional Forces
Another important aspect of friction relates to the lateral or side-force friction that occurs as a vehicle changes direction or compensates for pavement cross-slope and/or cross wind effects. 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:
Fs = V2 / 15R-e Eq. 4
where: FS = Side friction.
V = Vehicle speed, mi/hr.
e = Pavement super-elevation, ft/ft.
R = Radius of the path of the vehicle's centre of gravity (also, the radius of curvature in a curve), ft.
This equation is based on the pavement-tire steering/cornering force. It shows how the side-force friction factor act as a counterbalance to the centripetal force developed as a vehicle performs a lateral movement.
Dynamics of a vehicle travelling around a constant radius curve at a constant speed, and the forces acting on the rotating wheel.
Where;
W=Weight of vehicle
P=Centripetal force (horizontal)
FS=Friction force between tires and roadway surface (parallel to roadway surface)
α = Angle of super-elevation (tan α = e)
R = Radius of curve W Weight of vehicle
Factors Affecting Available Pavement Friction
The factors that influence pavement friction forces can be grouped into four categories-pavement surface characteristics, vehicle operational parameters, tire properties, and environmental factors. Because each factor in this table plays a role in defining pavement friction, friction must be viewed as a process instead of an inherent property of the pavement. It is only when all these factors are fully specified that friction takes on a definite value.
Pavement Surface Characteristics
Surface Texture
Pavement surface texture is characterized by the asperities present in a pavement surface. Such asperities may range from the micro-level roughness contained in individual aggregate particles to a span of unevenness stretching several feet in length. The two levels of texture that predominantly affect friction are micro-texture and macro-texture (Henry, 2000).
Micro-texture is the degree of roughness imparted by individual aggregate particles, whereas macro-texture is the degree of roughness imparted by the deviations among particles. Micro-texture is mainly responsible for pavement friction at low speeds, whereas macro-texture is mainly responsible for reducing the potential for separation of tire and pavement surface due to hydroplaning and for inducing friction caused by hysteresis for vehicles travelling at high speeds.
Vehicle Operating Parameters
Slip Speed
The coefficient of friction between a tire and the pavement changes with varying slip. It increases rapidly with increasing slip to a peak value that usually occurs between 10 and 20 percent slip. The friction then decreases to a value known as the coefficient of sliding friction, which occurs at 100 percent slip.
Tire Properties
Tire Tread Design and Condition
Tire tread design (i.e., type, pattern, and depth) and condition have a significant influence on draining water that accumulates at the pavement surface. Water trapped between the pavement and the tire can be expelled through the channels provided by the pavement surface texture and by the tire tread. The depth of tread is particularly important for vehicles driving over thick films of water at high speeds. Some studies (Henry, 1983) have reported a decrease in wet friction of 45 to 70 percent for fully worn tires, compared to new ones.
Tire Inflation Pressure
Tire under-inflation can significantly reduce friction at high speeds. Under-inflated tires allow the centre of the tire tread to collapse and become very concave, resulting in the constriction of drainage channels within the tire tread and a reduction of contact pressure. The effect is for the tire to trap water at the pavement surface rather than allow it to flow through the treads. As a consequence, hydroplaning speed is decreased.
Environment
Water
Water, in the form of rainfall or condensation, can act as a lubricant, significantly reducing the friction between tire and pavement. The effect of water film thickness (WFT) on friction is minimal at low speeds (<20 mi/hr [32 km/hr]) and quite pronounced at higher speeds (>40 mi/hr [64 km/hr]). The coefficient of friction of a vehicle tire sliding over wet pavement surface decreases exponentially as WFT increases. The rate at which the coefficient of friction decreases generally becomes smaller as WFT increases. In addition, the effect of WFT is influenced by tire design and condition, with worn tires being most sensitive to WFT. (Henry, 2000)
PAVEMENT SURFACE TEXTURE
Pavement texture is primarily associated with safety conditions, user comfort, and road surroundings. In terms of safety, texture directly affects how well tires stick to pavement in wet conditions and indirectly affects skid resistance. Texture is also associated with noise emissions caused by traffic. From a pavement management perspective, texture depth is important since it can be controlled by maintenance activities and even trigger maintenance treatments. Pavement texture has been categorized into three ranges based on the wavelength of its components.
The three levels of texture, as established in 1987 by the Permanent International Association of Road Congresses (PIARC) are microtexture, macrotexture and megatexture.
MICROTEXTURE
Microtexture is a surface texture irregularity which is measured at the micro scale of harshness and is known to be a function of aggregate particle mineralogy for given conditions of weather effect, traffic action and pavement age. As microtexture irregularities are classified between 0.005mm to 0.3mm the lower limit reflects the smaller size of surface irregularities which affect wet friction. Irregularities greater than 0.3mm cannot penetrate into the soft rubber material of the tire and thus do not affect tire pavement friction. A harsh surface pavement has an average microtexture depth of 0.05mm.
MEASUREMENT OF MICROTEXTURE
Currently there is no system capable of measuring microtexture profiles at high speed. The portion of pavement surfaces that contact the tires are polished by traffic and it is the microtexture of the surface of the exposed aggregate that comes into contact with the tire that influences the friction. The valleys are not subjected to polishing and their contribution to overall microtexture should not be included in prediction of friction.
Because of the difficulty in measuring microtexture profiles, a surrogate for microtexture is generally preferred. Wet pavement friction at low speeds is primarily influenced by microtexture. According to a research at the Pennsylvania State University (), a high correlation was found between the parameter of Penn State Model and the root mean square of the microtexture profile height. The parameter is the zero speed intercept of the friction speed curve and characterizes the friction at low slip seeds. It was also found that the British Pendulum Numbers (BPN) was highly correlated with the parameter. The slider of the British Pendulum engages only the portion of the asperities that are subjected to polishing by traffic and therefore the BPN values could be considered as the surrogate for microtexture.
Accordingly, there is currently no practical procedure for directly measuring microtexture profile. Even if there is such a procedure, it will probably enable testers to avoid measuring microtexture altogether by measuring microtexture and macrotexture in order to predict the wet pavement friction as a function of speed.
MACROTEXTURE
Macrotexture is a surface texture irregularity which is measured in millimetres and is mainly attributed to the size, shape, angularity, spacing and distribution of coarse aggregates (bigger than 2.0mm). Inadequate macrotexture, as a result of faulty construction practice or wear (worn or removed aggregates, embedded aggregates and surface bleeding) drops skid resistance, especially in the medium to high speed range, thus enhancing accident risk. This is because the harsh asperities of the aggregates penetrate the thin film of water, (what remains after bulk water has been dispelled by the combination of pavement drainage capabilities and tire tread) contact and harsh the tire thus obstructing skidding. Furthermore, deep macrotexture means that the pavement surface has a large void area, which is capable of draining excess water from the tire pavement contact region. As macrotexture are considered irregularities between 0.3mm and 5.0mm. Larger irregularities are more or less considered as pavement surface defaults. The sand patch method is the earliest quantitative method of assessing pavement macrotexture. A pavement surface is considered rough if the average depth of macrotexture is more than 1.0mm.
Macrotexture measurements can be divided into two main classes: static measurements and dynamic measurements. Common static macrotexture measurement methods include the sand patch method, the outflow meter, and the circular texture meter. The sand patch method is a volumetric approach of measuring pavement macrotexture. A known volume of sand is spread properly on a pavement surface to form a circle, thus filling the surface voids up with sand. The diameter of the circle on which the sand material has been spread is measured and used to calculate Mean Texture Depth (MTD). Because of operator dependency, the test results have poor repeatability. However, since there is great deal of past research, this volumetric test is still used as the reference standard throughout the word.
The outflow meter indirectly estimates pavement texture based on the time for a fixed volume of water to escape from a measured cylinder with a rubber bottom. The Circular Track Meter, or CTMeter, has a laser displacement sensor mounted on an arm that rotates on a circumference with a 142mm radius and measures the texture with a sampling interval of approximately 0.9mm.
SKID RESISTANCE
Skid resistance is the force developed when a tire that is prevented from rotating slides along the pavement surface (Pavement Management Committee, 1977). Skid resistance is a measure of the resistance of the pavement surface to sliding or skidding of the vehicle. It is a relationship between the vertical force and the horizontal force developed as a tire slides along the pavement surface. Therefore, the texture of the pavement surface and its ability to resist the polishing effect of traffic is of prime importance in providing skidding resistance. Skid resistance is an important pavement evaluation parameter because:
Skid resistance depends on a pavement surface's microtexture and macrotexture (Haas et al, 1994) Microtexture refers to the small-scale texture of the pavement aggregate component (which controls contact between the tire rubber and the pavement surface). Macrotexture refers to the large-scale texture of the pavement as a whole due to the aggregate particle arrangement (which controls the escape of water under the tire and hence the loss of skid resistance at high speeds) Skid resistance changes over time. Typically it increases in the first two years following construction as the roadway is worn away by traffic and rough aggregate surfaces become exposed, and then decreases over the remaining pavement life as aggregates become more polished.
CHARACTERISTICS OF SKID RESISTANT PAVEMENT
The ideal pavement surface has the following characteristics, which, however, are not necessarily all compatible with one another:
FACTORS AFFECTING SKID RESISTANCE
General
Skid resistance is usually described as the ability of a surface to provide friction to a reference type or slider, usually measured wet. Friction is dependent upon the pavement macro- and microtexture
Micro-texture has greater influence on friction at the low speeds encountered in residential areas. Macrotexture becomes dominant at higher speeds, although micro-texture is still important. Macro-texture supplies the paths through which water can escape from between the tyre and road surface, thereby allowing the micro-texture to provide resistance to the relative movement between the tyre and the road surface.
Abrasion resistance
Good abrasion resistance of the pavement slows the rate of decrease of skid resistance with time and trafficking. A minimum compressive strength, typically specified at 32 MPa, generally ensures durability to allow the retention of texture during the design life of the pavement.
Concrete surfacing practice has been primarily directed to provide a surface that is naturally safe in terms of resistance to skidding and also to maintaining the required friction throughout the life of the pavement.
Surface texture
Surface texture is dependent upon the aggregate type. The complete frictional characteristics of the road can be found if both the microtexture and macrotexture are known. Early on, it was established that good microtexture is important at low speeds and good macrotexture is important at high speeds. At higher speeds on wet roads, the surface must contain in addition to fine surface texture, sufficient drainage paths for the water to be dispersed before the fine texture can come into play. However, some researchers believe that the microtexture of the aggregate is important at all speeds.
Coarse aggregate
Durability of the coarse aggregate also can contribute to, and slow the rate of decrease of, skid resistance. The higher the Polished Stone Value (PSV) of the aggregate, the greater the retardation of any reduction in skid resistance.
Weather
Skid resistance is generally considered a wet pavement concern because dry pavements are believed to provide enough skid resistance to avoid skidding problems. Therefore, testing procedures and previous skid related safety studies have focused on wet pavement conditions. It is known that vehicles operating at low speeds on wet pavements develop full hysteretic friction force with the surface. This is because the water in the surface is squeezed out from under the vehicle tire keeping it in full contact with the surface. Skid resistance properties deteriorate when vehicle speeds increase. In this scenario, the hysteretic friction is reduced because a water film is developed between the vehicle tire and the surface thus decreasing skid resistance and potentially causing hydroplaning, even when friction levels are adequate
Purpose for Measuring Skid Resistance
The primary purpose of measuring pavement friction is for quality control during construction and for asset management thereafter. Skid friction values are used in network surveys for pavement management, evaluation of surface restoration, specifications for new construction, accident investigations, and winter maintenance on highways, amongst other purposes Henry J. J. (2000). Skid friction is also used at airports for evaluating runway conditions and determining the need for pilot advisories and maintenance activities. Recent developments for evaluating available friction on highways during winter weather maintenance activities include research by Iowa, Michigan and Minnesota where incorporation of friction measuring instrumentation (SALTAR) has been used with snowploughs to determine the necessary rate of salt application.
Skid friction values are also taken when a site reveals potential road safety problems (aging, bleeding, water accumulation, and surface contamination) and when the road surface has recently been treated to correct skid resistance problems World Road Association. (2003).
Skid Resistance Measuring Devices
Skid resistance is generally measured by the force generated when a locked tire slides along a pavement surface (3). The use of a locked tire is necessary because if the tire is freely rotating, no skidding can be detected since the point of contact between the tire and the pavement is at rest (with respect to the surface). Methods of measuring skid resistance vary and often prevent direct comparisons of values between different testing organizations. In 2000, Henry listed 23 devices currently in use for field friction testing purposes. Henry J. J. (2000)
These devices can be grouped in four categories:
Differences in friction using the same device could be in the range of 5% between two consecutive measurements of the same road surface. Therefore, results from one device are not equivalent or directly comparable to those obtained with another device. Friction is also sensitive to the test tire (ribbed or smooth) and measurements can also differ from two tires of the same type.
Locked wheel testers (American Society for Testing and Materials, ASTM E-274) are the most commonly used device in the U.S. In this method, the relative velocity between the surface of the tire and the pavement surface is equal to the vehicle speed. Usually the left wheel path in the travel lane is tested. The operator applies the brakes and measures the torque for one second after the tire is fully locked then computing the correspondent friction value. In the U.S., the use of a ribbed tire (ASTM E-501) predominates but the use of the smooth tire (ASTM E-524) has been increasing recently. Some prefer ribbed tires because they are less sensitive to water film thickness than the smooth tire.
Research completed by the Florida Department of Transportation indicates that the smooth tire provides skid measurements that are better indicators of safety than measurements with ribbed tires. Henry J. J. (2000)
For laboratory measures of pavement skid friction, Henry described the two main devices currently in use as the British Portable Tester (BPT, ASTM E-303) and the Japanese Dynamic Friction Tester (DFTester, ASTM E-1890). The BPT functions by measuring the loss in kinetic energy of a pendulum with a rubber slider at its edge that has been released over a sample. Contact speed with this method is low and therefore microtexture tends to dominate the readings. The DFTester has three rubber sliders mounted on a disk that is driven by a motor above the pavement surface. Friction is measured by a transducer as the disk spins into the sample. This method is credited with the advantage of measuring friction as a function of speed.
Regardless of the methodology used, the numerical skid resistance value associated with a specific pavement is usually presented as a two-digit constant, determined by multiplying the measured friction coefficient by 100 (though sometimes the number is left as a decimal). This number is described as the friction number (FN) or skid number (SN), note that FN rather than SN is the preferred abbreviation (13). FN is usually followed by the speed value at which the friction measurement was taken and the type of tire (e.g., FN50S represents the friction measurement taken at 50 mph with a smooth tire). Representative values for skid numbers obtained with a skid trailer, and the associated recommendations for each value.
SUMMARY OF FINDINGS
There is evidence to suggest that low skid resistance results in increased numbers of wet pavement crashes. Some studies have found a linear relationship of increasing wet weather crashes rates with decreasing skid resistance. Other studies suggest that the relationship may be non-linear, with the slope increasing with decreasing skid resistance. The common point is that a decrease in pavement skid resistance will likely result in an increase in crash risk. Pavement management strategies need to be developed to integrate skid resistance in the mix design and safety considerations. Maintaining high levels of skid resistance is important especially where there is frequent braking in response to unexpected events, such as on the approaches to intersections.
Skid resistance is an important consideration in highway safety. There exist sufficient studies to indicate that two main characteristics of pavement surface affect skid resistance: Micro-texture and macro-texture. The role of each in providing sufficient friction varies depending on the speed. In addition the materials variables that affect each type of texture are different.
It is well recognized that microtexture is a function of the initial roughness on the aggregate surfaces and the ability of aggregates to resist polishing. Selection of aggregate mineralogy and measuring its polishing resistance has been used widely as a measure of potential micro-texture. Microtexture is considered a controlling factor for skid resistance at low speeds but not high speeds. Because of the difficulty of quantifying aggregate roughness and the resistance to polishing, microtexture is best measured using surrogate tests that allow measuring wet friction at low speeds using small scale devices such as the British Pendulum and the Japanese DFTester. It is indicated in many studies that skid coefficient measurements correlate mostly with micro-texture but not macrotexture.
There are well-developed models for wet pavement friction (Skid resistance). The most widely accepted models indicate that pavement friction, which is a measure of the force generated when a tire slides in pavement surface, is a string function of speed (velocity of the tire to surface). Friction increases from zero (rolling tire) to a peak value and then decreases rapidly as the speed increases. When a brake in a vehicle is first applied, the slip speed is initially high and if breaking continues after the looked wheel condition is reached, the vehicle speed will be equal to the slip speed decreases until vehicle stops. It has been shown in experimental studies that good microtexture results in high friction at low slip speed (< 60 km/hr) but low friction at high speeds. Good macrotexture, on the other hand, results in low friction at low speeds but higher friction at high speeds (> 60 km/hr). To provide for sufficient friction it is important to design for good micro-texture and good macrotexture.
Macrotexture is mainly a function of surface texture, the large-scale roughness that is present due to arrangement of aggregate particles or the grooving created intentionally on surface. The importance of this surface roughness is it allows reducing the thickness of the water film during wet conditions and thus reduces the possibility for hydroplaning.
Macrotexture can be measured by a volumetric method using sand or glass beads, or by using a laser profiler. The volumetric method can be used in the laboratory as well as the field and is considered a practical method but requires some time. The laser method can be done at reasonable travel speed and thus is suitable for continuous pavement network monitoring.
Based on the literature review conducted in this project, it appears reasonable to attempt the development of a mixture design procedure that is considerate of skid resistance based on measuring microtexture and macrotexture.
The microtexture will be estimated using the British Pendulum will be considered.
The macrotexture will be measured using the sand patch method.
