Highway pavements are known of its two main types which are flexible pavement and rigid pavement. Flexible pavement is the pavement made of bitumen, whilst rigid pavement is the pavement made of Portland cement concrete. Concrete in nature are known of its long-lasting lifetime in structures, likewise, concrete pavement too, of its long structural life. However, the main concern for rigid pavement is to provide a durable surface with adequate friction that is economical and safe. Surface friction may possibly reduce over time due to polishing effect of surface due to passing traffic and weathering. In road design, surface friction and pavement texture are essential qualities to be highlighted on as their characteristic correlates with the latter road pavement performance in meeting the high way design criteria and safety. This is important as it could cause loss of life.
Many factors affect the surface friction at the tire pavement interface such as the type of tire, tread pattern, tire pressure, tire condition and so on. Pavement surface types and vehicle tires are major contributors to pavement friction. Tires in poor condition with worn treads, or improper inflation, will not have adequate braking friction on any pavement surface.
Furthermore, the research done by Kuttesch (2004) cited by Davis (2001) explained that accidents occurred from numerous contributing factors including driver error, poor geometric alignment of the roadway, and a lack of sufficient friction at the tire pavement interface during wet weather. In further research by Davis (2001) mentioned about the moisture on the pavement surface may prevent vehicle tires from making adequate contact with the road surface.
Surface dressing is a road surface treatment if sufficient care is taken in the planning and execution of the work. In pavement design there are two important characteristics need to consider which skid resistance and texture depth. Accidents occurred due to several reasons such as driver error, vehicle failure, and friction deficiencies at the tire pavement interface. Accidents occur due to friction deficiency of the pavement are more likely to occur with the accumulation of precipitation such as rain (John , 1996). The moisture on the pavement surface may prevent vehicles ties from making adequate contact with the road surface (Davis, 2001).
2.2 Highway Safety
Safety is a necessity in every day's doing. Thus, designing and constructing a danger free highway is a must and guidelines to follow are provided by the guidelines from Geometric Road Design provided by the Ministry of Works Malaysia.
2.3. Pavement Type and Structure
Rigid pavement is constructed of Portland cement concrete. Its material provides the rigid surface and longer lasting characteristic whilst flexible pavement is usually constructed out of bitumen, thus has given it the flexibility characteristic, however subjected to numerous types of defects in its service.
2.3.2 Rigid Pavement (PCC)
According to Kresge (2009), Concrete pavement gained its reputation this recent years due to asphalt decreased in availability. Sustainability of pavement is a must considering the amount of time taken to construct or maintain, and eventually the costing. For that reason, numerous countries opted for PCC.
Comparing asphalt and concrete (Kresge, 2009), asphalt required three times more energy than concrete for life cycle. Extremly, in term of global warming potential a (CO2 Equivalents) Aspalt give result 738 t/km compare with concrete which is produce only 674 t/km CO2 Equivalents. A rigid pavement structure is composed of a hydraulic cement concrete surface course, and underlying base and subbase courses.
High modulus of elasticity of PCC materials gives low deflections when it is under loading. The rigid pavements can be analyzed by the plate theory. Rigid pavements can have reinforcing steel, which is generally used to handle thermal stresses to reduce or eliminate joints and maintain tight crack widths. Figure 2.2 shows a typical section for a rigid pavement.
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Figure 2.1 Rigid Pavement (kresge 2009)
2.3.3 Rigid and flexible Pavement Characteristic
The primary structural difference between a rigid and flexible pavement is the manner in which each type of pavement distributes traffic loads over the subgrade. A rigid pavement has a very high stiffness and distributes loads over a relatively wide area of subgrade - a major portion of the structural capacity is contributed by the slab itself (Thomas, 2008 in Sufian 2011). The load carrying capacity of a true flexible pavement is derived from the load-distributing characteristics of a layered system (Thomas,2008 in Sufian 2011). Figure 2.2 shows load distribution for a typical flexible pavement and a typical rigid pavement.
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Figure 2.2 Typical stress distributions under a rigid and a flexible pavement.
Pavement distributes wheel loads over an area of subgrade. The distribution of loads are much larger than the wheel contact area thus to reduce the maximum subgrade stress to a level which the soil can tolerate without unacceptable deformation during the life of the pavement (Kamarulzaman, 2010 in Sufian 2011 ). This is explained in Figure 2. 3.
Figure 2.3: Concept of load spreading and the ideal pavement
2.4 Skid resistance
Generally skid resistance is known as force developed when tire prevented from rotating slides long the pavement surface. It could also come associated with vehicle tire air pressure, temperature, composition, treads pattern and depth. These factors contribute to the level of strength in the interaction generated between the tire and the surface (David, 2005). Factor influencing road surface frictions are shown below.
Table 2.1: Factors Influencing Road Surface Friction, David (2005)
Skid resistance changes over time as it increases in the first two years following construction as the roadway is worn away by traffic and rough aggregate surfaces become exposed, then decreases over the remaining pavement life as aggregates become more polished.
2.5 Factor Affecting Skid Resistance
Friction is dependent upon the pavement macro and microtexture. Microtexture has greater influence on friction at the low speeds areas. Macrotexture becomes dominant at higher speeds, though microtexture is still important. Macroctexture supplies the paths through which water can escape from between the tire and road surface, thereby allowing the microtexture to provide resistance to the relative movement between the tire and the road surface. (Cement and Concrete Association of Australia)
Specifically, the skid resistance properties of Portland Cement Concrete (PCC) pavement are affected by several key factors including aggregate type, aggregate gradation (size), water cement ration, air content, curing method as well as the surface finishing method to form texture. According to Meyer (1982) cited by Kelvin 2003, friction levels developed by a pavement in contact with a given tire are largely dependent on the pavement on the pavement surface texture, which can be classified into two sub-groups according to its scale; macrotexture and microtexrture. Factors influencing road surface are shown below
Table 2.2: Factors Influencing Road Surface Friction (Sufian, 2011)
Road
Contaminant (fluid)
Type
Micro-texture
Macro-texture
Unevenness/Mega-texture
Chemistry of material
Temperature
Thermal conductivity
Specific heat
Chemical structure
Viscosity
Density
Temperature
Thermal conductivity
Specific heat
Film thickness
Tread pattern design
Rubber composition
Inflation pressure
Rubber hardness
Load
Sliding velocity
Temperature
Thermal conductivity
Specific heat
According to CCAA (Cement Concrete and Aggregate Australia) 2002, skid resistance in concrete structure describes as the ability of concrete surface to provide friction to a reference tyre or slider usually measured wet. Skid resistance is primarily dependent upon the surface macro and microstructure. In the other hand, factor affecting skid resistance or friction and other parameter in concrete pavement are:
Environmental condition including temperature and surface
contaminant;
Physical properties of paving material;
Age of pavement and traffic volume;
Seasonal condition such as amount of rain.
2.6. Road Surface and Texture
Macrotexture and microtexture are two type of road surface texture that influences skidding. Macrotexture is required to eliminate water from the contact area between the tire and the road surface especially at higher vehicle speeds and is determined by the size of the aggregate particles at the road surface. The micro texture is determined by the roughness and angularity of the surface of the aggregate particles. The microtexture ensures high contact pressures between the aggregate and the tire. (Molenaar et al. 2004, in Sufian 2011). Texture of both macro and micro as shown below.
Figure 2.6 Comparison between macrotexture and microtexture
Microtexture is the term given to the finer texture which can be felt by running a finger over the surface of a stone particle. Microtexture can help improve friction since the sharp peaks can break through water films and thus allow some degree of adhesion friction to be restored. When a surfacing is newly laid, the aggregate particles will commonly have acceptable microtexture. This is reduced with time over the first few months or years due to the polishing effect of vehicle tires, particularly those of heavy vehicles. It is therefore necessary to assess the suitability of an aggregate for a particular environment by subjecting it in the laboratory to a simulated road polishing regime.
2.7 Pavement-Tire Interaction
Pavement skid resistance is defined as the ability of a travelled surface to prevent the loss of traction (ASTM E 867). The term "skid resistance" can be applied to any measurement taken concerning the frictional properties of pavement surfaces. To be able to fully comprehend the skid resistance of a tire interacting with the pavement surface, it is important to understand the forces at the tire-pavement interface. These forces are complex and dependent upon tire size, type of braking system, vehicle weight, and other vehicle specific characteristics.
A simple block with a known velocity can be used to model frictional forces. The value of friction can be established using the force normal to the block and the counteractive force occurring as a result of the frictional interaction between the object and the pavement as shown in Figure 2.8. The resistant force FR is a function of both the frictional properties of the surface and the change in velocity of the sliding block. For a constant speed, the coefficient of friction can be computed using the following equation (Davis, 2001)
Figure 2.8: Simplified Friction Diagrams
2.8 Friction and Texture Measurements
There are many different methods and equipment to measure the surface properties of pavements. The frictional characteristics of pavement surfaces have been measured quantitatively for many years. The results of friction testing are used to compare changes in skid resistance over time or to determine the level of safety of pavement surfaces.
2.8.1 Measurement of Skid Resistance
British Pendulum Test is used to test skid resistance. It is a pendulum impact type tester specified in ASTM E303. The British pendulum tester is one of the simplest and cheapest instruments used in the measurement of friction characteristics of pavement surfaces.
According to Fwa et al., 2003, Although it is widely suggested that the measurement is largely governed by the microtexture of the pavement surface, experience has shown that the macrotexture can also affect the measurements.
Fwa et al. (2003) claims that the British pendulum measurements could be affected by the macrotexture of pavement surfaces, aggregate gap width, or the number of gaps between aggregates. It can also lead to misleading results on coarse-textured test surfaces. Skid resistance is generally quantified using some form of friction measurement such as friction factor or skid number. According to Jayawickrama et al., 1996, cited by Davis, 2001, the measurement is as follow:
Friction factor (like a coefficient of friction):
f = F/L equation 2.1
Skid number: SN = 100(f) equation 2.2
Where: F = frictional resistance to motion in plane of interface
L = load perpendicular to interface
Portable testers are available to measure the frictional properties of pavement surfaces. These testers use a pendulum or slider theory to measure friction in a laboratory or in the field. During testing with the British Portable Tester, a pendulum is released from a specified height and a rubber slider attached to the end of the pendulum contacts the pavement surface. The retardation of the pendulum motion resulting from the frictional properties of the test surface is used to establish the British Pendulum Number (BPN). The values of the BPN vary from zero to 140. The BPN is recorded using a specially constructed scale located on the tester, which measures the height of the pendulum after contacting the surface (ASTM E303).The table below shows some typical Skid Numbers (the higher the SN, the better).
Table 2.5: Typical Skid Numbers
Skid Number
Comments
< 30
Take measure to correct
> 30
Acceptable for low volume roads
31 - 34
Monitor pavement frequently
> 35
Acceptable for heavily traveled roads
2.8.3 Surface Texture Measurement
Surface texture measurement is tested with Sand Patch Test (ASTM E 965). Quantity of sand is poured onto dry road surface and spread into a circular pattern with straightedge. As the sand is spread, it fills the low spots in the pavement surface. When the sand cannot be spread further, the diameter of the resulting circle is measured. The diameter can be correlated to an average depth, which can be correlated to skid resistance. A texture depth of about 1.5mm (0.06 inches) is normally required for heavily trafficked areas.
Standard on Skid Resistance
British Portable Skid Resistance Tester is used in determining the skid resistance value of a pavement taken on site or in laboratory. The impetus for this research came from the Transport and Research Laboratory (TRRL) of United Kingdom. According to TRRL, the device simulates the friction of road surface to travelling vehicle of 50 km/hr. The test gives the mean skid resistance value (SRV) which is percentage of coefficient of friction.
Accelerated Polishing Machine on the other hand stimulates the polishing action of tires on road surfaces. Specimens are polished by rubber tire, and then measured with pendulum to get the Polished Stone Value (PSV). Different sites may require different values of PSV, thus, table below, as provided by TRRL specifies the minimum values of PSV for the aggregate used in the surfacing.
Table 2.7: Suggestion minimum values of 'skid resistance' that is measured with the portable tester (TRRL)
Category
Type of site
Minimum skid resistance (surface wet)
A
Difficult sites such as:
Roundabouts
Bends with radius less than 150m on unrestricted roads
Gradients 1 in 20 or steeper of lengths > 100m
Approaches to traffic lights on unrestricted roads
65
B
Motorways, trunk and class 1 roads and heavily trafficked roads in urban areas (carrying > 2000 vehicles per day)
55
C
All other sites
45
Temperature is also known to influence the measurement of skid resistance. Taking hat into consideration, figure below are to be used for corrected value.
Figure 2.14: Skid resistance/ temperature correction relationship (TRRL, 1969)
