Constructed immediately above the base course, usually consists a mixture of aggregates and asphalt. It should be capable of withstanding high tire pressures, resisting abrasive forces due to traffic, providing a skidresistant driving surface, and preventing the penetration of surface water into the underlying layers.
Assumed initially that the subgrade layer is infinite in both the horizontal and vertical directions, whereas the other layers are finite in the vertical direction and infinite in the horizontal direction.
In the design of flexible pavements, the pavement structure usually is considered as a multilayered elastic system, with the material that may include the modulus of elasticity (E), the resilient modulus (R), and the Poisson ratio ( ).
Applying of a wheel load causes a stress distribution; the maximum vertical stresses are compressive and occur directly under the wheel load. These decrease with an increase in depth from the surface. In the AASHTO design method, the traffic load is determined in Terms of the number of repetitions of an 18,000-lb (80 KN) the Single-axle load applied to the pavement. This is usually referred to as: Ex 1 : Traffic (AADT) in both directions on the Highway during the first year of operation will be 12,000 with the following vehicle mix and axle loads. Single unit trucks
Ex 2:
The present AADT (in both directions) of 6000 vehicles is expected to grow at 5% per annum. Assume SN =4 and the percent of the traffic on the design lane is 55%, the design life is 20 years.
If the vehicle mix is:
Materials selected should satisfy the general requirements for base course materials, A structural layer coefficient, a2, for the material used also should be determined.
The structural layer coefficient (a1) relates to a dense graded asphalt concrete surface with its resilient modulus at 68°F. The effects of temperature on asphalt pavements include stresses induced by thermal action and the impact of freezing and thawing water in the subgrade.
The effect of temperature, particularly about to the weakening of the underlying material during the thaw period, is considered a significant factor in determining the strength of the underlying materials used in the design. The effect of rainfall is due mainly to the penetration of the surface water into the underlying material, if penetration occurs, the properties of the underlying materials may be altered significantly.
The resilient modulus of materials susceptible to frost action can reduce by 50 percent to 80 percent during the thaw period, and it is likely that the strength of the material will be affected during the periods of heavy rains.
The AASHTO guide suggests a method for determining the effective, resilient modulus. In this method, a relationship is then used to determine the resilient modulus for each season based on the estimated in situ moisture content and Relative damage during the period of time.
The relative damage ?? ð??"ð??" for each period is determined from the following chart, using the vertical scale or the equation given in the chart. The mean comparable damage ?? ð??"ð??" then computed, and the effective subgrade resilient modulus is determined using the Chart and value of ?? ð??"ð??" . The effect of drainage on the performance of flexible pavement is considered to the effect water has on the strength of the base material and roadbed soil, and The approach used is to provide for the rapid drainage of the water from the pavement structure by providing a suitable drainage layer and by modifying the structural layer coefficient.
The modification is carried out by adding a factor ?? ?? for the base and subbase layer coefficients (?? ?? and ?? ?? ). The ?? ?? factors are based both on the percentage of time during which the pavement structure will be nearly saturated, and on the quality of drainage, which is dependent on the time it takes to drain the base layer to 50 percent of saturation.
A flexible pavement takes U one dayU for water to be drained from within it and the pavement structure will be exposed to moisture levels approaching saturation for 7% of the time. Find the pavement drainage coefficient? The cumulative ESAL is an essential input to any pavement design method. However, the determination of this input is usually based on assumed growth rates which may not be accurate.
AASHTO guide proposes the use of a reliability factor that considers the possible uncertainties in traffic prediction and pavement performance prediction. For example, a 50% reliability design level implies 50% chance for successful pavement performance. Table 19.7 shows suggested reliability levels based on the AASHTO guide.
Analytical Study & Design of Flexible Pavement | |||
Table 19.4: Growth Factors | |||
table 19.3a | |||
FEi | Single Axle | ||
when pt = 2.5 ESAL = AADT ×N × ?? ???? × ?? ???? × 365 × ð??"ð??" ?? table 19.3b Tandem Axle | |||
Where: | |||
Year 2022 | ESAL = Equivalent Accumulated 18,000-lb (80 KN) single-axle load. AADT = First year annual average daily traffic. | ||
58 | N = Number of axles on each vehicle. | ||
Volume Xx XII Issue II V ersion I | ?? ???? = load equivalency factor for Single and Tandem axle. ?? ???? = Growth factor for a given growth rate r and design period n. 365 = Convert from day to year. ð??"ð??" ?? = Design lane factor. Name Equation ESAL Design lane AADT × N × ?? ???? × ?? ???? × 365 × ð??"ð??" ?? Total ESAL AADT × N × ?? ???? × ?? ???? × 365 | ||
Global Journal of Researches in Engineering ( ) E | EX: If * From Equation | First Year ESAL Design lane Total ESAL First Year Daily ESAL Design Lane Total Daily ESAL ?? ð??"ð??"ð??"ð??" = ( ??+ ð??"ð??" ) ð??"ð??" ??? ð??"ð??" = | AADT × N × ?? ???? × 365 × ð??"ð??" ?? AADT × N × ?? ???? × ð??"ð??" ?? AADT × N × ?? ???? ( ??+ ??.???? ) ?? ??? ??.???? = 11.03 AADT × N × ?? ???? × 365 |
Analytical Study & Design of Flexible Pavement |
1-By Equation | |
?????? 10 ( ( ???? +1 ) 5.19 ??????? 2.7 ) 0.4+ 1094 | +2.32?????? 10 (?? ?? )-8.07 |
Where: |