# Surface Course 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. # I. General Principles of Flexible Pavement Design 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 # II. AASHTO Design Method Considerations # 2-axle AADT1 AADT2 AADT3 N1 N2 N3 F Ei1 F Ei2 F Ei3 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: # ii. Base Course Construction Materials Materials selected should satisfy the general requirements for base course materials, A structural layer coefficient, a2, for the material used also should be determined. # iii. Surface Course Construction Materials 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. # Ex 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. # Ex1: 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. ![Performance ? Structural performance is related to the physical condition of the pavement with respect to factors that hurt the capability of the pavement to carry the traffic load. ? These factors include cracking, faulting, unraveling, and so forth.Functional performance is an indication of how effectively the pavement serves the user. ? The main factor considered under functional performance is riding comfort. ? To quantify pavement functional performance, a concept known as the serviceability performance was developed.And Under this concept, a procedure was developed to determine the present serviceability index (PSI) of the pavement, based on its roughness and distress, which were measured in terms of the extent of cracking, patching, and rut depth for flexible pavements. ? Two serviceability indices are used in the design procedure:1. The initial serviceability index (pi) is the serviceability index immediately after the Construction of the pavement. 2. The terminal serviceability index (pt) is the minimum acceptable value before reconstruction is necessary.Global Journal of Researches in EngineeringVolume Xx XII Issue II V](image-2.png "") ![single-unit trucks (2000 lb/axle) = 50% 2-axle single-unit trucks (6000 lb/axle) = 33% 3-axle single-unit trucks (10,000 lb/axle) = 17% And the traffic growth rate is 4% for all vehicles, design period of 20 years, assume SN = 5, and the percent of the traffic on the design lane is 45%; determine the design & Total ESAL?2-AADT 1 = 50% * 12000 = 6000 AADT 2 = 33% * 12000 = 3960 AADT 3 = 17% * 12000 = 2040 Find ?? ???? from table 19.3a 1-2000 lb/axle = 2 kips/axle F Ei1 = 0.0002 6000 lb/axle = 6 kips/axle F Ei2 = 0.01 10000 lb/axle = 10 kips/axle F Ei3 = 0.088 3-n = 20 years r = 4% from table 19.4, ?? ???? = 29.78 Design ESAL = [(6000 × 2 × 0.0002) + (3960 × 2 × 0.01) + (2040 × 3 ×0.088)] × 29.78 × 365 × 0.45 = 3033429 = 3.03×10 6](image-3.png "") ![(3-axle tandem, 4-single axle) trucks (10000 lb/axle) = 60% 2-axle single-unit trucks (5000 lb/axle) = 30% (3-axle single-unit, 2-axle tandem) trucks (7000 lb/axle) = 10% Determine : a-Daily Design ESAL b-Total ESAL for the First year c-Total ESAL d-Design ESAL for (10000 lb/axle) e-Daily Total ESAL for (7000 lb/axle) Solution : 1-10000 lb/axle = 10 kips/axle F Ei1 single = 0.102 F Ei1 tandem = 0.009 5000 lb/axle = 5 kips/axle F Ei2 single = = 60% × 6000 = 3600 AADT2 = 30% × 6000 = 1800 AADT3 = 10% × 6000 = 600 3-n = 20 years r = 5% ?? ???? = 33.06 a-Daily Design ESAL Daily Design ESAL = AADT × N × F Ei × f d DDESAL = [ 3600(3× 0.009 + 4 × 0.102) + 1800(2 × 0.008) + 600(3×0.027+2× 0.0025) ] * 0.45 = 741 b-Total ESAL for First year Total ESAL the First Year = AADT × N × F Ei × 365 = [ 3600(3× 0.009 + 4 × 0.102) + 1800(2 × 0.008) + 600(3×0.027+2× 0.0025) ] * 36 = 600936 c-Total ESAL Total ESAL = AADT × N × F Ei × G rn × 365 = [ 3600(3× 0.009 + 4 × 0.102) + 1800(2 × 0.008) + 600(3×0.027+2× 0.0025) ] * 365 * 33.06 = 19866944 = 19.9 × 10 6](image-4.png "") ![Figure 19.3](image-5.png "Figure") ![Figure 19.4](image-6.png "Figure") 1![Find Roadbed Resilient Modulus ?? ð??"ð??" When Relative Damage ?? ?? 7.23? 2-Find Roadbed Resilient Modulus ?? ð??"ð??" For mean Relative damage ?? ?? ? -Using the Equation: ?? ?? = 1.18 × ???? ?? × ?? ð??"ð??" ???.???? 1-0.723=1.18 × 10 8 × M ?? ?2.32 ?? ð??"ð??" = 1285 lb/???? ?? 2-Mean ?? ?? = U 0.133U = 1.18 × 10 8 × M ?? ?. ( ?? ?? )](image-7.png ": 1 -") ![Pavement drain the base layer to 7% saturation.](image-8.png "U") ![Ex: Designing a Flexible Pavement Using the AASHTO Method A flexible pavement for an urban interstate highway R =95%, and Standard deviation ?? ?? =0.45 is to be designed to carry a design ESAL of 2×???? ð??"ð??" . It is estimated that it takes about a week for the water to be drained from within the pavement, and the pavement structure will be exposed to moisture levels approaching saturation for U 30%U of the time. The following additional information is available: Initial serviceability index ?? ?? =4.5 Final serviceability index ?? ?? =2.5 Resilient modulus of asphalt concrete at 68°F = 450,000 lb/in 2 CBR value of base course material = 100, M r = 31,000 lb/in 2 CBR value of subbase course material = 22, M r = 13,500 lb/in 2 CBR value of subgrade material = 5 Solution: 1-Find Drainage Coefficient ?? ?? from table 19.5, 19.6 Drainage Coefficient ?? ?? for base and subbase layer = 0.8 M r = 1500 * CBR = 1500*5 = 7500 lb/in 2](image-9.png "") ![](image-10.png "") ![](image-11.png "") ![](image-12.png "") ![](image-13.png "") ![](image-14.png "") ![](image-15.png "") ![](image-16.png "") ![](image-17.png "") Analytical Study & Design of Flexible PavementTable 19.4: Growth Factorstable 19.3aFEiSingle Axlewhen pt = 2.5 ESAL = AADT ×N × ?? ???? × ?? ???? × 365 × ð??"ð??" ?? table 19.3b Tandem AxleWhere:Year 2022ESAL = Equivalent Accumulated 18,000-lb (80 KN) single-axle load. AADT = First year annual average daily traffic.58N = 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 × ?? ???? × ?? ???? × 365Global Journal of Researches in Engineering ( ) EEX: If * From EquationFirst 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-?? ???? From table 19.3a & 19.3b 19.3a: Axle Load Equivalency Factors for Flexible Pavements, Single Axles, and p t of 2.5 193b19 195 196Analytical Study & Design of Flexible Pavement 1971-By Equation?????? 10 ( ( ???? +1 ) 5.19 ??????? 2.7 ) 0.4+ 1094+2.32?????? 10 (?? ?? )-8.07Where:W18 = predicted number of 18,000-lb (80 KN) single-axle load applications ?PSI = ?? ?? -?? ?? SN = structural number indicative of the total pavement thickness ?? ?? = standard normal deviation for a given reliability ?? ?? = overall standard deviation 198: Standard Normal Deviation (Z R ) Values Corresponding to Selected Levels of Reliability Analytical Study & Design of Flexible Pavement © 2022 Global Journals 19.5: Definition of Drainage QualityTable 19.6: Recommended m i Values Analytical Study & Design of Flexible Pavement Analytical Study & Design of Flexible Pavement © 2022 Global Journals © 2022 Global Journals ( ) © 2022 Global Journals * Principles of Pavement Design Yoder Ej (ch1 2 * TrafficHighway NEngineering Garber 20 19