Lecture 20 Flexible Pavement DesignA flexible road is composed of successive layers of weaker materials. For an asphalt pavement this is a layer of asphalt followed by layers of base and sub base over the sub grade soils. An example is shown in Typical Section.English weights2-axle Truck factor = (.57*365)/150 = .57ESAL/year for each=985ESAL/year for each 3 axle truck= 902ESAL/year for each 5 axle truckTruck factor = (282.95-0.36)*365/260Considering all traffic= 81.97ESAL/year for each vehicle per dayAADT = Annual Average Daily TrafficGi = growth factorLecture 20 Flexible Pavement Design A flexible road is composed of successive layers of weaker materials. For an asphalt pavement this is a layer of asphaltfollowed by layers of base and sub base over the sub grade soils. An example is shown in Typical Section.Flexible pavement design is based on the concept that stress developed by the surface load is dissipated with depth and it is necessary to provide material that have adequate strength to resist the stress provided by the load. This usually results in a layered system with progressively weaker materials.Since there are so many types of vehicles with different loads, axle configuration, and tire combination, it is standardpractice to convert all loading to a standard loading using load factors.Load factors were developed from the Ottawa road test completed in the late 1950's and are given in Table 20.3 (partial Table). There are other ways of developing load factors such as using load or fatigue relationships. For a load factor directly proportional to an 18 kip load the factor would be1181181818xxxxWLFLFwithLFWLF The Ottawa Road test used a different approach to obtain the load factors. In this case the load factors were determined from the ratio of repeated loads to cause anequal increment of reduction in Pavement Serviceability Index (PSI). It looks like the following:xKipKipxKipkipxPSIPSIwithpNoLoadpNoLoadLF1818ReReTo convert traffic to ESAL (Equivalent single axle loads) use the following equationESALx = No axle loads (load x) X load factor(load x) X 365 X Growth factorWhere the load factor is from table 20.3 or is approximated by the equations.44)34(tan)18(sinxfactorxfactorWESALdemWESALgle English weightsUsing the approximation, at 10 kips the single axle value is 0.095 compared to 0.0877 from Table 20.3 and for tandem axle is .0075 compared to 0.00688 in the tableThe Growth factor reflects the annual growth of traffic and is calculated asn is the number of years and r is the annual growth rate.Typical example using the axle weight and distribution from the AASHTO road test is:Calculation of Equilivent Single Axle LoadsVehicle Vehicle/day Single Axle TandemAxle FactorType No Weight No Weight single Tandem ESAL(18Kip)21 100 200 2 0.0002 0.0422 50 50 2 0.0002 0.01 50 6 0.0104 0.5231 25 25 4 0.0021 0.05 25 12 0.189 9.4532 30 30 6 0.0104 0.31 60 24 0.26 15.641 10 10 6 0.0104 0.1 20 18 1 2042 10 10 9 0.0562 0.56 20 32 0.857 17.1451 5 5 6 0.0104 0.05 10 22 2.18 21.852 20 20 9 0.0562 1.12 40 40 2.08 83.261 5 5 9 0.0562 0.28 10 30 6.97 69.762 5 5 12 0.189 0.95 10 48 4.17 41.7Passenger1000 2000 2 0.0002 0.361260 282.94It is possible to calculate truck factors for 2,3 or more axles by the same technique. For example the following:2-axle Truck factor = (.57*365)/150 = .57ESAL/year for each2 axle truck(note the units)3-axle truck factor = (9.5+20.1+21.05+69.98)*365/(25+10+5+50)=985ESAL/year for each 3 axle truck5-axle truck Factor = (15.91+17.70+84.32+42.65)*365/(30+10+20+5)= 902ESAL/year for each 5 axle truckThe overall truck factor is Truck factor = (282.95-0.36)*365/260= 396.71ESAL/year for each truckConsidering all trafficTraffic factor = 282.95*365/1260= 81.97ESAL/year for each vehicle per dayA more general equation for calculating ESAL isAADT = Annual Average Daily TrafficFd = directional distribution Gi = growth factorTFi = Truck factor for axle number i.PI = Percent of trucks with axle number i.A Calculator for ESAL determination is available at Calculator for