7 PHILOSOPHY OF PAVEMENTS Pavements are subjected to moving traffic loads that are repetitive in nature.Each traffic load repetition causes a certain amount of damage to the pavement structure that gradually accumulates over time and eventually leads to the pavement failure.Thus, pavements are designed to perform for a certain life span before reaching an unacceptable degree of deterioration.In other words, pavements are designed to fail. Hence, they have a certain design life.
8 PAVEMENT TYPES Flexible Pavement Hot mix asphalt (HMA) pavements Called "flexible" since the total pavement structure bends (or flexes) to accommodate traffic loadsThe load transmit to the subgrade through particle to particle contact.Rigid PavementPortland cement concrete (PCC) pavementsCalled “rigid” since PCC’s high modulus of elasticity does not allow them to flex appreciablyThe load transmit to subgrade through beam action.
10 RIGID PAVEMENTSIn rigid pavements the stress is transmitted to the sub-grade through beam/slab effect. Rigid pavements contains sufficient beam strength to be able to bridge over localized sub-grade failures and areas of inadequate support.Thus in contrast with flexible pavements the depressions which occur beneath the rigid pavement are not reflected in their running surfaces.
12 Design of Pavement Structure Section - 2Design of Pavement Structure
13 PAVEMENT THICKNESS DESIGN Pavement Thickness Design is the determination of required thickness of various pavement layers to protect a given soil condition for a given wheel load.Given Wheel Load150 Psi3 PsiGiven In Situ Soil ConditionsAsphalt Concrete Thickness?Base Course Thickness?Subbase Course Thickness?
15 Picture from University of Tokyo Geotechnical Engineering Lab SUBGRADECharacterized by strength and/or stiffnessCalifornia Bearing Ratio (CBR)Measures shearing resistanceUnits: percentTypical values: 0 to 20Resilient Modulus (MR)Measures stress-strain relationshipUnits: psi or MPaTypical values: 3,000 to 40,000 psiPicture from University of TokyoGeotechnical Engineering Lab
16 SUBGRADE Some Typical Values Classification CBR MR (psi) Typical DescriptionGood≥ 1020,000Gravels, crushed stone and sandy soils.Fair5 – 910,000Clayey gravel and clayey sand, fine silt soils. Poor3 – 55,000Fine silty sands, clays, silts, organic soils.
17 TRAFFIC LOADS CHARACTERIZATION Pavement Thickness Design Are DevelopedTo Account For The EntireSpectrum Of Traffic LoadsCarsPickupsBusesTrucksTrailers
18 Axle loads bigger than 8.2 tons cause damage greater than one per pass EQUIVALENCY FACTOREquivalentStandard ESALAxle LoadIbs(8.2 tons)Damage per Pass = 1Axle loads bigger than 8.2 tons cause damage greater than one per passAxle loads smaller than 8.2 tons cause damage less than one per passLoad Equivalency Factor (L.E.F) = (? Tons/8.2 tons)4
19 Consider two single axles A and B where: EXAMPLEConsider two single axles A and B where:A-Axle = 16.4 tonsDamage caused per pass by A -Axle = (16.4/8.2)4 = 16This means that A-Axle causes same amount of damage per pass as caused by 16 passes of standard 8.2 tons axle i.e.,16.4 Tons Axle=8.2 Tons Axle
21 SERVICEABILITY CONCEPT OF PAVEMENT Serviceability is the ability of a pavement to serve the commuters for the desired results for which it has been constructed within the designed life and without falling the Terminal level (TSI).Present Serviceability Index (PSI)Present Serviceability is defined as the adequacy of a section of pavement in its existing condition to serve its intended use.Terminal Serviceability Index (TSI)It is defined as that stage of the pavement condition after which it is not acceptable for the road users.
22 SERVICEABILITY CONCEPT OF PAVEMENT Defined by users (drivers)Develop methods to relate physical attributes to driver ratingsResult is usually a numerical scaleFrom the AASHO Road Test(1956 – 1961)
23 Present Serviceability Index (PSI) Values from 0 through 5Calculated value to match PSRSV = mean of the slope variance in the two wheel paths (measured with the profile meter)C, P = measures of cracking and patching in the pavement surfaceC = total linear feet of Class 3 and Class 4 cracks per 1000 ft2 of pavement area A Class 3 crack is defined as opened or spilled (at the surface) to a width of in. or more over a distance equal to at least one-half the crack length A Class 4 is defined as any crack which has been sealed.P = expressed in terms of ft2 per 1000 ft2 of pavement surfacing.
24 PSI vs. Timep0Serviceability (PSI)p0 - ptptTime
25 PAVEMENT THICKNESS DESIGN Comprehensive Definition Pavement Thickness Design is the determination of thickness of various pavement layers (various paving materials) for a given soil condition and the predicted design traffic in terms of equivalent standard axle load that will provide the desired structural and functional performance over the selected pavement design life i.e. the serviceability may not fall below the TSI.
27 Flexible PavementsA flexible pavement absorbs the stresses by distributing the traffic wheel loads over much larger area, through the individual layers, until the stress at the subgrade is at an acceptably low level. The traffic loads are transmitted to the subgrade by aggregate to aggregate particle contact. A cone of distributed loads reduces and spreads the stresses to the subgrade.
28 TYPICAL LOAD & STRESS DISTRIBUTION IN FLEXIBLE PAVEMENTS. Wheel LoadBituminous LayerVertical stressFoundation stressSub-grade
29 EFFECT OF PAVEMENT THICKNESS ON STRESS DISTRIBUTION
30 BASIC EQUATION OF AASHTO PROCEDURE FOR FLEXIBLE PAVEMENT DESIGN. The various terms/parameters which are used in the basic equation of AASHTO Procedure for the Design of flexible pavements are:i). W18 (ESAL): It is the accumulated traffic load converted to 18-kips or 8.2 tons. This is also known as 18-kips Equivalent Standard Axle Load (ESAL). That the pavement will experience over its design life.
31 ii). Standard Deviation (S0): Standard deviation accounts for standard variation in materials and construction, the probable variation in traffic prediction and variation in pavement performances for a given design traffic application. The recommended value of S0 for flexible pavement is 0.4 to 0.5.iii) Reliability (R):Design Reliability refers to the degree of certainty that a given pavement section will last for the entire design period with the traffic & environmental condition. The recommended level of reliability for freeways in rural areas varies from 80% to 95%. A high reliability value will increase the thickness of pavement layer and will result in expensive construction.
32 TABLE FOR REALABLILITY Recommended Level of Reliability ( R ) Functional ClassificationReliability (%)UrbanRuralInterstate and Other FreewaysPrincipal Arterial80-9975-95Feeders80-95Local50-80
33 Standard Normal Deviate (ZR) iv). Standard Normal Deviate (ZR): It is defined as the probability that serviceability will be maintained at adequate levels from a user’s point of view throughout the design life of the facility. This factor estimates the probability that the pavement will perform at or above the TSI level during the design period and it accounts for the inherent uncertainty in design. The relationship of reliabilities with ZR is given in the table:Value of (ZR)Reliability R (%)Standard Normal Deviate (ZR)500.00060-0.25370-0.52475-0.67480-0.84185-1.03790-1.28291-1.34092-1.40593-1.47694-1.55595-1.64596-1.75197-1.88198-2.05499-2.32799.9-3.09099.99-3.750
34 Definition of Structural Number v). Structural No (SN):Structural No is the total structural strength value required to cater for the cumulative equivalent standard axles load (CESAL) during design life so that the serviceability may not fall below the Terminal serviceability Index (TSI)Definition of Structural NumberSubgradeStructural Coefficient (a):a = fnc (MR)SN = SN1 + SN2 + SN3
35 vi) Loss of Serviceability Index ∆ PSI. ∆ PSI = Initial Serviceability Index – Terminal Serviceability IndexThe recommended value for initial serviceability index is 4.2 while for terminal serviceability index it is to 2 to 2.5.∆ PSI = 4.2 – 2.5 = 1.7p0Serviceability (PSI)p0 - ptptTime
36 vii). Resilient Modulus (MR): It is defined as repetitive or cyclic stress divided by recoverable strain. Resilient Modulus is a measure of stiffness of the soil.MR = Repetitive stress / recoverable strainMR can be determined from the resilient modules test in thelaboratory or from the following equations:MR = 1500 * CBR for CBR < 10 %MR = 2555 (CBR)0.63 for any value of CBR
37 viii). Computation of Required Pavement Thicknesses The structure Number (SN) requirement as determined through adoption of design parameters as discussed above is balanced by providing adequate pavement structure. Under AASHTO design procedure the following equation provides for the means for converting the structural number into actual thickness of surfacing, base and sub base materials.SN = a1D1 + a2D2m2 + a3D3m3 _______________ (2)a1, a2, a3 = Layer coefficients representative of surface, baseand subbase courses respectively. It depends upon the modulus of resilient.D1. D2, D3 = Actual thicknesses (in inches) of surface, base and subbase courses respectively.m2, m3 = Drainage coefficients for base and subbase layers respectively.
38 This equation does not have a single unique solution. There are many combinations of layer thicknesses that can be adopted to achieve a given structural number. There are, however, several design, construction and cost constraints that may be applied to reduce the number of possible layer thicknesses combinations and to avoid the possibility of constructing an impractical design. According to this approach, minimum thickness of each layer is specified to protect the under lying layers from shear deformation.ix). Recommended Value of Layer CoefficientsAsphaltic Wearing Course, a1 = 0.44/inch (0.1732/cm)Asphaltic Base Course, a1 = 0.40/inch (0.1575/cm)Water Bound Macadam, a2 = 0.14/inch (0.0551/cm)Granular Subbase, a3 = 0.11/inch (0.0433/cm)OR Nomograph can be used to work out SN.
41 How to DesignStep 1. Fix the design life of the pavement.Step 2. Work out MR value of the subgradeMR = CBR for CBR <10%ORMR = (CBR)0.63 for CBR > 10Work out MR in the laboratory.Step 3. Conduct 7-days traffic count.Step 4. Classify the traffic and consider the commercial vehicles i.e. Bus, Tractor , Trolley, 2-Axle, 3-Axle, 4-Axle, 5-Axle and 6-Axle Trucks.Step 5. Take Growth rate from the table on the next slide.
42 Growth Rate S. No Vehicle Class Growth Rate 1 Bus 8.4% 2 Tractor Trolley7.9%3Mini Truck42-Axle7.0%53-Axle (Single)63-Axle (Tandem)74-Axle85-Axle96-Axle
43 CONVERT THE TRAFFIC TO EQUIVALENT STANDARD AXLE LOAD. ESAL = TRAFFIC X EQUILLANCY FACTOR , EQUIVALENCY FACTOR FOR VARIOUS CLASSES OF VEHICLES ARE GIVEN IN THE FOLLOWING TABLE.S. NoVehicle ClassEquivalency Factor (Empty)Equivalency Factor (Loaded)1Bus0.9392Tractor Trolley0.11.193Mini Truck0.01722.59642-Axle0.0436.4953-Axle (Single)0.07216.6263-Axle (Tandem)18.4874-Axle0.20619.0085-Axle0.08419.5996-Axle0.16527.96
44 Total Traffic for 10 Years Calculation of CESALVehicle TypeADTAnnual TrafficGrowth Rate %Growth FactorTotal Traffic for 10 YearsESA FactorCESAL for 10 Years80%20%LoadedEmptyBuses207300613.1896,2140.93972,276Tractor Trolly13950735668,6871.190.1649,964Trucks2XL5001825002,405,3506.490.04312,509,263Trucks 3XL2509125018.480.07217,797,666
45 Cumulate the future traffic throughout the design life with the help of the selected growth rate. Following is the simple relation to project the traffic to any selected year.Pn = (1 + r)n – 1WherePn = Projected traffic for nth yearr = Growth raten = year of considerationAdd all the yearly traffic from base year to the last year of the design life.
46 Step 6.Fix the parameter like R, ZR, So, ∆ PSI etc.The generally taken value of the above parameters is listed below:∆ PSI = 1.7R = 90%So = 0.45ZR =Step 7.Put these values in equation 1 and use trial & error method or Nomograph to work out the SNSN = a1D1+a2D2m2+a3D3m3Step 8.Take the value of m2 and m3 from the table on the next slide.
47 TABLE FOR QULALITY OF DRAINAGE QUALITY OF DRAINAGEQuality of DrainageWater Removed withinExcellent2 hoursGood1 dayFair1 weekPoor1 monthVery Poorwater will not drain
48 Quality of DrainagePercent of Time Pavement Structure is exposed to Moisture Levels Approaching SaturationLess ThanGreater Than1%1 - 5%2 - 25%25%Excellent1.20Good1.00Fair0.85Poor0.60Very Poor0.40
49 Select the most appropriate and economical combination of thicknesses. Put the above values in equation at step No. 07, to find out the various combination of thicknesses, keeping in view the minimum thicknesses requirements as mentioned below:Minimum Asphalt wearing course thickness = 5 CmMinimum asphaltic base course thickness = 7.5 CmMinimum unbound base course thickness = 15 CmMinimum unbound sub base thickness = 15 CmSelect the most appropriate and economical combination of thicknesses.
51 Practical ExampleLet us work out the thicknesses of various layers for the Pavement of Topi Bypass Road.The ADT is given as follow.Vehicle TypeADTCOASTER/ FLYING COACH250BUSSES25Tractor Trolley36Trucks2XL110Trucks 3XL2Trucks 4XL5
52 THE CESAL IS WORKED OUT AS FOLLOW:- Vehicle TypeADTAnnual TrafficGrowth Rate %Growth FactorTotal Traffic for 10 YearsESA FactorESAL for 10 Years80%20%LoadedEmptyCOASTER/FLYING COACH250912508.414.781,348,6750.9391,013,125BUSSES259125134,868101,312Tractor Trolly36131407.914.423189,5181.190.1184,212Trucks2XL11040150713.82554,8736.490.0432,885,673Trucks 3XL273018.480.072149,295Trucks 4XL51825190.385385,309Total4,718,925ESAL by taking 100 % of directional factor=4.719millionESAL by taking 80 % lane factor3.775CESAL =3.775 million
53 California Bearing Ratio (CBR)= 30 % at 95% MDD MR= (CBR)0.63 for CBR > 10Putting the value CBR , MR= PsiKeeping the value of various parameters as follow.R= 90%So=0.45∆Psi=1.7Using Nomograph to work out the SN
55 Using the following the equation SN = a1D1+a2D2m2+a3D3m3Given Data:a1=0.44,a2=0.14 ,a3=0.11 andm2,m3=1from Nomograph SN= 3.35Putting these values and assuming d1=2 inch, d2=10 inch and d3=10 inch3.35=0.44*2+0.14* *103.35≈3.38Hence the Design thickness areACWC= 5cmWBM=25cmGranular sub base=25cm
57 SERVICEABILITY AND PERFORMANCE CONCEPT • AASHO Road Test performance based on user assessment:– Difficult to quantify (subjective)– Highly variable– Present Serviceability Rating (PSR)Performance Measurements0-1 – V. Poor1-2 – Poor A panel of experts drove around in standard2-3 – Fair vehicles and gave a rating for the pavement3-4 – Good4-5 – V. Good• Measurable characteristics (performance indicators):– Visible distress (cracking & rutting)– Surface friction– Roughness (slope variance)