CEE 320 Winter 2006 Combination of various layers between road top surface / Finished Road Level (FRL) and the subgrade is known as pavement structure. Pavement Structure: PAVEMENT :
CEE 320 Winter 2006 PAVEMENT PURPOSE Load support Skid Resistance Good ride Less VOC Time Saving Drainage CHAPPAR - DARBAND ROAD (30 KM) PHASE-I
CEE 320 Winter 2006 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. PHILOSOPHY OF PAVEMENTS
CEE 320 Winter 2006 PAVEMENT TYPES Flexible Pavement –Hot mix asphalt (HMA) pavements –Called "flexible" since the total pavement structure bends (or flexes) to accommodate traffic loads –The load transmit to the subgrade through particle to particle contact. Rigid Pavement –Portland cement concrete (PCC) pavements –Called “rigid” since PCC’s high modulus of elasticity does not allow them to flex appreciably –The load transmit to subgrade through beam action.
CEE 320 Winter 2006 RIGID PAVEMENTS Thus in contrast with flexible pavements the depressions which occur beneath the rigid pavement are not reflected in their running surfaces. In 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.
CEE 320 Winter 2006 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 Load 150 Psi 3 Psi Given In Situ Soil Conditions Asphalt Concrete Thickness? PAVEMENT THICKNESS DESIGN
CEE 320 Winter 2006 Equivalent Standard ESAL Axle Load Ibs (8.2 tons) Damage per Pass = 1 Axle loads bigger than 8.2 tons cause damage greater than one per pass Axle loads smaller than 8.2 tons cause damage less than one per pass Load Equivalency Factor (L.E.F) = (? Tons/8.2 tons) 4 EQUIVALENCY FACTOR
CEE 320 Winter 2006 Consider two single axles A and B where: A-Axle = 16.4 tons Damage caused per pass by A -Axle = (16.4/8.2) 4 = 16 This means that A-Axle causes same amount of damage per pass as caused by 16 passes of standard 8.2 tons axle i.e., 8.2 Tons Axle 16.4 Tons Axle = EXAMPLE
CEE 320 Winter 2006 Serviceability 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. SERVICEABILITY CONCEPT OF PAVEMENT
CEE 320 Winter 2006 Defined by users (drivers) Develop methods to relate physical attributes to driver ratings Result is usually a numerical scale From the AASHO Road Test (1956 – 1961) SERVICEABILITY CONCEPT OF PAVEMENT
CEE 320 Winter 2006 Present Serviceability Index (PSI) Values from 0 through 5 Calculated value to match PSR SV = 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 surface C = total linear feet of Class 3 and Class 4 cracks per 1000 ft 2 of pavement area. A Class 3 crack is defined as opened or spilled (at the surface) to a width of 0.25 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 ft 2 per 1000 ft 2 of pavement surfacing.
CEE 320 Winter 2006 PSI vs. Time Time Serviceability (PSI) p0p0 ptpt p 0 - p t
CEE 320 Winter 2006 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. PAVEMENT THICKNESS DESIGN Comprehensive Definition
CEE 320 Winter 2006 Flexible Pavements A 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.
CEE 320 Winter 2006 Vertical stress Foundation stress TYPICAL LOAD & STRESS DISTRIBUTION IN FLEXIBLE PAVEMENTS. Bituminous Layer Wheel Load Sub-grade
CEE 320 Winter 2006 EFFECT OF PAVEMENT THICKNESS ON STRESS DISTRIBUTION
CEE 320 Winter 2006 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). W 18 (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.
CEE 320 Winter 2006 ii). Standard Deviation (S 0 ): 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 S 0 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.
CEE 320 Winter 2006 Recommended Level of Reliability ( R ) Functional Classification Reliability (%) UrbanRural Interstate and Other Freeways Principal Arterial Feeders Local50-80 TABLE FOR REALABLILITY
CEE 320 Winter 2006 iv). Standard Normal Deviate (Z R ): 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 Z R is given in the table: Value of (Z R ) Reliability R (%) Standard Normal Deviate (Z R )
CEE 320 Winter 2006 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 Number Subgrade Structural Coefficient (a): a = fnc (M R ) SN = SN1 + SN2 + SN3
CEE 320 Winter 2006 vi) Loss of Serviceability Index ∆ PSI. ∆ PSI = Initial Serviceability Index – Terminal Serviceability Index The 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 = 1.7 Time Serviceability (PSI) p0p0 ptpt p 0 - p t
CEE 320 Winter 2006 vii). Resilient Modulus (M R ): It is defined as repetitive or cyclic stress divided by recoverable strain. Resilient Modulus is a measure of stiffness of the soil. M R = Repetitive stress / recoverable strain M R can be determined from the resilient modules test in the laboratory or from the following equations: M R = 1500 * CBR for CBR < 10 % M R = 2555 (CBR) 0.63 for any value of CBR
CEE 320 Winter 2006 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 = a 1 D 1 + a 2 D 2 m 2 + a 3 D 3 m 3 _______________ (2) a 1, a 2, a 3 = Layer coefficients representative of surface, base and subbase courses respectively. It depends upon the modulus of resilient. D 1. D 2, D 3 =Actual thicknesses (in inches) of surface, base and subbase courses respectively. m 2, m 3 =Drainage coefficients for base and subbase layers respectively.
CEE 320 Winter 2006 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 Coefficients Asphaltic Wearing Course, a 1 = 0.44/inch (0.1732/cm) Asphaltic Base Course, a 1 = 0.40/inch (0.1575/cm) Water Bound Macadam, a 2 = 0.14/inch (0.0551/cm) Granular Subbase, a 3 = 0.11/inch (0.0433/cm) OR Nomograph can be used to work out SN.
CEE 320 Winter 2006 NOMOGRAPH
CEE 320 Winter 2006 Section - 4 How to Design
CEE 320 Winter 2006 How to Design Step 1.Fix the design life of the pavement. Step 2.Work out M R value of the subgrade M R = 1500 CBR for CBR <10% OR M R = 2555 (CBR) 0.63 for CBR > 10 OR Work out M R 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.
CEE 320 Winter 2006 S. NoVehicle Class Equivalency Factor (Empty) Equivalency Factor (Loaded) 1Bus Tractor Trolley Mini Truck Axle Axle (Single) Axle (Tandem) Axle Axle Axle 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.
CEE 320 Winter 2006 Vehicle TypeADT Annual Traffic Growth Rate % Growth Factor Total Traffic for 10 Years ESA Factor CESAL for 10 Years 80%20% LoadedEmpty Buses , ,276 Tractor Trolly , ,964 Trucks2XL ,405, ,509,263 Trucks 3XL ,797,666 Calculation of CESAL
CEE 320 Winter 2006 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. P n = (1 + r) n – 1 Where P n = Projected traffic for n th year r = Growth rate n = year of consideration Add all the yearly traffic from base year to the last year of the design life.
CEE 320 Winter 2006 Step 6. Fix the parameter like R, Z R, So, ∆ PSI etc. The generally taken value of the above parameters is listed below: ∆ PSI = 1.7 R = 90% So = 0.45 Z R = Step 7. Put these values in equation 1 and use trial & error method or Nomograph to work out the SN SN = a 1 D 1 +a 2 D 2 m 2 +a 3 D 3 m 3 Step 8. Take the value of m 2 and m 3 from the table on the next slide.
CEE 320 Winter 2006 QUALITY OF DRAINAGE Quality of DrainageWater Removed within Excellent2 hours Good1 day Fair1 week Poor1 month Very Poorwater will not drain TABLE FOR QULALITY OF DRAINAGE
CEE 320 Winter 2006 Quality of Drainage Percent of Time Pavement Structure is exposed to Moisture Levels Approaching Saturation Less Than Greater Than 1%1 - 5%2 - 25%25% Excellent Good Fair Poor Very Poor
CEE 320 Winter 2006 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 Cm –Minimum asphaltic base course thickness = 7.5 Cm –Minimum unbound base course thickness = 15 Cm –Minimum unbound sub base thickness = 15 Cm Select the most appropriate and economical combination of thicknesses.
CEE 320 Winter 2006 Section - 5 Practical Example
CEE 320 Winter 2006 Practical Example Let us work out the thicknesses of various layers for the Pavement of Topi Bypass Road. 1.The ADT is given as follow. Vehicle TypeADT COASTER/ FLYING COACH250 BUSSES25 Tractor Trolley36 Trucks2XL110 Trucks 3XL2 Trucks 4XL5
CEE 320 Winter 2006 THE CESAL IS WORKED OUT AS FOLLOW:- Vehicle TypeADT Annual Traffic Growth Rate % Growth Factor Total Traffic for 10 Years ESA Factor ESAL for 10 Years 80%20% LoadedEmpty COASTER/FLYING COACH ,348, ,013,125 BUSSES , ,312 Tractor Trolly , ,212 Trucks2XL , ,885,673 Trucks 3XL ,295 Trucks 4XL ,309 Total4,718,925 ESAL by taking 100 % of directional factor =4.719million ESAL by taking 80 % lane factor =3.775million CESAL =3.775 million
CEE 320 Winter 2006 California Bearing Ratio (CBR)= 30 % at 95% MDD M R= 2555 (CBR) 0.63 for CBR > 10 Putting the value CBR, M R = Psi Keeping the value of various parameters as follow. R= 90% S o= 0.45 ∆Psi=1.7 Using Nomograph to work out the S N
CEE 320 Winter 2006 Required SN =
CEE 320 Winter 2006 Using the following the equation SN = a 1 D 1 +a 2 D 2 m 2 +a 3 D 3 m 3 Given Data: a1=0.44, a2=0.14, a3=0.11 and m2,m3=1 from Nomograph SN= 3.35 Putting these values and assuming d1=2 inch, d2=10 inch and d3=10 inch 3.35=0.44*2+0.14* * ≈3.38 Hence the Design thickness are ACWC= 5cm WBM=25cm Granular sub base=25cm
CEE 320 Winter 2006 Thank You
CEE 320 Winter 2006 SERVICEABILITY AND PERFORMANCE CONCEPT AASHO Road Test performance based on user assessment: – Difficult to quantify (subjective) – Highly variable – Present Serviceability Rating (PSR) Performance Measurements 0-1 – V. Poor 1-2 – PoorA panel of experts drove around in standard 2-3 – Fairvehicles and gave a rating for the pavement 3-4 – Good 4-5 – V. Good Measurable characteristics (performance indicators): – Visible distress (cracking & rutting) – Surface friction – Roughness (slope variance)