Engr. Ejaz Ahmad Khan Deputy Director Pakhtunkhwa Highways Authority

1. PRESENTATION ON ROAD PAVEMENT DESIGN BY Engr. Ejaz Ahmad Khan Deputy Director Pakhtunkhwa Highways Authority

2. CEE 320Steve Muench

3. OUTLINE Section - 1 • Pavement Structure Section - 2 • Design of Pavement Structure Section - 3 • Flexible Pavement Design Section - 4 Section - 5 • How to Design • Practical Example

4. Section - 1 • Pavement Structure

5. PAVEMENT : • Combination of various layers between road top surface / Finished Road Level (FRL) and the subgrade is known as pavement structure. Pavement Structure:

6. PAVEMENT PURPOSE • Load support • Skid Resistance • Good ride • Less VOC • Time Saving • Drainage CHAPPAR - DARBAND ROAD (30 KM) PHASE-I

7. PHILOSOPHY OF PAVEMENTS • Pavements are subjected to moving traffic loads that are repetitivein nature. • Each traffic load repetition causes a certain amount of damageto the pavement structure that gradually accumulatesover 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 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.

9. FLEXIBLE PAVEMENT • Structure • Surface course • Base course • Subbase course • Subgrade

10. RIGID PAVEMENTS • 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. • Thus in contrast with flexible pavements the depressions which occur beneath the rigid pavement are not reflected in their running surfaces.

11. RIGID PAVEMENT • Structure • Surface course • Base course • Subbase course • Subgrade

12. Section - 2 • Design of Pavement Structure

13. Given Wheel Load 150 Psi Asphalt Concrete Thickness? Base Course Thickness? 3 Psi Subbase Course Thickness? Given In Situ Soil Conditions 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.

14. DESIGN PARAMETERS • Subgrade • Loads • Environment

15. SUBGRADE • Characterized by strength and/or stiffness • California Bearing Ratio (CBR) • Measures shearing resistance • Units: percent • Typical values: 0 to 20 • Resilient Modulus (MR) • Measures stress-strain relationship • Units: psi or MPa • Typical values: 3,000 to 40,000 psi Picture from University of Tokyo Geotechnical Engineering Lab

16. SUBGRADE Some Typical Values

17. Pavement Thickness Design Are Developed To Account For The Entire Spectrum Of Traffic Loads Cars Pickups Buses Trucks Trailers TRAFFIC LOADS CHARACTERIZATION

18. EQUIVALENCY FACTOR Equivalent Standard ESAL Axle Load 18000 - 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

19. = 16.4 Tons Axle 8.2 Tons Axle EXAMPLE 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.,

20. AXLE LOAD & RELATIVE DAMAGE

21. SERVICEABILITY CONCEPT OF PAVEMENT • 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.

22. SERVICEABILITY CONCEPT OF PAVEMENT • 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)

23. 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 ft2 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 ft2 per 1000 ft2 of pavement surfacing.

24. PSI vs. Time p0 Serviceability (PSI) p0 - pt pt Time

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.

26. Section - 3 • Flexible Pavement Design

27. 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.

28. TYPICAL LOAD & STRESS DISTRIBUTION IN FLEXIBLE PAVEMENTS. Wheel Load Bituminous Layer Vertical stress Foundation stress Sub-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

33. 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:

34. 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 (MR) SN = SN1 + SN2 + SN3

35. 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 p0 Serviceability (PSI) p0 - pt pt Time

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 strain MR can be determined from the resilient modules test in the laboratory 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, base and 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 Coefficients Asphaltic 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.

39. NOMOGRAPH

40. Section - 4 • How to Design

41. How to Design Step 1. Fix the design life of the pavement. Step 2. Work out MR value of the subgrade MR = 1500 CBR for CBR <10% OR MR = 2555 (CBR)0.63 for CBR > 10 OR Work 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

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.

44. Calculation of CESAL

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 – 1 Where Pn = Projected traffic for nth year r = Growth rate n = year of consideration Add 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 = 1.7 R = 90% So = 0.45 ZR = -1.282 Step 7. Put these values in equation 1 and use trial & error method or Nomographto work out the SN SN = a1D1+a2D2m2+a3D3m3 Step 8. Take the value of m2 and m3 from the table on the next slide.

47. TABLE FOR QULALITY OF DRAINAGE

49. 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.

50. Section - 5 • Practical Example