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TRANSPORTATION ENGINEERING-II

AASHTO 1993 Flexible Pavement Design Equation. TRANSPORTATION ENGINEERING-II. AASHTO DESIGN METHOD.

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TRANSPORTATION ENGINEERING-II

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  1. AASHTO 1993Flexible Pavement Design Equation TRANSPORTATION ENGINEERING-II

  2. AASHTO DESIGN METHOD • The basic objective of this test was to determine significant relationship between the no. of repetition of specified axle loads (of different magnitude and arrangement) and the performance of different thickness of pavement layers.

  3. AASHTO DESIGN METHOD CONSIDERATIONS • Pavement Performance • Traffic • Roadbed Soil • Materials of Construction • Environment • Drainage • Reliability • Life-Cycle Costs • Shoulder Design

  4. STEPS FOR DESIGNING • The AASHO design method states that: • “The function of any road is to carry the vehicular traffic safely and smoothly from one place to another”. • Following are the different steps followed in AASHTO design method while designing the pavement. • Measuring Standard Axle Load • Predicting Serviceability • Performance • Present Serviceability Rating (PSR)

  5. Present Serviceability Index • Terminal Serviceability • Regional Factor • Structural Number • Soil Support • Reliability • Over all Standard Deviation • Resilient Modulus

  6. Standard Axle Load (ESAL’s) • “An axle carrying a load of 18Kips and causing a damaging effect of unity is known as Standard Axle Load”. Serviceability • “Ability of a pavement to serve the traffic for which it is designed”. Performance • “Ability of a pavement to serve the traffic for a period of time”. Performance is interpreted as trend of serviceability with time.

  7. Very Good Good Fair Poor Very Poor Present Serviceability Rating • To define PSR, the AASHO constituted a panel of drivers belonging to different private and commercial vehicles. They were asked to • Rate the serviceability of different section on a scale of 0-5. • Say whether the sections were acceptable or not.

  8. Present Serviceability Index (ISI) • The prediction of PSR from these physical measurements is known as PSI and defined as “Ability of a pavement to serve the traffic for which it is designed”. Normally the value is taken as 4. • PSI value depends on the following factors; • Measurement of longitudinal surface irregularities • Degree of cracking • Depth of rutting in the wheel paths

  9. Terminal Serviceability Index (ISI) • “The lowest serviceability that will be tolerated on the road at the end of the traffic analysis period before resurfacing or reconstruction is warned”. • Its usual value is 2 for roads of lesser traffic volume and 2.5 for major highways.

  10. Basic design equation for Terminal Serviceability is Pt= Gt-{log (Wt)-log (p)} • =0.4+{0.081(L1+L2)3.23}/{(1+SN)5.19+L23.23} • log (p)= 5.93 + 9.36log(SN+1)-4.79log (L1+L2)+ 4.33log(L2) • Gt=a logarithmic function of the ratio of the loss in serviceability at time t to the potential loss taken to a point where pt=1.50 • p=a function of design and load variables that denotes the expected number of axle load applications to a pt=1.5 • = a function of design and load variables that influence the shape of the p Vs W serviceability curve. • Wt=axle load applications at the end of the time t • L1=load on one single axle or on one tendon axle set, in kg • SN= Structural Number of pavement

  11. Regional factor It is a factor which helps the use of the basic equations in a climatic condition other than the ones prevailing during the road test. Its values are: • Road bed material frozen to a depth of 5 in or more (winter) • Road bed material dry (Summer and fall) • Road bed material wet (spring thaw)

  12. Structural Number An index number that represents the overall pavement system structural requirements needed to sustain the design traffic loading for the design period. Analytically, the SN is given by: SN=a1D1M1+a2D2M2+a3D3M3 Where • D1,D2,D3 = thickness in inches respectively of surfacing, base and sub-base. • a1,a2,a3 = coefficients of relative strength.

  13. a1 = 0.2 for road bricks 0.44 for plant mix 0.45 for the sand asphalt a2 = 0.07 for sandy gravel 0.14 for crushed stone a3 = 0.11 for sandy gravel 0.50 to 0.10 for sandy soil M1, M2,M3 = drainage coefficients M1 = 1 shows good drainage conditions Soil Support • Its value depends on the CBR value of the layer.

  14. Reliability It is defined as “probability that serviceability will be maintained at adequate levels from a user point of view, through out the design life of the facility” • Overall Standard Deviation It takes in to account the designer’s ability to estimate the variation in 18K Equivalent Standard Axle Load. • Resilient Modulus It is defined as Mr = Repeated Axial Stress / Total Recoverable Axial Strain Mr=CBR x 1500

  15. AASHTO DESIGN EQUATION This equation is widely used and has the following form: Log10(W18)=Zr x So+ 9.36 x log10(SN + 1)-0.20+(log10((ΔPSI)/(4.2-1.5)) /(0.4+(1094/(SN+1)5.19)+2.32x log10(MR)-8.07 where: W18=predicted number of 80 KN (18,000 lb.) ESAL’s ZR=standard normal deviate So=combined standard error of the traffic prediction and performance prediction

  16. SN=Structural Number (an index that is indicative of the total pavement thickness required) SN=a1D1M1 + a2D2m2 + a3D3m3+... ai =ith layer coefficient di =ith layer thickness (inches) mi =ith layer drainage coefficient Δ PSI =difference between the initial design serviceability index, po, and the design terminal serviceability index, pt MR =sub-grade resilient modulus (in psi)

  17. Nomo-graph

  18. 1993 AASHTO Structural Design Step-by-Step

  19. Step 1: Traffic Calculation Total ESALs • Buses + Trucks • 2.13 million + 1.33 million = 3.46 million

  20. Step 2: Get MR Value • CBR tests along Kailua Road show: • CBR ≈ 8 • MR conversion AASHTO Conversion NCHRP 1-37A Conversion

  21. Step 3: Choose Reliability Arterial Road • AASHTO Recommendations Choose 85%

  22. Step 3: Choose Reliability Choose S0 = 0.50

  23. Step 4: Choose ΔPSI Somewhat arbitrary • Typical p0 = 4.5 • Typical pt = 1.5 to 3.0 • Typical ΔPSI = 3.0 down to 1.5

  24. Step 5: Calculate Design Decide on basic structure

  25. Step 5: Calculate Design

  26. Step 5: Calculate Design Preliminary Results • Total Required SN = 3.995 • HMA/ACB • Required SN = 2.74 • Required depth = 6.5 inches • UTB and aggregate • Required SN = 1.13 • Required depth = 9 inches

  27. Step 5: Calculate Design Apply HDOT rules and common sense • HMA/ACB • Required depth = 6.5 inches • 2.5 inches Mix IV (½ inch Superpave) • 4 inches ACB (¾ inch Superpave) • UTB and aggregate • Required depth = 9 inches • Minimum depths = 6 inches each • 6 inches UTB • 6 inches aggregate subbase

  28. Comparison

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