# TRANSPORTATION ENGINEERINGII AASHTO 1993 Flexible Pavement Design Equation

TRANSPORTATION ENGINEERING-II AASHTO 1993 Flexible Pavement Design Equation

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.

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

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)

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

Standard Axle Load (ESAL’s) • “An axle carrying a load of 18 Kips 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.

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

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

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.

Basic design equation for Terminal Serviceability is Pt= Gt- {log (Wt)-log (p)} • =0. 4+{0. 081(L 1+L 2)3. 23}/{(1+SN)5. 19+L 23. 23} • log (p)= 5. 93 + 9. 36 log(SN+1)-4. 79 log (L 1+L 2)+ 4. 33 log(L 2) • 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 • L 1=load on one single axle or on one tendon axle set, in kg • SN= Structural Number of pavement

• 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)

• 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=a 1 D 1 M 1+a 2 D 2 M 2+a 3 D 3 M 3 Where • D 1, D 2, D 3 = thickness in inches respectively of surfacing, base and sub-base. • a 1, a 2, a 3 = coefficients of relative strength.

a 1 = a 2 = a 3 = M 1, M 2, M 3 = M 1 = 0. 2 for road bricks 0. 44 for plant mix 0. 45 for the sand asphalt 0. 07 for sandy gravel 0. 14 for crushed stone 0. 11 for sandy gravel 0. 50 to 0. 10 for sandy soil drainage coefficients 1 shows good drainage conditions Soil Support • Its value depends on the CBR value of the layer.

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 18 K Equivalent Standard Axle Load. • Resilient Modulus It is defined as Mr = Repeated Axial Stress / Total Recoverable Axial Strain Mr=CBR x 1500

AASHTO DESIGN EQUATION This equation is widely used and has the following form: Log 10(W 18)=Zr x So+ 9. 36 x log 10(SN + 1)0. 20+(log 10((ΔPSI)/(4. 2 -1. 5)) /(0. 4+(1094/(SN+1)5. 19)+2. 32 x log 10(MR)-8. 07 where: W 18=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

SN=Structural Number (an index that is indicative of the total pavement thickness required) SN=a 1 D 1 M 1 + a 2 D 2 m 2 + a 3 D 3 m 3+. . . 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)

Nomo-graph

1993 AASHTO Structural Design Step-by-Step

Step 1: Traffic Calculation Total ESALs • Buses + Trucks • 2. 13 million + 1. 33 million = 3. 46 million

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

Step 3: Choose Reliability Arterial Road • AASHTO Recommendations Functional Classification Recommended Reliability Urban Rural WSDOT 85 – 99. 9 95 Principal arterials 80 – 99 75 – 95 85 Collectors 80 – 95 75 Local 50 – 80 75 Interstate/freeways Choose 85%

Step 3: Choose Reliability ZR 99. 9 -3. 090 99 -2. 327 95 -1. 645 90 -1. 282 85 -1. 037 80 -0. 841 75 -0. 674 70 -0. 524 50 0 Choose S 0 = 0. 50

Step 4: Choose ΔPSI Somewhat arbitrary • Typical p 0 = 4. 5 • Typical pt = 1. 5 to 3. 0 • Typical ΔPSI = 3. 0 down to 1. 5

Step 5: Calculate Design Decide on basic structure Resilient Modulus (psi) Layer a Typical Chosen HMA 0. 44 500, 000 at 70°F 500, 000 ACB 0. 44 500, 000 at 70°F 500, 000 UTB 0. 13 20, 000 to 30, 000 25, 000 Aggregate 0. 13 20, 000 to 30, 000 25, 000

Step 5: Calculate Design

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

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

Comparison Layer California AASHTO HMA Surface 2. 5 inches ACB 7. 0 inches 4. 0 inches UTB 6. 0 inches Aggregate subbase 6. 0 inches

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