TRANSPORTATION ENGINEERING-II

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AASHTO 1993Flexible 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 AASHTO 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 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.

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.

Very Good

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(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

• Regional factorIt 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 NumberAn 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.

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.

ReliabilityIt 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 DeviationIt takes in to account the designer’s ability to estimate the variation in 18K Equivalent Standard Axle Load.

• Resilient ModulusIt 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:

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

SN=Structural Number (an index that is indicative of the total pavement thickness required)

SN=a1D1M1 + a2D2m2 + a3D3m3+...ai =ith layer coefficientdi =ith layer thickness (inches)mi =ith layer drainage coefficientΔ 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

psiCBRM R 000,12815001500

psiCBRM R 669,9825552555 64.064.0

AASHTO Conversion

NCHRP 1-37A Conversion

Step 3: Choose Reliability

Arterial Road• AASHTO Recommendations

Functional ClassificationRecommended Reliability

Urban Rural

Interstate/freeways 85 – 99.9 85 – 99.9

Principal arterials 80 – 99 75 – 95

Collectors 80 – 95 75 – 95

Local 50 – 80 50 – 80

Choose 85%

Step 3: Choose Reliability

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

Choose S0 = 0.50

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

Step 5: Calculate Design

Decide on basic structureResilient 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

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

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 2.5 inches

ACB 7.0 inches 4.0 inches

UTB 6.0 inches 6.0 inches

Aggregate subbase 6.0 inches 6.0 inches

TRANSPORTATION ENGINEERING-II

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