TRANSPORTATION ENGINEERING IIAASHTO DESIGN METHOD

AASHTO DESIGN METHODThe 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.

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 LoadPredicting ServiceabilityPerformancePresent Serviceability Rating (PSR)

Present Serviceability IndexTerminal ServiceabilityRegional FactorStructural NumberSoil SupportReliabilityOver all Standard DeviationResilient Modulus

Standard Axle Load An axle carrying a load of 18Kips and causing a damaging effect of unity is known as Standard Axle Load.ServiceabilityAbility of a pavement to serve the traffic for which it is designed.PerformanceAbility of a pavement to serve the traffic for a period of time. Performance is interpreted as trend of serviceability with time.

Present Serviceability RatingTo 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.

Present Serviceability IndexThe 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.Value 4PSI value depends on the following factors;Measurement of longitudinal surface irregularitiesDegree of crackingExtent of patchingDepth of rutting in the wheel paths

The final equation becomesPSI=5.03-1.91(1+SV)-1.3 (RD)2 -0.01(C+P)1/2SV=Slope variance x 106 (average of both wheel tracks) a measure of longitudinal cracks.RD=rut depth measurement in inches over a 4 ft span embracing each wheel track.C=cracking; expressed as the area of pavement in ft2P=area of patching per 1000ftTerminal ServiceabilityThe 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=1.50p=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 tL1=load on one single axle or on one tendon axle set, in kgSN= 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=a1D1+a2D2M2+a3D3M3Where 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 mix0.45 for the sand asphalta2=0.07 for sandy gravel0.14 for crushed stonea3=0.11 for sandy gravel0.50 to 0.10 for sandy soilM2,M3= drainage coefficientsM1= 1 shows good drainage conditionsSoil SupportIts 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 facilityOverall Standard DeviationIt takes in to account the designers ability to estimate the variation in 18K Equivalent Standard Axle Load.Resilient ModulusIt is defined as Mr = Repeated Axial Stress / Total Recoverable Axial StrainMr=CBR x 1500

AASHTO DESIGN EQUATIONThis equation is widely used and has the following form:Log10(W18)=Zr x So+ 9.36 x log10(SN + 1)-0.20+(log10((PSI)/(4.2-1.5)) /(0.4+(1094/(SN+1)5.19)+2.32x log10(MR)-8.07where:W18=predicted number of 80 KN (18,000 lb.) ESALs ZR=standard normal deviateSo=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=a1D1 + a2D2m2 + a3D3m3+...ai=ith layer coefficientdi=ith layer thickness (inches)mi=ith layer drainage coefficient PSI=difference between the initial design serviceability index, po, and the design terminal serviceability index, ptMR=sub-grade resilient modulus (in psi)

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