ce 481 lecture 7 v01

03-09-2014

1

Flexible Pavement Design – Indian Roads Congress Method

CE481A

LECTURE 7

REFERENCE: IRC : 37‐2012

Recap

Asphalt institute pavement design method for flexible pavements

Different modes of failure

Critical responses associated with individual failure modes

Recap – Failure Criteria

Fatigue

Account for 20% cracking in the wheel path during AASHTO road test

Permanent Deformation

0.5‐inches of rutting in the total pavement

Related to stress in upper subgrade

854.0*291.30796.0 EN tf

477.4910365.1 cd xN

03-09-2014

2

Recap – Temperature Effects on Hot Mix AsphaltAir temperature data used to account for effect of temperature on HMA modulus

Pavement temperature varies with depthTemperature at upper third portion of each layer used as weighted average pavement temperatureZ value selected accordingly

inchesindepthz

etemperaturairmonthlymeanM

eqnModulusAsphaltinusedTetemperaturpavementmeanMwhere

zzMM

a

p

ap

.)(

64

34

4

11

Design Subgrade MR

Modulus value smaller than 62, 75, or 87.5% of all test values (difference based on ESAL)

Recap – Environmental Effects on ModulusLowest modulus values for different temperature regimes given in table below

Monthly changes can be estimated if moduli values at start and end of season is known

Used as input for calculating damage during different times of the year

03-09-2014

3

Recap –Modulus of Unbound Aggregate Base

Instead of using the predictive models (K ‐ θ or Uzan Model), AI Method uses a regression equation to find aggregate base resilient modulus

0.8681

0.2873

-0.1391

–0.0412

–0.47112 KE Eh h 10.447 E

Recap – Using AI Design Charts to Estimate Pavement Thicknesses

Tk = 7.75 in

Objectives

Understand the basics of IRC design method

Understand the design inputs required to perform pavement design using IRC methodTraffic inputs

Material inputs

Perform pavement design using IRC method

03-09-2014

4

IRC Specification for Mechanistic Empirical Pavement Design

IRC 37: 2012Mechanistic empirical approach

Based on research conducted by research organizations in India

Performance data collected on pavements across the country under different temperature and climatic conditions

Applicability

Design of new pavements

For high volume roads – NH, SH, Express ways etc.

Not applicable for overlay design

Not applicable for low volume roads

Deign Principles

03-09-2014

5

Pavement Composition

Figure: Different layers of bituminous pavement

Multiple layers with different properties

Bound ‐ Stabilized layers (lime,

cement, fly‐ash addition)

Unbound ‐Mixture of course and fine

aggregate compacted at Optimum Moisture Content to attain Max.

Dry Density

Optional Layer – used (if needed) when

cemented materials are used below

Natural Soil/Chemically modified soil

Pavement Design

Selecting pavement type

Selecting layers to be used (materials)

Selecting layer thicknesses

Selecting combinations to be used

Performance Criteria

Designed to perform satisfactorily without causing unacceptable levels of distress during the design life

Unacceptable distress levelsFatigue cracking of 20 % for traffic up to 30msa

Fatigue cracking of 10 % for traffic beyond 30msa

Rutting of 20 mm in 20 % of the length for traffic up to 30msa

Rutting of 20 mm in 10 % of the length for traffic beyond 30msa

Objective of design is to ensure that these conditions does not occur during the service life of pavements

03-09-2014

6


Performance – No of repetitions of standard axle can be sustained before excessive rutting or fatigue damage occurs

Correlate performance with critical parameters2 types of distresses

2 critical parameters

General form of performance equations

2

11

K

StrainInitialKN

Distress Criteria

Fatigue cracking in bituminous layers

Rutting due to permanent deformation in subgrade

Rutting due to permanent deformation in bituminous layers

http://www.pavementinteractive.org/article/flexural-fatigue/ http://www.pavementinteractive.org/article/rutting/

Alligator (Fatigue) Cracking

Rutting

Mechanistic parameters

Pavement performance indices

εt – horizontal tensile strain at the bottom of bituminous mixtures: indicator of fatigue cracking in HMA layers

εc – vertical compressive strain on top of subgrade: indicator of permanent deformation in subgrade layers

03-09-2014

7

CL R

tAC

tBase

q

RAC

Aggregate Base

Z, Z

Subgrade ===> Rutting

Critical Pavement Responses

===> Fatigue

Failure

Mechanistic Parameters

Performance explained based on mechanistic behaviour of components

Strain Calculations

Pavement section analysed for critical parametersIRC method is based on linear layered elastic theory

Pavements modelled as a multilayer system

Layer interfaces considered as rough

Assumptions

oTop two layers – Infinite horizontally with finite thicknessoSubgrade – Semi‐infinite

Strain Calculations

Inputs required

h2, E2, µ2

h3, E3, µ3

hn = , En, µn

h1, E1, µ1

P = Loading

q = P / Area

03-09-2014

8

Mechanistic parameters

Once magnitude of critical parameters are obtained limits can be set so that the pavement performs as per requirement

Limiting Strains

Computed strains (mechanistic parameters εt, εc) should be less than limiting strains (estimated based on traffic selected – fatigue and rutting equations)

Limiting strains corresponds to initial condition of pavement

Limiting strains should be smaller for higher design traffic volume

Pavement model

Figure: Different layers of a flexible pavement

03-09-2014

9

Fatigue Life

Critical parameters correlated to performance of pavementsBased on equation for rutting and fatigue failures

Fatigue model

VG30 for traffic upto 30msa

Fatigue Life

20% fatigue cracking is achieved

Class ProblemFor a pavement to sustain 15msa and 25msa loading, determine the maximum permissible tensile strains in the HMA layer (Assume MR for HMA = 1,500 Mpa)

For 15msa loading = 2.464 x 10‐4

For 25msa loading = 2.161 x 10‐4

Permissible strains (limiting) smaller for higher design traffic volume

03-09-2014

10

Fatigue Life

Standard mixes used4.5 % asphalt content

4.5% air voids

Correction factor “C” used if mix properties are different from standard

Effect of Air void and Bitumen Content on fatigue

VG 40 traffic more than 30msa

contentvoidAirV

bitumenofVolumeV

Where

a

b

Rutting in PavementRutting model

(80 per cent reliability)

(90 per cent reliability)

03-09-2014

11


To ensure unacceptable levels of distress do not occur during design life, the critical mechanistic parameters (identified as pavement indices for performance) are kept within acceptable limits

Acceptable limits are different for different conditions

Design Options for Other Distresses in pavement

Permanent deformation in HMA mixes Due to secondary compaction and shear deformation under heavy traffic load and higher temperature

Top down crackingBut due to heavy axle load, excessive tensile stresses developed at the top surface

High modulus rut and fatigue resistant mix to be used in top layers

Chemically Modified (Cementitious) Layers – Base and Sub‐base

Use poor quality in‐situ materials in pavement construction

Engineering properties modified by addition of stabilizers (Cement, Lime, Fly‐Ash etc.)

Strength improvements – time dependentCuring time

AdvantagesCost effectiveEnvironment friendly

03-09-2014

12

Chemically Modified (Cementitious) Layers Cont’d…DisadvantagesAdequate time to be provided for strength gain (weeks)Pavement cannot be opened for traffic immediatelyShrinkage cracks may form on stabilized layers

oReflects through to the pavement surface

oHigh strength/stiffness not always preferable

Mandates proper designOptimize strength/stiffness to limit shrinkage cracks

Design procedures are often soil specific

Chemically Modified (Cementitious) Layers – Fatigue Equations

Two types(1) Based on standard axles

(2) Based on cumulative damage analysis

Fatigue equation based on cumulative damage is used only when heavy traffic is operating

Fatigue Life in Cementitious layersFatigue life in terms of standard axles

03-09-2014

13

Possible Correction to Fatigue Equation

Possibly a typo

Correction NOT yet approved by IRC committee

113000 0.804

Fatigue Life in Cementitious layersFatigue equation for cumulative damage analysis

<1 (to satisfy fatigue criteria)

= number of axles of axle load of class i.

Design Inputs

03-09-2014

14

Estimating Design traffic load

Cumulative number of standard axle load (80kN) – ESAL

Initial traffic after construction (Commercial Vehicles Per Day‐CVPD)Axle gross weight of 30 kN or more considered (only)

7 days 24hr traffic count as per IRC : 9‐1972

Estimating Design traffic load Cont’d…

Traffic growth rate (r)Based on past trends

Based on economic parameters (GDP)

Changes in land use pattern

Expected demand due to specific development

Minimum 5% used (IRC:SP:84‐2009)

Assumed to be 5% if data not available


Design lifeNational highway and State highway ‐ 15 yearsExpressway and urban roads ‐ 20 yearsOthers ‐ 10 to 15 years

03-09-2014

15


Vehicle damage factor (VDF)Multiplier to convert different axle loads and axle configurations into the number of repetition of standard axle load of magnitude 80kN

Based on Fourth power law

4

s

a

W

WLEF

IRC Recommendation for Calculating LEF

Single axle with single wheel on either side

Single axle with dual wheels on either side

Tandem axle with dual wheels on either side

Tridem axle with dual wheels on either side

4

65

kNinloadAxle

4

80

kNinloadAxle

4

148

kNinloadAxle

4

224

kNinloadAxle


Axle loads for VDF calculationEstimated based on axle load survey

Direction wise estimation needed (if traffic significantly different)

oLeads to different pavement thicknesses in each direction

oPossible with divided highwaysoDesigned for higher VDF for two lane roads

Total CVPD Minimum percentage to be surveyed

< 3000 20

3000 to 6000 15

> 6000 10

03-09-2014

16


Indicative VDF values (if data not available)

Initial traffic volume (CVPD)Terrain

Rolling/ Plain Hilly

0‐150 1.5 0.5

150‐1500 3.5 1.5

>1500 4.5 2.5


Lane Distribution Factor (LDF)

IRC recommendations for designFor single lane – 1 (total CV – both directions)

Two lane road single carriage way – 0.50 (total CV – both directions)

Four lane single carriage way – 0.4 (total CV – both directions)

Dual two lane carriage way – 0.75 (CV – in each direction)

Dual three lane carriage way – 0.60 (CV – in each direction)

Dual four lane carriage way – 0.45 (CV – in each direction)

Design traffic

factordamageVehicleF

factorondistributiLaneD

CVPDonconstructiofcompletionofyeartheintrafficInitialA

factorGrowthr

lifeDesignn

FxDxAxr

rxTrafficDesignN

n

)(

]11[365

nyearthn rAxTraffic 1365

03-09-2014

17

Design traffic Cont’d…

onconstructiofyearandcountlastbetweenyearsofNox

countlastperasCVofNoP

rPA x

1

Material Properties

Subgrade

Soil compacted at 97% of laboratory dry density (minimum)

Minimum CBR value 8% (If CVPD > 450)

Dynamic cone penetrometer (60 degree cone)

Where, N = mm/blow Upper layers

Subgrade layer500 mm

NCBRLog 1010 log12.1465.2

03-09-2014

18

Material Properties ‐ Subgrade Cont’d…

Six to eight CBR values

Design CBR ‐ 90th percentile (Expressway, NH and state highway)

Design CBR 80th percentile (other categories of roads)

CBR(%) Maximum permissible variation in CBR Value

5 ±1

5‐10 ±2

11‐30 ±3

31 and above ±5

Effective CBR of Subgrade Material Used

CBR of compacted Borrow Material 500 mm thick

Effect of natural soil accounted for in design

Resilient modulus of SubgradeResilient modulus is the measure of subgrade elastic behaviour determined from recoverable deformation in the laboratory tests

5)(*6.17

5*10)(64.0

CBRforCBR

CBRforCBRMPaM R

ce 481 lecture 7 v01

Documents