ce 481 lecture 7 v01
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Filename: CE 481 Lecture 7 V01.pdfTRANSCRIPT
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
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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
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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
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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
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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
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Performance Criteria
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
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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
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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
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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
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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)
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Performance Criteria
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
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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
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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
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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
Estimating Design traffic load Cont’d…
Design lifeNational highway and State highway ‐ 15 yearsExpressway and urban roads ‐ 20 yearsOthers ‐ 10 to 15 years
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Estimating Design traffic load Cont’d…
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
Estimating Design traffic load Cont’d…
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
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Estimating Design traffic load Cont’d…
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
Estimating Design traffic load Cont’d…
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
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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
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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