Pavement Design1

Rasta - Center for Road Technology

(Resource Centre for Asphalt and Soil Training Academy)

Department, Name, Security Class

This is the first slide of the presentation

R A S T ACenter for Road Technology

Principles of Pavement Design

G. Kavitha, Faculty

RASTA, Center for Road Technology

VTU Extension Center

Bangalore


In the second slide, the tiltle of the presentation and the name of the speaker is provided, many such slides can be included with the content of the subjects


Outline
Requirements of PavementsPavement types and their choiceDesign factors for flexible pavementsDesign of flexible pavements by CBR method


Base

Black Topping

Road Composition

450

Sub Base

Road Crust

Vehicle

Sub Grade

300mm

Embankment

Ground Level


Whenever the pavement has to be raised above the existing ground level, embankment has to be constructed. This is essential to keep the subgrade above the high water table, tp prevent damage of the pavement due to surface water and capillary water and to maintain the design standards of highway with respect to vertical alignment


Structural Requirements

Traffic loads

Load repetition

Climatic variables (rainfall & temperature)

Environmental factors (water table, embankment)


The total thickness of the pavement and the thickness of the individual pavement layers should be sufficient to withstand the heavy traffic loads and the repetition of wheel loads of different magnitudes so that the stresses induced on the pavement are within the permissible limits, under the existing climatic and environmental conditions. The climatic conditions could be the variation in the rainfall and temperature and the environmental factors like height of embankment, depth of water table can also affect the structural capacity of the pavements. We will study these factors in detail in the subsequent slides.


Functional Requirements

Riding comfort

Economic operation

Safe operation


The pavement should provide good riding quality for the vehicles moving at design speed through out its service life. The unevenness / undulations cause vertical oscillations of the vehicles resulting in poor riding comfort.

The undulations also increase the vehicle operation cost. Also if the pavement surface is very slippery then it results in skidding of the vehicles and results in unsafe operations. therefore the functional requirement of the pavements is to provide safe, comfortable and fast movement of vehicles and goods at reasonable low vehicle vehicle operation cost.


Pavement Types

Flexible pavements

Rigid pavements / Cement Concrete (CC) Pavements

Semi-rigid / Composite pavements


Flexible pavements are those pavements which have low flexural strength. They are made up of a number of granular and bituminous layers which transmit the vertical or compressive stresses to the lower layers by grain to grain and thus distribute the stresses to a larger area in the shape of a truncated cone. The rigid pavements have good flexural strength and the stresses are transmitted through a wider area below through the slab action.


Stresses in Flexible pavements


This is how the stresses are distributed in the flexible pavement structure. The vertical compressive stress is maximum on the pavement surface directly under the wheel load and is equal to the contact pressure under the wheel. Due to the ability to distribute the stresses to a larger area the stresses get decreased at the lower layers is very low / negilgible.


Stress distribution through granular layers



Factors affecting the dispersion of compressive stresses
Characteristics of materials in each pavement layer Thickness of each layer Loading characteristics



As seen in the sketch, the influence of tyre pressure is predominant in the upper layers. At a greater depth the effect of tyre pressure dimishes and the total load exhibits a considerable influence on the vertical stress magnitudes. Tyre pressure of high magnitude therefore demand high quality of materials in the upper layers in pavements. The total depth of pavement is however not influenced by the tyre pressure.



In the cc pavements the load carrying capacity is mainly due to the rigidity and high modulus of elasticity of the slab itself.. Slab action. We provide a drainage layer above that DLC subbase course, which mainly prevents pumping under the pavement, to control differential frost action and to provide a strong structural support during for construction equipment during concreting of slabs


Semi Rigid Pavements
Intermediate class of materials used in base / sub base courseThe intermediate materials are bonded materials like lime fly ash aggregate mix (puzzolanic concrete), lean cement concrete or soil cementThey have slightly high flexural strength than the flexible pavementsThey do not possess as much flexural strength as cement concrete pavementsThese materials have low resistance to impact and abrasion


Choice Of Pavement Type

Initial cost

Maintenance cost

Total transportation cost

Availability of funds



Design Factors For Flexible Pavements

Design Wheel Load

Sub-grade Support

Materials in Pavement Component layers

Climatic and Environmental Factors

Drainage Characteristics


Various factors to be considered for the design of flexible pavements are as follows..


Load

Gross load, P

Tyre and Contact pressure, p or the area of contact, A

Multiple wheel load and ESWL

Repeated application of wheel loads and EWL factors

- P1 N1 = P2 N2

Cumulative standard axles, CSA in msa

Other factors - pavement width lane distribution factor, speed etc.


The various wheel load factors to be considered are maximum wheel load or the gross load which influences the thickness requirements of pavements. As per IRC the legal single axle load is 8170 kg which is now revised to 10.2 tonnes.

Contact pressure when the tyre pressure is higher, higher is the contact area and hence lesser is the contact pressure. When the contact pressure is lesser, higher tyre pressure demand high quality of materials in the upper layers as the surface pressure is high. To maintain the maximum wheel load within the specified limit and to carry greater loads it is necessary to dual and multiple wheel load assembly to the rear axles of the vehicles. In doing so the effect of the dual wheel assembly is not same as two times the load on any one wheel, but the effect is in betn single load and the two times the load carried by any one wheel. The dual or multiple wheel loads is converted to ESWL which is used in the pavement design. This ESWL is defined as that single wheel load which produces the same value of maximum deflection or stress at the desired depth z of the pavement.

Repeated application of wheel loads the deformation of the pavement due to the single application of a wheel load is small but due to repeated application of the wheel loads, there is increased magnitude of plastic and elastic deformation. This accumulated permanent deformation of the pavement results in pavement failure. The repeated application of wheel loads of different magnitudes are converted to a single load equivalent to the repeated application of any particular wheel load, using equivalent load factors.

CSA as per IRC 37-2001, flexible pavements are designed based on the cumulative number of standard axle loads and not the total number of the commercial vehicles. The mixed commercial vehicles with different axle loads are converted to CSA.


Subgrade

Soil type and index properties

Strength properties (CBR, K - value or E-value)

Drainage characteristics


Before designing the pavement, it is essential to know the type of soil (wet sieve analysis) available in the particular vicinity and also study some of the properties of the soil like index properties which includes LL, PL and P.I. since subgrade soil serves as the foundation of the pavement, we should ensure that the locally available soil fulfills the specific requirements. The strength of the soil like CBR helps in deciding the thickness requirement of the pavements. A subgrade with low CBR requires thicker pavement and vice versa. Since CBR is only a strength number and doesnot relate to any fundamental properties of the soil it is essential to also determine K value modulus of subgrade reaction by plate load test and E value by direct shear or triaxial compression test. Performance of the pavement also depends on the drainage characteristics of the subgrade. Soils like silt and silty sand have poor permeability and therefore help in frost action in regions where temperature can fall below zero degrees


Pavement Materials

Materials characteristics in different layers

(Stress distribution, drainage, strength factor etc)

Durability

Fatigue effects


It is essential to evaluate the properties of the various materials used in different layers of the pavement in order to ensure good strength, durability under adverse conditions of weather and fatigue behaviour of these materials for better performance of the pavements. The stress distribution characterisitics of the pavement component layers depend on the characteristics of the material Some of the properties determined are: CBR, Elastic moduli, subgrade modulus


Climatic And Environmental Factors

Rain fall

Depth of water table and relative height of formation

Sub-grade moisture content for design

Temperature variations - daily and seasonal

Frost action


Pavement performance is affected by the variation in moisture due to variation in rainfall, soil type, ground water level, drainage conditions etc. the surface water during rains may enter the subgrade through the pavement edges or through the porous pavement surface itself. The subgrade moisture may also vary due to fluctuations in the ground water table. It also depends on capillary action. This variations in moisture can be controlled by providing suitable surface and sub surface drainage system. Frost action the alternate cycles of frost heaving and thaw is called frost action. The held water in the subgrade forms ice crystals, when the temperature reduces below zero degrees. These grow in size with the continuous supply of water due to capillary action. This results in raising of the pavement structure known as frost heave. Subsequent increase in temperature would result in melting or thawing of the frozen ice crystals and soften the road bed, hence voids are created by the melted ice crystals. Temperature stresses of high magnitude are induced in cement concrete pavements due to daily and seasonal variations in temperature. In bituminous pavements the bituminous mix softens in hot weather and brittle in cold weather


Drainage characteristics

effective functioning of :
surface drainage system subsurface drainage system

Good drainage facilities both surface and sub surface should be provided in order to prevent the damage cause to the pavement due to accumulation of water. These drainage structures should also be well maintained inorder to continue working effectively.


Flexible Pavement Design

Basic Principles

Vertical stress or strain on sub-grade

Tensile stress or strain on surface course


Two critical stresses considered in the design of flexible pavements are vertical compressive stress on the subgrade which results in permanent deformation or rutting and horizontal tensile strain on the surface course resulting in cracks.


Evaluation Of Pavement Component Layers
Sub-grade
To Receive Layers of Pavement Materials Placed over it

Plate Bearing Test

CBR Test

Triaxial Compression Test



Evaluation Of Pavement Component Layers

Sub-base And Base Course

- To Provide Stress Transmitting Medium

To distribute Wheel Loads

To Prevent Shear and Consolidation Deformation

In case of rigid pavements to

Prevent pumping

Protect the subgrade against frost action

-Plate Bearing Test

CBR Test



Wearing Course

High Resistance to Deformation

High Resistance to Fatigue; ability to withstand high strains - flexible

Sufficient Stiffness to Reduce Stresses in the Underlying Layers

High Resistance to Environmental Degradation; durable

Low Permeability - Water Tight Layer against Ingress of Surface Water

Good Workability Allow Adequate Compaction

Sufficient Surface Texture Good Skid Resistance in Wet Weather

- bituminous materials used in wearing course tested by Marshall test



Flexible Pavement Design Using CBR Value Of Sub-grade Soil

California State Highways Department Method

Required data

Design Traffic in terms of

cumulative number of standard

axles(CSA)

CBR value of subgarde



Traffic Data
Initial data in terms of number of commercial vehicles per day (CVPD)Traffic growth rate during design life in %Design life in number of yearsDistribution of commercial vehicles over the carriage way


Traffic In Terms Of CSA (8160 Kg) During Design Life
Initial Traffic
In terms of Cumulative Vehicles/day

Based on 7 days 24 hours Classified Traffic
Traffic Growth Rate Establishing Models Based on Anticipated Future Development or based on past trends
Growth Rate of LCVs, Bus, 2 Axle, 3 Axle, Multi axle, HCVs are different

7.5 % may be Assumed



Design Life
National Highways 15 YearsExpressways and Urban Roads 20 YearsOther Category Roads 10 15 Years


Vehicle Damage Factor (VDF)

Multiplier to Convert No. of Commercial Vehicles of Different Axle Loads and Axle Configurations to the Number of Standard Axle Load Repetitions indicate VDF Values

Normally = (Axle Load/8.2)n

n = 4 - 5



VEHICLE DAMAGE FACTOR (VDF)
AXLE LOAD, tNo. of AxlesTotalAxlesEq.FACTORDamage Factor0-23034640.00020.01282-43662916570.0149.1984-6141220416161616213.3126-8136228716491649857.488-10985136111.044637.884


VEHICLE DAMAGE FACTOR (VDF)
AXLE LOAD, tNo. of AxlesTotal AxlesEquivalency.FACTORDamage Factor10-1287958033.52810.512-1408048047.165756.6414-16227427613.353684.616-182565623.171297.5218-202171936.5693.5


Vehicle Damage Factor (VDF)

Vehicle Damage Factor = 16225

-------

(No. of Veh. Weighed) 3280

= 4.95 (Sample size = 86 %)
AXLE LOAD, tNo. of AxlesTotal AxlesEq.FACTORDamage Factor20-2205553265Total Damage Factor 16255


Vehicle Damage Factors

LCV - 0.259

2-Axle Trucks-4.95

3- Axle Trucks-7.587

BUS-1.027

MULTI-AXLE TRUCKS - 9.535



INDICATIVE VDF VALUES
Initial Traffic in terms of CV/PDTerrainPlain/RollingHilly0 1501.50.5150 15003.51.5> 1500 4.52.5


Single Lane Roads

Total No. of Commercial Vehicles in both Directions

Two-lane Single Carriageway Roads

75% of total No. of Commercial Vehicles in both Directions

Four-lane Single Carriageway Roads

40% of the total No. of Commercial Vehicles in both Directions

Dual Carriageway Roads

75% of the No. of Commercial Vehicles in each Direction

Distribution Of Traffic


An assessment of the distribution of commercial traffic by direction and by lane is necessary as it affects the total equivalent std axle load used in the design


Computation of Traffic for Use of Pavement Thickness Design Chart

365 xA[(1+r)n 1]

N = --------------------------- x D x F

r

N = Cumulative No. of standard axles to be catered for the design in terms of msa

D = Lane distribution factor

A = Initial traffic, in the year of completion of construction, in terms of number of commercial vehicles per day

F = Vehicle Damage Factor

n = Design life in years

r = Annual growth rate of commercial vehicles



Computation Of CSA For Different Vehicle Classes
Veh.NGF,%DFVDFCSALCV50070.750.2590.89BUS20050.751.0316.642-axle300060.754.9594.623-axle50040.757.5820.7M-axle20030.759.540.29CSA for design life of 15 Yrs.133.14


Flexible pavement design chart (IRC) (for CSA< 10 msa)



Flexible Pavement Layers (IRC) (CSA< 10 msa)



Thickness & composition (mm)

Flexible Pavement Layers (IRC) (CSA< 10 msa)



Flexible pavement design chart (IRC)



Flexible pavement layers (IRC)



Flexible pavement layers (IRC)



Subgrade
Subgrade to be Well Compacted to Utilize its Full StrengthTop 500 mm to be Compacted to 97% of MDD (Modified Proctor)Material Should Have a Dry Density of 1.75 gm/ccCBR to be at Critical Moisture Content and Field DensityStrength Lab. CBR on Remoulded Specimens and NOT Field CBR


Subgrade
Soak the Specimen in Water for FOUR days and CBR to be DeterminedUse of Expansive Clays NOT to be Used as Sub-gradeNon-expansive Soil to be PreferredIf ARF < 500 mm, Soaking is NOT required


Permissible Variation in CBR Value
CBR (%)Maximum Variation in CBR Value5 +_ 15-10+_ 211-30+_ 331 and above+_ 4


Sub-base
Material Natural Sand, Moorum, Gravel, Laterite, Kankar, Brick Metal, Crushed Stone, Crushed Slag, Crushed ConcreteGSB- Close Graded / Coarse GradedParameters Gradation, LL, PI, CBRStability and Drainage Requirements


Sub-base
Min. CBR 20 % - Traffic up-to 2 msaMin. CBR 30 %- Traffic > 2 msaIf GSB is Costly, Adopt WBM, WMMShould Extend for the FULL Width of the FormationMin. Thickness 150 mm - 10 msa


Sub-base
Min. CBR 2 %If CBR < 2% - Pavement Thickness for 2 % CBR + Capping layer of 150 mm with Min. CBR 10% (in addition to the Sub-Base)In case of Stage Construction Thickness of GSB for Full Design Life


Base Course
Unbound Granular Bases WBM / WMM or any other Granular ConstructionMin. Thickness 225 mm < 2 msaMin. Thickness 250 mm - > 2 msaWBM Min. 300 mm ( 4 layers 75mm each)


Bituminous Surfacing
Wearing Course Open Graded PMC, MSS, SDBC, BCBinder Course BM, DBMBM- Low Binder, More Voids, Reduced Stiffness,


Bituminous Surfacing
Provide 75 mm BM Before Laying DBMReduce Thickness of DBM Layer, when BM is Provided ( 10 mm BM = 7 mm DBM)Choice of Wearing Course Design Traffic, Type of Base / Binder Course, Rainfall etc


Choice Of Wearing Courses
BASE/BINDERWEARING COURSEARFTRAFFICWBM, WMM, CRM, BUSGPMC+SC (B)PMC + SC (A)MSSL and ML,M,HL,M,H< 10BMSDBCPMC (A)MSSL,M,H510>100


Appraisal Of CBR Test And Design
Strength Number and Cannot be Related Fundamental PropertiesMaterial Should Pass Through 20 mm SieveSurcharge Weights to Simulate Field Condition Soaking for Four Days- UnrealisticCBR Depends on Density and Moisture Content of Sub-grade SoilDesign Based on Weakest Sub-grade Soil Encountered


Example Of Pavement Design For A New Bypass



DATA:

Two-lane single carriageway= 400 CV/day

(sum of both directions)

Initial traffic in a year of completion of construction

Traffic growth rate per annum = 7.5 percent

Design life= 15 years

Vehicle damage factor= 2.5 (standard axles

per commercial vehicle)

Design CBR value of sub-grade soil= 4 %



Distribution factor = 0.75

Cumulative number of standard axles to to be catered for in the design

365 x [(1+0.075)15 1]

N = ----------------------------- x 400 x 0.75 x 2.5 0.075

=7200000 = 7.2 msa

Total pavement thickness for= 660 mm

CBR 4% and Traffic 7.2 msa



Pavement Composition interpolated

From Plate 1, CBR 4% (IRC37-2001)

Bituminous surfacing=25 mm SDBC + 70 mm DBM

Road base, WBM=250 mm

Sub-base=315 mm



Example Of Pavement Design For Widening An Existing 2-lane NH To 4-lane Divided Road



Data:

i)4-lane divided carriageway

Initial traffic in each directions in the year of= 5600cv / day

Completion of construction

iii)Design life= 10/15yrs

iv)Design CBR of sub-grade soil= 5 %

v)Traffic growth rate= 8 %

vi)Vehicle damage factor= 4.5 (Found out from axle road survey axles per CV on existing road)



Distribution factor = 0.75
VDF = 4.5
CSA for 10 Years = 100 msa

CSA for 15 years = 185 msa

Pavement thickness for CBR 5% and

100 msa for 10 Years = 745 mm

For 185 msa for 15 years = 760 mm

Provide 300 mm GSB + 250 mm WMM + 150 mm DBM + 50 mm BC (10 years)

Provide 300 mm GSB + 250 mm WMM + 170 mm DBM + 50 mm BC (15 years)



References

1.Yoder and Witczak Principles of Pavement Design

John Wiley and Sons , second edition

2.IRC :37-2001, Guidelines of Design of

Flexible Pavements

3.IRC:81 - 1997 Tentative Guidelines for Strengthening

of Flexible Road Pavements Using Benkelman Beam Deflection Technique


Volvo Construction Equipment


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