Pavement Design

3.1 Introduction, types and structural components of pavement208220802073

Definition

• A relatively stable layer with even surface constructed over the natural soil for the movement of traffic
• Functions:
• To support and distribute wheel load of vehicle over a wide underlying area of subgrade soil
• To keep the temporary deformation within the permissible range

Structural Components of Pavement

Pavement Layers — Indian practice (left) and American practice (right):

Surface course

• Top layer
• Should be smooth, abrasion resistant, dust free, reasonably water proof and strong

Base and Subbase course

• Distribute wheel load evenly to the underlying layers
• Granular materials
• Base layer use superior material compared to subbase layer
• Subbase layer good drainage property

Subgrade

• Natural soil
• Should be well-compacted/provided with suitable treatment

Binder course

• Replace part of surface course

Types of Pavement

Depending on structural behavior:

• Flexible Pavement
• Rigid Pavement
• Semi-Rigid Pavement
• Composite Pavement

Flexible Pavement

• Low flexural strength

• Deflects momentarily under load but rebounds on removal of load
• Load transmission to the subgrade by lateral distribution with grain to grain point of contacts
• Designed in layers, empirical method

Rigid Pavement

• Cement concrete slab

• High flexural strength
• Load distribution by slab action
• Permanent settlements
• Design by structural analytical techniques

Semi-Rigid Pavement

• Base, subbase of bonded materials such as pozzolanic cement, lean cement concrete, soil cement
• Flexible pavement surface course
• Higher flexural strength than flexible pavement but much lower than that of a rigid pavement

Composite Pavement

• PCC bottom layer, HMA top layer
• Ideal pavement

• Very expensive, rarely used in new construction
• Mostly rehabilitation of PCC using asphalt overlays

Difference between Flexible and Rigid Pavement

3.2 Factors controlling pavement design20812078

Factors Controlling Pavement Design

1. Traffic and loading factors
2. Material properties
3. Environment factor
4. Failure criteria

1. Traffic and Loading Factors

• Axle load
• Number of load repetitions
• Contact area
• Vehicle speed

Axle load

• Wheel configuration:
  • Most commercial vehicles are provided with dual rear wheels

• Affects the stress distribution and deflection
  • Design:

Flexible pavement: Wheels on only one side is considered

Rigid pavement: Wheels on both sides are considered

• Dual wheel load is converted into equivalent single wheel load to simplify the analysis.

• Axle configuration:
  • Commercial vehicles are provided with multiple axles to further enhance the load carrying capacity.

Number of Load Repetitions

• Repeated application of the wheel load → pavement distress → pavement failure
  • Total number of repetitions of the axle loads is computed as Cumulative Number of Standard Axles for the design life.

Contact Area

Vehicle Speed

• Vehicle speed is directly related to duration of loading.
  • The damage to the pavement is much higher if the vehicle is moving at creep speed.
  • The resilient modulus of the pavement layers should be selected carefully considering the combination of loading and vehicle speed.

2. Material Properties

• Must be specified to determine pavement responses (stresses, strains and displacements)
  • Linearly elastic: Elastic moduli and Poisson’s ratios
  • For consideration of moving load/repetition of load, resilient modulus is a better representative.
  • Nonlinear property of material can be characterized by a stress dependent resilient modulus.

3. Environment Factor

• Temperature
• Precipitation
• Road geometry (small effect)

Temperature

Both temperature and precipitation affect elastic moduli of pavement layers.

Flexible pavement:

• Affects resilient modulus of asphalt layer.
  • Low temperature can cause asphalt pavement to crack.

Rigid pavement:

• Create curling stress and affect slab subgrade contact and the stress values.
  • Determine joint and crack openings and affect the efficiency of load transfer.
• Frost penetration is another temperature effect that affects the subgrade strength in cold climate.

Precipitation

• Quantity of surface water infiltration affects the moisture content of the subgrade and the depth of GWT.
  • Poor drainage may bring lack of shear strength, pumping, loss of support etc.

4. Failure Criteria

• Thermal cracking:
  • Low temperature cracking
  • Thermal fatigue cracking
  • Mix stiffness and fracture strength relationship to temperature, tensile strain due to daily temperature cycle

Rigid pavement:

• Fatigue cracking:
  • Major failure criteria in rigid pavement design.
  • Due to load and temperature stress.
  • Allowable number of load repetitions depends on ratio of flexural tensile stress and concrete modulus of rupture.
• Pumping, faulting/spalling/joint deterioration
  • Difficult to analyze mechanistically

Practice Tasks from the Lecture

• Highway pavement, structural components of pavement
• Types of pavement
• Difference between flexible and rigid pavement
• Factors controlling pavement design
• Traffic and loading factors, material characteristics, environmental factors, failure criteria
• Explore DOR flexible pavement design guidelines, 2021

3.3 Historical development of pavement structures (Flexible pavement design): Theoretical (Boussinesq and Burmister approach); Semi-empirical (Tri-axial approach); and empirical methods

Flexible Pavement Design: Historical Development

1. Theoretical / mechanistic methods
2. Semi-empirical method
3. Empirical methods
4. Mechanistic-empirical method

Theoretical/Mechanistic Design Method

• Materials forming the flexible pavement layers and the soil supporting the pavement have non-uniform properties.
• No completely satisfactory method of design based on theoretical considerations
• Approaches:
  • Boussinesq’s theory
  • Burmister approach

Boussinesq’s Theory

• A layer theory first derived by Joseph Valentin Boussinesq in 1885
• First attempt towards analytical solution.
• Using the layer theory as a structural model, stresses, strains and deflections at any point in the layered system can be obtained.
• One layered system

Assumptions The soil mass is:

• Homogenous
  • Isotropic
  • Ideally elastic (constant elastic parameters E and μ for all the pavement layers and the subgrade soil)
  • Semi-infinite

Drawbacks

• Soils are not perfectly elastic and homogenous.
  • Modulus of Elasticity E of all pavement layers are not constant.
  • Load may not be uniformly distributed.

Burmister’s Theory

• By Donald M. Burmister 1945
• Stresses and strains in two layered system (a pavement layer and a subgrade layer)

Assumptions

• Materials in the layers are homogeneous, isotropic and elastic.
  • Surface layer is infinite in extent in the lateral direction but of finite depth, whereas the underlying layer is infinite in both the horizontal and vertical direction.
  • Layers are in continuous contact and full friction is developed between the interface (perfectly rough interface)
  • Stress solutions are characterized by two material properties E and μ.
  • Surface layer is free of shearing and normal stresses outside the loaded area.

Displacement factor(𝑭𝒘)

Burmister’s Theory

Burmister’s Theory

Problem Plate bearing tests were conducted with a 75cm diameter plate on soil subgrade and a granular base. The stress was 0.02MN/m2 when the deflection on the subgrade soil was 0.25cm. The same plate yielded 0.25cm deflection on the base course of 15cm under a stress of 0.05MN/m2. Design the pavement for an allowable deflection of 0.5cm under a wheel load of 40KN and tire pressure of 0.5MN/m2.

Semi-Empirical Method

• Basically, theoretical but includes empirical modifications
• Approach:
  • Triaxial Method

Triaxial Method

• Employs empirical modifications
  • Traffic coefficient (X)
  • Saturation coefficient (Y)
• X is based on the ADT of design traffic
• Y is based on the annual rainfall

Empirical Method

• Based on Soil Classification (Group Index Method)
• Based on Arbitrary Soil Strength Test (CBR Methods: IRC:37-1970, Road Note 29, Road Note 31)
• Based on Pavement Performance (AASHTO Method)

Mechanistic-Empirical Methods

• Considers mechanics of materials together with observed performance
• Pavement configuration, structural model (Stress/strain), distress model/failure criteria
• Asphalt Institute Method, IRC 37-2001 and onwards, DoR guidelines

Practice Tasks from the Lecture

• Check mechanistic and semi-empirical methods for design of flexible pavements (Boussinesq’s Theory, Burmister’s Theory, Triaxial Method) and practice design problems

3.4 Design of flexible pavement by IRC, DoR guidelines, Road Note 31, AASHTO method20822080207620752074207220712068

Empirical Methods: CBR Methods

• Based on California Bearing Ratio (CBR) of subgrade soil and subsequent pavement layers
• Originally developed in California State Highway Department, has been developed and modified by various organizations for different conditions and wheel loads

CBR Method Recommended by

California State Highway Department

• Based on extensive CBR test data collected on pavement, provided a design chart with curves correlating the soaked CBR value and the total pavement thickness
• Includes curves for three classes of traffic with different wheel loads: Light (31.75kN), Medium (40.82kN) and Heavy (54.43kN)
• Higher the load, greater thickness of the pavement is required

IRC (37-1970)

• Adopts UK based traffic classification based on the number of heavy vehicles
• Design chart include correlation between soaked CBR values and pavement thickness for 7 curves for different number of commercial vehicles per day

• Recommendations:
  • CBR test should be performed in the laboratory following standard test procedure.
  • Subgrade soil sample should be compacted at OMC to Proctor density whenever possible or to dry density achievable in the field.
  • CBR test samples may be soaked in water for four days period before testing.
  • At least three samples should be tested for each type of soil at the same density and moisture content.

• Design steps:

1. Evaluate the soak CBR value of the subgrade
2. Choose appropriate curve considering the design traffic
3. Determine the pavement thickness for the CBR value of the subgrade
4. To determine thickness of sub-base, base and surface course, use the curves repeatedly

Road Note 31 /Catalogue Method

• Based on research conducted in countries throughout the world by Transport Research Laboratory UK
• First published in 1962, revised versions: 1966, 1977, 1993, 2024 (Fifth Edition)
• Cumulative number of standard axles (Road Note 29), catalogue based on full scale tests and empirical observations made in field.
• Considers variability in material properties, uncertainty in traffic forecast and effect of climate and high axle loads

• Overseas Road Note 31 (Fifth Edition) in 2024
  • Caters traffic up to 80 million cumulative ESAL in one direction
  • Guidance on design of Climate resilient surfacing

Pavement Layers:

Design steps:

1. Estimating the amount of traffic and the cumulative number of equivalent standard axles that will use the road over the selected design life;

• Only the total number and the axle loading of the heavy vehicles (unladen weight exceeding 3000 kg) need to be considered.
  • Design life of 15 and 20 years to match that of project analysis period (Overseas Road Note 5).

• Determination of cumulative number of equivalent standard axles: a. Determine the daily traffic flow for each class of vehicle weighed, using the results of traffic surveys or from other recent traffic count information that is available. [Forecast of the one-directional traffic flow for each class of vehicle on the day it is opened to traffic including Normal, Generated and Diverted traffic)

Normal Traffic: Traffic that would pass along the existing road or track even if no new pavement were provided

Diverted Traffic: Traffic that changes from another route (or mode of transport) to the project road because of the improved pavement, but still travels between the same origin and destination

Generated Traffic: Additional traffic which occurs in response to the provision or improvement of the road.

b. Determine the average daily one-directional traffic flow for each class of vehicle. c. Determine the mean equivalency factor of each class of vehicle and for each direction from the results of axle load survey and any other surveys that have recently been carried out to convert the daily cvpd to 80kN equivalent standard axle load. Corrections are suggested to consider distribution of heavy vehicles between lanes.

d. The products of the cumulative one-directional traffic flows for each class of vehicle over the design life of the road and the mean equivalence factor for that class added together is the cumulative ESAL for each direction. The larger of the two directional values should be used for design purposes.

[Determine the one-directional number of commercial vehicles per day for each class of vehicle for each year during the design life. The sum of number of commercial vehicles per day multiplied by 365 for each year of life is the cumulative traffic flow for the class in the design life. The sum of product of this cumulative traffic flow and corresponding equivalence factor for each class is the cumulative number of axles over the design life.] e) Data from the lane with the highest traffic loading, as determined, are then used to calculate the total traffic over the design life.

2. Assessing the strength of the subgrade soil and foundation layer over which the road is to be built.

• The CBR test results taken along the alignment, and /or on material from borrow areas, are the principal means of empirical assessment. When a statistical analysis of CBR data is used to determine the design CBR, this should be undertaken by calculating the lower 10th percentile, or 5th percentile (depending on the reliability level chosen), of the laboratory CBR test results.
  • Capping can be provided where the in situ untreated subgrade is weaker than is required by the selected pavement design charts. For very weak materials with CBR < 5.0%, there are specifically recommended capping.
  • The subgrade (and capping layer, where required) is recognized as the foundation layer for the pavement.

3. Selecting the most economical combination of pavement materials and layer thicknesses that will provide satisfactory service over the design life of the pavement using structural catalogues. For subgrade classes S1 and S2, capping layers using table must be applied before using the subsequent pavement design catalogues. This means that they must be converted to an effective Subgrade Class S3 (also known as Foundation Class F1) or higher. Subgrades of classes S3 to S5 may also be converted to higher effective classes using the capping options.

Charts and catalogues

Empirical Method

• Based on Soil Classification (Group Index Method)
• Based on Arbitrary Soil Strength Test (CBR Methods: IRC:37-1970, Road Note 29, Road Note 31)
• Based on Pavement Performance (AASHTO Method)

AASHTO Method

• Approximately 80% of all states in US use AASHTO pavement design and majority of states use AASHTO 1993 version.
• Empirical method based on field performance data measured at AASHO road test
• Design pavement based on specific total traffic volume and a minimum level of serviceability desired at the end of the performance period.

• Serviceability
  • A subjective measure reflecting the ability of a pavement to serve the traffic
  • Expressed in terms of the Present Serviceability Index (PSI) which varies from 5 (best) to 0 (worst) depending on the pavement condition
  • During AASHO road test, a panel of road users made a subjective rating Present Serviceability Rating (PSR) of the pavement based on visual and sensory observations; 0 < PSR <5. PSRs were correlated to the PSIs which can be determined from physical measurements of pavement.

Design steps:

1. Evaluate design traffic (W18):

• Determine the analysis period/design life Analysis period (AASHTO, 1993)

• Determine the cumulative ESAL for the design life in the design lane considering the growth factor (r), equivalent factor (truck factor), directional distribution factor (50% for most roadways) and lane distribution factor
  • The guideline provides truck factor tables for flexible pavement for varying SN of pavement, terminal serviceability index and axle configuration

2. Determine reliability factors (S0, ZR):

• Reliability (R) is the probability that the pavement section designed will perform satisfactorily over the traffic and environmental conditions for the design period. Suggested reliability for various functional classification (AASHTO, 1993)

4. Evaluate resilient modulus (MR):

• AASHTO uses resilient modulus (MR) for material characterization.
  • MR can be determined through triaxial laboratory tests detailed in AASHTO T307 method.
  • Correlation between CBR value and the MR of soil: MR psi = 1500 × CBR (%) (Reasonable for fine-grained soil with a soaked CBR of 10 or less)

5. Determine the overall structural number (SN)

• A measure of the structural strength of the pavement section based on type and thicknesses of the layers.
  • Determine the structural numbers required above the base, subbase, and subgrade layers SN1, SN2, SN3 using design equation or nomograph.

6. Determine structural layer coefficients (𝛼1, 𝛼2, 𝛼3)

• The relative structural value of each pavement layer per inch of thickness
  • Depends on material properties (Resilient modulus and CBR values)
  • For asphalt concrete surface course:

7. Determine drainage coefficients (𝑚2, 𝑚3)

• To consider local drainage characteristics Drainage coefficients for untreated bases and subbases (AASHTO, 1993)

• The values of D1, D2 and D3 have to meet certain minimum practical thicknesses. Minimum thickness (in.) (AASHTO, 1993)

• The 3 equations allow for thickness compensations among layers.
  • Depending on material availability in the location, the designer can make use of the thickness compensation concept to obtain the most economic pavement section.
  • Can be done with trial and error.

Problem Design a pavement for a rural highway using AASHTO flexible design procedure. The expected design ESAL is 13x106. The pavement structure is to consist of asphalt concrete with an elastic modulus of 400,000 psi, a granular base with an elastic modulus of 26,000 psi and a granular sub base with an elastic modulus of 12,000 psi. The elastic modulus of the subgrade is 8,000 psi. The drainage conditions of base and subbase are fair. It is estimated that the pavement will be saturated about 15% of the time. Assume that a reliability level of about 90% is required and that the initial serviceability index is 4.2 and the final serviceability index is 2.5.

IRC Flexible Pavement Design Guidelines

• IRC: 37-1984 (First rev., empirical)
• IRC: 37-2001 (Second rev.), IRC: 37-2012 (Third rev.), IRC: 37- 2018 (Fourth rev.) are mechanistic empirical designs.
• Structural model: Multi-layered linear elastic system
• IRC 37-2012 ad IRC 37-2018 uses revised failure criterions, include use of effective resilient modulus/CBR of subgrade concept.
• IRC 37-2018 has improved material characterization with recommendations for better performing bituminous mixes and binders for each layers.

IRC (37-2018)

Scope: Design of pavements for roads with design traffic >2 msa. Design Approach and Criteria

• Pavement type selection
• Structural model and pavement analysis
• Reliability Criteria
• Performance criteria

Pavement Type Selection:

• Based on traffic volume, soil characteristics, availability of local materials etc.
• Typical configurations: a. Bituminous surface course with granular base and sub-base b. Bituminous surface course with CTSB, CTB and granular crack relief layer c. Bituminous surface course with CTSB, CTB and SAMI d. Bituminous surface course with CTSB and emulsion/foam bitumen stabilized RAP/ virgin aggregate e. Bituminous surface course with GSB, CTB and granular crack relief layer f. Bituminous surface course with CTSB and granular base course.

Structural Model and Pavement Analysis

• Multiple layer system, linear elastic layered theory
• Analysis of critical stress strains using IITPAVE
• Analysis is performed for performance of pavement in terms of
  • Subgrade rutting
  • Bottom-up fatigue cracking of bituminous layers
  • Fatigue cracking of Cement Treated Base (CTB) layer

Critical Stress/Strain:

Reliability Criteria:

• 90 % reliability is recommended for all important roads such as National highways, State highways and Urban roads. For other roads, 90% reliability is recommended for design traffic volume of 20 msa or more.
• 80% reliability for design traffic volume of less than 20 msa.

Vehicle Damage Factor (VDF):

⁻ A multiplier for converting the number of commercial vehicles of different axle loads and axle configurations to the number of standard axle-load repetitions ⁻ Equivalent number of standard axles per commercial vehicle ELAF ⁻ The conversion of one repetition of a particular type of axle carrying a specific axle load into equivalent repetitions of 80 kN single axle can be done using a) b) c) d).

⁻ VDF for a typical vehicle can be obtained as the sum of the equivalency factor of all the axles in the vehicle.

• Design procedure:

1. Selecting a trial composition.
2. Bituminous mix design and the mix resilient modulus.
3. Selecting layer thickness
4. Structural analysis of the selected pavement structure (IITPAVE)
5. Computing the allowable strains/stresses
6. Doing the iterations
7. Check for cumulative fatigue damage (For CTB)

DoR (2021): Flexible Pavement

• DoR (2014) and DoR(2021) Guidelines for the design of flexible pavements.
• DoR (Second Ed. 2021) major volume is much similar to IRC 37-2018 in terms of design approach: pavement configurations, structural model and analysis, reliability and performance criteria.
• Guidelines in major volume for design of highways with more than 2 msa follow mechanistic empirical approach.
• Guidelines for design of roads (low volume roads) with traffic upto 2 msa in Annex E (IRC SP72) and follow empirical approach.

ELAF

DoR (2021): Flexible Pavement (>2msa)

• Design procedure: using IITPAVE software or Software developed by DoR (https://fpd.dor.gov.np/)
• Design table for flexible pavement with bituminous surface course with granular base and sub-base

Excel sheet for main volume design (>2msa):

Practice Tasks from the Lecture

• Check and practice IRC recommended CBR method and Road Note 31.

• Check and practice AASHTO Method

3.5 Rigid pavement design (Westergaard theory)2081207620722071

Design of Rigid Pavements

• Layered theory not suitable for analysis:
  • Difference in material properties of rigid and flexible pavement
  • Presence of joints in rigid pavements
• Analyzed using plate theory
  • Assumes concrete slab as thin plate with a plane before bending to remain plane after bending

Types of Rigid Pavements

1. Jointed Plain Concrete Pavement (JPCP)
2. Jointed Reinforced Concrete Pavement (JRCP)
3. Continuously Reinforced Concrete Pavement (CRCP)
4. Prestressed Concrete Pavement (PCP)

Jointed Plain Concrete Pavement (JPCP)

• Closely spaced contraction joints
• Dowel bars may be provided for load transfer across the joints
• Joint spacing 4.6 to 9.1m
• Recommended maximum joint spacing
  • 6.1m for doweled system
  • 4.6m for undoweled system

Jointed Reinforced Concrete Pavement (JRCP)

• Steel reinforcement in the form of wire mesh or deformed bars to allow longer joint spacing
• Dowels for load transfer across the joints
• Joint spacing from 9.1m to 30m
• Most economical joint spacing 12.2 m considering the cost of mesh, dowels, joint sealants etc.

Continuously Reinforced Concrete Pavement (CRCP)

• Continuous reinforcement and do not require transverse contraction joints
• Preferred in heavy traffic roads or weak subgrades
• Thickness reduced by 1 to 2 inch compared to conventional pavement
• Transverse cracks are expected in the slab in close intervals but are held together tightly by the reinforcing steel.

Prestressed Concrete Pavement (PCP)

• Prestressing reduces the tensile stress under traffic loads.
• Less probability of cracking, fewer transverse joints and less maintenance requirement

Stresses in Rigid Pavement

• Mainly due to:
  • Applied wheel load
  • Changes in temperature

Wheel Load Stresses

• Maximum stress induced by wheel load depends on location of load on the pavement surface

• Critical loading positions:
  • Interior
  • Edge and
  • Corner

Westergaard’s Theory

• Westergaard (1926) developed equations for critical wheel load stresses.
• The wheel load stresses and deflections depend on wheel load (P), slab thickness (h), radius of relative stiffness (l) and equivalent radius of resisting section (b)

• Assumptions:

1. Concrete slab acts as homogeneous, isotropic, elastic solid in equilibrium.
2. Thickness of slab is uniform.
3. Rigid pavement is considered to be a thin elastic plate resting on soil sub-grade which is assumed as a dense liquid. The subgrade reaction is vertical only and is proportional to the deflection of slab.

4. The load at the interior and at the corner of the slab is distributed over a circular area of contact
5. The load at the edge of the slab is distributed uniformly over semi-circular area of contact.

Radius of Relative Stiffness (l )

Modulus of Subgrade Reaction(k)

Equivalent Radius of Resisting Section (b)

Equations for Wheel Load Stress

Temperature Stresses

• Stresses due to variation in slab temperature
• Daily temperature variation → Warping stresses
• Seasonal variation → Frictional stresses

Warping Stress

• Daily temperature variation results temperature gradient across the slab thickness causing slab to curl and develops warping stress.

Frictional Stress

Practice Tasks from the Lecture

• Review types of rigid pavement and stresses in rigid pavement and Westergaard theory

3.6 Design of rigid pavement by latest IRC and AASHTO method2081207820752074

DoR (2021) and IRC 58-2015 Rigid Pavement Design

• DoR (2021) Pavement Design Guidelines (Rigid Pavement) is very much similar to IRC 58-2015.
• Mainly focused for design of Jointed Plain Cement Concrete Pavement
• Applicable for roads having daily commercial traffic (vehicle laden weight exceeding 3T) of over 450.

Design of Slab Thickness (DoR, 2021, IRC 58-2015)

• Design Factors: Subgrade and Subbase Characteristics, Concrete Characteristics, Axle Load and Traffic Considerations, Temperature Differential
• Features:
  • Design of pavements considering the combined flexural stress under the simultaneous action of load and temperature gradient for different categories of axles.
  • Design for bottom-up fatigue cracking caused by single and tandem axle load repetitions.
  • Design for top-down fatigue cracking caused by single, tandem and tridem axle load applications.
  • Consideration for with and without tied concrete shoulders
  • Consideration for with and without dowel bars

• Steps:
  • Consider a pavement configuration and specify design values for the various parameters.
  • Estimate the axle load repetitions critical to BUC.
  • Estimate the axle load repetitions critical to TDC.
  • Select a trial design thickness of pavement slab
  • Compute the flexural stresses at the bottom of the slab for different axle loads. Determine stress ratios and evaluate Cumulative Fatigue Damage (CFD)s. Sum of CFDs should be less than 1 to be safe in BUC.
  • Compute the flexural stresses at the top of the slab for different axle loads. Determine stress ratios and evaluate CFD. The sum of CFDs should be less than 1 to be safe in TDC.
  • The sum of CFDs for the BUC and TDC should be less than 1 to be safe in fatigue cracking.

Typical Cross Sections of Concrete Pavements

Subgrade

• The strength of subgrade is expressed by modulus of subgrade reaction k, which is the pressure per unit deflection (1.25 mm) as determined by plate load tests.
• Design k-value is often estimated from the soaked CBR test value

Effective CBR

• Minimum subgrade CBR of 8 percent is recommended for design.

Sub-Base

• K value for granular and cement treated subbases:

• K value for Dry Lean Concrete (DLC) subbases with separation and

drainage layers:

Characteristic of Concrete

Axle Load Characteristics

• Day time night time traffic analysis is important.
• Axle load survey of sample commercial vehicles is recommended.
• When data availability is limited (Appendix VII, DoR, 2021): For conversion of average number of axles for commercial vehicles: Busy highways 3.2 Feeder road 2.1 Percentage of front single, rear single, tandem and tridem axle can be taken as 31, 9, 28 and 32 respectively.

• Axle load spectrum can be taken as:

Design Traffic

Design Traffic (BUC)

• Commercial vehicles with tyre imprint of the outer wheel touching the longitudinal edge during six hours in day (10am to 4pm).
• For bottom-up cracking, single axle load causes the largest edge stress followed by tandem and tridem axles. So tridem axles are not considered for analysis.
• It is recommended that 25% of the total two-way commercial traffic may be considered as design traffic for two-lane two-way roads for the analysis of bottom-up cracking.
• For four lane and other multi-lane divided highways, 25% of the total traffic in the direction of predominant traffic may be considered for design of pavement for bottom-up cracking.

Design Traffic (TDC)

• Six hour night time (0am to 6am) commercial vehicles with the spacing between the front axle and the first rear axle (single/tandem/tridem) less than the spacing of transverse joints should be considered for top-down cracking analysis.
• A default value of 50% of the design traffic used for bottom-up cracking analysis may be considered for the analysis of top-down cracking when data availability is limited.

Temperature Differential

Calculation of Flexural Stresses (BUC)

• Finite Element Analysis of a large number of concrete pavements with different pavement configurations subjected to various combinations of axle loads and temperature differentials have been presented in the form of charts in Appendix

IV (chart) & Appendix V(Equations). Sample charts:

• Equations:

Calculation of Flexural Stresses (TDC)

• Critical top flexural stress is caused by a specified magnitude of single/tandem/tridem axle in combination with a specified negative temperature differential

Calculation Fatigue Damage (CFD) Analysis

• For each axle load group, flexural stress is divided by the design flexural strength or modulus of rupture of the cement concrete to obtain stress ratio (SR).
• If SR is less than 0.45, allowable number of cycles of axle load (N) is infinity.
• Otherwise:

• The cumulative fatigue damage caused to the slab during its service life should be equal to or less than one.
• If the sum of fatigue damages due to wheel load and curling stresses at the bottom and at the top is equal to or less than 1, the pavement is

safe from large scale cracking if: CFD (BUC) + CFD (TDC) < 1

Design of Slab Thickness (DoR, 2021, IRC 58-2015)

AASHTO 1993 Rigid Pavement Design

• Evaluate concrete properties (modulus of rupture 𝑆𝑐′, elastic modulus Ec) and subgrade strength (modulus of subgrade reaction k)
• Drainage Coefficient (Cd)
  • Depending on expected level of drainage. Recommended drainage coefficients (Cd)

• Load Transfer Coefficient (J)
  • Ability of concrete pavement to transfer load across discontinuities such as joints or cracks. Recommended load transfer coefficients (J)

• Iteratively solve the AASHTO equation or use nomograph for slab thickness

Practice Tasks from the Lecture

• Review design of rigid pavement by IRC 58-2015, DoR 2021 and AASHTO method

3.7 Pavement design (Surface dressing method) for low cost road, IRC method

IRC SP72-2015 and DoR, 2021

Subgrade Strength Classification:

• Traffic is classified as per the equivalent standard axle load calculated for the design. The category of traffic class is shown the table below:

Thickness

Design (Design

Chart):

Thickness

Design (Design

Chart):

Design of Surface Dressing (DoR, 2021)

• Nominal Size of Chipping:
  • Based on volume of commercial vehicle with unladen weights more than 1.5 tonnes and hardness of existing pavement.

• Selection of Binder:
  • Selection of viscosity graded (VG) paving bitumen based on climatic condition.

• Determination of overall weighing factor : If the existing surface of the road is rough, it should be rated as 'very lean bituminous' even if its overall colour is dark with bitumen. Similarly, when determining the rate of spread of binder for the second layer of a double surface dressing, the first layer should also be rated 'very lean bituminous.

• Determination of bitumen spray rate:

• For slow traffic or climbing grades with gradients steeper than 3 percent, the basic rate of spread of binder should be reduced by approximately 10 percent.
• For fast traffic or downgrades steeper than 3 percent the basic rate of spread of binder should be increased by approximately 10 percent.
• Emulsion binders: multiply the rate of spread by 90 / bitumen content of the emulsion (per cent).

• Determination of spread rate of chippings:

DOR (2021): Flexible Pavement (Upto 2msa)

Practice Tasks from the Lecture

• Check and practice IRC (37: 2018) and DOR (2021)
• Check and practice DOR (2021) low volume roads, IRC SP- 2015

End of Chapter 3 — Pavement Design