CSE40407-Topic 5 - Pavement Design_Part1

Introduction to Pavement Design

Contents

• Introduction
• Factors Affecting Pavement Design
• Pavement Material Design
• Pavement Structural Design: Local Practices
• Foundation Design
• Design of Flexible Pavement
• Design of Rigid Pavement

Pavement Overview

• Pavement is a widely used transport infrastructure.
• Flexible pavement is dominant due to difficulties in repairing rigid pavement in busy areas.
• Cost-effectiveness:
  • Flexible pavement is not always more cost-effective.

Advantages of Rigid Pavement

• Relative: Inert to chemical attack and far less susceptible to surface distresses, such as raveling and pothole.
• Elastic/shear modulus: Much larger

Pavement Structural Layers

Flexible Pavement

• Wearing Course (Bituminous)
• Base Course
• Roadbase
• Sub-base
• Subgrade (Foundation)

Rigid Pavement

• Concrete Slab
• Sub-base
• Subgrade (Foundation)
• Example layers:
  • WC (Wearing Course): 40 mm (built in 2006)
  • BC (Base Course): 60 mm (built in 1977)
  • RB1, RB2, RB3 (Roadbase): 50 mm each (built in 1977)
  • OGFC (Open Graded Friction Course): 30 mm (built in 2006)
  • Mesh reinforcement
  • Separation membrane

Foundation Design: Subgrade

• Granular soils:
  • California Bearing Ratio (CBR) test is the most common way for determining the elastic modulus of subgrade.
• Cohesive soils or clays:
  • Plasticity index is always used for the determination of the elastic modulus of the subgrade.

Factors Affecting Pavement Design

• Climate
• Materials
• Response
• Damage
• Time
• Traffic
• Structure
• Damage Accumulation
• Distress

Bituminous (asphalt) mixtures in Hong Kong

Test Methods for Aggregates

• Gradation and size
• Strength, toughness, and abrasion resistance
• Durability and soundness
• Particle shape and surface texture
• Cleanliness and deleterious materials
• Moisture content
• Stiffness
• Volumetric

Strength, Toughness, and Abrasion Resistance Tests

• Los Angeles (LA) value test
• Aggregate crushing value (ACV) test
• Aggregate impact value (AIV) test
• 10 percent fines aggregate crushing value test (10 % FACT)

Durability and Soundness Test

• The soundness test repeatedly submerges an aggregate sample in a sodium sulfate or magnesium sulfate solution.

CBR: California Bearing Ratio Test

• Developed by the California State Highways Department in 1930
• Characterize the resistance of a material to uniaxial penetration
• Measure of soil shear strength relative to standard crushed stone material

CBR Test Procedure

• Plot the graph of force vs. penetration with smooth curve drawn through the relevant points
• If the curve is uniformly convex upwards (no correction needed), if not some correction need be applied
• Record the force at 2.5mm and 5mm, calculate CBR at 2.5mm and 5mm using:

CBR Calculation

• Loads Corresponding to 2.5 mm and 5.0 mm Penetration for the Standard Rocks
• Penetration
  • 2.5 mm: 13.24 KN
  • 5.0 mm: 19.96 KN
• The larger of the CBR at 2.5mm and 5.0mm deformation will be taken as the design CBR

Resilient Modulus Test

• Test unbound materials using repeated load under confining pressure.
• The elastic modulus based on the recoverable strain under repeated loads is called the resilient modulus $M_R$, defined as
  • $M<em>R = \frac{\sigma}{\&</em>{\text{Recoverable}}}$
    • $\sigma$ = Deviator Stress
    • $σ_1$ = Major Stress
    • $σ_3$ = Confining Stress
    • $M_R$ = Resilient Modulus

Resilient Modulus for Granular Materials

• The resilient modulus of granular materials increases with the increase in the first stress invariant.
• Constitutive Relationship: A simple relationship between resilient modulus and the first stress invariant can be expressed as
  • $E = K<em>1\theta^{K</em>2}$
    • $K<em>1$ and $K</em>2$ are experimentally derived constants.
    • $\theta$ is the stress invariant, which can be either the sum of three normal stresses $σ<em>x, σ</em>y, and σ<em>z$, or the sum of three principal stresses $σ</em>1, σ<em>2, and σ</em>3$:
      • $\theta = σ<em>1 + σ</em>2 + σ<em>3 = σ</em>x + σ<em>y + σ</em>z$
• This relationship does not apply to non-granular materials.

Bitumen vs. Tar

Bitumen (asphalt binder):

• Soluble in petroleum products
• Generally a by-product of petroleum distillation process
• Can be naturally occurring
• The earliest record of human use of bitumen to date, is as a hafting material 180,000 years ago in the El Kowm Basin in Syria, where it was applied to stick flint implements to the handles of various tools in a way that persisted until Neolithic time.

Tar:

• Resistant to petroleum products
• Generally by-product of coke (from coal) production
• Potentially Carcinogenic

First US Hot Mix Asphalt (HMA) Construction

• Constructed in 1870’s Pennsylvania Ave.
• Used naturally occurring asphalt from surface of lake on Island of Trinidad
• Two sources
  • Island of Trinadad
  • Bermudez, Venezuela

Petroleum Refining

• Key components: residuum, flasher bottoms, heavy gas oil, light gas oil, flasher tops, lighter hydrocarbons, heavier hydrocarbons.
• End products: gasoline, coke, asphalt, waxes, lubricating oils, greases, kerosene, jet fuel, diesel fuel, home-heating fuel.

Penetration Testing

• Sewing machine needle
• Specified load, time, temperature (25 oC)
• 100 g Initial Penetration in 0.1 mm After 5 seconds
• The depth of penetration is measured in units of 0.1 mm and reported in penetration units (e.g., if the needle penetrates 8 mm, the asphalt penetration number is 80)

Penetration Specification

• Five Grades:
  • 40/50
  • 60/70
  • 85/100
  • 120/150
  • 200/300

Penetration Grading Specification

• Uses penetration results to specify
• Adds:
  • Flash point test
  • Ductility
  • Solubility
  • Thin film oven (TFO) aging
  • Penetration
  • Ductility

Flash Point (Safety)

Ductility

• Stretching rate: 5 cm/min
• Temperature: 25 °C

Solubility (Purity)

• Commonly used solvent: Trichloroethylene (hazardous)

Thin Film Oven Test

• ASTM D1754 and AASHTO T179 – Effects of Heat and Air on Asphalt Material
• Simulate Short Term Aging
• Place a layer of asphalt on a pan in an oven at 163 oC.
• The pan rotates at 5-6 RPM for 5 hours
• The asphalt is tested after aging
• Limit change in penetration and viscosity

Typical Penetration Specifications

• S.G.@25/25 DEG C: 1.039
• SOFTENING POINT R & B: 46.2 DEG C
• PENETRATION @25 DEG C: 68 (Top) 0.1mm, 68 (Bot) 0.1mm
• RETAINED PENETRATION AFTER TFO TES: >52 % ORIG.
• FLASH POINT - COC: >232 DEG C
• DUCTILITY @25 DEG C: >100 ст
• DUCTILITY @25 DEG C AFTER TFO TEST: >50 cm
• LOSS ON HEATING: <0.20 %wt.
• SOLUBILITY IN 1,1,1 - TRICHLOROETHANE: >99.50 %wt.
• VISC@100°C (300mmHg Vac): 3.216 Pa.Sec
• VISC@135'C (300mmHg Vac): 0.380 Pa. Sec
• WAX CONTENT: 1.6 %wt.

Drawbacks

• Empirical
• High Sensitivity

Superpave System (SHRP program)

• Addresses:
  • Fatigue
  • Rutting
  • Cracking
• Tests:
  • RTFO: Short Term Aging, Construction [RV] [DSR]
  • PAV: Long Term Aging, Low Temp Cracking [BBR] [DTT]

Superpave Binder

Superpave Asphalt Binder Specification

• The grading system is based on climate
• PG 64 - 22 Performance Grade
  • Average 7-day max pavement temperature
  • Min pavement temperature

Superpave Performance Grade

• Examples:
  • PG 52-16, PG 58-16, PG 64-16, PG 70-16, PG 76-16
  • PG 52-22, PG 58-22, PG 64-22, PG 70-22, PG 76-22
  • PG 52-28, PG 58-28, PG 64-28 70-28, PG 76-28
• Modifier Required

Superpave Test

• Rolling thin film oven (RTFO)
• Pressure aging vessel (PAV)
• Rotational viscometer (RV)
• Dynamic shear rheometer (DSR)
• Bending beam rheometer (BBR)
• Direct tension tester (DTT)

Rolling Thin Film Oven

• Simulates short-term aging by heating a moving film of asphalt binder in an oven for 85 minutes at 163°C.

Pressure Aging Vessel

• Simulates the effects of long-term asphalt binder aging that occurs as a result of 5 to 10 years HMA pavement service
• 50 grams RTFO aged binder at 2070kPa pressure and 90-110 oC temperature for 20 hours

Rotational Viscosity Test

• Test high temperature viscosities (the test is conducted at 135°C).

Dynamic Shear Rheometer

• Test medium to high temperature viscosities

Bending Beam Rheometer

• Test asphalt binders at low temperatures where the chief failure mechanism is thermal cracking
• A curve, called the master stiffness curve, is then fit to these points and is of the form:
• The key reporting values were creep stiffness at 60 seconds and the slope of the master stiffness curve at 60 seconds (commonly called the “m-value”).
  • Since a higher creep stiffness value indicates higher thermal stresses, a maximum creep stiffness value (300 MPa) was specified.
  • Since a lower m-value indicates a lesser ability to relax stresses, a minimum m-value (0.300) was specified.

Direct Tension Test

• Compliment the BBR in testing asphalt binders at low temperatures

Flash Point Test

• For Safety Consideration

Performance Grade of Asphalt Binder

• The required physical properties remain constant for all the performance grades (PG).
• However, the temperatures at which these properties must be reached vary depending on the climate in which the binder is expected to be used.

Performance Grades

• Rotational Viscosity: <3 Pa-s @ 135 °C
• Flash Point: >230°C
• DSR G*/sin δ (Dynamic Shear Rheometer): > 1.00 kPa
• (Rolling Thin Film Oven) RTFO, Mass Change < 1.00%
• DSR G*/sin δ (Dynamic Shear Rheometer): ≥ 2.20 kPa
• (Pressure Aging Vessel) PAV 20 hours, 2.10 MPa
• DSR G*sin δ (Dynamic Shear Rheometer): < 5000 kPa
• BBR S (creep stiffness) & m-value (Bending Beam Rheometer): S< 300 MPa, m≥ 0.300
• DTT (Direct Tension Tester): Eₜ ≥ 1.00%

Superpave Binder Specs & Selections

• Physical Hardening
• How the PG Spec Works
• Test Temperature Changes Spec Requirement Remains Constant

Permanent Deformation

• Addressed by high temp stiffness G*/sin δ on unaged binder > 1.00 kPa
• G*/sin δ on RTFO aged binder > 2.20 kPa
• Early part of pavement service life
• Heavy Trucks

Fatigue Cracking

• Addressed by intermediate temperature stiffness G*sin δ on RTFO & PAV aged binder < 5000 kPa
• Later part of pavement service life

Low Temperature Cracking

• Miscellaneous Spec Requirements

Superpave Binder

Effect of Loading Rate on Binder Selection

• Dilemma
  • Specified DSR loading rate is 10 rad/sec
  • What about longer loading times ?
• Use binder with more stiffness at higher temps
  • Slow - - increase one high temp grade
  • Stationary - - increase two high temp grades
  • No effect on low temp grade 90 kph
• Example:
  • for toll road: PG 64-22
  • for toll booth: PG 70-22
  • for weigh stations: PG 76-22

Summary of How to Use PG Specification

• Determine
  • 7-day max pavement temperatures
  • 1-day minimum pavement temperature
• Use specification tables to select test temperatures
• Determine asphalt cement properties and compare to specification limits

“Rule of 90” Effect of Loading Rate

• Effect of Traffic
• Rounding
• Reliability
• Example: PG 64 - 34 has a temperature range of 64 to - 34 or 98 C. Therefore, this binder is probably modified !! (Depends on Asphalt Source!)
• Is a PG a Modified Binder ?

Typical mixture types

• Dense-graded
• Open-graded
• SMA (Gap-graded)

Marshall Mix Design

• Developed by Bruce Marshall for the Mississippi Highway Department in the late 30’s
• Waterway Experiment Station (WES) began to study it in 1943 for WWII
  • Evaluated compaction effort
    • No. of blows, foot design, etc.
    • Decided on 10 lb. Hammer, 50 blows/side
    • 4% voids after traffic

Marshall Mix Design - Steps

• Select and test aggregate
• Select and test asphalt cement
• Develop trial blends at different asphalt contents
  • Heat and mix asphalt cement and aggregates
  • Compact specimen (100 mm diameter) using Marshall hammer
• Conduct Marshall tests and determine the optimum asphalt content

Key Information

• Theoretical maximum density
• Actual density
• Voids in total mix (Air void) %
• Voids in mineral aggregate %
• Voids filled with binder %
• Marshall stability (kN)
• Marshall flow (mm)

Marshall Stability and Flow

• Marshall Test
  • Specimen is preheated and placed in the testing head
  • Load is applied at the rate of 50 mm/min
  • A dial gauge is used to measure the vertical deformation
• Marshall Stability is the Max Load (in kN) required to produce failure
• Marshall Flow is the deformation depth (in mm) at the failure load

HK Specifications for Typical Bituminous Materials

Properties

Marshall Design Use of Data Asphalt Institute Procedure

• Air Voids, %
• Asphalt Content, %
• Stability
• Asphalt Content, %
• Unit Wt.
• Asphalt Content, %
• Target optimum asphalt content = average 4%
• Flow
• Asphalt Content, %
• VMA, %
• Asphalt Content, %
• Use target optimum asphalt content to check if these criteria are met Maximum Minimum OK OK

Marshall Design Method

• Advantages
  • Attention on voids, strength, durability
  • Inexpensive equipment
  • Easy to use in process control/acceptance
• Disadvantages
  • Impact method of compaction
  • Does not consider shear strength
  • Load perpendicular to compaction axis

Superpave Mix Design

• Optional Tests
  • Asphalt Pavement Analyzer
  • Hamburg Wheel Tracking (HWT) Test

Foundation Design: Subgrade

• For granular soils
  • $E_s = 10 \times CBR$
    • $E_s$ = elastic modulus of the subgrade [MPa]
    • CBR = California Bearing Ratio [%]
• For cohesive soils or clays
  • $E<em>s = 70 - I</em>p$
    • $E_s$ = elastic modulus of the subgrade [MPa]
    • $I_p$ = Plasticity Index [%]

Typical Values of Elastic Modulus of Subgrades

Foundation Design: Sub-base

• Material: Granular material
• Lean concrete is generally NOT recommended for sub-base application
  • Flexible: localized shrinkage cracks developed in the lean concrete sub-base would likely propagate upwards through the bituminous surfacing causing reflective cracking at the pavement surface
  • Rigid: granular material can be used for controlling pumping

Capping Layer/Sub-base Recommendations

Flexible Pavement Design

• Bituminous Layers
• Sub-base Layer
• Subgrade
• Wheel Load - tensile strain in bituminous layers, compressive strain in Subgrade

Assumptions for Flexible Pavement Design

• Three-layered system: i.e., all bituminous layers: wearing course, base course, & roadbase, are combined into one layer.
• Materials are homogeneous and isotropic, characterized by modulus of elasticity (E) and Poisson’s ratio (ν).
• A constant value of 0.35 for the Poisson’s ratios of all layers.

Design Criteria

• Fatigue cracking
  • Fracture under repeated or fluctuating stress having a maximum value generally less than the tensile strength of the materials
  • Initiation governed by the horizontal tensile strain at the bottom of the bituminous road base
• Permanent deformation
  • Pavement under the wheel path continually consolidating and settling under repeated traffic loading to form a groove/rut
  • Primarily depend on the vertical compressive strain at the surface of subgrade

Design Life

• Target: low life-cycle cost
• Design life: 40 years
• No structural maintenance is required under normal circumstances and the service life of the pavement structure can be sustained by minor repairs coupled with resurfacing at appropriate intervals.

Traffic Load

• Combined damaging effect of traffic load is collectively expressed as a cumulative number of equivalent standard axles with a 80kN single axle dual-wheel configuration with tyre pressure of 0.577 MPa

Commercial Vehicle Forecast

• Medium/heavy goods vehicle and bus
• Other light vehicles, eg: motorcycle, private car and public light bus, are ignored
• Forecast: on-site traffic count data, traffic census or other traffic studies or planning data

Commercial Vehicle Damage Factors

• Commercial vehicle damage factors (CVDF) are the numbers of equivalent standard axles per class of commercial vehicles, taking into account the cumulative damage effects arising from different axle loads of vehicles.

Distribution of Commercial Vehicles among Lanes

• For design purpose, 65% of the commercial vehicles be assumed travelling in the slow lane.
• The estimated number of vehicles should be checked to ensure that it does not exceed the capacity of the lane.

Lateral Wander Factor

Calculation of Design Traffic Load

Step 1: Determine the design initial average daily traffic flow ($AADT_d$)

• $AADT_d = AADTh \times (1+r)^m$
  • $AADTh$ = base annual average daily traffic flow [vehicle/day]
  • $r$ = annual traffic growth rate [in decimal], from past traffic figures or from Transport Department (typical values ranging from 0.01 to 0.04)
  • $m$ = length of period between timing in $AADT_h$ and the time that the road is expected to open to traffic [years]

Step 2: Determine the initial daily number of commercial vehicles ($C_e$) in the slow lane in one direction

• $C<em>e = P</em>s \times P<em>v \times D</em>r \times AADT_d$
  • $P_s$ = percentage of commercial vehicles using slow lane
    • 1 where there is only 1 traffic lane in the direction concerned
    • 0.65 for other cases
  • $P<em>v$ = percentage of commercial vehicles in $AADT</em>d$
  • $D_r$ = directional split factor
    • 0.55 for 1-way roads
    • 0.5 for 2-way roads (assuming 55%/45% split)

Step 3: Determine the cumulative number of commercial vehicles ($C_v$) using the slow lane during the design life

• $C<em>v = 365 \times C</em>e \times \frac{(1+r)^n - 1}{r}$
  • $r$ = annual traffic growth rate [in decimal]
  • $n$ = design life [years]

Step 4: Check that $C<em>v$ does not exceed the design flow capacity of the traffic lane ($C</em>d$)

• $C<em>v \leq C</em>d = 365 \times n \times \frac{D<em>f}{K</em>p}$
  • $n$ = design life [years]
  • $D_f$ = maximum design flow [vehicles per hour per lane] recommended in the TPDM, Volume 2 Chapter 2
  • $K_p$ = peak hour factor recommended in Table 5
    • Expressways: 0.05
    • Rural Trunk Roads: 0.065
    • Rural Roads: 0.08
    • Road Type: Expressway/trunk road, Primary distributor no frontage crossings, no standing vehicles, negligible cross traffic , District distributor frontage development, side roads, pedestrian crossings, bus stops. loading restrictions at peak hours.
    • 4 lane Dual carriageway Peak hourly flow : 3600, 5700, 2000, 3000, 2550
    • One direction of flow : 2800 , 3050 , 2950 , 3200 , 4800
    • Expressway/trunk road, Primary distributor no frontage crossings, no standing vehicles, negligible cross traffic :3600, 5700, 2000 3000, 2550

Step 5: Determine the design traffic load ($C_n$) for flexible pavements

• $C<em>n = C</em>v \times CVDF \times W_f$

  • $CVDF$ = weighted mean of commercial vehicle damage factors [standard axle / commercial vehicle] recommended in Table 3

  • $W_f$ = lateral wander correction factor recommended in Table 4

  • Class of commercial vehicle

    • Medium & heavy goods vehicle: 3.3 CVDF(No. of standard axles / vehicle)
    • Bus: 2.9 CVDF(No. of standard axles / vehicle)

Note: These commercial vehicle damage factors are for reference only. Measurements of local axle loads should be undertaken in major road pavement projects for determination of the CVDF appropriate to the particular road project.

Exercise - Calculation of Design Traffic Load

• Given:
  • dual two-lane expressway (7.3m-wide each direction)
  • current AADT is 10,000 vehicles/day
  • traffic grow rate r = 3%
  • 10% commercial vehicle composed of 100% medium/heavy goods vehicle
  • Design life of 40 years with expected traffic opening in 2020.

#

Properties of Bituminous Materials

• Mechanical properties of bituminous materials are temperature-, and loading- dependent.
• Elastic modulus is adopted as one of the input parameters

Bounds for Total Thickness of Bituminous Pavement

• Lower bound: indicates the bituminous pavement thickness below which a pavement structure would unlikely manifest long-life behavior due to the substantial tensile strain at the bottom of the road base under wheel load.
• Upper bound: set for the bituminous pavement thickness so that a pavement structure would not be over-designed. When the total thickness of a bituminous pavement reaches a certain value, the tensile strain at the bottom of road base would be low enough to avoid fatigue failure no matter how large is the traffic volume.

Selection of Surfacing Material

• Role of surfacing layer:
  • to resist repeated traffic load and environmental weathering
  • to provide necessary skid resistance and riding comfort to serve the vehicular traffic.
• Options:
  • Ordinary wear course (WC): dense-graded mixture; impermeable and smooth surfacing layer.
  • Stone mastic asphalt (SMA): gap-graded mixture; strong rut resistance and suitable for road sections with heavy axle loads and frequent stop-and-go traffic.
  • Porous friction course (FC): open-graded mixture; permeable to facilitate drainage and reduce traffic noise; non-structural layer from a conservative design perspective.

Foundation Design: Subgrade (Cont.)

*Example: pavement structural composition and thickness of each layer traffic Load: Cn = 27 million axles

• Subgrade modulus: 80MPa
• Road type: district distributor
• Pavement type: flexible pavement

Solution

Design steps provided charts for variable conditions.

Composition and Thickness of Bituminous Layers

Material

• Polymer modified friction course (Optional)Thickness (non-structural layer) 30
• Wearing course (Bituminous): 40
• Bose course (Bituminous): 65
• Rood base (max. 395)
• Sub-base (Granular)Designed to RD/GN/042

Thickness to be designed to RD/GN/042

Rigid Pavement Design

• Length, Width, Thickness details
• Thickness - Modulus of Elasticity, E Stress σ Strain ε
• Modulus of Subgrade Reaction, k Wheel Load

Design Criteria

• Traffic induced stresses
• Thermal stresses
• Fatigue failure

Traffic induced stresses

• In general, thickness of the slab will be governed by maximum tensile stress.
• The critical loading point is along the slab edges in both longitudinal and transverse directions.

Types of Rigid Pavements

• JPCP Jointed Plain Concrete Pavement
• JRCP Jointed Reinforced Concrete Pavement
• CRCP Continuously Reinforced Concrete Pavement

Thermal Stresses

• Two components: uniform longitudinal stresses over the cross- section of the concrete due to seasonal temperature variations and warping stresses due to daily temperature gradient change.
• Longitudinal tensile stresses develop when the concrete cools and its contraction is prevented by the friction between the concrete slab and sub-base. Stresses are greatest in the centre of the slab and increase with longer slabs.
• Warping stresses are the result of an uneven temperature distribution over the cross-section of the slab. If the top surface of a slab is warmer than the bottom surface, the slab becomes convex but its own gravity opposes such stress-free distortion, resulting in compressive stresses at the top and tensile stresses at the bottom of the slab.

Fatigue Failure

• The fatigue behaviour of concrete depends on the stress ratio which is the quotient of tensile stress and modulus of rupture of concrete.
• Individual damage of axle loads is accumulated using Miner’s rule to assess the pavement failure.

Design Life

• Target: low life-cycle cost
• Design life: 40 years

Average Number of Axles per Commercial Vehicle

• The predicted number of commercial vehicles is converted to number of axles by multiplying the number of commercial vehicles by the average number of axles (Aa) per commercial vehicle, which is recommended to be 3.1.

Distribution of Commercial Vehicles among Lanes

• The procedures and considerations in forecasting the distribution of commercial vehicles among traffic lanes for rigid pavement design are identical to those for flexible pavements.

--- ## Step-by-step Calculation of Design Traffic Load

• The steps for determining the anticipated number of axles for structural design of rigid pavement is identical to those for flexible pavement, except Step