Chapter 2: Mix Type Selection - Sabita

Chapter 2:

Mix Type Selection

South African Asphalt Mix Design

Seminar

Tuesday, 18 November 2014, CSIR ICC, Pretoria

Benoît Verhaeghe

Outline

• Asphalt mix types

• Factors impacting on mix type selection

• Mix design considerations and mix type selection

Asphalt Mix Types

Typical mix types

Mixture Types & Constituents

Sand Skeleton Stone Skeleton

Packing mechanisms

• Substitution (sand skeleton mixes)

– Medium/fine continuously-graded asphalt

– Semi-gap graded asphalt

– Gap-graded asphalt

• Filling (stone skeleton mixes)

– Coarse continuously-graded asphalt

– Stone-mastic asphalt

– Semi-open and open-graded asphalt

– Ultra-thin friction courses

0

100 20 40 60 80 100

80

60

40

20

0 100

80

60

40

20

0

% of fine aggregate

FILLER SKELETON

STONE

SKELETON

SAND

SKELETON

0

Bailey Method: Primary Control Sieve (PCS)

PCS

PCS = NMPS x 0.22 (average condition)

0.16 (all round) < 0.22 < 0.28 (all flat)

NMPS

(create voids)

(fill the voids)

Definition of coarse and fine aggregate

(Bailey Method)

10 mm

5 mm

2 mm

1 mm

Factors Impacting on Mix Type Selection

Factors impacting on mix type selection

• Traffic considerations

• Maximum particle size

• Climate

• Other considerations

Traffic considerations

• Heavy vehicles – Axle loads

– Traffic speed

– Tyre type, inflation pressures and loading

• Light vehicles

• Breaking and traction

• Fuel spillage

• Wander

Heavy vehicles

• Number

– Support layer stiff: rut resistance

– Support layer flexible: fatigue resistance

– Intensity of traffic at early age

• Axle Loads

– Restricted by law - potential for overloading?

– Equivalency factors

Pavement type* Range** Recommended

Granular/Granular 3-6 4

Granular/Cemented 2-4 3

Cemented/Granularpre-cracked***post-cracked***

4-103-6

5

Cemented/Cementedpre-cracked***post-cracked***

3-62-5

4,5

Hot-mix asphaltbase/Cemented

2-5 4

*Type of base over type of subbase

**Higher values for cracking, lower values for rutting

***Pre-cracked usually <10 of structural life

Traffic Classification

Design traffic (E80s) Traffic Class

< 0.3 million Light

0.3 to 3 million Medium

3 to 30 million Heavy

> 30 million Very Heavy

Pavementclass*

Pavementdesign

bearing capacity

(million 80 kNaxles/lane)

Volume and type of traffic **

Approximate

v.p.d.per lane***

Description

ES1 < 0,003 < 3 Very lightly trafficked roads; very fewheavy vehicles. These roads couldinclude the transition from gravel topaved roads and may incorporatesemi-permanent and/or all weathersurfacings.

ES2 0,003 - 0,01 3 - 10

ES3 0,01 - 0,03 10 - 20

ES4 0,03 - 0,10 20 - 75

ES5 0,10 - 0,30 75 - 220

ES6 0,3 - 1 220 - 700

Lightly trafficked roads, mainly cars,light delivery and agriculture vehicles;very few heavy vehicles.

ES7 1 - 3 > 700 Medium volume of traffic; few heavyvehicles.

ES8 3 - 10 > 700**** High volume of traffic and/or manyheavy vehicles.

ES9 10 - 30 > 2200**** Very high volume of traffic and/or ahigh proportion of fully laden heavyvehicles.ES10 30 - 100 > 6500****

Traffic Class Selection

• Consider increasing the traffic class if:

– Heavy vehicles are close to upper limit for the class AND

– There is a high % of fully laden heavy vehicles (e.g. mine haul roads) OR

– There is a high % of overloaded vehicles OR

– The expected traffic growth rate is more than 10% per year

Traffic speed

• High speeds reduce fatigue and rutting potential – Reduced bending and deflection

– Tensile strains may reduce by 50% for creep speed to 80 km/h

• Low speeds increase rutting potential – Increased loading time

– More rapid closure of voids

– Climbing lanes and intersections and others where speed < 30 km/h require special consideration

Time of Loading, t (log)

Stiffness (

log)

Elastic behaviour

Viscous

behaviour

Elastic behaviour

Viscous

behaviour

Mix Temperature

Stiffness (

log)

• Tyre type:

– Move from cross-ply to radial reduces fuel consumption by up to 30%, BUT result in increased contact stresses

– Fewer tyres (e.g. wide-based tyres) and heavier cargo result in higher contact stresses

Tyres

• Tyre pressure

– Under-inflation increases non-uniformity of contact stress

– Contact stresses exceeding 900 kPa are not uncommon

Tyres

CO

NTA

CT

ST

RE

SS

(kP

a)

Maximum vertical stress at CONSTANT

LOAD and various inflation pressures

1000

1200

1400

1600

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 0

200

400

600

800

720 kPa 620 kPa 520 kPa 420 kPa

Tyre width: 220 mm

PIN NUMBER ACROSS VRSPTA

INFLATION PRESSURE::

Tyre Load = 18 kN

INFLATION PRESSURE = 420 kPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 0

200

400

600

800

1000

1200

1400

1600

PIN NUMBER ACROSS VRSPTA

Tyre width: 220 mm

20kN 30kN 40kN 50kN

WHEEL LOAD:

CO

NTA

CT

ST

RE

SS

(kP

a)

Maximum vertical stress at CONSTANT

INFLATION PRESSURE at various loads

Other traffic considerations

• Light Vehicle Volumes

– Consideration for friction, noise reduction and riding quality

– High macro-texture required for speeds > 60 km/h

– Impermeability is an important consideration if the road is to be trafficked by predominantly light vehicles

• Braking and traction (at intersections or steep upgrades)

– Increased shear stresses: risk for PD/tearing

– Some mixes may not be appropriate


• Fuel spillage

– May cause softening of the asphalt

– Difficult to predict at the design stage

• Wander

– Increases with increase in speed and lane width

– Channelisation (e.g. BRT routes) increase rut and fatigue potential


0

50

100

150

200

250

0 50 100 150 200 250 300

Wander Stad. Dev. (mm)

Avg

. Ten

sile

Str

ain

(mic

rost

rain

)

• Traffic


• Climate



Maximum Particle Size

• Fundamental property of aggregate grading and asphalt mix type selection

• At most one-third of layer thickness (for compactability and to counter segregation during paving)


Maximum

particle size [mm]

Minimum layer thickness (mm)

Absolute minimum Preferred minimum

7,1 20 25

10 30 35

14 45 50

20 80 90

25 100 110

Recommended minimum layer thickness:


Typical MPSs for various applications:

Mix type Application Traffic Typical mix size

Sand skeleton

wearing

course

Light / Low 7,1 mm, 10 mm

Medium to

heavy¹ 10 mm, 14 mm

Very heavy 14 mm, 20 mm

Base course² All traffic

conditions 10 mm, 14 mm, 20 mm, 25 mm

Stone skeleton

Wearing

course

All traffic

conditions 10 mm, 14 mm

Base course All traffic

conditions 14 mm, 20 mm, 25 mm

1 14mm generally preferred to 10mm 2 Better to use the largest practicable size that is economically justifiable

• Traffic


• Climate



Climate

• Maximum temperature

- Key determinant for rutting potential

• Intermediate and minimum temperatures

- Determinant for fatigue and low-temperature

cracking potential

• Temperature differentials - Extreme temperature fluctuations increases the

demand for a balanced design

• High rainfall areas: - Increased potential for stripping and durability problems

- May have greater waterproofing requirements

High Temperature Zones

7-day average max asphalt temperatures:

Low Temperature Zones

Minimum asphalt temperatures:

PORT SHEPSTONE

CHIPINGE

DURBAN

BEIRA

FRANCISTOWN

GABORONE

WINDHOEK

WALVISBAY

KEETMANSHOOP

BLOEMFONTEIN

ROUXVILLE

WEPENER

KLERKSDORP

ZEERUST

JOHANNESBURG MAPUTO

LADYSMITH

VOLKSRUST

EAST LONDON

PORT ELIZABETH MOSSEL BAY

OUDTSHOORN

CERES

CAPE TOWN

MESSINA

TZANEEN

SOMERSET WEST

Macroclimatic regions of Southern Africa

PRETORIA

Wet

Moderate

Dry

Rainfall Zones

Moisture damage

• Aggregate factors

– Surface texture

– Aggregate type

• Binder factors

– Viscosity

– Chemical composition

• Environment

– Temperature

– Rainfall

Moisture damage

• Mix characteristics

– Permeability

– Variability

• Construction

– Compaction

– Joints

• Traffic volume

– Post-construction densification

– Hydrostatic pressure and hydraulic action

• Traffic


• Climate


– Functional requirements

– Geometric conditions

– Material availability and project specification


Functional requirements

• Special functional requirements include:

– Consideration for noise reduction in urban areas where light traffic volumes are high

– High skid resistance requirements at low speeds and MPD requirements at high speeds, especially for high rainfall areas

Vehicle Speed

Low macro-and-micro texture

Sk

id R

es

ista

nc

e

Low macro texture, high micro-texture

High macro texture, low micro-textureHigh macro texture, low micro-texture

High macro texture, high micro-texture

The Interaction between Texture Type and Vehicle Speed (after Visser

and Marais, 1984; and Sabey, 1966)

Facility

Texture Depth

(MPD, mm)

New Surfacings Existing Surfacings

Urban; legal and operating

speed equal or less than 50

km/h

0.5 0.5

Urban; legal speed less than 70

km/h 0.7 0.5

Rural; legal speed 70 km/h or

higher 0.9 0.5

Note: Values represent minimum threshold levels and not investigatory levels

Relative Importance of

Friction Components

for Different Network

Types

Typical Macro-texture Requirements

(Cook, 2005)

Geometric Considerations

• Situations where braking, acceleration and turning of heavy vehicles frequently occurs – Increased resistance to rutting, shoving, skidding and ravelling

• Situations where some difficulty in achieving specified tolerances and compaction could occur (e.g. intersections and steep grades) – Maintaining a minimum layer thickness would require special

attention

Material availability and project specifications

• Assess availability of binder and aggregate of the required quality upfront – Exploration of alternative/innovative approaches in the interest

of cost-effectiveness

– Alert tenderers to non-standard project specifications that may impact on material availability

– Alert clients to additional costs that may be incurred

• Assess whether materials of the required quality can be supplied consistently (i.e. same source, limited variability)

• To promote equitable tendering, client should indicate nominal proportions of component materials – Could be based on a preliminary design

Mix Design Considerations and Mix Type Selection

Design Objectives

• High Rutting Resistance required for: – High temperature regions

– Heavy and very heavy Traffic Classes

– Heavy vehicles traveling at low speeds

– Climbing lanes and intersections

• High Fatigue Resistance required for: – Heavy and very heavy Traffic Classes

– Low support stiffness

– Low temperature regions

Mix type selection

• In selecting the type of aggregate packing (sand or stone skeleton) and mix type, consider the following: – Selected mix type ultimately determines the grading

– Thin-layer asphalts for light traffic volume applications are normally sand-skeleton type mixes

– For high traffic volume applications, where friction and resistance to permanent deformation are key considerations, the preferred option is a stone-skeleton type mix

– Friction and noise are opposing properties, except for open-graded asphalt, SMA and UTFCs

– For the same maximum aggregate size, a continuously graded asphalt mix can be designed as either a stone or a sand-skeleton mix

Mix Type for rutting resistance

• High Rutting Resistance – SMA with / without modified binder

– Open-graded with modified binder

– Semi-open with A-R binder

– A-P/A-E2/A-H modified mixes

• Low Rutting Resistance – Semi-gap graded mixes

– Gap-graded mixes

Mix Type for fatigue/durability

• High Fatigue Resistance – Semi-open with A-R binder

– Continuously-graded with A-E binder

– Continuously-graded with A-R binder

• Low Fatigue Resistance – Open-graded with unmodified binder

Mix Type: Other Considerations

• Noise Reduction – Open-graded mixes and SMA

• Friction – Stone-skeleton mixes (open-graded, SMA, UTFC)

– Sand-skeleton mixes with rolled-in chips

• Impermeability – Semi-gap & gap-graded mixes

Mix type Binder type1 Typical

application

Performance rating (1 = Poor; 4 = Excellent)

Rut

resistance

Durability/

fatigue

resistance

Skid

resistance2

Impermeabi

lity to water

Noise

reduction

Sand

skeleton

Neat binder

Wearing

course

2 2 2 3 2

AR 3 4 2 3 2

AE 3 3 2 3 2

AP 4 3 2 3 2

AH 3 3 2 3 2

Rejuvenated

(RA) 3 3 2 3 2

Stone

skeleton

Neat binder

(Open-graded) 3 3 4 13 4

AE, AP (SMA) 4 4 3 3 4

AE (Open-

graded) 4 3 4 13 4

AR (Open

graded) 4 4 4 13 4

Mix Types & typical performance ratings

Mix Types & typical performance ratings

Mix type Binder

type1

Typical

application

Performance rating (1 = Poor; 4 = Excellent)

Rut

resistance

Durability/

fatigue

resistance

Skid

resistance2

Impermeabi

lity to water

Noise

reduction

Sand

skeleton

Neat binder

Base layer

3 3

N/A

3

N/A

AE 4 4 3

AP 4 3 2

Stone

skeleton

10/20 pen

(EME) 4 4 4

15/25 pen

(EME) 4 4 4

AE 3 4 2

AP 4 3 2

Thank You…

End of presentation

Documents

Chapter 2: Mix Type Selection - Sabita