chapter 2: mix type selection - sabita
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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
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)
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
Other traffic considerations
• 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
Other traffic considerations
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
• Maximum particle size
• Climate
• Other considerations
Factors impacting on mix type selection
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
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:
Maximum Particle Size
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
• Maximum particle size
• Climate
• Other considerations
Factors impacting on mix type selection
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
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
• Maximum particle size
• Climate
• Other considerations
– Functional requirements
– Geometric conditions
– Material availability and project specification
Factors impacting on mix type selection
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
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
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