Asphalt(Mix(Design( · 2016-04-12 · Mix Design Process Volumetrics only y ry Select mix type Select components Determine proportions of aggregates to meet target grading Determine
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Asphalt Mix Design
Asphalt Mix Design
Outline of presentation • Selection of mix type and component materials • Combining and proportioning of aggregates • Mixing conditioning and compaction of samples • Volumetric characteristics • Mechanical testing • Design of specialised mixes Reference: • AUSTROADS: Guide to Pavement Technology - Part
4B: Asphalt
Mix Design Process
Volumetrics only
Rec
ipe
only
Uns
atis
fact
ory
Select mix type
Select components
Determine proportions of aggregates to meet target grading
Determine binder content(s)for tests
JOB MIX
Mix and compact specimens
Determine volumetrics
Adjust or reselect components and/or proportions
Mechanical tests
Production trial
Test Additional tests
Uns
atis
fact
ory
Uns
atis
fact
ory
Selection of Mix Type
• Selecting mix type sets parameters for: – Aggregate quality, shape and grading – Filler requirements – Binder type and range of binder content – Volumetric requirements (voids, etc.)
• Design can be a compromise between: – Stiffness and rutting resistance for heavy traffic
applications using coarse gradings, stiff binders, etc. – Durability for light traffic (soft binders, low air voids,
high binder contents, etc.)
Recipe Criteria for Asphalt Mix Design
• Used – Where characteristics of mix not readily defined or
optimised by conventional tests – For simple mixes and minor applications
• Examples include: – Grading limits in open graded asphalt – Binder content for above mix types – Setting binder type or additives to achieve particular
attributes – Cold mixes
Volumetric properties
• Bulk density • Maximum density • Voids in mineral aggregate (VMA) • Air voids • Absorbed binder • Effective binder content • Voids filled with binder • Binder film thickness
Constituents of Compacted Asphalt
AIR VOIDS
EFFECTIVE BINDER CONTENT
VOIDS INMINERAL AGGREGATE
TOTAL AGGREGATEVOLUME(PARTICLE DENSITY ON A DRY BASIS)
TOTAL BINDER VOLUME
VOLUME OF MIX EXCLUDING AIR VOIDS(MAXIMUM DENSITY)
TOTAL VOLUME OF COMPACTED ASPHALT(BULK DENSITY)
ABSORBED BINDER
AGGREGATEAGGREGATE VOLUME (EFFECTIVE DENSITY)
VOIDS FILLED WITH BINDER
VMA, Air voids and Effective Bitumen
Aggregate volume(effective density)
Aggregate volume(apparent particledensity)
Effective bitumen binder
Air void
Aggregate
Absorbed bitumen
Water permeableporosity, not filledwith bitumen
Volume of water permeable porosity(water absorption)
Aggregate volume(particle density ona dry basis)
Dense Graded Asphalt • Dense graded asphalt mixes use a continuous
grading to facilitate packing of aggregate particles to achieve:
• flexibility • durability • structural stiffness • deformation resistance • low permeability.
• Fuller equation P = (d/D)n Where: P = percentage passing each sieve
d = size of sieve opening D = maximum size of aggregate n = grading exponent
Dense Graded Asphalt
TYPICAL GRADING CURVE FOR 14 mm MIX
60
80
100
40
20
0
Perc
enta
ge p
assi
ng
0.075 0.30 1.18 4.75 9.5 19.0Sieve size (mm)
Grading envelope forlight traffic mixes
Grading envelope for heavy duty mixes
n = 0.45 maximum density curve
Mechanical Testing • Fundamental tests for stiffness and deformation
resistance, and fatigue: - repeated load indirect tensile test - repeated flexural bending
• Simulative tests for deformation resistance and other properties: - wheel tracking test - moisture sensitivity
• Empirical tests : - Marshall test
Austroads Mix Design Procedure
Select materials and grading
At x compaction cycles
Design bitumen content
Level 1 Level 2 Level 3
Gyropac for 250 cycles*
Measure creep
Check % film index for durability*
Mix specimens at design bitumen content
Condition 1 h at 150C
Compact for appropriate cycles
Measure modulus
Moisture sensitivity if required
End Level 3
Report wheel tracking rate
End Level 1 Check compliance End Level 2
Compact wheel track slabs
Voids
Mix specimens at design bitumen content
Voids > spec. minimum
* testing normally carried out in Level 1
Compact slab
Compact trial mixes at 4 or 5 binder contentsin Gyropac
Evaluate plots of voids,density and VMA vs bitumen content
Fatigue test if required
Compaction of Laboratory Mixes
• Gyratory compaction • Marshall compaction • Preparation of slabs and large samples
Gyratory Compaction
Principle of Gyratory Compaction
Force
Gyratory angle
Gyratory Compaction
• Light traffic 50 cycles • Medium traffic 80 cycles • Heavy traffic 120 cycles • “Refusal Density” 250 cycles
Asphalt Stiffness
• Stiffness is response to load and will vary with: – temperature – rate of loading (traffic speed)
• Stiffness may be increased by: - decreasing the binder content - increasing the angularity of the aggregate - adjusting the filler content - using a harder binder.
• Stiffness may be estimated from: – Shell Nomographs – Resilient modulus test – Fatigue test
Indirect Tensile Test
Deformation
• Worst conditions for rutting are: – High temperatures – Heavily loaded vehicles – Channelised traffic – Slow moving vehicles
• Deformation resistance is controlled by – Aggregate grading and shape – Binder content – Appropriate field compaction
• Deformation resistance is measured by – Wheel tracking test
Fatigue • Fatigue is caused by repeated bending and
tensile stress • Fatigue life may be determined:
– From Shell formula – Laboratory test
• Laboratory test conditions for fatigue are: – Constant stress (constant load)
• Increasing stiffness increases fatigue life – Constant strain (constant deflection)
• Fatigue life is increased by softer bitumen, higher binder content or polymer modified bitumen
Fatigue Test Apparatus
Marshall Stability and Flow
Marshall Test Results
Bitumen
Bitumen
Bitumen
5.5
Stab
ility
(kN
)
4.0
6.0
8.0
10.0
12.0
Flow
(mm
)
5
4
3
05.5
14.0
Bitumen
1
2
Bitumen
Air v
oids
(%)
VMA
(%)
VFB
(%)
Bulk
Den
sity
(t/m
3)
10
8
6
4
2
05.5
2.380 50
20
18
16
14
12
105.54.5 6.5 4.5 6.5
6.55.54.56.55.54.5
4.5 6.5 4.5 6.5
2.480
2.460
2.440
2.420
2.400
Bitumen
70
60
90
80
100
Min
Min
Min
Max
Target
Min
Min
Max
Max
Design of Other Mix Types
• Open graded asphalt • Stone mastic asphalt • Fine gap graded asphalt
These mixes are generally designed as a combination of targets for grading and binder content together with determination of volumetric properties at defined compaction levels. Mixes may also be subject to mechanical tests where appropriate.
Special Applications
• Recycled asphalt • Cold mixes • Polymer modified binders • Crumb rubber asphalt • Ultra thin asphalt
References
• AUSTROADS AGPT04B/07 : Guide to Pavement Technology - Part 4B: Asphalt
• AUSTROADS Asphalt Guide • AUSTROADS Framework for Specifying Asphalt • AAPA National Asphalt Specification • AS 2150 Hot Mix Asphalt • AS 2891 Methods of sampling and testing
asphalt • AUSTROADS Manual of Test Methods
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