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Marshall Mix Design
CIVL 3137 46
Asphalt Concrete Properties
Stability
Workability
Skid Resistance
Durability
Stripping
Fatigue Cracking
Thermal Cracking
Bleeding
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Stability
The ability to withstand traffic loads withoutdistortion or deflection, especially at highertemperatures.
To get good stability, use strong, rough, dense-graded, cubical aggregatewith just enough binder to coat the aggregate particles. Excess asphaltcement lubricates the aggregate particles and lets them slide past eachother more easily (which reduces stability). But a thick asphalt coatingprovides good flexibility to resist cracking, which is desirable. Hmmm…
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Workability
The ability to be placed and compacted withreasonable effort and without segregation ofthe coarse aggregate.
Too much asphalt cement makes the mix tender. Too little asphalt cementmakes it hard to compact. Too much natural sand can also make the mixtender because natural sand has smooth, round grains. Hmmm…
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Skid Resistance
Proper traction in wet and dry conditions.
To get good skid resistance, use smaller aggregate so there are lots ofcontact points, use hard aggregate that doesn’t polish and make sure youhave enough air voids to prevent bleeding.
Some states now use an open-graded friction course (OGFC) that allowswater to drain to the sides of the pavement, eliminating hydroplaning. ButOGFC is not very durable because of the open pores. Hmmm…
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Durability
The ability to resist aggregate breakdown dueto wetting and drying, freezing and thawing,or excessive inter-particle forces.
To get good durability, use strong, tough, nonporous aggregate andenough asphalt cement to completely coat all of the aggregate particles(to keep them dry) and fill all of the voids between particles (to slow theoxidation of the asphalt cement). But this reduces stability. Hmmm…
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Stripping
Separation of the asphalt cement coating fromthe aggregate due to water getting betweenthe asphalt and the aggregate.
To reduce stripping, use clean, rough, hydrophobic aggregate and addenough asphalt cement to provide a thick coating of asphalt on everyaggregate particle. This improves durability but decreases stability.
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Bleeding
The migration of asphalt cement to the surfaceof the pavement under wheel loads, especiallyat higher temperatures.
To prevent bleeding, incorporate enough air voids so the asphalt cancompress by closing air voids rather than by squeezing asphalt cementout from between the aggregate particles.
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Fatigue Cracking
Cracking resulting from repeated flexure ofthe asphalt concrete due to traffic loads.
To minimize fatigue cracking, use the proper asphalt cement grade andhave a thick asphalt cement coating to make the concrete flexible. Thisimproves durability but decreases stability.
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Thermal Cracking
Cracking that results from an inability toacclimate to a sudden drop in temperature.
To minimize thermal cracking, use the proper asphalt cement grade.
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Mix Design Basics
The right grade of asphalt cementRelates to fatigue cracking, thermal cracking, stability
The right type of aggregateRelates to stability, durability, stripping, skid resistance
The right mix volumetricsRelates to stability, durability, stripping, bleeding, skid resistance
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Right Type of Asphalt Cement
767064585246-4-10-16-22-28
PG70-22
Reliability = (1.00)(0.98) = 98%
~0% ~2%~100% ~98%
Lowest 1-dayPavement
Temperature
Highest 7-dayPavement
Temperature
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The Right Type of Aggregate
1. Strong and Durable2. No Deleterious Substances3. Cubical (Angular and Equidimensional)4. Low Porosity5. Clean, Rough, and Hydrophobic6. Hard7. Dense-Graded
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The Right Mix Volumetrics
BleedingLow stabilityTender mix
StrippingLow durability
Fatigue cracking
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Marshall Mix Design
Bruce Marshall, Mississippi Highway Department
Marshall Mix Design Steps
1. Create aggregate blend to meet gradation specifications.
2. Establish mixing and compaction temperatures from the viscosity-temperature chart.
3. Compact three specimens at each of five asphalt contents spanning the expected optimum asphalt content.
4. Determine the relative density of each specimen and the mix volumetrics (mb, VTM, VMA, VFA).
5. Measure the performance properties of the each specimen at 60ºC (140ºF).
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Marshall Specimens
Traffic Blows / Side
Light 35
Medium 50
Heavy 75
10#18"
Make 3 specimens at each of5 different asphalt contents
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Marshall Hammer
Mold
Hammer
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Stability and Flow Test
140oF
flow
stability
deflection(0.01 in)
load(lbs)
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Stability and Flow Test
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Marshall Mix Design
159
160
161
162
163
164
3.5 4.0 4.5 5.0 5.5 6.0 6.5
Asphalt Content (%)
Uni
t Wei
ght (
pcf)
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1500
1600
1700
1800
1900
2000
3.5 4.0 4.5 5.0 5.5 6.0 6.5
Asphalt Content (%)
Mar
shal
l Sta
bilit
y (lb
s)
Marshall Mix Design
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1
2
3
4
5
6
3.5 4.0 4.5 5.0 5.5 6.0 6.5
Asphalt Content (%)
Air
Voi
ds (%
)
Marshall Mix Design
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0
5
10
15
20
25
3.5 4.0 4.5 5.0 5.5 6.0 6.5
Asphalt Content (%)
Flow
Marshall Mix Design
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50
60
70
80
90
100
3.5 4.0 4.5 5.0 5.5 6.0 6.5
Asphalt Content (%)
Void
s Fill
ed w
ith A
spha
lt (%
)
Marshall Mix Design
CIVL 3137 77
Marshall Mix Design
%7.43
3.47.41.5AC
MaximumDensity
MaximumStability
4% AirVoids
(The Asphalt Institute Procedure)
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Marshall Mix Design
5.1
AC 4.7
4.3
4.3%1
MaximumDensity
MaximumStability
4% AirVoids
(The NAPA Procedure)
ThisiswhatTDOTuses
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Marshall Mix Design(Asphalt Institute Criteria)
Criteria Min. Max. Min. Max. Min. Max.
Number of Blows
Stability (lbs) 500 750 1500
Flow 8 20 8 18 8 16
Air Voids (%) 3 5 3 5 3 5
VMA (%) See Next Slide
Light Traffic Medium Traffic Heavy Traffic
35 50 75
16 14
1800TDOTuses2000
18
750 1200
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VMA CriteriaM
inim
um V
MA
(%)
10
Nominal Maximum Particle Size
20
30
40
50
2"1½"1"¾"½"4 3/8"840
Deficient in either
asphalt or air voids
(Assuming 4% Design Air Voids)
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VMA CriteriaM
inim
um V
MA
(%)
10
Nominal Maximum Particle Size
20
30
40
50
2"1½"1"¾"½"4 3/8"840
Deficient in either
asphalt or air voids
(Assuming 5% Design Air Voids)
1%
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VMA CriteriaM
inim
um V
MA
(%)
10
Nominal Maximum Particle Size
20
30
40
50
2"1½"1"¾"½"4 3/8"840
Deficient in either
asphalt or air voids
(Assuming 3% Design Air Voids)
1%
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1500
1600
1700
1800
1900
2000
3.5 4.0 4.5 5.0 5.5 6.0 6.5
Asphalt Content (%)
Mar
shal
l Sta
bilit
y (lb
s)
Marshall Mix Design
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0
5
10
15
20
25
3.5 4.0 4.5 5.0 5.5 6.0 6.5
Asphalt Content (%)
Flow
Marshall Mix Design
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1
2
3
4
5
6
3.5 4.0 4.5 5.0 5.5 6.0 6.5
Asphalt Content (%)
Air
Voi
ds (%
)
Marshall Mix Design
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50
60
70
80
90
100
3.5 4.0 4.5 5.0 5.5 6.0 6.5
Asphalt Content (%)
Void
s Fill
ed w
ith A
spha
lt (%
)
Marshall Mix Design
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VMA CriteriaM
inim
um V
MA
(%)
10
Nominal Maximum Particle Size
20
30
40
50
2"1½"1"¾"½"4 3/8"840
Deficient in either
asphalt or air voids
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13
14
15
16
17
18
3.5 4.0 4.5 5.0 5.5 6.0 6.5
Asphalt Content (%)
VM
A (%
)
Marshall Mix Design
TDOT requiresthe AC to be lessthan that whichminimizes VMA
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Ways to Increase VMA
1. Reduce the dust content2. Open the aggregate gradation3. Gap-grade the aggregate blend4. Increase manufactured sand5. Reduce flat-and-elongated particles