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Asphalt concrete mix design
Francisco Aguiga
Assistant ProfessorCivil Engineering ProgramTexas A&M University Kingsville
CEEN 3144 Construction Materials
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Hot-mix asphalt
Asphalt cement + aggregatesmixed at high temperature placedand compacted while still hot
93% of the US roads (2 M miles)
Asphalt concrete
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Provide Stability avoid permanent deformation
under traffic loads
Fatigue resistance avoid fatigue cracking
Resistance to thermal cracking
Resistance to hardening
Resistance to moisture damage
Provide skid resistance
Provide workability
Objectives of asphalt concretemix design
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Figure 1 Stable structure with
considerable internalfriction
High resistance todeformation under load
Figure 2 Less stable structure with
less internal friction
Less resistance todeformation under load
Fundamentals of mix design
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Figure 3 Aggregate skeleton with
asphalt binder
Some air
Aggregate internal frictionavailable + asphalt binding
Too little air brittle pavement
Figure 4 No air rutting, bleeding
Too much asphalt Aggregate floats on asphalt
Easily deforms under loadsince there is no internalfriction
Fundamentals of mix design
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1. Select the grading max aggregate size
2. Select the aggregate to use
3. Select the proportion of each aggregate touse
4. Find specific gravity of aggregatecombination and asphalt cement
5. Make trial specimens
6. Find specific gravity of each specimen
7. Conduct stability tests
8. Compute percentage of voids, VMA, andpercent voids filled with asphalt
9. Select optimum asphalt content
Asphalt concrete mix design
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1. Select the grading
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Aggregates almost always selected bythe contractor
Select most economical materials Have as few aggregate piles as possible
(usually 2 to 5) $ transportation > $ aggregate Uncrushed river gravel can be
inexpensive, but yield a poorperformance Select crushed stone whenever possible
2. Select the aggregate to use
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In the lab trial and error
Mathematically
aA + bB = T
When b is zero
A = T/a
3. Proportioning of aggregates
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Asphalt specific gravity Usually provided by manufacturer
Aggregate specific gravity Obtained in the laboratory Average specific gravity
ga = Specific gravity of combined aggregate W1, W2, W3 = Respective percents by weight of
aggregate 1, 2, and 3 g1, g2, g3 = respective specific gravities of
aggregate 1, 2, and 3
4. Specific gravities
3
3
2
2
1
1
100
gW
gW
gW
ga
++
=
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4. Specific gravities
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4. Specific gravities
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4. Specific gravities
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4. Specific gravities
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4. Specific gravities
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4. Specific gravities
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4. Specific gravities
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Objective
Optimize the asphalt content to obtain the highest stabilityvalue (load) and highest density
Try asphalt contents in increments of 0.5 %
Preparation of mix
Optimize the asphalt content to obtain the highest stabilityvalue (load) and highest density Heat aggregates (350 to375 F)
Heat asphalt (250 to 280 F) stir constantly
Discard hot asphalt sitting longer than 1 hour
Weight size fractions and add aggregate, mix
Temperature of mix should not be less than 225 F beforecompaction
No reheating of mix
5. Preparation of compacted
specimens Marshall Method
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Compaction of specimens
Clean and heat compaction mold and hammer to200 to 300 F in a bath of boiling water
Place mold on base and insert paper filter, place1200 g of hot batch mix and spade the mixture 15times on perimeter and 10 times in the middle
Remove collar and smooth surface
Replace collar and apply 50 blows
Reverse specimen and apply another 50 blows
Release specimen from mold and allow to cool atroom temperature over night
5. Preparation of compactedspecimens
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6. Specific gravity of compacted
specimens
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6. Specific gravity of compactedspecimens
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6. Specific gravity of compacted
specimens
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Marshall flow test Weigh and measure specimens
Heat specimen to 140 F in a water bath for 30 to 40 min
Clean and lubricate guide rods
Remove specimen from bath and place in lower segment ofhead, place top segment of head
Place and zero flow meter
Load specimen at 50.8 mm (2 in.) per minute until the loaddecreases (max reached)
Record the flow in twenty-five hundredths of a millimeter(hundredths of an inch)
Total elapsed time not to exceed 30 sec
7. Stability test of compactedspecimens
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8. Compute percentage of voids
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8. Compute percentage of voids
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9. Select optimum asphalt
content
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9. Select optimum asphaltcontent
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9. Select optimum asphalt
content
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9. Select optimum asphaltcontent
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9. Select optimum asphalt
content
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9. Select optimum asphaltcontent
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9. Select optimum asphalt
content
