ceen 3144 l11 asphalt concrete mix design
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Asphalt concrete mix design
Francisco AguíñigaAssistant Professor
Civil Engineering ProgramTexas A&M University – Kingsville
CEEN 3144 Construction Materials
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Hot-mix asphaltAsphalt cement + aggregates mixed at high temperature placed and compacted while still hot93% of the US roads (2 M miles)
Asphalt concrete
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ProvideStability – avoid permanent deformation under traffic loadsFatigue resistance – avoid fatigue crackingResistance to thermal crackingResistance to hardeningResistance to moisture damageProvide skid resistanceProvide workability
Objectives of asphalt concrete mix design
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Figure 1Stable structure with considerable internal frictionHigh resistance to deformation under load
Figure 2Less stable structure with less internal frictionLess resistance to deformation under load
Fundamentals of mix design
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Figure 3Aggregate skeleton with asphalt binderSome airAggregate internal friction available + asphalt bindingToo little air – brittle pavement
Figure 4No air – rutting, bleedingToo much asphaltAggregate floats on asphaltEasily deforms under load since there is no internal friction
Fundamentals of mix design
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1. Select the grading – max aggregate size2. Select the aggregate to use3. Select the proportion of each aggregate to
use4. Find specific gravity of aggregate
combination and asphalt cement5. Make trial specimens6. Find specific gravity of each specimen7. Conduct stability tests8. Compute percentage of voids, VMA, and
percent voids filled with asphalt9. Select optimum asphalt content
Asphalt concrete mix design
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1. Select the grading
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Aggregates almost always selected by the contractorSelect most economical materialsHave as few aggregate piles as possible (usually 2 to 5)$ transportation > $ aggregateUncrushed river gravel can be inexpensive, but yield a poor performanceSelect crushed stone whenever possible
2. Select the aggregate to use
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In the lab – trial and errorMathematically
aA + bB = TWhen b is zeroA = T/a
3. Proportioning of aggregates
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Asphalt specific gravityUsually provided by manufacturer
Aggregate specific gravityObtained in the laboratoryAverage specific gravity
ga = Specific gravity of combined aggregateW1, W2, W3 = Respective percents by weight of aggregate 1, 2, and 3g1, g2, g3 = respective specific gravities of aggregate 1, 2, and 3
4. Specific gravities
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3
2
2
1
1
100
gW
gW
gWga
++=
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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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ObjectiveOptimize the asphalt content to obtain the highest stability value (load) and highest densityTry asphalt contents in increments of 0.5 %
Preparation of mixOptimize the asphalt content to obtain the highest stability value (load) and highest density Heat aggregates (350 to 375 °F)Heat asphalt (250 to 280 °F) – stir constantlyDiscard hot asphalt sitting longer than 1 hourWeight size fractions and add aggregate, mixTemperature of mix should not be less than 225 °F before compactionNo reheating of mix
5. Preparation of compacted specimens – Marshall Method
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Compaction of specimensClean and heat compaction mold and hammer to 200 to 300 °F in a bath of boiling waterPlace mold on base and insert paper filter, place 1200 g of hot batch mix and spade the mixture 15 times on perimeter and 10 times in the middleRemove collar and smooth surfaceReplace collar and apply 50 blowsReverse specimen and apply another 50 blowsRelease specimen from mold and allow to cool at room temperature over night
5. Preparation of compacted specimens
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6. Specific gravity of compacted specimens
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6. Specific gravity of compacted specimens
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6. Specific gravity of compacted specimens
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Marshall flow testWeigh and measure specimensHeat specimen to 140 °F in a water bath for 30 to 40 minClean and lubricate guide rodsRemove specimen from bath and place in lower segment of head, place top segment of headPlace and zero flow meterLoad specimen at 50.8 mm (2 in.) per minute until the load decreases (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 compacted specimens
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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 asphalt content
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9. Select optimum asphalt content
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9. Select optimum asphalt content
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9. Select optimum asphalt content
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9. Select optimum asphalt content
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9. Select optimum asphalt content