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Asphalt concrete mix

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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

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