Asphalt Mix Design
J. Murali Krishnan
Department of Civil EngineeringIIT Madras
HINCOL Training Program - 2013
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 1/36
Outline
Basics of Mix Design
Evaluation of Aggregate Gradations
Volumetric Properties of Compacted Paving Mixtures
Marshall Method of Mix Design
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 2/36
Basics of Mix Design
Evaluation of Aggregate Gradations
Volumetric Properties of Compacted Paving Mixtures
Marshall Method of Mix Design
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 3/36
Objectives
What is mix design?
Selecting and proportioning materials to obtain the desiredproperties in the finished construction product.
The overall objective for the design of asphalt paving mix is todetermine a cost-effective blend and gradation of aggregatesand asphalt that yields a mix having
1 Sufficient Asphalt - Durability
2 Sufficient Mix Stability - Strength
3 Sufficient Voids - Traffic compaction, flushing, bleeding
4 A maximum void content - permeability
5 Sufficient workability - Efficient placement of mix
6 Proper aggregate texture and hardness - skid resistance
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 4/36
Important
Fundamental performance properties are not measured in amix design.
How do we pick an asphalt content on the basis of ameasured parameter that reflects all the above objectives?
AIR VOID CONTENTAn air void content of 3 to 5 percent is most often used.When should this air void content be measured?
Four percent air voids is often considered the best initialestimate for a design that balances the desired performanceproperties.
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 5/36
Mix Type Selection
Dense graded HMA mixtures are divided into three types:1 Surface mixtures - Finest aggregate particles2 Binder or intermediate mixtures - Smaller aggregate particles3 Base mixtures - Larger aggregate particles
ASTM D3515 for various gradations (dense, open graded)
Gradation influences the lift thickness.
Rule of Thumb: Minimum thickness for each lift is around 2to 3 times the maximum aggregate size.
How much a roller can compact?
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 6/36
Design Method and Requirements
Marshall and Hveem Method have criteria empiricallydeveloped by correlating the results of laboratory tests on thecompacted paving mixes under service conditions.
All mix design procedures involve preparing a set of trialmixture specimens using materials proposed for use on theproject.
The three key components:1 Laboratory compaction of trial mix specimens
2 Strength (Stability!!!) and volumetric testing and
3 Analysis of results
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 7/36
Design Method and Requirements - Compaction
The compaction technique is intended to simulate thein-place density of HMA after it has endured severalyears of traffic!!!
Four compaction methods are currently used:1 Impact compaction (Marshall method of mix design)2 Kneading compaction (Hveem method of mix design)3 Several forms of gyratory compaction (Superpave)4 Compaction using vibratory impact hammers
Five trial mixtures and for each trial mixture at least threereplicate specimens.
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 8/36
Evaluation and Adjustment of Mix Design
For many engineering materials, the strength of the materialfrequently denotes quality;
This is not necessarily the case for asphalt mixtures.
Extreme high stability results in lowered durability andvice-versa
Aggregate gradation and asphalt content must strike afavorable balance between the stability and durabilityrequirements.
Grading curves
Fullers equation: p = 100(dD
)0.5p=total percentage passing given sieve, d=sieve size openingand D=largest size (sieve opening) in gradation
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 9/36
Fuller Curve
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 10/36
Fuller Curves
Fuller equation - mix conditions of maximum density andminimum voids in mineral aggregates (VMA)
Fuller gradations are easily compacted but tend to pack verytight and have low air voids
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 11/36
FHWA - 0.45 power
FHWA chart is currently followed: gradings that closelyapproach this straight line must be adjusted away from itwithin acceptable limits to increase the VMA values.
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 12/36
FHWA - 0.45 power
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 13/36
Aggregate Size Fractions
Coarse aggregate - retained on 2.36 mm
Fine aggregate - passing 2.36 mm
Mineral filler - passing 75m
Asphalt content may be expressed either as percentage byweight of total mix or as percentage by weight of dryaggregate
Most common is to express asphalt content as percentage byweight of total mix.
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 14/36
Basics of Mix Design
Evaluation of Aggregate Gradations
Volumetric Properties of Compacted Paving Mixtures
Marshall Method of Mix Design
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 15/36
Steps in Evaluating Aggregate Gradations1 Secure representative samples from each aggregate stockpile,
including fillers
2 Dry all aggregate samples to constant weight at 105 - 110C
3 Perform washed sieve analysis and specific gravity tests
4 Compute the blend of aggregates required to produce thedesired mix gradation using the full gradation for eachindividual aggregate
5 Adjust cold aggregate feeder controls to obtain the desiredaggregate blend and combined gradation
6 Prepare test specimens of the design mix. Analyze the testspecimens to determine that the mixture has the sameproperties
7 Verify, verify, verify at every stage the aggregate gradations
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 16/36
Adjusting for sufficient void space
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 17/36
Adjusting for sufficient void space
Adequate space for minimum amount of asphalt and air voidsas well as adequate stability under traffic
What is the role of particle shape and texture in creatingadequate void space?
The catch: Same aggregate gradation with the samecompaction effort, but with different shaped particles canproduce different void space!!!
Surface properties are not however considered for voidpurposes, but rather for stability and skid resistance
Question: What is the different between air voids and crackin asphalt pavements?
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 18/36
Basics of Mix Design
Evaluation of Aggregate Gradations
Volumetric Properties of Compacted Paving Mixtures
Marshall Method of Mix Design
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 19/36
Introduction
1 Air voids (Va)
2 Voids in mineral aggregates (VMA)
3 Voids filled with asphalt (VFA)
4 Effective asphalt content (Pbe)
5 The intent of laboratory compaction is to simulate thein-place density of HMA after it has endured severalyears of traffic
6 Air voids analysis is the most important part of mix design
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 20/36
Different Specific Gravities
Bulk Specific Gravity: Includesboth permeable and impermeablevoids normal to the surface
Apparent Specific Gravity:Volume of an impermeable material(to asphalt and water)
Effective Specific Gravity:Volume of permeable material(excludes voids permeable toasphalt)
Specific gravities should be determined to three decimalplaces - Errors in air void value of as much as 0.8 percent(corresponds to almost one year of field compaction) canoccur.
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 21/36
Volumetric Representation
VMA: Volume of inter-granular voidspace between the aggregate particlesthat includes air voids and effectiveasphalt content
Pbe: Total asphalt content minus theportion of asphalt that is lost byabsorption into the aggregate particles
Va:Total volume of small packets ofair between the coated aggregateparticles
VFA: Portion of the volume ofinter-granular void space between theaggregate particles that is occupied bythe effective asphalt content
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 22/36
Outline of Procedure
1 Measure bulk specific gravity of coarse and fine aggregate
2 Measure specific gravity of mineral filler and asphalt cement
3 Calculate bulk specific gravity of aggregate combination
4 Calculate the maximum specific gravity of mixtures withdifferent asphalt content
5 Fabricate samples, measure the bulk specific gravity of thecompacted paving mixture
6 Calculate asphalt absorption, effective asphalt content
7 Calculate VMA, Va, VFA
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 23/36
Basics of Mix Design
Evaluation of Aggregate Gradations
Volumetric Properties of Compacted Paving Mixtures
Marshall Method of Mix Design
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 24/36
Outline
Formulated by Bruce Marshall, Mississippi State HighwayDepartment
The original Marshall method is only applicable for pavingmixtures with maximum aggregate size of 25 mm or less
Modified Marshall method accounts for maximum sizes up to38 mm
Marshall method is empirical in nature, hence, the meaning ofthe results are lost when any modification is made to thestandard procedure - Does not work for reheated or remoldedmaterials
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 25/36
Outline
Use specimens of 64 mm height and 102 mm diameter
Density-voids analysis and stability-flow test
Stability - Maximum load resistance in Newtons that thestandard specimen will develop at 60 degree C
Flow - Total movement in units of 0.25mm occurring betweenno-load and the point of maximum load during stability test
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 26/36
Preparation of test specimens
At least two asphalt content above the expected design valueand two asphalt content below the expected design value
Three test specimens for each asphalt content
Dry aggregate to constant weight at 105 degree C
Separate the aggregate by dry sieving into the following sizefractions
1 25.0 to 19.0 mm2 19.0 to 9.5 mm3 9.5 to 4.75 mm4 4.75 to 2.36 mm5 passing 2.36 mm
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 27/36
Preparation of test specimens
Mixing temperature - Viscositycorresponding to 170 20centistokes (kinematic)
Compaction temperature -Viscosity corresponding to 280 30centistokes (kinematic)
How do we age the samples?
How many blows on each side?
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 28/36
Test Procedure
Measure bulk specific gravity for each asphalt content(measure all the 15 samples and average for each asphaltcontent)
Immerse specimen in water bath at 60 degree C for 30 to 40minutes before test
Apply constant deformation rate of 2 inches per minute tillfailure
Note down the stability and flow values
Repeat for all samples (total 15 samples for each batch)
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 29/36
Hot-mix design data
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 30/36
Trends in the mix design data
Stability value increases withincreasing asphalt content upto a maximum value afterwhich the stability decreases
Flow value increases withincreasing asphalt content
Percentage of air voidsdecreases with increasingasphalt content ultimatelyapproaching a minimum airvoids content
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 31/36
Trends in the mix design data
VMA generally decreases toa minimum value thenincreases with increasingasphalt content
VFA increases withincreasing asphalt content
Unit weight of total mixfollows the same trend asStability - asphalt contentexcept that maximum unitweight occurs at a slightlyhigher asphalt content thanthe maximum stability
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 32/36
Preliminary Design Asphalt Content
Choose the asphalt content corresponding to 4 percent airvoids
Check for the other values at this percent asphalt content
The final mix design (essentially asphalt percent) is based on1 VMA2 Compaction level3 Air voids4 Voids filled with asphalt
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 33/36
Hot-mix design data
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 34/36
The Laboratory Report for Marshall Mix Design
Section 1: Sieve Analysis (the 0.45 power plots along with themix design requirements from MOSTRH)
Section 2: Aggregate Blending Calculations
Section 3: Determination of specific gravity for coarse, fineaggregates, fillers and asphalt
Section 4: Step-by-step procedure for making samples forMarshall Mix Design
Section 5: Volumetric Calculations
Section 6: Conclusions (your recommendation for JMF, yourobservations, comparison with the design values . . . )
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 35/36
AICTE-STTP-2009
A Sample Marshall Mix Design
Volumetric Calculation
AICTE-STTP-2009
Marshall Method of Mix Design: Analyzing compacted mixture
AICTE-STTP-2009
Marshall Method of Mix Design: Analysing compacted mixture
AICTE-STTP-2009
Marshall Method of Mix Design: Analyzing compacted mixture
Specific gravity determination
Bulk specific gravity of aggregate: For mineral filler, apparent Sp. Gravity is enough.
AICTE-STTP-2009
Marshall Method of Mix Design: Analyzing compacted mixture
Effective Specific gravity of aggregate:
Includes all the void spaces in the aggregate particles except those that absorb asphalt.
The volume of bitumen absorbed by an aggregate is almost invariably less than the volume of water absorbed.
AICTE-STTP-2009
Marshall Method of Mix Design: Analyzing compacted mixture
Maximum specific gravities of mixtures with different asphalt contents:
Maximum Sp. Gr. are needed
at different asphalt content.
AICTE-STTP-2009
Marshall Method of Mix Design: Analyzing compacted mixture
Asphalt Absorption: Expressed as a percentage
by weight of aggregate rather than as percentage by total weight of mixture.
AICTE-STTP-2009
Marshall Method of Mix Design: Analyzing compacted mixture
Effective Asphalt Content: Total asphalt content minus
the quantity of asphalt lost by absorption into the aggregate particles.
This is the asphalt content on which the performance depends.
AICTE-STTP-2009
Marshall Method of Mix Design: Analyzing compacted mixture
Percent VMA in Compacted mixtures:
Inter-granular void space between the aggregate particles in a compacted mixture that includes the air voids and the effective asphalt content expressed as percent of the total volume.
AICTE-STTP-2009
Marshall Method of Mix Design: Analyzing compacted mixture
Air voids in compacted mixtures:
Small air spaces between the coated aggregate particles.
AICTE-STTP-2009
Marshall Method of Mix Design: Analyzing compacted mixture
VOIDS FILLED WITH ASPHALT:
Percentage of Intergranular void space between the aggregate particles (VMA) that are filled with asphalt.
VFA does not include absorbed asphalt.
AICTE-STTP-2009
Marshall Method of Mix Design: Analyzing compacted mixture
Thank you for your patience.
Murali KrishnanIIT Madras
J. Murali Krishnan, IIT Madras HINCOL-2013, Lecture IV 36/36
Basics of Mix DesignEvaluation of Aggregate GradationsVolumetric Properties of Compacted Paving MixturesMarshall Method of Mix Designmix_design_2.pdfSlide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12
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