MARSHALL MIX DESIGN 1.0 INTRODUCTION The main purpose of the design process is to determine the optimum bitumen content (OBC) of each asphaltic mixture. Before any asphalt mixes can be placed and laid on the road, the aggregate and the binder types are generally screened for quality and requirement. Approximately 15 samples are required Optimum Asphalt Content (OAC). The aggregates blend that will be used for mixtures preparation must fall within the specification requirements. Properties such as density and bulk specific gravity of aggregate and bitumen used for each mixture must be determined earlier before carrying out Marshall Test. By using the Asphalt Institute Method, the Optimum Asphalt Content are determined from the individual plots of bulk density, voids in total mix and stability versus percent asphalt content. The average of the 3 OAC values is taken for further sample preparation and analysis. 2.0 THEORY The mix design determines the optimum bitumen content. There are many methods available for mix design which varies in the size of the test specimen, compaction and other test specifications. Marshall Method of mix design is the most popular one. The Marshall Stability and flow test provides the performance prediction measure for the Marshall Mix design method. Load is applied to the specimen till failure, and the maximum load is designated as stability. During the loading, an attached dial gauge measures the specimen’s plastic flow (deformation) due to the loading. Page | 1
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MARSHALL MIX DESIGN
1.0 INTRODUCTION
The main purpose of the design process is to determine the optimum bitumen content
(OBC) of each asphaltic mixture. Before any asphalt mixes can be placed and laid on the road,
the aggregate and the binder types are generally screened for quality and requirement.
Approximately 15 samples are required Optimum Asphalt Content (OAC). The aggregates blend
that will be used for mixtures preparation must fall within the specification requirements.
Properties such as density and bulk specific gravity of aggregate and bitumen used for each
mixture must be determined earlier before carrying out Marshall Test. By using the Asphalt
Institute Method, the Optimum Asphalt Content are determined from the individual plots of bulk
density, voids in total mix and stability versus percent asphalt content. The average of the 3
OAC values is taken for further sample preparation and analysis.
2.0 THEORY
The mix design determines the optimum bitumen content. There are many methods
available for mix design which varies in the size of the test specimen, compaction and other test
specifications. Marshall Method of mix design is the most popular one. The Marshall Stability
and flow test provides the performance prediction measure for the Marshall Mix design method.
Load is applied to the specimen till failure, and the maximum load is designated as stability.
During the loading, an attached dial gauge measures the specimen’s plastic flow (deformation)
due to the loading.
The amount of binder to be added to a bituminous mixture cannot be too excessive or too
little. The principle of designing the optimum amount of binder content is to include sufficient
amount of binder so that the aggregates are fully coated with bitumen and the voids within the
bituminous material are sealed up. As such, the durability of the bituminous pavement can be
enhanced by the impermeability achieved. Moreover, a minimum amount of binder is essential
to prevent the aggregates from being pulled out by the abrasive actions of moving vehicles on
the carriageway. However, the binder content cannot be too high because it would result in the
instability of the bituminous pavement. In essence, the resistance to deformation of bituminous
pavement under traffic load is reduced by the inclusion of excessive binder content.
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MARSHALL MIX DESIGN
3.0 OBJECIVES
(i) To prepare standard specimens of asphalt concrete for the determination of the optimum
asphalt content based upon ASTM D 1559, “ Resistance to Plastic Flow of Bituminous
Mixtures Using Marshall Apparatus”.
(ii) To determine the combination of bitumen and aggregate that will give durable road
surfacing.
4.0 EQUIPMENT AND MATERIAL
(i) Marshall compactor
(ii) Mixer
(iii) Water Bath
(iv) Oven
(v) Thermometer
(vi) Marshall Compression Machine
(vii) Marshall Mould
(viii) Sieve Shaker
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MARSHALL MIX DESIGN
5.0 PROCEDURE
In the Marshall Test method of mix design, three compacted samples are prepared for each
binder content. At least four binder contents are to be tested to get the optimum binder content.
All the compacted specimens are subjected to the following test:
a) Bulk specific gravity determination test (ASTM D 2726)
b) Theoretical Maximum Specific Gravity (AASHTO T 209)
c) Marshall Stability and Flow Test (ASTM D 1559)
d) Density and voids analysis (ASTM D 2726)
5.1 Preparation of the Test Specimens
(i) The aggregate (about 1200g), graded according to the ASTM standard are over dried
at 170 – 180 ºC (not more than 280 ºC).
(ii) The required quantity of asphalt is weighted and heated to a minimum temperature of
about 135 ºC (maximum ± 160 ºC ± 5 ºC).
(iii) The thoroughly cleaned mould is heated on a hot plate or in an oven to a temperature
of about 135 – 150 ºC. The mould is 101.6 mm diameter by 76.2 mm high and
provided with a base plate and extension collar.
(iv) A crater is formed in the aggregate, the binder poured in and mixing carried out until all
the aggregate is coated. The mixing temperature shall be within the limit set for the
binder temperature.
(v) A piece of filter paper is fitted in the bottom of the mould and the whole mix poured in
three layers. The mix is then vigorously trowel 15 times round the perimeter and 10
times in the centre leaving a slightly rounded surface.
(vi) The mould is placed on the Marshall Compactor and given 75 blows.
(vii) After compaction, the base plate is removed and the same blows are compacted to the
bottom of the sample that has been turned around.
(viii) The specimen is then carefully removed from the mould and then marked.
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MARSHALL MIX DESIGN
5.2 Bulk density gravity determination test (ASTM D2726)
5.2.1 Introduction
Bulk specific gravity of compacted Hot Mix Asphalt (HMA) specimens, Gmb,is
theratio of the weight in air of a unit volume of a compacted specimen of HMA
(including permeable voids) at a standard temperature to the weight of an equal
volume of water at a stated temperature. The bulk specific gravity can be
calculated by using the following equation;
Bulk Specific Gravity, Gmm = (A
B−C)
Where:
A = mass of specimen in air (g)
B = saturated surface dry (SSD) mass (g)
C = mass of specimen in water (g)
5.2.2 Apparatus
Apparatus that used in this test are:
(i) Balance; and
(ii) Water Bath.
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MARSHALL MIX DESIGN
5.2.3 Procedure
(i) The specimen is cooled to a room temperature at 25 ± 1 ºC and the dry
mass is recorded as A.
(ii) The specimen is immersed in a 25 ± 1 ºC water bath and saturated at 4 ± 1
minute.
(iii) The specimen is then, placed in a basket and its mass is determined to
nearest 0.1 g while immersed in water at 25 ± 1 ºC.(C)
(iv) The immersed saturated specimen is removed from the water bath and
damp dried with a damp absorbent cloth as quickly as possible. The
specimen is then weighted, (B). Any water that seeps from the specimen
during the weighing operation is considered as part of saturated specimens.
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MARSHALL MIX DESIGN
5.3 Theoretical Maximum Specific Gravity (AASHTO T 209)
5.3.1 Introduction
Theoretical maximum specific gravity, Gmm, is the ratio of the weight in air of
the a unit volume of uncompacted bituminous paving mixture at a stated
temperature to the weight of an egual amount of water at a stated temperature. It
is also called Rice Specific Gravity. The theoretical maximum specific gravity can
be calculated by using the following equation;
Theoretical Maximum Specific Gravity , Gmm= (A
A+B−C)
Where:
A= sample mass in air (g)
B= mass of container filled with water (g)
C= mass of container and sample filled with water (g)
5.3.2 Apparatus
Apparatus that used in this test are:
(i) Vacuum Container;
(ii) Balances;
(iii) Vacuum lid;
(iv) Vacuum pump or water aspirator;
(v) Manometer or vacuum gauge;
(vi) Thermometer;
(vii) Water bath;
(viii) Bleeder valve; and
(ix) Timer.
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MARSHALL MIX DESIGN
5.3.3 Procedure
(i) Separate the particles of the sample, taking care not to fracture the mineral
particles, so that the particles of the fine aggregate portion are not large
than 6.3mm (1/4in). If the mixture is not sufficiently soft to be separated
manually, place it in a large flat pan and warm in an oven only until it is
pliable enough to separation.
(ii) Cool the sample to room temperature.
(iii) Determine and record the mass of the flask, including the cover.
(iv) Place the sample in the flask.
(v) Determine and record the mass of the flask, cover, and the sample.
(vi) Add sufficient water at approximately 25˚ ±1˚C to cover the sample by
25mm.
(vii) Place the lid on the flask and attach the vacuum line. To ensure a proper
seal between the flask and lid, wet the O-ring or use a petroleum gel.
(viii) Remove entrapped air by subjecting the contents to a partial vacuum of 3.7
± 0.3 kPa residual pressures for 15 ±2 minutes.
(ix) Agitate the container and contents, either continuously by mechanical
device or manually by vigorous shaking, at 2-minute interval. This agitation
facilitates the removal of air.
(x) Slowly open the release valve, turn off the vacuum pump and remove the
lid.
(xi) Suspend and immerse the flask and contents in water at 25˚ ±1˚C for 10 ±1
minutes. The holder shall be immersed sufficiently to cover it and the flask.
(xii) Determined and record the submerged weight of the flask and contents.
(xiii) Empty and re-submerge the flask following step (xi) to determined the
submerged weight of the flask.
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MARSHALL MIX DESIGN
5.4 Marshall stability and flow test (ASTMD1559)
5.4.1 Introduction
The most widely used method of asphalt mix design is the Marshall method
developed by the U.S. Corps of Engineers. The Marshall Flow and stability test
provides the performance prediction measure for the Marshall Mix design
method. The stability portion of the test measure the maximum load supported by
the test specimen at a loading rate of 51mm/min. Stability and flow, together with
density, voids and percentage of voids filled with binder are determined at
varying binder contents to determine an optimum for stability, durability, flexibility,
fatigue resistance, etc.
The mechanism of failure in the Marshall Test apparatus is complex but it is
essentially a type of unconfined compression test. This being so, it can only have
limited correlation with deformation in a pavement where the material is confined
by the tire, the base and the surrounding surfacing. Wheel tracking tests have
shown that resistance to plastic flow increases with reducing binder content
whereas Marshall Stability decreases. Improvement on the assessment, based
on stability, is possible by considering flow and most agencies (e.g. Asphalt
Institute, Malaysia’s JKR) set minimum for stability and maximum for flow for
various purposes (roads, airports, etc)
In addition to binder content, stability and flow being the prime variables in
the performance of an asphalt sample, the type of binder, grading of aggregate,
the particle shape, geological nature of parent rock (most importantly; porosity),
degree of compaction, etc. also play an importance role.
5.4.2 Apparatus
Apparatus that used in this test are:
(i) Marshall Stability and Flow Machine;
(ii) Water bath; and
(iii) Rubber glove.
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MARSHALL MIX DESIGN
5.4.3 Procedures
The dimension and specifications of the Marshall apparatus are explained in
ASTM D 1559. The diameter of the specimen is 101.6 mm and nominal thickness
is 63.5 mm.
(i) Three specimens, were prepared according to the Standard, are immersed
in a water bath for 30 to 40 minutes or in an oven for 2 hours at 60 ± 1.0º C.
(ii) The testing heads and guide rods are thoroughly cleaned; guide rods
lubricated and head maintained at a temperature between 21.1 and 37.8ºC.
(iii) A specimen is removed from the water bath or oven, placed in the lower
jaw and the upper jaw placed in position. The complete assembly is then
placed in the Marshall Stability and flow machine and the flow meter
adjusted to zero.
(iv) The load is applied to the specimen at a constant strain rate of 50.8
mm/min until the maximum load is reached. The maximum force and flow at
that force are read and recorded. The maximum time that’s allowed
between removal of the specimens from the water bath and maximum load
is 30 second.
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MARSHALL MIX DESIGN
5.5 Density and Voids Analysis (ASTM D 2726)
5.5.1 Bulk Density
The bulk density of the specimen is simply determined by weighing in air and
water.
Bulk Density, d = Gmb x ρw
Bulk Specific Gravity, Gmb = (A
B−C)
Where:
d = Bulk density (g/ cm³)
Gmb = Bulk Specific Gravity of the mix
ρw = density of water (1 g/ cm³)
A = mass of specimen in air (g)
B = saturated surface dry (SSD) mass (g)
C = mass of specimen in water (g)
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MARSHALL MIX DESIGN
5.5.2 Percentage of Air Voids (%AV)
The percentage of air voids in the mix is determined by firstly calculating the
maximum theoretical density TMD (zero voids) and then expressing the
difference between it and the actual bulk density d as a percentage of total
volume.
AV (%) = (1−GmbGmm ) x100
Where:
Gmb = Bulk Specific Gravity of the mix
Gmm = maximum theoretica
5.5.3 Theoretical Maximum Specific Gravity
Gmm = ( 100
([100−PbGse ]+[ PbGb ])
) Or Max specific gravity test (AASHTOT209)
Where:
Gse = ([ 100−Pb
[( 100Gmm )]−( PbGb ) ])
Gmm = maximum theoretical Specific Gravity of the mix
Pb = asphalt content, percent by the weight of the mix
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MARSHALL MIX DESIGN
Gse = effective specific gravity of the mix
Gb = Specific Gravity of asphalt cement
5.5.4 Voids in the Mineral Aggregate (VMA)
The volume of void in mineral aggregate VMA is an important factor for the
mixture design.
VMA = 100 – (1 – [ Gmb(1−Ps)Gsb
] )
Where:
Gmb = Bulk Specific Gravity of the mix
Ps = asphalt content, percent by weight of the mix
Gsb = bulk specific gravity of the aggregate
5.5.5 Voids filled with Asphalt (VFA)
VFA = (VMA−AVVMA )x 100
Where:
VFA = voids filled with asphalt
VMA = void in mineral aggregate
AV = the percentage of air voids
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MARSHALL MIX DESIGN
6.0 ANALYSIS OF RESULT
7.1 Gradation
The Aggregate Grading
Sieve Size Control Point % Passing % Retained Weight (g) Accumulated