DESIGN OF BITUMINOUS MIX-II
Design methods of bituminous mixes are
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Marshall
Hveem
Superpave
Marshall method of mix design:
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Developed by bruce marshall in the late 30’s for Mississippi highway department
Asphalt institute procedure recommended by MOSTRH.
The original procedure for designing bituminous mixes that were originally developed by
Marshall especially in terms of the compaction effort used underwent a lot of changes
over all these years so that the mix design corresponds to the actual conditions of traffic,
different climatic conditions that are prevalent now.
For dense graded bituminous mixes.
Main steps:
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Selection of mix type
Selection of maximum aggregate size and aggregate gradation
Minimum Layer thickness > 2 to 3 times Max. aggregate size
Aggregate Fractions Coarse (Retained on 2.36mm), Fine Aggregate (Pass 2.36mm and
Retained on 0.075mm), Mineral Filler (Passing 0.075mm)
Selection of Binder and Aggregates that satisfy the specifications for the given traffic and
climatic conditions - Testing source materials
Selection of Design Aggregate Gradation considering the traffic and climatic conditions
Preparation of test specimens of bituminous mixes using aggregates and binder with
appropriate compaction effort
Testing the specimens
Making additional trials if required
Selection of Optimum binder content as per specified criteria
Steps:
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Grading of Commercially available Mineral Aggregate
Proportioning of Mineral Aggregate Blend
Specific Gravities of binder and aggregates
Preparation of Marshall Specimens
Testing of Marshall Test Specimens
a) Bulk Specific gravity of compacted specimen
b) Stability
c) Flow
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d) Specific gravity of loose mix
Computation of percent Air Voids in Compacted Mix
Computation of percent Voids in Mineral Aggregates Filled by Binder
Selection of Optimum Binder Content
Checking Specification Values at Optimum Binder Content
Testing of samples:
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Determine Bulk Specific Gravity of specimens
Condition the specimens in Water bath at 60°C for 30 to 40 minutes
Conduct Marshall Stability Test - Determine Stability and Flow for each specimen
Correct the Stability Values for (non-standard height or volume)
Test should be completed within 30 seconds after removing from the water bath
Preparation of test specimen:
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standard Marshall Method involves preparation of a 4 inch dia (102mm), 2.5 inch
(64mm) thick specimen of bituminous mix with a selected gradation of aggregates and
binder content.
Standard Compaction effort is used 4.5kg (10lb) mass and a free fall of 457mm (18
inches)
Maximum size of aggregate = 25.4 mm (one inch)
6-inch (150mm) mould for larger aggregate size
Series of test specimens are prepared with the selected aggregate blend and different
binder contents
Selection of Trial Binder Content
P = 0.035a + 0.045b + Kc + F
Where, P = approx. Binder content (% by wt. of mix)
a = % of aggregate retained on 2.36mm sieve
b = % of aggregate passing 2.36mm and retained on 0.075mm sieve
c = % of aggregate passing 0.075mm sieve
K=0.15 for 11-15% passing 0.075mm sieve (if the filler content is 11 to 15% the value of
k is taken as 0.15)
= 0.18 for 6-10% passing 0.075mm sieve
= 0.20 for <= 5% passing 0.075mm sieve
F = 0 to 2% (based on absorption of aggregates)
specimens at 6 binder contents (specimens have to be tested are prepared at six different
binder contents. And at each binder content normally three specimens have to be
prepared. if take about 25 kg of blended aggregate and about four liters of binder that
would be normally be sufficient to cover these six binder contents and three specimens at
each binder content.)
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Three specimens at each binder content
About 25 kg of blended aggregate and 4 It of binder
Determine viscosity of binder at different temperatures using Rotational Viscometer and
selection of Mixing and Compaction Temperatures
viscosity should be ranging from 0.17 +/– 0.02 Pascal seconds and viscosity range for
compaction is 0.28 +/– 0.03 Pascal seconds.
Compaction of loose hot mix to obtain a test specimen of standard dimensions
Mixing temperature corresponding to a viscosity of 0.17 Pa-s
Dry aggregates heated to a temperature not exceeding (mixing temp. + 28)
Binder heated to mixing temperature (different for different types of binders)
Place the heated mix in compaction mould (pre-heated) and compact.
Compaction of specimen:
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ompaction of loose hot mix to obtain a test specimen of standard dimensions
Standard Compaction effort is used 4.5kg (10 lb) mass and a free fall of 457mm (18
inches)
Maximum size of aggregate = 25.4 mm (one inch)
6-inch (150mm) mould for larger aggregate size
Bulk sp.gravity of compacted mix(Gmb) = dry mass of mix/volume of water replaced by the
saturated surface dry specimen.
The specimen has to be saturated then the surface water has to removed and its weight has to be
taken and then its weight in air has to be taken. Therefore the difference in weights will give the
volume of water replaced by saturated surface dry specimen so the dry mass of the specimen
divided by volume of water replaced by the saturated surface dry specimen which is bulk
specific gravity of the compacted mix.
MORTH specifies for heavy compaction 75 blows are to be applied using Marshall compaction
hammer on both faces. First compact it on one side then the specimen has to be reversed then
again 75 blows of Marshall hammer have to be applied on the other face also. So this is the
standard compaction that is recommended for all the mixes that use for highways.
The specimen is put in a Marshall testing machine and Marshall Test is conducted. Marshall Test
is nothing but the breaking head put on both sides of the specimen so a compressive load is
applied along the diameter of the specimen at a rate of 51 mm per minute, the temperature of the
specimen is going to be maintained at 60 degree centigrade and the inside radius of the breaking
head is going to be approximately equal to that of the specimen which is 51 mm so load at this
rate is applied. The load at which the specimen breaks. so either in a proving ring or in a dial
case or in any automatic measurement see the load increasing then after a certain stage once the
specimen fails the load starts decreasing. So observe the failure load is and also observe the
deformation of this specimen undergoes when this specimen fails. Starting from an initial
deformation of zero the deformation at failure condition has to be observed. So the breaking load
is known as stability and the deformation at failure is known as flow. So these are known as
Marshall Stability and Marshall Flow.
Correction factor for stability:The stability that is obtained from Marshall testing machine as to be corrected for nonstandard
volume. If the dimensions attained are 4 inch dia and 2 ½ inch height there would not be any
correction that is required but if mostly the height varies there is certain correction that is to be
applied. For example, if the volume is within 509 to 522 cc there would not be any correction but
if it is more the stability will be reduced and if the volume is less the stability will be increased.
VOLUME(CC)
432-443
444-456
457-470
471-482
483-495
509-522
523-535
536-546
CORRECTION
FACTOR
1.32
1.25
1.19
1.14
1.09
1.00
0.96
0.93
VOLUME(CC)
547-559
560-573
574-585
586-598
599-610
611-625
CORRECTION
FACTOR
0.89
0.86
0.83
0.81
0.78
0.76
Volumetric analysis:After the Marshall test is done volumetric analysis is to carried out. This is to estimate important
volumetric parameters such as air void content, voids and mineral aggregate, mineral aggregate
voids filled with bitumen and so on for each of these specimens. And for each binder content
need to determine the maximum specific gravity. This is the specific gravity of the void-less
loose mix this has to be determined and using all this information calculate the effective specific
gravity of aggregates and carry out the volumetric analysis to compute air voids VMA and VFB.
Volumetric Analysis – Example
Initial Data
Three sources of aggregates (A, B, C) have been blended to obtain the desired gradation. The
proportion of each source in the blend and the corresponding bulk specific gravities are given
below.
Aggregate Source
Proportion in blend
Bulk sp. gravity
A
25%
2.954
B
45%
2.896
C
30%
2.835
Bulk Specific gravity of combined aggregate
=
= 100/ (25/2.954 + 45/2.896 + 30/2.835) = 2.8915
Bitumen content = 5% (by weight of mix)
Specific Gravity of bitumen, Gb = 1.03
Bulk Specific Gravity of the specimen (Gmb) = 2.552
Max. Sp.Gr .of loose mix for 5% binder (Gmm) = 2.729
Effective Specific Gravity of Aggregate (Gse)
(including all voids except those that absorb bitumen)
Gse (Pmm-Pb)/ (Pmm/ Gmm -Pb/Gb)
=(100-5.0)\{100/2.729 - 5.0 /1.03} = 2.9884
Pb= proportion of binder
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Pmm= percentage of total loose mix (100)
Ps= proportion of aggregate
Maximum Specific gravity of loose mix for other binder contents (approximately) Gmm
=Pmm/(Ps/Gse + Pb/Gb)
(eg: for 6% binder content, Gmm = 100/(94/2.9884+6/1.03)
2.6823 (2.729 for 5%)
Bitumen absorption (for 5% binder content case)
Pba = 100 *((Gse-Gsb)/(Gsb*Gse))*Gb
100((2.9884-2.8915)/(2.8915-2.9884))*1.03
= 1.155%
Effective Bitumen = Pb-Pba *Ps/100
=5.0-1.155-95/100=3.90%
Voids in Mineral Aggregate (VMA)
=100-Gmb* Ps/Gsb
=100-2.552*95/2.8915 16.15%
Air Voids, Va
=100* (Gmm-Gmb)/Gmm
=100 (2.729-2.552)/2.729
=6,49%
Voids Filled with Bitumen, VFB
=100 (VMA-VA)/VMA
= 100*(16.15-6.49)/16.15
=59.81%
Plot different trends:Variation of different mix parameters with binder content
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Stability VS Binder content
Flow VS Binder content
Percent air Voids VS Binder content
Percent Voids in Mineral Aggregate VS Binder content
Percent Voids Filled with Bitumen VS Binder content
Selection of optimum binder content:
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Binder content that satisfies all the mix requirements (specifications) should be selected
The specifications should normally be developed on the basis of performance mixes
under specified conditions
Asphalt Institute main criterion is a median air void content of 4%. Binder content
corresponding to 4% air voids should be selected as OBC if other criteria are satisfied
MoSRT&H Specifications for heavy traffic
No. of hammer blows on each face of specimen-
75
Minimum Marshall Stability, kg-
900
Marshall Flow, mm-
2-4
Volds in Compacted Mix, %-
3-6
Voids in Mineral Aggregates Filled by Bitumen, %-
65-75
Betained Stability on Immersion in water at 60°C
min 80%
Voids in Mineral aggregates, VMA (%) on the basis of max. aggregate size
Nominal agg
Max.size(mm)
9.5
12.5
19.0
25.0
3%
14%
13%
12%
11%
Min VMA for Air Void Content Of
4%
15%
15%
13%
11%
5%
16%
16%
14%
12%
Advantages of marshall method:
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Relatively Inexpensive
Convenient for design and quality control
Importance given to air void content
Also accounts for the strength and durability requirements of mix
Can be used on site also
Limitations:
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Impact method of compaction
Does not consider shear strength
Load perpendicular to compaction axis
Marshall test data can not predict fatigue or permanent deformation behaviour of in
service pavements. This method does not give guidelines for judging the
quality of bitumen
The laboratory mix design (aggregate gradation, optimum binder content and the
corresponding mix parameters like stability, density, air voids, etc) is normally
considered as the target to be attained in the field within permissible tolerance
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MOSRTH specifications refer to mix parameters after several years of traffic.
6-8% air void content if obtained soon after compaction should be OK assuming that
minimum of 98% laboratory density is to be attained.