Lect 18-conventional asphalt mix design

CONVENTIONAL ASPHALT
MIX DESIGN
Lecture 18
Prof. P. K. Bhuyan
Dept. of Civil Engg.
NIT Rourkela

Asphaltic Concrete
Asphaltic concrete is a mixture of
Coarse Aggregate
Fine aggregate
Mineral filler and
Bitumen
Well graded aggregates and mineral filler
resulting in maximum density when mixed with
optimum quantity of bitumen results in a mix
with very high stability

Stability
 Enough resistance to deformation under sustained or repeated
loads
Durability
Resistance to disintegration by weathering or abrasive forces of
traffic
Flexibility
Ability of a bituminous mix to bend repeatedly with out cracking
and to conform to changes in shape of the base course
Skid Resistance
Offer enough resistance to the skidding of tyres
Impervious Layer
Should be highly impervious to water
Desirable Properties of AC Mix

Steps Involved in Deriving
the Job Mix Formula
Selection of aggregates
Selection of aggregate gradation
Proportioning of aggregates to meet the
required gradation
Selection of bitumen
Preparation of specimen
Density – void analysis
Measurement of stability and flow
Determination of optimum bitumen content

Selection of Aggregates
 The aggregates should satisfy the specifications laid
down for the mix in respect of the following
Cleanliness
Percent passing 0.075 mm sieve
Particle shape
Combined flakiness and elongation index
Strength
Los Angeles abrasion value / Impact value
Polishing
Polished stone value
Durability
Soundness test
Water absorption
Stripping

Selection of Aggregate Gradation
Densely graded aggregate offers
High frictional resistance
Greater area of load transfer
The gradation that results in maximum density
would offer high stability to the final mix
Theoretical gradations could be used as a
starting point to arrive at the required gradation
by trial and error method

Theoretical Gradation
 Theoretical gradations generally take the following
form
P = 100 (d/D)x
Where, P = percent passing
d = size of sieve opening
D = largest size in gradation
 The basic idea of the theory is that the amount of
material of a given size should be just sufficient to fill
the voids between aggregates of larger size
 Fuller suggested a value of 0.5 for x
 However, a value of 0.45 for x is being used in
Superpave gradations

Specified Gradation
Specified gradations are worked out starting
from the theoretical gradations
Lower and upper limits of gradation for each
sieve size are arrived at for allowing window of
variation by examining the changes in density
and the resulting stability in the final mix
The specified gradations are also related to the
thickness of construction and the nominal size
of aggregate used

Sieve Size
mm
Grading I Grading II
50-65 mm
(19 mm nominal
aggregate size)
30-45 mm
(13 mm nominal
aggregate size)
26.5 100 -
19 79-100 100
13.2 59-79 79-100
9.5 52-72 70-88
4.75 35-55 53-71
2.36 28-44 42-58
1.18 20-34 42-58
0.60 15-27 26-38
0.30 10-20 18-28
0.15 5-13 12-20
0.075 2-8 4-10
Specified Gradation for BC

Specified Vs Theoretical Gradation
When theoretical Gradations are adopted in
actual practice, the smaller particles tend to
wedge between the larger ones, increasing
the voids that must be filled with the smaller
ones
As a result maximum densities are actually
achieved by gradations having an excess of
the small sizes compared with the theoretical
amounts

Specified Vs Theoretical Gradation
0
20
40
60
80
100
26.5
19
13.2
9.5
4.75
2.36
1.18
0.6
0.3
0.15
0.075
Sieve Size (mm)
PercentPassing
Specified Gradation
Theoretical Gradation

Proportioning of Aggregates
Normally, the aggregates from the quarry are
available in three nominal sizes viz., 19 mm, 9.2
mm and 2.36 mm (Grit and dust)
Sieve analysis is carried out on each of these
aggregates and their individual gradation is
determined
Sieve analysis is also carried out on lime which
will be used as filler
The proportion in which each of these
aggregates are to be mixed to get the specified
gradation is to be obtained

Typical gradation of Aggregates
and Lime
Sieve
Size
19 mm
Aggregate (A)
9.2 mm
Aggregate (B)
Grit +
Dust (C)
Lime (D)
26.5 100 100 100 100
19 75.08 100 100 100
13.2 1.48 100 100 100
9.2 0.00 90.00 100 100
4.75 0.00 5.00 100 100
2.36 0.00 0.00 70.50 100
1.18 0.00 0.00 50.55 100
0.6 0.00 0.00 40.50 100
0.3 0.00 0.00 30.00 100
0.15 0.00 0.00 16.54 100
0.075 0.00 0.00 6.00 97.00

Methods of Proportioning
 Graphical Methods
Rothfutch’s Method
 Trail and Error Method
 Analytical Method
For each sieve we write the constraints
aFA + b  FB + c  FC + d  FD <=UL
a  FA + b  FB + c  FC + d  FD >=LL
Where, a, b, c and d are the proportions of aggregates A, B, C and D
respectively
FA, FB, FC,and FD are respectively the percent fines of aggregates A,
B, C and D passing the sieve
Solve the above keeping a+b+c+d = 1 and d=0.02 (2%)

Worksheet for Proportioning of
Aggregates

Selection of Bitumen
 A proper grade should be selected as per specifications
 Bitumen should satisfy all the specifications laid down relating
to the following (BIS: 73)
Penetration
Softening point
Ductility
Flash point
Wax content
Loss on heating and retained penetration
Solubility
Viscosity at 60 OC and 135 OC
 If modified bitumen is used then additional tests (elastic
recovery, etc) should be performed as specified

Marshall Method of Mix Design
The basic concepts of the Marshall mix design
method were originally developed by Bruce
Marshall of the Mississippi Highway
Department around 1939 and then refined by
the U.S. Army.
The Marshall stability of the mix design is
defined as a maximum load carried by a
compacted specimen at a standard temperature
of 60oC.

Preparation of Specimen
 The coarse aggregates, fine aggregates and the filler
material should be proportioned and mixed as per the
dry mix design
 The required quantity of the dry mix is taken so as to
produce a compacted bituminous mix specimen of
thickness 63.5mm approximately
 Considering the specific gravities of aggregates in this
region, approximately 1200gm of aggregates and filler
would be required to get a standard specimen

 The dry mix of aggregates and filler is heated to a
temperature of 150 to 170oC
 The compacted mould assembly and rammer are
cleaned and kept preheated to a temperature of 100oC
to 145oC
 The bitumen is heated to a temperature of 150oC to
165oC and the required quantity of the first trial
percentage of bitumen is added to the heated
aggregates and thoroughly mixed.
 The mixing temperature of the 60/70 grade is about
165oC.

Marshall Mould
For preparing specimens
of 10.16 cm diameter and
6.35 cm height for Marshall
testing.
Consists of base plate,
forming mold and collar.
Interchangeable base
plate and collar can be used
on either end of compaction
mold.

Compaction of the
Specimen
 The mix is placed in the
mould and compacted by a
rammer with about 75 blows
on each side.
 The weight of hammer is
4.54 kg and height of fall is
45.7 cm
 The compacting temperature
may be about 135oC for
60/70 grade bitumen.
 The compacted specimen
should have a thickness of
63.5 ± 3.0mm.

Sample Extraction
The compacted
specimens are extracted
using a Sample Extractor
after the curing time
Sample extractor is
designed for fast
extrusion of samples from
compaction molds.

 At least two (preferably three) specimens should be
prepared at each trial bitumen content which may be
varied at 0.5% increments from 4.5 to 6.5 percent.

Density Void Analysis
The following quantities are worked out by
carrying out density voids analysis
Bulk density ( ) / specific gravity (G) of the
specimen
Average specific gravity of aggregates (Ga)
Theoretical maximum specific gravity (Gt)
Percent air voids in the final mix (VV)
Percent air voids in mineral aggregates (VMA)
Percent aggregate voids filled with bitumen (VFB)

Bulk Density Determination
 Bulk density of the specimen could be determined by three methods
 From dimensions: if the specimen is of true size whose dimensions can
be accurately determined
 = W / V
Where, W = weight of the specimen, g
V = (/4) d2 h, h=height and d = diameter of the specimen in cm
 By weighing in air and water: if the specimen has impermeable surface
G = W / (W - Ww)
Where, Ww = weight of the specimen in water
 By weighing paraffin coated specimen in air and water : if the specimen
has open impermeable surface
 W / = weight of the paraffin coated specimen in air
 Ww
/ = weight of the paraffin coated specimen in water
P
w
G
WW
WW
W
G
)( /
// 



Weights and Volumes in a
Compacted Specimen
Voids
Bitumen
Coarse
Aggregate
Fine
Aggregate
Mineral
Filler
Pmf
Pfa
Pca
Pb
0
Vmf
Vfa
Vca
Vb
Vv
%Volumes %Weights Specifc
Gravities
Gmf
Gfa
Gca
Gb

Theoretical maximum Specific Gravity
 Average specific gravity (Ga) of the aggregate mix
mf
mf
fa
fa
ca
ca
mffaca
a
G
P
G
P
G
P
PPP
G



 Theoretical maximum specific gravity (Gt) of the AC mix
b
b
mf
mf
fa
fa
ca
ca
t
G
P
G
P
G
P
G
P
G


100

Vv, VMA and VFB
 Voids in the final mix
 Voids in mineral aggregates
Pa = Pca + Pfa + Pmf
 Aggregate voids filled with bitumen
a
a
G
GP
VMA 100
t
t
V
G
GG
V
)(
100


VAM
VVMA
VFB V )(
100



Marshall Stability and Flow
 The specimens to be tested are kept immersed in
water in a thermostatically controlled water bath
at 60 ± 1oC for 30 to 40 minutes.

Take out the
specimen from the
water bath and place it
in the breaking head
Place the breaking
head in Marshall
testing machine

Load is applied on the breaking head by the loading
machine at the rate of 5 cm per minute.

Stability value is the load taken by the
specimen at failure.
Flow value is the deformation of the specimen
at failure
Record stability either by proving ring or load
cell display unit.
Record the flow by the dial gauge or
displacement cell attached to the breaking head
Apply correction factor to the stability value if
the height of specimen is different from 6.35 cm

Correction Factors
Volume of
specimen in
cm3
Approximate
Thickness of
Specimen in mm
Correction
Factors
457-470 57.1 1.19
471-482 58.7 1.14
483-495 60.3 1.09
496-508 61.9 1.04
509-522 63.5 1.00
523-535 65.1 0.96
536-546 66.7 0.93
547-559 68.3 0.89
560-573 69.9 0.86

Sample No
Bitumen content
percent
Stability Value, kg
Flow Value, mmMeasured Corrected
1 4 670 641.56 2.51
2 4 690 634.62 1.82
3 4 650 591.117 2.66
Average 622.43
1 4.5 770 730.97 2.28
2 4.5 770 700.1225 2.54
3 4.5 550 500.775 2.61
Average 643.95
1 5 800 756 2.8
2 5 720 651.105 2.77
3 5 910 831.512 2.59
Average 746.20
1 5.5 890 842.718 2.22
2 5.5 970 880.032 2.58
3 5.5 970 1063.362 5.01
Average 928.70
1 6 900 900.9 3.53
2 6 880 876.26 3.09
3 6 800 875.5 5.42
Average 884.22
1 6.5 810 773.55 6.11
2 6.5 850 823.225 4.01
3 6.5 730 744.6 4.34
Average 780.45

Optimum Content of Bitumen
The following graphs are plotted
Unit weight vs. bitumen content
Marshall stability vs. bitumen content
Percent voids in mix vs. bitumen content
Percent aggregate voids filled with bitumen vs.
bitumen content
Flow Values vs. bitumen content

Unit weight vs. Bitumen content
Bitumen
content
Unit
Weight
4 2.25
4.5 2.33
5 2.35
5.5 2.36
6 2.37
6.5 2.36
2.24
2.26
2.28
2.3
2.32
2.34
2.36
2.38
0 2 4 6 8
% of Bitumen
Unitweight
2.2
2.25
2.3
2.35
2.4
4 4.5 5 5.5 6 6.5
Bitumen content
Unitweight

Stability vs. Bitumen Content
Bitumen
Content Stability
4 622.432
4.5 643.955
5 746.205
5.5 928.704
6 884.22
6.5 780.458
0
200
400
600
800
1000
3.5 4 4.5 5 5.5 6 6.5
Bitumen %
Stability

% Voids in Mix vs. Bitumen Content
Bitumen
content vv
4 12.85
4.5 10.16
5 8.35
5.5 6.51
6 4.26
6.5 3.96
0.00
4.00
8.00
12.00
16.00
3.5 4 4.5 5 5.5 6 6.5
Bitumen content
%Voidsinmix

VFB vs, Bitumen Content
Bitumen
content VFB
4 41.43
4.5 51.41
5 59.14
5.5 67.40
6 77.80
6.5 80.26
0.00
20.00
40.00
60.00
80.00
100.00
3.5 4 4.5 5 5.5 6 6.5
Bitumen Content (%)
VFB

Flow Value vs. Bitumen Content
Bitumen
Content
Flow
Value
4 2.33
4.5 2.48
5 2.72
5.5 3.27
6 4.01
6.5 4.82
0
1
2
3
4
5
3.5 4 4.5 5 5.5 6 6.5
Bitumen Content (%)
FlowValue

Optimum Bitumen Content
 Bitumen content corresponding to maximum
stability = 5.5 %
 Bitumen content corresponding to maximum bulk
density = 6.0%
 Bitumen content corresponding to 4% air voids =
6.34
 Optimum bitumen content of the mix
(5.5+6.0+6.34)/3= 5.95%
Flow Value corresponding to 5.95 % bitumen
content = 4 mm
 And VFB at 5.94% = 78%

Brittle Mixes
Mixes with very high Marshall stability values
and very low Flow values are not desirable as
the pavements of such mixes may be brittle and
are likely to crack under heavy traffic

Lect 18-conventional asphalt mix design

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Lect 18-conventional asphalt mix design