68
CHAPTER 2: HOT MIX DESIGN
CHAPTER 2: HOT MIX DESIGN
Premix - Materials
3
Aggregate Gradation
• Distribution of particle sizes expressed as a percentage of
total weight (total % passing various sieve sizes)
• Determined by sieve analysis
• Gradation affect stiffness, stability, durability, permeability,
workability, fatigue, skid, and moisture damage resistance >>
limits on the agg gradation to be used in HMA
• HMA need to have sufficient air voids in grading mix for
durability (permits enough AC to be incorporated) and avoid
bleeding and rutting (yet still have enough air space in
mixture)
• Graphically presented on semi-log graph
• 4 gradations : well (dense), uniform (single), gap graded ,
open graded (air space in mixture)
4
2. Uniformly graded (single size)• Few points of contact
• Poor interlock (shape dependent)• High permeability•Surface dressing
1. Well graded (dense)• Good interlock• Low permeability• Near maximum density• AC14/AC10/AC28
3. Gap graded• Small % of agg in the mid size range • Only limited sizes• Good interlock • Low permeability• Stone Mastic Asphalt (SMA)
Types of Gradations
NCAT 5
0.45 Power Grading Chart
0 .075 .3 .6 1.18 2.36 4.75 9.5 12.5 19.0
Sieve Size (mm) Raised to 0.45 Power
0
20
40
60
80
100
maximum density line
Percent Passing
max
size
Aggregate Gradation
6
7
Aggregate Size Definitions
• Two designation for max size
1. Max size
2. Nominal max size
• Mix designations in spec typically use nominal max size
• Nominal Maximum Aggregate Size Largest sieve size that retains some of the
agg, but not more than 10%
• Maximum Aggregate Size Smallest sieve size which 100% of the agg
pass
1001009072654836221594
100998972654836221594
8
Steps in Gradation Analysis
Mechanical sieve analysis
– Place dry aggregate in standard stack of sieves
– Place sieve stack in mechanical shaker
– Determine mass of aggregate retained on each sieve
9
Mechanical Sieve
Individual Sieve Stack of Sieves
10
Mechanical Sieve
Stack in Mechanical
Shaker
Exercise 2.9.1
12
Sieve Analysis
Analisis Ayakan
0
10
20
30
40
50
60
70
80
90
100
0.01 0.10 1.00 10.00 100.00
Sieve Size, mm
Perc
ent
Passin
g,
%
Aggregate Stockpile
15
Aggregate Blending
• Two or more stockpile need to be blended to get max density and desired void for HMA (or meet spec envelope)
• Reasons for blending:
1. Obtain desired gradation
2. Single natural or quarried material not enough
3. Economical to combine natural and processed materials
• Normally three or more stockpiles plus mineral filler
• Most common method for determining proportion – trial & error
• Blended aggregate specific gravity
16
Blending Stockpiles
• Basic formula for combining stockpiles to achieve a target gradation is:
p = Aa + Bb + Cc + ….
where:p = percent of material passing given sieve size for the combined agg
A, B, C, .. = percent passing given sieve for each agg.a, b, c, … = proportion (decimal fraction) of A, B, C, …
to be used in blend, a + b + c +… = 1.00
17
Trial and Error
• Aided by experience and plots of indiv. gradation curves and spec limits
• Calculated grading compared with spec – adjust until pass
• Guided by reasoning, maths, experience
• Use of spreadsheet now common
18
Blending of Aggregates
Agg. BAgg. A
Blend Target
Material
%
Passing
%
Passing
% Used
U.S. Sieve%
Batch
%
Batch
No. 4
No. 8
No. 16
No. 30
No. 50
No. 100
No. 200
3/8 “
90
30
7
3
1
0
0
100
100
100
88
47
32
24
10
100
19
Blending of Aggregates
Agg. BAgg. A
Blend Target
Material
%
Passing
%
Passing
% Used
U.S. Sieve%
Batch
%
Batch
No. 4
No. 8
No. 16
No. 30
No. 50
No. 100
No. 200
3/8 “
45
15
3.5
1.5
0.5
0
0
100
100
100
88
47
32
24
10
100
50 %50 %
First Try
(remember trial & error)
90
30
7
3
1
0
0
50
90 * 0.5 = 45
30 * 0.5 = 15
7 * 0.5 = 3.5
3 * 0.5 = 1.5
1 * 0.5 = 0.5
0 * 0.5 = 0
0 * 0.5 = 0
100 * 0.5 = 50
80 - 100
65 - 100
40 - 80
20 - 65
7 - 40
3 - 20
2 - 10
100
20
Blending of Aggregates
Agg. BAgg. A
Blend Target
Material
%
Passing
%
Passing
% Used
U.S. Sieve%
Batch
%
Batch
No. 4
No. 8
No. 16
No. 30
No. 50
No. 100
No. 200
3/8 “
80 - 100
65 - 100
40 - 80
20 - 65
7 - 40
3 - 20
2 - 10
100
45
15
3.5
1.5
0.5
0
0
100
50
50
44
23.5
16
12
5
50
50 %50 %
90
30
7
3
1
0
0
50
95
65
47.5
25
16.5
12
5
100
100
100
88
47
32
24
10
100
Let’s Try
and get
a little closer
to the middle of
the target values.
21
Blending of Aggregates
Agg. BAgg. A
Blend Target
Material
%
Passing
%
Passing
% Used
U.S. Sieve%
Batch
%
Batch
No. 4
No. 8
No. 16
No. 30
No. 50
No. 100
No. 200
3/8 “
80 - 100
65 - 100
40 - 80
20 - 65
7 - 40
3 - 20
2 - 10
100
27
9
2.1
0.9
0.3
0
0
100
70
70
61.6
32.9
22.4
16.8
7
70
70 %30 %
90
30
7
3
1
0
0
30
97
79
63.7
33.8
22.7
16.8
7
100
100
100
88
47
32
24
10
100
22
Blended Aggregate Specific Gravities
• Once the percentages of the stockpiles have been established, the combined aggregate specific gravities can also be calculated
Combined G =100
P1 + P2 + ……. Pn
G1 G2 Gn
Exercise 2.9.2 and 2.9.3
Mix Design
Design objectives – Develop an economical blend of aggregates
and asphalt that meet design requirements
Historical mix design methods
1. Marshall – use impact hammer
2. Hveem – use kneading compactor, Hveem Stabilometer
New
1. Superpave gyratory – use gyratory compactor to simulate
field compaction, able to accommodate large size aggregate
Mix design methods
Requirements in Common
• Sufficient asphalt to ensure a durable pavement
• Sufficient stability under traffic loads
• Sufficient air voids
–Upper limit to prevent excessive environmental
damage
–Lower limit to allow room for initial densification
due to traffic
• Sufficient workability
Design bitumen content (JKR, 2008)
Mix type Bitumen content (%)
AC10 5-7
AC14 4-6
AC28 3.5-5.5
Volumetric Properties of Asphalt Mix
29
Aggregate Specific Gravity
• Ratio weight of mat. to water of equal volume at 23C,
useful in making weight-vol conversion
• In metric units, G simply:
G = weight / vol
• Four Gs – apparent, bulk, effective, bulk impregnated
1. Apparent – weight / vol solid …… Dry
2. Bulk – weight / overall vol …… SSD
3. Effective – weight / (overall vol – asp asorb. pores)
4. Bulk impregnated – eff. but immerse in asphalt
Gsb < Gse < Gsa
30
Gsa =
Mass of oven dry agg
Vol of agg
Apparent Specific Gravity
31
Gsb =
Vol of agg, + perm. pores
Vol. of water-perm. pores
Surface Voids
Bulk Specific Gravity
Mass of oven dry agg
Gs, eff =Mass oven dry agg
Vol of agg, + perm. pores not absorb. asphalt
Effective Specific Gravity
Surface Voids
Solid Agg.
Particle
Vol. of water-perm. voids
not filled with asphalt
Absorbed asphalt
Property When Asphalt cement is varied in mix design
Near optimum asphalt cement
VTM 2.5-8 3-5
VMA 10-16 12-15
VFA 50-85 65-85
•VTM 3-5%
•<3 – rutting, bleeding, loss of friction
•> 5- permebility, oxidation (aging, asphalt get stiffened)
•VMA 13-14
•Low VMA- loss of durability, not enough asphalt.Change in asphalt
cement content could be more critical
•High VMA- loss of strength, increased cost
•VFA- flip of VTM, VTM<3VFA goes up, VTM>5 VFA goes down
MARSHALL
MIX
DESIGN
Marshall Design Method
• Bruce Marshall, 1939, Mississipi Highway Department, refined-US army.
• WES began to study it in 1943 for WWII
– Evaluated compaction effort
• No. of blows, foot design, etc.
• Decided on 10 lb.. Hammer, 50 blows/side
• 4% voids after traffic
• Initial criteria were established and upgraded for increased tire pressures and loads
Lab Mix - Material
•Aggregate and bitumen test•Aggregate blend
Course Aggregate
Fine Aggregate
Lab Mix - compaction
•Sample preparation•Mix 160 ˚C•4 sample at each bitumen content•75 blows/face compaction•Compact 145˚C
Lab Mix - density
Bulk SG ASTM D 2726
Calculations
• Gmb = A / ( B - C )
Where:
A = mass of dry sample
B = mass of SSD sample
C = mass of sample under water
Lab Mix – Marshall Test
Stability- max load (kN) , loading rate 50.8mm/min Flow- diff sample height (mm)
Lab Mix – Marshall Form
SG and voids analysis
Exercise 2.9.4
OBC determination
• 4 graphs (JKR, 2008):
– Peak stability
– Peak bulk SG
– VFB= 75% WC, 70% BC
– VTM= 4% WC, 5% BC
Lab Mix – OBC Determination
2.320
2.330
2.340
2.350
2.360
2.370
4.0 4.5 5.0 5.5 6.0 6.5 7.0
Bit. Content (%)
Density (
g/c
u.c
m)
800
900
1000
1100
1200
1300
1400
4.0 4.5 5.0 5.5 6.0 6.5 7.0
Bit. Content (%)
Sta
bili
ty (
kg)
2.0
3.0
4.0
5.0
6.0
7.0
8.0
4.0 4.5 5.0 5.5 6.0 6.5 7.0
Bit. Content (%)
VTM
(%
)
55.0
60.0
65.0
70.0
75.0
80.0
85.0
4.0 4.5 5.0 5.5 6.0 6.5 7.0
Bit. Content (%)
VF
B (
%)
c
a
d
b
Lab Mix – OBC Determination
OBC =(a + b + c + d)/4 = e
Check parameters @ OBC
- Stability- Flow
- Stiffness
- VTM
- VFB
Lab Mix – Value @ OBC
800
900
1000
1100
1200
1300
1400
4.0 4.5 5.0 5.5 6.0 6.5 7.0
Bit. Content (%)
Sta
bili
ty (
kg)
2.0
3.0
4.0
5.0
6.0
7.0
8.0
4.0 4.5 5.0 5.5 6.0 6.5 7.0
Bit. Content (%)
VTM
(%
)
55.0
60.0
65.0
70.0
75.0
80.0
85.0
4.0 4.5 5.0 5.5 6.0 6.5 7.0
Bit. Content (%)
VF
B (
%)
e
e e
Lab Mix – Value @ OBC
3.00
3.50
4.00
4.50
5.00
5.50
6.00
4.0 4.5 5.0 5.5 6.0 6.5 7.0
Bit. Content (%)
Flo
w (
mm
)
100
150
200
250
300
350
400
4.0 4.5 5.0 5.5 6.0 6.5 7.0
Bit. Content (%)
Stiffn
ess (
kg/m
m)
Compare parameters with specification
Pass? >> OBC = e
Fail? >> redesign
e e
Mix design test and analysis parameters (JKR, 2008)
Parameter WC BC
Stability, S >8000N >8000N
Flow, F 2-4mm 2-4mm
Stiffness, S/F >2000N/mm >2000N/mm
VTM 3-5% 3-7%
VFB 70-80% 65-75%
Exercise 2.9.5
WHAT IS PREMIX PRODUCTION?
• Premix production is a process of mixing the aggregates and asphalt in the hot mix facilities, to be used as road material regardless whether it’s an ACW, ACB or DBM.
HOT MIX
FACILITES
a) Drum mix
b) Batch mix
Stock Pile Asphalt (Bitumen)
Agregate Stockpile
Premix - Materials
HOT MIX ASPHALT FACILITIES
Purpose of an HMA facility is to properly proportion, blend, and heat aggregate and asphalt to produce
an HMA that meets the requirements of the job mix formula.
Types Of Plants
BATCH DRUM MIXER
DIFFERENCE
• drum mix plants dry the aggregate and blend it with asphalt in a continuous process and in the same piece of equipment.
• batch plants dry and heats the aggregate and then in a separate mixer blend the aggregate and asphalt one batch at a time
Drum Mixer Planttypical layout
Storage
Silo
Dryer
Conveyor BeltsAggregate
Bins
Burner
Asphalt Storage
Batch planttypical layout
Storage
Silo
Batch
TowerDryer
Conveyor BeltAggregate
Bins
Burner
Asphalt Storage
Hot
elevator
61
Drim Mix Plant
62
DRUM MIX
Drum Dryer Mixer
65
DRUM MIX
66
67
BATCH PLANT
THE END
