CHAPTER 2: HOT MIX DESIGN · Mix Design Design objectives –Develop an economical blend of...

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CHAPTER 2: HOT MIX DESIGN

Premix - Materials

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)

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

maximum density line

Percent Passing

Aggregate Gradation

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

1001009072654836221594

100998972654836221594

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

Mechanical Sieve

Individual Sieve Stack of Sieves

Mechanical Sieve

Stack in Mechanical

Shaker

Exercise 2.9.1

Sieve Analysis

Analisis Ayakan

0.01 0.10 1.00 10.00 100.00

Sieve Size, mm

Passin

Aggregate Stockpile

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

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

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

Blending of Aggregates

Agg. BAgg. A

Blend Target

Material

Passing

% Used

U.S. Sieve%

No. 16

No. 30

No. 50

No. 100

No. 200

3/8 “

Agg. BAgg. A

Blend Target

Material

Passing

% Used

U.S. Sieve%

No. 16

No. 30

No. 50

No. 100

No. 200

3/8 “

50 %50 %

First Try

(remember trial & error)

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

100 * 0.5 = 50

80 - 100

65 - 100

40 - 80

20 - 65

7 - 40

3 - 20

2 - 10

Agg. BAgg. A

Blend Target

Material

Passing

% Used

U.S. Sieve%

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

50 %50 %

Let’s Try

and get

a little closer

to the middle of

the target values.

Agg. BAgg. A

Blend Target

Material

Passing

% Used

U.S. Sieve%

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

70 %30 %

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

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

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

Mass of oven dry agg

Vol of agg

Apparent Specific Gravity

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

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

4.0 4.5 5.0 5.5 6.0 6.5 7.0

Bit. Content (%)

Density (

4.0 4.5 5.0 5.5 6.0 6.5 7.0

Bit. Content (%)

4.0 4.5 5.0 5.5 6.0 6.5 7.0

Bit. Content (%)

4.0 4.5 5.0 5.5 6.0 6.5 7.0

Bit. Content (%)

Lab Mix – OBC Determination

OBC =(a + b + c + d)/4 = e

Check parameters @ OBC

- Stability- Flow

- Stiffness

Lab Mix – Value @ OBC

4.0 4.5 5.0 5.5 6.0 6.5 7.0

Bit. Content (%)

4.0 4.5 5.0 5.5 6.0 6.5 7.0

Bit. Content (%)

4.0 4.5 5.0 5.5 6.0 6.5 7.0

Bit. Content (%)

Lab Mix – Value @ OBC

4.0 4.5 5.0 5.5 6.0 6.5 7.0

Bit. Content (%)

4.0 4.5 5.0 5.5 6.0 6.5 7.0

Bit. Content (%)

Stiffn

Compare parameters with specification

Pass? >> OBC = e

Fail? >> redesign

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

Conveyor BeltsAggregate

Burner

Asphalt Storage

Batch planttypical layout

Storage

TowerDryer

Conveyor BeltAggregate

Burner

Asphalt Storage

elevator

Drim Mix Plant

DRUM MIX

Drum Dryer Mixer

DRUM MIX

BATCH PLANT

THE END

CHAPTER 2: HOT MIX DESIGN · Mix Design Design objectives –Develop an economical blend of...

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