New trends in HMA design ( Highway Engineering Dr. Sherif El-Badawy )

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Mansoura University - Faculty of Engineering – Public Works Engineering Dept.

New Trends in HMA Mixes


The GOAL of Mix Design


Getting this to

Carry this without

To much of This.

The GOAL is…


• Design of an asphalt-aggregate mixture

consists of…

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1 • Select the type and gradation of

aggregate

2 • Select the type and grade of asphalt

binder, with or without modification

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• Select the amount of asphalt binder to satisfy the project-specific requirements…

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Properties of a Good

Asphalt-Aggregate Mixture

Stiffness

Durability

Stability

Fatigue Resistance

Fracture Resista

nce

Low Skid

Permeability

Work-ability


Special HMA Mixes

• SuperPave

• Reclaimed Asphalt Pavement (RAP)

• Warm Mix Asphalt (WMA)

• Stone Matrix Asphalt (SMA)

• Open Graded Friction Course (OGFC)

• Pervious Pavement


Gyratory Compaction

• Gyratory compaction of specimens in mix design is one of the key differences with the Marshall method.


• Simulate field densification • traffic

• climate

• Accommodate large aggregates

• Measure compactability

Goals of Lab Compaction

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• 150 mm diameter • Up to 37.5 mm nominal

size

• Height recordation

Superpave Gyratory Compactor

150 mm

diameter

ram pressure

600 kPa

1.25 degrees 30 gyrations

per minute


SGC Types

Troxler Pine Brovold Interlaken

4141

4140

AFG1A

AFGC125X GYR-001 HM-293

Rainhart

144


SGC Compaction Curve

% Gmm

Log (Gyrations)

10 100 1000

The increase in density

(%Gmm) with each gyration

is used to graph this

compaction curve.


Hot-Mix Asphalt (HMA) Recycling

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Hot-Mix Asphalt (HMA) Recycling

Hot-Mix Asphalt Plant

Milling an Asphalt Pavement

Reclaimed Asphalt Pavement (RAP)


Why Recycle Asphalt?

• Pavement Benefits

• Economic Payoff

• Energy Savings

• Natural Resource Conservation

Milling (Cold Planing) of an Asphalt Pavement


Benefits of Milling

• Removes cracked and aged

pavement layers

• Improves pavement

smoothness and cross-slopes

• Maintains curb heights,

drainage inlets, and bridge

clearances

• Creates a rough texture that

bonds better with the overlay


Materials savings will depend on...

• Virgin binder cost

• Asphalt content of the mix design

• Aggregate cost

• RAP cost

• Asphalt content of the RAP

• Percentage of RAP

Economics of HMA Recycling

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Recycling Economics Example

• Aggregate: LE 60.00/ton

• Asphalt: LE 3000.00/ton

• Mix Design AC Content: 5.0%

LE 60 x .95 = LE 57

LE 3000 x .05 = LE 150

Total Mix LE 207



• Aggregate: LE 60.00/ton

• Asphalt: LE 3000.00/ton

• RAP: LE 40.00/ton (5% AC in RAP)

• Mix Design • AC Content: 5.0%

• RAP content 20% (19% rock, 1% AC)

LE 60.00 x .76 = LE 45.60

LE 3000.00 x .04 = LE 120.00

LE 40.00 x .20 = LE 8.00

Total Mix LE 173.6



• Virgin Mix: LE 207.0/ton

• Recycled Mix: LE 173.6/ton

@20% RAP: Savings = 16.1%


Current Practices

for HMA Recycling

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Surface Mixes: % RAP Allowed

0%

10%

15%

20%

25%

30%

n/a


Base Mixes: % RAP Allowed

0%

10%

15%

20%

25%

30%

n/a


Testing RAP

• General properties:

asphalt content

aggregate gradation

aggregate bulk specific gravity

fine aggregate angularity

fractured face count

flat & elongated percentage

deleterious materials


Current Superpave Guidelines for Selecting the Virgin Binder Grade

• Tier 1: Up to 15% RAP

• use the normal binder grade

• Tier 2: 15%-25% RAP

• use one binder grade lower, e.g., PG

58-28 instead of PG 64-22

• Tier 3: Over 25% RAP

• use blending charts to determine

required binder grade

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Warm Mix Asphalt


Q. Which project is which?

A: Hot-Mix Asphalt (HMA)?

B: Warm Mix Asphalt (WMA)?

Project No. 1 29

Project No. 2


What is WMA?

• Process or additive that allows for the production and compaction of asphalt pavements at temperatures lower than traditional HMA

• Reduced mixing temperature

can range between 135 - 85°C

• Different mixes and technologies dictate mixing temperature


Three General Categories for

WMA

• Foaming • Example, double Barrel Green

• Organic Additive

• Example, Sasobit

• Chemical Additive

• Example, Evotherm

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Mansoura University - Faculty of Engineering – Public Works Engineering Dept. Mansoura University - Faculty of Engineering – Public Works Engineering Dept.

Benefits of WMA

• Energy savings

• Reduced emissions

• Extended paving season

• Increased haul distance

• Less oxidized binder • Less brittle, may be more resistant to cracking

• Softer binder may be better for high RAP content mixes


Similarities With HMA

• Same plants, just modified in many

cases

• Same hauling and laydown equipment

• Same compaction equipment


Concerns About WMA

• Moisture susceptibility

• Rutting

• Coating of aggregate

• Drying aggregate

• Use with RAP and RAS binder

• Compatibility with polymer modified binder

• Expense

• One more material/process to control

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How Are The Concerns Being

Addressed?

• Moisture susceptibility

• Anti-stripping agents

• Monitoring of pavement performance

• Checking moisture content of mix

• To date no issues with moisture damage in the

field

• Rutting

• Binder is less oxidized but lab and early field

results indicate it is not an issue

• To date no issues with rutting in the field


How Are The Concerns Being

Addressed?

• Mixing with RAP and RAS binders

• Field and laboratory studies underway

• Evaluating blending of virgin and RAP binder for HMA and WMA

• Compatibility with polymer modified binder

• Test sites constructed

• May require higher temperature than unmodified mixes

• So far no observed problems

• Expense

• Offset by energy savings

• Seems feasible to use higher RAP


Stone Matrix Asphalt

(SMA)


SMA

• Stone mastic asphalt (SMA), also

called stone-matrix asphalt, was developed

in Germany in the 1960s.

• It is:

• Deformation resistant,

• Durable surfacing material,

• suitable for heavily trafficked roads.

• Good skid resistance

• Noise reduction

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SMA and Conventional AC


Introduction • SMA Has Been Used >30 Years in Europe

• First Project in U.S. ~ 1991 in Wisconsin


Results of the 1996 Survey study:

• Over 70% with rutting < 2 mm

• 25% with no measurable rutting

• No thermal or reflective cracking

• No evidence of raveling

SMA Performance in USA


How is SMA Different?

• Gradation

• Asphalt Content

• Dust Content

• Stabilizing Additives

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Asphalt Contents


Dust Content

Superpave

2 to 10 percent

SMA

8 to 12 percent

Mansoura University - Faculty of Engineering – Public Works Engineering Dept. SMA & OGFC

Increased Film Thickness

SMA has 25% thicker film coating than conventional dense-graded mix.


Additives

• Polymers

• Mineral Fillers

• Fibers

• Hydrated Lime

• Liquid Anti-Strip

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Stabilizing Additives

– Cellulose:

0.3% to 0.5%

– Mineral fiber:

0.4 to 0.5%


SMA Mixtures

• Consists of two parts

– Coarse aggregate

matrix

– Binder rich mortar

Typical SMA:

• 70−80% coarse aggregate,

8−12% filler,

• 6.0−7.0% binder,

• and 0.3 per cent fiber.


SMA Mixtures

• Mix properties

– 50 blow Marshall or

100 gyrations SGC

– VTM - 4%

– VMA - 17% Min.


Disadvantages

• Increased material cost associated with

higher binder and filler contents, and fiber additive.

• Increased mixing time and time taken to add extra filler,

may result in reduced productivity.

• Possible delays in opening to traffic as the SMA mix

should be cooled to 40°C to prevent bleeding.

• Initial skid resistance may be low until the thick binder

film is worn off the top of the surface by traffic

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Open-Graded Friction Courses


History of OGFC

• First use in 1944

• FHWA design procedure in 1974

• Stripping of underlying layer

• Quick failure mode (raveling)

• Needed improvements


Mix Design Method

• FHWA procedure – 1974

• Modified Marshall design


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Estimated Average Service Life

Based on 1998 Survey

Per

cen

t of

Sta

tes

Mansoura University - Faculty of Engineering – Public Works Engineering Dept. SM & OGFC

Use of OGFC

• 38% still use

• 38% discontinued use

• 8% do not use

• 16% did not respond

Based on 1998 Survey


Advantages

• Provides water drainage

–Reduce hydroplaning

–Improve friction

• Improves visibility

–Reduce splash/spray

–Improve visibility of traffic stripes

–Reduce headlight glare

• Reduces noise

• Mitigate Urban Heat Island Effect


Benefits of OGFC

Improved

Nighttime Visibility

& Friction

Reduced

Splash/Spray

and Hydroplaning

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Texas Dense-Graded Mix


OGFC in France


Accident Data: Travis County,

Texas

Year 2001 2002 2003 2004 %

Change

Total # Accidents 29 51 44 17 -58.9

Dry Weather Accidents 10 23 13 15 -2.2

Wet Weather Accidents 19 28 21 2 -91.2

Fatalities 0 1 5 0 -100

Total Injuries 25 16 21 0 -100

Annual Rainfall (in) 42.9 36.0 21.4 52.0 55.5

Total Rain Days 57 56 37 70 40.0


Design Air Voids

at 18 – 22%

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Variation of Road Noise

to Surface Type

Cement Concrete

Asphalt Concrete

OGFC/Porous

72-79

76-85

67-77

72-79

68-73

69-76

65 70 75 80 85

Internal Noise

External Noise



Noise Reduction - OGFC

Effect of -3dB(A)

Equivalent to 20% Speed Reduction

60 48

Equivalent to 1/2 the Intensity


Recommended Design Procedure

• Step 1 ~ Materials selection

• Step 2 ~ Selection of design gradation

• Step 3 ~ Determine opt. binder content

• Step 4 ~ Evaluate moisture susceptibility