Hot Mix Asphalt Production and Placement

Hot Mix AsphaltProduction and Placement

Hot Mix AsphaltProduction and Placement

Gary L. Fitts, P.E.Sr. Field EngineerAsphalt Institute

Texas A&M University, CVEN 342Wednesday, April 19, 2023

ASPHALT INSTITUTE

www.asphaltinstitute.org

ASPHALT INSTITUTE

• International association of petroleum asphalt producers, manufacturers, and affiliated businesses, established in 1919

• Promotes the use, benefits and quality performance of petroleum asphalt through engineering, research and educational activities.

• HQ office-Lexington, KY

ASPHALT INSTITUTE


References

• Asphalt Institute– www.asphaltinstitute.org– 859-288-4960

• Texas Asphalt Pavement Association– www.txhotmix.org– 512-312-2099

• National Asphalt Pavement Association– www.hotmix.org– 888-468-6499

• Asphalt Pavement Alliance– www.AsphaltAlliance.com

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HMA Plant Functions

• Aggregate and asphalt storage

• Aggregate drying

• Dust collection, air pollution control

• Aggregate and asphalt proportioning

• Mixing

• Mixture discharge/storage

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Topics

• Basic information on:– HMA Production– HMA Placement– HMA Compaction

• Considerations for developing HMA specifications

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Batch PlantBatch Plant

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Batch Plants-Features

• Aggregates dried, separated by size

• Aggregates recombined by weight in weigh hopper

• Aggregates introduced into pugmill, briefly mixed

• Asphalt introduced by weight, mixed with aggregates

• Completed HMA discharged or stored

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Batch Plant Layout

DryerDryer BatchBatchTowerTower

DustDustCollectorCollector

AsphaltAsphalt

Cold Feed BinsCold Feed Bins

Asphalt Trucking, Inc

ColdColdElevatorElevator Hot ElevatorHot Elevator

Storage SiloStorage Silo(optional)(optional)

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Drum Plants

• Aggregates are dried, mixed with asphalt in a continuous operation

• Quality control entirely dependent on:– stockpile management– plant calibration

• Mixture must be stored in surge bin or silo

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Drum Plant Layout

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Multipav, S.A., El Salvador

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Drum Plant-Knippa, Texas

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Drum Plant Configurations

• Parallel flow

• Counter flow

• “Coater” (continuous dryer/pugmill)

• Drum in a drum– double– triple

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Weigh Bridge

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HMA Production

• Objective is to produce a mixture meeting the specified design requirements– Volumetric– Mechanical

• Samples are normally obtained from trucks at the plant, taken to the laboratory, and tested to confirm these qualities

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Surface Preparation

Prime Coats

• Applied to unbound surface

• Moderate, uniform application (0.15-0.35 gal/sy) of low viscosity liquid asphalt

• Example materials– MC-30– MC-70– AEP

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Surface Preparation

Tack Coats• Applied to bound surface• Light, uniform application of liquid asphalt

(0.03-0.05 gal/sy residual) • Example materials

– Asphalt emulsions• SS-1h, CSS-1h, MS-2• “Special Tack Emulsions”

– Paving grade asphalt• PG 64-22

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Tack Coat Application

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Paving Equipment

Paving Machine Components

• Tractor unit

• Screed

• Electronic grade controls

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HMA Delivery

• Paver pulls up to meet the truck– DON’T BUMP THE PAVER!

• Break the load before opening tailgate

• Charge the hopper before it’s empty

Tractor and Screed Units

MS-22, Fig. 5.11 (Courtesy of Blaw-Knox)

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Forces Acting on Screed

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Automatic Screed Controls

• Electronic adjustment to screed height using sensing and reference system

• Sensor detects elevation changes, adjusts height of tow point

• Slope (transverse) controls

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Paving Operations

• Maintain uniform resistance to face of screed!– Keep uniform head of material at the

face of the screed

– Operate paver within a narrow range of forward speed

• Coordinate mixture delivery, paver speed and compaction operations

Uniform Head of Material

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Compaction

The process of compressing a material into a smaller volume

while maintaining the same mass.

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Compaction

• Essential to good performance!

• Need to compact to desirable air voids level– Fine graded mixtures: 4-8%– Coarse or gap-graded mixtures: 3-6%

• Compaction can only achieved if:– Mixture is confined– Mixture is hot (workable)

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Factors Affecting Compaction

• Mixture properties

• Base/subgrade support (confinement)

• Ambient conditions

• Lift thickness

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Compaction-Lift/Layer Thickness

• Coarse-graded mixtures, mixtures using modified asphalts– Minimum 4X nominal maximum size– ½ in NMS – minimum 2” lift thickness

• Fine-graded mixtures– Minimum 3X nominal maximum size

• Thicker lifts also conserve heat, providing more time to complete compaction

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100

0

Sieve Size, mm, raised to 0.45 power

.075 .3 2.36 12.5 19.0

Pe

rce

nt

Pa

ss

ing

control pointcontrol point

restrictedrestricted zonezone

max density linemax density line

maxmaxsizesize

nomnommaxmaxsizesize

Aggregate Gradation

ASPHALT INSTITUTE


100

0

Sieve Size, mm, raised to 0.45 power

.075 .3 2.36 12.5 19.0

Pe

rce

nt

Pa

ss

ing

maxmaxsizesize

nomnommaxmaxsizesize

Design Aggregate Structure

Fine graded

Coarse graded

30

20

10

0

1 2 3 4

Tim

e av

ail.

for

Com

pact

ion,

min

Compacted Thickness, in

30F60F90F

30

20

10

0

~ 6 min

Mix Temp. = 275F

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Rolling Phases

• Breakdown• Intermediate• Finish• Different equipment and different

techniques for each phase

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Compaction Equipment

• Screed unit– weight of screed– external force applied to

screed– vibratory unit

• 35 Hz (2100 VPM)

– tamper bar

• Rollers– vibratory steel-wheeled– pneumatic– static steel-wheeled– combination

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Vibratory Rollers

• Commonly used for initial (breakdown) rolling

• 8-18.5 tons, 57-84 in wide (“heavy” rollers)– 50-200 lbs/linear inch (PLI)

• Frequency: 2700-4200 impacts/min.• Amplitude: 0.016-0.032 in.

– For thin overlays (≤ 2 in.) use low amplitude or static mode

• Operate to attain at least 10 impacts/ft– 2-4 mph

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Amplitude & Frequency

Amplitude

Time between blows, t

Frequency = 1/t

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Roller Eccentrics

Low amplitude

High amplitude

Example from Dynapac CC 501Example from Dynapac CC 501

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Amplitude vs. Frequency

• High amplitude generates most force

• At same frequency, high amplitude does more work

• Is it logical to use high amplitude with high frequency?

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Vibratory Frequency

• Frequency is drum impacts per minute

• Working speed must match frequency

• Best results when impact spacing is 10-14 per foot

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Frequency

Impacts/ft = 1/Impact Spacing

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Impacts per Foot of TravelVibratory Rollers

VPM 2 mph 2.5 mph 3 mph 3.5 mph 4 mph

2000 11.4 --- --- --- ---

2500 14.2 11.4 --- --- ---

3000 17.0 13.6 11.4 --- ---

3500 19.9 15.9 13.3 11.4 10.0

4000 22.7 18.2 15.2 13.0 11.4

Reed Tachometer-used to check frequency of vibratory rollers

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Static Steel-Wheeled Rollers

• 10-14 ton rollers normally used for HMA compaction– Commonly use vibratory

rollers operated in static mode

• Lighter rollers used for finish rolling

• Drums must be smooth and clean

• For initial compaction, drive wheel must face paver

AI MS-22, Figure 6.05

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Static Steel-Wheeled Rollers

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Pneumatic Tire Manipulation

• Overlap manipulates mat under and between tire

• Tight finish resists moisture penetration

• Manipulation increased by lowering tire pressure

• Static force increased by high tire pressure

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Rolling Pattern

• Speed & lap pattern for each roller

• No. of passes for each roller

• Min. temperature by which each roller must complete pattern

IMPORTANT:Paver speed must not exceed that of

the compaction operation!!!

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Typical Pattern, 2 inch Overlay

• 2-4 passes, tandem vibratory roller– Mix temperature above 250F– Lowest amplitude setting, highest frequency– Vibration on one or both drums– Travel speed to obtain 10 impacts/ft (~3 mph)

• 2-4 passes, pneumatic roller– Mix temperature 180-230F– Roller speed ~ 3 mph

• 2-4 passes, static rolling– Mix temperature 140-180F– As necessary to remove blemishes

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HMA Specifications

• General types of project specifications:– Method– Statistically-based acceptance

• “QC/QA”

– End result– Warranty

• Short-term (workmanship related)• Long-term (design-build)

• No one type is appropriate for all conditions

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Method Specifications

• Contractor is required to follow specific instructions during all phases of construction– Owner provides mixture design– Mixture is accepted on the basis of owner’s (or

contractor’s) quality monitoring tests– Delivery, placement, and compaction accepted based

on inspection by owner/owner’s agent

• Examples of method spec:– TxDOT, Item 340 (all Standards Spec’s through 1982)– 2004 Standard Specifications to include method-type

Item 340

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Statistically-Based QC/QA

• Requires a minimum established level of contractor quality monitoring tests– Contractor usually furnishes the mixture design

• Defines lots and sublots for production and placement, requires stratified random sampling of materials

• Acceptance/payment performed on a lot-to-lot basis– Usually includes penalty/bonus provisions on key materials

qualities– Many agencies applying percent-within-limits (PWL) criteria,

whereby upper and/or lower limits of key criterion are defined• Most commonly applied specification type for highways

and airports• 1993 TxDOT Standard Specifications, all 2004 HMA

items (except 340)• Most common PWL specification: FAA P-401

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Short-Term Warranty

• Acceptance based on observed performance under limited term

• Performance defined through measurement of:– Roughness/ride quality– Cracking (non-wheelpath)– Rutting– Surface friction

• Contractor controls materials selection, mixture design, and all construction requirements

• Term may vary from 1 to 7 years, becoming more popular for use on highway projects– Indiana, Wisconsin, Michigan, other states

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Long-Term Warranties

• Contractor is required to deliver a pavement with performance characteristics at defined levels at the end of an extended period (15-30 years)

• Contractor responsible for pavement structural design and construction

• Most often used outside USA (design/build/operate/concessions projects in Central and South America, Europe)

• Best US example, NM 44 project

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Comparison of Different Types of Specifications

Relative Risk

Type Owner Cntrctr Comment

Method ++ - Requires diligent inspection

QC/QA + + Best applicable to large-scale projects

S-T Warranty

- + Project selection is crucial

L-T Warranty

-- ++ Bonding a concern, full control must be given to

contractor

Thanks for your attention!

Date post:	30-Dec-2015
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Hot Mix Asphalt Production and Placement

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