Hot Mix Asphalt Production and Placement Gary L. Fitts, P.E. Sr. Field Engineer Asphalt Institute Texas A&M University, CVEN 342 Tuesday, July 14, 2015

ASPHALT INSTITUTE A SPHALT I NSTITUTE 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 – – Texas Asphalt Pavement Association – – National Asphalt Pavement Association – – Asphalt Pavement Alliance –

ASPHALT INSTITUTE HMA Plant Functions Aggregate and asphalt storage Aggregate drying Dust collection, air pollution control Aggregate and asphalt proportioning Mixing Mixture discharge/storage

ASPHALT INSTITUTE Topics Basic information on: –HMA Production –HMA Placement –HMA Compaction Considerations for developing HMA specifications

ASPHALT INSTITUTE Batch Plant

ASPHALT INSTITUTE 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

ASPHALT INSTITUTE Batch Plant LayoutDryer BatchTower DustCollector Asphalt Cold Feed Bins Asphalt Trucking, Inc ColdElevator Hot Elevator Storage Silo (optional)

ASPHALT INSTITUTE 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

ASPHALT INSTITUTE Drum Plant Layout

ASPHALT INSTITUTE Multipav, S.A., El Salvador

ASPHALT INSTITUTE Drum Plant-Knippa, Texas

ASPHALT INSTITUTE Drum Plant Configurations Parallel flow Counter flow “Coater” (continuous dryer/pugmill) Drum in a drum –double –triple

ASPHALT INSTITUTE Weigh Bridge

ASPHALT INSTITUTE 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

ASPHALT INSTITUTE Surface Preparation Prime Coats Applied to unbound surface Moderate, uniform application ( gal/sy) of low viscosity liquid asphalt Example materials –MC-30 –MC-70 –AEP

ASPHALT INSTITUTE Surface Preparation Tack Coats Applied to bound surface Light, uniform application of liquid asphalt ( gal/sy residual) Example materials –Asphalt emulsions SS-1h, CSS-1h, MS-2 “Special Tack Emulsions” –Paving grade asphalt PG 64-22

ASPHALT INSTITUTE Tack Coat Application

ASPHALT INSTITUTE

ASPHALT INSTITUTE Paving Equipment Paving Machine Components Tractor unit Screed Electronic grade controls

ASPHALT INSTITUTE 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 (Courtesy of Blaw-Knox)

ASPHALT INSTITUTE Forces Acting on Screed

ASPHALT INSTITUTE 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

ASPHALT INSTITUTE 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

ASPHALT INSTITUTE

A SPHALT I NSTITUTE Compaction The process of compressing a material into a smaller volume while maintaining the same mass.

ASPHALT INSTITUTE 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)

ASPHALT INSTITUTE Factors Affecting Compaction Mixture properties Base/subgrade support (confinement) Ambient conditions Lift thickness

ASPHALT INSTITUTE Asphalt Binder Properties Binder grade –increase high temperature grade  stiffer binder –neat or modified? PG grades with 92C or more temp. difference are usually polymer modified Temperature –must complete compaction while mix temp exceeds: 85C (185F) for neat binders 115C (240F) for modified binders

ASPHALT INSTITUTE 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

ASPHALT INSTITUTE Sieve Size, mm, raised to 0.45 power Percent Passing control point restricted zone zone max density line maxsizenommaxsize Aggregate Gradation

ASPHALT INSTITUTE Sieve Size, mm, raised to 0.45 power Percent Passing maxsizenommaxsize Design Aggregate Structure Fine graded Coarse graded

Time avail. for Compaction, min Compacted Thickness, in 30F60F 90F ~ 6 min Mix Temp. = 275F

ASPHALT INSTITUTE Rolling Phases Breakdown Intermediate Finish Different equipment and different techniques for each phase

ASPHALT INSTITUTE 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

ASPHALT INSTITUTE Vibratory Rollers Commonly used for initial (breakdown) rolling tons, in wide (“heavy” rollers) – lbs/linear inch (PLI) Frequency: impacts/min. Amplitude: in. –For thin overlays (≤ 2 in.) use low amplitude or static mode Operate to attain at least 10 impacts/ft –2-4 mph

ASPHALT INSTITUTE Amplitude & Frequency Amplitude Time between blows, t Frequency = 1/t

ASPHALT INSTITUTE Roller Eccentrics Low amplitude High amplitude Example from Dynapac CC 501

ASPHALT INSTITUTE 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?

ASPHALT INSTITUTE Vibratory Frequency Frequency is drum impacts per minute Working speed must match frequency Best results when impact spacing is per foot

ASPHALT INSTITUTE Frequency Impacts/ft = 1/Impact Spacing

ASPHALT INSTITUTE Impacts per Foot of Travel Vibratory Rollers VPM2 mph2.5 mph3 mph3.5 mph4 mph Reed Tachometer-used to check frequency of vibratory rollers

ASPHALT INSTITUTE Static Steel-Wheeled Rollers 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

ASPHALT INSTITUTE Static Steel-Wheeled Rollers

ASPHALT INSTITUTE 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

ASPHALT INSTITUTE 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!!!

ASPHALT INSTITUTE 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 F –Roller speed ~ 3 mph 2-4 passes, static rolling –Mix temperature F –As necessary to remove blemishes

ASPHALT INSTITUTE 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

ASPHALT INSTITUTE 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

ASPHALT INSTITUTE 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

ASPHALT INSTITUTE 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

ASPHALT INSTITUTE 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

ASPHALT INSTITUTE Comparison of Different Types of Specifications Relative Risk TypeOwnerCntrctrComment 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

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