Extending the Performance Life of - Rocky Mountain Asphalt … · 2021. 1. 28. · fatigue life, resistance to deformation, strength and moisture damage.” –C. S. Hughes, NCHRP

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Extending the Performance Life of Milling and Overlay

Dave Johnson, Asphalt Institute

Manny Herrera, CDOT

•Project Selection•Overlay Design•Milling for Performance

• The Importance of Tack Coats

•Density Goals

Extending the Performance Life of Milling and Overlay Outline



•Density Goals


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Select a pavement that is structurally sound!

Be realistic! Pavement Distortions

Generic Deterioration Model

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0 5 10 15 20 25 30 35

Pre

sen

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Years

40% drop in quality

75% of life

40% drop in quality

12% of life

$1.00 for Renovation here

Will cost $5.00 - 10.00 here

Pavement deterioration withoutmaintenace or rehabilitation

Pavement deterioration withmaintenace or rehabilitation

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•Density Goals


Designing a winning pavement

•Design the mix for long life

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Balance the Mix Design

Strength/Stability

Rut Resistance

Shoving

Flushing Resistant

Durability

Crack Resistance

Raveling

Permeability

Smooth Quiet RideSkid Resistance

DON’T ATTACK ONE HALF AT THE EXPENSE OF THE OTHER HALF!!

Balanced Mix Design Basic Concept

Volumetric Criteria

Rutting Criteria

Cracking Criteria

Remember Superpave Levels 2 & 3?

Target area of balanced performance

Lift Thickness’ Effect on Compaction

• Aggregates need room to densify

• Too thin vs. NMAS leads to:◦ Roller bridging◦ Aggregate lockup◦ Aggregate breakage◦ Compaction Difficulties

• NCHRP Report 531 (2004)◦ Fine Graded Mix—Minimum Thickness = 3 X NMAS◦ Coarse Graded Mix—Minimum Thickness = 4 X NMAS◦ SMA Mix—Minimum Thickness = 4 X NMAS

Minimum NOT MAXIMUM!

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NMAS definition review

Nominal Maximum Aggregate SizeOne sieve size larger than the first sieve to retain (cumulative) more than 10%

Mix Types

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Mixture Types• Dense Graded ‐ a well‐graded, even distribution of aggregate particles from coarse to fine.

• SMA ‐ gap‐graded with a stable stone‐on‐stone skeleton w/ AC, filler, and stabilizing agents.

• Open Graded Friction Courses (OGFC) – Surface mix designed to be permeable to water.

Pa = 4%

Pa = 15‐20%

Pa = 3‐4%

Surface Mix Type Characteristics

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• Surface mixes use the highest quality materials• Aggregate and binder

• Dense graded and SMA mixes are designed to have low permeability• 4 % design air voids• Use finer (smaller aggregate) mix types

• OGFC or PFC mixes are designed to be permeable• 15% + design air voids• Heavy tack coat required• Must be day‐lighted for drainage• Not below the C & G elevation• No sealers at joints

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Aggregate Size vs. Strength

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Mixture Facts:

• Larger Aggregate Size ≠ Increased Strength• Higher coarse aggregate = lower asphalt content• Finer aggregate mixtures are less permeable,

more compactible and more durable

Mixture Strength is determined by:• Aggregate shape, strength and texture• Proper binder type and quantity• Field placement and compaction

•Binder◦ Hi‐Mod• Highly polymer‐modified binder

• Notably higher initial costs• Excellent long‐term performance◦ Rutting

◦ Reflection cracking

◦ Studded snow tires

•Binder◦ Fiber‐modified Mixtures• Various fiber types• Requires good and uniform dispersion

• Higher initial costs• Excellent performance has been seen◦ Rutting

◦ Reflection cracking

Alternative Materials to Consider



•Density Goals


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Cold Milling

MS‐22Section 7.3.2.3

Cold Milling

•Mill below depth of distress (rutting, surface‐initiated cracking)

•Don’t leave “scabs” of asphalt◦ Avoid milling to within ½ inch of interface w/granular base

•Consider properties of existing asphalt before milling◦ Increasing value of RAP obtained

Scabs

Advantages of Cold Milling

• Efficiently removes deteriorated pavement.

•Provides opportunity to improve smoothness.

•Provides RAP for recycling operations.•Provides a highly skid resistant surface.

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Cold Milling

• Surface texture produced by milling is a function of:◦ Carbide bit spacing and condition◦ Depth of cut◦ Rotational speed of head◦ Speed of travel

Cold Milling

•How will grade be controlled?◦ Ski, string line, laser, other

•Will ride quality be measured on the milled surface?◦ Profilograph, profiler, none

Surface After Milling

Beautifully milled surface. Note the consistent chevron pattern that matches the pattern of the teeth on the milling drum.

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Profile Milling

• Profile milling ‐ the depth of the milling will vary and is controlled by grade control

• How will grade be controlled?◦ Ski, string line, laser, other

•Will ride quality be measured on the milled surface?◦ Profilograph, profiler, none

Cleaning the Surface

Cleaning the Surface

•Goals:◦ Free of debris◦Minimal or no dust◦ Dry surface

•Ready for tack coat

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•Density Goals


Discuss the Importanceof

Tack Coats

Tack Coat

Apply past full‐width of mat to minimize movement of unsupported edge

Tack not wide enough

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Highly inconsistent tack application.

Nicely tacked.

Nicely tacked.

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Why do we use Tack Coat?

Importance of Tack Coat

• To promote the bond between old and new pavement layers

• Vital for structural performance

• A 30% loss in bonding can result in the loss of up to 70% of the expected pavement service life

• To prevent slippage between pavement layers

• To provide an additional moisture barrier, especially when applied along the transverse and longitudinal vertical surfaces MS‐22

Section 7.4

Unbonded

0 50 100 150 200‐50‐100‐150‐200

0 50 100 150 200‐50‐100‐150‐200

BondedHORIZONTAL MICROSTRAIN

compression

compression

tension

tension

HORIZONTAL MICROSTRAIN

Consequences of Debonding

Courtesy of NCAT

Tack Coat Summary

• Tack Coat plays a significant role in the compaction process.

• Tack coat creates the bond between asphalt layers.• The bond “confines” the asphalt layer and holds it in place while it is being compacted.

•A poorly bonded pavement will fatigue significantly faster.

• It is good practice to place the tack coat should be 3‐6 inches wider than the lane being placed when there is an unsupported edge.

• Tack coat is vital for performance but low in cost.

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•Density Goals


Importance of Compaction

“Compaction is the single most important factor that affects pavement performance in terms of durability, fatigue life, resistance to deformation, strength and moisture damage.” – C. S. Hughes, NCHRP Synthesis 152, Compaction of Asphalt Pavement, (1989)

“The amount of air voids in an asphalt mixture is probably the single most important factor that affects performance throughout the life of an asphalt pavement. The voids are primarily controlled by asphalt content, compactive effort during construction, and additional compaction under traffic.” – E. R. Brown, NCAT Report No. 90‐03, Density of Asphalt Concrete—How Much is Needed? (1990)

Reasons for CompactionReasons for Compaction

• To minimize prevent further consolidation

• To provide shear strength and resistance to rutting

• To improve fatigue cracking resistance

• To improve thermal cracking resistance

• To ensure the mixture is waterproof (impermeable)

• To minimize oxidation of the asphalt binder

Compaction also provides a smooth, quiet driving surface

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All are elements of durability

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Research on Critical Air Void Level for Impermeability

“…to ensure that permeability is not a problem, the in‐place air voids should be between 6 and 7 percent or lower. This appears to be true for a wide range of mixtures regardless of NMAS and grading.” – NCHRP 531

Improved Compaction = Improved Performance

A BAD mix with GOOD density out‐performed a GOOD mix with POOR density

for ride and rutting.

WesTrack Experiment

“A 1% decrease in air voids was estimated to improve the fatigue performance of asphalt pavements between 8.2 and 43.8%, to improve the rutting resistance by 7.3 to 66.3%, and to extend the service life by conservatively 10%.”

NCAT Report 16‐02 (2016)

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Effect of Temperature on Compaction

Temperature Control is Critical

Mat Temperature

•Compacting asphalt in the correct temperature range is very important

• Temperatures must be neither too hot nor too cold

•Optimum compaction temperatures vary depending on many factors◦ Start compaction: 310 – 280° F◦ Stop compaction: 180 – 175° F

Environmental Factors and CompactionEnvironmental Factors and Compaction

Several factors come into play regarding how fast the mix cools onsite, affecting time available for compaction:

• Ambient air temperature

• Temperature of the existing surface

• Wind speed

• Lift thickness

• Mix temperature

• Solar Radiation45

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Material Cooling

• Thicker = More Time for Compaction

• Free tools for estimating compaction time◦ PaveCool—single lift (generation 1)• PC• iOs App• Google App

◦MultiCool—multiple lifts (generation 2)• PC • Google App• Mobile Web

http://www.dot.state.mn.us/app/pavecool/

http://www.asphaltpavement.org/multicool

PaveCool ExamplePaveCool Example

•Key Inputs◦ Temperature• Air• Base• Mix Delivery

◦Wind Speed◦ Lift Thickness

•Output◦ Cooling Curve◦ Estimated Compaction Time

2 Inch Lift50°F Air, Surface TempMix Delivery temp ‐ 300°F28 minutes to complete compaction operations

2.5 Inch Lift50°F Air, Surface TempMix Delivery temp ‐ 300°F39 minutes to complete compaction operations

PaveCool ExamplePaveCool Example

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Temperature and Weather LimitationsTemperature and Weather Limitations

260‐220° F

Up to 200 ft

300‐260° F

Up to 200 ft.

180‐150° F

150 ft. & more

Breakdown

Approx. delivered temp: 310‐275° F

Distance

FinishIntermediate

• Typically, the roller train consists of three phases of rolling

• Increase density in breakdown and intermediate phases

• Smooth pavement and remove roller marks with finish rolling

• The goal is to adapt to the asphalt mix type, jobsite conditions in order to achieve or exceed required density and smoothness in fewest number of passes

Typical Rolling Train

Roller Train?

Approx. 92‐94%

Up to 200 ft.

Approx. 90‐92%

Up to 200 ft.

Approx. 92‐94%

150 ft. & more

Breakdown

Approx. density behind paver: 83%

Distance

FinishIntermediate

• Initial compaction with the paver vibratory screed (≥ 80% TMD)

• Breakdown rolling right behind paver on hot mat; rapid density increase

• Intermediate rolling close behind breakdown; obtain specification density

• Finish rolling many be well behind the other roller(s) on warm mat to remove roller marks; not intended to increase density

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Typical Compaction Sequence

• This drone photo shows the paver and the breakdown roller

• Breakdown roller is a double drum vibratory roller

• Roller operates in a roller zone that is in a specific temperature range

• Breakdown roller is applying a target number of passes to the mat by moving forward and then reversing directions

• One pass is defined as the movement of the roller over an area of the mat in one direction

• Notice that when the roller reverses that the operator stops at an angle.

Paving Goals

Continuous Operations• Hot plant running nonstop• Paver running at constant speed nonstop

Production = Hauling = Paver Processing = Compaction Speed

Specify Increased Compaction—Asphalt Institute Recommends

• Shoot for 94% TMD◦ Regularly achieved throughout the country.

• Use Percent Within Limit specifications◦ A 92% LSL demands 93 – 94% compaction target

◦ Use a one sided test – LSL only

◦ Consider high side outlier testing

• Assure Density is achieved on the road◦ Consider Cores for acceptance

◦ Require adequate gauge calibration

◦ Regularly determine Gmm on plant produced mix

• Pay for increased compaction – 5% Bonus

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Proper Overlap:

• 1.0 + 0.5 inches

• Exception:Milled or sawed joint should be 0.5 inches

Don’t starve the joint!

Do NOT Rake Across the Joint

Lute the Longitudinal Joint

This lute person isdoing a great job

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How does greater durability affects LCCA?

• First Cost◦ More attention to density likely to increase first cost slightly

•Maintenance Costs◦ Higher density should reduce maintenance ◦ Higher density should extend maintenance periods • Example: longer time to first overlay

• Rehabilitation◦ Higher Density should extend or eliminate rehabilitation cycles

Questions?

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Documents

Extending the Performance Life of - Rocky Mountain Asphalt … · 2021. 1. 28. · fatigue life, resistance to deformation, strength and moisture damage.” –C. S. Hughes, NCHRP