Update on Balanced Mix Design - Asphalt; WV - Understanding BMD.pdf · Cell Key Mix Factors...

transcript

Update on Balanced Mix Design

Randy West, Ph.D., P.E.Director, National Center for Asphalt Technology

Balanced Mix Design – a definition

An asphalt mix design that uses practical performance tests on appropriately conditioned specimens to ensure resistance to common distresses and considers mix aging, traffic, climate and location within the pavement structure.

Why change?

Most asphalt technologists are not satisfied with the current long term performance of our pavements. There is a desire to significantly improve the life of asphalt pavements.

Why change?

• Volumetric properties do not tell us anything about the quality of the binder, or about the interactions of different binder components and additives.

• Vbe = the volume of effective asphalt = VMA – Va

• Vbe is dependent on Gsb which is not a reliable property• Gsb of source materials are subject to change over time, but not often verified.

• Gsb has a low level of precision

• Gsb of RAP aggregate is questionable

With the current volumetric mix design system…

WMA additives

Recycled Shingles

Fractionated RAP

Recycled Tire RubberRecycled Plastic

Recycling agents

BMD Optimum Asphalt Content

Binder Content

Min Max

BMD Optimum Asphalt Content

BMD Performance Diagram

Numerous options to adjust mixes

Gradation Asphalt Content

Modifiers

RAP Content

RAS Content

Rejuvenator

BMD Implementation Status

California

⚫ Bending Beam Fatigue

⚫ Superpave Shear Test

Illinois

⚫ Illinois Flexibility Index

⚫ Hamburg Wheel Tracker New Jersey

⚫ Overlay Test

⚫ Hamburg Wheel Tracker

⚫ Overlay Tester

Louisiana

⚫ Semi-Circular Bend Test

BMD Pilot Projects

2019 BMD Workshops

Sacramento, CA

⚫ October 1, 2019

Pewaukee, WI

⚫ December 5-6, 2019 King of Prussia, PA

⚫ July 16-17, 2019

Taylorsville, UT

⚫ Sept. 26, 2019

Atlanta, GA

⚫ June 26, 2019

Springfield, MO

⚫ August 21, 2019

Culpepper, VA

⚫ Oct. 29, 2019

Completed BMD Research Studies

Completed & OngoingBMD Research Studies

The BIG questions

1. What performance tests will be used in BMD for each state?

2. How will the performance tests be used? Where will they fit in the mix design process (i.e. the Framework)?

3. What criteria should be used in specifications?

4. What aging/conditioning protocols should be used for mixtures?

5. How will the performance tests be used in Quality Assurance?

6. What should you do to get ready for BMD?

Cracking Group Experiments

NCAT Test Track

Top-down cracking

MnROAD

Low-temperature cracking

NCAT Test Track

America’s asphalt pavement proving ground

NCAT Cracking Group Sponsors

Selected Top Down Cracking Tests

SCB-LAEnergy Ratio OT-NCATIFIT OT-TX

All tests have been conducted on:1. lab prepared mix after short-term aging2. lab prepared mix after short-term and critical aging3. plant mix samples that were reheated 4. plan mix samples that were reheated and critically aged

IDEAL-CT

critical aging for Auburn, AL = loose mix oven aging at 135C for 8 hours

Section N8May 21, 2019

NCAT CG Field Performance

Section Description

Cracking(% of lane area)

Crit. AgedCT Index*

Start of this Cycle 12/6/19

N1 20% RAP (Control) 10.3 10.6 8.1

N2 Control w/ High Density 6.9 7.5 10.8

N5 Low AC, Low Density 3.5 9.3 7.6

N8 20% RAP 5% RAS 16.6 70 2.4

S5 35% RAP PG 58-28 0 0 16.3

S6 Control w HiMA 0 0 18.7

S13 AZ Rubber Mix 0 0 68.4

* Specimens compacted to 7.0±0.5% air voids

SCB-LA IFIT

Unable to identify worst and best performing mixes

IDEAL-CTEnergy Ratio OT-NCATOT-TX

SCB-LAEnergy Ratio OT-NCATIFIT OT-TX

Not practical enough for routine mix design

IDEAL-CT

IFIT IDEAL-CT

MnROAD Cracking Group Test Sections

Test sections constructed August 2016

MnROAD Cracking Group Sponsors

MnROAD Cracking Group

Low temperatures January 30, 2019

MnROAD Cracking Group Field Performance through April 2019

Cell Key Mix Factors

Transverse Cracking

Load Related Cracking

(% of lane area)16 Moderate RAP + RAS 58 1.5

17 Low RAP + RAS 70 6.3

18 Moderate RAP 35 3.8

19 Moderate RAP, extra AC 61 0.4

20 High RAP, softer binder 0 0.2

21 Moderate RAP, softer binder 28 1.1

22 Limestone agg. and 9.5 mm NMAS 50 4.4

23 Moderate RAP, highly mod. binder 43 14.9

MnROAD Cracking Group Tests

IFIT Low Temp. SCBOT-NCAT IDT Creep Compliance & Strength

Cantabro IDEAL-CT DCT

Intermediate Temperature Tests Low Temperature Tests

other tests are being performed by other research organizations

Ongoing Decisions & Work Ahead

• Selection of Tests

• Ruggedness and Precision studies

• Benchmarking current mixes

• Setting criteria

• Training

• Pilot Projects

Questions

Thank You

Randy Westrandy.west@auburn.edu

UnderstandingBond Strength

Randy West, Ph.D., P.E.Director, National Center for Asphalt Technology

1. What is the function of tack coat?2. What can happen when tack coat is not properly applied?3. How can you tell if your tack coat is sufficient?

The purpose of a tack coat is…

• to bond layers of a pavement together.

Slippage Failures

Forensic Investigations

Orlando Airport

US 45 Michigan

City of Auburn

I-65 Alabama

I-10 Florida

I-94 Minnesota

I-70 Colorado

Forensic Investigations - Cores

Orlando Airport

US 45 Michigan

City of Auburn

I-65 Alabama

I-10 Florida

I-94 Minnesota

I-70 Colorado

NCAT Test Track Section S5, 2007

Evolution of Cracking in Section S5

1. Weak bond between binder and base layers

2. Debonding between binder and base layers

3. “Middle-up” crack initiated at bottom of binder

4. “Middle-up” crack reaches surface of pavement

5. Full depth crack extends to bottom of base

1 2 3 4 5

COMPRESSION

TENSION

Bonded

COMPRESSION

TENSION

Unbonded

• 5 plywood strips (layers)

• 48” x 4” x 11/32”

• 60 and 160 pound loadings

• Bonded (glued) and Unbonded

Bonding Demonstration(courtesy of FHWA/AI Tack Workshop)

½” Deflection,

60lb Load

¼” Deflection,

160lb Load

Fully BondedUnbonded

Bonding Demonstration(courtesy of FHWA/AI Tack Workshop)

NCAT First Tack Coat Study

• What is the best type of tack coat material?

• What is the best application rate?

Literature on Bond Strength Testing

Juris-diction

Switzer-land

Germany AustriaUnited

KingdomQuébec Florida

Standard SN 671 961 DIN 1999ÖNORM 3639-1

SG3/05/234

unknown FM-599

Type Shear Shear Tensile TorqueTensile (in-situ)

Criteria 12 to 15 kN unknown1.0 to 1.5 N/mm²

N.A. N.A. N.A.

Bond Strength Experiment Design

Tack Coat Type:

CRS-2, CSS-1, PG 64-22

Application Rate:

0.02, 0.05, & 0.08 gal/yd2

Gradation:

19.0 mm Coarse & 4.75 mm Fine

Test Temperature:

50, 77, & 140°F

Normal Load:

0, 10, and 20 psi

Sample Preparation and Testing

• SGC specimens cut in half for specimen base

• Tack coat was applied to uncut surface of base

• Tacked specimen base was put back in SGC mold, loose mix compacted on top

• Bond strength test determined after four hours at the test temperature

• A servo-hydraulic testing machine loaded at 2”/min, maintained the temperature, and captured load and strain data.

Bond Strengths @ 140°F

Low Medium High Low Medium High Low Medium High

0 psi 10 psi 20 psi

Application Rates and Normal Pressure

Fine-graded,

PG 64-22

PG64-22

0 psi 10 psi 20 psi

Coarse-graded,

PG 64-22

Coarse-Graded Mixes Fine-Graded Mixes

PG64-22

0 psi 10 psi 20 psi

Fine-graded,

PG 64-22

0 psi 10 psi 20 psi

Coarse-graded,

PG 64-22

Bond Strengths @ 77°F

PG64-22

Coarse-Graded Mixes Fine-Graded Mixes

PG64-22

Effects of Temperature & Normal Pressure

0 psi 10 psi 20 psi 0 psi 10 psi 20 psi 0 psi 10 psi 20 psi

50°F 77°F 140°F

Test Condition

NCAT (ALDOT) Bond Strength Test

77°F, no normal pressure Sensitive to key variables including tack coat

type, application rates, surface textures

Ability to evaluate bond strength of pavement cores

Low cost, quick simple

For most conditions, the PG 64-22 provided better bond strengths than the CSS-1 and CRS-2

Lower application rates yielded higher bond strengths for all tack coat types

Field Validation Work

• Test sections set up on seven field projects

• Each section had a different tack coat application rate

• Cores taken after HMA compacted and cooled.

Measurement of Application Rate, ASTM D 2995

~100 m Geotextile pads for checking tack rate

Low Rate Medium Rate High Rate

Test Section Layout

Application rates measured using ASTM D 2995

~100 m Geotextile pads for checking tack rate

Test Section Layout

Application rates measured using ASTM D 2995

Test Section Layout

HMA overlay

3 random cores + 2 at pads per section

Test Section Layout

Recommendations

• The bond strength test is capable of measuring effects of tack coat type, application rate and surface texture.

• 100 psi proposed as a preliminary criterion

• Tack coat application rates should be specified in terms of residual asphalt.

• Milled/micro-milled surfaces have better bond than unmilledsurfaces

• Bond develops quicker for PG 67-22 and NTSS-1HM

The GoodThe Bad

The Ugly

This is what happens to tack picked up on truck tires.

NJDEMD

Do not perform bond strength testing

Do perform bond strength testing

States that Perform Interface Bond Strength Testing

2018 NCHRP Synthesis 916

WVDOH Bond Strength Summaries

Year Lots No Bond <100 psiAvg. BS

Std. Dev. of BS (psi)

2013 146 40 (27%) 81 (55%) 113.1 37.6

2014 216 75 (35%) 97 (45%) 152.6 48.0

2015 439 86 (20%) 201 (46%) 117.0 37.7

2016 346 59 (17%) 116 (34%) 150.0 63.3

2017 667 130 (19%) 238 (36%) 103.6 70.6

2018 401 122 (30%) 184 (46%) 115.2 98.8

Factors that Affect Bond Strength

• Milled vs Unmilled: milled typically much higher

• Cleanliness of Surface: cleanliness is next to godliness

• Tack Coat Type: emulsions, trackless, PG binder

• Tack Coat Quality: how & where is it checked?

• Tack Coat Appl. Rate: check residual rate, how much stays?

• Overlay gradation: coarse vs. fine?, literature is inconsistent

• Time and Traffic: bond strength typically improves

Key Takeaways

• A good bond between asphalt layers is critical to long-term performance of flexible pavements.

• Measuring bond strength is the only way to ensure a good bond exists.

• WV bond strength data clearly show problems exist.

• Numerous factors can impact bond strength.

Further Analysis Recommended

• How are those pavements with failing bond strengths performing?

• Are bond strengths of wheelpath cores ≤ non wheelpath cores?

• Evaluate tack coat materials and application practices.

• Examine practices for coring.

Thank You

Randy Westrandy.west@auburn.edu

NCAT Reports

Available Reference Materials

Gierhart, D., and Johnson, D. NCHRP Synthesis 516:

Tack Coat Specifications, Materials and Construction Practices. National Cooperative Highway Research Program,

Federal Highway Administration. Tack Coat Best Practices – TechBrief.

Office of Asset Management, Pavements, and Construction, 2016.

Decker, Dale S. Best Practices for Emulsion Tack Coats. Quality Improvement Publication 128,

National Asphalt Pavement Association, 2013

Mohammad, L. N., Elseifi, M. A., Bae, A., Patel, N., Button, J., and Scherocman, J. A.

NCHRP Report 712 Optimization of Tack Coat for HMA Placement.

National Cooperative Highway Research Program, 2012.

Recommended Application Rates

Storing Asphalt Emulsions

• Do not heat the emulsion above 185°F to avoid damaging the product.

• Do not let the emulsion freeze to avoid separation of the asphalt and water.

• Do not use forced air to agitate the emulsion, as it will break the emulsion.

• Do not allow excessive agitation by mixing or pumping.

Handling Asphalt Emulsions

• Agitate gently when heating emulsion to minimize skin formation in the tank

• Clean out lines and leave drain plugs open when not in service.

• Ensure that tanks and equipment contain accurate thermometers. Gradually warm up the pump to about 150°F to facilitate start up.

• Dilution of the asphalt emulsion should only occur at the emulsion manufacturing facility. Dilution should not be done by the contractor in the distributor tank

• Polymer-modified and non-tracking emulsions should not be diluted for use as tack coat

Effect of water (rain or damp surface)?

“For the effect of water on the tacked surface, the majority of the cases showed no statistically significant difference between dry and wet conditions. This data indicates that a small amount of water can be flashed away by the hot HMA mat and, thus, have inconsequential effects on the quality of the tack coat. This study used only hot mix as the overlay material; the use of warm mix may change this finding. In addition, these results are based on using a small quantity of water to simulate rainy conditions. Therefore, a dry and clean surface is recommended to avoid the negative effects of water on the bonding at the interface.”

NCHRP Project 712

Effect of water (rain or damp surface)?

NAPA QIP-128

“A small amount of moisture on the pavement surface should not be detrimental to long-term tack coat performance, although a damp pavement will slow the cure and break time of the tack coat emulsion. If the pavement surface layer is saturated with water and the existing pavement surface is damp or has standing water, the ability of the tack coat emulsion to provide adequate bond between the existing and the new pavement layers will be significantly compromised”.

Update on Balanced Mix Design - Asphalt; WV - Understanding BMD.pdf · Cell Key Mix Factors...

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