Tack Coat Performance and Materials Study

Bryan Wilson, Arif Chowdhury, Sheng Hu (TTI)Sang-Soo Kim, Munir Nazzal (OU)

Ala Abbas (Akron)

State Job #135097Final Research Results

July 10, 2017

Presentation Overview• Introduction• Methodology and Results

• Bond Test• Field Work• Lab Testing and Analysis• Performance Prediction• Life-Cycle Cost Analysis

• Findings and Recommendations• Implementation

Introduction

Importance of Bonding• HMA overlay life largely dependent on bond quality

• Critical for thin overlays• Poor bonding can lead to:

Fatigue cracking Slippage cracking Delamination

www.eng.auburn.edu www.dot.state.fl.uswww.pavementinteractive.com

Simplified physics of bonding• Improves load transfer• Decreases pavement deflection• Lowers stress concentrations

High stress

Low stress

Non-bonded layers

Bonded layers

Tack coats are a critical part of achieving a good pavement layer bond.

Recent Technologies• Trackless tack

VS

Recent Technologies• Spray pavers

(www.roadtec.com)

Research Goals• Justify or reject higher-cost tack coat strategies.• Select a performance-based or method-based

specification.

• Develop layer bond test.• Evaluate bond performance and predict long-term

performance of different tack coat strategies.• Assess minimum bond criteria.• Evaluate cost-effectiveness.

Research Objectives

Bond Test

Shear bond strength test• Reasonably represents loading scenario in the field.• Intuitive test method.• Ease of sample prep.• Ease of testing.• Inexpensive.

Field Work

Testing PlanSurface Type

Tack Material

Application Equipment

Tack Application RateTracking

Rank (gal/sy)

Milled Surface (Intermediate

Course)

None NA None 0 NA

Rubberized tack

Spray Paver

Mod. 0.08

NHigh 0.12

V. High 0.16

SS-1h Distributor truck

Low 0.04 NY

Moderate 0.08 N

Y

High 0.12NY

New Surface (Surface Course)

None NA None 0 NA

SS-1h Distributor truck

Low 0.04 N

Mod. 0.07 NY

High 0.10 NY

Rubberized Tack

Spray paver

Mod. 0.07NHigh 0.10

V. High 0.13

Trackless tack Distributor truck

Low 0.04 NY

Mod. 0.07 NY

High 0.10 NY

SS-1h Spray paver

Low 0.04NMod. 0.07

High 0.10

Test Section Variables

Property Values

Surface Type New HMA,  Milled

Tack Type SS‐1H Trackless Rubberized

Application Method Distributor Spray paver

Tack Application Rate, (gal/sy) 0.04 – 0.13Tracking Yes and NoBond age Initial and 8‐months

FRA-270 Location

US 40

I-70

Section Layout

Intermediate Course (Milled surface)

Low Mod.V. High

Rubberized Tack (Spray Paver) SS‐1H (Distributor) 

~700 ft

Highw/tracking

Loww/tracking

Mod. w/tracking

HighMod. High

Livingston Ave.

Bridge

Section LayoutSurface Course (New surface)

Surface Course (continued)

SB Mainline

Rubberized Tack (Spray Paver) SS‐1H (Spray Paver) SS‐1H (Distributor)

LowMod. Loww/ tracking

Mod. w/ tracking

High

~500 ft

Project Start Right lane divides

HighMod. V. High V. High Mod.US 40

SS‐1H (Distributor)                              Trackless Tack (Distributor)

Highw/ tracking

High Low Mod. High Bridge

~500 ft

Loww/ tracking

Highw/ tracking

Mod.w/ tracking

Livingston Ave.

Construction

Construction

Lab Testing and Analysis

Bond Testing

Bond Energy

Bond Strength

130 Samples

Testing Rate: 2-inch/min

Bond Results AnalysisVariable Comparison Sample Size

Testing Agency TTI laboratory.OU laboratory. 40

Tack Type and Application

SS‐1H‐Spray paver.SS‐1H‐Distributor.Trackless‐Distributor.Rubberized‐Spray paver.No tack.

49

Tack Rate Residual rates (0.03 – 0.11 gal/sy). 69

Surface Type New HMA surface.Milled HMA surface. 60

Tracking Undisturbed tack coat.Tracked tack coat. 59

Bond Age Immediately after construction8 months after construction 53

Bond Results

0

50

100

150

200

250

300

350

400

450

Bond

 Stren

gth (psi)

High

bond

Median

Max.

Min.

Low

bond

0

1

2

3

4

5

6

7

Bond

 Ene

rgy (ft‐lb/in2)

High

bond

Median

Max.

Min.

Low

bond

Stats Analysis

VariableBond Strength (psi) Bond Energy (ft‐lb/in2)

Statistical Significancep‐value1

Model R2 

Valuep‐value1

Model R2

ValueTesting Agency 0.345 0.02 0.877 0.00 NoTack Type and Application

<0.001 0.75 <0.001 0.85 Yes

Tack Rate 0.429 0.75 0.124 0.80 NoSurface Type 0.042 0.53 0.821 0.72 NoTracking 0.103 0.48 0.888 0.25 NoBond Age <0.001 0.90 <0.001 0.87 Yes

1 – The reported p‐value is for the variable in question and not the overall model. 

Tack Type and Application Method

361

252226

140109

320286

302

229 208

050

100150200250300350400450

TracklessDistributor

SS‐1HDistributor

None RubberSpray paver

SS‐1HSpray paver

Bond

 Stren

gth (psi)

Tack Type and Application Method

Low

bond

High

bond

95% Confidence

Initial

8‐Months

Tack Type and Application Method

4.0

2.9

1.8

1.10.7

4.4

2.6 2.4

1.91.5

0.00.51.01.52.02.53.03.54.04.55.0

TracklessDistributor

SS‐1HDistributor

None RubberSpray paver

SS‐1HSpray paver

Bond

 Ene

rgy (ft

‐lb/in2)

Tack Type and Application Method

Low

bond

High

bond

95% Confidence

Initial

8‐Months

Testing Agency

2.2 2.1

0.00.51.01.52.02.53.03.54.04.5

TTI OU

Bond

 Ene

rgy (ft‐lb/in2)

Testing Agency

Tack Rate (Application)

3.13.3

1.51.0

2.9

3.8

1.2

0.5

2.5

4.0

0.61.0

0.00.51.01.52.02.53.03.54.04.5

Rubber SS‐1H SS‐1H Trackless

Spray Paver Spray Paver Distributor Distributor

Bond

 Ene

rgy (ft‐lb/in2)

Tack Type_Application

LowMod.HighVery High

Application Rate

Surface Type

1.91 1.88

0.00.51.01.52.02.53.03.54.04.5

New Milled

Bond

 Ene

rgy (ft‐lb/in2)

Application Type

Tracking

3.7

2.9 2.8

4.0

2.6 2.7

0.00.51.01.52.02.53.03.54.04.5

New_Trackless New_SS1h Milled_SS1h

Bond

 Ene

rgy (ft

‐lb/in2)

Surface_Tack Type

No Tracking

Tracking

Life-Cycle Cost Analysis

Overview• Long-Term Performance Prediction

• Model input parameters• Model development• Performance prediction outputs

• Life-Cycle Cost• 15-yrs• Material costs• Routine maintenance cost• Rehabilitation cost

Pavement and Bond Modeling

Bond Energy

Bond Strength

1. Test bond 2. Convert bond to shear modulus

3. Input modulus in BISAR model.

Bond-to-Log(Ks) Transformation

Bond Strength* (psi)

log(Ks) Description/Justification

0 2 No Bond

15 2Lowest bond strength observed. Samples below this could not be cored.

30 2.1From NCAT, lowest bond strength observed near delaminated areas.

35 2.2 Upper range of samples from similar low‐bond locations.

65 2.5From NCAT, highest bond strength observed near delaminated areas.

85 3From NCAT, lowest bond strength observed on projects away from delaminated areas. Consideration given as a minimum bond‐strength criteria.

175 5.5Generally, the maximum observed shear strength from Texas field samples.

250 6Lowest internal HMA shear strength among ODOT's samples

400 6Highest observed bond strength and also highest internal HMA shear strength from ODOT.

*Loading rate = 2‐inch/min

0

1

2

3

4

5

6

7

0 50 100 150 200 250 300 350 400

Shear Ind

ex (log

(mod

ulus))

Bond Strength (psi)

Models

Layer Thickness (mm) Modulus (MPa) Poisson RatioSurface AC 30 5,000 0.35

Intermediate AC 40 7,000 0.35

Existing AC 50 4,000 0.35

Existing PCC 250 30,000 0.15

Subbase 150 240 0.35

Subgrade - 120 0.35

Fatigue and RuttingReflection Cracking

Crack tipBottom of layers under load

Reflective Cracking Prediction

* Assumption that both layers had the same bond condition.

Rutting Prediction

Full Slip

Half Bond

Full Bond

* Assumption that both layers had the same bond condition.

Fatigue Prediction

1

10

100

0 1 2 3 4 5 6 7

Fatig

ue Life (Years)

Shear Index

No Bond

Partial Bond

Full Bond

* Assumption that both layers had the same bond condition.

Performance Prediction by Treatment –New HMA

0102030405060708090100

0 2 4 6 8 10 12 14 16

Refle

ction Cracking

 (%)

Years

Distributor TracklessDistributor SS‐1HDistributor NoneSpraypaver RubberSpraypaver SS‐1H

Performance Prediction by Treatment –Milled HMA

0102030405060708090

100

0 2 4 6 8 10 12 14 16

Refle

ction Cracking

 (%)

Years

Spraypaver, Rubber

Spraypaver, SS‐1H

Distributor TracklessDistributor SS‐1HNone

Paving Equipment Costs

Equipment Manufacturer Cost ($1,000)Standard Paver NA $400 ‐ $450

Spray Paver

Roadtec $875 ‐ $950Vogel $925

Caterpillar (Integral dx) $740 ‐ $800

Spray Paver Retrofit*

Caterpillar (Integral dx) $350

*Available for limited models

Life-Cycle Costs – New HMA

$212 $209 $205 $210 $209

$0

$50

$100

$150

$200

$250

$300

TracklessDistributor

 SS‐1HDistributor

None RubberSpray paver*

SS‐1HSpray paver*

Presen

t Cost

($1000/la

ne‐m

ile)

* Additional  equipment cost

Life-Cycle Costs – Milled HMA

$171 $169 $166 $166 $161

$0

$50

$100

$150

$200

$250

$300

TracklessDistributor

 SS‐1HDistributor

None RubberSpray paver*

SS‐1HSpray paver*

Presen

t Cost

($1000/la

ne‐m

ile)

* Additional  equipment cost

Findings & Recommendations

Key Findings• Most bond measurements were acceptable.• Tack type, application method, and bond age had a

significant impact on bond performance.• High bond: Trackless, SS-1H w/ Distributor, No tack• Mod-low bond: SS-1H and rubberized w/ spray paver (initial)• After 8-months: Bond doubled for spray paver sections

• Testing agency, tack rate, surface type, and tracking did not significantly influence bond performance.

Key Findings• Better bonding reduces reflection cracking and fatigue

cracking. There may be an optimal bond.• Better bonding reduces rutting.• Based on 8-month data, all test sections had near “Full

Bond” conditions and similar long-term life.• Life-cycle costs

• New HMA: ~$210,000/lane-mile.• Milled HMA: ~$165,000/lane-mile• Differences among treatment types about $3,000 to $10,000,

between 2 and 6 percent of overall cost.

Recommendations• Use the shear bond strength test to evaluate bond

performance.

Method-Based Specification• Slight modifications.• Better incorporate spray pavers.• Allow slightly higher rates for

HMA.• Prescribe use of tack even

though results suggest it’s not always needed.

Performance-Based Specification• Responsibility to select tack rate moved to

contractor.• The application rate is chosen by the contractor

and must conform to the minimum rates specified in Table 407.06-1.

• Bond strength test added.• The Department will measure the bond strength of

the new overlay to the existing surface on the first day of paving. Core locations will be selected randomly and cores will be tested in accordance to the shear bond strength test (ODOT XXX). The average shear bond strength of three cores should be 70 psi or greater, with no single test result below 60 psi. If the contractor fails to meet this criteria, they must make changes to the tack material type, surface preparation procedures, and/or application procedures. Paving may be suspended until the contractor constructs a 1,000-ft test section meeting the criteria.

Long-Term Data Collection Plan• Bond Testing

• Initial• 8-month• 6-year

• FWD• Spring and Fall• Every 3 years

• Distress survey• Winter• Annually (Yrs 1-5)• Biennially (Yrs 7, 9, 11)

Overlay Age (yr) Month/Year Action Test 

Sections

0 July 2016 ConstructionBond testing All

0.6 Mar. 2017 Bond testing Group A*1.25 Oct. 2017 FWD measurements Group B1.5 Jan. 2018 Distress survey Group B2.5 Jan. 2019 Distress survey Group B2.75 Apr. 2019 FWD measurements Group B3.25 Oct. 2019 FWD measurements Group B3.5 Jan. 2020 Distress survey Group B4.5 Jan. 2021 Distress survey Group B5.5 Jan. 2022 Distress survey  Group C5.75 Apr. 2022 FWD measurements Group B

6.25 Oct. 2022 Bond testingFWD measurements

Group A*Group B

7.5 Jan. 2024 Distress survey Group B8.75 Apr. 2026 FWD measurements Group B9.25 Oct. 2026 FWD measurements Group B9.5 Jan. 2026 Distress survey Group B11.5 Jan. 2028 Distress survey Group C

*Also include no‐tack locations

Recommendations• Which tack coat strategy is best?

• Most cost-effective: SS-1H w/ spray paver, but savings were small.• The Department should consider allowing the contractor to chose

the tack material, application methods, and rates most convenient for their operations, considering they meet minimum application rate and uniformity requirements.

• Note: Initial bond from a spray paver can be much lower than from a distributor application. The bond will increase with time, but this may be a concern when paving in severe stopping/accelerating traffic conditions, where high bond performance is needed immediately after construction.

Implementation

What could be implemented?• Shear bond strength test.• Revised tack coat specification (Item 407), method-based

or performance-based.• General best-practices for tack coat construction.• Long-term data collection plan.

Possible Activities• Define scope of desired implementation. • Identify leadership group in Ohio to help with

implementation.• Develop presentation materials (powerpoint) about test

method and specification.• Present materials in key meetings/conferences/webinars.• Incorporate research findings into journal articles and

conference proceedings.

• Identify team for long-term data collection.• Schedule data collection activities.

Benefits• Eliminate poor layer bonding and associated distresses.• Save ODOT money on premature maintenance and

rehabilitation work.• Better understanding of long-term performance of different

tack coat strategies.

Potential Risks• Confirming relationship between bond shear strength and

actual short- and long-term performance.• Resolving differences in initial and long-term bond

performance. Can we rely on initial bond results? Particularly important for resolving cases with failed bond tests (i.e. spray paver construction) but adequate long-term field performance.

• Push-back from contractors about changes in specifications and additional shear testing.

• Too much load on labs to perform additional testing.

Principle InvestigatorBryan Wilson, PEb-wilson@tti.tamu.edu

Email administrative questions to: Research@dot.ohio.gov

Final report will be available online in a couple weeks at: www.dot.state.oh.us\research