Importance of Tack Coat th Annual 39 39 th al Asphalt lt Paving ing Confer eren ence ce Charles rlesto ton, n, WV February ruary 28, 2019 19 Jason son Dietz, tz, Pavem vemen ent and d Materi erials als Engin gineer eer Denver, nver, CO
Tack Coat Workshops AK AK WA WA VT VT ME ME MT MT ND ND MN MN NH NH OR OR ID ID MA MA WI WI SD SD NY NY MI MI RI RI WY WY PA PA IA IA CT CT NE NE OH OH NV NV NJ NJ IN IN IL IL UT UT DE DE WV WV CA CA CO CO MD MD VA VA KS KS MO MO KY KY DC DC NC NC TN TN OK OK AZ AZ AR AR Guam NM NM SC SC GA GA AL AL MS MS VI VI TX TX LA LA FL FL HI HI 2014 – Pilot, VA PR PR Completed (47) Not Requested (4) 2
FHWA Best Practices Tech Brief AASHTO SOM Specification Was Submitted NCHRP 20-05 Synthesis - Tack Coat Study Increasing Application Rates DOTs Specification Revisions 3
Going to Stiffer Base Asphalts (SS-1h CSS-1h) Verifying Calibration of Distributor Treat Tack as Separate Pay Item vs. Incidental Item Tack Coat Pocket Guide (August 2019) 4
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Survey results courtesy of Dave Johnson and Danny Gierhart of the Asphalt Institute 6
Far too frequent practices. 7
Uniform, complete, and adequate coverage 8
Terminology Purpose of Tack Coats Specifications Materials/Products Construction Practices Testing and Acceptance 9
What Wh at we ar are tal alking ing ab about: ut: Undilute iluted d Emulsi lsion on — an emulsion which consists of a paving grade asphalt binder, water, and an emulsifying agent. Diluted ted Emul ulsion sion — an emulsion with additional water added to it. The most common dilution rate is 1:1 (one part undiluted emulsion and one part additional water). Residual ual Asphal alt — the remaining asphalt after an emulsion has set, typically 57-70 percent of the undiluted emulsion. 10
Tack Coat Break — the moment when water separates enough from the asphalt to show a color change from brown to black. Tack Coat Se Set — when all the water has evaporated, leaving only the residual asphalt. Some refer to this as completely broken. 11
T/F A Broken Emuls lsion ion is the remaining asphalt after an emulsion has set. 12
What’s wrong (if anything) with the following specification regarding application rate?: “Apply the tack coat at a rate of 0.05 gallons/yd 2 ” 13
If the example spec intended 0.05 gal/yd 2 of residual asphalt: Undiluted emulsion applied at 0.05 gal/yd 2 using an emulsion with 60% residual asphalt, leaves 0.03 gal/yd 2 on the roadway? 40% less than intended 14
If the example spec intended 0.05 gal/yd 2 of residual asphalt: Diluted Emulsion using the same emulsion diluted 1:1 with water and applied at 0.05 gal/yd 2 leaves 0.015 gal/yd 2 on the roadway? 70% less than intended 15
If the example spec intended 0.05 gal/yd 2 of residual asphalt: To receive Residual Asphalt at 0.05 gal/yd 2 using an emulsion with 60% residual asphalt, the contractor would need to apply: 0.083 gal/yd 2 of Original Emulsion or 0.167 gal/yd 2 of 1:1 Diluted Emulsion 16
To promote the bond between pavement layers. ◦ To prevent slippage between pavement layers. ◦ Vital for structural performance of the Why do we use pavement. Tack Coats? ◦ All layers working together. ◦ Apply along all transverse and longitudinal vertical surfaces. 17
Poor pavement performance ◦ Early fatigue life Bottom up Top down ◦ Slippage cracks Direction of traffic? ◦ Shoving Costly pavement repairs ◦ Shorter than expected pavement life can be devastating for agency budgets ◦ Removal and replacement of a portion or the entire pavement structure is very expensive 18
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Shoving Surface Core Days later – significant slippage Courtesy of Road Science TM 20
Courtesy of MoDOT 21
Most st agencie ncies s sp specify ify tack coat t ma materials rials us using ing the he followin llowing g AAS AASHTO TO sp spec ecific ifications ations or r so some me form rm of the hem: m: Asphalt Emulsions ◦ M140 anionic emulsions ◦ M208 cationic emulsions ◦ M316 polymer-modified cationic emulsions Straight Binders ◦ M320 Performance Graded Asphalt Binder ◦ M322 Performance-Graded Asphalt Binder Using Multiple Stress Creep Recovery (MSCR) Test 22
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CSS SS-1h 1h (52%) 2%) 1. 1. Non on-standard* standard* (48% 8%) 2. 2. SS SS-1h 1h (46% 6%) 3. 3. SS SS-1 (36%) 6%) 4. 4. CSS SS-1 (28% 8%) 5. 5. • Non-standard means that a state has come up with its own nomenclature for an emulsion, outside those specified in AASHTO M 140 (Anionic Emulsions), M 208 (Cationic Emulsions), or M 316 (Polymer-Modified Cationic Emulsions) Examples: SS1hp, CRS-2h, EBL 24
NTSS SS-1hm 1hm (24% 4%) 1. 1. Others hers specified ecified (all ll by less ss than an 3% of the states tes) CBC-1H CBC-1HT EM-50-TT E-Tac NTCQS-HH NTCQS-1HM NTCRS-1HM NTCRS-1HSP NTHAP NTT PATT UltraFuse UltraTack “Non -tracking tacks are designed to improve the pavement performance by avoiding the tracking problems associated with traditional tacks. This material is typically manufactured to harden quickly and adhere minimally to tires. When a hot lift of asphalt is subsequently placed over the tack, the hardened tack is reactivated by the heat, and bonds the new overlay with the existing surface.” (Seo, 2016) 25
Most agencies accept material using one or more of the following methods: Approved Source List ◦ Specifies pre-approved products and sources ◦ Typically maintained on the agency’s website Supplier Certification ◦ Agency relies on supplier testing ◦ Requires document certifying material meets agency specs Testing of Samples ◦ Samples from supplier or field project ◦ Verify product meets AASHTO specs, some agencies also verify performance via some form of bond strength test 26
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* Ho How w is ta tack measure ured d fo for pay? y? ? 28
Emulsion are the most commonly used materials for tack coats Care needs to be taken to make sure material doesn’t separate ◦ Insulated storage tanks ◦ Gentle agitation ◦ Proper temperature range ◦ Avoid contamination ◦ Consult supplier for compatibility information 29
Provides detailed findings on agencies’ and contractors’ best practices Discusses: ◦ Training and communication ◦ Tack coat application rates ◦ Dilution issues ◦ Surface preparation ◦ Application best practices ◦ Asphalt distributors ◦ Vehicle tracking of tack coat 30
Agencies in the US are fairly evenly split regarding whether or not dilution is allowed Dilution Pros ◦ Larger liquid volume allows easier flow ◦ Larger liquid volume allows easier overlap on “fans” Dilution Cons ◦ Risk of losing control of the product ◦ Requires additional time to break ◦ Increases risk of tack flowing of roadway surface 31
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The FHWA/Asphalt Institute Tack Coat Workshops recommends dilution occur at the supplier’s terminal 33
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Power brooming is the most commonly-used method of cleaning the roadway surface 35
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Know the desired Check truck setup. application and ◦ Spray bar height residual rates. (~12”) ◦ Appropriate nozzles Visually inspect ◦ Nozzle orientation application (15-30°) Verify application. ◦ Check application ◦ Volume (most popular) rate gauge in truck ◦ Mass ◦ Check application ◦ ASTM D2995 temperature Collect samples. 51
Layer Bonding is Vital Surface Preparation ◦ Clean ◦ Dry Milling Improves Field Performance ◦ Shear ◦ Cleaning 52
When to Re Re-Tack? ◦ Tracking ◦ Contamination If in doubt … Re Re-Tack Tack 53
Wh What is the Optimal mal Appl plication cation Rate? ◦ Surface Type ◦ Surface Condition Wo Works kshop p Recommende ended d Ranges ges Appx. Bar Rate Residua ual l Rate Ap Appx. Bar Rate Surface ace Type Diluted ted 1:1 * uted * (gsy) (gsy) y) Undilute y) (gsy) y) New Asphalt 0.020 – 0.045 0.030 – 0.065 0.060 – 0.130 Existing Asphalt 0.040 – 0.070 0.060 – 0.105 0.120 – 0.210 Milled Surface 0.040 – 0.080 0.060 – 0.120 0.120 – 0.240 Portland Cement 0.030 – 0.050 0.045 – 0.075 0.090 – 0.150 Concrete * Assume emulsion is 33% water and 67% asphalt. 54
Application Quality Vital ◦ Proper Rate ◦ Consistency Distributor Truck ◦ Setup ◦ Calibration/Verification ◦ Maintenance Tacking of Longitudinal Joints ◦ Bonding ◦ Confinement Excessive Tack is Bad 55
Tack Coat Rate Depends on Surface Condition ◦ Fresh ◦ Weathered ◦ Raveled ◦ Milled Need for Research ◦ Field Performance ◦ Field Testing Bond strength Application amount Treat at Tack as Separ arat ate e Pa Pay y Item m vs vs. Incidental idental Item 56
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