Tack Coats Greg Hainsworth Materials Engineer
DelDOT Specifications • 401.07 Application of Tack Coat • A tack coat diluted with 50% water shall be applied on all dry and broom cleaned Portland cement concrete and bituminous pavement surfaces. Tack coat shall be applied at a rate of 0.05 to 0.15 gal/SY at a temperature of 70F to 160F.
What is tack? • Thin layer of material needed to bond the new surface to the existing. • Typically emulsions (water + asphalt) • CSS 1h is most common • Other grades RS1/2, SS1/2, PMXX## • Occasionally neat asphalt cement PG 64-22 • Must be applied hot • No wait time to pave • Important in thinner lifts to prevent flushing of the surface
What is tack? (cont) • Emulsion terminology • C – cationic as opposed to anionic • Cationic emulsions tend to give up their water faster than anionic formulations and are more forgiving in handling • SS – slow set • RS – rapid set • QS – quick set • 1 – low viscosity • 2 – high viscosity • h – hard binder grade • PM – polymer modified • LM – latex modified • HF – high float
When do we use tack? • Between layers of HMA • 401.08 paragraph 9 “if more than ten days elapse, a fog coat of RS-1 or CSS-1h shall be sprayed uniformly on the exposed base course before placing the wearing course of [HMA]” • Longitudinal joint surfaces • Full Depth Recycling • Undiluted CSS-1h is used as a curing membrane
Before Application • Surface preparation is very important • Broom surface • Air Pressure • Remove all loose material and dust from the surface that would prevent bonding between the tack and the existing pavement surface.
Surface Preparation • Pickup of tack on the wheel of the paving train. • Note the poor surface preparation contributing to the pickup of material.
Application Rates • Emulsions • AASHTO requirements for emulsions • Residual AC content (minimum) – 59% • 41% water • Generally accepted that 0.15 gal/SY is the maximum that should be applied. • Residual AC rates 0.03 – 0.08 gal/SY • Diluted emulsions can be useful • Easier to apply, lower residual AC rates • CAUTION: Diluting emulsions is a complex process • RS emulsions should not be diluted after delivery
Application Rates (cont) • Surface conditions • Rougher surfaces have more area per SY than smooth surfaces. • Oxidized surfaces need more than newer surfaces. • Surfaces shall be cleaned from any dust using brooms and / or compressed air. • Keep traffic off of tacked surfaces. • Only tack what is going to be paved in that shift.
Application Rates (cont) • WSDOT: Tack Coat
Application Rates (cont) • What affects the application rate? • Ground speed of the truck • Pressure of the pump • Nozzle opening (size and condition) • Nozzle orientation • Nozzle height above the pavement
Application • Bar Height Too High • Nozzles are overlapping 3X • Wide bands on the outside with minimal tack applied
Application (cont) • Bar height too low • No overlap between the nozzle patterns • Bands between the nozzles with no material
Application (cont) • Bar height is optimum • Double overlap between the nozzle patterns • Most uniform coating of tack applied across the mat
Application (cont) • Nozzle orientation
Application (cont) Proper nozzle orientation; Spray is overlapping without contacting adjacent nozzle spray before reaching the pavement surface.
Application (cont) Fresh tack coat will resemble the consistency and color of Chocolate Milk until it breaks, turns black and is tacky to the touch.
Application (cont) Thin and uneven distribution likely caused by clogged nozzles. Note the pavement markings are still visible.
Application (cont) Application temperature should be between 70F and 140F. Broken tack – dark black like fresh asphalt. DON’T PAVE OVER UNBROKEN TACK DON’T USE THE HMA TO BREAK THE TACK.
Application (cont) The absence of tack and bond between the layers will inhibit the transferring of stresses between the layers of HMA – the result will be bottom cracking starting in the overlay and not at the bottom of the pavement section.
Application (cont) • Tracking of the tack on the wheels of the truck. • Frequently occurs when traffic traverses the tack before it has broken. • The weakest bond between the layers occurs in the wheel paths.
Application (cont) Sliding in the wheel path
Application (cont) • Possible causes: • Insufficient tack applied • Tack not broken before HMA placement?
Application (cont) Be aware of surrounding conditions that could adversely affect the tack coat before it gets paved.
How to sample tack • Polyethylene bottles available from M&R • Sample from a port on the spray bar. • Draw off a small portion into another container until fresh material is evident. • Fill ½ gallon plastic bottle and return to lab in Dover. • Include job #, contractor, truck number, date and time if possible, approximate volume represented.
How to test tack • In the lab: • Determine the residual AC content • AC is graded just like typical AC binder • Typical grade of AC is a PG 64-22 • Deleterious contamination will be evident • Heavy oils • Insufficient residual AC • Total time: Approximately 3 working days
How to test tack (cont) • The shoe test • No ASTM standard • Instant results
How Much? • Blot test • Weigh a measured piece of absorbent paper • Lay down paper in front of the tack truck • Weigh the paper/tack together • Calculate the gal/SY • Theoretical Yield Calculation or…
How Much? (cont) No Yes
How Much? (cont) GOOD Even uniform coverage No puddles No stripes BAD Uneven coverage Clogged nozzles / improper orientation Stripes are clearly visible
Why the interest in tack coats? • Sampled at the job site. • Sampled at the storage tank same day.
What Next? • Revision to current specifications. • Prohibit dilution of emulsion after delivery. • Implementation of QC plans for producers that are supplying tack material to the paving contractors. • More frequent sampling in the field • Warranties?
Thanks to…. • Photo credits to: • Asphalt Institute • Washington State DOT • Ohio DOT • Pavementinteractive.org • CalTrans • U. of Cal / Pavement Research Center • Additional technical references: • CalTrans, NJDOT, NYSDOT, OHDOT, ADOT, LTRC, WSDOT, TXDOT
Questions, comments, difficulties…. Greg Hainsworth Gregory.firstname.lastname@example.org 302-760-2401