Tack Coats

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Tack Coats

• Greg Hainsworth
• Materials Engineer

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Tack Coats
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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.

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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

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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

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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

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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.

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Surface Preparation

• Pickup of tack on the wheel of the paving train.
• Note the poor surface preparation contributing to
  the pickup of material.

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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

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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.

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Application Rates (cont)

• WSDOT Tack Coat

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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

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Application

• Bar Height Too High
• Nozzles are overlapping 3X
• Wide bands on the outside with minimal tack
  applied

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Application (cont)

• Bar height too low
• No overlap between the nozzle patterns
• Bands between the nozzles with no material

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Application (cont)

• Bar height is optimum
• Double overlap between the nozzle patterns
• Most uniform coating of tack applied across the
  mat

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Application (cont)

• Nozzle orientation

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Application (cont)
Proper nozzle orientation Spray is overlapping
without contacting adjacent nozzle spray before
reaching the pavement surface.
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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.
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Application (cont)
Thin and uneven distribution likely caused by
clogged nozzles. Note the pavement markings are
still visible.
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Application (cont)
Application temperature should be between 70F and
140F.
Broken tack dark black like fresh
asphalt. DONT PAVE OVER UNBROKEN TACK DONT USE
THE HMA TO BREAK THE TACK.
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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.
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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.

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Application (cont)
Sliding in the wheel path
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Application (cont)
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Application (cont)

• Possible causes
• Insufficient tack applied
• Tack not broken before HMA placement?

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Application (cont)
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Application (cont)
Be aware of surrounding conditions that could
adversely affect the tack coat before it gets
paved.
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How to sample tack

• Polyethylene bottles available from MR
• 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.

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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

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How to test tack (cont)

• The shoe test
• No ASTM standard
• Instant results

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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

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How Much? (cont)
Yes
No
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How Much? (cont)
GOOD Even uniform coverage No puddles No stripes
BAD Uneven coverage Clogged nozzles / improper
orientation Stripes are clearly visible
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Why the interest in tack coats?

• Sampled at the job site.
• Sampled at the storage tank same day.

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Why the interest in tack coats?
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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?

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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

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• Questions, comments, difficulties.
• Greg Hainsworth
• Gregory.hainsworth_at_state.de.us
• 302-760-2401