SR 240 Slipform Paving Case Study

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SR 240 Slipform Paving Case Study
Alisa Cotter, Mike Hansen, and Eric McGarrah
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Where is SR 240?
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SR 240 is a Major Tri-Cities Route

• 60,000 ADT
• 110,000 ADT in 2025
• Original Alignment of 4 HMA Lanes (inadaquate)
• Project will replace with 8 PCC Lanes with a
  depth of 11 inches.

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Project to Date

• Westbound lanes were closed and reconstructed
  first.
• Westbound lanes closed and traffic diverted to
  eastbound lanes.
• 2 temporary lanes were constructed to maintain
  capacity.

• Westbound lanes opened to allow the construction
  of the eastbound lanes.

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Batching

• Temporary batch plant located within ½ mile of
  project.
• Plant relocated from Spokane.

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Batching

• Aggregate was stored on site.
• Loaded into bins that feed the plant the
  appropriate gradation for the mix.
• A weighing system is used to ensure the correct
  gradation.

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Batching

• Once aggregate is proportioned it goes into the
  horizontal drum followed by
• Water, cement, and admixtures.
• The horizontal drum mixes the components.
• The mix is then transferred to the tilt drum.

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Batching

• The tilt drum then transfers the mix to the end
  dump trucks.

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The Mix

• A standard WSDOT mix was used.
• Maximum water/cement ratio of 0.44.
• Minimum cement content of 564 lbs/yrd3.
• Type 1 cement.
• 4,416 psi average 28 day compressive strength.
• 718 psi average 14 day flexural strength.

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The Mix

• Pave-Air, a proprietary air entraining agent, was
  added for improved workability.
• Masterpave, a proprietary water reducing agent,
  was added to decrease water/cement ratio.

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Plant Production

• Materials are constantly delivered to the site.
• Two drums allow a second batch to be produced
  while a truck is being loaded.

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Plant Production

• Ten end dump trucks were used.
• 9 yd3 per truck
• 1.5 minutes to load.
• 3 minutes from arrival to departure.
• 300 - 350 yrd3/hr
• 33 - 39 trucks/hr

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Noted Project Plant Issues

• WSDOT inspector noticed that the mix was 20-40
  lbs low per batch.
• Contractor was instructed to increase cement
  content or batches would be rejected.
• Resulting in a temporary plant shut down.
• We dont know what caused this . . . . ?
• Mechanical issue
• Cement Shortage at the plant.

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Mix Delivery

• Short delivery time allowed the use of end dump
  trucks (2-3 min).
• Trucks had to avoid the paving wire.
• Turn around point was provided.
• Trucks had to back up 500 -1000ft.

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Mix Delivery

• Concrete trucks had priority on and off the
  project.

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High Productivity of Delivery and Placement

• Unusually high productivity was achieved with
  only ten trucks because of . . .
• Proximity of the plant.
• Thoughtful and efficient traffic control plan.
• Width of the paving.
• Use of a slip from paver.

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Set Up for Slipform Paving

• Surveyors must determine the elevation of the HMA
  at 50 intervals to set the guide wire.
• Number 1 tripping hazard for 1rst year engineers,
  isnt that right Alisa.

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Set Up for Slipform Paving

• Bundles of dowel bars and containers of curing
  compound must be laid out.

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Set Up for Slipform Paving

• Water is sprayed ahead of the paving to cool the
  HMA base and keep water from being sucked out of
  the mix.

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The Slipform Paver Needs Space

• The new alignment of this project provide a lot
  of space.
• Typical spatial needs include
• 2-3 feet on either side for tracks.
• Loading dowel bars.
• Access for laborers.
• Finishing

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Start-up of Slipform Paving

• The transverse construction joint must be
  constructed.
• 40-50 full width spot grinding was done at the
  construction joints.

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Start-up of Slipform Paving

• A wetter mix is usually associated with the
  initial batches making it difficult to maintain
  the pavement edge.

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Start-up of Slipform Paving

• The paver has not developed the head that is
  required, possibly resulting in surface voids.

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Concrete in the Slipform Paver

• The concrete is moved from side to side with the
  bucket on the front of the paver.

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Concrete in the Slipform Paver

• The concrete is then vibrated by a series of
  vibrators.

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Dowel Bar Insertion
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Dowel Bar Insertion
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Dowel Bar Insertion
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Loading of Dowel Bars
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Possible Issue of Dowel Bar Insertion

• Dowel bars need to be placed appropriately.
• Dowel bars are inserted using vibration and the
  vibration continues when the inserters are pulled
  back.
• Does this alter the dowel bars alignment?
• Are the paint marks denoting the dowel bar
  location in the right location?

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Mid-Slab Tie Bar Insertion

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Side Tie Bar Insertion
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Noted Paving Issues

• Shims were used at the construction joint because
  the form was not as tall as the depth of the
  paving. This might result in stress point that
  could induce cracking.

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Noted Paving Issues

• There was 3 observed stoppages in the paving due
  to the dowel bar bundles not being in the proper
  location. Probably resulting in imperfection in
  smoothness.
• WSDOT allowed a deviation in the dowel bar
  placement.
• Only outside lanes had dowel bars across entire
  width.
• Inside lanes only had dowel bars in the wheel
  path.
• Saved the project about 56,000.
• Will this bite them later?

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Finishing

• Initial finishing is done by the side forms and
  profile pan followed by a float and carpet drag
  on the paver.

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Finishing

• Edge finishing requires 4 laborers (2 on each
  side), one to remove the valley from invert on
  the profile pan, the other to finish.
• The finishing quality of this edge is very
  important if there is an adjacent lane of PCC to
  be paved.

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Transverse Tining
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Curing
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Noted Issues of the Curing

• The curing compound was tested twice and failed
  twice.
• WSDOT allowed the use at twice the application
  rate.
• During testing the contractor found it difficult
  to apply a uniform cure in windy conditions. The
  wind was not an issue during the case study.

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Compressive Strength Testing

• Minimum 28 day compressive strength of 3,216
  psi.

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Air Entraining Testing

• Air entrainment between 3 and 7.

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Ride Testing and Pavement Depth

• The pavements smoothness was tested using a
  profilograph.
• Pavement depth was determined from coring.

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Contractor Bonus

• The contractor received a 3 bonus for gradation,
  cylinder breaks, and air entrainment. Totaling
  107,500
• The contractor also received 64,300 for the
  pavements ride smoothness and 86,500 for
  additional pavement thickness.