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How Local Agencies Decide to Use PCC Overlays


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A presentation given during the 39th Annual NCPA Concrete Paving Workshop. By Brad Skinner, Montgomery County Engineer

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How Local Agencies Decide to Use PCC Overlays

  1. 1. HOW LOCAL AGENCIES DECIDE TO USE PCC OVERLAYS NCPA Concrete Paving Conference January 16, 2018
  2. 2. Who am I, and how would I know the answer to this question?  Today, 37 years out of Iowa State in 1980 with a BSCE  Registered PE in CO, and Iowa (PLS too), Previously in OK, though about it in several other states, but not really wanting to collect registrations … , but ….  17 years in Consulting (Land Development, City and IDOT type projects)  20th year in County Engineering. That said, …
  3. 3. Specific PCC Overlay Experience?  Hired as Dallas County Engineer in 1989, after less than two years in Montgomery County  Within a month of arrival, before I turned 30, Contractors started on 18 miles of PCC Overlays that had been let before my arrival. All over old HMA.  It was kind of a “throw the baby in the pool and see if it swims” approach, but the contracts were in place and the paving plant was being moved in.  I didn’t make the call, but I learned why it was decided and lived with it ever since.  Since then, I have managed another ten miles or so of PCC Overlays, 16 lane- miles of widening, plenty of full depth highways and City streets - overall more than sixty miles of PCC Pavements  I’ve also done about thirty miles of HMA Overlays over HMA or Seal Coat – because it was the right thing to do for the situation at hand
  4. 4. So first, decide if an overlay is appropriate, then we will get to what type.  Existing pavement (PCC or HMA) not too bad, but not enough.  Can the existing hold up during hauling of the mix?  Good enough structure present that you don’t need to start from scratch?  Future traffic can be accurately estimated.  Adjacent access and road-top width can accommodate the additional thickness?  Funding is available to do something more than routine surface maintenance.  Is a recycled in place pavement covered with a new slab really “just another type of overlay”?
  5. 5. Also, need to know what you want out of an overlay, if a reasonable choice.  Add structure?  Correct grades? If so, isolated or wide spread?  Generate smoothness?  Good enough condition for an HMA Overlay? PCC Overlay can accommodate more underlying problems – long term. Less need for prep patching with PCC onto HMA  Minor wheel tracks? Problem to be addressed for HMA, beneficial for PCC.  You can use a little black stuff to buy time.  Or, go for an enduring pavement with a true addition of structure
  7. 7. Bottom line is Overlays are not the cookbook solution we tend to get into with standard pavement replacement or new development.  Nice to use some engineering judgement once in a while, with or without punching a bunch of numbers.  Seldom a toss-up based upon pavement condition, but PCC and HMA industry, as well as ASHTTO and others have structural number calcs that you can use to compare outcomes. Please be aware each formula tends to favor their sponsor!***
  8. 8. HMA Pluses: 1. Shallow “T” potential because strength isn’t an issue a. Maybe in a residential subdivision b. Facelift on an already substantial section 2. A Band-aid will do … for now. . Ease of maintenance in ensuing years (but always doing maintenance?) 3. Maybe save $ short term with a thin overlay – budget pinch with a pending disaster (avoid “losing a pavement”), but you’ll be back again sooner than you want. 4. Fewer adjacent grade adjustments, if a couple inches matters? 5. If over a Seal Coat, creates an actual maintainable pavement, as a product of several Overlays
  9. 9. PCC Pluses: 1. Minimal thickness (<4”) might work in those residential subdivisions or in a “bonded” situation 2. Longer life cycle which normally means better life cycle costs (totally market dependent) 3. Less maintenance over the life of slab 4. Annualized cost is almost always less if using honest future maintenance projections 5. Project access usually much better than on a fresh grade 6. An extra inch will buy years more performance – geometric improvement on performance 7. Doubtful you need to go back during your career to repave – not so much with HMA
  10. 10. Ultimately, HMA is a placeholder, short term, or Band-aid – sorry to my friends that sell the black stuff. Or it can be a way of life, a default process, seldom the engineered solution. PCC, on the other hand, can only perform well if it has a proper foundation. Another point, HMA makes a great base to place PCC on! If you can afford it, and it fits geometrically, PCC overlays have been a good life- cycle cost choice (where an improvement is needed) throughout my thirty some year career, and I don’t see that changing.
  11. 11. $
  12. 12. $ STRUCTURE
  13. 13. $ STRUCTURE (Fit the) SITUATION?