Then we’ve got the high density mineral bonds (HA5). We’ve got slurry seals; we’ve got micro-surfacing which at one time was referred to as Polymer Modified Slurry (PMs). We’ve got ultra-thin bonded wearing courses. Thin overlays, now we’re talking about the hot-mix family here, and thin overlays with RAS (Recycled Asphalt Shingles) and RAP (Recycled Asphalt Product).
Now Mr. Gerry Huber talked about this. I remember we were at Purdue together a couple years back, and he had a great discussion about RAS/RAP mixtures with upwards of 57% recycled product. I think there was like 18% Recycled Asphalt Shingles and 39% Recycled Asphalt Product. Also, I want to point out that Dr. Mike Heitzman from NCAT also did a really good IPMA Academy live session for us just a while back on hot-mix asphalt thin overlays in pavement management.
Again, this list above is not meant to be all inclusive. If I missed something during the recording of the session, know that there are maybe a few others out there that you are considering or your agency already currently uses. So here is where the biggest bang for your buck comes for your city or county and the third leg in the three-legged stool system of pavement management, in-place recycling.
Since the majority of our four million miles of paved roads in USA are hot-mix asphalt pavements, we focus on the three main disciplines for hot-mix asphalt pavements, namely Hot In-place Recycling (HIR), Cold In-place Recycling (CIR), and Full-Depth Reclamation (FDR). However, on the concrete pavement side of the rehabilitation and preservation fence, there are plenty of requisite treatments for PCC as well. One of those we discuss quite often for rehabilitation of PCC is rubblization.
So following are the in-place asphalt recycling disciplines and the respective sub-disciplines.
We’ve got Hot In-place Recycling with the sub-disciplines being surface recycling, remixing and repaving, and then we’ve got this hybrid combo called Re-HEAT. Now simply speaking, the surface recycling technique involves a series of sequential heaters that heat up the old asphalt surface in place. The train moves down the roadway one lane at a time adding polymer modified rejuvenating agent to the scarified surface to a nominal depth of about one inches.
Now this is heated just like normal asphalt would be upwards of 300 degrees, and the rejuvenating mix is then laid out with an attached screed, like an actual paving screed on the back-end of the last sequential heater, or that we’ve got preheater, preheater and then the final heater with the integral screed attached to it. The mix is compacted with a conventional hot-mix asphalt type pavement equipment.
You couldn’t think about this process, the surface recycling or it used to be called scarification, as giving you basically a new leveling course where some of the wearing surfaces to go down from the list above that we talked about in preservation.
Next, we have the remixing sub-discipline which actually heats up the old in-situ asphalt pavements, similar to the former method I just discussed, but this time, having the ability to add upwards of 30% of new version hot-mix asphalt at the same time. Now imagine, if you will, using that 57% super-duper RAP/RAS combo that I mentioned earlier in the session as your supplemental admixture for this remixing technique.
Now in this technique, the crew can do two to three inches in depth by doing sequential heating, milling, and mixing. This technique is used on all types of roads, including the Oklahoma turnpike which I think I just drove over a week ago in the Better Roads bus. In the case of the turnpike, they put ultra-thin bonded wearing course on as the final wearing course or the final asphalt layer.
Now the third sub-discipline of Hot In-place (HIR) asphalt recycling industry is, what we call, the repaving method. When we talk about this in class, I use the peanut butter and jelly sandwich analogy. So picture this, if you will.
Take the old asphalt that gets heated and scarified, much like the first sub-discipline, with the jelly being into new pavement placed on the top of the peanut butter or the one inches of heated old pavement. Both layers are laid out with a double screed and compacted as one new homogenous mixture. Now the Re-HEAT reprocess, kind of a hybrid between the remixing and the surface recycling, basically heats up about two inches of the in-situ asphalt mix and scrapes it off to the road bed, basically picks it up and puts it into an onboard batch plant.
The batch plant runs right up inside the heater unit, and the mix gets polymer modified rejuvenating attitude. Nothing is being added, per se, other than the recycling agent and the old asphalt, but it’s getting mixed inside of a barrel which gives a really nice homogenous mixture. It now comes out behind the heater in the final preheating and heating stages, and it gets put down with basically the exact same paver. But it’s integral; it’s attached to the machine itself.
So it’s as good or better than a standard paver that you would see on a conventional hot-mix asphalt job. The mix goes down through the paver and gets laid out and compacted with conventional compaction equipment. What makes this a little bit different out of the Re-HEAT is that it does not require a wearing surface.
So in terms of Cold In-place Recycling (CIR), you can have single unit trains, dual unit trains, and multi unit trains. The contractors are still performing partial depth reclamation and rely on you as the agency to have at least four inches of pavement depth. Now often, the contractors can strip off the entire pavement section and recycle it in place with engineered emulsion or foamed asphalt, provided there is a good structure underneath.
The contractors coin this phrase “following the rock.” It’s worth noting here that as we go through this list of in-place recycling techniques, we are also following the declining PCI ratings. So we’re following the curve as we go down through these treatments. So while a Hot In-Place candidate pavement may have a PCI of 62, a CIR candidate may present itself as a PCI equals 52 and have a thicker pavement section for the contractor to deal with. That brings us to FDR (Full-Depth Reclamation) where a typical candidate selection may be a PCI of 21 or less.
Now the good news is whether it’s a 21 or an 11 or a 1, it’s still going to require FDR with some sort of wearing course. So don’t struggle once the pavements get down below 30 and think you have to do everything all at once worst first. No. In fact, you should take two or three years to budget for those types of roads and make sure you have enough money in the budget to cover those costs. Know that you can do these FDR projects for a third of the cost of conventional dig out and reconstruction.
So know that there will never likely be a time when you’ll have to ever, ever, ever dig up your old road and haul it away to a landfill site. In fact, for those of you who’ve seen me do a live conference event, I’ve been known to get the entire audience to stand up and make that pledge with me. FDR (Full-Depth Reclamation) can be done with mechanical stabilization, chemical stabilization. or bituminous stabilization. The pavement structure – it can be of any combination of pavement, thickness, granular or dirt – as a requisite treatment will take care of most every situation you may have.
Again, experts know to go out in the field and get proper mixed designs and soil samples, so they can align the right treatment with the right road at the right time for the right reason with the right contractor. At times, soil stabilization can be used in conjunction with the FDR to take care of the very worst case scenarios, and I think back to the 30 miles in Natchez Trace Parkway that we did for federal highways and absolutely took a full advantage of all the treatments that we talked about today with FDR and soil stabilization.
Note, this method is a fraction of the cost of conventional undercut and provides the same or better surface life, probably the biggest no-brainer of all time. For more information on any of the above, please refer to your copy of the book On Better Roads. Again, if you’d like to have your city or county staff get more detailed information on saving your crumbling roads for less money and less carbon footprint, have them check out www.ipmaacademy.com, a partnership between IPMA, the International Pavement Management Association, and Auburn University
Wherever you go, thanks for reading the Pavement Management Primer today. Hope to see you real soon. This is Blair Barnhardt, signing off.
The Barnhardt Group, LLC.
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Blair Barnhardt, CEO