posted originally at www.ericstlouis.com
Maximum financial and environmental benefits can be derived, and more interestingly, compounded by manipulating existing asphalt as little as possible while introducing as few new elements as possible. Recyclers that use 100% of the existing asphalt in place without burning the AC, look particularly interesting. An even more interesting benefit of recycling in place is the fact that we can eliminate our old nemesis the dreaded ‘’cold joint’’ seam between the patch and the surrounding asphalt.
Think of it as a healthy lifestyle change for our roads: Eliminate asphalt joints and you will add years to pavement life. In the last episode I wrote about asphalt cold joint heaters and their long term environmental stewardship benefits. Taking that one step further, by eliminating vertical or horizontal breaks in any asphalt mat, it becomes stronger and requires less maintenance. We should favour construction methods which minimise or eliminate joints.
Sustainable recycling of asphalt must therefore include in-place recyclers. These are mobile units which can heat asphalt in place, without excavating it. Once again, when recycling, the key to maintaining the integrity of the AC is to soften it gently. If there is such a thing as a golden rule to reheating whole asphalt, it is this: Avoid overheating the asphalt cement no matter how you chose to heat it. Forced or improperly applied heat sadly results in burnt AC and clouds of toxic smoke, the antithesis of good asphalt stewardship.
For a lasting in-place full depth patch, the asphalt must be heated such a way as to protect the AC as much as possible. If we apply too much heat we will burn the surface before the heat has had time to soak into the asphalt. This is often seen as a pale grey crust which forms on the surface of the patch while heating. This crust of burnt asphalt has two very bad effects on heat transfer: One, because as it forms it bubbles up from the road surface and the subsequent air gap acts as an insulator preventing further heat transfer to the patch. This results in insufficient patch depth. Two, the heat which is no longer transferred in depth to the patch overheats the crust and often results in actually setting fire to the patch. That is inexcusable. This burnt material will also need to be replaced with fresh material adding to the cost of the repair.
If we are to affect a lasting repair, we must soften the area at least 2 inches deep, usually the full depth of the course. The heated material must be scarified and blended to correct the profile while removing any foreign material or contaminants. Full depth scarification is more consistent if done mechanically. While scarifying we should leave a roughly 2-3’’ inch wide untouched border of heated asphalt; this border is called a transition zone. This transition zone between the patch and the surrounding asphalt will insure that the whole patch is well sealed and level with the surrounding road surface. Once scarified, fresh asphalt may be added to bring the patch up to grade and levelled. If required, any additives or rejuvenants are usually sprayed on at this stage. Crew experience counts in finishing; over the years I’ve seen some virtuoso performances on the finishing lute; true artists are a joy to behold in any medium.
Compaction is best performed by a small vibratory roller. Vibratory plates will, by their rocking motion, cause uneven compaction and should be avoided. Compaction should start with the roller drum on the cold asphalt and slightly overlapping the patch, rolling the complete perimeter of the patch first to take full advantage of the transition zone. A perfect seamless patch can be thus executed while maximising our investment in existing asphalt. In my opinion, this is a prime example of good asphalt stewardship.
For more articles on asphalt recycling and the infrared process visit www.ericstlouis.com