Wednesday, August 31, 2011

Strip Patching

The photo below got me thinking about strip patching and when it can be ineffective.  I'm not sure when each of the strip patches below were performed but you can see on the right lane that there are two different patches in the same lane.  If these patches were put in at the same time then it seems to me that they should have taken the whole lane out and repaved the lane.

One of the biggest issues that arise from this type of patching is not reaching compaction.  Contractors will commonly look at a cracked road surface and try to save money by only digging out and paving the damaged area.  This can sometimes be an effective road treatment but only if it is done correctly.  The first step to a successful strip patch is to determine if a strip patch is really the right treatment.  If you are having problems with both sides of the lane like in the photo above, it is a better idea to repave the entire lane.  If you just have one section of the road that needs to be fixed then a strip pave may be the answer.  The next step is to cut out the existing pavement.  The cut lines should be at least a foot beyond the visible cracking on all sides and should be AT LEAST as wide as your roller or compactor.  If your compacting equipment cannot get into the patch area to compact the material then your patch will end up raveling or you could even end up with a cracked surface again.  Either of those situations are going to cost you more money and in the end, the money you THOUGHT you saved by skimping on the width of your patch will be spent on yet another patch. 

Once you have the pavement dug out you should take a look at the base course.  If you had alligator cracking on the pavement before you dug it out you will have to fix up the base course by adding more base rock if necessary and re-compacting.  Even if you didn't have alligator cracking you should still check the compaction of the base rock and make sure that everything is compacted, dry, and ready to be paved on before you add your new hot mix. 

Next you'll want to make sure that you tack coat all of the vertical faces in your patch.  The older pavement will serve as a difficult to adhere your new hot mix to.  Tack coat will help the new hot mix stick to the old pavement to reduce the chances of you getting new cracks along the joints of your patch after a month or two as the new mix and old pavement pull apart.

Once you start putting your new hot mix into the patch make sure that you leave the material around 25% higher than the existing pavement.  This is so that when your roller compacts the material it will come down to grade and allows a good compaction percentage.  If you don't have enough material in the patch to start with, the roller will be able to compact it down to grade but once it hits the height of the existing pavement it will not be able to compact it further due to the roller being hung up on the existing pavement.  It will look nice to start with but after a few months the patch will start to ravel.  This is the part of patching that is really an art.  I've told you that you should have around 25% higher hot mix material than the existing pavement but this will greatly depend on the type of mix you are using on the patch.  Just remember that compaction is just as important in a patch as it is when you are paving an entire lane. 

Once you've added the new hot mix it is time to rake the material up on top of the patch area and off of the existing pavement.  A common mistake that contractors make is by using the bump and fling technique.  This is where the raker bumps up the material on the edge and then flings the excess material across the mat so it can be compacted into the middle of the mat.  The raker should indeed bump the material up on the edge of the patch but then the material should remain there, on the inside of the crack, until it is compacted.  The patch should look like the diagram below right before you start compacting.

Leaving the excess material on the edge of the patch like above allows for the material to be pushed down into the joint between the existing pavement and new patch.  This will help with adhesion and compaction on the joint. 

The first thing that the roller should do when you're ready to compact is "pinch the edges".  The roller drives on the existing pavement on one side of the patch with part of the roller hanging over onto the patch.  The compactor drives over the "bumped up" areas of your patch and pushes it down into existing pavement.  If you had too much bumped up in this area it's ok.  The excess material will be pushed out towards the middle of the mat where you might have flung it anyways, but now you have a well compacted joint instead.  The roller then should do the same thing on the other side of the patch and then roll the middle of the patch.

The photo I showed at the beginning of this post is an example of a poor strip patch job.  Not only should the contractor have repaved the entire lane but if you look closely you will see that there is a gap between the new patch and the old pavement, probably due to any number of the issues I've discussed today.  Patching and maintenance is where the owner can really lose a lot of money or save a lot of money. 

If you don't have the money to do it right the first time, where will you get the money to do it over?