Friday, July 22, 2011

Alligator Cracking Continued...

Over the last few days I received the following email in response to my entry Alligator Cracking:

Could the consistency/mixture of the pavement be different? Or maybe the thickness of the pavement is less toward the shoulder than the center. Or flooding or excessive rain at the time it was laid or before the pavement had time to dry, could that cause this? Maybe an underground spring or just ground moisture. Or maybe the ground wasn’t properly prepared before the pavement was laid. Could tree/plant roots cause something like this? How about an earthquake or tremor?

Just a thought.

Yes!  Almost all of those are potentials for this type of cracking!  Typically most of those problems will cause the alligator cracking to appear in various other parts of the pavement as well, not just on the joint. 

Mixture variation in the pavement can cause a variety of different issues in the pavement, depending on how the mixture varied.  However, alligator cracking is typically a structural problem instead of a material problem.  A material issue typically means that the mixture of asphalt binder (oil) and/or rock has problems.  When I refer to structural issues it is referring to the "structure" below the mixture that is being laid down.  The structure is typically old pavement, base rock, or soil.  So when I say that there is a structural problem, that means that there is something wrong with the underlying pavement, base rock, or soil below the new mixture that you're putting down. 

The thickness of the pavement is an interesting idea, if this part of the pavement had a thinner layer of pavement than was designed to hold the load, it could easily crack under the pressure. 

Rain and moisture at the time of paving could cause this type of cracking but more likely it will cause raveling or shoving instead.  The one time that it could definitely be an issue is if the paving crew is paving over aggregate base or soil instead of existing pavement.  If the base course is too wet at the time of paving it will tend to move and adjust under the weight of vehicles, sort of like playdough would.  This would also happen if there were an underground spring or ground moisture.  I would consider this a way of poorly preparing the ground before laying the pavement because it is up to the contractor to assure that his base course is prepped and ready for the asphalt layer.  This includes obtaining the correct moisture and compaction of the underlying structure.  Another way that the alligator cracking could occur would be if they didn't compact the base rock enough before paving on it. 

Roots and earthquakes could absolutely cause these types of cracks as well, however, if it were caused by roots you would tend to see a raised portion in the alligator cracking, where the root is pushing up on the pavement.  I hope that answers your questions!  Keep them coming!

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