Wednesday, July 13, 2011

Alligator Cracking

Welcome to post one of disaster roads!  The pictures I'll be using for this series were taken by our readers over the fourth of July weekend.  If you have a road you are curious about email it to me at and I'll talk about it in the blog. 

This post will focus on alligator cracking based on the photo below:

For those of you who might not know, alligator cracking is the scaly looking pattern you see on the right side of the photo.  Here the cracks have been filled in with a crack sealer which can help to slow the cracking from spreading by keeping out additional moisture.  Alligator cracking is one type of fatigue cracking that can happen to a road for a variety of reasons but is most often related to overloading the pavement. 

Roads are designed for a variety of variables, two of the biggest are the amount of traffic and the type of traffic.  A road in a subdivision may be designed for a lighter load than a Wal-mart loading dock because it is assumed that only small vehicles will be driving there instead of heavy trucks.  If a road is designed for a lighter load than is applied to it, the pavement can form a pattern of these types of cracks.  Often roads are designed with a lighter load in mind but some vehicles are overlooked.  For instance, it seems like a fair assumption that a subdivision will only see small vehicles but the subdivision is still visited by heavy garbage trucks weekly and if there is a bus stop in the area their roads will be overloaded from the bus's weight as well. 

However, the road shown above does not seem to be a result of poor pavement design but more poor pavement construction.  Take a look at the photo above and notice that the black crack sealant fills in a patch of alligator cracks but then continues up the road in a straight line with a couple more patches of alligator cracks as it goes.  The long straight crack is the joint in which one length of pavement was laid down next to another length of pavement.  Joints are common places for distresses on roads because if they are not properly installed, the joint can allow for moisture and other debris to wriggle in between the two sections of pavement making the joint a weaker part of the road way.  For this very reason, Caltrans and several other government entities specify that joints be lain on lane dividers like the yellow dashed line you see on the left and the white line you see on the white. 

The next thing you might have noticed is that the joint is a foot or two to the left of the edge of the lane.  The wheels of vehicles travel in this exact area of the lane, placing half of the weight of a vehicle on the joint in this road.  The joint is already a weaker section of the road and the additional weight of the vehicle tires overload this section of the road and create a variety of small cracks in the joint that radiate out from the joint line.  Over time moisture will accumulate in these cracks.  Cold weather will cause the moisture to expand and the cracks will grow until they form a pattern like you see above.          

Can you think of any other reasons this road might look like this?

1 comment:

  1. alligator cracking is also a result of poor preventive maintenance by the owner.