Alligator cracking of asphalt pavements

In contrast with ice-wedge polygons, current theories for the development of alligator cracking in asphalt pavements do not appear to acknowledge any significant contribution from thermal ratchet processes. It will be one of the aims of this posting, and probably a few to follow, to establish some possibly close analogies in the processes responsible for the formation of alligator crack patterns and those that are widely agreed to cause the formation of ice-wedge polygons in permafrost. Just as for my earlier explanations for the origins of pavement blisters, see posting of 23 March, 2010, the possible causes to be outlined below do not conform to accepted wisdom within the pavement engineering community. But as I will argue that does not mean they are necessarily wrong!

And if these alternative explanations for the physical causes of alligator cracking are correct the practical and financial implications could be very serious. In case you are not clear what I am talking about here it might be helpful to provide a few examples of alligator cracking. Without necessarily being aware of the fact you have very probably been walking or driving over many forms of alligator cracking without really noticing. I hope if you do actually read it you will never again be able to walk or drive over alligator cracking without being aware of the fact and possibly even recalling this blog!

Some examples of this all too familiar form of asphalt failure are shown in the Figures below. These make clear the association of these crack patterns with an alligator hide. They also suggest close morphological relationships with ice-wedge polygons that can form in permafrost, discussed in an earlier posting. They also show close similarities to the crack formations that occur in drying mud, the crazing experienced by ageing paint, lacquer or the glazes on ceramics, and many other related phenomena. To emphasize what I will argue to be the common cause for the formation of alligator crack patterns and those of ice-wedge polygons in permafrost, I will also sometimes refer to them as “detritus-wedge polygons”, This nomenclature will emphasize my belief in their common physical origins with ice- and the later discussed sand-wedge polygons.
(a)
(b)
(c)

Examples of detritus-wedge polygons (alligator cracking) on asphalt pavements.

You may legitimately ask why I claim that a correct understanding of the causes of alligator could have profound economic consequences? It may take a number of years for the alligator cracks to induce break-up of the road surface, usually resulting in pot-holes (as shown in Figure (c)). Annual worldwide budgets for repairing asphalt pavements, with a good deal of this it has to be said originating from the development of alligator cracks, involve quite staggering sums. It is clear therefore that any improvements in our understanding of what causes them cannot but be helpful to the task of actually preventing them from occurring and as a consequence reducing this serious drain on national economies.