Why might current theories be incomplete?

While local overloading leading to a form of localised flexural failure could undoubtedly be a contributing cause to many alligator crack patterns, there are strong indications that it cannot be the only reason, or, indeed, even the major reason. The first indication that some other cause may be at work is the sequence in which the crack patterns typically form. Recent observations have revealed that just like permafrost polygonal crack formation, the first sign of an incipient alligator crack pattern is often a largely linear central crack. Gradually, secondary cracks begin to bifurcate from this irregular central crack. These in turn bifurcate into tertiary cracks, etc. Eventually the cracks spread out to form an essentially oval region with the defining outer circumferential crack often being one of the last to form.

Were the cause of the alligator cracks to be a local patch overload, the cracks would reflect a largely bending form of failure. In these circumstances the first cracks would more likely be visible around the periphery of the failing region where flexural cracks would be evident on the top surface. Cracking in the central region, associated with tensile bending stresses would at least initially be on the bottom face of the asphalt, and would not become visible from above until the failure had reached a very advanced stage. Cracking on the top face over the central regions would become apparent only when significant tensile membrane action had developed. This would not begin until deformations had become large and would be associated with a fairly advanced stage in the growth of the failure pattern. This does not appear to be the case. What is more, a load related flexural failure mode would most likely exhibit bottom-up radial cracks emanating from the location of the local over-loading, connected by largely orthogonal rings of flexural failure. Any top-down cracking would at the early stages be more prominent at the outer regions of the crack network. Again this does not appear to be the case.

Over the past few years I have been observing the development of alligator cracking in a number of areas of newly laid asphalt. In many cases these observations are in places where no overload could possibly have occurred by virtue of the fact that it is physically impossible for heavy vehicles to reach the areas within which the alligator cracking has been developing. Even repeated loading well below that required to induce failure would have been impossible.

Largely linear cracks often initiating bifurcations into polygonal forms

(cannot figure out how to rotate!) 

Examples of alligator cracking on road asphalt pavement

(1p 20mm diameter coiun for scale)

Examples of alligator cracking on asphalt footpaths

Proliferation of the cracked region is also often inconsistent with an explanation based upon local static or fatigue overload. As noted above, it has often been observed that cracking grows outwardly from the initial, essentially longitudinal, cracks (see typical example above). Even within the areas of these initial cracks the crack widths tend to be greatest at the top surface. Once the crack network has been largely developed it is found that the cracks continue to open-up through the thickness of the asphalt. Furthermore, this gradual surface widening of the cracks is accompanied by the accumulation of detritus within the cracks, most likely resulting from the settling of the surface dirt being washed into the puddles that tend to occur in the regions developing alligator cracking. This is particularly apparent shortly after rain when the asphalt between cracks has dried but the detritus within the cracks remains wet. Some of the above photographs were taken a short time after rain to enable the wet and therefore darkened cracks to stand out more clearly. None of this is really consistent with a local overloading explanation of their origin.