Alligator cracking as a form of thermal ratchet process

Characteristic dimensions of the polygonal shapes that emerge to form typical alligator crack patterns have a close relationship with the thickness of the asphalt sheet. These dimensions also seem to depend upon the shear strength between the base of the asphalt and its underlying sub-grade. As in permafrost, where the asphalt is relatively thick, the characteristic dimensions of the asphalt polygons also appear to relate to the depth of penetration of the thermal waves into the asphalt. All of this argues against the growth of these forms of asphalt failure being attributable to the effects of local overloading [21,26].

But while there appear to be many reasons for supposing that flexural overload is generally not the cause of alligator cracking, there are compelling reasons for suggesting the origins to so often be in some form of thermal cracking processes. Rates of formation tend to be at their greatest during periods of extremes in temperature variation. During spring to early summer there are many large variations in temperature. Hot days are often followed by extremely cold nights. There are more frequent alternations in intense sunlight followed by heavy cloud cover. It is during these periods of frequent and extreme variations of temperature that growth of alligator crack patterns seems to be at its greatest. A factor that is sometimes referred to in support of a load related cause is the correspondence between areas of alligator cracking and areas of depression. It is certainly the case that alligator cracking is usually associated with areas of relatively low elevation. However, this correspondence could have another equally plausible explanation. It is in these low lying areas where puddles occur following rain. This will have a number of effects that could be conducive to crack growth. Runoff from surrounding asphalt into these, low lying, areas will mean that surface deposits will accumulate in the depressions. Any cracks will tend to fill with this detritus inhibiting their closure when temperatures rise. Longer than average exposure to water will mean that the asphalt in these areas will be more prone to leaching-out of some of the volatile substances that give the asphalt its ductility. Concentration of the thermal processes associated with latent heat transfer and evaporation in these puddle areas, is likely to accelerate these leaching processes. Relative to the surrounding asphalt, that in the puddle areas will be much more prone to becoming increasingly brittle. Tensile strains caused by restrained thermal contraction are consequently most likely to be relieved of their strain energy through selective crack formation in these relatively brittle areas. Once the cracks form, water will be preferentially leaked through the asphalt layer into the possibly moisture sensitive ground beneath. This will be especially the case if these cracks were to penetrate through the asphalt thickness, which they would if their origin were to be in the membrane tensions caused by thermal contraction. It would be less likely if the cracks had a flexural origin where the cracks may only partially penetrate the asphalt thickness. Water reaching the ground beneath the asphalt layer would have the effect that even small loads would cause any pre-existing depression to be further depressed.

Apart from this evidence, the geometry of the crack networks is much more consistent with a thermal shrinkage origin than with a flexural failure pattern caused by local overload. It seems no accident that the forms of ice- and detritus-wedge polygons are so often similar. They would appear to have common cause. Indeed, at even larger scales some of the surface features being recorded on the surface of Mars [27,28], and some of the other planets and larger satellites within the solar system, also appear to have their origins in fluctuations of insolation levels reaching their surfaces. The sizes of these polygons reflect the depth of penetration of the temperature waves, that depend in turn upon the periodicities of the thermal fluctuations, that of course in their turn depend upon the orbital characteristics of the planets with respect to the sun. Surface features reported on Mars [27,28] exhibit a number of well defined clusters in the average diameters of the polygons. These dimensional clusters suggest that thermal cycles of different periodicities could be at work.

There have even been suggestions that some of the tectonic cracking within the Earth’s lithosphere could have as its origins in the thermal variations occurring at periodicities of tens to hundreds of thousand years – the same time-scales that drive the periodic glacial-interglacial cycles [29]. In this latter case it would not be ice or detritus that prevents the cracks from closing during the subsequent warming phase. Molten magma from beneath the lithosphere would be extruded into and extended from these crust fractures, solidified and subsequently compressed to form basaltic and igneous rock. In this way new crust is borne and old crust compressed to form mountain ranges or ocean trenches. Over even longer time periods processes similar to those responsible for the formation of pingos could be at work in the uplifting of crust to form new continental land mass or thrusting it down to form new ocean floor [29]. But all this is a story for later postings.


There are grounds for hypothesising that a form of thermal ratchet process is a much more likely explanation for the development of many forms of alligator cracking, involving as it does the growth of top-down cracking. When the temperature is dropped a thin sheet of asphalt will develop high levels of tension as a result of the constraint to the thermal contraction wanting to occur. Why these tensions selectively start to cause cracking in the areas eventually to form alligator crack patterns is not fully understood. But one possible explanation would be that the areas of asphalt experiencing high levels of traffic loading are more likely to suffer stress and age related strain hardening. Another factor where high loading is not present, could be that the areas of slightly lower than average elevation will attract puddles after each shower of rain. The thermo-chemical processes associated with the eventual evaporation of this puddle water would result in an accelerated ageing of the bituminous binders and a leaching away of certain volatile substances responsible for asphalt retaining its ductile properties. Each of these factors would result in a form of localised asphalt embrittlement so that contraction induced tensile stressing would produce selective cracking in these areas. During the warming phase the asphalt would try to expand. Being generally restrained from doing so compressive stresses will develop. Even within the newly cracked areas any detritus accumulating within the cracks would tend to prevent the cracks from fully closing. Over repeated heating and cooling cycles the detritus within the cracks would be consolidated so that the cracking occurring when temperatures drop, perhaps during the next wet period, will cause the cracks to reopen and attract new detritus deposits when the puddles eventually evaporate. Once cracking has started any water ingress beneath the asphalt would weaken the subsoil, tending to increase the depression and accelerate the cyclic processes outlined above. Where sub-zero temperatures occur regularly, low temperature embrittlement could also be a major factor. Because of the effects from ice, and especially the cryostatic pressures associated with phase change, there will be a whole new range of thermo-mechanical phenomena introduced. For this reason the following focuses on processes where ice is not necessarily involved.

Before moving on to a more detailed description of how alligator cracking could be propagating a few brief comments upon a possible mechanism for the initiation of the cracks would seem to be in order. I have observed the cracks very frequently first start at the interfaces between the aggregates (stones) and the bituminous binders that make up the asphalt. At the early stages of the crack growth these are often barely visible without the use of some magnification. But over time the micro-cracks at the edges of individual stones begin to widen and to propagate through the binder to the nearest adjacent stone. Eventually, a jagged but at macroscopic level essentially linear crack starts to appear. Now the bitumen binder generally has a very high coefficient of linear expansion compared with the rock making up the aggregate. This means that an increase in temperature would if there were no bond between the aggregate and the binder see the hole in the binder expanding in size much more than the stone occupying that hole. To bring them back into a compatable state in which they are still bound together would require some fairly high tensile and shear stresses to develop at and near the interface. With the jagged nature of the interface some serious stress concentrations would seem to be very likely. So it is perhaps not surprising that it is at these locations of hight htermal stress concentration that cracks start to initiate. The picture below shows fairly well established cracks following the jagged paths between adjacent stones and binder boundaries.   



Cracks often follow the jagged stone/bitumen boundaries

But let me return to the mechanism here being suggested to be responsible for the development of macroscopic asphalt cracking and eventually the patterns known as alligator cracking. Figure 1 attempts to summarise the above thermal ratchet process in a situation where the cracking has not yet penetrated the full thickness of the asphalt layer. When the temperature is lowered the asphalt will develop in-plane tensile stresses whose depth profile will reflect the contraction strains resulting from the thermal wave. The attenuation of the thermal wave with depth will itself be governed by the heat conduction properties of the asphalt and, crucially, the period of the thermal cycle. At a potential crack location this relationship between tensile stress and temperature gradient would be similar to that depicted in figure 1(a). At some location a tensile crack may eventually form, figure 1(b), possibly following a particularly low temperature excursion, indicated by point ‘b’ in figure 1(g).

Crack initiation is often observed to occur at the aggregate binder interfaces. Due to the very large differences in their coefficients of thermal expansion temperature induced stresses will generally be concentrated at these interfaces, making it likely that it will be at these locations any thermal fatigue effects would be at their greatest. Over repeated thermal cycles these interfacial micro-cracks are observed to eventually join-up with one another to propagate into a form of essentially linear crack. Once this has occurred a low temperature excursion will be accompanied by a redistribution of stress, similar to that shown in figure 1(b). The horizontal distance, hc, from the single crack at which the in-plane stress is sufficient to again induce a discrete crack will be largely governed by the depth of penetration of the crack, dc, which in its turn depends upon the propagation depth of the thermal wave, dt. How these pairs of cracks associate with other pairs to form the various preferred polygonal patterns is an intriguing subject in its own right, but not one into which present space allows further digression. Similar behaviour has been described for the larger scale, ice-wedge polygon crack formations in regions of permafrost (Lachenbruch 1962). When the geometry of the polygonal network has been defined a low temperature excursion will result in inwards radial deformations within each polygon, with any residual resistance to further inward contraction being resisted by shear stresses developing at the lower asphalt depths, as indicated in figure 1(c). Detritus washing into the cracks, shown in figure 1(c), will provide increasing resistance to outward expansion as the detritus becomes consolidated during periods of warming, figure 1(d). During this phase there will be some shear reversal at the lower boundary. The next cooling cycle will see reduced resistance to the inward contraction related deformation, figure 1(e), and early cracking of the consolidated detritus within the previous crack. Depending upon weather conditions there may be many thermal cycles in the course of a single day to cause a repetition of this process. But it is likely to be the thermal waves induced by longer term, circadian or greater time periods, that will cause the material breakdown at depth and eventually the break-up of the surface layers.

Figure 2 suggests how a similar process could be responsible for the break-up of asphalt used for wear layers or the thinner sheets used on footpaths. At a potential crack location the asphalt develops tensile stress, figure 1(a), when the contraction strains are resisted during a period of temperature reduction. Tensile cracks may eventually form, figure 2(b), possibly following a particularly low temperature excursion, indicated by point ‘b’ in figure 2(g). An inward radial deformation will result, with any residual resistance to further inward contraction being transferred into shear stress development at the lower interface with the subgrade, particularly near the boundary crack. Detritus washing into the cracks, shown in figure 2(c), will provide increasing resistance to outward expansion as the detritus becomes consolidated during periods of warming, figure 2(d). During this phase there will be some shear reversal at the lower boundary. The next cooling cycle will see reduced resistance to the inward contraction related deformation, figure 2(e), and early cracking of the consolidated detritus within the previous crack. Again, depending upon weather conditions there may be many thermal cycles in the course of a single day to cause a repetition of this process. After many repetitions it might be anticipated that a breakdown of base shear resistance will occur near the crack. Subsequent cycles will see a shift in the zone of high shear towards the centre of the asphalt polygon as suggested in figure 2(f). Eventually this will cause a debonding of the asphalt polygon to the subgrade with the result that the polygonal blocks will break loose. When this starts to occur a rapid breaking-up of the integrity of the crack network will result, with individual blocks eventually being physically removed from the area.

Figure 3(a) and (b) show areas of alligator cracking at various stages of break-up. Within the area shown in figure 3(b) a new layer of asphalt has been laid over an area that had previously suffered serious alligator cracking. Within a very short time this newly laid asphalt also started to show a reflective crack pattern that followed those previously developed within the older, underlying, asphalt layer. An equivalent process within ice-wedges has been termed ‘syngenetic ice-wedges’ (Mackay 2000). It would appear that human created syngenetic detritus-wedges can occur in asphalt.