AAT Report, Report to NCHRP Proj 1-42: Top-down Fatique Cracking of Hot-mix Asphalt Layers, May, 2004, pp105.
Bunge, A. A. Naturhistorische beobachtungen und fahrten im Lena-delta, Akad. Imp. Sci. St Petersbourg Bull, 3rd series, 29, 1884, 422-476.
Croll, J. G. A. From asphalt to the Arctic: New insights into thermo-mechanical ratchetting processes, IIIrd Euro Conf on Computational Mechanics, Lisbon, Portugal, 5-8 June, 2006.
Croll, J. G. A. A new hypothesis for for the development of blisters in asphalt pavements, Int J Pavement Engineering, 2007.
Hoque, Z. Chpt 19, in Handbook of Highway Engineering, Taylor Francis Publ., Twa, T. F. (Ed), 2006.
Lachenbruch, A. H. Mechanics of thermal contraction cracks and ice-wedge polygons in permafrost, Geol. Soc. America Spec Paper, 70, 1962, pp69.
Leffingwell, E. de K. Ground-ice wedges, the dominant form of ground-ice on the north coast of Alaska, J of Geology, 23, 1915, 635-654.
Lytton, R L, Uzan, J, Fernando, E G, Roque, R, Hiltunen, D, Stoffels, S. Development and validation of performance prediction models and specifications for asphalt binders and paving mixtures, Report SHRP-A-357, Washington DC, National Research Council, 1963.
Mackay, J. R. Ice-wedge cracks, Garry Island, NWT. Canadian J. Earth Sci., 11, 1974, 1366-1383.
Mackay, J. R. Thermally induced movements in ice-wedge polygons, western Arctic coast: A long term study. Geographie physique et Quarternaire, 54, 2000, 41-68.
MATES, (1988) Michigan Department of Transportation, Flexible pavement distress – part 1, 21, July, 1988.
Matsuna, S and Nishizawa, T. Mechanism of longitudinal surface cracking in asphalt pavements, Proc. 7th Int Conf on Asphalt Pavements, Vol. 2, Nottingham, UK, 1992, 277-291.
Myers, L. R., Roque, R. and Ruth, B. E. Mechanisms of surface initiated longitudinal wheel path cracks in high bituminous pavements, J Association of Asphalt Paving Technologists, 67, 1998, 401-428.
NCHRP, National Cooperative Highways Research Program, Project 1-42, Models for predicting top-down cracking of hot mix asphalt layers, 2004.
Shell, Bitumen Handbook, Thomas Telford Publ., 2003.
Hoque, Z. Chpt 19, appearing in Handbook of Highway Engineering, Taylor Francis Publ., Twa, T. F. (Ed), 2006.
Wamburga, J. H. G., Maina, J. N. and Smith, H. R. Kenya asphaltic materials study, submitted to the Transport Research Board 78th Annual Meeting, Jan., Washington DC, 1999.
Washburn, A. L. Geocryology: A survey of periglacial processes and environments, second edition, Edward Arnold Ltd, London, 1979.
Tuesday 21 February 2012
Sand-wedge Polygons
In the so-called dry valleys of Antarctica, and indeed in other cold arid regions, a variant of ice wedge polygons is often observed. Instead of the wedges forming the periphery of the polygons being gradually filled with ice, they are for the sand wedge polygons gradually filled with wind blown sand. For this reason they are even closer in form and origin to the detritus wedge polygons forming as alligator cracks in asphalt pavements.
I mention these in passing in order that the extremely close analogies in the genesis of all these processes might be appreciated.
I mention these in passing in order that the extremely close analogies in the genesis of all these processes might be appreciated.
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.
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.
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.
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.
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.
Current theories for the formation of alligator crack patterns
Up till fairly recently explanations for the cracking of asphalt pavements were focused upon the stress concentrations that arise from wheel loading. It is widely believed that this cracking has its origins in the effects of heavy localised loading, especially in areas where subgrade supports are either inherently weak or have been weakened by the ingress of water beneath the asphalt sheet (Hoque 2006). Or the cracking could result from a process of fatigue cracking under repeated loading (MATES 1988). In either case the likelihood of cracking is recognised to be increased by any age or strain hardening that might have occurred in the asphalt. It is relatively recent recognition that this cracking is usually initiated from the upper surface of the asphalt that has inspired an increase in research activity to better understand the causes of “top down” cracking.
In a report (AAT 2004) commissioned by the US National Cooperative Highway Research Programme (NCHRP 2004), a review of the literature revealed that the “primary mechanism of top-down cracking” in asphalt pavements is “accumulated damage associated with repeated traffic loading”. This finding reflected the great weight of the reported literature which indicated that it was only “likely that thermal stresses could (also) contribute significantly”. In view of some interesting observations in Kenya (Wamburga et alia 1999), that solar radiation could be a major factor in accelerating surface age hardening which in turn could contribute to top-down cracking, surprise was expressed that “this mode of distress has not been acknowledged earlier” (AAT 2004). Indirect evidence of the potential importance of solar radiation had been given in a Japanese study (Matsuno et al 1992) reporting that in pavements experiencing top-down cracking this cracking was generally absent where the pavements were shaded by overpasses. Others agree that there appears to be a thermal stress component to top down cracking (Lytton et alia 1993; Myers et alia 2001). However, despite the growing evidence to the contrary, the dominant weight of opinion remains that cracking, and alligator cracking in particular, is generally caused by repeated traffic loading.
In a report (AAT 2004) commissioned by the US National Cooperative Highway Research Programme (NCHRP 2004), a review of the literature revealed that the “primary mechanism of top-down cracking” in asphalt pavements is “accumulated damage associated with repeated traffic loading”. This finding reflected the great weight of the reported literature which indicated that it was only “likely that thermal stresses could (also) contribute significantly”. In view of some interesting observations in Kenya (Wamburga et alia 1999), that solar radiation could be a major factor in accelerating surface age hardening which in turn could contribute to top-down cracking, surprise was expressed that “this mode of distress has not been acknowledged earlier” (AAT 2004). Indirect evidence of the potential importance of solar radiation had been given in a Japanese study (Matsuno et al 1992) reporting that in pavements experiencing top-down cracking this cracking was generally absent where the pavements were shaded by overpasses. Others agree that there appears to be a thermal stress component to top down cracking (Lytton et alia 1993; Myers et alia 2001). However, despite the growing evidence to the contrary, the dominant weight of opinion remains that cracking, and alligator cracking in particular, is generally caused by repeated traffic loading.
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.
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.
Monday 20 February 2012
Where was I?
I am horrified to find that it has been nearly 2 years since my last entry on the blog. To the vast numbers of you avidly following this blog (sic) my apologies, but it has been a busy 2 years in areas that perhaps I might at some time in the future have a chance to elaborate.
My last posting dealt with the difficulties experienced by Henry Moseley in having accepted his ideas on what could and could not be powering the motion of glacial ice, and how through a weird echo I found myself quite independently putting forward a very similar explanation for at least some forms of glacial motion some 150 years later. There is I am sure a lot more to be said on this subject, and I am sure to return in later postings to some of the issues raised. But for the moment I would like to turn to some of the events which encouraged another of my little excursions into the realms pavement engineering.
Soon after my initiation into the subject of pingos I started what has been a lengthy correspondence with Professor Ross Mackay, undoubtedly the world’s foremost authority on the subject of pingos (see some of the earlier postings). I cannot recall now whether it was his suggestion or whether the subsequent correspondence with Professor Chris Burn (an ex graduate student of Ross) was initiated independently. But one of the useful aspects to have emerged from the correspondence with Chris was his suggestion that as an analogue for the thermal ratchet model I had been arguing to form part of the thermal buckling explanation for the emergence of pingos, I would do well to look at the literature dealing with ice-wedge polygons. Again, this was a subject with which I was not at all familiar. But a quick reading of the fabulous book "geocryology” by Professor Washburn [8] soon made it apparent that there was indeed a direct link. For those of you who like me at the time are unfamiliar with what ice-wedge polygons are I have included a few pictures below. There genesis is
(a)
(b)
(c)
(d)
Examples of ice-wedge polygons in permafrost regions. Note the relic formation in (c).
The process whereby over periods measured in millennia – indeed over glacial periods – the ice wedges grow to become as wide as a few metres in width at the surface, is roughly as follows. During the extreme cooling of late autumn through to mid-winter the upper few meters of permafrost will undergo cooling. The restraint of the contraction that would otherwise take place when cooled (and permafrost has a very high coefficient of thermal expansion so the thermal strains are really quite high) induces high horizontal tensile stresses. Well they would were it not for the fact that permafrost like ice has a low tensile strength so that it cracks at a fairly early stage of the cooling. Moisture getting into these cracks quickly turns to ice. So that when the ice is warmed from early spring the cracks do not close, but instead there is left an additional vertical slither of ice with thickness variation vertically that reflects the original crack width. Over many years this repeated cycle results in the gradual widening of the ice wedge.
The above pictures show a few examples of both current and relic ice-wedge polygons. Current ice-wedge polygons often show-up as distinct contrasts in colour, sometimes as a result of the differences in vegetation supported by the very different ground conditions within the wedge areas compared with the rest of the ground. They, but especially the relic forms, also show up as differences in elevation between the central regions and the polygonal boundaries. For a more complete account of the different forms taken by active and relic ice-wedge polygons see for example [8,9,17]. While the permafrost literature is relatively silent on the subject, it would seem that many of the differences in elevation of the ground within the ice-wedge polygons could be accounted for in terms of the variety of compression related failures that occur during the constrained expansion that accompanies the subsequent warming.
As I have been arguing as the basis of a possible genesis of pingos it has even been suggested that these compressions are capable of causing the upheaval failure into the characteristic dome shaped mode. There is some evidence that at the rather smaller scale of typical ice-wedge polygons a similar form of general buckling uplift could be occurring. Washburn [8] reports cases where massive ground ice, in the form of lenses, exist beneath the centres of ice-wedge polygons. This would be consistent with a form of seasonally induced compressive uplift buckling at wavelengths consistent with the inter-crack spacing. In other cases there appears to be an accumulation of material either side of the troughs within which the ice-wedges occur. These so called “ramparts” have all the characteristics of a local form of compression related shoving failure, much like that forming when a rug on a smooth floor is pushed up against a wall. They would appear to have origins in the shoving action of the seasonally frozen, or even the perennially frozen, ground as it expands during warming. This outward expansion resulting in an effective outward transport of material is similar to the development of ice shove ridges around the edges of lake-ice [22,23,24]. Just how the compressive energy is released during the warming phase is complicated by the interactions between the permafrost and the overlying “active layer “ – the active layer being the soil above the top of the permanently frozen ground that is only seasonally frozen. Excellent long-term field evidence of this cyclical outward motion of the active layer is summarised by Mackay [17]. The mechanism at work in this outward mass movement would appear to be closely related to that recently hypothesised as being responsible for the mass movements involved in the formation of sorted and non-sorted, stone and rock, circles, polygons, nets etc [25]. There appear to be few explicit references to this form of compression related failure accompanying the release of compression energy generated when restrained expansion occurs during warming.
So while temperature induced fluctuations in tensile strain are widely used to account for the development of the ice-wedges of ice-wedge polygons, curiously, there appears to be relatively little acknowledgement that compressive straining during warming might account for some of the other observed features of ice-wedge polygons.
Anyway, the reason for this diversion into ice wedge polygons is by way of introduction to a return in my next posting to asphalt in order to explore whether similar processes might be responsible for what is one of the most common and costly forms of asphalt pavement failure – that of alligator (or in N America, crocodile) cracking.
My last posting dealt with the difficulties experienced by Henry Moseley in having accepted his ideas on what could and could not be powering the motion of glacial ice, and how through a weird echo I found myself quite independently putting forward a very similar explanation for at least some forms of glacial motion some 150 years later. There is I am sure a lot more to be said on this subject, and I am sure to return in later postings to some of the issues raised. But for the moment I would like to turn to some of the events which encouraged another of my little excursions into the realms pavement engineering.
Soon after my initiation into the subject of pingos I started what has been a lengthy correspondence with Professor Ross Mackay, undoubtedly the world’s foremost authority on the subject of pingos (see some of the earlier postings). I cannot recall now whether it was his suggestion or whether the subsequent correspondence with Professor Chris Burn (an ex graduate student of Ross) was initiated independently. But one of the useful aspects to have emerged from the correspondence with Chris was his suggestion that as an analogue for the thermal ratchet model I had been arguing to form part of the thermal buckling explanation for the emergence of pingos, I would do well to look at the literature dealing with ice-wedge polygons. Again, this was a subject with which I was not at all familiar. But a quick reading of the fabulous book "geocryology” by Professor Washburn [8] soon made it apparent that there was indeed a direct link. For those of you who like me at the time are unfamiliar with what ice-wedge polygons are I have included a few pictures below. There genesis is
(a)
(b)
(c)
(d)
fairly well established as being due to cycles of warming and cooling of the upper few metres of permafrost as a result of the annual seasonal cycles of surface temperature. Initial formation of basically discrete ice-wedge cracks within permafrost is fairly quickly (at least in geological timescales) followed by the growth of bifurcated cracks that start to link-up with other discrete cracks and their branches [17-19]. Eventually, networks of polygonal cracks will form, that for fairly self-evident reasons are often referred to as “ice-wedge polygons”. Ice-wedge polygons are ubiquitous in current regions of permafrost. They are also common in relic form in areas that experienced permafrost conditions during some past extended period of colder climate; such a relic formation is shown in (c) above. Like pingos there is considerable interest in the geomorphic relics of ice-wedge polygons that remain after the retreat of permafrost. They provide significant indicators as to the extent and the intensity of previous fluctuations in climate. The existence of relics as shown in picture (c) demonstrates the distance reached by permafrost conditions during the most recent glacial period – or even an earlier glacial period if it has not been wiped-out by the advance of subsequent and even more extensive ice sheets. The characteristic diameter of the random, but in an averaged sense, polygonal shapes, indicates the intensity of the seasonal variations in temperature experienced at that latitude. It would seem that for a given form of ground material conditions the depth to which the thermal wave extends into the ground has a strong determining influence on the distance between adjacent cracks. For anyone brought up on St Venant’s theorem within the subject of elasticity this relationship between the depth of stress relief due to cracking and distance to which this localize stress disturbance propagates would make it clear as to why the crack separation is directly linked to the crack depth. And of course the depth of the crack has a direct relationship to the depth of penetration of the thermal disturbance which in its turn relates to the severity of the maximums and minimum seasonal temperatures. So that the study of the morphologies of crack wedge polygons has taken on great significance in attempting to understand palaeoclimates, both on earth and some of the other planets and their moons. For this reason there is growing interest in the research being carried out on the growth and decline of ice-wedge polygons.
The process whereby over periods measured in millennia – indeed over glacial periods – the ice wedges grow to become as wide as a few metres in width at the surface, is roughly as follows. During the extreme cooling of late autumn through to mid-winter the upper few meters of permafrost will undergo cooling. The restraint of the contraction that would otherwise take place when cooled (and permafrost has a very high coefficient of thermal expansion so the thermal strains are really quite high) induces high horizontal tensile stresses. Well they would were it not for the fact that permafrost like ice has a low tensile strength so that it cracks at a fairly early stage of the cooling. Moisture getting into these cracks quickly turns to ice. So that when the ice is warmed from early spring the cracks do not close, but instead there is left an additional vertical slither of ice with thickness variation vertically that reflects the original crack width. Over many years this repeated cycle results in the gradual widening of the ice wedge.
The above pictures show a few examples of both current and relic ice-wedge polygons. Current ice-wedge polygons often show-up as distinct contrasts in colour, sometimes as a result of the differences in vegetation supported by the very different ground conditions within the wedge areas compared with the rest of the ground. They, but especially the relic forms, also show up as differences in elevation between the central regions and the polygonal boundaries. For a more complete account of the different forms taken by active and relic ice-wedge polygons see for example [8,9,17]. While the permafrost literature is relatively silent on the subject, it would seem that many of the differences in elevation of the ground within the ice-wedge polygons could be accounted for in terms of the variety of compression related failures that occur during the constrained expansion that accompanies the subsequent warming.
As I have been arguing as the basis of a possible genesis of pingos it has even been suggested that these compressions are capable of causing the upheaval failure into the characteristic dome shaped mode. There is some evidence that at the rather smaller scale of typical ice-wedge polygons a similar form of general buckling uplift could be occurring. Washburn [8] reports cases where massive ground ice, in the form of lenses, exist beneath the centres of ice-wedge polygons. This would be consistent with a form of seasonally induced compressive uplift buckling at wavelengths consistent with the inter-crack spacing. In other cases there appears to be an accumulation of material either side of the troughs within which the ice-wedges occur. These so called “ramparts” have all the characteristics of a local form of compression related shoving failure, much like that forming when a rug on a smooth floor is pushed up against a wall. They would appear to have origins in the shoving action of the seasonally frozen, or even the perennially frozen, ground as it expands during warming. This outward expansion resulting in an effective outward transport of material is similar to the development of ice shove ridges around the edges of lake-ice [22,23,24]. Just how the compressive energy is released during the warming phase is complicated by the interactions between the permafrost and the overlying “active layer “ – the active layer being the soil above the top of the permanently frozen ground that is only seasonally frozen. Excellent long-term field evidence of this cyclical outward motion of the active layer is summarised by Mackay [17]. The mechanism at work in this outward mass movement would appear to be closely related to that recently hypothesised as being responsible for the mass movements involved in the formation of sorted and non-sorted, stone and rock, circles, polygons, nets etc [25]. There appear to be few explicit references to this form of compression related failure accompanying the release of compression energy generated when restrained expansion occurs during warming.
So while temperature induced fluctuations in tensile strain are widely used to account for the development of the ice-wedges of ice-wedge polygons, curiously, there appears to be relatively little acknowledgement that compressive straining during warming might account for some of the other observed features of ice-wedge polygons.
Anyway, the reason for this diversion into ice wedge polygons is by way of introduction to a return in my next posting to asphalt in order to explore whether similar processes might be responsible for what is one of the most common and costly forms of asphalt pavement failure – that of alligator (or in N America, crocodile) cracking.
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