Some further comments on pavement cracking.

One of the purposes of the recent blogs has been to suggest that alternations in solar radiation may be contributing to what is one of the most common forms of asphalt pavement failure – alligator cracking. It has been argued that certain forms of asphalt become relatively brittle at low temperatures, and especially where age, strain or work hardening has been experienced, or moisture related leaching or solar embrittlement, have occurred. During the cooling phase restrained contraction will induce tension stresses which can often cause cracking. Due to ingress of detritus, cracks may not be closed during the subsequent heating phase. Over many repetitive cycles the cracks are observed to open up and propagate to eventually form well defined alligator crack patterns, in a process that could be regarded as thermal ratchetting. This process would appear to be responsible for crack patterns other than alligator cracking. Furthermore, low temperature embrittlement would seem to be not the only cause of thermal ratchet induced cracking.

Spatial inhomogeneity in levels of solar radiation experienced by the asphalt can often result in differential thermal stresses that sometimes induce cracking. This can be caused by the use of light coloured road markings that reflect a greater proportion of the incoming radiation than the adjacent black surfaced asphalt, often resulting in wide cracks adjacent to the edges of the road markings. An example taken from my cycle route into work allowed long term observations on the growth of such crack formations. A section of this cracking is shown in Figure 1. While the yellow lines have started to fade, the differences in solar reflection over time have meant that the areas beneath the road marking experienced different thermal conditions to the rest of the road surface. On very cold days these cracks could be as wide as 10mm. Eventually the cracks started to affect the integrity of the road surface and a new one was laid.  

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Figure 1: cracking along edge of road marking caused by differential solar reflection.

During a lively discussion following presentation at in International Conference of results like those discussed above, and also in some previous blogs, my attention was drawn to another way in which non-homogenous insolation can cause pavement cracking. If some insulating material is laid upon an asphalt surface it can change the temperatures beneath the insulation compared with that of the adjacent road surface. This too can cause differential stresses that may result in cracking. Figure 2, shows a fascinating example of cracks due to this cause. Over a period of weeks a pad of fairly well indurated horse manure was observed in a quiet lane in Devon to eventually result in cracks developing around the periphery of the horse pad (this is a polite description of a pile of horse dung). In this case the horse pad has provided a localised difference in the level of heating and cooling induced in the asphalt. It is apparent that the cracks again preferentially form at the boundaries between the aggregate stones and the bituminous binder.

 

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Figure 2: Development of peripheral cracking around a pad of horse manure caused by differential insulation to heating of the asphalt. [Photo courtesy of Dr E A W Maunder]