Some more observations of thermal cracking of pavements:

Another source of thermal ratchet induced cracking seems to be inhomogeneity of thermal properties either within the asphalt mix or with respect to any inclusions within the asphalt layer. Figure 3a shows a concrete manhole surround that was originally embedded integrally into an asphalt layer. Figure 3b and 3c show steel pegs that were driven into an existing asphalt layer. Over a period of years some intriguing thermal ratchet stress cracking has developed. As a consequence of the high thermal expansion coefficient of the asphalt, and especially the bituminous binder, relative to the concrete or steel intrusion, any heating will result in the holes in the asphalt trying to pull away from the relatively stiff inclusions. Associated thermal tension stresses normal to these boundaries have eventually led to cracks between the stiff inclusions and the asphalt.

The loss of the thermally induced tension normal to these boundaries means that subsequent increases in temperature will have no restraining tension normal to the boundary developed in the asphalt. This release of tension could be thought of as adding to the original stress state an outward compression stress around the periphery which will develop large tension stresses parallel to the boundary. Associated thermal tension stresses parallel to the boundaries have induced the cracks radiating from the corners of the square inclusion and at regular intervals around the circular inclusion. As previously discussed, detritus entering these cracks and those parallel with the boundary will prevent them from fully closing when the asphalt is cooled. Figure 3(c) shows a variant in which binder has selectively

 (a)

(b)

 

 

(c)

 

To follow

  

(d)

Figure 3: Examples of thermally induced cracks caused by differential thermal expansion coefficients between asphalt layer and an inclusion consisting of: (a) a concrete manhole; (b) (c) steel pegs driven into pre-existing asphalt; and (d) a hole drilled into a pre-existing asphalt layer.

 

leeched into the stress cracks to form an intricate pattern within the asphalt. An extreme example of the effects of this form of thermal inhomogeneity is shown in Figure 3(d); here a hole drilled into the original asphalt layer can be seen to have induced seemingly disproportionate crack consequences.  Although it has not been possible to produce photographs, the writer has also observed at microscopic levels similar crack patterns developing around aggregates used in some asphalts. Could the differential expansion coefficients between the binder and the aggregates be an initiating factor in many forms of asphalt cracking? And could the ingress of detritus preventing the closure of these micro-cracks be a factor in their development and eventual propagation into fully formed thermal ratchet crack patterns? These are all intriguing questions the answers to which are beyond the scope of this blog; they would though appear to be worthy of more detailed research.