And some further examples of push-pull failure:

The examples of push-pull motion discussed in the previous blog were the result of differences in material properties in tension and compression. There are other forms of push-pull motion (sometimes referred to as pulsatile motion) that result from some other type of differential material property. For example, many materials exhibit relatively high rates of visco-plastic creep flow at elevated temperatures compared with those at low temperatures. If significant differences occur over the typical temperature ranges experienced by a solid sheet, then again a gradual outward motion can occur.

Figure 5 shows evidence of such motions occurring in asphalt sheets covering essentially horizontal pavement at a location subject to fairly extreme circadian temperature cycles. That these motions occur is often best seen at the outer edges of any rigid constraint that impedes motion; at these locations there is a form of compression pile-up to form ridges. These are often observed to occur together with adjacent tension induced furrows. Sometimes the ridges take the form of an accumulating single large ridge and at other times there is a sequence of many ridge-furrow formations. Each ridge and furrow is generally the result of accumulations of failure occurring over many alternations in temperature. Figure 5(a) captures the clear effects of the gradual motions of the asphalt from right to left around a relatively rigid obstruction in the path of the outward motion. To describe this process as flow could be misinterpreted to imply some form of continuous motion, whereas what appears to take place is a discontinuous motion powered by compression and tension pulses originating from alternations in temperature. It is for this reason that the characterisation “thermal pulsatile motion” has sometimes been preferred.  

                                                                                                                   

Assuming that a given ridge line represents motions occurring over a particular period of time it can be seen that the relatively unimpeded motions between the adjacent obstructions have moved a considerably greater distance forward than have those directly blocked at the upstream edge of the obstacle. On the upstream edge, shown by the corner detail of Figure 5(b), the accumulation of material in the form of multiple ridges is clearly visible. The ridges, or ramparts, result from the compression related shoving action during warming, and may be viewed as forms of incremental, local, elastic-plastic-creep buckling. Between the ridges, the furrows are associated with the tensile action developed during cooling. In some of these furrows there are cracks which at times show signs of long term build-up of detritus. At the upstream corners the motion rates are clearly increasing with distance from the edge of the obstacle. Motion lines around the edge reflect the drag imposed upon the pulsatile motion. Downstream the motion induced ridges and furrows could be viewed as relating to a form of flow separation that might occur in a fluid flow undergoing separation to form a downstream turbulent wake. This can be seen in the detail at a downstream corner, shown lower left of the obstacle in Figure 5(c). It is almost possible to visualize a form of flow separation around this downstream corner and the formation of a stagnation region around the downstream face. As elsewhere the direction of flow is normal to the lines of the ridges and furrows. The ridges and furrows on the upstream corner are shown in greater detail in Figure 5(b).

A second clear example of thermal ratchet motion in asphalt is shown in Figure 6. This depicts a step to a shop in North London which had an asphalt sheet laid over its surface to prevent leaks into a basement area beneath. The top darker section is an essentially horizontal tread of the step and the bottom, lighter, section is the vertical riser. Over a period of years a high proportion of the asphalt from the horizontal surface has been extruded over the lip to form a distinctive tongue having clear down-slope convexity. That alternations of temperature have been powering this motion is again made clear by the bands of ridges and furrows. On the horizontal surface the directions of the asphalt motions feeding this overflow, are orthogonal to the bands of ridge-furrow formations. Comparing this with the ogives in the glacial flow it becomes possible to envisage a very similar form of thermally powered, pulsatile motion, driving each of these processes.

(a)

(b)

(c)

Figure 5: Evidence of a push-pull form of asphalt motion around a  relatively rigid concrete skylight.

Figure 6: A tongue of asphalt extruded from an ostensibly horizontal step tread.

These forms of pulsatile motion due to the differences in material properties when hot and cold have been discussed elsewhere (Croll, J. G.  A.,  Proc. Roy. Soc. Xxx)

One last example before I stop boring you with asphalt pavement failures, is that of Figure 7; this shows an area of asphalt that is experiencing a rather unusual form of alligator cracking. Within most of the central zones of the irregular crack polygons, the asphalt displays an increase in elevation relative to its original position. It is believed that the process responsible for developing this form of high centred detritus-crack polygon is similar to that previously suggested to be responsible for the development of asphalt blisters (blog of March 2010). It will occur when the compressive stresses developed during the warming phase of the temperature cycle are great enough to induce an upward blister buckling. For this to occur the bond between the

 

(a)

(b)

 

Figure 7:  Areas of detritus-wedge polygons with blister uplift deformation occurring within many of the polygons.

 

asphalt sheet and the subgrade would need to be low and the thickness of the asphalt relatively thin. It is interesting to observe in Figure 7(b) an area in the background where a field of incipient blisters is occurring without any evidence of an associated network of discrete thermal cracks.

 

Observations of thermal shoving failures in pavements:

Recent blogs have concentrated upon the cracking failures occurring in asphalt as a result of thermal stresses. But generally if a change in temperature causes tensile cracking failure then a temperature change in the opposite sense is likely to induce compression and a number of associated shoving forms of failure mode. Here are just a few examples where pavements exhibit signs of compression related shoving failures.  

Figure 4(a) shows paving slabs on a footpath in Camden Town, London. Over a period of time the staggered longitudinal cracks between adjacent slabs have been widened due to the opening up of these cracks during cooling, followed by an infill of detritus preventing the cracks from closing. Having started with a few mm of spacing between adjacent slabs many have a year or so later reached widths of more than 25 mm. Temperature increases have resulted in a gradual outward motion of the concrete paving slabs. The outward motion of the slabs would have been even greater were it not for the adjacent asphalt section of the pavement partially preventing this motion. In Figure 4(b) can be seen a ridge of asphalt that has been pushed up by the outward expansion forces from the concrete slabs.

There are situations where the shear transfers between paving slabs are sufficient to induce tensile stresses within alternating slabs great enough to cause a tensile fracture of the slabs. In these cases it is quite common for the cracking to follow a generally straight rather than staggered path.  

 

 

(a)

 

pictures to follow

(b)

Figure 4 : Compression related failures as a result of expansion, (a) outward movement of pavement slabs and associated widening of inter-slab cracks, and (b) the shoving-up of adjacent asphalt to form an asphalt ridge. 

I am currently writing this blog while waiting a decent weather window to allow my sailing on the south coats of Italy to continue. In this part of Calabria there are extremes of temperature. Sadly I do not have a camera to record some of the effects on the railway station platforms. Many of the stations have beautifully tiled surfaces - well they were beautiful before recent increases in temperature forced rows of tiles to pop-up to form rather unsightly ridges – in much the same way as laminate floors are heaved up by temperature or moisture changes inducing compression.      

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 3 shows a concrete manhole surround that was originally embedded integrally into an asphalt layer. Figure 4 shows a steel peg that was 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)

 

  

(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) (d) steel pegs driven into pre-existing asphalt; and (c) 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 5(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.

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.  

picture to follow

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.

 

picture to follow

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]

And now another year has passed – quite unbelievable!

Having been on a bit of a role in relation to ice, detritus and sand wedge polygons last February, a number of events conspired to take my mind from matters related to this blog. The first occurred at the end of February last year when “I Beatrice” an earlier contributor to this blog died fairly suddenly. While I would of course have been saddened by such an event it would normally not have thrown me off course quite so much. But this blogger was very special - for “I Beatrice” was my dear sister Gillian and the last remaining member of the nuclear family within which I grew up in New Zealand. As I write more than 12 months later a persistent lump still comes to the throat.

The second reason why my attention had been so diverted, relates to another little project I have been working on for the past few years and which now seems to be coming to some sort of fruition. As a student I spent one of my summer breaks working as a labourer at the Benmore, Hydroelectricity project being developed near the remote and rather unremarkable little town of Otematata, at the centre of the South Island of New Zealand. Civil Engineering students in NZ were in the middle decades of the 20^th C often supported by bursaries provided by the Ministry of Works. In exchange for maintenance during our studies we were expected to work for the MoW in each of our vacations and for at least a 3 year period following graduation. My Otematata work training period was officially defined as working as a labourer to ensure that future engineers understood a little of what it was like to be grinding the concrete of walls where shuttering had been deficient, laying hot steel bars into fiendishly complex reinforcing schedules for floor slabs, laying post-stressing cables in penstocks, … Part of this particular training at Benmore was spent working within a scaffold gang constructing massive scaffold structures within what would eventually become the spillway gates. Looking back I cringe at the risks being taken with at the time H&S taking a pretty low priority. But having survived you may well be asking what on earth this has to do 50 years later with my neglect of a blog dealing with phenomena such as pingos, asphalt blisters, … Well curiously it has.

For the reason for my distraction has been to develop a new form of scaffold structure based not upon the use of heavy and normally rusty old steel tubes and couplers but upon lightweight and non-corrosive composite tubes and couplers. The last year has been particularly challenging for this project not least because one of the companies responsible for the maintenance of the offshore structures providing oil and gas from the N sea, have been trialling these new products in anticipation of replacing their current reliance upon steel products with those based upon our new composite ones. But perhaps I might provide more on all this in a later blog.