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.