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.
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.
Been waiting for 3 years for these photos! Any chance they could be added?
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