For the last few years I been exploring (in it has to be said a rather dilatory manner) the possibility that the upward growth of characteristic blisters in asphalt and other materials might involve a form of two dimensional, thermal buckling. This envisages the working of a thermal ratchet process powered by fluctuations in the solar energy reaching the earth’s surface. For the case of the asphalt pavement blisters the alternations in hot and cold seem to involve timescales of circadian periodicities, or even less, with wavelengths characterising the thermal buckles that are of the order of a few centimetres.
This newly proposed explanation for the growth of blisters suggests that each time the heat input is sufficient to raise the average temperature through the thickness of the asphalt above the critical level required to induce enough compressive stress to power an increment in buckling deformation, there appears to be a small upward growth of the blister. That this upward growth is not fully recovered on subsequent cooling seems to be the result of differential properties of the asphalt when it is hot compared with when it is cold. While the laterally restrained asphalt is hot and under a state of compression any incremental upheaval buckling will be accompanied by high levels of creep deformation - meaning that the asphalt will continue to deform without the addition of further load. But when the temperature drops the asphalt has much higher elasto-visco-plastic stiffness, which means there will be less creep recovery when the compression is removed. The result is that not all the upward growth of the buckle blister will be recovered when the asphalt cools. There is growing evidence to support the idea that a gradually accumulating level of upward buckling deformation will be experienced. Each time the surface temperature is raised to a high enough level, for a time sufficient for the thermal wave to penetrate the thickness of the asphalt, another increment of uplift deformation will be experienced.
In giving talks about the mechanics underlying this process I have sometimes used the simple little model shown below. This consists of a silicon rubber sheet just a few mm thick, cast within a 150mm diameter, relatively stiff, circular jubilee clip around the circumference. A series of parallel black lines are drawn on the surface to enable simple visualisation of any subsequent deformations. To provide a heat source a standard reading lamp is placed a few cm above the disc of silicon and the lamp turned on; the base of this lamp can be seen behind the disc. It normally takes about 20 minutes for the temperature to reach the level required for the compression resulting from the constrained expansion to induce a sudden uplift buckle. Inevitably someone in the audience will witness the moment when the originally flat disc pops-up into the buckle configuration shown in the second photo. I have to say I have yet to witness the moment of the upheaval buckle during a talk.
One of the attractive features for this thermal upheaval buckling providing the explanation of the formation of asphalt blisters is that a heavy sheet when subject to an isotropic in-plane membrane stress (that is the forces developed would be the same in all directions) will have a preferred buckling mode that takes the form of a circular dome, like those shown in my previous blog. The stress required to produce this axisymmetric buckle mode is considerably lower than that required to produce other buckle shapes. This certainly conforms to the observation that most of the uplift blisters take this form. Furthermore, theory predicts that there will be a direct relationship between the amplitude of the buckle and the diameter of the characteristic buckle dome. In other words the thermal buckle hypothesis overcomes all the objections to gas pressure being the cause. And of course the growth would be barely affected by a tiny hole drilled through the thickness of the dome!
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