Capacitance-based stress self-sensing in asphalt without electrically conductive constituents, with relevance to smart pavements

The capacitance-based stress self-sensing in asphalt without device or functional constituent incorporation is reported for the first time, with relevance to smart pavements. Stress sensing enables weighing. Without the incorporation, the technology is applicable to existing and new pavements, and gives low cost, high durability, large sensing volume and maintained mechanical properties. Linearity of the capacitance with the normal compressive stress occurs for stress ranging from 2 Pa to 60 Pa. The ability to sense such low stress enables the sensing of even children. Nonlinearity at > 60 Pa involves the slope of the capacitance-stress curve decreasing. Monotonic and reversible capacitance increase with stress occurs for stress <= 600 Pa, with complete reversibility up to stress >= 2100 Pa. The electrodes are aluminum foil bonded to the asphalt with a thin bitumen interlayer. With coplanar electrodes (the stress not applied to the electrodes), the in-plane capacitance increases upon compression, presumably due to the compaction of the part of the asphalt near the plane of the coplanar electrodes. With sandwiching electrodes (the stress applied to the electrodes), the through-thickness capacitance increases reversibly upon compression, due to the decrease in the thickness of the bitumen interlayer. The coplanar electrodes enable self-sensing, and, compared to the sandwiching electrodes, they give much greater sensitivity and are more amenable to practical implementation. The sensing is based on the fractional change in capacitance, so the thickness of the asphalt or bitumen interlayer does not matter. The capacitance-based stress sensing is superior for asphalt than cement-based materials of prior work, due to the greater stiffness of the latter.