A pavement-quality indicator sits on a stack of simulated asphalt, materials with known dielectric constants.

A pavement-quality indicator sits on a stack of simulated asphalt, materials with known dielectric constants.


Simulation results from Femlab show predicted electric fields in and around the equipment sensing head.

Simulation results from Femlab show predicted electric fields in and around the equipment sensing head.


Engineers at TransTech Systems Inc. hoped an existing capacitor sensor would measure the density of 0.75-in.-thick asphalt with help from a few spacers. But lab tests showed that adding spacers would not produce the required effect. Engineers at the Schenectady-based company (www. transtechsys.com) then simulated the altered sensor with Femlab, a finite-element modeling tool from Comsol Inc., Burlington, Mass. (www.consol.com).

FE modeling agreed with the lab tests in that the spacers would not work. The FEA program, however, also predicted the effect of altering the sensing geometry. It showed that 99% of the density reading would come from less than 0.86 in. of depth, values acceptable to the client. The model's capacitance was calculated using Femlab's sub-domain integration function.

The reconfigured sensor and an impedance analyzer then measured material slabs of known dielectric constant to simulate asphalt. Experimental data agreed well with the simulation.