Fiber-optic traffic classification and weigh-in-motion sensors were developed, constructed, and deployed in Florida. As vehicles pass over the sensors, deformations occur, causing light in the fibers' glass core to be refracted into the fibers' surrounding media. A data-acquisition system records change in the sensors' light intensity and uses computerized algorithms to determine axle presence or weights. Various encapsulation materials were available for securing sensors. Field trials indicated that the stiffness of these materials would greatly affect the sensors' output signal. This laboratory study focused on determining suitable candidate encapsulants on the basis of their elastic modulus. The study was conducted on six candidate encapsulants. Fifty-four sensors 200 mm (8 in.) in length were tested at pneumatic pressures of 665 kPa (95 psi) to evaluate the encapsulants. Polymers with elastic moduli from 3500 to 70 000 kPa (500 to 10,000 psi) were evaluated to determine their effects on the sensors' response in rigid materials. To determine relationships between encapsulant modulus and stress attenuated to the sensor, sensors were placed in the grooved bottom of an aluminum testing block with 12.6-, 25.4-, and 38.0-mm (0.5-, 1.0-, and 1.5-in.) grooves and tested pneumatically in a pressure vessel. Pressure transmitted to the sensors at the various depths was evaluated, showing that as the elastic modulus increased from 3500 to 10 500 kPa (500 to 1,500 psi) there was a significant decrease in the percent stress transferred. Moduli in excess of 10 500 kPa (1,500 psi) yielded minimal increase in percent stress transferred.
Abstract