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This research evaluates a new concept to measure the tensile strains at the bottom of the asphalt concrete layer in flexible pavements. The concept consists of using the Hall Effect sensor in an H-gage configuration to measure the dynamic strains in flexible pavements. The evaluation program included both laboratory and field experiments to evaluate the fundamental and operational properties of the recommended gage design. The laboratory experiment showed that the Hall Effect gage can withstand the temperature and moisture conditions that are encountered during the construction and operation stages of hot mixed asphalt concrete pavements. It also indicated that the dynamic characteristics of the gage are suitable for measuring pavement strains under moving vehicle loads. The field experiment evaluated the in-service characteristics of the Hall Effect gage under a large number of combinations of vehicle speed, axle load, and tire pressure. The field data showed that the Hall Effect gage has good survivability and repeatability and it compares favorably with other strain gages that have been used in flexible pavements. more...

The use of a laser-Doppler anemometer to measure velocities in a fluid with a larger variation in its refractive index perpendicular to the laser beams may introduce appreciable errors. This situation is commonly encountered in convective heat transfer studies. The beams are refracted as they pass through the fluid and the distance of the intersection of the beams, control volume, from the surface may be significantly different than the distance of the beams from the surface when they entered the fluid. If the flow is unsteady or turbulent, the relative movement of the beams may be such that the size and location of the control volume is constantly changing causing appreciable errors. more...