The Benkelman Beam was developed at the Western Association of State Highway Organizations (WASHO) Road Test in 1952. It is a simple device that is used to measure the rebound deflection of a flexible pavement. The Benkelman Beam is used with a loaded truck – typically on a single axle (8.2 tonne). Measurement is made by placing the tip of the beam between the dual tyres and measuring the pavement surface rebound as the truck is moved away. The standard Benkelman Beam is supplied with a mechanical Dial Gauge which is suitable for measuring peak deflections but not easily used for measuring deflection bowls. CAPTIF has developed an electronic kit that can be attached to a standard Benkelman Beam in order to capture full deflection bowls accurately and easily.
CAPTIF use a Transverse Profile Beam to measure the rut depth of a pavement during testing. Rut depth is a primary indicator of the strength of the pavement. The rigid beam straddles the pavement and a carriage is driven from one side to the other. Attached to the carriage is a wheel that is free to move up and down to follow the contour of the pavement surface. The carriage’s horizontal position and the wheel’s vertical displacement are measured with digital rotary encoders. The data from each encoder is captured in a microprocessor then transferred to a portable computer running a Labview(external link) program. Once the data has been captured a Labview analysis program is used to calculate the rut depths.
The FWD machine drops a known weight from a specified height on to the pavement surface. Attached to the trailer are a row of geophone sensors which sit on the surface of the pavement while the weight is dropped. The Geophones measure the velocity of ground waves that are produced by the impact of the falling weight. From this velocity, back calculations are made to determine the deflection of the pavement surface and hence information about the strength of the pavement. CAPTIF contracts in FWD services as required.
The SLP is a device that measures the surface micro-texture of the pavement. A rigid beam (1.67m long) holds a carriage that can be driven from one end to the other. Mounted on the carriage is a Selcom Optocator Laser Displacement Module. The laser has a range of 32mm and measurements are sampled every 0.8mm of carriage travel. From this data a texture value known as the Mean Profile Depth (MPD). The SLP is used on the CAPTIF tests tracks and around the country on NZ Transport Agency’s Long Term Pavement Profile test sections.
