Strips and bars WIM sensors were first introduced in the 70s to provide low cost and less intrusive solutions. Such sensors are installed in grooves made in the upper pavement layer and fixed with epoxy resins. It takes a few hours to install them. Both the manufacturing and installation cost are reduced compared to the scales. The typical dimensions of a bar are: 3 to 4 cm (width and height) and the length vary between 0.75 to 1 m for a half lane (wheel weighing) or 3 to 4 m for a whole lane (axle weighing). They are mounted in grooves of 6 to 8 cm in depth and width. A strip is flat (below 2 cm in height) and 3 to 5 cm in width. Both strips and bars contain sensitive elements, capacitive, piezoelectric, strain gauges or fiber optics sensors, which measure pressure or strains variations while a wheel or an axle is passing on them. Strips and bars are only used for HS-WIM.
Such sensors are less extended than the tire imprint, and deliver a signal during all the time when the wheel or axle is on them, or even close to them (because of the pavement deflection). Therefore it is necessary to integrate the signal with time to get the wheel/axle force, knowing the vehicle speed. These sensors are more sensitive to the pavement response (e.g. modulus) than the scales, and thus may be sensitive to the temperature. Most of them are used with a self-calibration procedure to compensate the effects of external factors. The prices of these sensors vary from less than 1,000 US$ to more than 7,500 US$ depending on the technology, but remains cheaper than scales, above all after the installation.
The most common technologies available on the market are:
- Piezo-polymer strips and bars: the cheapest technology, but also the less accurate and quite sensitive to the temperature;
- Piezo-ceramic bars (in some cases nude cables are also used for lower accurate measurements): are an intermediate solution (accuracy and cost), which was widely used in the 80s and 2000s;
- Piezo-quarz bars: the most accurate and expensive technology, which provide on a smooth pavement an accuracy comparable to the bending plates; this technology is the mostly used since 2010.
- Strain-gauge strip sensors behave as a bending structure/beam but in the form of a bar; this technology was introduced in 2014.