Following a request from a manufacturer of concrete products, Applied Measurements has developed a user-friendly, low-cost device to rapidly and accurately measure tension in pre-stressed wire and cable.
To rapidly and accurately measure the tension within pre-stressed steel wire or cable using a repeatable process that will serve to increase the confidence of the end product. To manufacturers of concrete products, the ability to make these measurements using a simple process that removes operator uncertainty from the measurement is paramount. This was the challenge set to Applied Measurements, by Bison Concrete.
Bison is one of Britain’s leading producers of structural, pre-cast concrete products. Consistent, quality control is vital to maintain the company’s position at the forefront of the sector. The company is particularly renowned for pre-stressed elements, including hollow-core floors, solid composite floors and beams.
This development was prompted by a customer asking for a non-contact method for determining the tension in reinforcing cables and bars for pre-stressed concrete beams. Peter Lewis, the managing director of Applied Measurements investigated the dynamics of the measurement and discovered that a vibrating steel wire or bar would have on an externally applied transverse magnetic field.
After significant development the ‘Digiforce’ was born and the patented device has now been supplied to many other concrete manufacturers’ including RMC, Concrete, Tarmac, Spiroll Precast, Coltman Precast, Milbank in Britain, Burns in the USA and Impact in Australia – and other blue chip construction companies all who share the need to make rapid and accurate tension measurements on-site.
The equipment used for this tensioning process must be tested on a regular basis, but current methods are cumbersome and time-consuming and accuracy is in part dependent on the skill of the operator. The Digiforce developed by Applied Measurements has enabled Bison to achieve rapid, repeatable, non-contact tension measurement using cutting-edge technology.
Integral to the durability and structural performance of these products is the correct tensioning of the pre-stressing steel tendons. There are approximately 30 tendons in the average 1200mm-wide hollow-core floor slab, which may be up to 160m in length. By striking the wire or cable under tension and observing their fundamental natural frequency of vibration, it has proved possible to develop a battery powered hand held instrument capable of accurately determining the tensile forces applied to them.
Although simple in theory, the low frequencies – Hz to a few tens of Hz and complex sound spectra have up to now, prevented their accurate analysis on-site. The Digiforce addresses this issue via the integral mathematical software, to process only those frequencies of interest that contributes to the measurement. This means that the user no longer has to refer to look-up charts or return to the office to run complex calculations pertinent to that piece of equipment or installation.
The instrument is first programmed with the length of the wire or cable whose tension is to be measured. Also inputted is its weight per unit length. The magnetic sensor at the end of the instrument is then placed adjacent to the part to be measured. The measurement process is triggered and the part is struck lightly with the mallet supplied. After a few moments, the tension appears on the LCD display on the instrument.
Tensions in stainless steel or other non-ferromagnetic wires can be measured by attaching clips (also supplied) made of ferromagnetic materials, provided that their masses are insignificant compared to that of the cable to be measured.
The vibrations being analysed are complex and the fundamental frequencies that the method uses are in the range 3Hz to 70Hz and not the metallic note heard by the ear. The tension is determined from the formula, T = 4 x length2 x Hz2 x weight per unit length. Because of the very low frequencies, tuned circuit filter methods are not an option. Instead, the device makes an initial 20 measurements of each oscillation and discards these. The next five measurements from 10 oscillations are then used to establish a tolerance band to establish which measurements are reasonable to contribute to the calculation and which should be rejected as spurious. Subsequent measurements are then made and those that fall within the tolerance band are used to calculate the final measurement value.
Because the principal business of Applied Measurements is in designing and manufacturing precision load cells and force sensors, the measurements are traceable back to NPL calibration standards.
The Digiforce boasts an on-board data-logging facility for collecting results and an RS232 port / USB converter for later downloads to a host PC, driver software, aluminium striker, rugged carrying case and full instructions. Additionally the device is supplied with a 3-year warranty.
Following the success of the Digiforce in pre-stressed concrete, other applications are now being realised. One such success story is Stage Technologies in London, who use the product to monitor the loading on cables used to support various stage elements.