Digiforce measures tension in pre-stressed cable

Applied Measurements has developed Digiforce, a user-friendly and inexpensive device that rapidly and accurately measures tension in pre-stressed wire and cable.

It developed the device for Bison Concrete, a producer of structural, pre-cast concrete products.

One of Bison's customers asked for a non-contact method to determine the tension in reinforcing cables and bars for pre-stressed concrete beams.

Peter Lewis, managing director of Applied Measurements, investigated the dynamics of the measurement and eventually developed Digiforce, which has enabled Bison to achieve rapid and repeatable non-contact tension measurement.

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, Applied Measurements developed a battery-powered handheld instrument capable of accurately determining the tensile forces applied.

Although simple in theory, the low frequencies - Hz to a few tens of Hz - and complex sound spectra have up until now prevented accurate analysis onsite.

The Digiforce addresses this issue via the integral mathematical software to only process frequencies of interest.

This means the user no longer has to refer to 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 to be measured and 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 with that of the cable to be measured.

The vibrations being analysed are complex and the fundamental frequencies that the method uses are in the 3 to 70Hz range 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 ascertain which measurements should 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.

The measurements are traceable to NPL calibration standards.

The Digiforce boasts an onboard 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.

The device is supplied with a three-year warranty.

Following the success of the Digiforce in pre-stressed concrete, other applications are now being realised.

Digiforce has now been supplied to concrete manufacturers including RMC, Tarmac, Spiroll Precast, Coltman Precast, Milbank, Burns and Impact.

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