The reinforcement bar is detected by magnetising it and inducing a circulating “eddy current” in it. After the end of the pulse, the eddy current dies away, creating a weaker magnetic field as an echo of the initial pulse. The strength of the induced field is measured by a search head as it dies away and this signal is processed to give the depth measurement. The eddy current echo is determined by the depth of the bar, the size of bar and the orientation of the bar. The detection re-bars is essential as a pre-process for core cutting.
The rebars can also be detected by GPR ( radar ) method. The detection depth of rebars with GPR can be high upto 400 – 500 mm. Recently some small hand held Radar based rebar locators were introduced by some companies. But we have observed that, these rebar scanner are not reliable, and that’s why probably they are taken back from the market
Reliability & Limitations :
With this instruments a cover to reinforcement can be measured up to 70 to 100 mm with an accuracy of ±15 %. Manufacturers claim accuracy of a bar diameter measurement to be less than 2 to 4 mm, but it has been observed that, most of the available detectors do not accurately measure the bar diameter. Proper calibration of these instruments is very essential.
The factors which affect the accuracy are –
Very closely spaced bars or bundled bars or bars in layers, Binding wire, aggregate containing iron or magnetic properties
Some Observations –
- Accuracy of Rebar locator for bar diameter measurement is very less. Hence it is not recommended for predicting the bar diameter
- If the bars are closely spaced, less than 40 to 50 mm in case of beams, the two bars may not be differentiated.
- Similarly if the bars are in layers with a spacing of 40 to 50 mm incase of beams, the bars may not be differentiated
- In general bars can be detected upto 70 to 100 mm deep by standard magnetic detectors. Beyond this depth, only higher dia bars more than 25 mm may be detected upto 150 mm.
- For more cover depth use “Radar” based detectors