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Measurement of rebar stress

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Does anyone have first hand experience of measuring the stress in reinforcement in existing concrete structures using strain gauges?
 
I have been given some (limited) information about a test comprising the exposure of a reinforcing bar and attachment of a strain gauge.  A small hole is then drilled in the bar resulting in the cross-sectional area being reduced.  The increase in stress results in a (presumably) measurable strain.
 
I can see how this may, theoretically, enable the calculation of the E value for the steel but I have not yet established how the actual stress can be calculated.  Reservations about the method include the possibility of load transfer to adjacent bars (I'm assuming the calculation relies upon the force in the bar remaining constant) and temperature influences due to drilling the steel.
 
If anyone has any experience of this test I'd appreciate their input.
 
Thanks,
 
Peter Hirst