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Re: Measurement of rebar stress

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It sounds like you are talking about blind hole drilling techniques for measurement of in-situ strain/stress.  A small rosette gage is laid on the surface, and then a tiny shallow hole is drilled in the center which relieves the surface strain at the boundary of the hole.
Be aware that what you will get is a measurement of the release of total in-situ surface strain, regardless of what it results from.  So if there are significant residual stresses/strains at the surface, these will be indistinguishable from the strain/stress associated with in-service loading.
I would suggest a search of Micro Measurements or online with the phrase "blind hole drilling techniques" if you are looking for more information.
Mark D. Anderson  PE
----- Original Message -----
From: Pete
Sent: Thursday, December 12, 2002 10:00 AM
Subject: Measurement of rebar stress

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.
Peter Hirst