Measuring Axial Shortening using Electronic Strain Gages

[HarshanaM]

I am quite new to concrete testing and I have received an opportunity to work with a company that carries out testing in cast concrete structures. Their latest job includes measuring differential settlement in selected columns in a high rise building. I have given the responsibility of finding out which type of strain gage is best suited for the job in order for them to import it.

 

[HarshanaM]

So above are the catalog I received from the supplier but I have found it difficult to find out which one suits our need. We basically want to measure the strain difference in two 900X900mm concrete columns. I think embedded strain gage is more accurate rather than those mounted on to the column face or welded to rebar. or is it?
Please any experts out there please give me some tips.
Thanks in advance

 

[Ron]

Using the supplied manufacturer's nomenclature (it is different per manufacturer), for your application I would select the following:

KFG-80-350-C1
or
KFG-120-350-C1

The gage length for measuring concrete strain should be at least 3 times the nominal diameter of the coarse aggregate. As for the resistance, usually 120 ohms and 350 ohms are used, but this will depend on the equipment you are using to excite and process the strain in a Wheatstone bridge arrangement.

I have had no issue with surface bonded strain gages and have used them for steel, aluminum, concrete, and composites. Keep in mind that the larger the strain gage, the more difficult it will be to apply.

For your application, if you use embedded gages, you have only one shot at getting it right. There's too much activity around the gages after you install them to assure yourself it is protected.

If you use welded strain gages on the rebar, the rebar response might not initially mimick the concrete response.....the concrete might move more than the steel initially.

Using surface bonded gages, I would put one gage on each face of the column at each elevation you want to monitor. If you have round columns, put them 90 degrees apart. This will give you redundancy and an accuracy check.

 

[HarshanaM]

Thanks a lot Ron. Also how can I relate the column axial load and the resistance of the strain gage?