Residual strength of concrete and steel depend upon your material models .If you are using confined material model which means your transverse reinforcement spacing satisfies the modern seismic criteria for which researcher have used 0.58 of the ultimate strength other wise 0.2 is used for the other case ( unconfined material model ) residual strength normally defines your gradual or sudden strength drop same is the explanation implies to the steel 

There are many techniques used to measure residual stresses, which are broadly categorised into destructive, semi-destructive and non-destructive techniques.

Destructive Techniques

The techiques include are

1 Contour Method- measures the residual stress on a 2D plane section through a specimen, in a uniaxial direction normal to a surface cut through the specimen with wire EDM.

2 Slitting (Crack Compliance) - measures residual stress through the thickness of a specimen, at a normal to a cut "slit".

3 Block Removal/Splitting/Layering

4 Sach's Boring

Semi-destructive Techniques

The techiques include are

1 Deep Hole Drilling - measures the residual stresses through the thickness of a component by relaxing the stresses in a "core" surrounding a small diameter drilled hole.

2 Centre Hole Drilling - measures the near surface residual stresses by strain release corresponding to a small shallow drilled hole with a strain gauge rosette.

3 Ring Core - similar to Centre Hole Drilling, but with greater penetration, and with the cutting taking place around the strain gauge rosette rather than through its centre.

Non-destructive Techniques

The techiques include are

1 Neutron Diffraction

2 Synchrotron Diffraction

3 X-Ray Diffraction

4 Ultrasonic

5 Magnetic

Use above any technique to find residual strength.

Dear Linh,

your question caused some confusion. Maybe you can specify it a little.

I suppose, you are not interested in residual stresses, for which Nitinkumar listed a variety of options for testing.

Regards,

Alex