The best bet for this application by far is impact echo. GPR (with a 1.5 GHz antenna) has difficulty penetrating reliably beyond about 18 in., particularly with the two mats of steel. Infrared thermography could work to identify front side delaminations, but no way could you see back side delams...
Check out the procedures outlined in "Technical Notes 16B - Calculated Fire Resistance" published by the Brick Industry of America (http://www.brickinfo.org). It has a pretty good, and accepted, methodology for confirming that your wall is way more than 3 hrs.
The recommendations made above to use higher quality, low permeable concrete while providing adequate cover are the two most important provisions to enhancing the durability of a reinfoced concrete structure, especially for this exposure.
I question the use of an epoxy coating on one face of...
Equally off the direct topic, but some owners are especially interested in the robustness of the vault to sustain some pretty high levels of attack. For instances, one agressive way to break into a vault is the use of a 'burn bar,' which is a thermal tube burning oxygen and solid fuels at an...