1. Monitoring Steel Corrosion Challenges
Direct inspection of the rebar or corrosion coupons, sensors is almost impossible as these are embedded in concrete.
Retrofitting of corrosion probes/sensors is difficult.
Concrete is actually a complex composite material, whose structure and properties tend to change with time. It does not "behave" like simple aqueous corrosion systems.
Cracks in the concrete may represent a major factor in the severity of corrosion damage.
Chloride induced rebar corrosion tends to be of the localized variety.
Corrosion damage is typically manifested over many years of exposure.

2. Corrosion Monitoring Methods
ER probes
Extracted core samples
Chloride concentration profiles or carbonation depths
Soundness
Permeability of concrete
Visual inspection.

3. Maximum Load Monitor
The TRIP steel sensor unit (in foreground) is attached to a bridge, and the maximum strain experienced by the bridge is permanently recorded by a change in the magnetic character of the steel. The sensor is read by the portable instrumentation unit shown in the background.
TRIP steel is a special steel with a special chemical formulation, and it undergoes a permanent change in crystal structure in proportion to peak strain. It changes from a non-magnetic to a magnetic steel. The change can be easily measured.