The first step in the LPR data collection is to expose the steel reinforcement and makepositive electrical contact (tap). After confirming the continuity of the electrical conductivity between the contact point and reinforcement in the concrete area being tested, the LRP test can be performed using either manual or automated methods.
For the manual method, readings are collected using a three-electrode polarization resistance (3LP) system, and the reinforcement is polarized through manual operations. First, the electrical potential is monitored until it is stable, with potential variations of no more than +/- 5 mV per minute. If readings are not stable, it may indicate inadequate moisture or interference by an external electrical source, which should be evaluated and rectified before proceeding. Once a stable potential is established, the electrical potential value is recorded. The system is set to report a relative potential, versus the open-circuit potential, and the meter is zeroed. Electrical current is then applied to polarize the reinforcement at a slow, steady rate, such that the offset potential reaches +4, +8, and +12 mV from the open-circuit potential. At each noted potential increment, the corresponding current required to achieve that offset is recorded. The complete polarization procedure should take no longer than 2 minutes. Once complete, the current is removed, and the reinforcement is allowed to de-polarize. Within 3 minutes, the potential should return to the open-circuit potential. If it does not, then the associated “drift” in open-circuit potential will influence the measured corrosion rate, so the measurement should be re-taken. A period of 5 to 10 minutes should pass before repeating readings to ensure adequate de-polarization.
Some field instruments and laboratory potentiostats can be programmed (or are pre-programmed) to perform the polarization process automatically. Such systems should be programmed to linearly perturb the potential to at least +12 mV and no more than +20 mV from open-circuit potential at a rate of 10 mV per minute. It is preferred that the system monitors and collects the open-circuit potential, recording potentials in 1 second intervals for at least 1 minute before and after the test. Some devices are equipped with a “guard ring” electrode that induces a separate current into the electrode that surrounds and attempts to “contain” the primary counter electrode current into a specified polarization area. Such instruments are known to give significantly different corrosion rate values than the methods outlined above and are believed to underestimate the actual corrosion rate.