Electrical testing of corrosion protection
This annex describes the measurement of electrical resistance between an anchor and surrounding soil or structure to determine the effectiveness of the applied corrosion protection system.
Two tests are described;- the first measures the isolation of the anchor from the soil and structure and the second measures the isolation of the head of the anchor from the structure alone.
A.2 Electrical resistance measurement I (ERM I)
For the method of execution of ERM I (Isolation of anchor from soil/structure) see Figures A.1 a) and A.1 b).
The following equipment specification is adopted:
- measurement voltage 500 V d.c;
- measuring range > 10 kΩ (0,01 MΩ).
During measurement the anchor shall be connected to the positive pole with the earth connection at the negative pole of the measuring circuit. Generally, the moist soil of the site is used as the earth.
Lengths of reinforcing steel in reinforced concrete construction in contact with the soil, metallic pipes buried in the soil, or soil or rock nails may also be used as the earth connection.
During measurement the points of contact are kept clean.
The ERM I may be executed in two separate phases:
This tests the integrity of a plastic sheath on the free anchor length and the bond length after the subsequent stages of the installation of the anchor i.e. before lock-off of the anchor:
- after setting the anchor into the borehole;
- after the primary grouting;
- after a possible secondary grouting;
- after the acceptance test.
The execution of the measurement in these stages is shown in Figure A.1a).
A measured electrical resistance between tendon and ground ≥ 0,1 MΩ demonstrates an acceptable level of plastic sheath integrity.
NOTE 1 A faultless watertight plastic sheath gives values of RI > 100 MΩ.
NOTE 2 It is recommended that these measurements are used to observe the effects of the different actions during the installation of the anchor on the integrity of the plastic sheath.
This tests the overall electrical isolation of the anchor from ground and structure:
- after lock-off of the anchor;
- after the anchor head injection;
- at any time during the working life of the anchor.
A resistance RI between anchor and ground/structure ≥ 0,1 MΩ demonstrates overall electrical isolation of the anchor from the ground and structure.
The execution of the measurement on the finished anchor is shown in Figure A.1b).
- 1 Ohmmeter
- 2 Structure (concrete)
- 3 Ground
- 4 PE-sheath
- 5 Tendon
7 Bearing plate
8 Anchor head
9 Isolating plate
NOTE The top of the PE Sheat shall be clean and dry
- 1 Ohmmeter
NOTE ERM II is only executed if Rl after lock off is lower than 0,1 MΩ.
A.3 Electrical resistance measurement II (ERM II)
This measurement is only executed if RI after lock-off of the anchor is lower than 0,1 MΩ in order to prove that, at least, there is no direct contact between anchor head and reinforcing steel of the anchored structure.
ERM II is executed on the stressed anchor. For the method of execution see Figure A.2. The following equipment specification is adopted:
- measurement voltage circa 40 V a.c;
- measuring range 0-200 kΩ (0-0,2 MΩ).
Generally, the bearing plate is used as the earth connection.
The steel in the anchored structure may be used if the bearing plate is coated by an electrically isolating material.
During measurement the anchor head and in particular the isolating plate between anchor head and bearing plate is kept dry. The electrical contacts are kept clean and the metal bare. In order to ensure good contacts either clamps or strong magnets are used. Pins are not used for this type of measurement.
Measurement ERM II is sensitive to climatic influences such as air humidity in the area of the anchor head and also possibly to stray currents in the ground.
If several measurements are carried out on an anchor then, given the correct execution of the measurement, the greatest resistance measurement should be adopted.
A resistance RII between anchor head and bearing plate or reinforcing steel of the structure > 100 Ω demonstrates no direct contact between anchor head and reinforcing steel of the anchored structure.