Question:
How do you measure electrical continuity of steel in concrete, and why is it important? BG
Answer:
As reinforced concrete structures age, the steel can become increasing vulnerable to corrosion due to the ingress of chlorides or atmospheric carbon dioxide [1]. Electrochemical techniques can be used both to measure the corrosion and to control it. Reference electrode potential mapping is widely used on bridges exposed to deicing salts and structures exposed to marine conditions to map the extent and risk of corrosion [2]. Cathodic protection, along with lesser used techniques such as realkalisation and chloride extraction, are all well documented treatments with ISO and European standards [3,4,5]. All of these processes require that for the area under investigation or treatment, there is a direct metal-to-metal contact between all the steel bars in the reinforcing cage that is being assessed or protected. In the absence of such connections, under cathodic protection, stray currents may occur, leading to the formation of anodes where current leaves a disconnected reinforcing bar, leading to corrosion. If electrochemical measurements are being taken then any separation between rebars can lead to a cell with its own potential, giving misleading measurement of the steel to reference electrode potential.
This may seem a simple thing to measure, but out in the field with limited equipment it is important that operatives and engineers have clear method statements for carrying out measurements and criteria for defining continuity, whether carrying out steel potential mapping with a reference electrode and high impedance voltmeter, or installing a cathodic protection system. The problem is that concrete is a damp medium, with high resistivity, and there are multiple parallel connections between reinforcing bars. Before a steel cage or other structure is embedded in concrete or immersed in water, it is easy to measure the electrical continuity accurately with a digital multimeter or a resistance meter such as a Megger or a Nilsson meter. Once embedded in concrete, and especially when corrosion is initiated, it is harder to be sure that there is metal to metal contact. Stirrup steels round the main bars in beams can be a particular problem once corrosion initiates. Some older structures have very light reinforcement and even electrically separated mats of steel. Considerable effort may be needed to establish continuity both for assessment and when applying cathodic protection.
This issue was addressed in 1990 by Jack Bennett. Jack invented the ‘Elgard’ anodes for cathodic protection of steel in concrete along with many other products. As part of the development work, he carried out laboratory and field studies to ensure that steel bars were adequately bonded when impressed current cathodic protection was applied. Bennett presented his study and findings at a NACE conference committee meeting in the early 1990s, but never published it, however, he did circulate an internal Eltech memo of the work.
In researching the literature, I became aware that no one else had published anything on this subject, but Jack’s findings were being used in the standards on cathodic protection of steel in concrete such as
BS EN ISO 12696. I contacted Jack, who has now retired, and he, along with his former employers agreed that the memo should be published.
I therefore transcribed it into a Structural Concrete Alliance Technical Note [6].
The memo states that Bennett found the use of a Nilsson meter gave inaccurate measurement, indicating continuity where none existed. This is fortuitous, as, using a high impedance Fluke multimeter he got more accurate measurements. High impedance meters are always available on site when taking reference electrode potential measurements for investigation purposes and when installing cathodic protection systems.
It is interesting to note that Bennett found the most accurate method of determining continuity was to measure the DC potential difference between bars, which should be less than 1 mV. A slightly less accurate method was to measure the (DC) resistance which should be less than 1 ohm. In both cases, the leads should be reversed and the readings repeated. In BS EN 12696, the resistance technique appears to be given priority over the potential technique, which is not the priority that Bennett recommended. I would always recommend using both techniques, especially is there is any doubt about continuity.
There has been discussion in standards about whether the criterion for potential difference or resistance should be higher or lower. My reaction has always been that when anyone can offer hard data we should consider it, but until someone does so, then these are the criteria we should use. It would be good to see someone repeat or improve on Bennett’s work. but until then it is what we have to go on to ensure we have electrical continuity in our reinforcement cages.
References
1. J. P. Broomfield, Corrosion of steel in Concrete, 2nd Edition, Taylor and Francis, 2007.
2.ASTM C876 (2015) Standard test method for corrosion potentials of uncoated reinforcing steel in concrete.
3. BS EN ISO 12696 (2016), Cathodic protection of steel in concrete.
4.BS EN 1504-1 (2016) Electrochemical realkalization and chloride extraction treatments for reinforced concrete Part 1 realkalization.
5.BS EN 1504-2 (2021) Electrochemical realkalization and chloride extraction treatments for reinforced concrete Part 2 Chloride Extraction.
6.J. P. Broomfield (2021) The measurement of electrical discontinuity for steel in concrete subject to cathodic protection and other electrochemical treatments. Technical Note 29 Structural Concrete Alliance, Bordon, Surrey.
John Broomfield
