Cathodic protection - how and why it is used

Remediation of concrete using cathodic protection

An investigation by The Times found that in April 2019, 4,000 of around 9,000 bridges and large culverts on England’s motorways or A-roads showed evidence of defects or damage that may significantly affect their capacity.

There’s a huge cost involved in maintaining these structures with National Highways (NH) having spent a mammoth £4.5 billion operating, maintaining and improving its main road networks in 2019-2020 and allocating £0.3 billion for maintenance and repairs.  

Over the next five years NH intends to spend £1.5bn on structural maintenance, £200M more than in the previous five-years. Therefore, any reductions in the frequency of maintenance of concrete structures will play an important role in cost saving.

With concrete structures at constant risk of attack from water and salt ingress, deterioration can occur quickly yet take several years for the effects of corroding reinforcement to appear.

When specifying products used in repairs where cathodic protection is being used, resistivity is a very important characteristic to take into account. 

Resistivity vs. cathodic protection

As a specialist in civil and structural durability engineering, Mott MacDonald provides consultancy on cathodic protection, specifying appropriate products for repairs. Chris Atkins, technical principal at Mott MacDonald, summarises how and why cathodic protection is used:

“Steel corrodes in concrete either due to excess chlorides at the steel surface or due to carbonation. Cathodic protection is typically used in chloride contaminated concrete to minimise the removal of concrete.

“Cathodic protection passes a current through concrete with a voltage. The steel is made to be a cathode by creating an anode somewhere else. This can either be by using a material that naturally makes steel an anode or by using a material connected to a DC power supply in an impressed current cathodic protection system.

“Resistance relates how much voltage is needed to pass the current. Resistivity is the bulk material property that can be used to calculate the resistance produced by a set amount of material, so the resistance equals the resistivity multiplied by the length divided by the area.

 “For galvanic cathodic protection, the voltage that pushes the current is generated by the material selected and typically cannot be adjusted. High resistivities will limit the output (but will also limit corrosion). For impressed current systems the voltage to a given area can be adjusted.

“Standards require ranges of resistivity to be within 50-200% of the parent concrete. However, the parent concrete, for example, a bridge being repaired, will have a variable resistivity. Wet concrete has a lower resistivity than dry concrete and chloride contaminated concrete has a lower resistivity than uncontaminated concrete. Additionally, measuring the resistivity of concrete on site is a challenge due to the steel, which has a very low resistivity. Resistivity measurements on site can sometimes just indicate the concrete cover to the steel and where drainage is leaking.

“Variable resistivity will be considered as part of the design and in some cases, it may not be significant, however, in others such as a repair area with 50% low resistivity mortar, it will.”

The effects of resistivity

A focus of our research and development has been creating specialist products that address resistivity. NH standards require repair products to be within resistivity range of 5,000 to 15,000 Ohm.cm. 

In recent testing by Ryan Cobbs, materials engineer at Mott MacDonald, resistivity was examined in a short-term test (lasting 48 days) where anodes were cast into concrete, and the drive voltage and resultant current were measured when connected to an external cathode.

“The aim was to assess the effects of resistivity on the anodes’ current output,” explains Ryan. “We used a product with a low resistivity in the experiment alongside another Class R4 concrete repair material for comparison purposes. Concrete resistivity increases with time, and the specialist product exhibited a slower resistivity growth rate. The product was also 25% stronger under compression which would improve the quality and longevity of repairs.”

Similar testing commissioned by Weber on its spray concrete range of products showed the same trend in resistivity increase over a short period of time (see graph for details).

Cathodic protection in action: New Elvet Bridge, Durham

A good example of a cathodic protection project is New Elvet Bridge in Durham which has reopened following 15 months of complex repair and maintenance works.

The bridge, which carries around 17,000 vehicles a day, has undergone major works to prevent metal corrosion using an impressed current cathodic protection system with titanium mesh and specialist low resistivity concrete repair material. The system has been installed across entire beams and the bridge deck soffits to provide a comprehensive, long-term solution.

The reduction of wastage and the provision of long-term protection of the bridge have improved the project’s sustainability and will reduce the need for maintenance. Use of a low resistivity repair product will reduce the voltage of current needed in the cathodic protection further improving its sustainability. Use of dry spray technology can also mean lower rebound and waste which also aids efficiency on projects.

Sources:

https://nationalhighways.co.uk/media/twemiuaj/highways_england_web_1109.pdf

https://www.newcivilengineer.com/latest/4000-of-uks-busiest-road-bridges-are-in-poor-condition-04-12-2020/