Ask the Expect – Issue 165

The questions in this issue feature preventing corrosion of rebars in concrete and when to test for salt contamination of a substrate before painting.

Question:

What is the best way to prevent corrosion of reinforcing bars in concrete? CL

Answer:

By far the best way to prevent corrosion of reinforcement is to ensure the design and construction is carried out correctly so as to achieve the required depth and quality of cover. The majority of reinforcement corrosion problems can be traced to poor design  or detailing, lack of proper control of the concrete mix, its placement and curing, or mis-location of reinforcement resulting in low and inadequate cover.

Portland cement-based concretes protect reinforcement and other ferrous components by generating a low permeability and highly alkaline environment in which the steel protects itself through the formation of a stable passive oxide film. Provided these conditions are maintained then the steel remains protected. The  cover can be thought of as a thick barrier coating that contains chemical species that actively protects the steel. In the early stages, the concrete cover can even heal itself if there are fine cracks resulting from shrinkage as the concrete completes its curing and reaches its full strength.

Continuing along the protective coating analogy, if the thickness is inadequate or the barrier is impaired in any way, then corrosion of the steel becomes a significant risk.

There are two main initiators of corrosion in reinforcement, chloride ions and carbonation. Above a certain critical level (which can depend on many factors) the reinforcement can suffer from severe pitting corrosion, even in the presence of high levels of alkalinity. Chloride ions can be present due to accidental contamination (for example, using unwashed dune sand), purposeful addition (until relatively recently, calcium chloride was widely used as a set accelerator), and through ingress from external sources such as marine environments and de-icing salts.

The other important cause of reinforcement corrosion is carbonation. This is where carbon dioxide from the atmosphere is able to enter the concrete cover and dissolve in the moisture present in the concrete to produce carbonic acid which in turn neutralises the alkalinity generated by the cement as it reacts and hardens. For a good quality concrete with adequate cover depth, this effect is slow and means the neutralised zone may not reach the depth of the steel for many tens or even hundreds of years. Where the cover concrete is of poor quality (for example, through the addition of too much water to the mix) or of inadequate thickness, then the time taken for the steel to be in neutralised concrete can be a matter of a very few years. Once no longer protected by the alkalinity, the steel can corrode in the presence of moisture, resulting in the production of expansive corrosion products and the subsequent cracking and spalling of the cover. If maintained in a dry condition (for example, indoor exposure) reinforced concrete can survive carbonation with little or no consequence.

Current standards and codes of practice provide the guidance required to limit the risk of corrosion from either chlorides or carbonation for a range of commonly encountered exposure conditions.

Where the exposure is particularly aggressive, such as offshore applications, additional measures may be required. The same approaches can also be used to reinstate the required durability of existing structures where it has been compromised through historic shortfalls in design, construction practices or maintenance.

The application of coatings to the concrete surface to resist chlorides and carbonation are widely used to extend the service life of existing structures, with periodic recoating further extending the time before further measures are needed. Should corrosion of the reinforcement have occurred then removal of loose and delaminated concrete, cleaning of the steel and reinstatement with fresh concrete (often a modified repair mortar with enhanced properties) can be effective for the treatment of carbonated or mechanically damaged concrete where chloride levels are low but are generally less than satisfactory where chloride induced corrosion has occurred.  

Remediation of chloride contaminated concrete requires all residual chloride to be removed or, more practically, dealt with in some other way, such as by the use of corrosion inhibitors or cathodic protection. Corrosion inhibitors can be applied to the surface of the concrete and added to the repair mortar to enhance the ability of the fresh alkaline repair so it can better resist the ongoing influence of the remaining chlorides. Where the residual chloride level is too high for inhibitors to be affective or where a very long additional life is required, then cathodic protection may be the only viable alternative to demolition and reconstruction. 

Cathodic protection, dating back to 1824 (coincidentally the same year Portland cement was patented), is widely used  to protect buried or submerged structures such as pipelines, offshore facilities, and shipping but for many decades has also proved invaluable for the corrosion protection of steel reinforcement in concrete. Cathodic protection (CP) requires specialised design and should be carried out by suitably certificated personnel to ISO 15257: 2017 (see ICorr website for more details).

In summary, the best way to prevent corrosion of reinforcing bars in concrete is to keep them within a good covering of alkaline, chloride-free concrete. Where that is not possible, the build-up of chlorides and carbonation can be controlled by surface coatings, and steel that has already suffered corrosion can be rescued by returning it to an alkaline, chloride-free environment. Where that is not possible or practicable then additional measures can be taken to control the corrosion of the steel such as cathodic protection. 

Paul Lambert