Steel reinforcing bars (or rebars for short) play a vital role as tension devices to reinforce structures and materials such as concrete. Tucked away out of sight, strengthening and aiding the material under stress and strain, they are an important hidden asset. But their invisibility is also a key part of their potential liability from the enemy within: rebar corrosion. This is where these reinforcing bars come into contact with chloride ions or acidified concrete over time, initiating and sustaining corrosion. Most concerning of all? Rebar corrosion, in extreme circumstances, can lead structural failure.
Typically, the reinforcement embedded in the concrete is protected by the high pH environment created by the cement paste around the rebar, which prevents corrosion from occurring [typically with pH levels around 12.5 or higher], according to Surtreat Solutions, which supplies surface-applied concrete solutions.
However, when chlorides begin infiltrating the concrete through cracks and gel pores, it reduces the pH [from ~13 to <6] to levels where corrosion can occur on the reinforcement. When the reinforcing steel corrodes, the corrosion products (rust) that accumulate around the steel creates outward pressure on the concrete surrounding the reinforcement. As this corrosion product is 10 times the volume of steel, it occupies more space than the original rebar. This pressure causes the concrete to crack, spall (pictured below) and fall away.
The best way to prevent corrosion in concrete, states Surtreat, is by employing a corrosion-inhibiting admixture. Additional measures recommended to mitigate corrosion include the coating of reinforcement (for example, with an epoxy resin), and use of sealers and membranes on the concrete surface, periodically reapplied.
There are two main causes of reinforcement corrosion in reinforced concrete structures. They both result in the breakdown of a protective layer on the steel surface which, when present, prevents corrosion. First is carbonation, in which carbon dioxide (CO2) from the air permeates the concrete and progressively acidifies the cement paste. Once the carbonation depth reaches the reinforcement, the acidity of the concrete causes corrosion. “Carbonation is a natural process that affects all exposed concrete,” points out Steve Holmes, technical manager - specialist construction solutions, Sika. “However, it is exacerbated by low concrete cover to the reinforcement and poor quality/damaged concrete.”
COVER BLOWN
The second is exposure to chloride salts. Chlorides are present in sea water, de-icing salts and chlorinated water. “Chloride ions penetrate into the concrete from the exposed surface. Once a certain concentration of chloride ions is reached at the reinforcement surface, corrosion will begin, often leading to significant steel section loss,” Holmes explains.
Tell-tale signs to look out for include:
Concrete that has broken away, leaving behind corroded reinforcementAreas of concrete that sound hollow when tapped with a hammerCracks on rises or lintelsRust-stained water emanating from cracks or jointsAreas of failed concrete surrounding previous concrete repairs.So, how do you fix these issues? Holmes identifies several key steps.
First, identify what is causing the reinforcement corrosion and treat it first. “Treat the cause, not the symptom,” he says: “chloride-induced corrosion is generally found in wet or historically wet environments – look for water staining, failed waterproofing/drainage and pooling water. Carbonation is exacerbated by drier conditions and poor-quality concrete or low cover – look for honeycombing/exposed aggregate and cracking.”
Secondly, conduct specialist testing to confirm the extent of the corrosion risk to the structure, he advises, and, thirdly, work with a specialist such as Sika to correctly specify the repair methodology and the materials to be used. “The aim is to complete a high-quality repair [pictured, above] that stops the corrosion and prevents it from spreading further,” says Holmes. “A repair system might include reinforcement primers, structural repair mortars [whether hand-placed, spray-applied or flowable], corrosion inhibitors, anti-carbonation protective coatings, hydrophobic impregnants, and galvanic, hybrid or impressed current anodes.”
He strongly recommends using an experienced concrete repair contractor with appropriately-qualified staff. “BS EN 1504 defines the requirements of the materials and specifies the repair methodologies to be used.” He also recommends having a plan. “Regular inspection and maintenance of your concrete structures should help prevent the causes of corrosion and spot issues before they become large and expensive.”
QUANTIFYING THE DAMAGE
Although reinforcement is buried in concrete, it is possible to quantify the extent of ongoing corrosion and the risk of future corrosion through a variety of specialist testing techniques, including:
Hammer tap survey: de-bonded areas of concrete, which indicate as-yet invisible corrosion problems, can be located by a hammer tap test. Sound areas of concrete produce a dull sound, whereas a de-bonded area will produce a hollow sound. Half-cell potential surveying: this involves measuring the voltage difference between the corrosion reaction on the reinforcement surface, which changes, and a stable corrosion reaction in an external sensor. The voltage measured is an indicator of the corrosion risk to the reinforcement.Two other methods both require a cover depth survey to determine the risk:
Chloride penetration testing: it is possible to determine the chloride content of the concrete by extracting dust samples from different locations. This is done by collecting progressive drillings and analysing them to obtain free chloride content, whose magnitude is proportional to the corrosion risk
Carbonation depth testing: by spraying a freshly broken concrete surface with phenolphthalein indicator (which shows pH), it is possible to determine how deeply the concrete is carbonated, and with it the corrosion risk. Reinforcement corrosion can happen anywhere, states Sika’s technical manager, but there is often a lack of awareness about the causes and potential consequences of untreated/unrepaired damage, he says. “The damage caused is sometimes treated as an aesthetic issue. However, delaminated concrete can present a falling object risk and reinforcement section loss can result in a weakened structure.”
How pervasive is the problem? “Reinforcement corrosion is a widespread issue, particularly in the UK’s coastal and temperate environment, and is often neglected in maintenance and repair budgets. Once it takes hold, the extent of corrosion damage can grow quickly, along with the repair bill,” cautions Holmes. “Education, along with regular inspection and maintenance, can go a long way to mitigating the scale and costs related to any mitigation works.”
BOX: Lowering the corrosion odds
Corrosion of embedded metals in concrete “can be greatly reduced by placing crack-free concrete with low permeability and sufficient concrete cover”, according to the Portland Cement Association. “Low-permeability concrete can be attained by decreasing the water-to-cementitious materials ratio of the concrete, and the use of pozzolans and slag,” it advises. “Pozzolans and slag also increase the concrete resistivity, thus reducing the corrosion rate, even after it initiates.” ACI 318-11, Building Code Requirements for Structural Concrete provides minimum concrete cover requirements, states the PCA, that will help protect the embedded metals from corrosive materials.