Corrosion in concrete structures

by Teresa Acklin
Share This:

A chemical engineer explains how to detect, repair and protect against corrosion in concrete grain storage

By Rick Montani

   Concrete is far from the modern construction material it is often thought to be. Evidence of its use exists in ancient Greece and the Roman Empire. But it was not until the 20th century that concrete's full potential really came to be explored.

   Generally, concrete structures are designed to serve their purpose for as long as possible with minimum deterioration and maintenance. Reinforced concrete is a very robust material; its performance, however, depends on the conditions of exposure and the quality of design and workmanship.

   Concrete may be subjected to various aggressive influences, the most common being:

   Atmospheric pollution. Levels of carbon and sulfur dioxide in the atmosphere have increased, particularly in urban areas, from the combustion of gasoline and other fossil fuels.

   Immersion. In reservoirs, tunnels and other water-retaining structures, concrete may have to be capable of withstanding permanent immersion.

   The marine environment. Because of its ability to survive the testing conditions of the marine environment, sea walls, piers, harbors, etc., are constructed from concrete: but in the presence of chlorides there is a greatly increased risk of reinforcement corrosion.

   De-icing salts. Use of de-icing salts containing chlorides has increased dramatically in the past 20 years. The effects on roads and bridges are becoming increasingly evident.

   Other chemical contact Many industrial structures have to withstand chemical attack from industrial production processes.

   Loading and dynamic loading A major reason for selecting reinforced concrete is its ability to withstand both static and dynamic loading. However, if these are not correctly qualified at the design stage or are changed in use, serious problems will occur — deformation, cracking or even collapse.

   Impact.Some structures, such as multi-level parking garages and bridges, must be built to withstand heavy impact and remain safe.

   Abrasion. Frequent trafficking of concrete floor areas or gravel carried by wave action, etc., will have detrimental effects on the surface of the concrete.

   Temperature extremes. Exposed concrete may have to endure a wide range of weather conditions and temperature variations.

   These exposure conditions lead to defects, which generally can be classified into four main categories: spalls, cracks, joints and deteriorated surfaces. Each of these could have a number of possible root causes as outlined in the table on page 12.

   Of all these, corrosion generally presents the greatest threat to today's concrete structures. In order to address corrosion properly, the cause and extent of corrosion must be identified.

   The first step in any concrete repair and protection project is the survey and diagnosis; repair should never take place before a diagnosis. When corrosion is present, the repair of visible damage and protection against latent damage must address the root cause of this corrosion or the results will be short-term only.

Root Causes

    In reinforced concrete, the steel bars are surrounded by a cement matrix, normally with a pH of more than 12.5. This level of alkalinity is created by the setting reaction (hydration) of the Portland cement. The high pH has a passivating effect, inhibiting the electrochemical reactions of the corrosion process.

   Loss of passivity on the steel surface allows rust to form if the steel is in the presence of water and oxygen. The volume of this rust can increase by up to 12 times that of the original steel, resulting in progressive expansive stresses.

   The first visible signs of damage are usually unsustainable tensile stresses seen as cracking and spalling, perhaps with associated rust staining on the surface. These are the symptoms in over 90% of deterioration cases. Rust stains or non-structural, non-shrinkage cracks on the concrete surface are nearly always an indication of latent damage. They give the first visible signs that the reinforcement has begun to corrode.

   It is crucial to identify the root causes of damage — most defects will initially become evident at locations where there is low cover and/or poor quality concrete. The reasons for damage, however, usually require greater investigation. It is important to carry out technically correct repairs, not just treat the symptoms, but also to treat the latent damage to prevent continued deterioration and further extensive repairs in the future.

   The most common cause of loss of passivating alkalinity is carbonation — a process whereby atmospheric carbon dioxide reacts with the soluble alkaline calcium hydroxide and other cement hydrates in the concrete. These are then converted into insoluble calcium carbonate.

   This process is completely natural and helps gradually increase the compressive strength of the concrete. However, the alkalinity of the cement matrix is reduced and its passivating ability is lost progressively from the surface inwards. The speed of penetration depends largely upon the permeability of the concrete and the atmospheric humidity.

   Once the concrete in contact with reinforced steel has carbonated, the reinforcing steel is no longer protected. In the presence of moisture and oxygen, corrosion damage is inevitable.

   The potential for reinforcement corrosion is greatly enhanced if chlorides are present in the concrete. Chloride ions in water form a strong electrolyte, and the effect is to increase the flow of corrosion currents and electrochemically to disrupt the passivating iron oxide film on the steel surface. Sources of chlorides may include exposure in marine environments, exposure to de-icing salts and the use of accelerating admixtures based on calcium chloride.

   It is important, for the future performance of a structure, to determine whether damage results entirely from chloride-induced corrosion, or from chloride-induced corrosion following carbonation or in areas of low cover.

   In addition to their electrochemical influence on the initiation and rate of corrosion, chlorides can also cause physical damage to the concrete surface. This is in effect because of "thermal shock" resulting from accelerated freeze/thaw cycles. Progressive surface spalling can occur, reducing or eliminating cover to steel reinforcement. Repeated exposure of chlorides — such as the application of de-icing salts or salt water — accelerate this process.

Repair and Protection Basics

    Understanding the root cause of corrosion is essential if our repair and protection strategy is to be most effective. If our approach is simply to fill the cracks and patch the spalls without addressing the latent damage elsewhere, our restoration work will be short-lived. For a long-term solution, we must repair the visible damage and protect against the latent damage we cannot see.

   To ensure long-term protection, our basic approach must include the following:

   • any corroding reinforcement should be cleaned and then protected, preferably in an impervious alkaline environment;

   • a strong homogeneous bond should be created between the repair materials and the parent concrete;

   • where necessary, repair materials should have a similar thermal expansion coefficient to the original concrete (especially at extremes of temperature):

   • water vapor diffusion resistance should be similar to that of the original concrete;

   • the treatment should offer a high resistance to future carbon dioxide and chloride ion ingress;

   • materials should be physically compatible with structural requirements; and

   • all of the repair materials should be designed for on-site application.

   In addition to the materials selection and assessment, a thorough evaluation of prospective contractors must be undertaken. This should include taking up references of similar works and assessment of technical competence, management structure, relevant experience, financial standing and past performance.

Repair Materials

    It seems that with all the different repair and protection materials on the market today, a specifier must be a chemist of sorts to be able to make sense of them. It is really not that difficult when these are put into perspective.

   First, there are no magic materials that will substitute for good standard concrete repair practices. But there are proven materials that, if used properly, can achieve the long-term results desired. The trade-off with materials is generally cost versus performance. The key is understanding when the high performance product is necessary, thus justifying the higher cost.

   Cement-based repair materials consist of conventional mixes of sand, cement, admixtures or other enhancers and are quite appropriate for many repair applications. However, if the original concrete was unable to perform properly, it may be desirable to use higher performance materials if a long-term solution is desired.

   The other consideration involves the use of pre-packaged materials versus site-mixed materials. Pre-packaged products offer greater control of final properties, but they do increase the cost.

   Silica fume, another repair material, is an extremely fine pozzolan material which reacts with excess calcium hydroxide in the concrete or mortar to create a more dense, higher-strength matrix. It is commonly used to produce extremely high strength concrete structures, and it is used in repair materials for its abrasion and chemical resistance. Pre-packaged repair materials containing silica fume are now available and have demonstrated excellent long-term performance.

   The addition of a latex polymer into the cement-based mortar also adds some important properties. Polymer-modified mortars develop higher bond strengths because of increased adhesion. In addition, physical strengths are increased and permeability is decreased. These products are often used in areas where there is minimal cover (fewer than 2.54 centimeters) over reinforcing steel.

   Epoxy-modification uses a water-based epoxy resin combined with a cement-based mortar mixture. The epoxy adds even better chemical resistance properties to the mortar or coating.

   Epoxy materials are useful in reinforcing steel coatings and as bonding agents prior to patching. Their slow reaction rates give the contractor a long time in which to place the repair mortar after priming, and they develop excellent bond strengths and protect the reinforcing from corrosion.

   Pure epoxy resins used without any cement are widely accepted as the best adhesives in construction. These are used for structural repairs of cracks, as epoxy mortars when mixed with sand, as bonding agents, as anchoring or base plate grouts and as chemically-resistant coatings. Pure epoxy resins are the most versatile of all the polymer materials available, but, once again, the cost goes up.

   A number of other polymer-based materials are used as repair materials or protective coatings: polyesters, vinylesters, methacrylates, silanes, siloxanes, polyurethanes — the list goes on. What is important is that the polymer chosen as part of the repair and protection system be proven to meet the requirements of the project. Independent tests from the manufacturer are the best proof of performance.

   The latest in materials for repair and protection involve corrosion inhibitors. These chemicals can be added to new concrete as admixtures or can be surface-applied to existing concrete structures. Some have been proven to protect reinforcing steel and reduce the effects of corrosion. While none will completely stop corrosion, they can slow it dramatically and add years of service life to the structure.

   Always remember that "high-tech" materials do not replace the need for good concrete repair practices. Proper diagnosis, surface preparation, mixing and application procedures are the most important steps to a successful repair project.

   Rick Montani is national marketing manager for concrete restoration with Sika Corp., a worldwide producer of concrete admixtures and repair materials. This article is based on his presentation to the Grain Elevator and Processing Society at the GEAPS '95 Exchange conference in Seattle, Washington, U.S.

Possible root causes of visible concrete defects


Possible cause


Corrosion of rebars


Excess loading


Improper joint




Corrosion of rebars


Freeze/thaw damage


Excess loading


Reactive aggregates

Failed joints

Improper design/spacing


Excess movement


Sealant failure


Nosing failure


Excess abrasion


Salt/chloride exposure


Chemical exposure


Poor quality concrete