Arab Construction World – November 2008
In September 2006, the de la Concorde overpass near Montreal, Canada, collapsed onto Highway 19, killing five people and seriously injuring six others. A public inquiry later determined that among the contributing factors for the collapse, corrosion of the reinforcing steel likely played a role by contributing to localized cracking in the concrete.
The concrete had never been properly waterproofed, and repair work carried out in 1992 included extensive concrete removal and resulted in rebar exposure that actually weakened the structure.
On its own, concrete has low strength when loaded in tension; hence, it is common practice to reinforce concrete for improved tensile mechanical properties. In fact, many buildings, roads, bridges, and parking structures would be impossible to build without reinforced concrete.
Steel is one of the many building materials used in construction today; it is generally the preferred option for reinforcing concrete due to its durability and strength, although, when steel corrodes and degrades, the structure can become compromised and tragedy can occur.
Concrete is a highly alkaline material with a pH of 12-13. When steel rebar is used in construction, the high alkalinity of concrete creates a passive film around the rebar, which protects it from corrosion. However, when water containing chloride ions penetrate concrete and reach the steel, or the the pH level falls it can destroy the protective barrier. This can cause the steel to rust and expand, cracking concrete and degrading the strength and quality of the structure. Because of this, it is important to consider taking extra precautions to waterproof the concrete in order to protect the steel.
Reasons for corrosion of steel in concrete
The two most common causes of reinforcement corrosion are chloride ions causing a localized breakdown of the passive film on the steel and a general breakdown of the passive film through a process called carbonation involving the neutralization of the concrete, predominantly through a reaction with atmospheric carbon dioxide. In either case, water is required to carry contaminants and facilitate the reaction that causes steel corrosion.
Reinforced steel can be used in any situation and be susceptible to corrosion. In environments that are repeatedly exposed to salt by way of marine water, salt spray, or road de-icing salt, the steel is particularly at risk of corrosion by chloride ions. The chloride ions from salt slowly penetrate concrete through the ingress of water into pores in the hydrated cement paste or through cracks in the cement. When a concentration of the chloride ions reaches the steel, they destroy the protective film and corrode the steel.
Carbonation is a natural process that begins the moment the concrete is poured. Carbon dioxide from the air reacts with the lime present in all cement paste. The reaction produces calcium carbonate, which lowers the pH level of the concrete. Carbonation begins at the surface and moves its way deeper into the pores of the concrete. If the concrete is cracked, the carbon dioxide from the air will penetrate the concrete more quickly, resulting in a lower pH level and greater chance of steel degradation.
The greatest defense against the corrosion of steel in concrete is to start with high-quality concrete. Concrete should have a water-to-cementitious-material ratio (w/c) that is low enough to slow down the penetration of chloride salts and the development of carbonation. The w/c ratio should be less than 0.50 to slow down the rate of carbonation and less than 0.40 to minimize the risk of chloride infiltration.
There are various means of reducing the permeability of concrete and producing concrete with a low w/c ratio, such as increasing the cement content in the concrete, adding a crystalline waterproofing product to the concrete mix, using large amounts of fly ash or other cementitious materials that act as corrosion inhibitors, and limiting the amount of ingredients that contain chloride.
In addition to using the proper amount of steel, it is important to use plenty of material to cover the steel. Corrosion takes place from the outside in. Applying a defensive coating on the steel will provide an initial layer of protection. In addition, applying double the minimum required amount of concrete can delay the penetration of chloride ions by as much as four times. A standard recommendation is one and a half inches of concrete cover for most structures, two inches for structures exposed to de-icing salts, and two and a half inches for marine environments.
When concrete is properly designed and carefully produced, placed, and cured; it is an inherently durable material that is resistant against corrosion. However, if one of these factors is missing, the result will be non-durable concrete. Therefore, it is as important to allow time for the concrete to be adequately cured, as it is to have well-designed and well-constructed concrete. Studies have shown that concrete porosity is significantly reduced through proper curing. However, the time required for curing varies greatly depending on the w/c ratio and the relative humidity. The use of certain concrete additives can also play a large role in protecting against corrosion. Fly ash, silica fume, and blast-furnace slag all help to reduce the permeability of concrete and resist the penetration of chloride ions. Calci. um nitrate also helps prevent corrosion in the presence of chloride ions.
The results of corrosion are not only unattractive, but they can present a major threat to public safety. Protecting against corrosion provides a long-term cost savings and ensures the sustained use of bridges, roads, and free-standing concrete structures.