Reinforced concrete takes damages from the environment slowly over time, gradually compromising its structural capacity. This is the degradation of reinforced concrete. The principle of durability design is to slow down the damage process sufficiently so that the reinforced concrete sustains the damage over the required period of time.
As we know reinforced concrete consists of two parts – concrete and steel reinforcement. Relatively speaking, concrete is much less vulnerable and more durable than steel reinforcement. Steel reinforcement is typically placed close to the surface of the concrete, with a layer of concrete cover. When the concrete cover is penetrated, and the steel reinforcement is exposed, degradation rate goes exponentially. When the steel is damaged, the first to suffer is the bond between concrete and steel. Since reinforced concrete relies on the composite action between steel and concrete, thus relying on the shear capacity at the interface (so does any composite structure), when the interface shear is gone, the composite action is gone, which hugely compromises the overall structural capacity. The picture below shows a piece of degraded concrete in a coastal environment. Both concrete and steel reinforcement are heavily damaged due to cyclic wet-and-dry conditions and aggressive chemical content of the environment.
This post outlines the common sources of degradation hazards, and the solutions for improving durability of reinforced concrete.
Chemical damage
Carbonation – penetration of CO2. Concrete under the microscope has lots of mini voids, even when perfectly compacted. This is because there is usually more water in the concrete mix than necessary for the curing (hardening) of concrete, because we need water to make concrete workable for pumping it into the right place. The CO2 in the atmosphere can very slowly creep into the water in the mini voids of concrete, and make it gradually more acid. This will eventually start to corrode the reinforcement steel wrapped inside the concrete.
Acid. Other acid could also penetrate the concrete cover and reach reinforcement steel in similar fashion as CO2.
Chloride penetration. Once getting into concrete, chloride can affect the chemical content of the water in the mini voids of concrete, starting the corrosion of reinforcing steel. Chloride mostly come from salt, contained in sea water, coastal air and de-icing salt on highways, etc.
Alkali-aggregate reaction. There are many micro voids invisible to the naked eye in hardened concrete. These voids usually contain alkaline solution water (pH>7). Some aggregates used in the concrete may react with the alkaline, generating products that swell when in contact with water, cracking the concrete.
Sulphate attack. Sulphate can be found in some groundwater and gases with odours, such as sewage. When sulphate enters concrete, it reacts with the cement in concrete creating something called ettringite, which is a kind of crystal. As these crystals form, they squeeze the concrete causing it to crack.
Physical damage
Abrasion. Abrasion can come from vehicle wheels, grit or particles carried by wind or water current.
Freeze-thaw. There are many micro voids in concrete that could be saturated with water. The volume of water gets slightly bigger when frozen, and this is why ice always float above water. When water-saturated concrete is subject to cyclic freezing and thawing, the water within the micro voids expands and contracts, cracking the concrete.
Fire. Concrete itself is reasonably immune to fire, since it is a very good insulation material. However, the reinforcement steel can melt like cheese once fire reaches it, and concrete will deform and crack due to the loss of section capacity. Extreme high temperature could lead to explosive spalling of concrete cover.
Cracks. Cracks in concrete could be caused by many reasons including structural loading, shrinkage and thermal actions. The cracks themselves do not deteriorate concrete directly, but leave an opening for chemical deteriorations happen more directly and deal damage faster.
Common solutions
Increase of cover to reinforcement. Generally speaking, most of the chemical reactions that affect reinforcement steel take time to penetrate the concrete cover. Therefore having sufficient concrete cover can defer the damage and ensure the useful life of concrete. More cover = more durability. This point is also mentioned in this previous post on durability:
Adjusting concrete mix. Cement replacement such as Fly Ash or GGBS can effectively reduce the attack targeted at the cement. Minimum strength should be achieved to ensure strong enough cement paste. Reducing water content can reduce the amount and size of the mini voids, thereby reducing the attack through the chemical changes of water in the mini voids. When reasonable workability of concrete must be ensured, plasticisers are required to be able to reduce water content lower than the minimum practical threshold.
Control of cracks. Cracks can happen both during the curing process and over the useful life of the structure. Cracks can be controlled with structural measures such as reinforcement and prestress. Careful planning of joints, such as expansion/contraction and movement joints can help alleviate the structure from unnecessary stress and lower the risk of cracking.
Treatment of reinforcement. Steel is the weaker linked compared to concrete itself in the whole of reinforced concrete composite. As can be seen from the degradation mechanisms, although a lot of deterioration starts in concrete, the ultimate failure of most of them is reinforcement steel. To protect reinforcement at the ‘receiving end’, reinforcement can be galvanised or coated (such as epoxy) in order to be protected from corrosion, even if the surrounding concrete is heavily cracked or spalled. The extreme would be to use stainless steel, but this option is rarely used, and even if used it would be for a very local area, due to the hefty price tag of stainless steel.
Maintenance and repair. In addition to the above preventative approaches by design, a good maintenance and repair regime during the service life of reinforced concrete would substantially increase the actual useful life, regardless of how robust the design is. In fact, the nominal design life is based on the implicit assumption that a reasonable maintenance and repair regime is in place. Repairs such as injecting cracks and patching up spalled concrete would largely prevent the need for more extensive and expensive repairs such as local re-build.
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