Any concrete structure that is in close proximity with water faces a myriad of life shortening processes. Reinforced concrete infrastructure, found in marine environments, commonly face reduced life spans due to exposure to extreme environmental conditions, which allow water and waterborne chlorides to penetrate through the concrete to the reinforcing steel. This contact results in corrosion and expansive cracking, which leads to premature deterioration.
In order to ensure concrete structure do not face a reduced life span due to water, steps must taken to protect them. The first step is understanding how water damages concrete in the first place.
1. Corrosion of steel reinforcement
There are three essential components necessary for corrosion to take place in reinforced concrete: electrolyte for ion transfer (water), conductor for electron transfer (steel reinforcement), and oxygen. Eliminating one of the above will mitigate the damages due to corrosion. This is why there is no corrosion in dry concrete; furthermore, this is also why it’s important to have low permeability concrete to prevent the movement of water and the harmful chemicals in solution from reaching the steel reinforcements.
Overall, Concrete is a great host for the rebar. Due to the high-alkalinity of concrete, the steel reinforcing bars develop a passive layer that provides a protective barrier to the steel. In this state, concrete normally provides reinforcing steel with excellent corrosion protection.
However, the passive layer can be broken down over time due to atmospheric carbon dioxide, causing carbonation, which lowers the pH of the concrete and destabilizes the passive layer. However, carbonation is a slow process and the overall rate depends on the density of concrete and humidity of the exposed environment. Durable concrete with low permeability can reduce the rate of carbonation, in addition to slowing down the rate of water penetration necessary for corrosion to occur.
2. Chloride attack
Poor quality concrete has more connected pores and larger capillaries which increase the potential for the ingress of detrimental substances into the concrete. Substances such as chlorides can enter into the concrete through the pore network, leading to the breakdown of the passive protection layer around the rebar. Without the passive iron oxide film protecting the steel, corrosion is able to commence at a much higher rate.
3. Sulfate attack
The most common type of Sulfate attack is through external means, whereby water containing dissolved sulfate penetrates the concrete. This is usually the result of high-sulfate soils and ground waters, but can also be caused by atmospheric or industrial water pollution, bacteria in sewers, or even just regular seawater.
A sulfate attack will typically change the composition and microstructure of the concrete and lead to:
Loss of bond between the cement paste and the aggregate
4. Alkali aggregate reaction (AAR)
Occasionally, certain aggregates can react with the alkali hydroxides in concrete, causing slow deterioration of the concrete through expansion and cracking. These hairline cracks which develop are an invitation for water to cause corrosion of the rebar even in above-grade structures.
There are two forms of alkali-aggregate reaction, Alkali-Silica reaction (ASR) and Alkali-carbonate reaction (ACR). ASR is the more concerning type of reaction, as it is more common to find aggregates that contain reactive silica materials, and the latter, ACR, is relatively rare.
With ASR, the silica in these aggregates react with alkali hydroxide in concrete and forms a gel that swells by absorbing the water in the surrounding cement paste, or any water that finds its way into the concrete. As the gel absorbs more moisture, the swelling effect can cause long-term damage to the concrete by inducing expansive pressure. Cracking is often an indicator that ASR is present, with the cracking often located in areas with a frequent supply of water or moisture.
5. Freeze/thaw cycles
Freeze/thaw actions will likely cause deterioration to non-air entrained concrete. When water freezes to ice, it occupies 9% more volume then that of water. With no available space for this increase in volume, freezing can cause distress to concrete leading to hairline cracks. Thawing will then allow water to penetrate through the cracks and with each freeze/thaw cycle increase the number and size of hairline cracks, resulting in greater damage to the concrete.
Some noticeable signs of freeze/thaw damage are spalling and scaling of the concrete surface, surface parallel cracking, or exposed aggregate.
In keeping the water out of concrete, damage to the structure as a whole – from corrosion, freezing and other water caused effects – can be eliminated.