Concrete Thoughts

4 ways water deteriorates concrete

A concrete structure that is constantly exposed to water in form of humidity, torrential rain, or water bodies are at an extremely high risk of shortened life. Reinforced concrete infrastructure, found in dams, or structures around the marine area and also in high rainfall areas 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.

To ensure that there is increased durability in concrete structures and that they do not face a reduced life span due to water, steps must taken to protect them.

This starts with understanding how water can damage concrete.

1. Corrosion of steel reinforcement

Electrolyte for ion transfer (water), conductor for electron transfer (steel reinforcement), and oxygen are the three essential elements that cause corrosion. Eliminating any one of the these mitigates damages that may occur due to corrosion.

We never see any corrosion in dry concrete, and that is because one of the three elements essential for corrosion is missing (water). This is why it is 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 or low quality concrete has more connected pores and larger capillaries which increases the ingress of detrimental substances such as Chlorides into the concrete. These chlorides can enter through the pore network, breaking down the passive protection layer around the rebar, and without this passive iron oxide film protecting the steel, corrosion is able to commence at a much faster and higher rate.

3. Sulfate attack

The most common type of Sulfate attack is through external means, whereby water containing dissolved sulfate penetrates the concrete. Occurring as a result of high-sulfate soils, ground waters, and also atmospheric or industrial water pollution it typically changes the composition and microstructure of the concrete and leads to:

• Extensive cracking
• Expansion
• 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, this may cause slow deterioration of the concrete through expansion and cracking. These hairline cracks are an invitation for water to cause corrosion of the rebar even in above-grade structures.

The two forms of alkali-aggregate reaction are:

1. Alkali-Silica reaction (ASR)
2. 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.
(Lead author Sarah Coull)