What is Concrete Cancer?
Concrete cancer is a broad colloquial term used to describe several defects that can be found in concrete. Generally, this defect is defined as corrosion (rusting) of reinforcement inside the concrete. Alkali-silica reaction (ASR) is another phenomenon described by the term ‘concrete cancer’. In this article, we will explain how this damaging reaction affects structures and how to identify it.
Alkali-Silica Reaction (ASR)
Alkali-silica reaction occurs over time in concrete. It causes the formation of a swelling gel which exerts an expansive pressure inside the concrete. This leads to cracking of the concrete, and often discolouration on the surface.
ASR occurs when the alkaline solutions present in cement paste attack siliceous aggregate in the concrete mix. This reaction dissolves calcium ions into the cement pore water. Calcium ions react with the gel to convert it to calcium silicate hydrate. The alkaline solution converts reactive siliceous materials into an alkali silicate gel. The gel is hydroscopic (water loving) in nature and absorbs water, which causes expansion. This eventually cracks the aggregate and surrounding cement when the internal pressure exceeds the strength of the concrete.
For ASR to occur, there must be enough moisture present to transport and react chemicals, a high alkali content in the cement, and enough reactive minerals in the aggregate. Aggregates containing feldspars, micas, and some glassy minerals can release alkali compounds, while beach sands used in the mix can release chloride ions into the concrete. The reaction takes time, so it may not manifest for some years after the concrete is cast.
Steel rusts when exposed to air or water over time. Rusting causes steel to lose strength and expand. The reason it can be used as a structural element in reinforced concrete is because the reinforcement is protected by the alkaline nature of the concrete (the pH is usually more than 10). However, concrete is porous, and chemicals like carbon dioxide and water can work their way to the reinforcement. The less cover (depth the reinforcement is installed at) there is over the reinforcement, the faster this can occur.
Carbon dioxide is responsible for the carbonation in concrete. This process involves to the production of carbonic acid from atmospheric water and carbon dioxide that then reacts with the cement paste. The result of these reactions lower the pH of the concrete. Once this process reaches the reinforcement layer and sufficient water and oxygen is supplied, the once protected steel begins to corrode. Chloride ions (such as those from sea water) accelerate the rate of corrosion, which commonly occurs in seaside areas.
As the steel reinforcement begins to corrode, the structure loses strength. The concrete may also begin to swell and crack where the reinforcement has expanded. Material may also break away from the slab (a phenomenon called ‘spalling’). The surface of the concrete may become stained with the products of the rusting. This collective group of rust related concrete problems is called ‘concrete cancer’.
What to Look for?
The damage from concrete cancer is as follows:
- Concrete breaking away from surface (spalling)
- Concrete fallen off
- Steel reinforcement exposed
- Rust staining to concrete, especially
- Cracking (and staining in cracks)
- Surface looks almost like elephant skin
In some cases, there is no sign of damage on the surface. Frequent monitoring of the structure is important to observe any changes. A non-destructive half-cell potential test can be undertaken to determine the extent of reinforcement corrosion, and other non-destructive testing options can be performed to find other key signs of deterioration.
Treatment choice depends highly on the cause of the damage. These processes may include but are not limited to:
- Breaking out the old concrete, cleaning and patching up
- Chemical treatments
- Cathodic protection (passive/active)
- Additional waterproofing
The best way to mitigate ASR is with correct material choice. Alkali and reactive silica contents of the concrete should be controlled. Further improvements can be achieved from reducing exposure to water, especially salt water. Treatment for existing structures is limited in effectiveness as the reaction will always continue.
Rapid Consulting Engineers offer a range of testing solutions to determine the extent of carbonation, cover to reinforcement, likelihood of corrosion and other specialised non-destructive scanning services.