Chemical reactions in concrete and chemical reactions in reinforcement lead to cracks in concrete and are called chemical attack on concrete. Furthermore, environmental conditions have a great influence on the formation of cracks, as they provide the necessary components for the reactions.
Chemical reactions create additional tensile stresses in the concrete and cause cracking. There are three main causes that can cause cracks due to chemical attack on concrete.
- Corrosion of reinforcements
- Alkaline aggregate reaction
- Cement carbonation shrinkage
Influence of reinforcement corrosion on the formation of cracks in concrete
Reinforcement corrosion can occur in any environment and does not necessarily need to occur in extreme environmental conditions. If the basic requirements such as oxygen, moisture and electron movement are present, the reinforcement will crack.
Therefore, it is necessary to block them to prevent their combination. It is difficult to prevent the movement of electrons; however, contact of the reinforcement with moisture and oxygen could be minimized to reduce corrosion.
Chemical reactions produce byproducts of iron oxides and hydroxides, which increase the volume of the concrete. The increase in volume creates additional stresses in the concrete and when the tensile stresses are exceeded, the concrete begins to crack.
It is essential to resolve these cracks, as they expose the reinforcement to the environment that provides oxygen and moisture, which in turn accelerates corrosion of the reinforcement.
Steel corrosion can be caused by chlorides or carbonation in concrete. This can happen with both cracked and uncracked concrete. The following figure (from the Internet) shows the level of damage that can be expected at each stage of chemical attack on concrete.
For more information, see the article “ Carbonation Test ” to learn more about carbonation and the article “ Chloride Test for Concrete ” or chlorination and its effects. Furthermore, to prevent cracks due to corrosion of reinforcement, measures must be taken to prevent corrosion of reinforcement. For this purpose, the following methods can be used.
- Use anticorrosive agents
- Use anti-corrosion coating, which is not practiced when dealing with large quantities
- Use waterproofing agents
- Cathodic protection
- Use anti-corrosion reinforcement or corrosion-resistant reinforcement
Influence of the alkali-aggregate reaction on the formation of cracks in concrete
Alkaline aggregate reactions are not common in all structures. This chemical attack on concrete occurs in certain situations, as explained here.
Most of the time, aggregates are chemically inert. However, they sometimes react with the alkaline hydroxides in concrete.
It causes expansion and cracks in concrete. N/A 2 Ó and K 2 The alkaline substances present in the cement react with the silica fume in the aggregate. The following factors support the alkali-silica reaction.
- Aggregate reactivity
- Alkaline content in cement
- Continuous moisture availability
- Temperature conditions
What happens during the alkaline aggregate reaction?
- The soluble alkali in the cement dissolves in the mixing water and forms a caustic liquid
- The liquid reacts with the reactive aggregate and forms an expansive alkaline silica gel
- With constant availability of water and the right temperature, silica gel continues to form, resulting in an increase in volume.
- The reaction can lead to separation of the aggregate (detachment).
- As silica gel is included in the slurry, its continuous growth increases the internal hydraulic pressure
- Causes a pattern of cracks
- Cracks reduce the strength of concrete
- Cracking accelerates the decomposition process similar to carbonation.
The following figure shows the typical crack pattern that can be observed due to the alkaline aggregate reaction.
Various methods are used in the construction industry to minimize or control the alkali-silica reaction.
- Avoid using reactive aggregates
- Use low alkali cement in the range of 0.4-0.6%.
- Continue monitoring water availability
- Control the ideal temperature
- Add finely powdered reactive silica to the concrete mix. This creates alkaline calcium silicate, which does not expand.
Reduction of cement carbonation as a chemical attack on concrete
Carbonation shrinkage occurs when hardened concrete is exposed to air containing carbon dioxide.
When concrete hardens, carbon dioxide reacts with the concrete in the presence of water.
Calcium hydroxide is converted to calcium carbonate and other components of the cement are decomposed. This process is also possible in a low carbon environment.
During this process, the weight of the concrete increases and it undergoes irreversible carbonation shrinkage. During carbonation contraction, calcium hydroxide crystals dissolve and their place is taken by calcium carbonate.
The volume of calcium carbonate (replacement material) is less than the volume of calcium hydroxide (previous material), causing shrinkage. This can cause the concrete to crack.
Furthermore, carbonation offers another advantage, as it increases the strength of the concrete and increases permeability.
