When a concrete structure is prone to chemical actions its durability gets affected. The chemicals may cause cracking of concrete, volume change and deterioration of structure. The life of structure reduces and it can lead to failure of structures. Different types of chemical attacks and their effects on concrete structures are explained below.
Types of chemical attacks on concrete structures
Following are the different chemical actions on concrete structures
- Sulphate attack
- Chloride attack
- Alkali aggregate reaction
- Acid attack
Sulphate attack on concrete
Most of the soil types contains sulphates in the form of calcium, magnesium, sodium, ammonium and potassium. They occur in soil or ground water. When a concrete structure is built on these types of soils, they may attack the concrete.
Generally sulphates in solid form do not attack the concrete severely but when they are in liquid form they pass into the voids of concrete and react with hydrated cement products. Calcium sulphate causes minimum damage because of its low solubility while magnesium sulphate causes maximum damage.
Most of the sulphates attacks calcium hydroxide and hydrated calcium aluminates present in the concrete and results in changing the volume of cement paste in concrete. Hence deterioration of concrete structure takes place. Along with calcium hydroxide, Magnesium sulphate also reacts with hydrated calcium silicate and makes concrete into powdered mass.
Concrete with low water cement ratio is less affected by magnesium sulphate while high water cement ratio concrete is highly affected.
Sulphate-resisting Portland cement should be used where sulphates are present in the soil, water or atmosphere and come into contact with the concrete.
Super-sulphated cement, made from blast furnace slag, can also be used although it is not widely available. This cement can resist the highest concentrations of sulphates.
Chloride attack on concrete
Chloride attack on concrete is one of the important aspects of durability of concrete. It primarily affects the reinforcement of concrete and cause corrosion. Chlorides can be introduced into the concrete either during or after construction as follows.
- Before construction Chlorides can be admitted in admixtures containing calcium chloride, through using mixing water contaminated with salt water or improperly washed marine aggregates.
- After construction Chlorides in salt or sea water, in airborne sea spray and from de-icing salts can attack permeable concrete causing corrosion of reinforcement.
The chloride in the presence of water and oxygen reacts with alkaline protected layer around the reinforcement and removes it.
Alkali-Aggregate reaction on concrete
Alkali aggregate reaction is the chemical reaction between alkali in cement and silica content of aggregates. Hence it can also be called as Alkali Silica reaction. When this reaction takes place, a gel like substance is formed which absorbs water and volume of concrete will increase. This increasing volume develops cracking and disintegration of concrete.
BS8110: Part 2, clause 22.214.171.124, states that the Alkali Silica reaction only occurs when the following are present together:
- Concrete with high moisture level.
- When cement contains high alkali content in it.
- Aggregate with Alkali reactive constituents.
The code recommends that the following precautions be taken if uncertainty exists:
- Reduce the Saturation of concrete.
- Usage of Low alkali Portland cement.
- Use replacement cementitious materials such as blast furnace slag or pulverized fuel ash. Most normal aggregates behave satisfactorily.
Carbonation in concrete
When the carbon dioxide from the atmosphere penetrates into concrete and reacted with calcium hydroxide to form calcium carbonate then this process is called carbonation.
In general concrete with high alkali content form a protective layer around the reinforcement. But when the carbon dioxide changes into dilute carbonic acid it reduces the alkalinity as a result the corrosion of reinforcement takes place.
Carbonated concrete has a pH value of 8.3 while the passivation of steel starts at a pH value of 9.5. The depth of Carbonation in good dense concrete is about 3 mm at an early stage and may increase to 6–10 mm after 30–40 years. Poor concrete may have a depth of Carbonation of 50 mm after say 6–8 years.
The rate of Carbonation depends on
- Depth of cover
- Concrete density
- Cement content
- Water-to-cement ratio
- The presence of cracks
The depth of carbonation is determined using the solution of phenolphthalein in diluted alcohol. When the solution is applied the Non – carbonation zones becomes pink in color and remaining uncolored portion is termed as carbon affected zone.
Acid attack on concrete
Acids can attack concrete easily since concrete is not fully resistant against acids. Some acids like oxalic acid, phosphoric acids are not harmful to the concrete. Calcareous aggregates are more affected by acids while siliceous aggregates are good resistant.
The damage level is purely depends upon the pH of the acid solution. Damage is very severe if the pH value is very low. If they reach reinforcement through crack or pores, they will cause corrosion of bars and cracking of concrete will occur.
Prevention of acid attack
To prevent acid attack good dense concrete with adequate cover is required and sulphate-resistant cements should be used.