Introduction
Concrete structures can suffer from a reduction in durability when exposed to chemical actions. This exposure can cause various negative effects such as cracking, volume change, and deterioration of the structure. These issues can lead to a decrease in the life of the structure and potentially cause it to fail. It is important to note that there are different types of chemical attacks that can affect concrete structures, each with their own specific effects. Understanding these types of attacks and their impact is crucial in ensuring the long-term durability of concrete structures.
Types of chemical attacks on concrete structures
Concrete structures can be subjected to various chemical actions that can cause damage and affect their durability. One of these actions is sulphate attack, where sulphate ions react with the components of the cement paste, leading to the formation of expansive products that can cause cracking and deterioration of the concrete.
Another type of chemical attack is chloride attack, which occurs when chloride ions penetrate the concrete and come into contact with the reinforcing steel. This can lead to corrosion of the steel, which can weaken the structure and reduce its load-carrying capacity.
Alkali aggregate reaction is another chemical action that can affect the durability of concrete. This reaction occurs when alkalis from the cement paste react with certain types of reactive aggregates, leading to the formation of an expansive gel that can cause cracking and deformation of the concrete.
Carbonation is a chemical process that occurs naturally over time as carbon dioxide in the air reacts with the calcium compounds in the cement paste, leading to a gradual reduction in the alkalinity of the concrete. This can cause corrosion of the reinforcing steel and deterioration of the concrete.
Lastly, acid attack can occur when concrete is exposed to acidic substances, such as acids or acidic salts, which can react with the cement paste and cause it to dissolve or deteriorate. This can lead to significant damage to the concrete structure if not properly addressed.
Sulphate attack on concrete
Sulphates are commonly found in soil and groundwater, with various forms such as calcium, magnesium, sodium, ammonium, and potassium. When constructing a concrete structure on these types of soil, the sulphates present may potentially cause damage to the concrete. While solid sulphates generally do not cause severe damage, liquid sulphates can penetrate the concrete voids and react with the hydrated cement products. Among the different types of sulphates, calcium sulphate causes the least amount of damage due to its low solubility. On the other hand, magnesium sulphate causes the most damage to the concrete structure.
The sulphates primarily attack the calcium hydroxide and hydrated calcium aluminates present in the concrete, resulting in changes to the volume of cement paste in the concrete. As a result, deterioration of the concrete structure may occur. Magnesium sulphate also reacts with hydrated calcium silicate, which can turn the concrete into a powdered mass. Therefore, it is essential to take into account the presence of sulphates in the soil and groundwater when constructing a concrete structure to avoid potential damage.
Precautions
Concrete that has a low water cement ratio is more resistant to damage from magnesium sulphate compared to concrete with a high water cement ratio. When sulphates are present in the environment and come into contact with concrete, it can cause damage. Therefore, it is recommended to use sulphate-resisting Portland cement in such situations. This type of cement is designed to resist the effects of sulphates that may be present in the soil, water or atmosphere.
In cases where there are high concentrations of sulphates, it may be necessary to use super-sulphated cement. This type of cement is made from blast furnace slag and has the ability to resist even higher concentrations of sulphates than sulphate-resisting Portland cement. However, super-sulphated cement is not widely available in the market.
Overall, the choice of cement should be made based on the level of sulphates that the concrete is likely to be exposed to. Using the appropriate cement can ensure that the concrete remains durable and can withstand the effects of sulphates for an extended period.
Chloride attack on concrete
Concrete durability is a crucial factor in ensuring the longevity of structures. One of the significant aspects of concrete durability is its resistance to chloride attack. Chloride attack mainly affects the reinforcement of concrete and can cause corrosion, leading to structural damage.
Chlorides can enter the concrete during or after construction. Before construction, chlorides may be present in admixtures containing calcium chloride, in mixing water contaminated with salt water, or in improperly washed marine aggregates.
After construction, chlorides can enter the concrete from various sources, including salt or sea water, airborne sea spray, and de-icing salts. Permeable concrete is particularly vulnerable to chloride attack, which can result in the corrosion of reinforcement.
In the presence of water and oxygen, chlorides react with the alkaline protected layer around the reinforcement, leading to its removal. As a result, the reinforcement becomes exposed and vulnerable to corrosion, potentially causing significant damage to the structure.
Alkali-Aggregate reaction on concrete
Alkali aggregate reaction, also known as Alkali Silica reaction, is a chemical process that occurs between the alkali present in cement and the silica content in aggregates. This reaction leads to the formation of a gel-like substance that absorbs water, causing an increase in the volume of concrete. As a result, the concrete may develop cracks and eventually disintegrate.
According to BS8110: Part 2, clause 6.2.5.4, the Alkali Silica reaction only occurs when certain conditions are met. These conditions include concrete with high moisture levels, cement with high alkali content, and aggregates containing alkali reactive constituents. When these three factors are present together, the Alkali Silica reaction can occur, leading to the potential failure of the concrete structure. It is, therefore, important to carefully consider these factors when designing and constructing concrete structures to ensure their durability and longevity.
Precautions
If there is any uncertainty regarding the concrete being used, certain precautions are recommended to ensure its quality. One such precaution is to reduce the saturation of the concrete. Another option is to use low alkali Portland cement, which can help maintain the stability of the concrete. Additionally, replacement cementitious materials like blast furnace slag or pulverized fuel ash can be used to improve the quality of the concrete. Fortunately, most normal aggregates are suitable and will perform adequately in most situations. By following these recommended precautions, it is possible to improve the quality and durability of concrete, even in situations where uncertainty exists.
Carbonation in concrete
Carbonation is a process that occurs when carbon dioxide from the atmosphere enters concrete and reacts with calcium hydroxide to form calcium carbonate. In concrete with high alkali content, a protective layer forms around the reinforcement. However, the presence of dilute carbonic acid, which is formed when carbon dioxide changes, reduces the alkalinity of the concrete. This reduction in alkalinity leads to the corrosion of the reinforcement. Thus, carbonation can have a negative impact on the durability and strength of concrete structures. It is important to take steps to prevent or mitigate carbonation in concrete to ensure the longevity and safety of buildings, bridges, and other infrastructure.
Carbonated concrete typically has a pH value of 8.3, while the passivation of steel begins at a higher pH value of 9.5. The depth of carbonation in good, dense concrete usually starts at around 3 mm during its early stage and may gradually increase to 6–10 mm after a period of 30–40 years. However, poor quality concrete may experience a depth of carbonation of up to 50 mm within a period of 6–8 years. The rate of carbonation is influenced by various factors such as time, depth of cover, concrete density, cement content, water-to-cement ratio, and the presence of cracks.
To determine the depth of carbonation, a solution of phenolphthalein in diluted alcohol is applied to the concrete surface. As a result, the non-carbonated zones become pink in color, while the remaining uncolored area is referred to as the carbon-affected zone. By understanding the depth of carbonation in concrete, it is possible to predict the potential impact on the reinforcement steel, which could eventually lead to the deterioration of concrete structures. Therefore, it is essential to ensure that concrete is constructed using quality materials and appropriate construction techniques to minimize the risk of carbonation.
Acid attack on concrete
Concrete is not completely immune to acid attacks, which can cause significant damage. Certain types of acids, such as oxalic and phosphoric acids, may not harm the concrete. However, the susceptibility of concrete to acids varies depending on the type of aggregates used in its composition. Calcareous aggregates are more vulnerable to acids than siliceous aggregates, which have better resistance.
The extent of damage caused by acid attacks is directly related to the pH level of the acid solution. In cases where the pH level is very low, the damage to concrete is severe. If the acid solution penetrates the concrete and reaches the reinforcement through cracks or pores, it can corrode the bars and cause the concrete to crack.
Therefore, it is important to protect concrete structures from acid attacks, especially in industrial environments where acids are commonly used. Protective coatings or sealers can be applied to the surface of the concrete to prevent acid penetration. Additionally, selecting the appropriate type of aggregate for the concrete mix can enhance its resistance to acid attacks.
Prevention of acid attack
In order to prevent acid attacks, it is important to ensure that the concrete used is of good density and has sufficient cover. Additionally, the use of sulphate-resistant cements can also help to mitigate the risk of acid attacks.
Acid attacks can cause significant damage to concrete structures, leading to deterioration and potentially compromising the integrity of the entire structure. Therefore, it is crucial to take steps to prevent acid attacks from occurring in the first place.
One way to do this is by using concrete that is dense and well-constructed, as this can help to resist the corrosive effects of acid. It is also important to ensure that there is adequate cover, meaning that the concrete is protected by an appropriate layer of material.
Furthermore, the use of sulphate-resistant cements can also be beneficial in preventing acid attacks. These types of cements are specifically designed to be more resistant to the corrosive effects of sulphates and acids, making them a suitable choice for structures that may be at risk of acid attacks.