Assessment of potential exposure before construction or repair is essential to prevent premature deterioration of concrete. Specific cement types, water repellent sealants, and chemical resistant barrier coatings are common preventive measures to protect concrete against chemical attack. Steel reinforcement is often used in concrete applications to make the structure more solid, stronger, and safer. However, steel can be susceptible to corrosion, especially in cold climates.
Once corrosion starts to spread, it is almost impossible to stop it and repairing an isolated area will not solve the problem. To prevent corrosion of steel reinforcement, make sure there is at least 1.5 to 2 inches of concrete over the reinforcement. A waterproof concrete mix with a low water-cement ratio is best for protecting the steel. In addition, other corrosion inhibiting materials such as an epoxy coating and penetrating sealants help ensure that the steel remains effective. Corrosion is the deterioration of steel reinforcement in concrete caused by chloride or carbonation.
Corrosion can lead to the appearance of cracks in the concrete cover, delamination in concrete covers, etc. Concrete degradation can have several causes such as fire, aggregate expansion, seawater effects, bacterial corrosion, calcium leaching, physical damage, and chemical damage (from carbonation, chlorides, sulphates and non-distilled water). This process adversely affects concrete exposed to these damaging stimuli. Durability begins with a concrete mix design appropriate to the service conditions to which the concrete will be exposed.
The water-cement ratio and cement content must provide sufficient paste to fill the voids in the compacted concrete. Chloride ions in reinforced concrete can cause pitting corrosion of the reinforcing steel bars (rebar). Due to its low thermal conductivity, a concrete coating is often used for fire protection of steel structures. Concrete in buildings that have suffered a fire and have been standing for several years shows an extensive degree of carbonation due to carbon dioxide being reabsorbed. Ground penetrating radar is an accurate method that can be used along with other methods such as concrete radiography and electromagnetic conductivity. Corrosion can decrease the load-bearing capacity of the structure by causing loss of concrete, loss of bond between the reinforcing bars and the concrete, and decrease in the thickness of the reinforcing bars themselves.
Although concrete is very durable, every effort must be made to stop premature deterioration of concrete structures. Sulphates in solution in contact with concrete can cause chemical changes in the cement which can cause significant microstructural effects leading to weakening of the cement binder (sulphate chemical attack). Steel formwork pinches the top surface of a concrete slab due to the weight of the next slab being built. When the concrete temperature exceeds 65 °C for too long at an early age, ettringite (AFt) crystallisation does not occur due to its higher solubility at elevated temperature and monosulphate (AFm) is formed. Exposure of concrete structures to neutrons and gamma radiation in nuclear power plants and high flux material test reactors can induce radiation damage to their concrete structural components.
Carbon dioxide (CO2) from the atmosphere diffuses easily into leachate and causes a chemical reaction that precipitates (deposits) calcium carbonate (CaCO3) on the outside of the structure. The use of aggregates with similar particle sizes normally results in a weakly consolidated concrete.