Concrete is a strong and durable material, but it is not indestructible. Over time, concrete can suffer from deterioration due to a variety of causes, including scaling, disintegration, erosion, reinforcement corrosion, delamination, spalling, alkali-aggregate reactions and concrete cracking. These processes can be caused by fire, aggregate expansion, seawater effects, bacterial corrosion, calcium leaching, physical damage and chemical damage (from carbonation, chlorides, sulphates and non-distilled water). In addition to these external factors, concrete also shrinks as it cures.
This shrinkage can lead to cracking. When exposed to water, concrete also creeps or deforms over time. The cause of this creep is still unknown. Corrosion of steel reinforcement in concrete can also lead to cracks in the concrete cover and delamination in concrete decks.
Disintegration is the physical deterioration (such as flaking) or breakdown of concrete into small fragments or particles. Erosion is the deterioration of the concrete surface as a result of moving water particles rubbing against the surface. Inadequate curing can lead to the development of micro-cracks perpendicular to the surface of the concrete due to drying shrinkage. Mixing and segregation can be assessed by searching for domains in the cement paste with less or no (fine) aggregate at the microscopic level.
If the concrete is exposed too quickly to very high temperatures, explosive spalling of the concrete can occur. To protect against increased levels of chemicals such as chlorides, a sealer, polymer coating or latex-modified concrete coating can be used. Patching localised areas of concrete deterioration has proven to be an effective means of increasing the life expectancy of a structural element. Finite element models have been developed to assess expansive behaviour of concrete gravity dams due to alkali-aggregate reactions.
Carbon fibre cast in concrete using the VARTM method has been found to increase shear and flexural strength of a beam. Carbonation due to carbon dioxide being reabsorbed can occur in buildings that have suffered a fire and remained standing for several years. Microindentation and vertical scanning interferometry have been used to determine mechanisms for deterioration. When one area of the concrete is stressed (crushed, stretched or bent), C-S-H compounds dissolve in the concrete and are deposited in a nearby non-stressed area.
To maximise the benefits of repairs incorporating patches that are the result of corrosion, the reinforcing steel should have the surrounding concrete completely removed.