Concrete is a strong and durable material, but it can be damaged by a variety of factors. Fire, aggregate expansion, seawater effects, bacterial corrosion, calcium leaching, physical damage, and chemical damage from carbonation, chlorides, sulphates, and non-distilled water can all cause concrete to disintegrate. This process weakens the concrete and can lead to cracks forming after two to three years. Building components that are more susceptible to sulphate attack include concrete exposed to these damaging stimuli.
Sulphates react with the tricalcium aluminate in cement in the presence of moisture to form products that occupy a much larger volume than the original constituents. This expansive reaction causes disintegration and weakening of the concrete, masonry, plaster, and cracks. Alkalis such as sodium oxide (Na2O) and potassium oxide (K2O) are present in ordinary Portland cement (OPC). These alkalis react chemically with certain siliceous minerals (components of some aggregates) and cause expansion, cracking, and disintegration of the concrete.
The decrease in alkalinity also promotes oxidation of the reinforcement when moisture is present. Carbonation is another factor that can cause concrete to deteriorate. Carbon dioxide in the atmosphere reacts with calcium hydroxide in the concrete to form calcium carbonate. This reaction is accelerated by moisture and can lead to corrosion of the reinforcement if the concrete is permeable or if the reinforcement is too close to the surface due to inadequate coating.
Carbonation is more rapid in a dry atmosphere but corrosion of steel requires an alternation of dry and wet weather. Cracks and voids in concrete contribute to early carbonisation, particularly in industrial cities with higher levels of pollution. Chloride solutions can also attack cement paste and cause disruptive action on concrete similar to sulphate attack. Concrete deterioration can cause major headaches for building owners.
It is important to correctly identify these defects early and plan appropriate repair strategies. Disintegration is one type of deterioration that involves physical breakdown into small fragments or particles. To avoid these issues, consider using portland cement in combination with alternative cementitious materials such as slag or low alkali fly ash to decrease permeability and reduce the amount of alkali in the concrete. When placing concrete, ensure ambient conditions are not conducive to rapid evaporation of bleed water and avoid finishing the slab prematurely.
Create a highly impermeable concrete mix by using a low water-cement ratio mix (typically no more than 0.40). Examining the specific causes of concrete degradation will help create ideas on how to avoid them. Michael Brainerd from Simpson Gumpertz & Heger details five common ways concrete can come to an end and offers advice on how to avoid them. For critical structures, Brainerd often requires that not only the total air content of the fresh concrete be checked but also the air void system in the hardened concrete. When the concrete starts to degrade or reinforcing steel starts to degrade, it cannot withstand so much stress.
Structural concrete may no longer be up to its job once deterioration begins. It is important for building owners to identify these defects early and plan appropriate repair strategies.