Concrete can be damaged by 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. Soluble 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, resulting in an expansive reaction that causes disintegration and weakening of the concrete, masonry, plaster and cracks to form. The reaction is very slow and cracks start to show after 2 to 3 years.
Therefore, the following building components are more susceptible to sulphate attack. The severity of sulphate attack depends on In OPC, alkalis, i.e. sodium oxide (Na2O) and potassium oxide (K2O), are present to some extent. These alkalis react chemically with certain siliceous minerals (components of some aggregates) and cause expansion, cracking and disintegration of the concrete.
Due to the decrease in alkalinity, oxidation of the reinforcement is also promoted in the presence of moisture. Like sulphate attack, this reaction is also very slow and takes several years to develop cracks. In good quality dense concrete, carbonation is mainly limited to the surface layers of the concrete and the depth of carbonation may not exceed 20 mm in 50 years. Therefore, when the concrete is permeable or when the reinforcement is too close to the surface due to inadequate coating, carbonation results in corrosion of the reinforcement, which eventually leads to cracking and disintegration of the concrete.
Carbonation is more rapid in a dry atmosphere, but since the presence of moisture is necessary for galvanic action to occur and thus for corrosion of the steel, an alternation of dry and wet weather is more conducive to corrosion. Cracks and voids in concrete contribute to early carbonisation. In industrial cities, which have a higher percentage of carbon dioxide in the atmosphere due to pollution, the cracks caused in concrete due to carbonation are comparatively much more. A high concentration chloride solution can attack the cement paste of concrete and can cause a 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. Concrete deterioration can occur through scaling, disintegration, erosion, reinforcement corrosion, delamination, spalling, alkali-aggregate reactions and concrete cracking. Disintegration is the physical deterioration (such as spalling) or breakdown of concrete into small fragments or particles.
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. The alkali-silica reaction occurs when certain silica-containing aggregates react to form an expansive gel that causes the concrete to crack. Also, contractors should place concrete when ambient conditions are not conducive to the rapid evaporation of bleed water, and should avoid finishing the concrete slab prematurely. In addition, create a highly impermeable concrete mix by using a low water-cement ratio mix (typically no more than 0.40) "so that chlorides or carbonation take longer to reach the steel, says Brainerd.
The resulting pressure created within the concrete will lead to cracking and severe deterioration of the structure over time. Examining the specific causes of concrete degradation will help create some ideas on how to avoid them. This reaction known as carbonation also decreases the alkalinity of the concrete and reduces its effectiveness as a means of protecting the reinforcement. Scaling occurs when the hydraulic pressure of freezing water within the concrete exceeds the tensile strength of the concrete.
Chemical attack takes place at the microscopic level, where sulphate ions weaken the pasty compounds that bind the concrete together. This means that structural concrete may no longer be up to the job it was built to do once deterioration begins. Michael Brainerd, principal of Boston-based 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 the reinforcing steel starts to degrade, it cannot withstand so much stress.