Causes of Concrete Decay
Concrete, the building material of today has been in existence for more than a hundred years.
It was the idea of the French gardener Josef Monier to make flower pots of cement mortar more durable by using wire netting - the earliest form of concrete reinforcement. The combination of the compressive strength properties of concrete with the tensile strength of steel, provides the ideal construction material. Reinforced concrete is typically made on site . Unlike factory made products, variations can lead to poor quality and questionable durability of thismanufacturing method. These potential quality problems of concrete are increased by environmental influences, such as water ingress, carbonation and chloride attack.
Carbonation: Concrete hardens chemically. During the reaction between the clinker and the mixing water, crystalline hydrate phases ( including calcium silicate hydrate) and calcium hydroxide are formed, giving an alkaline environment with a pH-value of 12 to 13.
In a stable, highly alkaline concrete, a passivation layer on the reinforcing steel is formed which protects it from corrosion.
Under atmospheric conditions, the alkalinity of the concrete is reduced due to carbonation. The calcium hydroxide of the cement reacts with the carbon dioxide from the atmosphere to form calcium carbonate, which reduces th pH-value of the concrete. When the pH-value falls below 9.5 the passivation layer around the steel will be destroyed.
Due to the presence of oxygen and moisture, the steel will begin to rust. When steel rusts, it increases its volume considerably. This expansion causes cracking of the concrete and eventually spalling. The rate of carbonation depends on the concrete quality. For example, precast concrete having a strength of 45 N mm (B45) has a typical carbonation depth of 3 mm after a period of 20 years. However, lower quality concrete (B25) can have a carbonation depth in excess of 15 mm after th same period.
Poor quality concrete: Potential causes of concrete decay can beintroduced during the manufacturing process. For example, incorrect water/cement ratios, poor compactions, leaking shuttering and insufficient curing cause an increased porosity. This means that concrete has a higher permeablility to water, oxygen and carbon dioxide.
Depth of cover: The compaction of the concrete and corect depth of cover are decisive facts in the longevity of reinforced concrete.
Water: The water absorption of concrete is a function of its porosity. Water penetration accelerates rusting in the neutral zone of the concrete. Also cracks may occu due to freeze/thaw action.
Other causes: Recent tests have shown, that the presence of harmful substances in the atmosphere, such as sulpher trioxide, do not greatly affect concrete.
This process described as sulphurisation will ometo a halt after a penetration depth of 1-2 mm into the concrete. The danger of decay is drastically increased by the prsence of chlorides. The use of de-icing salts or rapid chloride acceleration can lead to steel corrosion. Because of the various causes of decay it is essential that a detailed diagnosis of the structure be carried out to ascertain causes and cures.
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