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Why protect structures in a concrete and masonry mixture, reinforced or pre-stressed concrete?

The concept of the concrete cover

Buildings for industrial use and also some other residential and other buildings are built using prefabricated systems or elements assembled on site. There is widespread use of reinforced or pre-stressed concrete, as is a masonry and concrete mix for flooring. The combination of steel and concrete and masonry makes these elements particularly sensitive to fire.
 
The behaviour of structures in reinforced or pre-stressed concrete.
Reinforced concrete has two components, both of them non-combustible but with different results and mechanical behaviour in terms of temperature that can prejudice their load-bearing function. Concrete is not affected by fire but the steel bars that provide the internal reinforcement can deform when exposed to heat.
The concrete in reinforced concrete structures therefore performs a dual function, that of bearing external loads and of insulating the steel, protecting it from thermal stress. The steel within the reinforced concrete can also have two functions, those being load-bearing and maintaining stability in adverse seismic conditions.
The behaviour of reinforced or pre-stressed concrete structures when exposed to fire is influenced by a number of factors, the most relevant being the thickness of the concrete covering the reinforcing bars, known as concrete cover, the degree of internal stress, the static framework and the thermal and mechanical properties of the materials in the face of temperature variations. 
It is rare for non-pre-stressed reinforced concrete structures to suffer major collapse in fire but this is not the case for fires involving pre-stressed reinforced concrete where the reinforcement also plays a major role in mechanical resistance and when the degree of stress typical of this type of concrete makes it highly sensitive to heat. If this type of structure has not been properly designed or protected it frequently collapses in fire, the result of the concrete breaking up, or spalling.
 
The behaviour of masonry and concrete structures.
Masonry and concrete flooring generally consists of beams in reinforced or pre-stressed concrete which function mainly as load-bearing elements, and of masonry used to lighten or fill in with an insulating function. This type of flooring, in addition to the requirement for fire resistance over time (R) also has a compartmentalisation function, requiring E and I rating. The fire-resistance function is provided by guaranteeing the load-bearing capacity of the reinforced or pre-stressed concrete beams, while the compartmentalisation function is guaranteed by the integrity over time of the masonry.
 
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