A fire in a concrete structure causes damage. The extent of which depends upon the intensity and duration of the fire.
The principle types of damages are
ü Reduction in strength of concrete
ü Cracking and spalling of concrete
ü Deflection and deformation of members
Concrete structures are determined by three main factors:
ü The capacity of concrete itself to withstand heat
ü The conductivity of the concrete to heat
ü The coefficient of thermal expansion of concrete
A large number of reinforced concrete structures salvaged from destruction in fires by timely fire fighting operations can be put to further service after strengthening and providing some cosmetic repairs since the cost of restoration of such structures less than
that for dismantling and construction of new ones. The fire may cause different degrees of damage to the structure: the structure may be completely burnt or destroyed; its surface may be slightly damaged or slight deformation may occur. In the first case, the whole of damaged portion has to be replaced during restoration of structure while in the latter, only repair and finishing may be required. The extent of damage caused th the structure during a fire depends on the duration of fire, and the temperature to which the structure was subjected during the fire.
High temperature during a fire reduces the strength of reinforced concrete structures due to change in the strength and deformability of materials, reduction in cross sectional dimensions, weakening of bond between the reinforcement and concrete which determines structural action under the load.
When assessing the effects of a fire on a building structure, it is important to recognize that the huge expansion that occurs in the members subjected to the fire temperature may cause damage in other members remote from the fire.
Shear cracking can occur in columns and cracking resulting from inversion of moment may occur if detailing is not adequate
Restoration of fire Damaged Elements
The eccentrically loaded columns fail when reinforcement bars in tension heat up. The fire resistance of such elements can be increased by increasing the thickness of protective layer. Heat transmission and temperature of bottom reinforcement are keys to the behavior of reinforced concrete slab exposed to fire. The reinforcing bars are assumed to retain one half of their original strength. Carrying capacity of slabs can be enhanced by increasing their thickness. For beams, depth and width can be increased. It should be kept in mind that in beams, weakening of bond between transverse reinforcement and concrete on account of heating reduces the residual shear load carrying capacity considerably.
The carrying capacity of axially loaded depends upon the cross section of the column coefficient of change in strength of concrete under high temperature and corresponding critical temperature. The carrying capacity can be restored by increasing the cross section with suitable increase in the longitudinal steel.
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