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Damage due to fire on RCC Structure

1.1: Fire resistance 

       A  concrete element of construction when subjected to fire undergoes a gradual reduction in strength and rigidity. The fire resistance of an element of construction is expressed as the time for which the element resist any change, damage or spalling.

1.2: Factor that influences the fire resistance of the concrete structure

                The factor that influences the fire resistance of the concrete structure is as follows;-

1) the size and shape of an element.

2)Type of concrete.

3)Type of reinforcement.

4)Provision of protective concrete cover to the reinforcement.

5)Load supported by the unit.

1.3: Principle cause of fire in the building

                The three basic essential requirements before the fire can start are:-

1)Air

2)Fuel

3)Heat

 

               The process of combustion of burning requires all three factors to be present. Removal of any one or there will result in extinguishing of the fire and this is the principle on which all form of fire fighting are based.

              Two requirements fuel and air are always present so only heat is required to start a fire.

1.4: Effects of temperature on reinforced concrete 

             During a change of temperature, the small difference in the rate of thermal expansion between cement gel, aggregate and steel bar will cause one component tie with another which prevent free temperature deformation between them.

             Internal strength caused by temperature up to 100 º c is negligible and will not lower the efficiency of the reinforced concrete but the higher temperature will have adverse effect 25% of the strength is lost  at                                        

200 º c to 250 º c. If heated to 500 º c to 600 º c and then cooled the aggregate will fail completely.

             The reason for failure under higher temperature are:-

1)Great internal stress due to temperature deformation between cement gel and aggregate.

2)Chemical dehydration of cement minerals which lead to segregation of free lime followed by ruptured of concrete.

1.5: Repair of RCC structure damage due to fire 

     A damaged structure is usually found spalling of concrete from reinforcement due to improper cover or due to exceeding of fire resisting period of concrete.

                    through a survey of damage of structure is required to collect much information from the site and locality. After the survey following actions is carried out.

1)Duration of fire

              The fire in the zone might have lasted for a long period but individual member might have been under the influenced of fire for a much shorter period.

2)If spalling has occurred the thickness of concrete of each member need to be measured.

3)Ascertain if the depth of spalling had penetrated into the concrete beyond the cover.

4)The strength of the concrete is to be certain by testing a core concrete after removing the spalling and loose concrete.

5)Ascertain that the cement gel of the concrete is still alive, if destroyed it should be reestablished.

6)If reinforcement is exposed due to spalling, the strength of the steel should be tested by extracting by a piece of steel of about 300 mm long in a laboratory to determine the present state.

                             On receiving all the information the concrete up to the depth of cement gel has been damage, shall be removed and surface should be clean properly, the damaged concrete with cover may be re-establish if necessary  reinforcement may be provided as required, however if the concrete  has been damaged beyond the repair that is the cement gel of the standing damage structure has suffered destruction of the steel has lost strength, the structure will have to be demolished and repair.

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