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The release of high quantities of heat from a fire have an effect on the mechanical properties of a construction, as well as the consistency of concrete-reinforcement. In the design phase the engineer is required to give the construction the required degree of fire protection (active-passive protection). Active protection is a way to stop the ignition and spread of a fire. This depends on the location and layout of the building, proper use, the fireproof partitions, thermal insulation and the efficiency of systems and fire alarm. While the passive protection refers to the "resilience" of the construction. Ruggedness is defined:
The load capacity
Maintaining Integrity Fire paneling (slabs, walls) prevent the flames to penetrate
Maintaining Competency insulating paneling (slabs, walls) prevent the heat from transferring to the unexposed surface in order to avoid the fire spreading to neighboring houses .
1. Characteristics of Fire
A fire in a typical compartment (i.e. any area which is surrounded from all sides by either walls or doors and windows) consists of four stages :
a) Incubation: After initial ignition, hot gas layer is formed on the roof of the building. (Figure 1)
b) Aggravation: The accumulated gas layer in the roof causes inflammation of the available fuel in the compartment. (Figure 2)
c) Fully Developed: All the solid fuel of the compartment is burning whereas the flames and gases are transported through openings in adjacent compartment. (Figure 3)
d) Depreciation: The depletion of fuel accompanied by the removal of most of the hot gas. (Figure 4)
The aggravation lasts 15-30 minutes and the temperatures reach the 800-900 C. The comprehensive development burns anything that can be burned. The temperatures rise a bit more and go to 1000-1100 C, depending on the amount of flaming objects. While extinguishing the fire lessens and the temperatures drop quickly.
1.1Assessment of values of temperature by the colors formed on the walls
To estimate of the temperature in the different parts of the building it is of importance to check it assess the residual strength of the components. When the concrete is combusted, as the temperature drop becomes different colors. Experimental studies have shown that when the temperatures go up to 300-600 C the color of concrete is red or pink. But if the color is gray, this means that the temperature reached was 600-900 C. The colors are a first indication of the residual strength, so for example at a temperature of 600-800 C concrete loses 50% -80% of its endurance.
2. Materials of reinforced concrete under high
The following parameters for concrete and steel are the results of experimental processes conducted either during the exposure of specimens at high temperatures or 'after thawing. This variation is because there is partial recovery of the mechanical properties of materials, which had fallen during the fire. Concrete does not recover, however its mechanical properties significantly as opposed to steel. The graphs below specify if the measurements are made â€‹â€‹or not during the fire.
2. 1 Concrete
The concrete in terms of fire behavior is more resistant than the usual building materials for the following reasons:
a) The components are built of concrete is generally much more massive than the corresponding wood or steel, so delaying the temperature increase inside the element.
b) Even in a dry environment concrete contains water and this is because at 600 C concrete decomposes its external layers.
c) An important role in the high endurance of the concrete in fire play the type of aggregate. Limestone aggregates (such as those used mainly in Greece) are very strong and lose their strength at 900 C. ''Reference: Rehabilitation, Renovation and Repairs of Structures''
2. 2 STEEL
Steel is a non-combustible material that cannot withstand in temperatures that rise during a fire. Laboratory tests of the standard mild steel show that the tensile strength in the beginning increases initially heated up to 250 C and return to its initial at 400 C, where it falls back to 550 C and finally reaches the allowable stress in accordance with the usual safety factors. Steels which have become highly resistant to cold rolling fall faster because the annealing occurring lose the extra strength so the critical temperature is 400C - 450C. The reinforcement of concrete is of course the properties of steel, but in order to have the advantages of concrete adequate coverage should be made under regulations.
2. 3 INFLUENCE OF STRENGTH OF CONCRETE
CONSISTENCY IN STEEL-CONCRETE AFTER EXPOSURE TO HIGH TEMPERATURES
Relevance is a key feature of reinforced concrete dependind on a number of factors which are taken into account in calculating the anchorage length and are the following:
a) The quality of concrete (compressive-tensile strength)
b) The type and size of aggregates in concrete
c) Type of loading applied to the concrete
d) Surface area of â€‹â€‹rebar
e) Coating reinforcement
f) Presence of transverse reinforcement
g) The specific temperature has a direct relevance because of the diversity of thermal expansion coefficients of concrete - steel.
3. THE DAMAGE TO EXIST IN CONCRETE AND STEEL CONSTRUCTION AS PARTS OF THE CONSTRUCTION OF HIGH TEMPERATURES
The behavior of materials (in terms of inflammation, preservation of structure and properties of mechanics fire) depends on the size and the attachment point which is a party. We cannot just talk about fire resistant materials, and components for durability. The major damage of concrete due to fire are the extrusion and the relaxation of the structure. Due to push the ties are released, resulting in faster heat up and fail. In many normally developing fires of buildings of reinforced concrete, the damage is limited due to the fact that in the first 3 to 5 cm of concrete is directly exposed to fire, although they lose their strength, at the same time act as a protective layer of the nucleus, and thus the bearer body loses only one part of load capacity. The steel in the top stretches due to the heating and ejects the coating and the concrete continues to increase as the temperature lowers the yield greatly. The fire causes serious problems in building up and can make them collapse. The damage and the risk of a building to collapse are due to the following causes:
a) the strength of steel is significantly reduced and parallel elongate steel.
b) the concrete subject blocked due to thermal expansion trends of coercion and breaks
c) The compressive zone bending operators fail because elongation of the steel.
d) The concrete exploded due to volumetric change components of quartz.
e) The concrete developed uneven thermal voltages.
The behavior of structural elements against fire depends not only on the material, but also the shape, size and type of connection and cooperation with other structural elements.
The main risk is the peeling of concrete especially in the corners, so the reinforcement is exposed to flames when heated above 600 C reaches for normal loads, the limit leakage. The strength of columns is mainly due to concrete that is slow to warm up inside so even if you peel the surfaces, although these are large, the column does not collapse. Columns with cross over 40x40cm withstand a fire of 1 Â½ hours with static load calculation. Columns 25x25cm withstand a fire of 1 hour.
Essential role in resistance to fire, plays the section width of the beam, the depth of the reinforcement from the surface and the dense surface reinforcement (stirrups). The systematic function of the beam plays an important role: beams or frames of an opening is most vulnerable, while continuing column frames beams and frames are safer because of the heat affecting the lower arms (exposures), and the supports of the reinforcement is near the floor ( of the upper floor), where the incoming air circulation and temperatures are lower. So if the reinforcement of openings reached the yield will be redistribution of moments with an increase in torque support, where the steel and materials were there cooler is able to undertake. For this reason, a good measure to increase the capacity of continuous beams is to continue some of the mounts to the bars open. So in figure 17 we observe that while the heating caused by plastic hinge arms at mid-span beam, the reinforcement of the top two divisions which cut the beam because of the plastic hinge, now operating as a slide.
The heating of the reinforcements created a plastic hinge at mid-span beam. The existence of those reinforecements allows two pieces to act as a cantilever.
Cracks appear along the support beams. Increasing the strength of continuous plates, and if I put the arms and openings at the top thus part of the weapons mounts.
4. REPAIR METHODS - APPLICATION EXAMPLE
Repairs are defined as actions that restore the building to its original state, for example: the initial capacity to carry loads safely. The repair methods used are presented below:
- Cast concrete
- Enclosed concrete
- Bonding glues
- Paste Plate
- Welding of new weaponry to the old
- Add external fasteners
- Mortar cement and plastics
4.1 Repair Hurt fire building in Bangalore city of India
Due to arson a building which was both school and shop in the western part of Bangalore city in India, suffered heavy losses due to arson. A detailed examination revealed that most of the columns, beams and slabs suffered from significant losses. The concrete was peeled in many components, and a few beams and plates revealed the weapons. Non-destructive testing and loading tests were conducted to assess the residual strength of the components after the fire. The building that suffered from the fire was repaired again to meet the requirements for durability and use.
The building consisted of a basement and two upper floors. The main body was constructed of reinforced concrete and the partition walls of brick. The arsonists started the fire with petrol in the middle area of â€‹â€‹the ground floor (show room), which consists of many fabrics of a highly flammable material, so the fire spread very quickly. The building was burning for more than 8 hours when the fire finally stopped.
Here are all the steps taken to repair.
4. 1. 1 Investigations carried out
Done in 3 phases:
1) Physical examination of the building struck by the fire
2) Non-destructive testing
3) Tests on typical load beams and plates
4. 1. 1. 1 Comments physical examination
Damages to the ground floor:
Walls: The walls were burned significantly everywhere. The coating is crumbling in many places. Roof slab floor: As shown in figure18 the underside of the plate was badly damaged, mainly cracks in several places. The plaster of the ceiling along with some concrete had peeled in many places and to large extent (fig20). In some places, the reinforced concrete was exposed to the fire a little while in others much more. In the southern part (between grids 1 and 3) the reinforced structure and materials had been discovered completely at the same time there was complete disorder of the concrete in many places. In the northern part of the room where the fire started jagged cracks occurred in the ceiling. However, there were no bent or extended bending arrows at any point of the plate.
Schematic presentation of the damage to roof slab floor
Schematic presentation of the damage to the first floor
Damage to the roof slab floor (a,b)
Beams Roof floor: Schematic presentation of the damage to floor beams of the roof we can see in figure 18. The majority of the beams were affected by high temperatures as shown in figure 21. The beam that was mostly affected was the FB2 (fig.18 and fig.21). In this block many vertical cracks were observed and in the middle of opening the concrete had peeled at a length of about 500-750 mm, leaving completely exposed the arms. In the remaining parts of the beam only the coating was gone. The detachment of the coating was very common in all the other beams. The thickness of the plaster had been singled out from the base of the concrete and was about to fall. On the block FB5 the plaster fell down partially and the remaining length had too many vertical cracks revealed. These cracks may be due to tensile forces developed in the beam during the fire.
Damage to ground floor ceiling joists
Eaves (canopy) to the south: The marquee had turned sharply and was on the verge of collapse. This was claimed during the investigation, immediately and temporarily. The minimum eaves supported the opening of the beam lying between the columns A1 and B1 (fig.18). During the fire, the area A1, A3, B3, B1 (fig.18) was the most affected area because there was concentrated the largest amount of fabric. The fire lasted for more than 8 hours which heated the beam that supported the eaves. The temperature developed in this area exceeded 300C and was the cause of the weapons mounts to leak, allowing them to turn. The beam that was parallel to the eaves bent and twisted to such a degree to be a danger and require immediate attention. Columns: The columns were affected significantly. Typical of these losses at different floor levels shown in fig.22. The columns A3, A4 and A7 had crack. The cracks were inclined 450 starting with the coronation of the eaves and continued downward (Fig.22). The other capillary columns had cracks that were not visible to the naked eye. Inside the building, almost all the columns, the plaster has become detached, but the concrete was intact. The weapons were not stripped of any column either internally or externally. The node beam - column of column A3, the coating of concrete had detached, leaving the arms a bit exposed. Because of this we saw the concrete core node. Reference:"Fire-Service-Aid Construction''
Typical stress in columns
Damage to the first floor. Considerable damage was caused to the first floor as well. In fig.19 shown schematically typical damage and the photo in fig.23 confirms them. Walls: Virtually all vertical walls between the openings to the West (main street) had serious flaws in all directions. Some of these were wider than 50 mm. The exterior walls which are parallel to the openings of the same direction above, had deteriorated and were on the verge of collapse. The walls of the classrooms also had a crack. Just below the main beams, some walls had cracked together and had become detached from them. Cracks in the walls of classrooms were due to extensive bending of the beams that supported the floor of the first floor.
First floor slab: tall flames from the ground floor of the plate reached the first floor and burned significantly. The plate was one breach seriously on both sides of the floor beams and cracks had formed along them. This is the standard way reinforced concrete slab cracks due to fire. The width of the cracks were 1-2 mm
Hurted walls of 1st floor
4. 1. 1. 2 Comments non Destructive Testing
Non-destructive testing showed that the residual strength of concrete was smaller than M15.
4. 1. 1. 3 comments on standard test charge beam and slab
Loading of the beams and FB2 FB5, and all the plates that were supported by the beams. Measured bending arrows which were created directly after 24 hours of loading and after 24 hours after the load was removed.
The results of physical examination, non-destructive testing and load testing showed that the building was unfit for use. However it appeared that the damage could be repaired. The repair plan provided:
- Repair boxed columns with concrete.
- Repairs of beams and slab on the first floor with concrete.
- Repair of wall filler.
- Repair of inclined ledges.
- Other non-construction repairs.
Repair of concrete columns with embedded (Fig. 24)
Repairs of the columns on the ground floor which had deteriorated were made. The detached top layer of concrete and also due to the disruption of the cement the structure and materials were revealed. The steps followed were:
1) Discharge column with appropriate support beams that ran it, at all levels of the floors.
2) Lowering disorganized and concrete cleaning and buffing the surface, sand blasting, so much so that there is no sudden changing the thickness of the concrete.
3) Incorporation of shear links in diameter by 12 mm to 1000mm in diameter holes opened in 16 mm.
4) The gap around the shear links were covered with non-shrinking mortar.
5) Installing required additional reinforcement around the columns.
6) Introduction twin U around the new weapons and welding them to educate a rectangular connector.
7) Once formed, the formwork was injected with concrete M20 with good workability.
8) On beam-column junctions the plate was tapped, creating around the square hub 150 mm, and concrete was injected from the top of the slab within the formwork, thus ensuring smooth concrete.
9) The boxed concrete was allowed to "rest" for a minimum period of 14 days.
10) The shuttering of the columns was removed just 24 hours after concreting.
Repair beams and slab first floor with concrete shot (Fig. 10b)
The slabs and beams were fatigued too. The detachment of the coating of concrete and cracks exposed the body. So the repair was made with the shooting of concrete in these steps:
1) Support plate-beams where necessary.
Reference :"Fire-Service-Aid Construction"
2) Lowering disorganized and concrete cleaning and buffing the surface, sand blasting, was made to such a degree so that no sudden changing the thickness of the concrete can occur.
3) The gap around the shear links were covered with non-shrinking mortar.
4) Applying concrete by shooting it in a thickness of 40 mm around the exposed body of the beams and 25 mm thick plate for the roof. The pressure used for the jet was about 0.6 N/mm2.
5) The longer splashing concrete was allowed to "rest" for a minimum period of 7 days. 
Repair of inclined ledges
The eaves on the south side of the building shifted due to the shift of the beam that supports it. But they were intact and had not suffered stress. For this reason, it was put in an upright position and was based on the free end with two new columns measuring 230mm x 230mm.
Repair of wall filler
Where the walls were faults and the coating was detached, followed the following plan of repair: All damaged materials were removed and the surface was cleaned with steel wool. It became resistant and the cracks were sealed. The new coating was joined and left to "rest" for a minimum period of 14 days. The building was now secure and ready for use.
More Boxing fatigued with concrete pole
Rehabilitation of beam and plate
Proposed recovery plan
6. Repayment methods
As aid is defined as an effort to further increase the resistance against the building of a strain (for example earthquake, fire, subsidence, etc.). We are interested in supporting the construction of a fire. The engineer in the design phase of construction, is taking into account a number of existing provisions to protect the building from fire. Significant improvement in durability can be achieved by using special construction details, which can be seen as an aid in planning stages. The construction details of some components are:
Shape and dimensions of beam cross section
As the temperature increases rapidly within minutes and small
profiles from EOS, the finest components are less resistant to fire than the thicker.
Comparison sectional shape in terms of vulnerability fire
On the left we see a beam cross section less vulnerable to fire and at the right a beam cross section vulnerable to fire.
Position of reinforcement in beam profiles
It is best not to aggregate the reinforced rods in the corners of the sections, because the temperature increase is faster in these places and there is increased likelihood of peeling. It is preferable for the main body to consist of more than two bars, and even better to place a part of the main weapons in a second layer.
Compare how reinforcement arrangements in terms of vulnerability Fire.
In the first figure on the right we see a little resistance to fire
In the middle figure we see an average resistance to fire
In the right figure we see a maximum resistance to fire
If the plate is prestressed with tendons without grouting special attention should be given to the anchorages.
A column is generally preferable to have large dimensions and a low percentage of structure and materials, because the minimum width is a key factor for the resistance to fire.
Comparing ways to reinforce columns in terms of vulnerability to Fire
In the right figure we see a column less vulnerable to fire
In the right figure we see a column more vulnerable to fire
Provision for expansion joints
An expansion joint is designed to accept the expansion caused by the fire in a compartment, so as not to create any damage to adjacent compartment. The thickness of the joint depends on many factors such as size and shape plates, beams and columns, type of connection between these components, required resistance to fire. As a minimum thickness joints can take 0.001xL for durability and 0.0015xL an hour for larger resistances, where L is the distance between adjacent joints. In general, joints should be filled with non combustible, fiber compressor and sealed. Moreover there should be regular monitoring and maintenance. It should be noted that there is absolutely no filler compressor.
Fire can have a very harmful effect on a structure of reinforced concrete and may even cause its the discarding. Most of the time the damage is repaired by appropriate methods. The conclusions we made from the above brief study of the effect of fire on structures made of reinforced concrete are:
1) The lightweight concrete has greater resistance to fire of concrete with siliceous aggregates.
2) Normal concrete has higher thermal elongation of the lightweight concrete.
3) The steel when treated in heated when hot is more durable than that suffered by cold treatment. This is true whether the experiment is "during" fire or "after thawing.
4) The reinforced concrete of higher strength have a lower rate of fall of the compressive strength under high temperatures, in contrast to the relatively lower strength concrete.
5) The increased compressive durable concrete is more pronounced for reinforcing bars with the smaller diameter of 8 mm and less concrete durability of 22.5 MPa as a result of experimental procedure.
6) The experimental procedure is observed that the temperatures developed in the reinforcement coating specimen 20 mm, is about the same as those used in
surface of the specimen. The weapons are affected by the temperatures that are developed and we must conclude that greater resistance to fire weapons need more overlap.
7). The static preferred continuous beams and columns frames.
8) Best solution to prevent the adverse effects of the fire is to take into account the appropriate provisions in the construction design phase.