Repair of Fire Damage Structure

Repair of Fire Damage Structure

Contents   

Introduction Effects of fire on material Repair to fire damage structure involves 

 



evaluation or the assessment of the fire f ire damage structure Selection of repair material Method of placing the repair material

Case Study

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Repair of Fire Damage Concrete Structure

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Introduction The assessment of fire history and the residual strength of a structure, is complex and requires skill and experience to achieve. The normal purpose of a repair is to restore in the required structure the performance it had before the fire, both in respect of strength and of fire resistance in a future fire.

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Introduction cont…. 

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The decision whether the fire damaged structure could be repaired and reused r eused depends on : 

The temperature and duration of fire



Properties of concrete and steel used



Residual strength of concrete element affected by fire


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Recommendations for efficient planning of repair 1. To assess the damage 2. Determine the feasibility of repair 3. Decide the best method to be used for repair 4. Prepare a scheme for reconstruction 5. Consult with the local authority 6. Schedule the sequence of operations 7. Prepare a scheme for propping and bracing including a schedule of prop loads 8. Specify the extent extent of of repair work in detail. SD0107


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Assessment of Damage 

For proper assessment of the structure we need to consider the effect of fire on material as well as structural member.

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Change in Compressive strength of concrete: 



In most of the cases strength loss is pronounced when subjected to above 300 ‘C temperature.

Colour changes in concrete:  

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Concrete on heating undergoes colour changes. Observation made on concrete after heat, can give information about heat penetration into concrete mass. Which can be seen from the table given below. Repair of Fire Damage Concrete Structure 6

Colour changes in concrete with temperature

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TEMPERATURE IN CONCRETE

COLOUR

0 TO 300

NORMAL

300 TO 600

PINK, RED OR REDDISH BROWN

600 TO 900

WHITISH GREY

900 TO 1000

BUFF


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Depth of Heat penetration into Concrete Depth of Heat penetration into Concrete during Exposure to Fire Test

Test Period (hour)

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Surface Temper ature at end of Test ('C)

Distance from surface of colour change Position corresponding to temperature

300 'C

650 'C

1000 'C

mm

mm

mm

1

950

57

18

-

2

1050

79

25

6

3

1150

120

44

3


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Cycle of effects of fire on reinforced concrete structure 

1.

Stage on heating



Probable effects

Surface cracking

Rise in surface temperature

2.

3.

Heat transfer to interior surface

Loss of concrete strength,

Heat transfer to reinforcement (accelerated if spalling occurs)

Reduction of yield strength,

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cracking and spalling

possible buckling and/or increase in deflection


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Cycle of effects cont…… 

On cooling

4.

Reinforcement cools

5.

Recovery of yield strength to practically original value, any buckled bars remained buckled Cracks close up; further

Concrete cools

reduction in strength; Deflection recovery incomplete for severe fire

6.

After concrete cools

Very dry concrete absorbs moisture from atmosphere results in further deformations and cracking

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Collection of data (Diagnosis) 1.

Debris Examine relevant debris to determine the duration of fire or temperature reached.

2.

Concrete colour Estimate the equivalent exposure from the depth of pink coloration.

3.

Visual classification There should be detailed examination and classification of damage for each structural member. We should use clearly stated description for each classification

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Damage classification for a typical reinforced concrete framed structure Class of damage

Description of damage (Column)

Class 1

Undamaged except for some peeling of plaster finish, smoke deposit

Class 2

Substantial loss of plaster finish - Concrete surface having extensive micro cracking and pink buff colour - minor spalling only

Class 3

Plaster finish almost entirely removed. Concrete surface buff coloured and elsewhere locally spalled to reveal reinforcement -Separation of concrete cover- concrete may give hollow sound.

Class 4

Sever damage including extensive spalling revealing considerable areas of steel reinforcement. One or more bars buckled and column may show sign of distortion.

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Description of damage (Floor Panels)

Class 1

Suspended ceiling extensively damaged but some panels may still be in place; few hollow tiles damaged but reinforced concrete ribs intact except for smoke soot.

Class 2

Substantial damage to hollow tiles - concrete ribs spalled with reinforcement revealed over small areas

Class 3

Reinforced Concrete ribs - extensively spalled of, but reinforcement generally still adhering to concrete; concrete seems smoke covered or pink; No severe Deflection.

Class 4

Sever damage including extensive spalling revealing considerable areas of steel reinforcement. Deflection may be severe.

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Description of damage (Beams)

Class 1

Smoke deposit; minor spalling only and no exposed reinforcement

Class 2

Substantial spalling along adjacent planes revealing main reinforcement of outer surface of corner bars, micro cracking of surface, cover concrete to soffit may have "hollow" ring. Concrete colour - Black/Pink

Class 3

Substantial spalling revealing reinforcement; concrete colour buff, cracks several millimeters in width. No severe Deflection. Sever damage including extensive spalling revealing considerable areas of steel reinforcement. Deflection may be severe and/or Fractures; Main reinforcement buckled. Concrete buff/grey coloured.

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Prognosis (Feasibility of repair ) Classification of repair Class of repair

Description

Class 1

Superficial gunite repair of slight damage not required fabric reinforcement.

Class 2

Non structural repair over a large area, e.g. restoring cover to reinforcement where this has been partly lost. The gunite will be reinforced with a nominal light fabric.

Class 3

Principal strengthening repair reinforced in accordance with the load carrying requirement of the member. Concrete and reinforcement strength may be significantly reduced.

Class 4

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Strengthening repair with original concrete and reinforcement written down to zero strength or complete demolition according to the following factors. (a) If member is badly distorted and or there is loss of tension in prestressing tendons or concrete is weakened throughout - demolish and replace. (b) If member is unsound structurally but removal would cause inconvenient disruption of adjoining member – add new materials to support original design load. Repair of Fire Damage Concrete Structure

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Schedule for damage classification Typical Section of schedule for damage classification Ground floor columns and first floor beams and slabs Columns Class of damage

Member reference no.

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Beams

Slabs

1

2

3

4

1

2

3

4

1

2

3

2

5

3

14

111

21

131

31

101

102

104

11

12

4

211

121

231

41

201

202

204

21

23

13

311

331

141

301

203

31

33

23

112

152

221

401

302

41

35

24

212

452

241

303

42

43

33

312

321

304

412

132

403

44 45

4

404


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Plot of Fire Damage v/s Temperature Fire Damage factor v/s Temp 1.2

r o 1 t c a F 0.8 e g 0.6 a m a 0.4 D e r 0.2 i F

w/c ratio 0.40

w/c ratio 0.65

0 0

100

200

300

400

500

600

700

Temperature 'c

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Damage classification: Typical assessments and decision Damage Classification: Typical summary of assessments and decisions

Memb er no.

203

141

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Class of Damag e

Fire damage factors

Concrete

Steel

Quality of original construction

Feasibility of repair

Effect of adjoin ing memb ers

Time for repair: Cost of repair

Decision

Remarks

Adjoining beams and staircase too badly damaged for repaired

2

1

1

1

1

0

NA

Demolish and Reconstruct

3

Compres sion 0.93 Shear 0.92

Main bars 0.77 Links 1

1

1

1

1

Repair as redesign

0.85

Main bars 0.60 Links 1

1

Repair as redesign

3

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1

1

1


Note that redesign would be same if column had been classed 4B

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Repair Material 

The basic principle of repair is that the repair medium should be as close as possible in all physical characteristics (Elastic modulus, Coefficient of expansion, Strength) to the base material or/and the properties of the new and old work are similar to facilitate maintaining a good bond by limiting the boundary stresses.

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Repair material contd…. 

A good repair material should have the best combination of following properties. (It should be compatible with the old material) 1. Mechanical properties as close to the base material. 2. Good adhesion in dry, damp or wet condition. 3. Low shrinkage (during curing and long term)

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Method of placing the repair material 

This can be done by: 1. Recasting in formwork 2. Spraying (Guniting) 3. Hand applied mortars. Each method will give satisfactory results, providing the specification, material and techniques are appropriate and the work competently done by experienced operatives.

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Method of placing contd…. 1.Repair by recasting in formwork This method is particularly used when •

Larger volume of material is to be placed.

•

Repetition of use of formwork

•

The whole length of beam and column required repair

2.

Repairs by spraying (Guniting)

•

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A mixture of cement, aggregate and water is projected into a place with high velocity.


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Method of placing contd…. •

•

• •

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Typically the materials used are coarse sand and Portland cement, the aggregate less than 10 mm size maximum Compaction to produce a dense homogeneous mass is achieved by its own velocity, with as little subsequent working as possible being done. The material can be placed on vertical as well overhead surfaces with limiting thickness. Typical characteristics are good density, low permeability, high strength and good bond. Repair of Fire Damage Concrete Structure

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Method of placing contd…. 3. Repaired by hand applied mortars •

Technique used will be similar to good rendering practice, but using a slurry bond coat. A polymer latex admixture is frequently added, to both bond coat and repair mortar.

•

This act as water reducing agent allowing a lower water cement ratio to be used.

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CASE STUDY A shopping cum school complex in the western part of Bangalore city was considerably damaged due to fire by some miscreants. The building comprises of Basement Floor  Used as boys hostel Ground Floor  ‘Co-optex’ a handloom fabric show room First Floor  Used as Library and Classrooms 

The fire spread very fast and last for more than Eight

hours. 

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The entire fabric in the showroom caught fire. Repair of Fire Damage Concrete Structure

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Building at the time of Investigation

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Columns & Beams Layout

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Effects of Fire on Structure 

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The fire had reached every part of the building namely walls, columns, beams and roof ceiling.  Walls had cracked everywhere  Plaster of walls, ceiling of roof, beams and columns had spalled off.  Reinforcement in most parts of slab and few beams was exposed considerably.  Cracks had developed in slabs and beams. of Fire Damage Concrete Structure more than 28 width ofRepair the cracks being

Schematic representation of damages on Ground Floor

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Schematic representation of damages on First Floor

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Reinforcement were exposed in Ground Floor slab

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Reinforcement were exposed in Ground Floor slab

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Reinforcement were exposed due to spalling of concrete cover (beam FB2)

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Typical Distress in Column

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Distress walls (115mm thick) of cupboard on First Floor

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Investigation 

In order to assess the existing strength of hardened concrete of different structural members and to detect any cracks the investigation was done. Investigation was carried out in three steps: 1. Physical Examination 2. Non-Destructive Testing of Structural element (a) Rebound Hammer Test (b) Ultrasonic Pulse Velocity Test 3. Load Test on First Floor slabs and Beams

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Loading of slab using sand

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Checking of deflection of slab

Dial gauges are mounted at midspan to measure the deflection Reading were taken immediate after the loading After 24 hour of loading SD0107


After 24 hour from removal of load to measure the recovery

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Rectification scheme Investigation revealed that the structure was not structurally sound and all structural damages required to be repaired. Following repaired were done; 1. Encasement of Column by concreting 2. Guniting of First Floor Beam and Slab 3. Post Grouting of Columns and beams 4. Treatment to brick masonry walls 5. Restoring rotating canopy 6. Other non structural repair SD0107


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Encasement of column by Concreting 

Encasement of entire column was done in view of structural stability, though they were disintegrated on the ground floor only.

1.

Unloading of Columns

2.

Surface Preparation All loose materials were removed by chipping and cleaned by Sand Blasting

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Encasement of column cont… 3. Use of Bonding agent A coating of NITO-BOND or its equivalent was applied on the clean surface as per manufacturer’s specifications. 4. Shear connectors of 12 mm dia at 1000 mm c/c were inserted in drilled holes of 16 mm dia. 5.The gap around shear connectors were sealed by appropriate non-swing sealing compound.

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Encasement of column cont… 6. The new reinforcement cage was positioned around the column as per requirement. 7. The twin U-ties were inserted around the new reinforcement and welded to form a rectangular type. 8. After the formwork has been completed M25 grade concrete with good workability possessing high slump is poured and well compacted.

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Encasement of column cont… 9. At beam column junction slab was punctured for 150 mm square and concrete was poured from top of the slab into the formwork to ensure good concreting at joints. 10. Encasement concrete is cured for a minimum period of 14 days. 11. The column formwork is stripped only 24 hrs after concreting.

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Grouting of columns

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Grouting of columns Single layer epoxy coat over roughened and cleaned surface of column. 12 Dia connectors at 1000 c/c with 16 Dia holes (Staggered)

1000

Existing beam 12 Dia Bars 8 Dia ties at 200 c/c 100 mm thk concrete with M20, 12.5 mm down size aggregates

Existing Column

A

A Existing Footing

100

8 Dia ties at 200 c/c

12 Dia connectors at 1000 c/c staggered 12 Dia 8 nos Epoxy silica putty 1:6 or Rendroc

All dimensions are in mm

Sectional plan AA

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Grouting of columns

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Guniting of Slabs and Beams 

Large scale distress was observed in slabs and beams with spalling of concrete cover exposing the reinforcement. Cracks were also of common sight everywhere.



Repair methodology adopted is as follows:

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Guniting of Slabs and Beams cont… 1.

Slab/beam was supported wherever necessary.

2.

Concrete cover was chipped off and all loose materials were removed by sand blasting.

3.

A thin layer of NITO-BOND was applied on the cleaned surface.

4.

Shear connector of 12 mm dia at 1000 mm c/c were inserted in drilled hole of 16 mm dia in a zigzag manner.

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Guniting of Slabs and Beams cont… 5. The gap around shear connectors was sealed by an appropriate non shrink sealing component. 6. Weld-mesh of 75x75x3 mm was wrapped on to the

exposed surface of rib of beam and to slab bottom and tack welded to the exposed reinforcement at close intervals and to shear connectors.

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Guniting of Slabs and Beams cont… 7.

Gunite mortar mixed with gunite aiding admixture of 40 mm thick around rib of beam and of 25 mm thick for roof slabs was applied under an operating pressure of around 0.6 N/sq.mm

8.

The gunited surface was cured for a minimum period of seven days

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Typical c/s of beam and slab

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Post grouting of columns and beams It is always possible that some cracks, cavities and voids are unfilled. Post grouting fulfill this task. In addition it ensures homogeneity of encasement concrete/gunite with old concrete. The different steps followed are as follows:

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Post grouting of columns contd… 1.

16 mm dia holes were drilled to a depth of 100 to 200 mm at 1000 mm c/c in a zigzag manner on all the vertical surface of column and beams.

2.

12 mm dia PVC nozzle was inserted into each hole and the gap around the nozzle was sealed using sealing agent.

3.

Pressure grout was applied through every nozzle with a free flowing neat cement grout mixed with the expansive agents (CONBEX-100)

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Post grouting of columns contd… 4.

The operating pressure for post grouting was around 1 N/sq.mm

5.

Grouting of every hole was continued until refusal.

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Grouting of beams

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Conclusion The distressed building was thoroughly investigated through physical examination along with NDT and Load Test. The deficiencies and distress were identified and documented to best of one’s ability. A feasible restoration scheme was proposed and executed carefully and efficiently. Thereafter a building was put into normal service as it was certified as structurally sound. SD0107


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References  

A book of ‘Structural Failure’ by R. Jagdish. ‘FIRE SAFETY IN BUILDING’ by Mr. V.K. Jain.



Concrete Society Technical Report no. 15, May 1978



CONCRETE, volume 18 number, 5 May 1984

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Repair of Fire Damage Structure

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