repair of fire damage structure

SD0107 Repair of Fire Damage Concrete Structure 1

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 damage structure

Selection of repair material Method of placing the repair material

Case Study


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.


Introduction cont….

The decision whether the fire damaged structure could be repaired and reused depends on : The temperature and duration of fire Properties of concrete and steel used Residual strength of concrete element

affected by fire


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 of repair work in detail.


Assessment of Damage

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

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:

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.


Colour changes in concrete with temperature

TEMPERATURE IN CONCRETE

COLOUR

0 TO 300 NORMAL

300 TO 600PINK, RED OR

REDDISH BROWN

600 TO 900 WHITISH GREY

900 TO 1000 BUFF


Depth of Heat penetration into Concrete during Exposure to Fire Test

Test Period (hour)

Surface Temperature 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

Depth of Heat penetration into Concrete


Cycle of effects of fire on reinforced concrete structure

Stage on heating1. Rise in surface

temperature

2. Heat transfer to interior surface

3. Heat transfer to reinforcement (accelerated if spalling occurs)

Probable effectsSurface cracking

Loss of concrete strength,

cracking and spalling

Reduction of yield strength,

possible buckling and/or

increase in deflection


Cycle of effects cont…… On cooling4. Reinforcement

cools

5. Concrete cools

6. After concrete cools

Recovery of yield strength to

practically original value, any

buckled bars remained buckled

Cracks close up; further

reduction in strength; Deflection

recovery incomplete for severe

fire

Very dry concrete absorbs

moisture from atmosphere

results in further deformations

and cracking


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


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 3Plaster 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 4Sever damage including extensive spalling revealing considerable areas of steel reinforcement. One or more bars buckled and column may show sign of distortion.

Damage classification for a typical reinforced concrete framed structure



Class of damage Description of damage (Floor Panels)

Class 1Suspended 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.



Class of damage 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.


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 3Principal strengthening repair reinforced in accordance with the load carrying requirement of the member. Concrete and reinforcement strength may be significantly reduced.

Class 4

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.


Schedule for damage classification

Typical Section of schedule for damage classification

Ground floor columns and first floor beams and slabs

Columns Beams Slabs

Class of damage 1 2 3 4 1 2 3 4 1 2 3 4

Member reference

no.

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

44 412 132 403

45 404


Plot of Fire Damage v/s Temperature

Fire Damage factor v/s Temp

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400 500 600 700

Temperature 'c

Fire

Dam

age

Fact

or w/c ratio 0.40

w/c ratio 0.65


Damage classification: Typical assessments and decision

Damage Classification: Typical summary of assessments and decisions

Member no.

Class of

Damage

Fire damage factors

Quality of original

construction

Feasibility of repair

Effect of

adjoining

members

Time for

repair: Cost of repair

Decision Remarks

Concrete Steel

203 2 1 1 1 1 0 NADemolish and Reconstruct

Adjoining beams and staircase too badly

damaged for repaired

141 3

Compression -

0.93 Shear -

0.92

Main bars - 0.77

Links - 1

1 1 1 1Repair as redesign

13 3 0.85

Main bars - 0.60

Links - 1

1 1 1 1Repair as redesign

Note that redesign would be same if column had been

classed 4B


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.


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)


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.


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)• A mixture of cement, aggregate and water is

projected into a place with high velocity.

Method of placing contd….


• 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.



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.



CASE STUDYA 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.

The entire fabric in the showroom caught fire.


Building at the time of Investigation


Columns & Beams Layout


Effects of Fire on Structure

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. width of the cracks being more than acceptable

limits. Some beams and slabs had even deflected.


Schematic representation of damages on Ground Floor


Schematic representation of damages on First Floor


Reinforcement were exposed in Ground Floor slab


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


Typical Distress in Column


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


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


Loading of slab using sand


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

After 24 hour from removal of load to measure the recovery


Rectification schemeInvestigation 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


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


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.


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.


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.


Grouting of columns


A A

1000

Existing Footing

Existing Column100 mm thk concrete with M20, 12.5 mm down size aggregates

Existing beam

8 Dia ties at 200 c/c

12 Dia Bars

12 Dia connectors at 1000 c/c with 16 Dia holes (Staggered)

Single layer epoxy coat over roughened and cleaned surface of column.

Grouting of columns

12 Dia 8 nosEpoxy silica putty 1:6 or Rendroc

8 Dia ties at 200 c/c

Sectional plan AA

Epoxy silica putty 1:6 or Rendroc

12 Dia connectors at 1000 c/c staggered

100

All dimensions are in mm


Grouting of columns


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:


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.



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.



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


Typical c/s of beam and slab


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:


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)


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.


Grouting of beams


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.


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

Thank you

repair of fire damage structure

Documents

concrete colourestimate

concrete mass

respect of strength

feasibility of repair

classification of damage

cases strength loss

colour changes

damage structurerepair