Repair of Fire Damage Structure

Repair of Fire Damage Concrete Structure

ContentsIntroductionEffects of fire on materialRepair to fire damage structure involvesevaluation or the assessment of the fire damage structureSelection of repair material Method of placing the repair materialCase Study

IntroductionThe 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 fireProperties of concrete and steel usedResidual 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 DamageFor 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

Depth of Heat penetration into Concrete

Depth of Heat penetration into Concrete during Exposure to Fire TestTest Period (hour)Surface Temperature at end of Test ('C)Distance from surface of colour change Position corresponding to temperature300 'C650 'C1000 'Cmmmmmm19505718-210507925631150120443

Cycle of effects of fire on reinforced concrete structureStage on heatingRise in surfacetemperature

Heat transfer to interior surface

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/orincrease in deflection

Cycle of effects contOn coolingReinforcement cools

Concrete cools

After concrete coolsRecovery of yield strength topractically original value, anybuckled bars remained buckled

Cracks close up; furtherreduction in strength; Deflectionrecovery incomplete for severefireVery dry concrete absorbsmoisture from atmosphereresults in further deformationsand cracking

Collection of data (Diagnosis)DebrisExamine relevant debris to determine the duration of fire or temperature reached.2.Concrete colourEstimate the equivalent exposure from the depth of pink coloration.3.Visual classificationThere should be detailed examination and classification of damage for each structural member. We should use clearly stated description for each classification

Damage classification for a typical reinforced concrete framed structure

Class of damageDescription of damage (Column)Class 1Undamaged except for some peeling of plaster finish, smoke depositClass 2Substantial loss of plaster finish - Concrete surface having extensive micro cracking and pink buff colour - minor spalling onlyClass 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 2Substantial damage to hollow tiles - concrete ribs spalled with reinforcement revealed over small areasClass 3 Reinforced Concrete ribs - extensively spalled of, but reinforcement generally still adhering to concrete; concrete seems smoke covered or pink; No severe Deflection. Class 4Sever damage including extensive spalling revealing considerable areas of steel reinforcement. Deflection may be severe.

Damage classification for a typical reinforced concrete framed structure

Class of damage Description of damage (Beams)Class 1Smoke deposit; minor spalling only and no exposed reinforcementClass 2Substantial 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/PinkClass 3Substantial 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 repairDescriptionClass 1Superficial gunite repair of slight damage not required fabric reinforcement.Class 2Non 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 4Strengthening repair with original concrete and reinforcement written down to zero strength or complete demolition according to the following factors.If member is badly distorted and or there is loss of tension in prestressing tendons or concrete is weakened throughout - demolish and replace.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 classificationGround floor columns and first floor beams and slabsColumnsBeamsSlabsClass of damage123412341234Member reference no.25314111211313110110210411124211121231412012022042123133113311413012033133231121522214013024135242124522413034243333123213044441213240345404

Plot of Fire Damage v/s Temperaturew/c ratio 0.40w/c ratio 0.65

Repair of Fire Damage Concrete Structure

Chart1

11

0.90.986

0.8250.95

0.70.8

0.5680.63

0.5040.58

0.40.4

0.30.3

0.20.2

0.1820.182

0.170.17

Temperature 'c

Fire Damage Factor

Fire Damage factor v/s Temp

Sheet1

Typical Section of schedule for damage classification

Ground floor columns and first floor beams and slabs

ColumnsBeamsSlabs

Class of damage123412341234

Member reference no.253141112113131101102104

1112421112123141201202204

212313311331141301203

313323112152221401302

413524212452241303

424333312321304

44412132403

45404

Damage Classification: Typical summary of assessments and decesions

Member no.Class from tabel 3Fire damage factorsQuality of original constructionFeasibility of repairEffect of adjoining membersTime for repair: Cost of repairDecesionRemarks

ConcreteSteel

Evidence required listed under

Sheet2

Damage Classification: Typical summary of assessments and decesions

Member no.Class of DamageFire damage factorsQuality of original constructionFeasibility of repairEffect of adjoining membersTime for repair: Cost of repairDecesionRemarks

ConcreteSteel

203211110NADimolish and ReconstructAdjoining beams and staircase too badly damaged for repaired

1413Compression - 0.93 Shear - 0.92Main bars - 0.77 Links - 11111Repair as redesign

1330.85Main bars - 0.60 Links - 11111Repair as redesignNote that redesign would be same if column had been classed 4B

Damage Classification: Typical summary of assessments and decesions

Member no.Class from tabel 1Fire damage factorsQuality of original constructionFeasibility of repairEffect of adjoining membersTime for repair: Cost of repairDecesionRemarks

ConcreteSteelEvidence required listed under

203211110NA

Temp of FireTemp reachedLow strength

Duration of FireReductio of yield point

Hammer soundingSample of steel

Colour and depth of pink zoneUltimate strength

spallingYoung's modulus

cores

Sheet3

0101

100.91290.986

200.8252000.95

300.73000.8

400.5683700.63

500.5044000.58

600.44500.4

700.35000.3

800.25890.2

900.1826100.182

1000.176200.17

Sheet3

Temperature 'c

Fire Damage Factor

Fire Damage factor v/s Temp

Damage classification: Typical assessments and decision

Damage Classification: Typical summary of assessments and decisionsMember no.Class of DamageFire damage factorsQuality of original constructionFeasibility of repairEffect of adjoining membersTime for repair: Cost of repairDecisionRemarksConcreteSteel203211110NADemolish and ReconstructAdjoining beams and staircase too badly damaged for repaired1413Compression - 0.93 Shear - 0.92Main bars - 0.77 Links - 11111Repair as redesign1330.85Main bars - 0.60 Links - 11111Repair as redesignNote that redesign would be same if column had been classed 4B

Repair MaterialThe 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 of Fire Damage Concrete Structure

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)

Repair of Fire Damage Concrete Structure

Method of placing the repair materialThis can be done by: 1.Recasting in formwork2.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.

Repair of Fire Damage Concrete Structure

Method of placing contd.1.Repair by recasting in formworkThis method is particularly used when Larger volume of material is to be placed.Repetition of use of formworkThe whole length of beam and column required repairRepairs by spraying (Guniting)A mixture of cement, aggregate and water is projected into a place with high velocity.

Repair of Fire Damage Concrete Structure

Method of placing contd.Typically the materials used are coarse sand and Portland cement, the aggregate less than 10 mm size maximumCompaction 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

Method of placing contd.3.Repaired by hand applied mortarsTechnique 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.

Repair of Fire Damage Concrete Structure

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 hostelGround Floor Co-optex a handloom fabric show roomFirst Floor Used as Library and ClassroomsThe 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 StructureThe 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 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

InvestigationIn order to assess the existing strength ofhardened concrete of different structural members and to detect any cracks the investigation was done.Investigation was carried out in three steps:Physical ExaminationNon-Destructive Testing of Structural element(a) Rebound Hammer Test(b) Ultrasonic Pulse Velocity Test3. Load Test on First Floor slabs and Beams

Loading of slab using sand

Checking of deflection of slabDial gauges are mounted at midspan to measure the deflection Reading were taken immediate after the loadingAfter 24 hour of loadingAfter 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 concreting2. Guniting of First Floor Beam and Slab3. Post Grouting of Columns and beams 4. Treatment to brick masonry walls5.Restoring rotating canopy6.Other non structural repair

Encasement of column by ConcretingEncasement of entire column was done in view of structural stability, though they were disintegrated on the ground floor only.Unloading of ColumnsSurface PreparationAll loose materials were removed by chipping and cleaned by Sand Blasting

Encasement of column cont3. Use of Bonding agentA coating of NITO-BOND or its equivalent was applied on the clean surface as per manufacturers 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 cont6. 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 cont9.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

Repair of Fire Damage Concrete Structure

Grouting of columnsAll dimensions are in mm

Repair of Fire Damage Concrete Structure

Grouting of columns

Repair of Fire Damage Concrete Structure

Guniting of Slabs and BeamsLarge 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 contSlab/beam was supported wherever necessary. Concrete cover was chipped off and all loose materials were removed by sand blasting.A thin layer of NITO-BOND was applied on the cleaned surface. Shear connector of 12 mm dia at 1000 mm c/c were inserted in drilled hole of 16 mm dia in a zigzag manner.

Guniting of Slabs and Beams cont5. The gap around shear connectors was sealed by an appropriate non shrink sealing component.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.

Guniting of Slabs and Beams contGunite 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 The gunited surface was cured for a minimum period of seven days

Typical c/s of beam and slab

Repair of Fire Damage Concrete Structure

Post grouting of columns and beamsIt 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 contd16 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.12 mm dia PVC nozzle was inserted into each hole and the gap around the nozzle was sealed using sealing agent.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 contdThe operating pressure for post grouting was around 1 N/sq.mmGrouting of every hole was continued until refusal.

Grouting of beams

Repair of Fire Damage Concrete Structure

ConclusionThe 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 ones 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.

ReferencesA book of Structural Failure by R. Jagdish.FIRE SAFETY IN BUILDING by Mr. V.K. Jain.Concrete Society Technical Report no. 15, May 1978CONCRETE, volume 18 number, 5 May 1984

Repair of Fire Damage Concrete Structure

Thank you

Repair of Fire Damage Concrete Structure