assessment of fire damage and structural rectification process

Presented by Eng. Jude Aruna Gayan

Based on the

Fire Damage Assessment Report

Submitted by Structural Design Unit –

Defence Head Quarter Complex Project, Sri Lanka

Date: 15th of February 2014

Time: 4.45 PM

Location: Lower Level 02 of Block 06 , Defence Head Quarter Complex

(DHQC) construction site at Akuregoda, Battharamulla

Cause: Electrical leakage in the building (Possible)

Damage: Stored materials and structure of the main tower of Block 06 LL2

and LL1 structural elements

The Incident ...

Fire Damage

• Severity of fire is influenced by three parameters.

Fire load (Quantity, Type And Distribution)

Ventilation (Area, Height, Location)

Compartment (Floor Area, Surface Area, Shape, Thermal Characteristics)

• In building fires, there are two regimes of combustion,

Ventilation controlled fire - Occur where availability of air is limited

Fuel surface controlled fire - Limit is imposed by availability of

combustible materials

The fire at the Block 06 Building could be categorized as

'Ventilation Controlled' type of fire.

• The spread of fire in Block 06 had been controlled due to the lack of

combustible materials at other areas and block work constructions that had

been completed around the fire initiated area.

• The surrounding block works constructed around the fire initiated zone had

concentrated fire in to a limited area, which had controlled the free access of air.

• Remaining debris in affected area shows that, there were combustible materials

such as sponge, rigifoam, safety nets stacked in a confined area of the floor

slab.

• This had caused fire got intensified gravely within a short period of time.

Effect of Fire on Reinforced Concrete

Compressive Strength of Concrete

• Depend on temperatures, mix proportions, Couse aggregate used and loading

conditions at time of exposure.

• Temperatures up to 300 °C do not seriously affect residual strengths of structural

concrete.

• Temperatures greater than

300 °C, Compressive strength

of concrete reduces very rapidly.

• A significant loss in

Compressive Strength of

Concrete when the temperature

reaches up to 500 °C mark.

Approximate

TemperatureProcess

100°CSimple Dilatation / Hydrothermal reactions – loss of chemically bound water

begins.

300°CStart of temperature loss for siliceous concretes – some flint aggregates

dehydrate.

100 – 400 °CCritical range for explosive Spallation / above 300 °C, large reduction in

density

400- 500 °CDecomposition of calcium hydroxide / At 500°C Reduction of 50% of the

concrete strength Ca(OH)2 -----> CaO + H2O

600°C Marked increase in ‘basic’ creep

700°C Dissociation of calcium carbonate

800°C Ceramic binding. Total loss of water of hydration

1200°C Melting starts

Mineralogical Changes In Concrete Caused By Heating

Effect of Fire on Reinforced Concrete cont.…

Elastic Modulus of Concrete

• Elastic modulus of concrete is drastically reduced if heated to temperatures in excess

of 300 °C.

• Elastic deflection due to this effect is not significant in relation to other effects of fire.

Effect of Fire on Reinforced Concrete cont.…

Loss of Bond

• Exposure to high temperatures weaken bond strength of reinforcement bars with

concrete.

• Loss of bond directly affects

crack-width control and consequently

reduce durability of the structure.

Spallation of Concrete

• Spallation involves the breaking off of layers of concrete from the exposed surface at high

and rapidly rising temperatures.

• The main parameter influencing the process is Vapor pressure.

Released from physically and chemically bound water in concrete pores Pressure builds up Lead to spallation

• Three main types of Spallation can be identified.

Explosive Spallation occurs early in the fire and proceeds with a series of disruptions, each locally

removing layers of shallow depth.

Sloughing off / Aggregate Spallation, also occurring in the early stages, involves the expansion and

decomposition of the aggregate at the concrete surface causing pieces of the aggregate to be ejected from

the surface. internal cracking due to different thermal expansion of aggregate and cement paste

Corner Spallation occurs in the later stages of the fire when temperatures are lower. Occurs mainly in

beams and columns, tensile cracks develop at planes of weakness such as the interface between the

reinforcement and the concrete.

Reinforcing Steel

• Steel loses its strength at high temperatures and is usually the reason if Excessive

deflections are observed after a fire.

• Exposure to temperatures less than 600 °C for mild steel has no significant effect in

the yield strength after cooling.

• If temperatures in excess of 700 °C the determination of the strength id critical to

assessment.

• Loss of Ductility may

occur after exposure to high

temperatures.

The assessment could be followed in Two methodologies,

1. Test the fire damaged concrete to directly assess the concrete quality.

2. Estimate the fire severity so as to deduce temperature profiles and hence to calculate the

residual strength of the concrete and the reinforcement.

The first methodology to directly assess the concrete quality.

Visual inspection

Non-destructive testing (E.g. rebound hammer, ultrasonic pulse velocity (UPV))

Destructive Testing (E.g. strength testing of concrete and reinforcement samples)

The second methodology involves three steps to assess the residual strength and the

outcome shall be verified by appropriate testing.

Evaluation of fire severity – This can be performed based on debris or applying numerical

evaluation methods.

Determination of temperature profiles – This may be performed applying numerical methods or

simpler calculation techniques

Assessment of residual strength of the concrete

Assessment Of Fire Damage

Proposed testing methods to determine the fire damage

Test

Location

Test

Type

Test

Method

Information Gained

Colour

changes

Lateral

extent of

damage

Depth of

Damage

Compressivestrength ofundamaged

concrete

Tensile strength

of r/fbars

On-SiteNon -

Destructive

Visual inspection √ √ √

Rebound Hammer √

Ultrasonic Pulse

Velocity √

Laboratory Destructive

Core Test √Reinforcement

test √

Assessment Of Fire Damage cont…

Class of

Damage

Element

Surface Appearance of

concreteStructural condition

Condition of

FinishColour Crazing Spallation

Exposure and condition

of main reinforcementCracks

Deflection /

Distortion

0 Any Unaffected or beyond extent of fire

1

ColumnSome

peeling Normal Slight MinorNone exposed

None NoneWall

FloorVery minor exposureBeam

2

Column

Substantial

lossPink/red Moderate

Localised to cornersUp to 25% exposed, none

buckled

None None

WallLocalised to patches

Up to 10% exposed, all

adheringFloor

Beam

Localised to

corners, minor to

soffit

Up to 25% exposed, none

buckled

3

Column

Total loss

Pink/Red

Whitish

grey

Extensive

Considerable to

corners

Up to 50% exposed, not

more than one bar buckledMinor None

WallConsiderable to

surface Up to 20% exposed,

generally adheringSmall Not significantFloor

Considerable to

soffit

BeamConsiderable to

corners, sides, soffit

Up to 50% exposed, not

more than one bar buckled

4

Column

DestroyedWhitish

grey

Surface

lost

Almost all surface

spalled

Over 50% exposed, more

than one bar buckledMajor

Any

distortion

WallOver 20% exposed, much

separated from

concreteSevere and

significant

Severe and

significantFloor

BeamOver 50% exposed, more than one

bar buckled

Visual Assessment Go to Slide No 36

More than 25 % R/F Exposure Condition

More than 50 % R/F Exposure Condition

Discoloration

Discoloration

Visual Assessment Of Fired Area

Initial Repair Classification

Class of Damage

Repair Classification

Repair Requirements

0 Decoration Redecoration if required

1 Superficial Superficial repair of slight damage not needing fabric reinforcement

2 General repairNon-structural or minor structural repair restoring cover to reinforcement where

this has been partly lost.

3 Principal repair

Strengthening repair reinforced in accordance with the load- carrying requirement

of the member. Concrete and reinforcement strength may be significantly reduced

requiring check by design procedure.

4 Major repairMajor strengthening repair with original concrete and reinforcement written down

to zero strength, or demolition and recasting.

Location Material ConditionsApproximateTemperature

(°C)

1 Timber Plank(2’*4’) Ignites 240

2 Aluminium Melted 650


4 Piece of concrete No colour change Below 350

5Steel

Aluminium

Not melted

Melted1100 - 650

6 Timber plank Ignites 240


8 Piece of concrete Pink colour dots 350

9 Steel Not melted 1100-650

10 Plywood Ignites 240

12Piece of concretePVC

Pink colour dotsCharred

350500

13 Iron Not melted Below 1100

14Aluminium

Steel

MeltedNot Melted

1100 - 650

15 Piece of concrete Pink colour dots 350

Survey on Fire Severity

• An assessment of the materials burnt and the

disposition of the fire provide information

about likely temperatures developed and the

duration at any location.

• This Evaluation provides useful guide in

planning more specific examination and

testing for the damage area.

Temperature in fired area was greater than 650°C (Melting temperature of Aluminum), but

should less than 1100°C (Melting temperature of Steel).

Survey on Fire Severity cont…

Aluminum melted

Binding wire not melted Iron piece not melted

Fully burnt timber plank

• Provides a rapid indication of the Compressive strength of concrete.

• The Rebound of an elastic mass depends on the hardness of the surface against which

its mass strikes.

• The rebound is taken to be empirically related to the compressive strength of the

concrete.

• The rebound value is read from a graduated scale and is designated as the rebound

number or rebound index.

• The compressive strength can be read directly from the graph provided on the body of

the hammer.

• The results are significantly affected by :

Mix characteristics

Angle of inclination of direction of hammer

Member characteristics

Test On Structural Element In The Fire Affected Area

Rebound Hammer Test

Graph vs. Rebound Index & Compressive Strength of Concrete

For Good quality / gravel & sand aggregate / Age 14 to 56 days / smooth and dry surfaces

Rebound Hammer – Schematic Diagram

Mechanism of Rebound Hammer

Procedure

• Surface preparation - Using abrasive Stone

No Plaster

No Paint or Dust

No Irregularity / Aggregates

No spalled surfaces,

• The results of this test on fire-damaged concrete, even on flat surfaces, are somewhat

variable and this is perhaps due to skin hardening effects that appear to occur.

• The survey is carried by dividing the member into well-defined grid points.

• Take the average of about 10 readings

• Should be tested against the Anvil.

Ex: Type N test hammer – Nominal value (79 ± 2)

Rebound Hammer Test

Interpretation of Results

The rebound reading on the indicator scale has been calibrated by the

manufacturer of the rebound hammer for horizontal impact.

Average Rebound Number Quality of Concrete

> 40 Very good hard layer

30 to 40 Good layer

20 to 30 Fair

< 20 Poor concrete

0 Delaminated

• Requires a Flat Surface and only appropriate for

Unspalled surfaces.

• Can be used to give an indication of Depth Of

Seriously Weakened Concrete.

Ultrasonic Pulse Velocity (UPV) Measurement - Part 4 of BS EN 12504

• Based on the Pulse Velocity Method

• Provide information on the Uniformity Of Concrete, Cavities, Cracks And

Defects, Presence Of Voids, Honeycombing or other discontinuities.

• The pulse velocity in a material depends on its Density And its Elastic

Properties which is related to the quality and the compressive strength of the

concrete.

• It is also applicable to indicate Changes In The Properties Of Concrete, and

in the survey of structures, to estimate the Severity Of Deterioration Or

Cracking.

• The UPV equipment (e.g. PUNDIT)

Transmitter

Receiver

Indicator

• Indicator shows the time for the ultrasonic pulse to

travel from the Transmitter to the receiver through

the concrete.

• The transducer is firmly attached to concrete surface

using a Gel or Grease to vibrate the concrete.

• The pulse velocity can be determined from V = L / T

• The velocity of sound in a concrete is related to the

concrete density & modulus of elasticity. V ~ √E/ρ

V = pulse velocity (km/s)

L = path length (cm)

T = transit time(µs)

E = modulus of elasticity

ρ = density of the concrete

• There are three basic ways in which the transducers may be arranged.

Opposite faces (Direct transmission)

Adjacent faces (Semi-direct transmission)

Same face (Indirect transmission)

Different Test Methods

• Direct transmission is the Most sensitive, and indirect transmission the Least

sensitive.

• Indirect transmission should be used when only one face of the concrete is accessible,

when the depth of a surface defect or crack is to be determined or when the quality of

the surface concrete relative to the overall quality is of interest.

• The results are influenced by;

• Type of cement

• Type and size of aggregate

• Presence of reinforcement

• Moisture condition

• Compaction

• Age of concrete

• Comparatively Higher velocity

indicate Concrete Quality is Good

in terms of density, uniformity,

homogeneity etc.

Concrete Quality Accordingly to Pulse Velocity.

• Uniformity and Relative quality of concrete.

• To indicate the Presence of voids and cracks, and to evaluate the effectiveness of

crack repairs.

• When used to monitor changes in condition over time, test locations are to be

marked on the structure to ensure that tests are repeated at the same positions.

• The Degree of saturation of the concrete affects the pulse velocity.

• The pulse velocity is independent of the dimensions of the test object provided

reflected waves from boundaries

Significance & Use

Core Test

• The most direct method of estimating strength of in-situ concrete is by testing cores cut

from the structure.

• A limited number of test cores were extracted from the fire damaged area to minimize

further damage.

Tensile Test on Reinforcement Steel

• Rebar samples were taken from representative elements of damaged structural members.

• The samples were tested for yield, elongation , ductility and tensile strength.

StructuralComponent

Unaffected by fire Affected by fire

Rebound Hammer (N/mm2)

UPV Test(N/mm2)

Core Test Result

(N/mm2)


UPV Test(N/mm2)

Core Test Result

(N/mm2)

Slab 1 53-55 - - 30-33 47.3 -

Slab 2 55-57 51.8 39.1 22-48 51.8 39.7

X Direction Beam 1

35-44 - - 28-42 42 33.8

Y Direction Beam 1

46-57 - - 37-46 - -

Column 1 38-48 - - 32-44 - -

Wall 1 37-42 - 40.9 37-39 47.3 35.8

Comparison of Results

Core Test Results

Element LocationCore Test Results

(N/mm2)

W 1 35.8

W 2 40.9

S 6 39.7

S 9 39.1

B 4 33.8

Element LocationUPV test Results

(N/mm2)

B 4 42

W 1 47.3

S 3 47.3

S 6 51.8

S 9 51.8

UPV Test Results

Rebound Hammer Test ResultsTensile Testing

Results

Comparison of damage class according to Visual Inspection with UPV,

Schmidt Hammer and Core Test Results

Visual Inspection

Class

Structural Element

UPV Test (N/mm2)

Core Test (N/mm2)Rebound

Hammer(N/mm2)

Tensile Strength of R/F

(N/mm2)

Class 04

S1 - - 35.3 - 55.4 -

S2 - - 38.7 - 49.7 460 - 350

S3 47.3 - 30.1 – 33.0 460 - 350

S4 - - 22.1 -47.9 460 - 350

S5 - - 30.1 – 31.8 350 - 250

S6 51.8 39.7 46.0 - 49.7 > 460

Class 03

B2 - - 46.0 -

B3 - - 22.1 – 37.0 460 - 350

B4 42.0 33.8 28.5 -42.4 > 460

B5 - - 37.0 – 46.0 -

W1 47.3 35.8 37.0 – 38.7 460 - 350

C1 - - 31.8 – 44.1 -

Class 02

B1 - - 35.2 – 42.4 -

B6 - - 35.3 – 44.1 -

B7 - - 37.0 – 44.1 -

B8 - - 38.7 – 40.5 > 460

S7 - - - -

S8 - - - -

Class 01

B9 - - - -

W2 - 40.9 38.7 – 40.5 -

C2 - - 38.7 – 47.9 -

Class 00 S9 51.8 39.1 55.4 – 57.3 -

Slide No 14

Comparison of final damage class according to Visual Inspection, UPV, Rebound

Hammer, tensile strength and Core Test Results

Final Damage

Class

Structural Element

Visual Inspection

Class

UPV Test (N/mm2)

Core Test (N/mm2)


Tensile Strength of R/F

(N/mm2)

Class 04

S2 Class 04 - - 38.7 - 49.7 460 - 350

S3 -do- 47.3 - 30.1 – 33.0 460 - 350

S4 -do- - - 22.1 -47.9 460 - 350

S5 -do- - - 30.1 – 31.8 350 - 250

B2 Class 03 - - 46.0 -

B3 -do- - - 22.1 – 37.0 460 - 350

B8 Class 02 - - 38.7 – 40.5 > 460

Class 03

S1 Class 04 - - 35.3 - 55.4 -

S6 -do- 51.8 39.7 46.0 - 49.7 > 460

B4 Class 03 42.0 33.8 28.5 -42.4 > 460

B5 Class 03 - - 37.0 – 46.0 -

B6 Class 02 - - 35.3 – 44.1 -

B7 -do- - - 37.0 – 44.1 -

W1 Class 03 47.3 35.8 37.0 – 38.7 460 - 350

C1 -do- - - 31.8 – 44.1 -

Class 02

S7 Class 02 - - - -

S8 -do- - - - -

B1 Class 02 - - 35.2 – 42.4 -

W2 Class 01 - 40.9 38.7 – 40.5 -

C2 -do- - - 38.7 – 47.9 -

Class 01 B9 Class 01 - - - -

Class 00 S9 Class 00 51.8 39.1 55.4 – 57.3 -

Final Class of Damage in LL1 of

Block 06

Rectification method for Structural elements

Class of Damage Rectification Methodology Construction Process

Class 00

Slabs Beams, columns and shear walls

Redecoration if required Surface cleaning

Mortar application

Class 01


Superficial repair of slight

damage not needing fabric

reinforcement

Surface cleaning

Breaking out damaged area

- Hammer and chisel

Mortar application



Class 02


Non-structural or minor

structural repair restoring cover

to reinforcement where this has

been partly lost.

Surface cleaning

Breaking out damaged area

- Hammer and chisel ( Small areas)

- Electrically or Pneumatically powered

breakers (Large areas)

Mortar application

Concreting

- Non-Shrinkage, flowable construction grout

Rectification method for Structural elementsClass of Damage Rectification Methodology Construction Process

Class 03

Slabs

Strengthening repair reinforced in

accordance with the load- carrying

requirement of the member. Concrete

and reinforcement strength may be

significantly reduced requiring check by

design procedure.

Surface cleaning Breaking out the entire damaged slab area

- Electrically or Pneumatically powered breakers . - Hydro-demolition Connection Reinforcement

- Lapping- Coupling- Welding - Not recommended

Concreting - Conventional concrete

Beams, columns and shear walls

Strengthening repair reinforced in

accordance with the load- carrying

requirement of the member. Concrete

and reinforcement strength may be

significantly reduced requiring check by

design procedure.

Surface cleaning Breaking out damaged area- Hammer and chisel ( Small areas)- Electrically or Pneumatically powered breakers

(Large areas) Connection Reinforcement- Lapping- Coupling- Welding - Not recommended Concreting - Non-Shrinkage, flowable construction grout

Strengthening Beams, columns and shear walls of Class 03



Class 04

Slabs

Major strengthening repair with original

concrete and reinforcement written

down to zero strength, or demolition and

recasting.

Surface cleaning Breaking out entire damaged slab area

- Electrically or Pneumatically powered breakers- Hydro-demolition

Connection Reinforcement- Lapping- Coupling- Welding - Not recommended

Concreting - Conventional Concrete

Beams, columns and shear walls

Major strengthening repair with original

concrete and reinforcement written

down to zero strength, or demolition

and recasting.

Surface cleaning Breaking out the entire damaged area

- Electrically or Pneumatically powered breakers Connection Reinforcement

- Lapping- Coupling- Welding - Not recommended

Concreting - Conventional Concrete

Repair Methods..

Main process to be undertaken in repair methods of reinforced concrete are

• Removal of damaged or weakened concrete

• Replacement of weakened reinforcement

• Replacement of concrete to to provide adequate structural capacity, durability and

fire resistance.

Surface Cleaning

• Pressure water jetting and in some areas power wire brushing and cementitious Paint

coat were used dependent upon the degree of discoloration.

• Surface cleaning may be required prior to the commencement of any repair works to

enable the clear identification of areas.

Repair Methods..

Breaking Out

• The objectives of breaking out were to remove all the deteriorated concrete and to

deepen the repair area without damage to the concrete and reinforcement that are to

remain in place

• Hammer and chisel, electrically powered breakers were used for breaking out.

• The Braking pattern was determined to avoid any sudden collapse and to un-effect to

the sound concrete.

• Sledgehammer and Chemical blasting were prohibited.

Repair Methods..

Flowable Micro-Concrete and Concrete

• The concrete for section enchasing in structural elements and large filling volumes

were rectified using a Concrete mix of Construction grout and chip concrete with

proportion of 3:1.

• The filling area less than 50 mm were rectified with an Construction grout mortar mix

with specified water cement ratio.

Special Thank to

• Senior Design Eng. S.S.A. Kalugaldeniya

• Senior Design Eng. (Ms). Kalani Sammandapperuma

• Eng. B.A.C. Batepola - In Charge / Site Laboratory

THANK YOU !!!

assessment of fire damage and structural rectification process

Engineering