Fire resistance of high-performance concrete columns

(1)

Publisher’s version / Version de l'éditeur:

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.

Questions? Contact the NRC Publications Archive team at

PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.

https://publications-cnrc.canada.ca/fra/droits

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.

Internal Report (National Research Council of Canada. Institute for Research in Construction), 2001-12-01

READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright

NRC Publications Archive Record / Notice des Archives des publications du CNRC : https://nrc-publications.canada.ca/eng/view/object/?id=8571f2d3-d4e8-408b-83f9-8995cbcfbf16 https://publications-cnrc.canada.ca/fra/voir/objet/?id=8571f2d3-d4e8-408b-83f9-8995cbcfbf16

NRC Publications Archive

Archives des publications du CNRC

For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.

https://doi.org/10.4224/20378506

Access and use of this website and the material on it are subject to the Terms and Conditions set forth at

Fire resistance of high-performance concrete columns

(2)

http://irc.nrc-cnrc.gc.ca

Fire Re sist a nc e of H igh-Pe rfor m a nc e

Conc re t e Colum ns

K o d u r , V . K . R . ; C h e n g , F . P . ;

W a n g , T . C . ; L a t o u r , J . C . ; L e r o u x ,

P .

I R C - I R - 8 3 4

December 2001

(3)

FIRE

RESISTANCE OF HIGH-PERFORMANCE GQNCRETE COLUMNS

V.K.R.

Kodur, F.P.

Cheng,

T.C.

Wang,

J.C.

Latour and

P. Leroux

ABSTRACT

Experimental

studies were carried out to

deternine the

fire resistance

performance

of high-performance

concrete

columns.

The

results of

fire

resistance experiments

on

fifteen

full-scale

columns are

described in

this

report.

The test variables included concrete

strength ,

type

af aggregate

in

concrete,

addition of fiber reinforcement, type of

fiber,

load intensity and eccentricity of loading.

These

studies were

conducted

as

part of

a

collaborative

research

program,

between National Research Council of

Canada

(NRCC) and

National

Chiao Tung

University

(NCTU),

Taiwan,

aimed

at studying fire

resistance

(4)

FllRE RESISTANCE OF HIGH-PERFORMANCE CONCRETE

COLUMNS

V.K.R.

Kodurl

F.P.

Cheng,

T.C. Wang, J.C.

Latour

and P.

Leroux

ACKNOWLEDGEMENTS

The research presented in this

report is part

of a Joint

Research Project

between the National Research

Council

of

Canada

(NRCC) and the

National

Chiao Tung

University (NCTU),

Taiwan.

The

authors

appreciate the

technical

and financial

contributions from Architecture and Building Research Institute

(5)

FIRE RESISTANCE OF HIGH-PERFORMANCE CONCRETE

COLUMNS

V.K.R.

Kodur,

F.P. Cheng, T.C. Wang, J.C. Latour and

P.

Leroux

INTRODUCTION

In recent years, the construction industry has shown significant interest in the use of high-performance concrete (HPC). This is due to the improvements in structural performance such as high strength and durability that it can provide, compared to traditional normal

strength

concrete (NSC). The use at high-performance concrete, which was previously in applications

such

as bridges, offshore structures and infrastructure projects, is becoming more popular in high-rise buildings. One of the major uses of HPC in buildings is for columns.

In buildings, HPC structural members are designed to satisfy the requirements of serviceability and safety limit states. One of the major safety requirements in building design is the provision of appropriate fire safety measures for structural members [I]. The basis for this requirement can be attributed to the fact that, when other measures for containing the fire fail, structural integrity is

the

last line of defence.

Generally, concrete up to a compressive strength of 55 MPa is referred to as

normal strength concrete (NSC), while concrete with compressive strength in excess of 55

MPa is classified

as

high strength concrete (HSC). HPC is typically characterised by high strength, good workability and durability. HSC is as a subset of HPC.

With the increased use of high-performance concrete, concern has developed regarding the behaviour of such

concrete

in fire. In particular, the occurrence of spalling

at elevated temperatures, when HSC is subjected to rapid heating, as in the case of a fire, is one of the reasons

for

this concern [2]. Further, resu!ts of fire tests in a number of laboratories [3,4,5,6] have shown that there are welj-defined differences between the properties of HSC and NSC

at

elevated temperatures.

Studies are in progress at the National Research Council of Canada (NRCC) to develop fire resistance design guidelines for the use of high-performance concrete and for

possible incorporation in codes and standards [1,7j. The main objective of this study, being undertaken in partnership with the National Chiao Tung University (NCTU) in Taiwan, is to determine the behaviour of HPC at elevated temperatures

and

to evaluate its

fire endurance. AS part of this joint research project, full-scale fire resistance tests on

three NSC

coiurnns

and twelve HSC columns were

conducted.

The full experimental details, together with the test results, are presented in this report.

(6)

TEST SPECIMENS

The experimental program consisted of fire resistance tests on fifteen reinforced

concrete columns. Three of these columns were of normal strength concrete (NSC) and

the remaining twelve columns were of high performance concrete (HPC).

The NSC calumns were designated as

TNCI-TNC3,

while HPC columns

were

designated based

on the main test variable. WPC eolumns made

with

plain concrete were

designated TWC4-THC9. HPC

columns

made

with steel fiber reinforced concrete were

designated as THSI 0-THSI 2 and HPC columns made with polypropylene fiber reinforced

concrete were designated as THPl3-TH P 15.

Dimensions

AIB columns were 3810 mm long from end plate to end plate

and

were

of

305

mrn

square

cross

section. The end-plate size was 61 0 x 508 x 25 mm. Materials

Cement, Silica Fume and Admixture

ASTM CISO, Type 1 Portland Cement,

a

general-purpose cement for construction

of

reinforced concrete structures, was used. For HPC mix, silica furrre, supplied from Grace Company*, was

added

to the concrete mix. Also, G-type superplasticizer supplied from Sun

Tech

Company" was used in the fabrication of HPC columns.

Two

types d coarse aggregate, namely siliceous and carbonate aggregates, were

used.

When concrete is made with normal weight

coarse

aggregate, having

constituents

composed mainly of silica and silicates

(quartz),

it is referred to as siliceous: aggregate concrete.

Whereas,

when concrete is made with coarse, aggregate consisting mainly of calcium carbonate or

a

combination

af

calcium and magnesium carbonate (for example

limestone and

dolomite), it is referred to as carbonate aggregate concrete. The fine

aggregate was siliceous

sand.

The maximum sire of

coarse

aggregate was 12

mm.

The physical properties of coarse

and

fine

aggregates

are given in Table 1.

- - - -

-* Certain commercial products are identifred in this paper in order to adequately specify the experimental procedure. In na case does such identification imply recommendations or

(7)

Fiber and Reinforcement

Steel and polypropylene fibers were added to the concrete mix for fabrication of fiber reinforced concrete columns, THSI 0-THS12 and THPI 3-THP15.

The steel fibers were

of

0.5

mrn

diameter and 30 rnm length and have

hooks

at

both ends. The fibers

were

of Dramix ZP3W .50 type and produced by BEKAERT'

GO.

Polypropylene fibers were a mixture of 10 mrn and 25 mm length and

of

very

small

diameter (hairline fibers) and

were

produced by FIBERMESH*

CO.

Reinforcement

Deformed steel bars were used

for

main reinforcing bars and ties. The longitudinal steel in

the

column were symmetrical arrangements of 4-25 rnm diameter (No. 8) bars and

the

ties

were

of 8 rnm diameter

(No.

3 ) bars.

The

spacing of ties was 145 mrn in the mid- range length of 2465 mm. Towards the ends of the column a spacing of

75

mrn

was

used (figure 1). The percentage of longitudinal steel was 2.18%. The yield and ultimate strength of

rebars

are listed

in

Table 2.

Concrete Mix

Five

batches

of concnete mix, Batches 1 to 5 ,

were

used fur fabricating the

columns. The five batches were needed to obtain required test variables, such as concrete strength, aggregate type and fiber reinforcement. Batch 3 was made with

carbonate aggregate, while the remaining batches were made with siliceous aggregate.

The

Batch

I mix was made of plain NSC to fabricate columns TNC1-TNC3.

Batches 2 and 3 were of plain HPC

and

were used to fabricate columns THC4-THC9. Batch 4

concrete

mix was made by adding steel fiber reinforcement to fabricate columns THS10-THS12, while

the

Batch

5

mix was made with polypropylene fiber reinforcement to fabricate columns THPI 3-THPI 5.

Batch quantities and measured properties of

the

concrete

mix

are

given in Table

3.

The slump and flow of fresh concrete in Batches 2 to 5 were above 200 rnm and 500 mm, respectively, and this meets the minimum requirements of HPC mix (See Table. 3). The 28-day cylinder compressive strength of NSG was 28 MPa, while for HPG the compressive strength ranged

from

73 MPa to I 00 MPa.

Certain commercial products are identified in this paper in order to adequately specify the

experimental procedure. In no case does such identification imply recommendations or

(8)

Fabrication

Columns

were fabricated and cured at NCTU, Taiwan, and then shipped to NRC,

Canada.

She colunans were cast in specially designed forms. The reinforcement cage was

assembled by welding Ilongitudinal bars to a steel end plate. Chromel-alumel

thermocouples were secured to the reinforcing steel at specific locations before the cage

was

properly positioned in the form. In order to avoid

any

possible dislocation of the

thermacouples during

casting,

a

careful working pfan

was

followed as described below.

Reinforcinq E3ars and Steel Plates .

The length of the column was

3818

mm,

measured from

end

plate to

end

plate.

The longitudinal reinforcing bars were cut to 381 0 mrn and machined at both ends to the

diameters, as shown in Figure 1.

The details

of

end plates, including dimensions, are shown in Figure 1. Holes with a diameter 1.6 rnm greater than that of the machined ends were drilled through the plates to accommodate longitudinal bars.

The main bars and ties were tied together to

complete

the

steel

cage.

The

cage

was then placed vertically on a leveled

end

plate.

Weldinq

Standard provisions were followed

for

welding bars to end plates.

Special

attention

was given

to the centering and perpendicularity of the end plates during welding. Mild steel welding rods were used to fill the holes on the outer face

of

the plates, drilled to

accommodate the reinforcing bars. The rough surface of the welded joints on the drilled

router face of the bottom end plate was ground to a smooth finish.

The top end plate was welded to the top end of the

columns

after

curing of the columns. The rough surface of the welded joints on the

outer

face

of

the

top end

plate was ground

to

a

smooth finish. The gap between concrete and end plate was filled with

high strength

gypsum

to facilitate

smooth

transfer of loads.

Nine

K-type chrornel-alumel thermocouples were used to measure the

temperatures

of

concrete

and

steel. Seven of these therrnocouptes

were

installed in concrete at mid-height section

and

the remaining two thermocouples

were

installed at mid-height of the column on reinforcement bars.

The

thermocouples were secured to the reinforcement bars after the fabrication of the reinforcement cage. The exact Iocations and numbering of the thermocouples are shown in Figure 2.

(9)

Thermocouples were installed on 5 columns, TNC1, TMC4, THC7, THS10, THP13, to measure cross-sectional temperatures representative in each type of column.

Forms and

Concrete

placement

The

colerrnns

were cast in the forms in a vertically uptight position. The forms were made of smooth plywood and were assembled with the front side left open for depositing fresh concrete. At the beginning of placing

the

concrete only

the

battam farm, at front

side,

was bolted and the concrete was carefully placed under the themocouples.

As

the

casting progressed upright, the center form

(one

at front side) was bolted to the bottom form. Careful attention was paid to avoid dislocation of the thermocouples. When this section

was

finished, the top form, at front side, was bolted to the other t o m s and concrete was cast

in

the last section. After this, the top section of

the

column

was finished by a smaIl trowel. During casting, inner and outer vibrators were used to cansolidate the

concrete.

In general, a year or more elapsed between the time a column was fabricated and

the time it was tested.

The forms

were

removed

on

the third day after placing of the

concrete,

aftewhich

the columns were cured by covering

with

wet sacks for seven days. After this the columns were stored indoors until shipping ta NRC.

TEST APPARATUS

The fire resistance experiments were carried out by exposing the columns to heat in a furnace specially built for testing the loaded columns. The test furnace was designed

to produce the conditions to which a member might be exposed during a fire, i.e. temperatures, structural loads and heat transfer. It

consists

of a steel framework supported by four steel columns, wlth the furnace chamber inside the framework (Figure

3). The characteristics

and

instrumentation of the furnace are described in detail in Reference 8. Only a brief description of the furnace and the main components are given here.

Loading Device

A hydraulic jack produces forces along

the

axis of the test column. The jack is located at the

bottom

of

the

furnace

chamber and has a loading capacity of 9778 kN (1000 1). The plate on

top

of this jack can be used as a plaiform to which the column can

be

attached-

Eccentric loads

can

be applied by means of hydraulic

jacks,

one at the top and

one at the bottom of the column, located at a distance of 5508 mrn from the

axis

of the column. The capacity of the top

jack

is 587 kN (1 33 kips) and the bottom jack is 489 kN (1 1 0 kips).

(10)

Furnace Chamber

The

furnace chamber has a floor of 2642 x 2642

mrn

size and is 3048 rnrn high. The interior faces of the chamber are lined with insulating materials that will efficiently transfer heat to the specimen. The ceiling and the floor insulation protect the column end plates from fire. It should be noted that only 3200 rnm of length of the

cotumn

is exposed

to fire.

There are

32

propane gas burners in the furnace chamber, arranged in eight columns containing four burners each. The total capacity

oli

the burners is 4700 kW (I 6 million Btulh). Each burner

can

be

adjusted individually, which allows a high degree of temperature unifomrty in the furnace chamber. The pressure in the furnace chamber is also adjustable. It was set somewhat lower than atmospheric pressure.

Instrumentation

The furnace temperatures are measured with the aid of eight chrornel-alumel

thermomuples. The junction of each thermocouple was located 305 rnrn away from the test specimen, at various heights. Two Zhemocauples were placed opposite each other at an internal of 610 rnm along the height of the furnace chamber. The locations of their junctions and their numbering are shown in Figure 4. Thermocouple Nos. 4 and 6

were

located at a height of 610 mrn from the floor, thermocouples

Nos.

2 and 8 at 1220 mm, thermocouples Nos. 1 and

7

at 2440 mm. The tempemtures measured by the thermocouples are averaged automatically and the average temperature is used as the criterion for controlling the f umace temperature.

The loads are controlled by

servo-controllers

and are measured using pressure transducers. The accuracy of controlling and measuring Ioads is about 4 kN (1 kip) at

lower load levels and relatively better at higher loads.

The axial deformation of the test specimen is determined by measuring the

displacement of the jack that supports The column. The rotation of the end plates of the columns are determined by measuring the displacement of the plates at a distance of 500

mm from the

center

of the hinge, at

the

top and

battorn

respectively. The displacement is measured with the aid of transducers with an accuracy of 0.002

mm.

TEST CONDITIONS AND PROCEDURES

The columns, with fixed

ends,

were

installed in the furnace by bolting their end

plates

to a loading head at the top and a hydraulic

jack

at the bottom.

For

columns with pinned ends, they were installed by bolting the top plate to the loading head and by securing the bottom plate to the hydraulic jack using carbide teeth.

(11)

Before

each

column test, the moisture condition in the center of the column was measured by inserting a Vaisala* moisture sensor into a hole drilled in the concrete. In general, the moisture

content

in columns varied approximately from 50 percent to 99 percent relative humidity. The relative humidity of columns is given in Table 4.

End Conditions

Eleven columns were tested with both ends fixed, i,e., restrained

against

rotation

and horizontal translation. For this purpose, 8 19-rnm (3/4 in.) diameter bolts, spaced regularly around the column, were used at each end to bolt the end plate to the loading head at the top

and

to the hydraulic jack at the bottom. Columns TNC3, THC9, TMS12

and

THPI

5

were tested under pinned end conditions, i-e.,

with

restraint against horizontal translation only. The pinned end condition was obtained by bolting the end plates to the

receiving plates with roller bearings at each end. Loading

The applied load on the columns ranged from 50 percent to 125 percent of full

service load (factored compressive resistance of the column) determined according to ACI

31 8-89 [7J. All columns

were

tested under a concentric load, except columns TNC3, THCS, THS12 and THP15, where the loads were eccentric by 25.4 mm. The factored compressive resistance

of

each column, as well as the applied loads, are given in table 4. The factored compressive resistances of the columns were calculated using the effective length factor, K (0.65 for fixed ends

and

1 for pinned ends). The

factored

resistances,

computed using

the

PCACOL computer program [13], together

with

applied loads

on

the column, are given in Table 4.

All loads were applied at least 40 minutes before starting the fire endurance test and

were

maintained until a condition was reached at which

no

further increase of the axial deformation and rotational deformation could be measured. This condition was

selected as the initial condition of the column deformations. The toad was constant

throughout the fire endurance test.

Fire

Exposure

The ambient temperature at

the

st& of each test was approximately 20°C. During the test, each column was exposed to heating controlled in such a way that the average

temperature in the furnace followed, as closely as possible, the CANNLC-S101 [9]

or

ASTM El 19 [ I

01

standard temperature-time curve. This curve can

be

approximately expressed by the following equation:

T, =

20 +750[1 -exp(-3.79553Ji)]+17~.41Ji

Where

T,

= temperature

of furnace

in

" C

-

_-

P rn hn11r.r:

Certain commercial products are identified in this paper in order to adequately specify the experimental procedure. In no case does such identification imply recommendations or

(12)

Recording

of

Results

The furnace, concrete and steel temperature and axial defarmations were measured at lninute intervals. In the case

of

the eccentrically loaded columns, the rotations of the end plates of the

column were

also

measured with varying frequencies, depending

on

the

rate

of change of the measured quantities. The crack propagation and occurrence of spalling in columns were monitored, during the test, through the obsewation

windows

in

the column furnace.

In column SHC7, since the failure did not occur even after 6 hours, ?he test load

was

increased by 15 psi (about 4

percent)

te

400 psi,

and then

by

another 10 psi (about

2.5 percent) to 410 psi.

Also

in

column

TflC8, since the

failure did

not

occur even

after 5

hours,

the test

load was increased 'by 15 psi (about 2.5 percent) to 600 psi. Failure Criterion

The column was considered to have Tailed, and the test was

terminated

when

the

axial hydraulic jack, which has

a

rnaxim~rn speed

of 76

m d m i n (3 in./rnin), could

no

longer maintain the load. RESULTS AND DISCUSSION

The results of the fifteen column tests are summarized in Table 4,

in

which the column characteristics, test conditions, fire

endurance

and

failure

modes

are given for each column. The furnace, concrete and

steel

temperatures recorded during the tests, as

well

as the axial deformations of the column specimens, are given in

Tables

A.1 ta A.15 in Appendix A. The temperatures, axial deformations are also plotted in Figures A.1 to A.15

in

Appendix A, where positive axial deformation values indicate the expansion of the

column. For Calumns TNC3, THC9, THS12

and

THP15, which had pinned ends, the

rotation at the ends are also given in the relevant tables. A typical view of few columns after fire resistance tests is

shown

in Figures E.1

to

B.6 in Appendix

B.

General Observations

During

the

fire tests, visual observations were made to record spalling, as well as crack propagation,

in

the columns. Also, after

the

completion of fire tests, post-test observations

were made

to analyse the failure pattern, extent and nature of spalling, and condition of rebars and ties. The following

are

some of the abservations recorded:

Column

TNCI

Time hr:rnin

0:00

The column was preload& far about 45 minutes before exposure to fire. 0:10

No

spalling in any face of

the

mlurnn.

(13)

1 :00 Hairline cracks propagate, no spalling. 1:30 Mairline

cracks

progress to minor cracks.

2:OQ Cracks

widen

on faces, but still

no

spalling.

3:QO No spalling and no loss of concrete cover to rebars.

4:00

No major cracks at corners, only

cracks

on faces widen.

4:15

Cracks

widen further at the bottom one-third height

in

the nortMeast face

of

the column.

4 3 8 The axial deflection increases at a faster rate and

results

in failure of

the

column

Post-fail u re:

Huge cracks develop al boottom one-third of the

ealumn

resulting in buckling of

longitudinal rebars. No significant spalling was obsewed. No

loss

of concrete cover throughout the column length. Failure of column is in compression mode.

Column TNC2

Observations

The

column

was

subjected

to desired load for about 45 minutes

befare

exposure to fire.

No spalling in any face of the column.

No cracks or spalling in the column.

Some

hairline cracks appear on the face of column. Axial deformation starts contracting.

Hairline cracks progress on the face, and propagate towards comers of the coyumn.

No spalling occurred on any face. Hairline cracks widen focally.

The rate of deformation increases.

Minute cracks widen at corners of the column.

The rate

of

axial deformation increases at a faster rate.

Failure occurred as the column can no longer maintain the load. Past-fail u re:

Some

spalling occurred just 'before failure. Rebars at mid-height are exposed

locally. Some huge cracks

were

observed. The failure mode was in compression. Column TNC3

f

ime

hr: min

0:00

The column was subjected to desired Ioad for

about

45 minutes before

exposure

to fire.

0:10 No spalling in any face of the column. 0:30 No cracks

or

spalling in the column.

1 :15

Some

minute

cracks

appear in easvnorth face.

(14)

Post-failure:

No Spalling occurred even at the end

of

the test and no loss of concrete

cover

to rebar.

Time hr:rnin

Observations

The column was subjected to desired load for about

40

minutes before exposure to fire.

No spalling in any face of the column.

Spalling occurs; a big piece of concrete falls off at mid-height of the column exposing longitudinal bar and ties.

Expansion of column reaches maximum value and stabilizes.

Hairline cwcks

were

observed

on

all faces. Some minute cracks

were

seen an the east face.

A number

of

larger cracks (about

3

to 5

mrn

in width) formed at the centerline on each face.

Four large cracks (about 10 to I5 mrn in wi&h) developed at the centerline on each face.

The

rate

of contraction of the column increases rapidly.

The column failed suddenly as it was unable to maintain the applied load, with failure

mode

being in compression and slight bending.

Post-failure:

There were wider cracks on

all

faces, and rebars and ties were exposed locally.

Observations

The colurnn was subjected to desired load for about 40 minutes before exposure

to fire.

No spalling in

any

face of the column. No spalting or hairline cracks.

A

number

of shad hairline cracks appear an all faces.

More visible cracks (about 100 to 150 mm in

length and about 2

mrn

in

width)

were observed at mid-height on the west face.

Expansion of column reached its maximum value and stabilized.

The number of short hairline cracks increased on

all

faces.

Axial deformation

begin

to decrease.

A new crack (B), 100 mrn

in

length,

developed

at mid-height

on

the east face.

Crack

(8)

lengthened to 200 ia 300 rnm and widened to about

2

mm.

Crack (B) developed to 300 to 400 mm in length. A number of new visible cracks about 100 mrn in length appeared at mid-height

on

the north face.

The rate of contraction of the column increased rapidly.

(15)

Post-failure:

Failure

mode

was mainly in compression with slight bending. Wide

cracks

in

t h e

column, and chunks of concrete on the floor, resulting from spalling before failure, were observed.

Column

THC6

t i m e

hr: min

Observations

The column was subjected to desired load for about 45 minutes before exposure to fire.

No spalling in any face of the column.

No spalling or minw cracks. Column expands.

Minor hairline cracks appeared on the north face. Axial defamation changes from expansion to contraction.

Hairline cracks appeared on all four faces. No spalling was observed. Cracks propagate over the face to corners, still no spalling occurred.

Hairline

cracks

widen to minute cracks. Axial deformation increases at a faster rate.

Small

chips of concrete fall off

along

the length of the column. Still no major spalling occured.

Cracks widen further. Axial deformation increases at a faster rate.

A

number

of

medium

size cracks appear on all four faces. No spalling was

observed.

Axial deformation increases at a very high rate. A big bang was heard and failure

of column

occurs

with the failure mode being in compression and bending.

Post-failure:

Lot of sspalling occurred in the bottom one-third height of the column.

Column THC7 Time

hr:min

The column was preloaded,

with

the desired load, for about 45 minutes before

exposure to fire.

A bang was heard, no spalling was

obsewed.

Still no cracks or spalling in the column.

Hairline cracks observed on all four faces, but no spalfing.

More hairline cracks. Little bit spalling on the

south

face. Cracks widen slightly but no major spalling.

Cracks widen, still no spalling.

Cracks further widen locally on the east face.

Huge longitudinal cracks, running from about one-fourth height to three-fourth

height, develop on the east face.

Since the failure did not occur even after 6 hours, the test load was Increased by 15 psi (about 4 percent) to 400 psi. Axial deformation increases suddenly.

(16)

6:02 The

load

was fuaher increased by another 10 psi (about 2.5 percent) to 41

0

psi. 6:03 Failure occurs in the

column

in compression and bending

mode.

Post-failure:

Huge

cracks

were

observed on

all

faces

and

only some spalling was noticed.

Column THC8

f irne hr:

min

Obse wations

0:00

The column was subjected to desired

load

for about 45 minules prior to fire exposure.

0:05 Very

little spalling

was obsewed (small bang

was

heard).

0:30 No spalling on any of the

four

faces of the column. 1:00 No hairline crack or spalling on any face

d

the column,

1 :10 The expansion in the column reaches maximum value.

1:30 Axial deformation begins to contract.

230

No

major crack or spalling was obsenred. The

column

contracts

gradually.

3:40 Hairline cracks appeared on the east face.

500 Test load

was

increased by 15 psi

(about

2.5

percent) to 600 psi.

5:05

Failure occurs in the column

in

compression and bending

mode.

A big bang was heard just before failure.

Post-failure:

Large chunk of concrete falls off, loss of concrete cover exposing rebars and ties.

Column

THC9

0:00 The column was subjected to

desired

load for

about

45 minutes prior to fire

exposure.

0:10 No spalling or hairline cracks.

6:3Q Very small chunk of concrete fall off. 1 :00 No further spalling in the column.

1 :I 5 Axial deformation reaches maximum

value.

2:OO Minor

cracks

appear at various levels on all four faces.

206 Column

failure

in ductile

mode.

Past-failure:

Very little spalling in the

column,

No loss of concrete cover to rebars and ties. Column THSlO

(17)

The cdumn was subjected to desired load for

about

45 minutes before exposure to fire.

Minor

chunks of concrete

fall

off on a!! faces

o

f

_column. _This_{phenomenon, small} amount of spalling, continued for about 3Q minutes. However no hairline cracks were

obsenred.

Spalling stops. Na hairline crack observed

an

the

surface

of

column. The axial expansion of column reached its maximum value and stabilized.

The column begins to contract.

A few hairline cracks (about 30 mrn in length) appeared on the north and the west

sidesI.

A longitudinal crack (A) appeared (about 100 mrn in length, 1 to 2 mrn in width) at mid-height on the north side.

New longitudinal crack (B) formed, at one-third height from the bottom on

the

north

side. Crack A widens to about 3 mm.

The column failed suddenly, as it was unable to maintain the applied load. Post-failu re:

The failure made was in compression and bending. Steel fibers, after coaling,

were completely black in colour, and could easily be broken

into

pieces. Large

cracks could be seen on all faces, and the

rebars,

and ties were exposed locally. Column THS11

Time

hr:min

Observations

0:00 The column

was

subjected to desired load for about 45 minutes before exposure

to fire.

0:10 No spalling or hairline cracks.

1:00 Small chunk

of

concrete

fall off on the east face.

2:00

Few

minor discontinuous hairline cracks appear. 3100 Axial deformation increases

at

a faster rate.

3:27 Failure occurs in colurnn through compression and bending. Post-failure:

Spalling in the column at about two-third height

from

the bottom.

Column

THS12

nrne Observations

hr:min

0:OO The column was loaded with test load for about 45 minutes before exposure to fire.

0:10 No spalling or cracks on any face of the column. Lot of water oozing out through top and bottom holes.

6:15 Localized minor spalling in some

spots

on north, east and

wesf

faces. 0:30 Liffle more spalling on all faces of the

column.

0:56 Expansion reaches peak value.

No

hairline cracks appeared.

(18)

1:29 A loud bang was heard. Failure

occurs

as

the column

cannot

maintain applied load. Not much spalling occurred even towards the

end

d test. The failure

mode

was ductile. After the test, only very small chunks of concrete fell off. Post-fail u re:

Ties and rebars are not exposed even locally. Some cracks appeared at mid- height on all fourfaces.

Column THP13

Time

hr:min

0:00

The column was loaded with test load for about 45 minutes

before

exposure to fire.

0:10

No spalling or cracks on any face of the column. 0:45 No spalling

or

cracks on any face of the column.

1:OO Very small hairline cracks, but no spalling. 1:30 Still no

major

cracks, and no spalling.

2:30 Hairline cracks on all four faces of the column.

3:30 The rate of axial deformation increases. Hairline

cracks

widen in the east face of the column.

4:30 Axial defomaiton increases at a

very

high rate.

No

major cracks appeared in the

column.

431 Failure of column occurs in compression made.

Post-failure:

Huge longitudinal cracks on the east face.

Column THP14

Observations

The column

was subjected

to

desired

load

for about

45

minutes before exposure

to fire.

A

loud

bang

was

heard.

No spalling on any face of the column.

No

crack

or spalling was observed on any face. Small hairline cracks appeared, but no spalling.

Axial deformation increases at a gradual rate. Still

only

hairline cracks appeared. Hairline cracks enlarge to minor cracks on all four

faces.

Failure of column occurs in compression

and

bending

mode.

A loud bang was

heard just prior to failure. Post-failure:

Some rebars were fully exposed locally.

Chunks

of concrete, resulting from spalling,

wetme

seen

on the floor of the furnace.

(19)

Column THP15

Observations

0:00 The

column

was subjected to desired test load for about 45 minutes before

exposure to fire.

0:07

A loud bang was heard, but no spalling.

0:30 No hairline crack

or

spalling

on

any face

of

the column. 1

5

Hairline cracks extend towards corners.

1 :28

Failure occurs

in compression and bending mode. Post-failu re:

Very

little spalling was observed.

Variation of Temperatures

The measured

furnace temperatures are compared with standard temperature

for

fifteen columns in Figures A. 1 to A. 15. From the figures it can be seen that the measured ternperature

in

the furnace followed closely the standard temperature-time

cunre.

Columns

TNC1, THC4,

THC7,

THS10

and

THP13 had thermocouples

in concrete

and steel to measure representative cross-sectional temperatures. The temperatures at various depths in concrete and on rebas are shown for columns TNC1, THC4, THC7,

THSIO

and THP13 in Figures A.1, A.4, A.7, A.1

O

and A.13, respectively. The

cross-

sectional temperature plats for column THC7 show some erratic trends. This might be

due to the damaged thermocouples, which may have occurred during fabrication or transportation.

In all these columns, the temperatures inside the column rose

rapidly

to abolrt 100°C and then the rate of increase of temperature decreased, Lie [I 11 has shown that this temperature behavior is due to the thermally-induced migration af moisture toward the centre of the column and the transformation of water in the concrete from liquid to vapor phase. The influence of moisture migration Is the highest at the center of the column. While there

are

slight differences in the temperature propagation between NSC and HPC columns, the ternperature propagation for various types of HPC columns was found ta be very similar.

To illustrate the relative thermal behavior of concrete columns wi€h concrete strength, the variation

af

crosssectional temperatures for a typical NSC column (TNCI ) concrete and a typical HSC columns (THC4) are

compared

in Fig. 5

as

a function of

exposure time. The temperatures are shown for various depths from the surface

along

the radial and at mid-height of the column. Except for the concrete strength the NSC and HPC columns had similar characteristics and were subjected to comparable load levels. It

can be seen

from

Fig.

5

that the temperatures in the NSG column are generally lower than the corresponding temperatures in the HPC column throughout the fire exposure. This variation can be attributed partly to the variation in thermal properties of the

two

concrete

(20)

temperature until the

cracks

widen and spalling occurs.

NSC,

wifh higher water content, requires more heat to achieve transformation of the capillary pore water and chemically bound water.

Variation

of

Deformations

The variations of axial deformation with time far fifteen colurnns are shown in Figs. A.1 to A.15. Both the NSC and HPC columns expanded until

the

reinforcement yields and then contracted leading to failure. R can be seen from the figures that the overall deformation behavior of HPG columns was similar to that

of

the NSC columns. However, in the expansion zone, the deformation in HPC columns was less than that for NSC columns.

The defamation in the columns resulted from several factors, such as load, thermal expansion and creep. The initial deformation of the column was mainly due to thermal expansion of concrete and steel. While the effect of

load and

thermal expansion is significant in the intermediate stages, the effect of creep becomes pronounced in the later stages due to the high fire temperature and

long

fire duration. This is one of the main reasons that the deformations are quite

large

before the failure of the columns.

To

illustrate the relative fire performance of different

types

of

columns

the axial

deformations for

columns

TNCI, THC4, THC8, THSIO and THP13 are compared in

Figure 6.

The

axial

deformation

in the expansion stage is very much similar for ail

HPC

columns. THC4 failed at 8 mm axial deformation and all other HPC colurnns failed around

24 mrn axial deformation. The axial deformation curves for

THC4

and THSI

0

are similar, except THS10 have larger axial deformation due to the presence of steel fiber that increased the ductility of the column. The presence of polypropylene fibers in HPC column, THP13, increased the ductility of the column as can be seen from the higher deformations attained before failure.

The axial deformation-time response of HPC

columns

is different from

that

of

the

NSC column. In the case of HSC columns, the deformation is significantly lower than

That

of

the NSC column in the expansion

zone.

This can be attributed partly to the lower thermal expansion and higher elastic

modulus

of HSC.

When the steel reinforcement in the column gradually yields, because of

increasing temperatures, the column

contracts.

At this stage, the column behavior is dependent on the strength of the concrete. Another significant difference that

can

be observed from Figure

6

is that the NSC column maintained the expansion

plateau

for

a

considerable

duration

before contracting, while the cantraction starts much earlier in the HPC columns. There is significant

contraction

in the NSC column leading to gradual ductile failure.

Spalling Pattern and Failure Mode

Majorii of columns failed in compression mode wRh slight bending. Columns THC9 and

THS12

failed in ductile mode. While there was no spalling

in

the

NSC

(21)

HPC columns was not significant in early stages. At higher load levels and under eccentric loads, lot of spalling

occurred

just prior to failure of HPC columns. In HPC columns, THPI 3, THP14 and THPI 5, the presence

of;

polypropylene fibers reduced spalling, both at early stage and just prior to failure.

The spalling resulted from crack propagation in the columns. The cracks in the HPC columns progressed, with time, at the corners of the cross-section, and led to spalling of chunks of concrete just before failure. This spalling was significant at about mid-height. While minute (hairline) cracks cauld be noticed in about 20 to 30 minutes, the widening of these cracks occurred after about 60 minutes or so. Large cracks occurred in HPC colurnns after about 3 hours of fire exposure. In the case of Column

THC8

(carbonate aggregate concrete), the spalling, as noticed at

the

end of the test, was less compared to Column THC4 (siliceous

aggregate

concrete). This could be attributed to the

effect of aggregate

in

the concrete mix. Due to the endothermic reaction in carbonate aggregate, the specific heat of carbonate aggregate concrete, above 600°C temperature, is generally much higher than that of siliceous aggregale concrete. This heat is

approximately ten times the heat needed to produce the same temperature rise in

siliceous aggregate

concrete.

This increase in specific heat is caused by the dissclciatbn of the dolomite in the carbonate concrete and is beneficial for fire resistance and also in reducing spalling

OF

concrete [l2].

When steel reinforcement yields, the concrete carries a progressively increasing

portion of the load. The

strength

of the concrete also decreases with time and, ultimaiely,

when

the

column c a n

no longer support the lead, failure occurs.

At

this stage, the column behavior is dependent an the strength of the

concrete.

There is significant contraction in the NSC columns leading 20 gradual ductile failure. The contraction in

HPC

columns is

much lower. This can be attributed to the facl that HPC becomes brittle at elevated temperatures and the strain attained at any stress level is lower than

that

attained in NSC for

any

given temperature [2]. This is especially applicable to the descending portion of the stress-strain

cuwe

of HPC at elevated temperatures [2].

The effect of high relative humidity in columns was not significant. There was no severe spalling in HPC

columns,

even when the relative humidity

was

high as 99%. This might be attributed to the presence of steel fiber in columns THS10 to TMS12, and polypropylene fiber in THP13 te THPI 5,

Fire Resistance

A

comparison

of

fire resistance far all columns is given in

Table

4. The time to

reach failure is defined as fire resistance of the column. NSC columns, TNCI and TNC2, attained higher fire resistance as compared to similar HPC columns.

The

decreased fire resistance for HPG columns, as compared to the NSC columns, can be attributed to the thermal and mechanical properties of HPC. Further, spalling which results in the decrease

in the cross-section at later stages of fire exposure, also contributed to lowering the fire

resistance of HPC columns.

Columns made of siliceous aggregate concrete typically have lower fire resistance as compared to those made with carbenate aggregae concrete 15,121. HPC columns, THC7-THC9, composed of carbonate aggregate, have higher fire resistance than H PC columns composed of siliceous aggregate. This cauld be attributed to the effect of

(22)

aggregate in the concrete mix, as explained

in

the previous

section.

Due

to the endothermic reaction, the specific heat of carbonate aggregate concrete, above

600°C

temperature, is generaFly much higher than of siliceous

aggregate

concrete.

This heat is approximately ten times the heat needed to produce the same tempemure rise in siliceous aggregate concrete. This increase En specific heat is caused by the dissociation of the dolomite in

the

carbonate concrete and 1s beneficial

in

enhancing fire resistance

and in minimizing spalling

of

concrete [I 21.

In general, the

presence

of fiber reinforcement improved fire resistance. Column THSIO made with steel fiber reinforced

concrete

has a slightly higher fire resistance, about

30

minutes, than similar column without fiber reinforcement, Column THC4.

Column THP13

made

with polypropylene fiber reinforced concrete has a significant higher

fire resistance, about 70 minutes, as compared to

Column

THC4. In addition, the presence of fibers also improved ductility in HPC columns,

THSIQ

and THP13, as can

be

seen

from

time-axial deformation plot jn Figures A.10 and A.13, respectively.

Summary

Based

on the studies reported in this report, it was found

that:

1 . The behavior of HPC

columns

a1 high temperstures is different from that of NSC columns. The fire resistance of HPC columns is lower than that

of

NSC

columns. 2. The type of aggregate has a visible

influence

on the performance of HPC columns at

elevated temperatures. The presence of carbonate aggregate in HPC increases fire

resistance,

3. The addition of steel and polypropylene fibers in HBC column increases fire resistance and improves the ductility of HPC column.

4. The presence of polypropylene fibers in HPC column can reduce spalling and enhance its fire resistance.

5.

The studies, currently in progress, will generate data on the fire resistance

of

HPC columns and will identify the conditions under which their columns can safely be used.

(23)

REFERENCES

Natjonal Building Code

of

Canada, National Research Council of Canada, Oltawa,

ON., 1995.

Phan,

L.T. "Fire Performance of High-Strength Concrete: A Report of the StaZe-of-

the-Art", National Institute of Standards

and

Technology, Gaithersburg, MQ, 1996. Diederichs, U., dumppanen, U.M. and Schnelder, U., "High Temperature Properties

and Spalling Behaviour of High-Strength Concrete", Proceedings of Fourth Weimar Workshop on High Performance Concrete, HAB Weimar, Germany, pp, 21 9-235,

1995.

Kodu r, V.K. R., "Behaviour of 'High

Pedormance

Concrete-Filled Steel Columns Exposed to Fire1', Canadian Journal of Civil Engineering, 1998.

Kodur, V.K.R. and Sultan, M.A.S, "Thermal Properties of High-Strength Concrete at Elevated Temperatures", CANM ET-ACI -JCI l nternational Conference, Takushima,

Japan, June d 998,

Kodur, V.K.R. and SuRan, M.A.Si "Behavious of High-Strength Concrete

Columns

Exposed to Fire", Concrete Canada International Conference Symposium,

Sherbrooke, Canada, Vol. 4, pp. 21 7-232, 1998.

~men'can Concrete Institute, "Building Code Requirements for

Structural

Concrete", ACI 318-89, 1989.

Lie, T.T., "New Facility to Determine Fire Resistance of Columns", Canadian Journal of Civil Engineering, 7(3), 1980, pp. 551 -558.

Underwriters' Laboratories of Canada, "Standard Methods of Fire Endurance Tests of Building Construction and Materialsi', CAN/ULC-S101 -M89, Scarborough, ON, 49

pp., 1989.

American Society for Testing

and

Materials, "Standard Methods of Fire Endurance Tests of Bui'lding Construction and Materials", ASTM

El

1 9-88, Philadelphia, PA,

1990.

Lie, T. T.

and

Celikkol, B., "Method to Calculate the Fire Resistance of Circular Reinfomed Concrete Columns". ACI Materials Journal, Vol. 88, No. 1 ,

pp.

84-91, 1991.

Kodur, V.K.R., "Fibre-Reinforced Concrete for Enhancing the Structural Fire

Resistance a f Columnsn. (in press), ACI Special Publication, 2001

.

Canadian PorZland Cement Association, *Strength Design of Reinforced

Concrete

(24)

List

of

Tables

Table 1.

Physical properties of

coarse and

fine aggregates

Table

2. Yield

and

ultimate strengths

of

rebars

Table 3. Mix proportions

and

compressive strength of

concrete

Table 4.

Summary

of test parameters and results

List of Figures

Figure

1 .

Elevation and cross-section

of reinforced

concrete

columns

Figure

2. Location

and numbering

of

the

thermocouples

in

concrete Figure

3. Column test

furnace

Figure

4. Location

and

numbering

of

the

thermocouples in

column f

usnace

chamber

Figure

5.

Temperature distribution at

various

depths

in

mlumns

TNCI

(NSC)

and

THC4 (HSC)

Figure

6. Axial deformation

for

typical

columns

TNCI, THC$, THC8,

THSI

6

and

THP13

(25)

Table

1

.

Physical properties

of coarse

and

fine aggregates

Table 2.

Yield

and

ultimate strength

of

rebars

Fine

Aggregate

7

2.62

2.57 2.04~

,

6.350 Saturated

surface dry

specific gravity

Dry

specific gravity

Absorption

rate

Sutface

absorption

rate

Table

3. Mix

proportion

and

compressive strength

of

concrete

batch

mix

Coarse

aggregate

Average

3.9

18235

29535

20.25

1

354.5

574.5

I T V D ~

of

concrete

Siticeous

2.65 Test

2

3.9

1

8320

29570

21.5

Unit

weight

(kg/

m) Yield load

(kg)

Ultimate

load

(kg]

Extension

rate (%) l ~ o l u m n s

fabricated

Carbonate

2.77

356 Ultimate

stress 575

Test

1

3.9

18150

29500

19 . . .

Compressive

strength

28 day

90 dav

1

[ ~ e s t

day

*

S: siliceous aggregate C : carbonate aggregate

2.68

1.055

Batch 1 Batch 2

1

Batch

3

1 Batch

4

2.78 0.1 9U

NSC

HPC-SX

HPC-CX HPC-SS

T N C I -

T H C 4 - T H C 7 -

THS1O-

Batch

5

12.1

90 MPC-SP

THP

13 -

-0.1

71

THP f

5

483

42

(26)

Table. 4 Parameters and

Results

of Fire Resistance Tests

on

High Performance Reinforced Concrete Concrete

Note:

1. Aggregate type: S

-

Slllceous C

-

Carbonale

2. Failure mode: 0

-

Buckling C

-

Compresston

3. Flbar type: Sf. - Steel fiber P.P -Polypropylene llber 4. End sbndltton: F-F

-

Flxed

-

Flxed P-P - Plned - PInsd 5. R.H, Relative Hurnldlty In the Column at Test Day

---

-_I---

-8. 2Bd 28 days

7. Factor Rss.

-

Rctor Aesfslance

THP13

SHP14

THPISi

Cotumn length:

Ellectlvo length lnclor K: KxO.65 lor lExed-!\xed end condllions

K=l lor pinned-pinned end conditions

P.P P.P P.P 305x305 305X305 3MX30.5 610x508 ~ 2 5 610X508 K 2 5 0 1 0 X 5 0 8 X 2 5 354 354 354 ~ g , 9 61.9 61.0 88.9 68.9 68.9 86.6 88.8 0 5 6 S S S 4-25-2.18 4 3 5 - 2 . 1 8 4-25-2.78 9.5 -75,150 9.5-75.150

---

9.5 -75.150 40 40 40 27108195 27108198 27/08/96 21/05/98 2 W 0 8 M 2810WlW 94 895 97 F-F F-F P-P

-

--

25.4 2118 2116 2116 3265 3286 3266 1BW 2W0 7500 0.83 1,04 0.71 0.54 057 0 4 6 C C. 0 G . 8 271 233 88