Fires in tunnels and their effect on rock

8/10/2019 Fires in tunnels and their effect on rock

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2006:10

R E S E A R C H R E P O R T

Fires in tunnels and their effect on rock

- a review

Kristina Larsson

Lule University of Technology

Department of Civil and Environmental Engineering

Division of Soil Mechanics and Foundation Engineering

2006:10 - ISSN: 1402-1528 - ISRN: LTU-FR--06/10--SE


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PREFACE

This report is a literature review of the knowledge of fires in tunnels at present. It summarizes

part of the fire dynamics necessary to understand the development of fire in tunnels, state-of-

the-art of concrete research concerning fire behaviour, and tries to collect what is known

about how rock behaves at high temperatures. The report will form a basis for deciding the

direction of continued research in the field of rock behaviour at high temperatures. The report

is funded by Banverket.

Lule, February 2006

Kristina Larsson


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SUMMARY

During the past 15 years there have been several major fires in tunnels all over the world,

most of them caused by accidents. Many of these fires have occurred in road tunnels, and

many have involved heavy goods vehicles carrying goods not classified as dangerous. The

heat developed by these fires has been on par with that expected from a fire in a fuel tanker,

and the damage to the tunnel structure has in some cases been severe. Sweden has been spared

disasters so far, but since many tunnels are being built as parts of major rail and road routes

around and under cities, more transportation will take place underground so the risks are

increasing.

A brief investigation of Swedish railroad tunnels has shown that reinforcement is used in

almost all tunnels; only 6 % does not have any reinforcement at all. Selective reinforcement(the tunnel carries all load and is only reinforced with spot bolting and/or shotcrete in a few

places) is used in 38 % of the tunnels. In 61 % of the tunnels interacting reinforcement is

used, which refers to systematic bolting, mesh and shotcrete that together with the rock mass

carries the load. Concrete constructions that carries load (supporting reinforcement) is used in

26 % of the tunnels, and then usually only on short sections. In Sweden most tunnels are

constructed in relatively strong rock, and at shallow depths, which makes concrete lining

excessive. It is therefore interesting to study the behaviour of strong, hard rocks subjected to

high temperatures from a tunnel fire.

The aim of this literature review was to find the state of the art of how rock behaves at high

temperatures caused by a fire, but very little information was found. Some large scale fire

tests have been performed in rock tunnels, but only a few of the reports from these tests

mention anything at all about what happened to the rock during the test. In a few cases it has

been noted that rock fell from the roof during and after the fire, but no investigation of the

cause of these rock falls has been made. During most of the large scale fire tests performed in

rock tunnels, the rock has been protected from the heat to prevent fall-outs. It seems no tests

have been performed to check the actual behaviour of rock during a fire.

Two areas of interest for future research can be noted. The behaviour of some typical Swedish

rock types found in tunnels should be investigated with focus on the behaviour at high

temperatures. For concrete there is a maximum temperature defined that the concrete can be

exposed to, to avoid spalling. A similar type of maximum temperature limit should if possible

be defined for different rock types. The other area of interest is the behaviour of typical rock

reinforcement used in Swedish tunnels. This reinforcement is usually a combination of rock

bolts and shotcrete, but the amount of reinforcement used varies. The effect of rock bolts must

be investigated, since they are mainly made of steel, which transfers heat very well. The bolts


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will conduct heat into the rock mass, forming local hot spots at some depth. The bolts will

also loose the majority of their strength at temperatures of about 300-400C, which meansthat they will no longer carry load. The effect of this must be considered and the risk

estimated for the people escaping the tunnel, and the fire fighters entering the tunnel. It has

been shown by several authors that shotcrete can be used for fire protection of concrete. It

should be investigated if shotcrete can fill the same function for rock, and still have the

reinforcing capabilities required, and if so what the minimum thickness is to prevent spalling

of rock.


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SAMMANFATTNING

Under de senaste 15 ren har det intrffat flera svra brnder som en fljd av olyckor i tunnlar

ver hela vrlden. Mnga av dessa brnder har intrffat i vgtunnlar, och flera har omfattat

lngtradare med en last som inte klassificerats som farligt gods. Vrmeutvecklingen i dessa

brnder har varit av den storleksordning som man frvntar sig frn en brand i en

brnsletanker, och i vissa fall har det blivit omfattande skador p tunneln. Sverige har hittills

klarat sig undan strre brnder, men med den utbyggnad av infrastruktur i form av vg- och

jrnvgstunnlar runt och under strre stder och den kning av transporter under jord som

fljer, s kar riskerna fr att ngot ska intrffa.

En underskning av svenska jrnvgstunnlar i berg har visat att frstrkning av ngot slag

anvnds i nstan alla tunnlar, bara 6 % av tunnlarna r ofrstrkta. Selektiv frstrkninganvnds i 38 % av tunnlarna och innebr att tunneln br all last och att bultning och/eller

sprutbetong anvnds p korta strckor. Samverkande frstrkning anvnds i 61 % av

tunnlarna, vilket innebr att bergmassan tillsammans med frstrkning i form av systematisk

bultning, nt och sprutbetong br lasten. Betongkonstruktioner som br last (brande

frstrkning) anvnds i 26 % av tunnlarna, och d vanligtvis bara p kortare strckor (t.ex. i

portaler). I Sverige r de flesta tunnlar byggda i relativt h llfast berg och ganska ytligt, vilket

innebr att betonglining r verfldig. Det r drfr intressant att studera hur hghllfast berg

beter sig vid de hga temperaturer som kan uppkomma vid en tunnelbrand.

Mlet med denna litteraturstudie var att inventera kunskapsnivn om bergs beteende vid hga

temperaturer (vid brand), men endast begrnsad information har hittats. Ngra storskaliga

brandfrsk har utfrts i bergtunnlar, men enbart ngra f av rapporterna frn dessa tester

nmner ngot alls om vad som hnde med berget under frsken. I ngra frsk har man

noterat att det fll ned berg frn taket under och efter branden, men man har inte underskt

orsakerna till nedfallen. Under de storskaliga brandfrsken har man skyddat berget mot

hettan fr att frhindra utfall. Det verkar inte som att ngra frsk alls har utfrts fr att

underska hur berget verkligen beter sig under en brand

Tv omrden har bedmts som intressanta fr vidare forskning. Man br bestmma

egenskaper och beteende fr ngra av de typiska svenska bergarter man kan frvnta sig i

tunnlar med avseende p hga temperaturer. Liksom fr betong s br man ta reda p den

maximala temperatur olika bergarter kan utsttas fr utan att spjlkning initieras, om en sdan

temperatur existerar. Det andra forskningsomrdet r hur typisk bergfrstrkning i en svensk

tunnel beter sig vid hga temperaturer. Frstrkningen r ofta en kombination av bergbultar

och sprutbetong, men mngden frstrkning varierar. Bergbultars pverkan mste underskas

eftersom de tillverkas av stl som r en vldigt god vrmeledare. Bultarna kommer att leda in


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vrme i bergmassan s att man fr lokala vrmekllor p djupet. Vid temperaturer p ca 300-

400C har bultarna frlorat det mesta av sin hllfasthet, och kan allts inte lngre ta ngonlast. Vad detta innebr fr tunnelns stabilitet och vilken risk det innebr fr mnniskor som

ska utrymma tunneln i hndelse av brand och fr rddningspersonalen br underskas. Flera

forskare har visat att sprutbetong kan anvndas som brandskydd fr betong. Det br

underskas om sprutbetong kan fylla samma funktion fr berg och nd ha den frstrkande

effekt man efterstrvar, och i sfall vilken minsta tjocklek man mste ha fr att frhindra

spjlkning.


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TABLE OF CONTENTS

Preface ......................................................................................................................................... i

Summary.................................................................................................................................... iii

Sammanfattning.......................................................................................................................... v

Table of contents ......................................................................................................................vii

1 Introduction ........................................................................................................................ 11.1 Background.................................................................................................................11.2 Swedish rock tunnels ..................................................................................................1

1.3 Aim............................................................................................................................. 21.4 Limitations..................................................................................................................3

2 Fires ....................................................................................................................................52.1 General ....................................................................................................................... 52.2 Smoke stratification in tunnels ................................................................................... 6

2.3 Flame length...............................................................................................................82.4 Large fires in tunnels with high longitudinal air flow..............................................102.5 Fire spread in tunnels................................................................................................11

2.6 Vehicle fire load ....................................................................................................... 112.7 Heat release rate (HRR)............................................................................................ 122.8 Case studies of tunnel fires.......................................................................................13

2.8.1 Channel tunnel..................................................................................................142.8.2 Mont Blanc tunnel ............................................................................................14

2.8.3 Tauern tunnel....................................................................................................152.8.4 Hong Kong ....................................................................................................... 162.8.5 St Gotthard Tunnel ...........................................................................................17

2.8.6 Kaprun tunnel ...................................................................................................17

3 High temperature properties of rock................................................................................. 19

4 Concrete............................................................................................................................254.1 Effects of high temperatures..................................................................................... 25

4.2 Failure modes ...........................................................................................................264.3 Prevention of spalling ...............................................................................................284.4 Test methods............................................................................................................. 30

4.5 Shotcrete as fire protection....................................................................................... 304.5.1 Shotcrete on concrete .......................................................................................30

4.5.2 Shotcrete on rock..............................................................................................31

5 High temperature properties of steel ................................................................................ 33

6 Fire tests............................................................................................................................356.1 Fire resistance tests ...................................................................................................356.2 Fire behaviour tests................................................................................................... 37

6.3 Full scale tests...........................................................................................................386.3.1 Repparfjord tunnel............................................................................................39

6.3.2 Lappeenranta tunnel .........................................................................................396.3.3 Runehamar tunnel............................................................................................. 406.3.4 Blasted rock tunnel, Sweden ............................................................................42

7 Swedish tunnel specifications...........................................................................................45

8 Discussion and conclusions ..............................................................................................47


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9 Recommendations for future research..............................................................................51

10 References ........................................................................................................................ 53


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1 INTRODUCTION

1.1 Background

Fire has been used by mankind for thousands of years, and its ability to break rocks is wellknown and was used early for tunnelling and mining. The method of using fire to break rock

is called fire-setting and works as follows; a fire was started at the wall, and when the rock

was hot the embers were removed and water was thrown onto the wall, which cracked the

rock. Wedges were driven into the cracks and blocks of rock could be removed with an iron-

bar. The Egyptians built a tunnel in this way as early as 2000 BC (www.pbs.org,2005). The

method was used in Europe up to the 18 th century, when blasting with black powder took over

as the main mining method.

During the past 15 years there have been several major fires in tunnels all over the world.

Some have been caused by accidents, and some have been arsons. Many of these fires have

occurred in road tunnels, and many have involved heavy goods vehicles carrying goods not

classified as dangerous. These fires have turned into disasters and have killed several people.

The heat developed by these fires has been on par with that expected from a fire in a fuel-

tanker, and the damage to the tunnel structure has in some cases been severe. Sweden has

been spared disasters so far, but since many tunnels are being built as parts of major rail and

road routes around and under cities, more transportation will take place underground so the

risks are increasing. The major part of European road and rail tunnels has concrete lining,

partly because of the weak rock under high pressure they are constructed in. In Sweden most

tunnels are constructed in relatively strong rock, and at shallow depths, which makes lining

excessive. Instead many of our tunnels are reinforced by a layer of shotcrete, which prevents

rocks from falling into the tunnels and also prevents water leakage into the tunnel.

After the tunnel fires in Europe several research projects have studied the behaviour of

concrete lining during and after a fire, and much have been learned on how to make the

concrete perform better. Addition of polypropylene fibres is one of the most efficient ways ofimproving the spalling properties of the concrete, protecting the surface with fibre reinforced

(polypropylene) shotcrete is another.

1.2 Swedish rock tunnels

Some statistics on Swedish railroad tunnels (Lundman, 2005) are summarized in

Table 1-1. A total of 125 rock tunnels were included in the statistics. There are more tunnels

in the railnet, but only rock tunnels and tunnels with information on length have been

included. The column with longest concrete construction refers to reinforced concrete

portals. Almost 50% of the tunnels have concrete portals with a length between 1 and 9 m,
http://www.pbs.org/http://www.pbs.org/


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and only 6 % have concrete portal constructions longer than 50 m. Reinforcement is used in

almost all tunnels, only 6 % does not have any reinforcement at all, and 38 % has selective

reinforcement. Selective reinforcement means that the rock mass around the tunnel carries all

the load and that the tunnel is only reinforced with spot bolting and/or shotcrete in a few

places. In 61 % of the tunnels interacting reinforcement is used, which refers to systematic

bolting, mesh and shotcrete that together with the rock mass carries the load. Reinforced

concrete constructions that carries load (supporting reinforcement) is used in 26 % of the

tunnels, and then usually only over short sections. Over 50 % of the tunnels are grouted, but

38 % are not, and for the remaining tunnels no information on grouting was found. The whole

list of tunnels can be viewed in Appendix 1.

Table 1-1. Reinforcement statistics for rock tunnels in the Swedish railnet.

Longest concrete constr [m]

0 1-9 10-49 > 50

No reinf. /selective

Interactingreinf

Supportingreinf

Grouting

no of

tunnels

22 62 20 7 7 / 47 76 32 66 / 48

18 % 50 % 16 % 6 % 6 % / 38 % 61 % 26 % 53 % / 38 %

The Swedish road administration (Vgverket) operates 28 rock tunnels on the national road

network (Freiholtz, 2005). Only two of these have concrete lining, and the rest are reinforced

with bolts and shotcrete.

1.3 Aim

Little research has been made on the behaviour of rock during and after a fire. There are many

tunnels in Sweden that are unreinforced or that have only bolts or plain shotcrete as

reinforcement. What would happen in these tunnels during a fire? How much work would it

require afterwards to make them safe for traffic again? This depends on how the rock and

shotcrete reacts to the high temperatures developed by a fire. The behaviour of shotcrete has

been studied in several European research projects, and it has been stated that polypropylene

reinforced shotcrete is a cheap and easy way of improving the fire resistance of concrete. The

behaviour of rock at these high temperatures does not seem to have been studied at all,

probably because it has not been considered necessary since the rock in European tunnels is

always protected by thick layers of concrete. The aim of this literature review is to summarize

the knowledge of the effects of fires in rock tunnels. Current status of research regarding

concrete fire behaviour is summarized. The main focus is road and rail tunnels. Basic fire

dynamics in enclosures and tunnels is also covered, but not in great detail. Cases of recent

tunnel fires are summarized to provide a background for the theory of fire dynamics and to

show which parameters are important to know about a tunnel fire. Some of the large scale


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tunnel fire tests performed during the past 10 years are described to see what has been studied

and learned from them.

1.4 Limitations

The limitations of the literature review are that fluid dynamics of a fire, smoke flow, andchemistry are not covered. Numerical simulations of fires are not covered either, since these

are mostly concerned with smoke propagation and the development of exact temperature

gradients of the fire gases.

The focus of the review is rock tunnels and the use of shotcrete for fire protection. This

literature review is not a complete state-of-the-art document of tunnel fire research and

knowledge, but it should provide a basis for understanding fire theory, summarize the most

important results of concrete research regarding fire behaviour, and should provide a startingpoint for further research on the behaviour of rock during a tunnel fire.

Many Swedish rock tunnels are grouted to prevent water leakage. The effect of fire on the

grout is not covered in this literature review, and neither are the effects on frost insulation

used in tunnels today. Other materials for fire protection than shotcrete have not been

reviewed.


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2 FIRES

2.1 General

A fire is a manifestation of a chemical reaction, but the mode of burning may depend more onthe physical state and distribution of the fuel, as well as its environment, than its chemical

nature (Drysdale, 1999). The understanding of fire behaviour in general requires knowledge

of chemistry, heat transfer, fluid dynamics, etc., and the behaviour of a fire in a tunnel is even

more complex. When comparing a tunnel fire to a fire in the open air, there are two important

differences (Ingason, 2005); the heat reflection to the burning vehicles is more efficient in a

tunnel fire because of the confinement, and the interaction of the ventilation with the growing

fire. The heat reflection causes an increase in intensity of the burning vehicle, and may

increase the heat release rate by up to a factor of three (Carvel et al., 2004), see section 2.7.

The interaction of ventilation and fire generates aerodynamic disturbances in the air flow

through the tunnel, which may cause changes in the ventilation pattern, such as throttling of

airflow, and reverse flow of hot gases and smoke from the fire into the ventilation stream

(backlayering) (Ingason, 2005). These effects complicate the fire-fighting, and also transport

toxic fumes and gases far away from the fire.

Ingason (2005) also compared tunnel fires to compartment fires (rooms in buildings) and

stated three major differences. First, the maximum heat release rate (HRR) of a compartment

fire depends on the natural ventilation, which is determined by the area and height of the

openings into the compartment. In tunnels the natural ventilation depends on the fire size,

slope of the tunnel, cross-sectional area, length of tunnel, type of tunnel (concrete lined, rock),

and meteorological conditions at the entrance to the tunnel. Tunnels often also have forced

longitudinal ventilation, which has an effect on the combustion efficiency as well (Ingason,

2005). Secondly, compartment fires can grow to flash-over within a few minutes, but this is

unlikely to happen in a tunnel fire because of great heat losses to the surrounding walls, and a

lack of containment of the hot fire gases. Inside a truck cabin or train compartment located

inside a tunnel, however, flash-over can occur. Thirdly, in the early stages of compartmentfires an upper layer of buoyant smoke is formed, with a cold smoke-free layer below. If there

is very low longitudinal ventilation in a tunnel the same type of smoke layer can be formed in

the early stages of a fire. Further away from the fire source, however, the smoke will descend

to the floor. The distance at which this occurs depends on the fire size, tunnel type,

circumference and height of the tunnel cross-section. If the ventilation is increased, the

stratification of the smoke will dissolve, and backlayering is formed on the upstream side of

the fire, and the stratification of the smoke downstream of the fire is determined by the heat

losses to the surrounding walls, and by the turbulent mixing of the buoyant smoke layer and

the opposite moving cold air below.


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There are two modes of combustion that are important to know and distinguish between

(Ingason, 2005), i.e., fuel-controlled and ventilation-controlled fire. Fuel-controlled fire

means that oxygen is in unlimited supply, and that the rate of combustion is independent of

the mass flow rate of air (oxygen supply rate), but is instead determined by the mass flow rate

of vaporised fuel (fuel supply rate). A ventilation-controlled fire has a limited oxygen supply,

and the combustion rate depends on both air and fuel supply rates. At the precise oxygen level

that enables complete combustion, the mixture is said to be stoichiometric. To determine

whether a fire is fuel- or ventilation-controlled, the air-to-fuel equivalence ratio ()

f

a

mr

m

&

&= , (2-1)

can be used, where am& is the mass flow rate of air (oxygen) supply, fm& is the fuel mass loss

rate (fuel supply), and ris the stoichiometric coefficient for complete combustion. If 1,the fire is fuel-controlled, and if < 1, the fire is ventilation-controlled.

2.2 Smoke stratification in tunnels

In fuel-controlled fires the smoke stratifies, which is of importance for those trying to escape

the fire. The smoke spread is highly dependent on the air velocity in the tunnel, which can be

illustrated by using three typical air velocity ranges (Ingason, 2005):

- low or no forced ventilation (0-1 m/s),

- moderate forced ventilation (1-3 m/s), and- high forced ventilation (> 3 m/s).

When the air velocity is low, e.g., in tunnels with natural ventilations, the stratification of

smoke is usually high around the fire source (Ingason, 2005). The backlayering distance can

be quite long, and sometimes the smoke travels almost equal distances in both directions, see

Figure 2-1a. When the velocity is close to 1 m/s, backlayering occurs upstream from the fire

source, for a distance up to 17 times the tunnel height.

When the tunnel has moderate forced ventilation, the stratification of the smoke near the fire

source is highly dependent on the air velocity, the higher the ventilation velocity, the shorter

the backlayering distance. For air velocities of 1-3m/s the backlayering distance can vary

between from 17 to zero times the tunnel height (Ingason, 2005), see Figure 2-1b.


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Figure 2-1. Schematic of smoke stratification, from Ingason (2005).

In the third group, the smoke stratification is usually low downstream from the fire, and there

is little or no backlayering, see Figure 2-1c and d. The air velocity required to prevent

backlayering is called the critical velocity (Ingason, 2005).

Newman (1984) showed that for duct fires there is a correlation between local temperature

distribution and the local mass concentration of chemical compounds. Ingason and Persson

(1999) showed a correlation between local smoke optical density (visibility), temperature and

the oxygen concentration. Ingason (2005) then finds it reasonable to assume that there is a

correlation between the local temperature stratification, gaseous composition and smoke

stratification in tunnels. The temperature stratification depends on the parameters air velocity,

heat release rate (HRR) and height of the tunnel, which can be related through the local

Froude number (Fr) (Ingason, 2005). Newman (1984) defined three different temperature

stratification regions based on the Froude number, see Figure 2-2. In the first region (Fr0.9), the stratification is severe, and the hot combustion products travel along the ceiling. The

temperature near the floor is close to ambient. The temperature stratification in this region is

buoyancy-dominated. In the second region (0.9 Fr10) the stratification is not severe, but

still involves vertical temperature gradients and is largely mixture-controlled (Ingason, 2005).

There is strong interaction between the ventilation velocity and the fire-induced buoyancy. In


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the third region (Fr> 10) there is little or no vertical temperature gradient, and consequently

little or no stratification.

Figure 2-2. Temperature stratification, from Ingason (2005).

Newman (1984) presented a formula for calculating the Froude number, which is valid for

ducts but have not been validated for tunnels

gHT

T

uFr

avg

avg

avg

=

5.1

2

, (2-2)

whereHis the ceiling height,gis the gravitational constant, Tavgis the average temperature

over the entire cross-section at a given position (K), Tavg= Tavg- Ta (average gas temperature

rise above ambient over the entire cross-section at a given position (K)), and uavg= uTavg/ Ta,

where u is the ventilation velocity. presents The equations for calculating Tavgand Tavgare

presented in detail by Ingason (2005).

2.3 Flame length

The height of the luminous flames depends on the fuel supply rate, the entrainment rate, and

the properties and geometry of the fuel (Ingason, 2005). It is important to know the flame

length to be able to consider fire spread between vehicles. Most fuels (solid and liquid) burn

with a luminous diffusion flame where about 70 % of the total energy is released as

convective heat and about 30 % is released as radiation. The net emissive power of the flame

depends on the concentration of soot particles and the thickness of the flame. For most

hydrocarbon fuel fires, the highest radiative flux is measured when the diameter of the flames

is 3 m or larger (Ingason, 2005), which is also the size when the fire becomes optically thick.

A large portion of the radiative flux from hydrocarbon fires can be absorbed by the smoke

surrounding the flames, which results in lower radiative fluxes to the surroundings (Ingason,

2005). The simplest relation for flame height in the open is

5/22.0 Qhfree = , (2-3)


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where hfreeis the flame height, and Q is the heat release rate, and is valid for axisymmetric

fires. In tunnels there are two factors that must be considered to determine the flame length:

the presence of the ceiling, and the ventilation. When a non-combustible ceiling is present the

horizontal extension of the flames, hhor, can be related to the flame cut-off height, hcut, see

Figure 2-3.

Figure 2-3. Horizontal length of flames under a ceiling, from Ingason (2005).

Babrauskas (1980) calculated the ratio hhor/hcutfor an unbounded horizontal ceiling and for a

corridor. The ratio was 1.5 for the unbounded ceiling. For the corridor the ratio was found to

be highly dependent on the width of the corridor, for a corridor of 3 m width the ratio was

1.81 and for a corridor width of 2 m, the ratio was 2.94. Assuming that a tunnel behavessimilarly to a corridor, Ingason (2005) states that the flames would extend along the ceiling to

a horizontal length of 1.5 3 times the cut-off height. The effect of the ventilation is not taken

into account in these calculations (Ingason, 2005).

Depending on the velocity of the air flow the effect on the flame length differs. High

longitudinal velocity creates a better mix of the oxygen supply with the fuel supply, and

thereby increases the efficiency of the combustion, which can lead to a shortening of the

flames. At moderate air velocities, an increase in the air velocity may cause a lengthening ofthe flames, because at low velocities the volatiles must spread over a larger area before there

is sufficient oxygen to allow complete combustion (Ingason, 2005). At velocities slightly

below moderate, the flames will again grow shorter and interact to a larger degree with the

tunnel ceiling, which decreases the inflow of oxygen to the fire source. At air velocities lower

than moderate, the flames will become less horizontal and will interact with the ceiling to a

greater degree, which reduces the flow of oxygen into the fire source. At zero forced

ventilation (natural convection only) the flames will hit the ceiling and spread in both

directions.


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2.4 Large fires in tunnels with high longitudinal air flow

In a tunnel where the vehicle density is high, and the longitudinal ventilation is high, a fire

can potentially spread between the vehicles and become ventilation-controlled. Ingason

(2005) states, based on research on duct fires in the 1970s by de Ris, that ventilation-

controlled fires occur more readily when the passage is narrow, the air flow (ventilation) is

high, the fuel load is high, and when the ignition source is large. This indicates that the

turbulent mixing in the combustion zone has to be high enough to completely exhaust the

oxygen supply before the fire becomes ventilation-controlled. This in turn means that a tunnel

fire has to involve at least two large vehicles with large fire load before it can become

ventilation-controlled (Ingason, 2005). Figure 2-4 shows a schematic of a ventilation-

controlled fire in a tunnel with relatively high forced longitudinal ventilation.

Figure 2-4. Burning zones in a tunnel with high longitudinal ventilation, from Ingason (2005).

The burning process itself can be seen as stationary, but to explain the process, five different

zones are assumed in Figure 2-4;

1)

burnt-out cooling zone,

2) glowing ember zone,

3) combustion zone,

4)

excess fuel zone, and

5)

preheating zone.

The zones move forward in a dynamic manner, provided that the vehicle density is high

enough in the area of the initial fire. In the burnt-out zone, the vehicles have been completely

consumed by the fire and the fire gases have already cooled down. In the glowing ember

zone, the vehicles have stopped burning and are in a late stage of the decay phase, and are

literally a pile of glowing embers. In the combustion zone (x = 0 in Figure 2-4) the vehicle

fire is fully developed, and flaming combustion is taking place in the whole zone. The flames

cause high heat transfer rates from the gas to the fuel, which leads to high fuel vaporisation


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rates (Ingason, 2005). The temperature of the gas phase just beyond x = 0 increases rapidly,

and reaches a maximum at x = x1, where the oxygen level reaches a minimum. The excess

fuel zone starts at x = x1, at which the oxygen is depleted. In this zone, fuel vaporises from the

vehicles, but no combustion takes place due to a lack of oxygen. The fuel vaporisation takes

place up to the point where the temperature drops below the pyrolysis temperature, (Tvap300C for most solid materials). Beyond this point (at x = x2) no vaporisation takes place, butthe gas flows on and loses its heat to the tunnel walls and preheats the vehicles within this

zone (preheating zone) (Ingason, 2005).

2.5 Fire spread in tunnels

Vehicles stopped in a tunnel where there is a fire may ignite although they are not in contact

with the fire source. The possibility of ignition of a material is determined by evaluation of

whether or not the exposed surface will reach a critical ignition temperature, which dependson the mode of ignition and also on the mode of heat transfer (Ingason, 2005). There are two

modes of ignition: spontaneous and piloted ignition. The difference between the two modes is

that a spontaneous ignition takes place when the vaporised fuel has reached a high enough

temperature without having a flame nearby (Drysdale, 1999). For spontaneous ignition to take

place, the critical temperature is 600C for radiant exposure, and 500C for convectiveexposure. For piloted ignition the critical temperatures are lower, 300 - 410C for radiantexposure, and 450C for convective exposure (Ingason, 2005).

2.6 Vehicle fire loadThe fire load of a vehicle can be estimated by summing the fire load of all constituent

materials, both of the truck itself and the cargo. Fire load (for a compartment) is defined as the

combustible content per unit floor area (Drysdale, 1999). The fire load is related to the

potential fire severity, and can thus be correlated to the fire resistance required for a specific

construction component. Examples of fire loads for road vehicles are shown in Table 2-1.

Table 2-1. Fire loads and HRR of road vehicles, from Egger (2005) and Firetun (1995).

Vehicle typeTypical fire

load [GJ]

Typical fire

power [MW]Remarks

Passenger car 3 3.9 2.5 5.0 Fire loads used in fire tests in Finland

Bus 41 20 Fire loads used in EUREKA fire tests

Truck load 65 20 30

Heavy goods vehicle 88 30HRR without very combustible goods

Railway car 41 77 12.5 22 Passenger cars made of steel

Tanker with 50 m3

gasoline1500 300

Level assumed by Dutch authorities for fires of

extreme dimensions


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2.7 Heat release rate (HRR)

The most commonly used parameters for describing a fire is the heat release rate (HRR) and

the temperature of the fire. It has been noted by several authors that a fire in a tunnel has a

much higher HRR than would be expected from the same fire in unrestrained conditions.

Carvel et al. (2004) studied how the HRR varied with tunnel geometry and ventilation. To be

able to describe the influence of tunnel geometry on the size of a fire, they defined an HRR

enhancement coefficient (Eq. 2-4.)

open

tunnel

Q

Q= , (2-4)

where Qtunnelis the HRR of a tunnel fire, and Qopenis the HRR of a similar open air fire. To

find a relationship between tunnel geometry and HRR, Carvel et al., (2004) used results from

fire tests from different types of tunnels and performed with different kinds of fuels. As an

example a car fire test in the open in Finland gave HRR-values of 1.5-1.8 MW, while burning

the same size of car in a tunnel gave HRR-values between 3.6 and 6 MW. This gives an

enhancement factor () of 2-3. Carvel et al. (2004) found that two factors influence the

enhancement factor:

- a small fire dimension compared to the tunnel dimensions means that re-radiation from the

tunnel walls dominates the process, and

- a large fire dimension compared to the tunnel dimensions means that there probably is an

insufficient amount of oxygen available for the fire to burn at a maximum HRR.

The effect of this is that will increase with fire dimension up to a point, then the fire

becomes ventilation controlled and will decrease, see Figure 2-5. The position of the

maximum and the rate of decrease towards zero depend on the nature of the fuel and the

geometry of the tunnel.

Figure 2-5. Variation of the enhancement coefficient with fire dimension, from Carvel et al.

(2004).


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HRR is difficult to estimate for a fire test in a tunnel even with a full set of measuring

equipment, and for a real fire it is even more difficult to estimate when the fire has already

occurred. To address this problem Carvel et al. (2004) presented an empirical relationship

(Eq. 2-5)

124

3

+

=

T

F

W

W (2-5)

where WFis the width of the fire object and WT is the width of the tunnel. The relationship is

valid for tunnels with flat roofs and all fuels (petrol, heptane, cars, etc.) except methanol. It is

possible that Eq. 2 underestimates the enhancement factor for a concave roof by up to 10 %,

since a concave roof is thought to give a lens-effect, thus concentrating the heat back toward

the source (Carvel et al., 2004).

Carvel and Beard (2005) states that the HRR is the most important factor contributing to the

severity of a fire. The higher the HRR the more severe the fire.

2.8 Case studies of tunnel fires

Fires in tunnels are not new phenomena and neither are fatalities in tunnel fires. The ten worst

tunnel fires by November 2001 were tabulated by Turner (2001), and are shown in Table 2-2,

with subsequent severe fires added. The table makes no pretence of being complete. Duringthe past 10 years several serious incidents with fires in tunnels have occurred. Some of them

are briefly summarized belo w, with information regarding duration of the fire, damage to the

tunnel structure, necessary rehabilitation of the tunnel, and total time of closure of the tunnel.

Table 2-2. Tunnel fires the from 1978 to present day, modified after Turner (2001).

Tunnel Country Type of tunnel Lengt Fatalities Year

Daegu metro South Korea metro - ~ 200 2003

Gleinalm Austria road 8.3 km 5 2001Kaprun Austria funicular 3.3 km 155 2001

St Gotthard Switzerland road 16.3 km 11 2001

Tauern Austria road 6.4 km 12 1999

Salerno Italy rail 9 km 4 1999

Mont Blanc France / Italy road 11.6 km 40 1999

Gueizhou tunnel China rail 0.8 km > 80 1998

Channel Tunnel UK / France rail 49.6 km - 1996

Isola Delle Italy road 0.15 km 5 1996

Pfnder Austria road 6.7 km 3 1995

Baku underground Azerbaijan metro - 289 1995

Great Belt Denmark during construction 8 km - 1994

Serra Ripoli Italy road 0.44 km 4 1993


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Table 2-3 (concluded).

Tunnel Country Type of tunnel Lengt Fatalities Year

Moscow Russia metro - 7 1991

New York USA metro - 2 1990

London United Kingdom metro - 31 1987

Gumefens Switzerland road 0.34 m 2 1987

Caldecott USA road 1.02 km 7 1982

London United Kingdom metro - 1 1981

Kajiwara Japan road 0.74 km 1 1980

Nihonzaka Japan road 2.04 km 7 1979

Velsen Netherlands road 0.77 km 5 1978

2.8.1 Channel tunnel

The Channel tunnel is a high speed railway tunnel crossing the English Channel, and it

consists of two parallel tubes, with a smaller service tunnel running in between. It was

finished in 1993. In November 1996 a fire in a HGV (Heavy Goods Vehicle) occurred on

board the shuttle train (Kirkland, 2002). Arson was suspected since no natural causes to the

fire could be found. The train was stopped in the tunnel and the passengers were evacuated.

There were no fatalities. The fire lasted about 7 hours, and can be considered severe since the

maximum temperature at the centre of the fire was estimated to 1000C. The tunnel liningsuffered severe damage over 46 m nearest to the fire source, serious damage over a length of

about 280 m, and was affected to some extent over 500 m (Kirkland, 2002). The remedial

measures were to install steel arches as a temporary support to ensure safe working conditionsduring the removal of debris from the site. Damaged concrete was removed by grit blasting,

and then damaged reinforcing steel was replaced. The damaged lining was replaced by 680

tons of plain shotcrete and 630 tons of fibre reinforced sho tcrete. The tunnel was closed for a

total of six months.

2.8.2 Mont Blanc tunnel

The Mont Blanc tunnel was built as a joint project between France and Italy and was opened

for traffic in 1965 (LaCroix, 2001). The tunnel is 11.6 km long, and the roadway is 7 m wide

with meeting traffic. The tunnel has a concrete lining, which carries the load and at lay-bys a

larger cross-section, see Figure 2-6. These lay-bys occur every 300 m, on alternating sides,

and every second lay-by is a shelter with a fresh air supply.

The fire accident occurred in March 1999, and was caused by a HGV that caught fire and

stopped halfway into the tunnel from the Italian side. This HGV had a refrigerated trailer with

margarine and flour. The fire started in the cab, but spread quickly to the trailer, which burned

emitting very toxic gases. The smoke was black and heavy. The flow of the ventilation was

from Italy towards France, making the air fairly clear upstream from the burning vehicle, and

filling the entire cross-section with smoke towards France (downstream). From the Italian side


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eight HGVs and several passenger cars entered the tunnel before the entrance was closed, but

since they could see the smoke coming from the burning vehicle they could stop their vehicles

and escape. All passenger cars turned and drove out, while the eight HGVs were left behind

and later burned. During the same time 18 HGVs, 9 passenger cars, a motorcyclist and a pick-

up van entered the tunnel from the French side. Four of the HGVs managed to pass the

burning HGV and make it to safety, but all the others were trapped in the smoke, and later

burned. Totally 39 people died in the accident. The fire lasted for 53 hours. The tunnel vault

was severely damaged over a length of 900 m, and there was also damage to the roadway

pavement and slab. The secondary lining was also destroyed or badly damaged over a

considerable distance because of the high temperatures (LaCroix, 2001). The temperatures in

the tunnel during the fire have been estimated to between 800 and 1000C, with a maximumtemperature of 1200C (Abraham and Derobrt, 2003). The average concrete facing thickness

was 0.5 m, with no reinforcement and water tightness. Spalling does not seem to haveoccurred, despite the high temperatures, which Abraham assumes can be explained by the

high permeability of the concrete (bad quality). The high permeability allowed water to

escape the concrete, which kept the stresses low enough for thermal spalling not to occur

(Abraham and Derobrt, 2003). The damage to the tunnel lining is instead caused by the

decreased strength of the concrete due to the high temperatures.

Figure 2-6. Cross section at a lay-by, from LaCroix (2001).

2.8.3 Tauern tunnel

The Tauern Tunnel is a 6.4 km long single tube tunnel, which was opened for traffic in 1975.

The roadway is 7.5 m wide, and the height is 4.7 m. The fire accident occurred in May 1999,

and was caused by a lorry running into some stopped vehicles from behind at relatively high

speed. A schematic of the incident is shown in Figure 2-7. One lorry and four passenger cars

had stopped in the tunnel at a stop sign. The stop sign was there because there was aconstruction in progress in the tunnel. The ramming lorry shoved two of the passenger cars in


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under the first lorry, and the other two smaller cars were pushed up against the tunnel wall.

The ramming lorry ended up with its cab right up against the back of the first lorry. In the

collision itself eight people were killed, and another four died as a result of deciding to stay in

their car during the ensuing fire (Eberl, 2001).

Figure 2-7. Schematic of accident causing the fire in the Tauern Tunnel.

The fire lasted for about 17 hours. During the extinction phase, engineers entered the tunnel

ahead of the fire fighters to ensure the stability of the roof. The tunnel (inner) ceiling sagged

in places and had to be propped. Soot cleaning was necessary in the entire tunnel, and about

350 m of inner ceiling had to be replaced (Eberl, 2001). The sidewalls over a length of 100 m

in the area of highest temperatures had spalled to a depth of 400 mm, and to a depth of 50 mm

over the whole sidewall surface or locally along another 450 m length. Totally some 600 m3

of spalled concrete was removed (Leitner, 2001). About 800 m of the roadway had to be

rehabilitated due to spalling. Here 5-8 cm of the surface was removed and a new pavement

added. The rescue niches in the tunnel worked as planned during the fire (radio and lighting).

The tunnel was closed to traffic for three months, during which also the ventilation system

was improved.

2.8.4 Hong Kong

The tunnel is a cross-harbour tunnel that was opened for traffic in 1972. It consists of twodouble tubes, with two lanes each, having a roadway width of 6.6 m and a height of 5.1 m

(Chow and Li, 2001). The fire accident occurred in May 2000, and was caused by a passenger

car that caught fire. The driver tried to extinguish the fire himself, but failed. After about four

minutes firemen were on the scene, and started to evacuate people into the other tunnel tube.

After evacuation the fire was rapidly extinguished. The fire lasted for about 45 min, and 15

min after that the tunnel was reopened for traffic. No damage to the ceiling was reported, and

no remedial measures beyond soot cleaning were necessary.

stop

stop


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2.8.5 St Gotthard Tunnel

The St Gotthard Tunnel is a 16.3 km long tunnel that was opened for traffic in 1981. The

tunnel is a single tube two-lane tunnel, 10 m wide and 4.5 m high. Above the roadway are two

ventilation ducts, one for exhaust and one for fresh air (Turner, 2001). The fire accident

occurred on 24 October 2001, and was caused by a heads-on collision between two lorries,

one carrying tyres and tarpaulin while the other had no load. The total number of fatalities

was 11. The fire took 36 hours to extinguish, and the maximum temperature has been

estimated to about 1000C. The high temperatures caused the inner reinforced concreteceiling to collapse over some length. Several of the survivors escaped by crossing into the

service tunnel running parallel to the main tunnel. Those that died are thought to have

suffocated from smoke inhalation during the first few minutes of the fire (Turner, 2001).

2.8.6 Kaprun tunnel

The Kaprun funicular tunnel is 3.3 km long, and has an average slope of 43 %. The cross

section is circular with a diameter of 3.4 3.6 m. The tunnel is designed so that the trains can

meet in the middle, where also a rescue tunnel connects with the train tunnel. The fire

accident occurred on 11 November, 2001, and was (probably) caused by faulty heater in the

lower drivers cabin (Ingason, 2003). The accident train was ascending, when the fire caused

the train to stop about 600 m into the tunnel. The descending train also stopped, about 1.5 km

uphill from the accident train. The fire developed rapidly, partly because of the fire load

associated with the passengers (skis and winter clothing) and partly because of the high

natural ventilation in the tunnel blowing in through a smashed window and fanning the fire

(Ingason, 2003). The natural draft in the tunnel is normally going upward, and on the day of

the accident the upward draft was about 10 m/s. About 30 minutes into the fire the lower

hauling rope (steel cable) snaps and shoots downhill. Since there was a risk of the emergency

brakes failing, the rescue attempt was abandoned at that time (Schupfer, 2001).The total fire

duration was about 3 hours, leaving the train totally burned out, and only 12 survivors who

had managed to escape downhill out of the tunnel. A total of 155 people perished in this

accident, 150 of them in the ascending train, two on the descending train, and three in the

upper terminal who died from the toxic fumes generated by the fire (Schupfer, 2001).


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3 HIGH TEMPERATURE PROPERTIES OF ROCK

Several mechanical and physical properties of rock change with temperature. Heuze (1983)

published the result of an extensive literature study of the effect of high temperatures on

mechanical, physical and thermal properties of granitic rocks. The studied properties are listed

in Table 3-1.

Table 3-1. Mechanical, physical and thermal properties studied by Heuze (1983).

Mechanical properties Physical properties Thermal properties

Youngs modulus Density Melting temperature

Poissons ratio Permeability Heat of fusion

Tensile strength Specific heat

Compressive strength Thermal conductivityViscosity Thermal diffusivity

Thermal expansion

The most important mechanical properties to consider in regard to fires are Youngs modulus,

compressive strength and thermal expansion. Youngs modulus and compressive strength are

affected by both temperature and pressure. The variation of the normalized Youngs modulus

with temperature is shown in Figure 3-1. Increasing the surrounding pressure tends to delay

the decay of the normalized modulus when the temperature increases (Heuze, 1983).

Figure 3-1. Normalized Youngs modulus versus temperature, from Heuze (1983).


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The compressive strength decreases with increasing temperature, as shown in Figure 3-2. At a

surrounding pressure of 500 MPa, the difference in strength is 700 MPa between samples at

300C and 900C, respectively.

Figure 3-2. Variation of compressive strength with temperature, from Heuze (1983).

Heating of rock leads to thermal expansion, which can be reversible or irreversible depending

on the heating rate and the maximum temperature. Richter and Simmons (1974) studied the

thermal expansion behaviour of igneous rocks, and found that for heating rates less than

2C/min, and a maximum temperature of 250C, the expansion curves are reproducible andno permanent strains are induced. The thermal expansion of a rock type depends on its

constituent minerals. Quartz-rich rocks have a larger thermal expansion than rocks with low

quartz content. The thermal expansions of some minerals are shown in Figure 3-3.


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Figure 3-3. Thermal expansion of typical minerals found in granitic rocktypes, from Simmons

and Cooper (1978). The minerals are a) quartz, b) olivine, c) pyroxene, d) orthoclase, and e)

plagioclase.

A schematic of the variation of the linear thermal expansion with pressure and temperature is

shown in Figure 3-4. Increasing the pressure increase the --transition temperature, andlowers the peak thermal expansion (Heuze, 1983). The --transition of quartz is animportant event, that occurs at a temperature of 573C. At this temperature quartz changes

from one form to another with an accompanying increase in volume. The physical properties

are not dependent on temperature in the short time interval considered here, i.e., the duration

of a fire.

Figure 3-4. Coefficient of thermal expansion versus temperature, from Heuze (1983).


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Dry granite melts at about 1050C, but partially or fully saturated rock melts at lowertemperatures. The specific heat of granite is discontinuous around the --transition, but thegeneral trend is that the specific heat increases with increasing temperature (Heuze, 1983), see

Figure 3-5. The thermal conductivity decreases with increasing temperature, see Figure 3-6, in

the interval 20 to 300C. The thermal diffusivity also decreases with increasing temperature,and shows a minimum at the --transition, see Figure 3-7 (Heuze, 1983).

Figure 3-5. Specific heat versus temperature, from Heuze (1983).

Figure 3-6. Thermal conductivity versus temperature, from Heuze (1983).


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Figure 3-7. Thermal diffusivity versus temperature, from Heuze (1983).

The temperature related failures are spalling due to differential expansion of adjacent minerals

leading to build up of thermal stresses that are added to the compressive stresses already

acting. If the combined stress level exceeds the tensile strength of the rock, cracking (failure)

will occur, and spalling of material is the result.


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4 CONCRETE

This chapter explains the behaviour of concrete when it is subjected to high temperature due

to fires. Some of the topics are the effects of raised temperatures, typical failure modes, and

how spalling can be prevented. Shotcrete is also covered as a means of protecting the

structural concrete from heat.

4.1 Effects of high temperatures

Two things happen in concrete subjected to high temperatures due to fire; 1) the mechanical

properties deteriorate, and 2) explosive spalling (Khoury, 2000). The deterioration of the

mechanical properties is caused by physicochemical changes in the cement paste and in the

aggregates and thermal incompatibility between the aggregates and the cement paste. These

are all influenced by environmental factors, such as temperature level, heating rate, applied

loading, and sealing (prevents moisture loss from the surface) (Khoury, 2000). The

physicochemical changes with temperature are listed in Table 4-1. The temperatures are for

the concrete material, not the fire. Explosive spalling occurs when the surface temperature of

the concrete is between 300C and 400C, and depends on type of concrete and heating rate(Khoury, 2000).

Table 4-1. Physicochemical changes of concrete with temperature, after Khoury (2000).

Temperature (C) Event100 Hydrothermal reactions

300 Calcium hydroxide dissociates

600 Marked increase in creep

700 Dissociation of calcium carbonate

800 Ceramic binding

1200 Melting starts

1300-1400 Concrete melted

The compressive strength of concrete decreases when the temperature is increased from room

temperature to about 80C, however, this decrease is largely reversible upon cooling. Above300C most concretes show a strength reduction, how large depends on the aggregates andcement paste used. Above temperatures of 550-600C the concrete has lost enough strength soas not to be structurally useful (Khoury, 2000). In a fire, usually only the first few centimetres

closest to the surface are subjected to temperatures above 330C due to the low thermaldiffusivity of the concrete. This layer provides insulation to the inner concrete and the

reinforcing steel, although its structural strength is greatly diminished. This layer is usually

replaced after the fire.


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Bostrm (2003) noted that the results from fire tests on concrete varied greatly with regard to

spalling. Several tests on three different kinds of concrete (normal, high strength, and self

compacting concrete) were performed to find the relation between spalling and water content,

water-powder ratio, and also to see the effect of addition of polypropylene fibres. The results

of testing of concrete columns showed a reduction of spalling when polypropylene fibres were

used, but a higher proportion of fibre did not give a better performance (Bostrm, 2003). The

optimum performance and proportion of fibres seemed to vary with concrete quality and

possibly with curing (in air or under water). The results from testing of slabs show a clear

reduction of spalling when polypropylene fibres were added. The conclusions from the tests

were that self compacting concrete spall much more than normal concrete with the same w/c-

ratio, but that addition of polypropylene fibres or some kind of insulation reduces spalling

significantly.

4.2 Failure modes

Spalling is a common form of failure for fire heated concrete. The spalling can be either

violent (fast) or non-violent (slow), and can be grouped into four categories; aggregate

spalling, explosive spalling, surface spalling, and corner/sloughing-off spalling (Khoury,

2000). Khoury (2000) states that the first three types occur during the first 20-30 min of a fire

and that they depend on the heating rate (20-30C/min), while the last type occurs after 30-60min of fire and depends on the maximum temperature. The damage caused by spalling can

vary from only superficial to severely reducing the safety of the structure in case of fire. The

most important factors that influence spalling are heating rate, permeability of the material,

pore saturation level, presence of reinforcement, and level of applied loading (Khoury, 2000).

Several different theories have been proposed to explain explosive spalling. There are three

main causes of spalling, namely:

- pore pressure,

- thermal stress, and

- combined pore pressure and thermal stress.

Characteristics of the different types of spalling are shown in Table 4-2, and their main

influencing factors in Table 4-3.

Table 4-2. Characteristics of spalling, modified after Khoury (2000).

Aggregate Corner Surface Explosive

Time of occurrence (min) 7-30 30-90 7-30 7-30

Nature splitting non-violent violent violent

Sound popping none cracking loud bang

Damage superficial can be serious can be serious Serious


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Table 4-3. The types of spalling and their influencing factors, modified after Khoury (2000).

Aggregate Corner Surface Explosive

Aggregate thermal expansion X X X

Aggregate thermal diffusivity X

Shear strength of concrete X

Tensile strength of concrete X X

Age of concrete X

Heating rate X X

Loading/restraint X

Heating profile X

Permeability X X

Section shape X

Reinforcement X X

Aggregate size X X

Maximum temperature XMoisture content X X X

Section size X

Pore pressure spalling is influenced by the permeability of the concrete, the initial water

saturation level, and the heating rate (Khoury, 2000). Thermal stress spalling is explained by

the fact that heating of a material with low conductivity (like concrete or some ceramics)

creates temperature gradients that induce compressive stresses close to the heated surface and

tensile stresses in the cooler regions. The compressive stresses at the surface can be enhanced

by load. The theory behind the combined pore pressure thermal stress is that explosive

spalling is a result of pore pressure, compression in the region closest to the surface (caused

by thermal stresses and external loading), and internal cracking, see Figure 4-1. During

heating, cracks form in the material as the sum of the stresses exceeds the tensile strength of

the material. The formation of cracks is accompanied by a sudden release of energy and the

sudden failure of the heated surface material.

Tests performed by Jansson (2005) have shown that the pore pressures are not very high at the

onset of spalling. A fire test of a concrete panel showed that when spalling initiated 15 min

into the test, the temperature at 10 mm depth was about 200C and the pressure in theconcrete about 0.7 MPa. This pressure is lower than the tensile strength of concrete at the

measured temperature, and indicates that pore pressures probably are less important than

thermal stresses as a cause of spalling (Jansson, 2005).


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Figure 4-1. Forces in heated concrete, after Khoury (2000).

4.3 Prevention of spalling

Great efforts have been made on trying to prevent spalling. In the past the only effective way

of doing this was to create a thermal barrier to insulate the concrete from the heat. The

problem with the thermal barrier was that it had to be made of a material that in itself would

not burn easily. Since the invention of the polypropylene fibre, the thermal barriers are not as

important anymore. The addition of polypropylene fibres into the concrete mix increases the

permeability during heating above 160C (melting temperature of the fibres). The meltedfibre leaves a channel in the concrete that allows moisture to escape, preventing build-up of

pore pressures (Khoury, 2000). The results of Jansson (2005) seem to contradict this theory,

but still concrete with addition of polypropylene fibres tend to spall less than a concrete

without polypropylene fibres. A summary of different preventive methods is shown in Table

4-4.

Pore pressure, p

Pores

Load, L, andthermal stress, t

Spall

Concrete

L+ t


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Table 4-4. Methods for preventing spalling of concrete, modified after Khoury (2000).

Method Effectiveness Comments

Polypropylene fibresVery effective, even in high strength

concrete

May not prevent spalling in expansive UHPC.

Does not reduce temperatures, only pore

pressures

Air-entraining agent Very effective Can reduce strength

Thermal barrier Very effectiveReduces concrete temperature, increases fire

resistance

Moisture content

controlReduces pore pressures

Normal moisture content is usually above the

no spalling limit for most buildings

Compressive stress

controlReduce explosive pressures Not economical as sections sizes increases

Choice of aggregateBest to use low expansion and small

size aggregates

If low moisture lightweight concrete is used,

additional fire resistance is possible, but in

high-moisture conditions violent spalling is

promoted

Reinforcement Reduces spalling damage Presence of reinforcement limited spread ofspalling in the Channel Tunnel fire

Supplementary

reinforcementReduces spalling damage Difficult to use in small and narrow sections

Choice of section

shape

Thicker sections reduce spalling

damageImportant for I-beams and ribbed sections

After a fire incident the damage to the concrete can be assessed by concrete petrography

(Nijland and Larbi, 2001). High temperatures results in changes of some phases of both the

cement paste and the aggregates which may alter the colour of the concrete and the original

mineralogical composition of both the cement paste and the aggregates. These changes can beused to find isograds (horizons of similar composition and/or appearance) in the concrete.

Since these isograds depend on the temperature they will coincide with isotherms and can

therefore be used to trace the temperature variations with depth from the surface (Nijland and

Larbi, 2001). Concrete petrography can be done on three levels of increasing accuracy: visual

and stereomicroscopic inspection, fluorescent macroscopic analysis (FMA), and polarising

and fluorescent microscopy (PFM). The first is performed on drilled cores, FMA on flat-

polished sections, and PFM on fluorescent thin sections. The visual inspection is aimed at

finding the colour variations with depth from the surface, patterns of cracking in and around

the aggregates, width and depth of cracks, dissolution and loss of bonding to the aggregate

particles, and the integrity of the cement paste (Nijland and Larbi, 2001). The isograds that

can be identified by the naked eye together with the corresponding temperatures are listed in

Table 4-5.


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Table 4-5. Isograds in concrete with corresponding temperatures, after Nijland and Larbi

(2001).

Temperature (C) Comment

< 300 Normal, no apparent macroscopic changes in concrete. The colour remains grey.

300-350

Oxidation of iron hydroxides like FeOOH in aggregate and cement paste to hematite,

-Fe2O3, causing a permanent change of colour of the concrete from grey to pinkishbrown.

573

Transition of -quartz to -quartz, accompanied by an instantaneous increase involume of about 5 %, resulting in a radial cracking pattern around the quartz grains in

the aggregate. This phase transition itself is reversible, but the radial cracking

provides a diagnostic feature that remains after cooling.

> 800

Complete disintegration of calcareous constituents of the aggregate and cement paste

due to both dissociation and extreme thermal stresses, causing a whitish grey

coloration of the concrete.

4.4 Test methodsThe testing methods most commonly used are furnace tests, both in small and large scale.

Testing in a furnace has the advantage of controlling the temperature quite exactly, and the

tests can also be observed and documented as they proceed. The disadvantage is that they can

be quite expensive, since experienced personnel are required for the operation of the furnace.

Chapter 6 provides a more detailed description of fire tests, both furnace and full-scale tests.

4.5 Shotcrete as fire protection

The most common way of providing fire protection for tunnel elements made of concrete is to

add a layer of shotcrete. Shotcrete has several advantages over other insulation materials. It is

easy to apply in the desired thickness, the properties can be adapted to fit the requirements for

each project, and it can be applied economically with a machine. If a fire occurs, the shotcrete

can also easily be replaced.

4.5.1 Shotcrete on concrete

Fire testing of concrete slabs to be used as lining in tunnels is a way of ensuring that the fire

protection is sufficient. In the test described here, three different shotcrete qualities were

tested (Bostrm, 2005). The differences in shotcrete recipe were the amount of steel fibres,

amount of polypropylene fibres, and applied thickness (60 or 90 mm). All the cast concrete

slabs were of the same quality, K45. Before application of the shotcrete the surface was either

washed or water-jeted. To provide better contact between concrete and shotcrete different

mechnical bonds (bolts) were used. The slabs were subjected to the RWS-curve (see Figure

6-1). Already after 4 minutes some samples start to spall, and after about 22 minutes all

samples have had some spalling of the shotcrete. The maximum spalling depth for the

samples with shotcrete thickness 90 mm ranges between 50 and 91 mm. The spalling depth is

measured from the concrete-shotcrete interface, with the positive direction towards the


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shotcrete. The samples with shotcrete thickness of 60 mm had a maximum spalling depth of

about 40 mm, i.e., spalling of the concrete had started (Bostrm, 2005).

4.5.2 Shotcrete on rock

Mangs and Keski-Rahkonen (1990) tested granite slabs protected by shotcrete according tothe ISO834 fire curve. The slabs had an area of about 1 m2, and a thickness of 20 - 30 cm. The

shotcrete was applied in different thicknesses and with different reinforcement. In about half

of the tests the shotcrete was anchored to the rock slabs with bolts, which in some cases were

insulated, as indicated in Table 4-6. A summary of the composition of the samples is shown in

Table 4-6.

Table 4-6. Composition of samples, from Mangs and Keski-Rahkonen (1990).

Sample Process ReinforcementThickness

[mm]Anchored

1 dry mix - 20 30 -

2 dry mix 3.4 mm steel wire mesh 60 80 -

3 wet mix - 60 100 +, insulated

4 wet mix 3.4 mm steel wire mesh 50 110 +, insulated

5 wet mix steel fibre, 18 mm, 75 kg/m3 80 100 -, sample edges fixed to furnace

6 wet mix steel fibre, 25 mm, 70 kg/m3 75 115 +, not insulated

7 wet mix plastic fibre, 900 g/m3 20 60 -, sample edges fixed to furnace

8 wet mix plastic fibre, 900 g/m3 40 - 120 +, not insulated

Changes of the shotcrete surface were observed visually (Mangs and Keski-Rahkonen, 1990).

The duration of the tests varied from 15 min to 4 hours. A summary of the results of the tests

and the observations made is shown in Table 4-7.

Table 4-7. Summary of shotcrete tests, from Mangs and Keski-Rahkonen (1990).

Sample Location Duration of test Test results

1 ceiling 16 min Granite-shotcrete bond breakage at 11 min, the whole shotcretelayer fell to the floor. Granite cleavage at 15 min.

2 wall 3 h No spalling observed. Cracking of granite at 1 h 28 min.

3 ceiling 4 hSlight spalling between 45 min and 1 h. Fissures in granite at 3

h 15 min.

4 wall 4 hSlight spalling between 25 min and 40 min. Fissures in granite

at 2 h 20 min, cleaving of granite at 3 h 15 min.

5 wall 2 h 35 min No spalling observed.

6 ceiling 53 min

Granite-shotcrete bond breakage at 17 to 23 min. Breakage of

anchor bolts at 53 min and the whole shotcrete layer fell to the

floor. No spalling observed.

7 ceiling 2 h 35 minVariations in shotcrete thickness, thinner parts fell to the floor

during the test. No spalling observed in thicker part.8 wall 55 min Spalling in a poorly insulated corner at 21 min.


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Temperatures in the samples were measured at the shotcrete-rock interface and at half-

thickness of the shotcrete layer. A summary of the temperatures for the different locations for

each sample are shown in Table 4-8. In some cases spalling of the shotcrete in the vicinity of

the thermocouples give anomalous temperature readings, these have been discarded in Table

4-8.

Table 4-8. Summary of temperature measurements by Mangs and Keski-Rahkonen (1990).

SampleDuration

of test

Interface

temp [C]

Half-thickness

temp [C]Comment

1 16 min 80 100 Temperature at 11 min.

2 3 h 410-460 650-680

3 4 h 360-440 420-470Lower temp refers to thermocouple at centre of

slab.

4 4 h 300-540 520-700Thermocouple 4-5 mm from interface. Lower

temp refers to thermocouple at centre of slab.

5 2 h 35 min 240-290 270-420 Thermocouple 2-5 mm from interface

6 53 min 80-100 140-220

7 2 h 35 min 650-870 720-900

8 55 min 90-110 110-150

The temperature on the outside of the granite slabs (at the centre) was measured on samples 3

and 4. After 2 hours the temperature was about 30C, after 3 hours about 50C, and after 4

hours about 60C (Mangs and Keski-Rahkonen, 1990). No note of the exact thickness of theseslabs can be found in the article.

Mangs and Keski-Rahkonen (1990) summarized their observations from the tests:

- bond breakage occurred in all tests (both with and without mechanical anchoring),

-

slight spalling of the shotcrete caused by water pressure was observed,

-

when the shotcrete layer fell off, the hot fire gases caused a thermal shock to the granite

which started spalling, and- large thermal stresses broke the shotcrete layer and the granite slab in sample 7.

As a conclusion Mangs and Keski-Rahkonen (1990) say that mechanical anchoring of the

shotcrete to the rock is important, since bond breakage occurred in all samples, but that the

steel bolts have to be insulated from the heat.


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5 HIGH TEMPERATURE PROPERTIES OF STEEL

Construction steel, e.g. steel used as reinforcement of concrete and as rockbolts (rebar), has

high tensile strength. When the steel is used for concrete reinforcement it can sometimes be

pre-tensioned, to provide a compressive force to the concrete beam. If the steel is heated to a

temperature of about 300C the steel looses the pre-tensioning and almost all strength. This isa concern in building construction, and the concrete is therefore designed to insulate the steel

from the heat of a fire. This is accomplished by ensuring that the steel is covered by a thick

concrete cover. When steel is used as rock bolts to reinforce a rock mass, no such protection

from heat can be ensured, since the bolts are installed perpendicularly to the rock surface. To

provide the best possible reinforcement, the shotcrete is installed first, and then the bolts are

installed through the shotcrete, which leaves the ends of the bolts unprotected from heat.


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6 FIRE TESTS

There are generally two types of fire tests, which will both be described in this chapter. The

first type is performed to determine the fire resistance of construction components such as

beams with regard to spalling, load carrying capacity and ability to function as a barrier. This

type of fire test is described in chapter 6.1. The second type of fire tests are performed to test

the behaviour of a fire with regard to different tunnel cross-sections, and can be performed on

different scales. These types of tests are described in chapter 6.2. Full scale tests performed

during the 90s and beginning of the 21stcentury are described in chapter 6.3. For more

details concerning tunnel fire experiments from the 1960s until present day, the reader is

referred to Carvel and Marlair (2005).

6.1 Fire resistance tests

Fire resistance is defined as the ability of an element of building construction to continue to

perform its function as a barrier or structural component during the course of a fire

(Drysdale, 1998). The fire resistance is determined by subjecting a full-scale sample to a

standard fire. The sample can be loaded to model its function in the structure, and it can be

tested to failure. The standard fire is a time-temperature relationship ensured by a furnace. To

determine the fire resistance of different materials some different fire curves have been

developed. For building materials it is common to use the ISO 834 fire curve, which is based

on a typical fire source of cellulose materials, e.g., wood, paper and fabric, such that can be

found in any home. The curve has a moderate temperature rise up to 1000C over a timeperiod of 120 min (Khoury, 2000), see curve ISO834 in Figure 6-1. The off-shore and

petrochemical industries use a different standard fire, in which the temperature rises rapidly

up to 900C over the first 5 min, and peaks at 1100C (Khoury, 2000). This curve is based onhydrocarbon fires, and is shown as curve HC in Figure 6-1. After the tunnel fires during the

90s it was realized that a fire in a tunnel could produce a more severe fire scenario than the

hydrocarbon curve. The Netherlands, which has many under-water tunnels, has developed a

more severe fire curve than the hydrocarbon curve. The curve is based on a petrol tanker with

a fire load of 300 MW that causes a fire that lasts for 2 hours. The curve (RWS in Figure

6-1) has a very rapid temperature rise up to 1200C, a peak of 1350C after 60 min, and thena gradual fall to 1200C after 2 hours (Khoury, 2000). The peak temperature represents themelting temperature of concrete.

Two fire curves representing less severe tunnel fires than the RWS curve are the German

RABT and EBA curves. These curves has the same rapid temperature increase up to 1200C,which is then sustained for 30 min (RABT) and 60 min (EBA), and then a cooling phase

follows (Carvel, 2005).


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The importance of including the cooling-off phase was noted by Wetzig (2001). Heating of a

concrete sample up to 1600C for two hours produced no cracking or collapse, but 30 minutesafter the end of the test the sample exploded. Wetzig (2001) noted that as the sample cooled

glazing of the surface took place, thus preventing water vapour from escaping, which led to

pressure build-up and finally the explosive destruction of the sample.

Figure 6-1. European fire curves, after Khoury (2000) and Carvel (2005).

In 2004 concrete panels of four different qualities were fire tested at SP (Wickstrm, 2004). A

summary of the different qualities is shown in Table 6-1. A total of 16 samples were tested,

samples 1 to 10 were tested without load, and samples 11 to 16 were tested with a load of 6.25

MN, which gives a theoretical compressive stress of 9.6 MPa. The samples were tested

according to the RWS fire curve, see Figure 6-1

Table 6-1. Sample numbering and concrete qualities, after Wickstrm (2004).

Concrete Sample

K65, no fibres 1, 2, 9 and 10

K65, 1.0 kg/m3 3, 4, 11 and 12

K65, 1.5 kg/m3 5, 6, 13 and 14

K45, 1.0 kg/m3 7, 8, 15 and 16

0

200

400

600

800

1000

1200

1400

0 20 40 60 80 100 120 140 160 180

Time [min]

Furnacetemper

ature[C]

RWS

HC

ISO 834

EBA

RABT


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The unloaded samples (1 to 8) were tested first. It was noted that extensive spalling started

after only 2 minutes in samples 1 and 2 and after 3 minutes also sample 3 starts to spall

extensively. Spalling of sample 3 stopped after about 11 minutes. Spalling of samples 1 and 2

stopped after 27 minutes. After the test was concluded glazing of the surface as well as

formation of craters (diameter 5 30 mm) and stalactites (maximum diameter 10 mm and

maximum length 50 mm) was observed on all samples (Wickstrm, 2004). Craters and

stalactites were more common on the samples that did not spall.

The loaded samples (11 to 16) and samples 9 and 10 were tested together. It was noted that

extensive spalling started after only 1.5 minutes in samples 9 and 10 and after about 2 minutes

light spalling starts in sample 11. After about 2.5 minutes also samples 13 and 16 started to

spall lightly. After 3 minutes there is extensive spalling of sample 11. Spalling of sample 11

stopped after about 12 minutes. Spalling of samples 9 and 10 stopped after 23 minutes. Afterthe test was concluded glazing of the surface as well as formation of craters (diameter 5 30

mm) and stalactites (maximum diameter 10 mm and maximum length 50 mm) was observed

on all samples (Wickstrm, 2004). Craters and stalactites were more common on the samples

that did not spall.

The conclusion from these tests is that K65 concrete without fibres and with 1.0 kg/m3of

polypropylene fibres spall more easily than K65 concrete with 1.5 kg/m3of polypropylene

fibres. The influence of loading is uncertain since only one loaded sample (no. 11) spalled,but the loaded sample showed spalling to a greater depth when compared with the unloaded

sample of the same quality. The samples with concrete K45 showed no spalling when

unloaded, and only light spalling (not measureable) with the load applied.

6.2 Fire behaviour tests

In this section tests on smaller scales than full scale are described. These types of tests are

mainly performed to test smoke flow and control. The tunnels in the reduced scale tests have a

height of 1.2 3 m, a width of 1.08 5.4 m and a length of 11 366 m. The advantage of

testing at a reduced scale is that it is cheaper than a full scale test, which means that more tests

can be performed at a lower cost. In these tests the heat source is often some kind of fuel pool,

(petrol, kerosene, heptane, or methanol) or wooden cribs. There are some limitations to small

scale tests that have to be considered. The results of a reduced scale test are only useful if

there is a similarity between the scale model and the full scale tunnel of interest (Carvel and

Marlair, 2005). If there is a strong similarity, the scale model can be used to test specific

aspects of the behaviour of the fire or smoke, and if the similarity is less strong the scale

model can only be used to gain general information. To scale between reality and a scale

model of strong similarity, the gas flow at and around the fire must be considered (Carvel andMarlair, 2005). Gas flow can be described with non-dimensional numbers, such as the Froude


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number, the Reynolds number, the Richardson number, and the Grashof number. In the ideal

case all of these numbers should be the same for the scale model and the real case. Typically

only conservation of the Froude number (see Eq. 6-1) is considered, since it is impossible to

conserve all the numbers (Carvel and Marlair, 2005). The Froude number is proportional to

HRR2/L5, whereLis a characteristic dimension (usually the tunnel height) and is formally

defined as

gL

UFr

2

= , (6-1)

where Uis the velocity of the gases,gis the acceleration due to gravity andLis a

characteristic dimension of the system (I

Fires in tunnels and their effect on rock

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