Session 3 Going public – selling theunderground solution

North American Tunneling 2006 – Ozdemir (ed.)© 2006 Taylor & Francis Group, London, ISBN 0 415 40128 3

Concrete tunnel lining fire resistance and protection

W. ChenJacobs Civil Inc, Boston, USA

ABSTRACT: For transportation tunnel projects, either new design or rehabilitation, owners always seek foradequate and economic solutions for fire resistance and protection of concrete lining should a fire occurs. Thebehavior and impact of a concrete lining during a fire depend on the tunnel’s designed fire load and its associatedfire time-temperature curve, besides its means of tunnel ventilation. Overstress and explosive spalling in concretelining may occur through differential temperature gradients and water vapor pressure in the lining. Until recent,fire design loads and time-temperature curves for tunnels are based on available building standards, which maynot be adequate. This paper assesses existing fire design standards, codes, and guidelines for tunnel concretelinings; reviews the latest active and passive methods for tunnel fire protection; summarizes the physical andmechanical properties of concrete lining exposed to fires; evaluates high performance concrete tunnel linings’behavior when it is exposed to fires; reviews the latest tunnel fire time-temperature design curves; proposes aloading combination for tunnel design in fire; and provides a simplified manual approach, considering timedependent and temperature gradient factors within the lining, for concrete tunnel lining design in fire.

1 INSTRUCTION

1.1 Fire design standards and guidelines fortunnels

Universal design standard or guideline for fire designof concrete tunnel lining does not exist (Beeston,2002), though intensive researches have been invested,in recent years, on the material behavior of tunnelstructure components exposed to fire, on the fire time-temperature curve in tunnels, and on the fire protectionmethods and material. Most fire design codes are onlysuitable for building structures. They can’t be directlyapplied to tunnel lining designs, since the fire loadand its associated fire time-temperature curve are notrepresentative to fire events in tunnels.

Very few countries have fire design standards fortunnel linings. The Netherlands Ministry for Pub-lic Works and Water Management (RWS) has setprescriptive standards for fire design curves and tem-perature limits for tunnel linings, since tunnels inNetherlands are below groundwater level and any leak-age, as a result from fire events, is not acceptable.Another country has similar prescriptive standards isGermany, the ZTV-RABT. International Tunnel Asso-ciation (2004) has published a document in an attemptto provide guidelines for structural fire resistance forroad tunnels.

The intent of this paper is to review these fire designstandards and guidelines and to derive a simplified

manual calculation approach for fire design and reha-bilitation of concrete tunnel lining for transportationtunnels.

1.2 Active and passive fire protection

Active fire protection systems are means to positivelyfighting fire and protecting life and tunnel struc-tures within tunnel when fire events occur. Typicalactive fire protection systems include tunnel ventila-tion and water suppression systems. These systems arebeyond the scope of this paper and won’t be furtherdiscussed.

Passive fire protection systems are means to add ontunnel lining’s fire resistance capacity. They are gener-ally provided by the following means (Carvel, 2005):

• A secondary layer of concrete or cementitiousmaterial applied to the inner tunnel surface.

• Protective panels attached to tunnel ceiling andwalls.

• Addition of polypropylene fibers in concrete mix toimprove its fire resistance.

The focus of this paper is on the evaluation of tunnelconcrete lining’s fire resistance capability when thelining is exposed to fires. Details of the fire protectionmeans and methods are not further discussed.

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2 TUNNEL BEHAVIOR WHEN EXPOSEDTO FIRE

2.1 Cause of fire and its impact to tunnelstability

Two main causes of tunnel fires are from the leakageof petroleum, from vehicles, and vehicles accidentsin tunnels. Spilled petroleum is often the reason of afast fire development. In general, the fire in a concretelined tunnel does not impair its support capacity. Thiscould be concluded from the fires in Channel Tunnel,StorebæltTunnel, and Mont-BlancTunnel. None of thefire caused the tunnels to collapse, though concretespall was observed in all cases. Also, concrete liningcompressive strength loss does not penetrate beyondthe spalled section. As a consequence, tunnel liningafter a fire may continue to provide sufficient supportcapability to avoid a general collapse; however, cautionmust be taken for underwater tunnels, where liningcracks may result in excessive water inflow into thetunnel and present life threat issues.

2.2 Spalling

At high temperature and when water vapor pressurebuild-up beyond the tensile capacity of the concrete, afast relief of chunks of concrete may explode, at fastvelocity, away from the wall. Cause of the vapor phe-nomenon is from the dehydration of calcium hydroxidein the concrete cement when the temperature is above400◦C, as shown in Equation (1).

Spalling rate is influenced by (Lance 1998,Gustaferro 2002):

• Moisture content – The higher the moisture contentthe higher the likelihood the spalling would occur.

• Steel reinforcement – Closely spaced reinforcementcan minimize concrete spalling.

• Type of aggregates – Gravel concrete undergoesmore spalling in a fire because of different coeffi-cient of thermal expansion that result in the develop-ment of high stresses in the concrete mix. Carbonateaggregates, such as limestone and dolomite, whenexpose to fire will calcine, a phenomenon thatcarbon dioxide is driven off and calcium (or mag-nesium) oxide remains. This reaction absorbs fireheat and, therefore, the carbonate aggregate per-forms better than other normal weight aggregates,siliceous aggregates, in a fire. Quartz aggregate isalso not recommended for fire resistant concrete. Itchanges state at about 573◦C and expands suddenlyby 0.85% in volume and affects the concrete.

• Density and permeability – Concrete that is notallowed to dry may spall, especially in highly

Table 1. Maximum temperature from vehicle fire in atunnel.

Vehicle type Temperature ◦C

Passenger 400◦CBus/small lorry 700◦CHeavy lorry (HGV) with combustible 1,350◦Cgoods

Petrol tanker (general case) 1,350◦CPetrol tanker (extreme case, such as 1,400◦C

for ITT)

Figure 1. Fire time-temperature curves (Annica Nordmark,1998).

impermeable concrete, with silica fume or lowwater-cement ratio. More permeable concrete gen-eral perform better in a fire. Lightweight concreteperforms better than normal weight concrete in afire.

• Rate of heating – Spalling can be more severe inrapidly growing fires.

• Thickness – The thicker the concrete, the better itwill behave when exposed to fire.

3 DESIGN PARAMETERS

3.1 Fire load and fire time-temperature curve

The fire load and fire time-temperature curve in a tun-nel from vehicle fire depend upon the type of vehicleinvolved, the number of the vehicle involved, the tun-nel geometry, and the tunnel ventilation system used(PIARC, 1999). Table 1 lists the maximum tempera-tures at the ceiling or wall of a tunnel that could bedeveloped based on the vehicle types (PIARC, 1999).

Figure 1 shows common fire time-temperaturecurves generally used in European countries. Curve 1is the Netherlands’ Rijkswaterstaat (or RWS) curve;Curve 2 is the Germany RABT curve; Curve 3 isbased on Eurocode 1: Basis of Design and Actionson Structures, Part 2.2; and Curve 4 is the standardISO curve, normally seen for building structures. Fromthese curves, it could be seen that heat release in

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tunnels is higher and faster than in building structures(ISO curve), because of its confined space nature andless natural ventilation ability in general.

ITA (2004) proposed a classification method fortunnel fire design loads, as a function to time and tem-perature, based on vehicle type and the criticality ofthe tunnel when exposed to fire and resulting crackand leakage. For design purpose, it classifies the tun-nel into four categories, cars only, heavy lorries, petrotankers, and special cases, such as immersed tube tun-nel (ITT). Its fire time-temperature curves basicallyfollow those curves presented in Figure 1.

3.2 Temperature gradient through lining section

It is difficult task to determine the temperature dis-tribution through the cross section of concrete lining.Concrete properties, when exposed to fire, vary withthe heat release rate of the fire, which is a functionof the time and its associated energy release capacityof the associated burning goods. One relative accurateapproach may be through 2-D or 3-D computationalfluid dynamics (CFD) program which has the capabil-ity of performing thermal-structure coupling analysisand can capture the structure properties with the vari-ation of temperature and time. However, this approachwould be tedious and time consuming and the accu-racy of its result is depending upon the accuracy ofthe time and temperature dependent thermal propertiesprovided. A simplified approach is therefore proposedhere.

Wickström (1986) proposed a simplified methodto predict the temperature variation through concreteslabs, based on many results of CFD analyses. In thismanual calculation, the concrete temperature, Tc, isgiven by:

where, Tf is the designed fire temperature in ◦C; ηxis a function of time, th in hours, that relates the con-crete surface temperature to the concrete temperatureat depth x, in meter, into the slab; ηw is a function oftime, th, that relates the fire temperature to the concretesurface temperature.

Figure 2 shows temperature variation curves in con-crete sections, which are calculated based on Equation(2) for the ISO standard fire time-temperature curve.

3.3 Thermal properties and modulus of elasticity

The density of the concrete depends on the aggregatesand its other mixtures. Typical density of concrete

Figure 2. Temperature gradient in concrete, depth in mm,based on ISO standard fire time-temperature and derivedfrom Equation (2) (Wade, 1991).

is about 2,300 kg/m3. When exposed to fire at tem-perature above 100◦C, the density of most concretewould drop 100 kg/m3 from the evaporation of freewater. This phenomenon has little effect to concrete’sthermal response. Other than the change of its mois-ture content, concrete’s density does not change muchfrom thermal impact, except for limestone aggre-gates. Limestone aggregates would decompose whenits exposure temperature is greater than 800◦C.

Concrete’s thermal conductivity and specific heatalso vary with its degree of exposure to thermal loads.These properties are important for 2-D and 3-D CFDand structure analyses; however, for the simplifiedmanual calculation to be presented in this paper, theyare not explicitly required and are not further dis-cussed. EC2 (1993) and BS 8110 (BSI, 1985) presentthese properties in details.

3.4 Concrete strength

Concrete compressive strength is reduced with theincrease of temperature. Figure 3 illustrates designvalues of concrete compressive strength at differenttemperature state in accordance with BS 8110.

In Figure 3, the line of normal weight concrete isgiven by (R being the relative factor to concrete’s nor-mal temperature strength and T being the temperature)the following equations:

The line for light weight concrete in Figure 3 isgiven by:

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Figure 3. BS 8110 design values for concrete compressivestrength vs. temperature (BSI, 1985).

Figure 4. Design values for yield strengths vs. temperature.

3.5 Reinforcing steel strength

Standards or codes for steel yield strength reductionof reinforcing steel vary, though they are very similar.For simplified calculation purpose, the curve from BS8110 is adopted here, as shown in Figure 4. The line ofthe relative factor of yield strength of reinforcing steelis given by:

4 SIMPLIFIED DESIGN OF TUNNELCONCRETE LINING EXPOSED TO FIRE

4.1 Design approach

Under a fire event, structures may behave beyond itselastic limit. It is more appropriate adopting ultimate

strength design or load and resistance factor designmethodologies than working strength design approachfor tunnel concrete lining design in fire.

The fundamental philosophy of the ultimatestrength design of concrete is its nominal capacity(R), nominal strength of a design member, shall begreater than the factored demand forces, U. Normallya strength reduction factor, ϕ, is applied to R to accountfor the uncertainty in estimating the material strengthand cross sectional properties; therefore,

For fire design, the strength and the member’s sec-tional properties are based on the most likely expectedsituation; therefore, most international codes specifythe strength reduction factor ϕ to 1 for fire designpurpose, and

4.2 Design loads

Design loads for transportation tunnels include deadloads (D), thermal load (T), ground and ground-water pressures (H), and lives loads (L) from vehi-cles. Wind and snow loads do not have noticeableimpact to tunnels. In accordance with ACI 318(2005), Equation 9-2, the loading combination, Lc, isgiven by:

Equation (12) is applicable for building structuresin normal temperature condition. Its application totunnel structures in extreme thermal condition shouldbe revised, since in a fire, the likelihood of all theseloads to occur is rare and the thermal load is in aextreme state. For fire design of buildings, a load fac-tor of 0.5 is normally used for live load, L, in manynational codes. Load factor for ground and groundwa-ter has not been seen from those codes. Since fire ina tunnel is an extreme event, the author recommendsusing a load factor of 1 to ground and groundwaterpressures. Also, thermal expansion may benefit theconcrete lining in compression. It is, therefore, rec-ommend a load factor of 1 to be used for temperatureloads. The load combination for fire design, Lcfire, canthen be expressed as:

Please note that New Zealand (SNZ, 1992) andEurocode (EC1, 1994) even further reduce the loadfactor of dead load to 1.

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4.3 Simplified design approach

The proposed simplified manual calculation is basedon design principles byACI 318 (2005), with the inputsof fire design time-temperature curve, the reducedmaterial strengths, and the revised loading combina-tion described in previous sections. Details steps aredescribed below.

The first step is the calculation of the requireddesign demand based on the loading combination pro-posed in Equation (13) and general structural analysisprincipals.

The second step is the selection of a design firetime-temperature curve, such as those curves shownin Figure 1 or adopting those recommendations fromITA (2004) guidelines.

The third step is the calculation of the temperatureacross the tunnel lining section. This can be calculatedbased on Equation (2) or from Figure 2. The temper-ature of the steel reinforcement is assumed to be thesame as that of concrete’s at that location.

The fourth step is the calculation of the reducedstrengths of the tunnel concrete lining components.For concrete, it can be derived from Equations (5) to(8), or using Figure 3. For reinforcing steel, it can bederived from Equation (9), or using Figure 4.

The derivation of the reduced concrete compressivestrength from the fourth step would be tedious andunpractical, since the concrete section is continuous.For practical design purpose, EC2 (1993) specifies theconcrete strength in two regions: full capacity for con-crete section with temperature less than 500◦C; andzero strength for concrete section with temperaturegreater than 500◦C. Another approach is to dividethe concrete into several layers and use mean val-ues at the sections for design. From past experience,it is found that the refined approach obtains similarresult as that by EC2; therefore, the EC2 approach isrecommended here.

The last step is the calculation of the lining’s nom-inal capacity, Rfire, using ACI 318 (2005) designprinciples and the verification of Rfire is equal orgreater than the calculated demand, Ufire.

5 FIRE PROTECTION FOR EXTREMETUNNELS

5.1 Underwater tunnels

For underwater tunnels, crack and leakage of tunnelmay lead to inundation of the tunnel, which can causelife safety issue and social and economic impacts to theoperation of the tunnel owner. More stringent criteriaare required for these tunnels; however, the universalstandard does not exit. The Netherlands RWS requires:• The temperature of the steel reinforcement in con-

crete shall not exceed 280◦C, to prevent sagging andtunnel collapse, and

• The temperature at the surface of the concrete shallnot exceed 380◦C, to prevent spalling.

To achieve these requirements, special coating orcladding (passive protection) to divert temperatureaway from the concrete surface of the lining isrequired. Caution need to be made for this approach tolife safety issue, since diverting temperature from tun-nel ceiling and wall means diverting the heat back tothe tunnel. Another approach is the use of a sacrificiallayer of lining to the design lining.

6 CONCLUSION

Several conclusions can be drawn from this paper:

• Universal design for tunnel concrete lining exposedto fire does not exist.

• High performance concrete lining performs poorthan normal concrete lining in a fire event (lowerpermeability tends to cause explosive spalling).

• Building standards or codes for fire design must bemodified for tunnel lining designs.

• Load factors, for loading combination in ultimatestrength design, for tunnel lining design in fire canbe lower than that in normal temperature condition.

• A simplified manual calculation approach for tunnellining design exposed to fire is derived.

• For underwater tunnels, the use of additional passivefire protection means should be considered.

REFERENCES

ACI. 318. 2005. Building code requirements for structuralconcrete and commentary.

Annica Nordmak, A. 1998. Fire and life safety for under-ground facilities: present status of fire and life safetyprinciples related to underground facilities. Tunnellingand Underground Space Technology, Vol. 13, No. 3:217–269.

Beeston, A. 2002. Refractory solutions for fire protectionof tunnel structures. Proceedings of the 4th InternationalConference on Tunnel Fires, Basel, Switzerland.

BSI. 1985. Structure use of concrete, BS 8110, BritishStandards Institution, UK.

Carvel, R. 2005. Fire protection in concrete tunnels. TheHandbook of Tunnel Fire Safety: 110–124.

EC1. 1994. Eurocode 1: Basis of design and design actionson structures. Part 2-2: Actions on structures exposed tofire. ENV 1991-2-2.

EC2. 1993. Eurocode 2: Design of concrete structures. ENV1992-1-2: General Rules – Structural Fire Design.

Lance, G.A. (1998) “Effects of Fire on Tunnel LiningStability,” Tunnels & Tunnelling International, October.

Gustaferro,A.H. (2002) “Fire-Resistant Concrete,” MA Mag-azine, Spring.

ITA. Working Group No. 6 Maintenance and Repair 2004.Guidelines for structural fire resistance for road tunnels.

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SNZ. 1992. Code of practice for general structural design anddesign loadings for buildings. NZS4203.

Wade, C.A. 1991. Method for fire engineering design ofstructural concrete beams and floor systems. BRANZTechnical Recommendation No.8.

Wickström, U. 1986. A very simple method for estimatingtemperatures. Fire Exposed Structures, New Technologyto Reduce Fire Losses and Costs: 186–194.

World Road Association (PIARC). 1999. Fire and smokecontrol in road tunnels.

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