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Concrete Tunnel Lining Fire Resistance and Protection

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Concrete Tunnel Lining Fire Resistance and Protection
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  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. Chen  Jacobs Civil Inc, Boston, USA ABSTRACT: For transportation tunnel projects, either new design or rehabilitation, owners always seek for adequate and economic solutions for fire resistance and protection of concrete lining should a fire occurs. The behaviorandimpactofaconcreteliningduringafiredependonthetunnel’sdesignedfireloadanditsassociated firetime-temperaturecurve,besidesitsmeansoftunnelventilation.Overstressandexplosivespallinginconcretelining 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 and mechanical 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 INSTRUCTION1.1  Fire design standards and guidelines for tunnels Universal design standard or guideline for fire designof concrete tunnel lining does not exist (Beeston,2002),thoughintensiveresearcheshavebeeninvested,in recent years, on the material behavior of tunnelstructurecomponentsexposedtofire,onthefiretime-temperaturecurveintunnels,andonthefireprotectionmethods and material. Most fire design codes are onlysuitable for building structures.They can’t be directlyapplied to tunnel lining designs, since the fire load and its associated fire time-temperature curve are notrepresentative to fire events in tunnels.Very few countries have fire design standards for tunnel linings. The Netherlands Ministry for Pub-lic Works and Water Management (RWS) has set prescriptive standards for fire design curves and tem- perature limits for tunnel linings, since tunnels in Netherlandsarebelowgroundwaterlevelandanyleak-age, as a result from fire events, is not acceptable.Another country has similar prescriptive standards isGermany, the ZTV-RABT. InternationalTunnelAsso-ciation (2004) has published a document in an attemptto provide guidelines for structural fire resistance for road tunnels.Theintentofthispaperistoreviewthesefiredesignstandards 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-tionandwatersuppressionsystems.Thesesystemsare beyond the scope of this paper and won’t be further discussed.Passive fire protection systems are means to add ontunnellining’sfireresistancecapacity.Theyaregener-ally provided by the following means (Carvel, 2005): ã  A secondary layer of concrete or cementitiousmaterial applied to the inner tunnel surface. ã  Protective panels attached to tunnel ceiling and walls. ã  Addition of polypropylene fibers in concrete mix toimprove its fire resistance.Thefocusofthispaperisontheevaluationoftunnelconcrete lining’s fire resistance capability when thelining is exposed to fires. Details of the fire protectionmeans and methods are not further discussed. 209  2 TUNNEL BEHAVIOR WHEN EXPOSEDTO FIRE2.1  Cause of fire and its impact to tunnel  stability Two main causes of tunnel fires are from the leakageof petroleum, from vehicles, and vehicles accidentsin tunnels. Spilled petroleum is often the reason of afastfiredevelopment.Ingeneral,thefireinaconcretelined tunnel does not impair its support capacity.Thiscould be concluded from the fires in Channel Tunnel,StorebæltTunnel,andMont-BlancTunnel.Noneofthefire caused the tunnels to collapse, though concretespall was observed in all cases. Also, concrete liningcompressive strength loss does not penetrate beyond the spalled section. As a consequence, tunnel liningafter a fire may continue to provide sufficient supportcapabilitytoavoidageneralcollapse;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 pressure build-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-nomenonisfromthedehydrationofcalciumhydroxidein the concrete cement when the temperature is above400 ◦ C, as shown in Equation (1).Spalling rate is influenced by (Lance 1998,Gustaferro 2002): ã  Moisture content –The higher the moisture contentthe higher the likelihood the spalling would occur. ã  Steelreinforcement–Closelyspacedreinforcementcan minimize concrete spalling. ã  Type of aggregates – Gravel concrete undergoesmore spalling in a fire because of different coeffi-cientofthermalexpansionthatresultinthedevelop-mentofhighstressesintheconcretemix.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 suddenly by 0.85% in volume and affects the concrete. ã  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  ◦ CPassenger 400 ◦ CBus/small lorry 700 ◦ CHeavy lorry (HGV) with combustible 1,350 ◦ CgoodsPetrol tanker (general case) 1,350 ◦ CPetrol tanker (extreme case, such as 1,400 ◦ Cfor 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 concrete performs better than normal weight concrete in afire. ã  Rate of heating – Spalling can be more severe inrapidly growing fires. ã  Thickness – The thicker the concrete, the better itwill behave when exposed to fire.3 DESIGN PARAMETERS3.1  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 is based on Eurocode 1: Basis of Design and Actionson Structures, Part 2.2; and Curve 4 is the standard ISOcurve,normallyseenforbuildingstructures.Fromthese curves, it could be seen that heat release in 210
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