Tailor Made Concrete Structures – Walraven & Stoelhorst (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-47535-8

Fire resistance of concrete tunnel linings

Jan L. VitekMetrostav a.s, and Czech Technical University, Prague, Czech Republic

ABSTRACT: Final tunnel linings are often designed made of only plain concrete. If such lining is subjected tofire, there is a substantial danger that a spalling would reduce the thickness of the lining more significantly thanin the case of reinforced lining and also that the danger of spalling would threaten the rescue activities. The twoseries of tests when large scale concrete specimens were subjected to the fire according to the RWS temperatureloading curve for a period of 180 minutes are described. The performance of concrete elements without and withpolypropylene fibres was observed. After the tests the compression strength of concrete was measured on thecores drilled from the specimens.

1 INTRODUCTION

The tunnels were usually equipped with a final liningmade of reinforced concrete. The resistance againstfire used to be dependent on the temperature of steelreinforcement. Now also a lining which is made onlyof plain concrete becomes used more often. The spec-ification of the fire resistance becomes more complexif the reinforcement is missing. The effect of fire mayresult in substantial spalling, which would reduce thethickness of the lining, and also in reduction of thestrength of concrete. In many cases the polypropylenefibres are used to reduce the effect of fire on the lin-ing. Fibres in concrete represent a cheap improvementof the fire resistance of the lining, which can reducespalling and degradation of concrete. More effectiveprotection of the lining using special materials whichcover the concrete surface used to be more expen-sive and also it requires more space; which means thatthe cross-section of the tunnel must be larger. On theother hand such materials can protect concrete surfacevery well and in the case of fire the repair becomessignificantly easier.

The construction of the road tunnel Libouchec in theCzech Republic required checking of the performanceof the plain concrete tunnel lining also against fire. Aseries of tests were carried out, which should help tounderstand the effect of fire on the plain concrete, onthe concrete with polypropylene fibres and on concretewhich is protected using a sprayed coating.

2 EXPERIMENTAL RESEARCH

2.1 Objective of the experimental program

The different possibilities exist how to improve theresistance of the tunnel lining against fire. Their

protection ability varies and also the costs may bevery different. The aim of the proposed research wasto compare several alternatives and to evaluate theirreal performance in the fire. Also the costs and otherfactors like an influence on the flow of works in thetunnel or labour consumption were considered.

2.2 Experimental program

Experiments started in 2005, when the first tunnelwith the secondary lining made of plain concrete wasunder construction. The client required to check theperformance of the concrete lining in fire. At that timethe national code was applied and the specimen madeof the concrete identical to that applied in the tun-nel was exposed to the fire test. It should be notedthat the plain concrete was not mixed with any fibresand any other protection was not used. The nationalcode assumed that the temperature curve according tothe ISO was used. This curve, also called “cellulosiccurve” has a relatively slow increase of temperature(about 800 deg.C after 30 minutes) and the maximumtemperature slightly exceeds 1000 deg.C. The resultsof this test were rather promising; almost no damageafter 180 minutes of fire exposure has been observed.The reasons were two fold: i) the slow growth oftemperature enabled the water to evaporate from theconcrete and ii) the maximum temperature was nothigh enough to make a serious damage of the concretesurface. Based on this experience a new series of testswas carried out.

The fire tests were carried out in the testing institutein Veseli n. L. in Southern Bohemia. This institute hasa long tradition in research of fire on different buildingmaterials. The furnace which was used is designed forthe tests of specimens of the size 3 × 3 m. This sizewas too large for our tests, since the specimens had to

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be transported from Prague, where they were cast, toVeseli n. L. Therefore the specimens had the dimen-sions only 2.4 × 3 m and they were 0.3 m thick. Thereinforcement was only in the central plane and it wasused preferably for manipulation with the specimens.The second series of specimens involved 3 elements.The first one was made of plain concrete without anyprotection. In the second element the polypropylenefibres were added to the concrete mix. The third ele-ment was made of the same plain concrete as the firstone and its surface was protected with a sprayed layercalled Meyco Fix Fireshield 1350.This layer was about50 mm thick. It was sprayed on the surface of concretewhich had to be roughened using a water jet.

In the second series which was tested one yearlater, again the specimen made of plain concrete with-out protection and the two specimens with the twotypes of polypropylene fibres were exposed to fire.The specimens of the second series were prestressedin the vertical plane. The stress in concrete reachedabout 4 MPa which should represent the compressioninduced by soil pressure.

All the specimens were exposed to the fire accord-ing to the RWS temperature curve, which is consideredas a very severe temperature loading. The temperature1000 deg.C is achieved very quickly – in 10 minutesand the maximum temperature reaches 1350 deg.C.Allthe specimens were exposed to fire for 180 minutes,since this is a general requirement for testing of thetunnel lining of the tunnels longer than 300 m.

2.3 Water content in concrete

Water content in concrete is one of the factors whichsignificantly influences the damage of concrete underthe fire. More water in concrete results in more severedamage. Some experimental techniques require stor-age of the specimens in water for several days beforethe fire tests. Such approach has an advantage that theconditions for testing of various concretes are reallyequal. However, in the real situation in tunnels, theconcrete does not contain so big amount of water. Theaim of the tests was to model the situation which is asclose as possible to the reality.Therefore the water con-tent in the final lining of three tunnels was measured.The results are illustrated in the Table 1.

Water content in tested specimens was controlled,so that a similar range would have been achieved. Thewater content in the tested specimens is shown in theTable 2.

It is quite clear that the water content in specimenswas in accordance with the water content in tunnellining and it may be assumed that the test’s conditionsare similar to the conditions in the tunnel.

2.4 Results of the tests in 2006

The plain concrete which was exposed to the fireaccording to the RWS temperature loading curve

Table 1. Measured water content in the final tunnel lining.

Range ofTunnel water content %

Valik 4.4–5.3Panenska 4.0–5.6Libouchec 3.9–5.4

Table 2. Measured water content in the tested specimens.

Range ofTunnel water content %

Specimens 2006 3.9–5.3Specimens 2007 3.4–5.7

Figure 1. Plain concrete specimen after the test.

exhibited a damage of the surface in the depth up to50 mm.The damage occurred partly due to the spallingof the surface layer and partly due to the melting ofsome aggregate. The surface after the test is illustratedin Fig. 1.

Although the damage of the surface is rather serious,the temperature drops inside the specimen rather fast.The temperature distribution in dependence on time invarious depths under the surface exposed to the fireis plotted in Fig. 2. The temperature 30 mm under thesurface exposed to the fire does not exceed 900 deg.C,which is a significant reduction of the surface temper-ature which is at the end of the test still 1200 deg.C. Inthe depth of 90 mm the temperature does not exceed400 deg.C.

The second specimen, where the polypropylene(PP) fibres were added (Fibrin 615) exhibited a verysmall damage of the surface. Only melting of someaggregate appeared among minor cracks (Fig. 3).

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Plain concrete

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Figure 2. Temperature distribution in time in differentdepths under the surface exposed to the fire.

Figure 3. Surface of the specimen with PP fibres.

The temperature distribution inside the specimenwas very similar to that of the specimen made of plainconcrete. The PP fibres improved the performance ofthe surface and reduced spalling of concrete, whichwas expected.

The third specimen was protected with a specialmaterial (Meyco Fix Fireshield 1350) sprayed on theconcrete surface. The thickness of the sprayed layeris about 50 mm. After a fire this protection layer isspoilt and has to be replaced with a new one. The firetest has shown a good performance of this protection.The concrete of the lining was heated only very lit-tle. The maximum temperature in the depth 30 mmunder the concrete surface was only 125 deg.C. Thetemperature variation in time is plotted in Fig. 4. Theapplication of the protection layer represents a reallyefficient way how to keep concrete of the lining unin-fluenced by the fire. However the inner space must belarger and the costs of the protection are not negligible.

After the tests, the cores were drilled from the spec-imens and the compression strength was measured. At

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Figure 4. Temperature distribution in time in differentdepths under the surface of concrete under the protectionlayer.

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Figure 5. Compression strength measured after the firetests.

the two specimens (plain concrete and plain concretewith fibres), a significant reduction of the strength wasobserved. In the last specimen no reduction appeared,but rather large statistical scatter was observed, mainlydue to the measurement on drilled cores.The measuredstrengths are plotted in the diagram in Fig. 5.

2.5 Results of the tests in 2007

The tests carried out in 2007 served as an extensionof the tests from 2006. In order to fit the performanceof the tunnel lining better, the compressive stress about4 MPa was induced using prestressing. Since the pro-tection layer was tested before the attention was paidmainly to application of fibres. Two elements with PPfibres (Fibrin 615 and Fibrin 315) were supplementedwith a reference specimen made of plain concrete.

The performance of the plain concrete withoutfibres was almost identical to the performance of thespecimen from the year 2006. The maximum temper-ature reached again about 800 deg.C and the reduction

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Figure 6. Surface of the specimen with PP fibres.

Compression strength

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Figure 7. Compression strength in plain concrete and inconcrete with PP fibres after the fire tests.

of temperature through the thickness of the slab wasvery similar to previous tests.

The specimens with PP fibres exhibited more seri-ous damage than that tested in 2006. The surface ofthe specimen with PP fibres showed more spalling,which is illustrated in Fig. 6. Also more melting of theaggregate was observed.

There was not a significant difference between theperformances of the two types of fibres.

After the tests the cores were drilled and the com-pression strength of concrete was measured. In thiscase the strengths of plain concrete and fibre rein-forced concrete were rather similar and it cannot bestated that the fibres would contributed to the reductionof the strength degradation (Fig. 7).

Tests carried out in 2007 showed that the fibreshad less favourable influence than in the test from2006. It is hard to explain this fact, but it has beenalready observed that fibre reinforced concrete canhave a larger statistical scatter of its properties than

it may be observed at ordinary reinforced concrete.Since the temperatures inside the concrete elementsare very similar in plain concrete and in concrete withfibres, it seems logical to conclude that the effect ofhigh temperature on concrete strength should be alsoin all the elements similar, what can be seen from thetests.

3 CONCLUSIONS

The tests showed that the spalling in plain concretewithout any protection does not exceed 50 mm duringa severe temperature exposure according to the RWStemperature loading curve for a period of 180 minutes.

The polypropylene fibres improved the resistanceagainst fire partly. The spalling was reduced but thetemperature distribution along the thickness of the ele-ment (lining) was not significantly improved.The dropof concrete strength is hardly to define, but it seemsthat it is similar to that which was observed at plainconcrete elements. The performance of concrete withPP fibres can also vary due to the natural statisticalscatter of such material.

The protection using Meyco Fix Fireshield 1350appeared as the most effective. On the other handsuch protection is rather expensive and also labourconsuming.

The design of protection against fire in a specifictunnel should be based on a detailed evaluation ofmany influencing factors. Taking the obtained resultsinto account, any solution can be found to be suitableincluding using an ordinary plain concrete.

ACKNOWLEDGEMENT

This research was carried out in the Research CentreCIDEAS (Centre for integrated design of advancedstructures) supported by the Ministry of education,project No. 1M6840770001. Partly the results of theresearch project 103/06/1627 supported by the GrantAgency of the Czech Republic were applied.

REFERENCES

Vitek, J.L. 2007. Fire resistance of concrete tunnel lin-ing – Experimental research. In Proc. of the 3rd CentralEuropean Congress on Concrete Engineering 2007 –Innovative materials and technologies, Visegrad, Hun-gary. G.L. Balazs, S.G. Nehme (eds.) 315–320, Budapest2007.

Bergmeister, K. 2006. Beton unter hohenTemperaturen – eineFrage derTunnelsicherheit. Beton- und Stahlbetonbau 101(2006), Heft 2, 74–80.

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