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Spalling in High Strength Concrete Exposed to Fire – Concerns, Causes, CriticalParameters and Cures

V.K.R. KodurInstitute for Research in Construction, National Research Council of Canada

Abstract

The increased use of high strength concrete (HSC) in buildings has raised concernsregarding the behaviour of such concrete in fire. Spalling at elevated temperaturesand the resulting reduction in fire resistance is of particular concern. In this paper, thevarious issues relating to spalling and its impact on fire resistance are discussed. Thespalling phenomenon and its main causes in HSC are presented. This includes thecritical parameters that influence spalling in HSC under fire conditions. Designsolutions (cures) to minimize spalling in HSC structural members are presented.Keywords: high-strength concrete; moisture; spalling; fire resistance; fireperformance; polypropylene fibres

Introduction

In recent years, the construction industry has shown significant interest in the use ofhigh strength concrete (HSC). This is due to improvements in structural performance,such as high strength and durability that it can provide compared to traditional normalstrength concrete (NSC). Recently, the use of HSC, which was previously inapplications such as bridges, offshore structures and infrastructure projects, has beenextended to high rise buildings.

In building design, the provision of appropriate fire resistance for structural membersis one of the major fire safety requirements. The fire resistance of a structuralmember is dependent on the geometry, the materials used in construction, the loadintensity and the characteristics of the fire itself.

Generally, concrete structural members exhibit good performance under fire situations(Harmathy, 1993; Lie and Woollerton, 1988). Studies have shown, however, that theperformance of HSC is different from that of NSC and may fail to exhibit goodperformance in fire (Kodur and Sultan, 1998a; Phan, 1996). Further, the spalling ofconcrete under fire conditions is one of the major concerns due in part to the lowwater-cement ratio in HSC (Diederichs et al., 1995). The spalling of concreteexposed to fire has been observed under laboratory and real fire conditions (Kodurand Sultan, 1998a; Sanjayan and Stocks, 1993; Phan, 1996).

In this paper, the basic phenomenon of spalling in HSC is explained. The variousissues related to spalling, such as reinforcement exposure and impact on safeenvironment in fire locations, are reviewed and the main causes for the occurrence ofspalling in HSC are explained. Results from a number of studies are reviewed toillustrate the critical parameters that govern spalling in HSC structural members underfire conditions. Design solutions (cures) to minimize spalling in HSC members arepresented.

Concerns

Spalling of concrete is defined as the breaking of layers (pieces) of concrete from thesurface of the structural elements when it is exposed to high and rapidly risingtemperatures such as those experienced in fires. The spalling can occur soon after

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exposure to heat and can be accompanied by violent explosions, or it may happenwhen concrete has become so weak after heating that, when cracking develops, piecesfall off the surface. The consequences may be limited as long as the extent of thedamages is small, but extensive spalling may lead to early loss of stability andintegrity due to exposed reinforcement and penetration of partitions.

Spalling, which often results in the rapid loss of concrete during a fire, exposes deeperlayers of concrete to fire temperatures, thereby increasing the rate of transmission ofheat to the inner layers of the member, including the reinforcement. When thereinforcement is directly exposed to fire, the temperatures in the reinforcement rise ata very high rate leading to a faster decrease in strength of the structural member. Ifthe reinforcement is of the fibre reinforced polymer (FRP) type, which has a muchlower melting and critical temperature compared to steel, it may lead to sudden loss ofstrength as well as burning of the reinforcement. The loss of strength in thereinforcement, added to the loss of concrete due to spalling, significantly decreasesthe fire resistance of a structural member.

Also, in a number of test observations it has been found that spalling is often of anexplosive nature (Phan, 1996; Bilodeau et al., 1998). Further, it has been reported inliterature that the spalling of concrete occurs in the initial stages of a fire. This earlyand explosive spalling is of great concern since this might endanger the safeenvironment for evacuation of occupants and might pose a risk for fire fightingpersonnel.

The following realistic incident illustrates the reasoning for the concern due tospalling: On November 18, 1996, a serious fire on a shuttle train transporting trucksdestroyed a section of the south tunnel of the railroad tunnel connecting England andFrance (Ulm et al., 1999; Fire Prevention, 1997). Nine trucks, ten train wagons andone locomotive burned for about 10 hours with temperatures up to 1000°C. Eightpeople were injured and the cost due to damage to trains, track and tunnel as well asdisruption to services was estimated to be as high as £50M. The fire caused severedamage to the tunnel rings by thermal spalling over a length of a few hundred metres.The spalling in the 45 cm precast RC concrete rings reached an average depth of 10 to20 cm. In some parts, thermal spalling of concrete destroyed the entire tunnel ring upto the chalk substratum. An analysis indicated that the concrete employed in theChunnel had typical features of HSC: a compressive strength of 80 to 100 MPa, and alow permeability (Ulm et al., 1999). Based on the detailed investigation, major workhad to be undertaken to repair the damage due to the spalling of the concrete.

Causes

A review of literature presents a conflicting picture on the occurrence of spalling andalso on the exact mechanism for spalling. While many research studies showexplosive spalling of HSC structural members, there are a few other studies whichreport little or no significant spalling. One possible explanation for this confusingtrend of observations is the large number of factors that influence the spalling and theinter-dependency of some of these factors. However, most researchers agree that themajor causes for spalling in concrete is due to low permeability and moisturemigration at elevated temperatures.

There are two broad theories on which the spalling phenomenon can be explained:

• Pressure build-up: Spalling is believed to be caused by the build-up of porepressure during heating. HSC is believed to be more susceptible to this pressure

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build-up than NSC because of its low permeability. The extremely high watervapour pressure, generated during exposure to fire, cannot escape due to the highdensity (and low permeability) of HSC and this pressure often reaches thesaturation vapour pressure. At 300°C, the pressure reaches approximately 8 MPa;such internal pressures are often too high to be resisted by the HSC mix having atensile strength of approximately 5 MPa (Diederichs et al., 1995). The drainedconditions at the heated surface, and the low permeability of concrete, lead tostrong pressure gradients close to the surface, in the form of the so-called“moisture clog” (Harmathy, 1993; Anderberg, 1997), as illustrated in Figure 1a.When the vapour pressure exceeds the tensile strength of concrete, chunks ofconcrete fall off from the structural member. The pore pressure is considered todrive progressive failure, i.e., the greater the spalling, the lower the permeabilityof concrete. This falling off can often be explosive in nature depending on the fireand concrete characteristics.

• Restrained thermal dilatation: This hypothesis (Bazant, 1997) considers that thespalling results from restrained thermal dilatation close to the heated surface,which leads to compressive stresses parallel to the heated surface. Thesecompressive stresses are released by brittle fractures of concrete, i.e., spalling.The pore pressure can play only a secondary role as far as the growth of a largercrack is concerned, because, due to the volume expansion of a growth crack, thepressure in the crack must rapidly decay after the crack begins to open. Thepressure may affect the bifurcation and the onset of instability in the form ofexplosive thermal spalling (see Figure 1b). This explanation seems consistentwith the order of magnitude of pore pressures, as well as with the experimentalobservation that the pre-dried specimens are less prone to thermal spalling.

Critical Parameters

At present, studies are in progress at the National Research Council of Canada(NRC), as well as a number of organizations world-wide, to determine the parametersthat influence the fire performance of HSC and to quantify the extent of theirinfluence on spalling and resulting fire resistance. Data from the studies show thatfire performance of HSC, in general, and spalling in particular, is affected by thefollowing factors:

Concrete Strength: Based on fire tests conducted at NRC and other laboratories,higher concrete strength is more susceptible to spalling and results in lower fireresistance (Kodur and Sultan, 1998a). This could be attributed to lower permeabilitywith increased strength of concrete: HSC with a compressive strength of 80 to100 MPa, will have a permeability of 10-15 to 10-16 m/s, compared to NSC of 30 to40 MPa compressive strength having a permeability of 10-12 to 10-13. While it is hardto specify the exact strength range at which concrete is susceptible to higher spalling,studies show that concrete strengths higher than 55 MPa are more susceptible. Thespalling performance of an NSC column is compared to an HSC column in Figure 2using the data obtained from full-scale fire tests on loaded columns (Kodur, 1998). Itcan be seen that the spalling is quite significant in the HSC column.

Concrete Density: The density of concrete mix influences the extent of spalling inHSC. Fire tests on normal density (made with normal weight aggregate) andlightweight (made with lightweight aggregate) HSC blocks have shown that the extentof spalling is higher when the lightweight aggregate is used in the HSC mix. Figure 3shows the extent of spalling in normal weight and lightweight HSC blocks exposed tohydrocarbon-based fire (Bilodeau et al., 1998). The higher spalling is mainly due to

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the presence of higher free moisture in the lightweight aggregate, which creates highervapour pressure under fire exposures.

Load Intensity and Type: The type of load and its intensity have significantinfluence on spalling and resulting fire resistance. A loaded HSC structural memberis susceptible to higher spalling than an unloaded member. This occurs because aloaded structural member is subjected to stresses due to load in addition to the porepressure generated by steam. Further, the extent of spalling is higher if the load is ofan eccentric (or bending) type since this will induce additional tensile stresses.

Moisture Content: The moisture content of concrete, expressed in terms of relativehumidity, influences the extent of spalling with higher RH levels leading to higherspalling. Fire resistance tests on full-scale HSC columns have shown that significantspalling occurred when the RH, at the time of fire tests, was higher than 80%. Theduration required to attain an acceptable RH level (below 75%) in HSC structuralmembers is longer than that for NSC columns due to the low permeability of HSC. Insome cases, such as offshore structures, RH levels can remain high throughout the lifespan of the structure and this should be accounted for in design.

Fire Intensity: The extent of spalling is much higher when the HSC specimens areexposed to faster heating rates or higher fire intensities. Hydrocarbon fires pose asevere fire scenario, as compared to building fires, and are characterized by higher fireintensity and a faster rise in temperature (Ulm, 1997). For applications such asoffshore structures and tunnels, HSC structural members should be evaluated forspalling under hydrocarbon fire conditions.

Aggregate Type: The type of aggregate used in the concrete mix influences spallingphenomenon and fire resistance. The two commonly used aggregate types in concreteare carbonate aggregate and siliceous aggregate. Tests on HSC columns have shownhigher spalling in siliceous aggregate concrete than that found with carbonateaggregate concrete (Kodur and Sultan, 1998a) This occurs mainly because carbonateaggregate has a substantially higher heat capacity, than siliceous aggregate, and isbeneficial in preventing spalling of the concrete (Kodur and Sultan, 1998b). Thisincrease in specific heat is likely caused by the disassociation of the dolomite in thecarbonate concrete.

Specimen dimensions: The specimen size also has an influence on the extent ofspalling. A review of literature shows that the risk of explosive thermal spallingincreases with the specimen size. This is due to the fact that the specimen size isdirectly related to the length-scales of heat and moisture transport through thestructure (prototype), as well as the capacity of larger structures to store more energy.Therefore, careful consideration must be given to the size of the specimens inevaluating spalling performance given that fire tests are often conducted on scaledspecimens.

Cures – Solutions

Based on the available information, it is possible to minimize spalling and enhance thefire resistance of HSC members by adapting the following guidelines:

Lateral Reinforcement: Test results on HSC columns have shown that tie spacingand their configuration have a significant effect on the fire performance of HSCcolumns. Closer tie spacing (at 0.75 times that required for NSC columns) andbending of ties at 135° back into the core of the column, as illustrated in Figure 4,

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minimizes spalling and thus enhances fire performance. Furthermore, the provisionof cross ties also improves fire performance. Fire tests on loaded HSC columns, withadditional confinement through cross ties and bending of ties at 135° back into thecore of the columns, have shown that spalling was significantly minimized and fireresistance was enhanced even under full service loads (Kodur, 1999).

Fibre Reinforcement: Based on the work carried out at NRC and otherorganizations, the addition of polypropylene fibres minimizes spalling in HSCmembers under fire conditions (Bilodeau et al., 1998; Kodur and Lie, 1997). There ishowever, still a debate on the exact mechanism by which the fibres minimize spalling.One of the more accepted theories is that, by melting at a relatively low temperatureof 170°C, the polypropylene fibres create “channels” for the steam pressure inconcrete to escape, and thus prevent the small “explosions” that cause the spalling ofthe concrete. From the studies, it was found that the amount of polypropylene fibresneeded to minimize spalling is about 0.1 to 0.25% (by volume). The effect ofpolypropylene fibres in minimizing spalling is illustrated in Figure 4, which showsHSC concrete blocks after exposure to two hours of hydrocarbon fire (Bilodeau et al.,1998). The polypropylene fibres were found to be most effective for HSC made withnormal weight aggregate. Further studies are in progress at NRC to determine theoptimum fibre content for different types of concrete.

Steel fibres also reduce the spalling in HSC and improve fire resistance (Kodur,1998). In this case, the presence of steel fibres enhances the tensile strength ofconcrete, even at high temperatures, and this helps to withstand the pore pressuregenerated due to melting of water under fire exposure. The tensile strength increasedto as high as 5-7 MPa and, in many cases, this may be sufficient to achieve two tothree hours of fire resistance without significant spalling. However, when the porepressure exceeds the tensile strength of concrete, spalling may still occur.

Other Factors: The extent of spalling is dependent on a number of factors asdiscussed above. Results from the literature show that the following factors provide afavourable condition for minimizing spalling and, where possible, some of thesefactors should be taken into consideration in the design of HSC mixes.

• Use of carbonate aggregate (limestone) in place of siliceous aggregate (quartz)• Use of normal density aggregate instead of lightweight aggregate• Providing sufficient concrete cover to facilitate protection for reinforcement for

maximum duration• Designing for lower load intensity

Summary

In HSC structural members, the occurrence of spalling is a major concern. Thespalling, which occurs due to the low permeability and low tensile strength of HSC,leads to lower fire resistance and might endanger the safe environment for evacuationof occupants in the event of fire. The extent of spalling in HSC is influenced byconcrete strength, concrete density, type of aggregate, load intensity, tie spacing andconfiguration. By adopting design guidelines, such as the addition of fibres and animproved tie configuration, spalling in HSC members can be minimized to asignificant extent and fire resistance can be enhanced.

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References

Anderberg, Y. (1997). "Spalling Phenomenon of HPC and OC", InternationalWorkshop on Fire Performance of High Strength Concrete, NIST SP 919,Gaithersburg, MD, pp. 69-75.

Bazant, Z.P. (1997). "Analysis of Pore Pressure, Thermal Stress and Fracture inRapidly Heated Concrete", International Workshop on Fire Performance ofHigh Strength Concrete, NIST SP 919, Gaithersburg, MD, pp. 155-164.

Bilodeau, A., Malhotra, V.M., and Hoff, G.C. (1998). "Hydrocarbon Fire Resistanceof High Strength Normal Weight and Light Weight Concrete IncorporatingPolypropylene Fibres", International Symposium on High Performance andReactive Powder Concrete, Sherbrooke, QC, pp. 271-296.

Diederichs, U., Jumppanen, U.M. and Schneider, U. (1995). "High TemperatureProperties and Spalling Behaviour of High Strength Concrete", Proceedings ofFourth Weimar Workshop on High Performance Concrete, HAB Weimar,Germany, pp. 219–235.

Fire Prevention (1997). "Channel Tunnel Fire Protection Measures Questioned afterFire on HGV Wagon", Fire Prevention, 296.

Harmathy, T.Z. (1993). "Fire Safety Design and Concrete", Longman Scientific &Technical, Essex, UK, pp. 412.

Kodur, V.K.R. (1998). "Performance of High Strength Concrete-Filled SteelColumns Exposed to Fire" Canadian Journal of Civil Engineering, 25(6),pp. 975-981.

Kodur, V.K.R. (1999). "Fire Performance of High Strength Concrete Columns",Presented at: ACI Spring Convention (SP in press), Chicago, IL.

Kodur, V.K.R. and Lie, T.T. (1997). "Fire Resistance of Fibre-Reinforced Concrete",Fibre Reinforced Concrete: Present and the Future, Canadian Society of CivilEngineers, pp. 189-213.

Kodur, V.K.R. and Sultan, M.A. (1998a). "Structural Behaviour of High StrengthConcrete Columns Exposed to Fire", International Symposium on HighPerformance and Reactive Powder Concrete, Sherbrooke, QC, pp. 217-232.

Kodur, V.K.R. and Sultan M.A. (1998b). "Thermal Properties of High StrengthConcrete at Elevated Temperatures", CANMET-ACI-JCI InternationalConference, ACI SP-170, Tokushima, Japan, pp. 467-480.

Lie, T.T. and Woollerton, J.L. (1988). "Fire Resistance of Reinforced ConcreteColumns: Test Results", Institute for Research in Construction InternalReport No. 569, National Research Council of Canada, Ottawa, ON, 302 pp.

Phan, L.T. (1996). "Fire Performance of High-Strength Concrete: A Report of theState-of-the-Art", National Institute of Standards and Technology,Gaithersburg, MD, pp. 105.

Sanjayan, G. and Stocks L.J. (1993). "Spalling of High-Strength Silica FumeConcrete in Fire", ACI Materials Journal, 90(2), pp. 170-173.

Ulm F.J., Acker P., Levy M. (1999). "Chunnel Fire. II: Analysis of ConcreteDamage", Journal of Engineering Mechanics, 125(3), pp. 283-289.

Danielsen, Ulf (1997). "Marine Concrete Structures Exposed to Hydrocarbon Fires",Report, SINTEF – The Norwegian Fire Research Institute, pp. 56-76.

Corresponding Author

For further information, please contact Venkatesh Kodur, Fire Risk ManagementProgram, Institute for Research in Construction, National Research Council ofCanada, Ottawa, ON, K1A 0R6, Canada. Phone: 613-993-9729. Fax: 613-954-0483.Email: venkatesh.kodur@nrc.ca.

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LIST OF FIGURES

Fig. 1 Mechanism for Spalling in Concrete Exposed to Fire(a) due to pressure build-up (b) due to restrained

thermal dilation

Fig. 2 View of NSC and HSC Columns after Fire Resistance Tests(a) Normal Strength Concrete Column (b) High Strength

Concrete Column

Fig. 3 Normal Density Aggregate and Light Weight Aggregate HSC Blocksexposed to Hydrocarbon Fires

(a) Normal Density Aggregate HSC Block (b) Light WeightAggregate HSC Block

Fig. 4 Conventional and Modified Tie Configuration for Reinforced ConcreteColumn

(a) Conventional Tie Configuration (b) Modified TieConfiguration

Fig. 5 View of HSC Blocks, with and with out fibres, after two hourHydrocarbon Fire Tests

(a) HSC Block with out fibres (b)HSC Blockwith polypropylene fibres

Fig. 6 Axial Deformation for HSC and NSC Columns

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Pressure Clog

X

I

Evaporization Zone

Tempera

(a) due to pressure build-up (b) due to restrained thermal dilation

Fig. 1 Mechanism for Spalling in Concrete Exposed to Fire

(a) Normal Strength Concrete Column (b) High Strength Concrete Column

Fig. 2 View of NSC and HSC Columns after Fire Resistance Tests

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(a) Normal Density Aggregate HSC Block

Fig. 3 Normal Density Aggregate and Light Weight Aggregate HSC Blocks exposed to Hydrocarbon Fires

(a) Conventional Tie Configuration (b) Modified Tie Configuration

Fig. 4 Conventional and Modified Tie Configuration for Reinforced Concrete Column

(a) HSC Block with out fibres rres

Fig. 5 View of HSC Blocks, with and with out fibres, after two hour Hydrocarbon Fire Tests

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