Effect of Fire on Concrete and Concrete Structures

Effect of fire on concrete andconcrete structuresGabriel Alexander KhouryImperial College, London, UK

SummaryThe behaviour of concrete in fire depends on its mixproportions and constituents and is determined bycomplex physicochemical transformations duringheating. Normal-strength concretes and high-performance concretes microstructurally followsimilar trends when heated, but ultra-high-performance concrete behaves differently. A keyproperty unique to concrete amongst structuralmaterials is transient creep. Any structural analysisof heated concrete that ignores transient creep willyield erroneous results, particularly for columnsexposed to fire. Failure of structural concrete in firevaries according to the nature of the fire; theloading system and the type of structure. Failurecould occur from loss of bending or tensile strength;loss of bond strength; loss of shear or torsionalstrength; loss of compressive strength; and spallingof the concrete. The structural element should,therefore, be designed to fulfil its separating and/orload-bearing function without failure for therequired period of time in a given fire scenario.Design for fire resistance aims to ensure overalldimensions of the section of an element sufficient tokeep the heat transfer through this element withinacceptable limits, and an average concrete cover tothe reinforcement sufficient to keep the

temperature of the reinforcement below criticalvalues long enough for the required fire resistanceperiod to be attained. The prediction of spalling` hitherto an imprecise empirical exercise ` is nowbecoming possible with the development ofthermohydromechanical nonlinear finite elementmodels capable of predicting pore pressures. Therisk of explosive spalling in fire increases withdecrease in concrete permeability and could beeliminated by the appropriate inclusion ofpolypropylene fibres in the mix and/or by protectingthe exposed concrete surface with a thermalbarrier. There are three methods of assessment offire resistance: (a) fire testing; (b) prescriptivemethods, which are rigid; and (c) performance-based methods, which are flexible. Performance-based methods can be classified into threecategories of increasing sophistication andcomplexity: (a) simplified calculations based onlimit state analysis; (b) thermomechanical finiteelement analysis; and (c) comprehensivethermohydromechanical finite element analysis. Itis only now that performance-based methods arebeing accepted in an increasing number ofcountries.

Prog. Struct. Engng Mater. 2000; 2: 429d447

Introduction

This paper presents a general brief outline of theeffect of fire on both concrete material andconcrete structures with emphasis being placedupon the subject areas receiving most attention in thepast few years, namely: (a) deterioration inmechanical properties of concrete and especiallyof high-performance and ultra-high-performanceconcretes in fire; (b) explosive spalling and theuse of polypropylene fibres; (c) the developmentof finite element structural analysis modelscapable of predicting pore pressures and spalling;and (d) fires in tunnels. The basic principlesof fire engineering are also presented on the

assumption that the reader is not a specialist in thisfield.

Background

Research into the effect of fire on concrete and concretestructures has been conducted since at least 1922[1],primarily in relation to buildings. The main areas ofinterest were: (a) the understanding of the complexbehaviour of the material itself; and (b) the structuralsafety and integrity of the building during, and after,fire. The introduction of the advanced gas-cooled(AGR) nuclear reactor system in the UK in the 1960sand 1970s provided added impetus and funding for

429

Copyright ^ 2000 John Wiley & Sons, Ltd. Prog. Struct. Engng Mater. 2000; 2:429}447

Fig. 1. Standard fire scenarios for buildings (ISO 834 or BS 476),offshore and petrochemical industries (hydrocarbon), and tunnels(RWS, RABT); idealized fire curves established from experience inreal fires which vary considerably from fire to fire

research into the effect of high temperatures onconcrete, albeit in relation to service and accidentnuclear reactor conditions[2], but nevertheless withuseful technology transfer to fire applications. Muchof the output from this research has been published ina series of biannual ‘SMiRT’ conferences since 1971,and in the journal Nuclear Engineering and Design. Upto about a decade ago, fire research was focused uponthe behaviour of ‘normal-strength’ concrete at hightemperatures, and engineers mainly employedprescriptive methods of design to ensure structuralstability in fire. Since then, there have been two majordevelopments in this field, namely: (a) the increasinguse of high-performance concrete (HPC) in buildings,tunnels and bridges; and (b) the growing acceptance ofthe use of performance-based structural analysis anddesign against fire. The employment of HPC in tunnellinings, with its greater tendency to explosively spallin fire, has resulted in a series of high-profile andcostly incidents of explosive spalling of concrete intunnel fires, which further fuelled urgent researchaimed at a better understanding and solution of thisserious problem. The impetus for the development ofperformance-based structural analysis came from thelimitations inherent in the traditional prescriptivemethods of design. Several countries have alreadydeveloped performance-based codes (e.g. UK,Sweden, Norway, New Zealand and Australia) andmany more countries are in the process of doing so.While the UK has led in this field with the publicationin 1992 of the Building Regulations Approved DocumentB3[3], performance-based structural design andanalysis methods are still not as widely used byengineers in the UK, as they are in Scandinavia. Thistopic is currently the focus of much discussion,research and development worldwide. This has led tonew ideas for improving fire safety, thus encouragingthe engineer to develop new creative solutions[4].

Fire scenarios

The temperature}time curves in ‘Standard’ fires(Fig. 1) used in testing, analysis and design wereestablished from experience in real fires and fall intothree main categories, depending upon the application(i.e. buildings, offshore/petrochemical, tunnels).

BUILDINGS: BS 476 [5] OR ISO 834 [6]

The standard furnace curve represents a typicalbuilding fire based upon a cellulosic fire in which thefuel source is wood, paper, fabric, etc. In BS 476, thetemperature increases from 20 to 842 3C after the first30 min (i.e. equivalent to an average heating rate of27.4 3C min!1). This fire profile has a slowtemperature rise up to 1000 3C over a period of120 min. This curve represents only one possibleexposure condition at the growth and the fullydeveloped fire stages, and does not include a final

decay stage. By comparison, real fires can havea slower or longer growth phase, and once they areestablished, temperatures can be higher than thefurnace temperatures, though they are rarelysustained because they are subject to pronouncedfluctuations. The standard temperature–time curve,therefore, corresponds to a severe fire, but not theseverest possible fire.

OFFSHORE AND PETROCHEMICAL INDUSTRIES [7`9]

In the 1970s, the oil company Mobil investigatedhydrocarbon fuel fires and developeda temperature–time profile with a rapid temperaturerise in the first 5 min of the fire up to 900 3C (i.e.176 3C min!1) and a peak of 1100 3C. This research laidthe foundation for test procedures to assessfire- protecting materials for the offshore andpetrochemical industries.

TUNNELS: RWS AND RABTMore recently, a spate of major tunnel fires haveindicated that an even more severe fire scenario needsto be considered. In the Netherlands, the Ministry ofPublic Works, the Rijswaterstaat (RWS), and the TNOCentre for Fire Research have established a fire curvefor the evaluation of passive protecting materials intunnels[10]. This RWS Dutch fire curve models a mostsevere hydrocarbon fire, rapidly exceeding 1200 3Cand peaking at 1350 3C (melting temperature ofconcrete) after 60 min and then falling gradually to1200 3C at 120 min, the end of the curve. RWS isintended to simulate tankers carrying petrol in tunnelswith a fire load of 300 MW causing a fire for 2 h, andwas established on the basis of Dutch experience intunnel fires. However, the maximum temperaturesattained in recent major fires did not reach RWS levels,e.g. Channel (1100 3C), Great Belt (800 3C), Mont Blanc(1000 3C), Tauern (1000 3C). The RWS fire curve,therefore, represents the severest form of tunnel fire interms of initial heating rates and maximumtemperatures. The RABT German fire curve, witha descending branch, represents a less severe fire

CONCRETE CONSTRUCTION430


Fig. 2. Physicochemical processes in Portland cement concreteduring heating: (a) temperatures are for the concrete material andnot the fire, except for explosive spalling where the temperaturerange is that of the concrete surface and depends upon the heatingrate and type of concrete; (b) unsealed cement paste (i.e. allowedto dry) behaves very differently from moist sealed cement pasted above 100 3C, the former is dominated by dehydrationprocesses, while the latter is dominated by hydrothermal chemicaltransformations and reactions

scenario in tunnels than the RWS curve, reachinga maximum temperature of 1200 3C (melting point ofsome aggregates) sustained up to 1 h before decayingto ambient.

Many standard (building) insulation materialsdecompose and lose their function above 1200 3C. Itmay be that insulation materials behave well in BS 476,ISO 834, Eurocode hydrocarbon fires and even theRABT fire, but may not withstand a fire under RWSconditions. For this reason, the manufacturer ofthermal insulation coatings for tunnel concrete linings,Lightcem, have recently developed two differentthermal types of insulation materials: one fortemperatures up to 1200 3C based on Portlandcements, and another for temperatures up to 1350 3Cbased on aluminous cements. It should, however, benoted that structural aluminous cement concreteshave been banned because of the risk of conversionand weakening that can take place under certaintemperature/humidity conditions.

Fire and concrete material

Fire resistance is a concept applicable to elements ofthe building structure and not a material, but theproperties of a material affect the performance of theelement of which it forms a part. Therefore, the effectof fire on concrete material will be discussed first inthis section and measures for improving materialperformance will be proposed. The influence of fire onconcrete structures will then be discussed in thefollowing section.

The advantages of concrete in a fire are two-fold. Itis: incombustible (e.g. when compared with wood);and a good insulating material possessing a lowthermal diffusivity (e.g. when compared with steel).However, there are two problems of concrete in fire.These are: deterioration in mechanical properties astemperature rises, caused by physicochemical changesin the material during heating (Fig. 2); and explosivespalling, which results in loss of material, reduction insection size and exposure of the reinforcing steel toexcessive temperatures. Consequently, both theseparating/insulating and load-bearing functions ofthe concrete member could be compromised.

DETERIORATION IN MECHANICAL PROPERTIES

Deterioration in mechanical properties of concreteupon heating may be attributed to three ‘material’factors:

� physicochemical changes in the cement paste;� physicochemical changes in the aggregate;� thermal incompatibility between the aggregate and

the cement paste;

and are influenced by ‘environmental’ factors, such as:

� temperature level;� heating rate;

� applied loading;� external sealing influencing moisture loss from the

surface.

These factors are described below in relation tocompressive and tensile strength.

Compressive and tensile strengthBefore explaining the changes in strength of concreteas a function of increasing temperature, two positiveaspects of heated concrete need pointing out.

Influence of transient creepUp to the 1970s, scientists were puzzled as to whyconcrete does not break up when heated above 100 3C,because the differential strain resulting from theexpansion of the aggregate and shrinkage of thecement paste is far too large to be accommodated byelastic strains. The discovery of the phenomenon of‘transient creep’ provided the answer. Transient creep(strictly, it should be called load-induced thermalstrain, LITS[11]) develops during first heating (and notduring cooling) under load. LITS is unique to concreteamongst structural materials. Above 100 3C it isessentially a function of temperature and not of timeand is, therefore, relatively easy to model

FIRE AND CONCRETE 431


Fig. 4. Effect of temperature and load level during heating-up upon the residual stressdstrain relation in uniaxial compression of unsealedC70 HITECO concrete cylindrical specimens; tests conducted at constant stress rate, descending branch not shown

Fig. 3. Load-induced thermal strains (or transient creep) ofconcrete during heating at 1 3C min!1 to 600 3C undercompressive loads up to 30% of the unheated strength; heating wasslow to reduce ‘structural’ effects; LITS was found to be morea function of temperature than of time

mathematically in a short-term heating scenario suchas fire. A further simplification, discovered by theauthor[11], is the existence of a ‘master LITS’ forPortland-cement-based concrete in general fortemperatures up to 450 3C, irrespective of the type ofaggregate or cement blend used (Fig. 3). LITS is muchlarger than the elastic strain (compare Figs 3 and 4 for20% stress level), and contributes to a significantrelaxation and redistribution of thermal stresses inheated concrete structures. Any structural analysis ofheated concrete that ignores LITS will, therefore, bewholly inappropriate and will yield erroneous results,particularly for columns exposed to fire. Thisphenomenon is still not fully appreciated by structuralengineers and should be incorporated more fully intostandards and design codes.

Influence of loading during heatingAnother positive aspect of concrete at hightemperature is the ‘beneficial’ influence of loadingwhich places the material into compression,‘compacts’ the concrete during heating and inhibitsthe development of cracks. Again, this influence is notfully appreciated by the practising engineer and is notadequately covered in the codes and standards. Anexample of this phenomenon is given in Fig. 4,comparing the stress–strain relation at levels of hightemperature 20–700 3C for concrete heated withoutload and under 20% load. The influence oftemperature can be markedly diminished. Both thecompressive strength and elastic modulus reduce farless with increase in temperature for the concreteheated under load. However, for concrete heatedwithout load, tests consistently show the modulus ofelasticity to reduce with increase in temperature bya larger proportion of the initial value than thecompressive strength[12].

Influence of temperatureCompressive strength, reduces from ambient to reacha minimum at 80 3C for unsealed concrete. This is an‘apparent’ strength loss, largely reversible upon

cooling, and is attributed to the weakening of thephysical van der Waals’ forces as the expanding watermolecules push the CSH layers further apart. Thisminimum, therefore, does not appear when the‘residual’ strength is measured after cooling andplotted against previous temperature. Frequently, the‘hot’ strength is shown to increase to a maximum atabout 200–300 3C greater than the initial strength priorto heating. Most concretes experience a strengthreduction above 300 3C, but this depends upon thetype of aggregate and cement blend used in the mix.Between 300 and 600 3C there is room for improvingthe performance of concrete by judicious mix design.



Fig. 5. Effect of temperature upon the residual compressivestrength of two high-performance concretes (C60-SF, C70) andtwo ultra-high-performance concretes (CRC, RPC-AF) after heatcycling without load at 2 3C min!1: (a) actual values; (b) percentageof initial strength at 20 3C

However, the author discovered a marked increase inthe ‘basic’ creep of Portland cement paste andconcrete[13] at about 550–600 3C, indicating thistemperature to be critical, above which concrete is notstructurally useful. Fortunately, in fire, only the firstfew centimetres from the concrete surface experiencetemperatures greater than 300 3C owing to the lowthermal diffusivity of concrete in general. The normalpractice after fire is to remove and replace this‘overheated’ layer. As long as the concrete does notspall during fire, this layer continues to providethermal protection to the steel and inner concrete, andwould effectively act as a thermal barrier, although itsstructural role becomes greatly diminished.

Another important phenomenon, also not fullyappreciated, is the major difference in behaviourbetween ‘unsealed’ and moist ‘sealed’ concrete attemperatures above about 100 3C. The dominantprocess for unsealed concrete relates to the loss of thevarious forms of water (free, adsorbed and chemicallybound), while the dominant process in sealed concreterelates to hydrothermal chemical reactions whichcould result in much weaker or stronger gel,depending upon the CaO/SiO2 ratio (C/S ratio). TheC/S ratio is influenced by the use of cementreplacements such as slag, pulverized fly ash (PFA), orsilica fume in the mix.

Given the large number of material andenvironmental factors that influence the compressivestrength of heated concrete, it is no surprise thatmeasurements of compressive strength at 150 3C canyield results ranging from as low as 30% to as high as120% of the original cold strength[14]. It is no wonderthat data for concrete strengths at elevatedtemperatures taken from different sources differsubstantially, and in several cases even seemcontradictory. Therefore, representing ‘typical’strength behaviour of all ‘normal-weight concrete’ athigh temperatures with a single ‘average curve’, asgiven in BS 8110[15], can be misleading unless thespecific mix and environmental conditions (e.g.heating rate, load during heating, moisture condition,cooling rate, etc.) are specified.

Recent research[12,16] has extended the testing ofcompressive strength from normal-strength concreteto high-performance concrete (HPC; strengths60–100 MPa) and even ultra-high-performanceconcrete (UHPC containing steel fibres; strengths100–300 MPa). The results shown in Fig. 5 show thatthe ‘hot’ strengths of the HPC concretes decline lesswith increase in temperature than those of the UHPCconcrete, but that the reverse is true for the residualstrength measured after cooling (possibly because ofthe steel fibres). The relatively poorer ‘hot’performance of the UHPC could be due to its verydense structure, from which moisture escapes lessreadily. This has two effects: a physical effect due toreduced van der Waals’ forces as water expands uponheating, and a chemical effect whereby detrimental

transformations can take place under hydrothermalconditions. This suggests that the UHPC hot strengthloss could have been more pronounced had thespecimens been kept for a longer time at appropriatetemperatures. Normal-strength concretes and high-performance concretes microstructurally followsimilar trends when heated, but ultra-high-performance concrete behaves differently, asevidenced by rate of moisture loss readings taken atconstant high temperatures.

Until very recently, all measurements of tensilestrength at high temperatures were performed byindirect means. However, there is sufficient evidenceto show that the tensile constitutive behaviour ofconcrete at high temperature cannot be adequatelydetermined from indirect ‘tension’ tests (i.e. splittingand bending). For this reason, stress–strain tests indirect tension were performed at high temperatures forthe first time last year at Imperial College incollaboration with Milan Polytechnic. The directtensile hot strength declines with increase intemperature. Also as expected, the results obtainedfrom the hot tests were closer to those obtained for theresidual tests for the HPC than for the UHPCconcretes, owing to the presence of steel.



Fig. 6. Concrete mix design for improved performance at hightemperature; some factors that improve performance at roomtemperature, e.g. low permeability from the use of SF or lowwater/cement ratio (w/c) increase risk of explosive spalling

Mix design against strength lossDeterioration of mechanical properties can be reducedfor the temperature range from ambient up to 600 3Cby judicious design of the concrete mix (Fig. 6). Thiswould take into consideration the behaviours of; (a)the aggregate; (b) the cement paste; and (c) theinteraction between them.

AggregateThe choice of aggregate is probably the moreimportant since some aggregates, such as flint orThames gravel, break up at relatively lowtemperatures (below 350 3C) while other aggregates,such as granite, exhibit thermal stability, up to 600 3C.The thermal stability of different aggregates increasesin the following order: flint, Thames gravel, limestone,basalt, granite, gabbro. Other desirable features inaggregates are: (a) low thermal expansion, whichimproves thermal compatibility with the cement paste;(b) rough angular surface, which improves thephysical bond with the paste; and (c) the presence ofreactive silica, which improves the chemical bond withthe paste.

Cement blendAs for the cement blend, an important feature is theC/S ratio. A low C/S ratio results in a low calciumhydroxide (Ca(OH)2) content in the original mix andensures a more beneficial hydrothermal reaction.Calcium hydroxide is not desirable because itdissociates at about 400 3C into CaO and CO

�. The

CaO rehydrates expansively and detrimentally uponcooling and exposure to moisture. The C/S ratio isreduced in practice by the use of slag, PFA or silicafume. Tests by the author show that the use of slagproduces the best results at high temperatures,followed by PFA and then silica fume. The relativelylow performance of the silica fume cement paste(contrary to its high durability performance at roomtemperature) may be attributed to the dense, lowpermeability structure of the paste which does notreadily allow moisture to escape from the heatedconcrete, thus resulting in high pore pressures and thedevelopment of microcracks.

EXPLOSIVE SPALLING

Spalling is the violent or non-violent breaking off oflayers or pieces of concrete from the surface ofa structural element when it is exposed to high andrapidly rising temperatures, as experienced in fireswith heating rates typically 20–30 3C/min��.

Spalling can be grouped into four categories:(a) aggregate spalling; (b) explosive spalling;(c) surface spalling; (d) corner/sloughing-off spalling.The first three occur during the first 20–30 min intoa fire and are influenced by the heating rate, while thefourth occurs after 30–60 min of fire and is influencedby the maximum temperature. Surface and explosive

spalling are violent while corner/sloughing-offspalling is non-violent. It could also be argued thatsurface spalling is really a subset of explosive spallingwhich is the most serious, and hence most researched,form of spalling.

The damage caused to a concrete structure byspalling can render fire safety design calculationsinaccurate and lead to significantly reduced levels ofsafety in fire. It is, therefore, important to: (a) betterunderstand the fundamental mechanisms responsiblefor explosive spalling of concrete; (b) developa realistic predictive model; and (c) optimise (in termsof cost and effectiveness) the methods for eliminatingexplosive spalling in practice.

Factors influencing explosive spallingMany material (e.g. permeability, saturation level,aggregate size and type, presence of cracking andreinforcement), geometric (e.g. section shape and size)and environmental (e.g. heating rate, heating profile,load level) factors have been identified fromexperiments as influencing spalling of concrete in fire(Table 1). The test results were used to producenomograms identifying spalling and non-spallingzones (e.g. Fig. 7). A detailed account of the factorsinfluencing spalling is beyond the scope of this paper,but the main factors are the heating rate (especiallyabove 3 3C min!1), permeability of the material, poresaturation level (especially above 2–3% moisturecontent by weight of concrete), the presence ofreinforcement and the level of external applied load.Low-permeability, high-performance concrete (HPC)is more likely to explosively spall, and to experiencemultiple spalling, than normal-strength concrete,despite its higher tensile strength. This is because



Table 1 Characteristics of the different forms of spalling

Spalling Time of Nature Sound Influence Mainoccurrence (min) influences

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Aggregate 7d30 Splitting Popping Superficial H, A, S, D, WCorner 30d90 Non-violent None Can be serious T, A, Ft, RSurface 7d30 Violent Cracking Can be serious H, W, P, Ft

Explosive 7d30 Violent Loud bang Serious H, A, S, Fs, G,L, O, P, Q, R,S, W, Z

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .A"aggregate thermal expansion, D"aggregate thermal diffusivity, Fs"shear strength of concrete, Ft"tensile strength of concrete, G"ageof concrete, H"heating rate, L"loading/restraint, O"heating profile, P"permeability, Q"section shape, R"reinforcement,S"aggregate size, T"maximum temperature, W"moisture content, Z"section size

Fig. 7. Explosive spalling empirical envelope for normal-strengthconcrete, showing influences of moisture content and appliedstress[24]

Fig. 8. Gradients of temperature, pore pressure and moisture innormal and high-performance concrete sections during heatingfrom one unsealed surface (Anderberg & Khoury)

greater pore pressures build up during heating(Fig. 8), owing to the material’s low permeability.Also, the peak in pore pressure occurs nearer to thesurface for HPC which explains why thinner concretesections spall repeatedly from HPC concrete in fire.

Mechanisms of explosive spallingThe mechanisms proposed to explain the explosivespalling of concrete fall under three categories:

� pore pressure spalling: as favoured by Shorter& Harmathy[17], Meyer-Ottens[18], Aktarruzzaman& Sullivan[19], Khoylou[20], Ichikawa[21];

� thermal stress spalling: as favoured by Saito[22],Dougill[23];

� combined pore pressure and thermal stress spalling:as favoured by Zhukov[24], Sertmehmetoglu[25],Connolly[26].

Pore pressure spallingThis has been predicted using a ‘moisture clogmodel’[17], ‘vapour drag forces model’[18] or an‘idealized spherical pore model’[19]. The main factorswhich influence pore pressure spalling are thepermeability of the concrete, the initial watersaturation level, and the rate of heating. Thegeneration of pore pressure in the heated concrete

(Fig. 8) is difficult to predict reliably. The models rangein complexity from the simple use of steam tables tofull solution of the equations of state using finiteelement analysis[27,28]. The majority of such modelspredict pore pressure levels which are substantiallyless than the tensile strength of concrete. Furthermore,test measurements of pore pressures in unsealedconcrete specimens also usually produced levels lessthan the tensile strength of concrete[25,29,30]. However,Khoylou[20], suggested that large hydraulic pressurescould be generated in heated saturated sealed sphericalpores to cause hydraulic pore pressure explosivespalling. This model, based on elastic theory, does notconsider the influence of moisture migration betweenpores, nor the role of creep, and may therefore greatlyoverestimate pore pressures. An analytical model byConnolly[26], based on the velocities of the moistureclog and the 100 3C isotherm also consistentlypredicted pore pressures greater than those measured.All this serves to illustrate how difficult it has been toaccurately predict pore pressures and pore pressureexplosive spalling by means of analytical methods.

Thermal stress spallingAt a sufficiently high heating rates, ceramics canexperience explosive spalling. This is attributed toexcessive thermal stresses generated by rapid heating,



Fig. 9. Calculated elastic thermal stress profiles in the centralcross-section of a 60 mm diameter gravel concrete cylinder heateduniformly on the curved surface at a constant rate of 13C min!1,without relaxation by creep

Fig. 10. Forces acting in heated concrete[24]

and demonstrates that factors other than porepressures may contribute to explosive spalling.Heating concrete generates temperature gradients(Fig. 8) that induce compressive stresses close to theheated surface (due to restrained thermal expansion)and tensile stresses in the cooler interior regions(Fig. 9). Surface compression may be augmented byload or prestress, which are superimposed upon thethermal stresses. However, very few concretestructures are loaded to levels where the necessaryfailure stress state is reached. This makes thermalstress spalling, by itself, a relatively rare (but notimpossible) occurrence. Predicting spalling based onthermal stresses would have the merit of simplicity, asreasonable estimates may be made of their magnitudeusing nonlinear constitutive models. However,although Dougill’s theory[23] is more realistic thanSaito’s[22], it still does not explain the obvious role ofmoisture in spalling.

Combined thermal stress and pore pressuresExplosive spalling generally occurs under thecombined action of pore pressure, compression in theexposed surface region (induced by thermal stressesand external loading) and internal cracking (Fig. 10).Pore pressure spalling may act by itself for smallunloaded specimens. For larger specimens, the porepressure will need to be considered, together withboth the thermal and load-induced stresses, before thelikelihood of explosive spalling, can be assessed.Cracks develop parallel to the surface when the sum ofthe stresses exceeds the tensile strength of the material.This is accompanied by a sudden release of energy anda violent failure of the heated surface region.Sertmehmetoglu[25] suggested that the effect of loadwas simply to create planes of weakness parallel to theheated face of the concrete. He demonstrated that theaction of relatively small pore pressures from withinsuch planes could initiate spalling. Pore pressurespalling and thermal stress spalling, both influencedby external loading, act singly or on combination,depending upon the section size, the type of concreteand the moisture content. In both normal and high-performance concretes, the evidence suggests thatpore pressure spalling is the more dominant of the two– hence the effectiveness of the polypropylene fibres.However, unpublished test results suggest that insome ultra-high-performance concretes, containinglarge proportions of expansive silica, thermal stressspalling may assume a greater importance – and theuse of large quantities of polypropylene fibres doesnot seem to prevent spalling. Further research isrequired to settle this issue.

Prediction of spallingUntil now, the prediction of spalling during heatinghas been largely an imprecise empirical exercise.Attempts to predict spalling by analytical methods

have failed[26] owing to the complex microstructureand multiphase nature of heated concrete, which doesnot readily lend itself to simplified analytical models.The inability to predict the occurrence of spalling hasbeen a limiting factor in the development of robustmodels capable of predicting the response of concretestructures to fire. It is only now that a fullycomprehensive and coupled nonlinear finite elementmodel is being developed to predict the pore pressuresand spalling in heated concrete structures (see below).

Design against spallingWhile a whole raft of measures (Table 2) have in thepast been proposed to combat explosive spalling, theonly really effective method has so far been the use ofa thermal barrier, protecting the surface of the concretefrom the fire. It is only now that polypropylene (pp)



Table 2 Evaluation of preventive measures for the spalling of concrete

Method Effectiveness Comments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Polypropylene fibres Very effective, even in high-strength concrete May not prevent spalling in expansive ultra- high-strength concrete. Does not reducetemperatures d only pore pressures

Air-entraining agent Very effective But can reduce strength

Thermal barrier Very effective Also reduces concrete temperatures andincreases fire resistance

Moisture content control Reduces vapour pressure Normal moisture content is usually above the ‘nospalling’ limit for most buildings

Compressive stress control Reduces explosive pressure Not economical as section sizes increase

Choice of aggregate It is best to use low expansion andsmall-size aggregate

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 reinforcement Reduces spalling damage Difficult to use in small and narrow sections

Choice of section shape Thicker sections reduce spalling damage Important for I-beams and ribbed sections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

fibres are being considered for use in the concrete mixfor the purpose of increasing permeability duringheating above 160 3C, thus reducing pore pressuresand the risk of spalling. Polypropylene fibres melt atabout 160 3C and provide channels in the concrete formoisture to escape. A further, recent advance has beenthe development by the company Fibrin of low-meltpp fibres (130 3C) which promise to be even moreeffective, particularly for low-permeability dense andsaturated concrete. Nevertheless, there remain twokey areas for further research and development,namely: (a) the exact mechanism by which pp fibresoperate is not properly understood (e.g. the influenceof the melting/evaporation process of the fibres uponthe microstructure of the concrete and the additionalrole of microcracking); and (b) the use of pp fibres inpractice is yet to be optimized (e.g. type and amount offibres in relation to applied load and concretestrength). There is unpublished evidence that evenlarge quantities of pp fibres may not prevent explosivespalling of some ultra-high-performance concretes infire. When such a material contains high levels ofexpansive silica, thermal stress spalling may becomemore dominant than in normal concrete.

CRITICAL TEMPERATURES

Three critical temperatures for structural concrete canbe identified (Fig. 2):

� spalling critical surface temperature: experimentalevidence indicates that the concrete surfacetemperatures when spalling occurs fall in therange of about 250–420 3C[19,26,31], dependingupon the heating rate and characteristics of theconcrete.

� initiation of strength loss critical temperature: thisdepends upon the type of concrete and for siliceousconcrete it is about 300 3C, but can be lower for flintaggregate concrete and higher for granite aggregateconcrete.

� generic loss of load-bearing capacity criticaltemperature: Portland-cement-based concretesexperience considerable creep at about 550–600 3C,at which temperature the material would not bestructurally useful in the ‘hot’ state.

Fire and concrete structures

Fire impacts upon concrete structures insofar as itgenerates a heat flow into the concrete’s exposedsurface, producing temperature, moisture and porepressure gradients within the concrete mass. Thermalstrains, stresses and cracking develop within theheated structure. Explosive spalling of the concretecan take place. Also, both the concrete and steel, aswell as the bond between them, experience strengthlosses upon heating. The designer should, therefore,ensure that all these factors combined do not prejudicethe structure’s primary separating and/or load-bearing functions for the duration of, and following,the fire.

TEMPERATURE DISTRIBUTION

Knowledge of the development of temperaturedistribution in concrete structures is the first key stepin the understanding of the structure’s behaviour infire. Air temperatures in fires frequently exceed900 3C. However, the good insulating properties ofconcrete mean that the temperature gradient is large



Fig. 11. Temperature contours in a quarter section of a concretebeam heated on three sides in ISO 834 (or BS 476) fire:(a) contours after 60 min; (b) 5003C isotherms after 30, 60 and 90min[9]

and only the temperature of the outside layer ismarkedly increased, while the temperature of theinternal concrete remains comparatively low. This factis illustrated by the example of a 160-mm, wide beamexposed on three sides to a standard ISO 834 (BS 476)fire[9]. The temperature after 1 h of fire exposure in theregion away from the influence of the corner will be asfollows: 600 3C at 16 mm depth and 300 3C at 42 mmdepth (Fig. 11). The 500 3C isotherm reaches 10 mm at30 min, 20 mm at 60 min, and 32 mm at 90 min. Thetemperature at a given distance from the exposedsurface will be higher at corners of an element due tothe transmission of heat from the two surfaces. Thus,the profile of equitemperatures of a cross-section isrounded at corners (Fig. 11).

FIRE RESISTANCE

The concept of ‘fire resistance’ has been for decades atthe heart of research, design and assessment ofconcrete structures exposed to fire. Fire resistance canbe defined as the ability of an element (not a material)of building construction to fulfil its designed functionfor a period of time in the event of a fire. Fire resistancetime of a structural element exposed to a standardfurnace test is defined as the time elapsed before a firelimit state is violated. These fire limit states are givenin BS 476: Part 20 as follows for separating (E and I)and load-bearing (R) functions[5]:

� limit state for insulation (I): A fire on one side ofa wall, or underside of a floor, acting asa compartment boundary, should not causecombustion of objects on the unexposed side. Limitsof temperature rise of 140 3C (average) or 180 3C(peak) above ambient temperature are specified inthe standard fire resistance test.

� limit state for integrity (E): A wall or floor acting asa compartment boundary should not allow passageof smoke or flame from one compartment to anotheras a result of breaks or cracks in the wall or floor.Both insulation and integrity criteria also apply tomembers embedded in walls or floors.

� limit state for load-carrying capacity (R): Themembers in a structural assembly should resist theapplied loads in a fire.

Hence, normally, each part of the structure will havea different function in fire, according to its type andposition. This function could be to contain a fire (aswith non-load-bearing walls), to support the designloads (as with beams and columns), or both (as witha floor). The fire design process for separatingstructures comprise only thermal analysis, whilst forload-bearing structures both thermal and structuralanalyses are required.

Design for fire resistance also aims to ensure:

� overall dimensions of the section of an elementsufficient to keep the heat transfer through thiselement within acceptable limits;

� average concrete cover to the reinforcementsufficient to keep the temperature of thereinforcement below critical values (500 3C forreinforcing steel and 350 3C for prestressing steel)before the required fire resistance period is attained.Concrete cover in fire applications is defined as thedistance between the nearest heated face of theconcrete and the surface of the main reinforcement,or an average value determined in BS 8110: Part 2[15].This cover is defined differently from the ‘nominalcover’ of BS 8110: Part 1[15].

The cover thickness for a specified steel temperaturelimit depends upon the thermal diffusivity (k/�Cwhere k"thermal conductivity, C"specific heat,and �"density) of aggregate used. The cover has toprovide lasting protection to the reinforcement fromboth fire and environmental attack. Choice of coverthickness should be on the basis of the more onerousrequirement[15]. Reinforced or prestressed membersgenerally fail in fire as a result of excessivetemperature rise in the steel. This failure appliesmainly to simply supported flexural members. Loss ofconcrete cover by spalling of the concrete, especiallyexplosive spalling in the region of the tensile steel,endangers the carrying capacity because of theincreased rate of heat transfer to the steel andreduction in overall thickness of concrete. Hence,a maximum cover to reinforcement is recommendedto reduce the potential for spalling, but a minimum



thickness is required for thermal insulation. Therefore,the actual thickness should be between these twolimits.

The fire resistance of a whole concrete structurewould not necessarily be that ascribed to its individualelements. Better fire behaviour could arise from suchfactors as robustness, adequate continuity ofreinforcement, reduced level of loading, compositeconstruction, and the availability of alternative pathsfor load support. The provision of continuity ofreinforcement in the design allows the redistributionof forces and moments to gradually take placetowards the parts of the structure not exposed tothe fire as the exposed parts are weakened, thusensuring an improved fire resistance relative to thesituation of the simply supported single element. It is,therefore, necessary to pay particular attention todetailing.

Factors affecting fire resistance of concrete elementsaccording to BS 8110[15] are:

� size and shape of elements;� disposition and properties of reinforcement or

tendon;� the load supported;� the type of concrete and aggregate;� protective concrete cover provided to reinforcement

or tendons;� conditions of end support.� the overall thickness of the section in order to keep

heat transfer through the floor or wall withinacceptable limits;

� provision of surface insulation.

Fire engineering calculations, or a full-scale test, allowinteraction between these factors to be taken intoaccount (see next section).

TYPES OF FAILURE IN FIRE

The failure of structural concrete in fire variesaccording to the nature of the fire (e.g. rate oftemperature increase and maximum temperature); theloading system; and the type of structure exposed tothe fire. Failure could occur due to:

� loss of bending or tensile strength;� loss of bond strength;� loss of shear or torsional strength;� loss of compressive strength;� spalling of concrete (discussed above).

Bending`tensile failureBending failure of load-bearing bending elementsgenerally occurs when the reinforcement failsas the tensile strength of the steel is reduced onheating. Failure due to bending in the centre of thespan of length L of simply supported load-bearingmembers is indicated by deflections of the order ofL/30.

Bond failureFailure of reinforced concrete members may occur infire when heating reduces the bond strength betweenthe steel and concrete. Bond failure is usuallycombined with concrete tensile failure as the tensilestrength decreases rapidly with heating.

Shear`torsion failureShear or torsion failure in fire is influenced by theconcrete tensile strength and is much morecomplicated to determine than bending failure,because of limited experimental experience. As torsionmembers with a defined fire resistance are rare, thesolution of the problem may be shifted to individualcases, if any. There is, however, practical interest inshear beams which are usually designed with a specialshear reinforcement.

Compressive failureReinforced concrete members under compression (e.g.columns) usually fail in fire due to concrete failure inthe compression zone as its strength diminishes withheating. The loss in strength of the reinforcement withheating is of minor importance in cases where the steelcontent is small, but not at high steel contents, whereboth the concrete and steel influence the load-bearingbehaviour.

METHODS OF ASSESSMENT OF FIRE RESISTANCE

The engineer has at his/her disposal three methods ofassessment of fire resistance (Fig. 12), namely:

� fire testing;� prescriptive methods;� performance-based methods.

The first two have been established for severaldecades. It is only now that performance-basedmethods based on fire engineering calculation arebeing accepted in an increasing number of countries.For any of the three methods of design, the detailing ofthe structure should be such as to implement thedesign assumptions for the changes during a fire in thedistribution of load and the characteristic strengths ofthe materials. In particular, the reinforcementdetailing should ensure that both elements and thestructure as a whole contain adequate supports, ties,bonds and anchorages for the required fireresistance[16].

Fire testingFire testing of an element or subassembly whenexposed to a ‘standard’ temperature–time regime (e.g.according to BS 476[5]), is the most expensive of thethree options, particularly for larger, more complexstructures. Any form of concrete element covered bya valid fire test report may be deemed to have the fireresistance ascribed to it by such a test, provided thatthe element has similar details of construction, stress



Fig. 12. Three main options of concrete fire assessment anddesign process; performance-based methods are gainingacceptance in an increasing number of countries

level, and support as the test specimen. Standard testsfor fire resistance are usually conducted on singlebuilding elements where it is not feasible to reproducein the test furnace the nature and magnitude ofrestraint and continuity of the adjoining construction.In some cases, therefore, the fire performance ofstructural elements in the building could be expectedto be much greater than that of the simple elementwhen tested in the furnace. In other cases, thermalmovement can reduce the fire resistance. Anotheradvantage of fire testing over prescriptive methods isthat it provides an indication of temperaturedistributions within, and deflections of, the elementduring heating as well as detailing weaknesses notdiscovered without tests. However, its accuracy issensitive to the testing apparatus and methodemployed, hence the considerable discussions atinternational committee level on harmonization oflaboratory testing between different countries. Thepreparation for testing and performing the test islengthy, and the cost of setting it up and execution ishigh. Furthermore, testing of a complete constructionin fire is a formidable task, as evidenced by thefull-scale fire tests undertaken on a building by theBuilding Research Establishment in the great airshiphangar in Cardington, England.

Prescriptive methodsCurrent building fire engineering practice is largelybased on the application of prescriptive codeswhereby the engineer designs in accordance with

predetermined requirements, based on genericoccupancies or classes of fire risk. Prescriptive codesare rigid and restrictive, and do not allow forengineering thinking. Although the cheapest toimplement, they are the least accurate of the threemethods. The safety level achieved by this method canvary significantly. In many cases it is veryconservative and not cost efficient, but sometimes itcan also be unsafe[4]. The fire resistance requirement inthe prescriptive method is expressed by target fireresistance ratings for members exposed to BS 476 orISO 834 fire. This means that a structural membershould be designed in such a way that it does notcollapse within 30, 60, 90 or may be 120 min. Noclasses in between are available. Hence, if a testspecimen collapses after 59 min, it will be categorizedin the R30 fire class, irrespective of the loading level.Such deficiencies have provided the driving force forthe development and wider acceptance ofperformance-based methods (see below).

Engineers apply prescriptive methods for the fireresistance of building elements or subassemblies fromtabulated data presented in the codes and standards.BS 8110: Part 2: 1985[15] gives tables specifyingminimum dimensions, including cover (in mm) forfire resistances ranging from 30 min to 4 h for concretebeams, columns and floors. The standard refers,erroneously, only to density of the concrete, hence thedistinction made in the tables just between ‘dense’ and‘lightweight’ (i.e. (2000 kg m!3) concretes and theirstrength change with temperature. The foregoingdiscussion on materials properties shows that thebehaviour of heated ‘dense’ concrete is verydependent upon the type of aggregate used. Someaggregates (e.g. Thames gravel) break up at 350 3Cwhile others (e.g. granite and gabbro) can be thermallystable even at 600 3C. Also, the behaviour of thelightweight aggregates depends upon their density.The data also distinguishes between simply supportedand continuous constructions for flexural members forboth reinforced and prestressed concrete. The tablesare based on the assumption that the elements aresupporting the full design load. The tabulated data forsimply supported elements are based on the steelreinforcement retaining a proportion of its strength athigh temperatures; reinforcing bars and prestressingtendons are considered to retain about 50% of theirambient strength at 550 and 450 3C respectively. If anydimension of a particular construction is less than theminimum specified in the tables and it is not possibleor desirable to increase it to meet the requirements, thefire resistance may be enhanced by the application ofa protective coating, system or membrane[32]. It isnecessary to appreciate that these periods do notsignify the duration of an actual fire. For examplea 60-min fire does not imply that a construction isexpected to withstand a fire of 60 min duration, butthat it will withstand a fire of a longer or a shorterduration whose severity corresponds to the 60-min



furnace test. Specific provisions of test for fireresistance of elements of structure in terms of the threeperformance criteria mentioned above are given in theBuilding Regulations[3]. The regulations also set outthe minimum periods of fire resistance in minutes forelements of structure at basement, ground and upperlevels of various types of building. The regulationsstate that where one element of structure supports orcarries or gives stability to another, the fire resistanceof the supporting element should be no less than theminimum period of fire resistance for the otherelement (whether that other element is load-bearing ornot). Circumstances for varying this principle are alsogiven[3].

Performance-based methodsPerformance-based methods are based on fireengineering calculations, and provide a cost-effectiveand flexible method of assessment superior toprescriptive methods. A given problem can be studiedfor different fire scenarios, geometries, materialproperties, loading or support conditions. This can beperformed in a relatively short period of time, thusallowing a better understanding of the behaviour ofthe structure subjected to fire until collapse. Moreover,computer programs can even simulate structuralconditions that are very difficult to study in a fire test.In the performance-based design, the structure is notallowed to collapse during the complete fire process,including the cooling phase[4].

Performance-based methods can be classified intothree categories of increasing sophistication andcomplexity. These are (Fig. 12):

� simplified calculations based on limit state analysis;� thermomechanical finite element analysis;� comprehensive thermohydromechanical finite

element analysis.

Thermal analysisFor separating functions, only thermal analysis isrequired. For the load-bearing function, thermalanalysis will need to be conducted in all threeperformance-based categories as part of the structuralanalysis. For the simplified limit state method, itwould be a stand-alone finite element calculation todetermine, for example, the location of the 500 3C (orother) contour. Without taking into considerationmoisture migration, thermal analysis would provideapproximate results, particularly in the temperaturerange 100–200 3C when moisture migration andevaporation play a significant role. Normally this canbe partly accounted for by introducing a latent heat ofevaporation component to the specific heat capacity,but this still does not entirely solve the problem.This component may not be significant in terms ofload-bearing capacity assessment, but in otherexamples (e.g. spalling or in nuclear reactor concrete)it is important.

Software for thermal analysis fall under twocategories:

� general programs developed by professionalsoftware houses. The most well-known are:ABAQUS, ADINA, and ANSYS in the USA, andPAFEC and LUSAS in the UK.

� fire-dedicated programs designed by researchworkers in the field of fire. The most reputed are:FIRES-T3 which was developed in 1977 at theUniversity of California, Berkeley, in the USA byBresler, Iding & Nizamuddin[33]; TASEF-2,developed in 1979 by Wickstrom[34] in Sweden; andTEMPCALC, developed in 1986 by Anderberg[35] inSweden. TEMPCALC continues to be improved andused in practice on a range of fire applications,including the simulation of the loss of surface-spalled material and the thermal behaviour ofintumescent thermal coatings.

Simplified limit state analysisHaving determined the temperature distribution forthe structural element, either from the publishedliterature on a similar element or from thermalanalysis, a simplified limit state analysis is carried out,using a technique first proposed by Anderberg[36]. Hesuggests a very simple method of analysis, based onthe hypothesis that the thickness of the damagedsiliceous concrete is assumed to equal the averagedepth of the 500 3C isotherm in the compression zoneof the cross-section. Damaged concrete (i.e. concretewith temperatures in excess of 500 3C) is not expectedto contribute to the load-carrying capacity of themember, whilst the residual concrete cross-section isassumed to retain its full initial values of strength andmodulus of elasticity. Anderberg suggests that thismethod, the ‘500 3C isotherm’, or ‘Effective cross-section’ method, is applicable to a reinforced concretesection with respect to axial load, bending momentand their combinations. For this method to apply,there should be minimum dimensions of the member,depending upon fire resistance time or fire loaddensity[37]. One note of caution is that the choice of thetemperature isotherm will depend upon the type ofconcrete used and its characteristic strength lossagainst temperature. For certain concretes, theisotherm temperature may well be below 500 3C, oreven below 400 3C.

Thermomechanical finite element analysisThe bulk of performance-based, fire-dedicatedsoftware dating back to the 1970s falls into thiscategory. The thermal and mechanical analysis arenormally interfaced and not integrated. In otherwords, the thermal calculation is carried out first forthe entire duration of the fire and then fed into themechanical analysis program to produce the stressesand strains for the member/structure – there being nointeraction between the two analyses – and moisture



Fig. 13. Simplified flow chart of thermohydromechanical finiteelement analysis of heated concrete structures including spalling;most fire-dedicated finite element software are thermomechanicaland exclude the hydral component. The thermal and mechanicalanalyses are usually interfaced and not integrated, hence the inputsand outputs are shown separately. With a fully integrated andinteractive model, the input data are not separated because they allcontribute to the overall analysis

effects are also absent. Nevertheless, validation of theresults from such programs has consistently shownthat the deformations of simple elements can bereasonably accurately predicted by such methods,providing transient creep (or LITS) is incorporatedinto the model, particularly for columns. Themodelling of LITS is relatively simple in fire-dedicatedprograms where the duration of the analysis is inminutes or a few hours. This is because LITS is largelya function of temperature rather than time. Therefore,time-dependent creep functions can be dispensedwith, without prejudicing the outcome. The absence ofa moisture migration analysis means that theevaporation plateau and explosive spalling cannot bepredicted.

The first such fire-dedicated concrete program wasinitially developed in 1974 at the University ofCalifornia and was called FIRES-RC by Becker& Bresler[38]. That program was improved later byAnderberg[39] who introduced a more realisticconcrete behavioural model. This program wasfollowed by CONFIRE, developed in 1982 byForsen[40] in Norway (valid so far only for siliceousconcrete); STABA-F, developed at the TechnicalUniversity of Braunschweig[41] in 1985; CEFICOSSdeveloped in 1987 by Franssen[42] of the University ofLiege; STRUCT, developed by the author and his PhDstudent at Imperial College in 1991[43]; and the mostrecent FIREXPO, developed in 1999 by the contractorBouygues, with input from the author, as part of theHITECO research programme funded by theEuropean Commission[28].

Comprehensive thermohydromechanical finite elementanalysisA comprehensive analysis would incorporate thermal,hydral and mechanical analyses in a fully integratedand interactive model (Fig. 13), capable of predictingexplosive spalling (e.g. in tunnels) or the moisturestate of the concrete containment of nuclear reactors(an important function as a biological radiationshield). The first such model was developed in the1970s by Bazant & Thonguthai[27], albeit withlimitations in the modelling. For example, the hydralcomponent is considered to be a single-phase smearedfluid and spalling is not incorporated.

A more advanced model was developed in 1999 atPadua University in co-operation with the author andENEA in Rome as part of a multinational programmeof research funded by the European Commission[28].Called HITECOSP (high-temperature concretespalling), it is a fully coupled nonlinear model,designed to predict the behaviour, and potential forspalling, of heated concrete structures for fire andnuclear reactor applications. Concrete is considered asa multiphase material consisting of a solid phase, twogas phases and three water phases.

Refinements (such as the effect of damage onpermeability and nonlinearities due to temperature)

are included and chemical–physical phase changes areincorporated, such as hydration–dehydration,evaporation–condensation andadsorption–desorption. Phase changes in concrete are,for the first time, incorporated directly in the transportmechanism of any concrete model. Melting andevaporation of pp fibres are modelled as phasechanges. The complete behaviour of concrete in theelastic, inelastic and plastic ranges up to fracture isdescribed. Previous models were based on elasticfracture only. The physical model is described withemphasis being placed upon the real processesoccurring in concrete during heating, based on testscarried out in several major laboratories aroundEurope as part of the wider HITECO researchprogramme. A number of experimental and modellingadvances are presented in this work. The stress–strainbehaviour of concrete in direct tension (determinedexperimentally for the first time) is input into themodel. The hitherto unknown microstructural, hygraland mechanical behaviour of HPC/UHPC weredetermined experimentally and the information is alsobuilt into the model. It is also the first time that sucha complex model has been developed with the aim ofpredicting the behaviour of concrete at hightemperatures in general and explosive spalling inparticular. The fluid phase is considered to consist of



Table 3 Concrete damage in recent tunnel fires

Tunnel Concrete strength Maximumtemp.(3C)

Fireduration (h)

Length affected Segment depthaffected

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Great Belt(1994) [50]

76 MPa, 28 day 800 7 16 segment rings(1.65 m long)damaged in crown

Up to 68% spalled inlayers along 10 segments

Channel (1996)[47,48]

110 MPa, mature 1100 9 500 m with 50 mseverely affectedby spalling

Up to 100% of segmentthickness spalled showinggrout

Mont Blanc(1999) [49]

Not reported 1000 50 900 m; tunnel crownmost affected

Not reported

............................................................................................................................................................................................................................................................

Fig. 14. Profiles of temperature (shown by depth of shading) and gas pressure (shown by undulations) in an HPC concrete beam 20 mininto a fire[28]

water, dry air and vapour. The water phase consists offree capillary and physically bound water. Chemicallybound water is considered part of the solid skeletonuntil it is released on heating. The solid is fullydeformable and able to experience elastic, damage,thermal, creep, shrinkage, plastic, cracking strainprocesses. Both ‘basic’ and, importantly, ‘transient’creep phenomena are included. Outputs arepresented in the form of three-dimensional colourdiagrams of temperature, vapour pressure, watersaturation and damage distributions. An exampleoutput for a tunnel lining segment subjected to fire isshown in Fig. 14.

Applications

BUILDING FIRES

Engineers and scientists have devoted attention to firein buildings for decades, and there is an abundance ofbooks[44,45] and other literature on the subject. Thewebsite of the Building Research Establishment(brebookshop.com) currently has 198 publicationsrelated primarily to fire in buildings, most of themavailable from The Stationery Office or directly from

BRE. The review presented above is derived mainlyfrom fire concrete research related to buildings andhas, therefore, been adequately covered within thescope of this paper.

TUNNEL FIRES

There have been at least nine major fires in Europeantunnels since 1990[46–52]. Two road-tunnel fires in 1999(Mont Blanc, France/Italy and Tauern, Austria)claimed 51 lives, and in November 2000 a funicular-railway-tunnel fire in Kaprun, near Salzburg, Austriaclaimed 159 lives. Two earlier fires in the Great BeltTunnel (Denmark, 1994) and the Channel Tunnel(UK/France, 1996) did not claim any lives, butresulted in significant damage and spalling to thetunnel segments, made of high-performance concrete(Table 3). The maximum temperatures reached inthese fires ranged from 800 3C (Great Belt) to 1100 3C(Channel), although the Dutch report even highertemperatures of 1350 3C in their tunnel fires[10]. Theduration of the fires (9 h for the Channel Tunnel) weresignificantly longer than those encountered inbuildings (normally up to 2 h). This is at least partlydue to the difficulties the fire services encounter in



reaching and extinguishing tunnel fires. Also, thecloseness of the tunnel roof above the fire means thatthe thermal plume has less height in which it canentrain cooler air, thus resulting in highertemperatures than in buildings.

The form of the tunnel also affects its ability to resistfire. Rectangular tunnels of the cut-and-cover type,formed of reinforced concrete, can be categorized asstructures whose principal loading condition iscontrolled by bending considerations. Thus, a fire insuch a structure only has to cause spalling in the soffitand loss of intrados rebar to seriously undermine thestability of the structure. Circular tunnels behavedifferently from rectangular ones as their principalloading condition is hoop compression. In fire, thehoop loads increase, owing to restrained expansionnear the heated surface. This loading could contributeto increased risk of concrete spalling.

The damage to the concrete extended forconsiderable lengths (500 m in the Channel Tunneland 900 m in the Mont Blanc tunnel) owing to the factthat the fire spread from one goods vehicle to another.The maximum depth of concrete damage to thesegment linings reached 68% of the thickness (i.e.270 mm out of 400 mm) in the Great Belt tunnel to100% of the thickness in the Channel Tunnel, causedby multiple spalling of the high-performance concrete,particularly in the lightly reinforced regions[46]. Twofactors limited the spread of multiple spalling: thepresence of reinforcement, and evaporation ofmoisture from the heated concrete. Had the ChannelTunnel not been bored (deliberately) in theimpermeable chalk layer, the loss of the concrete liningwould have resulted in flooding of the tunnel, witha much more serious outcome. Firemen reported that,on entering the fire zone in the Channel Tunnel, theywere showered with small, hot pieces of concrete in anenvironment that resembled ‘hell’. The cost of repairafter the Channel Tunnel fire was @45 million[47].Damage to the concrete segments would have beenlargely avoided had it been protected by a layer ofthermal insulation. Nowadays, designers havea second weapon in their armoury to protect concretefrom spalling, and that it the use of pp fibres in themix.

Despite the problems outlined above, even the latestversions of design guides for tunnels do notadequately cover this issue. For example, there is nomention of the need for adequate fire protectionagainst explosive spalling in the 1999 version of theDesign Manual[53]. The manual emphasizes whenprotection is not to be used, rather than recommendswhen protection should be used. The two primerecommendations of concrete cover and the use ofadditional mesh reinforcement are not fully adequatemeasures to protect concrete in the event of a mediumlarge fire. The statement in the manual that concrete is‘inherently fire resisting’ is negated by thephenomenon of explosive spalling.

The optimum passive fire protection design fortunnels could involve the use of both pp fibres anda thermal barrier. Without the thermal barrier, onlythe problem of explosive spalling would be addressedby the fibres, and temperature levels within theconcrete would not be reduced. Thermal barriers areeffective in reducing: (a) the risk of explosive spalling;(b) deterioration of concrete mechanical properties;and (c) steel temperatures. In Switzerland, a concretesurface temperature limit is set at 250 3C by the use ofprotective panels[46]. However, thermal barriers aredearer than pp fibres, and a cost-effective design fora new tunnel could, therefore, optimize on the use ofthe thermal barrier to the areas that need mostprotection (identified from knowledge of the firescenario, and from thermal calculations) while theconcrete would incorporate the fibres. The use offibres is, however, not possible in existing tunnels, andthermal barriers are the only effective option for theprovision of passive fire protection, if deemednecessary. A cost, benefit and risk analysis wouldidentify the optimum passive fire protection design.Some tunnels are so lightly loaded, and employ suchrelatively low-strength concrete, that designers mayfeel that an increased concrete cover combined withthe use of the fibres would be an adequate protectionagainst fire.

Assessment after fireAn immediate and thorough appraisal is normallyrequired after a fire. Such an appraisal should begin assoon as the building can be entered, and generallybefore the removal of debris.

After a fire, an estimate is made of the severity oftemperature exposure in terms of an equivalentstandard test. A visual examination and classificationof damage for each structural member is carried out.The maximum concrete temperature profile duringa fire can be estimated from results of previous tests,from computer simulations, and from post-fireassessment of the concrete (e.g. through its colourchange or by a thermoluminescent technique)[54]. Keydiagrams and schedules are then prepared. Followingthis, a general assessment of the likely repairs requiredmay be drawn up. Normally, concrete exposed totemperatures above 300 3C is replaced if possible.Otherwise the dimensions are increased (e.g.reinforced columns), depending upon the design load.

The fire resistance of a concrete structure isfrequently well above minimum requirements.Because of the structural continuity present in mostbuildings, there are reserves of strength whichmay enable the structure to survive fires and bereinstated. Reinstatement by repair will usually beeconomically preferable to demolition and rebuildingin terms of capital expenditure and earlierreoccupation.



Conclusions

MECHANICAL PROPERTIES

The behaviour of concrete material in fire dependsvery much on the specific concrete mix proportionsand constituents used, and is determined by complexphysicochemical transformations during heating.However, all Portland-cement-based concretes losetheir load-bearing capacity at temperatures above550–600 3C. At lower temperatures (i.e. that of the bulkof the concrete member during fire) the deteriorationin mechanical properties during heating can bereduced by judicious concrete mix design wherebythermally stable aggregates of low thermal expansionare employed, and cement blends are selected thatproduce a low CaO/SiO2 ratio, but not too lowa permeability (also important to reduce the risk ofspalling). A key property unique to concrete amongststructural materials is the load-induced thermal strain(LITS), also called transient creep. Any structuralanalysis of heated concrete that ignores LITS will bewholly inappropriate and will yield erroneous results,particularly for columns exposed to fire. Normal-strength concretes and high-performance concretesmicrostructurally follow similar trends when heated,but ultra-high-performance concrete behavesdifferently.

EXPLOSIVE SPALLING

The risk of explosive spalling in fire is significantlyreduced in high-permeability concrete. In low-permeability concrete, spalling could be eliminated bythe appropriate inclusion of polypropylene fibres inthe mix (this requiring further research) and/or byprotecting the exposed concrete surface with a thermalbarrier. Until now, the prediction of spalling duringheating has been largely an imprecise empiricalexercise. Attempts to predict spalling by analyticalmethods have failed, owing to the complexmicrostructure and multiphase nature of heatedconcrete. The inability to predict the occurrence ofspalling has been a limiting factor in the developmentof robust models capable of predicting the response ofconcrete structures to fire. This prediction is nowbecoming possible with the development ofthermohydromechanical, nonlinear finite elementmodels capable of predicting pore pressures andhence spalling in heated concrete structures. Expertsdiffer as to the mechanisms responsible for explosivespalling. The balance of evidence suggests that it is thecombination of pore pressure spalling and thermalstress spalling, with pore pressure spalling being thedominant mechanism in both normal-strength andhigh-performance-concretes. However, thermal stressspalling may assume greater importance in ultra-high-performance concretes containing a high proportion ofexpansive silica. Further research is required to settlethis issue.

FIRE AND CONCRETE STRUCTURES

Failure of structural concrete in fire varies according tothe nature of the fire, the loading system and the typeof structure exposed to the fire. Failure could occurdue to loss of bending or tensile strength; loss of bondstrength; loss of shear or torsional strength; loss ofcompressive strength; and spalling of the concrete.The structural element should, therefore, be designedto fulfil its separating and/or load-bearing functionwithout failure for the required period of time ina given fire scenario. Design for fire resistance aims toensure overall dimensions of the section of an elementsufficient to keep the heat transfer through thiselement within acceptable limits, and an averageconcrete cover to the reinforcement sufficient to keepthe temperature of the reinforcement below criticalvalues long enough for the required fire resistanceperiod to be attained.

METHODS OF ASSESSMENT OF FIRE RESITANCE

The engineer has at his/her disposal three methods ofassessment of fire resistance: (a) fire testing;(b) prescriptive methods; and (c) performance-basedmethods. The first two have been established forseveral decades. Prescriptive methods are rigid andrestrictive, and do not allow for engineering thinking,nor can they be applied to whole structures. Althoughthe cheapest to implement of the three methods, theyare the least accurate. The safety level achieved canvary significantly. Such deficiencies have provided thedriving force for the development and wideracceptance of performance-based methods.Performance-based methods employ fire engineeringcalculations and provide a cost-effective and flexiblemethod of assessment of fire resistance that is superiorto prescriptive methods. Performance-based methodscan be classified into three categories of increasingsophistication and complexity: (a) simplifiedcalculations based on limit state analysis;(b) thermomechanical finite element analysis; and(c) comprehensive thermohydromechanical finiteelement analysis, also capable of predicting moisturemigration, pore pressures and explosive spalling. It isonly now that performance-based methods, firstintroduced in the UK, are being accepted in anincreasing number of countries.

TUNNEL FIRES

A spate of major tunnel fires in the past few years hasresulted in significant spalling of the concrete lining,up to 100% of its thickness. Based on experience in theNetherlands, the Dutch authorities proposed a firescenario, RWS (more severe than the normalhydrocarbon fire) with temperatures rapidly risingand peaking at 1350 3C (melting temperature ofconcrete) to simulate fire in tankers carrying petrol.However, the maximum temperatures attained inrecent major fires in the UK, France, Denmark and



Austrian tunnels did not reach RWS levels, but thedurations of those fires far exceeded anything in thestandards. Guidelines for tunnel design urgentlyrequire updating to incorporate the latestdevelopments in fire protection engineering,particularly against spalling.

Acknowledgements

The author wishes to express his appreciation toDr Yngve Anderberg, of Fire Safety Design, forkindly reading the manuscript, and for his usefulcomments.

References and recommended reading

[1] Lea F & Stradling R. The resistance to fire of concrete and reinforcedconcrete. Engineering. 1922: 114(2959).

[2] ACI Seminar. Concrete for nuclear reactors, Berlin 1970. SpecialPublication SP34, Farmington Hills, MI: American Concrete Institute.

[3] Building Regulations. Approved Document B 3: Internal fire spread(structure). The Building Regulations Approved Document B, Fire Safety, 1992.

[4] Anderberg Y. Fire engineering design of structures based on designguides. 2nd International Conference on Performance-Based Codes and Fire SafetyDesign Methods, Maui, USA, 7d9 May, 1998.

[5] British Standards Institution. BS 476, Fire tests on building materialsand structures. London: BSI. 1987.

[6] ISO 834. Fire resistance test d elements of building construction.International Standard 834, 1975-11-01.

[7] Eurocode 1: Basis of design and actions on structures, Part 2.2: Actions onstructures exposed to fire. EN 1991. Brussels, Belgium: European Committee forstandardization.

[8] Eurocode 2: Design of concrete structures, Part 1: General rulesd Structural fire design. EN 1992. This is currently being revised, see [9].

[9] Anderberg Y. Temperature Profiles. Annex A CEN/TC92/SC2/PT1-2Doc 58, Lund Sweden: Institute of Technology. 2000.

[10] Varley N & Both C. Fire protection of concrete linings in tunnels.Concrete May 1999: 27d30.

[11] Khoury GA, Sullivan PJE & Grainger BN. Strain of concrete duringfirst heating to 600 3C under load. Magazine of Concrete Research 1985: 37(133):195d215.

[12] Khoury GA & Algar S. Advanced mechanical characterisation of HPCand UHPC concretes at high temperatures. Proceedings of the InternationalWorkshop on Durable Concrete Structures, Weimar, October 1998.

[13] Khoury GA, Sullivan PJE & Dias WPS. Deformation of concrete andcement paste loaded at constant temperatures from 140 3C to 720 3C. Materials andStructures 1986: 19(110): 97d104.

[14] Khoury GA. Compressive strength of concrete at high temperatures:a reassessment. Magazine of Concrete Research 1992: 44(161): 291d309.

[15] British Standards Institution. BS 8110: Part 1:1985, Structural use ofconcrete. Code of practice for design and construction. Part 2:1985, Structural use ofconcrete. Code of practice for special circumstances. Section four, Fire resistance.London: BSI. 1985.

[16] Khoury GA, Algar S, Felicetti R & Gambarova P. Mechanicalbehaviour of HPC and UHPC concretes at high temperatures in compression andtension. ACI International Conference on State-of-the-art in High PerformanceConcrete, Chicago, March 1999.

[17] Shorter GW & Harmathy TZ. Moisture clog spalling. Proceedings oflnstitution of Civil Engineers 1965: 20: 75d90.

[18] Meyer-Ottens C. The question of spalling of concrete structural elementsof standard concrete under fire loading. PhD Thesis. Technical University ofBraunschweig, Germany. 1972.

[19] Akhtarruzaman AA & Sullivan PJ. Explosive spalling of concreteexposed to high temperature. Concrete Structures and Technology Research Report.London: Imperial College. 1970.

[20] Khoylou N. Modelling of moisture migration and spalling behaviour in non-uniformly heated concrete. PhD Thesis. University of London. 1997.

[21] Ichikawa Y. Prediction of pore pressures, heat and moisture transferleading to spalling of concrete during fire. PhD Thesis. University of London. 2000.

[22] Saito H. Explosive spalling of prestressed concrete in fire. OccasionalReport No. 22. Japan: Building Research Institute. 1965.

[23] Dougill JW. Modes of failure of concrete panels exposed to hightemperatures. Magazine of Concrete Research 1972; 24(79): 71d76.

[24] Zhukov VV. Explosive failure of concrete during a fire. Translation No. DT2124. Borehamwood, England: 1975. Joint Fire Research Organisation.

[25] Sertmehmetoglu Y. On a mechanism of spalling of concrete under fireconditions. PhD Thesis. King’s College, London. 1977.

[26] Connolly RJ. The spalling of concrete in fires. PhD Thesis. AstonUniversity. 1995.

[27] Bazant Z & Thonguthai W. Pore pressure in heated concrete wallsd theoretical prediction. Magazine of Concrete Research 1979: 31(107): 67d75.

[28] Khoury GA, Majorana C, Kalifa P, Aarup B, Giannuzzi M & CorsiF. The HITECO BRITE project on the effect of heat on high performance concrete.Proceedings of the International Workshop on Durable Concrete Structures,Weimar, Germany, October 1998.

[29] Bremer F. Multi-layer (double-wall) prestressed concrete pressure vessel.Nuclear Engineering and Design 1967: 5: 183d190.

[30] Thelanderson S. Mechanical behaviour of heated concrete undertorsional loading at transient high temperature conditions. Bulletin No. 46, Institute ofTechnology. Lund, Sweden: 1974.

[31] Jumpannen U-M. Effect of strength on fire behaviour of concrete. NordicConcrete Research Publication No. 8. 1989.

[32] Morris WA, Read REH & Cooke GME. Guidelines for the constructionof fire-resisting structural elements. Building Research Establishment Report BR 128.1988.

[33] Iding R, Bresler B & Nizamuddin Z. FIRES-T3, A computer programfor the fire response of structures-thermal. Report No. UCB FRG 77-15 Berkeley:University of California. 1977.

[34] Wickstrom U. TASEF-2, A computer program for temperature analysis ofstructures exposed to Fire. Lund, Sweden: Institute of Technology. 1979.

[35] Anderberg Y. TEMPCALC User Manual. Lund, Sweden: Institute of FireSafety Design (IFSD). 1986.

[36] Anderberg Y. Analytical fire engineering design of reinforced concretestructures based on real fire characteristics. Proceedings of the FIP Congress, London1978. Part 1: 112d121.

[37] Anderberg Y. Simplified cross-section calculation methods to be used forbending moments and axial forces. Annex B CEN/TC92/SC2/PT1-2 Doc 58 Lund,Sweden: Institute of Technology. 2000.

[38] Becker J & Bresler B. FIRES-RC, A computer program for the fireresponse of structures-reinforced concrete frames. Report No. UCB FRG 74-3Berkeley: University of California. 1974.

[39] Anderberg Y. Fire-exposed hyperstatic concrete structures. Anexperimental and theoretical study. Bulletin 55 Lund, Sweden: Institute of Technology.1976.

[40] Forsen NE. A theoretical study on the fire resistance of concretestructures. SINTEF Report No: STF65 A82062, Cement and Concrete ResearchInstitute, Norwegian Institute of Technology. 1982.

[41] Rudolph K, Richter E, Hass R & Quast U. Principles for calculationof load-bearing and deformation behaviour of composite structural elementsunder fire action. Proceedings of 1st International Symposium on Fire Safety Science,1985.

[42] Franssen JM. A study of the behaviour of composite steel-concretestructures in fire. PhD. Thesis, Liege University, Belgium. 1987. (in French).

[43] Terro M. Computer modelling of the effect of fire on structures. PhDThesis, University London. 1991.

[44] Anchor RD, Malhotra HL & Purkiss JA (eds). Design of structuresagainst fire. UK: Elsevier. 1986, p. 251.

[45] Shields TJ & Silcock GW. Buildings and Fire. London: Longman. 1987.[46] Lance GA. Effect of fire on tunnel lining stability. Tunnels and Tunnelling

International 1998: October, pp. 39}41.[47] Coates J. Chunnel: They have been told. Construction News No. 6494,

5 December 1996: 1.[48] Arches brace Chunnel for six month repair schedule. New Civil Engineer,

28 Nov 1996: 3d4.[49] French Minister of the Interior, Equipment, Transportation and

Housing. Task force for technical investigation of the 24 March 1999 fire in the MontBlanc vehicular tunnel, Report of 30 June 1999.

[50] Tait JC & Hoj NP. Storebaelt eastern railway tunnel: Dania tunnel boringmachine fire d analysis and recovery. Proceedings of the Institution of Civil Engineers1996: Supplement, 114 Special Issue 1: pp. 40d48.



Gabriel Alexander Khoury Bsc MSc PhD DIC Eurlng CEngMIStructE MINucE MIFE MRAeSCivil Engineering Department,Imperial College, London SW7 2BU, UKE-mail: [email protected]

[51] PIARC Committee on Road Tunnels (C5). Fire and smoke control inroad tunnels. PIARC World Road Association, France, 1999. (ISBN 2-84060-064-1).

[52] Proceedings of the International conference on fires in tunnels,Boras, Sweden, October 1994, Swedish National Testing and Research Institute FireTechnology, SP Report 1994: 54.

[53] The Highways Agency (and other UK organizations). Design Manualfor Roads and Bridges: Design of Road Tunnels. Document BD 78/99, London: TheStationery Office. 1999.

[54] Placido F. Thermoluminescence test for fire-damaged concrete. Magazineof Concrete Research 1980: 32(11): 112d116.



Date post:	24-Oct-2014
Category:	Documents
Upload:	jpl
View:	176 times
Download:	4 times

Effect of Fire on Concrete and Concrete Structures

Documents