QianFaculty

h

Factors affecting thermally mechanical properties of concrete were reviewed.

a r t i c l e i n f o

Article history:

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2.4.5. Aggregates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3742.5. Spalling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374

3. Factors influencing the performance of concrete subjected to high temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3753.1. w/b and moisture content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3753.2. Type of aggregate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3753.3. SCMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377

Corresponding author. Tel.: +86 13095358933.E-mail address: maqianmin666@163.com (Q. Ma).

Construction and Building Materials 93 (2015) 371383

Contents lists available at ScienceDirect2.4.4. Microstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3732.4.1. Water evaporation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.4.2. Hydration products. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2.4.3. Pore structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .http://dx.doi.org/10.1016/j.conbuildmat.2015.05.1310950-0618/ 2015 Elsevier Ltd. All rights reserved.. . 373

. . 373

. . 373Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3722. Mechanical properties of concrete at high temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372

2.1. Compressive strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3722.2. Flexural strength, splitting tensile strength and modulus of elasticity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3722.3. Stressstrain relationship . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3722.4. Physical and chemical changes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373Received 9 October 2014Received in revised form 11 May 2015Accepted 14 May 2015

Keywords:ConcreteHigh temperatureMechanical propertiesa b s t r a c t

High temperature is well known for seriously damaging concrete micro- and meso-structure, whichbrings in a generalised mechanical decay of the concrete and even detrimental effects at the structurallevel, due to concrete spalling and bar exposure to the ames, in case of re. Because of the relevanceof concrete behaviour at high temperature and in re, many studies have been carried out, even veryrecently, on cementitious composites at high temperature, and the most relevant parameters have beenidentied and investigated. Within this framework, the authors provide a comprehensive and updatedreport on the temperature dependency of such parameters as the compressive strength, modulus of elas-ticity, strength in indirect tension (bending and splitting tests), stressstrain curves and spalling, but theroles played by the waterbinder ratio (w/b), aggregate type, supplementary cementitious materials(SCMs) and bres are investigated as well. Among the objectives of the paper, the approaches currentlyadopted to improve concrete mechanical properties at high temperature are treated as well. Meanwhile,the inuence of test modalities on the mechanical properties of concrete at high temperature is also dis-cussed in the paper.

2015 Elsevier Ltd. All rights reserved.Mechanical properties of concrete at high temperature were reviewed. Physical and chemical changes of concrete at high temperature were reviewed.i g h l i g h t smin Ma , Rongxin Guo, Zhiman Zhao, Zhiwei Lin, Kecheng Heof Civil Engineering and Mechanics, Kunming University of Science and Technology, 727, Jingming South Road, 650500 Kunming, ChinaReview

Mechanical properties of concrete at high temperatureA reviewjournal homepage: www.elsevier .com/locate /conbui ldmatConstruction and Building Materials

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378e at high temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381

stenate tin Fig.specimcs

(1)

Residual exural strength, residual splitting tensile strengthand residual modulus of elasticity of concrete after exposure to ele-vated temperatures are shown in Figs. 24, respectively. Same datacollection regime with compressive strength is used. Similar to thecompressive strength reviewed in the previous section, exuralstrength, splitting tensile strength andmodulus of elasticity of con-crete decreases with the increase of temperature, but at a nearlylinear rate.

2.3. Stressstrain relationship

Stressstrain relationship of concrete at elevated temperatureshas been investigated by many researchers [2,12,30,37,48,6473].It has been found that with the increase of temperature, stress

1.4

1.6

1

1.2

f,20

ilding Materials 93 (2015) 371383: 0 200 400 600 800 1000 1200Temperature (oncrete after heating to high temperature experiences three maintages

0he possible effect caused by these factors, the data collection1 is carried out only on the residual results of unstressed cubeens. It can be seen that the residual compressive strength of

0.2

0.4tates, also inuence the mechanical properties of concrete at highmperature (details are in Section 4). Therefore, in order to elimi-

0.6

f f,T

/ fas specimen size, stressed/unstressed conditions and hot/residualstrength of concrete when it is exposed to high temperature (seeFig. 1). In spite of concretemixtureproportions, testmodalities, such 0.83.4. Fibres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4. Influence of test modalities on the mechanical properties of concret

4.1. Hot and residual tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4.2. Stressed and unstressed tests . . . . . . . . . . . . . . . . . . . . . . . . . .4.3. Uni-axial and multi-axial tests . . . . . . . . . . . . . . . . . . . . . . . . .4.4. Specimen size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction

Under the pressure of population boom and land limitation, inorder to effectively resolve housing and transportation issues, theneed for high-rise buildings and underground construction is rapidincreasing. Such civil engineering is facing tremendous challengeof re damage during its constructing and service. Fire on theseengineering is frequently reported worldwide in recent years, seri-ously threatening personal and property safety. High temperatureis well known for seriously damaging concrete micro- andmeso-structure, which brings in a generalised mechanical decayof the concrete and even detrimental effects at the structural level,due to concrete spalling and bar exposure to the ames, in case ofre. Because of the relevance of concrete behaviour at high tem-perature and in re, many studies have been carried out, even veryrecently, on cementitious composites at high temperature, and themost relevant parameters have been identied and investigated.Within this framework, the authors provide a comprehensive andupdated report on the temperature dependency of such parame-ters as the compressive strength, modulus of elasticity, strengthin indirect tension (bending and splitting tests), stressstraincurves and spalling, but the roles played by the w/b, aggregatetype, SCMs and bres are investigated as well. Among the objec-tives of the paper, the approaches currently adopted to improveconcrete mechanical properties at high temperature are treatedas well. Meanwhile, the inuence of test modalities on themechanical properties of concrete at high temperature is also dis-cussed in the paper. Electrical furnace heating and gas/oil heating(re), these two different heating models, are used in the studiesto investigate the thermal behaviour of concrete at high tempera-ture. Furnace heating is usually used for the studies on the thermalchanges of concrete characteristics, while re is usually consideredwhen the studies are at a structurally elemental level. This papermainly focuses on the discussion on the thermal changes of con-crete characteristics at high temperature, the effect of re on thebehaviour of concrete is exclusive in this paper.

2. Mechanical properties of concrete at high temperature

2.1. Compressive strength

It is unavoidable that there is a reduction for compressive

372 Q. Ma et al. / Construction and BuRoom temperature300 C, compressive strength of con-crete keeps constant or even increases slightly.(2) 300800 C, compressive strength of concrete decreasesdramatically.

(3) 800 C afterwards, almost all the compressive strength ofconcrete has been lost.

2.2. Flexural strength, splitting tensile strength and modulus ofelasticity

0

0.2

0.4

0.6

0.8

1

1.2

0 200 400 600 800 1000 1200f cu

/f cu,

20Temperature ( C)

Fig. 1. Residual compressive strength of concrete at elevated temperatures (dataadapted from [146]).Fig. 2. Residual exural strength of concrete at elevated temperatures (dataadapted from [26,42,4755]).

1.2

ildin0.6

0.8

1

f t,T

/ ft,2

01.4

Q. Ma et al. / Construction and Bustrain curves become atter, and the peak stress shifts downwardsand rightwards, as shown in Fig. 5. These indicate that the peakstress and the modulus of elasticity of concrete decrease with theincrease of temperature, but the strain at peak stress increaseswith temperature.

2.4. Physical and chemical changes

With the elevation of temperature, concrete would experiencethe following physical and chemical changes and these changes

0

0.2

0.4

0 200 400 600 800 1000 1200Temperature (

Fig. 3. Residual splitting tensile strength of concrete at elevated temperatures (dataadapted from [9,52,54,5663]).

0

0.2

0.4

0.6

0.8

1

1.2

0 200 400 600 800 1000

E T/ E

20

Temperature (

Fig. 4. Residual modulus of elasticity of concrete at elevated temperatures (dataadapted from [1,6,10,13,17,18,37,38,48,58,59,63]).

Fig. 5. Residual stressstrain relationship of concrete at elevated temperatures[68].are considered to be responsible for the changes of the mechanicalproperties:

2.4.1. Water evaporationHydration products lose their free water and physically

absorbed water completely, and start to lose their chemicallybonded water at 105 C [74]. Capillary water is lost completely at400 C [75].

Up to 300 C, hydration of unhydrated cement grains isimproved due to an internal autoclaving condition as a result ofthe high temperature and the evaporation of water [76]. This isparticularly true for high strength concrete as its low permeabilityresists moisture ow. This can be used to explain the constantcompressive strength when the temperature is below 300 C asdiscussed in Section 2.1.

2.4.2. Hydration productsAFt/AFm dehydrates at 110150 C [77]. Above 350 C, calcium

hydroxide either decomposes into lime and water or further con-verts into CSH due to the accelerated pozzolanic reaction at ahigh temperature [7880]. The decomposition of Ca(OH)2 has nocritical inuence on the reduction of strength for concrete.However, if concrete is water cooled after exposure to high tem-perature, the rehydration of lime will cause a great reduction ofstrength for concrete due to a considerable expansion will becaused due to such a reaction [81]. CSH starts to decompose ataround 560 C [79] and it decomposes into b-C2S at around 600700 C [77,79]. CSH (I) decomposes at 800 C, which, however,only results in a slight reduction of strength for concrete [81].During 580900 C, decarbonation of carbonates occurs[64,78,8183].

2.4.3. Pore structureAs a result of the water evaporation and the chemical changes

of hydration products, elevation of temperature increases porosityand pore size of cement and concrete [11,21,23,64,75,76,78,8391]. The coarsening of the pore structure is mainly responsible forthe reduction of the mechanical properties as discussed in the pre-vious sections.

2.4.4. MicrostructureUp to 200 C, no micro-cracks are observed in either hardened

cement matrix or interfacial transition zone (ITZ) [81,92]. Whenthe temperature rises to 400 C, micro-cracks in cement matrixand ITZ start to propagate and their intensity increases with tem-perature [3,21,23,26,28,9399].

It is considered that the different thermal strains for hardenedcement matrix and aggregates have resulted in the developmentof the micro-cracks at high temperature. From Fig. 6 it can be seenthat with the increase of temperature, the hardened cement matrixexpands rst and then shrinks as a result of the loss of water, whileaggregates keep expansion during the whole heating. Similarresults have also been found by Fu et al. [100]. Such differentstrains will produce a stress between cement matrix and aggre-gates, causing micro-cracks in the ITZ. This is also responsible forthe reduction of the mechanical properties of concrete at hightemperatures.

When temperature is very high, such as above 1000 C, porosityand microstructure of concrete are smaller and better than those ata lower temperature due to concrete has been sintered at such ahigh temperature [83,85]. However, it does not indicate that themechanical properties of concrete at the very high temperaturewas better than those at a lower temperature as the relationship

g Materials 93 (2015) 371383 373between mechanical properties and pore structure is not trueany further due to the syntherization has changed the characteris-tic of concrete material [85].

Distance from heat

Temperature

Pore pressure

Distance from heat

Temperature

Pore pressure

Temperature

Pore pressureTemperature

Pore pressure

ildinFig. 6. Thermal strains of cement matrix and aggregates [102].374 Q. Ma et al. / Construction and Bu2.4.5. AggregatesAt around 573 C, siliceous aggregates transform from a-phase

to b-phase causing expansion of concrete [81,83]. Disintegrationof calcareous aggregates, such as limestone, occurs at a tempera-ture above 600 C [101].

2.5. Spalling

Spalling may occur for concrete at high temperature, which willgreatly reduce mechanical properties of concrete structure andeven cause collapse of the structure [103]. The mechanisms of spal-ling of concrete at high temperature could be mainly explainedfrom vapour pressure in pores and thermal stresses these twoaspects [103].

Hardened concrete is saturated with water in its pores at differ-ent extents. The moisture content in concrete is dependent on w/b,age of concrete and environment. When concrete surface is sub-jected to sufciently high temperature, a portion of water will bevaporised and move out from concrete into atmosphere. There isalso certain amount of water will be vaporised and move oppositeto the inner part of concrete. Due to thermal gradient, the innerpart of concrete is cooler and the vapour there will be condensed.With the accumulation of the condensed water, a saturated layer isgradually formed. This layer will resist the further movement ofvapour into the inner of concrete, but move towards the dry regionof the concrete surface with an attempt to escape out of concreteinto atmosphere. If the pore structure of the concrete is sufcientlydense and/or the heating rate is sufciently high, the escape of thevapour layer would be not fast enough, resulting in a large increaseof pore pressure in the concrete. If the tensile stress of concretecould not resist the pore pressure, spalling of concrete would occur[104]. Fig. 7 illustrates the whole process of the thermal spalling ofconcrete as a result of the pore vapour pressure.g Materials 93 (2015) 371383Fig. 8 shows the maximum pore pressures of concrete at hightemperature. From Fig. 8 it can be seen that the maximum porepressure is generally observed in the inner part of concrete.Compared to the inner part, vapour in the outer part is easier toescape out from concrete. This would reduce the pore pressure inconcrete at the near surface zone. Furthermore, the maximum porepressure in high strength concrete is generally larger than that inthe normal strength concrete [105108]. The high strength of con-crete is usually achieved by densifying its pore structure to lowerits permeability. Due to the low permeability, when the high

Distance from heat Distance from heat

Fig. 7. Spalling of concrete induced by pore vapour pressure [104].

0

1

2

3

4

5

6

0 10 20 30 40 50 60

Max

imum

por

e pr

essu

re (M

Pa)

Distance from the heated surface (mm)

Ref. [106]Ref. [107]Ref. [108]

Fig. 8. Pore pressure in concrete at high temperature (radiant heating to 600 C).

ildinstrength concrete is exposed to high temperature, the vapour gen-erated is not easy to escape out from the concrete, therefore result-ing in the larger maximum pore pressure. Fig. 7 also simulates the

Temperature

Tensile stress

Compressive stress

Compressive stress

Fig. 9. Spalling of concrete induced by thermal stresses.Tensile stress

Q. Ma et al. / Construction and Budevelopment of pore pressure in the concrete at high temperature,and which is corresponded to the steps of the pore vapour pressureinduced spalling of concrete.

Simultaneously, thermal gradient will also be formed betweenthe heated surface and the inner part of concrete when the con-crete is subjected to high temperature. This is particularly truewhen temperature increases very fast, which is always named asthermal shock. With temperature increases faster at the surfaceof concrete, compressive stress is generated parallel to the heatedconcrete surface, while tensile stress is generated in the inner con-crete in a perpendicular direction. When the compressive stressexceeds the tensile stress, spalling of concrete occurs [109], asshown in Fig. 9.

Both the above two causes would result in cracking of concreteat high temperature. Besides, the cracking of concrete at high tem-perature would also be caused by the decomposition of hydrationproduct, shrinkage of cement matrix and expansion of aggregates.The different thermal response between cement matrix and aggre-gates is also considered to distribute cracks in the ITZ between thetwo phases, damaging concrete meso-structure. Finally, all thecauses mentioned above make the spalling of concrete at high tem-perature to occur in the models of aggregate spalling, surface spal-ling, corner spalling and explosive spalling [103].

3. Factors inuencing the performance of concrete subjected tohigh temperature

3.1. w/b and moisture content

The study carried out by Chan et al. [7] has illustrated that up tothe temperature of 1000 C, the compressive strength loss of thehigh w/b concrete (w/b = 0.6) was higher than that of the loww/b concrete (w/b = 0.28, 0.35). Phan et al. [10] found that com-pared to the concrete with w/b of 0.22, the losses of both compres-sive strength and modulus of elasticity were higher for theconcrete with w/b of 0.57. Similar results have been found for con-crete containing slag [86,110], y ash [86,111] and metakaolin[111] when w/b ranged from 0.3 to 0.5 [86,111] and from 0.23 to0.71 [110]. Lightweight concrete also gave similar results when dif-ferent w/b of 0.43 and 0.46 was studied [27].

However, a lower w/b is prone to cause spalling of concrete athigh temperature. As reported by Phan et al. [10], spalling occurredfor the concrete with w/b of 0.22 when temperature was elevatedto 450 C, while the concrete with w/b of 0.33 was still intact at thesame temperature. As discussed in the previous section, spallingoccurs when pore vapour pressure in concrete accumulates to acertain extent. It is considered that such an accumulation wouldbecome faster when the pore structure is denser, which could becaused by using a lower w/b. That is why spalling of concrete iseasy to occur at high temperature when a lower w/b is used.Despite of w/b at the beginning of concrete mixing, spalling is alsomuch dependent on the moisture content of concrete at the time ofits exposure to high temperature. Fig. 10 gives an example of spal-ling of concrete at different moisture contents. It is clear to see thatthe possibility and the extent of spalling increase with moisturecontent of concrete as a result of the increased pore vapourpressure.

3.2. Type of aggregate

Effects of type of aggregate on compressive strength, exuralstrength, splitting tensile strength andmodulus of elasticity of con-crete at high temperatures are presented in Figs. 1114, respec-tively. The scatter from data to regression line may be caused bydifferent mixes and different test modalities. Generally speaking,the concretes made of siliceous aggregates, such as granite, expressunfavourable mechanical properties at high temperature com-pared to the concretes manufactured by using dolomite and lime-stone these calcareous aggregates. Furthermore, Cheng et al. [16]also found that the increase in strains for the concrete made of cal-careous aggregates was larger than that for the siliceous aggregatesconcrete. It is also found that spalling occurs at a higher tempera-ture and a later time for limestone concrete [112]. As stated inSection 2.4, calcareous aggregates decompose at a higher temper-ature than siliceous aggregates. This could be used to explain thebetter performance of the concrete with calcareous aggregates athigh temperature.

Lightweight aggregates, such as expanded clay, pumice and cer-amsite, are formed by volcano eruption or incineration. As a result,they have low heat conductivity and exhibit a high resistance toheat. Therefore, the concrete manufactured by using such aggre-gates should deliver improved mechanical properties at high tem-perature in comparison to normal aggregates concrete. Sun et al.[113] used high alumina cement to manufacture normal refractoryconcrete (normal aggregates), ceramsite refractory concrete I (cer-amsite as coarse aggregates), ceramsite refractory concrete II (cer-amsite as coarse and ne aggregates) and refractory brick concrete(broken refractory brick as coarse aggregates). The concrete speci-mens were heated to 1000 C. After the heating, ceramiste refrac-tory concretes I and II still had 3350% compressive strengthremained, which was much higher than that of normal refractoryconcrete of 17%. In the studies carried out by both Sancak et al.[27] and Tanyildizi and Coskun [29], pumice was used as coarseaggregates to manufacture lightweight concretes. The lightweightconcrete specimens had 2838% compressive strength remained

g Materials 93 (2015) 371383 375after exposure to 800 C, which was higher than the value of 1316% for normal reference concrete. In addition, the lightweightconcrete specimens still had 18% splitting tensile strength

ildin376 Q. Ma et al. / Construction and Buremained [29]. Cao et al. [114] compared the residual compressivestrength among lightweight concrete I (ceramiste as coarse aggre-gates), lightweight concrete II (ceramiste as both coarse and neaggregates) and normal concrete at high temperature. The resultsshowed that the normal concrete specimens had lost all the

Fig. 10. Relationship between moisture conte

00.20.40.60.8

11.21.41.61.8

0 200 400 600 800 1000 1200 1400

f cu,T

/f cu,

20

Temperature (C)dolomite limestone granitegravel basalt regression for dolomiteregression for limestone regression for granite regression for gravelregression for basalt

Fig. 11. Inuence of type of aggregate on residual compressive strength of concretesubjected to elevated temperatures (data for dolomite was adapted from[41,66,96]; data for limestone was adapted from [2,17,20,23,24,32,42,57,60,98];data for granite was adapted from [7,11,18,21,25,30,31,35,46,48,54,86]; data forgravel was adapted from [5,8,9,13,19,22,33,34,44,45,73,98]; data for basalt wasadapted from [15,49,97]).g Materials 93 (2015) 371383compressive strength at temperature of 1000 C, whilst 20.5%and 21% of the compressive strength was left for lightweight con-crete I and II, respectively. Turkmen and Findik [115] usedexpanded clay to replace natural sand at a replacement of 25% toproduce lightweight mortar. Such mortar still had 38% of compres-sive strength and 23% of exural strength remained after exposure

nt and possibility and extent of spalling.

0

0.2

0.4

0.6

0.8

1

1.2

0 200 400 600 800 1000 1200

f f,T/f f

,20

dolomite limestonegranite gravelbasalt regression for dolomiteregression for limestone regression for graniteregression for gravel regression for basalt

Temperature (C)

Fig. 12. Inuence of type of aggregate on residual exural strength of concretesubjected to elevated temperatures (data for dolomite was adapted from [53]; datafor limestone was adapted from [24,42,50]; data for granite was adapted from[21,54]; data for gravel was adapted from [19]; data for basalt was adapted from[47,49]).

ildin0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 200 400 600 800 1000 1200 1400

f t,T/

f t,20

limestone granitegravel basaltregression for limestone regression for graniteregression for gravel regression for basalt

Temperature (C)

Fig. 13. Inuence of type of aggregate on residual splitting tensile strength ofconcrete subjected to elevated temperatures (data for limestone was adapted from[45,56,57,60,61]; data for granite was adapted from [7,30,54]; data for gravel wasadapted from [8,58]; data for basalt was adapted from [15,49]).

1.2

Q. Ma et al. / Construction and Buto 800 C. In the study carried out by Jiang et al. [116], compared tonormal concrete which had 10% of the compressive strengthremained at the temperature of 1000 C, the value was 20% forlightweight concrete manufactured by using ceramiste. BothJiang et al. [117] and Wang et al. [118] used industrial sewagesludge ceramsite to manufacture lightweight concrete. After theexposure to 800 C, 46.9% of compressive strength and 40% of split-ting tensile strength remained for the lightweight concrete [117].In addition, 20.2% of initial modulus of elasticity and 18.4% of peakdeformation modulus remained for the lightweight concrete,which was higher than the normal reference concrete [118].

The study carried out by Jiang et al. [116] points out that hightemperature induced spalling did not occur when moisture contentin normal concrete was below 75%. However, for lightweight con-crete, when its moisture content was above 25%, spalling occurredat high temperature. This indicates that spalling of lightweightconcrete at high temperature is much more sensitive than normalconcrete to moisture content. It is known that the porosity of light-weight aggregate is much higher than that of normal aggregate,and so is the water absorption consequently. Therefore, in practice,in order to minimise the water absorption of lightweight aggre-gates and its effect on fresh concrete workability and subsequentsetting and hardening, lightweight aggregate is usuallypre-saturated before being used to mix concrete. However, suchtreatment will bring extra water into lightweight concrete to

0

0.2

0.4

0.6

0.8

1

0 200 400 600 800 1000 1200

MT/

M20

dolomite limestonegranite gravelregression for dolomite regression for limestoneregression for granite regression ofr gravel

Temperature (C)

Fig. 14. Inuence of type of aggregate on residual modulus of elasticity of concretesubjected to elevated temperatures (data for dolomite was adapted from [41]; datafor limestone was adapted from [16,17]; data for granite was adapted from[16,18,21,48]; data for gravel was adapted from [8,9,73]).increase its moisture content, and then increasing the possibilityof spalling for lightweight concrete at high temperature. This willextremely limit the super resistance of lightweight aggregate toheat. In literatures [27,29,115], the authors did not follow the prac-tical process to pre-saturate the lightweight aggregates. In litera-tures [113,114,117,118], the authors dried the lightweightconcrete specimens at 100 C before exposing them to high tem-perature, which minimised the possible spalling at a large extent.Therefore, from above it can be seen that further studies areneeded to investigate the effect of high temperature on lightweightconcretes in a condition similar to the practice. In such case, anovel pre-treatment should be applied to lightweight aggregatesto reduce the possibility of spalling of lightweight concrete, andthen to allow the super resistance of lightweight aggregates to heatto serve well.

3.3. SCMs

Table 1 summarises the literatures on the effect of SCMs on theresidual mechanical properties of concrete at high temperatures,including compressive strength, splitting tensile strength, exuralstrength and modulus of elasticity.

The incorporation of pulverised y ash (PFA) and slag in PC cangenerally remain the mechanical properties of concrete at a higherlevel after heating to high temperature up to 900 C and 1050 C,respectively. Compared to PC, the residual compressive strength,splitting tensile strength, exural strength and modulus of elastic-ity of PC blended with PFA increase by 1.2270%, 1.180%, 4.5200% and 338%, respectively. The values for PC blended with slagare 1.5510%, 1.243%, 1180% and 1.3117%, respectively. Thevalues vary mainly with different temperatures, replacementsand types of aggregates. In the research carried out by Wang[110], PC paste had lost its compressive strength and modulus ofelasticity completely at the temperature of 1050 C. However,18% of the compressive strength and 81% of the modulus of elastic-ity were still remained for PC blended slag paste with the replace-ment of 80% at the same temperature. Furthermore, PCs blendedwith PFA and slag also exhibit a high resistance to spalling at hightemperatures [86,91,124,122].

Aydin and Baradan [97] and Aydin [123] detected the formationof gehlenite in the PC samples incorporated PFA and slag at thetemperature of 900 C by using XRD analysis. Such phase may llin the pores caused by the high temperature. Therefore, the cementmatrix could be rened and the ITZ between cement matrix andaggregate could be enhanced so that the values of the mechanicalproperties for PCs blended with PFA and slag retain at a higherlevel. Furthermore, Karakurt and Topcu [120] found that thermalcracking did not occur in PFA and slag blending samples and thatthe degradation of CSH decreased compared to PC sample byusing SEM analysis. Moreover, the incorporation of slag signi-cantly reduces the amount of portlandite in PC so that decreasingthe degradation of portlandite at high temperatures [124,125]. Asa result of the above three aspects, the total porosity and the aver-age pore diameter of PCs blended PFA and slag are smaller thanthose of PC at high temperatures [86]. This could explain the higherresistance of PCs blended PFA and slag to high temperature.

On the other hand, the incorporation of silica fume (SF) appar-ently reduces the resistance of PC to high temperatures. Comparedto PC, the residual compressive strength, splitting tensile strength,exural strength and modulus of elasticity of PC blended SF at hightemperatures decrease by 1100%, 212%, 225% and 27%,respectively. The values also vary mainly with different tempera-tures, replacements and types of aggregates. Furthermore, severe

g Materials 93 (2015) 371383 377spalling was detected for PC blended SF in several studies[10,86]. Behnood and Ziari [128] explained that due to the llereffect and pozzolanic reactions provided by SF, cement matrix

cal

; ff,

ildinTable 1Summary of the researches carried out on the effect of SCMs on the residual mechani

Refs. Type of specimen Replacement (%)

PFA [11] Concrete with granite 0, 25, 55[17] Concrete with limestone 0, 10, 30[18] Concrete 0, 30[80] Mortar 0, 25, 35, 45[86] Concrete with granite 0, 20, 30, 40[97] Pumice mortar 0, 20, 40, 60[119] Lightweight concrete 0, 10, 20, 30[120] Concrete with limestone 0, 30[121] Mortar 0, 5, 10, 15, 20[122] Concrete with granite 0, 25, 55

Slag [38] Concrete 0, 10, 30, 50[63] Concrete 0, 20, 40, 60[86] Concrete with granite 0, 30, 40[110] Paste 0, 5, 10, 20, 50, 80[120] Concrete with limestone 0, 30[123] Pumice mortar 0, 20, 40, 60, 80[124] Paste 0, 35, 50, 65[125] Mortar 0, 20, 50, 80[63] Concrete 0, 20, 40, 60[126] Concrete with limestone 0, 30, 40, 50[127] Concrete 0, 30, 40, 50

SF [10] Concrete with limestone 0, 10[27] Lightweight concrete 0, 5, 10[32] Concrete with limestone 0, 10[61] Concrete with limestone 0, 10[80] Mortar 0, 2.5, 5, 7.5[86] Concrete with granite 0, 5, 10[91] Paste 0, 5, 10, 15, 20[121] Mortar 0, 5, 10, 15, 20[128] Concrete with limestone 0, 6, 10

Note: fcu, compressive strength; E, modulus of elasticity; ft, splitting tensile strength

378 Q. Ma et al. / Construction and Buand ITZ of PC blended with SF would be much denser than those ofPC. This, however, could restrain the expansion of aggregates whensubjecting to high temperatures and then reduce the mechanicalproperties noticeably. Poon et al. [86] also found that the totalporosity and the average pore diameter of PC with 10% SF weremuch larger than those of PC at the temperature of 800 C. Thiscould be the result of the restraint effect mentioned above and con-sequently inuence the retaining of the mechanical properties ofPC blended with SF at high temperature.

3.4. Fibres

A number of studies have been carried out on the effect of breon the mechanical properties of concrete after exposure to hightemperatures, and a summary is presented in Table 2.Polypropylene and steel bres are usually used in these studies.

Polypropylene bre generally has no signicant inuence onthe improvements of residual compressive strength and residualmodulus of elasticity for concrete after heating to high tempera-ture. However, such improvement is clearer to a certain extentwhen residual exural strength and residual splitting tensilestrength are considered. This is particularly at the temperaturebelow 400 C. Polypropylene bre can increase the resistance ofconcrete to cracking, improving its behaviour under tension.However, the melting and ignition points of polypropylene breare around 150 C and 400500 C, respectively. That is why theimprovement of residual exural and residual splitting tensilestrengths of polypropylene bre reinforced concrete reduces whenthe temperature is above 400 C due to the bre has been meltedup at such high temperature and the pores left are disadvantagefor the performance of concrete under tension [49,51,58,129].However, also due to the melting and ignition of polypropylenewhich is randomly distributed in concrete, at a relatively lowproperties of concrete.

Test temperatures (C) Mechanical properties tested

20, 250, 450, 650, 800 fcu20, 100, 300, 600, 750 fcu, E20, 100, 200, 400, 600 fcu, E20, 400, 700 fcu, ff20, 200, 400, 600, 800 fcu20, 300, 600, 900 fcu, ff20, 200, 400, 800 fcu, ft20, 100, 300, 450, 600 fcu20, 150, 300, 450, 600, 750 fcu20, 200, 400, 600, 800 ft

20, 150, 300, 400, 500, 600, 700 fcu, E20, 100, 200, 350 fcu, ft, E20, 200, 400, 600, 800 fcu20, 105, 200, 440, 580, 800, 1050 fcu, E20, 100, 300, 450, 600 fcu20, 300, 600, 900 fcu, ff20, 100, 200, 300, 400, 500, 600, 700, 800 fcu20, 150, 300, 600, 900 fcu, ft20, 100, 200, 350 fcu20, 400 ft20, 400 ff

20, 100, 200, 300, 450 fcu, E20, 100, 400, 800, 1000 fcu20, 100, 200, 300, 600 fcu20, 100, 200, 300, 600 ft20, 400, 700 fcu, ff20, 200, 400, 600, 800 fcu20, 250, 450, 600 fcu20, 150, 300, 450, 600, 750 fcu20, 100, 200, 300, 600 fcu

exural strength.

g Materials 93 (2015) 371383temperature, the left pores radiate out to form microcracks, con-necting the existing capillary pores to provide channels for theescaping of water vapour. Consequently, it is found that thepolypropylene bre reinforced concrete has much better resistanceto thermal spalling compared to the concrete without bre[47,52,60,130133]. This is particularly true for high performanceconcrete as water vapour is more difcult to escape in a densermatrix. An optimum dosage of polypropylene bre around 0.10.5% by volume of mix is recommended for concrete to obtain aproper high temperature resistance [134136], and it is found thatthe resistance of polypropylene bre reinforced concrete to hightemperature increases with the increase of the length of the bre[131].

The addition of steel bre can generally improve the residualmechanical properties of concrete at high temperature when com-pressive strength, exural strength and splitting tensile strengthare considered. The improvement in the residual modulus of elas-ticity is not clearly observed. The reason for such improvementscould be attributed to the fact that the testing temperatures arenot high enough to allow steel bre to be melted so that its ductil-ity could effectively contribute to concrete resisting the failureunder tension during the whole test period. Furthermore, steelbre has higher thermal conductivity than cement matrix andaggregates. Consequently, heat can transmit more uniformly inthe concrete reinforced with steel bre to reduce the cracks causedby thermal gradient in concrete, improving the performance ofconcrete under both compression and tension [55,57,136]. Alsodue to the reduced thermal gradient, the steel bre reinforced con-crete shows resistance to thermal spalling [49,137]. However, theresistance to spalling provided by steel bre is weaker than thatprovided by polypropylene bre, which may indicate that watervapour is the primary reason to cause spalling of concrete at hightemperature [57].

ical

e)

; ff,

ildinTable 2Summary of the researches carried out on the effect of bres on the residual mechan

Refs. Dimension of bre Replacement (% by volum

PP bre [43] L: 19 mm; D: 45 lm 0, 0.05, 0.1, 0.15[45] L: 12 mm; D: 18 lm 0, 0.3[47] N/A 0, 0.15, 0.2[48] L: 19 mm 0, 0.1[51] L: 19 mm; D: 35 lm 0, 0.1[53] L: 15 mm; D: 100 lm 0, 0.6[55] L: 6 mm, 30 mm; D: 60 lm 0, 0.25, 0.5[57] L: 12 mm 0, 0.1, 0.2, 0.3[63] L: 19 mm; D: 53 lm 0, 0.22[67] L: 30 mm 0, 0.6[126] L: 12 mm; D: 18 lm 0, 0.5, 1.0, 1.5, 2.0[127] L: 13 mm; D: 20 lm 0, 0.05, 0.1, 0.15, 0.2[128] L: 3, 6, 12, 19, 30 mm; D: 40 lm 0, 0.05, 0.1, 0.15[129] L: 15 mm; D: 100 lm 0, 0.5, 1[130] L: 20 mm; D: 20 lm 0, 0.1, 0.3[131] L: 12 mm; D: 50 lm 0, 0.1, 0.2, 0.3, 0.4[132] L: 6 mm; D: 18 lm 0, 0.1[133] L: 15 mm; D: 45 lm 0, 0.2[134] L: 19 mm; D: 45 lm 0, 0.1, 0.2, 0.3

Steel bre [44] L: 35, 60 mm; D: 440, 750 lm 0, 0.5, 1[45] L: 30 mm; D: 600 lm 0, 0.6[51] L: 30 mm; D: 550 lm 0, 0.4[53] L: 25 mm; D: 500 lm 0, 0.6[55] L: 30 mm; D: 600 lm 0, 0.25, 0.5[56] L: 2 mm; D: 2000 lm 0, 1[58] L: 25 mm; D: 400 lm 0, 0.5, 1, 1.5, 2[63] L: 25 mm; D: 42 lm 0, 1[67] N/A 0, 0.5[132] L: 12 mm; D: 50 lm 0, 1[135] L: 32.6 mm; D: 950 lm 0, 1[136] L: 30 mm; D: 500 lm 0, 2

Note: fcu, compressive strength; E, modulus of elasticity; ft, splitting tensile strength

Q. Ma et al. / Construction and Bu4. Inuence of test modalities on the mechanical properties ofconcrete at high temperature

4.1. Hot and residual tests

Bamonte and Gambarova manufactured a self-compacting con-crete [37] and a very high strength durable concrete [138], andtested the compressive strengths of both the concrete specimensat hot state and after heating. According to the results, when tem-perature was below 300 C, the compressive strength of both theconcretes at hot condition was lower than the residual ones.However, when temperature increased up to 600 C, a contrarytrend was observed. Qin and Zhao [139] and Hager [75] also foundsimilar results where hybrid bre reinforced slag concretes andhigh performance concrete were heated to 800 C and 600, respec-tively. Normal and self-compacting concretes were investigated inthe study carried out by Seshu and Pratusha [46]. The authors didnot test the compressive strength of the concretes below the tem-perature of 400 C, but afterwards till 800 C, the compressivestrength results also showed a similar trend with the previousstudies. Similar trend was also observed for the modulus of elastic-ity of high strength concrete when temperature was up to 450 C[14]. It is believed that when the temperature is below 400 C,the primary mechanism for the declines of compressive strengthand modulus of elasticity is the vapour pressure caused by theevaporation of the free water in capillary pores. The pores arepressed during the compressive test at hot state, increasing thevapour pressure and then intensifying the damage of the concrete.Consequently, the compressive strength and modulus of elasticityof concrete at hot state decrease at a larger rate than the residualones [138,139]. 400 C afterwards, cracks in the ITZ caused bythe different thermal responses between aggregates (expansion)and cement matrix (shrinkage) dominate the declines of compres-sive strength and modulus of elasticity. During cooling, expandedproperties of concrete.

Test temperatures (C) Mechanical properties tested

20, 200, 400, 600, 800 fcu, ff20, 200, 400, 600, 800 fcu, ft, ff20, 200, 300, 400, 800 fcu, ft20, 200, 300, 400, 600, 800 fcu, ft, ff20, 200, 400, 600, 800 fcu, ff20, 200, 400, 600, 800 fcu, ft20, 200, 400 fcu, ft, E20, 100, 200, 300, 600 fcu, ft20, 600, 800 fcu, E20, 100, 300, 500, 700 fcu20, 100, 450, 650 fcu, ffISO 834 fcu, EISO 834 fcu20, 200, 400, 600 fcu20, 200, 400, 600, 800 fcu, ft20, 600, 900 fcu, ff, E20, 200, 400, 600 fcu20, 100, 200, 300, 400, 500, 600, 700, 800, 900 fcu, ff20, 200, 300, 400, 500, 600, 700, 800, 900 fcu, ft, ff

20, 150, 500 fcu, E20, 200, 400, 600, 800 fcu, ft, ff20, 200, 400, 600, 800 fcu, ff20, 200, 400, 600, 800 fcu, ft20, 200, 400 fcu, ft, E20, 400, 600, 800 fcu, ft20, 300, 500, 800 fcu, ft20, 600, 800 fcu20, 100, 300, 500, 700 fcu20, 600, 900 fcu, ff, E20, 200, 400, 600, 800 ft20, 350, 500, 600, 700 fcu, ft

exural strength.

g Materials 93 (2015) 371383 379aggregates appear to shrink, further spreading the cracks in theITZ. As a result, the residual compressive strength and modulusof elasticity are much lower than the ones tested in hot state[138,139].

Bamonte and Gambarova [37] also studied the compressivestressstrain relationship of self-compacting concretes at the twotesting conditions. It was found that when the temperature wasbelow 400 C, the peak stress of the specimens after cooling washigher than the hot tested ones. However, when the temperaturewas above 400 C up to 600 C, the trend was contrary. Duringthe whole period of heating, the peak stress of the hot tested spec-imens was always observed at a later stage.

In the study carried out by Watanabe et al. [132], it was foundthat the bending strength of concrete specimens at hot state waslower than that after cooling during the whole heating period upto temperature of 600 C. The authors attributed the reason for thisto the fact that tensile stresses increased during the heating, butdid not exist any further in the residual state.

4.2. Stressed and unstressed tests

In the study carried out by Castillo and Durrani [1], during thewhole heating process up to temperature of 800 C, a stress of40% of the ultimate compressive strength at room temperaturewas loaded onto the high strength concrete cylinder specimens.The results showed that the compressive strength of the stressedspecimens was comparable to the unstressed ones during thewhole heating process. However, according to the results reportedby Phan and Carino [14] and Fu et al. [18], during the whole heat-ing process up to temperatures of 450 C and 600 C, respectively,the compressive strength of the specimens at stressed state washigher than the unstressed ones when a stress of 40% of the ulti-mate compressive strength at room temperature was applied ontothe stressed specimens. In the study carried out by Tao et al. [140],

cylinder specimens. The authors attributed this to the friction

ildin20% of the ultimate compressive strength at room temperature wasloaded onto the self-compacting concrete cylinder specimens dur-ing the whole heating process up to temperature of 800 C. Thestressed results were compared to the unstressed ones, and itwas also found that the compressive strength of the specimenswas higher for the stressed test. In the study carried out by Fuet al. [18], modulus of elasticity of high strength concrete atstressed (40% of the ultimate compressive strength at room tem-perature) and unstressed states was tested during heating processup to temperature of 600 C. It was found that the stressed modu-lus of elasticity was higher than the unstressed ones during thewhole heating process. The reason for the higher compressivestrength and modulus of elasticity at the stressed state could beattributed to the fact that the pre-loading induced friction betweenthe ends of specimens and the heads of testing machine limits thethermal stress in expansion and then restrains the thermal crack-ing [18]. In addition, the coarsened pores caused by high tempera-ture could be compressed under the pre-loading, densifying thepore structure of concrete. This could also be benecial for theimprovement of the compressive strength and modulus of elastic-ity of the concrete under stressed state [18].

The stressstrain relationship of concrete at stressed (40% of theultimate compressive strength at room temperature) andunstressed states during heating process was also studied in theresearch carried out by Fu et al. [18]. It was found that during theheating process up to temperature of 600 C, the peak stress of thestressed specimens was higher than the unstressed ones and wasobserved at an earlier stage. In the study carried out by Kim et al.[130], two levels of pre-loading of 20% and 40% of the ultimate com-pressive strength at room temperaturewere applied onto bre rein-forced concrete cylinder specimens during the whole heatingprocess (the heating regime was in accordance with ISO834).Stressstrain relationship of the specimens was studied and theresults were compared to the unstressed ones. The ndings weresimilar to the ones reported previously [18] when 20% pre-loadingis considered. However, the data for 40% pre-loading was invalidas spalling occurred for most specimens under such pre-loadinglevel, which could be used to indicate that spalling of concrete athigh temperature is more prone to occur under stressed condition.

4.3. Uni-axial and multi-axial tests

In the study carried out Ehm and Schneider [141], strength ofconcrete under bi-axial condition was tested during a heating pro-cess, and the results were compared to the ones tested underuni-axial condition. The stresses applied were in a tensile directionfor both axes. It was found that the concrete specimens were dam-aged more seriously under bi-axial condition during the wholeheating process up to temperature of 600 C. In addition, it wasfound that no matter the fraction between the horizontal stressapplied and the perpendicular one, compared to the uni-axialstrength at room temperature, the strength loss in the perpendic-ular direction was smaller than that in the horizontal direction.At temperature of 600 C, when the ratio between the horizontalstress and the perpendicular stress was 1:5, only 5% of the ultimateuni-axial strength at room temperature was remained in the hori-zontal direction, while the value was 25% for the perpendicularone. Similar results were also reported by Theinel and Rostasy[142].

In the study carried out by He and Song [143], bi- and tri-axialtensile-compressive tests were performed on high performanceconcrete specimens at different stress ratios after heating to hightemperature up to 600 C. The results showed that the strength

380 Q. Ma et al. / Construction and Buloss of concrete specimens under tri-axial state was greater thanthat under bi-axial state during the whole heating process. In addi-tion, it was found that the tensile strength increased with theeffect between the press platens and the specimen.Arioz [145] also tested the residual splitting tensile strength of

concrete cubes with sizes of 100 100 100 mm,150 150 150 mm and 200 200 200 mm after their expo-sures to temperatures from 20 C to 1200 C. It was found thatbelow 400 C, the residual splitting strength of the larger speci-mens was higher than that of the smaller specimens. Afterwards,the difference was not pronounced. The author attributed the rea-son for this to the fact that the temperature in the centre of thespecimens was lower than the temperature at the surface duringheating process due to concrete is poorly heat conducted, and sucheffect was more signicant for the larger specimens, especiallyduring the earlier stage of the heating.

5. Conclusion

Deterioration of mechanical properties of concrete occurs athigh temperature.

During the high temperature exposure, concrete experiences aseries of physical and chemical changes, such as water evapora-tion, disintegrations of hydration products and aggregates,coarsening of microstructure and increase of porosity. Thesechanges are considered to be responsible for the deteriorationof mechanical properties of concrete at high temperature.

Spalling may occur for concrete at high temperature. Watervapour pressure and thermal stress at high temperature mayinduce the spalling.

The residual compressive strength and modulus of elasticity ofthe concrete with lower w/b are higher than the concrete withhigher w/b. A lower w/b at the beginning of mixing and/or ahigher moisture content at the time when concrete is exposedto high temperature is prone to induce spalling of concrete athigh temperature as a result of high vapour pressure.

Calcareous aggregates provide greater high temperature resis-tance to concrete compared to siliceous aggregates.Lightweight concretes have a high resistance to heat due tothe natural characteristics of lightweight aggregates. However,the pre-saturation regime of lightweight aggregates which isdecrease of stress ratio for any given temperature, while thechange of compressive strength was contrary.

4.4. Specimen size

In the study carried out by Barnagan et al. [144], residual mod-ulus of elasticity of concrete cylinder specimens of 150 300 mmand prism specimens of 75 105 430 mm after heating to tem-perature of 500 C was tested. The results showed that the loss ofmodulus of elasticity caused by the heating was comparablebetween the two types of concrete specimens. Arioz [145] alsofound that the difference of the residual compressive strengthbetween the concrete cubes of 100 100 100 mm and the cubesof 150 150 150 mm was not signicant after the exposures totemperatures from 20 C to 1200 C. Similar results were alsoreported in the study carried out by Erdem [146] when cylinderspecimens with sizes of 50 100 mm, 100 200 mm and150 300 mm were studied during heating process up to tem-perature of 800 C.

Bamonte and Gambarova [138] tested the residual compressivestrengths of concrete cubes (40 40 40 mm) and concrete cylin-ders (36 110 mm) after their exposures to elevated tempera-ture up to 750 C. It was found that the cube specimens alwaysexhibited higher residual compressive strength compared to the

g Materials 93 (2015) 371383usually used in practice would induce spalling of lightweightconcretes at high temperature.

ildin The addition of PFA and slag in concrete could increase its resis-tance to high temperature, while the addition of SF wouldreduce such resistance.

Polypropylene bre generally has no signicant inuence on theimprovements of residual compressive strength and modulus ofelasticity for concrete after heating to high temperature. Itsimprovement on residual splitting tensile strength and exuralstrength would be greatly lost after around 400 C. However,polypropylene reinforced concrete has great resistance to spal-ling due to the release of vapour pressure.

Steel bre could generally improve the residual mechanicalproperties of concrete after heating to high temperature. Itcould also increase the resistance of concrete to spalling, butthe extent of such increase is less than that provided bypolypropylene bre.

When temperature is below 400 C, the compressive strength ofconcretes tested at hot state is lower than the one tested afterthe heating. 400 C afterwards, the residual compressivestrength is lower than the one tested at hot condition. Theresidual bending strength of concretes is higher than the onetested at hot state.

The compressive strength of concretes at high temperaturetested under stressed state is higher than the one tested underunstressed state.

Compared to uni-axial test, bi-axial and tri-axial tests bringmore serious damage for concretes at high temperature.

When the difference of specimen size is signicant enough, thespecimens with smaller size exhibits higher residual compres-sive strength than the larger specimens at high temperature.

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Mechanical properties of concrete at high temperatureA review1 Introduction2 Mechanical properties of concrete at high temperature2.1 Compressive strength2.2 Flexural strength, splitting tensile strength and modulus of elasticity2.3 Stressstrain relationship2.4 Physical and chemical changes2.4.1 Water evaporation2.4.2 Hydration products2.4.3 Pore structure2.4.4 Microstructure2.4.5 Aggregates

2.5 Spalling

3 Factors influencing the performance of concrete subjected to high temperature3.1 w/b and moisture content3.2 Type of aggregate3.3 SCMs3.4 Fibres

4 Influence of test modalities on the mechanical properties of concrete at high temperature4.1 Hot and residual tests4.2 Stressed and unstressed tests4.3 Uni-axial and multi-axial tests4.4 Specimen size

5 ConclusionReferences