Mechanical properties of concrete at high temperature—A review

Construction and Building Materials 93 (2015) 371–383

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Review

Mechanical properties of concrete at high temperature—A review Qianmin Ma ⇑, Rongxin Guo, Zhiman Zhao, Zhiwei Lin, Kecheng He Faculty of Civil Engineering and Mechanics, Kunming University of Science and Technology, 727, Jingming South Road, 650500 Kunming, China

h i g h l i g h t s  Mechanical properties of concrete at high temperature were reviewed.  Physical and chemical changes of concrete at high temperature were reviewed.  Factors affecting thermally mechanical properties of concrete were reviewed.

a r t i c l e

i n f o

Article history: Received 9 October 2014 Received in revised form 11 May 2015 Accepted 14 May 2015 Available online 14 June 2015 Keywords: Concrete High temperature Mechanical properties

a b s t r a c t High temperature is well known for seriously damaging concrete micro- and meso-structure, which brings in a generalised mechanical decay of the concrete and even detrimental effects at the structural level, due to concrete spalling and bar exposure to the flames, in case of fire. Because of the relevance of concrete behaviour at high temperature and in fire, many studies have been carried out, even very recently, on cementitious composites at high temperature, and the most relevant parameters have been identified and investigated. Within this framework, the authors provide a comprehensive and updated report on the temperature dependency of such parameters as the compressive strength, modulus of elasticity, strength in indirect tension (bending and splitting tests), stress–strain curves and spalling, but the roles played by the water–binder ratio (w/b), aggregate type, supplementary cementitious materials (SCMs) and fibres are investigated as well. Among the objectives of the paper, the approaches currently adopted to improve concrete mechanical properties at high temperature are treated as well. Meanwhile, the influence of test modalities on the mechanical properties of concrete at high temperature is also discussed in the paper. Ó 2015 Elsevier Ltd. All rights reserved.

Contents 1. 2.

3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical properties of concrete at high temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Compressive strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Flexural strength, splitting tensile strength and modulus of elasticity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Stress–strain relationship . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Physical and chemical changes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1. Water evaporation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2. Hydration products. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.3. Pore structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.4. Microstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.5. Aggregates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Spalling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors influencing the performance of concrete subjected to high temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. w/b and moisture content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Type of aggregate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. SCMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. Tel.: +86 13095358933. E-mail address: [email protected] (Q. Ma). http://dx.doi.org/10.1016/j.conbuildmat.2015.05.131 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.

372 372 372 372 372 373 373 373 373 373 374 374 375 375 375 377

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4.

5.

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3.4. Fibres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Influence of test modalities on the mechanical properties of concrete at high temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Hot and residual tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Stressed and unstressed tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Uni-axial and multi-axial tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Specimen size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction

2. Mechanical properties of concrete at high temperature

1.6 1.4 1.2 fcu/fcu,20

Under the pressure of population boom and land limitation, in order to effectively resolve housing and transportation issues, the need for high-rise buildings and underground construction is rapid increasing. Such civil engineering is facing tremendous challenge of fire damage during its constructing and service. Fire on these engineering is frequently reported worldwide in recent years, seriously threatening personal and property safety. High temperature is well known for seriously damaging concrete micro- and meso-structure, which brings in a generalised mechanical decay of the concrete and even detrimental effects at the structural level, due to concrete spalling and bar exposure to the flames, in case of fire. Because of the relevance of concrete behaviour at high temperature and in fire, many studies have been carried out, even very recently, on cementitious composites at high temperature, and the most relevant parameters have been identified and investigated. Within this framework, the authors provide a comprehensive and updated report on the temperature dependency of such parameters as the compressive strength, modulus of elasticity, strength in indirect tension (bending and splitting tests), stress–strain curves and spalling, but the roles played by the w/b, aggregate type, SCMs and fibres are investigated as well. Among the objectives of the paper, the approaches currently adopted to improve concrete mechanical properties at high temperature are treated as well. Meanwhile, the influence of test modalities on the mechanical properties of concrete at high temperature is also discussed in the paper. Electrical furnace heating and gas/oil heating (fire), these two different heating models, are used in the studies to investigate the thermal behaviour of concrete at high temperature. Furnace heating is usually used for the studies on the thermal changes of concrete characteristics, while fire is usually considered when the studies are at a structurally elemental level. This paper mainly focuses on the discussion on the thermal changes of concrete characteristics at high temperature, the effect of fire on the behaviour of concrete is exclusive in this paper.

1 0.8 0.6 0.4 0.2 0 0

200

400 600 800 Temperature ( C)

1000

1200

Fig. 1. Residual compressive strength of concrete at elevated temperatures (data adapted from [1–46]).

(2) 300–800 °C, compressive strength of concrete decreases dramatically. (3) 800 °C afterwards, almost all the compressive strength of concrete has been lost. 2.2. Flexural strength, splitting tensile strength and modulus of elasticity Residual flexural strength, residual splitting tensile strength and residual modulus of elasticity of concrete after exposure to elevated temperatures are shown in Figs. 2–4, respectively. Same data collection regime with compressive strength is used. Similar to the compressive strength reviewed in the previous section, flexural strength, splitting tensile strength and modulus of elasticity of concrete decreases with the increase of temperature, but at a nearly linear rate. 2.3. Stress–strain relationship Stress–strain relationship of concrete at elevated temperatures has been investigated by many researchers [2,12,30,37,48,64–73]. It has been found that with the increase of temperature, stress–

2.1. Compressive strength

1.2 1 0.8 ff,T / ff,20

It is unavoidable that there is a reduction for compressive strength of concrete when it is exposed to high temperature (see Fig. 1). In spite of concrete mixture proportions, test modalities, such as specimen size, stressed/unstressed conditions and hot/residual states, also influence the mechanical properties of concrete at high temperature (details are in Section 4). Therefore, in order to eliminate the possible effect caused by these factors, the data collection in Fig. 1 is carried out only on the residual results of unstressed cube specimens. It can be seen that the residual compressive strength of concrete after heating to high temperature experiences three main stages:

378 379 379 379 380 380 380 381

0.6 0.4 0.2 0 0

200

400

600

800

1000

1200

Temperature (

(1) Room temperature—300 °C, compressive strength of concrete keeps constant or even increases slightly.

Fig. 2. Residual flexural strength of concrete at elevated temperatures (data adapted from [26,42,47–55]).

Q. Ma et al. / Construction and Building Materials 93 (2015) 371–383

are considered to be responsible for the changes of the mechanical properties:

1.4 1.2

ft,T / ft,20

1 0.8 0.6 0.4 0.2 0 0

200

400

600 Temperature (

800

1000

1200

Fig. 3. Residual splitting tensile strength of concrete at elevated temperatures (data adapted from [9,52,54,56–63]).

1.2 1 0.8 ET / E20

373

0.6 0.4 0.2 0 0

200

400 600 Temperature (

800

1000

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

Fig. 5. Residual stress–strain relationship of concrete at elevated temperatures [68].

strain curves become flatter, and the peak stress shifts downwards and rightwards, as shown in Fig. 5. These indicate that the peak stress and the modulus of elasticity of concrete decrease with the increase of temperature, but the strain at peak stress increases with temperature. 2.4. Physical and chemical changes With the elevation of temperature, concrete would experience the following physical and chemical changes and these changes

2.4.1. Water evaporation Hydration products lose their free water and physically absorbed water completely, and start to lose their chemically bonded water at 105 °C [74]. Capillary water is lost completely at 400 °C [75]. Up to 300 °C, hydration of unhydrated cement grains is improved due to an internal autoclaving condition as a result of the high temperature and the evaporation of water [76]. This is particularly true for high strength concrete as its low permeability resists moisture flow. This can be used to explain the constant compressive strength when the temperature is below 300 °C as discussed in Section 2.1. 2.4.2. Hydration products AFt/AFm dehydrates at 110–150 °C [77]. Above 350 °C, calcium hydroxide either decomposes into lime and water or further converts into C–S–H due to the accelerated pozzolanic reaction at a high temperature [78–80]. The decomposition of Ca(OH)2 has no critical influence on the reduction of strength for concrete. However, if concrete is water cooled after exposure to high temperature, the rehydration of lime will cause a great reduction of strength for concrete due to a considerable expansion will be caused due to such a reaction [81]. C–S–H starts to decompose at around 560 °C [79] and it decomposes into b-C2S at around 600– 700 °C [77,79]. C–S–H (I) decomposes at 800 °C, which, however, only results in a slight reduction of strength for concrete [81]. During 580–900 °C, decarbonation of carbonates occurs [64,78,81–83]. 2.4.3. Pore structure As a result of the water evaporation and the chemical changes of hydration products, elevation of temperature increases porosity and pore size of cement and concrete [11,21,23,64,75,76,78,83–9 1]. The coarsening of the pore structure is mainly responsible for the reduction of the mechanical properties as discussed in the previous sections. 2.4.4. Microstructure Up to 200 °C, no micro-cracks are observed in either hardened cement matrix or interfacial transition zone (ITZ) [81,92]. When the temperature rises to 400 °C, micro-cracks in cement matrix and ITZ start to propagate and their intensity increases with temperature [3,21,23,26,28,93–99]. It is considered that the different thermal strains for hardened cement matrix and aggregates have resulted in the development of the micro-cracks at high temperature. From Fig. 6 it can be seen that with the increase of temperature, the hardened cement matrix expands first and then shrinks as a result of the loss of water, while aggregates keep expansion during the whole heating. Similar results have also been found by Fu et al. [100]. Such different strains will produce a stress between cement matrix and aggregates, causing micro-cracks in the ITZ. This is also responsible for the reduction of the mechanical properties of concrete at high temperatures. When temperature is very high, such as above 1000 °C, porosity and microstructure of concrete are smaller and better than those at a lower temperature due to concrete has been sintered at such a high temperature [83,85]. However, it does not indicate that the mechanical properties of concrete at the very high temperature was better than those at a lower temperature as the relationship between mechanical properties and pore structure is not true any further due to the syntherization has changed the characteristic of concrete material [85].

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Temperature Temperature Pore pressure Pore pressure

Distance from heat Temperature Fig. 6. Thermal strains of cement matrix and aggregates [102].

Temperature

Pore pressure Pore pressure

2.4.5. Aggregates At around 573 °C, siliceous aggregates transform from a-phase to b-phase causing expansion of concrete [81,83]. Disintegration of calcareous aggregates, such as limestone, occurs at a temperature above 600 °C [101].

Distance from heat

Distance from heat

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

6 Maximum pore pressure (MPa)

2.5. Spalling Spalling may occur for concrete at high temperature, which will greatly reduce mechanical properties of concrete structure and even cause collapse of the structure [103]. The mechanisms of spalling of concrete at high temperature could be mainly explained from vapour pressure in pores and thermal stresses these two aspects [103]. Hardened concrete is saturated with water in its pores at different extents. The moisture content in concrete is dependent on w/b, age of concrete and environment. When concrete surface is subjected to sufficiently high temperature, a portion of water will be vaporised and move out from concrete into atmosphere. There is also certain amount of water will be vaporised and move opposite to the inner part of concrete. Due to thermal gradient, the inner part of concrete is cooler and the vapour there will be condensed. With the accumulation of the condensed water, a saturated layer is gradually formed. This layer will resist the further movement of vapour into the inner of concrete, but move towards the dry region of the concrete surface with an attempt to escape out of concrete into atmosphere. If the pore structure of the concrete is sufficiently dense and/or the heating rate is sufficiently high, the escape of the vapour layer would be not fast enough, resulting in a large increase of pore pressure in the concrete. If the tensile stress of concrete could not resist the pore pressure, spalling of concrete would occur [104]. Fig. 7 illustrates the whole process of the thermal spalling of concrete as a result of the pore vapour pressure.

Distance from heat

Ref. [106] Ref. [107]

5

Ref. [108] 4 3 2 1 0 0

10

20 30 40 50 Distance from the heated surface (mm)

60

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

Fig. 8 shows the maximum pore pressures of concrete at high temperature. From Fig. 8 it can be seen that the maximum pore pressure is generally observed in the inner part of concrete. Compared to the inner part, vapour in the outer part is easier to escape out from concrete. This would reduce the pore pressure in concrete at the near surface zone. Furthermore, the maximum pore pressure in high strength concrete is generally larger than that in the normal strength concrete [105–108]. The high strength of concrete is usually achieved by densifying its pore structure to lower its permeability. Due to the low permeability, when the high

Q. Ma et al. / Construction and Building Materials 93 (2015) 371–383

Tensile stress Compressive stress

Temperature

Compressive stress

Tensile stress

375

w/b concrete (w/b = 0.28, 0.35). Phan et al. [10] found that compared to the concrete with w/b of 0.22, the losses of both compressive strength and modulus of elasticity were higher for the concrete with w/b of 0.57. Similar results have been found for concrete containing slag [86,110], fly ash [86,111] and metakaolin [111] when w/b ranged from 0.3 to 0.5 [86,111] and from 0.23 to 0.71 [110]. Lightweight concrete also gave similar results when different w/b of 0.43 and 0.46 was studied [27]. However, a lower w/b is prone to cause spalling of concrete at high temperature. As reported by Phan et al. [10], spalling occurred for the concrete with w/b of 0.22 when temperature was elevated to 450 °C, while the concrete with w/b of 0.33 was still intact at the same temperature. As discussed in the previous section, spalling occurs when pore vapour pressure in concrete accumulates to a certain extent. It is considered that such an accumulation would become faster when the pore structure is denser, which could be caused by using a lower w/b. That is why spalling of concrete is easy to occur at high temperature when a lower w/b is used. Despite of w/b at the beginning of concrete mixing, spalling is also much dependent on the moisture content of concrete at the time of its exposure to high temperature. Fig. 10 gives an example of spalling of concrete at different moisture contents. It is clear to see that the possibility and the extent of spalling increase with moisture content of concrete as a result of the increased pore vapour pressure. 3.2. Type of aggregate

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

strength concrete is exposed to high temperature, the vapour generated is not easy to escape out from the concrete, therefore resulting in the larger maximum pore pressure. Fig. 7 also simulates the development of pore pressure in the concrete at high temperature, and which is corresponded to the steps of the pore vapour pressure induced spalling of concrete. Simultaneously, thermal gradient will also be formed between the heated surface and the inner part of concrete when the concrete is subjected to high temperature. This is particularly true when temperature increases very fast, which is always named as ‘thermal shock’. With temperature increases faster at the surface of concrete, compressive stress is generated parallel to the heated concrete surface, while tensile stress is generated in the inner concrete in a perpendicular direction. When the compressive stress exceeds the tensile stress, spalling of concrete occurs [109], as shown in Fig. 9. Both the above two causes would result in cracking of concrete at high temperature. Besides, the cracking of concrete at high temperature would also be caused by the decomposition of hydration product, shrinkage of cement matrix and expansion of aggregates. The different thermal response between cement matrix and aggregates is also considered to distribute cracks in the ITZ between the two phases, damaging concrete meso-structure. Finally, all the causes mentioned above make the spalling of concrete at high temperature to occur in the models of aggregate spalling, surface spalling, corner spalling and explosive spalling [103].

3. Factors influencing the performance of concrete subjected to high temperature 3.1. w/b and moisture content The study carried out by Chan et al. [7] has illustrated that up to the temperature of 1000 °C, the compressive strength loss of the high w/b concrete (w/b = 0.6) was higher than that of the low

Effects of type of aggregate on compressive strength, flexural strength, splitting tensile strength and modulus of elasticity of concrete at high temperatures are presented in Figs. 11–14, respectively. The scatter from data to regression line may be caused by different mixes and different test modalities. Generally speaking, the concretes made of siliceous aggregates, such as granite, express unfavourable mechanical properties at high temperature compared to the concretes manufactured by using dolomite and limestone these calcareous aggregates. Furthermore, Cheng et al. [16] also found that the increase in strains for the concrete made of calcareous aggregates was larger than that for the siliceous aggregates concrete. It is also found that spalling occurs at a higher temperature and a later time for limestone concrete [112]. As stated in Section 2.4, calcareous aggregates decompose at a higher temperature than siliceous aggregates. This could be used to explain the better performance of the concrete with calcareous aggregates at high temperature. Lightweight aggregates, such as expanded clay, pumice and ceramsite, are formed by volcano eruption or incineration. As a result, they have low heat conductivity and exhibit a high resistance to heat. Therefore, the concrete manufactured by using such aggregates should deliver improved mechanical properties at high temperature in comparison to normal aggregates concrete. Sun et al. [113] used high alumina cement to manufacture normal refractory concrete (normal aggregates), ceramsite refractory concrete I (ceramsite as coarse aggregates), ceramsite refractory concrete II (ceramsite as coarse and fine aggregates) and refractory brick concrete (broken refractory brick as coarse aggregates). The concrete specimens were heated to 1000 °C. After the heating, ceramiste refractory concretes I and II still had 33–50% compressive strength remained, which was much higher than that of normal refractory concrete of 17%. In the studies carried out by both Sancak et al. [27] and Tanyildizi and Coskun [29], pumice was used as coarse aggregates to manufacture lightweight concretes. The lightweight concrete specimens had 28–38% compressive strength remained after exposure to 800 °C, which was higher than the value of 13– 16% for normal reference concrete. In addition, the lightweight concrete specimens still had 18% splitting tensile strength

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Fig. 10. Relationship between moisture content and possibility and extent of spalling.

1.8

1.2

1.6

1

1.4

ff,T/ff,20

fcu,T/fcu,20

1.2 1 0.8

0.8 0.6 0.4

0.6 0.4

0.2

0.2

0

0 0

200

dolomite gravel regression for limestone regression for basalt

400

600

800

Temperature (˚C) limestone basalt regression for granite

1000

1200

1400

granite regression for dolomite regression for gravel

Fig. 11. Influence of type of aggregate on residual compressive strength of concrete subjected 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 for gravel was adapted from [5,8,9,13,19,22,33,34,44,45,73,98]; data for basalt was adapted from [15,49,97]).

remained [29]. Cao et al. [114] compared the residual compressive strength among lightweight concrete I (ceramiste as coarse aggregates), lightweight concrete II (ceramiste as both coarse and fine aggregates) and normal concrete at high temperature. The results showed that the normal concrete specimens had lost all the

0

200

400

600 800 1000 1200 Temperature (˚C) dolomite limestone granite gravel basalt regression for dolomite regression for limestone regression for granite regression for gravel regression for basalt

Fig. 12. Influence of type of aggregate on residual flexural strength of concrete subjected to elevated temperatures (data for dolomite was adapted from [53]; data for 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]).

compressive strength at temperature of 1000 °C, whilst 20.5% and 21% of the compressive strength was left for lightweight concrete I and II, respectively. Turkmen and Findik [115] used expanded clay to replace natural sand at a replacement of 25% to produce lightweight mortar. Such mortar still had 38% of compressive strength and 23% of flexural strength remained after exposure

Q. Ma et al. / Construction and Building Materials 93 (2015) 371–383

1.4 1.2

ft,T/ft,20

1 0.8 0.6 0.4 0.2 0 0

200 400 600 800 Temperature (˚C) limestone gravel regression for limestone regression for gravel

1000

1200

1400

granite basalt regression for granite regression for basalt

Fig. 13. Influence of type of aggregate on residual splitting tensile strength of concrete 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 was adapted from [8,58]; data for basalt was adapted from [15,49]).

1.2

MT/M20

1 0.8 0.6 0.4 0.2 0 0

200

400

600 800 1000 1200 Temperature (˚C) dolomite limestone granite gravel regression for dolomite regression for limestone regression for granite regression ofr gravel

Fig. 14. Influence of type of aggregate on residual modulus of elasticity of concrete subjected to elevated temperatures (data for dolomite was adapted from [41]; data for 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]).

to 800 °C. In the study carried out by Jiang et al. [116], compared to normal concrete which had 10% of the compressive strength remained at the temperature of 1000 °C, the value was 20% for lightweight concrete manufactured by using ceramiste. Both Jiang et al. [117] and Wang et al. [118] used industrial sewage sludge ceramsite to manufacture lightweight concrete. After the exposure to 800 °C, 46.9% of compressive strength and 40% of splitting tensile strength remained for the lightweight concrete [117]. In addition, 20.2% of initial modulus of elasticity and 18.4% of peak deformation 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 high temperature induced spalling did not occur when moisture content in normal concrete was below 75%. However, for lightweight concrete, when its moisture content was above 25%, spalling occurred at high temperature. This indicates that spalling of lightweight concrete at high temperature is much more sensitive than normal concrete to moisture content. It is known that the porosity of lightweight 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 aggregates and its effect on fresh concrete workability and subsequent setting and hardening, lightweight aggregate is usually pre-saturated before being used to mix concrete. However, such treatment will bring extra water into lightweight concrete to

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increase its moisture content, and then increasing the possibility of spalling for lightweight concrete at high temperature. This will extremely limit the super resistance of lightweight aggregate to heat. In literatures [27,29,115], the authors did not follow the practical process to pre-saturate the lightweight aggregates. In literatures [113,114,117,118], the authors dried the lightweight concrete specimens at 100 °C before exposing them to high temperature, which minimised the possible spalling at a large extent. Therefore, from above it can be seen that further studies are needed to investigate the effect of high temperature on lightweight concretes in a condition similar to the practice. In such case, a novel pre-treatment should be applied to lightweight aggregates to reduce the possibility of spalling of lightweight concrete, and then to allow the super resistance of lightweight aggregates to heat to serve well. 3.3. SCMs Table 1 summarises the literatures on the effect of SCMs on the residual mechanical properties of concrete at high temperatures, including compressive strength, splitting tensile strength, flexural strength and modulus of elasticity. The incorporation of pulverised fly ash (PFA) and slag in PC can generally remain the mechanical properties of concrete at a higher level after heating to high temperature up to 900 °C and 1050 °C, respectively. Compared to PC, the residual compressive strength, splitting tensile strength, flexural strength and modulus of elasticity of PC blended with PFA increase by 1.2–270%, 1.1–80%, 4.5– 200% and 3–38%, respectively. The values for PC blended with slag are 1.5–510%, 1.2–43%, 1–180% and 1.3–117%, respectively. The values vary mainly with different temperatures, replacements and types of aggregates. In the research carried out by Wang [110], PC paste had lost its compressive strength and modulus of elasticity completely at the temperature of 1050 °C. However, 18% of the compressive strength and 81% of the modulus of elasticity were still remained for PC blended slag paste with the replacement of 80% at the same temperature. Furthermore, PCs blended with PFA and slag also exhibit a high resistance to spalling at high temperatures [86,91,124,122]. Aydin and Baradan [97] and Aydin [123] detected the formation of gehlenite in the PC samples incorporated PFA and slag at the temperature of 900 °C by using XRD analysis. Such phase may fill in the pores caused by the high temperature. Therefore, the cement matrix could be refined and the ITZ between cement matrix and aggregate could be enhanced so that the values of the mechanical properties for PCs blended with PFA and slag retain at a higher level. Furthermore, Karakurt and Topcu [120] found that thermal cracking did not occur in PFA and slag blending samples and that the degradation of C–S–H decreased compared to PC sample by using SEM analysis. Moreover, the incorporation of slag significantly reduces the amount of portlandite in PC so that decreasing the degradation of portlandite at high temperatures [124,125]. As a result of the above three aspects, the total porosity and the average pore diameter of PCs blended PFA and slag are smaller than those of PC at high temperatures [86]. This could explain the higher resistance of PCs blended PFA and slag to high temperature. On the other hand, the incorporation of silica fume (SF) apparently reduces the resistance of PC to high temperatures. Compared to PC, the residual compressive strength, splitting tensile strength, flexural strength and modulus of elasticity of PC blended SF at high temperatures decrease by 1–100%, 2–12%, 2–25% and 2–7%, respectively. The values also vary mainly with different temperatures, replacements and types of aggregates. Furthermore, severe spalling was detected for PC blended SF in several studies [10,86]. Behnood and Ziari [128] explained that due to the filler effect and pozzolanic reactions provided by SF, cement matrix

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Table 1 Summary of the researches carried out on the effect of SCMs on the residual mechanical properties of concrete.

PFA

Slag

SF

Refs.

Type of specimen

Replacement (%)

Test temperatures (°C)

Mechanical properties tested

[11] [17] [18] [80] [86] [97] [119] [120] [121] [122]

Concrete with granite Concrete with limestone Concrete Mortar Concrete with granite Pumice mortar Lightweight concrete Concrete with limestone Mortar Concrete with granite

0, 0, 0, 0, 0, 0, 0, 0, 0, 0,

25, 55 10, 30 30 25, 35, 45 20, 30, 40 20, 40, 60 10, 20, 30 30 5, 10, 15, 20 25, 55

20, 20, 20, 20, 20, 20, 20, 20, 20, 20,

250, 100, 100, 400, 200, 300, 200, 100, 150, 200,

450, 300, 200, 700 400, 600, 400, 300, 300, 400,

650, 800 600, 750 400, 600

fcu fcu, fcu, fcu, fcu fcu, fcu, fcu fcu ft

[38] [63] [86] [110] [120] [123] [124] [125] [63] [126] [127]

Concrete Concrete Concrete with granite Paste Concrete with limestone Pumice mortar Paste Mortar Concrete Concrete with limestone Concrete

0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,

10, 30, 50 20, 40, 60 30, 40 5, 10, 20, 50, 80 30 20, 40, 60, 80 35, 50, 65 20, 50, 80 20, 40, 60 30, 40, 50 30, 40, 50

20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20,

150, 100, 200, 105, 100, 300, 100, 150, 100, 400 400

300, 200, 400, 200, 300, 600, 200, 300, 200,

400, 350 600, 440, 450, 900 300, 600, 350

[10] [27] [32] [61] [80] [86] [91] [121] [128]

Concrete with limestone Lightweight concrete Concrete with limestone Concrete with limestone Mortar Concrete with granite Paste Mortar Concrete with limestone

0, 0, 0, 0, 0, 0, 0, 0, 0,

10 5, 10 10 10 2.5, 5, 7.5 5, 10 5, 10, 15, 20 5, 10, 15, 20 6, 10

20, 20, 20, 20, 20, 20, 20, 20, 20,

100, 100, 100, 100, 400, 200, 250, 150, 100,

200, 400, 200, 200, 700 400, 450, 300, 200,

300, 800, 300, 300,

600, 900 800 450, 450, 600,

800

600 600, 750 800 500, 600, 700 800 580, 800, 1050 600 400, 500, 600, 700, 800 900

450 1000 600 600

600, 800 600 450, 600, 750 300, 600

fcu, fcu, fcu fcu, fcu fcu, fcu fcu, fcu ft ff

E E ff ff ft

E ft, E E ff ft

fcu, E fcu fcu ft fcu, ff fcu fcu fcu fcu

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

and ITZ of PC blended with SF would be much denser than those of PC. This, however, could restrain the expansion of aggregates when subjecting to high temperatures and then reduce the mechanical properties noticeably. Poon et al. [86] also found that the total porosity and the average pore diameter of PC with 10% SF were much larger than those of PC at the temperature of 800 °C. This could be the result of the restraint effect mentioned above and consequently influence the retaining of the mechanical properties of PC blended with SF at high temperature. 3.4. Fibres A number of studies have been carried out on the effect of fibre on the mechanical properties of concrete after exposure to high temperatures, and a summary is presented in Table 2. Polypropylene and steel fibres are usually used in these studies. Polypropylene fibre generally has no significant influence on the improvements of residual compressive strength and residual modulus of elasticity for concrete after heating to high temperature. However, such improvement is clearer to a certain extent when residual flexural strength and residual splitting tensile strength are considered. This is particularly at the temperature below 400 °C. Polypropylene fibre can increase the resistance of concrete to cracking, improving its behaviour under tension. However, the melting and ignition points of polypropylene fibre are around 150 °C and 400–500 °C, respectively. That is why the improvement of residual flexural and residual splitting tensile strengths of polypropylene fibre reinforced concrete reduces when the temperature is above 400 °C due to the fibre has been melted up at such high temperature and the pores left are disadvantage for the performance of concrete under tension [49,51,58,129]. However, also due to the melting and ignition of polypropylene which is randomly distributed in concrete, at a relatively low

temperature, the left pores radiate out to form microcracks, connecting the existing capillary pores to provide channels for the escaping of water vapour. Consequently, it is found that the polypropylene fibre reinforced concrete has much better resistance to thermal spalling compared to the concrete without fibre [47,52,60,130–133]. This is particularly true for high performance concrete as water vapour is more difficult to escape in a denser matrix. An optimum dosage of polypropylene fibre around 0.1– 0.5% by volume of mix is recommended for concrete to obtain a proper high temperature resistance [134–136], and it is found that the resistance of polypropylene fibre reinforced concrete to high temperature increases with the increase of the length of the fibre [131]. The addition of steel fibre can generally improve the residual mechanical properties of concrete at high temperature when compressive strength, flexural strength and splitting tensile strength are considered. The improvement in the residual modulus of elasticity is not clearly observed. The reason for such improvements could be attributed to the fact that the testing temperatures are not high enough to allow steel fibre to be melted so that its ductility could effectively contribute to concrete resisting the failure under tension during the whole test period. Furthermore, steel fibre has higher thermal conductivity than cement matrix and aggregates. Consequently, heat can transmit more uniformly in the concrete reinforced with steel fibre to reduce the cracks caused by thermal gradient in concrete, improving the performance of concrete under both compression and tension [55,57,136]. Also due to the reduced thermal gradient, the steel fibre reinforced concrete shows resistance to thermal spalling [49,137]. However, the resistance to spalling provided by steel fibre is weaker than that provided by polypropylene fibre, which may indicate that water vapour is the primary reason to cause spalling of concrete at high temperature [57].

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Q. Ma et al. / Construction and Building Materials 93 (2015) 371–383 Table 2 Summary of the researches carried out on the effect of fibres on the residual mechanical properties of concrete.

PP fibre

Steel fibre

Refs.

Dimension of fibre

Replacement (% by volume)

Test temperatures (°C)

Mechanical properties tested

[43] [45] [47] [48] [51] [53] [55] [57] [63] [67] [126] [127] [128] [129] [130] [131] [132] [133] [134]

L: 19 mm; D: 45 lm L: 12 mm; D: 18 lm N/A L: 19 mm L: 19 mm; D: 35 lm L: 15 mm; D: 100 lm L: 6 mm, 30 mm; D: 60 lm L: 12 mm L: 19 mm; D: 53 lm L: 30 mm L: 12 mm; D: 18 lm L: 13 mm; D: 20 lm L: 3, 6, 12, 19, 30 mm; D: 40 lm L: 15 mm; D: 100 lm L: 20 mm; D: 20 lm L: 12 mm; D: 50 lm L: 6 mm; D: 18 lm L: 15 mm; D: 45 lm L: 19 mm; D: 45 lm

0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,

0.05, 0.1, 0.15 0.3 0.15, 0.2 0.1 0.1 0.6 0.25, 0.5 0.1, 0.2, 0.3 0.22 0.6 0.5, 1.0, 1.5, 2.0 0.05, 0.1, 0.15, 0.2 0.05, 0.1, 0.15 0.5, 1 0.1, 0.3 0.1, 0.2, 0.3, 0.4 0.1 0.2 0.1, 0.2, 0.3

20, 200, 20, 200, 20, 200, 20, 200, 20, 200, 20, 200, 20, 200, 20, 100, 20, 600, 20, 100, 20, 100, ISO 834 ISO 834 20, 200, 20, 200, 20, 600, 20, 200, 20, 100, 20, 200,

400, 400, 300, 300, 400, 400, 400 200, 800 300, 450,

600, 600, 400, 400, 600, 600,

ff ft, ff ft ft, ff ff ft ft, E ft E

400, 400, 900 400, 200, 300,

600 600, 800

fcu, fcu, fcu, fcu, fcu, fcu, fcu, fcu, fcu, fcu fcu, fcu, fcu fcu fcu, fcu, fcu fcu, fcu,

[44] [45] [51] [53] [55] [56] [58] [63] [67] [132] [135] [136]

L: 35, 60 mm; D: 440, 750 lm L: 30 mm; D: 600 lm L: 30 mm; D: 550 lm L: 25 mm; D: 500 lm L: 30 mm; D: 600 lm L: 2 mm; D: 2000 lm L: 25 mm; D: 400 lm L: 25 mm; D: 42 lm N/A L: 12 mm; D: 50 lm L: 32.6 mm; D: 950 lm L: 30 mm; D: 500 lm

0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,

0.5, 1 0.6 0.4 0.6 0.25, 0.5 1 0.5, 1, 1.5, 2 1 0.5 1 1 2

20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20,

500 400, 400, 400, 400 600, 500, 800 300, 900 400, 500,

fcu, fcu, fcu, fcu, fcu, fcu, fcu, fcu fcu fcu, ft fcu,

E ft, ff ff ft ft, E ft ft

150, 200, 200, 200, 200, 400, 300, 600, 100, 600, 200, 350,

800 800 800 600, 800 800 800

300, 600 500, 700 650

600 300, 400, 500, 600, 700, 800, 900 400, 500, 600, 700, 800, 900 600, 800 600, 800 600, 800 800 800 500, 700 600, 800 600, 700

ff E

ft ff, E ff ft, ff

ff, E ft

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

4. Influence of test modalities on the mechanical properties of concrete at high temperature 4.1. Hot and residual tests Bamonte and Gambarova manufactured a self-compacting concrete [37] and a very high strength durable concrete [138], and tested the compressive strengths of both the concrete specimens at hot state and after heating. According to the results, when temperature was below 300 °C, the compressive strength of both the concretes at hot condition was lower than the residual ones. However, when temperature increased up to 600 °C, a contrary trend was observed. Qin and Zhao [139] and Hager [75] also found similar results where hybrid fibre reinforced slag concretes and high performance concrete were heated to 800 °C and 600, respectively. Normal and self-compacting concretes were investigated in the study carried out by Seshu and Pratusha [46]. The authors did not test the compressive strength of the concretes below the temperature of 400 °C, but afterwards till 800 °C, the compressive strength results also showed a similar trend with the previous studies. Similar trend was also observed for the modulus of elasticity 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 strength and modulus of elasticity is the vapour pressure caused by the evaporation of the free water in capillary pores. The pores are pressed during the compressive test at hot state, increasing the vapour pressure and then intensifying the damage of the concrete. Consequently, the compressive strength and modulus of elasticity of concrete at hot state decrease at a larger rate than the residual ones [138,139]. 400 °C afterwards, cracks in the ITZ caused by the different thermal responses between aggregates (expansion) and cement matrix (shrinkage) dominate the declines of compressive strength and modulus of elasticity. During cooling, expanded

aggregates appear to shrink, further spreading the cracks in the ITZ. As a result, the residual compressive strength and modulus of elasticity are much lower than the ones tested in hot state [138,139]. Bamonte and Gambarova [37] also studied the compressive stress–strain relationship of self-compacting concretes at the two testing conditions. It was found that when the temperature was below 400 °C, the peak stress of the specimens after cooling was higher than the hot tested ones. However, when the temperature was above 400 °C up to 600 °C, the trend was contrary. During the whole period of heating, the peak stress of the hot tested specimens was always observed at a later stage. In the study carried out by Watanabe et al. [132], it was found that the bending strength of concrete specimens at hot state was lower than that after cooling during the whole heating period up to temperature of 600 °C. The authors attributed the reason for this to the fact that tensile stresses increased during the heating, but did 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 the whole heating process up to temperature of 800 °C, a stress of 40% of the ultimate compressive strength at room temperature was loaded onto the high strength concrete cylinder specimens. The results showed that the compressive strength of the stressed specimens was comparable to the unstressed ones during the whole heating process. However, according to the results reported by Phan and Carino [14] and Fu et al. [18], during the whole heating process up to temperatures of 450 °C and 600 °C, respectively, the compressive strength of the specimens at stressed state was higher than the unstressed ones when a stress of 40% of the ultimate compressive strength at room temperature was applied onto the stressed specimens. In the study carried out by Tao et al. [140],

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20% of the ultimate compressive strength at room temperature was loaded onto the self-compacting concrete cylinder specimens during the whole heating process up to temperature of 800 °C. The stressed results were compared to the unstressed ones, and it was also found that the compressive strength of the specimens was higher for the stressed test. In the study carried out by Fu et al. [18], modulus of elasticity of high strength concrete at stressed (40% of the ultimate compressive strength at room temperature) and unstressed states was tested during heating process up to temperature of 600 °C. It was found that the stressed modulus of elasticity was higher than the unstressed ones during the whole heating process. The reason for the higher compressive strength and modulus of elasticity at the stressed state could be attributed to the fact that the pre-loading induced friction between the ends of specimens and the heads of testing machine limits the thermal stress in expansion and then restrains the thermal cracking [18]. In addition, the coarsened pores caused by high temperature could be compressed under the pre-loading, densifying the pore structure of concrete. This could also be beneficial for the improvement of the compressive strength and modulus of elasticity of the concrete under stressed state [18]. The stress–strain relationship of concrete at stressed (40% of the ultimate compressive strength at room temperature) and unstressed states during heating process was also studied in the research carried out by Fu et al. [18]. It was found that during the heating process up to temperature of 600 °C, the peak stress of the stressed specimens was higher than the unstressed ones and was observed 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 compressive strength at room temperature were applied onto fibre reinforced concrete cylinder specimens during the whole heating process (the heating regime was in accordance with ISO834). Stress–strain relationship of the specimens was studied and the results were compared to the unstressed ones. The findings were similar to the ones reported previously [18] when 20% pre-loading is considered. However, the data for 40% pre-loading was invalid as spalling occurred for most specimens under such pre-loading level, which could be used to indicate that spalling of concrete at high 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 of concrete under bi-axial condition was tested during a heating process, and the results were compared to the ones tested under uni-axial condition. The stresses applied were in a tensile direction for both axes. It was found that the concrete specimens were damaged more seriously under bi-axial condition during the whole heating process up to temperature of 600 °C. In addition, it was found that no matter the fraction between the horizontal stress applied and the perpendicular one, compared to the uni-axial strength at room temperature, the strength loss in the perpendicular direction was smaller than that in the horizontal direction. At temperature of 600 °C, when the ratio between the horizontal stress and the perpendicular stress was 1:5, only 5% of the ultimate uni-axial strength at room temperature was remained in the horizontal direction, while the value was 25% for the perpendicular one. Similar results were also reported by Theinel and Rostasy [142]. In the study carried out by He and Song [143], bi- and tri-axial tensile-compressive tests were performed on high performance concrete specimens at different stress ratios after heating to high temperature up to 600 °C. The results showed that the strength loss of concrete specimens under tri-axial state was greater than that under bi-axial state during the whole heating process. In addition, it was found that the tensile strength increased with the

decrease of stress ratio for any given temperature, while the change of compressive strength was contrary. 4.4. Specimen size In the study carried out by Barnagan et al. [144], residual modulus of elasticity of concrete cylinder specimens of Ø150  300 mm and prism specimens of 75  105  430 mm after heating to temperature of 500 °C was tested. The results showed that the loss of modulus of elasticity caused by the heating was comparable between the two types of concrete specimens. Arioz [145] also found that the difference of the residual compressive strength between the concrete cubes of 100  100  100 mm and the cubes of 150  150  150 mm was not significant after the exposures to temperatures from 20 °C to 1200 °C. Similar results were also reported in the study carried out by Erdem [146] when cylinder specimens with sizes of Ø50  100 mm, Ø100  200 mm and Ø150  300 mm were studied during heating process up to temperature of 800 °C. Bamonte and Gambarova [138] tested the residual compressive strengths of concrete cubes (40  40  40 mm) and concrete cylinders (Ø36  110 mm) after their exposures to elevated temperature up to 750 °C. It was found that the cube specimens always exhibited higher residual compressive strength compared to the cylinder specimens. The authors attributed this to the friction effect 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 exposures to temperatures from 20 °C to 1200 °C. It was found that below 400 °C, the residual splitting strength of the larger specimens was higher than that of the smaller specimens. Afterwards, the difference was not pronounced. The author attributed the reason for this to the fact that the temperature in the centre of the specimens was lower than the temperature at the surface during heating process due to concrete is poorly heat conducted, and such effect was more significant for the larger specimens, especially during the earlier stage of the heating.

5. Conclusion  Deterioration of mechanical properties of concrete occurs at high temperature.  During the high temperature exposure, concrete experiences a series of physical and chemical changes, such as water evaporation, disintegrations of hydration products and aggregates, coarsening of microstructure and increase of porosity. These changes are considered to be responsible for the deterioration of mechanical properties of concrete at high temperature.  Spalling may occur for concrete at high temperature. Water vapour pressure and thermal stress at high temperature may induce the spalling.  The residual compressive strength and modulus of elasticity of the concrete with lower w/b are higher than the concrete with higher w/b. A lower w/b at the beginning of mixing and/or a higher moisture content at the time when concrete is exposed to high temperature is prone to induce spalling of concrete at high temperature as a result of high vapour pressure.  Calcareous aggregates provide greater high temperature resistance to concrete compared to siliceous aggregates. Lightweight concretes have a high resistance to heat due to the natural characteristics of lightweight aggregates. However, the pre-saturation regime of lightweight aggregates which is usually used in practice would induce spalling of lightweight concretes at high temperature.

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 The addition of PFA and slag in concrete could increase its resistance to high temperature, while the addition of SF would reduce such resistance.  Polypropylene fibre generally has no significant influence on the improvements of residual compressive strength and modulus of elasticity for concrete after heating to high temperature. Its improvement on residual splitting tensile strength and flexural strength would be greatly lost after around 400 °C. However, polypropylene reinforced concrete has great resistance to spalling due to the release of vapour pressure.  Steel fibre could generally improve the residual mechanical properties of concrete after heating to high temperature. It could also increase the resistance of concrete to spalling, but the extent of such increase is less than that provided by polypropylene fibre.  When temperature is below 400 °C, the compressive strength of concretes tested at hot state is lower than the one tested after the heating. 400 °C afterwards, the residual compressive strength is lower than the one tested at hot condition. The residual bending strength of concretes is higher than the one tested at hot state.  The compressive strength of concretes at high temperature tested under stressed state is higher than the one tested under unstressed state.  Compared to uni-axial test, bi-axial and tri-axial tests bring more serious damage for concretes at high temperature.  When the difference of specimen size is significant enough, the specimens with smaller size exhibits higher residual compressive strength than the larger specimens at high temperature.

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