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The effects of high temperature on compressive and flexural strengths of ordinary and high-performance concrete
ARTICLE IN PRESS
Fire Safety Journal 41 (2006) 155–163 www.elsevier.com/locate/firesaf
The effects of high temperature on compressive and flexural strengths of ordinary and high-performance concrete Metin Husem Department of Civil Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey Received 29 April 2004; received in revised form 10 October 2005; accepted 5 December 2005
Abstract The variation of compressive and flexural strengths of ordinary and high-performance micro-concrete at high temperatures was examined. Compressive and flexural strengths of ordinary and high-performance micro-concrete which were exposed to high temperatures (200, 400, 600, 800 and 1000 1C) and cooled differently (in air and water) were obtained. Compressive and flexural strengths of these concrete samples were compared with each other and then compared with the samples which had not been heated. On the other hand, strength loss curves of these concrete samples were compared with the strength loss curves given in the codes. Experimental results indicate that concrete strength decreases with increasing temperature, and the decrease in the strength of ordinary concrete is more than that in high-performance concrete. The type of cooling affects the residual compressive and flexural strength, the effect being more pronounced as the temperature increases. Strength loss curves obtained from this study agree with strength loss curves given in the Finnish Code. r 2006 Elsevier Ltd. All rights reserved. Keywords: High-performance concrete; Ordinary concrete; High temperature; Compressive strength; Flexural strength; Loss of strength
1. Introduction Concrete has been defined as a composite material obtained using cement, aggregate, water and when necessary chemical and/or mineral additives, placed into moulds of various sizes and shapes and hardened under convenient conditions [1–3]. Today concrete has been used with an increased strength and durability in connection with the developments in technology in pre-stressed concrete, concrete and reinforced concrete structures. Pre-stressed concrete, concrete and reinforced concrete structures are sometimes exposed to fires and many structures become damaged and/or out of use [4]. As it is known, high temperatures caused as a result of fire decreases the concrete strength and durability of such structures. Fire resistance of concrete is affected by factors like the type of aggregate and cement used in its composition, the temperature and duration of the fire, Tel.: +90 462 377 2622; fax: +90 462 377 2606.
E-mail address: [email protected]. 0379-7112/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.firesaf.2005.12.002
sizes of structure members, and moisture content of concrete [5–7]. Fire resistance of the aggregates is generally high. However, having non-uniform high temperature effect of aggregate or cooling the heated aggregate using water spray may cause internal pressure in the aggregates. This pressure may make the aggregate spall. Some of the deformation in the concrete is due to the expansion of cement in its composition. Hydrated Portland cement contains a significant amount of free calcium hydroxide and will decompose into calcium oxide due to loss of water at 400–450 1C . If this calcium oxide is wetted after being cooled or is kept in a moist environment, it transforms into calcium hydroxide again. The concrete may crumble as a result of such changes in volume [2,3,8,9]. An increase in the size of structural members increases fire resistance. The effects of high temperature on the mechanical properties of concrete have been investigated since the 1940s [8,10–34]. These studies examined the behavior of cement paste, mortar, concrete samples and reinforced concrete members exposed to high temperature [14]. Results of these studies constituted the technical basis for
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the provisions and recommendations for determining concrete strength of elevated temperature in many codes [14]. For design purpose, mechanical properties of concrete at elevated temperature may be obtained using design curves prescribed in the codes [33–38]. Many of these studies were realized for properties of ordinary concrete exposed to high temperature. Fewer studies have been carried out with high-performance concrete than with ordinary concrete [23,27–32]. Concrete structures are sometimes exposed to the effects of fire. Although there are different ways to extinguish the fire, it is generally done with water spray. This causes different stresses in reinforced concrete members at high temperature and the structural member can lose load bearing capacity. In this study, the behavior of ordinary and high-performance concretes exposed to high temperatures is examined after cooling in air or water. In the tests, temperatures of 23, 200, 400, 600, 800, and 1000 1C were chosen for ease of observation of the test results. All series were exposed to same temperatures. To determine the resistance of concrete samples exposed to high temperature, many test methods are used. Three test methods are commonly referred in most experimental programs on the fire performance of concretes [14]. These are named as stressed, unstressed and unstressed residual strength tests. In stressed tests, a preload (20–40% of the compressive strength at 23 1C) is applied to the specimen prior to heating and is sustained during the heating period. Heat is applied at a constant rate until a target temperature is reached, and is maintained for a time until a thermal steady state is achieved. Stress or strain is then increased at a prescribed rate until the specimen fails. In the unstressed test, the specimen is heated, without preload, at a constant rate to the target temperature, which is maintained until a thermal steady state is achieved. Stress or strain is then applied at a prescribed rate until failure occurs. In unstressed residual strength test, the specimen is heated without preload at a prescribed rate to the target temperature, which is maintained until a thermal steady state is reached within the specimen. The specimen is then allowed to cool, following a prescribed rate, to room temperature. Load or strain is applied at room temperature until the specimen fails. In this study, unstressed residual strength test is used to obtain the effects of high temperature on compressive and flexural strengths of ordinary and high-performance concrete. Research has indicated that [20,28,29] strength loss and explosive spalling occur in the concrete exposed to high temperatures. High-performance concrete has been found
to have greater strength loss in the intermediate temperature range than ordinary concrete when exposed to the same heating condition. High-performance concrete specimens are prone to explosive spalling, even when heated at a relatively slow heating rate (p5 C/min). According to the test results obtained from this study, strength loss of highperformance micro-concrete is more than that of ordinary micro-concrete. But explosive spalling in high-performance micro-concrete does not occur in ordinary micro-concrete. 2. The aim of study The main objective of this research is to examine the variation according to cooling type of compressive and flexural strength of high-performance micro-concrete and ordinary micro-concrete at high temperatures. Compressive and flexural strengths of ordinary and high-performance micro-concrete exposed to high temperatures (200, 400, 600, 800, and 1000 1C) and cooled in various environments (in air and water) were obtained and the results compared. Tests were also carried out on samples which had not been exposed to high temperatures. The study also examines the effects of minerals and chemical additives used in the production of high-performance concrete, on the mechanical properties of concrete exposed to high temperature. 3. Experimental study 3.1. Materials Because it is commonly used in concrete production in the region, the limestone aggregate was used in ordinary and high-performance concrete production. The maximum aggregate size used was 16 mm. The physical properties of this aggregate are given in Table 1. The petrographic structure of ordinary aggregate includes only opaque minerals ðo1%Þ and micritic cemented limestone comprising partially old microfossils. In the production of high-performance concrete and micro-concrete, PC 42.5 Portland cement was used. The number 42.5 denotes its characteristic compressive strength in MPa. In the production of ordinary concrete and microconcrete, PC 32.5 Portland cement was used. Some properties of these cements are given in Tables 2 and 3, respectively. In the production of high-performance concrete and micro-concrete, silica fume and superplasticizer
Table 1 The physical properties of aggregate Aggregate size
Loose density (kg=m3 )
Dry density(kg=m3 )
Saturated density (kg=m3 )
Water absorption (%)
Course(44 mm) Fine(o4 mm)
1445 1485
2706 2675
2720 2682
0.43 0.50
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Table 2 Some properties of PC 42.5 cement Physical properties
Mechanical properties
Density (g=cm3 )
3.10
Age (day)
Flexural strength (MPa)
Compressive strength (MPa)
Specific surface (Blaine) (cm2 =g) Setting Initial (h) time (vicat) Final (h)
3680 2.10 4.15
2 7 28
5.75 7.55 8.75
29.05 43.65 52.97
Table 3 Some properties of PC 32.5 cement Physical properties
Mechanical properties
Density (g=cm3 )
3.05
Age (day)
Flexural strength (MPa)
Compressive strength (MPa)
Specific surface (Blaine) (cm2 =g) Setting time (vicat) Initial (h) Final (h)
3260 2.10 3.40
2 7 28
4.01 5.35 6.65
15.20 28.2 36.17
Table 4 Chemical compositions of silica fume Component
SiO2
Fe2 O3
Al2 O3
CaO3
MgO3
CrO3
Loss on ignition
Free CaO
(%)
82
1.8
3.2
1.4
5
3
2.2
1.2
(ASTM C-494 F type) admixture were used. Chemical properties of silica fume are given in Table 4. In the examination of the effects of high temperatures on the compressive and flexural strengths of high performance and ordinary concrete, micro-concrete samples were used. High performance and ordinary concrete were produced first and their compressive strengths were determined. Micro-concrete having the same properties with the mortar phase in the concrete was then produced. Micro-concrete is used in the study of the effect of high temperature on flexural and compressive strength.
Table 5 The gradation of aggregate Gradation class
%, total weight
0 mm–0.25 mm 0.25 mm–0.50 mm 0.50 mm–1.00 mm 1.00 mm–2.00 mm 2.00 mm–4.00 mm 4.00 mm–8.00 mm 8.00 mm–16.00 mm
4 8 8 12 21 19 28
3.2. Mixture and production of concrete The gradations of aggregate used in the high performance and ordinary concrete were the same. The aggregate proportion is given in Table 5. In the mixture proportioning of high performance and ordinary concrete, water to cement ratios of 0.30 and 0.50 were used, respectively. The mixture proportions of concretes are given in Table 6. In the production of high-performance concrete, silica fume was added (10% of weight of cement) and superplasticizer admixture was used (2% of total weight of cement and silica fume). In mixing concrete, a concrete mixer having 80 l capacity and inclined axes was used. Each granulometric aggregate was weighed and placed into the concrete mixer moistened in
advance and mixed for 3 min with the addition of saturation water, for 3 min with the addition of cement (together with silica fume, if any), and thereafter, mixed for another 3 min without stopping to add the mixing water (together with the superplasticizer admixture, if any). The resulting concrete was placed in standard cylinder (150 mm 300 mm) moulds at 3 stages and 12 samples were prepared in each production (12 samples for high-performance concrete and 12 samples for ordinary concrete). The samples, which were taken out the day after, were kept in water at 22 2 C for 21 days. They were kept at 23 1C and 65% relative humidity until the time of the experiment. The specimens were 28 days old at the time of the tests.
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Table 6 Mix design of high performance and ordinary concrete Concretes
HPC OC
W/C
0.30 0.50
Cement ðkg=m3 Þ
500 350
Water ðkg=m3 Þ
150 175
Total aggregate ðkg=m3 Þ
1789 1829
Absorbed water ðkg=m3 Þ
9.5 11.70
Admixtures SP ðkg=m3 Þ
SF ðkg=m3 Þ
22 —
50 —
HPC, high-performance concrete; SP, superplasticizer admixture; OC, ordinary concrete; and SF, silica fume.
Table 7 Mix design of micro-high performance and ordinary micro-concrete Components
Cement Water Sand Silica fume Superplasticizer
Micro-highperformance concrete ðkg=m3 Þ
Micro-ordinary concrete ðkg=m3 Þ
786 236 1286 78.6 25.9
572 286 1345 — —
3.3. Mixture and production of micro-concrete The mix proportions of micro-high performance (HPMC) and ordinary micro-concrete (OMC) are given in Table 7. The maximum aggregate size used was 4 mm. The gradation of aggregate is given in Table 8. On examining the effects of high temperature on the mechanical properties of high performance and ordinary concrete, prismatic micro-concrete experimental samples with dimensions of 40 mm 40 mm 160 mm were produced to represent the mortar in the composition of such concrete. Water–cement ratios (in high-performance concrete it is 0.30, in ordinary concrete 0.50) and cement–sand ratios used in the production of concrete (in high-performance concrete it is 0.620, in ordinary concrete 0.425) are the same in order to obtain the mortars in high performance and ordinary concrete in the production of such samples. They are placed into prismatic moulds in three stages, applying vibration for 15 s. The samples were taken out of their moulds after 1 day and kept in water at 22 2 C for 21 days. Until the time of the experiment, they were kept at 23 1C, and 65% relative humidity. For each micro-concrete type, 180 test samples were produced. The samples were 28 days old at the time of the experiment. 3.4. Test procedure In this study, uniaxial compressive tests on high performance and ordinary concrete samples (for each concrete 12 samples) were performed with a constant loading rate of 0.15 MPa/s. The reason of choosing 0.15 MPa/s is to keep the loading rate to a minimum in the comparison of test results.
Table 8 The gradation of aggregate for micro-concrete Gradation class
%, total weight
0 mm–0.25 mm 0.25 mm–0.50 mm 0.50 mm–1.00 mm 1.00 mm–2.00 mm 2.00 mm–4.00 mm
7 15 15 23 40
In determining the effects of high temperature on the compressive and flexural strength of HPMC and OMC, a flexural test was done on the prismatic samples with dimensions 40 mm 40 mm 160 mm. Compressive tests were done on the samples which had been broken before. For each micro-concrete, 36 samples were used at each test temperature. The tests were performed at five different temperatures (200, 400, 600, 800 and 1000 1C) in order to have practical measurements. Twelve of these samples were kept at 23 1C and the other 24 samples were put in the oven. They were removed 1 h after the desired temperature was reached. Twelve of these samples were cooled in water and the other 12 were cooled in air (at room temperature) until they were at 23 1C. The flexural and compressive tests were performed on the cooled samples and 12 others which were kept at 23 1C. 4. Results and discussion Compressive strength tests are done on the concrete and micro-concrete samples. Twelve samples were used for each series of experiments. The average compressive strengths and standard deviations are given in Table 9. In this study, ordinary concrete with an average compressive strength of 34 MPa and high-performance concrete with an average compressive strength of 71 MPa have been produced using the composition ratios in Table 6. Micro-concrete samples are produced to represent the mortar phase of the produced ordinary and high-performance concrete. The samples produced for the examination of the behavior of HPMC and OMC exposed to high temperature are placed into an oven with a heating capacity of 1200 1C. The variation of the oven temperature as a function of time is given in Fig. 1. As it is seen in the figure, the rate of
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was performed on 24 samples for each series. The compressive and flexural strengths obtained in this way are given in Table 10. Change with temperature of flexural strength of HPMC and OMC concrete is given in Fig. 2, and change with temperature of compressive strength of micro-concretes is given Fig. 3. In these figures, ordinary micro-concrete is denoted as OMC, high-performance micro-concrete as HPMC, air cooling as AC, and water cooling as WC. The flexural strength of OMC cooled in air after being exposed to the effect of different temperatures is lower than that of the reference samples: 21% for 200 1C, 33% for 400 1C, 58% for 600 1C, 63% for 800 1C. Flexural strength of OMC cooled in water after being exposed to the effect of different temperature is also lower than that of reference samples: 22% for 200 1C, 36% for 400 1C, 68% for 600 1C, 84% for 800 1C. The compressive strength of OMC cooled in air is less than that of the reference samples: 7% for 200 1C, 12% for 400 1C, 27% for 600 1C, 47% for 800 1C. Compressive strength of OMC cooled in water is also less than that of reference samples: 27% for 200 1C, 29% for 400 1C, 44% for 600 1C. Because the samples disintegrated in water, compressive tests on samples could not be completed for 800 and 1000 1C. The flexural strength of HPMC cooled in air after being exposed to the effect of different temperature is lower than that of reference samples: 36% for 200 1C, 27% for 400 1C, 36% for 600 1C, 60% for 800 1C, 71% for 1000 1C. The flexural strength of HPMC cooled in water after being exposed to the effect of different temperature is also lower than that of reference samples: 30% for 200 1C, 28% for 400 1C, 45% for 600 1C, 70% for 800 1C. Because the samples disintegrated in water, flexural tests on samples could not be completed for 1000 1C. Compressive strength of HPMC cooled in air is smaller than that of reference samples: 32% for 200 1C, 23% for 400 1C, 26% for 600 1C, 51% for 800 1C and 75% for 1000 1C. Compressive strength of HPMC cooled in water is also less than that of the reference samples: 33% for 200 1C, 29% for 400 1C, 34% for 600 1C, 56% for 800 1C. Because the samples
achieving an oven temperature of 1000 1C is 6.67 1C/min in OMC and 5.5 1C/min in HPMC. The slope of time– temperature curve is higher for HPMC than for OMC. Because HPMC has a structure with fewer pores than OMC, therefore thermal conductivity of HPMC is higher than that of OMC. Twenty-eight days after their production, experimental samples were kept in the oven until the temperature is 200, 400, 600, 800, and 1000 1C, respectively. Then, they were taken out of the oven and some of them were left to cool in air and some of them in water till they reached 23 1C. Cooled in air or water, samples were first exposed to flexural and then to compressive experimentation on broken pieces. The flexural test was carried out on 12 samples for each series, but the compressive strength test
Table 9 The mechanical properties of concretes Concretes
Average compressive strength (MPa)
Standard division(MPa)
HPC OC
71 34
1.3 0.8
1200
Temperature (C)
1000 800 HPMC OMC
600 400 200 0 0
50
100
159
150
Time (minute)
Fig. 1. Variation of oven temperature by time.
Table 10 The mechanical properties of micro-concrete in different temperatures Temperature ( C)
Ordinary micro-concrete Flexural strength (MPa)
High-performance micro-concrete Compressive strength (MPa)
Flexural strength (MPa)
Compressive strength (MPa)
Type of cooling
23 200 400 600 800 1000
in air
in water
in air
in water
in air
in water
in air
in water
7.2 6.1 3.8 3.4 0.7
9.1 7.1 5.8 2.9 1.5 0.2
55 52.2 43.4 31.5 6.5
59.4 43.2 42.1 33.0 0 0
6.2 7.1 6.2 3.9 2.8
9.7 6.8 7.0 5.3 2.9 0
58 65.4 62.9 41.3 21
85.1 57.1 60.1 56.2 37.6 0
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Flextural strength (MPa)
12 OMC-AC
10
OMC-WC
8
HPMC-AC HPMC-WC
6 4 2 0 0
200
400
600
800
1000
1200
Temperature
Compressive strength (MPa)
Fig. 2. Variation of flexural strength with temperature.
100 OMC-AC
80
OMC-WC HPMC-AC
60
HPMC-WC
40 20 0 0
200
400
600 800 Temperature
1000
1200
Fig. 3. Variation of compressive strength with temperature.
disintegrated in water, compressive test on samples could not be completed for 1000 1C. As one will see from these figures, flexural and compressive strength of high-performance concrete cooled in air and water after being exposed to the effect of high temperature decrease up to 200 1C compared to the flexural and compressive strengths of the reference samples and the strength rapidly decreases after showing a certain increase at 200–400 1C. This decrease is more in those samples cooled in water. Decrease in flexural and compressive strength of ordinary concrete is much more compared to that of high-performance concrete. And this shows that high-performance concrete is more resistant to the effect of high temperature. However, above 600 1C, decrease in the strength of both concretes rapidly increases. The reason for this is that there is no significant change in concretes up to a temperature of 300 1C in aggregate and mortar phases, and that there are significant changes in aggregate and mortar phases after this temperature. These results are in harmony with the results of various research studies [17,18,22]. Flexural and compressive strengths of OMC and HPMC exposed to a high temperature, then cooled in water are less than those cooled in air. It is apparent that strength losses of OMC cooled in water compared to OMC cooled in air is more than the losses of HPMC (see Table 10). In the former studies, it is stated that, when ordinary concrete is heated to 300 1C, strength loss changes by 10–20% and up to 600 1C it changes by 60–70%. In addition, high-performance concrete heated over 450 1C
has a 40% strength loss [14,28]. In this study, test results show that compressive strength loss of OMC heated to 400 1C and cooled in air is 12%, in water is 29%. Compressive strength loss of samples heated up to 600 1C and cooled in air is 27%, in water is 44%. Compressive strength loss of HPMC heated to 400 1C and cooled in air is 23%, in water is 29%. Compressive strength loss of same concrete heated to 600 1C and cooled in air is 26%, in water is 34%. In the test results it is seen that compressive strength of OMC under the effect of high temperature and cooled in air and water has a decreasing trend when the temperature increases. Strength loss curves obtained from this study for the ordinary concrete have the same trend—but different changing ratio—obtained from Abrams [33], Morita et al. [34], and Furumura et al. [39] and from studies in NIST [14,29]. In the present study, it is seen that the strength of high-performance concrete exposed to high temperature and cooled in air and water decreases until 200 1C, an increase between 200 and 400 1C and a continuous decrease after 400 1C (see in Fig. 3 and Table 10). The strength increase at 200–400 1C in the samples cooled in air is more than for those cooled in water. These results have a similar trend to those of Khoury [22] and Hammer [40,41] and to the studies at NIST [14,24,27,29]. However, in Khoury’s study, strength decreases until 100 1C, increases between 100 and 400 1C and decreases continuously above 400 1C. In the studies performed at NIST, strength in highperformance concrete decreased until 100 1C, increased between 100 and 200 1C for some concrete mixtures and decreased rapidly after 200 1C. In Hammer’s study, the strength decreased until 300 1C (except the concrete produced without silica fume), increased from 300 to 450 1C and decreased rapidly after 450 1C. According to the results obtained from the tests, when the temperature is increased from 23 to 200 1C, compressive strength loss of HPMC is 32% for the specimens which were cooled in air and 33% for the specimens which were cooled in water. When the temperature is 400 1C, compressive strength loss is 23% for cooling in air and 29% for cooling in water. As it can be seen in Fig. 3, when the temperature is increased from 200 to 400 1C, compressive strength gain is 13% for the specimens cooling in air and 5% for those cooled in water. The former studies indicated that the increase is caused by evaporation of free water and removal of water of crystallization from the cement paste [42]. This result is supported by the experiments in that the specimens cooled in air have more strength gain than the specimens cooled in water because while they were being cooled in water some of the evaporated water is regained. It was reported that when explosive spalling occurs, the temperature range is between 300 and 650 1C [14,25,27,29]. Many factors were identified as affecting explosive spalling. These factors include age, moisture content, type of gravel and sand used, curing method, rate of heating [15]. In this study, it has been observed that when the temperature is
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Table 11 The mechanical properties of micro concrete with silica fume or superplasticizer admixture Temp. ð CÞ
23 400 600
Micro-concrete with superplasticizer (without silica fume)
Micro-concrete with silica fume (without superplasticizer)
High-performance micro-concrete (in Table 10)
Flexural strength (MPa)
Compr. strength (MPa)
Flexural strength (MPa)
Compr. strength(MPa)
Flexural strength(MPa)
Compr. strength(MPa)
9.6 8.5 8.3
63.4 66.0 66.7
5.7 6.8 5.6
47.8 42.3 41
9.7 7.1 6.2
85.1 65.4 62.9
Compressive strength (MPa)
Flextural strength (MPa)
12 10 8 6 4 2
SPMC
SFMC
HPMC
HPMC-WC
0 0
200
400 Temperature
600
800
90 80 70 60 50 40 30 20 10 0 0
Fig. 4. Variation of flexural strength of high-performance micro-concrete with temperature.
SPMC
SFMC
HPMC
200
400 Temperature
HPMC-WC
600
800
Fig. 5. Variation of compressive strength of high-performance microconcrete with temperature.
400–500 1C, approximately 30% of samples produced from HPMC spalled in the experiment. To examine the effect of admixed material used in concrete producing the explosive spalling, micro-concrete was first produced without using the superplasticizer admixture (which is used for the production of HPMC), and then without using silica fume (Table 11). These concretes were exposed to a heat process of 400 and 600 1C and cooled in air. The results are given in Fig. 4. As seen in the figure, flexural strength of micro-concrete produced using superplasticizer admixture is 1% less than the flexural strength of HPMC for 23 1C, 20% more for 400 1C, and 34% more for 600 1C. As for flexural strength of micro-concrete produced using silica fume (without using superplasticizer admixture), it is 41%, 4% and 10% less than that of HPMC for 23, 400 and 600 1C, respectively. Likewise, the compressive strength of microconcrete produced using superplasticizer admixture (without using silica fume) is 25% less than the compressive strength of HPMC for 23 1C, and 1% more for 400 1C, 6% more for 600 1C. Compressive strength of micro-concrete produced using silica fume is less than the compressive strength of HPMC by 44% for 23 1C, 35% for 400 1C and 35% for 600 1C. And this shows that the decrease in the flexural and compressive strength of micro-concrete produced using only silica fume is more than that of micro-concrete produced using only superplasticizer admixture and that the concrete produced using silica fume is affected more from high temperature. According to the results of experimentation conducted, it is considered that
the burst in HPMC is caused by the expansion of silica fume (Fig. 5). Strength loss curves of micro-concrete used in this study are given in Fig. 6 with the design curves given in Codes [35–38]. As it is seen in this figure, the Finnish Code is more suitable than the other Codes for high-performance concrete (except cooled in water) until 400 1C. CEB [35] and the Finnish Code [38] are more suitable than Eurocode [36,37] for ordinary and high-performance concrete between 400 and 600 1C. After 600 1C, Eurocode design curves are the most suitable ones except for ordinary concrete cooled in water. 5. Conclusions The conclusions drawn from the results obtained in this study are as follows:
Ordinary concrete with an average compressive strength of 34 MPa and high-performance concrete with an average compressive strength of 71 MPa have been produced. In order to represent their mortars microconcrete samples have been prepared. In ordinary and HPMC exposed to high temperature, flexural and compressive strength decreases with the increase of temperature. Such decrease is greater in those cooled in water. The compressive strength of HPMC cooled in air and water decreased up to 200 1C. The compressive strength of
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1.2 Eurocode Desing Curve (Siliceous aggregate) Eurocode Design Curve (Calcareous aggregate) CEB Desing Curve Finnish Design Curve (rakMK B4 K70-K100) OMC (Cooling in air) OMC (Cooling in water) HPMC (Cooling in air) HPMC (Cooling in water)
1
fc/f'c(23)
0.8
0.6
0.4
0.2
0 0
200
400
600
800
1000
1200
1400
Temperature (C) Fig. 6. Comparison of design curves and experimental loss of strength curves.
HPMC was increased between 200 and 400 1C. The compressive strength of OMC was decreased continuously. The compressive strength gain was 13% for specimens cooling in air. For the specimens cooling in water the strength gain was 5%. The compressive test was not done for OMC in the temperature above 600 1C, because of concrete samples disintegrated. For HPMC, the compressive test was not done at temperatures above 800 1C. It has been observed that some samples produced from HPMC spall explosively at temperatures between 400 and 500 1C and it has been seen that the cause for such explosive spalling in high-performance concrete is expansion of silica fume used in the production of such concretes. The explosive spalling was not observed for OMC specimens. Experimental studies indicated that OMC and HPMC produced using limestone aggregate caused loss of strength in high percentages in those cooled in water after being exposed to high temperature. Studies show that experimental samples have been damaged to a great extent and they have lost their compressive strengths if high-performance concrete is cooled in water after being exposed to the temperature of 800 1C, and ordinary concrete is cooled in water after being exposed to the temperature of 600 1C. The concrete may completely lose its strength as a result of the immediate expansions that will form during the expansion of mineral admixture used in the production of high-performance concretes in high temperature and/ or water spray-cooling of a reinforced concrete building element exposed to high temperature as a result of a fire. The CEN Eurocode and the CEB’s design curves for properties of fire-exposed concrete are not applicable to
high strength micro-concrete. Finnish Code is more suitable for high-performance concrete especially until 400 1C temperature. These codes are not applicable to OMC and HPMC cooled in water.
References [1] Neville AM. Properties of concrete. 3rd ed. Harlow Essex, England: Longman Scientific & Technical, Burnt Mill (Co-published in the United States with New York: Wiley); 1993. [2] Akman MS. Materials of construction. Istanbul: Istanbul Technical University Press; 1987. [3] Erdog˘an TY. Aggregates, the Turkish ready mixed concrete association. Istanbul, Turkey; 1985. [4] Cioni P, Croce P, Salvatore W. Assessing fire damage to reinforced concrete elements. Fire Safety J 2001;36(2):181–99. [5] Phan LT, Lawson JR, Davis FL. Effects of elevated temperature exposure on heating characteristics, spalling, and residual proportion of high-performance concrete. Mater Struct 2001;34(236):83–91. [6] Diederchs U, Schneider U. Bond strength at high temperature. Mag Concr Res 1981;33(115):75–84. [7] Noumowe AN, Clostres P, Debicki G, Bolvin M. High temperature effects on high-performance concrete (70–600 1C) strength and porosity. In: Malhotra VM, editor. Proceedings of the third CANMET/ACI international conference on durability of concrete. Nice, France; 1994. p. 157–172. [8] Malhotra HL. The effects of temperature on the compressive strength concrete. Mag Concr Res 1956;8(3):85–94. [9] Ako¨z F, Yu¨zer N, Koral S. The influence of high temperature on the physical and mechanical properties of ordinary Portland cement and silica fume mortar. Tech J Turk Chamber Civ Eng 1995;6(1):919–31. [10] Menzel CA. Tests of the fire resistance and thermal properties of solid concrete slabs and their significance. Proceeding, American Society for Testing and Materials (ASTM) 1943;V43:1099–153. [11] Binner CR, Wilkie CB, Miller P. Heat testing of high density concrete, supplement. Declassified Report HKF-1 U.S. Atomic Energy Commission; June 1949.
ARTICLE IN PRESS M. Husem / Fire Safety Journal 41 (2006) 155–163 [12] Saemann JG, Washa GW. Variation of mortar and concrete properties and temperature. ACI J Proc 1957;54(5):385–97. [13] Gustaferro AH, Sevaggio SL. Fire endurance of simply supported prestressed concrete slabs. J Prestressed Concr Inst 1967;12(1):37–52. [14] Phan LT. Fire performance of high-strength concrete: a report of the state-of-the- art. Gaithersburg, Maryland: NISTIR 5934, Building and Fire Research laboratory, National Institute of Standards and Technology; 1996. [15] Short NR, Purkiss JA, Guise SE. Assessment of fire damaged concrete using color image analysis. Constr Build Mater 2001;15(1):9–15. [16] Safi M. Effects of fire on FRP reinforced concrete members. Compos Struct 2002;58(1):11–20. [17] Janotka I, Nurnbergerova T, Nad L. Thermo-mechanical properties of penly reactor envelope at temperature up to 200 1C. Mater Struct 1999;32(224):719–26. [18] Saad M, Abo EI Enein SA, Hanna GB, Kotkata MF. Effects of temperature on physical and mechanical properties of concrete containing silica fume. Cem Concr Res 1996;26(5):669–75. [19] Schneider U. Concrete at high temperatures—a general review. Fire Safety J 1988;1381:55–68. [20] Phan LT, Carino NJ. Review of mechanical properties of HSC at elevated temperature. J Mater Civ Eng ASCE 1998;10(1):58–64. [21] Xiao J, Ko¨nig G. Study on concrete at high temperature in China— an overview. Fire Safety J 2003;39(1):89–103. [22] Khoury GA. Compressive strength of concrete at high temperature: a reassessment. Mag Concr Res 1992;44(161):291–309. [23] Castillo C, Durrani AJ. Effects of transient high temperature on high strength concrete. ACI Mater J 1990;87(1):47–53. [24] Kodur VKR, Lie TT. Fire performance of concrete-filled hollow steel columns. J Prot Eng 1995;7(3):89–98. [25] Ali F. Is high strength concrete more susceptible to explosive spalling than normal strength concrete in fire. Fire Mater 2002;26:127–30. [26] Kodur VKR, Lie TT. Evaluation of fire resistance of rectangular steel columns filled with fibre-reinforced concrete. Can J Civ Eng 1997;24: 339–49. [27] Kodur VKR, Sultan MA. Effect of temperature on thermal properties high-strength concrete. J Mater Civ Eng (ASCE) 2003;15(2): 101–7. [28] Phan LT, Carino JN. Fire performance of high strength concrete: research needs. In: Mohammed E, editor, ASCE/SEI structures congress 2000 proceedings, May 8–10, Philadelphia, PA; 2000. [29] Phan LT, Carino NJ. Mechanical properties of high strength concrete at elevated temperatures. Gaithersburg, Maryland: NISTIR 6726,
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Building and Fire Research laboratory, National Institute of Standards and Technology; 2001. Phan LT, Lawson JR, Davis FR. Effects of elevated temperature exposure on heating characteristics, spalling and residual properties of high-performance concrete. RILEM Mater Struct J 2001;34(236):83–91. Castillo C, Durrani AJ. Effects of transient high temperature on high strength concrete. ACI Mater J 1990;87(1):47–53. Diederchs U, Jumppanen UM, Schneider U. High temperature properties and spalling behavior of high strength concrete. Proceedings of the fourth Weimar workshop on high-performance concrete: material properties and design, Hochschule fu¨r Architektur und Bauwesen (HAB), October 4th and 5th, Weimar, Germany: 1995. p. 219–236. Abrams MS. Compressive strength of concrete at temperature to 1600 F. American Concrete Institute, Temperature of Concrete, SP 25 Detroit, Michigan; 1971. Morita T, Saito H, Kumagai H. Residual mechanical properties of HSC members exposed to high temperature—Part 1: test on mechanical properties, summaries of annual meeting. Architectural Institute of Japan, Niigata; 1992. Comite Euro-International Du Beton, Fire design of concrete structures-in accordance with CEB/FIB Model Code 90. CEB Bulletin D’Information, No. 208, Lausanne, Switzerland, July 1991. Comite´ Europe´en de Normalisation (CEN), prENV1992-1-2: Eurocode 2: Design of Concrete Structures. Parts 1–2: Structural Fire Design, CEN/TC 250/SC 2; 1993. Comite´ Europe´en de Normalisation (CEN), Eurocode 4: Design of Composite Steel and Concrete Structures. Parts 1–2: General RulesStructural Fire Design, CEN ENV; 1994. Concrete Association of Finland, High strength concrete supplementary rules and fire design RakMK B4; 1991. Furumura F, Abe T, Shinohara Y. Mechanical properties of high strength concrete at high temperature. Proceedings of the fourth Weimar workshop on high-performance concrete: material properties and design, Hochschule fu¨r Architektur und Bauwesen (HAB), October 4th and 5th, Weimar, Germany; 1995. p. 237–254. Hammer TA. High-strength concrete phase 3, compressive strength and E-modulus at elevated temperatures, SP6 fire resistance. Report 6.1, SINTEF Structures and Concrete, STF 70 A9 5023; 1995. Hammer TA. High-strength concrete phase 3, spalling reduction through material design, SP6 fire resistance. Report 6.2, SINTEF Structures and Concrete, STF 70 A9 5024; 1995. Lie TT. Fire and buildings. London: Applied Science Publishers Ltd.; 1972.
Fire Safety Journal 41 (2006) 155–163 www.elsevier.com/locate/firesaf
The effects of high temperature on compressive and flexural strengths of ordinary and high-performance concrete Metin Husem Department of Civil Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey Received 29 April 2004; received in revised form 10 October 2005; accepted 5 December 2005
Abstract The variation of compressive and flexural strengths of ordinary and high-performance micro-concrete at high temperatures was examined. Compressive and flexural strengths of ordinary and high-performance micro-concrete which were exposed to high temperatures (200, 400, 600, 800 and 1000 1C) and cooled differently (in air and water) were obtained. Compressive and flexural strengths of these concrete samples were compared with each other and then compared with the samples which had not been heated. On the other hand, strength loss curves of these concrete samples were compared with the strength loss curves given in the codes. Experimental results indicate that concrete strength decreases with increasing temperature, and the decrease in the strength of ordinary concrete is more than that in high-performance concrete. The type of cooling affects the residual compressive and flexural strength, the effect being more pronounced as the temperature increases. Strength loss curves obtained from this study agree with strength loss curves given in the Finnish Code. r 2006 Elsevier Ltd. All rights reserved. Keywords: High-performance concrete; Ordinary concrete; High temperature; Compressive strength; Flexural strength; Loss of strength
1. Introduction Concrete has been defined as a composite material obtained using cement, aggregate, water and when necessary chemical and/or mineral additives, placed into moulds of various sizes and shapes and hardened under convenient conditions [1–3]. Today concrete has been used with an increased strength and durability in connection with the developments in technology in pre-stressed concrete, concrete and reinforced concrete structures. Pre-stressed concrete, concrete and reinforced concrete structures are sometimes exposed to fires and many structures become damaged and/or out of use [4]. As it is known, high temperatures caused as a result of fire decreases the concrete strength and durability of such structures. Fire resistance of concrete is affected by factors like the type of aggregate and cement used in its composition, the temperature and duration of the fire, Tel.: +90 462 377 2622; fax: +90 462 377 2606.
E-mail address: [email protected]. 0379-7112/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.firesaf.2005.12.002
sizes of structure members, and moisture content of concrete [5–7]. Fire resistance of the aggregates is generally high. However, having non-uniform high temperature effect of aggregate or cooling the heated aggregate using water spray may cause internal pressure in the aggregates. This pressure may make the aggregate spall. Some of the deformation in the concrete is due to the expansion of cement in its composition. Hydrated Portland cement contains a significant amount of free calcium hydroxide and will decompose into calcium oxide due to loss of water at 400–450 1C . If this calcium oxide is wetted after being cooled or is kept in a moist environment, it transforms into calcium hydroxide again. The concrete may crumble as a result of such changes in volume [2,3,8,9]. An increase in the size of structural members increases fire resistance. The effects of high temperature on the mechanical properties of concrete have been investigated since the 1940s [8,10–34]. These studies examined the behavior of cement paste, mortar, concrete samples and reinforced concrete members exposed to high temperature [14]. Results of these studies constituted the technical basis for
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the provisions and recommendations for determining concrete strength of elevated temperature in many codes [14]. For design purpose, mechanical properties of concrete at elevated temperature may be obtained using design curves prescribed in the codes [33–38]. Many of these studies were realized for properties of ordinary concrete exposed to high temperature. Fewer studies have been carried out with high-performance concrete than with ordinary concrete [23,27–32]. Concrete structures are sometimes exposed to the effects of fire. Although there are different ways to extinguish the fire, it is generally done with water spray. This causes different stresses in reinforced concrete members at high temperature and the structural member can lose load bearing capacity. In this study, the behavior of ordinary and high-performance concretes exposed to high temperatures is examined after cooling in air or water. In the tests, temperatures of 23, 200, 400, 600, 800, and 1000 1C were chosen for ease of observation of the test results. All series were exposed to same temperatures. To determine the resistance of concrete samples exposed to high temperature, many test methods are used. Three test methods are commonly referred in most experimental programs on the fire performance of concretes [14]. These are named as stressed, unstressed and unstressed residual strength tests. In stressed tests, a preload (20–40% of the compressive strength at 23 1C) is applied to the specimen prior to heating and is sustained during the heating period. Heat is applied at a constant rate until a target temperature is reached, and is maintained for a time until a thermal steady state is achieved. Stress or strain is then increased at a prescribed rate until the specimen fails. In the unstressed test, the specimen is heated, without preload, at a constant rate to the target temperature, which is maintained until a thermal steady state is achieved. Stress or strain is then applied at a prescribed rate until failure occurs. In unstressed residual strength test, the specimen is heated without preload at a prescribed rate to the target temperature, which is maintained until a thermal steady state is reached within the specimen. The specimen is then allowed to cool, following a prescribed rate, to room temperature. Load or strain is applied at room temperature until the specimen fails. In this study, unstressed residual strength test is used to obtain the effects of high temperature on compressive and flexural strengths of ordinary and high-performance concrete. Research has indicated that [20,28,29] strength loss and explosive spalling occur in the concrete exposed to high temperatures. High-performance concrete has been found
to have greater strength loss in the intermediate temperature range than ordinary concrete when exposed to the same heating condition. High-performance concrete specimens are prone to explosive spalling, even when heated at a relatively slow heating rate (p5 C/min). According to the test results obtained from this study, strength loss of highperformance micro-concrete is more than that of ordinary micro-concrete. But explosive spalling in high-performance micro-concrete does not occur in ordinary micro-concrete. 2. The aim of study The main objective of this research is to examine the variation according to cooling type of compressive and flexural strength of high-performance micro-concrete and ordinary micro-concrete at high temperatures. Compressive and flexural strengths of ordinary and high-performance micro-concrete exposed to high temperatures (200, 400, 600, 800, and 1000 1C) and cooled in various environments (in air and water) were obtained and the results compared. Tests were also carried out on samples which had not been exposed to high temperatures. The study also examines the effects of minerals and chemical additives used in the production of high-performance concrete, on the mechanical properties of concrete exposed to high temperature. 3. Experimental study 3.1. Materials Because it is commonly used in concrete production in the region, the limestone aggregate was used in ordinary and high-performance concrete production. The maximum aggregate size used was 16 mm. The physical properties of this aggregate are given in Table 1. The petrographic structure of ordinary aggregate includes only opaque minerals ðo1%Þ and micritic cemented limestone comprising partially old microfossils. In the production of high-performance concrete and micro-concrete, PC 42.5 Portland cement was used. The number 42.5 denotes its characteristic compressive strength in MPa. In the production of ordinary concrete and microconcrete, PC 32.5 Portland cement was used. Some properties of these cements are given in Tables 2 and 3, respectively. In the production of high-performance concrete and micro-concrete, silica fume and superplasticizer
Table 1 The physical properties of aggregate Aggregate size
Loose density (kg=m3 )
Dry density(kg=m3 )
Saturated density (kg=m3 )
Water absorption (%)
Course(44 mm) Fine(o4 mm)
1445 1485
2706 2675
2720 2682
0.43 0.50
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Table 2 Some properties of PC 42.5 cement Physical properties
Mechanical properties
Density (g=cm3 )
3.10
Age (day)
Flexural strength (MPa)
Compressive strength (MPa)
Specific surface (Blaine) (cm2 =g) Setting Initial (h) time (vicat) Final (h)
3680 2.10 4.15
2 7 28
5.75 7.55 8.75
29.05 43.65 52.97
Table 3 Some properties of PC 32.5 cement Physical properties
Mechanical properties
Density (g=cm3 )
3.05
Age (day)
Flexural strength (MPa)
Compressive strength (MPa)
Specific surface (Blaine) (cm2 =g) Setting time (vicat) Initial (h) Final (h)
3260 2.10 3.40
2 7 28
4.01 5.35 6.65
15.20 28.2 36.17
Table 4 Chemical compositions of silica fume Component
SiO2
Fe2 O3
Al2 O3
CaO3
MgO3
CrO3
Loss on ignition
Free CaO
(%)
82
1.8
3.2
1.4
5
3
2.2
1.2
(ASTM C-494 F type) admixture were used. Chemical properties of silica fume are given in Table 4. In the examination of the effects of high temperatures on the compressive and flexural strengths of high performance and ordinary concrete, micro-concrete samples were used. High performance and ordinary concrete were produced first and their compressive strengths were determined. Micro-concrete having the same properties with the mortar phase in the concrete was then produced. Micro-concrete is used in the study of the effect of high temperature on flexural and compressive strength.
Table 5 The gradation of aggregate Gradation class
%, total weight
0 mm–0.25 mm 0.25 mm–0.50 mm 0.50 mm–1.00 mm 1.00 mm–2.00 mm 2.00 mm–4.00 mm 4.00 mm–8.00 mm 8.00 mm–16.00 mm
4 8 8 12 21 19 28
3.2. Mixture and production of concrete The gradations of aggregate used in the high performance and ordinary concrete were the same. The aggregate proportion is given in Table 5. In the mixture proportioning of high performance and ordinary concrete, water to cement ratios of 0.30 and 0.50 were used, respectively. The mixture proportions of concretes are given in Table 6. In the production of high-performance concrete, silica fume was added (10% of weight of cement) and superplasticizer admixture was used (2% of total weight of cement and silica fume). In mixing concrete, a concrete mixer having 80 l capacity and inclined axes was used. Each granulometric aggregate was weighed and placed into the concrete mixer moistened in
advance and mixed for 3 min with the addition of saturation water, for 3 min with the addition of cement (together with silica fume, if any), and thereafter, mixed for another 3 min without stopping to add the mixing water (together with the superplasticizer admixture, if any). The resulting concrete was placed in standard cylinder (150 mm 300 mm) moulds at 3 stages and 12 samples were prepared in each production (12 samples for high-performance concrete and 12 samples for ordinary concrete). The samples, which were taken out the day after, were kept in water at 22 2 C for 21 days. They were kept at 23 1C and 65% relative humidity until the time of the experiment. The specimens were 28 days old at the time of the tests.
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Table 6 Mix design of high performance and ordinary concrete Concretes
HPC OC
W/C
0.30 0.50
Cement ðkg=m3 Þ
500 350
Water ðkg=m3 Þ
150 175
Total aggregate ðkg=m3 Þ
1789 1829
Absorbed water ðkg=m3 Þ
9.5 11.70
Admixtures SP ðkg=m3 Þ
SF ðkg=m3 Þ
22 —
50 —
HPC, high-performance concrete; SP, superplasticizer admixture; OC, ordinary concrete; and SF, silica fume.
Table 7 Mix design of micro-high performance and ordinary micro-concrete Components
Cement Water Sand Silica fume Superplasticizer
Micro-highperformance concrete ðkg=m3 Þ
Micro-ordinary concrete ðkg=m3 Þ
786 236 1286 78.6 25.9
572 286 1345 — —
3.3. Mixture and production of micro-concrete The mix proportions of micro-high performance (HPMC) and ordinary micro-concrete (OMC) are given in Table 7. The maximum aggregate size used was 4 mm. The gradation of aggregate is given in Table 8. On examining the effects of high temperature on the mechanical properties of high performance and ordinary concrete, prismatic micro-concrete experimental samples with dimensions of 40 mm 40 mm 160 mm were produced to represent the mortar in the composition of such concrete. Water–cement ratios (in high-performance concrete it is 0.30, in ordinary concrete 0.50) and cement–sand ratios used in the production of concrete (in high-performance concrete it is 0.620, in ordinary concrete 0.425) are the same in order to obtain the mortars in high performance and ordinary concrete in the production of such samples. They are placed into prismatic moulds in three stages, applying vibration for 15 s. The samples were taken out of their moulds after 1 day and kept in water at 22 2 C for 21 days. Until the time of the experiment, they were kept at 23 1C, and 65% relative humidity. For each micro-concrete type, 180 test samples were produced. The samples were 28 days old at the time of the experiment. 3.4. Test procedure In this study, uniaxial compressive tests on high performance and ordinary concrete samples (for each concrete 12 samples) were performed with a constant loading rate of 0.15 MPa/s. The reason of choosing 0.15 MPa/s is to keep the loading rate to a minimum in the comparison of test results.
Table 8 The gradation of aggregate for micro-concrete Gradation class
%, total weight
0 mm–0.25 mm 0.25 mm–0.50 mm 0.50 mm–1.00 mm 1.00 mm–2.00 mm 2.00 mm–4.00 mm
7 15 15 23 40
In determining the effects of high temperature on the compressive and flexural strength of HPMC and OMC, a flexural test was done on the prismatic samples with dimensions 40 mm 40 mm 160 mm. Compressive tests were done on the samples which had been broken before. For each micro-concrete, 36 samples were used at each test temperature. The tests were performed at five different temperatures (200, 400, 600, 800 and 1000 1C) in order to have practical measurements. Twelve of these samples were kept at 23 1C and the other 24 samples were put in the oven. They were removed 1 h after the desired temperature was reached. Twelve of these samples were cooled in water and the other 12 were cooled in air (at room temperature) until they were at 23 1C. The flexural and compressive tests were performed on the cooled samples and 12 others which were kept at 23 1C. 4. Results and discussion Compressive strength tests are done on the concrete and micro-concrete samples. Twelve samples were used for each series of experiments. The average compressive strengths and standard deviations are given in Table 9. In this study, ordinary concrete with an average compressive strength of 34 MPa and high-performance concrete with an average compressive strength of 71 MPa have been produced using the composition ratios in Table 6. Micro-concrete samples are produced to represent the mortar phase of the produced ordinary and high-performance concrete. The samples produced for the examination of the behavior of HPMC and OMC exposed to high temperature are placed into an oven with a heating capacity of 1200 1C. The variation of the oven temperature as a function of time is given in Fig. 1. As it is seen in the figure, the rate of
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was performed on 24 samples for each series. The compressive and flexural strengths obtained in this way are given in Table 10. Change with temperature of flexural strength of HPMC and OMC concrete is given in Fig. 2, and change with temperature of compressive strength of micro-concretes is given Fig. 3. In these figures, ordinary micro-concrete is denoted as OMC, high-performance micro-concrete as HPMC, air cooling as AC, and water cooling as WC. The flexural strength of OMC cooled in air after being exposed to the effect of different temperatures is lower than that of the reference samples: 21% for 200 1C, 33% for 400 1C, 58% for 600 1C, 63% for 800 1C. Flexural strength of OMC cooled in water after being exposed to the effect of different temperature is also lower than that of reference samples: 22% for 200 1C, 36% for 400 1C, 68% for 600 1C, 84% for 800 1C. The compressive strength of OMC cooled in air is less than that of the reference samples: 7% for 200 1C, 12% for 400 1C, 27% for 600 1C, 47% for 800 1C. Compressive strength of OMC cooled in water is also less than that of reference samples: 27% for 200 1C, 29% for 400 1C, 44% for 600 1C. Because the samples disintegrated in water, compressive tests on samples could not be completed for 800 and 1000 1C. The flexural strength of HPMC cooled in air after being exposed to the effect of different temperature is lower than that of reference samples: 36% for 200 1C, 27% for 400 1C, 36% for 600 1C, 60% for 800 1C, 71% for 1000 1C. The flexural strength of HPMC cooled in water after being exposed to the effect of different temperature is also lower than that of reference samples: 30% for 200 1C, 28% for 400 1C, 45% for 600 1C, 70% for 800 1C. Because the samples disintegrated in water, flexural tests on samples could not be completed for 1000 1C. Compressive strength of HPMC cooled in air is smaller than that of reference samples: 32% for 200 1C, 23% for 400 1C, 26% for 600 1C, 51% for 800 1C and 75% for 1000 1C. Compressive strength of HPMC cooled in water is also less than that of the reference samples: 33% for 200 1C, 29% for 400 1C, 34% for 600 1C, 56% for 800 1C. Because the samples
achieving an oven temperature of 1000 1C is 6.67 1C/min in OMC and 5.5 1C/min in HPMC. The slope of time– temperature curve is higher for HPMC than for OMC. Because HPMC has a structure with fewer pores than OMC, therefore thermal conductivity of HPMC is higher than that of OMC. Twenty-eight days after their production, experimental samples were kept in the oven until the temperature is 200, 400, 600, 800, and 1000 1C, respectively. Then, they were taken out of the oven and some of them were left to cool in air and some of them in water till they reached 23 1C. Cooled in air or water, samples were first exposed to flexural and then to compressive experimentation on broken pieces. The flexural test was carried out on 12 samples for each series, but the compressive strength test
Table 9 The mechanical properties of concretes Concretes
Average compressive strength (MPa)
Standard division(MPa)
HPC OC
71 34
1.3 0.8
1200
Temperature (C)
1000 800 HPMC OMC
600 400 200 0 0
50
100
159
150
Time (minute)
Fig. 1. Variation of oven temperature by time.
Table 10 The mechanical properties of micro-concrete in different temperatures Temperature ( C)
Ordinary micro-concrete Flexural strength (MPa)
High-performance micro-concrete Compressive strength (MPa)
Flexural strength (MPa)
Compressive strength (MPa)
Type of cooling
23 200 400 600 800 1000
in air
in water
in air
in water
in air
in water
in air
in water
7.2 6.1 3.8 3.4 0.7
9.1 7.1 5.8 2.9 1.5 0.2
55 52.2 43.4 31.5 6.5
59.4 43.2 42.1 33.0 0 0
6.2 7.1 6.2 3.9 2.8
9.7 6.8 7.0 5.3 2.9 0
58 65.4 62.9 41.3 21
85.1 57.1 60.1 56.2 37.6 0
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Flextural strength (MPa)
12 OMC-AC
10
OMC-WC
8
HPMC-AC HPMC-WC
6 4 2 0 0
200
400
600
800
1000
1200
Temperature
Compressive strength (MPa)
Fig. 2. Variation of flexural strength with temperature.
100 OMC-AC
80
OMC-WC HPMC-AC
60
HPMC-WC
40 20 0 0
200
400
600 800 Temperature
1000
1200
Fig. 3. Variation of compressive strength with temperature.
disintegrated in water, compressive test on samples could not be completed for 1000 1C. As one will see from these figures, flexural and compressive strength of high-performance concrete cooled in air and water after being exposed to the effect of high temperature decrease up to 200 1C compared to the flexural and compressive strengths of the reference samples and the strength rapidly decreases after showing a certain increase at 200–400 1C. This decrease is more in those samples cooled in water. Decrease in flexural and compressive strength of ordinary concrete is much more compared to that of high-performance concrete. And this shows that high-performance concrete is more resistant to the effect of high temperature. However, above 600 1C, decrease in the strength of both concretes rapidly increases. The reason for this is that there is no significant change in concretes up to a temperature of 300 1C in aggregate and mortar phases, and that there are significant changes in aggregate and mortar phases after this temperature. These results are in harmony with the results of various research studies [17,18,22]. Flexural and compressive strengths of OMC and HPMC exposed to a high temperature, then cooled in water are less than those cooled in air. It is apparent that strength losses of OMC cooled in water compared to OMC cooled in air is more than the losses of HPMC (see Table 10). In the former studies, it is stated that, when ordinary concrete is heated to 300 1C, strength loss changes by 10–20% and up to 600 1C it changes by 60–70%. In addition, high-performance concrete heated over 450 1C
has a 40% strength loss [14,28]. In this study, test results show that compressive strength loss of OMC heated to 400 1C and cooled in air is 12%, in water is 29%. Compressive strength loss of samples heated up to 600 1C and cooled in air is 27%, in water is 44%. Compressive strength loss of HPMC heated to 400 1C and cooled in air is 23%, in water is 29%. Compressive strength loss of same concrete heated to 600 1C and cooled in air is 26%, in water is 34%. In the test results it is seen that compressive strength of OMC under the effect of high temperature and cooled in air and water has a decreasing trend when the temperature increases. Strength loss curves obtained from this study for the ordinary concrete have the same trend—but different changing ratio—obtained from Abrams [33], Morita et al. [34], and Furumura et al. [39] and from studies in NIST [14,29]. In the present study, it is seen that the strength of high-performance concrete exposed to high temperature and cooled in air and water decreases until 200 1C, an increase between 200 and 400 1C and a continuous decrease after 400 1C (see in Fig. 3 and Table 10). The strength increase at 200–400 1C in the samples cooled in air is more than for those cooled in water. These results have a similar trend to those of Khoury [22] and Hammer [40,41] and to the studies at NIST [14,24,27,29]. However, in Khoury’s study, strength decreases until 100 1C, increases between 100 and 400 1C and decreases continuously above 400 1C. In the studies performed at NIST, strength in highperformance concrete decreased until 100 1C, increased between 100 and 200 1C for some concrete mixtures and decreased rapidly after 200 1C. In Hammer’s study, the strength decreased until 300 1C (except the concrete produced without silica fume), increased from 300 to 450 1C and decreased rapidly after 450 1C. According to the results obtained from the tests, when the temperature is increased from 23 to 200 1C, compressive strength loss of HPMC is 32% for the specimens which were cooled in air and 33% for the specimens which were cooled in water. When the temperature is 400 1C, compressive strength loss is 23% for cooling in air and 29% for cooling in water. As it can be seen in Fig. 3, when the temperature is increased from 200 to 400 1C, compressive strength gain is 13% for the specimens cooling in air and 5% for those cooled in water. The former studies indicated that the increase is caused by evaporation of free water and removal of water of crystallization from the cement paste [42]. This result is supported by the experiments in that the specimens cooled in air have more strength gain than the specimens cooled in water because while they were being cooled in water some of the evaporated water is regained. It was reported that when explosive spalling occurs, the temperature range is between 300 and 650 1C [14,25,27,29]. Many factors were identified as affecting explosive spalling. These factors include age, moisture content, type of gravel and sand used, curing method, rate of heating [15]. In this study, it has been observed that when the temperature is
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Table 11 The mechanical properties of micro concrete with silica fume or superplasticizer admixture Temp. ð CÞ
23 400 600
Micro-concrete with superplasticizer (without silica fume)
Micro-concrete with silica fume (without superplasticizer)
High-performance micro-concrete (in Table 10)
Flexural strength (MPa)
Compr. strength (MPa)
Flexural strength (MPa)
Compr. strength(MPa)
Flexural strength(MPa)
Compr. strength(MPa)
9.6 8.5 8.3
63.4 66.0 66.7
5.7 6.8 5.6
47.8 42.3 41
9.7 7.1 6.2
85.1 65.4 62.9
Compressive strength (MPa)
Flextural strength (MPa)
12 10 8 6 4 2
SPMC
SFMC
HPMC
HPMC-WC
0 0
200
400 Temperature
600
800
90 80 70 60 50 40 30 20 10 0 0
Fig. 4. Variation of flexural strength of high-performance micro-concrete with temperature.
SPMC
SFMC
HPMC
200
400 Temperature
HPMC-WC
600
800
Fig. 5. Variation of compressive strength of high-performance microconcrete with temperature.
400–500 1C, approximately 30% of samples produced from HPMC spalled in the experiment. To examine the effect of admixed material used in concrete producing the explosive spalling, micro-concrete was first produced without using the superplasticizer admixture (which is used for the production of HPMC), and then without using silica fume (Table 11). These concretes were exposed to a heat process of 400 and 600 1C and cooled in air. The results are given in Fig. 4. As seen in the figure, flexural strength of micro-concrete produced using superplasticizer admixture is 1% less than the flexural strength of HPMC for 23 1C, 20% more for 400 1C, and 34% more for 600 1C. As for flexural strength of micro-concrete produced using silica fume (without using superplasticizer admixture), it is 41%, 4% and 10% less than that of HPMC for 23, 400 and 600 1C, respectively. Likewise, the compressive strength of microconcrete produced using superplasticizer admixture (without using silica fume) is 25% less than the compressive strength of HPMC for 23 1C, and 1% more for 400 1C, 6% more for 600 1C. Compressive strength of micro-concrete produced using silica fume is less than the compressive strength of HPMC by 44% for 23 1C, 35% for 400 1C and 35% for 600 1C. And this shows that the decrease in the flexural and compressive strength of micro-concrete produced using only silica fume is more than that of micro-concrete produced using only superplasticizer admixture and that the concrete produced using silica fume is affected more from high temperature. According to the results of experimentation conducted, it is considered that
the burst in HPMC is caused by the expansion of silica fume (Fig. 5). Strength loss curves of micro-concrete used in this study are given in Fig. 6 with the design curves given in Codes [35–38]. As it is seen in this figure, the Finnish Code is more suitable than the other Codes for high-performance concrete (except cooled in water) until 400 1C. CEB [35] and the Finnish Code [38] are more suitable than Eurocode [36,37] for ordinary and high-performance concrete between 400 and 600 1C. After 600 1C, Eurocode design curves are the most suitable ones except for ordinary concrete cooled in water. 5. Conclusions The conclusions drawn from the results obtained in this study are as follows:
Ordinary concrete with an average compressive strength of 34 MPa and high-performance concrete with an average compressive strength of 71 MPa have been produced. In order to represent their mortars microconcrete samples have been prepared. In ordinary and HPMC exposed to high temperature, flexural and compressive strength decreases with the increase of temperature. Such decrease is greater in those cooled in water. The compressive strength of HPMC cooled in air and water decreased up to 200 1C. The compressive strength of
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1.2 Eurocode Desing Curve (Siliceous aggregate) Eurocode Design Curve (Calcareous aggregate) CEB Desing Curve Finnish Design Curve (rakMK B4 K70-K100) OMC (Cooling in air) OMC (Cooling in water) HPMC (Cooling in air) HPMC (Cooling in water)
1
fc/f'c(23)
0.8
0.6
0.4
0.2
0 0
200
400
600
800
1000
1200
1400
Temperature (C) Fig. 6. Comparison of design curves and experimental loss of strength curves.
HPMC was increased between 200 and 400 1C. The compressive strength of OMC was decreased continuously. The compressive strength gain was 13% for specimens cooling in air. For the specimens cooling in water the strength gain was 5%. The compressive test was not done for OMC in the temperature above 600 1C, because of concrete samples disintegrated. For HPMC, the compressive test was not done at temperatures above 800 1C. It has been observed that some samples produced from HPMC spall explosively at temperatures between 400 and 500 1C and it has been seen that the cause for such explosive spalling in high-performance concrete is expansion of silica fume used in the production of such concretes. The explosive spalling was not observed for OMC specimens. Experimental studies indicated that OMC and HPMC produced using limestone aggregate caused loss of strength in high percentages in those cooled in water after being exposed to high temperature. Studies show that experimental samples have been damaged to a great extent and they have lost their compressive strengths if high-performance concrete is cooled in water after being exposed to the temperature of 800 1C, and ordinary concrete is cooled in water after being exposed to the temperature of 600 1C. The concrete may completely lose its strength as a result of the immediate expansions that will form during the expansion of mineral admixture used in the production of high-performance concretes in high temperature and/ or water spray-cooling of a reinforced concrete building element exposed to high temperature as a result of a fire. The CEN Eurocode and the CEB’s design curves for properties of fire-exposed concrete are not applicable to
high strength micro-concrete. Finnish Code is more suitable for high-performance concrete especially until 400 1C temperature. These codes are not applicable to OMC and HPMC cooled in water.
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