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Thermo-physical properties of concrete exposed to high temperature
Construction and Building Materials 45 (2013) 157–161
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Thermo-physical properties of concrete exposed to high temperature Ahmet B. Kizilkanat, Nabi Yüzer, Nihat Kabay ⇑ Yıldız Technical University, Department of Civil Engineering, Construction Materials Division, Davutpasa Campus, Esenler, 34210 Istanbul, Turkey
h i g h l i g h t s Thermo-physical properties of concrete at elevated temperatures are investigated. Thermal conductivity of concretes did not significantly change up to 300 °C. Moisture resistance factor of all concrete series reduced at 300 °C. Thermal conductivity and moisture resistance factor had the lowest values at 600 °C. Thermo-physical properties of concrete with pozzolans can be predicted.
a r t i c l e
i n f o
Article history: Received 5 April 2011 Received in revised form 21 March 2013 Accepted 22 March 2013 Available online 3 May 2013 Keywords: High temperature Aggregate Pozzolan Concrete Thermal conductivity Moisture resistance factor
a b s t r a c t The processes of heat penetration into a concrete mass are extremely important. Under high temperature effect, chemical composition and physical structure of concrete changes. These changes are primarily observed at the cement paste and then at the aggregates as well. In this paper, the effect of high temperature on thermal conductivity and moisture resistance factor of concrete with different types of pozzolans and aggregates is investigated. These parameters are analyzed in eight different types of concrete mixes depending on the presence of silica fume, granulated blast furnace slag and fly ash with siliceous and calcareous aggregates. In the scope of this study, simplified formulas of second order with high correlation factors have been derived that give thermal conductivity and moisture resistance factor as a function of temperature in the range of 0–600 °C for the concrete series separately. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Exposure to fire is undoubtedly the most destructive process that a concrete structure can be subjected to during its service life [1]. However, concrete as a non-combustible material does not emit toxic fumes, produce smoke, and suffer from fire for a specific period of time. The resistance of concrete depends on the duration of fire and the exposure temperature [2]. The processes of heat penetration into a concrete mass are extremely important [3]. Under high temperature effect, chemical composition, physical structure and moisture content of concrete changes. These changes are primarily observed at the cement paste and then at the aggregates as well. Heating to high temperatures causes the dehydration of hardened cement paste and conversion of calcium hydroxide into calcium oxide in which chemically bound water is gradually released to become free water. Aggregates also lose their evaporable water and hydrous aggregates dehydrate at high tempera-
⇑ Corresponding author. Tel.: +90 212 3835245; fax: +90 212 3835133. E-mail address: [email protected] (N. Kabay). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.03.080
tures, and undergo crystalline transformation accompanied by a significant volume expansion [4]. The factors influencing the penetration of heat into concrete relate to its properties at the onset of the fire, combined with changes in its physical and chemical composition as a result of fire. The thermal conductivity of concrete depends on the conductivity of its constituents, namely the cement paste and the aggregate. Concrete conductivity in general is known to decrease with increased temperature [3]. The thermal conductivity of concrete at elevated temperatures is affected by two main factors as the type of aggregate and the moisture content of the concrete. Experimental results indicate that for siliceous and calcareous aggregate concretes there is an approximately linear reduction with temperature when heated from 20 to 500 °C [4]. According to Zoldners’ [5] research on sandstone aggregate concrete, a 33% decrease in the conductivity of was observed when the temperature increased from 100 to 400 °C. Moisture diffusion and permeability of concrete at moderate temperatures is extremely slow. With a further increase of temperature, the moisture diffusivity continues to increase [4]. Fischer [6] reported that the permeability of concrete to gases
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(air) up to 600 °C increased many times from the value at 20 °C and that the main increase occurred from 450 to 600 °C. At temperatures of up to 500 °C, expulsion of water from both capillary and gel pores leads to a drastic increase in the average pore volume and to changes in the pore system. As temperature exceeds 500 °C, calcium hydroxide and calcium silicates hydrate (C–S–H) in the cement paste start decomposing, until C–S–H collapse at 900 °C. The type of aggregate in the concrete is also an important factor that determines its resistance to heat [7]. Haddad and Shannis [7] reported that while siliceous aggregates decompose at temperatures slightly above 500 °C, those of carbonic composition decompose at considerably higher temperatures. Above 1200 °C, aggregate melting or fusion occurs, and as a result concrete collapses. Industrial and natural pozzolanic materials are usually incorporated in concrete for improving the mechanical properties, decreasing the rate of hydration, decreasing the alkali-aggregate reactivity and decreasing the permeability of concrete [8]. As well stipulated, the contribution of these materials comes through their involvement in the pozzolanic reactions in addition to their filling of large capillary pores. Therefore, pozzolan use is expected to enhance fire resistance of concrete as it depletes calcium hydroxide and at the same time contributes to producing more C–S–H [7]. Concrete is a porous material, with its pores filled with water in both liquid (absorbed and free) and vaporized form and air. Therefore, elevated temperatures that develop in a concrete structure during fire affect not only its mechanical properties such as strength and modulus of elasticity, but also affect moisture migration in concrete. In the presence of liquid water, heat supplied from the fire to the concrete structure is partly absorbed by the evaporation of the liquid water. If the evaporation rate is higher than the vapor migration rate, increasing pore pressures may develop; and if they exceed the tensile strength of the material composing the solid matrix, explosive spalling may occur, resulting in the collapse of the structure [1]. Particularly, for high performance concrete (HPC) exposed to high temperature, moisture content in material or structure is one of the crucial factors, because spalling may occur due to the dense internal microstructure, which makes it difficult for water vapor transport and release in concrete [9]. Spalling of concrete at high temperature is attributed to three major factors; vapor pressure of capillary and gel water, decomposition of cement hydration products, and possible collapse of filling aggregate [10]. The higher the moisture content, the higher the possibility and tendency of spalling for HPC [9]. Thus vapor transport properties are significant in such concretes exposed to high temperature. According to literature survey it was seen that there is no sufficient information about high temperature effect on thermo-physical properties of concrete with pozzolans. In this paper, in order to fill this gap in the literature, the effect of high temperature on thermal conductivity and moisture resistance factor of concrete with different types of pozzolans and aggregates is investigated. These parameters are analyzed in eight different types of concrete mixes depending on the presence of silica fume, granulated blast furnace slag and fly ash with the basic physical and mechanical properties. 2. Experimental study The experimental study consisted of four parts; production of concrete specimens, curing, heating and cooling duration, and control testing. 2.1. Materials and specimen preparation A total of eight series of concrete namely S1, S2, S3, S4, C1, C2, C3 and C4 were made with ordinary Portland cement (CEM I 42.5 R), two types of aggregates (with same granulometry); siliceous and calcareous, three types of pozzolans; silica fume (SF), granulated blast furnace slag (GBFS) and fly ash (FA). Pozzolans were replaced
in 10% of cement by weight. The water-cementitious material ratio was kept at 0.50, and the workability was adjusted by using a superplasticizer at required dosages (0.5–1.15%). Physical and chemical properties of cement and pozzolans are given in Table 1. Concrete series were coded according to the aggregate types. S series were made of siliceous aggregates and C series were made of calcareous aggregates. Mix proportions and fresh and hardened concrete properties are given in Table 2. In each concrete series in order to determine thermal conductivity (TC) and moisture resistance factor (MRF), plate specimens with dimensions of 300 300 40 mm and cylinder specimens with dimensions of 100/40 mm were cast respectively. Specimens were demoulded 24 h after the production and then stored in a water tank at 20 ± 2 °C until testing. 2.2. High temperature effect and control tests Heating and cooling procedures were planned as follows: specimens were heated to specified temperature level without loading, cooled to room temperature, and then tested. After 28 days of curing period, specimens were dried in an oven at 100 ± 5 °C for 48 h to provide similar moisture conditions. Thereafter, the specimens of each concrete series were heated up to 200, 300 and 600 °C with 6 ± 2 °C/min heating rate and exposed at target temperature for 2 h in a furnace. After the heating process, the specimens were removed from the furnace and allowed to cool naturally to the room temperature (20 ± 2 °C). After the heating and cooling processes, TC test was performed by using a guarded hot plate (GHP) apparatus in accordance with TS ISO 8302. The basic GHP method consists principally of a hot plate and a cold plate. In this test, the specimen is placed on a flat plate heater assembly consisting of an electrically heated inner plate surrounded by a guard heater. The guard heater is controlled to maintain the same temperature on both sides of the gap separating the main and the guard heaters to ensure that heat flows in the direction of the specimen. On the opposite side of the specimen are additional flat plate heaters (cold plate) that are controlled at a fixed temperature. For a given heat input to the main heater, the hot plate assembly rises in temperature until the system reaches equilibrium [11]. The average TC (k, W/mK) of the specimen is determined from the Fourier heat flow in the following equation:
k¼
W d A DT
ð1Þ 2
In this equation, W (Watt) is the electrical power input to the main heater, A (m ) is the main heater surface area, DT (°K) is the temperature difference across the specimen, and d (m) is the specimen thickness. MRF was determined according to TS EN 12086 which also complies with DIN 52615. In this test, the specimens were placed in vapor-tight cups containing a sorbent (CaCl2). The cups were then placed in a controlled atmosphere cabinet at constant air temperature and relative humidity and weighed at 24 h time intervals Table 1 Chemical and physical properties of Portland cement and pozzolans. Oxide (%)
CEM I 42.5 R
SF
GBFS
FA
CaO SiO2 Al2O3 Fe2O3 MgO SO3 Loss on ignition Insoluble residue Specific density (g/cm3) Specific surface (Blaine, cm2/g)
64.05 20.95 4.98 3.24 1.29 2.59 1.35 0.44 3.19 3620
1.42 92.73 1.30 0.28 0.75 0.19 1.44
34.20 41.11 13.74 1.16 5.81 2.23 0.00
1.90 60.13 19.00 8.98 4.77 0.95 1.69
_
2.33 _
_
_
2.91 3335
2.21 3545
Table 2 Mix proportions (kg/m3), density of fresh concrete and compressive strength. Series
S1
S2
S3
S4
C1
C2
C3
C4
Cement Siliceous aggregate Calcareous aggregate SF GBFS FA Fresh density (kg/m3) Compressive strength (MPa)
355 1907
315 1943
315 1943
313 1929
350
321
326
330
_
_
_
_
_
_
_
_
1908
1881
1908
1934
_
35
_
_
_
36
_
_
_
_
35
_
_
_
36
_
_
_
_
_
_
2507
2507
35 2488
_
2503
2451
2419
2451
37 2483
34.9
40.1
33.1
31.8
35.0
38.7
35.0
32.4
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Dm d R T S s M Dpp
ð2Þ
where Dm (kg) is the amount of water vapor diffused through the sample, d (m) the sample thickness, S (m2) the specimen surface, s (s) the period of time corresponding to the transport of mass of water vapor Dm, Dpp (Pa) the difference between partial water vapor pressure in the air under and above specific specimen surface, R (J mol1 K1) the universal gas constant, M (kg mol1) the molar mass of water and T (K) is the absolute temperature. The water vapor diffusion resistance factor l which is more common in building physics was then determined using the following equation:
l ¼ Da =D
80
S1
60
S2 S3
40
S4
20 0
20
200
300
600
Temperature (°C) Fig. 1. Effect of temperature on TC of concrete with siliceous aggregate.
ð3Þ 2
100
1
where Da (m s ) is the diffusion coefficient of water vapor in the air. Moisture diffusion measurements were applied on five specimens, the TC measurements were applied on at least three specimens and the average of these values is given in Table 3.
3. Results and discussion
Relative TC (%)
D¼
100
Relative TC (%)
in order to determine the quantity of moisture diffused through the specimen. Weighing was repeated until the mass per unit time was no longer subject to changes and steady state values of mass gain were determined for the last five readings. The water vapor diffusion coefficient D [m2 s1] was calculated from the measured data according to the following equation:
3.1. Thermal conductivity
80
C2 C3
40
C4
20 0
3.1.1. Effect of aggregate The TC of concretes with siliceous aggregates (S1) was found as 2.05 W/mK (Table 3), a bit higher (about 4%) than the concretes made with calcareous aggregates (C1) with a TC of 1.98 W/mK. The reduction in the TC can be explained by the fact that the TC of calcareous aggregates is lower than that of the siliceous aggregates [4]. This is due to the higher crystallinity of the siliceous aggregates as compared with that of carbonate aggregates [12,13]. According to Neville [2], the mineralogical character of the aggregate greatly affects the conductivity of the concrete with it. He also mentioned that basalt and trachyte have a low conductivity, dolomite and limestone are in the middle range, and quartz exhibits the highest conductivity, which depends also on the direction of heat flow relative to the orientation of the crystals.
C1
60
20
200
300
600
Temperature (°C) Fig. 2. Effect of temperature on TC of concrete with calcareous aggregate.
300 °C in the areas of Ca(OH)2 concentration and the temperature in the range of 400–600 °C may activate a series of reactions in the hardened cement paste. These reactions commence with the complete desiccation of the pore system, followed by decomposition of hydration products and the destruction of C–S–H gels [14]. The reduction in TC of concrete series with calcareous aggregates is found to be more than that of siliceous aggregates. The highest reduction was observed in calcareous aggregate concrete with GBFS as 24% at 600 °C. The reduction of TC of concretes with any type of pozzolan is higher than that of concretes without pozzolan.
3.1.2. Effect of pozzolans The effect of pozzolans on TC of concretes was particularly observed in concretes with calcareous aggregates. The addition of SF and GBFS had no significant effect on TC of concretes with siliceous aggregates but caused a minor increase of about 5–8% in TC of concretes with calcareous aggregates. Concretes with FA had the lowest TC values of 2.03 and 1.96 W/mK in S and C series respectively. Demirbog˘a and Gül [8] reported that the effect of FA on the thermal conductivity is greater than that of SF. In their study, it can also be seen that the effect of 10% replacement of SF and FA with Portland cement had no significant effect on TC, when compared with that of 20% and 30% replacement.
3.2. Moisture resistance factor
3.1.3. Effect of high temperature TC of all concrete series decreased with the increase in exposure temperature (Figs. 1 and 2). The results show that the decreases in TC started after 200 °C and became more relevant at 600 °C. This can be explained by the fact that microcracks appear first at about
3.2.2. Effect of pozzolans As a result of the experimental study, it was seen that all types of pozzolans used in this study increased the MRF of concrete. The increase in MRF was particularly observed in concretes with SF and GBFS rather than FA. Table 4 clearly demonstrates that concretes
Table 3 Thermal conductivity (k, W/mK).
Table 4 Moisture resistance factor (l).
3.2.1. Effect of aggregate Table 4 shows the results of the MRFs of the concrete series. Concrete with calcareous aggregates had higher MRF values when compared with that of siliceous aggregates. Ollivier et al. [15] reported that the microstructure of the interfacial transition zone may be improved in the vacinity of calcareous aggregate, which reacts with calcium aluminates of Portland cement paste, forming calcium carboaluminates. The improvement in the interfacial transition zone would therefore decrease moisture diffusion and increase the MRF of concrete [16].
Temperature (°C)
S1
S2
S3
S4
C1
C2
C3
C4
Temperature (°C)
S1
S2
S3
S4
C1
C2
C3
C4
20 200 300 600
2.05 2.04 2.03 1.83
2.07 2.05 2.02 1.78
2.05 2.02 2.01 1.80
2.03 2.02 2.02 1.80
1.98 1.98 1.95 1.87
2.06 1.96 1.89 1.82
2.13 2.07 1.95 1.61
1.96 1.96 1.94 1.65
20 200 300 600
35 29 25 20
46 33 25 23
45 34 26 20
38 30 25 19
67 35 27 21
103 48 32 23
87 46 27 20
66 34 25 19
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with SF provide the highest MRF values, particularly in concrete series with calcareous aggregates. This is due to the fineness and higher pozzolanic activity of SF. The conversion of the calcium hydroxide to C–S–H by the pozzolanic reaction generally results in a decrease in porosity both in cement paste and cement pasteaggregate interface [17]. As reported by several researchers, moisture permeability of concrete is a function of porosity and density. An increase in porosity results in an increase in moisture permeability [18,19].
Table 5 Constants for proposed equation.
3.2.3. Effect of high temperature The MRF of all concrete series decreased dramatically with the increase of exposure temperature (Figs. 3 and 4). According to test results the decrease of MRF started at 200 °C. Although the MRFs of concretes with calcareous aggregate were higher than the concretes with siliceous aggregate at unheated conditions, the reduction in concretes with calcareous aggregate was higher at elevated temperatures (Table 4). It should also be noted that the MRFs of all concrete series at 600 °C were almost equal (21 ± 2). Moisture permeability of concrete is a function of porosity and density [18,19]. When exposed at high temperatures, significant changes occur in the pore structure of concrete [20].
Y ¼ a T2 þ b T þ c
ð4Þ
Relative MRF (%)
100 80
S1 S2
60
S3
40
S4
20 0
20
200
300
a (105)
b (103)
Thermal conductivity (k, W/mK)
S1 S2 S3 S4 C1 C2 C3 C4
0.09 0.1 0.08 0.01 0.03 0.05 0.1 0.1
0.2 0.1 0.1 0.3 0.02 0.8 0.3 0.4
Moisture resistance factor (l)
S1 S2 S3 S4 C1 C2 C3 C4
3 10 7 4 20 40 30 20
43 105 87 58 218 384 310 221
Predicted thermal conductivity (λ, W/mK)
Thermo-physical properties of concrete have been investigated by many investigators and a numerous number of equations were proposed [12,21,22]. However relatively little work has been done on the TC and MRF of concrete with pozzolans exposed to high temperature [16]. To provide the use of the thermo-physical properties as input data for the calculation of the concrete constructions exposed to heat, simplified formulas of second order have been derived that give TC and MRF as a function of temperature in the range of 0–600 °C with high correlation (Eq. (4)). In the current study, the thermo-physical relationships are developed for plain and pozzolan added concrete series and for two commonly used aggregates separately.
Series
2.044 2.067 2.045 2.021 1.982 2.078 2.141 1.951 35.994 48.420 47.072 39.298 70.927 110.190 93.546 70.097
2,2 2,1 R2 = 0,9149
2,0 1,9 1,8 1,7 1,6 1,5 1,5
1,6
1,7
1,8
1,9
2,0
2,1
2,2
Fig. 5. Predicted thermal conductivity values against experimentally obtained values.
120 100 R2 = 0,9832
80 60 40 20 0
0
600
Temperature (°C)
c
Actual thermal conductivity (λ, W/mK)
Predicted moisture resistance factor (μ)
4. Relationships for thermo-physical properties
Property
20
40
60
80
100
120
Actual moisture resistance factor (μ)
Fig. 6. Predicted moisture resistance factor values against experimentally obtained values.
Fig. 3. Effect of temperature on MRF of concrete with siliceous aggregate.
Relative MRF (%)
100 80
C1 C2
60
C3
40
C4
20 0
20
200
300
600
Temperature (°C) Fig. 4. Effect of temperature on TC of concrete with calcareous aggregate.
In this equation Y indicates TC (k, W/mK) or MRF (l); T indicates the exposed temperature (°C); a, b and c indicates constants (Table 5). To verify the proposed numerical models and investigate the effect of temperature on TC and MRF of concrete calculated by the proposed numerical models were compared with those obtained by the experiments. As seen in Figs. 5 and 6 the proposed models for concretes with different pozzolans and aggregates show high correlation factors of 0.92 and 0.98 for TC and MRF respectively. 5. Conclusions Based on the results obtained in this study, the following conclusions can be summarized.
A.B. Kizilkanat et al. / Construction and Building Materials 45 (2013) 157–161
Thermal conductivity of concretes with siliceous aggregates was found to be a bit higher than the concretes made with calcareous aggregates except for the series with granulated blast furnace slag at room temperature. Moisture resistance factor of concretes were highly affected by the type of aggregates used in concrete. Calcareous aggregates provided higher moisture resistance factor values for all series. Thermal conductivity and moisture resistance factor of concrete have decreased by the increasing exposure temperature. Although no considerable change was observed in thermal conductivity of concretes up to 300 °C, moisture resistance factor values of all series have begun to reduce drastically at 300 °C. Both thermal conductivity and moisture resistance factor had the lowest values at 600 °C. In the scope of this study, simplified formulas of second order with high correlation factors have been derived that give thermal conductivity and moisture resistance factor as a function of temperature in the range of 0–600 °C for each concrete series with pozzolans replaced 10% of cement by weight separately. Further investigations should be performed to develop the proposed equations for concretes with different ratios of pozzolans.
Acknowledgements This research was carried out in the Faculty of Civil Engineering at Yıldız Technical University. The authors wish to acknowledge the financial supports provided by Yıldız Technical University Research Fund (Project No. 22-05-01-03) and The Scientific and Technological Research Council of Turkey (TUBITAK-Project No. 109M008). References [1] Selih J, Sousa ACM, Bremner TW. Moisture and heat flow in concrete walls exposed to fire. J Eng Mech 1994;120:2028–43. [2] Neville AM. Properties of concrete. 4th ed. New York: Longman Scientific and Technical; 2000. [3] Riley MA. Possible new method for the assessment of fire damaged concrete. Mag Concrete Res 1991;43:87–92.
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[4] Bazant ZP, Kaplan MF. Concrete at high temperatures. 1st ed. London: Longman Group Limited; 1996. [5] Zoldners NF. Effect of high temperatures on concretes incorporating different aggregates. Mines branch research report R64. Department of mines and technical surveys, Ottawa; 1960. [6] Fischer R. On the behavior of cement mortar and concrete at high temperatures. Deutscher ausschuss für stahlbeton. Heft 214. Berlin, Germany: W. Ernst & Sohn; 1970. [7] Haddad RH, Shannis L. Post-fire behavior of bond between high strength pozzolanic concrete and reinforcing steel. Constr Build Mater 2004;18: 425–35. [8] Demirbog˘a R, Gül R. The effects of expanded perlite aggregate, silica fume and fly ash on the thermal conductivity of lightweight concrete. Cement Concrete Res 2003;33:723–7. [9] Chan YN, Lou X, Sun W. Compressive strength and pore structure of highperformance concrete after exposure to high temperature up to 800 °C. Cement Concrete Res 2000;30:247–51. [10] Haddad RH, Al-Saleh RJ, Al-Akhras NM. Effect of elevated temperature on bond between steel reinforcement and fiber reinforced concrete. Fire Safety J 2008;43:334–43. [11] Othuman MA, Wang YC. Elevated-temperature thermal properties of lightweight foamed concrete. Constr Build Mater 2011;25:705–16. [12] Kodur VKR, Sultan MA. Effect of temperature on thermal properties of highstrength concrete. J Mater Civil Eng 2003;15:101–7. [13] Xing Z, Beaucour AL, Hebert R, Noumowe A, Ledesert B. Influence of the nature of aggregates on the behaviour of concrete subjected to elevated temperature. Cement Concrete Res 2011;41:392–402. [14] Aydın S, Baradan B. Effect of pumice and fly ash incorporation on high temperature resistance of cement based mortars. Cement Concrete Res 2007;37:988–95. [15] Ollivier JP, Maso JC, Bourdette B. Interfacial transition zone in concrete. Adv Cement Based Mater 1995;2:30–8. [16] Kızılkanat AB. Investigation of the relationship between compressive strength and color change of concrete subjected to high temperatures. PhD thesis, Yıldız Technical University, Istanbul; 2010. [17] Mehta PK, Monteiro PJM. Concrete microstructure, properties and materials. 3rd ed. New York: McGraw-Hill; 2006. [18] Kabay N. The effect of pore structure on mechanical and physical properties of lightweight aggregate concrete. PhD thesis, Yıldız Technical University, Istanbul; 2009. [19] Kearsley EP, Wainwright PJ. Porosity and permeability of foamed concrete. Cement Concrete Res 2001;31:805–12. [20] Alonso C, Andrade C, Khoury GA. Relating microstructure to properties. International Centre for Mechanical Sciences, Course on effect of heat on concrete, Udine; 2003. [21] Kim KH, Jeon SE, Kim JK, Yang S. An experimental study on thermal conductivity of concrete. Cement Concrete Res 2003;33:363–71. [22] Kodur V, Khaliq W. Effect of temperature on thermal properties of different types of high strength concrete. J Mater Civil Eng 2011;23:793–801.
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Thermo-physical properties of concrete exposed to high temperature Ahmet B. Kizilkanat, Nabi Yüzer, Nihat Kabay ⇑ Yıldız Technical University, Department of Civil Engineering, Construction Materials Division, Davutpasa Campus, Esenler, 34210 Istanbul, Turkey
h i g h l i g h t s Thermo-physical properties of concrete at elevated temperatures are investigated. Thermal conductivity of concretes did not significantly change up to 300 °C. Moisture resistance factor of all concrete series reduced at 300 °C. Thermal conductivity and moisture resistance factor had the lowest values at 600 °C. Thermo-physical properties of concrete with pozzolans can be predicted.
a r t i c l e
i n f o
Article history: Received 5 April 2011 Received in revised form 21 March 2013 Accepted 22 March 2013 Available online 3 May 2013 Keywords: High temperature Aggregate Pozzolan Concrete Thermal conductivity Moisture resistance factor
a b s t r a c t The processes of heat penetration into a concrete mass are extremely important. Under high temperature effect, chemical composition and physical structure of concrete changes. These changes are primarily observed at the cement paste and then at the aggregates as well. In this paper, the effect of high temperature on thermal conductivity and moisture resistance factor of concrete with different types of pozzolans and aggregates is investigated. These parameters are analyzed in eight different types of concrete mixes depending on the presence of silica fume, granulated blast furnace slag and fly ash with siliceous and calcareous aggregates. In the scope of this study, simplified formulas of second order with high correlation factors have been derived that give thermal conductivity and moisture resistance factor as a function of temperature in the range of 0–600 °C for the concrete series separately. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Exposure to fire is undoubtedly the most destructive process that a concrete structure can be subjected to during its service life [1]. However, concrete as a non-combustible material does not emit toxic fumes, produce smoke, and suffer from fire for a specific period of time. The resistance of concrete depends on the duration of fire and the exposure temperature [2]. The processes of heat penetration into a concrete mass are extremely important [3]. Under high temperature effect, chemical composition, physical structure and moisture content of concrete changes. These changes are primarily observed at the cement paste and then at the aggregates as well. Heating to high temperatures causes the dehydration of hardened cement paste and conversion of calcium hydroxide into calcium oxide in which chemically bound water is gradually released to become free water. Aggregates also lose their evaporable water and hydrous aggregates dehydrate at high tempera-
⇑ Corresponding author. Tel.: +90 212 3835245; fax: +90 212 3835133. E-mail address: [email protected] (N. Kabay). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.03.080
tures, and undergo crystalline transformation accompanied by a significant volume expansion [4]. The factors influencing the penetration of heat into concrete relate to its properties at the onset of the fire, combined with changes in its physical and chemical composition as a result of fire. The thermal conductivity of concrete depends on the conductivity of its constituents, namely the cement paste and the aggregate. Concrete conductivity in general is known to decrease with increased temperature [3]. The thermal conductivity of concrete at elevated temperatures is affected by two main factors as the type of aggregate and the moisture content of the concrete. Experimental results indicate that for siliceous and calcareous aggregate concretes there is an approximately linear reduction with temperature when heated from 20 to 500 °C [4]. According to Zoldners’ [5] research on sandstone aggregate concrete, a 33% decrease in the conductivity of was observed when the temperature increased from 100 to 400 °C. Moisture diffusion and permeability of concrete at moderate temperatures is extremely slow. With a further increase of temperature, the moisture diffusivity continues to increase [4]. Fischer [6] reported that the permeability of concrete to gases
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(air) up to 600 °C increased many times from the value at 20 °C and that the main increase occurred from 450 to 600 °C. At temperatures of up to 500 °C, expulsion of water from both capillary and gel pores leads to a drastic increase in the average pore volume and to changes in the pore system. As temperature exceeds 500 °C, calcium hydroxide and calcium silicates hydrate (C–S–H) in the cement paste start decomposing, until C–S–H collapse at 900 °C. The type of aggregate in the concrete is also an important factor that determines its resistance to heat [7]. Haddad and Shannis [7] reported that while siliceous aggregates decompose at temperatures slightly above 500 °C, those of carbonic composition decompose at considerably higher temperatures. Above 1200 °C, aggregate melting or fusion occurs, and as a result concrete collapses. Industrial and natural pozzolanic materials are usually incorporated in concrete for improving the mechanical properties, decreasing the rate of hydration, decreasing the alkali-aggregate reactivity and decreasing the permeability of concrete [8]. As well stipulated, the contribution of these materials comes through their involvement in the pozzolanic reactions in addition to their filling of large capillary pores. Therefore, pozzolan use is expected to enhance fire resistance of concrete as it depletes calcium hydroxide and at the same time contributes to producing more C–S–H [7]. Concrete is a porous material, with its pores filled with water in both liquid (absorbed and free) and vaporized form and air. Therefore, elevated temperatures that develop in a concrete structure during fire affect not only its mechanical properties such as strength and modulus of elasticity, but also affect moisture migration in concrete. In the presence of liquid water, heat supplied from the fire to the concrete structure is partly absorbed by the evaporation of the liquid water. If the evaporation rate is higher than the vapor migration rate, increasing pore pressures may develop; and if they exceed the tensile strength of the material composing the solid matrix, explosive spalling may occur, resulting in the collapse of the structure [1]. Particularly, for high performance concrete (HPC) exposed to high temperature, moisture content in material or structure is one of the crucial factors, because spalling may occur due to the dense internal microstructure, which makes it difficult for water vapor transport and release in concrete [9]. Spalling of concrete at high temperature is attributed to three major factors; vapor pressure of capillary and gel water, decomposition of cement hydration products, and possible collapse of filling aggregate [10]. The higher the moisture content, the higher the possibility and tendency of spalling for HPC [9]. Thus vapor transport properties are significant in such concretes exposed to high temperature. According to literature survey it was seen that there is no sufficient information about high temperature effect on thermo-physical properties of concrete with pozzolans. In this paper, in order to fill this gap in the literature, the effect of high temperature on thermal conductivity and moisture resistance factor of concrete with different types of pozzolans and aggregates is investigated. These parameters are analyzed in eight different types of concrete mixes depending on the presence of silica fume, granulated blast furnace slag and fly ash with the basic physical and mechanical properties. 2. Experimental study The experimental study consisted of four parts; production of concrete specimens, curing, heating and cooling duration, and control testing. 2.1. Materials and specimen preparation A total of eight series of concrete namely S1, S2, S3, S4, C1, C2, C3 and C4 were made with ordinary Portland cement (CEM I 42.5 R), two types of aggregates (with same granulometry); siliceous and calcareous, three types of pozzolans; silica fume (SF), granulated blast furnace slag (GBFS) and fly ash (FA). Pozzolans were replaced
in 10% of cement by weight. The water-cementitious material ratio was kept at 0.50, and the workability was adjusted by using a superplasticizer at required dosages (0.5–1.15%). Physical and chemical properties of cement and pozzolans are given in Table 1. Concrete series were coded according to the aggregate types. S series were made of siliceous aggregates and C series were made of calcareous aggregates. Mix proportions and fresh and hardened concrete properties are given in Table 2. In each concrete series in order to determine thermal conductivity (TC) and moisture resistance factor (MRF), plate specimens with dimensions of 300 300 40 mm and cylinder specimens with dimensions of 100/40 mm were cast respectively. Specimens were demoulded 24 h after the production and then stored in a water tank at 20 ± 2 °C until testing. 2.2. High temperature effect and control tests Heating and cooling procedures were planned as follows: specimens were heated to specified temperature level without loading, cooled to room temperature, and then tested. After 28 days of curing period, specimens were dried in an oven at 100 ± 5 °C for 48 h to provide similar moisture conditions. Thereafter, the specimens of each concrete series were heated up to 200, 300 and 600 °C with 6 ± 2 °C/min heating rate and exposed at target temperature for 2 h in a furnace. After the heating process, the specimens were removed from the furnace and allowed to cool naturally to the room temperature (20 ± 2 °C). After the heating and cooling processes, TC test was performed by using a guarded hot plate (GHP) apparatus in accordance with TS ISO 8302. The basic GHP method consists principally of a hot plate and a cold plate. In this test, the specimen is placed on a flat plate heater assembly consisting of an electrically heated inner plate surrounded by a guard heater. The guard heater is controlled to maintain the same temperature on both sides of the gap separating the main and the guard heaters to ensure that heat flows in the direction of the specimen. On the opposite side of the specimen are additional flat plate heaters (cold plate) that are controlled at a fixed temperature. For a given heat input to the main heater, the hot plate assembly rises in temperature until the system reaches equilibrium [11]. The average TC (k, W/mK) of the specimen is determined from the Fourier heat flow in the following equation:
k¼
W d A DT
ð1Þ 2
In this equation, W (Watt) is the electrical power input to the main heater, A (m ) is the main heater surface area, DT (°K) is the temperature difference across the specimen, and d (m) is the specimen thickness. MRF was determined according to TS EN 12086 which also complies with DIN 52615. In this test, the specimens were placed in vapor-tight cups containing a sorbent (CaCl2). The cups were then placed in a controlled atmosphere cabinet at constant air temperature and relative humidity and weighed at 24 h time intervals Table 1 Chemical and physical properties of Portland cement and pozzolans. Oxide (%)
CEM I 42.5 R
SF
GBFS
FA
CaO SiO2 Al2O3 Fe2O3 MgO SO3 Loss on ignition Insoluble residue Specific density (g/cm3) Specific surface (Blaine, cm2/g)
64.05 20.95 4.98 3.24 1.29 2.59 1.35 0.44 3.19 3620
1.42 92.73 1.30 0.28 0.75 0.19 1.44
34.20 41.11 13.74 1.16 5.81 2.23 0.00
1.90 60.13 19.00 8.98 4.77 0.95 1.69
_
2.33 _
_
_
2.91 3335
2.21 3545
Table 2 Mix proportions (kg/m3), density of fresh concrete and compressive strength. Series
S1
S2
S3
S4
C1
C2
C3
C4
Cement Siliceous aggregate Calcareous aggregate SF GBFS FA Fresh density (kg/m3) Compressive strength (MPa)
355 1907
315 1943
315 1943
313 1929
350
321
326
330
_
_
_
_
_
_
_
_
1908
1881
1908
1934
_
35
_
_
_
36
_
_
_
_
35
_
_
_
36
_
_
_
_
_
_
2507
2507
35 2488
_
2503
2451
2419
2451
37 2483
34.9
40.1
33.1
31.8
35.0
38.7
35.0
32.4
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Dm d R T S s M Dpp
ð2Þ
where Dm (kg) is the amount of water vapor diffused through the sample, d (m) the sample thickness, S (m2) the specimen surface, s (s) the period of time corresponding to the transport of mass of water vapor Dm, Dpp (Pa) the difference between partial water vapor pressure in the air under and above specific specimen surface, R (J mol1 K1) the universal gas constant, M (kg mol1) the molar mass of water and T (K) is the absolute temperature. The water vapor diffusion resistance factor l which is more common in building physics was then determined using the following equation:
l ¼ Da =D
80
S1
60
S2 S3
40
S4
20 0
20
200
300
600
Temperature (°C) Fig. 1. Effect of temperature on TC of concrete with siliceous aggregate.
ð3Þ 2
100
1
where Da (m s ) is the diffusion coefficient of water vapor in the air. Moisture diffusion measurements were applied on five specimens, the TC measurements were applied on at least three specimens and the average of these values is given in Table 3.
3. Results and discussion
Relative TC (%)
D¼
100
Relative TC (%)
in order to determine the quantity of moisture diffused through the specimen. Weighing was repeated until the mass per unit time was no longer subject to changes and steady state values of mass gain were determined for the last five readings. The water vapor diffusion coefficient D [m2 s1] was calculated from the measured data according to the following equation:
3.1. Thermal conductivity
80
C2 C3
40
C4
20 0
3.1.1. Effect of aggregate The TC of concretes with siliceous aggregates (S1) was found as 2.05 W/mK (Table 3), a bit higher (about 4%) than the concretes made with calcareous aggregates (C1) with a TC of 1.98 W/mK. The reduction in the TC can be explained by the fact that the TC of calcareous aggregates is lower than that of the siliceous aggregates [4]. This is due to the higher crystallinity of the siliceous aggregates as compared with that of carbonate aggregates [12,13]. According to Neville [2], the mineralogical character of the aggregate greatly affects the conductivity of the concrete with it. He also mentioned that basalt and trachyte have a low conductivity, dolomite and limestone are in the middle range, and quartz exhibits the highest conductivity, which depends also on the direction of heat flow relative to the orientation of the crystals.
C1
60
20
200
300
600
Temperature (°C) Fig. 2. Effect of temperature on TC of concrete with calcareous aggregate.
300 °C in the areas of Ca(OH)2 concentration and the temperature in the range of 400–600 °C may activate a series of reactions in the hardened cement paste. These reactions commence with the complete desiccation of the pore system, followed by decomposition of hydration products and the destruction of C–S–H gels [14]. The reduction in TC of concrete series with calcareous aggregates is found to be more than that of siliceous aggregates. The highest reduction was observed in calcareous aggregate concrete with GBFS as 24% at 600 °C. The reduction of TC of concretes with any type of pozzolan is higher than that of concretes without pozzolan.
3.1.2. Effect of pozzolans The effect of pozzolans on TC of concretes was particularly observed in concretes with calcareous aggregates. The addition of SF and GBFS had no significant effect on TC of concretes with siliceous aggregates but caused a minor increase of about 5–8% in TC of concretes with calcareous aggregates. Concretes with FA had the lowest TC values of 2.03 and 1.96 W/mK in S and C series respectively. Demirbog˘a and Gül [8] reported that the effect of FA on the thermal conductivity is greater than that of SF. In their study, it can also be seen that the effect of 10% replacement of SF and FA with Portland cement had no significant effect on TC, when compared with that of 20% and 30% replacement.
3.2. Moisture resistance factor
3.1.3. Effect of high temperature TC of all concrete series decreased with the increase in exposure temperature (Figs. 1 and 2). The results show that the decreases in TC started after 200 °C and became more relevant at 600 °C. This can be explained by the fact that microcracks appear first at about
3.2.2. Effect of pozzolans As a result of the experimental study, it was seen that all types of pozzolans used in this study increased the MRF of concrete. The increase in MRF was particularly observed in concretes with SF and GBFS rather than FA. Table 4 clearly demonstrates that concretes
Table 3 Thermal conductivity (k, W/mK).
Table 4 Moisture resistance factor (l).
3.2.1. Effect of aggregate Table 4 shows the results of the MRFs of the concrete series. Concrete with calcareous aggregates had higher MRF values when compared with that of siliceous aggregates. Ollivier et al. [15] reported that the microstructure of the interfacial transition zone may be improved in the vacinity of calcareous aggregate, which reacts with calcium aluminates of Portland cement paste, forming calcium carboaluminates. The improvement in the interfacial transition zone would therefore decrease moisture diffusion and increase the MRF of concrete [16].
Temperature (°C)
S1
S2
S3
S4
C1
C2
C3
C4
Temperature (°C)
S1
S2
S3
S4
C1
C2
C3
C4
20 200 300 600
2.05 2.04 2.03 1.83
2.07 2.05 2.02 1.78
2.05 2.02 2.01 1.80
2.03 2.02 2.02 1.80
1.98 1.98 1.95 1.87
2.06 1.96 1.89 1.82
2.13 2.07 1.95 1.61
1.96 1.96 1.94 1.65
20 200 300 600
35 29 25 20
46 33 25 23
45 34 26 20
38 30 25 19
67 35 27 21
103 48 32 23
87 46 27 20
66 34 25 19
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with SF provide the highest MRF values, particularly in concrete series with calcareous aggregates. This is due to the fineness and higher pozzolanic activity of SF. The conversion of the calcium hydroxide to C–S–H by the pozzolanic reaction generally results in a decrease in porosity both in cement paste and cement pasteaggregate interface [17]. As reported by several researchers, moisture permeability of concrete is a function of porosity and density. An increase in porosity results in an increase in moisture permeability [18,19].
Table 5 Constants for proposed equation.
3.2.3. Effect of high temperature The MRF of all concrete series decreased dramatically with the increase of exposure temperature (Figs. 3 and 4). According to test results the decrease of MRF started at 200 °C. Although the MRFs of concretes with calcareous aggregate were higher than the concretes with siliceous aggregate at unheated conditions, the reduction in concretes with calcareous aggregate was higher at elevated temperatures (Table 4). It should also be noted that the MRFs of all concrete series at 600 °C were almost equal (21 ± 2). Moisture permeability of concrete is a function of porosity and density [18,19]. When exposed at high temperatures, significant changes occur in the pore structure of concrete [20].
Y ¼ a T2 þ b T þ c
ð4Þ
Relative MRF (%)
100 80
S1 S2
60
S3
40
S4
20 0
20
200
300
a (105)
b (103)
Thermal conductivity (k, W/mK)
S1 S2 S3 S4 C1 C2 C3 C4
0.09 0.1 0.08 0.01 0.03 0.05 0.1 0.1
0.2 0.1 0.1 0.3 0.02 0.8 0.3 0.4
Moisture resistance factor (l)
S1 S2 S3 S4 C1 C2 C3 C4
3 10 7 4 20 40 30 20
43 105 87 58 218 384 310 221
Predicted thermal conductivity (λ, W/mK)
Thermo-physical properties of concrete have been investigated by many investigators and a numerous number of equations were proposed [12,21,22]. However relatively little work has been done on the TC and MRF of concrete with pozzolans exposed to high temperature [16]. To provide the use of the thermo-physical properties as input data for the calculation of the concrete constructions exposed to heat, simplified formulas of second order have been derived that give TC and MRF as a function of temperature in the range of 0–600 °C with high correlation (Eq. (4)). In the current study, the thermo-physical relationships are developed for plain and pozzolan added concrete series and for two commonly used aggregates separately.
Series
2.044 2.067 2.045 2.021 1.982 2.078 2.141 1.951 35.994 48.420 47.072 39.298 70.927 110.190 93.546 70.097
2,2 2,1 R2 = 0,9149
2,0 1,9 1,8 1,7 1,6 1,5 1,5
1,6
1,7
1,8
1,9
2,0
2,1
2,2
Fig. 5. Predicted thermal conductivity values against experimentally obtained values.
120 100 R2 = 0,9832
80 60 40 20 0
0
600
Temperature (°C)
c
Actual thermal conductivity (λ, W/mK)
Predicted moisture resistance factor (μ)
4. Relationships for thermo-physical properties
Property
20
40
60
80
100
120
Actual moisture resistance factor (μ)
Fig. 6. Predicted moisture resistance factor values against experimentally obtained values.
Fig. 3. Effect of temperature on MRF of concrete with siliceous aggregate.
Relative MRF (%)
100 80
C1 C2
60
C3
40
C4
20 0
20
200
300
600
Temperature (°C) Fig. 4. Effect of temperature on TC of concrete with calcareous aggregate.
In this equation Y indicates TC (k, W/mK) or MRF (l); T indicates the exposed temperature (°C); a, b and c indicates constants (Table 5). To verify the proposed numerical models and investigate the effect of temperature on TC and MRF of concrete calculated by the proposed numerical models were compared with those obtained by the experiments. As seen in Figs. 5 and 6 the proposed models for concretes with different pozzolans and aggregates show high correlation factors of 0.92 and 0.98 for TC and MRF respectively. 5. Conclusions Based on the results obtained in this study, the following conclusions can be summarized.
A.B. Kizilkanat et al. / Construction and Building Materials 45 (2013) 157–161
Thermal conductivity of concretes with siliceous aggregates was found to be a bit higher than the concretes made with calcareous aggregates except for the series with granulated blast furnace slag at room temperature. Moisture resistance factor of concretes were highly affected by the type of aggregates used in concrete. Calcareous aggregates provided higher moisture resistance factor values for all series. Thermal conductivity and moisture resistance factor of concrete have decreased by the increasing exposure temperature. Although no considerable change was observed in thermal conductivity of concretes up to 300 °C, moisture resistance factor values of all series have begun to reduce drastically at 300 °C. Both thermal conductivity and moisture resistance factor had the lowest values at 600 °C. In the scope of this study, simplified formulas of second order with high correlation factors have been derived that give thermal conductivity and moisture resistance factor as a function of temperature in the range of 0–600 °C for each concrete series with pozzolans replaced 10% of cement by weight separately. Further investigations should be performed to develop the proposed equations for concretes with different ratios of pozzolans.
Acknowledgements This research was carried out in the Faculty of Civil Engineering at Yıldız Technical University. The authors wish to acknowledge the financial supports provided by Yıldız Technical University Research Fund (Project No. 22-05-01-03) and The Scientific and Technological Research Council of Turkey (TUBITAK-Project No. 109M008). References [1] Selih J, Sousa ACM, Bremner TW. Moisture and heat flow in concrete walls exposed to fire. J Eng Mech 1994;120:2028–43. [2] Neville AM. Properties of concrete. 4th ed. New York: Longman Scientific and Technical; 2000. [3] Riley MA. Possible new method for the assessment of fire damaged concrete. Mag Concrete Res 1991;43:87–92.
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