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The effect of high temperatures on the mechanical properties of concrete made with different types of aggregates
Fire Safety Journal 46 (2011) 425–430
Contents lists available at ScienceDirect
Fire Safety Journal journal homepage: www.elsevier.com/locate/firesaf
The effect of high temperatures on the mechanical properties of concrete made with different types of aggregates Ivanka Netinger 1, Ivana Kesegic 1, Ivica Guljas n University of Osijek, Faculty of Civil Engineering, Crkvena 21, 31000 Osijek, Croatia
a r t i c l e i n f o
abstract
Article history: Received 14 January 2010 Received in revised form 1 July 2011 Accepted 15 July 2011 Available online 6 August 2011
The main objective of this paper was to assess the benefits of using materials that were formed at high temperatures as an aggregate for concrete that was exposed to high temperature. The fire resistance of concrete made with some locally available, potential ‘‘fire-resistant’’ aggregates, such as diabase, steel slag, crushed bricks and crushed tiles, was investigated. The specimens of measurements 4 4 16 cm3 were kept in molds for 24 h and, after demolding, were kept in water at room temperature of about 20 7 2 1C until testing. At the age of 28 days, the specimens, with moisture content within the limits of 3–5%, were exposed to high temperatures in a previously heated test furnace. The residual mechanical properties (compressive and flexural strengths) of these concretes after natural cooling were compared with the residual mechanical properties of concrete made with commonly used river and dolomite aggregates. The replacement of natural concrete aggregates with brick and steel industry waste materials was justified, not only in terms of increased fire resistance, but also in terms of more responsible waste disposal. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Concrete High temperature Slag Crushed brick/tile Diabase
1. Introduction Concrete is well known for its capacity to endure high temperatures and fires owing to its low thermal conductivity and high specific heat [1]. However, it does not mean that fire, or high temperatures, do not affect concrete at all. Its color changes, while its compressive strength, modulus of elasticity, concrete density and the appearance of its surface become significantly affected by high temperatures [2–5]. Therefore, many researchers have recently become interested in the possibility of increasing the fire resistance of this material. According to studies, the fire resistance of concrete can be improved in several ways. The replacement of cement with slag or fly ash, for example, is a very efficient measure [6–11], as well as the addition of polypropylene fibers into a concrete mix, which is also found to be useful [12–14]. Since thermal properties of concrete are mainly interrelated with the type of aggregates used [15], this paper is based upon the presumption that all materials formed at high temperatures and usable as aggregates can improve the fire resistance of concrete. A special emphasis has been placed on the possibility of using slag as a concrete aggregate, which would not only be a
n
Corresponding author. Tel.: þ385 31 540 083; fax: þ385 31 540 071. E-mail addresses: [email protected] (I. Netinger), [email protected] (I. Kesegic), [email protected] (I. Guljas). 1 Tel.: þ385 31 540 082. 0379-7112/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.firesaf.2011.07.002
great contribution to fire engineering, but also to waste material handling within the Republic of Croatia.
2. Potential ‘‘fire-resistant’’ aggregates As previously mentioned, it has been assumed in this paper that materials developed at high temperatures are potential ‘‘fire-resistant’’ aggregates. At the same time, it was intended to use those that are widely available within the Republic of Croatia. For this purpose, the potential of domestic steel slag and waste material from local brick and tile industries was researched. In addition to steel slag and crushed bricks/tiles, this study also deals with diabase, a sort of eruptive rock, which is supposed to be fire resistant. 2.1. Waste material from the clay brick and tile industry Waste material from the brick and tile industry is the common name for waste material created by damaged brick and tile elements in the final phase of their production. As pre-fabricated bricks and tiles damaged over tolerance limits cannot be placed into the market, they get crushed at a location near to the factory. This type of waste material is often used as filling material for lower and upper layers of sports terrains. However, worldwide research [15–21] implies the possibility of using crushed brick as a concrete aggregate, which would be a valuable contribution to the solution of the ecological handling problem, as well as an
I. Netinger et al. / Fire Safety Journal 46 (2011) 425–430
attempt to preserve natural resources. Depending on the type of brick products used to make the aggregate and its strength, this kind of material might be used to produce concrete of comparable compressive strength to concrete made of natural aggregates [16,17]. The thermal conductivity coefficient of such concrete is lower than the thermal conductivity coefficient of concrete containing natural aggregate [21], which also explains the better fire resistance of concrete with crushed bricks and tiles as an aggregate [22]. 2.2. Steel slag availability in Croatia Slag is a waste product made by metal purification, casting or alloying. The type of accrued slag depends on the metal type that is processed (black or colored metals). Slag from the Republic of Croatia originates from colored and black metallurgy. Colored metallurgy slags are exported or installed in dikes, and, until now, have been disposed off into the surrounding environment. In this way, the issues of slag from local steel factories (e.g. Sisak and Split steel factories) remain unsolved. Steel slag deposited near Sisak, which is spread out in the nearby area, covers 25 hectares and is heterogeneously compounded of blast-furnace and electric-furnace steel slag. The quantity of waste material on this territory is estimated to be 1.5 million tons. This steel slag is used in road construction (as a stabilization layer) and in agriculture (smaller size fractions are used for soil improvement). The previously mentioned steel slag quantity is even bigger when the recently formed slag deposit, amounting to 300,000 tons from the production of seamless pipes in the Sisak steel factory, which are now deposited within the plant area, is taken into consideration. Steel slag disposed in Split, and estimated at around 30,000 tons, has not been exploited until now. Since an aggregate makes up 60–80% of the concrete volume, the use of steel slag as a concrete aggregate would be a greater contribution to waste material handling than when it is used in cement production. In addition, steel slag increases the fire resistance of concrete [23], and can be used as an appropriate replacement for commonly used aggregates [24,25], contributing to the preservation of natural resource aggregates and environment protection.
3. Experimental work 3.1. Materials In order to determine the behavior of some of the abovementioned, potential fire-resistant aggregates at high temperatures, seven sample groups of concrete were formed. For the mixture preparation, the CEM I 52.5N cement type has been used, according to European Standards, with a specific gravity of 3.0 kg/dm3. All the aggregates had the same sieving curve, which was adjusted to the sieving curve of the river aggregate (Fig. 1). The chemical characteristics of all the materials used are presented in Table 1.
100 90 80 70 passing (%)
426
60 50 40 30 20 10 0 0
0.125
0.25
0.5 1 sieve size (mm)
2
4
8
Fig. 1. Sieving curve.
its fire resistance (in comparison with the river aggregates) [2,3], it was not expected that any other mixture used in this research would show better fire resistance than the one with dolomite. The mixtures were prepared with the same cement content (450 kg/m3) and with the same water–cement ratio (w/c ¼0.5). Table 2 presents the proportions of all mixture constituents. The properties of these mixtures in their fresh state are given in Table 3. All the tests on fresh micro-concrete were performed according to the relevant European Standards: density was measured according to EN 1015-6:2006 [26], air content according to EN 1015-7:1998 [27] and consistency according to EN 1015-3:1999 [28].
3.3. Curing conditions and heating/cooling regimes The specimens of measurements 4 4 16 cm3 were demoulded 24 h after casting, placed in a water tank and kept there at a room temperature of about 2072 1C until they were tested. At the age of 28 days, the specimens, with moisture contents within the limits of 3–5%, were exposed to high temperatures. The specimens had been put into a test furnace that was preheated to certain temperatures (200, 400, 600, 800 and 1000 1C). After 1.5 h inside the furnace at certain temperature, the samples were taken out and left to cool down at room temperature. After the samples had been cooled down, their flexural and compressive strengths were tested. Flexural strength was tested according to EN 1015-11:1999 [29], whereas compressive strength was tested on the prism halves that remained after the flexural strength test, again according to EN 1015-11:1999. Strength properties at room temperature and residual strength properties are discussed in the following section.
4. Results and discussion
3.2. Mix proportions and mix details
4.1. Strength properties at room temperature
The mixture containing a river aggregate was used as the reference mixture (M). The M1 and M2 mixtures consisted of an aggregate made with steel slags found within the Republic of Croatia, whereas the M3 mixture was made of an aggregate of an eruptive origin, namely diabase. M4 and M5 mixtures consisted of a brick and tile industry waste material (crushed bricks and crushed tiles) used as an aggregate. For the M6 mixture, a calcareous aggregate, dolomite, was used, which is also quite commonly used in concrete mixtures. Since dolomite is known for
Flexural and compressive strengths of the observed specimens at the age of 28 days, tested according to the relevant European Standards, are given in Table 4. Each result was obtained as the mean value of the three samples tested for flexural strength, that is, of the six samples tested for compressive strength. It became obvious that the steel slag from Sisak (M1) gives concrete with significantly lower mechanical properties compared to the reference mixture (M) made with river aggregate. However, such results were expected, taking into account the noticeable low quality of steel slag grains.
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Table 1 Chemical properties of materials (% of mass). Compound/material
Cement
Aggregates
19.74 63.35 5.33 2.25 2.72 0.18 0.30 0.84 – 1.34
SiO2 CaO Al2O3 Fe2O3 MgO MnO Na2O K2O TiO2 Loss on ignition (LOI)
River aggregate
Slag Sisak
Slag Split
Diabase
Crushed brick
Crushed tile
Dolomite
81.03 0.58 8.24 3.44 1.10 – 0.14 1.62 0.33 3.30
17.08 24.98 5.40 25.45 10.58 8.91 0.12 0.13 – 4.99
14.24 31.52 7.60 25.74 7.42 3.80 0.13 0.08 – 4.71
46.81 8.60 16.09 9.39 10.78 0.13 1.94 0.62 1.07 4.11
60.53 7.23 14.30 4.99 0.80 – 1.18 0.76 0.45 9.60
64.49 1.86 13.67 5.38 1.76 0.04 1.18 2.07 0.95 8.51
0.49 31.78 0.10 0.10 20.85 – 0.01 0.01 – 46.64
Table 2 Mix design for 1 m3 of concrete. Mixture
Aggregate type
Mass of aggregate (kg)
Densitya (kg/m3)
Water content (%)
Mass of cement (kg/m3)
w/c ratio
M M1 M2 M3 M4 M5 M6
River aggregate Slag Sisak Slag split Diabase Crushed brick aggregate Crushed tile aggregate Dolomite
1572 1639 2225 1727 1317 1405 1668
2.65 2.90 3.87 2.90 2.53 2.42 2.80
225 225 225 225 225 225 225
450 450 450 450 450 450 450
0.5 0.5 0.5 0.5 0.5 0.5 0.5
a
Particle density on a saturated and surface-dried basis.
Table 3 Properties of fresh concrete.
M1
12 Consistency, Remarks by flow table (cm)
M M1
2228 2276
3 6
16 16
M2 M3 M4 M5 M6
2820 2361 1862 1944 2358
5 3 10.5 4.4 3
16 15 16 15 16
– Ruinous grains – – – – –
M3
M4
M5
M6
M
10 flexural strength (MPa)
Mixture Density of fresh Air content (%) concrete (kg/ m3)
M2
8 6 4 2 0 20
Table 4 Flexural and compressive strengths of concrete at the age of 28 days.
200
400 600 temperature (°C)
800
1000
Fig. 2. Temperature impact to flexural strength of the samples. Mixture
Flexural strength (MPa)
Compressive strength (MPa)
M M1 M2 M3 M4 M5 M6
10.1 4.7 8.1 8.6 6.6 6.0 8.4
46.9 26.2 43.3 38.3 35.7 31.6 45.9
Flexural and compressive strengths of the specimens with the steel slag from Split (M2) are approximately the same as for the reference specimens. According to such results, this kind of slag could be an appropriate replacement for conventionally used aggregates. Although diabase (M3) is an aggregate that is commonly used in concrete mixtures and it greatly contributes to the strength of
concrete, it was included in the research because of its origin. As it is an eruptive rock, it was expected to ensure good fire resistance to concrete. Strength properties of the specimens with crushed brick (M4) and tile aggregate (M5) are lower compared to the same properties for the reference mixture. Such results are expected due to the lower grain hardness of a crushed brick/tile aggregate compared to a river aggregate. Although dolomite (M6) is also frequently used as an aggregate in concrete mixtures and it gives concrete good strength properties, it should again be pointed out that it was included in this research only in order to prove that a local calcareous aggregate has a higher fire resistance than a river aggregate, as is indicated in EN 1992-12:2004þAC:2008 and EN 1994-1-2:2005þ AC:2008.
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M1
M2 M5
M4
M3 M6
Table 5 Flexural and compressive strengths of concrete—statistical parameters of measurement.
M
100 residual/initial flexural strength (%)
Mixture
80
60
400 600 temperature (°C)
800
M2
M3
M4
M5
M6
M1
20 200 400 600 800 1000
4.7 3.6 2.6 1.0 1.0 0.9
0.13 0.26 0.24 0.41 0.13 0.21
26.2 25.1 20.0 15.0 8.0 7.5
1.99 0.65 2.44 0.63 1.05 1.73
M2
20 200 400 600 800 1000
8.1 7.1 5.3 1.3 0.8 0.6
0.27 0.66 0.33 1.46 0.27 0.36
43.3 41.5 31.7 21.2 12.9 13.4
5.80 1.30 2.37 4.00 0.96 0.85
M3
20 200 400 600 800 1000
8.6 8.3 5.7 0.5 0.3 0.3
1.90 0.60 0.31 0.03 0.19 0.20
38.3 37.3 37.5 20.6 12.5 7.3
3.55 2.95 1.25 4.34 0.55 0.26
M4
20 200 400 600 800 1000
6.7 4.4 4.0 1.4 0.7 0.3
1.07 0.56 0.82 0.20 0.08 0.07
35.7 32.8 28.3 20.4 10.5 5.8
5.29 0.94 1.44 3.15 0.38 0.62
M5
20 200 400 600 800 1000
6.0 4.3 4.6 1.9 1.4 0.4
0.38 0.12 0.86 0.16 0.39 0.06
31.6 30.2 26.2 16.1 13.2 6.7
1.74 4.43 1.48 0.65 2.05 0.38
M6
20 200 400 600 800 1000
8.4 7.2 5.6 2.1 2.3 0.27
0.80 0.70 0.28 0.63 0.96 0.18
45.9 40.7 37.5 26.3 21.5 8.3
1.36 3.97 0.10 0.88 5.60 1.14
50 40
20 10 0 20
200
400 600 temperature (°C)
800
Standard deviation 2.25 2.72 3.90 2.97 1.31 0.55
M
30
Mean
46.9 42.5 36.9 17.3 10.6 4.6
Fig. 3. Ratio of residual and initial flexural strength upon exposure to high temperatures.
M1
Standard deviation 0.54 0.76 0.55 1.48 0.07 0.00
1000
60
Mean
10.1 7.8 5.7 1.1 0.2 0.0
20
200
Compressive strength (MPa)
20 200 400 600 800 1000
40
20
Flexural strength (MPa)
M
0
compressive strength (MPa)
Temperature (1C)
1000
Fig. 4. Temperature impact to compressive strength of the samples.
M1 M4 M
M2 M5 M6-ENV
M3 M6 M-ENV
residual/initial compressive strength (%)
100
4.2. Residual strength properties
80
60
40
20
0 20
200
400 600 temperature (°C)
800
1000
Fig. 5. Ratio of residual and initial compressive strength upon exposure to high temperatures.
Figs. 2–5 show the effects of high temperatures on flexural and compressive strengths of the samples. Figs. 2 and 4 also show the standard deviations of the measurement results. Each curve point represents the mean value of the results obtained by testing flexural strength (three samples) and compressive strength (six samples). In order to gain insight into the statistical distribution of results, Table 5 shows the statistical parameters of the measurements, the mean and standard deviation values. Despite the relatively small number of samples to which the individual variations seen in Figs. 3 and 5 can be attributed, one can notice the expected trends in the behavior of some of the mixtures described below. In order to compare the behavior of the mixtures observed here, with a river and dolomite aggregate at high temperatures, with the expected behavior of such concretes according to EN 1992-1-2:2004þAC:2008 and EN 1994-12:2005þAC:2008, two more curves are added to Fig. 5: the first one corresponds to the behavior of concrete with a siliceous
I. Netinger et al. / Fire Safety Journal 46 (2011) 425–430
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the separation of substantial amounts of carbon dioxide (CO2). It should be noted that the temperature of 700 1C, to which a calcareous aggregate is stable, refers to the temperature of the sample, while in this paper, the temperatures are measured in a furnace, and not in the samples.
aggregate (M-ENV) and the second one corresponds to the behavior of concrete with a calcareous aggregate (M6-ENV) under fire conditions according to the Standards mentioned. According to these figures, the following can be concluded:
In this research, the obtained behaviors of mixtures with river
(siliceous (M)) and dolomite (calcareous (M6)) aggregates at high temperatures are in accordance with those provided by the European Standards: a dolomite aggregate ensures better fire resistance to concrete than a river aggregate (see Fig. 5). Dolomite is beneficial in improving fire performance because a calcination process absorbs heat, and the lower density of calcined material provides a greater insulating effect [6]. At the same time, the quartz present in siliceous aggregates and sand is subjected to a number of physical changes at elevated temperatures, which results in weakening [6] and consequently in the weakening of the concrete itself. Up to a temperature of 200 1C, the mixtures with a crushed brick (M4) and a tile aggregate (M5) show a greater reduction in flexural strength, but a smaller reduction in the compressive strength compared to the reference mixture (see Figs. 3 and 5). From 200 up to 1000 1C, the mixture with crushed brick lost both of its strengths (flexural and compressive) at a slower rate than the reference mixture (M), which is even more observable in the case of the mixture with a crushed tile aggregate. According to [21], concretes with crushed bricks/tiles have lower thermal conductivity than concretes with natural aggregates, which might explain the better fire resistance of such mixtures in this research. In addition, the strength–temperature relationships for the unstressed residual strength tests can be characterized by two stages: the initial strength gain or minor strength loss stage and the permanent strength loss stage [30]. The temperature range for each of these phases depends on the type of concrete, which implies that the first phase of this investigation was noticed only in the mixture with the crushed tile aggregate and perhaps could also have been observed in other mixtures if smaller increments in the temperature range had been applied. The diabase mixture (M3) showed better fire properties when considering flexural strength up to 400 1C compared to the reference mixture (Fig. 3). For compressive strength (Fig. 5), this mixture was more fire resistant over the whole temperature range. Considering its origin, such results were expected. Diabase has a high melting point of 1227 1C at normal atmospheric pressure [31], which is why good fire resistance was expected. However, diabase has not shown improved fire resistance in the high-temperature range in comparison with dolomite. The mixture with steel slag from Sisak (M1) and the reference mixture behaved in a similar way considering the flexural strength up to 400 1C, and, above that temperature, the steel slag mixture was superior to the reference mixture (Fig. 3). When considering compressive strength (Fig. 5), the mixture with this type of slag was more fire resistant than the reference mixture over the whole temperature range. The percentage residual flexural strength of the mixture with steel slag from Split (M2) was significantly greater over the whole temperature range than the same property for the reference mixture (Fig. 3). The percentage residual compressive strength of this mixture was significantly greater than the reference mixture over the higher temperature range (Fig. 5). Both slag mixtures proved to be stable at temperatures between 800 and 1000 1C, in contrast to the dolomite mixture, which started to lose its strength in this temperature range. According to [6], carbonate aggregates, such as limestone and dolomite, are stable up to about 700 1C when the calcium carbonate (CaCO3) begins to disassemble to lime (CaO) with
5. Conclusions Using the same type and amount of cement in all concrete mixtures, the results achieved have shown the crucial contribution of the aggregates used to show the concrete’s mechanical properties at room temperature and to show its behavior when exposed to high temperatures. This paper has been based upon the presumption that all high-temperature materials usable as aggregates can produce concrete, which is more resistant to high temperatures in comparison to concrete made of a river aggregate. For this purpose, a high-temperature diabase aggregate was researched, as well as a group of waste materials from the steel and brick industry. In addition to improving the fire resistance of concrete, the use of these waste materials as concrete aggregates can make a significant contribution to waste management, recycling and environmental protection. Test results of the fire resistance of the mixtures are described above in relation to the fire behavior of the mixture made with commonly used natural aggregates (dolomite and river aggregate) given by the European Standards. It should be emphasized that better fire resistance of the mixture made of these aggregates in comparison to the one made of dolomite, which is known for having better fire resistance than the mixture of the river aggregate, was not expected. Given the results presented in this paper, the following can be concluded:
Diabase ensures good mechanical properties of concrete at
room temperature and better mechanical properties at temperatures up to 600 1C in comparison with the river aggregate and dolomite mixtures. Concrete made with brick industry waste shows satisfactory mechanical properties at room temperature, better fire resistance than the one made with river aggregates, and only slightly lower fire resistance than the one made with dolomite. Steel slag concrete shows similar fire resistance to the river aggregate mixture up to 400 1C, and much improved fire resistance at high-temperature ranges. The fire resistance of these mixtures is lower than the fire resistance of the dolomite mixtures up to 800 1C, above which it increases as it coincides with the temperature range within which the dissolution of CaCO3 in dolomite occurs. This leads to the conclusion that slag could find its application in higher temperature areas, for example, in the production of refractory concrete. According to the above conclusions, the replacement of a river aggregate in concrete with steel and brick industry waste material can not only be justified in terms of fire resistance, but also in terms of waste management. However, before making a final conclusion on the recommended areas of application for the particular mixture, the authors suggest making additional studies that will include the monitoring of temperature within the samples and monitoring of temperature effects on other concrete properties. It is also advisable to monitor the selected properties on coarse aggregate concrete.
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The effect of high temperatures on the mechanical properties of concrete made with different types of aggregates Ivanka Netinger 1, Ivana Kesegic 1, Ivica Guljas n University of Osijek, Faculty of Civil Engineering, Crkvena 21, 31000 Osijek, Croatia
a r t i c l e i n f o
abstract
Article history: Received 14 January 2010 Received in revised form 1 July 2011 Accepted 15 July 2011 Available online 6 August 2011
The main objective of this paper was to assess the benefits of using materials that were formed at high temperatures as an aggregate for concrete that was exposed to high temperature. The fire resistance of concrete made with some locally available, potential ‘‘fire-resistant’’ aggregates, such as diabase, steel slag, crushed bricks and crushed tiles, was investigated. The specimens of measurements 4 4 16 cm3 were kept in molds for 24 h and, after demolding, were kept in water at room temperature of about 20 7 2 1C until testing. At the age of 28 days, the specimens, with moisture content within the limits of 3–5%, were exposed to high temperatures in a previously heated test furnace. The residual mechanical properties (compressive and flexural strengths) of these concretes after natural cooling were compared with the residual mechanical properties of concrete made with commonly used river and dolomite aggregates. The replacement of natural concrete aggregates with brick and steel industry waste materials was justified, not only in terms of increased fire resistance, but also in terms of more responsible waste disposal. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Concrete High temperature Slag Crushed brick/tile Diabase
1. Introduction Concrete is well known for its capacity to endure high temperatures and fires owing to its low thermal conductivity and high specific heat [1]. However, it does not mean that fire, or high temperatures, do not affect concrete at all. Its color changes, while its compressive strength, modulus of elasticity, concrete density and the appearance of its surface become significantly affected by high temperatures [2–5]. Therefore, many researchers have recently become interested in the possibility of increasing the fire resistance of this material. According to studies, the fire resistance of concrete can be improved in several ways. The replacement of cement with slag or fly ash, for example, is a very efficient measure [6–11], as well as the addition of polypropylene fibers into a concrete mix, which is also found to be useful [12–14]. Since thermal properties of concrete are mainly interrelated with the type of aggregates used [15], this paper is based upon the presumption that all materials formed at high temperatures and usable as aggregates can improve the fire resistance of concrete. A special emphasis has been placed on the possibility of using slag as a concrete aggregate, which would not only be a
n
Corresponding author. Tel.: þ385 31 540 083; fax: þ385 31 540 071. E-mail addresses: [email protected] (I. Netinger), [email protected] (I. Kesegic), [email protected] (I. Guljas). 1 Tel.: þ385 31 540 082. 0379-7112/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.firesaf.2011.07.002
great contribution to fire engineering, but also to waste material handling within the Republic of Croatia.
2. Potential ‘‘fire-resistant’’ aggregates As previously mentioned, it has been assumed in this paper that materials developed at high temperatures are potential ‘‘fire-resistant’’ aggregates. At the same time, it was intended to use those that are widely available within the Republic of Croatia. For this purpose, the potential of domestic steel slag and waste material from local brick and tile industries was researched. In addition to steel slag and crushed bricks/tiles, this study also deals with diabase, a sort of eruptive rock, which is supposed to be fire resistant. 2.1. Waste material from the clay brick and tile industry Waste material from the brick and tile industry is the common name for waste material created by damaged brick and tile elements in the final phase of their production. As pre-fabricated bricks and tiles damaged over tolerance limits cannot be placed into the market, they get crushed at a location near to the factory. This type of waste material is often used as filling material for lower and upper layers of sports terrains. However, worldwide research [15–21] implies the possibility of using crushed brick as a concrete aggregate, which would be a valuable contribution to the solution of the ecological handling problem, as well as an
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attempt to preserve natural resources. Depending on the type of brick products used to make the aggregate and its strength, this kind of material might be used to produce concrete of comparable compressive strength to concrete made of natural aggregates [16,17]. The thermal conductivity coefficient of such concrete is lower than the thermal conductivity coefficient of concrete containing natural aggregate [21], which also explains the better fire resistance of concrete with crushed bricks and tiles as an aggregate [22]. 2.2. Steel slag availability in Croatia Slag is a waste product made by metal purification, casting or alloying. The type of accrued slag depends on the metal type that is processed (black or colored metals). Slag from the Republic of Croatia originates from colored and black metallurgy. Colored metallurgy slags are exported or installed in dikes, and, until now, have been disposed off into the surrounding environment. In this way, the issues of slag from local steel factories (e.g. Sisak and Split steel factories) remain unsolved. Steel slag deposited near Sisak, which is spread out in the nearby area, covers 25 hectares and is heterogeneously compounded of blast-furnace and electric-furnace steel slag. The quantity of waste material on this territory is estimated to be 1.5 million tons. This steel slag is used in road construction (as a stabilization layer) and in agriculture (smaller size fractions are used for soil improvement). The previously mentioned steel slag quantity is even bigger when the recently formed slag deposit, amounting to 300,000 tons from the production of seamless pipes in the Sisak steel factory, which are now deposited within the plant area, is taken into consideration. Steel slag disposed in Split, and estimated at around 30,000 tons, has not been exploited until now. Since an aggregate makes up 60–80% of the concrete volume, the use of steel slag as a concrete aggregate would be a greater contribution to waste material handling than when it is used in cement production. In addition, steel slag increases the fire resistance of concrete [23], and can be used as an appropriate replacement for commonly used aggregates [24,25], contributing to the preservation of natural resource aggregates and environment protection.
3. Experimental work 3.1. Materials In order to determine the behavior of some of the abovementioned, potential fire-resistant aggregates at high temperatures, seven sample groups of concrete were formed. For the mixture preparation, the CEM I 52.5N cement type has been used, according to European Standards, with a specific gravity of 3.0 kg/dm3. All the aggregates had the same sieving curve, which was adjusted to the sieving curve of the river aggregate (Fig. 1). The chemical characteristics of all the materials used are presented in Table 1.
100 90 80 70 passing (%)
426
60 50 40 30 20 10 0 0
0.125
0.25
0.5 1 sieve size (mm)
2
4
8
Fig. 1. Sieving curve.
its fire resistance (in comparison with the river aggregates) [2,3], it was not expected that any other mixture used in this research would show better fire resistance than the one with dolomite. The mixtures were prepared with the same cement content (450 kg/m3) and with the same water–cement ratio (w/c ¼0.5). Table 2 presents the proportions of all mixture constituents. The properties of these mixtures in their fresh state are given in Table 3. All the tests on fresh micro-concrete were performed according to the relevant European Standards: density was measured according to EN 1015-6:2006 [26], air content according to EN 1015-7:1998 [27] and consistency according to EN 1015-3:1999 [28].
3.3. Curing conditions and heating/cooling regimes The specimens of measurements 4 4 16 cm3 were demoulded 24 h after casting, placed in a water tank and kept there at a room temperature of about 2072 1C until they were tested. At the age of 28 days, the specimens, with moisture contents within the limits of 3–5%, were exposed to high temperatures. The specimens had been put into a test furnace that was preheated to certain temperatures (200, 400, 600, 800 and 1000 1C). After 1.5 h inside the furnace at certain temperature, the samples were taken out and left to cool down at room temperature. After the samples had been cooled down, their flexural and compressive strengths were tested. Flexural strength was tested according to EN 1015-11:1999 [29], whereas compressive strength was tested on the prism halves that remained after the flexural strength test, again according to EN 1015-11:1999. Strength properties at room temperature and residual strength properties are discussed in the following section.
4. Results and discussion
3.2. Mix proportions and mix details
4.1. Strength properties at room temperature
The mixture containing a river aggregate was used as the reference mixture (M). The M1 and M2 mixtures consisted of an aggregate made with steel slags found within the Republic of Croatia, whereas the M3 mixture was made of an aggregate of an eruptive origin, namely diabase. M4 and M5 mixtures consisted of a brick and tile industry waste material (crushed bricks and crushed tiles) used as an aggregate. For the M6 mixture, a calcareous aggregate, dolomite, was used, which is also quite commonly used in concrete mixtures. Since dolomite is known for
Flexural and compressive strengths of the observed specimens at the age of 28 days, tested according to the relevant European Standards, are given in Table 4. Each result was obtained as the mean value of the three samples tested for flexural strength, that is, of the six samples tested for compressive strength. It became obvious that the steel slag from Sisak (M1) gives concrete with significantly lower mechanical properties compared to the reference mixture (M) made with river aggregate. However, such results were expected, taking into account the noticeable low quality of steel slag grains.
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Table 1 Chemical properties of materials (% of mass). Compound/material
Cement
Aggregates
19.74 63.35 5.33 2.25 2.72 0.18 0.30 0.84 – 1.34
SiO2 CaO Al2O3 Fe2O3 MgO MnO Na2O K2O TiO2 Loss on ignition (LOI)
River aggregate
Slag Sisak
Slag Split
Diabase
Crushed brick
Crushed tile
Dolomite
81.03 0.58 8.24 3.44 1.10 – 0.14 1.62 0.33 3.30
17.08 24.98 5.40 25.45 10.58 8.91 0.12 0.13 – 4.99
14.24 31.52 7.60 25.74 7.42 3.80 0.13 0.08 – 4.71
46.81 8.60 16.09 9.39 10.78 0.13 1.94 0.62 1.07 4.11
60.53 7.23 14.30 4.99 0.80 – 1.18 0.76 0.45 9.60
64.49 1.86 13.67 5.38 1.76 0.04 1.18 2.07 0.95 8.51
0.49 31.78 0.10 0.10 20.85 – 0.01 0.01 – 46.64
Table 2 Mix design for 1 m3 of concrete. Mixture
Aggregate type
Mass of aggregate (kg)
Densitya (kg/m3)
Water content (%)
Mass of cement (kg/m3)
w/c ratio
M M1 M2 M3 M4 M5 M6
River aggregate Slag Sisak Slag split Diabase Crushed brick aggregate Crushed tile aggregate Dolomite
1572 1639 2225 1727 1317 1405 1668
2.65 2.90 3.87 2.90 2.53 2.42 2.80
225 225 225 225 225 225 225
450 450 450 450 450 450 450
0.5 0.5 0.5 0.5 0.5 0.5 0.5
a
Particle density on a saturated and surface-dried basis.
Table 3 Properties of fresh concrete.
M1
12 Consistency, Remarks by flow table (cm)
M M1
2228 2276
3 6
16 16
M2 M3 M4 M5 M6
2820 2361 1862 1944 2358
5 3 10.5 4.4 3
16 15 16 15 16
– Ruinous grains – – – – –
M3
M4
M5
M6
M
10 flexural strength (MPa)
Mixture Density of fresh Air content (%) concrete (kg/ m3)
M2
8 6 4 2 0 20
Table 4 Flexural and compressive strengths of concrete at the age of 28 days.
200
400 600 temperature (°C)
800
1000
Fig. 2. Temperature impact to flexural strength of the samples. Mixture
Flexural strength (MPa)
Compressive strength (MPa)
M M1 M2 M3 M4 M5 M6
10.1 4.7 8.1 8.6 6.6 6.0 8.4
46.9 26.2 43.3 38.3 35.7 31.6 45.9
Flexural and compressive strengths of the specimens with the steel slag from Split (M2) are approximately the same as for the reference specimens. According to such results, this kind of slag could be an appropriate replacement for conventionally used aggregates. Although diabase (M3) is an aggregate that is commonly used in concrete mixtures and it greatly contributes to the strength of
concrete, it was included in the research because of its origin. As it is an eruptive rock, it was expected to ensure good fire resistance to concrete. Strength properties of the specimens with crushed brick (M4) and tile aggregate (M5) are lower compared to the same properties for the reference mixture. Such results are expected due to the lower grain hardness of a crushed brick/tile aggregate compared to a river aggregate. Although dolomite (M6) is also frequently used as an aggregate in concrete mixtures and it gives concrete good strength properties, it should again be pointed out that it was included in this research only in order to prove that a local calcareous aggregate has a higher fire resistance than a river aggregate, as is indicated in EN 1992-12:2004þAC:2008 and EN 1994-1-2:2005þ AC:2008.
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M1
M2 M5
M4
M3 M6
Table 5 Flexural and compressive strengths of concrete—statistical parameters of measurement.
M
100 residual/initial flexural strength (%)
Mixture
80
60
400 600 temperature (°C)
800
M2
M3
M4
M5
M6
M1
20 200 400 600 800 1000
4.7 3.6 2.6 1.0 1.0 0.9
0.13 0.26 0.24 0.41 0.13 0.21
26.2 25.1 20.0 15.0 8.0 7.5
1.99 0.65 2.44 0.63 1.05 1.73
M2
20 200 400 600 800 1000
8.1 7.1 5.3 1.3 0.8 0.6
0.27 0.66 0.33 1.46 0.27 0.36
43.3 41.5 31.7 21.2 12.9 13.4
5.80 1.30 2.37 4.00 0.96 0.85
M3
20 200 400 600 800 1000
8.6 8.3 5.7 0.5 0.3 0.3
1.90 0.60 0.31 0.03 0.19 0.20
38.3 37.3 37.5 20.6 12.5 7.3
3.55 2.95 1.25 4.34 0.55 0.26
M4
20 200 400 600 800 1000
6.7 4.4 4.0 1.4 0.7 0.3
1.07 0.56 0.82 0.20 0.08 0.07
35.7 32.8 28.3 20.4 10.5 5.8
5.29 0.94 1.44 3.15 0.38 0.62
M5
20 200 400 600 800 1000
6.0 4.3 4.6 1.9 1.4 0.4
0.38 0.12 0.86 0.16 0.39 0.06
31.6 30.2 26.2 16.1 13.2 6.7
1.74 4.43 1.48 0.65 2.05 0.38
M6
20 200 400 600 800 1000
8.4 7.2 5.6 2.1 2.3 0.27
0.80 0.70 0.28 0.63 0.96 0.18
45.9 40.7 37.5 26.3 21.5 8.3
1.36 3.97 0.10 0.88 5.60 1.14
50 40
20 10 0 20
200
400 600 temperature (°C)
800
Standard deviation 2.25 2.72 3.90 2.97 1.31 0.55
M
30
Mean
46.9 42.5 36.9 17.3 10.6 4.6
Fig. 3. Ratio of residual and initial flexural strength upon exposure to high temperatures.
M1
Standard deviation 0.54 0.76 0.55 1.48 0.07 0.00
1000
60
Mean
10.1 7.8 5.7 1.1 0.2 0.0
20
200
Compressive strength (MPa)
20 200 400 600 800 1000
40
20
Flexural strength (MPa)
M
0
compressive strength (MPa)
Temperature (1C)
1000
Fig. 4. Temperature impact to compressive strength of the samples.
M1 M4 M
M2 M5 M6-ENV
M3 M6 M-ENV
residual/initial compressive strength (%)
100
4.2. Residual strength properties
80
60
40
20
0 20
200
400 600 temperature (°C)
800
1000
Fig. 5. Ratio of residual and initial compressive strength upon exposure to high temperatures.
Figs. 2–5 show the effects of high temperatures on flexural and compressive strengths of the samples. Figs. 2 and 4 also show the standard deviations of the measurement results. Each curve point represents the mean value of the results obtained by testing flexural strength (three samples) and compressive strength (six samples). In order to gain insight into the statistical distribution of results, Table 5 shows the statistical parameters of the measurements, the mean and standard deviation values. Despite the relatively small number of samples to which the individual variations seen in Figs. 3 and 5 can be attributed, one can notice the expected trends in the behavior of some of the mixtures described below. In order to compare the behavior of the mixtures observed here, with a river and dolomite aggregate at high temperatures, with the expected behavior of such concretes according to EN 1992-1-2:2004þAC:2008 and EN 1994-12:2005þAC:2008, two more curves are added to Fig. 5: the first one corresponds to the behavior of concrete with a siliceous
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the separation of substantial amounts of carbon dioxide (CO2). It should be noted that the temperature of 700 1C, to which a calcareous aggregate is stable, refers to the temperature of the sample, while in this paper, the temperatures are measured in a furnace, and not in the samples.
aggregate (M-ENV) and the second one corresponds to the behavior of concrete with a calcareous aggregate (M6-ENV) under fire conditions according to the Standards mentioned. According to these figures, the following can be concluded:
In this research, the obtained behaviors of mixtures with river
(siliceous (M)) and dolomite (calcareous (M6)) aggregates at high temperatures are in accordance with those provided by the European Standards: a dolomite aggregate ensures better fire resistance to concrete than a river aggregate (see Fig. 5). Dolomite is beneficial in improving fire performance because a calcination process absorbs heat, and the lower density of calcined material provides a greater insulating effect [6]. At the same time, the quartz present in siliceous aggregates and sand is subjected to a number of physical changes at elevated temperatures, which results in weakening [6] and consequently in the weakening of the concrete itself. Up to a temperature of 200 1C, the mixtures with a crushed brick (M4) and a tile aggregate (M5) show a greater reduction in flexural strength, but a smaller reduction in the compressive strength compared to the reference mixture (see Figs. 3 and 5). From 200 up to 1000 1C, the mixture with crushed brick lost both of its strengths (flexural and compressive) at a slower rate than the reference mixture (M), which is even more observable in the case of the mixture with a crushed tile aggregate. According to [21], concretes with crushed bricks/tiles have lower thermal conductivity than concretes with natural aggregates, which might explain the better fire resistance of such mixtures in this research. In addition, the strength–temperature relationships for the unstressed residual strength tests can be characterized by two stages: the initial strength gain or minor strength loss stage and the permanent strength loss stage [30]. The temperature range for each of these phases depends on the type of concrete, which implies that the first phase of this investigation was noticed only in the mixture with the crushed tile aggregate and perhaps could also have been observed in other mixtures if smaller increments in the temperature range had been applied. The diabase mixture (M3) showed better fire properties when considering flexural strength up to 400 1C compared to the reference mixture (Fig. 3). For compressive strength (Fig. 5), this mixture was more fire resistant over the whole temperature range. Considering its origin, such results were expected. Diabase has a high melting point of 1227 1C at normal atmospheric pressure [31], which is why good fire resistance was expected. However, diabase has not shown improved fire resistance in the high-temperature range in comparison with dolomite. The mixture with steel slag from Sisak (M1) and the reference mixture behaved in a similar way considering the flexural strength up to 400 1C, and, above that temperature, the steel slag mixture was superior to the reference mixture (Fig. 3). When considering compressive strength (Fig. 5), the mixture with this type of slag was more fire resistant than the reference mixture over the whole temperature range. The percentage residual flexural strength of the mixture with steel slag from Split (M2) was significantly greater over the whole temperature range than the same property for the reference mixture (Fig. 3). The percentage residual compressive strength of this mixture was significantly greater than the reference mixture over the higher temperature range (Fig. 5). Both slag mixtures proved to be stable at temperatures between 800 and 1000 1C, in contrast to the dolomite mixture, which started to lose its strength in this temperature range. According to [6], carbonate aggregates, such as limestone and dolomite, are stable up to about 700 1C when the calcium carbonate (CaCO3) begins to disassemble to lime (CaO) with
5. Conclusions Using the same type and amount of cement in all concrete mixtures, the results achieved have shown the crucial contribution of the aggregates used to show the concrete’s mechanical properties at room temperature and to show its behavior when exposed to high temperatures. This paper has been based upon the presumption that all high-temperature materials usable as aggregates can produce concrete, which is more resistant to high temperatures in comparison to concrete made of a river aggregate. For this purpose, a high-temperature diabase aggregate was researched, as well as a group of waste materials from the steel and brick industry. In addition to improving the fire resistance of concrete, the use of these waste materials as concrete aggregates can make a significant contribution to waste management, recycling and environmental protection. Test results of the fire resistance of the mixtures are described above in relation to the fire behavior of the mixture made with commonly used natural aggregates (dolomite and river aggregate) given by the European Standards. It should be emphasized that better fire resistance of the mixture made of these aggregates in comparison to the one made of dolomite, which is known for having better fire resistance than the mixture of the river aggregate, was not expected. Given the results presented in this paper, the following can be concluded:
Diabase ensures good mechanical properties of concrete at
room temperature and better mechanical properties at temperatures up to 600 1C in comparison with the river aggregate and dolomite mixtures. Concrete made with brick industry waste shows satisfactory mechanical properties at room temperature, better fire resistance than the one made with river aggregates, and only slightly lower fire resistance than the one made with dolomite. Steel slag concrete shows similar fire resistance to the river aggregate mixture up to 400 1C, and much improved fire resistance at high-temperature ranges. The fire resistance of these mixtures is lower than the fire resistance of the dolomite mixtures up to 800 1C, above which it increases as it coincides with the temperature range within which the dissolution of CaCO3 in dolomite occurs. This leads to the conclusion that slag could find its application in higher temperature areas, for example, in the production of refractory concrete. According to the above conclusions, the replacement of a river aggregate in concrete with steel and brick industry waste material can not only be justified in terms of fire resistance, but also in terms of waste management. However, before making a final conclusion on the recommended areas of application for the particular mixture, the authors suggest making additional studies that will include the monitoring of temperature within the samples and monitoring of temperature effects on other concrete properties. It is also advisable to monitor the selected properties on coarse aggregate concrete.
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