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Mechanical properties of lightweight concrete made with coal ashes after exposure to elevated temperatures
Cement and Concrete Composites 72 (2016) 27e38
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Cement and Concrete Composites journal homepage: www.elsevier.com/locate/cemconcomp
Mechanical properties of lightweight concrete made with coal ashes after exposure to elevated temperatures Y.B. Ahn, J.G. Jang, H.K. Lee* Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, Daehak-ro 291, Yuseong-gu, Daejeon 34141, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 October 2015 Received in revised form 28 March 2016 Accepted 31 May 2016 Available online 1 June 2016
This study investigated the thermal resistance of lightweight concrete with recycled coal bottom ash and fly ash. Specimens were exposed to temperatures up to 800 C then cooled to room temperature before conducting experiments. Compressive strength test, FF-RC test, TG analysis, and XRD analysis were performed to analyze the physicochemical effects of coal ashes on the thermal resistance of concrete. Test results indicated that both bottom ash and fly ash were associated with a substantial increase in the residual strength of thermal exposed concretes. The results were attributed to the surface interlocking effect and the smaller amount of SiO2 for bottom ash. For fly ash, the formation of pozzolanic C-S-H gel and tobermorite retained water at high temperatures, and the consumption of Ca(OH)2 lowered stress from rapid recrystallization after exposure to 600 C. It was concluded that the incorporation of coal ashes allows for lightweight concrete with good thermal resistance. © 2016 Elsevier Ltd. All rights reserved.
Keywords: High temperature Lightweight concrete Mechanical properties Fly ash Bottom ash
1. Introduction In response to the persistent pursuit of taller, longer, and larger structures around the world, the search for technologies that allow lighter and cheaper construction materials has attracted attention for decades now. One such technology is structural lightweight concrete, which has close to 20% lower density compared to normal weight concrete, and exhibits strength that is adequate enough for many structural purposes. The primary advantage of lightweight concrete is the reduction in the dead load of structures, allowing smaller cross-sections and less reinforcing materials. Lightweight concrete is also used because of its low transportation cost, resistance to earthquake loading, easier handling, and low overall cost [1]. Lightweight concrete, which can be in a form of polymer concrete or lightweight aggregate concrete, can achieve a greater strength-to-density ratio than ordinary concrete and only slightly compromises the strength of ordinary concrete [2]. The most commonly used lightweight aggregates are artificially calcined aggregates, such as expanded shale, expanded clay and expanded vermiculite, and natural lightweight aggregates, such as scoria and
* Corresponding author. E-mail address: [email protected] (H.K. Lee). http://dx.doi.org/10.1016/j.cemconcomp.2016.05.028 0958-9465/© 2016 Elsevier Ltd. All rights reserved.
pumice [3]. These aggregates have many benefits but also drawbacks, including relatively high cost, the large environmental impact of the energy-intensive calcination process, or limited local availability of material. One prospective material to overcome the drawbacks is coal bottom ash, a byproduct of the coal-fired electrical power plant. This chemically stable material is very affordable, abundant worldwide with one million tons produced annually in South Korea alone [4], and does not require any additional energy for production. In addition, recycling bottom ash will prevent serious ground and water pollution caused by the common practice of landfill; it has been verified in previous studies that heavy metals along with other pollutants from bottom ash can be solidified in cementitious forms [5e8]. Bottom ash accounts for 10e30% of total coal ash, with the rest mainly comprised of fly ash. The pozzolanic reaction of fly ash, along with its many advantages as a cementitious binder, has been extensively studied [9e13]. One of the most destructive causes of concrete failure is fire [14]. Fire, unlike many other threats to concrete, may cause complete failure with a single case of exposure, making it difficult to take necessary measures when a threat or the resulting damage is first detected. The application of lightweight concrete must therefore be conducted with a good understanding of its behavior at elevated temperatures, especially in relation to the properties of the aggregates and binder used. Numerous studies have been conducted on moisture flow, heat flow, and the main causes of spalling,
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especially inside normal and heavy-weight concrete [15e20]. However opinions vary on how concrete density, water content, and thermal conductivity affect its thermal resistance. For example, Sakr and EL-Hakim reported that dense matrix of concrete increased residual strength of concrete subjected to elevated temperature, whereas Hossain and Lachemi concluded otherwise [21,22]. In addition, Li et al. reported that oven-dried specimens showed lower strength than air-dried specimens after exposure to 800 C, whereas Lau reported that higher moisture contents resulted in greater strength loss [23,24]. Most importantly, although numerous studies have investigated the application of coal ashes in lightweight concrete, little attention has been given to its effect on thermal resistance. This study focused on the thermal resistance of lightweight concrete using recycled coal ashes, with bottom ash as aggregate, and fly ash as a portion of the binder. The behavior of the lightweight concrete with coal ashes at elevated temperatures was investigated in comparison with the behavior of lightweight concrete with expanded shale, a commonly used lightweight aggregate. A comparative analysis was conducted in terms of physical changes in the interface between aggregate and paste, the chemical composition of paste, and the degradation process of paste through dehydration and phase transformation. It is projected that the results of this study can deepen the understanding of lightweight concrete with coal ashes, leading to safer design and application of this technology.
that of expanded shale. Water-to-binder ratio of 0.635 and aggregate-to-paste ratio of 0.942 by weight were used for all mixtures. A relatively high water to binder ratio was adopted to achieve a rapid and extensive pozzolanic reaction of fly ash [25]. Viscosity modifier was used in amounts of less than 0.2 cement weight percent for adequate workability. Mix proportions of the specimens are given in Table 3. Paste was made before incorporating aggregates to reduce the separation of materials and to lower the amount of materials absorbed into the porous aggregates [5]. Aggregates were air dried for improvement in internal curing [26] and to reduce the film of water on the aggregate surface that is accused of creating a weak interface with higher water content [13]. Cylindrical (f10 cm 20 cm) and cubic (5 cm 5 cm 5 cm) specimens were fabricated. Fresh specimens were wrapped in plastic wrap for a day, then demolded and cured in water at 20 C for 28 days. Air-dry densities of the cured specimens were measured after being dried for 24 h at room temperature, and the results are summarized in Table 4. All specimens showed significantly lower densities, corresponding to 75e80% the density of normal weight concrete (2300 kg/m3). The incorporation of coal bottom ash and fly ash increased the air-dry density of the lightweight concrete in comparison with the incorporation of expanded shale and type I cement alone, respectively, at the same water-to-binder and aggregate-to-paste ratios. This may be explained by the Blaine fineness of fly ash, which is higher in value compared to that of cement, resulting in tighter packing.
2. Materials and experimental methods 2.3. Testing and characterization 2.1. Materials used Materials used in this study were class F fly ash from Dang-jin thermal power plant in South Korea, bottom ash from Seo-cheon thermal power plant in South Korea, expanded shale from Hanya Raw-material, Ltd. in China, and type I Portland cement. The chemical compositions of the fly ash and bottom ash were obtained by X-ray fluorescence (XRF) using MiniPal 2 from PANanalytical, and that of expanded shale and cement were given by the manufacturer. The chemical composition of materials used in this study is presented in Table 1, which indicates that SiO2 alone comprises 49.9% of the bottom ash and 65% of the expanded shale. Blaine finenesses of the fly ash and cement were 290 m2/kg and 280 m2/ kg, respectively. Physical properties of the aggregate materials were tested in accordance with standard test methods including ASTM C128 for density and absorption, ASTM C29 for bulk density, and ASTM C136 for sieve analysis. The results of the standard tests are organized in Table 2. The maximum aggregate size for both bottom ash and expanded shale was 10 mm. 2.2. Specimens preparation Six different mix proportions were designed to investigate the effect of bottom ash and fly ash on thermal resistance compared to Table 1 Chemical composition of materials. Chemical composition (%)
Fly ash
Bottom ash
Expanded shale
SiO2 Al2O3 CaO Fe2O3 K2O TiO2 ZrO2 Cl
51.00 25.00 6.24 12.70 1.70 2.14 0.14 e
49.90 29.30 1.64 10.50 4.69 2.83 0.15 0.57
65 18 2 7 e e e e
2.3.1. Thermal exposure Prior to thermal exposure testing, the specimens were first dried at 100 C until a constant weight was achieved, to remove capillary water and reduce the risk of spalling. The specimens were cooled to room temperature, then placed inside a furnace, whose internal temperature was increased from room temperature to 200, 400, 600, or 800 C at the rate of 10 C/min. The maximum temperature was maintained for 2 h to attain thermal equilibrium at the center of the specimens [27]. After the 2 h the furnace was turned off, and the specimens were allowed to cool slowly for 24 h inside the furnace. Finally, all specimens were sealed with plastic wrap to prevent rehydration until further experiments. Weight loss during the thermal exposure was measured as an indication of the amount of water loss [28]. It is recognized that free water is evaporated at temperatures up to 200 C, adsorbed water at temperatures up to 400 C, and chemically bound water at temperatures up to 1500 C [28,29]. 2.3.2. Free-free resonant column test (FF-RC test) The FF-RC test is a non-destructive testing method for estimating Young’s modulus within the elastic range of a material, in which the principle of elastic wave propagation is applied [30]. A cylindrical specimen is suspended from a support frame using two pieces of string to create free-free boundary conditions, as shown in Fig. 1. One end of the specimen is hit with a small mallet, and the resonant frequency is measured and recorded by the waveform analyzer at the other end. Young’s modulus is estimated using Eq. (1) [30]: 2
E ¼ r$ðf1 $lÞ ;
(1)
where r is the unit weight of the specimen, f1 is the resonant frequency, and l is the first mode wavelength, which is twice the length of the specimen [30]. Using a single r value and using twice the length of the specimen for l signify that the concrete is assumed
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Table 2 Physical properties of aggregate materials.
Bottom ash Expanded shale
Apparent specific gravity
Bulk specific gravity SSD
Water absorption ratio (%)
Fineness modulus
1.87 2.29
1.63 1.2
4.06 8.09
5.63 6.31
Table 3 Mix proportions of the specimens. Unit weight (kg/m3)
Specimen code
Water
EF0 EF20 BF0 BF5 BF15 BF20
390.6 385.5 357.4 356.3 354.1 353.1
Binder
Aggregate
Cement
Fly ash
Expanded shale
Bottom ash
615.6 486.0 563.2 533.4 474.4 445.2
0.0 121.5 0.0 28.1 83.7 111.2
948.1 935.6 0.0 0.0 0.0 0.0
0.0 0.0 867.4 864.8 859.6 857.0
Table 4 Air-dry densities of specimens (kg/m3). BF0
BF5
BF15
BF20
EF0
EF20
1789.1
1823.8
1830.9
1798.1
1721.4
1742.8
Support frame
Suspending strings
Small mallet
Cylindrical Specimen
Waveform analyzer
Fig. 1. Schematic diagram of FF-RC test setup.
to be a homogeneous material with a constant r value, and that the wave propagation throughout the specimen is assumed to be not hindered by characteristics of materials including aggregates, pastes, water, and crack. These assumptions are prospected to have a significant effect on the calculated dynamic Young’s modulus value itself.
2.3.3. Compressive strength test and stress-strain curve Residual strength, defined as the strength of specimens after exposure to a heating and cooling scheme, is one of the most frequently investigated performances of concrete used to evaluate its thermal resistance. The residual compressive strength of thermal exposed specimens in this study were tested in accordance with ASTM C39 using a 3000 kN UTM [31]. The speed of the cross head was set to 0.01 mm/s. To obtain the stress-strain curve, the
Viscosity modifier
0.6 1.0 0.0 0.0 0.0 0.5
strain of a specimen was acquired using the average value of displacement collected with three equally spaced LDVTs. The Young’s modulus of a concrete specimen was defined as the secant modulus using the strain corresponding to maximum strength, and the strain corresponding to 40% of the maximum strength.
2.3.4. Microscope observation Cross-sections of specimens at interfaces between aggregate and paste were compared before and after thermal exposure. A digital microscope manufactured by Shenzhen Technology (magnification up to 200) was used to measure the width of cracks created on interfaces during the thermal exposure. The degree of damage for each crack was classified as 1 for cracks with a maximum width smaller than 24 mm, 2 for cracks with a maximum width between 24 mm and 44 mm, and 3 for cracks with a maximum width larger than 44 mm. The average of the classification number for cracks in a sample does not have any physical meaning, but rather provides a measure for comparative studies.
2.3.5. Thermo gravimetric (TG) analysis TG analysis was conducted to evaluate the content of Ca(OH)2, from which the degree of pozzolanic reaction can be assessed. Powdered paste samples with fly ash contents of 0, 5, 15, and 20 wt % of total binder were heated to 800 C, with a temperature increment rate of 10 C/min. The residual weight of each test specimen was recorded every three seconds. The degree of carbonation reaction was minimized by conducting the TG analysis immediately after unwrapping and grinding the test specimens.
2.3.6. X-ray diffractometer (XRD) analysis XRD analysis was conducted to determine the presence of crystalline products of the dehydration process formed during the thermal exposure. Larnite (C2S) from the dehydration of C-S-H gel, b-phase quartz, and lime (CaO) from the dehydration of Ca(OH)2 were the major products that were anticipated [13,17,18]. The amount of crystalline products was evaluated from the crystalline to non-crystalline ratio obtained with the integration method. The scan speed was set to low, i.e., 2 /min for this purpose. The degree of carbonation reaction was minimized by conducting the XRD analysis immediately after unwrapping and grinding the test specimens.
Fig. 2. Surfaces of thermal exposed specimens after 800 C: (a) discoloration, (b) surface spalling, and (c) crack formation.
3. Experimental results 3.1. Surface observations and loss in weight Changes on the concrete surfaces before and after thermal exposure included discoloration, surface spalling, and crack formation, as shown in Fig. 2. This study focused on crack formation, as discoloration and surface spalling are commonly seen and extensively studied phenomena on concrete after exposure to high temperatures [28,32]. It was observed that concretes exposed to 600 C develop more cracks on their surfaces than concretes exposed to 800 C for all cylindrical specimens as shown in Fig. 3. This observation may be far from conventional belief, since one can safely argue that the increase of exposure temperature is closely related to the increase in the degree of thermal damage and crack formation on concrete. No research has reported such incoherence between the exposure temperature and crack formation. In addition, the results indicated that the incorporation of coal ashes reduced crack damage, as shown in Fig. 3. The ratio of residual weight to original weight for specimens after exposure to each temperature is shown in Fig. 4. At temperatures above 400 C, the ratio did not closely represent the lost amount of water, since spalling or the disintegration of cement particles undoubtedly decreased the residual weight. The largest weight decrement occurred between 100 C and 200 C for all specimens, while the magnitude of decrement varied significantly among the specimens. The magnitude decreased as the amount of fly ash increased for the BF0, BF5, and BF15 specimens. However, BF20 showed the largest loss in weight among all BF specimens. By comparing the residual weight of BF0 and EF0, it was concluded that the incorporation of bottom ash as aggregate reduces the magnitude of weight decrement caused by thermal exposure. 3.2. Compressive strength and stress-strain curve The 28 days compressive strengths of each specimen type are summarized in Table 5. The air-dry densities and compressive strengths of the BF0, BF5, and BF15 specimens satisfy the ACI criteria on structural lightweight concrete, with strengths of at least 17 MPa and densities of at most 1842 kg/m3 [33]. The ratio between the residual strength and the control strength (strength after exposure to 100 C) of each specimen is presented in Fig. 5. This ratio is more subjective for comparison, as many studies have shown that this ratio is not strongly affected by the change in initial strength [34]. In the present study, numerous interesting results were obtained. First, all specimens showed significant decrease in strength, preserving a mere 19.5%e51.7% of control strength after exposure to 800 C. Nevertheless, the residual strength ratio is considerably high, compared to that of normal weight concrete with similar testing conditions, which exhibit values ranging from 10% to 35% of
Fig. 3. Surface cracks of specimens exposed to 600 C vs. 800 C of (a) EF0, (b) BF0, and (c) BF20.
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Fig. 4. Residual weight of specimens after thermal exposure.
Table 5 28 days compressive strengths of specimens (MPa). BF0
BF5
BF15
BF20
EF0
EF20
18.8
17.6
20.7
16.8
20.6
22.2
control strength [13,23,29,35]. Secondly, for the thermal exposure regime and specimen size used in this study, the addition of coal ashes significantly improved the residual strength, i.e., thermal resistance, especially of the specimen exposed to 600 C, resulting in a difference between stability and failure under the slight stress exerted by the test setup. Thirdly, the BF5 and BF15 specimens showed an increase in compressive strength as the exposure temperature increased from 100 C to 200 C. Similar observations were reported previously, where the trapped vapor inside the concrete specimen accelerated the hydration process in the temperature range [13,29,36]. The increase in strength of these particular specimens is theorized to be attributed to the accelerated
pozzolanic reaction, as those specimens with little expected pozzolanic reactivity, such as EF0 and BF0, do not show such behavior. The stress-strain curves of the thermal exposed cylindrical specimens are presented in Fig. 6, and the calculated secant moduli are summarized in Table 6. The EF0 specimen exposed to 600 C did not retain adequate strength for the setup of LVDTs and thus was exempt from the strength test. From the test results, it is immediately observed that the Young’s modulus tends to decrease with increase in temperature, except in the range between 600 C and 800 C where an increase in the Young’s modulus value is observed. This is consistent with the results based on surface observations. In addition, the strain corresponding to the maximum strength of the specimen increased as exposure temperature increased. One unusual phenomenon observed in this study was the pseudo-plastic deformation behavior, i.e., ductility characteristic in compression of specimens incorporating bottom ash after exposure to temperatures 400 C and higher; all specimens showed such behavior after exposure to 800 C.
Fig. 5. Residual strength of specimens after thermal exposure.
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Y.B. Ahn et al. / Cement and Concrete Composites 72 (2016) 27e38 Table 6 Calculated secant Young’s modulus from Fig. 6 (GPa).
BF0 BF20 EF0 a
100 C
200 C
400 C
600 C
800 C
27.5 32.5 23.7
18.2 32.9 17.5
12.2 11.3 9.1
1.7 6.6 ea
8.2 4.9 3.5
Specimen did not retain enough strength for the test setup.
the hanging setup and thus was exempt from this test. It was observed from the test results that the resonant frequency and corresponding dynamic Young’s modulus value decreased as the exposure temperature increased. Although the absolute values of the calculated dynamic Young’s modulus in Fig. 7 were considerably different from the static Young’s modulus calculated from the stress-strain curve, the overall trend according to the exposure temperature was similar in both cases. In addition, the anomaly at 600 C observed with the visual examination of cracks and the obtained stress-strain curves is repeated here: the Young’s modulus value decreases very sharply at 600 C, then partly recovers at 800 C. Moreover, the transfer function has more notable peaks other than the peak at the resonant frequency for specimens with lower Young’s modulus value. The number and intensity of these noise peaks may signify the degree of disintegration in the structure of a test specimen, and these properties can collectively be represented by the area under the transfer function plot. As shown in Fig. 8, it is reasonable to conclude that the area under the FF-RC test curve and the Young’s modulus values from the stress-strain curves have good correlation. 3.4. Crack formation Representative cross-sectional images of thermal exposed specimens are shown in Fig. 9. Cement paste had penetrated into the open voids of the bottom ash, creating a very jagged and integrated interface. No physical changes or visible damages were observed until the exposure to 600 C when hairline cracks appear around the edges of both bottom ash and expanded shale. After exposure to 800 C, very few aggregates had a completely intact interface. Using measurements at 65 different sites, the average of the given number for each crack’s degree of severity was calculated to be 1.6 for specimens with bottom ash and 2.2 for specimens with expanded shale. Thus, it is concluded that specimens with bottom ash show less fractural damage in the interfaces than specimens with expanded shale. 3.5. TG analysis
Fig. 6. Stress-strain curves of specimens after thermal exposure: (a) BF0, (b) BF20, and (c) EF0.
3.3. FF-RC test results The results of the FF-RC test are presented in Fig. 7, along with the calculated Young’s modulus values, which are marked on the peak representing each specimen’s resonant frequency. The EF0 specimen exposed to 600 C did not retain adequate strength for
The results of TG analysis are presented in Fig. 10. All specimens showed a rapid and significant decrease in weight around 450 C, which can be attributed to decomposition of Ca(OH)2 [37]. The weight loss percentage for each specimen has a large correlation with the fly ash content of each specimen; specimens with higher fly ash content suffered smaller weight loss around 450 C. The decrease in residual weight can be divided into four sections. The first section for all specimens is roughly from 30 C to 250 C, where capillary and absorbed water are released, and the third section is where the decomposition of Ca(OH)2 takes place. The last section is roughly from 500 C to 800 C, in which a noticeable change in the rate of decrement takes place around 690 C. This subtle drop is inferred to indicate the decomposition of CaCO3 to CaO and CO2. The evaporation of capillary water, adsorbed water, and chemically bound water comprises most of the weight loss from 30 C to 800 C [38]. The total weight loss ranged from 18.4% to 21.0% per specimen, and specimens with higher fly ash content suffered smaller total weight loss.
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Fig. 7. FF-RC test results: (a) BF0, (b) BF20, and (c) EF0.
3.6. XRD analysis
4. Discussion
The results of the XRD analysis are shown in Fig. 11. The XRD peaks for all BF5 specimens exposed to temperatures up to 800 C showed an amorphous hump between 15 and 40 , which indicates that a significant amount of amorphous C-S-H gel remained hydrated even after exposure to high temperatures. A crystalline form of C-S-H, tobermorite, was also included in the BF5 specimens. On the other hand, crystalline larnite was the most common compound in the EF0 specimen, which showed 100% crystalline structure after exposure to 800 C. As shown in Fig. 11(b), the C-S-H gel hump is not found for the EF0 specimen, which agrees with the measured degree of crystallinity. The major compounds detected for the BF15 specimen exposed to 800 C were very similar to those for the BF5 specimen exposed to 800 C. This reveals that the type of aggregate used in a specimen has a large influence on the dehydration process and the resulting chemical composition.
4.1. Effect of bottom ash aggregates on thermal resistance After exposure to high temperatures, specimens with bottom ash retained a higher residual strength ratio compared to specimens with expanded shale. It was theorized that bottom ash possesses three properties that contribute to retaining residual strength, which may be considered under the following headings: (1) dense grading; (2) relatively small SiO2 content; and (3) high roughness. First, the size gradation of aggregate has a large effect on the arrangement of aggregates, the arrangement of voids, and the size of the voids inside concrete [39]. The bottom ash used in this study had a denser grading than expanded shale, which allowed for better dispersion of the aggregates and for smaller sized voids. At high temperatures, such internal structure is more advantageous as
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Young's modulus (GPa)
35 30
25 20 15 10
5 0 0
500
1000
1500
2000
2500
3000
3500
4000
Calculated area under spectrum of transfer function
(a)
Young's modulus (GPa)
35 30 25
20 15 10 5 0 0
500
1000
1500
2000
2500
3000
3500
4000
Calculated area under spectrum of transfer function
(b)
Young's modulus (GPa)
35 30 25 20 15
stress inside the concrete at temperatures around 600 C. At high temperatures aggregates expand due to thermal expansion, while paste shrinks from dehydration [22,29]. This thermal incompatibility causes significant internal stress that has been found in many studies to decrease the integrity of concrete and ultimately the strength of concrete [28,29]. In addition, SiO2 goes through a phase transformation around 600 C, which results in volume expansion of 5% [18]. For concrete with siliceous aggregates, the volume expansion from the phase transformation would add to the internal stress from thermal expansion. As a result, it is expected that concrete with bottom ash, composed of small amount of SiO2, is much more stable at temperatures above 600 C than concrete with expanded shale, which is composed of large amount of SiO2. Thirdly, the rough surfaces of the bottom ash may ultimately lead to slower fracturing of concrete from cracks and to slower dehydration of paste. As the microscope images indicate, the bottom ash exhibited an interlocking effect which leads to a stronger bond between aggregate and paste, resulting in less severe damage in the interfaces at high temperatures (see Fig. 9). The effects of cracks on the compressive strength of concrete subjected to elevated temperatures may be explained in two ways. For one thing, applied stress is concentrated around these cracks, leading to the enlargement and bridging of cracks. Stress is further concentrated around these larger cracks, leading to a prompt failure of concrete. Moreover, the cracks in the interfaces can serve as pathways for vapor to escape out of the specimen. More cracks enable easier and faster release of vapor, facilitating the dehydration process of the hydration products in concrete. The degree of dehydration, often signified by the weight of water lost, has the largest effect on the decrease of compressive strength after exposure to high temperature [21]. In this study, the BF0 specimens showed significantly smaller weight loss following exposure to high temperatures than the EF0 specimens, as shown in Fig. 4. This signifies that the type of aggregate, and thus the amount of cracks created in the interfaces, had a large effect on the amount of water lost. The cracks in the interfaces are deduced to account for the pseudo-plastic deformation behavior as well. Especially after exposure to 800 C, almost all interfaces showed cracks, as can be seen in Fig. 9. This large number of cracks may allow for concrete to absorb a large amount of energy before failure, resulting in what may be depicted as plastic deformation.
10
4.2. Effect of fly ash on thermal resistance
5 0 0
500
1000
1500
2000
2500
3000
3500
4000
Calculated area under spectrum of transfer function
(c) Fig. 8. Area under FF-RC transfer function vs. secant Young’s modulus: (a) BF0, (b) BF20, and (c) EF0.
it allows for smaller concentration of stress and slower release of water vapor. This was especially beneficial in this study because the risk of spalling was significantly reduced by first drying the specimens at 100 C. In contrast to bottom ash, such dense grading is hard to obtain using expanded shale, because artificially manufactured aggregate such as expanded shale has limited range and little variety in the size of the aggregate. Therefore, with limited grading, expanded shale cannot achieve the density of grading that provides expected advantages in the thermal resistance of concrete, unlike bottom ash. Secondly, the relatively smaller amount of SiO2 in bottom ash, compared to expanded shale, reduced the increase in internal
For the particular mix design used in this study, the cement-tofly ash replacement ratio of 5 or 15 wt% was considered superior to the ratio of 0 or 20 wt% at 28 days. This is based on the suspected amount of unreacted fly ash and the degree of accelerated hydration at temperatures between 100 C and 200 C. As mentioned earlier, the BF20 specimen lost the largest amount of water in this temperature range, while the BF15 specimen lost the smallest amount. This signifies that a large amount of water in BF20 was not chemically bound (see Fig. 4), which indicates that much of the fly ash in BF20 had not formed bonds with water to produce additional C-S-H gel at the age of 28 days. In addition, accelerated hydration from pozzolanic activities was not evident for the BF20 specimen, unlike the BF5 and BF15 specimens, indicated by the increase in compressive strength after thermal exposure to 200 C (Fig. 5). In conclusion, the addition of fly ash has obvious benefits for thermal resistance of concrete, while the replacement ratio of 20 wt% was excessive and ineffective in this study. Chemical reactions involving fly ash improved the residual strength of concrete exposed to high temperatures. Pozzolanic activities of fly ash involving Ca(OH)2 decrease the size of voids inside the concrete matrix. This can impede the escape of vapor by
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Fig. 9. Cross-sectional images after thermal exposure of concrete with (a) bottom ash after exposure to 100 C, (b) expanded shale after exposure to 100 C, (c) bottom ash after exposure to 600 C, and (d) expanded shale after exposure to 600 C.
Fig. 10. TG analysis results for paste with fly ash replacement ratio of (a) 0%, (b) 5%, (c) 15%, and (d) 20%.
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Fig. 11. Results of XRD analysis: (a) total and (b) amorphous hump for C-S-H gel. The symbols A, B, C, D, E, and F signify Ca(OH)2, moganite-SiO2, CaCO3, larnite-Ca2SiO4, CaO, and tobermorite, respectively.
narrowing the pathways, thus delaying the dehydration process. The comparison of loss in weight for EF0 and EF20 specimens, and the comparison of the results of TG analysis, support that the addition of fly ash has a large effect on reducing the amount of escaped water (see Figs. 4 and 10). It is also shown in Fig. 11 that the C-S-H gel has been completely dehydrated for the EF0 specimen, unlike the BF5 or BF15 specimens, supporting that the addition of fly ash impedes the dehydration process of hydration products. Such dense matrix may have provided more benefits in this study because the risk of spalling was low. In addition, fly ash and CaO
react to produce tobermorite, which is dehydrated at a much higher temperature compared to C-S-H gel [40]. The retention of water due to the formation of tobermorite may significantly contribute to the retention of strength after exposure to temperatures up to 800 C. The last observation to be discussed is the anomaly at the exposure temperature of 600 C, where concrete specimens showed larger surface cracks and lower Young’s modulus values compared to exposure at 800 C. Cracks were formed subsequent to the completion of the thermal exposure, cooling, and wrapping
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procedures, indicating that some form of reaction that does not require any extra elements from the surrounding environment took place. Such a reaction may be a phase transformation, which is described in a previous study done by Maciejewski and Reller [41]. In the study, they found that when CaCO3 was exposed to the temperature of 540 C for 100 min, it decomposed to highly active CaO, which at lower temperatures carbonates to form amorphous CaCO3 [41]. This metastable CaCO3 was rapidly crystallized at temperatures around 300 C. If this phase transformation occurred inside concrete, it would exert a degree of internal stress [34]. With thermal exposure to 600 C for 120 min, the experimental setup used in this study may be analogous enough to that in Maciejewski and Reller’s study [41]. The thermal profile may have allowed for the formation of active CaO, which would then carbonate inside the furnace to form metastable CaCO3. With the core of the specimen still at high temperatures due to low thermal conductivity, the described phase transformation may have occurred while cooling down to room temperature. The accompanying internal stress may be sufficient to cause cracking. As Maciejewski and Reller explained, the conditions required for the formation of active CaO and thus for the phase transformation of CaCO3 are hard to meet [41]. Consequently, an observation of the phase transformation may be less probable. However attributing this phase transformation to the crack formations and decreased Young’s modulus allows us to interpret two observations: (1) the change in the specimens after being wrapped in plastic only occurred for specimens exposed to 600 C; (2) specimens with an expected deficiency of Ca(OH)2, thus CaCO3 due to pozzolanic reaction, suffered less damage, as shown in Fig. 3. 5. Conclusions In this study, lightweight concrete with air dry densities between 1790 and 1825 kg/m3 were produced using coal ashes. The concrete specimens were subjected to temperatures up to 800 C, and the changes in their physical and chemical properties were investigated. The following conclusions were made on the basis of this study. (1) The strength of lightweight concrete decreased as the exposure temperature increases, retaining 19.5%e51.7% of the original strength after exposure to 800 C. From the FF-RC test results, it is observed that the resonant frequency and corresponding Young’s modulus value decrease as exposure temperature increases, which agrees with results of the stress-strain curve. It was concluded that the usage of bottom ash as aggregate and the incorporation of fly ash in the binder increased the thermal resistance. (2) The effects of using bottom ash on the thermal resistance of lightweight concrete were investigated through measurement of weight loss during thermal exposure, and examination of the interface between the aggregate and paste. In short, 1) denser gradation of bottom ash increased the consistency of the concrete matrix and created narrower pathways for vapor; 2) rougher surfaces of bottom ash increased the interlocking effect and decreased the degree of damage at interfaces; and 3) smaller SiO2 content of bottom ash resulted in smaller volume expansion due to phase transformation when exposed to high temperatures. (3) The effects of incorporating fly ash on the thermal resistance of lightweight concrete were investigated using TG analysis and XRD analysis. Test results lead to the conclusion that fly ash hindered the dehydration process of paste and increased the amount of chemically bound water in tobermorite or C-S-
37
H gel. In addition, the notable formation of cracks after exposure to 600 C was attributed to the phase transformation of amorphous CaCO3 described in a previous report [41]. It was concluded that the addition of fly ash can decrease the extra stress from phase transformation. Acknowledgments This research was supported by a grant from the National Research Foundation of Korea (NRF) (2015R1A2A1A10055694) funded by the Korean government, and by a grant from the Energy Technology Development Program (2013T100100021) funded by the Ministry of Trade Industrial and Energy of the Korean government. References [1] H.K. Kim, H.K. Lee, Use of power plant bottom ash as fine and coarse aggregates in high-strength concrete, Constr. Build. Mater. 25 (2011) 1115e1122. [2] G. Martínez-Barrera, E. Vigueras-Santiago, O. Gencel, Hagg Lobland HE. Polymer concretes: a description and methods for modification and improvement, J. Mater. Ed. 33 (2011) 37e52. [3] F. Koksal, O. Gencel, W. Brostow, H.E. Hagg Lobland, Effect of high temperature on mechanical and physical properties of lightweight cement based refractory including expanded vermiculite, Mater. Res. Innov. 16 (2012) 7e13. [4] T.H. Koh, Tire Shred-bottom Ash Mixtures: Mechanical Properties and Use as Construction Material, PhD thesis, Purdue University, Indiana, 2008. [5] J.G. Jang, Y.B. Ahn, H. Souri, H.K. Lee, A novel eco-friendly porous concrete fabricated with coal ash and geopolymeric binder: heavy metal leaching characteristics and compressive strength, Constr. Build. Mater. 79 (2015) 173e181. [6] J. Davidovits, Geopolymers: man-made rock geosynthesis and the resulting development of very early high strength cement, J. Mater. Ed. 16 (1994) 91e139. [7] J.G. Jang, H.J. Kim, H.K. Kim, H.K. Lee, Resistance of coal bottom ash mortar against the coupled deterioration of carbonation and chloride penetration, Mater. Des. 93 (2016) 160e167. [8] E. Toraldo, S. Saponaro, A. Careghini, E. Mariani, Use of stabilized bottom ash for bound layers of road pavements, J. Environ. Manag. 121 (2013) 117e123. [9] W.D.A. Rickard, A.V. Riessen, Performance of solid and cellular structured fly ash geopolymers exposed to a simulated fire, Cem. Concr. Compos. 48 (2014) 75e82. [10] J.G. Jang, H.K. Lee, Effect of fly ash characteristics on delayed high-strength development of geopolymers, Constr. Build. Mater. 102 (2016) 260e269. [11] M. Narmluk, T. Nawa, Effect of curing temperature on pozzolanic reaction of fly ash in blended cement paste, Int. J. Chem. Eng. Appl. 5 (2014) 31e35. [12] M. Nisnevich, G. Sirotin, Y. Eshel, T. Schlesinger, Structural lightweight concrete based on coal ashes (containing undesirable radionuclides) and waste of stone quarries, Mag. Concr. Res. 58 (2006) 233e241. [13] A. Nadeem, S.A. Memon, T.Y. Lo, Qualitative and quantitative analysis and identification of flaws in the microstructure of fly ash and metakaolin blended high performance concrete after exposure to elevated temperatures, Constr. Build. Mater. 38 (2013) 731e741. [14] M.R. Bangi, T. Horiguchi, Effect of fibre type and geometry on maximum pore pressures in fibre-reinforced high strength concrete at elevated temperatures, Cem. Concr. Res. 42 (2012) 459e466. [15] G. Debicki, R. Haniche, F. Delhomme, An experimental method for assessing the spalling sensitivity of concrete mixture submitted to high temperature, Cem. Concr. Compos. 34 (2012) 958e963. [16] Y. Ichikawa, G.L. England, Prediction of moisture migration and pore pressure build-up in concrete at high temperatures, Nucl. Eng. Des. 228 (2004) 245e259. [17] C. Alonso, L. Fernandez, Dehydration and rehydration processes of cement paste exposed to high temperature environments, J. Mater. Sci. 39 (2004) 3015e3024. [18] T.G. Nijland, J.A. Larbi, Unraveling the temperature distribution in firedamaged concrete by means of PFM microscopy: outline of the approach and review of potentially useful reactions, HERON 46 (2001) 253e264. [19] P.J.E. Sullivan, A probabilistic method of testing for the assessment of deterioration and explosive spalling of high strength concrete beams in flexure at high temperature, Cem. Concr. Compos 26 (2004) 155e162. [20] J. Selih, A.C.M. Sousa, T.W. Bremner, Moisture and heat flow in concrete walls exposed to fire, J. Eng. Mech. 120 (1994) 2028e2043. [21] K. Sakr, E. EL-Hakim, Effect of high temperature or fire on heavy weight concrete properties, Cem. Concr. Res. 35 (2005) 590e596. [22] K.M.A. Hossain, M. Lachemi, Mixture design, strength, durability, and fire resistance of lightweight pumice concrete, ACI Mater. J. 104 (2007) 449e457. [23] M. Li, C.X. Qian, W. Sun, Mechanical properties of high-strength concrete after fire, Cem. Concr. Res. 34 (2004) 1001e1005. [24] A. Lau, Effect of High Temperatures on Normal Strength Concrete and High
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Y.B. Ahn et al. / Cement and Concrete Composites 72 (2016) 27e38 Performance Concrete Containing Steel Fibers, The Hong Kong Polytechnic University, Hong Kong, 2003. S.E. Chidiac, D.K. Panesar, Evolution of mechanical properties of concrete containing ground granulated blast furnace slag and effects on the scaling resistance test at 28 days, Cem. Concr. Compos. 30 (2008) 63e71. A. Bentur, S. Igarashi, K. Kovler, Prevention of autogenous shrinkage in highstrength concrete by internal curing using wet lightweight aggregates, Cem. Concr. Res. 31 (2001) 1587e1591. H.J. Yim, S.J. Park, J.H. Kim, H.G. Kwak, Nonlinear ultrasonic method to evaluate residual mechanical properties of thermally damaged concrete, ACI Mater. J. 11 (2014) 399e409. O.A. Abdulkareem, A.M.M.A. Bakri, H. Kamurudin, I.K. Nizar, A.A. Saif, Effects of elevated temperatures on the thermal behavior and mechanical performance of fly ash geopolymer paste, mortar and lightweight concrete, Constr. Build. Mater. 50 (2014) 377e387. C. Castillo, A.J. Durrani, Effect of transient high temperature on high-strength concrete, ACI Mater. J. 87 (1990) 47e53. J.G. Vaghela, K.H. Stokoe, Small-strain Dynamic Properties of Dry Sand from the Free-free Resonant Column. Geotechnical Engineering Report GT95-1, University of Texas, Austin, 1995. ASTM C39e12a, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens, ASTM Int., 2012.
[32] J. Temuujin, A.V. Riessen, Effect of fly ash preliminary calcination on the properties of geopolymer, J. Hazard Mater. 164 (2009) 634e639. [33] ACI 213R-87, Guide for Structural Lightweight Aggregate Concrete, Am Concr Inst, 1987. [34] J. Kropp, J. Seeberger, H.K. Hilsdorf, Chemical and physical properties of cement paste and concrete containing fly ash after hydrothermal exposure, Am. Concr. Inst. Spec. Publ. 91 (1986) 201e218. [35] J. Xiao, G. Konig, Study on concrete at high temperature in China-an overview, Fire Saf. J. 39 (2004) 89e103. [36] K.W. Nasser, H.M. Marzouk, Properties of mass concrete containing fly ash at high temperatures, ACI J. 76 (1979) 537e550. [37] Q. Zhang, G. Ye, Dehydration kinetics of Portland cement paste at high temperature, J. Therm. Anal. Calorim. 110 (2012) 153e158. [38] O. Arioz, Effects of elevated temperatures on properties of concrete, Fire Saf. J. 42 (2007) 516e522. [39] A. Torres, J. Hu, A. Ramos, The effect of the cementitious paste thickness on the performance of pervious concrete, Constr. Build. Mater. 95 (2015) 850e859. [40] Y. Xiuli, C. Chong, C. Xiaoyu, T. Guodong, M.A. Hailong, High-temperature phase transition and the activity of tobermorite, J. Wuhan. Univ. Technol. 29 (2014) 298e301. [41] M. Maciejewski, A. Reller, Formation of amorphous CaCO3 during the reaction of CO2 with CaO, Thermochim. Acta 142 (1989) 175e188.
Contents lists available at ScienceDirect
Cement and Concrete Composites journal homepage: www.elsevier.com/locate/cemconcomp
Mechanical properties of lightweight concrete made with coal ashes after exposure to elevated temperatures Y.B. Ahn, J.G. Jang, H.K. Lee* Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, Daehak-ro 291, Yuseong-gu, Daejeon 34141, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 October 2015 Received in revised form 28 March 2016 Accepted 31 May 2016 Available online 1 June 2016
This study investigated the thermal resistance of lightweight concrete with recycled coal bottom ash and fly ash. Specimens were exposed to temperatures up to 800 C then cooled to room temperature before conducting experiments. Compressive strength test, FF-RC test, TG analysis, and XRD analysis were performed to analyze the physicochemical effects of coal ashes on the thermal resistance of concrete. Test results indicated that both bottom ash and fly ash were associated with a substantial increase in the residual strength of thermal exposed concretes. The results were attributed to the surface interlocking effect and the smaller amount of SiO2 for bottom ash. For fly ash, the formation of pozzolanic C-S-H gel and tobermorite retained water at high temperatures, and the consumption of Ca(OH)2 lowered stress from rapid recrystallization after exposure to 600 C. It was concluded that the incorporation of coal ashes allows for lightweight concrete with good thermal resistance. © 2016 Elsevier Ltd. All rights reserved.
Keywords: High temperature Lightweight concrete Mechanical properties Fly ash Bottom ash
1. Introduction In response to the persistent pursuit of taller, longer, and larger structures around the world, the search for technologies that allow lighter and cheaper construction materials has attracted attention for decades now. One such technology is structural lightweight concrete, which has close to 20% lower density compared to normal weight concrete, and exhibits strength that is adequate enough for many structural purposes. The primary advantage of lightweight concrete is the reduction in the dead load of structures, allowing smaller cross-sections and less reinforcing materials. Lightweight concrete is also used because of its low transportation cost, resistance to earthquake loading, easier handling, and low overall cost [1]. Lightweight concrete, which can be in a form of polymer concrete or lightweight aggregate concrete, can achieve a greater strength-to-density ratio than ordinary concrete and only slightly compromises the strength of ordinary concrete [2]. The most commonly used lightweight aggregates are artificially calcined aggregates, such as expanded shale, expanded clay and expanded vermiculite, and natural lightweight aggregates, such as scoria and
* Corresponding author. E-mail address: [email protected] (H.K. Lee). http://dx.doi.org/10.1016/j.cemconcomp.2016.05.028 0958-9465/© 2016 Elsevier Ltd. All rights reserved.
pumice [3]. These aggregates have many benefits but also drawbacks, including relatively high cost, the large environmental impact of the energy-intensive calcination process, or limited local availability of material. One prospective material to overcome the drawbacks is coal bottom ash, a byproduct of the coal-fired electrical power plant. This chemically stable material is very affordable, abundant worldwide with one million tons produced annually in South Korea alone [4], and does not require any additional energy for production. In addition, recycling bottom ash will prevent serious ground and water pollution caused by the common practice of landfill; it has been verified in previous studies that heavy metals along with other pollutants from bottom ash can be solidified in cementitious forms [5e8]. Bottom ash accounts for 10e30% of total coal ash, with the rest mainly comprised of fly ash. The pozzolanic reaction of fly ash, along with its many advantages as a cementitious binder, has been extensively studied [9e13]. One of the most destructive causes of concrete failure is fire [14]. Fire, unlike many other threats to concrete, may cause complete failure with a single case of exposure, making it difficult to take necessary measures when a threat or the resulting damage is first detected. The application of lightweight concrete must therefore be conducted with a good understanding of its behavior at elevated temperatures, especially in relation to the properties of the aggregates and binder used. Numerous studies have been conducted on moisture flow, heat flow, and the main causes of spalling,
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especially inside normal and heavy-weight concrete [15e20]. However opinions vary on how concrete density, water content, and thermal conductivity affect its thermal resistance. For example, Sakr and EL-Hakim reported that dense matrix of concrete increased residual strength of concrete subjected to elevated temperature, whereas Hossain and Lachemi concluded otherwise [21,22]. In addition, Li et al. reported that oven-dried specimens showed lower strength than air-dried specimens after exposure to 800 C, whereas Lau reported that higher moisture contents resulted in greater strength loss [23,24]. Most importantly, although numerous studies have investigated the application of coal ashes in lightweight concrete, little attention has been given to its effect on thermal resistance. This study focused on the thermal resistance of lightweight concrete using recycled coal ashes, with bottom ash as aggregate, and fly ash as a portion of the binder. The behavior of the lightweight concrete with coal ashes at elevated temperatures was investigated in comparison with the behavior of lightweight concrete with expanded shale, a commonly used lightweight aggregate. A comparative analysis was conducted in terms of physical changes in the interface between aggregate and paste, the chemical composition of paste, and the degradation process of paste through dehydration and phase transformation. It is projected that the results of this study can deepen the understanding of lightweight concrete with coal ashes, leading to safer design and application of this technology.
that of expanded shale. Water-to-binder ratio of 0.635 and aggregate-to-paste ratio of 0.942 by weight were used for all mixtures. A relatively high water to binder ratio was adopted to achieve a rapid and extensive pozzolanic reaction of fly ash [25]. Viscosity modifier was used in amounts of less than 0.2 cement weight percent for adequate workability. Mix proportions of the specimens are given in Table 3. Paste was made before incorporating aggregates to reduce the separation of materials and to lower the amount of materials absorbed into the porous aggregates [5]. Aggregates were air dried for improvement in internal curing [26] and to reduce the film of water on the aggregate surface that is accused of creating a weak interface with higher water content [13]. Cylindrical (f10 cm 20 cm) and cubic (5 cm 5 cm 5 cm) specimens were fabricated. Fresh specimens were wrapped in plastic wrap for a day, then demolded and cured in water at 20 C for 28 days. Air-dry densities of the cured specimens were measured after being dried for 24 h at room temperature, and the results are summarized in Table 4. All specimens showed significantly lower densities, corresponding to 75e80% the density of normal weight concrete (2300 kg/m3). The incorporation of coal bottom ash and fly ash increased the air-dry density of the lightweight concrete in comparison with the incorporation of expanded shale and type I cement alone, respectively, at the same water-to-binder and aggregate-to-paste ratios. This may be explained by the Blaine fineness of fly ash, which is higher in value compared to that of cement, resulting in tighter packing.
2. Materials and experimental methods 2.3. Testing and characterization 2.1. Materials used Materials used in this study were class F fly ash from Dang-jin thermal power plant in South Korea, bottom ash from Seo-cheon thermal power plant in South Korea, expanded shale from Hanya Raw-material, Ltd. in China, and type I Portland cement. The chemical compositions of the fly ash and bottom ash were obtained by X-ray fluorescence (XRF) using MiniPal 2 from PANanalytical, and that of expanded shale and cement were given by the manufacturer. The chemical composition of materials used in this study is presented in Table 1, which indicates that SiO2 alone comprises 49.9% of the bottom ash and 65% of the expanded shale. Blaine finenesses of the fly ash and cement were 290 m2/kg and 280 m2/ kg, respectively. Physical properties of the aggregate materials were tested in accordance with standard test methods including ASTM C128 for density and absorption, ASTM C29 for bulk density, and ASTM C136 for sieve analysis. The results of the standard tests are organized in Table 2. The maximum aggregate size for both bottom ash and expanded shale was 10 mm. 2.2. Specimens preparation Six different mix proportions were designed to investigate the effect of bottom ash and fly ash on thermal resistance compared to Table 1 Chemical composition of materials. Chemical composition (%)
Fly ash
Bottom ash
Expanded shale
SiO2 Al2O3 CaO Fe2O3 K2O TiO2 ZrO2 Cl
51.00 25.00 6.24 12.70 1.70 2.14 0.14 e
49.90 29.30 1.64 10.50 4.69 2.83 0.15 0.57
65 18 2 7 e e e e
2.3.1. Thermal exposure Prior to thermal exposure testing, the specimens were first dried at 100 C until a constant weight was achieved, to remove capillary water and reduce the risk of spalling. The specimens were cooled to room temperature, then placed inside a furnace, whose internal temperature was increased from room temperature to 200, 400, 600, or 800 C at the rate of 10 C/min. The maximum temperature was maintained for 2 h to attain thermal equilibrium at the center of the specimens [27]. After the 2 h the furnace was turned off, and the specimens were allowed to cool slowly for 24 h inside the furnace. Finally, all specimens were sealed with plastic wrap to prevent rehydration until further experiments. Weight loss during the thermal exposure was measured as an indication of the amount of water loss [28]. It is recognized that free water is evaporated at temperatures up to 200 C, adsorbed water at temperatures up to 400 C, and chemically bound water at temperatures up to 1500 C [28,29]. 2.3.2. Free-free resonant column test (FF-RC test) The FF-RC test is a non-destructive testing method for estimating Young’s modulus within the elastic range of a material, in which the principle of elastic wave propagation is applied [30]. A cylindrical specimen is suspended from a support frame using two pieces of string to create free-free boundary conditions, as shown in Fig. 1. One end of the specimen is hit with a small mallet, and the resonant frequency is measured and recorded by the waveform analyzer at the other end. Young’s modulus is estimated using Eq. (1) [30]: 2
E ¼ r$ðf1 $lÞ ;
(1)
where r is the unit weight of the specimen, f1 is the resonant frequency, and l is the first mode wavelength, which is twice the length of the specimen [30]. Using a single r value and using twice the length of the specimen for l signify that the concrete is assumed
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29
Table 2 Physical properties of aggregate materials.
Bottom ash Expanded shale
Apparent specific gravity
Bulk specific gravity SSD
Water absorption ratio (%)
Fineness modulus
1.87 2.29
1.63 1.2
4.06 8.09
5.63 6.31
Table 3 Mix proportions of the specimens. Unit weight (kg/m3)
Specimen code
Water
EF0 EF20 BF0 BF5 BF15 BF20
390.6 385.5 357.4 356.3 354.1 353.1
Binder
Aggregate
Cement
Fly ash
Expanded shale
Bottom ash
615.6 486.0 563.2 533.4 474.4 445.2
0.0 121.5 0.0 28.1 83.7 111.2
948.1 935.6 0.0 0.0 0.0 0.0
0.0 0.0 867.4 864.8 859.6 857.0
Table 4 Air-dry densities of specimens (kg/m3). BF0
BF5
BF15
BF20
EF0
EF20
1789.1
1823.8
1830.9
1798.1
1721.4
1742.8
Support frame
Suspending strings
Small mallet
Cylindrical Specimen
Waveform analyzer
Fig. 1. Schematic diagram of FF-RC test setup.
to be a homogeneous material with a constant r value, and that the wave propagation throughout the specimen is assumed to be not hindered by characteristics of materials including aggregates, pastes, water, and crack. These assumptions are prospected to have a significant effect on the calculated dynamic Young’s modulus value itself.
2.3.3. Compressive strength test and stress-strain curve Residual strength, defined as the strength of specimens after exposure to a heating and cooling scheme, is one of the most frequently investigated performances of concrete used to evaluate its thermal resistance. The residual compressive strength of thermal exposed specimens in this study were tested in accordance with ASTM C39 using a 3000 kN UTM [31]. The speed of the cross head was set to 0.01 mm/s. To obtain the stress-strain curve, the
Viscosity modifier
0.6 1.0 0.0 0.0 0.0 0.5
strain of a specimen was acquired using the average value of displacement collected with three equally spaced LDVTs. The Young’s modulus of a concrete specimen was defined as the secant modulus using the strain corresponding to maximum strength, and the strain corresponding to 40% of the maximum strength.
2.3.4. Microscope observation Cross-sections of specimens at interfaces between aggregate and paste were compared before and after thermal exposure. A digital microscope manufactured by Shenzhen Technology (magnification up to 200) was used to measure the width of cracks created on interfaces during the thermal exposure. The degree of damage for each crack was classified as 1 for cracks with a maximum width smaller than 24 mm, 2 for cracks with a maximum width between 24 mm and 44 mm, and 3 for cracks with a maximum width larger than 44 mm. The average of the classification number for cracks in a sample does not have any physical meaning, but rather provides a measure for comparative studies.
2.3.5. Thermo gravimetric (TG) analysis TG analysis was conducted to evaluate the content of Ca(OH)2, from which the degree of pozzolanic reaction can be assessed. Powdered paste samples with fly ash contents of 0, 5, 15, and 20 wt % of total binder were heated to 800 C, with a temperature increment rate of 10 C/min. The residual weight of each test specimen was recorded every three seconds. The degree of carbonation reaction was minimized by conducting the TG analysis immediately after unwrapping and grinding the test specimens.
2.3.6. X-ray diffractometer (XRD) analysis XRD analysis was conducted to determine the presence of crystalline products of the dehydration process formed during the thermal exposure. Larnite (C2S) from the dehydration of C-S-H gel, b-phase quartz, and lime (CaO) from the dehydration of Ca(OH)2 were the major products that were anticipated [13,17,18]. The amount of crystalline products was evaluated from the crystalline to non-crystalline ratio obtained with the integration method. The scan speed was set to low, i.e., 2 /min for this purpose. The degree of carbonation reaction was minimized by conducting the XRD analysis immediately after unwrapping and grinding the test specimens.
Fig. 2. Surfaces of thermal exposed specimens after 800 C: (a) discoloration, (b) surface spalling, and (c) crack formation.
3. Experimental results 3.1. Surface observations and loss in weight Changes on the concrete surfaces before and after thermal exposure included discoloration, surface spalling, and crack formation, as shown in Fig. 2. This study focused on crack formation, as discoloration and surface spalling are commonly seen and extensively studied phenomena on concrete after exposure to high temperatures [28,32]. It was observed that concretes exposed to 600 C develop more cracks on their surfaces than concretes exposed to 800 C for all cylindrical specimens as shown in Fig. 3. This observation may be far from conventional belief, since one can safely argue that the increase of exposure temperature is closely related to the increase in the degree of thermal damage and crack formation on concrete. No research has reported such incoherence between the exposure temperature and crack formation. In addition, the results indicated that the incorporation of coal ashes reduced crack damage, as shown in Fig. 3. The ratio of residual weight to original weight for specimens after exposure to each temperature is shown in Fig. 4. At temperatures above 400 C, the ratio did not closely represent the lost amount of water, since spalling or the disintegration of cement particles undoubtedly decreased the residual weight. The largest weight decrement occurred between 100 C and 200 C for all specimens, while the magnitude of decrement varied significantly among the specimens. The magnitude decreased as the amount of fly ash increased for the BF0, BF5, and BF15 specimens. However, BF20 showed the largest loss in weight among all BF specimens. By comparing the residual weight of BF0 and EF0, it was concluded that the incorporation of bottom ash as aggregate reduces the magnitude of weight decrement caused by thermal exposure. 3.2. Compressive strength and stress-strain curve The 28 days compressive strengths of each specimen type are summarized in Table 5. The air-dry densities and compressive strengths of the BF0, BF5, and BF15 specimens satisfy the ACI criteria on structural lightweight concrete, with strengths of at least 17 MPa and densities of at most 1842 kg/m3 [33]. The ratio between the residual strength and the control strength (strength after exposure to 100 C) of each specimen is presented in Fig. 5. This ratio is more subjective for comparison, as many studies have shown that this ratio is not strongly affected by the change in initial strength [34]. In the present study, numerous interesting results were obtained. First, all specimens showed significant decrease in strength, preserving a mere 19.5%e51.7% of control strength after exposure to 800 C. Nevertheless, the residual strength ratio is considerably high, compared to that of normal weight concrete with similar testing conditions, which exhibit values ranging from 10% to 35% of
Fig. 3. Surface cracks of specimens exposed to 600 C vs. 800 C of (a) EF0, (b) BF0, and (c) BF20.
Y.B. Ahn et al. / Cement and Concrete Composites 72 (2016) 27e38
31
Fig. 4. Residual weight of specimens after thermal exposure.
Table 5 28 days compressive strengths of specimens (MPa). BF0
BF5
BF15
BF20
EF0
EF20
18.8
17.6
20.7
16.8
20.6
22.2
control strength [13,23,29,35]. Secondly, for the thermal exposure regime and specimen size used in this study, the addition of coal ashes significantly improved the residual strength, i.e., thermal resistance, especially of the specimen exposed to 600 C, resulting in a difference between stability and failure under the slight stress exerted by the test setup. Thirdly, the BF5 and BF15 specimens showed an increase in compressive strength as the exposure temperature increased from 100 C to 200 C. Similar observations were reported previously, where the trapped vapor inside the concrete specimen accelerated the hydration process in the temperature range [13,29,36]. The increase in strength of these particular specimens is theorized to be attributed to the accelerated
pozzolanic reaction, as those specimens with little expected pozzolanic reactivity, such as EF0 and BF0, do not show such behavior. The stress-strain curves of the thermal exposed cylindrical specimens are presented in Fig. 6, and the calculated secant moduli are summarized in Table 6. The EF0 specimen exposed to 600 C did not retain adequate strength for the setup of LVDTs and thus was exempt from the strength test. From the test results, it is immediately observed that the Young’s modulus tends to decrease with increase in temperature, except in the range between 600 C and 800 C where an increase in the Young’s modulus value is observed. This is consistent with the results based on surface observations. In addition, the strain corresponding to the maximum strength of the specimen increased as exposure temperature increased. One unusual phenomenon observed in this study was the pseudo-plastic deformation behavior, i.e., ductility characteristic in compression of specimens incorporating bottom ash after exposure to temperatures 400 C and higher; all specimens showed such behavior after exposure to 800 C.
Fig. 5. Residual strength of specimens after thermal exposure.
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Y.B. Ahn et al. / Cement and Concrete Composites 72 (2016) 27e38 Table 6 Calculated secant Young’s modulus from Fig. 6 (GPa).
BF0 BF20 EF0 a
100 C
200 C
400 C
600 C
800 C
27.5 32.5 23.7
18.2 32.9 17.5
12.2 11.3 9.1
1.7 6.6 ea
8.2 4.9 3.5
Specimen did not retain enough strength for the test setup.
the hanging setup and thus was exempt from this test. It was observed from the test results that the resonant frequency and corresponding dynamic Young’s modulus value decreased as the exposure temperature increased. Although the absolute values of the calculated dynamic Young’s modulus in Fig. 7 were considerably different from the static Young’s modulus calculated from the stress-strain curve, the overall trend according to the exposure temperature was similar in both cases. In addition, the anomaly at 600 C observed with the visual examination of cracks and the obtained stress-strain curves is repeated here: the Young’s modulus value decreases very sharply at 600 C, then partly recovers at 800 C. Moreover, the transfer function has more notable peaks other than the peak at the resonant frequency for specimens with lower Young’s modulus value. The number and intensity of these noise peaks may signify the degree of disintegration in the structure of a test specimen, and these properties can collectively be represented by the area under the transfer function plot. As shown in Fig. 8, it is reasonable to conclude that the area under the FF-RC test curve and the Young’s modulus values from the stress-strain curves have good correlation. 3.4. Crack formation Representative cross-sectional images of thermal exposed specimens are shown in Fig. 9. Cement paste had penetrated into the open voids of the bottom ash, creating a very jagged and integrated interface. No physical changes or visible damages were observed until the exposure to 600 C when hairline cracks appear around the edges of both bottom ash and expanded shale. After exposure to 800 C, very few aggregates had a completely intact interface. Using measurements at 65 different sites, the average of the given number for each crack’s degree of severity was calculated to be 1.6 for specimens with bottom ash and 2.2 for specimens with expanded shale. Thus, it is concluded that specimens with bottom ash show less fractural damage in the interfaces than specimens with expanded shale. 3.5. TG analysis
Fig. 6. Stress-strain curves of specimens after thermal exposure: (a) BF0, (b) BF20, and (c) EF0.
3.3. FF-RC test results The results of the FF-RC test are presented in Fig. 7, along with the calculated Young’s modulus values, which are marked on the peak representing each specimen’s resonant frequency. The EF0 specimen exposed to 600 C did not retain adequate strength for
The results of TG analysis are presented in Fig. 10. All specimens showed a rapid and significant decrease in weight around 450 C, which can be attributed to decomposition of Ca(OH)2 [37]. The weight loss percentage for each specimen has a large correlation with the fly ash content of each specimen; specimens with higher fly ash content suffered smaller weight loss around 450 C. The decrease in residual weight can be divided into four sections. The first section for all specimens is roughly from 30 C to 250 C, where capillary and absorbed water are released, and the third section is where the decomposition of Ca(OH)2 takes place. The last section is roughly from 500 C to 800 C, in which a noticeable change in the rate of decrement takes place around 690 C. This subtle drop is inferred to indicate the decomposition of CaCO3 to CaO and CO2. The evaporation of capillary water, adsorbed water, and chemically bound water comprises most of the weight loss from 30 C to 800 C [38]. The total weight loss ranged from 18.4% to 21.0% per specimen, and specimens with higher fly ash content suffered smaller total weight loss.
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Fig. 7. FF-RC test results: (a) BF0, (b) BF20, and (c) EF0.
3.6. XRD analysis
4. Discussion
The results of the XRD analysis are shown in Fig. 11. The XRD peaks for all BF5 specimens exposed to temperatures up to 800 C showed an amorphous hump between 15 and 40 , which indicates that a significant amount of amorphous C-S-H gel remained hydrated even after exposure to high temperatures. A crystalline form of C-S-H, tobermorite, was also included in the BF5 specimens. On the other hand, crystalline larnite was the most common compound in the EF0 specimen, which showed 100% crystalline structure after exposure to 800 C. As shown in Fig. 11(b), the C-S-H gel hump is not found for the EF0 specimen, which agrees with the measured degree of crystallinity. The major compounds detected for the BF15 specimen exposed to 800 C were very similar to those for the BF5 specimen exposed to 800 C. This reveals that the type of aggregate used in a specimen has a large influence on the dehydration process and the resulting chemical composition.
4.1. Effect of bottom ash aggregates on thermal resistance After exposure to high temperatures, specimens with bottom ash retained a higher residual strength ratio compared to specimens with expanded shale. It was theorized that bottom ash possesses three properties that contribute to retaining residual strength, which may be considered under the following headings: (1) dense grading; (2) relatively small SiO2 content; and (3) high roughness. First, the size gradation of aggregate has a large effect on the arrangement of aggregates, the arrangement of voids, and the size of the voids inside concrete [39]. The bottom ash used in this study had a denser grading than expanded shale, which allowed for better dispersion of the aggregates and for smaller sized voids. At high temperatures, such internal structure is more advantageous as
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stress inside the concrete at temperatures around 600 C. At high temperatures aggregates expand due to thermal expansion, while paste shrinks from dehydration [22,29]. This thermal incompatibility causes significant internal stress that has been found in many studies to decrease the integrity of concrete and ultimately the strength of concrete [28,29]. In addition, SiO2 goes through a phase transformation around 600 C, which results in volume expansion of 5% [18]. For concrete with siliceous aggregates, the volume expansion from the phase transformation would add to the internal stress from thermal expansion. As a result, it is expected that concrete with bottom ash, composed of small amount of SiO2, is much more stable at temperatures above 600 C than concrete with expanded shale, which is composed of large amount of SiO2. Thirdly, the rough surfaces of the bottom ash may ultimately lead to slower fracturing of concrete from cracks and to slower dehydration of paste. As the microscope images indicate, the bottom ash exhibited an interlocking effect which leads to a stronger bond between aggregate and paste, resulting in less severe damage in the interfaces at high temperatures (see Fig. 9). The effects of cracks on the compressive strength of concrete subjected to elevated temperatures may be explained in two ways. For one thing, applied stress is concentrated around these cracks, leading to the enlargement and bridging of cracks. Stress is further concentrated around these larger cracks, leading to a prompt failure of concrete. Moreover, the cracks in the interfaces can serve as pathways for vapor to escape out of the specimen. More cracks enable easier and faster release of vapor, facilitating the dehydration process of the hydration products in concrete. The degree of dehydration, often signified by the weight of water lost, has the largest effect on the decrease of compressive strength after exposure to high temperature [21]. In this study, the BF0 specimens showed significantly smaller weight loss following exposure to high temperatures than the EF0 specimens, as shown in Fig. 4. This signifies that the type of aggregate, and thus the amount of cracks created in the interfaces, had a large effect on the amount of water lost. The cracks in the interfaces are deduced to account for the pseudo-plastic deformation behavior as well. Especially after exposure to 800 C, almost all interfaces showed cracks, as can be seen in Fig. 9. This large number of cracks may allow for concrete to absorb a large amount of energy before failure, resulting in what may be depicted as plastic deformation.
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it allows for smaller concentration of stress and slower release of water vapor. This was especially beneficial in this study because the risk of spalling was significantly reduced by first drying the specimens at 100 C. In contrast to bottom ash, such dense grading is hard to obtain using expanded shale, because artificially manufactured aggregate such as expanded shale has limited range and little variety in the size of the aggregate. Therefore, with limited grading, expanded shale cannot achieve the density of grading that provides expected advantages in the thermal resistance of concrete, unlike bottom ash. Secondly, the relatively smaller amount of SiO2 in bottom ash, compared to expanded shale, reduced the increase in internal
For the particular mix design used in this study, the cement-tofly ash replacement ratio of 5 or 15 wt% was considered superior to the ratio of 0 or 20 wt% at 28 days. This is based on the suspected amount of unreacted fly ash and the degree of accelerated hydration at temperatures between 100 C and 200 C. As mentioned earlier, the BF20 specimen lost the largest amount of water in this temperature range, while the BF15 specimen lost the smallest amount. This signifies that a large amount of water in BF20 was not chemically bound (see Fig. 4), which indicates that much of the fly ash in BF20 had not formed bonds with water to produce additional C-S-H gel at the age of 28 days. In addition, accelerated hydration from pozzolanic activities was not evident for the BF20 specimen, unlike the BF5 and BF15 specimens, indicated by the increase in compressive strength after thermal exposure to 200 C (Fig. 5). In conclusion, the addition of fly ash has obvious benefits for thermal resistance of concrete, while the replacement ratio of 20 wt% was excessive and ineffective in this study. Chemical reactions involving fly ash improved the residual strength of concrete exposed to high temperatures. Pozzolanic activities of fly ash involving Ca(OH)2 decrease the size of voids inside the concrete matrix. This can impede the escape of vapor by
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Fig. 9. Cross-sectional images after thermal exposure of concrete with (a) bottom ash after exposure to 100 C, (b) expanded shale after exposure to 100 C, (c) bottom ash after exposure to 600 C, and (d) expanded shale after exposure to 600 C.
Fig. 10. TG analysis results for paste with fly ash replacement ratio of (a) 0%, (b) 5%, (c) 15%, and (d) 20%.
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Fig. 11. Results of XRD analysis: (a) total and (b) amorphous hump for C-S-H gel. The symbols A, B, C, D, E, and F signify Ca(OH)2, moganite-SiO2, CaCO3, larnite-Ca2SiO4, CaO, and tobermorite, respectively.
narrowing the pathways, thus delaying the dehydration process. The comparison of loss in weight for EF0 and EF20 specimens, and the comparison of the results of TG analysis, support that the addition of fly ash has a large effect on reducing the amount of escaped water (see Figs. 4 and 10). It is also shown in Fig. 11 that the C-S-H gel has been completely dehydrated for the EF0 specimen, unlike the BF5 or BF15 specimens, supporting that the addition of fly ash impedes the dehydration process of hydration products. Such dense matrix may have provided more benefits in this study because the risk of spalling was low. In addition, fly ash and CaO
react to produce tobermorite, which is dehydrated at a much higher temperature compared to C-S-H gel [40]. The retention of water due to the formation of tobermorite may significantly contribute to the retention of strength after exposure to temperatures up to 800 C. The last observation to be discussed is the anomaly at the exposure temperature of 600 C, where concrete specimens showed larger surface cracks and lower Young’s modulus values compared to exposure at 800 C. Cracks were formed subsequent to the completion of the thermal exposure, cooling, and wrapping
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procedures, indicating that some form of reaction that does not require any extra elements from the surrounding environment took place. Such a reaction may be a phase transformation, which is described in a previous study done by Maciejewski and Reller [41]. In the study, they found that when CaCO3 was exposed to the temperature of 540 C for 100 min, it decomposed to highly active CaO, which at lower temperatures carbonates to form amorphous CaCO3 [41]. This metastable CaCO3 was rapidly crystallized at temperatures around 300 C. If this phase transformation occurred inside concrete, it would exert a degree of internal stress [34]. With thermal exposure to 600 C for 120 min, the experimental setup used in this study may be analogous enough to that in Maciejewski and Reller’s study [41]. The thermal profile may have allowed for the formation of active CaO, which would then carbonate inside the furnace to form metastable CaCO3. With the core of the specimen still at high temperatures due to low thermal conductivity, the described phase transformation may have occurred while cooling down to room temperature. The accompanying internal stress may be sufficient to cause cracking. As Maciejewski and Reller explained, the conditions required for the formation of active CaO and thus for the phase transformation of CaCO3 are hard to meet [41]. Consequently, an observation of the phase transformation may be less probable. However attributing this phase transformation to the crack formations and decreased Young’s modulus allows us to interpret two observations: (1) the change in the specimens after being wrapped in plastic only occurred for specimens exposed to 600 C; (2) specimens with an expected deficiency of Ca(OH)2, thus CaCO3 due to pozzolanic reaction, suffered less damage, as shown in Fig. 3. 5. Conclusions In this study, lightweight concrete with air dry densities between 1790 and 1825 kg/m3 were produced using coal ashes. The concrete specimens were subjected to temperatures up to 800 C, and the changes in their physical and chemical properties were investigated. The following conclusions were made on the basis of this study. (1) The strength of lightweight concrete decreased as the exposure temperature increases, retaining 19.5%e51.7% of the original strength after exposure to 800 C. From the FF-RC test results, it is observed that the resonant frequency and corresponding Young’s modulus value decrease as exposure temperature increases, which agrees with results of the stress-strain curve. It was concluded that the usage of bottom ash as aggregate and the incorporation of fly ash in the binder increased the thermal resistance. (2) The effects of using bottom ash on the thermal resistance of lightweight concrete were investigated through measurement of weight loss during thermal exposure, and examination of the interface between the aggregate and paste. In short, 1) denser gradation of bottom ash increased the consistency of the concrete matrix and created narrower pathways for vapor; 2) rougher surfaces of bottom ash increased the interlocking effect and decreased the degree of damage at interfaces; and 3) smaller SiO2 content of bottom ash resulted in smaller volume expansion due to phase transformation when exposed to high temperatures. (3) The effects of incorporating fly ash on the thermal resistance of lightweight concrete were investigated using TG analysis and XRD analysis. Test results lead to the conclusion that fly ash hindered the dehydration process of paste and increased the amount of chemically bound water in tobermorite or C-S-
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H gel. In addition, the notable formation of cracks after exposure to 600 C was attributed to the phase transformation of amorphous CaCO3 described in a previous report [41]. It was concluded that the addition of fly ash can decrease the extra stress from phase transformation. Acknowledgments This research was supported by a grant from the National Research Foundation of Korea (NRF) (2015R1A2A1A10055694) funded by the Korean government, and by a grant from the Energy Technology Development Program (2013T100100021) funded by the Ministry of Trade Industrial and Energy of the Korean government. References [1] H.K. Kim, H.K. Lee, Use of power plant bottom ash as fine and coarse aggregates in high-strength concrete, Constr. Build. Mater. 25 (2011) 1115e1122. [2] G. Martínez-Barrera, E. Vigueras-Santiago, O. Gencel, Hagg Lobland HE. Polymer concretes: a description and methods for modification and improvement, J. Mater. Ed. 33 (2011) 37e52. [3] F. Koksal, O. Gencel, W. Brostow, H.E. Hagg Lobland, Effect of high temperature on mechanical and physical properties of lightweight cement based refractory including expanded vermiculite, Mater. Res. Innov. 16 (2012) 7e13. [4] T.H. Koh, Tire Shred-bottom Ash Mixtures: Mechanical Properties and Use as Construction Material, PhD thesis, Purdue University, Indiana, 2008. [5] J.G. Jang, Y.B. Ahn, H. Souri, H.K. Lee, A novel eco-friendly porous concrete fabricated with coal ash and geopolymeric binder: heavy metal leaching characteristics and compressive strength, Constr. Build. Mater. 79 (2015) 173e181. [6] J. Davidovits, Geopolymers: man-made rock geosynthesis and the resulting development of very early high strength cement, J. Mater. Ed. 16 (1994) 91e139. [7] J.G. Jang, H.J. Kim, H.K. Kim, H.K. Lee, Resistance of coal bottom ash mortar against the coupled deterioration of carbonation and chloride penetration, Mater. Des. 93 (2016) 160e167. [8] E. Toraldo, S. Saponaro, A. Careghini, E. Mariani, Use of stabilized bottom ash for bound layers of road pavements, J. Environ. Manag. 121 (2013) 117e123. [9] W.D.A. Rickard, A.V. Riessen, Performance of solid and cellular structured fly ash geopolymers exposed to a simulated fire, Cem. Concr. Compos. 48 (2014) 75e82. [10] J.G. Jang, H.K. Lee, Effect of fly ash characteristics on delayed high-strength development of geopolymers, Constr. Build. Mater. 102 (2016) 260e269. [11] M. Narmluk, T. Nawa, Effect of curing temperature on pozzolanic reaction of fly ash in blended cement paste, Int. J. Chem. Eng. Appl. 5 (2014) 31e35. [12] M. Nisnevich, G. Sirotin, Y. Eshel, T. Schlesinger, Structural lightweight concrete based on coal ashes (containing undesirable radionuclides) and waste of stone quarries, Mag. Concr. Res. 58 (2006) 233e241. [13] A. Nadeem, S.A. Memon, T.Y. Lo, Qualitative and quantitative analysis and identification of flaws in the microstructure of fly ash and metakaolin blended high performance concrete after exposure to elevated temperatures, Constr. Build. Mater. 38 (2013) 731e741. [14] M.R. Bangi, T. Horiguchi, Effect of fibre type and geometry on maximum pore pressures in fibre-reinforced high strength concrete at elevated temperatures, Cem. Concr. Res. 42 (2012) 459e466. [15] G. Debicki, R. Haniche, F. Delhomme, An experimental method for assessing the spalling sensitivity of concrete mixture submitted to high temperature, Cem. Concr. Compos. 34 (2012) 958e963. [16] Y. Ichikawa, G.L. England, Prediction of moisture migration and pore pressure build-up in concrete at high temperatures, Nucl. Eng. Des. 228 (2004) 245e259. [17] C. Alonso, L. Fernandez, Dehydration and rehydration processes of cement paste exposed to high temperature environments, J. Mater. Sci. 39 (2004) 3015e3024. [18] T.G. Nijland, J.A. Larbi, Unraveling the temperature distribution in firedamaged concrete by means of PFM microscopy: outline of the approach and review of potentially useful reactions, HERON 46 (2001) 253e264. [19] P.J.E. Sullivan, A probabilistic method of testing for the assessment of deterioration and explosive spalling of high strength concrete beams in flexure at high temperature, Cem. Concr. Compos 26 (2004) 155e162. [20] J. Selih, A.C.M. Sousa, T.W. Bremner, Moisture and heat flow in concrete walls exposed to fire, J. Eng. Mech. 120 (1994) 2028e2043. [21] K. Sakr, E. EL-Hakim, Effect of high temperature or fire on heavy weight concrete properties, Cem. Concr. Res. 35 (2005) 590e596. [22] K.M.A. Hossain, M. Lachemi, Mixture design, strength, durability, and fire resistance of lightweight pumice concrete, ACI Mater. J. 104 (2007) 449e457. [23] M. Li, C.X. Qian, W. Sun, Mechanical properties of high-strength concrete after fire, Cem. Concr. Res. 34 (2004) 1001e1005. [24] A. Lau, Effect of High Temperatures on Normal Strength Concrete and High
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