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Effect of moisture migration and water vapor pressure build-up with the heating rate on concrete spalling type
Cement and Concrete Research 116 (2019) 1–10
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Cement and Concrete Research journal homepage: www.elsevier.com/locate/cemconres
Effect of moisture migration and water vapor pressure build-up with the heating rate on concrete spalling type
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⁎
Gyeongcheol Choea, Gyuyong Kima, , Minho Yoona, Euichul Hwanga, Jeongsoo Nama, Nenad Guncunskib a b
Department of Architectural Engineering, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Republic of Korea Department of Civil and Environmental Engineering, Rutgers University, Piscataway, NJ 08855-0909, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: High-strength concrete (HSC) Heating rate Moisture migration Spalling type Water vapor pressure
In this study, the effect of moisture migration and water vapor pressure build-up within high-strength concrete (HSC) on its spalling property is empirically investigated at various heating rates and compressive strengths with different water-binder ratios (W/B). An experiment is conducted on concrete specimens with various W/Bs by applying two different heating rates: fast and slow heating. It is confirmed that the moisture migration and water vapor pressure within the concrete differ, depending upon the W/B and heating rate. For HSC with W/B less than 0.33, surface spalling occurs due to the formation of moisture clogs on the surfaces of the concrete specimens, under the fast heating condition. For HSC with dense microstructures, although moisture clogs are not formed at specific positions within the concrete specimens, explosive spalling occurs due to boiling liquid expanding vapor explosion (BLEVE) in the concrete pores, under the slow heating condition.
1. Introduction HSC has several advantages such as excellent structural performance and high durability; however, it can explode when exposed to heat such as that from fire. Concrete spalling is difficult to predict due to its extremely irregular nature and can significantly lower the structural strength by causing cross-sectional losses in its members [1–3]. The spalling mechanism of concrete can be classified into spalling by water vapor pressure, by thermal stress, and by a combination of water vapor and thermal pressures. The factors affecting concrete spalling include internal ones, such as the air permeability, water content, compressive strength, and water/cement ratio, and external ones, such as the rate of temperature increase, maximum heating temperature, heating source, and external force [4]. All concrete exposed to rapid heating conditions such as fire are at the risk of spalling. However, according to various studies [5–10], the HSC is more sensitive to spalling than the NSC. Several studies on the causes of spalling [11–14] have compared the water vapor pressure within the concrete with the saturated water vapor pressure of water and confirmed that moisture migration and water vapor pressure buildup within the concrete have a significant influence on concrete spalling [15]. Thus, HSC, which has a relatively lower air permeability than
NSC, is more vulnerable to spalling due to less internal moisture migration and greater water vapor pressure on the surface. Methods for preventing spalling by reducing the internal water vapor pressure within the concrete, based on the internal moisture behavior, have been proposed [14,15–21]. P. Kalifa [19], in particular, mixed polypropylene fibers with concrete and experimentally confirmed that water vapor was discharged through the pores generated by the melted fibers and cracks in the cement matrix; this reduced the water vapor pressure and prevented spalling. H. D. Hertz [22] mentioned that, in general, the possibility of spalling increased with high heating temperatures and fast heating rates, but reported that dense concrete mixed with silica fume, under a 1 °C/min heating rate, experienced spalling at 350–400 °C, when it was not mixed with fibers and at 450 °C, when mixed with the fibers. L. T. Phan [23] confirmed that concrete with a compressive strength of 70 MPa or less did not experience spalling at a 5 °C/min heating rate, even without fiber mixing, whereas concrete with a compressive strength of 80 MPa or more experienced certain cases of spalling and that with a compressive strength of 100 MPa experienced a higher rate of spalling. Some spalling cases have been reported to abruptly explode, owing to the sudden release of considerable energy; L. T. Phan defined such spalling as “explosive spalling.” He also defined the stepwise and
Abbreviations: HSC, high-strength concrete; NSC, normal-strength concrete; W/B, water-binder ratio; BLEVE, boiling liquid expanding vapor explosion ⁎ Corresponding author. E-mail address: [email protected] (G. Kim). https://doi.org/10.1016/j.cemconres.2018.10.021 Received 23 August 2017; Received in revised form 4 October 2018; Accepted 22 October 2018 0008-8846/ © 2018 Published by Elsevier Ltd.
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continuous peeling of pieces from the surface as “surface spalling”. This type of spalling mainly occurred in experiments with fast heating rates. As HSC spalling is extremely irregular and the spalling property varies depending upon the heating conditions or the compressive strength of concrete, it is necessary to examine concrete-spalling characteristics with respect to the heating rate and W/B in order to improve the fire safety performance of structures using HSC. However, previous studies have mostly focused on examining the spalling of concrete exposed to sudden heating conditions. Furthermore, although experimental results on explosive spalling, which occurred when HSC with a compressive strength of 80 MPa or more was subjected to slow heating, have been reported, moisture migration and water vapor pressure, which are considered to be the cause, have not been sufficiently investigated. Therefore, in this study, the effect of moisture migration and water vapor pressure build-up within concrete on its spalling property, with respect to the heating rate and W/B, is empirically investigated by examining the spalling properties and internal water vapor pressures of concrete specimens with various W/Bs of 0.55, 0.33, 0.18, and 0.125, respectively, on exposure to slow and fast heating conditions.
Table 2 Mixture proportions and properties of fresh and hardened concrete. Property 3
Cement content-Type 1 (kg/m ) Silica fume-dry weight (kg) Slag (kg/m3) Gysum (kg/m3) Fine aggregate (kg/m3) Coarse aggregate (kg/m3) Water (kg/m3) Water:Cement ratio Water:Cementitious ratio Fresh concrete Slump (mm) Slump-flow (mm) Air contents (%) Hardened concrete Compressive strength (MPa) 28 days 56 days 300 days
B55
B33
B18
B12
336 0 0 0 797 956 185 0.550 0.550
475 25 0 0 792 950 165 0.347 0.330
596 133 133 0 617 740 160 0.268 0.180
660 240 300 60 389 736 150 0.227 0.125
180 – 4
– 650 2
– 700 2
– 700 2
31 32 34
72 75 83
112 116 126
173 182 205
and B12) were used, depending upon the concrete mixture proportions. River sand with a density of 2650 kg/m3, water absorption ratio of 1%, and fineness modulus of 2.6 was used as the fine aggregate. Table 2 shows the concrete mixture proportions and properties of both fresh and hardened concrete.
2. Experimental 2.1. Experimental outline Table 1 lists the experimental plan for this study. Heating tests were conducted using concrete specimens with W/Bs of 0.55, 0.33, 0.18, 0.125, respectively, for two types of heating rates: fast heating (ISO-834 standard heating) and slow heating (1 °C/min). During these tests, the temperatures on the surface of the concrete specimen and within were measured, and the pressure of the water vapor discharged through pressure pipes installed at depths of 30 mm and 50 mm within each specimen were measured using pressure sensors. In addition, the spalling properties were examined with the naked eye, 24 h after completion of the heating tests; the weight loss ratio was calculated by measuring the weight before and after heating. For the heating tests, rectangular pillar-shaped specimens with 100 × 100 mm cross sections and 200 mm heights were used. For evaluating the compressive strengths, cylindrical specimens with 100mm diameters and 200 mm heights were used. The compressive strengths of the concrete specimens were evaluated in accordance with ASTM C 39. The specimens were cured in a water bath at a temperature of 20 ± 2 °C for 28 days and further cured at a temperature and relative humidity of 20 ± 2 °C and 60 ± 5%, respectively, for 300 days in order to attain internal moisture equilibrium.
2.3. Heating method The heating method for the concrete specimens is shown in Fig. 1. Two types of heating rates, fast and slow heating, were used, as specified in the experimental outline. For fast heating, the ISO-834 standard heating curve that is usually used for fire heating tests was applied to create extreme temperature differences within concrete specimens, heated up to 900 °C for 50 min. For slow heating, the heating rate was set to 1 °C/min to minimize the temperature differences within the concrete specimens; further, the specimens were heated up to 700 °C. A duration of 840 min (14 h) was needed for the specimens to be heated up to 700 °C. Although the heating rate was set to 1 °C/min, the actual heating in the experiment occurred at a rate of 0.83 °C/min. Furthermore, in this study, the concrete specimens were heated using an electric resistance heating coil, and the maximum temperature and rate of temperature increase were controlled by a temperature controller.
2.2. Materials ASTM Type I cement (density: 3150 kg/m3 and fineness: 320 m2/kg) was used for the experiment. Fly ash (density: 2210 kg/m3 and fineness: 300 m2/kg), blast-furnace slag powder (density: 2500 kg/m3 and fineness: 600 m2/kg), gypsum (density: 2900 kg/m3 and fineness: 355 m2/ kg), and silica fume (density: 2500 kg/m3 and fineness: 20,000 m2/kg) were used as admixtures. Crushed granite with a density of 2700 kg/m3 and water absorption ratio of 0.9% was used as the coarse aggregate. Maximum dimensions of 20 mm (for B55 and B33) and 13 mm (for B18 Table 1 Experimental plan. ID
W/B
Heating rates
B55 B33 B18 B12
0.55 0.33 0.18 0.125
▪ Fast heating ISO-834 standard heating ▪ Slow heating 1 °C/min
Tests conducted ▪ ▪ ▪ ▪
Spalling property Rate of weight loss Temperature Water vapor pressure
Fig. 1. Heating method for experiment. 2
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Fig. 2. Setup for measuring the water vapor pressure within a concrete specimen.
surface. In particular, the B12 specimen failed to maintain the original shape, before heating, due to spalling. In the case of slow heating, spalling did not occur in the B55 and B33 specimens but occurred in the B18 and B12 specimens. As it was not possible to view the interior of the heating furnace used in this experiment, the spalling time was confirmed by the sound, during the tests. In the case of fast heating, the initial popping sound was confirmed at 10 min for the B33, 9 min for the B18, and at 6 min for the B12 specimen; for the B33 and B18 specimens, after several initial popping sounds, no further popping sounds were heard. However, for the B12 specimen, the popping sounds continued, until the completion of the heating test. In the case of slow heating, unlike the popping sounds of fast heating, concrete spalling occurred in the form of an explosion with a loud sound. In this study, the rate of weight loss, as calculated by Eq. (1), was used to quantitatively evaluate the degree of concrete spalling. Fig. 3 shows the calculated results.
2.4. Test setup for the measurement of the water vapor pressure in a concrete specimen Fig. 2 depicts the setup for measuring the water vapor pressure within a concrete specimen. Before the concrete was poured for fabricating the specimen, pressure pipes and thermo-couples were buried at depths of 30 mm and 50 mm, respectively, and the pressure of the water vapor discharged through these pipes were measured using pressure sensors, during the heating tests. The pressure pipes were fabricated using SUS304 material with inner and outer diameters of 1 mm and 2 mm, respectively and the portion to be buried was bent at a 90° angle to prevent the metal pipes from being pulled out of the concrete specimens. To prevent clogging in the metal-pipe inlets due to the inflow of aggregate or paste before the hardening of concrete, the inlets were sealed with paraffin. As paraffin, with a melting point of 62 °C, melted before the generation of water vapor pressure, the pipe inlets were opened, enabling water vapor to be discharged without restriction. In addition, the effect by vaporization of paraffin was not considered because the amount of paraffin used this study was just 0.05 g. Portions of the pressure pipes protruding from the concrete specimens were insulated with glass wool to minimize the influence of direct heat from the heating device. In addition, portions of the pressure pipes protruding out of the heating furnace were covered with a heating coil maintained at 100 °C to prevent water vapor condensation.
Wweight loss (%) =
W1 − W2 × 100 W1
(1)
where, Wweight loss: Rate of weight loss (%) W1: Weight of the concrete specimen before the heating test (g) W2: Weight of the concrete specimen after the heating test (g) From the results, a weight loss rate of 100% was defined as the case where the specimen could not stand independently after the heating test. Specimens without spalling exhibited a weight reduction of 5–6% compared to the weight before the test, due to the evaporation of the internal free water and the chemical bound water [24–26]. Under the fast heating condition, concrete specimens with lower W/Bs demonstrated greater weight loss due to the increase in concrete peeling. The weight loss rates of B18 and B12 specimens that experienced spalling, under the slow heating condition, were defined as 100% because their shapes were not maintained due to sudden explosions. Therefore, under
3. Test results 3.1. Spalling properties Table 3 depicts the spalling properties of the concrete specimens with respect to the heating rate and W/B. In the case of fast heating, spalling occurred in all the specimens, except for B55. Concrete specimens with lower W/Bs exhibited greater concrete peeling on the 3
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Table 3 Spalling property of concrete according to heat rate and W/B. B55
B33
B18
B12
Fast heating ISO-834
Slow heating 1 °C/min
different from the surface temperature. The largest temperature difference between the surface and center (50 mm depth) temperatures was 543 °C at a heating time of 10 min and the temperature difference was 323 °C towards the end of heating. For a B12 specimen that experienced spalling, the evaporation of moisture discharged outside, absorbed the thermal energy transmitted to the surface, resulting in a section with slightly lower temperatures. Under the fast heating condition, a plateau was observed at the center of the concrete specimen (50 mm depth). This plateau is owing to a phenomenon in which the temperature is temporarily maintained without increasing, due to the delay in heat transfer to the specimen because of the consumption of thermal energy by the endothermic reaction, during the phase change (evaporation) of water [13]. A plateau was observed at 100 °C for both B33 and B12 specimens. Water evaporates at 100 °C under 1 atm. Therefore, the pressure affecting the phase change of the moisture in the center can be considered equal to the atmospheric pressure, owing to the influence of cracks in both the specimens. There was a similar increase in temperatures on the surfaces and within the specimens, under the slow heating condition. The temperatures at the surface and center differed slightly as the heating time increased, but the difference was considerably less at approximately 20 °C compared to that of the fast heating. For a B12 specimen that experienced spalling, the temperature increase was similar to that of a B55 specimen before spalling, but the external and internal temperatures became equal at 5.8 h. Therefore, it was confirmed that, in the case of a specimen subjected to slow heating, spalling occurred in the form of sudden-specimen-collapse, unlike the fast heating condition.
Fig. 3. Weight loss of concrete specimen after heating test.
the fast heating condition, spalling occurrence and concrete weight loss differ in accordance with the W/B of concrete; whereas, under the slow heating condition, spalling occurrence is more significant than the weight loss. 3.2. Temperature
3.3. Water vapor pressure
Fig. 4 shows the measured temperatures on the surface and at depths of 30 mm and 50 mm, during the heating tests. The measured temperatures at each position were not significantly different with respect to the W/B of concrete but varied, depending upon the occurrence of spalling. Fig. 4 shows the temperatures of the B55 and B12 specimens as representative cases, with respect to the heating time. Under the fast heating condition, the B33 specimen showed an increase in the surface temperature similar to the ISO-834 standard heating curve; however, the internal temperature was significantly
Fig. 5 shows the water vapor pressure in concrete versus the heating time with respect to the heating rate and W/B of concrete. Under the fast heating condition, the water vapor pressures of the B55 and B33 specimens increased sharply after 10 min of heating, reached peak values at around 15 min, and then, decreased gradually. On the other hand, the B18 and B12 specimens showed irregular water vapor pressures and no particular trend was observed. The B55 and B33 specimens 4
Cement and Concrete Research 116 (2019) 1–10
G. Choe et al.
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required for the specimen to be completely dry and 160 g of moisture was reduced. Based on the completely dried condition, the water content of the specimen before drying was determined as 4.16%. As mentioned earlier in the spalling properties, the non-dried B12 concrete specimen was destroyed to an extent where its original shape could not be maintained due to continuous spalling during heating. However, for the completely dry specimen, concrete peeling did not occur despite numerous large and small cracks. Shorter and Harmathy [28] stated that spalling does not occur in a dry specimen, even in environments with large temperature imbalances within the specimen. The B12 HSC case investigated in this study confirmed that moisture has a significant influence on concrete spalling and that spalling occurs due to the water vapor pressure generated by moisture evaporation within the concrete, although cracks may occur due to the differences in the thermal expansion of the constituent materials or the dehydration of bound water in heated concrete.
clearly showed a tendency, where lower W/Bs led to higher water vapor pressures. However, in the case of B18 and B12 specimens that experienced severe spalling, crack expansion occurred due to severe spalling and the water vapor pressure was not significant at the measurement positions. Therefore, the behavior of water vapor pressure before spalling could not be confirmed. All the specimens subjected to slow heating demonstrated an increase in the water vapor pressure, after 3 h of heating; the maximum pressure of the B55 specimen was 288 kPa, which was not considerably different from that of the B55 specimen (388 kPa) subjected to fast heating. However, in the case of the B33 specimen, a pressure of 3110 kPa was measured at the center, even though there was no spalling. The water vapor pressures of the B18 and B12 specimens at the time of spalling were less than 400 kPa and they decreased rapidly.
4. Discussion 4.1. Effect of moisture on concrete spalling
4.2. Moisture migration with the heating rate
Fig. 6 shows the cross section of a B12 specimen dried in an oven, fast heated for 50 min, and cooled for 24 h. The method for drying the specimen has been referred to the Rilem committee [27]. The specimen was stated to be completely dry, immediately after its weight change was less than 0.2%, after placement in a 110 °C drier. Eighty days were
Figs. 7 and 8 compare the water vapor pressure in concrete with the saturated water vapor pressure curve, with respect to the heating rate and W/B. Saturated water vapor pressure is a function of the temperature and is the maximum pressure of the water vapor at a certain temperature. From the relationship between the saturated water vapor 5
Cement and Concrete Research 116 (2019) 1–10
G. Choe et al.
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Fig. 6. Cross section of B12 concrete specimen after oven dried during 80 days.
the saturated water vapor pressure, called the unsaturated state; the amount of water vapor is insufficient. Therefore, by comparing the saturated water vapor pressure and the water vapor pressure in concrete, it is possible to confirm the state change of moisture in concrete subjected to heating, the amount of water vapor, and moisture migration [29]. Moisture migration within concrete specimens, during fast heating, can be examined by comparing the water vapor pressure and the saturated water vapor pressure of the B55 and B33 specimens, respectively. For both specimens, the water vapor at 30 mm was in an
pressure curve and the water vapor pressure, the water vapor condition can be divided into three states: First is the state in which the water vapor pressure is higher than the saturated water vapor pressure, called the supersaturated state; there is more water vapor than the amount that can be contained in the measurement position. Hence, there is an inflow of moisture from outside. Second is the state in which the water vapor pressure is equal to the saturated water vapor pressure, called the saturated state; there is no inflow of water vapor from the outside nor moisture migration from the measurement position to another position. The third is the state in which the water vapor pressure is lower than
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Fig. 7. Comparison between water vapor pressure and saturated vapor pressure curve under fast heating. 7
Cement and Concrete Research 116 (2019) 1–10
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formed on the surfaces, rather than at the 30-mm and 50-mm positions measured in this study because moisture migration due to pressure gradient may have been difficult. Fig. 8 shows the comparison between the water vapor pressure and saturated water vapor pressure of concrete specimens subjected to slow heating. Unlike the specimens subjected to fast heating, the water vapor within these specimens was found to be in an unsaturated state, regardless of the depth and time. In particular, it is interesting to note that even for the B18 and B12 specimens, which experienced explosive spalling, spalling occurred, when the water vapor pressure was in an unsaturated state (no moisture clog was formed). This implies that there was no inflow of water vapor at any position, considering that there were no supersaturated regions and it can be determined that moisture migration did not occur within the concrete specimens during heating. It also indicates that the amount of water vapor was insufficient to reach a saturated state in all the positions, where the water vapor pressure was measured. This phenomenon can be explained by the composition of the pores within the concrete [13]. The pores in concrete consist of water in the form of free water, water vapor, and dry air. In a situation, where a pressure gradient is not generated between the pores due to uniform temperature distribution within the concrete, such as in a slow heating environment, there is an increase in the water vapor pressure in the pores, increasing the boiling point of the water; thereby, water remains in a liquid state without evaporation and results in water vapor pressure in an unsaturated state.
Fig. 9. Water vapor pressure of B55, B33 specimen by depth from heated face.
unsaturated state and that at 50 mm was in a supersaturated state. Due to the variation in temperature with depth, caused by fast heating, the moisture at 30 mm boiled and formed water vapor pressure and migrated inside due to pressure flow. The migrated moisture from the 30mm position added to the moisture at 50 mm to cause a supersaturated state and formed a moisture clog [30,31]. However, in the case of B18 and B12 specimens with dense pore structures, moisture clogs were
4.3. Surface spalling and explosive spalling Fig. 9 compares the water vapor pressure and saturated water vapor 8
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pressure of the B55 and B33 specimens, respectively, at the 30-mm and 50-mm positions, 9 and 15 min after heating, respectively, considering the spalling time. After 9 min of heating, the water vapor pressures at 30 mm and 50 mm were lower than the saturated water vapor pressures, resulting in an unsaturated state. However, after 15 min, the water vapor at 50 mm reached a supersaturated state and a moisture clog was formed. It was confirmed that the water vapor pressure of the B33 specimen at 30 mm was higher than that of the B55 specimen at 30 mm and that it was closer to the saturated water vapor pressure. Therefore, for specimens subjected to fast heating, moisture clogs were formed at positions closer to the surface as the W/B ratio decreased; the water vapor pressure caused by the moisture clog might have resulted in surface spalling, wherein the peeling of thin concrete layers occurred on the surface. Furthermore, as described in Section 3.1, the repeated spalling, which occurred in specimens subjected to fast heating, suggests that this moisture migration and water vapor pressure formation occurred repeatedly. Fig. 10 shows the stress-strain and water vapor pressure-thermal expansion strain curves of the heated concrete specimens. The stressstrain curve is the result of Choe and Kim's study [32] and the thermal expansion strain was obtained by installing a quartz tube on the central axis and measuring the concrete expansion using a displacement meter. The ultimate strain of concrete specimens subjected to high temperatures of 300 °C or less in the stress-strain curve was 0.0025–0.0030. From the relationship between the water vapor pressure and thermal expansion strain, it was found that the water vapor pressure began to decrease in the thermal strain range, similar to the ultimate strain of the specimens, during heating. Therefore, it was confirmed that the water vapor pressure was discharged outside through cracks that were generated, when the free thermal expansion deformation of concrete attained the ultimate strain. The concrete explosive spalling process, which occurs under the slow heating condition, can be summarized as shown in Fig. 11, with respect to water vapor pressure formation and moisture migration. The internal pressure of the concrete pores before heating is the same as the atmospheric pressure and the pores contain liquid water, vapor, and dry air (Fig. 11a). As the concrete temperature rises, the water vapor pressure within the pores increases; due to the thermodynamic properties of water, its boiling point increases, resulting in the existence of water in a liquid state (superheated water) at temperatures of 100 °C or higher. More thermal energy is required to evaporate such liquid water
Fig. 10. Relationship of stress-strain and water vapor pressure – thermal expansion strain of B33.
Fig. 11. Phase change of water in concrete with slow heating. 9
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(Fig. 11b). Finally, due to the expansion of cracks, the pressure in the pores becomes equal to the atmospheric pressure and the superheated water instantaneously evaporates because of the lowered boiling point, resulting in explosive spalling, which causes sharp volume expansion and instantaneous energy release (Fig. 11c). In the theory of explosion, this phenomenon is called “BLEVE (Boiling liquid expanding vapor explosion)”. Although, in general, this is an explosion phenomenon, which occurs in sealed vessels subjected to high temperatures, it is similar to the explosive spalling in HSC, in which moisture migration is difficult.
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J. 26 (2) (2004) 163–175, https://doi.org/10. 1016/S0958-9465(03)00085-4 (GC). [10] K.D. Hertz, Limits of spalling of fire-exposed concrete, Fire Saf. J. 38 (2003) 103–116, https://doi.org/10.1016/S0379-7112(02)00051-6. [11] P. Kalifa, F.D. Menneteau, D. Quenard, Spalling and pore pressure in HPC at high temperatures, Cem. Concr. Res. 30 (2000) 1915–1927. [12] R.B. Mugume, T. Horiguchi, Pore pressure development in hybrid fibre-reinforced high strength concrete at elevated temperatures, Cem. Concr. Res. 41 (2011) 1150–1156. [13] J.-C. Mindeguia, P. Pimienta, A. Noumowe, M. Kanema, Temperature, pore pressure and mass variation of concrete subjected to high temperature- experimental and numerical discussion on spalling risk, Cem. Concr. Res. 40 (2010) 477–487. [14] M. Ozawa, H. Morimoto, Effects of various fibres on high-temperature spalling in high-performance concrete, Constr. Build. Mater. 71 (2014) 83–92. [15] L.T. Phan, Pore pressure and explosive spalling in concrete, Mater. Struct. 41 (2008) 1623–1632. [16] G.A. Khoury, F.P. Majorana, B.A. Schrefler, Modelling of heated concrete, Mag. Concr. Res. 54 (2002) 77–101. [17] G.A. Khoury, B. Willoughby, Polypropylene fibres in heated concrete, part 1: molecular structure and materials behavior, Mag. Concr. Res. 60 (2008) 125–136. [18] G.A. Khoury, Polypropylene fibres in heated concrete, part 2: pressure relief mechanisms and modelling criteria, Mag. Concr. Res. 60 (2008) 189–204. [19] P. Kalifa, G. Chene, C. Galle, High-temperature behaviour of HPC with polypropylene fibres from spalling to microstructure, Cem. Concr. Res. 31 (2001) 1487–1499. [20] S.L. Suhaendi, T. Horiguchi, Effect of short fibers on residual permeability and mechanical properties of hybrid fibre reinforced high strength concrete after heat exposition, Cem. Concr. Res. 36 (2006) 1672–1678. [21] A. Bilodeau, V.K.R. Kodur, G.C. Hoff, Optimization of the type and amount of polypropylene fibres for preventing the spalling of lightweight concrete subjected to hydrocarbon fire, Cem. Concr. Compos. 26 (2) (2004) 163–174. [22] K.D. Hertz, Heat Induced Explosion of Dense Concretes, Report 166, CIB W14/84/ 33(DK) Institute of Building Design (Now Department of Buildings and Energy), Technical University of Denmark, 1984, p. 20. [23] L.T. Phan, J.R. Lawson, F.L. Davis, Effects of elevated temperature exposure on heating characteristics, spalling, and residual properties of high performance concrete, Mater. Struct. 34 (2) (2001) 83–91. [24] A.N. Noumowe, R. Siddique, G. Debicki, Permeability of high-performance concrete subjected to elevated temperature (600 °C), Constr. Build. Mater. 23 (5) (2009) 1855–1861. [25] P. Sukontasukkul, W. Pomchiengpin, S. Songpiriyakij, Post-crack (or post-peak) flexural response and toughness of fiber reinforced concrete after exposure to high temperature, Constr. Build. Mater. 24 (10) (2010) 1967–1974. [26] X. Hou, W. Zheng, Kodur V. K. R., Response of unbonded prestressed concrete continuous slabs under fire exposure, Eng. Struct. 56 (2013) 2139–2148. [27] U. Schneider, P. Schwesinger, G. Debicki, U. Diederichs, R. Felicetti, J.M. Franssen, ... L. Phan, RILEM recommendations: part 4: tensile strength for service and accident conditions, Mater. Struct. 33 (2000) 219–223. [28] G.W. Shorter, T.Z. Harmathy, Discussion on the article “the fire resistance of concrete beams” by Ashton and Bate, Proc. Inst. Civ. Eng. 20 (1976) 313. [29] Y. Ichikawa, G.L. England, Prediction of moisture migration and pore pressure build-up in concrete at high temperatures, Nucl. Eng. Des. 228 (1) (2004) 245–259. [30] T.Z. Hermathy, Effect of moisture on the fire endurance of building materials, Moisture in relation to fire tests, ASTM Spec. Tech. Publ. 385 (1965) 74–95. [31] R. Jansson, Fire spalling of concrete–a historical overview, MATEC Web of Conferences, 6 2013, p. 01001 (EDP Sciences). [32] G. Choe, M. Yoon, T. Lee, S. Lee, Y. Kim, Evaluation of properties of 80, 130, 180 MPa high strength concrete at high temperature with heating and loading, J. Korea Concr. Inst. 25 (6) (2013) 613–620.
5. Conclusion 1) Moisture migration and water vapor pressure formation, under high temperatures, are significantly affected by the heating rate and cement matrix structure. In particular, the position and water vapor pressure that affect spalling vary, depending upon the type of moisture migration within the concrete. 2) For concrete subjected to fast heating, moisture migration may occur due to uneven temperature distribution across the cross section. Moisture that migrates inside forms a moisture clog in a supersaturated state, increasing the water vapor pressure. In addition, concrete with higher compressive strength experiences the formation of moisture clogs closer to the surface because moisture migration inside is difficult; it is highly probable that repeated spalling will occur on the concrete surface due to water vapor pressure. 3) For slow heating, moisture migration due to temperature differences does not occur because the temperature is evenly distributed across the cross section; thus, a moisture clog is not formed. When HSC with a dense matrix structure is exposed to slow heating, BLEVE may occur owing to explosive spalling, depending upon the thermodynamic properties of the moisture that exists in the concrete pores and the high-temperature characteristics of concrete. 4) In order to effectively control the various types of concrete spalling, it is necessary to comprehensively understand the thermodynamic properties of concrete and the thermodynamic behavior of internal moisture, which vary depending upon environmental conditions such as the heating rate and heating temperature, and to apply a spalling control method considering these characteristics. Acknowledgement This work (NRF-2015R1A2A2A01007705) was supported by the Mid-Career Researcher Program through the NRF grant funded by MEST. References [1] G. Choe, G. Kim, N. Gucunski, S. Lee, Evaluation of the mechanical properties of 200 MPa ultra-high-strength concrete at elevated temperatures and residual strength of column, Constr. Build. Mater. 86 (2015) 159–168. [2] V.K.R. Kodur, Spalling in high strength concrete exposed to fire-concerns, causes, critical parameters and cures, Proceedings of Structures Congress, Advanced Technology in Structural Engineering, Philadelphia, USA, May 8–10, 2000. [3] L. Bostrom, R. Jansson, Spalling of self-compacting concrete, Proceedings of SIF'06 workshop, Structures in Fire, Aveiro, Portugal, 2006 (May). [4] Y. Fu, L. Li, Study on mechanism of thermal spalling in concrete exposed to elevated temperatures, Mater. Struct. 44 (1) (2011) 361–376.
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Contents lists available at ScienceDirect
Cement and Concrete Research journal homepage: www.elsevier.com/locate/cemconres
Effect of moisture migration and water vapor pressure build-up with the heating rate on concrete spalling type
T
⁎
Gyeongcheol Choea, Gyuyong Kima, , Minho Yoona, Euichul Hwanga, Jeongsoo Nama, Nenad Guncunskib a b
Department of Architectural Engineering, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Republic of Korea Department of Civil and Environmental Engineering, Rutgers University, Piscataway, NJ 08855-0909, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: High-strength concrete (HSC) Heating rate Moisture migration Spalling type Water vapor pressure
In this study, the effect of moisture migration and water vapor pressure build-up within high-strength concrete (HSC) on its spalling property is empirically investigated at various heating rates and compressive strengths with different water-binder ratios (W/B). An experiment is conducted on concrete specimens with various W/Bs by applying two different heating rates: fast and slow heating. It is confirmed that the moisture migration and water vapor pressure within the concrete differ, depending upon the W/B and heating rate. For HSC with W/B less than 0.33, surface spalling occurs due to the formation of moisture clogs on the surfaces of the concrete specimens, under the fast heating condition. For HSC with dense microstructures, although moisture clogs are not formed at specific positions within the concrete specimens, explosive spalling occurs due to boiling liquid expanding vapor explosion (BLEVE) in the concrete pores, under the slow heating condition.
1. Introduction HSC has several advantages such as excellent structural performance and high durability; however, it can explode when exposed to heat such as that from fire. Concrete spalling is difficult to predict due to its extremely irregular nature and can significantly lower the structural strength by causing cross-sectional losses in its members [1–3]. The spalling mechanism of concrete can be classified into spalling by water vapor pressure, by thermal stress, and by a combination of water vapor and thermal pressures. The factors affecting concrete spalling include internal ones, such as the air permeability, water content, compressive strength, and water/cement ratio, and external ones, such as the rate of temperature increase, maximum heating temperature, heating source, and external force [4]. All concrete exposed to rapid heating conditions such as fire are at the risk of spalling. However, according to various studies [5–10], the HSC is more sensitive to spalling than the NSC. Several studies on the causes of spalling [11–14] have compared the water vapor pressure within the concrete with the saturated water vapor pressure of water and confirmed that moisture migration and water vapor pressure buildup within the concrete have a significant influence on concrete spalling [15]. Thus, HSC, which has a relatively lower air permeability than
NSC, is more vulnerable to spalling due to less internal moisture migration and greater water vapor pressure on the surface. Methods for preventing spalling by reducing the internal water vapor pressure within the concrete, based on the internal moisture behavior, have been proposed [14,15–21]. P. Kalifa [19], in particular, mixed polypropylene fibers with concrete and experimentally confirmed that water vapor was discharged through the pores generated by the melted fibers and cracks in the cement matrix; this reduced the water vapor pressure and prevented spalling. H. D. Hertz [22] mentioned that, in general, the possibility of spalling increased with high heating temperatures and fast heating rates, but reported that dense concrete mixed with silica fume, under a 1 °C/min heating rate, experienced spalling at 350–400 °C, when it was not mixed with fibers and at 450 °C, when mixed with the fibers. L. T. Phan [23] confirmed that concrete with a compressive strength of 70 MPa or less did not experience spalling at a 5 °C/min heating rate, even without fiber mixing, whereas concrete with a compressive strength of 80 MPa or more experienced certain cases of spalling and that with a compressive strength of 100 MPa experienced a higher rate of spalling. Some spalling cases have been reported to abruptly explode, owing to the sudden release of considerable energy; L. T. Phan defined such spalling as “explosive spalling.” He also defined the stepwise and
Abbreviations: HSC, high-strength concrete; NSC, normal-strength concrete; W/B, water-binder ratio; BLEVE, boiling liquid expanding vapor explosion ⁎ Corresponding author. E-mail address: [email protected] (G. Kim). https://doi.org/10.1016/j.cemconres.2018.10.021 Received 23 August 2017; Received in revised form 4 October 2018; Accepted 22 October 2018 0008-8846/ © 2018 Published by Elsevier Ltd.
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continuous peeling of pieces from the surface as “surface spalling”. This type of spalling mainly occurred in experiments with fast heating rates. As HSC spalling is extremely irregular and the spalling property varies depending upon the heating conditions or the compressive strength of concrete, it is necessary to examine concrete-spalling characteristics with respect to the heating rate and W/B in order to improve the fire safety performance of structures using HSC. However, previous studies have mostly focused on examining the spalling of concrete exposed to sudden heating conditions. Furthermore, although experimental results on explosive spalling, which occurred when HSC with a compressive strength of 80 MPa or more was subjected to slow heating, have been reported, moisture migration and water vapor pressure, which are considered to be the cause, have not been sufficiently investigated. Therefore, in this study, the effect of moisture migration and water vapor pressure build-up within concrete on its spalling property, with respect to the heating rate and W/B, is empirically investigated by examining the spalling properties and internal water vapor pressures of concrete specimens with various W/Bs of 0.55, 0.33, 0.18, and 0.125, respectively, on exposure to slow and fast heating conditions.
Table 2 Mixture proportions and properties of fresh and hardened concrete. Property 3
Cement content-Type 1 (kg/m ) Silica fume-dry weight (kg) Slag (kg/m3) Gysum (kg/m3) Fine aggregate (kg/m3) Coarse aggregate (kg/m3) Water (kg/m3) Water:Cement ratio Water:Cementitious ratio Fresh concrete Slump (mm) Slump-flow (mm) Air contents (%) Hardened concrete Compressive strength (MPa) 28 days 56 days 300 days
B55
B33
B18
B12
336 0 0 0 797 956 185 0.550 0.550
475 25 0 0 792 950 165 0.347 0.330
596 133 133 0 617 740 160 0.268 0.180
660 240 300 60 389 736 150 0.227 0.125
180 – 4
– 650 2
– 700 2
– 700 2
31 32 34
72 75 83
112 116 126
173 182 205
and B12) were used, depending upon the concrete mixture proportions. River sand with a density of 2650 kg/m3, water absorption ratio of 1%, and fineness modulus of 2.6 was used as the fine aggregate. Table 2 shows the concrete mixture proportions and properties of both fresh and hardened concrete.
2. Experimental 2.1. Experimental outline Table 1 lists the experimental plan for this study. Heating tests were conducted using concrete specimens with W/Bs of 0.55, 0.33, 0.18, 0.125, respectively, for two types of heating rates: fast heating (ISO-834 standard heating) and slow heating (1 °C/min). During these tests, the temperatures on the surface of the concrete specimen and within were measured, and the pressure of the water vapor discharged through pressure pipes installed at depths of 30 mm and 50 mm within each specimen were measured using pressure sensors. In addition, the spalling properties were examined with the naked eye, 24 h after completion of the heating tests; the weight loss ratio was calculated by measuring the weight before and after heating. For the heating tests, rectangular pillar-shaped specimens with 100 × 100 mm cross sections and 200 mm heights were used. For evaluating the compressive strengths, cylindrical specimens with 100mm diameters and 200 mm heights were used. The compressive strengths of the concrete specimens were evaluated in accordance with ASTM C 39. The specimens were cured in a water bath at a temperature of 20 ± 2 °C for 28 days and further cured at a temperature and relative humidity of 20 ± 2 °C and 60 ± 5%, respectively, for 300 days in order to attain internal moisture equilibrium.
2.3. Heating method The heating method for the concrete specimens is shown in Fig. 1. Two types of heating rates, fast and slow heating, were used, as specified in the experimental outline. For fast heating, the ISO-834 standard heating curve that is usually used for fire heating tests was applied to create extreme temperature differences within concrete specimens, heated up to 900 °C for 50 min. For slow heating, the heating rate was set to 1 °C/min to minimize the temperature differences within the concrete specimens; further, the specimens were heated up to 700 °C. A duration of 840 min (14 h) was needed for the specimens to be heated up to 700 °C. Although the heating rate was set to 1 °C/min, the actual heating in the experiment occurred at a rate of 0.83 °C/min. Furthermore, in this study, the concrete specimens were heated using an electric resistance heating coil, and the maximum temperature and rate of temperature increase were controlled by a temperature controller.
2.2. Materials ASTM Type I cement (density: 3150 kg/m3 and fineness: 320 m2/kg) was used for the experiment. Fly ash (density: 2210 kg/m3 and fineness: 300 m2/kg), blast-furnace slag powder (density: 2500 kg/m3 and fineness: 600 m2/kg), gypsum (density: 2900 kg/m3 and fineness: 355 m2/ kg), and silica fume (density: 2500 kg/m3 and fineness: 20,000 m2/kg) were used as admixtures. Crushed granite with a density of 2700 kg/m3 and water absorption ratio of 0.9% was used as the coarse aggregate. Maximum dimensions of 20 mm (for B55 and B33) and 13 mm (for B18 Table 1 Experimental plan. ID
W/B
Heating rates
B55 B33 B18 B12
0.55 0.33 0.18 0.125
▪ Fast heating ISO-834 standard heating ▪ Slow heating 1 °C/min
Tests conducted ▪ ▪ ▪ ▪
Spalling property Rate of weight loss Temperature Water vapor pressure
Fig. 1. Heating method for experiment. 2
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Fig. 2. Setup for measuring the water vapor pressure within a concrete specimen.
surface. In particular, the B12 specimen failed to maintain the original shape, before heating, due to spalling. In the case of slow heating, spalling did not occur in the B55 and B33 specimens but occurred in the B18 and B12 specimens. As it was not possible to view the interior of the heating furnace used in this experiment, the spalling time was confirmed by the sound, during the tests. In the case of fast heating, the initial popping sound was confirmed at 10 min for the B33, 9 min for the B18, and at 6 min for the B12 specimen; for the B33 and B18 specimens, after several initial popping sounds, no further popping sounds were heard. However, for the B12 specimen, the popping sounds continued, until the completion of the heating test. In the case of slow heating, unlike the popping sounds of fast heating, concrete spalling occurred in the form of an explosion with a loud sound. In this study, the rate of weight loss, as calculated by Eq. (1), was used to quantitatively evaluate the degree of concrete spalling. Fig. 3 shows the calculated results.
2.4. Test setup for the measurement of the water vapor pressure in a concrete specimen Fig. 2 depicts the setup for measuring the water vapor pressure within a concrete specimen. Before the concrete was poured for fabricating the specimen, pressure pipes and thermo-couples were buried at depths of 30 mm and 50 mm, respectively, and the pressure of the water vapor discharged through these pipes were measured using pressure sensors, during the heating tests. The pressure pipes were fabricated using SUS304 material with inner and outer diameters of 1 mm and 2 mm, respectively and the portion to be buried was bent at a 90° angle to prevent the metal pipes from being pulled out of the concrete specimens. To prevent clogging in the metal-pipe inlets due to the inflow of aggregate or paste before the hardening of concrete, the inlets were sealed with paraffin. As paraffin, with a melting point of 62 °C, melted before the generation of water vapor pressure, the pipe inlets were opened, enabling water vapor to be discharged without restriction. In addition, the effect by vaporization of paraffin was not considered because the amount of paraffin used this study was just 0.05 g. Portions of the pressure pipes protruding from the concrete specimens were insulated with glass wool to minimize the influence of direct heat from the heating device. In addition, portions of the pressure pipes protruding out of the heating furnace were covered with a heating coil maintained at 100 °C to prevent water vapor condensation.
Wweight loss (%) =
W1 − W2 × 100 W1
(1)
where, Wweight loss: Rate of weight loss (%) W1: Weight of the concrete specimen before the heating test (g) W2: Weight of the concrete specimen after the heating test (g) From the results, a weight loss rate of 100% was defined as the case where the specimen could not stand independently after the heating test. Specimens without spalling exhibited a weight reduction of 5–6% compared to the weight before the test, due to the evaporation of the internal free water and the chemical bound water [24–26]. Under the fast heating condition, concrete specimens with lower W/Bs demonstrated greater weight loss due to the increase in concrete peeling. The weight loss rates of B18 and B12 specimens that experienced spalling, under the slow heating condition, were defined as 100% because their shapes were not maintained due to sudden explosions. Therefore, under
3. Test results 3.1. Spalling properties Table 3 depicts the spalling properties of the concrete specimens with respect to the heating rate and W/B. In the case of fast heating, spalling occurred in all the specimens, except for B55. Concrete specimens with lower W/Bs exhibited greater concrete peeling on the 3
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Table 3 Spalling property of concrete according to heat rate and W/B. B55
B33
B18
B12
Fast heating ISO-834
Slow heating 1 °C/min
different from the surface temperature. The largest temperature difference between the surface and center (50 mm depth) temperatures was 543 °C at a heating time of 10 min and the temperature difference was 323 °C towards the end of heating. For a B12 specimen that experienced spalling, the evaporation of moisture discharged outside, absorbed the thermal energy transmitted to the surface, resulting in a section with slightly lower temperatures. Under the fast heating condition, a plateau was observed at the center of the concrete specimen (50 mm depth). This plateau is owing to a phenomenon in which the temperature is temporarily maintained without increasing, due to the delay in heat transfer to the specimen because of the consumption of thermal energy by the endothermic reaction, during the phase change (evaporation) of water [13]. A plateau was observed at 100 °C for both B33 and B12 specimens. Water evaporates at 100 °C under 1 atm. Therefore, the pressure affecting the phase change of the moisture in the center can be considered equal to the atmospheric pressure, owing to the influence of cracks in both the specimens. There was a similar increase in temperatures on the surfaces and within the specimens, under the slow heating condition. The temperatures at the surface and center differed slightly as the heating time increased, but the difference was considerably less at approximately 20 °C compared to that of the fast heating. For a B12 specimen that experienced spalling, the temperature increase was similar to that of a B55 specimen before spalling, but the external and internal temperatures became equal at 5.8 h. Therefore, it was confirmed that, in the case of a specimen subjected to slow heating, spalling occurred in the form of sudden-specimen-collapse, unlike the fast heating condition.
Fig. 3. Weight loss of concrete specimen after heating test.
the fast heating condition, spalling occurrence and concrete weight loss differ in accordance with the W/B of concrete; whereas, under the slow heating condition, spalling occurrence is more significant than the weight loss. 3.2. Temperature
3.3. Water vapor pressure
Fig. 4 shows the measured temperatures on the surface and at depths of 30 mm and 50 mm, during the heating tests. The measured temperatures at each position were not significantly different with respect to the W/B of concrete but varied, depending upon the occurrence of spalling. Fig. 4 shows the temperatures of the B55 and B12 specimens as representative cases, with respect to the heating time. Under the fast heating condition, the B33 specimen showed an increase in the surface temperature similar to the ISO-834 standard heating curve; however, the internal temperature was significantly
Fig. 5 shows the water vapor pressure in concrete versus the heating time with respect to the heating rate and W/B of concrete. Under the fast heating condition, the water vapor pressures of the B55 and B33 specimens increased sharply after 10 min of heating, reached peak values at around 15 min, and then, decreased gradually. On the other hand, the B18 and B12 specimens showed irregular water vapor pressures and no particular trend was observed. The B55 and B33 specimens 4
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required for the specimen to be completely dry and 160 g of moisture was reduced. Based on the completely dried condition, the water content of the specimen before drying was determined as 4.16%. As mentioned earlier in the spalling properties, the non-dried B12 concrete specimen was destroyed to an extent where its original shape could not be maintained due to continuous spalling during heating. However, for the completely dry specimen, concrete peeling did not occur despite numerous large and small cracks. Shorter and Harmathy [28] stated that spalling does not occur in a dry specimen, even in environments with large temperature imbalances within the specimen. The B12 HSC case investigated in this study confirmed that moisture has a significant influence on concrete spalling and that spalling occurs due to the water vapor pressure generated by moisture evaporation within the concrete, although cracks may occur due to the differences in the thermal expansion of the constituent materials or the dehydration of bound water in heated concrete.
clearly showed a tendency, where lower W/Bs led to higher water vapor pressures. However, in the case of B18 and B12 specimens that experienced severe spalling, crack expansion occurred due to severe spalling and the water vapor pressure was not significant at the measurement positions. Therefore, the behavior of water vapor pressure before spalling could not be confirmed. All the specimens subjected to slow heating demonstrated an increase in the water vapor pressure, after 3 h of heating; the maximum pressure of the B55 specimen was 288 kPa, which was not considerably different from that of the B55 specimen (388 kPa) subjected to fast heating. However, in the case of the B33 specimen, a pressure of 3110 kPa was measured at the center, even though there was no spalling. The water vapor pressures of the B18 and B12 specimens at the time of spalling were less than 400 kPa and they decreased rapidly.
4. Discussion 4.1. Effect of moisture on concrete spalling
4.2. Moisture migration with the heating rate
Fig. 6 shows the cross section of a B12 specimen dried in an oven, fast heated for 50 min, and cooled for 24 h. The method for drying the specimen has been referred to the Rilem committee [27]. The specimen was stated to be completely dry, immediately after its weight change was less than 0.2%, after placement in a 110 °C drier. Eighty days were
Figs. 7 and 8 compare the water vapor pressure in concrete with the saturated water vapor pressure curve, with respect to the heating rate and W/B. Saturated water vapor pressure is a function of the temperature and is the maximum pressure of the water vapor at a certain temperature. From the relationship between the saturated water vapor 5
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Fig. 6. Cross section of B12 concrete specimen after oven dried during 80 days.
the saturated water vapor pressure, called the unsaturated state; the amount of water vapor is insufficient. Therefore, by comparing the saturated water vapor pressure and the water vapor pressure in concrete, it is possible to confirm the state change of moisture in concrete subjected to heating, the amount of water vapor, and moisture migration [29]. Moisture migration within concrete specimens, during fast heating, can be examined by comparing the water vapor pressure and the saturated water vapor pressure of the B55 and B33 specimens, respectively. For both specimens, the water vapor at 30 mm was in an
pressure curve and the water vapor pressure, the water vapor condition can be divided into three states: First is the state in which the water vapor pressure is higher than the saturated water vapor pressure, called the supersaturated state; there is more water vapor than the amount that can be contained in the measurement position. Hence, there is an inflow of moisture from outside. Second is the state in which the water vapor pressure is equal to the saturated water vapor pressure, called the saturated state; there is no inflow of water vapor from the outside nor moisture migration from the measurement position to another position. The third is the state in which the water vapor pressure is lower than
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formed on the surfaces, rather than at the 30-mm and 50-mm positions measured in this study because moisture migration due to pressure gradient may have been difficult. Fig. 8 shows the comparison between the water vapor pressure and saturated water vapor pressure of concrete specimens subjected to slow heating. Unlike the specimens subjected to fast heating, the water vapor within these specimens was found to be in an unsaturated state, regardless of the depth and time. In particular, it is interesting to note that even for the B18 and B12 specimens, which experienced explosive spalling, spalling occurred, when the water vapor pressure was in an unsaturated state (no moisture clog was formed). This implies that there was no inflow of water vapor at any position, considering that there were no supersaturated regions and it can be determined that moisture migration did not occur within the concrete specimens during heating. It also indicates that the amount of water vapor was insufficient to reach a saturated state in all the positions, where the water vapor pressure was measured. This phenomenon can be explained by the composition of the pores within the concrete [13]. The pores in concrete consist of water in the form of free water, water vapor, and dry air. In a situation, where a pressure gradient is not generated between the pores due to uniform temperature distribution within the concrete, such as in a slow heating environment, there is an increase in the water vapor pressure in the pores, increasing the boiling point of the water; thereby, water remains in a liquid state without evaporation and results in water vapor pressure in an unsaturated state.
Fig. 9. Water vapor pressure of B55, B33 specimen by depth from heated face.
unsaturated state and that at 50 mm was in a supersaturated state. Due to the variation in temperature with depth, caused by fast heating, the moisture at 30 mm boiled and formed water vapor pressure and migrated inside due to pressure flow. The migrated moisture from the 30mm position added to the moisture at 50 mm to cause a supersaturated state and formed a moisture clog [30,31]. However, in the case of B18 and B12 specimens with dense pore structures, moisture clogs were
4.3. Surface spalling and explosive spalling Fig. 9 compares the water vapor pressure and saturated water vapor 8
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pressure of the B55 and B33 specimens, respectively, at the 30-mm and 50-mm positions, 9 and 15 min after heating, respectively, considering the spalling time. After 9 min of heating, the water vapor pressures at 30 mm and 50 mm were lower than the saturated water vapor pressures, resulting in an unsaturated state. However, after 15 min, the water vapor at 50 mm reached a supersaturated state and a moisture clog was formed. It was confirmed that the water vapor pressure of the B33 specimen at 30 mm was higher than that of the B55 specimen at 30 mm and that it was closer to the saturated water vapor pressure. Therefore, for specimens subjected to fast heating, moisture clogs were formed at positions closer to the surface as the W/B ratio decreased; the water vapor pressure caused by the moisture clog might have resulted in surface spalling, wherein the peeling of thin concrete layers occurred on the surface. Furthermore, as described in Section 3.1, the repeated spalling, which occurred in specimens subjected to fast heating, suggests that this moisture migration and water vapor pressure formation occurred repeatedly. Fig. 10 shows the stress-strain and water vapor pressure-thermal expansion strain curves of the heated concrete specimens. The stressstrain curve is the result of Choe and Kim's study [32] and the thermal expansion strain was obtained by installing a quartz tube on the central axis and measuring the concrete expansion using a displacement meter. The ultimate strain of concrete specimens subjected to high temperatures of 300 °C or less in the stress-strain curve was 0.0025–0.0030. From the relationship between the water vapor pressure and thermal expansion strain, it was found that the water vapor pressure began to decrease in the thermal strain range, similar to the ultimate strain of the specimens, during heating. Therefore, it was confirmed that the water vapor pressure was discharged outside through cracks that were generated, when the free thermal expansion deformation of concrete attained the ultimate strain. The concrete explosive spalling process, which occurs under the slow heating condition, can be summarized as shown in Fig. 11, with respect to water vapor pressure formation and moisture migration. The internal pressure of the concrete pores before heating is the same as the atmospheric pressure and the pores contain liquid water, vapor, and dry air (Fig. 11a). As the concrete temperature rises, the water vapor pressure within the pores increases; due to the thermodynamic properties of water, its boiling point increases, resulting in the existence of water in a liquid state (superheated water) at temperatures of 100 °C or higher. More thermal energy is required to evaporate such liquid water
Fig. 10. Relationship of stress-strain and water vapor pressure – thermal expansion strain of B33.
Fig. 11. Phase change of water in concrete with slow heating. 9
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(Fig. 11b). Finally, due to the expansion of cracks, the pressure in the pores becomes equal to the atmospheric pressure and the superheated water instantaneously evaporates because of the lowered boiling point, resulting in explosive spalling, which causes sharp volume expansion and instantaneous energy release (Fig. 11c). In the theory of explosion, this phenomenon is called “BLEVE (Boiling liquid expanding vapor explosion)”. Although, in general, this is an explosion phenomenon, which occurs in sealed vessels subjected to high temperatures, it is similar to the explosive spalling in HSC, in which moisture migration is difficult.
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5. Conclusion 1) Moisture migration and water vapor pressure formation, under high temperatures, are significantly affected by the heating rate and cement matrix structure. In particular, the position and water vapor pressure that affect spalling vary, depending upon the type of moisture migration within the concrete. 2) For concrete subjected to fast heating, moisture migration may occur due to uneven temperature distribution across the cross section. Moisture that migrates inside forms a moisture clog in a supersaturated state, increasing the water vapor pressure. In addition, concrete with higher compressive strength experiences the formation of moisture clogs closer to the surface because moisture migration inside is difficult; it is highly probable that repeated spalling will occur on the concrete surface due to water vapor pressure. 3) For slow heating, moisture migration due to temperature differences does not occur because the temperature is evenly distributed across the cross section; thus, a moisture clog is not formed. When HSC with a dense matrix structure is exposed to slow heating, BLEVE may occur owing to explosive spalling, depending upon the thermodynamic properties of the moisture that exists in the concrete pores and the high-temperature characteristics of concrete. 4) In order to effectively control the various types of concrete spalling, it is necessary to comprehensively understand the thermodynamic properties of concrete and the thermodynamic behavior of internal moisture, which vary depending upon environmental conditions such as the heating rate and heating temperature, and to apply a spalling control method considering these characteristics. Acknowledgement This work (NRF-2015R1A2A2A01007705) was supported by the Mid-Career Researcher Program through the NRF grant funded by MEST. References [1] G. Choe, G. Kim, N. Gucunski, S. Lee, Evaluation of the mechanical properties of 200 MPa ultra-high-strength concrete at elevated temperatures and residual strength of column, Constr. Build. Mater. 86 (2015) 159–168. [2] V.K.R. Kodur, Spalling in high strength concrete exposed to fire-concerns, causes, critical parameters and cures, Proceedings of Structures Congress, Advanced Technology in Structural Engineering, Philadelphia, USA, May 8–10, 2000. [3] L. Bostrom, R. Jansson, Spalling of self-compacting concrete, Proceedings of SIF'06 workshop, Structures in Fire, Aveiro, Portugal, 2006 (May). [4] Y. Fu, L. Li, Study on mechanism of thermal spalling in concrete exposed to elevated temperatures, Mater. Struct. 44 (1) (2011) 361–376.
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