On the explosive spalling behavior of ultra-high performance concrete with and without coarse aggregate exposed to high temperature

Construction and Building Materials 226 (2019) 932–944

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On the explosive spalling behavior of ultra-high performance concrete with and without coarse aggregate exposed to high temperature Juan Yang a,b, Gai-Fei Peng b,⇑, Jie Zhao b, Guo-Shuang Shui b a b

College of Materials Science and Technology, Beijing Forestry University, 100083, China Faculty of Civil Engineering, Beijing Jiaotong University, 100044, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Coarse aggregates mitigate the

explosive spalling of UHPC slightly.  PP fibers significantly alleviate the

explosive spalling of UHPC.  The combination of vapor pressure

and thermal stress causes explosive spalling.  "Pressure releaser" work of PP fibers in UHPC may not play the most important role.

a r t i c l e

i n f o

Article history: Received 2 March 2019 Received in revised form 24 July 2019 Accepted 25 July 2019

Keywords: Ultra-high performance concrete Coarse aggregate Explosive spalling Vapor pressure Temperature difference Polypropylene fiber

a b s t r a c t Experimental investigations are conducted on the explosive spalling behavior of two types of ultra-high performance concrete (UHPC), i.e., UHPC with coarse aggregate (UHPC-CA) and UHPC without coarse aggregate (UHPC-no-CA) under high temperature slow heating conditions. The influences of polypropylene (PP) fiber and moisture content on the spalling behavior of UHPC are studied. The inner vapor pressure and temperature in the UHPC specimens are measured to analyze the driving force of explosive spalling of UHPC during heating. Results show that UHPC-CA exhibits a slightly better resistance to explosive spalling than UHPC-no-CA and coarse aggregate has a beneficial effect on minimizing the explosive spalling of UHPC. Also, Moisture content of UHPC specimen remarkably affects the explosive spalling behavior of both UHPCs, i.e. the higher the moisture content, the severer the explosive spalling is. Hybrid fibers (steel fiber and PP fiber) significantly improve the resistance to explosive spalling of UHPC and can prevent some UHPC specimens from explosive spalling. These results prove that vapor pressure is the primary driving factor on the explosive spalling of UHPC under the heating conditions in this work. Additionally, layered explosive spalling occurs to both UHPCs. The temperature gradient in UHPC-no-CA is much higher than that in UHPC-CA. Many coarse aggregates without cracking damage peel off from the UHPC-CA matrix. These results indicate that thermal stress is an important factor driving the explosive spalling. However, when the UHPC specimens are fully saturated (100% moisture content), the ‘‘pressure releaser” function of PP fiber may not play the most important role in alleviating the explosive spalling in UHPCs. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction ⇑ Corresponding author at: Faculty of Civil Engineering, Beijing Jiaotong University, 100044, China. E-mail addresses: [email protected] (J. Yang), [email protected] (G.-F. Peng). https://doi.org/10.1016/j.conbuildmat.2019.07.299 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

Ultra-high performance concrete (UHPC) includes two types, i.e., ultra-high performance concrete with coarse aggregate

J. Yang et al. / Construction and Building Materials 226 (2019) 932–944

(UHPC-CA) [1] and ultra-high performance concrete without coarse aggregate (UHPC-no-CA) [2]. UHPC is widely used in many types of structures due to its ultrahigh strength and excellent durability [3-6]. However, when exposed to high temperature, UHPC is highly prone to explosive spalling due to the high level of compactness and low permeability, which hinders the application of UHPC. To overcome this obstacle, the spalling behavior characteristics and the effective improvements in the resistance to explosive spalling of UHPC should be further investigated. In as early as 2000, Khoury [7] concluded that the spalling behaviors of ordinary concrete and high performance concrete (HPC) at high temperature are similar, but UHPC showed distinct spalling behavior, and the addition of polypropylene (PP) fiber cannot prevent the explosive spalling of UHPC specimens. Klingsch [8] also found that UHPC-no-CA is more likely to encounter explosive spalling than HPC. By comparing the microscopic changes in UHPCno-CA and HPC after exposure to high temperature, Alonso [9] concluded that the resistance to explosive spalling of HPC is better than that of UHPC-no-CA. These findings indicate that the conclusions regarding the explosive spalling of HPC cannot be directly applied to UHPC, and the explosive spalling behavior of UHPC needs to be further studied. Regarding the UHPC-CA spalling behavior, Sakura [10] reported that serious explosive spalling occurred in heated UHPC-CA with a compressive strength of approximately 100 MPa. Li [11] observed that when the UHPC-CA specimens were heated to temperatures above 500 °C, explosive spalling occurred only on the surface of several specimens, while in most of the specimens, explosive spalling occurred only when the temperatures were above 800 °C. Fujinaka [12] concluded that the greater the compressive strength of UHPC-CA, the more severe the explosive spalling in prism specimens. Sohn [13] classified the explosive spalling of UHPC-CA with different compressive strengths into four degrees. For the explosive spalling of heated UHPC-no-CA, Behloul [14] found that when the UHPC-no-CA column of 200 mm  200 mm  900 mm was exposed to ISO heating, no explosive spalling occurred. Sanchayan [15] also revealed that UHPC-no-CA is highly prone to explosive spalling when the temperature is higher than 400 °C. Liu [16] reported that UHPC-no-CA specimens spalled at a furnace temperature of 400 °C, and the maximum vapor pressure inside the concrete specimens reached to 3.4 MPa. PP fibers significantly improve the resistance of UHPC-no-CA to explosive spalling [14,17] and small PP fibers are more efficient in resisting fire [18]. Kahanji [19] reported that the explosive spalling of UHPCno-CA beam is affected by the loading levels, and the beam under the 60% loading level of the ultimate flexural strength of the beam at ambient temperature experienced much less spalling compared to those under 20% and 40% loading levels. For comparative research on the explosive spalling behavior of UHPC-CA and UHPC-no-CA, Hosser [20] found that UHPC-CA specimens spalled less than UHPC-no-CA specimens, and coarse aggregates can alleviate explosive spalling. Lai [21] studied the influence of polyvinyl alcohol (PVA) fiber and steel fiber on the high temperature behavior of UHPC-no-CA and UHPC-CA, but the effects of the coarse aggregate were not analyzed. The effects of polymer fibers on the compressive strength and explosive spalling of UHPC-no-CA and UHPC-CA were reported, but the study was limited to only the effect of fibers [22]. Therefore, reported studies on the explosive spalling of UHPC were aimed at UHPC-CA or UHPC-no-CA individually, and a comprehensive study on the explosive spalling resistance of both UHPC-CA and UHPC-no-CA is needed. The basic difference between UHPC-CA and UHPC-no-CA in materials is the coarse aggregate. The dosage and maximum size of coarse aggregate have a significant effect on the high temperature properties of heated UHPC-CA [23]. Hosser [20] reported that the explosive spalling severity of prismatic UHPC-CA specimen is

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much less than that of prismatic mortar specimen and coarse aggregate alleviates the explosive spalling to same extent. However, slight effect of coarse aggregate as negative parameters on spalling degree of high strength concrete (HSC) was observed [24]. Additionally, high level of iron or other metal elements in coarse aggregate will speed up the appearance of explosive spalling [25]. Pan [26] reported that large size coarse aggregates exhibit greater benefit to improve the resistance to spalling of concrete than small size aggregate. The quartzitic gravel, dolomite, sandstone and granite aggregate are in decreasing order of spalling resistance of HSC [27]. But basalt and granite aggregate has no clear bearing on concrete spalling because they have similar thermal expansion coefficients [28]. Due to the significant influence of aggregate type on the thermal expansion of concrete, coarse aggregate with different weight exhibits different effect on the high temperature behavior of concrete [29]. The result by Yoon [30] indicated that normal weight concrete displayed many cracks between the aggregates and cement matrices, whereas light weight concrete showed none. Compared to normal aggregate, the heavyweight aggregates such as magnetite aggregate with higher thermal conductivity would lessen the risk of explosive spalling significantly [31]. In the lightweight self-compacted concrete, the effect of coarse aggregate on the high temperature behavior of concrete is also significant [32]. Not only coarse aggregate, fine aggregate also has a preponderate effect on the fire performance as it affects the volume stability, porosity, mass loss and microstructure [33]. Liang [34] reported that steel slag played a key role in improving the fire resistance of UHPC-no-CA, because steel slag could reduce thermal incompatibility between the fine aggregate and the cement paste. In a word, aggregate (coarse aggregate or fine aggregate) plays a significant role in the behavior and properties of concrete when it is exposed to high temperature, but there are a few researches on UHPC. Regarding the explosive spalling mechanism of UHPC, Klingsch [8] noted that the vapor pressure mechanism governed explosive spalling, and the thermal stress influence is negligible in most cases. Ju [35] used a ‘‘thin-walled spherical shell” model to describe the vapor pressure mechanism that causes UHPC explosive spalling. In the latest report, Ju [36] noted that thermal stress is an important factor in UHPC explosive spalling. Other researcher suggested that the UHPC explosive spalling can be attributed to a coupling of vapor pressure, thermal stress and random cracks [37]. Lai [21] reported that the UHPC explosive spalling was caused by vapor pressure and changes in concrete microstructure. Therefore, the UHPC explosive spalling mechanism is not reasonably clear, and the inner vapor pressure, thermal stresses and other impacts inside UHPC specimens need to be further analyzed. More importantly, suitable measures to prevent UHPC from explosive spalling are closely related to the explosive spalling mechanism. The effective action of PP fibers on alleviating the explosive spalling of UHPC verifies the vapor pressure factor [19,38–39]. Liang [31] found that the built-up vapor pressure inside UHPC-no-CA specimens is the important contribution to the explosive spalling and PP fiber is an effective additive to control explosive spalling, but failed to eliminate it completely. Steel fiber could reduce the probability and intensity of spalling resulting from thermal stress. Another result indicated that PP fiber could play a negative role in preventing explosive spalling between 320 and 380 °C because PP fibers decomposed into various volatiles between 360 °C and 400 °C, increasing pore pressure [40]. Due to many complicated factors on the explosive spalling behavior of UHPC, the above different results are observed. Zhao [41] reported that the spalling mechanisms are different at different heating rates because the vapor pressure distributions under different heating conditions are quite different. The magnitude of pressure build-up inside the samples depends on the heating rate: quicker heating implies

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faster vaporization and higher peak pressure, while thermallyinduced cracks tend to enhance gas permeability and pressure release [42]. Additionally, the melted PP fibers can also reduce the inner temperature of concrete due to the pore increase inside concrete [43,44] and steel fibers also play a certain role in the pore pressure reduction in deeper regions of concrete [44,45]. Therefore, the addition of hybrid fibers (PP fiber and steel fiber) is an effective method to improve the resistance to explosive spalling of UHPC. In this paper, an experimental investigation is conducted on the explosive spalling behavior of UHPC-CA and UHPC-no-CA. To explore the explosive spalling mechanism of UHPC and the improvements in explosive spalling resistance, the effects of hybrid fibers (steel fiber and PP fiber) and moisture content on the explosive spalling behavior of UHPC are investigated, and the inner vapor pressure and temperature differences inside UHPC specimens are determined. 2. Experiment details 2.1. Raw materials P
II 52.5-R Portland cement (C), silica fume (SF*), fly ash (FA) and ground granulated blast furnace slag (GGBS) are used as binders, properties of which are listed in Table 1. Basalt aggregates with two particle size ranges (5–10 mm and 10–16 mm) at a mass ratio of 3:7 are used as coarse aggregate (CA), and its physical properties are presented in Table 2. Machine-made sand (MS) is used as fine aggregate and its physical properties are listed in Table 3. Steel fiber (S-F) is a flat-straight copper-coated steel fiber with a length of 16 mm, a diameter of 0.20 mm, and the tensile strength of 2200 MPa. In this study, the dosage of steel fiber is 0.5% by volume (39 kg/m3). Straight polypropylene fiber (PP-F) with a length of 19 mm and diameter of 0.02 mm is added at the dosage of 0.15% by volume (1.35 kg/m3) and its melting point is 165 °C. Polyacrylate superplasticizer with 50% solid content is used to maintain concrete workability. In this research, the slump and slump flow of both concretes are approximately 220 mm and 540 mm, respectively. 2.2. Preparation Four types of UHPC with the water to binder ratio (W/B) of 0.18 were prepared and designated as SF/CA, HF/CA, SF/no-CA and HF/no-CA. The water to cement ratio of all UHPC is 0.3. The mix proportions, compressive strength and splitting tensile strength at the age of 56 days are shown in Table 4. The UHPC-no-CA mix proportion is based on that of reactive powder concrete (RPC), and the mass ratio of aggregate to binder in RPC is 0.8–1.3

Table 2 Physical properties of coarse aggregate. Type

Apparent density (kg/m3)

Packing density (kg/m3)

Crushing index

Basalt

3030

1790

3.1%

[2,46–48]. To obtain the desired compressive strength, the ratio of aggregate to binder for the UHPC-no-CA was determined to be 1.2 in this experiment. All raw materials were sequentially added and mixed. Machinemade sand and 20% (by mass) of the total mixing water were initially mixed for approximately 2 min. Then, all binders were added and remixed for approximately 6 min. Next, coarse aggregates were added and remixed for another 3 min. Subsequently, steel fibers or PP fibers were gradually added to obtain an even fiber distribution. Finally, the remaining water and plasticizer were added and remixed for approximately 5 min to achieve the desired concrete workability. After vibration, all the specimens were immediately plastic-wrapped to minimize moisture loss. All the specimens were demolded after 24 h of mixing with water and then cured in water at 20 °C for 56 days. The curing regime enables the UHPC-CA and UHPC-no-CA specimens to obtain enough saturated humidity. Concrete cubes of 100 mm  100 mm  100 mm in size were prepared for compressive strength test, explosive spalling test and inner temperature measurement test. Concrete cylinders with a diameter of 100 mm and a height of 210 mm were utilized for the vapor pressure test. 2.3. Explosive spalling test 2.3.1. Establishment of moisture content in specimens Moisture content (wmc) is defined as the ratio of evaporable water within a specimen at the time of test to the original amount of evaporable water when the specimens were initially retrieved from a curing water tank [49]. Specimens were dried at 105 °C to yield different moisture contents, i.e., 0%, 25%, 50%, 75% and 100%. The 105 °C drying temperature is line with other reported studies [50–52]. When specimens were removed from curing water tanks and the water on the specimen surfaces was immediately wiped off, the moisture content of these specimens was considered to be 100% (m1). Moisture content was considered to be 0% when the weight (m0) of the specimens was stable. Using m0 and m1, the target weight (mi) of a specimen with any moisture content can be calculated. The designed moisture content is an average value of three specimens. Due to the dense microstructure of UHPC, water migration inside concrete is exceedingly difficult. Thus, the moisture content

Table 1 Properties of binders. Components

C

SF*

FA

GGBS

SiO2 (%) Al2O3 (%) Fe2O3 (%) CaO (%) MgO (%) SO3 (%) K2O (%) Na2O (%) Alkali content (%) Loss on Ignition (%) Density (g/cm3) Specific surface (m2/kg) Compressive strength (MPa) 3 days 28 days

20.44 4.57 3.02 64.48 1.8 2.7 0.8 0.13 0.66 1.92 3.14 384

90.62 0.62 1.2 1.36 2.96 0.01 0.78 0.48 0.99 1.73 2.20 22,205

50.34 30.64 6.01 5.24 1.53 0.22 1.56 0.54 1.57 1.98 2.52 —

32.65 14.16 0.02 37.21 9.09 0.36 — — — 0.22 2.90 1020

37.4 62.0

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J. Yang et al. / Construction and Building Materials 226 (2019) 932–944 Table 3 Physical properties of machine-made sand. Fineness modulus

Apparent density (kg/m3)

Packing density (kg/m3)

Mud content (%)

Clay pieces content (%)

Stone power content (%)

2.94

2600

1650

5.5

0.5

5.5

Table 4 Mix proportions and 56-day strengths of ultra-high performance concretes. Type

SF/CA HF/CA SF/no-CA HF/no-CA

W/B

0.18 0.18 0.18 0.18

Quantity (kg/m3) Cement

SF*

FA

GGBS

MS

CA

S-F

PP-F

540 540 540 540

90 90 90 90

180 180 180 180

90 90 90 90

620 620 1080 1080

930 930 0 0

78 78 78 78

0 1.35 0 1.35

Compressive strength (MPa)

Splitting tensile strength (MPa)

154.2 156.5 145.0 145.4

14.6 11.2 14.0 14.1

Note: SF is the abbreviation for steel fiber. HF is the abbreviation for hybrid fiber (steel fiber and polypropylene fiber). SF/CA is the ultra-high performance concrete with coarse aggregate and only steel fiber is added. HF/CA is the ultra-high performance concrete with coarse aggregate and hybrid fiber (steel fiber and polypropylene fiber) are added. SF/no-CA is the ultra-high performance concrete without coarse aggregate and only steel fiber is added. HF/no-CA is the ultra-high performance concrete without coarse aggregate and hybrid fiber (steel fiber and polypropylene fiber) are added.

of UHPC specimen is calculated based on the weight of the specimens and it is hypothesized that all the tested specimens have a constant humidity state.

2.3.2. Heating method Six specimens were heated in a muffle furnace from room temperature to 800 °C at a heating rate of 10.0 °C/min for the explosive spalling test. The heating setup is shown in Fig. 1.

2.4. Determination of residual compressive strength Residual compressive strengths of UHPC specimens after exposure to different target temperatures (100 °C, 200 °C, 400 °C, 600 °C and 800 °C) were determined using the cubic specimen of 100 mm  100 mm  100 mm. All data presented in the study are the average value of three specimens.

Fig. 1. Heating setup for the explosive spalling test.

Before heating, all the specimens at the age of 56 days were predried at 105 °C in order to reduce moisture content of concrete to a lower degree so that the specimens would not encounter explosive spalling during heating. The specimens were heated in an electric furnace at a rate of 2.0 °C/min, and the target temperature was maintained for 2 h. After the specimens cooled naturally to the room temperature, their compressive strengths were measured. The temperature–time curves during heating are given in Fig. 2. According to the Chinese standard GB/T 50081-2002 [53], the compressive strength of UHPC specimens were determined using a testing machine with a capacity of 200 tons under a load rate of 1.0 MPa/s.

2.5. Measurement of inner vapor pressure A vapor pressure testing system developed by Ju Y [35] was used to determine the inner vapor pressure of UHPC specimens during heating from room temperature to 800 °C at a rate of 10 °C/min. Cylinders with a diameter of 100 mm and a height of 210 mm were prepared for the vapor pressure test, as shown in Fig. 3 (25 mm and 50 mm points). To identify the vapor pressure distribution inside the UHPC specimen, three measuring points were chosen at different locations with depths of 25 mm, 50 mm and 100 mm from the end of the cylinder specimen. However, only two measuring tubes were placed in one specimen because of the

Fig. 2. The temperature-time heating curves of specimens at different target temperatures.

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2.7. Microstructure test To identify a correlation between the changes in micromorphology and explosive spalling behavior of the UHPC specimens, scanning electron microscope (SEM) observations were carried out to determine the variations in the concrete microstructure after exposure to high temperature. The samples were small slices with thicknesses of 8 mm from the hardened cement paste at the same location in the cube concrete specimens. The samples were completely dried at 105 °C in an oven for 1 day and then sealed. Finally, prior to the SEM observations, the samples were coated with gold for the purpose to strengthen electrical signal and then obtain the clearer observations.

3. Results and discussion 3.1. Residual compressive strength Fig. 3. Illustration of the test specimen for measuring vapor pressure.

Fig. 5 show the residual compressive strength of UHPCs after exposure to different high temperatures, i.e. 100 °C, 200 °C, 400 °C, 600 °C, 800 °C. At ambient temperature, the original compressive strengths of all the UHPCs at 56 days were higher than

small cross-section dimension of the cylinder specimen. Additionally, to avoid no data or inaccurate data, two vapor pressure determinations was conducted at each measuring point inside two cylinders. Thus, there are three cylinder specimens for one group test. The tubes are placed at various depths of 25 and 50 mm, 50 and 100 mm, 25 and 100 mm. At the same location, a thermocouple was placed around the measuring tubes to obtain the temperature. The used data is obtained based on the overall analysis on the pressure-time curves, explosive spalling occurrence, the peak vapor pressure and the corresponding temperature. 2.6. Measurement of inner temperature Three thermocouples were placed inside a cube specimen to determine the temperatures of the measuring points at depths of 15 mm, 25 mm and 50 mm. The temperature differences between two points were obtained to analyze the thermal stress, which was induced by the temperature gradient inside the specimen. The locations of the measuring points are shown in Fig. 4.

Fig. 4. Location of thermocouples inside a specimen.

Fig. 5. Residual compressive strength of UHPC after exposure to different temperatures.

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140 MPa, and the UHPC-CA shows the higher compressive strength than UHPC-no-CA by around 10 MPa, indicating that the coarse aggregates with high strength play an important granular skeleton role on improving the original compressive strength of UHPC. Additionally, the original compressive strengths of UHPCs with hybrid fibers are not less than those with only steel fiber, implying that PP fibers were uniformly distributed in concrete and don’t play a negative role in the compressive strength of UHPC. After exposure to high temperature, the residual compressive strength of all the UHPCs increased first and then decreased with the increasing high temperature and the peak value was obtained at 300 °C or 400 °C. The increase in compressive strength can be attributed to the secondary hydration of the unhydrated cement and reactive mineral admixtures which can continue with the increasing temperature. The microstructure analysis on the ground hydrated cement paste exposed to high temperature indicated the secondary hydration process [54]. In the recent research on UHPC, Peng [55] found that when the UHPC undergoes dry air heating, a high temperature steam environment could be established in the inner part of UHPC specimens and resulted in the transformation of C-S-H gels to crystals, leading to the improvement of concrete compressive strength. While the reduction or disintegration of CS-H gels and the phase transformation from Ca(OH)2 to CaO inside the specimens due to high temperature lead to the compressive strength loss of UHPC [54]. After 100 °C, the compressive strengths of UHPCs are slightly higher than those at ambient temperature. PP fibers can be melted at about 165 °C and then created many channels in concrete, which can be considered as a disadvantage effect on concrete compressive strength. However, after 200 °C, the compressive strength of all the UHPCs increased significantly and those of the UHPC with PP fibers show higher improvements, indicating that 200 °C high temperature cannot cause a severe damage of the created channels. After 400 °C, SF/CA and HF/no-CA show increased compressive strength, HF/CA shows unchanged compressive strength, but SF/no-CA decreased slightly. The different results can be attributed to the counteraction of two conflicting factors, i.e. the great improvement due to the secondary hydration, the reduction due to the damaged channels and interfacial transition zone (ITZ) between coarse aggregate and mortar caused by high temperature. After 600 °C, the compressive strengths of all the UHPCs except for HF/CA began to decrease, but still higher than 120 MPa and the residual percentage based on the original compressive strength are still higher than 87%, implying that UHPC exhibits a better resistance to high temperature in aspect of concrete compressive strength. After 800 °C, the compressive strengths of all the UHPCs were about 30% of their original strengths. Overall, SF/CA and HF/no-CA have the higher residual compressive strength and exhibit the better resistance to high temperature in compression. Additionally, it is noted that HF/CA shows a fast decreasing trend on the residual compressive strength at 600 °C. Coarse aggregates, steel fibers and PP fibers are included in the HF/CA matrix and may cause a problem that their overlay cannot be avoided completely. After high temperature, the overlays became great detects resulting in the reduction of concrete compressive strength. But the overlay in other UHPCs can be fewer due to the inexistence of PP fibers or coarse aggregates. SF/CA shows the better resistances to high temperature than SF/no-CA while the conclusion is opposite if PP fibers are added, especially after the higher target temperature. This may be attributed to the coarse aggregate content. It is clearly shown that the optimal dosage of coarse aggregate in UHPC sample is 25% by volume with respect to compressive strength of UHPC sample [56]. But in this work, the volume content of coarse aggregate is 30% and need further tailor to explore the optimal UHPC-CA with higher compressive strength.

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Overall, SF/CA and HF/no-CA exhibits the better resistance to high temperature and is more appropriate to be used in the compression elements. For the hybrid fiber reinforced UHPC-CA, the coarse aggregate content should be an optimal value for the purpose to avoid the poor properties resulted by the overlay in the matrix. 3.2. Explosive spalling behavior When the explosive spalling of UHPC specimens occurred during heating, a burst noise can be heard. After heating, severe damage in the spalled specimen were observed, including large amounts of great cracking in specimens and many broken mortar fragments. If only fine cracks are observed in the specimen surfaces, which cannot be defined as explosive spalling. 3.2.1. Explosive spalling occurrence The first spalling temperature and final spalling temperature are the corresponding temperature during heating when the first and final explosive spalling occurred, respectively. In this case, the explosive spalling occurrence is identified by an experimenter who heard the explosive spalling noise. The explosive spalling resistant limit time (RLT), temperature range (TR) from the first to the final temperature and the number of explosive spalling occurrences were recorded during heating, as shown in Fig. 6. The appearances of the spalled UHPC specimens are observed in Fig. 7. Exposed to high temperature, all types of UHPC specimen experienced explosive spalling except the HF/CA specimens with 25% moisture content. UHPC-CA had a smaller TR and encountered less explosive spalling than UHPC-no-CA, but the explosive spalling RLT between both UHPCs was not significantly different. It can be concluded that the explosive spalling of UHPC-no-CA specimens exposed to high temperature is slightly more severe than that of UHPC-CA. Additionally, as the moisture content of the UHPC specimen increased, both the TR and number of spalling occurrence increased. The explosive spalling RLT was significantly reduced. Thus, moisture content significantly affects the explosive spalling behavior of UHPC due to the inner vapor pressure increment induced by water vaporization in the specimens [57,58]. Fig. 7 indicates that when the moisture content of specimens is 75% or less, some of hybrid fiber-reinforced UHPC specimens did not spall. Obviously, the addition of hybrid fibers significantly alleviated the explosive spalling of both UHPC-CA and UHPC-no-CA, indicating that PP fibers plays a key role in improving the resis-

Fig. 6. Temperature ranges, explosive spalling resistant limit time and numbers of explosive spalling occurrence in the UHPC specimens.

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Fig. 7. Appearances of UHPC specimens with different moisture contents after heating.

tance to explosive spalling of UHPC. After PP fibers were melted at 165 °C, the continuous or disconnected channels were formed inside the concrete. The channels allow the vapor pressure to release or migrate into the low-temperature region, and then the vapor pressure inside the concrete decreased significantly thus lessening the likelihood and severity of explosive spalling [43,44]. When the heating temperature is lower than the melting point of PP fiber, the micro-cracks resulted by the mismatch in thermal expansion coefficients of the matrix is responsible for enhancing permeability, thereby reducing the susceptibility of explosive spalling of UHPC [50]. Additionally, due to the created channels, the porosity of UHPC fast increased, leading to the lower temperature [59]. However, when the moisture content of specimens is 100%, both UHPC-CA and UHPC-no-CA specimens experienced extremely severe explosive spalling, all specimens spalled into many fragments, and no obvious difference can be observed. This result indicates that adding PP fibers cannot improve the resistance to explosive spalling of UHPC with the moisture content of 100%. This can be attributed to two factors. Large amounts of water yield high vapor pressure, but the high vapor pressure cannot be released completely in time through the channels created by melted PP fibers due to the less dosage of PP fibers. Mitsui K [60] reported that PP fiber with a volume amount of 0.11% can significantly improve the fire resistance of UHPC-CA columns with a compressive strength of 100 MPa, but the volume must be increased to 0.33 vol% for the columns with a compressive strength of 150 MPa. In this experiment, the compressive strength of the UHPC is approximately 150 MPa and the amount of PP fibers is 0.15% by volume. The channels created by melted PP fibers may be not sufficient to release the vapor pressure of these UHPC specimens. On the other hand, high vapor pressure is not the most important factor driving explosive spalling, and the ‘‘pressure releaser” function of the PP fiber cannot be effective. Khoury [7] also proposed that PP fibers may not be able to prevent some UHPCs from explosive spalling. Some researchers found that the temperature gradientinduced thermal stress is the primary factor inducing the explosive spalling of concrete based on numerical simulations [61,62]. Ju

[36] developed a concept model to elucidate the explosive spalling of UHPC-no-CA from the viewpoint of the thermal stress mechanism and the theory of energy release based on numerical and experimental studies. 3.2.2. Layered explosive spalling There were some specimens of which the top parts spalled into small pieces or peeled off. Also, in other specimens, only the bottom parts remained in one piece. HF/CA specimens with 75% moisture content experienced two large cracking spalling along the horizontal direction. Explosive spalling occurrences in these forms are considered as layered explosive spalling. Similar explosive spalling behavior was observed in the UHPC-no-CA cylinder specimens [63]. Fig. 8 details the appearance of the layered specimens after explosive spalling occurrence. Generally, vapor pressure and thermal stress are the main factors that induce explosive spalling in concrete. The vapor pressure exists in the matrix or the interfacial zone between the matrix and coarse aggregate rather than inside the coarse aggregate, while the temperature gradientinduced thermal stress can exist in the zone parallel to the heated surface except for the interfacial zone. Therefore, the vapor pressure cannot independently induce explosive spalling layer-bylayer, and the resultant explosive spalling is caused by a combined effect of vapor pressure and thermal stress [42]. 3.2.3. Observations of the spalled UHPC-CA specimens The observations of a spalled SF/CA specimen with 25% moisture content are shown in Fig. 9. During explosive spalling, many coarse aggregates peeled off from mortar and maintained integrity. Due to the interlocking of steel fibers, some coarse aggregates did not break away from the main block. This phenomenon is also observed in the SF/CA specimens with other moisture contents and the spalled HF/CA specimens. This result can be attributed to two factors. First, vapor pressure and thermal stress existed along the ITZ between the matrix and coarse aggregates, leading to a decrease in the interfacial bonding force. Additionally, the different thermal expansion coefficients of coarse aggregate and mortar

J. Yang et al. / Construction and Building Materials 226 (2019) 932–944

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Fig. 8. The UHPC specimens that experienced layered spalling.

Fig. 9. Coarse aggregates from the SF/CA specimens (wmc = 25%).

caused the thermal mismatch, leading to the loss of bonding force between coarse aggregate and the matrix. By observing and analyzing the fracture surfaces of the two types of specimen, i.e., the spalled SF/CA specimens after the explosive spalling test and the specimens without heat treatment after the splitting tensile strength test, it can be found that during the splitting tensile strength test, the splitting failure of the UHPC-CA specimen without heat treatment was through the coarse aggregate rather than along the ITZ between the matrix and coarse aggregate [64]. This result is because the UHPC-CA has a much denser microstructure and the tensile stress along a tensile zone

in the splitting surface is evenly distributed. However, during explosive spalling, the failure of the SF/CA specimen occurred along the ITZ rather than through the coarse aggregate. This significantly differs from the failure mode of the specimens after the splitting tensile test. Vapor pressure exists in the matrix and the ITZ and does not present in the coarse aggregate, while the thermal stresses caused by the temperature gradient exist in all concrete components. In this experiment, the explosive spalling caused no ruptures through the coarse aggregate but ruptured through the mortar and along the ITZ. This result indicates that thermal stresses alone are not

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sufficient enough to cause explosive spalling. The combination of thermal stress and vapor pressure induces the explosive spalling of UHPC-CA. Thus, vapor pressure is a critical factor that controls the explosive spalling of UHPC-CA. However, the quantitative analysis regarding vapor pressure, thermal stress and their effects on explosive spalling must be further studied. 3.3. Inner vapor pressure The above results in listed 3.2.1 show that when the moisture content of the UHPC specimen was 100%, all types of UHPC suffered severe explosive spalling, and PP fibers cannot improve the UHPC resistance to explosive spalling. To further probe the problem, the inner vapor pressure inside UHPC specimens was determined. Inner vapor pressures at three points inside the UHPC specimens with 100% moisture content were measured. The vapor pressure-time and temperature-time curves at the measuring points in different types of UHPC specimen during heating are shown in Fig. 10. At meantime, the explosive spalling behaviors of the UHPC cylinder specimens were also investigated. Fig. 11 shows the appearances of all UHPC cylinder specimens exposed to high temperature. The results in Fig. 10 indicate that the inner vapor pressure of HF/CA was higher than that of HF/no-CA, and the SF/CA specimen had a much higher vapor pressure than SF/no-CA. However, Fig. 11 shows that the resistance to explosive spalling of the HF/CA cylinder specimens was greater than that of HF/no-CA, and no significant difference in the severity of explosive spalling occurrences are observed between SF/CA and SF/no-CA. The results imply that no clear link exists between the built-up of vapor pressure and

explosive spalling occurrence in the UHPC specimens with 100% moisture content. The same results were reported by Mindeguia [65]. The result is caused by a complex interplay of multiple factors, but coarse aggregate could be the domain factor. Inner vapor pressure in HF/CA is lower than that in SF/CA and the trend is also observed in UHPC-no-CA. This result confirms the ‘‘pressure releaser” function of PP fibers. PP fibers significantly improved the spalling resistance of the UHPC-CA specimens, but the improvement in the UHPC-no-CA specimens was slight. Maybe, it is due to the higher dispersion density of PP fibers in UHPC-CA mortar than that in UHPC-no-CA. However, the inner vapor pressure in UHPC-CA specimen is higher than that in UHPC-no-CA specimen. This result further confirmed that the ‘‘pressure releaser” function of the PP fibers in alleviating the spalling of UHPC-no-CA specimens with 100% moisture content is not significant and could not play the domain role in improving the explosive spalling of UHPC. The result is not in good accordance with the accepted theory, which underlines the primary factor of vapor pressure in inducing explosive spalling [66–68]. The inner vapor pressure gradually decreases with the increase of the depth of the measuring points due to the different position of the measuring points from heat source and the similar result has been reported by Ju Y [37]. The occurrence of explosive spalling could make concrete specimen spall into small fragments, then the shortened distance between the measuring point and heat resource could result in the fast reduce of the vapor pressure. However, the vapor pressure at the measuring point is related to the distance from point to spalling location and the severity of explosive spalling occurrence of UHPC specimen. If the spalling location is close to the measuring point and the more serious spalling occurred, the vapor pressure would be lower due to the fast release

Fig. 10. Vapor pressure -time and temperature-time curves at different positions in different UHPC specimens with 100% moisture content during heating.

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Fig. 11. Appearances of cylinder UHPC specimens with 100% moisture content after heating.

of vapor pressure and moisture. As shown in Fig. 10(b) and (d), the vapor pressure in the 100 mm point is much higher than anther both points because the explosive spalling in HF/CA and HF/noCA is slight compared with SF/CA and SF/no-CA, especially HF/CA.

mal stress vapor pressure is the main factor inducing explosive spalling of UHPC-no-CA specimens with 100% moisture content, rather than the vapor pressure, as the ‘‘pressure releaser” function of the PP fibers is not the primary factor to alleviate the explosive spalling of concrete.

3.4. Inner temperature-difference 3.5. SEM observations The inner temperature difference – heating time curves at three test points inside the UHPC specimens during heating are shown in Fig. 12. The temperature differences inside the UHPC-no-CA specimen were much higher than those of UHPC-CA. This may be due to the high thermal conductivity of CA [69]. A temperature gradient can cause thermal stresses with a positive correlation. Thermal stresses are prone to causing cracking damage in concrete [70]. Thus, thermal stresses in the UHPC-no-CA specimens were higher than those in the UHPC-CA specimens, resulting in the more severe cracking in the UHPC-no-CA specimens. The results by Ju Y [38] proved that a high temperature difference caused high thermal stress inside the UHPC-no-CA specimen, where the void in the skeleton is initially filled with liquid water, the maximum tensile stress inside can reach up to 6.2 MPa and the maximum compressive stress is approximately 11.5 MPa, showing that thermal stress can be an important factor causing the explosive spalling of UHPC-no-CA exposed to high temperature. Fig. 7 and Fig. 11 show that when the moisture content is 100%, PP fiber cannot alleviate the explosive spalling of UHPCno-CA. It is plausible that the temperature gradient-induced ther-

SEM micrographs of the ITZ between the CA and matrix of SF/CA specimens with different moisture contents are given in Fig. 13. When the moisture content of the SF/CA specimens was 0%, damage cracks could be observed along the ITZ only and no cracks existed in the mortar. The damage in the ITZ is attributed to the mismatch of thermal expansion between CA and mortar. The mortar and ITZ of the SF/CA specimen with 75% moisture content exhibited more severe cracking damage than the specimen with 25% moisture content. However, the damage to CA was not observed. Therefore, a large number of coarse aggregates peeled off from the UHPC-CA specimens during explosive spalling. Special damage in mortar is shown in Fig. 14. The mortar between two CA experienced serious cracking damage, and many small fragments formed due to cracking. However, no cracking damage existed inside the two CA, which prevented the crack in the motar from growing. Therefore, during heating, large driving forces resulting from vapor pressure and thermal stress cause large cracks in the matrix and ITZ, but CA can alleviate the growth of cracking to the same extent.

Fig. 12. Temperature differences at three test points inside different types of UHPC during heating from room temperature to 800 °C.

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Fig. 13. SEM micrographs of mortar, coarse aggregate and the ITZ of SF/CA specimens.

Fig. 14. Damage of mortar between two coarse aggregates.

(3) Moisture content significantly affects the explosive spalling of both UHPC-CA and UHPC-no-CA with a positive correlation, and polypropylene fiber significantly improved their spalling resistance, which indicates that vapor pressure is the main factor inducing the explosive spalling of UHPC. A large number of peeled coarse aggregates, layered explosive spalling occurrence and the different failure modes of coarse aggregate were observed, suggesting that the combination of vapor pressure and thermal stress causes explosive spalling. (4) When the moisture content of UHPC-no-CA specimens is 100%, PP fibers cannot alleviate the explosive spalling of UHPC-no-CA. Hybrid fiber-reinforced UHPC-CA cylinder specimens have higher vapor pressure but also exhibits better resistance to explosive spalling. These results suggest that inner vapor pressure may not be the primary factor driving the explosive spalling of UHPC-no-CA, and the ‘‘pressure releaser” function of PP fiber may not play the most important role in alleviating the explosive spalling of UHPC.

Declaration of Competing Interest

Fig. 15. SEM image of channels after PP fiber melted.

The SEM image of channels and cracks is shown in Fig. 15. After the PP fibers melted, the channels formed in the UHPC matrix with a random distribution, providing channels to release the internal vapor pressure. Large cracks can also be observed clearly. This result shows that PP fibers can release the vapor pressure but cannot avoid the crack growth in concrete in this experiment. 4. Conclusions (1) UHPC-CA has a higher original compressive strength than UHPC-no-CA. SF/CA and HF/no-CA exhibit the better resistance to high temperature than other UHPCs in compression. (2) Ultra-high performance concrete without coarse aggregate (UHPC-no-CA) encountered a little more severe explosive spalling than the ultra-high performance concrete with coarse aggregate (UHPC-CA), which is attributed to the fact that coarse aggregates can reduce the inner thermal stresses inside UHPC-CA. Steel fiber-reinforced UHPC specimens seriously spalled, while the hybrid fiber-reinforced UHPC specimens exhibited a better resistance to explosive spalling.

This manuscript is approved by all authors for publication and no conflict of interest exits in the submission of this manuscript. We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. The work described was original research and has not been published in any journals previously, and not under consideration for publication elsewhere. Acknowledgements The authors gratefully acknowledge the financial support of the National Science Foundation of China (Grant Nos. 51878032 and 51278048) and the Natural Science Foundation of Beijing (Grant Nos. 8172036 and 8192033). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.conbuildmat.2019.07.299. References [1] Z.Y. Huang, S.G. Li, Study on mechanical properties of ultra-high performance concrete with coarse aggregate, J. Hunan Univ. (Nat. Sci.) 45 (3) (2018) 47–54. [2] K.M. Ng, C.M. Tam, V.W.Y. Tam, Studying the production process and mechanical properties of reactive powder concrete: a Hong Kong study, Mag. Concr. Res. 62 (9) (2010) 647–654. [3] M. Kojima, K. Mitsui, M. Wachia, Application of 150 N/mm2 advanced performance composites to high-rise R/C building, in: in: Japan 8th

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