Thermal behavior of siliceous and ferro-siliceous sacrificial concrete subjected to elevated temperatures

    Thermal behavior of siliceous and ferro-siliceous sacrificial concrete subjected to elevated temperatures Hong-yan Chu, Jin-yang Jiang, Wei Sun, Mingzhong Zhang PII: DOI: Reference:

S0264-1275(16)30127-7 doi: 10.1016/j.matdes.2016.01.127 JMADE 1328

To appear in: Received date: Revised date: Accepted date:

3 November 2015 8 January 2016 26 January 2016

Please cite this article as: Hong-yan Chu, Jin-yang Jiang, Wei Sun, Mingzhong Zhang, Thermal behavior of siliceous and ferro-siliceous sacrificial concrete subjected to elevated temperatures, (2016), doi: 10.1016/j.matdes.2016.01.127

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ACCEPTED MANUSCRIPT Thermal behavior of siliceous and ferro-siliceous sacrificial concrete subjected to elevated temperatures

School of Material Science and Engineering, Southeast University, Nanjing 211189, China

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Hong-yan Chu1,2, Jin-yang Jiang1,2*, Wei Sun 1,2, Mingzhong Zhang3

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Jiangsu Key Laboratory of Construction Materials, Nanjing 211189, China

Department of Civil, Environmental and Geomatic Engineering, University College London,

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London WC1E 6BT, UK

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Abstract: Siliceous and ferro-siliceous sacrificial concrete (SC) are designed to reduce the leakage potential of radioactive materials in case of severe nuclear accidents. This paper

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presents an investigation on thermal behavior and damage evolution of SCs subjected to high temperatures. In this study, the microstructure, porosity, high-temperature integrity, mass loss,

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compressive strength, splitting tensile strength, and thermal diffusivity of SCs were investigated at different elevated temperatures up to 1000 ºC. Using ultrasonic testing

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technique, variations of ultrasonic pulse velocity (UPV) propagation in SCs exposed to different high temperatures were obtained. According to definition of damage, a relationship between damage of SC and UPV was derived, eventually concluding a correlation between the damage of SC and high temperatures that SC subjected to. It was found that, (1) the SCs designed have very good performances, and are suitable for use in practice; (2) with temperature increasing, the thermal diffusivity of SCs decreases continually, and the damage evolution of SCs can be described by a Weibull distribution model. Key words: sacrificial concrete; high temperature; porosity; mechanical properties; thermal *

Corresponding author. Tel.: +86-025-52090667; E-mail address: [email protected] (J.-y. Jiang) 1 / 43

ACCEPTED MANUSCRIPT diffusivity; damage 1. Introduction

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With ever-increasing energy and environmental crisis, nuclear power has been drawn more

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attention since it is cleaner and more efficient compared to fossil fuels. However, there are deficiencies, such as high project cost, problem of nuclear wastes disposal, and potential leakage of radioactive materials. The latest severe nuclear accident took place in

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Fukushima, Japan in 2011, which brought the safety of nuclear power plant (NPP) into public

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focus again. As a key component of the third generation of NPP, European Pressurized Water Reactor can substantially reduce the leakage potential of radioactive materials in case of

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severe nuclear accidents through the encasing function of siliceous and ferro-siliceous

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sacrificial concrete (SC) [1]. SCs have characteristics of both self-consolidating concrete

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(SCC) and high strength concrete (HSC). On the one hand, SCs should have high fluidity and deformability for the requirement of construction. On the other hand, the compressive

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strength of SCs is higher than 70 MPa at 28 day because of their low water/powder ratio. Thermal behavior of siliceous and ferro-siliceous SC should be in particular evaluated, as required by the increasing application of the SCs in NPPs and nuclear accident mitigation. In the case of fire or nuclear meltdown, concrete is subjected to elevated temperatures, which results in material deterioration [2]. The reasons for concrete degradation are summarized as follows: (1) chemical changes of the cement paste, such as dehydration of C －S－H and CH at approximately 150 ºC and 450 ºC, respectively, and decarbonation of CaCO3 at about 700 ºC [3-5]; (2) occurrence of thermal micro cracks owing to the incompatible deformation between aggregate and cement paste or thermal gradient [6]; (3) 2 / 43

ACCEPTED MANUSCRIPT occurrence of spalling due to pressure build-up [7], thermal stress [8], or a combination of them [2]. When concrete is exposed to high temperatures, there is a significant reduction for

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the strength of concrete (reviewed by [9]). Specifically, investigations on HSC or SCC have

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shown that both compressive strength [10-21] and tensile strength [5, 14, 22, 23] decline with the increase of temperature in residual strength testing. Comparative studies on the properties of HSC and normal strength concrete (NSC) have indicated that their strength is reduced in a

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similar pattern [5, 24]. According to Khoury [25], the constituents and mix proportions of

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concrete exposed to high temperatures have an effect on its residual strength. In general, HSC or SCC containing carbonate aggregate [6, 14, 16, 22] causes more thermal damage at high

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temperatures than that containing basalt, baritic, or siliceous aggregate due to decarbonation

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[26]. Nevertheless, the strength of HSC or SCC containing basalt aggregate [23], baritic

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aggregate [27], and siliceous aggregate [16] still declines sharply when subjected to elevated temperatures. To prevent spalling of HSC or SCC at high temperatures, the addition of

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polypropylene (PP) fibers which melt at about 170 ºC and thus can release the internal vapor pressure [7], has been proved to be an effective measure [12, 16, 22]. It has been testified that high temperatures affect a wide range of concrete properties, such as Poisson’s ratio [3, 28, 29], Young’s modulus [3, 4, 28-31], UPV [3, 29, 32], porosity [3, 20, 31, 34], and permeability [31, 33, 35, 36]. For HSC or SCC, research has indicated that an increase in temperature results in a decrease in Poisson’s ratio [3, 29], Young’s modulus [3, 29-31], UPV [3, 29], and in an increase in porosity [3, 31], permeability [31, 33, 35, 36]. In many of these studies, the maximum temperature does not exceed 800 ºC [3, 4, 20, 31, 36], and the permeability is measured by empirical formula depending on the injection pressure 3 / 43

ACCEPTED MANUSCRIPT [31]. An investigation by Kou et al. [20] reported the porosity of concrete at 25, 500, and 800 ºC, which does not show a complete evolution of porosity with the increase of temperature.

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The knowledge of how thermal properties change in siliceous and ferro-siliceous SC with the

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increased temperature is paramount for nuclear accident mitigation, but the thermal behaviors of SCs are not known when they are exposed to elevated temperatures. As a new material, information on microstructure, mechanical properties, thermal diffusivity, and damage

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evolution of SCs at high temperatures is rather limited and needs to be updated, driven by the

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extension service life of existing NPPs, by the construction of new NPPs, and by the increasing awareness of the leakage potential of radioactive materials.

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To give an answer to the above mentioned needs, a technical development project on the

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preparation of SCs has been recently completed at Southeast University (Nanjing, China),

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and the SCs designed will be used in TAISHAN NPP. According to the project requirements, core technical parameters of SCs are as follows: (1) the silica sand should contain SiO2 83

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wt.%, CaCO3 8.5 wt.%, and MgCO3 1 wt.%, and the chemical composition of iron ore is comprised of Fe2O3 85 wt.% and CaCO3 1 wt.%, respectively; (2) in the mixtures of SCs, the content of silica sand (including dry parts of admixtures and including additives) should reach 85% of total dry mass of siliceous SC, and the content of Fe2O3 SiO2 should be more than 59.3% of ferro-siliceous SC; (3) the slump flow of SCs is supposed to be in the range of 55 to 65 cm for fresh concretes; (4) after curing, the free water content and compressive strength should be less than 5%, and more than 30 MPa, respectively; (5) when exposed to high temperatures, the SCs should guarantee the integrity up to 1000 ºC. The paper aims to disclose the technical development project on the preparation of 4 / 43

ACCEPTED MANUSCRIPT siliceous and ferro-siliceous SC, and to publish the mixture proportions of SCs designed by our research group, based on which the thermal behaviors of SCs subjected to high

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temperatures were extensively investigated. To this end, the microstructure, porosity,

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high-temperature integrity, mass loss, compressive strength, splitting tensile strength, thermal diffusivity, and damage evolution of SCs were comprehensively studied up to 1000 ºC. 2. Materials

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2.1. Cement and mineral admixture

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The chemical composition of cement and mineral admixture used is presented in Table 1. The compressive strength of cement at 28 day was 62.8 MPa. Fly ash and silica fume were

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used as mineral addition in the experiment. Moreover, the fly ash used can be graded ClassⅠ

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according to the Chinese standard GB/T 1596―2005.

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2.2. Aggregate

The aggregates used in the paper were silica sand and iron ore supplied by Nuclear Science

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and Technology (Tongchang) Co., Ltd. The silica sand contained 99.88 wt.% SiO2, 0.058 wt.% CaCO3, and 0.062 wt.% MgCO3. The chemical composition of iron ore was Fe2O3, SiO2, and CaCO3 and their weight percent was 92.22%, 7.61%, and 0.17%, respectively. Therefore, both silica sand and iron ore fulfilled requirements of the project. 2.3. Superplasticizer A superplasticizer of polycarboxylate obtained from local supplier was utilized to gain a satisfactory fluidity of SC. Water-reducing rate, density, and air content of this superplasticizer were, 33.9%, 1050 kg/m3 at 20 ºC, and 3.8%, respectively. 2.4. Polypropylene fiber 5 / 43

ACCEPTED MANUSCRIPT Table 2 presents some physical and mechanical properties of PP fibers used in this investigation.

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2.5. Mixture proportions of SCs

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The mixes of siliceous and ferro-siliceous SC investigated in the paper are shown in detail in Table 3.

As shown in C1 mixture in Table 3, the slump flow and the dry mass proportion of silica

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sand (including dry parts of admixtures and including additives) were 58cm and 85.03%,

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respectively, both of which met the requirements of the project for siliceous SC. The slump flow and the content of Fe2O3 SiO2 in C2 mixture were 63cm and 72.07%, respectively, both

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of which also fulfilled the project requirements for ferro-siliceous SC.

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Based on the above mixture proportions, the specimens (shape: cubic; size: 100×100×100

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mm) of siliceous and ferro-siliceous SC were cast. After casting, the molds were covered with plastic sheets and cured 24 hours at ambient condition, and then the molds were removed,

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and the specimens were placed into a concrete curing room for curing over a span of 28 days with a temperature range of 21 1 ºC and relative humidity of above 95%. 80 cubic specimens were prepared for each mixture. 3. Experimental methodology 3.1. High temperatures test An electric furnace was used to heat specimens from ambient temperature (25 ºC) to target temperatures (200, 400, 600, 800, and 1000 ºC) with an average heating rate of 5 ºC/min. In order to guarantee homogeneous temperature throughout each specimen, the specimens were held at the target temperature for two hours in the furnace. After that, the specimens were 6 / 43

ACCEPTED MANUSCRIPT cooled to ambient temperature in the furnace. At each target temperature, 12 specimens were heated for each mixture. Note that the moisture in the specimens can freely escape from the

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furnace during the heating process.

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3.2. Microstructure

In order to analyze the microstructure of siliceous and ferro-siliceous SC subjected to high temperatures, scanning electronic microscopy (SEM) was undertaken.

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3.3. Porosity

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Mercury intrusion porosimetry (MIP) measurement was used to characterize the pore evolution of SCs quantitatively.

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3.4. Mass loss

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The free water contents of siliceous and ferro-siliceous SC were measured by placing the

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specimens inside a drying oven at 105 ºC for 20 days, and every other day the mass of the specimens was weighed. As for the mass loss of SCs, the mass of siliceous and ferro-siliceous

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SC was determined at ambient temperature and at each target temperature by three cubic specimens (about 25×25×25 mm) which was obtained from 100×100×100 mm specimens. It needs to be pointed out that those specimens were heated from ambient temperature (25 ºC) to 200, 400, 600, 800, and 1000 ºC in turn at an average heating rate of 5 ºC/min. After the specimens were weighed, they were placed into the drying oven or the furnace immediately, so as to avoid absorbing moisture in the air. 3.5. Mechanical tests The compressive strength and the splitting tensile strength of specimens for two mix proportions were measured by a universal testing machine with loading rate of 0.8 MPa/s and 7 / 43

ACCEPTED MANUSCRIPT the universal testing machine equipped with a splitting tensile setup at the loading rate of 0.08 MPa/s, respectively. For each test, 3 specimens were tested, and average values were

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reported.

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3.6. Thermal diffusivity

In steady or quasi-steady conditions, heat transmission via conduction is controlled by the thermal diffusivity that is the ratio of the heat transmitted to the heat stored by the unit mass

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of the material. The higher the thermal diffusivity, the lower the insulation capability. The

the specific heat, and

is density.

, where

is the thermal conductivity,

is

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thermal diffusivity is defined as:

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A thermal constant analyzer was used to determine the thermal conductivity and the

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specific heat of SCs subjected to high temperatures (200, 300, 400, 500, and 600 ºC), since

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600 ºC is the maximum temperature that the thermal constant analyzer can reach. Supposing that elevated temperatures have no impact on the volume of SCs, the density of SCs at 200,

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400, and 600 ºC could be obtained from its initial density. In this work, the density of SCs at 300 and 500 ºC was calculated simply by linear interpolation. 3.7. Damage

Damage was initiated after SC exposed to high temperature, which lead to a variation in UPV. According to definition of damage, the relationship between damage modulus

and Young’s

can be expressed as [37]: (1)

Assuming that SC is homogeneous material, the Young’s modulus longitudinal wave velocity

correlates its

of ultrasonic, and their relationship can be expressed by Eq. (2) 8 / 43

ACCEPTED MANUSCRIPT as follows [38], (2)

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is Poisson’s ratio.

is SC density, and

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where

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Supposing that high temperature has a negligible effect on the Poisson’s ratio of SC. According to Eq. (2), and then ,

(3)

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Substituting Eq. (3) into Eq. (1), the damage expression of SC can be concluded as

respectively.

are density of SC at ambient temperature and high temperatures, and

are UPV of SC at ambient temperature and high temperatures,

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respectively.

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and

(4)

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where

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follows,

The damage of siliceous and ferro-siliceous SC exposed to high temperatures can be

the UPV

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calculated through Eq. (4), as long as the initial UPV and the density

and the initial density

, and

after heating to high temperatures are determined via

experiment. In the actual measuring procedure, the experiment on UPV was implemented according to GB CECS02―88 [39]. 4. Results and discussion 4.1 Chemical composition of SCs The chemical composition of siliceous and ferro-siliceous SC is shown in Table 4. The content of Fe2O3 in ferro-siliceous SC was higher than that of hematite-containing concrete used in VULCANO experiment [40], while the content of SiO2 in the former was a 9 / 43

ACCEPTED MANUSCRIPT little bit lower than that of the latter. Furthermore, the content of Fe2O3 and SiO2 in ferro-siliceous SC were higher than those of hematite-containing concrete utilized in

composition

(Fe2O3 and

SiO2)

of

ferro-siliceous

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similar

to

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chemical

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HECLA-5 experiment [41], respectively. These results indicated that the content of key

hematite-containing concrete used in VULCANO experiment, but is slightly higher than that utilized in HECLA-5 experiment. In addition, the chemical compositions of SCs are

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important input data for MCCI calculations [40-42].

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4.2. Microstructure

SEM investigations illustrated distinct changes in the morphology of siliceous and

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ferro-siliceous SC subjected to elevated temperatures. Fig.1 presents SEM micrographs of

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specimens before and after exposure to 200, 400, 600, 800, and 1000 ºC.

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The matrices of SCs represented a continuous structure without micro cracks and pores, and the PP fibers could be clearly found in the matrices at ambient temperature (25 ºC), while emerged at 200 ºC. A

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two long channels left by molten PP fibers with diameter about 30

small amount of micro cracks was observed at 400 ºC, while a fairly large number of micro cracks emerged in the specimens with the temperature up to 600 ºC (Fig.1a C1―600 ºC). At 800 ºC, a rather high density of cracks was detected, and a micro crack appeared in the siliceous aggregate (Fig.1a C1―800 ºC) due to the volume change of aggregate [43]. When specimens heated up to 1000 ºC, the matrices of siliceous and ferro-siliceous SC became amorphous structure, and connected cracks spread all over the specimens. Compared to the results obtained in references [3] and [35], the matrix of SC is more compact than that of NSC ([35]), but is similar to that of HSC ([3]) after exposure to elevated temperatures. 10 / 43

ACCEPTED MANUSCRIPT Both the cement paste and the aggregate of SCs were affected by high temperature, which resulted from pressure build-up due to moistures and carbon dioxide release [7], and form

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thermal stress due to thermal gradient [6] or thermal incompatibility [8]. SEM micrographs

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indicated the influence of elevated temperatures on the microstructures of siliceous and ferro-siliceous SC. With the increase of temperature, the microstructures were highly damaged leading to the deterioration of SCs.

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4.3. Porosity

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For the two mixtures, the porosity increased continually with the increase of temperature, which is accordant with HSC subjected to high temperatures [44]. The porosity of SC is

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lower than that of NSC (compared to [34]), and is similar to that of HSC (compared to [44])

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after subjecting to elevated temperatures. This increasing porosity of SCs can be attributed to

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the loss of adsorbed water in the capillary pores and bound water of the hydration products [45], and cracks was resulted from incompatible deformation between cement paste and

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aggregate [8], and the channels were left by the molten PP fibers. As shown in Fig.2a, in the range of 25―400 ºC, the porosity increased slowly, and the increasing amplitudes of siliceous and ferro-siliceous SC at 400 ºC were 23.55% and 23.82%, respectively. However, the porosity increased rapidly in the range of 400-1000 ºC, and the increasing amplitudes of SCs at 1000 ºC were 212.70% and 147.40%, respectively, the results of which were consistent with the increasing cracks in the microstructures of SCs during the same temperature range. In the whole range of 25―1000 ºC, the porosity increasing amplitudes of siliceous and ferro-siliceous SC were 289.00% and 206.40%, respectively. In addition, the porosity of siliceous SC was lower than that of ferro-siliceous SC at any 11 / 43

ACCEPTED MANUSCRIPT target temperature. On the one hand, the water/ powder ratio of the former was lower than the latter (Table 3), which indicated the microstructure of the former was denser than the latter at

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ambient temperature. On the other hand, the porosity increasing amplitudes for both siliceous

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and ferro-siliceous SC were basically the same when they were exposed to elevated temperatures.

As for pore throat size distribution, there were multi peaks in the range of 25―1000 ºC,

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and the threshold pore diameter increased with the increase of temperature (Fig.2b, c). In

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siliceous SC (Fig.2b), most of the pores throat size at 25, 400, and 1000 ºC were within two peaks interval (0.02, 0.04), (0.03, 1.61), and (0.23, 24.18) respectively, all in

. A similar

at 400 ºC, to 0.46―30.22

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at 25 ºC, to 0.08―2.08

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trend was observed in ferro-siliceous SC (Fig.2c), but to a larger degree, from 0.03―0.05 at 1000 ºC. These results

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could be used to explain why the porosity of SC increased slowly in the range of 25―400 ºC, while the porosity increased rapidly during 400―1000 ºC.

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4.4. High-temperature integrity

During the elevated temperatures (25―1000 ºC) test, no spalling was observed for all the specimens of siliceous and ferro-siliceous SC, which indicated that the SCs could guarantee the integrity up to 1000 ºC, and could meet the demand of the project. Furthermore, the absence of spalling is consistent with observations with MCCI [42]. Fig.3 presents the photographs of SCs before and after elevated temperatures treatment. The red color of the ferro-siliceous SC was caused by iron ore, and the red color faded gradually due to high temperatures. 4.5. Mass loss 12 / 43

ACCEPTED MANUSCRIPT For both C1 and C2 mixtures, the stabilized mass was reached in 20 days (Fig.4a) with a mass loss of 4.12% and 4.45%, respectively, which indicated the free water contents of two

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mixes. These results suggested that the free water contents of siliceous and ferro-siliceous SC

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could meet the requirements of the project. The mass loss of free water content of siliceous SC was always lower than that of ferro-siliceous SC, the reason of which was that the initial water content of the former was lower than the latter (Table 3).

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As for mass evolution at high temperature test, Fig.4b presents the mass loss of siliceous

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and ferro-siliceous SC as a function of temperature. It was observed that the mass evolution was very similar for the two studied concretes, the reason of which was that the initial water

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contents in two mixtures were nearly the same (6.35% for siliceous SC, and 6.65% for

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ferro-siliceous SC). Between the ambient temperature and 200 ºC, a significant increase in

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mass loss corresponding to around 3.70% of the initial mass could be found. This result is in line with published literature [31]. The loss of mass in this range is due to the departure of

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free water and part bound water in SCs. Assuming that elevated temperature has a negligible impact on the volume of SCs, and then the evolution of their density can be calculated from their initial density. The density evolution of siliceous and ferro-siliceous SC is presented in Fig.4c. The total decrease in density of two mixes was around 7.00%. The decrease in density of HSC in the range of 105―400 ºC has been investigated by Kalifa et al. [46], and the results obtained in the paper are similar with this published data. The decrease in density is mainly due to the departure of water, the dehydration of hydration products, and the thermal expansion of concrete [2]. However, the density evolution of SCs is also correlated to their porosity evolution since all 13 / 43

ACCEPTED MANUSCRIPT these phenomena are related. 4.6. Mechanical properties

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The normalized plots of the compressive strength and the splitting tensile strength of SCs

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are reported in Fig.5.

The compressive strength of SCs decreased at a steady rate (the slopes of curves in Fig.5a) as the increase of temperature, which is consistent with the results of SCC with PP fibers

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subjected to high temperatures [15, 31, 33]. This could be interpreted by the increasing

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porosity for SCs in the same temperature range. The compressive strength of siliceous and ferro-siliceous SC at ambient temperature were 86.23 and 77.82 MPa, both of which were

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much higher the required 30 MPa in the project. The compressive strength of siliceous SC

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was higher than that of ferro-siliceous SC in the range of 25―600 ºC, while the strength of

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the former was slightly lower than the latter at 800 and 1000 ºC. In addition, the residual compressive strength of SC is still higher than that of NSC (compared to [28]), and is similar

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to that of HSC (compared to [18]) after exposure to high temperatures. Similar to the compressive strength, the splitting tensile strength of SCs also declined continually with the temperature increasing, while a rapidly decreasing process was observed in the range of 200―600 ºC, which was in agreement with that the porosity of SCs increased in the range of 25―1000 ºC. The changing trends and magnitudes of the splitting tensile strength of SCs are in line with the results of NSC and HSC exposure to elevated temperatures [16, 18, 22, 29, 33]. The splitting tensile strength of siliceous SC was higher than that of ferro-siliceous SC at ambient temperature and 200 ºC, while the strength of the former was a bit lower than the latter in the range of 400―1000 ºC. 14 / 43

ACCEPTED MANUSCRIPT Furthermore, the normalized compressive strength of SCs at 400 and 600 ºC was about 0.7 and 0.4, respectively, but their normalized splitting tensile strength at the same temperatures

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was approximately 0.5 and 0.3, respectively. These results suggested that the splitting tensile

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strength of SCs was more temperature sensitive than the compressive strength. At ambient temperature, the strength (both compressive and splitting tensile strength) of siliceous SC was higher than that of ferro-siliceous SC, which was because the water/powder

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ratio of the former was lower than that of the latter. The decrease in strength of SCs can be

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attributed to the damaged microstructure (Fig.1), the increased porosity (Fig.2), and the induced thermal damage.

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4.7. Thermal diffusivity

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According to Fig.4c, the density of SCs at 200, 300, 400, 500, and 600 ºC could be

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determined, the result of which is shown in Table 5. The thermal conductivity and the specific heat of SCs exposed to the same temperature were obtained simultaneously by the thermal

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constant analyzer, and are presented in Table 6. Based on these results, the thermal diffusivity of SCs could be determined, which is also shown in Table 6, and is plotted in detail in Fig.6. As illustrated in Fig.6, the thermal diffusivity of SCs decreased continually with the increase of temperature, while a rapidly decreasing process was observed in the range of 200―500 ºC. These changing trends of SCs are similar to those of Xing et al. [34], although the maximum temperature in this study is 300 ºC. According to the definition of the thermal diffusivity:

, where

is the thermal conductivity,

is the specific heat, and

is density. The thermal diffusivity is directly proportional to the thermal conductivity, but inversely proportional to the specific heat and the density. Since the density of SCs did not 15 / 43

ACCEPTED MANUSCRIPT change a lot (Table 5), and the thermal conductivity of SCs decreased with the increase of temperature (Table 6), and the specific heat of SCs increased as the increasing of temperature

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(Table 6), thus the thermal diffusivity of SCs declined with the increase of temperature.

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In addition, the thermal diffusivity of siliceous SC was higher than that of ferro-siliceous SC in the whole range of 200―600ºC. As shown in Table 6, the thermal conductivity and the specific heat of siliceous SC were always higher than those of ferro-siliceous SC during

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200―600 ºC, and the former was on average 16.64% and 24.39% higher than the latter,

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respectively. Besides, the density of ferro-siliceous SC was constantly bigger than that of siliceous SC during each target temperature (Table 5), and the former was on average 14.83% than

the

latter.

Therefore,

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bigger

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, that is, the thermal diffusivity of siliceous SC was higher than that of

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ferro-siliceous SC in the whole range of 200―600 ºC. With the increase of temperature, the loss of thermal diffusivity is due to the departure of

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water, the destruction of conductive bonds caused by decomposition of hydration production, and the thermal cracks [2, 34]. Compared to literature [34], the thermal diffusivity of SC is lower than that of NSC, but is nearly the same as that of HSC in the range of 25―300 ºC, the results of which indicate the SC has better insulation property than that of NSC, while is similar to that of HSC. 4.8. UPV The normalized UPV of siliceous and ferro-siliceous SC before and after heating to high temperatures is presented in Fig.7. As shown in Fig.7, the UPV of SCs decreased monotonically with the increase of 16 / 43

ACCEPTED MANUSCRIPT temperature, which is in accordance with the results of literature [3]. An accelerating process in the decrease of UPV of SCs was observed in the range of 200 ―600 ºC, which

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corresponded with the changing trends of the splitting tensile strength (Fig.5b) and the

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thermal diffusivity (Fig.6), but did not matched well with that of the compressive strength (Fig.5a) and the porosity (Fig.2a). This result suggested that utilizing UPV to evaluate the degradation of SCs is, in a way, feasible. In the range of 800―1000 ºC, the UPV of SCs did

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not change much. The decline of UPV with the increase of temperature is related to the

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damaged microstructure (Fig.1), the increased porosity (Fig.2a), the decreased thermal diffusivity (Fig.6), and the induced thermal damage.

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The UPV of siliceous SC was higher than that of ferro-siliceous SC in the whole range of

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25―1000 ºC, because the water/ powder ratio of the former was lower than the latter (Table 3)

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leading to that the microstructure of the former was denser than the latter at 25 ºC, and because the microstructures of two mixes were damaged in the same degree when they were

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subjected to high temperatures. In addition, the porosity of the former was constantly lower than that of the latter (Fig.2), which was another aspect to verify the phenomenon. 4.9. Damage

Damage of siliceous and ferro-siliceous SC subjected to high temperatures is shown in Fig.8. As illustrated in Fig.8, with the increase of temperature, the damage of SCs flattened out gradually after a sharp rise. Furthermore, the damage of siliceous SC was smaller than that of ferro-siliceous SC in the whole range of 25―1000 ºC, which was consistent with that the porosity of the former was lower than that of the latter at any temperature (Fig.2a). 17 / 43

ACCEPTED MANUSCRIPT When SCs were exposed to elevated temperatures, the damage evolution of them was accordant, and could be described by a Weibull distribution model (the full curve in Fig.8).

where

is the damage of SCs, and

(5)

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The Weibull distribution model could be expressed by an equation as follows,

is the temperature that SCs subjected to.

The R-Square of the Weibull distribution fitting was 0.9981, which indicated that the

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fitting result agreed very well with the experimental data, and that Eq. (5) could be applied to

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characterize the damage evolution of SCs subjected to high temperatures. It should be noted that the damage of SCs increased rapidly in the range of 200―600 ºC, which matched well

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with the changing trends of the splitting tensile strength, the thermal diffusivity, and the UPV

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of SCs in the same range, but accorded not so well with the porosity and the compressive

CE P

strength during the same temperature. These findings might slightly jeopardize the damage evaluation. On the whole, however, the model established in the paper was reasonably precise

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to describe the damage evolution of SCs exposed to elevated temperatures. In practice, the established model can be utilized in damage evaluation of SCs and assessment of fire hazard or nuclear accident. It needs to be pointed out that the empirical formula may not accurately capture the behavior of SCs with different compositions (e.g. different aggregate types). Furthermore, the correlation between the damage and the residual mechanical properties (both compressive and splitting tensile strength) of SCs is presented in Fig.9, which indicated that the relationship between the damage and the residual splitting tensile strength is better than that between the damage and the residual compressive strength. In a word, siliceous and ferro-siliceous SCs are shown to have more compact matrix, lower 18 / 43

ACCEPTED MANUSCRIPT porosity, higher residual compressive strength, similar residual splitting tensile strength, and better insulation properties than those of NSC (compared to literatures [18], [28], [34], [35]),

IP

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when subjected to elevated temperature. On the whole, however, the high temperature

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performance of SCs is similar to that of high-quality HSC (as indicated in references [3], [16], [18], [22], [29], [34], [44]). 5. Conclusions

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In the work, the technical development project on the preparation of siliceous and

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ferro-siliceous SC is disclosed, and the thermal behaviors of SCs subjected to high temperatures are comprehensively investigated. The main conclusions are summarized as

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follows,

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1) The siliceous and ferro-siliceous SC designed by our research group can fulfill all the

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technical parameters of the project, and can be applied to TAISHAN NPP. 2) As illustrated in SEM micrographs, both the cement paste and the aggregate of SCs are

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affected by high temperature, and the microstructures of SCs are highly damaged due to elevated temperatures. 3) For siliceous and ferro-siliceous SC, the porosity increases continually with the increase of temperature, and the porosity of the former is lower than that of the latter in the whole range of 25―1000 ºC. 4) The free water content of the siliceous and ferro-siliceous SC is 4.12% and 4.45%, respectively. The mass evolution is very similar for the two studied SCs, and the total decrease in density of two mixes is around 7.00%. 5) Both the compressive strength and the splitting tensile strength of SC decrease 19 / 43

ACCEPTED MANUSCRIPT monotonically with the increase of temperature, but the splitting tensile strength of SCs is more temperature sensitive than the compressive strength.

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6) The thermal diffusivity of SCs decreases continually with the increase of temperature,

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while a rapidly decreasing process is observed in the range of 200―500 ºC. The thermal diffusivity of siliceous SC is higher than that of ferro-siliceous SC in the range of 200― 600 ºC.

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7) With the increase of temperature, the UPV of SCs decreases monotonically, and an

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accelerating process in the decrease of UPV is observed in the range of 200―600 ºC. The UPV of siliceous SC is bigger than that of ferro-siliceous SC at any temperature.

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8) With temperature increasing, the damage of SCs flattens out gradually after a sharp rise,

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and can be described by a Weibull distribution model. In practice, the established model

accident.

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can be utilized in damage evaluation of SCs and assessment of fire hazard or nuclear

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Acknowledgements

The investigation is finically supported by China National Natural Science Funding Committee (No. 51378114), National Basic Research Program of China (973 program, 2015CB655105), Transformation of Major Scientific and Technological Achievements of Jiangsu Province Funded Projects (No. 85120000220), and Scientific and Technological Research and Development Plan of China Railway Corporation (No. 2013G001-A-2), which are gratefully appreciated. References [1] Konings RJM, Allen TR, Stoller RE, et al. Comprehensive nuclear materials. Amsterdam: 20 / 43

ACCEPTED MANUSCRIPT Elsevier; 2012. [2] Bazant ZP, Kaplan MF. Concrete at High Temperatures: Materials Properties and

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Mathematical Models. London: Longman Group Limited; 1996.

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[3] Luzio GD, Biolzi L. Assessing the residual fracture properties of thermally damaged high strength concrete. Mech Mater 2013; 64: 27-43.

[4] Pan Z, Sanjayan JG, Collins F. Effect of transient creep on compressive strength of

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geopolymer concrete for elevated temperature exposure. Cem Concr Res 2014; 56:

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182-189.

[5] Chan YN, Peng GF, Anson M. Residual strength and pore structure of high-strength

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Compos 1999; 21: 23-27.

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concrete and normal strength concrete after exposure to high temperatures. Cem Concr

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[6] Phan LT, Carino NJ. Effects of test conditions and mixture proportions on behavior of high-strength concrete exposed to high temperatures. ACI Mater J 2002; 99 (1) 54-66.

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[7] Castillo C, Durrani AJ. Effect of transient high temperature on high-strength concrete. ACI Mater J 1990; 87(1):47-53. [8] Ulm FJ, Coussy O, Bazant ZP. The chunnel fire: chemoplastic softening in rapidly heated concrete. J Eng Mech 1999; 125(3):272-282. [9] Ma QM, Guo RX, Zhao ZM, et al. Mechanical properties of concrete at high temperature—A review. Constr Build Mater 2015; 93:371-383. [10] Poon CS, Azhar S, Anson M, Wong YL. Comparison of the strength and durability performance of normal- and high-strength pozzolanic concretes at elevated temperatures. Cem Concr Res 2001; 31(9):1291-1300. 21 / 43

ACCEPTED MANUSCRIPT [11] Bamonte P, Monte FL. Reinforced concrete columns exposed to standard fire: Comparison among different constitutive models for concrete at high temperature. Fire

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Saf J 2015; 71: 310-323.

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[12] Poon CS, Shui ZH, Lam L. Compressive behavior of fiber reinforced high-performance concrete subjected to elevated temperatures. Cem Concr Res 2004; 34:2215-2222. [13] Gernay T, Franssen JM. A plastic-damage model for concrete in fire: Applications in

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structural fire engineering. Fire Saf J 2015; 71: 268-278.

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[14] Chen B, Liu J. Residual strength of hybrid-fiber-reinforced high-strength concrete after exposure to high temperatures. Cem Concr Res 2004; 34:1065-1069.

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[15] Le HT, Muller M, Siewert K, et al. The mix design for self-compacting high

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performance concrete containing various mineral admixtures. Mater Des 2015;

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72:51-62.

[16] Xiao J, Falkner H. On residual strength of high-strength concrete with and without polypropylene fibers at elevated temperatures. Fire Saf J 2006; 41(2):115-121.

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[17] Zhai Y, Deng ZC, Li N, et al. Study on compressive mechanical capabilities of concrete after high temperature exposure and thermo-damage constitutive model. Constr Build Mater 2014; 68: 777-782. [18] Suhaendi SL, Takashi H. Effect of short fibers on residual permeability and mechanical properties of hybrid fibre reinforced high strength concrete after heat exposition. Cem Concr Res 2006; 36:1672-1678. [19] Uygunoglu T, Topcu IB. Thermal expansion of self-consolidating normal and lightweight aggregate concrete at elevated temperature. Constr Build Mater 2009; 23: 3063-3069.

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ACCEPTED MANUSCRIPT [20] Kou SC, Poon CS, Etxeberria M. Residue strength, water absorption and pore size distributions of recycled aggregate concrete after exposure to elevated temperatures.

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Cem Concr Compos 2014; 53: 73-82.

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[21] Akcaozoglu K, Fener M, Akcaozoglu S, et al. Microstructural examination of the effect of elevated temperature on the concrete containing clinoptilolite. Constr Build Mater 2014; 72: 316-325.

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[22] Behnood A, Ghandehari M. Comparison of compressive and splitting tensile strength of

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high-strength concrete with and without polypropylene fibers heated to high temperatures. Fire Saf J 2009; 44:1015-1022.

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[23] Li M, Qian CX, Sun W. Mechanical properties of high-strength concrete after fire. Cem

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Concr Res 2004; 34: 1001-1005.

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[24] Chan SYN, Peng GF, Chan JKW. Comparison between high strength concrete and normal strength concrete subjected to high temperature. Mater Struct 1996; 29: 616-619.

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[25] Khoury GA. Effect of fire on concrete and concrete structures. Progr Struct Eng Mater 2001; 2: 429-447. [26] Chen LJ, He J, Chao JQ, et al. Swelling and breaking characteristics of limestone under high temperatures. Min Sci Technol 2009; 503-507. [27] Monte FL, Gambarova PG. Thermo-mechanical behavior of baritic concrete exposed to high temperature. Cem Concr Compos 2014; 53: 305-315. [28] Bahr O, Schaumann P, Bollen B, et al. Young’s modulus and Poisson’s ratio of concrete at high temperatures: Experimental investigations. Mater Des 2013; 45:421-429. [29] Heap MJ, Lavallee Y, Laumann A, et al. The influence of thermal-stressing (up to 1000 23 / 43

ACCEPTED MANUSCRIPT ºC) on the physical, mechanical, and chemical properties of siliceous-aggregate, high-strength concrete. Constr Build Mater 2013; 42:248-265.

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[30] Andic-Cakir O, Hizal S. Influence of elevated temperatures on the mechanical properties

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and microstructure of self consolidating lightweight aggregate concrete. Constr Build Mater 2012; 34:575-583.

[31] Fares H, Noumowe A, Remond S. Self-consolidating concrete subjected to high

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temperature mechanical and physicochemical properties. Cem Concr Res 2009;

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39:1230-1238.

[32] Omer SA, Demirboga R, Khushefati WH. Relationship between compressive strength

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and UPV of GGBFS based geopolymer mortars exposed to elevated temperatures.

El-Dieb

AS.

Mechanical,

durability

and

microstructural

characteristics

of

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[33]

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Constr Build Mater 2015; 94:189-195.

ultra-high-strength self-compacting concrete incorporating steel fibers. Mater Des 2009;

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30:4286-4292.

[34] Xing Z, Beaucour A-L, Hebert R, et al. Aggregate’s influence on thermophysical concrete properties at elevated temperature. Constr Build Mater 2015; 95:18-28. [35] Jalal M, Mansouri E, Sharifipour M, et al. Mechanical, rheological, durability and microstructural properties of high performance self-compacting concrete containing SiO2 micro and nanoparticles. Mater Des 2012; 34:389-400. [36] Bosnjak J, Ozbolt J, Hahn R. Permeability measurement on high strength concrete without and with polypropylene fibers at elevated temperatures using a new test setup. Cem Concr Res 2013; 53: 104-111. 24 / 43

ACCEPTED MANUSCRIPT [37] Murakami S. Continuum damage mechanics: a continuum mechanics approach to the analysis of damage and fracture. Berlin: Springer; 2012.

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[38] Meyers MA. Dynamic behavior of materials. New York: Wiley Interscience; 1994.

Architecture & Building (in Chinese); 2005.

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[39] China Association for Engineering Construction Standardization. Beijing: China

[40] Sevon T, Journeau C, Ferry L. VULCANO VB-U7 experiment on interaction between

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oxidic corium and hematite-containing concrete. Ann Nucl Energy 2013; 59: 224-229.

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[41] Sevon T, Kinnunen, Virta J, et al. HECLA experiments on interaction between metallic melt and hematite-containing concrete. Nucl Eng Des 2010; 240:3586-3593.

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[42] Journeau C, Bonnet JM, Boccaccio E, et al. European experiments on 2-D molten core

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concrete interaction: HECLA and VULCANO. Nucl Technol ANS 2010; 170:189-200.

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[43] Akca AH, Zihnioglu NO. High performance concrete ueder elevated temperatures. Constr Build Mater 2013; 44: 317-328.

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[44] Pliya P, Beaucour A-L, Noumowe A. Contribution of cocktail of polypropylene and steel fibres in improving the behaviour of high strength concrete subjected to high temperature. Constr Build Mater 2011; 25:1926-1934. [45] Galle C, Sercombe J. Permeability and pore structure evolution of silicocalcareous and hematite high-strength concretes submitted to high temperatures. Mater Struct 2001; 34: 619-628. [46] Kalifa P, Menneteau F-D, Quenard D. Spalling and pore pressure in HPC at high temperatures. Cem Concr Res 2000; 30:1915-1927.

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CE P

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D

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SC R

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ACCEPTED MANUSCRIPT

(a)

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CE P

TE

D

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SC R

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(b) Fig.1. SEM micrographs for SCs specimens before and after exposure to different temperatures: (a) siliceous SC and (b) ferro-siliceous SC

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ACCEPTED MANUSCRIPT

C1 C2

20

At 25C C1: 5.5777% C2: 8.5147%

10 5 0 0

200

T

At 1000C C1: 21.6999% C2: 26.0875%

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15

400

600

SC R

Porosity/(%)

25

800

1000

Temperature/(C)

0.03

TE

0.02

Log differential intrusion/(mL/g)

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C1

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0.04

25C 200C 400C 600C 800C 1000C

D

0.05

0.01

0.00 1E-3

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Log differential intrusion/(mL/g)

0.06

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(a)

0.08

0.06

0.04

0.01

0.1

1

10

100

1000

Pore throat size diameter/(m)

(b)

25C 200C 400C 600C 800C 1000C

C2

0.02

0.00 1E-3

0.01

0.1

1

10

100

1000

Pore throat size diameter/(m)

(c) Fig.2. MIP results of SCs specimens after heating to different temperatures 28 / 43

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ACCEPTED MANUSCRIPT

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Fig.3. Photographs of SCs before and after elevated temperature treatment

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ACCEPTED MANUSCRIPT

5 C1 C2

IP

T=105C

2

After 20 days drying: C1=4.12%, C2=4.45%

1 0 0

5

10

15

t/(day)

7.5

20

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(a)

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C1 C2

6.0 4.5 3.0

After heating to 1000C: C1=6.79%,C2=7.09%

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Mass loss/(%)

T

3

SC R

Mass loss/(%)

4

TE

1.5 0.0

200

400

600

800

1000

Temperature/(C)

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0

(b)

C1 C2

 



1.00 0.95



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1.05

0.90

25C1=2348 kg/m3 25C2=2706 kg/m3

0.85 0.80 0

200

400

600

800

1000

Temperature/(C)

(c) Fig. 4. Mass loss and density evolution of SCs: (a) mass loss due to free water expulsion, at 105 ºC, as a function of time, (b) mass loss at high temperatures, as a function of temperature, and (c) density evolution as a function of temperature 30 / 43

ACCEPTED MANUSCRIPT

1.0

C1 C2

T IP

0.6 0.4

f25 = 86.23 MPa cC1 f25 = 77.82 MPa cC2

0.2 0.0

200

400

600

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0

SC R

fTc/f25 c

0.8

800

1000

Temperature/(C)

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(a)

C1 C2

TE

1.0 0.8

fTs/f25 s

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0.6 0.4

f25 = 4.862 MPa sC1

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0.2

f25 = 4.614 MPa sC2

0.0 0

200

400

600

800

1000

Temperature/(C)

(b) Fig. 5. Normalized plots of the residual mechanical strength of SCs at ambient and after exposure to high temperatures: (a) the compressive strength and (b) the splitting tensile strength

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a25 =1.203 C1

1.2

C1 C2

25 =1.085 C2

a

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1.0

0.8

a600 =0.5628 C1

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Thermal diffusivity/(mm2/s)

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a600 =0.5439 C2

0.6

0.4 300

400

500

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200

600

Temperature/(C)

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Fig. 6. Thermal diffusivity of SCs after exposure to elevated temperatures, as a function of

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temperature

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ACCEPTED MANUSCRIPT

1.0

C1 C2

T IP

0.6

SC R

vTu/v25 u

0.8

0.4

v25 = 4.832 km/s uC1

0.2 0.0 0

200

400

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v25 = 4.445 km/s uC2 600

800

1000

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Temperature/(C)

Fig. 7. Plots of the normalized ultrasonic pulse velocity of SCs at ambient and after heating to

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high temperatures, as a function of temperature

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ACCEPTED MANUSCRIPT

1.0

T

C1 C2 Weibull fitting

0.4 0.2 0.0 0

200

400

600

IP

0.6

SC R

Damage

0.8

800

1000

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Temperature/(C)

Fig. 8. Damage evolution of SCs exposure to high temperatures and fitting of Weibull

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distribution, as a function of temperature

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ACCEPTED MANUSCRIPT

1.0

T IP

0.6

C1-fc C1-fs C2-fc C2-fs

0.4 0.2 0.0 0.0

0.2

0.4

0.6

0.8

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Damage

SC R

fT/f25

0.8

1.0

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CE P

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D

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Fig. 9. The relationship between the residual mechanical properties and damage of SCs

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ACCEPTED MANUSCRIPT Table 1 The chemical composition and physical properties of cement and mineral admixture Cement Fly ash Silica fume

Chemical composition

Weight percentage (%)

CaO

64.70

SiO2

20.40

Al2O3

4.70

Fe2O3

3.38

5.91

0.85

MgO

0.87

2.60

0.74

1.88

1.32

0.50

0.83

1.61

CE P

TE

D

K2O

Loss

IP 0.77

SC R

8.38 47.96

96.18

30.46

0.96

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SO3

Na2O

T

Materials

1.76 3.24

Physical properties 3.15

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Specific gravity

Specific surface (m2/kg) 362.20

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2.31

2.22 2.79 104

ACCEPTED MANUSCRIPT Table 2 Some physical and mechanical properties of PP fiber PP fiber

Purity (%)

100

Length (mm)

12

Diameter (μm)

18

Density (kg/m3)

910

IP

169

Burning temperature (oC)

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590

Young’s modulus (MPa)

>3500 386 16

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Tensile strength (MPa) Elongation at yield (%)

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Melting temperature (oC)

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Technical specification

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ACCEPTED MANUSCRIPT Table 3 Mix proportions and fresh properties of siliceous and ferro-siliceous SC Siliceous SC (C1)

Fly ash (kg/m3)

182

Silica fume (kg/m3)

20

Silica sand 0－5 mm (kg/m3)

935

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Silica sand 5－8 mm (kg/m3)

735

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Iron ore 0－4 mm (kg/m3) Iron ore 4－8 mm (kg/m3) Water (kg/m3)

338

T

331

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Cement (kg/m3)

Ferro-siliceous SC (C2)

SC R

Mixture

135 20 594 466 764 216 181

7.20

7.90

1.00

1.30

Water/cement

0.45

0.54

Water/powder

0.28

0.37

58

63

Unit weight (kg/m3)

2356

2715

Air content (%)

2.20

2.30

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PP fibers (kg/m3)

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Superplasticizer (kg/m3)

Slump flow (cm)

D

150

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ACCEPTED MANUSCRIPT

CaO

SiO2

Al2O3

Fe2O3

H2O

C1

9.72

78.01

3.01

NM

3.31

C2

8.45

44.49

2.10

T

Mixture

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Table 4 The chemical composition of siliceous and ferro-siliceous SC (wt.%)

SC R

33.91

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NM means not measured. Elements with < 0.1 wt.% have been omitted.

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3.42

ACCEPTED MANUSCRIPT

200 ºC

300 ºC

400 ºC

500 ºC

600 ºC

C1

2260

2247

2233

2226

2219

C2

2598

2581

2564

2546

T

Mixture

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Table 5 The density of SCs at 200, 300, 400, 500, and 600 ºC (kg/m3)

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TE

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SC R

2555

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ACCEPTED MANUSCRIPT Table 6 The thermal parameters of SCs at 200, 300, 400, 500, and 600 ºC Thermal conductivity Specific heat

Thermal diffusivity

parameters

[W/(m·K)]

[J/(kg·K)]

(mm2/s)

C1

C2

C1

200

2.34

1.92

861.10

300

1.94

1.58

895.40

400

1.65

1.41

500

1.49

1.33

600

1.42

1.28

C2

1.20

1.09

726.90

0.96

0.85

987.90

785.50

0.75

0.70

1113

898.20

0.60

0.58

1137

921.40

0.56

0.54

685.10

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CE P

TE

D

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(ºC)

C2

SC R

C1

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Temperature

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T

Thermal

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Graphical abstract

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SC R

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ACCEPTED MANUSCRIPT

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ACCEPTED MANUSCRIPT Highlights The SCs designed have very good performance, and are suitable for use in practice.



The porosity of SCs increases continually with the increase of temperature.



The mechanical properties of SCs are highly deteriorated due to high temperature.



The thermal diffusivity of SCs displays a rapidly decreasing process in 200―500 ºC.



The damage evolution of SCs can be described by a Weibull distribution model.

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CE P

TE

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SC R

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

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