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Investigation of gamma ray shielding efficiency and physicomechanical performances of heavyweight concrete subjected to high temperature
Construction and Building Materials 195 (2019) 574–582
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Investigation of gamma ray shielding efficiency and physicomechanical performances of heavyweight concrete subjected to high temperature Elzbieta Horszczaruk ⇑, Piotr Brzozowski West Pomeranian University of Technology Szczecin, Faculty of Civil Engineering and Architecture, Szczecin, Poland
h i g h l i g h t s Effects of high temperature on the gamma ray shielding for HWC were investigated. The HWC with magnetite aggregate presented the best high-temperature resistance. The HWC with magnetite aggregate presented the best shielding properties. High temperature has little effect on the linear attenuation coefficient and HVL.
a r t i c l e
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Article history: Received 14 February 2018 Received in revised form 13 September 2018 Accepted 18 September 2018
Keywords: Gamma ray shielding Heavyweight concrete High temperature Compressive strength
a b s t r a c t The results of durability investigation of two heavyweight concretes containing barite and magnetite aggregates are presented in the paper. The concretes, designed for gamma-ray shielding, were subjected to high temperature from the range of 300–800 °C. The effect of temperature on the mass, apparent density, compressive strength, coefficient of gamma-ray absorption and the superficial changes in the structure of the tested specimens were evaluated. The results of testing were compared to the results obtained for the specimens made from the ordinary concrete containing natural aggregate. The influence of temperature on the linear attenuation coefficient has been observed, as well as the strong relation between occurring of the superficial defects in the structure and the properties of the aggregate under high temperature. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction The use of heavyweight concretes (HWC) is the most common and relatively cheapest method of shielding against the harmful gamma radiation. HWC are used in the construction of the nuclear objects and the medical infrastructure (radiology) particularly for two reasons: good strength and effective ability to protect against radiation [1–3]. The main challenges for the concrete used in the nuclear power plants are: the breakdowns inside the object (particularly involved with the damages of the heat exchanging systems), external accidents, like earthquakes or other unintentional situations (e.g. airplane disaster, terrorist attacks) and the phenomena caused by material ageing [4]. The aggregates of the high density, containing metal compounds (iron oxide, titanium iron oxide and barium sulphate),
⇑ Corresponding author at: West Pomeranian University of Technology in Szczecin, Faculty of Civil Engineering and Architecture, 70-311 Szczecin, Al. Piastów 50, Poland. E-mail address: [email protected] (E. Horszczaruk). https://doi.org/10.1016/j.conbuildmat.2018.09.113 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
are used for HWC production since they are able to slow down the fast neutrons [1,5–9]. While both barite and magnetite HWC have good gamma shielding efficiency [7–9] as it is strongly related to density, the shielding against neutrons is more complex. It is depending on the elementary (atomic) composition and it is proved that barite is not good aggregate for neutron radiation concrete shields and magnetite and hematite is [10–13]. However, the high temperature inside the reactor evidently worsens the concrete durability and its parameters with time. The internal shielding layer of the concrete is often subjected to the heat from the reactor. In the case of the nuclear electric plant as well as the other objects exposed to the radiation, the resistance to the high temperature is required. The structure fire is often in the emergency or exceptional situations, therefore, the problems of durability of the structural concretes under high temperature are very important, regardless of the object destiny. Concrete is more resistant to fire than other construction materials. It can resist the temperature up to 200 °C. The aggregate plays a significant role in providing the highest thermal resistance of the cement concretes [2–9] as it usually makes up from 60 to 80% of
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the concrete volume. The aggregates for the fire-resistant concretes should show the low mass losses, low thermal expansion coefficient and small deformations under high temperature [2,10]. The crushed aggregate (mainly from the igneous rocks) is preferred, due to the improved adhesion of the cement paste to the aggregate grain, as well as the aggregate with high content of the reactive silica [15]. In general, the natural aggregates, like river sands and gravels, are thermally stable within the range of temperature 500–600 °C. Some aggregates containing high amounts of the flint, however, are stable only up to 300–350 °C. The quartzite or granite aggregates have lower thermal resistance than the limestone or dolomite ones [3,5,11]. In the case of quartzite this is connected with the crystalline transition of quartz occurring at a temperature of about 573 °C (low-quartz to highquartz). This process causes the increase of the volume [11]. The limestone aggregates are destroyed at 700–900 °C as a result of decomposition of CaCO3 [2,5,13]. The thermal resistance of the cement matrix depends on many factors. The most important are: w/c (water to cement) ratio, C/S (calcium oxide to silicon oxide) ratio and the content of portlandite (CH) in the matrix. The cement paste with low C/S ratio, therefore with the low content of CH, allow for obtaining the heat-resistant cement matrix [3,9,13]. In general, the process of degradation of the cement matrix starts from the very beginning of the concrete heating, however, the changes observed up to 300 °C are relatively small or even reversible (re-hydration) [13]. The biggest changes of the matrix porosity, observed at 300–600 °C, are the results of dehydration of the hydrated calcium silicates of C-S-H type. Dehydroxylation of the portlandite Ca(OH)2 is completed up to 600 °C. Within the range 600–800 °C, the dehydration of the hydrated calcium silicates of C-S-H type continues and the partial decomposition of calcium carbonate takes place. The majority of researchers accept 1000 °C as the temperature of the total decomposition of calcium carbonate. Within the range of temperature 800–1000 °C, the bonds aggregate-matrix are totally broken and concrete losses its strength [10,13]. A number of researches [14–17] confirms that the most dangerous for the concrete durability is the range of temperature from 400 °C to 600 °C. Decreasing of concrete strength at the high temperature is connected to such phenomena as: macro- and micro-cracking, aggregate expansion and progressive dehydration of the cement paste. Concrete in this temperature range is 40–60% of its initial strength [1,18,19]. The dehydration of C-S-H itself is not as harmful for the concrete as the cracking of the matrix, which is the result of this process. The temperature 600 °C seems to be a critical for the concrete, causing the extreme growth of the cracks observed on the concrete surface, as well as the significant increae of porosity leading to the dramatic loss of concrete strength (loss of 80% of the strength) [14,18]. At the temperature above 800 °C concretes lose their bearing capacity regardless of the initial strength. This is connected to the irreversible changes in the cement matrix [19]. A number of factors should be taken into consideration when evaluating the properties of concrete in the structure exposed to high temperature. Among them are the material factors, like: age of concrete, moisture content, chemical composition of cement, water to cement ratio (w/c); external factors connected to the loading, like: time of exposition to temperature, rate of temperature rising and its extremal values, dimensions of the element and other external actions (e.g. ionizing radiation). Concrete has relatively low thermal conductivity; as a consequence, there is a problem with removing the heat from the shield [20]. Under moderately high temperature the temperature gradient causes migration of the moisture and possibility of occurring
of tri-axial stresses. This phenomenon in the case of HWC is explained same as in the case of ordinary concretes – by difference in the thermal expansibility of the various materials (aggregate and cement paste) and dehydration of the cement paste, leading to decrease of strength [21]. In recent years there is noted a growing number of researches focused on searching for the materials, which could enable the efficient protection against gamma rays. Part of these works deals with evaluation of usability of various types of aggregates used in the shielding concretes [5,22–25]. Also, the mineral concrete additions are considered, including the waste materials, like silica fumes [26], fly ashes and blast-furnace slag [27,28] or EAF steel slag [29]. Another direction of researches is use of heavy metals oxides, like PbO or BiO3, in the shielding concretes [30]. All these studies are carried out for the concretes cured and exploited within the range of temperature (20 ± 2 °C). Few researches of the shielding concretes subject to high temperature are focused mainly on description of the changes of mechanical properties and defects in the microstructure of these concretes [17,31–33]. The aim of the presented research was evaluation of durability of the shielding concretes, containing the typical heavyweight natural aggregates (used commonly in the concretes of this type), subjected to temperature from the range of 300–800 °C. The physico-mechanical properties of the concretes and their ability to absorb the gamma radiation after heating were analyzed. 2. Preparation of concrete samples 2.1. Materials Portland cement CEM I 42.5R according to EN 197-1 [34] was used for preparation of the concrete specimens. Three various aggregates were used: natural aggregate consisting of sand (0–4 mm) and river gravel (4–16 mm) as well as crushed barite and crushed magnetite, both with grain size 0–16 mm. All aggregate mixtures were designed in such a way that the share of the coarse aggregate 4–16 mm was at least 50% of the total aggregate volume. The grain composition of the aggregates is presented in the Table 1. The chemical composition of the cement and aggregates is presented in the Table 2. Table 3 contains the basic physical properties of the aggregates. The superplasticizer (polycarboxylate ether) with density 1.06 g/cm3 was used for making the concrete mix. The content of superplasticizer was adjusted for obtaining the consistence class S3 according to EN 206 [35]. 2.2. Mix proportions The concrete mixes were designed in such a way that the volume of aggregate, cement and water was constant in all concretes, and the content of the coarse aggregate in the total aggregate volume was at least 50%. The percentage contents of the particular Table 1 Grain composition of the aggregates used for making the concrete specimens. Sieve size [mm]
Natural aggregate
Barite aggregate
Magnetite aggregate
16 8 4 2 1 0.5 0.25 0.125
100 54.8 36.6 27.2 25.1 12.3 2.3 2.2
100 55.0 40.6 27.5 20.1 10.7 5.1 4.6
100 54.6 30.0 25.4 18.1 9.8 4.9 4.7
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Table 2 Chemical composition of cement and aggregates [wt%] Material
CaO
SiO2
Al2O3
Fe2O3
Fe3O4
MgO
Na2O
K2O
SO3
BaSO4
L.O.I.
CEM I 42.5 R Natural aggregate Barite aggregate Magnetite aggregate
64.1 0.1 0.04 2.5
20.2 95.3 0.1 3.0
5.0 1.5 1.7 0.4
2.6 1.0 0.01 –
– 0.5 0.1 90.8
1.6 0.2 0.06 –
0.2 – – 0.3
0.7 – – 0.2
2.8 0.03 4.9 –
0.1 0.04 90.3 –
2.8 0.2 2.0 1.8
Table 3 Physical properties of the aggregates. Material
Natural aggregate
Particle size (mm) Density (g/cm3) Water absorption (%) (EN 1097-6 [32]) Los Angeles coefficient (%) (EN 1097-2 [33])
0–4 2.62 1.20 –
Barite aggregate 4–16 2.64 0.90 20
components in the concrete mix volume is presented in the Fig. 1. Due to the fact that obtaining of the heavyweight concretes with apparent density above 2600 kg/m3 (according to EN 206 [35]) was assumed when designing the concrete mixes, the weight con-
0–4 4.18 1.49 –
Magnetite aggregate 4–16 4.20 1.47 87
0–4 4.77 0.67 –
4–16 4.80 0.60 26
tent of the aggregate in the tested concretes was accepted as minimum 70%. The weight content of the components in the particular concrete mixes are presented in the Fig. 2, while the mix proportions are presented in the Table 4. After preparing the concrete mixes, the specimens for testing were made in the form of cylinders with height and diameter equal to 80 mm. As a consequence of the way of concrete mixes designing, the specimens of concretes with different apparent densities were obtained after 28 days of curing: two heavyweight concretes – barite concrete, BC, with density 3441 kg/m3, and magnetite concrete, MC, with density 3939 kg/m3, and reference concrete with natural aggregate, OC, with density 2373 kg/m3 (according to the apparent density it is an ordinary concrete).
2.3. Storage of specimens
Fig. 1. Percentage content of the components in the volume of the tested concretes.
The cylindrical specimens were stored in the moulds on the grate over the water and demoulded after 24 h. During the next days or up to the time of testing the specimens were stored in the special chamber with humidity 95% and constant temperature 20 ± 2 °C. After 28 days the specimens were removed from the
Fig. 2. Weight content of the components in the volume of the tested concretes.
Table 4 Mix proportions of concrete [kg/m3]. Concrete specimen
OC BC MC *
SP: superplasticizer.
Cement
450 450 450
SP*
6.8 8.2 8.0
Water
158 158 158
w/c
0.35 0.35 0.35
Fine aggregate (0–4 mm)
Coarse aggregate (4–16 mm)
Natural
Barite
Magnetite
Natural
Barite
Magnetite
470 – –
– 730 –
– – 440
1370 – –
– 2190 –
– – 2880
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chamber, weighed and dried in the laboratory dryer at the temperature 105 °C over 7 days. After drying, the specimens were again weighed and stored in the desiccator until the time of placing them in the oven for heating. There were 30 specimens prepared for each concrete type. 3. Methodology of research 3.1. Procedure of specimens heating When the age of concretes was 28 days, all specimens destined for heating were weighed and transferred from the climatic chamber to the laboratory dryers for 7 days. The specimens were dried at the temperature 105 °C to the constant mass. After removing from the dryers, the specimens were weighed and measured for determining the apparent density. Next, they were heated in the assumed temperature; four values of temperature were applied: 300 °C, 450 °C, 600 °C and 800 °C. The specimens were heated in the medium-temperature electric oven with the special structure (Fig. 3a). The oven has a working chamber in the shape of a cuboid, adjusted to the loading from the front. The insulation of the walls is made from the fiber mat, while the insulation of the bottom is made from the insulation plates. The circulation of the atmosphere inside the oven is forced by the ventilator. The heating elements are placed on the sides of the oven in the special heating panels. The temperature is controlled using the programmable controller by HtIndustry with embedded real-time clock, recording the temperature changes into the internal memory. The controller enables uploading of 30 controlling programs to the memory. The single program can have up to 15 steps. The system of the oven makes possible to control the cooling of the working chamber using cooling ventilator and the system of the cooling flaps (top and bottom flap), controlled by the HtIndustry device. The heating was conducted following the RILEM procedure [36]. In the first stage, the rate of temperature rising was 1 °C/min. Then for 1 h the maximum assumed temperature was maintained. The cooling of the specimens was conducted with the rate 1 °C/min.
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Fig. 3b presents the heating and cooling procedure of the test samples. The specimens were placed in the oven in such a way that they did not contact each other or the heating elements of the oven (Fig. 3c). Six specimens of every type of concrete was tested within one cycle of heating and cooling. After heating the specimens were visually inspected and again weighed for determining the mass loss and apparent density. Then, the strength were determined for 3 specimens from each series, while the remaining 3 specimens were stored in the desiccator until testing of the gamma-ray absorption. 3.2. Gamma radiation shielding measurement The testing of gamma radiation damping by concrete specimens was carried out using the specially constructed testing stand; the scheme of the stand is presented in the Fig. 4. The source of radiation (1) is radioactive element 137Cs with the photons energy
Fig. 4. The stand for testing the gamma-ray resistance of the building materials (described in the text).
Fig. 3. Apparatus and procedure for heating samples: a) medium-temperature electric stove, b) heating and cooling regimes of the concrete specimens, c) samples after the end of heating.
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0.662 MeV and radioactivity 10 mCi. The radiation passing through the specimen (2) was measured using scintillation counter (3) connected to the spectrometer (4) and registered by computer (5). The uninterruptible power supply, UPS, (6) provided the continuity of the equipment functioning during testing. The distance of the counter from the gamma radiation source was 350 mm. The subject of testing were the cylindrical concrete specimens immediately after heating described in Section 3.1. The height and diameter of the concrete cylinders were equal to 80 mm. As it was reported in literature [23], at the height of specimens 80– 100 mm the influence of the height on the coefficient of radiation damping is negligible. Every tested specimen was exposed to the gamma-rays for 60 min. The gamma radiation beam passing through the medium is decreased due to the interaction with the atoms of this medium. This phenomenon is described by the known law of radiation attenuation, which for monochromatic and narrow radiation beam takes the form of Lambert-Beer law:
I ¼ I0 eðlxÞ
ð1Þ
where: x – thickness of the absorbing layer, I0 – number of particles emitted within the unit of time by the radioactive source (intensity of radiation before the absorber), I – number of particles registered within the unit of time after passing through the layer with the thickness x (intensity of radiation after passing through the absorber), m – linear coefficient of attenuation of the radiation beam.
perature 20 ± 2 °C, they were compressed in the strength machine (Toni Technik, Germany). At the moment of the testing the age of the specimens was between 38 and 40 days. The specimens which were not heated (reference samples), after demoulding were stored in the climatic chamber with humidity 95% and stable temperature 20 ± 2 °C. The compressive strength of the reference specimens was determined after 38 days of curing in the climatic chamber. The detailed results of the tests are presented and discussed in the following section. 4. Results and discussion of the effect of high temperature 4.1. Mass and apparent density Fig. 5 presents the relative mass losses and Fig. 6 presents the relative density losses of the tested concretes after heating. All specimens before heating were dried to the constant mass at the temperature 105 °C for removing free water. A systematic decrease of the mass and apparent density of the tested concretes was observed together with rising temperature of heating. The concretes were designed in such a way that the content of the aggregate in the unit volume of the concrete mix was constant for all tested concretes and equal to 69% (Fig. 1).
The linear coefficient of attenuation of radiation means the relative decreasing of intensity of the narrow beam of monoenergetic radiation on the unit path. It is most often expressed in cm1. The value of the linear coefficient of attenuation depends on the radiation energy, E, and density of the medium, through which the rays are passing, q. Therefore, the concept of the mass coefficient of attenuation, mm [cm2/g], is used in the researches:
lm ¼
l q
ð2Þ
This coefficient is a quotient of the linear coefficient of attenuation, m [cm1], and density of the medium q [g/cm3]. In the case of gamma radiation, the so-called half-value layer (HVL) is useful for the concrete and other materials. The halfvalue layer is such thickness of the material, for which the attenuation of radiation is 50%:
HVL ¼ x1=2 ¼
ln2
l
¼
0:693
l
Fig. 5. Relative losses of mass of the specimens as a function of the heat temperature.
ð3Þ
On the basis of measurements of gamma radiation intensity after passing through the concrete specimens, which were previously heated (according to the procedure described in Section 3.1), the values of the linear coefficient of radiation attenuation have been determined for the tested heavyweight concretes. The values of mm and HVL have been calculated as well. The shielding properties of the heavyweight concretes were then compared to the ordinary concretes containing natural aggregates. 3.3. Strength measurements Because of changeable duration of specimens heating, the schedule of heating and strength measurements was adjusted in such a way that the age of specimens after heating was nearing and did not influenced the compressive strength determination. Immediately after heating and cooling the specimens to the tem-
Fig. 6. Relative changes of density of the specimens as a function of the heat temperature.
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The properties of the aggregate influenced significantly the physicochemical phenomena occurring in the tested concretes when exposed to high temperature. The biggest decrease of the mass and density within the whole temperature range was observed for the concrete containing the barite aggregate (BC). The advantage of the barite aggregate is high density and high melting temperature. However, the barite demonstrates the significant cleavage in two directions and anisotropic relationship of the thermal expansion [37]. Microfractured structure brings in intersecting surfaces, that act as preferential paths in favoring the splitting of the aggregate particles [38]. The barite aggregate, depending on its origin, can contain – besides the barium sulfate – small amounts of chalcedony, quartz and zeolites. These substances are sensitive to heat and as a result of crystallization they release water at the temperature 200–500 °C [38,39]. The smallest mass losses have been observed for the concretes with magnetite aggregate (MC). The magnetite aggregate demonstrates high density and hardness, as well as good thermal conductivity. The magnetite maintains the phase stability up to the temperature 570 °C [40]. The loss of mass and density in MC specimens within the temperature range 300– 800 °C was caused mainly by the changes inside the cement matrix, which in turn were the result of releasing of the chemically bound water [41].
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Fig. 8. Relative residual compressive strength of the tested concretes as a function of the heat temperature.
4.2. Compressive strength The results of compressive strength testing as a function of the heat temperature are presented in the Fig. 7. The highest strength has been obtained, regardless of heat temperature, for the concrete containing magnetite aggregate (MC). At the temperature 20 °C the lowest strength had the concrete with barite aggregate (BC). This is caused by the high water demand of the barite aggregate (all concrete mixes were made using dry aggregates), as well as the large cleavage of the aggregate. Decrease of strength of BC concrete comparing with the concrete with ordinary aggregate (OC) at the temperature 300 and 450 °C can be explained by the variable thermal conductivity of the barite aggregate. The detailed analysis of the strength changes has been carried out on the basis of the Fig. 8, in which the relative compressive strength as a function of temperature are presented for three tested concretes. The compressive strength changes for OC and BC concretes comply, within the majority of temperature range, to the provisions of EN 1991-1-2 [42] regarding the thermal actions of the concretes with silica aggregate. Within the temperature range 300–600 °C, the
Fig. 7. Compressive strength of the tested concretes as a function of the heat temperature.
Fig. 9. Linear attenuation coefficient of tested concrete types at different temperatures.
Fig. 10. Half-value layer (HVL) of tested concrete types at different temperatures.
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specimens of concretes with magnetite aggregate (MC) have shown significantly less decrease of compressive strength than the other two concretes. Within the range of temperature 100–300 °C no decrease of strength for MC concretes was observed. In the initial period of heating (up to 300 °C) the concrete strength depends mainly on the aggregate strength. The magnetite aggregate used contains more than 90% of Fe3O4 (Table 2). At the temperature below 570 °C and under the atmospheric pressure the magnetite (Fe3O4) maintains the phase stability [40]. Therefore, no decrease
of strength of MC concrete specimens was observed at the temperature 300 °C. At the temperature 450 °C the MC concrete has kept 90% of its initial strength, and at the temperature 600 °C more than 75%, that is on average by 30% more than in the case of the other two tested concretes. Compressive strength of the cement concretes (including HWC) under high temperature depends on the interaction between the cement paste and the aggregate. The general tendency towards decrease of strength with the rising temperature is reflected in the negative influence of the cement paste and growing significance of the aggregate. The factors, which can
Fig. 11. Surface state of concrete heated at different temperatures.
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contribute to the loss of compressive strength with the rising temperature are: aggregate damages, weakening of the matrixaggregate bond, weakening of the cement paste caused by increase of its porosity and partial decomposition of C-S-H phase as well as chemical transformation of portlandite [43–45]. At the temperature 800 °C all tested concretes have demonstrated about 20% of their initial compressive strength. The concrete strength at this temperature is mainly ruled by the matrix strength. At the temperature 800 °C the loss of adhesion of the matrix to the aggregate takes place. 4.3. Gamma radiation shielding efficiency Fig. 9 presents the results of measurements of the linear attenuation coefficient of gamma-rays, for the tested concretes as a function of temperature. The results show that the coefficient slightly depends on the temperature. For the specimens heated at the temperature 800 °C this coefficient has decreased, as compared to the laboratory conditions (temperature 20 °C), by: 9.3% for OC concrete, 6.8% for BC concrete, and 4.7% for MC concrete. The MC concrete has demonstrated the highest attenuation of gamma radiation. At the temperature 20 °C the attenuation by MC concrete was by 11.7% higher than by BC concrete and by 44.8% higher than by OC concrete. At the temperature 800 °C the attenuation by MC concrete was by 14.2% higher than that by BC concrete and by 69.8% higher than for OC concrete. The reduction of the linear attenuation coefficient is attributed to the influence of temperature on the concrete properties, particularly decrease of density, which is caused by the loss of water chemically bound in the matrix (see Fig. 6). Because the aggregate took up to 69 percent of the volume of the tested concretes, the density of aggregate and its resistance to high temperature decides on the shielding ability of concrete. The results of calculation of HVL values for the tested concretes are presented graphically in the Fig. 10. These values are only slightly lowered under the action of temperature. Taking into consideration the HVL values, the concrete with magnetite aggregate (MC) has attenuated the gamma-rays in most effective way. The HVL values for MC concrete were from 2.3 cm (at 20 °C) up to 2.5 cm (at 800 °C). For the concrete containing the barite aggregate the values of HVL were slightly higher: from 2.6 cm (at 20 °C) up to 2.8 cm (at 800 °C). The highest values of HVL have been obtained for the concrete with ordinary concrete (OC): from 3.8 cm (at 20 °C) up to 4.2 cm (at 800 °C). The results of calculations of HVL values for the tested concretes have confirmed that the ability to attenuate the gamma-rays depends mainly on the density of concrete. The densities of the tested concretes exposed to the given temperature were mostly influenced by the chemical processes inside the cement matrix, and then the thermal properties of the aggregate. The matrix constituted about 30% of the tested concretes volume, hence such small changes of and HVL values under the influence of the high temperature. 4.4. Changes in the microstructure of the tested specimens Fig. 11 presents the images of the tested concretes specimens after heating at the given temperature. On the surface of the specimens of the concrete with barite aggregate (BC), the fragments of delaminated concrete with large grains of aggregate were visible already at the temperature 300 °C. The damages were deepened with the rising temperature. The explosive spalling observed on the surface layer of BC concrete can be attributed to the thermal expansion of barite concrete [38,46]. According to [47], the thermal expansion coefficient of the barite aggregate (20.6 106 K1) is nearly twice that of the granite aggregate (8.6 106 K1). The
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number of the cracks on the surface of the BC concrete specimens grew with rising temperature, however, their length and opening width were significantly smaller than in the case of the OC concrete specimens. On the surface of the specimens of the concrete with natural aggregate (OC), the single tiny cracks were observed at the temperature 450 °C. These specimens were seriously cracked at the temperature 600 °C, and at the temperature 800 °C the cracks running over the whole height of the specimens were observed. Such an intensive cracking is characteristic for the natural silica aggregates, which was confirmed by the research described in [48]. On the surface of the specimens of the MC concrete at the temperature 300, 450 and 600 °C single tiny cracks were observed. Only at the temperature 800 °C larger number of the cracks occurred, and in some cases a crack with large width running over the whole specimen was observed. The magnetite aggregate, due to its excellent thermal conductivity and stability up to the temperature 570 °C, drained the heat outside, this way limiting cracking in the initial stages of heating. The concretes containing magnetite aggregate have shown the highest cracking resistance at the temperature 300–600 °C, what was reflected by the relative compressive strength of these concretes. 5. Conclusions The results of testing of the heavyweight concretes with magnetite and barite aggregate, as well as the ordinary concrete with natural aggregate, are presented in this paper. The concretes were exposed to high temperature and then the gamma-rays. The following conclusions have been formulated on the basis of the presented results: 1. Under high temperature, a loss of mass and apparent density adequate to the rise of temperature was observed for all concretes with constant volume of the cement paste. The highest decreases of the apparent density was observed for the concrete containing barite aggregate, what is related to its high water demand and strongly differentiated thermal deformability of the aggregate in axis. 2. The type of the used aggregate, including its physical and mechanical properties, had overwhelming effect on the compressive strength of the tested concretes within the range of temperatures from 300 up to 600 °C. Because of significant share of the aggregate in the volume of the tested concretes (69%), the distribution of stresses and cracks in concrete exposed to high temperature was ruled by the thermal properties of the used aggregates. These properties have also influenced the compressive strength. At the temperature 800 °C, the compressive strength of the tested concretes was mainly influenced by the cement matrix. As a result of the destructive chemical processes taking place in the matrix at the temperature 800 °C, the compressive strength of the concretes decreased up to the level of 20% of the initial value, regardless of the used aggregate. The loss of bond between the paste and aggregate have occurred. 3. The temperature slightly affected the ability of the tested concretes to attenuate the gamma-ray. The linear attenuation coefficient and HVL value of the tested concretes exposed to high temperature depend mainly on the density of the concretes. 4. The concrete containing magnetite aggregate is more suitable as the shielding material due to its good physical, mechanical and shielding properties. The heavyweight concrete with magnetite aggregate is more resistant to the high temperature than the barite and ordinary concrete.
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5. The heavyweight concretes with barite aggregate demonstrate the high ability to attenuate the gamma-ray and compressive strength. The barite concretes are not resistant to high temperature. The spalling caused by the anisotropic thermal properties of the barite aggregate is observed at the temperature 300 °C and intensifies with the rise of temperature. Due to the aforementioned properties of the barite aggregate, it should not be used for making the shielding concretes, which can be exposed to high temperature. Conflict of interest None declared. Acknowledgment This research was funded by the National Science Centre, Poland project No. 2014/13/B/ST8/03875 (OPUS 7). References [1] D.L. Fillmore, Literature Review of the Effects of Radiation and Temperature on the Aging of Concrete, Idaho National Engineering and Environmental Laboratory, 2004. [2] M.H. Kharita, S. Yousef, M. AlNassar, The effect of the initial water to cement ratio on shielding properties of ordinary concrete, Prog. Nucl. Energy 52 (5) (2010) 491–493. [3] Q. Ma, G. Rongxin, Z. Zhao, Z. Lin, K. He, Mechanical properties of concrete at high temperature – a review, Constr. Build. Mater. 93 (2015) 371–383. [4] T.T.C. Hsu, C-L. Wu, J-L. Lin, Infrastructure Systems for Nuclear Energy, John Wiley & Sons, 2014. [5] E.-S.A. Waly, M.A. Bourham, Comparative study of different concrete composition as gamma-ray shielding materials, Ann. Nucl. Energy 85 (2015) 306–310. [6] B. Oto, A. Gür, Gamma-ray shielding of concretes including magnetite in different rate, Int. J. Phys. Sci. 8 (2013) 310–314. [7] I. Akkurt, C. Basyigit, S. Kilincarslan, et al., Radiation shielding of concretes containing different aggregates, Cem. Concr. Compos. 28 (2) (2006) 153–157. [8] I. Akkurt, C. Basyigit, S. Kilincarslan, et al., The shielding of c-rays by concretes produced with barite, Prog. Nucl. Energy 46 (1) (2005) 1–11. [9] T.A. Almeida Jr. et al., Mass attenuation coefficients of X-rays in different barite concrete used in radiation protection as shielding against ionizing radiation, Radiat. Phys. Chem. 140 (2017) 349–354. [10] I. Akkurt, A.M. El-Khayatt, The effect of barite proportion on neutron and gamma-ray shielding, Ann. Nucl. Energy 51 (2013) 5–9. [11] T. Piotrowski, D.B. Tefelski, A. Polan´ski, J. Skubalski, Monte Carlo simulations for optimization of neutron shielding concrete, Cent. Eur. J. Eng. 2 (2) (2012) 296–303. [12] M.H. Kharita, M. Takeyeddin, M. Alnassar, S. Yousef, Development of special radiation shielding concretes using natural local materials and evaluation of their shielding characteristics, Prog. Nucl. Energy 50 (2008) 33–36. [13] T. Piotrowski T, D.B. Tefelski, J.J. Sokołowska, B. Jaworska, NGS-concrete – new generation shielding concrete against ionizing radiation – the potential evaluation and preliminary investigation, Acta Phys. Pol. A 128 (2B) (2015) 9–13. [14] A.F. Bingöl, R. Gül, Residual bond strength between steel bars and concrete after elevated temperatures, Fire Saf. J. 44 (6) (2009) 854–859. _ Sezer, Effect of elevated temperatures and time on compressive [15] Sß. Yazıcı, G.I. and tensile properties of concretes, Indian J. Eng. Mater. Sci. 15 (1) (2008) 23– 28. _ Sezer, H. Sßengül, The effect of high temperature on the [16] Sß. Yazıcı, G.I. compressive strength of mortars, Constr. Build. Mater. 35 (2012) 97–100. [17] E. Horszczaruk, P. Sikora, P. Zaporowski, Mechanical properties of shielding concrete with magnetite aggregate subjected to high temperature, Proc. Eng. 108 (2015) 39–46. [18] E. Horszczaruk et al., Properties of cement composites modified with silicamagnetite nanostructures, Proc. Eng. 196 (2017) 105–112. [19] J. Lee, Y. Xi, K. William, Y. Jung, A multi scale model for modulus of elasticity of concrete at high temperatures, Cem. Concr. Res. 39 (2009) 754–762. [20] A. Samarin, Use of Concrete as a biological shield from ionising radiation, Energy Environ. Eng. 1 (2) (2013) 90–97.
[21] J.F. Lamond, J.H. Pielert, Significance of tests and properties of concrete and concrete-making materials STP169D,, ASTM Int. (2006). [22] I. Akkurt, H. Akyildirim, B. Mavi, S. Kilincarslan, C. Basyigit, Gamma-ray shielding properties of concrete including barite at different energies, Prog. Nucl. Energy 52 (7) (2010) 620–623. [23] O. Gencel, A. Bozkurt, E. Kam, T. Korkut, Determination and calculation of gamma and neutron shielding characteristics of concretes containing different hematite proportions, Ann. Nucl. Energy 38 (12) (2011) 2719–2723. [24] B. Oto, A. Gür, E. Kavaz, T. Çakır, N. Yaltay, Determination of gamma and fast neutron shielding parameters of magnetite concretes, Prog. Nucl. Energy 92 (2016) 71–80. [25] A.S. Ouda, Development of high-performance heavy density concrete using different aggregates for gamma-ray shielding, Prog. Nucl. Energy 79 (2015) 48–55. [26] D. Rezaei-Ochbelagha, S. Azimkhania, H. Gasemzadeh Mosavinejad, Shielding and strength tests of silica fume concrete, Ann. Nucl. Energy 45 (2012) 150– 154. [27] K. Singh, S. Singhb, A.S. Dhaliwal, G. Singh, Gamma radiation shielding analysis of lead-flyash concretes, Appl. Radiat. Isot. 95 (2015) 174–179. [28] H. Binici, O. Aksogan, A.H. Sevinc, A. Kucukonder, Mechanical and radioactivity shielding performances of mortars made with colemanite, barite, ground basaltic pumice and ground blast furnace slag, Constr. Build. Mater. 50 (2014) 177–183. [29] M.A. González-Ortega et al., Radiological protection and mechanical properties of concretes with EAF steel slags, Constr. Build. Mater. 51 (2014) 432–438. [30] Y. Yao, X. Zhang, M. Li, R. Yang, T. Jiang, J. Lv, Investigation of gamma ray shielding efficiency and mechanical performances of concrete shields containing bismuth oxide as an environmentally friendly additive, Radiat. Phys. Chem. 127 (2016) 188–193. [31] K. Mohammed Haneefa, M. Santhanam, F.C. Parida, Review of concrete performance at elevated temperature and hot sodium exposure applications in nuclear industry, Nucl. Eng. Des. 258 (2013) 76–88. [32] E. Horszczaruk, P. Sikora, K. Cendrowski, E. Mijowska, The effect of elevated temperature on the properties of cement mortars containing nanosilica and heavyweight aggregates, Constr. Build. Mater. 137 (2017) 420–431. [33] K. Sakr, E. EL-Hakim, Effect of high temperature or fire on heavy weight concrete properties gamma-ray shielding of concretes including magnetite in different rate, Cem. Concr. Res. 35 (2005) 590–596. [34] EN 197-1, Cement. Composition, Specifications and Conformity Criteria for Common Cements, Comité Européen de Normalisation (CEN), Brussels, Belgium, 2000, p. 50. [35] EN 206, Concrete. Specification, performance, production and conformity, Comité Européen de Normalisation (CEN), Brussels, Belgium, 2013, p. 94. [36] Rilem Technical Committees 129-MHT, Test methods for mechanical properties of concrete at high temperatures, part 1: introduction, part 2: stress–strain relation, part 3: compressive strength for service and accident conditions, Materials and Structures 28/1995. [37] F. Lo Monte, P.G. Gambarova, Thermo-mechanical behavior of baritic concrete exposed to high temperature, Cem. Concr. Compos. 53 (2014) 305–315. [38] D. Józ´wiak-Niedz´wiecka, A.M. Brandt, K. Gibas, P. Denis, The alkali – aggregate reaction hazard in the case of barite concretes, Cement, Wapno, Beton 19 (2014) 234–242. [39] R. Felicetti, Gambarova PG. Effects of high temperature on the residual compressive strength of siliceous HSCs, ACI – Mater. J. 95 (4) (1998) 395–406. [40] R.J. Hussey, G.I. Sproule, D. Caplan, M.J. Graham, The growth and structure of oxide films formed on Fe in O2 and CO2 at 550°C, Oxid. Met. 11 (1977) 65–79. [41] G.A. Khoury, Effect of fire on concrete and concrete structures, Prog. Struct. Eng. Mater. 2 (4) (2000) 429–447. [42] EN 1992-1-2, Design of concrete structures – Part 1-2: General rulesStructural fire design, Comité Européen de Normalisation (CEN), Brussels, Belgium, 2004, p. 102. [43] Y.N. Ghan, G.F. Peng, M. Anson, Residual strength and pore structure of highstrength concrete and normal strength concrete after exposure to high temperatures, Cem. Concr. Compos. 21 (1999) 23–27. [44] P.Z. Bazant, M.F. Kaplan, Concrete at High Temperatures, Material, Properties and Mathematical Models, Longman, Harlow, England, 1996. [45] D.J. Naus, The Effect of Elevated Temperature on Concrete Materials and Structures — A Literature Review, U.S. Nuclear Regulatory Commission, Washington, 2006, p. 204. [46] T.-C. Ling, C.-S. Poon, High temperatures properties of barite concrete with cathode ray tube funnel glass, Fire Mater. 38 (2014) 279–289. [47] L.P. Witte, J.E. Backstrom, Properties of heavy concrete made with barite aggregates, ACI Mater. J. 51 (1954) 65–88. [48] I. Hager, T. Tracz, J. S´liwin´ski, K. Krzemien´, The influence of aggregate type on the physical and mechanical properties of high-performance concrete subjected to high temperature, Fire Mater. 40 (2016) 668–682.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Investigation of gamma ray shielding efficiency and physicomechanical performances of heavyweight concrete subjected to high temperature Elzbieta Horszczaruk ⇑, Piotr Brzozowski West Pomeranian University of Technology Szczecin, Faculty of Civil Engineering and Architecture, Szczecin, Poland
h i g h l i g h t s Effects of high temperature on the gamma ray shielding for HWC were investigated. The HWC with magnetite aggregate presented the best high-temperature resistance. The HWC with magnetite aggregate presented the best shielding properties. High temperature has little effect on the linear attenuation coefficient and HVL.
a r t i c l e
i n f o
Article history: Received 14 February 2018 Received in revised form 13 September 2018 Accepted 18 September 2018
Keywords: Gamma ray shielding Heavyweight concrete High temperature Compressive strength
a b s t r a c t The results of durability investigation of two heavyweight concretes containing barite and magnetite aggregates are presented in the paper. The concretes, designed for gamma-ray shielding, were subjected to high temperature from the range of 300–800 °C. The effect of temperature on the mass, apparent density, compressive strength, coefficient of gamma-ray absorption and the superficial changes in the structure of the tested specimens were evaluated. The results of testing were compared to the results obtained for the specimens made from the ordinary concrete containing natural aggregate. The influence of temperature on the linear attenuation coefficient has been observed, as well as the strong relation between occurring of the superficial defects in the structure and the properties of the aggregate under high temperature. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction The use of heavyweight concretes (HWC) is the most common and relatively cheapest method of shielding against the harmful gamma radiation. HWC are used in the construction of the nuclear objects and the medical infrastructure (radiology) particularly for two reasons: good strength and effective ability to protect against radiation [1–3]. The main challenges for the concrete used in the nuclear power plants are: the breakdowns inside the object (particularly involved with the damages of the heat exchanging systems), external accidents, like earthquakes or other unintentional situations (e.g. airplane disaster, terrorist attacks) and the phenomena caused by material ageing [4]. The aggregates of the high density, containing metal compounds (iron oxide, titanium iron oxide and barium sulphate),
⇑ Corresponding author at: West Pomeranian University of Technology in Szczecin, Faculty of Civil Engineering and Architecture, 70-311 Szczecin, Al. Piastów 50, Poland. E-mail address: [email protected] (E. Horszczaruk). https://doi.org/10.1016/j.conbuildmat.2018.09.113 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved.
are used for HWC production since they are able to slow down the fast neutrons [1,5–9]. While both barite and magnetite HWC have good gamma shielding efficiency [7–9] as it is strongly related to density, the shielding against neutrons is more complex. It is depending on the elementary (atomic) composition and it is proved that barite is not good aggregate for neutron radiation concrete shields and magnetite and hematite is [10–13]. However, the high temperature inside the reactor evidently worsens the concrete durability and its parameters with time. The internal shielding layer of the concrete is often subjected to the heat from the reactor. In the case of the nuclear electric plant as well as the other objects exposed to the radiation, the resistance to the high temperature is required. The structure fire is often in the emergency or exceptional situations, therefore, the problems of durability of the structural concretes under high temperature are very important, regardless of the object destiny. Concrete is more resistant to fire than other construction materials. It can resist the temperature up to 200 °C. The aggregate plays a significant role in providing the highest thermal resistance of the cement concretes [2–9] as it usually makes up from 60 to 80% of
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the concrete volume. The aggregates for the fire-resistant concretes should show the low mass losses, low thermal expansion coefficient and small deformations under high temperature [2,10]. The crushed aggregate (mainly from the igneous rocks) is preferred, due to the improved adhesion of the cement paste to the aggregate grain, as well as the aggregate with high content of the reactive silica [15]. In general, the natural aggregates, like river sands and gravels, are thermally stable within the range of temperature 500–600 °C. Some aggregates containing high amounts of the flint, however, are stable only up to 300–350 °C. The quartzite or granite aggregates have lower thermal resistance than the limestone or dolomite ones [3,5,11]. In the case of quartzite this is connected with the crystalline transition of quartz occurring at a temperature of about 573 °C (low-quartz to highquartz). This process causes the increase of the volume [11]. The limestone aggregates are destroyed at 700–900 °C as a result of decomposition of CaCO3 [2,5,13]. The thermal resistance of the cement matrix depends on many factors. The most important are: w/c (water to cement) ratio, C/S (calcium oxide to silicon oxide) ratio and the content of portlandite (CH) in the matrix. The cement paste with low C/S ratio, therefore with the low content of CH, allow for obtaining the heat-resistant cement matrix [3,9,13]. In general, the process of degradation of the cement matrix starts from the very beginning of the concrete heating, however, the changes observed up to 300 °C are relatively small or even reversible (re-hydration) [13]. The biggest changes of the matrix porosity, observed at 300–600 °C, are the results of dehydration of the hydrated calcium silicates of C-S-H type. Dehydroxylation of the portlandite Ca(OH)2 is completed up to 600 °C. Within the range 600–800 °C, the dehydration of the hydrated calcium silicates of C-S-H type continues and the partial decomposition of calcium carbonate takes place. The majority of researchers accept 1000 °C as the temperature of the total decomposition of calcium carbonate. Within the range of temperature 800–1000 °C, the bonds aggregate-matrix are totally broken and concrete losses its strength [10,13]. A number of researches [14–17] confirms that the most dangerous for the concrete durability is the range of temperature from 400 °C to 600 °C. Decreasing of concrete strength at the high temperature is connected to such phenomena as: macro- and micro-cracking, aggregate expansion and progressive dehydration of the cement paste. Concrete in this temperature range is 40–60% of its initial strength [1,18,19]. The dehydration of C-S-H itself is not as harmful for the concrete as the cracking of the matrix, which is the result of this process. The temperature 600 °C seems to be a critical for the concrete, causing the extreme growth of the cracks observed on the concrete surface, as well as the significant increae of porosity leading to the dramatic loss of concrete strength (loss of 80% of the strength) [14,18]. At the temperature above 800 °C concretes lose their bearing capacity regardless of the initial strength. This is connected to the irreversible changes in the cement matrix [19]. A number of factors should be taken into consideration when evaluating the properties of concrete in the structure exposed to high temperature. Among them are the material factors, like: age of concrete, moisture content, chemical composition of cement, water to cement ratio (w/c); external factors connected to the loading, like: time of exposition to temperature, rate of temperature rising and its extremal values, dimensions of the element and other external actions (e.g. ionizing radiation). Concrete has relatively low thermal conductivity; as a consequence, there is a problem with removing the heat from the shield [20]. Under moderately high temperature the temperature gradient causes migration of the moisture and possibility of occurring
of tri-axial stresses. This phenomenon in the case of HWC is explained same as in the case of ordinary concretes – by difference in the thermal expansibility of the various materials (aggregate and cement paste) and dehydration of the cement paste, leading to decrease of strength [21]. In recent years there is noted a growing number of researches focused on searching for the materials, which could enable the efficient protection against gamma rays. Part of these works deals with evaluation of usability of various types of aggregates used in the shielding concretes [5,22–25]. Also, the mineral concrete additions are considered, including the waste materials, like silica fumes [26], fly ashes and blast-furnace slag [27,28] or EAF steel slag [29]. Another direction of researches is use of heavy metals oxides, like PbO or BiO3, in the shielding concretes [30]. All these studies are carried out for the concretes cured and exploited within the range of temperature (20 ± 2 °C). Few researches of the shielding concretes subject to high temperature are focused mainly on description of the changes of mechanical properties and defects in the microstructure of these concretes [17,31–33]. The aim of the presented research was evaluation of durability of the shielding concretes, containing the typical heavyweight natural aggregates (used commonly in the concretes of this type), subjected to temperature from the range of 300–800 °C. The physico-mechanical properties of the concretes and their ability to absorb the gamma radiation after heating were analyzed. 2. Preparation of concrete samples 2.1. Materials Portland cement CEM I 42.5R according to EN 197-1 [34] was used for preparation of the concrete specimens. Three various aggregates were used: natural aggregate consisting of sand (0–4 mm) and river gravel (4–16 mm) as well as crushed barite and crushed magnetite, both with grain size 0–16 mm. All aggregate mixtures were designed in such a way that the share of the coarse aggregate 4–16 mm was at least 50% of the total aggregate volume. The grain composition of the aggregates is presented in the Table 1. The chemical composition of the cement and aggregates is presented in the Table 2. Table 3 contains the basic physical properties of the aggregates. The superplasticizer (polycarboxylate ether) with density 1.06 g/cm3 was used for making the concrete mix. The content of superplasticizer was adjusted for obtaining the consistence class S3 according to EN 206 [35]. 2.2. Mix proportions The concrete mixes were designed in such a way that the volume of aggregate, cement and water was constant in all concretes, and the content of the coarse aggregate in the total aggregate volume was at least 50%. The percentage contents of the particular Table 1 Grain composition of the aggregates used for making the concrete specimens. Sieve size [mm]
Natural aggregate
Barite aggregate
Magnetite aggregate
16 8 4 2 1 0.5 0.25 0.125
100 54.8 36.6 27.2 25.1 12.3 2.3 2.2
100 55.0 40.6 27.5 20.1 10.7 5.1 4.6
100 54.6 30.0 25.4 18.1 9.8 4.9 4.7
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Table 2 Chemical composition of cement and aggregates [wt%] Material
CaO
SiO2
Al2O3
Fe2O3
Fe3O4
MgO
Na2O
K2O
SO3
BaSO4
L.O.I.
CEM I 42.5 R Natural aggregate Barite aggregate Magnetite aggregate
64.1 0.1 0.04 2.5
20.2 95.3 0.1 3.0
5.0 1.5 1.7 0.4
2.6 1.0 0.01 –
– 0.5 0.1 90.8
1.6 0.2 0.06 –
0.2 – – 0.3
0.7 – – 0.2
2.8 0.03 4.9 –
0.1 0.04 90.3 –
2.8 0.2 2.0 1.8
Table 3 Physical properties of the aggregates. Material
Natural aggregate
Particle size (mm) Density (g/cm3) Water absorption (%) (EN 1097-6 [32]) Los Angeles coefficient (%) (EN 1097-2 [33])
0–4 2.62 1.20 –
Barite aggregate 4–16 2.64 0.90 20
components in the concrete mix volume is presented in the Fig. 1. Due to the fact that obtaining of the heavyweight concretes with apparent density above 2600 kg/m3 (according to EN 206 [35]) was assumed when designing the concrete mixes, the weight con-
0–4 4.18 1.49 –
Magnetite aggregate 4–16 4.20 1.47 87
0–4 4.77 0.67 –
4–16 4.80 0.60 26
tent of the aggregate in the tested concretes was accepted as minimum 70%. The weight content of the components in the particular concrete mixes are presented in the Fig. 2, while the mix proportions are presented in the Table 4. After preparing the concrete mixes, the specimens for testing were made in the form of cylinders with height and diameter equal to 80 mm. As a consequence of the way of concrete mixes designing, the specimens of concretes with different apparent densities were obtained after 28 days of curing: two heavyweight concretes – barite concrete, BC, with density 3441 kg/m3, and magnetite concrete, MC, with density 3939 kg/m3, and reference concrete with natural aggregate, OC, with density 2373 kg/m3 (according to the apparent density it is an ordinary concrete).
2.3. Storage of specimens
Fig. 1. Percentage content of the components in the volume of the tested concretes.
The cylindrical specimens were stored in the moulds on the grate over the water and demoulded after 24 h. During the next days or up to the time of testing the specimens were stored in the special chamber with humidity 95% and constant temperature 20 ± 2 °C. After 28 days the specimens were removed from the
Fig. 2. Weight content of the components in the volume of the tested concretes.
Table 4 Mix proportions of concrete [kg/m3]. Concrete specimen
OC BC MC *
SP: superplasticizer.
Cement
450 450 450
SP*
6.8 8.2 8.0
Water
158 158 158
w/c
0.35 0.35 0.35
Fine aggregate (0–4 mm)
Coarse aggregate (4–16 mm)
Natural
Barite
Magnetite
Natural
Barite
Magnetite
470 – –
– 730 –
– – 440
1370 – –
– 2190 –
– – 2880
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chamber, weighed and dried in the laboratory dryer at the temperature 105 °C over 7 days. After drying, the specimens were again weighed and stored in the desiccator until the time of placing them in the oven for heating. There were 30 specimens prepared for each concrete type. 3. Methodology of research 3.1. Procedure of specimens heating When the age of concretes was 28 days, all specimens destined for heating were weighed and transferred from the climatic chamber to the laboratory dryers for 7 days. The specimens were dried at the temperature 105 °C to the constant mass. After removing from the dryers, the specimens were weighed and measured for determining the apparent density. Next, they were heated in the assumed temperature; four values of temperature were applied: 300 °C, 450 °C, 600 °C and 800 °C. The specimens were heated in the medium-temperature electric oven with the special structure (Fig. 3a). The oven has a working chamber in the shape of a cuboid, adjusted to the loading from the front. The insulation of the walls is made from the fiber mat, while the insulation of the bottom is made from the insulation plates. The circulation of the atmosphere inside the oven is forced by the ventilator. The heating elements are placed on the sides of the oven in the special heating panels. The temperature is controlled using the programmable controller by HtIndustry with embedded real-time clock, recording the temperature changes into the internal memory. The controller enables uploading of 30 controlling programs to the memory. The single program can have up to 15 steps. The system of the oven makes possible to control the cooling of the working chamber using cooling ventilator and the system of the cooling flaps (top and bottom flap), controlled by the HtIndustry device. The heating was conducted following the RILEM procedure [36]. In the first stage, the rate of temperature rising was 1 °C/min. Then for 1 h the maximum assumed temperature was maintained. The cooling of the specimens was conducted with the rate 1 °C/min.
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Fig. 3b presents the heating and cooling procedure of the test samples. The specimens were placed in the oven in such a way that they did not contact each other or the heating elements of the oven (Fig. 3c). Six specimens of every type of concrete was tested within one cycle of heating and cooling. After heating the specimens were visually inspected and again weighed for determining the mass loss and apparent density. Then, the strength were determined for 3 specimens from each series, while the remaining 3 specimens were stored in the desiccator until testing of the gamma-ray absorption. 3.2. Gamma radiation shielding measurement The testing of gamma radiation damping by concrete specimens was carried out using the specially constructed testing stand; the scheme of the stand is presented in the Fig. 4. The source of radiation (1) is radioactive element 137Cs with the photons energy
Fig. 4. The stand for testing the gamma-ray resistance of the building materials (described in the text).
Fig. 3. Apparatus and procedure for heating samples: a) medium-temperature electric stove, b) heating and cooling regimes of the concrete specimens, c) samples after the end of heating.
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0.662 MeV and radioactivity 10 mCi. The radiation passing through the specimen (2) was measured using scintillation counter (3) connected to the spectrometer (4) and registered by computer (5). The uninterruptible power supply, UPS, (6) provided the continuity of the equipment functioning during testing. The distance of the counter from the gamma radiation source was 350 mm. The subject of testing were the cylindrical concrete specimens immediately after heating described in Section 3.1. The height and diameter of the concrete cylinders were equal to 80 mm. As it was reported in literature [23], at the height of specimens 80– 100 mm the influence of the height on the coefficient of radiation damping is negligible. Every tested specimen was exposed to the gamma-rays for 60 min. The gamma radiation beam passing through the medium is decreased due to the interaction with the atoms of this medium. This phenomenon is described by the known law of radiation attenuation, which for monochromatic and narrow radiation beam takes the form of Lambert-Beer law:
I ¼ I0 eðlxÞ
ð1Þ
where: x – thickness of the absorbing layer, I0 – number of particles emitted within the unit of time by the radioactive source (intensity of radiation before the absorber), I – number of particles registered within the unit of time after passing through the layer with the thickness x (intensity of radiation after passing through the absorber), m – linear coefficient of attenuation of the radiation beam.
perature 20 ± 2 °C, they were compressed in the strength machine (Toni Technik, Germany). At the moment of the testing the age of the specimens was between 38 and 40 days. The specimens which were not heated (reference samples), after demoulding were stored in the climatic chamber with humidity 95% and stable temperature 20 ± 2 °C. The compressive strength of the reference specimens was determined after 38 days of curing in the climatic chamber. The detailed results of the tests are presented and discussed in the following section. 4. Results and discussion of the effect of high temperature 4.1. Mass and apparent density Fig. 5 presents the relative mass losses and Fig. 6 presents the relative density losses of the tested concretes after heating. All specimens before heating were dried to the constant mass at the temperature 105 °C for removing free water. A systematic decrease of the mass and apparent density of the tested concretes was observed together with rising temperature of heating. The concretes were designed in such a way that the content of the aggregate in the unit volume of the concrete mix was constant for all tested concretes and equal to 69% (Fig. 1).
The linear coefficient of attenuation of radiation means the relative decreasing of intensity of the narrow beam of monoenergetic radiation on the unit path. It is most often expressed in cm1. The value of the linear coefficient of attenuation depends on the radiation energy, E, and density of the medium, through which the rays are passing, q. Therefore, the concept of the mass coefficient of attenuation, mm [cm2/g], is used in the researches:
lm ¼
l q
ð2Þ
This coefficient is a quotient of the linear coefficient of attenuation, m [cm1], and density of the medium q [g/cm3]. In the case of gamma radiation, the so-called half-value layer (HVL) is useful for the concrete and other materials. The halfvalue layer is such thickness of the material, for which the attenuation of radiation is 50%:
HVL ¼ x1=2 ¼
ln2
l
¼
0:693
l
Fig. 5. Relative losses of mass of the specimens as a function of the heat temperature.
ð3Þ
On the basis of measurements of gamma radiation intensity after passing through the concrete specimens, which were previously heated (according to the procedure described in Section 3.1), the values of the linear coefficient of radiation attenuation have been determined for the tested heavyweight concretes. The values of mm and HVL have been calculated as well. The shielding properties of the heavyweight concretes were then compared to the ordinary concretes containing natural aggregates. 3.3. Strength measurements Because of changeable duration of specimens heating, the schedule of heating and strength measurements was adjusted in such a way that the age of specimens after heating was nearing and did not influenced the compressive strength determination. Immediately after heating and cooling the specimens to the tem-
Fig. 6. Relative changes of density of the specimens as a function of the heat temperature.
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The properties of the aggregate influenced significantly the physicochemical phenomena occurring in the tested concretes when exposed to high temperature. The biggest decrease of the mass and density within the whole temperature range was observed for the concrete containing the barite aggregate (BC). The advantage of the barite aggregate is high density and high melting temperature. However, the barite demonstrates the significant cleavage in two directions and anisotropic relationship of the thermal expansion [37]. Microfractured structure brings in intersecting surfaces, that act as preferential paths in favoring the splitting of the aggregate particles [38]. The barite aggregate, depending on its origin, can contain – besides the barium sulfate – small amounts of chalcedony, quartz and zeolites. These substances are sensitive to heat and as a result of crystallization they release water at the temperature 200–500 °C [38,39]. The smallest mass losses have been observed for the concretes with magnetite aggregate (MC). The magnetite aggregate demonstrates high density and hardness, as well as good thermal conductivity. The magnetite maintains the phase stability up to the temperature 570 °C [40]. The loss of mass and density in MC specimens within the temperature range 300– 800 °C was caused mainly by the changes inside the cement matrix, which in turn were the result of releasing of the chemically bound water [41].
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Fig. 8. Relative residual compressive strength of the tested concretes as a function of the heat temperature.
4.2. Compressive strength The results of compressive strength testing as a function of the heat temperature are presented in the Fig. 7. The highest strength has been obtained, regardless of heat temperature, for the concrete containing magnetite aggregate (MC). At the temperature 20 °C the lowest strength had the concrete with barite aggregate (BC). This is caused by the high water demand of the barite aggregate (all concrete mixes were made using dry aggregates), as well as the large cleavage of the aggregate. Decrease of strength of BC concrete comparing with the concrete with ordinary aggregate (OC) at the temperature 300 and 450 °C can be explained by the variable thermal conductivity of the barite aggregate. The detailed analysis of the strength changes has been carried out on the basis of the Fig. 8, in which the relative compressive strength as a function of temperature are presented for three tested concretes. The compressive strength changes for OC and BC concretes comply, within the majority of temperature range, to the provisions of EN 1991-1-2 [42] regarding the thermal actions of the concretes with silica aggregate. Within the temperature range 300–600 °C, the
Fig. 7. Compressive strength of the tested concretes as a function of the heat temperature.
Fig. 9. Linear attenuation coefficient of tested concrete types at different temperatures.
Fig. 10. Half-value layer (HVL) of tested concrete types at different temperatures.
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specimens of concretes with magnetite aggregate (MC) have shown significantly less decrease of compressive strength than the other two concretes. Within the range of temperature 100–300 °C no decrease of strength for MC concretes was observed. In the initial period of heating (up to 300 °C) the concrete strength depends mainly on the aggregate strength. The magnetite aggregate used contains more than 90% of Fe3O4 (Table 2). At the temperature below 570 °C and under the atmospheric pressure the magnetite (Fe3O4) maintains the phase stability [40]. Therefore, no decrease
of strength of MC concrete specimens was observed at the temperature 300 °C. At the temperature 450 °C the MC concrete has kept 90% of its initial strength, and at the temperature 600 °C more than 75%, that is on average by 30% more than in the case of the other two tested concretes. Compressive strength of the cement concretes (including HWC) under high temperature depends on the interaction between the cement paste and the aggregate. The general tendency towards decrease of strength with the rising temperature is reflected in the negative influence of the cement paste and growing significance of the aggregate. The factors, which can
Fig. 11. Surface state of concrete heated at different temperatures.
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contribute to the loss of compressive strength with the rising temperature are: aggregate damages, weakening of the matrixaggregate bond, weakening of the cement paste caused by increase of its porosity and partial decomposition of C-S-H phase as well as chemical transformation of portlandite [43–45]. At the temperature 800 °C all tested concretes have demonstrated about 20% of their initial compressive strength. The concrete strength at this temperature is mainly ruled by the matrix strength. At the temperature 800 °C the loss of adhesion of the matrix to the aggregate takes place. 4.3. Gamma radiation shielding efficiency Fig. 9 presents the results of measurements of the linear attenuation coefficient of gamma-rays, for the tested concretes as a function of temperature. The results show that the coefficient slightly depends on the temperature. For the specimens heated at the temperature 800 °C this coefficient has decreased, as compared to the laboratory conditions (temperature 20 °C), by: 9.3% for OC concrete, 6.8% for BC concrete, and 4.7% for MC concrete. The MC concrete has demonstrated the highest attenuation of gamma radiation. At the temperature 20 °C the attenuation by MC concrete was by 11.7% higher than by BC concrete and by 44.8% higher than by OC concrete. At the temperature 800 °C the attenuation by MC concrete was by 14.2% higher than that by BC concrete and by 69.8% higher than for OC concrete. The reduction of the linear attenuation coefficient is attributed to the influence of temperature on the concrete properties, particularly decrease of density, which is caused by the loss of water chemically bound in the matrix (see Fig. 6). Because the aggregate took up to 69 percent of the volume of the tested concretes, the density of aggregate and its resistance to high temperature decides on the shielding ability of concrete. The results of calculation of HVL values for the tested concretes are presented graphically in the Fig. 10. These values are only slightly lowered under the action of temperature. Taking into consideration the HVL values, the concrete with magnetite aggregate (MC) has attenuated the gamma-rays in most effective way. The HVL values for MC concrete were from 2.3 cm (at 20 °C) up to 2.5 cm (at 800 °C). For the concrete containing the barite aggregate the values of HVL were slightly higher: from 2.6 cm (at 20 °C) up to 2.8 cm (at 800 °C). The highest values of HVL have been obtained for the concrete with ordinary concrete (OC): from 3.8 cm (at 20 °C) up to 4.2 cm (at 800 °C). The results of calculations of HVL values for the tested concretes have confirmed that the ability to attenuate the gamma-rays depends mainly on the density of concrete. The densities of the tested concretes exposed to the given temperature were mostly influenced by the chemical processes inside the cement matrix, and then the thermal properties of the aggregate. The matrix constituted about 30% of the tested concretes volume, hence such small changes of and HVL values under the influence of the high temperature. 4.4. Changes in the microstructure of the tested specimens Fig. 11 presents the images of the tested concretes specimens after heating at the given temperature. On the surface of the specimens of the concrete with barite aggregate (BC), the fragments of delaminated concrete with large grains of aggregate were visible already at the temperature 300 °C. The damages were deepened with the rising temperature. The explosive spalling observed on the surface layer of BC concrete can be attributed to the thermal expansion of barite concrete [38,46]. According to [47], the thermal expansion coefficient of the barite aggregate (20.6 106 K1) is nearly twice that of the granite aggregate (8.6 106 K1). The
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number of the cracks on the surface of the BC concrete specimens grew with rising temperature, however, their length and opening width were significantly smaller than in the case of the OC concrete specimens. On the surface of the specimens of the concrete with natural aggregate (OC), the single tiny cracks were observed at the temperature 450 °C. These specimens were seriously cracked at the temperature 600 °C, and at the temperature 800 °C the cracks running over the whole height of the specimens were observed. Such an intensive cracking is characteristic for the natural silica aggregates, which was confirmed by the research described in [48]. On the surface of the specimens of the MC concrete at the temperature 300, 450 and 600 °C single tiny cracks were observed. Only at the temperature 800 °C larger number of the cracks occurred, and in some cases a crack with large width running over the whole specimen was observed. The magnetite aggregate, due to its excellent thermal conductivity and stability up to the temperature 570 °C, drained the heat outside, this way limiting cracking in the initial stages of heating. The concretes containing magnetite aggregate have shown the highest cracking resistance at the temperature 300–600 °C, what was reflected by the relative compressive strength of these concretes. 5. Conclusions The results of testing of the heavyweight concretes with magnetite and barite aggregate, as well as the ordinary concrete with natural aggregate, are presented in this paper. The concretes were exposed to high temperature and then the gamma-rays. The following conclusions have been formulated on the basis of the presented results: 1. Under high temperature, a loss of mass and apparent density adequate to the rise of temperature was observed for all concretes with constant volume of the cement paste. The highest decreases of the apparent density was observed for the concrete containing barite aggregate, what is related to its high water demand and strongly differentiated thermal deformability of the aggregate in axis. 2. The type of the used aggregate, including its physical and mechanical properties, had overwhelming effect on the compressive strength of the tested concretes within the range of temperatures from 300 up to 600 °C. Because of significant share of the aggregate in the volume of the tested concretes (69%), the distribution of stresses and cracks in concrete exposed to high temperature was ruled by the thermal properties of the used aggregates. These properties have also influenced the compressive strength. At the temperature 800 °C, the compressive strength of the tested concretes was mainly influenced by the cement matrix. As a result of the destructive chemical processes taking place in the matrix at the temperature 800 °C, the compressive strength of the concretes decreased up to the level of 20% of the initial value, regardless of the used aggregate. The loss of bond between the paste and aggregate have occurred. 3. The temperature slightly affected the ability of the tested concretes to attenuate the gamma-ray. The linear attenuation coefficient and HVL value of the tested concretes exposed to high temperature depend mainly on the density of the concretes. 4. The concrete containing magnetite aggregate is more suitable as the shielding material due to its good physical, mechanical and shielding properties. The heavyweight concrete with magnetite aggregate is more resistant to the high temperature than the barite and ordinary concrete.
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5. The heavyweight concretes with barite aggregate demonstrate the high ability to attenuate the gamma-ray and compressive strength. The barite concretes are not resistant to high temperature. The spalling caused by the anisotropic thermal properties of the barite aggregate is observed at the temperature 300 °C and intensifies with the rise of temperature. Due to the aforementioned properties of the barite aggregate, it should not be used for making the shielding concretes, which can be exposed to high temperature. Conflict of interest None declared. Acknowledgment This research was funded by the National Science Centre, Poland project No. 2014/13/B/ST8/03875 (OPUS 7). References [1] D.L. Fillmore, Literature Review of the Effects of Radiation and Temperature on the Aging of Concrete, Idaho National Engineering and Environmental Laboratory, 2004. [2] M.H. Kharita, S. Yousef, M. AlNassar, The effect of the initial water to cement ratio on shielding properties of ordinary concrete, Prog. Nucl. Energy 52 (5) (2010) 491–493. [3] Q. Ma, G. Rongxin, Z. Zhao, Z. Lin, K. He, Mechanical properties of concrete at high temperature – a review, Constr. Build. Mater. 93 (2015) 371–383. [4] T.T.C. Hsu, C-L. Wu, J-L. Lin, Infrastructure Systems for Nuclear Energy, John Wiley & Sons, 2014. [5] E.-S.A. Waly, M.A. Bourham, Comparative study of different concrete composition as gamma-ray shielding materials, Ann. Nucl. Energy 85 (2015) 306–310. [6] B. Oto, A. Gür, Gamma-ray shielding of concretes including magnetite in different rate, Int. J. Phys. Sci. 8 (2013) 310–314. [7] I. Akkurt, C. Basyigit, S. Kilincarslan, et al., Radiation shielding of concretes containing different aggregates, Cem. Concr. Compos. 28 (2) (2006) 153–157. [8] I. Akkurt, C. Basyigit, S. Kilincarslan, et al., The shielding of c-rays by concretes produced with barite, Prog. Nucl. Energy 46 (1) (2005) 1–11. [9] T.A. Almeida Jr. et al., Mass attenuation coefficients of X-rays in different barite concrete used in radiation protection as shielding against ionizing radiation, Radiat. Phys. Chem. 140 (2017) 349–354. [10] I. Akkurt, A.M. El-Khayatt, The effect of barite proportion on neutron and gamma-ray shielding, Ann. Nucl. Energy 51 (2013) 5–9. [11] T. Piotrowski, D.B. Tefelski, A. Polan´ski, J. Skubalski, Monte Carlo simulations for optimization of neutron shielding concrete, Cent. Eur. J. Eng. 2 (2) (2012) 296–303. [12] M.H. Kharita, M. Takeyeddin, M. Alnassar, S. Yousef, Development of special radiation shielding concretes using natural local materials and evaluation of their shielding characteristics, Prog. Nucl. Energy 50 (2008) 33–36. [13] T. Piotrowski T, D.B. Tefelski, J.J. Sokołowska, B. Jaworska, NGS-concrete – new generation shielding concrete against ionizing radiation – the potential evaluation and preliminary investigation, Acta Phys. Pol. A 128 (2B) (2015) 9–13. [14] A.F. Bingöl, R. Gül, Residual bond strength between steel bars and concrete after elevated temperatures, Fire Saf. J. 44 (6) (2009) 854–859. _ Sezer, Effect of elevated temperatures and time on compressive [15] Sß. Yazıcı, G.I. and tensile properties of concretes, Indian J. Eng. Mater. Sci. 15 (1) (2008) 23– 28. _ Sezer, H. Sßengül, The effect of high temperature on the [16] Sß. Yazıcı, G.I. compressive strength of mortars, Constr. Build. Mater. 35 (2012) 97–100. [17] E. Horszczaruk, P. Sikora, P. Zaporowski, Mechanical properties of shielding concrete with magnetite aggregate subjected to high temperature, Proc. Eng. 108 (2015) 39–46. [18] E. Horszczaruk et al., Properties of cement composites modified with silicamagnetite nanostructures, Proc. Eng. 196 (2017) 105–112. [19] J. Lee, Y. Xi, K. William, Y. Jung, A multi scale model for modulus of elasticity of concrete at high temperatures, Cem. Concr. Res. 39 (2009) 754–762. [20] A. Samarin, Use of Concrete as a biological shield from ionising radiation, Energy Environ. Eng. 1 (2) (2013) 90–97.
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