Application of the Heavy-Weight Concrete as a Fire-Resistance Nuclear Concrete

Abstract

The application of ionising radiations became necessary and valuable for various reasons, i.e., electricity generation, medical treatment, agriculture, industry and scientific research. Nuclear power plants are one of the most complex radiation-shielding structures. Special design and building materials are required to enhance safety and reduce the risk of harmful radiation emissions. The construction of nuclear buildings must fulfil radiation attenuation, strength, fire resistance and durability which are cost-effective properties. Therefore, heavy-weight concrete (HWC) can fulfil these requirements due to its cost-effectiveness and good physical, mechanical and thermal properties. The research aims to introduce nuclear buildings, their application and their behaviour under elevated temperatures. Also, the research aims to review the heavy-weight concrete and heavy aggregate and their essential role in developing neutron-shielding and fire-resistant materials and prove this fact through investigations. However, the aim of this research was to investigate heavy-weight concrete’s physical, mechanical and thermal properties at different elevated temperatures. Whereas magnetite heavy-weight concrete is the main concern. Result showed the good thermal resistance capability of magnetite concrete up to 800°C, compared to the basalt and quartz concrete. Raising the water-cement ratio (w/c ratio) of the heavy-weight magnetite concrete reduced the risk of explosive spalling at 800°C. Whereas adding metakaolin and boron carbide improved the mechanical properties of magnetite concrete up to 500°C.

1. Introduction

The construction of radiation-shielding structures is one of the most critical issues, where the designing process and selection of accurate building materials are the key factors. Nuclear power plants, diagnostics therapy, industrial and some scientific research buildings are patterns of radiation-shielding structures. Therefore, nuclear buildings are exposed to harmful radiation and high temperatures. Gamma radiation and neutron flux raise the temperature of the reactor shield.

Due to its good shielding capability, concrete has been used in the construction of nuclear buildings. There are 449 nuclear reactors in 30 countries in operation and 60 in 15 countries under construction [1, 2, 3]. However, it indicates a growing demand for the construction of nuclear buildings. Attenuation against ionising radiation, thermal stability and high strength are essential in constructing nuclear buildings.

Moreover, thermal properties and behaviour under elevated temperatures are vital in designing nuclear buildings [4]. Under normal conditions, nuclear reactors are exposed to temperatures below 100°C during their service life. However, it can melt down in the case of an accident. Recommendation from the American Concrete Institute (ACI) indicates that the temperature limitation in concrete shields is about 65°C; furthermore, other international organisations allow temperatures up to 90°C [5]. Bertero and Polivka reported that cyclic heat treatment between 20 and 150°C is more damaging to concrete [6].

2. Properties of heavy-weight concrete at high temperatures

The fire resistance of heavy-weight concrete is excellent compared to other construction materials. Therefore, it can be used as a fire-resistant material in nuclear buildings. Its good fire resistance is owing to the aggregate and cement components. At the same time, the concrete matrix has low thermal conductivity and high heat capacity and a relatively low degradation rate at high temperatures. Therefore, concrete’s heat transfer rate, mass loss and strength loss are slow. Correspondingly, the residual mechanical, thermal and deformation properties of concrete are essential to identify the fire resistance of concrete. However, constituents of concrete, such as aggregate, cement type, and chemical, mineral or nanoparticle admixture, directly affect these properties. In nuclear buildings, shielding properties are directly affected by the iron contents and the amount of fixed water [7, 8, 9].

Identifying the residual physical, mechanical, shielding and thermal properties of heavy-weight concrete under elevated temperatures is prerequisite for designing nuclear concrete (Figure 1).

Figure 1.

Properties of heavy-weight concrete (HWC) under elevated temperatures [8, 9].

Fire or heat influences the mechanical, physical, thermal, deformation, radiation-shielding and other material-precise properties, such as spalling in concrete. Physical changes include visual observation, porosity, absorption rate and mass loss [10, 11].

Thermal properties measured the heat transfer, while mechanical properties measured changes in the strength and stiffness of concrete. The strain behaviour determines deformation properties. Other specific material properties, such as spalling and colour change, are also important. The properties mentioned above are varied with the function of temperature. Although concrete and steel are the main constituents affected by heat, the concrete ingredients and the type of steel fibre are essential [9].

2.1 Mechanical properties

Mechanical properties determining the fire resistance of concrete are compressive strength, flexural strength, modulus of elasticity and stress-strain response at elevated temperatures. In the case of fire, the remaining (residual) properties were considered [10, 11].

Compressive strength is a critical property of concrete. It depends on the mix design, aggregate type, grading and size of aggregate, water-cement (w/c) ratio and curing condition at ambient temperature. Moreover, at high temperatures, it is affected by the temperature level, heating rate and duration of heating.

The tensile strength is an essential property in concrete damage and leads to the appearance of microcracks. However, it represents 10% of the compressive strength in ordinary concrete and less in heavy-weight or high-strength concrete. Therefore, it is influenced by the same factor of compressive strength. Simultaneously, the modulus of elasticity of concrete decreases at elevated temperatures due to the degradation and breaking of the bond of the cement gel [12, 13].

Many authors study the mechanical properties of heavy-weight concrete at elevated temperatures. According to Demir et al. [14], about 58–64 MPa compressive strength was achieved using limestone, baryte and siderite aggregates. However, these values are satisfactory with the available kinds of literature. Results showed no significant loss in compressive strength at 300°C. Correspondingly, the residual compressive strength declined by 59% at 600°C [14]. Brandt and Jozwiak-Niedzwiedzka (2013) reported that the compressive strength declined by 30–40% above 100°C, while the tensile strength reduced even more [15]. Sakr and El-Hakim studied the mechanical, physical and shielding properties of baryte, gavel and ilmenite concrete at 25 up to 950°C, elevated temperatures. The study reported that ilmenite concrete has better residual mechanical properties than gravel and baryte [16]. Beaucour et al. investigated electric arc furnaces (EAFs) and steel slag as heavy-weight aggregates appropriate for radiation-shielding structures. Results demonstrated that EAF steel slag aggregates have better thermal behaviour than baryte aggregates [17].

2.2 Thermal properties

The thermal properties represent the thermal conductivity, specific heat capacity, thermal diffusivity, mass loss and density. Thermal conductivity is the capability of the materials to conduct heat, which can be measured by steady-state or transient methods [18]. Alternatively, the amount of heat required to change the temperature of the material by 1° is the specific heat. Therefore, the thermal conductivity $\lambda$ and the specific heat capacity Cp are measured using the ISOMET 2104 (heat transfer analyser) device [19, 20]. Several factors influence the heat capacity of concrete: the moisture contents, density and the type of aggregate. High thermal conductivity concrete has better thermal stability due to decreasing thermal stress between the inner and outer surfaces [21, 22]. Generally, the thermal conductivity of different concrete mixes ranges between 1.4 and 3.6 w/(m K), and the heat capacity between 840 and 1170 (J/kg K). Therefore, the growth in the moisture contents increased the heat capacity of concrete. According to the literature, the thermal conductivity of high-strength concrete is higher than that of standard-strength concrete [23, 24].

Investigation of the thermal properties of nine shielding concrete mixes was studied by Glinicki et al. [25]. However, the concrete mixes contained different magnetite, baryte, serpentine and sand aggregates (Figure 2). His study concluded that the thermal conductivity of proper concrete falls between 1.0 and 3.6 w/(m K). Serpentine CS- 1 mix showed the lowest thermal conductivity at 1.83 w/(m K), while magnetite CS- 6 mix showed the highest thermal conductivity at 3.07 w/(m K). Therefore, adding baryte or magnetite to the serpentine aggregate improved the thermal conductivity of concrete [25].

Figure 2.

Saturated and dry thermal conductivity and the density for different radiation-shielding concrete mixes [25].

Change in the mass of concrete occurred due to the release of the evaporable water, dehydration and decomposition of the cement paste and aggregates, and the spalling of concrete [26]. These changes develop cracks, surface spalling or even explosions of concrete [27]. Several factors influence the mass loss of concrete: the composition, porosity and thermal stability of the concrete mix. Different water-cement ratios and heavy-weight aggregate ratios directly influenced the mass loss of nuclear concrete [28]. A study was carried out by Ling, which indicated that loss in the mass of baryte-based concrete is high at 300°C. However, it is owing to the high absorption capability of baryte aggregates [29].

2.3 Deformation

When the material expands or shrinks because of the heat, it causes thermal expansion. Therefore, thermal expansion, creep and transient strains are different forms of material deformation. Thus, the thermal expansion is explained by the coefficient of thermal expansion. The coefficient of thermal expansion is a percentage deformation, while creep and transient strains are time-dependent deformations [24].

2.4 Spalling

Due to the heat treatment procedures, some pieces were separated from the concrete. Therefore, these separations are known as thermal spalling [30, 31]. The separation appears sudden, violent, prolonged and quiescent, and it can be classified into general or destructive spalling, local spalling and sloughing off spalling [32]. During the heat loading test, the thermal stress increased due to the growth in the pore pressure buildup [21]. Different factors, such as the density of aggregates, the heating rate, thermal gradients and thermal stress, can develop the risk of explosion of concrete. The divergence between the cement paste and aggregates is reduced by the thermal expansion of the heavy-weight aggregates [33]. High binder content can also increase the risk of explosive spalling, so high cement amounts require more water, while adding supplementary cementitious material such as silica fume can reduce the permeability of the cement gel [31].

2.5 Radiation shielding

Concrete can be used as shielding materials for both gamma and neutron radiations. Attenuation against hazardous radiation requires high-density materials, while protection against neutrons (neutron energy/speed) depends on the atom’s nuclei. In a nuclear reactor, the neutron flux is 1016–1018 n/cm2/s. Heavy elements in concrete are required to slow down the fast neutron, and hydrogen ion is necessary to absorb a neutron. Gradual heating from the shielding influences the neutron attenuation properties of concrete [34].

There is a direct correlation between the parameters of compressive strength, density, thermal conductivity/fire resistance and attenuation coefficient. However, developing these parameters can warrant the best shielding capability of the structure.

For more details, an experimental programme was carried out to investigate the magnetite heavy-weight concrete’s physical, thermal and mechanical properties at different water-cement ratios and with and without metakaolin and boron carbide additives. Furthermore, it compared the thermal stability of magnetite heavy-weight concrete with those of other types of concrete, such as basalt and magnetite concrete.

3. Experimental programme

3.1 Applied material

The experimental programme tested the heavy-weight magnetite concrete’s mechanical, thermal and physical properties based on different parameters. The parameters were the different types of aggregates, different water-cement ratios and different supplementary materials. The selection of ingredients for different concrete mixes relied on two main properties: hydrous aggregates’ capability to slow down high-energy neutrons and the high atomic weight and amount of fixed water to attenuate gamma rays and absorbed neutrons [35, 36, 37, 38]. The magnetite aggregates were the main heavy-weight aggregates applied to produce the heavy-weight concrete. Other aggregates, such as basalt and quartz, were tested and compared to the magnetite-heavy aggregates. Three different water/cement ratios of 0.42, 0.47, and 0.52 were examined and compared, and metakaolin and boron carbide supplementary materials were also investigated for the magnetite heavy-weight concrete. The main concrete ingredients are Portland cement, aggregates, water and admixtures. The type of applied cement is the CEM I 52.5 SR. However, it has high early ultimate strength and considerable heat evolution properties [39, 40, 41, 42]. Figure 3 illustrates the applied aggregates with their physical properties [37].

Figure 3.

Different mineral aggregates and their physical properties according to BS EN 1097 [37]. * S.D. = specific density [g/cm³], W.A = water absorption [%].

Heavy-weight magnetite aggregates have grain sizes of about 0–4, 4–8 and 8–16 mm, while quartz concrete mix contained 22% and 36% of coarse quartz aggregates with 4–8 and 8–16 mm and 42% of sand aggregates.

Fifty percent of quartz aggregates were replaced by basalt for the designing mix of basalt concrete. Table 1 shows the design mix of different concrete types according to the different aggregates, different water-cement ratios and different supplementary materials.

Parameters	Mixes	CEM I 52.5 N SR (kg/m^3)	Water (kg/m^3)	w/c (%)	Aggregate (kg/m^3)	Grain size [kg/m^3]					Admixture (kg/m3)	Air (l/m^3)	Design Mix Density (kg/m^3)
						0/4 (mm)	4/8 (mm)	8/14 (mm)	11/22 (mm)	Total	Glenium C300 60% & Glenium 51 40%
Aggregates	MG-R	380	160	0.42	Magnetite 100%	3310				3310	13.3	15	3863
	QU-R				Quartz & sand	784	411	672		1866	2.28	10	2408
	BZ-R				Basalt, normal & sand	1098	128	183	943	2353	9.5	10	2902
W/C %	MG-I	380	160	0.42	Magnetite 100%	3310				3310	13.3	15	3863
	MG-II		179	0.47		3237				3237	9.5		3805
	MG-III		198	0.52		3158				3158	6.48		3743
Additives	MG-R	380	160	0.42	Magnetite 100%	3310				3310	13.3	15	3863
	MG-MK					3271				3271	13.3		3844
	MG-BC					3233				3233	13.3		3826

Table 1.

Concrete mix proportions.

3.2 Laboratory experiments

The experimental programme includes two parts, sample preparation and laboratory experiments. Cubes and prisms of concrete specimens were manufactured using a standard mould. After the demoulding process, each specimen was cured in water for seven days. Afterwards, the samples were kept at ambient temperature until the 28th day and tested.

Every piece was subjected to temperatures of 20, 150, 300, 500 and 800°C for 2 h, with a heat rate of 1°C/min. At the same time, the heat treatment curve was similar to the ISO fire curve. Afterwards, the physical, mechanical and thermal properties were evaluated for different samples (Figure 4).

Figure 4.

(a) Mixing, (b) casting, (c) demoulding, (d) heat treatment, (e) compressive strength and (f) thermal properties’ measurements.

Physical properties include visual observations, mass loss and density measurements. After the heat treatment, pictures were taken from cooled samples; however, several measurements were taken for each specimen before and after the fire treatment to determine the difference in mass. The thermal conductivity $\lambda$ and specific heat Cp measurements for the different samples were taken using a heat transfer analyser (ISOMET 2104) instrument (Figure 4f).

4. Discussion and results

4.1 Visual observation

Visible changes were recognised in the different concrete mixes after the heat treatment procedures. Visual changes include cracks, colour changes and spalling. As shown in Table 2, there are no visible changes in different concrete mixes at 150°C. However, only evaporation water was released from the sample.

Table 2.

Concrete mix proportions.

At 300 C, there were no significant changes in different concrete mixes; however, only minor spalling spots were recognised in basalt and quartz concrete. Accordingly, adding metakaolin to magnetite heavy-weight concrete caused explosive spalling at 300°C; therefore, it increases the internal pressure and decreases the permeability of concrete. At 500°C, there are no apparent changes in the magnetite-based concrete and at different water-cement ratios. On the other hand, cracks were observed on the basalt concrete; however, these cracks were relatively more and were accompanied by explosive spalling in quartz concrete. Therefore, it explains the low thermal resistance of quartz concrete compared to those of basalt and magnetite concrete. Alternatively, at 500°C and 800°C, adding metakaolin and boron carbide caused explosive spalling in magnetite heavy-weight concrete. Therefore, supplementary materials decreased the permeability of the cement paste and rapid growth in the heat raised the thermal stress and caused the spalling of heavy-weight concrete (Table 2).

At 800°C, extra cracks, colour changes and surface spalling appeared on the quartz and basalt concrete. Consequently, explosive spalling was recognised on magnetite-heavy concrete, where 20% of the mass was lost. Raising the water-cement ratio of magnetite concrete reduced the risk of explosion spalling by increasing the porosity of the concrete (Table 2).

4.2 Thermal properties

According to the results, the thermal properties of different concrete mixes were varied, specifically in mixes with different aggregates. On the other hand, supplementary materials influence the thermal behaviours of heavy-weight concrete. As shown in Figure 5, magnetite concrete has a relatively high density compared to basalt and quartz concrete. Therefore, the density of magnetite concrete ranged between (3700–3750 kg/m3), and between (2400–2450 kg/m3) in basalt concrete, while it was in the range of (2350–2400 kg/m3) in normal quartz concrete. Therefore, adding metakaolin and boron carbide additives increased the density of magnetite concrete. There was a direct influence on the density and the thermal conductivity of different concrete mixes and their thermal stability (Figure 5).

Figure 5.

Density and thermal conductivity of different concrete mixes.

However, as shown in Figure 5, magnetite reference concrete has relatively high thermal conductivity and density, which explains its good thermal resistance against fire up to 800°C. Accordingly, the thermal conductivity and density of basalt concrete were relatively low. Therefore, they enhanced the thermal stability of basalt concrete up to 500°C. In contrast, quartz concrete has a relatively high thermal conductivity, but lower density, which therefore weakened its thermal resistance against fire above 300°C. Adding metakaolin and boron carbide raised the thermal conductivity of magnetite heavy-weight concrete. However, it increased the risk of explosive spalling at high temperatures (Figure 5).

There was a strong correlation between thermal conductivity, density and heat capacity. Concrete with low heat capacity has relatively good thermal resistance. As shown in Figure 6, the heat capacity of magnetite concrete was low; therefore, it explained its good thermal resistance. Accordingly, the heat capacity of basalt concrete is high despite its low thermal conductivity; therefore, it can resist heat/fire up to 500°C. On the other hand, quartz concrete has relatively high heat capacity and thermal conductivity compared to its low density. Therefore, it has weakened its thermal stability. Raising the water/cement ratio of magnetite heavy-weight concrete reduced the heat capacity. Consequently, it improved its thermal resistance, specifically at 800°C. Adding metakaolin and boron carbide increased the heat capacity of magnetite concrete; however, it directly worked on the rapid growth of the thermal stress and caused the spalling of concrete (Figure 6).

Figure 6.

Heat capacity and thermal conductivity of different concrete mixes.

4.3 Mechanical properties

As shown in Figure 7, the compressive strength of concrete mixes varied at different concrete mixes. Therefore, at ambient temperatures, the compressive strength of quartz concrete was relatively high compared to those of basalt and magnetite concrete. In contrast, magnetite reference concrete has a relatively lower compressive strength than basalt. Therefore, it was due to the moisture content in the property of magnetite concrete. Thus, raising the water-cement ratio of magnetite concrete improved the compressive strength at ambient temperatures. Accordingly, adding metakaolin and boron carbide improved the compressive strength of magnetite concrete at ambient temperatures. Therefore, it was due to the decreasing permeability of the heavy-weight concrete (Figure 7).

Figure 7.

Compressive strength of different concrete mixes at the ambient temperatures.

As shown in Figure 8, the compressive strength of concrete mixes gradually declined after the heat treatment. Therefore, at 150°C, the compressive strength of quartz and basalt concrete declined by 11 and 10%, respectively, whereas the declination rate in magnetite concrete was relatively highest by 22%. Therefore, it was due to the release of unbounded water. At 300°C, there was no significant declination in the compressive strength of different concrete mixes. Therefore, it became stable and sometimes increased due to the continuous evaporation and the beginning of the dehydration of the cement gel. Up to 500°C, the compressive strength of different concrete mixes significantly developed due to a decline in the moisture content caused by the maximum dehydration of the calcium silicate hydrates (CSH).

Figure 8.

Relative residual compressive strength of different concrete mixes at high temperatures.

Moreover, the development was significant by adding metakaolin and boron carbide additives and at high w/c ratios in concrete mixes. In contrast, at 500°C, the compressive strength of quartz concrete immensely declined by 60% due to the decomposition process of the CSH. At 800°C, the compressive strength of quartz and basalt concrete steeply declined by 87 and 77%, respectively, due to the second stage of the CSH decomposition (Figure 8).

However, magnetite concrete has a relatively stable compressive strength value of up to 800°C. However, increasing the water-cement ratio and adding metakaolin and boron carbide declined the compressive strength at 800°C due to the high decomposition level of the cement CSH. Furthermore, metakaolin and boron carbide raised the thermal stress while raising the water/cement ratio developed the pore volume, though together, they accelerated the CSH decomposition.

As shown in Figure 9, at 150°C, the flexural strength of different concrete mixes gradually declined due to the evaporation of unbounded water. However, an exception was recognised in quartz concrete, due to the high evaporation level of chemically bounded water. At 300°C, the flexural strength of different concrete mixes developed; therefore, it indicates the high dehydration level of the cement paste. At 500°C up to 800°C, the flexural strength of different concrete mixes gradually declined due to the dehydration and decomposition of the CSH. However, the decomposition level of the CSH was relatively high at 500 up to 800°C in quartz concrete and at 800°C in basalt concrete. On the other hand, raising the w/c ratio improved the flexural strength of magnetite concrete by reducing the CSH decomposition level (Figure 9).

Figure 9.

Relative residual flexural strength of different concrete mixes at high temperatures.

5. Conclusion

The properties of the heavy-weight concrete at elevated temperatures were reviewed and explained. Heavy-weight concrete can attenuate hazardous radiation and has good thermal/fire resistance. In addition, it can retain the physical and mechanical properties at high temperatures. For more details, the physical, thermal and mechanical properties were studied before and after the heat treatment for different concrete mixes. Therefore, visual observation, thermal properties, compressive strength and flexural strength were investigated for the magnetite, quartz and basalt concrete in addition to magnetite at different w/c ratios and with and without metakaolin and boron carbide additive.

Due to its high density and good thermal properties, magnetite heavy-weight concrete can resist heat/fire up to 800°C. On the other hand, quartz concrete has good thermal properties up to 300°C. Afterwards, it suffers from cracks, colour changes and explosive spalling in addition to the steep declination in the compressive and flexural strength at 500 and 800°C, respectively. However, it is owing to the high and rapid decomposition level of the CSH. Basalt concrete can resist heat/fire up to 500°C due to its low thermal conductivity. Hence, its physical and mechanical properties dropped at 800°C. Therefore, it indicates a high CSH decomposition level at 800°C.

Raising the water/cement ratio of the heavy-weight magnetite concrete eliminates the risk of explosive spalling at 800°C. At the same time, it reduced the compressive strength at 800°C. In contrast, adding metakaolin and boron carbide to the heavy-weight magnetite increased the explosive spalling risk at 500 and 800°C, respectively. Therefore, it was due to the influence of the supplementary materials on raising the heat conductivity and heat capacity of the heavy-weight magnetite concrete. Alternatively, at 800°C, adding metakaolin and boron carbide accelerated the decomposition level of the CSH.