Investigation of the Mechanical Property of Low-Activation Concrete

This paper presents the strengths and fracture toughness of low-activation concrete (LAC) which can be used in the nuclear shielding structures and possibly in the nuclear decommissioning project. The research used four mixtures which include high alumina cement with 20%, 40% and 60% aluminum oxide as fine aggregate replacement and compared with those of normal concrete. All of the concrete strengths were evaluated and compared. The test results showed that LAC has high early strength and decays later due to conversion effect. The other fundamental mechanical properties will also be presented in this paper.


Introduction
Although the Fukushima nuclear power plant accident in 2011 has attracted plenty attentions in nuclear safety issue, nuclear energy still plays a significant role in the world due to its high cost-efficiency and little carbon emission.Therefore, the structural technology dealing with the nuclear facilities still needs to be innovated.Conventionally, heavy concrete is mostly used in biological shields or shielding walls in a reactor vessel [1] .Kan and Pei [2] have found that the compressive and tensile strengths of heavy concrete did not differ from those of regular mortar at the same water to cement ratio; however, the modulus of elasticity increases with the heavy aggregate aggregates.Kan and Yang [3] later also investigated the toughness of heavy concrete strengthened by the inclusion of steel fibers.Recently, M. Kinno et al. [4][5] has attempted to develop many mixtures of low-activation concrete and reported that the concrete including fused alumina aggregate and high alumina cement contribute led to the least activation among all the designated mixtures.Thus, it seems reasonable and necessary to access the feasibility to use this type of material in shielding structure design.Thus, the research investigated many engineering properties and strengths of low-activated concrete influenced by the various aggregates of high aluminum inclusion

Materials and Mixtures
A Type I Portland cement and an alumina cement produced in France were used in this study.The typical constituent of both cements are shown in Table 1.The mechanical properties of concretes made were tested and compared.Mixtures of both concrete have a same water/cement ratio of 0.5.Table 2 shows the all mixtures.The amount of boron carbide B4C is fixed at 10 % by weight for all LAC mixtures; while the amount of Al2O3 aggregates is varied.The low-activated concretes were designated with 20 %, 40 % and 60 % fine aggregate replacement by fused aluminum oxide (Al2O3 aggregates) by weight, to examine the effect of the high alumina cement and various amounts of Al2O3 aggregates on the mechanical properties of LAC.The LAC mix with 40% sand replacement by Al2O3 aggregates, for example, is specified as LAC40.It is noted that the unit weight of concrete increases with the Al2O3 aggregates, as shown in Fig. 3. And, it is found that the placement in concrete with 60% of Al2O3 aggregates by weight produced a harsh mix in workability.

High Alumina Cement
In high alumina cement, the predominant compounds are calcium aluminates; calcium silicates account for no more than a few percent.The calcium aluminates react with water and the primary product is calcium aluminate decahydrate (CAH10) which has essentially a hexagonal crystal form.This material is unstable and changes to tricalcium aluminate hexahydrate (C3AH6) spontaneously.
The process occurs at room temperature and is accelerated by an increase in temperature.The effect of conversion, as it is known, is accompanied by an increase in porosity and a decrease in strength.

Fused Alumina Aggregates
Unlike normal concrete, the main feature of heavy concrete is the inclusion of metallic fillers, which are usually ilmenite, limonite-goethite, serpentine, magnetite, barite, ferrophosphorous, steel aggregate and iron shot.
In this study, fused aluminate aggregates were adopted as part of fine aggregates in concrete.It has an average grain size of 1 mm (ANSI No. 20) with a specific gravity is 3.90 and a hardness of 9 in Moh's scale.

Boron carbide
The ability of boron carbide to absorb neutrons without forming long-lived radionuclides makes it attractive as an absorbent for neutron radiation arising in nuclear power plants.Nuclear applications of boron carbide include shielding, control rod and shut down pellets.Within control rods, boron carbide is often powdered, to increase its surface area.In this study, boron carbide powder was used as part of concrete inclusion.It has an average grain size of 0.045 mm (ANSI No. 240) with a specific gravity is 2.52 and a hardness of 9.6 in Moh's scale.

Testing Specimens
Four types of specimens made from the above four mixtures were fabricated in this testing program.The  10 x 20 cm cylinders were used to determine the mechanical properties of concrete, including the compressive strength and tensile strength, modulus of elasticity and wave velocity.

Results and Discussion
The fundamental mechanical properties of concrete including compressive strength fc', splitting tensile strength ft'and elastic modulus Ec are presented in Table 3 and discussed as following.

Compressive Strength
. The compressive strength of LAC exceeds that of normal concrete and increases with Al2O3 aggregates for a given water/cemnt ratio.The compressive strengths for all mixes are shown in Fig. 2. It reveals that an increase in Al2O3 aggregates causes the increase in the CMPSE 2017 Fig. 2 Compressive strength of concretes compressive strength.However, the mixture with an inclusion of 60% of Al2O3 aggregates by weight turns out the lower compressive strength and can be attributed to the bad workability for placing the concrete.The mixture with 40% of Al2O3 aggregates appears the highest strengths in all the curing duration.
The compressive strength of LAC turns out higher in its earlier age (before 7 days of curing), but decays later due to the effect of conversion.

Wave Velocity
The mechanical properties of concrete can be estimated from its wave velocity by means of an ultrasonic wave tester.The test results revealed that the longitudinal wave velocity of LAC increases as the Al2O3 aggregates increases, as shown in Table 4 and Figure 3, which show a consistence with that of compressive strength of LAC.The mixture with 60% of Al2O3 aggregates by weight turns out the lower wave velocity and can be still attributed to the bad workability in placing the concrete.The mixture with 40% of Al2O3 aggregates appears the highest wave velocity before 28 days of curing duration.It is noticed that the wave velocity of all mixtures of LAC turn out decay in 91 day, which can be explained by the conversion effect.

Splitting Tensile Strength
The splitting test was used to determine the tensile strengths of concrete with various amounts of Al2O3 aggregates.Figure 4 shows that the tensile strength of NC is lower than LAC in 28 days, but becomes higher in 91 days of age.Furthermore, the tensile strength of LAC decreases as the Al2O3 aggregates increase.Obviously, the voids resulted from conversion dominate the tensile strength of LAC.The tensile strength reduces in 91 days of curing duration and decays more severe as the Al2O3 aggregates increases.

Elastic Modulus
Test results showed that the modulus of elasticity of LAC exceeds that of normal concrete and increases with Al2O3 aggregates for a given water/cemnt ratio.Figure 5 reveals that an increase in Al2O3 aggregates causes an increase in the elastic modulus, which appears consistent with that of compressive strength.LAC have higher compressive strength than normal for a given water/cement ratio.The compressive strength seems to be higher in the earlier age and decays after 7 days of curing.Concrete including 40% of Al2O3 aggregates as sand replacement performs the highest strength and workability as well.Elastic modulus of concrete appears to increase with the Al2O3 inclusion.The tensile strength seems to be decreasing with the increase of Al2O3 aggregates in concrete.The voids due to the effect of conversion do affect the mechanical properties of concrete.
§All units are in kg/m 3 .*Type I Portland cement concrete; **high aluminum concrete.

Table 3
Mechanical Properties of concrete

Table 4
Wave velocity of concrete