In this article we will discuss about:- 1. Meaning of High Density Concrete 2. Types of Radiation in High Density Concrete and Their Hazards 3. Shielding Ability 4. Aggregates to be Used in Shielding 5. Requirements of Radiation Shielding Concrete 6. Properties.
Meaning of High Density Concrete:
As the name suggests, the density of this concrete varies from 3360 kg/m3 to 3840 kg/m3, whereas the density of normal concrete is of the order of 2400 kg/m3. The density of light weight concrete is about 1900 kg/m3 and that of normal concrete about 2400 kg/m3. Thus the density of high density concrete is about 50% more than the density of conventional concrete. However this concrete can be produced of density upto 5200 kg/m3 using iron as both fine and coarse aggregate.
With the advent of the nuclear energy, there is a considerable demand of the concrete technologists in the market. Due to the use of nuclear energy producing reactors, large scale production of penetrating radiation and radioactive materials also has taken place.
Thus all nuclear energy producing units such as nuclear reactors, particle accelerator, industrial radiography, x-ray and gamma ray therapy units require nuclear shielding material for the protection of the operating personnel against the biological hazards such as radiation. The normal as well as high density concrete is effective and economical for the construction of permanent shield against radiation.
Types of Radiation in High Density Concrete and Their Hazards :
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The radiation can be classified into two groups as follows:
These radiations are considered in the design of biological shields.
1. Electro-magnetic waves.
2. Nuclear particles.
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1. Electro-Magnetic Waves:
These waves are of high frequency and have high energy. These waves are known as X and gamma rays. These are the only electro-magnetic waves which need shields for the protection of personnel. Though they are similar to high rays, but possess higher energy and greater penetrating power. X and gamma rays are identical, but their sources of production are different. Both these rays have high penetration power, but they can be adequately absorbed by an appropriate thickness of concrete shield.
2. Nuclear Particles:
Nuclear particles consist of nuclei of atoms or their fragments. These fragments are known as neutrons, protons, alpha and beta particles. Except neutrons all the other particles possess an electric charge. On the other hand neutrons are un-charged and remain un-affected by electric field, until they interact by collision with a nucleus. They have no definite range and some of them may penetrate any shield.
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Alpha, beta and proton particles carry electrical charge, which interact with electric field, surrounding the atom of the shielding material and lose their energy considerably. Generally these particles do not create a separate shielding problem, though accelerated protons at high energy levels may require heavy shielding comparable to that required for neutrons.
Thus X and gamma rays, and neutrons need protection shield. As stated above X and gamma rays are similar except in energy and origin. The biological hazards of radiation arise from the fact that the radiation interact with human tissues. In the process of interaction some of the energy of the human tissues is lost.
The energy loss is sufficient to ionize the atoms of the cells, upsetting the delicate chemical balance and causing the death of cells. If enough cells are affected, the organism dies. Thus the radiation must be reduced or weekend sufficiently, so that the remainder or left out radiation may not cause permanent damage to the persons exposed to it.
Apart from biological hazards, a very high temperature is also generated by the nuclear reaction. Thus the shielding is necessary to protect the electronic and other sensitive equipment in the vicinity.
Shielding Ability of High Density Concrete:
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Due to the following characteristics, concrete has been found an excellent shielding material:
1. It has sufficient capacity to absorb the radiation both of neutron and gamma rays, reducing the radiation to a very weak state.
2. It has good mechanical properties as strength and durability.
3. When green, it can be moulded into any shape. Thus the ease of construction makes concrete a specially suitable material for radiation shielding.
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4. Its initial and maintenance cost is also relatively low.
Disadvantages:
Its disadvantages are as follows:
1. As the sections of the structure are heavy, they need more space. Thus the use of concrete as shielding against radiation needs more space.
2. The weight of shielding concrete is very high in the range of 3360 to 3840 kg/m3.
Aggregates to be Used in Shielding High Density Concrete:
For making shielding concrete heavy weight aggregate having a specific gravity between 3.5 to 4.0 is needed. There are many aggregates whose specific gravity is more than 3.5 for making a heavy weight concrete.
Some of natural commercially used aggregates are as follows:
1. Barite
2. Magnetite
3. Ilmenite
4. Limonite, and
5. Hematite etc.
Barite is the most common natural aggregate having a specific gravity of 4.1 with 95% purity. Steel and iron aggregates in the form of shots and punching scrap for use as a heavy weight aggregate are also available in the market. They are known as artificial aggregate.
While selecting the aggregate to be used, the availability of the aggregate locally and their physical properties should be considered. In general the heavy weight aggregate should be strong, clean, inert and relatively free from deleterious materials which might impair the strength of concrete.
The capacity of various heavy aggregates to absorb gamma rays is directly proportional to their density. Also the heavier elements are more effective in absorbing fast neutrons by inelastic collisions than the lighter one. Therefore as heavy aggregate as possible should be used for the construction of shield.
However, density is not the only factor to be considered in the selection of an aggregate for neutron concrete shield. The desired increase in hydrogen content required to slow down the fast neutrons, can be accomplished by the use of hydrous ores. These materials contain a high percentage of water of hydration. On heating the concrete, some of this fixed water in the aggregate may be lost. Lemonite and goethite are reliable sources of hydrogen as long as shield temperature does not exceed 200°C, whereas serpentine is good upto 400°C. The physical properties of high density aggregate are shown in Table 23.1 below.
Requirements of Radiation Shielding Concrete:
The important requirements of radiation shielding concrete are as follows:
1. High density of the concrete ― the higher the density of the concrete, higher the absorption of radiation. The radiation shielding quality of concrete can be increased by increasing its density.
2. The other important requirement of radiation shielding concrete is its structural strength even at high temperature.
Thus to produce high density and high strength concrete, it is necessary to control the water/cement ratio very strictly. Appropriate admixture and proper vibrators for good compaction should be employed, after that good quality control should be followed.
High density concrete used for shielding differs from normal weight concrete in the following respects.
High density concrete should contain sufficient material of light atomic weight, which produces hydrogen. Sometimes in high density concrete serpentine aggregates are used due to their ability to retain water of crystallisation at high temperatures which assure a source of hydrogen. The availability of water of crystallisation in all heavy weight aggregate is not necessary.
High density concrete may contain high content of cement-and may exhibit increased creep and shrinkage. Due to the high density of aggregate, it has a tendency of segregation. Coarse aggregate used may be of only high density mineral aggregate, or a mixture of mineral aggregate and steel particles or only steel particles, Experiments have shown that if only smooth cubical pieces of steel or iron ore are used as coarse aggregate, the compressive strength will not be more than 21 MPa, regardless of the water/cement ratio or grout mixture ratio.
If sheared reinforcing bars are used as aggregate along with good grout, normal strength will be obtained. The grout used in high density pre placed aggregate concrete should be somewhat richer than that used in normal density pre placed concrete. High modulus of elasticity, low thermal expansion, and low elastic and creep deformation are the ideal properties for both high density and normal concrete.
Properties of High Density Concrete:
Following properties of high density concrete have been observed:
1. The strength of this concrete measured on standard cylinders has been found 42 MPa at 28 days for a water/cement ratio 0.58 and 24 MPa at water/cement ratio 0.9.
2. The density of this concrete for a mix of 1:4.6:6.4 with water/cement ratio of 0.58 has been found as 3700 kg/m3.
3. The coefficient of thermal expansion of barite concrete measured in the range of temperature of 4°C to 38°C is found about twice that of normal concrete.
4. The modulus of elasticity and poisson’s ratio of high density concrete and normal concrete are approximately the same.
5. Shrinkage of high density concrete is about 1/4 to 1/3 of the normal concrete.
6. Thermal conductivity, diffusivity etc. of high strength concrete is considerably lower than corresponding values for normal aggregate concrete.
7. Concrete made with barite aggregate does not stand well to weathering.
8. Fine barite aggregate delay the setting and hardening process of the concrete, hence trial mixes are advisable.
9. No entrained air should be permitted in this concrete. However use of de-entrain agent is suggested.