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1、Assessment of natural radioactivity levels and radiation hazards due to cement industry Abstract The cement industry is considered as one of the basic industries that plays an important role in the national economy of developing countries. Activity concentrations of 226Ra, 232Th and 40K in Assiut ce
2、ment and other local cement types from different Egyptian factories has been measured by using -ray spectrometry. From the measured -ray spectra, specific activities were determined. The measured activity concentrations for these natural radionuclides were compared with the reported data for other c
3、ountries. The average values obtained for 226Ra, 232Th and 40K activity concentration in different types of cement are lower than the corresponding global values reported in UNSCEAR publications. The manufacturing operation reduces the radiation hazard parameters. Cement does not pose a significant
4、radiological hazard when used for construction of buildings. Keywords: Natural radioactivity; Cement; Raw materials; Radiation hazards 1. Introduction The need for cement is so great. That it considered a basic industry. Workers exposed to cement or its raw materials for a long time especially in mi
5、nes and at manufacturing sites as well as people, that spend about 80% of their time inside offices and homes (Mollah et al., 1986; Paredes et al., 1987) result in exposure to cement or its raw materials being necessary reality so we should know the radioactivity for cement and its raw material. The
6、re are many types of cements according to the chemical composition and hydraulic properties for each one. Portland cement is the most prevalent one. The contents of 226Ra, 232Th and 40K in raw and processed materials can vary considerably depending on their geological source and geochemical characte
7、ristics. Thus, the knowledge of radioactivity in these materials is important to estimate the radiological hazards on human health. The radiological impact from the natural radioactivity is due to radiation exposure of the body by gamma-rays and irradiation of lung tissues from inhalation of radon a
8、nd its progeny (Papastefanou et al., 1988). From the natural risk point of view, it is necessary to know the dose limits of public exposure and to measure the natural environmental radiation level provided by ground, air, water, foods, building interiors, etc., to estimate human exposure to natural
9、radiation sources (UNSCEAR, 1988). Low level gamma-ray spectrometry is suitable for both qualitative and quantitative determinations of gamma-ray-emitting nuclides in the environment (IAEA, 1989). The concentration of radio-elements in building materials and its components are important in assessing
10、 population exposures, as most individuals spend 80% of their time indoors. The average indoor absorbed dose rate in air from terrestrial sources of radioactivity is estimated to be 70 nGy h1. Indoors elevated external dose rates may arise from high activities of radionuclides in building materials
11、(Zikovsky and Kennedy, 1992). Great attention has been paid to determining radionuclide concentrations in building materials in many countries (Amrani and Tahtat, 2001; Rizzo et al., 2001; Kumar et al., 2003; Tzortzis et al., 2003). But information about the radioactivity of these materials in Egypt
12、 is limited. Knowledge of the occurrance and concentration of natural radioactivity in such important materials is essential for checking its quality in general and knowing its effect on the environment surrounding the cement producing factories in particular. Because of the global demand for cement
13、 as a building material, the present study aims to: (1) Assess natural radioactivity (226Ra, 232Th and 40K) in raw and final products used in the Assiut cement factory and other local factories in Egypt. (2) Calculate the radiological parameters (radium equivalent activity Raeq, level index Ir, exte
14、rnal hazard index Hex and absorbed dose rate) which is related to the external -dose rate. The results of concentration levels and radiation equivalent activities are compared with similar studies carried out in other countries. 2. Experimental technique 2.1. Sampling and sample preparation Fifty se
15、ven samples of raw materials and final products used in the Assiut cement factories were collected for investigation. Twenty five samples of raw materials were taken from (Limestone, Clay, Slag, Iron oxide, gypsum) which are all the raw material used in cement industry, 20 samples of final products
16、were taken from Assiut cement (Portland, El-Mohands, White, and Sulphate resistant cement (S.R.C). For comparison with products from other factories, 8 samples were taken from the ordinary Portland cement from (Helwan, Qena, El-kawmya, Torra) and 4 samples were taken of white cement (Sinai and Helwa
17、n). Each sample, about 1-kg in weight was washed in distilled water and dried in an oven at about 110 C to ensure that moisture is completely removed, The samples were crushed, homogenized, and sieved through a 200 mesh, which is the optimum size to be enriched in heavy minerals. Weighted samples we
18、re placed in a polyethylene beaker, of 350-cm3 volume. The beakers were completely sealed for 4 weeks to reach secular equilibrium where the rate of decay of the radon daughters becomes equal to that of the parent. This step is necessary to ensure that radon gas is confined within the volume and the
19、 daughters will also remain in the sample. 2.2. Instrumentation and calibration Activity measurements were performed by gamma ray spectrometry, employing a 33scintillation detector. The hermetically sealed assembly with a NaI(Tl) crystal is coupled to a PC-MCA (Canberra Accuspes). Resolution 7.5% sp
20、ecified at the 662 keV peak of 137Cs. To reduce gamma ray background a cylindrical lead shield (100 mm thick) with a fixed bottom and movable cover shielded the detector. The lead shield contained an inner concentric cylinder of copper (0.3 mm thick) to absorb lead X-rays. In order to determine the
21、background distribution in the environment around the detector, an empty sealed beaker was counted in the same manner and in the same geometry as the samples. The measurement time of activity or background was 43 200 s. The background spectra were used to correct the net peak area of gamma rays of m
22、easured isotopes. A dedicated software program (Genie 2000 from Canberra) analyzed each measured -ray spectrum. 3. Conclusion The natural radionuclides 226Ra, 232Th and 40K were measured for raw materials and final products used in the Assiut cement factory in Upper Egypt and compared with the resul
23、ts in other countries. The activity concentration of 40K is lower than all corresponding values in other countries. The activity concentration of 226Ra and 232Th for all measured samples of Portland cement are comparable with the corresponding values of other countries. The obtained results show tha
24、t the averages of radiation hazard parameters for Assiut cement factory are lower than the acceptable level 370 Bq kg1 for radium equivalent Raeq, 1 for level index Ir, the external hazard index Hex 1 and 59 (nGy h1) for absorbed dose rate. The manufacturing operation reduces the radiation hazard pa
25、rameters. So cement products do not pose a significant radiological hazard when used for building construction. The radioactivity in raw materials and final products of cement varies from one country to another and also within the same type of material from different locations. The results may be im
26、portant from the point of view of selecting suitable materials for use in cement manufacture. It is important to point out that these values are not the representative values for the countries mentioned but for the regions from where the samples were collected. Prestressed Concrete Concrete is stron
27、g in compression , but weak in tesion : its tensile strengh varies from 8 to 14 percent of its compressive strength . Due to such a low tensile capacity , flexural cracks develop at early stages of loading . In order to reduce or prevent such cracks from developing , a concentric or eccentric force
28、is imposed in the longitudinal direction of the structural element . This force prevents the cracks from developing by eliminating or considerably reducing the tensile stresses at the critical midspan and support sections at service load, thereby raising the bending , shear , and torsional capacitie
29、s of the sections . The sections are then able to behave elastically , and almost the full capacity of the concrete in compression can be efficiently utilized across the entire depth of the concrete sections when all loads act on the structure . Such an imposed longitudinal force is called a prestre
30、ssing force , i.e. , a compressive force that prestresses the sections along the span of the structual element prior to the application of the transverse gravity dead and live loads or transient horizontal live loads . The type of prestressing force involved , together with its magnitude , are deter
31、mined mainly on the basis of the type of system to be constructed and the span length and slenderness desired . Since the prestressing force is applied longitudinally along or parallel to the axis of the member , the prestressing principle involved is commonly known as linear prestressing . Tension
32、caused by the load will first have to cancel the compression induced by the prestressing before it can crack the concrete. Figure 4.39a shows a reinforced concrete simple-span beam cracked under applied load. At a relative low load, the tensile stress in the concrete at the bottom of the beam will r
33、each the tensile strength of the concrete , and cracks will form. Because no restraint is provided against upward extension of cracks, the beam will collapse. Figure 4.39b shows the same unloaded beams with prestressing forces applied by stressing high strength tendons. The force, applied with eccen
34、tricity relative to the concrete centroid, will produce a longitudinal compressive stress distribution varying linearly from zero at the top surface to a maximum of concrete stress, = , at the bottom, where is the distance from the concrete centroid to the bottom beam, and is the moment of the inert
35、ia of the cross-section, is the depth of the beam. An upward camber is then created. Figure 4.39c shows the prestressed beams after loads have been applied. The loads cause the beam to deflect down, creating tensile stresses in the bottom of the beam. The tension from the loading is compensated by c
36、ompression induced by the prestressing. Tension is eliminated under the combination of the two and tension cracks are prevented. Also, construction materials (concrete and steel) are used more efficiently. Circular prestressing , used in liquid containmeng tanks , pipes , and pressure reactor vessel
37、s , essentially follows the same basic principles as does linear prestressing . The circumferential hoop . or “hugging” stress on the cylindrical or spherical structure , neutralizes the tensile stresses at the outer fibers of the curvilinear surface caused by the internal contained pressure . From
38、the preceding discussion , it is plain that permanent stresses in the prestressed structural member are created before the full dead and live loads are applied in order to eliminate or considerably reduce the net tensile stresses caused by these loads . With reinforced concrete , it is assumed that
39、the tensile strength of the concrete is negligible and disregarded . This is because the tensile forces resulting from the bending moments are resisted by the bond created in the reinforcement process . Cracking and deflection are therefore essentially irrecoverable in reinforced concrete once the m
40、ember has reached its limit state at service load . The reinforcement in the reinforced concrete member does not exert any force of its own on the member , contrary to the action of prestressing steel . The steel required to produce the prestressing force in the prestressed member actively preloads
41、the member , permitting a relatively high controlled recovery of cracking and deflection . Once the flexural tensile strength of the concrete is exceeded , the prestressed member starts to act like a reinforced concrete element . Prestressed members are shallower in depth than their reinforced concr
42、ete counterparts for the same span and loading conditions . In general , the depth of a prestressed concrete member is usually about 65 to 80 percent of the depth of the equivalent reinforced concrete member . Hence , the prestressed member requires less concrete , and about 20 to 35 percent of the
43、amount of reinforcement. Unfortunately , this saving in material weight is balanced by the higher cost of the higher quality materials needed in prestressing . Also, regardless of the system used , prestressing operations themselves result in an added cost : formwork is more complex ，since the geome
44、try of prestressed sections is usually composed of flanged sections with thin webs . In spite of these additional costs, if a large enough number of precast units are manufactured, the difference between at least the initial costs of prestressed and reinforced concrete systems is usually not very la
45、rge. And the indirect long-term savings are quite substantial, because less maintenance is needed, a longer working life is possible due to better quality control of the concrete, and lighter foundations are achieved due to the smaller cumulative weight of the superstructure. Once the bean span of r
46、einforced concrete exceeds 70 to 90 feet (21.3 to 27.4 m), the dead weight of the beam becomes excessive, resulting in heavier members and,consequently,greater long-term deflection and cracking. Thus,for larger spans,prestressed concrete becomes mandatory since arches are expensive to construct and
47、do not perform as well due to the severe long-term shrinkage and creep they undergo.Very large spans such as segmental bridges or cable-stayed bridges can only be constructed through the use of prestressing . Prestressed concrete is not a new concept, dating back to 1872,when P.H. Jackson ,an engine
48、er from California, patented a prestressing system that used a tie rod to construct beams or arches from individual block. After a long lapse of time during which little progress was made because of the unavailability of high-strength steel to overcome prestress losses, R.E. Dill of Alexandria, Nebr
49、aska , recognized the effect of the shrinkage and creep(transverse material flow) of concrete on the loss of prestress. He subsequently developed the idea that successive post-tensioning of unbonded rods would compensate for the time-dependent loss of stress in the rods due to the decrease in the length of the member because of creep and shrinkage. In the early 1920s, W.H.Hewett of Minneapolis developed the principles of circular prestressing He hoop-stressing horizontal reinforcement around walls of concrete tanks through the use of turnbuckles to prevent cracking du