《Assessment of natural radioactivity levels and radiation hazards due to cement industry 土木专英论文.doc》由会员分享，可在线阅读，更多相关《Assessment of natural radioactivity levels and radiation hazards due to cement industry 土木专英论文.doc（10页珍藏版）》请在三一办公上搜索。
1、Assessment of natural radioactivity levels and radiation hazards due to cement industryAbstractThe 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 ceme
2、nt 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 cou
3、ntries. 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 ra
4、diological hazard when used for construction of buildings.Keywords: Natural radioactivity; Cement; Raw materials; Radiation hazards1. IntroductionThe 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 mines a
5、nd 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. There ar
6、e 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 characteristi
7、cs. 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 and it
8、s 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 radia
9、tion 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 popul
10、ation 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 70nGyh1. Indoors elevated external dose rates may arise from high activities of radionuclides in building materials (Zikovsk
11、y 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 is limi
12、ted. 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 as a bui
13、lding 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, external haza
14、rd 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 technique2.1. Sampling and sample preparationFifty seven samples o
15、f 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 were taken fr
16、om 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 Helwan). Each samp
17、le, about 1-kg in weight was washed in distilled water and dried in an oven at about 110C to ensure that moisture is completely removed, The samples were crushed, homogenized, and sieved through a 200mesh, which is the optimum size to be enriched in heavy minerals. Weighted samples were placed in a
18、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 daughters will
19、 also remain in the sample.2.2. Instrumentation and calibrationActivity 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% specified at the 66
20、2keV peak of 137Cs. To reduce gamma ray background a cylindrical lead shield (100mm thick) with a fixed bottom and movable cover shielded the detector. The lead shield contained an inner concentric cylinder of copper (0.3mm thick) to absorb lead X-rays. In order to determine the background distribut
21、ion 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 43200s. The background spectra were used to correct the net peak area of gamma rays of measured isotopes. A de
22、dicated software program (Genie 2000 from Canberra) analyzed each measured -ray spectrum.3. ConclusionThe 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 results in other countries. T
23、he 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 that the averages of radiat
24、ion hazard parameters for Assiut cement factory are lower than the acceptable level 370Bqkg1 for radium equivalent Raeq, 1 for level index Ir, the external hazard index Hex 1 and 59 (nGyh1) for absorbed dose rate. The manufacturing operation reduces the radiation hazard parameters. So cement product
25、s 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 important from the point of v
26、iew 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 ConcreteConcrete is strong in compression , but weak i
27、n 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 is imposed in the longitudina
28、l 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 capacities of the sections . The secti
29、ons 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 prestressing force , i.e. , a compres
30、sive 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 determined mainly on the basis of t
31、he 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 caused by the load will first h
32、ave 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 reach the tensile strength of th
33、e 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 eccentricity relative to the concret
34、e 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 inertia of the cross-section, is the
35、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 compression induced by the prestr
36、essing. 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 vessels , essentially follows the same
37、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 the preceding discussion , it is
38、 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 the tensile strength of the conc
39、rete 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 member has reached its limit stat
40、e 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 the member , permitting a relati
41、vely 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 concrete counterparts for the same sp
42、an 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 amount of reinforcement. Unfortu
43、nately , 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 geometry of prestressed sections is u
44、sually 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 large. And the indirect long-term
45、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 reinforced concrete exceeds 70 to
46、 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 do not perform as well due to th
47、e 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 engineer from California, patented a p
48、restressing 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, Nebraska , recognized the effect of
49、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 reinforceme
