【文章內(nèi)容簡(jiǎn)介】 her countries. The activity concentration of 226Ra and 232Th for all measured samples of Portland cement are parable with the corresponding values of other countries. The obtained results show that the averages of radiation hazard parameters for Assiut cement factory are lower than the acceptable level 370 Bq kg?1 for radium equivalent Raeq, 1 for level index Iγr, the external hazard index Hex ≤1 and 59 (nGy h?1) for absorbed dose rate. The manufacturing operation reduces the radiation hazard parameters. 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 important 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 strong in pression , but weak in tesion : its tensile strengh varies from 8 to 14 percent of its pressive 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 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 capacities of the sections . The sections are then able to behave elastically , and almost the full capacity of the concrete in pression 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 , . , a pressive 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 the type of system to be constructed and the span length and slenderness 濰坊學(xué)院本科畢業(yè)論文 5 desired . Since the prestressing force is applied longitudinally along or parallel to the axis of the member , the prestressing principle involved is monly known as linear prestressing . Tension caused by the load will first have to cancel the pression induced by the prestressing before it can crack the concrete. Figure shows a reinforced concrete simplespan 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 the concrete , and cracks will form. Because no restraint is provided against upward extension of cracks, the beam will collapse. Figure shows the same unloaded beams with prestressing forces applied by stressing high strength tendons. The force, applied with eccentricity relative to the concrete centroid, will produce a longitudinal pressive 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 crosssection, is the depth of the beam. An upward camber is then created. Figure 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 pensated by pression induced by the prestressing. Tension is eliminated under the bination 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 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 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 tensile stresses caused by these loads . With reinforced concrete , it is assumed that 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 member 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 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 concrete 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 , th