混凝土基本概念 专业英语

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The primary purpose of the steel reinforcement is to carry internal tensile forces.Reinforcement is therefore placed in beams near the tensile face,i.e,near the lower face in the in span regions of positive moment and near the upper face in regions near intemal supports,where negative moments act. This is illustrated in Fig.10.1,In reinforced concrete design,it is important to provide reinforcement in all regions of potential cracking.

In such members , bending is usually present in addition to axial compression and the longitudinal steel reinforcement at each face of the member may act either in tension or compression . transverse ties are used to maintain the longitudinal steel in position during casting of the concrete and later to prevent its outward buckling when it is subjected to compressive stress. Again , the reinforcing steel for the column may be preassembled into a cage .

In a floor slab , bars are usually laid in the two main span direction at right angles , to resist the tensile force produced by the bending actions in each direction. For ease of construction , welded mesh is frequently used as slab reinforcement .

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Thus a rectangular arrangement of vertical and horizontal steel bars is introduced into regions of a beam where inclined cracks can form as a result of combined shearing action and bending moment.The longitudinal steel,or main reinforcement,and the transverse bars,called stirrups,may be preassembled into a reinforcing cage for ease of construction.
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The tensile strength of concrete is extremely low ,about one-tenth of its compressive strength ,and this precludes the use of plain concrete for most structural members . The elastic modulus for concrete subjected to compressive stress of short duration is reasonably high ,in the range of 20，000 to 30，000 MPa (about one-tenth , of the elastic modulus of steel); however ,concrete undergoes large additional long-term deformation due to creep and shrinkage ,so that the effective stiffness is much lower-perhaps a third to a quarter of the instantaneous stiffness .
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The widespread use of concrete in engineering construction stems from its cheapness compared with other structural materials currently available .Its lack of tensile strength is overcome by including reinforcement ,usually in the form of steel bars, to produce a composite material known as reinforced concrete .Although the steel reinforcement does not prevent the cracks from widening ,and it provides need is usually quite small ,relative to the volume of concrete ,so that the total cost of reinforced concrete construction remains commercially very competitive .

Although steel reinforcement is used primarily to carry the internal tensile forces produced by external loading , it also has other uses . steel is much stronger than concrete in compression ,and it is sometimes used to boost to the resistance of zones of compression , when the overall dimensions of the member are restricted . longitudinal steel is placed in all compression members.
Basic Concepts of Reinforced Concrete
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Although concrete is used very extensively in the construction of buildings ,bridges and many other engineering structures ,its mechanical properties are far from ideal .For example ,it is not a particularly strong material .The compressive strength of structural grade concrete ranges typically from 20 to 40 MPa ,or about 3000 to 6000 lb/in2 .This is somewhat lower than the compressive strength range of most timbers used in structural work .
Biblioteka Baidu

Cracking in concrete may be caused not only by external loading ,but also by temperature gradients and differential or restrained shrinkage .Secondary reinforcement is therefore provided to control such cracking ,which may be unsightly and even dangerous .