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Reinforced concrete design 5

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The tensile strength of concrete is only about 10 per cent of the compressive strength. Because of this, nearly all reinforced concrete structures are designed on the assumption that the concrete does not resist any tensile forces. Reinforcement is designed to carry these tensile forces, which are transferred by bond between the interface of the two materials. 

If this bond is not adequate, the reinforcing bars will just slip within the concrete and there will not be a composite action. Thus members should be detailed so  that  the  concrete  can  be well compacted around the  reinforcement  during construction. In addition, some bars are ribbed or twisted so that there  is  an extra mechanical grip.

 In the analysis and design of the composite reinforced concrete section, it is assumed that there is  a perfect bond, so that the strain in the reinforcement is identical to the strain in the adjacent concrete. 

This ensures that there is what is known as 'compatibility of strains' across the cross-section of the member. The coefficients of thermal expansion for steel and for concrete are of the order of 10 x 10-6 per °C and 7-12 x 10-6 per °C respectively.

 These values are sufficiently close that problems with bond seldom arise from differential expansion between the two materials over normal temperature ranges. Figure  illustrates the behaviour of a simply supported beam subjected to bending and shows the position of steel reinforcement to  resist the tensile forces, while the compression forces in the top of the beam are carried by the concrete.herever tension occurs it is likely that cracking of the concrete will take place. 

This cracking, however, does not detract from the safety of the structure provided there is good reinforcement bond to ensure that the cracks are restrained from opening so that the embedded steel continues to be protected from corrosion. When the compressive or shearing forces exceed the strength of the concrete, then steel reinforcement must again be provided, but in these cases it is  only required to supplement the  load-carrying  capacity of the  concrete.

 For example, compression reinforcement is generally required in a column, where it takes the form of vertical bars spaced near the perimeter. To prevent these bars buckling, steel binders are used to assist the restraint provided by the surrounding concrete.