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This is a wonderful question. Yes, both the statements are valid assumptions, but the bond is not cracked. The crack formation actually depends on fracture mechanics. In simpler terms concrete can take up some tension, but at the design moment it is cracked chunks of concrete held by the steel. Though we make the assumption that concrete tension is neglected, it only for bending purposes at design or ultimate loads.

Consider a simply supported under-reinforced RCC beam, rectangular c/s with no pre-load. We start loading it gradually with a UDL. The portion above the NA is under pure compression and portion below is in tension, strain in steel at this stage is same as strain in concrete and both materials act homogeneously. When the strain in concrete reaches the tensile rupture of concrete, the first crack is formed. This usually occurs at the point of maximum curvature, which in our case will be the center of the beam span on the tension side. Now at this location concrete can no more carry any tension and all the strain will be in steel, just at that location. On increasing the load further, this crack starts opening up. Meanwhile at other places in the beam, cracks will start developing where similar phenomena occurs (tensile strain exceeding the rupture strain of concrete). Between two such cracks, which will be quite apart, concrete is capable of carrying tension and both steel and concrete are perfectly bonded.

For design purposes we design the cracked section assuming the concrete below the NA to be cracked. The perfect bond between concrete and steel is when there is sufficient development length for steel bars to transfer all the tensile force into the concrete.

Detailed explanation can be found in textbooks by Edward G Nawy or James McGregor.



Viswanath Urala,

Structural Engineer

Walter P Moore, USA

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