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Re: Effect of Two Grade of Concrete on Flexural Reinforcement

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Lets see what can happen:
  1. Shear: It may be safer to use Vc for C45 concrete in your design
  2. Flexure (top)    No Effect
  3. Flexure (bot)    Use C45 in calculation
  4. Deflection        Use composite section method. However, you can include flange effect of slab where possible.
  5. Laps (bottom)    No effect.
  6. Laps (top)        Use laps based on C45 in design
  7. Interface Crack    Check if dowelling effect from stirrups is enough to resist shear by shear friction method at this level. Intentionally roughened joining surface may be required.  
  8. Differential Shrinkage/Creep     If beams are continuous, this will have some effect. Should not be a problem in ULS but may be checked if crack control is important.
  9. Post-Tensioning system    If your beams are post-tensioned before pouring of slabs, it may complicate matters.
  10. Frames part of Seismic system    No problem as C45 is still higher than 3000psi requirement.  
On the whole it appears quite feasible.
----- Original Message -----
Sent: Tuesday, May 23, 2006 12:09 PM
Subject: Effect of Two Grade of Concrete on Flexural Reinforcement

Hi all!
Has anyone of you accepted heterogeneous beam construction using two grades of concrete?  Part of the beam (about 2/3 of its height) is poured with C60 and the rest of the beam (about 1/3 of its height) is poured with the slab using C45.  This construction method is suggested to speed up construction of a 50-storey building.  The beams were designed using C60.

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