Some of the engineers say that we do not have to increase concrete strength, which will make the section needs more reinforcement!
This is in no way true, especially for seismic prone regions...! High strength concretes need less confinement reinforcement, less shear reinforcement, and less flexural rebars. Compression part of the RC elements would be reduced in the case of high strength concrete and this means a pronounced increase in their curvature ductility. This is true for RC beams, columns, walls, and even reinforced masonry elements.
For a typical building if you adopt a concrete strength of 25 MPa, required steel weight would be about 40 to 50 kg per square meter. This value, however, would be reduced to about 30 to 40 kg per square meter, if you increase concrete strength up to 35 MPa. However, care should be exercised in adopting ultra high strength concretes (strength of more than 60 MPa), as their component-level behaviors are not well recognized yet.
Thank you for your answer, but don't forgot that as per ACI 318 code, the minimum reinf. ratio will increase with increasing the concrete strength !
A couple of things should be appreciated. While per ACI 318 minimum flexural and shear reinforcements increase with concrete strength, such provisions are mainly applicable for beams in which required reinforcement is typically governed by strength criteria rather than minimum reinforcement. This provision is intended to result in a flexural section capacity of more than its corresponding cracking moment. As suggested by ACI 318, such minimum value can be totally waived through placing 30% additional reinforcement in the beam.
In a typical RC building, the minimum reinforcement might be used only in floor slabs and sometimes in shear walls those which their minimum reinforcements are not depend on concrete strength. As a result, virtually in all projects, required steel would be reduced by increasing concrete strength. Less development and anchorage lengths, and improved durability should be also added to the advantages of high strength concretes.
yes, but if you look at the problem from another face, as you said the reinf depend on strength requirements (mainly on the effective depth "d"). So now I have a high strength concrete material, I can reduce the section dimension, d is small and you will need a lot of reinf to resist the external moment, so what is the use?
Am sure that what you talked about is applicable for shear and compression members, but for the flexural one?
I guess what they mean is, in a flexural framework :
if you have to increase the strengh of the concrete in order to cope with compressive strengh on one side of the beam, then you have to increase the quantity of steel in order to deal with tension on the other side.
Most of the rules don't even consider concrete in calculations for the part of the beam wich is under tension.
Not true. If you do that, that will lead to over reinforced section.
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