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Compressive strength of concrete

 
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a121s
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PostPosted: Wed Feb 06, 2013 5:47 am    Post subject: Compressive strength of concrete Reply with quote

Hello Sefians,

During the mix design of concrete we find out the target strength of a perticular grade which will be higher than the grade to be designed for i.e,

For M20 grade, the target strength will be = 20+4.6*1.65 = 27.6 N/mm2.

But during the design of any structural member its been said to take the compressive strength = 0.8*Fck = 0.8*20 = 16 N/mm2 for design purpose.

I want to know that, though we are increasing the grade to target strength then why is it that we need to reduce it again for design purpose?

With Regards
SHISHIR S. KADAM
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Dr. N. Subramanian
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Location: Gaithersburg, MD, U.S.A.

PostPosted: Fri Feb 08, 2013 1:41 am    Post subject: Re: Compressive strength of concrete Reply with quote

Dear Er Kadam,

The target strength is fixed higher so that 95% of the cube tests will be within the required strength. Hence the strength to be considered is only the design strength. Any  extra strength that may arise may be useful in resisting unforeseen loads and accidental loads.

In the 1940s tests conducted in USA revealed that the strength in the structure is only 0.85 f'c and hence we use it in the derivations of flexural strength equations. This reduction may be due to several factors. One Of the main ones is that the strength fck is determined in the laboratory under controlled environments whereas the structure is subjected to harsher environment. Moreover vibrating concrete in a cube can be done perfectly whereas it can not be done so in a structure.

Hope it is clear now.

Best wishes
NS

a121s wrote:
Hello Sefians,

During the mix design of concrete we find out the target strength of a perticular grade which will be higher than the grade to be designed for i.e,

For M20 grade, the target strength will be = 20+4.6*1.65 = 27.6 N/mm2.

But during the design of any structural member its been said to take the compressive strength = 0.8*Fck = 0.8*20 = 16 N/mm2 for design purpose.

I want to know that, though we are increasing the grade to target strength then why is it that we need to reduce it again for design purpose?

With Regards
SHISHIR S. KADAM


Last edited by Dr. N. Subramanian on Fri Feb 08, 2013 2:19 pm; edited 1 time in total
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a121s
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Joined: 13 Jun 2011
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PostPosted: Fri Feb 08, 2013 5:12 am    Post subject: Reply with quote

Hello Subramanian Sir,

Thank you for your kindful reply.Yes sir, my doubt is totally cleared now.

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SHISHIR S. KADAM
M.Tech(Structures),A.M.I.E
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