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Difference between cracked & Uncracked Concrete?
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satish wagh
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PostPosted: Tue May 03, 2011 6:44 am    Post subject: Difference between cracked & Uncracked Concrete? Reply with quote

Dear Sefians,

Can anybody guide on this.

What is the definition of cracked concrete and un-cracked concrete?

How do codes specifiy the difference for cracked and un-cracked concrete ?


Thanks & Regards  
Satish  


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PostPosted: Tue May 03, 2011 1:40 pm    Post subject: CRACKED & UNCRACKED SECTION Reply with quote

Concrete is brittle in nature and have roughly 10% of compressive strength of concrete. At low tensile stresses concrete does not crack and behave as an uncracked section. The water retaining structures are designed as uncracked section by limiting stress in concrete & steel within permissible limit. For normal admosphere, crack width in liquid retaining structure is limit to 0.2mm and for  aggressive environment it limits to 0.1mm.
If stresses in concrete and steel are low they are not efficient to withstand the bending moment and hence not economical to design as uncracked section. Stresses are kept high in steel and concrete. Under high stresses in steel, adjacent concrete section, which is under tensile cracks. The sectional area of concrete above neutral axis resists compression, the concrete below neutral axis cracks. The beam, slab subject to loading bends in sagging, reinforcement with cover is provided at bottom. This act as cracked section. If the section is not cracked it does not behave as an  RCC section. The crack width is limit to 0.3mmfor normal building concrete section. All building components like slab, beam, column and footing are designed as cracked section only.

V.M.RAJAN.
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bipinsh
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PostPosted: Tue May 03, 2011 2:22 pm    Post subject: Reply with quote

Cracked section are those in which the extereme tension fibre stress exceeds the modulus of rupture fcr specified by the code, Cracking moment could be obtained using formula Mcr= fcr*It/yt whre It=second moment of area of transformed RC section and yt=distance betwn NA and extreme tension fibre.
If beam is very lightly loaded and bending moment is less than Mcr in such a case the concrete and steel both participate in resisting tension.

code IS456 cl6.2.2 suggest formula for fcr=0.7*sqrt(fck)
corresponding suggested by ACI is =0.623*sqrt(fck)
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Dr. N. Subramanian
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PostPosted: Tue May 03, 2011 6:09 pm    Post subject: Reply with quote

Hi,

The recent issue of ACI 318-08 gives the following formula:
fcr= λ*0.55*sqrt(fck)

where λ modification factor for lightweight concrete; For normal weight concrete λ =1.0. For lightweight concrete λ = fct/(0.5 √fck) ≤ 1.0, where fct is splitting tensile strength of light weight concrete.

Note that λ is provided in each equation and hence we can design structures with light weight concrete also, using the ACI code!

Best wishes
Subramanian

bipinsh wrote:
Cracked section are those in which the extereme tension fibre stress exceeds the modulus of rupture fcr specified by the code, Cracking moment could be obtained using formula Mcr= fcr*It/yt whre It=second moment of area of transformed RC section and yt=distance betwn NA and extreme tension fibre.
If beam is very lightly loaded and bending moment is less than Mcr in such a case the concrete and steel both participate in resisting tension.

code IS456 cl6.2.2 suggest formula for fcr=0.7*sqrt(fck)
corresponding suggested by ACI is =0.623*sqrt(fck)
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vikram.jeet
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PostPosted: Tue May 10, 2011 7:01 am    Post subject: Difference between cracked & Uncracked Concrete? Reply with quote

Dear Er Satish  

The usage of cracked concrete and uncracked concrete sections emanate from
the two philosophies aimed to serve the intended function:

A ] Cracked and uncracked sections in relation to Liquid retaining structures
as per IS 3370
The tensile stresses in concrete are reduced to provide uncracked sections
leak proof against water/liquid.Deatails can be seen in IS3370 Parts

B ] Cracked and uncracked sections in relation to Elastic theory as per IS456
The basic principle of elastic theory is that under flexure .concrete area below
NA is in tension and cracks, and hence neglected in design.Tension is taken by
steel reinf only.However if tensile stresses donot exceed certain limit as prescribed
in clause B-4, the section may be treated as uncracked and safe without contribution
of reinf in strength (however min reinf needs to be provided there too.)

Uncracked in relation to GAS:
The uncracked concrete in relation to GAS leakage is not in possibility.Sludge digesters
top domes in sewage treatment plants show leakage ,if built only in concrete
but steel sheeting needs to be sandwiched to arrest leakage of gas which is a  
bye product in the an-aerobic process of sewage treatment in the Digester.

with best wishes and regds

vikramjeet



Dear Sefians,
Can anybody guide on this.
What is the definition of cracked concrete and un-cracked concrete?
How do codes specifiy the difference for cracked and un-cracked concrete ?
Thanks & Regards
Satish

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satish wagh
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PostPosted: Tue May 10, 2011 8:57 am    Post subject: Difference between cracked & Uncracked Concrete? Reply with quote

Dear Vikramjeet Sir,

Thanks for your valuable guidance and input on this.

Your replies in this forum are always educative and improve the understanding!!

With warm regards

Satish  


From: vikram.jeet [mailto:forum@sefindia.org]
Sent: Tuesday, May 10, 2011 2:15 PM
To: general@sefindia.org
Subject: [SEFI] Re: Difference between cracked & Uncracked Concrete?


Dear Er Satish

The usage of cracked concrete and uncracked concrete sections emanate from
the two philosophies aimed to serve the intended function:

A ] Cracked and uncracked sections in relation to Liquid retaining structures
as per IS 3370
The tensile stresses in concrete are reduced to provide uncracked sections
leak proof against water/liquid.Deatails can be seen in IS3370 Parts

B ] Cracked and uncracked sections in relation to Elastic theory as per IS456
The basic principle of elastic theory is that under flexure .concrete area below
NA is in tension and cracks, and hence neglected in design.Tension is taken by
steel reinf only.However if tensile stresses donot exceed certain limit as prescribed
in clause B-4, the section may be treated as uncracked and safe without contribution
of reinf in strength (however min reinf needs to be provided there too.)

Uncracked in relation to GAS:
The uncracked concrete in relation to GAS leakage is not in possibility.Sludge digesters
top domes in sewage treatment plants show leakage ,if built only in concrete
but steel sheeting needs to be sandwiched to arrest leakage of gas which is a
bye product in the an-aerobic process of sewage treatment in the Digester.

with best wishes and regds

vikramjeet



Dear Sefians,
Can anybody guide on this.
What is the definition of cracked concrete and un-cracked concrete?
How do codes specifiy the difference for cracked and un-cracked concrete ?
Thanks & Regards
Satish

-- ญญ  





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suresh_sharma
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PostPosted: Wed May 11, 2011 2:59 am    Post subject: Reply with quote

All RCC designs with the exception of water tank are based on cracked section but while analysing the structure we consider the full moment of inertia. Does it not sound absurd? Why not we use uncracked sectional proprties for working out the stresses?
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nmwhanna
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PostPosted: Sun Feb 24, 2013 8:52 am    Post subject: To Dr. N. Subramanian Reply with quote

Hi Dr. N. Subramanian


what is the reference of the formula in ACI 318-08


Thanks


Nabil Hanna
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logeshcivil
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PostPosted: Thu Apr 15, 2021 4:22 am    Post subject: Reply with quote

Dear Sefians

From reading above answers, i have inferred the following. Correct me if i am wrong.
1. Uncracked section design is usually used for design of liquid retaining structures.
2.Other RCC structures designed as cracked sections.

I have the following doubts.
1.What happens when all structures are designed as uncracked sections?
2. Can uncracked section design be used in seismic conditions?

Thank you

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Er. S.Logesh
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vikram.jeet
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PostPosted: Thu Apr 15, 2021 5:05 am    Post subject: Reply with quote

Dear Dr Logesh ji
Provision of uncracked section is made out of necessity  & not as a luxury. The requirement of cocrete section is very high to have control on crackwidth to have leak proof design of liquid retaining structures .
Basic concept of rcc design is that compression to be resisted by concrete and tension by steel reinf  i.e.  both members are provided as per their respective strengths.

Section requirement for singly reinfd section:
Moment of resistance = Qbd^2

For cracked sec  ( M25 mix & Fe 500 reinf)  Q=9.87
For uncracked sec (.          _  do _.               ).  Q= 3.0
( Permissible tension in bending =18 kg/cm2 for mix M25 as per IS3370,. M/Z=f , Z=bd^2/6 , MR=(18/6)bd^2=3bd^2

Due to presence of reinf the factor of 3.0 will increase slightly
Hence you can see that conc requirement is high.

The performance of only conc section in EQ will be poor . All liquid retaining strs, even though secs are uncracked the reinf is provided for strength requirements during normal & EQ cases for resisting thru ductility.
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