www.sefindia.org

[P.K.Mallick]

Table-3 of IS:456 2000 describes Environmental Exposure Condition. in the same table the MODERATE exposure condition is explained as follows:

Concrete surfaces sheltered from severe rain or freezing whilst wet
Concrete exposed to condensation and rain
Concrete continuously under water
Concrete in contact or buried under non-aggressive soil/ground water
Concrete surfaces sheltered from saturated salt area in coastal area.

As per Table -5 of IS:456-2000

Minimum Grade of Reinforced Concrete for Moderate exposure is M25.


I have observed that some Engineers suggest

M25 concrete for Foundation,External columns,Peripheral beams and Roof slab and M2O concrete for Internal columns,internal beams and slabs other than roof slab in case of Moderate Exposure.

Do you agree or disagree ?

[drnsmani]

Dear Er Mallick,

As per ACI the min concrete strength is 27.5 MPa and as per BS it is  M30(for moderate exposure). It may be difficult to adopt different grades of concrete for different elements of the building such as internal columns, beams, external beams, columns,etc. But it is common to have high strength column and normal strength beams and slabs.

Regards,
Subramanian

[thirumalaichettiar]

Dear Er Mallick,

Technically what you stated is viable and can be made use of but as Dr.N.S pointed out the use of different grade to different elements in a structure will be a confusing and trouble some to control. But careful execution is possible.

There is no question of AGREEING OR DISAGREEING. It is a technical solution as per CODE and acceptable one.

If the soil is a contaminated with sulfate or other salts  then use of SULPHATE RESISTANCE cement for footing and other grades for the rest of the structure is being used in GULF to take effective resistance to the sulfate contain soils.

It is good to use higher grade of concrete for axial members say columns and the lower grades for the beams and slabs. Specially use of higher grade for SLAB will be of little use due to the restriction of the spacing of reinforcement.


T.Rangarajan.

[hemal]

Dear Subramanian sir,

If we have different concrete grade for column and beam (say high grade for column and lower for beam), then how we can maintain higher grade for beam-column junction (joint) which is a critical part of column?

Regards

hemal

--- On Wed, 28/7/10, drnsmani <forum@sefindia.org> wrote:

[rajmane]

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Friday, 30 July 2010
Dear sefians

I feel it is too minor a point to discuss whether we should use M20 or M25. I feel we must use concretes with strengths as much higher as possible. Modern cement plants produce such good cements that we can get M35 and above easily with w/c ratio of about 0.50. Hence lower than M35 could mean higher w/c ratio which is not good for durability. To make the quality control measures meaningful particularly with reference to w/c, we must use very high strengths not from load carrying capacity but for denser microstructure considerations which would make the concrete to last for more number of years.

N P Rajamane
Secretary, Indian Concrete Institute, Tamil Nadu Centre, Retired Head, Advanced Materials Laboratory, CSIR-Structural Engineering Research Centre

--- On Thu, 29/7/10, drnsmani <forum@sefindia.org> wrote:

[vikram.jeet]

Respected PK Mallick sir,

Since at site ,it is the Foremen and labour which work under tiring conditions and
specifying different grades of concrete for internal and external columns
may increase the chances of error on part of human and may not be
encouraged. However for beams and slabs on floor ,one mix, different than
column mix may not trouble .
Even as regards reinforcements for slabs, some design engineers , prefer to use
only one type dia of bars in RCC slabs on a floor to avoid chances of mixing of bars
by labour. (If 10mm dia is used for slabs ,its spacing is varied ).

These cares are being taken by designers due to fact that supervision is never
upto the desired standards and it is considered as QA measure at design
stage itself.

with kind regards

vikramjeet
-- ­­

Posted via Email

[drnsmani]

Dear Dr Rajamane,

Glad to see your posting after a long gap. What you say is absolutely correct and important. Unless BIS specifies the minimum concrete strength as M30 or M35, people will continue to use M20 only.

Regards
Subramanian

Dear sefians

I feel it is too minor a point to discuss whether we should use M20 or M25. I feel we must use concretes with strengths as much higher as possible. Modern cement plants produce such good cements that we can get M35 and above easily with w/c ratio of about 0.50. Hence lower than M35 could mean higher w/c ratio which is not good for durability. To make the quality control measures meaningful particularly with reference to w/c, we must use very high strengths not from load carrying capacity but for denser microstructure considerations which would make the concrete to last for more number of years.

N P Rajamane
Secretary, Indian Concrete Institute, Tamil Nadu Centre, Retired Head, Advanced Materials Laboratory, CSIR-Structural Engineering Research Centre

[drnsmani]

Dear Er Hemal,

we have discussed about it earlier. Please browse.

You may also read my paper in ICJ
     Subramanian, N., Transmission of HSC column loads through NSC slabs, The Indian Concrete Journal, Vol.80, No.1, Jan 2006, pp.44-49.

Best wishes,
NS

[arunkashikar]

Different mix for column and beam/slab also creates one problem at our sites. Beam-column junction is often cast with lower grade concrete used for beam/slab which is not correct. Regards,Arun kashikar
- from VodafoneFrom: "vikram.jeet" <forum@sefindia.org>
Date: Sat, 31 Jul 2010 10:07:52 +0530
To: <general@sefindia.org>
ReplyTo: general@sefindia.org
Subject: [SEFI] Re: MODERATE EXPOSURE CONDITIONS AS PER IS:456-2000

     Respected PK Mallick sir,   Since at site ,it is the Foremen and labour which work under tiring conditions and  specifying different grades of concrete for internal and external columns  may increase the chances of error on part of human and may not be  encouraged. However for beams and slabs on floor ,one mix, different than  column mix may not trouble .  Even as regards reinforcements for slabs, some design engineers , prefer to use  only one type dia of bars in RCC slabs on a floor to avoid chances of mixing of bars  by labour. (If 10mm dia is used for slabs ,its spacing is varied ).   These cares are being taken by designers due to fact that supervision is never  upto the desired standards and it is considered as QA measure at design  stage itself.   with kind regards   vikramjeet -- ­­         --------------

Posted via Email

[Jalil A. Sheikh]

Respected subramanian sir,

By selecting different mixes for column and beam/slab, say m-35 for column and m-25 for beam/slab, which concrete mix is ok for junction between column & beam.
Some seniors says that lower grade is ok for such junctions( i.e. m-25). According your views what is advisable?

J.A.Sheikh



From: drnsmani <forum@sefindia.org>
To: general@sefindia.org
Sent: Thu, 29 July, 2010 12:16:54 AM
Subject: [SEFI] Re: MODERATE EXPOSURE CONDITIONS AS PER IS:456-2000

Dear Er Mallick,

As per ACI the min concrete strength is 27.5 MPa and as per BS it is M30(for moderate exposure). It may be difficult to adopt different grades of concrete for different elements of the building such as internal columns, beams, external beams, columns,etc. But it is common to have high strength column and normal strength beams and slabs.

Regards,
Subramanian
P.K.Mallick wrote:  Table-3 of IS:456 2000 describes Environmental Exposure Condition. in the same table the MODERATE exposure condition is explained as follows:

Concrete surfaces sheltered from severe rain or freezing whilst wet
Concrete exposed to condensation and rain
Concrete continuously under water
Concrete in contact or buried under non-aggressive soil/ground water
Concrete surfaces sheltered from saturated salt area in coastal area.

As per Table -5 of IS:456-2000

Minimum Grade of Reinforced Concrete for Moderate exposure is M25.


I have observed that some Engineers suggest

M25 concrete for Foundation,External columns,Peripheral beams and Roof slab and M2O concrete for Internal columns,internal beams and slabs other than roof slab in case of Moderate Exposure.[/color:94cba95561]

Do you agree or disagree ?

Posted via Email