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Some questions about concrete

 
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suraj
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PostPosted: Wed Dec 22, 2004 5:32 am    Post subject: Some questions about concrete Reply with quote

Structural concrete pour



Some simple practical questions about concrete for the budding engineers



Concrete of grade 35 or 40N is to be poured

Acceptable slump is 125+/- 25mm

Temperature of concrete 26 deg C

At delivery time slump is observed 200mm

What should be done? Reject or accept the truck?





Cubes fail on 7th day test. What should be done ?



What should be preferred for curing of concrete

Curing compound or water



Eng Suraj Singh PE

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kagarwal
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Joined: 26 Jan 2003
Posts: 3

PostPosted: Wed Dec 22, 2004 9:04 am    Post subject: Some questions about concrete Reply with quote

Dear members.
I am designing a 9 story building near noida. The site is rocky having
bearing capacity around 21t/m^2. I am trying to proceed with isolated
and combined footing as recommended by soil consultant. Is there any
codal provision, restricting the use of isolated/combined footing in
such cases.
Thanks
Kapish

-----Original Message-----
Message From  rccbuildings2@yahoo.co.uk [mailto:rccbuildings2@yahoo.co.uk]
Sent: Wednesday, December 22, 2004 4:32 PM
To: Kapish Agarwal
Subject: Some questions about concrete


Structural concrete pour



Some simple practical questions about concrete for the budding engineers



Concrete of grade 35 or 40N is to be poured

Acceptable slump is 125+/- 25mm

Temperature of concrete 26 deg C

At delivery time slump is observed 200mm

What should be done? Reject or accept the truck?





Cubes fail on 7th day test. What should be done ?



What should be preferred for curing of concrete

Curing compound or water



Eng Suraj Singh PE


          
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suraj
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Joined: 17 Apr 2008
Posts: 1981
Location: NCR Faridabad, E mail suraj_engineer@yahoo.co.uk

PostPosted: Wed Dec 22, 2004 10:17 am    Post subject: Some questions about concrete Reply with quote

It is suggested to design beams frame foundation for such case


kagarwal@desein.com wrote:
Dear members.
I am designing a 9 story building near noida. The site is rocky having
bearing capacity around 21t/m^2. I am trying to proceed with isolated
and combined footing as recommended by soil consultant. Is there any
codal provision, restricting the use of isolated/combined footing in
such cases.
Thanks
Kapish

-----Original Message-----
Message From  rccbuildings2@yahoo.co.uk [mailto:rccbuildings2@yahoo.co.uk]
Sent: Wednesday, December 22, 2004 4:32 PM
To: Kapish Agarwal
Subject: Some questions about concrete


Structural concrete pour



Some simple practical questions about concrete for the budding engineers



Concrete of grade 35 or 40N is to be poured

Acceptable slump is 125+/- 25mm

Temperature of concrete 26 deg C

At delivery time slump is observed 200mm

What should be done? Reject or accept the truck?





Cubes fail on 7th day test. What should be done ?



What should be preferred for curing of concrete

Curing compound or water



Eng Suraj Singh PE



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chordia1
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Joined: 26 Jan 2003
Posts: 11

PostPosted: Wed Dec 22, 2004 11:14 am    Post subject: Some questions about concrete Reply with quote

There is no codal provision one can easily go in for combination of Isolated
and Combined footing and to be more causious connect it with stiching slab.

Regards

sanjeevJain
----- Original Message -----
Message From  <rccbuildings2@yahoo.co.uk>
To: <chordia@del6.vsnl.net.in>
Sent: Wednesday, December 22, 2004 3:50 PM
Subject: Some questions about concrete


Quote:
It is suggested to design beams frame foundation for such case


kagarwal@desein.com wrote:
Dear members.
I am designing a 9 story building near noida. The site is rocky having
bearing capacity around 21t/m^2. I am trying to proceed with isolated
and combined footing as recommended by soil consultant. Is there any
codal provision, restricting the use of isolated/combined footing in
such cases.
Thanks
Kapish

-----Original Message-----
Message From  rccbuildings2@yahoo.co.uk [mailto:rccbuildings2@yahoo.co.uk]
Sent: Wednesday, December 22, 2004 4:32 PM
To: Kapish Agarwal
Subject: Some questions about concrete


Structural concrete pour



Some simple practical questions about concrete for the budding engineers



Concrete of grade 35 or 40N is to be poured

Acceptable slump is 125+/- 25mm

Temperature of concrete 26 deg C

At delivery time slump is observed 200mm

What should be done? Reject or accept the truck?





Cubes fail on 7th day test. What should be done ?



What should be preferred for curing of concrete

Curing compound or water



Eng Suraj Singh PE



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sidhudaljeet
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Joined: 26 Jan 2003
Posts: 110

PostPosted: Wed Dec 22, 2004 2:19 pm    Post subject: Some questions about concrete Reply with quote

Slump 200 is too much, 125 is also on heigher side, if you pump conc. it is all right.
Take sample of the mix, just check how much water it has. simply dry the conc, this way you would be able to know the water cement ratio. since slump is very rough idea of water cement ratio.
7 days sample gennerally fails due to curing at low temp, if the ambient temp is above 30, 7 days strength should be allright.
if water cement ratio comes much more than the specified, it might be poor conc.
it should be rejected.
Daljeet

rccbuildings2@yahoo.co.uk wrote:

Structural concrete pour



Some simple practical questions about concrete for the budding engineers



Concrete of grade 35 or 40N is to be poured

Acceptable slump is 125+/- 25mm

Temperature of concrete 26 deg C

At delivery time slump is observed 200mm

What should be done? Reject or accept the truck?





Cubes fail on 7th day test. What should be done ?



What should be preferred for curing of concrete

Curing compound or water



Eng Suraj Singh PE



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raju_goa
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Joined: 26 Jan 2003
Posts: 37

PostPosted: Wed Dec 22, 2004 3:56 pm    Post subject: Some questions about concrete Reply with quote

Hi Kapish,
There is no codal provision restricting use of isolated / combined footings.
You can opt for isolated footings and possibly combined footings if the
footings overlap. It would be ideal to tie the columns at plinth level with
plinth beams.

Shekhar Panandiker
Goa.

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James_Cohen
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Joined: 26 Jan 2003
Posts: 31

PostPosted: Wed Dec 22, 2004 4:10 pm    Post subject: Some questions about concrete Reply with quote

Dear Mr. Singh,

If the high slump and low strength results are for the same pour, the
concrete should be rejected before additional construction takes place. If
the specification called for a specific 7-day strength, this is usually done
to provide the contractor with "advance notice" that there may be a problem.
So, if the low-strength results are local, I would advise halting
construction in that area until additional strength tests are completed. It
is best to stop further construction until either 14-day test results can
establish the adequacy of the concrete or to remove and replace the affected
area. The contractor is unlikely to want to wait for 28-day results and you
and the owner are unlikely to want additional construction to occur which
may need to be removed or substantially altered. If the problem is endemic
to the entire structure, the available choices are not good, but still need
to be resolved.

An alternative approach is to re-analyze the structure with the lower
results. Design is typically predicated on a rather high factor of safety,
part of which is based on unknown material strengths and unknown
construction tolerances. If these are known, as should be the case in your
situation, these factors of safety are no longer required. It may be that
the actual strength is still adequate to provide the required residual
factor of safety. In situ testing may also help to establish that the
concrete does have the necessary strength to remain with little or no
strengthening.

James Cohen, PE
James Cohen Consulting, PC
http://expertpages.com/jccpc

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rajmane
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Joined: 26 Jan 2003
Posts: 160
Location: Chennai

PostPosted: Thu Dec 23, 2004 2:07 pm    Post subject: Some questions about concrete Reply with quote

23rd Dec 2004
Dear all
(1)" Concrete of grade 35 or 40N is to be poured,  Acceptable slump is
125+/- 25mm, Temperature of concrete 26 deg C,

At delivery time slump is observed 200mm"
I think the concrete need not be rejected provided the increased slump
does not cause any construction problems. It is almost to produce
concrete every time with the same slump. This is a fact. I think more
emphasis should be on parameters such as : (a) cohesiveness, (b)
non-seggregability, (c)no excessive bleeding, (c) Correct setting
property, etc.
(2) "Cubes fail on 7th day test". The reasons for this should be studied
in any particular case. Some parameters to be looked into are: Change in
cement and admixture type and content (b) Change in materials, (c)
Actual curing regime followed, (d) Method of testing, loading rate etc,
(d) Status of test specimen, defects if ant in specimen, (e) Calibration
of testing machine. etc. If all these can be satisfactorily looked into,
then, NDT on structures including core sampling should be considered. 7
day strength is only an indicator and not the final criterion for
acceptance or rejection of concrete as per IS:456-2000.
(3) "What should be preferred for curing of concrete, Curing compound or
water"
Water curing must be always preferred. However, with expert supervision
(not usual normal supervision), curing compound can be used. Any project
engineer/site supervisor should develop a proper quality check and
application methodology.

(4) The suggestion of "simply dry the conc, this way you would be able
to know the water cement ratio" is not rational. This way w/c can not be
determined. I do not know in which reference/standard published works,
this suggestion is available. Drying (in hot air oven) causes further
hydration of cement and this does not get reflected in weight loss. we
can send endlessly on the problems/difficulties in accepting this
suggestion. I draw the attention of Neville's one paper in Concrete
International where he hoped that some day some body will find
acceptable/correct way to find w/c of fresh concrete after it is mixed.


Thank you all

N P Rajamane
Deputy Director and Head, CCL, SERC, CSIR, Chennai
600113
Ph: 22549152 (O), 24417882 (R), 9840076821
Fax: 22541508

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suraj
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Joined: 17 Apr 2008
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PostPosted: Thu Dec 23, 2004 3:52 pm    Post subject: Some questions about concrete Reply with quote

Dear Sir,
Impressed by the your explanation
Eng Suraj Singh PE


rajmane@sercm.csir.res.in wrote:
23rd Dec 2004
Dear all
(1)" Concrete of grade 35 or 40N is to be poured, Acceptable slump is
125+/- 25mm, Temperature of concrete 26 deg C,

At delivery time slump is observed 200mm"
I think the concrete need not be rejected provided the increased slump
does not cause any construction problems. It is almost to produce
concrete every time with the same slump. This is a fact. I think more
emphasis should be on parameters such as : (a) cohesiveness, (b)
non-seggregability, (c)no excessive bleeding, (c) Correct setting
property, etc.
(2) "Cubes fail on 7th day test". The reasons for this should be studied
in any particular case. Some parameters to be looked into are: Change in
cement and admixture type and content (b) Change in materials, (c)
Actual curing regime followed, (d) Method of testing, loading rate etc,
(d) Status of test specimen, defects if ant in specimen, (e) Calibration
of testing machine. etc. If all these can be satisfactorily looked into,
then, NDT on structures including core sampling should be considered. 7
day strength is only an indicator and not the final criterion for
acceptance or rejection of concrete as per IS:456-2000.
(3) "What should be preferred for curing of concrete, Curing compound or
water"
Water curing must be always preferred. However, with expert supervision
(not usual normal supervision), curing compound can be used. Any project
engineer/site supervisor should develop a proper quality check and
application methodology.

(4) The suggestion of "simply dry the conc, this way you would be able
to know the water cement ratio" is not rational. This way w/c can not be
determined. I do not know in which reference/standard published works,
this suggestion is available. Drying (in hot air oven) causes further
hydration of cement and this does not get reflected in weight loss. we
can send endlessly on the problems/difficulties in accepting this
suggestion. I draw the attention of Neville's one paper in Concrete
International where he hoped that some day some body will find
acceptable/correct way to find w/c of fresh concrete after it is mixed.


Thank you all

N P Rajamane
Deputy Director and Head, CCL, SERC, CSIR, Chennai
600113
Ph: 22549152 (O), 24417882 (R), 9840076821
Fax: 22541508

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vikramjeet
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Joined: 26 Jan 2003
Posts: 7

PostPosted: Wed Dec 29, 2004 6:46 am    Post subject: Some questions about concrete Reply with quote

Dear kapish



Isolated Footings  are economical vis -a - other type of footings /
Raft.However, since the structure is
located in Zone - 4  It is advisable  to tie the  isolated footigs  at
foundatioin Level itself in both
directions  .Tie beams will also take care of  Moments  at footing LvL
due to gravity loads & Seismic.


Regards

Jeet

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