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Regarding Cracks in slab
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ramu_se
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Joined: 30 Oct 2017
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PostPosted: Tue Nov 29, 2022 4:21 pm    Post subject: Regarding Cracks in slab Reply with quote

Dear All,

Cracks developed in a 5' length cantilever slab. The thickness of the slab is 6" and reinforcement is 8mm @ 6" c/c at the top and 8mm @ 12" c/c as distributors.

It is a G+1 Residential building.

Cracks developed near the beam slab joint which is marked on the image attached.

I have attached the drawing files for your reference.

Kindly check and let me know the cause of this crack and how to rectify this cracks.
Thanks & Regards,
Ramu.



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sakumar79
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PostPosted: Wed Nov 30, 2022 3:55 am    Post subject: Reply with quote

Dear Er Ramu,
1. Even though you have noted that the main bar is 8mm @ 6" at top, the detailing of the cantilever slab top bar is shown in full line which is normally used for bottom bars. Did you check the reinforcement laid out at site before concreting?

2. If the cantilever slab main bar is at top, what is the need for the cranked rods from the adjacent slab to also be taken up for the full cantilever?

3. At S4 and S8 which are indicated to be sunk slab, the detailing of the rods is not shown. Was any instruction given separately or do you know how it was constructed at site?

4. For 5' cantilever, 6" thickness is okay but rods is a bit low - is it a balcony with only MS handrails? If there is brick wall at ends, this reinforcement is insufficient as far as I can tell. While this is unlikely to be the reason for the current problem, you should check it out also.

5. Have concrete cubes been cast? If you have 7 day strength, it might indicate the quality of concrete which may have an effect here.

Hope that helps
S Arunkumar
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vikram.jeet
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PostPosted: Wed Nov 30, 2022 4:33 am    Post subject: Reply with quote

Er Ramu,,

For a 5ft cantilever,, 6 " Thickness is ok,, but  8dia at 6" C/c is less ( though theoretically may be suffice) ,, but in cantilevers labour moves and top bars settle down and generally provision is done considering higher cover to account for practical problems of keeping bars in position.

Concrete mix may not be a problem as these are tension cracks

Since building is G+1 ,, pl instruct to support temporarily  and  do retrofitting
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ramu_se
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PostPosted: Wed Nov 30, 2022 5:33 am    Post subject: Reply with quote

Dear Arunkumar Sir,


Thank you for the detailed explanation.





1. Even though you have noted that the main bar is 8mm @ 6" at top, the detailing of the cantilever slab top bar is shown in full line which is normally used for bottom bars. Did you check the reinforcement laid out at site before concreting?


Reinforcement laid out properly at site. 8mm@6"c/c at top.

2. If the cantilever slab main bar is at top, what is the need for the cranked rods from the adjacent slab to also be taken up for the full cantilever?

Since the slab has continuity, the cranked bars extended to adjacent slab.

It is not necessary to extend the cranked bars?

3. At S4 and S8 which are indicated to be sunk slab, the detailing of the rods is not shown. Was any instruction given separately or do you know how it was constructed at site?

S4, S8 are sunk slabs and designed as one-way slab, and since it is
sunken there is no reinforcement extension to the adjacent slab.Detailing is shown in the Autocad drawing.

4. For 5' cantilever, 6" thickness is okay but rods is a bit low - is it a balcony with only MS handrails? If there is brick wall at ends, this reinforcement is insufficient as far as I can tell. While this is unlikely to be the reason for the current problem, you should check it out also.

Flyash brick wall as a parapet wall

5. Have concrete cubes been cast? If you have 7 day strength, it might indicate the quality of concrete which may have an effect here.

RMC was used.

If the reinforcement is not sufficient, how to rectify this? how to add additional support?

Thanks & Regards,
Ramu.
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ramu_se
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PostPosted: Wed Nov 30, 2022 5:41 am    Post subject: Reply with quote

Dear Vikram Sir,

Thank you.

The same reinforcement was given for a similar type of slab but there was no problem.

They have given temporary loose support, and after two days it became tight. This indicates that the slab is lowering down.

Is it a flexural crack?

Will it cause any serious problems?

What will be the proper solution for this?

Kindly guide me.

Thanks & Regards,
Ramu.
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sakumar79
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PostPosted: Wed Nov 30, 2022 10:14 am    Post subject: Reply with quote

Dear Er Ramu

1. If reinforcement is laid properly, then ok.
2. The concept of cranked bars is that 50% of bars from one slab and 50% of bars from other slab on either side of beam will cross over thereby an average of the bar provided on either side will be available for the flexural capacity of the slab at the support. If you already provide cantilever reinforcement at 100% anchoring into the supporting slab, you dont need to crank the bars of the supporting slab.
3. I agree that S4 and S8 bars dont extend into the cantilever. My question is how is the cantilever bar anchored into beam at these locations?
4. I would recommend MS handrails instead of brick parapet due to reduced moment capacity.
5. Even if RMC was used, we should be able to get the concrete cube test results right?

In addition, there are many possibilities for the cracks to occur. It could be due to unexpected early dislodging of the props, poor quality of material/concrete, etc. This will require knowledge of the site to eliminate options.

One suggestion is to carry out pressure/injection grouting (you may consult a local repair specialist agency) and allow concrete to gain full strength and then carry out load testing to ensure that the slab can handle the load - the procedure for load testing is given in IS 456 a little vaguely and you can look up some literature on the net also.

Hope that helps
S Arunkumar

ramu_se wrote:
Dear Arunkumar Sir,


Thank you for the detailed explanation.


1. Even though you have noted that the main bar is 8mm @ 6" at top, the detailing of the cantilever slab top bar is shown in full line which is normally used for bottom bars. Did you check the reinforcement laid out at site before concreting?


Reinforcement laid out properly at site. 8mm@6"c/c at top.

2. If the cantilever slab main bar is at top, what is the need for the cranked rods from the adjacent slab to also be taken up for the full cantilever?

Since the slab has continuity, the cranked bars extended to adjacent slab.

It is not necessary to extend the cranked bars?

3. At S4 and S8 which are indicated to be sunk slab, the detailing of the rods is not shown. Was any instruction given separately or do you know how it was constructed at site?

S4, S8 are sunk slabs and designed as one-way slab, and since it is
sunken there is no reinforcement extension to the adjacent slab.Detailing is shown in the Autocad drawing.

4. For 5' cantilever, 6" thickness is okay but rods is a bit low - is it a balcony with only MS handrails? If there is brick wall at ends, this reinforcement is insufficient as far as I can tell. While this is unlikely to be the reason for the current problem, you should check it out also.

Flyash brick wall as a parapet wall

5. Have concrete cubes been cast? If you have 7 day strength, it might indicate the quality of concrete which may have an effect here.

RMC was used.

If the reinforcement is not sufficient, how to rectify this? how to add additional support?

Thanks & Regards,
Ramu.
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vikram.jeet
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PostPosted: Wed Nov 30, 2022 11:02 am    Post subject: Reply with quote

Retrofitting is generally not easy, especially for rcc works and in case of cantilevers it is further.

Yes it is flexural tensile cracking .
It can fail if balcony overcrowded  ,, or planters put at edge. This situation can come sny time when str in use.

Some suggestion on retrofitting:

Steel beams : assuming some nominal bottom mesh available
Provide ISMC sections ( about 5 +7=12') on both sides of bslcony slab at its top cantilevring out from sides . Can't portion of this channel besm will be on top whereas at back, 7' inserted in wall and front col if any, just by chipping col cover. Welding of steel beams with col bars also to be done.

Further provide another channel beam at free end of balcony joining two channel beams and weld. Provide L shaped clams made of steel flats at say 2feet centers, with hora leg under balcony slab and vertical leg to be welded to front channel section .

Second option :

Please chisel top cover in about 900mm (if not full slab) cant length from beam in entire width of can't slab.
Please provide 10 dia chimta at 6"-8" Crs. Chimta must pass through top bars of beam  for which beam top bars to be exposed thru chiseling.. Weld this 10dia reinf with top disb reinf . Provide rich mix  with prior application of epoxy, using jeera aggregate.
Pl note that this already cast beam will be subject to some torsion and reinf in it to be checked accordingly.
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pvgraju
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PostPosted: Thu Dec 01, 2022 8:44 am    Post subject: Regarding Cracks in slab Reply with quote

Dear Sir,

Due to corrosion also cracks may develop. so try to scan the concrete to find the intensity of corrosion. Then adopt suitable strengthening  methods such as additional support below the cracked slab, closing the cracks by grouting, treatment for corrosion etc.

Regards,

P.V.Gavarraju.
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vikram.jeet
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PostPosted: Thu Dec 01, 2022 2:06 pm    Post subject: Reply with quote

Second option :
In case beam supporting cantilever slab is not adequate in torsion ,, then back slab also need top reinf for at least 1.5*5' = 7.5 ' AND then instead of Chimta bars ,, 12.5 ' long 10 dia bars , are reqd to pass over the beam ,, welded to beam bars as well as existing distb bars meeting this 12.5' longbars.


As regards corrosion angle  as discussed by Er Gvarraju saheb ,, since str is just under construction  ,, it seemgly , not a possibility .
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ramu_se
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PostPosted: Fri Dec 02, 2022 5:03 am    Post subject: Reply with quote

Dear Arun & Vikram Sir,

Thank you for the explanation.

The crack is under observation, to check if it is getting expanding or not.

Can we provide the steel section from the column to support the cantilever slab?

Image of the steel section is attached.


Thanks & Regards,
Ramu.



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