Flexural cracks in beams

[Muhammad Hashmi]

Assalam O Alykum 

Can anybody give there perspective

Flexural cracks are appeared based on the self weight of beam and slab of 150mm thickness after 1 month of casting 11.24m span in one of the site under construction

designed by third party consultant. Beam size is 60x50 ( Width x Depth) Bottom & top reinforcement 12T20 and rings T10@150c.c through out the length of span. Compressive strength 

of concrete 30 MPA cylindrical.

I estimate the following causes 

1.       Depth of the beam provided 500mm does not full fill the criteria given by ACI code for providing minimum thickness of beam. Say L/16 = 700mm is required.

 2.       Bending / cracks appeared at the center , that can be find out by plastering the beam (the most critical one), if the material coming at the center is less

than the other areas, it can confirm the deflection problem.

3.       Structure of the beam might work as a frame. However the column orientation at grid line 2/5,3/5 and 5/5 along the minor direction, that will not work as a proper fixation at the column (Top tension zone) location that can increase the bending moment / deflection along the tension zone at the center and possible reason of cracks.

Structural drawing is also attached for comment 

 

 

[UmarMakhzumi]

WSalaam,

Cracking is not always bad. If J were you, I would re-check the design, get site reports on concrete strengths, confirm site placed rebars met development lengths and if all that works would let it go. Also other items you can do is to monitor deflection and get some NDT done if site information can’t be trusted.

I am not sure about your comments on column orientation.

Thanks.

 

 

[Muhammad Hashmi]

6 hours ago, UmarMakhzumi said:

WSalaam,

Cracking is not always bad. If J were you, I would re-check the design, get site reports on concrete strengths, confirm site placed rebars met development lengths and if all that works would let it go. Also other items you can do is to monitor deflection and get some NDT done if site information can’t be trusted.

I am not sure about your comments on column orientation.

Thanks.

 

 

 

Assalam O Alykum 

Thanks Umer for your reply.

I checked the casted beams the critical once and found that reinforcement provided on the beams are less than required. 

Compressive strength found Ok by performing Schmidt hammer test. 

I suggest them to plaster the beam through out the length of beam, if the material at the center coming less than it can confirm the deflection problem. 

[UmarMakhzumi]

2 hours ago, Muhammad Hashmi said:

I suggest them to plaster the beam through out the length of beam, if the material at the center coming less than it can confirm the deflection problem.

You can measure the deflection by using a thread and putting it across the floor and then seeing if floor is lower than the thread. Anyway, if the reinforcement works, that's fine. If it doesn't you should retrofit the beam.

Thanks.

[Muhammad Hashmi]

Assalam O Alykum 

Dear Umar. Reinforcement looks fine on the beams and can easily carry self weight of slab and self weight of beam, Cant understand the reason for flexural cracks may be reinforcement placed in two layers so the main bars near to centroidal. 

Or may be the column orientation is perpendicular to the direction of beam of having span 11.24m as I text earlier.

What do you say ?  

[UmarMakhzumi]

Hairline cracking is not bad but actually good. It means that load is transferring from concrete to steel.

You can measure the crack width. If it is surface or harline cracks, then it should be fine.

Thanks.

[Muhammad Hashmi]

Excellent brother

[abbaskhan2294]

On 6/13/2021 at 7:17 PM, Muhammad Hashmi said:

Assalam O Alykum 

Can anybody give there perspective

Flexural cracks are appeared based on the self weight of beam and slab of 150mm thickness after 1 month of casting 11.24m span in one of the site under construction

designed by third party consultant. Beam size is 60x50 ( Width x Depth) Bottom & top reinforcement 12T20 and rings T10@150c.c through out the length of span. Compressive strength 

of concrete 30 MPA cylindrical.

I estimate the following causes 

1.       Depth of the beam provided 500mm does not full fill the criteria given by ACI code for providing minimum thickness of beam. Say L/16 = 700mm is required.

 2.       Bending / cracks appeared at the center , that can be find out by plastering the beam (the most critical one), if the material coming at the center is less

than the other areas, it can confirm the deflection problem.

3.       Structure of the beam might work as a frame. However the column orientation at grid line 2/5,3/5 and 5/5 along the minor direction, that will not work as a proper fixation at the column (Top tension zone) location that can increase the bending moment / deflection along the tension zone at the center and possible reason of cracks.

Structural drawing is also attached for comment 

 

 

There can be certain conclusions to this. 

 

1) crack as mentioned by Umar bhai are not bad and nothing to worry as such unless they are within acceptable ranges of the code. Structural cracks maximum size is 0.40mm or 0.016 inch and crack widths shall be monitored regularly. 

 

2) The development length if not provided adequately can also be a reason for these cracks giving a feeling that the positive reinforcement provided is less than the actual case as shown here. 

       The cracks are flexural cracks originating at extreme fibres with clear visibility to fainter ones at the mid. It should certainly not be an issue but shall be monitored for avoiding any inconvenience in future. 

Edited June 23, 2021 by abbaskhan2294

[Kamranullah]

On 6/15/2021 at 9:04 AM, UmarMakhzumi said:

You can measure the deflection by using a thread and putting it across the floor and then seeing if floor is lower than the thread. Anyway, if the reinforcement works, that's fine. If it doesn't you should retrofit the beam.

Thanks.

what will be the allowable deflection for this beam?

and Can Negative Reinforcement reduced the Deflection ?

 

On 6/13/2021 at 8:17 AM, Muhammad Hashmi said:

Assalam O Alykum 

Can anybody give there perspective

Flexural cracks are appeared based on the self weight of beam and slab of 150mm thickness after 1 month of casting 11.24m span in one of the site under construction

designed by third party consultant. Beam size is 60x50 ( Width x Depth) Bottom & top reinforcement 12T20 and rings T10@150c.c through out the length of span. Compressive strength 

of concrete 30 MPA cylindrical.

I estimate the following causes 

1.       Depth of the beam provided 500mm does not full fill the criteria given by ACI code for providing minimum thickness of beam. Say L/16 = 700mm is required.

 2.       Bending / cracks appeared at the center , that can be find out by plastering the beam (the most critical one), if the material coming at the center is less

than the other areas, it can confirm the deflection problem.

3.       Structure of the beam might work as a frame. However the column orientation at grid line 2/5,3/5 and 5/5 along the minor direction, that will not work as a proper fixation at the column (Top tension zone) location that can increase the bending moment / deflection along the tension zone at the center and possible reason of cracks.

Structural drawing is also attached for comment 

 

 

500mm is clear depth or it include the 150mm of slab ?

 

[UmarMakhzumi]

On 6/26/2021 at 6:54 AM, Kamran ullah said:

what will be the allowable deflection for this beam?

Based on building code.

On 6/26/2021 at 6:54 AM, Kamran ullah said:

and Can Negative Reinforcement reduced the Deflection ?

Yes.