Hair Line Cracks in Roof Slab

24m x 24m is a huge slab with just a perimeter beam. Are there no columns in the space between? If you have not already done so, re-shore the slab until you figure out what needs to be done.

we hv columns at 4 to 5 m spacingwith extra rods of 20mm alround the column in the roof slab

Okay, that sounds better. Concrete cracks for a variety of reasons. Shrinkage is one possibility. Early removal of forms is another. Re-shoring between columns and maintaining the shores for another 14 days is probably a good idea to control cracking. Good curing practices should also be followed.

I'm going to be direct here in my observations and comments.

First off, did a Licensed Structural Engineer design this suspended slab before it was constructed? Any inspection performed during construcion?

Second, where are these columns located? Along the perimeter beams?

Third, is there only a single floor bay? Are there more bays in this particular building (adjacent ones)?

I'm very concerned with regard to the structural viability of this suspended floor slab when I read that there is only a double layer of steel mesh for reinforcement. 24 m x 24 m is roughly 78.74 feet square, which makes it a long span two-way slab. I fear that this slab may be under-reinforced.

I'm equally concerned when I read that the cracks first appeared on the underside of the slab after the formwork was removed. Of course there was no mention of where the cracks where occurring in this floor bay. If located in the center of the slab where the positive moments are the greatest (with tensile stresses located at the bottom of the slab), then these cracks may be indicative of premature slab failure.

Also, no mention of slab depth, concrete compressive strength, if any type (and magnitude) of construction load is present, etc etc etc.

yes it was designed and inspected by certified strutural engineer .columns are located not only in perimeter beams but also inside the slab also.the concrete slab thickness is 23 cm .the crack is not every where but at some portions only.this is having just groundfloor only.xtrar rods in top layer alround the column of 20 mm used.the slab is not entirely suspended.columns r there in the inner areas.with spacing of column 4 to 6 m.concrete compressive strength is 40 n

Without performing a review of the approved construction plans, calculations, field tests of the concrete mixes (if done) and the inspector's filed notes + perform field observations there isn't much more to comment on here. There's just too many variables involved to ascertain the cause of the slab cracks.

As ba suggested, there's a possibility that the cracks are shrinkage cracks, but no one can absolutely guarantee that they are indeed due to concrete shrinkage.

From what you have described so far, the slab appears to me to be a flat plate type slab with fairly closely-spaced interior columns. I'm assuming that they are spaced either 4m or 6m each way, correct? If so, the slab is still a 2-way slab. Two-way slabs are usually susceptible to high shear forces and stress in the regions around an interior column. So far you haven't given us any information where in the slab these cracks are occurring, and if their locations are consistent throughout the slab, or if they're randomly located. Also, are the cracks full depth of the slab? Also, what are the typical crack widths (actually measured)? If they're located all around the top perimeter of the interior or corner column(s), then there's a strong possibility too that the shear stresses are very high there......slab may be under-reinforced or there's insufficient slab depth. Typically with plate type suspended slabs the slab is usually thickened around at the top of the column/bottom of slab interface (like a column cap/haunch) to help resist the high 2-way shear and punching shear stresses. If there's no column cap present, then additional rebar is usually required to resist the shear stresses that the concrete alone cannot resist.

I think you need to actually sit down with that structural engineer, face-to-face, and have a heart-to-heart discussion with him/her about why these cracks are occurring in your slab. Also, are you the Contractor or the building's Owner (or his representative)? You haven't told us how you ft into the scheme of things regarding this building. Anyhow, the engineer alone has the valuable information pertaining to the slab & can give you the answers you seek. In the end, ask him to certify to you in writing that the slab was properly designed and is structurally adequate now and into the foreseeable future.

Sorry, we cannot give you a more definitive answer to your problems, but it's nearly impossible for any of us to do an adequate engineering review from thousands of miles away + it's doubtful that you could afford my engineering services that I'd charge to you (even if I did take the job).

Signed,

CaptMoosie, PE/PhD

Civil, structural & environmental engineer