Concrete Cracks
Concrete Cracks
(OP)
During the construction of a multistory concrete building (bays 25 x 28 ft) we noticed many long concrete cracks at the botom of the slabs. Some of the cracks are large enough for water to pass through from one floor to the other. The cracks are always located at either the midspand or at 1/3 points. In addition to the cracks the slabs appear to have deflected. The concrete slab thickness is 9". The shoring plan calls for (3) three floors at 7, 14 & 21 days to support (1) one freshly poured floor. However, at this point I am not sure this is what really takes place. I can only think of two posibilities here:
(1)The slabs have been overloaded during construction.
(2)The slabs may have been underdesigned.
Any comments or recommendations will be appreciated?
(1)The slabs have been overloaded during construction.
(2)The slabs may have been underdesigned.
Any comments or recommendations will be appreciated?






RE: Concrete Cracks
RE: Concrete Cracks
RE: Concrete Cracks
RE: Concrete Cracks
RE: Concrete Cracks
f'c(slab)=6000 psi in areas with f'c(columns = 7000 psi
Upper levels f'c(slab)= 5000
Largest bay = 26.33' span
Rebar: #5 at 12" O.C.
Rebar: #4 perp to the span.
Camber: +1/2"
LL = 40 psf. This is the load teh shoring plan is based on.
Many thanks to everyone
RE: Concrete Cracks
.31 sq in / 108 sq in = 0.00287. Not much more than needed just to support temperature and shrinkage.
RE: Concrete Cracks
Is it a Post Tensioned slab?
At what point is the slab stripped of the supporting formwork?
Daniel
RE: Concrete Cracks
if you use DL = slab wt. plus 10 psf
with LL = 40 psf
I get a required area of flexural steel at about 0.44 sq. in per foot. (assuming a wL^2/11 moment for a rough calc.)
This is #6 @ 12"o.c., not #5's.
So much for the factored design....
for the servicability -
Dead load only moment comes to about 6.9 ft-kips which creates a tension stress on the bottom of 512 psi. fr = 7.5sqrt(f'c) = 530 psi so you are very close to the cracking stress based on the 28 day strength...the formwork removal may be occurring at a lower concrete strength.
Without the actual spans, support fixities, in-place dead loads, etc. I can't do much more than this but it may be that you could do further analysis and see what the actual stresses are on the slab at form removal/reshoring activities and get a handle on what's going on.
RE: Concrete Cracks
No it is not a Post Tensioned slab
RE: Concrete Cracks
I would be wary about using anything less than these limits. I also suspect the reinforcing is light.
RE: Concrete Cracks
By ACI recommendations, the strength at which the structure may strip should be specified by the EOR or professional of record for the building. Are these recommendations being followed?
My personal gut check, sans calculator says the rebar quoted sounds light. I'm working with flat plate structures with that same mild steel for temp steel with PT main reinforcing. The printer didn't omit the P-series drawings in the structural set, did they?
Also, for superimposed loading, is there a MEP or partition Dead Load included?
Concrete cracks, but this seems extreme. has construction been suspended?
Daniel Toon
RE: Concrete Cracks
An analysis will be performed. I just wanted to see if there is anything I cannot see before I ask for one.
One of the critical bays with cracks at every floor is as follows: 26.3 ft wide by 80+ ft long supported at exterior columns on one side and continoues over interior columns or walls on the other. There are no drop panels. Top column strips are used with #5 at 4 or 6 OC both ends of the span. Column strips are extending only to about a 1/4 of the span each side.
Bot. rebar #5 at 12 OC
Bot. rebar #4 at 12 OC (Perp. to the span)
Thank you
RE: Concrete Cracks
2. Concentrated prop loads at approx. 5.75 ft OC (approx. 33 SF of concrete = 3.712 kip each)
3. Striping shall not be before 85% of its strength (not sure if this has been followed).
4.No MEP or partiiton load was used. Just the 40 PSF LL
Thanks again everyone.
RE: Concrete Cracks
Also, this is a two way construction (25x28), and the minimum slab thickness obtain from the ACI table is only required if a deflection analysis has not been performed.
RE: Concrete Cracks
Then they have not checked crack control.
Judging by the crack locations and lack of attention to other design oints it also looks like they may have not extended bars far enough. Cracks at 1/3rd points shere the moment is low are not expected if the reinforcement is detailed properly.
How much restraint is there to shortening?
RE: Concrete Cracks
1. The slab thickness is not enough to prevent execive deformations (Span/depth).
2. Reinf ratio is too low- both strength and crack widths..
3. Reinf spacing is too big.
With one word.
Re design.
This is what my calculator is showing.
(to get it right assume you are wrong)