huangyhg

concrete cracks
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?

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before we jump to any conclusions, is the structure post-tensioned?
with the thickness and spans indicated, is the slab constructed with drop panels? i've encountered the problem with the slab rotating at the bottom of the drop panel and creating tension throughout the entire slab between drop panels at the middle of the slab. in one instance, the slab had opened enough to allow a business card to be passed through.
this sounds unusual to me. what other info can you provide? rebar size? known construction loads placed on the slabs? anything unusual about the concrete mix? are the cracks visible gaps? does the water flow straight through the cracks and drip on the other side? or does it form a damp patch on the underside?
is it one story or multi? was there re-shoring involved? are the slabs of one way or two way design? what is the reinforcing? what is the design superimposed dl and design ll?
no there are no drop panels.
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
this could be a bit light on the reinforcement steel. min required ratio to gross area oftemperature and shrinkage reinforcement based on aci-318-99 is .0018 for grade 60 rebar. you have
.31 sq in / 108 sq in = 0.00287. not much more than needed just to support temperature and shrinkage.
what type of shoring system is being used?
is it a post tensioned slab?
at what point is the slab stripped of the supporting formwork?
daniel
if this is a one-way slab system (spanning 25 to 28 feet), with a 9" slab -
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.
minimum 3 levels at 7-14-21+ are supporting 1 level above.
no it is not a post tensioned slab
unless you have edge beams or drop panels, you need ln/30 minimum slab thickness (aci table 9.5(c)) for exterior panels. for a 26.33 ft span, this comes out to 10.53 inches. note that the span is defined as face of column to face of column, so it might work out to a little less. for interior panels, the limit is ln/33 or 9.58 inches. if these aren't exceeded, they're very close.
i would be wary about using anything less than these limits. i also suspect the reinforcing is light.

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