Concrete Cracks 8

[sigma1]

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?

 

[JedClampett]

Before we jump to any conclusions, is the structure post-tensioned?

 

[dik]

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.

 

[Bagman2524]

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?

 

[minorchord2000]

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?

 

[sigma1]

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

 

[Bagman2524]

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.

 

[DTGT2002]

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

 

[JAE]

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.

 

[sigma1]

Minimum 3 levels at 7-14-21+ are supporting 1 level above.
No it is not a Post Tensioned slab

 

[JedClampett]

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.

 

[DTGT2002]

Well, I am looking at the application of load to the partial structure - is the shoring/formwork system a flying ganged truss type system, with larger concentrated loads, or a loose built repetative shore/prop system, with fairly uniform loading.

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

 

[sigma1]

Thank you JAE

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

 

[sigma1]

1. Edge beams but no drop panels
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.

 

[mitchelon]

Not having read all the comments, I think Bagman2524 hit the spot. For any type of flexural member, I would expect to see a reinforcement ratio around 0.01. I would even use this value for fairly symmetrical buildings up to about 7 stories without even looking back because I’ll know it’s enough to make sure the steel will yield before the concrete crushes (remember your undergrad reinforced concrete class?) and I’ll be in the economical range. Minimum steel = 0.38sqin/ft, steel provided = 0.31sqin/ft?????

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.

 

[rapt]

Looks to me like someonr designed it on a FEM program and accepted the gross property mshort term deflections as real values for a start. This is a good depth for a PT slab, not RC for these spans. I would estimate total long term deflection in the end bays of 80-90mm

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?

 

[Yavoran]

My 3 p.

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)