Cracks in a 8" continuous slab parking deck

[VYordan]

I designed the deck 8 years ago for a building that has an underground parking level with a roof that is also used as a an open parking deck. The structural system beams run from east to west and are 4'wide x 1.5'deep including the slab. The slab runs north to south traversing the beams with CLEAR spans being 24',20', 25'and 14'. Loading consists of the slab weight (100 psf) and live load (50 psf)
Initially, the system was designed as a one way beam using a frame analysis and design program. The actual reinforcement illustrated on the drawings is twice to three times the required from calculations to take into account impact. Temperature reinforcement was specified as #4@10" bottom. Recently the system was modeled using SAFE and the original design was corroborated. Cracked section analysis was performed and long term deflections were in the order of 0.19" for live loads and lower for the slab weight. The gravity system consists of shear walls running from north to south and a perimeter 8” basement wall under the slab.
Most of the cracks do not run parallel to the beams in midspans which would indicate a flexural problems, -especially for the longer spans- but parallel to the one way slab direction - there are a few parallel cracks –with signs of infiltration- running 7' to 10' from each other. Also what confuses me more is various angled cracks radiating near the beam support in one area of the 25' span. These crack patterns are not explained by the expected response of the slabs to the loads, being a one way system. The crack patterns described are in plan view seen from below. The slab on top is covered by a waterproofing membrane which was not given any maintenance by the owner thru out the years as evidenced by rips in it. Concrete was specified as 5,000 psi with a maximum WCR of 0.5. Reinforcement is A-615. Concrete cylinder tests and submittals during construction confirmed the design parameters. The total surface area is in the order of 80'x100'. Most of the cracks in are in the 24' and 25'spans. Some in the 20' and even 14' spans. Negative reinforcement over beams is #5@5”and positive reinforcement in the 24’and 26’ spans is $5@12”.
I suspect the following: 1. Top & bottom reinf. covers much more than specified. I specified a sloping form to account for the drainage slopes and it looks that the slab bottom is level and the top surface was sloped, probably leaving the top reinforcement as if it was a uniform 8" slab (there are cracks running on top of slab corresponding to the negative moment parallel to the beam support in the 25’span) 2. Actual WCR higher than specified. Recent core tests provided resistances varying from 2,500 to 4,500 psi. 3. Overloading of the slab with concentrated loads during construction with some type of crane loading or some other temporary loading such as concrete block stacking, which might explain the diagonal cracks near supports. 4. Have to check any embedded piping or conduits inside slab, might explain something…. There are some localized circular patterns also with signs of infiltration. NO deflections are apparent. Please also note that the beam supports are so rigid that the slabs behave as if they were single spans fixed at the supports.

 

[JAE]

So do you have a question?

 

[hokie66]

1. I doubt there is too much cover for the bottom bars. There never is.
2. Too much cover for top bars is a common problem. Inspection during construction should have prevented this.
3. The cracking parallel to the slab span is also common and largely unpreventable. The beams act as large restraining elements, and the slabs shrink more and earlier. More reinforcement can restrain the cracks, but not prevent them.
4. The cracks in the slab near the columns are evidence that the slab is trying to act as a two way spanning slab in that area. The slab doesn't know that it is supposed to only span in one direction.

 

[rapt]

You do not seem to have told us the slab thickness, or the beam spans.

As Hokie says in point 3, this cracking is common in band beam and slab floors. Shrinkage differential between the slab and the band can cause stress up to 300-500psi. Combine this with stresses in the slab in that direction that have been ignored due to the one way action assumption and you can expect cracks at column line and midspan perpendicular to the beams. These can be controlled with sufficient reinforcement but they will always occur.

Watch out for Safe's long term deflection numbers, there have been reports of serious bugs in the calculations and grossly underestimated long term deflection calculations. I understand they are fixing or have fixed the problem but you may not be using a fixed copy. But safe will not be allowing for these extra restraint stresses and they will affect the design.

 

[hokie66]

rapt,
He did say in the title that the slab is 8".

 

[VYordan]

I performed a simulation run with concentrated loads near the
the column where the radiating cracks are and this indicates no need for additional temperature bottom reinf. ($4@10")at these locations until said loads are in the order of 25K. That is a lot.
The point being, no simulation or analysis model really represents exactly the actual structure configuration and its response.

Thank you very much for your attention.

 

[dik]

What was your mix design? Slump? w/c=0.5 is high for a parkade slab. Sounds like shrinkage... any signs of deterioration? what is your spec'd concrete cover? what has been measured?

Your 8" slab with 100 DL has little allowance for any type of traffic topping, mechanical, electrical, sprinklers, etc.

Just for a start...

Dik