Suspended roof floor slab cracked at supports 1

[DrVG]

Hi I would appreciate if someone could advice me on this problem. I have a situation where a driveway suspended slab at the roof level cracked at every supports (spaced up to 5m apart). The crack pattern seems continued and extending over the length of the beams underneath. Is this type of crack associated due to thermal movement? Or poor construction esp. reinf placing? I have noticed there is no expansion joints despite the slab spanning more then 45m.

 

[hokie66]

It is not possible to say whether the cracking which you describe is serious without knowing the details of construction. How thick is the slab? Is it conventionally formed or cast on steel form deck? If deck, how deep? What are the beams made of and what size are they? Are the cracks centred over the beams or at the edge of the beams?

Do you have the structural drawings of the driveway? Do you know what the design loading was? And do you know how the slab has actually been loaded?

With a bit more information, we may be able to give you a bit of advice, but in the end you should have a structural engineer in your area assess the structure.

 

[DrVG]

Thank you Hokkie, Please find some replies to your questions. The thickness of slab is ranging from 125mm to 150mm and it is designed as one way slab. It is formed in a conventionally method using formwork and props. All the beams are R.C and the primary beams size is 450mm x 750mm spanning max of 5m. All the cracks are at the centred over the beams.

In terms of loading, they have used 5kN/m2 (i.e. standard loading for vehicles) and have considered all the dead load. The total service design dead load is 8.5kN/m2. As such there seems satisfactory in term of loading calculation. Well to answer your last question, i don't really have a complete report but it looks to me they have designed in very simplified method such as wl2/10 for bending moment and M.F for deflection checking, which is very conservative, I think.

Any help? Appreciate.

 

[hokie66]

Just some thoughts:

1) The 125 to 150 slab is a bit light for the 5000 spans. L/D is normally limited to about 28 for end spans and about 33 for internal spans.

2) The cracks definitely sound like flexural cracks rather than direction tension cracks. This could be due to either inadequate reinforcement quantity or improper placement of the reinforcement (too low).

3) 5kPa is more than adequate if just for automobile loading, but if there are any trucks or other heavy vehicles using the driveway, overload could be an issue.

4) Is there any cracking on the slab soffits near midspan? If not, this probably indicates the top steel has yielded at the supports, but the bottom steel has not yielded.

5) If both top and bottom steel was designed for Wl/10, and the live loads have not exceeded the 5 kPa design load, then the design is conservative and the cracks can be considered a serviceability issue rather than compromising the strength. As you said this was a roof driveway slab, you probably have a waterproofing issue.

I still recommend that you have a structural engineer assess the slab on site.

 

[DrVG]

Hi Hokie thanks again,

You have cleared most of my doubts. Actually I agreed with your advise as stated in item 2. It is a combination of poor workmanship esp. on the placement of top reinforcement and the design calculation with minimum thickness. Bytheway do you have any idea on the pattern of crack that likely may form in a suspended slab if it is cast without any movement joints? Your immediate reply and advises are very much appreciated.

 

[hokie66]

When you talk about cracking in slabs caused by lack of movement joints, this type of cracking is due to direct tension rather than bending. Direct tension forces are due to restraint. A concrete slab of indeterminate length will not crack as it shrinks if it is unrestrained. So the amount and pattern of cracking due to direct tension will depend on the restraint conditions and the amount of crack control reinforcement rather than just the length. Many slabs are cast longer than your 45 metres without significant cracking.

 

[csd72]

Sounds like you could have a durability issue here. Deicing salts may have been used on the carpark and therefore they will have ingressed into the cracks.