Crack Control in Exposed Concrete Roof Slabs 1

[sybie99]

Hi,

I am designing a roof slab of 17 x 25m. It is in an area where temperatures will routinely be 35C in the day and drop -5 degrees Celcius at night. I am concerned that such temperature fluctuations will lead to cracks developing. It is a 250mm thick suspended slab with minimal imposed loading so very little reinforcement is required to resist the Imposed Live Loads. I am not sure how to determine what amount of reinforcement to place in the slab to control temperature cracking as a result of the daily heating and cooling of the slab. Does anyone have a simple way of calculating the percentage of reinforcement to use in such instances?

Thanks

 

[tngv752]

Which part do you belong to ?

The slab has to be designed for (shrinkage + temperature difference) and crack control for waterproofing. Reinforcement need to be minimum to control crack and shrinkage in to direction (or maybe expnasion joint can be used).

Where and What is the support condition of the slab of 17m x 25m (supported by external and internal walls and columns)?

 

[ztengguy]

where do you live at? 95 to 23 in one day, wow wouldnt want to be there.

 

[sybie99]

Its the Namib desert. Slab is supported on precast beams that span 17metres onto walls

 

[Teguci]

Your volume to surface area ratio is pretty high and concrete doesn't transmit heat that quickly so your volume change calculation will require additional considerations than just the delta temperature. Unless your are dealing with a fire, the heat gradient will not be steep enough to be of concern through the thickness of concrete. Your concern should be isolated to the slab supports and connections. When the slab expands or contracts, whatever resists this motion will be punished. Bearings should be detailed to slide and reinforcement across any joints should be sleeved to allow some cross motion (pipe insulation works). If everrything is cast tight, you will need a flexible frame instead of shear walls.

 

[sybie99]

Thanks Teguci,

In this case we have 17m precast beams at 1.5m spacings. We have a prosuct called bond lok which is a steel shutter (permanent shutter) spanning the 1.5metres with a 250mm slab over. I was thinking of using a 10mm @ 200mm centres mesh at 50mm from the top surface. This amounts to 0.15% reinforcement. I am not sure to what extent the beams will restrain the slab (there will be reinforcement sticking out the top of the precast beams that will help to tie the cover slab to the beams)

 

[JedClampett]

It's not clear from your question, but if you're expecting the concrete to be crack proof enough to prevent stormwater from getting in the strcuture, you'll be much happier with a waterproofing membrane. Despite all our best efforts, concrete cracks, and when you depend on a concrete slab to not leak, it almost certainly will.
Add a nominal slope and some kind of roofing.

 

[sybie99]

JedClampett,

There will be a waterproofing membrane, so my feeling is therefore that I should not be too concerned about some cracking, I therefore think Y10's @ 200 c/c will be okay.

Regards,

Sybrand

 

[kieran1]

It would be prudent to detail the slab to allow for expansion and contraction. If you do this I wouldn't care too much about % reinforcement for cracking. Ensure the membrane can cope with some movement so it doesn't split

Kieran

 

[rollerich]

if you want no cracks or as minimal as possible use a post tensioned slab. or

I use Euclid tuff stand structural fibers plus micro fiber to control shrinkage cracks
Use normal amount of steel for the span.

mollerich@ats-sd.com 40 years PE