Suspended Deck Slab Rectifications

[engoaus]

Hi All,

Would love any thoughts/better ideas on this predicament, especially if anyone has dealt with a similar situation in the past:

Existing Scenario:

• 100mm thick suspended slab (combination of open balcony/partial kitchen above, low head-height garage below)
• Simply supported 3.7m span to single-leaf brick walls without any slip joints,
• Unknown f’c, reo and footings (however engaged brick piers in garage level below)
• Significant full-depth shrinkage cracking in secondary direction (unsure if due to lack of secondary direction reo or slip joints or both), which leaks through in the rain
• Lack of falls
• Unsound prop added at some stage under kitchen to SOG below (has noticeably settled here); assume due to excessive deflections with large SDL

Concerns:

• Strength/deflection concerns – obviously even with heavy reo, running this to current code doesn’t work as is, and the prop is not good
• External slab falls – needs to drain
• Waterproofing to garage below – electrical appliances in there!
• Rectification options are limited as removing half the kitchen or having an internal step with a new slab isn’t an option and limited existing head-room in garage below

Options:

• To achieve falls:
  • Lightweight tiles on stilts, less intensive stiffening/strengthening below but waterproofing issues difficult to solidly resolve (water would still end up getting to slab under and pond etc)
  • Topping slab to achieve falls, adding in waterproofed control joints, but heavy load increases so more intensive stiffening/strengthening
• To stiffen/strengthen:
  • Sika CarboDur strips (CFRP) to soffit, maintaining current slab schematic and enabling toppings without too much extra load onto existing footings, less intrusive on head room in garage below but very expensive,
  • Add steel beams under the slab to span between engaged piers, thus flipping the slab span direction, gaining the benefit of continuity but potentially won’t work if no reo in this direction and with the full-depth cracks – would need to have slab scanning performed to get a better idea? Also more intrusive into headroom below

Cheers all!

 

[Enable]

How large of a backslope is there currently? When we have backslope on balconies our first repair of choice is to build-up with a thin-applied polyurethane waterproofing membrane before laying down the UV stable topcoat. It's lightweight and will be what you use to waterproof the surface anyways, so it doesn't add much additional cost. If the backslope is large (>3/4") you can still use this method, but when making up large falls you generally want to lay down the topcoat (with sand) for padding instead of the basecoat. The basecoat is very elastic whereas the topcoat is much harder, so too much basecoat and you'll create a sponge for chairs and the like to go through. You should get the required slopes and talk to some local waterproofing manufacturers. Here we use MasterSeal (1500 / 2500), Tremco (Vulkem 350/351), NeoGard, or Qualideck.

As an alternative, if the garage below is pretty baron you could always punch some reasonably located drains and slope to them with the waterproofing as above. This may be better if the fall is quite large. Not least of which because you have the door threshold to contend with, and if you slope too much then you'll have to start raising doors / transitions and that seems annoying.

For strengthening I think you've covered the usual go-tos. Remove and replace with a more robust slab is also an option, but probably preferred less than your other two. With either of your options don't forget about the necessity of fireproofing. If the concerns are strictly serviceability in nature, I don't know if you still have to fireproof (I wouldn't think so?) but if the slab is shown to be deficient at ultimate, you'll most likely require fireproofing. Though, that is based on my experience repairing commercial parking garages, and may not apply to your jurisdiction for what looks like a residential build.

P.S. if you use the topcoat for padding use the regular topcoat, not the UV stable topcoat. UV stable is 2-4x times the price, which you don't need for padding. Only use UV stable for the very last topcoat.

 

[engoaus]

Hi Enable,

Appreciate the detailed reply!

The existing slope is random; water eventually drains due to the deflection of the slab over the southern edge. There is about 50mm (2") available for falls without affecting doors etc, hence worst-case 40mm (1.5") for a new system/topping etc at 1% on top of existing with a bit of margin to spare for membranes etc.

Agreed the drainage through the slab is a nice alternative; it is a wet area currently with the higher part of the subfloor (right in the photo) draining out hydrostatic pressure through weep holes in the base brick courses (I was looking to add a perimeter spoon drain or similar for this so combining these might be a good option).

Yes residential build and Sika software for their CarboDur system does indeed require the fire load case to be satisfied without the CFRP which is obviously fully exposed in that event.

Seems a slab scan will provide the most benefit in the first instant, regardless of approach.