Suspended slab topping reinforcement

[shei]

Good day!

I am currently doing a design for temperature reinforcement for a suspended slab topping. The scheme is to use welded wire mesh 6mm in diameter for temperature bars to control cracking. To compute for the bar requirement, I used
As = 0.002bt for Grade 33. However, my concrete topping on one floor varies from 8" maximum to 2" minimum since the existing slab has a certain slope and the new usage requires it to be level.

question:
a) What thickness should i use to compute for As?

b) Would an 8" thick topping require double layer
reinforcement?

c) Other methods or schemes to obtain an effective
concrete topping?

Thanks.

 

[JAE]

You may have to lay out your reinforcing on a plan showing different zones where different types/amounts of reinforcing are required due to thickness variations.

Also - 8" of topping is quite heavy - make sure you check the load carrying capacity of the original slab.

 

[shei]

Thanks JAE for your reply. I have checked on the original slab and its still adequate to carry the additional load.

Now, I'm trying to do some cost evaluation. Part of it is to reduce the strength of concrete if possible. My question is: What is the lowest possible concrete strength (fc') for slab topping of this purpose? Any relationship between slab thickness and concrete strength? If I reduced the concrete strength, would this affect my reinforcement? From the formula for temperature bars, As = 0.002bt, I see no relationship between concrete strength and reinforcement.

Hope for you response... shei

 

[denoid]

If the purpose of the mesh reinforcing is to control cracks from opening near surface of the topping, why not chair the mesh to within, say, the upper third of the topping, irregardless of depth.

If this is a floor surface, we have used F'c = 3500 psi to 4000 psi topping, keeping W/C ratio's low, also for resistance to shrinkage cracking.

Have you considered a lightweight aggregate for the thicker toppings?

 

[JAE]

shei,
denoid has a good point about chairing the mesh - it would also help ensure that the mesh stays up where it belongs.

Also, we have added topping slabs in the past where the underlying surface is sloping, or the thickness of the topping was getting a bit heavy. In these cases, we would specify a void form fill, such as the following:

RIGID CELLULAR, EXTRUDED POLYSTYRENE INSULATION COMPLYING WITH ASTM C578, TYPE IV, 25 PSI COMPRESSIVE STRENGTH MINIMUM.

We place this in strips, perhaps 24" wide with 4 to 6" gaps between to form a type of ribbed slab on top of your underlying structural slab. This reduces weight and keeps the topping slab the same thickness. As the topping gets thinner with the slope, we drop off the insulation in steps, maintaining a minimum thickness, perhaps 2 1/2" or 3" to a max. of 3 1/2 to 4 inches. Then, you can reinforce your "fairly constant thickness" topping accordingly. The topping actually spans between the ribs but such that the WWF can be enough for flexural bending.