Control Joint in Structural Slab on Voidform
Control Joint in Structural Slab on Voidform
(OP)
A pump house is designed sitting on concrete slab supported on piles (kind of common in the north area when the building is not considered heated and no uplift movement is allowed). This kind of slab is little bit special, from structural design viewpoint.
The slab is designed as the typical structural slab (like the flat-plate concrete floor), on the other hand it is a slab on “grade” - the undegradable void form. The slab concrete is reinforced to support the load on floor (with neglecting the voidform) and the frost heave pressure from the voidform. The pressure (somewhere around 600psf based on the locality and voidform property) is usually more critical.
Is control joint necessary for this slab? The house is about 30ftx20ft footprint with 9-24”dia piles. The slab is 18" thick with double reinf. The cracks (because of the restraint from the piles)on the slab may be a concern.
Thanks very much for shedding some light on this!
BTW, the punching shear from the middle pile (when frost heave occurs) seems special as well, since the effective area against the shear is much smaller than those in the typical punching shear check.
The slab is designed as the typical structural slab (like the flat-plate concrete floor), on the other hand it is a slab on “grade” - the undegradable void form. The slab concrete is reinforced to support the load on floor (with neglecting the voidform) and the frost heave pressure from the voidform. The pressure (somewhere around 600psf based on the locality and voidform property) is usually more critical.
Is control joint necessary for this slab? The house is about 30ftx20ft footprint with 9-24”dia piles. The slab is 18" thick with double reinf. The cracks (because of the restraint from the piles)on the slab may be a concern.
Thanks very much for shedding some light on this!
BTW, the punching shear from the middle pile (when frost heave occurs) seems special as well, since the effective area against the shear is much smaller than those in the typical punching shear check.






RE: Control Joint in Structural Slab on Voidform
For a structural slab..on forms or on void forms, the drag forces are less and the reinforcing is higher.
For an 18" thickness - a control joint wouldn't do you much good unless it was very deep and that would work counter to the structural behavior.
RE: Control Joint in Structural Slab on Voidform
See the thread by cdi12 on 'Concrete Mat Expansion Joint' that was started last Friday. Many of the same ideas apply.
If you use .5% reinforcing (.005 times gross cross sectional area of concrete) cracks will stay closed, and no joints are needed.
RE: Control Joint in Structural Slab on Voidform
lkjh345, I'm not sure if I have to go up to 0.5% reinf for better crack control. From stregth point of view, the minimum reinf ratio (0.2%) is good enough. Some cracks are okay for the slab. I just don't know whether control joint is necessary to locate the cracks.
RE: Control Joint in Structural Slab on Voidform
I suppose you could get some sort of spring coefficient and model the thing, but for a house that's probably overkill. I guess I'd go about it by making sure the concrete mix is chosen to minimize shrinkage (i.e. larger coarse aggregates, low water/cement ratio, water reducer, etc.) and just be careful how you detail the slab top reinforcing to keep cracks closed in likely crack areas.
Even though where you think the cracks will appear, they think differently.
RE: Control Joint in Structural Slab on Voidform