Basketball Court Slab on Grade

[KarlT]

Hello all.

I am looking at designing an exterior slab on grade for a 50 ft by 84 ft basketball court. The slab will be exposed to the elements, including freeze / thaw cycles.

I am considering a 5" concrete slab on 6" of well-compacted granular. I was going to provide a 10" by 10" thickened edge around the perimeter of the slab, with the bottom of the thickened edge tapering into the 5" typical slab. I am also looking at a 2'x2'x12" thickened section at the basketball net pole embed locations.

In terms of steel I was going to go with 2% of the slab cross-sectional area in each direction, with a maximum rebar spacing of say 12" to 16" to keep the crack widths small. The slab will have 1/4" vinyl applied to the top, and we were thinking of casting the slab without control joints to prevent curling at the joints.

Does that sound about right to everyone?

 

[KarlT]

Actually, I was looking at 0.002 Ag for the steel reinf..sorry!

 

[eric1037]

A couple questions:

Where are is the project located? Frost depths vary.

What are the underlying soils? Are they sand, silt, or clay?

We generally recommend a minimum of edge drains and possibly under-slab drains if there are silty or clayey soils.

 

[JAE]

Not having joints on an 84 ft slab - it will crack and your rebar may not keep them all that closed. If you control the water/cement ratio well, and cure the hell out of it, you can reduce the crack tension a bit.

The drain gravel could be placed on a sloping subgrade where the gravel thickness is minimum in the center of the court and sloping down (getting thicker) at the edges so any under-slab water will drain to the sides - your thickened edges will interfere with the water "getting out" so you might consider some way to accommodate this.

If you are avoiding joints for the vinyl topping, perhaps you could still install the joints, let the slab sit for a month, and then patch over the joints prior to placing the vinyl. Most shrinkage occurs in the first days of concrete's life...but you may end up with some ugly, irregular cracks if you don't put in the joints to at least control them....one joint at mid-court line would be a minimum.

 

[KarlT]

eric1037

Project is located in Winnipeg, Manitoba, Canada (Frost is typically experienced to about 6 ft depth from grade, and slabs on grade are expected to shift and move around somewhat)

Subgrade consists of brown/black silty clay to 4-5 ft depth, about 3 ft of soft saturated grey plastic silt, followed by 3-4 feet of brown clay with silt pockets and then blue clay with silt pockets for a large depth.

JAE - I did find something in the PCA Concrete Floors on Ground in which you could calculate the required area of steel based on a coeff. of subgrade friction for a certain size of slab. However, I do think that it was based on an indoor slab, and fluctuating temperatures may add to it? My thickened edge is there to prevent undermining of the edges of the slab from soil settlement, but they may also add to the restraint as well?

If I put one joint at the middle, then isn't there a possibility that it could open up a fair bit and tear the vinyl in the winter? - we can get up to -40 degrees celsius! (I'm assuming the joints you mention would be control joints with greased dowels) I'm thinking that with rebar at say 12" o/c, you would get a random network of small cracks versus one joint that opens and closes constantly due to temp & shrinkage...

 

[JAE]

I guess I don't have a large amount of experience in outdoor non-jointed slabs such as this.

The PCA formulas (formulae?) - there are a few, and each gives different amounts of steel...lots of variability so I don't have a whole lot of confidence in them.

Post-tensioning the slab is one sure-fire way to avoid cracks.

One joint in the middle...yes, you might be right that a lot of movement would be gathered there as opposed to distributed into smaller "micro" cracks.

 

[cap4000]

I generally use 1% steel(Wire Reinforcement Institute) in my crack resistant slab on grade. Gym Floors must have high Ff/Fl numbers(Almost a "superflat spec" finish). Your Pole Embedment looks SHAKY if its a true cantilever. Go Much Deeper if so. Allow a gap for expansion movement at the ends where the slab butts to the walls. Good Luck

 

[Ron]

For your 5" slab, the joints should be spaced at 10 to 12 feet on center, both ways. This will allow smaller amounts of movement at each joint. Joints should be cut to depth of 1.25 inches and should be cut the day of placement, not the day after.

Then do as JAE said ("cure the hell out of it") and let the slab sit for a couple of weeks. You'll be able to look into the joint and see the cracks they have caused to occur. Then apply a semi-rigid or even a flexible joint sealer. Then apply your coating.

A 1 to 2% steel ratio is fine for this purpose. Use welded wire fabric, preferably in sheets, not rolled.

 

[BFPartners]

The soils (clays) in Winnipeg swell under moisture and you could experience unwanted cracking. Use the recommendations in CSA 23.1 and A23.2 for maximum joints in slabs and floor flatness numbers. The question becomes if you are trying to stop the cracks then you must use 0.002Ag or if you are trying to stop the cracks from offsetting then you should use a Welded Wire Fabric or dowels.Are you sure you don't want to use asphalt and then tar patch the cracks that do occur? Asphalt is cheaper! The design guide as produced by PCA "Concrete Floors on Ground" is helpful. You should consider Type 50 cement.

 

[apetr26542]

If you have that kind of temperature change, with or with out joints, that slab is gonna move. With a length of 42ft, and a temperature change of 100 deg F. The slab will expand / contract by 1/4" total. or 1/8" at each edge. You may need an expansion joint in the middle, without any joints, the slab would shrink 1/2", assuming the ambient temperature is relatively warm when the concrete is cured.

 

[DaveFlax]

No amount of steel will help you. Your best choice would be to use Type-K cement, instead of Portland cement. Unlike Portland concrete that shrinks, Type-K concrete expands. Joint spacing of 100 feet and more is common.

 

[samdamon]

Karl-

What is the coeff of thermal expansion of the 1/4" vinyl compared to the concrete? Are control joints necessary in the vinyl? How is it bonded? Does the manufacturer say if the vinyl will bridge cracks of a certain in the substrate? The answer to those questions might help determine how heavily you need to reinforce the slab and the spacing of control/construction joints.

Even if the vinyl topping does not have any special requirements, the comments by others about using low w/c ratio and curing aggressively seem very appropriate. I would probably ask for a water cure of this slab (poly sheets laid over saturated burlap, kept wet for at least a week).

A 6' frost depth! Whew!

Regards

 

[KarlT]

SamDamon

Check out this link to see why Winnipeg has a 6ft frost depth!

Brrrrr!!!!!

http://weather.cnn.com/weather/forecast.jsp?locCode=YWG