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Patio Slab Thickness Question

Patio Slab Thickness Question

Patio Slab Thickness Question

We are building a house in north Texas.

Out builder is going to pour a patio slab for us that will be approx. 15' x 45'.

We are being told that they will pour a thichness of 3 1/2", with no reinforcement.

We are concerned that the thichness is sufficient, and that it can accommodate the weight of out ~1000 lb. grill and island.

Please advise.


Tom Jeffrey...

RE: Patio Slab Thickness Question

If you have a specific load that will be applied to your patio slab, you might ask your builder to provide a thickened slab in that specific area.  3 1/2" is thin, but probably OK for a patio slab.  With that thickness, however, you will most likely need concrete control joints at a spacing of about 12 feet o.c. at the most.  These would be either tooled joints or sawcut joints in the slab, done as soon as possible after the concrete can be walked on.

Make sure he cures it well.  Also, if you get freezing in your area (which I think you do in N. Texas) you should ask for air entrainment in the concrete mix.

The thickened slab could be 6 to 8 inches thick at your grill location, and I would extend that thickness about 6 inches beyond the perimeter of your island/grill.

RE: Patio Slab Thickness Question

My concern would not be the "design" thickness (3-1/2 inches), but moreso the constructed thickness.  With a slab this thin, the grade control has to be almost perfect or significant cracking (shrinkage) will occur.

This thickness is quite adequate for the loading, but I would specify at least a 4-inch thickness with a tolerance of -1/4", +3/8".  This means that the ground will have to be carefully prepared before receiving the concrete.

Have them sawcut the joints at 15-feet center-to-center along the 45-foot dimension.  This will give you 15x15 slabs which should provide for good crack control.  The sawcuts must be put in the same day as slab placement, and must be 1" deep.  Don't let the contractor talk you out of these items.  They are important to the performance of your slab.  He only has to live with it for a year...you'll have it 'till you sell the house!

RE: Patio Slab Thickness Question

In practice we never go under 6 in thick slabs, +95% reinforced 2 sides, and this even coming upon the preparatory layer of concrete itself, sometimes on some graded rooling-stone/sand underlying layer. Even when cutting the slabs more or less as Ron says (sometimes there is an added colouring layer) visible shrinkage cracks (usually minor and tolerable) appear.

With less care and in supermarkets where I have not at all lanything to do I have seen finishing slabs in roofs for vehicle use with cuts performed with very visible cracks (2-3 mm wide) outdoing any cut present and passing in perfect alignement through the cuts that purportedly would prevent such thing.

So take your conclusions.

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