How can I secure an aluminum shed to an existing concrete slab?

Foundation seal will work fine. Comes in 50' rolls.

if you want to seal the surface a epoxy paint is good !

but if the water come from under the concrete slab only a hole with aggregates ( small rocks ) and a swamp pump works fine !

How out of level is it? 1/8" from side to side, no problem. 2", problem. Spa needs to be pretty close to level.
You could add more concrete on top of slab and level that.

These are not the same, so cannot be directly converted.

Pounds per sq ft is a measure of pressure, while lbs / cu ft is a measure of density, weight per unit volume.

I think maybe what you are doing here is converting the weight of concrete in a 1" slab, where a sq ft would contain

1 x 1 x 1/12 = .0833 cu ft

The figure of 150 lb/ cu ft is correct for the average density of concrete, so the above slab would have

1/12 x 150 = 12.5 lbs/sq ft at 1" thickness

BTW that is a thin slab of concrete, insufficient even for a light-duty pathway. If you are referring to a screed for tiling or similar, that is cement.

Is this for vehicles? If so concrete won't work as it will be too brittle especially when it gets cold.

Things to consider are cut and replace -1. Concrete saw the affected area replace and re-concrete (impact hammer is another option but a concrete saw is neater and you can control the cut better.. 2. You can bypass the pipe. 3. Dig under the slab - you obviously would need to be near an edge.

It is a slab of usually Concrete that has Tension introduced upon it's Reinforcing rods/cables, AFTER it has been Poured, dried/set as opposed to a Pre-tension slab whereas the Tension is put on the reinforcing rod/cables inside the concrete and are Tensioned BEFORE or While it drys/sets

This is Absco Sheds and the page that gives you instructional videos for assembling their sheds.
(Plus preparing the site, and constructing the concrete slab)

http://www.absco.com.au/videos.php