Well, I'm not in the concrete business, but I do have a degree in civil engineering, so I'm kind of particular about my concrete. Â If the ground is undisturbed (not fill) and has a high clay content, you can remove the grass and pour right on the dirt. Â Heavy loam or sandy soil needs to be excavated about six inches and a bed of compacted gravel put down. Â Never move dirt around and then try to pour on top of it. Â Even if you compact it, it will settle. Â Four inches of concrete is minimum, six is better. Â The cost increase in marginal. Â Welded wire fabric will keep the slab together when it cracks. Â Concrete is going to crack, nothing you can do. Â Some people like fiber reinforced concrete, but it is better for high traffic areas where the top gets a lot of abuse. Â Wire gives the whole slab integrity. Â If you live in an area with a heavy freeze-thaw cycle, order air-entrained concrete, it will keep the top from spalling.
I notice you are putting your slab up against your house. Â I would not advise trying to pour it level. Â A very slight slope, maybe 1/2 inch per 10 feet will keep the water away from the house, and keep it from pooling under the tub. Â Your tub won't even notice that slight a slope.
Don't be temped by fancy "stamped" concrete. Â It is more important that the tub be evenly supported by a uniform surface. Â A medium "broom" finish in the direction of the slope will give an attractive, non-slip surface. Â Cover with plastic, but pull back to mist with water to keep the surface damp. Â Cure for two weeks before putting the spa down and filling, don't skimp. Â The only thing about concrete harder than doing it is doing it over.
All of this advice is worth exactly what you paid for it. Â Find a local concrete professional and ask their opinion. Â A company that employs a registered professional engineer is a good sign. Â A good place for a referral is one of the "backyard" stores that deals in storage buildings, gazebos, decks, grills, and sometimes, spas.
