Yet another slab/pad question

[calstar]

This is regarding a hot tub pad(not necessarily shop related but right next to my small back yard shop! Shop is raised floor on piers) so if this isnt appropriate for this forum please delete. Site of pad is 125' +- from the street; I'm having a pumper because the elevation is around 30' higher than the street and accessible only by walkways and stairs(did not want to transport bags of ready mix over that route, paying for the pumper is worth it to me, 71yo). 8x8' pad was going to be 5.5" but I screwed up the elevation and needed to add a 2x2(1.5") on top to get one corner above the grade. See the pic to see the site(I have yet to add kickers to the form).
The base is non-compacted filled earth(20yrs ago by previous owner) so I dug out soil to get a level base then jumping jacked it, replaced the excavated dirt and did the same. I didn't bring in any base material due to the route I'd have to transport pad. Soil is clay and non clay mixed fill. I'm not concerned about cosmetic surface cracking as it will be covered by the tub, but I will wet cure it for at least a week. Weight of filled tub and people 5000lbs +-.

Question:

Given that the fill isn't the best I put a #4 grid 12"oc, 3" dobies, is that too high for the dobies in the 7-8" pad to get any tension benefit in case the soil settles? Should I use shorter 2"dobies to get the rebar in the lower 1/4 to 1/3 of the slab? About 1.5yds of concrete.

Will pea gravel be an adequate aggregate?

Is 3500psi ok or 4000 better for this project?

thanks, Brian

 

[Stuart in MN]

Here's your picture - for imgur links, you need to change the 'https' part of the URL address to 'http'.

 

[ConCretin]

Your rebar location is fine. The rebar won't add strength to the pad directly with regard to cracking but it will hold things together if cracks happen to occur - which btw is unlikely in my opinion. That pad looks plenty stout and the base is basically undisturbed soil st this point.

Pea-stone will also be fine. 3/4" aggregate is more typical but the mix will pump better with the smaller aggregate.

I'm starting to sound like a broken record but 3,500 lb mix will also be fine too.

Good luck with placement and enjoy that hot tub.

 

[mike93lx]

1.5 yards is a small load. Make sure you coordinate on what to do with the extra. Would be a good time to pour anything additional that might be needed

 

[calstar]

1.5 yards is a small load. Make sure you coordinate on what to do with the extra. Would be a good time to pour anything additional that might be needed

Yes I've been thinking what else to pour, have a few ideas. It is a small load and accordingly a little pricey, I'm only doing the pump and ready mix because of the convenience and the time and effort it will save me given the location, at 71 I don't have the strength/energy I once had.

regards, Brian

 

[wssix99]

Given that the fill isn't the best I put a #4 grid 12"oc, 3" dobies, is that too high for the dobies in the 7-8" pad

Save some money and effort and skip the steel. It doesn't add structural strength in a slab (like it does in a beam or a column). As Doug reminds, ^ it's just a tool for crack control, which isn't important for you and shouldn't be a factor here, anyway.

The "strength" of this slab is in it's depth. The compressive strength of the concrete eclipses any force it will see from the hot tub, so the psi of the mix and the aggregate doesn't matter for your situation. (The higher psi mix many just give you a smoother surface or nicer finish - that's all.)

The pressure that your slab imparts on the dirt below generally decreases as a square of the depth. (A 2" slab puts a fourth-ish of the pressure as a 1" slab and a 3" slab puts a ninth-ish of the pressure of a 1" slab.) So, you don't need to worry about your base. A 7-8" slab is really stout and gentle on the ground below.

Typically you would use a gravel sub base to reduce the pressure on the dirt below without going to the expense of a thicker slab. However, the economics and other circumstances of your situation allow you to go thicker with the concrete. So, again - you are all good.