I am installing a hot tub, aka spa, aka Jacuzzi and want to make a concrete pad 10 x 10 feet x 6" deep (with #4 rebar on 12 inch spacing).

The spa when full will weigh 3700 pounds (plus people -so say 4000+lbs) and is about 7 feet square.

Can anyone advise me what type of ready mix concrete I should order and what "grade"/Specificagion?

I am in California (where else? :D ) and do not have ground frosts, but do get the occasion earthquake.

Thanks

You'd be building a bomb shelter with all those specs. I would remove any top soil and spread 4" of sand followed by a full bed of concrete block standing on end. I then cover the block with sheets or pressure treated plywood.

I'd tend to stick with concrete only because I'm lazy and don't want to carry blocks or plywood. :-) Follow Bob's suggestions for site preparation and then rent a jumping jack to tamp down the soil before the pour. Ensure you consider all wiring, water supply and drainage issues before you pour.

Bob is absolutely correct that this is overkill. People tend to think of hot tubs as heavy loads... but it really isn't compared to specs for driveways or building foundations. Your hot tub has a footprint of maybe 50 sq ft, so 4000lbs/50=80lbs/sq ft because the weight is evenly distributed. A heavy car is ~4000 lbs spread on 4 tires, so 1000lbs per tire. Each tire probably only has a 6"x6" area on the ground so that is 4000lbs/sq ft - a world of difference from your hot tub.

Thanks for the advice gents. I admit to over-engineering the pad, but did I mention that I live in California, and don't want the pad to start cracking every time we get a 5.0 quake? I also have some fearsome tree roots from a neighbour's property that will grow under my pad.

My question was really seeking information about what I'm led to believe are the different types and grade of concretes: -I've heard of "2500", "3000" and "3500" concrete grades, and wondered what these numbers refer to (Pounds per sq foot?). I've also heard of 'the cheaper pea gravel' type of RMC - presumably there are others?

I'm only going to do this once, so want to get it right the first time.

So, does anyone know what these numbers mean, and what type of concrete I need?

Thanks in advance

Jan

2500/ 3500 etc are in psi. Multiply by 144 to get lbs/sq ft. It's a big number, so pretty well anything will work. That is why Bob said plywood and cinder blocks.

a 10x10 pad shouldn't have a problem with earthquakes. It's too small and there's not much weight on it. In fact, your hot tub may be the place to run to during the next quake - bring a lifejacket for the waves! :-)

I would be a bit worried about tree roots though. They are insidious. How close are the trees and how big are both the roots and trees? Any way you can remove the roots from under the pad?

Concrete pads always crack over time even in Illinois. Concrete is brittle but a pad of gravel and block can move and absorb any changes in the earth. Either aooroach will work but many folks have no room to get a concrete truck behind their home and wheel barrowing that much cement isn't fun.