Not Diesel related – concrete advice sought!

[Digger-Bear]

Not Diesel related – concrete advice sought!



My wife and I are about to hire a contractor to pour a concrete slab behind our home. Our plan is, within the year, to add on a screen porch but for now we just want a spot to put our hot tub. The size will be about 12 x 30.



I don’t know beans about concrete – what questions should we be asking and what should we be looking for while they are working?



Appreciate any help!

 

[daveshoe]

Concrete 101

Hey,Digger

The first thing to realize is that adding a porch later may make this a project that requires footings and walls under that slab. Concrete slabs move up and down with freezing and thawing cycles. That is a problem if the future porch has a conventional roof that joins to your house roof. That roof requires a stable foundation. I would ask your town building regs department about this. While you are at it,find out what inspections,if any are required for a simple slab.

Concrete strength is determined by the amount of cement mixed with the sand and gravel. Too much water added to the mix is not a good thing. Four inches of slump is wet enough for a slab. I have found 4000 PSI rated to be more crack resistant and something that has a much harder surface than 3500 PSI. The finisher has to stay there for a while to get a nice surface. If you have a hot tub going in,you certainly do not want a broomed [while wet] finish--it will be hard on bare feet that are soft from your soak. You can get a nice traction finish that is trowelled to high density,but the wait is at least 4-5 hours after it is poured. You could also locate an example of a slab that is the correct texture to walk on barefoot,and show your finisher what you want in a finished slab.

I waold also spend the money on reinforcing bars set on benches in a 16''x16'' grid. That prevents or minimizes cracking. Spend your money here and you do not need the crack preventing ''fiber additive'' that is often sold to customers. Wire ''fence'' placed in the slab often is useless because the laborers do not properly embed it.

I tried to keep this short,so ask and I will try to clarify stuff for you.

 

[bmoeller]

I would add a footing anyways. Will help prevent rats and other varmits from digging underneath, and causing problems later.

 

[RON'S RAM]

I agree about using 4000PSI concrete and definately make sure they use steel reinforcing in the slab. A guy who specializes in floors & foundations said one of the most important things in prepping for a floor is to make the finished grade the same as you want the finished concrete. If you want the slab pitched at all, pitch the gravel underneath the slab also. If you want it level, level the gravel beneath it. Keeping the slab damp while curing will also make a stronger slab. Also remember the weight of all that water. If the hot tub is not in the center of the slab,the weight will most likely settle the slab into the earth more than where there is no weight and may crack the slab.

One other thing to think about. I found this out too late. You can ask the contractor to add a color to the cement in the truck before it is poured out. I didn't think the concrete company would do this beacause I thought it would leave coloring in the cement truck. I later asked one of the drivers and he said they do that often. Just add a color packet to the mix at the jobsite before pouring.

Good luck!