RV Concrete Pad Questions

I feel for the OP. He must be really confused at this point.

He should hire a real expert and not take anything written here to heart.

I'm not familiar with your soil and freeze/thaw, moisture or expansive soil conditions. but in looking at historical data for your area, you do have occasional freezing conditions and seasonal rain which needs some attention. The bottom of footings need to below grade to help offset those situations. The compacted fill allows for drainage and to help offset irregularities in soil conditions. It is cheap insurance, because someone may have buried a cactus or log under your slab.

Post tensioning of the slab is great, but a dumb slab with adequate fiber and mesh reinforcing is probably adequate. A 6" thick slab should be an absolute minimum.

It's not the strength of the concrete that matters. It's what is under it that really makes the difference. A highly compacted sub grade is what determines the viability of concrete. If your sub grade is weak, then the concrete will fail. 2400, 3000, 4000 it does not matter. Compact, compact, then compact some more.

That's really not true. It might be true in most cases, but there are places where you can't get the soil compacted and stable, regardless of how much you spend. There are places where the soil is going to give you problems.

In those cases, higher strength concrete and maybe post-tensioning the slab come into play. Houston and SE Arizona are classic examples. Expansive clay soils don't really care how much you compact them or the base course. They do what they please and will tear apart a structure if you don't build right for them.

A post-tensioned slab is extremely unlikely to crack because the concrete is under constant compression from the cables inside. A more typical example is bridges and elevated floor slabs where with post-tensioning, it doesn't matter if anything is under the slab. A raft slab does exactly what the name implies. It sits atop the bad soil like a boat on water.

Since none of us know what the OP's soil is like, we're throwing rules of thumb at the question. The OP might have perfectly stable soils where it doesn't take much engineering to get the slab right. If the OP's soil isn't good, there are ways to overcome the problem. Generally, that means engineering a slab, almost as if there is nothing under it. Sometimes, over excavating the pad area and filling it in with engineered fill is enough.

I did our concrete myself because it was what my grand father, my dad and what I did for many years. I did 5" with 1/2" rebar at 20"x20" squares. I rolled a design that had a straight line going across it every 9 ft. I then sawed on the straight lines. This was done in 1998 and no cracks but as was said above concrete cracks.
chevman

It's not the strength of the concrete that matters. It's what is under it that really makes the difference. A highly compacted sub grade is what determines the viability of concrete. If your sub grade is weak, then the concrete will fail. 2400, 3000, 4000 it does not matter. Compact, compact, then compact some more.

If in doubt, have a soils engineer make the final call.

In my project, I have 2400 PSI 6" over native. Highly compacted. I have RV's running through here all day long. No failures at all, and I have 180,000 sq ft of it. Will it crack at some point? Yes. Nothing you can do about that. That is the nature of concrete. Mostly hairline cracks.

the silverback wrote:
If you intend to park it for a long time put something under the tires between them and the concrete pad. Both Michelin and Goodyear say this needs to be done. Maybe there is something on their web sites. I remember a engineer from Michelin saying a piece of plywood is OK. Please check it out.

Yes. I am full-timing now and it was recommended that there be a barrier between the ground (concrete etc.) and the tires. I use wood and also have a vinyl place mat above the wood. It is said that the concrete will leach the oils from the tire. This is more important than tire covers.

I agree with the 4,000 pound mud as they use a lot of fly ash these days. Rebar and 6 inch mesh would be grand. Don't forget about expansion joints or have the contractor come back the next day and deep cut the slab with a power saw creating a place for expansion.

Dick

If you do that, you better pick your contractor well. Some may not be up to that task.

Thanks for all the replies and I will use the info to better quiz potential contractors.

I'm not going to spec anything but size and usage. I don't want to be put in the position of a contractor saying..."Well that's what you asked for".

I do wonder why so many use a gravel base. I sort of figured that since my ground is so hard it the slab would just be poured right on to of the soil. The only excavation I see necessary would be for the footers.

If the top of the slab is set so that the desired thickness is at the high spots of the site wouldn't that even eliminate any leveling???

FYI, my ground is so hard that driving a MH on it barely leave tire marks and will even support the jacks.

Thanks again for the info.

I would use 6 or 8 inch concrete 4000 psi air entrained with fiber mesh. Rebar where needed .Compaction is one of the most important parts. Can also use rolls of wire if preferred. Expansion, control joints and saw cuts where needed.