Rebar doesn't prevent cracking, but it will help hold the slab together if it happens plus, rebar will provide additional structural integrity especially when you have heavy traffic.

That said 12" -18" is good in a grid pattern, use 6" concrete slab and thicken the edges approximately 8-12" in from the edge.

Reinforcing steel for structural purposes is not a guessing game or for crowd sourcing. Engineering questions should be asked of an engineer, much like legal questions should be asked of an attorney.

It is not for the building structure. Only for the hardscape. The structural engineer did not put anything on the plans. Should he have?

By the way had an interesting observation when the old driveway concrete was demolished. There was about 6-7 inch thick concrete, no rebar, just some very thin iron bars in a few places, and no base rock. Very surprising how the driveway held up so well with hardly any cracks for so many years. The soil is somewhat like clay. The sideyard concrete had cracks.

For landscaping 12x12 is probably overkill but if it's not a huge space, it's not that expensive. Getting it centered in the slab is critical. Agree with GN Builders in beefing up the overall slab and thicker edges.

I've been involved with many commercial and industrial building projects. Concrete slabs were very often specified without reinforcing bars, except for those that receive large loading. A well-prepared base is critical, as is shrinkage control, very often being added fibers to the mix and control joints (usually sawn) at regular spacing.

Here in Michigan residential concrete slabs are usually reinforced with a fiber additive to the concrete mix or 6" x 6" wire woven mesh is set in place before the concrete is poured. Check to see what the common practice is in your area.

Same here Mark, they use re-bar if heavy trucks going in and out, like truck garages. Most driveway aprons have 6" concrete with wire mash and dumpster trucks going over without any issues and they are much heavier than RV.

Given how cheap rebar is compared to the total cost of a slab and how many broken slabs I’ve seen in Dallas where there was little to no rebar installed, I’d go with a 12” x 12” #4 grid and a 6” slab (that‘s what the structural engineer spec’d for my slabs. No problems with the oldest going on 10 years on expansive clay).

20ft grade 60 #4 costs around $7 and compared to substrate prep and forming, labor is minimal.

Labor is probably the primary cost of adding steel. It is measured, laid, tied together and set on spacers which are left. I switched most of my home slabs to #3 18-24" ea way about 1997 at an upcost of $1K per home vs 6x6/10/10 rebar (wire) but it immediately solved any tile cracking issues. Its probably 2-3x that cost now but that is my standard design unless plans are provided by others.

We've had to upgrade some driveways for 80K fire department loads to 5-1/2" 3000psi slabs with #4 24" ea way. Still cracking happens so we try to contain in with deep groove cutting in to smaller sections. A geotechnical (soils) engineer provided the specification. Gravel was not required but it helps. That upgrade was 6-8K. I doubt any fire trucks would ever drive on it.

For sidewalks, aprons, and driveways, we always use 4000 PSI concrete

+1 vote for rebars, I have seen too many instances where geniuses do not compact the soil. I do not expect compaction everywhere ... but oh come on, if you putting a driveway over dug trench... compact. With a gravel just basic walking on it could give you a 1% - 2% extra.

Remove the tree. You cannot prevent roots from growing.

Soil movement determines Reinforcing.

Is your contractor qualified? Are you? To determine what is required.

You asked about driveway, then hardscrape,

I am not qualified. I want to minimize maintenance work and cost. So my preference is concrete whereas my contractor strongly suggests pavers. I would prefer to not remove the tree as it gives great shade.

A friend of mine has excavation business he has 100' concrete driveway that was installed about 12 yrs ago with a 6" slab and wire-mash and there is not a single crack, and he has nothing but heavy equipment being moved constantly.

The most important thing is a good sub-base and a good mason specializing in concrete work and he will give you a slab that will never crack.

That said, grind the roots that in the way, remove loose soil, fill the entire driveway with good sub-base and leave it for 6 months to a year and keep driving over it, with the time, and rain, etc you will get a total settlement and after that pour your concrete. That is the best way to do it.

Good luck