Been calling around on getting a shop built. Had a couple of guys ask if I want rebar or if I fiber in my concrete. I currently haven2 slabs on my place. One was poured with rebar closet to 50 yrs ago. The other is fiber about 10 yes. Ago. The old floor still looks good where the new one is cracked every where. What would y’all recommend fiber or bar? Looking at 40x80 4-8 thick

I've poured two with fiber. No cracks yet, but the heaviest things I put in it are boats, and occasionally my truck.

Neither one will prevent cracks; they provide strength and a shop shouldn’t need either if you don’t have any really heavy point loading. . There are two types of concrete; the kind that has cracked and the kind that going to crack. The only way to minimize cracks is proper curing. As concrete sets, it generates a lot of heat. Controlling the heat differential between the core of the concrete and the constantly changeing ambient temperature is the key. Most people don’t bother with driveways and slabs because the thickness isn’t much. But, if you will keep it wet with burlap or something similar over the first 12-24 hours it will help tremendously. You can also have the ready mix company add ice but that’s usually for really thick pours and a skilled finisher. Pouring at night helps as well. Just do a little research on curing temperatures. And, I'll add that proper control joints help with controlling cracking.
Here's a good pedestrian explanation on curing:
https://www.thebalancesmb.com/highly-recommended-methods-to-cure-concrete-844449

To add to what Dallas said, you do not want your concrete to be self leveling as it is poured. This means there is too much water in the mix. This will create more curling and shrinkage as the concrete cures, which will cause more cracks.

I'm sure there are engineers on this board that could give you all the info in concrete strength and reinforcement requirements.

Though I have been out of the business a while, we used 12 inch centers on 4 inch pours and 18 inch centers on 6 inch for our rebar spacing. Wire mesh works too but is a pain to use on smaller runs.

If it was my shop I would run wire mesh and then tie it together with #4 rebar. Then the 2nd day after the pour I would cut groves every 10 feet.

Overkill, maybe but I never had to go back and redo any of my jobs.

30 x 40 shop, did mesh and rebar.

I am one of those engineers. Like most things, rule of thumb, with experience to back it up, will cover most situations, and if you are pouring a shop slab of 4 or 8 inches, it's not critical. I would use mesh over the entire slab and rebar just around the edge of the entire slab and fiber for more tensile strength, but I'm cautious. It will crack, as was stated above, so you put in seams to control where it cracks. Foundations and slabs for more than a boat or car or tractor in a shop need to be designed for point or distributed loads, and if that's the case, you need a civil engineer to design it, but doesn't sound like what you have there.

WOW Taylor, I just learned a lot. Good explanation and info that I may never need to know, but then again who knows. Truly, I didn't know any of that.

Jawbone, I'm on a red wine and concrete roll today Don't know too much about anything else though.

I'm NOT an engineer, just a damned good red-neck fixer of things, AND I stay in a Holiday Inn Express once before and I can tell you this is exactly what I would do.

Agree completely.
I'll further add that you can do everything by the textbook on the actual concrete mix, controlling temps and moisture, the wire/rebar/fiber placement, the forming, the pour, joints, and the finishing---and still have cracks because of other factors going on with the grade or site prep or water impacting the slab well after the pour or a host of other things.

I would use wire mesh sheets and fiber