I am building a forge in my backyard. What should I use for the flooring? I think that a base of concrete covered in 6-8 inches of pea gravel would be good, but I have no experience  in these matters. I live in Cumberland County ME if that makes a difference.

Lay concrete then cover it with gravel? . . .eh . . . uh . . . er. . . HUH? Only if you like walking on a shifting surface and losing anything smaller than a quarter if you drop it. If you're concerned about  your feet and knees on a concrete floor wear gum shoes or better, Z-Coils. 

If you want a dirt floor talk to a local gravel company and buy some D1 gravel. It's a crushed aggregate specifically graded to compact hard. 

If you don't have seasonal wet soil conditions and good sub soils clay compacted hard makes a good floor as well.

I prefer concrete myself but that's me.

Frosty The Lucky.

A good hard packed dirt floor would be fine, if your soil conditions allow it... otherwise concrete over packed gravel would be my choice.

Anything that gives you good, solid, safe footing. I may be new to blacksmithing but I do know a thing or two about putting a shop together in unusual and irregular locations...

 This subject has been discussed at some length with  numerous opinions and reasons pro and con for various floor systems .

The short answer is that  depends on your resources , location and intended use for the space, whether  it's for traditional or contemporary blacksmith work , general metal work ,including fabrication , machining and forged work .

Some types of work are best kept in separate spaces ,  wood work away from hot metal and sparks, machine tools away from  dust , grit and potential damage from the impact and vibration from heavy forge and fab work. There no reason that separate areas of the shop can't have different floors depending on use or as the building evolves over time.

 

I like concrete myself.  But then I also have cats, so a gravel or dirt floor isn't the best idea.

Eeewwww.
 

I have crushed limestone, can be packed hard or not. With these bad knees it's my favorite floor. Concrete is hard on them.

Compacted dirt floor is my choice. Easier on the feet and far easier to mount tools.

Personal choice rules here.

Ag lime is what I have. Worked on concrete for 45 years. Don't want  it at home. What oldgoaly and anvil said.