Could someone trip if the tip of their shoe caught where the wood and concrete meet? I can trip on air, so that would be my first concern.

If not, then consider trying your stain/distressing project on the currently exposed portion of wood. If you don't like it, you have only a small part of concrete to patch. If you wait to try it out until after all the concrete is chipped off, your repair job is a lot more substantial in the event you don't like the effect.

I would chip out the concrete and put tile in the gap. You could find something in a similar color if you want to be discrete, hit craigslist if you want to be frugal, spend the big bucks for something wonderful because it isn't a very large area, or use glass tile with lighting underneath if you want to go over the top.

Before chipping everything, make sure it doesn't affect the soundproofing of your home. We've had some units in our building that had similar problems and the owners had to patch the dammage for that reason. Be aware that a patching job will never be seemless. Mrs yow's tile idea sounds like a good option.

Much like the brick windowsill, once again, decoration is not the solution to a structural problem!

You say you're "not concerned about the structure of the building," but there's a reason the concrete has chipped away from the board, and that's what you need to address first. Has the wood gotten wet? Is there an expansion and contraction problem between winter and summer? Was the original concrete floor laid too thin over the boards?

If you're serious about your loft, you owe it to yourself to investigate the *failure* of every problem before you embark on some kind of DIY coverup. If the underlying problem isn't addressed, it will either get worse or cause problems for whatever kind of patch job you do.

Good luck!

I'm with nashdp. Concrete cracking says the underlayment should be stiffened, or the concrete around the area could also crack more.

If you remove the concrete the wood will probably flex as you step on it. Personally, I'd find this a bit disconcerting.

If you read the question, its only the thin layer of concrete over an underlying beam that has come delaminated - not the entire floor (which is apparently a thicker pour). A patch would look like a patch - I would fill in the area with colorful tile or thin wood paneling/flooring to bring up this area of the floor level with the rest of the concrete.

Your whole floor is unfinished concrete?

Doesn't seem right. This is a commercial loft or a residential loft?

Even if they had put expansion joints on either side of the beam it wouldn't make a difference. Concrete doesn't stick to wood.

Seems like the chipping concrete is a leveler that they skimmed over the surface of an existing slab. But that's not finished. For a residential property it has to have a finished surface of some sort--even if it is properly finished concrete that has been sealed.

This is obviously not a structural issue. The concrete topping is just for sound dampening purposes or in some cases to cover radiant floor heat. Whatever you do over those beams is going to be purely cosmetic. With that in mind a concrete patch is going to crack in exactly the same place that the last one did. Whether you choose tile, new concrete or a wood patch you want to decouple the new area from the existing concrete. Any decent tile store will sell a sanded silicone that looks similar to grout in a wide variety of colors that will provide the flex needed to keep the joint looking fresh and new.

Bondo?