What causes those red-brown-stained holes in concrete?

I see them most often in sidewalks. They’re a slightly-caved-in hole, about an inch or so in diameter, surrounded by rust-coloured stains that appear to have run off. Is it the end of a rebar rusting out, or something more interesting?

“Rebar rusting out” - I suspect that’s it. The caved-in-hole is where a small piece of concrete “spalled” off (popped out).

Technically not rebar-that’s a much heavier reinforcement typically reserved for flatwork thicker than sidewalks. In sidewalks and other lightweight concrete, it’s probably what is called welded wire mesh-the diameter of the wire being under .250". The trick when using it is to get in in the middle of the flatwork-not all the way at the bottom, nor too close to the surface. :wink:

I would agree it is likely the welded wire mesh. Sometimes it gets too near the surface and doesn’t have adequate cover and you will get this sort of staining.

There’s no trick if the mesh is installed properly, i.e., propped up on chairs to exactly mid-height of the pour. Unfortunately, most workers usually dislike this method because then they can’t walk or wheel equipment through the area. Instead they want to pour with the mesh laying flat on the sub-base and then try to pull the mesh up to the required height. Doing this is mostly guess-work and will often lead to the mesh being too near the bottom or top.

There’s one big “rust hole” in the concrete path around my house. I don’t know if the concrete is reinforced, though - I always assumed it was due to a stone in the concrete aggregate containing iron and leading to discoloration. But I guess I’m wrong.

That was my other guess. The only reason I was unsure was because of the number of these holes (I see many every day) and the fact that I don’t see as many (any?) on asphalt-based paths, like those beside the river in Saskatoon.

Quite true. No shortage of lazy buggers in construction, though.

Not sure if we’re talking about the same thing here, but I know of several in this small town near corners… Used to be signs years ago that got busted or torn out or cut off at the base or whatever… Some have been replaced a couple times and you’ll see a couple next to a sign.


Concrete aggregates with which I’m familiar are made from limestone/oolite. AFAIK, there’s little iron in that kind of calcitic rock.