We just placed a 6-inch-thick slab on grade with #4 rebars spaced at 12 inches on centers each way. The rebars were 1 1/4 inches from the slab surface. The concrete was placed by crane and bucket and had a 5- to 6-inch slump. It was hand-screeded without vibration. After the slab had hardened, visible lines marked the location of the rebars and the owner is upset. What caused the problem? Is there a way of removing the lines?
What you described is sometimes called steel shadowing. There are several theories as to how it happens. Some believe that rebar heats up when the sun hits it. This then causes the concrete to hydrate more rapidly over the rebar. When lines have shown on the underside of elevated slabs it's been blamed on form oil being chemically changed by the sun but unaffected in areas shaded by the rebar. In one case the shadow of a telephone pole on a flatwork job left a long visible streak in the concrete, seeming to confirm this possibility. Another theory holds that when rebar is too close to the surface and the concrete isn't properly vibrated, the concrete doesn't fully knit together over the bars. This causes the visible line. Like most discolorations that aren't stains, these marks are probably not erasable. You can cover them with an opaque coating, but that's a pretty expensive solution for what is essentially a cosmetic problem. Reader response: Reader Mike Jacobson with R.P. Carbone Construction Co., Cleveland, submitted the following comment on the November 1992 Problem Clinic item, "Rebar Near Surface Causes Marks in Slab" (page 846): During concrete placement and consolidation, larger aggregate particles will be displaced from the area directly over the bars. The concrete mix becomes nonuniform and hydrates at different rates, resulting in a different color, or shadowing. Shadowing can be minimized by maintaining rebar alignment and coordinating cover and maximum aggregate size.