Q.: We sometimes patch existing asphalt parking lots with concrete. On a couple projects, we sawcut the area to be patched, removed the bad material, and replaced it with air-entrained concrete. This seems to be economical. The one drawback is the patches are so noticeable. Has anyone else done this before, and how can we color the concrete to make the patches less noticeable?
A.: Although Concrete Construction has published articles about whitetopping, we don't recall anything specific on concrete used to patch asphalt lots. Maybe some other readers can share any similar experiences.
As far as coloring, talk to your ready mix supplier about supplying concrete colored black. Water-dispersible carbon black can be used to make black concrete. Don't exceed 10 percent by weight of cement. There also are emulsified carbon black or black iron oxide pigments formulated for use in concrete. Add 2 to 5 pounds of pigment per sack of cement. To obtain uniform color, careful proportioning and extended mixing may be needed. Blending dry cement and color compounds can help reduce streaking.
Since you are concerned with durability in these paving applications, we recommend running an air content test on these mixes. Unmodified carbon black substantially reduces air content. Most carbon black intended for coloring concrete contains an admixture to offset the effect on air content.