Q: At a regional shopping center in northern Illinois we noticed some good exposed-aggregate sidewalks which have been in place a number of years. Then we saw that a lot of the bands of plain concrete separating the exposed aggregate surfaces were in relatively poor condition. Is this happening elsewhere? If so, does it indicate superior durability for exposed aggregate work?

A: Without knowing more about the construction history of the two concretes, we don't have an answer to this question, but we'd like to know if others have observed similar conditions.

Reader Response:

One contractor reader, Milton Collins, responded that better durability should be expected in the exposed aggregate concrete. Collins believes this is because the cement-sand matrix that remains exposed between the aggregate particles has not been subjected to the finishing operations that often, or usually, increase the water-cement ratio. The higher water-cement ratio weakens the surface and makes it less durable.This may be true. Yet it does not explain why more material in the picture seems to have broken loose than just the top layer of cement-sand matrix. There appear to be some loosened aggregate particles as well. This suggests that a little farther down in the concrete the scaling resistance was still deficient, even though it was probably not affected by the trowel.Another reader, Edward M. Rizzo, of Preco Industries Ltd., replied that his company has test results on circular test specimens that were subjected to ASTM C 672 salt scaling exposures and then measured for weight loss after 50 cycles. Specimens whose surfaces had been exposed by retarders before the test lost only 60 to 80 percent as much weight as a control specimen. (The control specimen lost 80 milligrams per square centimeter of surface). Tests also showed that the weight loss of the exposed aggregate specimens could be greatly reduced by applying a good surface sealer.