Polished Lightweight Concrete?

"We can't polish the lightweight concrete floor slabs. Why can't I do that?" was my client's question. "It says in the spec, ' Normal weight concrete:  No lightweight aggregate.' "

Even though my wife insists that I believe I am an expert on virtually everything, I had to tell my client that I did not know the answer, and I would have to look into it.  I could think of some possibilities, but I could not recite two or more concrete reasons (pardon the pun).

I did some internet research and also called a product rep I know, and I ended up with several explanations but no real, simple solution. In my research I found that The National Ready Mixed Concrete Association has a very tidy overview called "CIP36- Structural Lightweight Concrete". This handy reference told me many things I knew on the subject and a few things I did not know.

Lightweight structural concrete is actually so because of the aggregate, and not the cement. Various aggregate recipes use various light (or "lighter") weight materials to reduce the weight of concrete by as much as 35 to 40%. This lightweight concrete still runs 90-115 pounds per cubic foot, which is still a heavy material. However, the reduced weight of the concrete in structural use means a reduced dead load, allowing reductions in the size of structural columns, footings, and other load bearing elements. Lightweight concrete also provides greater fire resistance and greater R-values than normal weight concrete, in part due to air entrainment, which also allows the internal retention of more water, which enhances internal curing and hence, the strength and durability of the concrete. All of this goes in the "plus" column for lightweight concrete.

When our client proposed a polished finish, which requires grinding, densifying, and penetrant coatings, the negatives for lightweight concrete began to emerge. The water retained by the air entrained concrete and the more porous, lightweight aggregates also greatly increases the cure time for fully dry material. The air entrainment also causes the concrete to exhibit a more porous surface after grinding, which can cause irregular results in finish applications. According to the product rep I consulted, there are also issues with the polishing process and the aggregates in the lightweight concrete. The more porous aggregates, such as shale, will often fracture during grinding. These imperfections can cause inconsistency in the way the concrete accepts applied finishes. The manufacturer did not completely rule out polishing the surface of lightweight concrete, but stated emphatically that the customer would not get the type of finish they were expecting, and must be made aware of the situation.

With no recommendation from the product rep on a substitute finish in the same genre, I returned to my client, who had also researched the issue. We compared notes, with my client noting the lack of viable finish options in the same genre.  My client decided to leave the polished finish in the spec and discuss it with his Owner. As the project is a somewhat rustic facility, the final look of the finish might be appropriate even though the resultant finish would not reach typical quality or sheen expectations.