A columnist for "Conncrete Surfaces" sister publication "The Concrete Producer" explains why concrete that will be ground and polished benefits from having a larger coarse aggregate content than conventional structural concrete.

Question: In our last project we needed to polish some concrete. There was a varied appearance to the surface after it was ground smooth. In some areas the rocks are visible, and in other areas just sand. To prevent this problem from occurring later, what can be done to avoid this?

Answer: The number of slab-on-ground and suspended slabs that are being polished continues to grow. This has revealed a problem where a clash of specifications causes a conflict of expectations, frequently resulting in the concrete subcontractor holding the bill for an architectural problem.

The conflict occurs where the architect has specified the polished finish while the structural engineer has specified the concrete to have certain properties. Beauty when polished is not one of them. As a result, the aggregate may contain soft particles, and the concrete may have a sand to coarse aggregate content that is set to ease placement and construction, not for exposing the aggregate. Concrete that will be ground and polished benefits from having a larger coarse aggregate content than conventional structural concrete. Producers and contractors should discuss what mix is best suited for placing and grinding.

The problems come in three types:

1.There may be “waves” in the slab where there are areas of higher and lower coarse aggregate content visible after grinding. There is often a regular pattern to these areas, as they occur due to the placement methods used. Using a jitterbug or other practices to bring “cream” to the surface during finishing can cause this problem or make it worse. Finishing activities should be performed to provide as uniform a surface as possible.

2. There are normal finishing defects at the surface that have a depth of more than 1/8 inch. In polishing and grinding those areas the tendency is to grind out the defect, resulting in a deeper local grind that reveals more aggregate. This leads to a non-uniform surface that you are told is not how the slab was envisaged in the design stage. This problem arises frequently when the surface has been densified by troweling to provide a “hard troweled” or burnished surface. These small defects may not be a problem in a conventional floor, but cause problems when polishing. A better way to deal with this issue is to fill the surface defects with a mortar made with the same cement source as the concrete. Place this into the void and steel trowel flat, then use warm water to cure for a few days before grinding.

3. Variability in the color of the paste. This arises from two sources: the curing methods employed and using chloride-based accelerators. Curing using plastic sheeting can result in mottled paste coloring that goes down a few sixteenths inch. Calcium chloride will mottle the paste even deeper. Avoid these practices for what is really architectural concrete.

The aggregate used for structural concrete may conform to ASTM C33 Class 3 or Class 4. Architectural concrete requires Class 5 material at least. The producers of structural aggregate don’t need to worry about color consistency or aggregate appearance. Use high-quality aggregate if it will be exposed.


Finally, there are problems that occur due to curling and shrinkage. Grinding and polishing a curled slab to be flat will expose more aggregate around the perimeter of the slab than the center. And the grinding will make cracks appear to be larger and more visible. Use the lowest possible shrinkage mixes for concrete intended for polishing.

Make sure that the expectations are set at prepour meetings and always make a mockup, have it approved, and keep it around as the acceptance standard. By taking these precautions uniform, good quality ground and polished concrete will be installed.

Kevin MacDonald is president of Beton Consulting Engineers. Contact him at kmacdonald@betonconsultingeng.com or visit www.betonconsultingeng.com.

The original article can be found here.