Polishing concrete is like painting art. Your tools are the brushes and your canvas is the slab. But no matter how good you are as a painter, if the canvas is substandard it’s difficult to produce beautiful art.

When working on a new slab, you have the opportunity to become involved early in the process. To get a uniformly polished finish, the slab needs to be placed and finished flat and level. The slab-on-grade concrete specification should contain both overall and local minimum FF and FL requirements.

Once the slab is placed flat and level, it should be kept as flat and level as possible. As we all know, slabs do not dry uniformly. The resulting curling in slabs is similar to what happens to mud as it dries. The top dries faster than the bottom and the corners of the slab panels tend to rise.

When a curled slab is polished, more paste is removed at the curled areas, resulting in more aggregate exposure than the owner might have desired. Many professionals recommend a well-graded aggregate system that can lower shrinkage potential. Mixtures can be graphically analyzed using a coarseness factor chart.

Closer joints reduce curling potential. Weigh the cost of cutting and filling joints against the non-uniformity created by polishing over curled joints. At CRT Concrete Consulting in Fishers, Ind., we recommend maximum joint spacing of 10 feet for a 4-inch-thick slab, 12.5 feet for a 5-inch slab, and 15 feet for a 6-inch (or greater) slab.

Many engineers and contractors mistakenly believe curing eliminates curling. Curing can delay curling, but eventually the top of the slab will be drier than the bottom, so curling will occur. Often, the benefits of curing are outweighed by issues resulting directly from the curing methods. Curing compound and polishing systems may not be compatible, and it can be expensive to fix. Water curing blankets can dry non-uniformly, leaving permanent discoloration in the polished floor.

Effects on polishing

Finishing procedures can have a dramatic impact on the polishing process, such as densification of the surface. Hard troweled surfaces have an extremely dense and hard surface layer. If the owner requests some level of aggregate exposure, the polisher must cut through this layer. The industry is working to determine what finishing techniques will minimize the thickness and hardness of this paste layer while still providing the flatness desired. Some believe synthetic blades assist in this effort. Plastic blades can also help by minimizing any surface color variation created by the finishing process.

No detail is too small when thinking about a slab that is about to be polished. Previously unseen bootprints will appear after a concrete floor is polished.
Christopher Tull No detail is too small when thinking about a slab that is about to be polished. Previously unseen bootprints will appear after a concrete floor is polished.

Understanding the canvas of an existing slab on grade is much more difficult. The polishing contractor should have an opportunity to inspect the existing slab after the floor covering, if any, is removed. Previous flooring products can leave permanent marks in the polished floor.

Contractors want their choice in diamond tools to be in alignment with the strength of the concrete that will be polished. Tests can be performed using hardness picks to scratch the concrete’s surface (see “Assessing Concrete Hardness Before Polishing” in CC, September 2011). This number is used to select the proper diamond tools. Generally, the harder the concrete, the softer the matrix that bonds the diamonds to the polishing pad.

Often when testing for strength, contractors drill and break cores. Cores provide the overall strength of the concrete, but offer little value in determining the strength or hardness of the concrete’s surface.

When polishing a space that contains more than a single concrete placement, variability can come into play. Different mixtures will likely be visible in the finished project. A mixture containing 20% fly ash and a gravel coarse aggregate will create a different look than a straight cement-limestone mixture.

Repairing defects

The extent of needed slab repairs must also be addressed. Most manufacturers have excellent repair means and methods, but little consideration is given to the appearance in a polished slab. While the defect will be repaired, you should know it likely will remain visible.

Also, the moisture content may play a role, particularly if dye is applied. The dye enters the pores of the concrete to produce a desired color. Drier slabs absorb dye differently than slabs with higher moisture content.

Moisture content can be tested in several ways. The two most common tests are ASTM F 1869, which is commonly called the calcium chloride test, and ASTM F 2170, the internal relative humidity test. Both tests require the space to be in an in-service condition for 24 hours before beginning.

Much debate is underway to define what moisture content and which test will provide the best information. CRT recommends using both tests. ASTM F 1869 provides an indication of the moisture content in the top of the slab, while ASTM F 2170 tells what the moisture content will look like once the slab has equilibrated with the environmental conditions in the building.

All of the information obtained before starting should be used to communicate expectations with the customer. First, present ideas in writing. The old adage that “if it’s not in writing, it never happened” is true. Second, always install a mockup large enough so that the contractor can use the equipment that will be used during the project.

Polishing concrete is like most things in life: The better the communication between all involved, the better the outcome. CS

Christopher Tull, P.E., LEED AP, is founder and owner of CRT Concrete Consulting Inc. Contact him atchris@crtconsulting.com.