|Anant Shekhar Sahay, Vice-president Business Development-South East Asia, Bomanite India.|
On new concrete installations, the concrete mix design is critical. Using a cement-rich concrete (typically 3,500 psi) provides a dense surface free of voids for the polishing process. We can also use many times water-reducing admixtures to keep a low water cement ratio without jeopardizing the workability during the concrete placement. Wet curing of the new slab is an excellent way of curing but many times it is not practical. If this is the case, dissipating liquid applied curing compounds are a good way to cure the slab. Any remaining curing compound that does not dissipate over time is usually easily removed during the coarse-grinding phase of the polishing process.
Of equal importance to the appropriate mix design is how the concrete is placed and finished. The flatness and levelness of the concrete substrate is crucial not only for a uniform polish, but also how the project is bid. Floors that have excessive highs and lows will require a tremendous amount more grinding, increasing your labor costs as well as the costs of your diamond tooling.
According to industry standards, finished concrete shall have a minimum Floor Flatness rating of at least 40. The industry standard for terrazzo is no more variance than 1/4 in. in a 10-ft. span, which can also be applied to polished concrete. If the slab is being machine troweled with walk-behind machines, it is imperative to not leave indentations or boot marks behind since they are not easily removed during the polishing phase.
The main methods of coloring polished concrete consist of integrally coloring the concrete mix or staining or dyeing the surface after the fact. By far the easiest and most controllable method is to integrally color your concrete. Most integral color is either in the liquid or powder form and added at the ready-mix batch plant and then mixed for a specified time. Color loads for integral color should never be smaller than 3 cu. yds. for color consistency. Colored aggregate or crushed glass can be added to the concrete mix or hand seeded into the top layer of the mix. The polishing process will reveal the underlying aggregate.
Adding colors to polished concrete, either integrally or with stains and dyes, can create a distinctive look for your floors.
Acid staining or dyeing is a great method of achieving color on polished floors. Different applicators have different methods and time frames of applying the stains and dyes. We have had excellent results with applying dyes around the 400-grit phase. Let's review the process:
- Coarse grind with 40-grit metal bond (if necessary)
- Grind with 80-grit metal bond
- Apply densifier
- Grind with 150 metal bond
- Remove scratch pattern from 150 grit with 100/200 resin bond
- Remove scratch pattern from 100/200 grit with 400 resin bond
- Apply color stains or dyes
- Polish with 800-grit resin bond
- Polish with 1,500-grit resin bond
- Polish with 3,000-grit resin bond
Dealing with edges up against walls is always an issue. If the polished surface is to terminate directly against the wall, there are walk-behind machines to address this. Most of the time, tooling by hand with grinders and diamond pads is necessary. Another way to address edges is to use different mediums and create a border making the edge an architectural feature. We have had great results saw-cutting a border in and then using cement based skims or colored epoxy.
If you are considering to pursue the polished concrete market, make sure you do your homework. The purchase of equipment can be a large investment. Talk with manufacturers of polishing equipment and ask why their equipment and services are better than others. Make sure you have a market to support the hefty investment. Polishing machines can also be used for other applications, such as adhesive and coatings removal or to prepare surfaces for decorative overlays, to name a few.