Polishing concrete can be a calamity of trials and errors. The process also can be perfectly smooth and trouble free. Most jobs are somewhere in between. Knowing the answers to certain questions before you start may help you get your project closer to the trouble free end of the spectrum. Perhaps most important—what does your client want? What can you and your client realistically expect from a slab? These questions must be answered before your first cut. Understanding the realities of the floor and the expectations of the client will help you create a procedure that delivers the floor the client wants, and the payment you deserve.
Here's an example. I recently got a call from an applicator in Ohio. The applicator, I'll call him Bill, had a slab he wanted me to check out. Bill had a shot at a new 15,000-square-foot warehouse floor. He'd already done a mock-up along with his competitor. When I arrived at the warehouse, Bill showed me the mock-ups done on an old floor. Next, we visited the new floor, a freshly poured, tight slab that had a cure-and-seal coating. This surface would be more difficult to grind than the old floor used for the mock-ups. I told both Bill and his potential client that the mock-ups done on the old floor would not work for the new warehouse floor. The exposure and look would be different.
Bill needed to create new mocks-ups on the new floor to establish an accurate price range and deliver the client's aesthetic expectations. Bill's job never got off the ground, which was probably for the best. If Bill had proceeded on the new slab with the procedures used for the old slab, he would likely have been in a world of hurt.
The moral is to make sure your demo areas are part of the slab that will be polished. If not, your initial grinding steps will cost you profits when the results are unexpected. Other important things to consider before you start grinding are:
- Age of slab
- Amount of traffic the slab gets
- The type and severity of the slab's stains and soiling
- Surface defects, such as cracks, spalls, voids, and popped aggregate
- Presence of surface or penetrating protective treatments
These conditions, as well as other not mentioned, give you an indication of how to prep the slab and what diamond grit to start with.
Understanding diamond grit
Let's look at a 60,000-square-foot new pour in Henderson, Nev., cured in the desert sun and left uncovered during the summer. Now the crunch is on for completion of the floor before equipment installation. The applicator, I'll call him Ted, is ready to go. He has two four-head electric grinders, two propane grinder-polishers, and a 30/40 metal grit diamond for the first grinding step.
The first 15,000-square-foot grind causes major scratch patterns. So Ted tries a 60/80 metal to, hopefully, remove the scratch patterns. What's wrong? Ted's equipment representative told him these metals should do the trick on the slab. Why are the scratches getting worse and not closing up? Well, this was Ted's first attempt at grinding and polishing concrete. The company he worked for had hired an established applicator and brought in an equipment representative to train the crew. The equipment rep said the diamonds would work. But neither the instructor nor the equipment rep ever visited this particular slab in Henderson and the instructor only knew one grinding and polishing system.
By the time I got to the jobsite, this first 15,000 square feet was in bad shape and definitely needing to be resurfaced. The first thing I noticed was the slab was very hard, due to improper curing, which indicated a need for a softer bond diamond to do the initial grind. This bond refers to the matrix holding the diamonds, which can be either metal, resin, or a combination/hybrid bond. The rule of thumb is "the harder the slab, the softer the bond;" and conversely, "the softer the slab, the harder the bond." Matching the hardness or softness of the bond to the hardness or softness of the slab helps prevent scratch problems.
So, to start the resurfacing, the grit was lowered to a 16/20 soft bond versus the 30/40 hard-bond diamond originally recommended for the first cut.
The resurfacing followed these steps.
1. Because the dry cutting left irreversible scratches that wouldn't close up, Ted needed to cut wet instead of dry.
2. The cutting began with a 16/20 soft-bond metal, then a 30/40 soft-bond metal, and a 60/80 soft-bond metal. Then a 70 medium-bond metal was used, followed by a 120-grit copper-bond hybrid diamond was applied to transition to the resin bonds.
3. Wet cutting continued with a 100 resin, then a 200 resin.
4. A lithium-silicate hardener-densifier was applied for faster, more effective polishing. The lithium-silicate product was chosen to avoid the lengthy scrubbing, rinsing, and wastewater collection procedures involved with sodium- and potassium-silicate products.
5. The floor was polished dry with a 400 resin.
6. A microthin coating of a lithium-silicate-containing protective guard was burnished onto the slab for additional gloss and ease of maintenance.
This sequence of soft-bond diamonds prepared the floor for the more standard 70-grit metal. From there, the 120-grit copper hybrid removed all the remaining scratches—at least those seen by an untrained eye. The resins, densifier, and guard brought the floor to a condition that pleased the client. Ted used this procedure on the remaining 45,000 square feet. The project, which started near the calamity end of the spectrum, ended a lot closer to the trouble-free end.
1. Understand your floor and how it reacts to diamonds. Hard-bond segments or metal discs will scratch a hard floor. Remember the rule of thumb: soft bond/hard floor and hard bond/soft floor.
2. Make sure your vendors understand completely what's happening at the jobsite. Get them to the project to ensure proper products and procedures.
3. Sometimes you need to cut wet instead of dry, depending on the equipment, diamonds, and condition of the slab.
4. Make sure to involve the client from the beginning to ensure that the floor is moving in the right direction.
All concrete floors have one thing in common: There is no common denominator. Always test and set your protocol based on the correct equipment and diamonds. Determine whether to grind wet or dry. Do what you can to work around the trades to deliver a finished, densified, polished, and protected floor that pleases everyone. Including you.
Jeff Dykstra is a Las Vegas-based concrete products and services specialist and consultant.
Gary Henry is a writer for PROSOCO, a maker of finished concrete flooring products.