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A guide to polishing concrete

polished concrete

The Process

Standard Procedure:

This is a GUIDE to the normal procedure involved in polishing concrete, including exposed aggregate concrete surfaces and using polished concrete tools. The procedure is virtually the same with terrazzo, and other natural stone products. Note that this is quite different to creating a polish effect by grinding and then coating concrete floors with a clear coating such as epoxy or polyurethane, these use the coating to produce the sheen. Firstly, a very important word: - no two slabs are the same, even on the one job, so talk to the experts before you buy polished concrete equipment!

Therefore you:

# Cannot predict what the slab will look like - whether the aggregate is going to be evenly exposed; or patchy, because the contractor who placed the concrete pushed it down with his boots. The great thing you can tell the customer before you quote the job is that “your slab will be totally unique to you.”

# Cannot predict how much topping will need to be ground off to expose the aggregate.

# Cannot always predict how hard the concrete will be to grind. Use polished concrete equipment to complete a test patch before you quote to give you some idea both on how hard and how much may need to come off. How readily the surface will take on the required sheen. This can change dramatically depending on how porous the slab is due to water being added to the mix when pouring or air bubbles from not being vibrated.

Typical Procedure

1. The FIRST CUT: is the grinding of the required amount off the top to expose the aggregate; some want ‘salt and pepper’ with the aggregate partially exposed, or the customer may prefer a heavy exposure. The first cut done with polished concrete tools is always completed in several passes and at an even speed so no part of the floor is overground and forms an ‘ocean wave’. The first cut is done with coarse production grinding disc or tooling, 30-60grit is common; 16grit is also used to do the first part of the first cut. In practice the amount that is removed is often 2- 3mm and is removed in .5mm per pass. This means that if 3mm is removed, this will be done in 6 even passes to ensure flatness. So to quote a six pass floor properly, you need to quote on grinding 6 times the floor area. A shotblaster in the hands a skilled operator can drastically reduce both the cost and time to get just the top off the slab; especially if the slab is highly troweled (burnished).

2. SECOND CUT: Following the first cut it is best to do a finer grind with 60-80grit diamond discs to remove the all score marks from the coarser discs before you move the final metal bond cut.

3. THIRD CUT: Following the second cut it is best to do one thorough fine grind with 120-150grit diamond discs to remove the all score marks from the previous cut before you move into the polishing process. The important point here is that the phenolic polishing pads are not intended to remove score marks and wear fast on a coarse ground surface. Guide to quoting: So far in this example you have taken 8 even passes to do the job. If you charge at $8 per m2 for a single pass grind, this job is 8 passes x $8 per sq m = $64:00m2 for this stage.

Tip: Know what it costs to grind each m2.

Using Polished Concrete Equipment and the Hardening Stage

Following the first and second cut, the floor is ready for the polishing and hardening stage. You will need to make an inspection at this point to decide what polished concrete tools you will require.

1. If the floor is very dense, with little or no porosity in the sand/cement matrix, then you may be best to start the polishing with 100grit and then 200grit Phenolic (resin bond) polishing pads or a similar piece of polished concrete equipment. Then apply Concrete Shield Densifier or DuraCore's Densi Crete; this will harden the surface, and help to ‘close’ the surface, and help create a sheen as you move further into the polishing process. In the case of a particularly dense slab, polishing can come up well without any treatment, but usually won’t be as well ‘sealed’. If you apply the hardener to an already hard dense floor, it can make the initial polish slow and time consuming.

2. If the floor is porous and not really a hard matrix it may be better to apply a generous application of the Concrete Shield Densifier before you begin the polishing process. Now do the 50, 100 and the 200grit polishing with the Phenolic pads on your polished concrete tools. Now that you have completed polishing 50, 100 then 200grit grades you may find that this is the time to apply Concrete Shield Densifier (or another application, if you have already applied a coat before the 100grit pads). Now you can do the 400grit pads, and then the 800grit. Many people will at this point have a pump bottle or pressure garden sprayer with them while polishing while doing the 800grit process (and finer grades) and spray a small amount of hardener on the surface as they go. This wetted surface helps to further close over the surface and develop the sheen. With 800grit, you should have a nice ’60-70%’ sheen, and if you need/want to go further, the 1500grit, 3000grit and buff grades will give exceptional levels of sheen.

*Hint - Ensure to thoroughly clean the floor in-between passes.

A Guide to Quoting:

As a guide, the various steps from 50, 100, 200, 400, 800, 1500, 3000 and then ‘Buff’ to produce the polished finish will take at least as long as each pass with the first cut grind. This means that there are 4x the time to polish the floor to 800grit grade, and six times the time to get to ‘BUFF’. This is where most contractors come unstuck; they forget to allow for every step. Interesting sales point: Polished concrete is very good value for money; if you charge $120 to $160 per m2 this not a lot different to a good quality vinyl, including cost of laying. The difference is that it last forever, it is easy to maintain, looks great and it is unique to the customer, ‘no-one else in the world has got a floor precisely the same as you.’

Grout Demonstration

Products Required

To buy polished concrete equipment to help you complete the job, call Con-Treat today!

* The above information is to be used as a guide only and Con-Treat accepts no responsibility for any consequence resulting from the advice contained herein or verbally.

 

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