Polished Concrete Explained

HOW TO GET POLISHED FLOORS

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Polished Concrete Explained

How to get a polished (or honed) finish on concrete floors….

Normally, there are about ten steps required to produce polished concrete floors with many grinding passes although there are other ways to get a similar polished concrete appearance as explained below.

The general rule of grinding is to double the diamond grit size for each subsequent pass, so you might start with very coarse, metal bond (see below) 16 or 32 grit diamonds, followed by 60 grit diamonds followed by 120.

Metal Bond Diamond Segments

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Metal bond segmented cup wheel

 

 

Metal bond refers to the segments found on most diamond grinding wheels or plates. They are metal blocks containing diamonds which are welded to a steel-backing plate or wheel. These segments are strong and flat so as to aggressively smooth the concrete and may contain diamonds from 10 grit to as fine as 250 grit. The higher the number…the smaller the size of the diamonds.

Resin Bond Diamond Wheels and Pads

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Resin bond diamond pads for polishing

To polish concrete, it is necessary to change from metal bond diamond segments at around 100 grit to resin bond pads which are softer and less aggressive. These pads consist of a resin matrix which contains diamonds ranging from 50 grit to extremely fine 10,000 grit. Finely polished concrete may be achieved at around 3,000 grit resin pads, although for many commercial, industrial and residential concrete floors, 800 grit is plenty sufficient and may well be recommended for achieving increased slip resistance and for hiding dust and dirt. Highly polished surfaces, above 800 grit for example, can show the dirt and dust too easily for many applications.

Changing from Metals to Resins

Most operators grind the floor up to 120 grit using metal bond segments and then change to resin pads starting with 50 grit and go up to to 3,000 or above in some cases.

 

Edge Grinding

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Edge Grinding

While there are many machines available to grind and polish concrete floors, a separate edging task is usually required after each pass. Often the edges are polished first and the rest of the floor is blended into the edges. Another option is to grind the floor to 100 grit and then feather the edges into the floor with the same grit finish.

Hardening the surface

Polished concrete floors typically have the surface hardened with a chemical densifier before the second, third or fourth grinding pass. The densifier soaks into the floor and causes a chemical reaction that hardens (densifies) the floor to produce a high shine when the polishing is finished and to increase the wear resistance of the concrete. This process may be repeated at different stages to obtain a high quality finish.

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Aggregate Finishes

The floor can either be ground to fully expose the aggregate embedded into the concrete for strength; or, the aggregate can be partially exposed for a “salt and pepper” look… or, the concrete grinding can expose no more than the fine sands closer to the surface. We have specialized, industrial, heavy-duty machinery (900+ lbs.) to make this job easier. Until we start grinding, we never know how far down the aggregate had been originally laid. It could be ¼” or ½” or more down into the concrete slab. It takes skill and experience to control the process of polished concrete floors and a pertinent, contributing factor of the outcome is the quality of the applicator’s original concrete laying work.

Marks in the Concrete

Marks visible in your concrete may be attributed to many things. When the concrete was originally laid, the applicator may have walked over the concrete while it was too soft which pushed down the aggregate so much that when the floor is now grinded down to expose the aggregate, there can be distinct and visible boot marks.

Some grinding machines can produce wavy or circular marks under foot. You may see flat areas where high spots have been grinded down while you may see wavy or circular marks as the result of lower areas having been grinded.

Non-Aggregate Finishes

If a surface will be polished without exposing the aggregate (a much less laborious task), then only finer resin pads need to be utilized so that the movement of the pads will better follow the contours of the existing substrate. This will result in a polished floor that will not show the pattern and texture of the rock aggregates within the concrete. This method is faster, less expensive for the consumer and quite common in large warehouses or trade depots. Densifying remains important to improve the durability of the surface and produce a final gloss.

 

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Polished & Densified Concrete: No aggregate exposure

Problems with exposed aggregate

Sometimes the finish of exposed aggregate can be uneven if the original mix of concrete was poured unevenly or finished off poorly. Boot marks or perhaps kneeling board marks can appear suddenly while grinding because aggregate has been pushed down which might require grinding down another 1/4” inch. Grinding this far into the aggregate is a risk and the end-product may be as envisioned.

Single-Head or Multiple-Head Machines

Original terrazzo grinding was done with single head grinders until the production of three-head planetary machines. Planetary simply means that each head turns in one direction while the turntable which holds the heads turns independently in either the same direction or the opposite direction. Some grinders can vary the direction of both the turntable and the heads, some can vary the speed of each, and some have counter-rotating heads.

The planetary heads may follow the contours of the substrate better than single or twin-head grinders and are faster to use. Single-head grinders should be moved in a circular motion to avoid grinding lips or shoulders.

Angle (Hand) Grinders & Dust Shrouds

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Dust shroud attachments for hand grinding edges

Dust shrouds are for use with angle grinders, smaller hand-held grinders which can grind & polish into tight corners and along edges.  Dust shrouds were developed to avoid excessive airborne dust as the result of grinding. Angle grinders have a cutting edge at the front to follow along the walls and another at the side for when a corner is reached. The dust shrouds enable polishing or grinding into a corner without restriction and without much dust escaping. Dust shrouds will fit all 5-inch grinders and most 7-inch angle grinders.

Look-A-Like Polished Concrete Floors Using Urethanes

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Grinded & Sealed Concrete

The number of steps for polishing concrete can be reduced by up to 60% while still achieving a similar appearance by diamond grinding and then coating with a clear, urethane sealer. Grinding with up to 400-grit pads followed by densification and sealing is optimal for a nice finish.

Polyurethanes are two-component tough sealers that often will have a high-gloss factor or wet-look. Do not use epoxy sealers as they are less scratch-resistant and may exhibit phototendering over time with repeated sunlight exposure.  Acrylic, solvent-based or water-based sealers may also be utilized; however, they tend to have less durability & longevity.

Typical Process for Achieving Polished Concrete

  • Remove cap
  • 30 Grit Diamond Metal
  • 80 Grit Diamond Metal
  • 50 Grit Diamond Resin – wet
  • 100 Grit Diamond Resin – wet
  • 200 Grit Diamond Resin – wet
  • Application of Densifier
  • 400 Grit Diamond Resin – wet
  • 800 Grit Diamond Resin – dry
  • Application of Stain Guard