Grinding and Leveling Concrete

How They Work

Grinders use horizontally rotating discs to perform many tasks, from light texturing to opening the pores of the surface to removal of paints and thin coatings. The grinding attachments, available in various types and grits to suit different applications, are the key to their versatility. Because grinders use rotary action rather than impact to remove material, the depth of material removal is limited to about 1/8 inch, depending on the type of attachment used. They generally leave behind a smoother profile than scarifying or shot blasting, and when working on hard, dense concrete, they may polish rather than abrade the surface.

For floor and slab surfaces, you can find walk-behind grinders ranging in size from single-disc units for working in small or restricted areas to dual–, triple– or even four-disc machines for high-production grinding of large slabs. A single-disc grinder has a working width of 10 to 12 inches, while a dual-disc unit covers 20 or more inches in one pass. Disc rotation speeds range from about 250 to over 3,000 rpm. On multiple-disc units, the discs usually are counter-rotating to provide balanced torque so the grinder won’t pull from side to side. For more even grinding, some machines are also equipped with floating heads that will follow the contour of the floor and adjustable rear wheels to keep the grinding discs level. Many manufacturers offer power options, including electric, gasoline, and propane. Most machines are equipped with vacuum ports for dust-free dry grinding.

Although walk-behind machines are well suited for profiling large slab surfaces, you can also buy handheld grinders for working in tight areas where larger units can’t maneuver, such as in corners and close to walls. These smaller workhorses have grinding diameters ranging from 5 to 12 inches. They can also be used to remove bumps, form marks, and graffiti from vertical surfaces or to grind concrete. Like their larger cousins, handheld units come with a selection of grinding accessories and can be hooked up to an industrial vac for dust control.

Typical Applications

Many of today’s grinders can serve multiple functions and are generally more adaptable than other types of surface prep equipment, especially for decorative work. They are a good solution for profiling floors before applying thin coatings or paints because they won’t create ridges in the surface, as can scarifiers. Other applications include breaking up deposits of grease, dirt, and industrial contaminants and leveling uneven joints or high spots. Many contractors also use grinders with finer-grit abrasives to polish concrete surfaces.

Attachment Options

Investing in various attachments for your grinder will maximize its versatility and allow you to perform a broader range of tasks with one machine. The three most common types of grinding attachments are silicon-carbide-impregnated grinding stones, tungsten-carbide inserts, and diamond-segmented grinding tools. Some grinders are also available with scarifying attachments for removing thicker coatings and mastics.

Silicon-carbide stones in various grits for coarse or fine polishing are economical for applications such as smoothing trowel marks or rough finishes and leveling high spots less than 1/16 inch. However, these stones can clog easily, making them inefficient at removing most coatings.

Tungsten-carbide inserts and diamond grinding accessories remove surface coatings faster and more efficiently. Tungsten-carbide inserts are carbide-tipped blocks with beveled edges that strip heavier buildups or coatings without digging into the concrete. Similar to using a scraper, their removal action works best in materials thicker than 1/16 inch. Applications include removing adhesives, thick paints, resins, tar, industrial buildups, and rubber deposits.

Diamond-segmented abrasives are generally the best solution for profiling work and removal of thin-film coatings or coatings with high hardness values, such as urethanes and epoxies. Other applications include polishing and removing minor surface imperfections. The diamond segments, bonded to a backing plate or removable block inserts or plugs, sit on the surface and are embedded in a metal or resin matrix. During grinding, the matrix wears away gradually to expose new diamonds. You can choose diamond tooling in various grit levels, ranging from fine to coarse, with different bond hardnesses, shapes, and diamond concentrations, allowing you to match the tool to your project requirements.

Regardless of your attachment, look for inserts or grinding discs that are easily replaceable. This will allow you to move more easily from coarser to finer grit levels.

Tips for Best Results

Despite their versatility, grinders generally aren’t designed for aggressive profiling jobs, and there are certain coatings a traditional grinder just can’t remove. They are most effective at removing thin coatings and paints or cleaning and lightly abrading floor surfaces.
When deciding between tungsten carbide and diamond attachments for coating removal, the main factors are the material type and thickness and the strength of the mechanical bond to the surface. One manufacturer offers this tip: If the material can be removed with a knife, use tungsten carbide. Diamond segments are more suitable for thinner coatings no greater than 1/16 inch.

When using diamond tooling, choosing the right bond hardness of the matrix (the material that holds the diamond segments) is important to achieve greater efficiency and maximize tool life. Generally, use a hard bond when grinding soft materials and a soft bond for hard materials. Softer materials wear away the diamonds more quickly.

The weight of walk-behind grinders can affect grinding performance. Heavier units permit more aggressive grinding because they place more weight on the discs, allowing them to make better contact with the surface. Some machines can add ballast weights to tackle more difficult material removal jobs.