Concrete Floor Prep: Grinding, Adhesive Removal and Moisture Checks

What is concrete floor prep, and why does it matter?

Concrete floor prep is the work that makes a slab ready for its next layer. The slab must be clean, sound, flat and dry. A polish, stain, seal, epoxy coating or new floor covering will only last if the surface under it is right.

Most floor failures trace back to the slab, not the product. Old glue left behind shows through. Oil stops a coating from bonding. A damp slab pushes moisture up through the new floor. High spots and low spots show through a gloss finish. Prep fixes these problems before they cost you a second job.

Prep has three jobs, and they come in this order:

1. Find out what is on the floor. On older buildings, test tile, mastic and coatings before anyone grinds or scrapes.
2. Remove and level. Grind, strip or shot blast the surface until it is sound and flat.
3. Measure moisture. Test the slab and decide whether a moisture barrier is needed.

AES, based at 3100 Gravois Ave in St. Louis, does all three. The company serves St. Louis, St. Charles and Columbia, Missouri. The rest of this guide explains each step so you know what to ask for in a bid.

When do I need to grind a concrete floor?

You need to grind when anything sits on the slab that the next finish cannot bond to, or when the slab is not level. AES lists four common reasons: removing adhesive and old layers, starting a polish, preparing for new flooring, and leveling.

Here is how those reasons play out on a real job.

AES says grinding can also remove surface chips and cracks along with the top layer of concrete. It is the first of two steps in concrete finishing. The second step is polishing, which brings out the final high-gloss finish.

If you are comparing finishes, read polished concrete vs epoxy. Prep needs differ a little, but both start with a ground, clean slab.

How does concrete grinding work?

Concrete grinding uses a heavy machine fitted with diamond discs. The crew starts with coarse discs to cut and level, then moves to finer discs for a smoother surface. The right grit depends on what you plan to put on top.

AES says its machines all use diamond grinding discs of various grits. That lets the crew grind the floor to the smoothness your project needs. The same approach carries into polishing. In AES’s own words, each stage of grinding and polishing uses a finer grade of diamonds. The final stage uses the finest grade to bring up the shine, and a polishing compound may be used to remove residue.

For a plain-language definition, see the glossary entries for concrete grinding and grit. If you plan to polish, densifier is a hardening step that comes into play after grinding.

Three things set professional grinding apart from a rental:

• Machine power. AES says its machines have a much higher capability than any you can rent. That means a faster job and a better result.
• Consistency. A crew works the whole floor to one standard, so you do not get a patchwork of rough and smooth areas.
• Dust control. Grinding makes a lot of dust. AES fits its equipment with dust suppression systems that remove dust before it enters the air. AES notes this is critical in medical or pharmaceutical manufacturing plants.

AES says it completes grinding in commercial kitchens, food preparation areas, pet care buildings and packaging plants with minimum disruption to the business. For the full service description, see concrete grinding contractors.

Can old floor tile or glue contain asbestos?

Yes. Older floor tile and the glue under it can contain asbestos, so test before you grind, scrape or sand. Disturbing asbestos-containing material is what releases fibers. Missouri DNR lists asphalt and vinyl floor tile, linoleum and flooring mastics among potential asbestos-containing materials.

DNR also explains the risk in one sentence: exposure may occur when the material is disturbed or damaged in a way that releases particles and fibers into the air. Grinding and scraping disturb floors. That is why testing comes first.

AES says that for many years asbestos was used in making floor tiles and flooring adhesive, and those materials may still exist in older homes and commercial buildings. AES says it can test, remove and dispose of such flooring or adhesive.

What this means for your project:

1. Do not start with a scraper. If the floor is old, hold off until you know what is in it.
2. Get a sample tested. An asbestos survey by a certified inspector samples suspect materials for lab analysis. See asbestos testing and asbestos survey.
3. If the lab finds asbestos, follow the rules. Removal must follow Missouri and federal requirements. See asbestos floor tile and asbestos tile removal.
4. If the lab finds none, move on to adhesive removal and grinding.

AES says its workforce is fully trained in asbestos floor tile removal, so a job that turns up asbestos can stay with one contractor. The glossary entry on mastic explains the old glue that often sits under tile.

Never try DIY removal of floor material you suspect contains asbestos.

How is old floor adhesive removed?

Old adhesive is removed by shot blasting or by grinding with diamond discs. Both methods strip glue, coatings and the top surface. Which one fits depends on the adhesive, the slab and the next finish.

AES says that for normal tile and floor adhesive removal it uses either shot blasting or the latest grinding machines with different grades of diamond encrusted discs, and that these are strong enough to remove any adhesive, floor coating or surface.

Why not just scrape it? AES gives the reason: after years of traffic, much of the adhesive and parts of the old flooring stay stuck to the concrete. If the work is done wrong, parts of the concrete can come loose or get damaged, and the floor ends up unfit for the new covering. Professional removal cleans the sub floor so the next layer can go down.

The AES page on floor adhesive removal shows before and after photos of finished floors.

How do I know if my concrete slab is too wet?

You find out with a moisture test. Looks and feel do not tell you. AES offers calcium chloride testing and relative humidity testing. These tests measure the moisture vapor transmission rate (MVER) so you can decide whether a moisture barrier is needed.

This matters because moisture moves up through concrete. It can lift tile, bubble coatings, peel vinyl and feed mold under a floor. AES describes liquid moisture barriers as a way to prevent water vapor intrusion under coolers, under floor tiles, on fast-track construction jobs and on projects done in wet weather.

Test results lead to a clear decision:

For more detail on the product, see moisture barriers and the glossary entry for moisture barrier.

What does a moisture barrier do, and when is it needed?

A moisture barrier is a liquid coating applied over the slab and under the floor covering. It reduces water vapor passing through the concrete. You need one when test results show the slab is too wet for the planned finish, or when the schedule cannot wait for the concrete to dry.

AES says it uses 2-component liquid epoxy moisture barriers made for concrete floors. AES lists these features for the product:

• Treats concrete up to 25 lb. MVER and 100% humidity
• Reduces vapor transmission to near zero
• Easy and quick installation
• Zero VOC rating, which contributes to LEED / GREEN building
• Compatible with most floor covering surfaces
• Available manufacturer’s 10 year guarantee

AES says the barrier can go under nearly any flooring surface, including sheet vinyl, tile, carpet and hardwood. It can be applied over dry or damp surfaces, even 5-day old concrete. AES installs it in industrial facilities, office buildings, retail buildings, warehouses, institutional buildings and residential buildings.

The surface must be ready too. AES says that after testing, the slab should be cleaned and judged sound, solid and absorptive. Cleaning can be done with a pressure washer or damp mop, followed by a thorough vacuuming. That is why moisture work belongs inside the prep plan and not after it.

What does the full prep sequence look like?

A well-run prep job follows a fixed order. Each step protects the next one. Here is the sequence a good contractor will walk you through.

1. Walk the floor. Look at the age of the building, what covers the slab, and what goes on top next.
2. Test suspect materials. On older floors, send samples to a lab before any disturbance.
3. Abate if needed. If asbestos shows up, follow the required removal process first.
4. Remove coatings and adhesive. Use grinding or shot blasting as the floor requires.
5. Grind and level. Cut high spots, clear chips and cracks at the surface, and open the slab.
6. Test moisture. Run calcium chloride and relative humidity tests.
7. Apply a barrier if the tests call for it. Clean the slab first.
8. Finish. Polish, stain, seal, coat or install the new covering.

Steps 2 and 3 are the ones people skip. Skipping them can expose workers and building users to fibers and can stop a job after it starts. Start with testing and you avoid that.

If your goal is a coating, review epoxy floor coatings. If your goal is color, see concrete staining and concrete sealing. For answers to common finish questions, see the concrete finishing FAQs.

How long does a prepped floor last?

A properly prepped and finished floor can last a long time, but the interval depends on traffic. AES says heavy-traffic places may need to redo their concrete finishing every 5 to 10 years, and it calls the process quick and affordable with very little downtime.

AES also says polished concrete does not become slippery and can be cleaned with a damp cloth or mop. It says the finish resists staining and chipping, even in high-traffic spaces. Those results rest on the prep steps above. Weak prep shortens everything that follows.

Where do these floors go? AES names these as common settings for concrete finishing: showrooms and malls, automobile showrooms, hospitals, garages, laboratories, schools and universities, commercial kitchens, churches, salons, warehouses and manufacturing facilities. Prep scope changes by setting. A commercial kitchen needs dust control and a short shutdown. A warehouse may need heavy-duty grinding and a barrier. A lab may need both.

For a deeper look at lifespan and care, read how long does polished concrete last.

What should I ask a floor prep contractor?

Ask questions that reveal whether the contractor will test, protect and document the job. A bid that lists only a square-foot price tells you little. Use this list.

• Will you test old tile, mastic and coatings for asbestos before you start?
• Can you remove asbestos-containing floor material if the lab finds it?
• How do you control dust during grinding?
• Do you offer both grinding and shot blasting?
• What moisture tests do you run, and what do the results mean for my finish?
• Can you apply a moisture barrier if the slab needs it?
• Who does the finishing step, and does the same crew handle prep?
• How will you limit disruption to my business?

AES does asbestos abatement and concrete finishing under one roof, so the testing, abatement, grinding and finishing steps can sit in one scope. AES quotes each job. Prices depend on floor size, the condition of the slab, what must come off, and the finish you choose.

Next step

If you plan to polish, stain, seal, coat or recover a concrete floor in St. Louis, St. Charles or Columbia, Missouri, start with the prep plan. Read about concrete grinding contractors, then call AES at (314) 865-2662 or send the quote form at /contact.