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Competencies covered

MSFFL2011: Select, operate and maintain grinding equipment

Machines used to prepare concrete

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The best machine for a concrete preparation job depends on the nature of the material you want to remove and the amount you need to take off.

It also depends on the surface 'profile' and flatness you're looking for.

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The sorts of contaminants you may need to remove include oil, grease, asphalt, curing compounds and adhesive residues.

Loose surface material may include old or cracked cement-based toppings.

It could also include laitance, which is a powdery or milky layer of cement and sand.

For more details on the nature of these problems and the effect they have on underlayments and adhesives, go to: 'Inspecting concrete subfloors' (in Inspecting and testing subfloors).

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Machine types

Set out below are the main machines used to prepare a concrete subfloor.

The 'CSP' numbers refer to the concrete surface profile of the floor once the machine has done its job.

Basically, the lower the number, the smoother the surface will be.

We'll explain CSP more fully in the next lesson, but for now you should keep in mind the following guide:

  • CSP 1 or 2 provides a surface smooth enough to lay a floor covering directly on top

  • CSP 3 to 5 is fine for self-levelling screeds and toppings, but not for directly applying a covering.
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Concrete grinding machines use rotating heads to smooth and level the concrete surface.

The process is called 'diamond grinding' when the abrasive discs contain diamond particles.

However, tungsten carbide discs can also be used, especially when thick membranes and glues need to be removed.

Diamond grinders provide a surface finish of CSP 2 and can take off high spots or uneven joints to a depth of about 1 to 3 mm.

They are also good at removing sealers, paints and adhesives.

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Shot blasters

Shot blasters throw thousands of steel shot particles at very high speed onto the concrete surface to remove weak or loose material.

The machines are very manoeuvrable, entirely dustless, and relatively low noise.

The surface profile achieved by shot blasters ranges from CSP 3 to 7, depending on the grade of shot used.

They are best at removing laitance and other weak surface materials, but not as successful with thick coatings and adhesives.

They are also unable to level a floor, because the blasting process tends to remove similar amounts from high areas and low areas.

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Scabblers are much more aggressive than grinders and shot blasters, and can remove up to 6 mm of surface thickness per pass.

They use a percussion action to hammer the scabbling bits into the surface with pistons powered by compressed air.

With a CSP ranging from 6 to 9, scabblers can cause a lot of surface damage.

They are typically used on footpaths, roads and runways, and can also be used to create non-slip surfaces on ramps.

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Chisel scrapers

Chisel scrapers are basically jack hammers with a wide chisel-shaped blade head.

Different heads can be fitted for different purposes, such as lifting tiles, vinyl and cork from the floor, removing residues or even breaking up concrete or sandstone.

Most scrapers are pneumatic (air operated), but some models use 240 volt or battery power.

Because they are not really designed for removing the concrete surface itself, a CSP number is not relevant to their action.

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Scarifiers are a form of milling machine with a rotating drum.

The cutters are made of tungsten carbide or hardened steel, and they plane the surface of the concrete to produce a roughened finish of varying profiles, depending on the cutter assembly used.

Most scarifiers produce a surface profile of CSP 6 to 9.

However, with a new attachment called 'flat-faced tungsten flails' these machines can achieve a CSP of 3 to 4.

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Learning activity

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As you can see from the range of machines described above, grinding is just one of the methods used to remove surface layers and coatings from a concrete subfloor.

However, it's generally the most suitable method for floor laying, because it can achieve the flatness and surface finish you're looking for as long as the original slab is in reasonable condition.

Let's say you had a subfloor with a heavy layer of laitance on the surface, and it was too deep to remove easily with a grinder.

What machine would you choose to clean up the laitance before you finished the floor with a grinder?

State the name of the machine and reason why you would choose it.

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