A Guide to Polished Concrete

Polished concrete floors, when done properly, can be wonderful.

They really have lots of advantages, for example, they can:

  • be beautiful, through finding the right mix, aggregate, depth of polish, etc,
  • be cost effective, as often a concrete screed maybe required for the floor build-up anyway,
  • be thermally efficient, through direct contact with the source of underfloor heating,
  • offer variety in colour, with huge choice available of sands, aggregates, additives, etc
  • hygenic, through their sealed nature and lack of joints to trap dirt
  • durable, though their solid hardwearing nature
  • be unique

On the other hand, a badly done concrete floor can look messy, unfinished and can be a disaster.

This is a guide on how to get it right.

So, when constructing a concrete floor, it is essential to consider the following:

SELECTING THE MIX

Concrete floors are normally made from three basic ingredients, sand, cement and aggregate.

The ratio of these ingredients to each other affects the appearance, colour, strength characteristics of the floor.

Sand is the base, the finer background material. The aggregate is the larger material, crushed stone chippings. The cement binds the mix together and affects its strength, colour and tone.

Other ingredients can be added to this core mix to alter the characteristics further.

A standard concrete mix has a ratio of 5 to 1 to 5 of sand, cement and aggregate.

The choice of sand, in itself, will hugely affect the appearance.

Its colour, grade and consistency will affect the base or background of the mix

There is an huge choice of concrete types available. Some options are more expensive than others. The most economical choice is to use a standard concrete mix or a mix with up to 70% Eco-cement (which is much lighter in appearance). The standard mix supplied by each factory will vary depending on the aggregate and sand types used.

The choice of aggregate, its colour, variety and size again will hugely affect the appearance of the floor.

Coloured aggregate can be applied to the concrete mix. The % and types of aggregates and sand in the concrete mix can be altered to lighten, darken or bring colour to the concrete floor. These will be exposed by the grinding and polishing process later. This is generally a more expensive way of altering the concrete colour.

A less expensive method of altering the colour of the concrete is to add a pigment. These are available in a wide range, with the cost varying depending on the selected colour.

Another option available is the seeding of glass into the mix. Seeding is where glass is sprinkled evenly into the freshly poured concrete. The grinding and polishing process will reveal the glass pieces. Nails, bolts, computer chips, or any other objects can be seeded into the mix and then polished smooth.

CONCRETE POURING TIPS

The ideal depth of the concrete screed over under-floor heating pipes is between 75mm and 100mm.

Ideally, polypropylene fibres should be used in the concrete mix to prevent tiny shrinkage cracks on the surface of the concrete.

Frost can damage concrete when the water content freezes causing blistering and cracking to the top of the concrete as it freezes. As a general rule Concreting should not be commenced until a temperature of at least 2°C has been reached on a rising thermometer and should be stopped when the temperature reaches 4°C on a falling thermometer. Every care should be taken to protect fresh concrete from the effects of frost by covering with layers of dry straw or other approved means.

Pour the concrete as is delivered from the concrete plant. Make sure not to add extra water to the concrete in the truck as this will weaken it.

When pouring the concrete, it is essential that the levels are correct.

If the floor is to be ground and polished the concrete should not be tamped as this will push the aggregate down from the surface resulting in patches in the floor where the aggregate won’t be seen.

Once the floor has been poured and leveled you should wait until it has sufficiently cured which allows the bleed water to evaporate from the concrete. If the bleed water is not allowed to evaporate completely the water is sealed in the concrete resulting in a weakened top layer of the concrete.

The next step is to pan float the concrete and power-float it to fill any air pockets on the surface with the fat from the pan floating.

Make sure the edges of the rooms are also well floated so that they blend in with the floor during the grinding and polishing process.

FINISHING THE CONCRETE FLOOR:

Expansion joints are necessary in the floor to avoid cracking. It is usually sufficient to place these at doorways where they will be covered by the door saddle and won’t be seen. In larger areas expansion joints should be set out to divide the floor into large squares, up to a maximum of 9m². They can be cut into the cured concrete floor and are approx. 6mm wide.

POLISHING AND GRINDING:

There are various levels of grinding and polishing available, but as a general rule the deeper you grind into the concrete the more aggregate you expose and therefore the glossier the finish.

The high gloss finish involves making several passes with the grinder to take a 3-5mm layer of concrete off the top. Once sealed and polished the result is an extremely durable floor. Truck wheels and substances leave no marks on the glossy surface and light reflects gently off the floor.

Alternatively you can choose not to expose the concrete aggregate. Instead, the top layer of the surface paste is polished but not removed. This process creates a stronger surface paste with a shiny finish.

Another option is to go for a hard troweled floor with a power float. The appearance of this is like raw concrete and so is not to everybody’s taste. A gloss finish is achieved using a solvent based sealer. This finish is completely dependant on the way the concrete is power-floated and it will have to be very well protected after pouring as scratches and fillers will be hard to disguise.

PROTECTION AND MAINTENANCE:

In general the concrete floor does not need to be covered until after it has been grinded.

The top layer acts as protection from scratches, bangs, spills etc.

Once the floor has been grinded it will need to be covered until the end of the project.

Maintaining the floor is relatively straightforward. The floor can be cleaned using soapy water and a micro fibre mop. To buff the floor an industrial type pad cleaner with specific cleaning pads can be used. A spray-on sealer is first applied and worked into the floor using the micro fibre mop before making a pass with the pad cleaner. This buffs the floor and removes any marks or scratches.

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