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FLOORING IT

Elaine Toogood shares her specification tips for achieving top-notch polished concrete floors

It sounds like a simple matter to describe the type of smooth concrete floor finish that you wish to achieve, but there are a number of variables to consider. Diamond-ground polished concrete floors are ground progressively smoother, a process which removes the surface of the concrete. The aggregate is usually exposed, the extent and size depending on the depth of surface removed. Alternatively, power-trowelled floors are also referred to as polished concrete, but give a smooth, dense surface and a “solid” colour finish with minimal aggregate exposure. Each requires a significantly different specification and installation and, importantly, has different programming implications.

Below is a set of key parameters to achieve best results:

  • Establish your aesthetic and visit examples to benchmark the type and quality required, bearing in mind that every polished concrete floor has a unique finish with variations across its surface.
  • Consider its function and likely maintenance regime. An industrial floor with wheeled traffic, for example, is likely to require greater crack control.
  • Where aesthetics are important, appoint a specialist installer to carry out the work and seek pre-tender advice. Most contractors have their own specification and method statements adapted to suit project particulars, including the concrete supply – because control of the delivery and quality of the concrete is critical. Typically the concrete should have a minimum 350kg/m3 CEM I, with well-graded aggregates.
  • For diamond-ground floors, the choice of aggregate is critical to the appearance and options should be discussed with the specialist installer. Unlike terrazzo floors, uneven aggregate distribution should be expected.
  • Allow 100mm depth for an unbonded concrete topping (screed) to reduce risk of curling. If heating pipes are embedded, this depth is to the top of insulation, ie it includes the depth of the pipes.
  • Don’t be too specific about the exact shade of grey of the floor. This will save time and cost. Colours vary around the country depending on source of local natural materials and colour of cement. The process of burnishing or trowelling also changes the tone when compared to a formed face cast using the same mix.
  • If you want to be specific about the tone or colour, consider using pigmented dry shake toppings. These proprietary powders combine surface hardeners, cement and pigments and are broadcast onto the fresh concrete to be floated or trowelled in. They can be specified to any RAL colour but there will still be tonal variation in the surface, as with any concrete surface. It is also possible to add pigment to the mix to provide a through colour. This is worth considering for a ground polished floor or where steps or other formed features are incorporated into the design.
  • Concrete shrinks as it cures, so non-structural cracks are to be expected. To minimise cracking, joints should be created either using screed rails to isolate areas of pour, or by saw-cutting into the concrete afterwards to induce crack location.

Designers should provide design intent drawings for discussion with installers. A useful rule of thumb for 100mm depth is to place a joint at approximate 4m centres, ideally creating as square an area as possible. Joints should also be placed to separate areas that are likely to dry at different rates, such as dog legs, ingress corners or between shaded or sunny areas. Underfloor heating needs to be commissioned at a steady slow rate, but only once the concrete has cured sufficiently. Anything that might inhibit shrinkage could lead to cracking so attention is required to perimeter details and any abrupt variations under the concrete. Reinforcement mats (mesh) can help limit cracking and sometimes fibres are used. Proprietary low-shrink mixes are available.

  • Joints can be left as a shadow gap or filled afterwards with steel or brass rods or more commonly with flexible grout or mastic. Linings can hide the perimeter isolation gap at wall junctions.
  • Since power trowelling takes place within hours of the concrete floor being placed, planning and timing of operations is essential. Careful scheduling, usually with early morning pours, will help reduce the need for power trowelling late into the night with the associated risk of sloppy workmanship. Diamond grinding takes place once the concrete is sufficiently cured, nominally after about a month.

  • The time taken to trowel or grind any floor will depend not only on area, but on the length of perimeter walls, where smaller equipment is needed. Ideally internal walls should be installed after the concrete is complete. It is worth noting that skill is required to limit the visual impact of different tools, and a difference in finish around the boundary is to be expected. A dry shake finish produces less tonal variation.
  • Recessed door tracks and floor boxes are best installed after the concrete floor, using knock-out ply-box spacers to create accurate voids in the floor. Attention to detail at thresholds is essential.
  • Once the floor is polished or trowelled smooth it will be left to cure. Some installers favour whole-room curing; others place mats on top for protection. The timing of each is critical and should be discussed with the contractor and programmed in advance. Be aware that any isolated item placed on fresh concrete can create a permanent dark patch due to differential curing.
  • Finally, the floor should be sealed to improve maintenance and possibly enhance the sheen.