It goes without saying, if you want a surface that is smooth, easy to maintain and suitable for polishing, you need to have your concrete installed by a professional contractor following a tried and tested concrete specification. We can of course give your installer our well respected specification to follow, unless they already have one they currently use.
We recommend that the mix for a polished concrete slab should be a minimum of C35 grade, which is a good hard concrete supplied to the building site via a concrete mixing truck. We would advise for this type of concrete floor that you do not use a volumetric mixing solution as every floor we’ve worked on that has used this method of delivery has been of substandard mix quality.
It is CARRcrete’s recommendation that you use major suppliers like Tarmac, Lafarge, Hanson etc as their quality control and documentation seems to be far superior to that of small companies.
100mm of slab thickness is a minimum over underfloor heating systems and insulation boards. Laying to this depth reduces the risk of cracking through shrinkage and movement.
The C35 concrete mix should be placed with no added water (which would reduce the strength). The mix should ideally be screeded into place and magnesium floated in multiple directions to remove any tamp lines and to flatten the floor out ready for the power trowel.
It is really important that no low spots, lines or gouges are left in the surface after the magnesium float work has been done. If the concrete is not flat and smooth the power trowel will not remove any lack of preparation.
The finish the client requests determines how much power trowel work is needed. For example an Cream finish concrete floor needs multiple passes with a trowel to tighten and burnish the surface to produce the cloudy, swirly patterns we often see. This is a highly specialised technique which requires years of experience to master.
Salt and Pepper floors need plenty of power trowel work to compress and tighten the mix but not nearly as much as the cream finish. This is mainly because the surface of a Salt and Pepper floor receives a light grinding before the polishing process.
The Stone finish needs the least amount of work with a power float, but it still needs to be done to ensure the floor is flat, smooth and compact. The act of trowelling the surface helps remove air, breaks down installation marks and lines and overall produces a much better polished concrete floor.
Once the concrete has been laid, is smooth and trowelled to our recommended specification for finished floors, you need to allow any surface bleed water to evaporate before curing the concrete to ensure that it hardens correctly. Curing of the concrete can be done either by covering with plastic sheeting (ensuring that there are no creases in the sheets which could cause curing marks) or by ponding water on the surface.
It is our recommendation that newly installed concrete is left for at least 28 days before any grinding, polishing, densification or sealing works are carried out. Special care should be taken during the colder months of the year as concrete will not cure properly in cold temperatures. Ideally the temperature should be at least 10degrees centigrade for best results. Concrete should never be left to cure on a non-weatherproof site or be left exposed to the elements.