Expert tips for laying concrete to achieve unique polished floors

Concrete is a natural product made up of sand, water, stone (aggregate) and cement.

Polished concrete is the process of removing surface pace from concrete and exposing the aggregate underneath.

Concrete is not difficult to lay but it is critical that it is laid correctly because it will pave the way for polishing as well as set what your floor will look like for the entirety of its life. Badly laid concrete floors will make having a beautiful finished product difficult to achieve.

Some contractors will have the skill set and awareness to lay the concrete correctly, while others do not want the responsibility and pressure of having to lay it but Renobuild can take ownership of laying, finishing and polishing your concrete, if you wish, call us on 053-9366444 to discuss further.

Before laying the concrete you must ensure that your insulation/insolation is correctly placed around external walls, so when the concrete is poured it cannot come into contact external walls, this is known as ‘cold bridging’, if this happens it can affect your heating long term. Insulation/isolation should also be put around rising walls, girders and pipes and any other movable objects as this allows the concrete to move and less likely to crack.

Slump testing should be done when the concrete is ready to be pumped, this involves samples of the wet concrete to be taken to ensure correct consistency or slump. If too loose or wet this will cause segregation within the aggregate in the floor and after polishing it may not be visible or could be patchy & expensive to get down to. If too dry it will not flow properly making it difficult to spread and place.

Click the link to check out our expert tips video on placing concrete to learn more about slump testing.

Make sure your local quarry use current materials when providing a sample and order it as close to laying as you can as aggregate can change within a 6-8 month period. We also recommend that the quarry is no more than 45 minutes away as any longer the mixture can begin to set in the lorry thus causing problems when it is time to lay the concrete.

When placing the concrete use shovels, scoops or the back of a rake through the mixture, do not use the tines of a rake as this causes lines to appear in the surface when polished in addition to disturbing the even spread of aggregate. Do not tamp the mixture as the downward hitting motion in the wet concrete pushes the aggregate away from the surface pushing it further into the concrete as well as creating patches or pulses. The mixture will need to be vibrated in order to remove air bubbles and help evenly spread it. Air pockets can affect the final polished surface as seen in the photo below it can be costly to remove when trying to achieve a polished concrete finish. If using a vibrating poker be careful not to drag it through the concrete as this will put visible lines or burn marks in the floor.

Check out placing concrete expert tips video for more information.

If the floor has under floor heating make sure you use the right amount of concrete, too thin you will see the pipe outline, if the concrete is too thick it will reduce the effectiveness of your heating system. Secure the pipes to the floor to allow for an even cover. Plastic needs to be laid between the insulation and the pipes but not covering the pipes as this creates a void around the pipes which reduces the effectiveness of your heating system.

Power floating is critical when concrete is placed there is a lot of bleed water on the surface and small amounts of trapped air helping to improve the overall finish of the floor. The surface must be panned and bladed, just note the areas that were laid first as this is where you should start and finish on the areas that were laid last as these areas are slightly wetter. Edges must also be treated the same as the power float machines cannot reach into the edges or corners they have to be done by hand, using a trowel.

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Concrete strength is also important and must be tested prior to laying the floor so that we can determine whether or not the floor can be polished. Strength is determined by water to cement ratio in the overall mixture. If the aggregate consists of a weak stone such as chalk this can also cause weakness in the concrete.

Strength is measured by newtons per square millimetre or N/mm2. It varies from 15N/mm2 up to 65N/mm2. Anything below 30N is considered too weak/soft and may not take a shine well, anything above 40N is too hard therefore will take longer to polish.

When people see cracks they automatically think there is something wrong, however they are a common factor in concrete floors because you are dealing with a natural organic material. We actually think cracks can enhance the look of some floors. Nevertheless not everyone will agree, some customers do not like them so there are a number of mechanisms that can be used to try minimise or control cracks such as:-

Placing Polypropylene Fibres into the concrete mix, it works by knitting the concrete together. Any fibres sticking out of the floor will be removed during the grinding process.

A mesh can also be used by pouring the concrete mix over the mesh this can help strengthen the concrete.

Saw cuts or expansion joints can help control the cracks and are mainly used on larger floor areas. Placing saw cuts into about 30% of the surface allows any cracks to run along the cuts and not randomly along the floor.

Crack inducers can encourage the concrete to crack in a hidden way.

Controlling moisture can also help prevent cracks, drying out too quickly can cause cracks to appear, by placing a plastic sheet over the floor, spraying a retardant spray over it after power floating or mist the floor with water all contribute to keeping it damp. If cracks appear you can also use a binder and dust from the floor to grout the cracks to try make them as invisible as possible.

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If concrete has not been laid correctly it can have some dips which cannot be ground down when polishing. Unevenness in the floor will cause unevenness in the aggregate exposure, the best way to prevent differing aggregate is to make sure the concrete has been poured to a floor level of SRI.

Communication is the key, clarity on the stages the floor is at and knowing what to expect from your polished concrete floor when completed is critical.

To learn more about polished concrete head over to our website and check out other blogs on this topic or have a look at our expert tips videos.

Topics: Polished Concrete