- Why and what kind of concrete needs to be sanded
- Preparing the floor for sanding
- Main area grinding
- Wet sanding of concrete
- Clearing dead zones
Uncoated concrete flooring is rarely suitable in residential premises, but in sales rooms, in warehouses or in garages, they are very appropriate and practical. In this article we will talk about how to properly sand the floor to give it strength, and how you can improve the appearance of the coating by polishing.
Why and what kind of concrete needs to be sanded
Concrete is a natural and environmentally friendly material with high hardness and strength. However, when almost any reinforced concrete product hardens, delamination of the array inevitably occurs, during which the filler of the coarse fraction settles down, displacing sand, dissolved cement and other impurities to the surface..
This defect has no significant effect on other structural elements. But on the floor, especially with high traffic, the top layer will wear out quickly, which is accompanied by abundant dust formation, the appearance of irregularities. In addition, the upper thin layer of concrete has high porosity and actively absorbs moisture. Water erosion does not stop at the surface, but moves deep into the pillar, forming new pores and destroying them.
Previously, the problem was solved by ironing the surface with a strong cement mortar. Now there are more advanced technical means that allow you to evenly remove the fragile concrete layer and expose the slice of the massif, which practically does not absorb water, has a pleasant appearance and serves for decades..
Sanding the concrete floor will also increase its adhesion to other finishing materials. In particular, when installing self-leveling floors, it is recommended to grind the concrete screed of the subfloor to eliminate small ripples and improve the quality of adhesion. It should be noted, however, that grinding does not eliminate pits, bumps and slopes, since the equipment exactly follows the topography of the surface on which it moves..
The technology of grinding and polishing concrete owes its appearance to the so-called mosaic floors, for which concrete is used with a filler of hard volcanic rocks: granite, quartzite, basalt and others. Removing a layer of 5–20 mm reveals a bizarre pattern of a concrete cut, which then appears more clearly due to fine grinding and polishing.
To carry out grinding work, the concrete must be cured after pouring for at least 14 days, otherwise the adhesion of the fragments will not be strong enough and they will simply be torn out of the massif. The grade of concrete must be at least M200, vibration shrinkage of the mixture is recommended to get rid of the pillar from voids and air bubbles.
Preparing the floor for sanding
To perform grinding, the concrete mass must be solid and durable, any cracks during processing will expand, which will render the coating unusable. If, due to uneven hydration, the concrete surface is cracked, the cracks must be widened and filled with a special repair compound based on epoxy resins.
When there are doubts about the strength of concrete, the surface is treated with a deep penetration primer, which will provide additional bond to the fragments of the massif..
Quite often, old concrete floors that have been in use for several decades are subjected to grinding. In such cases, the quality of concrete is checked by detailed tapping, or the surface is subjected to ultrasonic flaw detection..
Main area grinding
The area of concrete floors can reach hundreds of square meters; on such a scale, manual processing methods are not relevant. Applied mosaic grinders and polishers with electric drive for indoor work and with a gasoline engine for outdoor use..
The main working body of the machines is a grinding wheel, on which abrasive stones or discs, cutters or frankfurts are fixed. It is optimal to use machines with a planetary device: in them from 9 to 21 abrasive elements are divided into groups, rotating by themselves and among themselves. With this trajectory, small scratches on the floor have different directions and do not form streaks when light is incident at certain angles.
Most grinders are single-pass, but concrete is processed in several steps using increasingly gritty abrasives. First, the surface is roughly abraded with grinding stones with a grain of 200 to 125 microns in order to remove the upper fragile layer. Then preliminary leveling is carried out with discs with an abrasiveness of 20–32 microns and final grinding and polishing with diamond cones of grit from 4 to M14, depending on the desired smoothness and gloss.
Wet sanding of concrete
During the grinding process, a huge amount of fine dust is generated, which quickly fills the air space and interferes with work. To eliminate this drawback, a small amount of water is supplied to the grinding wheel, which moisturizes the discs and stones, and also serves as a binder for small particles and does not allow them to come off the floor. Although most modern technology comes with powerful vacuum cleaners, wet sanding has one major advantage..
The fact is that very small particles of solid fractions clog the pores of abrasive materials and reduce their grain size to almost zero. When moistened, fine dust softens and flows out of the pores under its own weight. In addition, the resulting emulsion serves as a cutting fluid, preventing diamond chips, corundum or silicon carbide from heating up..
Mosaic floors are famous for their attractiveness due to their use as a filler of hard volcanic rocks, which have a beautiful polished cut. The hardness of these materials is even higher and without wet grinding they cannot be processed properly..
Clearing dead zones
No sander can sand narrow areas along the corners between floors and walls. The grinding wheel has a protective cover that prevents personal injury and distributes moving stones and discs from obstacles. Because of this, a side a few millimeters high is formed at the edges of the floor. These areas will not disturb the appearance, as they are covered with a baseboard. However, for its correct installation, it is necessary to remove irregularities..
Thin and high sides can be easily cut with a diamond blade mounted on the angle grinder. The cut will inevitably be made at an angle with a slight depression. Therefore, the wide sills must be gradually grinded with a grinding wheel. When processing it is extremely important not to damage the already sanded surface, therefore, the dead zones are cleaned after each stage of machining.
Stripes near walls, the width of which is larger than the overlapping area of the plinth, are especially difficult to clean and sand. In the corners of rooms, such areas almost always appear, since the grinding wheel does not fit well into a right angle. These areas require a smaller mosaic sander or very careful hand finishing. Sometimes the last stage of grinding the dead zones is replaced by a longer polishing, in which manual angle grinders with felt wheels and a special abrasive paste of the appropriate grain size are used.