Professional preparation of the subfloor should also include surface leveling. If the base has significant unevenness and deviations from the horizontal plane, it may be necessary to create a leveling layer several centimeters thick. In other cases, on the contrary, a thin leveling layer 3 or 5 mm thick is required. Sometimes it may be necessary to level non-absorbent substrates. In these cases, the required absorbency of the leveling layer can only be achieved with a thickness of at least 3 mm. We must not forget that the most important condition for the use of leveling mixtures is the presence of a stable, pre-mechanically treated and primed base.
In accordance with GOST 31189-2003 “Dry mixtures for construction. Classification" floor leveling mixtures are divided into:
- According to the laying technology - compacted and self-compacting;
- According to the grain size of the filler – into mortar and dispersed;
- According to the thickness of the layer - into thick-layer (coarse-grained mortar mixtures), thin-layer (fine-grained dispersed mixtures) and universal (fine-grained dispersed mixtures).
Compactable mixtures require compaction during installation and are characterized by a mobility indicator “by cone immersion” (Pk). To compact them, tampers, surface or deep vibrators are used. Self-compacting mixtures can be laid without compaction and are characterized by the “ring spread” mobility indicator (Pk). In accordance with the requirements, self-compacting mixtures have greater mobility than compacted ones. Mortar mixtures contain aggregate with a particle size of no more than 5 mm. Dispersed mixtures contain aggregate with a particle size of no more than 0.63 mm.
As a rule, all dispersed floor mixtures are also self-compacting. Since they are most often used with a small layer thickness and have the greatest mobility (spreadability), they are also called leveling or self-leveling. Leveling mixtures are used to obtain the most even, smooth floor surface for laying floor coverings.
Main functions of leveling floor mixtures
Repair involves filling foundation defects: potholes, depressions, losses, cracks, and so on. For this purpose, it is advisable to use quick-hardening repair mixtures designed for a sufficiently large layer thickness and allowing the selection of consistency over a wide range. Preliminary leveling of the floor base is necessary not only to obtain a flat surface, but also to create a plane with minimal deviations from the horizon or a given slope. A horizontal floor surface or slope is created, as a rule, at the stage of manufacturing screeds using thick-layer mixtures. Final leveling is necessary for laying most types of floor coverings (for example, commercial linoleum or self-leveling polymer coatings), when not only an even, but also a fairly smooth seamless surface is required. The rough surface roughness and the presence of construction seams when laying such coverings can create certain problems, for example, unevenness on the surface of commercial linoleum. To create the smoothest, seamless floor surface, leveling mixtures are used. Ensuring absorbency - all floor leveling mixtures based on cement binders perform another important function - they form a surface with good absorbency, which is extremely necessary for drying water-dispersion adhesives used for gluing coatings.
Regardless of which mixture you choose, a number of conditions must be observed when using it. For example, the amount of mixing water used to prepare the mixture is an important factor. It must be taken into account that during the hardening of the mixture, approximately half of the required amount of mixing water should evaporate (the remaining moisture is bound by cement). For normal drying of the mixture, it is necessary to ensure gradual removal of moisture due to natural air circulation or ventilation. Drying the mixture too slowly always leads to an undesirable increase in work time.
On the other hand, serious problems can arise, on the contrary, due to the mixture drying out too quickly. Lack of water leads to the fact that the material does not have time to gain the required strength, and its uneven drying creates conditions for the formation of cracks. Therefore, after laying the leveling mixture, it is necessary to ensure its protection from drafts (windows must be closed!) and direct sunlight. For these reasons, work with leveling mixtures must be carried out under certain climatic conditions. Ideal conditions are room temperature (about +20°C) and relative air humidity of 50-60%.
In addition, the materials with which work is performed, including dry mixtures, must also be stored under certain climatic conditions. For example, in hot summer time, you need to ensure that the material is not exposed to direct sunlight for a long time and is not kept in a closed vehicle for a long time. Excessive heating of the mixture leads to a significant deterioration in leveling ability. Mixtures that are too cold harden slowly (at the same time they can lose moisture and delaminate, which leads to loss of strength). The optimal storage temperature for dry leveling mixtures is from +15 to +25°C.
Preparation of leveling mixtures
To prepare the leveling mixture, take a measured amount of clean water at a temperature of +15 to +20°C. The dry mixture is gradually added to the water while stirring, achieving a homogeneous mass without lumps. Mixing is done with a mixer or drill with an attachment at a rotation speed of 400-800 rpm. The amount of mixing water should be taken in accordance with the recommendations for each specific brand of leveling mixture.
Mixing at too high a speed can cause the mixture to separate and cause excessive air entrainment. For some brands of mixtures (see technical descriptions), after preparation it is necessary to maintain a technological pause for the mixture to mature (about 5 minutes) and mix again. The consumption time (potlife) of the mixture depends on the specific brand of product (in most cases, about 30 minutes). When working, it is necessary to take into account that this time depends on the temperature and in real conditions it may be more or less than the specified value. After consumption time, the mobility of the mixture begins to decrease and, accordingly, its leveling ability deteriorates.
Laying leveling mixtures
The prepared mixture is applied to the base of the floor and spread over it with a squeegee. Depending on the required layer thickness, serrated squeegees for layers up to 5 mm or smooth squeegees with adjustable supports for layers over 5 mm are used to distribute the mixture. The thickness of the mixture layer is determined by the size of the tooth or the height of the squeegee support. When working with a toothed squeegee (with a layer thickness of up to 5 mm), additional control of the level of the laid mixture is usually not required. If the thickness of the leveling layer is more than 5 mm, it is recommended to use special mobile beacons for this purpose.
The self-leveling effect is achieved starting from a certain layer thickness. Therefore, to obtain the most even surface possible, it is recommended to lay the mixture in a layer at least 3 mm thick. The laying of the leveling mixture begins from the depths of the room in a “pull-up” manner, gradually moving towards the exit. Laying must be carried out continuously. On large surfaces, it is necessary to work quickly enough so that the breaks between adjacent sections of the laid mixture are not too long (optimally - no more than 15 minutes). With longer breaks, unevenness may occur at the boundaries between adjacent pours due to the fact that the previously applied mixture begins to lose mobility and portions of the mixture do not fit well with each other. To avoid such problems, it is recommended to plan in advance the order of laying the mixture, taking into account the number of layers, the configuration and area of the room. To remove air bubbles and obtain a more even surface, the mixture should be treated with a needle roller immediately after laying (for a layer thickness of more than 20 mm, use a mesh roller). If continuous laying of the mixture is not possible, leveling is carried out in stages using limiting slats. If it is necessary to finish work indoors, it is recommended to install slats in doorways.
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Evaporation of water
A layer of leveling mixture 2 mm thick contains approximately 0.8 liters of water per 1 m2. If half of this amount of water evaporates in a room of 20 m2, this means that at least 8 liters of water must be released into the room air. However, at a temperature of +20°C, the entire volume of air in the room can absorb only about 1 liter of water. This example clearly shows how necessary natural ventilation or indoor air circulation is when working with floor leveling mixtures! But, nevertheless, the windows must be closed and drafts are not allowed!
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Mixing water quantity
An increase in the amount of mixing water inevitably leads to a decrease in the strength of the hardened mortar and an increase in its drying time. At the same time, shrinkage strains also increase, which can lead to the formation of cracks. In addition, an overdose of mixing water leads to delamination of the mixture, when the heaviest component (sand) settles down, and the lightest components (modifying additives), on the contrary, float to the surface. As a result of delamination, the homogeneity of the structure of the hardened solution is disrupted and its surface layer is significantly weakened. The main sign of mixture delamination is usually the presence of a chalking white coating on the surface of the leveling layer. In this case, the material should be considered not to comply with the declared characteristics, and the resulting floor base should not meet the requirements for it.
The main condition for the quality of work when laying leveling mixtures is strict control of the amount of mixing water!
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