Everything about everything. Volume 5 Likum Arkady

Why does cement harden?

Why does cement harden?

Cement is one of the most common materials in modern construction. By itself, it is a fine powder. But if it is mixed with water and allowed to harden, it, together with sand and gravel, turns into a solid, durable substance. Cement is the main constituent of mortar and concrete.

Mortar is a mixture of cement, sand and water. Concrete is the same mixture, but with the addition of gravel or crushed stone. Modern cement is made by heating limestone and clay or slag to very high temperatures. This mixture is heated until large, sintered lumps are formed. They are called clinkers. The clinkers are then ground into powder.

When water is added to cement powder, complex chemical reactions take place. The result is a durable artificial stone that is insoluble in water. What are these chemical reactions? What happens for the cement to harden? Chemists have no definitive answer to this question. There are four components to the composition of cement. It is believed that each of these components transforms into crystals upon addition of water. These crystals bond together and the cement hardens.

The type of cement that hardens under water is called hydraulic cement. Surprisingly, the Romans discovered the process of obtaining hydraulic cement in the III-II century BC. e. They made this cement by mixing volcanic ash with lime. This discovery was one of the outstanding achievements of the Romans.

Cement. Classification and labeling.

This is what no construction site can do without, it is without cement. It does not matter what kind of house is being built: wooden or brick. The only difference is in its quantity. Any home needs a foundation. And in brick, in addition, he goes to the masonry. With block construction, whole rooms are cast from it. And what about road construction? And what about protection from the elements of the sea? And what about the diversion of mudflows? And what about bridges and dams over turbulent rivers? This building material has been worked out through experience over the centuries, therefore it is reliable and has such a value.

Background

As soon as a person began to build a dwelling of stone, a tool was immediately needed to bind these stones. At first it was just clay. But such buildings did not differ in durability, and outwardly the building looked unpresentable. Then the binding properties of lime were noticed. At first, the ancient Greeks and Romans discovered this, and the Romans discovered that when pozzolana (volcanic ash) and the track (hardened volcanic ash) are added to lime, the dried masonry becomes almost monolithic. In Russia, from clay limestonesthe result was gray lime, setting in wet and damp masonry. In practice, both Rome and Russia almost approached the production of cement by experience: both the clays and the pozzolana contained iron and aluminum oxides, which, as a result of the action of water and lime, underwent a process of hydration. Then, for a long time, there were no changes in the composition of the binder (only the fillers in the solutions changed). And just recently, in 1822- 1824 g .G. almost simultaneously the Russian Cheliev and the Scotsman Aspind received building mixtures similar in composition to modern cements. Moreoverthe Scotsman came up with the idea of ​​obtaining clinker and producing cement from it. The name "Portland cement" also came from England, since the concrete from the Scotsman's cement resembled the color and strength of the stone mined in the mountains near the city of Portland.

What is cement?

By itself, it is not formed anywhere in nature. And, thank God, otherwise we would not have seen sand and grass, we would have walked on concrete. It is an artificial building material that, when mixed with water, forms a binding plastic mass. Over time, the mass hardens and becomes a stone-like body, a monolith. Cement differs from other binders in that it gains strength and solidityin conditions of high humidity and even under water. If we take air lime or gypsum as binders, then they harden only in air. The reason is that in concrete, cement hardens not so much due to the evaporation of water, but because the water reacts with the cement. In this case, only solid or crystalline substances are formed and heat is released. This is most likely why the process of mixing cement and water is called a seal, not a dissolution. The formation of a monolithic mass occurs as a result of cement hydration. Therefore, if you let the concrete dry quickly in the sun, it will "break", that is, it will crack and begin to collapse. To prevent this from happening, it is wetted until the concrete is completely hardened.

Cement production

First you need to prepare raw materials. Limestones are used as raw materials. The best limestones for cement production- these are marl, chalk and calcareous tuffs. Dolomites and gypsums, although they are limestones, deteriorate the quality of the cement. That is, the best cement is obtained from finely porous limestones with no silicon inclusions. Limestone is crushed and thoroughly mixed with clay. The resulting mixture contains about a quarter of clay, the rest is limestone. This composition is fed into a rotary kiln with a diameter of 2 to 7 meters and about 200 meters long. In the furnace - 1450 ° C, this is the "sintering temperature", at this time the particles of clay and limestone melt and diffuse into each other. The composition leaves the furnace after 2-4 hours in the form of sintered lumps of various sizes, this is the so-called cement clinker. Next, the clinker is crushed to a particle size of 1-100 microns. At the same time, up to 6% gypsum is added, this is necessary to prevent the process of cement setting from moisture in the air. Why is cement "in a hurry" to seize from atmospheric moisture? It's just that the sticking surface after grinding is very large: the surface area of ​​particles of just one gram reaches 5000 cm2. Are other mineral supplements added? Naturally, after all, cement is needed in the foundation, and for masonry, and for floors, you need, for example, water-repellent or quickly hardening cement. To obtain different properties, a different composition is needed, so mineral additives are designed to provide certain properties.

Classification of cements

There is no unified and comprehensive classification of cement, similar to the periodic system of Mendeleev or the classification of the flora of Carl Linnaeus. Therefore, there are several classifications, each of which takes into account some separate category of characteristics.

For example, there is clinker classification of cements, which is the basis of their production:

• - Portland cement clinker;
• - high-alumina and alumina clinker;
• - sulfatoferritic clinker;
• - sulfate aluminate clinker.

By appointment cements are subdivided into:

• - special;
• - general construction.

Some classifications are based on material composition.... Then the cements are subdivided as follows:

• - cements with mineral additives;
• - no additives cements.

There is a classification that takes into account the compressive strength:

• - cements where strength is not taken into account;
• - cements with strength M600, M550, M500, M400, M300, M200.

A couple of classifications generally allow for time intervals. One, taking into account the speed of hardening, divides the cements into:

• - normally hardening;
• - fast-hardening.

The other takes into account the setting time:

• - quick-setting (up to 45 min);
• - normally grasping (45 min-2 hours);
• - slow-setting (more than 2 hours).

Cement marking

Determination of the grade of cement is based on the determination of its strength. How is it defined? The cement is thoroughly mixed with sand in a ratio of 1: 3. The finished mixture is mixed with water. Water is taken in the amount of 40% of the weight of the cement. Cubes or parallelepipeds are molded from the resulting plastic mass. To correctly determine the strength, such a workpiece is kept in water for28 days. These pieces of concrete are then tested for flexural and compressive pressure. Most often, for testing the compressive strength, the halves are taken as a result of fracture from the bend test. And, attention! The amount of pressure that was needed to crush the workpiece is the brand of cement. Let's say that it took a pressure of 500 kg / cm 2 ... So this is cement with a 500 grade.

Now let's deal with the markings written, for example, on the bag. Inscription - MPTs400-D20. "M" means that structures with the use of this cement will be frost-resistant, the letters "PC" mean that this is Portland cement, the number 400 is a brand that means compressive strength, "D" is the presence of organic additives, and the number after it shows the percentage these additives. Thus, before us is a bag of frost-resistant Portland cement grade 400 with 20% organic additives.

Varieties of cement

At the head, you need to put high-quality Portland cement, which does not even contain mineral additives. Next are cements containing mineral additives to modify properties. The next group includes cements containing organic additives (usually resins). Slag cement is also distinguished, from which massive concrete elements of the building are made. Additional letters on the marking can say a lot about the types of cement.

1. 1. B. Fast setting, intended for repair work.
2. 2. BC. White cement for decoration and sculptural work.
3. 3. PPTs. Pozzolanic cement with finely ground silica. The main advantage is reduced heat generation. Because of this, the top and inner layers give off heat evenly, which means that the concrete will not crack.
4. 4. SC. Sulphate-resistant cement with protection against the destruction of concrete by salts. Therefore, it is perfect for hydraulic structures.
5. 5. Shopping center. Oil well cement for plugging gas and oil wells.
6. 6. SHTs. Slag cement produced without clinker base.
7. 7. TsTS. Colored cement obtained through the introduction of coloring pigments.
8. 8. PL means that plasticizers are used, GF - hydrophobic additives, due to which the effect of non-wetting and water repellency appears.

Process errors

When laying a sand-cement floor screed, we very often encounter the problem of its deformation: it can crack both immediately after drying and crack over several years.

Cracks in the concrete floor appear as a result of errors in the device

If the crackling can still be corrected, then the swollen areas will have to be dismantled and re-filled. Dismantling even a small damaged area of ​​the floor entails a lot of problems and costs. After all, even the smallest section spoils everything around during dismantling.

Reinforcement of the cement-sand screed avoids the destruction of the concrete floor.

When creating a screed with the wet method, always spread out the reinforced mesh and make beacons (you can do it in the opposite direction: first beacons, then the mesh). This work is done in one day. When the lighthouses have frozen (the next day), you can pour sand-cement mortar between them. The beacons fixed in this way will be the guiding support for the rule. Further, relying on the rule of lighthouses, you can remove the excess solution.

Following this technology, it is quite possible to achieve a relatively flat floor surface and that the concrete floor screed does not crack at all. But this action is not enough to lay thin linoleum on such a screed. In this case, you will have to additionally level the screed with self-leveling floors.

When dry, the cement slurry shrinks, and the beacons installed earlier have already shrunk. After laying a fresh screed over the settled lighthouses, the concrete will settle below the lighthouses.

The causes of cracks are as follows: as it matures, the cement loses a little in volume and gradually shrinks. If you put a fresh cement solution between the lighthouses and stretch it along the settled lighthouses, then, of course, there will be shrinkage. Moreover, it will turn out much more than in the usual case. The shrinkage will be so below the beacons that the peaks will be in their place, and large depressions will form between the beacons. The more the solution contains water, the lower the screed will settle.

If you want to speed up the construction process (put beacons and fill the floor screed in one day), then use gypsum building mixtures to fix the beacons. With the help of such mixtures (Rotband), beacons can be installed in 3-4 hours. But there are also disadvantages to this method. Rotband, in contrast to the sand-cement mixture, practically does not shrink; therefore, depressions will certainly appear on the entire surface.

Amount of water

A solution with an excessive water content dries longer, shrinks and deforms more, and also loses its strength.

Of course, too thin a solution is much easier to level over the surface of the floor. The rule is a perfectly smooth floor. But the problems will begin a little later.

A screed made from too liquid mortar will shrink and deform for a long time. The probability of screed cracking is 80%.

The strength mark drops several times when an excessive amount of water is added to the solution. The surface of the poured floor will be loose. When cleaning, you will regularly wash off or sweep away some of the top coat. Due to the constant dirtiness, you will not be able to use any decorative floor coverings. To somehow remedy the situation, you will have to work hard, for example, treat the floor with a special deep-penetrating primer.

Reinforcement

And the last mistake that leads to floor cracking is improper and poor-quality reinforcement. If you spent money on rebar, it should be useful and somehow work. If the reinforcement lies under the screed (practically by itself), then there is no sense from it. The reinforcing mesh must be in the body of the concrete pavement.

The most inexpensive and effective method is fiber reinforcement for mortar. Fiber does an excellent job of reinforcing screeds, thanks to which in many European countries, fiber reinforcement is adopted by national building standards.

The advantage of a semi-dry screed is the reduced amount of water for preparing the mortar, and as a result, the drying time and the risk of cracks and shrinkage are reduced.

Use a damper tape to avoid contact between the screed and other structures (columns, walls, partitions).

Do not place the sand-cement mixture on a wooden base. Such a base requires a special approach and the use of adjustable floor elements.

When carrying out the screed using semi-dry technology, try to use plastic wrap, with which you will cut off the screed from the concrete base. This technique will avoid the adsorption of moisture released from the solution. Therefore, you will ensure that the screed does not crack.

For screed, use only high-quality cement and sifted sand with a small admixture of clay.

To prevent the floor screed from cracking, approach the start of work responsibly, install a reinforced mesh with high quality, use a first-class self-leveling mortar, and you will definitely succeed!

• The causes of cracks
• Varieties of structural cracks
• Plastic shrinkage damage
• Temperature and shrinkage damage

Private developers, who are not professional builders, often do not understand why concrete cracks when it dries.

Often, if improperly prepared and poured, concrete cracks and crumbles after drying.

It seems that the components for the concrete were used of high quality, and the proportions were maintained correctly, and the pouring technology was followed, but cracks in the concrete monolith still appear. So why is this happening and are there any ways to avoid it?

Cracks in concrete can occur for a variety of reasons. Conventionally, these reasons can be divided into several large groups:

• structural;
• structural;
• the impact of external factors.

Structural cracks arise due to the miscalculations of the designers or due to the introduction of unjustified changes in the design calculations of the structure, such as replacing the M100 mortar with a lower grade when pouring it or erecting an additional floor that was not taken into account in the project.

Types of cracks in concrete: a) longitudinal cracks; b) transverse cracks; c) corrosion of concrete and reinforcement; d) buckling of compressed reinforcement bars.

Such cracks are a serious threat to the bearing capacity of the structure, up to and including its destruction. But to eliminate the causes of their appearance, very little is needed: to trust the performance of design calculations only to reputable companies and not to deviate from these calculations either during the pouring of concrete or during further construction.

Cracks in concrete can also appear under the influence of external factors: fire, floods, soil movements due to an earthquake or nearby explosions. The reason for their appearance is practically beyond the control of human will, so their prediction is impossible.

Structural cracks are the most common and most diverse group of cracks in concrete. Often the danger of such cracks is underestimated and sufficient measures are not taken to eliminate them, which leads to the loss of the concrete monolith of its strength characteristics and its gradual destruction.

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Varieties of structural cracks

Structural cracks in concrete are the most common and varied group of cracks in concrete. In fact, these are shrinkage cracks. The reason for their appearance is the natural physical and chemical processes taking place in concrete. They are especially active at the initial stage of the ripening of the concrete monolith, then their speed slows down, but the processes themselves do not stop until the concrete is fully ripe.

Reasons for the formation of cracks in concrete.

In other words, this damage appears in concrete due to drying and shrinkage of the concrete mixture after pouring. It is generally known that a concrete mix consists of 4 main components: cement (binder), sand and gravel or crushed stone (fillers) and water. Each of the components plays its own strictly defined role in the creation of a concrete monolith.

A freshly prepared concrete solution has a plastic or even liquid consistency. The mixture poured into the mold begins to solidify. The further this process goes, the more the cement and water that are part of the concrete are reduced in volume. As a result, the poured mixture shrinks, and due to the compaction of the mass, loads arise in the body of the forming concrete monolith, which the cement mortar that has not yet gained sufficient strength, which holds the mixed concrete components together, is simply not able to cope with.

As a result, shrinkage cracks are most often the result of processes occurring inside the hardening concrete monolith. They are conventionally divided into:

• damage from plastic shrinkage;
• temperature and shrinkage damage;
• shrinkage damage from drying out of the solution.

It is very important to correctly determine the cause of damage in a concrete monolith, because the method of their repair directly depends on this.

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Plastic shrinkage damage

Shrinkage cracking diagram.

This type of damage usually appears as a result of intense moisture loss from the exposed surface of the laid concrete, resulting in uneven shrinkage and compaction of the concrete mass.

This process takes place at the very beginning of the setting of the poured concrete mixture. Due to the evaporation of moisture, the surface of the mortar is actively losing volume, and the middle and bottom layer of the laid concrete remain in their original dimensions. The result of this shrinkage is the appearance on the surface of the concrete mixture of a mesh of small (human hair-width) and shallow cracks.

Phenomena similar to the described ones occur with concrete during precipitation. When it rains, the concrete surface gets wet, and a certain amount of moisture gets inside the monolith. At the end of the rain and the appearance of the sun, the wet concrete surface heats up, expands, and cracks may appear on it.

This type of damage also includes cracks that appear in concrete under the influence of gravity. Such cracks are caused by insufficient compaction of the concrete to be placed. In this case, the following is obtained: the gravitational forces act on the setting concrete monolith, and if insufficiently compacted areas remain in its body, then the mixture in these areas will continue to compact, breaking the integrity of the concrete monolith.

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Temperature and shrinkage damage

Diagram of concrete hardening processes, formation of structure and formation of properties.

Such deformations occur because the cement used for bonding, in contact with water, enters into a hydration reaction, the result of which is the release of a large amount of heat and, in accordance with physical laws, an increase in the volume of the solution.

In the mortar being laid, this heating and increase occurs evenly, but in hardening concrete in the hardened areas, hydration slows down, and in non-hardened areas it continues with the same force. This unevenness becomes the cause of damage to the drying concrete.

The hydration reaction also has the opposite effect, which is no less dangerous for the integrity of the concrete monolith. In the hardening upper layers of the poured concrete mixture, hydration stops, and they decrease in volume, while in its deep layers, the process continues, and, accordingly, they increase their volume. The result of such an impact on the monolith of multidirectional forces often leads to ruptures of the concrete monolith.

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Shrinkage damage from drying out of concrete

This type of damage usually occurs because the already set, but not yet fully matured concrete monolith continues to decrease in volume.

This is a feature not only of concrete, but also of any cement and adhesives, such as cement screed, plaster, etc.

This is the most common type of shrinkage damage, and it is very difficult to prevent the formation of such cracks. In addition, from such thermal damage, small cracks in concrete, which appeared from the first two types of shrinkage damage, expand and deepen.

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How to prevent and repair cracks in concrete

Components for the preparation of concrete mix.

It is clear to any sane person that it is better to prevent a problem from arising than to eliminate its consequences. All this is completely true for cracks in a concrete monolith. To save yourself from unnecessary work in the future, when preparing a concrete mixture, you need to adhere to a few simple rules.

When mixing the mixture, it is necessary to maintain the recipe and strictly observe the proportions between its components. Keep in mind that cracks can appear not only from an excess of water in the mixture, but also from an excess of cement in it.

When pouring, the concrete mixture must be compacted as much as possible. This will protect the poured mixture from the appearance of damage in it due to the influence of the forces of gravity. Also, to prevent the appearance of cracks in the concrete being laid, reinforced belts are arranged.

After pouring, concrete must be cared for. Its main task is to prevent excessively rapid or uneven evaporation of moisture from the body of the poured concrete mixture. For this, the mixture is covered with a moisture-proof film or burlap, periodically - after 4-8 hours - its surface is moistened with water until it sets completely.

Expansion joints in concrete floors.

For large pouring areas, in order to avoid the appearance of cracks from temperature movements, it is imperative to arrange expansion joints. If necessary, the formwork can be insulated.

If cracks do appear, then you need to work as quickly as possible to eliminate them. Cracks need to be repaired with Portland cement-based cement mortar. Moreover, it is advisable to prepare the cement mixture of the same grade as the poured concrete, then the homogeneity of the concrete structure will not be disturbed.

After filling the cracks with cement mortar, the treated surface must be carefully smoothed with a brush. Then the surface is covered for 2-3 days with plastic wrap, fixed at the edges with strips or bars. The film should be periodically removed to moisten the treated surface with water.

Even the most highly professional builder will not be able to completely avoid the appearance of cracks in concrete, sooner or later they will appear. But their appearance can be delayed for a long time, and the cracks that have appeared can be quickly and efficiently repaired, preventing the destruction of the concrete monolith. Good luck!

Cracking of concrete structures is quite common. The reasons for this harmful phenomenon have been identified and systematized. However, regardless of the source of cracks, when this defect appears, immediate repair work is required.

Why do cracks occur in concrete?

There are two main reasons for the appearance of cracks in concrete structures - the influence of external factors and uneven internal stresses within the concrete mass.

Cracking appearing in concrete under the influence of external factors are subdivided into types:

• Bending cracks located perpendicular to the axis of the reinforcement in tension in bending;
• Shear cracks resulting from bending cracks. They are located in the zones of transverse stresses diagonally to the axis of the reinforcement;
• Fistula cracks (through). They arise under the influence of central tensile forces;
• Cracks in the places of contact of concrete with anchor bolts and reinforcing elements. Cause stratification of concrete products.

Causes of occurrence: improper anchoring and reinforcement in the corners of strip foundations, subsidence or swelling of the soil, "flimsy" or poorly fixed formwork, loading of concrete goods until the moment of admissible strength gain, incorrect choice of section and location of reinforcement, insufficient concrete compaction during pouring, exposure to chemically active liquids.

As practice shows, as a rule, the causes of cracks in concrete are several of the listed factors.

The causes of internal stresses, which literally "break" the concrete structure, are the significant temperature differences on the surface and in the thickness of the concrete. The temperature difference can be due to the following reasons:

• Rapid cooling of the concrete surface by wind, water or snow;
• Rapid drying of the surface under the influence of high air temperature and direct sunlight;
• Intensive release of heat during the hydration of large volumes of cement inside massive concrete products.

Such cracks caused by the temperature difference go into the depth of several tens of millimeters and, as a rule, completely close after the temperature of the concrete mass has equalized and the temperature of the surface layer. Only so-called "hairline" cracks remain on the surface, which are acceptable and can be easily removed by grouting or ironing.

Methods for eliminating cracking in freshly poured concrete

• Reinforced concrete cracks, which appeared before the beginning of material setting, can be eliminated by repeated vibration treatment;
• Cracks that have arisen during the setting and hardening process are eliminated by rubbing cement (iron) or repair mortar into cracking;
• The network of cracks, which appeared 8 hours after pouring, is eliminated in the following way. The surface is cleaned with a metal brush. The resulting cement dust is removed. The surface is treated with a repair compound and, after drying, is re-cleaned with a brush or foam glass.

Cracks that appear in concrete after complete hardening are repaired by injection with polyurethane compounds. The injection technology consists in feeding special compounds into the crack, which seal the crack and form an elastic "seam".

The latter effectively restrains further crack propagation under the influence of static and dynamic loads.

Having told in this article why does concrete crack, it is impossible not to mention how to prevent this very harmful process, which ultimately leads to the complete destruction of concrete structures.

• Very often, when mixing the material on their own, inexperienced builders add a large amount of water. This results in strong evaporation and very fast setting and hardening. The consequence is the formation of shrinkage cracks. In this regard, water must be added in small portions and the recommended consistency of the solution must be observed, even if it seems that it is too thick;
• Concrete structures poured in conditions of high air temperature and bright sunlight must be protected without fail with plastic wrap, wet cloth or special mats. If this is not possible, the concrete surface (at least four times during the day) is sprayed abundantly with water;
• In order to avoid the appearance of cracks from soil shrinkage, one should strictly adhere to the accepted technologies of concrete work: soil compaction, filling a pillow, laying reinforcing belts, etc.

In any case, before starting concrete work, you should carefully study and strictly follow the theoretical and practical recommendations of GOST and experts on: the choice of the brand and type of cement, the type and type of reinforcement, the composition of concrete and other features of concrete work.

Why does the floor screed crack?

Many builders claim that a narrow crack is acceptable and does not require repair, but this is not always the case. It is important why the floor screed is cracking, because if the reason is improper installation or an unreliable foundation, then the destruction will continue. In this case, the solution will crumble, the defects will increase, and as a result, the finishing layer and the whole repair in general will be disturbed. Thus, you need to know what to do if the floor screed is cracked.

Reasons for screed cracking

Gypsum plaster almost does not shrink when ripe, but the cement-sand mixture does. Consequently, despite the small time gap between the device of the beacons and the laying of the screed, cavities and peaks on the surface of the screed will still work. By its structure, any gypsum-containing mixture differs from cement mortar. They differ in plasticity, coefficient of linear expansion, adhesion. The probability that cracks are formed to the full depth of almost 100% at the places of abutment of the gypsum and cement mortar along the lighthouses.

The second common mistake is preparing the solution with too much water. The purpose of adding more water than necessary is to make it easier for yourself, since the solution becomes more convenient to work with and very plastic. Of course, this is very convenient in the process of pouring the solution, but after a while you will have problems with such a screed:

• Excess water in the solution exposes it to large shrinkage and deformation. Therefore, most likely the screed will crack and swell.
• An increased ratio of water to cement during the preparation of any cement mortar greatly reduces the strength grade. That is, the screed will not acquire the strength it needs and its surface will turn out to be loose. Accordingly, it will dust and sweep, which will adversely affect the installation of any floor covering. To add strength to the screed, you will have to cover it with a special deep penetration primer.

Another mistake that craftsmen make when laying a screed is improper reinforcement. The reinforcement should be in the body of the concrete, but not in any way under the screed. By and large, the use of a reinforced mesh is pointless. Fiber reinforcement will be much cheaper and more effective.

In order to prevent cracking of the screed, it is necessary:

• Use a damper tape that cuts off the screed from walls, columns, partitions. The screed must not come into contact with them.
• Do not pour cement-sand mortar on a wooden base. In this case, other floor technologies are used (adjustable floors, Knauf prefabricated floors).
• Use plastic wrap in the process of laying a semi-dry screed, which will isolate it from the concrete base. This is necessary in order to exclude the absorption of moisture from the laid mixture into the concrete.
• Purchase high-quality cement and river or quarry coarse sand with a minimum amount of clay.

How to repair cracks?

Sealing cracks in the screed will only help if we are talking about an old coating or cracks have formed in problem areas: the border of different communications, pipes or base materials, cracks above the lighthouses.

In this case, for repair, it is necessary to prepare a mixture of 1 part of cement and 6 parts of cement; it should be kneaded with PVA glue. Cracks need to be sewn up to the base and choose everything from them, but what can be crumbled. The surface should be putty and primed with repair mortar. It is very important to level it before solidification. It should also be noted that the repair of cracks in the screed is only the possibility of obtaining a smoother surface, and it does not at all guarantee the strength and integrity of the screed in the future.

How to strengthen the screed against cracking?

If you want to avoid problems and not resort to further repair work with the screed, you just need to follow the technology of its installation. The quality primarily depends on the proportions of the composition. With an excess of water or cement, cracks are guaranteed to form. The quality of the foundation is also important. If its surface is unreliable or absorbs moisture strongly, the screed must be reinforced.

Another important point is the drying of the solution. Most try to speed up this process and start creating drafts or heating the room. Because of this, an uneven and too rapid evaporation of moisture occurs, which also leads to cracking. The sand-cement mortar should dry out gradually at normal temperature and humidity, in addition, in windy and hot weather, they should be moistened and protected from drying out too quickly. For this, as a rule, a damp burlap is used.

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A cracked screed is easy to repair

Most craftsmen insist that a narrow, small crack is allowed in the floor screed in a new apartment and nothing needs to be done with it. In most cases, this is not the case. The question arises, what is the reason that it cracked? Such a defect may arise due to improper pouring or an unstable foundation, and if, without eliminating these defects, fill in a new screed or repair cracks on the old one, the base will continue to collapse. The next step will begin to crumble concrete along the edges of the crack, then the finishing layer will begin to warp, then the plinth. And you will have to do all the repairs again. Therefore, the first step is to find out why the floor screed cracked in the new building and what to do so that the cracks do not go further?

Whatever one may say, the floor screed is the best, and sometimes almost the only way to level the base for a finishing coating. She makes a smooth and even coating for the flooring of the finishing layer, hiding communications or any defects in the base under its thickness. But when pouring it, some people are faced with such insignificant difficulties as preparing the right mixture or installing the wrong beacons for the floor screed. It is these little things that lead to unwanted cracks when the screed dries up. But don't despair! Sometimes sealing cracks in a floor screed is not such a difficult task. Let's take a look at the reasons why the screed can burst and how to avoid it. And if we could not avoid this, we will analyze a specific example of how to repair cracks in the floor screed.

The causes of cracks

• Manufacturing technology violated
• The proportion of ingredients is not observed when mixing
• Poor quality or small amount of cement in the mix
• No expansion joint
• Improper reinforcement

Mixing proportion not observed

This is the most common cause of floor screed cracks. This is usually found in ready-made mixtures. First of all, those who first decided to do this fall into the risk group, go to the store and purchase a ready-made dry mixture. Manufacturers of dry mixes in production calculate the exact amount of necessary additives, which, dissolving in water, are uniformly distributed among themselves.

You probably know that the grout is better applied to the floor, and beginners will probably want to add a little water. In the end, such a move will only make the quality of the mixture worse. What the manufacturer writes on the pack of the mixture must be strictly followed according to the instructions.

It is not recommended to mix the solution manually, a construction mixer is most suitable for this purpose, and if you don’t want to shell out for this expensive miracle of technology, you can buy a simple nozzle for an electric drill and make a uniform kneading at low speeds.

For a high-quality screed, it is recommended to take medium-grained sand mined in a quarry and not river sand, which is more affordable. The optimal cement grade is M-400. First of all, the sand is sifted from lumps of clay and pebbles. Water is added already by eye, until the mixture gets sufficient viscosity and plasticity.

If these minimum conditions are not met, cracks are more likely to appear.

Lots of water in solution

Large amount of water in concrete subject it to shrinkage or deformation. In this case, the screed is also likely to crack. Flooding of the concrete mix also reduces the strength of the finished product (screed). In simple words, the screed will not be strong enough and the surface will be loose.

In this case, the screed will need to be covered with deep penetration soil in order to avoid dusting and sweeping after laying the topcoat. And this is again unnecessary costs.

Material difference

The second common mistake is the different material of the beacons and the screed itself. Gypsum-based plaster rarely shrinks after drying, which cannot be said about a cement-sand mixture. And since between the installation of the beacons and the pouring of the screed, not much time passes on the screed surface, either depressions or bumps will turn out.

They arise not only from the different compositions of the gypsum mixture of beacons and cement-sand screed. but also differences in plasticity, coefficient of linear expansion and adhesion. And in those places where the cement mortar is adjacent to the gypsum beacons, cracks may appear in the floor screed, so what to do. We'll have to fix everything.

No expansion joint

Another gross cause of a screed crack is improper placement of expansion joints or their complete absence. Namely the wall seam and the intermediate seams on the floor.

The wall expansion joint must be filled with elastic material (polypropylene, polystyrene), run along the entire thickness of the screed, thereby separating it from the effect of deformation loads of the walls. Some craftsmen also recommend placing expansion joints around columns, built-in furnishings and stairs.

Intermediate expansion joints, on the other hand, do not pass through the entire thickness of the screed, but only half. They divide the screed into equal parts, preventing it from cracking after shrinkage. The width of such seams is selected depending on the thickness, and the presence of a warm floor. Do not forget to make special marks in the area of ​​the reinforcing mesh if your screed is reinforced.

Expansion seams are provided in all types of screeds in premises with an area of ​​more than 30 m. Accordingly, the maximum area of ​​the fields into which the screed must be divided is the same 30 m. The sides of the area should not be more than 6 m. In due time, intermediate expansion seams must be cut in the corridors , and the distance between these types of seams should be less than six meters.

If ceramic tiles or porcelain stoneware are chosen as the topcoat, the notches from the expansion joints should be comparable between the tile joints.

Inside the premises, we leave the seams unfilled, but it is recommended to seal the seams on the outdoor area with silicone or waterproof glue in order to avoid water getting into them and at sub-zero temperatures did not break your screed.

Wall seams can usually be left blank. In the event that you decide to close them up, it is recommended to use only soft materials.

Reinforcement

Another of the common mistakes in which the screed cracks is improper, poor-quality reinforcement. If you decide to purchase reinforcement and make a high-quality base, then it should be in the body of the concrete and not lie under the thickness of the screed. It is not recommended to use a reinforcing mesh here, and there is simply no need to waste so much money, but fiber reinforcement will be very effective. The reinforcement should be in the body of the concrete, but not in any way under the screed. By and large, the use of a reinforced mesh is pointless. Fiber reinforcement will be much cheaper and more effective.

Before renovation

It does not matter if the screed is cracked, but by the beginning of resuscitation work, a number of procedures should be performed to simplify and speed up the work to eliminate the crack.

1. First of all, determine why they were formed. If the screed was not performed, you will determine the presence of expansion joints and how the floor was poured.
2. If the cracks in the floor screed look like some segments scattered all over the base, they are repaired with epoxy adhesives using the "force-locking" technology.
3. If cracks in the underfloor heating screed appeared due to the absence of an expansion joint between rooms or along walls, you should not close them up without making this very joint.

Before starting repair work, you should first identify the cause of the cracks. Otherwise, after a few months, they will again make themselves felt, and not only in old, but also in new places.

Before fixing cracks in the floor screed, you will need to look at the extent of the damage and highlight the areas that need repair.

You can easily find visible cracks. But the hidden voids will have to be looked for by tapping the whole base with a wooden mallet.

If during this procedure you hear a ringing sound, then you have found one of these voids. Found hidden flaws should be marked and, at the end of the work, calculate the area that needs repair.

If, as a result, it turned out that 30% of the area of ​​the room or more is needed to be repaired, it is recommended to dismantle the old base and fill in a new coating.

We repair small cracks

It is recommended to cut minor cracks in the floor screed with a grinder up to 20 mm. Remove debris after processing with a regular vacuum cleaner and wipe off the remaining dust with a damp cloth and let the surface dry before repairing. Once dry, the surface is ready for repair.

Cunning! If the premises are uninhabited, it is recommended to check cracks for possible subsequent deformation. To do this, the cracks in the floor screed are glued over with sheets of paper and left for a while. If the sheet is torn, the crack continues to expand and the repair requires a heavier approach.

We repair large cracks

It is not for nothing that a crack is one of the most severe damage to the screed, therefore, the repair of cracks in the screed must be done here and now. If you do not pay attention to this in time, then most likely it will grow, which will lead to the impossibility of repair and you will have to make a new screed.

To eliminate cracks in the floor screed, you need to make a lot of costs, both financial and physical. Therefore, we recommend that you follow all the technologies described above and strictly follow them, then the repaired floor will serve you for a long time and you will not have to start repairing it again.

To avoid repair work after drying, it is enough just to follow the technology of laying the screed. To summarize: First of all, we observe the proportions of the mixture. Excess water 100% will give us cracks on the dry floor. The preparation of the base also plays an important role. If it absorbs moisture, the screed will need to be reinforced in any case.

And the most important thing! There is no need to accelerate the drying of the solution by artificial drafts or heating the room. With such actions, moisture evaporates unevenly and quickly, and cracks will also appear from this.

The screed on the floor should dry on its own, gradually and at the same temperature. If the weather outside is hot or, on the contrary, windy, it will need to be moistened, thereby protecting it from rapid drying. To carry out this process, wet burlap is mainly used.

Observing these simple rules when pouring the screed, you will never have cracks on the floor

Video instruction

Why is the screed cracking?

Cracks in the floor screed - defect or permissible error. Many builders argue that if the crack is not wide, and the coating will be laid on top, then no repair is needed. Unfortunately, this is not always the case. It all depends on why the screed is cracking. If the reason is improper pouring or an unreliable foundation, then the destruction will continue, the solution will crumble, defects will increase, and subsequently the finishing layer will be disturbed, and the entire repair as a whole. To avoid troubles and unnecessary expenses in the future, consider the cases when and how to repair the floor screed.

Reasons and solution

‘’ Yandex ’’ is not found

• improperly prepared cement mortar;
• too fast or uneven drying;
• too thin or uneven layer;
• installation of beacons for gypsum-containing mixtures.

All the above cases can be ignored if there are few cracks and they are very thin. Usually such defects appear immediately after drying and do not change over time. They are not critical for most decorative coatings.

Deep cracks that diverge over time can form when installed on an unreliable or soft base without additional reinforcement. Cracking is also very likely if the substrate is porous. It "pulls" moisture out of the solution. In this case, echoing areas may appear (determined by tapping) - this means that the screed has peeled off in some places. Torn and deep cracks can also appear when using a "greasy" mortar - with a lot of cement. Unfortunately, this damage is very serious, and simple repair of the cracks will not solve anything. In this case, you will have to do everything again.

Sealing cracks in the screed will help if we are talking about an old coating, or if cracks have appeared in problem areas: this is the border of different base materials, pipes or communications, cracks above the lighthouses.

For repairs, a mixture of 6 parts of pure sand and one part of cement is prepared, and mixed with PVA glue. Cracks are sewn up to the base, and everything that can be crumbled is selected. The surface is primed and putty with repair mixture. It is very important to flatten it before hardening. It should be noted that the repair of cracks in the screed is just an opportunity to get a smoother surface, and it does not guarantee its integrity and strength in the future.

So that problems do not arise

‘’ Yandex ’’ is not found

Another important point is the drying of the solution. Many try to speed it up by arranging drafts or heating the room. Because of this, moisture evaporates unevenly and too quickly, which also leads to cracking. Cement-sand mortars should dry out gradually at normal humidity and temperature, moreover, in hot and windy weather they must be moistened and protected from rapid drying (for example, covered with a damp burlap).

In accordance with generally accepted technology, if a mortar or concrete is made in accordance with the rules and proportions, immediately after pouring into a mold, formwork or onto the surface, it begins to harden. However, its strength characteristics do not increase immediately, but over a certain period of time.

During this period, even if visually the mortar or concrete looks solid, no significant load can be applied to them - the material can crack and collapse.

In this regard, novice builders are interested in the question of how much cement (concrete or mortar) dries, as well as what factors affect the slowdown or acceleration of this process.

Stages of hardening of the cement mixture

In general, a 30-day exposure of the freshly poured structure will be sufficient to continue construction work. In some cases, when pouring powerful foundations for buildings, structures or industrial equipment, this period is increased to 90 days.

For small "household" construction - pouring a floor screed, laying ceramic tiles, arranging a concrete blind area or path and other similar work, you can walk and move objects on the surface after 72 hours from the moment the mortar or concrete was laid.

In this case, the material goes through two stages of hardening: setting and actual hardening.

• Grasping. This is a fairly quick process - no more than 24 hours from the moment the mixture is prepared. The main factor affecting the setting speed is the ambient temperature.

In the warm season, when the air temperature is in the range of 20-22 degrees Celsius, the mortar (concrete) begins to "set" approximately 2 hours after mixing. If the air temperature fluctuates around 0 degrees, this process can drag on for 20 hours.

At the same time, all this time the material remains "mobile", and if at this time you begin to perform any actions with it, the stage of "setting" can be significantly prolonged in time.

• Hardening. According to building codes and instructions, the mortar (concrete) hardens within 30 days after the structure is poured.

However, in this case, not complete hardening is meant, but hardening to such a value at which the next stage of construction work can be started. Complete hardening occurs within one, or even several years.

It should be noted that the indicated periods are valid while maintaining the optimum ambient temperature and humidity according to the instructions. Also, in order for the set mortar or concrete to gain its strength evenly and not crack, its surface must be protected from direct sunlight (usually with plastic wrap), on very hot days, pour it in the morning or evening, and during the day, spray the surface with water in within 72 hours.