Setting time of the cement mortar. Setting and hardening of concrete or cement Why cement hardens

Progress does not stand still, and technologies are constantly evolving, such as aluminum welding. The emergence of new technologies and materials leads to the inevitable improvement of the usual methods, to unexpected and interesting solutions in a variety of applications. New materials are beginning to be used in the most unusual ways, and interior designers are constantly coming up with new ways to use innovations.

Companies that produce new building materials, architects, builders, design specialists help make the world around us bright and beautiful, transforming it with the help of new interesting solutions from gray and featureless to colorful and varied.

Changes have not passed by what we are used to building material- cement. Cement is available in many types, both conventional and special. For each of the types there is a time-tested formula that determines the quality of the material. This formula is also constantly changing, and often changes in it occur for reasons of aesthetics. For this reason, white cement was born. Now they make bricks out of it, fake diamond, stucco for decoration, etc. White cement included in dry adhesives High Quality, both white and colored concretes are made from it, terrazite surfaces are arranged, self-leveling floors are used in the manufacture of small architectural forms, blocks, sidewalk curbs, balconies, stairs, etc.

White cement has a lot of advantages, which include the following:

● It has high compressive strength, as early dates hardening, and at optimal. Its daily strength, depending on the manufacturer, is on average 19 MPa, which allows it to be attributed to fast-hardening products.

● The degree of whiteness of such cement is close to the reference (96.3%). The first grade has a whiteness of 80%, the second - from 75 to 77%, the third - from 71 to 74 percent.

● White cement is a sulfate-resistant material, contains little alkali - their content is not higher than 5%.

● Has high level frost resistance.

White cement contains diatomaceous earth, clinker, mineral pigment or an inert mineral additive. Available in 300, 400 and 500 grades.

White cement is used for the manufacture of high-quality concrete, which is used to create finished facade elements of any shape. White cement concrete products have higher strength, smooth surface white color that does not require additional processing. During operation, white-concrete products can be washed with water, they are not subject to shrinkage and shedding.

Due to the fact that white cement has a “pure” color, concretes made from it have a high-quality colorimetric characteristic. If the composition is prepared perfectly and a high-quality coloring pigment is added to it, the finished product has the necessary color tone, without a broken shade. When using white cement, less coloring pigment is required for color preparation.

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 for several years.

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

If the crackles can also be corrected, then the swollen areas will have to be dismantled and refilled. The dismantling of even a small damaged area of the floor brings with it a lot of problems and cash costs. After all, even the smallest area during dismantling spoils everything around.

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

When creating a wet screed, always spread the reinforced mesh and make beacons (you can do it in the opposite direction: first the beacons, then the mesh). This work is done in one day. When the beacons are 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, leaning the rule on the beacons, you can remove the excess solution.

Following this technology, it is quite possible to achieve a relatively flat floor surface and ensure 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 drying, the cement mortar shrinks, and the beacons installed earlier have already shrunk. After laying a fresh screed over the settled beacons, the concrete will settle below the beacons.

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

If you want to speed up the construction process (place the beacons and pour the floor screed on the same 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, unlike the sand-cement mixture, practically does not shrink, in connection with this, cavities 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 strength.

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

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

The strength grade drops several times when an excess amount of water is added to the solution. The surface of the flooded floor will be loose. When cleaning, you will regularly wash off or sweep away part of the top coat. Due to constant contamination, you will not be able to use any floor decorative coverings. In order to somehow correct 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 incorrect and poor-quality reinforcement. If you spent money on fittings, 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.

most inexpensive and effective method- fiber reinforcement for mortar. Fiberglass does an excellent job of reinforcing screeds, thanks to this, in many European countries fiber reinforcement is accepted by national building standards.

The advantage of a semi-dry screed is the reduced amount of water used to prepare 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 lay sand-cement mixture on a wooden base. Such a base requires a special approach and the use of elements of adjustable floors.

When screeding using semi-dry technology, try to use a plastic film 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.

So that the floor screed does not crack, approach responsibly to the beginning of work, install a reinforced mesh with high quality, use a first-class self-leveling mortar, and you will definitely succeed!

Causes of cracks
Varieties of structural cracks
Plastic shrinkage damage
Temperature shrinkage damage

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

Often, with improper preparation and pouring, concrete cracks and crumbles after drying.

It seems that high-quality components for concrete were used, and the proportions are maintained correctly, and the pouring technology is observed, but cracks in the concrete monolith still appear. So why does this happen and are there 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 occur due to miscalculations by designers or due to unjustified changes in the design calculations of the structure, such as replacing the M100 grade mortar with a lower grade during pouring or erecting an additional floor 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 its destruction. But to eliminate the causes of their appearance, very little is needed: to trust the design calculations only to reputable firms and not deviate from these calculations either during concrete pouring or during further construction.

Cracks in concrete can also appear under the influence of external factors: fire, flood, 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 strength characteristics of the concrete monolith and its gradual destruction.

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Varieties of structural cracks

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

Causes of cracking in concrete.

In other words, these damages appear in concrete due to drying and shrinkage. concrete mix after filling. It is well known that the concrete mixture consists of 4 main components: cement (binder), sand and gravel or crushed stone (aggregates) and water. Each of the components plays its strictly defined role in the creation of a concrete monolith.

Freshly prepared concrete mortar 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 in the body of the emerging concrete monolith, due to the compaction of the mass, loads arise that the cement mortar, which has not yet gained sufficient strength, which holds together the mixed components of concrete, is simply unable to cope with.

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

damage from plastic shrinkage;
temperature shrinkage damage;
shrinkage damage from drying mortar.

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

Scheme of crack formation due to shrinkage.

This type of damage usually occurs due to intense loss of moisture by the exposed surface of the laid concrete, resulting in uneven shrinkage and compaction of the concrete mass.

This process occurs at the very beginning of the setting of the poured concrete mixture. Due to the evaporation of moisture, the surface of the solution actively loses volume, and the middle and bottom layer of laid concrete remain in their original dimensions. The result of such shrinkage is the appearance on the surface of the concrete mixture of a grid of small (the width of a human hair) and shallow cracks.

Phenomena similar to those described occur with concrete during precipitation. During rain, the surface of the concrete gets wet, and a certain amount of moisture gets inside the monolith. When the rain stops and the sun comes out, the wet surface of the concrete heats up, expanding, and cracks can appear on it.

Also, this type of damage includes cracks that appear in concrete under the influence of gravity. The reason for the appearance of such cracks is insufficient compaction of the laid concrete. In this case, the following happens: the forces of gravity act on the setting concrete monolith, and if insufficiently compacted areas remain in its body, the mixture in these areas will continue to compact, breaking the integrity of the concrete monolith.

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Temperature shrinkage damage

Scheme of processes during concrete hardening, formation of structure and formation of properties.

Such deformations occur because the cement used for the binder, 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 the hardening concrete, in the hardened areas, hydration slows down, and in the non-hardened areas it continues with the same force. This unevenness causes 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 congealing upper layers hydration stops in the poured concrete mixture, and they decrease in volume, while in its deep layers the process continues, and they, accordingly, increase their volume. The result of such an impact on the monolith of multidirectional forces is often ruptures of the concrete monolith.

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Shrinkage damage from concrete drying out

This kind of damage usually happens because the already set, but not yet fully mature, concrete monolith continues to shrink in volume.

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

This is the most common type of shrinkage damage, and preventing the formation of such cracks is a very difficult task. In addition, from such temperature damage, small cracks in concrete expand and deepen, which appeared from the first two varieties of shrinkage damage.

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How to prevent and eliminate 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 occurring 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 mix, you need to follow 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 composition of 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 laid concrete, reinforced belts are arranged.

Concrete after pouring necessarily needs care. Its main task is to prevent excessively rapid or uneven evaporation of moisture from the body of the poured concrete mixture. To do 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 completely sets.

Expansion joints in concrete floors.

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

If cracks nevertheless appear, then it is necessary to carry out work to eliminate them as quickly as possible. Cracks must be sealed with Portland cement mortar. Moreover, it is desirable to prepare the cement mixture of the same brand as the poured concrete, then the uniformity of the concrete structure will not be disturbed.

After sealing cracks with cement mortar, the treated surface must be carefully smoothed with a brush. Then the surface is covered for 2-3 days with a plastic film, fixed along the edges with planks 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 repaired quickly and efficiently, preventing the destruction of the concrete monolith. Good luck!

Cracking in concrete structures is a fairly common occurrence. The causes of this harmful phenomenon are identified and systematized. However, regardless of the source of cracks, when this defect occurs, immediate repair work is required.

Why do cracks occur in concrete?

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

Cracks that appear in concrete under the influence of external factors are divided into types:

Cracks on bends located perpendicular to the axis of reinforcement, working in tension during bending;
Shear cracks resulting from bending cracks. located in areas transverse stresses diagonally to the reinforcement axis;
Fistula cracks (through). Occur under the influence of central tensile forces;
Cracks at the points of contact of concrete with anchor bolts and reinforcing elements. Cause stratification of reinforced concrete products.

Causes of occurrence: incorrect anchoring and reinforcement in the corners strip foundations, subsidence or heaving of the soil, "flimsy" or poorly fixed formwork, loading of reinforced concrete products until the moment of permissible strength development, incorrect selection of the section and location of reinforcement, insufficient compaction of concrete during pouring, exposure to chemically active liquids.

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

Causes internal stresses that literally "break" the concrete structure is a significant temperature difference on the surface and in the thickness of the concrete. The temperature difference may be due to the following reasons:

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

Such cracks caused by a temperature difference go deep into several tens of millimeters and, as a rule, completely close after the temperature of the concrete thickness and the temperature of the surface layer equalize. Only the so-called "hairy" cracks remain on the surface, which are acceptable and can be easily eliminated by grouting or ironing.

Methods for eliminating cracking in freshly poured concrete

Reinforced concrete cracks that appeared before the material began to set can be eliminated by repeated vibration treatment;
Cracks that have arisen in the process of setting and hardening are eliminated by rubbing cement (iron) or repair mortar into the crack;
The network of cracks that 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, it is re-cleaned with a brush or foam glass.

Cracks that appear in concrete after complete hardening are eliminated by injection with polyurethane compounds. The injection technology consists in applying 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 said in this article why concrete cracks, 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 leads to strong evaporation and very fast setting and curing. The result is the formation of shrinkage cracks. In this regard, water must be added in small portions and the recommended consistency of the solution should be observed, even if it seems that it is too thick;
Concrete structures cast 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 surface of the concrete (at least four times during the day) is abundantly sprayed with water;
In order to avoid the appearance of cracks due to soil shrinkage, one should strictly adhere to the accepted concrete work technologies: soil compaction, cushion filling, laying of reinforcing belts, etc.

In any case, before starting concrete work, you should carefully study and strictly follow the theoretical and practical recommendations GOST and specialists in: 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 is the floor screed cracking?

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 cause is improper installation or an unreliable foundation, then destruction will continue. In this case, the solution will crumble, the defects will increase, and as a result, the finishing layer and, in general, the entire repair will be broken. Thus, you need to know what to do if the floor screed is cracked.

Causes of screed cracking

Gypsum plaster almost does not shrink when ripe, but the cement-sand mixture does. Therefore, despite the small gap in time between the installation of beacons and the laying of the screed, depressions and peaks on the surface of the screed will still be obtained. In its structure, any gypsum-containing mixture differs from cement mortar. They differ in plasticity, coefficient of linear expansion, adhesion. The probability that at the junction of gypsum and cement mortar along the lighthouses cracks form to the entire depth is almost 100%.

The second common mistake is to prepare the solution with excess water. The purpose of adding more water than necessary is to make it easier for yourself, because 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 and cement during the preparation of any cement slurry 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 out, which will adversely affect the laying 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 the masters make when laying the screed is the wrong reinforcement. The reinforcement must be in the body of the concrete, but not under the screed. By and large, the use of reinforced mesh is pointless. Fiber reinforcement will be much cheaper and more effective.

To prevent cracking of the screed, you must:

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 flooring technologies are used (adjustable floors, Knauf prefabricated floors).
Use a plastic film in the process of laying a semi-dry screed, which will isolate it from concrete base. This is necessary to prevent the absorption of moisture from the mixture being laid into the concrete.
Purchase high-quality cement and river or quarry coarse sand with a minimum amount of clay.

How to fix 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 beacons.

In this case, for repair, it is necessary to prepare a mixture of 1 part of cement and 6 parts of cement, knead it on PVA glue. Cracks need to be embroidered to the base and choose all of them, but what can be crumbled. The surface should be puttied and primed repair mix. It is very important to align it before solidification. It should also be noted that the repair of cracks in the screed is only an opportunity to obtain a more even surface, and it does not at all guarantee the strength and integrity of the screed in the future.

How to strengthen the screed from cracking?

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

Another important point- drying of the solution. Most try to speed up this process and start drafting or heating the room. Because of this, uneven and too rapid evaporation of moisture occurs, which also leads to cracking. Sand-cement mortar should dry 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, wet burlap is used.

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Cracked screed can be easily repaired

Most craftsmen say that a narrow, small crack is allowed in the floor screed in new apartment and you don't have to do anything about it. In most cases, this is not the case. The question arises, what is the reason that it cracked? Such a defect may occur due to improper pouring or a weak base, and if, without eliminating these defects, a new screed is poured or cracks are repaired 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 final finishing layer will begin to warp, then the plinth. And you will have to do all the repairs again. Therefore, it is necessary first of all to find out why the floor screed cracked in the new building and what to do so that the cracks do not go further?

Like it or not, a floor screed is the best, and sometimes almost the only way to level the base for a final finish. She makes a smooth and even coating for the flooring of the finishing layer, hiding communications or any defects in the base under her thickness. But when pouring it, some people face such minor difficulties as preparing the right mixture or putting the wrong beacons for the floor screed. Just the same, these little things lead to unwanted cracks when the screed dries. But don't despair! Sometimes sealing cracks in the floor screed is not the same difficult task. Let's take a look at the reasons why the screed can burst and how to avoid it. And if it was not possible to avoid this, we will analyze specific example how to repair cracks in floor screed.

Causes of cracks

Broken manufacturing technology
Incorrect mixing ratio of ingredients
Poor quality or low amount of cement in the mixture
No expansion joint
Wrong reinforcement

Mixing proportion not correct

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

You probably know that the liquid solution is better applied to the floor, and beginners will probably be tempted to add a little water. Such a move in the end result will only make the quality of the mixture worse. What the manufacturer writes on the pack of the mixture must be strictly observed according to the instructions.

Mixing the solution by hand is not recommended, for this purpose it is most suitable construction mixer and if you don’t want to fork out for this expensive miracle of technology, you can buy a simple nozzle for an electric drill and make a uniform batch 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 brand of cement will be M-400. First of all, the sand is sifted from lumps of clay and pebbles. Water is added by eye until the mixture has sufficient viscosity and plasticity.

If this minimum condition is not met, cracks are more likely to appear.

Lots of water in solution

Lots of water in concrete subject it to shrinkage or deformation. In this case, the screed is also likely to crack. The flooding of the concrete mixture 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 a deep penetration primer in order to avoid dusting and sweeping after the topcoat is laid. And this is again an extra cost.

Material difference

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

They arise not only because of 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 adjoins the gypsum beacons, cracks may occur in the floor screed, so what to do. We'll have to fix everything.

No expansion joint

Another rough cause of a crack in the screed is the incorrect location of the expansion joints or their complete absence. Namely wall seam and intermediate seams on the floor.

The wall expansion joint must be filled with an elastic material (polypropylene, polystyrene), pass through the entire thickness of the screed, thereby separating it from the influence of the deformation loads of the walls. Some craftsmen also recommend laying an expansion joint around columns, built-in interior items and stairs.

Intermediate expansion joints, per turn, do not pass through the entire thickness of the screed, but only half. They divide the screed into equal shares, 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 \u200b\u200bthe reinforcing mesh if your screed is reinforced.

Expansion joints are provided in all types of screeds in rooms with an area of more than 30 m. Accordingly, the maximum area of \u200b\u200bthe 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. At one time, intermediate expansion joints must be cut out in the corridors , and the distance between these types of seams should be less than six meters.

If the top coat is selected ceramic tile or porcelain stoneware, then the notches from the expansion joints should be comparable between the tile joints.

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

Wall joints can usually be left empty. In the event that you decide to repair them, it is recommended to use only soft materials.

Reinforcement

Another of the common mistakes in which the screed cracks is incorrect, poor-quality reinforcement. If you decide to purchase reinforcement and make a high-quality foundation, then it should be in the body of 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 transfer so much money for nothing, but fiber reinforcement will be very effective. The reinforcement must be in the body of the concrete, but not under the screed. By and large, the use of 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 the beginning of resuscitation work should carry out a number of procedures to simplify and speed up work to eliminate the crack.

First of all, determine for what reason they were formed. If the screed was not performed, you will determine the presence of expansion joints and how the floor was poured.
If cracks in the floor screed look like segments scattered over the entire base, they are repaired with epoxy adhesives using the “forced closure” technology.
If cracks in the underfloor heating screed appeared due to the absence of an expansion joint between rooms or along the walls, they should not be repaired without making this very joint.

Before starting repair work, you must 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 repairing cracks in the floor screed, you will need to see what the degree of damage is and highlight the areas that need to be repaired.

You can easily find visible cracks. But you will have to look for hidden voids 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.

Repairing small cracks

Minor cracks in the floor screed are recommended to be cut with a grinder up to 20 mm. Remove debris after processing with a conventional vacuum cleaner and wipe the remaining dust with a damp cloth and allow the surface to dry before repairing. After drying, the surface is ready for repair.

Cunning! If the premises are non-residential, it is recommended to check the cracks for possible subsequent deformation. To do this, cracks in the floor screed are sealed over with sheets of paper and left for a while. If the sheet is torn, the resulting crack continues to expand and repairs require a more difficult approach.

Repairing large cracks

It is not for nothing that a crack is one of the most severe damage to a screed, so repairing cracks in a floor 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.

In order to eliminate cracks in the floor screed, it is necessary to make quite a few 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 undertake its repair 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 dried floor. Ground preparation also plays a big role. If it absorbs moisture, the screed will need to be reinforced anyway.

And the most important thing! No need to speed up the drying of the solution by artificial drafts or warming up the room. With such actions, moisture evaporates unevenly and quickly, cracks will also appear from this.

The screed on the floor should dry itself, gradually and at an equal 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.

By following these simple rules when pouring screed cracks on your floor will never be

Video instruction

Why does the screed crack?

Cracks in the floor screed - a defect or an acceptable error. Many builders argue that if the crack is not wide, and a coating is 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 an incorrect filling or an unreliable foundation, then the destruction will continue, the solution will crumble, defects will increase, and subsequently the finishing layer will be broken, 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 on gypsum-containing mixtures.

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

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

It will help to seal cracks in the screed if we are talking about an old coating, or if cracks appeared in problem areas: this is the border different materials bases, pipes or communications, cracks over lighthouses.

For repair, a mixture of 6 parts of pure sand and one part of cement is prepared, and kneaded with PVA glue. Cracks are embroidered to the base, and everything that can be crumbled is selected. The surface is primed and puttied with a repair mixture. It is very important to level it before solidification. It is worth noting that the repair of cracks in the screed is just an opportunity to get a more even surface, and it does not guarantee its integrity and strength in the future.

To avoid problems

‘’yandex’’ is not found

Another important point is the drying of the solution. Many try to speed it up by creating drafts or heating the room. Because of this, moisture evaporates unevenly and too quickly, which also leads to cracking. Cement-sand mortars should dry 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 damp burlap).

Any repair work in the house or the construction of a building is not complete without the use of a cement mixture. It is impossible to establish a foundation, pour a floor, plaster walls without this material.

The cement composition, made in accordance with the technology and in compliance with the necessary proportions, begins to solidify from the first minutes of pouring into a mold or formwork. But it fully builds up its strength over a certain period of time. During this period, the mass is not able to withstand a significant load. A flooded screed can crack and collapse.

The usual curing time is 4 weeks. Strong foundations under high-rise building, industrial facilities are given 3 months for drying. A thin screed, for example, under laying a tiled floor or a concrete path, 72 hours is enough.

The mortar goes through two stages:

Grasping. It lasts from 1-2 to 24 hours from the moment of mixing. The mass retains a mobile state, preventing further work from being carried out.
hardening. According to SNiP, it occurs within 30 days from the moment of filling. This norm implies drying, allowing you to start a new stage of construction or finishing the building. Completely this process comes to an end after at least 1 year.

Different brands do not harden for equal periods of time. At a temperature of +10 ° C and sufficient humidity, cement M400 allows you to continue working after 12-15 days, and M500 - already for 9-10 days.

Setting conditions for concrete

Factors that affect drying time include:

Temperature. At +20+23°C this process takes 1-3 hours, at 0°C it takes up to 20-24 hours. Frozen water completely stops hardening.
Humidity. The optimal indicator is 65-70%. The level of moisture in the air rises by spraying the surface, covering it with a wet cloth or film, filling the screed with wet sawdust or straw.
brand of cement. Compositions differing in density and other characteristics require different times to gain brand strength. M400 sets in 1.5-2 hours, M500 takes 1 hour.
Compliance with filling technology. It is important to prepare the mixture by thoroughly mixing the components in accordance with the required proportions and compact the filled layer.
Care of the screed during hardening. During this period, direct sunlight and drafts are not recommended.

The introduction of various additives speeds up the process and increases quality characteristics structures. The most common type - Portland cement - incorporates minerals, an increase or decrease in the proportions of which affects the hardening time.

Ways to influence the drying of cement

Often, a sharp change in the weather does not allow to complete the concreting on time and ensure the necessary conditions for quality coverage. For such situations, there are the following methods:

When the ambient temperature drops below the permissible limits, the prepared solutions are heated different ways: electricity, steam, greenhouse equipment and others. The tricalcium silicate additive during the hydration of the mixture increases its temperature.
The need to finish concreting at low temperatures causes the use of antifreeze additives based on calcium and sodium salts.
The change in the composition is reflected in the duration of the screed hardening: sand and crushed stone reduce the time, porous fillers (slag and expanded clay) prolong the process.
Slow down the setting of surfactants. For example, adding bentonite or a soapy solution delays the evaporation of water and prevents the surface from drying out.

14 days after pouring under satisfactory conditions temperature regime and humidity, the M400 brand gains 50% strength, and the M500 - 75%. The use of various additives allows you to adjust the construction time.

When interacting with water, it hardens and turns into the so-called cement stone. However, few people know the essence of this process: how it hardens, why it hardens, what awareness of the ongoing reaction gives us and how we can influence it. To date, understanding all stages of hydration allows scientists to invent new additives in concrete or cement, in one way or another affecting the processes occurring during the setting of cement and the hardening of a concrete or reinforced concrete structure.

In general, there are two main stages in the process of curing concrete:

concrete setting a rather short stage occurring in the first day of the life of concrete. The setting time of concrete or cement mortar depends significantly on the ambient temperature. At the classical design temperature of 20 degrees, the cement begins to set approximately 2 hours after the cement mortar has been mixed, and the setting ends approximately three hours later. That is - the setting process takes only 1 hour. However, at a temperature of 0 degrees, this period stretches to 15-20 hours. What can I say, if the very beginning of cement setting at 0 degrees begins only 6-10 hours after the concrete mixture has been mixed. At high temperatures, for example, when steaming reinforced concrete products in special chambers, we accelerate the setting period of concrete up to 10-20 minutes!
During the setting period, concrete or cement solution remain mobile, they can still be influenced. This is where the thixotropy mechanism comes into play. While you "move" the concrete that has not set to the end, it does not go into the hardening stage, and the cement setting process is stretched. That is why delivery of concrete on concrete mixers, accompanied by constant mixing of the concrete mixture, is able to preserve its basic properties. If you wish, read the details about the main properties and concrete composition.

From personal experience, I can recall extraordinary cases when our mixers with concrete stood and "threshed" at the facility for 10-12 hours, waiting for unloading. Concrete in such a situation does not harden, but some irreversible processes occur that significantly reduce its quality in the future. We call it concrete welding. Such events are especially critical in the summer in the heat. Remember the shortened setting time of cement when high temperature, which we talked about above. Managers and dispatchers of the BESTO Company try to avoid such incidents, but sometimes unforeseen situations occur, mainly related to the collapse of low-quality formwork. Concrete is spilling, everyone is running around trying to collect it, restoring the formwork, and time goes by, and concrete mixers with concrete that have not yet been unloaded stand and thresh. Well, if there is where to redirect, but if not? In a word, trouble.
hardening of concrete This process occurs immediately after the end of the setting of the cement. Imagine that we are using concrete pump finally laid the concrete in the formwork, he safely grabbed, and then the process of concrete hardening actually begins. In general, the hardening of concrete and the curing of reinforced concrete products takes not a month, or two, but years. The 28 day period is regulated only in order to guarantee a certain brand of concrete for one period or another. The graph of curing of concrete or reinforced concrete products is non-linear and in the first days and weeks the process is most dynamic. Why so? And just let's figure it out. It's time to talk about the process of cement hydration.

Mineralogical composition and hydration of cement

We will not analyze the stages of obtaining Portland cement here, for this there is a special section describing cement production in details. We are only interested in the composition of cement and its main components that react with water when mixing cement mortar or concrete. So. Four minerals obtained as a result of all stages of cement production are considered as the basis of Portland cement:

C3S tricalcium silicate
C2S dicalcium silicate
C3A tricalcium aluminate
C4AF Tetracalcium aluminopherite

The behavior of each of them at different stages of concrete setting and hardening is significantly different. Some minerals react with mixing water immediately, others a little later, and still others - it's not at all clear why they "hang around" here. Let's look at them all in order:

C3S tricalcium silicate 3CaO x SiO2 a mineral involved in the process of increasing the strength of cement over time. Without a doubt, it is the main link, although, during the first days of the life of concrete, tricalcium silicate has a serious faster rival C3A, which we will mention later. The process of cement hydration is isothermal, that is, a chemical reaction accompanied by the release of heat. It is C3S that “heats” the cement mortar during mixing, stops heating from the beginning of mixing until the moment of setting, then heat is released during the entire setting period, and then a gradual decrease in temperature occurs.

Tricalcium silicate and its contribution to the strength development of concrete is most significant only in the first month of the life of a concrete or reinforced concrete structure. These are the same 28 days of normal hardening. Further, its influence on the set of strength of cement is significantly reduced.

C2S dicalcium silicate 2CaO x Si02 begins to act actively only a month after the cement has been mixed in the concrete mixture, as if taking a shift from its tricalcium silicate brother. During the first month of the life of concrete or concrete goods, he generally plays the fool and waits in the wings. This period of idleness and relaxation can be significantly reduced through the use of special additives in cement. But, its action lasts for years, during the entire period of increasing the strength of reinforced concrete, reinforced concrete or concrete.

C3A tricalcium aluminate 3CaO x Al2O3 the most active of the above. He begins vigorous activity from the very beginning of the process of grasping. It is to him that we owe for the set of strength, during the first days of the life of concrete or reinforced concrete. In the future, its role in hardening and curing is minimal, but in speed it has no equal. You can’t call him a marathon runner, but perhaps a sprinter, yes.

C4AF Tetracalcium aluminopherite 4CaO x Al2O3 x Fe2O3 this is just the one that - "it is not clear why he is hanging around here at all." Its role in the set of strength and hardening is minimal. A slight effect on the set of strength is noted only at the very latest stages of hardening.

All of the listed components, when mixed with water, enter into chemical reaction, due to which there is an increase, adhesion and precipitation of crystals of hydrated compounds. In fact, hydration can also be called crystallization. So it's probably clearer.

The BESTO company supplies ready-mixed concrete and mortar, made using the most modern additives, which make it possible to obtain concrete mixes and cement mortars with improved frost resistance, water resistance, mobility, etc. Modern dosing and concrete mixing equipment helps to achieve the best results in terms of the uniformity of the composition of the concrete mixture or cement mortar.

I hope I didn't hydrate your brains with my silicates and aluminates. With tricalcium greetings, Eduard Minaev.