Concrete is indispensable in construction when laying the foundation of future houses and structures. It often happens that it begins to crack in the process of drying. What could be the reasons for this not very pleasant event, we will consider in this article.
In the process of concrete drying, structural (non-structural) cracks appear. The reason for this is internal reactions occurring in concrete. It becomes strong and hydration occurs. If at this moment the causes are not eliminated, then structural cracks will appear, which pose a threat to the entire structure as a whole. In the latter case, the concrete will continue to crack from external factors.
If the concrete began to crack when it was first poured, then the whole thing is in excess moisture, which forms plastic shrinkage. Moisture evaporates from the surface, the mass is compacted and the solution is unevenly laid. This is not scary, since the inner layers usually remain the same size, and the top layer is covered with cracks.
A much more important and serious problem associated with compaction and shrinkage, when the concrete has already set, but you did not use a vibrating tool during concrete placement, and under the influence of gravity, the deformation of the structure continues.
Also, during the drying period due to temperature fluctuations, due to continuous variable expansions and contractions of substances, the concrete laying becomes uneven, which leads to cracks in already hardened areas.
Corrosion cracks are associated with rusting of reinforcing bars and mesh. Steel increases in volume and this leads to concrete rupture at any stage of drying.
Correct calculations of the composition of the mixture for concrete, the inadmissibility of errors in the calculation of soil movements, natural disasters, impacts, etc. are important. It is difficult to predict such reasons, but it is possible.
- It is necessary to follow the exact recipe for the preparation of the solution. Often people try to save money, but this is not the case when saving will be useful.
- Concrete must be poured in accordance with all standards, using vibrocompactors, ventilation, etc.
- It is not enough to lay concrete, you still need to be able to save it at the moment of drying. If necessary, protect from moisture, insulate the surface, heat, etc.
After pouring the foundation, the concrete needs time to gain strength, this moment was described in detail in the article about. However, it is not enough just to wait for the time necessary for this process, it is also necessary to provide optimal conditions for this period - temperature and humidity. This is precisely the care of freshly laid concrete during its maturation. Without care, concrete may not gain strength, and cracks will appear on its surface.
Why does concrete crack when it dries?
Dried concrete has a smaller volume than wet concrete, so when it dries, concrete shrinks - it decreases in volume. When a layer of concrete is thick enough, it dries unevenly: inside it is still wet, but on the surface it is already dry. While the deeper layer of concrete has not yet dried out and retains the same volume, the surface layer of concrete decreases in volume and surface tension is created between different sections, the concrete crust tends to shrink, decrease, it is not enough to cover a larger deep layer and it cracks.
In the same way, cracks can also form due to the temperature difference in different layers of concrete: one layer expands with a higher temperature, while the other has a low temperature and contracts; a crack appears at the junction of such changes.
Purpose of concrete care
In order for concrete not to crack, it needs care: it is necessary to ensure the uniformity of its temperature and humidity throughout the volume. The smaller the differences, the better the foundation will turn out.
It is also necessary to provide enough water so that all the cement reacts with it and binds the gravel and sand. If the concrete dries evenly but in conditions of lack of water, then part of the cement will remain unused. It is most reasonable to pour concrete in the summer, and at high temperatures and under the influence of the sun and wind, the water from the concrete will evaporate. These losses must be replenished - watering the concrete.
When pouring in winter, you need to use warm water, and maintenance is also very complicated: you need to constantly heat the concrete, because at low temperatures maturation slows down, and at negative temperatures it stops completely. With independent construction, it is unrealistic to create such conditions, so this option is not even considered in this article.
concrete care plan
Concrete care should be started within 2-3 hours after its completion. If possible, it is necessary to close the surface of the concrete from direct sunlight: this will avoid heating, and therefore increased evaporation of water. In the case of a strip foundation, this is quite simple to do, but when pouring a monolithic slab, the surface can be very large, and it is problematic to close it from the sun. In addition, the coating will need to be periodically removed in order to water the concrete with water. Therefore, when caring for a slab foundation, you can simply water it more often to prevent overheating in the sun.
How to water concrete?
Concrete should be watered with water through a sprayer, sprinkler or from a watering can, but not with a direct stream. The watering schedule is something like this:
- 2 hours after pouring and in the first 5-7 days of ripening - every 3-4 hours;
- in the next 7 days 2-3 times a day.
In hot and sunny weather, you can water more often as the concrete surface dries. If the concrete hisses during watering, a reaction occurs - this is already a sign that the concrete no longer has enough water and it absorbs it.
- After you have decided on the type of foundation, the place and depth of its laying, you have carried out all the earthworks (dug a trench for the foundation, made a sand and gravel cushion), installed the formwork, strengthened its walls with supports, assembled the reinforcing cage, installed it in the formwork and it was securely fixed there, it was time for the last and most important stage of laying the foundation - its pouring.
- The main characteristic of concrete is its compressive strength - this characteristic is reflected in the brand of concrete. But branded strength is not achieved immediately by concrete, concrete gradually gains strength within four weeks.
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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. They are located in the zones of 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 of strip foundations, subsidence or heaving of the soil, “flimsy” or poorly fixed formwork, loading of reinforced concrete products to the point of acceptable 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.
The causes of 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;
- Quick drying of the surface under the influence of high air temperature and direct sunlight;
- 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 "hair" cracks remain on the surface, which are acceptable and can be easily eliminated with grout or.
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, one should carefully study and strictly follow the theoretical and practical recommendations of GOST and specialists on: choosing the brand and type of cement, type and type of reinforcement, concrete composition and other features of concrete work.
Why does concrete crack after pouring?
It is difficult to imagine all types of modern construction activities without concrete work. Concrete is traditionally used for pouring monolithic structures, reinforced concrete elements, foundations. The concrete mass is the bearing part of any object, which determines its stability, strength, service life. Naturally, a serious concern for developers and homeowners arises when cracks appear in concrete.
Dangerous symptom
The appearance of cracks affects the further operation of the construction object:
- affects the ability of the structure to perceive the acting loads;
- significantly worsens the appearance;
- reduces the life of the structure;
- reduces strength properties.
Often, with improper preparation and pouring, concrete cracks and crumbles after drying.
The formation of any cracks is a concern. After all, they signal a violation of the strength of the building, and their results cannot always be predicted. In some cases, in new buildings, it is possible to prevent cracking of the monolith if the technological process of pouring is observed, and high-quality solutions are used.
Cracks also appear, taking into account the age factors of the construction object, when the concrete mass is covered with a network of dangerous "webs" over time. They are the result of internal stresses of the structure or evidence of deformation under the influence of external loads.
Monitoring the state of cracks, timely repair allows you to extend the life of structures. Let us consider in more detail why concrete cracks after pouring, how defects that appear on the surface can be repaired, and analyze the causes of their occurrence.
Defects in concrete structures are manifested as a result of the impact of forces on the concrete monolith and under the influence of internal stresses. Let's take a closer look at these two groups.
External factors
Deformations are the main reason for the formation of cracks and defects on the surface under the influence of external forces.
Such cracks are a serious threat to the bearing capacity of the structure, up to its destruction.
Cracks in concrete that occur as a result of external factors are associated with the following processes:
- Bend. They are formed perpendicular to the steel reinforcement bars, which are stretched under the influence of bending loads. Traces of deformation originate in the middle of the maximum deformed area and stop where there is no influence of bending processes.
- Shear, which acts diagonally with respect to the longitudinal axis of the bar. Externally, such defects coincide in direction with the transversely applied force and originate from traces of bending deformations.
- Structural tensile stresses that may coincide with the axis of the reinforcing bars or act parallel to them. Traces of central tension are transverse in nature, located perpendicular to the longitudinal axis of the structure.
- Violation of the requirements for the correct installation of reinforcing bars located at the corners of the tape base. The installation of anchors, performed with deviations, leads to delamination of the protective layer protecting the reinforcement, which manifests itself in the form of cracks located parallel to the embedded reinforcement elements.
Expert opinion: Cracks in concrete after pouring
There are many factors that can affect the formation of cracks in concrete after pouring. Most often, concrete begins to crack due to heat and sudden changes in temperature. It is possible to prevent the problem if freshly poured concrete is sprayed with water at a temperature slightly below the surface temperature of the concrete. Additionally, concrete can be covered with a film or a layer of wet sand.
Dmitry Orlov
Internal processes
The result of the internal deformation of the concrete mass is the stress associated with the following points:
- significant cooling of concrete due to a sharp temperature drop in depth and on the surface. Such defects occur when the mixture is quickly cooled with water, air and, at the same time, during the maturation of the cement, a significant heat release occurs. If the value of internal stresses is greater than the strength characteristics of the concrete mass, then surface cracks appear, extending into the depth of the monolith. When the temperature changes, they can disappear and reappear;
Shrinkage cracks are most often the result of processes occurring inside the hardening concrete monolith.
- accelerated drying under the influence of direct sunlight, high ambient temperature, wind.
Regulatory documents allow the presence of surface, shrinkage cracks up to 0.1 mm wide in concrete structures during cyclic freezing and thawing. They do not affect durability, but need timely termination.
Why does concrete crack?
The main causes of cracking of the tape type foundation are:
- Processes occurring in the soil associated with its subsidence or heaving.
- Excessive concentration of moisture in the soil, causing it to rise.
- Soil compression associated with the operation of the drainage system.
- Violation of the rules for the installation of formwork or errors in its calculation.
- Premature application of forces to a concrete mass that has not reached operational strength.
- Errors in choosing the optimal section of steel reinforcement.
- Violation of the placement points of steel rods.
- The presence of air cavities associated with poor sealing.
- Failure to comply with the exposure time of the solution.
- Deep corrosion of the reinforcement inside the monolith.
- Shrinkage and temperature processes occurring in the initial and final stages of mortar hardening.
- Causes of a seismic or man-made nature.
Comments:
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?
Causes of cracks
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 execution of design calculations only to reputable firms and not deviate from these calculations either during 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.
In other words, these damages appear in concrete due to the drying and shrinkage of the concrete mixture after pouring. 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
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 mortar actively loses volume, while the middle and lower layer of the 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.
Similar to the described phenomena 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
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 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 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
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, be sure to compact 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 completely set.
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