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Based on the assigned grade of concrete, you can understand what maximum load in kgf/cm2 a particular product is designed for. Of course, all reinforced concrete products are released from production already with a tempering strength, which is summer period should be at least 70% of the brand name, and in winter - at least 90%. Therefore, construction organizations can immediately put the product into operation.
But consumers who buy ready-mixed concrete for pouring a foundation or want to make it themselves will be interested to know How many days does concrete take to gain strength? and how to achieve this quickly?
28 days for brand control
For brand control, technologists use a period of 28 days. The first week, in warm weather, concrete intensively gains its strength, about 70 percent of the actual one. This occurs due to the interaction of cement grains and water, resulting in the formation of potassium hydrosilicates. The process may take more than one year. For example, for some reinforced concrete products for which concrete grade M 200 was specified, after a few years the strength reached concrete grade 400.
When to remove the formwork?
If you are pouring the foundation yourself, it is recommended to remove the foundation formwork after three days, but it is better to load the concrete structure after a week. Under winter conditions, strength growth is significantly reduced. If the structure is not covered, the concrete may freeze and not gain strength at all. For the summer period it is also required special care, that is, constant hydration and shelter from direct sun rays so as not to cause the concrete surface to dry out.
Heat and moisture treatment accelerates the strength gain of concrete
How many days will it take for concrete to gain strength if it is subjected to heat and moisture treatment? In a few hours. If the temperature in the steaming chamber is 80-90 degrees, then the structure gains strength to 60-70 percent of the original value after 12-14 hours. But under such conditions, concrete quickly loses water and begins to dry out. Therefore the most best concrete it is considered to be the one that gained strength under natural conditions.
To quickly gain strength, you can use special additives for concrete, which are used in the process of preparing the mixture. Dosing is based on the amount of cement. With the use of additives, concrete can gain grade strength in two weeks. Again, if hardening occurs in the warm season. For winter, antifreeze additives are used that maintain a positive temperature in the concrete during the setting period.
When pouring yourself strip foundation you can get a rough idea, How many days will it take for concrete to gain strength?- per month. Therefore, try to maintain this interval in order to prevent unpleasant consequences when the structure is loaded in the future.
Concrete strength gain schedule
- Stages of solution hardening
- What influences the gain of maximum strength?
- Acceleration of strength gain
The key stage of repair and construction work is drying the concrete. The poured composition hardens and gains strength over several weeks. The process is supervised by engineers and requires constant monitoring.
Specialists ensure compliance with standards and, if necessary, make adjustments to the schedule. The material is sensitive to temperature fluctuations and has a “seasonality coefficient” - in winter, concrete work is carried out using heating systems. To determine how long concrete takes to dry, various factors are taken into account.
Stages of solution hardening
Concrete work is part of any construction, from country houses to industrial and special ones. The material is used at various stages of construction of objects, for pouring foundations and load-bearing structures, flooring devices.
Builders are successfully using the property cement-sand mixture with the addition of crushed stone – the ability to take the formwork’s shape. They value the strength and durability of the material, the drying time of which is about 28 days.
Depending on operating conditions and the quality of the composition, the estimated service life of objects reaches 250 years, and on average is estimated at 50-100. For modern construction This is a solid period - technologies are constantly being improved, new materials and design solutions appear.
Strengthening continues to be emphasized Special attention and control each stage:
- Freezing. Occurs in the first hours of the “life” of the composition. The solution is delivered to the work site in a concrete mixer or prepared on site to maximize the preservation of the required properties.
Hardening time in summer at temperatures above 20°C is about an hour, in hot weather - 15-30 minutes. At “zero” – it begins 6-10 hours after preparing the mixture and extends to 20 hours from the moment of pouring;
- Hardening. The main stage takes 7-14 days. During this period, the structure gains up to 70% of the calculated value, which depends on the grade of concrete;
- Control value according to GOST 18105-86. The standard curing time is 28 days. Experts compare the results obtained with the standards in a special table.
Concrete strength development - temperature, humidity, hydration
There is a direct relationship between the hardening of the solution in different conditions and reaching the maximum value.
What influences the gain of maximum strength?
Absolute majority concrete works performed outdoors.
Weather conditions and temperature schedule are the key parameters that determine how long the solution hardens.
During the warm season, the mixture matures and gradually hardens naturally. The process depends on physical and chemical properties composition and has slight differences associated with the brand of concrete.
In the autumn-winter period, strength gain is provided in two ways:
- Antifreeze additives. Used to preserve the properties of the prepared solution. Special substances prevent water from freezing and loss of quality, making it easier to fill the structure and level the surface;
- Electric heating Performed using several methods common essence– ensuring uniform heating of the concrete thickness during the period required to gain strength.
At low temperatures apply PNSV wires or “implant” electrodes into the material, after which the voltage is connected. Less commonly used as heating element the formwork itself is covered with special mats.
The work requires compliance with electrical safety rules and is carried out in accordance with SNiP 3.03.01-87. If minimum temperature reaches 0°C, and the average per day does not exceed 5°C, concreting is initially planned with heating of the poured structure. If necessary, PMD is included in the solution.
Acceleration of strength gain
Concrete compositions are classified depending on their compressive strength. Light solutions are used for auxiliary work or structures that do not experience load.
Concretes M-200 – M-400 are considered basic. The compositions are used in the construction of most objects civil engineering. Solutions of a class higher than M-500 are intended for special objects and structures of increased strength.
The basic curing speed is calculated based on grades M-200 - M-300. The figures are based on a time period of four weeks. In practice, the required period is reduced under certain conditions:
- Use of special additives. These are auxiliary components that are mixed into the solution during preparation. Application reduces the time of complete hardening to 14 days. Such work is carried out in the summer - anti-frost additives do not have this property;
- Hydration.
In dry, hot weather, water quickly evaporates from the drying composition, which negatively affects the strength development schedule and the quality of the structure. Constant moistening helps to create conditions under which optimal hardening dynamics are achieved.
After completion of the calculation period, concrete tests and control measurements are carried out. If the indicators meet the standards, proceed to the next stages of work.
In order for construction to be completed according to plans, it is recommended to develop detailed project documentation taking into account the design features. In the calendar schedule, concrete work is planned, if possible, in the most favorable season.
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Concreting of strip and slab foundation at home is carried out according to long-developed technology. At first glance, there is nothing complicated in the work, but during pouring, during the process and after curing of the monolith, many questions arise related to various nuances. Some of them are so important that failure to comply with them may well lead to some kind of structural damage. For example, how long should it take after pouring before removing the formwork and how long should the concrete be cured before the next stage of work begins? Among experts you can hear different opinions, but the rules still exist.
What are the consequences of untimely removal of formwork?
As you know, to fill a strip or slab foundation of a house, a mushy mixture is used. concrete mortar. After laying it in the formwork, the processes of cement hydration and gradual hardening of concrete begin. To complete them correctly, you need to allocate a certain amount of time required so that the foundation can stand and gain design strength.
If the formwork is removed from the structure immediately after the cement has set, there will be a possibility of the monolith spreading in different directions. A fragile “body” will not only not be able to take loads, but also maintain its own shape. This is especially true for massive foundations.
If the dismantling of the formwork from the strip foundation is carried out after cement mortar will set, but before it gains a certain strength, cracks will appear in the structure. For the underground part of the house, which takes on and distributes all the loads onto the ground, this threatens to split and complete destruction already during the operation of the house.
How long should the foundation stand after pouring? There is no clear answer to this question. The average period of time is determined by 28 days, but in some cases 15-20 days are sufficient. IN difficult conditions deadlines need to be extended.
Professionals assure that the foundation of the house must stand for at least a month before it is loaded.
To prevent the structure from shrinking, warping or collapsing, you must strictly follow the instructions building regulations and technologies for constructing the underground part of the house.
Concrete - setting time and strength gain
The foundation is the support of the building, and therefore does not tolerate negligence, inability and lack of basic knowledge.
How long should the foundation settle?
The deadlines specified in the standards for the concrete structure to be able to withstand do not always correspond to real time. They are influenced by extraneous factors, such as:
- temperature environment;
- humidity conditions;
- presence of precipitation;
- season;
- ground conditions;
- terrain;
- dimensions and type of foundation - strip, slab, columnar;
- design strength of concrete;
- quality of materials;
- presence groundwater Location on;
- construction technology;
- presence of additives;
- magnitude of design loads.
In addition to the above points, situations may arise that affect the period during which the foundation of the house will have to settle before further work. In some cases, the concrete structure is left even for the winter, so that when the soil thaws it is easier to identify defects and correct shrinkage. At the same time, the monolith is securely covered. It is noteworthy that no standard can take into account all the nuances, so the question of how long the foundation of the house will be defended is decided on an individual basis.
When determining deadlines, the most appropriate worse conditions for the site. The stock, in this case, will play a positive role.
How is the foundation of a house built?
The concrete structure is allowed to cure for the first time immediately after pouring. This period lasts up to seven days, during which the surface is watered. The concrete sets and begins to harden. The foundation is covered from above plastic film, but you can also use:
- wet cloth;
- sawdust;
- straw.
Before watering, the polyethylene is lifted, and other materials are wetted from above. They perfectly retain moisture, preventing water from evaporating ahead of time. The duration of hardening of the monolith depends on the time of year. After about a week, and in the hot season - after 10-14 days, watering is stopped, but the cover layer is left until 28-30 days after the end of installation concrete mixture into the formwork. In this way, primary standing occurs, which is quite sufficient for foundations installed on a foundation buried below the freezing level of the soil.
But in practice, there is also a secondary standing. It concerns situations where shallow foundations have to be built on heaving soils. In this case, the hardened concrete structure is left to winter. With the arrival of spring, movements are recorded, and the base is strengthened by adding sand or gravel with obligatory layer-by-layer tamping.
Experts assure that it will be better if the foundation stands without load whole year. It turns out that in the first month after pouring, concrete gains strength up to 70-75 percent, and the remaining 25-30 percent in the next 11 months. From this we can conclude that if the construction time frame allows, then preference should be given to a longer time period. If the period of construction of the building is limited by rigid boundaries, then the installation of the walls of the house begins 28 days after pouring the foundation. Under favorable climatic conditions and the use of lightweight materials in enclosing structures, the period can be reduced to two weeks.
08/22/2016 at 13:08
Construction of reinforced concrete monolithic foundation requires knowledge and understanding 
many important points.
Before pouring the mixture into the formwork, a non-professional in the construction topic should prepare theoretically.
The time taken to dismantle the formwork is of considerable importance. How to control the strength and when can the foundation be loaded?
How long to wait for strength gain
As stated in paragraph.
How concrete gains strength and how to control these parameters
2.5 SNiP 2.03.01-84, concrete of at least M-200 should be used for the construction of foundations. Since BM-100 is used for preparation, the foundation body itself is most often made of M-200 concrete. 
The hardness of the mortar laid in the formwork is influenced by various factors, including the following:
- Correct ratio of ingredients;
- Air temperature;
- Air humidity;
- The period of time from preparation of the mixture to installation;
- Layer thickness;
- Compliance with technology, etc.
Curing is a chemical process that requires optimal conditions, the most important are heat and humidity. Depending on the ratio of these indicators, the process of achieving standard strength characteristics lasts up to 28 days.
If it is excessively hot, that is, the air temperature is above 25 degrees, then the mixture will crack, the moisture necessary for the normal course of the hardening reaction will quickly evaporate, and at temperatures below +5 degrees the processes will slow down, which negatively affects the hardening time.
The optimal temperature is +20 degrees Celsius. Already from the first hours, the strength of the mixture begins to increase: after 2.5 hours the mixture will set, but the hardness is still too low for the concrete to hold its shape. The foundation gains strength most intensively in the first week, reaching 70% of the design value. Solidification and hardening lasts up to 28 days.
Concrete setting control
In conditions of concrete work construction companies Quality control is carried out by testing concrete samples using the following methods:
- Compression using special equipment;
- Tapping the array with a Kashkarov hammer;
- Ultrasonic devices (non-destructive method).
To test for stationary machine prepare cubes: from one portion of the mixture, pour samples measuring 10x10 cm in quantities of at least 3, marking the samples themselves, as well as recording the date and time on them. 
The cubes are transferred to a special construction laboratory for testing, where, based on the load at which the cubes collapsed, calculations are made and the strength of the concrete is determined, taking into account the age of the cubes. This method is considered accurate.
Tapping with a hammer gives approximate results and is an imprecise method. There are hammers different types, and the device designed by Kashkarov is notable for the fact that the impact force is not reflected in the final strength readings. The hammer itself weighs 400-800 g.
Strength indicators are determined by the marks remaining on the concrete in accordance with the table given in the regulatory literature.
Ultrasonic devices are based on determining the speed of ultrasound passing through the thickness of concrete: the denser the concrete, the lower the speed. In addition to the strength value, the ultrasonic method makes it possible to determine the presence of voids, cavities in the foundation mass or other structural element.
Special methods should be used by professionals with experience in construction. laboratories, amateurs will not be able to determine the exact value of the material’s resistance to compression, that is, strength.
IN artisanal conditions The setting test is carried out as follows: simultaneously with laying the mixture into the formwork, a separate form of arbitrary size (with a plan size of 10 × 10 cm), but preferably the same height as the main structure, is poured.
On day 2, you need to remove the formwork on one side and see if the concrete holds its shape and how well it has set. If necessary, after a day, remove the formwork from the other side of the sample and analyze the dynamics of setting. You can try to break one of the samples to make sure it is hard.
It is important to understand that the sample is smaller in size than the foundation mass, and in a small volume the concrete hardens faster. Once you are sure that the sample has set, you should give the array an additional 2-5 days to get the desired result - a firmly hardened, set foundation.
When to remove formwork
Removal of formwork can be carried out in case of urgent need for 3-5 days, but it is better to wait 7-14 days. 
Well-set concrete that has gained 30-70% strength retains its shape and does not chip when disassembling the formwork.
Stripping is permissible in early dates, if shields and boards are needed to perform work on another occupation or on the next object.
In private construction, it is reasonable not to rush and allow the mixture to gain the required strength indicators, which will take 2 weeks.
How long can the foundation be loaded?
To put a load on the foundation means to perform next stage construction of a building, in the case of a foundation this is the construction of walls:
The load is acceptable when the concrete reaches 100% of its design strength properties. In this case, there is no need to fear deformation or destruction of the foundation, since the structure is already able to absorb loads from walls, ceilings, and roofs. 
This period occurs after 28-30 days from the moment the concrete is poured into the formwork.
This period can be reduced if you apply special means - chemical additives, or technological techniques such as warming up in the cold season, watering or covering with wet mats in the summer when it is hot.
If concrete sets naturally, it is better not to rush and remove the formwork no earlier than after one or two weeks, and build walls at least 4 weeks old.
There is nothing complicated in the design of the foundation, but it is better when it is done by professionals who have both experience and technical means concrete hardening control.
If, after all, the formwork is poured on your own, then it is better to remove the formwork after 7-14 days, and subject it to loading no earlier than 28 days from the date of pouring.
How does concrete gain strength?
The main characteristic of concrete is its compressive strength - this characteristic is reflected in its grade. But brand strength is not achieved immediately; concrete gradually gains strength over four weeks. Therefore, after pouring concrete, you need to wait some time.
Strengthening concrete
The most intensive strength gain occurs in the first 5-7 days after pouring - during this time it gains about 70% of its brand strength. Subsequently, its strength increases and reaches vintage after 28 days of ripening. Until this time, it is not recommended to load the concrete structure, i.e. if this is a foundation, then you can put a house on it only after it has gained its brand strength. Concrete gains minimum strength after 7 days; after this period, the formwork can be dismantled.
The graph of concrete strength gain over time is shown in the figure:
Concrete maturation schedule different temperatures.
The graph shows the dependence of the strength of concrete on time at different temperatures of its maturation: from 30 to 80 degrees. Strength is shown as a percentage of brand strength. However, these are theoretical data obtained in laboratory conditions; in practice, it is unrealistic to withstand such conditions: the temperature changes during the day and it is absolutely certain that it will not be constant and equal to 30 degrees. Therefore, when self-construction foundation, it is better to play it safe and let the concrete stand for a month, and only then dismantle the formwork and continue construction.
Cement is used as a binder in concrete, it chemical reaction with water leads to the appearance of hard rocky new formations, which bind together particles of filler - crushed stone and sand. Initial period This reaction is called setting, during which initial bonds between filler particles are formed in the concrete. Then a set of strength occurs when these bonds are strengthened. In order for this chemical reaction to occur, water is needed. But since the maturation of concrete is a long process, the water initially contained in the concrete mixture has time to evaporate. To prevent this from happening, the surface of the concrete structure is covered with plastic film or roofing felt, and also watered. It is important that the concrete dries evenly throughout the entire volume.
During the cold season, the water contained in the concrete mixture may freeze and the maturation of concrete will stop. Moreover, when freezing, the water will increase in volume and begin to destroy the concrete from the inside. At temperatures below 10 degrees, the strength gain slows down very much. Therefore, when pouring a concrete mixture at low temperatures, it must be heated throughout its maturation. Of course, with self-construction this is impossible (or at least very difficult), so you need to pour concrete with your own hands in the summer. The temperature required for its ripening is 20-25 degrees or higher.
The time it takes for concrete to gain strength can be reduced by using special additives that speed up this process. Such fast-hardening concretes gain strength in two weeks, but during self-construction their use is difficult, because they not only ripen faster, but also set faster. This means that after preparing such a quick-hardening concrete mixture, the time for pouring it will be significantly less. Another way to achieve accelerated maturation of concrete is to increase the temperature: the graph shows that the higher the temperature, the goes faster increase in strength. However, it is unrealistic to create such conditions during independent construction.
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Curing
During the maturation of freshly laid concrete, it requires care: it is necessary to ensure optimal temperature and humidity so that it gains its design strength and does not crack when drying.
Concrete for foundation
Concrete is stone material, which is formed as a result of hardening of the concrete mixture. The concrete mixture for pouring a monolithic foundation consists of cement, sand, gravel and water mixed in certain proportions.
Pouring the foundation of a house: how to properly pour a monolithic foundation?
After you have decided on the type of foundation, the location and depth of its laying, you have carried out everything earthworks(dug a trench under the foundation, made a sand and gravel cushion), installed formwork, reinforced its walls with supports, assembled reinforcement cage, installed it in the formwork and secured it securely there, it was time for the last and most important stage laying the foundation - pouring it.
Calculation of the amount of concrete for the foundation
The initial data for calculating the amount of concrete for pouring the foundation is the type of foundation (slab, strip, columnar) and its configuration. The type of foundation and parameters are selected depending on bearing capacity soil and foundation loads.
Brand and class of concrete
The main characteristics of concrete are its grade and strength class. A table of the relationship between brand and class is given in this article.
Date of publication: 10/29/2010 15:57:26
Concrete hardening
Concrete strength gain (in hours)
| Hardening time, hours | |||||||
|---|---|---|---|---|---|---|---|
| 0°C | 5°C | 10°C | 15°C | 20°C | 25°C | 30°С | |
| 4 | 6 | 7 | 8 | 10 | 12 | 13 | 14 |
| 8 | 10 | 12 | 13 | 16 | 18 | 20 | 22 |
| 12 | 13 | 16 | 18 | 21 | 23 | 25 | 27 |
| 16 | 16 | 19 | 22 | 24 | 27 | 30 | 32 |
| 20 | 18 | 21 | 24 | 27 | 31 | 33 | 36 |
| 24 | 20 | 23 | 27 | 30 | 34 | 37 | 39 |
| 28 | 22 | 25 | 29 | 32 | 37 | 30 | 42 |
| 32 | 23 | 27 | 31 | 34 | 38 | 42 | 45 |
| 36 | 24 | 28 | 32 | 36 | 40 | 43 | 47 |
| 40 | 25 | 29 | 33 | 37 | 42 | 44 | 48 |
| 44 | 25 | 29 | 34 | 38 | 43 | 46 | 49 |
| 48 | 26 | 30 | 34 | 39 | 43 | 47 | 50 |
Concrete strength gain (in days)
| Hardening time, days | Concrete curing temperature | ||||||
|---|---|---|---|---|---|---|---|
| 0°C | 5°C | 10°C | 15°C | 20°C | 25°C | 30°С | |
| concrete compressive strength % of 28-day | |||||||
| 1 | 20 | 23 | 27 | 30 | 34 | 37 | 39 |
| 2 | 26 | 30 | 34 | 39 | 43 | 47 | 50 |
| 3 | 30 | 35 | 41 | 45 | 50 | 52 | 56 |
| 4 | 34 | 40 | 46 | 50 | 55 | 58 | 63 |
| 5 | 39 | 44 | 51 | 55 | 60 | 63 | 68 |
| 6 | 42 | 48 | 54 | 59 | 64 | 68 | 72 |
| 7 | 45 | 52 | 58 | 63 | 68 | 72 | 76 |
| 10 | 53 | 60 | 67 | 72 | 77 | 82 | 85 |
| 14 | 60 | 68 | 74 | 81 | 86 | 690 | 95 |
| 21 | 70 | 76 | 83 | 91 | 97 | >100 | >100 |
| 28 | 75 | 83 | 90 | 100 | >100 | >100 | >100 |
What determined its wide distribution is its high strength. The material gains any strength in real conditions, since there are many reasons that contribute to the shortage of the value corresponding to concrete of a certain grade. Knowledge of these reasons and their characteristics contributes to the formation concrete foundations, structures with maximum performance.
Recruitment process
Physico-chemical hydration reactions create new monolithic compounds that give the material properties artificial stone. The new quality is formed over many days (finally after about six months) and ideally the strength properties of a concrete structure should correspond to concrete of a certain class and grade. In terms of time, the process of stone ripening has two successive stages: the initial stage - setting, and the final stage - hardening. Once completed, the concrete can be loaded.
Grasping
Scheme of possible stratification of the concrete mixture: a - during transportation and compaction, b - after compaction; 1 - direction in which water is squeezed out, 2 - water, 3, 4 - fine and coarse aggregates. Concrete is not used immediately after hardening, as it may take some time to transport the material to the site. The mixture should remain mobile, which is facilitated by mechanical stirring of the solution in the mixer of the automixer. Thixotropy allows you to preserve the basic properties of the mixture before it is poured, delaying the start of the initial stage of maturation. However, you should know that if time is delayed or the temperature rises, an irreversible process of “welding” of the solution develops, as a result of which its characteristics are reduced.
The setting time depends on the air temperature - from 20 minutes. until 20 o'clock. The longest duration of this process is in winter at temperatures around 0 degrees. Pouring the foundation during this period will be accompanied by an extension of the initial setting interval from 6 to 10 hours, and the stage itself will extend for 15 to 20 hours.
It is optimal to pour concrete into the mold at 20 degrees. Then, provided that the solution is mixed an hour before pouring, setting will begin after one hour and complete after 60 minutes. Hot weather promotes almost instant setting of the solution in 10 – 20 minutes.
Hardening
The optimal course of hydration during hardening of the solution: temperature range from 18 to 20 degrees, humidity close to 100%. Deviations from these parameters significantly change the hardening rate of the stone. Complete maturation of concrete takes several years.
At the same time, at this stage it naturally changes over time. For example, by the end of the 3rd day it reaches 50%, on the 14th day it reaches 90%, and on the 28th day - 100%. Then, after three months, the strength increases by another 20%, and after 3 years it can become 100% greater than it was by the end of 28 days after mixing.
Strengthening Features
A decrease in the temperature of the environment leads to a slowdown in hardening. The zero mark on the thermometer stops the process due to the freezing of water in the stone (the quality of the concrete decreases), and a rise in values resumes it again. The mixture begins to dry out when there is little or no moisture, but this can slow down and stop proper curing, which will prevent the concrete from gaining the desired properties. But autoclave hardening of mixtures is significantly accelerated at elevated temperatures and humidity conditions: 80 – 90 degrees. and 100% humidity, which leads to an accelerated increase in strength indicators. Due to moisture in the air, the strength-building interval of a mortar that is laid openly may be shortened.
Concretes of higher grades (consist of more cement best quality) harden and gain strength faster, so they should be processed more quickly. In the interval from the 3rd to the 10th day after laying, the standard strength gain of concrete is ensured close to ideal conditions excerpts. In warm weather, the solution is covered with moisture-absorbing materials, through which the stone is moistened around the clock 6 - 7 times, and covered with a dense film.
IN sunny weather it is protected from direct rays. In winter, concrete can be artificially heated from the inside, insulated, heated by heat generators to prevent water from freezing, and insulated from precipitation. An important parameter for continuation of work there is a normatively safe period for the development of strength properties. Table 1 shows the dependence of the strength indicators of concrete on the average daily temperature after the corresponding number of days.
The standard safe period for concrete maturation can be considered 50%, and safe - from 72% to 80% of the brand value, which, for example, is important to know when working on the foundation.
What does strength gain depend on?
Factors that control the set of strength properties of stone include: how much time has passed after pouring, temperature and humidity conditions, quality (activity) and grade of cement, ratio of water and cement in the solution, proportions of components in the mixture, mixing technology, method and speed of installation , quality and regularity of moistening, the presence of plasticizers (hardening accelerator additives) in the mixture in winter, etc. Raising the grade of concrete depends on an increase in the proportion and higher grade of cement in the mixture, the proportions of the components. The brand directly affects the strength gain of concrete. For low grades, critical strength is more important. Table 2 reflects this pattern.
Therefore, strength determines the reliability and durability of the building structure. In cold weather, stone gains strength due to its own heat generation, but to normalize the stone formation schedule, it is advisable to use appropriate additives that accelerate hardening and reduce the temperature at which hydration stops. With them, the mixture gains brand strength after 14 days. A good decision There will also be a change in the components in concrete. For example, aluminous cement gains strength even in cold weather, as it releases approximately 7 times more of its own heat compared to Portland cement.
The shape and grain fraction of natural fillers play a significant role in achieving this property. Their irregular shape and increased roughness provide Better conditions adhesion and quality of concrete. It is known that an increase in the proportion of water in a concrete mixture can lead to separation of the material mass. The consequence of this is also that with a relative increase in the proportion of water in the solution by 60% of the optimal value (w/c = 0.4), a decrease in strength by 50% of the brand value occurs. However, with a water/cement ratio of 1/4, the hardening (hardening) period is halved.
To speed up the process and minimize the curing of concrete, it is advisable to use sand concrete with a low water/cement ratio. An uncompacted concrete solution has a chance to mature only to 50% of the standard strength, even with an optimal water/cement ratio. At the same time manual seal can increase its strength by 30 - 40%, and vibrating rammer increases the strength to the standard 95 - 100%.
During construction, there are cases when 1-6 months have passed after concreting and it turns out that the concrete has not gained the required strength, and its strength class falls short of the norm by 10%-20%.
Most often this occurs after “winter” concreting or concreting in hot weather.
What to do? Dismantling concrete structures and rebuilding them is extremely expensive and requires a lot of time and effort. Leaving it “as is” and not paying attention in the case of load-bearing structures is unacceptable, because further construction works associated with the load of such structures.
There is a solution!
Applicability of the method
The method of secondary launch (activation) of the strength gain of cement stone is applicable to achieve the strength possible specifically for this concrete. That is, the strength that provides:
- the condition of the cement stone that arose during the hardening of concrete (the cement stone should not be unfrozen or crack);
- the actual concrete recipe obtained during concreting, taking into account the actual content and grade of cement, including all water added or included in the concrete mixture.
The working “window” for using the method of secondary launch (activation) of strengthening of cement stone is up to one year from the end of laying the concrete mixture. Moreover, the sooner you start using the method, the more intense the activation of concrete strength takes place and the less time is needed to achieve the required concrete strength. It is optimal if no more than 3-4 months have passed since the concrete mixture was laid (minus the duration of concrete hardening at a temperature below plus 10°C).
For example, when practical application the method increases the strength of the foundation slabs, load-bearing walls, columns up to 24%, which ensured their compliance with the project requirements and allowed further construction work to be carried out as usual.
Strength appears. If you look at the requirements state standards, then you can find information that the strength can vary from M50 to 800. However, some of the most popular grades of concrete are from M100 to 500.
Strengthening schedule
The concrete solution will acquire the desired performance properties within a certain time after pouring. This time interval is called the soaking period, after which the protective layer can be applied. The concrete strength development curve reflects the time during which the material will reach highest level strength. If saved normal conditions, then it will take 28 days.
The first five days are the time during which intensive hardening will occur. But 7 days after completion of the work, the material will reach 70% strength. It is recommended to begin further construction work after reaching one hundred percent strength, which will happen after 28 days. The time schedule for concrete strength gain may differ for individual cases. In order to determine the timing, control tests are carried out on samples.
What else you need to know
If work on monolithic house construction is carried out in warm weather, then to optimize the process of keeping the mixture and acquiring physical and mechanical properties it will be necessary to support the structure in the formwork and leave it to mature after dismantling the fence. The schedule for concrete strength gain in cold weather will be different. In order to achieve brand strength, it is necessary to ensure concrete heating and waterproofing. This is due to the fact that low temperatures help slow down polymerization.
In order for the strength gain to occur as quickly as possible, and the curing time of concrete to be minimized, it is necessary to add sand concrete, which has a minimum water percentage ratio, to the ingredients. If cement and water are added in a ratio of four to one, then the time will be halved. To obtain this result, the composition must be supplemented with plasticizers. The mixture can ripen faster if its temperature is artificially increased.
Strengthening control
In order for the concrete strength gain schedule to be observed, for some time - up to a week - it is necessary to carry out measures to ensure conditions for curing the solution. It must be heated, moistened, and also covered with moisture- and heat-insulating materials.
Quite often used for this heat guns. Experts recommend paying special attention to surface moisturizing. 7 days after completion of pouring under such conditions, if the external temperature varies between 25 and 30 °C, the structure can be loaded.
Classification of concrete
If in the process of mixing the solution cement and traditional dense aggregates are used, which make it possible to obtain heavy compositions, then these mixtures belong to the M50-M800 grades. If you are looking at this, porous fillers were used to prepare it, making it possible to obtain light compositions. Concrete has a grade within M50-M150, if it is particularly light or lightweight, as well as cellular.
The design must be determined at the stage of drawing up documentation for the construction of the facility. This characteristic is given based on the resistance to axial compression in cube samples. In structures under construction, axial tension is the main one, and the grade of cement is determined by it.
Concrete strength gain (tensile strength gain graph) will take longer when the compressive strength grade increases. But in the case of high-strength materials, the increase in tensile strength slows down. Depending on the composition and area of use of the mixture, the strength class and grade are determined.
The most durable materials are considered to be those with the following grades:
- M100.
They are used in the construction of critical structures. When structures and buildings that require great strength are erected, M300 grade concrete is used. But when arranging the screed, it is best to use the composition of the M200 brand. The strongest cements are those whose grade starts with M500.
Dependence of strength gain on temperature
If you are going to use the mortar in construction, then you should know the graph of the dependence of the strength gain of concrete on temperature. As mentioned above, setting occurs within the first few days after mixing the solution. But to complete the first stage, it will take time, which is influenced by the ambient temperature.
For example, when the thermometer is kept at 20 °C and above, it takes an hour to set. The process begins 2 hours after the mixture is prepared and is completed after 3 hours. The time and completion of the stage will shift when it gets colder; it will take more than a day to set. When the thermometer is kept at zero, the process begins 6-10 hours after preparing the solution, and it lasts up to 20 hours after pouring.
It is also important to know about reducing viscosity. At the first stage, the solution remains mobile. During this period, mechanical influence can be exerted on it, giving the structure the required shape. The setting stage can be prolonged using the thixotropy mechanism, exerting a mechanical effect on the mixture. Mixing the solution in a concrete mixer prolongs the first stage.
Percentage of concrete strength from branded concrete depending on temperature and time
Novice builders are usually interested in the graph of concrete strength gain at 25 °C. In this case, everything will depend on the brand of concrete and the curing period. If you use Portland cement grade up to 500 when mixing, you will eventually be able to get M200-300 concrete. After a day at the specified temperature, its percentage of compressive strength from the brand one will be 23. After two, three days, this figure will increase to 40 and 50%, respectively.
After 5, 7 and 14 days, the percentage of brand strength will be 65, 75 and 90%, respectively. The schedule for the strength gain of concrete at 30 °C changes slightly. After a day or two, the strength will be 35 and 55% of the brand value, respectively. After three, five and seven days the strength will be 65, 80 and 90%, respectively. It is important to remember that the standard safe period is 50%, while work can begin only when the strength of concrete has reached 72% of the brand value.
Critical strength of concrete depending on brand: review
Immediately after pouring, the solution will gain strength due to heat generation, but after the water freezes, the process will stop. If the work is to be carried out in winter or autumn, then it is important to add anti-frost mixtures to the solution. Once laid, it generates 7 times more heat than ordinary Portland cement. This indicates that the mixture prepared on its basis will gain strength even at low temperatures.
The speed of the process is also influenced by the brand. The lower it is, the higher the critical strength will be. The graph of concrete strength gain, a review of which is presented in the article, indicates that the critical strength for concrete grades from M15 to 150 is 50%. For prestressed structures made of concrete grades from M200 to 300, this value is 40% of the grade. Concrete grades from M400 to 500 have a critical strength within 30%.
Concrete hardening in perspective
The concrete strength gain schedule (SNiP 52-01-2003) is not limited to a month. The curing process may take several years to complete. But you can determine the brand of concrete after 4 weeks. The structure will gain strength at different rates. This process occurs most intensively in the first week. After 3 months, the strength will increase by 20%, after which the process slows down, but does not stop. The indicator may double in three years, this process will be influenced by:
- time;
- humidity;
- temperature;
- brand of concrete.
Quite often, novice builders wonder in which GOST the concrete strength gain schedule can be found. If you look at GOST 18105-2010, you can find out more about this. These documents mention that temperature directly affects the duration of the process. For example, at 40 °C the brand value is reached within a week. Therefore, it is not recommended to carry out work in winter. After all, heating concrete on your own is problematic; for this you need to use special equipment and first familiarize yourself with the technology. But heating the mixture by more than 90 °C is completely unacceptable.
Conclusion
After reviewing the strength gain schedule, you can understand that formwork is removed when the strength of the structure exceeds 50% of the brand value. But if the external temperature drops below 10 °C, then the brand value will not be reached even after 2 weeks. Such weather suggest the need to heat the poured solution.
In this article we will talk about how long it takes concrete to gain strength and what methods of controlling this parameter are available today.
The topic of the article is not accidental, since most construction projects built year after year using concrete. The popularity of this material is not decreasing, but rather increasing, despite the widespread introduction of alternative technologies in the construction of objects for various purposes.
This is why the question is so relevant: how long does it take for concrete to gain strength and how does this affect the timing of construction work?
Average rate of strength gain
Before answering the question, when does concrete gain 70 strength, let’s figure out what the number 70 means. Essentially, this is a percentage designation of brand parameters. When this parameter is achieved, the structures and structures conditionally comply with the requirements of GOST.
It is no secret that in accordance with the assigned grade of concrete, not only the price is determined, but also maximum load based on kgf/cm², which can be applied to reinforced concrete products without compromising the integrity of the product. That is why all industrial reinforced concrete products are produced with a selling strength of 70% of the brand norm in the summer and 90% in the winter.
Since all industrially produced concrete products meet the requirements of GOST by default, construction organizations can use it for its intended purpose immediately after receiving the ordered product.
Unlike construction organizations Those who order reinforced concrete products from the factory, private users of the mortar when pouring formwork should have a clear idea of how long it takes for concrete to gain strength.
In the photo - working with concrete in the cold season
On average, technologists carry out brand control 28 days after the mortar is poured into the formwork. It can be assumed that this is the average time indicator necessary to set the optimal hardness parameters.
In warm weather, during the first week after installation, the material intensively gains strength up to a conditional 70% of the brand norm. During this process, the interaction of cement grains and the liquid medium occurs until the formation of potassium hydrosilicates.
Important: The hardening process can continue after reaching the conditional 70% of the brand norm.
For example, some reinforced concrete products with the initial concrete grade M 200, after several years, acquire strength corresponding to materials with the M 400 grade.
Time to remove the formwork
Now that we have decided how many days it takes for concrete to gain strength, we will decide when we can start dismantling the formwork.
- If you do it yourself, but taking into account technological requirements and recommendations, then you can start dismantling the formwork within three days.
During this time, optimal hardness parameters will be achieved at which cutting reinforced concrete with diamond wheels is possible. But, despite this, the structure can be loaded no earlier than in a week. - If the pouring of structures and structures is carried out in winter time, the growth of strength slows down significantly. Therefore, the formwork can be removed no earlier than after a week. It is possible to load structures of this type and carry out diamond drilling of holes in concrete no earlier than after 2 weeks.
Important: Pouring formwork in winter should be done using special covering materials, since an uncovered solution will freeze and will not gain the required strength at all.
You must understand that these instructions are important, since if dismantling is carried out ahead of time, there is a high probability of cracks appearing in the thickness finished design. But we must take into account that it is also undesirable to overcook the formwork, since it prevents the free access of air, as a result of which the concrete dries unevenly.
Setting rates and methods for controlling these parameters
The photo shows the foundation after timely dismantling of the formwork
Returning to how long it takes for concrete to gain strength, let’s consider the rate of gradual hardening:
- During the first three days after installation under normal temperature conditions, the material gains about 30% of its brand strength.
- After 7-14 days after laying under normal temperature conditions, the material gains over 60% of the brand norm.
- Within 28 days after installation, concrete can reach 100% of the brand norm.
- Within 90 days after installation, the material can gain up to 120% of the brand norm.
- Further hardening and strengthening of structures with access to moisture also occurs, but the intensity of the process is an order of magnitude lower.
The rate of hardening of cement-containing mortars is most inhibited by a decrease in temperature. As a result of cooling, cement particles interact less actively with water. As a result, chemical reactions proceed extremely slowly.
Reducing the temperature to minus values completely stops the hardening process. With a subsequent increase in ambient temperature, the material will harden, but in this case one cannot count on a brand-name strength gain.
The photo shows the result of the solution drying out during the setting process
At the same time, an increase in temperature in the thickness of the material makes it possible to sharply accelerate the rate of hardening. But when increasing the temperature, you should ensure that the solution in the formwork does not dry out before the allotted time.
For example, when concrete is heated with water steam to a temperature of 80°C, it will take at least 16 hours to gain 70% of the brand strength. Thus, industrial steaming is performed in the manufacture of piles and a number of other reinforced concrete products.
Important: Concrete cannot be heated above 90 °C, since at the boiling point of water a chemical reaction occurs, at which hardening of the cement-containing mortar becomes impossible.
Another point that should be paid special attention to is that the hardening of a cement-containing mortar is an exothermic process, during which the concrete generates heat. As a result, by increasing the temperature for a more intensive strength gain, you risk overdrying the concrete, since the heat released during the exothermic process will be added to the heating temperature.
Conclusion
Now you know how much strength concrete gains and what factors determine the intensity of this process. As a result, you will be able to ensure that everything is carried out within the framework of technological recommendations.
More useful and educational information you can discover by watching the video in this article.