If the concrete has not reached its design strength. What determines and how quickly does concrete strength gain
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By the assigned grade of concrete, you can understand for what maximum load in kgf / cm 2 a particular product is designed. Of course, all reinforced concrete products are already out of production with a tempering strength, which in summer should be at least 70% of the brand strength, and in winter - at least 90%. Therefore, construction organizations can immediately put the product into operation.
But consumers who buy ready-made concrete mix for pouring the foundation or want to make it on their own, it will be interesting to know for how many days does concrete 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 is intensively gaining its strength, about 70 percent of the actual. This is 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, in some reinforced concrete products, for which the concrete grade M 200 was presented, after a few years the strength reached the 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 in a week. In winter conditions, the increase in strength is significantly reduced. If the structure is not covered, then the concrete may freeze and not gain strength at all. For summer period also required special care, that is, constant moisture and shelter from direct sunlight, so as not to cause the concrete surface to dry out.
Heat and moisture treatment accelerates the curing of concrete
After how many days will concrete 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 up to 60-70 percent of the branded one after 12-14 hours. But in such conditions, concrete quickly loses water, and at the same time begins to dry out. Therefore, the most best concrete it is considered the one that gained strength under natural conditions.
For a speedy set of strength, you can use special additives for concrete, which are used in the process of preparing the mixture. The dosage is based on the amount of cement. With the use of additives, concrete can gain brand strength in two weeks. Again, if hardening occurs during the warm season. For winter, anti-freeze additives are applicable, which maintain a positive temperature in the concrete for the setting period.
At self-fill strip foundation, you can roughly orient yourself, in how many days will concrete gain strength- per month. Therefore, try to maintain this interval in order to prevent unpleasant consequences in the future when the structure is loaded.
Concrete Strength Graph
- Mortar hardening stages
- What affects the set of maximum strength
- Acceleration of strength gain
The key stage in the repair construction works- drying of concrete. The cast composition hardens and gains strength for several weeks. The process takes place under the supervision of engineers and requires constant monitoring.
Experts ensure compliance with the standards and, if necessary, make adjustments to the schedule. The material is sensitive to temperature fluctuations and has a "seasonality factor" - in winter, concrete work is carried out using heating systems. Various factors are taken into account to determine how much concrete dries.
Mortar hardening stages
Concrete work is a part of any construction, from country houses to industrial and special ones. The material is used at various stages of the construction of objects, for pouring the foundation and load-bearing structures, floor devices.
Builders successfully use the property of a cement-sand mixture with the addition of crushed stone - the ability to take the form of formwork. The strength and durability of the material is appreciated, the drying time of which is about 28 days.
Depending on the operating conditions and the quality of the composition, the estimated service life of the facilities reaches 250 years, and the 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.
Strength is still being given Special attention and control each stage:
- Freezing. It occurs in the first hours of the "life" of the train. The solution is delivered to the place of work in a concrete mixer or prepared on site to maximize the preservation of the required properties.
The solidification time in summer at temperatures above 20 ° C is about an hour, in the heat - 15-30 minutes. At "zero" - it starts 6-10 hours after preparation of the mixture and stretches up to 20 hours from the moment of pouring;
- Hardening. The main stage takes 7-14 days. During this period, the structure is gaining up to 70% of the design 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 result obtained with the standards of a special table.
Strength of concrete - temperature, humidity, hydration
There is a direct relationship between the solidification of the solution in different conditions and reaching the maximum value.
What affects the set of maximum strength
The vast majority of concrete work is performed outdoors.
Weather conditions and temperature graphs are key parameters that determine how long the solution hardens.
In the warm season, the maturation of the mixture and gradual hardening occurs naturally. The process depends on the physical and chemical properties of the composition and has slight differences associated with the concrete grade.
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 freezing and loss of quality, facilitate the pouring of the structure, leveling the surface;
- Electric heating. It is carried out by several methods with a common essence - ensuring uniform heating of the concrete thickness during the period necessary for curing.
At low temperatures use PNSV wires or "implant" electrodes into the material, after which the voltage is connected. Less commonly used as heating element the formwork itself, cover the surface with special mats.
The work requires compliance with electrical safety rules and is performed in accordance with SNiP 3.03.01-87. If the minimum temperature reaches 0 ° C, and the average per day does not exceed 5 ° C, concreting is initially planned with warming up the cast structure. If necessary, PMD is included in the solution.
Acceleration of strength gain
Concrete compositions are classified according to the compressive strength index. Light mortars are used for auxiliary work or structures that are not stressed.
The basic concretes are M-200 - M-400. Compositions are used in the construction of most civil engineering objects. Mortars of class above M-500 are intended for special objects and structures of increased strength.
The base rate of hardening is calculated on the basis of grades M-200 - M-300. Rates are based on a time span of four weeks. In practice, the required period is shortened under certain conditions:
- Use of special additives. These are auxiliary components that are mixed into the solution during preparation. Application shortens the full hardening time to 14 days. Such work is carried out in the summer - anti-frost additives do not have a similar property;
- Moisturizing.
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 hydration contributes to the creation of conditions under which optimal curing dynamics are achieved.
After the end 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 the construction to be completed according to plans, it is recommended to develop a detailed project documentation taking into account the design features. In the timetable, concrete work is planned, if possible, in the most favorable season.
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Concreting of belt and slab foundation at home is performed according to a long-established technology. At first glance, there is nothing complicated in the work, but during the pouring, during the process and after the solidification of the monolith, many questions arise related to various nuances. Some of them are so important that non-observance may well lead to one or another structural failure. For example, how long does it take after pouring before removing the formwork, and how long does the concrete need to cure before starting the next stage of work? You can hear different opinions among experts, but the rules still exist.
What is the threat of untimely removal of the formwork
As you know, for pouring a strip or slab foundation of a house, a mushy concrete mortar... After laying it in the formwork, the processes of cement hydration and the gradual hardening of concrete begin. For their correct completion, it is necessary to allocate a certain amount of time required for the foundation to be able to withstand and gain design strength.
If the formwork from the structure is removed immediately after the cement has set, there will be a possibility of the monolith creeping into different sides... A fragile "body" not only will not be able to take the load, but also keep its own shape. This is especially true for massive foundations.
If the dismantling of the formwork from the strip foundation is carried out after the cement mortar has set, but before it gains a certain strength, then cracks will appear in the structure. For the underground part of the house, which takes over and distributes all the loads to the ground, this threatens with split and complete destruction already during the operation of the house.
How much should the foundation stand after pouring? There is no definite answer to this question. The average period of time is determined by 28 days, but in some cases 15-20 days are enough. In difficult conditions, the deadlines need to be extended.
Professionals assure that the foundation of the house must stand at least a month before it is loaded.
So that the structure does not shrink, does not warp and collapse, you must strictly observe the implementation of building rules and technology for erecting the underground part of the house.
Concrete - setting and curing time
The foundation is the pillar of the building, therefore it does not tolerate negligence, inability and lack of basic knowledge.
How long should the foundation be defended
The terms specified in the regulations, provided for in order to concrete structure could withstand, do not always correspond to real time. They are influenced by extraneous factors, such as:
- ambient temperature;
- humidity regime;
- the presence of precipitation;
- season;
- soil conditions;
- terrain relief;
- dimensions and type of foundation - strip, slab, columnar;
- design strength of concrete;
- quality of materials;
- the presence of groundwater at the site;
- construction technology;
- the presence of additives;
- the value of the calculated loads.
In addition to the above points, situations may arise that affect the period during which the foundation of the house will have to be defended before further work begins. 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. In this case, the monolith is reliably covered. It is noteworthy that not a single standard will be able to take into account all the nuances, therefore, the question of how much the foundation of the house will be defended is decided on an individual basis.
In determining the timing, the worst conditions for the site should be accepted. The stock, in this case, will play a positive role.
How does the foundation of the house stand
For the first time, the concrete structure is allowed to stand immediately after pouring. This period lasts up to seven days, during which the surface is watered. The concrete sets and begins to harden. Cover the foundation from above plastic wrap but you can also use:
- absorbable cloth;
- sawdust;
- straw.
The polyethylene is raised before watering, and other materials are moistened from above. They perfectly retain moisture, preventing water from evaporating prematurely. The duration of the solidification of the monolith depends on the season. After about a week, and in the hot season - after 10-14 days, watering is stopped, but the casing layer is left up to 28-30 days after the end of laying concrete mix into the formwork. Thus, there is a primary standing, which is quite sufficient for foundations installed on a foundation buried below the level of soil freezing.
But in practice, there is also secondary standing. It concerns situations when on heaving soils shallow foundations have to be erected. In this case, the hardened concrete structure is left to winter. With the arrival of spring, the movements are recorded, and the base is strengthened by adding sand or gravel with the obligatory layer-by-layer ramming.
Experts assure that it will be better if the foundation stands idle for a 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 allows, then preference should be given to a longer time period. If the period for the construction of the building is limited by rigid frames, then the installation of the walls of the house is started 28 days after the foundation is poured. With favorable climatic conditions and the use of lightweight materials in the enclosing structures, the period can be reduced to two weeks.
08/22/2016 at 13:08
Reinforced concrete device monolithic foundation requires knowledge and understanding 
many important points.
Before pouring the mixture into the formwork, the layman in the construction topic should prepare theoretically.
The time of formwork dismantling is of considerable importance. How to control strength and when can the foundation be loaded?
How long to wait for strength gain
As indicated in p.
How concrete gains strength and how to control these parameters
2.5 SNiP 2.03.01-84, concrete at least M-200 should be used for the construction of foundations. Since BM-100 is used for a preparation device, 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 time period from preparation of the mixture to laying;
- Layer thickness;
- Compliance with technology, etc.
Strength gain is chemical process requiring optimal conditions the most important are warmth 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, moisture will quickly evaporate from it, which is necessary for the normal course of the hardening reaction, and at temperatures below +5 degrees, the processes slow down, which negatively affects the hardening time.
The optimum 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 keep its shape. The most intensively the foundation is gaining strength in the first week, reaching 70% of the design. Hardening, hardening lasts up to 28 days.
Control of concrete setting
In the conditions of concrete work by construction enterprises, quality control is carried out by testing concrete samples by the following methods:
- Compression with special equipment;
- By tapping the massif with Kashkarov's hammer;
- Ultrasonic devices (non-destructive method).
For testing on a stationary machine, cubes are prepared: from one portion of the mixture, samples of 10 × 10 cm in an amount of at least 3 are poured, marking the samples themselves, and also fixing the date and time on them. 
The cubes are transferred to a special construction laboratory to carry out tests, where, based on the load at which the cubes collapsed, they perform calculations and deduce the strength of the concrete, 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 different types of hammers, and the device designed by Kashkarov is notable for the fact that the impact force does not affect the final strength readings. The hammer itself weighs 400-800 g.
Strength indicators are determined by traces remaining on concrete, in accordance with the table given in the regulatory literature.
Ultrasonic devices are based on determining the speed of passage of ultrasound 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 establish the presence of voids, cavities in the foundation mass or other structural element.
Special techniques should be applied by professionals with operational experience. laboratories, amateurs will not be able to determine the exact value of the resistance of the material to compression, that is, strength.
In artisanal conditions, the setting is checked as follows: simultaneously with the laying of the mixture, the form is poured separately into the formwork arbitrary size(in terms of size 10 × 10 cm), but preferably the same height as the main structure.
On day 2, on one side, the formwork must be removed and see if the concrete holds its shape, how much it has seized. If necessary, after a day, remove the formwork from the other side of the sample and analyze the setting dynamics. You can try to break one of the samples to make sure it is hard.
It is important to understand that a sample is smaller than an array of foundation, and in a small volume, concrete hardens faster. After making sure that the sample has seized, you should give the array an additional 2-5 days to get desired result- firmly hardened, seized foundation.
When to remove the formwork
Removal of the formwork can be carried out in urgent need for 3-5 days, but it is better to withstand 7-14 days. 
Well-set concrete, which has gained 30-70% strength, retains its shape, does not chip when disassembling the formwork.
Removal of formwork is permissible at an early date, if shields, boards are needed to perform work on another grip or at the next object.
In private construction, it is reasonable not to rush and let the mixture gain the required strength indicators, which will take 2 weeks.
How long can the foundation be loaded
Giving a load to the foundation means performing the next stage of building a building, in the case of a foundation, this is a wall arrangement:
The load is acceptable when the concrete acquires 100% of the design strength characteristics. In this case, you can not be afraid of deformations, destruction of the foundation, since the structure is already able to perceive the loads from walls, floors, and roofs. 
This period occurs after 28-30 days from the moment of pouring concrete into the formwork.
This period can be shortened if you apply special means- chemical additives, or technological methods, such as warming up in the cold season, watering with water or covering with wet mats in summer when the heat is hot.
If concrete sets in natural conditions, it is better not to rush and remove the formwork no earlier than after one to two weeks, but to erect the walls at the age of at least 4 weeks.
There is nothing complicated in the construction of the foundation, but it is better when professionals who have both experience and technical means concrete hardening control.
If, nevertheless, the casting of the formwork is carried out on its own, then it is better to remove the formwork after 7-14 days, and subject it to load - not earlier than 28 days from the date of casting.
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 the concrete, it is necessary to wait for a while.
Strength of concrete
The most intensive set of strength occurs in the first 5-7 days after pouring - during this time it gains about 70% of its brand strength. In the future, its strength increases and reaches the brand 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 the expiration of this period, the formwork can be disassembled.
The graph of concrete strength versus time is shown in the figure:
Maturation of concrete at 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 will definitely not be constant and equal to 30 degrees. Therefore, at self-construction foundation, it is better to play it safe and let the concrete stand for a month, and only then disassemble the formwork and continue construction.
Cement is used as a binder in concrete, its chemical reaction with water leads to the appearance of solid stony neoplasms, which bind together filler particles - crushed stone and sand. The initial period of this reaction is called setting, during which the initial bonds between the filler particles are formed in the concrete. Then there is a set of strength, when these bonds are strengthened. In order for this chemical reaction to take place, 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. In order to prevent this from happening, the surface of the concrete structure is covered with plastic wrap or roofing felt, and also poured over it with water. It is important that the concrete dries evenly throughout the volume.
In the cold season, the water contained in the concrete mix can freeze and the maturation of the concrete will stop. Moreover, 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 the entire maturation. Of course, with independent construction, this is impossible (or at least very difficult), so you need to pour concrete with your own hands in the summer. The required temperature for its ripening is 20-25 degrees or higher.
The concrete curing time can be reduced by using special additives that accelerate this process. Such fast-hardening concretes gain strength in two weeks, but with independent construction, their use is difficult, because they not only ripen faster, but also set faster. This means that after the preparation of such a fast-hardening concrete mixture, the time for pouring it will be much less. Another way to achieve accelerated curing of concrete is to increase the temperature: it can be seen from the graph that the higher the temperature, the faster the increase in strength goes. However, with independent construction, it is unrealistic to create such conditions.
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Read also:
Curing
During the maturation of freshly laid concrete, it needs care: it is necessary to ensure optimal temperature and moisture so that it gains design strength and does not crack when dry.
Foundation concrete
Concrete is a stone material that forms as a result of the hardening of a concrete mixture. The concrete mix 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 fill a monolithic foundation?
After you decided on the type of foundation, the place and depth of its foundation, you 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 reinforcement cage, installed it in the formwork and securely fixed it there, the time has come for the latter and the very 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 load on the foundation.
Concrete grade and class
The main characteristics of concrete are its brand and strength class. A table of the relationship between brand and class is given in this article.
Date of publication: 10/29/2010 3:57:26 PM
Concrete hardening
Strength of concrete (in hours)
| Curing time, hours | |||||||
|---|---|---|---|---|---|---|---|
| 0 ° C | 5 ° C | 10 ° C | 15 ° C | 20 ° C | 25 ° C | 30 ° C | |
| 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 |
Strength of concrete (in days)
| Curing time, days | Concrete hardening temperature | ||||||
|---|---|---|---|---|---|---|---|
| 0 ° C | 5 ° C | 10 ° C | 15 ° C | 20 ° C | 25 ° C | 30 ° C | |
| concrete compressive strength% of 28 days | |||||||
| 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 |
Question: how long should you wait for the concrete to harden? How and for how long does concrete gain strength? Is it really necessary to wait 28 days after the concrete is poured? When can concrete structures be loaded?
It is more profitable for each developer or builder to build a structure, building or structure in the shortest possible time. But there is whole line opinions about what is necessary after completing work on concreting structures to wait until the structure "Hardens", to then proceed to the next stage of construction.
Do I need to wait 28 days after pouring the concrete?
For the correct conclusion, it is necessary to analyze the regulatory documents and determine the mode, stages and terms of construction.
When performing concrete work, they are faced with two pressing questions:
- How long does it take to remove the formwork?
- How long can it take to load reinforced concrete element or design?
Let us consider these questions one by one.
For precast concrete elements, it is very important to determine tempering strength.
Vacation strength- this is the gained strength of concrete, established by the standards, at which a reinforced concrete product can be delivered from the factory to the construction site.
The value of the tempering strength is set in accordance with GOST or other regulatory documents, depending on:
- the type and size of the structure;
- concrete composition;
- hardening conditions;
- ambient temperatures and climatic conditions of the region;
- the time and size of the load;
- conditions of transportation.
Below, in table 1 given depending on the type and class of concrete, the average values of the tempering strength as a percentage of the design.
Table 1
So, the tempering strength of precast concrete products, depending on a number of factors, is 50 ÷ 100% of the design. Conclusion number 1: when the tempering strength is reached, it is already possible to carry out installation and then load reinforced concrete structures, with the expectation that full loading (100%) will occur no later than 28 days from the date of manufacture of the products. A more specific procedure and timing for loading prefabricated structures is stipulated in the PPR (work production project).
Also in construction there is such a concept as stripping strength .
Stripping strength- this is the minimum gained concrete strength, at which it is possible to remove the formwork without damaging the concrete. For precast concrete elements, the formwork strength must be sufficient for safe transport. The conditions and rate of curing for each product or structure are determined by the manufacturer.
In the conditions of a construction site, in the manufacture of monolithic structures, stripping is usually performed immediately before the start of loading the structure.
SNiP 3.03.01-87 establishes the following conditions for stripping reinforced concrete structures (see Table 2).
table 2
| Parameter | Stripping strength (% of standard, for 28 days) |
| Strength of concrete (at the time of stripping of structures), not lower: | |
| - heat-insulating | 0.5 MPa |
| - structural and thermal insulation | 1.5 MPa |
| - reinforced | 3.5 MPa, but not less than 50% of the design strength |
| - pre-stressed | 14.0 MPa, but not less than 70% of the design strength |
| Stripping of reinforced concrete structures with subsequent processing of concrete (p. 2.34) | 70% of design strength |
Russian regulatory document TR 80-98 " Technical Recommendations on the technology of unheated concreting of monolithic structures using a thermos and an accelerated thermos " gives the following permits for stripping and loading structures, table 3.
Required concrete strength for stripping and structural loading:
Table 3
| Building construction | ||
| over 70% | 70% or less | |
| concrete strength,% of design | ||
| Side formwork panels on foundations and columns, walls, girders and beams are allowed under normal hardening conditions | Shoot after 6 - 72 hours | |
| Load bearing formwork panels | 100 | See below |
| Span length of bearing reinforced concrete slabs up to 3 m | 100 | 70 |
| Span length of bearing reinforced concrete slabs (except slabs) up to 6 m | 100 | 70 |
| Columns, supporting structures (beams, girders, slabs) with a span of 6 m and more | 100 | 80 |
| Structures with prestressing reinforcement | 100 | 80 |
Notes:
- It should be firmly remembered that a structure can be fully loaded to 100% only when the concrete has reached its full design strength.
- It is possible to remove the side panels of the non-bearing part of the formwork provided that the temperature difference between the concrete and the outside air meets the following condition:
- Dt = 20 ° C for structures with M p = 2 - 5;
- Dt = 30 ° С for structures with М п more than 5,whereM p - the modulus of the surface of the structure (the ratio of the sum of the areas of the cooled surfaces of the structures in m 2 to its volume in m 3), m -1.
Further measures for the performance of formwork and the movement of workers on reinforced concrete structures are allowed when the strength of the concrete is 1.5 MPa or more. ( SNiP 3.03.01-87, p. 2.17). Also, in this regulatory document there is an indication (clause 2.110) that when using intermediate supports (props) to bridge spans, with partial or sequential removal of the formwork, the permissible stripping strength can be reduced, which means a greater turnover of the formwork and a reduction in construction time ... More specific measures for early removal of the formwork should be determined based on the specific conditions of construction and covered in the PPR.
Some literary sources indicate the following values for demoulding reinforced concrete structures, tab. 4:
Table 4
Conclusion number 2: Based on all of the above and analyzing all tables of concrete stripping strength and loading, stripping strength is within 50 ... 80% of the design. Then:
- stripping of the structure is allowed when the actual concrete strength reaches 70% of the design, in which case it can be gradually loaded further;
- stripping of the structure is allowed to be carried out when actual strength 50% of the design, it is only necessary to install additional supports for insurance and elimination of deflections. In this case, you can also gradually load the structure (put formwork, masonry, etc.).
How long can it take for concrete to gain stripping strength, at which it is also possible to load the structure?
As already mentioned above, at different conditions(temperature, humidity, precipitation, etc.) different concrete gain strength in different ways. On the rice. 2 shows a graph of the rate of strength gain depending on the temperature of TVO (heat of moisture treatment).
The graph shows that under laboratory conditions at a constant temperature of 60 ° C, concrete (70%) acquires average stripping strength in 32 hours (1.3 days), and at a temperature of 30 ° C it acquires in about 4 days.
Since at construction sites, during the day, the ambient air temperature fluctuates, then the average daily temperature is taken into account, which in summer is 18 ... 28 ° С, and in autumn it reaches 5 ... 10 ° С. At these temperatures, concrete will gain strength much more slowly.
Rice. 1. Graph of the rate of concrete strength gain depending on the temperature of TVO (heat of moisture treatment)
At enterprises for the production of concrete and structures from it, there must be schedules for the strength gain of concrete of a certain composition. For an approximate determination of the strength of a particular concrete, you can use the strength development graphs depending on the type of cement, temperature and class of concrete ( rice. 2) from regulatory documents.
Below is the increase in the strength of concrete, depending on the temperature of the ambient air or TVO, (in% of R 28):
Strength development graphs (table 5-9)
Strength set of concrete of class C15 - C25 on Portland cement grade M400 (% of R 28):
Table 5
| Concrete age, days | Concrete temperature, ° С | ||||||||
| -3 | 0 | 5 | 10 | 20 | 30 | 40 | 50 | 60 | |
| 1/2 | — | 1 | 4 | 5 | 12 | 17 | 28 | 38 | 50 |
| 1 | 3 | 5 | 9 | 12 | 23 | 35 | 45 | 55 | 63 |
| 2 | 6 | 12 | 19 | 25 | 40 | 55 | 65 | 75 | 80 |
| 3 | 8 | 18 | 27 | 37 | 50 | 65 | 77 | 85 | — |
| 5 | 12 | 28 | 38 | 50 | 65 | 78 | 90 | — | — |
| 7 | 15 | 35 | 48 | 58 | 75 | 87 | 98 | — | — |
| 14 | 20 | 50 | 62 | 72 | 87 | 100 | — | — | — |
| 28 | 25 | 65 | 77 | 85 | 100 | — | — | — | — |
Strength set of class C30 concrete on Portland cement grade M500 (% of R 28):
Table 6
| Concrete age, days | Concrete temperature, ° С | ||||||||
| -3 | 0 | 5 | 10 | 20 | 30 | 40 | 50 | 60 | |
| 1 | — | 8 | 12 | 18 | 28 | 40 | 55 | 65 | 70 |
| 2 | — | 16 | 22 | 32 | 50 | 63 | 75 | 85 | 90 |
| 3 | 10 | 22 | 32 | 45 | 60 | 74 | 85 | 92 | 98 |
| 5 | 16 | 32 | 45 | 58 | 74 | 85 | 96 | — | — |
| 7 | 19 | 40 | 55 | 66 | 82 | 92 | 100 | — | — |
| 14 | 25 | 57 | 70 | 80 | 92 | 100 | — | — | — |
| 28 | 30 | 70 | 90 | 90 | 100 | — | — | — | — |
Strength set of concrete of class C15 - C25 on slag Portland cement grade M400 (% of R 28):
Table 7
| Concrete age, days | Concrete temperature, ° С | ||||||||
| -3 | 0 | 5 | 10 | 20 | 30 | 40 | 50 | 60 | |
| 1/2 | — | — | 2 | 4 | 7 | 20 | 25 | 32 | 42 |
| 1 | — | 3 | 6 | 10 | 16 | 30 | 40 | 50 | 65 |
| 2 | 3 | 8 | 12 | 18 | 30 | 40 | 60 | 75 | 90 |
| 3 | 5 | 13 | 18 | 25 | 40 | 55 | 70 | 90 | — |
| 5 | 8 | 20 | 27 | 35 | 55 | 65 | 85 | — | — |
| 7 | 10 | 25 | 34 | 43 | 65 | 70 | 92 | — | — |
| 14 | 12 | 35 | 50 | 60 | 80 | 96 | 100 | — | — |
| 28 | 15 | 15 | 65 | 80 | 100 | — | — | — | — |
Strength set of C40 class concrete on M600 Portland cement (% of R 28):
Table 8
| Concrete age, days | Concrete temperature, ° С | |||||
| 0 | 5 | 10 | 20 | 30 | 40 | |
| 1 | 8 | 13 | 21 | 32 | 45 | 59 |
| 2 | 17 | 25 | 36 | 52 | 65 | 75 |
| 3 | 23 | 35 | 46 | 62 | 74 | 83 |
| 7 | 42 | 57 | 68 | 83 | 90 | 98 |
| 14 | 58 | 73 | 82 | 94 | 100 | — |
| 28 | 71 | 83 | 92 | 100 | — | — |
Strengthening of concrete using antifreeze additives:
Table 9
| Antifreeze additive | Binder type | Concrete hardening temperature, ° С | Concrete strength,% ofR 28 when hardening in the cold after a number of days | |||
| 7 | 14 | 28 | 90 | |||
| 1) Sodium nitrite (in aqueous solution), NaNO 2 | Portland cement | -5 | 25 | 40 | 60 | 100 |
| -10 | 15 | 25 | 35 | 70 | ||
| -15 | 5 | 10 | 20 | 50 | ||
| 2) Crystalline sodium nitrite, NaNO 2 | Portland cement | -5 | 25 | 40 | 60 | 100 |
| -10 | 15 | 25 | 35 | 70 | ||
| -15 | 5 | 10 | 20 | 50 | ||
| 3) Nitrodap | slag Portland cement | -5 | 15 | 25 | 45 | 90 |
| -10 | 10 | 15 | 25 | 60 | ||
| -15 | — | 5 | 15 | 40 | ||
Conclusion number 3: The graphs and tables show that concrete based on Portland cement at an average daily temperature of 10 and above gains 50% of the design strength in 5 ... 7 days, and concrete on Portland slag cement gains under the same conditions - in 14 or more days. In winter, at subzero temperatures with the use of even antifreeze additives (Table 9), concrete gains its design strength in 90 days or more. To accelerate the time of the required strength during winter concreting, it is necessary to use electric heating.
For a quick set of strength, according to SNiP 3.03.01-87 "Bearing and enclosing structures . 2. Concrete works"(p. 2.15) concrete needs proper care. Concrete maintenance begins immediately after placing it in the formwork and continues until the moment it is removed. Concrete should be stored away from direct sunlight and atmospheric precipitation, wind, as well as create heat and moisture conditions for hardening (cover with a film). It is recommended to water concrete made on Portland cement for 7 days, and water on the basis of low-activity and slag Portland cement for at least 14 days. At an air temperature of 15 ° C, it is recommended to water the concrete after 3 hours during the first 3 days. At average temperature air from +5 to 0 ° С watering and wetting of concrete is not carried out. The full load (design) of reinforced concrete structures is allowed only after the concrete has the design strength.
Separately, I would like to focus on the foundation, since there are some features of its work:
- The best time to build a foundation is summer (good temperature).
- It is undesirable to expose the foundation to long-term downtime, because locking the pit, frost heaving, alternating freezing and thawing of the base soils leads to its destruction.
- The above factors lead to uneven shrinkage of the foundation.
- If, nevertheless, there is a need to leave the foundation for the winter, it is necessary to "mothball" it - to close it and protect it from atmospheric precipitation, to exclude blocking and flooding of the soil near the foundation (approximately 0.4 ... 0.5 m).
- Since concrete under favorable conditions gains 50 ... 80% of the design strength in 7 ... 14 days, then it is allowed to load the foundation after 7 ... 14 days, taking into account that full loading (100%) will occur only after 28 days from the moment the foundation is poured.
- When using hardening accelerators at normal temperatures, it is already possible to load the foundation even after 5 days.
- The foundation should be loaded evenly to avoid uneven settlement of the base.
For a more accurate safety net to control the strength of foundations or other reinforced concrete structures, a series of standard sample cubes 150x150x150 or 100x100x100 mm are made, which are then tested for compression.
Literature:
- How to build a house. How does concrete gain strength? Time of concrete hardening, graph of strength gain. Access mode:
- TR 80-98 Technical recommendations on the technology of unheated concreting of monolithic structures using a thermos and an accelerated thermos. MOSCOW - 1998.
- VSN 20-68 Instructions for concreting road bases for asphalt concrete pavements in Moscow in winter.
Alexander A. Konev
Curing
Stop hack! Very, very many country builders think that the next important operation after the completion of placing concrete in the formwork is stripping and enjoying the results of their labor. In fact, this is not the case. After the completion of the concrete placement in the formwork, the next serious construction technological process begins - concrete care. By creating optimal conditions for hydration in the process of concrete maintenance, the planned brand strength of the concrete stone is achieved. The absence of a concrete curing step can lead to deformations, cracking and a decrease in the rate of concrete curing.
Concrete care is a set of measures to create optimal conditions for the curing of concrete to the set of the established brand strength. The main goals of concrete care:
- Minimize plastic shrinkage of the concrete mix;
- Ensure sufficient strength and durability of the concrete;
- Protect concrete from temperature extremes;
- Protect the concrete from premature drying;
- Protect concrete from mechanical or chemical damage.
Maintenance of freshly poured concrete begins immediately after the completion of the concrete mix and continues until 70% of the design strength [clause 2.66 of SNiP 3.03.01-87] or another justified demolding period is reached.
At the end of concreting, it is necessary to inspect the formwork for the preservation of the specified geometric dimensions, leaks and breakdowns. All identified defects should be eliminated before the start of the concrete setting (1-2 hours after placing the concrete mixture). Hardening concrete must be protected from impacts, shocks and any other mechanical influences.
In the initial period of concrete maintenance, immediately after the end of its laying, in order to avoid erosion and damage to its surface, the concrete should be covered with plastic wrap, tarpaulin or burlap.
The temperature and humidity conditions of concrete hardening should be especially carefully maintained. Normal moisture for hardening is 90-100% in case of excess water. As shown in Table 52 above, the curing under humid conditions significantly increases the final strength. cement stone.
In case of premature dehydration (which can also occur when cement laitance leaks from non-waterproof formwork), concrete receives insufficient surface strength, a tendency to peeling sand from concrete, increased water absorption, reduced resistance to weather and chemical influences. Also, with premature dewatering, early shrinkage cracks occur and there is a risk of later shrinkage cracks. Premature shrinkage cracks are formed primarily due to the rapid decrease in the volume of freshly laid concrete by open areas surfaces due to evaporation and weathering of water. When concrete dries, it shrinks and shrinks. As a result of this deformation, structural and internal stresses arise, which can lead to cracks. Shrinkage cracks appear first on the surface of the concrete, and then they can penetrate deeper. Therefore, it is necessary to take care of the delayed drying of the concrete. It should only begin when the concrete has gained sufficient strength to withstand shrinkage stress without cracking. During the formation of early cracks, when the concrete is still plastic, the resulting shrinkage cracks can be closed using surface vibration.
Drying of concrete is accelerated in the wind, at low humidity and at an air temperature lower than the temperature of hardening concrete. Therefore, the surface of the concrete must be protected from drying out during the maintenance of the concrete. After the concrete gains a strength of 1.5 MPa (about 8 hours of hardening), the concrete surface should be regularly moistened with water by means of diffuse irrigation (not with a jet!). You can cover the surface with burlap, tarpaulin or sawdust and moisten them with water, covering the top with plastic wrap, creating conditions like a wet-drying compress. Moistening of concrete is not carried out at average daily temperatures below + 5 ° С. If there is a threat of freezing, concrete can be covered with additional heat-insulating materials (foam, mineral wool, rags, hay, sawdust, etc.).
Even if it is not possible to permanently moisten the concrete with water, the concrete should be covered with a polymer film with a thickness of at least 0.2 mm (200 microns). The sheets of the film should be laid in the largest possible single pieces with a minimum of seams. Connect the film panels with an overlap with an overlap of 30 cm with gluing with adhesive tape. The edges of the film must adhere tightly to the concrete in order to minimize the evaporation of water from under the film.
In order to avoid damage to freshly poured concrete by moving groundwater it is necessary to protect it from erosion until a strength of at least 25% is reached (1-5 days, depending on conditions at a positive temperature).
The term for the end of the maintenance of concrete coincides with the term for its safe demoulding.
Table number 69. Relative compressive strength of concrete at different hardening temperatures
Concrete |
Term |
Average daily temperature of concrete, ° С |
|||||
|---|---|---|---|---|---|---|---|
|
concrete compressive strength% of 28 days |
|||||||
|
M200 - M300 on |
|||||||
* Conditionally safe line starting work on the foundation.
Concrete care and temperature conditions
The temperature of the freshly prepared concrete mixture should not exceed 30 ° C. When concreting at an average daily air temperature from + 5 ° C to - 3 ° C, the temperature of the concrete mixture with a cement mass of more than 240 kg / m3 (concrete grade M200 and higher) should be at least + 5 ° C, and with a smaller amount of cement not less than + 10 ° C.
Safe concreting at an air temperature of less than - 3 ° C and a single freezing of concrete and its thawing is possible only when the temperature of the concrete mixture has been maintained at a level not lower than + 10 ° C for at least 3 days.
Cold Weather Concreting
In cold weather, there is a slowdown in setting and an increase in concrete strength. At an average daily temperature of + 5 ° C, it takes twice as long for the concrete to reach the same strength as at + 20 ° C. At a temperature close to the freezing point, the strength of concrete practically stops. If fresh concrete freezes, its structure can collapse. Excess water unused during cement hydration forms a system of capillary pores in the hardening concrete.
When exposed to frost, the water in the pores completely or partially freezes, and the ice formed as a result of freezing puts pressure on the walls of the pores, which can lead to the destruction of their structure. Freezing of concrete in early age entails a significant decrease in its strength after thawing and in the process of further hardening in comparison with normally hardened concrete. This is due to ice crystals breaking the bonds between the surface of the aggregate and the cement glue (cement stone).
Freeze resistance of freshly laid concrete can be achieved special composition concrete mix and the required time of concrete hardening at a positive temperature.
Table 70. Curing time of the concrete required to achieve sufficient frost resistance (RILEM directive *)
|
Concrete temperature (average daily temperature) |
|||
|---|---|---|---|
|
Strength class of cement |
5 ° C |
12 ° C |
20 ° C |
|
Required curing time (days) to achieve freeze resistance of concrete with a water-cement ratio of 0.60 |
|||
|
M400 D2032,5 H(32.5N) |
|||
|
32.5R(fast-hardening) |
|||
|
4 2,5N |
|||
|
45 , 5R(fast-hardening) |
|||
* International Union of Laboratories and Experts in Building Materials, Systems and Structures.
Table no. 71 Time of concrete hardening required to achieve sufficient frost resistance *
Class (grade) of concrete |
Strength of concrete of monolithic structures to the moment of freezing,% |
Number of days of curing of concrete at concrete temperature |
|
|---|---|---|---|
|
B7.5-B10 (M100) |
|||
|
B12.5-B25(M150 -M350) |
|||
|
B30 (M400) and higher |
|||
|
Water-saturated concrete with alternating freeze cycles |
|||
|
Concrete with anti-freeze additives designed for a specific temperature |
|||
* Adapted with simplification from table 6 SNiP 3.03.01-87
TO effective measures for the production of work on concreting in the winter include:
- the use of cement with a fast set of strength (letter “R” in the strength class),
- increasing the content of cement in the concrete mix,
- decrease in water-cement ratio,
- preheating of aggregates (up to + 35 ° C) and water (up to + 70 ° C) for concrete mix [table 6 SNiP 3.03.01-87],
- the use of antifreeze and air-entraining additives.
When using preheated concrete, do not heat it to temperatures above + 30 ° C. When applying hot water with a temperature of up to + 70 ° C, it should be pre-mixed with a granular aggregate (before the introduction of cement into the concrete mixture), so as not to “steam up” the cement. For this, the following sequence of loading materials into the concrete mixer is observed:
- simultaneously with the filler, the main part of the heated water is fed,
- after several revolutions, cement is fed and the rest of the water is poured,
- the duration of mixing is increased by 1.25 -1.5 times compared with summer regulations to obtain a more homogeneous mixture (minimum 1.5 - 2 minutes),
- the duration of vibration of the concrete mixture is increased by 1.25 times.
When preheating the concrete mixture, as well as when using concrete with antifreeze additives, it is allowed to lay the mixture on an unheated, non-porous base ( sand pillow) or old concrete if, according to the calculation, in the contact zone during the calculated period of curing the concrete does not freeze [paragraph 2.56 of SNiP 3.03.01-87]. After placing the concrete and vibrating it, it must be covered with a polymer film and heat-insulating materials (including the use of snow) in order to preserve the heat released during cement hydration (for 3-7 days under normal conditions). In case of frost, a greenhouse should be built over the foundation and heated.
For amateur summer cottage builders without experience, you can recommend adhering to the following rule: perform concrete work at expected average daily temperatures within28 daysfrom the moment the foundation is poured below+5
° CNot recommended.
It should also be remembered that it is not allowed to leave shallow (not buried) foundations unloaded for the winter period... If for some reason this condition turns out to be impracticable, temporarily insulating coatings of sawdust, slag, expanded clay, slag, straw and other materials that protect the soil from freezing should be arranged around the foundations [clause 6.6 VSN 29-85]. Outlets of reinforcement of concrete structures must be covered or insulated to a height (length) of at least 0.5 m.
Hot weather concreting
An increase in the temperature of the concrete activates the interaction of water and cement and accelerates the hardening of the concrete. On the other hand, excessive heating of the concrete mix leads to expansion, which is fixed during the setting of the concrete and the hardening of the cement stone. Subsequently, during cooling, the concrete shrinks, however, the resulting structure prevents this, and residual stresses and deformations appear in the concrete. Typically, concrete heats up more from the surface, therefore, excess stress primarily occurs at its surface, where cracks can form. The critical period of time when shrinkage cracks form often begins one hour after the concrete mix has been prepared and can last from 4 to 16 hours.
With a predicted average daily air temperature above + 25 ° C and a relative air humidity of less than 50%, it is recommended to use fast-hardening Portland cements for concreting, the grade of which must exceed the grade strength of concrete by at least 1.5 times. For concretes of class B22.5 and higher, it is allowed to use cements whose grade exceeds the brand strength of concrete by less than 1.5 times, provided that plasticized Portland cements are used or plasticizing additives are introduced [clause 2.63 of SNiP 3.03.01-87]. Or use additives that slow down the time of concrete hardening.
It may also be reasonable to lay concrete in the morning, evening or at night when the air temperature drops and the exposure of the concrete mixture to the sun's rays is excluded.
During concreting, the surface temperature of the concrete should not exceed + 30 + 35 ° C. When cracks appear on the surface of the laid concrete due to plastic shrinkage, its repeated surface vibration is allowed no later than 0.5-1 hours after the end of laying. In special cases, flake ice can be used to cool the concrete.
Freshly laid concrete must be protected from dehydration due to exposure to air temperature, sunlight and wind. After concrete gains strength of 0.5 MPa, concrete care should consist in ensuring a constant wet state of the surface by installing a moisture-absorbing coating and its constant moistening, keeping open concrete surfaces under a layer of water or continuously spraying moisture over the surface of structures using a lawn sprayer or perforated hose. At the same time, only periodic watering with water of open surfaces of hardening concrete and reinforced concrete structures is not allowed.
In order to avoid the possible occurrence of a thermally stressed state in monolithic structures at direct impact fresh concrete should be protected from sunlight with reflective (foil) plastic film or paper in combination with heat-insulating materials. When using wooden formwork, it also needs to be constantly watered.
Measures for cooling hardening concrete are especially relevant when minimum size cross-section of the foundation tape 80 cm or more. In this case, too much heat is generated during hydration and overheating of the concrete and subsequent cracking is possible even under normal temperature conditions.
Table No. 72. Concrete care measures depending on the air temperature.
Concrete care measures |
Air temperature ° C |
|||||
|---|---|---|---|---|---|---|
|
< -3°C |
from -3 ° C to + 5 ° C |
from + 5 ° C to + 10 ° C |
from + 10 ° C to + 15 ° C |
from + 15 ° C to + 25 ° C |
> + 2 5 ° C |
|
|
Cover with foil, moisten the surface, moisten the formwork, cover the concrete with moisture-preserving material |
Yes at strong wind |
|||||
|
Cover with foil, moisten the surface. |
||||||
|
Cover with foil, put thermal insulation |
||||||
|
Cover with foil, put thermal insulation, arrange a greenhouse, heat up to T + 10 ° C for 3 days |
||||||
|
Maintain a thin layer of water on the concrete surface at all times |
||||||
The concrete poured into the formwork may not set for a long time and not gain the design strength. Let's determine why this is happening, how to avoid it and, most importantly, what to do if the concrete does not harden.
Concrete characteristics
Concrete is a mixture of a coarse aggregate with a binder that has the ability to pass from a liquid to a solid phase. Currently there are different types concrete - asphalt concrete, polymer concrete and so on. However, the most widespread is concrete, in which Portland cement is used as a binder. Portland cement is a mixture of lime and clay, ground in a certain proportion and fired, capable of forming a hard and durable artificial cement stone when mixed with water.
Portland cement
It turned out that in nature there are often large deposits of a mineral called marl, consisting of clay and lime in the ratio required for the manufacture of cement. In the production of cement at the factory, this mineral is burned in special furnaces and crushed to a state of dust.
Various grades of cement are produced for different purposes. Grade is the characteristic of the cement stone, after hardening, to withstand a certain compressive load. When cement mixed with water sets, a chemical reaction and transformation occurs liquid composition into solid. The final strength of the material and the setting time (time of the chemical reaction) depend on the amount of water.
Concrete grades and classes
Essential lack of cement stone is its shrinkage, that is, the difference in volume during the transition from liquid to solid phase can be up to 10%. Uneven shrinkage leads to the appearance of so-called shrinkage cracks and internal stresses that reduce strength. The addition of large aggregates, such as sand and crushed stone, makes it possible to obtain concrete in which these disadvantages are significantly reduced and do not have a large effect on the strength of the structures erected from it. The coarse aggregate also saves on cement, the cost of which is much higher than the production of sand and crushed stone.
The strength characteristics of concrete are characterized by classes (image above), which also reflect the compressive strength of concrete. In the old fashioned way, they are sometimes also called stamps.
Important! The concrete grade and the concrete grade should not be confused - they are not the same thing.
Empirically, the proportions of water and cement have been developed, which make it possible to obtain, even from cement of the same brand.
Concrete gains its design strength during the first 28 days, then the reaction slows down greatly, continuing throughout the entire existence of the concrete structure, that is, over time, concrete becomes more and more durable, and with correct operation its service life can be from 100 years or more.
Another disadvantage of concrete is its low tensile or flexural strength, which is 15-20 times less than the compressive strength. Therefore, the Frenchman Monier came up with a way to place a metal (steel) frame in the tensile zone of a concrete structure, which perceives tensile stresses. This is how reinforced concrete appeared - the most important material used in construction until now.
How to avoid concrete problems
The reasons for the poor setting of concrete are commonplace and it is recommended to carefully avoid them, since it is much easier to do this than to suffer with the consequences of neglecting them. It is necessary to take a responsible approach to work and observe very simple rules, especially when it comes to your own home or building.
- Before concrete work, the customer needs to familiarize himself with their main stages and technology, as well as the properties and methods of choosing ingredients, that is, cement, sand, crushed stone. This will help control the work process and pause it in time if something does not go as planned.
- Only qualified experienced performers who already have experience in working with concrete should be invited for work.
- Materials should be purchased only from trusted suppliers, and check for quality certificates. It is better to have concluded contracts with suppliers so that in the event of a later discovery that the materials were of poor quality and did not comply with GOST or technical specifications, it would be possible to claim damages or losses incurred.
An example of a certificate for compliance with the requirements of GOST
- When purchasing cement - the most expensive and important material, you need to be especially careful. Purchasing loose cement of unknown origin should be avoided from unfamiliar suppliers, it is better if it is packaged in bags.
It is imperative to check the inscriptions on the bags and their compliance with quality certificates, which should not be photocopies, but have real "wet" stamps.
It is good to find an operating laboratory for testing building materials near the construction site. Such laboratories are usually located at serious construction organizations, factories of reinforced concrete products or construction universities. If you transfer a small amount of cement from the purchased batch (up to 0.5 kg) to such a laboratory, after 2-3 days, specialists can accurately answer whether it makes sense to buy this cement and what is its real strength (brand), they can also give recommendations on the proportions of crushed stone and sand for the preparation of the optimal composition of concrete of the desired class.
Unfortunately, the supply of cement was joined by a large number of scammers ... They usually appear in places of mass individual construction and carry out street sales directly from cars. So, for example, claiming that the darker the cement, the higher its strength, they, mixing the lowest-grade cement with coal dust or soot, tried to pass it off as high-quality and sell it at a high price.
A fake is easily detected by mixing a small amount of cement with water. If after that a film of particles poorly absorbed by water appears on the surface of the water, it is not recommended to purchase such cement.
The easiest way to determine the quality of cement: clench it in your fist. How less material stay in the fist, the better. If almost all of the cement "leaked" through your fingers, then this is a great product. If all the cement remained in your hand and turned into a lump, then you should refrain from working with it.
You should also stay away from unverified sellers who claim that their cement contains additives that increase the strength and setting time, most likely there are no additives there, and if there are, then rather, on the contrary, that worsen its properties. Of course, various additives to concrete exist, but their use in the preparation of a concrete mixture should be deliberate (know exactly what they are for and when they should be used) and strictly controlled.
Use cement that has been in an unheated room in winter, strongly discouraged ... The activity of such cement can be reduced by more than 90% and its use for any construction work is meaningless. Sometimes there are attempts to sell such cement. Usually, bags of such frozen cement are harder and denser to the touch, and the cement itself contains lumps that are easily kneaded by hands.
Why concrete does not freeze
Despite the fact that concrete work is not very difficult, and all the main technological processes have long been developed and applied at a huge number of construction projects, numbering hundreds of thousands around the world, various unforeseen situations may arise during the concreting process, the most common of which is the absence or retardation and curing.
Among the reasons that concrete does not harden are the following:
- Using too much water in the solution;
- Concrete laying at temperatures below + 5 ° С without heating it;
- The mixture froze in severe frosts;
- Too long kneading the mixture with a car mixer;
- Poor quality cement or concrete;
- Non-compliance or other mistakes when mixing concrete;
- The use of various untested or low-quality concrete admixtures;
- Poor concrete maintenance.
Whatever the reason, it is often quite difficult to fix it. Sometimes you even have to break the concrete and re-lay it. Read more about solving such problems below.
If, nevertheless, it happened that the work was completed, and the concrete did not set (on the second or third day it should already be quite hard), first of all, you should understand the reasons for what happened.
- For the convenience of laying, the performers used the amount of water much more than required, thereby violating the water-cement ratio. Such concrete will set one way or another, but its strength will be low, and it will also have strong shrinkage and be covered with a network of cracks.
For unloaded structures, this may not matter much (defects and surface curvatures can later be hidden by cement-sand plaster). When concreting critical load-bearing structures, for example, foundations, such concrete must be disassembled, and the laboriousness of disassembly will be the less, the sooner this disassembly begins. When using the reinforcement, it can be cleaned and its reuse is quite acceptable.
Ideally, the percentage of water in the concrete mix should be around 25-30% for good strength. However, this solution is quite thick and may not be suitable for certain purposes.
- The rule has been violated that concrete work is not carried out at a minimum daily temperature of less than 5 degrees Celsius. The setting time of such concrete will greatly slow down, however, in the absence of negative temperatures, it will gain the design strength for a period longer than 28 days.
- Concreting in conditions of negative temperatures. Such concreting can be carried out only in conditions of extreme necessity with the use of special additives containing calcium or magnesium salts, as well as with the use of special closed tents and air heat guns. Concreting without special measures in winter is unacceptable.
In winter, it is better to abandon concreting, or resort to special equipment and additives to the concrete solution.
- A situation may arise when immediately after concreting, frost hit and the mixture froze. In this case, any concrete work should be stopped immediately, and the concreted structure, without disassembling the formwork, should be left until the warm season.
During thawing, the concrete will continue to set, but its final strength will be 10-15% lower than the design one, which should be taken into account when erecting overlying structures, for which this structure will serve as a support. It is good if, before the onset of frost, the structure was completely concreted, otherwise, during pre-concreting, connecting embedded parts should be arranged - pins, brackets, since with a long break in concreting, individual fragments of the product will not be able to be connected together properly. Perhaps this design will require additional reinforcement.
- Sometimes it happens that when delivering concrete with an automobile mixer, the operator, for some reason, for a long time does not turn off the function of mixing the mixture (the time of which should be strictly limited), which has an extremely negative effect on the incipient chemical reaction between cement and water, as a result of which the reaction stops, the mixture poured into the formwork does not set, and after the water evaporates, the composition is easily disassembled by hand. Such concrete must be dismantled, and the work must be reworked. In this case, responsibility and compensation for damages are entirely imposed on the concrete supplier.
- Use of substandard or fake cement. How to maximally try to avoid such a situation has already been written above. It is almost impossible to deal with such a problem if the materials have already been laid, therefore there are two ways out - to wait and hope that the concrete will still harden (only for unloaded structures), but at the same time remember that such concrete will not last long in any case. Or break everything and lay a high-quality mortar (if the concrete structure is a supporting one, then this is the only option).
- Incorrectly designed concrete mix in case of self-production, non-observance of the proportions of the materials used. Such concrete can begin to set after a long time, but its strength will be insufficient for the required further use. The structure must be disassembled or reinforced, which can significantly increase its cost.
- Sand and crushed stone may contain inclusions of minerals that, when exposed to water, release chemicals that adversely affect the setting reaction of the cement. These aggregates for concrete must also be purchased from trusted suppliers and do not contain harmful chemical active ingredients.
- The use of untested, publicized, purported enhancing supplements, available in both dry and liquid form. IN best case such additives can be neutral, and at worst, harmful to concrete and affect its setting. Experimental lovers can always try to pre-manually make a small amount of concrete with such additives and see what happens.
- Lack or inadequacy of concrete care measures. If, after the end of concreting, the loss of moisture by concrete is not compensated for due to natural evaporation (drying), the water-cement ratio is violated and the reaction in the outer layer becomes either extremely slowed down or completely stops. In this case, in these places the concrete either does not gain the required strength, or dries up and crumbles when the slightest mechanical stress. That is why, after concreting, structures are usually wrapped with vapor-tight films - polyethylene or polypropylene, covered with rags and regularly watered with water several times a day for 10-14 days.
In most cases, concrete setting problems can be avoided. But if you are unlucky, and you are faced with such a situation, do not do anything rashly, but do not delay the solution of this issue for a long time.
If the concrete is to be dismantled - immediately, without delay, proceed to these works. If concrete for a long period does not gain the required design strength, consult with specialists about the possibility of further use of such a structure and about additional strengthening of its bearing capacity.
Do not regret the lost funds and decisively get rid of poor-quality building materials without any attempts to use them in further construction. Analyze in detail your actions and the actions of the performers in order not to repeat such mistakes in the future.
Concrete does not freeze: reasons, what to do, how to avoid problems
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Concrete is construction material from a mixture of filler and binder, which, when mixed with water, pass from a liquid to a solid state. Sand, expanded clay, crushed stone and other bulk materials with a granular structure are used as aggregates.
Binders for concrete are organic and inorganic in nature. Organic polymers include bitumen, glue. Inorganic - cement and its varieties, liquid glass, plaster, etc.
The composition of concrete - the selected aggregate and binder, as well as their ratio - affect its strength characteristics and field of application. If you are interested in direct deliveries of high-quality concrete, use the link to the concrete plant at https://betonnijzavod.ru.
Strength of concrete
The strength of concrete is its main physical and mechanical property. Strength gain is chemical reaction hydration - the interaction of water and binder, as a result of which a concrete stone is formed.
Strength set with concrete stone includes 2 stages:
- setting that occurs on the first day after the preparation of the concrete mixture. The higher the ambient temperature, the faster the concrete sets. Nevertheless, during this period, the mixture remains mobile and can be influenced, i.e. pour into the formwork and seal;
- hardening. This process takes 28 days. This is how long it takes for the concrete to harden to the design level. In other words, to become a full-fledged building structure.
Binders give concrete an additional property called shrinkage. The hardened concrete stone gradually gains its final strength over several years, slightly shrinking and cracking as it does so.
Concrete does not gain strength: reasons
Depending on the grade of concrete, as well as environmental conditions, there are regulated strength values at the stages:
- removing the formwork;
- the beginning of the operation of the structure;
- quality control of the resulting concrete.
The actual values obtained may not correspond to the normative ones for the following reasons:
- using too much water;
- the absence of antifreeze additives in the composition or neglect of the heating of the concrete mixture when working in conditions below +5 degrees;
- use of frozen components;
- insufficient compaction of the mixture;
- too long kneading;
- poor-quality binder, in particular, cement and its varieties;
- improperly selected aggregate-binder ratio;
- the use of low-quality additives;
- inadequate care of the poured concrete or lack of it.
Most often, to solve the problem, they resort to emergency heating of the concrete for several days. If after that the strength does not reach 70% of the design, then the question of dismantling the structure or the development of measures to strengthen it is decided.
