How to find out the grade of concrete after pouring. Handy ways to determine the grade of concrete

The result of work on the creation of concrete and reinforced concrete structures is highly dependent on both the quality of the components used to prepare the concrete mixture, and on the observance of technological conditions at each stage of concrete work.

Careful control must be carried out at the following stages:

receipt and storage of materials used in the course of concrete work - sand, cement, gravel, crushed stone, reinforcement, etc .;
creation and installation on site of elements of the reinforcement structure;
creation and assembly of formwork elements;
preparation of formwork and base for concrete placement;
preparation and transportation of the concrete mixture to the place of laying;
maintenance of a concrete structure during the period of its critical or design strength (curing).

All components of the future concrete structure are checked for compliance with GOST standards. Their characteristics are analyzed in accordance with a unified methodology specially designed for laboratories at construction enterprises.

Quality control of materials

In the course of reinforcement operations, the quality of work and materials is checked upon receipt of the reinforcement - the factory marking (presence of tags), the compliance of the brands with the declared requirements of the designers are checked. The processes of storage and transportation are accompanied by checking the correct placement of reinforcing steel by grades, grades and sizes, preserving its quality characteristics after delivery to the construction site. When constructing reinforcing structures and elements, the compliance with the geometric shape and dimensions, the correctness of welds and their quality is checked. Reinforcement elements placed in the concreting block and combined into a common structure are analyzed for compliance with the specified dimensions and position according to the tolerances.

Work on the installation of formwork elements is carried out with verification of the correctness of their installation, construction of fasteners, the tightness of the interfacing of the panels at the joints, the correspondence of the assembled formwork and the reinforcement structure (ensures the formation of a protective layer of a given thickness). The spatial position of the formwork is analyzed by leveling and linking to the axes in several separate sectors, the accuracy of the calculated dimensions is determined by measurements using a measuring tool. Formwork construction tolerances are specified in GOST R 52085-2003, GOST R 52086-2003 and reference literature. Before the concrete is placed, the surfaces of the formwork are checked for cleanliness and the quality of the application of lubricants.

Preparation and placement of concrete mix

The introduction of the components of the mixture into the mixer is accompanied by a thorough check of the dosed portions, the duration of mixing, the density and the degree of concrete mobility. The control of the mobility of the concrete mixture is carried out at least twice per work shift, its indicators should not be 10 mm less or more than the calculated ones, density tolerances - no more than 3%.

The procedure is performed with monitoring the parameters of the mixture - for the absence of seizure, delamination, loss of mobility due to drying.

At the place of concrete work, it is important to monitor the height of the dumping of the mixture, the duration of vibration with the achievement of uniform compaction, to prevent the stratification of the mixture, the formation of voids and cavities in its structure.

Vibration compaction of the concrete mixture is carried out under visual control, the criteria are the degree of its settlement, the formation of cement milk, the completion of the release of air bubbles. More precisely, the compaction results are analyzed using radioisotope densitometers, which calculate the density of the concrete mixture by measuring the degree of absorption of gamma radiation.

In the process of concreting structures of a large area, the compaction of the concrete mixture is determined using several cylindrical sensors, externally resembling probes, placed depending on the thickness of the mixture to be laid. The higher the density of the concrete, the less its resistance to the electric current passed through the concrete mixture - the operation of sensors is based on this principle. They are installed near vibration installations, informing the operator about the achievement of the required density by sound and light signals.

Evaluation of the strength of concrete by its samples

To find out the full quality characteristics of concrete is possible in only one way - by testing it for strength by compressing specially made concrete cubes until it is possible to completely destroy them.
The cubes are made at the same time the concrete is being poured and held in exactly the same conditions as the main concrete structures. Typically, 160 mm cubes are tested for compression.

Depending on the class of concrete, three test cubes of the same size are required. To assess the characteristics of foundations intended for various structures, cubes are formed from every 100 cubic meters of concrete mixture. When creating massive foundation structures designed for the installation of technological equipment, samples for strength tests are prepared from each next 50 cubic meters of concrete, and for foundations for frame and thin-walled (lightweight) structures, cubes must be made from each new batch of concrete with a volume of 20 cubic meters.

A relatively complete assessment of the strength of a concrete structure can be obtained by drilling cores in its body, followed by testing the samples for compressive strength.

Indestructible Methods for Testing Concrete for Strength

In addition to laboratory studies of the strength characteristics of concrete samples from specific batches, there are ways to indirectly assess concrete structures and structures without any destruction. Among them, the most popular is the mechanical method, based on the relationship between the surface hardness of concrete and its compressive strength, as well as pulse-ultrasonic, the use of which is based on measuring the speed of longitudinal ultrasound waves directed into the concrete structure and the degree of their complete attenuation.

Mechanical testing of the strength characteristics of reinforced concrete is carried out using an instrument called a sclerometer. Consider the versions of this device designed to determine the strength of concrete.

Kashkarov's hammer... It must be installed with the side with the ball on the surface of the concrete structure, then hit on the back with an ordinary metal hammer. After the impact, potholes will remain on the concrete surface and on the reference bar, the measurement of which will determine the surface compressive strength of the concrete. The design of Kashkarov's hammer must comply with the standards of GOST 22690-88.

Schmidt's hammer... An impact rod is located in its body - after removing the blocking, it is necessary to fully extend it, then press it against the concrete surface, pressing the impact rod into the body until it is completely immersed in it and hits the concrete. The impact of a rod hammer will cause the device to bounce and move the measuring mechanism along the scale with markings - in the process it is important to keep the tool strictly perpendicular to the surface of the concrete structure. Hammer bounce distance - depends on the surface strength of the concrete, i.e. the higher it is, the greater the distance the hammer will move. The principle of operation of modern analogues of the Schmidt hammer, equipped with an electronic measuring scale, does not differ from its mechanical counterparts.

Special devices for ultrasonic examination of concrete, for example, UKB-1, also allow you to determine the strength of concrete structures. They generate ultrasound, according to the speed of movement of which through the thickness of concrete its strength characteristics are determined. If the technological conditions meet certain requirements - the use of materials with similar characteristics, the technology compliance with the established standards, etc. - the accuracy of the data on the strength of concrete will be quite high.

Quality control of concrete work in winter

In low temperature conditions, following the procedures described above will not be sufficient. In addition to quality control measures, additional actions need to be taken, which will be discussed below.

The condition of the concrete mixture is checked during the entire preparation period of the next batch at least once every 120 minutes. When entering the concrete mixer, unheated filler (crushed stone, gravel and sand) should not contain snow and ice, frozen grains. In the process of obtaining concrete mixtures with antifreeze additives, it is necessary to measure the temperature of dry components and water before entering them into the mixer, to determine the salt content and the temperature of the finished mixture at its outlet.

The transportation of concrete is carried out with a one-time check for a change in the state of the covering and insulation materials, the quality of heating and thermal insulation of the containers in which the mixture is transported and into which it enters after delivery.

If it is performed before laying the concrete mixture, then it is required to control its temperature during the heating of each new portion.

At the construction site, immediately before starting work on laying the mixture, an examination of the inner walls of the formwork, the base of the concreted site and the reinforcement structure is carried out for the absence of snow and ice. The outer walls of the formwork must be thermally insulated in accordance with the technological conditions, it will heat the base of the concreted area and the zones of its interface at the joints with the formwork.

In the process of placing concrete, its temperature is monitored at the stage of unloading from the vehicle, then the temperature readings are taken again, but after the completion of concrete placement. Concreting areas not closed by formwork should also be assessed for technological compliance in terms of waterproofing and thermal insulation characteristics.

Measurements of the temperature of concrete passing the stage of curing in winter conditions are carried out in the following order:

when using the technologies of preheating, "thermos" and heating in the given temperature and humidity conditions (hot house), the temperature should be measured once every two hours during the first day, at least twice during the shift during the next three days and once every 24 hours during the further aging period;
when laying concrete containing antifreeze additives, its temperature must be measured three times during each day from the moment of completion of the work until it reaches the design strength;
when conducting electric heating of a concrete structure, during its temperature rise with an interval of up to 10 ° C per hour, the temperature should be measured every two hours, then at least twice during each shift.

After the concrete structure has passed the curing period and the design strength has passed, and the formwork has been dismantled, the air temperature is measured at least once during each work shift. Temperature data for a concrete structure is obtained by drilling narrow wells and immersing thermometers in them, as well as using special technical thermometers. It is extremely important to monitor temperature changes in sectors potentially subject to high cooling (protrusions and corners), as well as heating - areas close to heating electrodes, an area of direct contact with thermoactive formwork elements. Recording of information on temperatures is carried out in a special sheet.

If concrete is heated using electrodes, it is necessary to measure the current and voltage in the supply transformer twice for each shift, and enter these data into the log.

Strength laboratory testing of concrete specimens is carried out in accordance with the standard procedure outlined above. In addition, at the place of concrete work, additional sample cubes are created for testing the strength:

In a situation where test specimens are kept at low temperatures, it is necessary first to withstand them at temperatures from +15 to +20 о С, and then to check their strength characteristics.

If a set of strength characteristics of a concrete structure is provided using electrical elements, induction or infrared heating, or in a thermally active formwork, then obtaining samples for testing such concrete is most often impossible. The only way to monitor the strength of concrete in such situations is to strictly ensure the design temperature conditions.

In addition to assessing the strength by breaking sample cubes and drilled cores, it is necessary to check with non-destructive methods - for example, using Schmidt and Kashkarov hammers. It is important to carefully register each operation within the framework of quality control carried out in accordance with the technologies of concrete work, since upon acceptance of the object, this documentation will be presented to the commission. We remind you that the acceptance of the concrete base, the concreting block, where the concrete mixture is to be laid, is drawn up by an act, then the temperature control logs are kept in a given order and in accordance with the established forms.

The strength of concrete is the most important characteristic on which the operational parameters of the material depend. Strength means the ability of concrete to withstand external mechanical forces and aggressive environments. Methods for determining this value by non-destructive testing methods: mechanical or ultrasonic are especially relevant.

The rules for testing the strength of concrete in compression, tension and bending are determined by GOST 18105-86. One of the characteristics of concrete strength is the coefficient of variation (Vm), which characterizes the homogeneity of the mix.

According to GOST 10180-67, the compressive strength of concrete is determined when compressing control cubes with rib sizes of 20 cm at 28 days of age - this is the so-called cubic strength. Prismatic strength is defined as 0.75 cube strength for concrete class B25 and above and 0.8 for concrete class below B25

In addition to GOSTs, requirements for the design strength of concrete are set in SNiPs. So, for example, the minimum stripping strength of concrete of unloaded horizontal structures with a span of up to 6 meters should be at least 70% of the design strength, and over 6 meters - 80% of the design strength of concrete.

Mechanical non-destructive methods for determining the strength of concrete

Non-destructive methods of concrete for compression are based on indirect characteristics of instrument readings. Concrete strength tests are carried out using the main methods: elastic rebound, shock impulse, separation, chipping, plastic deformation, separation with chipping.

Consider the types of test devices of the mechanical principle of operation. In this way, the strength of concrete is determined by the depth of penetration of the working body of the device into the surface layer of the material.

The principle of operation of Fizdel's hammer is based on the use of plastic deformations of building materials. The impact of a hammer on the concrete surface forms a hole, the diameter of which characterizes the strength of the material. The place where typos are applied must be free of plaster, putty, paint layer. The tests are carried out with elbow strikes of medium force 10-12 times at each section of the structure with a distance between the prints of at least 3 cm. The diameter of the resulting holes is measured with a caliper in two perpendicular directions with an accuracy of a tenth of a millimeter. The strength of the concrete is determined using the average indentation diameter and a calibration curve. The calibration curve is based on a comparison of the obtained diameters of the prints and the results of laboratory tests on samples taken from the structure or manufactured using technologies similar to those used.

The principle of operation of the Kashkarov hammer is also based on the properties of plastic deformation. The difference between these devices lies in the presence of a hole between the hammer and the rolled ball, into which the control rod is inserted. The blow of Kashkarov's hammer results in the formation of two prints. One - on the surface of the inspected structure, the second - on the reference rod. The ratio of the diameters of the resulting indentations depends on the strength of the test material and the control rod and does not depend on the speed and force of the hammer blow. The concrete strength is determined by the average ratio of the diameters of the two indentations using a calibration schedule.

Pistols TsNIISK, Borovoy, Schmidt's hammer, KM sclerometer equipped with a rod impactor operate based on the principle of elastic rebound. Measurements of the rebound value of the striker are carried out at a constant value of the kinetic energy of the metal spring and are fixed by a pointer on the scale of the device. The cocking and descent of the striker occurs automatically when the striker touches the test surface. The KM sclerometer has a special striker of a certain mass, which, with the help of a pre-stressed spring with a given stiffness, strikes a metal striker, pressed by the other end to the surface being examined.

The shear pull test method is used to determine the strength of concrete in the body of a concrete member. Test areas are selected in such a way that there are no fittings in this area. For research, three types of anchor devices are used. Anchoring devices of the first type are installed in the structure during concreting. For the installation of the second and third types of anchor devices, boreholes are preliminarily prepared by drilling them into concrete.

Ultrasonic method for measuring the strength of concrete

The principle of operation of ultrasonic testing devices is based on the relationship that exists between the speed of propagation of ultrasonic waves in a material and its strength.

Depending on the sounding method, there are two calibration dependencies: "wave propagation speed - concrete strength", "ultrasonic wave propagation time - concrete strength".

The through sounding method in the transverse direction is used for prefabricated linear structures - beams, girders, columns. Ultrasonic transducers during such tests are installed on two opposite sides of the tested structure.

Surface sounding is tested for flat, ribbed, hollow-core floor slabs, wall panels. The wave converter is installed on one side of the structure.

To obtain reliable acoustic contact between the tested structure and the working surface of the ultrasonic transducer, viscous contact materials such as solidol are used. Installation of "dry contact" is possible using cone nozzles and protectors. Ultrasonic transducers are installed at a distance of at least 3 cm from the edge of the structure.

Devices for ultrasonic strength testing consist of an electronic unit and sensors. Sensors can be separate or combined for surface sounding.

The speed of propagation of an ultrasonic wave in concrete depends on the density and elasticity of the material, the presence of voids and cracks in it, which negatively affect the strength and other quality characteristics. Therefore, ultrasonic sounding provides information on the following parameters:

uniformity, strength, modulus of elasticity and density;
the presence of defects and the peculiarities of their localization;
the form of the A-signal.

The device records and converts the received ultrasonic waves into a visual signal. Equipping control equipment with digital and analog filters allows optimizing the signal-to-noise ratio.

Methods for destructive testing of concrete strength

Each developer can choose independently the methods of non-destructive testing, but according to the existing SNiPs, destructive testing is mandatory. There are several ways to organize the fulfillment of SNiP requirements.

Concrete strength control can be carried out on specially made samples. This method is used in the production of prefabricated reinforced concrete structures and for the final inspection of BSG (ready-mixed concrete) at the construction site.
The strength of concrete can be controlled on samples that were obtained by cutting and cutting out of the structure itself. Sampling locations are determined taking into account the reduction in bearing capacity depending on the stress state. It is advisable that these places are indicated by the designers themselves in the design documentation.
Testing of samples made at the work site under conditions determined by specific technological regulations. However, the placement of concrete in cubes for subsequent tests, its hardening and storage differ significantly from the actual conditions of laying, compaction and hardening of working concrete mixtures. These differences significantly reduce the reliability of the results obtained in this way.

Self-measurement of concrete strength

Professional methods for determining the strength of concrete are expensive and not always available. There is a way to independently conduct a survey for the strength of concrete structures.

For testing, you will need a hammer weighing 400-800 g and a chisel. A chisel placed on the surface of the concrete is hit with a medium force. Next, the degree of damage to the surface layer is determined. If the chisel left only a small mark, then the concrete can be attributed to the strength class B25. In the presence of a more significant notch, concrete can be attributed to classes B15-B25. If the chisel penetrates into the structure body to a depth of less than 0.5 cm, then the sample can be attributed to class B10, if more than 1 cm - to class B5. The strength class or grade of concrete is the main indicator of the quality of the concrete mixture, which determine the average strength of concrete. For example, the average strength of concrete B30 (M400) is 393 kgf / cm2.

It is possible to roughly determine the strength of concrete Rb at 28 days in MPa using the Bolomey-Skramtaev formula, which is the basic law of concrete strength. To do this, you need to know the brand of cement used - Rc and the cement-water ratio - C / W. Coefficient A with normal quality of aggregates is approximately 0.6.

Rb = A * Rts * (C / V-0.5)

In this case, the strength of concrete in time obeys the formula

n = Brand strength * (lg (n) / lg (28)), where n is at least 3 days,

on the 3rd day, concrete gains about 30% of the brand strength, on the 7th day - 60-80%, and 100% ultimate strength is reached on the 28th day. A further increase in the strength of concrete occurs, but very slowly. According to SNiP 3.03.01-87, fresh concrete maintenance continues until 70% of strength is achieved or until another period of stripping.

Methods for self-determination of the strength of concrete structures are simple and economical. However, in the case of the construction of important facilities, it is advisable to turn to the services of specialized laboratories.

If you have conceived the construction of your house, then you will hardly be able to do without concrete.

In order to choose a high-quality material that will guarantee the durability of the structure, it will be useful to know what indicators determine the quality of concrete, what the brand and class are, and how quality control is carried out.

We will also consider what methods can be used to control the quality of concrete: visually, contact or laboratory, and how to determine the quality of the hardened mixture.

Mixture quality indicators

There are several indicators to determine the quality properties of concrete, namely:

In laboratory conditions, to control and determine these indicators of concrete, special tests of samples are carried out by pressing or breaking them on special machines.

The material grade is the value that determines the average compressive strength of concrete. To express these indicators, the designation kgf / cm² is used. The class of material is the characteristic of compressive strength, expressed in MPa.

Areas of application of various types of concrete

Obviously, more stringent requirements are imposed on the class. The parameters specified for compliance with the class must be met in 95 cases out of 100. For compliance with the brand, these indicators can be slightly higher or lower. It is customary to use letters and numbers to designate a class in regulatory documents. B50 grade material must withstand a pressure of 50 MPa at 95%.

On the construction market, you can find materials, the class of which can vary from B7.5 to B40.

To designate the grade of concrete, it is customary to use the letter "M", in numerical terms, the grade can vary from 50 to 100. In most cases, materials from M100 to M500 are used. The ratio of grades and classes of concrete is presented in the table:

The ratio of grades and classes of concrete

Concrete compositions initially have a very low strength, therefore, it can be determined for 7-14 days, under normal hardening conditions.

Important! It must be remembered that the strength characteristics of the material depend on the physicochemical reactions that occur during the interaction of water and cement.

In different solidification media, the speed of interaction is different.

For example, rapid drying leads to the termination of these processes, which leads to a deterioration in the strength of the structure.

In order for the poured concrete to meet the strength characteristics indicated by the class, it is necessary to create optimal conditions for this. Namely, up to 28 days after pouring, the surface must be periodically moistened with water. A bitumen emulsion or a plastic film laid on top can act as protection.

The quality characteristics of hardened concrete are influenced by the properties and performance of the constituent materials.

Therefore, measures to determine the quality of concrete are carried out in the following areas of work:

when dosing all components of concrete;
while mixing the concrete mixture;
when pouring and sealing the solution;
when finishing the finished product for compliance with the developed projects.

In the construction business, two types of control are practiced:

Operational;
Receiving.

The first type of control is carried out at the end of each operation. Starting with the verification of the received materials and ending with the verification of the finished structure.

The second type of control is used in cases where the material has already frozen. The main task of such control is to determine the average density and strength of the solidified material. Based on these checks, compliance with the established standards is carried out. Samples from concrete or full-fledged structures are selected for checks.

How is quality defined?

Several methods can be used to determine the quality of concrete:

Visually, you can determine the following characteristics of concrete:

Consistency. It should be homogeneous, without lumps and clots;
Excess water. To determine the excess water, it is necessary to pour a small part of the concrete into the pit. Ideally, you should get a cake without layers and cracks;
Mix color. Good quality material should be gray in color. A brown or reddish tint indicates an excess of sand or aggregate. If the solution has an uneven color, then it is better to refuse it.

To determine the quality of the mixture in a laboratory way, it is necessary to carry out the following manipulations:

Special devices are used to check the quality of hardened concrete.

To check the quality of concrete by contact, you should purchase a special device - a sclerometer. Its principle of operation is the impact of a shock pulse on the sample.

But not everyone can afford such a check, since the price of the device is quite high.

Quality control of a frozen structure

To determine the quality of the hardened material, special cubes are used, which are poured simultaneously with the bulk of the material. In most cases, the determination of the quality of concrete is provided to laboratories, which record the results of the study in journals. At home, such a check is quite difficult and expensive to perform.

To perform such work, several methods are used:

destructive;
non-destructive.

Core drilling is a destructive method. For such work, you will need a special tool that will extract these samples. The resulting parts are then examined for strength.

The non-destructive method includes mechanical control based on the impact of Kashkarov's hammer on a concrete base.

The ball of such a hammer is pressed with one side against the concrete, and with the other side against the reference rod. When hit with a metal hammer, imprints will appear on the concrete and the bar, which are measured. With the ratio of the obtained figures, the strength of the material is determined according to the calibration curves.

The next non-destructive method for checking the quality characteristics of concrete is ultrasonic examination. The principle of such control is based on a change in the speed of passage of waves produced by such a device through a material of different strength. The quality and strength of the material is established on the basis of calibration curves.

To the table of contents

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How to check the quality of concrete and what is needed for this?

When purchasing or erecting a private house on a concrete foundation, it is necessary to very carefully control the quality of ready-made or poured concrete. Moreover, this operation can be done on your own without having special measuring equipment. How to check the quality of concrete using a multi-purpose tool will be described in this article.

Non-destructive methods of concrete quality control

First of all, you should carefully inspect the surface. The surface must be smooth. If the pouring was carried out in winter, there should be no "patterns" on the concrete. If they are, it means that during the pouring period the concrete froze through, which reduces the strength of the structure to 50-100 kg / cm2;
Quality control using a hammer weighing at least 0.5 kg. Knock on the concrete structure and hear the tone. Ringing tonality and absence of damage indicate the high quality of concrete and strength at a level of at least 200 kg / cm2; ringing sound and hammer prints identify the antinode at the level of 150-200 kg / cm2; dull sound in the absence of damage - concrete has serious defects; dull sound and damage from impacts - poor quality concrete, strength no more than 100kg / cm2;
Visible surface defects in the form of a significant number of pores "speak" of poor compaction. In addition, this is a one hundred percent guarantee of poor quality concrete preparation. If it is a concrete structure in the open air, there is a high risk of gradual destruction in the cycle: "moisture penetration, moisture freezing, destruction of a micro layer of concrete", etc.

Method of checking the quality of concrete with a hammer and chisel

To check the concrete, you need a hammer weighing 500-800 grams and a steel chisel.

Place the chisel on the surface to be tested at an angle of about 180 degrees and hit it with medium force. For a more accurate check, a similar operation must be done in different places of the structure. Assessing the impact mark:

The trace is barely noticeable - high quality concrete corresponding to the B25 brand;
The trace is very noticeable - concrete grade B15-B25;
Strong indentations formed. The concrete began to paint - B10 grade concrete;
The chisel entered the material by more than 10 mm - concrete grade no more than B5.

With ongoing construction, it makes sense to determine the quality of the concrete before pouring. To do this, it is necessary to fill in a sample with dimensions of 100x100x100 cm. Until the concrete has set, pierce it with a concrete rod to release air.

Then the sample is dried at an ambient temperature of 20-25 degrees Celsius and after 28 days it is taken to a specialized laboratory for analysis. Thus, you can get the most accurate technical characteristics and concrete grade.

Assumption! If the construction time is running out, the sample can be transported 7-14 days after pouring. At the same time, the exact holding time should be named in the laboratory.

In addition to determining the quality of concrete with improvised means, there are the following methods that require special tools, devices and installations:

Determination of concrete strength using Fizdel's hammer;
Determination of concrete strength using Kashkarov's hammer;
Ultrasonic method: determining the quality of concrete by determining the propagation time of a sound wave and its speed with special equipment.

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How to check the quality of concrete in a liquid and hardened state

With active work with building mixtures, sooner or later you have to learn to determine some characteristics by visual signs or with the help of special devices. If necessary, checking the quality of concrete can be carried out both in a liquid state and in a hardened state, when the structure is already completely ready.

The photo shows a liquid solution, the quality of which remains to be seen.

Determination of the parameters of the liquid mixture

Immediately before pouring a freshly prepared solution, it is recommended to verify the technological properties, especially if the batch was carried out by hand or the manufacturer does not inspire confidence. Having performed an independent control, you can learn a lot about the quality of products.

Density check

Having calculated the approximate mass of a substance in a certain unit of volume, one can judge which category a given composition belongs to. This parameter is particularly influenced by the type of placeholder. Detailed information on the density of mixtures is presented in the table.

Attention! The first two categories of solutions are mainly used to form an additional layer.

However, in some cases, small structures can be created with their help.

Preparatory activities must be carried out immediately before testing. To carry out the work, you will need: a two-liter container, a trowel, scales and a metal rod for sealing. The used container is immediately weighed, after which its volume is determined in cubic centimeters.

The main formula for memorization is highlighted.

Next, the finished mixture is applied with a trowel into the prepared dish to the brim. Bayonet is performed with a reinforcing pin. After the compaction process, the surface of the concrete slurry is leveled by cutting off the excess.

The filled container is weighed with an error of up to a gram.

First, the net mass of the mixture is determined by subtracting the weight of the used tare for verification. Example: 5000-400 = 4600 g.
The result is then divided by the volume of the two-liter vessel. The result is: 4600/2000 = 2.3 kg per 2000 cm3.
At the last stage of calculations, it remains to find out the density in one cubic meter: 2.3 × 1000 = 2300 kg / m3.

Note! The density of the composition can be increased by proper selection of the aggregate, by reducing the amount of water, as well as by high-quality vibration using special equipment.

Hardness testing

This parameter determines not only the convenience of laying, but also, to some extent, the strength characteristics of concrete. Officially, the test is carried out using a special device in accordance with GOST 10181.1-81. The device is a cylindrical metal vessel.

Determination using a special device.

In the process of determining the stiffness, the product is fixed on a vibration platform with an amplitude of movements of 0.35 mm and a frequency of 2800 to 3200 vibrations per minute. The final indicator is the arithmetic mean of two determinations at once, taken from the same sample.

However, the price of such equipment is quite high, so individual developers do not have the opportunity to carry out research in this way. Therefore, you can use a simplified version, which provides for the presence of one vibrator.

The cubic shape, with an edge of 20 cm, is installed on a vibrating table and fixed in one position. A standard cone is placed in it, intended for filling with a solution. The vibration continues until the liquid composition is horizontally distributed. The value is determined using a stopwatch.

Mobility assessment

Testing is carried out using a standard galvanized iron or sheet steel cone. The amount of settlement of a given object characterizes the mobility of the solution. If the readings are too low, water and an astringent are added.

Scheme for determining the mobility of concrete.

Control of hardened material

The most accurate assessment of the quality of concrete is made after its final hardening, when 28 days have passed since pouring. Inspection can be destructive and non-destructive. In the first case, a direct sample is taken, and in the other, testing is carried out with various devices, the readings of which are not absolutely accurate.

Non-destructive methods

Tearing off the disks involves relieving stress, which is produced during local destruction. The applied force in this case is divided by the quadrature of the surface projection.
Chipping an edge allows you to define the characteristics of linear structures such as columns, beams, and piles. The method cannot be applied if the protective layer does not exceed 2 cm.
Shearing off is the only non-destructive testing method for which calibration dependences are officially regulated. When testing, it is possible to achieve high accuracy.
Quality control with ultrasound involves determining the speed of wave propagation. Distinguish between through and surface sounding. The difference lies in the location of the sensors.

Carrying out ultrasonic testing for the foundation.

Resilient rebound provides an opportunity to measure the amount by which the striker will move after impact on the surface of the structure. The tests are carried out using spring hammers.
The impact pulse allows you to register the energy of the perfect impact, which is formed when the striker touches the plane. Such devices are compact in size.
Plastic deformation is based on measuring the dimensions of the imprint left after being hit with a steel ball. The method is a little outdated, but it is still used due to the low cost of the device.

This is how the plastic deformation method is carried out.

Destructive methods

Sawing a sample from a concrete structure is carried out with special equipment such as URB-175, equipped with a cutting tool like diamond discs.
Drilling is carried out using drilling machines IE 1806. They have a diamond or carbide drill.

The drilling process is shown.

As a conclusion

Before purchasing a ready-made composition from the manufacturer, you need to find out if he has a quality certificate for concrete. This is not a prerequisite, but gives an idea of the reliability of the company's intentions. More detailed instructions on the topic are reflected in the video in this article.

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Selection and quality control of concrete

The correct choice and quality control of concrete is a guarantee of the strength, durability and safety of the future structure. It is very scrupulous to approach the issue of choosing a supplier, a method of delivering concrete and using a concrete solution.

Concrete foundations

The main load-bearing element of the entire building is the foundation. It is divided into various types. There are pile or block foundations, but most often tape, columnar or combined structures are used in the construction of private houses. At the same time, the quality of the concrete used for the construction of foundations is very important. This method is very popular due to its affordability, ease of construction and high strength of the resulting structures.

The choice of the grade of concrete is very important, the strength of the structure depends on it. For private houses, concrete from the 200th grade can be used. However, the fact that you ordered concrete of the grade you need does not mean that such a solution will be delivered to you. Less important indicators when using concrete are the degree of its ramming, the conditions of laying and setting.

How to choose a supplier

Conscientious manufacturers always control the process of making concrete, its composition and the quality of components. But how do you choose such a supplier? There are several ways:

You can contact companies that supply concrete for large construction projects. You can find their contacts at the foremen at such facilities.
Owners of newly built houses can also help make the choice of concrete for the foundation. That being said, you should examine the condition of their foundations. If you notice defects in the form of salt, cracks and delamination, then this indicates a low quality of concrete.
When choosing supplier sites, pay attention to those where there is not only a contact phone number, but also the organization's addresses, information about the company, registration data, etc.
When buying concrete, ask for a product passport with stamps. Moreover, he must go to each car separately. There must be specified grades for strength, mobility, frost resistance, time and day of shipment. It is better not to take handwritten passports without seals. It is advisable to buy Gostovsky concrete, not commercial.
It is also necessary to save supply contracts. This way you can file a complaint with the manufacturer.

But the quality control of concrete must be carried out in any case.

Transportation can affect the fluidity of the mixture, its consistency. If the vehicle is driven over bumps at high speed, the homogeneity of the solution may be impaired. As a result, large concrete elements will settle to the bottom, and the liquid will rise. This solution cannot be used. Of course, the solution can be mixed, but it is better not to take it at all. It is also worth doing when transporting it open.

The choice of concrete should be made taking into account the time taken for transportation. If this time is exceeded, the concrete may delaminate. It should be within 2-3 hours. And in order to save time, it is better to unload directly into the formwork.

Concrete quality control

It can be done in several ways. The visual method is as follows:

When delivering concrete, the composition must be of a uniform consistency.
You can do a test fill. The concrete should not either run out of water or fall out in lumps. This indicates its heterogeneity.
A check for excess water is carried out as follows: when pouring a small part of concrete into the pit, a cake should form without cracks and layers. If the composition looks like lumps in a liquid substance, then it is of poor quality.
Good concrete is gray in color. Brown indicates an overabundance of sand, while red indicates poor aggregates. However, when using plasticizing additives, the color of the concrete may differ from the gray. The uneven coloring should also alert. It is better to refuse such a composition.

Also, a test can be performed by a concrete quality control laboratory:

To do this, you need to pour the brought concrete into a wetted wooden box. Seal the composition by puncturing it with fittings. Place the box in a room with a specified humidity level and standard temperature. After 28 days, the sample is taken for testing.
Do not forget that you need to take a sample and pour it into molds to check the quality of concrete directly from the machine. You also need the driver to sign a sample collection report.
The laboratory uses a special device to test the quality of concrete.

How to pour the mixture

The choice of the concrete class is just as important as the correct placement of the mix. It is done in this way:

First you need to check the base in the pit. The fill base must be free of dust and clean. To do this, before pouring the mixture, the bottom of the pit is cleaned with air pressure.
Before pouring, the entire formwork is moistened with milk of lime or emulsions. This protects the concrete from strong adhesion to the formwork and makes it easier to dismantle.
Filling should be carried out in full in one day. This is necessary to obtain a solid monolithic structure. At large facilities, work is carried out by several teams from different machines at the same time.

To supply concrete, use a gutter with a length of 1.5 m. It must move to different points of the foundation belt, otherwise you will not be able to evenly distribute the concrete along the belt. For the concrete to flow well, it is very important to make the right choice of cement for concrete. But if you cannot move the chute along the belt, you will have to lengthen it. This will allow you to move the end of the gutter to the desired location.

The easiest way to pour the mixture is with a concrete pump. The consumption will be higher, but the productivity will increase. Moreover, the unit can supply mortar to a length of 50 m. Thus, it is possible to fill from one machine at different points of the base.
The distance at which the mixture is dropped should not be more than 200 cm.
Concreted in layers of 5-10 cm, without interrupting the pouring.
After pouring, the concrete is tamped. You can find out how the choice of a vibrator for concrete is made in our other article.

It is best to compact concrete with a downhole device. Choosing a vibrator for concrete will help increase the strength of the structure. In this case, the device is immersed in each layer separately, moving it inside the layer. It is not necessary to buy such a unit, you can rent it or order a rammer from a specific company.
If you do not want to buy a rammer, you can simply pierce the concrete in different places with reinforcement. However, this is very difficult, since you need to poke into concrete almost every square centimeter.

To protect the solution from external factors, after pouring it is covered with a film. But for uniform drying, you will need to remove the film 4 times a day and pour water on the concrete. This is done for the first three days, then for another three days the procedure is carried out three times a day. However, instead of watering, you can choose a steam generator for steaming concrete, it is more efficient.
Installation of walls begins after complete setting of the concrete (after four weeks). By this time, the concrete strength will be maximum, as can be judged from the test results.

Quality control after pouring

You have already figured out how to choose a concrete grade for a foundation, but it is also worth learning about ways to control a solidified foundation. For this, an ultrasound or a sclerometer can be used.

You can do this yourself. To do this, you need a chisel and a hammer weighing in the range of 300-800 g. The procedure is carried out as follows:

The chisel is placed at 90 degrees to the base and hit on it.
If the chisel fits easily into the base, then grade 70 grout or even lower was used.
When the tool is deepened by 3-7 mm, it can be said that the concrete grade was used in the range of 70-100.
Small holes with a depth of 2-3 mm indicate a concrete grade of 100 or 200.
When using a loaf with a brand of 200 and above, a small mark will be left as a result of the impact.

To make the result more accurate, such tests are carried out on different parts of the base.

Standards

The grade of the concrete composition affects the strength of the base after the mortar has completely set. This indicator is marked with the letter M and a digital indicator ranging from 50 to 500. It is measured in kgf / cm3. The indicator shows the compressive strength of concrete. For small structures, 100-150 brand foundations are sufficient. For individual houses, concrete of grade 200-300 is needed. High grade concrete is used for the manufacture of load-bearing reinforced concrete structures.

The correct choice of plasticizer for concrete will increase its frost resistance. This figure indicates the total number of freeze and thaw cycles that the solution has held. It is marked with the letter F and a digital indicator in the range of 50-500. For our latitudes, concrete of medium frost resistance is suitable.

Another important indicator is water resistance. It is marked with the letter W and is in the range of 2-12 kgf / cm2. The number indicates the concrete's ability to withstand water pressure. For foundations for an individual house, you can take concrete with an indicator W 2-4. In the construction of swimming pools, concrete with a water resistance index of 8-12 is used.

The choice of crushed stone for concrete should be made taking into account the loads on the structure

How to check the quality (strength) of concrete and concrete mix yourself

Everything is important in construction, but of course, special attention should be paid to the supporting structures of the building. We already wrote about ways to check brick (masonry) masonry in , now it's time to talk about concrete structures and checking their quality.

The quality of this type of structure largely depends on the quality of the concrete used in the construction and the correctness of its placement. Its indicators testify to the strength and durability of buildings and structures. In the event that bad concrete was delivered to you or its laying was done incorrectly, the most serious consequences are possible, up to the destruction of structures. Therefore, it is important to check the quality of the resulting structure, especially the quality of the foundation.

Concrete structures are most often found outdoors. As a result, with poor-quality compaction or poor-quality concrete mix, a large number of pores remain in the structure, through which moisture penetrates into the structure. Moisture gets into the structure, freezes, and destroys the micro-layer of concrete. This is a serious defect, therefore the quality of the concrete of the supporting structures must be of the best quality.

To control (check) concrete, you caninvite a specialist our center to the object or try to carry out the research yourself using the tools at hand according to the rules and tips described below.

If the construction is just beginning, it makes sense to determine the quality of the concrete even before the start of its laying.

Checking the concrete mix before laying

First you need to make sure what color of the concrete mass: It should be clean, gray, uniform... If the shade is brown, most likely the amount of sand in the concrete is exceeded and this concrete is of poor quality.

It is important to distinguish between the brown shade of concrete and sand and the possible brown shade due to various additives.

Its next indicator is homogeneity in composition. If it is not, this is also a big drawback and problems in the construction process. The mixture should flow and not fall in chunks. Its consistency should be plate, but at the same time, if it is liquid, this is also not good. Such concrete is also not of high quality.

At this stage, we strongly advise you to take samples of the supplied concrete when pouring important load-bearing structures.

To do this, you need to make cubic forms from the boards for pouring concrete samples. The dimensions are small - 100x100x100 mm.

The poured concrete mixture must be compacted using a rod (in layers) or by vibrating. These samples are then dried. The ambient temperature should be between 20-25 degrees Celsius.

After 28 days, this sample is taken to a specialized laboratory. Here it will be analyzed for strength. The analysis procedure is standard. As a result of this study, you will receive the most accurate values and characteristics of the concrete supplied to you.

Ideally, it would be to draw up an act on the pouring of samples and ask the driver who supplied you with the concrete mixture to sign it.

Checking the quality of concrete of the finished structure

First you need to carefully inspect the surface. It should be smooth. If it was poured in winter, then there will be no patterns on the concrete. If there are any, most likely he froze during the pouring period, and this is bad. As a result, the strength of the structure decreases within the range of 50-100 kg / cm2. (i.e. if you poured concrete of the M300 grade, in fact, the concrete of the structure will have the M200-250 grade).

1) Checking the quality of concrete by the sound of impact

To check the quality of the finished structure, you must use a hammer (or a piece of heavy thick iron pipe) weighing at least 0.5 kg.

The principle of the research is similar to the "Schmidt's hammer" and "Kashkarov's hammer" devices.

You need to evaluate the ringing tonality. If the sound is dull, then the concrete has poor strength, and its compaction is rather poor and of poor quality. Such a study is suitable for concrete structures of the M100 grade and higher.

2) Checking the quality (strength) of concrete with a chisel

The strength (class, brand) of concrete of a finished structure can be determined with a chisel by the effect on it of an average hammer impact force, weighing 300-400 grams.

If the chisel is easily immersed (driven) into concrete, it is necessary to exclude getting into the filler (crushed stone, gravel, etc.) - the concrete grade is below M70
If the chisel is immersed in concrete to a depth of about 5 mm. - then most likely the concrete grade is M70-M100
In the case when thin layers are separated from the concrete surface upon impact, the concrete grade is in the range of M100 - M200
Concrete grade M200 and more, if a completely shallow trace remains from the chisel, or it does not exist at all, and there are no delaminations.

All these methods, with the exception of laboratory tests of manufactured samples, give a general idea. For more accurate values and confidence in your design, it is better to use specialist services with specialized measuring instruments. After all, there are a large number of methods for non-destructive testing of concrete (ultrasonic examination of concrete, shock-impulse method, etc.).

Provides the standard strength of concrete products. With the help of mixtures, high-quality pouring of foundations and monolithic building structures is carried out. Concrete of any brand can withstand samples and the maximum number of checks on the quality of cement and fillers, in mixtures and ready-made structures.

We offer solutions and concrete from the manufacturer with standard indicators that meet the requirements of GOST. Strict compliance with the standards is ensured by the use of special additives and plasticizers that meet the requirements for water resistance, frost resistance, etc. But is it possible to assess the quality of concrete during pouring or when setting without a laboratory study?

How to find out the quality of concrete before pouring

It is possible to find out exactly the specific characteristics of the mixture only in the laboratory, which works at every large enterprise. After all, concretes M350 or M400 look almost the same as M100. When ordering a batch, the client is forced to trust the passport and documents for the ordered delivery, which are presented by the mixer driver. To do this, you must carefully review the certificates provided.

Next, you should pay attention to the brand indicated by the manufacturer, as well as the time when the invoices were issued. After all, the car makes many trips per day, and the documents presented for unloading may not correspond to the actually delivered batch to the construction site. There are also signs by which experienced craftsmen can roughly evaluate a brand.

How to visually identify the grade of concrete?

Pay attention to the shade of the mixture. Light brown indicates an excess of sand, reddish - about slag additives or the presence of clay. The solution should be uniform, gray in color. However, the change in color of the mixture may depend on the presence of various additives.
You can determine the quality of concrete by pouring a part into a separate container (bucket, moistened mold). Puddles of water should not appear on the solution.
When pouring, no delamination or cracks should form on the surface.
If the concrete falls like a "cake", and cement milk is poured separately, this can be considered a sign of poor quality of the provided mixture.

But it is possible to accurately check the concrete mixture only in a specialized laboratory, and most often - after curing. Therefore, when pouring the main foundation, it is necessary to cast cubes with a size of 100x100x100 mm and leave to harden under standard conditions. After full maturation (28 days), the samples should be sent for testing to the laboratory.

How to determine the quality of concrete after hardening

To assess the quality of the resulting product, there are several ways that experienced builders know.

Carefully inspect the resulting surface of the product or foundation. High grade concrete should have a smooth surface, without pores and layers. If the pouring work was carried out at low temperatures in winter, characteristic patterns should not appear on the surface, which indicate the freezing of the mixture. The presence of patterns indicates freezing during pouring, they lower the concrete mark by 70-100 points (from M300 to M200-250);
With a hammer that weighs no more than 500 g, knock on the foundation after reaching 70% of the strength characteristics. The sound should be clear. If hammer imprints remain on the surface with a ringing sound during impact, this shows that the density of concrete is 150-200 kg / cm2. A dull sound indicates a low grade of the mixture, and says the strength of which does not exceed 100 kg / cm2. And if, upon impact, cracks form on the surface or the material crumbles, it is recommended to re-fill.

By striking the chisel of a hammer with a low weight (300-400 grams), you can also find out the quality of concrete. It is necessary to evaluate how and to what depth the chisel plunges into the concrete with moderate impacts.

If the tip sinks deeply and easily without hitting rubble or gravel, the grade is below M70.
With an immersion depth of up to 5 mm, it can be assumed that the grade is equivalent to M70-M100.
For concretes of the M100-M200 brand, when the chisel strikes, small thin layers are only separated on the surface.
If there are no traces from the chisel at all or there is a shallow trace and there are no delamination at all, it can be assumed that the concrete grade is higher than M200.

However, all these methods provide only a rough estimate. Only laboratory tests can accurately determine the quality of the poured concrete and the reliability of the structure. In this case, special equipment, measuring devices and tools are used. In addition to testing cast control samples (a cube with an edge of 10 cm), there are many non-destructive methods for checking the quality of the mixture - ultrasonic, shock-pulse, other devices and control methods. The methods listed above are "popular" and do not claim to be highly accurate. In addition, the reliability of the determination depends on the experience of the master, who is trying to determine the grade of concrete.

How to check concrete performance in a laboratory

If the technology and proportions are precisely observed in the manufacture of the mixture at the enterprise, components are selected, there are no low-quality fillers, the quality of the finished product will undoubtedly be ideal. But just in case, it is recommended to prepare samples for subsequent control. They will be checked in a laboratory by compression method and an expert opinion will be issued.

In order to fully accurately determine the quality of the finished concrete and its compliance with the TU, it is necessary to pre-fabricate formwork in the form of cubes with an edge size of 100 mm. After the concrete has matured, the samples must be delivered to the laboratory, where they know exactly how to check the grade of concrete. This procedure is carried out after compaction (vibration) and drying under the same conditions in which the general pouring was carried out.

Checking and clarifying the grade of the resulting concrete, as well as issuing a certificate, should be carried out after the mixture has fully matured, after 28 days.