Methods for checking the brand of concrete. Fraud when ordering concrete

If you are thinking about building your own house, then it is unlikely that you can do without concrete.

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

We will also consider in what ways it is possible to perform quality control of concrete: visually, contact or laboratory, and how to determine the quality of the hardened mixture.

Mixture quality indicators

To determine the quality properties of concrete, there are several indicators, namely:

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

The material grade is a 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 for different types of concrete

Obviously, the class has more stringent requirements. The parameters specified to match the class must be met in 95 cases out of 100. To match the brand, these indicators may be slightly higher or lower. To designate a class in regulatory documents, it is customary to use letters and numbers. Material grade B50 must withstand a pressure of 50 MPa in 95%.

In the construction market, you can find materials whose class can vary from B7.5 to B40.

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

The ratio of grades and classes of concrete

Concrete compositions initially have very low strength, so it can be determined on the 7-14th day, under normal conditions of solidification.

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 a different solidification medium, the interaction speed 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 bituminous emulsion or a polyethylene film laid on top can act as protection.

The quality characteristics of hardened concrete are influenced by the properties and indicators 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;
• during mixing of the concrete mixture;
• when pouring and compacting the solution;
• when finishing the finished product for compliance with the developed projects.

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

• postoperative;
• Acceptance.

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

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

How to define quality?

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 red tint indicates excess 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 the laboratory, 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. The principle of its 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 the frozen structure

To determine the quality of the hardened material, special cube samples 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 enter the results of the study into journals. At home, such a test 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 a Kashkarov 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 struck with a machinist's hammer, impressions will appear on the concrete and the rod, which are measured. With the ratio of the obtained figures, the strength of the material is determined from the calibration curves.

The next non-destructive method for testing the quality characteristics of concrete is ultrasound. The principle of such control is based on changing the speed of the waves produced by such a device through a material of different strength. The quality and strength of the material is set on the basis of calibration curves.

To the table of contents

Services of the online store "Russian Builder" on the quality of concrete

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Experts will help you choose the right materials for building a house. In addition, the online store "Russian Builder" provides its customers with services and quality control of concrete.

Poor quality consumables can cause problems from the very beginning of construction. To avoid such troubles, you can trust the specialists and order a technologist for your facility - he will control the quality of concrete and provide competent assistance in choosing the materials used.

High-class specialists will help to control the quality of not only freshly delivered concrete, but also poured. At the same time, the cost of work will pleasantly surprise you - the services of a technologist will cost 5 thousand rubles. The list of services provided includes:

Our specialists are competent in any issues related to construction, so we are ready to help you at any time. We will be glad to see you in our online store "Russian Builder".

Watch our video compilation:

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

When purchasing or building a private house on a concrete foundation, it is necessary to 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 universal tool will be described in this article.

Non-destructive methods for testing the quality of concrete

• First of all, you should carefully examine 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, which reduces the strength of the structure to 50-100 kg / cm2;
• Quality control using a hammer weighing at least 0.5 kg. Tap on the concrete structure and appreciate the tone of the sound. The ringing tonality and the absence of damage testify to the high quality of concrete and strength at a level of at least 200 kg/cm2; a ringing sound and imprints from a hammer 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 - concrete is of poor quality, strength is not more than 100 kg / cm2;
• Visible surface defects in the form of a significant number of pores "speak" of poor sealing. In addition, this is a 100% guarantee of poor quality concrete preparation. If this is a concrete structure located in the open air, there is a high risk of gradual destruction in the cycle: “moisture penetration, moisture freezing, destruction of the micro-layer of concrete”, etc.

Method for testing the quality of concrete with a hammer and chisel

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

We set the chisel on the surface to be checked at an angle of approximately 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. Evaluate the impact trace:

• The trace is barely noticeable - high quality concrete corresponding to the B25 grade;
• The trace is very noticeable - concrete grade B15-B25;
• Strong depressions formed. Concrete began to paint - concrete grade B10;
• 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 concrete before pouring. To do this, it is necessary to pour a sample with dimensions of 100x100x100 cm. Until the concrete has set, it should be pierced with a concrete rod to release air.

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

Assumption! If the construction time is running out, the sample can be transported 7-14 days after pouring. In this case, the exact exposure time should be indicated in the laboratory.

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

• Determination of the strength of concrete using a Fizdel hammer;
• Determination of the strength of concrete using a Kashkarov 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 liquid and hardened state

When actively working with building mixtures, sooner or later one has to learn to determine some characteristics by visual signs or with the help of special devices. If necessary, the quality of concrete can be checked both in the liquid state and in the hardened state, when the structure is already fully prepared.

The photo shows a liquid solution, the quality of which has yet to be verified.

Determining 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 done by hand or the manufacturer does not inspire confidence. Having performed independent control, you can learn a lot about the quality of products.

Density check

By calculating the approximate mass of a substance in a certain unit of volume, one can judge which category this composition belongs to. This parameter is particularly affected 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 measures must be taken 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 container used is immediately weighed, after which its volume in cubic centimeters is determined.

The main formula for memorization is highlighted.

Next, the finished mixture is applied with a trowel into the prepared dishes to the brim. Reinforcing pin is bayoneted. After the compaction process, the surface of the concrete mortar is leveled by cutting off the excess.

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

1. First, the net mass of the mixture is determined, for which the weight of the tare used for verification is subtracted. Example: 5000-400=4600 g.
2. Then the result is divided by the volume of a two-liter vessel. The result is: 4600/2000 = 2.3 kg per 2000 cm3.
3. 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 aggregates, reducing the amount of water, as well as high-quality vibration using special equipment.

Stiffness testing

Not only the convenience of laying depends on this parameter, 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 a range of motion 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.

A cubic mold with an edge of 20 cm is mounted on a vibrating table and fixed in one position. A standard cone is placed in it, designed to be filled with a solution. Vibration continues until the liquid composition is distributed horizontally. The value is determined using a stopwatch.

Mobility assessment

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

Scheme for determining the mobility of concrete.

Hardened Material Control

The most accurate assessment of the quality of concrete is made after its final solidification, when 28 days have passed since the moment of pouring. Control can be destructive or 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

• The separation of the disks involves the removal of stress, which is produced by local destruction. The applied force in this case is divided by the quadrature of the surface projection.
• Rib shearing 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 is the only non-destructive testing method for which calibration dependencies are officially regulated. When testing, it is possible to achieve high accuracy.
• Ultrasound quality testing involves determining the speed of waves. Distinguish between through and surface sounding. The difference lies in the location of the sensors.

Carrying out ultrasonic testing for the foundation.

• Elastic rebound provides an opportunity to measure the amount by which the striker will move after impact on the surface of the structure. Tests are carried out using spring hammers.
• The impact impulse allows you to register the energy of a perfect impact, which is formed when the striker comes into contact with the plane. Such devices are compact in size.
• Plastic deformation is based on measuring the size of the impression left after impact with a steel ball. The method is a bit outdated, but still used, due to the cheapness of the device.

This is how plastic deformation is carried out.

Destructive methods

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

The drilling process is shown.

As a conclusion

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

masterabeton.ru

Selection and quality control of concrete

The right choice and quality control of concrete is a guarantee of strength, durability and safety of the future structure. It is very scrupulous to approach the issue of choosing a supplier, the method of delivery of concrete and the use of concrete mortar.

concrete foundations

The main load-bearing element of the entire building is the foundation. It is divided into different types. There are pile or block foundations, but most often in the construction of private houses tape, columnar or combined structures are used. 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 brand of concrete is very important, the strength of the structure depends on it. For private houses, you can use concrete from the 200th brand. However, the fact that you ordered concrete of the brand you need does not mean that such a solution will be brought to you. Less important indicators when using concrete are the degree of compaction, laying and setting conditions.

How to choose a supplier

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

1. You can contact companies that supply concrete to large construction projects. You can find their contacts at the foremen at such facilities.
2. Owners of newly built houses can also help make the choice of concrete for the foundation. At the same time, you should study the condition of their foundations. If you notice defects in the form of salt, cracks and delaminations, then this indicates a low quality of concrete.
3. When choosing supplier websites, pay attention to those that contain not only a contact phone number, but also company addresses, company information, registration information, etc.
4. When buying concrete, ask for a product passport with stamps. And it should go to each car separately. There should be stipulated grades for strength, mobility, frost resistance, time and day of shipment. It is better not to take handwritten passports without stamps. It is advisable to buy Gostovsky concrete, and not commodity.
5. It is also necessary to save supply contracts. So you can complain to the manufacturer.

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

Transportation can affect the mobility 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 elements of concrete 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 in an open state.

The choice of concrete should be made taking into account the time spent on transportation. If this time is exceeded, the concrete may delaminate. It should be within 2-3 hours. And 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:

1. When delivering concrete, the composition must be of a homogeneous consistency.
2. You can test fill. Concrete should not either pour water or fall out in lumps. This speaks to its heterogeneity.
3. Checking for excess water is carried out as follows: when pouring a small part of the 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.
4. Good concrete is gray. Brown indicates an excess of sand, red indicates bad aggregates. However, when plasticizing additives are used, the color of the concrete may differ from gray. The uneven coloring should also alert. It is better to refuse such a composition.

Also, the laboratory for testing the quality of concrete can perform the test:

1. To do this, you need to pour the brought concrete into a moistened wooden box. Seal the composition by puncturing it with reinforcement. Put the box in a room with a given level of humidity and standard temperature. After 28 days, the sample is taken for testing.
2. 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. It is also necessary for the driver to sign the sampling act.
3. The laboratory uses a special device to test the quality of concrete.

How to fill the mixture

The choice of concrete class is as important as the correct placement of the mixture. It runs like this:

• First you need to check the base in the pit. The bulk 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 wetted with milk of lime or emulsions. This will protect the concrete from strong adhesion to the formwork and facilitate its dismantling.
• Filling should be carried out in full in one day. This is necessary to obtain a solid monolithic structure. On large objects, work is carried out by several teams from different machines at the same time.

• A 1.5 m long chute is used to supply concrete. It must move to different points of the foundation tape, otherwise you will not be able to evenly distribute the concrete along the tape. In order for 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 increase its length. So you can move the end of the gutter to the right place.

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

• It is best to tamp concrete with a deep device. The choice of internal vibrator for concrete will help to 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 in almost every square centimeter

• To protect the solution from the effects of external factors after pouring, it is covered with a film. But for uniform drying, it will be necessary to remove the film 4 times a day and water the concrete with water. 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 concrete steaming, it is more efficient.
• Installation of walls begins after the concrete has completely set (after four weeks). By this time, the concrete strength will be maximum, which can be judged by the test results.

Quality control after pouring

You have already understood how to choose the grade of concrete for the foundation, but it is also worth learning about ways to control the hardened base. For this, ultrasound or a sclerometer can be used.

You can do this check yourself. To do this, you will need a chisel and a hammer weighing between 300-800 g. The procedure is carried out as follows:

1. The chisel is placed at 90 degrees to the base and hit on it.
2. If the chisel easily enters the base, then a solution of brand 70 or even lower was used.
3. 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.
4. Small cracks 2-3 mm deep indicate a concrete grade of 100 or 200.
5. When using a 200 brand long loaf and higher, 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.

Regulations

The brand of concrete composition affects the strength of the base after the solution 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, foundations of grade 100-150 are sufficient. For individual houses, concrete grade 200-300 is needed. Concrete of high grades is used for the manufacture of load-bearing reinforced concrete structures.

The right choice of plasticizer for concrete will increase its frost resistance. This indicator indicates the total number of freeze and thaw cycles maintained by the solution. 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 ability of concrete to withstand water pressure. For foundations for an individual house, you can take concrete with an indicator W 2-4. When constructing 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 methods for checking brick (stone) 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 laying. Its indicators testify to the strength and durability of buildings and structures. In the event that bad concrete was delivered to you or it was laid 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 enters the structure. Moisture gets into the structure, freezes, and destroys the micro layer of concrete. This is a serious defect, so the quality of the concrete of the load-bearing structures must be the best.

To control (check) concrete, you caninvite a specialist of our center to the object or try to conduct a study on your own using the tools at hand according to the rules and tips described below.

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

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 tint of concrete from sand and the possible brown tint due to various additives.

Its next indicator is uniformity in composition. If it is not so, this is also a big drawback and problems in the construction process. The mixture should pour, not fall in chunks. Its consistency should be a 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 delivered concrete when pouring important load-bearing structures.

To do this, you need to make cube-shaped molds from boards for pouring concrete samples. The dimensions are small - 100x100x100 mm.

The poured concrete mixture must be compacted with 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 delivered to you.

It would be ideal to draw up an act of pouring samples and ask the driver who delivered the concrete mix to sign on it.

Checking the quality of finished concrete

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 it froze during the pouring period, which is bad. As a result, the strength of the structure is reduced within 50-100 kg/cm2. (i.e. if you poured concrete of grade M300, in fact, concrete of the structure will have grade M200-250).

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 study is similar to the devices "Schmidt hammer" and "Kashkarov's hammer".

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

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

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

• If the chisel is easily immersed (driven) into concrete, it is necessary to prevent getting into the filler (crushed stone, gravel, etc.) - the concrete grade is lower than M70
• If the chisel is immersed in concrete to a depth of about 5 mm. - then most likely the brand of concrete 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 or more, if the chisel leaves a very shallow mark 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 testing of concrete, shock-pulse method, etc.).

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 make the concrete mixture, and on the observance of technological conditions at each stage of concrete work.

Careful control should be exercised 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 reinforcing structure;
• creation and assembly of formwork elements;
• preparation of formwork and foundation for concrete laying;
• preparation and transportation of the concrete mixture to the place of laying;
• maintenance of a concrete structure during the period of its attainment of 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 single methodology, specially designed for laboratories at construction companies.

Material quality control

In the course of reinforcement operations, the quality of work and materials is checked upon receipt of the reinforcement - factory marking (presence of tags) is checked, the conformity of marks to the declared requirements of designers. The processes of storage and transportation are accompanied by checking the correct placement of reinforcing steel by grades, grades and sizes, maintaining 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 the welds and their quality are checked. The reinforcing 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 a check of the correctness of their installation, the construction of fasteners, the tightness of the pairing of panels at the joints, the compliance of the assembled formwork form and the reinforcing structure (ensuring the formation of a protective layer of a given thickness). The spatial position of the formwork is analyzed by leveling and binding to the axes in several separate sectors, the correctness 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 mixture is placed, the formwork surfaces are checked for cleanliness and the quality of the application of lubricants.

Composition and placement of concrete mix

The introduction of the mixture components into the mixer is accompanied by a thorough check of the dosed portions, the duration of mixing, the density and degree of concrete mobility. The control of the mobility of the concrete mix is ​​carried out at least twice per shift, its indicators should not be 10 mm less or more than the calculated ones, density tolerances should not exceed 3%.

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

At the place of concrete work, it is important to monitor the height of the mixture dropping, the duration of vibration to achieve uniform compaction, to prevent the mixture from separating, the formation of voids and shells in its structure.

Vibrocompaction of the concrete mixture is carried out under visual control, the criteria are the degree of its precipitation, the formation of cement laitance, the completion of the release of air bubbles. More precisely, 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 that look like probes, placed depending on the thickness of the mixture being laid. The higher the density of concrete, the lower its resistance to electric current passing through the concrete mixture - the operation of the sensors is based on this principle. They are installed near vibrating plants, informing the operator about the achievement of the required density by a sound and light signal.

Evaluation of the strength of concrete from its samples

It is possible to find out the full qualitative characteristics of concrete 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 that the concrete is placed, and they are kept under exactly the same conditions as the main concrete structures. Typically, cubes 160 mm long pass the compression test.

Depending on the class of concrete, it is required to produce three test cubes of the same size. To evaluate the characteristics of foundations intended for various structures, cubes are formed from every 100 cubic meters of concrete mix. When creating massive foundation structures designed for the installation of technological equipment, samples for strength testing are prepared from every 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.

Non-destructive 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 evaluate concrete structures and structures without any destruction. Among them, the most popular are the mechanical method, based on the relationship between the surface hardness of concrete and its compressive strength, as well as the pulse-ultrasonic method, the use of which is based on measurements of the velocity of longitudinal ultrasound waves directed into the concrete structure and the degree of their complete attenuation.

Tests of the strength characteristics of reinforced concrete by the method of mechanical action are 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 side with an ordinary metalworker's hammer. After the impact, the concrete surface and the reference rod will leave indentations, the measurement of which will determine the surface compressive strength of the concrete. The design of the Kashkarov hammer must comply with GOST 22690-88.

Hammer Schmidt. There is an impact rod in its body - after removing the lock, 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 the 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 rebound 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, the speed of which through the thickness of the concrete determines its strength characteristics. If the technological conditions meet certain requirements - the use of materials with similar characteristics, the compliance of the technology with established standards, etc. - the accuracy of data on the strength of concrete will be quite high.

Quality control of concrete work in winter

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

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

Transportation of concrete is carried out with a one-time check for a change in the state of covering and insulating materials, the quality of heating and thermal insulation of 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 the start of work on laying the mixture, an examination of the internal walls of the formwork, the base of the concreted site and the reinforcing structure for the absence of snow and ice is carried out. The outer walls of the formwork must be thermally insulated in accordance with the technological conditions, it will warm up the base of the concreted section and its junction areas at the joints with the formwork.

In the process of laying the concrete, its temperature is controlled at the stage of unloading from the vehicle, then the temperature readings are taken again, but after the completion of the work on placing the concrete. Concreting areas not closed by formwork should also be assessed for technological compliance in terms of waterproofing and heat-insulating characteristics.

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

• when using preheating, “thermos” and heating technologies under specified temperature and humidity conditions (green house), temperature measurements should be taken every two hours during the first day, at least twice during the shift over the next three days and once every 24 hours during the further period of aging;
• when laying concrete containing antifreeze additives, its temperature must be measured three times during each day from the moment the work is completed until it reaches the design strength;
• when conducting electrical heating of a concrete structure, during its temperature rise at intervals 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 period of curing and gaining design strength, as well as the dismantling of the formwork, the air temperature is measured at least once during each work shift. Temperature data on a concrete structure is obtained by drilling narrow holes 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, a zone of direct contact with thermosetting formwork elements. Accounting for 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 a log.

Laboratory testing of concrete samples for strength is carried out in accordance with the standard procedure given above. In addition, additional sample cubes are created at the site of concrete work, designed to test for strength:

In a situation where test specimens are kept at low temperatures, it is necessary to first keep them at a temperature of +15 to +20 ° C, and then check their strength characteristics.

If a set of strength characteristics of a concrete structure is provided with the help of 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 regimes.

In addition to the assessment of strength, carried out by destroying sample cubes and drilled cores, it is necessary to check by non-destructive methods - for example, using Schmidt and Kashkarov hammers. It is important to carefully record each quality control operation carried out in accordance with concrete work technologies, 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 temperature control logs are kept in the prescribed manner and in accordance with the established forms.

Question:

Asks Oleg from Rostov: “Hello! The foundation will be poured soon. Concrete will be ordered from the factory. The question is whether it is possible to determine the quality of concrete when it is brought to the construction site. It is clear that it is impossible to determine exactly without a laboratory, but at least approximately visually you can see a low-quality product?

Answer:

The question of how to check the quality of concrete for the foundation before it is laid inside the formwork is very relevant for private developers. In the absence of a laboratory, special instruments and chemical reagents, the characteristics of this structural material can be approximately estimated by several indirect signs:

• color - a bluish tint indicates a normal amount of cement, a brown color most likely indicates the predominance of sand;
• cement milk - in 85% of cases, the yellowness of this product indicates harmful impurities of clay, which is extremely dangerous for reinforced concrete structures, however, in the remaining 15% of options, some harmless slags give exactly the same color;
• consistency - ready-mixed heavy concretes must be mixed by RBI employees according to exact recipes on industrial equipment, therefore the water-cement ratio W / C is always lower (the mixture is thick and inactive) than when using a household concrete mixer, plasticity is provided by special additives, the formation of cement stone (hydration) occurs faster;
• uniformity - the mixture should not contain particles of crushed stone or lumps of sand that have not been deboned in cement laitance.

The last and most reliable method is to rent a concrete pump - a mixture of poor quality, even if the recipe at the RBI mortar unit is followed, will not go through the hoses of this special equipment. Concrete of class B10 and above (corresponds to grade M150) with mobility P3 and above is suitable for supplying material with a concrete pump.

The supply of material by a concrete pump will provide additional quality assurance.

After gaining 70% strength when hitting concrete with a reinforcing bar, the sound should be clear and resonant. If cracks form or the material begins to crumble, the structure should be completely or partially destroyed in order to fill it again or try to strengthen it with a clip.

Upon visual inspection, the specialist is able to determine from the “patterns” on the surface that the structural material has been subjected to freezing after pouring before curing. In poor concrete, the outer surface is never smooth. The simplest tools (hammer / chisel) can determine the strength class of a structural material with an accuracy of 70%:

• immersion of the chisel into concrete when hitting 400 g with a hammer more than 10 mm - approximately B5;
• recess within 7 mm - about B10;
• a noticeable scratch - a mixture of B15;
• almost imperceptible trace - B25.

Mechanical method for determining the strength class.

Important! All of these methods are "folk" and do not claim to be the exact method. Even in special laboratories, samples are examined on the 28th day after selection at the time of pouring the load-bearing structure. In any case, it is possible to reliably determine the class of concrete only after it has gained strength and only in laboratory conditions. Indirect signs will help to distinguish only concrete of very poor quality.

Concrete has the main characteristics - strength (brand or class), mobility, frost resistance, water permeability. It is possible to reliably measure plasticity in the building spot, which is also called workability or mobility, according to the cone settlement method:

• on a plate with a diameter of 0.5 m, a truncated cone is installed in the center with a funnel down;
• the size of the cone is 305 x 203 x 102 mm (height, diameter of the lower and upper holes, respectively;
• after filling the cone with concrete and compacting the structural material with a reinforcement bar, the mold is removed;
• concrete spreads over a metal platform.

After removing the shaping funnel, shrinkage is measured relative to the top of the pyramid:

• P1 - no more than 4 cm
• P2 - shrinkage 5 - 9 cm
• P3 - interval 10 - 15 cm
• P4 - within 16 - 20 cm
• P5 - more than 21 cm

Cast concrete mass shrinks from 16 cm, plastic by 5 - 15 cm, rigid - within 4 cm.

When the developer receives the mixture by truck mixers, it is unlikely that it will be possible to improve the characteristics of the material. The mixer is not designed to add cement and other components on site, it can only dump the product in parts and keep it in a plastic state.

If the mixture is made in the building spot with a concrete mixer, the situation can be corrected and the ratio of components for subsequent batches can be adjusted. In this case, you should take into account the nuances:

• the process of hydration (formation of cement stone) is an irreversible chemical reaction;
• cement, filler, water or additives can only be added before setting starts, which starts at 45 - 180 minutes, depending on the modifiers and additives used during mixing.

If the recipe is changed after the setting has begun, the chemical bonds of the structure that begins to form are destroyed, and the strength of the structural material is sharply reduced. Internal stresses contribute to the opening of cracks; it is impossible to withstand the class of concrete.

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The right choice and quality control of concrete is a guarantee of strength, durability and safety of the future structure. It is very scrupulous to approach the issue of choosing a supplier, the method of delivery of concrete and the use of concrete mortar.

concrete foundations

The main load-bearing element of the entire building is the foundation. It is divided into different types. There are pile or block foundations, but most often in the construction of private houses tape, columnar or combined structures are used. 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 brand of concrete is very important, the strength of the structure depends on it. For private houses, you can use concrete from the 200th brand. However, the fact that you ordered concrete of the brand you need does not mean that such a solution will be brought to you. Less important indicators when using concrete are the degree of compaction, laying and setting conditions.

How to choose a supplier

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

1. You can contact companies that supply concrete to large construction projects. You can find their contacts at the foremen at such facilities.
2. Owners of newly built houses can also help make the choice of concrete for the foundation. At the same time, you should study the condition of their foundations. If you notice defects in the form of salt, cracks and delaminations, then this indicates a low quality of concrete.
3. When choosing supplier websites, pay attention to those that contain not only a contact phone number, but also company addresses, company information, registration information, etc.
4. When buying concrete, ask for a product passport with stamps. And it should go to each car separately. There should be stipulated grades for strength, mobility, frost resistance, time and day of shipment. It is better not to take handwritten passports without stamps. It is advisable to buy Gostovsky concrete, and not commodity.
5. It is also necessary to save supply contracts. So you can complain to the manufacturer.

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

Transportation can affect the mobility 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 elements of concrete 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 in an open state.

The choice of concrete should be made taking into account the time spent on transportation. If this time is exceeded, the concrete may delaminate. It should be within 2-3 hours. And 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:

1. When delivering concrete, the composition must be of a homogeneous consistency.
2. You can test fill. Concrete should not either pour water or fall out in lumps. This speaks to its heterogeneity.
3. Checking for excess water is carried out as follows: when pouring a small part of the 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.
4. Good concrete is gray. Brown indicates an excess of sand, red indicates bad aggregates. However, when plasticizing additives are used, the color of the concrete may differ from gray. The uneven coloring should also alert. It is better to refuse such a composition.

Also, the laboratory for testing the quality of concrete can perform the test:

1. To do this, you need to pour the brought concrete into a moistened wooden box. Seal the composition by puncturing it with reinforcement. Put the box in a room with a given level of humidity and standard temperature. After 28 days, the sample is taken for testing.
2. 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. It is also necessary for the driver to sign the sampling act.
3. The laboratory uses a special device to test the quality of concrete.

How to fill the mixture

The choice of concrete class is as important as the correct placement of the mixture. It runs like this:

• First you need to check the base in the pit. The bulk 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 wetted with milk of lime or emulsions. This will protect the concrete from strong adhesion to the formwork and facilitate its dismantling.
• Filling should be carried out in full in one day. This is necessary to obtain a solid monolithic structure. On large objects, work is carried out by several teams from different machines at the same time.

• A 1.5 m long chute is used to supply concrete. It must move to different points of the foundation tape, otherwise you will not be able to evenly distribute the concrete along the tape. In order for 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 increase its length. So you can move the end of the gutter to the right place.

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

• It is best to tamp concrete with a deep device. The choice of internal vibrator for concrete will help to 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 in almost every square centimeter

• To protect the solution from the effects of external factors after pouring, it is covered with a film. But for uniform drying, it will be necessary to remove the film 4 times a day and water the concrete with water. 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 concrete steaming, it is more efficient.
• Installation of walls begins after the concrete has completely set (after four weeks). By this time, the concrete strength will be maximum, which can be judged by the test results.

Quality control after pouring

You have already understood how to choose the grade of concrete for the foundation, but it is also worth learning about ways to control the hardened base. For this, ultrasound or a sclerometer can be used.

You can do this check yourself. To do this, you will need a chisel and a hammer weighing between 300-800 g. The procedure is carried out as follows:

1. The chisel is placed at 90 degrees to the base and hit on it.
2. If the chisel easily enters the base, then a solution of brand 70 or even lower was used.
3. 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.
4. Small cracks 2-3 mm deep indicate a concrete grade of 100 or 200.
5. When using a 200 brand long loaf and higher, 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.

Regulations

The brand of concrete composition affects the strength of the base after the solution 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, foundations of grade 100-150 are sufficient. For individual houses, concrete grade 200-300 is needed. Concrete of high grades is used for the manufacture of load-bearing reinforced concrete structures.

The right choice of plasticizer for concrete will increase its frost resistance. This indicator indicates the total number of freeze and thaw cycles maintained by the solution. 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 ability of concrete to withstand water pressure. For foundations for an individual house, you can take concrete with an indicator W 2-4. When constructing 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