Fraud when ordering concrete. Determination of the quality of concrete upon acceptance

Construction is a rather laborious process. To eliminate unnecessary costs and not waste time, you should take good care of the quality of materials. First of all, you need to think about how to check the brand of concrete mix.

The ordered solution does not always correspond to the characteristics prescribed in the document. If the added raw materials for the manufacture of concrete do not meet the proper proportions, the quality of the solution automatically changes. To accurately recognize the brand, it is necessary to conduct a quality assessment.

Concrete grade is an indicator showing the compressive strength. M300-400 grades are suitable for construction. M100-250 have minimal strength, suitable only for auxiliary work. Much depends on the chosen supplier. It is worth looking for trusted companies with a good reputation that can provide the necessary documents for the products offered. If for some reason you doubt the honesty of the supplier, you should think about further research of the solution for compliance with the specified brand.

Determining the grade of concrete can be done by different methods:

• Laboratory expertise;
• Ultrasonic method;
• Self check.

Each method differs in percentage of accuracy and has certain subtleties.

Contact verification methods

Contact verification is carried out in two ways. The first is with the help of professional equipment - a sclerometer. The device determines the strength by shock impulse. The sclerometer can be mechanical and electronic, and its price ranges from 10 to 35 thousand, a purchase for a one-time use is simply not rational for an ordinary buyer.

The second method involves sending the sample to the laboratory. First you need to carry out a series of manipulations:

• Prepare a wooden box with a volume of 15 cm³;
• Pour the purchased solution into the mold directly from the concrete mixer tray, pre-moisten the box with water. Seal the filled solution by making several punctures with reinforcement;
• Place the sample for 28 days in the same conditions as the main structure;
• The frozen sample is taken to the laboratory for analysis. Evaluation can be made at intermediate stages of setting (3, 7 and 14 days).

The examination will issue a conclusion on the study of a sample of this brand, its compliance with established standards.

Sample testing in the laboratory

Ultrasonic technique

Ultrasonic devices, in addition to the study of strength, are used for flaw detection. The speed of propagation of ultrasound in concrete reaches 4500 m/s.

The calibration dependence between the speed of sound propagation and the compressive strength of concrete is fixed in advance for each composition of the mixture. In the case of using 2 dependencies for concretes of alternative or unknown compositions, inaccuracy may occur in determining the strength. The strength-ultrasound velocity ratio is influenced by a number of factors, the fluctuations of which in this case must be taken into account when applying ultrasonic testing:

• Method for making concrete mortar;
• Quantity and grain composition;
• Change in cement consumption by more than 30%;
• Possible cavities, cracks and defects in the finished structure;
• Concrete compaction level.

Ultrasonic testing is suitable for mass testing of structures of any shape, as well as for continuous monitoring of set or strength reduction. The disadvantage of the method is the error in the transition from acoustic to strength indicators. Ultrasonic equipment should not be used to check the quality of high-strength grades, the allowable range is limited to classes B7.5 ... B35 (10-40 MPa), in accordance with GOST 17624-87.

Ways to self-check

Testing in the laboratory or by special means does not always justify itself. This applies to those cases when a small building is being built on private land. The filled and frozen solution can be checked at home in several ways. If it does not meet the necessary requirements, you can use a paid examination and indemnify the supplier.

Smoothness test

Carefully consider the frozen structure. It should be smooth, the presence of patterns indicates non-compliance with the filling rules. Such a solution is likely to freeze, which will significantly reduce its strength. In fact, concrete of the M300 brand will, in terms of its properties, become like M200-250.

Voice test

You can check by the sound of impact. To do this, take a hammer or a piece of metal pipe, weighing no more than 0.5 kg. The ringing tonality when striking is important here. A dull sound indicates low strength and poor sealing. And when cracks, crumbs appear, it is necessary to completely or partially replace the structure.

visual assessment

The method involves checking the characteristics of the solution upon acceptance. You can highlight such points as:

• Color - a high-quality mixture is gray with a bluish tint, if yellowness is clearly manifested in the cement milk, clay impurities or slag additives are present in the mixture. Brown or red color is characterized by an excess of sand or aggregate in an unacceptable amount; it is more reasonable to refuse a solution with an uneven shade altogether;
• The correct consistency is homogeneous, without lumps and clots and resembles moist soil;
• Excess water - determined by pouring a small amount of the mixture into the pit, you should get a cake without layers and cracks;
• A purchased solution of inadequate quality begins to delaminate even during transportation, the mixture cannot be removed with a shovel or fed through a sleeve.

If a mixer is delivered, it is possible to determine the quality of concrete without inspection only according to the documents provided. In this case, everything depends on the honesty of the seller.

Checking concrete with a hammer and chisel

A hammer and chisel is the simplest answer to the question of how to check the quality of poured concrete. For this, a hammer impact test is performed. A chisel is attached to the surface of a completely dried foundation, and a blow is applied with medium force. If the resulting dent exceeds 1 cm, strength class B5 (M75), less than 0.5 cm - B10 (M150). A small dent remains on B15-25 (M200-250), a slight mark appears on B25 (M350).

It is necessary to take a hammer weighing 300-400 gr.

All the described methods have their advantages and disadvantages, for the accuracy of the result it is worth seeking help from specialists. Laboratory, ultrasound and shock-impulse studies are more reliable and comprehensive. The quality directly depends on the characteristics of the constituent components, compliance with proportions, storage and transportation conditions. Therefore, you can protect yourself by choosing a trusted supplier with a good reputation, this will significantly reduce the risk of problems in the future.

Many construction works are accompanied by concrete pouring. Not many people know what indicators of concrete quality to pay attention to when buying a ready-made solution or ready-made concrete blocks. Let's consider this question, because it is quite difficult to buy concrete, the price per cubic meter of which will not ruin your company.

What to pay attention to

So, to determine the quality of the mixture, pay attention to the following indicators:

• resistance to low temperatures or frost resistance. The concrete mixture must retain strength, both during freezing and after thawing;
• waterproof. Should be taken into account in conditions of high humidity or direct contact with water;
• corrosion resistance. This indicator is important in case of further contact of the screed with caustic chemicals;
• strength. An important parameter showing the resistance of building materials to all kinds of loads.

We check

To check the solution (blocks), strength tests are carried out and a conclusion is given based on their results. Examination is carried out both after pouring the solution and after kneading.

Methods for determining the quality of concrete:

1. in the laboratory (laboratory);
2. visual;
3. contact.

When visually inspecting, it is necessary to pay attention to such characteristics of the sample as consistency, presence of excess moisture, color.

Please note that high-quality concrete has a uniform gray color. If the technology is not followed, the color of the concrete will have red or brown shades.

When using the laboratory method, preliminary work is necessary:

• pouring the mixture into specially prepared wooden boxes;
• sealing mortar with parallel puncture fittings.

After the above steps, leave the prepared sample for 28 days in a room with conditions similar to the main room. After the expiration of the period, the sample is sent to the laboratory for analysis.

The contact method for analyzing the quality of the concrete mixture is carried out using a device - a sclerometer.

Solid solution analysis

If it is necessary to analyze already hardened concrete, the following methods are used:

• destructive;
• non-destructive.

With the destructive method, test samples (cores) are drilled out and an examination is carried out.

Non-destructive analysis is carried out using special equipment - a Kashkarov hammer, as well as an ultrasonic apparatus. The results obtained are analyzed based on the indicators of the calibration curves.

Concrete analysis is an important stage of construction work and it is advisable not to neglect it, so as not to redo the work later.

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 discussed 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.

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, the base of the concreted section and the areas of its interface at the joints with the formwork will be heated.

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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