Determination of the strength of concrete when inspecting buildings and structures. Concrete strength control by the method of shearing off with shearing Determination of strength by mechanical methods of non-destructive testing

A. V. Ulybin, Ph.D. S. D. Fedotov, D. S. Tarasova (PNIPKU "Venture", St. Petersburg)

This article discusses the main methods of non-destructive testing of concrete strength, used in the inspection of buildings and structures. The results of experiments on the comparison of data obtained by non-destructive methods of control and testing of samples are presented. The advantage of the method of separation with spalling over other methods of strength control is shown. The measures are described, without which the use of indirect non-destructive testing methods is unacceptable.

The compressive strength of concrete is one of the most frequently monitored parameters in the construction and inspection of reinforced concrete structures. There are a large number of control methods used in practice. More reliable, from the point of view of the authors, is the determination of strength not by control samples (GOST 10180-90) made from a concrete mixture, but by testing the concrete of the structure after it has set its design strength. The method of testing control samples allows you to assess the quality of the concrete mix, but not the strength of the concrete of the structure. This is due to the fact that it is impossible to provide identical conditions for strength gain (vibration, heating, etc.) for concrete in a structure and concrete cubes of samples.

Control methods according to the classification of GOST 18105-2010 ("Concrete. Rules for control and assessment of strength") are divided into three groups:

Destructive;
Direct non-destructive;
Indirect non-destructive.

Table 1. Characteristics of methods for non-destructive testing of concrete strength.

№	Method name	*Application range , MPa**	Measurement error**
1	Plastic deformation	5 - 50	± 30 - 40%
2	Elastic rebound	5 - 50	± 50%
3	Shock impulse	10 - 70	± 50%
4	Detachment	5 - 60	There is no data
5	Chipping off	5 - 100	There is no data
6	Chipping ribs	5 - 70	There is no data
7	Ultrasonic	5 - 40	± 30 - 50%

* According to the requirements of GOST 17624-87 and GOST 22690-88;

** According to the source without building a private calibration dependence

The methods of the first group include the mentioned method of control samples, as well as a method for determining strength by testing samples taken from structures. The latter is basic and is considered the most accurate and reliable. However, when examining him, they rarely run to him. The main reasons for this are a significant violation of the integrity of structures and the high cost of research.

Methods for determining the strength of concrete by non-destructive testing are mainly used. Moreover, most of the work is performed by indirect methods. Among them, the most common today are the ultrasonic method in accordance with GOST 17624-87, methods of shock impulse and elastic rebound in accordance with GOST 22690-88. However, when using these methods, the requirements of the standards for the construction of private calibration dependences are rarely met. Some performers are not aware of these requirements.

Others know, but do not understand, how big the error of measurement results is when using the dependencies embedded or attached to the device, instead of the dependency built on the specific concrete under study. There are “specialists” who know about the specified requirements of the norms, but neglect them, focusing on the financial benefit and the customer's ignorance in this matter.

Many works have been written about the factors influencing the error in measuring the strength without constructing private calibration dependencies. Table 1 shows the data on the maximum measurement error by various methods, given in the monograph on non-destructive testing of concrete.

In addition to the indicated problem of using inappropriate ("false") dependencies, let us designate one more problem that arises during the survey. According to the requirements of SP 13-102-2003, the provision of a sample of measurements (parallel tests of concrete by an indirect and direct method) in more than 30 areas is necessary, but not sufficient for the construction and use of a calibration dependence. It is necessary that the dependence obtained by paired correlation-regression analysis has a high correlation coefficient (more than 0.7) and a low standard deviation (less than 15% of the average strength). For this condition to be fulfilled, the measurement accuracy of both controlled parameters (for example, the speed of ultrasonic waves and the strength of concrete) must be sufficiently high, and the strength of the concrete used to construct the dependence must vary over a wide range.

When examining structures, these conditions are rarely met. Firstly, even the basic method of testing samples is often accompanied by a high error. Secondly, due to the heterogeneity of concrete and other factors, the strength in the surface layer (investigated by the indirect method) may not correspond to the strength of the same area at a certain depth (when using direct methods). And finally, with the normal quality of concreting and the conformity of the class of concrete to the design one, within one object, one can rarely find structures of the same type with strength varying over a wide range (for example, from B20 to B60). Thus, the dependence has to be built on the basis of a sample of measurements with a small change in the investigated parameter.

As an illustrative example of the above problem, consider the calibration dependence shown in Fig. 1. Linear regression dependence is built according to the results of ultrasonic measurements and press tests of concrete samples. Despite the large scatter of measurement results, the dependence has a correlation coefficient of 0.72, which is permissible according to the requirements of SP 13-102-2003. When approximated by functions other than linear (power, logarithmic, etc.), the correlation coefficient was less than indicated. If the range of the concrete strength under study were less, for example, from 30 to 40 MPa (the area highlighted in red), then the set of measurement results would turn into a “cloud” presented in the right part of Fig. 1. This point cloud is characterized by the lack of connection between the measured and the desired parameters, which is confirmed by the maximum correlation coefficient of 0.36. In other words, the calibration dependence cannot be plotted here.

RICE. 1. The relationship between the strength of concrete and the speed of ultrasonic waves

It should also be noted that at ordinary objects the number of strength measurement sites for building a calibration dependence is comparable to the total number of measured sites. In this case, the strength of concrete can be determined from the results of only direct measurements, and there will be no sense in the calibration dependence and the use of indirect control methods.

Thus, without violating the requirements of the current standards, in any case, it is necessary to use direct non-destructive or destructive control methods to determine the strength of concrete during inspection. Taking this into account, as well as the problems indicated above, we will further consider in more detail direct methods of control.

According to GOST 22690-88, this group includes three methods:

Tear-off method

The pull-off method is based on measuring the maximum force required to pull off a fragment of a concrete structure. The pull-off load is applied to the flat surface of the test structure by gluing a steel disc (Fig. 2) with a pull rod to connect to the instrument. Various epoxy-based adhesives can be used for bonding. GOST 22690-88 recommends adhesives ED20 and ED16 with a cement filler.
Today, modern two-component adhesives can be used, the production of which is well established (POXIPOL, Contact, Moment, etc.). In the domestic literature on concrete testing, the test method involves gluing the disk to the test site without additional measures to limit the separation zone. Under these conditions, the breakaway area is not constant and should be determined after each test. In foreign practice, before testing, the separation area is limited by a groove created by circular drills (crowns). In this case, the separation area is constant and known, which increases the measurement accuracy.

After tearing off the fragment and determining the force, the tensile strength of concrete (R (bt)) is determined, by which the compressive strength (R) can be determined by recalculating using the empirical dependence. For translation, you can use the expression specified in the manual:

For the pull-off method, various devices can be used that are also used for the pull-off method with spalling, such as ONIKS-OS, PIB, DYNA (Fig. 2), as well as old analogs: GPNV-5, GPNS-5. To carry out the test, it is necessary to have a gripper corresponding to the thrust located on the disc.

Rice. 2. Tear-off device with a disc for gluing to concrete

In Russia, the separation method is not widely used. This is evidenced by the absence of commercially available devices adapted for mounting to disks, as well as the disks themselves. There is no dependence in the regulatory documents for the transition from pull-out force to compressive strength. In the new GOST 18105-2010, as well as the previous GOST R 53231-2008, the tear-off method is not included in the list of direct non-destructive testing methods and is not mentioned at all. The reason for this, most likely, is the limited temperature range of application of the method, which is associated with the duration of hardening and (or) the impossibility of using epoxy adhesives at low air temperatures. Most of Russia is located in colder climatic zones than European countries; therefore, this method, which is widely used in European countries, is not used in our country. Another negative factor is the need to drill the furrow, which further reduces inspection performance.

Rice. 3. Testing of concrete by the method of prying off with spalling

This method has a lot in common with the tear-off method described above. The main difference is the way it is attached to concrete. Lapel anchors of various sizes are used to apply the breakout force. When inspecting structures, anchors are placed in a hole drilled in the measurement area. Just as with the pull-off method, the breaking force (P) is measured. The transition to concrete compressive strength is carried out according to the dependence specified in GOST 22690: R = m 1 .m 2 .P, where m 1- coefficient taking into account the maximum size of coarse aggregate, m 2- coefficient of conversion to compressive strength, depending on the type of concrete and hardening conditions.

In our country, this method has found, perhaps, the most widespread due to its versatility (Table 1), the relative ease of attachment to concrete, the possibility of testing practically on any section of the structure. The main limitations for its application are dense concrete reinforcement and the thickness of the test structure, which must be greater than twice the length of the anchor. The instruments specified above can be used to carry out the tests.

Table 2. Comparative characteristics of direct methods of non-destructive testing

Advantages	Method
Advantages	Detachment	Chipping off	Chipping a rib
Determination of the strength of concrete with a class of more than B60	-	+	-
Can be installed on uneven concrete surfaces (unevenness more than 5 mm)	-	+	-
Possibility of installation on a flat section of the structure (without the presence of an edge)	+	+	-
No need for a power supply for the installation	+*	-	+
Fast installation time	-	+	+
Work at low air temperatures	-	+	+
Availability in modern standards	-	+	+

* Without drilling the breakaway groove.

In addition to the simpler and faster fastening to the concrete of the structure in comparison with the pull-off method, the presence of a flat surface is not required. The main condition is the need for the curvature of the surface to be sufficient for the installation of the device on the anchor rod. As an example, Fig. 3 shows the POS-MG4 device installed on the destructed surface of the abutment of a hydraulic structure.

Rib cleaving method

The last direct method of non-destructive testing is a modification of the pull-off method - the method of chipping the rib. The main difference is that the strength of concrete is determined by the force (P) required to shear off the section of the structure located on the outer edge. In our country, for a long time, devices of the type GPNS-4 and POS-MG4 Skol were produced, the design of which assumed the obligatory presence of two adjacent external corners of the structure.

The grips of the device, like a clamp, were attached to the tested element, after which a force was applied through the gripping device to one of the ribs of the structure. Thus, the test could only be carried out on linear elements (columns, beams) or in openings at the edges of flat elements (walls, floors). Several years ago, a device was developed that allows it to be mounted on a test piece with only one outer edge. Fastening is carried out to one of the surfaces of the test element using an anchor with a dowel. This invention somewhat expanded the range of application of the device, but at the same time eliminated the main advantage of the chipping method, which consisted in the absence of the need for drilling and the need for a source of electricity.

The compressive strength of concrete when using the rib shearing method is determined by the normalized relationship: R = 0.058 .m .(30P + P 2) ,

where m- coefficient taking into account the size of the aggregate.

For clarity, the comparison of the characteristics of direct control methods are presented in table. 2.

According to the data given in the table, it can be seen that the greatest number of advantages is characterized by the method of separation with spallation.

However, despite the possibility of using this method according to the instructions of the norms without constructing a particular calibration dependence, many specialists have a question about the accuracy of the results obtained and the correspondence of their concrete strength, determined by the method of testing samples. To investigate this issue, as well as to compare the results of measurements obtained by the direct method with the results of measurements by indirect methods, the experiment described below was carried out.

Method comparison results

In the laboratory "Inspection and Testing of Buildings and Structures" of the Federal State Budgetary Educational Institution of Higher Professional Education "SPbSPU", studies were carried out using various control methods. A fragment of a concrete wall cut with a diamond tool was used as an object of research. The dimensions of the concrete sample are 2.0 × 1, O x 0.3 m.

Reinforcement was made with two reinforcement meshes with a diameter of 16 mm, located with a pitch of 100 mm with a protective layer of 15-60 mm. In the test sample, heavy concrete was used on aggregate from granite crushed stone of fraction 20-40.

The basic destructive control method was used to determine the strength of concrete. 11 cores of various lengths with a diameter of 80 mm were drilled from the sample using a diamond drilling unit. From the cores, 29 samples were made - cylinders meeting the size requirements of GOST 28570-90 ("Concrete. Methods for determining strength by samples taken from structures"). According to the results of testing samples for compression, it was revealed that the average value of concrete strength was 49.0 MPa. The distribution of strength values obeys the normal law (Fig. 4). At the same time, the strength of the concrete under study has a high heterogeneity with a coefficient of variation of 15.6% and an RMS of 7.6 MPa.

For non-destructive testing, the methods of separation, separation with shearing, elastic rebound and shock impulse were used. The rib shearing method was not used due to the close location of the reinforcement to the sample ribs and the impossibility of performing tests. The ultrasonic method was not used, since the strength of concrete is above the permissible range for the application of this method (Table 1). Measurements by all methods were carried out on the sample edge cut with a diamond tool, which provided ideal conditions in terms of surface evenness. To determine the strength by indirect control methods, we used the calibration dependencies available in the instrument passports or included in them.

In fig. 5. the process of measurement by the tear-off method is presented. The results of measurements by all methods are presented in table. 3.

Table 3. Results of strength measurement by different methods

№ p / p	Control method (device)	Number of measurements, n	Average value of concrete strength, Rm, MPa	Variation coefficient, V,%
1	Compression test in the press (PGM-1000MG4)	29	49,0	15,6
2	Tear-off method with chipping (POS-50MG4)	6	51,1	4,8
3	Detachment Method (DYNA)	3	49,5	-
4	Shock pulse method (Silver Schmidt)	30	68,4	7,8
5	Shock pulse method (IPS-MG4)	7 (105)*	78,2	5,2
6	Rebound Method (Beton Condtrol)	30	67,8	7,27

* Seven sites with 15 measurements each.

According to the data presented in the table, the following conclusions can be drawn:
the average value of the strength obtained by the compression test and direct methods of non-destructive testing differs by no more than 5%;
according to the results of six tests by the method of separation with chipping, the spread of strength is characterized by a low value of the coefficient of variation of 4.8%;
the results obtained by all indirect control methods exceed the strength by 40-60%. One of the factors that led to this overestimation is the carbonization of concrete, the depth of which on the investigated surface of the sample was 7 mm.

conclusions

1. The apparent simplicity and high productivity of indirect methods of non-destructive testing are lost when the requirements for constructing a calibration dependence and taking into account (eliminating) the influence of factors distorting the result are met. If these conditions are not met, these methods can be used when examining structures only for a qualitative assessment of strength according to the "more - less" principle.
2. The results of strength measurements by the basic method of destructive control by compressing the samples taken can also be accompanied by a large scatter caused by both the inhomogeneity of concrete and other factors.
3. Considering the increased laboriousness of the destructive method and the confirmed reliability of the results obtained by direct methods of non-destructive testing, it is recommended to change the latter during the examination.
4. Among direct methods of non-destructive testing, the method of separation with spalling is optimal in most parameters.

Rice. 4. Distribution of strength values according to the results of compression tests.

Rice. 5. Measurement of strength by the pull-off method.

A. V. Ulybin, Ph.D. S. D. Fedotov, D. S. Tarasova (PNIPKU "Venture", St. Petersburg), magazine "World of construction and real estate, No. 47, 2013.

The ability of concrete to resist mechanical and thermal stress is called strength. This is the most important characteristic that affects the operational parameters of the structure.

All rules relating to testing concrete for tensile, compression and bending are prescribed in GOST 18105-86. An important characteristic of material reliability is the coefficient of variation, which characterizes the homogeneity of the mixture (Vm).

where S m- squared deviation of strength, R m- the strength of the concrete in the batch.

According to GOST 10180-67, the cube compressive strength of the material is determined. It is calculated by compressing control cube samples with stiffeners at the age of 28 days. For class B25 and above, the prismatic indicator should be 0.75, for compositions with a class below B25 - 0.8.

Requirements for the design strength, in addition to GOSTs, are also spelled out in SNiPs. For example, the stripping indicator of unloaded horizontal structures with a span of less than 6 meters should be at least 70% of the design strength, if the span exceeds 6 meters - 80%.

Testing samples makes it possible to determine the quality of the mixture, but not the characteristics of the concrete in the structure. Such studies are carried out in accordance with GOST 18105-2010 and use the following methods:

destructive,
indirect destructive,
direct destructive.

Direct methods of non-destructive testing are very popular. The main methods of this type include ultrasonic or mechanical.

Methods for controlling the strength of concrete in accordance with GOST 22690-88

separation;
shearing off;
chipping of the rib.

Research Tools

the electronic unit;
a tear-off device with a device for gluing to concrete;
sensors;
dowels and anchors;
reference metal rod.

The graph reflects the strength of the material over time, while line A is vacuum treatment, B is natural hardening, C is the change in indicator after undergoing vacuum treatment.

Testing the strength of concrete by the pull-off method

This type of study is based on the measurement of the maximum force to tear off a part of a concrete structure. Moreover, the tear-off load should be applied to a flat surface by gluing the device disk. For gluing, epoxy-based adhesives are used. GOST 22690-88 specifies ED16 and ED20 adhesives with a cement filler. You can also use two-component formulations. The pull-off area is determined after each test. After pulling off and calculating the force, the tensile strength of the concrete (Rbt) is measured. Using the empirical relationship and this indicator, you can calculate the R indicator - compressive strength. To do this, use the formula:

Rbt = 0.5∛ (R ^ 2)

Chipping off

After the concrete has hardened, an anchor device is placed in the pre-drilled hole, after which it is pulled out with a part of the concrete. This method is very similar to the one described earlier. The main difference is the way the tool is attached to the surface. The breakout force is generated by the petal anchors. The anchor is placed in the hole and P - breaking force is measured. GOST 22690 indicates the transition of the compressive strength of the concrete composition according to the formula:

R = m1 * m2 * P,

where m2 is the coefficient of the transfer of compressive strength, depending on the hardening conditions and the type of concrete, m1 is the coefficient reflecting the maximum parameters of a large aggregate (loose stone materials).

The limitations for using this research method are dense reinforcement and insignificant thickness of the structure. The surface thickness must be greater than twice the length of the anchor.

Rib cleaving method

The strength of concrete with this method is determined by the force (P) required to shear a part of the structure located on the edge of the outer side. The device is attached to the surface using an anchor bolt with a dowel. The following formula is used to determine the indicator:

R = 0.058 * m * (30P + P2),

where m is understood as a coefficient reflecting the size of the aggregate.

Ultrasonic method

The action of ultrasonic testing devices is based on the relationship between the speed with which waves propagate through the structure and its strength. Based on this method, it was determined that the speed, as well as the time of wave propagation, correspond to the strength of concrete.

For prefabricated linear structures, the through-transmission method is used. In this case, ultrasonic transducers are located on opposite sides of the structure. Flat, hollow-core and ribbed floor slabs, as well as wall panels, are examined by surface transmission, in which a wave transducer (flaw detector) is placed on one side of the structure.

To ensure maximum acoustic contact with the working surface, viscous contact materials (for example, grease) are chosen. A dry version is possible with the use of protectors and cone nozzles. Installation of ultrasonic devices is carried out at a distance of at least 3 cm from the edge.

Tests are carried out in accordance with GOST 22690.2-77. Determination of concrete strength is carried out in the range of 5-50 MPa. An impact is applied to the flat test surface, resulting in two imprints: on the reference metal bar and on the base surface. With each blow, the rod is moved 10 mm into the hole in the hammer body. The blows to the base are applied through white carbon paper. An angular scale is used to measure prints on paper.

For studies on the basis of elastic rebound, a Schmidt hammer, Borovoy and TsNIISK pistols, and a KM sclerometer with a rod impactor are used. The platoon and launch of the striker occurs automatically at the moment the striker touches the test base. The size of the bounce of the striker is fixed by a special indicator on the scale of the apparatus.

The goals, basic principles and basic procedure for carrying out work on interstate standardization are established by GOST 1.0-92 “Interstate standardization system. Basic Provisions "and GOST 1.2-2009" Interstate Standardization System. Interstate standards, rules and recommendations for interstate standardization. Rules for Development, Acceptance, Application, Update and Cancellation "

1 DEVELOPED by the structural unit of JSC "Research and Development Center" Construction "Research, Design and Technological Institute of Concrete and Reinforced Concrete. A.A. Gvozdeva (NIIZHB)

2 INTRODUCED by the Technical Committee for Standardization TC 465 "Construction"

3 ADOPTED by the Interstate Council for Standardization, Metrology and Certification (Minutes dated June 18, 2015 No. 47)

Country short name according to MK (ISO 3166) 004-97	Code of the country according to MK (ISO 3166) 004-97	Abbreviated name of the national authority on standardization
Armenia		Ministry of Economy of the Republic of Armenia
Belarus		State Standard of the Republic of Belarus
Kazakhstan		Gosstandart of the Republic of Kazakhstan
Kyrgyzstan		Kyrgyzstandard
Moldova		Moldova-Standard
Russia		Rosstandart
Tajikistan		Tajikstandart

4 By order of the Federal Agency for Technical Regulation and Metrology of September 25, 2015 No. 1378-st, the interstate standard GOST 22690-2015 was put into effect as a national standard of the Russian Federation from April 1, 2016.

5 This standard takes into account the main regulatory provisions regarding the requirements for mechanical methods for non-destructive testing of concrete strength of the following European regional standards:

EN 12504-2: 2001 Testing concrete in structures - Part2: Non-destructive testing - Determination of rebound number.

EN 12504-3: 2005 Testing concrete in structures - Determination of pull-outforce.

Degree of Compliance - Non-Equivalent (NEQ)

Information on changes to this standard is published in the annual information index "National Standards", and the text of changes and amendments is published in the monthly information index "National Standards". In case of revision (replacement) or cancellation of this standard, the corresponding notice will be published in the monthly information index "National Standards". Relevant information, notice and texts are also posted in the public information system - on the official website of the Federal Agency for Technical Regulation and Metrology on the Internet

GOST 22690-2015

Concretes
Determination of strength by mechanical methods of nondestructive testing

Date of introduction - 2016-04-01

1 area of use

This standard applies to structural heavy, fine-grained, light and stress concretes of monolithic, precast and precast-monolithic concrete and reinforced concrete products, structures and structures (hereinafter referred to as structures) and establishes mechanical methods for determining the compressive strength of concrete in structures by elastic rebound, shock impulse , plastic deformation, pull-off, rib chipping and shear-off.

2 Normative references

This standard uses normative references to the following interstate standards:

Note - Standard test schemes are applicable over a limited range of concrete strength (see appendices and ). For cases not related to standard test schemes, calibration dependencies should be established according to general rules.

4.6 The test method should be selected taking into account the data given in the table and additional restrictions established by the manufacturers of specific measuring instruments. The use of methods outside the ranges of concrete strength recommended in the table is allowed with a scientific and technical justification based on the results of research using measuring instruments that have passed metrological certification for an extended range of concrete strength.

Table 1

Method name	Limiting values of concrete strength, MPa
Rebound and plastic deformation	5 - 50
Impact impulse	5 - 150
Detachment	5 - 60
Chipping a rib	10 - 70
Chipping off	5 - 100

4.7 Determination of the strength of heavy concrete of design classes B60 and above or with an average compressive strength of concrete R m≥ 70 MPa in monolithic structures must be carried out taking into account the provisions of GOST 31914.

4.8 The strength of concrete is determined in sections of structures that do not have visible damage (peeling of the protective layer, cracks, cavities, etc.).

4.9 The age of the concrete of the controlled structures and its sections should not differ from the age of the concrete of the structures (sections, samples) tested to establish the calibration dependence by more than 25%. Exceptions are strength control and construction of a calibration dependence for concrete that is more than two months old. In this case, the difference in the age of individual structures (sections, samples) is not regulated.

4.10 The tests are carried out at a positive concrete temperature. It is allowed to carry out tests at a negative concrete temperature, but not lower than minus 10 ° C, when establishing or linking a calibration dependence, taking into account the requirements. The concrete temperature during testing must correspond to the temperature specified by the operating conditions of the devices.

Calibration dependencies established at a concrete temperature below 0 ° C are not allowed to be used at positive temperatures.

4.11 If it is necessary to test concrete of structures after heat treatment at surface temperature T≥ 40 ° С (to control the tempering, transfer and stripping strength of concrete), the calibration dependence is established after determining the strength of concrete in the structure by an indirect non-destructive method at a temperature t = (T± 10) ° С, and concrete testing by direct non-destructive method or specimen testing - after cooling at normal temperature.

5 Measuring instruments, apparatus and instruments

5.1 Measuring instruments and instruments for mechanical testing, designed to determine the strength of concrete, must be certified and verified in the prescribed manner and must comply with the requirements for the application.

5.2 The readings of instruments, graded in units of concrete strength, should be considered as an indirect indicator of concrete strength. The indicated devices should be used only after establishing the calibration dependence "device reading - concrete strength" or binding the dependence set in the device in accordance with.

5.3 A tool for measuring the diameter of indentations (caliper in accordance with GOST 166), used for the method of plastic deformation, must ensure measurement with an error of not more than 0.1 mm, a tool for measuring the depth of an indentation (dial gauge in accordance with GOST 577, etc.) - with an error no more than 0.01 mm.

5.4 Standard test procedures for shear-off and rib-chipping tests provide for the use of anchoring devices and grips in accordance with applications and.

5.5 For the shear-off method, anchor devices should be used, the embedment depth of which should not be less than the maximum size of the coarse concrete aggregate of the structure under test.

5.6 For the tear-off method, steel discs with a diameter of at least 40 mm, a thickness of at least 6 mm and at least 0.1 of the diameter should be used, with the roughness parameters of the glued surface at least Ra= 20 microns according to GOST 2789. The adhesive for gluing the disc must ensure the strength of adhesion to concrete, at which destruction occurs along the concrete.

6 Test preparation

6.1.1 Preparation for testing includes checking the devices used in accordance with the instructions for their operation and establishing the calibration dependencies between the strength of concrete and the indirect characteristic of strength.

6.1.2 The calibration dependence is established on the basis of the following data:

Results of parallel tests of the same sections of structures using one of the indirect methods and a direct non-destructive method for determining the strength of concrete;

Results of testing sections of structures using one of the indirect non-destructive methods for determining the strength of concrete and testing core samples taken from the same sections of the structure and tested in accordance with GOST 28570;

Results of testing standard concrete samples by one of the indirect non-destructive methods for determining the strength of concrete and mechanical tests in accordance with GOST 10180.

6.1.3 For indirect non-destructive methods for determining the strength of concrete, the calibration dependence is established for each type of rated strength specified in for concretes of the same nominal composition.

It is allowed to build one calibration dependence for concretes of the same type with one type of coarse aggregate, with a unified production technology, differing in the nominal composition and the value of the normalized strength, subject to the requirements.

6.1.4 The permissible difference in the age of concrete of individual structures (sections, samples) when establishing the calibration dependence on the age of concrete of the controlled structure is taken according to.

6.1.5 For direct non-destructive methods, it is allowed to use the dependencies given in the appendices and for all types of rated concrete strength.

6.1.6 The calibration dependence should have a standard (residual) deviation S T. H. M, not exceeding 15% of the average value of the concrete strength of the plots or samples used in the construction of the dependence, and the correlation coefficient (index) not less than 0.7.

It is recommended to use a linear dependence of the form R = a + bK(where R- strength of concrete, K- an indirect indicator). The procedure for establishing, evaluating the parameters and determining the conditions for using a linear calibration dependence is given in the appendix.

6.1.7 When constructing the calibration dependence of the deviation of the unit values of concrete strength R i f from the average value of the concrete strength of the sections or samples used to construct the calibration dependence, should be within:

From 0.5 to 1.5 average values of concrete strength at ≤ 20 MPa;

From 0.6 to 1.4 average value of concrete strength at 20 MPa< ≤ 50 МПа;

From 0.7 to 1.3 average value of concrete strength at 50 MPa< ≤ 80 МПа;

From 0.8 to 1.2 average value of concrete strength at> 80 MPa.

6.1.8 Correction of the established dependence for concretes at intermediate and design age should be carried out at least once a month, taking into account the additionally obtained test results. The number of samples or sites for additional tests when making adjustments should be at least three. The correction technique is given in the appendix.

6.1.9 It is allowed to use indirect non-destructive methods for determining the strength of concrete, using the calibration dependences established for concrete that differs from the tested one in composition, age, hardening conditions, moisture, with reference in accordance with the procedure for the application.

6.1.10 Without reference to specific conditions according to the application, the calibration dependencies established for concrete that differ from the tested one may be used only to obtain approximate strength values. It is not allowed to use approximate strength values without reference to specific conditions for assessing the strength class of concrete.

Then, the sites are selected in the amount provided for, on which the maximum, minimum and intermediate values of the indirect indicator are obtained.

After testing by the indirect non-destructive method, the sections are tested by the direct non-destructive method or samples are taken for testing in accordance with GOST 28570.

6.2.4 To determine the strength at negative temperature of concrete, the areas selected for plotting or binding the calibration dependence are first tested by an indirect non-destructive method, and then samples are taken for subsequent testing at positive temperatures or warmed by external heat sources (infrared emitters, heat guns, etc. ) to a depth of 50 mm to a temperature not lower than 0 ° C and tested by a direct non-destructive method. The temperature control of the heated concrete is carried out at the depth of installation of the anchor device in the prepared hole or along the surface of the chip in a non-contact way using a pyrometer in accordance with GOST 28243.

The rejection of the test results used to construct the calibration dependence at negative temperatures is allowed only if the deviations are associated with a violation of the test procedure. In this case, the rejected result should be replaced by the results of a repeated test in the same area of the structure.

6.3.1 When constructing a calibration dependence for control samples, the dependence is established according to the unit values of the indirect indicator and the strength of concrete of reference sample cubes.

For a unit value of an indirect indicator, the average value of indirect indicators for a series of samples or for one sample (if the calibration dependence is established for individual samples) is taken. For a unit value of concrete strength, the strength of concrete in a series according to GOST 10180 or one sample (calibration dependence for individual samples) is taken. Mechanical tests of samples in accordance with GOST 10180 are carried out immediately after testing by an indirect non-destructive method.

6.3.2 When constructing a calibration dependence based on the test results of cube samples, at least 15 series of cube samples according to GOST 10180 or at least 30 separate cube samples are used. Samples are made in accordance with the requirements of GOST 10180 in different shifts, for at least 3 days from concrete of the same nominal composition, using the same technology, with the same hardening mode as the structure to be controlled.

The unit values of the concrete strength of the cube samples used to construct the calibration dependence must correspond to the deviations expected in production, while being within the ranges established in.

6.3.3 The calibration dependence for the methods of elastic rebound, shock pulse, plastic deformation, separation and spalling of the rib is established on the basis of the test results of the manufactured cube specimens, first by the non-destructive method, and then by the destructive method according to GOST 10180.

When establishing the calibration dependence for the method of separation with spalling, the main and control samples are made according to. An indirect characteristic is determined on the main samples, control samples are tested in accordance with GOST 10180. The main and control samples must be made of the same concrete and harden under the same conditions.

6.3.4 The dimensions of the samples should be selected in accordance with the largest aggregate size in the concrete mixture in accordance with GOST 10180, but not less than:

100 × 100 × 100 mm for rebound, shock impulse, plastic deformation methods, as well as for the shear-off method (control samples);

200 × 200 × 200 mm for the method of chipping the rib of the structure;

300 × 300 × 300 mm, but with a rib size of at least six installation depths of the anchor device for the shear-off method (main samples).

6.3.5 To determine the indirect strength characteristics, tests are carried out in accordance with the requirements of the section on the lateral (in the direction of concreting) faces of the cube specimens.

The total number of measurements on each sample for the method of elastic rebound, shock pulse, plastic deformation upon impact must be at least the specified number of tests per section according to the table, and the distance between the places of impacts must be at least 30 mm (15 mm for the shock pulse method). For the method of plastic deformation by indentation, the number of tests on each face should be at least two, and the distance between test sites should be at least two diameters of indentations.

When establishing the calibration dependence for the rib cleaving method, one test is carried out on each lateral rib.

When establishing the calibration dependence for the shear-off method, one test is carried out on each side face of the main sample.

6.3.6 When tested by the method of elastic rebound, shock impulse, plastic deformation upon impact, the samples shall be clamped in a press with a force of at least (30 ± 5) kN and not more than 10% of the expected value of the breaking load.

6.3.7 Samples tested by the pull-off method are installed on the press so that the surfaces on which the pull-out was carried out do not adjoin the support plates of the press. The test results in accordance with GOST 10180 increase by 5%.

7 Testing

7.1.1 The number and location of controlled sections in structures must comply with the requirements of GOST 18105 and be indicated in the design documentation for the structure or be installed taking into account:

Control tasks (determination of the actual class of concrete, stripping or tempering strength, identification of areas of reduced strength, etc.);

Type of construction (columns, beams, slabs, etc.);

Placement of grips and concreting order;

Reinforcement of structures.

The rules for assigning the number of test sites for monolithic and prefabricated structures when controlling the strength of concrete are given in the appendix. When determining the concrete strength of the structures under study, the number and location of the sections should be taken according to the survey program.

7.1.2 The tests shall be carried out on a section of the structure with an area of 100 to 900 cm 2.

7.1.3 The total number of measurements at each site, the distance between the measurement sites at the site and from the edge of the structure, the thickness of the structures at the site of measurements should be not less than the values given in the table, depending on the test method.

Table 2 - Requirements for test sites

Method name	Total number measurements Location on	Minimum distance between measurement sites on the site, mm	Minimum edge distance constructions to place measurements, mm	Minimum thickness structures, mm
Elastic rebound
Impact impulse
Plastic deformation
Chipping a rib
Detachment		2 diameters disk
Tear-off with spalling at working depth of anchor embedmenth:
≥ 40mm
< 40мм

7.1.4 The deviation of individual measurement results in each section from the arithmetic mean of the measurement results for this section should not exceed 10%. Measurement results that do not satisfy the specified condition are not taken into account when calculating the arithmetic mean of the indirect indicator for a given area. The total number of measurements in each section when calculating the arithmetic mean must meet the requirements of the table.

7.1.5 The strength of concrete in the controlled section of the structure is determined by the average value of the indirect indicator according to the calibration dependence established in accordance with the requirements of the section, provided that the calculated value of the indirect indicator is within the established (or tied) relationship (between the lowest and highest values of strength ).

7.1.6 The roughness of the surface of a section of concrete of structures when tested by the methods of rebound, shock impulse, plastic deformation should correspond to the roughness of the surface of sections of the structure (or cubes) tested when establishing the calibration dependence. If necessary, it is allowed to clean the surfaces of the structure.

When using the method of plastic deformation during indentation, if the zero reading is removed after the application of the initial load, there are no requirements for the roughness of the concrete surface of the structure.

7.2.1 The tests are carried out in the following sequence:

The position of the device when testing the structure relative to the horizontal is recommended to be taken the same as when establishing the calibration dependence. In a different position of the device, it is necessary to correct for the indicators in accordance with the instructions for use of the device;

7.3.1 The tests are carried out in the following sequence:

The device is positioned so that the force is applied perpendicular to the test surface in accordance with the instructions for use of the device;

When using a spherical indenter to facilitate measurements of indentation diameters, the test may be carried out through sheets of carbon and white paper (in this case, tests to establish the calibration dependence are carried out using the same paper);

The values of the indirect characteristic are recorded in accordance with the instructions for use of the device;

Calculate the average value of the indirect characteristic at the site of the structure.

7.4.1 The tests are carried out in the following sequence:

The device is positioned so that the force is applied perpendicular to the test surface in accordance with the instructions for use of the device;

It is recommended to take the position of the device during the test of the structure relative to the horizontal the same as in the test when establishing the calibration dependence. In a different position of the device, it is necessary to correct for the readings in accordance with the instructions for use of the device;

The value of the indirect characteristic is recorded in accordance with the instructions for use of the device;

Calculate the average value of the indirect characteristic at the site of the structure.

7.5.1 In the pull-off test, the sections shall be located in the zone of lowest stresses caused by the service load or the compression force of the prestressed reinforcement.

7.5.2 The test is carried out in the following sequence:

At the place where the disc is glued, remove the surface layer of concrete with a depth of 0.5 - 1 mm and clean the surface of dust;

The disc is adhered to the concrete by pressing the disc and removing excess glue outside the disc;

The device is connected to a disk;

The load is gradually increased at a rate of (1 ± 0.3) kN / s;

Measure the projection area of the separation surface on the plane of the disk with an error of ± 0.5 cm 2;

The value of the conditional stress in concrete during separation is determined as the ratio of the maximum separation force to the projection area of the separation surface.

7.5.3 The test results are not taken into account if, when the concrete was torn off, the reinforcement was exposed or the projected area of the tearing surface was less than 80% of the disc area.

7.6.1 When tested by the shear pull method, the sections shall be located in the zone of lowest stresses caused by the service load or the compression force of the prestressed reinforcement.

7.6.2 The tests are carried out in the following sequence:

If the anchor device was not installed before concreting, then a hole is made in the concrete, the size of which is selected in accordance with the instructions for use of the device, depending on the type of anchor device;

An anchor device is fixed into the hole to a depth specified in the instruction manual for the device, depending on the type of anchor device;

The device is connected to an anchor device;

The load is increased at a speed of 1.5 - 3.0 kN / s;

Record the reading of the force meter of the device R 0 and anchor slip Δ h(the difference between the actual tear-out depth and the embedment depth of the anchor device) with an accuracy of at least 0.1 mm.

7.6.3 Measured pull-out force R 0 is multiplied by the correction factor γ, determined by the formula

where h- working depth of the anchor device embedment, mm;

Δ h- the amount of slip of the anchor, mm.

7.6.4 If the largest and smallest dimensions of the torn-out part of the concrete from the anchor device to the boundaries of destruction along the surface of the structure differ by more than two times, and also if the tear-out depth differs from the embedment depth of the anchor device by more than 5% (Δ h > 0,05h, γ> 1.1), then the test results can be taken into account only for an approximate assessment of the strength of concrete.

Note - Approximate values of concrete strength are not allowed to be used for assessing concrete strength class and building calibration dependencies.

7.6.5 The test results are disregarded if the tear-out depth differs from the embedment depth of the anchor device by more than 10% (Δ h > 0,1h) or the reinforcement was exposed at a distance from the anchor device that is less than the depth of its embedment.

7.7.1 When testing the rib shear method, there should be no cracks, concrete gaps, sagging or cavities with a height (depth) of more than 5 mm in the test area. The sections should be located in the zone of the lowest stresses caused by the operational load or the compression force of the prestressed reinforcement.

7.7.2 The test is carried out in the following sequence:

The device is fixed to the structure, a load is applied at a rate of no more than (1 ± 0.3) kN / s;

The readings of the force meter of the device are recorded;

Measure the actual shear depth;

Determine the average shear force.

7.7.3 The test results are not taken into account if the reinforcement was exposed during spalling of concrete or the actual spalling depth differed from the specified one by more than 2 mm.

8 Processing and presentation of results

8.1 The test results are presented in a table, which indicates:

Type of construction;

Concrete design class;

Concrete age;

The strength of the concrete of each controlled area by;

Average concrete strength of the structure;

Zones of a structure or part of it, if the requirements are met.

The form of the test results presentation table is given in the appendix.

8.2 Processing and assessment of compliance with the established requirements of the actual concrete strength values obtained using the methods given in this standard is carried out in accordance with GOST 18105.

Note - The statistical assessment of the class of concrete based on the test results is carried out according to GOST 18105 (schemes "A", "B" or "C") in cases where the strength of concrete is determined by the calibration dependence built in accordance with section ... When using previously installed dependencies by binding them (by application ) statistical control is not allowed, and the assessment of the class of concrete is carried out only according to the scheme "G" GOST 18105.

8.3 The results of determining the strength of concrete by mechanical methods of non-destructive testing are drawn up in the conclusion (protocol), which contains the following data:

About tested structures, indicating the design class, the date of concreting and testing, or the age of the concrete at the time of testing;

On the methods used to control the strength of concrete;

On types of devices with serial numbers, information on instrument checks;

On the accepted calibration dependences (equation of dependence, parameters of dependence, compliance with the conditions for using the calibration dependence);

Used to construct a calibration dependence or its binding (date and results of tests by non-destructive indirect and direct or destructive methods, correction factors);

On the number of sites for determining the strength of concrete in structures with an indication of their location;

Test results;

Methodology, results of processing and evaluation of the data obtained.

Appendix A
(required)
Standard Shear Pull Test Arrangement

A.1 The standard shear peel test scheme provides for tests to be performed while meeting the requirements of -.

A.2 The standard test setup is applicable in the following cases:

Tests of heavy concrete with compressive strength from 5 to 100 MPa;

Tests of lightweight concrete with compressive strength from 5 to 40 MPa;

The maximum fraction of coarse concrete aggregate is not more than the working depth of anchoring devices.

A.3 The supports of the loading device should adhere evenly to the concrete surface at a distance of at least 2 h from the axis of the anchor device, where h- the working depth of the embedment of the anchor device. The test scheme is shown in the figure.

1 2 - support of the loading device;
3 - capture of the loading device; 4 - transitional elements, rods; 5 - anchor device;
6 - pulled out concrete (tear-off cone); 7 - tested structure

Figure A.1 - Schematic of a shear peel test

A.4 Three types of anchor devices are used in the standard shear-pull test (see figure). The Type I anchor device is installed in the structure during concreting. Anchoring devices of types II and III are installed in holes previously prepared in the structure.

1 - working rod; 2 - working rod with expanding cone; 3 - segmental grooved cheeks;
4 - support rod; 5 - working rod with a hollow expanding cone; 6 - leveling washer

Figure A.2 - Types of anchor devices for a standard test setup

A.5 The parameters of the anchor devices and the permissible ranges of the measured concrete strength for them under the standard test scheme are indicated in the table. For lightweight concrete, in the standard test scheme, only anchoring devices with an embedment depth of 48 mm are used.

Table A.1 - Parameters of anchor devices for the standard test setup

Anchor type devices	Anchor diameter devicesd, mm	Embedment depth of anchor devices, mm		Permissible for anchor device strength measurement range for concrete compression, MPa
Anchor type devices	Anchor diameter devicesd, mm	working h	complete h "	heavy	lung
				45 - 75
				10 - 50	10 - 40
				40 - 100
				5 - 100	5 - 40
				10 - 50

A.6 Designs of types II and III anchors should provide preliminary (before load application) compression of the hole walls at the working depth of embedment h and post-test slip control.

Appendix B
(required)
Standard rib shear test setup

B.1 The standard scheme for testing the rib shearing method provides for testing in compliance with the requirements -.

B.2 The standard test scheme is applicable in the following cases:

The maximum fraction of coarse concrete aggregate is not more than 40 mm;

Tests of heavy concrete with compressive strength from 10 to 70 MPa on granite and limestone crushed stone.

B.3 For testing, a device is used, consisting of a power exciter with a force-measuring unit and a gripper with a bracket for local cleavage of the rib of the structure. The test scheme is shown in the figure.

1 - a device with a loading device and a force measuring device; 2 - support frame;
3 - chipped concrete; 4 - tested structure; 5 - gripper with a bracket

Figure B.1 - Schematic of the rib shear test

B.4 In case of local spalling of the rib, the following parameters should be provided:

Cleavage depth a= (20 ± 2) mm;

Cleavage width b= (30 ± 0.5) mm;

The angle between the direction of action of the load and the normal to the loaded surface of the structure β = (18 ± 1) °.

Appendix B
(recommended)
Calibration dependence for the shear-off method

When carrying out tests by the method of pull-off with spalling according to the standard scheme according to the appendix, the cubic compressive strength of concrete R, MPa, it is allowed to calculate by the calibration dependence according to the formula

R = m 1 m 2 P,

where m 1 - coefficient taking into account the maximum size of the coarse aggregate in the tear-out zone, taken equal to 1 when the aggregate size is less than 50 mm;

m 2 - coefficient of proportionality for the transition from the force pulling out kilonewtons to the strength of concrete in megapascals;

R- pull-out force of the anchor device, kN.

When testing heavy concrete with a strength of 5 MPa or more and light concrete with a strength of 5 to 40 MPa, the values of the proportionality coefficient m 2 are taken according to the table.

Table B.1

Anchor type devices	Range measurable concrete strength compression, MPa	Anchor diameter devicesd, mm	Embedment depth of anchor device, mm	Coefficient valuem 2 for concrete
Anchor type devices	Range measurable concrete strength compression, MPa	Anchor diameter devicesd, mm	Embedment depth of anchor device, mm	heavy	lung
	45 - 75
	10 - 50
	40 - 75
	5 - 75
	10 - 50

Odds m 2 when testing heavy concrete with an average strength above 70 MPa, it should be taken in accordance with GOST 31914.

Appendix D
(recommended)
Calibration Constraint for the Rib Cleaving Method
with standard test setup

When performing the test by spalling ribs according to the standard scheme according to the appendix, the cubic compressive strength of concrete on granite and limestone crushed stone R, MPa, it is allowed to calculate according to the calibration dependence according to the formula

R = 0,058m(30R + R 2),

where m- coefficient taking into account the maximum size of the coarse aggregate and taken equal to:

1.0 - when the aggregate size is less than 20 mm;

1.05 - with aggregate size from 20 to 30 mm;

1,1 - for aggregate size from 30 to 40 mm;

R- shearing force, kN.

Appendix D
(required)
Requirements for instruments for mechanical testing

Table E.1

The name of the characteristics of the devices

Characteristics of instruments for the method

elastic
rebound

percussion
momentum

plastic
deformations

detachment

chipping
ribs

separation from
chipping

Striker, striker or indenter hardness HRCэ, not less

Roughness of the contact part of the striker or indenter, μm, no more

Diameter of striker or indenter, mm, not less

The thickness of the edges of the disk indenter, mm, not less

Conical indenter angle

30 ° - 60 °

Imprint diameter,% of indenter diameter

20 - 70

Perpendicularity tolerance when applying a load at a height of 100 mm, mm

Impact energy, J, not less

0,02

Load increase rate, kN / s The equation of dependence "indirect characteristic - strength" is taken linear by the formula

E.2 Rejection of test results

After constructing the calibration dependence according to the formula (), it is corrected by rejecting single test results that do not satisfy the condition:

where the average value of concrete strength according to the calibration dependence is calculated by the formula

here the values R i H, R i f,, N- see explications to formulas (), ().

E.4 Correction of the calibration dependence

Correction of the established calibration dependence, taking into account the additionally obtained test results, should be carried out at least once a month.

When adjusting the calibration dependence, at least three new results obtained at the minimum, maximum and intermediate values of the indirect indicator are added to the existing test results.

As data accumulates to build a calibration dependence, the results of previous tests, starting with the very first ones, are rejected so that the total number of results does not exceed 20. After adding new results and rejecting old ones, the minimum and maximum values of the indirect characteristic, the calibration dependence and its parameters are set again according to the formulas () - ().

F.5 Conditions for the application of the calibration dependence

The use of a calibration dependence for determining the strength of concrete according to this standard is allowed only for values of an indirect characteristic falling in the range from H min to H max.

If the correlation coefficient r < 0,7 или значение , then the control and assessment of strength according to the obtained dependence are not allowed.

Appendix G
(required)
Calibration dependence binding method

G.1 The value of concrete strength, determined using the calibration dependence established for concrete that differs from the tested one, is multiplied by the coincidence coefficient K with. Meaning K s is calculated by the formula

where R wasps i- concrete strength in i-m section, determined by the method of separation with chipping or testing of cores in accordance with GOST 28570;

R indirect i- concrete strength in i-th section, determined by any indirect method according to the used calibration dependence;

n- number of test sites.

G.2 When calculating the coincidence coefficient, the following conditions must be met:

The number of test sites taken into account when calculating the coincidence coefficient, n ≥ 3;

Each data point R wasps i /R indirect i must be at least 0.7 and no more than 1.3:

1 x 4 m length of linear structures;

1 by 4 m 2 of the area of flat structures.

Appendix K
(recommended)
Form of the table of presentation of test results

Designation of structures (construction batches), design strength class concrete, date of concreting or the age of concrete tested constructions	Designation 1)	No. of plot according to the scheme or location in axes 2)	Concrete strength, MPa		Strength class concrete 5)
			section 3)	average 4)






1) The brand, symbol and (or) location of the structure in the axes, structure zone, or part of a monolithic and precast-monolithic structure (capture), for which the concrete strength class is determined. 2) The total number and location of sites in accordance with . 3) The strength of the concrete of the site in accordance with . 4) Average strength of concrete of a structure, structure zone or part of a monolithic and precast-monolithic structure with a number of sections that meet the requirements . 5) The actual strength class of concrete of a structure or part of a monolithic and precast-monolithic structure in accordance with clauses 7.3 - 7.5 GOST 18105 depending on the selected control scheme. Note - The presentation in the column "Concrete strength class" of the estimated values of the class or the values of the required concrete strength for each section separately (assessment of the strength class for one section) is not permissible.

Key words: structural heavy and light concrete, monolithic and prefabricated concrete and reinforced concrete products, structures and structures, mechanical methods for determining compressive strength, resilient rebound, shock impulse, plastic deformation, separation, rib shearing, shearing off with shearing

Which determines its performance properties. Therefore, when erecting important supporting structures, builders carefully monitor this indicator. The most common control method is to determine the strength of the concrete by the shear-off method. However, there are many other ways.

Therefore, in this article we will take a closer look at how to determine the strength of concrete using the most common modern methods.

Types of methods for checking strength

The most reliable way to control the quality of concrete is to test the concrete structure after the material has reached its design strength.

With regard to testing separately made control samples, it allows you to determine only, but not the strength of the material in the structure. This is due to the impossibility of ensuring the same conditions for the strength of the prototype (vibration, heating, etc.) and the concrete product.

All existing control methods are divided into three groups:

Direct non-destructive;
Destructive;
Indirect non-destructive.

Non-destructive control methods are often used, however, most often the work is performed by indirect methods. The last group includes testing of control samples, as well as samples taken from a concrete structure.

Note! Compressive strength is used to determine the class of concrete. For this, concrete cubes are crushed using a hydraulic press, which produces the result.

I must say that destructive methods are also widespread in construction, but they are used less often, since they violate the integrity of the structure. In addition, the cost of such tests is very high.

Therefore, today the most common are the following methods for determining strength:

Rebound method;
Ultrasonic method;
Shock impulse method.

I must say that different verification methods have different errors:

Basic requirements for strength testing

According to the requirements set out in SP 13-102-2003, concrete sampling for research by indirect and direct methods must be performed in more than 30 areas, however, this is not enough for the construction and use of the calibration dependence.

It is also necessary that the dependence obtained by the paired correlation-regression study has a correlation coefficient of at least 0.7, and the standard deviation is less than 15 percent of the average strength. To meet these conditions, the measurement accuracy must be very high, while the concrete strength must vary over a wide range.

I must say that in the study of structures, these conditions are met quite rarely. The point is that the basic test method is accompanied by a significant error.

In addition, the strength of concrete at the surface may differ from the strength at some depth. However, if the concreting is done with high quality and the concrete corresponds to its design class, then the parameters of the same type of structures do not change over a wide range.

Direct non-destructive or destructive methods should be used to determine strength without violating applicable regulations.

According to GOST 22690-88, direct methods include:

Tear-off method;
Tearing off concrete with chipping;
Chipping the rib.

Now let's take a closer look at the most common technologies for determining the quality of concrete.

Strength determination technology

Tear off method

The principle of this method is based on measuring the force that must be applied to tear off a section of a concrete structure. The pull-off load is applied to the flat surface of the concrete structure. To do this, a steel disk is glued to it, which is connected to the measuring device by means of a rod.

The disc is glued with epoxy glue. GOST 22690-88 recommends using ED20 glue with a cement filler. True, in our time there are reliable two-component adhesives.

This technology implies gluing the disc without additional measures to limit the separation area. As for the separation area, it is not constant and is determined after each test.

True, in foreign practice, the separation section is preliminarily limited by a groove made with circular drills. In this case, the separation area is constant and known.

After determining the force required for separation, the tensile strength of the material is obtained.

According to it, using an empirical dependence, the compressive strength is calculated using the following formula - Rbt = 0.5∛ (R ^ 2), where:

Rbt - tensile strength.
R is the compressive strength.

For the study of concrete by the pull-off method, the same instruments are used as for the method of pull-off with spalling, these are:

ONYX-OS;
POS-50MG4;
GPNS-5;
GPNV-5.

Note! To carry out the test, you will also need a gripper, namely a disc with a rod attached to it.

In the photo - checking the quality of concrete by pulling off with chipping

Chipping off

This method has much in common with the above method. Its main difference lies in the way the device is mounted to a concrete structure. To apply a tearing force to it, petal anchors are used, which can be of different sizes.

Anchors are inserted into holes drilled in the measurement area. As in the previous case, the device measures the breaking force.

The calculation of the compressive strength is carried out using the dependence expressed by the formula - R = m1 * m2 * P, where:

m1 denotes the coefficient of the maximum size of the coarse filler;
m2 stands for the coefficient of conversion to compressive strength. It depends on the conditions of the type of concrete, as well as the conditions for curing.
P is the destructive force obtained as a result of research.

In our country, this method is one of the most popular, as it is quite versatile. It provides the ability to test anywhere in the structure, as it does not require a flat surface. In addition, it is not difficult to fix the petal anchor with your own hands in the thickness of concrete.

True, there are some restrictions, which consist in the following points:

Dense reinforcement of the structure - in this case, the measurements will be unreliable.
Thickness of the structure - it should be twice the length of the anchor.

Chipping a rib

This technology is the latest direct non-destructive testing method. Its main feature is the determination of the force that is applied to shear a section of concrete located on the edge of the structure.

The design of the device, which can be installed on a concrete product with one outer corner, was developed relatively recently. Installation of the device to one of the sides is carried out using an anchor with a dowel.

After receiving data from the device, determine the compressive strength according to the following normalized relationship, expressed by the formula - R = 0.058 * m * (30P + P2), where:

m - coefficient, takes into account the size of the aggregate.
P is the force applied to crush the concrete.

Ultrasonic determination

The ultrasonic method for determining the strength of concrete is based on the relationship between the strength of a material and the speed of propagation of ultrasonic waves in it.

Moreover, there are two calibration dependencies:

The propagation time of ultrasound waves and the strength of the material.
The propagation speed of ultrasound waves and the strength of the material.

Each method is designed for a specific type of structure:

Through sounding in the transverse direction - used for linear prefabricated structures. In such studies, the instruments are installed on both sides of the test structure.
Surface sounding - used to study ribbed, flat, hollow-core floor slabs and wall panels. In this case, the device is installed only on one side of the structure.

To ensure high-quality acoustic contact between the test structure and the ultrasonic transducer, viscous materials are used, for example, solid oil. Dry contact is also common, but in this case, tapered nozzles and protectors are used.

Ultrasound devices consist of two main elements:

Sensors;
Electronic unit.

Sensors can be:

Separate - for end-to-end sounding.
United - intended for surface sounding.

The advantages of this test method include simplicity and versatility.

Research with a Kashkarov hammer

The process of testing concrete with a Kashkarov hammer is regulated by GOST 22690.2-77. This method is used to determine the strength of the material in the range of 5-50 MPa.

Instructions for examining concrete by this method are as follows:

First, a flat section of the structure is sought.
If there is roughness or paint on its surface, then it is necessary to clean the area with a metal brush.
Then copy paper should be placed on the surface of the concrete and a sheet of plain white paper should be placed on top..

Further, a blow is applied to the concrete surface with a Kashkarov hammer of medium force perpendicular to the concrete plane. As a result of the impact, two prints are left - on the reference rod and on a sheet of paper.
After that, the metal rod is displaced by at least 10 mm and another blow is applied.... For greater accuracy of the study, the procedure must be repeated several times.
Then the prints on the reference rod and paper should be measured to the nearest 0.1 mm.
After measuring the prints, add the diameters obtained on paper and the diameters on the reference rod separately..

An indirect parameter of concrete strength is the average value of the ratio of indentations on the reference bar and on concrete.

Rebound method

This research method is the simplest. The test is carried out using a special electronic device. It has a hammer that pushes the ball into the concrete. Electronics determines the strength of the material by the rebound of the ball after being pressed.

To test concrete, you need to rest the device on the concrete surface and press the corresponding button. The results are displayed on the screen of the device. I must say that the process of material testing with the help of a shock-impulse type device takes place in almost the same way.

These are all the main methods for determining the quality of concrete, which are most often used in modern construction.

Output

As we found out, there are quite a few ways to determine the strength of concrete. Moreover, it is impossible to call one of them the best, since different methods, as a rule, are designed for different types of concrete structures, and also have different errors.

You can get more information on this topic from the video in this article.