Determination of concrete strength: methods, equipment, GOST. Monitoring and evaluation of concrete strength

When checking building structures, the determination of concrete strength is carried out to determine their condition at the current time. Actual indicators after the start of operation usually do not coincide with the design parameters. They are directly affected by deformation loads and external factors. In the diagnostic process, different methods can be used.

Basic terms and definitions

Before considering the main methods of monitoring and evaluating the strength of concrete, it is recommended that you familiarize yourself with some concepts so that no further questions arise. All terms and definitions necessary for a clearer understanding of the topic are presented below.

• Concrete is a building material obtained artificially as a result of hardening of a solution with a binder and fillers. The composition of the mixture to achieve the best performance can be introduced additional additives.
• Strength - the property of the hardened material to perceive mechanical loads, without collapsing at the same time. During operation, structures are subjected to compression and tension, as well as other influences.
• Strength limit - the highest value of the mechanical load exerted, reduced directly to a certain cross-sectional area, after which partial or complete destruction of the material occurs.
• Destructive methods for determining the strength of concrete are the control of the listed parameters by taking control samples taken from the test structure according to GOST 28570.

• Non-destructive testing - verification of the reliability of the basic properties of individual structural elements without dismantling. With this method, there is no need to decommission the object.
• Design test area - a certain fraction of the volume, length or area of ​​limited dimensions for which strength tests are carried out.

What is control done for?

In the construction of residential buildings, industrial or commercial facilities, the determination of concrete strength avoids many negative consequences. The material is used at various stages of the construction of buildings for various purposes. Depending on the type of structure, the requirements for mixtures can vary significantly. For example, for pouring foundations and walls, different types of concrete are used, determined by the strength characteristics.

The use of mixtures that do not meet the requirements can lead to the formation of cracks, deterioration in performance and premature destruction of the structure. Research is often needed to determine if the building can be used for any purpose.

Concrete Strength Chart: Grade and Grade Matching

Mortars are divided into categories in which various parameters are taken into account. The concrete strength in MPa is usually broken down into classes denoted by a capital letter with a number. Such marking in the professional environment is considered the most convenient. For example, a B25 solution will have a strength of 25 MPa.

As for the brand of concrete, it expresses an approximate value in kilograms per square centimeter. Designation is carried out according to the same principle. However, with a ratio of indicators, the normative coefficient of variation can be 13.5 percent.

For example, it is proposed to familiarize yourself with a special table of concrete strength, which gives the correspondence between the classes and brands of mixtures.

What affects strength?

When chemical processes occur, the concrete mix hardens. Water interacts with an astringent. Under the influence of some factors, the rate of a chemical reaction can accelerate or slow down. The final strength of concrete will depend to some extent on them.

Important factors include:

• initial binder activity;
• amount of water in the composition;
• compaction level;
• temperature and humidity;
• quality mixing components.

An important role is played by the quality of the fillers used. Components with a fine fraction and clay substances lead to a decrease in strength. Larger particles have better adhesion to the binder. Their use has a positive effect on strength indicators.

Classification of research methods

When determining the strength of concrete in building structures, it is necessary to solve complex technical problems. The development of theoretical and practical research in the field of quality control of building compounds has led to the emergence of numerous methods. Each of them has a specific scope, as well as its pros and cons.

If we take the method of influencing directly on the tested design, then we can distinguish three main methods.

1. Destructive. After conducting control operations, the sample cannot be used for its intended purpose.
2. Nondestructive. Testing does not affect the operational capabilities of the structure.
3. Locally destructive. After special events, repair of the building is required.

Inspection should be carried out only after a detailed familiarization with the design and technical documentation. Having received certain information about the composition used and the manufacturing technology of the structure, we can begin to work on determining the strength properties.

What factors determine the choice of method?

To find out the tensile strength of concrete, you must first determine the research methodology. The following factors influence her choice:

• state of the building mixture;
• availability of test sites;
• amount of information collected;
• the presence or absence of heterogeneous layers in the structure.

Despite the variety of methods, the results obtained by destructive methods are the most reliable, since during the tests the desired parameter is measured - the force applied during compression. In addition, a sample taken directly from the body of the structure, and not the upper part, is carefully studied.

Destructive testing methods

The essence of the methods is to study samples obtained by drilling or sawing from a finished structure. They are subjected to a static load with a gradual increase in the growth rate. As a result, it is possible to calculate the stresses with the applied efforts.

The dimensions and shape of the samples taken depend on the type of tests being conducted. They must meet the requirements of GOST 10180.

To determine the strength of concrete, samples are collected by drilling or sawing out individual parts.

1. Places are appointed after a preliminary inspection. The design test area should be at some distance from the joints and edges.
2. The remaining grooves after sampling are bricked up with fine-grained concrete.
3. During drilling or sawing, saws with diamond blades, special crowns or a suitable carbide tool are used.
4. There should be no reinforcement at the sampling sites. If this option cannot be implemented, then part of the concrete is taken with metal rods with a cross section of up to 16 mm for samples with dimensions greater than 10 cm.
5. The presence of reinforcement is unacceptable in studies of axial tension and compression. This negatively affects the end results. In addition, the rods should not be in prism-shaped samples during tensile tests under bending.
6. The places of extraction of samples, their quantity, as well as sizes are determined by the rules for monitoring the strength of concrete, taking into account the points of GOST 18105.

Each taken blank is marked and described in the protocol. After that, she undergoes thorough preparation for further tests. All samples should have a special scheme in which the orientation of the parts directly in the structure is clearly reflected.

Non-destructive testing mechanically

This method is based on calibration dependencies. They are based on indirect characteristics. These include:

• the bounce of the striker directly from the concrete surface;
• shock pulse energy parameters;
• sizes of prints left as a result of mechanical stress;
• stress leading to local failure during separation;
• the force in the implementation of the gap on the edge of the structure.

In the rules for monitoring the strength of concrete, it is proposed to use a certain set of measuring devices during testing: a vernier caliper, an angular scale, an hour indicator and some other tools. The number of tests carried out and the distances between work areas are given in the table.

The above activities should be carried out on a concrete structure with a total area of ​​100-600 square meters. see. After the implementation of the main tests, the data are recorded in a special journal for setting calibration dependencies between the indirect characteristics and the strength parameters of the hardened solution.

Non-destructive testing by physical methods

The category of such methods includes technologies of acoustic exposure and penetrating radiation. They provide an opportunity to judge the qualitative characteristics of the structure according to the internal structure, since the propagation velocity of waves of elastic vibrations is measured directly on the test material.

The most commonly used device for determining the strength of concrete is the ultrasonic method. It allows you to take readings without exerting a mechanical effect on the structure. With its help, the speed of passage of ultrasonic waves through a layer of concrete is measured. With a cross-cutting study, the sensors can be located on two sides, and with a surface study, on one side.

Control using ultrasound is considered the most informative and fairly simple. It allows not only to evaluate the strength parameters, but also to find possible defects inside the layers. The device used has several operating modes, which are presented in the table.

An ultrasonic concrete strength meter allows numerous tests to be carried out repeatedly, constantly monitoring changes in parameters. The disadvantage is the error in the ratio of acoustic characteristics with basic parameters.

On the hardening of cement-based building mixtures

There is a direct dependence of the strength of concrete on temperature during the solidification process. Normal conditions are considered to be from 15 to 20 degrees. With decreasing temperature, the increase in strength slows down. During freezing, hardening will occur if special additives have been added to the composition.

An increase in temperature speeds up the solidification process, especially if the humidity is sufficient. However, heating over 85 degrees is contraindicated, since it is difficult to protect the concrete mixture from drying out. The hardening process can be stimulated in two ways. The first of these is the use of internal heat, and the second - external.

On the analysis of possible problems in determining the strength

Using an ultrasonic concrete strength meter, special attention must be paid to establishing calibration dependencies. Without them, the data obtained cannot be considered evidence. To get more accurate results, you will have to take into account the amount and composition of the filler, the level of compaction, cement consumption and much more.