Quality control of the material used is mandatory in the installation of buildings and structures. To confirm the conformity of the declared characteristics with design standards, concrete is tested for strength, bending and tensile strength. This measure allows the contractor to report to the customer on the work in accordance with the project, and the manufacturer to confirm the quality of the products. Timely completed tests will allow you to make changes to the course of work and avoid mistakes.
Tests are carried out in certified laboratories on the basis of GOST 22690-2015, for which experts use various methods of measurement and impact on selected material samples. As these, concrete cubes are usually used, which are tested for compression, but there are other research methods.
During the verification, the following results are obtained:
- Determine the conformity of the quality of the material to the design documents. Tests are carried out at least three times over the entire construction period.
- In case of deviation of the characteristics, the replacement of structures made of rejected material is carried out, which allows to keep the overall performance of the structure within the project.
- Preliminary tests are required for repairs in technical rooms and basements.
- Testing of reinforced concrete structures allows you to decide on the fate of old buildings and structures.
What determines and what affects the strength of concrete
The ability of concrete to resist external influences due to internal stress depends on the composition of the mortar and the grade of cement. When confirming the strength of the material corresponding to a certain brand, signs of fracture in the form of chips, cracks, and delamination of the structure should not be detected on the sample.
Sometimes builders are trying to save on materials using cheaper concrete of low grades, but violation of design values can lead to serious consequences, so this means of saving is unacceptable.
In addition to the ratio of filler to cement, the strength of the composition is affected by additives and plasticizers used to impart special properties to the product (acid resistance, water resistance, rising speed, plasticity). To obtain structures that can withstand high loads, the elements are reinforced with metal wire of various sections without fail.
In addition to the composition of the mortar, the strength of concrete is influenced by the external conditions under which the pouring is carried out. With the qualitative removal of air bubbles from the concrete mass by compaction of the mixture, the strength of the products increases markedly.
It should also be borne in mind that when using the solution at negative temperatures, measures should be taken to heat the material by installing electrodes in the fill and connecting electricity to them. In such a situation, sheltering the base with sawdust is still used.
When working with concrete, it is important to maintain the necessary humidity to prevent cracking of the pouring surface during the rapid evaporation of moisture, which also affects the quality of the material and its strength. To avoid this process, it is necessary to cover the concrete with a film or other improvised means, as well as periodically moisten the surface.
As a result, it can be argued that the strength of concrete depends on many factors, and therefore quality control is especially important when installing load-bearing structures, since even if the technological processes are fully observed, there can always be factors that will affect concrete and cause problems in the future .
Classification of test methods
To test concrete, several methods are used:
- Verification of samples cast in laboratory conditions. This method involves the manufacture of cubes or cylinders from the test mixture with subsequent verification of the strength of the material on the press;
- Checking samples cut or cut from an already finished structure. Such samples are obtained by drilling with diamond crowns. Next, the obtained cores are sent to the laboratory to determine the strength characteristics, as in the first case, using a press. This method is associated with significant costs for obtaining the sample and with the threat of weakening the integrity of the element from which the core was obtained;
- A method of testing concrete for strength by a non-destructive method. In this case, tools and instruments are used with which you can study the characteristics of concrete without placing samples in special devices. For these studies, they can use ultrasound, check the quality of the foundation using the pulse-shock method of testing concrete, etc.
The most popular method for obtaining the most accurate indicators of concrete properties is to test samples for compression under pressure.
Testing Stages
Concrete testing is carried out by examining samples for strength by non-destructive and destructive methods.
Destructive methods
This method involves testing using a press, when a gradually increasing pressure is exerted on a sample obtained during laboratory casting or sawn from the base of an already finished structure. The exposure continues until the destruction of the sample is fixed.
This method is the most accurate and mandatory in the construction of critical structures.
Nondestructive methods
To obtain results when using non-destructive testing methods, special instruments and devices are used. Partial destruction is carried out by fixing a special tool on the concrete surface, which allows you to examine the concrete for separation, fixing the necessary force.
The reaction of the material to chipping is also studied, when the device is installed at the angle of the concrete base and the material is destroyed under load.
Under shock loads, the behavior of concrete is studied during the impact by a special device and the reaction to an elastic rebound is recorded - the value of the rebound of a metal ball released with a certain force is measured.
In ultrasonic quality control of concrete, a special device is used, which makes it possible to record the passage of waves within the structure. By reaction to reflection, they conclude about the quality of the material.
How to check the strength of concrete yourself? It is impossible to get a full-fledged study of the material at home. Quality control of the material can be done exclusively by visual methods. A high-quality mixture usually has a gray or gray-green color, the structure of the solution should be uniform, with normal viscosity.
If the material has a yellowish tint, this means that the quality of such a solution is low and impurities are present in its composition that reduce the strength characteristics. A good sign is the detection of a thick consistency on the surface of the cement milk mortar.
Under shock loads (hammer blow on a material that has gained full strength), the tool should bounce off the base without significant changes on the surface, leaving almost invisible dents.
Workability Test Procedure
To determine this characteristic, laboratory specialists use a viscometer. This device allows you to measure the time in seconds that is required for laying the material.
Using a viscometer, they begin styling, while simultaneously starting the countdown. At the end of the process, the actual time is recorded. The quality of concrete is determined by the time spent laying this method. The less time passes, the higher the quality of the material.
Tensile Test Procedure
For the production of tensile tests, it will be necessary to prepare an elongated specimen such as a prism. This sample is placed in a special device in a horizontal position, then in the middle of the sample there is a force action with increasing load. The step of the impact on the sample is 0.5 MPa / s.
The result is fixed after the destruction of the concrete structure in the central part of the sample.
Compression Test Procedure
This test method allows you to determine the grade of material. For the test, cubes are cast from concrete used in construction, or samples are cut from an already cast product. The size of the cubes for testing concrete varies from 100 to 300 mm along the edge. In addition to the cubic shape, the samples can be made in the form of cylinders or prisms.
When casting samples in laboratory conditions, use a vibrating table so that the mixture gets the maximum density. Tests are carried out on the 3rd, 7th and 28th day after the acquisition of strength sample. The main tests are carried out on day 28 after a full set of material strength.
The sample is placed under a press that presses on a cube with a capacity of 140 kgf / m2 in increments of 3.5 kgf / m2. The force vector is strictly perpendicular to the base of the sample. According to the testimony, the possibility of resistance of the material to compression is determined, and the concrete grade is recorded in the test report.
Concrete strength grades and their scope
To determine the characteristics of concrete, it is assigned a marking according to GOST: the letter M and a number indicating the compressive strength of the material. The higher the value, the more durable the product from this material is - strength depends on the amount of cement in the mixture.
According to the strength characteristics, concrete is divided into grades from M100 to M500 with a step of 50. Another characteristic, the class of concrete, determines the ability of a material to work in aggressive environments.
Concretes of the brand M100, M150, M200 and M250 are classified as light and cellular. They are used for pouring structures that do not carry a significant load. They are used in the installation of borders, foundations for small buildings, pedestrian paths.
Concrete M300 and M350 can be used for casting floor slabs, foundation devices in multi-story buildings, casting monolithic walls.
The most durable concrete grades, M400, M450 and M500, are used in the production of reinforced concrete structures operating in difficult conditions with increased load (for example, for the construction of hydraulic structures).