Non-destructive testing of concrete: direct and indirect

Building structures based on a mixture of binder, sand and aggregate need to be tested for reliability and safety. However, such studies should not be the cause of the interruption of operation of the test object, therefore, the determination of the strength of concrete by a non-destructive method is made. This allows you to reduce costs, reduce labor intensity and eliminate local damage.

Direct control methods

These methods are necessary for the formation of calibration dependencies and their subsequent adjustment for indirect methods carried out on the same parts of the structure. The technology for determining the strength properties of concrete can be applied during the examination at various stages of the construction of buildings, as well as during the operation and reconstruction of finished objects.

Chipping off

This operation is carried out in accordance with state standards, which reflects the basic information about the method of conducting. The results obtained are not affected by the surface condition.

For research used anchor devices of three types.

1. The working core equipped with an anchor head.
2. The device with the presence of expanding cone and grooved segment cheeks.
3. A device with a hollow expanding cone, which has a special rod for fixing the device in one position.

Note! When choosing the type of fixture and the depth of penetration of the anchor, one should take into account the assumed strength of the composition and the dimensions of the aggregate, which is reflected in the table below.

In monolithic structures, the testing of the strength of concrete by a non-destructive method, involving tearing, is carried out at three sites at once. When adjusting the calibration dependencies, three indirect tests are carried out together with this method.

Rib Chipping

This method involves cutting off the edge of the test structure. In the first place, it is used to control linear segments such as crossbars, columns, piles, lintels and support beams. The operation does not require additional preparation, however, in the presence of a protective layer with a thickness of less than 20 mm, the method cannot be applied.

Breakaway metal discs

Another event that allows non-destructive testing of concrete is not widely used in our country, which is associated with limited temperature conditions. Another negative factor is the need to drill the furrow, and this reduces the productivity of the study.

The method itself involves removing the voltage recording, which is required for local destruction of the hardened composition when the steel disk is torn off. In determining the strength properties, the applied force and the projected surface area are taken into account.

Indirect control methods

Подобные исследования проводятся, когда нужно оценить значение прочностных характеристик, используя их в качестве одного из нескольких факторов, дающих представление о техническом состоянии сооружения. Полученный результат не допускается использовать, если не была определена частная градуировочная зависимость (см.также статью «Защита бетона от влаги: способы и применяемые материалы»).

Ultrasound Testing

Widespread method of testing concrete non-destructive method, involving the use of ultrasonic waves. During the operation, a relationship is established between the rate of oscillation and the density of the hardened mixture.

Dependency can be affected by a variety of factors.

• The fraction of the aggregate and its amount in solution.
• The selected method of preparation of the composition.
• The degree of compaction and stress.
• Change in binder consumption by more than 30 percent.

Addition! Ultrasonic surveys provide the ability to carry out mass tests of virtually any design an unlimited number of times. The main drawback lies in the permissible error.

Elastic rebound

Non-destructive testing of concrete strength by this method allows to establish the relationship between compressive strength and material elasticity. In the study of the metal head of the main device after the impact moves away a certain distance, which is an indicator of the strength properties of the structure.

During the test, the fixture is fixed so that the steel element is in close contact with the concrete surface, for which special screws are used. After mounting the pendulum is mounted horizontally. In this case, it snaps directly on the trigger.

Attaching the device perpendicular to the plane, pull the trigger. The striker is automatically cocked, after which it is released independently and makes a blow under the action of a special spring. The metal element bounces off at some distance, which is measured by a special scale.

As a basic instrument for testing, an instrument of the KISS system is used, which has a rather complicated structure. The strength of the hardened mixture can be determined on the basis of the device data after conducting 6-7 tests according to a special schedule.

Giving a shock pulse

Thanks to this research method, it is possible to fix the impact energy released at the moment of contact between the striker and the concrete structure. A positive moment is the fact that non-destructive testing of concrete, operating on the principle of shock impulse, have compact dimensions. However, their price is quite high.

Plastic deformation

During the operation, measurements are made of the dimensions of the trace left on the concrete surface by the steel element. The method is considered somewhat outdated, but due to its low cost equipment, it continues to be actively used in the construction environment. After the strike, the remaining prints are measured.

Devices for determining the strength of this type are based on pushing the rod directly into the plane by static pressure of the desired force or conventional impact. Pendulum, hammer and spring products are used as main devices.

Below are the conditions of the operation.

• Tests should be carried out on the site, whose area ranges from 100 to 400 square meters. cm.
• When performing this operation, at least five measurements should be made with high accuracy.
• The impact force should be perpendicular to the plane being tested.
• To determine the strength characteristics requires a smooth surface, which is achieved by molding in the formwork of metal.

Note! If a concrete is measured by the strength of a non-destructive method using hammer-type devices, the samples should be installed on a perfectly flat base.

Comparative characteristics of the example

A well made of monolithic reinforced concrete is taken as an object. Its depth is 8 m, and the radius is 12 m. Filling of the side surfaces was carried out by hooks that divide the structure into 7 tiers in height.

The research results are presented in the table below.

Conclusion! From the table below it becomes clear that the minimum error in the research is characteristic of the ultrasonic method. The scatter during the shock pulse test is maximum.

Testing without the use of devices

The above studies have been considered using special devices, but if necessary, simple tests can be done by hand. It will not be possible to obtain exact information about the strength properties, but it is quite realistic to determine the class of concrete.

First, the necessary tool is prepared: a chisel and a hammer, whose weight ranges from 400-800 g. The impact-cutting device is installed perpendicular to the surface.

The blows of an average force are inflicted on it, in the wake of which analysis will be carried out.

• A barely noticeable imprint may indicate that the hardened mixture belongs to class B25 and higher.
• Highly noticeable marks on the surface of the structure usually remain when using B15 concrete.
• The significant indentations and the presence of crumbs allow us to attribute the applied composition to the class B10.
• If the tip of the tool entered the plane to a depth of more than 1 cm, then concrete B5 was possible for work.

Attention! It is possible to carry out a check in this way within a few minutes without any equipment. After that, there will already be an idea of ​​what strength the solidified composition has.

State standard

Non-destructive methods of monitoring the strength of concrete according to GOST 22690-88 are regulated, the clauses of which apply to light and heavy mixtures. However, it reflects only mechanical methods that do not include ultrasound. Their limit values ​​are presented in the table.

Working with concrete

• For the formation of structures on the basis of the construction mix, wooden or metal formwork is made, which is able to give the desired shape to the material.
• To improve the quality characteristics of the composition is placed mesh of steel reinforcement, bonded by welding or wire. Typically, the size of the cells ranges from 10 to 20 centimeters.
• If it is necessary to separate some part from the structure, then cutting of reinforced concrete with diamond circles is used.. Such an operation can be performed using water to avoid heavy dusting.
• The solution is poured, as a rule, at positive temperatures.. However, in the presence of special equipment for heating, it is permissible to carry out work with negative thermometer readings.
• To create ventilation inside the concrete structure (for example, for a foundation or attic) diamond holes are drilled in the concrete.
• It is allowed to load the finished structure only after the final hardening of the mixture, that is, after 28 days.

As a conclusion

Представленная инструкция позволяет получить представление о проведении проверки прочностных качеств бетонных конструкций не только при помощи специального оборудования, но и с использованием подручных средств. Однако самостоятельный метод даст возможность сделать лишь предварительную оценку характеристик (см.также статью «Коррозия бетона: причины и меры противодействия»).

More information about the control can be obtained after watching the video in this article.