Determination of concrete strength. separation method.

Durability is perhaps one of the main parameters of concrete, which determines its operational properties. Therefore, during the construction of important supporting structures, builders carefully monitor this indicator. The most common method of control is the determination of the strength of concrete by the method of separation with chipping. However, there are many other ways.

Therefore, in this article we consider in detail how to determine the strength of concrete by the most common modern methods.

Kinds of ways to test the strength

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

As for the testing of separately performed control samples, it allows to determine only the quality of the concrete mix, but not the strength of the material in the structure. This is due to the impossibility of providing the same conditions for the curing of a prototype (vibrating, heating, etc.) and a concrete product.

All existing methods of control are divided into three groups:

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

Often use non-destructive methods of control, however, most often the work is performed by indirect methods. The latter group includes testing of test samples, as well as samples taken from a concrete structure.

Note! In terms of compressive strength, they determine the class of concrete. For this, concrete cubes are crushed using a hydraulic press, which gives the result.

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

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

• The method of elastic rebound;
• Ultrasound method;
• Impulse Impulse Method

I must say that different methods of verification have different errors:

Basic Requirements for Strength Testing

According to the requirements set forth in SP 13-102-2003, concrete should be sampled for research by indirect and direct methods at more than 30 sites, however, this is not enough to build and use a calibration curve.

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

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

In addition, the strength of concrete on the surface may differ from the strength at a certain depth. However, if the concreting is done qualitatively and the concrete corresponds to its design class, then the parameters of homogeneous structures do not change in a wide range.

To determine strength without violating existing standards, one should use direct non-destructive or destructive methods.

According to GOST 22690-88 direct methods include:

• Detachment method;
• Breakaway бетона со скалыванием;
• Rib Chipping.

Now take a closer look at the most common technology for determining the quality of concrete.

Strength technology

Detachment method

Принцип данного метода базируется на измерении усилия, которое нужно приложить для отрыва участка бетонной конструкции. Breakawayающую нагрузку применяют  к ровной поверхности бетонной конструкции. Для этого к ней приклеивается стальной диск, который при помощи тяги соединяется с измерительным прибором.

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

This technology involves gluing the disc without additional measures to limit the separation area. As for the area of ​​separation, it is non-permanent and is determined after each test.

However, in foreign practice, the separation site is preliminarily limited to the furrow, performed by ring drills. In this case, the area of ​​separation is constant and known.

After determining the effort required for peeling, the resistance of the material to stretching is obtained.

According to it, using the empirical dependence to calculate the compressive strength 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 method of separation used the same devices as for the method of separation with chipping, it is:

• PIP;
• ONYX-OS;
• PIC-50MG4;
• ГПНС-5;
• GPNV Fifth

Note! To perform the test, you also need a gripping device, namely, a disk with a fastener attached to it.

Breakaway со скалыванием

This method has much in common with the above method. Its main difference lies in the method of mounting the device to a concrete structure. In order to attach 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 compressive strength is carried out using the dependence, expressed by the formula - R = m1 * m2 * P, where:

• m1 denotes the coefficient of maximum size of large filler;
• m2 refers to the conversion factor to compressive strength. It depends on the type of concrete conditions, as well as the conditions of curing.
• P - the destructive force resulting from research.

In our country, this method is one of the most popular, since it is quite versatile. It provides an opportunity to perform the test on any part of the structure, since it does not require a flat surface. In addition, it is easy to fix the petal anchor with your own hands in the thickness of the concrete.

True, there are some limitations that lie in the following points:

• Thick reinforcement structures - in this case, the measurement will be unreliable.
• The thickness of the structure - it must be twice the length of the anchor.

Rib Chipping

This technology is the latest direct method of non-destructive testing of control. Its main feature is the determination of the force that is applied to the spallation of 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 external angle, was developed relatively recently. Installation of the device to one of the parties is carried out using an anchor with a dowel.

After receiving data from the device, determine the compressive strength according to the following normalized dependence, 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 the splitting of concrete.

Ultrasound detection

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.

And there are two calibration dependencies:

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

Each method is designed for a specific type of structure:

• Cross-sounding in the transverse direction - used for linear prefabricated structures. In such studies, devices are installed on both sides of the test structure.
• Surface sounding - is used to study ribbed, flat, multi-hollow 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, grease. “Dry contact” is also common, but in this case conical nozzles and protectors are used.

Ultrasound instruments consist of two main elements:

• Sensors;
• Electronic block.

Sensors can be:

• Separate - for through sound.
• United - intended for superficial sound.

The advantages of this method of verification is simplicity and versatility.

Kashkarov hammer research

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 the study of concrete by this method are as follows:

• Initially, a level design is sought.
• If there is a roughness or paint on its surface, 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 on top..

• Further, a medium-sized Kashkarov hammer is applied to the concrete surface perpendicular to the concrete plane. As a result of the impact, two imprints are left - on the reference rod and on the sheet of paper.
• After that, the metal rod is shifted by not less than 10 mm and another blow is applied.. For greater accuracy of the study, the procedure must be repeated several times.
• Then measure the prints on the reference rod and paper with an accuracy of 0.1 mm.
• When measuring prints, the diameters obtained on the paper and the diameters on the reference rod should be folded separately..

An indirect parameter of concrete strength is the average value of the ratio of imprints on the reference rod and on concrete.

Rebound method

This method of research is the simplest. The test is performed using a special electronic device. It has a hammer that presses the ball into the concrete. Electronics determines the strength of the material on the rebound of the ball after pressing.

To test the concrete, you need to rest the device on the concrete surface and press the appropriate button. The results are displayed on the device screen. It must be said that the process of testing a material with the help of a shock-pulse type device is almost the same.

That's all the main ways to determine the quality of concrete, which are most often used in modern construction.

Conclusion

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

From the video in this article, you can get more information on this topic.