Concrete specimens: what are they and what are they for?

Concrete test tubes are a very important part of construction in this material, we explain why.

What are concrete specimens and what are they used for?

Much, if not most, of the structures of the buildings built in the last 70 years are made of concrete, and so will those that will continue to be made for a long time. Beyond telling us about the importance and diffusion of this material, this tells us that the material has very good mechanical properties; and to know exactly these mechanical properties, tests are carried out in the laboratory using concrete specimens.

Being so widely used, concrete has strict control over its properties and the agents that affect them, since it is essential to certify its correct operation as an essential material in structures of all kinds to ensure the safety of the people who inhabit and use the buildings. . This is known as concrete quality control and is planned and regulated by the administration and construction regulations of each country.

Concrete quality control

This is how the final objective of a series of control tests is known, whose objective is to verify that the characteristics of quality, resistance, and durability of the concrete are those foreseen in the calculation project carried out previously. The characteristics that the concrete must meet are specified in the specifications corresponding to each technical regulation and include the following criteria:

compressive strength

Consistency

aggregate size

Type of environment to which it will be exposed

Additive requirements

Tensile strength

Absorption

Specific weight

wear permeability

All these characteristics must be satisfied by all the manufacturing batches of the material, this is known as a product unit, that is, the amount of concrete manufactured at one time. For this reason it is so important that for each unit of product, concrete specimens are made, which are used to carry out the studies relevant to the different checks.

concrete specimens

The aforementioned criteria are verified through different resistance tests of the materials, which are carried out on concrete specimens, which are nothing more than cylindrical samples of the material. They must be a true reflection of the product unit, or the concrete production batch, so they must be carried out in a specific and standardized manner.

It is recommended to consult it in the technical regulations of each country, but in general, the test pieces must be cylindrical, 15 cm in diameter by 30 cm high, or 15 cm by 15 cm; even 10 cm by 10 cm for high resistance concrete.

How to make a concrete test tube

The first thing is to make sure we have enough space and controlled conditions so that it does not affect the setting process of the concrete, then a cylindrical mold capable of being hermetically sealed is prepared, which will contain the sample of the concrete mixture to be subjected to the relevant tests, pouring into the mold in three layers.

Each pouring layer is about 10 cm long, and once inside the mould, each layer must be shaken 25 times with a metal rod so that it is correctly compacted and the air bubbles lodged inside the mixture come out. At the end of the three layers of the concrete sample, the mold is struck about 10 times with a hammer for the same purpose as the rod.

Curing and conservation of a concrete specimen

Finally, wait 24 hours before removing the mold and identify the sample with its code, its date, and the expected resistance of the sample. The specimens are then completely immersed in potable water to cure. Inside the water, they must maintain a temperature between 20 and 2 °C in order to obtain a relative humidity of 95%. It is worth mentioning that it is also valid to reach these conditions using specialized cameras.

By law, the responsibility for ensuring stable conditions and control of the concrete specimens is assigned to the builder. In other words, the test pieces must be away from direct sunlight, protected from shocks or unexpected loads, as well as protected from the wind until the moment prior to carrying out the test.

Types of tests with concrete specimens

compression test

Compression test. Wikipedia) It is a destructive test, which yields the value of the compressive strength of the concrete from which the specimen is made, and which is carried out in cylindrical specimens of 15 × 30 cm, after 28 days of setting of the specimen. . It consists of applying a constant and progressively greater load until the point of fracture or plastic deformation of the concrete is found.

Flexotraction test

flexotraction-concrete-test It is done on prismatic square-section specimens, with a length that does not exceed four or five times on one side of a face, also being a destructive test. It consists of simulating the mechanical efforts of a beam by supporting the specimen horizontally on four support points, two near the center at the top and two at the ends at the bottom, in this way the tensile strength is determined. axial.

Indirect tensile test

Indirect tensile test.ICH trainingIn this case, the cylindrical specimen is placed horizontally inside a press that will apply a compressive stress, in the same way as in the compression test, but on the lateral faces of the cylinder . The force is applied to two diametrically opposite generatrices of the sample and the tensile strength is determined at its breaking point.