RU2545781C1 - Method for experimental determination of static-dynamic characteristics of concrete - Google Patents

Abstract

FIELD: construction.

SUBSTANCE: method is realised by fixation of an experimental concrete sample in the form of a prism in clamps of a test bench using an alignment device, providing for central application of stretching load in process of loading, and registration of a force and deformations of the sample in time using a dynamometer and a strain gauge station during loading executed via a lever system in two stages: at the first stage - stepped static loading of the sample to the specified level by means of laying of piece weights onto a load platform, at the second one - instant or stepped dynamic additional loading or unloading by means of short-term variation of the axis diameter in the point of force transfer from the lever to the compensating element, setting, if necessary, the value of movements in the elastic element.

EFFECT: simplified methodology and increased validity and reliability of test results.

5 dwg, 2 ex

Description

The invention relates to the construction, in particular, to the determination of concrete deformation parameters under static loading of concrete samples to a level not exceeding the compressive strength of concrete R _b and tensile R _bt , dynamic loading to failure with a constant loading speed and dynamic unloading.

The design of reinforced concrete structures is carried out taking into account the static application of the load and its further impact, while using the prismatic strength of concrete, determined during the gradual (steps) loading of concrete samples using a press [1]. The disadvantage of this method is the relatively low loading speed of concrete prisms, which does not allow judging about the deformation of the sample under high-speed loading. Determination of tensile strength of concrete is carried out using a tensile testing machine, which also does not allow to obtain the deformation characteristics of the sample under high-speed loading.

When calculating reinforced concrete structures for explosive and impact loads, the tensile strength and ultimate deformations of concrete samples are used, which are determined at the time of their destruction under dynamic loading and exceeding the similar values found during the static test.

One of the solutions that allow testing concrete for dynamic loading is a pneumodynamic installation for high-speed loading of concrete prisms [2].

The disadvantage of this solution is the inability to create a certain level of static loading, preceding the high-speed loading of a concrete prism.

The closest solution to the claimed invention is a method of experimental determination of static-dynamic diagrams of concrete, in which instant or step-by-step dynamic loading is carried out by a load falling with decreasing current strength in an electromagnet [3].

The disadvantage of this solution is the inconvenience, the need for an electromagnetic installation; the impossibility of performing dynamic loading at a predetermined movement; the impossibility of deforming the sample during high-speed unloading by a predetermined value other than the value of loading; the impossibility of multiple dynamic loading of the sample in alternation with unloading; in a high error of the data obtained during the experiment.

The technical result of the invention is the simplification of the test method, improving the accuracy of the data obtained, expanding the possibilities of experimental determination of the static-dynamic characteristics of concrete, which consists in the ability to pre-set the displacement in the compensating element during dynamic loading and unloading.

The technical result is achieved by the fact that in the method of experimental determination of the static-dynamic characteristics of concrete, which consists in securing the experimental concrete sample in the form of a prism in the clamps of the test bench using a centering device that provides a central application of load during loading, and recording the forces and deformations of the prism in time using a dynamometer and strain gauge, according to the invention, loading is carried out through the lever system in two stages: in the first - stepwise static loading of the sample to a predetermined level by stacking piece goods on a loading platform, on the second - instantaneous or stepwise dynamic loading or unloading by briefly changing the diameter of the axis at the junction of the lever and the compensating element.

On figa presents a diagram of a device for implementing the proposed method in a tensile test. On figb presents a diagram of a device for compression testing. On figa presents a diagram of the displacement of the axis during the dynamic loading during the tensile test. On figb explains the method of testing the sample in the case when pre-set movement in the compensating element under dynamic loading. Figure 3 presents a diagram of the loads acting on the lever during the tensile test.

A specially designed installation includes a frame 1, devices for centering and gripping the sample 2, a lever 4 for transmitting force to the test sample 3, connected through the rack 5 to the frame 1, a compensating element 6, supported on the frame 1 and connected to the lever 4 via the axis 7, a metal ball 10, a bolt 9, a loading platform 8 for applying a static load, piece weights 11 and a nut 12.

The compensating element 6 is a spring or a dynamometer ring, the rigidity of which is predetermined by calibration.

Axis 7 is a metal rod with different diameters of cross sections. At half the length of the shaft there is a thread for nut 12.

The diameter of the hole in the lever 4 exceeds the larger diameter of the section of the axis 7. The larger diameter of the section of the axis 7 exceeds its smaller diameter by the maximum amount of movement of the lever 4 along the axis of the compensating element 6 at the time of dynamic loading and unloading.

The metal ball 10 and the difference in the diameters of the cross section of the axis 7 are necessary for sharp dynamic loading and unloading of the sample 3. Nut 12 is necessary so that during testing the axis 7 does not move beyond a predetermined value.

A bolt 9 is necessary to limit the movement of the metal ball 10 at the time of sharp loading and unloading when the axis 7 is displaced, that is, at the time of reducing or increasing the diameter of the cross section of the axis 7 under the ball 10.

The method is as follows.

Loading is carried out through the lever system in two stages. At the first stage, a force is created in the compensating element 6 by laying piece goods 11 on the loading platform 8. In this case, the ball 10 rests on the axis 7 in the place of a larger section of the axis. At the second stage, the test sample 3 is fixed in the clamps 2, then the axis 7 is displaced so that the ball 10 is above a smaller section of the axis, while the load acting on the compensating element 6 will abruptly transfer to the sample 3 through the lever 4, by dynamically loading the concrete sample . If the movement of the lever 4 must be set in advance, the nut 12 is used.

Further displacement of the axis 7 will lead to an increase in the diameter of the axis under the ball 10 and the removal of the load from the test sample.

During testing, the force acting on the prism is measured with a dynamometer, and the deformation parameters of the prism itself under static loading and dynamic loading are measured using a strain gauge equipped with a built-in strain gauge that allows strain gauges to be connected without the use of intermediate amplifiers, and having the ability to connect to a computer and use a specialized software to record and display the converted signals of several input channels depending ing on time.

In the case of static loading during tensile testing, the load acting on the sample is determined by the formula:

, $$N=\frac{P\left(l-a\right)-Kb}{a}$$

,

where P is the applied load; K is the force in the compensating element; l is the length of the lever 4; a, b are the distances from the strut 5 to the sample 3 and to the elastic element 6, respectively.

In the case of dynamic loading, a sharp redistribution of the load occurs from the compensating element 6 to the sample 3.

Examples

Tensile tests were performed on rectangular samples with a length of 16 cm, a height of 4 cm and a width of 4 cm made of fine-grained concrete B20 with a ratio W / C = 0.741, C / P = 1: 3.789.

The distance from the sample to the strut 5 a = 0.1 m = 100 mm, from the strut 5 to the axis of the compensating element b = 0.1 m = 100 mm, the length of the lever 4 l = 0.6 m = 600 mm. The elements of the transmission of effort are made from Art. 3.

1) Platform loading P = 200 H. The load on the compensating element was K = 1000 N, the deformations of the compensating element are 0.2 mm. After fixing the sample in the clamps of the stand, axis 7 was shifted. The compensating element was unloaded, K = 0 N. The dynamic loading of the sample was N = 1000 N, the elongation of the sample was 0.2 mm.

2) Platform loading P = 100 H. The load on the compensating element was K = 500 N, the deformations of the compensating element are 0.1 mm. We fix the sample in the clamps of the stand. We load the platform to P = 200 N. The total deformation of the elastic element was 0.16 mm, the deformation of the sample was 0.06 mm (static loading). The forces in the elastic element are equal to 800 H, the forces in the sample were 200 H. We displace the axis 7, reducing its diameter. The forces in the sample were 1000 N (dynamic loading), the forces in the compensating element of steel were 200 H. The strains in the sample were 0.16 mm. With a sharp shift of axis 7 towards an increase in the cross-sectional diameter, the forces in the sample were 200 H (unloading), the strains in the sample were 0.06 mm, and the forces in the compensating element of steel were 800 H.

It can be seen from the examples that due to the use of an axis with different diameters of the cross sections, the sample is deformed under sharp loading by a given value. A technical result was achieved: the possibility of multiple dynamic loading of the sample in alternation with unloading, high accuracy of the data obtained during the experiment was achieved.

1. GOST 24452-80 Concrete. Methods for determining prismatic strength, elastic modulus and Poisson's ratio. - M .: NIIZHBD. 1982. - 15 p.

2. Bazhenov. Yu.M. Concrete under dynamic loading. - M.: Stroyizdat, 1970 .-- 272 p.

3. RF patent No. 2482480, cl. G01N 3/00, 2006.

Claims (1)

The method of experimental determination of the static-dynamic characteristics of concrete, which consists in fixing a prototype concrete sample in the form of a prism in the clamps of a test bench using a centering device that provides central application of load during loading, and recording the force and strain of the prism in time using a dynamometer and strain gauge, different the fact that the loading is carried out through the lever system in 2 stages: at the first - stepwise static loading of the sample to the rear level detection means for stacking piece goods load platform, the second - the instantaneous or stepwise dynamic dogruzhenie or unloading by means of a short axis diameter change in location of the lever connections and the compensating element, setting, if necessary, the amount of movement in a given element.

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