Introduction

For the reinforcement of reinforced-concrete structures, it is customary to use thermally hardened reinforcements and its welded joints [1]. The procedure of thermal hardening of reinforcements is realized by the method of rolling with separation performed by heating with subsequent abrupt cooling [2]. In the surface layer, we observe changes in the structure characterized by the presence of a mixture of bainite with martensite. In the central zone, we detect the formation of the structures of ferrite and pearlite. The technology of manufacturing of the framework of reinforced-concrete products includes the procedure of welding of transverse bars to the thermally hardened reinforcement. As a result, the mechanical characteristics change in a local volume of the surface layer of the load-bearing thermally hardened reinforcement. Note that it is impossible to evaluate these changes and determine the mechanical characteristics in the zone of welding by the standard methods. For this purpose, it is necessary to use an approach based on the determination of relations between the microhardness in a local volume of the material and its mechanical characteristics.

The investigations carried out in [2] demonstrate that the weldability of thermally hardened steel is satisfactory and, at the same time, the degree of softening is equal to 5–9%. It was also shown that the corrosion resistance of this reinforcement in marine environments decreases by 5–9%.

In what follows, we analyze and determine the behavior of microhardness of thermally hardened reinforcement over its cross section and in the zone of welded joint. We also find the mechanical characteristics of different zones of rebars and establish the correlation relations between these characteristics and microhardness.

Methods of Investigations

We investigated the strength and microhardness characteristics of different zones of thermally hardened А500С steel ∅ 20 mm. To determine the behavior of microhardness over the cross section, we produced polishing machine. The microhardness was found with the help of a PMT-3М device by analyzing fragments of the reinforcement and welded joint. The variations of microhardness were recorded in two directions (Fig. 1) across the cross section of the thermally hardened reinforcement (А–А), in the zone of welded joint in the reinforcement with ∅ 20 mm (В–В), and in the bar with ∅ 10 mm (С–С) welded perpendicularly to the reinforcement. The zones characterized by the changes in microhardness occurred were detected according to the results of microhardness measurements.

Fig. 1
figure 1

Schematic diagram of microhardness measurements in the thermally hardened reinforcement and its welded joint: (1) main reinforcement; (2) transverse reinforcement; (3) weld.

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In the next stage, cylindrical specimens with different zones of microhardness were subjected to tensile tests for the evaluation of the mechanical characteristics in local volumes with the use of the digital image correlation (DIC) method [2,3,4,5]. For this purpose, thermally hardened reinforcement was turned down to diameters corresponding to the changes in microhardness. The values of true strains were found by stretching turned specimens by a length of 100 mm. At the same time, we recorded tensile stresses in a computer and, simultaneously, recorded the images of the surfaces of turned areas. The recorded images were used to estimate the elongation and determine the relative strains according to the requirements of DSTU and recommendations proposed in [3,4,5,6].

The images of the deformed surface of the specimen were recorded by a Grasshopper 3 monochromatic digital camera with a Gomputar F25/2.8 lens, which was set up on a special device of an EUS-20 test unit, and tensile forces were recorded from indications of the pressure sensor of the test unit with the help of an Е14-440 digital transducer equipped with a Power Graph software complex. The total elongation of the specimen was determined and the levels of strains along two coordinates εх and εу were established. On the basis of the test results, equilibrium diagrams were constructed and tensile stresses were determined. The true stresses were determined as the ratio of tensile stresses to the running cross section at a given time calculated according to the DIC results [3]. According to this approach, the obtained values of true stresses and strains for different specimens were used to establish the correlation between stresses obtained by determination of the microhardness of the corresponding zone and stresses determined by stretching of specimens. On the basis of the established microhardness in the section of the weld joint, it is possible to evaluate the area of the near-weld local zone where we observe the changes in the mechanical characteristics. According to the variations of stresses in the welding zone, we can find the decrement of the strength of the rebar, which should be taken into account in the evaluation of the service life of the reinforced-concrete beam.

Results of Investigations

The distribution of microhardness for the thermally hardened А500С reinforcement in the cross-section of a rebar with ∅ 20 mm (line А–А, Fig. 1) is shown in Fig. 2а. The maximum value of microhardness (of up to 300 HV) was observed in the subsurface layer with a thickness of up to 2 mm, where the structure is characterized by a mixture of bainite and martensite, which corresponds to an area bounded by a diameter of 20–16 mm. The next transient layer with a thickness of ∼ 3 mm lies in the zone from 16 to 10 mm with a microhardness varying from 250 to 200 HV. The remaining area of the cross section belongs to the middle zone with a constant microhardness of 200 HV and is bounded by ∅ 10 mm, which corresponds to the structure of pearlite. The microhardness in the weld zone was investigated similarly (Fig. 2b).

Fig. 2
figure 2

Distributions of microhardness in the thermally hardened reinforcement (line A–A, Fig. 1) (a) and in the zone of welding (B–B) (b).

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The microhardness in this zone was measured to establish its influence on changes in the structure of the main thermally hardened reinforcement after welding of the transverse rebar. The results of measurements of microhardness in the weld zone (Fig. 2b) demonstrate that, at a depth of about 3 mm, the microhardness of the surface layer of the main (∅ 20 mm) thermally hardened reinforcement decreases down to 200 HV and attains the value typical of the middle zone (D = 10 mm), which corresponds to the structure of pearlite. In other words, the microhardness of the surface layer of thermally hardened reinforcement changes as a result of welding of the transverse reinforcement. Hence, the physicomechanical characteristics of reinforcement in the local volume become lower. A similar picture of changes in microhardness within the range of up to 200 HV was observed in the weld zone for the transverse reinforcement (С–С, Fig. 1). To determine the fracture resistance and deformation resistance characteristics for various zones of the thermally hardened reinforcement, we tested cylindrical specimens. One batch of specimens was made of continuous reinforcements, the second batch was turned down to ∅ 16 mm, and the third batch was turned down to D = 10 mm.

The results of the tests and the characteristics obtained for three batches of specimens are presented in the form of “true stress–true strain” fracture diagrams in Fig. 3 and in Table 1.

Fig. 3
figure 3

True fracture diagrams of the specimens of reinforcement: (1) rebar with D = 20 mm; (2) 16 mm; (3) 10 mm.

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Table 1 Mechanical Characteristics and Microhardness for Different Zones of the Reinforcement
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For the middle zone where the structure of the material and its mechanical characteristics are homogeneous over the cross section, we substantiate the relationship between the Vickers microhardness and the yield strength 3 HV = σ0.2 [7]. In particular, for the core, the tensile yield stress of the reinforcement turned down to D = 10 mm is σ0.2 = 598 МРа, which practically coincides with the values given by the approximate relation. The yield strength for the thermally hardened reinforcement determined according to its microhardness differs from the value established (in tension) for the solid bar and for the bar turned down to D = 16 mm because their strength and deformation characteristics vary over the cross section.

The obtained values of microhardness in the weld zone (В–В, Fig. 1) practically coincide with its values in the middle zone. This means that the strength and deformation characteristics of the metal in this region coincide with the corresponding characteristics for the middle zone of the reinforcement. The true stresses S determined in the local volume of this zone by the DIC method differ by 25–30% from the values obtained for the thermally hardened reinforcement. As a result of welding of transverse reinforcement to the load-bearing thermally hardened reinforcement, its strength decreases in a local volume, which results in the elevated stress concentration in this zone. This should be taken into account both in analyzing the stress-strain state and in the evaluation of the service life of reinforced-concrete structures with thermally hardened reinforcement.

Conclusions

We determined the distributions of microhardness in a thermally hardened А500S reinforcement and in its welded joint. We established the existence of correlation between the levels of microhardness and the values of yield strength determined by using the DIC method for the middle zone of the thermally hardened reinforcement. It was shown that the strength of thermally hardened reinforcement in the local volume of welding decreases by 25–30%.