Keywords

1 Introduction

Reinforced concrete structures are among the most common in industrial and civil engineering [1,2,3,4,5,6,7]. During operation, such structures may be damaged and become unusable [8,9,10,11,12,13,14,15,16]. Many works are devoted to the research of new materials [17,18,19,20,21,22,23,24] that will improve the characteristics of reinforced concrete structures [25,26,27,28,29,30]. One of them are steel concrete structures. Steel concrete structures with external tape reinforcement are characterized by effective joint work of steel and concrete [31]. Despite the disadvantages such as reduced corrosion resistance and fire resistance compared to reinforced concrete, the using of structures with external reinforcement in some cases gives a good economic effect provided care during operation and appropriate measures to prevent corrosion and increase fire resistance [31]. Steel concrete structures use high-strength strip reinforcement in a stretched (sometimes compressed) area, which is located mainly on the extreme edges of the cross section. Very often there is a problem of uneven distribution of loads on the structure within one structure. If it is a beam floor, then such beams should be reinforced with different frames. When using steel concrete beams, this causes additional difficulties, because the range of tape reinforcement is limited [32]. One of the ways out of this situation is the use of a package of fittings - sheet and rods within one beam, along with pre-stressed reinforcing rods arrange unstressed rods, which partially absorb the pre-stressing forces. The sheet is constant in all beams and the number of areas of the rod reinforcement varies depending on the need. The using of a package of reinforcement simplifies and accelerates the production of reinforced concrete elements, as well as more fully used rod reinforcement [33, 34]. The effect is further enhanced by the use of pre-stress [35]. In this work, prestressed steel concrete beams reinforced with a package of reinforcement with a different ratio of sheet and rod reinforcement in the area of pure moment.

The aim of the study is to determine the effect of the percentage of reinforcement prestressed steel concrete beams reinforced with a package of reinforcement on the loss of compression force, to assess the effectiveness of the use of rod reinforcement in the package.

2 Research Methodology

To conduct research, three steel concrete beams with combined reinforcement with different percentages of reinforcement with unstressed rod reinforcement were developed and constructed, namely:

  • Beam B-1 - the total percentage of reinforcement on the area of the working reinforcement 2.97%, including unstressed rod reinforcement 0.65%;

  • Beam B-2 - the total percentage of reinforcement by the area of the working reinforcement 3.51%, including unstressed rod reinforcement 1.16%;

  • Beam B-3 - the total percentage of reinforcement on the area of the working reinforcement 4.02%, including unstressed rod reinforcement 1.75%;

The purpose of the experimental study is

  • to determine the effect of the percentage of reinforcement with additional unstressed reinforcement of prestressed beams on the nature of the loss of compression forces

  • inspection of stress-strain states of such bending structures;

  • comparison of the obtained data to assess the behavior of bending structures with similar reinforcement and identify patterns of their work.

The research was carried out on - samples of beams with cross-sectional dimensions 270 × 135 mm with a calculated span of 2700 mm. The total length of the beams is 3000 mm. The dimensions are taken from the condition of providing the percentage of reinforcement within 2.97%…4.02%. Tape reinforcement used periodic profile with a cross section of 105 × 6 mm, with a tensile strength fyk = 450 MPa.

Concrete made of prototypes corresponded to classes C40/50…C45/55. This class of concrete is selected based on the results of the preliminary calculation, provided that the bearing capacity of the samples is exhausted by the fluidity of the stretched reinforcement without destroying the concrete of the compressed zone.

In all experimental beams, strip reinforcement made of 16G2AF steel of periodic profile with cross reefs, recommended by UkrNDIMet, with the main parameters developed at Lviv Polytechnic Institute.

The A400C-class rod rebar was reinforced by a power bench hood with hydraulic jacks with voltage control using dynamometers and a manometer. The rod reinforcement of class A400C of the stretched zone was strengthened to the strength limit of tape reinforcement fyk = 450 MPa, and the rod reinforcement of the compressed zone - to the maximum limit determined by regulations: fyk = 540 MPa [31].

Investigations of prestressing losses of prestressing of reinforced concrete beams reinforced with a package of reinforcement were performed with endurance for 60 days under the action of prestressing forces.

3 Test Specimen Description and Material Properties

Physico-mechanical characteristics of strip steel (yield strength, tensile strength and modulus of elasticity) were determined by testing specially manufactured samples, according to regulations, on a rupture machine brand R-20 with simultaneous recording of tensile diagram force - strain. Physico-mechanical characteristics of concrete (strength, modulus of elasticity, concrete class) were determined by testing cubes with an edge of 100 mm and a prism length of 400 mm with a cross section of 100 × 100 mm, respectively, on a hydraulic press with a capacity of 2500 kN. To implement the objectives and objectives of the study developed and manufactured three beams, the characteristics of the materials of the prototypes are given in [].

The presence of prestressed reinforcement in the upper zone of the beams is provided for the perception of tensile bending tensile forces and crack resistance of this zone when transmitting prestressing forces of strip reinforcement (transfer of prestressing forces to concrete was performed 28 days after concreting).

In the experimental beams used tape reinforcement of constant cross-section along the entire length of the beams, and rod reinforcement of different diameters is not laid on the entire length of the beams. The rod reinforcement in all beams is unstressed with a length of 1850, 2050, 2150 mm, respectively for beams B-1, B-2, B3, it was freely placed on top of the tape pre-stressed reinforcement. Due to the saving of material, the rod reinforcement is spaced along the length of the beam in different directions relative to the axis of symmetry by: 150 mm (beams B-1), and 125 mm (beam B-2, B-3). Anchoring of tape reinforcement is provided due to its coupling with concrete and transverse anchor rods.

In each of the beams in the upper compressed zone is designed one longitudinal rod by diameter 16 mm class A400C. Transverse reinforcement is designed by diameter 8 mm class A400C and placed along the length of the beam in the form of clamps with a step: in the area of pure bending in all beams step is taken structurally numerically equal to 180 mm; and in the zone of action of transverse forces, depending on the bearing capacity of the beam - 135 mm (B-1), 110 mm (beam B-2), 80 mm (beam B-3). Three pairs of transverse rods with a step of 50 mm are placed on the supports in all beams. Schemes of frameworks of experimental samples are given in Fig. 1.

Fig. 1.
figure 1

Construction of the samples.

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4 Results and Discussion

Experimental studies have shown that the percentage of reinforcement with unstressed rod reinforcement, prestressed steel concrete beams reinforced with a package of reinforcement, significantly affects the loss of compression force from prestressing tape reinforcement, as well as reducing relative deformation depending on the percentage of reinforcement.

The reduction of relative deformations depending on the percentage of reinforcement by unstressed rods in the experimental beams is:

  • at the stage from manufacture to dismantling (1st stage) is for beam B-1 (2.5%), for beam B-2 (2.1%), for beam B-3 (1.7%);

  • at the stage from stripping to the moment of release of tape reinforcement (2nd stage) is for beam B-1 (2.3%), for beam B-2 (1.6%), for beam B-3 (1.0%);

  • at the stage of release of tape reinforcement (3rd stage) is for beam B-1 (23.9%), for beam B-2 (21%), for beam B-3 (14.5%).

  • at the stage of relaxation before the beams are tested (stage 4) is for beam B-1 (11.3%), for beam B-2 (10%), for beam B-3 (7.8%).

Data comparing the development of deformations depending on the percentage of reinforcement with unstressed rods in combined reinforcement are shown in the graph of Fig. 2 and in Table 1.

Fig. 2.
figure 2

Development of deformation depending on the percentage of reinforcement by unstressed rod reinforcement in combined reinforcement.

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The minimum drop of initial deformations (loss of compression forces) in the prestressed strip reinforcement was observed in the beam B-3 (25.0%), with the highest percentage of reinforcement by unstressed rod reinforcement, which was 49.72%.

The drop in initial deformations (loss of compression forces) in the pre-stressed strip reinforcement of the B-2 beam was 34.7%, with the percentage of reinforcement by unstressed rod reinforcement 26.86%.

The maximum drop of initial deformations (loss of compression forces) in the prestressed strip reinforcement was observed in the beam B-1 (40.0%), with the lowest percentage of reinforcement by unstressed rod reinforcement, which was 26.86%. (Table 1).

Increasing the percentage of reinforcement with unstressed rod reinforcement by 22.86% reduces the drop in initial deformation (loss of compression force) by 37.5%, which proves the effectiveness of such reinforcement.

Table 1. Experimental results.
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The use of prestressed reinforced concrete structures reinforced with a package of reinforcement in construction is appropriate, such structures require more research.

5 Conclusions

External reinforcement in the form of strips, sheets or rolled profiles allows you to effectively perform steel concrete structures with a higher percentage of reinforcement with limited cross-sectional dimensions. The performed experimental researches allowed to investigate the fall of initial deformations (loss of compressive forces) before loading of experimental samples with measurement of deformations of concrete and reinforcement in the zone of action of pure bending. Increasing the percentage of reinforcement with unstressed rod reinforcement by 22.86% reduces the drop in initial deformation (loss of compression force) by 37.5%, such a disproportionate reduction proves the effectiveness of combined reinforcement.