Experimental and calculated evaluation of carbon fiber reinforcing for increasing concrete columns carrying capacity

The article presents the concrete columns tests results reinforced with canvases based on carbon fibers FibARM Tape 530/300 and FibARM Lamel 14/100 slats, the calculation of the use effectiveness for strengthening the compressed reinforced concrete elements. Three groups of columns were tested: unreinforced columns, columns reinforced with FibARM Tape 530/300 canvases in one layer, reinforced with FibARM Lamel 14/100 lamella (width is 100 mm), while one of the samples was reinforced with lamella only, and the other was reinforced both lamella and clamps. In addition, it was proposed calibration calculation of concrete columns reinforced with canvases based on carbon fibers FibARM Tape 530/300, the calculation purpose was to determine the bearing capacity for evaluating the suitability in the further structure operation after columns strengthening. According to the experiment results, data were obtained on the carrying capacity of centrally compressed concrete columns reinforced with carbon fiber both with a discrete arrangement of bands (strips) made of composite material along the height of the structures, as well as with their continuous wrapping (analogue of reinforced concrete holder). According to the experiment results, data were obtained on the carrying capacity of centrally compressed concrete columns reinforced with carbon fiber, both with a discrete arrangement of bands (strips) made of composite material along the height of structures, and with their continuous wrapping (analogue of reinforced concrete holder).


Introduction
Bearing columns are used as the main element for the perception of compressive loads in buildings. The reliability of the whole structure is preserved due to these elements. But often the calculated indicators of the columns are not suitable for the tasks that they have to perform in practice. Then the columns require reinforcement [1,2].
There are many reasons why there is a need for increasing the reliability of such elements. In most cases, the necessity for strengthening the columns arises if visible defects appear on them. In addition to ordinary cracks, the formation of a slope or roll should also be attributed to serious deformations. Another case for resorting to the strengthening of columns is the area building expansion. Load increasing on these elements may not be foreseen by the original project. Any major overhaul or reconstruction can lead to a reduction in the carrying capacity of columns. Then it is impossible to exploit the building without amplification [3][4][6][7][8].
Strengthening with composite materials in the form of carbon tapes and canvases is gluing the latter on a specially prepared surface of reinforced concrete structures, which gives them the necessary strength with virtually no weight gain. The fundamental characteristic of carbon fiber is its elastic deformation up to fracture. This material does not have plastic properties, its destruction is brittle, and this property allows not to redistribute stresses and not to change the calculated overlap scheme after its strengthening. The experience of Russian and foreign scientists and practitioners proves that concrete structures reinforcement using modern composite materials is more economical than traditional methods. The technology is simple and does not require the use of heavy equipment and temporary supports [5,9].

Experiments and methods
Experimental studies of the reinforced concrete elements carrying capacity using composite materials were launched abroad. The tests were subjected to concrete structures, reinforced with composite materials based on glass, aramid, carbon and other fibers. Subsequently, the efficiency of structures reinforcement with composite materials based on carbon fibers was revealed [10][11][12].
The paper presents the results of tests of reinforced concrete columns when reinforced with canvases based on carbon fibers FibARM Tape 530/300 and lamels FibARM Lamel 14/100, and also proposed the calculation of concrete columns reinforced with canvases based on carbon fibers FibARM Tape 530/300 [7]. The height of the columns is 2500 mm, the cross-section is square with a side of 400 mm. Concrete class for strength is not less than B20, frost resistance is F300 and waterproof is W6. The column reinforcement scheme is shown in Figure 1.  The load was applied to the column in steps of 5,000 kg at a speed of 500 kg / min [7]. The tests were carried out at a temperature of + (15-22) ° C on a CD 20/400 PU testing machine according to the axial compression scheme, while absolute deformations were recorded at the base of 500 mm in the middle of the column length. The load was applied to the column in steps of 5,000 kg at a speed of 500 kg / min [7]. Figure 5 shows the process of testing an unreinforced column (left) and reinforced with canvases (right). Figure 6 shows the testing process for concrete columns reinforced with lamella (the left figure) and both lamella and canvases (the right figure).  Table 1 shows the values of destructive loads for different groups of columns, obtained as a result of tests. According to test results, it was established that reinforcement of concrete columns working in compression by winding them with carbon canvases led to an increase in bearing capacity for compression by 54%, while strengthening them with lamella increased the bearing capacity of columns only by 8%, and only the addition of partial winding to the lamella with canvases led to an increase in the compressive capacity by 35%.

Calculations and results
The article proposed a calibration calculation of concrete columns reinforcement with canvases based on carbon fibers FibARM Tape 530/300, the purpose of which was determining the bearing capacity for evaluating the suitability in the further operation of the structure after the work on columns reinforcement. The purpose of the verification calculations is determining the bearing capacity to assess the suitability in the further operation of the structure after the work on the column's reinforcement.
Design class of columns concrete is B35. There are columns with a conditional class of concrete B20. Strengthening of the columns which have not gained durability to a design class is required.
The composite material based on carbon fibers FibARM Tape 530/300 is taken for strengthening columns, which has the following characteristics, presented in Table 2. The calculation of the concrete columns reinforced with a conditional class of concrete B20: Actual characteristics: R b = 11,5 (MPа) [B20] h = 400 (mm) b = 400 (mm). The distance between the transverse canvases in the light is S = 50 mm. The sticker transverse canvases were accepted in two layers [11].
С Еthe working conditions coefficient, it is taken 0.9. Relative deformations limit value of carbon canvas elongation: The calculated values of the tensile modulus: The bearing capacity of the reinforced column is checked by the formula: The following condition must be met for designing: 230000 · 0,0153=3519 <0,75 · 4900 = 3675 (condition is met).
Design concrete resistance of reinforced construction to compression:

Conclusions
According to the experiment results, data were obtained on the carrying capacity of centrally compressed concrete columns reinforced with carbon fiber, both with a discrete arrangement of bands (strips) made of composite material along the height of structures, and with their continuous wrapping (analogue of reinforced concrete holder) [13][14]. Test results analysis shows that reinforcement of concrete columns working in compression by winding them with carbon canvases led to an increase in bearing capacity for compression by 54%, while strengthening them with lamella increased the bearing capacity of columns only by 8%, and only the partial winding addition to lamella with canvases led to an increase in the compressive capacity by 35%.