Method of experimental research of concrete beams with fiberglass reinforcement for bending

. This article presents a methodology for experimental studies of bent concrete beams made of ordinary heavy concrete, equipped with fiberglass composite reinforcement, and data on methods for measuring the arising deformations, cracks, and deflections of prototypes of beams under the action of transverse forces.


Introduction
At the Fergana Polytechnic Institute, complex experimental and theoretical studies of the stress-strain state, the formation, and development of normal and oblique cracks, deformations of reinforcement and concrete, strength and forms of destruction of an element during transverse bending of concrete beams with fiberglass reinforcement have been carried out.

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To carry out experimental studies, prototype beams were made with a rectangular crosssection with dimensions of 16x30 cm and a length of 240 cm.For the beams, ordinary heavy concrete of class B20-B30 was used.For concrete, Portland cement, with an activity of 42.5 MPa of the Turon cement plant in the Besharyk district of the Fergana region, was used as a binder.The beams were made in wooden formwork.The inner surfaces of the formwork were covered with tin.The concrete compositions were designed so that their compressive strength corresponded to the strength of concrete class B20-B25 [9].
Together with sample beams from the same concrete batch, cubes with an edge of 10 cm and samples for pulling out fiberglass reinforcement tests were made.Sample cubes were tested at the age of 28 days and before testing the beams.The cubes were tested on a hydraulic press with a capacity of 50 tons following GOST 10180-2012 (Table 1).
As longitudinal working reinforcement in the tensioned zone, 2Ø12 or 2Ø16ShKA were installed, in the compressed zone 2Ø10ShKA, as clamps, fittings Ø 4 or Ø8ShKA were used with a pitch of 15 (10) cm.
The reinforcement of the clamps was connected to the longitudinal reinforcement by knitting with the help of a soft wire.Reinforcement cages were installed in the formwork in the design position and fixed.After installing all the necessary devices for measuring the deformations of reinforcement and concrete, the samples-beams were concreted by vibrating the concrete with deep vibrators.In the concreted state, the beams were kept for 5-6 days, then they were laid out, and the samples were kept under laboratory conditions [4].

Experimental research techniques
Beam samples of four series were made and tested on a power stand.The test bench was specially made so that, when loaded with two concentrated forces, it was possible to investigate both the zone of pure bending in the middle part of the beam and the shear zones at the support parts of the beams.Before testing, the beam was mounted on two hinged supports.One of the hinges was made hinge-movable, and the other was hingedfixed.The distance between the concentrated forces was 700 and 1300 mm, and the distance from the supports to the weights was 700 and 400 mm.In this case, the distance from the supports to the edges of the beams was equal to 150 mm.The loading was carried out by a hydraulic jack with a capacity of 40 tons.For this, a distribution traverse was used [2][3][4][5][6][7].
Before the start of the tests, readings were taken from all measuring instruments installed on the sample-beam.These readings of the instruments were taken as "conditional zero".Sample beams were loaded smoothly in several stages.The step load was approximately 10% of the design breaking load.In total, 8-12 loading stages were performed.After each loading with a step load, readings were taken from all devices, after which they were kept at this load for 15-20 minutes until the instrument readings stabilized.After that, the readings of the instrument readings were again made.Then this process was repeated at each loading stage.
During the entire test period from the beginning of loading to the end of the entire test, the deformations of compressed and stretched longitudinal reinforcement, concrete of compressed section, the moment of formation of normal and inclined cracks, the width of the crack opening, and deflections of characteristic points of beams along the length were measured.The fixation of the stage and the full load was recorded according to the hydraulic jack pressure gauge readings.All tested beam samples were brought to failure.
Deformations of concrete and reinforcement were measured using dial indicators with an accuracy of 0.01 mm on a 100 and 300 mm base.For this, portable measures were used, equipped with dial-type indicators.The deflections of the beams were measured with dial indicators with an accuracy of 0.01 mm and deflectors with an accuracy of 0.001 mm [4].Fig. 1.General views of the test stand of beams for bending from the front and side sides: 1 is test beam; 2 is distribution traverse; 3 is hydraulic jack; 4 is rack; 5 is support beam; 6 is base plate; 7 is base; 8 is hydraulic jack pressure gauge; 9 is rope safety.
After the end of the tests, normal and inclined cracks, crack tips, the height of the compressed zone, the distance between the cracks, and the thickness of the concrete cover were noted, and the beam specimens were photographed.During the test, the deformations of longitudinal reinforcement, concrete in the compressed zone, loads corresponding to the moment of occurrence of normal and inclined cracks, and deflections of beams at characteristic points along the length of the beams were measured.
To measure deformations in longitudinal fiberglass reinforcement in the manufacture of beam frames, clamps made of pipes with a 2-2.2 mm thickness were put on stretched and compressed rods (Fig. 2).Holes Ø5mm are made in the clamps in opposite directions.On the holes, Ø5mm nuts are electrically welded to the clamps.After putting on the clamps inside the formwork, screw it with a Ø5mm bolt until it stops against the working reinforcement.The same long Ø5mm bolt was loosely tightened on the outer side of the formwork.
These long bolts were left on the outer side of the formwork by 2-3 cm.On these bolts, on their parts, which remained in the formwork (in concrete), soft wires with an effervescent coating Ø1-2 mm were wound.On top of these wires, they were covered with a thin layer of plasticine.After concreting the beams, after 5-6 days, the samples were stripped, and the wires were removed from the long bolts of the clamps [4].
Before testing the beams, the long bolts from the clamps were unscrewed, and pins made of Ø10mm reinforcing wire with a Ø5mm cut at one end and a core at the flat end was installed in the holes.The rod of the indicator of the portable measure rested against the cores.As a result, it was possible to measure the deformations of the longitudinal working reinforcement on the basis of 100 or 300 mm.Such clamps were installed on the basis of 100 or 300 mm on working reinforcement in the zone of pure bending and in the shear zone of the beams (in the areas between the support and the load).
In the course of testing the beams, the lateral surfaces of the beams were carefully checked; when cracks were formed, they were immediately noted, their opening width was measured, and the corresponding load was recorded.
When the magnitude of the stage load of 85-90% of the breaking load was reached, all the main devices were removed, and the loading of the samples continued until failure.At the same time, the beam was observed, and intensive crack opening and sample destruction were recorded.In most cases, the destruction of prototypes occurred along inclined sections, and two beams collapsed along normal sections [1][2][3][4][5][6][7][8][9][10].
The experiments showed that the beam specimens failed at close to the calculated breaking loads.It should be noted that the experimental breaking loads, on average, exceeded the calculated ones by 0-20%.The performed experimental studies made it possible to obtain a complete picture of the stress-strain state of bending concrete beams with fiberglass reinforcement, the nature of the formation and development of normal and inclined cracks, and the form of destruction of prototypes, to determine the load at which the destruction occurred.
With the destruction of beams along the tensioned zone, the concrete crushing in the compressed zone was observed.With the destruction of beams along an inclined section, the onset of a state close to the limit state was also noted in the zone of pure bending.In this case, in most cases, there was a violation of the connection of the clamps with the longitudinal reinforcement, which was carried out by knitting the rods using a soft wire.This happened at loads (0.9-0.95) Q ult .
2.03.14.18, the indicators of strength, crack resistance, and stiffness of the experimental beams were calculated.
3. By processing and analyzing the obtained experimental data, scientifically substantiated proposals and recommendations were prepared on the stress-strain state, crack resistance, and stiffness of concrete beams with composite reinforcement, their calculation for force effects, and on the design of elements with fiberglass reinforcement.

Fig. 2 .
Fig. 2. Installation of clamping devices on longitudinal working armatures for measuring the average relative deformations from force effects.

Table 1 .
Experimental performance of conventional heavy concrete beams