Crack resistance of bent concrete structures with fiberglass reinforcement

. This article presents experimental data on the appearance and development of normal and oblique cracks in relation to the longitudinal axis of the element under the influence of transverse loads on bending concrete beams equipped with shishaplastic Composite fittings made of ordinary heavy concrete .


Introduction
The Fergana Polytechnic Institute conducted comprehensive experimental and theoretical studies of the stress-strain state, the formation and development of normal and inclined cracks in them, deformations of reinforcement and concrete, the strength and forms of destruction of the element during transverse bending of concrete beams with fiberglass reinforcement.

Materials, designs and production of prototypes
To carry out experimental studies, prototypes were made-beams with a rectangular crosssection with dimensions of 16x30cm and a length of 240 cm.Ordinary heavy concrete of class B20-B30 was used for the beams.For concrete, Portland cement with an activity of 42.5 MPa of the Turon cement plant of the Besharyk district of the Ferghana region was used as a binder.The beams were made in wooden forms.The inner surfaces of the formwork were covered with tin.
2Ø12 or 2Ø16 ShKA were installed as longitudinal working fittings in the stretched zone, 2Ø10 ShKA were installed in the compressed zone, Ø 4 or Ø8 ShKA fittings with a pitch of 15 (10) cm were used as clamps.
The armature of the clamps was connected to the longitudinal armature by knitting using soft wire.Reinforcing frames 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 concrete samples were concreted by vibrating concrete with deep vibrators.In the concreted state, the beams were kept for 5-6 days, then they were excavated and the samples were kept in a laboratory.

Experimental research methodology
Samples of beams of four series were made and tested on a power stand.The test bench was made specifically so that, when loaded with two concentrated forces, it would be possible to examine both the zone of pure bending in the middle part of the beam and the shear zones at the supporting parts of the beams.Before the test, the beam was mounted on two hinged supports.One of the hinges was made pivotally movable, and the other -pivotally fixed.The distance between the concentrated forces was 700 and 1300 mm, and the distance from the supports to the loads was 700 and 400 mm.The distance from the supports to the edges of the beams was equal to 150 mm.Loading is carried out by a hydraulic jack with a capacity of 40 tons.For this purpose, a distribution traverse was used.
Before the start of the tests, readings were taken from all measuring instruments installed on the sample beam.These readings were taken as "conditional zero".The beam samples were loaded smoothly in several stages.The stage load was approximately 10% of the calculated destructive load.A total of 8-12 loading stages were performed.After each loading with a stage load, readings were taken from all devices, after which they were maintained under this load for 15-20 minutes.until the instrument readings are stabilized.After that, the readings of the instruments were counted again.Then this process was repeated at each stage of loading.
During the entire test period from the beginning of loading until the end of the entire test, deformations of compressed and stretched longitudinal reinforcement, compressed concrete, the moment of formation of normal and inclined cracks, the width of crack opening, deflections of characteristic points of beams along the length were measured.The fixation of the stage and full load were recorded according to the readings of the hydraulic jack pressure gauge.All the tested beam samples were brought to destruction [3][4][5].
Deformations of concrete and reinforcement were measured using hour-type indicators with an accuracy of 0.01 mm based on 100 and 300 mm.For this purpose, portable headsets equipped with hour-type indicators were used.Deflections of beams were measured by hourtype indicators with an accuracy of 0.01 mm and deflection meters with an accuracy of 0.001 mm.

Research results
In experimental studies, the appearance of cracks was determined by the values of deformations in the longitudinal reinforcement of beams.In addition, during the loading of the prototypes, the lateral surfaces of the test beams were carefully examined.Such a combined observation and relatively small values of the stage load (~ 0,05Fult) allowed a fairly accurate determination of the load value when cracks appear.
On all tested samples of beams, normal cracks first formed in the zone of pure bending, then, as the load increased, inclined cracks appeared in the areas between the support and the load, in the so-called cut span.
In the samples of beams at the moment of crack formation, the value of the bending moments of the Msgs depended on the value of the cut-off span "а".
In the beams of the I series, а=70 сm ( The values of the calculated moments corresponding to the formation of the first normal cracks of theM crc ҳ were calculated according to the method described in [1].At the same time, the development of plastic deformations in stretched concrete was taken into account.
The ratio of the experimental values of the moments of crack formation to the calculated values for the series of samples I, II and IV was less than 1 and ranged from 0.61 to 0.84, their average value was 0.71.This means that in experiments, normal cracks are formed at loads almost 30% less than the calculated ones [3].
For samples of beams of the IV series, the ratio of the experimental moments of crack formation to the calculated ones was 1.04 and 1.07 (Table 1).
On the sample beams, after the start of loading at the 2nd and subsequent stages, first one or two normal cracks formed in the zone of pure bending, then as the load increased, new normal cracks formed.The opening width of the first cracks was 0.05-0.08mm, an increase in the load led to the development of normal cracks, the tops of the cracks rose higher and higher towards the compressed concrete zone, reducing it, at the same time their opening width increased [2].
Approximately at loads equal to half of the destructive one, the opening width of normal cracks reached the values of асrc = 0.2-0.35mm, a further increase in the load caused the intensive development of cracks and a significant increase in their opening width.
At loads amounting to 0.6-0.85 of the destructive ones, the crack opening width averaged 0.4-0.7 mm.A further increase in the load led to a strong opening of the trenches and the destruction of the experimental beams.
During the beams at all stages of loading, measurements were made of the width of the crack opening and their development was noted.Processing of experimental data made it possible to plot the width of crack openings depending on the relative load and compare them with theoretical ones (Fig. 1,2).Inclined cracks located at an angle to the longitudinal axis of the beams formed a little later than normal, at higher load values.
Experiments have established that the formation of inclined cracks, their development and the width of the opening depends on the reinforcement with clamps and their intensity, the diameter of the clamps, values of the ratio  ℎ 0 , depends on the number and diameter of the longitudinal reinforcement, the strength of concrete.
The theoretical transverse forces corresponding to the moment of formation of inclined cracks were determined by the formula: ҳ = 0,6  ,  ℎ 0 . (1) The test showed that for ratios  > 1,5ℎ 0 , if the value of the ratio is entered into the formula instead of the coefficient 0.6, then the convergence of experimental and theoretical forces of the formation of inclined cracks is significantly improved.In this case, the formula for determining the theoretical transverse forces corresponding to the moment of formation of inclined cracks has the form: At the same time, the maximum value of the force of formation of inclined cracks should not exceed 0,6 , ℎ 0 .
In the beams of the I series (a=70cm), the initial inclined cracks were formed at loads of   ҳ = 12,9 − 13,3 kN, while the A decrease in the size of the cut span from 70 cm to 40 cm led to an increase in the load of the formation of inclined cracks.At the same time, an increase in the   т value was found on average 2.18 times.
At the cross-section spans a = 40 cm, the formation of mainly one inclined crack was observed, and at a = 70 2-3 inclined cracks were formed.The increase in the load caused the intensive development of inclined cracks.Especially when the load value of 0.8 Qult and more was reached, inclined cracks developed strongly, their opening width reached 1.0 mm or more.Thus, the inclined cracks became critical and the destruction of the experimental beams.
At loads (0.5-0.7)Qult, the opening width of the inclined cracks was in the range of 0.2-0.5 mm.Calculations have shown that the experimental data obtained during the testing of beams are in good agreement with the theoretical (Fig. 3,4).longitudinal and transverse reinforcement and the compressed zone of concrete, deflections, the onset of the limiting state, the shape and nature of destruction in bent concrete structures with fiberglass reinforcement are the same with similar structures with steel reinforcement.
2. At low loads, i.e. when the tensile stresses in concrete in the stretched zone do not exceed the values of the calculated tensile resistance of concrete, elastic deformations and stresses develop in the beams, the elements work without cracks.After the tensile stresses in concrete have reached the values of the calculated tensile resistance, at first normal cracks form in the zone of pure bending of the beams, as the load increases, inclined cracks appear in the support zones of the beams.The opening width of the initial cracks was 0.05-0.1 mm.
3. In the future, a gradual increase in the load led to the development of normal and inclined cracks, a reduction in the height of the compressed cross-section zone, and the achievement of a crack opening width of 0.1-0.3mm.When the loading level of the experimental beams reached values of (0.6-0.85) Qult, there was an intensive development of cracks, the opening width of which reached   = 0,4 ÷ 0,7mm.A further increase in the load caused the onset of the critical-limit state and destruction.
4. Experimentally determined data on the formation, development and disclosure of normal and inclined cracks of beams with fiberglass reinforcement under load provide valuable material enriching the theoretical prerequisites for the calculation and design of concrete structures reinforced with composite reinforcement and can be used in practical calculations of similar structures.
5. The experimental values of the moments and transverse forces corresponding to the formation of cracks established experimentally satisfactorily converge with the theoretical ones calculated according to the provisions of "ШНҚ" 2.03.14.18 "Concrete structures with composite polymer reinforcement".
59), normal cracks were formed at bending moments of 9.0-9.3kNm;at the same time, the ratio of the moment of crack formation to the destructive moment was equal to   т   т = 0,205.For samples of the II series , these values were   т = 8,2 − 8,5 к,   т   т = 0,214.In the samples of the III series, the strength of , 060 (2023) E3S Web of Conferences IPFA 2023 https://doi.org/10.1051/e3sconf/20234520602323 452 concrete was lower than in the samples of other series, so the moments of crack formation were 8-11% lower, accordingly, the ratio of   т   т also decreased and its value averaged 0.163.The beam BKPA I-3A of series I and samples of beams of series IV were tested at values а=40см ( At the same time, for the BKPA I-3A beam, the Mcrcт values were 6.8 kN*m, and the magnitude of the ratio   т   т =0,24.In the beams of the IV series, the initial normal cracks were formed at moments of 11.7 and 12.3 kN * m.The average value of values was equal to 0.31.

Fig. 1 .Fig. 2 .
Fig.1.Opening width of normal cracks in beams of I, II series: 1-4 -sequence numbers of cracks; -experienced; -theoretical 90.In the samples of the II and III series (a=70cm), the   т values were 10.4-12.2kN, while the loads of the formation of inclined cracks were 0.35-0.38 of the destructive one.

Table 1 .
Formation of normal and inclined cracks in experimental beams.