Experimental Studies of Low-Reinforced Concrete Structures Containing Inter-Bay Construction Joints Strengthened with Prestressed Basalt Composite Reinforcements and External Transverse Reinforcements

Abstract

During the long-term operation of main run-of-river head powerhouses for hydroelectric power plants, technical changes that deteriorate the operational properties of their reinforced concrete structures can occur. Therefore, in order to substantiate the application of prestressed basalt composite reinforcements to strengthen reinforced concrete hydraulic structures in operation, a set of computational and experimental studies was carried out, taking into account their characteristic features. After 4 years of ageing, the serviceability and reliability of the beams with prestressed basalt composite reinforcements were demonstrated through stabilisation of the prestress losses and the values obtained for bearing capacity, deflection, and the width of the opening of the inter-bay construction joints and the deformations of the metal reinforcements and the basalt composite reinforcements. The bearing capacity of the investigated reinforced concrete beams reinforced with external transverse reinforcements was increased 1.4–2.5 times over that of the variants reinforced with longitudinal prestressed basal composite reinforcements. Furthermore, in this study, the impacts of static loads and seismic effects with a magnitude greater than 8 on the run-of-river hydroelectric power plant powerhouse were calculated based on dynamic design theory. Regarding applications to hydroelectric power plant structures and constructions, for which it is not always possible to determine the location of compressed or tensile zones during their operation nor under seismic action, our research results are suggestive of a reasonably positive effect.

1. Introduction

Run-of-river hydroelectric power plants (HPPs) constitute a significant proportion of all hydropower plants [1,2,3,4]; their powerhouses are located in river channels, which are an important element of the pressure front for hydroelectric facilities.

The head run-of-river powerhouse of the HPP includes reinforced concrete structures, such as piers, boom beams, head walls, and others (Figure 1). These structures are designed to absorb the complex of loads and influences that acts from the upstream side during operation while transferring forces to the slabs and internal structures of the HPP’s powerhouse at the same time.

This complex of loads comprises static loads; hydraulic loads, including hydrodynamic and wave loads; the effects of temperature, including seasonal effects; seismic effects; dynamic loads; water back pressure on the inter-bay construction joints; loads from ground pressure; non-design loads; and loads caused by changes in the rules of operation of the hydraulic units.

In the concrete structures of head powerhouses of run-of-river HPPs reinforced with bays, technical changes are caused during their long-term operation due to the following factors: settlement, displacement, and inclinations; opening of their inter-bay construction joints; a reduction in their strength and stiffness; the complex of loads that they experience, including temperature-related, seismic, and dynamic effects; variations in the opening and closing of their gates; and opening of the contact joints between the foot and the base due to upstream pools, among other things. These changes degrade the performance of reinforced concrete structures in HPP powerhouses.

During the design of hydroelectric facilities [5], seismic surveys are usually conducted to study the seismic activity in construction areas and determine the normal seismic hazard levels for subsequent calculations. However, at present, seismic activity is increasing in certain areas. For example, in Russia, seismic impacts in construction areas and their categories were previously determined using general seismic zoning (GSZ) maps according to the Construction Norms and Rules [6,7] applied at the time. New regulatory documents have emerged, such as the Code of Regulations [8], including GSZ-2015 maps, which must be taken into account in the design and calculations for construction. In these past contexts, the seismic impacts on hydraulic structures were calculated using linear spectral theory. However, according to modern regulations such as the Code of Regulations [8], calculations must be made for class I and II hydraulic structures on the basis of dynamic theory using accelerogram data.

The strength and stability of the powerhouses for run-of-river HPPs, as calculated when they were designed, met the requirements of the regulations in force at the time.

However, based on an analysis of the data accumulated for the last 50 years, the intensity of seismic impacts in Russia has increased by 1–2 points.

At the same time, during their long-term operation, hydraulic structures and building structures in hydroelectric power plants change significantly compared with how they were initially designed due to changes both in their physical and mechanical characteristics and external loads and impacts.

Considering these connections, engineers and researchers face the task of ensuring the safe operation of hydroelectric power plants and their reliability, even under a complex of loads and during high-intensity seismic events.

At present, strengthening reinforced concrete structures and constructions is being studied widely by various scientists, and composite materials have become widespread due to their ability to provide the necessary reserves in terms of bearing capacity without the need to change structures’ mass or dimensions.

The performance of reinforcing concrete with basalt composite reinforcements was evaluated and compared using carbon fibreglass and conventional steel reinforcements in previous research [9]. For comparison, reinforced concrete beams and columns were fabricated and tested under the action of bending and axial compressive loads, respectively. The thermal, chemical, and mechanical properties of individual reinforcements were also tested. In evaluating the main characteristics of the basalt fibre composite reinforcement (BFRP) and how they differed from CRFP and conventional steel reinforcements for concrete structures, the results of these studies showed an increase in the ultimate stresses sustained by both composite bars compared with those resisted by the steel bars, along with a decrease in ductility. Most importantly, both composite bars showed resistance to failure when exposed to acidic and alkaline environments.

Lightweight, non-corrosive basalt composite reinforcement bars, manufactured from continuous basalt filaments and epoxy and polyester resins using the pultrusion method, were studied in research [10]. Basalt composite reinforcements (BCRs) are bars with continuous spiral fins formed from basalt filaments for better bonding with concrete structures. The advantages of BCRs compared to steel reinforcements are as follows:

-

BCRs are more than twice as strong, in terms of their tension (to prevent the concrete from cracking), as steel reinforcements;

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BCRs are not susceptible to corrosion, as steel reinforcements inherently are;

-

BCRs are 89% lighter than steel reinforcements (and significantly faster to fabricate, install, and handle);

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BCRs have the same coefficient of thermal expansion as concrete;

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Basalt fibres do not absorb or transmit moisture and, therefore, do not create a pathway for water penetration and concrete failure;

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BCRs do not conduct electricity, which prevents electrolysis in marine environments;

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BCRs do not create magnetic fields when they are exposed to electromagnetic energy;

-

BCRs can operate in a temperature range of −70 °C to +100 °C, which is an important parameter determining their operation in fire-resistant rooms as well.

Thus, using basalt composite reinforcement increases the service life of reinforced concrete structures, including hydroelectric power plant structures, and ensures that they are long-lasting.

In [11], the performance of concrete beams reinforced with basalt fibre composite reinforcement (BFRP) was examined after exposing them to freeze–thaw cycles. The results of pull-out tests after 100 and 200 freeze–thaw cycles under a freezing temperature of −20 °C showed a negligible effect on the bond strength of the BFRP beams (within 10%), with important implications for hydraulic structures operating in different climatic conditions.

Studies on the bending of concrete beams reinforced with basalt fibre reinforcement (BFRP) in [12] indicated that the use of BFRP is an effective way to repair and strengthen structures whose performance characteristics have changed over their long-term operation. BFRP repair systems provide a cost-effective alternative to conventional repair systems and materials. It was noted that BFRP had high tensile strength and a lower modulus of elasticity than steel reinforcement. Indeed, the advantages of BFRP over steel reinforcement were also noted to be similar in [9].

BFRP is considered an ideal choice for use in buildings and structures that operate in water environments, including marine environments, given that BFRP increases the service life of reinforced concrete structures, resulting in their safe operation in the long term.

In article [13], the bending behaviour of concrete beams reinforced with ribbed basalt composite (BFRP) rods was studied experimentally. A total of eight concrete beams, with a length of 3100 mm and a cross-section of 200 × 300 mm, were fabricated and tested until their failure, six of which were reinforced with 8, 12, and 16 mm diameter ribbed BFRP reinforcements and two of which were reinforced with 10 M and 15 M steel reinforcements. All of the beams failed in the concrete in the compressed zone, and a high degree of deformability was recorded. From the nature of the cracking behaviour of the beams, it was evident that these investigations considered the action of the bending moment only, excluding the effects of the combined action of the bending moment and shear force.

The paper in question noted that the behaviour of the BFRP-RC beams was significantly affected by the axial bending stiffness of the basalt composite reinforcements ($E_f{A}_f$), and the studies therein are very useful in pointing out the need to stress BFRP in order to utilise its technical characteristics appropriately to improve structures’ strength and deformability.

In studying the performance and ultimate load capacity of concrete beams reinforced with basalt composite reinforcements, Ref. [14] presented test results for eight BFRP-reinforced beams of different diameters (10, 13, 16, and 25 mm). The beams were divided into three categories, with low, medium, and high reinforcement ratios. All the beams failed in the concrete in the compressed zone, but a high reinforcement ratio reduced their deflection to some extent. It was observed by the authors that the ultimate loads sustained by the beams were directly related to the reinforcement ratio. This paper also compared its prediction model with other experimental models and confirmed its validity. It should be noted that the BFRP was not prestressed in this concrete beam model.

In order to modernise the Haparanda railway line in northern Sweden such that it can sustain axle loads of up to 300 kN [15], the transverse shear strength of the slabs in existing concrete suspension bridges must be increased. Thus, in one study, the possibility of increasing the bearing capacity of these slabs using horizontal prestressing was discussed. The strengthening was designed according to the Eurocode design standards, and tests were carried out before and after its implementation. Deformations in the main transverse reinforcement caused by a train with an axle load of 215 kN were completely neutralised by using eight prestressing bars with a load of 430 kN/bar, demonstrating that the actual strengthening effect was greater than that predicted by the equations in the design.

Experimental results on the bending of prestressed beams reinforced with BFRPs, including estimation of the prestress losses, were presented in Ref. [16]. Five beams were fabricated, three of which were reinforced with 6 mm diameter BFRPs prestressed to 20%, 30%, and 40% of the ultimate stress level. The stress losses were monitored using strain gauges on the reinforcement bars. In addition, two control beams were reinforced with tensile BFRP and steel bars with the same cross-sectional area, respectively. The dimensions of all the specimens were 125 × 200 × 1900 mm.

Considering the results obtained from experimentally testing the bending of the beams to failure, the following conclusions were reached:

-

Prestressing the BFRP-reinforced beams by more than 30% of their ultimate tensile strength improved their performance to a level exceeding that of the steel-bar-reinforced specimens;

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The ultimate deflection in the prestressed BFRP beams was lower compared with that in the BFRP beams that were not prestressed;

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The formation of cracks on the surface of the beams was measured for the prestressed beams against the stress level.

This study did not contain data on strengthening the above beams with an external reinforcement system, however.

The aim of [17] was to study the bending of reinforced concrete beams strengthened with basalt composite reinforcement (BFRP) using near-surface mounted (NSM) technology to evaluate the strengthening efficiency. Near-surface mounted prestressing technology (PNSM) was considered given that, to some extent, it combines the benefits of the two well-known “external prestressing” and “NSM” technologies.

The variables considered in this study were as follows:

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The strengthening effect (not strengthened or strengthened);

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The type of composite reinforcement (BFRP or GFRP);

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The strengthening method (NSM or PNSM);

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The initial prestress level (30% and 50% of the ultimate tensile strength of the BFRP bars);

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The position of the NSM composite reinforcements (at the bottom or on the side).

The test results demonstrated the efficiency of applying BFRP bars to strengthen reinforced concrete beams using NSM and PNSM techniques. Additionally, the BFRP bars offered an additional advantage, compared with the GFRP bars, of a higher creep rupture strength when using the PNSM technique. Moreover, the PNSM technique was found to provide a greater enhancement in the pre-cracking stage, compared with the NSM technique, in strengthening the reinforced concrete beams, while their ductility was significantly reduced as a trade-off.

The article [18] considered the application of basalt composite reinforcement (BFRP) to reinforcing concrete beams and reached the following conclusion based on its results: as BFRP has a significantly lower bending stiffness than traditional steel reinforcements, it must be prestressed for the deflections of the beams to be controlled within the permissible values.

The study in question focused on evaluating the failure and shear behaviour of concrete beams reinforced with prestressed BFRP without clamp reinforcements, outlining both experimental and numerical results (obtained using ANSYS). All the beams tested exhibited identical failure mechanics: bending and shear. As is evident in the article, the experimental results and the numerical modelling results obtained using ANSYS were in reasonable agreement. Thus, the results obtained are highly applicable to the development of strengthening systems for reinforced concrete structures in operation.

In [19], experimental results obtained by studying concrete beams with prestressed reinforcements were analysed, and computational nonlinear models were created in the ANSYS software, with fairly good agreement between the experiments and calculations obtained regarding the nature of the fractures, the numerical values for the failure loads, and the stress distribution.

Finally, in [20], test results on the use of concrete beams reinforced with prestressed basalt composite reinforcements, both with and without external clamps, were analysed. Using steel reinforcements in concrete beams within water and saline environments and in the presence of chemical substances was deemed problematic due to their intensive corrosion. On this basis, it was necessary to use BFRP reinforcements instead, which must be prestressed to avoid beam cracking. Experiments were carried out on two beams, which failed at shear. Another two beams were shear-strengthened with external steel clamps and failed at bending due to the reinforcements yielding. As a result, it was noted that the distribution of the shear force and bending moment along the length of the beam should be considered when designing beams in order for them to meet the strength requirements.

Thus, studies [9,10,11,12,13,14,15,16,17,18,19,20] have shown that the materials referred to in this article are highly relevant and necessary for strengthening long-life HPP structures, especially in areas with high seismic activity.

The use of composite reinforcement bars over steel bars has a number of advantages:

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Composite reinforcements have a 2.5–3.0 times higher tensile strength than steel reinforcements of the same diameter and a strength comparable to high-strength carbon steel wire;

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Composite reinforcements are 4.0–4.5 times lighter than steel reinforcements of the same diameter;

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Composite reinforcements belong to the most chemically resistant group of materials (e.g., basalt composite reinforcements are highly resistant to the alkaline environment of concrete, do not corrode, and are also resistant to other aggressive environments);

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Currents cannot be induced in composite reinforcements (dielectric), as is inherent in high-strength steel reinforcements when HPP turbines, generators, and transformers are in operation.

Considering the optimal ratios between their strength characteristics and cost, the use of basalt composite reinforcements is deemed the most appropriate.

Thus, based on previous work by a number of scientists, the authors of this paper made calculations and carried out experimental tests with the aim of increasing the load-bearing capacity and seismic resistance of reinforced concrete structures for HPPs.

2. Materials and Methods

Through performing experimental–numerical studies, the strength, deflection, width of the opening of the inter-bay construction joints, and deformation of the metal and basalt composite reinforcements were determined, depending on the method used to strengthen the reinforced concrete structure.

The stress–strain state was calculated under the action of static loads and seismic effects with a magnitude greater than 8 in order to determine the need to strengthen HPP powerhouse structures using the methods presented in the experimental part of this research.

The phasing of the work was as follows:

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Developing a mathematical model for a run-of-river HPP powerhouse;

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Performing calculations using ANSYS mathematical software under the action of static loads (weight, water pressure from the upstream side, water back pressure at the base, etc.) and seismic impacts;

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Experimentally studying low-reinforced concrete models (analogues of hydraulic structures—for example, models of piers and the head wall of the powerhouse) containing operational cracks (including those at inter-bay construction joints) reinforced using prestressed metal and basalt composite reinforcements;

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Experimentally studying static loading of the reinforced concrete models strengthened with transverse reinforcements in the zone of the inter-bay construction joints (where the combined action of the bending moment and transverse forces applies);

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Experimentally studying static loading of the reinforced concrete models strengthened with transverse composite straps in the zone of the combined action of the bending moment and transverse forces.

The stress–strain state was studied computationally in the universal industrial software ANSYS based on the finite element method (FEM). The FEM is one of the most widely used methods in mathematical modelling to determine the stress–strain state of various industrial and civil objects, as well as complex energy facilities. The development of information technologies and industrial software has made it possible to use CUDA parallel computing hardware and software technology to solve interdisciplinary problems and increase the accuracy and speed of conducting research. In addition, this technology allows complex interdisciplinary problems to be solved while taking into account thermal, dynamic, and seismic effects.

Class B35 concrete, metal and basalt composite reinforcements, and carbon wrap tape (

Table 1. Characteristics of the materials used for the models.

Characteristic	Concrete, MPa	Reinforcement, MPa		Carbon Wrap Tape, MPa
		Metallic	Basalt Composite
Compressive strength	$R_{bn}=25.5$	-	-	-
Tensile strength	$R_{btn}=1.95$	$R_{sn}=400$	$R_{f\mu }=800$	>4900 (fibre)
Modulus of deformation	$E_b=34500$ (compression)	$E_s=200000$ (tension)	$E_{f\mu }=50000$ (tension)	>230000 (fibre tension)

) were used to fabricate and then experimentally study the reinforced concrete models.

The physical reinforced concrete models were experimentally studied in the laboratory of the NIIES branch of JSC Institute Hydroproject using a specialised 100-tonne metal power stand and hydraulic equipment:

• A DG-50 hydraulic jack (for static loads):
  • The force generated is 50 tonnes;
  • Stem extension (stroke length): 60 mm;
  • Working pressure: 45.5 MPa.
• An ENERPAC MP700 hydraulic pumping station:
  • Nominal pressure: 70.0 MPa;
  • Stem extension (stroke length): 26.5 mm.
• A “CONER” hydraulic jack no. 11920 (tensioning of the reinforcement bars; see Figure 2):
  • Rod stroke length of the hydraulic cylinder: 120 mm;
  • Maximum tensioning pressure: 400 bar;
  • Piston area of the hydraulic cylinder: 5100 mm².

The following measuring equipment was used:

• TML FLA-10-11 strain gauges with a 10 mm base and 120 Ohm resistance (for the reinforcement), Number 79148-20 in the Russian State Register of Measuring Instruments, and TML PL-60-11 strain gauges with a 60 mm base and a resistance of 120 Ohm (for the concrete), Number 79148-20 in the State Register of Measuring Instruments.
• The “NTP Gorizont” strain gauge station, Number 79484-20 in the Russian State Register of Measuring Instruments (

  Table 2. Characteristics of the “NTP Gorizont” strain gauge station.

  Characteristic	“NTP Gorizont” Strain Gauge Station
  The number of measuring channels	32
  Strain gauge switching schemes	Bridge, half-bridge, quarter-bridge
  The rated resistance of the strain gauge transducers	50–1000 Ohm
  Conversion factor measurement ranges	±5; ±10 MV/V
  Limits of the permissible relative error in the oscillation frequency measurements	±5 × 10−5
  External interfaces	RS—485, USB; Bluetooth

  ).
• “ZICHT” clock-type indicators, Number 76658-19 in the Russian State Register of Measuring Instruments (for measuring the width of the opening of the inter-bay construction joints and cracks and the deflections), with a division value = 0.005 mm and the largest difference in the indicator’s forward stroke errors = 0.005 mm within the working section of the scale and =0.003 mm within the normalised section of the scale.

To justify the use of prestressed basalt composite reinforcements in hydraulic structures, the prestress losses in the reinforced concrete elements were first studied experimentally, and their strength was determined subsequently [16,21,22].

Experiments were carried out on concrete prisms with a cross-section of 12 × 12 cm and a length of 48 cm reinforced with rod reinforcements, comprising 12 mm diameter class A400 steel rods and basalt composite reinforcements the same diameter.

In this research, reinforcing bars of class A400 steel were chosen, as they are used in the vast majority of hydraulic structures that need to be strengthened after their long-term operation.

In terms of the absolute values, the steel reinforcement bars were prestressed to an average value of 354 MPa (not exceeding the 400 MPa yield stress of A400 steel reinforcements), while the basalt composite bars were prestressed to 420 MPa (to a value close to the yield stress for the steel reinforcements).

Subsequently, the load–strain relationship was obtained for the reinforcing bars.

Further experimental investigations aimed to determine the prestress losses in the reinforcement bars, and for this purpose, the strain losses—that is, the relative elongation of the bars—were preliminarily determined.

In order to compare the data on the prestress losses (the relative elongation) in the reinforcement bars, “strain–time” relationships for different types of reinforcements were plotted—the A400 class steel reinforcements, high-strength carbon steel wire, and the basalt composite reinforcements—with the data on the deformation of high-strength wire included for comparison purposes.

The prestress losses were calculated by determining the reduction in deformation in the reinforcement bars investigated in terms of their relative elongation, taking the elastic moduli of the steel reinforcements (200,000 MPa) and basalt composite reinforcements (50,000 MPa) into account, as presented in Figure 3.

The experimental results showed that the prestressing loss in the steel reinforcement bars was in the range of 2.9–4.0% after 90 h, while it was 3.4% in the basalt composite bars.

Thus, taking into account the advantages of basalt composite reinforcement over steel reinforcement mentioned previously, as well as the experimental data on the prestressing losses in the reinforcements obtained, using prestressed basalt composite reinforcements to repair and strengthen hydraulic structures that have been in long-term operation was deemed a relevant practical task.

To substantiate the relevance of applying prestressed basalt composite reinforcements to strengthen reinforced concrete hydraulic structures in operation, a body of experimental research was carried out, taking into account the following features characteristic of reinforced concrete hydraulic structures:

• The use of a low class of concrete (up to B15) and steel reinforcements (up to A500), with a longitudinal coefficient of working reinforcements μ < 0.01;
• The presence of inter-bay construction joints;
• Violation of the bonds between working steel reinforcements and the concrete at inter-bay construction joints as a result of concrete shrinkage deformations during curing in the construction period, as well as due to the impacts of temperature, alternating loads, and seismic activity over the long-term operation of hydraulic structures;
• The specific nature of cracking in low-reinforced hydraulic structures.

This work is a continuation and development of research [23].

An approved methodology for the physical modelling of reinforced concrete hydraulic structures was used in this research.

Beam-type models (length: 2000 mm; height: 300 mm; width; 150 mm) with inter-block construction joints (the models were concreted in two blocks in order to create inter-block joints) and steel reinforcements applied in the 2Ø12 longitudinal direction in the tensile zone of the beams were studied experimentally under the action of static loads.

In order to confirm the long-term operational capability of concrete beams reinforced with prestressed basalt composite reinforcements (in the tensile and compressed zones of the beams) over time and substantiate the possibility and efficiency of reinforcing them in the transverse direction (under the action of transverse forces and bending moments), experimental studies were conducted in two phases: in 2020 and in 2024 (

Table 3. Stages of testing the reinforced concrete beams.

	2020		2024
	B1	B2	B1	B2
Stage 1	The presence of a Ø32 channel in the tensile zone of the beam for installing and prestressing the 1Ø12 basalt composite reinforcement	The presence of a Ø32 channel in the compressed zone of the beam for installing and prestressing the 1Ø12 basalt composite reinforcement
Stage 2	Prestressed 1Ø12 basalt composite reinforcements in the tensile zone of the beam	Prestressed 1Ø12 basalt composite reinforcements in the compressed zone of the beam
Stage 3			Prestressed 1Ø12 basalt composite reinforcements in the tensile zone of the beam and 2Ø12 transverse reinforcements installed on both sides of the inter-block joints along the shear span of the beam	Prestressed 1Ø12 basalt composite reinforcements in the compressed zone of the beam and 2Ø12 transverse reinforcements installed on both sides of the inter-block joints along the beam’s shear span
Stage 4			Prestressed 1Ø12 basalt composite reinforcements in the tensile zone of the beam and carbon wrap tape installed transverse to the inter-block joints along the shear span of the beam	Prestressed 1Ø12 basalt composite reinforcement in the compressed zone of the beam and carbon wrap tape installed transverse to the inter-block joints along the shear span of the beam

).

3. Results

In the present work, the impact of static loads and seismic effects with a magnitude greater than 8 on a run-of-river building in an HPP was calculated on the basis of dynamic theory.

As a result of these calculations, the stress–strain state at different time points (in seconds) was determined from the beginning of a seismic impact.

Figure 4a shows the stress state in the structures of the HPP building under static loads and seismic action, as calculated using dynamic theory, at time points of 2.62 s and 4.94 s from the beginning of the seismic action.

During the process of seismic action, the acting forces and, accordingly, the stress state are redistributed. Thus, at the time point 2.62 s from the beginning of the seismic impact, the highest tensile stresses (of up to 4.7 MPa) occur in the upper head wall of the HPP building (Figure 4a).

At the time point 4.94 s from the beginning of the seismic impact, the highest tensile stresses (of up to 3.5 MPa) occur in the piers in the HPP building (Figure 4b).

As observed in Figure 4, when the head structures in the run-of-river building in the HPP experience a seismic impact, significant tensile stresses occur in them, exceeding the tensile strength of the concrete and consequently causing the inter-bay construction joints to open and cracks to form, as well as opening of the contact between the foot and the base of the building, which reduces its stability margin.

On the basis of the results presented from the calculations performed within the framework of dynamic theory for the building in the HPP under the action of static loads and seismic impacts with a magnitude greater than 8, measures to strengthen the reinforced concrete structures in this building and increase its stability are definitely required. In this regard, using prestressed basalt composite reinforcements and transverse external reinforcements to ensure that its strength and stability are improved, as required, is advisable.

In the studies presented (from 2024), experiments on beams B1 (with prestressed basalt composite reinforcements placed in the tensile zone) and B2 (with prestressed basalt composite reinforcements placed in the compressed zone) were carried out in order to obtain new data on the strength, the deflection, the widths of the opening of the inter-bay construction joints, and the stresses in the metal reinforcements and basalt composite reinforcements, with 4 years of prestressing applied to the beams with basalt composite reinforcements, considering the prestressing loss curve presented above (Figure 3).

The types of tests conducted and the characteristics of experimental beams B1 and B2 are presented in Scheme 1.

Experimental beams B1 and B2 were fabricated in 2020 in two steps (block concreting) using steel reinforcements to construct the inter-bay construction joints. Channels were created for the installation of the basalt composite reinforcements, followed by prestressing of the basalt composite reinforcements “on the concrete”, caulking of the channels, and the subsequent release of basalt composite reinforcement “onto the beam concrete” after the channel concrete had gained enough strength.

The basalt composite reinforcement in beams B1 and B2 was prestressed to the value of 0.45 × 800 = 360 MPa (taking 0.45 as the coefficient—regarding clause 1.4.2 of code of practice [24]—and ${\mathit{R}}_{\mathit{f},\mathit{n}}$ = 800 MPa as the tensile strength of the basalt composite reinforcements (Scheme 1)—regarding clause 5.2.4 of the code of practice [25]), with subsequent concreting of the channels in the beams.

It should be noted that the Ø12 transverse prestressed reinforcement was installed in beams B1 (stage 3) and B2 (stage 3) in 2024 on both sides of the inter-bay construction joints; the carbon wrap tape 900/300 carbon fibre fabric was bonded in a single layer in 2024 in the span between the support and the point of application of the P/2 force after the removal of the transverse prestressed reinforcement (beams B1 (stage 4) and B2 (stage 4)).

The design of the experimental beams is shown in Figure 5.

The following should be noted:

(1)

During testing, the experimental beams were supported on movable (roller) and fixed (knife) supports located at a distance of 150 mm from the ends;

(2)

The load was applied vertically in stages by means of a hydraulic jack and transmitted through a horizontal crosshead at two points 320 mm from the centre of the beam, with a force spacing (the “pure bending” zone) of 640 mm and a force–support spacing (“shear span”) of 540 mm (Figure 5).

The experimental beams were equipped with strain gauges with a base of 20–50 mm and clock-type indicators to take the following measurements: deflections, the width of openings of the inter-bay construction joints, and the deformations of the steel reinforcements and basalt composite reinforcements.

The following main results were obtained during the experimental research.

In experimental beams B1 (stage 1) and B2 (stage 1) in 2020, the following character of their cracking was recorded: after the vertical inter-bay construction joints opened, inclined cracks formed (wedging out) in the direction of the applied forces P/2 (at a distance Xj = 0.24·hj–0.34·hj from the upper compressed face of the beams) with P = 44.2 kN and 39.0 kN, respectively.

Under the action of a load that was almost destructive, cracks formed along the longitudinal working reinforcement in the direction “from the inter-bay joint to the supports” (with the bond between the reinforcement and concrete violated in the tensile zone of the beams). A system of vertical normal cracks formed at the section between the forces in the “pure bending” zone (Figure 6).

In analysing the results of testing the beams, the following was found:

(1)

The force that caused beams B1 (stage 1) and B2 (stage 1)—which were not strengthened with prestressed basalt composite reinforcements but solely 2Ø12 longitudinal working reinforcements made of class A400 steel in the tensile zone and which contained two inter-bay construction joints—to fail was 46.8 kN and 41.6 kN, respectively. At the same time, in beam B1 (stage 1), the Ø32 channel for accommodating the basalt composite reinforcement was located in the tensile zone, while in beam B2 (stage 1), it was located in the compressed zone, explaining the difference in the magnitude of the failure force. Both of these beams experienced brittle failure in the formation of an inclined crack (wedged) in their inter-bay construction joints under the combined action of the bending moment and transverse force.

(2)

Tests of beams B1 (stage 2) and B2 (stage 2) with prestressed basalt composite reinforcements installed in the tensile and compression zones, respectively, showed that their load-carrying capacity increased to 67.6 kN and 55.12 kN, respectively.

(3)

Further experiments on beams B1 (stage 3) and B2 (stage 3) with prestressed transverse reinforcements of up to 50 MPa on both sides of their inter-bay construction joints showed an increase in their load-bearing capacity of up to 83.2 kN, with no signs of brittle failure.

(4)

Furthermore, after the removal of the transverse reinforcement, carbon fibre fabric was bonded in one layer along the shear span of beam B1 (stage 4), and its bearing capacity increased to 142.5 kN, with no signs of brittle failure.

(5)

After the removal of the transverse reinforcement, carbon fibre fabric was bonded in one layer along the shear span of beam B2 (stage 4), and its load-bearing capacity increased to 124.8 kN, while the working metal reinforcement in the tensile zone of the beam yielded.

Data on the magnitude of the breaking force and the failure patterns of the beams are presented in

Table 4. Data on the magnitude of the breaking force and the failure patterns of the beams.

No.	Experimental Values of Force P (kN)	Beam B1				Beam B2
		Stage 1	Stage 2	Stage 3	Stage 4	Stage 1	Stage 2	Stage 3	Stage 4
1	Force P corresponding to the beam’s collapse	46.8	67.6	83.2	142.5	41.6	55.12	83.2	124.8
2	Failure pattern of the beam	Brittle	Not brittle	Not brittle	Not brittle	Brittle	Not brittle	Not brittle	Not brittle (the steel reinforcement reached yield strength)

.

Through analysing the results of testing the beams, in terms of their deflection, the width of the openings of the inter-bay construction joints, and the deformation of the steel and basalt composite reinforcements, the following was observed:

(1) The deflection of beam B1 (stages 1, 2, 3, and 4) decreased (at a maximum by a factor of 1.54 with P = 67.6 kN) when it was strengthened using external transverse reinforcement with carbon fibre fabric compared with when using the option of longitudinal prestressed basalt composite reinforcement in the tensile zone of the beam (Figure 7).

The deflection of beam B2 (stages 1, 2, 3, and 4) decreased (at a maximum of 1.54 times over with P = 55.12 kN) when it was strengthened using external transverse reinforcement with carbon fibre fabric compared with the option of using longitudinal prestressed basalt composite reinforcement in the compressed zone of the beam (Figure 8).

Comparing the deflection of beams B1 and B2 indicates that the strengthening options applied have a positive effect on reducing this value while increasing the load-bearing capacity of the beams (Figure 7 and Figure 8).

(2) The width of the opening of the inter-bay construction joints in beam B1 (stage 2) (with prestressed basalt composite reinforcement located in the tensile zone of the beam) was fixed at 1.21 times smaller than that in beam B2 (stage 2) (with prestressed basalt composite reinforcement located in the compressed zone of the beam) with P = 55.12 kN (Figure 9 and Figure 10).

Whether beams B1 and B2 were strengthened using the external transverse prestressed reinforcement or external transverse reinforcement with carbon fibre fabric, the width of the opening of the inter-bay construction joints was noted to be within 10%, and comparison with regulatory documents [26] shows that the permissible values for a_cr,j were not exceeded.

(3) Strengthening beams B1 and B2 (stages 3) with the external prestressed transverse reinforcements resulted in increased deformation of the steel reinforcements (Figure 11 and Figure 12), while strengthening beams B1 and B2 (stages 4) with the external transverse reinforcement with the carbon fibre fabric significantly reduced the deformation of the steel reinforcements more than two times over.

The maximum tensile strain in the steel reinforcement was recorded in the range of (160–220) × 10⁻⁵, corresponding to stresses of 380 MPa and nearing the yield strength.

(4) The deformation of the prestressed basalt composite reinforcement located in the tensile zone of beam B1 (Figure 13) was reduced almost twice over (Figure 12) for the variant strengthened using the external transverse reinforcement with the carbon fibre fabric.

In the inter-bay construction joints, the deformation in the steel and prestressed basalt composite reinforcements was quite similar (Figure 11, Figure 12 and Figure 13), indicating their joint operation in these structures.

In terms of the deformation of the prestressed basalt composite reinforcement located in the compressed zone in beam B2 (Figure 14), compression occurred when using all of the strengthening options within 20–50 MPa.

In addition to the above,

Table 5. Values of beam deflections, width of opening of interlock joints, deformation of reinforcement under the action of destructive force, in comparison with the maximum permissible parameters of normative documents.

No.	Value	Beam B1				Beam B2
		Stage 1	Stage 2	Stage 3	Stage 4	Stage 1	Stage 2	Stage 3	Stage 4
1	f, mm	${\displaystyle \frac{2.7}{11.33}}$	${\displaystyle \frac{3.25}{11.3}}$	${\displaystyle \frac{2.8}{11.33}}$	${\displaystyle \frac{5.8}{11.33}}$	${\displaystyle \frac{3.4}{11.33}}$	${\displaystyle \frac{2.95}{11.33}}$	${\displaystyle \frac{3.55}{11.33}}$	${\displaystyle \frac{5.3}{11.33}}$
2	$a_{cr,j}$, m	${\displaystyle \frac{0.14}{0.5}}$	${\displaystyle \frac{0.37}{0.5}}$	${\displaystyle \frac{0.35}{0.5}}$	${\displaystyle \frac{0.59}{0.5}}$	${\displaystyle \frac{0.155}{0.5}}$	${\displaystyle \frac{0.26}{0.5}}$	${\displaystyle \frac{0.35}{0.5}}$	${\displaystyle \frac{0.485}{0.5}}$
3	${\epsilon }_{s,j}\times {10}^{-5}$	${\displaystyle \frac{42}{200}}$	${\displaystyle \frac{160}{200}}$	${\displaystyle \frac{160}{200}}$	${\displaystyle \frac{164}{200}}$	${\displaystyle \frac{86}{200}}$	${\displaystyle \frac{130}{200}}$	${\displaystyle \frac{222}{200}}$	${\displaystyle \frac{132}{200}}$
4	${\epsilon }_{f,j}\times {10}^{-5}$	-	${\displaystyle \frac{280}{1600}}$	${\displaystyle \frac{205}{1600}}$	${\displaystyle \frac{196}{1600}}$	-	${\displaystyle \frac{-60 *}{1600}}$	${\displaystyle \frac{-40 *}{1600}}$	${\displaystyle \frac{-108 *}{1600}}$

Note the following: Numerator: the current values of the parameters; denominator: maximum permissible parameters determined by the regulatory documents; *: compression strains of the basalt composite reinforcements in beam B2 (in stages 2, 3, and 4).

presents the values of the deflection f of beams B1 and B2; the width of the opening of the inter-bay joints $a_{cr,j}$; and the deformation of the steel and basalt composite reinforcements under the action of force P corresponding to the failure of beams (Table 4) against the maximum parameters allowed by the regulatory documents [26,27] for comparison.

In analysing the data from Table 5, we find that 93.3% of the parameters ${(f,a}_{cr,j}, {\epsilon }_{s,j}, {\epsilon }_{f,j})$ were below the maximum permissible values [25,27].

4. Conclusions

(1)

Studying low-reinforced concrete beams with cracks (2020) after strengthening them with prestressed basalt composite reinforcements and external transverse reinforcements (2024) yielded positive results, given that ageing the prestressed basalt-composite-reinforced beams for 4 years proved their serviceability and confirmed the correctness of experiments [22] in determining the prestress losses and their stabilisation.

(2)

The bearing capacity of beams B1 and B2 was increased 1.4–2.5 times as they were strengthened from the variant of prestressed basalt-composite longitudinal reinforcement to the variant of external transverse reinforcement.

(3)

The deflection of the beams, the width of the opening of the inter-bay construction joints, and the deformation of the steel reinforcements and prestressed basalt composite reinforcements decreased as the various reinforcement options mentioned above were applied. All of these values were within the limits permitted by the regulatory documents [24,25,26,27].

(4)

It was a very positive finding that, for the variant strengthened with a prestressed basalt composite longitudinal reinforcement located in the compressed zone of the reinforced concrete structure, the load-bearing capacity, deflection, width of the opening of the inter-bay construction joints, and deformations and stresses in the steel longitudinal and basalt composite reinforcements were within the limits permitted by the regulations.

Consequently, for structures in hydroelectric power plants for which it is not always possible to determine the location of the compressed or stretched zone, as well as in cases of seismic impacts, our results can facilitate a reasonably positive solution.