ABSTRACT

The article presents the results of experimental research on the strength and deformation of concrete with a strength class of B30 under short-term compressive loads, which relate to floor slabs of various lengths. An analysis of the results of full-scale tests of restressed multi-hollow slabs, reinforced with high-strength reinforcing wire of class Bp1400, is conducted in terms of load-bearing capacity, crack resistance of normal sections, and deformability under short-term static loading. For a comprehensive analysis of the behavior of restressed formwork-free floor slabs under short-term static loading, it is necessary to consider concrete stress-strain diagram parameters: average compressive strength, initial elastic modulus, and ultimate deformations. The question of assessing the reliability of the slabs is a complex probabilistic problem, especially when considering the seismic activity of the construction area and the variability of the properties of the concrete used and the characteristics of the reinforced concrete structure as a whole.

АННОТАЦИЯ

Представлены результаты экспериментальных исследований прочности и деформаций бетона класса по прочности на сжатие В30 при действии кратковременной сжимающей нагрузки, относящихся к плитам перекрытий различных длин. Выполнен анализ результатов натурных испытаний предварительно напряженных многопустотных плит, армированных высокопрочной проволочной арматурной проволокой класса Вр1400, по несущей способности, трещиностойкости нормальных сечений и деформативности при нагружении кратковременной статической нагрузкой. Для глубокого анализа работы предварительно напряженных плит перекрытий безопалубочного формования при нагружении кратковременной статической нагрузкой следует учитывать параметры диаграммы "напряжение-деформация" бетона: средняя прочность бетона на сжатие, начальный модуль упругости и предельные деформации. Вопрос оценки надежности плит является сложной вероятностной задачей, особенно если учесть сейсмоактивность районов строительства и изменчивость свойств используемого бетона и характеристик железобетонных конструкции в целом.

Keywords: concrete, strength, crack resistance, stiffness, plate, reinforcement, high-strength reinforcing wire.

Ключевые слова: бетон, прочность, трещиностойкость, жесткость, плита, арматура, высокопрочная арматурная проволока, перекрытие.

The objective of this work is an experimental study of the stress-strain state and an assessment of the strength, rigidity and crack resistance of typical reinforced concrete floor slabs of bench-type formwork-free formation under short-term static load.

To achieve this objective, the following tasks are solved sequentially:

1. Experimental study of the strength and deformability of concrete used for the manufacture of floor slabs;

2. Control of the technology of manufacturing floor slabs under production conditions according to the technical regulations for the manufacture of concrete samples for experimental studies;

3. Conducting standard tests of floor slabs under short-term static load in accordance with the requirements of current regulatory documents;

4. Processing and analysis of experimental research data to prepare a conclusion on the results obtained and assess the strength, rigidity and crack resistance of floor slabs of bench-type formwork-free formation.

Introduction. The system of any multi-story reinforced concrete frame building is formed by vertical load-bearing structures (columns, panels) connected into a unified spatial system with horizontal load-bearing structures (floor slabs). The load-bearing elements in these buildings include the reinforced concrete frame consisting of columns, beams, and foundations, as well as vertical stiffness elements in the form of reinforced concrete diaphragms and horizontal elements.

The floor slabs of multi-story frame buildings can be precast, cast-in-place, or a combination of both. Precast beam floor slabs typically consist of hollow panels supported by the frame's beams.

Approximately 60% of the total consumption of reinforced concrete structures is attributed to floor slabs. Therefore, it is essential to pay the most attention to their rational structural design. To reduce the weight of the slabs, they are designed with circular or oval voids and are often pre-stressed.

Precast restressed concrete multi-hollow slabs with high-strength wire reinforcement (referred to as slabs) are manufactured on forming stands using a continuous formwork-free technology. According to this technology, longitudinal reinforcement is laid on the stand, secured at both ends, and tensioned using a hydraulic jack. Concrete placement is done using a forming unit, which involves the sequential placement of a rigid concrete mix that includes provisions for fixing the tensioned reinforcement wires in their design positions. After forming, the slabs are subjected to heat treatment and covered with polyethylene film to protect the surface. The concrete ribbon is then cut into slabs of the required length using diamond saws. This technology is used to produce multi-hollow slabs with a thickness of 22 cm, a width of 120 cm, and lengths up to 12 m, designed to support various loads.

Properties of materials and testing methods. In the experimental research, heavy concrete specimens were prepared using Portland cement of grade PC400D20 from  the "Kyzylkumcement" joint-stock company with an activity of 38.2 MPa.

As coarse aggregate, granite crushed stone from the "Nerudnik" quarry with a particle size of 5-20 mm, conforming to GOST 8267 "Crushed Stone and Gravel from Dense Rocks for Construction. Technical Specifications", was used. The granite crushed stone has the following properties:

•Bulk density: 1380 kg/m³

•Void volume: 38%

•Water absorption after 1 hour: 1.4%, after 48 hours: 1.8%.

As fine aggregate, quartz sand from the "Nerudnik" quarry, complying with GOST 8736 "Sand for Construction. Technical Specifications", was used. The sand has the following properties:

•Specific gravity: 2.62 t/m³

•Bulk density: 1470 kg/m³

•Fineness modulus: 2.8.

The concrete mix for class B30 concrete used to manufacture the slabs had the following composition:

•Cement: 480 kg/m³

•Sand: 680 kg/m³

•Crushed stone: 1080 kg/m³

•Water: 210 liters/m³

The workability of the concrete mix was 50-60 seconds according to for each composition.

To determine the strength and deformation characteristics of the concrete, concrete cubes with edge sizes of 100 mm and prisms with dimensions of 100x100x400 mm were manufactured simultaneously with the slabs, using the same concrete mix. The tests on cubes and prisms were conducted at the age of 28-30 days in a hydraulic press IGМ-1000 in accordance with GOST 10180 and GOST 24452.

During the prism tests, the load was applied in increments of 10% of the expected ultimate load. The modulus of elasticity of concrete was determined under a load of 30% of the ultimate load. Based on the results of these tests, the prism compressive strength, initial modulus of elasticity, and ultimate compressibility of the concrete for each grade were determined.

To determine the mechanical properties of the reinforcing wire, samples were tested according to the requirements of GOST 12004. These tests were also carried out using a hydraulic press IGМ-1000.

The prestressed reinforcement is made of high-strength wire of class Bp1400 from the Beloretsk Metallurgical Plant (Russian Federation), characterized by a tensile strength of 1800-1820 N/mm² at 7% elongation, and an elastic modulus Es=196,000 MPa, in accordance with the requirements of GOST 7348 "Wire of Carbon Steel for Reinforcement of Prestressed Concrete Structures. Technical Specifications".

In accordance with [1], the load-bearing capacity of multi-hollow slabs was assessed through full-scale tests, checking strength, stiffness, and crack resistance of normal sections under the action of control uniformly distributed loads in accordance with the regulatory documentation.

The criteria for assessing load-bearing capacity include allowable deflections under control loads based on stiffness, crack width, and the load-bearing capacity with a strength safety factor [1] depending on the nature of failure of the tested reinforced concrete structure.

Results and discussions. High-strength prestressing reinforcement is used, consisting of 5mm diameter wire of class Bp1400, which is placed in the upper and lower zones of the slab within the spaces between voids. For all slab sizes, the upper zone reinforcement consists of 4 ∅ 5mm Bp1400 wire each. Heavyweight concrete of class B30 is used for all slab sizes. The design dimensions of these slabs are chosen to ensure their interchangeability with typical multi-hollow (six-hollow) floor slabs with a thickness of 22 cm (see Figure 1).

Figure 1. Cross-section of the floor slab with oval voids.

1 - floor slab; 2 - anchor projections;

One of the drawbacks of these slabs compared to typical ones is that during their formation, there is no opportunity to create keying slots (indentations) along the side faces and anchor protrusions on their ends, which are intended for carrying out seismic measures. Therefore, the formation of indentations is carried out after molding with fresh concrete, and anchor protrusions are installed after the slabs have been heated in the factory or on the construction site during their installation.

Currently, such slabs are produced by many enterprises in the republic. The LLC "BINOKOR TEMIRBETON SERVIS" produces similar slabs according to working drawings [2] on equipment imported from abroad.

As is well known, the quality and reliability of reinforced concrete products are generalized indicators that are evaluated by a set of characteristics dependent on many technological and structural factors. The reliability of such structures should be based on their trouble-free operation, i.e., on the preservation of their functionality throughout the prescribed service life. In the context of this work, tests were conducted to determine the concrete strength using both destructive and non-destructive methods, as well as the structural parameters of the slabs.

The determination of concrete strength was carried out by testing experimental concrete cubes with edge dimensions of 10 cm on a hydraulic press at 28 days of age and by using the method of elastic rebound of concrete slabs with the ONIKS-2.53 device [3]. In each case, six tests were conducted to ensure the consistency of the results.

The testing scheme of the slabs corresponded to the design scheme of the slab, representing a freely supported beam with a uniformly distributed load created by individual concrete weights (see Figure 2). These tests, in line with the set goal, were carried out until the load corresponding to the evaluation of crack resistance and stiffness of the slabs was reached. The deflection of the slabs was measured using the N.P. Maximov system with a dial gauge, taking into account the settlement of the supports measured with 0.01 mm dial indicators. The crack width was measured using the MPB-24 device.

Figure 2. Testing process of the floor slab

As is well known, the bond between the reinforcement and the concrete, not less than the strength of the concrete itself, ensures the reliability of restressed slabs. For this purpose, the concrete of such slabs must possess the necessary strength and density, which is achieved by appointing its optimal composition and proper placement and compaction of the concrete mix. If, for any reason, the transfer strength is reduced, there is a risk of increasing the length of the stress transfer zone and reducing the crack resistance of the support sections of the slab. In the slabs, after the release of tension, the wire tends to return to its initial position, resulting in a process of rapid creep of concrete around it, leading to the densification of the concrete structure. As a result, the concrete in the contact zones becomes denser and stronger compared to the peripheral layers. This is evidenced by the fact that after cutting the slabs, there is no slippage of the wires from both ends of the slab.

Table 1.

The test results for determining the strength and deformation of the concrete slabs.

The data in Table 1 show that, based on the test results, the composition of concrete used in production conditions is relatively homogeneous in structure with minimal deviations.

As indicated by the data in the table, the modulus of elasticity of concrete is related to its strength, and its actual values are typically evaluated through a straightforward dependency. Such a relationship is commonly accepted in normative documents for the design of reinforced concrete structures [4, 5]. One of these dependencies in the form of a regression equation is as follows:

                                    (1)

The ultimate compressibility of concrete under compression is typically assumed to be 200x10^-5 when calculating the crack resistance of reinforced concrete structures. However, its actual values vary within fairly wide limits and linearly increase with the increase in prism strength [6], which can be expressed by the following regression equation:

                               (2)

Where: R_b is the prism strength of concrete, in MPa.

As the results of the comparison between the calculated and experimental values of the modulus of elasticity and ultimate compressibility of the concrete slabs, computed using equations (1) and (2), show, the difference on average is 18% and 2.5%, respectively. This indicates that for the calculated determination of the modulus of elasticity of concrete slabs in the formula (1), an additional factor is needed to be included, such as the specific content of cement paste in the concrete mix or another parameter that characterizes the structural characteristics of such concretes.

The resistance of concrete for calculations is determined according to the current norms for the design of reinforced concrete structures [4, 5], based on the values of the normative resistance obtained by non-destructive methods before testing the slabs. With data on the strength of concrete in individual sections (at least six), it is possible to estimate it using statistical methods. The normative value of concrete strength is calculated using the formula:

, MPa                                                      (3)

Where:

- average actual strength of the concrete structure;

- root mean square deviation of test results;

- number of measurements;

- coefficient determined from the expression.

                                                         (4)

The calculation also includes coefficients for the concrete working conditions, as specified in the standards [5]. Depending on the obtained concrete strength class under compression by interpolating the tabular values provided in the standards, the calculated resistance of the concrete is determined.

Structural solutions for the supports of floor slabs on brick walls and reinforced concrete girders of buildings, taking into account seismic measures, are shown in Figure 3.

Figure 3. Support details of floor slabs on brick walls (a) and reinforced concrete girders (b).

1 - brick wall; 2 - floor slab; 3 - anchor projections; 4 - seismic belt; 5 - reinforced concrete girder; 6 - anchor pins of the girder; 7 - cement-sand mortar M100.

To withstand seismic forces, a rigid diaphragm is created on all levels of the floor slabs by installing monolithic reinforced concrete inserts between the lateral edges of the slabs, followed by anchoring them into the seismic belts of the buildings [2, 7, 8, 11]. 

Table 2.

 The characteristics of the multi-void slabs designed for testing are provided

During the testing of the slabs, the load was applied in steps, with each step constituting 10% of the design load, and there was a 10-minute hold at each step. After reaching the design load, there was a 1-hour hold to observe the processes of rapid creep of the concrete.

Figure 4 shows characteristic load-deflection graphs obtained during the testing of the floor slabs. Based on these graphs, it can be concluded that the deflection of the slabs increases as the load is raised, but as it approaches the ultimate load, there are no significant deformations indicative of concrete creep.

Figure 4. Typical load-deflection (deflection + bending) curves

1-stamp PB60.12-8, 2-stamp PB90.12-8.

The values of deflection under the design load for all tested floor slabs do not exceed the limits specified in the design documentation [2]. When subjected to the design loads, no cracks appeared in the tension zone of any of the floor slabs. Cracks with a crack width of 0.05 mm were only observed when loads exceeded the design load by 30-40%. This condition can be explained by the possibility that the strength of the floor slabs in the tension zone might be slightly higher compared to the compressive zone. This is due to the formation of a dense structure of rigid concrete mixture in these zones, achieved by the pressure applied by the pons (void formers) of the molding machine during the vibration compaction process.

Under the action of the design load, no slippage of the working reinforcement was observed at the edges of the slabs. This provides confidence in the reliability of the bond between high-strength wire and concrete under load in pre-stressed continuous formless floor slabs. This is facilitated by the high strength of the concrete itself, as well as the adhesion of the reinforcement to the concrete, the frictional forces between the reinforcement and concrete caused by shrinkage deformations, and the mechanical interlocking of the reinforcement in the concrete due to its surface shape.

During strength testing, the load-bearing capacity of the floor slabs was ensured at control loads with a safety factor of 1.6. Under these conditions, cracks with a crack width of 0.2-0.25 mm were observed at the level of the restressed reinforcement. Further increase in the load led to a significant increase in deflection and the opening of cracks in the slabs up to 1.0-1.5 mm, with simultaneous yielding of the reinforcement without fracturing the concrete in the compressive zone of the structure. Based on the test results, the tested floor slabs met the requirements specified in [1] and complied with the current standards [9].

The results of the experiments allow establishing the influence of the percentage of reinforcement of floor slabs on their load-bearing capacity. For slabs with a low percentage of reinforcement (1.2% and 2.0% for slabs with lengths of 4700 mm and 6000 mm, respectively), the height of the compressed zone is small before failure. For slabs with lengths exceeding 7200 mm (reinforcement percentage over 4%), the height of the compressed concrete zone increases, indicating that the reinforcement is not fully utilized (and stress in it does not reach the yield point). This condition confirms that the reinforcement is in the elastic stage when the floor slabs reach the ultimate state of load-bearing capacity. In connection with this, it can be noted that the incomplete utilization of the calculated resistance of high-strength wire reinforcement can lead to an inaccurate determination of the nature of the failure of the formwork-free floor slabs and their over-reinforcement.

All floor slabs exhibited flexural failure characteristics along normal cross-sections. The results of the comprehensive tests allow for the determination of the actual load-bearing capacity of the slabs and the operational loads. These factors are considered during the development of detailed drawings for pre-stressed multi-void formless floor slabs intended for use in construction in seismic zones [2, 10]. To fully realize the structural characteristics of such slabs under operational conditions and the impact of forces acting along their perimeter in longitudinal and transverse joints, the requirements outlined in the design documentation and current standards [2, 9] must be strictly adhered to.

Conclusion. The results of experimental studies on the strength and deformability of concrete with a compressive strength class of B30 and tests of multi-void formless floor slabs under short-term loads provide a basis for the potential use of concrete with a class of B25 to manufacture such structures under design loads of 4.5 kPa and 6.0 kPa. Additionally, the use of wire reinforcement as restressed reinforcement is in line with the recommendations of current standards for designing reinforced concrete structures. Forming restressed concrete floor slabs without the need for molds using a concrete mix with an optimal composition and proper consistency allows for the production of concrete with a strong and dense structure, ensuring a guaranteed concrete class for compressive strength. However, it is essential to take into account the actual strength of the concrete used in the construction by determining it through non-destructive testing.

Selective tests conducted on mass-produced and experimental floor slabs subjected to static loading have shown that they meet the requirements of regulatory documents in terms of stiffness and crack resistance and possess sufficient strength. To ensure the reliable and successful use of such floor slabs, strict adherence to the structural solutions and installation guidelines is necessary, especially concerning the implementation of seismic measures as outlined in regulatory documents.

For a reliable assessment of the operational reliability of pre-stressed floor slabs with high-strength wire reinforcement, which is a complex probabilistic problem, it is necessary to continue comprehensive experimental and theoretical research in this direction to obtain and accumulate new results.

References:

1. GOST 8829-2018. Interstate standard. Factory-made reinforced concrete and concrete products. Loading test methods. Rules for evaluating strength, rigidity, and crack resistance.
2. AO "OzOGIRSANOATLOYIHA" Code H4586. Restressed reinforced concrete multi-void floor slabs of stand-up molding for residential, public, and industrial buildings. Volume 1. Working drawings. Tashkent, 2018.
3. GOST 22690-15. Concretes. Determination of strength by non-destructive mechanical methods. Technical requirements.
4. KMK 2.03.01-21. Concrete and reinforced concrete structures. Tashkent, 2021.
5. SP 63.13330.2012. Concrete and reinforced concrete structures. Basic provisions. Updated edition. SNiP 52-01-2003. Moscow, 2012.
6. Popov V.M., Plyusnin M.G. Influence of variability of concrete and reinforcement characteristics on the load-bearing capacity of flexural elements. Bulletin of Civil Engineers. 2015, No. (50), pp. 80-84.
7. Босаков С.В. и другие. Расчет и экспериментальная оценка прочности многопустотных плит безопалубочного формования с учетом требований ЕN. Строительная наука, 2010, № 6, стр.47-54.
8. KMK 2.01.03-19. Construction in seismic regions. Tashkent, 2019.
9. OzDSt 2805-2018. Prestressed reinforced concrete multi-void floor slabs of stand-up molding.
10. Yusufkhojaev, S., Yusupov, R., Alimov, X., Makhmudov, J., & Choi, E. (2023). Crack Resistance of Prestressed Reinforced Concrete Beams with Wire Rope Reinforcement. Materials, 16(19), 6359.
11. Юсупов Р.Р., Азимов А.А. Қолипсиз қуйилган йиғма темирбетон плиталардан ташкил топган ораёпмаларни кучайтирилиши ва ҳисоби. Журнал “Архитектура, қурилиш ва дизайн” 2022, №2, 172-175 бетлар.