DETERMINING THE STRENGTH OF GLASS FIBER REINFORCED CONCRETE BEAMS

ABSTRACT

Today, special importance is given to the construction of buildings and structures that meet the requirements of construction, ensuring their strength, durability and seismic safety. Therefore, various fibers are used to increase the strength of the main load-bearing structures of buildings and structures. Modern construction requirements require not only the construction of strong, durable, safe and reliable buildings and structures. Research and scientific substantiation of the mechanical properties of fiber concrete, especially its tensile strength, is one of the important and urgent issues in the construction industry.

АННОТАЦИЯ

Сегодня особое значение придаётся строительству зданий и сооружений, отвечающих требованиям строительства, обеспечивающим их прочность, долговечность и сейсмобезопасность. Поэтому для повышения прочности основных несущих конструкций зданий и сооружений применяются различные волокна. Современные требования к строительству диктуют необходимость не только возведения прочных, долговечных, безопасных и надежных зданий и сооружений. В статье представлено исследование и научное обоснование механических свойств фибробетона, его прочности на растяжение, что является одной из важных и актуальных задач строительной отрасли.

Keywords: glass fiber, fiber reinforced concrete, strength, concrete, beam, strength.

Ключевые слова: стекловолокно, фибробетон, прочность, бетон, балка, прочность.

1. Introduction

Over the past years, the growing need of high-performance and durable construction materials preconditioned the growth of investigations about fiber-reinforced concrete (FRC). The glass fibers are one of the best-known fibers among the ones that have been examined to enhance the mechanical and structural behavior of concrete in relation to their high tensile strength, corrosion-resistance and lightweight qualities [1]. Glass Fiber Reinforced Concrete (GFRC) is a composite material and consists of the high compressive strength of cementitious matrices combined with the better tensile and flexural strength in an expanded form of the dispersed glass fibers. This combination gives the ductility, impact resistance and crack control capacity of structural elements and makes GFRC a good substitute to conventional reinforced concrete in structural and architectural use [2].

The structural performance of conventional concrete has been restrained by the brittleness of traditional concrete particularly in flexural or tensile loading [3]. To overcome this inadequacy, the use of glass fibers helps in closing the microcracks and retards the spread of microcracks thus increasing the post-cracking and energy absorption properties of concrete beams. Many experiments have proven that the introduction of glass fibers may dramatically enhance flexural and tensile strength of beams, their toughness, and ease of service. The extent of the same however is mostly dependent on the extent of aspects like volume fraction of fiber, aspect ratio, distribution, and bond properties with the cement matrix [4].

The strength of the glass fiber reinforced concrete beams has to be determined to know about the ability to carry loads and the possibility of structural uses. The flexural strength, ultimate load capacity and cracking behaviour should be accurately measured by experimental and analysis studies so to maintain reliability and consistency in performance [5]. The parameters give an insight of how glass fibers can be used to alter stress distribution, slow crack initiation and enhance overall ductility. This paper is concerned with experimental identification of the strength parameters of glass fiber reinforced concrete beams [6]. Through the experiment involving the behavior of beams of different content and lengths of fibers, the study seeks to develop the relationship between the fiber reinforcement properties and the mechanical properties produced. The results will be found to aid the optimization of GFRC mix design in the promotion of structural efficiency, sustainability and durability of the contemporary construction systems.

2. Method

For the experimental study, reinforced concrete beams with dimensions of b=100 mm, h=200 mm, and l=1200 mm were prepared. Glass fibers with a length of 40 mm were added to the concrete mixture at volume fractions of 0.1%, 0.2%, 0.3%, 0.4%, and 0.5% relative to the total volume of concrete.

For the beams, A-III grade reinforcement bars with a diameter of 12 mm were used as longitudinal (tensile) reinforcement, A-III grade bars with a diameter of 8 mm were used as constructive reinforcement, and B-I grade bars with a diameter of 5 mm were used as stirrups (transverse reinforcement). The reinforcement cage was fabricated in accordance with the requirements that the edge distances between the longitudinal and transverse bars should not be less than 0.5d₁ + d₂ or 0.5d₂ + d₁, and in no case less than 20 mm, thereby ensuring compliance with construction and durability standards. The testing process of samples is shown in Figure 1.

Figure 1. The testing process of samples

During the testing process, the initial static load was 500 kg/cm², and the time interval between each incremental load was 8–12 minutes. During this period, the indicator readings were recorded and documented in tables.

The formation of cracks and the development of existing ones were marked with a marker pen. At the same time, the corresponding load value was also noted. When the applied load reached approximately 60–70% of the ultimate (destructive) load, the measuring instruments were removed, and the specimen was loaded until failure while observing the nature of its failure. The cracks in the sample beams occurred in the middle spans and the failure occurred there. In structures, such failure is called a normal section. During the test, it was found that the failure of the samples occurred at values ​​close to the design loads.

3. Result and discussions

Table 2.

Strength indicators of reinforced concrete and fiber-reinforced concrete beams

The outcome of the test (Table 2) proves the obvious increase in the ultimate  load capacity of the beams that have fiberglass. The control beam (no fiber) was found to fail at load of 98kN, at the same time glass fiber reinforced specimens had load capacities of 102 kN to 108 kN, which is an increase of 12,09 % to 18,68 % compared to the conventional beam. The greatest enhancement was on the sample whose content of glass fiber reinforced concrete beam was 0.2 % and thus registered the highest ultimate force of 108 kN.

The initial cracks became visible during the experimental loading at around 60-70 % of the ultimate load. These cracks were majorly located in the midspan (l/2) of the beams, which is the location of maximum bending moment. As the load went on increasing, the cracks that were already found would extend vertically to the compression zone and new cracks would take their place. All specimens had similar crack patterns with the glass fiber reinforced concrete beams having narrower crack widths and the slower rate of crack propagation as compared to the control specimen. This is explained by the bridging effect of the fiberglass material which retarded crack propagation and enhanced the post cracking ductility of the beams.

Concerning the failure mode, the failure occurred in all the specimens along the normal section because of flexural stresses. Crushing of concrete in the compression zone and yielding of tensile reinforcement were the features of the failure, and the entire loss of the bearing capacity was observed. It is also important to note that the addition of fiberglass did not alter the basic failure mechanism but had a great effect on the deformation behavior of the beam and the energy absorption capacity.

Conclusion

The findings have shown that addition of the glass fiber to a maximum of 0.2% volume content significantly enhances the flexural behavior, crack resistance, and load bearing capacity of reinforced concrete beams. Increasing the limit, the benefits are expected to decrease, which is probably caused by uneven force of fibers and decreased workability of the concrete mixture. The results indicate that fiber reinforcement is an important factor in improving the structural performance, longevity, and crack management of bending elements in concrete material.

References:

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