Improving the compressive strength of lightweight cylindrical concrete column with basalt fiber reinforced polymer acting under imposed load

Article history Received: August 13, 2020 Revised: October 1, 2020 Accepted: October 19, 2020 Abstract Relevance. The brittleness of lightweight concrete has developed concern among structural engineers. This concern led to the search on how to improve the strength of lightweight concrete and still retain the weight lightness. Researches are ongoing to solve the strength challenges noticed in lightweight concrete, but at the moment there are few works on solving the issues regarding expanded clay concrete, thus it served as a motivation for studying this issue. The aim of the work is to analyze the effects of basalt fiber polymers on lightweight expanded clay concrete columns acting under imposed loads. Methods. To achieve this process, a total number of nine expanded clay cylindrical concrete columns were experimentalized and analyzed. 1.6 % of dispersed chopped basalt fiber was used in the concrete mixture which serves as reinforcement. Also, basalt fiber mesh was used in the experimental analysis. Results. The expanded clay cylindrical column without basalt fiber polymer withstood strength up to 19.6 tons at 58 minutes, the column with dispersed chopped basalt fiber withstood strength up to 26.67 tons at 61 minutes while the column with dispersed chopped basalt fiber and basalt mesh confinement got destroyed at 29 tons at 64 minutes. The results show that lightweight expanded clay cylindrical columns confined with basalt fiber mesh withstood higher load compared to the columns with just dispersed chopped basalt fiber and without it.


Introduction
Concrete is the widely used construction material throughout the world; however, it completely losses the load-carrying capacity once cracks/failures are initiated. To overcome these issues, the inclusion of short dispersed fibers in concrete has been practiced and studied by various researchers.
The randomly oriented short fibers control the propagation of microcrack and improve the overall crack resistance of the concrete. Also, the fiber-reinforced concrete (FRC) behaves like a composite material and this behavior is significantly different from the conventional concrete.
Lightweight concrete (LWC) is generally defined as concrete made of ordinary Portland cement (OPC), water, river sand (or lightweight sand), and lightweight coarse aggregates, and its density is typically below to 1950 kg/m 3 [1]. Consider the growing demand, including high-rise buildings, large-span concrete structures, and floating structures, lightweight concrete that is made by diverse types of aggregate has been widely studied and successfully developed and applied over the past two decades [2][3][4][5][6].
LWC offers several advantages, such as saving dead loads for foundations, high strength/weight ratio, and service as ideal filled materials for sandwich structures. Thus, lightweight concrete has many potential applications in the construction industry. Nevertheless, some drawbacks in lightweight concrete's natural mechanical properties have limited applications, especially as load-bearing structural members [3]. At the same mixing ratio and compressive strength, the brittleness of LWC is much higher than normal concrete (NC). Plus, the deformation capacity of lightweight concrete is also poor when compared with NC [7].  Figure 2. Mechanism of formation of plastic hinge in beams [8] In many recent earthquakes, it has been observed that buildings with relatively weak columns collapsed in a pancake fashion. This is due to the presence of soft stories and due to the presence of strong beams but comparatively weaker columns ( Figure 1). In multistory reinforced concrete buildings, it is desirable to form plastic hinges in beams rather than in columns to dissipate earthquake induced energy by yielding of the beams rather than the columns ( Figure 2). The columns are responsible for the overall strength and stability of the structure, during severe seismic jolt. Furthermore, columns are compression members and axial compression reduces the ductility of reinforced concrete columns, thus necessitating more stiff confining reinforcement. Therefore, it is preferable to control inelasticity in columns, to the extent possible, while dissipating most of the energy EXPERIMENTAL RESEARCHES through yielding of the beams as a measure against the total collapse of the structure [9; 10]. To achieve this "weak beam strong column" objective, the columns of a building can be made stiffer against deformation by wrapping them with fiber-reinforced polymers (FRP). Research work was done to conduct experimental studies of reinforced concrete retaining walls, including considering their reinforcement by inclined reinforcing bars [11]. The results obtained from the research showed the opening of horizontal interblock joints, the formation of inclined cracks emerging from the joints. An increase in the strength of reinforced concrete structures of retaining walls and a decrease in their deformability due to reinforcement by inclined rods in the interblock weld were recorded.
Confinement has been known to add both strength and ductility in the axial direction for concrete column and this idea originally developed back in the 1920s [12]. Numerous conventional techniques, e.g. ferrocement, concreting overlay, grout injection, external reinforcement, post-tensioning, near-surface retrofitting, etc., are available and getting popular for retrofitting of the concrete column. In recent years, the use of fiberreinforced polymers as an external strengthening has gained considerable popularity over conventional strengthening and repair of concrete structures. The FRP composites have been used successfully for rehabilitation and strengthening of existing reinforced concrete elements to meet the higher standard of seismic loading. One popular technique of FRP strengthening is the wrapping of reinforced concrete columns to increase their axial strength, shear strength, and seismic resistance.
Fiber-reinforced polymer composites have become a favorite material of professionals in both engineering and construction due to its advantages. Major ones include lightweight, high strength, and construction convenience [13][14][15][16][17][18][19]. FRP composites provide excellent corrosion resistance, which keeps costs down and increases the service life of structural materials [20][21][22][23][24][25][26][27][28][29]. The circumferential confinement of FRPs restrains the transverse expansion of concrete; thus, the strength and ductility of FRP-confined concrete are notably enhanced when the concrete is subjected to a triaxial compressive load [30][31][32][33]. Therefore, it can be inferred that the problem of high brittleness and the poor ductility of lightweight aggregate concrete can be effectively solved by using FRP confinement. Thus, FRP application can make it an effective method to reduce self-weight in structural design. The advantage of the composite structure is that it can fully use the characteristics of the multi-materials [34][35][36].
Concrete elements are known for their brittleness therefore the need to strengthen the elements. The column is a very important load-bearing element in structures. Though a lot of researches have been done on the strength improvement of conventional concrete columns, very few researches have been done on the strength of lightweight expanded concrete with or without basalt fiber (BF). Based on the above, this research work has the task to suggest or solve the problem associated with the brittleness of lightweight concrete structural elements.

Materials and methods
This experimental study of concrete is carried out by using the state standard GOST 10180-2012 [37]. The materials for the lightweight concrete mix and the production of the concrete for this study are listed below for better illustration.
1. Lightweight expanded clay aggregate of 5-8 mm fraction as coarse aggregate. The lightweight expanded clay was washed to remove the dust in the aggregate. After washing the aggregate, it was spread on the metal surface for 48 hours to dry up. Expanded clay aggregate is a lightweight aggregate from clay. The clay is dried, heated, and burned in rotary kilns at 1100-1300 °C inhabiting essential properties like lightweight, insulating, strong, non-combustible, and fire-resistant, extremely stable and durable, natural material for sustainable construction, versatility, and high drainage capacity [38][39][40][41][42].
2. Quartz sand of 0.6-1.2 mm fraction as fine aggregate. A feature of the proposed quartz is the presence of coarse-grained sand, with a large modulus of fineness up to M3,5. Quartz sand has a rounded part with a low content of clay inclusions and inclusions of soft rocks. The resulting quartz sand undergoes additional enrichment and drying. The moisture content is up to 0.2% [43].
3. Mineral filler Silverbond quartz flour of 50 µm. Quartz flour is produced by grinding chemically pure, natural quartz sand to a finely divided state. The technology used guarantees the stability of the chemical composition during grinding and allows to get a constant particle size distribution of quartz flour. The crushed quartz flour is represented by rounded particles with uneven, broken edges. Quartz differs from other mineral fillers in hardness, abrasion and chemical resistance, anti-corrosion, and low coefficient of thermal expansion. Quartz is a chemically stable mineral, it is soluble only in hydrofluoric acid. With a low oil absorption and a small surface area of the particles, the use of quartz flour will enable to get a system with a high degree of filling.
4. Binder Holcim Portland cement M500 D20 CEM II 42.5 N. The characteristics of Holcim Portland cement M500 D20 CEM II 42.5 N: M -brand, 500 is a figure showing the average compressive strength for 28 days in kg/cm², D -additives, 20 -allowable number of additives in % (up to 20%), CEM II -cement containing additives, and the content of additives is 6-20%, I-type additives, limestone, 42.5-class compressive strength for 28 days, must be at least this value, and B-quick hardening.
5. Organic mineral-based additives: silica fume, and fly ash. 6. Super plasticizing and water-reducing additive Sika Plast concrete. 7. Tap water at room temperature. Generally, water that is suitable for drinking is satisfactory for use in concrete.  The columns will be molded in a Euro cylinder of dimensions ∅150 mm × height 300 mm. The concrete columns are molded in Euro cylinder forms according to Eurocode 1 and 2 [44;45] as shown in Figure 3. After pouring the ECC in cylindrical columns molds, the molds were covered with polytelin and kept at room temperature (20 ± 5) ℃ and relative air humidity (95 ± 5)%. On the 76 th hour, the ECC columns were removed from the molds and kept in the curing bath till the 28 th day then the columns were tested for deformation on a compressive test on a hydraulic press PG-100 (Figure 4).
After the test, a comprehensive strength comparison of the three sets of concrete will be done. From the analysis, the stress deformation and time will be analyzed from the average results of three columns from each of the three sets for comparison. Three strain gauge sensors are clued on the body of the ECC cylindrical column and numbered 1, 2, 3 as shown in Figure 5. Sensors 1 and 2 are for measuring the stress-strain deformation on the vertical zone of the column while sensor 3 is for measuring the horizontal deformation of the column.

Results
The 1.6% dispersed chopped basalt fiber used in this research experiment was derived from the compressive test results in Table where ECC cube with 1.6% BF showed better compressive strength.  Figure 9 shows the comparison of the maximum compressive load strength of the columns. Figure 11, it is seen the effect of the load on the strain gage. In Figure 11 8, 9, 10, and 13 show the results of ECC + BF + Basalt mesh confined cylindrical columns after imposed loads. The ECC + BF + basalt mesh sustained a load of 29 tons. The compressive test started at time 16:56 and noticeable deformation started at 17:50 and complete test destruction ate 18:00. It took 64 minutes for the complete deformation. Figure 13 shows that strain gage 1 (in green line graph) sustained from 16:59 -17:59, strain gage 2 (in blue) sustained from 16:59 -17:59 while strain gage 3 (in red) sustained from 17:00 -18:00.