Abstract
The use of steel fibers in a certain ratio in the concrete mixture can provide more energy dissipation to transitioning the concrete from a brittle material to a ductile material. The effects of steel fibers on the interlocking force behavior between concrete and reinforcement, as well as on concrete compressive strength, were investigated experimentally in this study. Two different water/cement ratios were prepared in the concrete mixtures for the creation of concrete samples. In the preparation of concrete, the volume of steel fibers was treated as 0.00%, 0.25%, 0.50%, and 1.00%. To evaluate the effects of steel fibers on the interlocking force between reinforcement and concrete, ribbed steel reinforcement bars were embedded by fastening the samples vertically at interlocking lengths of 5 cm, 10 cm, and 15 cm during casting. Pull-out tests and concrete compression tests were performed on the concrete samples that had been cured for 28 days. As a result of the increase in steel fiber volumetric ratios from 0.25% to 1.00%, the compressive strength increased and it was found out that the steel fibers contribute negatively to the increase in compressive strength. Nevertheless, the tensile strength increases in parallel with the increase in interlocking length between concrete and reinforcement, but the tensile strength of the steel fiber samples is generally lower than that of nonfibrous samples. The principal findings of this study demonstrate that the steel fibers have a beneficial effect on flexural behavior, cracking performance, and postcracking residual stresses.
1. Introduction
Reinforcement and concrete steel can only work together if the steel does not move in the concrete. An interlocking is necessary between concrete and steel to prevent independent movement. Interlocking refers to the shear stresses that exist between the concrete and reinforcement making up this interlocking [1–5]. These binding forces between the reinforcement and the concrete grow or decrease in parallel with the reinforcement’s stress. The pull-out test measures the required forces by applying tensile force to concrete-interlocked reinforcing. Hence, the interlocking force is defined as the measured force. Owing to its interlocking force, the steel transfers axial force from steel bars to concrete, thus ensuring that the two materials work synchronously together [6]. In the previous studies, the effect of steel fiber length, volume ratio, and slenderness on the interlocking strength was investigated and it was observed that the effect of the volume ratio of steel fibers on the interlocking strength between concrete and steel bars was limited. Given that the compressive strength of the concrete surrounding the steel bars is an important factor in the interlocking strength, it has been found that the contribution of steel fibers on the compressive strength was limited [7]. Utilizing a higher steel fiber ratio reduces the interlocking strength between concrete and reinforcement [8]. So, the effect of the ratio of steel fiber volume on the interlocking strength between concrete and steel bars could be investigated. Furthermore, fiber-reinforced concrete has been the subject of numerous research studies to enhance postcracking tensile behavior. For this purpose, by adding fibers to the concrete mixtures, it is possible to increase flexibility and reduce inclined shear cracks or concrete pours after crack formation. These results in a more ductile material that can withstand greater amounts of stresses without fracturing cause some recovery on the tensile, shear, torsion, and flexural properties of fiber-reinforced concrete. So, there have been a lot of studies reported for each mentioned characteristic test [9–17].
The use of steel fiber-reinforced concrete (SFRC) has been widely studied by researchers. However, there is no specific research considering the interlocking length of steel fibers and its effect on interlocking strength. Therefore, this study aims to investigate the effect of interlocking size on different types of steel fibers, water/cement ratios, and interlocking lengths. Hence, in this study, novel mixtures were manufactured by keeping the ratios of the materials used in concrete preparation as constant and additionally mixing the steel fiber at 0.00% (empty), 0.25%, 0.50%, and 1.00% by volume. By placing this mixture in the mold, a 10 mm diameter ribbed steel bars were also perpendicularly placed in the mold. The effects of different percentages of steel fibers and interlocking lengths of 5 cm, 10 cm, and 15 cm on the interlocking strength and concrete compressive strength were explored. Interlocking tests were conducted by performing pull-out tests on prepared samples embedded at 5 cm, 10 cm, and 15 cm distances into 28-day concrete prismatic specimens of 15 × 15 × 15 cm dimensions of 10 mm diameter perpendicular ribbed steel bars. In addition, compression tests were managed by measuring the strength of the 28-day concrete prismatic samples of 15 × 15 × 15 cm in the compression test machine. In the study, the peeling (stripping) behavior between concrete and reinforcement was also explored.
2. Methodology
The experimental test procedure consists of testing prepared samples with different steel fiber ratios, different concrete mixtures, and different interlocking (adherence) lengths in tensile and compression tests. The following sections provide details of the experimental test procedure adopted in the present study.
2.1. Mechanical Properties of the Concrete
The properties of aggregate, water, cement, concrete, steel bars, and steel fibers utilized in the fabrication of test specimens were initially determined. The CEM I 42.5R cement supplied from Konya Cement Factory was utilized in the concrete mixture. Table 1 lists the physical, chemical, and mechanical properties of the utilized cement. The water in the concrete mixture was utilized in compliance with TS EN 1008 specifications, and it was sourced from the city’s water network which has potable water quality.
Concrete samples were fabricated using three different types of aggregates: sand, crushed stone I, and crushed stone II. Table 2 shows the physical properties of the aggregates utilized as well as the quantities used. A new generation hyperplasticizer is utilized in the manufacture of high-strength, high-performance concrete. A high-performance superplasticizer chemical additive material having a specific gravity of 1.070 g/cm3 (0.03) was employed in this study. Low carbon steel fibers with an average tensile stress of 1200 MPa and an elastic limit of 0.2 percent were employed in the preparation of concrete samples. Also, Table 2 lists the brand, physical, and chemical properties of the steel fibers utilized. In the test specimens, the ribbed class is B420C steel, the steel diameter is 10 mm, the tensile strength is 500 MPa, the yield strength is 420 MPa, the minimum elongation at break is 10%, and the reinforcing steel utilized is 1.1 meters long.
2.2. Construction of the Test Samples
As before mentioned, this study intends to investigate the effects of water/cement ratio and the amount of steel fibers on the interlocking strength between concrete and steel bars and on the compressive strength of the concrete. Tensile extraction tests were conducted on ribbed steel bars anchored into concrete samples of 10 mm diameter, and compression tests were performed on cube and cylindrical concrete samples. The water/cement ratio for two different types of mixtures was 0.70 and 0.63, respectively. The steel fiber was added to each combination at a rate of 0.25%, 0.5%, and 1.0% by volume. The concrete mixture is displayed as K1 and K2 coding in the samples. While coding is treated based on the proportion of fiber content, the terms “TEMPTY” for nonsteel fiber, “T25” for 0.25 percent, “T50” for 0.50 percent, and “T100” for 1.0 percent are used. Table 3 lists the mixing codes. For example, K1 concrete mixture, a sample containing 0.25% steel fiber content, was dubbed K1T25.
As previously stated, the main aim of this study is to investigate how steel fibers affect the interlocking strength between reinforcing steel and concrete and concrete compressive strength. The sample molds with the size of 15 × 15 × 15 cm were prepared to produce concrete specimens. To investigate the behavior at varied interlocking lengths, an apparatus (mechanism) was designed using plastic electrical pipes such that the contact surfaces of the reinforcing bars with the concrete were 5 cm, 10 cm, and 15 cm (Figure 1).
Nine chambers are constructed in such a way that they give interlocking length as 5 cm by three of them, 10 cm by three of them, and 15 cm by three of them, and they are ready for concrete casting by fastening plastic pipes to the underside of the reinforcements. To explore how steel fibers affect concrete compressive strength, a total of 96 cube and 24-cylinder samples were collected. The number of concrete samples is given in Table 4.
The concrete mixture was put into the molds that had been prepared in accordance with Table 3. Figure 2 depicts the producing process of concrete specimens. New concrete tests were performed during the production of concrete samples to get information regarding the consistency and workability of fresh concrete. In one of these tests, after placing the concrete in the slump cone by inflating it in three stages, the slump cone was lifted, and the amount of slump was determined in line with the TS EN 12350-2 specifications [18] by measuring the height difference between the initial position and the position after the concrete was spread by its own weight. The results of the fresh concrete slump tests are given in Table 5. After 24 hours, the concrete samples were taken out from the mold and kept in the curing settings until they attained the desired wetness. Samples held in the curing pool for at least 28 days were taken out and utilized in experimental tests. Following the completion of the pouring and storage stages of the concrete samples, the samples for reinforcement for the tensile test were performed, while the other cube and cylinder samples for the compression test were performed.
For the reinforced samples with steel bars, the pull-out test was performed using the apparatus (mechanism) as shown in Figure 3. The loading process was performed with a manual hydraulic hand pump (maximum working tensile: 100 MPa). During pulling (tensile), the iron claws on the load-bearing section are grabbed to keep steel bars stable. The force exerted during pulling, and the lower and upper displacement values were read in mm at time by recording via a data logger. The compressive strength test was performed on three 15 × 15 × 15 cm cube and cylinder test specimens. The compressive strength test was performed in accordance with TS EN 12390-3 provisions [19]. A continuous loading rate of 0.2 MPa/s–1.0 MPa/s should be selected according to TS EN 12390-3 specifications. The chosen pressure loading rate is 0.3 MPa/s. The compressive strength values of cube and cylinder concrete specimens were obtained by pressing in an automatically controlled press instrument with a loading capacity of 3000 kN (Figure 3). In the United States, Japan, Australia, and New Zealand, the cylindrical specimens are generally regarded as practice code provisions. However, in Europe, the cube specimens are often utilized in most of such kind of experimental test studies.
3. Results and Discussion
The achieved results from the pull-out tests for concrete samples of K1 and K2 mixtures were discussed in this section. For this reason, Figures 4 and 5 provide comparative graphs of the maximum loaded steel fiber ratio and maximum loaded interlocking length variations of different concrete samples. When the average values are examined according to Figure 4, it has been observed that the loads in the K1 concrete mixture increase with the increase in the interlocking (adherence) length in the steel fiber samples. When the average values are examined according to Figure 4 and Table 6, it has been observed that the 15 cm interlocking strength length carries a higher load than the other interlocking strength lengths in the K1 concrete mixture in all steel fiber ratios given.
The values of maximum load-bearing capacity for concrete samples in the K2 mixture increase as the interlocking length increases in the tensile test. In K2 mixture concrete samples, when the percentages of steel fibers increase, the load values of the maximum tensile capacity increase in samples with a clamping length of 5 cm but decreases in the case of 10 cm and 15 cm interlocking lengths (Figure 5).
The way the average values of the data are obtained is shown as an example in the line graphs. Figure 6 illustrates the lower displacement values of three different samples at different load levels for 15 cm interlocking length. From Figure 6, the concrete specimens including steel fibers demonstrate more ductile behavior, but brittleness is clearly observed in those without steel fibers.
Different steel fiber ratios were employed in the mixture of concrete samples to examine the effect of steel fibers on compressive strength. Compression tests on concrete samples with varying steel fiber ratios were performed. Compressive strength test was carried out on three samples for each group by taking the average of these samples (Figure 7).
According to the pressure column graphs in Figure 7, it has been observed that samples without steel fiber endure higher compressive strength in all concrete mixtures and it has been observed that the strength is high in samples without steel fiber. The values of load-bearing capacity of steel bars with 15 cm interlocking length and different fiber ratios were achieved close to each other in the K1 concrete mixture. The compressive strength of nonfiber concrete was found to be greater in cube and cylindrical pressure samples of the K1 concrete mixture. When the percentage of steel fibers in cube samples with K1 and K2 mixtures increases, so does the concrete compressive strength. From these graphs, it was found out that when the applied force reached its maximum level and then reduced gradually, the sample once again showed stability against the applied pressure. Despite not rising as much as the maximum force, this situation reached an upmost value and released the load. The second peak occurred in the graph was observed with the involvement of steel fibers because this situation was not found in the graphs of the samples without fiber. In samples of K2 concrete mixture with 10 cm and 15 cm interlocking lengths, it was observed that the fiber ratio of 0.25% was higher than the other ratios. Even though the tensile strength of the K1 concrete mixture was better in nonfiber samples even at diverse interlocking strengths, the tensile strength of the samples in K2 concrete mixture was close in different fiber ratios, but the nonfiber samples performed better than the fiber-reinforced samples.
Looking into the final physical appearance of the specimens after the pull-out tests, different bond-slip behaviours were observed. Although the fibers lay in a mixture of reinforcement and the concrete was carefully prepared, it exhibited variances in the samples with the same mixture and properties, which can be observed more clearly in the cracking samples. The reason is that the steel fibers are not spread uniformly and that the reinforcement varies depending on the position of the fibers where they intersect. The concrete cracking is the most common in samples with an interlocking length of 15 cm (Figure 8).
4. Conclusions
In this study, an experimental investigation has been conducted to investigate the influence of steel fibers on the interlocking length. The following crucial conclusions can be drawn within the principal scope of this study:(i)The increase in the energy absorption capacity of the steel fiber under pressure loads is important in terms of preventing sudden and explosive collapse under static loads and in terms of energy absorption under dynamic loads. The mechanical performance of steel fiber concrete under tensile stress is lower than its mechanical performance under pressure stress. The type, geometry, usage rate (by volume) of the steel fiber and the placement of the fibers in the concrete can vary according to the preparation methods of the fiber concrete.(ii)As a result of the increase in steel fiber volumetric ratios from 0.25% to 1.00%, the compressive strength increased. However, the compressive strength was higher in samples than those of without steel fiber.(iii)It has been clearly seen that increasing the interlocking length in the concrete mixtures prepared has a positive effect on interlocking strength. Moreover, the compressive strength of the cube and cylinder specimens increased when the water/cement ratios in the K1 and K2 mixtures were reduced. In addition, when the percentage of steel fibers in the concrete cylinder with K1 mixture increases, so does the concrete compressive strength, but these values fall in concrete cylinder samples with K2 mixture. The compressive strength of the samples without steel fiber was higher than the compressive strength of the samples with steel fiber, according to the test findings of the cube samples with varied combinations. In addition, these values were achieved comparable to each other in the cylindrical samples. As a result of the experiments, a ductile fracture was observed when the tensile and compressive strengths of steel fiber-reinforced concrete were examined. It was determined that the fracture was brittle in the samples without steel fiber additives.
Eventually, it has been clearly deducted from this study that the steel fibers were more effective at preventing crack growth. Therefore, results are improved for slope stability, reinforced concrete pipelines and infrastructure materials, factory warehouse and hangar floors, airport, port and road pavements, shotcrete applications, thin shell constructions, and extremely high-strength concrete.
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.