Steel Fibres: Enhancing Concrete Structures' Strength & Ductility - Research Paper

Abstract

The use of steel fibres in concrete structures has enabled constructors to design strong structures. Many studies have been conducted to test the advantages of using steel fibres, whereby both long and short steel fibres have advantages in structures. The use of short and long steel fibres have been exercised in beam structures to enhance the ductility and strength of the structure. Cracking, which is common in building structures, has also been reduced in beams through the use of steel fibres. The results of the 12 tested beams show that steel fibres have enhanced the performance of beams and other concrete members.

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Introduction

The use of steel fibre in enhancing the strength of building materials has become significant in the contemporary world. Steel fibre in reinforced concrete beams has also been influenced by modern technology and has become one of the best ways to strengthen the beams. Initially, reinforced concrete beams involved materials such as straws and horsehair. However, researchers invented the use of steel fibres, which forced the use of straws and horsehair to give way to steel due to its strength. Typically, the addition of steel fiber in reinforced concrete beams is mainly to improve the ductility of the beams and enhance the shear resistance. Nevertheless, the strain rate and loading type may influence the strength of the beams.

Furthermore, post-cracking is one of the common issues that is experienced in various structures. Therefore, the use of steel fibres in reinforced concrete beams has helped in reducing the extent of cracking. In this case, the fibres bridging actions helps in curbing crack propagation more efficiently due to their strength compared to other reinforcement materials. The idea of using steel fibres has also been reported by ACI, whereby various concrete structural members have been tested, and those with steel fibres have resisted higher shear stresses.

On the extreme, glass and synthetic fibres have been used in various structures as a method of curbing corrosion in the reinforced concrete members. Nonetheless, researchers have found that glass and synthetic fibres were less resistance compared to steel fibres. Besides, the use of stiff hooked-end steel fibres with high tensile strengths has been termed as one of the reasons why the modern reinforced structural members are more reliable compared to the past structures that used the straight fibres with limited strengths. Thus, advancing steel fibres has also helped in attaining better and more durable structures. ACI has also recommended a minimum amount of steel fibres of 0.75% of concrete volume. Nevertheless, lack of enough support to show the most effective and suitable types and length of steel fibres either with standard or lightweight concrete has forced more researchers to study steel fibres in reinforced concrete beams. Energy absorption of concrete members has also been studied by various researchers, whereby the use of steel fibres have been termed as one of the ways to improve energy absorption in these structures. In essence, the addition of randomly oriented steel fibres into the concrete matrix is essential since it enhances multiple crucial characters in concrete structures, such as flexural strength, shear strength, and ductility.

Research Significance

Many researchers have studied and documented the use of steel fibres on concrete members. Moreover, studies have been conducted to show the shear behavior of steel fibre reinforced high strength lightweight concrete beams. Additionally, past experimental investigations have shown that integration of sufficient steel fibre help in strengthening the concrete beams. Conversely, it is essential to conduct further research to obtain more information about the use of steel fibres in concrete members. Equally, the investigation will be significant since it will provide information that can be combined with previously obtained results to determine the significance of steel fibres in reinforced concrete beams.

Test Programs

Test Specimens

The test programs involved the determination of the decisive role of adding two different sizes of steel fibres into both standard and high strength reinforced concrete beams on the shear behavior. Twelve specimens, which were fully controlled and entirely monitored under the four-pointy loading test, were used to help in obtaining the required information of the research. The three investigation parameters in this research, include the length of the steel, fibres, type of aggregate, and concrete compressive strength. Besides, the beams were subjected to two concentrated symmetrical and vertical downward forces. The concrete trial mixes will be tested after approximately 28 days to ensure that apposite mixtures for the experiment have been selected. A 500 psi (35 MPa) is the target average concrete compressive strength for 28 days for regular strength concrete reinforced beams. On the extreme, 10000, psi (70 MPa) average concrete compressive strength is the target for the high strength reinforced concrete beams. The steel fibres fraction (Vf) and shear span to depth ratio (a/d) are also essential in this experiment, whereby they will remain fixed for all samples. The volume of steel fibres will be 0.75% of the concrete amount, and all specimens will be tested at a shear span-to-depth ratio of 3.

Materials

Concrete

The experiment was carried out using four types of concrete that were based on the types of aggregates and concrete compressive strength and types of steel fibres. These kinds of aggregates were blended with ordinary Portland cement. Moreover, superplasticizer was added to those beams with steel fibres to enhance the workability of the mixture. Short steel fibres were also used, whereby they were poured in the concrete laboratory at the Memorial University of Newfoundland.

Steel Reinforcing Bars

10M deformed steel double-legged stirrups were used in the experiment, whereby they were used in the clamping of the longitudinal bars within their precise spacing. Moreover, deformed steel reinforcing bars of 15M and 20M were also used against the flexural moment. 15M was perpendicularly placed after they were cut and used as spacers on the foot reinforcing bars to help in hanging the second reinforcing row and distinguish it from the first reinforcing row. The average yield strength was taken as 430 MPa for both reinforcing bars after the reinforcing bars showed yield strength of 440MPa for 20M and 420MPa for 15m bars.

Steel Fibres

The steel fibres that were used had two different lengths and two different ends. Four 35 mm long single hooked-end steel fibres were cast, and other four 60mm long double hooked-end steel fibres were poured into demonstrating the different pull out behaviours of the two steel fibres used. Long steel fibres were more durable since they showed a higher tensile strength that might be influenced by the length and the end shape. Strengths of 1345 MPa and 2300 MPa and aspect ratios of 64 and 67, while the modulus of the elasticity values was 185000 MPa and 210000 MPa, respectively.

Test Results

Stress vs strain curves showed that the tensile strength of long double-hooked fibres is higher than the single-hooked fibres with 35 mm long. On the other hand, 35mm long and 0.55 mm in diameter and 60mm and 0.99mm in diameter reported strengths of 1345 MPa and 2300 MPa and aspect ratios of 64 and 67, while the modulus of the elasticity values was 185000 MPa and 210000 MPa, respectively.

Test Behavior

Load-Deflection Behavior

The deflection was measured using LVDT's, which showed a typical load versus deflection curves that were acquired from three LVDT's. The deflections from the LVDT's also were almost identical to ensure that the load on the beams was asymmetrical. The loads-deflection curves for the specimens indicated that they failed in flexure and could be discussed into three stages. Besides, the beam was not cracked and behaved in a linear elastic manner. Nonetheless, a formation of a crack was recorded after the load was increased, and more hairline cracks appeared as more weight was added. Also, the stage ended after the load-deflection curve started to change the slope.

On the other hand, the specimens that were in group 2 containing 35mm long fibres showed lower capacity with higher deformation as per the ultimate displacement, compared to average weight specimen. In this case, beams LNB35 and NNB35 had a capacity of 146.1kN and 155.7Kn, respectively. On the other hand, the deflection values of 38mm and 17mm were recorded respectively. Therefore, the load-deflection behavior of the specimen shows that the weight presented primarily affected the results. The ductile shear mode was also tested, whereby specimens NNB35 and NHB35 with average weight aggregate failed in flexible shear mode, which shows they did not develop sufficient ductility.

The beams in the third group with double-hooked end with 60mm fibres showed different trends than those in group 2. In this case, the standard weight beams reported more flexibility and higher capacity than the lightweight concrete specimens. For instance, LNB60 showed a capacity of 164.1 kN and NNB60 showed a capacity of 165.5 kN. Therefore, the double-hooked end 60mm fibres are more efficient compared to the single hooked-end fibres.

Effects of Length of Fibres

After comparing the load and deflection of LNB35 vs LNB60, NNB35 vs NNB60, LHB35 vs LHB60, and NHB35 vs NHB60, it was noted that the double-hooked end long fibres improved the load-carrying capacity and deflection at ultimate charge for beams made with a different type of aggregates and concrete strengths. See Figure 1.1, Figure 1.2, and Figure1.3. Moreover, the steel fibres whose length was 60mm enhanced deflection peak for all members at an average of 45% more compared to deflections of the beamed that had short fibres. Also, the deflection failures for the beams that had long steel fibres was recorded at a range of 7% and 61%. However, the deflection of LNB60 decreased by 23% in contrast with the NNB60 member displacement, which might be influenced by the weak intertwine resistance across lightweight concrete aggregates.

Figure1.1 Load-deflection curves (group 3)

Figure 1.2. Load-deflection curves (group 2)

Figure 1.2. Load-deflection curves (Group 1)

Load-Brain Behavior

The tests showed that steel strains developed faster compared to concrete strains, which might have been influenced by the high tensile stresses in the extreme extension fibre against the 60 small compressive stresses on the excessive compression fibre. Moreover, the steel strains were inconsiderable at the first linear stage; nonetheless, this changed after the load was increased and after cracks were observed in the beams. Linearity and elasticity of both concrete and steel strains were also expected to appear as the specimens were underlying until fist cracks appeared. Conversely, the loads on the concrete beams went down as the cracks progressed, which shows that the beams grew weaker as the cracks grew and an indication that the beams failed in shear.

Crack Patterns

The essential effect of the presence of the steel fibres in concrete entails the cracking behavior. In this case, the progress and the appearance of the cracks were monitored in every step to ensure that detailed information was acquired. Moreover, this was performed to categorize flexural cracks from the diagonal shear cracks and to define the general mode of failure and crack patterns of the beams. Therefore, it was recorded that verti...