The use of steel fibre in enhancing the strength of building materials

The use of steel fibre in enhancing the strength of building materials

Introduction

The use of steel fibre in enhancing the strength of building materials has become significant in the contemporary world. The use of 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 fibreswith 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 about 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 fibrereinforced high strength lightweight concrete beams. Additionally, past experimental investigations have shown that integration of sufficient. 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 lengthof 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 appositemixtures 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. See table 1.1

Table 1.1: Shear strength analysis

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 perpendicularlyplaced 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 44OMPa 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 fibresused.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.

TestBehavior

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 thy 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 steelfibres 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 vertical hairline cracks developed from the extreme tension fibre upward towards the neutral axis within the constant moment. Also, the crack spacing values at the serviceability limit state was tested, and the number of cracks was recorded, whereby the highest number of cracks was 8. See table 3.Additionally, the load was increased, and the vertical flexural cracks progressed. On the other hand, the flexural cracks within the shear span on each side of the specimen appeared.Furthermore, the vertical cracks started to propagate in an inclined manner towards the loading points, which was initiated by the increase of shear stresses.

Inclined flexural cracks developedand eventually formed a diagonal tension crack in beams with the shear span-to-depth ratio between 2.5 and 6.0 (ASCE-ACI Committee 318, 2014). In this case, the addition of steel fibres showed that they strengthened the beams since those that had steel fibresdid not fall in shear.  Typically, steel fibres produce higher shear capacity compared to glass and synthetic fibres (Imam, Vandewalle, &Mortelmans, 1994). The cracks that developed in beams with steel fibre were also smaller in width. See table 1.2. Typically, short steel fibres reduce the width of the cracks than ling steel fibres (Hamrat et al., 2016). Therefore, one can argue that the use of fibre steels in concrete beams is essential as it enhances their strength. See figure 1.4.

Figure 1.4. pull out action of short steel fibres at faiilure

Table 1.2: Crack widths analysis.

Table 1.3: Crack spacing values at the serviceability limit state.

Ps † Service Load = 0.4 Pu.

Effect of Types of Aggregates

            All the lightweight concrete beams with steel fibres failed in flexure. On the other hand, the two standard weight beams with short steel fibres showed diagonal ductile shear modes of failure irrespective of concrete grade. These results might be steered by the good bond interaction between lightweight aggregates and steel fibres. NNB and NHB specimens gave a good correlation with LNB and LHB members in terms of normalized shear and flexure strengths. Normalized shear and flexure strengths of normal strength lightweight concrete beams with 35 mm long steel fibres dropped by 11% to 18%, respectively, compared to same specimens with normal-weight aggregates. Furthermore, NHB35 beam agreed with LHB35 regarding normalized strengths either shear or flexure. The normalized flexure strength of 71 NNB60 matched the value obtained from LNB60. However, LNB60 showed lower standardized shear strength by about 3% than the force produced by NNB60. The normalized shear strength of LHB60 improved by 4% while the normalized flexural strength enhanced by 18% compared to the experimental values of NHB60. According to Gebreyohannes and Muzeyin (2016), the type of aggregates may not primarily affect the normalized strengths of beams. However, the length of steel fibres and concrete grade factors might affectively lead the kind of aggregates to show unpredicted behavior.

Ductility

Typically, the ductility ratio of beams is determined by dividing the deflection at failure on the deflection at yield stage. m =¶f/ ¶y. In this case, NNB35 and NHB35 specimens failed in shear but showed some flexibility. On the other hand, all beams failed in flexure and showed the most significant ductility ratios. Also, short and long steel fibres in LWC beams with either concrete grade showed similar ductility ratios. The ductility ratio that was recorded for LNB35 was 3.5, LNB60 was 3.0, and LHB35 was 4.9, and 4.9 for LHB60. Nevertheless, it was discovered that double-hooked steel fibres with 60 mm long were more efficientthan single-hooked fibres with 35 mm long in NWC. Also, NHB60 showed the highest ductility ratio of 5.6. The effectiveness of 35mm long single hooked steel fibresshowed an improvement in ductility in NWC members. In this case, LNB35 beam displayed an increase in its elasticity by about 14% compared LNB60 specimen. According to Biolzi and Cattaneo (2017), the higher the flexural capacity beams can resist, the higher the elasticity can be attained.

Energy Absorption

Determiningenergy assumption is also essential in concrete structures. Therefore, the research showed how several aspects could influence energy absorption in the beams. In this case, the area under the load-deflection curve from the origin point up to failure point represented that flexural energy of the beam. The energy absorption of the specimen showed that the addition of steel fibres improved energy absorption. Therefore, steel fibres are recommendable in concrete beams regarding energy absorption (Nasvik, 2012). The energy absorption was also tested using the addition of long and short steel fibres, whereby the energy absorption was improved by about 87% in both NNB60 and LNB60 beams after addition of long steel fibres. On the other hand, the energy absorption of NNB35 and LNB35 specimens increased by about 80% on average after the addition of short steel fibres. Therefore, both long and short steel fibres had an impact on the energy absorption of the beams. Also, double-hooked long fibres and single-hooked short fibres were used, whereby the presence of double-hooked long steel fibres was more effective than the presence of single-hooked short fibres by about 10%.

Also, both short and long fibres showed higher energy absorption in standard concrete beams than in high strength beams irrespective of the types of aggregates. However, long steel fibres can be recommendable for NWC beams and LWC beams with both standard and high strength concrete. Moreover, the short steel fibres can also be a second recommendable choice for NWC beams with high strength and LWC beams irrespective of concrete grade.

Stiffness

The stiffness was increased by the steel fibres, whereby both un-cracked (stage 1) and cracked section (stage2) increased the stiffness. However, the long steel fibres mostly increased as the stiffness by more than 50% in stage 1 and by more than 30% in stage 2. The highest ration that was recorded was with NNB60 beam, which showed a stiffness enhancement of 91% compared to the reference beam NNB. On the other hand, LNB60 showed the lowest increase due to the addition of long steel fibres by about 26% in contrast with the other specimens contained long steel fibres. Correspondingly, improvement of the stiffness due to the addition of short steel fibres ranged from 5% to 31%. The lowest improvement rate was recorded in NNB35 beam compared to NNB member, whereas the greatest was in LNB35 in contrast with LNB specimen. Therefore, the long steel fibres were more efficient in enhancing the stiffness compared to the short steel fibres. According to Thomas and Ramaswamy (2006), steel fibresregularly increase toughness. Therefore, the steel fibres can be used in enhancing the toughness of concrete beams.

Conclusion and Recommendation

Steelfibre reinforced concrete can be a tremendous composite material. The researcher has revealed that steel fibres have a significant role in improving the stiffness, energy absorption, and flexibility of the concrete beams. Also, the research provided the preliminary evidence of the improvement on shear behavior of steel fibres with lightweight concrete beams with sufficient satisfactory depths.  However, long steel fibres were more efficient than short steel fibres in load-deflection responses, stiffness, energy absorption, flexibility, flexural resistance, and shear capacity. Therefore, it is recommendable for one to use steel fibres as the first choice. However, in terms of crack width, the short steel fibres were more efficient compared to the long steel fibres. The research also showed that adding the endorsed minimum amount of 0.75% of either short or long steel fibres to lightweight concrete members can efficiently replace stirrups and change the mode of failure from brittle to ductile form just like in standard weight beam. However, using short fibres is not highly recommended in NWC beams to avoid the possibility of ductile shear failures. In essence, steel fibres are recommendable in concrete beams and other concrete members.

References

ACI Committee 318, (2014) “Building Code Requirements for Structural Concrete (ACI318-14)             and Commentary.” American Concrete Institute, Farmington Hills, Michigan, USA,     473pp.

Biolzi, L., &Cattaneo, S. (2017). Response of Steel Fibre Reinforced High Strength Concrete       Beams: Experiments and Code Predictions. Cement and Concrete Composites, 77, 1-13.             doi:10.1016/j.cemconcomp.2016.12.002

Gebreyohannes, E., &Muzeyin, S. (2016). Effect of Aggregate Size and Type on Shear Capacity of Normal Strength Reinforced Concrete Beams. AAU, School of Civil and Environment      Engineering. AAU.

Hamrat, M., Boulekbache, B., Chemrouk, M., &Amziane, S. (2016). Flexural Cracking     Behaviour of Normal Strength, High Strength and High Strength Fibre Concrete Beams,             Using Digital Image Correlation technique. Construction and Building Materials , 678–         692

Imam, M., Vandewalle, L., &Mortelmans, F. (1994). Shear Capacity of Steel Fibre High-  Strength Concrete Beams. Special Publication. 149, 227 – 242.

Nasvik Joe (2012). “How To Use Steel Fibres in Concrete.” Retrieved from             http://www.concreteconstruction.net/how-to/materials/how-to-use-steel-fibres-       inconcrete_o

Thomas, J.,&Ramaswamy, A. (2006). Shear Strength of Prestressed Concrete TBeams with          Steel FibresOver Partial/Full Depth. ACI Structural Journal, 103(3). doi:10.14359/15321