Influence of cellulose nanofibers and metakaolin for sustainable HPC beam-column joints under cyclic loads

ABSTRACT

This research examines the effectiveness of cellulose nanofibers (CNFs) in reinforcing beam-column joints against repeated, reversed stresses (cyclic loading), a crucial factor in earthquake resistance. By exploring various combinations of silica fume and CNF content, the study aimed to develop a high-performance concrete mix. The hypothesis centered on enhancing joint ductility, the ability to absorb energy, through the combined effect of CNFs with varying lengths and volumes alongside silica fume. The results were remarkably positive. When CNFs were incorporated, the joints exhibited significant improvements in multiple areas: deformability (capacity to deform under stress), ductility (energy absorption), and overall ability to dissipate energy during cyclic loading. Furthermore, the CNFs led to a reduction and better distribution of cracks within the joints, while also increasing the load required for initial cracking and the overall load-bearing capacity. These findings highlight the promise of cellulose nanofibers as a reinforcement material for beam-column joints in earthquake-prone structures. Further research can optimize their use with silica fume for broader adoption in sustainable and resilient construction practices.

Keywords:
Cellulose nanofibers; Silica fumes; Metakaolin; Concrete; Cyclic loads

1. INTRODUCTION

Beam-column Joints couplings, which are found in buildings with a framework, are among the most important sites when the building is subjected to devastating lateral pressures, like those induced by an earthquake, these connections are what keep it together and prevent it from falling apart. The designers wanted to prevent a disastrous collapse, so they put a lot of emphasis on making the beam-column couplings as flexible as they possibly could be. When beam-column connections are exposed to reverse cyclic loads, ductile failure can be achieved by making use of stirrups that are closely distanced from one another. However, when stirrups are employed in a manner that is too close together, the flowability of the concrete within the joint, as well as its compaction, is often affected. In circumstances such as these, the use of high-performance concrete (HPC) is strongly recommended [¹,²,³,⁴]. HPC refers to concrete that exceeds the typical levels of strength and durability [⁵]. As a direct consequence of this, flowability is enhanced, segregation is reduced, and joint compaction is made more effective [⁶,⁷,⁸,⁹,¹⁰]. Previous studies have shown that incorporating rubber particles into concrete can boost the material’s impact resistance, as well as its capability to withstand the strain, dissipate energy, and be deformed. A failure brought on by the utilization of rubber particles. The effects of adding silica fumes (SF) to regular concrete columns and then exposing them to cyclic load [⁷]. They discovered that the material’s ductility, toughness, and deformation capacity are all raised by up to 20% SF [¹¹, ¹²]. The risk of mechanical property and shear strength losses must be considered when creating high-performance reinforced concrete (HPRC) mixtures. Many studies found that under certain conditions, concrete’s shear strength decreased when a significant amount of SF is utilized in its composition [¹³,¹⁴,¹⁵]. When compared to traditional concrete, HPC blends often contain fewer coarse particles. The result may be a smooth fragmented surface or the development of a mechanism for poor aggregate interlock. Due to their fragility, beam-column junctions are particularly susceptible to damage from shear forces [¹⁶,¹⁷,¹⁸].

One of the most modern clay innovations is called metakaolin. Calcined clay, or alumina-silicate portland cement, is created by roasting kaolinite to a certain temperature. Incorporating metakaolin into concrete is a viable option. Due to its pozzolanic qualities, it can be used to replace certain elements of cement [¹⁹,²⁰,²¹,²²,²³]. Better mechanical characteristics, less heat generated during the hydration process, less concrete contraction, less concrete permeation, more resilience to chemical impact, and, most importantly, less alkali-silica response leading to denser cementitious materials are just some of the benefits that using metakaolin throughout the concrete. Kaolinite-rich igneous rocks are commonly referred to this as china clay as well as Kaolin. Metakaolin granules are finer than cement ones nevertheless not as thin as those of cementitious materials. It is a powerful puzzolana that quickly undergoes a pozzolanic reaction well with residual calcium hydroxide left over after ordinary portlant cement (OPC) hydrolysis to create the hydrates of calcium silicate as well as calcium alumina silicate hydrate [²³,²⁴,²⁵,²⁶,²⁷,²⁸].

There are several approaches taken to mitigate the negative effects that HPC mixtures have on mechanical properties and shear strength. Adding rubber to concrete might lessen the material’s shear strength, mechanical characteristics, and weight-carrying ability. There is evidence from earlier studies that combining fibers with SF can mitigate this effect. Adding 0.5% cellulose nanofibers (CNF) and 15% shredded rubber to HPC mixes increased the initial fracture load, ultimate load, and energy absorption by 2.6, 1.06, and 2.96 times, respectively [²⁸,²⁹,³⁰,³¹,³²]. To get the intended outcomes, this is done. Unlike HPC compounds, which are devoid of these rubber and fiber additives, HPC compounds are devoid of these elements. Ismail and Hassan researched to find out how changes in the length of the fibers or their proportion in the mixture affected the performance of concrete parts. When a lot of CNFs are utilized to build the beam, its moment capacity improved by 14%. In addition, the beam’s ductility rose by 72% and its hardness multiplied by 2.35. The maximum shear load increases by 2.37 times for CNFs that are 1% longer and by 2.53 times for CNFs that are 1% shorter as compared to rubberized beams without CNFs [³³]. The failure mechanism of rubberized concrete beams is also found to alter from shear failure to flexural-shear failure with longer CNFs and flexural failure with shorter CNFs [³⁴,³⁵,³⁶]. The only known failure mechanism before this point is a shear failure. To put it another way, the shear failure became a flexural-shear failure as the length of the CNF increased. It is shown that longer CNFs increased the ductility and toughness of rubberized concrete beams, albeit to a lesser extent than shorter CNFs did. Although there is undeniable proof of progress being achieved, this is the situation. Yet, there is a significant hurdle to go over when SF and CNF components are combined to enhance the innovative aspects of HPC combinations. The fresh properties of HPC combinations must be improved to get beyond this obstacle. This research is conducted to find if adding fly ash (FA) or metakaolin (MK), two examples of supplemental cementitious materials (SCMs), will improve the consistency of the mixture. Different HPC mixtures with various percentages of CNF and MK with the addition of SFs are tested. The effect of combining different volumes of CCNs and MK with the addition of SFs on enhancing the ductile behavior of the tested joints is investigated. The main parameters are the percentage of CNFs (1%, 2.5%, 3.5%, 4%, 5% by weight of cement), MK (5%, 7.5%, 10%, 12.5%, 14% by weight of cement), SF (2.5%, 5%,7.5%,10%12.5% by weight of cement), (coarse aggregate size (20 mm), concrete type M 60 (HPC) and are denoted as BCJ1CN01MK05SF2.5, BCJ5CN05MK15SF12.5, BCJ3CN3.5MK10SF7.5, BCJ4CN04MK12.5SF10, and BCJ5CN05MK15SF12.5. Comparing and contrasting the fatigue resistance of these samples using external beam-column joints while they are being subjected to reverse cyclic stress is the objective of this study.

2. MATERIALS AND METHODS

During our investigation, we made use of two different types of CNFs (DramiX 3D) and MK. MK as cement replacement materials and SF as an addition for M60 grade of HPC having a w/b ratio of 0.31 based on the above-formulated mix design procedure. Dramix steel fibers’ high aspect ratio, anchor mechanism and tensile strength provide ultimate performance and concrete ductility. It is the perfect alternative to installing mesh or rebar, saving you time, and reducing costs while providing better performing concrete. MK as well as cellulose nanofibers have been combined or utilized separately in a variety of ways as cement substitutes. The initial prototype of CNF had dimensions of 35 mm in length, 0.55 mm in diameter, and an aspect ratio of 65. In every variety of CNF, the tensile strength is 1150 MPa, The density is 7.85 kg/m³, as well as Young’s modulus, approximated 210 GPa. A higher moisture admiXture assisted in accomplishing the necessary flowability of said tested mixes (HRWRA), which is based on polycarboxylate and is identical to ASTM C494 Type F. The diameter of the reinforcing bars employed in the samples put through the tests is 20 mm, while that of the stirrups is 10 mm. The typical yield stress of the steel used in both the bars and the stirrups is 400 MPa.

Figure 1 shows the SEM view of the cellulose nanofiber fiber from a plant throwing light on the fact that it is a cellular fiber. Cellulose nanofiber vegetable stuff is still present in fiber, and it features a scaly crystalline structure. The fibers have been bundling structures with each bundle consisting of several fibrils. The horizontal markings on the fiber surface are explained by this.

Figure 1
SEM micrographs cellulose nanofiber of CNF.

SEM micrographs cellulose surface of an untreated CNF is shown in Figure 2, which confirms the multi-cellular structure. The cross-section of the image represents the walls of the fiber cells along with their lumen. These cellulose nanofibers would include it is going to possess high insulating and absorption qualities. The elimination of some contaminants and dirt from the cellulose nanofiber fiber made the surface appear clearer. Based on the SEM analysis, the nanostructures had a size of 88.67 nm. The SEM image reveals that the particles are present in a spherical shape.

Figure 2
SEM micrographs irregular surface of cellulose CNF.

2.1. Details about the casting as well as the specimen

For BCJ1CN01MK05SF2.5 and BCJ1CN01MK05SF2.5 mixtures to flow well and avoid segregation, 550 kg/ m³ of binder is required, and the proportion between water-to-binder (w/b) needed to be 0.4. To achieve the optimal level of fluidity control, the binder weight consisted of 50% GU Portland cement, 50% FA, as well as 20% MK. BCJ1CN01MK05SF2.5 could include 0.35 percent CNFs, which, when present in higher percentages, led to a decrease in freshness. Table 1 shows testing mixture and mix design. The following is an explanation of each of the produced mixtures:

• Joints A1 through A3 and joints A4 through A5 The mixtures used in these joints are chosen to investigate the impact that varying proportions of SF have on how well beam-column couplings perform under cyclic loading. The A2-A3 joints seem to be more movable when contrasted with the A4-A5 joints. Also, the mixtures employed in such joints are selected to allow for the investigation of the impact of a larger coarse maximum particle size upon the effectiveness of the connections that are evaluated.

• A8 combined with A2 for the comparison. The effect of incorporating CNF into BCJ1CN01MK05SF2.5 is investigated using these two mixtures, which are chosen for the study.

• The joint A4 is higher than the A3. To investigate the influence that different types of concrete have, these two mixtures are chosen (BCJ5CN05MK15SF12.5 compared with BCJ1CN01MK05SF2.5).

• Comparing A5 with A4 as a joint effort, such two mixtures are chosen to study the impact of varying the fiber length (CNF25 compared with CNF15).

• A5 combined with A1 is compared. These two mixtures are chosen so that an investigation could be conducted into the effect that optimizing the proportion of SF and CNFs would have upon the evaluation of beam-column joints.

Table 1
Testing mixture and mix design.

UNIT	MIXTURE ID	CEMENT (kg/m3)	SCM (kg/m3)	CA (kg/m3)	FA (kg/m3)	SF (kg/m3)	HRWRA (l/m3)	DENSITY (kg/m3)
A1	BCJ1 CN01MK05SF2.5	245	MK (100) + SF (150)	590.67	570.67	1.2	3.45	2256.4
A2	BCJ2 CN2.5MK7.5SF05	245	MK (100) + SF (150)	590.67	650.5	38.3	2.78	2276.5
A3	BCJ3 CN3.5MK10SF7.5	245	MK (100) + SF (150)	590.67	674.6	54.3	3.65	3452.2
A4	BCJ4 CN04MK12.5SF10	245	MK (100) + SF (150)	590.67	598.7	47.45	3.45	2278.6
A5	BCJ5 CN05MK15SF12.5	245	MK (100) + SF (150)	590.67	540.4	63.7	3.97	2176.7

Each tested mixture and joint is identified by its concrete type (HPC or BCJ5CN05MK15SF12.5), SF percentage, coarse aggregate size, and either CNF percentage or CNF length. An example label would read HPC-15SF-0.35CNF35 if the specimen included 15% SF and 0.35% CNF15. These joints are created to break in flexure whilst displaying ductile behavior to comply with CSA regulations. The BCJ1CN01MK05SF2.5, as well as BCJ1CN01MK05SF2.5 joints, proved capable of moving under the weight as well as approaching the other side without really being reverberated or compressed as they are cast from with a single route. Every specimen is cast, dried for 24 hours, wetted, covered using plastic over 7 days, and would then allowed to air-cure through the day of the testing. Both the specimen sizes and the reinforcements are shown in Figure 3.

Figure 3
Test setup.

2.2. Analyses of mechanical and freshness attributes

To evaluate the flowability of the mixtures, both slump flow and V-funnel timings are performed. Comparing the diameters of the slump and the J-rings is how the L-boX test determined whether or not the subject passed the qualification. To ensure accuracy, the tests are carried out following European Guidelines. Tests for compressive strength (ASTM (2011a) C39/C39M) and splitting tensile strength (STS) (ASTM (2011b) C496/C496M) are conducted, respectively, on cylinder-shaped specimens that are 200 mm tall and 100 mm in diameter. The curing conditions for these cylinders are the same as those used on the joints that are being examined. Table 2 displays the findings for all of the materials that are put through their paces in terms of their fresh and mechanical ­qualities.

Table 2
Properties of fresh mixtures that have been tested.

UNIT	MIXTURE ID	TIME (sec)	H2/H1 (L – BOX)	DIA. OF SLUMP J RING	FUNNEL V	% OF SR	% OF AIR	DAYS (28)	DAYS (28) STS	STD. DEV.	STD. DEV. STS
A1	BCJ1 CN01MK05SF2.5	1.56	0.34	20	6	2.3	1.78	76.67	4.76	0.12	0.03
A2	BCJ2 CN2.5MK7.5SF05	2.76	0.67	35	10.4	4.23	4.23	56.98	3.87	0.34	0.04
A3	BCJ3 CN3.5MK10SF7.5	2.87	0.76	40	12.6	6.98	4.32	39.76	4.12	0.4	0.06
A4	BCJ4 CN04MK12.5SF10	1.45	0.95	46	6.98	5.98	5.34	48.67	3.58	0.56	0.03
A5	BCJ5 CN05MK15SF12.5	1.53	0.34	47	4.87	7.98	5.23	48.23	3.87	0.65	0.03

2.3. Setup for testing

Here, we discuss a study that examined the impact that gravity on an experiment involving beams of light. To apply an axial load at the top of the column that is equivalent to 10% of the column’s carrying capacity, a hydraulic jack is used. Following the recommendations provided by the ACI, a reverse model cyclic quasi-static load is provided, and the displacement is monitored. Figure 4, which may be seen by following this link, depicts the test setup that is used in the inquiry. The displacement is applied in a series of successive load stages until it is no longer possible. At a frequency of 0.08 Hz, each load step went through three complete cycles in the opposite direction. Both Figure 4 and Table 3 illustrate the displacement sequence that is used. This particular pattern of loading is chosen so that there would be a steady increase in displacement that is neither excessive nor insufficient up until the point where the specimen broke. The LVDTs are used to measure the beam’s curvature at both the beam’s extremities and its zenith to provide an accurate reading. Crack width is analyzed using a crack microscope after each step of the displacement load. Cracks are marked after each phase of the load (60 × magnification with a lower value of 0.02 mm).

Figure 4
The order of the displacements applied.

Table 3
Sequence of applied displacement.

SETUP LOAD	NO. OF CYCLES	AMPLITUDE (mm)
1	4	0.34
2	4	1.92
3	4	2.65
4	4	3.13
5	4	4.76

3. RESULTS

3.1. Deflections of the load curves

The pace at which the material’s strength degrades when it is subjected to its ultimate load is slowed down when SF is present, which increases the likelihood that ductile failure will occur. Both the random placement of steel reinforcements and careless geometry are considered to be potential culprits. The envelope among joints load-tip deflection curves having varying percentages of SF are depicted in Figure 5. The lowering branch among the curve of envelop load-deflection corresponds to 90% of the ultimate load, which can be found in Table 3. It is discovered that an SF of 15% in the coarse aggregate mixtures allowed for the achievement of maximum and ultimate deflection (A1-A3). It is essential to keep in mind that the elasticity of SF raises the malleability of the concrete matrix, which may result in a greater degree of joint deflection.

Figure 5
Envelopes of load deflection for joints with varying SF contents in coarse aggregate mixtures.

When the size of the coarse aggregate is 20 mm in joints with the same percentage of SF, there is a modest decrease in the amount of deflection at the ultimate load as well as the ultimate deflection. There is a possibility that the mixture’s mechanical characteristics may suffer as a result of the increased volume among the interfacial zone of cement mortar as well as the higher size of coarse aggregate (see Table 2).

When CNF35 is utilized in a BCJ1CN01MK05SF2.5 mixture that also contained 15% SF, deflection tips on ultimate load as well as the ultimate deflection are both improved by 8.8% and 5.8%, respectively (A3 compared with A2). The transition from BCJ1CN01MK05SF2.5 to BCJ5CN05MK15SF12.5 led to a little increase in the final deflection as well as the deflection at the ultimate load (A4 vs A3). Figure 6A, 6B and Table 3 demonstrate that there is not a significant difference in the levels of SF and CNF that may be achieved by employing 60 mm CNFs as opposed to 35 mm CNFs.

Figure 6
For the tested joints, the load deflection (A) A4 and (B) A3 curve under hysteresis loading conditions.

3.2. Deterioration of stiffness

In the elastic region, the initial stiffness is calculated by dividing the peak force felt during the first cycle of loading by the resulting displacement. This is done so to arrive at the value for the initial stiffness. The rise in the number of repetitions had quite a negative impact on the stiffness of such beam-column joints since microcracks began to form and spread. Combinations with either a coarse aggregate measuring 20 mm experienced an early stiffness drop of 27.7% whenever the SF fraction is raised from 10% to 25%.

After thirty cycles of loading, the stiffness of beam-column connections made of different types and lengths of concrete begins to decrease. Figure 7 and Table 4 and Table 5 demonstrate that the addition of 35 mm CNFs results in an increase of 11.6% in the initial stiffness of the BCJ1CN01MK05SF2.5 mixture that is made with 15% SF (A3 compared with A2). The joint that contained CNFs is able to resist a larger load in comparison with the joint that did not contain CNFs.

Figure 7
Envelopes of load deflection for (A) joints of varying CNF lengths and (B) concrete types maximized proportion of CNFs and SF.

Table 4
Effects of cyclically loading in reverse.

UNIT	MIXTURE ID	CEMENT (kg/m3)	SCM (kg/m3)	CA (kg/m3)	FA (kg/m3)	CRACK WIDTH
						THE INTERFACE OF THE COLUMN BEAM (mm)	PANEL JOINT (WITHIN)
A1	BCJ1 CN01MK05SF2.5	43.34	198.0	B	14.87	6.98	–
A2	BCJ2 CN2.5MK7.5SF05	23.87	56.87	B	12.76	4.87	–
A3	BCJ3 CN3.5MK10SF7.5	21.6	56.9	BJ	8.96	3.98	0.83
A4	BCJ4 CN04MK12.5SF10	24.87	27.56	B	12.76	6.32	–
A5	BCJ5 CN05MK15SF12.5	31.87	45.8	B	13.87	5.34	–

Table 5
Measures of yield strength, ultimate strength, ductility and brittleness.

UNIT	MIXTURE ID	YIELD DEFLECTION (mm)	LOAD ULTIMATE (mm)	DEFLECTION ULTIMATE (mm)	DUCTILITY	INDEX BRITTLENESS
A1	BCJ1 CN01MK05SF2.5	6.89	23.87	30.2	5.98	0.34
A2	BCJ2 CN2.5MK7.5SF05	7.34	26.98	34.2	6.98	0.23
A3	BCJ3 CN3.5MK10SF7.5	6.98	24.98	38.97	4.98	0.43
A4	BCJ4 CN04MK12.5SF10	5.98	27.56	34.8	3.98	0.45
A5	BCJ5 CN05MK15SF12.5	4.98	21.98	35.98	2.87	0.56

3.3. Failure mechanisms

Four different types of failure could occur when beam-column connections are allowed with reverse-cyclic loading. Failure within the joint shear mode after the beam has reached its yield point is referred to as “BJ-mode,” and it occurs whenever joint shear stress is greater than the joint strength, depicted in Figure 8A and 8B. When the SF percentage is increased to 20%, a transition from B-mode to BJ-mode failure appears to take place, which is followed by the appearance of diagonal cracks. The utilization of relatively high percentages of SF likely led to an excessive drop in the material’s compressive strength. This is one of the possibilities.

Figure 8
Joints with varying SF % in coarse aggregate mixtures (A) and joints with varying coarse aggregate sizes (B) degraded in stiffness during testing.

When the proportion of SF increased to 20%, the failure mode changed from BJ mode - B-mode. Maybe this is because the aggregate interlock between larger coarse aggregate is stronger, hence increasing the joint’s shear capabilities (A3 compared with A5). As can be observed in Table 4, B-mode failure occurred in all CNF-containing joints (A4-A5), although some of these joints are BCJ1CN01MK05SF2.5 and others are BCJ5CN05MK15SF12.5. In addition, the findings showed that a B-mode failure is placed if 25% SF is paired with 1% CNFs (35 mm). The inclusion of the 1% CNFs in A5 helped to halt the fractures, increased the joint shear strength, as well as switched failure to B-mode failure, despite the high percentage of SF (25%). Figure 9 depicts the loss of stiffness in the examined specimens (a) joints with varying CNF lengths and (b) joints with the maximum percentage of CNFs and SF.

Figure 9
Loss of stiffness in the examined specimens (A) joints with varying CNF lengths and (B) joints with the maximum percentage of CNFs and SF.

4. DISCUSSION

It has been demonstrated that the percentage of Cr (SF) in the joint is directly related to the crack patterns found in beams and columns. Joints with larger percentages of SF (A4-A5) demonstrate that the degree of SF causes an increase in the number of cracks that are randomly distributed throughout the joint. Figure 10 provides a visual representation of the cracking pattern, and Table 4 provides information regarding the maximum fracture diameter at the beam-column interface. This is because more rubber particles will be encountered by cracks as they spread if the SF % is raised. As a result of rubber’s pliability and resistance to high tensile deformation, carbon black (SF) particles have the potential to be an effective crack repair agent. As the proportion of SF increased, there is a corresponding decrease in the fracture beam-column width contact and the interface with the metal.

Figure 10
The effect of the percentage of SF, and the volume and length of the CNFs on the first crack load, the ultimate load, the ductility, and the brittleness of the material, as well as the energy dissipation.

4.1. Ductility

The displacement ductility of the beam-column junctions improved due to the material’s increased compressive strength, anchorage strength, and bonding capacity. At 90% of the ultimate load, the permissible displacement is defined as the load-tip deflection that completely encircles the descending branch of the curve and reaches the horizontal line. This information is revealed by the equation. Because of its high deformability and low stiffness, SF has an improved strain capacity and deformability in beam-column joints. The substance plays a role in this. It is determined that reaching the maximum allowable SF%, which is 20% in mixtures with coarse aggregates of 20 mm, increased displacement ductility. The ideal percentage for SF allowed us to make this observation. For instance, overall displacement ductility for 20 mm coarse aggregate blends is improved by 6.8% by raising the SF between 10% to 20%. When comparing the mixture with 20% SF to a higher SF percentage (A2), the displacement ductility decreases, as shown in Figure 10. When the coarse aggregate size of 20 mm showed that the displacement ductility decreased to some degree. It is possible that the decreased shear strength of the joints is a contributing factor in this, as this decreased the joints’ capacity to deform as well as changed the failure mode from B-mode to BJ-mode.

The addition of CNFs to the BCJ1CN01MK05SF2.5 mixture with 15% SF led to a considerable improvement in the joint’s displacement ductility, which ultimately reached as high as 22.4%. CNF bridging seems to be a likely contributor toward the joint’s increased ultimate load and large displacement further than the yield point. If so, this would be one possible explanation. According to the findings, shifting from BCJ1CN01MK05SF2.5 to BCJ5CN05MK15SF12.5 provided a slight improvement in displacement ductility as well. BCJ5CN05MK15SF12.5 is used instead of BCJ1CN01MK05SF2.5.

4.2. Brittleness

Elastic energy output before failure divided by plastic energy capacity at failure is a good example of what is meant by the term “brittleness index” (BI), which is shown in Figure 10 Because concrete is a brittle material, it is important to stress that its BI value needs to be greater than that of ductile materials. According to the findings, the BI went down with the proportion of SF rising to the optimum limit within combinations using coarse aggregates that are either 20 mm in size. Whether the total size is measured in mm or inches, this is true. By increasing the SF content from 10% to 20% in combinations that included coarse aggregate that is 20 mm in size, it is possible to reduce the BI by 11.1%. (A4-A5). On the other hand, a larger proportion of SF (more than 20%) produced a higher BI value (in comparison to a join with 20% SF). It’s possible that the same mechanism at play in the ductility section also contributes to this behavior. In addition, the BI of tested joints increased somewhat when the size of the coarse aggregate slightly decreased ductility.

The BI outcomes in junctions are evaluated with varying CNF lengths/volumes as well as various kinds of concrete (A3-A5). When compared to the mixture that did not contain CNFs, the value of the BI decreased by 21.7% when CNFs are paired with 15% SF. This is in comparison to the mixture that did not contain CNFs. According to the findings, there is a little drop in the BI value brought about by switching from BCJ1CN01MK05SF2.5 concrete to BCJ5CN05MK15SF12.5 concrete while maintaining the same ratios of CNFs to SF throughout the mix. It’s for the same reason that ductility is important, thus this is included here as well. Joints with no CNFs exhibited a lower BI value (higher ductility) although longer CNFs (60 mm) are used compared to shorter CNFs (35 mm) (A5 compared to A4) (A5 compared with A2). Moreover, Figure 10B shows that the BI value is reduced by as much as 41.4% when the highest percentages of SF (25%) and CNFs (1%), respectively, are applied to the comparison between the test joint (A5) and the control joint (A1).

4.3. Dissipation of energy

Both elastic and post-elastic energy can be dissipated by the structure; the former is represented by viscous damping energy, and the latter by hysteretic damping energy. In Table 6 and Figure 10C, the amount of dissipated energy for joints assessed with varying SF% and coarse aggregate size is displayed for each load step as well as for the entire evaluation. According to the data that is shown in the table, the hysteretic energy dissipation capacity rose by as much as 20% in combinations that included a coarse aggregate of 20 mm. For example, raising the SF in mixture proportions containing coarse aggregate with a size of 20 mm from 10% to 20% led to a 10.3% improvement in the mixture’s ability to disperse energy. (A4 opposite A5) This is because SF particles aren’t very rigid, so they bend and stretch more easily and lose energy as a result. The reduced shear strength that resulted from employing a high fraction of SF (more than 20%) in mixture joints is the cause of the transition from B-mode failure to BJ-mode failure. This change is attributed to the use of a high proportion of SF. The capability for energy dissipation is reduced when the dimension of the coarse aggregate is increased from 10 to 20 mm. Comparing A5 and A2, for example, reveals that the energy dissipation capacity fell by 12.2%; this occurred despite there being no change in the proportion of SF or the failure mode (B-mode). It’s for the same reason that CNFs impact ductility and BI, which we’ve already discussed. Energy dissipation is also found to be greatly improved by 6.5% when BCJ1CN01MK05SF2.5 is replaced with BCJ5CN05MK15SF12.5 concrete. A4 is superior to A3 in every way. When compared to BCJ1CN01MK05SF2.5 joints, BCJ5CN05MK15SF12.5 joints are capable of withstanding a greater ultimate load and deflection. The bigger the number of fibers that are dispersed throughout the mixture at a given CNF volume, the higher the probability that the fibers will be aligned in a direction that is perpendicular to the fissures. When a high percentage of CNFs and SF are utilized, Joint A5’s energy dissipation capability rose by 53.6% in comparison to that of the control joint A1.

Table 6
Total energy dissipation.

UNIT	A1 ED (kN.mm)	A2 ED (kN.mm)	A3 ED (kN.mm)	A4 ED (kN.mm)	A5 ED (kN.mm)
1	0.35	0.64	1.96	1.32	0.23
2	1.76	2.45	4.56	3.56	5.67
3	6.98	14.98	16.9	24.09	26.98
4	27.9	34.9	12.98	26.98	27.09
5	45.87	65.98	34.98	12.98	34.87
6	110.8	234.8	32.98	108.5	12.65
7	1675.98	345.9	456.9	376.9	456.98
8	4356.9	456.8	1324.98	1567.5	4567.9
9	7545.9	4568.0	6508.9	8762.0	5677.9
10	12456.97	14563.97	14569.76	1687.94	12347.76

4.4. Initial stress and failure stress

Table 4 and Figure 10E illustrate the first crack load and ultimate load (A1-A5) for beam-column joints exposed to reverse cyclic stress. These tables and figures provide the results of an analysis. The initial fracture load, as well as the ultimate load, are both reduced when the quantity of SF that is present is increased. The utilization of larger coarse aggregate (twenty mm) in beams with a higher SF percentage (twenty percent) led to an improvement in the joint shear strength as well as a change in the mode of failure. Joints subjected to BCJ1CN01MK05SF2.5 and BCJ5CN05MK15SF12.5 testing exhibit the initial fracture load and ultimate load in Figure 10F and Table 4, respectively. A comparison of joints with and without CNFs (35 mm) reveals that using CNFs appears to considerably rise within first fracture load and load-carrying capability (A8 compared with A2). This is because CNFs delay the beginning of interior micro fractures and cause the cracks to take a winding route, increasing the energy needed for further crack propagation and hence the load-bearing capacity. Both the initial fracture load and the ultimate load in BCJ5CN05MK15SF12.5 joints exhibited behavior that is comparable to that of BCJ1CN01MK05SF2.5 joints (A4 compared with A3). When compared to shorter CNFs, longer CNFs (60 mm) provided a little reduction within the first fracture load as well as the load-carrying capacities (35 mm). The joint’s load-carrying capacity is decreased due to the large amount of SF (25%) that is present in A5 (A1). When 1% CNFs are added to A11, the ductility, BI, deformability, and energy dissipation are all significantly improved.

5. LIMITATIONS AND APPLICATIONS

While cellulose nanofibers (CNFs) and metakaolin (MK) offer promising advantages for sustainable HPC beam-column joints under cyclic loads, there are certain limitations to consider:

5.1. Cost

CNFs are a relatively new material, and their production process can be expensive compared to conventional fibers like steel or glass. This can significantly increase the overall cost of the concrete mix.

5.2. Workability

A high content of CNFs can significantly increase the mix viscosity, making it more difficult to handle and pour. This can lead to challenges during construction and pumping, requiring optimizations in the mix design to achieve a balance between workability and desired properties. Techniques like using high-range water reducers or optimizing the CNF content can be explored.

5.3. Long-term performance

The long-term performance of CNFs in concrete, particularly their bond durability with the cement matrix, is still under investigation. There is limited data on how CNFs behave over extended periods under real-world conditions. More research is needed to ensure their effectiveness over the entire lifespan of the structure.

5.4. Compatibility

Optimizing the mix design for CNF- and MK-incorporated concrete might require adjustments compared to conventional HPC mixes. The absorption characteristics of CNFs, along with pozzolanic activity of MK, can influence setting time and workability. Finding the right balance for desired properties requires careful consideration.

5.5. Ongoing research

Further research is needed to optimize the use of CNFs and MK in concrete, including their long-term performance and assurance of consistent quality control measures during production.

6. SUMMARY AND CONCLUSION

In this analysis, we saw how beam-column junctions fared when subjected to cyclic loads in the other direction. Stiffness, stiffness degradation, energy dissipation, load-carrying capability, and brittleness index are some of the criteria that have been evaluated for joints. Multiple characteristics, one of which is ductility, are assessed to highlight the effect that coupling SF with different amounts of CNF of variable lengths, as well as volumes, had on the structural nature of the joints that are examined. A wide range of SF percentages, CNF lengths, and CNF volumes is utilized in the construction of the joints. The following inferences can be made in light of the experimental findings presented in this work:

• The research concluded that, for mixtures having 20 mm coarse aggregate, 20% SF is ideal, equivalent to the load-bearing capacity of 20 mm fine aggregate. This enhanced brittleness index, ductility, and energy dissipation as well as, deformability.

• As the SF is increased beyond the optimal level (15 percent for joints with 10-millimeter coarse aggregates and 20 percent for joints with 20-millimeter coarse aggregates), The change from B-mode to BJ-mode failure has detrimental effects on the initial crack load, carrying capacity, ductility, BI, deformability, and energy dissipation.

• Due to the increasing aggregate size’s shear strength, joint failure mode changed from BJ-mode to B-mode when the SF is increased to 20%. This improved beam-column junction performance.

• The incorporation of CNFs into BCJ1CN01MK05SF2.5 and BCJ5CN05MK15SF12.5 beam-column joints helped facilitate an improvement in the load-carrying capacity, deformability, ductility, energy dissipation, initial stiffness, cracking behavior, and first crack load. Increases of 11.1% in initial stiffness, 42.2% in first crack load, 15.6% in load-carrying capacity, 22.4% in ductility, and 50.5% in energy dissipation are seen when 0.35% CNFs (35 mm) are added to BCJ1CN01MK05SF2.5 mixture with 15% SF.

• 60-mm CNFs have the lower load-carrying capability, deformability, ductility, and energy dissipation than shorter ones. BCJ5CN05MK15SF12.5 mixtures utilizing CNFs performed similarly to BCJ1CN01MK05SF2.5s but are more susceptible to temperature and humidity fluctuations than those using short CNFs - for example, at temperatures as low as 20 °C.

• CJ5CN05MK15SF12.5 mixtures improved beam-column junctions and changed the failure mode from BJ-mode to B-mode, minimizing the load-bearing capacity reduction caused by high SF and no CNFs.

7. ACKNOWLEDGMENTS

Thanks for Nooorul Islam university providing lab facilities to this research.

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Publication Dates

• Publication in this collection
  29 July 2024
• Date of issue
  2024

History

• Received
  05 Apr 2024
• Accepted
  07 May 2024