Enhancing concrete properties with bamboo and jute fibers: a response surface methodology approach

ABSTRACT

The Design of Experiment (DOE) technique was used to assess the impact of factors such as silica fume, bamboo fibers, and jute fibers on concrete strength. The Box–Behnken design of Response Surface Methodology (RSM) identified the optimal combination of variables and their effects on split tensile and compressive strength at 14 and 28 days. Pareto charts and Analysis of Variance (ANOVA) were used to analyze regression models for these responses. In this study, the jute and bamboo fibers with Silica Fume mixed concrete (each 0.5%) provides the maximum compressive strength of 30.27 MPa and split tensile strength of 3.19 MPa after 28 days of curing. After determining each progression variable’s statistical significance, second-order polynomials were used to create the resulting models. The quality of concrete strength was increased by adding bamboo and jute fibers along with silica fume and further addition of these fibers may reduce the strength of the concrete. The Response Surface and Pareto chart recommended the most significant and influential element for spilt tensile strength is jute and bamboo fibers, and for compressive strength is a jute fiber. Regarding split and compressive strength, the validation test percentage error is less than 3% and 4%, respectively.

Keywords:
Silica fume; Jute and bamboo fibers; Concrete; Strength analysis; Response surface analysis

1. INTRODUCTION

In the building sector, there is an increasing need for concrete reinforced with sustainable resources such as natural fibers. The use of natural fibers in concrete guarantees its increased strength while having no negative environmental effects. To accomplish this goal, numerous researchers have employed natural fibers as a highly efficient reinforcing material. These natural fibers contribute significantly to the concrete’s capacity for high-energy absorption, resistance following cracking, and heightened fatigue resistance [¹]. The most common ways to address the lack of fiber-induced concrete are using natural fibers and non-natural polymer-based fibers. The drawbacks of artificial fibers are their high cost and potential risks to human health and the environment [²]. Concrete reinforced with natural fibers, such as jute and bamboo, features small-diameter, randomly dispersed fibers within the concrete [³]. This approach offers notable benefits in terms of energy efficiency, resource conservation, cost-effectiveness, and environmental preservation [⁴]. Cement composites may be strengthened by adding natural fibers like jute, hemp, coconut, sisal, and bamboo, which are often used in them. This incorporation reduces crack propagation and simultaneously bolsters the material’s mechanical properties [⁵]. The addition of natural fibers to the cement matrix improves the material’s mechanical qualities, impact resistance, resistance to fracture propagation, and energy absorption capacity [⁶]. Research has demonstrated that the inclusion of plant-based natural fibers significantly enhances concrete’s mechanical properties, toughness, crack behavior, impact resistance, and strain capacity [⁷]. Add a comparison of natural fibers (jute and bamboo) and hybrid polypropylene fibers. Include a brief mention of the benefits of hybrid polypropylene fibers in reducing shrinkage cracks and enhancing mechanical properties. Connect this to the goal of the study: exploring the potential of natural fibers for similar benefits [⁸]. The combined utilization of steel and silica fume fibers in concrete has notably heightened its mechanical properties, although this collective effect leads to a decrease in the material’s elastic modulus. Furthermore, the potential of employing silica fume alongside sisal fibers to bolster concrete strength has been explored, revealing increased mechanical properties in both M30 and M40 concrete compositions, as indicated by test results [⁹].

Introducing short and low jute fiber into concrete enhances its mechanical properties, particularly when used with higher cement content. The superior qualities of jute fibers find extensive applications across sporting industries, marine, automotive, and aerospace sectors due to their exceptional properties [¹⁰]. Ten percent rice husk ash and five percent 50 mm length by five mm diameter jute fibers were added to concrete to improve its mechanical properties and impact resistance. Concrete that contains 1% bamboo fibers has improved mechanical qualities such split tensile strength and compressive strength by 17% and 22%, respectively; Epoxy-infused bamboo composites are more attractive for structural material applications because they improve concrete’s strength qualities when compared to untreated fibers [¹¹]. Adding natural fibers like jute and bamboo to concrete is a part of the broader field of fiber-reinforced concrete (FRC), which aims to improve certain properties of concrete, including tensile strength, ductility, and crack resistance [¹²]. Both jute and bamboo are considered sustainable, cost-effective, and environmentally friendly alternatives to synthetic fibers. When these fibers are added to concrete, they can enhance its tensile strain capacity to some extent, but the overall effectiveness depends on several factors such as fiber content, aspect ratio (length to diameter ratio), and the bonding between the fibers and the concrete matrix [¹³]. Combining silica fume and natural fibers in concrete offers several practical applications and potential commercial viability due to the enhanced properties they impart to the concrete mix. While silica fume is a byproduct of silicon metal production and may add to the initial cost of concrete, the improved performance and durability can lead to long-term cost savings by reducing maintenance, repair, and replacement expenses. The practical applications and commercial viability of using silica fume and natural fibers in concrete offer significant opportunities for improving construction practices, enhancing durability, and meeting the evolving needs of the construction industry.

The process of the response surface, a technique for optimization called box-Behnken design combines experimental and mathematical models. Researcher utilized to test both the quantity of experimental cycles and the adaptability of models [¹⁴]. Cement has a more significant impact than other factors, according to the analysis of variance, because of its larger linear effect [¹⁵]. In generating a suitable empirical model, the prediction of optimal performance for minimizing flexible pavement failure in an asphaltic mixture is effectively accomplished through the use of a response surface approach [¹⁶]. When compared to response surface models deployed onshore, those created to assess the steel towers’ dependability index exhibit the appropriate level of accuracy [¹⁷]. Response surface methodology (RSM) and artificial neural networks (ANNs) are valuable techniques for predicting the compressive strength of concrete under compression, particularly when employed in three-variable modelling procedures [¹⁸]. Comparing the RSM model to other models such as the double exponential model, the power-power model, the logarithmic model, and the bilinear model, it has been found that nondestructive testing may accurately predict compressive strength [¹⁹]. After validating with experimental data, it was found that the experiments designed for Geopolymer concrete using the Response Surface Methodology’s Box-Behnken design showed a deviation of 2.34%. The relationship and certainty between the response and the parameters were provided using the response surface models for compressive strength after 7 days, flexural strength after 28 days, and both. The study’s findings highlight the paucity of research on the combined effects of silica fume, bamboo, and jute fibers on the mechanical qualities of concrete.

Examining the impact of varying ratios of bamboo, jute fibers, and silica fume on the split and compressive strength of concrete and its variations in characteristics was the main goal of this investigation, which was conducted 14 and 28 days following solidification. Using the response surface method, the mechanical characteristics of natural fiber-reinforced concrete added with fibers produced from jute and bamboo may be predicted. The design of experimentation (DOE) method is used to create concrete that has the ideal amounts of bamboo, jute, and silica fume. The DOE approach can be used to investigate how independent factors affect experimental outcomes [²⁰]. DOE allows for the optimization of the test variables and yields an ideal response for the experimental data by establishing a link between the independent variables and the empirical model [²¹]. It is possible to use the statistical and mathematical Design of Experiment (DOE) approach, especially response surface methodology, to assess how autonomous variables affect results while conducting the fewest number of tests [²²]. The broad utilization of concrete technology is attributed to its effectiveness in producing accurate outcomes. The Box–Behnken design (BBD) was employed alongside Response Surface Methodology (RSM) for statistical analysis to identify the optimal combination of crucial factors of jute fibers, silica fume, and bamboo fibers. The study aimed to understand how these progressive variables influenced split tensile and compressive strength. The surface behavior was analyzed for concrete’s compressive and tensile strength using response surface analysis and other DoE approaches.

2. MATERIALS AND METHODS

2.1. Materials

The materials used in this study were cement, fine aggregate, coarse aggregate, water, bamboo fiber, jute fiber and silica fume. Ordinary Portland cement (OPC), potable water, fine aggregates (between 5 and 10 mm), and coarse aggregates (between 10 and 20 mm) were the elements used to make plain concrete. The same chemicals were added to Jute Fiber (JF) and Bamboo Fiber (BF) to create fiber-reinforced concrete. These fibers were bought from regional Indian vendors. Table 1 displays the physical properties of the BF and JF that were employed in the concrete mix. Specific gravity and 30 minutes of first setting time are characteristics of Ordinary Portland Cement of grade 53, which meets Indian code standard IS 12269:2013 [²³]. Incorporating riverbed sand with a specific gravity of 2.69 from zone III, the mix included crushed stone coarse particles measuring twenty millimeters, following the specifications in IS 10262:2019 [²⁴]. Concrete is mixed with dark grey, 2.2 specific gravity powdered silica fume. The concrete specimen is prepared using untreated locally specified bamboo fibers with a diameter of 1 mm and jute fibers with a diameter of 0.2 mm, as shown in Figure 1.

Table 1
Mechanical characteristics of jute and bamboo fibers.

TYPE OF PROPERTY	BAMBOO FIBERS	JUTE FIBERS
Length in mm	40	40
Diameter in mm	0.9	0.2
Tensile strength in MPa	410	310
Density in kg/cm3	0.5	1.39
Aspect ratio	45	245

Figure 1.
Materials used for this study (a) bamboo and (b) jute fibers.

The M25 grade has been used to construct the concrete mixture, per IS10262:2019 [²⁴]. With a water-to-cement ratio of 0.48, the percentage of the concrete mixture is 1:1.61:3.04. To create fiber-reinforced concrete (FRC), silica fume with concentrations ranging from 0% to 10% by cement weight and 50 mm-long jute and bamboo fibers were taken into consideration for assessing the characteristics of concrete. Before being mixed with concrete, jute and bamboo fibers are air dried for thirty minutes after being submerged in water for twenty-four hours to create fiber-reinforced concrete. Fibers can be evenly dispersed when introduced in layers during mixing to avoid a balling effect. Moreover, all mixes maintain the same amount of water, fine and coarse aggregate, and both. Concrete cubes and cylinders with dimensions of 150 × 150 mm and 300 × 150 mm, respectively, were cast to meet the parameters. By IS 516:1959 [²⁵], the split tensile strength of the cylinder specimen and the compressive strength of the cast cube specimen were measured at 14 and 28 days after curing. Testing of spilt and tensile strength of concrete was done by using 1000 KN of UTM and CTM testing machines. As per ASTM C150 [²⁶] requirements, Type II Ordinary Portland cement was used in this investigation. According to ASTM C 128-07 [²⁷], ASTM C 29-09 [²⁸], and ASTM C 127-07 [²⁹] standards, Table 2 describes the properties of coarse and small particles found in nature. Coarse particles were created using crushed rocks that came from Isfahan City, Iran; the biggest diameter was 19 mm. The grading of coarse aggregate and natural aggregate is shown in Figure 2.

Table 2
Properties of Type – II Portland cement.

CHEMICAL PROPERTIES	VALUE IN %	PHYSICAL PROPERTIES	VALUE
SiO2	25	Absorption of water	–
Al2O3	7	Density	3048 kg/m3
Fe2O3	6	Surface area	310 m2/kg
CaO	65	Autoclave Expansion	0.1%
MgO	0.4		26% in 3 days
K2O	0.9	Compressive Strength	30 MPa in 7 days
Na2O	0.6		44 MPa in 28 days
SO3	1.3	Setting time	90 min (Initial)
Loss of Ignition	1.1		150 min (Final)
C3S	52	–	–
C2S	18	–	–
C3A	8	–	–
C4AF	10	–	–

Figure 2.
Size distributions of (a) fine and (b) coarse aggregate.

2.2. Concrete mix design

The parameters of the concrete mixes examined in the experimental investigation are shown in Table 3. Fifteen concrete mixtures were made, consisting of one control sample (CS) and fourteen samples with silica fume (SF), bamboo fiber (BF), and jute fiber (JF). With a water-to-cement ratio (W/C) of 0.40, the control sample (CS) contained 1755 kg of aggregate and 450 kg of cement. Samples including BF, JF, and SF were made using different fractions, and 1% to 5% of SF was added to each mix according to the weight of cement. In every instance, the addition of SF occurred shortly after the addition of water, but the addition of BF and JF occurred once the concrete mixture was prepared. To guarantee equitable dispersion in the concrete mix, the mixture was stirred for five minutes. A decrease in the workability of the mixture was noted as the amount of Jute Fiber (JF) and Bamboo Fiber (BF) increased. Several variables contribute to this decrease, one of which is the fibers’ increased surface area, which increases the viscosity of the concrete and causes it to clump together around the fibers by absorbing extra hydration water. In addition, the high aspect ratio of the fibers may cause them to cluster and obstruct the flow of concrete. Table 3 displays the nomenclatures SFBJ1 through SFBJ15, which correspond to the concrete mixes that incorporate Bamboo Fiber (BF), Jute Fiber (JF), and Silica Fume (SF). At 28 days of curing age, a total of 15 concrete samples were ready for split tensile and compressive testing. Each sample was prepared in three layers, and to release trapped air, each layer was vibrated on the vibrating table for 15 to 20 seconds. After removing any extra concrete, the top surface was smoothed and given time to settle at room temperature. Samples were de-moulded after a day and allowed to cure for a total of 28 days at 22°C and 100% relative humidity in a water bath.

Table 3
Mix proportion of concrete used for experimental studies.

TYPE OF MIX	CEMENT (kg/m3)	CA (kg/m3)	FA (kg/m3)	LENGTH OF FIBER (mm)	SILICA FUME (%) (X1)	JUTE FIBERS (%) (X2)	BAMBOO FIBERS (%) (X3)
SFBJ 1	360	1215	643	40	10	0.5	0
SFBJ 2	400	1215	643	40	0	1	0.5
SFBJ 3	400	1215	643	40	0	0.5	1
SFBJ 4	400	1215	643	40	0	0	0.5
SFBJ 5	380	1215	643	40	5	1	1
SFBJ 6	380	1215	643	40	5	0	1
SFBJ 7	380	1215	643	40	5	1	0
SFBJ 8	400	1215	643	40	0	0.5	0
SFBJ 9	360	1215	643	40	10	0	0.5
SFBJ 10	380	1215	643	40	5	0.5	0.5
SFBJ 11	380	1215	643	40	5	0.5	0.5
SFBJ 12	360	1215	643	40	10	1	0.5
SFBJ 13	380	1215	643	40	5	0.5	0.5
SFBJ 14	380	1215	643	40	5	0	0
SFBJ 15	360	1215	643	40	10	0.5	1

2.3. Testing methods for fresh and harden concrete

In compliance with IS 1624-1986 [³⁰] guidelines, a slump cone workability test was used to assess the concrete’s fresh qualities. The slump cone was filled using a foundation plate to create a pancake-like spread of new concrete. After that, the graded plate was taken out, and the concrete was lifted around 300 mm above the floor. This made it possible to calculate the greatest diameter average and the perpendicular diameter that corresponded to it. For the hardened concrete test, the compressive, split tensile and flexural strength of each concrete mix were conducted. Compressive strength tests were done on 150 × 150 × 150 mm cast cubes after 28 days to determine their compressive strength to avoid segregation and bleeding. Using a computerized Universal Testing Machine (UTM) set at a rate of 1 mm/min, the load was delivered gradually by the usual procedure. When the specimens were 28 days old, they were dried in an oven for 24 hours at 85°C to remove any remaining water before calculating how quickly the mortar samples absorbed water. This guaranteed reliable evaluation of the characteristics and behavior of the specimens.

2.4. Response surface method

When multiple influencing variables impact the outcome variables, the response surface approach becomes a statistical and mathematical method employed for the optimization and development of issues [³¹]. Output parameters are heavily impacted by several factors, The response surface approach facilitates the recognition and successful application of the interactions among a group of autonomous variables. The influence of mix parameters, including bamboo fiber, jute fiber, and silica fume, on the mechanical properties of concrete was investigated using the Box–Behnken design (BBD). The computed answers were the split tensile strength (f_STS14 and f_STS28) and compressive strength (f_cs14 and f_cs28), considering the independent variables as silica fume (X₁), jute fibers (X₂), and bamboo fibers (X₃). Equation 1 can be formulated based on the obtained responses. To delineate variations in the mechanical attributes of concrete can be expressed in equation 2. The combined factors and their functional responses have been evaluated using this model.

$$y=f\left(X_1,X_2,X_3\right)$$ (1)

$$y=k_o+\sum k_i{x}_i+\sum k_i{x}_i^2+\sum \sum k_{ij}{x}_i{x}_j$$ (2)

where k_i, k_ij, k₀, and k_i are the regression coefficients and y is the necessary response variable. The precision of the obtained equation may be ascertained with the aid of the coefficient of determination (R²). Factors and variable levels for each of the four answers under consideration must be supplied in the DOE of RSM autonomous variables. The outcomes of implementing the three-factor Box–Behnken design (BBD) approach on 15 distinct mixes are presented in Table 4. The impact of the addition of bamboo and jute fibers and silica fume and their strength characteristics were discussed in the following experimental works and discussions.

Table 4
Variable levels.

TYPE OF VARIABLES	MAXIMUM	MINIMUM
Jute fiber	1	0
Silica fume	10	0
Bamboo fiber	1	0

2.5. Microstructural analysis of concrete specimens

It is crucial to stress the importance of comparing the experimental results with the findings of microstructural analyses in order to guarantee the precision of the experiments and a thorough analysis of the internal microstructure of the produced mixtures. Using a variety of methods, such as energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and Fourier transform infrared radiation (FT-IR) under various operating conditions, the surface behavior of both regular concrete and concrete mixes containing silica fume (SF) and jute fiber (JF) was examined. To get a better knowledge of the behavior and performance of the concrete, these investigations offer insightful information about its composition, structure, and qualities. In order to increase the mechanical strength of the concrete mixes, the analyses were carried out to look at how well the SFBJ concrete mixes performed and how they interacted with the concrete particles. Based on the findings of each concrete mix’s mechanical strength test, microstructural analysis was done in this study to determine the best mixtures. Understanding the processes behind the improved performance of the SFBJ concrete mixes was obtained by establishing a correlation between the mechanical qualities and the microstructural features. This methodology facilitated a thorough comprehension of the ways in which the microstructure impacts the concrete’s total mechanical strength.

3. RESULTS AND DISCUSSION

3.1. Slump value of the mix

As seen in Figure 3(a), the combinations of mixes including Jute Fiber (JF) and Silica Fume (SF) exhibit outstanding workability. The workability of the mixes rises irrespective of the kind of addition and replacement percentages. As a consequence, a water-to-liquid (w/L) ratio of 0.4 was determined to have reasonable workability; however, a ratio greater than this caused bleeding. This highlights how crucial it is to properly regulate the water quantity to preserve the best workability without jeopardizing the consistency of the concrete mixture. Researchers state that when mixed with water, Jute Fiber (JF) and Bamboo Fiber (BF) usually show friction because of the rise in viscosity during the fermentation process, which reduces flowability [³²]. On the other hand, after fermentation, neither viscosity nor flowability significantly changes for Jute Fiber (JF) or Bamboo Fiber (BF). Consequently, combinations that included Silica Fume (SF) showed better workability than blends made only of BF and JF. It is hypothesized that the addition of both fractionated and frictionless additives BF and JF may have balanced each other out to produce somewhat greater workability than standard combinations. SFBJ11 is unique among the many concrete mixes in that it has a high slump value, which indicates that the concrete is more workable.

Figure 3.
(a) Slump value, (b) compression strength and (c) split tensile strength of concrete mix with SF and fibers.

3.2. Compression strength

The compressive strength of concrete and its strength variations was investigated in this study by adding silica fume with jute and bamboo fibers. Figure 3(b) shows the impact of compressive strength of concrete by adding the above-mentioned materials. Although the mechanical properties of concrete have been improved by the addition of jute and bamboo fibers, these properties decrease as the fiber concentration increases [³³]. Concrete’s compressive strength is very little affected by the addition of fiber; very slight changes have been noted. The test findings show that SFBJ11 has the highest compressive strength, rising over SFBJ01’s starting value by 15.24% and 8%, respectively, at 14 and 28 days. Jute fiber inclusion was seen to create a falling trend in compressive strength because of the low specific gravity and high porosity of the JFRCC relative to the reference concrete [³⁴].

3.3. Split tensile strength

The results of the examination into split tensile strength after 14 and 28 days of curing are shown in Figure 3(c), which also displays fiber-reinforced concrete with silica fume. In comparison with SFBJ01, SFBJ11 exhibits a substantial increase in split tensile strength. Concrete with 0.5% silica fume and each 0.5% of jute and bamboo fibers by weight of cement display improvements of 13.87% and 14.61% after 14 and 28 days of curing. Based on the statistics, there is clear evidence that the utilization of jute and bamboo fibers has a substantial positive impact on split tensile strength [³⁵]. In addition, the uneven distribution of fibers in the material leads to a reduction in split tensile strength for concrete specimens with jute and bamboo fibers when the weight percentage surpasses 0.5%.

3.4. RSM modelling

3.4.1. Examination and discussion of observations

In the present study, Box-Behnken was taken into consideration to understand the effects of components. Bamboo fibers, jute fibers, and silica fume, when included, progressively impact the concrete’s compressive and split tensile strength. Table 5 lists the fifteen experiments that were taken into consideration for each answer. The responses that were discovered were expressed in equations (3) through (6). Figure 4 displays each response’s residual fit for normal probability plots. The distribution of errors is uniform when each response’s residuals almost exactly follow a straight line.

$$f_{cc14}=27.18+0.256x_1+0.09x_3-2.74x_2-2.38x_3^2-1.42x_2^2+0.25x_2\ast x_3$$ (3)

$$f_{cs28}=32.07+0.227x_1-0.58x_3-3.11x_2-2.59x_3^2-1.52x_2^2+0.39x_2\ast x_3$$ (4)

$$f_{STS14}=2.351+0.0268x_1-0.0957x_3+0.039x_2-0.086x_3^2-0.161x_2^2+0.039x_2\ast x_3$$ (5)

$$f_{STS14}=2.351+0.0268x_1-0.0957x_3+0.039x_2-0.086x_3^2-0.161x_2^2+0.039x_2\ast x_3$$ (6)

Table 5
Expected and predicted strength of concrete mix.

TYPE OF MIX	COMPRESSIVE STRENGTH (MPa)				SPLIT TENSILE STRENGTH (MPa)
	14 DAYS		28 DAYS		14 DAYS		28 DAYS
	EXPECTED	RSM	EXPECTED	RSM	EXPECTED	RSM	EXPECTED	RSM
SFBJ 1	27.76	27.22	32.19	31.86	2.58	2.42	2.91	2.82
SFBJ 2	21.64	22.65	25.24	25.18	1.97	2.05	2.4	2.51
SFBJ 3	22.65	21.67	25.6	25.92	2.14	1.93	2.5	2.57
SFBJ 4	26.8	25.37	30.91	29.67	1.9	2.08	2.6	2.71
SFBJ 5	20.2	21.6	23.6	24.37	2.1	1.95	2.21	2.38
SFBJ 6	26.8	25.1	30.8	29.23	1.85	2.04	2.54	2.37
SFBJ 7	26.81	23.67	31.06	29.16	2.2	2.11	2.31	2.51
SFBJ 8	26	24.18	29.8	27.66	2.2	2.14	2.74	2.82
SFBJ 9	28.27	28.18	32.92	32.19	2.46	2.34	2.74	2.82
SFBJ 10	30.5	25.15	34.6	30.27	2.39	2.21	3.08	3.19
SFBJ 11	30.48	25.11	33.5	30.27	2.59	2.40	3.08	3.19
SFBJ 12	23.52	24.82	27.92	28.67	2.59	2.41	2.65	2.72
SFBJ 13	30.5	25.15	34.2	30.19	2.18	1.92	3.2	3.41
SFBJ 14	25.3	27.16	29.6	31.23	1.98	2.14	2.24	2.67
SFBJ 15	24.61	24.92	27.61	28.36	2.31	2.08	2.47	2.56

Figure 4.
Normal probability plot of concrete mixes.

Table 6 summarizes the use of a set of statistical models and the evaluation procedure known as Analysis of variance (ANOVA) to investigate the relationship between responses and progression factors. The appropriateness of the models is strongly supported by the fact that p < 0.005, as outlined in Table 7. It is clearly shows that the model answers were correct because the discrepancy between each response’s adjusted R² and expected R² was less than 20%. The R² values for f_cs14, f_cs28, f_STS14, and f_STS28 were also 97.23%, 98.27%, 92.23%, and 96.29%, in that order. The correlation between the experimental and anticipated values is displayed in Figure 5. f_cs14, f_cs28, f_STS14, and f_STS28 can all be predicted using the model that has been built. Predicted values closely match experimental data. The models’ F values, which are important at higher levels, can be applied to evaluate the correctness of the model [³⁶]. Table 5 indicates that the models are more substantial since the answers of f_cs14, f_cs28, f_STS14, and f_STS28 have F values of 39.39, 43.50, 16.81, and 21.70, respectively. Table 6 indicates that the models are more substantial since the answers of f_cs14, f_cs28, f_STS14, and f_STS28 have F values of 39.39, 43.50, 16.81, and 21.70, respectively. Significant and highly substantial can be used to describe the progression variable of bamboo fiber if its P value is more than that of silica fume, almost equal to that of jute fibers, and more than the optimal value of 2.23 for both f_STS14 and f_STS28. The tensile strength of the concrete mix was increased as a result of the fibers and concrete spanning each other [³⁷]. The findings from f_cs14, f_cs28, f_STS14, and f_STS28 make it evident that adding bamboo fibers significantly improves the concrete’s tensile strength while adding jute fibers affects or increases the compressive strength characteristics. The P value for lack of fit, or model P value, should be minimized. If the P value is < 0.004 it can be considered significant and if P is < 0.001 it is a highly significant value. The insignificant P value was considered when the value was> 0.005. In ANOVA Table 6, the linear X₁ p-values were greater than 0.005, while the p-values of X₂, X₃, and X₂² for f_cs14 and f_cs28 were less than 0.005. The analysis suggests that bamboo fibers had no discernible effect, and the p-values for the quadratic X²₃ and the linear X₃ are both more than 0.005. This indicates that the effects of silica fume and bamboo fibers on compressive strength at 14 and 28 days are not as significant.

Table 6
Analysis of Variance (ANOVA) for concrete mixes.

SOURCE	CS – fcs14			CS – fcs28			STS – fSTS14			STS – fSTS28
	DF	F	P	DF	F	P	DF	F	P	DF	F	P
Model	6	10.05	0.001	6	11.21	≤0.001	6	4.92	0.003	6	7.11	0.004
Linear	3	22.64	<0.001	3	25.53	≤0.001	3	10.18	0.004	3	15.07	0.002
X1	1	1.98	0.181	1	1.48	0.247	1	1.92	0.211	1	1.82	0.241
X2	1	38.34	<0.001	1	44.28	≤0.001	1	12.08	0.005	1	19.73	0.001
X3	1	21.10	0.001	1	32.51	≤0.001	1	17.25	0.003	1	21.18	0.001
Square	2	1.51	0.268	2	1.58	0.252	2	0.68	0.471	2	1.42	0.283
X22	1	0.71	0.003	1	0.78	0.001	1	1.21	0.004	1	2.36	0.006
X32	1	2.41	0.172	1	2.37	0.161	1	0.42	0.003	1	0.49	0.004
Two-way Interaction	1	0.02	0.854	1	0.08	0.744	1	0.12	0.722	1	0.41	0.552
X2 * X3	1	0.02	0.854	1	0.08	0.744	1	0.12	0.722	1	0.41	0.552

Table 7
Regression (R²) and proportion of variance analysis.

RESPONSE	PREDICTED R2	ADJUSTED R2	R 2	DIFFERENCE (PRE. – ADJ.)	P VALUE
fcs14	98.42	93.25	96.12	5.17	≤0.001
fcs14	99.18	96.87	97.28	2.31	≤0.001
fSTS14	93.35	89.69	93.18	3.66	0.004
fSTS28	97.24	90.82	95.38	6.42	0.002

Figure 5.
(a) Compressive strength and (b) split tensile strength of concrete mix – actual vs predicted.

3.4.2. Pareto and lack of fit analysis

The Pareto chart in Figure 6 indicates that jute fibers are more important for compressive strength after 14 and 28 days of curing than bamboo and silica fume. The linear ‘C’ value was greater than that of linear (A & B). This suggests that jute fibers could be the most crucial component in determining the compressive strength, supported by the P value of linear X2 in ANOVA Table 5 being greater than that of X1 and X3. The results are consistent with other studies [³⁸, ³⁹], showing that adding fibers to concrete may significantly improve its tensile strength but has no discernible impact on its compressive strength. Factors X2, X3, X22, and X23 show that both jute and bamboo fibers contribute to tensile strength after 14 and 28 days; the p-value for bamboo fibers is less than 0.005. The silica fume was deemed unimportant since the linear X1 was more than 0.005. The bamboo fiber provides a high impact when compared to the jute fiber by referring linearity value of “C” as shown in Figure 6. These values were more than the optimal values of 2.23 for both f_STS14 and f_STS28. The fibers and concrete and their binding action improved the tensile strength of the concrete mix [⁴⁰]. The responses for f_cs14, f_cs28, f_STS14, and f_STS28 indicate that the adding of bamboo fibers pointedly improves the tensile strength of the concrete while adding jute fibers affects or increases the compressive strength properties [⁴¹].

Figure 6.
Pareto chart for different concrete mix and its compressive and split tensile strength.

3.4.3. Progression variable optimization and analysis using surface plots

Figures 7 and 8 provide three-dimensional (3D) surface plots to help you understand how progression factors impact the responses. As displayed on the surface plot, the “z” axis represented the response, while the factors for silica fume, jute fiber, and bamboo fiber progressed along the “x” and “y” axes. According to Figure 7, when 5% silica fume is added to the cement together with a percentage of bamboo fibers (from 0% to 0.5%) and jute fibers (from 0% to 0.5%), the greatest compressive strength is reached at 14 and 28 days of curing. More than 0.5% causes the strength to decrease. However, on days 14 and 28, the influence of jute fibers was stronger than that of bamboo fibers and silica fume. Including silica fume along with jute fibers in concrete boosted its compressive strength [⁴²]. OPC 53 grade cement is employed in this study, adhering to the Indian code regulation IS 12269:2013 [²³], was used. With a specific gravity of 2.91, it validates 30 minutes of setting time. The 3D surface plot reveals that the optimal combination, yielding the highest compressive strength for both f_cs14 and f_cs28, involves the lowest percentages: 0.5% bamboo fibers, 5% silica fume and 0.5% jute fibers. In Figure 7(a), the optimal compressive strength of f_cs14 and f_cs28 is illustrated. The optimum strength and progression of desirability values are represented by “y” and “d,” respectively as shown in Figure 8(a). In this representation, zero signifies an undesirable combination, while one indicates a favorable one [⁴³]. The figure indicates that the optimal concentrations for achieving maximum compressive strength at both 14 and 28 days were 6.060% for silica fume, 0.4141% for bamboo fibers, and 0.2826% for jute fibers. For f_cs28 and f_cs14, Table 8 shows a percentage of error of 3.03% and 3.35%, respectively. The relationship between the tensile strength at 14 and 28 days and the escalating quantity of bamboo and jute fibers with concrete are depicted in Figure 7(b). Nonetheless, a substantial increase in tensile strength occurred due to the synergistic effect of the addition of these two fibers [⁴⁴]. However, it is noteworthy that when the weight fraction containing 0.5% of each bamboo and jute fibers is exceeded, the split tensile strength of the concrete decreases. In Figure 9, the maximum tensile strength at 14 and 28 days was found to be achieved with the following optimal proportions of jute, bamboo, and silica fume: 7.2727%, 0.3838%, and 0.4646%, respectively. The results were validated through the implementation of a test, as shown in Table 8, and the results showed an error percentage of 2.18% and 2.93% for 14 and 28 days tensile strength, respectively.

Figure 7.
Response surface plots of (a) f_cs14 and (b) f_cs28 for concrete mixes and its compressive strength.

Figure 8.
Response surface plots of (a) f_STS14 and (b) f_STS28 for concrete mixes and its split tensile strength.

Table 8
Error and validation outcomes in %.

STRENGTH TYPE	SF	JF	BF	PREDICTION (RSM)	CONFIRMATION	ERROR (%)
fcs14	6.07	0.29	0.42	27.68	26.48	4.33
fcs28	6.07	0.29	0.42	31.72	30.57	3.62
fSTS14	7.31	0.47	0.39	2.41	2.33	3.31
fSTS28	7.31	0.47	0.39	2.81	2.72	3.20

Figure 9.
(a) Compressive strength and (b) split tensile strength of concrete mixes and its optimization plots using RSM.

3.5. Microstructural analysis

Cement-based materials’ microstructure features have a major impact on the hardened properties of concrete. Scanning electron microscopy (SEM) was used to analyze the microstructure of the concrete and the structure of the matrix. Upon 28 days, two SEM micrographs were obtained, one including the regular mix (M25) and the other featuring the SFBJ11 mixture. In contrast to the control and SFBJ11 mix micrograph displayed in Figure 10(a,b), has a denser and more homogenous microstructure. The aforementioned observation implies that the SFBJ11 combination exhibits a more compact and homogenous particle dispersion, potentially leading to enhanced hardened qualities including durability and strength. Concrete’s pore structure is refined by the hydration mechanism, and the addition of smaller conventional mix grains greatly lowers the size of the pores. Based on the mechanical property trial findings, the mixture containing SFBJ11 performed exceptionally well in terms of compressive strength. Its increased strength is attributed to the Calcium-Silicate-Hydrate (C-S-H) gel that is included in this combination. The SEM examination of SFBJ11 revealed an increase in the area of Calcium Hydroxide (CH) compared to the control mix. SEM analysis showed a denser and more homogeneous microstructure in concrete mixes with bamboo and jute fibers, aligning with the improved compressive strength. This refinement in pore structure suggests enhanced stress distribution, which positively influences both fresh properties (workability) and mechanical properties (compressive strength). This suggests that a more thorough hydration process occurs in the SFBJ11 combination, resulting in the creation of more CH and the potential for enhanced mechanical characteristics. This conclusion is supported by the EDX results, which are shown in Figure 10(c,d) show the quantity of calcium hydrate in SFBJ11 and conventional concrete mixes. The rise in the SF ratio led to a drop in the amount of silicate hydrate (Si) and an increase in calcium hydroxide (CH), which in turn caused a reduction in the content of calcium-silicate-hydrate (C-S-H). Porosity increased as the C-S-H concentration decreased. Previous studies have also arrived at comparable findings concerning the use of silica fume (SF) in the reduction of C-S-H bonds [⁴⁵, ⁴⁶]. In addition, the outcomes of the EDX and SEM studies agree with the compressive strength and porosity test carried out using the methanol exchange technique. This indicates that the conclusions drawn from the mechanical testing align with the microstructural characteristics observed through EDX and SEM analyses.

Figure 10.
(a,b) SEM images, (c,d) EDX images and (e,f) XRD images of (a,c,e) Normal and (b,d,f) SFBJ11 concrete mix with SF.

To investigate the corrosive products in the concrete caused by sulphate attack, X-ray diffraction (XRD) analysis was carried out for the SFBJ11 and control mixes using silica fume (SF). The XRD diffractograms are shown in Figure 10(e,f). The intensity peaks of portlandite and calcium-silicate-hydroxide (C-S-H) in the 200-cycle-exposed SFBJ11 sample were comparatively lower than those in the control sample. The XRD diffractogram’s lower intensity peaks for portlandite and C-S-H show that the SFBJ11 combination may be more resistant to sulphate attack than the control mix. There were tiny amounts of thenardite and gypsum visible in Figure 10(e). The material’s intensity peaked after exposure, suggesting that cyclic sulphate exposure led to the synthesis of gypsum, thenardite, and other compounds. This implies that the sulfate attack mechanism consumes portlandite and C-S-H. When the XRD patterns of the SFBJ11 mix and the conventional mix are compared, it is clear that the SFBJ11 mix’s crystalline structures are noticeably more prominent. High intensity is seen at 140, 210, 260, 291, 370, and 450 in the SFBJ11 mix, which corresponds to 80, 110, 60, 40, 30, and 20 hkl planes. This confirms the significantly higher intensity of peaks in the SFBJ11 mix compared to the control mix, indicating the presence of more crystalline structures and potentially greater strength and durability.

4. CONCLUSION

The objective of this study was to maximize the mechanical properties of concrete reinforced with bamboo and jute fibers and silica fume using the Box-Behnken design of Response Surface Methodology (RSM).

• The results demonstrate that adding 0.5% of bamboo and jute fibers, along with silica fume, significantly enhances compressive and tensile strengths. Jute fibers, with a higher aspect ratio, proved more effective in improving split tensile strength, while bamboo fibers contributed to compressive strength due to their structural interlocking capability. However, exceeding 0.5% fiber content negatively affected strength due to increased porosity and clustering effects.

• Silica fume’s filler properties refined the concrete’s microstructure, reducing porosity and enhancing homogeneity, as confirmed by SEM and XRD analyses. This improvement in microstructure corresponded to the observed enhancements in mechanical properties.

• Reliable predictive models were developed using RSM, validated by low error percentages, showing strong agreement between experimental and predicted values. These models provide a robust framework for forecasting the mechanical behavior of fiber-reinforced concrete.

• The study has limitations, including its focus on fixed curing conditions, the exclusion of hybrid fiber combinations, and the absence of long-term durability analysis under environmental exposure. Future research should explore these aspects to optimize fiber-reinforced concrete further and investigate its long-term performance in diverse conditions.

• This research contributes valuable insights into the potential of natural fibers and silica fume for sustainable and high-performance concrete applications, addressing environmental and material challenges in the construction industry.

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Publication Dates

• Publication in this collection
  31 Jan 2025
• Date of issue
  2025

History

• Received
  05 Nov 2024
• Accepted
  12 Dec 2024