# Mechanical Performance of Jute Fiber-Reinforced Micaceous Clay Composites Treated with Ground-Granulated Blast-Furnace Slag

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Materials

#### 2.1. Micaceous Clay

_{opt}= 19.84% and 23.52%, along with maximum dry densities of ρ

_{dmax}= 1.63 g/cm

^{3}and 1.56 g/cm

^{3}, for K and MC, respectively. Such trends can be attributed to the spongy nature (i.e., elastic/rebound response to compaction energy) and high water demand of the mica mineral [12,20,64].

_{2}) and aluminum trioxide (Al

_{2}O

_{3}) with mass fractions of 64.9% and 22.2% for K, and 49.5% and 29.2% for GM, respectively. The pH for slurries of K and GM was, respectively, found to be 7.4 and 7.8, from which both materials were classified as neutral substances. Other material properties included a specific surface area of SSA = 11.2 m

^{2}/g and 5.3 m

^{2}/g for K and GM, respectively.

#### 2.2. Jute Fibers

_{D}= 30–40 μm; they were cut into segments of approximately F

_{L}= 15 mm, thus resulting in an aspect ratio of F

_{AR}= F

_{L}/F

_{D}= 375–500 (see Figure 1a,b). The scanning electron microscopy (SEM) technique was used to observe the fiber’s surface morphology, and the results are illustrated in Figure 1c. The fiber’s surface embodies a highly-irregular shape comprising of a series of peaks and troughs of varying heights, depths and spacing, thus signifying a rough surface texture. Such surface features may potentially promote adhesion and/or induce frictional resistance at the soil–fiber interface, and thus amend the soil fabric into a coherent matrix of induced strength and improved ductility (see Section 4.3). The physical and mechanical properties of JF, as supplied by the distributor, are provided in Table 3. The specific gravity of JF was found to be 1.30–1.46, which is approximately two-fold less than that of the MC blend.

#### 2.3. Ground-Granulated Blast-Furnace Slag

^{2}/g; the latter is approximately two-fold greater than that of ordinary Portland cement [67]. The chemical composition of GBFS is mainly dominated by calcium oxide or lime (CaO) and silicon dioxide (SiO

_{2}) with mass fractions of 44.7% and 27.1%, respectively. The former, the calcium oxide, acts as a precursor agent, initiating a series of short- and long-term chemical reactions in the soil–water medium, i.e., cation exchange, flocculation–agglomeration and pozzolanic reactions, thereby amending the soil fabric into a unitary mass of enhanced mechanical performance (see Section 4.3).

## 3. Experimental Program

#### 3.1. Mix Designs and Sample Preparations

_{x}= x% JF; S

_{y}= y% GBFS; and T

_{z}= z days of curing.

_{c}= JF content; S

_{c}= GBFS content; w

_{c}= water content; m

_{JF}= mass of JF; m

_{GBFS}= mass of GBFS; m

_{MC}= mass of micaceous clay (or natural soil); and m

_{W}= mass of water.

_{c}= 23.52%, the standard Proctor optimum water content of the natural soil (ASTM D698–12), was added to each blend and thoroughly mixed by hand for approximately 15 min. Extensive care was taken to pulverize the clumped particles, targeting homogeneity of the mixtures. A special split mold, similar to that described in the literature, was designed and fabricated from stainless steel to accomplish static compaction [33,43,49]. The mold consisted of three segments, namely the top collar, the middle section, and the bottom collar. The middle section measures 50 mm in diameter and 100 mm in height, and accommodates the sample for the unconfined compression test (see Section 3.2). The moist blends were statically compacted in the mold in five layers; each layer achieved a dry density of ρ

_{d}= 1.56 g/cm

^{3}(i.e., the standard Proctor maximum dry density of the natural soil, obtained as per ASTM D698–12). The surface of the first to fourth compacted layers was scarified to ensure adequate bonding between adjacent layers of the mixture. Samples containing GBFS were enclosed in multiple layers of cling wrap and transferred to a humidity chamber, maintained at 70% relative humidity and a temperature of 25 ± 2 °C, where curing was allowed for 7 and 28 days prior to testing.

_{0}S

_{0}T

_{0}(natural soil), F

_{1.0}S

_{0}T

_{0}, F

_{0}S

_{6}T

_{0}and F

_{1.0}S

_{6}T

_{0}were examined, and the results are provided in Figure 2. The variations of both dry density and water content were found to be marginal, as evident with the low standard deviations (SD), thus corroborating the suitability of the adopted sample preparation technique.

#### 3.2. Unconfined Compression Test

_{0}S

_{0}T

_{0}(natural soil), F

_{1.0}S

_{0}T

_{0}, F

_{0}S

_{6}T

_{28}and F

_{1.0}S

_{6}T

_{28}. In this regard, the standard deviation (SD) and the coefficient of variation (CV) for the triplicate peak UC strength data were found to range between SD = 3.74 kPa and 11.19 kPa, and CV = 3.23% and 5.15%; these low values corroborate the repeatability of the adopted sample preparation technique, as well as the implemented UC testing procedure.

#### 3.3. Scanning Electron Microscopy Studies

_{0}S

_{0}T

_{0}(natural soil), F

_{1.0}S

_{0}T

_{0}, F

_{0}S

_{6}T

_{28}and F

_{1.0}S

_{6}T

_{28}were examined. The desired samples, prepared as per Section 3.1, were first air-dried for approximately 14 days. The desiccated samples were then carefully fractured into small cubic-shaped pieces measuring approximately 1000 mm

^{3}in volume, and were further subjected to SEM imaging at various magnification ratios ranging from 250× to 20,000×.

## 4. Results and Discussion

#### 4.1. Effect of JF on UC Strength

_{x}S

_{y}T

_{z}where x = {0, 0.5, 1.0, 1.5}, y = {0}, and z = {0}—are provided in Figure 3. The stress–strain relationship for the natural soil sample demonstrated a rise–fall response with a visually-detectable peak point, thereby indicating a strain-softening behavior accompanied by a brittle sample failure. As a result of JF-reinforcement, the stress–strain locus progressively transitioned towards a strain-hardening character. In this case, the greater the JF content the more prominent the strain-hardening effect and hence the less dramatic (or the more ductile) the failures.

_{c}= 1%, beyond of which JF-reinforcement was found to adversely influence strength development in the composite. The natural soil exhibited a peak UC strength of q

_{u}= 82.15 kPa, while the samples reinforced with F

_{c}= 0.5% and 1% resulted in higher values of q

_{u}= 119.35 kPa and 138.21 kPa, respectively. The higher JF inclusion of 1.5% changed the peak UC strength to 132.24 kPa, which still holds a notable advantage over the natural soil, as well as the sample reinforced with 0.5% JF. The axial strain at failure, denoted as ε

_{u}, is an indication of the material’s ductility; higher ε

_{u}values manifest a more ductile (or a less brittle) character. Improvement in ductility is often quantified by means of the deformability index I

_{D}[70]:

_{u}

^{S}= axial strain at failure for the stabilized soil sample; and ε

_{u}

^{N}= axial strain at failure for the control (or natural soil) sample.

_{u}

^{N}= 4.73%). As a result of JF-reinforcement, the deformability index exhibited a monotonically-increasing trend, and resulted in I

_{D}= 1.24, 1.39 and 1.81 (ε

_{u}

^{S}= 5.88%, 6.57% and 8.55%) for F

_{c}= 0.5%, 1% and 1.5%, respectively.

_{50}, is a measure of the material’s stiffness in the elastic compression domain [22,71]. The variations of E

_{50}, as given in Figure 3, exhibited a trend similar to that observed for the peak UC strength, peaking at F

_{c}= 1% and then slightly decreasing for the higher JF content of 1.5%. The natural soil and samples reinforced with 0.5%, 1% and 1.5% JF resulted in E

_{50}= 2.27 MPa, 3.35 MPa, 3.70 MPa and 3.67 MPa, respectively. The area under a typical stress–strain curve up to the peak point, defined as the energy stored by a sample undergoing deformation and referred to as peak strain energy, serves as a measure of the material’s toughness [22,72]. Unlike strength and stiffness, the development of toughness, similar to ductility, was consistently in favor of the JF inclusions, and displayed a monotonically-increasing trend with respect to JF content (see the E

_{u}values in Figure 3). An increase in toughness warrants an increase in the peak UC strength and/or the axial strain at failure [41,57]. With regard to JF-reinforcement, both q

_{u}and ε

_{u}contribute to the development of toughness; however, the greater the JF content the less prominent the strength’s contribution and hence the more significant the role of ductility. The natural soil resulted in E

_{u}= 2.36 kJ/m

^{3}, while the samples reinforced with F

_{c}= 0.5%, 1% and 1.5% resulted in higher values of E

_{u}= 4.49 kJ/m

^{3}, 6.11 kJ/m

^{3}and 8.32 kJ/m

^{3}, respectively.

#### 4.2. Effect of JF + GBFS on UC Strength

_{0}S

_{0}T

_{0}) and various GBFS-treated samples—F

_{x}S

_{y}T

_{z}where x = {0}, y = {3, 9}, and z = {7, 28}—are provided in Figure 4a. Unlike the JF-reinforced samples (see Figure 3), the stress–strain responses for all GBFS-treated composites were seemingly strain-softening and hence accompanied by brittle failures. In general, the greater the GBFS content and/or the longer the curing period, the higher the developed strength and stiffness, and the more prominent the strain-softening character. Stress–strain curves for the natural soil (F

_{0}S

_{0}T

_{0}) and various JF-reinforced samples treated with 6% GBFS—F

_{x}S

_{y}T

_{z}where x = {0, 0.5, 1.0, 1.5}, y = {6}, and z = {7}—are provided in Figure 4b. Much like the natural soil reinforced with JF (see Figure 3), for any given GBFS content, an increase in JF content progressively transitioned the stress–strain locus towards a strain-hardening character. In this case, the greater the JF content the more pronounced the strain-hardening effect and hence the more ductile the failures.

_{c}= 1%; beyond 1% JF, the effect of JF-reinforcement adversely influenced strength development in the composite. For instance, the sample F

_{0}S

_{6}T

_{28}resulted in q

_{u}= 191.32 kPa, while the inclusions of 0.5%, 1% and 1.5% JF, with the same 6% GBFS content and the same 28-day curing condition, resulted in q

_{u}= 250.08 kPa, 327.42 kPa and 302.76 kPa, respectively. Moreover, for any given JF content, the greater the GBFS content and/or the longer the curing period, the higher the developed peak UC strength, following a monotonically-increasing trend. The sample F

_{1.0}S

_{0}T

_{0}, for instance, exhibited a peak UC strength of q

_{u}= 138.21 kPa. As a result of 3%, 6% and 9% GBFS inclusions, along with the same 1% JF content and a 7-day curing condition, the peak UC strength increased to 203.56 kPa, 273.68 kPa and 330.06 kPa, respectively. Similar mix designs cured for T

_{c}= 28 days exhibited significant improvements over their 7-day counterparts, as the aforementioned values increased to 248.65 kPa, 327.42 kPa and 443.21 kPa, respectively. The ASTM D4609–08 standard suggests a minimum improvement of 345 kPa in the natural soil’s peak UC strength (at T

_{c}= 28 days) as a criterion for characterizing an effective stabilization scheme [34]. As demonstrated in Figure 5b, the sample F

_{1.0}S

_{9}T

_{28}promotes a 361.06 kPa improvement in the peak UC strength and hence satisfies the aforementioned criterion.

_{c}= 7 and 28 days, respectively. Similar to the natural soil reinforced with JF, for any given GBFS content and curing time, the greater the JF content the higher the deformability index, following a monotonically-increasing trend. For any given JF content, however, the greater the GBFS content and/or the longer the curing period, the lower the developed ductility. The deformability index for various JF + GBFS blends was cross-checked with that of the natural soil (or I

_{D}= 1) to arrive at the optimum cases. In this regard, nine cases (out of 28) manage to satisfy the I

_{D}≥ 1 criterion, and thus are deemed as optimum with respect to ductility improvement. The nine optimum cases and their corresponding I

_{D}values include F

_{0.5}S

_{3}T

_{7}(I

_{D}= 1.10), F

_{1.0}S

_{3}T

_{7}(I

_{D}= 1.34), F

_{1.5}S

_{3}T

_{7}(I

_{D}= 1.68), F

_{1.0}S

_{3}T

_{28}(I

_{D}= 1.09), F

_{1.5}S

_{3}T

_{28}(I

_{D}= 1.34), F

_{1.0}S

_{6}T

_{7}(I

_{D}= 1.16), F

_{1.5}S

_{6}T

_{7}(I

_{D}= 1.32), F

_{1.5}S

_{6}T

_{28}(I

_{D}= 1.10), and F

_{1.5}S

_{9}T

_{7}(I

_{D}= 1.08).

_{50}against JF content for the natural soil and various GBFS-treated samples tested at 7 and 28 days of curing, respectively. The variations of E

_{50}exhibited a trend similar to that observed for the peak UC strength given in Figure 5. As such, for any given JF content, the development of stiffness was in favor of both the GBFS content and the curing time. As typical cases, the samples F

_{1.0}S

_{0}T

_{0}, F

_{1.0}S

_{3}T

_{7}, F

_{1.0}S

_{3}T

_{28}, F

_{1.0}S

_{9}T

_{7}and F

_{1.0}S

_{9}T

_{28}resulted in E

_{50}= 3.70 MPa, 5.39 MPa, 7.81 MPa, 12.30 MPa and 18.92 MPa, respectively. Moreover, for any given GBFS content and curing time, stiffness enhancements were only notable for samples with up to 1% JF inclusions. In this regard, the samples F

_{0}S

_{6}T

_{28}, F

_{0.5}S

_{6}T

_{28}, F

_{1.0}S

_{6}T

_{28}and F

_{1.5}S

_{6}T

_{28}, for instance, resulted in E

_{50}= 8.25 MPa, 9.47 MPa, 11.21 MPa and 10.23 MPa, respectively.

_{0}S

_{6}T

_{28}, F

_{0.5}S

_{6}T

_{28}, F

_{1.0}S

_{6}T

_{28}and F

_{1.5}S

_{6}T

_{28}resulted in peak strain energies of E

_{u}= 3.99 kJ/m

^{3}, 6.30 kJ/m

^{3}, 9.71 kJ/m

^{3}and 10.70 kJ/m

^{3}, respectively. Similarly, for any given JF content, the greater the GBFS content and/or the longer the curing period, the higher the developed toughness. As typical cases, the sample F

_{1.0}S

_{0}T

_{0}resulted in E

_{u}= 6.11 kJ/m

^{3}, while the aforementioned value increased to 8.02 kJ/m

^{3}, 8.22 kJ/m

^{3}, 8.78 kJ/m

^{3}and 9.88 kJ/m

^{3}for F

_{1.0}S

_{3}T

_{7}, F

_{1.0}S

_{3}T

_{28}, F

_{1.0}S

_{9}T

_{7}and F

_{1.0}S

_{9}T

_{28}, respectively.

_{50}and E

_{u}against q

_{u}for various JF + GBFS mix designs, respectively. The variations of E

_{50}were situated within the 0.054q

_{u}< E

_{50}< 0.025q

_{u}domain (E

_{50}in MPa, and q

_{u}in kPa). For E

_{u}, however, a broader domain in the form of 0.063q

_{u}< E

_{u}< 0.018q

_{u}(E

_{u}in kJ/m

^{3}, and q

_{u}in kPa) was noted. The former, the E

_{50}, exhibited a rather strong correlation with q

_{u}. On the contrary, the peak strain energy was poorly correlated with the peak UC strength. In this regard, simple correlative models in the forms of E

_{50}= 0.038q

_{u}(with R

^{2}= 0.836) and E

_{u}= 0.029q

_{u}(with R

^{2}= 0.449) can be derived; the former can be implemented for indirect estimations of E

_{50}.

#### 4.3. Stabilization Mechanisms and Microstructure Analysis

**i**) frictional resistance generated at the soil–fiber interface, owing to the fiber’s rough surface texture; and (

**ii**) mechanical interlocking of soil particles and fibers [1,22,40,45,48,51,57,66]. The interfacial frictional resistance is a function of the soil–fiber contact area, with greater contact levels providing a higher resistance to bear the external loads. Consequently, this amending mechanism can be ascribed to the fiber content, meaning that the greater the number of included fiber units, i.e., increase in fiber content, the greater the contact levels achieved between the soil particles and fibers, and thus the higher the generated interfacial frictional resistance against UC loading. The second amending mechanism, the mechanical interlocking of soil particles and fibers, is achieved during sample preparation/compaction, and induces the composite’s adhesion by immobilizing the soil particles undergoing shearing. Quite clearly, the more effective/pronounced the achieved mechanical interlocking the higher the permanence against UC loading. Consequently, this amending mechanism is in line with the fiber content, and more importantly the fiber’s elongated form factor. In general, the greater the number of included fiber units, i.e., increase in fiber content, the greater the number of interlocked or enwrapped soil aggregates, and thus the higher the developed peak UC strength. It should be noted that the soil–fiber amending mechanisms, as described above, only hold provided that the fiber units do not cluster (or adhere to each other) during mixture preparation and compaction [22,54,56,73]. At high fiber contents, the behavior of the composite, at some points, may be governed by a dominant fiber-to-fiber interaction; this effect, commonly referred to as fiber-clustering, leads to a notable improvement in the sample’s ductility/deformability and toughness (see Figure 6 and Figure 8) while offsetting the desired soil-to-fiber interaction capable of improving the sample’s peak UC strength and stiffness. Fiber-clustering effects were evident for all samples containing 1.5% JF, as the previously-improved peak UC strength and stiffness manifested a notable decrease compared with similar mix designs containing 1% JF (see Figure 5 and Figure 7).

^{+}< K

^{+}<< Mg

^{2+}< Ca

^{2+}[77]. GBFS-treatment supplies the clay–water medium with additional calcium cations (Ca

^{2+}), which immediately substitute cations of lower valence (e.g., sodium Na

^{+}) and/or same-valence cations of smaller ionic radius (e.g., magnesium Mg

^{2+}) in the vicinity of the clay particles. These cation exchanges lead to a decrease in the thickness of the Diffused Double Layers (DDLs), owing to the development of strong van der Waals bonds between adjacent clay particles in the matrix, which in turn promote aggregation and flocculation of the clay particles [76,78,79]. Long-term chemical reactions, commonly referred to as pozzolanic reactions, are strongly time- and often temperature-dependent, meaning that their commencement and evolution require a certain and often long period of curing. During pozzolanic reactions, ionized calcium (Ca

^{2+}) and hydroxide (OH

^{–}) units, released from the water–binder complex, gradually react with silicate (SiO

_{2}) and aluminate (Al

_{2}O

_{3}) units in the soil, thereby leading to the formation of strong cementation products/gels, namely Calcium–Silicate–Hydrates (CSH), Calcium–Aluminate–Hydrates (CAH) and Calcium–Aluminate–Silicate–Hydrates (CASH); these products encourage further solidification and flocculation of the soil particles, which in turn accommodate the development of a dense, uniform matrix coupled with enhanced strength performance [31,33,76,79]. It should be noted that the short- and long-term amending reactions, as described above, are generally in favor of a higher binder content; this general perception also complies with the results outlined in Figure 5, Figure 7 and Figure 8.

_{0}S

_{0}T

_{0}(natural soil), F

_{1.0}S

_{0}T

_{0}, F

_{0}S

_{6}T

_{28}and F

_{1.0}S

_{6}T

_{28}, respectively. The microstructure of the natural soil sample manifested a partly-dense, non-uniform matrix, accompanied by a notable number of large inter- and intra-assemblage pore-spaces, respectively, formed between and within the soil aggregates; such morphological features warrant the existence of an edge-to-edge dispersed fabric (see Figure 10a). The microstructure of the JF-reinforced sample or F

_{1.0}S

_{0}T

_{0}exhibited a partly-dense but more uniform matrix, accompanied by a limited number of small intra-assemblage pore-spaces mainly distributed along the soil–fiber connection interface. In essence, the fiber units acted as physical anchors within the matrix, interlocking the neighboring soil aggregates and hence withstanding compressive stresses during shearing (see Figure 10b). As a result of GBFS-treatment (see sample F

_{0}S

_{6}T

_{28}in Figure 10c), the microstructure became even more uniform in nature, indicating aggregation and flocculation of the soil particles and hence the development of a fully-dense matrix with a dominant edge-to-face flocculated fabric. Prevalent cementation products were clearly visible between and within the soil aggregates, which portrayed a major role in eliminating the inter- and intra-assemblage pore-spaces in the matrix. As a result of JF-reinforcement and GBFS-treatment (see sample F

_{1.0}S

_{6}T

_{28}in Figure 10d), the soil–fiber connection interface was markedly improved, as evident with the presence of fully-clothed fibers strongly embedded between and within the soil aggregates, which in turn led to a further improvement in the composite’s strength and stiffness.

## 5. Modeling

#### 5.1. Model Development

_{u}(in kPa), as evident with the experimental results discussed in Section 4, can be categorized as: (

**i**) JF content F

_{c}(in %); (

**ii**) GBFS content S

_{c}(in %); and (

**iii**) curing time T

_{c}(in days). Therefore, the peak UC strength problem for various JF + GBFS blends can be expressed as:

_{0}to β

_{9}= model/fitting parameters (or regression coefficients); and β

_{0}= peak UC strength of the natural soil, since setting F

_{c}= 0, S

_{c}= 0 and T

_{c}= 0 leads to q

_{u}= β

_{0}.

^{2}(dimensionless), the root-mean-squared error RMSE (in kPa), the normalized root-mean-squared error NRMSE (in %) and the mean-absolute-percentage error MAPE (in %), were adopted to assess the model’s predictive capacity [82,83]:

_{u}

^{A}= actual peak UC strength, as presented in Figure 5; q

_{u}

^{P}= predicted peak UC strength; b = index of summation; and N = number of experimental data points used for model development (N = 28, as outlined in Table 5).

^{2}(=0.964) and low RMSE (=17.28 kPa), NRMSE (=4.78%) or MAPE (=6.19%) values warrant a strong agreement between actual and predicted peak UC strength data. The R

^{2}index merely surpassed 0.95, thus indicating that leastwise 95% of the variations in experimental observations are captured and further explained by the proposed regression model. The NRMSE index was found to be slightly less than 5%, thus signifying a maximum offset of 5% associated with the predictions. However, the P-value associated with some of the regression components, namely S

_{c}, T

_{c}, S

_{c}

^{2}, T

_{c}

^{2}and F

_{c}T

_{c}, was found to be greater than 5%, implying that these components are statistically-insignificant and hence make no or little contribution towards the predictions. Statistically-insignificant terms can be eliminated to accommodate the derivation of a simplified model with unanimously-significant regression components [59]. As such, Equation (7) can be simplified as:

^{2}= 0.951, RMSE = 20.00 kPa, NRMSE = 5.54% and MAPE = 7.28%, which are on par with that observed for Equation (7). In essence, Equation (11) suggests a more practical path towards predicting the peak UC strength while maintaining a performance similar to that offered by the more complex Equation (7). Moreover, the p-values associated with all of the regression components were unanimously less than 5% (see Regression Outputs in Table 7), thus corroborating their statistical significance (and contribution) towards the predictions. Figure 11 illustrates the variations of predicted, by Equation (11), against actual peak UC strength data, along with the corresponding 95% prediction bands/intervals, for various JF + GBFS blends. Despite the existence of some scatter, all data points cluster around the line of equality and firmly position themselves between the 95% upper and 95% lower prediction bands, thereby indicating no particular outliers associated with the predictions. The proposed regression model given in Equation (11) contains a total of four fitting parameters, i.e., β

_{1}, β

_{4}, β

_{7}and β

_{9}(β

_{0}is equal to the peak UC strength of the natural soil), all of which can be calibrated by little experimental effort, as well as simple explicit calculations, and hence implemented for preliminary design assessments, predictive purposes and/or JF + GBFS optimization studies. Assuming that the peak UC strength of the natural soil (or β

_{0}) is at hand, the four fitting parameters can be adequately calibrated by a total of four UC tests carried out on four arbitrary JF + GBFS mix designs.

#### 5.2. Sensitivity Analysis

_{c}, S

_{c}and T

_{c}, on the dependent variable q

_{u}. The overall relative impact, both positive and negative, of an independent variable, i.e., x

_{a}= F

_{c}, S

_{c}or T

_{c}, on the dependent variable q

_{u}, commonly referred to as sensitivity, can be defined as:

_{a}= partial derivative of q

_{u}or Equation (11) with respect to x

_{a}= F

_{c}, S

_{c}or T

_{c}; σ(x

_{a}) = standard deviation of x

_{a}data; σ(q

_{u}) = standard deviation of predicted q

_{u}data; b = index of summation; and N = number of observations (N = 28, as outlined in Table 5).

_{a}= dq

_{u}/dx

_{a}in Equation (12), measures the likelihood of q

_{u}increasing or decreasing as a result of an increase in x

_{a}. As such, the likelihood of increase or decrease in q

_{u}as a result of an increase in x

_{a}can be, respectively, defined as:

_{P}(x

_{a}) = number of observations where D

_{a}≥ 0; and M

_{N}(x

_{a}) = number of observations where D

_{a}< 0.

_{a}= F

_{c}, S

_{c}or T

_{c}, on the dependent variable q

_{u}can be, respectively, defined as:

_{P}(x

_{a}) and S

_{N}(x

_{a}) are, respectively, positive and negative fractions of the sensitivity parameter, S(x

_{a}) or Equation (12), meaning that for any given x

_{a}, S(x

_{a}) = S

_{P}(x

_{a}) + S

_{N}(x

_{a}).

_{P}(x

_{a}) or Equation (15), is of interest for further analysis. The positive-sensitivity parameter can be expressed in terms of percentage to facilitate a more practical comparison between the independent variables [84]:

_{P}(x

_{a}) = positive contribution offered by an increase in x

_{a}resulting in an increase in q

_{u}(in %); and K = number of independent variables (K = 3, namely F

_{c}, S

_{c}and T

_{c}).

_{c}≤ 1.5%, exhibits favorable improvements only up to a particular/optimum fiber content, beyond of which marginal improvements or adverse effects, owing to fiber-clustering, can be expected (see the discussions in Section 4.3). As for GBFS content and curing time, the likelihood of increase was found to be 100% for both variables, thus indicting that GBFS-treatment, where 3% ≤ S

_{c}≤ 9%, consistently leads to favorable improvements which can be further enhanced by means of curing. The positive contribution offered by an increase in JF content resulting in an increase in the peak UC strength was obtained as 35%. For GBFS content and curing time, however, the positive contribution was found to be 38% and 27%, respectively. These results imply that for a given JF + GBFS blend without curing, F

_{c}and S

_{c}would theoretically portray an equally-significant role towards strength development. With curing, however, the overall contribution offered by GBFS-treatment profoundly outweighs that of JF-reinforcement, as F

_{P}(S

_{c}) + F

_{P}(T

_{c}) = 65% >> F

_{P}(F

_{c}) = 35%.

## 6. Conclusions

- For any given GBFS content and curing time, the greater the JF content the higher the developed strength and stiffness up to F
_{c}= 1%; beyond 1% JF, the effect of JF-reinforcement adversely influenced the development of strength and stiffness. The composite’s ductility and toughness, however, were consistently in favor of JF-reinforcement, meaning that the greater the JF content the higher the developed ductility and toughness. - For any given JF content, the greater the GBFS content and/or the longer the curing period, the higher the developed strength, stiffness and toughness, following monotonically-increasing trends. The composite’s ductility, however, was adversely influenced by GBFS-treatment, meaning that the greater the GBFS content and/or the longer the curing period, the lower the developed ductility.
- The addition of GBFS to JF-reinforced samples improved the soil–fiber connection interface or bonding, as the fiber units became fully embedded between and within the soil aggregates; this in turn led to a further improvement in the composite’s strength and stiffness. The ASTM D4609–08 strength criterion was used to assess the efficiency and hence applicability of the proposed JF + GBFS mix designs. In this regard, the sample F
_{1.0}S_{9}T_{28}managed to satisfy ASTM’s criterion and hence can be taken as the optimum design choice. - A non-linear, multivariable regression model was developed to quantify the peak UC strength q
_{u}as a function of the composite’s basic index properties, i.e., JF content F_{c}, GBFS content S_{c}, and curing time T_{c}. The predictive capacity of the suggested model was examined and further validated by statistical techniques. A sensitivity analysis was also carried out to quantify the relative impacts of the independent regression variables, namely F_{c}, S_{c}and T_{c}, on the dependent variable q_{u}. The proposed regression model contained a limited number of fitting parameters, all of which can be calibrated by little experimental effort, as well as simple explicit calculations, and hence implemented for preliminary design assessments, predictive purposes and/or JF + GBFS optimization studies.

## Author Contributions

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**JF at different magnification ratios: (

**a**) Raw fibers (no magnification); (

**b**) Processed fibers (no magnification); and (

**c**) Processed fibers (1500× magnification).

**Figure 2.**Variations of dry density along the height of the compacted samples: (

**a**) F

_{0}S

_{0}T

_{0}; (

**b**) F

_{1.0}S

_{0}T

_{0}; (

**c**) F

_{0}S

_{6}T

_{0}; and (

**d**) F

_{1.0}S

_{6}T

_{0}.

**Figure 3.**Stress–strain curves for the natural soil and various JF-reinforced samples, i.e., F

_{x}S

_{y}T

_{z}where x = {0, 0.5, 1.0, 1.5}, y = {0}, and z = {0}.

**Figure 4.**Typical stress–strain curves for the natural soil (F

_{0}S

_{0}T

_{0}) and various stabilized samples: (

**a**) F

_{x}S

_{y}T

_{z}where x = {0}, y = {3, 9}, and z = {7, 28}; and (

**b**) F

_{x}S

_{y}T

_{z}where x = {0, 0.5, 1.0, 1.5}, y = {6}, and z = {7}.

**Figure 5.**Variations of peak UC strength q

_{u}against JF content for the natural soil and various GBFS-treated samples: (

**a**) T

_{c}= 7 days; and (

**b**) T

_{c}= 28 days.

**Figure 6.**Variations of deformability index I

_{D}against JF content for the natural soil and various GBFS-treated samples: (

**a**) T

_{c}= 7 days; and (

**b**) T

_{c}= 28 days.

**Figure 7.**Variations of E

_{50}against JF content for the natural soil and various GBFS-treated samples: (

**a**) T

_{c}= 7 days; and (

**b**) T

_{c}= 28 days.

**Figure 8.**Variations of peak strain energy E

_{u}against JF content for the natural soil and various GBFS-treated samples: (

**a**) T

_{c}= 7 days; and (

**b**) T

_{c}= 28 days.

**Figure 9.**Variations of (

**a**) E

_{50}and (

**b**) peak strain energy E

_{u}against peak UC strength q

_{u}for various JF + GBFS blends.

**Figure 10.**SEM micrographs for the tested samples: (

**a**) F

_{0}S

_{0}T

_{0}(natural soil); (

**b**) F

_{1.0}S

_{0}T

_{0}; (

**c**) F

_{0}S

_{6}T

_{28}; and (

**d**) F

_{1.0}S

_{6}T

_{28}.

**Figure 11.**Variations of predicted, by Equation (11), against actual peak UC strength data for various JF + GBFS blends.

Properties | K | GM | MC | Standard Designation |
---|---|---|---|---|

Specific gravity of solids, G_{s} | 2.69 | 2.80 | 2.73 | ASTM D854–14 |

Clay fraction [<2 μm] (%) | 51 | - | 39 | ASTM D422–07 |

Silt fraction [2–75 μm] (%) | 48 | - | 55 | ASTM D422–07 |

Fines fraction [<75 μm] (%) | 99 | 93 | 94 | ASTM D422–07 |

Sand fraction [0.075–4.75 mm] (%) | 1 | 7 | 6 | ASTM D422–07 |

Natural water content, w_{n} (%) | 2.14 | 0.41 | 1.67 | ASTM D2216–10 |

Liquid limit, LL (%) | 44.67 | - | 48.67 | AS 1289.3.9.1–15 |

Plastic limit, PL (%) | 23.72 | - | 36.94 | AS 1289.3.2.1–09 |

Plasticity index, PI (%) | 20.95 | - | 11.28 | AS 1289.3.3.1–09 |

Linear shrinkage, LS (%) | 7.06 | - | 8.84 | AS 1289.3.4.1–08 |

Shrinkage index, SI (%) ^{1} | 37.61 | - | 39.83 | Sridharan and Nagaraj [65] |

USCS classification | CI ^{2} | - | MI ^{3} | ASTM D2487–11 |

Optimum water content, w_{opt} (%) | 19.84 | - | 23.52 | ASTM D698–12 |

Maximum dry density, ρ_{dmax} (g/cm^{3}) | 1.63 | - | 1.56 | ASTM D698–12 |

Unconfined compression strength, q_{u} (kPa) ^{4} | 137.62 | - | 85.14 | ASTM D2166–16 |

Splitting tensile strength, q_{t} (kPa) ^{4} | 21.76 | - | 14.62 | ASTM C496–17 |

^{1}SI = LL–LS;

^{2}Clay with intermediate plasticity;

^{3}Silt with intermediate plasticity; and

^{4}Tested at standard Proctor optimum condition.

Properties | K | GM |
---|---|---|

SiO_{2} (%) | 64.9 | 49.5 |

Al_{2}O_{3} (%) | 22.2 | 29.2 |

K_{2}O (%) | 2.7 | 8.9 |

TiO_{2} (%) | 1.4 | 0.8 |

Fe_{2}O_{3} (%) | 1.0 | 4.6 |

MgO (%) | 0.6 | 0.7 |

Na_{2}O (%) | 0.2 | 0.5 |

CaO (%) | 0.1 | 0.4 |

Acidity, pH [20% slurry] | 7.4 | 7.8 |

Oil absorption (mL/100 g) | 34.0 | 36.0 |

Loss on ignition, LOI [at 1000 °C] (%) | 6.5 | <6 |

Specific surface area, SSA (m^{2}/g) | 11.2 | 5.3 |

Properties | Value |
---|---|

Specific gravity, G_{s} | 1.30–1.46 |

Length, F_{L} (mm) | 15 |

Diameter, F_{D} (μm) | 30–40 |

Aspect ratio, F_{AR} = F_{L}/F_{D} | 375–500 |

Young’s modulus (GPa) | 10–30 |

Tensile strength (MPa) | 400–900 |

Tensile elongation at break (%) | 1.5–1.8 |

Water absorption (%) | 12 |

Properties | Value |
---|---|

Specific gravity of solids, G_{s} | 2.87 |

Fines fraction [<75 μm] (%) | 96 |

Sand fraction [0.075–4.75 mm] (%) | 4 |

Natural water content, w_{n} (%) | <1 |

Acidity, pH [20% slurry] | 9.6 |

Loss on ignition, LOI [at 1000 °C] (%) | <3 |

Specific surface area, SSA (m^{2}/g) | 0.7 |

CaO (%) | 44.7 |

SiO_{2} (%) | 27.1 |

Al_{2}O_{3} (%) | 13.6 |

MgO (%) | 5.1 |

Fe_{2}O_{3} (%) | 3.5 |

TiO_{2} (%) | 1.7 |

K_{2}O (%) | 0.7 |

Na_{2}O (%) | 0.2 |

Group | Designation | JF Content (%) | GBFS Content (%) |
---|---|---|---|

Control ^{1} | F_{0}S_{0}T_{0} | 0 | 0 |

JF-reinforced | F_{0.5}S_{0}T_{0} | 0.5 | 0 |

F_{1.0}S_{0}T_{0} | 1.0 | 0 | |

F_{1.5}S_{0}T_{0} | 1.5 | 0 | |

GBFS-treated | F_{0}S_{3}T_{7,28} | 0 | 3 |

F_{0}S_{6}T_{7,28} | 0 | 6 | |

F_{0}S_{9}T_{7,28} | 0 | 9 | |

JF + GBFS | F_{0.5}S_{3}T_{7,28} | 0.5 | 3 |

F_{1.0}S_{3}T_{7,28} | 1.0 | 3 | |

F_{1.5}S_{3}T_{7,28} | 1.5 | 3 | |

F_{0.5}S_{6}T_{7,28} | 0.5 | 6 | |

F_{1.0}S_{6}T_{7,28} | 1.0 | 6 | |

F_{1.5}S_{6}T_{7,28} | 1.5 | 6 | |

F_{0.5}S_{9}T_{7,28} | 0.5 | 9 | |

F_{1.0}S_{9}T_{7,28} | 1.0 | 9 | |

F_{1.5}S_{9}T_{7,28} | 1.5 | 9 |

^{1}Natural soil.

R ^{1} | R^{2} | Adjusted R^{2} | RMSE (kPa) | NRMSE (%) | MAPE (%) |
---|---|---|---|---|---|

0.982 | 0.964 | 0.946 | 17.28 | 4.78 | 6.19 |

^{1}Coefficient of correlation.

Source of Variation | DF ^{1} | SS ^{2} | MS ^{3} | F-Value | Significance F |
---|---|---|---|---|---|

Regression | 9 | 2.20 × 10^{5} | 2.44 × 10^{4} | 52.62 | 4.26 × 10^{−11} < 5% (S) |

Residual | 18 | 8.36 × 10^{3} | 4.64 × 10^{2} | ||

Total | 27 | 2.28 × 10^{5} |

^{1}Degree of freedom;

^{2}Sum of squares;

^{3}Mean squares; and (S) = Significant.

Variable | Coefficient | SE ^{1} | t-Value | p-Value |
---|---|---|---|---|

Intercept | β_{0} = 64.75 | 16.19 | 4.00 | 8.42 × 10^{−4} < 5% (S) |

F_{c} | β_{1} = 171.31 | 28.76 | 5.96 | 1.23 × 10^{−5} < 5% (S) |

S_{c} | β_{2} = 2.43 | 13.06 | 0.19 | 8.55 × 10^{−1} > 5% (NS) |

T_{c} | β_{3} = 1.48 | 6.68 | 0.22 | 8.27 × 10^{−1} > 5% (NS) |

F_{c }^{2} | β_{4} = −85.99 | 16.29 | −5.28 | 5.10 × 10^{−5} < 5% (S) |

S_{c }^{2} | β_{5} = 0.26 | 1.04 | 0.25 | 8.02 × 10^{−1} > 5% (NS) |

T_{c }^{2} | β_{6} = −0.04 | 0.20 | −0.22 | 8.31 × 10^{−1} > 5% (NS) |

F_{c} × S_{c} | β_{7} = 6.65 | 2.53 | 2.63 | 1.70 × 10^{−2} < 5% (S) |

F_{c} × T_{c} | β_{8} = −0.17 | 0.68 | −0.25 | 8.09 × 10^{−1} > 5% (NS) |

S_{c} × T_{c} | β_{9} = 0.61 | 0.17 | 3.55 | 2.28 × 10^{−3} < 5% (S) |

^{1}Standard error; (S) = Significant; and (NS) = Not Significant.

R ^{1} | R^{2} | Adjusted R^{2} | RMSE (kPa) | NRMSE (%) | MAPE (%) |
---|---|---|---|---|---|

0.976 | 0.951 | 0.943 | 20.00 | 5.54 | 7.28 |

^{1}Coefficient of correlation.

Source of Variation | DF ^{1} | SS ^{2} | MS ^{3} | F-Value | Significance F |
---|---|---|---|---|---|

Regression | 4 | 2.17 × 10^{5} | 5.43 × 10^{4} | 111.49 | 1.04 × 10^{−14} < 5% (S) |

Residual | 23 | 1.12 × 10^{4} | 4.87 × 10^{2} | ||

Total | 27 | 2.28 × 10^{5} |

^{1}Degree of freedom;

^{2}Sum of squares;

^{3}Mean squares; and (S) = Significant.

Variable | Coefficient | SE ^{1} | t-Value | P-Value |
---|---|---|---|---|

Intercept | β_{0} = 89.14 | 9.70 | 9.19 | 3.69 × 10^{–9} < 5% (S) |

F_{c} (%) | β_{1} = 148.90 | 27.51 | 5.41 | 1.69 × 10^{–5} < 5% (S) |

F_{c }^{2} | β_{4} = −85.99 | 16.68 | −5.16 | 3.17 × 10^{–5} < 5% (S) |

F_{c} × S_{c} | β_{7} = 10.52 | 1.69 | 6.22 | 2.40 × 10^{–6} < 5% (S) |

S_{c} × T_{c} | β_{9} = 0.65 | 0.06 | 11.08 | 1.07 × 10^{–10} < 5% (S) |

^{1}Standard error; and (S) = Significant.

Variable, x_{a} | D_{a} = dq_{u}/dx_{a} | S(x_{a}) | P_{P}(x_{a}) (%) | P_{N}(x_{a}) (%) | S_{P}(x_{a}) | S_{N}(x_{a}) | F_{P}(x_{a}) (%) |
---|---|---|---|---|---|---|---|

JF content, F_{c} | ${\beta}_{1}+2{\beta}_{4}{F}_{\mathrm{c}}+{\beta}_{7}{S}_{\mathrm{c}}$ | 0.639 | 71 | 29 | 0.548 | 0.090 | 35 |

GBFS content, S_{c} | ${\beta}_{7}{F}_{\mathrm{c}}+{\beta}_{9}{T}_{\mathrm{c}}$ | 0.605 | 100 | 0 | 0.605 | 0 | 38 |

Curing time, T_{c} | ${\beta}_{9}{S}_{\mathrm{c}}$ | 0.427 | 100 | 0 | 0.427 | 0 | 27 |

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Zhang, J.; Soltani, A.; Deng, A.; Jaksa, M.B.
Mechanical Performance of Jute Fiber-Reinforced Micaceous Clay Composites Treated with Ground-Granulated Blast-Furnace Slag. *Materials* **2019**, *12*, 576.
https://doi.org/10.3390/ma12040576

**AMA Style**

Zhang J, Soltani A, Deng A, Jaksa MB.
Mechanical Performance of Jute Fiber-Reinforced Micaceous Clay Composites Treated with Ground-Granulated Blast-Furnace Slag. *Materials*. 2019; 12(4):576.
https://doi.org/10.3390/ma12040576

**Chicago/Turabian Style**

Zhang, Jiahe, Amin Soltani, An Deng, and Mark B. Jaksa.
2019. "Mechanical Performance of Jute Fiber-Reinforced Micaceous Clay Composites Treated with Ground-Granulated Blast-Furnace Slag" *Materials* 12, no. 4: 576.
https://doi.org/10.3390/ma12040576