An Experimental Investigation of the Effects of Adding Polymer and Basalt Fibers on the Mechanical Properties and Durability of Lightweight Concrete

Abstract

Despite extensive research on fiber-reinforced lightweight concrete, the synergistic effects of combining different types of fibers, such as polymer and basalt fibers, on the mechanical properties and durability of lightweight concrete have not been fully investigated. This study aims to fill this scientific gap by examining the combined use of polymer and basalt fibers to enhance the performance of lightweight concrete (LWC). Lightweight concrete is widely used to reduce the weight of structures and improve seismic performance. However, its brittle nature and lower mechanical properties compared to normal-weight concrete (NWC) limit its application in high-stress environments. This study seeks to overcome these limitations by optimizing the use of polymer and basalt fibers to improve the mechanical properties and durability of lightweight concrete. In this research, 320 cylindrical samples were prepared, and the results show that adding 1% polymer fibers significantly improved the compressive and tensile strengths of lightweight concrete by 24.4% and 66.13%, respectively, at 28 days. Additionally, the combination of polymer and basalt fibers showed a positive synergistic effect, leading to improved mechanical properties and durability of the concrete, including a 45.38% reduction in final water absorption and a 43.15% reduction in chloride ion penetration at 90 days. This study provides new insights into the synergistic effects of polymer and basalt fibers in lightweight concrete and proposes a practical solution for improving its mechanical properties and durability. The findings of this research contribute to the development of lightweight concrete structures with greater reliability and flexibility.

1. Introduction

Much of the damage to buildings in the Nepal and Bam (Iran) earthquakes was due to the heavy weight of the buildings and the materials used in their construction. Therefore, it can be summarized that heavy structures are unreliable in terms of safety and strength. The weight of a structure is directly related to the seismic load applied to it. Using lightweight materials like LWC can reduce the weight and seismic load, thereby increasing the structure’s stability and health and reducing the risk of sudden failure [1]. ACI 213R-87 states that the density of structural LWC is between 1120 and 1920 kg/m³, and its 28-day compressive strength is 17 MPa [2]. Additionally, LWC is a good thermal insulator, and using this building material can save up to 15% in energy and cause less harm to the environment [3,4]. Therefore, it can be said that the main motivation for using LWC and lightweight elements is to reduce the weight of the building, reduce the dead load and loads caused by earthquakes, increase the reliability of the structure’s stability and health, and reduce the risk of sudden structural failure.

However, many researchers have reported that due to the use of lightweight materials, the mechanical properties and durability of LWC are weaker than those of normal concrete [5,6].

Researchers have reported that the higher the number of lightweight materials used in the mixing design, the weaker the mechanical properties and durability of LWC [7,8]. Tedjditi et al. [9] stated that the compressive strength, flexural strength, and modulus of elasticity of LWC containing 25% virgin cork as a lightweight material are 73%, 42%, and 78% lower than those of control concrete without lightweight materials, respectively. Also, with the increasing amount of virgin cork, the compressive strength, flexural strength, and modulus of elasticity decreased to a greater extent than in the sample without lightweight materials. However, as seen in the stress–strain curves, the samples containing high values of cork showed pre-rupture of the plastic phase, indicating a good inelastic deformation capacity of LWC with a high percentage of cork [9].

Researchers have stated that the use of lightweight materials that have a lot of pores in the structure of LWC creates voids, so the structure of LWC is porous. This increases the absorption of water and also the penetration of destructive factors such as chloride ions into LWC [10,11].

In an experimental study, Hasan et al. [12] examined the rate of water absorption and RCP of the LWC, which is a measure of concrete durability. Researchers have reported that due to the presence of voids and the high porosity of the LWC structure, the rate of water absorption and penetration of chloride ions into LWC are 12.5% and 0.35% higher compared to conventional concrete, respectively [12].

A suitable solution to improve the mechanical properties and durability of LWC is to add fibers as an additive to the LWC mix. Natural fibers include asbestos, cellulose, sisal, animal wool, hair, etc., which are of natural origin. For synthetic fibers, we can name steel fibers, carbon fibers, polypropylene (PP) fibers, basalt fibers, and polymer fibers, which have a synthetic origin [13,14]. Adding fibers to LWC enhances its properties significantly. It boosts energy absorption, increases resistance to cracking, and improves post-cracking behavior by bridging voids. This helps prevent crack widening during early hydration. Fibers also increase the impact resistance, splitting tensile strength, flexural strength, flexural stiffness, and modulus of elasticity [15,16].

Some researchers have also stated that the addition of fibers has little effect on improving the compressive strength of LWC, and if high doses of fibers are used, the compressive strength of LWC is reduced compared to the fiber-free sample [17,18]. Most researchers also report that adding a variety of fibers to an LWC mix reduces workability and slump. The reason for this is the clumping of fibers and blockage of LWC structures [19,20]. Nahab et al. [21] stated that the addition of steel fibers to an LWC mix made of lightweight expanded clay aggregates (Leca) reduced workability and had little effect on improving compressive strength, but flexural strength improved by 55% compared to the fiber-free sample. Also, with increasing Leca values, the mechanical properties of LWC were further weakened [21]. In a study by Altalabani et al. [22], adding 0.22% PP fiber to LWC made of Leca improved the compressive strength by 0.12%, tensile strength by 4.5%, and modulus of elasticity by 2.55% compared to control concrete. However, increasing the fiber dose to 0.33% decreased the compressive strength by 5.30%, while the splitting tensile strength and modulus of elasticity increased by 4.7% and 3.9%, respectively. Sardar et al. [18] also stated that the addition of basalt fibers to LWC made of Leca has little effect on improving the compressive strength and, if high amounts of fibers are used, this reduces the compressive strength compared to the fiber-free sample. Unlike other researchers, Zeng et al. [23] reported that adding 0.5% and 1.5% of basalt fibers to LWC made of stag ceramic increases the compressive strength by 11.5% and 18.5%, respectively. The reason for this is the higher resistance of lightweight ceramic LWA, which makes the effect of basalt fibers on compressive strength more prominent. The researchers also stated that an LWC mixture containing 1.5% basalt fibers has a 34% higher shear strength than the sample without fibers [23]. The researchers reported that steel fibers, due to their long length and diameter and hard appearance, create voids in the LWC structure and increase the rate of penetration of water flow and destructive factors into the sample.

In a previous study, Shah et al. investigated the effect of steel fibers at dosages of 2% and 4% on the mechanical properties of concrete containing expanded polystyrene (EPS) at replacement levels of 0%, 15%, 30%, and 45%, along with 10% micro-silica. The researchers reported that incorporating 45% EPS reduced the concrete density by 53%. Additionally, Shah et al. stated that the mixture containing 15% EPS and 2% steel fibers provided the best balance between weight reduction and strength retention. Finally, the researchers concluded that adding 2% steel fibers improved crack resistance and increased the flexural strength by 5% [24].

Othuman Maydin et al. [25] stated that the addition of 0.45% lignocellulosic fibers to lightweight foamed concrete reduced workability. However, the use of lignocellulosic hemp fibers had the most significant effect on improving strength and reducing porosity. In contrast, lignocellulosic jute fibers had the greatest impact on reducing water absorption [25].

In a study, Kadela et al. [26] obtained a type of steel fiber by recycling wires from worn-out tires. The researchers reported that incorporating these recycled steel fibers into lightweight concrete increased the compressive strength by up to 48%, tensile strength by up to 52%, and flexural strength by up to 41% [26].

Ma et al. [27] concluded that adding 0.5%, 1%, and 1.5% basalt fibers with a length of 9 mm to the mix design of lightweight concrete containing epoxy spheres reinforced with calcium carbonate increased the compressive strength by 22.8% compared to the reference specimen. Additionally, the researchers stated that the use of basalt fibers had a negligible effect on increasing the concrete density [27].

In another study, Behera et al. [28] investigated the performance of ultra-high-strength lightweight concrete made with fly ash aggregates, oil palm shell, and supplementary cementitious materials, reinforced with single and hybrid hooked-end steel and PVA fibers. The researchers reported that incorporating these fibers improved the compressive strength, tensile strength, and elastic modulus by 55.98% compared to the fiber-free specimen. However, the cost of LWHFRC was 16.46% higher than that of the fiber-free mix [28].

Xue et al. [29] also stated that adding 0.3% basalt fibers with a length of 12 mm exhibited the best performance in enhancing compressive strength, tensile strength, and toughness while effectively preventing crack propagation to an acceptable extent [29].

Chen et al. [30] reported that the use of hybrid micro-steel and polypropylene (PP) fibers enhanced tensile strength, improved crack resistance, increased ductility, and effectively controlled crack propagation. The researchers also stated that the combination of these fibers, along with the biomineralization process, contributed to the enhancement in self-healing and the overall strength of the concrete [30].

Jiang et al. [31] investigated the performance and engineering applications of lightweight concrete reinforced with shaped plastic–steel fibers under freeze–thaw cycling conditions. The researchers reported that incorporating 6 kg of these fibers exhibited the best performance in enhancing compressive strength and toughness, increasing them by 13.7% and 22.9%, respectively. Furthermore, they stated that the compressive strength of conventional concrete decreased by 25% after 150 freeze–thaw cycles, whereas the fiber-reinforced specimen experienced the least reduction in strength [31].

Wang et al. [32] concluded that adding 1% steel fibers to a lightweight concrete mix enhanced the strength and stiffness of columns made from this type of concrete. Additionally, this strength improvement reduced column damage and deterioration while preventing excessive displacement and acceleration [32].

In a study, Golewski [33] reported that adding 20% coal fly ash (CFA) to concrete increased its compressive strength; however, this strength enhancement was accompanied by an increase in water absorption. Nevertheless, Golewski concluded that when the CFA content was increased to 30%, water absorption decreased, but this was followed by a reduction in compressive strength [33].

The researchers concluded that incorporating 0.75% bamboo fibers effectively prevented crack propagation, reduced the rate of load decline, and increased the residual load-bearing capacity of LWAC beams after fracture. Bamboo fibers significantly enhanced the fracture mechanical properties of lightweight aggregate concrete, with the most notable improvement observed when the fiber length reached 30 mm and the fiber content was 0.75%. Additionally, Li et al. [34] reported that the inclusion of bamboo fibers increased bending strength by 51.5%, fracture energy by 298.4%, initial toughness by 39.66%, and unstable toughness by 75.69% compared to the fiber-free specimens [34].

Zinkaah et al. [35] stated that the water absorption rate of LWC containing steel fibers is 10% higher than that of fiber-free samples. Some researchers found that adding fibers like carbon fiber, basalt fiber, and PP fiber to LWC helps maintain its structure by bridging cracks and blocking voids. This reduces water absorption and permeability in lightweight concrete. In one study, Loh et al. [36] reported that by adding 0.285% of PP and PVA fibers individually to an LWC mix made from oil palm shell (OPS), the water absorption of LWC was 17.2% and 21.5%, respectively, compared to the lightweight sample without fibers, which is reduced. Loh et al. Further stated that the addition of PP + PVA hybrid fibers reduced the water absorption of the sample by 24% compared to the sample without fibers due to the positive synergistic effect of both fibers on each other [36]. Dawood et al. [37] also believe that adding 0.5%, 1%, and 1.5% of carbon fiber reduced the water absorption of LWC by 18.7%, 38.5%, and 40% compared to the fiber-free sample, respectively.

In an experimental study, Mohseni et al. [38] reported that the electric charge passing through LWC containing 10% and 20% scoria was 2% and 5.5% higher than that of normal-weight concrete, respectively. However, the addition of 1% PP fiber to the LWC mixture containing 10% scoria reduced the amount of electric charge passing through the sample by 1.2% compared to the LWC sample without the fibers. Hossain et al. [39] stated that adding 0.25% PVA fibers to an LWC mix reduced the rapid permeability of chloride by 16% compared to a sample containing 1% crushed rubber grain. However, some researchers have noted that steel fibers, due to their conductivity, can act as pathways for electric charge and ions, leading to an increased permeability of concrete chloride and reduced electrical strength of LWC, and sometimes interfering with test results [40].

In this experimental study, the effects of adding different amounts of polymer fibers and basalt fibers, both individually and in combination (polymer + basalt), on the mechanical properties and durability of LWC made of Leca have been thoroughly investigated. The study specifically examines how these fibers influence key mechanical properties, including compressive strength and splitting tensile strength, as well as durability characteristics such as final water absorption, RCP, and electrical resistance at 28 and 90 days.

The novelty of this research lies in the simultaneous evaluation of polymer and basalt fibers, both separately and in combination, to enhance not only the mechanical performance but also the long-term durability of LWC. While previous studies have primarily focused on the mechanical properties of LWC, research on its durability remains limited. By exploring the synergistic effects of polymer and basalt fibers, this study provides valuable and up-to-date insights into optimizing the performance of lightweight concrete for practical applications. The experimental program of this study is presented in the block diagram below.

2. Materials and Methods

2.1. Materials

2.1.1. Cement

The cement used in this study is ordinary Portland cement, produced by Saman Cement Factory in Kermanshah, Iran, with a 28-day compressive strength of 52 MPa, a specific weight of 3120 kg/m³, and a specific surface area of 3200 cm²/g, which is manufactured according to the ASTM C150 standard [41].

2.1.2. Micro-Silica

According to the American Concrete Institute (ACI), micro-silica is non-crystalline silica, a very fine, gray dye produced in arc furnaces that is a by-product of the production of siliceous or metallic siliceous alloys [42]. In this research, micro-silica has been used as a cementitious additive (pozzolan), which has been produced following ASTM C1240 [43]. The chemical and physical properties of micro-silica are listed in

Table 1. Chemical properties of cement and micro-silica.

Chemical Specification	Cement (%)	Micro-Silica (%)
SiO2	20.7 ± 0.3	90–95
CaO	65 ± 0.5	0.5–1
Al2O3	5.2 ± 0.2	0.6–1.2
Fe2O3	4.6 ± 0.2	1.2–1.8
Na2O	0.15 ± 0.2	-
MgO	1.8 ± 0.2	0.6–1.2
So3	2.2 ± 0.4	-
K2O	0.5 ± 0.05	-
C	-	0.8–2
Cl	-	0.05–0.07
C3S	59.47	-
C2S	14.48	-
C3A	6	-
C4AF	14	-
Free CaO	1.3 ± 0.2	-

.

2.1.3. Coarse and Fine Aggregates

In this study, natural gravel with a size of 6–12 mm was used as coarse aggregate, and natural sand with a size of 0–6 mm was used as fine aggregate. The granulation of coarse and fine aggregates has been carried out according to ASTM C136 [44]. Figure 1 shows the gravel granulation diagram, and Figure 2 and Figure 3 shows the sand granulation diagram. Also, the water absorption and specific gravity of consumed gravel and sand were obtained according to the ASTM C127 standard [45]. The specifications of the consumed gravel and sand are listed in

Table 2. Physical and mechanical properties of coarse and fine aggregates and Leca.

Properties	Coarse Aggregates (Gravel)	Fine Aggregates (Sand)	Leca
Size (mm)	6–12	0–4.75	4–10
Bulk Density (kg/m3)	2720	2713	370
Dry Density (kg/m3)	2659	2585	320
Fineness Modulus	6.14	3.00	-
Water Absorption (%)	0.79	1.83	18
FM	6.14	3.00	-
SE (%)	-	79	-

.

2.1.4. Lightweight Expanded Clay Aggregates (Leca)

The lightweight materials used in this research are structural lightweight expanded clay aggregates (structural Leca) with a size of 4–10 mm, (Figure 4) which is granulated by the ASTM C330 standard [46]. Leca are produced from the expansion of clay in rotary kilns with a temperature of about 1200 °C. The lightweight materials used in this study are produced by the Leca Iran Factory, located in Mamonieh city, Iran.

Table 3. Physical and mechanical properties of fibers.

Fiber	Length (mm)	Diameter (µm)	Aspect Ratio (L/D)	Tensile Strength (MPa)	Elastic Modulus (MPa)	Water Absorption (%)	Density (kg/m3)
Polymer Wave (PF)	55	200	275	450–700	3500–7000	0	910
Basalt (BF)	12	18.5	64.86	2800–3200	82,000–92,000	8–10	2800

presents the physical characteristics of Leca.

2.1.5. Fibers

In this research, basalt fibers with a length of 12 mm (Figure 5), which are of volcanic origin, and polymer fibers (KORTTA Wave) with a length of 55 mm (Figure 6), which are made of modified nano-copropylene materials, have been used singly and in combination. The specifications for basalt fibers and polymer fibers are shown in Table 3.

2.1.6. Admixtures

To achieve optimal workability in LWC mixtures, a high workability superplasticizer and water reducer based on polycarboxylate with a specific gravity of 1100 kg/m³ according to ASTM C1017 [47] and ASTM C494 type standards F [48] were produced and used.

2.1.7. Water

Kermanshah drinking water, which does not contain any contamination, was used to prepare LWC mixtures and also to process the samples.

2.2. Methods

2.2.1. Mix Proportions

In this experimental study, 31 mixing designs were created based on ACI 211 [49]. They include the following:

• One design with fiber-free LWC (control LWC).
• Ten designs with basalt fibers.
• Ten designs with polymer fibers.
• Ten designs with a mix of basalt and polymer hybrid fibers.

To observe the effects of adding basalt and polymer fibers individually and in combination on the mechanical properties and durability of LWC, the amounts of cement, micro-silica, coarse and fine aggregates, Leca, superplasticizer, and water are fixed in all designs, and only the amounts of fibers are variable. The amounts of polymer and basalt fibers are 0.25%, 0.5%, 0.75%, 1%, 1.25%, 1.5%, 1.75%, 2%, 2.25%, and 2.5% of the total weight of cementitious and pozzolanic materials (cement + micro-silica). Also, the mentioned values of fibers are true for designs containing composite fibers, and the share of each fiber is 1/2. For all designs, the ratio of water to cement (W/B) is constant and equal to 0.38, the amount of micro-silica consumed is equal to 15% by weight of cement, and the amount of superplasticizer consumed is equal to 0.8 L per 100 kg of cement and pozzolanic materials.

Table A1. Mixing designs of control LWC and LWC containing different amounts of polymer and basalt fibers alone and in combination.

Mix ID	Cement (kg/m3)	SF * (kg/m3)	Coarse Aggregates (kg/m3)	Fine Aggregates (kg/m3)	Leca (kg/m3)	Water (kg/m3)	SP ** (kg/m3)	Polymer Fibers (%)	Basalt Fibers (%)
C	460	69	240	650	220	201	4.25	-	-
P0.25%	460	69	240	650	220	201	4.25	0.25	-
P0.5%	460	69	240	650	220	201	4.25	0.5	-
P0.75%	460	69	240	650	220	201	4.25	0.75	-
P1%	460	69	240	650	220	201	4.25	1	-
P1.25%	460	69	240	650	220	201	4.25	1.25	-
P1.5%	460	69	240	650	220	201	4.25	1.5	-
P1.75%	460	69	240	650	220	201	4.25	1.75	-
P2%	460	69	240	650	220	201	4.25	2	-
P2.25%	460	69	240	650	220	201	4.25	2.25	-
P2.5%	460	69	240	650	220	201	4.25	2.5	-
B0.25%	460	69	240	650	220	201	4.25	-	0.25
B0.5%	460	69	240	650	220	201	4.25	-	0.5
B0.75%	460	69	240	650	220	201	4.25	-	0.75
B1	460	69	240	650	220	201	4.25	-	1
B1.25%	460	69	240	650	220	201	4.25	-	1.25
B1.5%	460	69	240	650	220	201	4.25	-	1.5
B1.75%	460	69	240	650	220	201	4.25	-	1.75
B2%	460	69	240	650	220	201	4.25	-	2
B2.25%	460	69	240	650	220	201	4.25	-	2.25
B2.5%	460	69	240	650	220	201	4.25	-	2.5
H0.25%	460	69	240	650	220	201	4.25	0.125	0.125
H0.5%	460	69	240	650	220	201	4.25	0.25	0.25
H0.75%	460	69	240	650	220	201	4.25	0.375	0.375
H1%	460	69	240	650	220	201	4.25	0.5	0.5
H1.5%	460	69	240	650	220	201	4.25	0.625	0.625
H1.5%	460	69	240	650	220	201	4.25	0.75	0.75
H1.75%	460	69	240	650	220	201	4.25	0.875	0.875
H2%	460	69	240	650	220	201	4.25	1	1
H2.25%	460	69	240	650	220	201	4.25	1.125	1.125
H2.5%	460	69	240	650	220	201	4.25	1.25	1.25

In the above table, SF *: micro-silica; SP **: superplasticizer; C: LWC without fibers (control sample); P: mixing design containing the mentioned amounts of polymer fibers; B: mixing design containing the mentioned amounts of basalt fibers; H: the design of mixtures containing polymer + basalt hybrid fibers.

(Appendix A) provides information on all designed mixing schemes.

2.2.2. Mixing, Preparing, and Processing Samples

First, dry materials such as cement, micro-silica, coarse aggregates (gravel), fine (sand), and Leca were added in stages to the mixer boiler. Water and superplasticizer were then added to the dry mixture. Then, the concreting process was performed according to the ASTM C192 standard [50]. The molds used in this study were the American Society for Testing and Materials (ASTM) cylindrical molds with a size of 150 × 300 mm for compressive strength and splitting tensile strength tests and cylindrical molds with a diameter of 100 mm and a height of 200 mm for water absorption, RCP, and electrical resistance tests. After 24 h, LWC samples were removed from the molds and transferred to the curing pond (Figure 7). LWC samples were cured with water according to ASTM C192 [46] for 28 and 90 days.

3. Test Methods

3.1. Mechanical Properties

3.1.1. Compressive Strength

A compressive strength test according to ASTM C39 [51] was performed on cylindrical specimens with a diameter of 150 mm and a height of 300 mm at 28 and 90 days. According to ASTM C39, the compressive strength of concrete samples is obtained by dividing the maximum applied load (Pmax) in kg/cm² by the cross-sectional area of the sample (A) in square centimeters.

$$\mathbf{F}\mathbf{c}={\displaystyle \frac{{\mathbf{P}}_{\mathbf{m}\mathbf{a}\mathbf{x}}}{\mathbf{A}}}$$

(1)

3.1.2. Splitting Tensile Strength

The splitting tensile strength test of concrete according to ASTM C496 [52] was performed on cylindrical specimens with dimensions of 150 × 300 mm at 28 and 90 days. The splitting tensile strength of concrete is obtained according to the following equation:

$$\mathbf{T}={\displaystyle \frac{\mathbf{2}\mathbf{P}}{(\mathsf{\Pi }.\mathbf{L}.\mathbf{D})}}$$

(2)

where T is the splitting tensile strength in terms of kg/cm², P is the maximum applied load in terms of kg, L is the test length in terms of cm, and D is the test diameter in terms of cm.

Figure 8 shows the cylindrical sample with a size of 150*300 mm before the compressive strength test. Additionally, Figure 9 shows the cylindrical sample after compressive loading. Figure 10 and Figure 11 show the placement of the cylindrical sample in the concrete compression testing machine for the compressive strength test and the tensile strength test, respectively.

In this study, a concrete crusher jack with a capacity of 200 tons and a loading rate of 0.25 MPa was used for testing compressive and tensile strength. For the tensile strength test, the same jack used in the compressive strength test was employed along with a mold measuring 150 × 300 mm, which had a solid rod at the top to withstand the stress applied to the specimen.

3.2. Durability

3.2.1. Final Water Absorption

The final water absorption test of concrete according to ASTM C642 [53] was performed on cylindrical samples with a diameter of 100 mm and a height of 200 mm at 28 and 90 days. The final water absorption percentage of concrete is obtained by the following equation:

$$\mathit{W}\mathit{a}\mathit{t}\mathit{e}\mathit{r}\mathit{A}\mathit{b}\mathit{s}\mathit{o}\mathit{r}\mathit{p}\mathit{t}\mathit{i}\mathit{o}\mathit{n}\left(\mathbf{\%}\right)={\displaystyle \frac{\mathit{B}-\mathit{A}}{\mathit{A}}}\times 100$$

(3)

where B is the weight of the saturated sample in water and A is the weight of the dry sample in kg.

3.2.2. Rapid Chloride Permeability Test (RCPT)

The RCPT according to the ASTM C1202 [54] standard was used to quickly measure the permeability of chloride ions in concrete. Briefly, 50 mm thick specimens were cut from cylindrical specimens 100 mm in diameter and 200 mm high. The samples were then saturated for 18 h under a vacuum in a vacuum chamber. After placing the specimen inside the cell, one side of the sample was exposed to sodium chloride solution, and the other side was exposed to sodium hydroxide solution. The passing electric charge of saturated concrete samples was recorded under a potential difference of 60 V for 6 h. Finally, the total passing electric charge was calculated according to the following equation. Figure 12 shows how to perform the RCPT. Figure 13 shows the cut sample for the RCPT test.

$$\mathit{Q}=\mathbf{900}\times \left({\mathit{I}}_{\mathbf{0}}+{\mathbf{2}\mathit{I}}_{\mathbf{30}}+{\mathbf{2}\mathit{I}}_{\mathbf{60}}+\dots +{\mathbf{2}\mathit{I}}_{\mathbf{330}}+{\mathbf{2}\mathit{I}}_{\mathbf{360}}\right)$$

(4)

In the above relation, Q is the total amount of electric charge passing through the test in terms of coulombs, and I is the current measured at different times after the application of voltage and in terms of mA.

3.2.3. Electrical Resistance

One of the characteristics of concrete that shows resistance to electric current due to the movement of ions in concrete is electrical resistance. There is a strong relationship between chlorine ion penetration and electrical resistance in concrete mixes. In this research, electrical conductivity has been performed according to the ASTM C1760 standard [55]. The test procedure is that cylindrical specimens with a diameter of 100 mm and a height of 200 mm were placed between the two cells of the device, and the cells were filled with a 3% sodium chloride solution. The 60 V voltage was then applied to both ends of the sample for 60 s. Also, the temperature of the samples during the test was maintained at 20–25 °C. Finally, the electrical conductivity was calculated according to Equation (5), and the electrical resistance was calculated according to Equation (6). In previous studies, this method has been used to calculate the electrical resistance of concrete [56,57].

$$\mathit{\sigma }={\displaystyle \frac{\mathit{K}\times \mathit{I}\times \mathit{L}}{\mathit{V}\times {\mathit{D}}^{\mathbf{2}}}}$$

(5)

In this regard, the electrical conductivity of concrete with σ is shown in terms of m⁄(Ω.m, k is a fixed number equal to 1273.2, I is the measured current intensity after 60 s in terms of Ma, V is the applied voltage in volts, D is the sample diameter in millimeters, and L is the sample length in millimeters.

$$\mathit{R}={\displaystyle \frac{\mathbf{1000}}{\mathit{\sigma }}}$$

(6)

R is the electrical resistance in terms of Ω.m and σ is the electrical conductivity.

For the RCPT and electrical resistance test, the RCT3 device from Tosee Payadar Salman Iran was used. This device has a voltage range of 0 to 60 volts, a current range of 0 to 600 milliamps, and a temperature range of 0 to 100 degrees Celsius, and it is capable of testing 12 samples simultaneously according to the selected standard.

4. Discussion

4.1. Compressive Strength

Table A2. Results of compressive strength and splitting tensile strength.

Mix ID	Compressive Strength (MPa)		Splitting Tensile Strength (MPa)		Density (kg/m3)
	28 Days	90 Days	28 Days	90 Days
C	37.5	44.2	3.1	3.6	1721
P0.25%	42.2	48.9	4.2	4.95	1733
P0.5%	42.5	49.7	4.25	5	1791
P0.75%	45.55	51	4.85	5.1	1793
P1%	46.65	56.1	5.15	6.2	1830
P1.25%	44.98	52.4	4.65	5.5	1863
P1.5%	44.45	51.5	4.15	4.9	1850
P1.75%	41.65	47.9	4	4.7	1876
P2%	41.1	47.3	3.85	4.6	1899
P2.25%	35	40.3	3.75	4.4	1894
P2.5%	32.75	37.7	3.6	3.9	1900
B0.25%	31.25	35.6	3.15	3.7	1730
B0.5%	32.9	37	3.3	3.8	1739
B0.75%	33.55	37.8	3.38	3.9	1746
B1	34.45	38.6	3.7	4.2	1752
B1.25%	36.65	42.9	3.8	4.5	1767
B1.5%	41.65	48.3	4.1	4.86	1773
B1.75%	44.3	50.8	4.4	5.3	1790
B2%	39.45	45	3.5	4.1	1798
B2.25%	28.35	32.6	2.8	3.2	1825
B2.5%	24.35	27.7	2.5	2.9	1831
H0.25%	32.2	36.95	3.4	3.81	1738
H0.5%	33.25	38	3.53	3.93	1746
H0.75%	34.8	38.35	3.70	3.97	1752
H1%	35.9	39.4	3.80	4.3	1760
H1.5%	37.9	44.5	3.91	4.8	1771
H1.5%	42.6	49	4.29	4.94	1788
H1.75%	45.2	51.3	4.8	5.65	1804
H2%	33.3	38.2	3.4	3.85	1815
H2.25%	31.1	36.1	3.3	3.74	1826
H2.5%	30.55	35.2	3.15	3.68	1851

(Appendix B) presents the results related to the compressive strength test of the LWC sample without fibers and samples containing the mentioned values of polymer fibers, basalt fibers, and polymer + basalt composite fibers at 28 and 90 days. The results show that the addition of optimal amounts of polymer and basalt fibers in a single and hybrid form improves the compressive strength of LWC and increases it compared to the control sample. According to Table A2 (Appendix B), it is found that polymer fibers have the most effect and basalt fibers have the least effect in improving the compressive strength of LWC. The addition of 1% of polymer fibers to the LWC mixture has the greatest effect on improving the compressive strength and increases it by 24.4% and 26.92% compared to the fiber-free samples at 28 and 90 days of age, which is a significant amount. Also, the optimal amount of basalt fibers in this study is 1.75%, which increases the compressive strength of LWC by 18.13% and 14.93% at 28 and 90 days compared to the control sample. Using a combination of polymer and basalt fibers in LWC mix designs results in slightly higher compressive strength than using basalt fibers alone due to their positive synergistic effect. However, adding more than 2% of these fibers (2.25% or 2.5%) as either single or composite doses decreases compressive strength compared to the fiber-free samples. The reason for this can be the improper distribution of fibers in the concrete matrix, high aggregation, clumping of fibers, and improper compaction of the mixture.

4.2. Splitting Tensile Strength

The results related to the splitting tensile strength of control LWC and lightweight samples containing different amounts of polymer and basalt fibers are presented in Table A2 (Appendix B). According to Table A2 (Appendix B), adding polymer and basalt fibers, whether individually or in combination, significantly increases the splitting tensile strength of LWC compared to the fiber-free samples. Initially, short basalt fibers fill the cracks, preventing their expansion. As loading continues, polymer fibers—being stronger and longer—bridge the cracks, further preventing them from widening. This results in a notable increase in splitting tensile strength. Therefore, polymer fibers have the greatest effect, and basalt fibers have the least effect in improving splitting tensile strength. The highest splitting tensile strength is related to the sample that contains 1% of polymer fibers. In this sample, the 28-and 90-day splitting tensile strength is 66.13% and 72.23% higher than that of the fiber-free sample, respectively. Using more than 1% of polymer fibers gradually decreases splitting tensile strength, though it remains higher than that of the control sample. Among samples with basalt fibers, the one with 1.75% has the highest splitting tensile strength, showing an increase of 41.93% and 47.22% at 28 and 90 days, respectively, compared to the control sample. Unlike polymer fibers, the use of doses greater than 1.75% of basalt fibers impedes the splitting tensile strength of LWC and even reduces it compared to the control sample. It should be noted that the effect of polymer + basalt composite fibers in improving the splitting tensile strength of LWC is slightly greater than that of single basalt fibers. The reason for this is the positive synergistic effect of polymer fibers on basalt fibers, which results in the weaknesses of basalt fibers being partially compensated. For example, the 28-and 90-day splitting tensile strength of a sample containing 1.75% composite fibers is 9.09% and 6.60% higher than that of a sample containing the same amount of single basalt fibers. Figure 10 shows the failure and rupture of the control sample, the sample containing 1% polymer fibers, the samples containing 1.75% basalt fibers, and the combination fibers after the application of splitting tensile load. As can be seen from the figure, the samples containing the optimal amounts of polymer and basalt fibers, individually and in combination, are less damaged than the control sample, and their crack resistance is much higher than that of the control sample. Among the mentioned samples, the sample containing 1% of polymer fibers has the highest resistance to rupture and breakdown. The reason for this is the mechanism of bridging the polymer fibers in the cracks and preventing the spread of cracks, which prevents the sudden failure of the lightweight sample. Therefore, it can be said that the best performance of fibers in improving the mechanical properties of LWC is related to a significant increase in splitting tensile strength as well as crack resistance. Figure 14 shows the images of the cylindrical samples after the tensile strength test.

4.3. Water Absorption

Water movement in lightweight concrete, due to its porous structure, is influenced by factors such as porosity level, pore distribution and size, type and amount of additives, and the fibers used. In fiber-reinforced samples, the pathways of water movement can be affected by the mechanical blockage caused by the presence of fibers. In this study, the addition of basalt and polymer fibers to lightweight concrete reduced the final water absorption percentage, indicating a decrease in water mobility within the concrete structure. According to previous research, water movement in both normal and lightweight concrete can occur through three main mechanisms:

• Capillary absorption: When concrete comes into contact with water, it is drawn into the structure through fine pores. The presence of fibers, especially basalt fibers, can reduce this effect by filling small cavities.
• Pressure-driven penetration: In conditions where concrete is exposed to water pressure, water moves through continuous pores in the concrete structure. Reducing porosity and blocking these pathways through an optimal combination of fibers and micro-silica has resulted in a lower permeability of lightweight concrete in this study.
• Water vapor transmission: This typically occurs during the drying process of concrete and depends on environmental relative humidity and pore size. The presence of fibers can control the diffusion of water vapor and prevent rapid moisture loss from concrete.
• Therefore, in this study, fiber incorporation not only reduced the final water absorption percentage but also restricted water movement pathways within the concrete, leading to enhanced durability of lightweight concrete.

Figure 15 shows the final water absorption at 28 and 90 days of the LWC control sample and samples containing the mentioned amounts of polymer and basalt fibers individually and in combination. In LWC, there is greater porosity and more voids than in normal-weight concrete, which makes the amount of water absorption of LWC higher than in normal-weight concrete. According to Figure 15, increasing the curing age reduces the water absorption percentage of all samples and improves the durability of LWC. Also, adding all three types of polymer fibers, basalt fibers, and polymer + basalt composite fibers to the LWC mixture reduces the final water absorption percentage and has a positive effect in improving the durability of LWC. In this study, basalt fibers, due to their delicate appearance and small length-to-diameter ratio, fill cavities and voids and show the best performance in reducing the percentage of water absorption compared to the other fibers used. Adding 1.25% of basalt fibers to the LWC mix reduces the final water absorption percentage by 31.93% and 45.38% at 28 and 90 days compared to the control sample, which is a significant amount. If more than 1.25% of basalt fibers are used, the final water absorption of LWC will gradually increase but still be less than that of the control sample. Researchers found that using fibers with long lengths and diameters can create voids in concrete, increasing water absorption. However, in this study, using small amounts of polymer fibers improved water absorption rates. This is due to the fibers bridging voids in LWC, preventing crack spread and the formation of new cracks. Adding 1% of polymer fibers to the LWC mixture has the most positive effect in reducing the final water absorption of LWC and reduces the final water absorption percentage by 16.77% and 26.93% compared to the fiber-free sample. When more than 1% of polymer fibers are used, the final water absorption of LWC gradually increases so that in samples containing 2.25% and 2.5% of polymer fibers, the water absorption is higher than in the control sample. Also, in the samples that contain the mentioned doses of polymer + basalt composite fibers, due to the positive synergistic effect of basalt fibers, the percentage of final water absorption is lower than in both the control sample and the samples containing single polymer fibers. The use of 1% composite fibers in the LWC mixing design reduces the final water absorption percentage of LWC by 20% and 29.61%. Another reason for the positive effect of fibers in reducing the percentage of water absorption is the use of the optimal amount of micro-silica. The use of micro-silica fills the voids and blocks the cavities in the LWC structure and contributes significantly to the better performance of the fibers in improving the durability of LWC.

In Figure 15, the vertical axis represents the final water absorption percentage of the lightweight concrete samples. The horizontal axis displays the mix design groups along with the fiber percentages used in each group.

In this figure, the following abbreviations are used:

• The letter P denotes polymer fibers.
• The letter B represents basalt fibers.
• The letter H indicates hybrid polymer + basalt fibers.

Additionally, the subscript numbers of P, B, and H indicate the percentage of fibers used in the mix design groups.

4.4. Rapid Chloride Permeability Test (RCPT) and Electrical Resistance

The results of the RCPT and the electrical resistance test of fiber-free LWC samples and samples containing the mentioned amounts of polymer fibers and basalts in single and composite forms are shown in Figure 16 and Figure 17 at 28 and 90 days. Adding the optimal percentage of all three types of polymer fibers, basalt fibers, and polymer + basalt composite fibers to the LWC mixture reduces the amount of electric charge passing through the samples and increases the electrical resistance; thus, the penetration of chloride ions decreases under the influence of the electric field at 28 and 90 days. Among the fibers tested in this study, the use of basalt fibers in the LWC mixing design, due to their delicate physical properties and small diameter, causes the gaps and the cavities in the LWC structure to be filled, exerting the most positive effect in improving electrical resistance and reducing the chloride ion penetration rate into LWC samples. Adding all the mentioned amounts of basalt fibers to the LWC mixture increases the electrical resistance and thus reduces the penetration of chloride ions into the LWC at 28 and 90 days. The optimal amount of basalt fibers is 1.25%, which increases the electrical resistance of LWC by 46.54% and 46.42% and decreases the penetration of chloride ions into LWC by 39.67% and 43.15% compared to the sample without fibers at 28 and 90 days, which is a significant amount. Researchers have reported that the use of fibers that have a large length and diameter increases the electrical charge and reduces the electrical resistance due to the creation of space and holes in the structure of LWC, thus increasing the penetration of chloride ions into the concrete sample. The use of the optimal amount of polymer fibers (1%) increases the electrical resistance by 22.06% and 25.24% and thus reduces the penetration of chloride ions and ions affected by the electric field into LWC by 28.52% and 31.5% compared to the control sample at 28 and 90 days. The reason for this is the prevention of the opening and expansion of existing cracks due to the shrinkage of LWC caused by the bridging of polymer fibers in the openings of cracks and voids. In the case of using large amounts of polymer fibers (2.25%, 2.5%) in the LWC mix, the amount of electrical resistance compared to the sample without fibers is reduced, and the amount of chloride ion penetration into the sample is higher than in the control sample. Also, the use of the optimal percentage of polymer + basalt composite fibers has a greater effect in improving the electrical resistance and reducing the permeability of chloride into LWC than the use of polymer fibers alone. The reason for this is the positive synergistic effect of basalt fibers on polymer fibers. The addition of 1% of composite fibers to the LWC mixture improves the electrical resistance by 23.80% and 27.94% compared to the fiber-free sample, causing the number of chloride ions that penetrate into LWC to be reduced by 33.28% and 34.02% at 28 and 90 days compared to the control sample. The amount of electric charge passing through the control sample at 28 and 90 days is equal to 2503 and 2301 Columbus. According to the table provided in ASTM C1202, the penetration of chloride ions into LWC is reported for the medium control sample. By adding the optimal amounts of basalt fibers, polymer fibers, and composite fibers to LWC, the amount of electric charge passing through the sample is reduced, and the penetration of chloride ions into LWC is reduced from medium to low. It is also necessary to mention that the use of the optimal percentage of micro-silica has a significant effect and improves the fibers’ performance by improving their electrical strength and reducing the penetration of chloride ions into LWC. Micro-silica reacts with calcium hydroxide to produce hydrated calcium silicate, which fills cavities and voids in LWC and prevents the movement of chloride ions and electric field ions.

In Figure 16, the vertical axis represents the passing charge measured in coulombs for the lightweight concrete samples. The horizontal axis displays the mix design groups along with the fiber percentages used in each group.

In this figure, the following abbreviations are used:

• The letter P denotes polymer fibers.
• The letter B represents basalt fibers.
• The letter H indicates hybrid polymer + basalt fibers.

Additionally, the subscript numbers of P, B, and H indicate the percentage of fibers used in the mix design groups.

In Figure 17, the vertical axis represents the electrical resistivity of the lightweight concrete samples. The horizontal axis displays the mix design groups along with the fiber percentages used in each group.

In this figure, the following abbreviations are used:

• The letter P denotes polymer fibers.
• The letter B represents basalt fibers.
• The letter H indicates hybrid polymer + basalt fibers.

Additionally, the subscript numbers of P, B, and H indicate the percentage of fibers used in the mix design groups.

5. Conclusions

In this study, the mechanical properties, such as compressive strength, splitting tensile strength, and durability, including final water absorption, RCP, and electrical resistance, of fiber-free LWC as a control sample and LWC samples containing values of 0.25%, 0.5%, 0.75%, 1%, 1.25%, 1.5%, 1.75%, 2%, 2.25%, and 2.5% of polymer and basalt fibers in single and hybrid forms at 28 and 90 days were evaluated. To achieve the objectives of this study, 310 cylindrical specimens of LWC were constructed using lightweight expanded clay aggregates (Leca). To investigate the effects of fibers on the mechanical properties and durability of LWC, the amounts of all materials and additives used in this study were constant, and only the amounts of fibers were variable. After the experiments and data analysis, the following results were obtained:

• Increasing the curing age of LWC samples from 28 to 90 days improved their mechanical properties and durability due to the retention of moisture in the samples by water and the completion of the hydration process.
• Adding the optimal amount of all three types of polymer fibers, basalt fibers, and polymer + basalt composite fibers to the LWC mixture improved the compressive strength and tensile strength at 28 and 90 days. Adding 1% polymer fibers had the greatest impact on the mechanical properties of lightweight concrete (LWC), increasing the compressive and tensile strength of the lightweight sample by 24.4% and 26.92%, and 66.13% and 72.23% at 28 and 90 days, respectively, compared to the reference sample. Additionally, adding 1.75% basalt fibers had the greatest impact on the mechanical properties of lightweight concrete (LWC), increasing the compressive strength by 18.13% and 14.93% and the tensile strength by 41.93% and 47.22% at 28 and 90 days, respectively, compared to the reference sample.
• The best performance of using single and composite polymer and basalt fibers in LWC mixes is related to the significant increase in tensile strength of the LWC, meaning that LWC made of a brittle material is a more malleable material with a greater tensile capacity.
• Considering the failure and rupture of the control sample and the samples containing the optimal amounts of polymer fibers, basalt fibers, and composite fibers after applying compressive and tensile loads, it is concluded that the resistance of the samples containing fibers to failure and rupture due to loading the control sample is much larger, and the samples containing the mentioned fibers are less damaged and cracked. The sample containing 1% of polymer fibers had the least amount of damage and cracking.
• The addition of optimal amounts of polymer and basalt fibers, both individually and in combination, reduced the final water absorption percentage and improved the durability of lightweight concrete (LWC).
  • Basalt fibers, due to their ability to fill voids and pores, had the greatest effect on reducing water absorption.
  • Adding 1% polymer fibers reduced water absorption by 16.77% and 26.93% at 28 and 90 days, respectively.
  • Adding 1.25% basalt fibers decreased water absorption by 31.93% and 45.38% at the same ages.
  • Adding 1% polymer + basalt composite fibers reduced water absorption by 33.28% and 34.02% at 28 and 90 days, respectively.

This reduction is attributed to the fiber bridging mechanism of polymer fibers in cracks and the role of micro-silica in filling voids within the LWC matrix.

6.

Increasing the electrical resistance of lightweight concrete (LWC) reduced chloride ion penetration and, consequently, corrosion. Adding optimal amounts of polymer, basalt, and composite fibers to the LWC mix enhanced electrical resistance while reducing electrical charge and chloride ion penetration.

• Adding 1% polymer fibers increased electrical resistance by 22.06% and 25.24% and reduced chloride ion penetration by 28.52% and 31.5% at 28 and 90 days, respectively.
• Adding 1.25% basalt fibers increased electrical resistance by 46.45% and 46.42% and reduced chloride ion penetration by 36.67% and 43.15% at 28 and 90 days, respectively.
• Using 1% composite fibers improved electrical resistance by 23.80% and 27.94% and decreased chloride ion penetration by 33.28% and 34.02% at 28 and 90 days, respectively.
• These improvements are due to the fiber bridging mechanism in cracks and the role of micro-silica in filling voids, enhancing the durability of LWC.

7.

The use of optimal amounts of polymer fibers, basalt fibers, and micro-silica in the mixing design of LWC improves the mechanical properties and durability of LWC. It can be stated that this increases the reliability of the structure’s stability and health and reduces the risk of sudden structural failure. Additionally, by installing sensors and using various models, the health of the structure can be monitored and controlled throughout its operational life.

This study demonstrates that combining polymer and basalt fibers provides the best performance in enhancing mechanical properties, reducing water absorption, and improving the durability of lightweight concrete. Compared to other studies, the synergistic effect of these fibers and the role of micro-silica in reducing chloride ion penetration and increasing electrical resistance are particularly noteworthy.

Future Recommendations: The present study investigates the effects of polymer and basalt fibers on the mechanical properties and durability of LWC. Based on the obtained results, the following recommendations for future research are proposed:

It is suggested that future research should examine the long-term effects of polymer and basalt fibers on the mechanical properties and durability of LWC to better determine the sustainability of these effects. Additionally, researchers could use microscopic analysis methods to investigate structural and microstructural changes in fiber-reinforced LWC to gain a better understanding of the effects of these fibers. It is recommended that researchers examine environmental impacts, such as temperature fluctuations and humidity, on the mix design and the fire resistance of polymer and basalt fiber-reinforced concrete to obtain better and more coherent results in this area.