A Study on the Applicability of Waste Glass Wool and Waste Mineral Wool as Fiber Reinforcement

Abstract

Recently, the handling of waste industrial resources has become an issue, and the importance of sustainable resources has increased. Among these waste industrial materials are glass wool and mineral wool, which are fibrous materials used as insulation materials with characteristics such as sound absorption, insulation, and non-flammability. However, after their service life, glass wool and mineral wool used for insulation are generally buried or incinerated, causing problems such as air and soil contamination. This research was conducted to examine the applicability of waste glass wool and mineral wool obtained from expired insulation as fiber reinforcement in cement concrete. The research aimed to evaluate the fresh concrete properties, strength properties, and durability properties by adding waste glass wool and waste mineral wool up to 0.5–2.0% of the cement weight. Regarding the slump and air content of fresh concrete, the results showed that the addition of waste fibers within this range did not significantly affect the air content. However, the slump decreased as the addition amount increased due to the high absorption, which is a characteristic of the fibers. In addition, the evaluation of strength revealed that the incorporation of fibers decreased the compressive strength compared to the reference concrete. However, the tensile strength increased due to the load-supporting function of the waste fibers. In the evaluation of freezing–thawing resistance and chloride ion penetration resistance, it was confirmed that the freezing–thawing resistance improved in all cases where waste glass wool was added. The chloride ion penetration resistance was found to be similar to that of the reference concrete. However, in the case of waste mineral wool, it was observed that an addition rate of more than 2.0% of fibers was required to ensure freezing–thawing resistance. As the addition rate increased, the total charge passed (permeability) increased significantly, leading to a decrease in chloride ion penetration resistance.

1. Introduction

Glass wool and mineral wool fibers are materials used in inorganic insulation, and are widely used in various industrial fields such as automobiles, ships, and appliances due to their excellent sound absorption and soundproofing performance. However, these fiber materials require periodic replacement in order to maintain good performance, as their insulation and sound absorption performance decrease, respectively, over the period of use. In general, materials with significantly poor performance or a replacement cycle are sent to factories or companies where they can be reprocessed. However, the types of recycled fibers are also limited to synthetic fibers such as polyester, and a large amount of waste fibers are still processed through landfill and incineration. During this process, the incineration process discharges air pollutants such as dioxin and carbon dioxide, and in the landfill process, it is accompanied by soil pollution. Recently, as interest in resource recycling has increased, studies on various methods for recycling these waste fibers have been continuously conducted, and the developed technology is being used according to the use in each industry.

In the civil engineering industry, concrete has advantages in various aspects such as economic and durability, and is used as the most representative structural material used in various infrastructure such as roads, buildings, dams, and railroads. However, concrete generally shows good resistance to compressive loads, but supports only up to 12% of tensile or bending loads compared to compressive loads [1]. As a result, concrete is known to be vulnerable to tensile and easily cracks due to tensile load [2,3,4,5]. This low resistance of concrete to tensile and bending loads leads to brittle behavior of concrete. In addition to brittleness behavior, concrete has the disadvantages of low fracture resistance, impact resistance, and high weight [6]. Therefore, studies have been continuously conducted to overcome these problems. One of the ways developed to improve the brittle matrix of ordinary concrete is the addition of high tensile fibers to concrete as reinforcing materials [7,8].

Fiber-reinforced concrete (FRC) was first introduced and developed in the 1960s, and it was found to have advantages such as reducing crack generation and improving long-term mechanical properties through reform of internal structure [9]. The results of these fiber reinforcement are achieved by increasing the bonding force of the cement matrix and ultimately increasing the bending capacity of concrete due to the high tensile strength and elastic modulus characteristics of the fiber [1]. After fiber was found to have an excellent effect on concrete reinforcement, various types of fibers, such as steel fibers, artificial fibers, glass fibers, and natural fibers, have been used to reinforce concrete, and research on the characteristics of the concrete mixed with the fibers has been actively conducted [10]. In addition, among the studies on various reinforcing fibers, there are research cases in which adding polypropylene fibers to concrete reduces shrinkage cracks and improves impact resistance [11]. In another study, basalt fibers that are free from environmental pollution and have high elasticity and destructive energy were added to concrete, and it was confirmed that reinforced concrete with basalt fibers suppresses initial micro crack generation and improves mechanical properties such as permeability and freezing–thawing resistance [12]. The residual strength of reinforced concrete with latex and rock wool fibers after high temperature exposure was evaluated to examine the resistance to fire that could occur depending on the use of the structure, and it was found that fiber could improve the resistance of high temperature [13].

As the insulation performance of the structure becomes more important, the amount of waste mineral wool generated during the construction and demolition process also increased. In the study to recycle waste mineral wool, it was confirmed that the mixing of waste mineral wool improves explosive behavior of mortar [14]. A study to evaluate the applicability of steel fiber-reinforced concrete (SFRC) to tunnel lining concrete, which is essential for crack control, confirmed that SFRC can be applied to tunnel lining due to its excellent residual strength after cracking and structural spalling according to shrinkage [15]. In recent research cases on fracture properties among the excellent reinforcement effects of fiber-reinforced concrete, the distribution form of steel fibers and volume fracture were applied as variables to analyze double-K fracture model (DKFM), boundary effect model (BEM), fictitious crack model (FCM), effective crack model(ECM), and numerical simulation model (NSM). It was confirmed that the distribution form of the aligned steel fibers had better fracture properties than the random dispersion [16]. In a study to characterize the fracture process of fiber-reinforced concrete, an FE model was proposed based on the mixed-mode cohesive zone model. Experiments and simulation analysis of the model consisting of the interface between fiber and concrete, the potential fracture surface, and the interface between rebar and concrete were conducted. This research mentioned the importance of concrete friction coefficients and failure modes that affect the analysis of the numerical models, and de-bonding behavior induced by diagonal cracks that can reduce fiber reinforcement effects [17]. In addition to steel and glass fibers, which are representative reinforcing fibers, there is a study to examine the effect of various types and shapes of fibers on fiber-reinforced concrete such as PAN, PP, and PVA. Through the study, it was reviewed that the length of the fiber affects the amount of dry shrinkage reduction and the start of temperature cracks [18]. In a recent study on the reinforcement effect of carbon nanofibers, known as materials that can improve the mechanical and electrical properties of concrete, if fibers in cement are evenly dispersed by ultrasonic dispersion, carbon nanofibers show a bridging and pullout effect. It was inferred that this not only delays the cracking time of a mortar but also affects the improvement of fracture performance [19]. As a result of reviewing the characteristics of high-performance concrete mixed with carbon fiber, PP fiber, and aramide fiber according to volume fracture, it was confirmed that all three types of fibers had a positive effect on the tensile strength and bending strength of concrete. However, it was reviewed that the compressive strength decreased linearly. Mechanical properties of the hybrid fiber-reinforced concrete mixed with three types of fibers were the best. In addition, a uniaxial compression mathematical model for predicting stress and strain of HFRC was reviewed and presented [20]. Since the introduction of FRC, many research cases have been reported to mix various types of fibers into concrete. Discussions on these experimental or theoretical findings have mentioned the effect of steel fiber addition on concrete strength properties, variability in basalt fiber quality that can be problem in mixing concrete, and need to comprehensively study the hybrid fiber for concrete [21]. There are various research cases to apply coconut fibers extracted from coconuts to concrete as sustainable resources in accordance with the recent issue of sustainable construction industry promotion, and the proper addition rate of coconut fibers and reinforcement effect of concrete strength and durability were reviewed [22]. In other FRC-related studies using coconut fiber, studies were conducted to use coconut fiber as a concrete reinforcement while applying crushed door glass as an aggregate. There was no change in the amount of C-S-H gel produced through Fourier transform infra-spectroscopy analysis, and it was confirmed that adding waste glass or coconut fiber at an appropriate level was effective in improving compressive strength [23]. Moreover, various studies like this have been conducted to improve the properties of concrete through fiber reinforcement in the past, but research on use of waste fibers suitable for environmental issues as reinforcement is relatively insufficient. In addition, the use of waste fiber as a reinforcement can be proposed as a solution to environmental issues, and at the same time, economic effects can be secured due to the reduction in other reinforcements used in concrete. Therefore, this study was conducted with the aim of reviewing the reinforcement effects that may occur when waste fibers obtained from insulation are used in concrete, and in addition to physical characteristics such as workability and strength, durable factors to be considered in the actual design process were reviewed.

2. Materials and Sample Preparation

2.1. Materials

In this study, Ssangyong C&E’s ordinary portland cement (Seoul, Korea) was used, and as aggregates, 20 mm of coarse aggregate and crushed sand suitable for the particle size stipulated in the Korean Concrete Standards were used as shown in the Figure 1 and Figure 2.

Table 1. Physical properties of aggregate.

Aggregate	Gmax (mm)	Finess Modulus	Density (g/cm3)		Absorption (%)	Criteria of South Korea
			Drying	Saturated Surface Dry		Drying Density (g/cm³)	Absorption (%)
Sand	-	3.0	2.63	2.65	0.65	Over 2.5	Under 3.0
Gravel	20	6.6	2.60	2.62	0.71	Over 2.5	Under 3.0

represent the physical properties of the aggregates. The coarse aggregate exhibited a density of 2.65 g/cm³ under saturated surface dry conditions, with an absorption rate of 0.65%. The sand was found to have a density of 2.62 g/cm³ and an absorption rate of 0.71%. As for the waste fiber wool used in this research, waste glass wool and mineral wool that had reached the end of their service life were used, as depicted in Figure 3. The chemical compositions of each waste fiber are detailed in

Table 2. Chemical composition of waste wool fiber.

Type	Content (%)
	SiO2	Al2O3	CaO	Fe2O3	MgO	MnO	Na2O
Waste Glass Fiber	67.3	2.33	8.76	0.52	3.25	0.01	14.9
Waste Mineral Wool	39.8	14.9	34.9	0.62	5.41	0.34	0.00

. Additionally, an air entraining agent and a water reducer were used to evaluate the changes in the physical properties of waste fiber-reinforced concrete in its fresh concrete condition.

2.2. Sample Preparations

In order to evaluate the properties of concrete according to the addition of waste fibers, reference concrete and waste fiber-reinforced concretes were prepared according to the type of waste fiber wool and addition rate, and

Table 3. Mix design for concrete.

Sample No.	Fiber Type	Unit Weight (kg/m3)
		Fiber	Cement	Water	Sand	Coarse	AE	WR
OPC	-	-	318	175	783	967	0.032	1.59
GW 0.5	Waste Glass Fiber Wool	1.59
GW 1.0		3.18
GW 2.0		6.36
MW 0.5	Waste Mineral Fiber Wool	1.59
MW 1.0		3.18
MW 2.0		6.36

shows the name and mixing ratio of the specimens used in this research. The waste fiber wool with addition rate of 0.5, 1.0, and 2.0% were added based on the weight of the cement, respectively. The design slump and air content for each concrete were determined to be 150 mm and 6%, and the same water/cement ratio of 55% was applied to all mix design. The amount of air entraining agent and water reducer used to satisfy each design value was applied with the addition rate determined from the previous studies.

The mixing of concrete with added waste fiber wool for specimen preparation was conducted using a forced mixer, following the sequence depicted in Figure 4.

First, fine aggregate and coarse aggregate were mixed for 30 s, and then, waste fiber wool was added and mixed for 30 s. After that, cement was added and mixed for 30 s, and finally, blending water and additives were added, and mixing was carried out for 2 min. After mixing, properties of fresh concrete mixture were evaluated according to KS F 2402 “slump test method of concrete” [24] and KS F 2421 “air flow test of concrete” [25], respectively. Then, specimens were created in a circular cylinder type of 100 × 100 × 200 mm and a prism type of 100 × 100 × 400. Specimens were cured in the atmosphere by covering a tarpaulin to prevent rapid evaporation of moisture in the atmosphere at 20 °C for 24 h, and after atmospheric curing, the samples were demolded and cured at 20 ± 2 °C in a constant temperature water bath.

3. Methodology

3.1. Slump Test and Air Content Test of Fresh Concrete

Slump is a property that is directly related to the workability of concrete and affects the compactibility and pumpability of the mixture. Therefore, excessive compression energy may be required when placing concrete mixtures that do not satisfy the designed slump, which may induce material separation of concrete, resulting in a decrease in strength and durability. As the addition rate of fibers in concrete increases, the viscosity of the mixture increases and the slump decreases [26]. Therefore, it can be said that fiber-reinforced concrete shows a different slump behavior from ordinary concrete, and as the fiber addition rate increases, measures such as increasing the unit water or increasing the use of water reducer are applied to satisfy the design slump. This test was conducted for the purpose of evaluating flow properties of fresh concrete according to the type and addition rate of waste fiber wool by KS F 2402. The air content of concrete has a great influence on durability such as freezing–thawing resistance as well as strength. If the air content is excessively dispersed in concrete, the strength of the concrete will decrease significantly, making it difficult to meet the strength standards, and it may also have a negative impact on durability. Thus, when concrete is placed in the cold region, an air entraining agent is added to most concrete to ensure freezing–thawing resistance [27]. It is commonly known that the freezing–thawing resistance of concrete is the best at the air content within 6% [28]. The air content is a measure to evaluate durability of concrete when it is not hardened. This test was conducted to examine the change in air content when waste glass wool and mineral wool were added to the concrete. The test method was conducted according to KS F 2421.

3.2. Compressive Strength Test

Concrete is known as a material with excellent compressive strength, which can be used as the most representative structural material. Various fiber reinforcement materials (polymer fiber, steel fiber, glass fiber, etc.) are applied to reinforce concrete generally and improve the bending strength of mortar, but it is known that the compressive strength decreases at different rate depending on the type of fiber and addition rate [29,30]. However, there are a some studies in the case of fiber-reinforced concrete that differ in regard to the decrease in compressive strength. Kaushik’s study shows that the compressive strength increases when 20 to 30 percent of cement was substituted by fly ash and glass and steel fiber were added to concrete [31]. The compressive strength test was conducted to evaluate the change in compressive strength that may occur when fibers are added, the test was conducted according to KS F 2405 “Test method for compressive strength of concrete” [32]. The wet curing was performed on a manufactured cylindrical specimen with diameter of 100 × 200 mm, for 28 days and 90 days. After curing time of designed age, the surface of specimens were polished to prevent eccentric load and placed on a uniaxial compression test equipment. Then compression load was applied until the specimen was destroyed at a speed of 0.6 ± 0.4 MPa to measure the compressive strength.

3.3. Flexural Strength Test

The compressive strength and splitting tensile strength of concrete are considered as the most important parameters. Flexural strength on the other hand is of relatively low importance, but a critical factor that can predict the load that can lead to defects such as the start of cracks [33]. Fibers added during the mixing process of concrete are randomly dispersed while being meshed and entangled around the aggregate. Through this process, fiber-reinforced concrete has a high tensile strength compared to plain concrete [34]. This test was conducted to evaluate the flexural strength of concrete added with waste fibers, and it was conducted according to KS F 2408 “Standard test method for flexural strength of concrete” [35]. The prismatic specimens of 100 × 100 × 400 mm wet cured during the designed age were used. The specimen was placed on equipment of the same model as the compressive strength equipped with the jig for measuring flexural strength, and the load was applied at a rate of 0.06 ± 0.04 MPa/s until the specimen was destroyed by applying the three-element loading method as shown in Figure 5.

3.4. Freezing and Thawing Resistance

Concrete is a porous and brittle material, and defects may occur due to reduced strength or crumbling when constructed in a site where freezing and thawing environments are repeated, such as in a cold area [36]. In particular, when water absorbed into the concrete structure is frozen by the surrounding environment, volume expansion occurs, and there is a larger space for moisture to move into the concrete. After then, as the freezing and thawing process repeatedly occurs, the deterioration of concrete accelerates, significantly reducing overall durability such as penetration resistance of chlorine ions and harmful substances. Therefore, evaluating the freezing–thawing resistance of concrete is one of the parameters to evaluate the durability of concrete during the service life. This experiment was performed to evaluate whether the addition of waste fibers to concrete had an effect on the freezing–thawing resistance. The experiment was conducted according to ASTM C 666 “Standard Test Method for Resistance of Concrete to Rapid Freezing and Thawing” [37]. After the specimens in the form of a prism of 100 × 100 × 400 mm at the age of 14 days was placed in the equipment, the specimens were subjected to freezing–thawing in one cycle of 4 h in the temperature range of −18 °C to 4 °C as shown in Figure 6. The evaluation was conducted through visual rating and weight retention ratio every 30 cycles, and the reduction ratio of the relative dynamic modulus of elasticity was reviewed. The dynamic elasticity modulus of elasticity was measured using the equipment as shown in Figure 7. The test was conducted until the relative dynamic modulus of elasticity ratio was reduced to 60% or less or until the freezing–thawing action reached 300 cycles as specified in ASTM C666.

3.5. Chloride Ion Penetration Resistance

The penetration of foreign substances inside concrete is mainly affected by the fine pore structure of concrete, construction site, curing time, etc. [38]. Materials such as water, chloride, and acid can reduce durability and strength when penetrating into concrete [39]. Among the above materials, chlorine ions not only adversely affect the internal matrix of concrete, but also induce corrosion rebar when contacting reinforced concrete, and when combined with freezing and thawing, the problem of durability accelerates. According to a related research case, fibers added to concrete can alter several physical properties of concrete, including voids and permeability [40]. The diameter of harmful pores defined by researchers varies, but in general, the pore structure inside concrete is known to directly affect the penetration of ions or moisture. The chloride ion penetration test was conducted according to ASTM C 1202 “Electrical Indication of Concrete’s Ability to Resistance Chloride Ion Penetration” [41] to evaluate the resistance to chloride ion penetration of concrete reinforced with waste fibers. A side surface of specimens at the age of 28 days was coated with epoxy, cut to a thickness of 50 mm, and ion diffusion cells were attached to both ends thereof, and each cell was filled with calcium chloride solution and calcium hydroxide solution, and then a voltage of 30 V was energized for 8 h. Figure 8 shows the experiment foreground and schematic diagrams of the experiment, respectively. The current flowing through the specimens were collected through a data logger at 30 min intervals, and the passing charge amount for each specimen was calculated, and the evaluation was performed according to the criteria specified in ASTM C 1202 as shown in

Table 4. Standard for chloride ion penetration resistance.

Charge Passed (Coulombs)	Chloride Ion Penetrability
>4000	High
2000~4000	Moderate
1000~2000	Low
100~1000	Very Low
<100	Negligible

.

4. Results and Discussion

4.1. Slump Test and Air Content Test of Fresh Concrete

Slump and air content tests were conducted to evaluate the effect of waste fibers on the properties of fresh concrete, and the results are as follows: First, the results of slump according to the addition of waste fiber wool are shown in Figure 9. The design slump of the reference concrete was set to 150 mm, but a result after measuring the slump of the reference concrete was 135 mm. In Korea, the design slump of 80 mm or more has an allowable range of ±25 mm, so it was found that the reference concrete satisfies the allowable range of the slump. The results of slump test when waste glass wool was added in concrete were 130, 130, and 100 mm for 0.5, 1.0, and 2.0%, and when waste mineral wool results were 135, 125, and 100 mm, respectively. This means that at the addition rate of 2.0% or more, a slump change appeared to affect the workability of fresh concrete. In addition, these results are similar to those of studies related to fiber-reinforced concrete, which consistently showed that generally show that workability decreases with higher fiber addition rates in fiber-reinforced concrete [42,43,44]. Therefore, the low fractions of waste fiber wool do not significantly affect the flow properties of concrete, so it can be applied without changes in the existing mix design, but at the addition rate of 2% or more, measures are needed to offset the increase in adhesion due to waste fiber wool addition. Figure 10 shows the result of air content according to the type and addition rate of waste fiber wool, and the air content of reference concrete added with air entraining agent was 6.0%. The result of air content for concrete with waste glass wool and waste mineral wool fiber addition rates of 0.5, 1.0, and 2.0% were showed 6.0, 5.5, and 6.0% (waste glass wool) and 6.5, 6.5, and 6.5% (waste mineral wool), respectively. There was no significant change in the total air content of concrete according to the addition rate of waste fiber wool, so the effect of the addition rate of waste fiber wool on the air content was found to be insignificant compared to the slump characteristics. These means that the two types of waste fibers used in this study did not affect the computing process of addition rate of air entraining agent.

4.2. Compressive Strength of Waste Fiber-Reinforced Concrete

Figure 11 and Figure 12 show the results of measuring compressive strength for 28 days and 90 days according to the type and addition rate of waste fiber wool. As shown in Figure 11, the results of compressive strength at 28 days for concrete to which waste fiber wool was added decreased compared to the reference concrete. Reference concrete showed compressive strength of 38.4 MPa at 28 days, and concrete to which waste glass wool and waste mineral wool were added showed compressive strength of 36.6, 36.4, and 34.9 MPa (waste glass wool) and 32.0, 33.5, and 34.7 MPa (waste mineral wool), respectively, depending on the addition rate. The compressive strength of the waste glass wool decreased as the addition rate increased, but the compressive strength of the waste mineral wool fiber also increased as the addition rate increased. However, in the case of waste mineral wool fibers, the maximum compressive strength was 34.7 MPa (2.0%), while for the specimen with added glass fiber wool, the lowest compressive strength was found to have a compressive strength similar to that of waste mineral wool at 34.9 MPa (2.0%), despite the decrease in compressive strength as the addition rate increased. This could be confirmed more clearly in terms of strength reduction rate, and the maximum compressive strength reduction rate of the waste glass wool-added specimen was 10%, but the maximum compressive strength reduction rate of the waste mineral wool specimen was about 17%. Figure 12 is a graph showing the results of measuring compressive strength for 90 days under the same conditions, and reference concrete showed compressive strength of 43.7 MPa at 90 days, and in specimens added with waste glass wool and waste mineral wool fibers, concrete showed compressive strength of 43.0, 41.8, and 41.4 MPa (waste glass wool) and 38.6, 38.5, and 40.7 (waste mineral wool), respectively. The compressive strength of 90 days also showed a similar tendency to 28 days, but the reduction rate of compressive strength was found to decrease significantly. The maximum reduction rate of compressive strength of waste glass wool was found to be about 5% at the addition rate of 2%, and for waste mineral fiber wool was confirmed to be 12% at addition rate of 0.5%. The reason why the compressive strength decreased as the addition rate of waste fiber wool increased is judged to be the result of the high absorption property of fibers. In general, when fibers are mixed into concrete, the mixed water is absorbed by fibers due to the high absorption, which is the characteristic of the fiber itself, and as a result, a number of interfaces with poor adhesion between cement paste and fibers are formed, which adversely affected the compression load.

4.3. Flexural Strength of Waste Fiber-Reinforced Concrete

In general, the flexural strength of concrete is known to be very low, approximately less than 10%, compared to the compressive strength, and for this reason, the propagation of cracks under flexural load is very fast. So, controlling cracks due to such flexural load may play an important role in the durability life of concrete [45]. Reinforcement of concrete through fibers can increase the flexural capacity of concrete and reduce the generation of tensile cracks [46]. This experiment was conducted to evaluate the effect of improving the flexural strength of concrete through the addition of waste glass wool and waste mineral wool. The results of measuring the flexural strength of concrete according to the waste fiber wool addition rate are shown in Figure 13 and Figure 14. As shown in Figure 13, flexural strength at 28 days for the reference concrete was 6.12 MPa and for the specimen with the addition rate of 0.5% of waste glass wool and mineral wool was 6.16 and 6.13 MPa, respectively, so it was confirmed that the improvement effect on the flexural strength was not significant at the addition rate of 0.5%. However, at the addition rate of 1.0% or more, it was found that the flexural strength was 6.42 MPa (waste glass wool) and 6.47 MPa (waste mineral wool), and when the addition rate was increased to 2.0%, the strength increase was 6.77 MPa (waste glass wool) and 6.76 MPa (waste mineral wool), about an 11% increase. Similarly, as shown in Figure 14, the strength of the reference concrete was 6.34 MPa at 90 days, and in concrete with 0.5% addition rate of waste glass wool and mineral wool, it was 6.52 and 6.67 MPa, respectively, and no significant improvement effect was confirmed. This means that the appropriate addition rate of the waste fiber wool for the flexural strength improvement effect is 0.5% or more, and at the addition rate of 1.0%, it is 7.09 MPa (waste glass wool) and 6.73 MPa (waste mineral wool), respectively, which shows a clear strength improvement effect compared to the reference concrete. In addition, when the addition rate is increased to 2.0%, it is 7.56 and 7.03 MPa, which have a strength improvement effect of 19 and 11%, respectively, compared to the reference concrete. In particular, waste glass wool was found to have the highest strength increase between 28 and 90 days at a 2.0% addition rate, and at the addition rate of 1.0% or more, it was confirmed that the effect of improving flexural strength was more pronounced than that of waste mineral wool. This effect of improving flexural strength is considered possible because fiber strands dispersed in concrete form a 3D network structure to support a part of flexural load and performed a role of a bridge until destruction by flexural load.

4.4. Freezing and Thawing Resistance of Waste Fiber-Reinforced Concrete

Concrete is a structural material constructed under various environments and is directly exposed to various environmental conditions such as rainfall, heat waves, and snowfall. In particular, in the case of a cold region, a phenomenon in which moisture due to rainfall penetrates into the concrete and freezes may occur. At this time, moisture inside the concrete is frozen and expands to about 9%, and accordingly, expansion pressure is applied to the concrete [47]. Continuous application of expansion pressure to concrete can cause cracks and, in severe cases, lead to defects such as destruction of concrete. Various types of fibers, such as steel fiber, artificial fiber, and glass fiber, are widely used as reinforcing materials that can be an alternative to securing durability such as freezing–thawing resistance of concrete through many research cases [48]. This experiment was conducted to evaluate whether the freezing–thawing resistance, one of parameter of concrete durability evaluation, can be improved by the addition of waste fiber wool. Figure 15 and Figure 16 show the results of the measurement of relative dynamic modulus of elasticity by cycle of freezing–thawing according to the addition rate of waste glass wool and waste mineral wool. Most of the waste glass wool-added specimens in Figure 15 showed high resistance to freezing–thawing compared to reference concrete. For reference concrete, the relative modulus of elasticity dropped below 60% in 210 cycles, while in waste glass wool-added specimens subjected to the same cycle, it was found to be 62 (0.5%), 66 (1.0%), and 66% (2.0%), respectively. It was confirmed that the addition of waste glass wool affects the improvement of the freezing–thawing resistance of concrete, and this effect is because the waste glass wool dispersed in concrete shares the expansion pressure with freezing pressure to delay the generation of cracks and prevent the cement paste from being separated. Figure 16 is the result of measuring the relative modulus of elasticity for the waste mineral wool-added specimens, and it was found that the relative modulus of elasticity was higher up to the initial 120 cycles compared to the reference concrete. However, after 150 cycles, specimens with 0.5% and 1.0% added waste mineral wool decreased sharply to 71 (0.5%) and 66% (1.0%), respectively, particularly in specimens with 0.5% added waste mineral wool that were destroyed in 180 cycles as it can be shown in the Figure 16. The 1.0% addition specimen showed a relative modulus of elasticity of 58% in the same cycles, which did not meet the criteria. When 210 cycles of freezing–thawing were applied, the relative dynamic modulus of elasticity of reference concrete fell below the criterion, but the 2.0% waste mineral wool-added specimen maintained above the criterion at 64%, and it was confirmed that it fell below the standard in 240 cycles to 54%. This means that in the case of waste mineral wool, unlike waste glass wool, the appropriate fiber addition rate required to improve the durability in terms of freezing–thawing resistance is high.

Figure 17 and Figure 18 shows the weight retention rate of specimens to the application of freezing–thawing cycles by type of waste fiber and addition rate. As a result of the measurement, it showed a similar tendency to the result of the relative dynamic modulus of elasticity. Until the initial 150 cycles, both specimens to which two types of waste fiber wool are added had a weight loss due to freezing–thawing less than reference concrete, but after 150 cycles, the weight retention rate was decreased sharply in specimens with low addition rate of waste fibers. In the case of waste mineral wool, like the relative dynamic modulus of elasticity, the weight retention rate was 83% in the specimen to which 1.0% was added which was 4% lower than 87% of the reference concrete. These results represent that the addition of waste fiber wool can improve the freezing–thawing resistance, but the appropriate addition rate varies depending on type of waste fiber wool, and at a low addition rate of 1.0%, it cannot be expected to improve the freezing–thawing resistance through waste mineral wool.

Figure 19, Figure 20, Figure 21, Figure 22, Figure 23, Figure 24, Figure 25, Figure 26 and Figure 27 are photographs taken to perform visual evaluation of each sample every 30 cycles, and little scaling of the surface occurred from the initial 0 to 30 cycles. After 30 cycles, scaling occurred radically and exposing the coarse aggregate to the sample surface, and after 90 cycles, some coarse aggregates fell off. In particular, similar to the results of the weight retention rate in 150 cycles, coarse aggregate raveling occurred remarkably in the 0.5% addition sample of waste glass wool and in 0.5 and 1.0% addition samples of waste mineral wool, and thereafter, coarse aggregate fell off from the corners of all freezing–thawing specimens. In addition, in 210 cycles, the mineral wool 0.5% addition specimen was completely destroyed, and defects were observed in all specimens to the extent that there was a severe shape change compared to initial specimens. As a result, it was confirmed that the improvement on freezing–thawing resistance of concrete added with waste glass wool fiber and waste mineral wool fiber varies slightly depending on the addition rate and type of fiber. This means that the amount of addition required to secure appropriate resistance by freezing pressure in concrete differs because the tensile properties vary depending on the type of waste fiber.

4.5. Chloride Ion Penetration Resistance of Waste Fiber-Reinforced Concrete

The penetration of chloride into concrete structures can lead to corrosion and reduction in strength of rebar, resulting in early repair of the structure [49]. Therefore, the chloride ion penetration resistance of concrete is an important factor for evaluating the durability, and in general, the easier it is to penetrate chloride ions, the higher the total charge passed is measured. This experiment was conducted to evaluate the chloride ion penetration resistance of concrete in which waste fiber wool was added. Figure 28 shows a graph depicting the result of deriving the total charge passed by accumulating the charge passed measured in units of 30 min for 8 h on the 28-day specimens. First, the reference concrete was 3423 Coulomb, showing a moderate level of charge passed based on ASTM. The specimen to which waste glass wool was added showed passing charge amounts of 3619, 3582, and 3468 Coulomb with the addition rates of 0.5%, 1.0%, and 2.0% respectively, and it was confirmed that the total charge passed was similar as it showed a difference of up to 6% compared to reference concrete. However, the specimen to which waste mineral wool was added showed total charge passed of 3716, 4304, and 4547 Coulomb by addition rate, and as the addition rate increased, the total charge passed was also increased. When more than 1.0% of waste mineral wool was added, the level of total charge passed was higher than reference concrete, which sharply decreased the chloride ion penetration resistance. In general, when fibers were dispersed in concrete, they reduced the size of the pores in the cement matrix and formed micro pores that increased the density of the internal structure. As a result, it is known that durability may be improved through impermeability. However, if the fiber dispersion in the concrete is uneven or other substances in the fiber were present and mixed together, pores that may inhibit permeability may be formed, thereby deteriorating the concrete pore structure. Therefore, in the case of waste glass wool, there was no improvement in chloride ion penetration resistance through formation of fine pore structures in concrete, but it was expected that it can be applied as a fiber-reinforcing material according to similar permeability to reference concrete. But, specimens to which waste mineral wool was added, the total charge passed increased by up to 33% compared to reference concrete, which means that as the addition rate increased, harmful pores with a size that is easy for chloride ions to penetrate were formed in the cement matrix. Thus, it is considered that the addition of waste mineral wool to concrete cannot be expected to improve the durability aspect such as chloride ion penetration resistance.

5. Conclusions

This research was conducted to evaluate the applicability of using concrete reinforcement of waste glass wool and waste mineral wool secured from waste insulation that has passed the use period. The strength properties of concrete mixed with waste wool fiber and durability related to freezing–thawing and chlorine ion penetration were evaluated, and the overall conclusions are as follows.

• As a result of reviewing the characteristics of fresh concrete mixed with waste fiber wool, it was found that when the amount of waste fiber wool mixed is more than 2%, the slump decreases sharply compared to the reference concrete. It can be concluded that this is due to the high absorption characteristic of waste wool fiber. However, it has been confirmed that the addition of waste fiber wool has little effect on air content. In conclusion, if more than 2% of waste fiber is added, it is assumed that it is necessary to add a water or a water reducing agent to satisfy the design slump.
• In the measurement of the compressive strength of the waste fiber wool-added concrete, the compressive strength tended to decrease slightly because an interface that could affect the compressive strength of concrete was formed, and it was confirmed that up to 5% was added to waste glass wool fiber and 12% was added to waste mineral wool fiber.
• As a result of the tensile strength measurement, the tensile strength increased for both types of waste fiber wool, and it was confirmed that the tensile strength increased as the addition rate of waste fiber wool increased. The maximum tensile strength increase rate at 28 days was found to be about 11% at 2.0% addition rate. As a result of measuring the tensile strength at 90 days by adding 2% waste fiber wool, the waste mineral wool maintained a strength increase rate of 11%, but in the case of the waste glass wool, the tensile strength further increased, resulting in an improvement effect in tensile strength of about 19%.
• As a result of reviewing the freezing–thawing resistance of the waste fiber wool-added concrete, regardless of the type of waste fiber wool, it has been confirmed that the freezing–thawing resistance of concrete through the relative dynamic modulus of elasticity and weight retention rate improves when more than 2% is added. At a low addition rate of less than 2%, it was confirmed that there was no or insignificant improvement effect depending on the type and addition rate of waste wool fiber. This means that more than an appropriate amount must be added to form a network structure and support the expansion pressure generated during the freezing process.
• As a result of the chloride ion penetration test, which is an element of durability, it was found that when waste glass wool was added, the total charge passed was similar to that of reference concrete regardless of the addition rate. But, in the case of waste mineral wool, the total charge passed increased as the addition rate increased, and in case of addition rate above 1%, the total level of charge passed was high. Therefore, it was confirmed that the addition of waste mineral wool had no effect on improving the permeability of concrete.

The overall conclusions of this research are that, in the case of waste glass wool, the effect of reducing the compressive strength was insignificant even at high fraction, and the effect of improving the tensile strength was clear. Additionally, it was determined that waste glass wool can be used as a fiber-reinforcing material based on the results of the experiment assessing durability such as freezing–thawing resistance and chloride ion penetration. However, in the case of waste mineral wool, the improvement in tensile strength is clear, but the reduction in compressive strength is larger than that of waste glass wool, and the durability was disadvantageous. Therefore, it is believed that follow-up studies on other matters such as additional methods or application after reprocessing are needed.