Thermal Properties of Carbon Fiber-Reinforced Lightweight Substrate for Ecological Slope Protection

Abstract

A new ecological substrate is proposed to achieve a desired electric conduction and heating to protect the slope plants from freeze injury. Expanded polystyrene (EPS), cement, carbon fiber, graphite, and raw soil are the main components of the ecological substrate. The electrical conductivity, heating efficiency, thermo-sensitivity, and heat preservation of the substrate are experimentally investigated. The result shows that the addition of carbon fiber could significantly decrease resistivity of substrate, but the effect of fiber content exceeding 3% on the resistivity of substrate becomes insignificant. Conductive fine materials (i.e., carbon fiber and graphite powder) covering the surface of EPS would result in a significant reduction of the global resistivity of non-dry substrate, but it could only slightly affect the counterpart of the completely dry substrates. The substrate could hardly be formed when the EPS content exceeds 4%. As EPS content increases, the contact surface decreases and the resistivity of the substrate increases. The peak temperature at 30 min of substrate without root is higher than that of substrate with plant roots. Nevertheless, the temperature alteration ratio below 40 °C of substrate with plant root is nearly the same as its counterpart in the substrate without roots. The resistance of the substrate with plant roots increases with the temperature. The resistance of rootless substrate decreases by the heat action of the loosely bound water. EPS particles improve the heat preservation performance of substrate, but the heat preservation performance of substrate degrades with the growth of plants.

1. Introduction

Exposed slopes are most prone to water erosion, particularly in areas with a high rainfall, thereby often leading to disastrous landslides [1]. As reported by the U.S. Geological Survey (USGS), thousands of major landslides were triggered by rainfall each year [2]. Over the past decades, various solutions have been proposed to prevent the occurrence of such landslide accidents. For example, bolt anchorage has been widely recognized as one of the most effective and economic measures to stabilize the slope. Nevertheless, it has been reported that the concrete for bolt anchorage could often age with the elapsed time due to steel corrosion, which often results in the failure of a bolt-anchorage stabilization system [3,4]. Similar issues also occur for other civil engineering measures in preventing the landslide accidents, such as gravity retaining wall [5] and anti-slide pile [6]. To address the above issues, various attempts have been undertaken to seek alternative measures that could prevent landslide effectively in the long term.

Instead of traditional civil engineering measures, the use of plants on slopes to prevent landslides and slope instability has received increasing attentions by many researchers over the past years [7,8]. It has been widely reported that the coverage of vegetation with the plant roots could significantly reduce the slope surface erosion and decrease the soil and water loss [9,10,11]. In particular, as the plant roots grow, it would provide anchoring into bedrock or stable soil layers, which would be further favorable for the slope stability [12,13,14].

Due to the high production efficiency, soil-spraying technology, which was first developed by Japanese scholars [15], has been widely used in vegetation engineering for slope restoration. Nevertheless, it was reported that it is difficult to facilitate vegetation on the steep slopes [16]. To address this issue, various researchers [17] attempted to spray the ecological substrates on the slope surface for the slope restoration. The ingredients of the ecological substrate are usually composed of fibers, loam, cement, and a pH-adjusting agent, etc. literature [18] indicated that the ecological substrate can greatly promote the plants’ growth, thereby effectively restoring the slope. However, it has been reported that the young plants for slope protection, such as cynodondactylon and zoysiagrass, would stop growing and be fatal to the cold weather, especially when it encounters continuous snow in the winter [19,20]. However, to date, little attention has been paid to the functionality of ecological substrate in protecting the plant growth in extremely cold weather, in contrast to the studies on the vegetative properties of ecological substrate [21,22].

Therefore, the current work aims to propose a new type of ecological substrate with the capability of promoting the plants’ growth in extremely cold winter. Expanded polystyrene (EPS), cement, carbon fiber, graphite, and raw soil are considered as the main components of the ecological substrate. Then, a series of laboratory tests are conducted to investigate (1) the effect of carbon fiber and EPS particles on conductivity; (2) the effect of curing period on substrate resistivity; (3) the thermal properties (e.g., heating efficiency and thermo-sensitivity) of proposed ecological substrate; and (4) the heat preservation performance of proposed ecological substrate. Detailed results are presented and discussed in the following sections.

2. Materials and Methods

2.1. Materials

The ingredients of substrate include EPS particles, cement, carbon fiber, graphite powder, and soil. Carbon fiber is adopted as conductive heating material to reduce the resistance of the substrate to electric heating and undertakes the role of reinforcement to prevent substrate cracking. The graphite powder is also used to assist carbon fiber in electric heating [23,24,25], as well as other conductive materials, such as steel fibers [26]. Carbon fiber was purchased from Toho Chemical Industrial (Shanghai) Co. Ltd., Shanghai, China, and the physical-mechanical properties of the carbon fiber used here are shown in

Table 1. Physical-mechanical properties of carbon fiber.

Parameter	Value or Grade
Electrical resistivity/(Ω·cm)	1.5 × 10−3
Single core diameter/μm	7
Length/mm	6
Tensile strength/MPa	4900
Elastic modulus/GPa	240
Acid and alkali resistance	Strong

. The graphite powder used in this work was purchased from Xieli Graphite Co. Ltd., Dongguan, China, and the physical properties of the graphite powder are shown in

Table 2. Physical properties of graphite powder.

Parameter	Value or Grade
Electrical resistivity/(Ω·cm)	30 × 10−2
Diameter/μm	30
Carbon content/%	99
Thermal conductivity/W/(m·K)	151
Acid and alkali resistance	Strong

.

Considering the reduction of heat loss and energy consumption, the EPS that is often used in eco-geotechnical engineering [27,28] is added into the substrate for heat preservation. EPS particles used in substrate could make itself lightweight [29,30]. In the current study, EPS particles with diameter varying from 1 mm to 3 mm were used. The particle density and packing density of EPS particles are 0.024 g/cm³ and 0.016 g/cm³, respectively.

Silty soil used for the tests was excavated at the depth of 30 cm~50 cm below the course surface, from the bank of Xunsi River in Wuhan city. After a series of laboratory tests, the physical properties of the studied soils were obtained and are shown in

Table 3. Physical properties for studied soil.

Parameter	Value
Natural density/(g/cm3)	2.03
Water content/%	21.5
Liquid limit/%	43.7
Plastic limit/%	23.45
Optimum moisture content/%	24.85
pH	6.23

. All the above materials are granular materials. Thus, the regular Portland cement were used as solidified agent.

Prior to resistivity tests and electric heating tests, the soil was firstly dried up through the oven at the temperature of 50 °C. Then, the dried soil was grounded into powder and mixed with the cement according to the prescribed proportion. Thereafter, water was added into the mixture of cement and soil to achieve the global moisture content of 50%. Subsequently, carbon fiber, graphite powder, and EPS particles were added to the mixture to form the substrate via slow stirring.

Sixteen mix ratios were prepared. It is noted that the mix ratio used here is defined in terms of the ratio of weight of each ingredient to the total weight of dry soil. A more detailed procedure for the specimen preparations can refer to the literature [31,32]. The testing program in this work is shown in

Table 4. Testing program for the specimens with different mix ratios.

Test ID	EPS (%)	Carbon Fiber (%)	Cement (%)	Graphite Powder (%)
1	1	1	2.5	5
2	1	2	5	10
3	1	3	7.5	15
4	1	4	10	20
5	2	1	7.5	10
6	2	2	10	5
7	2	3	2.5	20
8	2	4	5	15
9	3	1	10	15
10	3	2	7.5	20
11	3	3	5	5
12	3	4	2.5	10
13	4	1	5	20
14	4	2	2.5	15
15	4	3	10	10
16	4	4	7.5	5

The percentage (%) shown in the table is the weight of ingredients to the total weight of dry soil.

.

2.2. Resistivity Test

Before test, the substrate was poured into a mold (diameter = 39.1 mm, height = 80.0 mm) in 5 layers, and each layer was compacted with 15 blows. Then, the prepared specimens were cured for 7 days. The above procedure was applied to all specimens. During the specimen preparation, it was found that when the content of EPS particles is over 3%, it makes it difficult to conduct the compaction due to the fact that the substrate disintegrates significantly as the EPS content increases. Therefore, in order to ensure that the substrate gains sufficient strength, the suggested EPS content could not exceed 4%.

In this study, the TH2817A Precision LCR Meter manufactured by Langpu electronics technology Co. LTD, Shenzhen, China was used to measure the resistivity of the specimen. The two-electrode method which has also been successfully used by other researchers [33] was employed here. The measurement of resistance of substrate specimen in the field is shown in Figure 1a. It is noted that the measuring instrument had been preheated for 30 min before tests. A contact transducer was used at both ends of the specimen. To ensure a good contact between the contact transducers and the ends of specimens, a 250 g counterweight block was placed on the upper end of the specimen. The resistivity of the specimens was recorded when the value as shown on the instrument screen was stable. Three measurements were conducted for each specimen and the average value was taken. The electron micro-structure of specimen is shown in Figure 1b.

2.3. Electric Heating Test

For electric heating tests, the test mold with dimension of 20 cm × 20 cm × 5 cm was manufactured. In order to reduce the heat loss during heating and heat preservation process, EPS board with thickness of 1 cm was adhered to the external sides of the mold. Besides, a self-made electrode (20 cm × 5.5 cm) made of 304 steel wire mesh (wire diameter = 0.4 mm, mesh dimension = 2.8 mm × 2.8 mm) was inserted in the substrate at a distance of 3 cm from the mold boundary, as shown in Figure 2.

The top view of field measurement to temperature of substrate is indicated in Figure 3a. Three temperature sensors were connected with the concrete thermometer, which were placed in the monitoring points at surface of the substrate to monitor the temperature. The detailed information is indicated in Figure 3b. Point 1 is at the left bottom corner, 2 cm away from the boundary and the electrode, respectively. Point 1 and Point 3 are centrosymmetric. Point 2 is at the center of the substrate. To meet real-world engineering conditions, the plants were grown in the developed substrates, as shown in Figure 3c. Details of the planting can refer to literature [32]. Variation of temperatures with the elapsed time of substrate with and without plant roots was monitored, respectively.

In order to effectively reflect the heating efficiency and changes of heat sensitivity in the heating process, the form of constant voltage was applied. The DC power supply with maximum power of 300 W (60 V, 5 A), which was purchased from Maisheng electronics technology Co. LTD, Dongguan, China, was used here. The power supply was linked with the electrodes, and electric heating test was conducted under the constant voltage of 60 V. During the test, the variation of real-time temperature values and current values were monitored every minute. The thermometer used herein for temperature monitoring was JDC-2, which was purchased from Meiyu instrument Co. LTD, Shanghai, China.

3. Results and Discussion

3.1. Effects of Carbon Fibre and EPS Content on Substrate Resistivity

Figure 4 shows the variation of resistivity of the specimen with the increasing fiber content corresponding to different EPS contents (1%, 2%, 3%, and 4%) after seven days of curing. It is observed that the resistivity decreases exponentially with the increase of carbon fiber content for the specimens with all EPS contents, especially when fiber content is less than 3%. As the carbon fiber content increases from 1% to 4%, the average resistivity of the specimen decreases significantly from 210 Ω·cm to 37 Ω·cm. This phenomenon might be attributed to the fact that the excess carbon fiber that has ultra-low resistivity is distributed in the substrate and forms a conductive circuit with graphite powder, which exceeds the percolation threshold and results in the reduction of the global resistivity of the substrate. When the content of carbon fiber less than 3%, the volume fraction is less than the percolation threshold. The conductive circuit can not fully contact, and the resistivity of the substrate is controlled by other base ingredients. However, with the increase of carbon fiber content, the volume fraction of carbon fiber approaches the percolation threshold indefinitely. The resistivity of the substrate increases rapidly, so a nonlinear variation occurs. When the volume fraction of car×bon fiber exceeds the percolation threshold to dominate the resistivity of the substrate, the variation of resistivity is not so obvious with the increase of carbon fiber. The above observations highlight that the variation of substrate resistivity is sensitive to the change of carbon fiber content when it is at low content levels (<3%), beyond which the effects of carbon fiber content on the variation of resistivity become insignificant. Besides, in contrast to EPS content, the effect of carbon fiber content on the change of substrate resistivity is more significant.

3.2. Effect of Curing Period on Substrate Resistivity

Figure 5 presents the resistivity of specimens with curing periods of 7 and 55 days corresponding to the mixture of EPS, carbon fiber, cement, and graphite power at different mixing ratios (Table 3). It is observed that the resistivity of all specimens with a curing period of 55 days is greater than its counterpart of specimens with a curing period of 7 days, even though some fluctuations occur due to the variation of mix ratios. This highlights that the resistivity of specimens would increase significantly with the increase of the curing period. This phenomenon is probably due to the different moisture contents of the specimens. As observed, the surface of the specimen with curing period of seven days remains wet, and the internal moisture content is around 60%, where the conductive circuit of carbon fiber and graphite powder maintains good conductivity. In contrast, at the curing period of 55 days, the specimen was dried up and the moisture of the specimen significantly decreased, and its internal porosity increased, which would lead to a reduction of conductivity of the specimen.

3.3. Heating Efficiency

Figure 6a presents the variation of temperatures with the elapsed time at different monitoring points of the substrate with and without plant roots (as shown in Figure 3). It is noted that the mix ratio of 1% EPS, 4% carbon fiber, 10% cement, and 20% graphite powder for the substrate was used here. Interestingly, it is observed that the heating process of the substrate with and without plant roots could be divided into two stages: Stage I (0~15 min) and Stage II (15~30 min). In Stage I, the temperature of the substrate increases significantly as the increase of time at a relatively high increasing rate at all of the three measured points. In contrast, in Stage II, the temperature–time curve tends to be flat, showing that the temperature of substrate becomes relatively stable after 15 min. In addition, by comparing the temperature at three measurement points (Points 1, 2, and 3), it is found that the temperature at the bottom of substrate (Point 1) rises at the highest increasing rate with the elapsed time among the measurements at three points at the State I (<15 min), with the result that the temperature at Point 1 remains higher than its counterparts (at Point 2 and Point 3) at the stable stage (State II). For example, at a time of 15 min, the temperature at Point 1 is 89 °C, while the temperatures at Point 2 and Point 3 are 75 °C and 70 °C, respectively.

Interestingly, it is found that the variation of temperature in the substrate with plants roots with the elapsed time follows the same pattern as its counterpart in the substrate without plant roots. Nevertheless, the results indicate that the temperature measured at the substrate with plant roots is much lower compared with that at the substrate without plant roots. For example, at the time of 15 min, the temperatures at Point 1, Point 2, and Point 3 are 71 °C, 64 °C, and 58 °C, respectively, at the substrate with plant roots, in contrast to 89 °C, 75 °C, and 70 °C at the corresponding measuring points at the substrate without plant roots. This indicates that the heating efficiency would decrease significantly with the growth of plant roots in the substrate. Considering the practical engineering application, reaching 40 °C could protect the slope plants from freezing injury. Figure 6b shows the temperature alteration ratio below 40 °C of substrate with and without plant roots. It can be observed that the average increasing rate of the temperature in the substrate with plant roots is nearly the same as its counterpart in the substrate without plant roots. This indicates that the heating efficiency of the substrate does not change with the growth of plant roots. Although the global heating efficiency of substrate with plant roots has decreased, it is still over the requirement.

3.4. Thermo-Sensitivity of Substrate

Figure 7a presents the variation of resistance with the increase of temperature at the curing period of 175 days for substrate with plant roots. It is observed that, for the substrate with plant roots (Figure 7a), the resistance of the substrate increases significantly with the rise of the temperatures, especially when the temperature exceeds 50 °C. This phenomenon might be due to that fact that, as the temperature increases, the moisture content of the specimen decreases and the internal porosity of the substrate increases, thereby leading to the significant increase in the resistance of the substrate. In contrast, for the substrate without plant roots (Figure 7b), the resistance of the substrate increases significantly with the rise of the temperatures when the temperature is lower than 36 °C. However, it is interestingly found that the resistance of the substrate starts to decrease when the temperature is beyond 36 °C. That is, the variation pattern of resistance with the rise of temperature for the substrate without plant roots (Figure 7b) differs greatly from its counterpart for the substrate with plant roots (Figure 7a).

This observation might be explained in terms of the thermal motion of water molecules in the substrate. Usually, there is an electric double layer formed by orientation of the negative charge on the soil particles surface and the cation in the water [34]. The water in and outside the electric double layer is defined as bound water and free water, respectively. The water most closely attached with soil particles in the double electric layer is firmly bound water, and the left water is loosely bound water [35], as illustrated in Figure 8. For the substrate with plant roots, free water evaporates quickly as the temperature rises. As a result, there is only firmly bound water in soil [36], which is wrapped on soil particle surfaces or absorbed by plant roots. In current tests, the water that can migrate in the soil is free water, and the bound water does not migrate, thereby leading to the increase of substrate resistance [37]. For the substrate without plant roots, the interchangeable cation in diffusion layer and free water will drive polar water molecules to move under the action of applied electric field. As the temperature increases, the kinetic energy of water molecules increases, the directional arrangement is further disrupted, and the resistivity increases. As the temperature further increases, the kinetic energy continues to increase, and the loosely bound water becomes free water and begins to move, which could dissolve or drive the soluble salt in the soil particles into the free water, thereby increasing electrical conductivity and reducing resistivity.

3.5. Heat Preservation Properties

Figure 9 presents the variation of temperature of substrate with the elapsed time after stopping heating corresponding to the specimens at curing period of 0 day (without plant roots) and 175 days (with plant roots), respectively. It is observed that the temperature of substrate without EPS decreases faster than that of substrate with EPS, which indicates that the addition of EPS contributes to improving the heat preservation performance of the substrate. In view of the temperature alteration ratio at a given temperature, it can also be observed the average temperature of substrate with plant roots decreases faster than its counterpart without plant roots. The temperature alteration ratios of substrate with and without plant roots are 1.6 °C/min and 0.9 °C/min, respectively. These observations indicate that the heat in substrate with plant roots after 175 days has escaped much more than its counterpart without plant root (curing period = 0 days). That is, the heat preservation performance of the substrate degrades with the growth of plant roots.

4. Conclusions

(1) In order to exceed the percolation threshold to reduce the resistivity, the carbon fiber content must be more than 2%, but fiber content exceeding 3% on the resistivity of substrate becomes less significant. Thus, carbon fiber content with 3% is recommended for the substrate to be applied in practice.

(2) For non-dry substrate, conductive fine materials composed of carbon fiber and graphite powder cover the surface of EPS and form a conductive circuit, which would result in a significant reduction of the global resistivity of the substrate. Nevertheless, it should be noted that the substrate could hardly be formed when the content of EPS is beyond 4%.

(3) For completely dry substrates, the conductive circuit composed of carbon fiber and graphite powder could only slightly affect the global resistivity of the substrate. As EPS content increases, the contact surface decreases and the resistivity of the substrate increases.

(4) The peak temperature at 30 min of substrate without root is higher than that of substrate with plant roots. Nevertheless, at the temperature lower than 40 °C, the temperature alteration ratio of the substrate with plant root is nearly the same as its counterpart in the substrate without plant roots.

(5) The resistance of the substrate with plant roots increases with the temperature. Nevertheless, the heat action of the loosely bound water in the rootless substrate causes the substrate resistance to decrease.

(6) EPS particles could significantly improve the heat preservation property of substrate, but the heat preservation performance of substrate would decrease with the growth of plants.

In this paper, the test results demonstrate that the substrate has attractive thermal properties that could promote the plant’s growth in different weather conditions (including extremely cold weather), which could be a promising substrate for the ecological slope protection.