# Characterization of PVA Fiber-Reinforced Pervious Concrete with Blended Recycled Ceramic Aggregates and Natural Aggregate

Authors: Qidan Xiao; Liuyang Feng; Yapei Xia; Fandong Xu
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

Effective industrial waste management is one of the important challenges facing modern society. Ceramic wastes occupy an important proportion of industrial waste. The utilization of waste of ceramics is becoming more and more attractive for many researchers to alleviate the environmental impact of industrial waste. As a low-impact development method, pervious concrete (PC) received more and more attention from researchers in recent years. However, the relatively low strength compared with ordinary concrete restricts the application of pervious concrete. This research used recycled ceramic coarse aggregates (RCCA) to partially replace natural coarse aggregates (NCA) to prepare recycled ceramic coarse aggregate pervious concrete (RCCAPC) and fiber-reinforced RCCAPC incorporating polyvinyl alcohol (PVA) fibers, and the effect of the replacement rate of RCCA and the amount of PVA fiber on the mechanical properties and permeability was investigated. The results show that RCCA can improve the mechanical properties of PC at a low substitution rate, as well as PVA fiber, and when the PVA content is about 0.3%, the enhancement effect is the best. Both RCCA replacement rate and PVA fiber content have little effect on the permeability coefficient of PC. Based on described studies, using RCCA to partially replace NCA at a low substitution rate is suitable for the preparation of PC, and PVA fibers can enhance the strength of PC, which may help to broaden the application scenarios of PC.


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## Body

## 1. Introduction

With the rapid development of industry and economy, more and more ceramic products are manufactured [1]. However, the manufacturing process of ceramic products inevitably produced a certain proportion of defective products that were not suitable for use [2, 3]. According to statistics, wastes of ceramics generated during the production phase account for 3%–30% of its total output each year [2, 4, 5]. In addition, discarded sanitary ware and ceramic tiles generated from the construction and demolition wastes (CDW) also produced a large amount of ceramic wastes. Coincidentally, ceramic wastes also account for 30% of CDW [4]. That means ceramic wastes account for a large proportion of solid waste production. Ceramic wastes are durable and highly resistant to chemical and physical degradation, and the biodegradation period of ceramic is very long (up to 4 thousand years) [2, 6]. A large number of ceramic wastes were sent to landfills, putting tremendous pressure on the environment. The method of landfill treatment is difficult to face with such a huge number of ceramic wastes, and it is urgent to find an alternative treatment method to reduce the big pressure of environmental protection [7–9]. Fortunately, ceramic wastes may be potentially suitable for reuse in the mortar or concrete components due to the aforementioned properties (hard, durable, and highly resistant) [2, 9, 10]. As a way of sustainable development, the use of ceramic wastes in the construction industry can reduce energy consumption and greenhouse gas emissions. It can also reduce the consumption of natural resources such as natural stone [6, 11].In recent years, pervious concrete has received more and more attention from researchers as a low-impact development method [12, 13]. The components of PC generally include single-sized coarse aggregate, without or with a small amount of fine aggregate and cement. Because of the small content of cement, a large number of interconnected pores are formed between the coarse aggregates. PC pavements can allow water to permeate freely so that groundwater can be replenished on rainy days, or surface runoff can infiltrate the drainage system in the practice of rainwater management [12, 14, 15]. In addition, PC pavement also has the functions of reducing road noise and alleviating the heat island effect, and adding some photocatalysts with PC can achieve the function of air purification [16–20]. PC has huge application potential. However, the strength of PC is greatly reduced compared with ordinary concrete, which severely limits the application of PC [21, 22]. Researchers generally believe that PC made from aggregates with a single particle size of 5–10 mm can better achieve the balance of mechanical strength and water permeability [23, 24]. A small amount of fine aggregate can enhance the strength and durability of PC. Ibrahim et al. [25] reported that the addition of 10% river sand in pervious concrete mixture with a W/C ratio of 0.3–0.4 resulted in a 50 percent increase in compressive strength. Some researchers have studied the fatigue performance of PC; pervious concrete may be susceptible to fatigue failure under traffic loading [26–28]. AlShareedah et al. [26] reported that the fatigue life of pervious concrete is controlled by the stress ratio, and the effect of porosity is statistically insignificant. As for freeze-thaw durability, in the lack of a standard test method for freeze-thaw durability of pervious concrete, researchers used the standard test method used for conventional concrete. It was reported that pervious concrete performed well in a freeze-thaw environment with sufficient drainage under pervious concrete pavement. However, it is more susceptible to freeze-thaw damage when installed in cold weather areas with no air-entrainment [27, 28].Many researchers have studied the use of ceramic wastes to partially replace coarse aggregates to make concrete. A study conducted by Pacheco-Torgal and Jalali [11] reported a slight increase in chloride penetration depth and higher compressive strength in recycled concrete with the replacement ratio of NCA with RCCA rose from 0% to 25%. Saz et al. [29] utilized RCCA as partial replacement of NCA to enhance both compression and tensile behavior of high strength concrete; the results showed that the use of RCCA improved the mechanical properties of high strength concrete when the replacement ratio is not higher than 60%. But another study conducted by Miguel et al. [30] pointed out that mechanical performance of the recycled concrete concretes decreases as the replacement percentage of NCA by RCCA increases, and the compression, flexure, and tensile splitting strength have presented a reduction of 11.1%, 5.8%, and 22.2% for 75% replacement, respectively, compared with the control group. It means that the effect of RCCA on the mechanical properties of cement-based materials needs further research. Using of ceramic wastes as coarse aggregate for the manufacture of PC not only meets the requirements of sustainable development, but also combines the advantages of low-impact development. Unfortunately, there is not much research focusing on this.PVA fiber is widely used in cement-based materials to improve mechanical properties. And the content of PVA fiber has a great influence on the mechanical properties of cement-based composite materials [31–36]. Some studies have shown that PVA fibers can improve the bending behavior and impact toughness of cement-based composites and can improve the durability of cement-based composites [32, 34, 37–39]. However, at present, the influence of PVA fibers on the properties of pervious concrete incorporating RCCA is not clear.The present study explored the effect of partially replacing natural coarse aggregates with recycled ceramic coarse aggregate on the strength and water permeability of pervious concrete, as well as the effect of PVA fiber content. To that end, the compressive strength, flexural strength, and water permeability coefficient of RCCAPC and PVA fiber-reinforced RCCAPC were studied, respectively.


## 2. Experimental Program

### 2.1. Raw Materials

Raw materials used in this study include cement, NCA, RCCA, PVA fibers, silane coupling agent, and water. The ordinary Portland cement (P.O42.5) produced by Tongli Cement Co., Ltd. (Henan, China) was used in this study, and its apparent density, 28-day flexural strength, and compressive strength are 3.1 g/cm3, 8.9 MPa, and 48.9 MPa, respectively. Figure 1 shows the appearance of NCA, RCCA, and PVA fiber. The NCA from basalt gravel was produced by Tianzhong Qingshi Factory (Xinyang, China). The RCCA in this paper was obtained after crushing and screening from the waste ceramic tiles. The performance indicators of these two kinds of aggregates are shown in Table 1, and the performance index met the requirements of CJJ/T253-2016 “Technical Specification for Application of Recycled Aggregate Pervious Concrete” [40]. Both of the two kinds of aggregates were soaked in KH-550 silane coupling agent made by Xi'an Wande Chemical Co.,Ltd. (Xi’an, China) for 24hours to enhance the bond strength between ceramic aggregate and cement matrix. The PVA fiber was produced by Xi’an Hanlong Chemical Technology Co.,Ltd. (Xi’an, China), with a length of 12 mm, and its ratio of length density is 200, and the density and tensile strength of PVA fiber are 1.12 g/cm3 and 980 MPa∼1200 MPa, respectively.Figure 1
Recycled ceramic coarse aggregate (RCCA), natural coarse aggregate (NCA), and PVA fiber. (a) RCCA. (b) NCA. (c) PVA fiber.
(a)(b)(c)Table 1
Basic properties of RCCA and NCA.
Aggregate typeAggregate size/(mm)Apparent density/(kg.m−3)Bulk density/(kg.m−3)24 h water absorption/(%)Crushing index/(%)RCCA5–10232913273.428.7NCA5–10254715121.0513.1


### 2.2. Mix Design

In order to reveal the impact of RCCA replacement rate and PVA fiber content on properties of PC, 6 mix designs were determined with RCCA replacement rate and PVA fiber content as variables, respectively. Total 12 sets of mix ratios are shown in Table2, the ratios of water to binder were selected as 0.3, and the mix proportions in theory were obtained by changing RCCA replacement rate (0%, 20%, 40%, 60% 80%, and 100%) and PVA fiber content (0%, 0.10%, 0.15%, 0.20%, 0.25%, 0.30%, and 0.35%). Among them, the replacement rate of RCCA of the 6 mix designs used to explore the effect of PVA fiber content was selected as 40%.Table 2
Mix proportions of RCCAPC and PVA fiber-reinforced RCCAPC.
sampleVolume replacement rate/(%)Cement/(kg·m−3)RCCA/(kg·m−3)NCA/(kg·m−3)Water/(kg·m−3)PVA fiber content/(%)Mixing amount/(kg·m−3)RCCA003510148110500RCCA2020358260118510700RCCA404036552088910900RCCA606037378059211200RCCA8080380104029611400RCCA1001003881300011700RCCA40F0.1403737805921120.11.12RCCA40F0.15403737805921120.151.68RCCA40F0.2403737805921120.22.24RCCA40F0.25403737805921120.252.8RCCA40F0.3403737805921120.33.36RCCA40F0.35403737805921120.353.92RCCA20-volume replacement ratio of RCCA is 20%; RCCA40F0.1-volume replacement ratio of RCCA is 40%, and the PVA fiber volume content is 0.1%.


### 2.3. Test Scheme

#### 2.3.1. Sample Preparation

The raw materials were mixed using a compulsory single horizontal shaft concrete mixer. To ensure uniform dispersion of the fibers in the matrix, the fibers (if it is included in the mix designs), cement, and aggregates were dry mixed for 3 min. Then, half of the water was added into the mixture and stirred for 2 min. The mixture was followed by mixing with the rest half of the water for 2 min. Altogether, after mixing for 7 min, the homogeneous fresh cementitious composite with good workability and well fiber dispersion was obtained. After that, the discharging process was carried out twice, a half mold high per discharge. The mechanical vibration was adopted for 2s after each discharge was completed, and the top of the mold was rolled by a smooth roller. The specimens were demolded after 24 hours of maintenance and then cured in the condition of 20°C and 95% humidity until 28 days.


#### 2.3.2. Test Method

The porosity and density of the prepared sample were measured based on the requirements of Chinese National Standard CJJ/T253-2016[40]. The permeability coefficient was tested by a self-made constant water head (200 mm) permeable device (utility model patent, shown in Figure 2). In the constant head method, four sidewalls of the samples were sealed with cement slurry and smeared with paraffin. The head is maintained constant, and the flow rate is recorded and used to calculate the hydraulic conductivity as follows:(1)K=QLAh,where Q is the flow rate, L and A are the length and cross-sectional area of the specimen, respectively, and h is the constant head maintained as 200 mm.Figure 2
Schematic diagram of self-made constant water head permeability coefficient testing device.The compressive strength and bending strength of the specimens were tested following the Chinese National Standard GB/T 50081–2019[41] using a universal testing machine made by Jinan Dongfang Testing Instrument Co.,Ltd.


## 2.1. Raw Materials

Raw materials used in this study include cement, NCA, RCCA, PVA fibers, silane coupling agent, and water. The ordinary Portland cement (P.O42.5) produced by Tongli Cement Co., Ltd. (Henan, China) was used in this study, and its apparent density, 28-day flexural strength, and compressive strength are 3.1 g/cm3, 8.9 MPa, and 48.9 MPa, respectively. Figure 1 shows the appearance of NCA, RCCA, and PVA fiber. The NCA from basalt gravel was produced by Tianzhong Qingshi Factory (Xinyang, China). The RCCA in this paper was obtained after crushing and screening from the waste ceramic tiles. The performance indicators of these two kinds of aggregates are shown in Table 1, and the performance index met the requirements of CJJ/T253-2016 “Technical Specification for Application of Recycled Aggregate Pervious Concrete” [40]. Both of the two kinds of aggregates were soaked in KH-550 silane coupling agent made by Xi'an Wande Chemical Co.,Ltd. (Xi’an, China) for 24hours to enhance the bond strength between ceramic aggregate and cement matrix. The PVA fiber was produced by Xi’an Hanlong Chemical Technology Co.,Ltd. (Xi’an, China), with a length of 12 mm, and its ratio of length density is 200, and the density and tensile strength of PVA fiber are 1.12 g/cm3 and 980 MPa∼1200 MPa, respectively.Figure 1
Recycled ceramic coarse aggregate (RCCA), natural coarse aggregate (NCA), and PVA fiber. (a) RCCA. (b) NCA. (c) PVA fiber.
(a)(b)(c)Table 1
Basic properties of RCCA and NCA.
Aggregate typeAggregate size/(mm)Apparent density/(kg.m−3)Bulk density/(kg.m−3)24 h water absorption/(%)Crushing index/(%)RCCA5–10232913273.428.7NCA5–10254715121.0513.1


## 2.2. Mix Design

In order to reveal the impact of RCCA replacement rate and PVA fiber content on properties of PC, 6 mix designs were determined with RCCA replacement rate and PVA fiber content as variables, respectively. Total 12 sets of mix ratios are shown in Table2, the ratios of water to binder were selected as 0.3, and the mix proportions in theory were obtained by changing RCCA replacement rate (0%, 20%, 40%, 60% 80%, and 100%) and PVA fiber content (0%, 0.10%, 0.15%, 0.20%, 0.25%, 0.30%, and 0.35%). Among them, the replacement rate of RCCA of the 6 mix designs used to explore the effect of PVA fiber content was selected as 40%.Table 2
Mix proportions of RCCAPC and PVA fiber-reinforced RCCAPC.
sampleVolume replacement rate/(%)Cement/(kg·m−3)RCCA/(kg·m−3)NCA/(kg·m−3)Water/(kg·m−3)PVA fiber content/(%)Mixing amount/(kg·m−3)RCCA003510148110500RCCA2020358260118510700RCCA404036552088910900RCCA606037378059211200RCCA8080380104029611400RCCA1001003881300011700RCCA40F0.1403737805921120.11.12RCCA40F0.15403737805921120.151.68RCCA40F0.2403737805921120.22.24RCCA40F0.25403737805921120.252.8RCCA40F0.3403737805921120.33.36RCCA40F0.35403737805921120.353.92RCCA20-volume replacement ratio of RCCA is 20%; RCCA40F0.1-volume replacement ratio of RCCA is 40%, and the PVA fiber volume content is 0.1%.


## 2.3. Test Scheme

### 2.3.1. Sample Preparation

The raw materials were mixed using a compulsory single horizontal shaft concrete mixer. To ensure uniform dispersion of the fibers in the matrix, the fibers (if it is included in the mix designs), cement, and aggregates were dry mixed for 3 min. Then, half of the water was added into the mixture and stirred for 2 min. The mixture was followed by mixing with the rest half of the water for 2 min. Altogether, after mixing for 7 min, the homogeneous fresh cementitious composite with good workability and well fiber dispersion was obtained. After that, the discharging process was carried out twice, a half mold high per discharge. The mechanical vibration was adopted for 2s after each discharge was completed, and the top of the mold was rolled by a smooth roller. The specimens were demolded after 24 hours of maintenance and then cured in the condition of 20°C and 95% humidity until 28 days.


### 2.3.2. Test Method

The porosity and density of the prepared sample were measured based on the requirements of Chinese National Standard CJJ/T253-2016[40]. The permeability coefficient was tested by a self-made constant water head (200 mm) permeable device (utility model patent, shown in Figure 2). In the constant head method, four sidewalls of the samples were sealed with cement slurry and smeared with paraffin. The head is maintained constant, and the flow rate is recorded and used to calculate the hydraulic conductivity as follows:(1)K=QLAh,where Q is the flow rate, L and A are the length and cross-sectional area of the specimen, respectively, and h is the constant head maintained as 200 mm.Figure 2
Schematic diagram of self-made constant water head permeability coefficient testing device.The compressive strength and bending strength of the specimens were tested following the Chinese National Standard GB/T 50081–2019[41] using a universal testing machine made by Jinan Dongfang Testing Instrument Co.,Ltd.


## 2.3.1. Sample Preparation

The raw materials were mixed using a compulsory single horizontal shaft concrete mixer. To ensure uniform dispersion of the fibers in the matrix, the fibers (if it is included in the mix designs), cement, and aggregates were dry mixed for 3 min. Then, half of the water was added into the mixture and stirred for 2 min. The mixture was followed by mixing with the rest half of the water for 2 min. Altogether, after mixing for 7 min, the homogeneous fresh cementitious composite with good workability and well fiber dispersion was obtained. After that, the discharging process was carried out twice, a half mold high per discharge. The mechanical vibration was adopted for 2s after each discharge was completed, and the top of the mold was rolled by a smooth roller. The specimens were demolded after 24 hours of maintenance and then cured in the condition of 20°C and 95% humidity until 28 days.


## 2.3.2. Test Method

The porosity and density of the prepared sample were measured based on the requirements of Chinese National Standard CJJ/T253-2016[40]. The permeability coefficient was tested by a self-made constant water head (200 mm) permeable device (utility model patent, shown in Figure 2). In the constant head method, four sidewalls of the samples were sealed with cement slurry and smeared with paraffin. The head is maintained constant, and the flow rate is recorded and used to calculate the hydraulic conductivity as follows:(1)K=QLAh,where Q is the flow rate, L and A are the length and cross-sectional area of the specimen, respectively, and h is the constant head maintained as 200 mm.Figure 2
Schematic diagram of self-made constant water head permeability coefficient testing device.The compressive strength and bending strength of the specimens were tested following the Chinese National Standard GB/T 50081–2019[41] using a universal testing machine made by Jinan Dongfang Testing Instrument Co.,Ltd.


## 3. Results and Analysis

### 3.1. Effect of Replacement Rate of RCCA

#### 3.1.1. Compressive Strength and Bending Strength

Figure3 shows the relationship between the replacement rate of RCCA and the compressive strength and bending strength, respectively. This study shows that using RCCA to partially replace NCA can improve the mechanical properties of PC at low substitution rate, which was similar to the results obtained by Pacheco-Torgal and Jalali [11] and Saz et al. [29]. The compressive strength and bending strength gradually increased with the increase of the replacement rate of RCCA and reached the maximum values of 21.35 MPa and 2.74 MPa, respectively at the replacement rate of 40%.Figure 3
Relationships between compressive strength, bending strength, and replacement rate of RCCA.Figure4 shows the relationship between the porosity, density, and the substitution rate of RCCA, respectively. The changing trend of the strength and density of the PC is very similar. The increase in strength may be due to the decrease of porosity and the resulting increase in density. Another possible reason for the enhancement may be the higher water absorption of RCCAs. The RCCAs were soaked for 24 hours before preparation, and the internal water was volatile due to the high porosity of the samples, and the external water was difficult to enter the interface between the aggregate and cement matrix during the curing process. The loss of moisture content at the interface was compensated by the water released inside the RCCA. The amount of released internal moisture increased with the increment of RCCA replacement rate; it ensured the complete hydration reaction of cement. Therefore, the surface of RCCA and NCA was more tightly wrapped by the cement matrix, and the aggregate-cement bond was further enhanced. Similar results have been reported by Kevern and Nowasell [42].Figure 4
Relationships between porosity, density, and replacement rate of RCCA.Figure5 shows the fracture morphology of the specimen after the compression test, which may verify the abovementioned speculation. Figure 5(a) shows that the hydration of cement at the bonding surfaces between aggregates and cement matrix is incomplete, which led to the spalling of aggregates in the process of compression tests. On the contrary, Figure 5(b) shows that the aggregate-cement bond is tight, and the destroyed section runs through the RCCA and NCA without spalling.Figure 5
Fracture morphology of RCCA20 and RCCA40 after compression test. (a) RCCA20. (b) RCCA40.
(a)(b)The strength of the sample showed a downward trend when the substitution rate continued to increase based on 40%. When the substitution rate was 80%, the compressive strength of PC decreased by 11.35%, which is similar to the result of Miguel et al. [30], which reported a reduction of 11.1% for 75% replacement. The compressive and bending strength reached the minimum values of 17.52 MPa and 1.98 MPa, respectively, when the replacement rate was 100%. The reason may be that the total water storage of ceramic aggregate increased due to the increment of the substitution rate, but the hydration of cement at the aggregate-matrix interface did not continue indefinitely with the increase of the amount of released water. As can be seen from the sample fracture in Figure 5(b), hydration of cement at the interface was fully reacted when the replacement rate reached 40%. On the other side, the increment of substitution rate based on 40% resulted in the growth of porosity and the decline of the density. This also led to a decrease in compressive strength and bending strength.It is necessary to point out that many RCCAs are flakes. When the replacement rate is too high, the thickness of cement slurry between ceramic aggregates was relatively reduced, which led to a more uneven distribution of contact stress between ceramic aggregates. On the other hand, recycled ceramic aggregates showed certain damages in the crushing process. Therefore, there were a small number of microcracks on the ceramic surface, which affected the strength of recycled aggregate, because these microcracks are easy to continue to develop under the action of load. These factors also affect the strength of PC.


#### 3.1.2. Permeability Coefficient

Figure6 shows the relationship between replacement rate, porosity, and water permeability coefficient, respectively. The permeability coefficients of the samples all meet the requirements of the specification [40], and all the porosities are between 23% and 25%. The variations of the permeability coefficient and porosity are not consistent. This may be caused by the changes of the internal pore shape of the pervious concrete. Figure 7 shows the black and white binarized section of the RCCA100 sample, and there are three main types of pore shape at the cross-section: I-type, U-type, and branch type. The water flow can quickly pass through the I-type pores, and the time for passing through the U-type and branch type pores was relatively long, which resulted in a decrease of the permeable quantity Q per unit time, and that led to a reduction of the permeability coefficient K. Therefore, when the substitution rate is too high, although the porosity of the sample increased slightly, the permeability coefficient did not increase; it even showed a downward trend. This may be due to that there are too many U-type and branch type pores generated.Figure 6
Relationships between porosity, permeability coefficient, and replacement rate of RCCA.Figure 7
Black and white binarized section of RCCA100 sample I-type U-type branch type.


### 3.2. Effect of PVA Fiber Content on the Properties of Fiber-Reinforced RCCAPC

#### 3.2.1. Compressive Strength and Flexural Strength

Figure8 shows the effect of PVA fiber content on the compressive strength and bending strength of PVA fiber-reinforced RCCAPC. The compressive strength and bending strength of fiber-reinforced RCCAPC gradually increased with the increment of the PVA fiber content. When the content of PVA fiber was 0.25% and 0.3%, the compressive and flexural strengths reached the maximum values of 23.51 MPa and 3.41 MPa, respectively, which were increased by 10.1% and 24.5%, relative to the maximum value of RCCAPC. And the strength of the fiber-reinforced RCCAPC showed a downward trend with the continuous increase of PVA fiber content. The existing literature [36, 40, 43, 44] shows that, within a certain dosage range, PVA can improve the compressive strength and flexural strength of cement-based materials. The results of this test are consistent with this.Figure 8
Relationships between compressive strength, bending strength, and PVA-fiber volume.Figure9 shows macroscopic damage form of the compressive strength test both of the RCCAPC and fiber-reinforced RCCAPC. Figure 9(a) shows more serious failure morphology and a large area of scattered cracking and shedding of the RCCAPC sample. As for the fiber-reinforced RCCAPC sample in Figure 9(b), the appearance of penetrating cracks occurred on the surface, and the PVA fibers were exposed on the cross-section, with no cracking or shattering. The reason for the difference between these two kinds of failure morphologies is that three-dimensional fiber network structures formed due to the uniform and disorderly arrangement and horizontal overlap of fibers in fiber-reinforced RCCAPC cement matrix, and the network limited the displacement of aggregate. The overall mechanical properties of RCCAPC were enhanced. Under the effect of vertical positive pressure, the specimen produced transverse displacement and deformation. The largest transverse displacement occurred in the middle part of the specimen; therefore, the corresponding stress in this area is relatively larger than any other area. For this reason, the RCCAPC sample formed the bursting crack from the middle to both ends, and the internal collapse depth was relatively deep in the middle part (Figure 9(a)). The three-dimensional PVA fiber mesh structure in the fiber-reinforced RCCAPC enhanced the integrity of the sample due to the “cyclo-hoop effect”, which led to weakened horizontal deformation and longitudinal penetrating crack (Figure 9(b)). The compressive strength of fiber-reinforced RCCAPC improved with a small margin compared to that of RCCAPC. It should be pointed out that, different from ordinary concrete, in the process of compression test, due to the existence of a large number of pores, the frictional force of the contact surfaces of the upper and lower plates of test machine is not enough to hinder the lateral deformation of the contact surface of PC. Therefore, the compressive failure of pervious concrete is caused by excessive lateral deformation.Figure 9
Macroscopic damage pattern of RCCAPC and fiber-reinforced RCCAPC after compressive test. (a) RCCA40. (b) RCCA40F0.3.
(a)(b)Figure10 shows the macroscopic damage form of the bending test both of the RCCAPC and fiber-reinforced RCCAPC. The RCCAPC section is relatively flat, the ceramic aggregates on the section were broken, and there was a small number of shedding particles but no obvious fracture sign in the test (Figure10(a)). The fracture section of the PVA fiber-reinforced RCCAPC was uneven, the RCCAs were broken, and the failure process had obvious signs (Figure10(b)). With the gradual increase of the test load, microcracks appeared at the bottom of the samples, and then the cracks gradually expanded to the top, with fine particles falling off. The compression area at the upper end of the section was not completely broken. It suggests that the bending strength of fiber-reinforced RCCAPC was significantly enhanced compared to that of RCCAPC.Figure 10
Macroscopic damage pattern of RCCAPC and fiber-reinforced RCCAPC in bending test. (a) RCCA40. (b) RCCA40F0.3.
(a)(b)Due to the characteristics of the dry-wet mixing method in this study, the aggregate surface was more easily covered by cement paste, because the NCA and RCCA were rougher than PVA fiber. As the fiber content continued to increase, the number of high-fineness PVA fibers per unit volume increased rapidly. When the fiber content was more than 0.25%, it was found that some of the exposed fibers were not completely wrapped by the cement matrix, and some fibers were located between the aggregates, which led to a reduction of the contact area and bonding force between the aggregates, leading to weakened integrity of the spatial PVA fiber network as well as its crack inhibition effect. Under these circumstances, the compressive strength and flexural strength of the sample decreased. Furthermore, the decrease in compressive strength is more obvious.Figure11 shows stress-strain curves of RCCAPC and fiber-reinforced RCCAPC in compression tests. The curves can be divided into three stages: approximate elastic stage, crack development stage, and yield failure stage. With the increase of the content of PVA fiber, the peak value of the curve increased slightly, and the decrease of the curve in the yield failure stage tended to be smooth. The area of the parts below the stress-deflection curve from large to small is RCCA40F2.5 > RCCA40F3.5 > RCCA40F1.5 > RCCA40. It indicated that the toughness of the sample increased with the addition of PVA fiber. When the volume content of PVA fiber exceeded 0.25%, the peak value decreased slightly, but the sample still showed good toughness. When the test load was small, the curves were similar to straight lines, which was mainly due to the tiny elastic deformation of the mixed aggregates and cement matrix. And the PVA fibers have little effect on the mechanical properties of pervious concrete. With the development of the cracks, the curves began to enter the descending stage, which means that the three-dimensional PVA fiber network restrained the crack and alleviated the transverse deformation.Figure 11
Complete stress-strain curves of RCCA40, RCCA40F1.5, RCCA40F2.5, and RCCA40F3.5.Figure12 shows stress-deflection curves of the bending test of RCCAPC and fiber-reinforced RCCAPC. With the addition of PVA fiber, the areas of the parts below the stress-deflection curve increased gradually, which means the bending strength and toughness of the sample were improved. When the volume content of PVA fiber exceeded 0.25%, the flexural strength of the RCCAPC decreased, and the bending deformation did not increase significantly after reaching the peak value and the area of the part below the curve reduced, indicating that the bending toughness of the sample decreased. Therefore, the addition of PVA fiber can improve the compressive strength and flexural strength of recycled aggregate pervious concrete, but the content should not be too much.Figure 12
Stress-deflection curves RCCA40, RCCA40F1.5, RCCA40F2.5, and RCCA40F3.5.


#### 3.2.2. Permeability

Figure13 shows the permeability of fiber-reinforced RCCAPC. With the increase of PVA fiber content, the permeability coefficient and porosity showed an overall downward trend but decreased by no more than 5%. When the PVA content continued to increase, the permeability coefficient and porosity increased slightly. When the content of PVA fiber was less than 0.25%, the fracture of the sample showed that the cement paste can completely wrap the PVA fibers, and the volume of the cement increased, and the internal pore volume of RCCAPC decreased. On the other hand, some PVA fibers extended into the pores and occupied the part of space in the pores, and that led to the decrease of the flow velocity. Due to the above two factors, the porosity of the sample decreased, and the permeability coefficient decreased slightly. When the content of PVA fiber was too high, part of PVA fibers could not be completely wrapped by cement paste. It was observed that part of exposed PVA fibers was distributed between NCAs and RCCAs, which resulted in the decrease of the density of cementation layer between aggregates, and permeable channels were formed in PVA fiber-aggregate bonding and PVA fiber-cement bonding, which is helpful to improve water permeability. On the other hand, the uncoated PVA fibers extended into the pores of the sample, the volume of uncoated PVA fibers was smaller than that of the pulped PVA fibers, and the softness of uncoated PVA fibers was relatively good. So, when the water pass through, uncoated PVA fibers could bend in the direction of water flow. Therefore, when the volume content of PVA fiber exceeded 0.25%, the permeability coefficient had a slight upturn. From the above analysis, it can be seen that PVA fiber had little effect on the permeability coefficient of RCCAPC. Considering the mechanical properties of the samples, the content of PVA fiber should be controlled at about 0.25%.Figure 13
Relationships between porosity, permeability coefficient, and PVA-fiber volume.


## 3.1. Effect of Replacement Rate of RCCA

### 3.1.1. Compressive Strength and Bending Strength

Figure3 shows the relationship between the replacement rate of RCCA and the compressive strength and bending strength, respectively. This study shows that using RCCA to partially replace NCA can improve the mechanical properties of PC at low substitution rate, which was similar to the results obtained by Pacheco-Torgal and Jalali [11] and Saz et al. [29]. The compressive strength and bending strength gradually increased with the increase of the replacement rate of RCCA and reached the maximum values of 21.35 MPa and 2.74 MPa, respectively at the replacement rate of 40%.Figure 3
Relationships between compressive strength, bending strength, and replacement rate of RCCA.Figure4 shows the relationship between the porosity, density, and the substitution rate of RCCA, respectively. The changing trend of the strength and density of the PC is very similar. The increase in strength may be due to the decrease of porosity and the resulting increase in density. Another possible reason for the enhancement may be the higher water absorption of RCCAs. The RCCAs were soaked for 24 hours before preparation, and the internal water was volatile due to the high porosity of the samples, and the external water was difficult to enter the interface between the aggregate and cement matrix during the curing process. The loss of moisture content at the interface was compensated by the water released inside the RCCA. The amount of released internal moisture increased with the increment of RCCA replacement rate; it ensured the complete hydration reaction of cement. Therefore, the surface of RCCA and NCA was more tightly wrapped by the cement matrix, and the aggregate-cement bond was further enhanced. Similar results have been reported by Kevern and Nowasell [42].Figure 4
Relationships between porosity, density, and replacement rate of RCCA.Figure5 shows the fracture morphology of the specimen after the compression test, which may verify the abovementioned speculation. Figure 5(a) shows that the hydration of cement at the bonding surfaces between aggregates and cement matrix is incomplete, which led to the spalling of aggregates in the process of compression tests. On the contrary, Figure 5(b) shows that the aggregate-cement bond is tight, and the destroyed section runs through the RCCA and NCA without spalling.Figure 5
Fracture morphology of RCCA20 and RCCA40 after compression test. (a) RCCA20. (b) RCCA40.
(a)(b)The strength of the sample showed a downward trend when the substitution rate continued to increase based on 40%. When the substitution rate was 80%, the compressive strength of PC decreased by 11.35%, which is similar to the result of Miguel et al. [30], which reported a reduction of 11.1% for 75% replacement. The compressive and bending strength reached the minimum values of 17.52 MPa and 1.98 MPa, respectively, when the replacement rate was 100%. The reason may be that the total water storage of ceramic aggregate increased due to the increment of the substitution rate, but the hydration of cement at the aggregate-matrix interface did not continue indefinitely with the increase of the amount of released water. As can be seen from the sample fracture in Figure 5(b), hydration of cement at the interface was fully reacted when the replacement rate reached 40%. On the other side, the increment of substitution rate based on 40% resulted in the growth of porosity and the decline of the density. This also led to a decrease in compressive strength and bending strength.It is necessary to point out that many RCCAs are flakes. When the replacement rate is too high, the thickness of cement slurry between ceramic aggregates was relatively reduced, which led to a more uneven distribution of contact stress between ceramic aggregates. On the other hand, recycled ceramic aggregates showed certain damages in the crushing process. Therefore, there were a small number of microcracks on the ceramic surface, which affected the strength of recycled aggregate, because these microcracks are easy to continue to develop under the action of load. These factors also affect the strength of PC.


### 3.1.2. Permeability Coefficient

Figure6 shows the relationship between replacement rate, porosity, and water permeability coefficient, respectively. The permeability coefficients of the samples all meet the requirements of the specification [40], and all the porosities are between 23% and 25%. The variations of the permeability coefficient and porosity are not consistent. This may be caused by the changes of the internal pore shape of the pervious concrete. Figure 7 shows the black and white binarized section of the RCCA100 sample, and there are three main types of pore shape at the cross-section: I-type, U-type, and branch type. The water flow can quickly pass through the I-type pores, and the time for passing through the U-type and branch type pores was relatively long, which resulted in a decrease of the permeable quantity Q per unit time, and that led to a reduction of the permeability coefficient K. Therefore, when the substitution rate is too high, although the porosity of the sample increased slightly, the permeability coefficient did not increase; it even showed a downward trend. This may be due to that there are too many U-type and branch type pores generated.Figure 6
Relationships between porosity, permeability coefficient, and replacement rate of RCCA.Figure 7
Black and white binarized section of RCCA100 sample I-type U-type branch type.


## 3.1.1. Compressive Strength and Bending Strength

Figure3 shows the relationship between the replacement rate of RCCA and the compressive strength and bending strength, respectively. This study shows that using RCCA to partially replace NCA can improve the mechanical properties of PC at low substitution rate, which was similar to the results obtained by Pacheco-Torgal and Jalali [11] and Saz et al. [29]. The compressive strength and bending strength gradually increased with the increase of the replacement rate of RCCA and reached the maximum values of 21.35 MPa and 2.74 MPa, respectively at the replacement rate of 40%.Figure 3
Relationships between compressive strength, bending strength, and replacement rate of RCCA.Figure4 shows the relationship between the porosity, density, and the substitution rate of RCCA, respectively. The changing trend of the strength and density of the PC is very similar. The increase in strength may be due to the decrease of porosity and the resulting increase in density. Another possible reason for the enhancement may be the higher water absorption of RCCAs. The RCCAs were soaked for 24 hours before preparation, and the internal water was volatile due to the high porosity of the samples, and the external water was difficult to enter the interface between the aggregate and cement matrix during the curing process. The loss of moisture content at the interface was compensated by the water released inside the RCCA. The amount of released internal moisture increased with the increment of RCCA replacement rate; it ensured the complete hydration reaction of cement. Therefore, the surface of RCCA and NCA was more tightly wrapped by the cement matrix, and the aggregate-cement bond was further enhanced. Similar results have been reported by Kevern and Nowasell [42].Figure 4
Relationships between porosity, density, and replacement rate of RCCA.Figure5 shows the fracture morphology of the specimen after the compression test, which may verify the abovementioned speculation. Figure 5(a) shows that the hydration of cement at the bonding surfaces between aggregates and cement matrix is incomplete, which led to the spalling of aggregates in the process of compression tests. On the contrary, Figure 5(b) shows that the aggregate-cement bond is tight, and the destroyed section runs through the RCCA and NCA without spalling.Figure 5
Fracture morphology of RCCA20 and RCCA40 after compression test. (a) RCCA20. (b) RCCA40.
(a)(b)The strength of the sample showed a downward trend when the substitution rate continued to increase based on 40%. When the substitution rate was 80%, the compressive strength of PC decreased by 11.35%, which is similar to the result of Miguel et al. [30], which reported a reduction of 11.1% for 75% replacement. The compressive and bending strength reached the minimum values of 17.52 MPa and 1.98 MPa, respectively, when the replacement rate was 100%. The reason may be that the total water storage of ceramic aggregate increased due to the increment of the substitution rate, but the hydration of cement at the aggregate-matrix interface did not continue indefinitely with the increase of the amount of released water. As can be seen from the sample fracture in Figure 5(b), hydration of cement at the interface was fully reacted when the replacement rate reached 40%. On the other side, the increment of substitution rate based on 40% resulted in the growth of porosity and the decline of the density. This also led to a decrease in compressive strength and bending strength.It is necessary to point out that many RCCAs are flakes. When the replacement rate is too high, the thickness of cement slurry between ceramic aggregates was relatively reduced, which led to a more uneven distribution of contact stress between ceramic aggregates. On the other hand, recycled ceramic aggregates showed certain damages in the crushing process. Therefore, there were a small number of microcracks on the ceramic surface, which affected the strength of recycled aggregate, because these microcracks are easy to continue to develop under the action of load. These factors also affect the strength of PC.


## 3.1.2. Permeability Coefficient

Figure6 shows the relationship between replacement rate, porosity, and water permeability coefficient, respectively. The permeability coefficients of the samples all meet the requirements of the specification [40], and all the porosities are between 23% and 25%. The variations of the permeability coefficient and porosity are not consistent. This may be caused by the changes of the internal pore shape of the pervious concrete. Figure 7 shows the black and white binarized section of the RCCA100 sample, and there are three main types of pore shape at the cross-section: I-type, U-type, and branch type. The water flow can quickly pass through the I-type pores, and the time for passing through the U-type and branch type pores was relatively long, which resulted in a decrease of the permeable quantity Q per unit time, and that led to a reduction of the permeability coefficient K. Therefore, when the substitution rate is too high, although the porosity of the sample increased slightly, the permeability coefficient did not increase; it even showed a downward trend. This may be due to that there are too many U-type and branch type pores generated.Figure 6
Relationships between porosity, permeability coefficient, and replacement rate of RCCA.Figure 7
Black and white binarized section of RCCA100 sample I-type U-type branch type.


## 3.2. Effect of PVA Fiber Content on the Properties of Fiber-Reinforced RCCAPC

### 3.2.1. Compressive Strength and Flexural Strength

Figure8 shows the effect of PVA fiber content on the compressive strength and bending strength of PVA fiber-reinforced RCCAPC. The compressive strength and bending strength of fiber-reinforced RCCAPC gradually increased with the increment of the PVA fiber content. When the content of PVA fiber was 0.25% and 0.3%, the compressive and flexural strengths reached the maximum values of 23.51 MPa and 3.41 MPa, respectively, which were increased by 10.1% and 24.5%, relative to the maximum value of RCCAPC. And the strength of the fiber-reinforced RCCAPC showed a downward trend with the continuous increase of PVA fiber content. The existing literature [36, 40, 43, 44] shows that, within a certain dosage range, PVA can improve the compressive strength and flexural strength of cement-based materials. The results of this test are consistent with this.Figure 8
Relationships between compressive strength, bending strength, and PVA-fiber volume.Figure9 shows macroscopic damage form of the compressive strength test both of the RCCAPC and fiber-reinforced RCCAPC. Figure 9(a) shows more serious failure morphology and a large area of scattered cracking and shedding of the RCCAPC sample. As for the fiber-reinforced RCCAPC sample in Figure 9(b), the appearance of penetrating cracks occurred on the surface, and the PVA fibers were exposed on the cross-section, with no cracking or shattering. The reason for the difference between these two kinds of failure morphologies is that three-dimensional fiber network structures formed due to the uniform and disorderly arrangement and horizontal overlap of fibers in fiber-reinforced RCCAPC cement matrix, and the network limited the displacement of aggregate. The overall mechanical properties of RCCAPC were enhanced. Under the effect of vertical positive pressure, the specimen produced transverse displacement and deformation. The largest transverse displacement occurred in the middle part of the specimen; therefore, the corresponding stress in this area is relatively larger than any other area. For this reason, the RCCAPC sample formed the bursting crack from the middle to both ends, and the internal collapse depth was relatively deep in the middle part (Figure 9(a)). The three-dimensional PVA fiber mesh structure in the fiber-reinforced RCCAPC enhanced the integrity of the sample due to the “cyclo-hoop effect”, which led to weakened horizontal deformation and longitudinal penetrating crack (Figure 9(b)). The compressive strength of fiber-reinforced RCCAPC improved with a small margin compared to that of RCCAPC. It should be pointed out that, different from ordinary concrete, in the process of compression test, due to the existence of a large number of pores, the frictional force of the contact surfaces of the upper and lower plates of test machine is not enough to hinder the lateral deformation of the contact surface of PC. Therefore, the compressive failure of pervious concrete is caused by excessive lateral deformation.Figure 9
Macroscopic damage pattern of RCCAPC and fiber-reinforced RCCAPC after compressive test. (a) RCCA40. (b) RCCA40F0.3.
(a)(b)Figure10 shows the macroscopic damage form of the bending test both of the RCCAPC and fiber-reinforced RCCAPC. The RCCAPC section is relatively flat, the ceramic aggregates on the section were broken, and there was a small number of shedding particles but no obvious fracture sign in the test (Figure10(a)). The fracture section of the PVA fiber-reinforced RCCAPC was uneven, the RCCAs were broken, and the failure process had obvious signs (Figure10(b)). With the gradual increase of the test load, microcracks appeared at the bottom of the samples, and then the cracks gradually expanded to the top, with fine particles falling off. The compression area at the upper end of the section was not completely broken. It suggests that the bending strength of fiber-reinforced RCCAPC was significantly enhanced compared to that of RCCAPC.Figure 10
Macroscopic damage pattern of RCCAPC and fiber-reinforced RCCAPC in bending test. (a) RCCA40. (b) RCCA40F0.3.
(a)(b)Due to the characteristics of the dry-wet mixing method in this study, the aggregate surface was more easily covered by cement paste, because the NCA and RCCA were rougher than PVA fiber. As the fiber content continued to increase, the number of high-fineness PVA fibers per unit volume increased rapidly. When the fiber content was more than 0.25%, it was found that some of the exposed fibers were not completely wrapped by the cement matrix, and some fibers were located between the aggregates, which led to a reduction of the contact area and bonding force between the aggregates, leading to weakened integrity of the spatial PVA fiber network as well as its crack inhibition effect. Under these circumstances, the compressive strength and flexural strength of the sample decreased. Furthermore, the decrease in compressive strength is more obvious.Figure11 shows stress-strain curves of RCCAPC and fiber-reinforced RCCAPC in compression tests. The curves can be divided into three stages: approximate elastic stage, crack development stage, and yield failure stage. With the increase of the content of PVA fiber, the peak value of the curve increased slightly, and the decrease of the curve in the yield failure stage tended to be smooth. The area of the parts below the stress-deflection curve from large to small is RCCA40F2.5 > RCCA40F3.5 > RCCA40F1.5 > RCCA40. It indicated that the toughness of the sample increased with the addition of PVA fiber. When the volume content of PVA fiber exceeded 0.25%, the peak value decreased slightly, but the sample still showed good toughness. When the test load was small, the curves were similar to straight lines, which was mainly due to the tiny elastic deformation of the mixed aggregates and cement matrix. And the PVA fibers have little effect on the mechanical properties of pervious concrete. With the development of the cracks, the curves began to enter the descending stage, which means that the three-dimensional PVA fiber network restrained the crack and alleviated the transverse deformation.Figure 11
Complete stress-strain curves of RCCA40, RCCA40F1.5, RCCA40F2.5, and RCCA40F3.5.Figure12 shows stress-deflection curves of the bending test of RCCAPC and fiber-reinforced RCCAPC. With the addition of PVA fiber, the areas of the parts below the stress-deflection curve increased gradually, which means the bending strength and toughness of the sample were improved. When the volume content of PVA fiber exceeded 0.25%, the flexural strength of the RCCAPC decreased, and the bending deformation did not increase significantly after reaching the peak value and the area of the part below the curve reduced, indicating that the bending toughness of the sample decreased. Therefore, the addition of PVA fiber can improve the compressive strength and flexural strength of recycled aggregate pervious concrete, but the content should not be too much.Figure 12
Stress-deflection curves RCCA40, RCCA40F1.5, RCCA40F2.5, and RCCA40F3.5.


### 3.2.2. Permeability

Figure13 shows the permeability of fiber-reinforced RCCAPC. With the increase of PVA fiber content, the permeability coefficient and porosity showed an overall downward trend but decreased by no more than 5%. When the PVA content continued to increase, the permeability coefficient and porosity increased slightly. When the content of PVA fiber was less than 0.25%, the fracture of the sample showed that the cement paste can completely wrap the PVA fibers, and the volume of the cement increased, and the internal pore volume of RCCAPC decreased. On the other hand, some PVA fibers extended into the pores and occupied the part of space in the pores, and that led to the decrease of the flow velocity. Due to the above two factors, the porosity of the sample decreased, and the permeability coefficient decreased slightly. When the content of PVA fiber was too high, part of PVA fibers could not be completely wrapped by cement paste. It was observed that part of exposed PVA fibers was distributed between NCAs and RCCAs, which resulted in the decrease of the density of cementation layer between aggregates, and permeable channels were formed in PVA fiber-aggregate bonding and PVA fiber-cement bonding, which is helpful to improve water permeability. On the other hand, the uncoated PVA fibers extended into the pores of the sample, the volume of uncoated PVA fibers was smaller than that of the pulped PVA fibers, and the softness of uncoated PVA fibers was relatively good. So, when the water pass through, uncoated PVA fibers could bend in the direction of water flow. Therefore, when the volume content of PVA fiber exceeded 0.25%, the permeability coefficient had a slight upturn. From the above analysis, it can be seen that PVA fiber had little effect on the permeability coefficient of RCCAPC. Considering the mechanical properties of the samples, the content of PVA fiber should be controlled at about 0.25%.Figure 13
Relationships between porosity, permeability coefficient, and PVA-fiber volume.


## 3.2.1. Compressive Strength and Flexural Strength

Figure8 shows the effect of PVA fiber content on the compressive strength and bending strength of PVA fiber-reinforced RCCAPC. The compressive strength and bending strength of fiber-reinforced RCCAPC gradually increased with the increment of the PVA fiber content. When the content of PVA fiber was 0.25% and 0.3%, the compressive and flexural strengths reached the maximum values of 23.51 MPa and 3.41 MPa, respectively, which were increased by 10.1% and 24.5%, relative to the maximum value of RCCAPC. And the strength of the fiber-reinforced RCCAPC showed a downward trend with the continuous increase of PVA fiber content. The existing literature [36, 40, 43, 44] shows that, within a certain dosage range, PVA can improve the compressive strength and flexural strength of cement-based materials. The results of this test are consistent with this.Figure 8
Relationships between compressive strength, bending strength, and PVA-fiber volume.Figure9 shows macroscopic damage form of the compressive strength test both of the RCCAPC and fiber-reinforced RCCAPC. Figure 9(a) shows more serious failure morphology and a large area of scattered cracking and shedding of the RCCAPC sample. As for the fiber-reinforced RCCAPC sample in Figure 9(b), the appearance of penetrating cracks occurred on the surface, and the PVA fibers were exposed on the cross-section, with no cracking or shattering. The reason for the difference between these two kinds of failure morphologies is that three-dimensional fiber network structures formed due to the uniform and disorderly arrangement and horizontal overlap of fibers in fiber-reinforced RCCAPC cement matrix, and the network limited the displacement of aggregate. The overall mechanical properties of RCCAPC were enhanced. Under the effect of vertical positive pressure, the specimen produced transverse displacement and deformation. The largest transverse displacement occurred in the middle part of the specimen; therefore, the corresponding stress in this area is relatively larger than any other area. For this reason, the RCCAPC sample formed the bursting crack from the middle to both ends, and the internal collapse depth was relatively deep in the middle part (Figure 9(a)). The three-dimensional PVA fiber mesh structure in the fiber-reinforced RCCAPC enhanced the integrity of the sample due to the “cyclo-hoop effect”, which led to weakened horizontal deformation and longitudinal penetrating crack (Figure 9(b)). The compressive strength of fiber-reinforced RCCAPC improved with a small margin compared to that of RCCAPC. It should be pointed out that, different from ordinary concrete, in the process of compression test, due to the existence of a large number of pores, the frictional force of the contact surfaces of the upper and lower plates of test machine is not enough to hinder the lateral deformation of the contact surface of PC. Therefore, the compressive failure of pervious concrete is caused by excessive lateral deformation.Figure 9
Macroscopic damage pattern of RCCAPC and fiber-reinforced RCCAPC after compressive test. (a) RCCA40. (b) RCCA40F0.3.
(a)(b)Figure10 shows the macroscopic damage form of the bending test both of the RCCAPC and fiber-reinforced RCCAPC. The RCCAPC section is relatively flat, the ceramic aggregates on the section were broken, and there was a small number of shedding particles but no obvious fracture sign in the test (Figure10(a)). The fracture section of the PVA fiber-reinforced RCCAPC was uneven, the RCCAs were broken, and the failure process had obvious signs (Figure10(b)). With the gradual increase of the test load, microcracks appeared at the bottom of the samples, and then the cracks gradually expanded to the top, with fine particles falling off. The compression area at the upper end of the section was not completely broken. It suggests that the bending strength of fiber-reinforced RCCAPC was significantly enhanced compared to that of RCCAPC.Figure 10
Macroscopic damage pattern of RCCAPC and fiber-reinforced RCCAPC in bending test. (a) RCCA40. (b) RCCA40F0.3.
(a)(b)Due to the characteristics of the dry-wet mixing method in this study, the aggregate surface was more easily covered by cement paste, because the NCA and RCCA were rougher than PVA fiber. As the fiber content continued to increase, the number of high-fineness PVA fibers per unit volume increased rapidly. When the fiber content was more than 0.25%, it was found that some of the exposed fibers were not completely wrapped by the cement matrix, and some fibers were located between the aggregates, which led to a reduction of the contact area and bonding force between the aggregates, leading to weakened integrity of the spatial PVA fiber network as well as its crack inhibition effect. Under these circumstances, the compressive strength and flexural strength of the sample decreased. Furthermore, the decrease in compressive strength is more obvious.Figure11 shows stress-strain curves of RCCAPC and fiber-reinforced RCCAPC in compression tests. The curves can be divided into three stages: approximate elastic stage, crack development stage, and yield failure stage. With the increase of the content of PVA fiber, the peak value of the curve increased slightly, and the decrease of the curve in the yield failure stage tended to be smooth. The area of the parts below the stress-deflection curve from large to small is RCCA40F2.5 > RCCA40F3.5 > RCCA40F1.5 > RCCA40. It indicated that the toughness of the sample increased with the addition of PVA fiber. When the volume content of PVA fiber exceeded 0.25%, the peak value decreased slightly, but the sample still showed good toughness. When the test load was small, the curves were similar to straight lines, which was mainly due to the tiny elastic deformation of the mixed aggregates and cement matrix. And the PVA fibers have little effect on the mechanical properties of pervious concrete. With the development of the cracks, the curves began to enter the descending stage, which means that the three-dimensional PVA fiber network restrained the crack and alleviated the transverse deformation.Figure 11
Complete stress-strain curves of RCCA40, RCCA40F1.5, RCCA40F2.5, and RCCA40F3.5.Figure12 shows stress-deflection curves of the bending test of RCCAPC and fiber-reinforced RCCAPC. With the addition of PVA fiber, the areas of the parts below the stress-deflection curve increased gradually, which means the bending strength and toughness of the sample were improved. When the volume content of PVA fiber exceeded 0.25%, the flexural strength of the RCCAPC decreased, and the bending deformation did not increase significantly after reaching the peak value and the area of the part below the curve reduced, indicating that the bending toughness of the sample decreased. Therefore, the addition of PVA fiber can improve the compressive strength and flexural strength of recycled aggregate pervious concrete, but the content should not be too much.Figure 12
Stress-deflection curves RCCA40, RCCA40F1.5, RCCA40F2.5, and RCCA40F3.5.


## 3.2.2. Permeability

Figure13 shows the permeability of fiber-reinforced RCCAPC. With the increase of PVA fiber content, the permeability coefficient and porosity showed an overall downward trend but decreased by no more than 5%. When the PVA content continued to increase, the permeability coefficient and porosity increased slightly. When the content of PVA fiber was less than 0.25%, the fracture of the sample showed that the cement paste can completely wrap the PVA fibers, and the volume of the cement increased, and the internal pore volume of RCCAPC decreased. On the other hand, some PVA fibers extended into the pores and occupied the part of space in the pores, and that led to the decrease of the flow velocity. Due to the above two factors, the porosity of the sample decreased, and the permeability coefficient decreased slightly. When the content of PVA fiber was too high, part of PVA fibers could not be completely wrapped by cement paste. It was observed that part of exposed PVA fibers was distributed between NCAs and RCCAs, which resulted in the decrease of the density of cementation layer between aggregates, and permeable channels were formed in PVA fiber-aggregate bonding and PVA fiber-cement bonding, which is helpful to improve water permeability. On the other hand, the uncoated PVA fibers extended into the pores of the sample, the volume of uncoated PVA fibers was smaller than that of the pulped PVA fibers, and the softness of uncoated PVA fibers was relatively good. So, when the water pass through, uncoated PVA fibers could bend in the direction of water flow. Therefore, when the volume content of PVA fiber exceeded 0.25%, the permeability coefficient had a slight upturn. From the above analysis, it can be seen that PVA fiber had little effect on the permeability coefficient of RCCAPC. Considering the mechanical properties of the samples, the content of PVA fiber should be controlled at about 0.25%.Figure 13
Relationships between porosity, permeability coefficient, and PVA-fiber volume.


## 4. Conclusion

In this study, RCCAPC and fiber-reinforced RCCAPC samples were prepared by using PVA fibers and RCCA partially instead of NCA. The effects of RCCA replacement rate and PVA content on the mechanical properties and permeability of RCCAPC and fiber-reinforced RCCAPC were studied. The conclusions are as follows:(1)
Using RCCA to partially replace NCA at an appropriate replacement rate can improve the strength of PC. When the ceramic substitution rate was 40%, the compressive strength and flexural strength of RCCAPC reached the maximum values of 21.35 MPa and 2.74 MPa, respectively, and the permeability coefficient was 3.551 mms−1, only reduced by 2.5% compared to the maximum permeability coefficient of 3.64 mms−1. With the continuous increase of the ceramic aggregate substitution rate, the compressive strength and flexural strength of the RCCAPC decreased, and the permeability coefficient increased slightly. When the ceramic substitution rate was 100%, the strength of the RCCAPC samples was reduced to a minimum value. Therefore, considering the high utilization rate of waste ceramic aggregate, this study suggests that the replacement rate of recycled ceramic aggregate should be selected as 40%.(2)
The addition of PVA fibers can improve the mechanical properties of RCCAPC, and PVA fiber-reinforced RCCAPC showed the corresponding plastic deformation in the cube compression test and bending test. When the content was 0.25% and 0.3%, the compressive strength and bending strength reached the maximum value of 23.51 MPa and 3.41 MPa, respectively. And the enhancement effect of PVA fiber on flexural strength of fiber-reinforced RCCAPC was more obvious than compressive strength, and bending strength was 24.5% higher than the maximum value of RCCAPC. The stress-strain curve and stress-deflection curve reflected the toughening effect of PVA fiber. But too much content would lead to a decrease of mechanical properties. The optimum volume content should not exceed 0.3%.(3)
The permeability coefficient of RCCAPC is mainly related to effective porosity and pore shape; ceramic substitution rate and PVA fiber content have little influence on the permeability coefficient, and the range of influence is within 2.4% and 4.5%, respectively.



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*Source: 4375043-2022-03-22.xml*