Performance Evaluation of Waste Crumb Rubber/Silica Fume Composite Modified Pervious Concrete in Seasonal Frozen Regions

State Key Laboratory of Mechanical Behavior and System Safety of Traffic Engineering Structures, Shijiazhuang Tiedao University, Shijiazhuang 050043, China Key Laboratory of Roads and Railway Engineering Safety Control (Shijiazhuang Tiedao University), Ministry of Education, Shijiazhuang 050043, China Guangxi Transportation Science & Technology Group Co. Ltd., Nanning 5300007, China Guangxi Key Lab of Road Structure and Materials, Nanning 5300007, China


Introduction
In the process of urbanization, the unprecedented levels of urbanization not only brings great convenience to people's life but also causes many social and environmental problems [1][2][3]. Due to the excessive consumption of natural resources and the increase of carbon emissions, global climate change is becoming more and more obvious. Extreme severe weather conditions such as high temperature and flood frequently occur [4][5][6]. At the same time, the urban surface is covered by a dense concrete structure, which blocks the flow of water and gas between underground and ground, causing many urban problems, such as urban waterlogging, groundwater recession, noise pollution, and urban heat island phenomenon [7][8][9][10]. With the increasing prominence of urban problems and people's awareness of environmental protection, it has become the focus of people's attention to construct green and sustainable development of city-peopleenvironment.
us, China has proposed the concept of sponge city in 2012 [11]. In 2015, the first batch of sponge city construction pilot city was determined, and 16 cities were shortlisted. Meanwhile, state finance provided 10 billion CNY as support.
e most important component of sponge city construction is urban pervious pavement; it is a kind of pavement structure with connected pores that can realize the functions of being permeable and breathable. It can well solve the urban problems brought by impervious pavement, and it is an ecological and environmental protection pavement structure [12,13]. Compared with the traditional impervious pavement structure, the pervious pavement has several advantages: (1) raising the groundwater level and replenish groundwater resources, (2) reducing urban surface runoff and alleviate urban waterlogging, (3) reducing urban noise pollution and improving urban living environment, and (4) alleviating urban heat island phenomenon and improving the urban ecological environment. At present, PC is widely used in parks, sidewalks, light vehicle roads, and squares [14][15][16]. Generally speaking, the permeability and compressive strength of PC can meet the engineering requirements [17][18][19][20]. However, when PC is used as road surface or base layer, it will often be subjected to the repeated vehicle loads, which requires that PC not only has sufficient strength but also has certain deformability to resist fatigue [21]. Moreover, the porous structure of PC makes the adverse effect of F-T more serious than that of ordinary concrete. So, the F-T resistance of PC is also very important [22,23]. erefore, good deformability and F-T resistance for PC in seasonal frozen regions are as important as permeability and strength property.
WCR has been widely used in road engineering for its high toughness and antiaging properties. e application of WCR in ordinary concrete shows that WCR can effectively improve the deformation property, F-T resistance, toughness, vibration, and noise reduction performance of concrete, but it has a negative impact on its strength [24][25][26][27][28]. SF, as a byproduct of the smelting industry with abundant silica, is an excellent cement-based modified material and has a broad application prospect in cement concrete [29,30]. e research has already verified that the addition of SF can significantly improve the strength and F-T resistance of concrete [31,32].
Although mature and extensive research has been conducted on the application of WCR or SF in ordinary concrete, their application in PC is rarely reported, especially the modification with two kinds of materials simultaneously. e researches on rubberized PC showed that the addition of WCR significantly decreased the mechanical properties and abrasion resistance, but it had a crucial positive effect on the ductility and vibration reduction performance of PC [33,34]. In addition, the properties of rubberized PC were pertinent to the particle size of WCR. e adverse effect of fine rubber on mechanical properties was less than that of coarse rubber. At the same time, the positive influence of fine rubber on deformability was better than that of coarse rubber [35,36]. erefore, the WCR with relatively small particles is the preference in modifying pervious concrete, which is different from ordinary concrete. In terms of SF, it had been verified that the SF-reinforced PC had great advantages in mechanical strength and durability, but the deformability was not improved [37][38][39]. e compressive strength of PC, with 10% SF replacement, had been greatly improved by more than 80% [38]; this great increase in compressive strength was worthy of further study to verify. e properties of PC modified or reinforced with WCR or SF, on the whole, need to be studied extensively and indepth. Based on the current research progress, a large number of studies on WCR or SF single modification concrete have been conducted by domestic and foreign scholars, and considerable research findings were achieved. It can be seen that, due to the high elasticity of WCR, the addition of it can significantly improve the deformability of concrete under load, thus improving concrete toughness. At the same time, the addition of WCR will weaken the bonding between concrete components, resulting in the reduction of concrete strength.
e rich silica content of SF leads to a further chemical reaction between SF and cement hydration products to produce a stronger gel, which improves the strength and durability of concrete. PC, as pavement material in practical engineering, requires not only sufficient strength to bear the action of load but also good deformability to bear the action of repeated load. In addition, the frost resistance of PC is the most important index of durability when it is used in seasonal frozen regions. erefore, the development of PC with good deformability, superior frost resistance, and high strength is an important basis for the application. Considering the advantages of WCR or SF, in order to give full play to the characteristics of both materials, combined with previous research performed by our group [30,35], this article conducted the laboratory investigation on the mechanical property and F-T resistance of RSFPC. e research outline is shown in Figure 1.

Raw Materials.
e ordinary Portland cement of 42.5 and SF were used as cementitious materials for RSFPC. e coarse aggregate with 4.75-9.5 mm and WCR with a particle size of 40 mesh, obtained from a local factory, were selected in the article. e technical properties of all the above materials can be found in [30,35]. Besides, a superplasticizer was applied and its properties are listed in Table 1. e experiment water is tap water.

Mix Design.
With equal volume replacement of cement, the effect of different SF contents on the properties of PC has been studied by our group [30]. e research indicated that 12% SF presented better modification effectiveness, so in this study, the SF incorporation level was selected as 12%. In addition, the effect of particle size of WCR on the properties of PC indicated that the fine WCR showed better improvement than that of the coarse WCR [35], so the fine WCR was selected and the incorporation level of WCR for RSFPC was set as 4%, 6%, and 8% of cementitious material quality.
e volumetric method was adopted to design RSFPC in accordance with the Chinese national standard [40]. e water-to-binder ratio was 0.3, and the designed porosity was 15%. e content of the superplasticizer was 0.8%. e mix design is shown in Table 2.

Specimen Preparation and Test Methods.
e preparation method, the production, and curing condition of RSFPC specimens have been reported in detail in [21]. e sizes of the specimens are as follows: cube specimen with 100 × 100 × 100 mm for compressive strength and F-T cycles tests; prism specimen with 100 × 100 × 400 mm for flexural experiments. All experiments were conducted according to GB/T 50081-2002 and 50082-2009 [41,42]. e test method for rapid freeze-thaw was adopted. e freezing time was about 2.5 hours and the thawing time was about 1.5 hours. e lowest and highest temperatures in the center of the specimen were −18°C and 5°C, respectively. As shown in Figure 2, a three-point bending test is adopted for the flexural experiment. In order to accurately determine the flexural failure strain and flexural elastic modulus of RSFPC, the midspan displacement of the three-point bending specimen under load was recorded by a micrometer gauge placed on the bottom of the specimen. e flexural failure strain and flexural elastic modulus can be calculated by where ε is the flexural failure strain; ∆ is the deflection corresponding to flexural failure load (mm); E is the flexural elastic modulus (GPa); F 0.5 is the load that equals 50% flexural failure load (kN); F 0 is the initial load of the specimen and its value can be taken as the actual load of the specimen nearest to 2 kN (kN); and ∆ 0.5 and ∆ 0 are the displacements of the specimen corresponding to F 0.5 and F 0 (mm).

Experiments Results.
All the property indexes of RSFPC are expressed as the average value of three specimens; the experiments results are listed in Tables 3-7.

Properties Analysis of RSFPC.
In order to analyze the compound modification effectiveness of WCR and SF, the control group (without WCR and SF), WCR group (WCR modification group), and SF group (SF modification group) are introduced as comparative groups. e data of the control group, WCR group, and SF group are obtained from the previous research conducted by our team. Figure 3 shows the compressive strength of RSFPC and comparative groups. It indicates that the compressive strength of RSFPC decreases with the increase of WCR contents, which means the addition of WCR has an adverse impact on the compressive strength of PC. e result is consistent with that of the WCR single modification PC reported in [35]. is is because the WCR on the interfacial transition zone weakens the cementation between cement and coarse aggregate and thus decreases the bond force. Compared with WCR groups, due to the

Flexural Failure Strain.
WCR is a kind of elastic material; the purpose of using WCR modification is to improve the deformability of PC. Figure 5 describes the flexural failure strain of RSFPC. It is obvious that the flexural failure strains of the WCR group and RSFPC are significantly higher than those of the control group and SF group. Due to the high elasticity, the addition of rubber makes the deformation of the PC better under the action of vehicle load, reducing the rigidity and improving the toughness of the PC. With the same amount of WCR, the flexural failure strain of the WCR group and RSFPC group are basically the same, which indicates that the WCR is the fundamental factor that creates the improvement of the flexural failure strain. Compared with the control group, the flexural failure strain of RSFPC is enhanced by 12%, 59%, and 76%, respectively. It means that the deformability of PC has been greatly improved, and when used as the road surface or base, PC has better fatigue properties under repeated loads. Figure 6 shows the flexural elastic modulus of RSFPC. e flexural elastic modulus of RSFPC decreases with the increasing WCR contents and it reveals that the addition of WCR makes the flexural elastic modulus aggravated. Based on the effects of WCR on the flexural strength and flexural failure strain above, on the one hand, the WCR reduces the flexural strength of PC and improves its flexural failure strain, on the other hand, thus reducing its flexural elastic modulus. RSFPC has higher flexural elastic modulus than that of the WCR group at the same WCR content because the flexural failure strain of the WCR group and RSFPC is basically the same at the same level of WCR content. However, due to the incorporation of SF, the flexural strength of RSFPC increases, resulting in a higher flexural elastic modulus. Compared with the control group, the WCR4 + SF12 group has a flexural elastic modulus of 24 GPa with a 5.3% increase, although there is a 15% decrease compared with the SF group. e flexural elastic modulus is closely related to the flexural strength and flexural failure strain, so the modification materials and its contents of RSFPC should be considered comprehensively based on its property indexes in practical application.

F-T Resistance.
e F-T resistance is another vital index for RSFPC in seasonal frozen regions. e compressive strength loss rates of the control group, WCR group, RSFPC group, and SF group after different F-T cycles are presented in Figure 7. It is obvious that compared with the control group, the compressive strength loss rates of the WCR group, RSFPC group, and SF group under different F-T cycles are lower, which indicates that WCR modification, SF modification, and WCR and SF compound modification are all beneficial to F-T resistance of PC. e improvement of F-T resistance of PC by WCR is mainly attributed to the increase of PC deformation during the frost WCR4 + S12 WCR6 + S12 WCR8 + S12 Advances in Materials Science and Engineering heaven. Moreover, the improvement of F-T resistance of PC by SF is because SF improves the strength of cementitious material and reduces the effectiveness of F-Tdamage. RSFPC group has lower compressive strength loss rates than that of WCR group with the same WCR content, which indicates that WCR and SF compound modification is better than the WCR single modification. However, compared with SF single modification, the compressive strength loss rate of WCR and SF composite modification is higher, indicating that the WCR and SF composite modification had poor F-T resistance. Although both SF and WCR can improve the F-T performance of PC, the WCR and SF composite modification is not the superposition of their single modification effect. certain F-T durability; it requires that the material has comprehensive properties rather than being superior in one area and inferior in others. Due to the requirement of permeability, the porosity of PC is usually large. However, high porosity will cause a reduction in mechanical properties, such as compressive strength, flexural strength, and elastic modulus. In addition, too large porosity is very unfavorable to F-T resistance, which will accelerate the damage of PC. Meanwhile, under the action of repeated load, PC is prone to fatigue damage and failure, and its deformability is an important index that affects its service life.

Discussion
erefore, PC applied in practical engineering must have a certain strength, deformability, and F-T resistance.
In Section 3, the mechanical properties and F-T resistance are discussed in detail; all kinds of property indexes for RSFPC and other PC are presented in Figure 8 with a radar map. It can comprehensively compare the differences between various property indexes of different types of PC and select PC with superior comprehensive performance. Except for the compressive strength loss rate under F-T cycles, "the larger, the better" is another property index for PC. Figure 8 shows that RSFPC with 6% WCR content has the best comprehensive performance, followed by RSFPC with 4% WCR content. ese two groups have better strength, greater deformability, and higher F-T resistance. e strength and F-T resistance of SF group are better, but the deformability is poor. e control group has poor F-T resistance and deformability, while WCR groups express lower strength and elastic modulus. erefore, it is necessary to select the

Conclusions
In this article, the compressive strength, flexural strength, flexural failure strain, flexural elastic modulus, and F-T resistance of RSFPC were investigated. e composite modification effects of WCR and SF were analyzed.
(1) e flexural failure strain of PC indicates that WCR is the determining factor in improving the flexural deformability of PC. (2) WCR has adverse effects on the strength and elastic modulus of PC, but these adverse effects can be improved by the addition of SF. Compared with the control group, the WCR and SF composite modification can not only guarantee the strength characteristics of PC but also enhance the F-T resistance to some extent and, at the same time, significantly improve deformability of PC. (3) Although both SF and WCR are advantageous to the F-Tresistance of PC, the improvement effect of WCR and SF composite modification is not the superposition of both, which is mainly because of their different effects on the mechanical properties of PC.
(4) Considering the mechanical properties, deformability, and F-T resistance of PC, the systematic comparison of different modification PC shows that RSFPC has a more balanced and comprehensive performance and is suggested to be the first choice in engineering application.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.

Authors' Contributions
G. B. Luo contributed to conceptualization, formal analysis, and investigation. P. Zhao was responsible for the methodology and writing, reviewing, and editing of the manuscript. Y. P. Zhang wrote and prepared the original draft. Z. Z. Xie was responsible for funding acquisition and formal analysis.  Advances in Materials Science and Engineering would like to express their appreciation to the Road and Bridge Laboratory of Jilin University for support during the experiment.