Comprehensive Benefit Evaluation of Pervious Pavement Based on China’s Sponge City Concept

Abstract

Sponge cities provide broad hydrological functions to alleviate urban flooding and other water-related problems in China. Conventional impervious paving cannot meet contemporary sustainable city goals. The permeable paving technology offers primary benefits such as increasing stormwater infiltration, drainage, purification, groundwater recharge, and microclimatic amelioration. Few studies have evaluated the embracive range of benefits and the social functions holistically. This study aimed to develop a comprehensive benefit evaluation system to cover a broad range of indicators. Nineteen indicators were selected based on the literature review, field studies, and research experience. Organized in a three-tiered hierarchical structure, they were divided into environmental, economic, and social benefits. A grey intuitionistic fuzzy comprehensive evaluation model was built by combining intuitionistic fuzzy analysis with a grey comprehensive evaluation. The computational tools could determine the differential weights of indicators and benefit scores. Taking an example of a permeable pavement project in Quanzhou City, the comprehensive benefits were assessed and validated using our evaluation model. The results show that (1) the comprehensive benefits of the project met the economic feasibility criteria with advantages over conventional paving; (2) the environmental benefits were prominently expressed; (3) the social benefits were assessed and confirmed. The results verified the feasibility and applicability of the quantitative-qualitative model. The method could permit the integrated and systematic benefit assessment of permeable paving designs. It also provides guidance and reference to evaluate the performance of permeable pavements and their comprehensive range of benefits. The findings could reference choosing and refining designs, optimizing the benefits, and promoting a science-oriented development of permeable paving.

1. Introduction

Recent expansion and intensification of urbanization have aggravated environmental problems such as the heat island effect, air and water pollution, and urban flooding [1]. Particularly in compact urban areas, the urban land surface is increasingly sealed by impermeable pour concrete and asphalt [2]. Such conventional paving technology can exacerbate the undesirable effects and compromise the drive towards urban sustainability. A permeable paving offers a better alternative, allowing rainwater to infiltrate, store, and be purified in its passage through the substrate below the land’s surface. It has been advocated as a substitute cover for roadside footpaths and other paved areas to reduce rainwater runoff, increase infiltration, replenish groundwater, and alleviate urban climate [3].

Permeable paving is a new form of land cover material equipped with sufficient pores with a relatively large diameter and continuity to offer good permeability to air and water [4,5]. It allows rainwater to infiltrate in situ through the pavement into the soil base or internal drainage facilities for off-site discharge. The essential function of supporting the foot traffic pressure should not be compromised despite its high porosity [6]. The mechanical strength and walkability of the surface have to be maintained for regular pavement use while serving additional hydrological functions [7]. Compared to conventional paving, permeable paving can provide a broad range of environmental services as an environmental indicator [8]. Considered an integral component of the urban green infrastructure, the technology has been keenly advocated to tap its multiple benefits, extending from hydrological to other environmental domains [9,10]. Regarded as a low-impact development installation, it permits fast rainwater infiltration to replenish soil moisture and groundwater [11]. A portion of the rainwater can be stored in its structural pore space. Rainwater moves into and through the internal voids into the covered soil. The excess soil moisture can drain further downwards to join the groundwater. If necessary, subsurface pipe drains can efficiently collect and discharge the drainage water away from the site. The increased infiltration component can bring a corresponding reduction in surface runoff. In places with exceptionally long and heavy rainfalls, the permeable paving may increase substrate retention and detention, curtail the stormwater runoff considerably, and suppress the probability of urban flooding [12,13].

The stormwater may contain pollutants in particulate (abiotic and abiotic) and dissolved forms picked up in the atmosphere or from the ground. In the process of infiltration and drainage, the water can be cleaned by the natural purification action of the soil environment [14]. The pollutants in the water experience adsorption, decomposition, migration, transformation, etc., in the soil environment mainly by aerobic and anaerobic microorganisms. Thus, the pollutants’ concentration, toxicity, or activity may decrease [15]. The water could be cleansed of harmful substances before joining the subsurface drainage system or the groundwater [16,17,18].

Hitherto, most research studies on the benefits of a permeable paving focus on environmental and economic aspects [19,20]. Few studies have analyzed the social benefits due to their potentially complex nature. Most studies mainly focused on either environmental or economic benefits, with a few taking an integrated approach for appraising the overall benefits. The dominant research direction concentrated on environmental benefits through case studies. For instance, Kazemi et al. studied the water quality benefit of permeable pavement in Louisville, KY [21]. The pollutant concentration of runoff from the permeable pavement was better than impervious. Clark et al. quantified the thermal benefits of green infrastructure and found that permeable pavement can effectively mitigate the urban heat island [22]. Some studies investigated the economic benefits of permeable paving. Hao et al. applied the net present value theory to calculate economic effects [23].

The results indicated that the economic benefits of permeable paving exceeded conventional paving, and the benefits would grow in time. Few studies adopted a broad perspective on the multiple benefits of permeable paving. Zhou et al. used the integrated value of ecosystem service trade-offs to model permeable pavements. They analyzed the stormwater rainfall, rainwater purification, and carbon emissions through a cost-benefit comparison. The results showed that permeable paving had slightly higher initial input costs than conventional paving. However, its ecological and environmental benefits and economic value were much higher from the sustainable development perspective [24]. The Center for Neighborhood Technology and American Rivers jointly published a guide to assess the performance and benefits of green infrastructure. The document outlined the critical factors in evaluating the economic value of green infrastructure practices in urban environments. It demonstrated the calculation method for assessing the economic benefits of permeable paving to enhance the understanding of its value [25]. Valuating the benefits of permeable paving can improve understanding and raise recognition and adoption [26]. Despite multiple benefits, the uptake of this stormwater best practice has remained limited mainly due to some economic and technical barriers, which could be overcome by the propagation of knowledge and skill [27].

In China, the research and construction of sponge cities are still in their infancy. An integrated scientific system for sponge city development has not been developed. There is a lack of systematic analysis of the most effective way to achieve the sponge city goals using a combination of measures [28]. Few studies have evaluated the contributions of permeable paving to sponge city objectives. The many studies conducted on the topic did not assess the holistic benefits of permeable paving. Research in this orientation would demand developing a comprehensive benefit indicator system.

This study aims at the following objectives: (1) Constructed a comprehensive benefit evaluation system for permeable paving. A grey intuitionistic fuzzy comprehensive evaluation model was built by combining intuitionistic fuzzy analysis [29] with a grey comprehensive evaluation [30]. The method could determine the differential weights of the evaluation indicators and the benefit scores of the evaluation objects. (2) The evaluation levels were delineated according to the corresponding criteria. (3) A case study of permeable pavement was selected to validate the comprehensive benefits by applying our evaluation model. Their comprehensive benefits were calculated to verify model practicality and applicability. From the findings, recommendations were distilled to improve the benefit yields of permeable paving.

2. Methods and Data

2.1. Establishing a Comprehensive Benefit Evaluation System for Permeable Paving

The specific research steps adopted in this study are presented in Figure 1.

2.1.1. Objectives and Principles of Benefit Evaluation

The permeable paving technology offers important technical means to achieve sponge city objectives, including increasing stormwater infiltration, reducing runoff to alleviate urban flooding, replenishing groundwater, and purifying rainwater [31]. Establishing a comprehensive benefit evaluation system can provide a conceptual basis with practical applications to tackle the design problems of sponge cities in China. The technology is a low-impact development, reducing unnecessary construction costs and maximizing the effects [32].

Three groups of benefits could be evaluated: environmental, economic, and social. These broad-based benefits were assessed by initially selecting the representative evaluation indicators, which furnished the fundamental building blocks to compose a comprehensive benefit evaluation system. To ensure the evaluation system’s reliability, accuracy, and credibility, it was necessary to adhere to pertinent objectives, scientific rationality, independence, and representativeness. A combination of qualitative and quantitative analytical techniques and a broad and hierarchical structure was followed to develop a refined evaluation system [33].

2.1.2. Selecting Evaluation Indicators

Selecting benefit indicators constituted the basic step in developing the benefit evaluation system. The choice was guided by the performance evaluation indicators of sponge cities, combined with the inherent characteristics of permeable paving [34]. This study gleaned detailed information on potential indicators using different methods. They included an extensive literature review, analysis of national policies, standards and norms, and the authors’ field studies and research. The indicators with strong relevance to our research objectives and high use frequency in published articles were targeted. Kendall’s harmony coefficient and Cronbach’s coefficient were calculated using SPSS software using the Delphi method. The potential indicators were screened and tested. Finally, 19 indicators were selected to build a comprehensive benefit indicator system for permeable paving. The hierarchical structure of the indicator system is shown in

Table 1. The hierarchical structure of the comprehensive benefit evaluation indicators for permeable pavement.

Target Layer	Guideline Layer	Indicator Layer
Comprehensive benefits of permeable pavement A	Environmental benefits A1	Lifting total annual runoff control rate A11 Raising pollution removal rate of stormwater runoff A12 Enriching groundwater resources A13 Alleviating urban flooding A14 Relieving heat island effect A15 Absorbing sound and reducing noise A16 Furnishing landscape effect A17 Conserving resources A18 Improving light environment A19
	Economic benefits A2	Saving investment A21 Augmenting rainwater recycling and reuse benefit A22 Reducing the cost of pollution removal A23 Saving cost of urban drainage facilities A24
	Social benefits A3	Lifting resident satisfaction A31 Enhancing living environment A32 Elevating city image A33 Serving demonstration effect A34 Providing employment A35 Advancing technical specification and construction standard A36

.

2.1.3. Calculating the Weights of Evaluation Indicators

The studies on the benefits of permeable pavement tend to focus on a specific aspect, lacking a comprehensive evaluation of benefits. This study used the gray intuitionistic fuzzy method to evaluate and weigh selected indicators. Traditional evaluation models usually used hierarchical analysis [35], fuzzy hierarchical analysis [36], and other related techniques. However, such evaluation models were deemed unsuitable for our research purpose due to the notable influence of subjective factors on evaluation results. Therefore, we adopted an alternative approach: the gray intuitionistic fuzzy comprehensive evaluation method. Based on the characteristics of each indicator, the evaluation criteria were established. After threshold processing, the gray theory was introduced to construct the gray evaluation power matrix. The grey system theory was developed to solve the problem of data inadequacy and uncertainty. A grey relational model is a research tool for grey relational analysis, which can quantitatively analyze the dynamic development process of a system. It can investigate the closeness of the relationship between various factors in a system and then identify the primary and secondary factors driving the development of a system. Finally, the comprehensive evaluation method was enlisted to determine the comprehensive benefits.

The proposed method incorporated the decision maker’s subordinate and non-subordinate degree of agreement for indicators and the hesitation degree into the evaluation. The proposed system was based on a new judgment matrix, which determined the weights of the evaluation indicators. Once the weights of the indicators were determined, the effectiveness scores of the indicators were established. Gray theory was combined with a comprehensive scale index to determine the overall effectiveness of permeable paving. Finally, the gray theory was combined with a comprehensive scale index to determine the overall benefits of permeable paving.

The gray intuitionistic fuzzy comprehensive evaluation model determined the weights of the indicators in a series of steps. Firstly, a two-by-two comparison of the importance of the indicators was performed by using a questionnaire survey. A new judgment matrix was established according to the indicator scale of the intuitionistic fuzzy set. The indicator weights are calculated hierarchically; finally, a one-off test was conducted. The steps are shown in Figure 2.

The intuitionistic fuzzy analysis method used a new set of scales to quantify the importance of indicators.. After determining the evaluation scale, experts in a specific field were selected to compare their importance by using evaluation indicators to obtain the intuitionistic fuzzy judgment matrix A = (a_ij)m × n, where a_ij = (g_ij, f_ij), i, j = 1, 2…n, and the matrix satisfied the following properties.

$$g_{ii}=f_{ii}=0.5,i=1,2,3\cdots n$$

a_ij = (g_ij, f_ij), a_ji = (g_ji, f_ji), i, j = 1, 2…n, then g_ij + g_ji = 1.

The intuitionistic fuzzy judgment matrix A obtained according to the above method is as follows.

$$A=\left[\begin{array}{cccc}\left(g_{11},f_{11}\right)& \left(g_{12},f_{12}\right)& \cdots & \left(g_{1n},f_{1n}\right)\\ \left(g_{21},f_{21}\right)& \left(g_{22},f_{22}\right)& \cdots & \left(g_{2n},f_{2n}\right)\\ ⋮& ⋮& \cdots & ⋮\\ \left(g_{n1},f_{n2}\right)& \left(g_{n2},f_{n2}\right)& \cdots & \left(g_{nn},f_{nn}\right)\end{array}\right]$$

(1)

After establishing the intuitionistic fuzzy judgment matrix, it was transformed, and the intuitionistic fuzzy numbers of the evaluation indicators were obtained after transformation by the following equations.

$${\omega }_i=\left[\frac{{\displaystyle \sum _{j=1}^n{g}_{ij}}}{{\displaystyle \sum _{i=1}^n{\displaystyle \sum _{j=1}^n{g}_{ij}}}},\frac{{\displaystyle \sum _{j=1}^n{f}_{ij}}}{{\displaystyle \sum _{i=1}^n{\displaystyle \sum _{j=1}^n{f}_{ij}}}}\right]\left(i=1,2,3\dots n\right)$$

(2)

$$w_i=\left(g_i,f_i\right)$$

(3)

$$\left({\omega }_1, {\omega }_2\cdots {\omega }_{\mathrm{n}}\right)=\left[\left(g_1,f_1\right),\left(g_2,f_2\right)\cdots \left(g_n,f_n\right)\right]$$

(4)

The new intuitive fuzzy numbers obtained above were used to calculate the score weights of the first-level indicators with the following.

$$H_{\left({\omega }_I\right)}=\frac{1-f_i}{1+{\pi }_i}$$

(5)

The normalization process was used to obtain the weights of the required evaluation indicators with the following.

$${\sigma }_i=\frac{H_{\left({\omega }_i\right)}}{{\displaystyle \sum _{i=1}^n{H}_{\left({\omega }_i\right)}}}$$

(6)

The relevant literature [37,38,39] indicated that the consistency of the direct fuzzy matrix could be determined by calculating the indicator compatibility [40]. In this study, an improved compatibility test was employed to perform the consistency test of the matrix. Suppose A and B are two fuzzy judgment matrices: $A={\left(a_{ij}\right)}_{m\times n},B={\left(b_{ij}\right)}_{m\times n}$. The compatibility index of A and B was calculated with the following equation.

$$I\left(A,B\right)=\frac{{\displaystyle \sum _{i=1}^n{\displaystyle \sum _{j=1}^n\left|a_{ij}+b_{ij}-1\right|}}}{n^2}$$

(7)

The calculation had to determine whether the value of I (A, B) was 1. If it is, the two fuzzy matrices were compatible and passed the consistency test, signifying that the expert’s fuzzy matrix was constructed reasonably [41]. If it is not 1, the data must be corrected, and the fuzzy judgment matrix must be reconstructed.

According to the steps of the indicator compatibility test in Equation (6), the weight vectors σ and σ of the matrix were determined with the following equation.

$$\sigma ={\left({\sigma }_1,{\sigma }_2\cdots {\sigma }_n\right)}^T,{\displaystyle \sum _{i=1}^n{\sigma }_i}=1,{\sigma }_i\ge 0$$

(8)

The eigenvector A* of the matrix A, A* = (σ_ij*)m × n, and σ_ij*, σ_ij*, were calculated with the following equation.

$${\sigma }_{ij}{}^{\ast }=\frac{{\sigma }_i}{{\sigma }_i+{\sigma }_j}$$

(9)

Based on Equation (9), the I value (A, A*) was calculated to see whether it was less than or equal to 0.1. If I ≤ 0.1, the requirements of the consistency test would be satisfied, signifying that the experts’ evaluation of the indicators was consistent and the calculated indicator weights met the requirements, which could be used in the next stage of comprehensive evaluation. If I > 0.1, the experts would be required to re-score the importance of the indicators, i.e., the parameters of the indicators would be revised until they could pass the consistency test.

2.2. Establishing a Comprehensive Benefit Evaluation Model

Once the weights of the evaluation indicators were determined, an evaluation method was established to aggregate multiple indicators into a single evaluation value. In this study, the threshold comparison method was used to dimensionlessly process the evaluation results obtained by different methods, transforming them into individual indicators. Their scores could be added, subtracted, multiplied, and divided. Finally, the integrated scalar index method was applied to determine the comprehensive benefits of permeable paving [42]. After the calculation using Equation (10), a standard evaluation grade was determined following the corresponding specification standards. The evaluation grade of the comprehensive benefits was finally determined after comparison.

$$T={\displaystyle \sum _{i=1}^n{\omega }_i}•{\tau }_i$$

(10)

Here, we have the following parameters:

T—overall evaluation score for the indicator;

ω_i—indicator weights;

τ_i—score of the indicator after dimensionless treatment.

After determining the evaluation criteria, the scores of the indicators were calculated using different scoring criteria. To facilitate constructing the grey evaluation weight vector for a comprehensive evaluation, we used a threshold for the dimensionless processing of indicators. The product of the processed value and the full score value offered the scores of the evaluation indicators. The comprehensive evaluation weight vectors of the indicator layers and the criteria layers were obtained separately according to the above calculation method. According to the principle of maximum affiliation, we obtained the effectiveness of each indicator. We proposed specific improvement measures and suggestions for indicators with average or below-average effectiveness levels. The comprehensive evaluation matrix B of the criteria layers was constructed from the indicator layers’ grey intuitionistic fuzzy evaluation vector, and the construction method is as follows.

$$B_i={\sigma }_i\times R_i$$

(11)

$$B=\left[\begin{array}{c}{B}_1\\ ⋮\\ B_i\end{array}\right]$$

(12)

Using the intuitionistic fuzzy analysis to obtain the weight σ of the criterion layer and the comprehensive evaluation matrix B of the criterion layer, the gray intuitionistic comprehensive evaluation vector D of the criteria layers was calculated by Equation (13). According to the principle of maximum affiliation, the broad benefit of the evaluation object was determined.

$$D=\sigma \times B$$

(13)

The evaluation set constructed by integrating the grey intuitionistic comprehensive evaluation vector D, with the evaluation grey class V = (9, 7, 5, 3, 1)T, was quantified as a single value. The influence of each factor on the rating target results was integrated to obtain the integrated benefit evaluation value, which is calculated by the following.

$$C=D\times V$$

(14)

2.3. Project Overview

Riverside Park was selected to test the proposed benefit evaluation method as a case study. The site is located in the lower reaches of the Jinjiang River in Quanzhou city in the Fujian Province of China. The Park occupies a river-bank zone with a flat terrain prone to rainwater accumulation, which are features that are suitable for permeable paving installation.

The highly permeable pavement had a layered design (Figure 3 and Figure 4). At the bottom, the soil base layer was compacted to 95% of maximum density. A 300 mm thick graded gravel bedding layer was laid above the base. A 30 mm thick sand filter layer was applied above the bedding layer to serve as an additional rainwater filter. Then, two layers of pervious concrete were laid. The 150 mm lower layer was a relatively less costly ordinary permeable concrete with large aggregate size. The 30 mm upper layer forming the pavement surface was colored permeable concrete with slightly smaller aggregate size. The pavement in the park adopted a colorful mosaic pattern. Finally, after drying the colored concrete, a double surface coating of propylene polyurethane sealant was applied. The sealant reacted with the colored concrete to form a durable transparent coating. The sealant could prevent aggregate dislodgement, protect the pavement, and improve its wear resistance.

Unlike conventional impervious pavement, permeable paving demanded more rigorous construction methods. It should meet both the load-bearing capacity and the permeability requirements. The project contractor recruited experienced professionals to implement the work to ensure delivery quality. The work had received good reviews.

The project’s permeable paving materials were inspected and endorsed by the official Fujian Provincial Construction Engineering Quality Inspection Company. The materials’ technical data derived from laboratory tests are listed in

Table 2. Technical data of the permeable pavement materials installed at the case study’s site: Riverside Park in Quanzhou, Fujian Province, China.

Test Item	Technical Requirement	Test Result
Flexural strength (MPa)	Average value ≥ 4	Average value 4.4
	Single block minimum ≥ 3.2	Single block minimum 4.2
Compressive strength (28 d, MPa) a	25	27.6
Permeability coefficient (cm/s)	≥2.2 × 102	2.4 × 102
Wear resistance (mm)	≤35	21
Frost resistance (%)	Single block mass ≤ 5 loss rate	2.3
	Strong loss rate ≤ 20	17
Free of formaldehyde (mg/kg)	≤0.5	0.41
Visual inspection of double propylene polyurethane sealant coating film	Flatness, no brush wrinkles, pinholes, bubbles, and other defects	No listed defects
Impact resistance of double propylene polyurethane sealant(500 g steel ball)	No cracking or peeling of the coating film	The requirements are satisfied

^a The compressive strength was measured 28 days after installation.

.

2.4. Determining the Valuation Grade

The evaluation grade of the comprehensive benefit of permeable pavement was established following the “Performance Evaluation and Assessment Index of Sponge City Construction”, “Guidelines for the Evaluation of Water Ecological Civilization City Construction,” and other relevant documents. The comprehensive benefits of permeable pavement were divided into five grades, which were excellent (8, 10], good (6, 8], fair (4, 6], poor (2, 4], and very poor [0, 2]. According to expert opinions, the grading was determined using the score interval corresponding to each grade [43].

3. Results and Analysis of the Case Study

3.1. Assigning Weights to Evaluation Indicators

This study selected evaluation indicators from the triple dimensions of environmental, economic, and social benefits to construct the benefit evaluation system. The intuitionistic fuzzy and grey theories were combined to construct a comprehensive benefit evaluation model. The weights of the indicators calculated by the judgment matrix were blended with the grey weight vector theory to analyze and calculate the benefits of permeable pavement comprehensively.

The evaluation system consisted of three criteria layers and 19 evaluation indicators. The intuitionistic fuzzy analysis method was used to determine the weights of the criteria layer and indicators. Ten experts in the permeable pavement were invited to assign weights to the chosen indicators. The importance of the three criteria layers was compared in pairs, and the intuitionistic fuzzy matrix of the criterion level was constructed after computing the experts’ evaluation results. The three criteria sets were qualifications, expert judgment (three levels), and familiarity with relevant knowledge (five levels, i.e., 1.0, 0.8, 0.5, 0.2, 0.0).

$$A=\left[\begin{array}{ccc}\left(0.50,0.50\right)& \left(0.85,0.10\right)& \left(0.90,0.10\right)\\ \left(0.15,0.80\right)& \left(0.50,0.50\right)& \left(0.70,0.20\right)\\ \left(0.10,0.90\right)& \left(0.30,0.60\right)& \left(0.50,0.50\right)\end{array}\right]$$

We then calculated the intuitionistic fuzzy number of the criteria layers.

$$\left[{\omega }_1 {\omega }_2 {\omega }_3\right]=\left[\left(\mathrm{0.50.0.16}\right),\left(0.30,0.36\right).\left(0.20,0.48\right)\right]$$

The score weight of the criteria layers was calculated.

$$H\left({\omega }_1,{\omega }_2,{\omega }_3\right)=\left(0.63,0.47,0.38\right)$$

Finally, the normalization process was applied to obtain the criteria layers. The environmental, economic, and social benefits weights were 0.43, 0.32, and 0.25, respectively. After determining the indicator weights, the consistency test was performed by calculating the compatibility value I and the eigenvector matrix A* of A according to the construction moment A. The matrices of A* composed of the membership degree of the intuitionistic fuzzy judgment matrix are as follows.

$$A^{*}=\left[\begin{array}{ccc}0.50& 0.57& 0.63\\ 0.43& 0.50& 0.56\\ 0.37& 0.44& 0.50\end{array}\right]$$

$$A=\left[\begin{array}{ccc}0.50& 0.85& 0.90\\ 0.15& 0.50& 0.70\\ 0.10& 0.30& 0.50\end{array}\right]$$

This step was followed by calculating I(A, A*) and obtaining I(A, A*) = 1. The results indicated passing the consistency check, signifying that the calculated indicator weights were acceptable.

Using the same method, the weights of the evaluation indicators of the three criteria layers were calculated. Statistical data were computed from the expert scores. The intuitionistic fuzzy matrices were A1, A2, and A3. The final weight coefficients of the entire indicator system were obtained and shown in

Table 3. The weight coefficients of the evaluation indicators.

Target Layer	Guideline Layer	Weight (σ)	Indicator Layer	Weight (σ)
Comprehensive benefits of permeable pavement A	Environmental benefits A1	0.43	Lifting total annual runoff control rate A11 Raising pollution removal rate of stormwater runoff A12 Enriching groundwater resources A13 Alleviating urban flooding A14 Relieving heat island effect A15 Absorbing sound and reducing noise A16 Furnishing landscape effect A17 Conserving resources A18 Improving light environment A19	0.121 0.119 0.114 0.118 0.107 0.105 0.109 0.111 0.096
	Economic benefits A2	0.32	Saving investment A21 Augmenting rainwater recycling and reuse benefit A22 Reducing the cost of pollution removal A23 Saving cost of urban drainage facilities A24	0.271 0.256 0.241 0.232
	Social benefits A3	0.25	Lifting resident satisfaction A31 Enhancing living environment A32 Elevating city image A33 Serving demonstration effect A34 Providing employment A35 Advancing technical specification and standard construction A36	0.170 0.166 0.172 0.170 0.160 0.162

.

3.2. Determining Indicator Benefits and Calculating Grey Evaluation Weight Vector

The case study’s benefit indicators were scored according to the developed evaluation criteria. The project data were obtained from the authors’ field measurements; the contractor, the Fujian Fifth Construction Company; and the permeable paving material supplier, the Shiquanshimei New Material Co. The three sets of benefits, namely environmental, economic, and social, are explained in the following subsections (

Table 4. The hierarchical structure of the comprehensive benefit evaluation indicator system for permeable pavement.

Target Layer	Guideline Layer	Indicator Layer
Comprehensive benefits of permeable pavement A	Environmental benefits A1	Lifting total annual runoff control rate A11 Raising pollution removal rate of stormwater runoff A12 Enriching groundwater resources A13 Alleviating urban flooding A14 Relieving heat island effect A15 Absorbing sound and reducing noise A16 Furnishing landscape effect A17 Conserving resources A18 Improving light environment A19
	Economic benefits A2	Saving investment A21 Augmenting rainwater recycling and reuse benefit A22 Reducing the cost of pollution removal A23 Saving cost of urban drainage facilities A24
	Social benefits A3	Lifting resident satisfaction A31 Enhancing living environment A32 Elevating city image A33 Serving demonstration effect A34 Providing employment A35 Advancing technical specification and construction standard A36

).

3.2.1. Environmental Benefits

The following steps calculated the benefit score and gray evaluation weight vector of nine environmental benefit indicators (A₁₁ to A₁₉).

(1)

Lifting total annual runoff control rate A₁₁

This benefit was evaluated by a statistical method based on its definition. The total annual runoff control rate is the core indicator proposed in the Technical Guide to Sponge City Construction—Construction of Low Impact Development Stormwater Systems (under trial implementation). Based on multi-day rainfall through natural and artificially enhanced infiltration, storage evaporation, etc. The proportion of the cumulative annual rainfall controlled on the site to the total annual rainfall. In this study, the area of the site and the type of permeable pavement were evaluated using the green building design evaluation software [44]. The value of the total annual runoff control rate was 63.35%. The software evaluation results were given to the evaluation experts identified by the evaluation criteria. The experts performed the scoring, and the final average of the experts’ scores was taken.

(2)

Raising pollution removal rate of stormwater runoff A₁₂

The benefits of the pollution removal rates of stormwater runoff were measured by collecting rainwater samples from impervious surfaces and rainwater filtered through permeable pavement on site. Then, the content of pollutants in the rainwater was measured, and ten eligible rainfall events in June–July 2019 were selected to ensure accuracy. With a minimum interval of 3 days between two rainfall events, rainwater samples were collected in 250 mL wide mouth bottles. Suspended solid (SS) was measured by the gravimetric method, using GB 11901-89 assay using an electronic balance to measure the mass of suspended matter after drying the two different rainwater samples. The SS concentration differed between rainfall levels. Eight data groups were selected, and their average SS concentrations are shown in

Table 5. Suspended solid (SS) content of experimental rainwater samples.

Rainwater Water Sample	SS (mg/L)	SS Removal Rate (%)
Rainwater from impervious pavement	142.6	64.3
Rainwater filtered by permeable pavement	50.9

.

The indicator’s benefit score was calculated to be 7.1. The grey evaluation weight vector of the pollution removal rate of stormwater runoff was calculated according to Equations (1)–(9) as [0.33, 0.42, 0.25, 0.00, 0.00].

(3)

Enriching groundwater resources A₁₃

This benefit was determined according to expert opinions. Five experts with rich experience in urban planning and environmental management were invited to score the indicator according to the assessment rules, yielding 7.0, 7.5, 8.0, 7.5, and 7.0. The grey evaluation weight vector of connotation groundwater resources was calculated as [0.36, 0.41, 0.23, 0.00, 0.00].

(4)

Alleviating urban flooding A₁₄

Different rainfall levels and the pavement’s antecedent moisture could notably influence the test results [45]. We monitored ten medium-intensity rainfall events to ensure the dryness of the permeable pavement before the rainfall. Based on the benefit evaluation criteria for this indicator, a benefit score of 4.34 was calculated for this indicator. The grey evaluation weight vector for urban flooding was calculated as [0.19, 0.25, 0.35, 0.21, 0.00].

(5)

Relieving heat island effect A₁₅

In this study, we compared the surface temperatures of the permeable pavement in the case study park and the pavement in the nearby Sunken Island Park. A clear and calm (windless) day was selected to ensure accuracy. An infrared thermometer was used to measure the surface temperature. The measurement method followed the Specification for Surface Meteorological Observation [46]. The temperatures of the two pavements were measured at 14:00 h for five days. Five temperature readings were taken at a one-minute interval on each measurement day, and the average value was calculated and shown in

Table 6. Surface temperature records of permeable pavement at the case study Riverside Park, compared with the impermeable pavement at the nearby Sunken Island Park.

Experimental Group	1	2	3	4	5	Average
Permeable pavement surface temperature (°C)	50.8	49.6	51.9	46.5	48.4	49.44
Surface temperature of ordinary concrete (°C)	52.6	51.9	54.7	49.3	50.6	51.82

.

Table 6 shows that the permeable pavement in the Riverside Park was reduced by 2.38 °C on average. The benefit score of the indicator was 5.3. The grey evaluation weight vector was calculated as [0.23, 0.30, 0.37, 0.10, 0.00].

(6)

Absorbing sound and reducing noise A₁₆

This study measured the noise values of the case study park and the nearby Shenzhou Park at 10:00 h. An Aihua AWA5636-0 high-precision handheld portable noise meter (Hangzhou, China; 40 dBA–130 dBA, Level 2 ± 1) was used. The method developed by GB/T14623-93 [47] was adopted. Five groups of results were selected. The noise reduction in permeable pavements in Riverside Park compared with the conventional pavement was 2.9 dB. The comprehensive benefit score of the indicator was 6.2. The evaluation weight vector was calculated as [0.29, 0.39, 0.32, 0.10, 0.00].

(7)

Furnishing landscape effect A₁₇

The five experts used a questionnaire to set the scores at 8.2, 8.6, 8.3, 9.2, and 8.7. The gray evaluation weight vector was calculated as [0.48, 038, 0.14, 0.00, 0.00].

(8)

Conserving resources A₁₈

The project’s material supplier provided the data for this indicator. The project officially started construction in 2013, using some construction waste as the raw material. The practice followed China’s “Construction Waste Resource Utilization Industry Specification Conditions” promulgated in 2016. The amount of construction waste used for the entire project attained 17%. The benefit score of this indicator was calculated as 3.4. The grey evaluation weight vector of resource saving was calculated as [0.16, 0.20, 0.28, 0.36, 0.00].

(9)

Improving light environment A₁₉

Improving the light environment was mainly evaluated by a questionnaire survey of the Riverside Park residents who were chosen randomly. Of the 20 distributed questionnaires, 16 valid ones were used to calculate the mean score of 8.1. The grey evaluation weight vector was calculated as [0.42, 0.40, 0.18, 0.00, 0.00].

(10)

Overall grey evaluation weight matrix

Overall, the grey evaluation weight matrix R₁ of the nine indicators was constructed as follows.

$$R_1=\left[\begin{array}{ccccc}0.39& 0.41& 0.20& 0.00& 0.00\\ 0.33& 0.42& 0.25& 0.00& 0.00\\ 0.36& 0.41& 0.23& 0.00& 0.00\\ 0.19& 0.25& 0.35& 0.20& 0.00\\ 0.23& 0.30& 0.37& 0.10& 0.00\\ 0.29& 0.39& 0.32& 0.10& 0.00\\ 0.49& 0.38& 0.14& 0.00& 0.00\\ 0.16& 0.20& 0.28& 0.36& 0.00\\ 0.42& 0.40& 0.18& 0.00& 0.00\end{array}\right]$$

3.2.2. Economic Benefits

The benefit score and grey evaluation weight vector of four economic benefit indicators (A₂₁ to A₂₄) were calculated in the following four steps.

(1)

Saving investment A₂₁

Investment saving was related to two aspects. The construction cost could be reduced mainly through the cost items of labor, material, machinery use, and other measures. The construction cost of permeable pavement was primarily hinged on the material and construction cost [48]. This study is based on the technical quantity information on construction materials in Fujian Province, Fujian Province C25 pervious concrete, in accordance with the case of structural material costs with the construction of the unit price of 148 Yuan/m² (the exchange rate was USD 1:00 = 6.36 Yuan Chinese Renminbi currency on 30 January 2022). With some construction waste included as raw materials, the construction unit price of the permeable paving was 132 Yuan/m². Permeable pavement needed daily maintenance with high-pressure water gun flushing at 5% of the unit price. The difference between the construction cost of permeable pavement and conventional impermeable pavement was not significant. The permeable pavement could save about 6% of the investment. According to the scoring criteria, the indicator score was calculated as 7.2. The grey evaluation weight vector of the investment savings was calculated as [0.34, 0.42, 0.24, 0.00, 0.00].

(2)

Augmenting rainwater recycling benefits A₂₂

According to this indicator’s evaluation criteria, the runoff coefficient of the permeable pavement should be measured. The data were provided by the project’s testing unit, which set up three rainfall observation points and ten runoff observation points to measure the simultaneous and continuous rainfall and runoff data in real-time. A set of data was recorded every 5 min. Finally, the runoff coefficient of the area was calculated as 0.393, and the score of the index benefit was 7.4. The grey evaluation weight vector of the saved investment was calculated as [0.35, 0.42, 0.23, 0.00, 0.00].

(3)

Reducing the cost of pollution removal A₂₃

Reducing the cost of pollution removal depends on pollutant concentrations in stormwater runoff. According to the evaluation criteria, there is a relationship between stormwater pollution and chemical oxygen demand (COD). Therefore, the indicator’s benefit score was based on the measured COD. This indicator was measured with the stormwater runoff pollution removal (A₁₂). The same 250 mL bottles were used to collect ten sets of rainwater samples before and after passing through the permeable pavement. A UV-VIS spectrophotometer HJ-013 (Shanghai, China: Shanghai Yidian Holdings, Upper Score Analysis Instrument) measured the COD based on the method specified in HJ/T 399-2007 [49]. The average COD values of the eight data groups are shown in

Table 7. COD content in rainwater samples.

Rainwater Water Sample	COD (mg/L)	COD Removal Rate (%)
Natural rainwater	43.6	31.7
Rainwater filtered by permeable pavement	29.8

. The COD removal rate of the permeable pavement was 31.7%, and the benefit score was calculated as 6.3. The grey evaluation weight vector was calculated as [0.30, 0.38, 0.32, 0.00, 0.00].

(4)

Saving the cost of urban drainage facilities A₂₄

The evaluation data of this index was provided by the construction unit of the project, URA. With the fully permeable pavement adopted for the project, the drainage network arrangement and maintenance costs are lower than those of the conventional impermeable pavement. After the construction unit’s statistics, the cost of drainage facilities in the area was saved by 13.52%. The benefit score of this indicator was calculated according to the evaluation criteria of this indicator as 8.7. The grey evaluation weight vector for saving the cost of urban drainage facilities was calculated as [0.49, 0.38, 0.13, 0.00, 0.00].

(5)

Overall grey evaluation weight matrix

Overall, the grey evaluation weight matrix R₂ of the economic indicators was compiled as follows.

$$R_2=\left[\begin{array}{ccccc}0.34& 0.42& 0.24& 0.00& 0.00\\ 0.35& 0.42& 0.23& 0.00& 0.00\\ 0.30& 0.38& 0.32& 0.00& 0.00\\ 0.49& 0.38& 0.13& 0.00& 0.00\end{array}\right]$$

3.2.3. Social Benefits

The benefit score and grey evaluation weight vector of six social benefit indicators (A₃₁ to A₃₆) were calculated in the following steps.

The social benefit evaluation indicators were assessed by a questionnaire survey of relevant professional experts. The respondents scored according to the evaluation criteria of social benefit indicators. The experts’ scores for the six social indicators were calculated as follows.

$$A_{31}=\left[\begin{array}{ccccc}8.1& 8.7& 8.2& 8.5& 8.9\end{array}\right]$$

$$A_{32}=\left[\begin{array}{ccccc}6.8& 7.2& 7.9& 8.0& 7.5\end{array}\right]$$

$$A_{33}=\left[\begin{array}{ccccc}8.8& 9.0& 8.2& 8.5& 8.0\end{array}\right]$$

$$A_{34}=\left[\begin{array}{ccccc}8.0& 7.2& 8.3& 7.5& 8.0\end{array}\right]$$

$$A_{35}=\left[\begin{array}{ccccc}6.0& 5.6& 4.8& 5.0& 6.5\end{array}\right]$$

$$A_{36}=\left[\begin{array}{ccccc}6.5& 7.0& 6.3& 6.6& 6.8\end{array}\right]$$

The grey evaluation weight vectors of the social benefit indicators were computed. The matrix R₃ was compiled as follows.

$$R_3=\left[\begin{array}{ccccc}0.46& 0.38& 0.15& 0.00& 0.00\\ 0.38& 0.40& 0.22& 0.00& 0.00\\ 0.47& 0.38& 0.15& 0.00& 0.00\\ 0.41& 0.42& 0.17& 0.00& 0.00\\ 0.27& 0.34& 0.39& 0.00& 0.00\\ 0.31& 0.41& 0.28& 0.00& 0.00\end{array}\right]$$

3.3. Comprehensive Benefits Evaluation

According to Equations (10)–(14), the weights of the evaluation indicators at the criterion level were multiplied with the grey evaluation weight matrix to obtain the comprehensive evaluation weight vector of the indicators. The following three steps were taken to complete the computations.

(1)

The comprehensive evaluation weight vector B₁ for environmental benefit evaluation indicators is calculated as follows.

$$B_1={\sigma }_1\times R_1=\left[\begin{array}{ccccc}0.32& 0.35& 0.25& 0.08& 0.00\end{array}\right]$$

(2)

The comprehensive evaluation weight vector B₂ for economic efficiency evaluation indicators is calculated as follows.

$$B_2={\sigma }_2\times R_2=\left[\begin{array}{ccccc}0.37& 0.40& 0.23& 0.00& 0.00\end{array}\right]$$

(3)

The comprehensive evaluation weight vector B₃ for social benefit evaluation indicators is calculated as follows.

$$B_3={\sigma }_3\times R_3=\left[\begin{array}{ccccc}0.39& 0.39& 0.22& 0.00& 0.00\end{array}\right]$$

The weights of the evaluation indicators at the criterion level were multiplied with the grey evaluation weight matrix to obtain the comprehensive evaluation weight vector of the indicators.

$$D=\sigma \times B=\left[\begin{array}{ccc}0.43& 0.32& 0.25\end{array}\right]\times \left[\begin{array}{ccccc}0.32& 0.35& 0.25& 0.08& 0.00\\ 0.37& 0.40& 0.23& 0.00& 0.00\\ 0.39& 0.39& 0.22& 0.00& 0.00\end{array}\right]$$

$$D=\left[\begin{array}{ccccc}0.35& 0.38& 0.24& 0.03& 0.00\end{array}\right]$$

The comprehensive benefits of permeable pavement were integrated into a single value process to make the evaluation results intuitive. The evaluation value C of the comprehensive benefits of permeable pavement in the case was obtained.

$$C=D\times V=\left[\begin{array}{ccccc}0.35& 0.38& 0.24& 0.03& 0.00\end{array}\right]\times \left[\begin{array}{c}9\\ 7\\ 5\\ 3\\ 1\end{array}\right]=7.1$$

4. Discussion

The evaluation model calculated the comprehensive benefit score of the permeable pavement case at 7.1, showing a good benefit grade. The main research discussion and results of this study are summarized in Figure 5.

The environmental benefits of permeable pavement contributed the largest proportion, indicating that the environment should be emphasized in infrastructure construction while focusing on sustainable urban ecology.

(1)

Improving environmental benefits perspective

The evaluation results of environmental-benefit indicators of the case showed that two were rated as excellent, four were good, two were average, and one was poor. Among them, the relatively poor benefit of saving resources could be attributed to the early start of the project when the awareness of resource-saving was relatively weak, resulting in subdued material savings. After promulgating the relevant policy on construction waste recycling in China, construction waste began to be used as raw materials in permeable paving, bringing a progressive improvement in resource saving in recent years.

To improve the environmental benefits of permeable paving, the foremost concern is changing the ingrained construction practice. The conventional focus on construction speed or expediency can switch to long-term environmental considerations. Permeable pavement can follow the enlightened construction management practices of integrated planning, process control, BIM technology, and enhanced governance. In the project’s entire life cycle, attention should focus on optimizing environmental benefits, protecting the environment, and strengthening maintenance [50]. Permeable pavement should bear the expected load and maintain the design permeability, especially to prevent pore-clogging [51]. The environmental benefits will be significantly reduced if poor design and workmanship fail to achieve the desired performance, lead to a fast decline in performance, or damage quickly in regular use [52]. The environmental performance is contingent critically on sustaining the hydraulic and structural performance [53]. Damage could be prevented by using high-quality materials to build durable products and protecting and maintaining them to extend the service life. Additionally, environmentally friendly and recycled materials can save resources and protect the environment [54]. Similarly, innovative techniques and materials can enhance environmental benefits [55].

(2)

Improving economic efficiency perspective

The evaluation of economic-benefit indicators of the case showed that one was excellent and three were good. The results verified that the construction cost of permeable pavement was not much different from the traditional pavement. Moreover, the permeable pavement brought good economic benefits in other aspects, providing the incentives and motives for developers to adopt permeable pavement instead of traditional pavement.

Improving economic efficiency is mainly concerned with reducing costs and enhancing revenue. Cost reduction is linked to all phases of a construction project. The pre-project planning should carefully assess the site’s conditions, including the limitations and challenges to construction processes, in order to allow site-oriented planning and design. Some critical issues should inform design innovations, including the permeable pavement structure, materials, and technical specifications for different layers. The underground utility network at the site should be assessed and arranged and, if necessary, relocated or re-aligned to avoid conflicts with the permeable paving installation. Pervious pavement systems play a vital role in reducing pollutants from stormwater runoff. It enhances the storage and reuse of rainwater and preserves or reinstates the site’s hydrological functions [56]. The method should be applied jointly with other stormwater management measures, helping cities tackle the thorny issue of urban flooding and achieve multiple sponge-city goals. Construction costs can be trimmed by combining permeable pavement with other stormwater management measures to form an efficient rainwater capture, storage, and use system. Revenue can be enhanced by increasing the utilization rate of rainwater. The collected rainwater can be assigned for irrigation, pavement cleaning, and other uses to save natural water resources. Construction efficiency and quality with cost reduction implications can be achieved by training professional and technical personnel at different levels. Innovative construction techniques can be continually acquired, improved, explored, and developed to contribute to cost saving [57]. Measures to prevent pavement deformation and clogging are critical to sustain performance and reduce the repair and re-paving costs [58]. New and lower-cost materials, including recycled ones, can be evaluated for application to permeable paving. For maintenance, new and more effective cleaning techniques can be investigated to improve cleaning efficiency and reduce the frequency [59,60].

(3)

Improving social benefits perspective

The evaluation of the social benefits of the case showed that two indicators were excellent, three were good, and one was average. The construction of the innovative permeable pavement demands more complex planning, design, testing, material supply, training, implementation, and maintenance than traditional pavement. Therefore, it requires more well-trained and experienced professional and technical construction personnel. Its adoption can bring the collateral benefit of creating job opportunities and enhancing employment in the construction sector to usher social benefits to the community.

Improving social benefits is reflected in enhancing the community’s landscape quality and promoting environmental awareness and protection. The permeable pavement design could align with the city’s temperament and genius loci. Attractive and pragmatic design details can be introduced, such as color choice and patterns, associated signage, and other roadside and greenspace paraphernalia to enhance the city’s image. Beautiful, stable, durable, clean, and safe permeable pavements should enliven the ambiance and draw more people to walk and exercise in the outdoor environment. More activities in green spaces should improve physical and mental health and bring other collateral social benefits [61]. Extensive installation and associated maintenance requirements could demand workers and technical staff training to raise the human resource quality and create employment opportunities.

5. Conclusions

Permeable pavement as a part of the sponge city idea deserves to be explored in detail concerning its multi-dimensional benefits. This study investigated the broad range of benefits by conducting field research and a comprehensive literature review. Initially, 19 basic evaluation indicators were selected regarding environmental, economic, and social benefits. Indicator screening was conducted, and an indicator system for comprehensive welfare was established. A gray intuition fuzzy comprehensive evaluation model was established to analyze and evaluate the comprehensive benefits of permeable pavement. An engineering case study of permeable paving was selected to test the new model.

(1)

Clarify the comprehensive benefits of permeable pavement by establishing a benefit evaluation indicator system

The comprehensive benefits of permeable pavement were systematically organized and elaborated by performing an extensive literature review. Fieldworks were conducted to strengthen the conceptual and knowledge base. The benefit evaluation system was developed, comprising environmental, economic, and social dimensions. A benefit evaluation system was created to incorporate three guideline layers and 19 evaluation indicators taken from the three perspectives.

(2)

Construction of a comprehensive evaluation model of permeable pavement benefits

A gray intuitionistic fuzzy comprehensive evaluation model was enlisted to evaluate the chosen benefits. The model first established the intuitionistic fuzzy analysis method, introducing the affiliation degree, non-affiliation degree, and fuzzy set to determine the indicator weights. It then established the expert scoring standard of the indicators and applied the gray theory to transform the indicator scores into the gray evaluation vector. Finally, the benefit scores were computed.

(3)

Empirical verification of the practicality and applicability of the evaluation model

After establishing the evaluation model, the practicality and applicability of the model were verified. A permeable pavement project in an urban park in Quanzhou (Fujian, China) was selected to test the evaluation model. The multiple benefits were calculated and rated good. The evaluation process of the case study was analyzed to understand the underlying factors affecting the benefits. The study provided recommendations and measures to improve the benefits.

(4)

Improve the understanding of permeable paving

Most studies of permeable pavement benefits recognized the ecological and environmental benefits. This study extended the evaluation to the economic and social benefits and developed a model to assess the triple benefit dimensions in a unified system. We believe that such an approach and the enhanced understanding are conducive to promoting this promising innovation. The computation method can allow systematic and integrated benefit assessment of different permeable pavement designs. The findings can improve and refine designs aiming at optimizing benefits. They can facilitate the development of a science-based permeable paving programs to suit different socio-economic and site conditions.

Although the objectives of this study have been achieved, some limitations need to be addressed. Firstly, the evaluation criteria of some indicators used he data measured by a collection of Chinese and overseas researchers. Due to the differences between investigations in relation to experimental design and real-world situations, using the evaluation criteria reported in the literature may affect the accuracy of our results. Secondly, the questionnaire surveys set the weights and benefit scores of some indicators. However, the limited size of our respondent sample size may affect the accuracy of our results. Factors such as geographical location and respondents’ socio-demographic background could affect the quality of the responses. Thirdly, this study’s comprehensive indicator evaluation system was validated on a permeable pavement case. The testing could be expanded using more cases. Future studies may consider verifying permeable pavements at different times of the year, under different climatic conditions, and in other climatic regions. Fourth, this study was based on the context of the Fujian Province, China. The findings may need to be adjusted for application to other places.

Most studies focus on the environmental and economic aspects of permeable paving. Due to the inherent complexity of the potential factors, few studies have encompassed the social benefits. Our work established a comprehensive benefit evaluation system including the full spectrum of environmental, economic, and social benefits. It is hoped that future studies could explore deeper to enlarge the knowledge base. First, studies could aim at improving and refining evaluation indicators. The selection of indicators can more thoroughly consider the actual traits of permeable pavements. The industry’s continued development may change the relative importance of benefit indicators. The more representative indicators can be incorporated to evaluate comprehensive benefits. Secondly, improved industry-wide technical standards and guidelines can be established and promulgated for common, if not mandatory, adoption. The quality of design, material, maintenance, and workmanship can be continually upgraded to meet the discerning demands. Thirdly, additional research associated with earnest knowledge transfer efforts will increase the scientific rigor and quality of the permeable-pavement practice. As permeable pavement performance is affected by geographical and environmental factors, the scope of the field studies should be expanded. In addition, more authoritative experts can be invited to participate in questionnaire surveys to improve the veracity and accuracy of the assessments and judgments.