1. Introduction
Pervious pavement plays an important role in urban rainwater management system. At the same time, it is of great significance to the future development of the urbanization [
1]. In urban construction, large area of the city's original soil surface has been coated with cement, concrete, and other impervious surfaces [
2]. The public places such as sidewalks, open-air parking lots and square, were also covered by stone or cement brick. Impervious surface has a significant adverse impact on urban ecology and climate environment while improving traffic and road conditions and beautifying the environment [
3,
4,
5]. In general, summer is a season of heavy rains in China. The serious water accumulation has been occurred in many cities, which caused many disasters, such as traffic jams, power outages, and flooded houses [
6,
7,
8]. The main reason for this is that the urban road surface is impervious [
9]. The pervious pavement is an effective method to balance the urban ecosystem. The rainwater can penetrate into the ground from the pervious pavement and cause the groundwater level to rise rapidly [
10]. The pervious pavement is the connecting passage between the underground and the ground, so that the ground is warm in winter and cool in summer, which can increase the comfort of urban living [
11]. In addition, due to the large number of pores and large specific surface area of pervious pavement, it has strong adsorption capacity for dust, which can reduce dust pollution. However, in order to prevent voids from being clogged, regular cleaning is needed. On the other hand, the noise caused by vehicles can be well absorbed by the pores of pervious pavement [
12,
13]. Some developed countries began to investigate and develop pervious pavement materials in the 1970s and applied them to sidewalks, bicycle lanes, public square, open-air parking lots, park roads, shoulder and central divider [
14]. It has increased the permeable space of the city and has achieved good results in regulating the urban climate and maintaining the ecological balance. After the 1990s, paving the pavement with pervious materials instead of traditional materials has become a common experience in urban construction in developed countries [
15]. For example, Germany, which is known for its environmental protection technology, proposes to transform 90% of the roads in the country into pervious pavements by 2010. It can be seen that the pervious pavement is of great significance to the future development of the city.
Cement is the most basic raw material for engineering construction. At present, there is no material at home and abroad that can replace its status. Despite the rapid development of the cement industry, many problems have arisen, such as serious waste of resources, huge energy consumption, and serious pollution to the environment [
16]. In addition, the carbon dioxide produced by the cement production causes global warming [
17]. On the other hand, the dust emission in cement production is high, which will pollute the air, produce haze, and cause environmental pollution [
18]. Reducing the amount of cement and finding possible cement substitutes is an important way to achieve sustainable development.
Fly ash is a kind of industrial by-product waste. It is the fine ash obtained from the coal combustion [
19]. In China, the annual output of fly ash has reached 30 million tons. With the development of the power industry, the amount of fly ash discharged from coal-fired power plants has increased year by year. If the large amount of fly ash is not treated reasonably, it will generate dust and pollute the atmosphere; if it is discharged into the water system, it will cause river blockage, and the toxic chemicals will cause harm to human and other creatures [
20]. Therefore, how to dispose and utilize fly ash has become a widely concerned issue. In China, as early as the 1950s, fly ash began to be used as a concrete and mortar admixture and pavement base material in construction and road engineering, especially in hydropower construction and dam projects [
21]. In the 1960s, the utilization of fly ash began to shift to wall materials and the comprehensive utilization of fly ash was booming in the 1980s. Fly ash has also been used in other countries with different degrees. The UK has developed a high-quality commercial fly ash for reinforced concrete. Poland focused on the use of fly ash in building materials. The comprehensive utilization of fly ash in France started earlier, especially in the application of cement and concrete [
21,
22]. Australia attached great importance to the quality control system of fly ash concrete industry, and had a company specializing in high quality fly ash products [
23]. Therefore, the utilization of fly ash and its application in roads and building materials cannot only realize waste utilization, but also reduce the amount of cement, which has high economic and environmental protection value.
The application of fly ash in cement concrete has many advantages, such as reducing cement consumption; improving the workability of concrete mixture; reducing creep, hydration heat, and thermal expansion of concrete; and improving the impermeability of concrete [
24]. The effects of fly ash on cement concrete have been reported by many researchers. Wu et al. [
25] investigated the effect of fly ash on the fresh and aged mechanical properties and durability of coral aggregate concrete, the compressive strength, splitting tensile strength, chloride penetration, and micro-hardness of the interfacial transition zone (ITZ) were measured, compared with the control group, the addition of fly ash improved the cohesiveness and water retention. On the other hand, fly ash had a negative effect on early-age strength, but significantly enhanced the long-term strength. Zhu et al. [
26] conducted experiments on fly ash modified engineered cementitious composites (ECC), the ductility and compressive strength properties were tested. The results showed that the addition of fly ash reduced the compressive strength about 40%, but it is positive in improving the ductility of ECC. Uthaman et al. [
27] conducted experiments and investigated the strength and durability of fly ash concrete. The results indicated the early-age strength and durability of fly ash concrete were insufficient, but the long-term strength and durability were improved. The freeze–thaw resistance of high-performance concrete (HPC) modified by fly ash was investigated by Ma et al. [
28], the result indicated the fly ash improved the freeze–thaw durability of HPC. In summary, fly ash has positive effect on the long-term mechanical property and durability of concrete, but it is negative for early-age strength.
However, the application of fly ash in pervious concrete is not as extensive as in ordinary concrete and the related researches are few. Muthaiyan and Thirumalai [
29] studied the strength properties of pervious concrete modified by fly ash. The fly ash was used to replace cement at 10% and 20% levels, and the results showed that 28 d compressive strength and flexural strength of fly ash modified pervious concrete were significantly reduced, especially the mixture with 20% fly ash. Aoki et al. [
30] investigated the compressive strength and permeability of pervious concrete modified by fly ash and the cement replacement levels were 20% and 50%. The compressive strengths of 20% and 50% fly ash modified pervious concrete decreased 12.7% and 43.7%, respectively. The permeability of pervious concrete was not affected by the addition of fly ash. Zaetang et al. [
31] studied the fly ash modified pervious cement concrete. The cement was replaced by fly ash at different levels (85, 90, 95, and 100%), the compressive strength and splitting tensile strength at the age of seven days were tested. The results indicated that the compressive strength and splitting tensile strength of fly ash modified pervious concrete decreased with the increasing fly ash content. When the fly ash content was 100%, the maximum compressive strength and splitting tensile strength were 5.9 MPa and 0.74 MPa, respectively. Peng et al. [
32] investigated the compressive strength of fly ash modified pervious concrete at the age of 28 and 60 days with fly ash content of 30%. The obvious decrease in compressive strength at 28 d was observed compared to the control mixture. Compressive strength at 60d was comparable to the control group. The adverse effects of fly ash on the strength of pervious concrete were also reported by other researchers [
33,
34].
At present, the application of fly ash in ordinary concrete has been extensively studied. However, studies on the effects of fly ash on the properties of pervious concrete are relatively few, and the existing studies were mainly focused on the early-age (generally 28 d) properties of fly ash modified pervious concrete. The long-term (60d, 90d, etc.) performance of fly ash modified pervious concrete has rarely been reported. Additionally, the freeze–thaw durability of fly ash modified pervious concrete was not studied extensively. Therefore, it is necessary to carry out research to investigate the strength properties at different ages and freeze–thaw durability of fly ash modified pervious concrete. Based on the previous researches conducted by our group [
35,
36], in order to evaluate the strength time–varying and freeze–thaw durability of fly ash modified pervious concrete, pervious concrete modified by fly ash was prepared with equivalent volume method, the porosity, permeability, strength at different ages and freeze–thaw durability of fly ash modified pervious concrete with different fly ash incorporation levels were studied. The conclusions obtained in this paper are helpful to understand the strength development with age and freeze-thaw durability of fly ash modified pervious concrete, and will provide reference for the research and application of fly ash in pervious concrete.