Abstract

This study assessed the water drainage capability of porous asphalt material (PAM) based on laboratory rainfall simulator tests. A series of tests were conducted using a range of slopes and rainfall intensities. The results showed that as the rainfall intensity increased, the water subsurface drainage steadily decreased. When the slope increased, the subsurface outflow of the porous asphalt decreased. Nevertheless, the slope of PAM insignificantly affected the water subsurface drainage. For the PAM specimen with a porosity of 15%, at a rainfall intensity of 2.5 L/min, when the slope increased from 0% to 8%, the subsurface outflow reduced by 2.8%. The investigation of the effect of porosity on subsurface drainage showed that the porous asphalt with a higher porosity displayed a higher subsurface drainage. At the slope of 4%, at a rainfall intensity of 4.9 L/min, for the PAM specimen with porosity of 10% and 15%, the subsurface outflow was 72.3% and 79.1%, respectively. It could be seen that the porosity had a strong effect on the drainage capability of PAM. The above results imply that the utilization of PAM depended more on the porosity than the slope. In the future, further experiments evaluating the water drainage of PAM should be adopted.

1 Introduction

With the high speed of industrialization and the urbanization process, natural permeable surfaces have been changed to artificial impermeable ones, such as roads and buildings (Dai et al. 2021). For instance, in Shenyang, China, the increase in urbanization contributed to the artificial impermeable surface by about 52.42% over a period of 15 years (1984–2000) (Li et al. 2019). The results of this progress produced remarkable negative impacts to water hydrology, such as flooding and pollution (Chu et al. 2018; Wu et al. 2020; Zhang et al. 2020). To decrease these negative impacts, green infrastructure has been introduced as a potential solution. These solutions include green roofs, green gardens, green ponds, and permeable pavements. The permeable pavements are constructed with a high porosity; therefore, the water could go through its body and effectively drain out vertically through the bottom side, and laterally through the edge side (Eisenberg et al. 2015). Since a permeable pavement system could be infiltrated by water through its interconnected pores, it could reduce the peak flow and the pollutants in the rainwater during rainfall. It is well-known that a permeable pavement system could be effectively used for water management in cities for low impact development (Eck et al. 2012; Berbee et al. 1999; Roseen et al. 2009). Porous asphalt material (PAM) has been commonly used in permeable pavement systems. It has been used as a surface material, laid over the conventional asphalt material or the permeable base/subbase material to construct the permeable pavement system (Manrique-Sanchez and Caro 2019). The characteristics of porous asphalt material when used in the permeable pavement systems were figured depending on several factors, such as material property (porosity, permeability), design geometric (longitudinal slope, cross slope, pavement length), and environmental factors (rainfall intensity, freezing and thawing, etc.) (Tan et al. 2004; Ranieri 2002; Pal and Das 2019). 

Several studies have been conducted to assess the water drainage capability of porous asphalt material during rainfall while considering slope and rainfall intensity (Yang et al. 2024; Chen et al. 2024). The literature showed that there were two common methods to achieve the water drainage for porous asphalt material, the experimental method, and numerical modeling method. In the laboratory, rainfall simulation tests for the porous asphalt material have been adopted to assess the water drainage capability. Ranieri observed the water drainage of porous asphalt material using the rainfall simulation method (Ranieri 2002). The porous asphalt sample with a thickness of 70 mm and dimensions of 1450 x 750 mm was prepared and tested at different slopes and rainfall intensities. The PAM samples were designed with the porosity of 41%, 222%, and 42%. The test device included water supply, rainfall simulator, test box, and flowmeter. Along the samples, piezometers were attached to inform the height of water flow. Nevertheless, this study did not measure the subsurface drainage outflow. The results of the experiment showed that the water flow in the PAM was not a laminar steady flow. The results of the experiment were used to calibrate with those of the analytical solution. As the slope increased, the water flow head in the body of the porous asphalt decreased. The author indicated that the water flow in the porous asphalt material was a function formulation of rainfall intensity, permeability, and geometric design. The water drainage capability was not mentioned in the study. Later, the study by Tan et al. (2004) was conducted to fill the gap since they evaluated the subsurface drainage of PAM. They conducted laboratory experiments to observe the water drainage for the PAM. One PAM specimen was fabricated with a width of 150 mm, thickness of 100 mm, and length of 500 mm. Another PAM was fabricated with a width of 150 mm, thickness of 100 mm, and length of 1000 mm. The results of the experiment were validated well with those of the finite element method. For the PAM with the length of 500 mm, when the slope increased, the subsurface drainage of the PAM decreased. In contrast, for the PAM with the length of 1000 mm, there was a negative relationship between two variables. The length of PAM specimen had a remarkable effect on the water drainage. The researchers introduced graphs which included the relationship between allowable rainfall intensity and ratio of thickness to width of pavement. They were used as a tool for pavement engineering to design pavements with PAM. In the study by Afonso et al. (2020), porous asphalt materials with different porosities were tested in a laboratory rainfall simulation. These PAM specimens were designed with different porosities. The water drainage was investigated at the before and after scenarios. The results showed that the infiltration rate of the PAM was constant after 20 mins of testing before clogging. They figured that clogging material was caused by rubber in the PAM, which significantly affected its water drainage. However, clogging material consisting of sand and soil did not affect much. They recommended that to maintain the water drainage capability of PAM, a sweeping and vacuuming method should be used. They concluded that there was a strong relationship between the permeability test and rainfall simulation test. The clogging was a consideration when using PAM in the field.

In another study by Madrazo-Uribeetxebarria et al. (2019), the drainage of PAM was evaluated based on laboratory experiments. In the study, a rainfall simulator experiment was carried out using a PAM sample with the dimension of 0.5 x 0.5 m. When the rainfall intensity values were 35 mm/h and 70 mm/h, the slope of the PAM samples were 1% and 6%, respectively. The water drainage was measured according to the time of rainfall. The results indicated that there was no surface ponding for the PAM in the study. At the higher slope, a higher peak water flow and a higher water drainage volume were recorded. They recommended using PAM as a low impact development function for the cities. 

Previous studies showed that the water drainage of PAM strongly depended on the specimen properties, rainfall intensity, and slope (Muttuvelu and Kjems 2021; Kuruppu et al. 2019; Santhanam and Majumdar 2020). To date, there is a lack of studies and experiments on the water drainage of PAM considering rainfall intensity and pavement slope. As such, this study was conducted to fill the research gap to observe the behaviour of the water drainage of PAM considering slope and rainfall intensity. To meet the scope of the study, a series of rainfall simulation experiments were conducted to extract the subsurface water drainage and surface drainage. Based on the results, the discussion and recommendations were conducted.

2 Laboratory rainfall simulation experiment

2.1 Porous asphalt material specimen

In this study, rainfall simulation experiments were performed with PAM specimens that met the requirements of the Vietnamese standard for asphalt material (TCVN 8819:2011) (MOST 2011). See Figure 1 for samples of the PAM specimens used in the study. The aggregate of the mixture was originated from An Giang City, Vietnam. The specimens were fabricated with a porosity of 10% and 15%, namely PA10 and PA15, respectively. These porosity values were used based on previous PAM design studies (Chu and Fwa 2019; Hongchang et al. 2011). Their dimensions were 30 cm in length, 30 cm in width, and 5 cm in thickness (30 x 30 x 5 cm). They were fabricated in the field. The mixture was mixed with binder PG60-70, and the nominal maximum aggregate size was 19 mm. The proportions of mixture components in the current study are displayed in Table 1.

Table 1 Components of the PAM mixture.

Figure 1 Two PAM specimens used in the current study.

2.2 Testing equipment

The rainfall simulation equipment was fabricated based on the study by Castro-Fresno et al. (2013). It consists of a rainfall distribution system, a water flowmeter, a testing frame, and water tanks. The rainfall distribution system was connected directly to the inflow and was responsible for generating water above the testing frame. The testing frame consisted of a box containing the specimen. Water tanks were equipped to measure water volume for surface and subsurface outflow volume, separately. During the experiment, the inflow rate according to time was continuously recorded. The box was made from steel with an area of 0.5 m x 0.5 m, and a height of 3.6 m, was used for containing the specimen. To ensure that the water could not flow through the space between the specimen and the steel box, the glue was used. The angle of slope for pavement can be controlled from 0% to 8%. The PAM specimen during the experiment is shown in Figure 2.

Figure 2 Rainfall simulation equipment.

2.3 Testing cases

The literature has shown that the water drainage of PAM strongly depended on the rainfall intensity, slope, and porosity of the PAM. The current study investigated the outflow volume of water drainage of the PAM specimen with a porosity of 10% and 15%. Details of the testing cases are presented in Table 2.

Table 2 Testing cases.

3 Results and discussion

The water from rainfall drops over the PAM can drain out vertically through the bottom side and/or laterally through the edge side. The rainfall simulation test in the current study could provide the outflow volume for the water surface drainage and subsurface drainage. The surface drainage outflow was collected using a tray which is connected to the PAM specimen at the top. The subsurface and surface outflow were totally collected in the water tanks. During the experiment, the surface and subsurface outflow volumes were continuously recorded. Generally, after 2 minutes of rainfall, the water drainage rate of the surface and subsurface was constant. Hence, the outflow volume of the PAM after 2 minutes was chosen to report. Figures 3–7 show the results from the rainfall simulation tests for the two PAM specimens.

3.1 Effect of slope on the water drainage of porous asphalt specimens

The results of water subsurface outflow of the PA10 and PA15 specimen according to different slopes (SR) and rainfall intensities are presented in Figures 3 and 4. Generally, PA10 and PA15 specimens at different slopes resulted in varied subsurface outflow.

Figure 3 Subsurface outflow of PA10.

Figure 4 Subsurface outflow of PA15.

The curves in Figure 3 and Figure 4 indicated that the slope significantly affected the water subsurface drainage of the PAM specimen. As the slope increases, the subsurface outflow of the PAM decreases. For instance, for the PA15 specimen, at the rainfall intensity of 2.5 L/min, when the slope increased from 0% to 8%, the subsurface outflow reduced by 2.8%. A possible explanation for this behaviour is that at the high slope, rainwater started a dual drainage as surface and subsurface in the PAM (Eck 2010; Elkhateeb et al. 2022; Liu et al. 2016). Hence, the subsurface drainage was found to be lower at the higher slope scenario. It referred that the surface and subsurface drainage performance of PMA are strongly dependent on the designed slope. This conclusion is consistent with that in the study by Nguyen and Ahn (2021).

The current study found that at the slope of 4% and 8%, the subsurface drainage behavior of PAM is similar, and much different than at the slope of 0%. For example, at a rainfall intensity of 3.5 L/min, at the slope of 4% and 8%, the difference of the subsurface outflow is only 1%. Nevertheless, the difference for the subsurface outflow at the slope of 0% and 4% is 13%. It could be concluded that the increase of slope has a remarkable effect on the subsurface drainage of PAM.

3.2 Effect of porosity on the water drainage of porous asphalt specimens

The results for water drainage of PA10 and PA15 specimens are illustrated in Figures 5–7. The results showed that most of the water drainage went through the body of the PAM specimen. In other words, the subsurface drainage of PAM is higher than the surface drainage. It indicated that the PAM could help to reduce the runoff during rainfall. The PAM specimens with different porosity produced different subsurface outflow. It could be seen that the porosity of PAM played an important role in the subsurface drainage.

Figure 5 Comparison of outflow volume for PA10 and PA15 at the slope of 0%.

Figure 6 Comparison of outflow volume for PA10 and PA15 at the slope of 4%.

Figure 7 Comparison of outflow volume for PA10 and PA15 at the slope of 8%.

When observing the effect of porosity on subsurface drainage, it could be seen that the PA10 specimen performs at a lower subsurface outflow than specimen PA15 does. When the porosity of the PAM specimen increases, the subsurface outflow increases. The reason was that the PAM with higher porosity could store more water than the one with lower porosity (Vujovic et al. 2021). For example, at the slope of 4%, the rainfall intensity was 4.0 L/min, while for the PA10 specimen, the subsurface outflow was 72.3%; and for the PA15 specimen, the subsurface outflow was 79.1%.

Another highlighted point is that even the subsurface drainage of the PA10 specimen is lower than that of the PA15 specimen. However, the reduction of subsurface drainage of the PA10 specimen according to rainfall intensity was higher than that of PA15 specimen. For instance, when the rainfall intensity jumped from 2.5 L/min to 4.0 L/min for the PA10 specimen, the subsurface outflow only dropped by 5.95%. Nevertheless, for PA15 specimen, the subsurface outflow only dropped by 15.9%. This behaviour indicates that the PAM with a higher porosity is more sensitive to rainfall intensity than one with a lower porosity.

3.3 Effect of rainfall intensity on the water drainage of porous asphalt specimens

The figures above show that rainfall intensity has a high impact on the water drainage performance of PAM. As the rainfall intensity increased, the subsurface outflow of PAM decreased. For instance, for the PA10 specimen, at the slope of 8%, when the rainfall intensity was 2.5 L/min, the subsurface outflow reached 75.4%. When the rainfall intensity was 4.0 L/min, the subsurface outflow dropped down to 66.9%. It could be explained that at the high rainfall intensity, the surface and subsurface drainage process happened together. Hence, as the rainfall intensity increases, the subsurface drainage of water decreases. For the PAM with low porosity, the rainfall intensity seems to not affect the subsurface drainage very much. For example, for the PA10 specimen at the slope of 4%, the difference in rainfall intensity caused a difference of 5.9%. However, for PA15, with the same condition, there was a difference of 15.9%. It can be concluded that the drainage of the PAM with a higher porosity is strongly dependent on the rainfall intensity.

As mentioned above, due to urbanization and industrialization, natural permeable surfaces were being replaced by artificial impermeable ones. Hence, it is necessary that urban engineering must find the solution for sustainable development. The current study showed that the use of PAM could increase the subsurface drainage and reduce the surface runoff from the roads. In other words, the use of PAM could increase the artificial permeable surfaces. Hence, it is recommended to widely use PAM as a solution to enhance the sustainability of the urbanization process.

4 Conclusion

This study evaluated the water drainage of PAM via a rainfall simulation experiment in the laboratory. A series of experiments were carried out using PAM specimens with different porosities at different rainfall intensities and slopes. Based on the results, the conclusions were that slope, porosity, and rainfall intensity significantly affected the subsurface water drainage of the PAM specimen. When the rainfall intensity increased, the subsurface outflow of PAM tended to decrease. As the slope increases, the subsurface outflow of the PAM decreases. The porosity of the PAM specimens also remarkably affected the subsurface drainage. The PAM specimen with a higher porosity performed better than one with lower porosity in the reduction of flooding. This study highlighted that the drainage of PAM with higher porosity is strongly dependent on rainfall intensity. It is noted that the results in this study were extracted from a laboratory experiment. Further study on the durability and the drainage performance of PAM should be adopted.

Acknowledgments

We acknowledge Ho Chi Minh City University of Technology (HCMUT), VNU-HCM for supporting this study. This paper is from SCNSU 2024.

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