Study of the Permeability Coefficient of Pervious Concrete with Various Aggregate Grades

Pervious concrete(PC) is a kind of green and environmentally friendly material with good permeability. It has the function of relieving the pressure of urban drainage system and promoting the harmonious coexistence between human and nature. The purpose of this study was to evaluate the effect of porosity and aggregate size on the water permeability and mechanical properties of PC. Through the volumetric method, three kinds of single-grained gravel aggregates of 5∼10mm, 10∼15mm and 15∼20mm were used to design and prepare PC with porosity of 15%, 20% and 25%. A linear fitting function and a power fitting function are found to demonstrate the correlation between the permeability coefficient and the porosity, respectively. The results showed that porosity and aggregate size are both important factors affecting the performance of PC. When the porosity is the same, the larger the aggregate size is used, the higher the permeability coefficient of PC is obtained. The permeability coefficient increases with the increase of the measured porosity. The smaller aggregate size is beneficial to increase the 28d compressive strength of PC. A power fitting function can better to characterize the correlation between the permeability coefficient and the measured porosity, and the fitting equation is Kt = 121.24 PM239 (R2=0.97).


Introduction
With the rapid development of modern cities, many cities are covered by a large number of closed infrastructure and impervious concrete pavement, which leads to the drainage of urban rainy days can only be eliminated through urban drainage systems. The annual precipitation in some cities in China is huge, and the drainage capacity of urban drainage systems is limited, which leads to the problem of surface gathered water on rainy days [1]. PC is an ecotype concrete with a certain permeability, which can make water naturally permeable [2]. Compared with ordinary concrete, PC has larger porosity, ranging from 15% to 25% and permeability coefficient from 1 to 15 mm/s [3]. It has the advantages of water permeability, temperature and humidity regulation. Therefore, pervious concrete is of great significance for the construction of sponge cities [4].
Meng [5] prepared PC by one-time feeding method and cement-wrapped stone method respectively. The results show that mixing with cement-wrapped stone method is beneficial to improve the permeability of permeable concrete. Wang et al. [6] compared continuous gradation with single particle size gradation, pointing out that using single particle size aggregate can ensure the mechanical properties and water permeability of concrete at the same time. Zhang [7] pointed out that the performance of PC is better when the water cement ratio is between 0.24 and 0.30. Liu [8] pointed out that in the early stage of hydration and hardening, moisture maintenance can reduce the risk of cracking.
In this study, through the preparation of pervious concrete with different aggregate sizes and different designed porosities, the influence of aggregate size, porosity on its permeability and

Samples Preparation
The PC samples were prepared as follows. Firstly, the cement-wrapped stone method was used to mix the raw materials. All the aggregate and water were mixed for 30 s. Then, the cement was added and mixed for 1 min to form a gelling material to evenly cover the outer shell of the aggregate. The layered interpolation method was used to tamp the mixture. The size of the samples was 100 × 100 × 100 mm. The samples were demolded after 1d and cured for 28d. The temperature and humidity were set to 20±2 °C and 96% respectively.

Testing Methods
The compressive strength of samples was measured according to the standard [9]. The failure load was recorded and the compressive strength was calculated according to formula (1).
where cu f is the compressive strength of samples (MPa); F is the failure load (kN); and A=10 4 mm 2 .
The internal pore of PC can be divided into closed pore and open pore. Only the open pore can make water pass through naturally. In this experiment, the porosity was measured by water balance and calculated according to formula (2).
where P M is the measured porosity (%); m 0 is the mass of the sample submerged in water (g); m 1 is the mass of the samples in the air; W  is the density of the water; and V is the volume of the samples.
In this experiment, the test principle and experimental device are shown in Figure 2. The permeability coefficient can be calculated by formula (3).

Perm
The tend sizes is porosity

Com
The tend is shown shown in  mpressive Str dency betwee n in Figure 5 n Figure 6.

5.
The tende th and the de agg m Figure 5, it increase of t egate size in same design of the aggre niform, the ef under press te size is incr ment paste c concentration inside the structure is obvious under pressure, and the compressive strength becomes lower. When the designed porosity increases from 15% to 20%, the 28d compressive strength decreases by 10.0%~14.3%; when the designed porosity increases from 20% to 25%, the 28d compressive strength decreases by 9.8%~13.4%. As the designed porosity increases, the amount of cement paste decreases, the cement paste bonding layer becomes thinner, and the stress concentration effect is more obvious under pressure, resulting in a decrease in the 28d compressive strength of the samples. From Figure 6, it can be concluded that when the measured porosity is from 13.8% to 24.7%, the 28d compressive strength is reduced from 24.6MPa to 16.1MPa, the linear correlation coefficient is 0.80.

Conclusion
(1)The permeability coefficient increases with the increase of the measured porosity. Power fitting function can better characterize the correlation between permeability coefficient and measured porosity, and the result of the fitting is K T = 121.24 P M 2.39 (R 2 = 0.97). (2)The smaller aggregate size helps to increase the 28d strength of the pervious concrete.