## 3 Results and Discussions
### Equilibrium Point of the Permeability and Strength of Pervious Concrete  
Figure 10 shows the equilibrium point of the compressive strength and permeability coefficient variation with the effective porosity of the pervious concrete. The permeability coefficient at 1 mm/s is the minimum value for the pervious concrete [38]. The pervious concrete with a permeability coefficient with above 1 mm/s can be used as a functional zone. Otherwise, the pervious concrete is called nonfunctional pervious concrete. Meanwhile, according to the compressive strength, functional pervious concrete can be divided into two types of pervious concrete [38]. When the compressive strength of pervious concrete is above 20 MPa, the pervious concrete is usually used in pavement zone. When the compressive strength of pervious concrete is below 20 MPa the pervious concrete is usually used at nonpavement zone, such as garden and greenbelt zones. The effective porosity of the equilibrium point of the compressive strength and permeability coefficient of the pervious concrete was approximately 16%, where the compressive strength was 21.5 MPa and the water permeability was 3.2 mm/s. It is worth noting that there is a difference between the effective porosity and target porosity. Therefore, when the mix design procedure here is used for actuarial engineering, it would be appropriate to increase the value of the target porosity. The optimum porosity ranged from 7.5% to 16.5%, which provides a theoretical basis for the mix design and performance for the preparation of pervious concrete.  
Figure 9: Permeability Coefficient of the pervious concrete and its relationships versus the effective porosity.  
Figure 11 shows the comparisons of the permeability coefficient (A) and compressive strength (B) of pervious concrete with different types of aggregates [21; 22; 23; 24; 25; 26; 27; 28]. Although different experimental conditions such as porosity type (fresh state, total, or open), test method and samples size may restrict the comparison of the properties of pervious with different types of aggregates, Some useful information can still be found by summarizing and analyzing these pervious concretes. As shown in Figure 11, compared with other types of aggregates, the values of both the permeability coefficient and compressive strength of iron tailings-based pervious concrete described here were located in the moderate position. For example, when the porosity is at the typical value of 15%-30%, the permeability coefficient and compressive strength of the iron tailings-based pervious concrete are 2\(\sim\)10 mm/s and 10\(\sim\)25 MPa, respectively, which are similar to the values of the pervious concretes using natural gravel, copper slag, electric arc furnace slag, and limestone as aggregates. In contrast, the compressive strengths of burnt brick-based pervious concrete and recycled concrete-based pervious concrete are sacrificed for higher permeability coefficients. For pervious concrete using the same ordinary Portland cement, such as burnt brick-based pervious concrete, recycled concrete-based pervious concrete and iron tailings-based pervious concrete (see Table 1), although their compressive strength are similar under the same porosity, but the permeability coefficient of burnt brick-based pervious concrete and recycled concrete-based pervious concrete is one order of magnitude higher than that of iron tailings-based pervious concrete, which may be mainly due to the good pore structure of burnt brick-based pervious concrete and recycled concrete-based pervious concrete, with fewer semi-connected pores.  
Figure 10: Equilibrium point of the compressive strength and permeability coefficient variation with effective porosity of the pervious concrete.