Cellular Porous Brick—A Novel Permeable Pavement Strategy

Permeable pavement is generally considered as one of the most effective practices of rainwater management. This paper analyses the defects of current permeable pavement. Inspired by the honeycomb bionic structure which has superior mechanical properties and structural efficiency, an innovative “hon-eycomb-like” model of permeable brick measuring 190 × 100 × 60 mm in size was proposed and constructed. The mechanical strength and permeability performance of the honeycomb permeable brick were experimentally assessed by universal testing machine and artificial rainfall equipment, respectively. Experiment results demonstrated an excellent performance in compressive tolerance and permeability. The critical damage load (Fc) and compressive strength (Ec) of honeycomb brick were 336.46 KN and 17.70 MPa, which were 18.06% and 11.04% higher than that of ordinary solid permeable bricks, respectively, compared with the conventional permeable bricks. The honeycomb brick is capable of increasing the permeability coefficient by 19.2% and 11.96% under rainfall repetition period of 20 and 30 years respectively. These results demonstrated that the permeable brick with honeycomb-like structure can provide a new paving strategy for the construction of sponge city.


Introduction
Rapid urban expansion in many cities has resulted in that vegetation cover was replaced with impervious surfaces for the construction of buildings and associated infrastructure [1] [2] [3]. The diminishing of permeable coverage in urban DOI: 10.4236/nr.2020.1112032 544 Natural Resources area, especially in rain-rich region such as south china, may frequently cause vast runoff of storm-water in summer [4].
Urban water-related issues have raised concerns worldwide among scientific communities, and water management is an essential aspect in the sustainable development of urban areas [5] [6]. A larger number of solutions have been suggested to address water problems for urban areas including the best management practices (BMPs) in the United States [7], sustainable urban drainage systems (SUDS) in the United Kingdom [8], the low impact development (LID) strategy in New Zealand as well [9], water sensitive city (WSC) or water sensitive urban design (WSUD) in Australia [10]. All of these novel concepts and strategies were aimed at preserving and maintaining the natural drainage runoff profile as much as possible [11] [12].
For developing countries like China, most urban areas have witnessed high population growth, intensive expansion of impermeable roads and rooftops [13].
In order to alleviate urban water-related problems, the urban water management strategy name as "sponge city" has been implemented as an adaptation measure to address urban water issues in China since 2013 [14]. It emerges that Sponge City has four main principles; these are: urban water resourcing, ecological water management, green infrastructures, and urban permeable pavement [15].
Among the above Sponge City strategies, many studies have found that the performance of permeable pavements in terms of flooding reduction was even better and more effective than other Sponge City technologies, including green roofs or rainwater resourcing [16] [17].
There are four main types of infiltration pavements: permeable concretes, permeable asphalts, grass pavement, and permeable interlocking concrete pavers available in current market [18]. However, these permeable pavements, have common disadvantage that cannot achieve the combination of pervious and structural strength. The permeability of permeable pavement, is greatly affected by the porosity, which also has an significant inverse relationship with the structural strength.
Therefore, how to improve the capacity of permeability under the premise of not reducing the bearing capacity and structural strength of the road and square has become one of the technical barriers in urban sponge transformation.
Extensive great inventions originated from the imitation of nature in human history. Due to its superior mechanical properties and structural efficiency, honeycomb structure has been widely used in chemical industry, aerospace, national defense and other fields [19]. However, despite that honeycomb structure has excellent mechanical properties and permeability, research has mainly fo- Therefore, the objective of this study is to assess the potential application foreground of honeycomb structures in the sponge transformation of urban hardened ground such as roads, squares, parking lots, and urban site walkways. A novel honeycomb-like permeable brick was designed and mechanical properties of which were tested by laboratory experiment. The performance in term of bearing capacity, permeability, cost and manufacturing process between honeycomb bricks and conventional permeable concrete bricks were also compared in this study.

Materials and Methods
This section presents the raw materials used in this experiment, the manufacturing process of honeycomb permeable brick, as well as the test and analysis methods used in this project.

Sample Manufacture
The manufacturing process of honeycomb permeable brick is shown in Figure 1. Concrete was prepared with a proportion of coarse aggregate:fine aggregate:cement:water 20:12:5:3, then poured the mixture into the resin model with hexagon prism and compacted manually after fully mixed. After that, the model was sealed and demoulded after 24 hours, and then placed in a ventilated circumstance for natural drying and waiting for testing. The materials, models and finished products were shown in Figures 2(a)-(c), respectively. The dimension of honeycomb permeable brick was 190 × 100 × 60 mm with honeycomb hole of 9 mm in diameter. The bricks without honeycomb hole were used as the reference and named W-1. Two kinds of bricks made of the same materials.

Test Methods and Equipments
After drying, the main physical parameters such as cell size, density, porosity, as well as permeability of the samples were tested during experiment.

Physical Parameters
The main physical parameters of honeycomb permeable brick were tested as follows: Density test was determined using the electronic balance (KF-H2 type, Zhejiang Jinhua Kaifeng Group Co., Ltd.) according to ASTM C271 standard [20]. The cell size was measured by vernier caliper according to BMS8-124AB standard.

Mechanical Properties Test
The compressive strength and shear strength of honeycomb were tested by elec-

Permeability Test
For permeability coefficient (mm/s) tests the method outlined in Zhao Fei et al. [23] was adopted. As shown in Figure 4, the artificial rainfall simulation system consists of an artificial rainfall equipment, a test tank, water collection equipment,   The Permeability coefficient of the Honeycomb brick was tested according to Darcy's law, which was calculated using the following formula: where Ψ N,tc is the runoff coefficient under repetition period of n years and rainfall duration of T C ; T C is the rainfall duration (min); P N,tc is the rainfall under N years repetition period and T C (mm) rainfall duration; W is the water capacity of permeable brick (mm); K j is the saturated conductivity of soil base (mm/min).

Results and Analysis
The follow sections presented the representative results obtained in current experiment.

Physical Parameters of Honeycomb Permeable Brick
The densities of bricks are inversely related to the porosity, which is also linked to their potential of permeability. As shown in Table 1, the density of sample Y-1 was 2037.37 kg/m 3 , 3.49% lower than that of W-1 (2108.42 kg/m 3 ). The porosity of Y-1 was 22.57%, 9.34% higher than that of W-1 (13.23%). These observations imply a considerable permeability differences between two samples. It can be seen from Figure 5 that each mechanical index of honeycomb brick is superior to that of solid bricks, e.g. the failure load and cracking load of sample Y-1 are 18.06% and 11.04% higher than W-1 respectively. These results suggest a potential linkage between the honeycomb structure and its mechanical properties. As reported in the previous researches, pore structure was able to prevent stress concentration occurred in the plate with arc-shaped hole lattice [19], which was also in accordance with the phenomenon that the arch structure     Figure 5. This behaviour can be imputable to the excellent elastic deformation resistance of the honeycomb structure, for which it would not possibly resulting in a collapse to the test block immediately even if the external force exceed the yield limit of honeycomb brick.

Mechanical Properties of Honeycomb Permeable Brick
As documented in previous studies, the deformation of honeycomb structure can be categorized into four stages, they are: elasticity, elastoplasticity, plasticity and compaction [24]. It is hypothesized that even if the honeycomb brick has cracked during the process of compressing, the wall of honeycomb hole may forms a strengthened column, which could effectively prevent the cracks to develop along a specific path [25], thus to delay a structural collapse take place in the brick. Figure 6 presented the stress deformation curve of honeycomb brick Y-1 and solid brick W-1, respectively. Figure 6(a) shows that the maximum deformation of sample Y-1 was less than 15 mm when subjected pressure increased from 0 to 145 KN. By contrast, the maximum deformation of sample W-1 was higher than 22.5 mm under pressure ranged from 0 to 90 KN (seen Figure 6(b)). It was obviously that, as compared to solid brick, smaller deformation occurred in the honeycomb brick when bearing identical pressure.
It can be seen from Figure Figure 7 shows the permeability of two samples. Permeability coefficient is often used as a technical index to evaluate the infiltration performance of permeable pavement [27]. As can be seen from Figure 7, the permeability coefficient of honeycomb brick (Y-1) was 5.72 mm/s under P20 (a designed rainfall repetition period of 20 years), and was 6.74 mm/s under P30, which was 19.2% and 11.96%

Permeability
higher than that of solid-brick (4.80 mm/s and 6.02 mm/s under P20 and P30, respectively). The design of porous structure in the honeycomb brick, leading to a higher porosity of the pavement, as a consequence, contributes to the improvement of permeability.

Comparison with Traditional Permeable Brick
As can be seen from strength, were mainly due to its porous structure, which could more effectively to diffuse pressure stress and anti-deformation [26]. As a result of the internal porous structure, leading to a higher porosity of the brick, would also contribute to the enhancement of permeability.
In addition, honeycomb brick has significant advantages in cost, which is linked to its porosity structures that substantial reduced consumption of material. As reported in Table 3, the manufacturing processes of the honeycomb brick are uncomplicated in comparison to permeable concrete, permeable asphalt, and sintered brick.

Conclusion
In this study, an innovative "honeycomb-like" permeable brick was proposed and the mechanical strength and permeability performance of this novel honeycomb permeable brick were assessed. The experiment results confirmed that the honeycomb porous permeable brick has superior compressive strength and permeability in comparison to the conventional permeable pavements. The compressive strength and failure load of honeycomb brick were 24.2% and 18.1% higher than that of solid brick respectively, and the permeability was 19.2% -53.8% higher than that of solid-brick. This study provided new methods for the sponge transformation of urban hardened ground, such as city square and pedestrian street.