Cement (P.C32.5R) was purchased from Sichuan Xing Chuan Cheng Cement Co., Ltd. Concrete early strength agent (DC-C13) was supplied by Beijing Dechang Weiye Construction Engineering Technology Co., Ltd. Coarse aggregate (3 - 5 mm) and Fine aggregate (bulk river sand) were obtained from Baiyun District, Guangzhou. HJW-30 forced single horizontal shaft concrete mixer was provided by Shanghai Juhong Instrument Equipment Co., Ltd. Resin model was provided by Wuhan Ruyi plexiglass products factory. Honeycomb socket model was designed and manufactured by Shenzhen Jingrui 3D Printing Technology Co., Ltd.

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.

After drying, the main physical parameters such as cell size, density, porosity,

and major mechanical properties as plane compressive strength, shear strength, as well as permeability of the samples were tested during experiment.

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/m³, 3.49% lower than that of W-1 (2108.42 kg/m³). 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.

Table 2 presents the results of strength tests of two permeable brick samples. It clearly shows that the compressive strength and failure load of W-1 were significantly lower than that of honeycomb brick Y-1. Because the two samples have

identical sizes, material and process methods, the hexagon honeycomb hole could be the main factor that contributed to the difference of compressive strength and failure load between W-1 and Y-1. As shown in Table 2, the compressive strength of honeycomb permeable brick (Y-1) was 24.21% higher than that of W-1, which indicated that the bearing capacity of porous brick increased instead of decline with reduced material consumption. Analogously, the failure load of W-1 was 285 KN, which was 15.29% lower than that of Y-1. These results demonstrated that the honeycomb structure has excellent mechanical properties of compression resistance and energy absorption.

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 presented more superior compressive strength. Furthermore, as plotted in Figure 5, the failure load (Fbc) of Y-1 was approximately 6.32% higher than its cracking load (Fsc), while the failure load of sample W-1 was equal to the cracking

load according to the test results shown in 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. 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 6, the sample of Y-1 presented a continuous and stable deformation curve under stress during the whole test period. In contrast, W-1 undergones two severe deformations under load of 18 KN and 82 KN respectively. These behaviours were mainly attributed to the unique porous structure of honeycomb brick. On the one hand, the honeycomb unit was able to effectively buffer the radial external force (lateral force), thus lead to a smaller

stress and deformation. On the other hand, as the honeycomb unit can acts as a spring to buffer the radial force (lateral force), thereby allow most of the deformation and stress concentrated in several honeycomb cells adjacent to the bearing point of external force, thus leading to a reduced stress and deformation for other parts of the bricks [26] .

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% 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.

As can be seen from Table 3, the porous honeycomb has significant advantages in terms of compressive strength, shear resistance, porosity and permeability in comparison to several conventional permeable pavements. The excellent performance of honeycomb brick in compressive strength and flexural (shear) 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.

Data cited from references [1] and [2] .