J. Cent. South Univ. (2021) 28: 1534-1545

DOI: https://doi.org/10.1007/s11771-021-4713-y

Improved designed method of pervious concrete based on optimal volume ratio of paste to aggregate

BA Ming-fang(巴明芳)¹, QI Xin-yu(齐新宇)¹, ZHENG Yu-hang(郑宇航)¹,

HUANG Guo-yang(黄国阳)¹, HE Zhi-min(贺智敏)¹, LIU Jun-zhe(柳俊哲)²

1. School of Civil and Environmental Engineering, Ningbo University, Ningbo 315211, China;

2. College of Civil Engineering and Architecture, Qingdao Agricultural University, Qingdao 266109, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021

Abstract: An improved design method of pervious concrete was proposed to lower the deviation between the designed and actual porosity and maintain both mechanical property and permeability of pervious concrete. The improved design method is mainly based on the optimal volume ratio of paste to aggregate (VRPA), which was determined by testing the average thickness of cement paste coating aggregate. The performances of pervious concrete designed by the traditional method and the improved one were compared. The results show that with the increase of designed porosity, the reduction of compressive strength and flexural strength of pervious concrete designed by the improved method is significantly smaller than those designed by the traditional one. The maximum deviation between the designed and actual porosity of the pervious concrete by the improved method is only 1.54%, which is far less than 8.7% obtained by the traditional one. Micro-structural analysis shows that the porous distribution of pervious concrete designed by improved method exhibits better uniformity.

Key words: pervious concrete; absolute volume method; volume ratio of paste to aggregate; mechanical properties; porous structures; permeability

Cite this article as: BA Ming-fang, QI Xin-yu, ZHENG Yu-hang, HUANG Guo-yang, HE Zhi-min, LIU Jun-zhe. Improved designed method of pervious concrete based on optimal volume ratio of paste to aggregate [J]. Journal of Central South University, 2021, 28(5): 1534-1545. DOI: https://doi.org/10.1007/s11771-021-4713-y.

1 Introduction

Pervious concrete is a kind of macro-porous concrete, which is characterized by use of specific particle size aggregate as the skeleton, and the cementitious material wrapped on the aggregate surface as the cemented layer. Therefore, the pervious concrete has a large number of continuous porosity, ranging from 15% to 35% [1-6], and the presence of interconnected pores allows the water and air to easily flow through it, which leads to its higher air and water permeability [7]. Since its various environmental benefits, application of pervious concrete has become a worldwide sustainable solution to the increasingly severe problem of urban rainwater management and control [8]. In China, pervious concrete structure has become an indispensable structural system in the construction of sponge city [9].

Many previous studies have reported that the performances of pervious concrete vary with the water-to-cement ratio, volume ratio of paste to aggregate (VRPA), aggregate size, mineral admixtures, etc [10-12]. The absolute volume method has been widely used for its convenience in actual design [13-15], which was firstly proposed by DEA et al [16] and YAHIA et al [17]. IBRAHIM et al [18] adopted absolute volume method to study the properties of the pervious concrete and concluded that the mix proportion has a direct impact on the mechanical properties of pervious concrete. LUND et al [19] used the absolute volume method to study the frost resistance of pervious concrete with different mineral contents. KAROLINA et al [20] applied the method to study the influence of different fine aggregate contents on the mechanical properties and water permeability of pervious concrete. XU et al [21] selected various parameters to improve the design method pervious concrete. JIANG et al [22] studied the influence of the water-to-cement ratio and the paste-aggregate ratio on the performance of the ordinary Portland cement-based pervious concrete. However, pervious concrete designed based on the absolute volume method often has the problem of paste sinking, hole plugging and insufficient cohesion of aggregate [23]. Consequently, in order to take into account both the mechanical properties and water permeability of pervious concrete, it is urgent to optimize the corresponding mix design based on absolute volume method. NGEYEN et al [24] put forward a model in which cement paste uniformly wraps the aggregate, and proposed the calculation method of paste-coating thickness. Then XIE [25] reported that the strength of matrix, the number of contact points, the bonding width between aggregates at the contact point and the thickness of paste between aggregates determine the mechanical performance and water permeability of pervious concrete. Although the accuracy of pervious concrete proportion has improved a lot, while many new design methods are relatively too complex to apply in civil engineering field.

In this paper, an improved absolute volume method for proportion design of pervious concrete is put forward based on the optimal VRPA, and the performances and porous structural characteristics of pervious concrete based on the absolute volume method and the improved one were compared and analyzed. This improved absolute volume method aims to lower the gap between the designed and actual porosity of pervious concrete and to consequently maintain the mechanical performance and the water permeability of pervious concrete.

2 Materials and methods

2.1 Materials

The ordinary Portland cement (P.O42.5) from local cement supplier was used in pervious concrete, and its chemical composition and physical properties are listed in Table 1. According to specification given in Chinese Standard (GB/175), the 3-day and 28-day compressive and flexural strength of cement P.O 42.5 are listed in Table 2. The water-reduction rate of polycarboxylate water reducer is 25%. The single sized crushed stone with particle size of 4.75-9.5 mm is adopted, and its surface area of unit mass of aggregate measured assuming that the aggregate is a uniform sphere. The volume of aggregate was obtained by the hydrostatic balance method [26]. Table 3 shows the physical properties of single-sized crushed stones.

Table 1 Chemical composition of cement (mass ratio, %)

Table 2 Physical properties of cement

Table 3 Physical properties of aggregate

2.2 Mix proportions of pervious concrete

2.2.1 Absolute volume method

The absolute volume method mainly includes three steps. Firstly, the dense bulk density of coarse aggregate is multiplied by the reduction coefficient (usually 0.97-0.99 according to the physical properties of aggregate), and the mass of coarse aggregate in the previous concrete is calculated. Secondly, by presetting the designed porosity and according to the volume of coarse aggregate, the volume of cement paste in the previous concrete is obtained, and the mass of paste is determined by measuring its corresponding density. Finally, the mass of cementitious material and water is calculated according to the preset water binder ratio.

In this study the water-to-cement mass ratio was preset as 0.2, and the pervious concrete proportions with the paste-aggregate volume ratio of 0.35, 0.39, 0.43 and 0.45 were designed by the above absolute volume method to maintain the same target porosity. The dosage of water reducing agent here is 2.5% of the cement content and the designed porosity of porous concrete is 25%. Table 4 shows the corresponding calculated proportions.

Table 4 Pervious concrete proportions based on absolute volume method

2.2.2 Improved absolute volume method

The aggregate and hardened cement paste coating aggregate form the skeleton structure of pervious concrete by the mutual adhesion between them, and the rest of the space forms the porous system [25]. There is an optimal average thickness for determined paste coating aggregates. When the thickness is less than the optimal value, the paste can be stably wrapped on the surface of aggregate without sliding or moving, while it is easy to be uneven, which seriously affects the mechanical properties of pervious concrete. When the thickness is greater than the optimum value, the excess paste will fill the pores between the aggregates, resulting in the decrease of the effective porosity, and eventually causing the hole-plugging at the bottom of pervious concrete [26]. Therefore, it is necessary to determine the optimal thickness of paste coating aggregate, and then get the most appropriate paste-aggregate volume ratio. Thus the improved absolute volume method includes the determination of paste components content with preset water-to-cement ratio, the optimal VRPA, and the aggregate content.

2.2.2.1 Paste components content by fluidity of cement paste

For certain cement paste with a fixed water-to-cement ratio, the optimal average thickness of paste coating aggregate with a certain specific surface mainly depends on the fluidity of cement paste. Here the appropriate fluidity of cement paste is determined according to the Chinese Standard (GB/T8077-2012) by adjusting dosage of water reducing agent. Table 5 shows the fluidity and plastic viscosity of paste with different content of water reducing agent. Plastic viscosity of paste was tested by rotational viscometer produced by Rongjida company. Considering fluidity and plastic viscosity, the paste with 2% dosage of superplasticizer is more suitable [27, 28].

2.2.2.2 Optimal volume ratio of paste-to-aggregate in terms of optimum average thickness of paste coating aggregate

The determination process of the optimal thickness of paste coating aggregate is as follows:

1) Weigh 1 kg of 5-10 mm sized aggregates saturated with dry surface.

2) Weigh 1 L of cement paste with 2% superplasticize as shown in Table 5 and mix it fully with above weighed aggregates, and then lay the single layer of fresh mixture evenly in the middle of sample sieve with the mesh size of 4.75 mm. The sample sieve was fixed in the skip table.

3) Start the skip table and keep it jumping for 15 s , and then part of the cement paste will fall into the bottom of the sieve screen.

4) Use tweezers to carefully and quickly pick up 200 particles of aggregates on the top layer of the sample sieve screen, and weigh them as m_A1.

Table 5 Properties of paste fluidity with different super-plasticizer content

5) Wash above removed aggregates with water and dry them until the surface is dry and then weighing the mass of m_A2.

Thus the total volume (V_P) and optimal thickness (W) of the cement paste coating the above measured aggregate surface can be calculated as:

                           (1)

                    (2)

where V_P is the total volume of cement paste coating the measured aggregates; ρ_s is the density of paste coating the measured aggregate surface, g/cm³; S is the surface area of above measured aggregates, 10^-4 m²; S₁ is mass specific surface area of aggregate, cm²/g.

Up to now, the optimal volume ratio of paste-to-aggregate can be calculated to be 0.39 according to the following formula:

                          (3)

where V_a is the volume of measured aggregate, cm³.In order to verify the accuracy of above determined optimal VRPA, the thickness of paste coating aggregate with different VRPA was measured and calculated. Figure 1 illustrates the results. It can be seen from this figure that with the increase of VRPA, the thickness of paste coating aggregate shows an increasing trend. However, when the VRPA is greater than 0.39 or so, the corresponding thickness of paste is basically unchanged. Therefore, the proposed improved design method to determine the optimal VRPA is accurate and feasible.

Figure 1 Thickness of paste coating aggregate varying with VRPA

2.2.3 Improved design method

The water cement ratio is preset as 0.20, and the optimal VRPA is determined to be 0.39. Pervious concrete proportions with different VRPA are shown in Table 6 and their performances are compared with those of pervious concrete with the same VRPA designed by the traditional absolute volume method in Table 4.

Table 6 Pervious concrete proportions by improved design method

2.3 Experimental methods

Pervious concretes according to the mix proportions in Tables 4 and 6 were mixed, and specimens were prepared, including 12 sets of cube specimens with sizes of 100 mm×100 mm×100 mm for compressive strength testing, 12 sets of rectangle specimens with sizes of 100mm×100 mm×500 mm for flexural strength testing, and 24 sets of cylindrical specimen with sizes of Φ100 mm×50 mm for permeability coefficient testing. After demolding, all the specimens were placed in the indoor curing room with temperature of (20±3) °C, relative humidity of (70±5)% until the scheduled 7 and 28 d ages.

The compressive strength was measured according to the Chinese testing standard (GB/T50081-2002). The three-point bending testing method was adopted for flexural strength testing, and the loading method was controlled with displacement. The constant water head method was used to measure the permeability coefficient of pervious concrete, referring to the Chinese Standard (JC/T 945-2005).

In order to evaluate the problem of paste settling and hole plugging in pervious concrete, ink-printing method was adopted to measure the meso-porous characteristics of the specimen surfaces by applying image processing software named Image pro plus. Specifically, in terms of the image gray processing, the area ratio of the meso-pores in pervious concrete can be calculated. The more the gray-scale photos are processed, the more representative the pore area ratio is calculated. The white part of the ink-printing image obviously is meso-pores in pervious concrete (see Figure 2). So there is no need for binary processing, and thus it is very convenient to use this image processing method.

Figure 2 Porous structures of surface of pervious concrete

By the age of 28 d, the corresponding specimens were crashed and the cement paste coating aggregate was scraped by a scraper ground into powder for the X-ray diffraction (XRD) analysis and Fourier transform infrared spectroscopy (FTIR) analysis.

3 Results and discussions

3.1 Mechanical properties of pervious concrete designed by two methods

3.1.1 Compressive and flexural strength

The compressive strengths of pervious concrete designed by the traditional and improved methods are shown in Figure 3.

It can be seen that the compressive strength of specimens designed by two methods increases basically with the increase of the VRPA except the 7 d specimens with 0.43 VRPA designed by traditional way. It can also be seen that before reaching the optimal VRPA, the compressive strength of pervious concrete designed by the improved method is significantly higher than that designed by the traditional one. However, when the VRPA exceeds the optimal value, with the increase of VRPA, the compressive strength of the pervious concrete designed by traditional method is significantly higher than that designed by the improved one. This further indicates that there is excess cement paste to settle and plug the pore in pervious concrete after the VRPA exceeds the optimal value. That is to say, the contradiction between compressive strength and permeability of pervious concrete begins to increase after the VRPA exceeds the optimum.

Figure 3 Comparison of compressive strengths of pervious concrete designed by two design methods

The flexural strength of the pervious concrete specimen designed by two methods is shown in Figure 4. It can be seen that the flexural strength of the pervious concrete designed by the two methods at age of 28 d has little difference before reaching the optimal VRPA. However, after the VRPA exceeds the optimum, the flexural strength of specimen designed by the improved design method increases much more slowly than that of specimen designed by the traditional method. This further implies that the pervious concrete with the optimum VRPA can better take into account the mechanical properties and the permeability.

Figure 4 Flexural strength of pervious concrete designed by two design methods

3.1.2 Deformation and flexural toughness of pervious concrete designed by improved method

Both curves of compressive-force and flexural-force to deformation are shown in Figure 5. It can be seen from this figure that the deformation under the ultimate compressive or flexural loading decreases with the increase of VRPA. When the VRPA is 0.45, the compressive and flexural displacement reaches the minimum value of 1.87 and 1.05 mm, respectively. This is attributed to the fact that with the increase of VRPA, the porosity of pervious concrete decreases, and the buffer space decreases under loading, resulting in the increase of brittleness and the decrease of ductility. From Figure 5(b), it can be seen that the flexural displacement decreases from 2.48 to 1.05 mm with the increase of VRPA, and the corresponding brittleness increases sharply.

Figure 5 Load-displacement curve of pervious concrete designed by improved design method:

In this study energy ratio method was adopted to evaluate the flexural toughness of pervious concrete designed by improved method [29]. As shown in Figure 5(b), taking O-G45 curve as an example, the first deformation peak point A corresponding to the initial crack deflection B is named as δ, and the area enclosed by OAB is named as T₁, and then the abscissa of point D is three times the displacement of point B named as 3δ, and the area enclosed by OACD is T₃. The ratio of T₃ to T₁ is defined as the flexural toughness index I₃, which means the toughness of the pervious concrete, shown in formula (4) as:

                                   (4)

where T₁ and T₃ are the areas (mm²) of OAB and OACD respectively in Figure 5(b).

According to Eq. (4), the toughness indexes of pervious concrete with different VRPA were calculated, as shown in Figure 6. It can be seen from this figure that the flexural toughness index of the pervious concrete designed by the improved method increases firstly and then decreases with the increase of the VRPA. It also can be seen that the maximum flexural toughness index is 2.28, when the paste-aggregate ratio is 0.39. This is because the cement paste coatings aggregate unevenly when the VRPA is lower, which easily causes the problem of aggregate exposure and lower energy absorbed by the paste. However, the internal porosity of the specimen becomes smaller when the VRPA exceeds the optimum, which easily leads to decreased flexural toughness index subjected to flexural forces.

Figure 6 Flexural toughness index of pervious concrete designed by improved design method

3.2 Water permeability of pervious concrete based on two design methods

The permeability coefficient of pervious concrete designed by the absolute volume method and the improved one are shown in Figure 7. It can be seen from Figure 7 that the permeability coefficient of the specimen designed by the improved method is nearly 5 times that designed by the absolute volume method. This further indicates that the pervious concrete designed by the improved method possesses the optimal water permeability than that designed by the traditional method. It can also be seen from Figure 7 that the water permeability coefficient designed by the two methods decreases significantly with the increase of VRPA, which is attributed to the fact that the increase of the VRPA leads to settlement of excess paste to plug the inter-connected pores.

Figure 7 Water permeability of pervious concrete designed by two methods:

The relationship between strength and permeability of pervious concrete designed by two methods is shown in Figure 8.

Figure 8 Relationship between mechanical and permeability of pervious concrete designed by two methods

It can be seen that with the increase of permeability coefficient, the change trend of compressive strength and flexural strength of pervious concrete designed by the absolute volume method is obvious sharp compared with that designed by the improved method. This is mainly due to the fact that the pervious concrete designed by the improved method can better take the mechanical properties and the permeability into account by considering the optimal VRPA.

3.3 Porous characteristics of pervious concrete designed by two methods

3.3.1 Characteristics of effective porosity

The results of effective porosity of specimens designed by two methods are shown in Figure 9.

It can be seen that the change trend of effective porosity of the specimens designed by two methods obviously decreases with the increase of VRPA. However, it can also be seen from Figure 9 that the difference between the designed and measured effective porosity of the specimens designed by the improved design method is much smaller than that designed by the absolute volume method. Specifically, the maximum deviation between designed and measured effective porosity of the pervious concrete by improved method is only 1.54%. While the corresponding maximum difference by the absolute volume method is 8.7%. Therefore, it is further proved that the improved method is much applicable to realize the designed objectives of the pervious concrete.

Figure 9 Effective porosity of pervious concrete designed by two methods:

3.3.2 Characteristics of meso-surface pores

Figure 10 shows the bottom-surface pores of specimen with 0.39 VRPA designed by two methods. It can be seen from this figure that the bottom surface of the specimen designed by absolute volume method exhibits obvious paste settlement and pore plugging, while the porosity of bottom surface of specimen designed by improved method is obviously higher. Therefore, it further testifies that pervious concrete specimen designed by the improved design method possesses better permeability than that designed by the absolute volume method.

Figure 10 Bottom-surface pores of pervious concrete with same VRPA designed by two methods:

The bottom-surface pore structures of specimens with different VRPA designed by improved method are shown in Figure 11.

Figure 11 Bottom-surfaces pore structure of specimen pervious concrete designed by improved design method:

It can be seen that the interconnected porosity of specimen presents an obvious trend of decrease with the increase of the VRPA. When the VRPA exceeds 0.39, the excess cement paste begins to appear, even flows to the bottom surface, which eventually results in paste settlement, just as shown in Figures 11(d) and (e).

The results of meso-surface porosity of the specimen designed by two methods are shown in Figure 12. It can be seen that the meso-porosity of the top and bottom surface of the specimen designed by improved method is larger than that with the same VRPA designed by traditional method. It can also be seen that the changing trend of bottom-surface porosity is much sharper than that of top-surface porosity. This is because with the increase of VRPA, the cement paste among aggregates increases, and the redundant paste will also partly settle to result in the decrease of the bottom-surface porosity. To some extent, this can help to explain why the pervious concrete designed by the improved method can give better consideration to both mechanical and permeable properties.

Figure 12 Meso-surface porosity of pervious concrete designed by two methods

3.4 Microscopic characteristics of cement paste coating aggregate in pervious concrete designed by improved methods

3.4.1 SEM and energy spectrum analysis

Taking the sample from specimen cured for 28 d with 0.39 VRPA for example, the SEM image is exhibited in Figure 13(a), and Figure (b) shows the corresponding EDS result by surface scanning. It can be determined that the closely arranged crystal is CaCO₃, which can be deduced according to mass and atom fractions of each element. This is mainly caused by carbonization in the process of indoor curing. From Figure 13 it can also be found that a small amount of tabular Ca(OH)₂ crystal is distributed in the system and a small amount of silicate calcium appears according to the energy spectrum analysis in Table 7, which makes the cement paste structure much dense.

Figure 13 SEM image (a) and EDS analysis (b) of specimen cured for 28 d with 0.39 VRPA

3.4.2 Phase components analysis

The phase components of pervious concrete sample with different VRPAs designed by improved method are exhibited in Figure 14. It can be seen that the crystallization of C—S—H is low, so the peak value of diffraction peak is not obvious. However, from this figure, it can also be seen that the samples with different VRPAs exhibit higher peak of phase CaCO₃ as the pervious concrete is easily carbonated in air condition due to the rapid diffusion of carbon dioxide in cement paste. This finding from Figure 14 is consistent with that from Figure 13.

The phase components of cement paste sample with different VRPA by FTIR spectrum analysis are shown in Figure 15. It can be seen that the absorption peak at 3630 cm^-1 was O—H stretching vibration of Ca(OH)₂ which increases a little with the increase of VRPA. This indicates the increase of Ca(OH)₂ content in samples. It also can be seen that the absorption peaks near 1490 and 1295 cm^-1 are anti-symmetric stretching vibration and plane bending vibration of CO₃^2- in calcite, respectively, which indicates that calcite is formed in cement paste of pervious concrete. This finding is consistent with the XRD analysis.

Figure 14 XRD spectra of pervious concrete samples by improved design method

Figure 15 FTIR analysis of specimen designed by improved method

4 Conclusions

1) The reduction of compressive and flexural strength of pervious concrete designed by improved method is significantly smaller than that designed by the traditional one; the maximum deviation between the designed porosity and the actual porosity of the pervious concrete designed by the improved method is only 1.54%, which is far less than 8.7% obtained by the traditional absolute volume method. Furthermore, the improved design method can better take the mechanical properties and permeability of pervious concrete into account.

2) The diffracted peaks of the hydration products of paste are not different, but there are obvious calcium carbonate peaks in all samples, which indicates the pervious concrete is easily carbonated in air condition due to the rapid diffusion of carbon dioxide in cement paste. Thus, in later report the effects of early carbonation on properties of the pervious concrete will be presented.

Contributors

BA Ming-fang provided the concept, conducted the literature reviews and edited the draft of manuscript. QI Xin-yu conducted the experiments and wrote the first draft of the manuscript. ZHEN Yu-hang and HUANG Guo-yang cooperated with QI Xin-yu for the experiments and analysis. HE Zhi-min advised some experimental methods. LIU Jun-zhe designed the experimental program.

Conflict of interest

BA Ming-fang, QI Xin-yu, ZHEN Yu-hang, HUANG Guo-yang, HE Zhi-min and LIU Jun-zhe declare that they have no conflict of interest.

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(Edited by FANG Jing-hua)

中文导读

基于最优浆骨比参数的透水混凝土优化设计方法

摘要：为了减小透水混凝土设计孔隙率与实际孔隙率的偏差，保持其良好的力学性能和渗透性，提出了一种改进的透水混凝土设计方法。改进后的透水混凝土设计方法主要是基于最优浆骨比(VRPA)的参数提出来的，而最优浆骨比是通过测试包裹集料水泥浆体的平均厚度来确定的。通过对比两种传统方法设计透水混凝土性能，发现随着设计孔隙率的增加，采用改进方法设计的透水混凝土的抗压强度和抗折强度折减量明显小于传统设计方法设计的透水混凝土的折减量。采用改进方法制备的透水混凝土孔隙率设计值与实际值的最大偏差仅为1.54%，远小于采用传统方法的最大偏差8.7%。微观结构分析表明，采用改进方法设计的透水混凝土中孔隙分布具有较好的均匀性。

关键词：透水混凝土；绝对体积法；浆体体积比；力学性能；孔结构；渗透性

Foundation item: Projects(51978346, 51778302) supported by the National Natural Science Foundation of China; Project(202002N3117) supported by the Ningbo Science and Technology Project, China

Received date: 2020-06-10; Accepted date: 2020-10-26

Corresponding author: LIU Jun-zhe, PhD, Professor; Tel: +86-13819825409; E-mail: liujunzhe@163.com; ORCID: https://orcid.org/ 0000-0003-4686-5336