Analysis of the porous system structure, permeability coefficient and mechanical resistance of pervious concreteABSTRACT
When used as a surface layer on permeable pavements, pervious concrete promotes water percolation, thus helping urban water runoff management. Water percolation occurs due to its porous structure, and interconnected pores are fundamental for its efficiency. To better understand pervious concrete, this study aimed to analyse the porous structure and mechanical and hydraulic properties of pervious concrete. Compressive strength, flexural tensile strength, porosity and permeability coefficient tests were performed. The porous structure was characterised using three approaches: ImageJ software, Sketchup software and scanning electron microscopy. According to hardened state results and Brazilian technical standards, pervious concrete can be used cast-in-place for pedestrian traffic or light vehicular traffic areas. Pervious concrete pores’ length, perimeter, Feret’s diameter and width increase, while circularity and the number of pores decrease as the void volume increases, indicating that as the volume of voids in pervious concrete increases, the pores become larger, more elongated and smaller in quantity. Good to excellent correlations were found between the concrete’s compressive strength, flexural tensile strength and permeability coefficient and the concrete’s porosity, pore area, pore volume and geometric tortuosity, although different image analysis methodologies were used to obtain the porous structure data.
Keywords
Pervious concrete; Image analysis; Pore characteristics; Porosity
1. INTRODUCTION
The increase in urban conglomerates and impervious areas associated with climate change makes permeable pavement properties stand out. As they can provide runoff infiltration and retention, their use has a prominent positive environmental impact. The surface layer of permeable pavements can be made of permeable interlocking pavement (concrete pieces with widened joints, concrete pieces with hollow areas and pervious concrete pieces such as pavers), pervious concrete plates, moulded pervious concrete in loco [1], permeable asphalt concrete [2, 3] and concrete pieces filled with grass [4]. These possibilities for the surface layer result in a porous material that allows water percolation [5] and contributes to urban water runoff management. Highlighting pervious concrete, studies have analysed and discussed its environmental benefits and durability [2], maintenance [6], effects on temperature reduction, noise absorption and water quality [7,8,9,10] and incorporation of solid waste into its composition [11, 12].
Pervious concrete has porosity of 15%–35% and compressive strength of 2.8–28 MPa [13]. Its flexural tensile strength is generally 1–3.8 MPa, as it is influenced by compaction energy, porosity and the aggregate/cement ratio. Its permeability coefficient is between 2 ms-1 and 5.4.10-3 ms-1, depending on the mix and materials used [14], although values around 15.10-2 ms-1 have also been attained [15].
Porosity is one of the main characteristics of pervious concrete once rainwater percolates through this porous structure, which is defined by pore size, pore distribution and geometric tortuosity [16]. Pores can be categorised as effective pores, which allow the passage or storage of water, and isolated pores, which are not permeable to water [17]. Thus, the permeability coefficient is used to characterise the water flow capacity of pervious concrete, in which the permeability coefficient and mechanical strength are largely dependent on the pore structure of the pervious concrete [18].
The pore structure of pervious concrete can be characterised using image analysis techniques aiming to correlate this information with other properties of pervious concrete [19, 20]. Equipment such as X-ray tomography and scanning electron microscopy (SEM) are used to obtain digital images from which it is possible to determine porosity, size, distribution and connectivity of the pores [21], but if, on the one hand, these tools do not destroy pervious concrete samples, on the other hand they are expensive [22].
Another way to characterise the internal structure of pervious concrete is through images obtained from specimens’ sections studied using image processing software, a method that is an economically accessible option [19, 23,24,25,26,27,28].
In this context, this study aimed to characterise the porous structure of pervious concrete to better understand its internal structure and its correlation with its physical, mechanical and hydraulic properties, thus improving its performance. In this is way, this study contributes to improving the performance of pervious concrete pavements, which, in turn, can contribute positively to the environment and society by mitigating the problems of soil waterproofing and flooding.
2. MATERIALS AND METHODS
Three different mixes of pervious concrete were chosen. The characterisation of the materials, the physical-mechanical tests and the analysis of the porous structure of the pervious concrete specimens were carried out as described in the following sections.
2.1. Composition of pervious concrete and the tests carried out on hardened pervious concrete
Pervious concrete was mixed with 93.5% coarse aggregate, 6.5% fine aggregate and Portland cement V–ARI (equivalent to CEM II 52.5 R). The w/c (water/cement) relation was determined by the binder drainage test [8] with three different void volumes of 15%, 20% and 25%.
The coarse aggregate (4.75–12.5 mm) had a specific mass of 2,750 kg m-3 and a water absorption index of 0.83%. The fine aggregate (0.6–2.0 mm) had a specific mass of 2,120 kg m-3 and a water absorption index of 2.11% (Table 1). Portland cement V–ARI was chosen because of its high initial resistance, which contributes to the demoulding of test specimens and to the demoulding of prefabricated pervious concrete elements, such plates or pavers. Nevertheless, as Portland cement V-ARI cement has higher levels of C3A and C3S which leads to higher heat of hydration which, in turn, can lead to greater evaporation of water creating more pores, thus special attention must be paid to the curing process so that its durability is not compromised [29].
With the characteristics of coarse and fine aggregates and cement known, as well as the optimum w/c obtained through the binder drainage test for the different void volumes adopted, the composition of pervious concretes was stablished (Table 2).
Compressive strength (carried out following the Brazilian standard NBR 5739 [33]), flexural tensile strength (Brazilian standard NBR 12142 [34]), permeability coefficient C1754 [35] and porosity tests ACI 522R-10 [13] were then performed. A permeability coefficient test was first conducted on the same 100 mm × 200 mm cylindrical specimens in which compressive strength would be determined later. Both tests were conducted in quadruplicate. Flexural tensile strength test was carried out in triplicate in prismatic specimens 40 cm long and with a cross-section of 10 cm × 10 cm. The porosity test was performed in triplicate in 50 mm × 120 mm cylindrical specimens extracted from 350 mm × 350 mm × 120 mm plates.
2.2. Analysis of the pervious concrete porous system
The analysis of pervious concrete as a porous system was performed on prismatic and cylindrical specimens. Data about the pores’ characteristics were obtained using free ImageJ software, while the geometric tortuosity and volume of the pores were obtained from a virtual specimen created using Sketchup software. Scanning electron microscopy (SEM) was also used to analyse the pervious concrete pores.
2.2.1. Pervious concrete pore characteristics
The number of pores, average pore area, average pore width and length, average Feret’s diameter of the pores, average pore perimeter and circularity (Table 3) were determined.
A plate (350 mm × 350 mm × 120 mm) was made for each pervious concrete mix. From each mix, a prism (350 × 120 × 120 mm) was extracted and then sectioned vertically and horizontally with a circular diamond table saw. Eleven images were analysed in the vertical direction, while six images were analysed in the horizontal direction. A total of 17 images per mixture were obtained (Figure 1). Each section was painted, scanned, and processed using ImageJ software.
Initially, each section (Figure 2a) was completely covered with matte black paint; later, the sections were covered with white paint to differentiate the solid part (white) from the pores (black) (Figure 2b). The sections were then scanned in grayscale at 300 dpi resolution (Figure 2c), imported into ImageJ software and converted into a binary image whose imperfections were smoothed. In the latter step, on the binary smoothed image, an 80 mm x 80 mm square was delimited in the centre of the image because in some cases, there could be loss of material on the sides of the sections due to the cutting process (Figure 2d).
Treatment of the pervious concrete sections: From painting to the processed images: (a) natural section, (b) section painted in white and pores painted in black, (c) scanned image, (d) bounded and smoothed binary image (corresponds to an 80 mm × 80 mm square of the central region shown in c).
Throughout the process, care was taken to follow the sequence and orientation of each section to reproduce correctly the real prism that was extracted from each plate. The scanning of the vertical and horizontal sections of the pervious concrete specimens and their processing in the ImageJ software made it possible to extract individual information from each section, resulting in an average value per mixture.
2.2.2. Determination of the geometric tortuosity and pore volume of pervious concrete
For the construction of virtual specimens, a cylindrical specimen measuring 75 mm × 120 mm was extracted from the pervious concrete plates of different mixes. The cylindrical specimen of each pervious concrete mix was filled with liquefied red paraffin (Figure 3a) before grinding [37]. After each grinding, which removed layers with a thickness close to 1 mm, the section was measured and photographed (Figure 3b). As a white reference mark was made in the specimen (Figure 3a), the section surface of the specimen could always be photographed in the same position.
The photos (Figure 3b) were exported to the Sketchup software (Figure 3c), and the part corresponding to the solids (Figure 3d) and pores (Figure 3e) was drawn on each image. After each grinding, the height of the remaining specimen was measured and imported into the Sketchup software, thus forming the virtual specimen (Figure 3f) from which the solid volume (Figure 3g) and pore volume were determined. On average, 55 photos were taken for each composed virtual sample, a procedure that resulted in a virtual specimen with a diameter of 75 mm and a height of 64 mm.
Geometric tortuosity, which is the ratio between the shortest distance obtained when going through the connected pores from the entrance to the exit of a porous medium and the length of the straight line along the axis of this sample [38], was determined using the Sketchup software’s line command. With this command, it was possible to traverse the connected pores of the virtual sample and create a continuous path from the top to the bottom, which is the geometric path.
For each pervious concrete mixture, six geometric paths were obtained. The arithmetic mean of their lengths was calculated, and geometric tortuosity was obtained once the virtual specimen’s height of 64 cm was already known.
2.2.3. Analysis of pervious concrete pores using SEM
SEM was used to analyse the shape of pores and how they form and to determine their size in some pervious concrete samples.
A cylindrical specimen with a diameter of 50 mm and a height of 120 mm was extracted from a plate of each pervious concrete mixture, and three sections were made: one close to the surface, one in the middle and one close to the cylinder base. The samples were reduced in size manually or with the aid of a diamond saw and submitted to the SEM after being adequately prepared.
To verify whether there was a significant difference between the results of each evaluated parameter, an analysis of variance test with a confidence interval of 95% and Tukey’s test were performed.
3. RESULTS AND DISCUSSION
3.1. Physical-mechanical properties of pervious concrete
The compressive strength varied from the maximum value of 20.9 MPa for pervious concrete with a 15% void volume to the minimum value of 12.0 MPa for pervious concrete with a void volume of 25%, with a statistically significant difference between the medium values of all three mixtures (Table 4). As compressive strength usually varies from 2.8 MPa to 28.0 MPa [13] [24], the results are within the range of expected values, reaching the highest values in the best results.
The mean flexural tensile strength (Table 4) showed a statistically significant difference only between the pervious concrete with a 15% void volume and the other two mixtures and assumed values between 8.3 MPa for the pervious concrete with a 15% void volume and 4.2 MPa for the pervious concrete with a 25% void volume. These high values of flexural tensile strength stand out because as mentioned in the literature, they usually vary between 1.0 and 4.9 MPa [39, 40]. In the case of vibrating table compaction, which was the method used in this study, the cement paste flows downwards, generally resulting in a low volume of voids at the bottom [41] which may explain the increase in flexural tensile strength.
The mean porosity corresponded to 25.0%, 29.3% and 33.4% for the mixture with a void volume of 15%, 20% and 25%, respectively, while the permeability coefficient had a mean value of 9.0 mms-1, 10.6 mms-1 and 21.2 mms-1 for the pervious concrete mixture with a void volume of 15%, 20% and 25%, respectively (Table 4).
The porosity results were within the range of porosities found in other studies, from 20.8% [41] to 36.8% [42], while the permeability coefficient assumed relatively high values, although they were still within the range reported in other studies (0.16–22.0 mm.s-1) [14, 15, 43].
The same letter (a, b or c) is used to indicate there is no statistically significant difference in the same characteristic for different concrete mixtures (p < 5%).
3.2. Pervious concrete porous system characterisation
3.2.1. Characterisation and formation of pervious concrete pores
The mean length of pores was 0.54 cm for pervious concrete with a void volume of 15% and 0.78 cm and 0.84 cm for pervious concrete with a void volume of 20% and 25%, respectively (Table 5). Analysing these results in light of the results of other studies must consider the dosage methods, the consequent formation of the void volume and porosity, the image acquisition and the processing techniques once they vary from study to study.
Considering these variable conditions, previous studies have found pores with mean lengths of 3.38–7.47 mm [22], 2.62–4.40 mm [24], 1.60–2.40 mm [19], 0.81–0.97 mm [18, 42], 2.19–5.49 mm [44], 4.02–6.24 mm [16], 3.49–17.81 mm [41], 0.50–4.30 mm [15] and a characteristic size of 2.0 mm to a maximum of 12.2 mm [45]. The results of this study are within these ranges.
Aside from length, the mean perimeter (2.02–3.77 mm), Feret’s diameter (0.68–1.08 mm) and width (0.55–0.86 mm) were determined (Table 5). They increased as the void volume increased, indicating that as the void volume of pervious concrete increased, the pores became larger (Table 5). The width and length have close average values, but this does not mean that the shape of the pores is close to a square. Instead, they are limited within a geometry close to a square, varying irregularly in all directions within this space, as shown in Figures 2 and 3 and as the average circularity demonstrates.
The average circularity results indicated that the pores are not circular, as perfect circles have a circularity equal to 1. As the void volume in the pervious concrete increased, the more elongated the pores became once circularity ranged from 0.69 cm to 0.59 cm (Table 5).
Statistical analysis showed no significant difference between the three pervious concrete mixes when comparing the perimeter, Feret’s diameter and width. In terms of the length and pore circularity, a significant difference was found only between pervious concrete with a 15% void volume and the other pervious concrete mixes (Table 5).
The pore area decreased from 0.7 cm2 to 0.3 cm2 as the void volume decreased from 25% to 15% (Table 5) once greater compaction energy was needed to achieve the void volume of the theoretical pervious concrete mix. As the pore area decreased when the void volume decreased, the pore number increased. CHANDRAPPA and BILIGIRI [18] also reached this finding by analysing three-dimensional samples using computed tomography. The comparison of means showed a statistically significant difference between the three different pervious concretes mixes with regard to the pore area, while the mean number of pores showed a difference only between the pervious concrete with a void volume of 15% and the others.
How the pores were distributed along the vertical profile of the sample was also examined. In analysing the number of pores, a greater number of pores near the bottom and the top of the plate were observed in the horizontal sections (Figure 4) and a smaller number of pores were observed in the section corresponding to the centre of the plate.
It must be noted that compaction was realised using the vibrating table. Conversely, compacting pervious concrete with a compaction hammer resulted in a greater number of pores but with a smaller size in the upper region of the specimen and a smaller number of pores with a larger size in the lower region. Therefore, different compaction methods promote different porous characteristics in pervious concrete [41]. In the case of vibrating table compaction, the cement paste flows downward toward the bottom, generally resulting in a low void volume at the bottom, while static compaction without vibration results in incomplete compaction, which produces a different porous structure [41].
3.2.2. Geometric tortuosity and pore volume of pervious concrete
The geometric tortuosity decreased as the void volume of the pervious concrete increased, varying from an average value of 1.8 for the pervious concrete with a void volume of 15% to 1.3 for the mixture with a void volume of 25% (Table 6). Moreover, there was a significant difference between the average geometric tortuosity of the pervious concrete with a void volume of 15% compared with the other samples.
Pore volume, solid volume and geometric tortuosity obtained from the pervious concrete virtual specimen.
Tortuosity decreases with increasing porosity. When the compaction energy of pervious concrete increases, increasing contact between materials, tortuosity increases, thus contributing to a decrease in the permeability coefficient [46]. As tortuosity has an important relationship and impact on the permeability coefficient, studies have been carried out looking for its characterization and modelling [47, 48].
The mean pore volume varied from 64.7 cm3 to 86.0 cm3 as the void volume varied from 15% to 25% (Table 6). This was obtained by taking the sum of the volume of each horizontal section of the virtual specimen.
Porosity was calculated by considering the volume of the pores and the solid volume obtained from the virtual specimen (Table 6), assuming the values of 22.9%, 25.9% and 30.5% for pervious concrete with a void volume of 15%, 20% and 25%, respectively. Comparing these porosity values with those obtained from real specimens (Table 6), there is a variation of 8.4%, 11.6% and 8.7% for pervious concrete with a void volume of 15%, 20% and 25%, respectively, highlighting a difference of 10%.
3.2.3. Analysis of pervious concrete pores using SEM
Images from the top of the pervious concrete sample (Figure 5) with a void volume of 15% show pore sizes varying between 4.1 mm2 and 23 mm2. In the concrete with a 20% void volume, they ranged between 3.0 mm2 and 73.9 mm2 while in the case of the 25% void volume concrete they ranged from 0.5 mm2 to 59.4 mm2. It must be noted that due to the size reduction, the samples had different sizes.
The pervious concrete sample with a void volume of 15% had more circular pores and fewer elongated pores on the top image of the specimen. The comparison of the top images of the other pervious concrete specimens indicates that as the void volume of the pervious concrete increased, the pores became less rounded and more elongated. In the middle part of the specimen, it is possible to identify pores with a similar shape that are also more circular in the three pervious concrete specimens.
At the bottom of the specimens, the pores were usually more elongated. Even in the samples with different sizes and amounts of pores, the increase in the void volume of the pervious concrete specimens generated pores with larger areas, which were less circular and more elongated. In some of the analysed images (not shown), mainly in the pervious concrete mixes with a void volume of 20% and 25%, the samples had pores that covered a large part of the sample area. In some cases, the pores showed a tendency towards continuity, although it was only possible to quantify a part of it.
3.3. Pervious concrete porous system related to compressive strength, flexural tensile strength and coefficient of permeability
The correlation between compressive strength and flexural tensile strength with porosity (Figure 6a), total pore area (Figure 6b) and total pore volume (Figure 6c) was high, presenting a coefficient of determination above 0.92. As these parameters increased, both compressive strength and flexural tensile strength decreased (Figure 6a, b, c), corroborating the results found by other researchers [5, 15, 18, 19, 24].
Compressive strength and flexural tensile strength related to (a) porosity, (b) total pore area, (c) pore volume and (d) geometric tortuosity.
The correlation of compressive strength and flexural tensile strength with geometric tortuosity indicates that both factors increased as the geometric tortuosity increased. The correlation was very good, with a coefficient of determination of 0.91 for compressive strength and 0.99 for flexural tensile strength (Figure 6d). Compressive strength had a more significant variation as porosity, total pore area, pore volume and geometric tortuosity varied more than flexural tensile strength. Deo and Neithalath [24] found that a 10% increase in porosity reduced compressive strength by around 50%. In this study, when comparing pervious concrete with a void volume of 15% and 25%, the reduction was smaller, at 38% in compressive strength and 41% in flexural tensile strength.
The permeability coefficient increased when porosity, total pore area and pore volume increased (Figure 7 a, b, c respectively) and decreased as geometric tortuosity increased (Figure 7d). When observing the permeability coefficient of pervious concrete with void volumes of 20% and 25%, the permeability coefficient doubled, possibly due to the greater connectivity between the pores, which would result in a greater permeability coefficient [49].
Permeability coefficient as a function of (a) porosity, (b) total pore area, (c) pore volume and (d) geometric tortuosity.
YU et al. [50] have analysed permeability coefficient and porosity data from various authors and also found an exponential correlation.
The correlation between permeability and porosity was also presented in the power function, with a correlation coefficient of 0.9081 [51]. The same authors correlated permeability with the volume of effective pores through a linear correlation with a correlation coefficient of 0.6934, while in this study the correlation coefficient was equal to 1.0 for an exponential correlation.
4. CONCLUSION
The study aimed to contribute to knowledge about the porous structure of pervious concrete and analyse the correlations between the porous structure properties and the compressive strength, flexural tensile strength and permeability coefficient of pervious concrete. These properties were evaluated according to Brazilian standard criteria that consider using pervious concrete as a surface layer of permeable pavements.
Regarding the mechanical resistance and the coefficient of permeability and according to Brazilian standards, the three different mixes of pervious concrete can be used as a surface layer for permeable pavements when concrete is cast on site and when the pavement is subject to stresses from pedestrian traffic or light vehicular traffic.
Image processing allowed the identification of important information about the porous system of pervious concrete. In addition to evaluating the length and area of the pores, the width, perimeter, Feret’s diameter and circularity of the pores, as well as the number of pores per section, were also analysed. The length, perimeter, Feret’s diameter and width increased as the void volume increased, indicating that as the void volume of pervious concrete increases, the pores become larger.
The virtual sample made it possible to determine geometric tortuosity and pore volume. The determination of porosity through the virtual sample presented a very good result, as for the three different mixes, it presented a 10% difference from the porosity determined in the real specimen. The possibility of determining geometric tortuosity through image processing is important due to its correlation and impact on the pervious concrete permeability coefficient, this one being fundamental for the efficiency of a permeable pavement.
It is important to highlight that the porosity data were obtained by testing the pervious concrete specimens, the total pore area was obtained with image analysis using ImageJ software and the pore volume and geometric tortuosity were obtained through the virtual specimen treatment using Sketchup. Moreover, although different methodologies were used, good to high correlations were found between these characteristics and compressive strength, flexural tensile strength and the permeability coefficient.
The results of this work may help determine the best mix for the execution of pervious concrete in practice, whether in the case of slabs or pavers or pervious concrete moulded in situ.
5. ACKNOWLEDGMENT
This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001 and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPp) – process 304475/2020-3.
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