Pervious concrete is mainly composed of cement, water, and coarse aggregate. The basic characteristic of pervious concrete is permeability, with a high porosity between 15% and 35%, which is achieved by the fact that there is little or no fine aggregate in the composition of the concrete [1
]. Due to its high porosity, pervious concrete can develop compressive strength in the range of 3.5 to 28 MPa [7
] and flexural strength in the range of 1 to 3.5 MPa [6
]. Zhong and Wille [8
] have shown on pervious concrete with an ultra-high-performance matrix that compressive strength exceeding 50 MPa can be achieved without reducing hydraulic conductivity.
Pervious concrete, also known as porous, no-fine, gap-graded, permeable, draincrete, was first used in 1852 in the United Kingdom [9
]. Today, it is widely used, especially in the United States, Europe, and Japan, due to the increased awareness of environmental protection [10
]. Pervious concrete was traditionally used in greenhouses, tennis courts, driveways, parking lots, streets, zoo areas, road shoulders, and swimming pool decks [11
]. It was used in China in the spongy city plan to alleviate overloaded drainage systems [13
Researchers consider three key parameters that affect the quality of this concrete, namely the size of the aggregate, the amount of cement/binder, and the compaction method. Sonebi and Bassuoni [14
] found that the increase in the amount of cement reduces the number of voids and the permeability of concrete. The origin and grain-size distribution of aggregate affected the properties of pervious concrete. Usually, a single-sized coarse aggregate or grading between 9.5 and 19 mm is used. Yang et al. [1
] investigated the influence of the 4.75 mm sieve-passing percentage on pervious concrete compressive strength and concluded it increases with the increased sieve-passing percentage. The percentage of passages on the sieve varied from 2.55 to 20% and the highest compressive strength was obtained for the passage of 20%. According to Croush et al. [15
], the compaction method has a significant impact on strength and permeability. In study [16
], the authors recommended using impact methods for compaction
Some authors used construction and demolition waste and post-consumer urban waste in pervious concretes [17
]. Zaetan et al. [17
] investigated the properties of pervious concrete containing recycled concrete-block aggregate and recycled concrete aggregate and found that the replacement of natural aggregate with both recycled aggregates increased compressive strengths and abrasion resistance. Lu et al. [18
] studied an eco-friendly pervious concrete product using a waste glass cullet and recycled concrete aggregate. As a replacement for fine aggregates, the authors used crushed waste glass from post-consumer beverage bottles and, as a replacement for natural coarse granite aggregate, they used recycled concrete aggregate. Even though it was found that using crushed waste glass reduced the compressive strength, high permeability, and thermal insulating properties, this eco-friendly pervious concrete with post-consumer waste seems to be promising for use on building construction. Gesoğlu et al. [19
] found that the replacement of natural aggregate with rubber particles resulted in an increase of the toughness and ductility of concrete, and suggested that this kind of pervious concrete can be used in constructing parking areas, walkways, and road shoulders.
Synthetic, glass, carbon, and even cellulose fibers can be used to improve the properties of pervious concrete [20
]. Although steel fibers are used to reinforce standard concrete, they are not used in pervious concrete due to the risk of corrosion. Kevern et al. [22
] found that the addition of macro synthetic fibers reduces the permeability of the pervious concrete. Liu et al. [23
] reinforced pervious concrete with basalt fibers and found that they can increase the flexural strength and toughness of pervious concrete. In study [20
], the authors used four different types of fibers, namely glass, polypropylene, hemp, and carbon fibers, and concluded that the addition of fibers generally improved the mechanical properties of concrete but negatively affected porosity and permeability.
Textile production has a noticeable impact on the environment because clothes production requires significant amounts of chemicals, water, and energy. Fast-fashion brands do not design their clothes to last, and with affordable prices, they encourage consumers to always get the latest clothes. As much as 85% of clothes, after being worn a few times, end up in landfills or incinerators. For some materials, it can take 200+ years to decompose in a landfill [24
]. Europeans recycle only 25% of discarded textile waste [25
]. Textile recycling has environmental benefits, such as the reduced consumption of energy and water, avoided pollution and decreased landfill space requirements, and the reduced use of virgin fibers and the usage of dyes [26
]. Textile recycling can be categorized as pre-production or post-consumption material. Pre-production includes textiles, fibers, yarns, and the production process residues, while post-consumption includes discarded materials, such as clothing, shoes, and furniture [27
]. The use of textile waste materials in building materials has great potential either as thermal insulation or as a reinforcement of composites.
Knowing that it takes 7000 liters of water just to grow the cotton for one cotton T-shirt and about 2.6 kg of CO₂ to produce it, and with the fact that the average number of wears a T-shirt has before being thrown away is falling [28
], the idea is to use old cotton T-shirts as reinforcement. Worldwide, over 10 billion tons of concrete is being produced each year, and with only a 1% replacement of the volume of concrete with textiles, 6.25 million tons of textile waste would be disposed of.
Anglade et al. [29
] found that concrete blocks with polyester textile waste were superior thermal insulators. Tran et al. [30
] have investigated concrete reinforced with nylon or polypropylene carpet fibers. Adding textile waste fibers to concrete usually improves its flexural strength, while the workability, compressive strength, and modulus of elasticity decrease [31
]. Bartulović et al. [36
] investigated the possibility of using small pieces of cotton knitted-fabric waste in concrete and concluded that the inclusion of waste cloth increases the tensile properties and ductility of concrete. In study [37
], the authors studied the mechanical and durability properties of short textile waste fiber-reinforced cement pastes and found that composites provided a better mechanical performance in respect to reference specimens after accelerated ageing conditions. Selvaraj [38
] cut waste cloths into small-sized pieces (approximately 20 mm × 20 mm) and dosed them in concrete in various percentages. The addition of these waste cloths in concrete increased the energy absorption and enhanced the flexural and tensile properties of concrete.
This study was conducted to investigate the possibility of using waste cotton as a reinforcement of pervious concrete and the influence of textile strips on the properties of pervious concretes with different aggregate gradations and maximum grain sizes.
3. Results and Discussion
The results obtained by the laboratory tests are shown in Table 2
shows the average values and standard deviation of the measured results. Since the infiltration rate on cubes, water absorption, porosity, and CT scanning were determined on the same specimens, only the measured values were given in Table 2
to establish a connection between the measured values. The characteristics of the pervious concrete that were investigated by analyzing the results of CT scanning and 3D reconstruction are shown in Table 3
and Figure 5
and Figure 6
The possibilities of using pervious concrete mixtures are for a concrete pavement, the parking areas for passenger cars with small axle loads, concrete curb units, concrete paving flags, and concrete paving blocks. According to Guide for the Design and Construction of Concrete Parking Lots
, ACI 330-08 by the American Concrete Association (ACI) [48
], HR EN 1338 [49
], HR EN 1339 [50
], and HR EN 1340 [51
]—for Class 1—the target flexural strength is 3.5 MPa. According to Table 2
, only the mixture M1 met this condition.
, Figure 8
and Figure 9
show the ratio of the properties of pervious concrete with the addition of a fine aggregate, textile strips, or both and the single-sized concrete, M1 and M2, respectively, in order to examine the impact of these additives on the properties of pervious concrete.
According to Table 2
, all mixtures have significantly lower compressive strength than mixture M1, even mixture M1-V, which has almost the same composition as referent single-size concrete except the addition of waste cloth strips. Since all eight mixtures have the same amount of cement and water, the cause may be in the particle size distribution of the aggregate and/or the compaction effort during placement. Mixtures M1-V, M3, and M3-V have 35–43% lower compressive strength compared to M1 (Figure 7
). According to the results from Figure 1
and Figure 7
, and the conclusion in study [1
], it seems that the higher the percentage of the sieve passage of 4 mm (higher than 20%), the more the compressive strength of the concrete begins to decrease. Additionally, with the increase of the maximum aggregate size, the compressive strength decreases, and the lowest value was achieved by the mixture M4, with only 43% of M1’s compressive strength. Since all mixtures have an equal consistency and water–cement ratio, in part to some previous cracking of the paste around larger pieces of aggregate, larger aggregates exhibit lower concrete strengths. M2 and M4-V have almost identical compressive strength. Similar results were obtained for flexural strength. Figure 7
shows that the addition of a fine aggregate, textile strips, or both, reduces the compressive and flexural strength of pervious concrete. Mixtures M3 and M3-V have a slightly higher value compared to the others (Table 2
) and have almost reached the required Class 1 condition but are still about 25% lower than M1’s flexural strength. The porosity values range from 28.9 to 36.1% (Table 2
), which is more than the usual 15 to 25% [7
]. All mixtures have a higher porosity than mixture M1, especially the mixtures with larger maximum grain size. Mixtures M1-V, M3, and M3-V have a higher porosity than M1, which may have caused lower compressive and flexural strength compared to the reference mixture.
According to Table 2
, the higher the compressive strengths, the better the abrasion resistance of the pervious concrete specimens (Figure 10
a). Sherwani et al. came to the same conclusion in study [52
] that by increasing the compressive strength of pervious concrete, higher abrasion resistance can be achieved. As in study [52
], as the size of aggregates increases, the loss of volume increases. According to abrasion resistance classes in EN 1339, only mixture M1 can be classified as Class 2, and the other mixtures have much higher volume losses/50 cm2
. Additions to concrete with a lower maximum grain size have a stronger influence on abrasion resistance than concrete with larger maximum grain size. According to Table 2
, EN 1338, EN 1339, and EN 1340, for water absorption results, mixtures M1, M1-V, M3, M3-V, and M4-V can be classified as Class 1, while M2, M2-V, and M4 can be classified as Class 2. Pervious concretes with lower water absorption have better resistance to freezing and thawing, which increases the durability of concrete and helps with achieving a possible longer service life during application. The density values range from 1709.9 to 1834.2 kg/m3
and mixture M1 has the highest value (Table 2
). As can be seen in Table 2
, Figure 7
, Figure 8
and Figure 10
b, concrete of higher porosity has a lower density, which coincides with the conclusions in studies [2
and Figure 9
show that the M1 has the lowest permeability, measured by both methods: infiltration rate on test slabs and infiltration rate on the cube specimens. Mixtures with larger grains have significantly higher permeability. According to study [56
], the specimens with small particles produce meandering paths for water permeation, and specimens consisting of large grains produce straight paths, which affect the infiltration rate of the water flowing out of the specimens. The typical permeability of pervious concrete is in the interval 1.4–12.2 mm/s [13
], while Gesoğlu et al. [19
] obtained permeability coefficients between 0.25 and 6.1 mm/s, which the authors said fell in the recommended limits for pervious concretes. Only the infiltration rate measured on the test slab of mixture M1 is significantly below that range of 1.4–12.2 mm/s but slightly below the value according to study [19
]. Values obtained on other mixtures are either in this range or higher.
Based on the results from Table 2
, it is possible to establish a linear relationship between the infiltration rate on test slabs and the infiltration rate on cube specimens (Figure 11
). Although Figure 11
shows a very strong positive linear relationship between the results, Table 2
shows that the measurements on the cube in relation to measurements on the slab can be not only lower, but almost the same, or higher. The obtained results confirm the conclusion of the authors Lederle et al. in reference [45
], in which the authors showed that there is a high specimen to specimen variation within the same mixture of concrete despite very similar values of global porosity. The established relationship from Figure 11
should be verified with the specimens installed in the field.
and Figure 5
and Figure 6
show the obtained values on MSCT for the same concrete cubes on which the infiltration rate was tested. As mentioned earlier, MSCT devices quantify the attenuation of radiation passing through the material using the Hounsfield scale within each rectangular shape. According to reference [58
], the HU range can be selected from −960 to 800 for pores, from 800 to 2000 for cement paste, and from 2000 to 2974 for aggregates. For cellulosic materials such as cotton the values are ranging from −750 HU to −430 HU [59
]. If these values are selected in the display settings to display the distribution of the cloth strips in the specimens, Figure 12
According to Figure 12
, it can be seen that especially in the specimens with larger grain sizes, the displays of cloth strips and pores overlap. Therefore, Figure 13
shows the cross-sectional appearance of the textile after the flexural strength test.
The variability of the local volume fraction of the constituents is visible by the value of the minimum and maximum value and the standard deviation (Table 3
). If the difference between the maximum and minimum values is higher, it can be assumed that the observed material has lower homogeneity. Likewise, a larger difference between mean and median values indicates greater inhomogeneity of the specimens. For example, a standard cement mortar was recorded on the same MSCT device and the difference between the mean value and the median was around 40 HU, while on the pervious concrete specimens this difference is up to nine times higher. The standard deviation of the data indicates the spread of scores from the mean values. The main characteristic of pervious concrete is that it contains voids that allow water to percolate through to the specimen, so it is to be expected that there is a correlation between porosity, permeability, and standard deviation. Figure 14
shows a strong positive linear relationship between the standard deviation and porosity and both infiltration rate measuring methods.
Looking at Figure 5
and Figure 6
, it can be seen that there are large holes in the cross sections of the samples that are caused by an insufficient compaction effort during the specimens’ placement. A similar conclusion was reached in studies [4
], where the authors observed that rod holes in specimens do not get completely filled in when the rod is removed. The specimens were cast into the cube in two equal layers, while slabs were compacted in one layer. Each layer of the cube was compacted by 15 strokes with a tamping rod and 10 strokes with a wooden tamper, used from the set for the flow table test of the fresh concrete. The strokes were of an intensity equal to those for the testing slump of the fresh concrete. As mentioned before, the compaction of the slab was executed in segments, so each segment was compacted by 15 strokes with a tamping rod and 10 strokes with a wooden tamper. Since the middle of the slab is significantly away from the edges of the mold, it is certainly better compacted compared to the cube. Furthermore, the thickness of the slab is less than the thickness of one layer in the cube. This is probably the reason why the results of the infiltration rate on the M1 slab are lower compared to the results obtained on the cube. The wooden tamper is made from a soft material and it is not suitable for the compaction of zero-slump concrete.
The effect of the addition of a fine fraction on the properties of pervious concrete was evaluated by the ratio of the properties of the pervious concrete mixtures containing 10% fine fraction and the reference single-size pervious concrete mixtures (Figure 15
, Figure 16
and Figure 17
). The following ratios of properties were considered in this way: M1/M3, M1-V/M3-V, M2/M4, and M2-V/M4-V. As can be seen in Figure 15
, contrary to the expectations and research in study [7
], the addition of the fine fraction did not increase compressive strength. The exception to this is M4-V. The flexural strength is increased in reinforced mixtures and the porosity is increased in mixtures with a lower maximum aggregate grain. In reference [1
], it was observed that porosity decreases with increasing fine aggregate content, which only applies to specimens with a larger maximum grain size in this study. In standard concrete, fine and coarse aggregate should be graded in such a way as to reduce the voids inside the concrete, while in pervious concrete the voids are desirable. According to the principle of geometric similarity in one fraction of aggregate grain size, equality in the content of cavities is valid. By mixing the aggregate fractions, the specific aggregate volume increases. If there is a gap between the fractions, the higher specific aggregate volume can be achieved, increasing the gap between the nominal sizes of the smallest and the largest fraction [61
]. The obtained results are probably influenced by the large number of undersized grains in the 4-8 mm fraction and insufficient compaction effort during placement. Moreover, consideration should be given to reducing the fine fraction maximum grain size to 2 mm.
The addition of a fine fraction generally reduces abrasion resistance, increases water absorption, has not significantly affected density (Figure 16
), and has had a larger impact on concretes with lower maximum grain size. As can be seen in Figure 10
a, the abrasion resistance of concrete increases as the compressive strength is increased. It is also known that abrasion resistance is increased as the percentage of sand is reduced [63
], which is in line with the results obtained. An excessive amount of the particle size of 0.125 mm is harmful to abrasion resistance, so it is recommended that this amount does not exceed 5% by the weight of cement for concrete with high resistance to abrasion [61
]. The content of particles of less than 0.125 mm in size was higher (Figure 1
); as such, reducing the content of these particles would ensure better results.
According to Figure 17
, the influence of the fine fraction mainly increases the permeability measured on the slabs and the decreases measured on the cubes. Bonicelli et al. [64
] found that too much sand and improper compaction energy may reduce the drainage features of pervious concrete, which is in accordance with the above-mentioned undersized grains and the method of installation.
The effect of the incorporation of waste cloth strips on the properties of pervious concrete was evaluated by the ratio of the properties of the pervious concrete mixtures containing strips and reference to pervious concrete mixtures without strips (Figure 18
, Figure 19
and Figure 20
). The following ratios of properties were considered as mixture/mixture-V.
and Figure 19
show that textile strips improved the compressive strength, flexural strength, and abrasion resistance of concrete with the addition of a fine fraction. It is possible that textile reinforcements can delay the formation and growth of cracks at the interface of the cement matrix and aggregate and thus increase strength. Textile strips do not adversely affect porosity, water absorption, and density. According to Figure 20
, the influence of textile strips mainly increases the permeability measured on the slabs and decreases the a infiltration rate on the cubes with the addition of a fine fraction.
The main reason for adding textile strips is for the purpose of reinforcing pervious concrete. Based on the values measured during the testing of prisms, σ–δ ratios are shown in Figure 21
The reference specimens M2, M3, and M4 break immediately after exceeding the flexural strength during testing, and thus, the σ–δ curves are terser then reinforced mixtures M2-V, M3-V, and M4-V. The inclusion of waste strips increases the ductility of pervious concrete due to the reinforcing effect of textile strips. The M1-V mixture did not achieve good results compared to M1. In general, the M1-V mixture has only better porosity and permeability compared to M1.