The components from which the pervious concrete is made correspond to those of normal concrete with a difference in the grain size gradation where, only one coarse grain fraction or two non-adjacent fractions are used to achieve a porosity of 11 to 35% (1-3). In addition, the fine aggregate is omitted completely or added in a very small percentage, so that it is known as no-fines concrete (4). Pervious concrete is characterized by its ability to drain water through the concrete mass (5), its high noise absorption and its lighter color, which has a positive effect on reducing the occurrence of heat islands in urban areas (6). Pervious concrete also has some disadvantages, namely poor abrasion resistance, poor resistance to freezing and thawing cycles and relatively low strength (7). The compressive strength of permeable concrete can be up to 30 MPa (5), and the bending tensile strength up to 3.5 MPa (3). Micro-reinforcement of concrete with steel, polymer or glass fibers can improve its properties, in particular its shrinkage, abrasion and impact resistivity and flexural strength (8-10). Since water permeability/drainage is the main purpose of pervious concrete, steel fibres, which are known for their tendency to corrode, would not be a good choice for incorporation in such concrete. Finally, due to their inflexibility, steel fibres could negatively affect the pore connectivity in pervious concrete and consequently its drainage capacity.

In accordance with the above, glass, carbon, synthetic and cellulose fibres can be added to improve the weaker properties of permeable concrete (3,11,12). After (12-14), the volume fraction of microfibres in pervious concrete, with which satisfactory properties are achieved, is between 0.07 and 0.2%. Kevern et al. (12) concluded that the addition of macrosynthetic fibres in pervious concrete reduces its permeability and infiltration coefficient, but, as in (15), does not have a significant effect on the results of compressive and tensile strength.

In contrast, Rangelov et al. (6) used carbon fibres to reinforce permeable concrete and achieved an increase in the strength properties of the concrete, and Liu et al. (16), who increased the flexural tensile strength with basalt fibre reinforcement. Amde and Rogge (11) investigated permeable concrete with various additives, including cellulose fibres, which have been shown to effectively improve the tensile strength and resistance of the concrete to abrasion and freeze-thaw cycles.

Since permeable concrete has poor workability, it must be adequately compacted to achieve satisfactory mechanical properties while maintaining the required degree of porosity (3). Numerous studies of compaction methods have been carried out in the laboratory to obtain samples identical to those produced in-situ. Shu et al. (17) applied compaction under laboratory conditions using a tamping rod to prepare permeable concrete samples. To consolidate the samples, in (1,18) compaction with a tamping rod and Proctor hammer was used, in (1) also compaction by dropping the mold from a certain height and in (19) the centrifugal compactor was used as the compaction method. Putman and Neptune (1) concluded that compaction with a Proctor hammer best simulates the compaction method on a construction site.

Although most researchers use a ram rod as the compaction method, samples of permeable concrete compacted in this way achieve an increased degree of variability in their properties, which is caused by the creation of rod holes (1). Li et al. (20) used a metal roller in the laboratory to compact permeable concrete. Zhuge (21) used two different compaction methods: a standard compaction method and a vibratory compaction method.

By vibrating, they increased the adhesion between the aggregate grains and the cement matrix without significantly reducing the permeability of the concrete. Yang et al. (4) also compacted permeable concrete on a vibrating table and the vibration process lasted 40 s for each sample. Juradin et al. (22) compacted the samples on a vibrating table. In three series of samples, the first two layers were vibrated for 5 seconds and the third for 30 seconds (the total vibration time was 40 s), while in four series of samples, three layers were vibrated evenly for three times each, so that the total vibration time was less than 12 s. The authors concluded that vibration favourably affects the permeability and strength of concrete, longer vibration gives better permeability and shorter vibration gives better strength.

Due to its lower strength, the application of pervious concrete is focused on construction of parking lots, pathways and lightweight traffic roads. Enhanced porosity, which influences structural and functional properties of pervious concrete (23), leads to applications where the acoustic absorption or storm water runoff control is needed. To widen the application of this kind of concrete, researches are continuously working on the development of porous concrete with enhanced properties such as high performance pervious concrete (24-26) or fibre reinforced pervious concrete (27-32). Zhong and Wille (33) analysed and discussed the influence of fibre reinforcement on the freeze-thaw durability of pervious concrete and concluded that the improvement can be achieved through the incorporation of fibres into high performance pervious concrete matrix. For pavement structures the flexural strength is a measure of the resistance to structural failure. As for the conventional concrete, the addition of fibres to pervious concrete can enhance its flexural strength. Oni et al. (32) investigated pervious concrete pavement bricks reinforced with kevlar, polyvinyl alcohol and ultra-high molecular weight polyethylene fibres and achieved 9.5% increase of flexural strength in comparison to control mix that shows the effectiveness of fibres for pavement applications which are regularly subjected to two dimensional flexural stress. AlShareedah et al. (34) investigated the potential of pervious concrete reinforced with fibres made of recycled cured carbon fibre composite material through a pervious concrete pavement demonstration project. The pavement sections with reinforcement had the same values of compressive and flexural strength as the control section, but higher infiltration rates and lower surface deflection. FORTA Technical Report (35) has carried out a number of project trials and applications of synthetic fibres in pervious concrete that led to the conclusion that longer fibre lengths and higher dosages are the best opportunity to increase pervious concrete toughness and durability. Novak et al. (36) investigated the mechanical properties of pervious recycled aggregate fibre reinforced concrete. The obtained findings showed that fibre reinforced pervious concrete has a very ductile behavior and a high post-cracking strength.

This paper deals with the possibility of the improvement of pervious concrete properties by incorporation of various types of fibres and studies the effect of short duration vibration on pervious concrete properties in comparison with incorporation by wooden lath and hammer.

Test results for the density, porosity, compressive and splitting tensile strengths, and permeability by both applied methods are shown in Table 2.

The effect of vibration on the porosity and permeability according to both methods used here was evaluated by the ratio of the properties of the vibrated mixtures and the mixtures of the same composition compacted with wooden lath and hammer (EV / E, GV / G, CV / C, PPV / PP, HV / H). Thus, the obtained relative values of porosity and permeability by both methods are given in Figure 5, while relative values of compressive and splitting tensile strengths are given in Figure 6.

The effect of incorporation of different types of fibres on the porosity and permeability according to both methods used here was evaluated by the ratio of the properties of the pervious concrete mixtures containing fibres and the reference pervious concrete mixtures, taking into consideration that in this case there are two reference mixtures: E mixture compacted with wooden lath and hammer and EV mixture compacted by short duration vibration. The following ratios of properties were considered in this way: G/E, C/E, PP/E, H/E, GV/EV, CV/EV, PPV/EV and HV/EV. Thus, the obtained relative values of porosity and permeability by both methods are given in Figure 7, while relative values of compressive and splitting tensile strengths are given in Figure 8.

The appearances of pervious concrete mixtures with incorporated carbon fibres, both vibrated (CV) and compacted with a wooden lath and hammer (C), are shown in Figure 9 and Figure 10.

The referent mixtures and the appearances of pervious concrete mixtures compacted with wooden lath and hammer or vibrated integrated with different types of fibres are shown in Figure 11.

From Table 2, it is clear that the density values range from 1755 to 1950 kg/m³ and the porosity from 29.8 to 35.8%, which is more than the usually achieved values of 15 to 25% for porosity of pervious concrete (5). This result for porosity is expected for single-sized aggregates and in accordance with the results in previous research that have shown that a larger grain ensures good permeability of concrete (45-47). Figure 12 shows the specimens of pervious concrete after the splitting tensile test. According to the visible large pores, it is clear why concretes have such high porosity and good permeability.

The values of permeability obtained by the FH method are slightly higher than the values obtained by the CH method for all of the mixtures (Table 2). The highest value of permeability was achieved in the case of the concrete mixture EV (31.28 mm/s according to the FH method and 25.27 mm/s according to the CH method), followed by E, GV, and PP. All values of permeability obtained herein are higher than 2-12 mm/s, the typical permeability value for pervious concrete (2, 5). A positive effect of vibration on the porosity and permeability according to the FH and CH methods (Figure 5) was observed for EV, GV, and CV mixtures, while it was absent for PPV and HV mixtures. Figure 12 shows a smaller proportion of pores in PPV concrete than in PP concrete which justifies such results.

From Table 2, it is clear that the compressive strengths of pervious concrete range from 8.3 to 18.6 N/mm² while the splitting tensile strengths range from 1.22 to 2.14 N/mm². Similar results were achieved in the research of Mahalingam and Mahalingam (48). In their study, the compressive strength and splitting tensile strength values varied from 5 MPa to 16 MPa and from 1.15 MPa to 1.7 MPa, respectively. A positive effect of vibration on the compressive and splitting tensile strengths (Figure 6) was observed for all the mixtures with fibres incorporated while it was absent for the reference concrete mixture, EV. The reason for the positive effect of vibration on the properties of pervious concrete may be found by comparing Figure 10 with Figure 11. Specifically, Figure 10 shows a viscous layer 23-26 μm thick formed at the contact surface between the aggregate grain and the cement matrix in the vibrated mixture CV while there is no such layer in the mixture C compacted with wooden lath and hammer (Figure 9). Formation of a viscous layer by vibration of concrete specimens was already confirmed in a study by Juradin and Krstulović (49). According to (21), vibration improves the quality of the interfacial zone between the cement paste and aggregates, which is usually the weakest link in terms of mechanical properties.

It can be seen from Figure 7 that, in this research, the fibres almost always adversely affect the porosity and, consequently, the permeability of pervious concrete. In contrast, Figure 8 shows the positive effect of fibres on the compressive and splitting tensile strengths of concrete. An exception here is H mixture which recorded a decrease in compressive and splitting tensile strengths (12% for compressive strength and 27% for splitting tensile strength) compared with the reference mixture (E). There is a positive effect of fibres on mechanical properties of pervious concrete because the fibres bridge the gap between the coarse aggregates and bind the pervious concrete mixture with the fibre-filled matrix (11, 50), and because fibre reinforcements may supress the generation and growth of cracks in the interface (51). Hesami et al. (29) incorporating glass, steel and polyphenylene sulphide (PPS) fibres and Geethanjali et al. (52) using polypropylene fibres obtained an improvement in strength in regards to the control mixtures. The authors in (52) concluded that the increase in strength is because of the polypropylene fibres in pervious concrete enhances the bonding between the coarse aggregate and cement paste. Rangelov et al. (6) used CCFCM elements (cured carbon fibre composite material) and improvements in mechanical properties have been observed on compressive and tensile strength. Infiltration rates were increased, especially on fibre reinforced slab specimens. Oni et al. (32) on specimens with Kevlar, PVA and UHMWPE fibres got an increase in the permeability but decrease in compressive and splitting tensile strength, when compared with the control group. Similar strength results but poorer permeability was obtained by Pils et al. (31) on specimens with PP fibres in regards to control mixture.

Of all the fibres studied herein, glass and carbon fibres improved the splitting tensile strength of pervious concrete mixtures the most. An explanation could be found in the number of fibres connecting the aggregate and the cement paste. Fibres are characterised by their aspect ratio, i.e. by the length to diameter ratio and aspect ratio for hemp and propylene differs significantly from the value for glass and carbon fibres, Table 1. Specifically, glass and carbon fibres have a smaller diameter than polypropylene and hemp fibres; thus, in the same volume of all fibres a higher number of fibres can be found in glass and carbon fibres than in propylene and hemp fibres (Figure 11). Furthermore, a higher number of fibres bridging the interfacial transition zone would imply better mechanical properties of pervious concrete.

The authors in (6) and (31) noticed the importance of compaction. In (31), in conclusion it stands: “… mechanical compaction should be used so that the mechanical properties are improved…”. As can be seen in Figure 8, the positive effect of fibre addition was more emphasised in the case of vibrated mixtures - i.e., mechanical properties of vibrated mixtures with incorporated fibres (HV, GV, CV, PPV) achieved higher relative values with respect to their reference mixture (EV) than mixtures compacted with wooden lath and hammer (H, G, C, PP) with respect to their reference mixture (E). After all, the value of the standard deviation for compressive strength is lower for vibrated specimens, Table 2.

In future research, the authors of this paper will explore the possibility of using such concrete in paving concrete blocks and flags according to (53) and (54). Since very high permeability of concrete was obtained in this paper, the addition of a fine aggregate is expected to improve the mechanical properties of concrete, with a permeability that is in the acceptable range. Given the drainage capability of pervious concrete, such blocks and flags would certainly contribute to flood protection in urban areas.

Ten mixtures of pervious concrete were prepared, five of which were compacted by wooden lath and hammer and five by short duration vibration. Fibres of various origins were added to the mixtures: polypropylene, glass, carbon, and hemp fibres. Density, porosity, permeability (by FH and CH methods), compressive strength and splitting strength were tested in hardened pervious concrete specimens. An examination of the structure of concrete was made using a polarisation microscope. According to the results obtained, the following can be concluded: