Investigation on mechanical properties and water stability of porous polyurethane concrete

Abstract

1. Introduction

Porous concrete, also known as pervious or permeable concrete, is a kind of concrete composed of aggregate and binder, and characterized by a porosity of 15%–30% [1]. It plays an important role in environmentally friendly pavements and brings broad benefits, such as water penetration and purification, noise and temperature reduction, skid resistance improvement, etc., which are mainly attributed to its diverse void structure [2]. However, the open voids bring the porous concrete into more frequent contact with air and water, which leads to a reduction in mechanical properties and durability due to the deterioration of the binder performance. Hence, to improve the durability of porous concrete, the property of binders is critical. The traditional binders of porous concrete usually are cement and asphalt, and they have been used in the surface layer of permeable pavement for decades. Porous cement concrete (PCC) usually has higher strength, better durability and stronger rigidity, but a long curing time, poor deformability and cracks easily [3]. Porous asphalt (PA) is convenient to construct and maintain, but is prone to peeling, rutting and ageing [4]. To improve the properties of porous concrete, a lot of work has been done to modify the cement paste [5] or asphalt [6], and some modifications have shown positive results. However, the application of porous concrete is still limited in heavy load traffic. Hence, it is imperative to develop a new kind of binder to ameliorate the properties of porous concrete.

Polyurethane (PU) has been applied in structural and infrastructural civil engineering for decades, such as repair, reinforcement and protection [7]. It has also been used as pavement materials, such as modified asphalt [8, 9], maintenance [10], polyurethane rubber particle mixture [11, 12] and functional pavements [13]. So far, porous concrete with PU as the binder, namely porous polyurethane concrete (PPUC), has been proposed to achieve water permeability [14], anti-icing properties [15], noise reduction [16] and so on. The basic mechanical properties, durability as well as road performance, have also been investigated. A summary of the relevant research is presented in Table 1.

As a new road material, there are no specific standards or specifications for PPUC. From Table 1, there are two different testing systems for PPUC. According to the standards of PCC, PPUC shows better performance in flexural strength, fatigue resistance, resistance to permanent deformation and weathering, but is not as good as PCC in compressive strength and freeze–thaw durability [17, 18]. When compared to PA, PPUC presents better stability, fatigue performance, compressive and tensile strength, and resistance to deformation [11, 14, 16, 19]. The abrasion resistance of PPUC measured by the Cantabro test results in opposite conclusions when compared with PA by different researchers [14, 19]. Lu et al. [14] showed that the abrasion resistance of PPUC after immersion in 25 °C water was better than PA, while Cong et al. [19] concluded that PPUC had worse abrasion resistance after immersion in 20 °C water. Cong et al. [21] also found that water and temperature had a great influence on the Cantabro loss of PPUC. Although many researchers have investigated the properties of PPUC and believed that it is a competitive alternative to PA or open-graded friction course (OGFC), a comprehensive evaluation of the mechanical properties of PPUC has still not been unveiled. Table 1 also provides the main factors affecting the properties of PPUC, namely different PU dosages and gradations. According to previous research, the gradation of PPUC is usually discontinuous, and the dosage of PU varies from 1.0% to 6.5%.

Furthermore, although many studies have investigated the water stability of PPUC, few have explained why water damage occurs. Generally, water damage to PPUC is attributable to the degradation of PU. Sun [11] suggested that the operating time, curing time, strength, and water resistance of PU binder were preliminary screening indicators. Cong et al. [19] suggested that the moisture content of aggregates should be reduced to prevent side reactions of the isocyanate groups, and they recommended that the hard segment content of PU should be between 41% and 51% to provide acceptable water stability [21]. Although the properties of PU have been restricted, studies on the mechanism of water damage of PPUC are still lacking.

As an alternative to PCC or PA, the functional properties of PPUC are also important. Due to the discontinuous gradation and less binder dosage, PPUC has a larger void ratio than conventional porous concrete (Table 1). Thus, PPUC possesses better permeability, anticlogging performance, temperature and noise reduction properties [14]. However, the functional properties of PPUC are highly dependent on the structure and mix design, and the laws also needs to be explored.

The objectives of this study are to comprehensively investigate the mechanical properties and water stability of PPUC, and to explore the failure state of PPUC as well as the mechanism of water damage. First, the compressive strength and flexural strength of PPUC with four different types of polyurethane were studied to select the type of polyurethane with better performance. Second, the comprehensive mechanical performance and water stability of PPUC with different PU dosages and gradations were investigated. The void ratio and permeability coefficient were also examined for the functional properties of PPUC. Third, the pull-off strength and infrared spectra of the PU binder under dry and wet conditions were measured to explain the mechanism of water damage. Finally, a correlation analysis was performed to establish the relationship between the mechanical properties and the physical properties of PPUC.

2. Materials and methods

2.1 Materials

Two-component polyurethanes with four types of component A were selected, and the basic properties of PU are listed in Table 2. The chemical structures of the different types of components A and B were measured by attenuated total reflection infrared spectroscopy (ATR-IR) as shown in Fig. 1. All component A were light yellow liquid polyol and component B was a brown–black liquid isocyanate. The ATR-IR spectrum also indicated that all component A had a hydroxyl group (wave number: 3 200–3 550 cm^–1) and an ester group (wave number: 1 743 cm^–1) and component B had an isocyanate group (wave number: 2 243 cm^–1). The dosage of PU binders was the percentage of aggregate weight, which in this study varied from 3% to 5%.

Fig. 1.

The ATR-IR spectra of PU components

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P·O 42.5 ordinary Portland cement combined with drinking water, styrene–butadiene–styrene-modified asphalt were selected as the other binders for comparison. The properties of cement and asphalt referred to Wang et al. [22] and Zhang et al. [23], respectively.

Basalt aggregates were used in the particle size range of 1.18–16 mm. Fine limestone (0.075–1.18 mm) and mineral powder (<0.075 mm) were used in the gradation of OGFC-13 (G5). The basic properties of the aggregates are listed in Table 3. G3 and G4 were graded using OGFC-13 without fine aggregates to obtain better bonding [11]. The gradations of PPUC are shown in Fig. 2.

Fig. 2.

Gradation curves

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2.2 Preparation of specimens

The aggregates were dried at 105 °C and cooled to room temperature, then mixed in a mixing pot. The two components of PU were weighed and mixed in a certain proportion, and stirred with a powerful mixer for 1–2 min to form a homogeneous mixture. The PU binder was immediately poured into the mixing pot and mixed at 25 °C for 180 s. Cubic specimens (100 mm×100 mm×100 mm) for compressive strength testing and cuboid specimens (100 mm×100 mm×400 mm) for flexural strength measurement were prepared according to the casting method of PCC [24]. Marshall specimens (φ: 101.6 mm×63.5 mm) were cast and compacted 75 times on one side according to Cong et al. [19]. The total mixing time was limited to 5 min, and then a set of six specimens was cast in 10 min. The specimens were left in the mould in an indoor environment for at least 24 h before demoulding. After demoulding, the specimens were placed in an oven at 40 °C for 3 days before testing [17].

The gradation of PCC was G1, and the water to cement ratio was 0.32. The binder (total weight of cement and water) to aggregate ratio was 33%. The specimens were covered with a wet cloth for 24 h before demoulding. The specimens were cured in a natural environment and covered with a wet cloth, and tested after 27 days of watering. The Marshall specimens of PA have the gradation of G5 and the asphalt to aggregate ratio was 4.6% by weight.

Porous concrete can be regarded as aggregates coated by binders gathered together, and then a compaction force is applied to form a tightly packed structure. Thus, binders could be treated as a coating. According to the standard test method for pull-off strength of coatings ASTM D4541 (Pull-off strength of coatings using portable adhesion testers), the interface bonding property was explored between PU and aggregate substrate, as shown in Fig. 3. The basalt substrates (100 mm×100 mm×10 mm) were washed for 10 min by an ultrasonic cleaner and dried in an oven at 105 °C. After the substrates were cooled to room temperature, a thin tape was attached to the edge of the substrate to limit the thickness of PU film to (0.12±0.02) mm (Fig. 3a). The thickness of the PU film was measured by a vernier caliper. Two components of PU binder were stirred with a drop of red pigment, and after uniformly mixing, the binder was placed on the surface of the substrate immediately. The red pigment was used to distinguish the binder and glue, and would not affect the mechanical properties of the PU film. A flat trowel was used to smooth the binder surface. Then, the specimens were cured in an oven at 40 °C for 3 days (Fig. 3b). After that, a specimen was placed in a 60 °C water bath for 48 h, and then put in an indoor environment for 24 h.

Fig. 3.

The preparation of interface specimens: (a) Basalt substrate with thin tapes on the edges; (b) Cured interface specimen; (c) Studs attached on the surface of PU film; (d) Schematic diagram of interface specimen; (e) PosiTest AT-A self-aligning tester

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2.3 Mechanical properties tests

The mechanical properties of PPUC tested in this study contained compressive strength and flexural strength, referring to GB/T 50 081 (Standard for test methods of concrete physical and mechanical properties); Marshall stability and Cantabro loss test, according to JTG E20 (Standard test methods of bitumen and bituminous mixtures for highway engineering); and pull-off strength, referring to ASTM D4541 (Pull-off strength of coatings using portable adhesion testers). The mix ID, specimen size and designed tests are listed in Table 4.

Compressive strength (f_c) was an important indicator to quantify the ability of PCC to withstand loading. All tests were conducted on a press testing machine at a loading rate of 1.0 kN/s. Flexural strength (f_f) was used to evaluate the flexibility of PPUC and PCC. All tests were conducted on a flexural strength testing machine at a loading rate of 0.1 kN/s. The results of each test were implemented with three replicate specimens.

Marshall stability tests under standard and immersion conditions were employed to evaluate the stability and resistance to water damage of PPUC. The specimens were put into a water bath at 60 °C for 30–40 min in standard conditions, and for 48 h in immersion conditions. Cantabro tests were employed to evaluate the raveling and water stability of PPUC. In standard conditions, specimens were put into a water bath at 20 °C for 20 h; after that, the surface-dried specimen was rotated for 300 times at a speed of 30 r/min; in immersion conditions, specimens were put into a water bath at 60 °C for 48 h, and cooled down to room temperature for 24 hours, then rotated for 100 times at the same speed.

The interface specimens were tested in a room environment and after water immersion (60 °C, 48 h), respectively. Aluminous studs (diameter: 20 mm) were stuck on the surface of interface specimens by a commercially available acrylic structure AB glue (A:B = 1:1), and cured in a room environment for 24 h (Fig. 3c). The schematic diagram of the interface specimen is illustrated in Fig. 3d. The pull-off strength test was carried out by a PosiTest AT-A self-aligning tester (Fig. 3e) with a loading rate of 0.2 MPa/s.

2.4 ATR-IR analysis

From the IR spectrum, the chemical structures of PU binders and the change of functional groups could be observed directly, explaining the decline of mechanical properties to some extent. The layer of PU binder on the interface specimens was peeled off, and the outer surface (connected with air) and the inner surface (connected with the aggregate substrate) of the PU layer were measured by a BRUKER TENSOR 27 Fourier transform infrared spectrometer with a diamond ATR accessory. The ATR-IR spectrums of PU layers under the room environment and water immersion (60 °C, 48 h) conditions were checked.

2.5 Functional properties tests

Unlike PPUC, the initial mass (M₀) of PCC were measured by drying the specimens in a 60 °C oven until the mass was stable. Before measuring the mass in water (M₁), PCC was immersed in water for 24 h.

3. Results and discussion

3.1 Compressive strength and flexural strength of PPUC

The physical characteristics and mechanical properties of PPUC and PCC are listed in Table 5. In order to optimize the compressive strength and flexural strength of PPUC, factors, such as type of binder, dosage and gradation were considered. In general, the flexural strength of PPUC was better than that of PCC, while the compressive strength of PPUC was not good enough compared to PCC. In the case of the single gradation (G1), the compressive strength of PPUC was about a quarter of that of PCC, as the cement paste dosage was about 8 times the PU binder dosage, which also resulted in a lower void ratio and permeability coefficient of PCC. For A2-4-G1 and A4-4-G1, the flexural strength of PPUC was comparable to PCC, but the void ratio and permeability coefficient were increased. The ratio of compressive strength to flexural strength (f_c/f_f) indicated that the toughness of PPUC was much higher than that of PCC.

3.1.1 Effect of PU binders

The compressive strength, flexural strength, void ratio and permeability coefficient of PPUC were significantly affected by the performance of the PU binder at the same gradation and PU dosage (Fig. 4), mainly due to the different chemical structures and viscosities of the four types of PU. Due to the highest void ratio, A1-4-G1 exhibited poor strength properties compared to the other PPUC. On the other hand, A2-4-G1 had the highest compressive strength and flexural strength, despite the relatively large void ratio. Because the difference in the total void ratio between A1-4-G1 and A2-4-G1 was not significant, the reason for the difference in strength properties of PPUC was mainly due to the chemical structure of the component A. In general, ester-based PU have better mechanical properties than ether-based PU because the ester groups show less flexibility than the ether groups. From the IR spectra of the component A (Fig. 1), the relative abundance of the ester group (wave number: 1 743 cm^–1) of the component A2 was the highest, and therefore, A2-4-G1 has the highest strength.

Fig. 4.

The properties of PPUC with different binder types

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Unfortunately, segregation was observed at the bottom of A3-4-G1 and A4-4-G1 (Fig. 4, inset pictures), which was related to the viscosity of PU during the operating time. According to Cong et al. [19], the viscosity of PU after mixing the two components remained at a relatively low level during the operating time, and then increased sharply. Because the moulding time in this study was within 10 min, the viscosity of PU has not increased much yet and appeared to be correlated with the initial viscosity of component A. However, the chemical structures (content of —NCO, —OH), molecular weight and ratio of the reactants all affected the viscosity of the PU binder, so the viscosity of the PU binder cannot be judged by the viscosity of component A alone. The segregation phenomenon also indicated that the aggregates of A3-4-G1 and A4-4-G1 were not uniformly coated by the PU binder, and as a result, the strength properties of PPUC were relatively low, and the void ratio as well as the permeability coefficient of PPUC decreased. Furthermore, the strength properties of PPUC were poorly correlated with the tensile strength of PU, which meant that the selection of PU type could not be determined by the tensile strength of PU.

In summary, the chemical structure of component A and the viscosity of the PU binders have great influence on the strength properties of PPUC. For different PU binders, the above-mentioned two factors need to be given priority in the selection. Because A2-4-G1 exhibited the highest mechanical strength and relatively high permeability, PU made from the component A2 was selected for the following tests.

3.1.2 Effect of PU dosage and gradation

Figure 5 illustrates the effect of PU dosage and aggregate gradation on the compressive strength and flexural strength of PPUC. As the dosage of PU binder increased (3%–5%), the thickness of the PU film wrapped around the aggregate in PPUC increased. As a result, the compressive strength and flexural strength of PPUC were improved, but the void ratio and permeability coefficient decreased. It should be noted that at a PU dosage of 5% and a single gradation (G1), segregation was observed at the bottom of A2-5-G1, which indicated that an excessive amount of binder was used in the G1 gradation.

Fig. 5.

The mechanical properties of PPUC with different PU dosage and gradation

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For different gradations of PPUC (A2-3-G1∼A2-3-G4), with the same PU dosage (3%), A2-3-G1 showed relatively low compressive strength and flexural strength, but had the largest void ratio and water permeability coefficient. For A2-3-G2 to A2-3-G4, both strength and void ratio showed a trend of decreasing, and then increasing. This is due to the fact that the total surface area and the bulk void of the aggregates changed with the increase of fine aggregate content. According to the technical specifications for design and construction of porous asphalt pavement (JTG/T 3350–03), the PU film thickness of PPUC was calculated. For A2-3-G1, the calculated PU film thickness was the thickest (73.2 μm), while the PU film thickness increased sequentially from A2-3-G2 to A2-3-G4 (45.7 μm, 47.2 μm and 53.8 μm, respectively). Although the PPUC with the single gradation (G1) presented the thickest PU film, the larger void ratio made it the lowest strength. For the other gradations, although A2-3-G3 had the middle PU film thickness, it had the smallest void ratio, resulting in the greatest strength. Therefore, it can be concluded that gradation affects the mechanical strength of PPUC by influencing the void ratio of the mixture, but not the PU film thickness. With the same PU dosage, the strength properties of PPUC can be improved by optimizing the gradation, but the improvement was relatively limited.

Because the strength and permeability coefficient of A2-3-G4 were relatively high, OGFC-13 without fine aggregate (G4) was selected as the gradation for further optimization. At 5% PU dosage, the compressive strength and flexural strength of A2-5-G4 was increased to 7.7 and 4.0 MPa, respectively, and the total void ratio (24.2%) and permeability coefficient (19.64 mm/s) were relatively high. For further optimization, PPUC with full gradation of OGFC-13 (G5) and 5% PU dosage was prepared. The compressive strength (10.0 MPa) and flexural strength (5.8 MPa) of PPUC were further improved. Unfortunately, the void ratio and permeability coefficient decreased dramatically, but still met the specification requirements (0.5 mm/s).

In summary, the compressive strength and flexural strength were greatly influenced by the gradation and PU dosage. The flexural strength of PPUC was more likely to meet the specification requirements of PCC (CJJ/T 135 Technical specification for pervious cement concrete pavement), while the compressive strength was more difficult to meet. Therefore, further research can focus on the compressive performance of PPUC. The optimized mix design of PPUC can meet different needs in applications, A2-5-G4 for higher permeability and A2-5-G5 for higher strength.

3.1.3 Failure characteristics of PPUC

The force-deformation curves for the flexural strength tests of PPUC and PCC are shown in Fig. 6. In general, PPUC exhibited greater load values and deformations than PCC. For PCC, the force-deformation curve was almost linear, which indicated that PCC underwent an elastic deformation process. However, for PPUC, the force-deformation curve was a nonlinear relationship. In particular, for A2-5-G4, it showed a greater ductile fracture behaviour than A2-5-G5. This difference was due to the use of different amounts of fine aggregates in the gradation. The area under the force-deformation curve was calculated to obtain the fracture energy, which was an indicator of the brittleness or toughness of the material. From the internal table in Fig. 6, the fracture energy ranking of PPUC was A2-5-G4>A2-5-G5>PCC. This also indicated that A2-5-G4 was more ductile while PCC was more brittle. Comparing A2-5-G4 and A2-5-G5, the latter has higher strength but less deflection, so the fracture energy was smaller and relatively more brittle than the former, which was also due to the use of more fine aggregates in the latter.

Fig. 6.

Force-deformation curves of PPUC until flexural failure

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Figure 7 shows the fracture interface of PCC and PPUC after the flexural strength testing. To make the fracture surface more easily recognizable, a few drops of red pigment were added to the PU binder. The main fracture characteristic of PCC and PPUC was the failure at weak bridge between the aggregates, which was a cohesion failure of the binder. This indicated that the bonding strength of both binders was insufficient or that the aggregates had a small joint area and a low contact number. Especially for PPUC, there were some broken bubbles on the fracture surface of the PU binder, which indicated that gas had entered during the mixing process or side reactions had occurred inside the binder. Therefore, there is a need to increase the number and area of contact points in PPUC to improve the mechanical properties.

Fig. 7.

Fracture interfaces of PCC and PPUC after flexural strength test: (a) PCC; (b) PPUC(A2-5-G4); (c) Enlarged view of PPUC

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For PCC, another common fracture feature was the fracture of the interfacial transition zone (ITZ). As shown in Fig. 7a, the exposure of dark aggregates was mainly due to the ITZ failure and less frequently due to the aggregate fracture. In contrast, the ITZ failure in PPUC was rarely found due to the strong adhesion between the PU binder and the aggregates. Aggregate fracture was more common than ITZ failure in PPUC, but it was still very rare compared to binder cohesion failure.

3.2 The Marshall stability of PPUC

The results of the Marshall stability of PPUC and PA are illustrated in Fig. 8. In general, the Marshall stability of PPUC was higher than that of PA, despite the different PU dosage, gradation and test conditions. The results indicated that PPUC had higher stiffness than PA, even with a lower amount of binder and a single gradation (A2-3-G1). It is worth mentioning that the Marshall stability of A2-5-G5 exceeded the upper limit of the instrument (85 kN) under standard and immersion conditions. This indicated that the stability of PPUC was so good that it was not even necessary to evaluate its stability. In the following study, Marshall stability tests were used to compare and optimize the performance of the PPUC with different mix designs.

Fig. 8.

Marshall stability of PPUC and PA

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As the amount of PU binder increased (A2-3-G1 to A2-5-G1), the Marshall stability of PPUC improved (14.54–21.38 MPa) and the void ratio decreased, which was due to the thickening of the PU film wrapped around the aggregate surface (73.2 μm, 97.6 μm and 122.0 μm, respectively). Both the stability and void ratio of PPUC were linearly related to the PU film thickness. At the same PU binder dosage, the Marshall stability was mainly influenced by the skeleton structure of PPUC, namely the gradation of the aggregates. The highest Marshall stability was found for A2-3-G4, indicating that the gradation (G4) was the most stable. Using the optimized gradation and PU dosage, the stability of A2-5-G4 and A2-5-G5 was significantly improved by increasing the amount of PU binder and fine aggregates. For different void ratios, the Marshall stability of PPUC showed an increasing trend with the decrease of void ratio. This was because the void ratio of PPUC was mainly influenced by the aggregate gradation and PU dosage.

The Marshall stability of PPUC under the immersion condition was used to demonstrate its resistance to water damage. As shown in Fig. 8, except for A2-5-G5, the residual stability of PPUC ranged from 56.1% to 85.7%, which was lower than the value of PA (88.5%). Moreover, the residual stability decreased with the increase of PU dosage, the reason of which is to be further explored in the subsequent study. However, there was no obvious pattern in the effect of different gradations on the residual stability of PPUC. Although the residual stability of PPUC was lower, the values of the immersion Marshall stability of PPUC were still higher than those of PA. To further explore the water damage mechanism of PPUC, the pull-off strength tests and ATR-IR analysis were performed on the PU binder under the same condition.

3.3 The Cantabro loss of PPUC

Figures 9 and 10 show the Cantabro losses of PPUC and PA under standard and immersion conditions. It can be seen from Fig. 9 that the Cantabro losses of PPUC increased with the number of rotations, especially at low PU dosage and single gradation (G1). Due to the severe immersion Cantabro losses of PPUC in some mix designs, the number of rotations of the specimens under immersion conditions was reduced to 100 in this study. In general, except for A2-5-G5, PPUC exhibited worse peeling resistance than PA. The reason for this result was that different PU dosage and gradation had a greater effect on the void structure of PPUC, and therefore the Cantabro losses were higher. This also showed that it is necessary to evaluate the Cantabro loss of PPUC. Using the same gradation, A2-5-G5 showed better peeling resistance and water resistance than PA, which was due to the slightly higher amount of PU binder than asphalt, in addition to the influence of the binder itself.

Fig. 9.

Immersion Cantabro losses of PPUC with different rotation times

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Fig. 10.

Cantabro losses of PPUC and PA

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Figure 10 shows that the Cantabro losses decreased significantly with increasing PU dosage in both conditions. The improvement of the peeling resistance of PPUC was due to the thickening of the PU film that encapsulated the aggregates and the increased adhesion of the PU binder. For different gradations, A2-3-G3 and A2-3-G4 exhibited better peeling resistance at room temperature, but showed the opposite behaviour under the immersion condition. We believe that the Cantabro losses of PPUC under standard conditions were mainly determined by the stability of the skeleton structure (gradation). However, the main factor contributing to the Cantabro losses at 60 °C water bath for 48 h was the water damage of the PU binder. By increasing the amount of PU binder (5%) and fine aggregates (G4, G5), the peeling resistance of PPUC was greatly improved, and the Cantabro losses of A2-5-G4 and A2-5-G5 were less than 20% under both conditions. In particular, the Cantabro loss of A2-5-G5 was 4.3% under standard condition and 4.6% under immersion condition (300 times), which was lower than that of PA [standard condition: 9.2%; immersion condition: 11.5% (300 times)]. The reasons for this result were explained above and will not be repeated here.

3.4 Pull-off strength of PU binder

The pull-off strength test was used to evaluate the interface bonding performance between the PU binder and the basalt substrate as a simplified characterization of the binder-aggregate interaction. For the PU consisting of component A2 and component B, the average pull-off strength was 4.37 MPa (4.17 MPa–4.93 MPa) at room temperature, and the fracture state is shown in Fig. 11a. The average adhesion failure area between the basalt substrate and PU film was 95%, recorded as 95% P/B (see Fig. 3d); the other adhesion failure was between PU film and glue, 5% G/P. For the pull-off strength of the specimen under immersion condition, the average pull-off strength was 2.04 MPa (1.72 MPa–2.28 MPa), and the average failure state was 40% P/B, 50% P and 10% G/P (Fig. 11b). The results of the pull-off strength tests showed that the bonding strength between the PU binder and the basalt substrate decreased under immersion condition. A new failure state appeared (cohesion failure, Fig. 11b), which indicated that the cohesion of the PU binder was destroyed to some extent by hot water. As a result, the Marshall stability of PPUC decreased and the Cantabro loss increased.

Fig. 11.

The failure state of the interface specimens under two conditions: (a) Room environment; (b) Water immersion

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As discussed in Section 3.1.3, the fracture state of the flexural strength test was the cohesion failure of the PU binder. However, in the indoor environment, the failure state of the interface specimens after the pull-off tests was the interfacial adhesion fracture. The main reason for this contradiction was the discrepancy between the surface roughness of the basalt substrate and the true roughness of the aggregate. Therefore, the failure state of the interface specimen could not represent the failure state of PPUC, but the adhesion and cohesion of the PU binder can still be verified by the pull-off test.

3.5 Preliminary exploration of water damage

The ATR-IR spectra of the PU films peeled from the interface specimens are shown in Fig. 12. Initially, we planned to explore the effect of water on the PU films by measuring the changes of different functional groups. However, no significant changes in the ATR-IR spectrum were observed on the outer surface, suggesting that the chemical structure of PU was relatively stable. Fortunately, when measuring the inner surface of the PU film attached to the substrate, a small peak was observed at 2 269.1 cm^–1 for the dried PU film, while no peak was observed for the immersed film. The peak at 2 269.1 cm^–1 belongs to the residual isocyanate functional group. This phenomenon indicated that water penetrated into the PU film and reacted with the inner isocyanate groups, which led to a decrease in the cohesion and adhesion of the PU films. This also explained the decrease in Marshall residual stability as the amount of PU binder increased. The increase in PU film thickness resulted in more residual isocyanate groups within it for side reactions, thus reducing the stability of PPUC.

Fig. 12.

ATR-IR spectra of PU films (A2:B) under different conditions: (a) Room environment, dry-outer surface; (b) Water immersion, wet-outer surface; (c) Room environment, dry-inner surface; (d) Water immersion, wet-inner surface

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3.6 Void ratio and permeability coefficient of porous concrete

The void ratio and permeability coefficients of porous concrete cast by different binders, dosage, methods and gradations are listed in Table 6.

The cubic specimens were cast by the surface vibration method, while the cylindrical specimens were cast by the Marshall compaction method. The density and void ratio of PPUC changed with different compaction energies. In most cases, for PPUC, the density of the cylindrical specimens was greater than that of the cubic specimens for the same mix design, while the void ratio and permeability coefficient presented the opposite conclusion. This was due to the fact that the energy of Marshall compaction method was higher than that of the surface vibration method. However, for A2-5-G5, the void ratio and permeability coefficients of cylindrical specimens were slightly higher than those of cubic specimens due to the larger size of cubic specimens. This was because the curing process of the PU binder generated air bubbles, which blocked the voids and reduce the permeability inside the cubic specimens. In addition, as the amount of PU binder and fine aggregates increased, the density of PPUC increased, and the void ratio and permeability coefficient decreased. Further analysis of the density and void ratio is presented in the correlation analysis in the next section.

3.7 Correlation analysis

The correlation analysis of the physical parameters and mechanical properties of PPUC using R Studio are shown in Fig. 13. The different parameters of PPUC are on the diagonals in the figure. The scatter plots are illustrated in the upper right panel, and the correlation coefficients are shown in the lower-left panel. As an example of the relationship between density (Density) and total void ratio (R_T), parallel and perpendicular lines are drawn from these two parameters (Fig. 13a, orange lines with arrows), and at the intersection of these two lines, the scatter plot is in the upper right corner, and the correlation coefficient R is in the lower left corner. The value shows a negative linear correlation between density and total void ratio with a correlation coefficient of –0.94. Similarly, the total void ratio and permeability coefficient show a positive linear correlation with a correlation coefficient of 0.96 (Fig. 13a, green lines with arrows).

Fig. 13.

Correlation analysis of physical properties (total void ratio: RT; connected void ratio: RC, permeability coefficient: K) and mechanical properties of PPUC: (a) compressive strength (Fc) and flexural strength (Ff); (b) Marshall stability (standard: MSS, immersion: MSI) and Cantabro loss (standard: SS, immersion: SI)

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From Fig. 13a, there is a good correlation between the parameters of PPUC formed by the surface vibration method. The independent variables are density and total void ratio, and the dependent variables are compressive strength, flexural strength and permeability coefficient. In this study, for each moulding method, the density and total void ratio of PPUC were influenced by gradations and PU dosage. Therefore, the strength properties and permeability coefficient of PPUC were affected by gradations and PU dosage. Similarly, for Fig. 13b, the dependent variables are Marshall stability, Cantabro losses and permeability coefficient. The results show that there was a good correlation between the Marshall stability and density or total void ratio of PPUC, regardless of the test conditions, but the Cantabro loss of PPUC was poorly correlated with them. Furthermore, the results also show that the permeability coefficient correlated well with the density and total void ratio of PPUC. In conclusion, the strength properties, stability and permeability coefficient of PPUC are influenced by the amount of PU dosage and aggregate gradation, while there is no obvious pattern in the influence of factors on the Cantabro losses.

The correlation between density, void ratio and permeability coefficient of porous concrete with different binder types was obtained by linear fitting according to Table 6, as shown in Fig. 14. Table 7 shows the fitted equations and coefficients of determination (COD) of these parameters. It can be seen that there was a linear relationship between the density, total void ratio, connected void ratio and permeability coefficient of porous concrete, regardless of the binder types. The correlation coefficient between total porosity and density was relatively worse (–0.8267), which was mainly due to the different densities of binder types; the density of cement paste is generally around 2.1 g/cm³, while the density of asphalt and PU is around 1 g/cm³. However, the effect of the binder types is not very significant due to the relatively large volume occupied by aggregates and total void ratio. The correlation between the permeability coefficient and the connected void ratio is better than the total void ratio, which is due to the fact that the total void ratio calculated the closed pores in the specimens, which did not contribute to the permeability coefficient. In this study, the density of porous concrete depended mainly on the gradations, the amount and types of binder and the compaction energy. This indicated that different binder types (cement paste, asphalt and PU) as well as dosage, gradations and compaction energy had significant effects on the void ratio and permeability of porous concrete.

Fig. 14.

Correlation analysis of physical parameters of porous concrete: (a) Total void ratio; (b) Connected void ratio; (c) Permeability coefficient

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4. Conclusions

In this study, the comprehensive mechanical properties and water stability of PPUC were investigated, and the water damage mechanism was explored. The main conclusions are summarized as follows.

1. The strength of PPUC was influenced by the chemical structure of component A, the viscosity and dosage of the PU binder and the aggregate gradation. The flexural strength and Marshall stability of PPUC were easier to meet the requirements, while the compressive strength and Cantabro loss were more difficult. Therefore, further research can focus on improving the compressive performance and peeling resistance of PPUC. The main fracture feature of PPUC was the bridge failure between the aggregates, followed by aggregate fracture, while interfacial failure was rarely observed.

2. The reason for the poor residual Marshall stability and immersion Cantabro loss of PPUC lay in the water damage to the PU film. The pull-off strength of the PU binder was reduced by hot water, and exhibited cohesion and adhesion failure. Infrared analysis showed that the residual isocyanate groups within the PU film reacted with water molecules and generated carbon dioxide gas, which reduced the cohesion and adhesion of PU film. Prolonging the curing time of PPUC and not being exposed to water prematurely are possible to improve the water resistance of PPUC.

3. The correlation analysis showed that the compressive strength, flexural strength, Marshall stability and permeability coefficient of PPUC were well correlated with the density and total void ratio of the specimens, while the pattern of effect of Cantabro losses was not clear. As for porous concretes of all kind, the void ratio and permeability coefficient were affected by the types and amount of binder, gradations, and compaction energy.

Further study will focus on improving the compressive performance, peeling and water resistance of PPUC, and promoting the application of PPUC in permeable pavements.

ACKNOWLEDGEMENTS

This work was supported by the Fundamental Research Funds for the Central Universities (No. 22120210027). The authors gratefully thank the Huntsman Chemistry R&D Center for technical support.

Author contributions

The authors confirm contributions to the paper as follows. Study conception and design: J. Yang, H. Li, Y. Zhang; data collection: J. Yang, X. Zuo, Y. Tian; analysis and interpretation of results: J. Yang, B. Yang; draft manuscript preparation: J. Yang, H. Li, John Harvey, Saifullah Mahmud. All authors reviewed the results and approved the final version of the manuscript.

Conflict of interest statement. None declared.

References