Unconfined mechanical properties and micromechanism of short-age recycled aggregate from road solid waste modified by waterborne polyurethane

In recent years, road waste treatment has become a hot topic, and how to solve and utilize these solid wastes is becoming an interesting research field. In order to explore the mechanical modification effect of waterborne polyurethane (PU) on short age road solid waste recycled aggregate, unconfined compression test and scanning electron microscope test were carried out on samples with different dry density and PU content. The dry density was 1.6 g cm−3, 1.7 g cm−3 and 1.8 g cm−3, and the PU content was 0%, 3%, 4%, 5% and 6%, respectively. The tested results show that, the strength of recycled aggregate (RA) increases with the increase of dry density. PU improves the strength and ductility of recycled aggregate (RA), and the residual strength and peak strength of RA increase with the increase of PU content. The peak strength and residual strength of waterborne polyurethane improved recycled aggregate (WPRA) increase with the increase of dry density, too. Polyurethane forms a three-dimensional network structure by film shape and linear shape, and its network structure can provide cohesion and effectively inhibit the crack propagation of the sample. The results demonstrate that adding appropriate amount of PU can effectively improve the mechanical properties of RA, which provides a technical reference for the recycling of RA.


Introduction
A large number of demolition construction wastes are generated in urban municipal reconstruction, including waste concrete, waste brick, solid waste residue of road demolition, waste asphalt and ceramics [1][2][3].At present, landfilling is the main treatment approach of such solid waste, but it not only takes up a lot of land, but also pollutes the environment [4,5].On the other hand, the field of civil engineering is facing gradual depletion of natural building materials resource [6], which requires a more sustainable source of material.Therefore, it is of great engineering significance to use the existing road solid waste (recycled aggregate broken into a certain particle size range) instead of natural building materials for pavement base [7,8].
In recent years, as a new type of building materials, polymer has been gradually used in construction engineering, water conservancy engineering, road subgrade engineering and other infrastructure engineering fields [9][10][11][12].Polymer modified concrete has also been widely applied and innovated, and scholars have conducted extensive research on new polymer building materials, thus demonstrating their superior mechanical properties [13][14][15].Polyurethane is a new kind of organic polymer material with excellent properties and diversified products, which is widely used in various engineering fields.It is a carbamate formed by the reaction of diisocyanate and polyol, which has dual properties of metal toughness and rubber elasticity.Because of its good biocompatibility and non-toxic degradation products [16][17][18], polyurethane has great development prospects in the fields of tissue repair and regeneration, drug delivery, furniture, construction and transportation [16].Polyurethane curing and controlling desertification [17], building polyurethane mortar [18] and polyurethane permeable pavement materials also have great application prospects.As one of the important application materials of polyurethane, waterborne polyurethane curing agent can enhance the mechanical properties and durability of soil as a soil stabilizer.In recent years, it has been gradually applied to pavement materials.
Liu et al conducted unconfined compression test, tensile test and shear test on PU treated sand, and found that the unconfined compressive strength, tensile strength and shear strength of PU treated sand were significantly improved.Samaila et al [20] discussed polyurethane modified marine clay through unconfined compressive strength, organic matter content and pH value, and discovered that the mechanical properties of polyurethane modified marine clay had good effect.Liu et al [21,22] found that the tensile strength of sand samples increased with the increase of polypropylene fiber and PU content through tensile test and SEM microscopic test.The tensile strength of sand was greately improved by the interaction between fiber and sand particles and the polymer binding force between sand particles.Li et al [23] studied the effect of polymer content on the unconfined compressive strength of the sludge, and revealed through microscopic tests that the mechanical properties of the sludge were improved by adding polyurethane mainly through adhesion, wrapping, filling and bridging.Chen et al [24] studied the effect of polyurethane polymer on the mechanical strength of sandy soil of slope vegetation through shear test, unconfined compressive strength and scanning electron microscope images.The results showed that the strengthening effect of sandy soil was more obvious with the increase of polymer content.
Although the mechanical properties of unimproved recycled road solid waste aggregate and the application of waterborne polyurethane in strengthening in situ engineering materials (e.g.sand and sludge) were extensively investigated, few studies were focused on the application of waterborne polyurethane in the modification of road solid waste, which may be treated and then used as foundation filling material.
Compaction quality is one of the most important indexes reflecting the construction quality of road engineering.Different dry density will affect the stability and strength of soil [25,26].In this experiment, different dry densities were designed to analyze the mechanical strength of WPRA.
This study aims to understand the short age mechanical properties and micro mechanisms of recycled aggregate samples from road solid waste with different PU contents through unconfined compression tests and scanning electron microscopy tests, providing technical reference for future green municipal infrastructure construction projects.

Test materials
The recycled fine aggregate used in the test is collected from the 2nd-Ring North-Road, Shaoxing City, Zhejiang Province.When the road subgrade is demolished, the asphalt layer of the road surface is removed first, and then the aggregate of the road base layer is broken by machine.According to the Test Methods of Materials Stabilized with Inorganic Binders for Highway Engineering (JTG e51-2019) [27], the part of fine particles with particle size less than 4.75 mm in the crushed material is selected as the recycled fine aggregate, as shown in figure 1.The geotechnical test of recycled fine aggregate is carried out, and the basic physical properties of soil samples are shown in table 1.
The PU solution used in this experiment is produced by Shenzhen JITIAN Chemical Co., Ltd, and the PU model is F0410.The main component indicators are shown in table 2.

Sample preparation and test plan 2.2.1. Sample plan
Materials with different densities will show elastic-plastic or atypical brittle characteristics, and different compaction methods will also affect the degree of particle close arrangement, which will have a significant impact on the mechanical properties.In order to study the effects of different density and different PU content on the strength and ductility of WPRA, three dry densities and four PU contents were designed in this experiment.The test is divided into 15 groups, with 6 samples in each group.The water content, PU content, different dry density and curing time design of the samples are shown in table 3. PU content in this experiment is mainly based on the solid content of the PU emulsion as a variable.After being diluted with water, it is added to the RA and fully stirred.The PU blending ratio in table 3 is the mass ratio of PU to dry RA.XPUY in table 3 represents the sample number of WPRA, X (%) represents the PU content, and Y (g cm −3 ) represents the dry density.

Sample preparation procedure
According to Test Methods of Soils for Highway Engineering (JTG 3430-2020) [28], the sample preparation steps are as follows: (1) Put RA into a constant temperature oven at 105 °C for 24 h.After drying the moisture, leave the RA to stand and cool to normal temperature.
(2) Weigh the corresponding amount of RA, PU and water according to the mixing ratio of each group of samples, and use a mixer to mix them evenly.The standard specimen size for unconfined compression test is a cylinder with 50 mm in height and 50 mm in diameter.(3) After the sample is made, it is put into the curing box for standard curing for 7 days, the curing temperature is 20 °C ± 2 °C and the humidity is higher than 95%.

Test method 2.3.1. Unconfined compression test
According to Test Methods of Soils for Highway Engineering (JTG 3430-2020) [28], after curing the sample for 7 days, the unconfined compression test is carried out.The fully automatic unconfined compression tester produced by Nanjing TKA Technology Co., Ltd is adopted.During the test, the sample is placed on a metal sample stand with smooth bottom and an epoxy resin indenter is placed on the top to minimize the end effect.The axial displacement loading speed of the test instrument is controlled to 1 mm min −1 .For each group of samples, six samples are tested under the same conditions to check the consistency of the test.

Microscopic test
In order to reveal the strengthening mechanism of WPRA reinforced by PU, SEM test was carried out.JSM-6360LV type tungsten filament high and low vacuum scanning electron microscope produced by Japan Electronics Co., Ltd was used.Before testing, a layer of conductive adhesive should be coated on the special aluminum tray of the SEM instrument, and the dry granular samples should be adhered to the conductive adhesive, so as to ensure that the samples are close to the adhesive surface.Due to the poor conductivity of geotechnical materials, the samples should be sprayed and coated with a layer of platinum film by ion diffraction technology.The platinum plated sample test stand is placed in the instrument to adjust the lens height.First, locate the approximate area of the sample at low magnification, then manually adjust the magnification, then adjust the definition, and finally analyze the microstructure of WPRA particles by SEM images obtained by scanning electron microscope.

Test results and analysis
3.1.Unconfined compressive strength 3.1.1.Effects of dry density on the unconfined compressive strength of RA Figure 2 shows the RA stress-strain curves with different dry densities, from which the peak strength of 0PU1.6, 0PU1.7 and 0PU1.8 are 438kPa, 488kPa and 593kPa, respectively.The peak strength of RA increases with the increase of dry density, and the corresponding strain values of the peak strength of three kinds of dry density RA are between 3% and 4%.The compressive strength of RA with 0% PU content is mainly provided by the bite force and friction between particles.

Effects of PU content on the unconfined compressive strength of WPRA
Figure 3 shows the stress-strain curves of WPRA with different contents of PU1.6, and figure 4 shows the variation of peak strength and residual strength.According to the geotechnical test specification (JTG 3430-2020) [28], the stress value corresponding to 3% peak strain is selected as residual strength.As shown in figure 4, when the content of PU is low, compared with RA, the strength of WPRA increases slightly, and the strength of 3PU1.6 is only 591kPa.The reason is that the content of PU is low, which does not provide strong cohesive force between RA particles.Compared with 3PUI1.6, the peak intensity of 6PU1.6 is increased from 591kPa to 1908kPa, with an increase of 223%.With the increase of PU content, the peak strength and residual strength of WPRA increases gradually.The peak strength of 3PU1.6, 4PU1.6,5PU1.6 and 6PU1.6 increases by 135%, 226%, 347% and 436% respectively compared with that of 0PU1.6.The strength of WPRA is mainly provided by the intercalation between particles and the bonding strength of PU curing agent.PU curing agent is easy to solidify when exposed to air.After hardening into polyurethane, RA is cemented as a whole.At the same time, PU reacts with water on the surface of RA particles.The polyurethane contains a large number of long molecular chains and isocyanate groups, which bond RA particles into a network structure, thus improving the strength of WPRA.The special polymer network structure of PU curing agent can improve the cohesion of RA [29].The stress-strain curve of WPRA in figure 3 is smoother than that of RA in figure 2, and its peak strain increases gradually with the increase of PU content, which indicates that RA after PU treatment shows good ductility, which is mainly due to the so-called incomplete fracture of bridges [30].RA after PU treatment will form connecting bridges between particles, which can inhibit the generation and development of cracks.The variation pattern of WPRA with different PU content in this experiment is consistent with the results of Jin Liu's polyurethane stabilized sand experiment [31].In the PU polymer stabilized sand experiment, the PU content is  1%, 2%, and 4%, corresponding to peak strengths of 78kPa, 234kPa, and 258kPa, respectively.As the PU content increases, its peak strain gradually increases.Due to the loose and porous physical properties of sand compared to road solid waste recycled aggregates, the peak strength of polyurethane modified recycled aggregates is more significant than that of polyurethane modified sand.Therefore, the strength and ductility of RA can be improved by adding PU.

Effects of dry density on the unconfined compressive strength of WPRA
Figure 5 shows the stress-strain curves of WPRA with different content of PU1.7, and figure 6 shows the variation of peak strength and residual strength.As shown in figure 5, the overall strength of different content of PU1.7 is higher than that PU1.6, and its peak strain is larger than that of PU1.6, and the ductility can be improved by increasing the dry density.As shown in figure 6, compared with the same content of PU1.6, the peak strength of 3PU1.7,4PU1.7,5PU1.7 and 6PU1.7 increases by 84%, 76%, 79% and 69% respectively, and the residual strength increases by 77%, 92%, 43% and 57%, respectively.The experiment on the tensile properties of polyurethane organic polymer and polypropylene fiber composite modified sand [21] also analyzed the influence of different dry densities on mechanical strength.The dry density values were set at 1.4 g cm −3 , 1.5 g cm −3 , and 1.6 g cm −3 , respectively.The experiment showed that the tensile strength also increased with the increase of dry density, with an increase of between 18% and 22%.For samples with lower dry density, the gaps between sand particles were larger, resulting in insufficient filling of PU.It is difficult to fully  utilize the bonding performance of PU, resulting in weak connection between sand particles and limiting the improvement of tensile strength.As the dry density increases, when the sample has a higher dry density, greater compaction is required during the preparation process, which will generate more contact.The polyurethane film will be more conducive to the internal connection force and interface mechanical interaction of the sample.The reason why increasing the dry density can increase the strength of WPRA is that higher dry density makes WPRA particles more compact, enhances the interlocking force and friction between particles.At the same time, particles are squeezed by external forces, which makes PU emulsion more permeable to WPRA particles and pores, thereby enhancing the mechanical properties of WPRA.
Figure 7 shows the stress-strain curves of WPRA with different contents of PU1.8, and figure 8 shows the variation of peak strength and residual strength.As shown in figures 6 and 7, the peak strength of WPRA increases with the increase of PU content.The peak strength of 3PU1.8 is 2536kPa, which is about 2.6 times higher than that of 3PU1.7.The peak strengths of 4PU1.8, 5PU1.8 and 6PU1.8 are 3002 kPa, 3296 kPa and 3430 kPa respectively; the residual strength is 179 kPa, 497 kPa and 805kPa respectively.When the dry density of WPRA is increased to 1.8 g cm −3 , the peak strength increases with the increase of PU content, but the increase is not significant.The reason for the increase of WPRA strength is the effect of dynamic compaction on the interfacial friction caused by the dense arrangement of RA particles.At the same time, PU emulsion can better fill the pores of recycled aggregate particles under strong compaction.The peak strength of WPRA tends to increase with the increase of PU content.However, when the dry density is relatively high, the extrusion phenomenon of Figure 7.The stress strain curve of WPRA with dry density of 1.8 g cm −3. .PU emulsion exists in the compaction process with high PU content.This is because the total volume of PU and water exceeds that of the voids of compacted RA particles.In this case, the excess PU will aggregate outside the sample and form a solid covering, but the internal structure cannot be better enhanced [32].In the case of higher dry density, when the PU content reaches a certain value, the compressive strength of WPRA will gradually tend to a certain value, and the peak strength of the sample does not increase significantly.Therefore, the dry density of 1.8 g cm −3 and PU content of 5% can achieve the maximum engineering value of WPRA.

Microstructure analysis
In order to further analyze the strength increasing mechanism of WPRA, scanning electron microscope (SEM) tests were carried out on WPRA with different PU content.The main purpose is to analyze the modification effect of PU material on WPRA microstructure, and to compare with WPRA macro mechanical properties, and to discuss its fracture mechanism at the same time [33].Figure 9 is SEM micrograph of RA at different multiples, figure 10 shows the SEM images of WPRA with different times of PU content of 3%, 4%, 5% and 6%.
Figure 9(a) is the SEM image of RA particles at 50 times, it can be seen that there are many pores and cracks on the microstructure surface of RA.Figures 9(b) and (c) are the SEM images of RA particles at 500 and 2000 times.It can be seen that many small particles are adhered to the surface of the particles.
Figure 10(a) is a SEM image of 500 times of 3% PU, which shows that linear polyurethane bridges the particles well, and its effect is similar to that of fiber reinforcement.Figure 10 (b) is a SEM image of 500 times of 6% PU, which shows the trace of tensile fracture of linear polyurethane.When the specimen is subjected to external load in the unconfined compression test, the linear polyurethane will provide excellent tensile resistance, thus inhibiting the failure of the specimen and hinder the crack propagation.Figures 10(c), (d), (e) and (f) are the SEM images of WPRA with different content of PU at 2000 times.It shows that there are a lot of polyurethane gels attached to the particle surface, and polyurethane encapsulates many fine powder particles and adheres to the particle surface.At the same time, polyurethane can accumulate on the surface of RA particles, better fill the pores, and reduce the porosity of WPRA.Polyurethane exists in the form of film and encapsulates the whole large particles.Based on this adhesion and encapsulation, it provides strong adhesive strength, and the connection between particles is provided by the elasticity and tensile strength of polyurethane.The adhesion between the particles is improved by the polyurethane, which provides the basis for the RA particles to connect with each other as a whole.From the SEM images of WPRA 2000 times with different PU content, it can be seen that with the increase of PU content, the polyurethane film becomes thicker and thicker, and the adhesive force between the particles becomes larger and larger, which can fully explain that the peak strength of the sample increases with the increase of PU content.
According to the above SEM image analysis, it can be concluded that polyurethane can fill the pores and particle gaps on the particle surface, and exist in the form of line and film, directly connecting the two distant particles.Therefore, the curing method of polyurethane can be attributed to the surface shape, pore filling and particle bridging.Combined with the research of Song et al [34], it can be concluded that the membrane polyurethane and linear polyurethane constitute three-dimensional network structure.The polyurethane network structure and RA particles form a cross structure which runs through and relies on each other to form a spatial cross structure.When the WPRA is subjected to loads, the structure will provide cohesion and inhibit the crack expansion.In order to better understand the interaction between polyurethane and RA, a simplified schematic diagram was drawn.The schematic diagram is obtained by SEM micrograph and mechanical test, as shown in figure 11.Compared with RA, the strength of WPRA mainly depends on the intercalation force between particles, friction and polyurethane network structure.

Conclusion
Unconfined compression test and SEM test were carried out on WPRA samples.Three kinds of dry density and four kinds of PU content were designed as variables to explore the effect of their interaction on the mechanical properties of WPRA.The microstructure and mechanism of WPRA were revealed by SEM.According to the test results, the main conclusions are drawn as follows: (1) The compressive strength of RA is mainly provided by the interlocking force and friction among particles.
The peak strength of 0PU1.6, 0PU1.7 and 0PU1.8 are 438kPa, 488kPa and 593kPa, respectively.The strain values corresponding to the peak stress of the three samples with different dry densities are between 3% and 4%.
(2) The addition of PU can significantly improve the mechanical properties of WPRA.The peak strengths of 3PU1.6, 4PU1.6,5PU1.6 and 6PU1.6 are 591 kPa, 990 kPa, 1521 kPa and 1907 kPa, respectively, which are 135%, 226%, 347% and 435% higher than that of 0PU1.6, respectively.The strength of WPRA increases with the increase of PU content, and the strain value of peak strength gradually increases with the increase of PU content, indicating that WPRA after PU treatment shows good ductility.
(3) Different dry density can significantly affect the mechanical properties of WPRA samples.The peak strength and residual strength of WPRA increase with the increase of dry density, but when the dry density reaches a certain limit, the peak strength of WPRA cannot be improved by continuously increasing the PU content.It is found that dry density of 1.8 g cm −3 and PU content of 5% can achieve the maximum engineering value of WPRA.
(4) The micro images of WPRA reveal the reinforcement mechanism of PU.The film like polyurethane and linear polyurethane can form a three-dimensional network structure together.The polyurethane network structure and RA particles form a three-dimensional intersection structure, which will provide cohesion and inhibit crack expansion under the load of WPRA.

Figure 1 .
Figure 1.The recycling production process of RA.

Figure 2 .
Figure 2. The RA stress-strain curves with different dry densities.

Figure 3 .
Figure 3.The stress strain curve of WPRA with dry density of 1.6 g cm −3 .

Figure 4 .
Figure 4.The peak strength and residual strength of WPRA with dry density of 1.6 g cm −3 .

Figure 5 .
Figure 5.The stress strain curve of WPRA with dry density of 1.7 g cm −3 .

Figure 6 .
Figure 6.The peak strength and residual of WPRA with dry of 1.7 g cm −3 .

Figure 8 .
Figure 8.The strength and residual strength of WPRA with dry density of 1.8 cm −3 .

Figure 9 .
Figure 9.The SEM micrographs of different multiples of RA.

Figure 10 .
Figure 10.The SEM micrographs of different multiples of WPRA.

Figure 11 .
Figure 11.The simplified diagram of (a) unimproved RA and (b) WPRA (the pores among particles are filled with polyurethane).

Table 1 .
The physical properties of RA.

Table 2 .
The physical properties of PU.

Table 3 .
The sample plan of WPRA.