J. Cent. South Univ. (2021) 28: 1829-1842

DOI: https://doi.org/10.1007/s11771-021-4733-7

Water-induced changes in strength characteristics of polyurethane polymer and polypropylene fiber reinforced sand

WANG Ying(王颖)¹, LIU Jin(刘瑾)¹, SHAO Yong(邵勇)², MA Xiao-fan(马晓凡)¹,QI Chang-qing(祁长青)¹, CHEN Zhi-hao(陈志昊)¹

1. School of Earth Sciences and Engineering, Hohai University, Nanjing 210098, China;

2. Jiangsu Taihu Lake Control Engineering Construction Administration, Department of Water Resources of Jiangsu Province, Wuxi 214000, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021

Abstract: As a new kind of air-hardening soil reinforcement material, polymer is being widely applied in river-bank slope reinforcement and ecological slope protection area. Thus, more attention should be paid to study the characteristics of reinforced soil after immersion. In this study, water-induced changes in strength characteristics of sand reinforced with polymer and fibers were reported. Several factors, including polymer content (1%, 2%, 3% and 4% by weight of dry sand), immersion time (6, 12, 24 and 48 h), dry density (1.40, 1.45, 1.50, 1.55 and 1.60 g/cm³,) and fiber content (0.2%, 0.4%, 0.6% and 0.8% by weight of dry sand) which may influence the strength characteristics of reinforced sand after immersion were analyzed. The microstructure of reinforced sand was analyzed with nuclear magnetic resonance (NMR) and scanning electron microscope (SEM). Experimental results indicate that the compressive strength increases with the increase of polymer content and decreases with the increase of immersion time; the softening coefficients decrease with the increase of the polymer content and immersion time and increase with an increment in density and fiber content. Fiber plays an active role in reducing water-induced loss of strength at 0.6% content.

Key words: polymer; fiber reinforced sand; immersion; compressive strength; softening coefficient

Cite this article as: WANG Ying, LIU Jin, SHAO Yong, MA Xiao-fan, QI Chang-qing, CHEN Zhi-hao. Water-induced changes in strength characteristics of polyurethane polymer and polypropylene fiber reinforced sand [J]. Journal of Central South University, 2021, 28(6): 1829-1842. DOI: https://doi.org/10.1007/s11771-021-4733-7.

1 Introduction

As a raw material, sand is widely used in construction engineering because of its vast distribution, high strength and low compression. However, sand is seldom separately used as a building material, as its loose structure, low cohesion and no plasticity. Therefore, it is usually made into a composite material by mixing other materials like cement, lime and fly ash with sand to meet the need for practical engineering [1-3]. These reinforced materials can make sand form a composite material with high strength. However, these composite materials are hard materials. Brittle failure always easily occurs with greater destructiveness, and the strength will be greatly reduced as corroded. Apart from these, some traditional reinforced materials may have a negative effect on the surrounding environment [4-6].

As a result, many scholars are devoted to studying some new materials to reinforce soil.Polymer as a brand-new material which can be utilized to reinforce soil has attracted the attention of numerous scholars. LIU et al [7] carried out laboratory tests and the results show that sand mixed with organic polymer can form composites with superior strength. SONG et al [8] studied the water properties, mechanical properties, durability and seed growth of soil reinforced with vinyl acetate polymer by laboratory tests. REZAEIMALEK et al [9] studied the impact of curing environment and polymer content on the performance of sand mixed with the soil stabilizer. LU et al [10] found the rule that the strength properties of alkali-activated geopolymers vary with the water-to-binder ratio and sodium sulfate content. Additionally, resins and gypsum are also good additives to make the soil form a composite material with great performance [11-13]. These studies demonstrate that the non-traditional chemical additives can be added into soil and change their strength and water stability characteristics.

Moreover, with the deepening of research on soil reinforcement, some scholars have found that the strength properties of soil can be further improved by adding fibers to the soil, and the composite materials can also have a positive degree of toughness, and can resist a certain degree of crack and earthquake damage [14-16]. CHEBBI et al [17] proposed the method of using short and randomly distributed fibers to evaluate its effect on tensile behavior of compacted clayey soil. TANG et al [18] mixed polypropylene fibers with clayey soil,and found that polypropylene fibers can reduce the desiccation cracking behavior of clayey soil. PHANIKUMAR et al [19] presented the swell-consolidation characteristics of fiber-reinforced expansive soil, and the results indicate that fibers can reduce swell potential and the secondary consolidation characteristics of expansive soil. TANG et al [20] found that fibers can effectively enhance the tensile strength of soil, and the interfacial mechanical behavior of fiber and soil was also affected by the shape of fiber. By adding fibers into soil can also obviously improve the compressive and tensile strength of soil, and the fibers can play a role of reinforcement, which can provide cohesion and pulling force [21].

These two types of soil reinforcement materials have their own advantages. Therefore, some scholars have made geopolymer-fiber-soil composite materials by adding geopolymers and fibers to the soil at the same time to ensure that they have higher strength and toughness [22-24]. CHEN et al [25] evaluated the effectiveness of polypropylene fibers on the strength of fiber-reinforced and cement-stabilized Shanghai soft clay by laboratory tests. LIU et al [26] studied the strength characteristics of polymer-basalt fiber-sand composite material, but the influence of water on the strength of this composite material was ignored. JAMES [27] investigated the potential of sugarcane press mud (PM) as a secondary additive in conjunction with lime for the stabilization of an expansive soil. LI et al [28] investigated the mechanical behaviors of fiber-reinforced fly ash-soil mixture, and found that 30% fly ash and 1% fiber provides the optimum content, and the inclusion of fiber reinforcement has positive benefits on the mechanical properties of the reinforced soil.

The above researches mentioned that the performances of reinforced soil were evidently influenced by geopolymer and fibers content. With the in-depth research, some studies based on the effect of complex environment on improved soil attracted many scholars’ attentions [29, 30]. Their findings show that dry-wet cycle and freeze-thaw cycle have great influence on the strength and deformation characteristics. Some scholars studied the immersion-induced changes in structure and properties of cement and the California bearing ratio (CBR) of lime improved expansive soil. The research proved that the strength of materials is strongly affected by immersion [31, 32]. Therefore, the studies on the performance of reinforced soil after immersion should also be valued.

The current researches mainly focus on the strength characteristics of improved soil under the dry condition, while the influence of water on the improved soil was ignored. Polymer is widely used in river bank slope reinforcement and ecological slope protection, and water has a significantly effect on the performance of reinforced soil. Thus, more attention should be given to the structure and properties of reinforced soil after immersed. The compressive strength is an important index which can affect the stability of slope especially under external loads. In this paper, a new environmental-friendly polymer was used to reinforced sand. The natural sand is mixed with polyurethane polymer (PUR) and polymer-polypropylene fiber (PPF), respectively, to form a composite material. And strength and softening coefficient of the composite material are investigated by methods of laboratory tests. Properties of the polymer-improved sand after immersion were studied by changing polymer content, immersion time and density. The polypropylene fiber was used to enhance the softening coefficient of improved sand, and the influence of polypropylene fiber on polymer-sand composite material after immersed was also evaluated by changing polypropylene fiber content. Finally, the microstructure change of the composite materials was analyzed in-depth by nuclear magnetic resonance (NMR) and scanning electron microscope (SEM). The research findings can provide reference for the studies of reinforced soil after immersion, and fibers can be adopted as a good material to reduce the water-softening characteristic of improved soil.

2 Materials and methods

2.1 Materials

Sand as a main fraction in the composite material was taken from Nanjing, China. The basic physical properties of sand are given in Table 1, and the particle size distribution curve was obtained by particle analysis experiment (Figure 1).

In this study, polyurethane polymer was used as a stabilizer to connect the loose sand into a whole.

Table 1 Basic physical properties of sand

Polyurethane polymer is a light-yellow transparent liquid consisting of a large number of long chains of macromolecule. After being mixed with water, the polymer reacts with the water molecule forming a milky white solution. The detailed reinforcement mechanism of polymer will be discussed in following section. And the structural formula of the prepolymer is as shown in Eq. (1). The basic physical and chemical properties of polyurethane polymer are shown in Table 2.

Polypropylene fiber (PF) shown in Figure 2 is used as a material to reinforce the polymer-sand material. Table 3 shows the physical properties of the polypropylene fiber in detail.

(1)

2.2 Sample preparation

The processes of preparing PUR-sand and PUR-PPF-sand composite material preparation are as follows. Firstly, the requisite weight of dry sand was mixed with different contents of PPF evenly first. Secondly, 10% of water (by the weight of dry sand) and the proposed PUR was mixed with a blender to obtain PUR-water mixture. After that, PUR solution was mixed with PPF-sand mixture to form PUR-PPF-sand mixture. Finally, the prepared mixture was transferred into the mold, and static pressure was applied to form the sample with a diameter of 39.1 mm and a height of 80 mm. The samples were kept in a 20 °C incubator, after cured for 48 h (the weight of samples almost unchanged). The sample were placed in a container filled with water (samples were completely immersed in water). After cured samples being immersed for different time, further tests will be carried out.

Figure 1 (a) Sand used in this study; (b) Particle size distribution curve

Table 2 Basic physical and chemical properties of polyurethane polymer

Figure 2 Polypropylene fiber and PUR-PPF-sand mixture

Table 3 Physical and mechanical indices of polypropylene fibers

2.3 Test program

In order to study the water-induced changes in structure and properties of sand mixed with different contents of PUR (p_c), 1%, 2%, 3%, 4% (by the weight of dry sand) contents of PUR were chosen. The immersion time (I_t) of 6, 12, 24 and 48 h was selected to investigate the effect of immersion time on the structure and properties of polymer-sand material. The properties of polymer-sand mixture with different densities (ρ_d) (1.40, 1.45, 1.50, 1.55 and 1.60 g/cm³) were also studied. To investigate the effect of PPF content (f_c) on the properties of polymer-sand material, PPF contents of 0.2%, 0.4%, 0.6% and 0.8% (by the weight of dry sand) are used in the experiments. The detail test program is given in Table 4.

Table 4 Parameters of test program

2.4 Tests of strength and structure of sample

The unconfined compression test was carried out to study the compressive strength of samples before and after immersion. Therefore, six samples (three parallel samples before and after immersion, respectively) were developed for each set of tests. YYW-2 strain controlled unconfined pressure gauge (Nanjing soil instrument factory Co. Ltd., Nanjing, China) was used in this test, and the loading rate was kept at 2.4 mm/min.

Scanning electron microscopy (SEM) was used to record the internal microstructure and fiber reinforcement surface of the PUR and PF-reinforced sand. The distribution of water among the sample was imaged by nuclear magnetic resonance (NMR).

Softening coefficient (K_s) was used to evaluate the water resistance of reinforced sand. The formula of softening coefficient is as following:

                                  (4)

where σ_w is the compressive strength of immersed sample, kPa; σ_d is the compressive strength of dry sample, kPa. The higher the K_s is, the greater the water resistance of composite materials is.

3 Results and analyses

A series of laboratory tests were carried out on the sand composite material after immersion. Several factors which may affect the strength and softening coefficient (K_s) were investigated in detail, including the polymer content (p_c), immersion time (I_t), dry density (ρ_d) and fiber content (f_c).

3.1 Compressive strength

Figure 3(a) displays the relationship between compressive strength and immersion time. The results presented in Figure 3(a) shows that the compressive strength of samples before immersion increased significantly with the increasing of polymer content. However, the strength of samples with different content of polymer reduced by 61.34%, 80.56%, 82.93% and 83.37%, respectively after being immersed for 6 h. It also can be seen from Figure 3(a) that immersion-induced loss of compressive strength is obviously, and it is more apparent as sand mixed with high content of polymer. In addition, the strength of the sample with 1% content of polymer decreases with the increase of immersion time, which is not under 40-50 kPa. And from Figure 3(a), it can also be observed that the immersion time has a slight effect on the strength reduction of samples (p_c³2%) after 6 h of immersion. Figure 3(a) also displays that, the higher the polymer content (p_c³2%), the higher the compressive strength curves appear at different immersion time, and the compressive strengths decrease slightly with the increase of immersion time.

Figure 3 (a) Relationship between compressive strength and immersion time; (b) Relationship between compressive strengths of samples and density;(c) Relationship between compressive strengths and fiber content

The compressive strengths of samples before and after immersion varying with density are displayed in Figure 3(b). The higher the density of the sample is, the higher compressive strength appears. The compressive strengths of samples before immersion increase linearly with the increase of density until the density increases to 1.55 g/cm³. With the increase of densities from 1.40 to 1.45 g/cm³, 1.45 to 1.50 g/cm³ and 1.50 to 1.55 g/cm³, respectively, the compressive strengths of samples before immersion increase by 11.02%, 11.72% and 8.32%. However, the compressive strength of samples increases only by 3.17% from 241.51 to 248.30 kPa as the density increases from 1.55 to 1.60 g/cm³. It can also be seen in Figure 3(b) that the compressive strengths of samples after being immersed for 24 h increase uniformly with density (ρ￡1.55 g/cm³), but as density increases from 1.55 to 1.60 g/cm³, the compressive strength of samples after being immersed 24 h has a greater increase compared to samples with smaller density.

Figure 3(c) demonstrates the variation in compressive strength of the samples mixed with different contents of fiber. As shown in the Figure 3(c), the compressive strengths of fiber-reinforced samples before immersion increase obviously with the increase of fiber content. In other words, fibers play an important role in improving the strengths of sand-polymer composite material. For instance, the compressive strength of samples at the fiber content of 0.2%, 0.4%, 0.6% and 0.8% increase by 16.34%, 26.91%, 66.75% and 83.36%, respectively, compared to that of samples without fibers. Moreover, the fiber-induced increments in the strength of immersed samples are more significantly than that of samples before immersion (shown in Figure 3(c)). According to Figure 3(c), it can also be noted that with the increase of fiber content from 0 to 0.2%, 0.2% to 0.4%, 0.4% to 0.6% and 0.6% to 0.8%, the compressive strengths of immersed samples increase by 194.42%, 16.70%, 36.59% and 10.41%, respectively. Based on these results, it can be summarized that the effect of fiber content on improving strength of samples becomes slight as fiber content reaches 0.6%, in agreement with the results in Ref. [31].

3.2 Stress-strain curve

The typical stress-strain curves of samples before immersion with different polymer contents obtained from unconfined compression tests are presented in Figure 4. As illustrated, the contribution of polymer increase to peak axial stress is significant. For samples with different polymer contents, the axial stress all increased with the increase of axial strain exponentially until reached peak axial stress, and decreased significantly after that (Figure 4). These samples behave with significant brittle failure as noted that strain softening characteristics as shown in stress-strain curves of samples. It can also be observed that samples with higher polymer content have small axial strain corresponding to the peak axial stress. The peak axial strain (ε_p) has a well linear relationship with polymer content: ε_p=-0.85p_c+11.25, ε_p is the peak axial strain. High polymer content contributes sand to form a more brittle composite as it can reach peak strength at small axial strain and have small residual strength.

Figure 5 shows the typical stress-strain curves of specimens treated with different polymer contents immersed for different time. It can be noted that an increase in immersion time leads to a decrease in axial stress. However, as can be observed in Figure 5, the stress-strain curves of samples with 1% content of polymer vary with immersion time. Longer immersion time results in the slight decreases of axial stress after peak axial stress. At higher polymer content (p_c≥2%), the stress-strain curves of samples immersed in different time share the similar overall shape the reduction in axial stress after peak stress is slight; and immersion time has no significant effect on the loss of post-peak stress. In addition, at higher polymer content, the peak axial strains increase with the increase of polymer content, the peak axial strains of samples mixed with 2%, 3% and 4% polymer are at 5%-6%, 6%-7% and 10%-13%, respectively. As shown in Figure 6, the difference in the size of the cracks in the surface of four immersed specimens after compressional failure is observed. The samples with relatively low polymer content (i.e., 1% and 2%) have an obvious crack that comes through the surface of samples (Figures 6(a) and (b)). In contrast, these obvious cracks dwindle with increasing polymer content and have relative intact shape after pressure relieved (Figures 6(c) and (d)), indicating that immersed samples with high polymer content present ductile failure characteristics. Thus, samples with high polymer content after immersion still have enough strain resistance, which is in agreement with stress-strain curves of corresponding specimens.

Figure 4 Typical stress-strain curves of specimens with varying polymer contents

The typical stress-strain curves of specimens with different density and mixed with different contents of fibers are represented in Figure 7. The curves of samples with the lower density lies below than that of higher density, and high-density results in an obvious increase in peak axial stress. The overall shape of stress-strain curves of samples before immersion is not significantly affected by the increase of density. However, stress-strain curves of immersed samples present completely different shape. An increment in density leads to small axial strains when samples achieve the peak axial stress. Apart from this, the loss of post-peak stress increased with the increase of density. It indicates that samples with higher density have more strength reductions after immersion. It can also be noted in Figure 7 that the contribution of increasing fiber content of axial stress is significant to samples before and after immersion. With the increase of fiber content,the samples change from strain softening to strain hardening. Additionally, axial stress performed an increasing trend with increasing fiber content, because the adding of fiber make sample become a ductile material. It can be explained that fiber plays the role of micro-anchor among sand particles which effectively limit the lateral deformation of the samples (Figure 8), leading to elastic deformation characteristic of reinforced sand.

Figure 5 Typical stress-strain curves of specimens treated with different polymer contents immersed for different time

Figure 6 Failure Characteristics of samples mixed with different polymer contents immersed for 24 h:

3.3 Softening coefficient

The relationship between immersion time and softening coefficient at same polymer content is presented in Figure 9(a). The lower the softening coefficient is, the greater the strength loss is. The curve of samples with high polymer content consistently lies above that of low polymer content at each immersion time, indicating that samples mixed with high content polymer have greater strength loss. However, the evolution curve of the softening coefficient of samples mixed with 1% content of polymer shows obvious difference to that of samples mixed with 2%, 3% and 4% polymer content. For instance, the softening coefficient of samples mixed with 1% content polymer decreases significantly with increasing immersion time. And the softening coefficients of samples mixed with high polymer content (i.e., 2%, 3% and 4%) share similar trends as immersion time increases. Immersion-induced decreases in softening coefficient are not very obvious at high polymer content. In addition, the curves approach each other with the increase of polymer content, indicating that the influence of polymer content on softening coefficient is reduced at higher polymer content.

Figure 7 Typical stress-strain curves of samples with different densities and different contents of fibers

Figure 8 Failure characteristics of samples treated with polymer and fiber:

Figure 9(b) shows the relationship between softening coefficient of samples and density and fiber content. It can be seen in Figure 9(b) that density has an obvious effect on the softening coefficient of samples. Increasing density effectively increases the softening coefficient. At higher densities (ρ>1.55 g/cm³), the density-induced increases in the softening coefficient are more significant than that at lower densities (ρ≤1.55 g/cm³). With the increase of densities from 1.40 to 1.45 g/cm³, 1.45 to 1.50 g/cm³ and 1.50 to 1.55 g/cm³, the softening coefficient increased by 11.97%, 16.51% and 12.15%, respectively. However, as density increases from 1.55 to 1.60 g/cm³, the softening coefficient increased by 47.38%. It indicates that samples with high density have better resistance to water softening. Figure 9(b) also displays the variation in softening coefficient of the samples mixed with different contents of fiber. As shown in Figure 9(b), by adding fiber into sand-polymer composite material, the softening coefficients of samples have apparent improvement. The softening coefficient of samples mixed with 0.2%, 0.4%, 0.6% and 0.8% increased by 147.37%, 163.16%, 173.68% and 178.95%, respectively, compared to samples without fiber. According to Figure 9(b), it can also be noted that as the fiber content reaches 0.6%, the role that fiber play in improving the softening coefficient is weakened (the softening coefficient increased only by 1.92% when fiber content increased from 0.6% to 0.8%). It can be concluded that fibers can enhance the ability of resisting immersion softening, and fiber has an obvious effect on improving the softening coefficient of samples at content of 0.6%.

Figure 9 (a) Variation of softening coefficients with immersion time for different content of polymer;(b) Softening coefficient varies with density and fiber content

4 Mechanism analyses, discussion and applications

4.1 Mechanism analyses and discussion

Polyurethane polymer is a self-emulsifying polymer with a large number of hydrophilic groups on its surface. The chemical reaction process is that the isocyanate group (R-NCO) of polyurethane prepolymer first reacts with water forming amine (R-NH₂) and carbon dioxide (CO₂). The active hydrogen of amine worked as curing components reacts with isocyanate groups, finally leading to a three-dimensional cross-linked insoluble polymer membrane. Polymer solution will form a uniform curing membrane among sand particles after 48 h incubator curing, and this curing membrane formed between sand particles became thicker with the increasing of polymer content (Figure 10). Therefore, the loose sand particles were joined together by these membranes, and the pores between sand particles were also filled with these membranes. The polymer membrane formed between particles was a flexible material which has the ability of absorbing water and volume expansion, and the more content of polymer is, the stronger this ability shows. Moreover, the longer time samples immersed in the water, polymer membrane has more time to react with the water, which absorbed more water.

Figure 11 shows the nuclear magnetic resonance (NMR) images of polymer-sand composite material (mixed with 2% content of polymer) after being immersed for different time. The water distribution in samples from different immersion time can also be observed in Figure 11. The bright spot in the NMR image represents water. Therefore, the brighter the color of Figure 11 is, the more the water is. As shown in Figure 11, with the increase of immersion time, the signal of NMR is becoming stronger. It indicates that the samples absorb more water. Apart from these, it can be noted that the signal intensity of water is uniformly distributed in the sample. It indicates that the polymer membrane well distributed among sand particles.

Figure 10 SEM images of sand mixed with different content of polymer:

Figure 11 NMR images of sand mixed with 2% polymer after immersed different time:

Sand particles are wrapped up by more polymer membranes as more polyurethane polymer was added (Figure 10), and therefore, the loose sand particles were connected to form a whole. As a result, the strength of the sample increases significantly with the increase of polymer (Figure 3). However, polymer membrane has well water absorption ability. Thus, the polymer membranes among sand particles become softer after samples immersed, and the higher the content of polymer is, the more water the membranes absorb. Finally, the bonding force of polymer membranes between sand particles greatly decreases. Therefore, the strength of samples mixed with more content of polymer obviously decreases after immersion. Thus, the softening coefficient of the sample decreases with the increase of polymer content (Figure 9(a)). With the increase of samples’ density, the connection between sand particles is closer, which makes the sand become a whole composite material with higher strength (Figure 3(b)). Even the sample is immersed in water, the compressive strength still increases with the increase of density. As a result, the softening coefficient increases with the increase of density (Figure 9(b)).

Schematic diagram of reinforcement mechanism of sand-polymer-fiber composite is illustrated in Figure 12. It can be seen in Figure 12 that fiber twines around and links sand particles, and fibers have an effect of reinforcement and form a spatial reticular structure among samples. This spatial reticular structure can restrict the lateral expansion of the sample when it is subjected to vertical pressure, so the adding of fiber can effectively increase the strength of sand-polymer composite material. In addition, fibers can reduce the voids among samples. Therefore, even the sand-polymer-fiber is immersed, a small amount of water enters the sample; As a result, the strength of fiber-reinforced polymer-sand composite material decreases slightly after immersed. Apart from this, expansion of specimens was bound by spatial reticular structure formed by fibers. Even though the polymer membrane was softened after samples immersed, fibers can play a role of increasing cohesion of samples, and fibers can provide tension, which limits the displacement of sand particles when samples have lateral deformation [33, 34]. Finally, the compressive strength of the immersed sample mixed with fiber is significantly higher than that of immersed samples without fibers. And the immersion-induced loss of strength of samples mixed with fiber is far less that of sand-polymer composite material. Thus, the softening coefficient of the sample increases with the increase of fiber content.

4.2 Applications

The application of polyurethane polymer and fiber in the actual field is constrained due to its technical and cost problems. The present cost of polyurethane polymer (i.e., 60-70 RMB/kg) and polypropylene fiber (i.e., 5-10 RMB/kg) used in our experiment would require higher budgets compared to the traditional construction materials (e.g., cement) to treat soil in a unit area. With reference to the test results, using 3% polyurethane polymer and 0.6% fiber for sand stabilization would cost between 120 and 180 RMB/m³. In terms of application technology, PUR and PPF can be mixed with sand first forming a mixture. Then this mixture is laid on river bank or roadbed, final, and the compaction is needed to achieve the required compactness. Besides, the whole construction time should be controlled within half an hour because of coagulation time of PUR, therefore, the process should be divided into sections.

Although the development and applications of polyurethane polymer and fiber treatment are restricted currently, especially for the large-scale applications, its economic feasibility will be gradually improved over the long term due to the rapid growth of the global polyurethane market. The current polyurethane industry occupies a huge market due to its wide application fields (mostly for coatings and adhesives). It is reported that the global polyurethane market should reach 68 billion USD by 2021 from 49.8 billion USD in 2016 at a compound annual growth rate (CAGR) of 6.4%, from 2016 to 2021 (BCC Research, 2017). In consideration of the increasing production and growing demand for polyurethane products, further reduction in the price of polyurethane will be possible. On the other hand, PPF may also be replaced by some plant fiber which has advantage of low price and eco friendliness.

5 Conclusions

Experiments on compressive strength of sand-polymer and sand-polymer-fiber composite material after immersion were carried out to determine the influence of polymer content (p_c),immersion times (I_t), dry density (ρ_d) and fiber content (f_c) on the structure and properties of samples. The conclusions obtained in this study can be summarized as follows:

1) The strength of sample has significantly decrease after immersion, the strength of samples mixed with 1%, 2%, 3% and 4% content of polymer reduced by 61.34%, 80.56%, 82.93% and 83.37% respectively, after immersed 6 h.

Figure 12 Schematic diagram of reinforcement mechanism of sand-polymer-fiber composite

2) The strength of samples after immersion 24 h are increasing with the increasing of density and fiber content. Immersion-induced decreases in softening coefficient are not very obvious at high polymer content (p_c=3% and 4%). At higher densities (ρ>1.55 g/cm³), the density-induced increases in the softening coefficient are more significant than that of at lower densities (ρ≤1.55 g/cm³). As fiber content reaches 0.6%, the role that fiber plays in improving the softening coefficient is weakened.

3) The loose particles were connected into a whole by polymer membranes, but the polymer membrane has the ability of absorbing water. Therefore, the softening coefficient is decreasing with the increasing polymer content and immersion time. Fiber has an effect of reinforcement and form a spatial reticular structure among the sample. Thus, fiber can weak the water absorption ability and increase the strength and softening coefficient of samples.

Contributors

WANG Ying wrote the draft of manuscript and plotted the figure. LIU Jin provided the concept and reviewed the manuscript. SHAO Yong provided the experimental sand and carried out tests. MA Xiao-fan carried out the NMR tests. QI Chang-qing conducted data analysis and conducted the literature review. CHEN Zhi-hao carried out the tests of strength and polished up the draft of manuscript.

Conflict of interest

WANG Ying, LIU Jin, SHAO Yong, MA Xiaofan, QI Chang-qing, and CHEN Zhi-hao declare that they have no conflict of interest.

References

[1] KERAMATIKERMAN M, CHEGENIZADEH A, NIKRAZ H. Experimental study on effect of fly ash on liquefaction resistance of sand [J]. Soil Dynamics and Earthquake Engineering, 2017, 93: 1-6. DOI: 10.1016/j.soildyn. 2016.11.012.

[2] OURIA A, MAHMOUDI A. Laboratory and numerical modeling of strip footing on geotextile-reinforced sand with cement-treated interface [J]. Geotextiles and Geomembranes, 2018, 46(1): 29-39. DOI: 10.1016/j.geotexmem.2017.09.003.

[3] SUJATHA E R, GEETHA A R, JANANEE R, KARUNYA S R. Strength and mechanical behaviour of coir reinforced lime stabilized soil [J]. Geomechanics and Engineering, 2018, 16(6): 627-634. DOI: 10.12989/gae.2018.16.6.627.

[4] CHANG I, IM J, PRASIDHI A K, CHO G C. Effects of Xanthan gum biopolymer on soil strengthening [J]. Construction and Building Materials, 2015, 74: 65-72. DOI: 10.1016/j.conbuildmat.2014.10.026.

[5] QURESHI M U, CHANG I, AL-SADARANI K. Strength and durability characteristics of biopolymer-treated desert sand [J]. Geomechanics and Engineering, 2017, 12(5): 785-801. DOI: 10.12989/gae.2017.12.5.785.

[6] NAQI A-li, JANG J. Recent progress in green cement technology utilizing low-carbon emission fuels and raw materials: A review [J]. Sustainability, 2019, 11(2): 537. DOI: 10.3390/su11020537.

[7] LIU Jin, FENG Qiao, WANG Yong, ZHANG Da, WEI Ji-hong, KANUNGO D P. Experimental study on unconfined compressive strength of organic polymer reinforced sand [J]. International Journal of Polymer Science, 2018, 2018: 1-18. DOI: 10.1155/2018/3503415.

[8] SONG Ze-zhuo, LIU Jin, BAI Yu-xia, WEI Ji-hong, LI Ding, WANG Qiong-ya, CHEN Zhi-hao, KANUNGO D P, QIAN Wei. Laboratory and field experiments on the effect of vinyl acetate polymer-reinforced soil [J]. Applied Sciences, 2019, 9(1): 208. DOI: 10.3390/app9010208.

[9] REZAEIMALEK S, HUANG Jie, BIN-SHAFIQUE S. Evaluation of curing method and mix design of a moisture activated polymer for sand stabilization [J]. Construction and Building Materials, 2017, 146: 210-220. DOI: 10.1016/ j.conbuildmat.2017.04.093.

[10] LU Q F, WANG Zi-shuai, GU Liu-yang, CHEN Yi, SHAN Xiao-kang. Effect of sodium sulfate on strength and microstructure of alkali-activated fly ash based geopolymer [J]. Journal of Central South University, 2020, 27(6): 1691-1702. DOI: 10.1007/s11771-020-4400-4.

[11] DWARI R K, MISHRA B K. Evaluation of flocculation characteristics of kaolinite dispersion system using guar gum: A green flocculant [J]. International Journal of Mining Science and Technology, 2019, 29(5): 745-755. DOI: 10.1016/j.ijmst.2019.06.001.

[12] ANAGNOSTOPOULOS C A. Strength properties of an epoxy resin and cement-stabilized silty clay soil [J]. Applied Clay Science, 2015, 114: 517-529. DOI: 10.1016/j.clay.2015. 07.007.

[13] HAMIDI S, MARANDI S M. Effect of clay mineral types on the strength and microstructure properties of soft clay soils stabilized by epoxy resin [J]. Geomechanics and Engineering, 2018, 15(2): 729-738. DOI: 10.12989/gae.2018.15.2.729.

[14] CORREIA A A S, VENDA OLIVEIRA P J, CUSTODIO D G. Effect of polypropylene fibres on the compressive and tensile strength of a soft soil, artificially stabilised with binders [J]. Geotextiles and Geomembranes, 2015, 43(2): 97-106. DOI: 10.1016/j.geotexmem.2014.11.008.

[15] DENG Zong-cai, GAO Lei, WANG Xian-yun. Glass fiber-reinforced polymer-reinforced rectangular concrete columns under simulated seismic loads [J]. Journal of the Brazilian Society of Mechanical Sciences and Engineering, 2018, 40(2): 1-12. DOI: 10.1007/s40430-018-1041-8.

[16] YANG Jia-qi, SMITH S T, WANG Zhen-yu. Seismic behaviour of fibre-reinforced-polymer- and steel-strengthened timber connections [J]. Advances in Structural Engineering, 2019, 22(2): 502-518. DOI: 10.1177/1369433218794001.

[17] CHEBBI M, GUIRAS H, JAMEI M. Tensile behaviour analysis of compacted clayey soil reinforced with natural and synthetic fibers: Effect of initial compaction conditions [J]. European Journal of Environmental and Civil Engineering, 2020, 24(3): 354-380. DOI: 10.1080/19648189.2017.138 4762.

[18] TANG Chao-sheng, WANG De-yin, ZHU Cheng, ZHOU Qi-you, XU Shi-kang, SHI Bin. Characterizing drying-induced clayey soil desiccation cracking process using electrical resistivity method [J]. Applied Clay Science, 2018, 152: 101-112. DOI: 10.1016/j.clay.2017.11.001.

[19] PHANIKUMAR B R, SINGLA R. Swell-consolidation characteristics of fibre-reinforced expansive soils [J]. Soils and Foundations, 2016, 56(1): 138-143. DOI: 10.1016/ j.sandf.2016.01.011.

[20] TANG Chao-sheng, LI Jian, WANG De-yin, SHI Bin. Investigation on the interfacial mechanical behavior of wave-shaped fiber reinforced soil by pullout test [J]. Geotextiles and Geomembranes, 2016, 44(6): 872-883. DOI: 10.1016/ j.geotexmem.2016.05.001.

[21] TANG Chao-sheng, WANG De-yin, CUI Yu-jun, SHI Bin, LI Jian. Tensile strength of fiber-reinforced soil [J]. Journal of Materials in Civil Engineering, 2016, 28(7): 04016031. DOI: 10.1061/(asce)mt.1943-5533.0001546.

[22] MALEKZADEH M, BILSEL H. Hydro-mechanical behavior of polypropylene fiber reinforced expansive soils [J]. KSCE Journal of Civil Engineering, 2014, 18(7): 2028-2033. DOI: 10.1007/s12205-014-0389-2.

[23] LIU Jin, WANG Ying, KANUNGO D P, WEI Ji-hong, BAI Yu-xia, LI Ding, SONG Ze-zhuo, LU Yi. Study on the brittleness characteristics of sand reinforced with polypropylene fiber and polyurethane organic polymer [J]. Fibers and Polymers, 2019, 20(3): 620-632. DOI: 10.1007/ s12221-019-8779-1.

[24] PARK S S. Unconfined compressive strength and ductility of fiber-reinforced cemented sand [J]. Construction and Building Materials, 2011, 25(2): 1134-1138. DOI: 10.1016/ j.conbuildmat.2010.07.017.

[25] CHEN Mu, SHEN Shui-long, ARULRAJAH A, WU Huai-na, HOU Dong-wei, XU Ye-shuang. Laboratory evaluation on the effectiveness of polypropylene fibers on the strength of fiber-reinforced and cement-stabilized Shanghai soft clay [J]. Geotextiles and Geomembranes, 2015, 43(6): 515-523. DOI: 10.1016/j.geotexmem.2015.05.004.

[26] LIU Jin, BAI Yu-xia, SONG Ze-zhuo, WANG Ying, CHEN Zhi-hao, WANG Qiong-ya, KANUNGO D P, QIAN Wei. Effect of basalt fiber on the strength properties of polymer reinforced sand [J]. Fibers and Polymers, 2018, 19(11): 2372-2387. DOI: 10.1007/s12221-018-8507-2.

[27] JAMES J. Sugarcane press mud modification of expansive soil stabilized at optimum lime content: Strength, mineralogy and microstructural investigation [J]. Journal of Rock Mechanics and Geotechnical Engineering, 2020, 12(2): 395-402. DOI: 10.1016/j.jrmge.2019.10.005.

[28] LI Li-hua, ZHANG Jiang, XIAO Heng-lin, HU Zhi, WANG Zhi-jie. Experimental investigation of mechanical behaviors of fiber-reinforced fly ash-soil mixture [J]. Advances in Materials Science and Engineering, 2019, 2019: 1-10. DOI: 10.1155/2019/1050536.

[29] BOZ A, SEZER A. Influence of fiber type and content on freeze-thaw resistance of fiber reinforced lime stabilized clay [J]. Cold Regions Science and Technology, 2018, 151: 359-366. DOI: 10.1016/j.coldregions.2018.03.026.

[30] ORAKOGLU M E, LIU Jian-kun, NIU Fu-jun. Experimental and modeling investigation of the thermal conductivity of fiber-reinforced soil subjected to freeze-thaw cycles [J]. Applied Thermal Engineering, 2016, 108: 824-832. DOI: 10.1016/j.applthermaleng.2016.07.112.

[31] JU C P, HUNG S H, CHEN Chang-keng, CHEN W L, LEE J W, LIN R M, CHEN Wen-cheng, CHERN LIN J H. Immersion-induced changes in structure and properties of a TTCP/DCPA/CSH cement [J]. Materials Chemistry and Physics, 2011, 130(1-2): 303-308. DOI: 10.1016/ j.matchemphys.2011.06.051.

[32] LIU Jin, BAI Yu-xia, LI Ding, WANG Qiong-ya, QIAN Wei, WANG Ying, KANUNGO D, WEI Ji-hong. An experimental study on the shear behaviors of polymer-sand composite materials after immersion [J]. Polymers, 2018, 10(8): 924. DOI: 10.3390/polym10080924.

[33] DIAMBRA A, IBRAIM E, MUIR WOOD D, RUSSELL A R. Fibre reinforced sands: Experiments and modelling [J]. Geotextiles and Geomembranes, 2010, 28(3): 238-250. DOI: 10.1016/j.geotexmem.2009.09.010.

[34] ZHANG Xi-dong, RUSSELL A R. Assessing liquefaction resistance of fiber-reinforced sand using a new pore pressure ratio [J]. Journal of Geotechnical and Geoenvironmental Engineering, 2020, 146(1): 04019125. DOI: 10.1061/ (asce)gt.1943-5606.0002197.

[35] BCC Research. Polyurethanes: New technologies and applications drive global market growth [R]. BCC Research, 2017.

(Edited by ZHENG Yu-tong)

中文导读

水对聚氨酯和聚丙烯纤维固化砂土强度特性的影响

摘要：聚合物作为一种新型气硬型土壤加固材料，在河道岸坡加固和生态护坡领域得到了广泛的应用。本文研究了水对聚合物和纤维加固后砂土强度特性的影响。对可能影响浸水后加固砂土的强度特征的几个因素进行了分析，包括聚合物含量(1%，2%，3%和4%与干砂的质量比)，浸水时间(6，12，24和48 h)，干密度(1.40，1.45，1.50，1.55和1.60 g/cm³)和纤维含量(0.2%，0.4%，0.6%和0.8%与干砂的质量比)。用核磁共振(NMR)和扫描电子显微镜(SEM)对加固砂土的微观结构进行了分析。试验结果表明，抗压强度随聚合物含量的增加而增大，随浸水时间的延长而减小；软化系数随聚合物含量的增加和浸泡时间的延长而减小，随密度和纤维含量的增加而增大。纤维含量为0.6%时，对减小水引起的强度损失有积极作用。

关键词：聚合物；纤维；浸水；固化砂土；抗压强度；软化系数

Foundation item: Project(41472241) supported by the National Natural Science Foundation of China; Project(KJXM2019028) supported by the Natural Resources Science and Technology Project of Jiangsu Province, China; Project(2019B17314) supported by the Fundamental Research Funds for the Central Universities, China

Received date: 2020-08-19; Accepted date: 2021-01-10

Corresponding author: LIU Jin, PhD, Professor; Tel: +86-13913976590; E-mail: jinliu920@163.com; ORCID: https://orcid.org/0000-0001-7654-255X