J. Cent. South Univ. (2017) 24: 560-568

DOI: 10.1007/s11771-017-3458-0

Strength characteristics of modified polypropylene fiber and cement-reinforced loess

YANG Bo-han(杨博瀚)¹, WENG Xing-zhong(翁兴中)¹, LIU Jun-zhong(刘军忠)¹, KOU Ya-nan(寇亚楠)¹,

JIANG Le(姜乐)¹, LI Hong-lei(李洪磊)¹, YAN Xiang-cheng(颜祥程)^{1, 2}

1. Department of Airfield and Building Engineering, Air Force Engineering University, Xi’an 710038, China;

2. PLA Unit 95616, Qionglai 611500, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2017

Abstract: The reinforcement and stabilization of loess soil are duscussed by using fibers as the reinforcement and cement as the stabilization materials. To study the strength characteristics of loess soil reinforced by modified polypropylene (MPP) fiber and cement, samples were prepared with six different fiber contents, three different cement contents, three different curing periods and three kinds of fiber length. The samples were tested under submergence and non-submergence conditions for the unconfined compressive strength (UCS), the splitting tensile strength and the compressive resilient modulus. The results indicated that combined reinforcement by PP fiber and cement could significantly improve the early strength of loess to 3.65–5.99 MPa in three days. With an increase in cement content, the specimens exhibited brittle fracture. However, the addition of fibers gradually modified the mode of fracture from brittle to ductile to plastic. The optimal dosage of fiber to reinforce loess was in the range of 0.3%–0.45% and the optimum fiber length was 12 mm, for which the unconfined compressive strength and tensile strength reached their maxima. Based on the analysis of failure properties, cement-reinforced loess specimens were susceptible to brittle damage under pressure, and the effect of modified polypropylene fiber as the connecting “bridge” could help the specimens achieve a satisfactory level of ductility when under pressure.

Key words: fiber-reinforced soil; cement-stabilized soil; loess; mechanical properties; fracture characteristics

1 Introduction

In order to effectively improve soil strength and stability, geotextile, geogrid and other traditional reinforcement materials have been widely used in present day geotechnical engineering [1]. However, traditional reinforcement materials are generally large-sized, and hence, must be laid out in a certain direction and with a proper spacing. As a result, they cannot be mixed evenly with the soil leading to the anisotropy in the mechanical properties of the reinforced soil. Fiber is a new reinforcement material, which offers several advantages as: high strength, good dispersity, ease of mixing in the form of isotropic uniform materials, and prevention of the formation of potential weak surfaces due to the specific reinforcement direction and distance, unlike the case of traditional reinforcement materials. In recent years, fibers are being increasingly utilized in geotechnical engineering, especially in combination with cement, lime and other soil stabilizing materials, as the addition of fibers can significantly improve the mechanical properties of soil. Thus, the fiber reinforcement technology has attracted a lot of research attention from scholars and engineers [2–7].

Polypropylene (PP) fiber is the most widely used fiber for soil reinforcement. It is a type of polymer made by a special process of polymerization. The fiber has high strength, thermal resistance, acid and alkali resistance and non-absorbance. Engineering properties of polypropylene fiber-reinforced soil, such as shear, compressive and tensile properties, have become the focus of many researchers. AHMAD et al [8] and IBRAIM et al [9] applied PP fibers as reinforcement material, which significantly improved the mechanical properties of soil. LI et al [10] concluded from an experiment that PP fibers could significantly improve the flexural properties of soil. Experiment results from MAHESHWARI et al [11] and YANG et al [12] indicated that the reinforcing effect of PP fibers can effectively improve the ultimate bearing capacity of soil and reduce soil settlement. TANG et al [13] investigated the shear strength characteristics of PP fiber-reinforced cement soil, and illustrated that the reinforcing effect of fibers could offer more contribution in cement soil than in regular soil. CAI et al [14] studied the compressive strength of PP fiber-reinforced calcareous soil, and analyzed the influences of fiber content, fiber length, lime content and curing period.

The above research results have revealed the engineering properties of PP fiber-reinforced soil from different aspects. The results illustrate that fiber reinforcement is an excellent soil improvement technology. However, it should be noted that existing studies basically focus on soil under non-submergence conditions. Furthermore, such studies often use sand as the study object, but they rarely investigate the collapsible loess. The difference in soil texture and types of fiber could markedly influence the effect of reinforcement. However, these factors are frequently ignored by many researchers. Besides, currently in a lot of emergency projects, the strength of collapsible loess should be improved in a short time, possibly under the influence of diverse conditions such as soaking by rainwater. Despite having some studies that focus on loess, there is still a lack of experimental investigation on fiber-reinforced and stabilized loess, especially a systemic study on the mechanical properties of fiber reinforced and stabilized loess under both soaking and non-soaking conditions in a short curing period.

In this work, a series of tests on mechanical properties were conducted in submergence and non-submergence conditions by controlling cement content, fiber content, curing age and fiber length. Representative collapsible loess in northern China was taken as the study object; modified polypropylene fiber was used as the reinforcement material. The influence of cement content and fiber content on the mechanical properties of loess was mainly analyzed, and a new understanding of the mechanism of fiber-reinforced cement stabilized loess was offered from a morphological analysis of damage characteristics.

2 Experimental material and methods

2.1 Experimental materials

Xi’an soil was selected as the experimental soil, which is representative of northern China. The sample was taken from the construction site of Metro Line 2 in the southern suburb of the city, and represented the wet collapsible loess. According to the Test Methods of Soils for Highway Engineering (JTG E40–2007), Xi’an soil is classified as clayey soil. The physical properties of soil sample are listed in Table 1. The cement used in the experiment was ordinary Portland cement, P·O 42.5R, from the brand of Qinling company, and had a density of 3.10 g/cm³. The cement content was varied as 6%, 8% and 10% (mass percentage of dry soil). Modified monofilament polypropylene fiber was chosen as the reinforcement material to investigate the improvement on the properties of samples. Table 2 gives the properties of the fiber. The fiber contents in the test were selected as 0.15%, 0.3%, 0.45%, 0.6%, 0.8% and 1.0% (mass percentage of dry soil). The fiber lengths were selected as 9 mm, 12 mm and 19 mm; the length of fibre is 12 mm without any special instructions.

2.2 Experimental method

To investigate the influence on the unconfined compressive strength (UCS) and splitting tensile strength (STS) after the addition of cement and fiber, the UCS and STS tests on both cement-stabilized soil and fiber-reinforced stabilized soil were performed by using the strength tester of pavement materials in accordance with relevant provisions of the Experiment Methods of Inorganic Binder Stabilizing Materials for Highway Engineering (JTG E51–2009). The size of cylindrical samples used was d50 mm×50 mm.

According to the Compressive Resilient Modulus Test Methods of Inorganic Binder Stabilizing Materials for Highway Engineering (JTG E51–2009), and the Indoor Compressive Resilient Modulus Test Methods of Inorganic Binders Stabilized Materials, the molding cylindrical samples, d100 mm×100 mm in size, were prepared in accordance with the standard of the maximum dry density and the optimum moisture content, and the compressive resilient modulus was determined. As GUAN’s study has already shown that this method produces results consistent with the field test results, the present study also adopted this method [15].

Table 1 Physico-mechanical properties of loess sample

Table 2 Physico-mechanical parameters of MPP fiber

3 Results and discussion

3.1 Unconfined compressive strength (UCS) test

The unconfined compressive strength is the main index to measure the effect of reinforcement and the stabilizing effect of fiber-reinforced soil. Experiments were conducted under both submergence and non- submergence conditions after three curing periods:3 d, 7 d and 14 d. Three specimens per property are tested for each strength. Corresponding axial stress-strain curves were collected. The results of the unconfined compressive strength (UCS) test are listed in Table 3.

3.1.1 Effects of cement content

It can be seen from Table 3 that the unconfined compressive strength of fiber-reinforced cement stabilized soil generally increased with increasing cement content. Compared to fiber-reinforced cement- stabilized soil (3.65–8.28 MPa), the fiber-reinforced plain soil possessed small strength (as low as 0.91– 1.46 MPa) and poor water stability. During the curing period, soaking could result in disintegration. Thus, fiber reinforcement alone could not improve the specimen’s water stability. With further addition of cement into the fiber-reinforced soil, the strength became significantly higher than that of plain soil and fiber-reinforced soil; the water stability coefficient increased to 40%–80% as well. The above results indicated that cement content did alter intrinsic soil property. The hydration of cement significantly increased soil strength via gluing soil particles and it greatly improved water stability of soil. The fiber effect became more obvious as the disperse fibers were embedded in soil body by large amount of soil particles glued by cement, resulting in greater fiber interface friction and cohesion. Under the dual actions of the chemical reinforcement by hydration and the physical reinforcement by fiber anchoring and stretching, both unconfined compressive strength and water stability were evidently improved. Previous studies have proved the improved fiber reinforcement effect of stable substance in soil body. For example, the results of CAI et al [14] showed that fiber could markedly increase the flexural performance of calcareous soil. Based on unconfined compressive strength test, TANG et al [13] reported that fiber reinforcement effect was more apparent in cement soil and calcareous soil. Meanwhile,SHI et al [16] found the evidence of polypropylene fiber effectively improving the shear strength of calcareous soil. Obviously, fiber reinforcement effect is more pronounced while adding stable substances such as cement and lime. Therefore, the future research direction would be the development of more effective soil stable substances to improve the mechanical properties of fiber reinforced soil.

Table 3 Results of UCS test

3.1.2 Effects of fiber content

It can be observed from Table 3 that specimen’s unconfined compressive strength gradually increased with increasing fiber content, but only up to 0.45% and above that decrease in strength started. For instance, for the specimen C10p with 10% cement content, when the fiber content increased from 0 to 0.45%, 7d non- submergence compressive strength (dry strength) was doubled (from 3.32 MPa to 7.4 MPa); 7d submergence compression strength (soaking strength) almost doubled from 2.49 MPa to 4.77 MPa. However, the strength significantly decreased after soaking, as shown in Fig. 1. This was mainly because of the random 3D network distribution of fibers in bulk mass. The more the fiber content added, the more was the spatial network distribution, resulting in a stronger ability to bear more tensile stress, in other words, to disperse and transmit loads [17]. However, after soaking, the interface friction between soil and fiber and the cohesion sharply dropped, resulting in a greatly reduced strength. Thus, for a certain unconfined compressive strength, the fiber content in cement-stabilized soil could be utilized to reduce the cement content, that is, to reduce large dosage of cement content even with a small dosage of fiber content.

Fig. 1 Variation of UCS with fiber content (7d curing time)

To further investigate the influence of fiber content on the unconfined compressive strength, seven fiber content groups were studied for 8% cement stabilized soil, and the results are also given in Table 3. It can be seen from Fig. 1 that, both the 7 d soaking and non- soaking unconfined compressive strength first increased and then decreased with increasing fiber content. It can be obtained from Table 3 that the unconfined compressive strength reached its peak when fiber content reached 0.45%, indicating that there is maximum fibre content for optimum strength properties and above that strength starts to decrease. Beyond the optimal range, the strength might decline as fiber content increased. A similar observation has also been reported by several other investigators. WELLKER et al [18] determined an optimal fiber content of 0.2% based on shear strength parameters. AKBULUT et al [19] reported an optimal fiber content of 0.2% for polyethylene fiber to maximize unconfined compressive strength. Besides, MOHAMED [20] reported that the compressive strength began to drop when the fiber content of hey was increased beyond 1%. Overall, there is an optimal fiber content based on different mechanical tests and fiber types. Beyond the optimal content, the increase of fiber content will reduce their disorderly distribution in soil but increase their orderly distribution, and the fibers are prone to conglomeration, thereby greatly reducing interface friction and cohesion. Based on the results of the present study, the optimal content of MPP fiber was 0.45%.

3.1.3 Effects of curing period

Figure 2 shows that the unconfined compressive strength increased with an increase in curing period. The compression strength of fiber-reinforced cement- stabilized soil of various cement contents increased sharply at an early age, and relatively slowly at a late age. However, it was always much higher when compared to plain soil with the same curing period. For example, the strength for the non-soaking specimen C10p3 was 5.99 MPa at three days and increased by 23.5% to reach 7.4 MPa in seven days. However, increase from 7 to 14 days was merely 11.9% for the strength to reach to 8.28 MPa. On the other hand, for the plain soil specimen, the 14 d strength was merely 1.46 MPa. Moreover, 3d UCS of soaking specimen C8p3 was 3.62 MPa, 7 d UCS reached 4.84 MPa, with an increase of 33.7%. The 14 d UCS was 5.36 MPa, with an increase of 8.7% compared to 7 d strength. It can be found by analyzing the results from TANG et al [13] that, the unconfined compressive strength of fiber reinforced lime soil increased slowly during early curing period, while the contrary was true for fiber reinforced cement stabilized soil. SHI et al [16] investigated polypropylene fiber-reinforced lime soil, and it was reported that curing period had a significant influence on specimen’s strength. Specifically, the strength increased slowly during an early curing period while sharply at a later curing period. It can be concluded that the strength of fiber-reinforced stabilized soil is improved with an increase in curing period. However, the rate of increase differs at the early and the late curing stages due to the differences in stabilizing substances. In the present study, given that PO 42.5R ordinary Portlandcement was adopted as the stabilizing substance, the strength increased rapidly in the early stage. For instance, 3 d strength for the soaking specimen exceeded 3 MPa at 8% cement content, a phenomenon that can be regarded as very important for emergency soil reinforcement projects.

Fig. 2 Variation of UCS with curing period (0.45% fiber content)

3.1.4 Effect of fiber length

In order to investigate the influence of fiber length on the unconfined compressive strength, cement content and fiber content were kept constant at 8% and 0.3%, respectively. MPP fibers were selected at 9, 12 and 19 mm to prepare fiber-reinforced cement-stabilized soil samples. Unconfined compressive strength test and tensile splitting strength test were conducted after 7 d curing period. The results are shown in Fig. 3.

Fig. 3 Variation in strength with different fiber lengths

It can be seen from Fig. 3 that the unconfined compressive strength and tensile splitting strength were significantly influenced by fiber length. With the same cement and fiber content, both unconfined compressive strength and tensile splitting strength first increased and then decreased with an increase in fiber length. The strength of fiber reinforced stabilized soil reached its peak at 12 mm fiber length, indicating the most effective physical reinforcement. The 7 d unconfined compressive strengths for the non-soaking and soaking specimens at 12 mm fiber length were respectively 1.05 and 1.39 times of those at 9 mm fiber length, and 1.2 and 1.47 times of those at 19 mm. Similarly, the 7 d tensile splitting strengths for the non-soaking and soaking specimens at 12 mm fiber length were respectively 1.19 and 1.31 times of those at 9 mm fiber length, and 1.26 and 1.17 times of those at 19 mm. The above results are consistent with previous studies. Using sisal fibers of length of 10, 15, 20 and 25 mm, PARBAKAR and SRIDHAR [21] found that the compressive strength of fiber-reinforced soil first increased, and then, decreased with an increase in fiber length; the optimal fiber length was determined to be 20 mm.

Among the three lengths of fibers considered in this work, the 9 mm long fibers mainly slid in the soil and were pulled out in failure mode during the unconfined compressive strength and the splitting tensile strength tests. The strength of fiber-reinforced cement-stabilized soil with 19 mm fibers was lower than that with 9 mm fibers for two reasons: 1) for the same fiber content, an increase in fiber length (19 mm in comparison to 9 mm) meant a reduction in the number of fibers, resulting in a weak spatial network structure and thus limiting tensile stress; 2) for a larger length/diameter ratio for the 19 mm fibers, a higher load would result in the failure of fibers due to tensile rupture. For the 12 mm long fibers, the sum of friction and cohesive force was close to the splitting tensile strength. As the load increased, the 12 mm long fibers were at the critical states of sliding, being pulled out and being ruptured. Thus, at an appropriate fiber length/diameter ratio, the splitting tensile strength would be close to the tensile strength of the MMP fibers, thus achieving the best physical reinforcement.

3.1.5 Analysis of stress–strain relationship

The stress–strain curve of fiber-reinforced cement- stabilized soil for different cement contents with 7 d curing period is shown in Fig. 4. There was a relatively significant improvement in the unconfined compressive strength with an increase in cement content. At 7 d curing period and for 0.3% fiber content, the unconfined compressive strengths of fiber-reinforced cement- stabilized soil with cement contents of 6% (5.13 MPa), 8% (5.51 MPa) and 10% (6.66 MPa) were, respectively, 4.24, 4.55 and 5.5 times of the specimen without cement content (1.21 MPa). Meanwhile, increase in cement content resulted in a sudden after-peak drop in the stress-strain curve, and the residual strength still increases with the cement content and thus toughness of material is higher for the higher cement content, showing a failure more similar to a brittle one.

With 7 d curing period, the stress-strain curves of fiber-reinforced unstabilized soil and fiber-reinforced stabilized soil are shown in Figs. 5 and 6. It can be seen from Fig. 5 that the addition of fibers resulted in a large increase in the unconfined compressive strength; the ultimate strength generally increased with an increase in fiber content. The initial stages of the stress–strain curves were essentially overlapped and exhibited steep slopes. The rate of increase of stress far outweighed the rate of increase of strain, indicating that the structural strength increased during initial loading stage due to the addition of fibers, presenting a strong anti-crack reinforcement. Figure 6 shows that, as the fiber content increased, the post-peak portion of the stress-strain curve declined slowly, while the residual strength became increasingly higher. TANG et al [13] also conducted a similar study on fiber-reinforced cement soil, and found that both the peak strength and the residual strength increased with an increase in fiber content. It can be concluded that fibers had improved the specimen’s failure mode, gradually from brittle to ductile to plastic failure.

Fig. 4 Stress-stain curves for fiber-reinforced stabilized soil at varied cement contents

Fig. 5 Fiber soil stress-strain curves

3.2 Splitting tensile strength test

Compared to the compressive strength, the tensile strength is usually small, and difficult to be measured accurately. However, the tensile strength, as small as it is,is an essential mechanical index in actual applications of geotechnical engineering. Thus, it has been widely studied by a number of scholars by either direct or indirect experimental methods. In this work, splitting tensile strength test (indirect tensile strength test) was performed on cement-stabilized soil and fiber-reinforced stabilized soil with 7 d curing period under both soaking and non-soaking conditions. The splitting tensile strength σ_t can be expressed as

                                   (1)

where F is the peak load (N); d is the diameter of a cylinder specimen (mm); l is the height of the cylinder specimen (mm). The results are shown in Figs. 7 and 8.

Fig. 6 Stress–strain curves for dry C8p specimens

Fig. 7 Variation in tensile strength with cement content

It can be seen from Figs. 7 and 8 that the addition of cement and fiber (addition of fibres up to 0.45%) greatly improved the specimens’ tensile splitting strength. The tensile strength increased constantly with increases in cement and fiber contents. Specifically, the tensile strength increased linearly with the increase in cement content. The tensile strength was severely influenced by the curing condition. The variation in tensile strength with the change in cement or fiber content was always lower for the soaking specimen than that for the non- soaking specimen. With an increase in fiber content, tensile strength of non-soaking specimens for 7 d curing ranged approximately between 0.5 and 1.0 MPa, while that for soaking specimens ranged between 0.3 and 0.6 MPa. However, it is to be noted that increased fiber content did not always mean an increase in tensile strength. The fiber reinforcement was the most effective at 0.45% fiber content, which was consistent with the previous conclusion on the influence of fiber content on unconfined compressive strength.

Fig. 8 Variation in tensile strength with fiber content

3.3 Compressive modulus of resilience test

Compressive modulus of resilience test was conducted after 3 d, 7 d and 14 d curing periods (soaking specimens were soaked for 24 h on the last day of curing period). The results are shown in Figs. 9 and 10.

Fig. 9 Variation in compressive modulus of resilience with cement content for cement-stabilized soil

It can be seen from Fig. 9 that cement imposed a significant influence on the compressive modulus of resilience of loess sample. The compressive modulus of resilience of cement-stabilized soil increased with an increase in cement content and an increase in curing period. The material used for stabilizing the soil was grade PO 42.5R cement, which contributed to the high early-age strength of the cement-stabilized soil. For instance, the compressive modulus of resilience of non-soaking cement-stabilized soil specimen was 651–837 MPa at 3 d curing period, 881–972 MPa at 7 d curing period, and 930–1083 MPa at 14 d curing period; the increase from 3 to 14 days was by 29.42%–42.76%. Meanwhile, compressive moduli of resilience with 3 d curing period for both soaking and non-soaking specimens with reinforcement were much higher than that for plain soil.

Fig. 10 Variation in compressive modulus of resilience with cement content for fiber-reinforced stabilized soil

It can be seen from Fig. 10 that the compressive modulus of resilience of fiber-reinforced stabilized soil increased with an increase in curing period. Besides, it first increased and then decreased with an increase in fiber content, showing the peak in compressive modulus of resilience at 0.3% fiber content. As the fiber content increased, the fibers conglomerated in soil, resulting in loose contact between soil particles and fibers. Meanwhile, the distinct difference in the stiffnesses of fiber and stabilized soil particles would cause sliding between them under load. Consequently, a thin weak surface would form in between, as a result of which the fiber frictional reinforcement could not be fully mobilized [22].

4 Analysis of compressive failure properties of fiber-reinforced stabilized soil

The compressive failure mechanisms of fiber- reinforced stabilized soil under loads are a macro embodiment of the slide and deflection of internal particles, the tensile drawing of fibers and the generation and development of microcracks. Thus, it is necessary to analyze the failure properties. The compressive failure mechanisms of fiber soil, cement-stabilized soil and fiber-reinforced stabilized soil are shown in Fig. 11.

Fig. 11 Sample failure pattern and fiber ‘bridge’ connection:

As shown in Fig. 11(a), the cement-stabilized soil formed longitudinal cracks under axial loads under the condition of no obvious signs, developing rapidly and running through the specimen. The cracks became longer and wider and released loose properties, accompanied by debris and soil blocks. During the test, the top and bottom surfaces were subjected to the actions of pressure blocks. The circular shear surface was obvious after the peeling off of the loose area intersected by surface cracks. Finally, the specimen showed inverted-cone brittle failure, as shown in Fig. 11(b), which was similar to the brittle failure of concrete under pressure. Thus, the addition of cement resulted in a transition from ductile to brittle material [23, 24].

It can be seen from Fig. 11(c) that, as the load increased, microcracks appeared first on the weak fiber-soil surface, and then developed longitudinally and obliquely without any obvious fracture plane. Meanwhile, the middle part of the specimen gradually extruded, and the specimen experienced drum-shape plastic failure, indicating that fibers had improved the specimen’s brittle failure characteristics. TANG et al [13] and SHI et al [16] obtained a similar failure pattern, suggesting that fibers could effectively prevent the development of cracks. As shown in Fig. 11(d), short, thin and dense cracks were developed; the specimen ruptured without disconnection, and fibers showed an apparent ‘bridging’ function, endowing the failure specimen possessing good ductility.

5 Conclusions

The unconfined compressive strength and water stability coefficient of cement-stabilized soil and fiber-reinforced stabilized loess soil generally increased as cement and fiber contents (up to 0.45%) were increased. With the reinforcement and stabilization, the early-age strength of loess soil was substantially improved; for example, strength after 3d curing period reached as high as 3.65–5.99 MPa from the original strength of 0.91–1.38 MPa for similar curing period. Increased cement content resulted in a sudden drop on the stress–strain curve, showing brittle failure characteristics. However, fibers induced a high residual strength, and improved the specimen’s brittle failure pattern, which presented a transition from brittle failure to ductile and plastic failure. It can be concluded from the tests that fibers could evidently improve the mechanical properties of collapsible loess, that the optimal content range for modified polypropylene fiber- stabilized loess was 0.3%–0.45%, and that the optimal length was 12 mm. The compressive modulus of resilience increased with increases in cement and fiber contents and also an increase in curing period. The variation in the compressive modulus of resilience was most sensitive to the change in cement content.

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(Edited by YANG Bing)

Cite this article as: YANG Bo-han, WENG Xing-zhong, LIU Jun-zhong, KOU Ya-nan, JIANG Le, LI Hong-lei, YAN Xiang-cheng. Strength characteristics of modified polypropylene fiber and cement-reinforced loess [J]. Journal of Central South University, 2017, 24(2): 560-568. DOI: 10.1007/s11771-017-3458-0.

Foundation item: Project(050101) supported by Horizontal Research Foundation of PLA Air Force Engineering University, China; Project(51478462) supported by the National Natural Science Foundation of China

Received date: 2015-07-13; Accepted date: 2015-12-16

Corresponding author: YANG Bo-han, PhD; Tel: +86-13227067638; E-mail: yangbohanpla@126.com