Strength Characteristics of Soil Cement Reinforced by Natural Hair Fiber

† Professor, Department of Civil Engineering, Daegu University (Corresponding Author : [email protected])

Strength Characteristics of Soil Cement Reinforced by Natural Hair Fiber

Moorak Son

･ Jaeyong Lee

Received: January 3

, 2018; Revised: January 31

, 2018; Accepted: March 26

, 2018

ABSTRACT : This study systematically examines the changes in the compressive and tensile strength of soil cement reinforced by natural hair fiber, which is regularly produced from human. Extensive experimental tests of various test specimens have been carried out in a laboratory. Several factors are considered, including the soil type, amount of cement, amount of fiber, fiber length, loading type, and curing age. The test results indicate that both the compressive and tensile　strengths are significantly affected by the fiber, either increasing or decreasing depending on the conditions. The increase in tensile strength is significant in the sand-based soil cement due to the tensile resistance of the fiber which is interlocked with the surrounding soil or cement particles. The natural fiber provides a larger strain to failure due to its extensibility, which allows greater deformation. Based on the test results, natural hair fibers can be an effective and environmentally friendly way to improve soil ground subjected to tensile loading, such as an embankment slope, road subgrade, or landfill, thus reducing the cost for cement and waste treatment. The study results provide a useful information of better understanding the mechanical behavior of natural hair fiber in soil cement and the practical use of waste materials in civil engineering. The findings can be practically applied for improving earth structures under tensile loading.

Keywords : Soil cement, Strength, Natural hair fiber, Experimental test, Waste material

ISSN 1598-0820 DOI https://doi.org/10.14481/jkges.2018.19.4.17

19(4): 17～26. (April 2018) http://www.kges.or.kr

1. Introduction

Various structures and earthworks are frequently constructed all over the world, such as buildings, roads, embankments, dams, and slopes. The most important factors for these con- structions are safety and economic feasibility, which should always be considered in engineering practice. Failure to these can cause significant problems, such as loss of life, unnecessary expenditure, construction delays, or legal disputes. For safe construction, soils should have enough bearing capacity when compressive loading is involved, in addition to sufficient tensile resistance when there is tensile loading. If the soil conditions are not safe for construction, measures should be taken to improve them.

Many methods have been developed to improve the strength conditions of soil, including mechanical, chemical, and bio- logical methods. These methods include chemical grouting, cement grouting or mixing, biological treatments, mechanical compaction, and water drainage. Cement or lime is pumped into the soil or mixed with it to increase the strength to a required level. The soil strength can be increased significantly,

depending on the amount of cement or lime used. However, as the amount increases, the construction cost increases as well. Furthermore, while this method can sufficiently increase the compressive strength of the soils, the effect on the tensile strength is relatively small. For this reason, fiber materials are considered to increase the tensile strength of the soils.

In earlier days, various natural fiber materials were used to increase soil strength, including straw, hay, and horse hair (Abtahi et al., 2009). Recently, synthetic fibers have been considered for increasing soil strength, such as polypropylene (PP), polyester (PET), polyethylene (PE), polyvinyl alcohol (PVA), nylon, glass fiber, and steel fiber. Many relevant studies have been carried out (Yang, 1972; Gray & Ohashi, 1983; Freitag, 1986; Craig et al., 1987; Maher & Ho, 1993

& 1994; Cho & Kim, 1995; Consoil et al., 1998; Kumar et al., 1999; Kim et al., 2002; Song & Im, 2002; Yetimoglu

& Salbas, 2003; Yetimoglu et al., 2005; Cai et al., 2006;

Kim et al., 2007; Tang et al., 2007; Sadek et al., 2010; Hejazi et al., 2012; Mirzababaei et al., 2013; Diambra & Ibraim, 2014; Gupta, 2014; Qu & Sun, 2016; Benessalah et al., 2016).

Reported results indicate that soil strength can be increased

Fig. 1. Grain size distribution of Kaolinite clay, Jumunjin sand, and Portland cement

Table 1. Physical properties of materials

Material Unit weight (kN/m³) Specific gravity Liquid limit (%) Plastic limit (%)

Kaolinite clay 16 2.4 50 26

Jumunjin sand (Korean standard sand) 15 2.6 - -

Portland cement 15 3.05 - -

Natural hair fiber 11~14 1.1∼1.4 - -

by using fibers with cement. However, some synthetic fibers have negative impacts on the environment and high cost.

Hejazi et al. (2012) presented a simple review of the use of various fibers for improving soils.

Despite many studies of the strength of soils reinforced with fibers, natural hair fibers didn’t get much attention as a material that can increase soil strength. Using natural hair fibers as a construction material could reduce construction costs by reducing the amount of cement or lime needed, and waste disposal costs can also be reduced by reusing the waste instead. A few studies were reported of the effect of natural hair fibers on the strength of concrete or asphalt (Ahmed et al., 2011; Jain & Kothari, 2012; Ganiron, 2014) and on the strength of soils (Son et al., 2015; Butt et al., 2016; Son & Lee, 2016). Nevertheless, there is still little information and systematic analysis of the effect of natural hair fibers on soil strength for different soil, cement, fiber, loading, and curing conditions.

Extensive experimental tests have been conducted on various test specimens in a laboratory while considering the soil type, amount of cement, amount of fiber, fiber length, loading type, and curing age. This study integrates all the test data from previous studies and new test results and

systematically examines and analyses the improvement of soil strength under various conditions. The study results provide a useful information of better understanding the mechanical behavior of natural hair fiber in a soil cement matrix and the practical use of waste material in civil engineering. The findings can be practically applied for improving earth structures under tensile loading, such as embankment slopes, road subgrades, or landfills.

2. Experiments and procedures

2.1 Materials

Soil (clay and sand), cement, water, and natural hair fiber were used to make test specimens. The physical characteristics of the materials are summarized in Table 1. The soils were Kaolinite clay and Jumunjin sand, and the cement was ordinary Portland cement. The particle size distribution of the materials is shown in Fig. 1. The clay and cement particles have similar size ranges but are much smaller than the sand particles.

Scanning electron microscope (SEM) images were obtained

for the different materials (see Fig. 2), which clearly show

the relative size and shape of each material. The clay and

Clay particles and hair fiber Sand particles and hair fiber Cement particles and hair fiber Fig. 2. Comparison of the relative magnitude of the used materials with SEM pictures

Table 2. Mixing design of materials for each specimen

Cases Soils (g) Cement (g) Water (g) Hair fiber

Clay Sand Clay specimen Sand specimen C S (g)

CH (0)*, SH (0)

400 (water content:

50%)

400 (water content:

0%)

25 (C=100 kgf/m³) 50 (C=200 kgf/m³) 75 (C=300 kgf/m³)

26.8 (C=100 kgf/m³) 53.2 (C=200 kgf/m³) 80.0 (C=300 kgf/m³)

20 40 60 (W/C 80%)

21.5 42.7 64 (W/C 80%)

-

CH (5_0.1)**, SH (5_0.1) 0.4

CH (5_0.5), SH (5_0.5) 2

CH (20_0.1), SH (20_0.1) 0.4

CH (20_0.5), SH (20_0.5) 2

*C: Clay specimen, S: Sand specimen, H: Hair fiber, **CH (5_0.1) represents the clay specimen with hair fiber length=5 mm and hair fiber content=0.1%

cement particles are much smaller than the fibers, while the sand particles are much larger. The images also confirm that the clay and cement particles have similar size ranges. The big difference in size of the clay and sand particles compared to the fiber is one of the possible reasons for different strength results which are examined in this study.

Natural hair fibers were tested and observed before using them in test specimens to remove fibers that had been chemically or physically treated. A direct tensile test was conducted on different natural hair fibers and test results showed that the fibers mostly had tensile strength higher than 0.2 kgf for a single fiber.

2.2 Preparation of test specimens

Test materials were prepared and specimens were fabricated according to the stand test method and procedure for soil cement. To make clay specimens reinforced with fibers, an amount of water equal to the liquid limit of the clay was added to dried clay, and a mixer was used to mix it sufficiently.

Then, the natural hair fibers were added to the water and mixed with a stirrer, and cement powder was added slowly.

The mixture of cement, fiber, and water was added to the clay and water mixture and mixed for 10 minutes. To make sand specimens reinforced with fibers, a mixture of cement, fiber, and water was mixed with dried sand for 10 minutes.

For sufficient mixing, the materials that had attached to the outer part of the mixing bowl were collected with a scraper and mixed again. The soil cement mixture with fibers was put in a mold in three layers, and each layer was compacted with a tamping rod 25 times to prepare the test specimens.

Three specimens were prepared for each curing age (7, 14, and 28 days) and cured at 20±3°C and 95% humidity using a thermo-hygrostat.

To identify the strength change according to the length and amount of fibers, fiber lengths of 5 mm and 20 mm and amounts of 0.1% and 0.5% relative to the soil weight were considered. To examine the effect of the cement amount, 100, 200 (typical in the field), and 300 kgf of cement per unit soil volume (m

) were considered. The water-to-cement ratio (W/C) was 80%, which is also typical in the field.

Table 2 shows the mix design in weight of each specimen

for different test conditions.

Fig. 3. Uniaxial compression and direct tensile tests for test specimen

Fig. 4. Compressive strength with curing age (clay, cement amount: 200 kgf/m

)

2.3 Test methods

Both uniaxial compressive and direct tensile strength tests (Fig. 3) were conducted with the displacement control method using a loading speed of 1 mm/min. I-shaped specimens were used for the direct tensile tests. The tests were carried out on three specimens (specimen size: 50mm in diameter, 100mm in height) for each set of conditions, and the mean value was used for comparison.

3. Test results and analysis

Fig. 4 shows the change in the compressive strength of the clay-based soil cement specimens for different amounts and lengths of natural hair fibers with 200 kgf/m

of cement

amount. The specimen strength with 200 kgf/m

of cement and 0.1% fibers (20 mm in length) increased by approximately 12% at a curing age of 28 days compared to the case with no fiber reinforcement. The less amount of cement with 100 kgf/m

resulted in insufficient bonding between the cement, soil and fibers. The effect of natural hair fibers on the strength thus became smaller. On the other hand, the more amount of cement with 300 kgf/m

increased the strength due to the cement itself so that the effect by hair fibers was less significant.

With short fibers (5 mm) or high amounts of fibers (0.5%),

an increase in compressive strength was not observed. The

results can be attributed to the fact that the short fibers can

have a small binding effect in a specimen and a high amount

of fibers can result in localized concentrations of fibers in

Fig. 5. Compressive strength with curing age (sand, cement amount: 200 kgf/m

) a specimen, which can cause a weak spot under loading. The

stress-strain curves for the fiber reinforced specimens had higher peak strengths and greater residual strength than no reinforced specimens. The failure planes of the clay-based soil cement specimens showed that the 0.5% content of 20-mm fibers was concentrated in a local zone, which caused a weak spot in the specimen under loading. The results show that increasing the length and amount of fibers can negatively affect the fiber distribution and decrease the strength of the soil cement.

The sand-based soil cement specimens were then tested with the same test procedure. Fig. 5 shows the change in the compressive strength for the sand-based soil specimens for different amounts and lengths of fibers. There was no pronounced difference for the different amounts of fiber when the length was 5 mm. The binding effect of the 5-mm fiber was small and did not influence the compressive strength.

In the case of the 20-mm fiber, the compressive strength was dependent on the amount of fiber. The specimen with 0.1% fiber showed the highest increase in the compressive strength (approximately 10% at 28 days) compared to the specimen with no fibers. However, when the amount was increased to 0.5%, the strength decreased. Overall, the trend was similar to that of the clay-based soil cement. It can be concluded that the effect of the fiber on increasing the compressive strength of the soil cement was not significant, although this was not the case for the tensile strength. As indicated for the clay-based soil cement, the stress-strain curves for the reinforced sand specimens had higher peak strengths

and residual strengths than no reinforced specimens.

The failure planes of the sand-based soil cement specimens also showed that the 0.5% amount of 20-mm fibers was concentrated in a local zone and caused a weak spot. However, the effect of the fiber concentration on the compressive strength was much more obvious in the sand-based soil cement than in the clay-based soil cement. This is presumably attributable to the difference in size between the fibers and soil particles, as well as the contact characteristics of the soil particles.

A sand particle is much bigger than a natural hair fiber (see Fig. 2), and the contacts of the sand particles, which show shear strength through the contact points, were considerably hindered by the fibers. For this reason, the strength under a compressive load was decreased significantly. In contrast, a clay particle is much smaller than a natural hair fiber (see Fig. 2), and the fiber does not affect the interaction between clay particles much under a compressive load, which reduces the influence of the amount of fibers on the strength.

Fig. 6 shows the change in the tensile strength of the clay-based soil cement specimens for different amounts and lengths of fibers with 200 kgf/m

of cement amount. At the curing age of 28 days, the tensile strength increased for certain fiber conditions compared to the case with no reinforcement.

The strength increase was most pronounced with the 0.1%

amount of 20-mm fiber, increasing by approximately 11%,

19%, and 21% for different cement amount of 100 kgf/m

,

200 kgf/m

, and 300 kgf/m

, respectively. However, the strength

was not affected much with the 0.1% amount of 5-mm fiber,

regardless of the cement amount. This is attributable to the

Fig. 6. Direct tensile strength with curing age (clay, cement amount: 200 kgf/m

)

Fig. 7. Direct tensile strength with curing age (sand, cement amount: 100 kgf/m

) small binding effect of short fibers in a specimen. The tensile

strength decreased when the amount of fiber increased to 0.5% due to the concentration of fibers in a local zone, which was most significant for the smallest amount of cement.

Overall, the change in tensile strength was generally more evident than in the compressive test. These results are attri- butable to the relatively small tensile strength of the soil cement compared with the compressive strength, which means the change in tensile strength due to the hair would be relatively large. The natural hair fiber played a more impor- tant role in tension than compression, which can be observed clearly in the sand-based soil cement under tensile load.

The stress-strain curves for natural fiber-reinforced specimens had a higher peak strength and greater deformation at the peak strength than no reinforced specimens. However, after

the peak, the strength suddenly dropped to zero without any residual strength, and the post-failure behavior was different from the compressive load conditions. It is clear from this result that the fiber acts as a reinforcing tension fiber that provides both higher peak strength and more deformation but no residual strength. The failure planes showed that the 0.5% amount of 20-m fibers was concentrated in a local zone and again caused a weak spot. The failure was mostly caused by the bond failure of the fiber to the cement rather than breaking failure. The tensile strength of the clay-based soil cement can be highly increased if the bond of the fibers to the soil cement matrix can be improved.

Figs. 7-9 show the change in the tensile strength of the

sand-based soil cement specimens for different amounts and

lengths of natural hair fibers with different cement amounts.

Fig. 8. Direct tensile strength with curing age (sand, cement amount: 200 kgf/m

)

Fig. 9. Direct tensile strength with curing age (sand, cement amount: 300 kgf/m

)

Hair fiber in clay cement matrix Hair fiber in clay cement matrix (SEM)

Hair fiber in sand cement matrix Hair fiber in sand cement matrix (SEM)

Fig. 10. Views of hair fibers in soil cement matrix

SH (5 mm, 0.1%) SH (5 mm, 0.5%) SH (20 mm, 0.1%) SH (20 mm, 0.5%) Fig. 11. Views on failure planes after direct tensile tests (sand, cement amount: 200 kgf/m

)

The strength increase was most pronounced with the 0.1%

amount of 20-mm fiber at the curing age of 28 days, increasing as much as 49% with 200 kgf/m

of cement amount. When compared with the clay-based soil cement, the increase in the tensile strength was significant in the sand-based soil cement. As mentioned, this is ascribable to the relative size between the fiber and soil particles and the contact charac- teristics of the soil particles. The sand particles are much bigger than the natural hair fibers (see Fig. 2), so the inter- locking between them is involved more (Fig. 10) as long as the fiber is sufficiently longer than the sand particles to provide higher resistance to a tensile load. The clay particles are much smaller than the fiber (see Fig. 2), so the fiber does not affect the interaction between clay particles much under tensile load, which decreases the influence of the amount of fibers on the strength.

With more cement (Fig. 9), the effect of the fiber was very slight with 0.1% fiber, but the strength was highly affected with 0.5% fiber, regardless of the fiber length. This result, on the one hand, is attributable to the amount of cement increasing the tensile strength of the soil cement without fibers whereby the effect of the fibers on the tensile strength decreases. On the other hand the result is attributable to the amount of fibers increasing the possibility of weak spots due to the concentration of fibers in a local zone whereby a failure is caused under a smaller tensile load.

As observed in the clay-based soil cement, the change in strength under a tensile load was much more evident than under compressive load. These results are also attributable to the smaller tensile strength of the soil cement compared with the compressive strength. In addition, the natural hair fiber played a more important role in tension than compression.

The stress-strain curves showed that the reinforced specimens

had higher peak strength and deformation at the peak than no reinforced specimens. After the peak, the strength suddenly dropped to zero without any residual strength with different post-failure behavior from the compressive load conditions.

The failure planes (Fig. 11) showed that the 0.5% amount of 20-mm hair was concentrated in a local zone and increased the vulnerability to loading. Again, the failure was mostly caused by bond failure rather than breaking failure, so increasing the bond strength would improve the tensile strength.

4. Conclusions

This study used environmentally friendly natural hair fiber to enhance the strength of clay and sand soil. A platform technology was also established for industrial use of the fiber materials. The results and technological implications are as follows.

(1) The inclusion of natural hair fibers in soil cement specimens changed both the compressive and tensile strengths of the soil cement specimens. The change in strength varied according to the soil type, the length and amount of fiber, and the amount of cement.

(2) The effect of natural hair fibers was more pronounced

in the sand-based soil cement than the clay-based soil

cement. In addition, the effect was more significant in

the tensile strength than the compressive strength. The

results are attributable to the contact and interlocking

characteristics of fiber with soil particles under different

soil and loading conditions. The higher contact stress

between sand particles and greater interlocking in tensile

loading are involved in the strength change. The tensile strength of the soil cement is also much smaller than the compressive strength, so the change in strength would naturally be larger for the tensile strength.

(3) The amount of cement affected the strength of the soil cement with the inclusion of natral hair fibers. The strength increase due to fibers was most effective with 200 kgf/m

of cement. The less amount of cement resulted in insufficient bonding between the cement, soil and fibers. The effect of fibers on the strength thus became smaller. On the other hand, the more amount of cement increased the strength due to the cement itself so that the bonding effect by fibers was less significant.

(4) The length and amount of natural hair fiber also affected the strength of the soil cement. The greatest increase of in strength occurred with the fiber included (20-mm length and 0.1% amount). The shorter fibers (5 mm) had a small binding effect in a specimen, while the high amount of fibers (0.5%) resulted in a localized concentration of fibers, which induced weak spots under loading and decreased strength.

(5) The increases in compressive and tensile strengths for the clay-based soil cement were approximately 12% and 19%, respectively, at a curing age of 28 days with natural hair fibers (20 mm in length and 0.1% in amount) and 200 kgf/m

of cement. The strength in the sand-based soil cement increased approximately 10% for the compressive strength and 49% for the tensile strength at 28 days with fibers (20 mm in length and 0.1% in amount) and 200 kgf/m

of cement. The increase in strength was most pronounced in the sand-based soil cement under tensile loading.

(6) The inclusion of natural hair fibers increased the residual strength under compressive loading and allowed for much larger deformation until failure under tensile loading. The results indicated that the natural hair fibers played a greater role as flexible reinforcing tension members within the soil cement matrix.

(7) The failure planes of the soil cement specimens under tensile load showed that the failure was mostly caused by the bond failure of natural hair fibers rather than breaking failure. It would be possible to increase the strength if the bond between the fibers and soil cement matrix could be improved.

(8) Reusing natural hair fibers to increase the strength of soils could have many benefits, including reduction of waste disposal costs, the amounts of cement required to improve ground strength, and the environmental impact compared to conventional methods using chemical fibers and treatments. The study results showed that there is potential for improving earth structures under tensile loading, such as embankment slopes, road subgrades, or landfills. The long-term behavior of soil cement reinforced by natural hair fiber should be examined further in the future.

Acknowledgement

This research was supported by Daegu University Research Grant 2014-0285. This support is gratefully acknowledged.

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