ejge paper 2005 -0520

 

 

Characterization of Tensile Strength
Behaviour of Coir Products

 

G. Venkatappa Rao

Department of Civil Engineering
Indian Institute of Technology, Hauz Khas, New Delhi, India
Email: gvrao@civil.iitd.ernet.in

and

R. K. Dutta

Department of Civil Engineering
National Institute of Technology, Hamirpur Himachal Pradesh, India
Email: rkd@recham.ernet.in

ABSTRACT

The present paper deals with the characterization of the tensile strength behaviour of four woven and four non-woven coir geotextiles. It has been concluded that the tensile strength of woven coir products is influenced by number of yarns and their runnage. However the behaviour of the non-woven coir geotextiles is influenced by the presence of the type of stitching, yarn used, coir/jute netting and the coir web weight.

Keywords: Geotextiles, coir, tesile strength, woven, non-woven

INTRODUCTION

India is well endowed with natural fibres like jute and coir. The products of coir and jute are available inexpensively in a processed form. It is the first largest country (66% of world production), producing coir fiber from the husk of coconut fruit. The coir fibres (50 to 150 mm long and 0.2 to 0.6 mm diameter) till recently were being spun into coir yarn and then woven to obtain woven nettings. The fibres are also now a days being air laid, needle punched or adhesive bonded to obtain non-woven products or blankets. Like their polymeric counterparts, natural fiber geotextiles are being synthesized primarily for erosion control application and these are comparatively inexpensive in Asian countries, Most of the present day products are being developed with an eye on erosion control applications (for vegetative growth), particularly, because among naturals they have much longer life. Their biodegradability has not encountered users for more permanent applications. In fact they are yet to be standardized , for their tensile behaviour and their biodegradability characteristics. Studies in this direction were initiated at Indian Institute of technology, Delhi (Venkatappa Rao and Balan (1994), Balan (1995) and Venkatappa Rao (1997)). Keeping this in view, over the years many varieties of woven and non-woven products have been developed in India and are now commercially available. The paper presents characteristion of tensile strength behaviour of four woven products and four non-woven products.

BACKGROUND

Extensive testing has been carried out at IIT Delhi (Balan (1995) and Venkatappa Rao (1997)) on jute and coir geotextiles in order to arrive at a rational procedure for evaluating the physical and strength characteristics as well as their biodegradability. Based on these studies, it was recommended that the thickness of natural geotextiles can be taken corresponding to a normal pressure of 2 kPa after one minute of application of pressure. The tensile strength of natural geotextiles can be taken as that corresponding to a wide width specimen (200  wide ´ 100 long) at a deformation rate of 10 mm/min determined in a constant rate of extension machine, and the overall life of jute and coir can have a life of more than one and two/three years respectively. It is also pertinent to point out that the tensile strength testing has been conducted on specimens of varying length of 25 mm to 200 mm keeping the width as 200 mm and also by varying the width from 25 mm to 200 mm and keeping a length of 100 mm constant. The deformation rate used varied from 2 to 300 mm/min. Five different varieties of jute and coir have been used in this study.

MATERIALS USED

The non-woven and woven coir geotextiles used in the present study were manufactured and supplied by M/S Aspinwall Geotech Ltd, Alappuzha and The Kerala State Coir Corporation, Alappuzha (Kerala State). The woven products are woven by power looms using specified coir yarns. On the other hand, for the non-woven products the coir web is first air laid and then stitched in machine direction either with HDPE yarn or jute yarn using a variety of scrims, as detailed in the following sections.

Woven Coir Geotextiles

Four different varieties of woven coir geotextiles designated as A, B, C and D were used in the present study. As evident they have widely varying aperture from 45 mm wide ´ 30 mm to 7 mm wide ´ 4 mm resulting in mass per unit area ranging from 335 gsm to 1335 gsm.

Material composition

The woven coir geotextiles Type A, B, C and D are netting composed of 100% coir fiber spun into yarn and woven in conventional flat bed looms in widths of 1, 2 or 4m.

Physical properties

The physical properties of these woven coir geotextiles are given in Table 1.

Table 1. Physical properties of woven coir geotextiles
Parameter Woven coir geotextile Type
A B C D
Roll width (m)1 1 1 1
Roll length(m)25 50 50 24
Runnage (warp)230 260 335 240
Runnage (weft)240 265 225 200
Number of yarns (warp)30 40 100 250
Number of yarns (weft)22 40 67 143
Aperture size (mm ´ mm) 45 ´ 30 25 ´ 25 15 ´ 10 7 ´ 4
Thickness (mm)7.3 6.7 8 9.5
Mass per unit area (gsm)335 360 610 1335

Non-woven Coir Geotextiles

Four different varieties of non-woven coir geotextiles designated as Type E, F, G and H were used in the present study.

Material composition

The Type E is composed of 100% de-curled coir fiber web of 400 gsm encased over top and bottom with brown PP netting. The mass per unit area of top and bottom netting is 7.1 gsm and 4.8 gsm. The matrix is stitched together on 50 mm centres with white PP thread dipped in black natural glue.

Type F is similar to Type E except that the coir fiber web is 750 gsm.

The non-woven coir geotextile Type G consists of 100% de-curled coir fiber web of 650 gsm encased over top and bottom with stable woven heavy jute netting. The matrix is stitched together on 50 mm centres with 2-ply jute yarn. The mass per unit area of the top and bottom jute netting is 100 gsm each.

The Type H comprises 100% de-curled coir web of 390 gsm encased over the top with heavy duty woven coir netting of 700 gsm and at the bottom with brown UV stabilized PP netting of 4.8 gsm. The matrix is stitched together on 50 mm centres with heavy 2 ply jute thread.

Physical properties

The physical properties of these non-woven products are given in Table 2. In addition, the physical properties of the top and bottom nettings of are given in Table 3.

Table 2. Physical properties of non-woven coir geotextiles
Parameter Non-woven coir geotextile Type
E F G H
Roll width (m)2.2 2.2 2.2 2.0
Roll length (m)25 42 25 25
Roll weight (kg)22 69 33 50
Thickness (mm)8.9 9 12 13.6
Mass per unit area (gsm)420 750 865 1175

 

Table 3. Physical properties of top and bottom scrim of non-woven coir geotextiles
Parameter Non-woven coir geotextile Type
ParameterE F G H
Mass per unit area of top PP netting (gsm)7.1 7.1 100 700
Aperture size of top PP netting (mm ´ mm) 20 ´ 35 20 ´ 35 20 ´ 25 10 ´ 15
Diameter of yarn of top netting in m/c direction (mm)0.25 0.25 - 3
Diameter of yarn of top netting in x-m/c direction (mm)0.22 0.22 - 3
Mass per unit area of bottom PP netting (gsm)4.8 4.8 100 4.8
Aperture size of bottom PP netting (mm ´ mm) 9 ´ 9 9 ´ 9 20 ´ 25 9 ´ 9
Diameter of yarn of bottom netting in m/c direction (mm)0.18 0.18 - 0.18
Diameter of yarn of bottom netting in x-m/c direction (mm)0.075 0.075 - 0.075

EXPERIMENTAL PROGRAM

Strength Testing

Laboratory tests were conducted on these geotextiles to determine their tensile strength. Five specimens each in m/c and x-m/c direction were tested at a deformation rate of 10 mm/min. The test procedures adopted were generally based upon the previous investigations carried out to standardize the testing and evaluation of the natural fiber geotextiles carried out at Indian Institute of Technology Delhi (Balan, 1995 and Venkatappa Rao, 1997).

TENSILE STRENGTH

Laboratory tests were conducted on woven and non-woven coir geotextiles to determine their tensile strength. The typical tensile stress-strain curves for woven coir geotextiles Types A, B, C, D and non-woven coir geotextiles Types E, F, G and H in machine and cross machine directions are presented in Figuress 1 to 2 respectively. A summary of these test results is presented in Tables 4 through 7. Being natural fiber products, one would expect that the scatter among the results will be very wide. A general comparison of all the stress-strain curves reveals that the variation in the results for a given non-woven geotextile is generally more than those for the woven geotextiles. These broadly indicate that the woven products exhibit higher strength compared to non-woven products and also the tensile elongation at failure is higher for the latter. The maximum strength exhibited is of the order of 30 kN/m at a failure strain of 40 %. Not withstanding this, the results indicate a definitive behaviour from which it is possible to draw conclusions and relate them with the overall composition, physical properties and manufacturing process of the products. A study of Table 1 and Tables 4 and 5 and Figs. 1 and 2 reveal the following.


Figure 1. Comparison of tensile strength among woven and
non-woven coir geotextiles in machine direction

 


Figure 2. Comparison of tensile strength among woven and
non-woven coir geotextiles in cross machine direction

Wide Width Tensile Strength of Woven Coir Geotextiles

For woven coir geotextile Type A, it is seen that the tensile strength in the machine direction is 3.86 kN/m at an elongation at failure of 19.64 % and the values for the cross-machine directions are 2.5 kN/m and 27.5 % respectively. It may be noted that the runnage of the yarns used in the machine and the cross-machine directions is nearly the same (230 mpk and 240 mpk respectively) but that the number of yarns are respectively 30 and 22 per metre. As such one may attribute higher strength in the machine direction to the large number of yarns. The smaller failure strain in the cross-machine direction could also be due to the same reason.

For woven coir geotextile Type B, it is seen that the tensile strength in the machine direction is 6.34 kN/m at an elongation at failure of 20 % and the values for the cross-machine directions are 6.1 kN/m and 20.83 % respectively. It may be recalled

that the runnage of the yarns used in the machine and the cross-machine directions is nearly same (260 mpk and 265 mpk respectively) and that the number of yarns are respectively 40 and 40 per metre. As such one may attribute similar strength as well as failure strain in the machine and the cross-machine direction to the same number of yarns. As expected, the strength of Type B is higher than Type A due to high mass per unit area.

For woven coir geotextile Type C, it is seen that the tensile strength in machine direction is 11.45 kN/m at an elongation at failure of 25.42 % and the values for cross-machine directions are 7.5 kN/m and 26.25 % respectively. For this geotextile the runnage of the yarns used in machine and cross-machine directions is 335 mpk and 225 mpk respectively and that the number of yarns are respectively 100 and 67 per metre. As such one may attribute higher strength in the machine direction to the larger number of yarns. Also the failure strain in the machine and the cross-machine direction is of the same order and it is due to the same reason. As expected, the strength of Type C is higher than Type A and Type B due to high mass per unit area.

For woven coir geotextile Type D, it may be observed that the tensile strength in the machine direction is 31.5 kN/m at an elongation at failure of 42 % and the values for the cross-machine directions are 12.73 kN/m and 18.15 % respectively. The runnage of the yarns used in the machine and the cross-machine directions is not significantly different (240 mpk and 200 mpk respectively) and that the number of yarns are respectively 250 and 143 per metre. As such one may attribute higher strength in the machine direction to the larger number of yarns. The smaller failure strain in the cross-machine direction could also be due to the same reason. Also the strength of Type D is predictably higher than those Types A, B and C due to the high mass per unit area.

In addition, from Tables 4 and 5, it may be noted that the coefficient of variation in the tensile strength of these woven coir geotextiles Types A, B, C and D were 0.03, 0.61, 3.11 and 1.81 in the machine direction and 0.03, 0.26, 0.91 and 0.17 in the cross-machine direction. Barring exceptions, these results indicate that there is a fair level of uniformity in the specimens, despite the variation that could be expected in a natural material.

Also the coefficient of variation in the tensile elongation of these woven coir geotextiles Types A, B, C and D are 1.29, 6.17, 9.90 and 0.625 in the machine direction and 4.43, 5.51, 12.96 and 0.70 in the cross-machine direction (Tables 4 and 5). Thus the coefficient of variation is more in the cross-machine direction than in the machine direction. Such variations may be expected as the power looms used in cottage industry may not be able to maintain the same level of tension in the weft direction, as is possible in the warp direction.

The initial tangent modulus of these woven coir geotextiles in the machine direction were determined corresponding to 5 mm deformation and reported in Table 4. From this found that coir geotextile Type D has the highest initial tangent modulus (being the heaviest and strongest) followed by geotextiles Types B and C and geotextile Type A the lightest and weakest has the lowest initial tangent modulus.

Table 4. Wide width tensile strength of woven geotextile specimens in machine direction
Type Tensile strength in machine direction (kN/m) Tensile elongation in machine direction (%) Initial tangent
modulus (kN/m)
Range Median Variance Range Median Variance
A3.64 to 4.09 3.86 0.03 17.68 to 20 19.64 1.29 30
B5.85 to 7.84 6.34 0.61 17.5 to 23.5 20 6.17 48
C9.79 to 13.75 11.45 3.11 20 to 26.67 25.42 9.90 46
D30.3 to 33.64 31.50 1.81 41 to 43 42 0.625 100

 

Table 5. Wide width tensile strength of woven geotextile specimens in cross machine direction
Type Tensile strength in cross machine direction (kN/m) Tensile elongation in cross machine direction (%)
Range Median Variance Range Median Variance
A2.39 to 2.84 2.50 0.03 25 to 29.92 27.5 4.43
B5.90 to 7.12 6.10 0.26 18.75 to 25 20.83 5.51
C5.83 to 8.92 7.50 0.91 20 to 28.25 26.25 12.96
D12.27 to 13.18 12.73 0.17 17 to 19.15 18.15 0.70

 

Table 6. Wide width tensile strength of non-woven geotextile specimens in machine direction
Type Tensile strength
in machine direction (kN/m)		 Tensile elongation
in machine direction (%)		 Initial tangent modulus (kN/m)
Range Median Variance Range Median Variance
E1.50 to 1.87 1.70 0.02 29.17 to 35 35 10.19 10
F2.24 to 3.13 2.76 0.20 27.50 to 32.50 31.67 7.18 13.8
G7.07 to 10.86 7.62 2.26 11.25 to 16.67 12.08 6.89 68
H12.73 to 16.57 15.38 2.39 24 to 30 26.75 4.55 49

 

Table 7. Wide width tensile strength of non-woven geotextile specimens in cross machine direction
Type Tensile strength
in cross machine direction (kN/m)		 Tensile elongation
in cross machine direction (%)
Range Median Variance Range Median Variance
E0.65 to 0.99 0.84 0.02 22.08 to 25 22.5 12.99
F0.52 to 0.67 0.65 0.004 10.83 to 19.17 16.67 11.61
G1.70 to 3.31 2.96 0.58 7.08 to 16.67 9.67 13.39
H7.46 to 9.36 8.25 0.54 24 to 31.5 26.5 11.83

 

Wide Width Tensile Strength of Non-Woven Coir Geotextiles

For the non-woven coir geotextiles, a study of Tables 2 and 3, Tables 6 and 7 and Figs. 1 and 2 reveal the following.

The mass per unit area of non-woven geotextile Type E is 420 gsm. It has a coir web of around 410 gsm with light PP netting of 7.1 gsm with aperture size 20 mm ´ 35 mm at the top and a netting of 4.8 gsm with aperture size 9 mm ´ 9 mm at the bottom. The matrix is stitch-bonded together on 50 mm centres with white PP thread in machine direction. The tensile strength in the machine direction is 1.7 kN/m at an elongation at failure of 35 % and the values for the cross-machine directions are 0.8 kN/m and 22.5% respectively. The contribution to the tensile characteristics of such products could comprise as that due to the coir web, the stitching and the netting in the decreasing order of magnitude. Thus one would expect a higher strength in the direction of stitching (i.e. in the machine direction).

The composition of the non-woven geotextile Type F is similar to Type E except that it is heavier, its mass per unit area being 750 gsm. Thus the higher strength of Type F could may be directly attributed to the nearly double the weight of the coir web.

The mass per unit area of non-woven geotextile Type G is 865 gsm. It has a coir web of 665 gsm placed inside two layers of heavy jute netting of 100 gsm with aperture size 20 ´ 25 mm. The matrix is stitch bonded together on 50 mm centres with 2 ply jute yarn in the machine direction. The tensile strength in machine direction is 7.6 kN/m at an elongation at failure of 12.08 % and the values corresponding to the cross-machine directions are 3.0 kN/m and 9.67%. The higher tensile strength in machine direction could be attributed due to the larger number of yarns in the jute netting in that direction. The stitching with jute thread along the machine direction may also a contributing factor.

Non-woven geotextile Type H has a mass per unit area of 1175 gsm with a coir web of 460 gsm and provided with woven coir netting of 700 gsm with aperture size 15 mm ´ 10 mm on one side and a brown UV stabilized PP netting of 4.8 gsm. and aperture size 9 mm ´ 9 mm on the other side. The matrix is stitched together on 50 mm centres with heavy 2 ply jute thread in the machine direction. The tensile strength in the machine direction is 15.38 kN/m at an elongation at failure of 26.75 % and the corresponding values for cross-machine directions are 8.25 kN/m and 26.5 %. The higher tensile strength in the machine direction could be due to the higher number of yarns in coir and the stitching with jute thread in the machine direction. The elongation at failure in case of Type H in machine direction and cross-machine direction is of the same order, the cause of the same being not directly evident.

The coefficient of variation in the tensile strength of non-woven coir geotextiles Types E, F, G and H were 0.02, 0.20, 2.26 and 2.39 in the machine direction and 0.02, 0.004, 0.58 and 0.54 in the cross-machine direction. These results indicate, despite the variation that could be expected in a natural material, there is a fair level of uniformity in the specimens. It could be noticed that Type G and Type H has more variation than the others possibly due to the variations in the characteristics of jute/coir netting and yarns. Also the very broadly, the variations are higher than those for woven geotextiles.

The initial tangent modulus of these non-woven coir geotextiles were determined corresponding to 5 mm deformation and it was found that coir geotextile Type G has the highest initial tangent modulus followed by geotextile Types H. Geotextiles Type E and F have the lowest initial tangent modulus. The presence of coir/jute netting and stitching by jute yarn could have greatly influenced the higher values of Types G and H.

CONCLUSIONS

On the basis of the results and discussion presented in this chapter, the following salient conclusions have been drawn.

All woven coir geotextiles used in the present investigation have more tensile strength in the machine direction than in the cross-machine direction. Their tensile strength is influenced by number of yarns and their runnage.

All non-woven coir geotextiles used in the present investigation have more tensile strength and tensile elongation in the machine direction than in the cross-machine direction. The behaviour of the non-woven coir geotextiles is influenced by the presence of the type of stitching, yarn used, coir/jute netting and the coir web weight.

The variance in tensile strength and tensile elongation of the non-woven coir geotextiles is generally more in comparison to woven coir geotextiles.

REFERENCES

  1. Balan, K. (1995) Studies on engineering behavior and uses of geotextiles with natural fibers, Ph.D Thesis submitted to Indian Institute of Technology Delhi (India).

  2. Venkatappa Rao, G. and K. Balan (1994) Design and development of a natural fiber strip drain, Proc. Int. Symp. on Biocomposites and Blends Based on Jute and Allied Fibers, pp. 87 – 95.

  3. Venkatappa Rao, G. (1997) Geosynthetic testing – recent developments, Proc. Geosynthetics Asia'97, 26-29 Nov, Bangalore, India, Vol. II., pp. A137 – A168.

 

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