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Comparative study of bubble point method and mercury intrusion porosimetry techniques for characterizing the pore-size distribution of geotextiles
Geotextiles and Geomembranes 13 (1994) 679 702 ")": 1994 Elsevier Science Limited Printed in Ireland. All rights reserved. 0266-1144/94/$7.00 ELSEVIER
Comparative Study of Bubble Point Method and Mercury Intrusion Porosimetry Techniques for Characterizing the Pore-Size Distribution of Geotextiles
Shobha K. Bhatiaa & Jennifer L. Smithb UDepartment of Civil and Environmental Engineering, 220 Hinds Hall, Syracuse University, Syracuse, New York, 13244, USA hO'Brien & Gere Engineers, 5000 Brittonfield Parkway, Syracuse, New York, 13221 USA (Received 15 October 1993; accepted 4 March 1994)
ABSTRA CT In this paper, two simple and rapid techniques for evaluating the pore-size distribution of geotextiles are compared." the bubble point method and mercury intrusion porosimetry. Both of these techniques have successfully been used to measure the pore-size distribution of various materials. These techniques, however, measure different and unique porometric characteristics of materials. The bubble point method measures through-flow pores while the mercury intrusion method measures the volume of pores. In this paper, test results of both bubble point and mercury intrusion methods for a wide variety of geotextiles are presented. In general, the mercury intrusion pore-size distributed results showed much larger pores in the geotextiIes than did the bubble point method. Overall, the mercury intrusion method was unable to distinguish between geotextiles of different manufacturing processes and various thicknesses. The bubble point method, on the other hand, was able to distinguish between geotextiles of different manufacturing processes and of various thicknesses, as long as the cross-sections of the geotextiles varied.
1 INTRODUCTION Geotextiles have become widely used in a variety of filtration applications, such as: in conventional pipe underdrains, behind rigid retaining walls, in 679
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Shobha K. Bhatia, Jennifer L. Smith
earth dams, beneath erosion control structures, and in landfill leachate collection systems. For the proper design of a geotextile as a filter, reliable information about the pore-size distribution of the geotextile is needed. Despite the importance of the pore-size distribution of a geotextile, it is a very difficult property to measure. Numerous techniques have been developed, such as: dry sieving (Calhoun, 1972; Gerry & Raymond, 1983), hydrodynamic sieving (Fayoux, 1977), wet sieving (Saathoff & Kohlhase, 1986), image analysis (Rollin et al., 1977; Faure et al., 1990; Bhatia et al., in preparation; Bhatia et al., 1994) and mathematical models (Lombard & Rollin, 1987); however, no method has been universally accepted. Varying results of pore-size distribution are often obtained within and between the existing test methods (Rollin, 1986; Smith, 1993). These methods, however, are generally only used for evaluating characteristic pore sizes of geotextiles, such as 095 , 050 and O15 (the pore sizes at which 95%, 50% and 15% of the pores in the geotextile are finer, respectively). In addition, these techniques are time consuming. In this paper, two fairly repeatable, reproducible, and rapid methods for evaluating the pore-size distribution of geotextiles are described: the bubble point method and mercury intrusion porosimetry. Results for a wide variety of geotextiles are presented. A detailed comparison is made between results obtained from the bubble point and mercury intrusion porosimetry methods. A comparative study of these methods is presented.
2 METHODS
2.1 Background The bubble point and mercury intrusion porosimetry methods are both standardized test methods for evaluating pore-size distributions of porous materials: membrane filters in the case of the bubble point method (ASTM, 1991) and soil and rock in the case of the mercury intrusion porosimetry method (ASTM, 1984). The bubble point method has been used for various materials, such as membranes (Bechhold, 1908; McBain & Kistler, 1930) and porous ceramic bodies (Knoll, 1940). The method was first applied to textile fabrics in 1949 (Schwertz, 1949) to test for water repellency of fabrics, and later in 1986 to find the pore-size distribution of textile fabrics (Miller et al., 1986). The mercury intrusion porosimetry method was first successfully used in 1945, to determine pore-size distributions of fritted glass, activated clays, diatomaceous earth and silica gels (Ritter & Drake, 1945). The method was later applied to correlate interfiber pore volume with air permeability (Burleigh et al., 1949; Wakeham &
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Spicer, 1949; Honold & Skau, 1951), and more recently to evaluate the porosity of Portland cement (Winslow & Diamond, 1970), the microstructure of clay (Griffiths & Joshi, 1989), and the pore-size distribution of geotextiles (Prapaharan et al., 1989).
2.2 Principles Both the bubble point and mercury intrusion porosimetry techniques are based on the principle of capillary flow, which states that a porous material will only allow a liquid to pass when the pressure applied exceeds the capillary attraction of the liquid in the largest pore. Because an equilibrium condition exists with the force of gravity, the diameters of the pores present in a porous material can be determined, based on the assumption that the pores are cylindrical: pwhere
4z cos 0 d
(1)
P = differential pressure being applied, z = surface tension of the liquid saturating the porous material, 0 = contact angle between the wetting liquid and the porous material (cos 0 = 1 for a completely saturated specimen), and d = diameter of the pore. In the bubble point test, a porous material is saturated with a liquid which completely wets the material. The saturating liquid must be easily absorbed into all pores of the material, so that the contact angle between the liquid and the porous material is, equal to zero (low surface tension liquids generally meet this condition). Gas pressure is then gradually increased at the upstream face of the wetted material and at a critical pressure, the first air bubble(s) will come through the largest pore(s) in the wetted material. The diameter of the largest pore may then be calculated. The bubble point test can also be used to measure the complete poresize distribution of a porous material by continually increasing the pressure and allowing progressively smaller and smaller pores to be emptied of liquid. This method is based on: (1) a dry specimen will pass air through all of its pores when any amount of air pressure is applied to one side of the specimen; and (2) a saturated specimen will only pass air through pores when the capillary attraction of the fluid is exceeded by the air pressure. As the air pressure is increased, smaller and smaller pores will pass air. By considering the flow rate of gas through pores of the specimen
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for both a dry and a saturated state, the percentage of gas flow passing through pores larger than or equal to a certain size may be calculated: Q -
wet flOWh wet flowt x 100% dry flow h dry flow t
(2)
where
Q : percentage of air flow passing through the material for a particular pore size range, h = higher pressure limit, and l = lower pressure limit. At each incremental increase in pressure during the bubble point test, a pore diameter present in the porous material is calculated. This procedure continues until the entire spectrum of pore sizes present in the material is determined. The number of pores of a particular size is determined from the percentage of total gas flow passing through the porous material for each pore size range. The complete pore-size distribution is then determined. In the mercury intrusion porosimetry test, a porous material is completely surrounded with mercury. Pressure is then applied to force the mercury into the pores of the material. Because mercury is a non-wetting liquid, it will not enter the pores of the material through capillary action. Unlike the bubble point method, however, the contact angle between the mercury and the pore wall is not equal to zero. The success of this method is dependent on the ability of the geotextile to retain its original structure during intrusion of mercury, any compression of fibers will lead to erroneous results (Prapaharan et al., 1989). With each incremental increase in pressure, a given pore volume present in the porous material is measured. This procedure continues until the entire spectrum of pores present in the material is measured. The number of pores of a particular size is evaluated by considering the percentage of the total volume of mercury intruded into the sample for each pore-size range. The complete pore-size distribution of the porous material is thus determined.
3 TESTING PROGRAM 3.1 Geotextiles
The bubble point and mercury intrusion porosimetry methods were used to evaluate the complete pore-size distribution of 28 different geotextiles from five different manufacturers. The geotextiles were selected based on polymer type, fiber type and manufacturing process: Type A are woven,
Bubble point method and mercury intrusion porosimetry techniques
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slit-film, plain weave; Types B and D are nonwoven, staple fiber, needlepunched geotextiles from two different manufacturers; Type C are nonwoven, continuous filament, needle-punched; and Type E are nonwoven, continuous filament, heat-bonded geotextiles. The staple fiber geotextiles, Types B and D, produced very similar results in all cases, therefore, results are only given for Type B. The physical properties of the geotextiles are given in Table 1.
3.2 Equipment and procedure The bubble point and mercury intrusion porosimetry tests were performed at Porous Materials Inc. (PMI), Ithaca, NY, with the PMI Automated Perm-Porometer (following ASTM, 1991) and with the PMI Automated Porosimeter (following ASTM, 1984), respectively. In the bubble point tests, porewick, a liquid with a much lower surface tension (0.0791 g/cm) than water (0.356 g/cm), was able to penetrate pores 0.065/~m in size, and was therefore used for the bubble point tests. Geotextile specimens were 3.96 cm in diameter and each test took approximately 20 minutes to perform. The computer program that controlled the Perm-Porometer provided the output data: (1) the differential pressure between the gas/liquid interface, (2) the calculated flow rate relating the wet and dry curves and (3) the calculated diameter of the pore at a specific pressure and flow rate, for each pressure increment. The data were analyzed with the following rationale: il~it takes a certain percentage of the total flow to pass through pores of a certain size and larger, then that cumulative filter flow percentage may be used to represent those numbers of pores coarser and finer than a particular size. F r o m this it was possible to plot the percentage of pores finer than a given size versus a particular pore diameter. In the mercury intrusion porosimetry tests, it was necessary to know the applicable contact angle between mercury and the geotextile pore walls. Despite the need for an accurate measurement of the contact angle, it has been difficult to obtain reliable measurements of contact angles (Neumann & Good, 1977). True contact angles for intrusion still remain unknown (Lowell & Shields, 1981). A contact angle of 135 ° between mercury and polyester geotextiles has, however, been measured by Prapaharan et al. (1989). No information regarding the contact angle between mercury and polypropylene geotextiles was found. Since most of the geotextiles included in this study were polypropylene geotextiles, a contact angle of 140° was used for all of the tests in order to be consistent with Prapaharan et al. (1989). For the mercury intrusion porosimetry tests, it is necessary that a measurable volume of mercury be intruded into a specimen. This is often
PP PP PP PP PET PET PP PP PP PP PP
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Table 1
Slit-film Slit-film Slit-film Stitched, Slit-film Multifilament Multifilament Staple Staple Staple Staple Staple
Fiber type
0-290-0-440 0-320-0-560 0-4204).531 0-2234).535 0.316-0.388 0-200 0-1804).220 0-1004).170 0-100-0.150 0.1374).141 0.100-0.111
0.64 0.56 0.81 1.20 0.73 1.08 0.90 1.06 2.08 3.13 4.54
Woven Woven Woven Woven Woven Woven Needle-punched Needle-punched Needle-punched Needle-punched Needle-punched
177.31 133-46 239.63 391.83 359.25 600.57 115.59 158.63 306.44 376.72 669.31
AOS (095) (mm) (ASTM D-4751)
Manufacturing process Average mass/area Average thickness (g/m2) (mm) (ASTM D-3776) (ASTM D-1777)
Geotextiles Used in this Study and their Basic Properties as Determined at Syracuse University
r~
PP
Cl C2 C3 C4 C5 C6 D1 D2 D3 D4 D5 E1 E2 E3 E4 E5 E6
Continuous Continuous Continuous Continuous Continuous Continuous Staple Staple Staple Staple Staple Continuous Continuous Continuous Continuous Continuous Continuous Needle-punched Needle-punched Needle-punched Needle-punched Needle-punched Needle-punched Needle-punched Needle-punched Needle-punched Needle-punched Needle-punched Heat-bonded Heat-bonded Heat-bonded Heat-bonded Heat-bonded Heat-bonded
Polypropylene; PET = polyester; NA = not available.
PP PP PP PP PP PP PET/PP PET/PP PET/PP PET/PP PET PP PP PP PP PP PP 131.33 220.20 311.15 443-54 562.36 430.89 257.05 322.42 524-37 184.46 198.64 57.74 68.30 105.68 144.03 215.31 271.04 1.32
1.17 0.34 0.32 0.39 0-49 0-61 0.63
1.57
2.04 2-55 3.50 4.44 3.46 2-41 2.72 3-72
0.219-0.292 0.131-0.204 0.136q9.182 0.140-0.160 0-190-0.220 0.194-0.255 0-070-0.130 0.098-0.104 0.055-0.085 0.138-0.151 0-090-0.110 0.546-0.596 0.432-0.546 0.174-0.231 0.140~.180 0.097 0.057-0.069
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difficult with thin geotextiles because of insufficient pore volume. For thin geotextiles such as heat-bonded, generally two or three specimens, approximately 1-5 cm in diameter, were stacked on top of one another to obtain sufficient pore volume. Since mercury completely surrounds all the specimens during the test, it is believed that there is minimum interaction between layers. Only one layer, approximately 1 cm in diameter, was used for the needle-punched geotextile specimens. Each test took approximately 35 minutes to perform. The computer program that controlled the porosimeter provided the output data: (1) the cumulative pore volume, (2) the percentage of total pore volume, (3) the average pressure, (4) the cumulative surface area and (5) the calculated diameter of the pore at a specific pressure, for each pressure increment. The data were analyzed according to the following rationale: if it takes a certain percentage of the total intrusion volume to pass through pores of a certain size and larger, then that percentage can be used to determine the percentage of pores of a particular size that are present in the specimen. From this it was possible to plot the percentage of pores finer than a given size versus a particular pore diameter.
4 RESULTS 4.1 Bubble point results Numerous bubble point tests were performed for 28 different geotextile specimens. In general, fairly good repeatability was found for multiple geotextile specimens of the same geotextile (Bhatia & Smith, 1994). 4.2 Mercury intrusion results Numerous mercury intrusion tests were performed with geotextile specimens. Excellent repeatability was found for multiple geotextile specimens of the same geotextile. Mercury intrusion results for several geotextile specimens also showed excellent agreement with results obtained by Prapaharan et al. (1989) for heat-bonded geotextile E4 (see Fig. l(a)) and Purdue Laboratories for needle-punched geotextile C4 (see Fig. l(b)). Since mercury is a very heavy liquid, there was concern as to possible deformation of fibers within the geotextile during the tests. Several mercury intrusion tests were repeated with identical geotextile specimens to observe any effects. Generally, results were very similar for the geotex-
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tiles tested, such as for needle-punched geotextile C5 (see Fig. 2(a)). There were, however, several cases where successive tests showed different larger and smaller pores for several geotextiles, such as for heat-bonded geotextile E4 (see Fig. 2(b)). The difference in large pores was most likely due to mercury remaining within the pores of the geotextile after extrusion, thus reducing the pore space. Whereas, t h e difference in smaller pores may have been due to either permanent deformation of fibers or to variations in contact angle resulting from the retention of mercury in the geotextile pores. The effect of the contact angle on mercury intrusion pore-size distribution results was also studied. A contact angle of 140 ° was used for the polypropylene geotextiles and an angle of 135.5 ° for the polyester geotextiles, it is quite possible, however, that the contact angle could range between 120 ° and 180 ° (Good, 1984). An analysis showing the effect of contact angle on the pore-size distribution results for geotextile C3 is given in Fig. 3. For a contact angle of 140 °, within an absolute range of 5 °, the results showed very little effect of contact angle on pore-size distribution (less than an absolute difference of 0-02 m m in 095 and in 050 pore sizes). Between contact angles of 120 ° and 180 ° , the difference in pore-size distribution results is quite pronounced, however, it is believed that the actual angle did not vary greatly from 140 °
5 C O M P A R I S O N OF M E T H O D S A N D DISCUSSION
5.1 Comparison of methods The bubble point and mercury intrusion porosimetry methods are both standardized test methods for evaluating pore-size distributions of many different types of materials, but not for geotextiles. They measure different and unique porometric characteristics of materials. Therefore, these methods do not produce similar pore-size distribution results for a given material. The bubble point test is a dynamic test where continuous air flow is used to remove liquid from the pores of a geotextile. Since air flow is only applied to one side of the geotextile, liquid is removed from those pores which form a continuous path through the geotextile. Those pores are emptied of liquid in order from largest to smallest. Because it is a flow through technique, the bubble point test measures the narrowest diameter of each pore channel present in the geotextile. The mercury intrusion test, on the other hand, is a static test. The pores of the geotextile are not measured as continuous pore channels through
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the geotextile, but rather as singular pores within the geotextile. These pores are measured from largest to smallest in terms of entire pore volume. Mercury will intrude the pores of a geotextile from all directions as the pressure is increased. Once mercury fills a pore, the test in that section of the pore channel stops. The bubble point method thus gives information on the number and sizes of the smallest effective through-flow pore channels in a geotextile and the mercury intrusion method gives information on the sizes of all pores found within a geotextile and a pore-size distribution based on total pore volume.
5.2 Comparison of results A comparison of bubble point and mercury intrusion pore-size distribution results is given for several different geotextiles of similar mass per unit area (131.33-158.63 g/m 2) in Fig. 4. Overall, the pore-size distribution curves given by the mercury intrusion method were much smoother than those given by the bubble point method. The mercury intrusion method was able to measure the complete pore-size distribution of a geotextile every time, unlike the bubble point method, where there were difficulties with the smallest one-third of total pores in several geotextiles (in the range of 0.06 m m and smaller). Good repeatability was found for multiple geotextile specimens for both methods. In general, there was more variability in bubble point results
Bubble point method and mercury intrusion porosimetry techniques
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than in mercury intrusion results, however, the differences were not significant. The variability which is seen in some tests for both methods is largely due to the heterogeneity of geotextile specimens. As with the bubble point method, the specimens which were used for the mercury intrusion tests were also visually inspected for similarity. The mercury intrusion method showed significantly larger pores for nearly all of the geotextiles tested than did the bubble point method. The bubble point and mercury intrusion results did, however, agree for the largest 5% of the pores present in two of the three woven, slit-film geotextiles tests (see Fig. 4(a)). No such agreement was found for any other geotextile. The finer pores of the slit-film geotextile were, however, quite different (for geotextile A2, the 05o was approximately 0.2 mm for the mercury intrusion method and approximately 0-03 mm for the bubble point method). The shapes of the pore-size distribution curves in the finer pore size range (less than 50% finer) were very similar for both methods. Both methods showed overall differences in pore-size distribution trends between woven (Fig. 4(a)) and nonwoven geotextiles (Fig. 4(b) (d)). Generally, both methods gave a more well-graded pore-size distribution for the woven geotextiles than for the nonwoven geotextiles. This was not, however, the case for the largest 40% of the pores given by the mercury intrusion method for the woven geotextiles. In the mercury intrusion method, a geotextile specimen is completely surrounded by mercury, mercury is then gradually intruded into the pores of the geotextile. For woven geotextiles, it takes very little pressure before most of the pores of the geotextile are intruded (typically, over 40% of the pores are intruded at a pressure of 0.341 kg/cm 2, which occurs within the first 20 seconds of a test). The intrusion rate for these pores is too fast to accurately measure the pore-size distribution of woven geotextiles. There are also several other problems with measuring large pores of geotextiles by mercury intrusion porosimetry. While intruding pores it is possible that at pressures below 0.21 kg/cm 2, mercury will not enter pores that it should because it clings together (Washburn, 1921). Even above these pressures, mercury may still not enter pores because the pressure is too low to force mercury in the areas of local roughness along the pore walls (Winslow, 1984). Another problem is due to the three-dimensionality of the pores. Because of this, the actual pressure on the mercury at the entrance of a pore may vary due to the differences in elevation, which is equivalent to different heads of mercury at various pore entrances (which can be as much as 25% of the gross pressure applied at low pressures) (Winslow, 1984). If the pressure reading is inaccurate then the calculated diameter of the pore will also be incorrect. Both methods also had difficulties distinguishing between pore-size
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distributions for nonwoven geotextiles of various manufacturing processes (needle-punched, Fig. 4(b) and (c); and heat-bonded, Fig. 4(d)). The mercury intrusion method showed a very uniform pore-size distribution for a majority of the pores present in the nonwoven geotextiles, excluding the largest and smallest 10% of pores in the geotextiles. This is most likely the result of elastic deformation of fibers upon the intrusion of mercury into the geotextile. The bubble point method did show a variety of pores present within the geotextile. In terms of the larger pores of the nonwoven geotextiles, the mercury intrusion method also gave a larger 095 for the continuous filament geotextile than for the staple fiber geotextile, which was consistent with the bubble point method. However, if the 095 results of several geotextiles of the same manufacturing processes of various mass per unit areas are compared, a relationship can be seen for the bubble point results, but not for the mercury intrusion results (see Fig. 5(a)). Similar trends can be seen for 050 results (see Fig. 5(b)). In general, 095 and 050 results were always greater for the mercury intrusion method in comparison to the bubble point method. 5.3 Geotextile thickness
The pore-size distribution results of geotextiles of various thicknesses of different manufacturing processes for the bubble point and mercury intrusion methods are shown in Fig. 6. Neither method showed any trends in pore-size distribution with increasing geotextile thickness for the slit-film geotextiles (see Fig. 6(a)). In fact, the mercury intrusion method showed little, if any, difference in pore-size distribution between any of the slit-film geotextiles tested. When comparing pore-size distribution results for the needle-punched geotextiles (staple fiber, Fig. 6(b); continuous filament, Fig. 6(c)), the bubble point method showed little effect of geotextile thickness on results, whereas the mercury intrusion method did. The mercury intrusion trends, however, showed increasing pore size with increasing thickness, which is not likely. In addition, the mercury intrusion method showed little difference in O9s pore sizes for these geotextiles. When comparing pore-size distribution results for the heat-bonded geotextiles (Fig. 6(d)), the bubble point method was able to distinguish between heat-bonded geotextiles of various thicknesses. The mercury intrusion method showed predominantly the same larger pores of the geotextiles (greater than 80%), however, it did show differences in the finer pore sizes of these geotextiles of increasing thickness. The trends for both methods showed decreasing pore sizes with increasing thickness.
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Bubble point and mercury intrusion pore-size distribution results were compared with results given by the dry sieving, wet sieving and hydrodynamic sieving methods. Generally, the bubble point method produced results which were similar to those given by the hydrodynamic sieving and wet sieving methods, where glass bead mixtures were used, while the mercury intrusion porosimetry method produced results which were similar to those given by the dry sieving method, where glass bead fractions were used (see Fig. 7). It is known from methods such as dry sieving, wet sieving and hydrodynamic sieving, that as the thickness of a geotextile increases, the
Bubble point method and mercury intrusion porosimetry techniques
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geotextile pore structure becomes effectively smaller, which is evident as glass beads meet more resistance in thicker geotextiles which have longer and more tortuous pathways. The bubble point method was only able to distinguish between heat-bonded geotextiles of increasing thickness, while the mercury intrusion porosimetry method was not able to distinguish between any geotextile tested (see Fig. 6). The bubble point method was able to distinguish between heat-bonded geotextiles of increasing thicknesses because the internal structure changes as heat-bonded geotextiles gain thickness, in needle-punched geotextiles (continuous filament and staple fiber) the internal structure remains the same for all geotextile thicknesses (Bhatia et al., 1993). Based on the results, it is believed that at this time the mercury intrusion porosimetry test does not show potential for characterizing pore sizes of geotextiles, because of its inability to distinguish between the pore structure of any geotextile tested. The bubble point method also had limitations, but was able to distinguish between geotextiles made by different manufacturing processes. The bubble point equipment is also capable of measuring the air and water permeability of virgin and clogged geotextiles, in addition to measuring the pore-size distribution of virgin and clogged geotextiles, as long as the geotextile manufacturing process varies (Bhatia & Smith, 1994). Therefore, the bubble point method shows potential for use in quality control applications.
CONCLUSIONS The bubble point and mercury intrusion methods are both simple and rapid techniques for evaluating the pore-size distribution of geotextiles. Both methods show repeatability in pore-size distribution measurements for all geotextiles. The methods are, however, very different in concept, and do not measure the same porometric characteristics of a geotextile. The bubble point method shows some difference in results for geotextiles of various thicknesses. This especially holds true for needle-punched geotextiles, where air flow will find the largest pore channels of the geotextile, which should be very similar for a particular manufacturing process. This method does, however, show differences in pore-size distribution between heat-bonded geotextiles of various thicknesses. This is due to melting of fibers in the heat-bonding process. The mercury intrusion method gave much larger pore-size distribution results than did the bubble point method. The mercury intrusion method, with the exception of the heat-bonded geotextiles, showed little difference
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ACKNOWLEDGEMENTS Research support from the National Science Foundation (EID9023915) and the federally sponsored Patricia Harris Fellowship program is gratefully acknowledged. The authors are very grateful to Dr Krishna Gupta of Porous Materials Inc., for giving them the opportunity to perform numerous bubble point and mercury intrusion tests and for his helpful comments. They would also like to thank Mr Jeff Pruitt and Mr Mark Meloni, of PMI, for their help in performing these tests.
REFERENCES Bechhold, H. (1908). Z. Physik. Chem., 21,328. Bhatia, S.K. & Smith, J.L. (1994). Application of the bubble point method in the characterization of the pore-size distribution of geotextiles. Amer. Soc. Test. Mater. Geotech. J., Submitted. Bhatia, S.K., Huang, Q. & Smith, J. (1993). Application of digital image processing in morphological analysis of geotextiles. In Proceedings of Conference on Digital Image Processing: Techniques and Applications in Civil Engineering, American Society of Civil Engineers, pp. 95-108. Bhatia, S.K., Huang, Q. & Mlynarek, J. (1994). Pore size characterization of nonwoven geotextile using image processing technique. Burleigh, E.G., Jr., Wakeham, H., Honold, E. & Skau, E.L. (1949). Textile Research Journal., 19, 547.
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Calhoun, C.C. Jr. (1972). Development of design criteria and acceptance specifications for plastic filter cloth. Technical Report F-72-7, U.S. Army Corps of Engineers Waterways Experimental Station, Vicksburg, Mississippi. Faure, Y.H., Gourc, J.P. & Gendrin, P. (1990). Structural study of porometry and filtration opening size of geotextiles. Geosynthetics: Microstructure and Performance, ASTM STP 1076, I.D. Peggs, editor, Philadelphia, pp. 102 19. Fayoux, D. (1977). Filtration hydrodynamique des sols par des textiles. In Proc. Int. Conf on the Use of Fabrics in Geotechnics, Paris, pp. 329-32. Gerry, B.S. & Raymond, G.P. (1983). Equivalent opening size of geotextiles. Geotechnical Testing Journal, 6(2) 53-63. Good, R.J. (1984). The contact angle of mercury on the internal surfaces of porous bodies. In Surface and Colloid Science, Vol. 13, eds E. Matijevic & R.J. Good, Plenum Press, New York, pp. 283-7. Griffiths, F.J. & Joshi, R.C. (1989). Change in pore size distribution due to consolidation of clays. Geotechnique, 39(1) 159 67. Honold, E. & Skau, E.L. (1951). Textile Research Journal, 21,419. Knoll, H. (1940). Kolloid Zhur., 90, 189. Lombard, G. & Rollin, A. (1987). Filtration behavior analysis of thin heatbonded geotextiles. In Proceedings of the Geosynthetics 1987 Conference, New Orleans, pp. 482-92. Lowell, S. & Shields, J.E. (1981). Powder Technology, 28, 201. McBain, J.W. & Kistler, S.S. (1930). Trans. Faraday Soc., 33, 157. Miller, B., Tyomkin, I. & Wehner, J.A. (1986). Quantifying the porous structures of fabrics for filtration applications. Fluid Filtration: Gas. Vol. 1, ASTM STP 975 R.R. Raber, editor, American Society of Testing and Materials, Philadelphia, PA, pp. 97 109. Neumann, A.W. & Good, R.J. (1977). Techniques of measuring contact angles. In Surface and Colloid Science, Vol. 11, eds R.J. Good & R.R. Stromberg, Plenum Press, New York, p. 31. Prapaharan, S., Holtz, R.D. & Luna, J.D. (1989). Pore size distribution of nonwoven geotextiles. Geotechnical Testing Journal, 12(4) 261-68. Ritter, J.L. & Drake, L.C. (1945). Ind. Eng. Chem., 17, 782. Rollin, A.L. (1986). Filtration opening size of geotextiles. ASTM Standardization News, pp. 50 2. Rollin, A.L., Masounave, J. & Dallaire, G. (1977). Etudes des proprietes hydrauliques des membranes non-tissees. In Proceedings of the International Conference on the Use of Fabrics in Geotechnics, Association Amicale des Ingenieurs Anciens Eleves del l'Ecole Nationale des Ponts et Chaussees, Paris, Vol. 2, pp. 295-9. Saathoff, F. & Kohlhase, S. (1986). Research at the Franzius-Institut on geotextile filters in hydraulic engineering. In Proceedings of the Fifth Congress Asian and Pacific Region Division, ADP/IAHR, Seoul, Korea. Schwertz, F.A. (1949). J. Applied Physics, 20, 1070. Smith, J.L. (1993). The pore-size distribution of geotextiles. Master's Thesis, Syracuse University, Syracuse, New York. ASTM (1984). Standard Test Method for Determination of Pore Volume and Pore Volume Distribution of Soil and Rock by Mercury Intrusion Porosimetry. ASTM Designation D 4404-84, PA, Vol. 4.08. ASTM (1991). Standard Test Method for Pore Size Characteristics of Membrane
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Filters by Bubble Point and Mean Flow Pore Test. ASTM Designation F 316-86, PA, Vol. 10.05. Wakeham, H. & Spicer, N. (1949). Textile Research Journal, 19, 703. Washburn, E.W. (1921). Proceedings of the National Academy of Science, Vol. 7, No. 115. Winslow, D.N. (1984). Advances in Experimental Techniques for Mercury Intrusion Porosimetry. In Surface and Colloid Science, Vol. 13, eds. E. Matijevic & R.J. Good, Plenum Press, New York. Winslow, D.N. & Diamond, S. (1970). A mercury porosimeter study of the evaluation of porosity in portland cement. Journal of Materials, ASTM, 5(3), 564-85.
Comparative Study of Bubble Point Method and Mercury Intrusion Porosimetry Techniques for Characterizing the Pore-Size Distribution of Geotextiles
Shobha K. Bhatiaa & Jennifer L. Smithb UDepartment of Civil and Environmental Engineering, 220 Hinds Hall, Syracuse University, Syracuse, New York, 13244, USA hO'Brien & Gere Engineers, 5000 Brittonfield Parkway, Syracuse, New York, 13221 USA (Received 15 October 1993; accepted 4 March 1994)
ABSTRA CT In this paper, two simple and rapid techniques for evaluating the pore-size distribution of geotextiles are compared." the bubble point method and mercury intrusion porosimetry. Both of these techniques have successfully been used to measure the pore-size distribution of various materials. These techniques, however, measure different and unique porometric characteristics of materials. The bubble point method measures through-flow pores while the mercury intrusion method measures the volume of pores. In this paper, test results of both bubble point and mercury intrusion methods for a wide variety of geotextiles are presented. In general, the mercury intrusion pore-size distributed results showed much larger pores in the geotextiIes than did the bubble point method. Overall, the mercury intrusion method was unable to distinguish between geotextiles of different manufacturing processes and various thicknesses. The bubble point method, on the other hand, was able to distinguish between geotextiles of different manufacturing processes and of various thicknesses, as long as the cross-sections of the geotextiles varied.
1 INTRODUCTION Geotextiles have become widely used in a variety of filtration applications, such as: in conventional pipe underdrains, behind rigid retaining walls, in 679
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Shobha K. Bhatia, Jennifer L. Smith
earth dams, beneath erosion control structures, and in landfill leachate collection systems. For the proper design of a geotextile as a filter, reliable information about the pore-size distribution of the geotextile is needed. Despite the importance of the pore-size distribution of a geotextile, it is a very difficult property to measure. Numerous techniques have been developed, such as: dry sieving (Calhoun, 1972; Gerry & Raymond, 1983), hydrodynamic sieving (Fayoux, 1977), wet sieving (Saathoff & Kohlhase, 1986), image analysis (Rollin et al., 1977; Faure et al., 1990; Bhatia et al., in preparation; Bhatia et al., 1994) and mathematical models (Lombard & Rollin, 1987); however, no method has been universally accepted. Varying results of pore-size distribution are often obtained within and between the existing test methods (Rollin, 1986; Smith, 1993). These methods, however, are generally only used for evaluating characteristic pore sizes of geotextiles, such as 095 , 050 and O15 (the pore sizes at which 95%, 50% and 15% of the pores in the geotextile are finer, respectively). In addition, these techniques are time consuming. In this paper, two fairly repeatable, reproducible, and rapid methods for evaluating the pore-size distribution of geotextiles are described: the bubble point method and mercury intrusion porosimetry. Results for a wide variety of geotextiles are presented. A detailed comparison is made between results obtained from the bubble point and mercury intrusion porosimetry methods. A comparative study of these methods is presented.
2 METHODS
2.1 Background The bubble point and mercury intrusion porosimetry methods are both standardized test methods for evaluating pore-size distributions of porous materials: membrane filters in the case of the bubble point method (ASTM, 1991) and soil and rock in the case of the mercury intrusion porosimetry method (ASTM, 1984). The bubble point method has been used for various materials, such as membranes (Bechhold, 1908; McBain & Kistler, 1930) and porous ceramic bodies (Knoll, 1940). The method was first applied to textile fabrics in 1949 (Schwertz, 1949) to test for water repellency of fabrics, and later in 1986 to find the pore-size distribution of textile fabrics (Miller et al., 1986). The mercury intrusion porosimetry method was first successfully used in 1945, to determine pore-size distributions of fritted glass, activated clays, diatomaceous earth and silica gels (Ritter & Drake, 1945). The method was later applied to correlate interfiber pore volume with air permeability (Burleigh et al., 1949; Wakeham &
Bubble point method and mercury intrusion porosimetry techniques
681
Spicer, 1949; Honold & Skau, 1951), and more recently to evaluate the porosity of Portland cement (Winslow & Diamond, 1970), the microstructure of clay (Griffiths & Joshi, 1989), and the pore-size distribution of geotextiles (Prapaharan et al., 1989).
2.2 Principles Both the bubble point and mercury intrusion porosimetry techniques are based on the principle of capillary flow, which states that a porous material will only allow a liquid to pass when the pressure applied exceeds the capillary attraction of the liquid in the largest pore. Because an equilibrium condition exists with the force of gravity, the diameters of the pores present in a porous material can be determined, based on the assumption that the pores are cylindrical: pwhere
4z cos 0 d
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682
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for both a dry and a saturated state, the percentage of gas flow passing through pores larger than or equal to a certain size may be calculated: Q -
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Q : percentage of air flow passing through the material for a particular pore size range, h = higher pressure limit, and l = lower pressure limit. At each incremental increase in pressure during the bubble point test, a pore diameter present in the porous material is calculated. This procedure continues until the entire spectrum of pore sizes present in the material is determined. The number of pores of a particular size is determined from the percentage of total gas flow passing through the porous material for each pore size range. The complete pore-size distribution is then determined. In the mercury intrusion porosimetry test, a porous material is completely surrounded with mercury. Pressure is then applied to force the mercury into the pores of the material. Because mercury is a non-wetting liquid, it will not enter the pores of the material through capillary action. Unlike the bubble point method, however, the contact angle between the mercury and the pore wall is not equal to zero. The success of this method is dependent on the ability of the geotextile to retain its original structure during intrusion of mercury, any compression of fibers will lead to erroneous results (Prapaharan et al., 1989). With each incremental increase in pressure, a given pore volume present in the porous material is measured. This procedure continues until the entire spectrum of pores present in the material is measured. The number of pores of a particular size is evaluated by considering the percentage of the total volume of mercury intruded into the sample for each pore-size range. The complete pore-size distribution of the porous material is thus determined.
3 TESTING PROGRAM 3.1 Geotextiles
The bubble point and mercury intrusion porosimetry methods were used to evaluate the complete pore-size distribution of 28 different geotextiles from five different manufacturers. The geotextiles were selected based on polymer type, fiber type and manufacturing process: Type A are woven,
Bubble point method and mercury intrusion porosimetry techniques
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slit-film, plain weave; Types B and D are nonwoven, staple fiber, needlepunched geotextiles from two different manufacturers; Type C are nonwoven, continuous filament, needle-punched; and Type E are nonwoven, continuous filament, heat-bonded geotextiles. The staple fiber geotextiles, Types B and D, produced very similar results in all cases, therefore, results are only given for Type B. The physical properties of the geotextiles are given in Table 1.
3.2 Equipment and procedure The bubble point and mercury intrusion porosimetry tests were performed at Porous Materials Inc. (PMI), Ithaca, NY, with the PMI Automated Perm-Porometer (following ASTM, 1991) and with the PMI Automated Porosimeter (following ASTM, 1984), respectively. In the bubble point tests, porewick, a liquid with a much lower surface tension (0.0791 g/cm) than water (0.356 g/cm), was able to penetrate pores 0.065/~m in size, and was therefore used for the bubble point tests. Geotextile specimens were 3.96 cm in diameter and each test took approximately 20 minutes to perform. The computer program that controlled the Perm-Porometer provided the output data: (1) the differential pressure between the gas/liquid interface, (2) the calculated flow rate relating the wet and dry curves and (3) the calculated diameter of the pore at a specific pressure and flow rate, for each pressure increment. The data were analyzed with the following rationale: il~it takes a certain percentage of the total flow to pass through pores of a certain size and larger, then that cumulative filter flow percentage may be used to represent those numbers of pores coarser and finer than a particular size. F r o m this it was possible to plot the percentage of pores finer than a given size versus a particular pore diameter. In the mercury intrusion porosimetry tests, it was necessary to know the applicable contact angle between mercury and the geotextile pore walls. Despite the need for an accurate measurement of the contact angle, it has been difficult to obtain reliable measurements of contact angles (Neumann & Good, 1977). True contact angles for intrusion still remain unknown (Lowell & Shields, 1981). A contact angle of 135 ° between mercury and polyester geotextiles has, however, been measured by Prapaharan et al. (1989). No information regarding the contact angle between mercury and polypropylene geotextiles was found. Since most of the geotextiles included in this study were polypropylene geotextiles, a contact angle of 140° was used for all of the tests in order to be consistent with Prapaharan et al. (1989). For the mercury intrusion porosimetry tests, it is necessary that a measurable volume of mercury be intruded into a specimen. This is often
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Fiber type
0-290-0-440 0-320-0-560 0-4204).531 0-2234).535 0.316-0.388 0-200 0-1804).220 0-1004).170 0-100-0.150 0.1374).141 0.100-0.111
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177.31 133-46 239.63 391.83 359.25 600.57 115.59 158.63 306.44 376.72 669.31
AOS (095) (mm) (ASTM D-4751)
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Geotextiles Used in this Study and their Basic Properties as Determined at Syracuse University
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PP PP PP PP PP PP PET/PP PET/PP PET/PP PET/PP PET PP PP PP PP PP PP 131.33 220.20 311.15 443-54 562.36 430.89 257.05 322.42 524-37 184.46 198.64 57.74 68.30 105.68 144.03 215.31 271.04 1.32
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difficult with thin geotextiles because of insufficient pore volume. For thin geotextiles such as heat-bonded, generally two or three specimens, approximately 1-5 cm in diameter, were stacked on top of one another to obtain sufficient pore volume. Since mercury completely surrounds all the specimens during the test, it is believed that there is minimum interaction between layers. Only one layer, approximately 1 cm in diameter, was used for the needle-punched geotextile specimens. Each test took approximately 35 minutes to perform. The computer program that controlled the porosimeter provided the output data: (1) the cumulative pore volume, (2) the percentage of total pore volume, (3) the average pressure, (4) the cumulative surface area and (5) the calculated diameter of the pore at a specific pressure, for each pressure increment. The data were analyzed according to the following rationale: if it takes a certain percentage of the total intrusion volume to pass through pores of a certain size and larger, then that percentage can be used to determine the percentage of pores of a particular size that are present in the specimen. From this it was possible to plot the percentage of pores finer than a given size versus a particular pore diameter.
4 RESULTS 4.1 Bubble point results Numerous bubble point tests were performed for 28 different geotextile specimens. In general, fairly good repeatability was found for multiple geotextile specimens of the same geotextile (Bhatia & Smith, 1994). 4.2 Mercury intrusion results Numerous mercury intrusion tests were performed with geotextile specimens. Excellent repeatability was found for multiple geotextile specimens of the same geotextile. Mercury intrusion results for several geotextile specimens also showed excellent agreement with results obtained by Prapaharan et al. (1989) for heat-bonded geotextile E4 (see Fig. l(a)) and Purdue Laboratories for needle-punched geotextile C4 (see Fig. l(b)). Since mercury is a very heavy liquid, there was concern as to possible deformation of fibers within the geotextile during the tests. Several mercury intrusion tests were repeated with identical geotextile specimens to observe any effects. Generally, results were very similar for the geotex-
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tiles tested, such as for needle-punched geotextile C5 (see Fig. 2(a)). There were, however, several cases where successive tests showed different larger and smaller pores for several geotextiles, such as for heat-bonded geotextile E4 (see Fig. 2(b)). The difference in large pores was most likely due to mercury remaining within the pores of the geotextile after extrusion, thus reducing the pore space. Whereas, t h e difference in smaller pores may have been due to either permanent deformation of fibers or to variations in contact angle resulting from the retention of mercury in the geotextile pores. The effect of the contact angle on mercury intrusion pore-size distribution results was also studied. A contact angle of 140 ° was used for the polypropylene geotextiles and an angle of 135.5 ° for the polyester geotextiles, it is quite possible, however, that the contact angle could range between 120 ° and 180 ° (Good, 1984). An analysis showing the effect of contact angle on the pore-size distribution results for geotextile C3 is given in Fig. 3. For a contact angle of 140 °, within an absolute range of 5 °, the results showed very little effect of contact angle on pore-size distribution (less than an absolute difference of 0-02 m m in 095 and in 050 pore sizes). Between contact angles of 120 ° and 180 ° , the difference in pore-size distribution results is quite pronounced, however, it is believed that the actual angle did not vary greatly from 140 °
5 C O M P A R I S O N OF M E T H O D S A N D DISCUSSION
5.1 Comparison of methods The bubble point and mercury intrusion porosimetry methods are both standardized test methods for evaluating pore-size distributions of many different types of materials, but not for geotextiles. They measure different and unique porometric characteristics of materials. Therefore, these methods do not produce similar pore-size distribution results for a given material. The bubble point test is a dynamic test where continuous air flow is used to remove liquid from the pores of a geotextile. Since air flow is only applied to one side of the geotextile, liquid is removed from those pores which form a continuous path through the geotextile. Those pores are emptied of liquid in order from largest to smallest. Because it is a flow through technique, the bubble point test measures the narrowest diameter of each pore channel present in the geotextile. The mercury intrusion test, on the other hand, is a static test. The pores of the geotextile are not measured as continuous pore channels through
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(b) Fig. 2. Successive mercury intrusion tests on the same specimen. (a) Nonwoven, needlepunched, continuous filament geotextile (geotextile); nonwoven, heat-bonded, continuous filament geotextile (geotextile E4).
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the geotextile, but rather as singular pores within the geotextile. These pores are measured from largest to smallest in terms of entire pore volume. Mercury will intrude the pores of a geotextile from all directions as the pressure is increased. Once mercury fills a pore, the test in that section of the pore channel stops. The bubble point method thus gives information on the number and sizes of the smallest effective through-flow pore channels in a geotextile and the mercury intrusion method gives information on the sizes of all pores found within a geotextile and a pore-size distribution based on total pore volume.
5.2 Comparison of results A comparison of bubble point and mercury intrusion pore-size distribution results is given for several different geotextiles of similar mass per unit area (131.33-158.63 g/m 2) in Fig. 4. Overall, the pore-size distribution curves given by the mercury intrusion method were much smoother than those given by the bubble point method. The mercury intrusion method was able to measure the complete pore-size distribution of a geotextile every time, unlike the bubble point method, where there were difficulties with the smallest one-third of total pores in several geotextiles (in the range of 0.06 m m and smaller). Good repeatability was found for multiple geotextile specimens for both methods. In general, there was more variability in bubble point results
Bubble point method and mercury intrusion porosimetry techniques
691
than in mercury intrusion results, however, the differences were not significant. The variability which is seen in some tests for both methods is largely due to the heterogeneity of geotextile specimens. As with the bubble point method, the specimens which were used for the mercury intrusion tests were also visually inspected for similarity. The mercury intrusion method showed significantly larger pores for nearly all of the geotextiles tested than did the bubble point method. The bubble point and mercury intrusion results did, however, agree for the largest 5% of the pores present in two of the three woven, slit-film geotextiles tests (see Fig. 4(a)). No such agreement was found for any other geotextile. The finer pores of the slit-film geotextile were, however, quite different (for geotextile A2, the 05o was approximately 0.2 mm for the mercury intrusion method and approximately 0-03 mm for the bubble point method). The shapes of the pore-size distribution curves in the finer pore size range (less than 50% finer) were very similar for both methods. Both methods showed overall differences in pore-size distribution trends between woven (Fig. 4(a)) and nonwoven geotextiles (Fig. 4(b) (d)). Generally, both methods gave a more well-graded pore-size distribution for the woven geotextiles than for the nonwoven geotextiles. This was not, however, the case for the largest 40% of the pores given by the mercury intrusion method for the woven geotextiles. In the mercury intrusion method, a geotextile specimen is completely surrounded by mercury, mercury is then gradually intruded into the pores of the geotextile. For woven geotextiles, it takes very little pressure before most of the pores of the geotextile are intruded (typically, over 40% of the pores are intruded at a pressure of 0.341 kg/cm 2, which occurs within the first 20 seconds of a test). The intrusion rate for these pores is too fast to accurately measure the pore-size distribution of woven geotextiles. There are also several other problems with measuring large pores of geotextiles by mercury intrusion porosimetry. While intruding pores it is possible that at pressures below 0.21 kg/cm 2, mercury will not enter pores that it should because it clings together (Washburn, 1921). Even above these pressures, mercury may still not enter pores because the pressure is too low to force mercury in the areas of local roughness along the pore walls (Winslow, 1984). Another problem is due to the three-dimensionality of the pores. Because of this, the actual pressure on the mercury at the entrance of a pore may vary due to the differences in elevation, which is equivalent to different heads of mercury at various pore entrances (which can be as much as 25% of the gross pressure applied at low pressures) (Winslow, 1984). If the pressure reading is inaccurate then the calculated diameter of the pore will also be incorrect. Both methods also had difficulties distinguishing between pore-size
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distributions for nonwoven geotextiles of various manufacturing processes (needle-punched, Fig. 4(b) and (c); and heat-bonded, Fig. 4(d)). The mercury intrusion method showed a very uniform pore-size distribution for a majority of the pores present in the nonwoven geotextiles, excluding the largest and smallest 10% of pores in the geotextiles. This is most likely the result of elastic deformation of fibers upon the intrusion of mercury into the geotextile. The bubble point method did show a variety of pores present within the geotextile. In terms of the larger pores of the nonwoven geotextiles, the mercury intrusion method also gave a larger 095 for the continuous filament geotextile than for the staple fiber geotextile, which was consistent with the bubble point method. However, if the 095 results of several geotextiles of the same manufacturing processes of various mass per unit areas are compared, a relationship can be seen for the bubble point results, but not for the mercury intrusion results (see Fig. 5(a)). Similar trends can be seen for 050 results (see Fig. 5(b)). In general, 095 and 050 results were always greater for the mercury intrusion method in comparison to the bubble point method. 5.3 Geotextile thickness
The pore-size distribution results of geotextiles of various thicknesses of different manufacturing processes for the bubble point and mercury intrusion methods are shown in Fig. 6. Neither method showed any trends in pore-size distribution with increasing geotextile thickness for the slit-film geotextiles (see Fig. 6(a)). In fact, the mercury intrusion method showed little, if any, difference in pore-size distribution between any of the slit-film geotextiles tested. When comparing pore-size distribution results for the needle-punched geotextiles (staple fiber, Fig. 6(b); continuous filament, Fig. 6(c)), the bubble point method showed little effect of geotextile thickness on results, whereas the mercury intrusion method did. The mercury intrusion trends, however, showed increasing pore size with increasing thickness, which is not likely. In addition, the mercury intrusion method showed little difference in O9s pore sizes for these geotextiles. When comparing pore-size distribution results for the heat-bonded geotextiles (Fig. 6(d)), the bubble point method was able to distinguish between heat-bonded geotextiles of various thicknesses. The mercury intrusion method showed predominantly the same larger pores of the geotextiles (greater than 80%), however, it did show differences in the finer pore sizes of these geotextiles of increasing thickness. The trends for both methods showed decreasing pore sizes with increasing thickness.
Bubble point method and mercury intrusion porosimetry techniques
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Bubble point and mercury intrusion pore-size distribution results were compared with results given by the dry sieving, wet sieving and hydrodynamic sieving methods. Generally, the bubble point method produced results which were similar to those given by the hydrodynamic sieving and wet sieving methods, where glass bead mixtures were used, while the mercury intrusion porosimetry method produced results which were similar to those given by the dry sieving method, where glass bead fractions were used (see Fig. 7). It is known from methods such as dry sieving, wet sieving and hydrodynamic sieving, that as the thickness of a geotextile increases, the
Bubble point method and mercury intrusion porosimetry techniques
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geotextile pore structure becomes effectively smaller, which is evident as glass beads meet more resistance in thicker geotextiles which have longer and more tortuous pathways. The bubble point method was only able to distinguish between heat-bonded geotextiles of increasing thickness, while the mercury intrusion porosimetry method was not able to distinguish between any geotextile tested (see Fig. 6). The bubble point method was able to distinguish between heat-bonded geotextiles of increasing thicknesses because the internal structure changes as heat-bonded geotextiles gain thickness, in needle-punched geotextiles (continuous filament and staple fiber) the internal structure remains the same for all geotextile thicknesses (Bhatia et al., 1993). Based on the results, it is believed that at this time the mercury intrusion porosimetry test does not show potential for characterizing pore sizes of geotextiles, because of its inability to distinguish between the pore structure of any geotextile tested. The bubble point method also had limitations, but was able to distinguish between geotextiles made by different manufacturing processes. The bubble point equipment is also capable of measuring the air and water permeability of virgin and clogged geotextiles, in addition to measuring the pore-size distribution of virgin and clogged geotextiles, as long as the geotextile manufacturing process varies (Bhatia & Smith, 1994). Therefore, the bubble point method shows potential for use in quality control applications.
CONCLUSIONS The bubble point and mercury intrusion methods are both simple and rapid techniques for evaluating the pore-size distribution of geotextiles. Both methods show repeatability in pore-size distribution measurements for all geotextiles. The methods are, however, very different in concept, and do not measure the same porometric characteristics of a geotextile. The bubble point method shows some difference in results for geotextiles of various thicknesses. This especially holds true for needle-punched geotextiles, where air flow will find the largest pore channels of the geotextile, which should be very similar for a particular manufacturing process. This method does, however, show differences in pore-size distribution between heat-bonded geotextiles of various thicknesses. This is due to melting of fibers in the heat-bonding process. The mercury intrusion method gave much larger pore-size distribution results than did the bubble point method. The mercury intrusion method, with the exception of the heat-bonded geotextiles, showed little difference
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ACKNOWLEDGEMENTS Research support from the National Science Foundation (EID9023915) and the federally sponsored Patricia Harris Fellowship program is gratefully acknowledged. The authors are very grateful to Dr Krishna Gupta of Porous Materials Inc., for giving them the opportunity to perform numerous bubble point and mercury intrusion tests and for his helpful comments. They would also like to thank Mr Jeff Pruitt and Mr Mark Meloni, of PMI, for their help in performing these tests.
REFERENCES Bechhold, H. (1908). Z. Physik. Chem., 21,328. Bhatia, S.K. & Smith, J.L. (1994). Application of the bubble point method in the characterization of the pore-size distribution of geotextiles. Amer. Soc. Test. Mater. Geotech. J., Submitted. Bhatia, S.K., Huang, Q. & Smith, J. (1993). Application of digital image processing in morphological analysis of geotextiles. In Proceedings of Conference on Digital Image Processing: Techniques and Applications in Civil Engineering, American Society of Civil Engineers, pp. 95-108. Bhatia, S.K., Huang, Q. & Mlynarek, J. (1994). Pore size characterization of nonwoven geotextile using image processing technique. Burleigh, E.G., Jr., Wakeham, H., Honold, E. & Skau, E.L. (1949). Textile Research Journal., 19, 547.
Bubble point method and mercury intrusion porosimetry techniques
701
Calhoun, C.C. Jr. (1972). Development of design criteria and acceptance specifications for plastic filter cloth. Technical Report F-72-7, U.S. Army Corps of Engineers Waterways Experimental Station, Vicksburg, Mississippi. Faure, Y.H., Gourc, J.P. & Gendrin, P. (1990). Structural study of porometry and filtration opening size of geotextiles. Geosynthetics: Microstructure and Performance, ASTM STP 1076, I.D. Peggs, editor, Philadelphia, pp. 102 19. Fayoux, D. (1977). Filtration hydrodynamique des sols par des textiles. In Proc. Int. Conf on the Use of Fabrics in Geotechnics, Paris, pp. 329-32. Gerry, B.S. & Raymond, G.P. (1983). Equivalent opening size of geotextiles. Geotechnical Testing Journal, 6(2) 53-63. Good, R.J. (1984). The contact angle of mercury on the internal surfaces of porous bodies. In Surface and Colloid Science, Vol. 13, eds E. Matijevic & R.J. Good, Plenum Press, New York, pp. 283-7. Griffiths, F.J. & Joshi, R.C. (1989). Change in pore size distribution due to consolidation of clays. Geotechnique, 39(1) 159 67. Honold, E. & Skau, E.L. (1951). Textile Research Journal, 21,419. Knoll, H. (1940). Kolloid Zhur., 90, 189. Lombard, G. & Rollin, A. (1987). Filtration behavior analysis of thin heatbonded geotextiles. In Proceedings of the Geosynthetics 1987 Conference, New Orleans, pp. 482-92. Lowell, S. & Shields, J.E. (1981). Powder Technology, 28, 201. McBain, J.W. & Kistler, S.S. (1930). Trans. Faraday Soc., 33, 157. Miller, B., Tyomkin, I. & Wehner, J.A. (1986). Quantifying the porous structures of fabrics for filtration applications. Fluid Filtration: Gas. Vol. 1, ASTM STP 975 R.R. Raber, editor, American Society of Testing and Materials, Philadelphia, PA, pp. 97 109. Neumann, A.W. & Good, R.J. (1977). Techniques of measuring contact angles. In Surface and Colloid Science, Vol. 11, eds R.J. Good & R.R. Stromberg, Plenum Press, New York, p. 31. Prapaharan, S., Holtz, R.D. & Luna, J.D. (1989). Pore size distribution of nonwoven geotextiles. Geotechnical Testing Journal, 12(4) 261-68. Ritter, J.L. & Drake, L.C. (1945). Ind. Eng. Chem., 17, 782. Rollin, A.L. (1986). Filtration opening size of geotextiles. ASTM Standardization News, pp. 50 2. Rollin, A.L., Masounave, J. & Dallaire, G. (1977). Etudes des proprietes hydrauliques des membranes non-tissees. In Proceedings of the International Conference on the Use of Fabrics in Geotechnics, Association Amicale des Ingenieurs Anciens Eleves del l'Ecole Nationale des Ponts et Chaussees, Paris, Vol. 2, pp. 295-9. Saathoff, F. & Kohlhase, S. (1986). Research at the Franzius-Institut on geotextile filters in hydraulic engineering. In Proceedings of the Fifth Congress Asian and Pacific Region Division, ADP/IAHR, Seoul, Korea. Schwertz, F.A. (1949). J. Applied Physics, 20, 1070. Smith, J.L. (1993). The pore-size distribution of geotextiles. Master's Thesis, Syracuse University, Syracuse, New York. ASTM (1984). Standard Test Method for Determination of Pore Volume and Pore Volume Distribution of Soil and Rock by Mercury Intrusion Porosimetry. ASTM Designation D 4404-84, PA, Vol. 4.08. ASTM (1991). Standard Test Method for Pore Size Characteristics of Membrane
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Filters by Bubble Point and Mean Flow Pore Test. ASTM Designation F 316-86, PA, Vol. 10.05. Wakeham, H. & Spicer, N. (1949). Textile Research Journal, 19, 703. Washburn, E.W. (1921). Proceedings of the National Academy of Science, Vol. 7, No. 115. Winslow, D.N. (1984). Advances in Experimental Techniques for Mercury Intrusion Porosimetry. In Surface and Colloid Science, Vol. 13, eds. E. Matijevic & R.J. Good, Plenum Press, New York. Winslow, D.N. & Diamond, S. (1970). A mercury porosimeter study of the evaluation of porosity in portland cement. Journal of Materials, ASTM, 5(3), 564-85.