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Interface morphologies in polyolefin fiber reinforced concrete composites
PII: s1359-835x(97)00114-0
ELSEVIER
Interface morphologies in polyolefin reinforced concrete composites
Linfa Yan, R. L. Pendleton
Composires Part A 29A ( 1998) 643-650 0 1998 Elsevier Science Limited Printed in Great Britain. All rights reserved 1359~835X/98/$19.00
fiber
and C. H. M. Jenkins*
Department of Mechanical Engineering, South Dakota School of Mines and Technology, Rapid City, SD 57701, USA (Received 3 March 1997; revised 22 October 1997; accepted 29 October 1997)
Morphologies of fiber-matrix interface in polyolefin fiber reinforced concrete composite (FRC) and fiber surface were observed by using a scanning electronic microscope (SEM). Interfacial roughness, alternate bulges and grooves with breadth varying from less than 1 pm up to 20 pm, oriented along interface longitudinal direction. Fibrils and fiber chips, with either end or both ends, were anchored in matrix. No patch of fiber material stuck on the interface, after fiber was peeled away from interface, was found. Fiber surface was roughened along fiber axial direction during mixing in cementitious mix and abraded again during pullout. It is concluded that the interfacial roughness was formed due to fiber surface roughness, and the bond between polyolefin fiber and concrete matrix is
mainly mechanical. 0 1998 Elsevier Science Limited. (Keywords: A. fibers; B. interface/interphase;
concrete matrix; roughness; mechanical bond)
INTRODUCTION Research has shown that post-cracking toughness of short synthetic fiber reinforced cement composites (FRC) is much higher than that of plain concretelm4. The increased toughness is believed attributable to the fiber bridging effect which resists further opening of initial cracks4. Resistance to crack opening and propagation is related to the strength of the bond between the fiber and the matrix and associated interfacial friction. This bond strength is therefore a primary factor in composite design. Much attention has been given to the bond strength and fiber pullout behaviors5-7 . In a study of interfacial debonding and sliding in brittle matrix composites, Mumm and Faber7 found that the fiber sliding was strongly dependent on the fiber surface roughness. Wang ef al.* studied single fiber pullout from a cementitious matrix, and found in most cases the pulling force for polypropylene and nylon fibers continued to increase after the fiber had begun to slip, due to fiber surface abrasion. It is generally recognized that the bond strength between the fiber and matrix, and the associated interfacial friction as debonding has taken place, constitute the main sources hindering fiber movement. Much attention, therefore, has also been given to the stress transfer through an interface’-12. Two basic models have been frequently used. Kelly and Tyson’ assumed that the matrix is rigid plastic and therefore the interfacial shear stress is a constant. Cox’” * Author to whom correspondence 2406; fax: (605) 394-6131.
should be addressed.
Tel: (605) 394.
assumed that the interfacial shear stress is linear elastic. These models indicate that the maximum fiber stress is located in the middle of fiber length. The maximum shear stresses, however, are located at the ends of the fiber in Cox’s model. As soon as the shear stress, on the interface exceeds the bond strength, the debonding will take place. Although these mathematical models can give approximate stress distributions along the interface, the bond strength remains unsolved until interfaces in individual composites have been carefully studied. Synthetic fibers have low Young’s modulus compared with a concrete matrix but they are usually chemically stable in acidic and basic environments, with polyolefin thought to be the least reactive”. No diffusion is possible for high weight molecules of solid polymeric fiber to inorganic cementitious mix at room temperature. The only possible mass transfer is lime solution infiltrating into the fiber and this seems very weak due to the compact ffiber surface layer and the water repellent property of the fibers”. Mineralization of calcium silicate hydrates formed during curing could not take place beneath the fiber surface unless there are microcracks underneath the fiber surface. The formation of transition layers with a chemical composition gradient between the fiber and matrix, or new substances resulting from chemical reaction on the fiber-matri# interface, seems impossible. In many tests, the synthetic reinforcement breaks clean from the concrete at the conclusion of the tests; this would suggest that there can be little or no chemical adhesion between the reinforcement and the concrete. There is virtually no information in the literature regarding the
643
Interface
morphologies
in polyolefin
fiber composites:
chemical bond between synthetic fibers and concrete. The dominating bond left seems to be primarily mechanical. Measures have been taken to partially remedy the lack of adhesive bond13. Fibrillated films of polypropylene, being opened in the form of continuous networks, may themselves be impregnated with cement mortar to form a composite sheet with high bending and impact strength. The chopped and twisted fibrillated polypropylene fibers, with their open structure, can partially remedy the lack of interfacial adhesion by making use of a wedge action at the slightly opened fiber ends and also by mechanical bonding through the fibrillation. Other measures have also been taken in principle of forming a mechanical interlock. The predesigned shapes and profiles that form mechanical interlocks have their specific applications and relatively higher costs. However, short monofilament fibers, with an as-received smooth surface, are desirable in large scale field applications due to their low cost and ease of use. Questions are immediately raised when using such short synthetic fibers as reinforcements, such as: what the interface bond will be? Fiber pullout tests provide a measure of an average bond strength in a composite. Direct observation of the interface has the potential of providing a much clearer picture of the mechanism involved in such bonding. No direct observation of such fiber-matrix interfaces has been found in literature. The objectives of this study are to investigate morphologies of the fiber surface immediately after mixing, and the fiber-matrix interface formed after mixing and hardening, and therefore to obtain a preliminary understanding of the interfacial bond between the fiber and concrete matrix in FRCs.
EXPERIMENTAL Two batches of short monofilament polyolefin fibers, categorized as No. 1 and No. 2 fibers from 3M Corp., spun and then drawn to 25 mils (0.635 mm) in nominal diameter (actually an elliptical cross section with 25 mils being the small diameter) and 2 inches long (50.8 mm), were used as reinforcing elements for FRCs; these were denoted as No. 1 FRC and No. 2 FRC, respectively, corresponding to fiber batch numbers. Interfaces between these fibers and concrete matrix were observed. To facilitate analysis of the formation of these interfaces, No. 1 fibers mixed with cementitious matrix materials (mix) for various mixing times were extracted from the mix for surface modification examination. Surface morphology of fibers pulled out from matrix of No. 1 FRC specimens which had been subjected to an impact test were also investigated. Observation of all samples was conducted under a JEOL JSM-840A scanning electron microscope (SEM) operating at 15, 20 or 25 kV, using a secondary electron imaging mode.
Preparation
of jber
No. 1 polyolefin
644
sueace for SEM observation fibers were mixed with cementitious
L. Yan
(mix of cement, fine and course aggregates, fly-ash and water; w/c = 0.5) in a 9 ft2 (0.25 m2) drum mixer for various times up to 88 min. Fibers for surface observation were picked out of the fresh mix at mixing times of 6, 35 and 88 min, respectively, and followed by washing in water to remove mix materials. The fibers were then kept in hydrochloric acid (37 wt%) solution to remove any alkali contaminants which might have stuck to their surfaces and concealed the true morphologies of fiber surfaces. An asmanufactured fiber was also soaked in the same solution to check acidic stability of the fiber material. All fibers were followed by rinsing them in water after soaking for 240 h. Fibers pulled out during impact testing were cut from the cracked surfaces. Impact specimens with mixing time of 35 and 88 min, respectively, were used to sample such fibers. No surface treatment, e.g. soaking by acidic solution, was conducted on these fibers. All fiber surfaces used for SEM observation were coated with carbon for electric conduction. The fibers used and their preparatory conditions are listed in Table 1, where Mf means mixed-only fiber, and Pf, the pulled out fiber, with the number following indicating mixing time in minutes.
Preparation
offiber-matrix
interfaces for SEM observation
No. 2 polyolefin fibers were mixed with mix (a mixture of cement, fine and course aggregates, fly-ash, water, plasticizer and air entertainment; w/c = 0.5) in the same drum mixer for casting of various test specimens of No. 2 FRC. Untracked parts of an impacted No. 1 FRC specimens with a mixing time of 35 min and 88 min, respectively, and a No. 2 FRC specimen with a mixing time of 5 min, were split with a chisel into smaller pieces. After cracking, the bridging fibers were cut with scissors to avoid further damaging the bulks. Bulks with untouched fibers inside were chosen for further sampling. Some were heated in an electric furnace up to 350°C to evaporate the fibers. The heated and remaining unheated bulks were split again along the plane where the fibers had lain. After splitting the unheated bulks, fibers stuck on the cleaved surface were peeled away by hand. All the split halves were checked, before sampling for SEM, under optical microscope to ensure the cleaved surface did in fact pass through the region where the fibers had lain, as shown in Figure I, leaving the interfaces undamaged. All the samples in this group chosen for SEM study were coated with gold for electrical conduction. Coating for each SEM specimen was conducted in several orientations,
Table 1
Preparatory
conditions
for individual
No. 1 fibers
Samples
Source
Mixing time (min.)
Surface treatment
MrO Mt-6 M& Pf-35
As-received Fresh mix Fresh mix Pulled out from impact specimen Pulled out from impact specimen
0 6 88 35
Hydrochloric As above As above None
88
None
Pf88
mix
et al.
acid
Interface
morphologies
in polyolefin
fiber composites:
L. Yan et al.
Figure 2 The surface of an as-manufactured No. I tiber (Sample M,-0). showing the surface texture due manufacturing and surface damage due to scratching of external particles. No acidic etching can be apparently found on the surface
Figure 1 (b) Pi-35
Table 2
Cleaved surfaces containing
Preparing
conditions
fiber-matrix
for individual
interfaces:
(a) HI-S:
interface samples
Samples
Mixing time (min)
Method to remove fibers
No.1 FRO: H,-35
35
P,-35 H,-88
35 88
Heating Peeling Heating
5 5
Heating Peeling
No. 2 FRC: H,-5 P,-5
usually 8, each for 30 sec. The preparation conditions various interface samples are summarized in Table 2.
for
RESULTS S@ace
morphologies
of mixed-only and pullout jibers
The surface of as-manufactured fibers is usually smooth when drawn, as shown in Figure 2. The texture along fiber longitudinal direction was seen, indicating the last processing of the fiber was a drawing operation. A groove with one
Figure 3 Damaged surface of Sample Mf6: Ca) fungus-like bugles. peeled up fib&. cavities and grooves formed during mixing: (b) grooves under fungus-like bulges lying along fiber axial direction
645
Interface
morphologies
in polyolefin
fiber composites:
L. Yan
et al.
Figure 4 Surface morphologies of pulled out fiber and mixed-only fiber: (a) difference in morphology between Pf35 (left) and M,-88 (right); (b) scratched surface of P,-35 having a landslide appearance, indicating it had been ploughed by matrix
particle at each of its ends was formed apparently by scratching of these particles, implying the fiber surface can be easily damaged longitudinally by external hard particles. No etching trace by hydrochloric acid on the surface was discernible, indicating the fiber material is chemically stable in the acid. There should be, therefore, no concern about changing surface morphologies of the as-mixed fibers due to the surface treatment in the acid. Therefore, the observed surface morphologies of acid treated fibers merely reflected fiber damage itself. As the fibers were being mixed, the hard particles of mix impacted and abraded the fiber surface, causing plastic deformation and even removal of part of surface layer, the degree and orientation of which were mainly dependent on the property of the fiber surface layer. Observation of sample MT6 showed that fungus-like bulges, pits, and grooves were formed on the surface; fibril peeled up from the surface, leaving grooves and ridges lying in the fiber axial direction, as shown in Figure 3(u). Further examination revealed that even in the ‘fungi’ covered area, surface
646
Figure 5 (c) P,-35
Interfacial morphologies
in No. 1 FRC: (a) H,-35; (b) Hi-88; and
damage took place along the fiber axial direction as seen from Figure 3(b). The different surface morphologies between samples M+-88 and Pr-35 were observed, as shown in Figure 4(u). The surface of M$38 was covered with fungus-like bulges (in the middle and right side of the surface), longitudinal grooves and long wire-like ridges, as well as torn-up fibrils
Interface
morphologies
in polyolefin
fiber composites:
L. Yan et al.
Figure 6 Morphologies of the interface in No. 2 FRC: (a) bulges and grooves lying along interfacial longitudinal direction with embGshment of nest-like clusters in the heated specimen of HI-S:(b) fibrils left on the interface in the peeled specimen of P,-5: (c)small needles planted on the interface in P,-5: and cdl needle-cement clusters in PI-5
and patches (on the left side of the surface). It is apparent that the wire-like ridges can be seen only after the surface layer has been peeled up. The surface of Pf35 seemed relatively neat and simple, longitudinal scratches being dominant on its surface. A close look at Pf35 revealed landslide morphology, as shown in Figure 4(b), implying that the surface was ploughed by hard matrix due to the matrix material incising into grooves or pits. A trunklike fibril (on the right) hints that part of surface material had been stretched to fracture during pullout because of anchoring of bulges in the matrix. Most of bulges were removed during fiber pullout process; only few ‘fungi’ could be found on fiber surface after pullout of the fiber.
Interjkes
in NO. I FRC
During the curing of FRC. the interface on the matrix side was molded by the surface of the fiber. The roughness, composed of bulges and grooves, apparently orienting in the direction where the fiber axis had lain, was observed in all
samples of FRC, as shown in Figure 5. In comparison of different mixing times. it was observed that the longer the mixing time, the finer the roughness in No. 1 FRC. Sample Hi-35 had a pine bark-like morphology (Figure S(a)), with breadth of about lo-20 pm. This was diff$rent from sample HI-88 (Figure 5(b)), which had worm-like or bamboo-like ridges with a diameter less than 10 pm. This indicates the removal of the fiber surface layer may gradually reveal the change in fiber surface morphology with the depth from the surface. Observation of samples with their interfaces exposed by peeling away fibers revealed that. in addition to the roughness of the interface. fibril segments and fiber chips were anchored in the matrix, as shown in Figure 5(c). No transition layer, or even trace of a patch of fiber material adhering to the interface. was observed. suggesting the interfacial bonding strength is very weak. It is noted that the orientation of roughness in P,-35 is not as clear as that in H,-35. This may be due to the damage which occurred while the fiber was being peeled, thus changing some of the original interfacial morphology.
647
Interface
morphologies
in polyolefin
fiber composites:
L. Yan
et al.
broken-off fibril
Figure
Figure 7 process
9
Schematic
of forming tendrils and grooves on fiber surface
Schematic drawing of fibrils formed in fiber due to cold drawing surface-grooved fiber embedded in matrix
cementitious matrix /
\ high strength direction (fiber axial direction) <
> matrix materials penetrating ce
low strength direction I Figure
Figure 8 Schematic drawn fiber
of microstructure
and anisotropic
properties
10
Schematic
of formation
of interfacial
roughness
in a
with the smallest width of crack shown in Figure 6(d) less than one third pm.
Interfaces in No. 2 FRC
The interfacial morphology of sample Hi-5 is shown in Figure 6(a). The ridge-like bulges and grooves are more apparent and much finer compared to those observed in No. 1 FRC. The breadth of ridges and grooves along the longitudinal direction of the interface are estimated to be of an order less than 1 pm. Nest-like clusters, which were not found in the No. 1 FRCs and obviously composed of cementitious sticks, were also observed. These were due to the sample having been subjected to heating, where some materials had been removed while others remained stuck on the interface. The inter-facial morphology of sample Pi-5 shows that fibril segments of different sizes were left on the surface. These were strips or tendrils with ends planted in the matrix, as shown in Figure 6(b). Higher magnification revealed that needles lay both on the interface and at angles to the interface, as shown in Figure 6(c). In addition, many had been pulled away during peeling off of the fiber, leaving holes on the interface. Further examination of the interface revealed that needle-like substances were stuck in the cement to form needle-cement clusters on the interface, as seen from Figure 6(d), confirming that nest-like clusters observed in Hi-5 originally had synthetic needles inside. The needles were also observed in other places, besides on the fiber-matrix interface, such as the cleaved surface and air bubble surfaces14. It is worth mentioning here that, from all SEM pictures shown, the concrete matrix is a cracking material. Cracks, small or large, can be observed everywhere in the pictures,
646
DISCUSSION Synthetic fibers of monofilament are usually manufactured by extruding (‘spun’ in textile industry jargon) the melt, or solution, of raw material through the fine hole of a die (spinneret). Slight surface texture along the fiber axis may be produced when the fiber is passing through the die. Spun polypropylene fibers do not have high strength in the axial direction due to poor crystallinity and texture (low orientation). Cold drawing is used to change fiber microstructure and thus to increase their strength15. The original small crystallites are arranged in spherulite superstructures. On cold drawing, the superstructures are greatly elongated and long crystallites (microfils) are formed from the original ctrystallites plus additional long chain polymer molecules from the spherulite. This stress crystallization increases the total polymer crystallinity while orienting the polymer chains and crystallites in the drawing direction. Microfils are further stuck together to form bundles or fibrils. The distribution of fibrils in fiber is schematically shown in Figure 7.
The crystallinity depends on the draw ratio, the ratio of the surface speed of feed and draw sets of roll, respectively. The greater the ratio, the higher the crystallinity, and the higher the fiber strength. Higher strength is achieved at the expense of plasticity, and therefore a general rule is that high strength fiber usually has low plasticity. Microscopically, high draw ratio also leads to fine crystallites, and thus fine fibrils, and high strength. However, one cannot expect that all regions in the fiber are crystalline. There are regions
Interface
crack in matrix
Figure 11
morphologies
penetrated matrix materials
Schematic showing forces acting on a fiber
of amorphous, non-crystallinity between crystallites, much like a composite of amorphous matrix embedded with fibrils of crystallites. The mechanical properties of the fibers manufactured in this way are anisotropic, strong in the longitudinal direction, but weak in the radial and circumferential directions, as shown in Figure 8. Alternating loading on the fiber surface, along with impact and abrasion from mix particles in this study, caused immediate plastic deformation where the load was high, and fatigue where the load was low. Some hard cementitious particles with sharp edges incising and wedging into vulnerable fiber surface kept moving with mix, trying to peel up or away the fiber surface layer. The relative weak bond between textured parts (or fibrils) which lay along the fiber longitudinal direction failed first. Some fibrils were lifted up, broken or tom away, leaving grooves and tendrils on the fiber surface, as schematically shown in Figure 9. The amorphous matrix of the fiber was more plastic and therefore stretched longer than the fibrils at fracture. Recovery of the stretched parts left ‘fungi’ on the fiber surface. During the hardening of FRC, the interface between the fiber and cementitious matrix was tightly matched due to the penetration of mix materials into grooves and pits on the fiber surface. A negative impression of the fiber surface was replicated on the interface on the matrix side; ridge-like bulges and grooves lying along interfacial longitudinal directions were thus formed, as shown in Figure 10. Care has to be taken to analyze these observations. Samples denoted Hi-(heated interface) had no polymeric materials left on them due to the fact that the samples had been heated to 350°C whereby the polymer materials may be melted or decomposed. Therefore. little mechanical damage on the interface took place and thus the inter-facial morphology, except for fiber chips and fibrils, was largely preserved. Samples marked Pi-(peeled interface) had the interface damaged to some extent because manual removal of fiber might bring away cementitious materials on the original interface, resulting in the orientation being not as conspicuous as those in the heated samples. However, fiber chips and fibrils which were anchored firmly in the matrix had been left for observation. This facilitated the analysis of true interfacial bond. A notable phenomenon found in the Pi-5 sample were needles and needle clusters existing on the interface, as well as in other areas of the samples. Compared to the morphology of H,-5. nest-like clusters should have been filled with needles, but having been removed, left the
in polyolefin
fiber composites:
L. Yan et al.
present nest-like cementitious skeleton. We then conclude that the needles in sample Hi-5 were removed by heating, and therefore those in sample Pi-5 were of Aber material. An assumption can be made that fibrils, produced due to the fibrillation of fibers during mixing, were further collapsed into needles, presumably due to the fact that the fibrils in the surface layer of No. 2 fiber were more brittle than those in the No. 1 fibers, which resulted from the high draw ratio. Finer ridges, compared to those of No. 1 FRCs, observed on the interface of Hi-5 sample, corresponding to finer fibrils removed from the No. 2 fiber surface, seem to support this assumption. The existence of needle clusters and needles themselves might affect the properties of the FRC. Nevertheless, this needs to be further investigated. The interfacial roughness existing between reinforcing fibers and the cementitious matrix as observed constituted an interlock between the fiber and matrix. When cracks initiated in the FRCs, the interlocked interface resisted the moving of the fiber; cementitious bulges, which had penetrated into the grooves and pits on the fiber surface, scraped the fiber surface; and polymeric bulges located on the fiber surface, which had been embedded in the matrix, were stretched, as schematically shown in Figure Il. When the pulling force on the fiber became greater than the interlock and frictional resistance, the liber was moved, and then new and/or deepened grooves were formed on the surface of the pulled out fiber. The surface morphology observed on the pulled out fiber. having a sculpted orientation with sharp edges, was apparently different from that of a merely mixed one, implying the fiber surface had been scraped by matrix along its axial direction. Although we did not conduct fiber pullout tests. it may be inferred from this observation that the slippage strength would be greater than bonding strength due to the surface abrasion. This was caincident with the results of fiber pullout’.
CONCLUSIONS An SEM study was conducted on the interfaces between polyolefin fibers and cementitious matrix. Surface morphologies of the synthetic fibers that were just mixed for various times, and that were pulled out during impact tests, were also studied. Ridge-like bulges and grooves oriented in the axial direction of the interface were found on the fiber-matrix interface in all specimens studied. Fibrils and fiber chips anchored in the interfaces were also observed in all FRC specimens. In No. 2 FRC, needles and needle clusters of fiber material were found to be distributed everywhere, on interfaces, air bubble surfaces, and in the matrix. Fibrillation of the drawn monofilament fibers characterized by fibril segments being striped up and away during mixing, leaving grooves and pits on the fiber surface, was observed. Fungus-like bulges covering most of the as-mixed fiber surface. formed by recovery of stretch-to-fracture of the surface layer of the fiber, were also identified. Damage of the pulled out fiber surface was characterized by landslide
649
Interface morphologies
in polyolefin
fiber composites:
L. Yan
et al.
morphology oriented along the fiber axis due to the abrasion of the hard matrix. It is concluded that the roughening interface between the reinforcing fibers and the matrix is formed due to the damage of fiber surface caused by mixing in cementitious mix. This roughening interface constructs a mechanical interlock which resists relative movement of fibers immediately after cracks initiated. Therefore, fiber surface damage caused by mixing plays an important role in forming this roughness. No attempt has yet been undertaken to determine the optimum degree of the damage to be chosen for field applications, though of course this is an important area to study. It is, however, suggested to investigate proper cold draw ratio of the fiber, which causes fibrillation in the fiber, especially in the surface layer. The properties of fibrils in the surface layer may not only directly affect the morphology of interfacial roughness, but the shape and distribution of fiber chips peeled off from the fiber. The effect and behavior of these chips and needles under loading are still unclear and therefore need investigation.
REFERENCES
ACKNOWLEDGEMENTS
10.
This work has been supported in part by the National Science Foundation under Grant No. ORS-9108773 and State of South Dakota Future Fund. Dr. E. F. Duke at South Dakota School of Mines and Technology is specially acknowledged for his guidance in SEM work. Dr. V. Ramakrishnan at the same university and Mr. C. N. MacDonald from 3M Corp. are appreciated for their support in providing laboratory facilities and fibers, respectively, for use in preparing specimens.
1.
2. 3.
4. 5.
6.
7.
8.
9.
11. 12. 13. 14.
15.
Wang, Y., Backer, S. and Li, V. C., An experimental study of synthetic fiber reinforced cementitious composites. J. Mufer. Sci., 1987,22,4281-4491. Wang, Y., Li, V. C. and Backer, S., Tensile properties of synthetic fiber reinforced mortar. Gem. Concrefe Compo., 1990,12,29-40. Wang, Y., Li, V. C. and Backer, S., Tensile failure mechanism in synthetic fiber-reinforced mortar. J. Mater. Sci., 1991, 26, 65656575. Shah, S. P., Do fibers increase the tensile strength of cement-based matrixes?. ACZ Mater. J., 1991,88(6), 595-602. Li, Z., Mobasher, B. and Shah, S., Characterization of interfacial properties in fiber-reinforced comentitious composites. J. Am. Ceram. Sot., 1991, 74(9), 2156-2164. Naaman, A. E. and Alwan, J. M., Comment on ‘Characterization of interfacial properties in fiber-reinforced comentitious composites’. J. Am. Ceram. Sot., 1993, 76(6), 1645-1646. Mumm, D. R. and Faber, K. T., Interfacial dedonding and sliding in brittle-matrix composites measured using an improved fiber pullout technique. Acta Mettall. Mater., 1995, 43, 1259-1270. Wang, Y., Li, V. C. and Backer, S., Analysis of synthetic fiber pullout from a cement matrix. In Bonding in Cemenfifious Composites, eds S. Mindess and S. P. Shah. Materials Research Society Symposium Proceedings, 1988, MRS., Pittsburgh, PA, pp. 159-165. Kelly, A. and Tyson, W. R., Tensile properties of fiber reinforced metals: copper/tungsten and copper/molybdenum. J. Mech. Phys. Solids., 1965, 13, 329-350. Cox, H. L., The elasticity and strength of paper and other fibrous materials. British J. App. Phys., 1952, 3, 72-79. Kim, J., Zhou, L. and Mai, Y., Stress transfer in fiber fragmentation test. J. Mater. Sci., 1993, 28, 6233-6245. Moncrieff, R. W., Man-Made Fibers. Wiley, New York, 1963, p. 123. Nanni, J., Fiber Cements and Fiber Concrete. Wiley, New York, 1978, pp. 81-91. Yan, L., Study of dynamic properties and interfaces of synthetic fiber reinforced cement composites. Ph.D. dissertation, South Dakota School of Mines and Technology, Rapid City, SD, 1996. Ahmed, M., Polypropylene Fibers-Science and technology. Elsevier, New York, 1982.
ELSEVIER
Interface morphologies in polyolefin reinforced concrete composites
Linfa Yan, R. L. Pendleton
Composires Part A 29A ( 1998) 643-650 0 1998 Elsevier Science Limited Printed in Great Britain. All rights reserved 1359~835X/98/$19.00
fiber
and C. H. M. Jenkins*
Department of Mechanical Engineering, South Dakota School of Mines and Technology, Rapid City, SD 57701, USA (Received 3 March 1997; revised 22 October 1997; accepted 29 October 1997)
Morphologies of fiber-matrix interface in polyolefin fiber reinforced concrete composite (FRC) and fiber surface were observed by using a scanning electronic microscope (SEM). Interfacial roughness, alternate bulges and grooves with breadth varying from less than 1 pm up to 20 pm, oriented along interface longitudinal direction. Fibrils and fiber chips, with either end or both ends, were anchored in matrix. No patch of fiber material stuck on the interface, after fiber was peeled away from interface, was found. Fiber surface was roughened along fiber axial direction during mixing in cementitious mix and abraded again during pullout. It is concluded that the interfacial roughness was formed due to fiber surface roughness, and the bond between polyolefin fiber and concrete matrix is
mainly mechanical. 0 1998 Elsevier Science Limited. (Keywords: A. fibers; B. interface/interphase;
concrete matrix; roughness; mechanical bond)
INTRODUCTION Research has shown that post-cracking toughness of short synthetic fiber reinforced cement composites (FRC) is much higher than that of plain concretelm4. The increased toughness is believed attributable to the fiber bridging effect which resists further opening of initial cracks4. Resistance to crack opening and propagation is related to the strength of the bond between the fiber and the matrix and associated interfacial friction. This bond strength is therefore a primary factor in composite design. Much attention has been given to the bond strength and fiber pullout behaviors5-7 . In a study of interfacial debonding and sliding in brittle matrix composites, Mumm and Faber7 found that the fiber sliding was strongly dependent on the fiber surface roughness. Wang ef al.* studied single fiber pullout from a cementitious matrix, and found in most cases the pulling force for polypropylene and nylon fibers continued to increase after the fiber had begun to slip, due to fiber surface abrasion. It is generally recognized that the bond strength between the fiber and matrix, and the associated interfacial friction as debonding has taken place, constitute the main sources hindering fiber movement. Much attention, therefore, has also been given to the stress transfer through an interface’-12. Two basic models have been frequently used. Kelly and Tyson’ assumed that the matrix is rigid plastic and therefore the interfacial shear stress is a constant. Cox’” * Author to whom correspondence 2406; fax: (605) 394-6131.
should be addressed.
Tel: (605) 394.
assumed that the interfacial shear stress is linear elastic. These models indicate that the maximum fiber stress is located in the middle of fiber length. The maximum shear stresses, however, are located at the ends of the fiber in Cox’s model. As soon as the shear stress, on the interface exceeds the bond strength, the debonding will take place. Although these mathematical models can give approximate stress distributions along the interface, the bond strength remains unsolved until interfaces in individual composites have been carefully studied. Synthetic fibers have low Young’s modulus compared with a concrete matrix but they are usually chemically stable in acidic and basic environments, with polyolefin thought to be the least reactive”. No diffusion is possible for high weight molecules of solid polymeric fiber to inorganic cementitious mix at room temperature. The only possible mass transfer is lime solution infiltrating into the fiber and this seems very weak due to the compact ffiber surface layer and the water repellent property of the fibers”. Mineralization of calcium silicate hydrates formed during curing could not take place beneath the fiber surface unless there are microcracks underneath the fiber surface. The formation of transition layers with a chemical composition gradient between the fiber and matrix, or new substances resulting from chemical reaction on the fiber-matri# interface, seems impossible. In many tests, the synthetic reinforcement breaks clean from the concrete at the conclusion of the tests; this would suggest that there can be little or no chemical adhesion between the reinforcement and the concrete. There is virtually no information in the literature regarding the
643
Interface
morphologies
in polyolefin
fiber composites:
chemical bond between synthetic fibers and concrete. The dominating bond left seems to be primarily mechanical. Measures have been taken to partially remedy the lack of adhesive bond13. Fibrillated films of polypropylene, being opened in the form of continuous networks, may themselves be impregnated with cement mortar to form a composite sheet with high bending and impact strength. The chopped and twisted fibrillated polypropylene fibers, with their open structure, can partially remedy the lack of interfacial adhesion by making use of a wedge action at the slightly opened fiber ends and also by mechanical bonding through the fibrillation. Other measures have also been taken in principle of forming a mechanical interlock. The predesigned shapes and profiles that form mechanical interlocks have their specific applications and relatively higher costs. However, short monofilament fibers, with an as-received smooth surface, are desirable in large scale field applications due to their low cost and ease of use. Questions are immediately raised when using such short synthetic fibers as reinforcements, such as: what the interface bond will be? Fiber pullout tests provide a measure of an average bond strength in a composite. Direct observation of the interface has the potential of providing a much clearer picture of the mechanism involved in such bonding. No direct observation of such fiber-matrix interfaces has been found in literature. The objectives of this study are to investigate morphologies of the fiber surface immediately after mixing, and the fiber-matrix interface formed after mixing and hardening, and therefore to obtain a preliminary understanding of the interfacial bond between the fiber and concrete matrix in FRCs.
EXPERIMENTAL Two batches of short monofilament polyolefin fibers, categorized as No. 1 and No. 2 fibers from 3M Corp., spun and then drawn to 25 mils (0.635 mm) in nominal diameter (actually an elliptical cross section with 25 mils being the small diameter) and 2 inches long (50.8 mm), were used as reinforcing elements for FRCs; these were denoted as No. 1 FRC and No. 2 FRC, respectively, corresponding to fiber batch numbers. Interfaces between these fibers and concrete matrix were observed. To facilitate analysis of the formation of these interfaces, No. 1 fibers mixed with cementitious matrix materials (mix) for various mixing times were extracted from the mix for surface modification examination. Surface morphology of fibers pulled out from matrix of No. 1 FRC specimens which had been subjected to an impact test were also investigated. Observation of all samples was conducted under a JEOL JSM-840A scanning electron microscope (SEM) operating at 15, 20 or 25 kV, using a secondary electron imaging mode.
Preparation
of jber
No. 1 polyolefin
644
sueace for SEM observation fibers were mixed with cementitious
L. Yan
(mix of cement, fine and course aggregates, fly-ash and water; w/c = 0.5) in a 9 ft2 (0.25 m2) drum mixer for various times up to 88 min. Fibers for surface observation were picked out of the fresh mix at mixing times of 6, 35 and 88 min, respectively, and followed by washing in water to remove mix materials. The fibers were then kept in hydrochloric acid (37 wt%) solution to remove any alkali contaminants which might have stuck to their surfaces and concealed the true morphologies of fiber surfaces. An asmanufactured fiber was also soaked in the same solution to check acidic stability of the fiber material. All fibers were followed by rinsing them in water after soaking for 240 h. Fibers pulled out during impact testing were cut from the cracked surfaces. Impact specimens with mixing time of 35 and 88 min, respectively, were used to sample such fibers. No surface treatment, e.g. soaking by acidic solution, was conducted on these fibers. All fiber surfaces used for SEM observation were coated with carbon for electric conduction. The fibers used and their preparatory conditions are listed in Table 1, where Mf means mixed-only fiber, and Pf, the pulled out fiber, with the number following indicating mixing time in minutes.
Preparation
offiber-matrix
interfaces for SEM observation
No. 2 polyolefin fibers were mixed with mix (a mixture of cement, fine and course aggregates, fly-ash, water, plasticizer and air entertainment; w/c = 0.5) in the same drum mixer for casting of various test specimens of No. 2 FRC. Untracked parts of an impacted No. 1 FRC specimens with a mixing time of 35 min and 88 min, respectively, and a No. 2 FRC specimen with a mixing time of 5 min, were split with a chisel into smaller pieces. After cracking, the bridging fibers were cut with scissors to avoid further damaging the bulks. Bulks with untouched fibers inside were chosen for further sampling. Some were heated in an electric furnace up to 350°C to evaporate the fibers. The heated and remaining unheated bulks were split again along the plane where the fibers had lain. After splitting the unheated bulks, fibers stuck on the cleaved surface were peeled away by hand. All the split halves were checked, before sampling for SEM, under optical microscope to ensure the cleaved surface did in fact pass through the region where the fibers had lain, as shown in Figure I, leaving the interfaces undamaged. All the samples in this group chosen for SEM study were coated with gold for electrical conduction. Coating for each SEM specimen was conducted in several orientations,
Table 1
Preparatory
conditions
for individual
No. 1 fibers
Samples
Source
Mixing time (min.)
Surface treatment
MrO Mt-6 M& Pf-35
As-received Fresh mix Fresh mix Pulled out from impact specimen Pulled out from impact specimen
0 6 88 35
Hydrochloric As above As above None
88
None
Pf88
mix
et al.
acid
Interface
morphologies
in polyolefin
fiber composites:
L. Yan et al.
Figure 2 The surface of an as-manufactured No. I tiber (Sample M,-0). showing the surface texture due manufacturing and surface damage due to scratching of external particles. No acidic etching can be apparently found on the surface
Figure 1 (b) Pi-35
Table 2
Cleaved surfaces containing
Preparing
conditions
fiber-matrix
for individual
interfaces:
(a) HI-S:
interface samples
Samples
Mixing time (min)
Method to remove fibers
No.1 FRO: H,-35
35
P,-35 H,-88
35 88
Heating Peeling Heating
5 5
Heating Peeling
No. 2 FRC: H,-5 P,-5
usually 8, each for 30 sec. The preparation conditions various interface samples are summarized in Table 2.
for
RESULTS S@ace
morphologies
of mixed-only and pullout jibers
The surface of as-manufactured fibers is usually smooth when drawn, as shown in Figure 2. The texture along fiber longitudinal direction was seen, indicating the last processing of the fiber was a drawing operation. A groove with one
Figure 3 Damaged surface of Sample Mf6: Ca) fungus-like bugles. peeled up fib&. cavities and grooves formed during mixing: (b) grooves under fungus-like bulges lying along fiber axial direction
645
Interface
morphologies
in polyolefin
fiber composites:
L. Yan
et al.
Figure 4 Surface morphologies of pulled out fiber and mixed-only fiber: (a) difference in morphology between Pf35 (left) and M,-88 (right); (b) scratched surface of P,-35 having a landslide appearance, indicating it had been ploughed by matrix
particle at each of its ends was formed apparently by scratching of these particles, implying the fiber surface can be easily damaged longitudinally by external hard particles. No etching trace by hydrochloric acid on the surface was discernible, indicating the fiber material is chemically stable in the acid. There should be, therefore, no concern about changing surface morphologies of the as-mixed fibers due to the surface treatment in the acid. Therefore, the observed surface morphologies of acid treated fibers merely reflected fiber damage itself. As the fibers were being mixed, the hard particles of mix impacted and abraded the fiber surface, causing plastic deformation and even removal of part of surface layer, the degree and orientation of which were mainly dependent on the property of the fiber surface layer. Observation of sample MT6 showed that fungus-like bulges, pits, and grooves were formed on the surface; fibril peeled up from the surface, leaving grooves and ridges lying in the fiber axial direction, as shown in Figure 3(u). Further examination revealed that even in the ‘fungi’ covered area, surface
646
Figure 5 (c) P,-35
Interfacial morphologies
in No. 1 FRC: (a) H,-35; (b) Hi-88; and
damage took place along the fiber axial direction as seen from Figure 3(b). The different surface morphologies between samples M+-88 and Pr-35 were observed, as shown in Figure 4(u). The surface of M$38 was covered with fungus-like bulges (in the middle and right side of the surface), longitudinal grooves and long wire-like ridges, as well as torn-up fibrils
Interface
morphologies
in polyolefin
fiber composites:
L. Yan et al.
Figure 6 Morphologies of the interface in No. 2 FRC: (a) bulges and grooves lying along interfacial longitudinal direction with embGshment of nest-like clusters in the heated specimen of HI-S:(b) fibrils left on the interface in the peeled specimen of P,-5: (c)small needles planted on the interface in P,-5: and cdl needle-cement clusters in PI-5
and patches (on the left side of the surface). It is apparent that the wire-like ridges can be seen only after the surface layer has been peeled up. The surface of Pf35 seemed relatively neat and simple, longitudinal scratches being dominant on its surface. A close look at Pf35 revealed landslide morphology, as shown in Figure 4(b), implying that the surface was ploughed by hard matrix due to the matrix material incising into grooves or pits. A trunklike fibril (on the right) hints that part of surface material had been stretched to fracture during pullout because of anchoring of bulges in the matrix. Most of bulges were removed during fiber pullout process; only few ‘fungi’ could be found on fiber surface after pullout of the fiber.
Interjkes
in NO. I FRC
During the curing of FRC. the interface on the matrix side was molded by the surface of the fiber. The roughness, composed of bulges and grooves, apparently orienting in the direction where the fiber axis had lain, was observed in all
samples of FRC, as shown in Figure 5. In comparison of different mixing times. it was observed that the longer the mixing time, the finer the roughness in No. 1 FRC. Sample Hi-35 had a pine bark-like morphology (Figure S(a)), with breadth of about lo-20 pm. This was diff$rent from sample HI-88 (Figure 5(b)), which had worm-like or bamboo-like ridges with a diameter less than 10 pm. This indicates the removal of the fiber surface layer may gradually reveal the change in fiber surface morphology with the depth from the surface. Observation of samples with their interfaces exposed by peeling away fibers revealed that. in addition to the roughness of the interface. fibril segments and fiber chips were anchored in the matrix, as shown in Figure 5(c). No transition layer, or even trace of a patch of fiber material adhering to the interface. was observed. suggesting the interfacial bonding strength is very weak. It is noted that the orientation of roughness in P,-35 is not as clear as that in H,-35. This may be due to the damage which occurred while the fiber was being peeled, thus changing some of the original interfacial morphology.
647
Interface
morphologies
in polyolefin
fiber composites:
L. Yan
et al.
broken-off fibril
Figure
Figure 7 process
9
Schematic
of forming tendrils and grooves on fiber surface
Schematic drawing of fibrils formed in fiber due to cold drawing surface-grooved fiber embedded in matrix
cementitious matrix /
\ high strength direction (fiber axial direction) <
> matrix materials penetrating ce
low strength direction I Figure
Figure 8 Schematic drawn fiber
of microstructure
and anisotropic
properties
10
Schematic
of formation
of interfacial
roughness
in a
with the smallest width of crack shown in Figure 6(d) less than one third pm.
Interfaces in No. 2 FRC
The interfacial morphology of sample Hi-5 is shown in Figure 6(a). The ridge-like bulges and grooves are more apparent and much finer compared to those observed in No. 1 FRC. The breadth of ridges and grooves along the longitudinal direction of the interface are estimated to be of an order less than 1 pm. Nest-like clusters, which were not found in the No. 1 FRCs and obviously composed of cementitious sticks, were also observed. These were due to the sample having been subjected to heating, where some materials had been removed while others remained stuck on the interface. The inter-facial morphology of sample Pi-5 shows that fibril segments of different sizes were left on the surface. These were strips or tendrils with ends planted in the matrix, as shown in Figure 6(b). Higher magnification revealed that needles lay both on the interface and at angles to the interface, as shown in Figure 6(c). In addition, many had been pulled away during peeling off of the fiber, leaving holes on the interface. Further examination of the interface revealed that needle-like substances were stuck in the cement to form needle-cement clusters on the interface, as seen from Figure 6(d), confirming that nest-like clusters observed in Hi-5 originally had synthetic needles inside. The needles were also observed in other places, besides on the fiber-matrix interface, such as the cleaved surface and air bubble surfaces14. It is worth mentioning here that, from all SEM pictures shown, the concrete matrix is a cracking material. Cracks, small or large, can be observed everywhere in the pictures,
646
DISCUSSION Synthetic fibers of monofilament are usually manufactured by extruding (‘spun’ in textile industry jargon) the melt, or solution, of raw material through the fine hole of a die (spinneret). Slight surface texture along the fiber axis may be produced when the fiber is passing through the die. Spun polypropylene fibers do not have high strength in the axial direction due to poor crystallinity and texture (low orientation). Cold drawing is used to change fiber microstructure and thus to increase their strength15. The original small crystallites are arranged in spherulite superstructures. On cold drawing, the superstructures are greatly elongated and long crystallites (microfils) are formed from the original ctrystallites plus additional long chain polymer molecules from the spherulite. This stress crystallization increases the total polymer crystallinity while orienting the polymer chains and crystallites in the drawing direction. Microfils are further stuck together to form bundles or fibrils. The distribution of fibrils in fiber is schematically shown in Figure 7.
The crystallinity depends on the draw ratio, the ratio of the surface speed of feed and draw sets of roll, respectively. The greater the ratio, the higher the crystallinity, and the higher the fiber strength. Higher strength is achieved at the expense of plasticity, and therefore a general rule is that high strength fiber usually has low plasticity. Microscopically, high draw ratio also leads to fine crystallites, and thus fine fibrils, and high strength. However, one cannot expect that all regions in the fiber are crystalline. There are regions
Interface
crack in matrix
Figure 11
morphologies
penetrated matrix materials
Schematic showing forces acting on a fiber
of amorphous, non-crystallinity between crystallites, much like a composite of amorphous matrix embedded with fibrils of crystallites. The mechanical properties of the fibers manufactured in this way are anisotropic, strong in the longitudinal direction, but weak in the radial and circumferential directions, as shown in Figure 8. Alternating loading on the fiber surface, along with impact and abrasion from mix particles in this study, caused immediate plastic deformation where the load was high, and fatigue where the load was low. Some hard cementitious particles with sharp edges incising and wedging into vulnerable fiber surface kept moving with mix, trying to peel up or away the fiber surface layer. The relative weak bond between textured parts (or fibrils) which lay along the fiber longitudinal direction failed first. Some fibrils were lifted up, broken or tom away, leaving grooves and tendrils on the fiber surface, as schematically shown in Figure 9. The amorphous matrix of the fiber was more plastic and therefore stretched longer than the fibrils at fracture. Recovery of the stretched parts left ‘fungi’ on the fiber surface. During the hardening of FRC, the interface between the fiber and cementitious matrix was tightly matched due to the penetration of mix materials into grooves and pits on the fiber surface. A negative impression of the fiber surface was replicated on the interface on the matrix side; ridge-like bulges and grooves lying along interfacial longitudinal directions were thus formed, as shown in Figure 10. Care has to be taken to analyze these observations. Samples denoted Hi-(heated interface) had no polymeric materials left on them due to the fact that the samples had been heated to 350°C whereby the polymer materials may be melted or decomposed. Therefore. little mechanical damage on the interface took place and thus the inter-facial morphology, except for fiber chips and fibrils, was largely preserved. Samples marked Pi-(peeled interface) had the interface damaged to some extent because manual removal of fiber might bring away cementitious materials on the original interface, resulting in the orientation being not as conspicuous as those in the heated samples. However, fiber chips and fibrils which were anchored firmly in the matrix had been left for observation. This facilitated the analysis of true interfacial bond. A notable phenomenon found in the Pi-5 sample were needles and needle clusters existing on the interface, as well as in other areas of the samples. Compared to the morphology of H,-5. nest-like clusters should have been filled with needles, but having been removed, left the
in polyolefin
fiber composites:
L. Yan et al.
present nest-like cementitious skeleton. We then conclude that the needles in sample Hi-5 were removed by heating, and therefore those in sample Pi-5 were of Aber material. An assumption can be made that fibrils, produced due to the fibrillation of fibers during mixing, were further collapsed into needles, presumably due to the fact that the fibrils in the surface layer of No. 2 fiber were more brittle than those in the No. 1 fibers, which resulted from the high draw ratio. Finer ridges, compared to those of No. 1 FRCs, observed on the interface of Hi-5 sample, corresponding to finer fibrils removed from the No. 2 fiber surface, seem to support this assumption. The existence of needle clusters and needles themselves might affect the properties of the FRC. Nevertheless, this needs to be further investigated. The interfacial roughness existing between reinforcing fibers and the cementitious matrix as observed constituted an interlock between the fiber and matrix. When cracks initiated in the FRCs, the interlocked interface resisted the moving of the fiber; cementitious bulges, which had penetrated into the grooves and pits on the fiber surface, scraped the fiber surface; and polymeric bulges located on the fiber surface, which had been embedded in the matrix, were stretched, as schematically shown in Figure Il. When the pulling force on the fiber became greater than the interlock and frictional resistance, the liber was moved, and then new and/or deepened grooves were formed on the surface of the pulled out fiber. The surface morphology observed on the pulled out fiber. having a sculpted orientation with sharp edges, was apparently different from that of a merely mixed one, implying the fiber surface had been scraped by matrix along its axial direction. Although we did not conduct fiber pullout tests. it may be inferred from this observation that the slippage strength would be greater than bonding strength due to the surface abrasion. This was caincident with the results of fiber pullout’.
CONCLUSIONS An SEM study was conducted on the interfaces between polyolefin fibers and cementitious matrix. Surface morphologies of the synthetic fibers that were just mixed for various times, and that were pulled out during impact tests, were also studied. Ridge-like bulges and grooves oriented in the axial direction of the interface were found on the fiber-matrix interface in all specimens studied. Fibrils and fiber chips anchored in the interfaces were also observed in all FRC specimens. In No. 2 FRC, needles and needle clusters of fiber material were found to be distributed everywhere, on interfaces, air bubble surfaces, and in the matrix. Fibrillation of the drawn monofilament fibers characterized by fibril segments being striped up and away during mixing, leaving grooves and pits on the fiber surface, was observed. Fungus-like bulges covering most of the as-mixed fiber surface. formed by recovery of stretch-to-fracture of the surface layer of the fiber, were also identified. Damage of the pulled out fiber surface was characterized by landslide
649
Interface morphologies
in polyolefin
fiber composites:
L. Yan
et al.
morphology oriented along the fiber axis due to the abrasion of the hard matrix. It is concluded that the roughening interface between the reinforcing fibers and the matrix is formed due to the damage of fiber surface caused by mixing in cementitious mix. This roughening interface constructs a mechanical interlock which resists relative movement of fibers immediately after cracks initiated. Therefore, fiber surface damage caused by mixing plays an important role in forming this roughness. No attempt has yet been undertaken to determine the optimum degree of the damage to be chosen for field applications, though of course this is an important area to study. It is, however, suggested to investigate proper cold draw ratio of the fiber, which causes fibrillation in the fiber, especially in the surface layer. The properties of fibrils in the surface layer may not only directly affect the morphology of interfacial roughness, but the shape and distribution of fiber chips peeled off from the fiber. The effect and behavior of these chips and needles under loading are still unclear and therefore need investigation.
REFERENCES
ACKNOWLEDGEMENTS
10.
This work has been supported in part by the National Science Foundation under Grant No. ORS-9108773 and State of South Dakota Future Fund. Dr. E. F. Duke at South Dakota School of Mines and Technology is specially acknowledged for his guidance in SEM work. Dr. V. Ramakrishnan at the same university and Mr. C. N. MacDonald from 3M Corp. are appreciated for their support in providing laboratory facilities and fibers, respectively, for use in preparing specimens.
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