Pullout Response of Ferrocement Members Embedded in Soil

INTRODUCTION

Generally, conventional reinforcement used for reinforcing soil contains only one type of material, such as geogrid, geosynthetic, or wire mesh. It is known that the material used in soil reinforcement applications must be safe against tension failure and adhesion failure for its effective use in the field and reliable design of earth structures.1 A single type of material can provide limited reinforcement capability in reinforced soil structures due to its low frictional resistance and poor cohesion. For an optimal response, therefore, different types of reinforcement that fulfill both requirements such as possessing adequate tensile strength and frictional resistance, are getting considerable attention lately. Ferrocement, a thin reinforced-mortar composite consisting of evenly distributed fine wire mesh as the reinforcement and cement-sand mortar as the matrix can be a prospective complementary material for this perspective. The enhanced performance of ferrocement over conventional reinforcement comes from its synergetic action of mesh with mortar and mortar with soil. In ferrocement, high-tensile steel wire mesh provides adequate tensile and pullout resistance, whereas the sand-cement mortar provides adequate frictional resistance and improved cohesion owing with its relatively greater surface area and roughness as compared with conventional soil reinforcing materials. If properly designed, the rough surface of ferrocement elements can grip the soil particles and the frictional resistance needed for optimal design against pullout failure can be significantly improved. Very little or no research attention has been given to investigating the pullout response of ferrocement elements embedded in soil. The author is aware of only two such investigations on ferrocement thus far.2,3 In the first investigation, ferrocement-soil interface shear behaviors were studied under shear tests of two types of ferrocement panels with varying shear speed. It was shown that the ferrocement panels with plain surfaces had less frictional resistance and cohesion than the ferrocement panels with rough surfaces both in sandy and clayey soils, and it was concluded that the ferrocement-soil interface shear strength was decreased with the increase in shear speed. The second investigation presented a comparative study on the fundamental behavior between shear and pullout tests of ferrocement elements. It was concluded that the pullout strength was higher than the shear strength of ferrocement elements embedded in soil. An in-depth clarification on any particular test method such as detailed failure mechanisms of ferrocement elements under pullout tests, however, have not been clarified yet. The lack of research interest of ferrocement in this concern may be due to the fact that ferrocement is usually used as compression, flexural, and tension members of building components and structures, which preclude pullout response of ferrocement embedded in soil. Another possible cause of the scarcity of research on ferrocement as soil reinforcement may be the due to the lack of a bridge between the two professions such as geotechnical and structural engineers or soil and building engineers. Using ferrocement for reinforcing soil is not a difficult task as compared with using conventional soil reinforcement materials such as geogrids or geosynthetics. Ferrocement, a composite material made of sand-cement mortar reinforced by mesh, is especially suitable for applications in reinforcing soil structures such as earth dams and embankments, which are usually constructed in a layer-by-layer method and when the layers are particularly made in horizontal plane. Generally, in such soil works, reinforcements are horizontally placed between the two soil layers with a vertical spacing of approximately 800 to 1000 mm (31.4 to 39.3 in.). In these construction works, ferrocement panels either in precast form or in-place fabrication can be easily used. For example, in the case of in-place fabrication, after compaction of a soil layer, thin mortar (sand-cement mixture) layer (first mortar layer) can be spread over the soil and then the mesh can be placed on the first mortar layer. After that, another mortar layer (second mortar layer) can be spread on the mesh layer placed. No formwork nor skilled labor would be needed for this task. Application of geocomposites made with two separate material such as strip and grid, strip and anchor, as well as steel bars and anchor plates are some of the examples.4-6 In this way, the ferrocement would provide a composite that derives benefits from each of the individual reinforcement and exhibits a synergetic action between soil and mortar, as well as between mortar and mesh. Reinforcement of soil with ferrocement, however, still remains a science in its infancy, and ideas are still evolving toward assessing the optimal technique for soil reinforcement applications. Nevertheless, owing to the aforementioned distinct advantages of ferrocement and recent developments that broaden the scope of application of ferrocement, pullout response of ferrocement elements embedded in soil may become a critical design consideration for soil reinforcement applications.

The main objective of this research was to optimize the design of ferrocement elements for soil reinforcement applications. In view of this objective, the pullout response of ordinary ferrocement elements embedded in two types of soil was studied initially and then these were optimized to obtain a maximum frictional resistance over ordinary ferrocement elements. For achieving the optimum frictional capability, one type of ferrocement element contained some small stones of 4 to 8 mm (0.15 to 0.31 in.) that were randomly placed over the surface of ferrocement during the casting process to obtain a rough surface thickness of approximately 2 to 4 mm (0.07 to 0.15 in.). Another type of ferrocement element containing some small parallel channels of 15 mm (0.59 in.) width, 5 mm (0.19 in.) depth, and 50 mm (1.96 in.) center-to-center spacing with varying numbers were made and were intended to grip the soil particles, therefore resulting in a substantial improvement in the frictional resistance. The paper reports the stress-displacement relationships of ferrocement and development of the technique that modify direct shear to evaluate pullout resistance as well as failure modes of ferrocement elements from the pullout test results.

RESEARCH SIGNIFICANCE

The problem of failure mechanism and bearing capacity of ferrocement under various loading conditions such as compression, tension, flexure, shear, torsion, and impact loads has been studied quite extensively. In spite of the volume of information available, relatively very little or no research work is reported in the technical literature on the pullout response of ferrocement elements embedded in soil, although it presents considerable versatility toward the development of ferrocement elements for reinforced soil applications. The investigation reported in this paper is aimed at studying the behavior and frictional resistance of ferrocement under pullout tests in soil. The major variables considered in this study are the surface properties of ferrocement, mortar strength, type of mesh, and their failure mechanism. The paper also deals with the optimal design of ferrocement elements from the pullout test results for effective application in soil reinforcement.

COST, STRENGTH, AND DURABILITY

The cost of ferrocement varies widely depending on the type of mesh used. According to the present market price, the cost of ferrocement varies from 200 to 500 yen (U.S. $1.67 to $4.17) for steel mesh-reinforced mortar and from 400 to 800 yen (U.S. $3.33 to $6.67) for high-performance carbon or polymeric mesh-reinforced mortar. The strength of ferrocement was found as high as 35 MPa (5075 psi), equivalent to 300 kN/m (1676.6 lb/in.) for 10 mm (0.39 in.) thickness ferrocement. On the other hand, the cost of geogrids or geosynthetics varies from 1000 to 2000 yen (U.S. $8.33 to $16.67) and depends on the type of material, thickness, grid size, and strength. The strength of geogrids or geosynthetics varies from 30 to 800 kN/m (167.6 to 4470.4 lb/in.). Corrosive mesh-reinforced ferrocement may be less durable than the geogrids or geosynthetics, but noncorrosive mesh-reinforced ferrocement is expected to be more durable because of its more frictional resistance in the ferrocement-soil interface.

FAILURE OF FERROCEMENT UNDER PULLOUT TESTS

The ferrocement elements used for soil reinforcements applications may possess the four possible modes of failure that need to be investigated under pullout tests by embedding in soil (Fig. 1). Among the four different failure modes, a most commonly occurring mode is the frictional failure between soil and reinforcement, which is shown in Fig. 1(a). This mode of failure occurs when the interfacial friction between reinforcement and soil is less than the pullout force and the tensile capacity of mortar and mesh. If the tensile capacity of mortar is more than the interfacial friction, then this mode of failure would be dependent mainly on the surface properties. The second possible and frequently occurring mode of failure can be noted as mortar failure, as shown in Fig. 1(b). This mode of failure occurs when the pullout force exceeds the tensile stress of mortar, but is less than the frictional resistance of ferrocement and tensile capacity of mesh. The third possible mode of failure may be mesh failure, as shown in Fig. 1(c). If the frictional capacity of interface exceeds the tensile capacity of mesh, then this mode of failure takes place. The forth possible, and comparatively less frequent, failure mode may be noted as bond failure between the mesh and mortar, as depicted in Fig. 1(d). When the bond force between the mesh and mortar is less than the pullout force, frictional resistance of interface, and tensile capacity of mesh, then this failure mode may occur.

EXPERIMENTAL PROGRAM

Materials, mixtures, and specimens

The specimens were prepared in the wooden molds with open tops. The requisite amount of sand and cement was drymixed in a pan, and then the requisite quantity of water was added gradually while the mixture was continuously stirred. The mortar was mixed manually. Ordinary portland cement and river sand passing through a No. 8 (2.38 mm [0.093 in.]) sieve having a fineness modulus of 2.33 were used for casting. The cement-sand ratio and water-cement ratio were both 0.5 by weight. The commercially obtained square mesh and chicken mesh were cut to obtain the desired size. For square mesh, the diameter of wire was 1.0 mm (0.039 in.) with a center-to-center opening of 10 mm (0.39 in.), and for chicken mesh, the diameter of wire was 0.8 mm (0.031 in.). The Young's moduli and Poisson's ratio of the square and chicken meshes were 138 kN/mm^sup 2^ (893.3 tsi-tsi indicates ton per square inch), 104 kN/mm^sup 2^ (6732.1 tsi) and 0.3, respectively. The other specifications and physical appearances of the meshes are shown in Fig. 2.

The sand-cement mortar layer was spread at the base of the mold; on this base layer, the first mesh was laid, and then it was covered by further application of the mortar. Ferrocement panels, made of stone, with ordinary plain surfaces and rough surfaces (thickness of the rough surface was approximately 2 to 4 mm [0.07 to 0.15 in.]) and small parallel channels (depth of channel was 5 mm [0.19 in.] and width of channel was 15 mm [0.59 in.]) of varying quantities were prepared. The channel-to-channel spacing was 50 mm (1.96 in.). The thickness and size of all the ferrocement panels were 10.0 mm (0.39 in.) and 315 x 380 mm (1.24 x 1.49 in.), respectively (Fig. 3). Five types of specimens such as ferrocement elements with smooth surface (FSS), ferrocement elements with rough surface made by small stone (FRS), ferrocement elements with rough surface made by two small channels (FR2), ferrocement elements with rough surface made by four small channels (FR4), and ferrocement elements with rough surface made by six small channels (FR6) were prepared and tested (Fig. 3).

The cost of the number of channels is mainly related to the materials used and labor needed during preparation of ferrocement elements. It should be noted herein that the channels were made on the surface of ferrocement elements by using the wooden bars of cross sections of 5 x 15 mm (0.19 x 0.59 in.) with the length equivalent to the width of the ferrocement panels. The bars were placed in the mold immediately before the application of mortar. The labor required for this task was negligible as compared with the time required for the entire works. Also, the same bars, after taken off from the completed ferrocement elements, were reused many times for casting the new ferrocement elements. Therefore, the cost for the bars is also considered to be negligible. On the other hand, total material (sandcement mortar) required for each panel was reduced due to the increased number of channels, which provided a significant savings of the sand-cement mortar. Consequently, the final cost of the ferrocement element due to the increased number of channels was minimized, owing to the savings of materials (mortar). Moreover, an additional advantage with the increase in the number of channels was that the total weight of the ferrocement panels was minimized.

Properties of soil

The particle size distribution curves of both soils is shown in Fig. 4. The curve for sandy soil reveals that more than 90% of the soil is in the silt and sand fraction. The particle size distribution curve for clayey soil indicates that more than 66% of the soil is in the clay and silt fraction. The other properties of sandy and clayey soils and the parameters used in these tests are depicted in Table 1.

Test apparatus

The apparatus used in this study is shown in Fig. 5. For convenience, the important components of the testing equipment are numerical labeled and a legend is provided in the figure. The testing equipment and methodology reported in this paper are within the standards of the Japanese Geotechnical Society (JGS: T941-199X). A test setup for testing various soil reinforcement materials such as geogrids, geosynthetics, high-density polyethelene (HDPE), steel plates, and other hard materials is recommended. It is noted herein that the Japanese Geotechnical Society recommended even smaller equipment than the one considered in this study for evaluating the pullout characteristics of not only the geogrid or geosynthetic but other hard materials such as steel plates as well. Some researchers used even smaller equipment, such as a circular pullout box of 60 mm (2.36 in.) diameter.7 It is evident that the ferrocement element is a composite material in which cement mortar is reinforced with layers of wire mesh, which is also a hard material acting compositely after cast. Therefore, the tests of such material can be carried out as of the steel plate. The tests setup used in this study has the shear area of 1000 mm^sup 2^ (39.3 in.^sup 2^) between the soil and the ferrocement with a box height of 200 mm (7.8 in.), which is of adequate size compared with other test setups found in the literature. Therefore, the results obtained in this study can be considered as good results because they have very little effect of boundary conditions due to large pullout equipment.

Pullout tests

The ferrocement panels were made to obtain rectangular pieces of 315 x 380 mm (12.40 x 14.96 in.) in size with 120 mm (4.72 in.) extended mesh. The specified length of the pieces was selected to facilitate clamping with the shear apparatus. The panels were clamped in the box in such a way that the embedded length of the panel was 380 mm (14.96 in.) in the loading direction and 315 mm (12.40 in.) in the transverse direction. Water was added gradually to the soil and mixed up to obtain the desired water content uniformly throughout the soil. After embedding the ferrocement panel on the lower box, the upper box was set on the panel, and then the soil was filled in the upper box. The pullout tests were carried out in the way of pulling out the panel from the soil with a constant selected speed of 1.0 mm (0.039 in.) per minute by means of screw jacks under electrically-operated constant pressure. The pullout forces were measured using a tension load cell with the minimum count of 5 N (1.1 lb). The displacements were measured by means of a mechanical dial gauge with a minimum count of 0.001 mm (0.000039 in.). All the pullout tests were conducted according to the T941-199X standard. During the test, the small parallel channels made on the ferrocement surface were transverse to the loading direction.

Criteria of normal stresses

It is noted that the strength parameters obtained under pullout tests would be more accurate if the tests were to be performed under vertical stresses that makes the soil in normally consolidated conditions. For the backfills used in this paper, it is observed that the vertical stresses should be above 30 kPa (4.35 psi) to get a normally consolidated condition. Depending on the type of soils, it can also be seen that the vertical stresses used in the pullout tests usually vary from 10 to 30 kPa (1.45 to 4.35 psi) for over-consolidated conditions and from 40 to 100 kPa (5.8 to 14.5 psi) for normally consolidated conditions. According to the Japanese Geotechnical Society, the stress levels selected in this paper can be considered typical vertical stresses for obtaining the accurate strength parameters in reinforced soils. It should be pointed out that, in calculating the interaction resistances such as cohesion and internal friction under pullout test, it is necessary to clarify the common method of finding out these important parameters. In general, the methods of failure envelope and Mohr circle are well known in determining cohesion and internal frictional resistances. In the first method, for obtaining a failure envelope, a number of identical specimens are tested under different normal stress. The pullout stress required to cause failure is determined for each normal stress. The failure envelope is obtained by plotting the points corresponding to the shear strength at different normal stresses and joining them by a straight line. The inclination of the failure envelope to the horizontal gives the angle of the frictional resistances and its intercept on the vertical axis is equal to the cohesion intercept. The Mohr circle method is needed when the stress on failure planes are not directly known. In the present research, the pullout test is carried out by pulling out the reinforcement from the soil under different normal stresses. The pullout stresses acted on the interface between the ferrocement and soil are measured directly equivalent to the frictional stress of the interface. In view of this objective, the repeated tests under different normal stress such as 40, 60, 80, and 100 kPa (5.8, 8.7, 11.6, and 14.5 psi) are conducted. To find out the statistical validation of the results obtained, the R^sup 2^ values between the normal stress and pullout stress are calculated.

RESULTS AND DISCUSSION

Pullout stress-displacement relationships for square mesh ferrocement

FSS specimen-The relationships between the pullout stress and displacement of square mesh-reinforced ferrocement panels with plain surfaces (FSS specimen) under normal stresses of 40, 60, 80, and 100 kPa (5.8, 8.7, 11.6, and 14.5 psi) in sandy soil with a water content of 14.8% (w^sub opt^ = 15.3%) are given in Fig. 6(a). It is observed from Fig. 6(a) that the pullout stresses are increasing almost linearly with the increase in displacement of approximately 1.5 to 2.0 mm (0.059 to 0.7 in.). After that, the pullout stresses are more or less horizontal at lower normal stress but increase slightly at higher normal stress. This happens due to the failure of bonding stresses between the ferrocement surfaces and the backfill soil. The frictional failure shown in Fig. 7 indicates that there is no crack on the surface of the ferrocement panels. It can be noted in Fig. 6(a) that the higher the normal stress, the higher the pullout resistance. For the four normal stresses of 40, 60, 80, and 100 kPa (5.8, 8.7, 11.6, and 14.5 psi), the corresponding ultimate pullout stresses are recorded as 57.2, 90.9, 121.2, and 155.5 kPa (8.2, 13.1, 17.5, and 22.5 psi), respectively.

FRS specimen-The pullout stress-displacement relationships for ferrocement of rough surfaces with stone (FRS specimen) are depicted in Fig. 6(b). The pullout stress-displacement curves can be considered as a trilinear shape especially under higher normal stress. Initial linearity can be taken within the displacement of 1.0 mm (0.039 in.), second linearity is within the displacement of 1.0 to 4.0 mm (0.039 to 0.15 in.) and final linearity after 6.0 mm (0.23 in.) displacement. Under higher normal stresses, such as 80 and 100 kPa (11.6 and 14.5 psi), a slight decrease in pullout stresses at the displacement of 6.0 to 6.5 mm (0.23 to 0.25 in.) is observed, indicating the occurrence of mortar failure of ferrocement. The mortar failures of ferrocement elements are shown in Fig. 8. Under lower normal stress (40 kPa), it is clearly evident from Fig. 6(b) that the pullout stress is increasing gradually with the increase in displacement, indicating the effect of small stone on the surface of ferrocement. Notice also a distinct difference between the curves, even at the initial stages. These features may be due to the effect of the roughness of ferrocement panels and the effect of stone on it. The ultimate pullout stresses in this case are obtained as 58.5, 95.2, 130.0, and 160.0 kPa (8.5, 13.8, 18.85, and 23.2 psi) for the corresponding four normal stresses, respectively.

FR2 specimen-An inspection of the plotted results of the pullout stress-displacement relationships for Specimen FR2 (shown in Fig. 6(c)) indicates that they are, in general, apparently bilinear characteristics having a curvilinear portion in between. Within the displacement of 1.0 mm (0.039 in.), all the curves are linear, but the difference between the pullout stresses under different normal stress conditions within this range is not clearly evident. After that, however, it shows a significant difference with the increase in displacement. This is due to the effect of existence of channels on the surface of ferrocement. The linear range at higher normal stresses such as 60, 80, and 100 kPa (8.7, 11.6, and 14.5 psi) can be extended up to a displacement of 2.5 mm (0.098 in.). It should be pointed out that, during experimentation of the pullout test of ferrocement elements, a drop of stress was observed when the mortar failure occurred. This drop of stress on the curve indicates that the mortar failure (Fig. 8) occurred at a pullout displacement of 7.0 mm (0.27 in) under normal stress of 80 kPa (11.6 psi) and at a displacement of 6.0 mm (0.23 in.) under normal stress of 100 kPa (14.5 psi). In this case, the ultimate pullout stresses obtained are 59.5, 102.4, 138.0, and 169.3 kPa (8.6, 14.8, 20.01, and 24.5 psi) for the four normal stresses, respectively.

FR4 and FR6 specimens-Figures 6(d) and (e) show the pullout stress-displacement relationships for ferrocement containing four and six channels (FR4 and FR6 specimens), respectively. Dissimilar to the previous figures shown for FSS, FRS, and FR2 specimens, all the curves for FR4 and FR6 belong to a group of curvilinear characteristics, even at the initial stage of pullout displacement. This is due to an increased number of channels on the surface of ferrocement. As expected, the increase in pullout stresses is remarkable with the increase in normal stress and number of channels. The failure of mortar (Fig. 8) at higher normal stress is also obvious from the drop of pullout stress-displacement curves. As seen in Fig. 6(d), the FR4 specimen shows its mortar failure at the displacement of 7.0 to 8.0 mm (0.27 to 0.31 in.) under normal stresses of 60, 80, and 100 kPa (8.7, 11.6, and 14.5 psi). The FR6 specimen, on the other hand, shows its mortar failure at the displacement of 4.0 to 5.0 mm (0.15 to 0.19 in.) under normal stresses of 60, 80, and 100 kPa (8.7, 11.6, and 14.5 psi) (Fig. 6(e)). The ultimate pullout stresses for FR4 specimen are 72.7, 110.5, 156.2, and 185.0 kPa (10.5, 16.0, 22.6, and 26.8 psi) and for FR6 specimen are 85.0, 115.0, 171.2, and 197.0 kPa (12.3, 16.6, 24.8, and 28.5 psi) corresponding to normal stresses of 40, 60, 80, and 100 kPa (5.8, 8.7, 11.6, and 14.5 psi), respectively.

Pullout stress-displacement relationships for chicken mesh ferrocement

FSS specimen-The relationships between the pullout stress and displacement of chicken mesh ferrocement panels with plain surfaces (FSS specimen) under normal stresses of 40, 60, 80, and 100 kPa (5.8, 8.7, 11.6, and 14.5 psi) in sandy soil with a water content of 14.8% (w^sub opt^ = 15.3%) are given in Fig. 9(a). It is interesting to note that the pullout stressdisplacement relationships that were observed in the case of square mesh-reinforced ferrocement elements are noticeably different in the case of chicken mesh-reinforced ferrocement elements. As can be seen, there is a clear fluctuation and oscillation behavior in the pullout stress-displacement relationships, especially after the occurrence of mortar cracking for chicken mesh-reinforced ferrocement elements. The mortar cracking is shown in Fig. 8. This is obvious because of the expansion of chicken mesh and the ease of the propagation of cracks along the expansion of mesh. The chicken mesh has diagonal wires, which facilitates expansion in the loading direction, whereas these characteristics remain absent in the case of square mesh, owing to its parallel wires. In cases of frictional failure, however, which are seen in the cases of lower normal stresses, the behavior of chicken mesh-reinforced ferrocement is almost similar to that of the square mesh-reinforced ferrocement elements.

FRS and FR2 specimens-As shown in Fig. 9(b) and (c), the pullout behavior of ferrocement for FRS and FR2 specimens showed that both the elements showed three types of failure modes depending on the applied normal stresses. Under lower normal stresses such as at 40 kPa (5.8 psi), frictional failure (Fig. 7) is observed, and at 60 kPa (8.7 psi), the mortar failure occurred (Fig. 8). On the other hand, under higher normal stresses such as at 80 and 100 kPa (11.6 and 14.5 psi), the mesh failure occurred (Fig. 10). These figures also present a clear distinction in the pullout stresses with the increase in pullout displacement under different normal stress conditions. At a normal stress of 40 kPa (5.8 psi), there is a gradual rise in the pullout stresses for the FRS specimen, whereas the pullout stresses are almost constant for FR2 specimens, indicating the effect of small stone on the surface of ferrocement panels. The pullout displacement at which the mesh failure occurred is also different for the different surface condition of ferrocement elements. For example, in the case of FRS specimens, the mesh failure occurred at approximately 9.0 to 10.0 mm (0.35 to 0.39 in.) pullout displacement, whereas in the case of FR2 specimens, the mesh failure occurred at the pullout displacement of nearly 12.0 mm (0.47 in.).

FR4 and FR6 specimens-The pullout response and failure modes of chicken mesh-reinforced ferrocement elements with four and six channels (FR4 and FR6 specimens) are shown in Fig. 9(d) and (e), respectively. Obviously, the fluctuation nature of the pullout stress-displacement relationships due to the failure of mortar and mesh is significantly noticeable with the increase in pullout displacement. Similar to the trend of square ferrocement elements, chicken mesh-reinforced ferrocement elements with four and six channels also show the improvement in the pullout stresses with the increase in number of channels. The fluctuation of the pullout stresses with the increase in pullout displacement, however, is more noticeable in the case of FR6 than that of FR4. The chicken mesh-reinforced FR4 panels show mortar failure at normal stresses of 40 and 60 kPa (5.8 and 8.7 psi) and mesh failure at normal stresses of 80 and 100 kPa (11.6 and 14.5 psi), whereas the chicken mesh-reinforced FR6 panels show mortar failure at normal stress of 40 kPa (5.8 psi) only and mesh failure at normal stresses of 60, 80, and 100 kPa (8.7, 11.6, and 14.5 psi).

It is noted herein that the pullout behavior in the form of stress-displacement relationships in clayey soil show an almost similar trend to that in the sandy soil. Therefore, the pullout stress-displacement relationships in clayey soil are not repeated herein. The results of ultimate pullout strength of ferrocement elements in clayey soil, however, are depicted in Table 2.

Ultimate pullout stress and failure modes of ferrocement

For the sake of clarity toward the cost-effective and optimum design of ferrocement elements, a summary of different modes of failure of the ferrocement elements under pullout tests in sandy and clayed soil are given in Table 2. It is evident from this table that the failure mode depends on ferrocement pullout resistance, which is relevant to the surface characteristics of the ferrocement; strength of the mortar, which is relevant to the cement/sand ratio; and the tensile capacity of the mesh, which is relevant to type of mesh. For ferrocement containing square mesh, only two failure modes, such as frictional failure (Fig. 1(a)) and mortar failure (Fig. 1(b)), occurred, whereas ferrocement containing chicken mesh, three failures modes, such as frictional failure (Fig. 1(a)), mortar failure (Fig. 1(b)), and mesh failure (Fig. 1(c)) occur. It should be pointed out that the bond failure (Fig. 1(d)) described in the possible failure modes was not observed.

For a clear understanding of the ultimate pullout stress of the ferrocement elements in both sandy and clayey soils, the ultimate pullout stresses corresponding to the different normal stresses of the ferrocement panels with smooth and rough surfaces are shown in Fig. 11 and 12. As can be seen, the ultimate pullout stresses increased constantly with an increase in the normal stress for all types of ferrocement panels in both the soils. The rate of increase of ultimate pullout stresses for ferrocement panels with rough surfaces is more than that of the ferrocement panels with smooth surfaces. It is also observed that the increase in ultimate pullout stresses for ferrocement of any type is more in clayey soil than in sandy soil. According to the Mohr-Coulomb failure criteria and from the straight lines plotted in Fig. 11 and 12, equations for pullout resistance of ferrocement can be obtained as follows.

In sandy soil

τ^sub FSS^ = 1.62σ^sub FSS^ + 7.62 (1)

τ^sub FRS^ = 1.69σ^sub FRS^ + 7.83 (2)

τ^sub FR2^ = 1.82σ^sub FR2^ + 10.45 (3)

τ^sub FR4^ = 1.91σ^sub FR4^ + 12.81 (4)

τ^sub FR6^ = 1.96σ^sub FR6^ + 14.78 (5)

In clayey soil

τ^sub FSS^ = 1.47σ^sub FSS^ + 9.10 (6)

τ^sub FRS^ = 1.69σ^sub FRS^ + 11.73 (7)

τ^sub FR2^ = 1.72σ^sub FR2^ + 13.24 (8)

τ^sub FR4^ = 1.78σ^sub FR4^ + 21.97 (9)

τ^sub FR6^ = 1.81σ^sub FR6^ + 30.03 (10)

where τ is the pullout resistance of ferrocement in kPa and σ is the normal stress on ferrocement in kPa. Therefore, the angle of friction of ferrocement for FSS, FRS, FR2, FR4, and FR6 specimens are calculated as 58.3, 59.3, 61.2, 62.3, and 62.9 degrees in sandy soil, and 55.5, 59.3, 59.8, 60.6, and 61.0 degrees in clayey soil, respectively. The cohesion values are obtained as 7.62, 7.83, 10.45, 12.81, and 14.78 kPa (1.10, 1.13, 1.51, 1.75, and 2.14 psi) in sandy soil, and 9.10, 11.73, 13.24, 21.97, and 30.03 kPa (1.31, 1.70, 1.91, 3.18, and 4.35 psi) in clayey soil for the ferrocement of FSS, FRS, FR2, FR4, and FR6, respectively. The equations shown previously are based on the straight lines that were drawn from the relationships between normal stresses and ultimate pullout stresses plotted in the graphs. One equation is obtained based on the four normal stresses and four corresponding ultimate pullout stresses. Therefore, it is actually the average value of the same four types of panels tested under different normal stresses. It is known that the coefficient of friction can be calculated from one normal stress and its corresponding ultimate pullout stress. For example, using equation F = µN, where F is the pullout force, N is the normal stress, and µ is the coefficient of friction. In the present research, the pullout test is carried out by pulling out the reinforcement from the soil under different normal stresses. The pullout stresses acted on the interface between the soil and the reinforcement are measured directly and plotted in Fig. 11 and 12 with the applied normal stresses as abscissa and pullout out stresses as ordinate. The least square linear lines obtained by the regression analysis for the four categories are similar to that of the method of failure envelope. It is noted herein that the R^sup 2^, or the coefficient of determination of the regression analysis, has values of 0.979, 0.9931, 0.9949, 0.998, and 0.9995 in sandy soil and 0.9719, 0.9898, 0.9981, 0.9974, and 0.999 in clayey soil for the ferrocement of FR6, FR4, FR2, FRS, and FSS, respectively; that is, the R^sup 2^ value for all the cases close to 1.0 indicates that the tests data are fitted well and almost all of the variability with the variables specified in this paper have been accounted for. This also indicates the reliability of the test results.

In view of comprehensible perception of the comparative study among the different types of ferrocement elements, frictional properties previously obtained are further analyzed and shown in Fig. 13. As can be seen, noticeable improvement in the frictional angle, a measure of frictional resistance of ferrocement elements, appears due to the presence of stone and transverse channels on the surface of the ferrocement elements. Also, the increase in the number of channel shows an increase in the frictional angle in both the soils. Among all the ferrocement elements tested, the FR6 specimen is the most effective in imparting the frictional resistances both in sandy and clayey soils. It is also revealed that the frictional angle of ferrocement is more effective in sandy soil than in clayey soil (Fig. 13(a)). The cohesion, a measure of bonding phenomena of ferrocement, on the other hand, is more effective in clayey soil than in sandy soil (Fig. 13(b)). This may be due the effect of surface roughness of the ferrocement panels as well as more frictional resistance of sandy soil and more cohesion of clayey soil.

Discussion regarding corrosion of ferrocement as embedded in soil

Air and moisture accelerate corrosion in ferrous materials unless they are protected. Acids tend to corrode steel, whereas alkalies, such as found in animal waste, portland cement, lime, as well as some soils, will cause rapid corrosion of ferrous materials. Electrolytic action caused by slight voltages set up when dissimilar metals are in contact with each other in the presence of water also encourages corrosion in some metals. Steel is particularly subject to electrolytic corrosion. Corrosion can be reduced by carefully selecting metal products for the application, reducing the time that the metal will be wet by preventing condensation and promoting good drainage, avoiding contact between dissimilar metals, and by using corrosion-inhibiting coatings. Stainless steels and cast iron tend to form oxide coatings that provide a considerable amount of self-protection from corrosion. Most other steels, however, require protective coatings if they are exposed to moisture and air. Methods used include zinc coating (galvanizing), vitreous-enamel glazing, and painting. Painting is the only method practical for field application, although grease and oil will provide temporary protection. Before painting, the metal surface must be clean, dry, and free of oil. Both bituminous and oil-based paints with metallic-oxide pigments offer a good protection if they are carefully applied in continuous layers. Two to three coats offer the best protection. It is noted herein that the research reported in this article deals with the ferrocement-soil interface behavior under pullout tests and development of the technique of the composite reinforcement system made of cement mortar and wire mesh. It is important to point out the corrosion of ferrocement sheet when embedded in soil because the ferrocement can be made of corrosion-resistant mesh. Researches have already shown that several kinds of noncorrosive meshes are already available in the market.4 Ceramic-clad mesh (using a micro-infiltrated macro-laminated coating), inorganic zinc silicate-clad mesh, hot-dip galvanized mesh, zinc-coated mesh (using the Delot process), zinc-rich clad mesh, nickel-clad mesh, copper-clad mesh, copper alloyclad mesh, 304 stainless steel-clad mesh, galvalum (aluminum and zinc) clad mesh, reactive copper in an organic coating mesh, titanium mesh, Type 304 stainless steel mesh, Type 316 stainless steel mesh, Type 317 stainless steel mesh, Type 304N stainless steel mesh, Type XM-19 stainless steel mesh, nitronic 33 stainless steel mesh, corrosion-resistant steel alloy mesh, and Type C613000 aluminum bronze mesh are some examples. In addition to these, other types of noncorrosive meshes made of high-density polyethylene and geosynthetics are also available. Moreover, corrosion-resistant ferrocement plates are also currently available. Although the results on corrosion of ferrocement are beyond the scope of this research article, it can be considered a good start to achieve the goal toward the development of the composite reinforcement system in soil reinforcement applications. The paper reported the results on pullout tests of ferrocement elements obtained under a short-term basis and, therefore, the corrosion of ferrocement was not taken into account. The authors suggest additional study of the corrosion of ferrocement as embedded in soil for more durability under a long-term basis.

CONCLUSIONS

A systematic study on the development of ferrocement elements made of high-tensile steel wire mesh embedded in a cement mortar as a viable system for soil reinforcement applications was conducted. The adequate frictional capacity requirement was provided by the interfacial friction between the mortar and soil whereas the tensile strength requirement was provided by the high-tensile steel wire mesh. Both requirements can be independently controlled, enabling an optimum design of the ferrocement elements from both considerations for a given situation. Synergetic action between mortar and soil is already apparent, especially with an increased number of transverse channels on the surface of ferrocement and, thus, it justifies the optimization attempts of ferrocement elements for soil reinforcement applications. Although finding an optimal number of channels is beyond the scope of this research paper, the results reported previously, however, are encouraging toward achieving the goal. Based on the pullout tests on ferrocement and modes of failure, the following conclusions can be drawn:

1. The use of small channels on the surface of ferrocement elements appears to be highly effective in enhancing the pullout performance of the ferrocement elements embedded in soil. Among the five types of ferrocement elements tested, the FR6 specimen appears to be the most effective. Ferrocement element with an increased number of channels possesses higher frictional resistances in sandy soil than in clayey soil, and higher cohesion value in clayey soil than in sandy soil. For square mesh-reinforced ferrocement elements, only frictional failure and mortar failure occurred under any normal stress in both soils. For chicken mesh-reinforced ferrocement elements, on the other hand, three modes of failure, such as frictional failure, mortar failure, and mesh failure, occurred, especially under higher normal stress conditions. The research reported in this study is rudimentary and the pullout resistances were studied for the variation of number of channels of up to six channels only. Although investigating the optimum value of the number of channels is beyond the scope of this study, it can be considered a good start to achieve the goal. From the trend of the results, one may argue that the pullout resistance may be more when the number of channels would be eight or 10. This, however, needs to be substantiated by evidence requiring further experimentation, which can be suggested for the future works; and

2. The results obtained in this study suggest that the inherent surface property of the ferrocement element has resulted in significant quality improvements, including enhancement of pullout strength and failure mechanism. These high-performance characteristics have positioned ferrocement panels as the ideal product when interfacial friction between soil and ferrocement, and strength characteristics are called for. Although field applications and theoretical models of this reinforcement have not been thoroughly examined, it is expected that ferrocement will perform much better than conventional reinforcement alone. Further investigations are suggested to study the strength characteristics and other basic properties for a wider range of number of channels as well as theoretical modeling. Nonetheless, the results depicted in this paper are fairly encouraging, especially for improving the engineering properties of reinforced soil and stability performance of soil structures.

ACKNOWLEDGMENTS

The research reported in this paper is partly supported by Research Grant No. 18580243 with funds from Grants-in-Aid for Scientific Research given by the Japanese Government. The author gratefully acknowledges this support. Any opinions, findings, and conclusions expressed in this paper are those of the author and do not necessarily reflect the views of the sponsor.