A. S. J. J. PANNIRSELVAM et al.: EXPERIMENTAL INVESTIGATION OF INTERFACIAL FRICTION BETWEEN ...
655–659
EXPERIMENTAL INVESTIGATION OF INTERFACIAL FRICTION
BETWEEN FRPs-WRAPPED STEEL SURFACE AND
COHENSIONLESS SOIL
EKSPERIMENTALNA RAZISKAVA TRENJA NA MEJNI PLOSKVI
MED POVR[INAMA S POLIMERI (FRP) OVITEGA JEKLA IN
NASUTE ZEMLJINE
Annie Sweetlin Jebarani Joshua Pannirselvam
1*
, Daniel Thangaraj Duraisamy
2
1
Velammal College of Engineering and Technology, Madurai to Rameshwaram Highway, Viraganoor, Madurai 625 009, India
2
Anna University College of Engineering Tindivanam, Tamil Nadu, India
Prejem rokopisa – received: 2019-10-31; sprejem za objavo – accepted for publication: 2020-06-17
doi:10.17222/mit.2019.264
Steel piles are nowadays commonly used as a foundation component. The soil and steel surface interaction plays a major role in
transferring the load to the soil. This study was carried out to examine the interfacial frictional behaviour of cohesionless soil
and a steel surface wrapped with different types of fibre-reinforced polymers (FRPs). Conventional direct-shear tests were con-
ducted on steel plates wrapped with FRP sheets. Fibre-reinforced polymers like unidirectional carbon, basalt, aramid and glass
were wrapped on steel plates and tested at different orientations with respect to the applied shear. Two different soil samples
were taken for the study, viz., well-graded and poorly graded sand. The experimental results showed that the interface friction
increased with the surface roughness and also with the orientation of the fibres to the loading direction. The study also showed
that the gradation of sand plays a vital role in the interfacial shear strength of the sand and steel surface wrapped with different
types of fibre-reinforced polymers.
Keywords: interface friction, fibre-reinforced polymers, surface roughness, shear strength
Jekleni piloti se danes pogosto uporabljajo za temeljenje razli~nih komponent. Interakcija med zemljino (na primer re~nim
peskom, mivko itd.) in povr{ino jekla, igra glavno vlogo pri prenosu obremenitve s pilotov na zemljino. V ~lanku avtorji
opisujejo {tudijo ugotavljanja trenja na meji med nasuto zemljino in jekleno povr{ino, oble~eno v razli~ne vrste z vlakni
oja~anih polimerov (FRP; angl.: Fiber Reinforced Polymers). Za meritve so uporabili direktni stri`ni preizkus z razli~no
orientacijo stri`ne obremenitve. Jeklene povr{ine so bile oble~ene v polimere oja~ane z ogljikovimi, bazaltnimi, aramidnimi ali
steklenimi vlakni. Uporabili so dve razli~ni vrsti zemljine, kakovosten in manj kakovosten pesek. Eksperimentalni rezultati so
pokazali, da trenje na mejni ploskvi nara{~a s pove~ano hrapavostjo povr{ine in prav tako v odvisnosti od orientacije vlaken
glede na obremenitev. [tudija je prav tako pokazala, da kakovost izbranega peska pomembno vliva na stri`no trdnost na mejni
ploskvi med peskom in jeklom, oble~enim z razli~nimi vrstami polimerov oja~anih z vlakni.
Klju~ne besede: trenje na mejni ploskvi, z vlakni oja~ani polimeri, povr{inska hrapavost, stri`na trdnost
1 INTRODUCTION
Steel piles transfer load to the adjacent soil by inter-
face friction. Hence the study of this interface shear
strength is necessary to increase the structural capacity
of the piles. This has been done basically with simple
shear studies like a direct-shear test on steel surfaces and
both cohesive and non-cohesive soils. The emerging
technology of fibre-reinforced polymers also added to
the strength and durability of steel piles. Fibre-reinforced
polymers have special physical characteristics like high
strength, non-corrosive, viscoelastic behaviour and light
weight. Hence, they find wide applications in civil engi-
neering. Fibres can be natural or artificial. Artificial
fibres are mostly used in civil-engineering applications
as natural fibres can decompose in the course of time.
The strength of these fibres vary with their orientation
with regard to the loading direction. Recently, studies
have been done to determine the behaviour of the fibres
wrapped on a steel surface and their interaction with the
adjacent soil; their results are discussed in this paper.
The roughness of the interacting surface is of crucial im-
portance in geotechnical terms. This paper also attempts
to study the effects of surface roughness and soil grada-
tion on the interface shear strength and friction angle of
fibres.
M. Uesugi and H. Kishida
1
did laboratory experi-
ments on the friction between dry sand and mild steel.
They also studied the possible influences of surface
roughness of steel and particle size on the interfacial
shear strength. T. D. O’Rourke et al.
2
did experiments on
sand-polymer interfaces using direct-shear tests and
found that the interface shear strength increased with the
soil density and surface hardness. J. D. Frost and J.
Hans
3
did experiments on the interface behaviour of
sand-FRP interfaces and found that the interface shear
strength depends on the relative roughness of surface,
initial density of soil mass, angularity of soil particles
Materiali in tehnologije / Materials and technology 54 (2020) 5, 655–659 655
UDK 620.1:67.017:677.021.16:620.191.31 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(5)655(2020)
*Corresponding author's e-mail:
jpas@vcet.ac.in (Annie Sweetlin Jebarani Joshua Pannirselvam)

and normal stress level. M. A. Pando
4
did experiments on
the interaction between FRP composite piles and sand
and found that the surface roughness along with surface
hardness and angularity of soil influenced the interface
friction angle. I. Nishizaki and S. Meiarashi
5
studied a
long-term deterioration of GFRP in water and moist at
various temperatures and in immersed conditions. The
study showed that at 60 °C cracks appeared in the speci-
men but no chemical changes occurred. D. Porcino et al.
6
did experiments on natural silicas and aluminium plates
of different roughness degrees under a constant normal
load and constant normal stiffness. They found that the
surface roughness influences the interface shear strength
between the soil and solid. Narayan Prasad Sahu et al.
7
studied the durability of aramid-fibre-reinforced poly-
mers wrapped on concrete specimens on the corrosion
and found that it is highly resistant. Y.-K. Li et al.
8
re-
searched the interfaces between soils and steel, focusing
on the friction resistance between them. The test results
showed that the peak and residual friction angle increase
with an increase in the surface roughness and decrease in
the surface hardness. The friction angle reaches the larg-
est value with the largest surface roughness. V. S.
Quinteros et al.
9
did experiments on the steel-sand inter-
face, using Bishop’s ring shear apparatus to obtain the
residual-friction angles. They found that the resid-
ual-friction angle depends on the surface roughness and
effective vertical stresses, but it is independent of fines
content. F. Han et al.
10
did experiments on the shear
strength of sand-steel interface using ten different types
of sand and found that the surface roughness, soil grada-
tion and particle size are influential parameters. H. S.
Aksoy et al.
11
performed a direct-shear test to find the in-
ternal friction angle, interface shear test and skin-friction
angle between made-up soil samples and steel-FRP piles.
They found that as the percentage of clay increases, the
skin-friction angle reduces. J. P. A. Jose et al.
12
studied
the performance of a basalt-fibre-reinforced polymer-
concrete specimen and found a significant increase in the
shear strength as the roughness of the interface in-
creased.
2 MATERIAL PROPERTIES
Two types of soil samples were tested: sand from a
field and ordinary river sand. Preliminary tests were con-
ducted to find the index properties of both types of
cohesionless soil, which are listed in Table 1. In addi-
tion, we chose carbon-fibre-reinforced polymer (CFRP),
glass-fibre-reinforced polymer (GFRP), basalt-fibre-rein-
forced polymer (BFRP) and aramid-fibre-reinforced
polymer (AFRP). All the fibres were unidirectional. The
properties of all the fibres are listed in Table 2. Mild-
steel plates with a size of 6 × 6 × 0.6 cm were used in the
experiments.
3 METHODOLOGY
The absolute surface roughness (R
a
in µm) of the
steel surface and steel plates wrapped with different
FRPs were found using a surface roughness meter –
Mitutoyo brand for both orientations. The meter has a
needle that moves over the surface and the running val-
ues are printed as a digital output in the wave form. It
also gives the average value of R
a
as a digital display,
considered to be the result. As the soil was classified as
the SP and SW sand, as per IS 2720 Part XIII-1983, the
direct-shear test was adopted to find the shear strength of
the soil and, hence, the interface-friction angle. A study
of the interface frictional strength was carried out in a
conventional direct-shear apparatus, as per IS 2720 Part
XIII-1983, with a shear box with a size of (6×6×2) cm
A. S. J. J. PANNIRSELVAM et al.: EXPERIMENTAL INVESTIGATION OF INTERFACIAL FRICTION BETWEEN ...
656 Materiali in tehnologije / Materials and technology 54 (2020) 5, 655–659
Table 1: Index properties of the sand
S. No. PROPERTIES RIVER SAND FIELD SAND
1 Particle size D
10
0.2 0.17
2 Particle size D
30
0.5 0.45
3 Particle size D
60
0.7 1.1
4 Uniformity coefficient, C
u
3.5 6.58
5 Coefficient of curvature, C
c
1.78 1.065
6 Type of soil (as per 1498-1970) Poorly graded sand (SP) Well graded sand (SW)
7 Specific gravity 2.62 2.58
8 Relative density 50 % 50 %
9 Maximum density (kN/m
3
) 19.42 13.83
10 Minimum density (kN/m
3
) 13.54 12.85
11 Test density (kN/m
3
) 16.40 13.32
Table 2: Properties of the fibres
S. No. Fibre property/type CFRP GFRP BFRP AFRP
1 Fibre thickness (mm) 0.3 0.6 0.33 0.5
2 Fibre orientation Unidirectional Unidirectional Unidirectional Unidirectional
3 Tensile strength (kN/m
2
) 3500 × 10
3
3400 × 10
3
1500 × 10
3
2500 × 10
3
4 Tensile modulus (kN/m
2
) 238 × 10
6
73 × 10
6
86 × 10
6
131 × 10
6

and the steel-plate specimen with the size of (6×6×0.6)
cm. The experimental set-up is shown in Figure 1. The
shear test was conducted for control samples SW
sand/SW sand, SP sand/SP sand, steel/SW sand and
steel/SP sand. Then the steel plates were wrapped with
different FRPs using epoxy resin as shown in Figure 2a.
The plates were placed in the apparatus at orientations of
0° and 90° to the loading direction. The direct-shear test
was done at normal stresses of (50, 100, 150, 200) kN/m
2
for all the samples. The sheared samples are shown in
Figure 2b. The horizontal displacement was measured
using a dial gauge. The interface friction angle was
found as the slope of line from the graph plotting the
normal stress as the abscissa and the shear stress as the
ordinate.
4 RESULTS AND DISCUSSION
4.1 Interface-friction angle and fibre orientation
The interface-friction angle was found for the plain
steel samples with well-graded and poorly graded sands
and also for the steel samples wrapped with different
FRPs. The results for the internal-friction angle for the
fibre orientation parallel and perpendicular to the loading
direction are presented in Table 3. The results showed
that the interface friction of the fibre-wrapped specimens
was higher than that of the control specimens for both
types of sand. It was also found that, on average, the in-
terface-friction angles of the fibres with the perpendicu-
lar orientation were about 10 % higher than those of the
fibres with the parallel orientation.
Table 3: Interface-friction angle and surface roughness
S. No. Material R
a (µm)
Interface-friction angle in
degrees
Poorly
graded sand
Well-graded
sand
1 STEEL 0.631 14 22
2 CFRP 0° 0.743 21 22
3 CFRP 90° 0.978 26 26
4 BFRP 0° 2.436 23 29
5 BFRP 90° 2.911 29 30
6 AFRP 0° 5.875 29 30
7 AFRP 90° 6.31 31 33
8 GFRP 0° 8.874 30 33
9 GFRP 90° 9.312 33 37
4.2 Influence of the surface roughness on the friction
angle
The absolute surface roughness (R
a
in μm) of the
plain steel surface and steel plates wrapped with differ-
ent FRPs and different fibre orientations was found using
a surface-roughness meter; the results are given in Table
3. They show that as the value of surface roughness in-
creased, the internal-friction angle also increased. Also,
the values of the surface roughness are slightly higher for
the fibres with the 90° orientation than those with the 0°
orientation, as represented in Figure 3. In addition, as
the gradation of the sand varies, the interface friction
A. S. J. J. PANNIRSELVAM et al.: EXPERIMENTAL INVESTIGATION OF INTERFACIAL FRICTION BETWEEN ...
Materiali in tehnologije / Materials and technology 54 (2020) 5, 655–659 657
Figure 1: Experimental set-up for the direct-shear test
Figure 2: FRPs wrapped on the steel plates before the test: a) aramid, b) basalt, c) carbon, d) glass and FRPs wrapped on the steel plates after the
test: e) aramid, f) basalt, g) carbon, h) glass

varies as well. Well-graded sand provides more resis-
tance to shear than poorly graded sand.
4.3 Variation in the shear stress and normal stress
The conventional direct-shear test was conducted for
various normal-stress values and the maximum shear
stress was found. The variation in the normal and maxi-
mum shear stress for poorly graded sand and for the par-
allel fibre orientation is shown in Figure 4, while Fig-
ure 5 refers to the perpendicular fibre orientation. It was
found that as the normal stress was increased, the maxi-
mum shear stress also increased. The tensile strength of
fibres also has a significant impact on the interface fric-
tion. As the tensile strength of the glass fibres increased,
it exhibited a higher value in both parallel and perpendic-
ular orientations.
In the well-graded sand, the values of the shear stress
were greater than in the poorly graded sand, clearly indi-
cating that its shear resistance was higher. Also, when
compared to the control specimen, the value of the
glass-fibre specimen was nearly twice as big, which is
more advantageous when used as the structural material.
The variation in the normal and shear stress in the
well-graded sand and for the parallel orientation is
shown in Figure 6, while Figure 7 refers to the perpen-
dicular orientation.
For both types of sand, the fibres with the perpendic-
ular orientation gave higher values than the fibres with
the parallel orientation. The maximum shear stress in-
creased by 5 % in the parallel direction and by 15 % in
the perpendicular direction with respect to the soil grada-
tion.
5 CONCLUSIONS
Conventional direct-shear tests were conducted to
find the interface-friction angle between well or poorly
graded sandy soils and steel-plate specimens wrapped
with CFRP, BFRP, AFRP and GFRP. From the results
obtained, it was seen that, in general, there was a larger
increase in the angle of interface friction with the FRP
wrapping in the perpendicular direction than in the paral-
lel direction to the shear loading for all the types of
fibres. There was also an increase in the shear strength at
the interface with an increase in the surface roughness
for all the types of fibre-wrapped steel specimens. The
shear strength increases with the increasing normal
A. S. J. J. PANNIRSELVAM et al.: EXPERIMENTAL INVESTIGATION OF INTERFACIAL FRICTION BETWEEN ...
658 Materiali in tehnologije / Materials and technology 54 (2020) 5, 655–659
Figure 3: Effect of the surface roughness on the interface-friction an-
gle
Figure 7: Normal versus shear stress at 90° in well-graded sand
Figure 4: Normal versus maximum shear stress for 0° in poorly
graded sand
Figure 6: Normal versus shear stress at 0° in well-graded sand
Figure 5: Normal versus shear stress for 90° in poorly graded sand

stress. The lowest results were obtained for the CFRP
specimen and the highest were obtained for the GFRP
specimen.
For the CFRP specimen, the increase in the inter-
face-friction angle for the parallel fibre orientation was
50 % higher than that of the control specimen with an in-
crease in the surface roughness in poorly graded sand.
Similarly, the increase in the interface angle of the speci-
men with the perpendicular fibre orientation was 85 %
higher than that of the control specimen with an increase
in the surface roughness in poorly graded sand, whereas
no significant change was noticed for the well-graded
sand.
The increase in the interface-friction angle of the
GFRP specimen with the parallel fibre orientation was
114 % higher than that of the control specimen in poorly
graded sand. Similarly, the increase in the interface angle
of the specimen with the perpendicular fibre orientation
was 135 % larger than that of the control specimen in
poorly graded sand; in well-graded sand, the increase in
the interface-friction angle of the specimen with the par-
allel fibre orientation was 135 % higher than that of the
control specimen in the well-graded sand, while the in-
crease in the interface angle of the specimen with the
perpendicular fibre orientation was 164 % higher than
that of the control specimen.
The experimental results show that the steel-FRP
composite material exhibits a high surface roughness and
interface friction, increasing its shear strength.
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