Park2008

Geotextiles and Geomembranes 27 (2009) 162–166
Contents lists available at ScienceDirect
Geotextiles and Geomembranes
journal homepage: www.elsevier.com/locate/geotexmem
Technical Note
Effect of fiber reinforcement and distribution on unconfined compressive
strength of fiber-reinforced cemented sand
Sung-Sik Park*
Division of Civil, Environmental and Urban Engineering, Wonkwang University, 344-2 Sinyongdong, Iksan, Jeonbuk 570-749, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 March 2008
Received in revised form 8 September 2008
Accepted 9 September 2008
Available online 8 November 2008
A series of unconfined compression tests were carried out to examine the effect of fiber reinforcement
and distribution on the strength of fiber-reinforced cemented sand (FRCS). Nakdong River sand, polyvinyl
alcohol (PVA) fiber, cement and water were mixed and compacted into a cylindrical sample with five
equal layers. PVA fibers were randomly distributed at a predetermined layer among the five compacted
layers. The strength of the FRCS increases as the number of fiber inclusion layers increases. A fiberreinforced specimen, where fibers were evenly distributed throughout the five layers, was twice as
strong as a non-fiber-reinforced specimen. Using the same amount of fibers to reinforce two different
specimens, a specimen with five fiber inclusion layers was 1.5 times stronger than a specimen with one
fiber inclusion layer at the middle of the specimen. The fiber reinforcement and distribution throughout
the entire specimen resulted in a significant increase in the strength of the FRCS.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Fiber-reinforced sand
Cemented sand
Fiber distribution
Unconfined compressive strength
1. Introduction
Clay or granular soils are mixed with cement or gypsum to
improve engineering properties such as cohesion and friction angle
(Abdulla and Kiousis, 1997; Clough et al., 1981; Ismail et al., 2002;
Kim et al., 2008; Liu et al., 2006; Saxena and Lastrico, 1978).
Cemented soils can be used for the reinforcement of soft ground,
the backfill materials of retaining walls, and for the sub-base of
roads and railroads because cemented soils are less compressible
and have an increased strength over that of natural soils. While
such cemented soils can significantly increase compressive
strength with an increasing cement ratio, this causes a brittle or
sudden failure without plastic deformation to occur. Cemented
sand that shows a brittle failure pattern can be reinforced with fiber
to prevent such brittle failure. Several researchers (Consoli et al.,
1998; Consoli et al., 2003; Kaniraj and Havanagi, 2001; Maher and
Ho, 1993; Tang et al., 2007) have attempted to study the combined
effect of fiber and cement on granular or clayey soils. Such fiberreinforced cemented specimens were prepared for laboratory
testing by manually mixing sand, cement and fibers with water. For
most of the experimental research that has been carried out on
fiber-reinforced soils (Babu et al., 2008; Chauhan et al., 2008;
Kaniraj and Gayathri, 2003), the fibers were considered to be
uniformly distributed throughout a small specimen. However, in
practice the fibers may be poorly distributed or may flow into one
* Tel.: þ82 63 850 6716; fax: þ82 63 850 7140.
E-mail address: [email protected]
0266-1144/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.geotexmem.2008.09.001
place when fiber-reinforced samples are prepared in the laboratory.
While applying such fiber-reinforced materials in the field, the fiber
concentration or difficulties with fiber mixing could be significant
and the anticipated strength increase of fiber-reinforced materials
may not be achieved.
The objective of this study is to examine the effect of fiber
distribution and concentration on the strength of fiber-reinforced
cemented sand. A series of unconfined compression tests were
performed on artificially cemented samples with layered fiber
reinforcement. The unconfined compressive strength (UCS) of the
samples was evaluated to quantify the uncertainties associated
with fiber distribution or concentration in the strength of fiberreinforced soils. The strength of fiber-reinforced soils could be
factored to consider the uncertainty induced by non-uniform fiber
distribution, which can be applied to a reliable design (e.g., limit
state design) of soil reinforcement.
2. Unconfined compression tests
2.1. Sand and cement
Clean Nakdong River sand is used in this study and its gradation
curve is shown in Fig. 1. As described in Table 1, an X-ray Fluorescence (XRF) analysis of Nakdong River sand shows that it consists of
silica sand with more than 76% silica. A microphotograph of Nakdong River sand is shown in Fig. 2. The sand was sieved to a size
ranging between 2 mm and 0.075 mm diameter and washed with
a No. 200 sieve (0.075 mm in diameter) to remove fines. It was then
oven-dried for testing. Ordinary Portland cement was used as
S.-S. Park / Geotextiles and Geomembranes 27 (2009) 162–166
163
100
90
Percent finer (%)
80
70
60
50
40
30
20
10
0
10
1
0.1
0.01
Grain size (mm)
Fig. 1. Grain size distribution of Nakdong River sand.
a cementing agent for the preparation of artificially cemented
samples.
Fig. 2. Microphotograph of Nakdong River sand.
2.2. Fiber
Polypropylene fiber is the most widely used inclusion in the
laboratory testing of soil reinforcement (e.g., Khattak and Alrashidi, 2006; Santoni et al., 2001; Tang et al., 2007; Viswanadham
et al., 2009; Yetimoglu and Salbas, 2003; Yetimoglu et al., 2005).
This fiber is lighter than water, and weak in alkalis. Polyvinyl
alcohol (PVA) fiber is a synthetic fiber that has recently been used
in fiber-reinforced concrete, since its weather resistance, chemical resistance, and tensile strength are superior to that of polypropylene fiber. PVA fiber has a significantly lower shrinkage
from heat than nylon or polyester. It has a specific gravity of 1.3,
a good adhesive property to cement, and high anti-alkali characteristics. For this reason, it is suitable for mixing with cement
and is widely used in concrete and cement reinforcement. The
type of PVA fiber used in this study is RECS100L (produced by
Kuraray in Japan) with a tensile strength of 1078 N/mm2. The PVA
fiber shown in Fig. 3 is 12 mm in length and 0.1 mm in diameter,
and is thicker than PVA fiber normally used for reinforced
cement.
three fiber-reinforced specimens (Case 1, Case 2, and Case 3) were
prepared for testing, as illustrated in Fig. 4. The number of fiber
inclusion layers for each specimen varied: zero (no fiber), one
(middle layer only), three (middle, top and bottom layers), and five
(all layers). Depending on the number of fiber inclusion layers, the
degree of fiber reinforcement or distribution was determined as 0%,
20%, 60%, and 100%.
One of the difficulties in preparing fiber-reinforced specimens is
the method used to mix fibers into a non-plastic soil such as sand
(Freitag, 1986). Cemented sand was prepared by mixing dry sand
and cement with an optimum water content of 14%. The cemented
sand was then divided into five portions and each portion was
mixed by hand with a predetermined amount of fiber. Therefore,
2.3. Sample preparation
The cement ratio is based on dry sand and defined as follows:
rc ¼
Wc
100ð%Þ
Ws
(1)
where Wc is the weight of cement and Ws is the weight of dry sand.
The cement ratio used in this study is 4%. The fiber ratio is based on
the amount of fiber used in each fiber inclusion layer. It is defined as
follows:
rf ¼
Wf
100ð%Þ
Ws
(2)
where Wf is the weight of fiber used in each fiber inclusion layer,
not in the entire specimen. One non-fiber-reinforced specimen and
Table 1
X-ray Fluorescence analysis of Nakdong River sand.
Classification
Content (%)
SiO2
76.62
Al2O
12.37
CaO
1.65
K2O
4.19
Na2O
2.88
Fe2O3
1.98
MgO
–
P2O5
–
TiO2
0.28
Fig. 3. PVA fiber RECS100L (diameter 0.1 mm).
164
S.-S. Park / Geotextiles and Geomembranes 27 (2009) 162–166
a
b
c
1000
140 mm
Layer 4
Cemented sand
Layer 3
Fiber-reinforced
cemented sand
Layer 2
Layer 1
70 mm
Case 1
Case 2
Case 3
Fig. 4. Fiber-reinforced specimen.
a uniform distribution of fibers throughout the specimen was
achieved for laboratory testing. However, the hand mixing method
will not be applicable to large-scale work in the field. After mixing,
each portion was compacted directly into a small-sized mold by
applying 55 blows per layer via a metal rammer that has a 65 mm
diameter circular face with a weight of 22 N. The metal mold was
separated and then the specimen was cured for 7 days. Each
specimen was 70 mm in diameter and 140 mm in height. Unconfined compression tests were carried out at a shearing rate of 1%/
min (1.4 mm/min).
3. Results of unconfined compression tests
Two series of unconfined compression tests were carried out to
investigate the strength of fiber-reinforced cemented sand (FRCS).
Each series consisted of three fiber-reinforced cases (Case 1, Case 2,
and Case 3), as shown in Table 2. The aim of the first series, the L
series, was to evaluate the effect of fiber reinforcement on an UCS.
In the L series, the fiber ratio of rf is constant at 0.2%. Fig. 5 shows
the results of the L series where the number of fiber inclusion layers
varied when using the same amount of fibers. As the degree of fiber
reinforcement increases, the UCS significantly increases, as
summarized in Table 3. The 100% (fully reinforced) fiber-reinforced
specimen was twice as strong as the 0% fiber-reinforced specimen,
as shown in Fig. 6 and Table 3.
The aim of the second series, the T series, was to evaluate the
effect of fiber distribution on an UCS. The effect of fiber distribution
or concentration on the UCS was accessed by increasing the fiber
inclusion layer and by using the same amount of fibers in the entire
specimen. In the T series, the rf of Case 1, Case 2, and Case 3 is 1%,
0.33%, and 0.2%, respectively. However, the total amount of fibers
used in the entire specimen is 1% of the weight of dry sand and
constant in all cases. L-3 and T-3 are essentially the same conditions
and their results show repeatability of test results. Fig. 7 shows the
results of the T series. As the degree of fiber distribution increases,
Table 2
Summary of unconfined compression tests.
Specimen
Reinforced layer
Fiber ratio, rf
Strength (kPa)
Secant elastic
modulus (MPa)
No fiber
No fiber
No fiber
440
37
L series
L-1
L-2
L-3
Layer 3
Layer 1, 3, 5
All layers
0.2%
0.2%
0.2%
530
696
857
47
35
41
T series
T-1
T-2
T-3
Layer 3
Layer 1, 3, 5
All layers
1%
0.33%
0.2%
609
741
894
31
35
40
Unconfined compressive stress (kPa)
Layer 5
800
600
400
No fiber
200
L-1
L-2
L-3
0
0
1
2
3
4
5
Axial strain (%)
Fig. 5. Results of L series.
the UCS significantly increases, as summarized in Table 4. The 100%
(fully distributed) fiber-distributed specimen was 1.5 times
stronger than the 20% fiber-distributed specimen, as shown in Fig. 8
and Table 4.
The same cases in each series (L-1 and T-1, L-2 and T-2, L-3 and
T-3) have the same degree of fiber reinforcement and distribution
of 20%, 60%, and 100%. With the exception of Case 3, the rf of the T
series is greater than that of the L series. By making a comparison
between the same cases in each series, it was confirmed that, for
the effect of the amount of fiber on the UCS, its strength increased
with an increase in fiber content (Ang and Loehr, 2003; Kaniraj and
Havanagi, 2001; Santoni et al., 2001; Tang et al., 2007). The secant
elastic modulus of each specimen was calculated from one half of
the axial strain at peak strength. As shown in Table 2, the
comparison shows that it does not seem to be influenced by the
fiber reinforcement and distribution. Park et al. (2008) conducted
unconfined compression tests on fully reinforced or distributed
(Case 3 in this study) cemented specimens using four different
cement ratios. Test results demonstrated that a secant elastic
modulus depended on a cement ratio. Fig. 9 shows photographs of
the non-fiber-reinforced specimen and other specimens in the L
series after failure. In the non-fiber-reinforced specimen, a vertical
crack throughout the entire specimen appeared after failure, as
shown in Fig. 9(a). In the fiber-reinforced specimens, cracks began
to appear in the non-fiber inclusion layer and spread to the fiber
inclusion layers, as shown in Fig. 9(b)–(d). The top plate slightly
penetrated into the specimen at the end of the tests. This may not
have significantly influenced the UCS of the specimen because it
occurred after peak stress. Such penetration of the plate may be
Table 3
Results of L series.
Specimen
Degree of fiber reinforcement (%)
Strength (kPa)
Ratio of strength
increase (%)
No Fiber
L-1
L-2
L-3
0
20
60
100
440
530
696
857
100
120
158
195
200
200
180
180
Strength Increase (%)
Strength Increase (%)
S.-S. Park / Geotextiles and Geomembranes 27 (2009) 162–166
160
140
120
100
160
140
120
0
20
40
60
80
100
100
0
20
40
60
Degree of Fiber Reinforcement (%)
Degree of Fiber Distribution (%)
Fig. 6. Effect of fiber reinforcement in L series.
Fig. 8. Effect of fiber distribution in T series.
1000
Unconfined compressive stress (kPa)
165
80
100
concentrated fibers. From a practical point of view, a fiber mixing
method, which can control the uniformity of fiber distribution
throughout the specimen, is very important especially when
applying fiber-reinforced soils in the field.
800
600
400
No fiber
200
T-1
T-2
T-3
0
0
1
2
3
4
5
Axial strain (%)
Fig. 7. Results of T series.
related to the ductile behavior of fiber-reinforced soils. It could be
partially attributed to the non-confinement of a specimen in an
unconfined compression test. The effect of confinement due to
rough plates may be insignificant especially in unconfined
compression tests because there is no confinement in the horizontal direction.
In summary, the strength of the FRCS depends on the degree of
fiber reinforcement and distribution. The UCS of cemented sands
reinforced with evenly distributed fibers can be up to 1.5 times
greater than those reinforced with poorly distributed or
Table 4
Results of T series.
Specimen
Degree of fiber
distribution (%)
Strength (kPa)
Ratio of strength increase (%)
T-1
T-2
T-3
20
60
100
609
741
894
100
122
147
Fig. 9. Specimens after failure: (a) non-fiber-reinforced specimen; (b) L-1 specimen;
(c) L-2 specimen; (d) L-3 specimen.
166
S.-S. Park / Geotextiles and Geomembranes 27 (2009) 162–166
4. Conclusions
The effect of fiber reinforcement and distribution on the
strength of the FRCS was investigated in terms of an UCS. PVA fibers
were randomly reinforced at a predetermined layer among five
compacted layers. The UCS of fiber-reinforced cemented specimens
gradually increases as the number of fiber inclusion layers
increases. The specimen where fibers were evenly distributed at
five layers was twice as strong as the non-fiber-reinforced specimen. When the same amount of fibers was reinforced in the entire
specimen, the specimen with five fiber inclusion layers was 1.5
times stronger than the specimen with one layer at the middle. The
strength ratio of 1.5 is a factor that can consider the strength
reduction of the FRCS induced by non-uniform fiber distribution.
The strength of fiber-reinforced cemented soils depends not only
on the degree of fiber reinforcement and distribution but also on
the amount of fiber used in the specimen.
These results can be used to quantify the uncertainties associated with fiber distribution or concentration in the strength of
fiber-reinforced soils. Using fiber-reinforced soils does not necessarily guarantee an anticipated strength increase or a safe design
unless the uncertainties associated with the fiber distribution or
concentration are adequately considered.
Acknowledgement
This paper was supported by Wonkwang University in 2008.
References
Abdulla, A.A., Kiousis, P.D., 1997. Behavior of cemented sands – I. Testing. International Journal for Numerical and Analytical Methods in Geomechanics 21 (8),
533–547.
Ang, E.C., Loehr, J.E., 2003. Specimen size effects for fiber-reinforced silty clay in
unconfined compression. Geotechnical Testing Journal 26 (2), 1–10.
Babu, G.L.S., Vasudevan, A.K., Haldar, S., 2008. Numerical simulation of fiber-reinforced sand behavior. Geotextiles and Geomembranes 26 (2), 181–188.
Chauhan, M.S., Mittal, S., Mohanty, B., 2008. Performance evaluation of silty sand
subgrade reinforced with fly ash and fibre. Geotextiles and Geomembranes 26
(5), 429–435.
Clough, G.W., Sitar, N., Bachus, R.C., Rad, N.S., 1981. Cemented sands under static
loading. Journal of the Geotechnical Engineering Division 107 (GT6), 799–817.
Consoli, N.C., Prietto, P.D.M., Ulbrich, L.A., 1998. Influence of fiber and cement
addition on behavior of sandy soil. Journal of Geotechnical and Geoenvironmental Engineering 124 (12), 1211–1214.
Consoli, N.C., Vendruscolo, M.A., Prietto, P.D.M., 2003. Behavior of plate load tests on
soil layers improved with cement and fiber. Journal of Geotechnical and Geoenvironmental Engineering 129 (1), 96–101.
Freitag, D.R., 1986. Soil randomly reinforced with fibers. Journal of Geotechnical
Engineering 112 (8), 823–826.
Ismail, M.A., Joer, H.A., Sim, W.H., Randolph, M.F., 2002. Effect of cement type on
shear behavior of cemented calcareous soil. Journal of Geotechnical and Geoenvironmental Engineering 128 (6), 520–529.
Kaniraj, S.R., Gayathri, V., 2003. Geotechnical behavior of fly ash mixed with
randomly oriented fiber inclusions. Geotextiles and Geomembranes 21 (3),
123–149.
Kaniraj, S.R., Havanagi, V.G., 2001. Behavior of cement-stabilized fiber-reinforced fly
ash–soil mixtures. Journal of Geotechnical and Geoenvironmental Engineering
127 (7), 574–584.
Kim, Y.T., Kim, H.J., Lee, G.H., 2008. Mechanical behavior of lightweight soil
reinforced with waste fishing net. Geotextiles and Geomembranes 26 (6),
512–518.
Khattak, M.J., Alrashidi, M., 2006. Durability and mechanistic characteristics of fiber
reinforced soil–cement mixtures. International Journal of Pavement Engineering 7 (1), 53–62.
Liu, H., Deng, A., Chu, J., 2006. Effect of different mixing ratios of polystyrene prepuff beads and cement on the mechanical behaviour of lightweight fill. Geotextiles and Geomembranes 24 (6), 331–338.
Maher, M.H., Ho, Y.C., 1993. Behavior of fiber-reinforced cemented sand under static
and cyclic loads. Geotechnical Testing Journal 16 (3), 330–338.
Park, S.-S., Kim, Y.-S., Choi, S.-G., Shin, S.-E., 2008. Unconfined compressive strength
of cemented sand reinforced with short fibers. Journal of the Korean Society of
Civil Engineers 28 (4C), 213–220.
Santoni, R.L., Tingle, J.S., Webster, S.L., 2001. Engineering properties of sand–fiber
mixtures for road construction. Journal of Geotechnical and Geoenvironmental
Engineering 127 (3), 258–268.
Saxena, S.K., Lastrico, R.M., 1978. Static properties of lightly cemented sand. Journal
of the Geotechnical Engineering Division 104 (GT12), 1449–1464.
Tang, C., Shi, B., Gao, W., Chen, F., Cai, Y., 2007. Strength and mechanical behavior of
short polypropylene fiber reinforced and cement stabilized clayey soil. Geotextiles and Geomembranes 25 (3), 194–202.
Viswanadham, B.V.S., Phanikumar, B.R., Mukherjee, R.V., 2009. Swelling behaviour
of a geofiber-reinforced expansive soil. Geotextiles and Geomembranes 27 (1),
73–76.
Yetimoglu, T., Salbas, O., 2003. A study on shear strength of sands reinforced with
randomly distributed discrete fibers. Geotextiles and Geomembranes 21 (2),
103–110.
Yetimoglu, T., Inanir, M., Inanir, O.E., 2005. A study on bearing capacity of randomly
distributed fiber-reinforced sand fills overlying soft clay. Geotextiles and Geomembranes 23 (2), 174–183.