Maria
Idrees
University
of Engineering and Technology Lahore, Pakistan
E-mail: mariaidrees@uet.edu.pk
Farah
Nawaz Qureshi
University
of Engineering and Technology Lahore, Pakistan
E-mail: fnq2105@gmail.com
Kafeel
Ahmad
University
of Central Punjab, Pakistan
E-mail: kafeel.ahmed@ucp.edu.pk
Sajjad
Mubin
University
of Engineering and Technology Lahore, Pakistan
E-mail: sajjadmubin@uet.edu.pk
Submission: 6/22/2019
Revision: 9/18/2019
Accept: 10/9/2019
ABSTRACT
Different
types of fibers impart specific characteristics to concrete, including crack
bridging, early age crack resistance, ductility, toughness, strength, and loss
of workability. It seems that if these fibers are combined, then specific
characteristics of each fiber may be imparted to concrete and the desired
characteristics of the concrete composite may be achieved. Thus, this
investigation has been conducted to study the properties of concrete composites
composed of four different types of fibers used singly or in hybrid form. The
effectiveness of hybrid fibers in cementitious composites to achieve better
characteristics; strengths, toughness, workability, and cost, was investigated
and compared. Composites made of carbon fiber, plain steel fiber, polypropylene
fiber, and glass fiber and their hybrid combinations (2, 3 and 4 fibers mixed),
at constant volume of fiber 1.25%, along 4% styrene-butadiene rubber latex and
1.5% superplasticizer, are prepared and tested. The composites are compared and
investigated for their feasibility in terms of their properties and cost. The
comparison showed the suitability of some bi-hybrid composites, and
incompatibility of tri-hybrid and tetra-hybrid composites in terms of
effectiveness and feasibility.
Keywords: Hybrid Composites, Polymers, Material properties, Mechanical properties, Toughness.
LIST OF NOTATIONS:
C Carbon Fiber
G Glass Fiber
FRC Fiber Polymer Cementitious Composites
P Polypropylene Fiber
S Steel Fiber
SBR Styrene butadiene rubber latex
1.
INTRODUCTION
The extensive research
on fiber-based cementitious composites is being carried out to gain maximum
benefits out of fibers at minimum cost. By optimizing them for maximum
efficiency, their properties like the aspect ratio of fiber, the volume of
fiber, shapes, and sizes of fibers were studied in the past. The research aims to study the effectiveness
of hybrid fiber in cementitious composites to achieve better characteristics,
i.e., strengths, toughness, and workability at the lower cost.
Plain concrete is
brittle and has a low tensile strength and strain capacity. These shortcomings
can be overcome by adding fibers and producing fiber cement composites (FRC).
Fibers may or may not increase the strength depending upon their types, volume,
aspect ratio, shapes, etc. however, they can bridge cracks and provide post
cracking ductility. (BENTUR; MINDESS, 2006)
The initial cracking of
concrete during the curing period can be effectively reduced by using organic
synthetic fiber. The high strength fibers like steel and carbon can efficiently
increase the tensile strength of concrete. However, the increase depends upon
the bond strength between concrete and fiber. The durability of fiber
reinforced concrete increases due to controlled cracking.
The main function of
fibers in a cementitious material is to control the cracking by bridging the
advancing crack. It results in an improvement in flexural toughness and other
properties like impact strength and ductility (BADR; ASHOUR; PLATTEN, 2006).
The problems associated
with fiber incorporation are; loss of workability, difficulty in casting, high
cost, and sometimes reduction in strength and durability due to the high volume
of entrapped air.
The workability problem
can be addressed by adding SBR latex in FRC. SBR latex in concrete increases
the workability and reduces water ingress. It enhances the durability and
improves the strength of concretes at 28 days. (SONI; JOSHI, 2014)
Additionally, bonding
between fibers and matrix plays a vital role in determining the strength of
concrete, durability, and energy absorption capacity. SBR provides excellent
bonding characteristics to the matrix. (WANG; LACKNER; WANG, 2011)
G. N. Shete showed that
SBR latex improves the internal structure of latex modified concrete at 28 days
and hence reduces water absorption. SBR modified concrete has lower strength
than conventional concrete at early ages, but it increases at 28 days. (SHETE, 2014)
Different fiber has
different properties like color, texture, elongation capacity, strength
parameters, resistance to chemicals and electrical conductivity, etc. and when
added to concrete, they behave differently in the matrix. They transfer their
properties to the concrete when added. Selection of fiber should be according
to the building type or requirement of construction. Hybridization of fiber may
be the most effective way to achieve cost-effective combination with high
toughness and strength properties.
By adding optimum
percentages of each fiber content, we may get the synergetic response in
properties and highest strength to cost ratio as well. Hybridization of fibers
may lead us towards a structure in which one type of fiber, which is stronger
and tougher, improves the peak load value and ultimate strength, where the
other type of fiber, which is more ductile, provides the improved toughness and
strain capacity in the post-cracking phase. Steel fiber gives the highest
toughness as compared to all other fibers, it gives pullout failure under
flexural loading, but due to higher density of these fibers, electrical
conductivity and magnetic fields associated with it, it may lead to
hybridization with other high strength fiber like carbon fiber.
Hybrid polymer fiber
cementitious composites are lightweight materials and have improved mechanical,
damping, and thermal properties. (SATHISHKUMAR; NAVEEN; SATHEESHKUMAR, 2014)
Hybrid fiber is used to
improve the workability and control the early-age cracking in fresh concrete.
It increases the toughness of the concrete by cracking bridging and reduction
in crack tip opening displacements CTOD. Hence, optimum performance can be
obtained by mixing of different fibers. (YAO; LI; WU, 2003, AFROUGHSABET; BIOLZI; MONTEIRO, 2018)
Bentur and Mindess
suggested the use of strong and stiff fiber for increasing first crack stress
and ultimate strength. The flexible and ductile fiber is used for improving
toughness and post-cracking strain capacity.
This research is being
carried out to explore combinations of feasible hybrids to obtain desired
properties. The research of Sivakumar A, Santhanam M. on hybrid fiber showed
that only one combination of polypropylene fiber (0.12%) with steel fiber
(0.88%) performed better than mono steel fiber. All other combinations showed a
decrease in flexural toughness when nonmetallic fiber was increased. Glass
fiber showed poor toughness performance because of its short length and reduced
bond with the matrix. Because of the lower density of nonmetallic fiber, a high
amount of fiber in the mix may be the reason for higher strength. However, they
cannot sustain high crack width and has more reduced post-peak performance than
steel.
It was concluded in a
study that only a small percent of nonmetallic fiber, with steel fiber, can
show similar toughness to mono steel fiber. Polymeric fibers, e.g.,
polypropylene and polyvinyl, can offer excellent early crack resistance. (SIVAKUMAR; SANTHANAM, 2007)
In an investigation on
polypropylene fiber, showed that 1.5 kg /m3 performed better in terms of compressive
strength, permeability and electrical resistivity (KAKOOEI et
al., 2012).
The carbon fiber of different
sizes increases the bearing capacity to crack, ultimate stress and Young’s
modulus of fiber cementitious composite. (HOSSAIN; AWAL, 2011)
Carried out a study on steel cord and synthetic fiber, it was found that
hybrid cementitious composites performed better by showing higher strength and
ductility in flexure. Multiple cracks and pseudo hardening strain under
uniaxial tension confirmed hybrid fiber sufficiency as high-performance fiber (KAWAMATA; MIHASHI; FUKUYAMA, 2003).
Incorporation of hybrid fiber in concretes increases ductility,
toughness, energy absorption capacity, and durability performance in comparison
to single fiber reinforced concrete (PAKRAVAN; LATIFI; JAMSHIDI, 2017).
Use of hybrid fibers
instead of mono-fiber can improve mechanical strength and performance synergy
of FRC (BANTHIA et
al., 2014).
Yao W, Li J, and Wu K
conducted a study, for constant fiber volume (0.5%), the hybrid fiber
composites; Polypropylene carbon hybrid composite, polypropylene steel hybrid
fiber composite, and steel carbon hybrid composites. Only carbon and steel
showed high strength.
According to the
author, similar modulus and synergistic interaction can be the reason. (YAO; LI; WU, 2003). Pakravan HR, Latifi M, and
Jamshidi M showed that hybrid fiber reinforcement showed improved toughness,
energy absorption, ductility, and durability (PAKRAVAN; LATIFI; JAMSHIDI, 2017).
Mobasher B and Li CY
investigated the tensile and flexural strength of carbon and alumina whisker
and polypropylene fiber, significant strength increase was observed for
whiskers, but ultimate strength and post-peak toughening was not much affected
by whiskers due to inability to bridge macrocracks. Fracture toughness was
increased by PP fibers. Hence hybrid composite performed better in terms of
strength and toughness. (MOBASHER; LI, 1996)
Silva ER, Coelho J,
Bordado JC (2013) found that the hybrid polyethylene and polypropylene fiber
composites showed higher mechanical strength for 24 mm long fibers and 2.9%
fiber. The strong fiber mechanical interlocking enhanced mechanical strength
properties. The post cracking ductility was improved for the composite. (SILVA; COELHO; BORDADO, 2013)
Fibers are used in FRC
depending on their application. For pipes and roofing asbestos fiber is used.
For precast panels, glass fibers may be used. For pavements, dams, and
shotcretes, steel fibers are preferable. Polypropylene fibers are used as a
secondary reinforcement to control plastic shrinkage cracking. Vegetable fibers
are used in low-cost building materials.
In this investigation,
single fiber composites are compared with each other, and their hybrid
combinations are studied to find out the feasible combinations. The main aim of
the study is to compare the single and hybrid fiber cementitious composites
(FRC) in terms of strength, toughness, workability, and cost.
Among different types
of fibers used in this study, i.e., plain steel, carbon, polypropylene and
glass fiber, Steel and carbon fibers are relatively expensive while
polypropylene and glass fibers are cheap. The purpose of the study is not to
attain high strengths but is to understand the roles of type of fiber (single
and hybrid) in deciding the mix design to obtain desired properties at an
affordable cost.
The total volume of
fiber was kept 1.25 % of concrete. Polycarboxylic based superplasticizer 1.5%
of cement weight, was used with a water-cement ratio of 0.5. SBR latex 4% was
added to compensate for the loss of workability due to the addition of fibers
and to provide protection against deterioration of fibers. SBR modified
concrete decreases the permeability and ingress of water and ions in concretes.
SBR increase the adhesive quality of concretes and mortars (beneficial for
pullout failure).
In plain concrete, strain-softening does not occur; this behavior is close to linear elastic fracture mechanics. It results in the accumulation of strain energy in the material. The fracture process zone in front of these longitudinal splitting bond cracks would be small, and all energy is available for fracture initiation. As soon as the strain energy becomes greater than fracture energy required, the cracks initiate at the interfacial transition zones. There is a zone of energy dissipation in front of this crack.
In the case of brittle materials like concrete, this zone is minimal, and all the available energy is used in the crack propagation.
It results in a reduction in strain energy; however, if the accumulated energy is sufficient enough to cover the fracture energy required for crack propagation, the crack starts moving. It results in a further decrease in strain energy. However as a fiber stops this crack then the stress is transferred to the fiber if the fiber is strong enough to carry that stress, like steel or carbon fiber, then crack stops propagating and fracture toughness of the concrete increases significantly. Moreover, the strain-softening behavior is also introduced that further results in an increase in fracture toughness.
After Eq. (1) to Eq (3) gave this behavior of High Strength Concrete (DAVID; BROEK, 1979); ACI 446.1 R-91, 1991) as shown in Figure 1 (BROEK, 1974).
Figure 1: Fracture process zone in high strength concrete pullout
samples (Kafeel,
2009)
σys = (1)
rp = (2)
rp α (3)
where
σys = Yield strength of the material
rp = Size of the fracture process zone
KI = Stress intensity factor
However, in case of FRHSC the presence of steel fibers, that are bridging the cracks, FRHSC needs more energy to pull out these fibers out of the matrix. Hence increased bond energy is required for the formation and propagation of bond splitting cracks, and this results in improved bond strength.
2.
MATERIALS AND METHODOLOGY
Ordinary Portland cement (ASTMC150
Type1), Lawrencepur sand (fineness modulus 2.4), and polycarboxylic based
superplasticizer (Chemrite sp303, conforming to ASTM C 494 Types A, D & F)
were used in the mix preparation. Styrene-Butadiene
Rubber (SBR) latex (Imporient Chemicals) was added to improve
durability, adhesive quality, and workability of fiber cementitious composites.
Polypropylene fiber (450MPa tensile strength, i.e., low strength), glass fiber,
steel fiber, and carbon fiber (TC36S12K) were used for reinforcing the
cementitious composite (See Figure 2). The physical and mechanical properties
of cement and fibers are given in Table 2 and Table 2.
Figure 2: Fibers (Before and after mixing)
Table 1: Properties of
Cement and Sand
Physical Properties of Cement |
|
Standard
consistency |
30 |
Initial
setting time |
1hr
31 min |
Final
setting time |
3 hr
32 min |
Color |
Grey |
Specific
gravity |
3.15 |
Blaine
cm2/g |
3090 |
Chemical
Properties of Cement |
|
SiO2
% |
20.25 |
Al2O3
% |
5.05 |
Fe2O3
% |
3.13 |
CaO
% |
62.13 |
MgO % |
2.29 |
K2O
% |
0.74 |
Na2O
% |
0.24 |
SO3
% |
2.57 |
LOI % |
4.42 |
Physical Properties of Sand |
|
Fineness
modulus |
2.7 |
Max.
aggregate size |
4.75mm |
Specific
gravity |
2.625 |
Table 2: Physical
Properties Of Fibers
Physical
properties |
Polypropylene
fiber |
Glass
fiber |
Carbon
fiber |
Steel
Fiber |
Length
(mm) |
12mm |
20 |
25 |
25 |
Diameter
(mm) |
7 um |
0.15 |
7 um |
0.27 |
Aspect
ratio |
1715 |
133 |
3571 |
93 |
Density(g/cm3) |
0.9 |
2.6 |
1.81 |
7.8 |
Tensile
Strength(MPa) |
300-450 |
1900-2500 |
4900 |
500-1500 |
Elastic
modulus (GPa) |
----- |
70 |
250 |
190-210 |
Crack
Elongation (%) |
15% |
2% |
1-2% |
0.5-3.5% |
3.
METHODOLOGY
All fibers (especially
polypropylene and Carbon fiber) were scattered by hand before mixing. Fibers
were added during dry mixing, and then superplasticizer (1.5% of cement
weight), water (w/c 0.5), and SBR latex (4% of cement weight) were added one by
one.
Fourteen batches were prepared. The total
volumetric fraction of fiber was kept 1.25% for all composites. The FRC
compositions are given in Table 3.
Table 3: Volumetric Fraction of Fibers
Mixture
ID |
Volumetric
Fraction of Fibers (%) |
|||
|
Polypropylene
fiber |
Glass
fiber |
Steel
fiber |
Carbon
fiber |
P |
1.25 |
|
|
|
G |
|
1.25 |
|
|
S |
|
|
1.25 |
|
C |
|
|
|
1.25 |
PG |
0.625 |
0.625 |
|
|
SC |
|
|
0.625 |
0.625 |
PC |
0.625 |
|
|
0.625 |
PGS |
0.5 |
0.5 |
0.25 |
|
PGC |
0.417 |
0.417 |
|
0.417 |
PGSC1 |
0.31 |
0.31 |
0.31 |
0.31 |
PGSC2 |
0.64 |
0.22 |
0.07 |
0.32 |
PGSC3 |
0.525 |
0.525 |
0.1 |
0.1 |
PGSC4 |
0.56 |
0.57 |
0.06 |
0.06 |
Each FRC mix was cast into
four 4" cubes for the compressive strength test (as shown in Figure 3),
four prisms (4"x4"x20") for flexure test and four
(4"x8") cylinders for the split tensile test. The slump was noted for
each mix. Samples were tested for the strengths on the 28th day of casting. The
compression test, flexural strength, and the split tensile test were conducted.
Figure 3: Fiber-Reinforced Composites
4.
RESULTS AND DISCUSSIONS:
Table 4 provides a general view of the results
obtained.
Table 4: Properties Of FRC
Mixture
ID |
Flexural
strength |
Tensile
strength |
Compressive
strength |
Toughness |
Workability |
|
(MPa) |
(MPa) |
(MPa) |
Joule |
Slump
(in) |
P |
2.60 |
2.57 |
25.2 |
17.26 |
6 |
G |
2.90 |
2.87 |
31.2 |
5.14 |
2.5 |
S |
6.31 |
4.44 |
39.1 |
58.06 |
9 |
C |
7.85 |
5.58 |
41.4 |
21.10 |
1.25 |
PG |
4.06 |
2.72 |
28.8 |
7.07 |
4.125 |
SC |
4.58 |
4.54 |
36 |
18.90 |
5 |
PC |
3.95 |
3.93 |
32.4 |
12.87 |
4.75 |
PGS |
3.86 |
3.21 |
32.4 |
14.12 |
4.5 |
PGC |
3.82 |
3.37 |
31.2 |
8.35 |
3.75 |
PGSC1 |
4.46 |
2.72 |
24 |
15.58 |
6.5 |
PGSC2 |
3.89 |
1.97 |
21.6 |
9.78 |
6.875 |
PGSC3 |
4.06 |
2.57 |
22.8 |
8.73 |
6.5 |
PGSC4 |
3.85 |
2.42 |
21.6 |
10.56 |
5.125 |
Figure 4: Compressive Strengths of Single and Hybrid FRC
Table 4 and Figure 4
show that at constant volume of fiber and water-cement ratio, the polypropylene
fiber showed minimum compressive strength and Carbon fiber showed the maximum.
The order of compressive strengths was as following:
Polypropylene < Glass < Steel
< Carbon
When fibers were mixed,
and hybrid FRC was prepared, then all tetra combinations were unsuccessful in
terms of compressive strength, as they showed much lower strengths than single
fiber composites. Trihybrid did not show good results too. In bi-hybrid composites,
Polypropylene glass and polypropylene carbon hybrid were found okay for use,
but steel carbon composites showed lower strength than its single fiber
composites. The loss in strength may be due to completely different fiber types
and incompatibility of fibers.
Figure 5:Tensile Strengths of Single and Hybrid FRC
Table 4 and Figure 5 reveal that at
constant volume of fiber and water-cement ratio, the polypropylene fiber showed
minimum tensile strength and Carbon fiber showed maximum. The order of tensile
strengths was as following:
Polypropylene < Glass < Steel <Carbon
Polypropylene fiber used in this
study had lower tensile strength as compared to other types of polypropylene
fibers available in the market.
All tetra combinations were
unsuccessful in terms of tensile strength, as they showed much lower strengths
than single fiber composites. Tri-hybrid did not show good results too. In
bi-hybrid composites, Polypropylene glass and polypropylene carbon hybrid were
found okay for use, but steel carbon composites showed lower strength than its
single fiber composites. The loss in strength may be due to incompatibility of
fiber types.
Figure 6: Flexural Strengths of Single and Hybrid FRC
Table 4 and Figure 6 refers that the
order of flexural strengths was as follows:
Polypropylene < Glass < Steel < Carbon
At a constant volume of fiber and
water-cement ratio, the polypropylene fiber showed minimum flexural strength,
and Carbon fiber showed the maximum.
In Bi-hybrid combinations, only
Poly-Glass performed better. Steel-Carbon showed lower flexural strength than
single FRCs. Poly-Carbon was just acceptable.
Tri-hybrid composites did not show
poor results. They were just okay for use. Tetra- hybrid composites performed
least as the average strengths of mixed single fiber composites were not
attained in tetra-hybrid composites.
Figure 7: Comparative Toughness of Single And Hybrid FRC
Table 5, Figure 7, and Figure 8
refer to comparative toughness of FRC. The toughness of composites was maximum
for steel fiber, followed by carbon, polypropylene, and glass fiber. Steel
fiber composite is the toughest. Polypropylene showed good toughness at low
cost, hence was a favorable composite.
Glass fiber showed the least toughness. Hybrid fiber composite did not
show better toughness than single fiber composites.
Table 5: Toughness Of FRC
Fiber
Composite |
Total toughness (Joule) |
Post crack toughness
(Joule) |
Pre-crack
toughness |
P |
17.26 |
15.61 |
1.65 |
G |
5.14 |
1.64 |
3.51 |
S |
58.06 |
52.88 |
5.19 |
C |
21.10 |
7.15 |
13.95 |
PG |
7.07 |
4.10 |
2.97 |
SC |
18.90 |
13.47 |
5.43 |
PC |
12.87 |
9.90 |
2.98 |
PGC |
8.35 |
4.53 |
3.81 |
PGS |
14.12 |
11.89 |
2.23 |
PGSC1 |
15.58 |
10.59 |
5.00 |
PGSC2 |
10.56 |
6.20 |
4.36 |
PGSC3 |
9.78 |
6.08 |
3.70 |
PGSC4 |
8.73 |
5.94 |
2.79 |
Figure 8: Comparative Load Deflection Curves of Single and
Hybrid FRC
Figure 9: Slumps of Single and Hybrid FRC
In terms of workability, i.e.,
slump value (Figure 9), Carbon fiber was the least workable, and the steel
fiber was the most workable. Polypropylene fiber was more workable than glass
fiber. Hybrid fibers showed better workability in all cases.
Figure 10: Comparative Costs of Single and Hybrid FRC
Cost
analysis (Figure 10) showed that in Pakistan, polypropylene and glass fiber
composites are almost of equal price for the constant volume of the concrete
composite. Carbon fiber is slightly expensive than steel. Carbon and steel
fibers are 3 times costlier than polypropylene and glass fiber.
The
comparison of the overall results and the suitability of the combination used
in terms of its properties and cost is determined.
5. CONCLUSIONS
The following
conclusions are made for the single and hybrid compositions of fibers and SBR
latex in cement composites. The plain steel fiber, glass fiber, low strength
polypropylene fiber, and carbon fiber were used in the investigation.
1) Carbon fiber showed the highest strengths in terms of compression, tension, and flexure in FRC, followed by steel fiber. Polypropylene fiber showed the lowest strengths.
2) Steel fiber showed the highest toughness, followed by carbon fiber in FRC.
3) Polypropylene fiber composite was low cost, low strength, but tough and workable.
4) Glass fiber composite was slightly stronger and workable, but less tough than polypropylene composite.
5) Carbon fiber was least workable while steel was the most workable (in terms of slump value).
6) Carbon fiber and steel fiber were expensive, while polypropylene and glass fiber were cheap.
7) In terms of strength, generally Bi-hybrid composites showed better results, almost average of their single fiber composites except for steel-carbon composite, the poly-glass composite was successful while poly-carbon composite was satisfactory for tensile and flexural capacities.
8) Tri-hybrid composites showed generally poor strength results as compared to two of its single fiber composites. Thus tri-hybrid composites, i.e., poly-glass-carbon and poly-glass-steel were not satisfactory for use.
9) Tetra-hybrid composites showed poor performance in terms of strengths.
10) The poor performance of tri and tetra hybrid fiber may be linked to the incompatibility of more fibers to be used together due to their different properties.
6. ACKNOWLEDGEMENT
The research is supported by the Faculty Research Project Grant (No. ORIC/101-ASRB/4454),“Effect of hybrid fibers on mechanical properties of fiber reinforced cementitious composites” by University of Engineering and Technology Lahore.
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