161
SP-224—12
Crack Growth Resistance of Thin Mortar
Layers with Hybrid Fiber Reinforcement
by L. Sorelli, N. Banthia, and G. A. Plizzari
Synopsis: Hybrid fiber reinforcement of cement composites is rapidly emerging as an
innovative and promising way of improving mechanical performance and durability of
cement-based materials.
In the present paper, fracture behavior of medium, high and very high strength mortars
reinforced with hybrid fibers was experimentally studied by using contoured double
cantilever beam specimens. Different combinations of small steel fibers and fibrillated
polypropylene micro-fibers are investigated. These composites are very suitable for thin
sheet products such as roofing sheets, tiles, curtain walls, cladding panels, permanent
forms, etc.
Aim of the paper was to study the influence of matrix strength, fiber type and fiber
combinations on the fracture toughness of the resulting fiber reinforced mortars.
Results indicate that some combinations of fibers and matrix strengths exhibit a higher
resistance to crack growth and evidence the contribution of polypropylene fibers to
mortar toughness.
Keywords: cement; fiber; reinforcement
162 Sorelli et al.
Luca Sorelli received his PhD at University of Brescia in
2003, Italy. His current research consist of hybrid fiber
reinforced concrete, structural applications and finite
element modeling of high performance cementitious
composite.
Nemkumar Banthia, FACI, is a Professor of Civil Engineering
at the University of British Columbia (Canada). He is a
member of several ACI committees and chairs the ACI
Committee 544 on Fiber Reinforced Concrete. His primary
research interests consist of cement-based materials, fiber
reinforced concrete and fiber reinforced polymers,
shotcrete, strain-rate effects and impact, use of fiber
reinforced plastics in repairs.
Giovanni A. Plizzari, ACI Member, is a Professor of
Structural Engineering at the University of Bergamo
(Italy). He is a member of the FIB TG 4.5 “Bond Models” and
the RILEM Committee “Hybrid Fiber Concrete”. His research
interests include material properties and structural
applications of High Performance Concrete.
INTRODUCTION
In the new breed of high performance cement based
materials, there has been great interest lately in the
development of Hybrid Fiber Reinforced Cementitious
Composites (HyFRCC) that combine different types of fibers
in a cementitious matrix [1]. The aim is to take
simultaneous advantages from the material properties of
each fiber type (multi-functionality) and from their
interaction (synergy) to optimize the mechanical and
physical performances of the composite [2-5].
A promising hybrid system of fibers concerns a combination
of steel fibers and polypropylene fibers. The former are
used to enhance strength and toughness properties [6] such
as flexural (modulus of rupture), shear [7], impact [8] and
fatigue strength [9]. The latter are commonly used to
reduce shrinkage cracking [10,11] and permeability [12] of
concrete; in fact, bundles of fibrillated polypropylene
fibers open during concrete mixing and separate into
millions of multistrand filaments that are able to mitigate
crack formation due to plastic shrinkage. Vondran and
Webster [12] found that a volume fraction (Vf) of 0.2% of
Thin Reinforced Cement-Based Products 163
polypropylene fibers markedly reduce both the permeability
and the plastic shrinkage cracking.
Fibers may influence the fracture mechanism in a concrete
structure [3,13]. In fact, small-diameter fibers, here
defined as micro-fibers, may delay the fracture process by
which the micro-cracks coalescence to form large
macroscopic cracks [3,14]. Furthermore, micro-fibers modify
the crack pattern by transforming the macro cracks into a
network of smaller and narrower cracks.
A combination of small synthetic (polypropylene) fibers and
steel fibers could be used to yield a hybrid system that
may prove to be an interesting material for thin concrete
or mortar overlays for structural repair and retrofitting
[15]. The enhanced toughness, the reduced plastic shrinkage
cracking and the lower water-permeability could be highly
advantageous in producing a durable thin repair or product.
The use of short fibers in substitution of conventional
reinforcement (reinforcing bars or welded mesh) may allow a
reduction of labor costs.
In the present work, fracture behavior of thin mortar
layers with a combination of small steel and polypropylene
fibers is experimentally investigated by performing
Countered Double Cantilever Beam test (CDCB) [16].
Furthermore, the research aims to study the influence of
matrix strength on the mechanical behavior of concrete with
hybrid fibers.
The chosen amount of polypropylene fibers was higher than
the amount commonly used for controlling plastic shrinkage
cracking (Vf=0.1-0.2%), with the aim of improving the mortar
toughness. A small thickness of the specimens was adopted
to better reproduce the fiber distribution in thin
cementitious elements.
Specimens with a relatively large size were tested to
reduce the size effects and to allow for a simpler
determination of the mortar toughness.
In order to better understand the fracture behavior and to
determine the constitutive laws for the materials adopted,
the experiments were simulated by Finite Element analyses
based on Non Linear Fracture Mechanics (NLFM) [17].
MATERIALS
The mix compositions include 854, 980 and 1019 kg/m3
of
cement ASTM Type I for Medium (MSM), High (HSM) and
Very High Strength Mortars (VHSM) respectively. The water-
cement-sand (with a maximum diameter size of 5 mm)
proportions, as well as the air entrainer and the
164 Sorelli et al.
plasticizer contents of the three different types of
mortars are reported in Table 1.
Steel fibers (SF) and fibrillated polypropylene micro-
fibers (PP) were combined as reported in Table 2. A
reference concrete without fibers (MSM0, HSM0 and VHSM0)
was also made. Properties of the adopted fibers are
reported in Table 3. The steel fibers have a circular cross
section and a straight shape. They are made from high
carbon steel and are coated with brass for corrosion
protection.
The cylindrical ( = 100 mm, h = 200 mm) compressive
strength (fc) determined after 28 days of curing is reported
in Figure 1; notice that the average compressive strength
for the MSM was approximately 60 MPa, for the HSM was
approximately 95 MPa and for the VHSM was approximately 115
MPa.
SPECIMEN DESCRIPTION AND TEST SET-UP
Figure 2a shows a schematic of the Contoured Double
Cantilever Beam specimen that was adopted for the
characterization of the crack growth resistance. According
to LEFM assumption and a model based on the crack
equivalent, the CDCB specimen is shaped in such a way that,
by using Linear Elastic Fracture Mechanics (LEFM), the
Stress Intensity Factor is independent of the crack length
and the specimen allows for a stable crack propagation
under constant load [18, 19]. The CDCB specimen also leads
to more reliable compliance measurements since the
displacements are large and the critical loads are small
compared with tests on other types of specimen [18]. A
groove reduced the thickness of the middle section from 40
to 15 mm to better control the crack path (Figure 2).
Four CDCB specimens were prepared for each material. The
direction of casting was perpendicular to the surface of
the double cantilever beam specimen and the fresh mortar
matrix was poured while the mould was externally vibrated.
The load was applied vertically by the hydraulic jack of
the Instron machine with a stroke rate of 0.1 mm/min on a
steel wedge placed between two rollers at the top of the
specimen (Figure 3a). The Splitting Load (SL) is the
horizontal components of the total load [19] (Figure 3b).
In order to limit the vertical component of the applied
load that may influence the fracture
behavior of the specimen, the angle of the wedge was chosen
equal to 15° [20]. The coefficient of friction between
the wedge and the rollers was ignored since the wedge
Thin Reinforced Cement-Based Products 165
surfaces were carefully machined (Figure 4). The Crack
Mouth Opening displacement (CMOD) was measured by means of
a resistive displacement transducer (clip gauge) which was
fixed at the level of the loading points (Figure 3a).
The applied load and the CMOD data were acquired using a
digital data acquisition system running at 5 Hz.
RESULTS AND DISCUSSION
The Splitting Load vs. CMOD curves are given in Figure 4
for MSM, in Figure 5 for HSM and in Figure 6 for VHSM
mortars. The plotted curves represent the average curves of
four specimens tested and were obtained by means of the
full least-squares fit Loess procedure (a locally weighted
regression smoothing algorithm).
Notice a significant toughness increase in medium strength
mortars (MSM) due to the presence of steel fibers (along
with a higher residual strength and a more stable behavior
during fracture). Furthermore, the marked differences in
the shapes of curves obtained for the steel fibers as
opposed to those obtained for the polypropylene fibers can
be observed (Figure 4). In the latter case, the curves are
characterized by a lower peak load followed by a steeper
post-peak branch. However, the general enhancement in the
performance of the polypropylene fibers when added to steel
fibers in hybrid materials should be noted.
The same trend is confirmed for the High and Very High
Strength Mortars (Figures 5 and 6). It can be observed that
the peak loads increase with the matrix strength,
especially for the steel fiber reinforced mortars.
The hybrid materials show higher toughness than the mortars
with 0.5% of steel fibers, but lower toughness than the
mortars with 1% of steel fibers. This is in substantial
agreement with other researches carried out on same types
of fibers under bending [5, 21].
While the steel fibers were pulled out from the three
different matrices, part of the polymeric fibers broke
during the fracture. However, the presence of the secondary
polymeric fibers enhanced the fracture energy (GF; defined
as the area under the load-CMOD curves divided by the
projected cracked area) of about 50% for all the
cementitious matrices considered (Figure 7).
This shows that polypropylene fibers in the matrix with
steel fibers allow for appreciable advantages in term of
toughness beside the expected reduction of shrinkage
cracking (synergy).
166 Sorelli et al.
The significant increase of peak load in specimens of HSM
as compared to specimens of MSM can be observed. On the
contrary, no major differences are observed when the matrix
strength further increases; this is probably due to the
bond strength between steel fibers and the mortar matrix
that did not increase in the VHSM (with respect to the
HSM).
Table 4 reports the number of steel fibers counted in the
cross section. In the same table it is also indicated the
expected number of steel fibers assuming either an uniform
3D distribution or a 2D distribution according to [22]:
f
Cf
A
AV
N
⋅
⋅
⋅α=
2
(1)
where N is the expected number of fibers bridging the cross
section, Vf is the volume fraction of fibers, Ac is the
concrete cross section, Af is the fiber cross section area,
is a constant that varies from 0.5 for a 3D distribution
to 0.64 for a 2D distribution [22]. The results show that
the number of fibers counted in the cracked section is
always closer to a 3D distribution.
MODELING
The experiments were numerically simulated by using a 2D
Finite Element model based on Non Linear Fracture Mechanics
(NLFM) to better comprehend the test results and to
identify the fracture parameters of the materials used in
the tests. A discrete crack approach based on the
fictitious crack model was adopted [17].
The Finite Element analyses were performed by using Merlin
[23] that considers the structure as many linear elastic
subdomains linked by interface elements that simulate the
cracks, whose position must be known a priori.
Interface elements initially connect the sub-domains (as
rigid links) and start activating (i.e. cracks start
opening) when the normal tensile stress at the interface
reaches the tensile strength (fct)of the material.
Afterwards, the crack propagates and cohesive stresses are
transmitted between the crack faces according to a stress-
crack opening ( -w) law (Figure 8) which is given as input
for the interface elements.
The CDCB was modeled by adopting 3148 three node triangular
elements (plane stress) for the elastic sub-domains (having
a thickness of 40 mm), linked by means of 67 interface
elements (having a thickness of 15 mm). By assuming a 2D
Thin Reinforced Cement-Based Products 167
model, the stress concentration present at the groove tip
was neglected (Figure 9).
Figure 10 shows the mesh adopted for the CDCB specimens.
The stress-crack opening displacement relationships ( -w)
were approximated with bilinear laws herein (Figure 8). The
tensile strength of this law (fct) was determined from the
experimental compressive strength according to the CEB
Model Code 90 [24]. The experimentally determined fracture
energies (GF) (Figure 7) were used as input data. The other
parameters, namely the stress at the knee point (1), the
crack opening at the knee point (w1) were identified by an
inverse analysis based on the best fitting procedure [25].
Eventually, critical crack opening (wcr) was determined.
Figure 11 shows a typical comparison between the numerical
and the experimental curves for the steel fibers (Vf=1%) in
the High Strength Mortar. The same figure exhibits the
deformed mesh at different loading stages as well as the
distribution of cohesive stresses over the ligament length.
It should be noticed that the crack tip opening
displacement at the peak load is around 0.14 mm and that
the fracture process zone involves most of the ligament
length. The large crack tip opening displacement explains
why, in the adopted specimens, the peak load is more
related to the fiber bridging mechanisms than to the matrix
strength.
The numerical and the experimental curves of the Splitting
Load versus the CMOD are plotted for all the MSM materials
in Figure 12; notice the excellent agreement between the
different curves. The same results are reported in Figures
13 and 14 for the HSM and VHSM mortars, respectively.
The best fitting parameters of the bilinear softening laws
as well as the modulus of elasticity are summarized in
Table 5.
CONCLUDING REMARKS
Splitting tests were carried out on Countered Double
Cantilever Beam specimens. Because of the cross section
thickness of 15 mm, these specimens seem suitable to
characterize the fracture behavior of thin concrete members
made of fiber reinforced concrete.
Experimental results indicated that steel fibers better
enhance the mortar toughness. However, the addition of
polypropylene fibers to a steel fiber reinforced mortar
increases the toughness of the composite for all matrix
strengths considered. In fact, the fracture energy (GF) of
the hybrid materials with 0.5% of steel fibers was improved
168 Sorelli et al.
from 35% to 64% by polypropylene micro-fibers in the three
different matrices strengths (medium, high and very high).
Considering the fact the polypropylene fibers better
control the cracks due to the plastic shrinkage, the
reduced permeability and the lower cost of polymeric
fibers, these hybrid composites seem very suitable for thin
concrete overlays for structural repair and retrofitting.
Furthermore, they can be conveniently adopted for thin
cementitious products, such as roofing sheets, tiles,
curtain walls, cladding panels, permanent forms, etc.
However, although synergy between the two fibers is already
apparent in the hybrids, further optimization attempts are
clearly warranted. Therefore, further research, which
considers plastic shrinkage permeability and thermal
effects, is necessary to optimize the combinations of these
fibers.
The fatigue resistance may also be improved by a hybrid
system where micro-fibers can be active as bridging
mechanism over the micro-cracks surrounding macro-fibers
and cause synergistic effects in the composite. In
addition, in case of a fire, when the free and chemically
bonded water is transformed in vapor, the polymeric fibers
will melt leaving canals through which water vapor can
escape from the boundary zones without spalling off the
concrete covers. This may guarantee the fire protection
required in structural applications.
Non Linear Fracture Mechanics is a satisfactory tool to
model the fracture behavior of these cementitious
composites where the fracture process zone involves most of
the ligament length of the specimen.
Acknowledgements
The authors would like to thank Mr. David Woomk for his
diligence and his enthusiasm in preparing the experimental
tests as well as the helpful support of the technicians of
University of British Columbia (Canada).
Thanks are also due to the Dow Chemical Company and the
Bekaert for supplying respectively the polypropylene and
the steel fibers.
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Thin Reinforced Cement-Based Products 169
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172 Sorelli et al.
Figure 1. Compressive strengths for different materials adopted.
Figure 2. Schematic of a Countered Double Cantilever Beam specimen (a); schematiccrack path (b).
Thin Reinforced Cement-Based Products 173
Figure 3. A CDCB specimen under loading (a); load transmitted by the steel wedge (b).
Figure 4. Splitting Load vs. CMOD curves experimentally determined from MSM fiberreinforced mortars.
174 Sorelli et al.
Figure 5. Splitting Load vs. CMOD curves experimentally determined from HSM fiberreinforced mortars.
Figure 6. Splitting Load vs. CMOD curves experimentally determined from VHSM fiberreinforced mortars.
Figure 7. Fracture energy Gf values for materials adopted.
Thin Reinforced Cement-Based Products 175
Figure 8. Constitutive laws for discrete crack model.
Figure 9. 3D stress distribution due to the groove.
Figure 10. Mesh of the specimen and pre-imposed crack line.
176 Sorelli et al.
Figure 11. Numerical and experimental curves in terms of Splitting Load and CMOD forthe Medium Strength Mortar with 1% of steel fibers.
Figure 12. Experimental and numerical Splitting Load versus CMOD curves for MSMmortars.
Figure 13. Experimental and numerical Splitting Load versus CMOD curves for HSMmortars.
Thin Reinforced Cement-Based Products 177
Figure 14. Experimental and numerical Splitting Load versus CMOD curves for VHSMmortars.
178 Sorelli et al.