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ORIGINAL ARTICLE
Effect of silica fume, steel fiber and ITZ on the strengthand fracture behavior of mortar
Xiao Hui Wang Æ Stefan Jacobsen ÆSiaw Foon Lee Æ Jian Ying He Æ Zhi Liang Zhang
Received: 25 September 2008 / Accepted: 22 January 2009 / Published online: 30 January 2009
� RILEM 2009
Abstract Two sets of parameters known to affect
the quality and thickness of the interfacial transition
zone (ITZ), i.e. water/binder ratio and content of silica
fume were varied in a series of mortars without and
with steel fiber. Compressive and three-point bending
tests were performed and the dissipated energies were
calculated. Nanoindentation characteristics of the
steel fiber–matrix and fiber–matrix-aggregate interfa-
cial zones in the steel fiber reinforced mortars were
studied. Influence of water/binder ratio, steel fiber,
silica fume and ITZ on the strength and toughness of
the mortar was analyzed, respectively. It is found that
mortar compressive strength can be increased with
low volume addition of steel fiber if the air content is
well controlled; the interfacial characteristic and
microstructural morphology near the fiber surface
play a critical role on the three-point bending strength
and the toughness of the steel fiber reinforced mortar.
Keywords Steel fiber � Silica fume �Strength and toughness � Nanoindentation �Interfacial transition zone (ITZ)
1 Introduction
Concrete is a highly complex and heterogeneous
composite material. The properties of the concrete
depend on the aggregate, matrix, steel reinforcement
and/or fiber, as well as the interfacial transition zone
(ITZ) between the aggregate and matrix, and the steel
and/or fiber and matrix. Those transition zones
determine many of the important properties of
concrete, such as strength, cracking and fracture
behavior. In studies [1–3] carried out to determine the
influence of aggregate, matrix and ITZ properties on
the tensile and compressive strengths of concrete, it
was found that the interfacial bond was the deciding
factor for the tensile strength and played a little role
on the compressive strength. It was concluded that
the properties of the ITZ may have a moderate
influence on mechanical properties of concrete and
have a drastic effect on the mechanical properties of
fiber reinforced cement composites [4].
In addition, the ITZ properties have a particular
importance on the cracking mechanism and fracture
behavior of concrete [3, 5, 6]. Fracture of concrete
was shown to be influenced by the presence of the
paste–aggregate interface [3, 6]. The strength of the
X. H. Wang (&)
Department of Civil Engineering, Shanghai Jiaotong
University, Shanghai 200240, China
e-mail: w_xiaoh@163.com
X. H. Wang � S. Jacobsen � S. F. Lee �J. Y. He � Z. L. Zhang
Department of Structural Engineering, Faculty
of Engineering Science and Technology, Norwegian
University of Science and Technology (NTNU),
7491 Trondheim, Norway
Materials and Structures (2010) 43:125–139
DOI 10.1617/s11527-009-9475-1
interface affected the fracture energy in different
ways depending on the shape of the particles; the
critical crack opening decreased when the interface
was strong, approaching the value corresponding to
the matrix, and it increased when the crack was rough
[6]. As the w/c ratio increased, the porosity in ITZ
was also increased, resulting in the initiation and
development of cracks in this zone [7].
The above mentioned properties and behaviors of
cementitious materials at the macroscale are all
significantly affected, if not dominated, by their
structural features and properties at the micro/nano-
scale where the deterioration and failure process
starts—properties at ITZ. The morphological charac-
teristic of the microstructure of ITZ was primarily
characterized via electron microscopy. Throughout
those studies, Scanning Electron Microscopy (SEM)
[3, 8], Environmental Scanning Electron Microscopy
(ESEM) [8] and Transmission Electron Microscopy
(TEM) [9] have been extensively used. In order to
investigate and quantify microstructural gradients
across the ITZ, backscattered electron imaging [10,
11] has been also used. However, for the above
mentioned microscopical techniques, only two
dimensional sections of a three-dimensional micro-
structure can be observed [11].
It is not until recent years that progress and
improved availability of advanced instruments and
techniques, such as depth-sensing micro/nanoinden-
tation, have made it possible to study mechanical
properties of various micro/nanoscale features in
cement-based materials. Microindentation testing
was firstly used to characterize gradients in mechan-
ical properties to determine the mechanical
characteristics of microsize zones in various cement
based composites [12–14]. Due to the limitations of
microindentation, such as larger indent comparing
with the thickness of ITZ [15], nanoindentation,
which has already been used for many other
materials [16], began to be used in the cementitious
composites. By using the nanoindentation tech-
nique, better representation of heterogeneous fea-
tures exhibited by cementitious materials can be
obtained [17]. Till now, nanoindentation was widely
used to measure elastic modulus and hardness of
cement paste cured at different conditions [18–20].
Few research works were focused on the studying
of the ITZ between a rigid inclusion and matrix
[12, 21, 22].
In the present paper, in order to link the nano-
mechanical properties of the ITZ with the macro-
scopic mechanical properties, mortars without and
with steel fiber were prepared; where water/binder
ratios of 0.3 and 0.5, and 0% and 10% contents of
silica fume were considered. Uniaxial compressive
strength testing and three-point bending strength
testing were performed. Dissipated energies Welastic
and Wpost-elastic before and after maximum elastic load
were calculated to analyze the effect of steel fiber on
the toughness of the mortar. Samples prepared from
the same steel fiber reinforced mortars were used for
nanoindentation test and the steel fiber–matrix and
fiber–matrix-aggregate interfaces were studied. The
whole sizes of each indented areas were determined
by Scanning Electron Microscopy (SEM) and bond
conditions across the interfaces were observed.
Characteristics of the profiles of the interfacial
transition zones were studied. It is found that the
compressive strength of the mortar can be increased
due to the low volume (0.3 vol%) addition of steel
fiber on condition that the air content of the hardened
mortar is well controlled; while the three-point
bending strength and the toughness of the steel fiber
reinforced mortar are mainly dependent on the
interfacial characteristics and microstructural mor-
phology near the fiber surface, which are mainly
related to the water/binder ratio, the presence of silica
fume, ITZ thickness as well as voids and microcracks
within those zones.
2 Experimental programme
2.1 Materials
Nine types of mortars were prepared. The detailed
information of the specimens can be seen in Table 1,
where specimens were identified with numbers
designation: the first two numbers indicating different
water/binder ratio, the second two numbers corre-
sponding to the different content of silica fume, the
last two numbers corresponding to the different
content of steel fiber. As an example, specimen
031003 implies a mortar with water/binder ratio 0.3,
having 10% silica fume and 0.3 vol% steel fiber.
The mortar mixtures were prepared with cement,
sands 0–4 and 0–2 mm, water, limestone powder,
and/or silica fume, and/or steel fiber. Norcem Anlegg
126 Materials and Structures (2010) 43:125–139
cement which is an ordinary Portland cement was
used. Silica fume (Elkem Microsilica grade 940-U) in
powder form with the content of SiO2 larger than
90% was used. The mass retained on 45 micron sieve
is small than 1.5%. The limestone powder, used as a
filler, has a fineness of 57.2%\24 lm, with a specific
weight of 2,730 kg/m3. Sands 0–4 and 0–2 mm have
fineness modulus of 2.37 and 1.51, respectively. The
percentages of particle \0.125 mm of sands 0–4 and
0–2 mm are 6.1% and 23.6%, respectively. Glenium
151 was incorporated in all mixtures as a superplast-
icizer, having 15% solids content and a density of
1,030 kg/m3. Straight high carbon steel fibre OL13/.16
has a length of 13 mm and a diameter of 0.16 mm,
and a tensile strength more than 2000 MPa.
2.2 Proportions of mixes used
The matrix volume of all mixes is 410 l/m3. The
characteristics of the mortar mixes are shown in
Table 2, where the numbers in the brackets of column
4 and 5 indicates the contents of the limestone and
Glenium 151 as percentages of the weight of cement,
respectively.
2.3 Mixing procedures
The mortars were mixed in a flat-bottomed mixer
with maximum volume of 12 l. For the mortars
without steel fiber, the mixing procedure was as
follows: (1) cementitious materials (including silica
fume) and sands were blended for 1 min at lower
speed; (2) following the next 4 min, half of the
mixing water was firstly added during the mixing,
then all the superplasticizer and the remaining water
were added; (3) stop the mixing, substances sticking
in the sides of the mixer were cleaned and mixed into
the mixes within 1 min; (4) the mixture was allowed
to rest for 5 min, then mixed for an additional 1 min
at lower speed. For the steel fiber mortar, after
finishing the adding of the water and superplasticizer
in Step 2, the steel fibers were added. It is suggested
Table 1 The detailed
information of the
specimens
Name Water/binder ratio Silica fume (percentage
of the weight of cement)
Steel fiber
(volume percentage)
030000 0.3 0.0 0.0
030003 0.0 0.3
030010 0.0 1.0
031000 10.0 0.0
031003 10.0 0.3
050000 0.5 0.0 0.0
050003 0.0 0.3
051000 10.0 0.0
051003 10.0 0.3
Table 2 Characteristics of the mortar mixes
Name Mix proportion (kg/m3)
Anlegg cement Free water Limestone Glenium 151 Sand 0–4 mm Sand 0–2 mm Silica fume Steel fiber
030000 542.5 153.1 47.2 (8.7%) 11.39 (2.1%) 1,406.8 248.3 0.0 0.0
030003 543.0 153.7 47.2 (8.7%) 10.86 (2.0%) 1,399.4 246.9 0.0 24.2
030010 543.9 153.5 47.3 (8.7%) 11.42 (2.1%) 1,382.1 243.9 0.0 78.0
031000 483.4 149.2 47.4 (9.8%) 12.09 (2.5%) 1,406.8 248.3 48.3 0.0
031003 483.8 149.4 47.4 (9.8%) 12.10 (2.5%) 1,399.4 246.9 48.4 23.4
050000 411.2 201.4 47.3 (11.5%) 4.93 (1.2%) 1,406.8 248.3 0.0 0.0
050003 411.5 201.5 47.3 (11.5%) 4.94 (1.2%) 1,399.4 246.9 0.0 23.4
051000 367.9 196.9 47.8 (13.0%) 6.36 (1.73%) 1,406.8 248.3 36.8 0.0
051003 368.2 197.1 47.9 (13.0%) 6.37 (1.73%) 1,399.4 246.9 36.8 23.4
Materials and Structures (2010) 43:125–139 127
that the fibers should be added in small batches to get
good dispersion of fibers in the mixture.
2.4 Rheology and curing
For each type of fresh mortar, slump test with a small
cone and rheological measurements with the ConTec
Viscometer were performed. Then, 50 9 50 9
50 mm cubes were cast for the compressive test and
40 9 40 9 160 mm prisms were cast for three-point
bending strength test and nanoindentation test. During
the casting, steel molds were vibrated in order to
evacuate parts of the entrapped air. Then, surfaces of
the specimens were carefully smoothed and covered
with plastic sheets. Specimens were demolded after
24 h, and then each specimen was weighed in air and
water, respectively. After that, the specimens were
cured at 20�C under water for 28 days.
2.5 Sample preparation for studying interfacial
properties
After 28 days curing in a water bath, the central part
of the 40 9 40 9 160 mm prism was taken out using
a diamond saw. This part was cut into small samples
of the size about 15 9 15 9 15 mm, which were
embedded in epoxy resin in air and prepared for
grinding and polishing.
For fiber-reinforced composites, due to the lack of
published grinding and polishing procedures in the
literature, the best possible grinding and polishing
procedures were finally determined after several trial
tests. The TegraForce-5 grinding and polishing
machine was used. The MD-Piano plates (220, 600
and 1200) were selected for the coarse grinding to
obtain flat surface. The Largo (9 lm), Dur (3 lm)
and Dac (1 lm) were selected for the fine polishing.
The Nap was used for the final polishing. At every
step, a microscope was used to check the effective-
ness of the grinding and polishing and ultrasonic bath
cleaning was performed to remove all dust and
diamond particles.
2.6 Nano-mechanical properties of ITZ
In order to evaluate the effect of nano-mechanical
properties of the ITZ on the macroscopic mechanical
properties of mortar, such as strength, cracking and
toughness, a Triboindenter was used to study the ITZ
properties. A typical representation of the indentation
load versus indentation depth or displacement for an
indentation experiment is shown in Fig. 1. The elastic
modulus E and hardness H of the sample was
calculated as follows [16]
S ¼ dP
dh¼ 2
ffiffiffi
pp Er
ffiffiffi
Ap
ð1Þ
E ¼ ð1� m2Þ � 1
Er
� ð1� m2i Þ
Ei
� ��1
ð2Þ
H ¼ Pmax
Að3Þ
where P is the indentation load; h is the indentation
depth; S is the initial unloading stiffness; Pmax is the
peak indentation load; A is the is the projected area of
the elastic contact; Er is the reduced elastic modulus;
E and m are Young’s modulus and Poisson’s ratio for
the specimen and Ei and mi are the same parameters
for the indenter. For the indenter used the present
experiments, the elastic modulus Ei = 1,140 GPa and
the Poisson’s ratio mi = 0.07.
For the microhardness profiles of the ITZ, it was
reported that in the vicinity of the inclusion surface
there is a gradient in the microhardness, but in the
bulk paste it becomes relatively constant. The trends
in the gradients can be classified into four types as
shown in Fig. 2 [13]. For the type I curve, there is an
Fig. 1 A typical representation of the indentation load versus
indentation depth [16]
128 Materials and Structures (2010) 43:125–139
increase in the microhardness of the matrix in the
vicinity of the inclusion. The reason may lie in (1)
the inclusion and the matrix are well bonded at the
interface; (2) the near surface ITZ is rich in massive
CH [13]. Deviations from these conditions can lead to
changes in the shape of the curve and can account for
shapes such as II, III, and IV in Fig. 2.
3 Results and discussion
3.1 Properties of fresh and hardened mortar
Properties of the fresh and hardened mortar are
shown in Table 3. In order to assess workability and
consistency of the fresh mortar, density of the fresh
mortar, air content, slump, flow diameter, yield value
and plastic viscosity were measured, respectively.
Just as summarized by Banfill [23], with the increase
of the water/binder ratio, the yield stress and plastic
viscosity decrease, indicating a better workability
and consistency for mortars with large water/binder
ratio. In addition, due to the addition of the silica
fume, the yield stress increases for mortars with low
water/binder ratio, indicating a workability loss due
to the addition of the silica fume. The workability
and consistency is obviously influenced by the
addition of steel fiber in mortars with w/b = 0.3.
For instance, with the increased addition of steel
fiber from 0.0 to 1.0 vol%, yield stress and plastic
viscosity increase greatly. For specimen 030010, it is
too stiff to be measured in the ConTec Viscometer.
However, for mortars with w/b = 0.5, the workabi-
lity and consistency is less influenced by the addition
of low content of steel fiber. The yield stress and
plastic viscosity of specimen 050003 are even
smaller than those of specimen 050000, mainly due
to the low content of steel fiber and partly insuffi-
cient dispersion of fibers.
The air content of fresh mortars with w/b = 0.3
increases slightly with the addition of silica fume.
Along with this effect is a decrease in density after
demoulding. However, due to the use of superplast-
icizer in the mortars with high w/c ratios (e.g.,
w/b = 0.5), segregation of the mixture may occur [24]
and the air content of fresh mortars with silica fume is
smaller than that of mortars without silica fume.
Fig. 2 Classification of microhardness profile in the interfacial
transition zone (ITZ) around a rigid inclusion in a cement paste
matrix [13]
Table 3 Properties of the fresh and hardened mortar
Name Fresh mortar Hardened mortar
Slump
(mm)
Flow diameter
(mm)
Yield value
(Pa)
Plastic viscosity
(Pa�s)
Air content
(%)
qtheor(voidfree) qdemould k(%)
030000 101 240/265 8 30.2 1.9 2,458 2,359 (7.62) 4.03
030003 94 180/205 11 43.0 2.3 2,473 2,358 (7.95) 4.67
030010 60 – – – 3.2 2,510 2,399 (19.26) 4.43
031000 105 230/235 30 14.5 4.0 2,445 2,323 (10.75) 4.99
031003 95 200/205 42 15.8 2.9 2,460 2,324 (7.85) 5.54
050000 100 265/265 8 6.5 3.4 2,367 2,293 (4.04) 3.14
050003 98 260/260 7 5.7 4.0 2,383 2,302 (3.58) 3.39
051000 110 230/245 10 2.8 2.5 2,358 2,244 (9.66) 4.84
051003 103 245/245 10 5.1 1.5 2,373 2,291 (4.92) 3.47
Note: Number in brackets is the standard deviation
Materials and Structures (2010) 43:125–139 129
In Table 3, the air content of the hardened mortar
k was calculated as follows [25]
kð%Þ ¼ 1� qdemould
qtheorðvoidfreeÞð4Þ
where qtheor(voidfree) is the theoretical void free density
of the mortar and qdemould is the measured density
after demoulding. It seems that the air content of the
hardened mortar does not agree well with the air
content measured on the fresh mortar. This may result
from the vibration of the molds during the casting. It
also can be seen from Table 3 that the air content of
the hardened mortars with both water/binder ratios is
increased by the addition of the silica fume.
3.2 Compressive and bending strengths
Uniaxial compressive strength testing of the mortars
was performed on 50 mm cubes after 28 days of
curing. Test results are shown in Table 4. Each result
presented the average of six companion specimens.
Three-point bending strength testing was performed
on 40 9 40 9 160 mm prisms after 28 days of
curing, where the support span is 100 mm. The
bending strength presented is the average of three
companion specimens.
It can be seen from Table 4 that the compressive
and bending strengths of the mortars are influenced by
the addition of silica fume and steel fiber. For the
mortars without steel fiber, both the compressive
strength and bending strength are increased by the
addition of silica fume, where the increase of
compressive strength is very obvious for mortars with
low and high water/binder ratio, as pointed out by
Kjellsen et al. [26]. For the steel fiber reinforced
mortar, the compressive strength is also increased by
the addition of silica fume. However, the bending
strength is little influenced or even reduced. For
mortars without and with silica fume, although it was
reported that the fibers are increasingly being used to
enhance the strength and toughness of the cementi-
tious composites [27], this enhancing effect of strength
is not obvious for the 0.3 vol% addition of steel fiber.
Similar test result was also obtained, where the density
and compressive strength of fiber-reinforced concretes
are about the same as those of the plain counterparts
due to 1.0 vol% addition of steel fiber [28].
Due to the expected positive effect of the interfa-
cial bond on the compressive strength [3] for the
same batch of aggregate, different matrix compres-
sive strength should result in different mortar
compressive strength. For fiber reinforced mortars
with low fiber volume content, if the contribution
strength of steel fiber outweighs the decrease of the
matrix compressive strength resulting from the
increase of air content, the compressive strength of
the fiber reinforced mortar may increase due to the
addition of the low content of steel fiber; otherwise,
the addition of the low content of steel fiber may have
little or adverse effect on the compressive strength of
the fiber reinforced mortar. With the increased fiber
volume content, the compressive strength of the fiber
reinforced mortars can greatly increase. For instance,
the 4.67% higher air content of the hardened mortar
and 0.3 vol% steel fiber results in a lower compres-
sive strength of mortar 030003, compared with
mortar 030000 with 4.03% air content; on the other
hand, the 3.47% lower air content of the hardened
mortar and 0.3 vol% steel fiber results in a higher
compressive strength of mortar 051003, compared
with mortar 051000 with 4.84% air content. When the
fiber volume content increases from 0.3 to 1.0 vol%,
such as mortar 030010, although the air content of
hardened mortar increases, the compressive strength
of mortar 030010 increases correspondingly.
3.3 Toughness of the mortar
In the three-point bending strength tests, failure
modes of mortars without and with steel fiber are
Table 4 Compressive and bending strengths of the mortar
Strength at 28 days (MPa) w/b = 0.3 w/b = 0.5
030000 030003 030010 031000 031003 050000 050003 051000 051003
Compressive 72.5 68.9 78.0 78.6 74.9 39.8 34.9 42.6 46.7
Bending 8.9 9.8 11.3 9.0 9.1 7.1 6.8 7.2 7.2
130 Materials and Structures (2010) 43:125–139
quite different, see Fig. 3. Mortar without steel fiber
failed immediately when the maximum elastic load
reached; whereas mortar with steel fiber showed good
ductile behavior. When the maximum elastic load
was reached, the load decreased and the crack width
increased gradually. For the mortar with 1.0 vol%
steel fiber, the load even increased after the reach of
the maximum elastic load. The typical load-flexure
extension (P–d) curves of mortars without and with
steel fiber are schematically shown in Fig. 4.
For each type of the mortar, three companion
specimens were used to determine the bending
strength of the mortar. The maximum elastic load
Pmax-elastic and the corresponding elastic flexure
deflection dmax-elastic of each specimen are summa-
rized in Table 5. Then, the dissipated energy Welastic
and Wpost-elastic before and after Pmax-elastic of each
specimen were calculated, where Welastic and Wpost-
elastic are the area under the load-deflection curve, see
Fig. 4; the total dissipated energy Wtotal is the sum of
Fig. 3 Typical failure
modes of the mortars: awithout steel fiber; b with
steel fiber
Fig. 4 Load deflection
(P–d) curves of specimens:
a without steel fiber; b with
steel fiber
Materials and Structures (2010) 43:125–139 131
the energy Welastic and Wpost-elastic. For the conve-
nience of comparing, the minimum flexure extension
dmin of all specimens with steel fiber was determined
to calculate the dissipated energy Wpost-elastic after
Pmax-elastic, where dmin = 0.3114 mm. The calculated
Welastic, Wpost-elastic and Wtotal of each type of the
mortar can be seen in Table 5.
Then, the influence of factors on the toughness of
the mortar was analyzed, where the toughness is
generally defined as the energy absorption capacity
[28]. It can be seen from Table 5 that the Welastic,
Wpost-elastic and Wtotal of mortars with low water/
binder ratio are larger than those of the counterparts
with larger water/binder ratio, indicating high
toughness of mortars with low water/binder ratio. In
addition, the toughness of the mortar is influenced by
addition of steel fiber. For the mortars with w/b = 0.3,
both the Welastic and Wpost-elastic increase with the
addition of steel fiber. As a result, the Wtotal of mortar
030010 is more than twice of that of mortar 030000
due to the addition of 1.0 vol% steel fiber. For the
mortars with w/b = 0.5, although Welastic is nearly
identical, Wpost-elastic is still increased due to the
addition of steel fiber, resulting in an increase of the
Wtotal. For steel fiber reinforced concrete, the tough-
ness is also remarkably improved by the addition of
1.0 vol% steel fiber [28]. Silica fume, however, has
little or even negative effect on the mortar without
Table 5 Dissipated energy of the mortar
Specimen Dissipated energy (kN�mm) Pmax-elastic (kN) dmax-elastic (mm)
Welastic Wpost-elastic Wtotal
Each Mean value Each Mean value Each Mean value
w/b = 0.3 030000 0.32990 0.3135 0.0391 0.1361 0.3690 0.4496 3.9680 0.1730
0.30070 0.0616 0.3623 3.8676 0.1603
0.31004 0.3076 0.6176 3.8428 0.1614
030003 0.3785 0.4102 0.2926 0.2729 0.6711 0.6831 4.3024 0.1830
0.5442 0.1700 0.7142 4.4272 0.2611
0.3081 0.3561 0.6642 4.0551 0.1554
030010 0.4278 0.4298 0.5416 0.5452 0.9694 0.9750 4.6952 0.1862
0.4125 0.5193 0.9318 4.5721 0.1805
0.4490 0.5748 1.0238 4.6458 0.1946
031000 0.2636 0.2642 0.0653 0.1311 0.3289 0.3953 3.9015 0.1377
0.2700 0.0639 0.3339 3.9160 0.1367
0.2589 0.2641 0.5231 3.9872 0.1296
031003 0.2927 0.2742 0.4915 0.4309 0.7842 0.7051 4.1888 0.1372
0.2524 0.3943 0.6467 3.8903 0.1295
0.2774 0.4068 0.6843 3.9871 0.1340
w/b = 0.5 050000 0.1990 0.2093 0.1342 0.1001 0.3332 0.3094 3.0585 0.1288
0.2375 0.0852 0.3227 3.3676 0.1406
0.1914 0.0809 0.2723 3.0319 0.1287
050003 0.1824 0.2002 0.2272 0.2401 0.4096 0.4402 2.8424 0.1280
0.1746 0.2788 0.4534 2.9023 0.1281
0.2436 0.2141 0.4577 2.9898 0.1673
051000 0.1780 0.1987 0.0436 0.0500 0.2216 0.2486 2.8456 0.1235
0.2160 0.0904 0.3064 3.2396 0.1334
0.2020 0.0159 0.2179 3.1757 0.1292
051003 0.1884 0.2176 0.2377 0.2497 0.4261 0.4673 3.0621 0.1234
0.2377 0.2709 0.5086 3.4266 0.1367
0.2266 0.2405 0.4671 3.0626 0.1472
132 Materials and Structures (2010) 43:125–139
steel fiber. For the steel fiber reinforced mortars, the
Wtotal is increased due to the 10% addition of silica
fume.
3.4 Properties of the interfacial transition zone
in steel fiber reinforced mortars
Nanoindentation tests were carried out to study the
steel fiber–matrix and fiber–matrix-aggregate inter-
faces in mortars 030003, 031003 and 050003. Firstly,
the P–h curves of each indented area were checked to
determine the validity of each indentation. Irregular
P–h curves and curves exhibiting large displacement
jump at any portion of the loading portion were
discarded. The irregular nature of those curves may
be due to the presence of a large void [20] while the
discontinuous load–displacement curves may lie in
the surface cracking during the force-driven indenta-
tion tests [18]. Since the contact stiffness is measured
only at peak load, and no restrictions are placed on
the unloading data being linear during any portion of
the unloading [16], curves showing nonlinear char-
acteristic in the unloading portion were adopted.
After the check of the data validation, the
secondary electron (SE) image of the each indented
area was used to determine the distribution of all
indents in each indented area. The whole indents
were divided into appropriate groups: indents in the
steel fiber, indents in the matrix, and/or indents in the
aggregate. There are also indents in partially hydrated
cement clinkers in some cases. The hardness profiles
of the steel fiber–matrix and steel fiber–matrix-
aggregate interfacial zones of mortars 030003,
031003 and 050003 are shown in Figs. 5, 6, 7, where
all values were determined as a function of the
distance from the fiber surface; the vertical axis
denotes the edge of the actual steel fiber and/or the
aggregate surface is schematically marked.
The hardness profile of the ITZ between steel fiber
and matrix in Fig. 5a shows the trend of type I in
Fig. 2. In the distance 5–35 lm approaching the steel
fiber surface, there is an obvious increase in hardness.
Away the steel fiber surface, i.e. in the 35–65 lm
zones, the hardness becomes relatively constant;
while in Fig. 5b, the bonds across the interfaces are
also efficient, as seen by the obvious rise in the
hardness profile as the steel fiber and aggregate are
approached. Low hardness is observed in the distance
of 20–40 lm away the steel fiber–matrix interface.
Within the steel fiber–matrix-aggregate zone, there
are also some partially hydrated cement clinkers near
the aggregate and fiber surfaces. As a whole, in
mortar 030003, the bonding across the interfaces is
quite efficient.
In Fig. 6a, the whole hardness profile of the ITZ
between steel fiber and matrix likes type III in Fig. 2,
but it is more uniform. Due to the addition of the
silica fume, the hardness profile of the steel fiber–
matrix interfacial zone is quite different from that of
the mortar 030003. It is interesting to note that there
is no obvious trough in the interfacial zone in Fig. 6a;
comparatively low hardness is observed just in the
10–30 lm zones. Compared with 0.1636 GPa at the
weakest point in Fig. 5a, this value reaches
0.2391 GPa in Fig. 6a, indicating an increase of the
hardness value at the weakest point owing to the 10%
addition of the silica fume. In Fig. 6b, small rise in
the hardness profile as the steel fiber is approached
indicates good bonding across the steel fiber-matrix
interface. However, this bond is not as efficient as
that of the steel fiber-matrix interface shown in
Fig. 5b; the hardness value at the weakest point in
Fig. 6b is very low, just 0.034 GPa. The influence of
the silica fume is not as obvious as that shown in
Fig. 6a. With the approaching of the aggregate, rise
in the profiles is observed, indicating at least partial
bonding at the aggregate–matrix interface, as sug-
gested by Igarashi et al. [13]. As a whole, the bond
behavior in 031003 is good, but not as efficient as that
in 030003. The addition of silica fume results in no
distinct presence of weak ITZ between steel fiber and
matrix in some parts of the mortar; however, in the
steel fiber–matrix-aggregate interfacial zone, the
effect of the silica fume is not obvious.
The hardness profile of the ITZ in Fig. 7a shows
typical characteristic of type IV in Fig. 2. Compared
with the comparatively high matrix hardness in
mortars 030003 and 031003, comparatively low
matrix hardness in mortar 050003 is observed due
to the larger water/binder ratio. The lowest hardness
is observed near the steel fiber surface, just 25.6 lm
from the fiber surface. This value is 0.1452 GPa,
indicating a decrease of the hardness at the weakest
point owing to the increase of the water/binder ratio.
A trough is shown in the 0–35 lm zones. This
indicates that the interfacial bonding at the actual
interface in specimen 050003 is poor and not as
effective as that obtained in specimens 030003 and
Materials and Structures (2010) 43:125–139 133
031003 with low water/binder ratio. In Fig. 7b, small
rise in the hardness profile as the steel fiber surface is
approached can be indicative of at least partial
bonding at the interface. Unlike the cases in mortars
030003 and 031003, the smallest hardness value
occurred near the fiber surface. It is very surprising to
note that in the distance 5–20 lm from the aggregate
surface, the hardness are quite similar to those of the
aggregate although it is part of matrix. It is doubted
that there maybe a piece of aggregate underneath this
area. As a whole, partial bond exhibits at the
interfaces in mortar 050003.
3.5 Relationship between the ITZ
and macroscopic mechanical properties
As pointed out above, the addition of the 1.0 vol%
ductile steel fiber can greatly improve the three-point
bending strength and fracture toughness of cementi-
tious materials even the compressive strength is little
0
1
2
3
4
5
6
7
8
9
10
11
-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70
Distance from fiber surface (µm)
H-h
ardn
ess
(GPa
)
Steel fiber
Matrix
(a)
01234567
89
101112131415
-30 -20 -10 0 10 20 30 40 50 60 70 80 90
Distance from fiber surface (µm)
H-h
ardn
ess
(GPa
)
Steel fiberMatrix
Partially hydrated cementAggregate
(b)
Aggregate surface
Fig. 5 Hardness profile in
the interfacial transition
zones of mortar 030003:
a steel fiber–matrix
interfacial zone; b steel
fiber–matrix-aggregate
interfacial zone
134 Materials and Structures (2010) 43:125–139
improved [28]. The magnitude of the bending
strength and fracture toughness increase of fiber-
reinforced cementitious composites is mainly con-
trolled by their interfacial characteristics and
microstructural morphology near the fiber surface.
Thus, the effect of interfacial characteristics and
microstructural morphology near the fiber surface on
the bending strength and toughness of the fiber
reinforced mortar are mainly discussed.
For mortar without and with silica fume, such as
030003 and 031003, different bond conditions in the
ITZ lead to different bending strength and fracture
toughness. Although efficient bonding across the steel
fiber–matrix interface is shown in both 030003 and
031003, in the steel fiber–matrix-aggregate zone, the
situation is different. In Fig. 5, efficient bonding
across the interfaces in 030003 is observed. Due to
the largest air content in mortar 031003, the thickness
of the ITZ in the steel fiber–matrix-aggregate zone is
very wide, approximate 60 lm; also, there are many
voids and microcracks in this zone, see the BSE
image in Fig. 8. Some microcracks even extend from
the fiber surface to the aggregate surface. The
comparative large and discontinuous pores may be
0
1
2
3
4
5
6
7
8
9
10
11
-30 -20 -10 0 10 20 30 40 50 60 70 80
Distance from fiber surface (µm)
H-h
ardn
ess
(GPa
)
Matrix
Steel fiber
(a)
0123456789
101112131415
-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Distance from fiber surface (µm)
H-h
ardn
ess
(GPa
)
Steel fiberMatrixAggregate
(b)
Aggregate surface
Fig. 6 Hardness profile in
the interfacial transition
zones of mortar 031003:
a steel fiber–matrix
interfacial zone; b steel
fiber–matrix-aggregate
interfacial zone
Materials and Structures (2010) 43:125–139 135
related to the consumption of calcium hydroxide
crystals by reaction with silica fume while the
increase in microcracks may result from autogenous
shrinkage in the presence of comparatively large
amounts of silica fume [29]. Those porosity and
microcracks may result in earlier initiation and
development of cracks in this zone. As a result,
compared with mortar 030003, comparatively low
bending strength and Welastic are obtained in mortar
031003.
For mortar with different water/binder ratio, such
as 030003 and 050003, bending strength and fracture
toughness differ a lot due to the different bond
characteristic in the interfaces. In mortar 030003, the
rise in the hardness profiles as the steel fiber and
aggregate are approached indicates efficient bonds
across the steel fiber–matrix and fiber–matrix-aggre-
gate interfaces; while in mortar 050003, due to the
large water/binder ratio, a trough is shown in the
distance 0–35 lm from the steel fiber surface in the
steel fiber–matrix interfacial zone or in the distance
5–40 lm from the steel fiber surface in the fiber–
matrix-aggregate interfacial zone, indicating poor
bond in those zones. From the BSE image shown in
0
1
2
3
4
5
6
7
8
9
10
11
-20 -10 0 10 20 30 40 50 60 70Distance from fiber surface (µm)
H-h
ardn
ess
(GPa
)
Matrix
Steel fiber
(a)
(b)
0123
456789
1011
12131415
-30 -20 -10 0 10 20 30 40 50 60 70 80 90 100
Distance from fiber surface (µm)
H-h
ardn
ess
(GPa
)
Steel fiberMatrixAggregate
Aggregate surface
Fig. 7 Hardness profile in
the interfacial transition
zones of mortar 050003:
a steel fiber–matrix
interfacial zone; b steel
fiber–matrix-aggregate
interfacial zone
136 Materials and Structures (2010) 43:125–139
Fig. 9, a clear, porous transition zone and discon-
tinuous bleeding voids underneath the fiber are
observed. Due to those reasons, the bending strength
and fracture toughness of mortar 050003 are lower
than those of mortar 030003.
4 Conclusions
The following concluding remarks can be made from
this experimental work:
(1) Properties of the fresh mortar are greatly
influence by water/binder ratio, the addition of
silica fume and steel fiber. With the increase of
the water/binder ratio, the yield stress and
plastic viscosity decrease corresponding. Due
to the addition of the silica fume, the yield stress
increases more for mortars with low water/
binder ratio. For mortars with low water/binder
ratio, yield stress and plastic viscosity increase
greatly resulting from the addition of steel fiber;
while for mortars with large water/binder ratio,
the workability and consistency is less influ-
enced by the low volume addition of steel fiber.
(2) At both water/binder ratios, compressive strength
increased with silica fume. If the air content of the
hardened mortar is well controlled, compressive
strength can be increased due to the low volume
(0.3 vol%) addition of steel fiber; otherwise, the
addition of the low steel fiber content may have
little or adverse effect on the increase of the
compressive strength of the mortar. With the
increased volume addition of steel fiber from 0.3
to 1.0 vol%, the compressive strength of the
mortar is remarkably increased.
(3) Failure modes of the mortar are influence by the
addition of steel fiber. Good ductile behavior is
shown by steel fiber reinforced mortars. The
toughness of the mortar is denoted by the
dissipated energy. Mortars with lower water/
binder ratio show higher toughness. For mortars
with low water/binder ratio, the toughness is
increased with the increased volume addition of
steel fiber; for mortars with large water/binder
ratio, the elastic energy is nearly identical while
the post elastic energy is still increased due to
the addition of steel fiber.
(4) The three-point bending strength and the tough-
ness of the steel fiber reinforced mortar are
related to the interfacial characteristics and
microstructural morphology near the fiber sur-
face. For mortar with the same water/binder
ratio, comparatively low bending strength and
Welastic in mortar with 10% silica fume result
from the wider thickness of the ITZ in the steel
fiber–matrix-aggregate interfacial zone and
voids and microcracks within this zone. Those
discontinuous pores and the increase in micro-
cracks may result from the reaction with silica
fume and autogenous shrinkage. For mortar
with different water/binder ratio, due to the poor
bonds across the steel fiber–matrix and fiber–
matrix-aggregate interfaces in mortar with
higher water/binder ratio, the bending strength
Fig. 8 Voids and microcracks in the steel fiber–matrix-
aggregate zone in mortar 031003
Fig. 9 Bleeding void and porous zone near the surface of steel
fiber in mortar 050003
Materials and Structures (2010) 43:125–139 137
and fracture toughness are obviously lower than
those of mortar with lower water/binder ratio.
The present work aims at linking the nano-
mechanical properties of the ITZ with the macro-
scopic mechanical properties. It will help to proceed
in the development of more ductile cement based
materials and obtain fiber reinforced cementitious
composites with high tensile strength, flexural
strength and fracture toughness by tailoring the
microstructure of the fiber–matrix interface and
fiber–matrix-aggregate interface.
Acknowledgements This research was done at the
Norwegian University of Science and Technology (NTNU),
when the first author is working at the Department of Structural
Engineering as a visiting Professor for one year. The first
author gratefully thank the support by the Norwegian Research
Councils a part of the Cultural Agreement between Norway
and China—Government scholarships 2007/2008 (No.
26X35003) and the National Natural Science Foundation of
China (No. 50508020). The invitation provided by Professor
Stefan Jacobsen in NTNU to cooperate with this research work
is gratefully appreciated. In addition, the financial support from
the COIN (Concrete Innovation Centre) Project 3-3.5—
Nanotechnology applied to Cement-based Materials is greatly
appreciated. Help given by Dr. Hilde Lea Lein, engineer Ove
Edvard Loraas, engineer Arild Monsøy and Wilhelm Dall in
NTNU is also acknowledged.
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