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The effects of silica fume and polypropylene fibers on the impact resistance and mechanical properties of concrete

Construction and Building Materials 24 (2010) 927–933

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Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat

The effects of silica fume and polypropylene fibers on the impact resistance
and mechanical properties of concrete
Mahmoud Nili *, V. Afroughsabet
Civil Eng. Dept., Bu-Ali Sina University, Hamedan, I.R, Iran

a r t i c l e

i n f o

Article history:
Received 21 August 2009
Received in revised form 21 November 2009
Accepted 21 November 2009
Available online 24 December 2009

Keywords:
Polypropylene fibers
Silica fume
Mechanical properties
Impact resistance

a b s t r a c t
Impact resistance and strength performance of concrete mixtures with 0.36 and 0.46 water–cement
ratios made with polypropylene and silica fume are examined. Polypropylene fiber with 12-mm length
and four volume fractions of 0%, 0.2%, 0.3% and 0.5% are used. In pre-determined mixtures, silica fume
is used as cement replacement material at 8% weight of cement. The results show that incorporating
polypropylene fibers improves mechanical properties. The addition of silica fume facilitates the dispersion of fibers and improves the strength properties, particularly the impact resistance of concretes. It
is shown that using 0.5% polypropylene fiber in the silica fume mixture increases compressive split tensile, and flexural strength, and especially the performance of concrete under impact loading.
Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction
It is well known that concrete is a quasi brittle material. Brittleness increases with increasing strength. This may be due to low
tensile strength and lack of bonding in the transition zone of the
cement matrix which obviously restricts utilization of high
strength concrete under static and, in particular dynamic loading
[1–3]. However, despite the defects in high strength concrete, demand for this material continues to grow. It is well understood that
silica fume, due to high pozzolanic activity, is inevitable material
when producing high strength concrete; however, it causes the
concrete to have a more brittle structure [4–5]. Therefore, ductility
improvement is a vital matter in concrete science that must be taken into account by researchers. One possible solution to improve
the ductility and resistance of concrete structures [6–10] to dynamic loading, such as impact, fatigue and earthquakes, is incorporating fibers in the concrete. Adding fibers to concrete increases the
energy absorption capacity of concrete and provides a more ductile
structure. The fibers are mainly made of steel, carbon or polymer
[11]. Among the polymer fibers, polypropylene (PP) has attracted
the most attention among researchers because of its low cost, outstanding toughness and enhanced shrinkage cracking resistance in
concrete reinforced with this type of fiber [11–16]. Many studies
have evaluated the ductility of fibrous specimens; the impact test
is a well known method for assessment of concrete ductility [17].
Hibbert and Hannant [18] designed an instrument to control the

* Corresponding author. Tel.: +98 9181112615; fax: +98 8118224205.
E-mail address: nili36@yahoo.co.uk (M. Nili).
0950-0618/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.conbuildmat.2009.11.025


impact resistance of fiber-reinforced concrete. A 100 Â 100 Â
400-mm specimen is supported in a Charpy test apparatus and
completely fractured by one blow; the fracture energy is measured
from the amplitude of the pendulum swing. The drop weight test
was also used to perform impact tests on plain and steel fiber-reinforced concrete beams by Mohammadi et al. [19]. The Committee
544 [20] ACI proposed a drop weight impact test to evaluate the
impact resistance of fiber concrete. Disc specimens that were of
150 mm in diameter and 64 mm in thickness were cut from
150 Â 300-mm cylinders. The number of blows required to cause
the first visible crack and to cause failure were recorded. Because
of the nature of the impact test, and especially because of the in
homogeneity of concrete, the data obtained from the impact test
can be scattered noticeably, as reported by Schrader [21]. This test
is widely used because of its simplicity and economy. The variation
in the impact resistance determined from this test is reported in
the literature for some types of FRC, but less data can be found
for polypropylene fiber-reinforced concrete [22]. Thus, several impact test methods have been used to demonstrate the relative brittleness and impact resistance of concrete. However, none of these
test methods have been standardized yet. In the present research,
the impact resistance and strength performance of fibrous and
non-fibrous specimens with and without silica fume are experimentally examined.

2. Test program and procedures
In this research, two series of concrete mixtures with 0.46 and 0.36 water–cement ratios, were prepared and labeled A1 and B1, respectively. Some specimens
were reinforced with 0.2%, 0.3% and 0.5% (by volume) polypropylene fibers. Silica


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M. Nili, V. Afroughsabet / Construction and Building Materials 24 (2010) 927–933
the specimens was determined as well in accordance with the ACI committee
544 proposal [20]. For this purpose, six 150 Â 64-mm discs, which were cut from
150 Â 300-mm cylindrical specimens using a diamond cutter were prepared and
placed on a base plate with four positioning lugs; they were then struck with repeated blows. The blows were introduced through a 4.45 kg hammer dropping frequently from a 45.7-cm height onto a 6.35-cm steel ball, which was located at the
center of the top surface of the disc. Figs. 2 and 3 show the specimens and impact
base plate and the test procedure. The numbers of blows producing the first visible
crack and cause ultimate failure were recorded. In each test, the number of blows to
produce the initial visible crack was recorded as the first crack strength, and the
number of blows to cause complete failure of the disc was recorded as the failure
strength.

2.1. Materials and mixing procedure

Fig. 1. Flexural test machine.

Ordinary Portland cement (ASTM Type 1) produced by Hekmatan Factory and
silica fume, a by-product of the silicon and ferrosilicon Semnan factory, were used
in this work. The cement and silica fume properties are given in Table 1. Coarse
aggregate with a maximum size of 19 mm and fine aggregate with a 3.4 fineness
modulus were used in this experiment. The specific gravity and water absorption
of the coarse and fine aggregates were 2.69 and 0.56% and 2.61 and 1.92%, respectively. A high range water reducer agent with a commercial name of Carboxylic
110 M (BASF) was used to adjust the workability of the concrete mixtures. The
mixing procedure for fresh concrete mixtures was as follows: the cement (or cement and silica fume) and fine aggregate were mixed initially for 1 min; and
superplasticizer with half mixing water were mixed for 2 min. Coarse aggregate
and the rest of water were added and mixed for 3 min. Finally fiber was added
to the mixtures and mixed for 5 min. The polypropylene fiber properties, as well
as the mix proportions of the mixtures, are provided in Tables 2 and 3,
respectively.

Table 1
Properties of cement and silica fume.
Composition (%)

Fig. 2. Disc type specimens for the impact test.

fume as a cement replacement was also added (8% by weight) to some specimens.
Compressive strength tests were performed at the ages of 7, 28 and 91 days on
100 Â 100 Â 100-mm cubic specimens and the flexural strength test was also performed (see Fig. 1) on 80 Â 100 Â 400-mm specimens. The tensile strength test was
also performed on 100 Â 200-mm cylindrical specimens. The Impact resistance of

Chemical compositions
Sio2
Al2O3
Fe2O3
MgO
Na2O
K2O
CaO
C3S
C2S
C3A
C4AF
Physical properties
Specific gravity
Specific surface (cm2/gr)

Cement
21.20
5.35
3.40
1.44


63.95
51.46
22.00
6.42
10.35
3.1
3000

Fig. 3. (a) Base plate within four positioning lugs and subjected to repeated blows and (b) procedure for the impact test.

Silica fume
85–95
0.5–1.7
0.4–2
0.1–0.9
0.15–0.2
0.15–1.02





2.21
14,000


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M. Nili, V. Afroughsabet / Construction and Building Materials 24 (2010) 927–933
Table 2
Properties of polypropylene fiber.
Length (mm)

Effective diameter (lm)

Density (kg/m3)

Shape

12

22

0.91

Straight

2.2. Specimen molding
Each type of freshly mixed concrete was cast into cubic (100 mm), cylindrical
(100 mm  200 mm specimens), prismatic and cylindrical cutting specimens for
compressive, splitting tensile, flexural and impact tests, respectively. All specimens,
before de-molding, were stored at 23 °C and 100% relative humidity for about 24 h.
The concrete specimens were then cured in lime-saturated water until the day of
testing.

3. Results and discussion
The compressive, tensile and flexural strength results are summarized in Table 4 and graphically illustrated in Figs. 4–6.
3.1. Compressive strength
The variations of the compressive strength versus fiber volume
fractions, at the ages of 7, 28 and 91 days, are illustrated in Fig. 4. It

can be generally seen that, for all specimens, as the fiber volume
increases the compressive strength increases. As shown, for 0.46
water–cement ratio specimens, the increase in compressive
strength are 3% at 0.2% fiber volume and 14% at 0.5% fiber volume.
Adding silica fume to non-fibrous specimens also improves compressive strength. Increase in compressive strength up to 13%,
21% and 23% are observed in No. 5 at the ages of 7, 28 and 91 days
compared to No. 1, respectively. Whereas in fibrous specimens, for
instance No 8, when silica fume was added to a 0.5% fiber specimen, an increase of 20% at 7 days, 27% at 28 days and 30% at the
ages of 91 days are obtained. In series B1, the specimens with a
water–cement ratio of 0.36 demonstrated a similar trend in the, results. In the case of specimen Nos. 10 and 12, increase of 1–6% in
compressive strength is observed as the fiber volume varies between 0.2% and 0.5%, respectively. On the other hand, introducing
silica fume to the specimens (No. 13) improves the compressive
strength by about 7–14% at the ages of 7 and 91 days, respectively.
When silica fume and polypropylene fiber are simultaneously
incorporated into the specimens (Nos. 14 and 16) an improvement
in compressive strength between 9–18% and 11–20% at the ages of
7–91 days, compare to reference specimen, No. 9, are observed.
This indicates that the pozzolanic properties of silica fume and also
the crack restriction effect of fiber can promote the compressive
strength of concrete.

Table 3
Mix proportions of the concrete mixtures.
Mix
no.

W/
(C + Sf)

Water (kg/
m3)

Cement (kg/
m3)

Silica fume (kg/
m3)

Fine agg. (kg/
m3)

Coarse agg. (kg/
m3)

Vf
(%)

Weight (kg/
m3)

Sp
(%)

Slump
(Cm)

A1
1
2
3
4
5
6
7
8

0.46
0.46
0.46
0.46
0.46
0.46
0.46
0.46

177
177
177
177
177
177
177
177

385
385
385
385
354.2
354.2
354.2
354.2





30.8
30.8
30.8
30.8

920
918
916
914
915
912
911
908

884
882
880
878
879
876
875
873


0.2
0.3
0.5

0.2
0.3
0.5


1.82
2.73
4.55

1.82
2.73
4.55

0.60
0.90
1.25
1.70
0.70
1.10
1.35
1.75

5.0
6.0
6.0
4.0
7.0
7.0
7.0
5.0

B1
9
10
11
12
13
14
15
16

0.36
0.36
0.36
0.36
0.36
0.36
0.36
0.36

162
162
162
162
162
162
162
162

450
450
450
450
414
414
414
414





36
36
36
36

912
910
908
906
906
903
902
899

877
874
873
870
871
868
867
864


0.2
0.3
0.5

0.2
0.3
0.5


1.82
2.73
4.55

1.82
2.73
4.55

1.10
1.50
1.60
1.90
1.20
1.55
1.65
1.95

6.5
6.5
8.0
6.0
10.0
10.0
8.5
5.0

Table 4
Compressive, tensile and flexural strength of the specimens.
Mix no.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16

Compressive strength (MPa)

Tensile strength (MPa)

Flexural strength (MPa) 28 days

7 days

28 days

91 days

7 days

28 days

91 days

32.95
33.88
36.15
37.56
37.28
37.51
38.12
39.41
47.58
48.12
49.01
49.64
51.19
51.88
52.30
52.68

41.30
42.32
44.05
46.09
49.88
50.29
50.88
52.61
55.58
55.97
56.46
58.24
63.34
65.93
66.16
66.33

46.65
48.96
50.21
53.56
57.44
58.71
59.43
60.49
61.01
62.27
63.43
64.54
69.48
72.28
72.41
73.26

2.67
2.81
2.85
3.01
2.95
2.97
3.13
3.24
3.56
3.63
3.73
3.90
3.98
4.04
4.15
4.27

3.22
3.49
3.66
3.68
3.52
3.69
3.87
4.09
4.39
4.41
4.49
4.68
4.71
5.05
5.09
5.43

3.89
3.97
4.03
4.16
3.97
4.01
4.06
4.39
4.74
4.93
5.04
5.22
5.52
5.36
5.71
5.86

4.45
4.48
5.17
5.58
5.09
5.46
5.68
6.14
6.30
6.58
6.63
6.36
6.97
7.06
7.56
7.83


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M. Nili, V. Afroughsabet / Construction and Building Materials 24 (2010) 927–933

3.2. Splitting tensile strength
Split tensile strength results versus fiber volume fractions are
shown in Fig. 5. The results show that for both water–cement ratios,
tensile strength rises as the fiber volume fractions increases. For
example, the tensile strength of A1 mixture, at the age of 28 day increases 8%, 14% and 14% when the fiber volume fractions in the
mixes are 0.2%, 0.3% and 0.5%, respectively. Adding silica fume to
the specimen (No. 5) the tensile strength increases by 9% compared
to reference ones. However, when silica fume is introduced to fibrous specimens, the rate of tensile strength increases by 15%,
20% and 27% in specimens Nos. 6–8, respectively. Although splitting
tensile strength is greatly affected due to a reduction in water–cement ratio, in B1 mixtures, the same tendency as A1 specimens is
observed. In other words, introducing the fiber and silica fume to
the mixtures improves tensile strength. Furthermore, the combined
effect of fiber and silica fume leads to increases of 15%, 16% and 23%
in tensile strength in specimens’ Nos. 14–16, respectively.

3.3. Flexural strength
The flexural strength results versus fiber volume fractions, at
the age of 28 days, carried out on sixteen different mixtures are
presented in Fig. 6. As explained in the tensile strength results,

3.4. Impact test
The impact resistance performance of the A1 and B1 series of
concrete are given in Table 5 and are also shown in Fig. 7. As it is
shown, the number of blows at the first crack (N1) and the number
of blows for failure (N2) are provided in the results. The percentage
increase in the number of post first crack blows to failure (N2–N1/
N1) is labeled the termed as PINPB and is also given in Table 5. As
the results suggest, by incorporating PP fibers into the A1 mixtures,
N1 is increased by 31%, 100% and 360% by adding 0.2%, 0.3% and
0.5%, fiber, respectively. When silica fume is introduced to the mixture (No. 5) N1 increases six times. However, in silica fume fibrous
specimens (Nos. 6–8), N1 increases about 6.6, 7.6 and 8.5 times,

(b)

80

70

Compressive Strength [MPa]

W/C=0.46
W/C=0.36
W/C=0.46-Sf
W/C=0.36-Sf

60

50

40

30

20
0

0.1

0.2

0.3

0.4

0.5

80

70

60

50

40
W/C=0.46
W/C=0.36
W/C=0.46-Sf
W/C=0.36-Sf

30

20

0.6

0

0.1

Fiber Volume Fraction [%]

(c)
Compressive Strength [MPa]

Compressive Strength [MPa]

(a)

the flexural strength of fibrous specimens increases compared to
the reference specimen. However, the rate of increase is higher
in A1 specimens. Silica fume, as in the tensile strength results, improves flexural performance. The combined effect of fiber and silica
fume is considerable, and typically, an improvement in flexural
strength of 22% in No. 6, 27% in No. 7 and 38% in No. 8 are observed.
In B1, the increase in specimens flexural strength is the same as A1,
but at a lower rate. However, the highest flexural strength value of
7.83 MPa is belongs to specimen No. 16 in series B1, which contain
silica fume and 0.5% polypropylene fiber.

0.2

0.3

0.4

0.5

Fiber Volume Fraction [%]

80

70

60

50

40
W/C=0.46
W/C=0.36
W/C=0.46-Sf
W/C=0.36-Sf

30

20
0

0.1

0.2

0.3

0.4

0.5

0.6

Fiber Volume Fraction [%]
Fig. 4. Compressive strength versus fiber volume fractions at the ages of: (a) 7 days, (b) 28 days and (c) 91 days.

0.6


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M. Nili, V. Afroughsabet / Construction and Building Materials 24 (2010) 927–933

(b)

8
W/C=0.46
W/C=0.36
W/C=0.46-Sf
W/C=0.36-Sf

Tensile Strength [MPa]

7

8
W/C=0.46
W/C=0.36
W/C=0.46-Sf
W/C=0.36-Sf

7

Tensile Strength [MPa]

(a)

6

5

4

3

6

5

4

3

2

2
0

0.1

0.2

0.3

0.4

0.5

0.6

0

0.1

Fiber Volume Fraction [%]

(c)

0.3

0.4

0.5

0.6

8
W/C=0.46
W/C=0.36
W/C=0.46-Sf
W/C=0.36-Sf

7

Tensile Strength [MPa]

0.2

Fiber Volume Fraction [%]

6

5

4

3

2
0

0.1

0.2

0.3

0.4

0.5

0.6

Fiber Volume Fraction [%]
Fig. 5. Splitting tensile strength versus fiber volume fractions at the ages of: (a) 7 days, (b) 28 days and (c) 91 days.

(a)

(b) 11

11

W/C=0.36

W/C=0.46

10

W/C=0.46-Sf

Felxural Strength [MPa]

Felxural Strength [MPa]

10
9
8
7
6
5
4
3

W/C=0.36-Sf

9
8
7
6
5
4

0

0.2

0.3

0.5

Fiber Volume Fraction [%]

3

0

0.2

0.3

0.5

Fiber Volume Fraction [%]

Fig. 6. Flexural strength and fiber volume fractions at the age of 28 days: (a) w/c = 0.46 and (b) w/c = 0.36.

respectively. This may be attributed to the fact that adding silica
fume improves dispersion of the fibers in the specimens [7,22].
A similar trend to that specified for N1 is observed for N2
values. On the other hand, PINPB values that indicate the ability

to absorb kinetic energy suggest that adding fiber delays failure
strength. On the other hand, the results also reveal that adding silica fume (No. 5) despite increment the strength, leads to higher
brittleness. However, initiation and cracks propagation under


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M. Nili, V. Afroughsabet / Construction and Building Materials 24 (2010) 927–933

Table 5
Test results for impact resistance of polypropylene fiber-reinforced concrete.
Mix no.

Impact resistance (blows)

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16

First crack (N1)

Failure (N2)

First crack

Failure

35
46
70
161
243
268
300
331
132
139
192
239
281
299
325
365

38
54
79
181
246
277
315
371
134
152
211
307
284
322
343
399

712.1
935.9
1424.2
3275.5
4943.8
5452.5
6103.5
6734.2
2685.5
2827.9
3906.2
4862.5
5716.9
6083.2
6612.1
7425.9

773.1
1098.6
1607.3
3682.4
5004.9
5635.6
6408.7
7547.9
2726.2
3092.4
4292.8
6245.9
5777.9
6551.1
6978.3
8117.7

8.6
17.4
12.9
12.4
1.2
3.4
5.0
12.1
1.5
9.4
9.9
28.5
1.1
7.7
5.5
9.3

Percentage increase in number of post-first-crack blows to failure.

Number of Blows at First Crack

(a)

(b)

450
W/C=0.46
W/C=0.36
W/C=0.46-Sf
W/C=0.36-Sf

400
350

450
W/C=0.46
W/C=0.36
W/C=0.46-Sf
W/C=0.36-Sf

400

Number of Blows at Failure

a

PINPB (blows)a

Impact energy (kN mm)

300
250
200
150
100
50

350
300
250
200
150
100
50

0

0

0

0.1

0.2

0.3

0.4

0.5

0.6

Fiber Volume Fraction [%]

0

0.1

0.2

0.3

0.4

0.5

0.6

Fiber Volume Fraction [%]

Fig. 7. Impact strength versus percentage of polypropylene fiber volume fractions at: (a) first crack and (b) failure strength.

impact loading are reduced in fibrous and nearly silica fume fibrous specimens. As the water–cement ratio decrease in the B1
mixtures, lower ductility and increased strength of the paste can
be attained. Adding of silica fume, despite increasing the strength,
led to higher brittleness. Although N1 increases compare to A1
mixtures the rate of increase resulting from fiber or silica fume,
in N1 and N2 decreases considerably. Adding fibers also increases
the PINPB value seven times over the specimens made by silica
fume and without fiber specimens. This means that fibers effectively reduced the brittleness of the specimens. In Fig. 8, a comparison of the failure pattern in the disc specimens with and without
fiber is shown. It can be concluded that, by adding fiber, the failure
crack pattern changed from a single large crack to a group narrow
cracks, which demonstrates the beneficial effects of fiber-reinforced concrete subjected to impact loading.
4. Conclusions

1. The increase of polypropylene fiber in the mixtures from 0.2% to
0.5%, generally increased the compressive strength. The compressive strength of fibrous specimens at the age of 91 days,
with 0.5% fiber, increased by 15% compared with those of the
reference.

2. When silica fume is added into the non-fibrous and fibrous mixtures, the compressive strength, at the age of 91 days, was
enhanced by 23% and 30%, respectively. On the other hand, adding of silica fume into the fibrous specimens led to an increased
in compressive strength up to 30% at the age of 91 days. This
may be due to pozzolanic effect of silica fume and crack restriction effect if fiber.
3. Splitting tensile and flexural strength of 0.5% fibrous silica fume
concretes was enhanced considerably.
4. The number of blows at first cracks and failure, as impact indices, increased considerably in fibrous specimens. Incorporating
0.2%, 0.3% and 0.5% polypropylene fiber into the 0.46 watercement ratio specimens led to an increase in the number of
blows by 31%, 100% and 360%, respectively at first crack and
42%, 107% and 376%, respectively, at failure compared to those
of the reference. Likewise, a similar trend was observed, but at a
lower rate, in 0.36 water-cement ratio specimens.
5. The results revealed that silica fume improved the fiber dispersion in the mixtures.
6. Adding silica fume to fibrous specimens improved the specimens strength more than adding silica fume by itself. These
results show that silica fume can strengthens the transition
zone and reduces crack initiation, and therefore, improves the
failure strength of polypropylene fiber concretes.


M. Nili, V. Afroughsabet / Construction and Building Materials 24 (2010) 927–933

933

Fig. 8. Fracture pattern of concrete with different fiber volume fractions under the drop weight test: (a) plain concrete, (b) 0.2% fiber, (c) 0.3% fiber and (d) 0.5% fiber.

7. A ductile failure, under impact loading, was observed in fibrous
specimens. When silica fume was used in non-fibrous concretes, it led to an increase in brittleness. However, incorporating silica fume and polypropylene considerably improved the
ability of concrete to absorb kinetic energy.
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