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.
Trang 1The effects of silica fume and polypropylene fibers on the impact resistance
and mechanical properties of concrete
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 disper-sion 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 ten-sile, 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
Brittle-ness 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,
de-mand 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
ta-ken into account by researchers One possible solution to improve
the ductility and resistance of concrete structures [6–10]to
dy-namic loading, such as impact, fatigue and earthquakes, is
incorpo-rating 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,
out-standing 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
impact resistance of fiber-reinforced concrete A 100 100 400-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-rein-forced 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
150 300-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 im-pact test methods have been used to demonstrate the relative brit-tleness 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 experi-mentally examined
2 Test program and procedures
In this research, two series of concrete mixtures with 0.46 and 0.36 water–ce-ment 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 0950-0618/$ - see front matter Ó 2009 Elsevier Ltd All rights reserved.
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E-mail address: nili36@yahoo.co.uk (M Nili).
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Trang 2fume 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
100 100 100-mm cubic specimens and the flexural strength test was also
per-formed (see Fig 1 ) on 80 100 400-mm specimens The tensile strength test was
also performed on 100 200-mm cylindrical specimens The Impact resistance of
the specimens was determined as well in accordance with the ACI committee
544 proposal [20] For this purpose, six 150 64-mm discs, which were cut from
150 300-mm cylindrical specimens using a diamond cutter were prepared and placed on a base plate with four positioning lugs; they were then struck with re-peated blows The blows were introduced through a 4.45 kg hammer dropping fre-quently 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 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%, respec-tively 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 ce-ment 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.
Fig 1 Flexural test machine.
Fig 2 Disc type specimens for the impact test.
Table 1 Properties of cement and silica fume.
Chemical compositions
Physical properties
Specific surface (cm 2 /gr) 3000 14,000
Trang 32.2 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
sum-marized inTable 4and graphically illustrated inFigs 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 inFig 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 com-pressive strength Increase in comcom-pressive 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 speci-men, 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, re-sults In the case of specimen Nos 10 and 12, increase of 1–6% in compressive strength is observed as the fiber volume varies be-tween 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/
m 3 )
Cement (kg/
m 3 )
Silica fume (kg/
m 3 )
Fine agg (kg/
m 3 )
Coarse agg (kg/
m 3 )
Vf (%) Weight (kg/
m 3 )
Sp (%) Slump (Cm) A1
B1
Table 4
Compressive, tensile and flexural strength of the specimens.
Mix no Compressive strength (MPa) Tensile strength (MPa) Flexural strength (MPa) 28 days
Table 2
Properties of polypropylene fiber.
Length (mm) Effective diameter (lm) Density (kg/m 3
) Shape
Trang 43.2 Splitting tensile strength
Split tensile strength results versus fiber volume fractions are
shown inFig 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
in-creases 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
fi-brous 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–ce-ment 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 inFig 6 As explained in the tensile strength results,
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, im-proves 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
3.4 Impact test The impact resistance performance of the A1 and B1 series of concrete are given inTable 5and are also shown inFig 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 inTable 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 mix-ture (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) (a)
(c)
20 30 40 50 60 70 80
0 0.1 0.2 0.3 0.4 0.5 0.6
Fiber Volume Fraction [%]
W/C=0.46 W/C=0.36 W/C=0.46-Sf W/C=0.36-Sf
20 30 40 50 60 70 80
0 0.1 0.2 0.3 0.4 0.5 0.6
Fiber Volume Fraction [%]
W/C=0.46 W/C=0.36 W/C=0.46-Sf W/C=0.36-Sf
20 30 40 50 60 70 80
0 0.1 0.2 0.3 0.4 0.5 0.6
Fiber Volume Fraction [%]
W/C=0.46 W/C=0.36 W/C=0.46-Sf W/C=0.36-Sf
Trang 5respectively 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 sil-ica fume (No 5) despite increment the strength, leads to higher brittleness However, initiation and cracks propagation under
(b) (a)
(c)
2 3 4 5 6 7 8
0 0.1 0.2 0.3 0.4 0.5 0.6
Fiber Volume Fraction [%]
W/C=0.46 W/C=0.36 W/C=0.46-Sf W/C=0.36-Sf
2 3 4 5 6 7 8
0 0.1 0.2 0.3 0.4 0.5 0.6
Fiber Volume Fraction [%]
W/C=0.46 W/C=0.36 W/C=0.46-Sf W/C=0.36-Sf
2 3 4 5 6 7 8
0 0.1 0.2 0.3 0.4 0.5 0.6
Fiber Volume Fraction [%]
W/C=0.46 W/C=0.36 W/C=0.46-Sf W/C=0.36-Sf
Fig 5 Splitting tensile strength versus fiber volume fractions at the ages of: (a) 7 days, (b) 28 days and (c) 91 days.
3 4 5 6 7 8 9 10 11
Fiber Volume Fraction [%]
W/C=0.46 W/C=0.46-Sf
3 4 5 6 7 8 9 10 11
Fiber Volume Fraction [%]
W/C=0.36 W/C=0.36-Sf
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.
Trang 6impact loading are reduced in fibrous and nearly silica fume
fi-brous 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
effec-tively reduced the brittleness of the specimens InFig 8, a
compar-ison 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-rein-forced 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
com-pressive 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 mix-tures, the compressive strength, at the age of 91 days, was enhanced by 23% and 30%, respectively On the other hand, add-ing 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 restric-tion 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 indi-ces, increased considerably in fibrous specimens Incorporating 0.2%, 0.3% and 0.5% polypropylene fiber into the 0.46 water-cement 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 disper-sion in the mixtures
6 Adding silica fume to fibrous specimens improved the speci-mens 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
Table 5
Test results for impact resistance of polypropylene fiber-reinforced concrete.
a
Percentage increase in number of post-first-crack blows to failure.
(b) (a)
0 50 100 150 200 250 300 350 400 450
0 0.1 0.2 0.3 0.4 0.5 0.6
Fiber Volume Fraction [%]
W/C=0.46 W/C=0.36 W/C=0.46-Sf W/C=0.36-Sf
0 50 100 150 200 250 300 350 400 450
0 0.1 0.2 0.3 0.4 0.5 0.6
Fiber Volume Fraction [%]
W/C=0.46 W/C=0.36 W/C=0.46-Sf W/C=0.36-Sf
Fig 7 Impact strength versus percentage of polypropylene fiber volume fractions at: (a) first crack and (b) failure strength.
Trang 77 A ductile failure, under impact loading, was observed in fibrous
specimens When silica fume was used in non-fibrous
con-cretes, it led to an increase in brittleness However,
incorporat-ing silica fume and polypropylene considerably improved the
ability of concrete to absorb kinetic energy
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