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Effect of steel and carbon fiber additions on the dynamic properties of concrete containing silica fume
Materials and Design 34 (2012) 332–339
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Effect of steel and carbon fiber additions on the dynamic properties of concrete containing silica fume V.T. Giner, F.J. Baeza, S. Ivorra ⇑, E. Zornoza, Ó. Galao Department of Construction Engineering, University of Alicante, Alicante, Spain
a r t i c l e
i n f o
Article history: Received 13 June 2011 Accepted 30 July 2011 Available online 3 August 2011 Keywords: A. Concrete F. Elastic behavior G. Non-destructive testing (NDT)
a b s t r a c t Fiber reinforced concretes (FRC) are technologically important due to their combination of good structural properties, durability and multifunctional properties. The behavior of a structure under dynamic actions is determined by its dynamic mechanical properties and its total overall damping. In this work the influence of the steel fiber (SF) and carbon fiber (CF) additions on the mechanical properties of concrete containing silica fume has been studied, focusing on its passive material damping ability (damping ratio) and dynamic properties. According to the obtained results, under dynamic loads of low magnitude and high frequency, CF additions are more effective than SF additions for reducing vibrations, as they increase the damping ratio of concrete. In all cases, the dynamic elastic properties of concrete present higher values than their static counterparts. Both CF and SF additions lead to slight decreases of the compressive strength of concrete. For the same volumetric fraction, this fact is more significant in the concrete types containing SF. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Concretes and, in general, cement-based composite materials reinforced with randomly distributed and uniformly dispersed discontinuous fibers in the cement matrix, are technologically important due to their combination of good structural properties [1–8], durability [9,10] and multifunctional properties [11,12]. Progress made in the fields of engineering and architecture has led to an increasing need of concretes with outstanding properties, such as high-performance concretes (HPC). During the study and development of this kind of concretes, the combination of silica fume with steel fibers (SF) or carbon fibers (CF) has become a common practice. The addition of silica fume to concrete mixes leads to a significant increase of the compressive strength, among other properties, due to its filler effect and pozzolanic activity [13–18]. However, it also causes concrete to have a more brittle structure. In order to counteract this negative effect, fibers are added to concrete [5,7]. The main improvements provided by the addition of fibers are higher flexural toughness and strength, higher tensile ductility and strength, and lower drying shrinkage [1–8]. Furthermore, the effectiveness of short fibers highly depends on their degree of dispersion in the mix. In this respect, it is known that silica fume acts like a dispersant of fibers, contributing to optimize their distribution in the concrete mass [1,3].
⇑ Corresponding author. E-mail address: [email protected] (S. Ivorra). 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.07.068
The enhancement produced by the addition of fibers to concrete results in more ductile structures with higher energy absorption capacity, which are very desirable properties from the point of view of structures under dynamic actions. The vibrations produced by dynamic actions usually cause service problems as they can reduce comfort up to unacceptable levels. But, in some extreme cases, they can cause real safety problems, compromising the stability of the whole structure [18–20]. This fact has motivated, during the last decades, new research lines focused on the study of the dynamic behavior of new materials and composite concretes [21–25]. The behavior of concrete under dynamic actions is determined by its dynamic properties (such as dynamic modulus of elasticity, modulus of rigidity, Poisson’s ratio, compressive strength or strain limits), which present different values compared with their static counterparts. These variations are normally expressed as a function of the strain or stress rates [19,26]. The characterization of these dynamic properties can be experimentally achieved, from concrete specimens, according to the standard test method ASTM C 215 [27]. The dynamic performance of a structure is also highly conditioned by its damping ability. In a vibrating structure, damping is understood as the dissipation of the mechanical energy, generally by converting it into thermal energy [19]. In order to describe the damping ability of a structure or material, the more often used parameter is the damping ratio n. This non-dimensional parameter can be defined as follows [19,28]:
n¼
c c c ¼ ¼ pffiffiffiffiffiffiffi ccr 2mx 2 km
ð1Þ
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ξ=0.025
ξ=0.050 ξ=0.100 ξ=0.150 ξ=0.200 ξ=0.250 ξ=0.500 ξ=1.000
Fig. 1. Dynamic magnification factor versus the frequency ratio for several values of the damping ratio.
where c is the damping coefficient that represents viscous or linear damping; ccr is the critical damping coefficient; m is the mass of the system; x is the natural frequency of the undamped system; and k is the stiffness of the system. The main role that the damping ratio plays on the dynamic behavior of a structure can be clearly noticed in Fig. 1, where the Dynamic Magnification Factor (DMF) is represented for different values of the damping ratio. This non-dimensional constant describes the ratio between the displacement under a dynamic (harmonic) load and the displacement under a static load of the same magnitude (amplitude). Its value is given by the following expression [19,28]:
1 DMF ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ð1 r Þ2 þ ð2rnÞ2
ð2Þ
where r ¼ x x is the frequency ratio (between the frequency of the excitation and the natural frequency of the undamped system). In real structures, the value of the damping coefficient is much smaller than the critical damping coefficient, usually ranging from 2 to 10% of the critical damping value, and consequently the damping ratio is also small (n < 0.1). Therefore, from the structural dynamics theory, it can be deduced that in practice the damped natural frequency for a structure with such a small damping ratio may be taken to be equal to the undamped natural frequency [28]. As can be observed in Fig. 1, for small values of the damping ratio the peak amplitude occurs at a frequency ratio very close to 1; that is, the damping ratio only affects the vibration behavior significantly near resonance (r = 1). Theoretically, the DMF reaches its maximum value at the resonance situation, and then it is inversely proportional to the damping ratio [19,28]:
DMF ðr ¼ 1Þ ¼
1 2n
ð3Þ
However, the total overall damping of a structure is a complex concept that depends on several different factors, and it is considered a sum of the contributions of the damping of the bare structure, the damping by the non-structural elements and the damping by the energy radiation to the soil. The damping of the bare structure contributes to the energy dissipation by the material damping and the damping at the bearings and joints. In most cases the material damping is predominant [19]. In concrete, defects such as dislocations, phase boundaries, grain boundaries and various interfaces contribute to the material damping, since defects may move slightly and surfaces may slip slightly with respect to
one another during vibration, thereby dissipating energy. Thus, the microstructure of concrete greatly affects its damping capacity [29]. Therefore, as the addition of fibers to the concrete mix modifies its microstructure by introducing new friction surfaces, it could be an interesting technique to improve the passive material damping of concrete structures. In many applications such as slabs for industrial buildings or roads where machines or vehicles can transmit their vibrations through the structure to other machines, working people or near structures, the increase of the material damping could reduce the transmitted vibrations. In this sense, the published literature about the damping ability of the fiber reinforced concrete (FRC) incorporating silica fume is rather short. For this reason, there is a need for studying the influence of the SF or CF additions on the material damping of concrete containing silica fume, which justifies the present paper. The measurement of the damping properties of structures can be mainly achieved by two methods, which are the decay curve method and the bandwidth method. In most practical cases with real structures, the most recommended and used method is the first one. This is due to the fact that non-linear behavior of structures makes it very difficult to evaluate the resonance curve. Hence, by the bandwidth method, any measuring error will severely affect the evaluation of damping [8,19]. Nevertheless, at a material design level where the structural complexity is reduced to a concrete specimen, there are standard test methods [27] to determine the resonance curve in an accurate way. Thus it is possible to use the resonance curve of concrete specimens, obtained according to these standard test methods, to evaluate the material damping by the bandwidth method with an adequate precision, as reported in previous work on concretes with increasing silica fume admixtures [30]. The main objective of the present work is to study the effect of the SF or CF additions on the mechanical properties of concrete containing silica fume, focusing on its passive material damping ability (damping ratio) and dynamic properties.
2. Experimental procedure 2.1. Materials and specimen preparation The Portland cement type used was CEM I 52.5 R, designated in accordance with UNE-EN 197-1:2000 [31]. The brands of the short carbon fibers and the hooked-end steel fibers used were Zoltek PANEX 35 and ArcelorMittal HE 55/35, respectively. Their properties, supplied by vendors, are given in Tables 1 and 2. Both CF and SF were proportioned by total volume fraction. A superplasticizer based on melamine formaldehyde was used as an admixture in 0.5–2% proportions of cement mass. Undensified silica fume (USF), supplied by Grupo FerroAtlántica SL, was used in addition to the admixture of all concrete types, with a constant dosage of 10% with respect to cement mass [30]. Three different types of silica fume, besides USF, semidensified
Table 1 Properties of carbon fibers. Type
Zoltek PANEX 35
Diameter (D) Length (L) Aspect ratio (D/L) Specific surface area Carbon content Tensile strength Elastic modulus Density
7.2 lm 3.5 mm 486 5.56 05 m2/m3 95% 3800 MPa 242 GPa 1.81 g/cm3
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V.T. Giner et al. / Materials and Design 34 (2012) 332–339 Table 2 Properties of hooked-end steel fibers. Type
ArcelorMittal HE 55/35
Diameter (D) Length (L) Aspect ratio (D/L) Specific surface area Tensile strength Density
0.55 mm 35 mm 64 7.27 03 m2/m3 1200 MPa 7.85 g/cm3
Table 3 Chemical composition and particle size of the undensified silica fume used in the tests. Parameters
Average%
Moisture Loss on ignition Cl SO3 SiO2 C Al2O3 Fe2O3 CaO MgO K2O Na2O Alkalines like Na2O Free silicon
0.60 3.08 0.010 0.11 94.11 2.85 0.266 0.118 0.407 0.343 0.563 0.204 0.57 0.12
Cumulative volume (%) under 1 lm Average particle diameter (lm)
90.78 0.67
Table 4 Mixture proportioning of the reference concrete specimens R. kg/m3
R
Cement
Water
Sand
Gravel
Silica fume
330
165
630
1170
33
Table 5 Mix types and proportions of each concrete type. Type designation Fig. 2. Particle size distribution of the three silica fume types previously tested. USF undensified silica fume, SDSF semidensified silica fume and DSF densified silica fume.
silica fume (SDSF) and densified silica fume (DSF) were analyzed. The USF was finally selected according to the results of previous research where the effect of silica fume particle size on mechanical properties of concrete was studied [4]. The particle size of the different silica fume types was analyzed by using a Beckman Coulter LS 230 laser diffraction particle size analyzer, obtaining the distributions shown in Fig. 2. Also SEM images were taken of each type as shown in Fig. 3. The chemical composition and particle size of the USF used are shown in Table 3. Reference concrete specimens, only with USF addition, were prepared according to the mix proportioning shown in Table 4. The reason for making the reference concrete specimens was to evaluate the effect of the different CF and SF additions. Based on the reference concrete proportioning, three other mix types with different dosage of CF and SF were fabricated as shown in Table 5. Three prismatic and three cylindrical specimens of each concrete type were casted. Prismatic specimens of 100 100 400 mm dimensions were used for flexural strength and dynamic
R R + 0.5CF R + 0.5SF R + 1.0SF
Total volume% CF
SF
– 0.5 – –
– – 0.5 1.0
CF = carbon fiber. SF = steel fiber.
properties tests. Cylindrical specimens of £150 300 mm dimensions were used for compressive strength and elastic properties tests. The complete process of mixing, casting, consolidating and curing concrete specimens was carried out according to the standard practice ASTM C 192/C 192 M [32]. Both prismatic and cylindrical specimens were cured in moist room (>95% RH, 23 °C) until tested. 2.2. Testing 2.2.1. Static properties tests At 28 days, two cylindrical specimens of every concrete type were tested to determine the compressive strength according to
Fig. 3. SEM images taken on different types of silica fume tested: undensified silica fume (left), semidensified silica fume (center), densified silica fume (right).
V.T. Giner et al. / Materials and Design 34 (2012) 332–339
(a)
(b)
335
(c)
Data Logger 4 3
1 2
Fig. 4. Gage distribution on the performance to obtain mechanical constant of concrete specimens (elastic modulus, modulus of rigidity and Poisson’s ratio). (a) Stress application and gage configuration. The sample cross section shows longitudinal strain gages e1 and e3, and transverse strain gages e2 and e4. (b) Stress cycles during elastic modulus tests. (c) Stress–strain curve corresponding to elastic modulus tests.
the standard test method ASTM C 39/C 39 M [33]. At the ages of 28 and 60 days, one cylindrical specimen of each concrete type was tested to evaluate the static modulus of elasticity, modulus of rigidity and Poisson’s ratio, as shown in Fig. 4. For this purpose, an HBM Spider 8–600 Hz equipment was used jointly with HBM strain gauges (120 O, k = 2.10) and Catman v.5.0 analysis software. Longitudinal and transverse strain values were obtained for each loading cycle up to a maximum value when the applied load is equal to 40% of the sample ultimate load, as specified by the standard test method ASTM C 469 [34]. A press machine of 3500 kN capacity was used for both compressive strength and elastic properties tests. Two prismatic specimens of each concrete type were tested at 28 days to determine the flexural strength according to the stan-
(A) Longitudinal
dard test method ASTM C 78 [35]. A hydraulic press machine of 1500 kN capacity was used for these tests.
2.2.2. Dynamic properties tests Three prismatic specimens of each concrete type were tested to evaluate their fundamental longitudinal, transverse and torsional resonant frequencies, following the specifications of the standard test method ASTM C 215 [27]. An Erudite MkIV electronic resonant frequency test system, supplied by CNS Farnell, was used. The configuration of the device corresponding to each type of measure can be seen in Fig. 5. The frequency sweep range used to determine the fundamental resonant frequency of concrete specimens, for a particular mode of vibration, mainly depends on their shape and dimensions. In this case, the frequency sweep ranges used in each mode of vibration were 4000–6000 Hz, 1000–3000 Hz and 2000– 4000 Hz, for the longitudinal, transverse and torsional modes, respectively. The experimental values of the dynamic modulus of elasticity, modulus of rigidity and Poisson’s ratio were calculated, from the different resonant frequencies, according to the formulation included in the standard test method [27]. The damping ratio was determined by the bandwidth method, using the resonance curves obtained in the longitudinal vibration mode.
(B) Tranverse
(C) Torsional
Shaker
Accelerometer
Fig. 5. Experimental performance to obtain dynamic characteristics of the concrete types tested. Longitudinal configuration (up), transverse measure (center), and torsional (down).
Fig. 6. Density of concretes for £150 300 mm cylindrical specimens and 100 100 400 mm prismatic specimens at 28 days.
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Table 6 Twenty-eight day static mechanical properties of the different concrete mixes. Type designation
Compressive strength (MPa ± SD)
Flexural strength (MPa ± SD)
Static modulus of elasticity (GPa ± SD)
Static modulus of rigidity (GPa ± SD)
Static Poisson’s ratio (m ± SD)
R R + 0.5CF R + 0.5SF R + 1.0SF
48.51 ± 0.71 47.85 ± 2.54 46.80 ± 3.39 46.44 ± 1.46
7.48 ± 0.07 7.65 ± 0.83 7.15 ± 0.22 7.71 ± 0.08
35.26 ± 0.05 36.16 ± 0.18 33.73 ± 0.10 35.46 ± 0.07
13.96 ± 0.02 15.34 ± 0.08 13.68 ± 0.07 14.69 ± 0.02
0.26 ± 0.00 0.18 ± 0.00 0.23 ± 0.00 0.21 ± 0.00
SD = standard deviation.
These tests were repeated ten times for every specimen, mode of vibration and concrete type at the ages of 28 and 60 days.
3. Results and discussion 3.1. Static properties 3.1.1. Compressive and flexural strength Previously to any mechanical test the density of each specimen was measured according to the standard test method indicated on ASTM C 39/C 39 M [33]. Fig. 6 shows the mean value of the density of the specimens tested, both prismatic and cylindrical ones. The effect of each type of fiber is totally different. For a CF addition the density decreases, probably due to a greater porosity caused by the workability loss of fresh concrete and harder vibration pro-
Fig. 7. Compressive strength of £150 300 mm cylindrical specimens at 28 days.
Fig. 8. Flexural strength of 100 100 400 mm prismatic specimens at 28 days.
cess [4,9,36]. However, SF tends to increase the composite density, as the specific gravity of SF is 3 times the concrete’s one (Table 2). The results of the 28 day compressive strength tests on the cylindrical specimens are shown in Table 6 and Fig. 7. According to these results, both CF and SF additions lead to slight decreases of the compressive strength compared with the reference concrete. For the same volumetric fraction, this fact is more significant in the concrete type with SF, since the decreases obtained are over the 3% and the 1% for types R + 0.5SF and R + 0.5CF, respectively. The reduction of the compressive strength obtained for type R + 1.0SF is over the 4% of the reference value. These results are consistent with the previous published literature [3,9], where the reduction of the compressive strength is explained by the increase of porosity and air content that is produced by the addition of fibers. Table 6 includes the test results of the 28 day flexural strength of prismatic specimens, which are also represented in Fig. 8. Although concrete types R + 0.5CF and R + 1.0SF show slight increments of the flexural strength, ranging between the 2% and 3% of the reference value, they should not be regarded as significant because they are small and do not follow a well-defined pattern. It is worth noting that the reported results refer to the first-crack strength, and under no circumstances the present work involved the determination of the flexural post-cracking strength and toughness. 3.1.2. Static elastic properties The values of the static elastic properties obtained from the cylindrical specimens, at 28 days, are shown in Table 6. These properties present lower values in comparison with their dynamic counterparts. The differences between the static and dynamic modulus of elasticity range between 12% and 18% for all concrete types. These results are similar than others reported on scientific literature [37]. The variations observed in the static modulus of elasticity for concrete types containing fibers do not exceed the 4% of the reference value, whether they are increments or
Fig. 9. Static modulus of elasticity of £150 300 mm cylindrical specimens at 28 and 60 days.
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reductions, and do not follow a clear tendency. Therefore, these variations should not be considered significant. Fig. 9 shows the temporal evolution of the static modulus of elasticity at the ages of 28 and 60 days. During this period of time, an increasing tendency can be noticed for concrete types containing fibers. These results are coherent with the increase of the resonant frequencies, as they depend on stiffness, which is directly proportional to the elastic modulus [30]. 3.2. Dynamic properties
Table 8 Twenty-eight day dynamic properties of different concrete prismatic specimens. Type designation
Dynamic modulus of elasticity (GPa ± SD)
Dynamic modulus of rigidity (GPa ± SD)
Dynamic Poisson’s ratio (m ± SD)
Damping ratio (% ± SD)
R R + 0.5CF R + 0.5SF R + 1.0SF
40.85 ± 0.54 42.51 ± 0.19 41.43 ± 0.60 40.52 ± 0.65
16.29 ± 0.35 16.52 ± 0.15 16.13 ± 0.25 15.58 ± 0.30
0.28 ± 0.01 0.29 ± 0.01 0.26 ± 0.00 0.30 ± 0.01
0.63 ± 0.04 0.67 ± 0.08 0.63 ± 0.07 0.58 ± 0.06
SD = standard deviation.
3.2.1. Resonant frequencies Table 7 shows the fundamental resonant frequencies of the prismatic specimens for each mode of vibration at 28 days. These results show a similar pattern in the three modes of vibration [30,38]. The highest resonant frequencies are obtained for type R + 0.5CF, followed by the reference concrete, and then tending to slightly decrease when the amount of SF increases. In any case, the variations in resonant frequencies produced by the additions of SF or CF are lower than 4%, compared with the reference concrete. The temporal evolution of the longitudinal resonant frequencies, at the ages of 28 and 60 days, is shown in Fig. 10. In this period of time, the longitudinal resonant frequency of the reference concrete remains constant, while it tends to slightly increase for the concrete types containing fibers. These small increments are lower than 1% of the 28 day values. The increase of the resonant frequencies is related to the temporal evolution of other mechanical properties, and especially to the elastic modulus (directly proportional to stiffness) [30]. 3.2.2. Dynamic elastic properties The values of the dynamic elastic properties obtained at 28 days are included in Table 8. These properties have been calculated from Table 7 Twenty-eight day resonant frequencies of concrete prismatic specimens for the three different modes of vibration. Type designation
Longitudinal resonant frequency (Hz ± SD)
Transverse resonant frequency (Hz ± SD)
Torsional resonant frequency (Hz ± SD)
R R + 0.5CF R + 0.5SF R + 1.0SF
5239 ± 25 5294 ± 8 5198 ± 19 5123 ± 24
2245 ± 13 2283 ± 13 2234 ± 9 2188 ± 26
3010 ± 25 3035 ± 4 2982 ± 9 2921 ± 17
SD = standard deviation.
Fig. 10. Longitudinal resonant frequency of 100 100 400 mm prismatic specimens at 28 and 60 days.
Fig. 11. Dynamic modulus of elasticity of 100 100 400 mm prismatic specimens at 28 and 60 days.
the different resonant frequencies and, consequently, they follow a similar trend. However, in the specific case of the dynamic modulus of elasticity some differences can be observed, as the value obtained for type R + 0.5SF is slightly higher than the reference value, unlike the rest of dynamic properties where R + 0.5SF presents lower values than the reference concrete. As a result, types R + 0.5CF and R + 0.5SF show increments of about 4% and 2%, respectively, compared with the reference value. On the other hand, the dynamic modulus of elasticity of R + 1.0SF is lower than the reference one, although this difference is under the 1%. The temporal evolution of the dynamic modulus of elasticity, showed in Fig. 11, is completely related to the one showed in Fig. 10 for the longitudinal resonant frequency. Thus, the dynamic modulus of elasticity slightly increases from 28 to 60 days [30]. These increments are under the 2% of the 28 day value in all cases.
3.2.3. Damping ratio The damping ratio values obtained for every concrete type are included in Table 8, and represented in Fig. 12. In the graph two horizontal lines have been drawn meaning the confidence interval for the mean of the reference concrete tests, with a confidence level of 95%, assuming a normal (Gaussian) statistical distribution. This confidence interval was determined according to standard UNE 66,040:2003 [39]. The reason for establishing this interval is to evaluate whether the presence of SF or CF means a significant change in concrete damping ratio or the experimental differences are due to the random nature of the tests results. As can be observed in Fig. 12, the damping ratio of the concrete type R + 0.5CF is slightly higher than the one of the reference concrete. The increment produced by the addition of CF is over the 6% of the reference value. On the other hand, the damping ratio of type R + 0.5SF does not present any significant variation with respect to
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Fig. 12. Damping ratio of 100 100 400 mm prismatic specimens at 28 days. Acceptability limits represent the confidence interval for the mean of the reference concrete tests, with a confidence level of 95%, assuming a normal (Gaussian) statistical distribution.
the reference value, but for type R + 1.0SF a decrease of the 8% can be noticed. This difference between the additions of CF and SF may be due to their different specific surface areas, i.e. the total surface area of a single fiber per unit of volume is 5.56 105 m2/m3 for CF, while for SF only reaches 7.27 103 m2/m3. The volume of a single steel fiber is about 58,000 times bigger than the volume of a single carbon fiber. As a result, for the same volumetric fraction, the number of carbon fibers added to the concrete mixture is much higher than the corresponding number of steel fibers. Consequently, the addition of CF introduces many more interfaces and transition zones in the cement matrix, which are responsible for dissipating energy by internal friction during vibration, thereby increasing the damping ratio of concrete. A similar pattern was reported by Park [40] for the mechanical properties of carbon fiber reinforced cement pastes with different cross sections. The fact that SF additions can reduce the damping ratio of concrete, which is related to its energy absorption capacity, may contrast with other previous researches [6–8]. However, this may be due to the magnitude and frequency of the dynamic loads applied in the tests. In prior work made by Wu [41] it is concluded that peak vibrations amplitudes between 1.0 and 1.5 g are needed to detect an increase in damping ratio of steel fiber reinforced concretes. In the present work, the intensity of the excitations is extremely low and their frequencies are high. In this situation, cracks are not developed. Nevertheless, other previous investigations studied the dynamic performance of steel fiber reinforced concrete (SFRC) structures by applying dynamic loads up to failure. In these works it was proved that the main contribution of SF to concrete occurs after matrix cracking. The results indicate that SF effectively restrained the initiation and propagation of cracks during the failure of SFRC structures, mitigated the stress concentrations at the tips of cracks, and delayed the damage process under impact and fatigue. Therefore, the addition of SF to concrete mixtures is useful if a large amount of energy absorption capacity is required to reduce brittle failure [6,7], hence improving the ductility of the structure. 4. Conclusions The object of the present investigation is to study the effect of steel or carbon fibers on the dynamic properties of concrete containing silica fume, focusing on its damping ratio and dynamic modulus of elasticity. Specimens of different concrete types, with variations in the type of fiber and proportioning, have been
fabricated and tested to determine the experimental values of their dynamic and static properties. After discussing the results, the following conclusions can be drawn: The addition of carbon fiber slightly increases the resonant frequencies of concrete specimens in the three modes of vibration. On the contrary, they tend to decrease when the amount of steel fiber increases. The dynamic elastic properties follow a similar trend, since they are increased by the addition of carbon fiber and slightly reduced, in most cases, by the addition of steel fiber. In all cases, the temporal evolution of both resonant frequencies and dynamic elastic properties follows an increasing tendency, although the increments observed are small. Under dynamic loads of low magnitude and high frequency, when cracks have not yet developed in the cement matrix, the addition of carbon fiber increases the damping ratio of concrete. However, under the same conditions, it is reduced by the addition of steel fiber. This fact makes carbon fiber more effective than steel fiber for reducing vibrations in these situations. This might be useful in some applications such as slabs for industrial buildings or roads where machines or vehicles can transmit their vibrations through the structure to other machines, working people or near structures. Both carbon and steel fibers additions lead to slight decreases of the compressive strength of concrete. For the same volumetric fraction, this fact is more significant in the concrete types containing steel fibers. The static elastic properties present lower values in comparison with their dynamic counterparts. The variations observed in the static modulus of elasticity for concrete types containing fibers should not be considered significant. The temporal evolution of the static modulus of elasticity shows an increasing tendency for concrete types containing fibers. These results are coherent with the increase of the resonant frequencies.
References [1] Chen P-W, Fu X, Chung DDL. Microstructural and mechanical effects of latex, methylcellulose, and silica fume on carbon fiber reinforced cement. ACI Mater J 1997;94(2):147–55. [2] Chen P-W, Chung DDL. A comparative study of concretes reinforced with carbon, polyethylene, and steel fibers and their improvement by latex addition. ACI Mat J 1996;93(2):129–33. [3] Chen P-W, Chung DDL. Concrete reinforced with up to 0.2 vol% of short carbon fibres. Composites 1993;24(1):33–52. [4] Ivorra S, Garcés P, Catalá G, Andión LG, Zornoza E. Effect of silica fume particle size on mechanical properties of short carbon fiber reinforced concrete. Mater Des 2010;31(3):1553–8. [5] Eren Ö, Çelik T. Effect of silica fume and steel fibers on some properties of highstrength concrete. Constr Build Mater 1997;11(7–8):373–82. [6] Yan H, Sun W, Chen H. The effect of silica fume and steel fiber on the dynamic mechanical performance of high-strength concrete. Cem Concr Res 1999;29(3):423–6. _ S [7] Köksal F, Altun F, Yig˘it I, ß ahin Y. Combined effect of silica fume and steel fiber on the mechanical properties of high strength concretes. Constr Build Mater 2008;22(8):1874–80. [8] Katzensteiner BO. Use of steel fiber-reinforced concrete in seismic design. PhD thesis. University of British Columbia: Canada; 1994. [9] Garcés P, Fraile J, Vilaplana-Ortego E, Cazorla-Amorós D, Alcocel EG, Andión LG. Effect of carbon fibres on the mechanical properties and corrosion levels of reinforced portland cement mortars. Cem Concr Res 2005;35(2):324–31. [10] Garcés P, Andión LG, De la Varga I, Catalá G, Zornoza E. Corrosion of steel reinforcement in structural concrete with carbon material addition. Corros Sci 2007;49(6):2557–66. [11] Chung DDL. Multifunctional cement-based materials. Marcel Dekker, Inc.; 2003. [12] Chen P-W, Chung DDL. Concrete as a new strain/stress sensor. Composites Part B 1996;27B(1):11–23. [13] Detwiler RJ, Mehta PK. Chemical and physical effects of silica fume on the mechanical behaviour of concrete. ACI Mat J 1989;86(6):609–14. [14] Xie J, Elwi AE, MacGregor JG. Mechanical properties of three high-strength concretes containing silica fume. ACI Mat J 1995;92(2):135–43. [15] Khatri RP, Sirivivatnanon V, Gross W. Effect of different supplementary cementitious materials on mechanical properties of high performance concrete. Cem Concr Res 1995;25(1):209–20.
V.T. Giner et al. / Materials and Design 34 (2012) 332–339 [16] Iravani S. Mechanical properties of high-performance concrete. ACI Mat J 1996;93(5):416–25. [17] Mazloom M, Ramezanianpour AA, Brooks JJ. Effect of silica fume on mechanical properties of high-strength concrete. Cem Concr Compos 2004;26(4):347–57. [18] Aïtcin PC. High-performance concrete. E & FN Spon; 1998. [19] Bachmann H. Vibration problems in structures: practical guidelines. Birkhäuser; 1997. [20] Ivorra S, Pallares FJ, Adam JA. Dynamic behaviour of a modern bell tower–a case study. Eng Struct 2009;31(5):1085–92. [21] Hernández-Olivares F, Barluenga G, Bollati M, Witoszek B. Static and dynamic behaviour of recycled tyre rubber-filled concrete. Cem Concr Res 2002;32(10):1587–96. [22] Zheng L, Huo XS, Yuan Y. Experimental investigation on dynamic properties of rubberized concrete. Constr Build Mater 2008;22(5):939–47. [23] Wong WG, Fang P, Pan JK. Dynamic properties impact toughness and abrasiveness of polymer-modified pastes by using nondestructive tests. Cem Concr Res 2003;33(9):1371–4. [24] Orak S. Investigation of vibration damping on polymer concrete with polyester resin. Cem Concr Res 2000;30(2):171–4. [25] Nili M, Afroughsabet V. The effects of silica fume and polypropylene fibers on the impact resistance and mechanical properties of concrete. Constr Build Mater 2010;24(6):927–33. [26] Ross CA, Tedesco JW, Kuennen ST. Effects of strain rate on concrete strength. ACI Mat J 1995;92(1):37–45. [27] ASTM C 215. Standard test method for fundamental transverse, longitudinal, and torsional resonant frequencies of concrete specimens. American Society for Testing Materials; 2008. [28] Paz M, Leigh W. Structural dynamics: theory and computation. Kluwer Academic Publishers; 2004.
339
[29] Chung DDL. Materials for vibration damping. J Mat Sci 2001;36(24): 5733–7. [30] Giner VT, Ivorra S, Baeza FJ, Zornoza E, Ferrer B. Silica fume admixture effect on the dynamic properties of concrete. Constr Build Mater 2011;25(8):3272–7. [31] UNE-EN 197-1:2000. Cement–Part 1: composition. Specifications and conformity criteria for common cements. AENOR; 2000. [32] ASTM C 192/C 192M. Standard practice for making and curing concrete test specimens in the laboratory. American Society for Testing Materials; 2007. [33] ASTM C 39/C 39M. Standard test method for compressive strength of cylindrical concrete specimens. American Society for Testing Materials; 2005. [34] ASTM C 469. Standard test method for static modulus of elasticity and Poisson’s ratio of concrete in compression. American Society for Testing Materials; 2002. [35] ASTM C 78. Standard test method for flexural strength of concrete (using simple beam with third-point loading). American Society for Testing Materials; 2009. [36] Xu X, Hu Y. The properties of oxidated carbon fibers reinforced cement. Adv Mat Res 2011;163–167:1203–6. [37] Xu Z, Hao H, Li HN. Experimental study of dynamic compressive properties of fibre reinforced concrete material with different fibres. Mater Des 2011;33:42–55. [38] Amick CH. Concrete damping properties and their modification. PhD thesis. University of California, Berkeley; 2004. [39] UNE 66040:2003. Statistical interpretation of test results. Estimation of the mean. Confidence interval. AENOR;2003. [40] Park SJ, Seo MK, Shim HB, Rhee KY. Effect of different cross-section types on mechanical properties of carbon fibers-reinforced cement composites. Mat Sci Eng A 2004;366:348–55. [41] Wu Q. Experimental investigation of damping ratios of steel fiber reinforced concrete. PhD thesis. University of Nevada: Reno; 1995.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Effect of steel and carbon fiber additions on the dynamic properties of concrete containing silica fume V.T. Giner, F.J. Baeza, S. Ivorra ⇑, E. Zornoza, Ó. Galao Department of Construction Engineering, University of Alicante, Alicante, Spain
a r t i c l e
i n f o
Article history: Received 13 June 2011 Accepted 30 July 2011 Available online 3 August 2011 Keywords: A. Concrete F. Elastic behavior G. Non-destructive testing (NDT)
a b s t r a c t Fiber reinforced concretes (FRC) are technologically important due to their combination of good structural properties, durability and multifunctional properties. The behavior of a structure under dynamic actions is determined by its dynamic mechanical properties and its total overall damping. In this work the influence of the steel fiber (SF) and carbon fiber (CF) additions on the mechanical properties of concrete containing silica fume has been studied, focusing on its passive material damping ability (damping ratio) and dynamic properties. According to the obtained results, under dynamic loads of low magnitude and high frequency, CF additions are more effective than SF additions for reducing vibrations, as they increase the damping ratio of concrete. In all cases, the dynamic elastic properties of concrete present higher values than their static counterparts. Both CF and SF additions lead to slight decreases of the compressive strength of concrete. For the same volumetric fraction, this fact is more significant in the concrete types containing SF. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Concretes and, in general, cement-based composite materials reinforced with randomly distributed and uniformly dispersed discontinuous fibers in the cement matrix, are technologically important due to their combination of good structural properties [1–8], durability [9,10] and multifunctional properties [11,12]. Progress made in the fields of engineering and architecture has led to an increasing need of concretes with outstanding properties, such as high-performance concretes (HPC). During the study and development of this kind of concretes, the combination of silica fume with steel fibers (SF) or carbon fibers (CF) has become a common practice. The addition of silica fume to concrete mixes leads to a significant increase of the compressive strength, among other properties, due to its filler effect and pozzolanic activity [13–18]. However, it also causes concrete to have a more brittle structure. In order to counteract this negative effect, fibers are added to concrete [5,7]. The main improvements provided by the addition of fibers are higher flexural toughness and strength, higher tensile ductility and strength, and lower drying shrinkage [1–8]. Furthermore, the effectiveness of short fibers highly depends on their degree of dispersion in the mix. In this respect, it is known that silica fume acts like a dispersant of fibers, contributing to optimize their distribution in the concrete mass [1,3].
⇑ Corresponding author. E-mail address: [email protected] (S. Ivorra). 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.07.068
The enhancement produced by the addition of fibers to concrete results in more ductile structures with higher energy absorption capacity, which are very desirable properties from the point of view of structures under dynamic actions. The vibrations produced by dynamic actions usually cause service problems as they can reduce comfort up to unacceptable levels. But, in some extreme cases, they can cause real safety problems, compromising the stability of the whole structure [18–20]. This fact has motivated, during the last decades, new research lines focused on the study of the dynamic behavior of new materials and composite concretes [21–25]. The behavior of concrete under dynamic actions is determined by its dynamic properties (such as dynamic modulus of elasticity, modulus of rigidity, Poisson’s ratio, compressive strength or strain limits), which present different values compared with their static counterparts. These variations are normally expressed as a function of the strain or stress rates [19,26]. The characterization of these dynamic properties can be experimentally achieved, from concrete specimens, according to the standard test method ASTM C 215 [27]. The dynamic performance of a structure is also highly conditioned by its damping ability. In a vibrating structure, damping is understood as the dissipation of the mechanical energy, generally by converting it into thermal energy [19]. In order to describe the damping ability of a structure or material, the more often used parameter is the damping ratio n. This non-dimensional parameter can be defined as follows [19,28]:
n¼
c c c ¼ ¼ pffiffiffiffiffiffiffi ccr 2mx 2 km
ð1Þ
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ξ=0.025
ξ=0.050 ξ=0.100 ξ=0.150 ξ=0.200 ξ=0.250 ξ=0.500 ξ=1.000
Fig. 1. Dynamic magnification factor versus the frequency ratio for several values of the damping ratio.
where c is the damping coefficient that represents viscous or linear damping; ccr is the critical damping coefficient; m is the mass of the system; x is the natural frequency of the undamped system; and k is the stiffness of the system. The main role that the damping ratio plays on the dynamic behavior of a structure can be clearly noticed in Fig. 1, where the Dynamic Magnification Factor (DMF) is represented for different values of the damping ratio. This non-dimensional constant describes the ratio between the displacement under a dynamic (harmonic) load and the displacement under a static load of the same magnitude (amplitude). Its value is given by the following expression [19,28]:
1 DMF ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ð1 r Þ2 þ ð2rnÞ2
ð2Þ
where r ¼ x x is the frequency ratio (between the frequency of the excitation and the natural frequency of the undamped system). In real structures, the value of the damping coefficient is much smaller than the critical damping coefficient, usually ranging from 2 to 10% of the critical damping value, and consequently the damping ratio is also small (n < 0.1). Therefore, from the structural dynamics theory, it can be deduced that in practice the damped natural frequency for a structure with such a small damping ratio may be taken to be equal to the undamped natural frequency [28]. As can be observed in Fig. 1, for small values of the damping ratio the peak amplitude occurs at a frequency ratio very close to 1; that is, the damping ratio only affects the vibration behavior significantly near resonance (r = 1). Theoretically, the DMF reaches its maximum value at the resonance situation, and then it is inversely proportional to the damping ratio [19,28]:
DMF ðr ¼ 1Þ ¼
1 2n
ð3Þ
However, the total overall damping of a structure is a complex concept that depends on several different factors, and it is considered a sum of the contributions of the damping of the bare structure, the damping by the non-structural elements and the damping by the energy radiation to the soil. The damping of the bare structure contributes to the energy dissipation by the material damping and the damping at the bearings and joints. In most cases the material damping is predominant [19]. In concrete, defects such as dislocations, phase boundaries, grain boundaries and various interfaces contribute to the material damping, since defects may move slightly and surfaces may slip slightly with respect to
one another during vibration, thereby dissipating energy. Thus, the microstructure of concrete greatly affects its damping capacity [29]. Therefore, as the addition of fibers to the concrete mix modifies its microstructure by introducing new friction surfaces, it could be an interesting technique to improve the passive material damping of concrete structures. In many applications such as slabs for industrial buildings or roads where machines or vehicles can transmit their vibrations through the structure to other machines, working people or near structures, the increase of the material damping could reduce the transmitted vibrations. In this sense, the published literature about the damping ability of the fiber reinforced concrete (FRC) incorporating silica fume is rather short. For this reason, there is a need for studying the influence of the SF or CF additions on the material damping of concrete containing silica fume, which justifies the present paper. The measurement of the damping properties of structures can be mainly achieved by two methods, which are the decay curve method and the bandwidth method. In most practical cases with real structures, the most recommended and used method is the first one. This is due to the fact that non-linear behavior of structures makes it very difficult to evaluate the resonance curve. Hence, by the bandwidth method, any measuring error will severely affect the evaluation of damping [8,19]. Nevertheless, at a material design level where the structural complexity is reduced to a concrete specimen, there are standard test methods [27] to determine the resonance curve in an accurate way. Thus it is possible to use the resonance curve of concrete specimens, obtained according to these standard test methods, to evaluate the material damping by the bandwidth method with an adequate precision, as reported in previous work on concretes with increasing silica fume admixtures [30]. The main objective of the present work is to study the effect of the SF or CF additions on the mechanical properties of concrete containing silica fume, focusing on its passive material damping ability (damping ratio) and dynamic properties.
2. Experimental procedure 2.1. Materials and specimen preparation The Portland cement type used was CEM I 52.5 R, designated in accordance with UNE-EN 197-1:2000 [31]. The brands of the short carbon fibers and the hooked-end steel fibers used were Zoltek PANEX 35 and ArcelorMittal HE 55/35, respectively. Their properties, supplied by vendors, are given in Tables 1 and 2. Both CF and SF were proportioned by total volume fraction. A superplasticizer based on melamine formaldehyde was used as an admixture in 0.5–2% proportions of cement mass. Undensified silica fume (USF), supplied by Grupo FerroAtlántica SL, was used in addition to the admixture of all concrete types, with a constant dosage of 10% with respect to cement mass [30]. Three different types of silica fume, besides USF, semidensified
Table 1 Properties of carbon fibers. Type
Zoltek PANEX 35
Diameter (D) Length (L) Aspect ratio (D/L) Specific surface area Carbon content Tensile strength Elastic modulus Density
7.2 lm 3.5 mm 486 5.56 05 m2/m3 95% 3800 MPa 242 GPa 1.81 g/cm3
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V.T. Giner et al. / Materials and Design 34 (2012) 332–339 Table 2 Properties of hooked-end steel fibers. Type
ArcelorMittal HE 55/35
Diameter (D) Length (L) Aspect ratio (D/L) Specific surface area Tensile strength Density
0.55 mm 35 mm 64 7.27 03 m2/m3 1200 MPa 7.85 g/cm3
Table 3 Chemical composition and particle size of the undensified silica fume used in the tests. Parameters
Average%
Moisture Loss on ignition Cl SO3 SiO2 C Al2O3 Fe2O3 CaO MgO K2O Na2O Alkalines like Na2O Free silicon
0.60 3.08 0.010 0.11 94.11 2.85 0.266 0.118 0.407 0.343 0.563 0.204 0.57 0.12
Cumulative volume (%) under 1 lm Average particle diameter (lm)
90.78 0.67
Table 4 Mixture proportioning of the reference concrete specimens R. kg/m3
R
Cement
Water
Sand
Gravel
Silica fume
330
165
630
1170
33
Table 5 Mix types and proportions of each concrete type. Type designation Fig. 2. Particle size distribution of the three silica fume types previously tested. USF undensified silica fume, SDSF semidensified silica fume and DSF densified silica fume.
silica fume (SDSF) and densified silica fume (DSF) were analyzed. The USF was finally selected according to the results of previous research where the effect of silica fume particle size on mechanical properties of concrete was studied [4]. The particle size of the different silica fume types was analyzed by using a Beckman Coulter LS 230 laser diffraction particle size analyzer, obtaining the distributions shown in Fig. 2. Also SEM images were taken of each type as shown in Fig. 3. The chemical composition and particle size of the USF used are shown in Table 3. Reference concrete specimens, only with USF addition, were prepared according to the mix proportioning shown in Table 4. The reason for making the reference concrete specimens was to evaluate the effect of the different CF and SF additions. Based on the reference concrete proportioning, three other mix types with different dosage of CF and SF were fabricated as shown in Table 5. Three prismatic and three cylindrical specimens of each concrete type were casted. Prismatic specimens of 100 100 400 mm dimensions were used for flexural strength and dynamic
R R + 0.5CF R + 0.5SF R + 1.0SF
Total volume% CF
SF
– 0.5 – –
– – 0.5 1.0
CF = carbon fiber. SF = steel fiber.
properties tests. Cylindrical specimens of £150 300 mm dimensions were used for compressive strength and elastic properties tests. The complete process of mixing, casting, consolidating and curing concrete specimens was carried out according to the standard practice ASTM C 192/C 192 M [32]. Both prismatic and cylindrical specimens were cured in moist room (>95% RH, 23 °C) until tested. 2.2. Testing 2.2.1. Static properties tests At 28 days, two cylindrical specimens of every concrete type were tested to determine the compressive strength according to
Fig. 3. SEM images taken on different types of silica fume tested: undensified silica fume (left), semidensified silica fume (center), densified silica fume (right).
V.T. Giner et al. / Materials and Design 34 (2012) 332–339
(a)
(b)
335
(c)
Data Logger 4 3
1 2
Fig. 4. Gage distribution on the performance to obtain mechanical constant of concrete specimens (elastic modulus, modulus of rigidity and Poisson’s ratio). (a) Stress application and gage configuration. The sample cross section shows longitudinal strain gages e1 and e3, and transverse strain gages e2 and e4. (b) Stress cycles during elastic modulus tests. (c) Stress–strain curve corresponding to elastic modulus tests.
the standard test method ASTM C 39/C 39 M [33]. At the ages of 28 and 60 days, one cylindrical specimen of each concrete type was tested to evaluate the static modulus of elasticity, modulus of rigidity and Poisson’s ratio, as shown in Fig. 4. For this purpose, an HBM Spider 8–600 Hz equipment was used jointly with HBM strain gauges (120 O, k = 2.10) and Catman v.5.0 analysis software. Longitudinal and transverse strain values were obtained for each loading cycle up to a maximum value when the applied load is equal to 40% of the sample ultimate load, as specified by the standard test method ASTM C 469 [34]. A press machine of 3500 kN capacity was used for both compressive strength and elastic properties tests. Two prismatic specimens of each concrete type were tested at 28 days to determine the flexural strength according to the stan-
(A) Longitudinal
dard test method ASTM C 78 [35]. A hydraulic press machine of 1500 kN capacity was used for these tests.
2.2.2. Dynamic properties tests Three prismatic specimens of each concrete type were tested to evaluate their fundamental longitudinal, transverse and torsional resonant frequencies, following the specifications of the standard test method ASTM C 215 [27]. An Erudite MkIV electronic resonant frequency test system, supplied by CNS Farnell, was used. The configuration of the device corresponding to each type of measure can be seen in Fig. 5. The frequency sweep range used to determine the fundamental resonant frequency of concrete specimens, for a particular mode of vibration, mainly depends on their shape and dimensions. In this case, the frequency sweep ranges used in each mode of vibration were 4000–6000 Hz, 1000–3000 Hz and 2000– 4000 Hz, for the longitudinal, transverse and torsional modes, respectively. The experimental values of the dynamic modulus of elasticity, modulus of rigidity and Poisson’s ratio were calculated, from the different resonant frequencies, according to the formulation included in the standard test method [27]. The damping ratio was determined by the bandwidth method, using the resonance curves obtained in the longitudinal vibration mode.
(B) Tranverse
(C) Torsional
Shaker
Accelerometer
Fig. 5. Experimental performance to obtain dynamic characteristics of the concrete types tested. Longitudinal configuration (up), transverse measure (center), and torsional (down).
Fig. 6. Density of concretes for £150 300 mm cylindrical specimens and 100 100 400 mm prismatic specimens at 28 days.
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Table 6 Twenty-eight day static mechanical properties of the different concrete mixes. Type designation
Compressive strength (MPa ± SD)
Flexural strength (MPa ± SD)
Static modulus of elasticity (GPa ± SD)
Static modulus of rigidity (GPa ± SD)
Static Poisson’s ratio (m ± SD)
R R + 0.5CF R + 0.5SF R + 1.0SF
48.51 ± 0.71 47.85 ± 2.54 46.80 ± 3.39 46.44 ± 1.46
7.48 ± 0.07 7.65 ± 0.83 7.15 ± 0.22 7.71 ± 0.08
35.26 ± 0.05 36.16 ± 0.18 33.73 ± 0.10 35.46 ± 0.07
13.96 ± 0.02 15.34 ± 0.08 13.68 ± 0.07 14.69 ± 0.02
0.26 ± 0.00 0.18 ± 0.00 0.23 ± 0.00 0.21 ± 0.00
SD = standard deviation.
These tests were repeated ten times for every specimen, mode of vibration and concrete type at the ages of 28 and 60 days.
3. Results and discussion 3.1. Static properties 3.1.1. Compressive and flexural strength Previously to any mechanical test the density of each specimen was measured according to the standard test method indicated on ASTM C 39/C 39 M [33]. Fig. 6 shows the mean value of the density of the specimens tested, both prismatic and cylindrical ones. The effect of each type of fiber is totally different. For a CF addition the density decreases, probably due to a greater porosity caused by the workability loss of fresh concrete and harder vibration pro-
Fig. 7. Compressive strength of £150 300 mm cylindrical specimens at 28 days.
Fig. 8. Flexural strength of 100 100 400 mm prismatic specimens at 28 days.
cess [4,9,36]. However, SF tends to increase the composite density, as the specific gravity of SF is 3 times the concrete’s one (Table 2). The results of the 28 day compressive strength tests on the cylindrical specimens are shown in Table 6 and Fig. 7. According to these results, both CF and SF additions lead to slight decreases of the compressive strength compared with the reference concrete. For the same volumetric fraction, this fact is more significant in the concrete type with SF, since the decreases obtained are over the 3% and the 1% for types R + 0.5SF and R + 0.5CF, respectively. The reduction of the compressive strength obtained for type R + 1.0SF is over the 4% of the reference value. These results are consistent with the previous published literature [3,9], where the reduction of the compressive strength is explained by the increase of porosity and air content that is produced by the addition of fibers. Table 6 includes the test results of the 28 day flexural strength of prismatic specimens, which are also represented in Fig. 8. Although concrete types R + 0.5CF and R + 1.0SF show slight increments of the flexural strength, ranging between the 2% and 3% of the reference value, they should not be regarded as significant because they are small and do not follow a well-defined pattern. It is worth noting that the reported results refer to the first-crack strength, and under no circumstances the present work involved the determination of the flexural post-cracking strength and toughness. 3.1.2. Static elastic properties The values of the static elastic properties obtained from the cylindrical specimens, at 28 days, are shown in Table 6. These properties present lower values in comparison with their dynamic counterparts. The differences between the static and dynamic modulus of elasticity range between 12% and 18% for all concrete types. These results are similar than others reported on scientific literature [37]. The variations observed in the static modulus of elasticity for concrete types containing fibers do not exceed the 4% of the reference value, whether they are increments or
Fig. 9. Static modulus of elasticity of £150 300 mm cylindrical specimens at 28 and 60 days.
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reductions, and do not follow a clear tendency. Therefore, these variations should not be considered significant. Fig. 9 shows the temporal evolution of the static modulus of elasticity at the ages of 28 and 60 days. During this period of time, an increasing tendency can be noticed for concrete types containing fibers. These results are coherent with the increase of the resonant frequencies, as they depend on stiffness, which is directly proportional to the elastic modulus [30]. 3.2. Dynamic properties
Table 8 Twenty-eight day dynamic properties of different concrete prismatic specimens. Type designation
Dynamic modulus of elasticity (GPa ± SD)
Dynamic modulus of rigidity (GPa ± SD)
Dynamic Poisson’s ratio (m ± SD)
Damping ratio (% ± SD)
R R + 0.5CF R + 0.5SF R + 1.0SF
40.85 ± 0.54 42.51 ± 0.19 41.43 ± 0.60 40.52 ± 0.65
16.29 ± 0.35 16.52 ± 0.15 16.13 ± 0.25 15.58 ± 0.30
0.28 ± 0.01 0.29 ± 0.01 0.26 ± 0.00 0.30 ± 0.01
0.63 ± 0.04 0.67 ± 0.08 0.63 ± 0.07 0.58 ± 0.06
SD = standard deviation.
3.2.1. Resonant frequencies Table 7 shows the fundamental resonant frequencies of the prismatic specimens for each mode of vibration at 28 days. These results show a similar pattern in the three modes of vibration [30,38]. The highest resonant frequencies are obtained for type R + 0.5CF, followed by the reference concrete, and then tending to slightly decrease when the amount of SF increases. In any case, the variations in resonant frequencies produced by the additions of SF or CF are lower than 4%, compared with the reference concrete. The temporal evolution of the longitudinal resonant frequencies, at the ages of 28 and 60 days, is shown in Fig. 10. In this period of time, the longitudinal resonant frequency of the reference concrete remains constant, while it tends to slightly increase for the concrete types containing fibers. These small increments are lower than 1% of the 28 day values. The increase of the resonant frequencies is related to the temporal evolution of other mechanical properties, and especially to the elastic modulus (directly proportional to stiffness) [30]. 3.2.2. Dynamic elastic properties The values of the dynamic elastic properties obtained at 28 days are included in Table 8. These properties have been calculated from Table 7 Twenty-eight day resonant frequencies of concrete prismatic specimens for the three different modes of vibration. Type designation
Longitudinal resonant frequency (Hz ± SD)
Transverse resonant frequency (Hz ± SD)
Torsional resonant frequency (Hz ± SD)
R R + 0.5CF R + 0.5SF R + 1.0SF
5239 ± 25 5294 ± 8 5198 ± 19 5123 ± 24
2245 ± 13 2283 ± 13 2234 ± 9 2188 ± 26
3010 ± 25 3035 ± 4 2982 ± 9 2921 ± 17
SD = standard deviation.
Fig. 10. Longitudinal resonant frequency of 100 100 400 mm prismatic specimens at 28 and 60 days.
Fig. 11. Dynamic modulus of elasticity of 100 100 400 mm prismatic specimens at 28 and 60 days.
the different resonant frequencies and, consequently, they follow a similar trend. However, in the specific case of the dynamic modulus of elasticity some differences can be observed, as the value obtained for type R + 0.5SF is slightly higher than the reference value, unlike the rest of dynamic properties where R + 0.5SF presents lower values than the reference concrete. As a result, types R + 0.5CF and R + 0.5SF show increments of about 4% and 2%, respectively, compared with the reference value. On the other hand, the dynamic modulus of elasticity of R + 1.0SF is lower than the reference one, although this difference is under the 1%. The temporal evolution of the dynamic modulus of elasticity, showed in Fig. 11, is completely related to the one showed in Fig. 10 for the longitudinal resonant frequency. Thus, the dynamic modulus of elasticity slightly increases from 28 to 60 days [30]. These increments are under the 2% of the 28 day value in all cases.
3.2.3. Damping ratio The damping ratio values obtained for every concrete type are included in Table 8, and represented in Fig. 12. In the graph two horizontal lines have been drawn meaning the confidence interval for the mean of the reference concrete tests, with a confidence level of 95%, assuming a normal (Gaussian) statistical distribution. This confidence interval was determined according to standard UNE 66,040:2003 [39]. The reason for establishing this interval is to evaluate whether the presence of SF or CF means a significant change in concrete damping ratio or the experimental differences are due to the random nature of the tests results. As can be observed in Fig. 12, the damping ratio of the concrete type R + 0.5CF is slightly higher than the one of the reference concrete. The increment produced by the addition of CF is over the 6% of the reference value. On the other hand, the damping ratio of type R + 0.5SF does not present any significant variation with respect to
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Fig. 12. Damping ratio of 100 100 400 mm prismatic specimens at 28 days. Acceptability limits represent the confidence interval for the mean of the reference concrete tests, with a confidence level of 95%, assuming a normal (Gaussian) statistical distribution.
the reference value, but for type R + 1.0SF a decrease of the 8% can be noticed. This difference between the additions of CF and SF may be due to their different specific surface areas, i.e. the total surface area of a single fiber per unit of volume is 5.56 105 m2/m3 for CF, while for SF only reaches 7.27 103 m2/m3. The volume of a single steel fiber is about 58,000 times bigger than the volume of a single carbon fiber. As a result, for the same volumetric fraction, the number of carbon fibers added to the concrete mixture is much higher than the corresponding number of steel fibers. Consequently, the addition of CF introduces many more interfaces and transition zones in the cement matrix, which are responsible for dissipating energy by internal friction during vibration, thereby increasing the damping ratio of concrete. A similar pattern was reported by Park [40] for the mechanical properties of carbon fiber reinforced cement pastes with different cross sections. The fact that SF additions can reduce the damping ratio of concrete, which is related to its energy absorption capacity, may contrast with other previous researches [6–8]. However, this may be due to the magnitude and frequency of the dynamic loads applied in the tests. In prior work made by Wu [41] it is concluded that peak vibrations amplitudes between 1.0 and 1.5 g are needed to detect an increase in damping ratio of steel fiber reinforced concretes. In the present work, the intensity of the excitations is extremely low and their frequencies are high. In this situation, cracks are not developed. Nevertheless, other previous investigations studied the dynamic performance of steel fiber reinforced concrete (SFRC) structures by applying dynamic loads up to failure. In these works it was proved that the main contribution of SF to concrete occurs after matrix cracking. The results indicate that SF effectively restrained the initiation and propagation of cracks during the failure of SFRC structures, mitigated the stress concentrations at the tips of cracks, and delayed the damage process under impact and fatigue. Therefore, the addition of SF to concrete mixtures is useful if a large amount of energy absorption capacity is required to reduce brittle failure [6,7], hence improving the ductility of the structure. 4. Conclusions The object of the present investigation is to study the effect of steel or carbon fibers on the dynamic properties of concrete containing silica fume, focusing on its damping ratio and dynamic modulus of elasticity. Specimens of different concrete types, with variations in the type of fiber and proportioning, have been
fabricated and tested to determine the experimental values of their dynamic and static properties. After discussing the results, the following conclusions can be drawn: The addition of carbon fiber slightly increases the resonant frequencies of concrete specimens in the three modes of vibration. On the contrary, they tend to decrease when the amount of steel fiber increases. The dynamic elastic properties follow a similar trend, since they are increased by the addition of carbon fiber and slightly reduced, in most cases, by the addition of steel fiber. In all cases, the temporal evolution of both resonant frequencies and dynamic elastic properties follows an increasing tendency, although the increments observed are small. Under dynamic loads of low magnitude and high frequency, when cracks have not yet developed in the cement matrix, the addition of carbon fiber increases the damping ratio of concrete. However, under the same conditions, it is reduced by the addition of steel fiber. This fact makes carbon fiber more effective than steel fiber for reducing vibrations in these situations. This might be useful in some applications such as slabs for industrial buildings or roads where machines or vehicles can transmit their vibrations through the structure to other machines, working people or near structures. Both carbon and steel fibers additions lead to slight decreases of the compressive strength of concrete. For the same volumetric fraction, this fact is more significant in the concrete types containing steel fibers. The static elastic properties present lower values in comparison with their dynamic counterparts. The variations observed in the static modulus of elasticity for concrete types containing fibers should not be considered significant. The temporal evolution of the static modulus of elasticity shows an increasing tendency for concrete types containing fibers. These results are coherent with the increase of the resonant frequencies.
References [1] Chen P-W, Fu X, Chung DDL. Microstructural and mechanical effects of latex, methylcellulose, and silica fume on carbon fiber reinforced cement. ACI Mater J 1997;94(2):147–55. [2] Chen P-W, Chung DDL. A comparative study of concretes reinforced with carbon, polyethylene, and steel fibers and their improvement by latex addition. ACI Mat J 1996;93(2):129–33. [3] Chen P-W, Chung DDL. Concrete reinforced with up to 0.2 vol% of short carbon fibres. Composites 1993;24(1):33–52. [4] Ivorra S, Garcés P, Catalá G, Andión LG, Zornoza E. Effect of silica fume particle size on mechanical properties of short carbon fiber reinforced concrete. Mater Des 2010;31(3):1553–8. [5] Eren Ö, Çelik T. Effect of silica fume and steel fibers on some properties of highstrength concrete. Constr Build Mater 1997;11(7–8):373–82. [6] Yan H, Sun W, Chen H. The effect of silica fume and steel fiber on the dynamic mechanical performance of high-strength concrete. Cem Concr Res 1999;29(3):423–6. _ S [7] Köksal F, Altun F, Yig˘it I, ß ahin Y. Combined effect of silica fume and steel fiber on the mechanical properties of high strength concretes. Constr Build Mater 2008;22(8):1874–80. [8] Katzensteiner BO. Use of steel fiber-reinforced concrete in seismic design. PhD thesis. University of British Columbia: Canada; 1994. [9] Garcés P, Fraile J, Vilaplana-Ortego E, Cazorla-Amorós D, Alcocel EG, Andión LG. Effect of carbon fibres on the mechanical properties and corrosion levels of reinforced portland cement mortars. Cem Concr Res 2005;35(2):324–31. [10] Garcés P, Andión LG, De la Varga I, Catalá G, Zornoza E. Corrosion of steel reinforcement in structural concrete with carbon material addition. Corros Sci 2007;49(6):2557–66. [11] Chung DDL. Multifunctional cement-based materials. Marcel Dekker, Inc.; 2003. [12] Chen P-W, Chung DDL. Concrete as a new strain/stress sensor. Composites Part B 1996;27B(1):11–23. [13] Detwiler RJ, Mehta PK. Chemical and physical effects of silica fume on the mechanical behaviour of concrete. ACI Mat J 1989;86(6):609–14. [14] Xie J, Elwi AE, MacGregor JG. Mechanical properties of three high-strength concretes containing silica fume. ACI Mat J 1995;92(2):135–43. [15] Khatri RP, Sirivivatnanon V, Gross W. Effect of different supplementary cementitious materials on mechanical properties of high performance concrete. Cem Concr Res 1995;25(1):209–20.
V.T. Giner et al. / Materials and Design 34 (2012) 332–339 [16] Iravani S. Mechanical properties of high-performance concrete. ACI Mat J 1996;93(5):416–25. [17] Mazloom M, Ramezanianpour AA, Brooks JJ. Effect of silica fume on mechanical properties of high-strength concrete. Cem Concr Compos 2004;26(4):347–57. [18] Aïtcin PC. High-performance concrete. E & FN Spon; 1998. [19] Bachmann H. Vibration problems in structures: practical guidelines. Birkhäuser; 1997. [20] Ivorra S, Pallares FJ, Adam JA. Dynamic behaviour of a modern bell tower–a case study. Eng Struct 2009;31(5):1085–92. [21] Hernández-Olivares F, Barluenga G, Bollati M, Witoszek B. Static and dynamic behaviour of recycled tyre rubber-filled concrete. Cem Concr Res 2002;32(10):1587–96. [22] Zheng L, Huo XS, Yuan Y. Experimental investigation on dynamic properties of rubberized concrete. Constr Build Mater 2008;22(5):939–47. [23] Wong WG, Fang P, Pan JK. Dynamic properties impact toughness and abrasiveness of polymer-modified pastes by using nondestructive tests. Cem Concr Res 2003;33(9):1371–4. [24] Orak S. Investigation of vibration damping on polymer concrete with polyester resin. Cem Concr Res 2000;30(2):171–4. [25] Nili M, Afroughsabet V. The effects of silica fume and polypropylene fibers on the impact resistance and mechanical properties of concrete. Constr Build Mater 2010;24(6):927–33. [26] Ross CA, Tedesco JW, Kuennen ST. Effects of strain rate on concrete strength. ACI Mat J 1995;92(1):37–45. [27] ASTM C 215. Standard test method for fundamental transverse, longitudinal, and torsional resonant frequencies of concrete specimens. American Society for Testing Materials; 2008. [28] Paz M, Leigh W. Structural dynamics: theory and computation. Kluwer Academic Publishers; 2004.
339
[29] Chung DDL. Materials for vibration damping. J Mat Sci 2001;36(24): 5733–7. [30] Giner VT, Ivorra S, Baeza FJ, Zornoza E, Ferrer B. Silica fume admixture effect on the dynamic properties of concrete. Constr Build Mater 2011;25(8):3272–7. [31] UNE-EN 197-1:2000. Cement–Part 1: composition. Specifications and conformity criteria for common cements. AENOR; 2000. [32] ASTM C 192/C 192M. Standard practice for making and curing concrete test specimens in the laboratory. American Society for Testing Materials; 2007. [33] ASTM C 39/C 39M. Standard test method for compressive strength of cylindrical concrete specimens. American Society for Testing Materials; 2005. [34] ASTM C 469. Standard test method for static modulus of elasticity and Poisson’s ratio of concrete in compression. American Society for Testing Materials; 2002. [35] ASTM C 78. Standard test method for flexural strength of concrete (using simple beam with third-point loading). American Society for Testing Materials; 2009. [36] Xu X, Hu Y. The properties of oxidated carbon fibers reinforced cement. Adv Mat Res 2011;163–167:1203–6. [37] Xu Z, Hao H, Li HN. Experimental study of dynamic compressive properties of fibre reinforced concrete material with different fibres. Mater Des 2011;33:42–55. [38] Amick CH. Concrete damping properties and their modification. PhD thesis. University of California, Berkeley; 2004. [39] UNE 66040:2003. Statistical interpretation of test results. Estimation of the mean. Confidence interval. AENOR;2003. [40] Park SJ, Seo MK, Shim HB, Rhee KY. Effect of different cross-section types on mechanical properties of carbon fibers-reinforced cement composites. Mat Sci Eng A 2004;366:348–55. [41] Wu Q. Experimental investigation of damping ratios of steel fiber reinforced concrete. PhD thesis. University of Nevada: Reno; 1995.