- Email: [email protected]
Measurement of crack length sensitivity and strain gage factor of carbon fiber reinforced cement matrix composites
Measurement 74 (2015) 21–30
Contents lists available at ScienceDirect
Measurement journal homepage: www.elsevier.com/locate/measurement
Measurement of crack length sensitivity and strain gage factor of carbon fiber reinforced cement matrix composites Egemen Teomete ⇑ Dokuz Eylul University, Civil Engineering Department, Kaynaklar, Buca, Izmir, Turkey
a r t i c l e
i n f o
Article history: Received 17 March 2015 Received in revised form 26 June 2015 Accepted 13 July 2015 Available online 23 July 2015 Keywords: Strain Crack length Self sensing Smart materials Cement
a b s t r a c t Five different carbon fiber reinforced cement matrix composites with fiber length of 13 mm were designed having different fiber volume fractions. From each mix, six samples of 5 cm cubes and three 4 4 16 cm prism samples were cast. Compression, split tensile and notched bending tests were applied to three samples from each mix with simultaneous electrical resistance measurements. Strong correlations between strain and electrical resistance change were reported. At percolation threshold, the strain caused shift of the system from post percolation to pre-percolation, changing the electrical resistance dramatically; causing highest gage factor. The microstructure controlled mechanisms were enlightened. The split tensile test and notched bending test with simultaneous electrical resistance, strain and crack length measurements have been applied as novel methods. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The performances of the structures are endangered by earthquakes, material degradations and other environmental effects. In USA concrete infrastructures have materials deteriorations and 30% of the bridges were found to be structurally not reliable [1]. Structural health monitoring is crucial to protect the people and also for asset planning. Commercial metal strain gages measure the strain of a point while they have low durability and short life time. Moreover, a vast number of them have to be used to monitor a structure which increases the cost [2]. Fiber optic Bragg grating sensors were developed for structural health monitoring [3]. This study is a contribution of developing self sensing smart materials and structures for the construction industry. Han et al. investigated the piezoresistivity of nickel powder reinforced cement composite under compression [4]. ⇑ Tel.: +90 232 301 7060; fax: +90 232 453 1192. E-mail address: [email protected] http://dx.doi.org/10.1016/j.measurement.2015.07.021 0263-2241/Ó 2015 Elsevier Ltd. All rights reserved.
Chung studied self sensing as well as mechanical and electrical properties of carbon fiber reinforced polymer and cement matrix composites [5–11]. Carbon fiber inclusion in the cement composite decreases electrical resistance. The application of load affects electrical resistance [12–16]. In this study, the relationships between compressive, tensile strain and crack length with electrical resistance change were investigated for carbon fiber reinforced cement based composites using compression; split tensile and notched bending tests. Split tensile test and notched bending tests have been used for the first time for cement based smart materials research in this project. Two and four electrode methods have been used to monitor the electrical resistance of the cement based composites. Current supply and voltage measurement were conducted using the same electrodes in two electrode method while these were achieved using two different electrode pairs in four electrode method. The measurements were affected by the sample cross section and the distance between electrodes in two electrode method while measurements were not affected in four electrode
22
E. Teomete / Measurement 74 (2015) 21–30
method [17,18]. Four electrode method was used in compression, split tensile and notched bending tests in this study. Measurement of electrical resistance was conducted using perimetral and embedded electrode configurations. Han et al. [19] proposed the use of embedded mesh electrodes for cement composites. A conductive plate or mesh is inserted in the material in embedded electrode configuration while a conductive wire or paint was attached to the perimeter in perimetral electrode method [1,19–22]. In this study, copper wire mesh was used in embedded electrode configuration by inserting in the material. While damage affects electrical resistance of carbon fiber reinforced cement based composites, simultaneous measurement of crack length with respect to electrical resistance has not been conducted [11]. Simultaneous measurement of crack length and electrical resistance has been achieved in this work. There have been various studies on smart cement based materials. Vehicle detection sensors were developed by nickel particle inclusions in the cement matrix and tested [23]. In order to investigate the nonlinear current–voltage relationship of carbon fiber reinforced cement composites, tunneling effect theory and Ohm’s law were used [24]. Piezo-electric properties of carbon nano tube reinforced cement based composites were investigated [25]. Dielectric, ferroelectric and piezoelectric properties of 0– 3 barium titanate–portland cement composite was investigated [26]. Strain sensing of carbon fiber reinforced geopolymer concrete was investigated by compression test and bending test [27]. The effect of low frequency mechanical load on cement matrix 2–2 piezoelectric composite was investigated. A linear relation between the piezoelectric coefficients and frequency was found while there was no relation between the magnitude of the applied load and piezoelectric coefficients [28]. Electrical impedance spectra of short conductive fiber-reinforced composites was studied and numerical simulations were verified by experimental results [29]. The intrinsic conductivities of short conductive fibers in cement matrix composites were determined by DC conductivity and impedance spectroscopy measurements. The fiber aspect ratio in randomly distributed fiber composites can be determined by conductivity versus aspect ratio relation [30]. Direct notched tension test was used to correlate DC and AC electrical properties with mechanical properties. The relations between crack bridging, fiber volume fraction and crack propagation with the electrical properties were reported [31]. Electrical impedance measurements were used to investigate the effects of debonding and pullout of single steel fiber from cement matrix and it was reported that electrical impedance measurements were sensitive to cracking [32]. Electrical impedance spectra was used to investigate the effects of fiber aspect ratio, fiber volume fraction, fiber orientation and fiber shape on the spectra and relations between microstructure property – impedance spectra were reported [33]. AC impedance spectra was used to investigate the fiber dispersion issues of orientation, segregation and clumping in fresh cement matrix composites [34]. It was reported that the interfacial transition zone did not affect the global
electrical conductivity while there were differences between the conductivities of transition zone and matrix pastes. When hydration was between 0.5 and 0.8, it had considerable effect on the difference [35]. Finite element method was used to model the coupled problem of propagation of moisture and electrical resistance. One electrical measurement and properties of saturated and unsaturated concrete were used to detect the position of moisture front in concrete [36]. Overlay current in a conductive concrete snow melting system was studied and an equation for overlay current was determined and calibrated with experimental results [37]. Han et al. [38], worked on multi walled carbon nanotube cement based vehicle detection sensors under cyclic compression and impulsive loads and reported that the sensors were sensitive for vehicle detection. Effect of moisture content on multi walled carbon nanotube (MWNT) cement composites’ piezoresistive sensitivity was investigated and the field emission effect of MWNT was used to enlighten the mechanism between moisture content and piezoresistive sensitivity [39]. Han et al. [40] presented a review of self sensing concrete and structures. In this study, five different carbon fiber reinforced cement matrix composites were designed with a fiber length of 13 mm and different fiber volume percents. From each mix, six samples of 5 cm cubes and three of 4 4 16 cm prismatic beam samples were cast. Compression test and split tensile test were applied to three samples from each mix while notched bending test was applied to three beam samples from each mix. The relations between compressive strain, tensile strain and crack length with electrical resistance change were investigated. The variations of gage factor, linearity, crack sensitivity and linearity for crack sensitivity with fiber volume percent were discussed. The split tensile test has been used for the first time in this project to investigate the relation between electrical resistance change and tensile strain for cement based composites. Also, simultaneous measurement of crack length and electrical resistance for cement based composites using notched bending test has been achieved for the first time in this project. The novel methods and findings are contributions to smart materials and structures research.
2. Experimental method In this study five different carbon fiber reinforced cement based composite were designed. In all mixes CEM I 42,5 R cement was used. The mass ratio of sand/cement is 1; silica fume/cement is 10%; water/cement is 0.4; super plasticizer Sika ViscoCrete High Tech 30/cement is 2% for all mixes. The mixes C02, C035, C05, C08 and C1 have 0.2, 0.35, 0.5, 0.8,1 volume percent of 13 mm carbon fibers, respectively. From each mix, six samples of 5 cm cube and three samples of 4 4 16 cm rectangular prisms were cast. A total of 45 samples were cast. The sand used was composed of silicone oxide, quartz and calcite as determined by XRD analyses. The maximum particle size of the sand was 2 mm. Silica fume had an average particle size of 100 nm. The main composition of
23
E. Teomete / Measurement 74 (2015) 21–30
silica fume was silicon-dioxide which was composed of silicon and oxygen atoms. High range superplasticizer Sika ViscoCrete High Tech 30 which is modified polycarboxylate based polymer was used in the mixes. Carbon fiber which was based on Polyacrylonitrile (PAN) and had a length of 13 mm diameter of 7 lm was used (product of DowAksa Co.). The carbon fiber properties are presented at Table 1. Pure copper wire mesh was used as electrode. The mesh opening was 5 mm and wire diameter was 600 lm. For this study, special molds were designed and manufactured. In order to pass the copper wire mesh electrodes, the 5 cm cube molds and 4 4 16 cm rectangular prism molds had 2 mm wide slots on either side as seen in Fig. 1a and b. A flat beater was used to prepare the mix and it was cast at two levels in the molds. In order to minimize the air in the mix, each levels were beat by a rod and the mold was vibrated. The samples were demolded and put in 100% moisture for 28 days of cure. The samples were removed from cure at 28th days and put in laboratory
environment for 7 days. A high speed electric saw was used to open the 5 mm deep notches in the middle of the rectangular prisms. The location of the crack growth was dictated by the notch. 2.1. Compression test Compression test was conducted at a load rate of 0.5 mm/min. Embedded four electrode method was used at the test. During the test, the sample and a reference resistance of 1000 Ohm were fed with a constant DC voltage of 15 V. Simultaneous to the test, the potential difference between voltage electrodes of the sample (Ev) was measured as Vs while the potential difference across the reference resistance (Rr = 1000 Ohm) was measured as Vr as seen in Fig. 2a and b. The strain of the sample in the force direction was measured by a strain gage. During the test, the voltages Vs and Vr, the load, the stroke of the loading head, strain gage data were recorded at a rate of 10 Hz (10 data in a second).
Table 1 The properties of carbon fibers used in this work (data supplied by carbon fiber manufacturer DowAksa Akrilik Co.). Diameter (lm)
Density (g/cm3)
Tensile strength (MPa)
Modulus of elasticity (GPa)
Strain at rupture (%)
7
1.75
3500
235
1.5
Fig. 1. (a) 5 cm cube mold with copper wire mesh electrodes. (b) Rectangular prism mold with copper wire mesh electrodes.
Fig. 2. (a) The test circuit diagram for compression test. (b) The sample at the compression test.
24
E. Teomete / Measurement 74 (2015) 21–30
2.2. Split tensile test Split tensile test was conducted at a load rate of 2.5 mm/min. Spherical loading heads conforming to standard EN 12390-6 [41] were used at the test. Similar to compression test, embedded four electrode method was used. The DC current was supplied using the outer current electrodes (Ec) while the potential difference between the inner electrodes (Ev) was measured as Vs (Fig. 3a and b). The voltage of the reference resistance (Rr = 1000 Ohm) was measured as Vr. Strain gages in the horizontal direction were used to monitor the tensile strain as seen in Fig. 3a and b. Tensile strain caused formation of a vertical crack which led to the failure of the sample (Fig. 3c). All samples had similar crack pattern as presented in Fig. 3c. The voltages Vs and Vr, strain gage data, the load and the stroke were recorded to a PC at a rate of 10 Hz.
two electrodes (Ev) as seen in Fig. 4a and b. The voltage of the reference resistance (Rr = 1000 Ohms) was measured as Vr. In order to monitor the real time crack length, two TML brand crack gages were attached on either sides of the sample (Fig. 4b). The crack formed at the test is presented in Fig. 4c. The load, the crack gage data, voltages Vs and Vr, and the stroke were recorded to a PC at a rate of 10 Hz. 2.4. Evaluation of the test results For the compression, split tensile and notched bending tests; the current on the circuit (Ic) and the resistance of the sample (Rs) at any time of the test were determined by using Ohms law using Eqs. (1) and (2), respectively.
Ic ¼
Vr Rr
ð1Þ
2.3. Notched bending test
Rs ¼
Vs Ic
ð2Þ
The notched three point bending test was conducted at a load rate of 0.2 mm/min with displacement control. Similar to the compression and split tensile tests, embedded four electrode method was used. At the test, DC current (Ic) was supplied from outer two electrodes (Ec) while voltage of the sample (Vs) was measured using inner
The percent change in the electrical resistance of the sample (%R) was determined by Eq. (3) where Rso is the electrical resistance of the sample without any load.
%R ¼
Rs 1 100 Rso
Fig. 3. (a) The circuit diagram of the split tensile test. (b) The split tensile test. (c) Sample after failure.
Fig. 4. (a) The circuit diagram of the bending test. (b) The bending test. (c) The crack formed at the test.
ð3Þ
E. Teomete / Measurement 74 (2015) 21–30
25
Fig. 5. Mix C08 having 13 mm carbon fiber 0.8% volume fraction, compression test. (a) %R–time graph. (b) Force–time graph. (c) %R–strain graph (K = 76; LE = 13).
Performance measures were defined and used in this study for strain and crack sensitivity. Gage factor (K) and linearity (LE) are the performance measures for a strain sensor. Gage factor is the change in electrical resistance by unit strain as in Eq. (4). The higher the gage factor, the more sensitive the strain gage is. Metal foil strain gages have a gage factor of two. Linearity (LE) is the percent of maximum difference between input and output curve (%R versus strain curve) and fitted linear regression line, to full scale output (Rfs), as presented in Eq. (5) by Ou and Han [42]. The error in measurement of strain decreases as linearity decreases.
ðR Rso Þ=Rso De Dmax 100 %LE ¼ %Rfs K¼
ð4Þ
ð5Þ
Crack sensitivity (SC) is the % change of electrical resistance per unit crack length in mm. To the best knowledge of the author, the crack sensitivity (SC) has been first defined in this work for cement based composites. Linearity for crack sensitivity (LEC) is the percent of maximum difference between input and output curve (%R versus crack length curve) and fitted linear regression line, to full scale output. The error in measurement of crack length decreases by decreasing LEC. Linearity for crack sensitivity has also been defined for the first time in this study, to the best knowledge of the author. 3. Results and discussions In this study five different carbon fiber reinforced cement matrix composites were designed. From each mix, three of 5 cm cubes were tested with compression test, three of 5 cm cubes were tested with split tensile test and 3 of rectangular prism samples were tested with notched bending test. The relations between tensile strain–electrical resistance change, compressive strain–electrical resistance change and crack length–electrical resistance change were determined. These results are presented in this section. 3.1. Compression test results The electrical resistance change (%R)–time and force– time graphs for C08 are presented in Fig. 5a and b. At the
beginning of the test until 26th second, the sample was not deformed due to the crushing of the micro asperity by the loading plate; the electrical resistance change (%R) and force were almost zero (Fig. 5a and b). After 26th second, the sample started to deform under compressive strain; the electrical resistance of the sample (Rs) started to decrease leading to a negative %R (Fig. 5a). The electrical resistance decreased due to the shortening of the fibers, closing of micro cracks and micro voids. The force started to increase after 26th second. When cracks started to form, the active area that could transfer electrons reduced; the electrical resistance of the sample (Rs) increased; the %R started to increase at 205th second. The formation of the cracks at 205th second did not affect the force time graph simultaneously due to the crack bridging effect of the fibers and also due to the nature of the compressive strain. At 220th second, the force was at its maximum and started to decrease due to the loss in stiffness of the sample caused by crack propagation. The %R–strain graph is presented in Fig. 5c. The gage factor of C08 was K = 76, which was almost 40 times more sensitive than metal foil strain gages. C08 had a linearity of LE = 13%. The electrical resistance change and strain had a correlation coefficient of 0.98 which testified the strong linear relationship (Fig. 5c). Similar results were obtained from the tests of other mixes so the same graphs for the other mixes are not presented. As the carbon fiber volume percent increased from 0.2 to 0.35 and from 0.5 to 1, strain could disrupt a smaller percent of the fiber–fiber and fiber–matrix contact; the effect of strain on the electrical resistance got smaller; the %R changed less; sensitivity decreased and gage factor decreased (Fig. 6a). At 0.5 volume %, the system was at percolation threshold; few direct electron conduction paths formed. Strain disrupted these few paths, shifted the system from post-percolation to pre-percolation situation, resulted in the highest sensitivity and gage factor. At percolation threshold fiber volume percent, the cement based composite had the highest gage factor. The %R–strain graph deviates from linearity at most at percolation threshold of 0.5% fiber volume fraction due to the shift of the system from post-percolation to pre-percolation. This led to the highest linearity (LE%) and lowest correlation coefficient (R2) at percolation threshold of 0.5% fiber volume fraction as seen in Fig. 6b and c. The deviation from linearity was not considerable as seen from the LE% and R2 values.
26
E. Teomete / Measurement 74 (2015) 21–30
Fig. 6. Compression test. (a) Gage factor–fiber volume %. (b) Linearity–fiber volume %. (c) Correlation coefficient–fiber volume %.
The highest gage factor obtained was K = 90 at percolation threshold of 0.5% fiber volume percent. The carbon fiber cement based composite is much more sensitive to compressive strain than metal foil strain gages. 3.2. Split tensile test results The split tensile test results for C02 mix are presented in Fig. 7. Displacement controlled test was conducted at a rate of 2.5 mm/min. The stiffness of the wooden bars between the loading plate and the sample was much less than the stiffness of the sample so most of the deformation was shared by the wooden bars at the beginning of the test (Fig. 3b and c). At 37th second, the wooden bars started to crush and caused a plateau in the force–time and strain– time graphs as seen in Fig. 7a and b. As the wooden bars crushed, their stiffness increased and the slope of the force–time graph increased as seen in Fig. 7a. At 143rd second, the wooden bars were almost totally crushed and their stiffness increased which led to an increase in the slopes of the force–time and strain–time graphs (Fig. 7a and b). After 143rd second, as the stiffness of the wooden bar increased, the sample started to deform more; there was a sudden increase in the slope of strain–time graph as seen in Fig. 7b. At 165th second, a vertical crack
formed as seen in Fig. 3c which decreased the stiffness of the sample ending in a sudden decrease in the applied load (Fig. 7a). The formation of crack decreased the cross section that the electrons could flow, the change in electrical resistance increased suddenly at 165th second (Fig. 7c). Due to the large value of ordinate axis in Fig. 7c, the variation of %R by strain before 165th second cannot be distinguished although there was a variation which is presented in Fig. 7d. The change in electrical resistance had a strong linear relationship with tensile strain with a correlation coefficient of 0.99. The mix C02 had a gage factor of 50 and linearity of 16%. The mix C02 was 25 times more sensitive to strain than commercial foil strain gages. Similar observations were obtained for the other mixes so the same graphs are not presented for the other mixes. As the fiber volume percent increased from 0.2% to 0.35%, tensile strain could disrupt a smaller amount of fiber–fiber and fiber–matrix contacts, resulting in a decrease in the sensitivity of the system which led to a decrease in the gage factor (Fig. 8a). At percolation threshold of 0.5% volume fraction of fibers, tensile strain disrupted the few direct electron conduction paths and shifted the system from post-percolation situation to pre-percolation situation which resulted in a local maximum gage factor. The gage factor decreased as volume
Fig. 7. Split tensile test results of C02 mix. (a) Force–time graph. (b) Tensile strain–time graph. (c) %R–time graph. (d) %R–strain graph (K = 50, LE = 16).
E. Teomete / Measurement 74 (2015) 21–30
27
Fig. 8. Split tensile test results. (a) Gage factor–fiber volume percent. (b) Linearity–fiber volume percent. (c) R2–fiber volume percent.
percent increased to 0.8% due to the same phenomenon described for the 0.2–0.35% increase. The maximum gage factor obtained was 328 at 1% volume fraction. The carbon fiber reinforced cement composite was 160 times more sensitive to strain than commercial foil strain gages. The %R versus strain graph had a strong linear relationship with low linearity values and high correlation coefficients as presented in Fig. 8b and c. At percolation threshold of 0.5%, the shift of the system from post-percolation to pre-percolation resulted in the highest deviation from linearity while the correlation coefficient was also one of the lowest values. The highest correlation coefficient obtained was 0.99 which testified the strong linear relationship between tensile strain and electrical resistance change. 3.3. Notched bending test results The crack length–time and force–time graphs of C02 mix are presented in Fig. 9a and b. When crack started to propagate from the tip of the notch at 72nd second, the stiffness of the beam sample decreased and the force started to decrease from maximum (Fig. 9a and b). In Fig. 10a, the percent change of electrical resistance (%R)– crack length is presented. The graph is subdivided into regions of A, B, C which are explained below. The time between the beginning of the test and crack initiation (72nd second) formed region A. In region A, the electrical resistance change (%R) oscillates between small positive and negative values (Fig. 10a). The negative values were due to the compressive strain region above the neutral axis of the beam which shortened the fibers, closed the micro cracks and micro voids. The positive %R values were due to the tensile strain below the neutral axis of the beam which elongated the fibers, developed micro cracks and micro voids. The opening of micro cracks and micro voids
decreased the cross section that the electrons could pass ending in an increase in the electrical resistance. Competition of these two events (compressive strain above neutral axis and tensile strain below neutral axis) until 72nd second in region A resulted in the oscillation of electrical resistance from negative to positive values (Fig. 10a). The region B started by initiation of the crack at 72nd second (Fig. 10a). As the crack propagated, the cross section that the electrons could pass decreased dramatically which led to an increase in the electrical resistance change (%R) as seen in Fig. 10a. The crack propagation and its result of decrease in cross section overcame the effect of compressive strain above the neutral axis. The maximum capacity of the crack gage was 21 mm above which the crack gage did not send signal as it would be divided into two parts. At region C, the crack length was over 21 mm, the crack gage did not send more signal while the crack propagated further to the top of the beam which increased the %R as seen in Fig. 10a. There was a linear relationship between the crack length and %R at region B. This linear relationship had a correlation coefficient of 0.95 as seen in Fig. 10b. For mix C02, the crack sensitivity (SC) was 4.8, linearity for crack sensitivity (LEC) was 15%. The results obtained from other mixes were similar so their graphs are not presented here. As the fiber volume percent increased, the percent of fiber–fiber and fiber–matrix contacts broken by the crack propagation decreased; the sensitivity of the system to the crack propagation decreased as seen in Fig. 11a. At percolation threshold of 0.5%, the crack propagation shifted the system from post-percolation to pre-percolation situation by breaking the few direct electron conduction paths which resulted in the highest crack sensitivity as seen in Fig. 11a. The deviation from a line of the %R–crack length data was highest at percolation threshold of 0.5% so the linearity was highest and R2 was lowest as seen in Fig. 11b and c.
Fig. 9. Notched bending test C02 mix with 0.2% volume fraction of 13 mm fibers. (a) Crack length–time graph. (b) Force–time graph.
28
E. Teomete / Measurement 74 (2015) 21–30
Fig. 10. Notched bending test C02 mix with 0.2% volume fraction of 13 mm carbon fibers. (a) %R–crack length whole experiment. (b) %R–crack length, linear relationship, crack sensitivity SC = 4.8; linearity for crack sensitivity LEC = 15%.
Fig. 11. Notched bending test results. (a) Crack sensitivity–fiber volume %. (b) Linearity–fiber volume %. (c) R2–fiber volume %.
This deviation from a line was due to the shift of the system from post-percolation to pre-percolation situation by crack propagation. The variation of electrical resistivity with fiber volume percent was presented in Fig. 12. There is a general trend of decrease of resistivity with fiber volume percent. The small increase at 1% volume fraction was due to the formation of voids between fiber–matrix and fiber–fiber interfaces due to high fiber volume fraction of 13 mm fibers. These voids inhibited electron transfer. A sharp decrease of resistivity at 0.5% was not observed which was the percolation threshold. At percolation threshold, the fibers start to form a few continuous electron transfer lines. These a few lines were not enough to decrease the resistivity excessively. It is obvious that the percolation threshold was between 0.5% and 0.8% from Fig. 12. There is a high probability that the percolation threshold was very close to 0.5% (i.e. 0.51–0.52%). Strong linear relationships between the strain–electrical resistance change and crack length–electrical resistance change are encouraging for developing on smart cement based sensors.
Fig. 12. Resistivity–fiber volume percent relationship for cement composites.
4. Conclusions Five different carbon fiber reinforced cement composites with fiber length of 13 mm were designed. From each mix six of 5 cm cube samples and three of 4 4 16 cm prismatic samples were cast and cured. From each mix, three samples were tested by compression test, split tensile test and notched bending test. A total of 45 tests were conducted. The correlations between tensile strain, compressive strain and crack length with electrical resistance change were determined. The performance measures were used to assess the self sensing performance of the composites. The results obtained from compression are: 1. Compressive strain shortened the carbon fibers, closed the micro voids and micro cracks and resulted in a decrease in electrical resistance. 2. There was a strong linear relationship between electrical resistance change and strain. A gage factor of 76 was obtained which was almost 40 times the gage factor of commercial foil strain gages. 3. As the fiber volume percent increased, the percent of fiber–fiber and matrix fiber contacts disrupted by the strain decreased which ended up a decrease in the gage factor. At percolation threshold of 0.5% fiber volume fraction at which few direct electron conduction paths formed, these few direct electron conduction paths were disrupted; system shifted from post-percolation to pre-percolation situation and highest gage factor obtained. 4. Due to the shift of the system from post-percolation to pre-percolation situation, the highest deviation from linearity was obtained at percolation threshold of
E. Teomete / Measurement 74 (2015) 21–30
0.5%, which resulted in the highest linearity (LE) and lowest correlation coefficient. The results of split tensile tests are presented as below: 1. Split tensile test was used for the first time in this study to assess the relationship between tensile strain–electrical resistance change of cement matrix composites. 2. The tensile strain caused elongation of the fibers, opening of micro cracks and micro voids which resulted in an increase of the electrical resistance. 3. Similar to the results of compression test, the gage factor had a local maximum at a fiber volume fraction of 0.5% due to the shift of the system from post-percolation to pre-percolation situation. The linearity had a maximum at 0.5% volume fraction due to the highest deviation from linearity. A strong linear relationship was observed between the tensile strain and electrical resistance change (%R). The results of notched bending tests are presented as below: 1. The simultaneous measurement of crack length and electrical resistance change to investigate their relationship for cement composites with notched bending test has been achieved for the first time in this work in the literature. Crack sensitivity (SC) and linearity for crack sensitivity (LEC) have been introduced in this work as novel concepts. 2. The propagation of the crack decreased the cross sectional area that the electrons could pass and increased the electrical resistance. A strong linear relationship between the crack length and electrical resistance change was observed. 3. The increase of volume percent of fibers decreased the percent of disrupted fiber–fiber and fiber–matrix contacts which resulted in a decrease of crack sensitivity. 4. At percolation threshold of 0.5%, crack propagation disrupted the few direct electron conduction paths shifting the system from post-percolation to pre-percolation situation which resulted in the highest crack sensitivity. The shift of the system from post-percolation to pre-percolation situation at percolation threshold resulted in the maximum deviation from linearity; highest linearity and lowest R2 were obtained at percolation threshold fiber volume fraction. Strain sensing, crack detection and damage assessment of concrete structures can be achieved by the carbon fiber reinforced cement based composites. The results presented can be used in future work for developing self sensing smart concrete. Acknowledgements This work is supported by The Scientific and Technological Research Council of Turkey (TUBITAK)
29
through Grant No: 110M221. The author would like to thank to Sika Construction Chemicals Co. for providing the silica fume and super-plasticizer and to DowAksa Co. for carbon fibers used in this study. The author would like to thank to the Dokuz Eylul University, Metallurgical and Materials Engineering Department for providing the mechanical testing system for bending tests. The author is thankful to Assoc. Prof. Dr. Tahir Kemal Erdem for his contributions to the work. References [1] F. Reza, G.B. Batson, J.A. Yamamuro, J.S. Lee, Resistance changes during compression of carbon fiber cement composites, J. Mater. Civil. Eng. 15 (5) (2003) 476–483. [2] D.D.L. Chung, Review functional properties of cement–matrix composites, J. Mater. Sci. 36 (2001) 1315–1324. [3] K.T. Lau, Structural health monitoring for smart composites using embedded FBG sensor technology, Mater. Sci. Technol. 30 (13a) (2014) 1642–1654. [4] B.G. Han, B.Z. Han, J.P. Ou, Novel piezoresistive composite with high sensitivity to stress/strain, Mater. Sci. Technol. 26 (7) (2010) 865–870. [5] P.W. Chen, D.D.L. Chung, Low-drying-shrinkage concrete containing carbon fibers, Compos. Part B – Eng. 27 (3–4) (1996) 269–274. [6] P.W. Chen, D.D.L. Chung, Concrete as a new strain stress sensor, Compos. Part B – Eng. 27 (1) (1996) 11–23. [7] X.J. Wang, D.D.L. Chung, Self-monitoring of fatigue damage and dynamic strain in carbon fiber polymer–matrix composite, Compos. Part B – Eng. 29 (1) (1998) 63–73. [8] S.K. Wang, D.D.L. Chung, Temperature/light sensing using carbon fiber polymer–matrix composite, Compos. Part B – Eng. 30 (6) (1999) 591–601. [9] S.K. Wang, D.D.L. Chung, Apparent negative electrical resistance in carbon fiber composites, Compos. Part B – Eng. 30 (6) (1999) 579–590. [10] X.C. Luo, D.D.L. Chung, Electromagnetic interference shielding using continuous carbon–fiber carbon–matrix and polymer–matrix composites, Compos. Part B – Eng. 30 (3) (1999) 227–231. [11] D.D.L. Chung, Cement reinforced with short carbon fibers: a multifunctional material, Compos. Part B – Eng. 31 (6–7) (2000) 511–526. [12] D.D.L. Chung, Self-monitoring structural materials, Mater. Sci. Eng. 22 (2) (1998) 57–78. [13] X. Fu, E. Ma, D.D.L. Chung, W.A. Anderson, Self-monitoring in carbon fiber reinforced mortar by reactance measurement, Cem. Concr. Res. 27 (6) (1997) 845–852. [14] X. Fu, D.D.L. Chung, Effect of curing age on the self-monitoring behavior of carbon fiber reinforced mortar, Cem. Concr. Res. 27 (9) (1997) 1313–1318. [15] E. Teomete, T.K. Erdem, Cement based strain sensor: a step to smart concrete, Cem. Wapno Beton 2 (2011) 78–91. [16] E. Teomete, O.I. Kocyigit, Tensile strain sensitivity of steel fiber reinforced cement matrix composites tested by split tensile test, Constr. Build. Mater. 47 (2013) 962–968. [17] M. Chiarello, R. Zinno, Electrical conductivity of self-monitoring CFRC, Cem. Concr. Compos. 27 (2005) 463–469. [18] B. Han, X. Guan, J. Ou, Electrode design, measuring method and data acquisition system of carbon fiber cement paste piezoresistive sensors, Sens. Actuators, A 135 (2007) 360–369. [19] B. Han, X. Yu, J. Ou, Self-Sensing Concrete in Smart Structures, first ed., Butterworth-Heinemann, 2014, ISBN 9780128005170. [20] B. Chen, J. Liu, Damage in carbon fiber–reinforced concrete, monitored by both electrical resistance measurement and acoustic emission analysis, Constr. Build. Mater. 22 (2008) 2196–2201. [21] H. Li, H. Xiao, J. Ou, Effect of compressive strain on electrical resistivity of carbon black-filled cement-based composites, Cem. Concr. Comp. 28 (2006) 824–828. [22] H. Li, H. Xiao, J. Ou, Electrical property of cement-based composites filled with carbon black under long-term wet and loading condition, Comp. Sci. Technol. 68 (2008) 2114–2119. [23] B. Han, K. Zhang, X. Yu, E. Kwon, J. Ou, Nickel particle based selfsensing pavement for vehicle detection, Measurement 44 (2011) 1645–1650. [24] X. Jing, Y. Wu, W. Ruiqing, Nonlinear conduction in carbon fiber reinforced cement mortar, Cem. Concr. Compos. 33 (2011) 444–448.
30
E. Teomete / Measurement 74 (2015) 21–30
[25] H. Gong, Y. Zhang, J. Quan, S. Che, Preparation and properties of cement based piezoelectric composites modified by CNTs, Curr. Appl. Phys. 11 (3) (2011) 653–656. [26] R. Rianyoi, R. Potong, N. Jaitonong, R. Yimnirun, A. Chaipanich, Dielectric, ferroelectric and piezo electric properties of 0–3 barium titanate–portland cement composite, Appl. Phys. A 104 (2011) 661–666. [27] S. Vaidya, E.N. Allouche, Strain sensing of carbon fiber reinforced geopolymer concrete, Mater. Struct. 44 (2011) 1467–1475. [28] Z.J. Li, D. Zhang, K.R. Wu, Cement matrix 2–2 piezoelectric composite Part 1 Sensory effect, Mater. Struct. 34 (2001) 506–512. [29] J.M. Torrents, T.O. Mason, A. Peled, S.P. Shah, E.J. Garboczi, Analysis of the impedance spectra of short conductive fiber-reinforced composites, J. Mater. Sci. 36 (2001) 4003–4012. [30] A.D. Hixson, L.Y. Woo, M.A. Campo, T.O. Mason, E.J. Garboczi, Intrinsic conductivity of short conductive fibers in composites by impedance spectroscopy, J. Electroceram. 7 (2001) 189–195. [31] A. Peled, J.M. Torrents, T.O. Mason, S.P. Shah, E.J. Garboczi, Electrical impedance spectra to monitor damage during tensile loading of cement composites, ACI Mater. J. 98 (4) (2001) 313–322. [32] J.M. Torrents, T.C. Easley, K.T. Faber, T.O. Mason, Evolution of impedance spectra during debonding and pullout of single steel fibers from cement, J. Am. Ceram. Soc. 84 (4) (2001) 740–746. [33] T.O. Mason, M.A. Campo, A.D. Hixson, L.Y. Woo, Impedance spectroscopy of fiber-reinforced cement composites, Cem. Concr. Compos. 24 (2002) 457–465.
[34] L.Y. Woo, N.J. Kidner, S. Wansom, T.O. Mason, Combined time domain reflectometry and AC-impedance spectroscopy of fiberreinforced fresh-cement composites, Cem. Concr. Res. 37 (2007) 89–95. [35] J.D. Shane, T.O. Mason, H.M. Jennings, Effect of the interfacial transition zone on the conductivity of portland cement mortars, J. Am. Ceram. Soc. 83 (5) (2000) 1137–1144. [36] F. Rajabipour, J. Weiss, J.D. Shane, T.O. Mason, P.S. Surendra, Procedure to interpret electrical conductivity measurements in cover concrete during rewetting, J. Mater. Civ. Eng. 17 (5) (2005) 586–594. [37] P.J. Tumidajski, P. Xie, M. Arnott, J.J. Beaudoin, Overlay current in a conductive concrete snow melting system, Cem. Concr. Res. 33 (2003) 1807–1809. [38] B. Han, X. Yu, E. Kwon, A self-sensing carbon nanotube/cement composite for traffic monitoring, Nanotechnology 20 (44) (2009) 1–5. [39] B. Han, X. Yu, J. Ou, Effect of water content on the piezoresistivity of MWNT/cement composites, J. Mater. Sci. 45 (2010) 3714–3719. [40] B. Han, S. Ding, X. Yu, Intrinsic self-sensing concrete and structures: a review, Measurement 59 (2015) 110–128. [41] European Committee for Standardization EN 12390-6: Testing hardened concrete – Part 6: Tensile splitting strength of test specimens. [42] J. Ou, B. Han, Piezoresistive cement-based strain sensors and selfsensing concrete components, J. Intell. Mater. Syst. Struct. 20 (2009) 329–336.
Contents lists available at ScienceDirect
Measurement journal homepage: www.elsevier.com/locate/measurement
Measurement of crack length sensitivity and strain gage factor of carbon fiber reinforced cement matrix composites Egemen Teomete ⇑ Dokuz Eylul University, Civil Engineering Department, Kaynaklar, Buca, Izmir, Turkey
a r t i c l e
i n f o
Article history: Received 17 March 2015 Received in revised form 26 June 2015 Accepted 13 July 2015 Available online 23 July 2015 Keywords: Strain Crack length Self sensing Smart materials Cement
a b s t r a c t Five different carbon fiber reinforced cement matrix composites with fiber length of 13 mm were designed having different fiber volume fractions. From each mix, six samples of 5 cm cubes and three 4 4 16 cm prism samples were cast. Compression, split tensile and notched bending tests were applied to three samples from each mix with simultaneous electrical resistance measurements. Strong correlations between strain and electrical resistance change were reported. At percolation threshold, the strain caused shift of the system from post percolation to pre-percolation, changing the electrical resistance dramatically; causing highest gage factor. The microstructure controlled mechanisms were enlightened. The split tensile test and notched bending test with simultaneous electrical resistance, strain and crack length measurements have been applied as novel methods. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The performances of the structures are endangered by earthquakes, material degradations and other environmental effects. In USA concrete infrastructures have materials deteriorations and 30% of the bridges were found to be structurally not reliable [1]. Structural health monitoring is crucial to protect the people and also for asset planning. Commercial metal strain gages measure the strain of a point while they have low durability and short life time. Moreover, a vast number of them have to be used to monitor a structure which increases the cost [2]. Fiber optic Bragg grating sensors were developed for structural health monitoring [3]. This study is a contribution of developing self sensing smart materials and structures for the construction industry. Han et al. investigated the piezoresistivity of nickel powder reinforced cement composite under compression [4]. ⇑ Tel.: +90 232 301 7060; fax: +90 232 453 1192. E-mail address: [email protected] http://dx.doi.org/10.1016/j.measurement.2015.07.021 0263-2241/Ó 2015 Elsevier Ltd. All rights reserved.
Chung studied self sensing as well as mechanical and electrical properties of carbon fiber reinforced polymer and cement matrix composites [5–11]. Carbon fiber inclusion in the cement composite decreases electrical resistance. The application of load affects electrical resistance [12–16]. In this study, the relationships between compressive, tensile strain and crack length with electrical resistance change were investigated for carbon fiber reinforced cement based composites using compression; split tensile and notched bending tests. Split tensile test and notched bending tests have been used for the first time for cement based smart materials research in this project. Two and four electrode methods have been used to monitor the electrical resistance of the cement based composites. Current supply and voltage measurement were conducted using the same electrodes in two electrode method while these were achieved using two different electrode pairs in four electrode method. The measurements were affected by the sample cross section and the distance between electrodes in two electrode method while measurements were not affected in four electrode
22
E. Teomete / Measurement 74 (2015) 21–30
method [17,18]. Four electrode method was used in compression, split tensile and notched bending tests in this study. Measurement of electrical resistance was conducted using perimetral and embedded electrode configurations. Han et al. [19] proposed the use of embedded mesh electrodes for cement composites. A conductive plate or mesh is inserted in the material in embedded electrode configuration while a conductive wire or paint was attached to the perimeter in perimetral electrode method [1,19–22]. In this study, copper wire mesh was used in embedded electrode configuration by inserting in the material. While damage affects electrical resistance of carbon fiber reinforced cement based composites, simultaneous measurement of crack length with respect to electrical resistance has not been conducted [11]. Simultaneous measurement of crack length and electrical resistance has been achieved in this work. There have been various studies on smart cement based materials. Vehicle detection sensors were developed by nickel particle inclusions in the cement matrix and tested [23]. In order to investigate the nonlinear current–voltage relationship of carbon fiber reinforced cement composites, tunneling effect theory and Ohm’s law were used [24]. Piezo-electric properties of carbon nano tube reinforced cement based composites were investigated [25]. Dielectric, ferroelectric and piezoelectric properties of 0– 3 barium titanate–portland cement composite was investigated [26]. Strain sensing of carbon fiber reinforced geopolymer concrete was investigated by compression test and bending test [27]. The effect of low frequency mechanical load on cement matrix 2–2 piezoelectric composite was investigated. A linear relation between the piezoelectric coefficients and frequency was found while there was no relation between the magnitude of the applied load and piezoelectric coefficients [28]. Electrical impedance spectra of short conductive fiber-reinforced composites was studied and numerical simulations were verified by experimental results [29]. The intrinsic conductivities of short conductive fibers in cement matrix composites were determined by DC conductivity and impedance spectroscopy measurements. The fiber aspect ratio in randomly distributed fiber composites can be determined by conductivity versus aspect ratio relation [30]. Direct notched tension test was used to correlate DC and AC electrical properties with mechanical properties. The relations between crack bridging, fiber volume fraction and crack propagation with the electrical properties were reported [31]. Electrical impedance measurements were used to investigate the effects of debonding and pullout of single steel fiber from cement matrix and it was reported that electrical impedance measurements were sensitive to cracking [32]. Electrical impedance spectra was used to investigate the effects of fiber aspect ratio, fiber volume fraction, fiber orientation and fiber shape on the spectra and relations between microstructure property – impedance spectra were reported [33]. AC impedance spectra was used to investigate the fiber dispersion issues of orientation, segregation and clumping in fresh cement matrix composites [34]. It was reported that the interfacial transition zone did not affect the global
electrical conductivity while there were differences between the conductivities of transition zone and matrix pastes. When hydration was between 0.5 and 0.8, it had considerable effect on the difference [35]. Finite element method was used to model the coupled problem of propagation of moisture and electrical resistance. One electrical measurement and properties of saturated and unsaturated concrete were used to detect the position of moisture front in concrete [36]. Overlay current in a conductive concrete snow melting system was studied and an equation for overlay current was determined and calibrated with experimental results [37]. Han et al. [38], worked on multi walled carbon nanotube cement based vehicle detection sensors under cyclic compression and impulsive loads and reported that the sensors were sensitive for vehicle detection. Effect of moisture content on multi walled carbon nanotube (MWNT) cement composites’ piezoresistive sensitivity was investigated and the field emission effect of MWNT was used to enlighten the mechanism between moisture content and piezoresistive sensitivity [39]. Han et al. [40] presented a review of self sensing concrete and structures. In this study, five different carbon fiber reinforced cement matrix composites were designed with a fiber length of 13 mm and different fiber volume percents. From each mix, six samples of 5 cm cubes and three of 4 4 16 cm prismatic beam samples were cast. Compression test and split tensile test were applied to three samples from each mix while notched bending test was applied to three beam samples from each mix. The relations between compressive strain, tensile strain and crack length with electrical resistance change were investigated. The variations of gage factor, linearity, crack sensitivity and linearity for crack sensitivity with fiber volume percent were discussed. The split tensile test has been used for the first time in this project to investigate the relation between electrical resistance change and tensile strain for cement based composites. Also, simultaneous measurement of crack length and electrical resistance for cement based composites using notched bending test has been achieved for the first time in this project. The novel methods and findings are contributions to smart materials and structures research.
2. Experimental method In this study five different carbon fiber reinforced cement based composite were designed. In all mixes CEM I 42,5 R cement was used. The mass ratio of sand/cement is 1; silica fume/cement is 10%; water/cement is 0.4; super plasticizer Sika ViscoCrete High Tech 30/cement is 2% for all mixes. The mixes C02, C035, C05, C08 and C1 have 0.2, 0.35, 0.5, 0.8,1 volume percent of 13 mm carbon fibers, respectively. From each mix, six samples of 5 cm cube and three samples of 4 4 16 cm rectangular prisms were cast. A total of 45 samples were cast. The sand used was composed of silicone oxide, quartz and calcite as determined by XRD analyses. The maximum particle size of the sand was 2 mm. Silica fume had an average particle size of 100 nm. The main composition of
23
E. Teomete / Measurement 74 (2015) 21–30
silica fume was silicon-dioxide which was composed of silicon and oxygen atoms. High range superplasticizer Sika ViscoCrete High Tech 30 which is modified polycarboxylate based polymer was used in the mixes. Carbon fiber which was based on Polyacrylonitrile (PAN) and had a length of 13 mm diameter of 7 lm was used (product of DowAksa Co.). The carbon fiber properties are presented at Table 1. Pure copper wire mesh was used as electrode. The mesh opening was 5 mm and wire diameter was 600 lm. For this study, special molds were designed and manufactured. In order to pass the copper wire mesh electrodes, the 5 cm cube molds and 4 4 16 cm rectangular prism molds had 2 mm wide slots on either side as seen in Fig. 1a and b. A flat beater was used to prepare the mix and it was cast at two levels in the molds. In order to minimize the air in the mix, each levels were beat by a rod and the mold was vibrated. The samples were demolded and put in 100% moisture for 28 days of cure. The samples were removed from cure at 28th days and put in laboratory
environment for 7 days. A high speed electric saw was used to open the 5 mm deep notches in the middle of the rectangular prisms. The location of the crack growth was dictated by the notch. 2.1. Compression test Compression test was conducted at a load rate of 0.5 mm/min. Embedded four electrode method was used at the test. During the test, the sample and a reference resistance of 1000 Ohm were fed with a constant DC voltage of 15 V. Simultaneous to the test, the potential difference between voltage electrodes of the sample (Ev) was measured as Vs while the potential difference across the reference resistance (Rr = 1000 Ohm) was measured as Vr as seen in Fig. 2a and b. The strain of the sample in the force direction was measured by a strain gage. During the test, the voltages Vs and Vr, the load, the stroke of the loading head, strain gage data were recorded at a rate of 10 Hz (10 data in a second).
Table 1 The properties of carbon fibers used in this work (data supplied by carbon fiber manufacturer DowAksa Akrilik Co.). Diameter (lm)
Density (g/cm3)
Tensile strength (MPa)
Modulus of elasticity (GPa)
Strain at rupture (%)
7
1.75
3500
235
1.5
Fig. 1. (a) 5 cm cube mold with copper wire mesh electrodes. (b) Rectangular prism mold with copper wire mesh electrodes.
Fig. 2. (a) The test circuit diagram for compression test. (b) The sample at the compression test.
24
E. Teomete / Measurement 74 (2015) 21–30
2.2. Split tensile test Split tensile test was conducted at a load rate of 2.5 mm/min. Spherical loading heads conforming to standard EN 12390-6 [41] were used at the test. Similar to compression test, embedded four electrode method was used. The DC current was supplied using the outer current electrodes (Ec) while the potential difference between the inner electrodes (Ev) was measured as Vs (Fig. 3a and b). The voltage of the reference resistance (Rr = 1000 Ohm) was measured as Vr. Strain gages in the horizontal direction were used to monitor the tensile strain as seen in Fig. 3a and b. Tensile strain caused formation of a vertical crack which led to the failure of the sample (Fig. 3c). All samples had similar crack pattern as presented in Fig. 3c. The voltages Vs and Vr, strain gage data, the load and the stroke were recorded to a PC at a rate of 10 Hz.
two electrodes (Ev) as seen in Fig. 4a and b. The voltage of the reference resistance (Rr = 1000 Ohms) was measured as Vr. In order to monitor the real time crack length, two TML brand crack gages were attached on either sides of the sample (Fig. 4b). The crack formed at the test is presented in Fig. 4c. The load, the crack gage data, voltages Vs and Vr, and the stroke were recorded to a PC at a rate of 10 Hz. 2.4. Evaluation of the test results For the compression, split tensile and notched bending tests; the current on the circuit (Ic) and the resistance of the sample (Rs) at any time of the test were determined by using Ohms law using Eqs. (1) and (2), respectively.
Ic ¼
Vr Rr
ð1Þ
2.3. Notched bending test
Rs ¼
Vs Ic
ð2Þ
The notched three point bending test was conducted at a load rate of 0.2 mm/min with displacement control. Similar to the compression and split tensile tests, embedded four electrode method was used. At the test, DC current (Ic) was supplied from outer two electrodes (Ec) while voltage of the sample (Vs) was measured using inner
The percent change in the electrical resistance of the sample (%R) was determined by Eq. (3) where Rso is the electrical resistance of the sample without any load.
%R ¼
Rs 1 100 Rso
Fig. 3. (a) The circuit diagram of the split tensile test. (b) The split tensile test. (c) Sample after failure.
Fig. 4. (a) The circuit diagram of the bending test. (b) The bending test. (c) The crack formed at the test.
ð3Þ
E. Teomete / Measurement 74 (2015) 21–30
25
Fig. 5. Mix C08 having 13 mm carbon fiber 0.8% volume fraction, compression test. (a) %R–time graph. (b) Force–time graph. (c) %R–strain graph (K = 76; LE = 13).
Performance measures were defined and used in this study for strain and crack sensitivity. Gage factor (K) and linearity (LE) are the performance measures for a strain sensor. Gage factor is the change in electrical resistance by unit strain as in Eq. (4). The higher the gage factor, the more sensitive the strain gage is. Metal foil strain gages have a gage factor of two. Linearity (LE) is the percent of maximum difference between input and output curve (%R versus strain curve) and fitted linear regression line, to full scale output (Rfs), as presented in Eq. (5) by Ou and Han [42]. The error in measurement of strain decreases as linearity decreases.
ðR Rso Þ=Rso De Dmax 100 %LE ¼ %Rfs K¼
ð4Þ
ð5Þ
Crack sensitivity (SC) is the % change of electrical resistance per unit crack length in mm. To the best knowledge of the author, the crack sensitivity (SC) has been first defined in this work for cement based composites. Linearity for crack sensitivity (LEC) is the percent of maximum difference between input and output curve (%R versus crack length curve) and fitted linear regression line, to full scale output. The error in measurement of crack length decreases by decreasing LEC. Linearity for crack sensitivity has also been defined for the first time in this study, to the best knowledge of the author. 3. Results and discussions In this study five different carbon fiber reinforced cement matrix composites were designed. From each mix, three of 5 cm cubes were tested with compression test, three of 5 cm cubes were tested with split tensile test and 3 of rectangular prism samples were tested with notched bending test. The relations between tensile strain–electrical resistance change, compressive strain–electrical resistance change and crack length–electrical resistance change were determined. These results are presented in this section. 3.1. Compression test results The electrical resistance change (%R)–time and force– time graphs for C08 are presented in Fig. 5a and b. At the
beginning of the test until 26th second, the sample was not deformed due to the crushing of the micro asperity by the loading plate; the electrical resistance change (%R) and force were almost zero (Fig. 5a and b). After 26th second, the sample started to deform under compressive strain; the electrical resistance of the sample (Rs) started to decrease leading to a negative %R (Fig. 5a). The electrical resistance decreased due to the shortening of the fibers, closing of micro cracks and micro voids. The force started to increase after 26th second. When cracks started to form, the active area that could transfer electrons reduced; the electrical resistance of the sample (Rs) increased; the %R started to increase at 205th second. The formation of the cracks at 205th second did not affect the force time graph simultaneously due to the crack bridging effect of the fibers and also due to the nature of the compressive strain. At 220th second, the force was at its maximum and started to decrease due to the loss in stiffness of the sample caused by crack propagation. The %R–strain graph is presented in Fig. 5c. The gage factor of C08 was K = 76, which was almost 40 times more sensitive than metal foil strain gages. C08 had a linearity of LE = 13%. The electrical resistance change and strain had a correlation coefficient of 0.98 which testified the strong linear relationship (Fig. 5c). Similar results were obtained from the tests of other mixes so the same graphs for the other mixes are not presented. As the carbon fiber volume percent increased from 0.2 to 0.35 and from 0.5 to 1, strain could disrupt a smaller percent of the fiber–fiber and fiber–matrix contact; the effect of strain on the electrical resistance got smaller; the %R changed less; sensitivity decreased and gage factor decreased (Fig. 6a). At 0.5 volume %, the system was at percolation threshold; few direct electron conduction paths formed. Strain disrupted these few paths, shifted the system from post-percolation to pre-percolation situation, resulted in the highest sensitivity and gage factor. At percolation threshold fiber volume percent, the cement based composite had the highest gage factor. The %R–strain graph deviates from linearity at most at percolation threshold of 0.5% fiber volume fraction due to the shift of the system from post-percolation to pre-percolation. This led to the highest linearity (LE%) and lowest correlation coefficient (R2) at percolation threshold of 0.5% fiber volume fraction as seen in Fig. 6b and c. The deviation from linearity was not considerable as seen from the LE% and R2 values.
26
E. Teomete / Measurement 74 (2015) 21–30
Fig. 6. Compression test. (a) Gage factor–fiber volume %. (b) Linearity–fiber volume %. (c) Correlation coefficient–fiber volume %.
The highest gage factor obtained was K = 90 at percolation threshold of 0.5% fiber volume percent. The carbon fiber cement based composite is much more sensitive to compressive strain than metal foil strain gages. 3.2. Split tensile test results The split tensile test results for C02 mix are presented in Fig. 7. Displacement controlled test was conducted at a rate of 2.5 mm/min. The stiffness of the wooden bars between the loading plate and the sample was much less than the stiffness of the sample so most of the deformation was shared by the wooden bars at the beginning of the test (Fig. 3b and c). At 37th second, the wooden bars started to crush and caused a plateau in the force–time and strain– time graphs as seen in Fig. 7a and b. As the wooden bars crushed, their stiffness increased and the slope of the force–time graph increased as seen in Fig. 7a. At 143rd second, the wooden bars were almost totally crushed and their stiffness increased which led to an increase in the slopes of the force–time and strain–time graphs (Fig. 7a and b). After 143rd second, as the stiffness of the wooden bar increased, the sample started to deform more; there was a sudden increase in the slope of strain–time graph as seen in Fig. 7b. At 165th second, a vertical crack
formed as seen in Fig. 3c which decreased the stiffness of the sample ending in a sudden decrease in the applied load (Fig. 7a). The formation of crack decreased the cross section that the electrons could flow, the change in electrical resistance increased suddenly at 165th second (Fig. 7c). Due to the large value of ordinate axis in Fig. 7c, the variation of %R by strain before 165th second cannot be distinguished although there was a variation which is presented in Fig. 7d. The change in electrical resistance had a strong linear relationship with tensile strain with a correlation coefficient of 0.99. The mix C02 had a gage factor of 50 and linearity of 16%. The mix C02 was 25 times more sensitive to strain than commercial foil strain gages. Similar observations were obtained for the other mixes so the same graphs are not presented for the other mixes. As the fiber volume percent increased from 0.2% to 0.35%, tensile strain could disrupt a smaller amount of fiber–fiber and fiber–matrix contacts, resulting in a decrease in the sensitivity of the system which led to a decrease in the gage factor (Fig. 8a). At percolation threshold of 0.5% volume fraction of fibers, tensile strain disrupted the few direct electron conduction paths and shifted the system from post-percolation situation to pre-percolation situation which resulted in a local maximum gage factor. The gage factor decreased as volume
Fig. 7. Split tensile test results of C02 mix. (a) Force–time graph. (b) Tensile strain–time graph. (c) %R–time graph. (d) %R–strain graph (K = 50, LE = 16).
E. Teomete / Measurement 74 (2015) 21–30
27
Fig. 8. Split tensile test results. (a) Gage factor–fiber volume percent. (b) Linearity–fiber volume percent. (c) R2–fiber volume percent.
percent increased to 0.8% due to the same phenomenon described for the 0.2–0.35% increase. The maximum gage factor obtained was 328 at 1% volume fraction. The carbon fiber reinforced cement composite was 160 times more sensitive to strain than commercial foil strain gages. The %R versus strain graph had a strong linear relationship with low linearity values and high correlation coefficients as presented in Fig. 8b and c. At percolation threshold of 0.5%, the shift of the system from post-percolation to pre-percolation resulted in the highest deviation from linearity while the correlation coefficient was also one of the lowest values. The highest correlation coefficient obtained was 0.99 which testified the strong linear relationship between tensile strain and electrical resistance change. 3.3. Notched bending test results The crack length–time and force–time graphs of C02 mix are presented in Fig. 9a and b. When crack started to propagate from the tip of the notch at 72nd second, the stiffness of the beam sample decreased and the force started to decrease from maximum (Fig. 9a and b). In Fig. 10a, the percent change of electrical resistance (%R)– crack length is presented. The graph is subdivided into regions of A, B, C which are explained below. The time between the beginning of the test and crack initiation (72nd second) formed region A. In region A, the electrical resistance change (%R) oscillates between small positive and negative values (Fig. 10a). The negative values were due to the compressive strain region above the neutral axis of the beam which shortened the fibers, closed the micro cracks and micro voids. The positive %R values were due to the tensile strain below the neutral axis of the beam which elongated the fibers, developed micro cracks and micro voids. The opening of micro cracks and micro voids
decreased the cross section that the electrons could pass ending in an increase in the electrical resistance. Competition of these two events (compressive strain above neutral axis and tensile strain below neutral axis) until 72nd second in region A resulted in the oscillation of electrical resistance from negative to positive values (Fig. 10a). The region B started by initiation of the crack at 72nd second (Fig. 10a). As the crack propagated, the cross section that the electrons could pass decreased dramatically which led to an increase in the electrical resistance change (%R) as seen in Fig. 10a. The crack propagation and its result of decrease in cross section overcame the effect of compressive strain above the neutral axis. The maximum capacity of the crack gage was 21 mm above which the crack gage did not send signal as it would be divided into two parts. At region C, the crack length was over 21 mm, the crack gage did not send more signal while the crack propagated further to the top of the beam which increased the %R as seen in Fig. 10a. There was a linear relationship between the crack length and %R at region B. This linear relationship had a correlation coefficient of 0.95 as seen in Fig. 10b. For mix C02, the crack sensitivity (SC) was 4.8, linearity for crack sensitivity (LEC) was 15%. The results obtained from other mixes were similar so their graphs are not presented here. As the fiber volume percent increased, the percent of fiber–fiber and fiber–matrix contacts broken by the crack propagation decreased; the sensitivity of the system to the crack propagation decreased as seen in Fig. 11a. At percolation threshold of 0.5%, the crack propagation shifted the system from post-percolation to pre-percolation situation by breaking the few direct electron conduction paths which resulted in the highest crack sensitivity as seen in Fig. 11a. The deviation from a line of the %R–crack length data was highest at percolation threshold of 0.5% so the linearity was highest and R2 was lowest as seen in Fig. 11b and c.
Fig. 9. Notched bending test C02 mix with 0.2% volume fraction of 13 mm fibers. (a) Crack length–time graph. (b) Force–time graph.
28
E. Teomete / Measurement 74 (2015) 21–30
Fig. 10. Notched bending test C02 mix with 0.2% volume fraction of 13 mm carbon fibers. (a) %R–crack length whole experiment. (b) %R–crack length, linear relationship, crack sensitivity SC = 4.8; linearity for crack sensitivity LEC = 15%.
Fig. 11. Notched bending test results. (a) Crack sensitivity–fiber volume %. (b) Linearity–fiber volume %. (c) R2–fiber volume %.
This deviation from a line was due to the shift of the system from post-percolation to pre-percolation situation by crack propagation. The variation of electrical resistivity with fiber volume percent was presented in Fig. 12. There is a general trend of decrease of resistivity with fiber volume percent. The small increase at 1% volume fraction was due to the formation of voids between fiber–matrix and fiber–fiber interfaces due to high fiber volume fraction of 13 mm fibers. These voids inhibited electron transfer. A sharp decrease of resistivity at 0.5% was not observed which was the percolation threshold. At percolation threshold, the fibers start to form a few continuous electron transfer lines. These a few lines were not enough to decrease the resistivity excessively. It is obvious that the percolation threshold was between 0.5% and 0.8% from Fig. 12. There is a high probability that the percolation threshold was very close to 0.5% (i.e. 0.51–0.52%). Strong linear relationships between the strain–electrical resistance change and crack length–electrical resistance change are encouraging for developing on smart cement based sensors.
Fig. 12. Resistivity–fiber volume percent relationship for cement composites.
4. Conclusions Five different carbon fiber reinforced cement composites with fiber length of 13 mm were designed. From each mix six of 5 cm cube samples and three of 4 4 16 cm prismatic samples were cast and cured. From each mix, three samples were tested by compression test, split tensile test and notched bending test. A total of 45 tests were conducted. The correlations between tensile strain, compressive strain and crack length with electrical resistance change were determined. The performance measures were used to assess the self sensing performance of the composites. The results obtained from compression are: 1. Compressive strain shortened the carbon fibers, closed the micro voids and micro cracks and resulted in a decrease in electrical resistance. 2. There was a strong linear relationship between electrical resistance change and strain. A gage factor of 76 was obtained which was almost 40 times the gage factor of commercial foil strain gages. 3. As the fiber volume percent increased, the percent of fiber–fiber and matrix fiber contacts disrupted by the strain decreased which ended up a decrease in the gage factor. At percolation threshold of 0.5% fiber volume fraction at which few direct electron conduction paths formed, these few direct electron conduction paths were disrupted; system shifted from post-percolation to pre-percolation situation and highest gage factor obtained. 4. Due to the shift of the system from post-percolation to pre-percolation situation, the highest deviation from linearity was obtained at percolation threshold of
E. Teomete / Measurement 74 (2015) 21–30
0.5%, which resulted in the highest linearity (LE) and lowest correlation coefficient. The results of split tensile tests are presented as below: 1. Split tensile test was used for the first time in this study to assess the relationship between tensile strain–electrical resistance change of cement matrix composites. 2. The tensile strain caused elongation of the fibers, opening of micro cracks and micro voids which resulted in an increase of the electrical resistance. 3. Similar to the results of compression test, the gage factor had a local maximum at a fiber volume fraction of 0.5% due to the shift of the system from post-percolation to pre-percolation situation. The linearity had a maximum at 0.5% volume fraction due to the highest deviation from linearity. A strong linear relationship was observed between the tensile strain and electrical resistance change (%R). The results of notched bending tests are presented as below: 1. The simultaneous measurement of crack length and electrical resistance change to investigate their relationship for cement composites with notched bending test has been achieved for the first time in this work in the literature. Crack sensitivity (SC) and linearity for crack sensitivity (LEC) have been introduced in this work as novel concepts. 2. The propagation of the crack decreased the cross sectional area that the electrons could pass and increased the electrical resistance. A strong linear relationship between the crack length and electrical resistance change was observed. 3. The increase of volume percent of fibers decreased the percent of disrupted fiber–fiber and fiber–matrix contacts which resulted in a decrease of crack sensitivity. 4. At percolation threshold of 0.5%, crack propagation disrupted the few direct electron conduction paths shifting the system from post-percolation to pre-percolation situation which resulted in the highest crack sensitivity. The shift of the system from post-percolation to pre-percolation situation at percolation threshold resulted in the maximum deviation from linearity; highest linearity and lowest R2 were obtained at percolation threshold fiber volume fraction. Strain sensing, crack detection and damage assessment of concrete structures can be achieved by the carbon fiber reinforced cement based composites. The results presented can be used in future work for developing self sensing smart concrete. Acknowledgements This work is supported by The Scientific and Technological Research Council of Turkey (TUBITAK)
29
through Grant No: 110M221. The author would like to thank to Sika Construction Chemicals Co. for providing the silica fume and super-plasticizer and to DowAksa Co. for carbon fibers used in this study. The author would like to thank to the Dokuz Eylul University, Metallurgical and Materials Engineering Department for providing the mechanical testing system for bending tests. The author is thankful to Assoc. Prof. Dr. Tahir Kemal Erdem for his contributions to the work. References [1] F. Reza, G.B. Batson, J.A. Yamamuro, J.S. Lee, Resistance changes during compression of carbon fiber cement composites, J. Mater. Civil. Eng. 15 (5) (2003) 476–483. [2] D.D.L. Chung, Review functional properties of cement–matrix composites, J. Mater. Sci. 36 (2001) 1315–1324. [3] K.T. Lau, Structural health monitoring for smart composites using embedded FBG sensor technology, Mater. Sci. Technol. 30 (13a) (2014) 1642–1654. [4] B.G. Han, B.Z. Han, J.P. Ou, Novel piezoresistive composite with high sensitivity to stress/strain, Mater. Sci. Technol. 26 (7) (2010) 865–870. [5] P.W. Chen, D.D.L. Chung, Low-drying-shrinkage concrete containing carbon fibers, Compos. Part B – Eng. 27 (3–4) (1996) 269–274. [6] P.W. Chen, D.D.L. Chung, Concrete as a new strain stress sensor, Compos. Part B – Eng. 27 (1) (1996) 11–23. [7] X.J. Wang, D.D.L. Chung, Self-monitoring of fatigue damage and dynamic strain in carbon fiber polymer–matrix composite, Compos. Part B – Eng. 29 (1) (1998) 63–73. [8] S.K. Wang, D.D.L. Chung, Temperature/light sensing using carbon fiber polymer–matrix composite, Compos. Part B – Eng. 30 (6) (1999) 591–601. [9] S.K. Wang, D.D.L. Chung, Apparent negative electrical resistance in carbon fiber composites, Compos. Part B – Eng. 30 (6) (1999) 579–590. [10] X.C. Luo, D.D.L. Chung, Electromagnetic interference shielding using continuous carbon–fiber carbon–matrix and polymer–matrix composites, Compos. Part B – Eng. 30 (3) (1999) 227–231. [11] D.D.L. Chung, Cement reinforced with short carbon fibers: a multifunctional material, Compos. Part B – Eng. 31 (6–7) (2000) 511–526. [12] D.D.L. Chung, Self-monitoring structural materials, Mater. Sci. Eng. 22 (2) (1998) 57–78. [13] X. Fu, E. Ma, D.D.L. Chung, W.A. Anderson, Self-monitoring in carbon fiber reinforced mortar by reactance measurement, Cem. Concr. Res. 27 (6) (1997) 845–852. [14] X. Fu, D.D.L. Chung, Effect of curing age on the self-monitoring behavior of carbon fiber reinforced mortar, Cem. Concr. Res. 27 (9) (1997) 1313–1318. [15] E. Teomete, T.K. Erdem, Cement based strain sensor: a step to smart concrete, Cem. Wapno Beton 2 (2011) 78–91. [16] E. Teomete, O.I. Kocyigit, Tensile strain sensitivity of steel fiber reinforced cement matrix composites tested by split tensile test, Constr. Build. Mater. 47 (2013) 962–968. [17] M. Chiarello, R. Zinno, Electrical conductivity of self-monitoring CFRC, Cem. Concr. Compos. 27 (2005) 463–469. [18] B. Han, X. Guan, J. Ou, Electrode design, measuring method and data acquisition system of carbon fiber cement paste piezoresistive sensors, Sens. Actuators, A 135 (2007) 360–369. [19] B. Han, X. Yu, J. Ou, Self-Sensing Concrete in Smart Structures, first ed., Butterworth-Heinemann, 2014, ISBN 9780128005170. [20] B. Chen, J. Liu, Damage in carbon fiber–reinforced concrete, monitored by both electrical resistance measurement and acoustic emission analysis, Constr. Build. Mater. 22 (2008) 2196–2201. [21] H. Li, H. Xiao, J. Ou, Effect of compressive strain on electrical resistivity of carbon black-filled cement-based composites, Cem. Concr. Comp. 28 (2006) 824–828. [22] H. Li, H. Xiao, J. Ou, Electrical property of cement-based composites filled with carbon black under long-term wet and loading condition, Comp. Sci. Technol. 68 (2008) 2114–2119. [23] B. Han, K. Zhang, X. Yu, E. Kwon, J. Ou, Nickel particle based selfsensing pavement for vehicle detection, Measurement 44 (2011) 1645–1650. [24] X. Jing, Y. Wu, W. Ruiqing, Nonlinear conduction in carbon fiber reinforced cement mortar, Cem. Concr. Compos. 33 (2011) 444–448.
30
E. Teomete / Measurement 74 (2015) 21–30
[25] H. Gong, Y. Zhang, J. Quan, S. Che, Preparation and properties of cement based piezoelectric composites modified by CNTs, Curr. Appl. Phys. 11 (3) (2011) 653–656. [26] R. Rianyoi, R. Potong, N. Jaitonong, R. Yimnirun, A. Chaipanich, Dielectric, ferroelectric and piezo electric properties of 0–3 barium titanate–portland cement composite, Appl. Phys. A 104 (2011) 661–666. [27] S. Vaidya, E.N. Allouche, Strain sensing of carbon fiber reinforced geopolymer concrete, Mater. Struct. 44 (2011) 1467–1475. [28] Z.J. Li, D. Zhang, K.R. Wu, Cement matrix 2–2 piezoelectric composite Part 1 Sensory effect, Mater. Struct. 34 (2001) 506–512. [29] J.M. Torrents, T.O. Mason, A. Peled, S.P. Shah, E.J. Garboczi, Analysis of the impedance spectra of short conductive fiber-reinforced composites, J. Mater. Sci. 36 (2001) 4003–4012. [30] A.D. Hixson, L.Y. Woo, M.A. Campo, T.O. Mason, E.J. Garboczi, Intrinsic conductivity of short conductive fibers in composites by impedance spectroscopy, J. Electroceram. 7 (2001) 189–195. [31] A. Peled, J.M. Torrents, T.O. Mason, S.P. Shah, E.J. Garboczi, Electrical impedance spectra to monitor damage during tensile loading of cement composites, ACI Mater. J. 98 (4) (2001) 313–322. [32] J.M. Torrents, T.C. Easley, K.T. Faber, T.O. Mason, Evolution of impedance spectra during debonding and pullout of single steel fibers from cement, J. Am. Ceram. Soc. 84 (4) (2001) 740–746. [33] T.O. Mason, M.A. Campo, A.D. Hixson, L.Y. Woo, Impedance spectroscopy of fiber-reinforced cement composites, Cem. Concr. Compos. 24 (2002) 457–465.
[34] L.Y. Woo, N.J. Kidner, S. Wansom, T.O. Mason, Combined time domain reflectometry and AC-impedance spectroscopy of fiberreinforced fresh-cement composites, Cem. Concr. Res. 37 (2007) 89–95. [35] J.D. Shane, T.O. Mason, H.M. Jennings, Effect of the interfacial transition zone on the conductivity of portland cement mortars, J. Am. Ceram. Soc. 83 (5) (2000) 1137–1144. [36] F. Rajabipour, J. Weiss, J.D. Shane, T.O. Mason, P.S. Surendra, Procedure to interpret electrical conductivity measurements in cover concrete during rewetting, J. Mater. Civ. Eng. 17 (5) (2005) 586–594. [37] P.J. Tumidajski, P. Xie, M. Arnott, J.J. Beaudoin, Overlay current in a conductive concrete snow melting system, Cem. Concr. Res. 33 (2003) 1807–1809. [38] B. Han, X. Yu, E. Kwon, A self-sensing carbon nanotube/cement composite for traffic monitoring, Nanotechnology 20 (44) (2009) 1–5. [39] B. Han, X. Yu, J. Ou, Effect of water content on the piezoresistivity of MWNT/cement composites, J. Mater. Sci. 45 (2010) 3714–3719. [40] B. Han, S. Ding, X. Yu, Intrinsic self-sensing concrete and structures: a review, Measurement 59 (2015) 110–128. [41] European Committee for Standardization EN 12390-6: Testing hardened concrete – Part 6: Tensile splitting strength of test specimens. [42] J. Ou, B. Han, Piezoresistive cement-based strain sensors and selfsensing concrete components, J. Intell. Mater. Syst. Struct. 20 (2009) 329–336.