Journal of Environmental Treatment Techniques PDF |
2017, Volume 5, Issue 3, Pages: |
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J. Environ. Treat. Tech.
ISSN:
Journal weblink: http://www.jett.dormaj.com
A Structural Health Monitoring Technique
S. A. Maqbool, A. Arshad
National University of Sciences and Technology, Islamabad, Pakistan
Received: 06/06/2017 |
Accepted: 28/08/2017 |
Published: 17/09/2017 |
Abstract
The addition of carbon microfibers has proved to be one of the most effective means of improving the electrical conductivity of cementations matrix. In the present study, 5x5x5cm carbon fiber reinforced mortar specimens were positioned in a 60Hz, ± 2.5V AC circuit with a data acquisition system in order to monitor the changes in its electrical resistivity under the influence of different parameters. It was seen that a high fiber fraction and low moisture content makes the specimen act like a pure resistor with negligible capacitance or inductance associated with it. It was also observed that electronic conduction was dominant over electrolytic conduction in a mix proportion with high fiber volume fraction and low water to cement ratio. The resistivity was found to steadily decrease under compressive loading and then increase during the formation of micro and
Keywords: Concrete, carbon fiber, resistivity, loading rate
1 Introduction 1
Structural health monitoring (SHM) is the most modern development in the field of civil engineering. It aims to develop automated systems for the continuous monitoring, inspection, and damage detection of structures. Civil engineering infrastructure is generally the most expensive national investment and asset of any country. In addition, civil engineering structures have long service life compared with other commercial products, and they are costly to maintain and replace once they are erected [1]. Further, there are few prototypes in civil engineering, and each structure leads to be unique in terms of materials, design, and construction. All civil structures age and deteriorate with time. Innovations like
Corresponding author: Arshad Ali, National University of Sciences and Technology, Islamabad, Pakistan, E- mail: aliarshad08@yahoo.com.
volume, and absence of mechanical property degradation due to the embedding of smart materials, etc [4].
Plain cement mortar is a semiconductor or insulator with resistivity of the very high order. Conductivity of a cement based specimen depends on the pore structure and the chemistry of the pore solution. Introduction of carbon fibers into cementations matrix significantly increases the electrical conductivity enabling one to monitor changes in resistivity under varying degrees of strain [5, 6]. Hence the measurement of variation in resistivity helps one to monitor the response of smart material structures under these varying conditions [7].
This study was carried out to develop an economic smart material using carbon microfibers that can be used in field of civil engineering for the purpose of structural health monitoring. The objective of this research was to develop smart
2 Materials and Methods
Cement was a Type 1 Ordinary Portland Cement (OPC) obtained from the local market. The chemical composition and properties of the cement used in the study is given in Table 1 and 2, respectively. The locally available, silica fume with bulk density 550 kg/m3 were used. The chemical composition of the silica fume used in the study is presented in Table 3. Polyacrylonitrile (PAN)
Two different water reducer i.e. Ultra High Range Sika
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Table 1: Percentage chemical composition of OPC used
in the study
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Chemical composition |
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Percentage |
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C3S |
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63 |
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C2S |
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13 |
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C3A |
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6 |
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C4AF |
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11 |
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CaO |
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65.4 |
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SiO2 |
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21.1 |
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Al2O3 |
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4.44 |
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Fe2O3 |
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3.68 |
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MgO |
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0.9 |
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SO3 |
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2.7 |
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Loss on ignition |
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1.16 |
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Alkalies |
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0.38 |
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Table 2: Properties of OPC used in the study |
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Laboratory tests |
Values |
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ASTM |
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Specification |
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Standard |
24% |
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consistency |
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Initial setting time |
120 min |
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60 min |
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(minimum) |
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Final setting time |
480 min |
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375 min |
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(maximum) |
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Fineness |
99% |
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90% |
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(minimum) |
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Soundness |
5.33 mm |
10 mm |
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(maximum) |
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Specific gravity |
3.12 |
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Table 3: Percentage chemical composition of silica fume
used in the study
Chemical composition |
Percentage |
SiO2 |
88.9 |
Al3O2 |
0.0065 |
TiO2 |
0.002 |
P2O5 |
0.08 |
CaO |
0.93 |
MgO |
0.54 |
Na2O |
0.52 |
Fe2O3 |
0.85 |
K2O |
0.60 |
SO3 |
0.25 |
Loss on ignition |
4.5 |
Table 4: Properties of carbon fiber used in the study
Parameter |
Amount |
Length |
5 mm |
Diameter |
7 µm |
Tensile strength |
4900 MPa |
Tensile modulus |
250 GPa |
Elongation |
2% |
Density |
1.81 g/cm3 |
Normal drinking water was used in specimen preparation and their curing. Mix proportions given in Table 6 is the final mix used for making specimens for different sets of experiments, this mix was finalized while aiming for compressive strength above 6000psi,
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for that different combination of mixes were used i.e., sand, water/cement ratio,
Table 5: Physical properties of sand used in the study
Physical properties |
Amount |
Fineness modulus |
2.83 |
Bulk specific gravity |
2.65 |
Bulk specific (oven dry) |
2.61 |
Water absorption |
1.5 |
Apparent specific gravity |
2.23 |
Specimens were casted with dimensions 5x5x5cm. Two copper electrodes of thickness 0.35mm and width 10mm were inserted through the entire depth of the specimen as shown in the Figure 1. The
technique was adopted. Electrical resistivity measurements were made by applying a known AC voltage of ±2.5V across the electrodes using a function generator at a frequency of 60Hz [12, 13].
Table 6: Mix proportions by weight of concrete used in
the study
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Silica |
Sand/ |
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Water/ |
fume/ |
Carbon fiber |
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cement |
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cement ratio |
cement |
ratio |
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ratio |
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ratio |
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0.40 |
- |
- |
0 |
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0.40 |
- |
- |
1 |
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0.40 |
0.2 |
1.5 |
2 |
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0.40 |
- |
- |
3 |
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0.40 |
- |
- |
4 |
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0.40 |
- |
- |
5 |
3 Results and Discussion
All systems in this universe are in a state of entropy. There are always elements at work that bring about changes in a system. In a system like carbon fiber reinforced cement mortar specimen, there are various factors that play a critical role in changing its resistivity. The following paragraphs enumerate the internal factors that affect the electrical resistivity of such a system.
There are basically two mechanisms of electrical conduction in moist specimens: electronic and electrolytic. Electronic conduction is through the motion of free electrons in the conductive phases, e.g. carbon fibers, and electrolytic conduction is through the motion of ions in the pore solution [14, 15].
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Fig 1: Specimen for resistivity measurements
The conductivity of mortar can be attributed to mainly three media: carbon fibers, the pore solution within the matrix and the physical interface between the fibers and matrix.
Carbon fibers are known to reduce the electrical resistivity of cementations composites. The conductivity of the mix is directly proportional to the volume fraction of carbon fibers and can be visualized as shown in Figure
2.The resistivity behavior for mixes with different fiber fraction volume further asserts this fact as illustrated in Figure 3. The conductivity in a carbon microfiber reinforced mortar specimen follows the phenomena of percolation theory. Percolation theory is basically a geometrical theory that describes the structure of random particles or filaments in a matrix as a function of their volume fraction [16].
Fig 2.0: Percolation Theory
0% CF
1% CF
2% CF
Resistivity (Ω - cm) |
Days |
Fig 3: Resistivity as a function of percentage of fiber volume
It postulates that it is only when the volume fraction of the particle or filament exceeds a certain critical value that the particle or filament can come into contact and form clusters. As a result, electrical conduction can occur due to the connection of the clusters. Percolation threshold thus is the critical fraction of lattice points that must be filled in order to create a continuous path of nearest neighbors from one side to another. Thus the electrical conductivity of the specimen also largely depends on the fiber's aspect ratio and material [17]. A higher volume fraction ensures that there exists more
0.6w/c ratio has lesser resistivity than 5% fiber volume and 0.5 w/c ratio.
This can be attributed to the large amount of pore solution available in the capillary pores of the mix with
0.5w/c ratio. However the combination of 5% fiber volume and 0.4 w/c ratio has a lesser resistivity value than the one with 0.5 w/c ratio and is lower than even the mix with 0.5 w/c ratio. This is because the former most has larger amount of fines and lesser pores with respect to both the latter ones thus allowing denser packing and more
0.3the conduction transfers from electrolytic to electronic and
Figure 4 and Figure 5 show the resistivity behavior and drop in voltage of various specimens subjected to compression, respectively.
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CF 1% |
CF 0% |
CF 2% |
CF 3% |
CF 4% |
CF 5% |
Resistivity (Ohm.cm)
Time (seconds)
0% CF
1% CF
2% CF
3% CF
4% CF
5% CF
Voltage drop (volts)
Time (seconds)
Fig 5: Drop in voltage of various specimens under compression
A significant increase in the maximum strain achieved prior to failure was observed. But the objective of conducting compressive test on smart material is to observe the change in electrical resistivity with increase in load to develop an effective SHM mechanism [19, 20]. All the specimens were loaded till failure at constant loading rate. Initially as load increases for specimen having 0 - 3% carbon fiber no change in resistivity observed which is represented by straight line in graph. Finally as the load reaches the full capacity of the specimen there was a sudden rise in the resistivity value. This can be attributed to the complete failure of the matrix. Although there was a significant decrease in
resistivity of specimen with the introduction of 1 - 3% carbon fibers in the specimen but results are indicating that percolation network was not established resulting in failure to detect any change in electrical resistivity under loading.
As we increase the fiber percentage further to 3 and
4% the resistivity decreases five times of plain matrix. And when placed under compression, initially there was no resistivity change observed but as the load reached 70% of ultimate strength it was seen that the resistivity too started increasing. This increase in the resistivity was a clear sign of formation of micro cracks in matrix. Finally as the load reached the full capacity of the
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specimen there was a sudden increase in the resistivity value which shows the complete failure of specimen.
CF 0% |
CF 1% |
CF 2% |
CF 3% |
CF 4% |
CF 5% |
Resistivity (Ω - cm)
Time (seconds)
Fig 6: Variation of resistivity under cyclic loading for various specimens
Finally on adding 5% fibers the resistivity decrease 52 times of plain matrix clearly indicating the development of percolation network within the specimens. When placed under the compressive loading, initially as load increases there is a steady fall in the resistivity value. This can be attributed to the increase in contact between carbon fibers by the closing of voids and thus decrease in the distance between the matrix and the copper plates. Then there is a relatively flat portion, which indicates that the closing of original micro cracks and opening of new ones reached a dynamic balance [21, 22]. Gradually as the load increased it was seen that the resistivity too started increasing. This increase in the resistivity was a clear indication of formation of new micro cracks within the matrix, which created distance between the fibers and weakened the
It can be clearly seen from the graph that as the fiber content increases resistivity decreases. Lowest value of resistivity corresponds to the 5% fiber reinforcement. Another important observation was the similar trend of 1% and 2%fiber reinforced mortar samples and same can be observed for 3% and 4% fiber reinforced samples. Samples with 5% fiber reinforcement clearly stand out showing the formation of percolation network. However when used to monitor strain the resistivity changes before the formation of micro cracks are relevant or in other words the resistivity changes that are reversible on load removal are the values that can really tell what is happening in the structure. During the experiment it was noted that the initial resistivity changes only in the range of 5 - 10 % of ultimate load was irreversible. The resistivity changes during load values higher than 10 % of maximum load and up to 45 % of maximum load were
reversible to a great extent. This contradicts to the existing literature [23, 24, 25] where an irreversible change in the resistivity values in the initial phase of loading due to flaw generation was noticed, and then a reversible decrease in the resistivity values till larger cracks was observed. The reason for this could be the low resistivity of the specimen itself, which makes the resistance changes too small to be conveniently detected. From the figures it can be seen that the slope of the resistivity in the elastic range in greater than the slope in the strain hardening and softening regions [26, 27]. Thus, indicating that such a smart material can more efficiently monitor strains in the elastic range of compressive stress application than in the post crack period.
As shown in Figure 6.0, the sensing of damage under increasing stresses was demonstrated in carbon fiber- reinforced mortar. The damage was found to be accompanied by a partially reversible increase in the electrical resistivity of the mortar. The greater the damage, the larger was the resistivity increase. As fiber breakage would have resulted in an irreversible resistivity increase, the damage is probably not due to fiber breakage, but due to partially reversible interface degradation. The interface could be that between fiber and matrix. This behavior was observed within the elastic regime.
In order to study the response of the cyclic loading, samples were cast using different percentage of carbon microfibers. All samples were cured for 28 days before testing. The samples were subjected to cyclic loading. The load during each cycle was gradually increased to 75 % of the ultimate capacity of the sample.
The behavior of the specimen cyclic loading was found to be similar to that under compressive loading. Resistivity decreased in the initial phase till it reached a relatively flat phase after which it increased. An increase in the resistivity towards the end, indicate that micro cracks were beginning to form at higher load values.
Specimens having
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under cyclic loading were unable to observe any significant change in resistivity and data was very noisy for lower fiber
As we increase the carbon fiber percentage to
4 Conclusions and Recommendations
The electrical resistivity mortar increases over time. But increase in fiber volume fraction at constant w/c ratio decreased the electrical resistivity significantly. The density of carbon fiber reinforced mortar is reduced due to addition of air voids. A mix proportion with lower w/c ratio and higher fiber volume fraction had lower electrical resistivity and
Further studies can be carried out to study the influence of carbon microfibers on other mechanical properties of mortar such as tensile strength, tensile ductility, flexure toughness, impact resistance and freeze thaw durability etc. Carbon microfiber reinforced matrix can further be used in the field of SHM to study temperature sensing ability, thermoelectric behavior, thermal insulation ability and improved resistance to earthquake.
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