Evaluation of Sensor Performance for Concrete Applications Sherif Yehia, Osama Abudayyeh, Ikhlas Abdel-Qader, Ammar Zalt, and Vijay Meganathan Structural health monitoring using sensor technology is one of the most
HEALTH MONITORING OESIGN PROCEOURES
promising ways that can provide an excellent means for protecting
The main steps in the process of designing a health monitoring system are summarized in Figure 1. The /lrst step is to identify the parameters that need to be monitored. Next, the sensors that will be used to detect these parameters must be determined. Then, a laboratory calibration of the sensors should be perfonned to verify its perfonnance and behavior. Depending on the results obtained from this step, a decision on deploying the sensor should be made.
important structures such as bridges. dams, and nuclear reactors. These sensors provide real-time information about structural conditions such
as strain, temperature, and vibration. The information obtained from these sensors can later he analyzed and compared with the design data; the process allows the eady detection of problems. This paper presents
experimental results on two types of sensors to verify their behavior, accuracy, and applicability for strain monitoring of concrete applieation.'i, The investigation included concrete curing-behavior test, thermal test, accuracy test, and other t€!sts to assess the mechanical properties of concrete with embedded sensors.
STRAIN MONITORING INSTRUMENTATION Monitoring of strains at critical locations in the bridge deck can provide cmcial infonnation about creep, shrinkage, and temperature effects as well as stress due to loading. Gathering this infonnation can provide a basis for predicting the behavior of similar bridges. Two types of strain should be considered: short- and long-tenn strains. Short-tenn strains are those changes that occur over a period of hours, while long-term strains are those changes occurring over a period of months or years. Short-term strain is generally caused by changes in dead and live loads, daily temperature cycles, or wind. Long-term strain is caused by seasonal temperature changes, creep, and shrinkage in the concrete structure (4). The most common types of sensors used in bridge health monitoring are strain sensors, measuring elements that translate force, pressure, and tension into strain readings. Various types of sensors have been used successfully in bridge health monitoring, such as VWSGs, electrical resistance strain gauges, fiber-optic sensors (FOSs), and wireless sensors. Each one of these sensors will be discussed below. 'The overall advantages and disadvantages of the four types of sensors mentioned above are summarized in Table 1.
Different inspection techniques have been used for bridge condition assessment. Traditional inspection techniques, such as visual inspection and chain drag, are mainly subjective and cannot detect the defects at their occurrence. According to a recent study by the FHWA, more than 56% of the average condition rating performed on bridges using visual inspection was wrong with a 95% probability (1). One way to overcome the shortages in the available inspection techniques is to use the concept of structural health monitoring (SHM) with state-of-the-art sensor technology. SHM is used 10 identify, record, and measure structural perfonnance. It helps in detecting and evaluating any problems that can affect a structw-e's safety, serviceability, and integrity (2). In other words, it is the ability to proactively manage structural health by diagnosing deterioration and damage at the time of its occurrence [0 guarantee public safety, to prolong the service life of the bridge, and to reduce future costs (3). Various types of sensors have been successfully used in bridge health monitoring applications, including strain sensors, which are the focus of this paper. The emphasis is on assessing the behavior and the performance of the two common types of strain sensors: vibrating wire strain gauges (VWSGs) and embeddable fiber-optic sensors (EFOSs). The main objective of the experimental evaluation is to understand the sensors' output and behavior when embedded inside real structures.
Vibrating Wire Strain Gauges According to FHWA Guidelines for Instrumentation of Bridges, VWSGs are recommended for monitoring long-term strains in concrete (4). A VWSG has a body composed of a steel tube with flanges or end plates attached to either end. Inside the tube a steel wire is held in tension between the two end plates. Any strain in concrete causes the plates (0 move relative to each other, decreasing or increasing the tension in the wire. This tension in the wire is then measured by plucking the wire with an electromagnetic coil and measuring the frequency of the resulting vibration (5. 13). The amount of time required to perfonn the pluck-read operation on a sensor is less than I s. Therefore, when numerous sensors are multiplexed to a data acquisition system, several seconds may be required to cycle through
s. Yehia and O. Abudayyeh, Civil and Construction Engineering Department, and I. Abdel-Qader and V. Meganathan, Electrical and Computer Engineering Departmerit, Westem Michigan University. 1903 West Michigan Avenue, Kalamazoo, MI480D8-5316. A. Zalt, Parsons Corporation, 100 Broadway, New York. NY 10005. Corresponding author: S. Yehia,
[email protected]. Transportation Research Record: ,journal of the Transportation Research Board. No. 2050. Transportation Research Board of the National Academies, Washington. DC, 2008, pp. 101-110. 001: 10.3141/2050-10
101
102
Transportation Research Record 2050 -------~
Parameter Identification
Step 1:
Step 2:
Step 3:
Sensor Selection
Laboratory
No
Calibration
Ye, Step 4: FIGURE 1
Real Structures Deployment
Procedure of sensor selection. The baseline system is based on a reference line provided by a teut piano wire, and the
measurement between the reference and the bridge girder or deck. all the VWSGs. Due to this limitation; VWSGs are not suitable for dynamic measurements (6),
Electrical Resistance Strain Gauges The design of electrical resistance strain gauges is based on the fact that the electrical resistance of a conductor will change when it is subjected to strain in either tension or compression. Because the resistance is directly proportional to the length of the conductor, any change in the length resulting from strain will translate to changes in its resistance. When the conductor is stretched, it becomes longer and narrower, which causes an increase in resistance. A Wheatstone bridge circuit then converts this change in resistance to a voltage (6, 7).
Fibe ....Optic Sensors An emerging technology for strain measurement in concrete is the use of FOSs. Main advantages of these sensors are their high sensitivity and their immunity to electromagnetic interference. They can be used to measure a wide variety of applications, including strain, temperature, corrosion. crack formation. and displacement. Two types of FOSs are commonly used for strain monitoring: fiber Bragg gratings (FBO) and Fabry~Perot sensors. FBG sensors have a series of engravings, or Bragg gratings, in their fiber core, The Bragg gratings reflect back an optical wavelength through diffraction. When there is a change in the grating, the peak wavelength of the reflected light is shifted. Any strain or change in mechanical property can thus be measured (8). FBG sensors can be connected in series or multi-
103
Yehia, Abudayyeh, Abdel-Qader, Zalt, and Meganathan
TABLE 1
Advantages Bnd Disadvantages of Electrical, Vibrating Wire, Fiber-Optic, and Wireless Sensors (5-12)
Sensor
Function
Advantages
Disadvanlages
Electrical resistance strain gauges
ShorHenn monitoring:
Operate over a wide range of temperatures Inexpensive Suitable for dynamic Loads Available in a wide variety of gauge lengths Provides an electrical signal that can be measured with a wide variety of circuits
Data readout equipment is expensive Tedious installation, time-consuming to install and to connect to data acquisition system Affected by electromagnetic interference Lead lengtb limitation
Vibrating wire gauges
Long-ternl monitoring
Long-term reliability Multiplexing ability Easy installation Low cost Immune to electromagnetic interference Rugged housing resistant to impact and corrosion Measures temperature as well as strain
May require long lengths of wire Cannot monitor live loads
Fiber-optic sensors
Long-tenn monitoring and short-tenn monitoring
Light weight Small size Multiplexing ability Immune to electromagnetic interference Environmental ruggedness Extremely accurate
The fiber wire should be handled carefully and gc-ntly Expensive hardware and software Long-telID behavior still under investigation Expensive installation
Wireless system
Short-tenn moniloring
No cables are required for data transfer Low cost of deployment Each mote works independently
Restricted battery life Still under investigation
plexed to reduce cable length. Fabry-Perot sensors consist of a tube containing optical fibers that create a reflective interface. These sensors are very accurate and have a low sensitivity to thermal effects (9).
Wireless System Conventional sensors, which depend on the use of cables to communicate their measurements to a central processing unit, have very high installation costs, and the wires themselves might be damaged, which will affect the output results. Being aware of these disadvantages, researchers have focused efforts on developing wirele-ss monitoring systems that have lowerinitial and installation costs and can ensure a greater degree of reliability in the communication of sensor measurements (10). A wireless sensor network consists of hundreds of small nodes, or "motes," which are independent sensing devices (strain gauges, accelerometers. or linear voltage displacement TABLE 2
transducers) that incorporaLe a microcontroller (computer on a chip to control electronic device), power unit, and a communication module (11). A wireless sensor network is designed to work on batteries, which limit the nem:ork life span to between 5 and 10 years, not long enough for real-time monitoring of a civil infrastructure. Hence. it is important to design and program the sensor node to minimize its overall power consumption so that the life span of monitoring can be maximized (12).
EXPERIMENTAL PROGRAM According to recent literature (5, 14-17), the most common sensor types used for long-tenn monitoring applications are VWSGs and FOSs, either Bragg grating or Fabry-Perot sensors. which were the focus of the experimemal study. Table 2 provides a general comparison found in the literature of different parameters of both EFOSs and VWSGs.
Comparison Between Parameters of Fiber Optic and Vibrating Wire Sensors (5. 14-171 EFOS
VWSG
Gauge length
t to 500 mm
50 to 300 mm
Resolution
0.01 % full scale
I microstrain
Measurement range
±2,OOO to ±I 0,000 microstrain
±2,000 to ±3,000 micrmtrain
Parameter
Remote operations possibility
Yes
Ye,
Working principle
Measuring the change in optical characteristics such as intensity. wave length, phase
Measuring the frequency of a taut wire
Availability for embedment and surrace mounting
Yes
Yes
Material of sensor
High-strength silica
High-strength steel piano wire
Structural response capabilities
Static and dynamic loads
Just static loads
Temperature range
-20°C to 60°C
-20"C to 80°C
Immunity to electromagnetic interface
Yes
Yes
Ability for multiple: Another reason could be the size difference between lhe two specimens, which led to a difference in strain. The overall strain during the 28 days of curing due to moisture loss, s.hrinkage, and temperature variation are summarized in Table 4.
Sensor Response to Temperature Change To evaluate the perfonnance of the EFOSs and VWSGs due to temperature variations. the- specimens labeled CVFl, CVF2, CVF3, and CVF4 were subjected to cycles offrcczing and thawing to calculate the coeft1cient of thennal expansion of the concrete. First, the four specimens were placed in a cold chamber and then in an area of ambient temperature, as shown in Figure 6. The temperature of the cold chamber varied belween --2"C and -20"C, while the ambient temperature during the test varied bet~veen t2"C and 25"c' To obtain strain due to lemperature variation from the VWSGs, a correction for the steel gauge effect was made as noted earlier, \vhile for EFOSs no temperature correction was needed. The reason for this is that the thcl1nal coefficient of the EFOSs is very close to that of concrete. The obtained mean value of the coefficient of thermal expansion is about 9.7f-l-E/"C for the VWSGs and about 8.1 flt'.I"C for
FIGURE 6 Concrete specimens in a
cold
Sensor Accuracy Sensor accuracy is the degree of conformity of the measured qmH1(ity to its actual or true value. This value is generally expressed as a maximum positive or l1egaLive percentage of the full-scale output. For in~lancc, if the specified sensor accuracy is ±4% of the full-scale output and the range of the sensor is 2,000 lb, the measurement can be expected to be within ±80 Ib of the tme reading (4,20). To demonstrate the accuracy parameters for the two types of sensors, four cylinders were tested. A known value of the load was applied and the corresponding strain was calculated. This value represented the known strain (theoretical strain) that \vas used for comparison with the resulting strain from both sensors. The resulls of the accuracy test are shown in Figure 7. Figme 7 shows the stress-strain relationship that was recorded during the- accuracy test. Both sensors and the theoretical data were in agreement up to 200-1--! strain. The experimental results also show that the strain measured \vith the EFOSs was (;loser to the theoreti~ cal strain thanlhat obtained \vith the V\VSGs. Both sensors deviated from the theoretical strain. The ovemll deviation from the theoretical strain was found to be 3% of full-scale output at the applied force for the VWSGs and 2.4% offun~scale output oftlle applied force for the EFOSs.
Sensor Repeatability: Precisian Sensor repeatability is the me-asure of agreement between the results of successive measurements ortlle same measurand under the same conditions (4,20). The most important factor that has to be taken into considel'alioll for measuting the repeatability of' a sensor is that the measurements should be carried out in a short period \:V'ith the same equipment, the same timing, and the same observer. The overall eITor was found to be 2.7% of the full-scale output of applied force for the VWSGs and 2.0% of the full-scale output of the applied
chamber and ambient temperature.
108
Transportation Research Record 2050 _VWSG
0
- - - EFOS , 4- ,- Theoretical
-200 "2
E
-400
1;;
e
.~
ec ~
-600 -800 -1000 -1200 0
1000
2000
4000
3000
5000
Stress (Psi)
FIGURE 7
Accuracy test results.
force for the EFOSs. A higher degree of agreement was found by using the EFOSs than by using the VWSGs.
of elasticity, and flexure. The results obtained from the sensors were then compared with those of traditional measUling techniques.
Sensor Reproducibility
Creep and Shrinkage Results Due to loss of water, concrete specimens undergo a change in volume knovm as shrinkage. When concrete is subjected to a sustained stress, creep strain will develop gradually with time. Creep and shrinkage strain measurements were perfonned on specimens with VWSGs and EFOSs. Readings were taken for comparison by using DEMEC points attached to two surfaces of the prisms at three points spaced at approximately 8 in. Creep tests were performed according to ASTM C512-02. Creep specimens were cast with a sensor embedded inside. The sensors were oricntcd longitudinally in the center of the specimen. Because the concrete prisms did not contain rebar, the sensors were suspended from above into the forms. The prisms were wrapped in moist burlap for curing and placed in a spring-loaded creep frame for the creep tests. Load was applied with a hydraulic jack and monitored with a load cell. The results of the creep and shrinkage tests are shown in Figure 8. Figure 8 shows that the DEMEC points gave higher strain than the sensor output. The reason could be that the surface of the concrete
Sensor reproducibility is the closeness of the agreement between the results of measurements ofthe same force carried out under differ ent conditions (4, 20). In this test, the measurements were taken on two different days with different operators and different loading machines. The results showed good agreement in the readings from the first and second days, with an overall error obtained from VWSGs to be 2.2% of the full-scale output of the applied force and 2.1 % of the full-scale output of the applied force from EFOSs, which was within the manufacturer's specified range. w
Use of Sensors to Determine Mechanical Properties of Concrete The hardened-stage properties of the concrete mix were tested by using the embedded sensors inside the concrete cylinders and prisms. Three tests were perfonned during this stage: creep and shrinkage, modulus
Variable --- Creep VWSG ..... Creep DEMEC •• _. Shrinkable VWSG - 4 - Shnnkable DEMEC
0
·200
:?
·400
~
·600
•
.~ ·aoo
·200
I
-400
·600
.0
e ·1000
~
Variable - - Creep EFOS -"··Creep DEMEC ._•. Shrinkable EFOS ~ Shrinkable DEMEC
0
I
·1200
,
·1000
'"
·1400 ·1600
o
..• ·aoo
~
.
;;
', ,--~---
·1200 ·1400
~-~.----::--,':"-~..,-"'~ -
·1600 10
20 Time (Days)
30
o
40
10
(aj
FIGURE 8
Creep and shrinkage response EFOS end VWSG Bnd DEMEC points:
30
20
40
TIme (Days)
(bj (8)
EFOS and DEMEC and
(b]
VWSG Bnd DEMEC.
109
Yehia, Abudayyeh, Abdel-Gader, Zalt, and Meganathan
4000
3000 ~
!!:.
w w 2000
& 1000
0 0.0000 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006 0.0007 O.OOOS 0.0009
Strain Value FIGURE 9 Stress-strain relationship: sampla of results of modulus of elasticity test at 14 days of curing.
was having more strain due to its direct contact with the surrounding environment. In addition, the surface was easier to deform under the load when compared with the core of the prisms, where the sensors were embedded. After finishing the creep and shrinkage tests, the frames were released and the strain was tracked. The elastic strain retained after releasing the frame with VWSG was about 60 I JlE. while the elastic strain after loading was about 620 JlE. For the EFOSs, the retained strain was around 450 1lE. while the elastic strain was around 475 flE. This means that most of the elastic strain was obtained after releasing the frame, while the other strain stayed as permanent deformation.
Figure 9 shows a sample of the results ofrnodulus of elasticity at 14 days of curing. The results showed that the EFOSs correlated better when compared with the results of conventional methods.
Flexure Test A concrete beam 9 in. x 11 in. x 8 ft (23 em x 28 em x 2.4 m) was cast with longitudinal rebar and minimum shear confinement. Two VWSGs were placed parallel to the tension steel at a distance 00 in. (7.6 em) from the bottom of the beam next to the support. The load was appliedgrndually up to 10,000 Ib (4,536 kg) at the middle ofthe beam by means of a hydraulic jack, and the strain output from the sensors was monitored and compared with the calculated strain later, as shown in Figure 10. The calculated stain was detennined by using the moment values at the sensor's location due to the applied load. The result showed good correlation between the calculated strain and the sensor output. This in fact demonstrated the ability of the
Modulus of Elasticity The modulus of elasticity test was conducted according to ASTM C 469-02. The test was conducted at 2,7, 14,21, and 28 days. The results obtained from each sensor were compared to the LVDT results.
10000
_'NJSG Strain ~. - Calculated Strain
,
8000
'S 0
•
,•
,
/
/
6000
0
!!:.
•
~ 0
4000
"-
2000
, ,"
0 0
10
20 Strain (Mlcrostraln)
FIGURE 10
Flexure test: load versus strain,
30
40
110 TABLE 5
Transportation Research Record 2050 Sensors Experimental Results Summary
Parameter
VWSG
FOS
Curing strain
Less consistent and stable behavior
More consistent and stable behavior
Themlal strain
IO)JifC the average value of the coefficient of thennal expansion of concrete
81lEI"C the average value of the coefficient of thermal
Accuracy
3% of full-scale output
Precision (repeatability)
2.7% of full-scale output 2.2% of full-scale output
2.4% of full-scale output 2% of full-scale output
Reproducibility Concrete mechanical property creep and shrinkage
CONCLUDING REMARKS Various tesls were conducted to evaluate the performance characteristics of strain sensors. The experimental study was intended to provide understanding of the sensor behavior and response when the sensors are deployed on real structures. Laboratory test results showed that both sensors had good load and temperature responses. Table 5 provides a summary of the experimental evaluation of these sensors.
ACKNOWLEDGMENTS The authors acknowledge the support of Dennis Randolph of the Calhoun County Road Commission in Marshall, Michigan, and Steve Kahl of the Experimental Studies Group, Michigan Department of Transportation (MDOT), for providing timely assistance and valuable feedback during this project. Partial support for this work provided by C~dhoun County Road Commission and MDOT is acknowledged and greatly appreciated.
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2. 3.
4. 5. 6.
2.1 % of full-scale output
Less strain comparing to the DEMEC points
strain sensors to function under different load conditions. The total difference at the maximum load applied was about 4.75%.
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expansion of concrete
Less strain compruing to the DEMEC points
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