Simple Smart Piezoelectric Bolt Sensor for Structural Monitoring of Bridges

Ageing and heavy traffic affect the structural condition of bridges and evaluation of their structural integrity is an important task to ensure their safety utilization. Structural health monitoring constitutes a method for continuous evaluation of the condition of a target construction and for this purpose many sensors and systems have been developed. In this research a simple smart piezoelectric sensor was develop and description of the proposed system, basic experiments to estimate the properties of the sensor and preliminary application to evaluate the dynamic characteristics of a target bridge are discussed. The applicability of this new sensor to estimate appropriately the structural condition of bridges was verified by comparing the measurements results with those obtained by means of more sophisticated sensors.


Introduction
After the Pacific War, in Japan many building were constructed as a part of the reconstruction program in itiated by the Japanese Govern ment. Therefore old buildings of that date are now in danger due to the deterioration of their structures. On the other hand in the decade of 1980 of the last century constructions of large pro jects of building and bridges were initiated. Then high demand of materials, in special for reinforced concrete structures, induced the used of sea sand which contains salts that in reaction with water and cement orig inates the corrosion of the steel reinforcement. Outside Japan an examp le of the necessity of health monitoring was evident in the collapse of the I-35W Mississippi River bridge. During the evening rush hour on August 1, 2007, the bridge suddenly collapsed, killing 13 people and injuring 145. In the case of Japan the Ministry of Land and Transportation has performed a study of the vulnerability of bridges and 121 cases of bridges in danger have been reported. The risk of the bridges is due to the deterioration of aged concrete bridges (weathering) and corrosion of steel bridges.
A continuous or real t ime structural health monitoring could help to prevent damages. Monitoring systems already exist however they are in general designed to be installed in n ew s tru ctu res and are exp ens ive. Fo r exis ting o ld structures, simp le and cheap sensor systems are required.
In this research a new sensor that is based on piezoelectric cable inserted into a bolt is developed and its applicab ility to perform health monitoring of structures is verified by means of a shaking table test series. Then the proposed deviceis emp loyedtocarryoutafield measurement of the dynamic characteristics of a provisional bridge located in Yu rihonjo city, Akita prefecture, Japan. The results obtained with this new simp le smart sensor are comparab le with those obtained with more sophisticated and expensive commercial sensor.

Development of the Simple Smart
PiezoelectricBolt Sensor

Characteristics of the Measurement Device
The piezoelectric-bolt sensor is shown in Figure 1, and contains in its interior a p iezoelectric cable which is covered by urethane resin. Then the no minal d iameter of the bolt sensor is 15 mm and its longitude is 80 mm. However to fix the sensor an external metal envelope, forming the bolt spiral, covers the system and the nominal diameter results in 20 mm (M20). The scheme of the piezoelectric cable is shown in Figure 2 which can be connected to a computer board by means of a 4m length cable with 5.6 mm of diameter. This proposed simple sensor can measure strain by the change of the voltage in the piezoelectric cable when this cable is deformed by an external act ion. Therefore relative displacements and vibration of bridge jo ints can be easily measured. Moreover, the characteristics of the sensor like sampling frequency permits to send the signal to a four channels computer board for automat ic data acquisition. In this way signals from four sensors can be recorded simu ltaneously with a d irect input computer board.  Piezoelectric cable sensor (9 ) First, the proposed sensor was tested under harmonic excitation using a v ibration mach ine, and response of the sensor is compared with the amp litude and frequency of the input signals. For in-situ test on bridge structure, the sensor was installed together with a high precisioncommercial accelero meter to co mpare both results. For the proposed sensor, data was recorded using a wireless system (Zigbee wireless module). Source energy (lithiu m battery pack Lipo2000mAh) is required for the board that receives the signal fro m the sensor and transmits it to the co mputer for data acquisition. Personal computer required its own source of energy for acquisition and posterior data processing. Figure 3 shows the general scheme of the data acquisition system using the proposed sensor.

Sensor Installati on and Measurement Method
In general, s mall v ibration and small d isplacements of bridges are measured by means of high precision devices like accelero meters, laser d isplacement transducers, optical fibre, etc. These measurements have an error of the order of 5 µm wh ich represents a high precision measurement. In the case of the proposed sensor which is intended to be used for continuous real time health monitoring, the p iezoelectric core cable of the bolt sensor emits a signal that corresponds to a change of the voltage when the bolts suffers a deformation. To permit the bolt to experiment a deformation it must be installed in the target structure inside a hole and then fixed by a nut. In principle, the change in voltage corresponds to a shear deformat ion of the bolt. Figure 4 shows the general layout of the vibration test of the bolt sensor using a small shaking table mach ine. Figure  5 shows a detail of the installation of the smart bolt sensor which is subjected to a shear deformation induced by the vibration devices. The test was performed at fixed amp litude and therefore it is supposed that the sensors behave under a fixed harmon ic force. For co mparison displacement of the vibration machine was measured by mean of a laser displacement transducer.  The proposed sensor is designed to be used to estimate the probable failure or collapse of structures and specifically the failure of bridge structures. That is, a difference than the high precision sensors which are used to estimate exactly the response of the bridge under normal loads, the bolt sensor is proposed to detect extremely behaviour and deformat ion that could indicate the imminent failure or collapse of bridges. For this purpose the bolt sensor must be installed in crit ical portion of the target structure and continuous real time monitoring using the wireless signal transmission and automatic data acquisition system should permit to tailor the objective. For in-situ measurements on real structure, the temporary bridge located at Yurihonjo city, Akita Prefecture, Japan was selected, and sensor was installed in a joint of the bridge as is shown in Figure 6. The bridge spans over the Koyoshi river and has 175 m in totallength. Fo r co mparison high precision accelero meters were also installed and measurements were performed during 3 days from 9:00 am to 4:00 p m. In addition video recorder was also installed to verify the type of vehicles which produce the vibration of the bridge.
The evaluation of the perfo rmance of the sensor has three steps: 1) Co mparison of the displacement measurements during vibration test with the output voltage of the sensors, 2) Analysis of the sensor output according to the amplitude variation during vibrat ion tests and 3) Co mparison of results fro m high precision devices and bolt sensor measurements during vibration of temporary Yuri bridge under vehicles excitations.

Discussion of Results
In the case of vibration test the results for 0.5 mm o f peak to peak amplitude and constant frequency of 2 Hz is shown in Figure 7. The figure shows the results obtained by the laser displacement transducer compared with the results from the bolt sensor. Good correlation can be observed from both results and the vibration characteristic is also clearly detected by the proposed simple s mart bolt sensor.
Vibrat ion test were performed for 0.25 mm, 0.5 mm and 1 mm of amp litude respectively.For each amp litude value the frequency was set up from 1 Hz to 15 Hz. The curves that relate the frequency, amp litude and the maximu m output voltage of the sensor are shown in Figure 8.   (1) where V out is the output voltage in mV, K is a coefficient equal to 4.5, f n is the frequency of the input load, and D is the amp litude of the vibrat ion in mm. Figure 9 shows the comparison of measurements performed at temporary bridge located in Yu rihonjo city, Japan. The results fro m simp le s mart bolt sensor and results fro m h igh precision 3D accelero meters are co mparable and general tendency which permits to identify the v ibration originated by various kind of vehicles were obtained. The types of vehicles were identified by the analysis of the video record that was synchronized with the measurements devices. For examp le, the pass of heavy vehicle like a garbage car was clearly detected. Therefore, by co mparison it can be stated that the remarkable vibration due to the vehicle passing is well captured by the proposed sensor and applicability for real time v ibration monitoring has been verified.

Conclusions
To investigate the structural condition of bridges or any other structure, usually high precision or high performance measurement devices are emp loyed. However, these devices are expensive and carefully handle must be taken into account since they are delicate instruments to be used for a permanent or long term monitoring. In this research a new smart simp le p iezoelectric sensor and its corresponding data acquisition system were developed to be used for real time structural health monitoring of structures. Installation of the system in selected portion of structures would permit to perform the monitoring of the structural condition, in special after an earthquake event and in the case of bridges to investigate their behaviour under vehicle loads and to detect potential failures due to ageing.
Proposed strain bolt sensor was installed in a joint of a temporary bridge and measurements of the output voltage have permitted to infer the amp litude and frequency of the vibration and therefore to obtain the dynamic characteristics of the structures under investigation.
The response of the proposed sensor for different type of vehicles passing the bridges is similar to that detected by means of high sensitivity accelero meters. Therefo re the applicability of the proposed sensor and measurement system was verified.