Load Testing of Bridges Using Sensors
Engineering and Construction
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Abstract
This report shows the application of fiber optic sensors in monitoring bridges. Fiber Bragg Grating (FBG) sensors and the Fabry–Pe´rot sensors have been used in measuring the static and dynamic loads on the columns and decks of bridges as well as composite repairs for maintenance purposes. These sensors can be bonded to the concrete bridges or they can be embedded into the composite rebar of the bridge. Two studies that involved the incorporation of fiber optic sensing to monitor the structural health of reinforced concrete bridges are described. Data that shows the reliability of the system on a specific bridge are presented in this report. Moreover, some advantages of fiber optic sensors are described.
KEYWORDS: Bragg gratings, fiber optics, concrete structures, sensors, long-gauges,
Introduction
Assessment of the safety of bridges is based on testing of strain and stress caused by the loads by considering the measurements of particular points across the bridge. Frequent assessment of the strength of the bridge not only provides safety measures on the uses of the bridge but also saves on the cost by providing maintenance intervention thereby extending the durability of the bridge (Glisic et al 2013). For these reasons, the standard structural health monitoring systems have been developed by the civil engineering infrastructure in the last decades. Some of the structural health monitoring point sensors include; crack-meters, tilt-meters, and strain gauges. However, these point sensors have recently raised concerns over their low understanding of the in-service conditions, poor management of service life and verification of upgrades. In this regard, there is increasing development and improvement of the sensing techniques by the use of integral fiber optics strain sensors (FOSS) to enhance the understanding of in-service conditions and improve the load testing of bridges. Therefore, this research is aimed at developing a more sensitive, high resolution and flexible fiber optic strain sensors for load testing of bridges techniques.
Studies That Incorporated Sensors in Bridges
The high rate of deterioration of bridges, especially those that are reinforced with concrete, has raised safety concerns as well as the cost incurred in repairing and replacing them. The increased failure of bridges has led to the development of more efficient methods of load testing. For instance, a research by Wu, Wu, Lu & Feng, (2017), developed a method that involved a dynamic Finite Element Model Updating (FEMU) that used spatially-distributed optical fiber sensors in the testing of load on a highway bridge. This method involved an establishment of static long-gauge strains and the first-order modal macro-strain parameter. The local bending stiffness, the boundary conditions and the density of the bridge were selected as the variables of the design. The study considered the relationship between the local element stiffness and the macro-strain whereby it was found that the macro-strain is inversely proportional to the local stiffness covered by the long-gauge strain sensor.
The corresponding relationship was used in the modification of the local stiffness based on the micro-strain. After doing a series of numerical simulations and experiments to verify the effectiveness of the method, the results showed that the static deformation, macro, and micro-strain modal could be determined using this method. This study reveals that the prediction response is almost similar to the actual values with the possibility of updating the dynamic and static characteristics. Notably, a single type of sensor was required for use in this method where the global and local parameters could be modified. However, this method was only suitable for use in medium-small span beam-like bridges in highways since it is unrealistic to long-gauge strain sensors on the entire bridge. In this regard, the study by Bitao and colleagues requires some improvements to make the proposed method more applicable and useful.
On the other hand, Minardo et al (2012) performed a static load test on a road-bridge with a span length of 44.40m using a stimulated Brillouin scattering in optical fibers for measurement of deformation and temperature. This sensor was able to determine the strain distribution along the supporting beam of a bridge with a strain resolution of ±15µ∈ and a spatial resolution of 3 meters. This experiment was done on a concrete road-bridge with a steel load-bearing structure. The optic fiber was embedded along the whole length of the beam at specific points after which the measurements were determined in connection with the final loading test of the bridge prior to traffic use of the bridge. This experiment tested the response of the bridge to static loadings and to provide a database on the undamaged bridge. In performing this test, Minardo and the colleagues placed 5 pre-weighed aggregate Lorries at strategic locations of the bridge after which the accurate measurement provided by the fiber optic sensor was compared to the data provided by other instruments.
A new distributed strain measurement was carried out along the unloaded bridge one day after the load test. However, the strain levels were comparable with the uncertainty of the sensor. The experiment showed the need to take care when interpreting the results of the installed optical fiber sensors because the sensor may respond to changes in the loading conditions on a temporal scale. Alternatively, the study recommends the use of a thinner fiber optic of about 250 micrometers. This research by Minardo and the colleague shows that the use of distributed sensors is effective in determining the load strain of a bridge because the strain at every point on the bridge can be determined rather than at just a single point. Unlike the other standard strain gauges and strain-meters, the distributed optical strain sensor detects the detail of the defect of all the proximity locations of the bridge. Moreover, the optical fibers are durable thus can be used for a long period before they are replaced. Nevertheless, the study gives room for further studies and analysis of the capabilities of the portable stimulated Brillouin scattering (SBS) sensor in monitoring the girder response to heavy traffic loading.
Significance of Optical Sensors
The development in optical fiber sensing technology has recently led to its application in monitoring the health of civil structures such as highway bridges. The major reason for the continuous development of the optical fibers is to improve the efficiency, functionality and to reduce the size and the weight of the sensors (Ye, Su, & Han, 2014). Other reasons may include the need to improve the flexibility for small-gauge, to improve the resistivity to environmental conditions and electromagnetic interference. The fiber optical sensors provide measurements with high resolutions and accuracy that cannot be provided by conventional techniques. Moreover, the fiber optic sensors can multiplex various form of sensors on a single fiber and makes such systems transfer data over a large area and longer distances. In this regard, the optical sensors have revolutionized the instrumentation technology especially on the recent development of optic sensors devices that can be used in monitoring the load strain, load measurements, and cracking of highway and railway bridges.
Application of Sensors in Structural Engineering
The major types of fiber optic sensors that are commonly used in the testing of load strain include Fiber Bragg Grating (FBG) sensors (figure 1and 4) and the Fabry–Pe´rot sensors. However, the FBG sensors tends to provide less precise measurements with low resolution compared to Fabry–Pe´rot sensor technology that provides more precise measurements with high resolution. The Fabry–Pe´rot sensors and the FBG can be fabricated in devices that can be embedded into the concrete and the reinforcing bars along a bridge to monitor the load strain (Ye, Su, & Han 2014). The load measurement is a very vital aspect of the safety and health of a bridge since an excess load on the bridge may strain the concretes and the bars leading to cracks that finally make the bridge collapse. In this regard, it is important to measure the load strain that significantly affects the serviceability and the safety of a bridge. Therefore, the sensors can be bonded at a point along with the rebar with the sensing zone bonded to a polished surface of the rebar. The sensing part may be protected using layers of rubber while the fiber jacket may be used to protect the output/input lead (figure 2). The device may be tied into the structural rebar while the fiber lead can be tied along the reinforcement cage of the ingress point. A test was conducted to demonstrate the feasibility of the sensors and the ease of embedding into the concrete without incurring damages on the device and found a good response when the sensor was tested to structural failure (Ye, Su, & Han, 2014). Alternatively, the fiber optic sensor may be embedded along a groove on the rib-free side of the reinforcing bars to prevent distortion of the bond properties.
Description of a Bridge Loading Test
Considering, for instance, a preliminary test carried out on a highway concrete bridge to determine the load strain where up to 7 pre-weighed Lorries were placed at strategic positions over the bridge for a span length of 45m. An FBG sensor was embedded along the whole length of the reinforcing beam of the bridge using epoxy adhesive. The optical fiber sensor provided measurements in relation to the final loading test of the bridge prior to its opening to traffic. The weights of the Lorries are reported in table 1. The results of the measurement provided by the FBG sensor were compared with the data from other conventional electrical gauges.
Load testing on a reinforced concrete bridge with fiber composite rebar can easily be done using the optical fiber technique (Wan, Hong, Wu, & Sato, 2013). In this technique, the optical fiber can be embedded into the rebar in a longitudinal direction during construction. Such structures with fiber composite rebar optical fiber sensors can easily fit into the material and get entirely integrated within the structure. Continuous monitoring of the structural response and operational loading of bridges using sensors has shown effective and promising results in the prediction of the structural performance of the bridges.
Conclusion
Although the sensors technology is relatively new in civil structural engineering, this field has developed due to the application of optical fibers in the sensing devices of structures such as bridges. The Fiber Bragg Grating (FBG) sensors and the Fabry–Pe´rot sensors have gained popularity in the determination of the load strain and the general health and safety of most highway bridges. Very few failure reports have been recorded over the use of these sensors. However, the use of advanced fiber composite structures has proved to be more effective in facilitating load testing because the sensors can easily be bonded within the composites of the bridge structure. Fiber optic sensors have shown good performance in load testing as opposed to the conventional electric strain gauges. These sensors are not only durable but also provide detailed information on the real-time condition of the bridge. The sensors have been proved effective in determining the health of bridges in relation to the environmental effects and the applied load. Additionally, the optic sensors have shown more advantages than the conventional electrical strain gauges. The high transmission bandwidth of the sensors facilitates the ability of the sensing devices to monitor the health of bridges remotely without the need for on-site inspection. Due to the various advantages of sensors, optical sensing technology has been effective in monitoring every phase of the bridge life.
Appendix (es)
Figure 1: Principle of operation of a fiber Bragg grating sensor (Casas, & Cruz 2003)
Figure 2: Scheme of the fiber Bragg grating strain sensor (Casas, & Cruz 2003).
Figure 3: Measurement principle of FBG sensor (Ye, Su, & Han 2014).
Figure 4: Measurement principle of the EFPI sensor (Ye, Su, & Han 2014).
Number of lorries The total weight (Tonnes)
1 44.3
2 36.6
3 46.2
4 41.2
5 38.3
6 47.5
7 45.8
Table 1: Weight of the Lorries Used For the Load Test
Figure 5: Steps/displacements diagram
Acknowledgments
The work described in this report was jointly supported by …………………………………………………………………………………………………
Disclaimers
This is a technical note on sensor technology and it involves research in progress. This paper represents the opinions of the authors, and is the product of professional research therefore, any errors are the fault of the authors. It is not meant to represent the position or opinions of the WTO or its Members, nor the official position of any staff members.
Notation list
µ = Population mean
∈ = is an element of load testing
References
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Wan, C., Hong, W., Wu, Z., & Sato, T. (2013). Testing and Monitoring for a Large Scale Truss Bridge Using Long-Gauge Fiber Optic Sensors. Key Engineering Materials, 569-570, 223-229. doi: 10.4028/www.scientific.net/kem.569-570.223
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