21 Bridge Assessment Using Weigh-in-Motion and Acoustic Emission Methods 207 its first generation products from 1980s [10], Bridge WIM (BWIM) started to regain momentum 20 years ago in Europe, when its development was reinitiated in a European Commission COST action [7]. The main research work was then funded at the end of the last century through the European Commission 4th Framework, WAVE project (WAVE 2002). New and improved algorithms were being developed to enhance its accuracy, especially of the individual axle loads, and to extend its applicability to different types of road bridges, the SiWIM bridge-WIM prototype was born. In an independent test performed within the WAVE project, the B-WIM systems at that time were shown to have accuracy comparable to other technologies such as bending plate and piezo quartz. WAVE project also generated the idea of a Free-of-Axle Detector system (FAD) that does not require any sensors (axle detectors) on the pavement. This results in no traffic congestions during installation and maintenance of the system. After conclusion of the WAVE developments of the bridge-WIM technology continued, both in Europe and in Asia [12]. In Europe, the main theoretical developments are taking place in Ireland and Slovenia, where the main focus is optimization of measurements and measuring parameters. Bridge weigh-in-motion (B-WIM) systems are applied on existing bridges or culverts from the road network which are transformed into undetectable weighing scales (WAVE 2001). For this purpose the structures are typically instrumented with the strain measuring gauges and, when necessary, with the axle detectors. Traditionally, strains are measured on the main longitudinal members of the bridge to provide response records of the structure under the moving vehicle load, but other locations can be used to improve the results. Measurements during the entire vehicle pass over the structure provide redundant data, which facilitates evaluation of axle loads. This is an advantage over the pavement WIM systems where an axle measurement generally lasts only a few milliseconds. Bridge WIM is particularly appropriate for: • Shorter term measurements as it can be easily installed and detached from the bridge. Unlike other WIM system, its accuracy of results is not affected due to portability of installation. • Measurements on sites, where cutting into the pavement is not allowed, is not feasible due to the heavy traffic or if permissions for road blocks are difficult to obtain. • Bridge assessments, as it provides supplementary structural data: the dynamic (impact) factors [3], the load distribution factor and the strain records. B-WIM is today an established WIM technology used in approximately 20 countries. Its main features are complete portability (without losing its accuracy as with portable pavement installations), accuracy, especially on smooth pavements and shorter bridges (below 20-m span), ease of installation and interventions, connectivity and price efficiency. SiWIM bridge WIM system efficiently deals with presence of several vehicles on the bridge (Znidaric et al. 2012). Still, bridges with influence lines longer than 30–40 m will likely provide less accurate results because number of multiple-presence vehicles can increase beyond reasonable amount. Collected data is used in various applications, from traffic studies, design and reconstruction of pavements, pre-selection of overloaded vehicles and bridge applications. There, the main advantages compared to the pavement WIM systems are the measured influence lines and load distribution factors of the structure being measured, as well as Dynamic Amplification Factors (DAF) of all loading events on the bridge. Details about decoupling the static and dynamic components of measured strain signals in the SiWIM software are given in [3] and (Znidaric & Lavric 2010). SiWIM can be also equipped with a number of additional sensors (accelerometers, displacement gauges, strain gauges. . . ) that provide correlated results between the traffic loading and the reactions at the points of measurements. 21.2.3 Acoustic Emission (AE) The acoustic technique is one of the most promising non-destructive techniques that can be used to monitor development of defects in the structural elements. Acoustic emission (AE) is a phenomenon of creating transient elastic waves resulting from local internal micro-material movements [11]. The main principle of this method consists in detecting fast discharge of elastic energy caused by micro-scale deformations or defects inside the loaded material. These waves can be detected by piezoelectric sensors (Fig. 21.1). AE method can be applied to different materials, processes and structures. An active crack can be detected much earlier by AE methods than with the unaided eye of an inspector. AE methods can be used for detecting active cracks, corrosion, leaking, etc. The location of the event can also be identified, using an array of sensors. The AE approach has been used for monitoring of bridges [5], but never in combination with soft load testing and WIM techniques. To date, no standards have been set for field monitoring using acoustic emission techniques on structures like bridges, but recommendations were issued by the RILEM (International Union of Laboratories and Experts in Construction Materials, Systems and Structures) technical committee TC212-ACD [13]. All on site AE measurements were as far as
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