# Fast Falling Weight Deflectometer Method for Condition Assessment of RC Bridges
## 1 Introduction  
Health monitoring of civil infrastructure is nowadays a pivotal point in civil engineering and in the maintenance program of the infrastructures. Nowadays the collapse of bridges or similar artwork, also induced by extreme climatic events [1, 2], plays a strategic role even in the emergency plan [2, 3]. The decay due to aging, deterioration of the materials and lack of maintenance can induce a sensible modification of the static and dynamic response of the infrastructure [4, 5]. A significant element of vulnerability of this kind of infrastructure is the hydraulic one [1, 6]. Recent collapses [7] highlighted a significant role of rainstroms in the safety evaluation of reduced-span bridges. Several strategies are available to determine the structural conditions and mechanical performances of bridges and viaducts. An interesting review of a method of damage assessment is presented in References [8, 9, 10, 11], in which the possibility to derive a structural condition index throughout the displacement and strain monitoring is investigated. At this purpose, the most effective approach is to perform a continuum investigation of the infrastructure during its life span [12], from the construction through the life cycle. Given that this is not possible for a large part of the existing infrastructures, other strategies should be developed. In References [13; 14], a fast method to relieve the geometry and furnish a safety evaluation of the infrastructure is proposed. Ongoing research is focused on the identification of the mechanical property of concrete throughout ultrasonic method and combined method [15; 16; 17; 18; 19]. Although the traditional non-destructive methods had a significant enhancement [20] in the last period, a benchmark Finite Element (FE) model for the evaluation of the structural vulnerability is often necessary also improved with several tuning methods [21]. Furthermore, full-scale tests are performed to determine bridge stiffness that derives from the stiffness of each structural component [22]. Tests consist essentially of non-destructive static and dynamic methodologies. Stiffness can be obtained by measuring strains and displacements resulting from static load tests. In this field, the research in dynamic [23] and earthquake engineering [24] furnishes interesting insight. Another branch of investigation is based on the modal identification of the structural response and the comparison of this with numerical results from FE models [25; 26]. The natural frequency of a bridge can be measured with the detection of the acceleration induced by vibrating mass or impacts. The Experimental Modal Analysis (EMA) [27] is an interesting way to investigate structural health [27]. This method encounters the principal limits in the interpretation of the dynamic response with an FE. In addition, the environmental actions, as well as temperature, wind and environmental vibration, affect the dynamic response in a sensible way [28]. Each technique is time-consuming in terms of setting the instrumentation and performing the tests and back calculation of the results. In many cases, tests must be performed by closing the bridge to traffic, causing severe disruption to the daily flow of traffic. For these reasons, new strategies should be introduced that are more rapid, accurate and cost-efficient [29; 30]. A crucial issue for stakeholders is monitoring infrastructures in a more accurate and rapid way, to achieve results in short time and address efficiently the economic resources for maintenance and/or rehabilitation works. In Italy, there are more than 1.5 million bridges (from short to large span), and approximatively only 4% of these are monitored. After the Morandi bridge disaster in Genova (Italy) on 14 August 2018, a dramatic collapse of a 210 m central section of the viaduct in which 43 people died, a vast public debate has focused on the needs of the bridges maintenance in Italy and Europe.