# Fast Falling Weight Deflectometer Method for Condition Assessment of RC Bridges
## 1 Introduction
In May 2020, the Italian Higher Council of Public Works of the Ministry of Infrastructure and Transport approved the \"Guidelines for the classification and management of risk, the assessment of safety and the monitoring of existing bridges\". The importance to develop efficient, low-cost, replicable, accurate and low time-consuming tools to monitoring bridges is clear. The original Falling Weight Deflectometer (FWD) allows us to perform one test in 5 min, while traditional impulsive methods need hours or days. The Fast-FWD is five times faster per drop than the original one. The original hydraulic load lifting system has been replaced by an electric motor. Its main field of application is in monitoring of pavement engineering, capable of performing tests on flexible (asphalt), rigid (concrete) and semi-rigid pavements (asphalt and concrete). The Fast-FWD is mounted on a trailer and towed by a vehicle especially instrumented (Figure 1): The operator can control the testing unit with a laptop computer. A test can be performed in less than one minute once the testing position has been reached. It is possible to reply to a high number of tests in a limited time, by applying impulsive loads on the structure surface and recording the deflections at different distances from the load application. The use of Fast-FWD to evaluate stiffness values of pavement structure [31] was introduced in the late 1970s. Its application gradually spread in the middle of the 1990s, becoming more used, especially in the USA. [32; 33]. Some case studies are available in the literature: in 1996, tests regarded two bridges in Virginia (USA) measuring deflections on bridge decks [33]; in 2001, it was used to measure natural frequency of a bridge and compared it with prior test results, to identify damage [34]. Moreover, it has been suggested to consider the use of a large impulse source, such as the FWD, for locating natural frequencies during preliminary vibrating testing of bridges [35]. In a recent study, the feasibility was explored of FWD as an impact force on two bridges and compared with an impact hammer test [36]. The application of a Fast Falling Weight Deflectometer to determine the modal shape of existing bridges is described in Reference [38], in which two case studies are considered: a Parker Pony Truss (Hartbarger Bridge) and a modern concrete deck on rolled steel beam (Baptist Ford Bridge). The modal shape is analyzed through an accelerometer. It is highlighted how the FWD can create proper impact forces that are useful for screening bridges and detecting performance and conditions. The aim of the research is to evaluate whether Fast-FWD can furnish bridge stiffness values, to help engineers to understand bridge performance and conditions, detecting also possible deteriorations replacing tests over time. This paper presents a case study in which the Fast-FWD used as a controlled excitation device was analyzed, performing impact dynamic testing on a single span-bridge approximately 12.0 m long and 15.5 m wide. It overpassed the Perdalonga canal (Figure 2), located in the city of Quartu Sant'Elena (Sardinia, Italy), along Colombo Avenue. Forty-eight tests were performed in less than one hour, on the bridge deck, abutment and also on the approach paved embankment. The results obtained were analyzed throughout an FE analysis in ANSYS (R18.1) environment. Figure 1: (**a**) Fast Falling Weight Deflectometer (Fast-FWD) with the main elements: falling mass and geophones of measure bar [37] and (**b**) on trailer towed by vehicle ready for testing. Figure 2: Bridge on Perdalonga canal Quartu Sant’Elena (Sardinia, Italy).