Giudizio immediato sullo stato di degrado dei ponti in c.a. basato su indagini low-cost

The structural engineer is often involved to express an opinion on the state of efficiency of the existing structures. This is partly because, in Italy, a large part of structures was built before the middle of the last century. Indeed, the 14th General Census of Population and Housing in 2001, released by Istat, that counted 12.774.131 buildings and 38.397 building complexes, for a total of 12.812.528 units, shows that 19,2% (2.150.259) was built before 1919, 12,3% between 1919 and 1945, 50,0% between 1946 and 1981; 11,5% from 1982 to 1991 and 7,0% since 1991. In addition to building, the field of road infrastructure requires as well special attention. In recent years the efficiency of infrastructure has been questioned due to various phenomena such as the increase in traffic loads, increasing the speed of travel, the changes of the relevant legislation relating to safety standards. The functionality of the bridges depends closely on a good inspection activities, and the lack of a proper and timely maintenance, entails an increase of the deterioration and therefore higher costs of repair. The periodic assessment of the conditions of stability of infrastructures is part of the duties of the company and Agencies owners of the roads. They are therefore directly responsible for any damage suffered by road users in dependence of instability or collapse of the bridges. There is therefore the need to assess the reliability of the infrastructure, with particular attention to those who, although they have far exceeded their threshold of useful life, are still in operation. In recent years, many companies of management of road networks have turned their interest to the definition of systems of Bridge Management System (BMS). BMS is the set of inspection, investigation, maintenance, repair of a group of bridges, organized according to priority, with the support of computer databases and algorithms officers. The state of conservation is characterized by assigning qualitative judgments. Some BMS provide for the detection of the extension and intensity of the defects, others associate to each element a qualitative index relative to the condition in which is each element. The bibliographic study, showed that no existing method considers the location of the damage within the element in which it manifests itself, although it a fundamental parameter for a correct diagnosis as closely related to the kinematic mechanism of collapse. The goal of my research work has been to provide a useful decision support tool during the data analysis of inspection, identifying a mode of low-cost and immediate investigation, for concrete bridges, which achieves a reliable judgment on the static condition of the structure inspected. It has been given special attention to concrete structures, the most widely used material in the field of civil construction. Forty years ago, it was presented as a material with low maintenance and almost unlimited duration. Quite different is the situation that presents itself today to the industry, it is remarkable the presence in Italy of reinforced concrete structures subjected to situations of instability for which you need to make thorough checks to assess their strength. The process that we propose is based: on visual observation of the bridge and on the analysis of the data obtained by means of instrumental tests carried out on the material to be inspected. Assuming that to do a good structural diagnosis, we must find the right compromise between reliability and invasiveness, it has been chosen, as a primary investigation, non-destructive tests, ie the complex of tests and surveys that do not alter the material and do not require the destruction of the structure. In particular, among the various non-destructive techniques, I chose to use the rebound hammer and ultrasonic testing techniques. In order to verify the reliability of non-destructive testing, it has been arranged a campaign of experimental investigation of a series of mixtures, packaged with concrete class different to be able to evaluate a wide range of resistances. They were made a total of 100 cubic test pieces of side 15 cm. They were then subjected to non-destructive tests for the measurement of the ultrasonic velocity and the rebound number, and in the end they were subjected to a breaking test for the determination of the compressive strength. It was then studied the correlation between the non-destructive testing and compression testing. The classical approach for the study of the correlation is to calibrate the regression lines between the indirect properties and the material characteristics and constructional of interest. The proposed methodology is based instead on the use of neural networks, which is a simple and quick tool of correlation between the measured and the mechanical properties of the material. From this practical application it has been identified neural networks as a way to better predict the value of concrete strength investigated, starting from the use of the results obtained by the low-invasive investigations. The proposed method of assessment can be summarized into two steps: • Visual inspection: identification and classification of any degradations and failures on the structure, it must be targeted to the identification of critical points that present and guide the subsequent phases of diagnostic. During the visual inspection is to be assessed the condition of each structural element, identifying extension, intensity and localization of all types of defect present. • Instrumental analysis: mechanical characterization of materials based on low-invasive investigations (NDT). For each significant parameter is assigned a numeric value that will form the basis for the calculation of the function of judgment of the single element (Fe). Adding the evaluation carried out for every individual element that make up the bridge and comparing this value to the sum of the defects that may potentially occur in the observed structure, you obtain the function evaluation of the whole structure (Fs). The parameters for the assessment of the condition of the structure are therefore: • data observable during visual inspections; technical arguments made by experts about the static scheme of the structure from which flows the importance that can have the location of the defect within the element on which it appears, • the results of instrumental tests carried out on the material constituting the bridge. The innovative aspect of the proposal, compared to the existing methods, is the introduction of two factors by which you can evaluate the location of the damage (Li), and use the mechanical characterization of the material as a parameter of judgment (Ti). In support of the calculation method I proposed the Evaluation Tables to be completed during the visual inspections to note down observations. In this way you can make it clear how, starting from what was found during the inspection, for each structural element, it was possible to issue the final judgment. The applicability of the method was verified by means of application on three real structures. The first two are located in Pixina Manna (Pula) and the third structure is located a few kilometres far from the city of Cagliari. Application examples show that the method is an important aid to the evaluation of the structure, providing a guide for the technician who must carry out the survey and directing a uniformity of opinion.