پیش بینی ظرفیت خمشی نهایی تیرهای بتن از "نسبت آرامش" تجزیه و تحلیل سیگنال انتشار آکوستیک

This paper presents an alternative approach to the problem, based on “testing” the real structure rather than trying to model it. Experiments on reinforced concrete (RC) beams, representative of bridge beams, are described. The beams were loaded in cycles up to failure whilst recording the acoustic emissions (AE) generated. The analysis of the AE signals was then undertaken based on a proposed new parameter, named the “relaxation ratio”. This quantifies the AE energy recorded during the unloading and loading phases of a cycle test and it showed a clear correlation with the bending failure load of the RC beams. A change in trend was noted when the load reached approximately the 45% of the ultimate bending load. The results appeared to be influenced by factors such as the concrete strength and loading rate and further work is needed to extend the results to full scale testing of bridge beams.

Bridges make up part of the asset of a country and their assessment and maintenance is a fundamental issue. The introduction of the 40 tonnes EU Directive in the UK has highlighted the matter even further [6]. For a long time, the assessment of a structure was considered a “special case” of the design and the first code that dealt directly with assessment dates back to 1990. Although studies have progressed, the majority of the existing codes assess the load carrying capacity of bridges by using theoretical methods and models. Bridge engineering is not only about designing but also includes “looking after” and maintaining the service-life of bridges. After a period of relative neglect in the 1970s there was a growing awareness of the necessity to safeguard and maintain the stock of bridges as part of the national asset. The concern about environmental and sustainability issues, started in the mid 1980s. The introduction of the EU Directive which states that “bridges on principal routes, have to be capable of taking 40 tonne vehicles by 1 January 1999”, highlighted this necessity even further. Before the mid-20th century, no analytical tools were available and the only way to assess the integrity of a bridge was based on visual inspection and intuitive judgement [12]. For a long time, the assessment was considered as a “special case” of the design and thus the same rules were applied. The “Assessment Code BD21/84” [1] represented a milestone in the start of introducing codes which dealt directly with bridge assessment and it was followed later on by the new protocol BD44, produced in 1990 [2]. Although these codes represent a huge improvement for the assessment of bridge structures, they are mainly based on theoretical and modelling calculations and therefore always include a certain degree of uncertainty and the use of safety factors. This paper describes a different approach that aims to predict the degree of damage and failure load of concrete beams by directly testing the structure. The final future aim would then be to implement this method in order to be applied to real bridge structures. The method herein proposed uses a newly developed type of acoustic emission (AE) data analysis, which utilises a newly defined parameter, named relaxation ratio [5]. This is based on the principle that the presence of AE energy during the unloading phase of an AE test is generally an indication of structural damage of the material and/or structure under study. The results of experiments on several reinforced concrete (RC) beams are described, the relaxation ratio is calculated and the results are discussed. The emerging trend of the values of the relaxation parameter seem able to provide an estimation of the bending failure load of the tested specimens, although some external factors (discussed later on) appeared to affect the results. The validity of the proposed method was finally validated by a comparison with the assessment criterion suggested by the Japanese Society for Nondestructive Inspections (JSNDI).

This paper described the application of a new type of analysis of AE signals (based on a proposed relaxation ratio parameter) to several specimens of RC beams. Although at this stage the conclusions are not definitive, due to some discrepancy in the results, the method appears very promising and suitable to practical applications. • The values of the relaxation ratio appeared to be related to the percentage of failure load reached in a specific cycle and are therefore related to the degree of damage of the beam. A value greater than one is indicative of dominance of relaxation phase and therefore of structural damage. • In the first data set of tests, the value of the relaxation ratio always became greater than one when approximately 45% of the ultimate bending load was reached. This gave rise to the possibility to use this method of analysis to predict the failure load of RC beams. • The results were affected by the concrete strength and loading rate used during the experiments. • Further work is needed to establish in which exact conditions the relaxation ratio analysis is successful. The limits of its application and the confirmation of its validity need to be investigated.