PSI - Issue 10

P.A. Kakavas-Papaniaros et al. / Procedia Structural Integrity 10 (2018) 311–318 P.A. Kakavas-Papaniaros et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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tures. Being a non-destructive procedure, the method is extremely popular when it comes to assessing monumental buildings. The strength of the structural material or member can be calculated by experimental or in-situ measure ments of the travel time of the dominant ultrasonic wave through the masonry (McCann and Forde (2001); Hola and Schabowicz (2010)). It is also possible to identify internal discontinuities in the structural element, such as cavities, cracks or other imperfections, by properly evaluating abnormalities in the distribution of the ultrasound travel times along the length of the member (Aggelis and Shiotani (2007); Aggelis et al. (2009); Ramamoorthy et al. (2004)). However, the accuracy and, most importantly, the applicability of the ultrasound method are strongly related with the thickness of the structural element at hand. It has been observed that when the structural member is very thick, the high attenuation of the transmitted pulse makes it practically impossible to detect the acoustic signal on its other side. Unfortunately, that is the case in many historical masonries, where thicknesses of about 1m are possible (e.g. ground floor of two or three-story historical buildings). In such cases the “classical” application of the ultrasonic method, which requires the transmitter and receiver placed on opposite sides of the wall, is obviously impossible. The same issue occurs when assessing buildings that are in direct contact with adjacent ones, which is common in densely occupied urban areas. Obviously, in this case the problem is related with the fact that one side of the masonry at the “border” of the building is inaccessible (the term “blind” wall is often used in praxis). Alternative techniques have been developed and can be employed in order to overcome such problems. Τ he flat jack method (Binda and Tiraboschi (1999)) is one procedure that is unaffected by the previously discussed issues. However, this method is semi-destructive, since the jacks are fitted in horizontal section cuts that are properly made in the masonry. Hence, some damage on the wall is unavoidable. As a result, it may be impossible to apply the method on buildings of historical value, where interventions on their elements are in many cases prohibited. Moreover, equipment and application costs for this method are substantially increased compared to the low-cost ultrasound measurements. Apparently, any intrusive method suffers from similar limitations when it comes to assessing a historical building. As a result, even the direct estimation of masonry strength using properly extracted specimens (cores) is in such cases practically inapplicable. It is noted that the extraction of a small number of cores from carefully selected masonry elements is always desired, since results of direct stress tests are required in order to properly calibrate those of the indirect procedures, such as the ultrasound method. However, an extensive sampling and stress-test program in order to obtain a reliable estimation of the required masonry strength according the modern codes is “out of discussion”, due to the unavoidable and undesirable significant damage on the elements of the historical building. Finally, it should also be noted that the equations provided in Eurocode 6 (CEN (2005)) for the approximate calculation of masonry strength based on material stress-tests may sometimes lead to results of questionable reliability, especially when considering layered stone masonries which are commonly encountered in historical buildings. Given the previously discussed drawbacks of all available procedures for the determination of the strength of historical masonries, we believe that it is important to investigate possible alternative setups for the ultrasound method that will allow its application in historical buildings and monuments. An alternative setup that battles the attenuation of the acoustic waves by technically “reducing” the element thickness is discussed in the present paper. Results from an experimental program, aiming to investigate the applicability and reliability of the proposed pro cedure and introduce proper “modifications” on the equations used in the “classical” ultrasonic method , are presented in next sections. The method is based on the use of properly placed “inserts” between the transducers and the structural elements. It is noted that, at the present stage, our work focused on building a solid foundation for the proposed method. The relation of elasticity with the strength of masonries is a complex issue suffering from a sufficient theoretical establishment of the interconnection between these parameters. Hence, concrete elements were selected and investigated to avoid the effects of this issue on the experimental results. However, the effects of material anisotropy and discontinuities are discussed using well established theoretical tools, i.e. slowness curves and surfaces.

2. Proposed procedure and experimental investigation of t he ultrasound “travel - times”

2.1. Setup and basic assumptions

Typical compressive strengths of masonry units are relatively high. They are usually in the order of 60 MPa and higher, but the strength of the mortar, on the other hand, is very low. Typical compressive strengths of Type “ S ” or Type ” N ” mortars are about 12.5 MPa and 9.5 MPa respectively according to standards (CEN (2005)). The modulus