Issue 41

S. K. Kourkoulis et alii, Frattura ed Integrità Strutturale, 41 (2017) 536-551; DOI: 10.3221/IGF-ESIS.41.64 537 I NTRODUCTION ontinuous structural health monitoring (CSHM) is perhaps the most effective tool in hands of structural engineers, in the direction of timely detecting accumulation of damage in structures of any kind. It offers, thus, the time interval, which is necessary in order for proper measures to be undertaken, preventing further damage accumulation that can lead to catastrophic failures. The topic is in the cutting edge of scientific research worldwide, since according to the ancient Greek physician and philosopher Hippocrates “ prevention is better than cure ”. Nowadays, research is focused on the development and practical implementation of new ideas (both in the hardware and the software level) for CSHM, which are flexible, user-friendly and also effective from the financial point of view. The field of restoration and conservation of cultural heritage monuments is among the ones for which CSHM is a “ conditio sine qua non ” for an intervention to be considered successful and completed. In the specific field, however, there are quite a few additional requirements (mainly of aesthetic origin) that do not permit the use of traditional sensing techniques. For example, the size of the devices used as sensors should be very small in order to avoid distorting the beauty and harmony of the external appearance of the monuments. Along the same lines, attaching the sensing devices to the structural members should be implemented very carefully in order to avoid harming the integrity of the member’s external surface. Besides the as above practical complexities, additional problems must be solved in case of CSHM of cultural heritage monu- ments, related to the proper interpretation of the data recorded by the sensing devices which are attached on restored structural elements. Indeed, in quite a few cases, restoration of damaged or fractured members is achieved with the aid of metallic reinforcing elements and suitable adhesive materials. As a result, a three-material complex with material interfaces is created, the constituent elements of which are usually of completely different mechanical behaviour and response to any kind of external stimuli. A typical example of such a restoration approach is the technique adopted nowadays for restoring the integrity of structural elements of the monuments of the Sacred Hill of the Acropolis of Athens. These monuments, which constitute a unique complex of buildings, including the Parthenon, Propylaia, Erechtheion and the temple of Athena Nike, were built by the ancient Greeks during the 5 th century B.C. using almost exclusively Pentelic marble. Nowadays, their historical and aesthetic magnificence is recognized worldwide and they are listed among the masterpieces of classical western civilization. The main restorative intervention on the Athenian Acropolis monuments was directed by N. Balanos in 1885. The interventions were of a major scale and their basic principle was that all fragments of structural elements, spread around the monument, should be re-positioned on the monument and the addition of elements made of new material should be as limited as possible. From this moment on, the term “anastylosis” was established as a reconstruction technique whereby ruined monu- ments are restored using the original architectural elements to the greatest degree possible. From a purely archeological point of view Balanos’ intervention was successful due to the minimum possible use of new marble, although the use of scattered ancient fragments as ordinary building material has been strongly criticized. Unfortu- nately from the structural and chemical points of view, the use of metallic reinforcing elements (which were made of steel) without prior verification of their mechanical and physico-chemical compatibility with the ancient material and without taking into account their chemical resistance against air pollution became the source of extensive damage of the monu- ment [1]. In the direction of curing these problems the “Committee for the Conservation of the Acropolis Monuments” (ESMA) was established in 1975 inaugurating a new approach to the restoration of the Acropolis of Athens monuments [1]. The scientific personnel of the committee developed a pioneer technique for the restoration of the structural integrity of the monuments’ structural members, according to which the fractured marble elements are connected by inserting titanium bars into pre-drilled holes. The adhesion between marble and bars is achieved by a proper white cement paste [2, 3]. The aforementioned technique is based on the basic restoration principles, dictated by the “Venice Charter” [4, 5]: Re- versibility (if needed, the monument could be brought to its state prior to the intervention), minimization of the inter- ventions to the extent that guarantees protection of the authentic material from further damage (for this reason the number of titanium bars required by the connection’s design, should be the smallest possible) and finally compatibility between the materials used for the restoration and the authentic ones. The restoration approach described above was assessed a few years ago both experimentally and numerically [6-10]. It has been indicated that, in spite of its efficiency and general acceptance, there are still some open issues that should be further studied, mainly in the direction of reducing the intervention on the authentic building material. In this context an experi- mental protocol was recently implemented [11, 12], during which accurate copies of fractured epistyles, restored according to the as above procedure, were submitted to multi-point bending in order to simulate the actual loading conditions that will be realized after the epistyle is re-placed in its original position. The main innovation of that study was the simultaneous use of traditional and innovative sensing techniques that permitted pumping data both from the outer surface of the speci- mens as well as from their interior and especially from the two interfaces (marble-to-cement and cement-to-metal) which C