PSI - Issue 3

Stavros K. Kourkoulis et al. / Procedia Structural Integrity 3 (2017) 316–325 S. K. Kourkoulis and I. Dakanali / Structural Integrity Procedia 00 (2017) 000–000

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Fig. 3. (a) Joining the two fragments; (b) Making the connection surface of both fragments rough; (c) Applying cement paste to the connection’s surfaces; (d) The bolted titanium bars used for joining the two fragments.

At this point it is recalled that the basic principle for the restoration of monuments is reversibility: If needed the monument must be returned to its state prior to the interventions. Previous conservations projects of the Athens Acropolis monuments used steel as reinforcing material for the connection of the fragments. Steel elements were oxidized, due to air pollution and caused significant damage to the surrounding stone. At the late seventies titanium was proposed as the most proper reinforcing material by Angelides (1976) due to its high strength and the fact that it has the same thermal expansion coefficient with marble. Also, it is a relatively light metal with high corrosion re sistance (Penelis 1996). Commercially pure titanium is used since then for the needs of the three worksites of the Acropolis hill and it was the material used also for the reinforcing bars of the present experimental protocol. The filling material is a cement paste (binder and water without any aggregate). It must absorb vibrations and minor deformations protecting the authentic building material, since it comes into direct contact with the marble. Its quality should assure coherence and durability over time while its strength must not exceed that of the marble since in case of overload it is be the restoration material that must fail rather than the authentic stone (Aggelakopoulou 2013). 2.3. Experimental set up After curing for 28 days the specimen was placed on two marble cubes (simulating the in situ supporting conditions, i.e. the capitals) on a very stiff (capacity 6 MN) servo-hydraulic loading frame (Fig.4). For the load transferred to the epistyle to be as uniform as possible, a steel construction of 3 layers was placed above the specimen. The first layer included 8 equidistant rods in contact with the marble. Above the rods 4 wide flange H beams (Fig. 4a) were placed. The second layer consisted of 4 equidistant rods and 2 wide flange H beams (Fig.4b). The last layer’s beam was sup ported on the previous layer through 2 rods and it was loaded by another rod in contact to the frame’s traverse (Fig.4c). Eight Linear Variable Differential Transformers - LVDTs were used to measure deflections (Figs.5a, b). Two clip gauges were used to measure the relative displacement of two knife-edges attached on the fault’s either side (Figs.5c,e). The overall specimen’s deformation was also monitored with the aid of innovative sensing techniques including:  Acoustic Emission (AE). Proper sensors are mounted on the specimen’s surface according to a proper arrange ment aiming to the determination of the location of sources of acoustic emissions, i.e. transient elastic waves emitted when a material is under mechanical loads. According to recent approaches spontaneous release of energy is attributed to initiation of cracks while continuous acoustic signals of high duration, are attributed to friction (Grosse & Ohtsu 2008, Miller & McIntire 1987). In the present protocol 8 acoustic sensors were attached around the fault’s area (Fig. 6).  Digital Image Correlation (DIC), a full-field image analysis method, based on a sequence of digital images. It determines the contour and the displacements of a mechanically loaded object in three dimensions. The object is observed by two cameras from different directions and the position of each object point is assigned to a specific pixel in the camera plane. During loading, the system takes digital images of the object comparing them against the image of the undeformed object. Nowadays DIC is extensively applied in many fields such as materials and components testing, fracture mechanics, electronic industry, biological engineering etc, (Sutton et al. 1986, Mc Cormick & Lord 2010). The system of the two cameras used for the DIC technique was applied focusing on one of the two longitudinal sides of the specimen providing an overview of the full field displacement field. The general view of the cameras arrange ment and the spots’ pattern, on which the technique based, are given in Fig.7.  In addition the Pressure Stimulated Currents (Triantis et al. 2008) and the Optical Fibers techniques were used for comparison/calibration reasons. The position of the electrical contracts for the PSC technique is shown in Fig.5d.