Issue 40

S. K. Kourkoulis et alii, Frattura ed Integrità Strutturale, 40 (2017) 74-84; DOI: 10.3221/IGF-ESIS.40.07 76 Monastery of Arkadi and the Fortezza fortress in Rethymno. It is also considered as a potential substitute stone for the restoration of the Epidaurus monuments. When Alfas stone is mined it is relatively soft; however a short while after it is exposed to the sunlight it hardens. It is a compact sub-white stone of low porosity and rather homogeneous structure (Fig.3b). It is composed by 99% of calcite, 0.5% of quartz and 0.5% of aragonite [4]. Its main mechanical properties are recapitulated in Tab. 1 [4-7]. Figure 3 : (a) Primary school at Scordilo village made of Alfas stone; (b) The compact texture of Alfas stone. Modulus of elasticity 2.5-10.0 GPa Uniaxial Compression Strength 15.0 - 36.8 MPa Bending strength 7.4 - 10.4 MPa Absolute density 2.45 g/cm 3 Bulk density 1.73 g/cm 3 Porosity ~30.0 (%) Table 1 : Mechanical properties of Alfas stone [4-7] (The values strongly depend on the point and depth of quarrying). Two more materials were tested in the present protocol, i.e. a cement-mortar and a cement-paste. The specific materials are used as filling materials of the holes drilled and the grooves sculptured in the marble structural elements of the Acropolis of Athens monuments for the placement of metallic elements which either connect the epistyles to each other or restore the monolithic nature of fragmented structural members. These materials ensure the adhesion between marble and metal while in addition they protect the authentic building stone in case of overload [8]. According to the practice followed by the scientists working for the Parthenon’s restoration project these filling materials are composed by coarse quartz sand (grain size: 1mm-2mm), fine quartz sand (grain size: 0.1-0.4 mm) and white Aalborg cement. The ratio for the mortar mix is 2 coarse quartz sand : 1 fine quartz sand : 1 cement. The quartz (silica) sand must be perfectly dry, clean and well graded. The sand’s proportion of the containing SiO 2 is a factor of its quality. Quartz is known for its piezoelectric properties. The grains of the sand extracted from rivers are of perfectly spherical shape due to natural friction. Products composed by this sand exhibit excellent workability and elegant finished surfaces. The proper ratio of grain size ratio can provide several advantages to the final product such as the reduction of the gap between the grains, the reduction of the water absorption resulting to high water resistance, increased strength of the mortar and minimized cracking. In addition, quartz sand provides protection against corrosion due to poor environmental conditions. The Aalborg cement is a rapid hardening Portland cement with high early (2 days) and standard (28 days) strengths. It is produced of extremely pure limestone and fine-ground sand. It is characterized by its white color, high consistency, extraordinarily low content of alkali (Na 2 O) and high sulphate resistance [9]. The composition for the cement-paste mix is 2.5 cement : 1 water, however it varies according to the needs of its specific application. Acoustic Emission technique Fracture is combined with release of stored elastic strain energy, consumed for the generation of new cracks. The elastic waves generated propagate in the material and can be detected by piezoelectric transducers mounted to the structure’s surface by means of proper viscous materials [10]. The sensors’ output is amplified through a low-noise preamplifier, filtered to remove any extraneous noise and further processed by proper electronic equipment. The AE method includes a wide range of applications such as laboratory experimental studies, field inspections, structural integrity evaluation, (a) (b)