Issue 40

Z.S. Metaxa et al, Frattura ed Integrità Strutturale, 40 (2017) 61-73; DOI: 10.3221/IGF-ESIS.40.06 63 utilizes the electrical resistance change method for the damaged areas of the matrix. The second type of sensor used was (b) Fiber Optical Bragg Gratings (FOBG) in order to detect damage on the surrounding medium through the transmission of light within the fiber. Density (kg/m 3 ) 3130 Phenomenological density (kg/m 3 ) 1100 Curing time of cement according ΕΝ 196-3 120 min Table 2: Physical characteristics of white cement. The PVA-CNT fibers were fabricated at the facilities of University of Bordeaux (CNRS-CRPP). Multi-wall carbon nanotubes (MWCNTs) with 98 % purity were used from Arkema, Lacq in Aquitaine province of France. Typical the fibers have a concentration of approximate 13 wt% MWCNTs. For specific applications, the manufactured fibers can be coated with MWCNTs in order to increase their surface conductivity [18]. In the present case, the coated PVA-CNT fibers had been coated with an immersion technique in an aqueous solution with dispersed MWCNTs. The solution contained 0.5 wt% MWCNTs and 0.7 wt% SDS; the latter is a common surfactant, widely used to stabilize MWCNTs in aqueous solutions. The fibers were dried before use and several images (macro photos as well as cross-sectional) can be seen in [19]. The coated and annealed PVA-CNT fibers were also coated fibers according to the already described procedure that had an additional heat treatment stage. The annealing of the polymer (approximately around 150 o C) well above the glass temperature transition ( T g ) temperature, makes the polymer softer, thus enabling the more efficient attachment of the MWCNTs on the surface of the fiber. Additionally, it improves the crystallinity as well as the mechanical properties of the manufactured fibers. Two types of cement-based prismatic specimens were manufactured: Type I specimen deals with sensors that were em- bedded during manufacturing. Type II deals with surface-attached sensors; they were attached on the specimens when the curing process of the cement-based specimens ended and just before mechanical testing. The same mixing procedure was followed for all the cement-based specimens according to ASTM C305 [22]. Mixing was performed using a standard 5 l mixer made from TECHNOTEST ® . Initially, cement and water were placed into the mixing bowl. Mixing started immediately at a slow speed of (140 ± 5 r/min) and lasted for 30 s. Following, both types of sand (course and fine), that were premixed, were slowly added to the paste over a 30 s period, while mixing at slow speed. Next, the materials were mixed for 30 s at medium speed (285 ± 10 r/min). The mixer was stopped for 90 s to let the mortar stand, during the first 30 s of this interval; any mortar that had collected on the side of the bowl was scraped down into the batch. To certify the homogeneity of the mortar, as a final step, the materials were mixed at medium speed for 90 s. (a) (b) Figure 1 : (a) Mold for the manufacturing of the prismatic specimens with already installed PVA-CNT fiber and specific oil and (b) casted mixture in the mold for two specimen’s preparation. Customized prismatic molds made from aluminum having a length of 80 mm and a cross-section of 20 mm × 20 mm, were used. Very thin holes of approximately 1 mm dimeter, suitable for the placement of PVA-CNT fibers / FOBGs, were drilled at both sides of the molds. Before casting, suitable concrete demolding oil was used to facilitate the speci- mens’ removal. Then the fibers were placed in the molds as shown in Fig. 1a and casting was performed (Fig. 1b). Immediately after casting, the specimens were covered with plastic wrap and were left to cure for 24 h at room temperature. After demolding, the saturated wet covering method was adopted for further curing of the samples. This was to make sure that specimens were kept in a wet condition during the curing period without damaging/affecting the fibers. The specimens were covered with wet cloths (water saturated with calcium hydroxide) and were sealed with plastic wrap.