PSI - Issue 10

K. Kaklis et al. / Procedia Structural Integrity 10 (2018) 129–134 K. Kaklis et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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Table 1 presents the pozzolanic mortar mix used in this series of experiments. The water to binder (W/B) ratio was 0.92 for all mixes. The mixing tools and materials were stored at a constant temperature of 23 o C for 24 h before mixing. The quantity of hydrated lime that would react with metakaolin was fixed in a weight ratio equal to 1.5, ensuring the pozzolanic reaction, while any unreacted quantity of hydrated lime, after its carbonation, contributes to the enhancement of plasticity to the final mortar (Costigan and Pavía, 2010). This excess of hydrated lime after its carbonation, enables the mortar to acquire a pore size distribution similar or compatible to porous stone, thus facilitating the homogeneous distribution of water and water vapor in the complex system (Veiga et al, 2009; Aggelakopoulou et al, 2011).

Table 1. Mortar mix (composition in mass %), B: binder, A: aggregates, W: water.

Binders

Sand

B/A

W/B

Lime

Metakaolin

50

30

20

1

0.92

2.2. Preparation of specimen for mechanical properties

Cylindrical specimens were utilized for the uniaxial and triaxial compression tests. Mortar specimens were cast in prismatic molds constructed out of melamine furniture boards. The mold was removed two days after casting and the mortar was allowed to cure in a curing chamber at a relative humidity (RH) of 90-95% and a temperature (T) of 20 °C with respect to the EN 196– 1 standard. Specimens were left to cure for 26 days in the above-specified controlled environment followed by 2 days in ambient conditions prior to testing as outlined in the methodology as described by Gameiro et al. (2014). Mortar cylinders were then cored out of these blocks by using a typical laboratory coring apparatus following the recommendations in the relevant literature for rock mechanics tests (Bieniawski and Bernede (1979); Vogler and Kovari (1978)). The flat surfaces of each core specimen used for uniaxial or triaxial testing were then ground based on the ISRM specifications. Mortar cylinders had diameters of 50 mm and heights of 100 mm. The height to diameter ratio ( h/D ) for the uniaxial and triaxial compression tests remained constant and equal to 2:1. The stress-strain behavior and the deformation characteristics of the pozzolanic lime mortar were investigated using the uniaxial and the triaxial compression test under cyclic loading conditions. For these tests, the pozzolanic mortar was considered as an isotropic material and, therefore, the orientation of the specimens was not taken into account. In uniaxial compression tests a cylindrical specimen of diameter D and height h is subjected to a uniform axial stress acting on the ends of the specimen. In addition to the peak stress value, the complete stress-strain curve is re corded in order to calculate Young’s modulus. Triaxial compression tests are employed to determine the mechanical properties of materials under multiaxial loading conditions. This test provides a quantitative measure of the compressive strength of a material under con finement as well as the corresponding stress-strain behavior of the specimen under such conditions. In rock or soil mechanics, triaxial testing is commonly used to estimate the in-situ material strength by simulating the corresponding confining (geostatic) pressures. In preparing specimens for this test, each sample is covered with a thin rubber membrane and subsequently placed inside a pressure vessel. The pressure vessel is used to apply the desired confining pressure, while the rubber membrane prevents the confining fluid (typically hydraulic oil or water) from entering the pore space of the specimen. In triaxial tests following the ISRM suggested methods, specimens are initially loaded hydrostatically to the desired confining pressure using a lateral pressure controller. The lateral stress is then kept constant, while the axial stress increases until failure. The uniaxial and triaxial compression tests were performed using a stiff 1600 kN MTS hydraulic testing frame (model 815) which applied the axial load through an external 500 kN load cell by Maywood. A Wykeham Farrance triaxial chamber with a maximum lateral pressure capacity of 14 MPa was employed for the triaxial tests; the 500 kN load cell was not used during the triaxial tests. 3. Experimental setup