Issue 50

A. Marinelli et alii, Frattura ed Integrità Strutturale, 50 (2019) 438-450; DOI: 10.3221/IGF-ESIS.50.37 441 Comparative study: the specimens and the experimental set-up Within the scope of this comparative study, the experimental investigation comprised a series of non-destructive (N-D) tests for the determination of the apparent density, real density, open porosity, total porosity and sound speed propagation for each material, as well as destructive testing for the determination of their compressive strength and flexural strength under concentrated load or constant moment. A range of prismatic samples (Fig.3), compliant to the specifications of the relevant standards, was cut as per the following schematic representation (Fig.4a) for the bending tests, with some of them carrying a central machined notch, 3.5 mm wide. More specifically, a series of both 3PB and 4PB tests were performed on three types of Corsehill sandstone specimens, with three repetitions each: specimens bearing no notch and specimens bearing a 10 mm or 20 mm long notch. The significant inhomogeneity of Portland limestone as well as the easier access to it for testing purposes, supported a decision for a more detailed experimental investigation for this material, based though on the same principles. A series of both 3PB and 4PB tests were performed on six types of Portland limestone specimens, with three repetitions each: specimens bearing no notch and specimens bearing a 5mm, 10mm, 15mm or 20mm long notch. Quasi-cubic specimens (Fig.4b) were used for all other types of tests, with three repetitions each. Figure 3 : (a) Corsehill sandstone and (b) Portland limestone specimens. Figure 4 : Dimensions for typical prismatic specimens for (a) bending tests and (b) compression and N-D tests. All experimental work took place at the ‘Heavy Structures’ laboratory, Edinburgh Napier University. The determination of apparent density, real density, open porosity and total porosity was carried out in accordance with BS EN1936: 2006, making use of an evacuation vessel which can maintain a pressure of 2 ± 0.7 kPa while providing gradual submersion of a test specimen, and a scale with an accuracy of 0.01% which is also capable of operating while submerged in water. The determination of sound speed propagation was carried out in accordance with BS EN 14579: 2004, making use of an electrical pulse generator, a pair of transducers, an amplifier and an electronic timing device capable of measuring the time taken for the transmitted pulse to reach the receiving transducer. For performing these non-destructive tests, a ventilated oven capable of maintaining a temperature of 70 ± 5 ⁰ C, a room maintained at a temperature of 20 ± 1 ⁰ C and a linear measuring device with an accuracy of 0.05 mm were used as needed, per the standards’ requirements. Both 3PB and 4PB tests (Fig.5), following BS EN 12372: 2006 (Natural stone test methods – Determination of flexural strength under concentrated load) and BS EN 13161: 2008 (Natural stone test methods – Determination of flexural strength under constant moment) respectively, were carried out to determine the flexural strength of Corsehill sandstone and Portland limestone under varying conditions. Using the principles discussed in ASTM E399-90 (Standard test method of plane-strain fracture toughness of metallic materials), machined notches of varying sizes as previously described, were made on the samples. However, some of the requirements of the above standard cannot be fulfilled for stone, while other size conditions can only be tested after the results have been obtained, hence promoting the trial-validating testing procedure. The direct scope of these bending tests was the collection of data in order to determine the relationship between the external loading and crack opening displacement (COD) and the dependence of COD on the relative dimensions of the notch with respect to the size of the specimens as well as the method of its determination. (a) (b) (a) (b)