Issue 50

A. Marinelli et alii, Frattura ed Integrità Strutturale, 50 (2019) 438-450; DOI: 10.3221/IGF-ESIS.50.37 440 purpose of definition of failure criteria for these stones used in new as well as in restoration projects, their mechanical behaviour and fracture characteristics are experimentally investigated by performing standard compressive tests, as well as three-point bending (3PB) and four-point bending (4PB) tests on appropriately cut prismatic samples, in the presence of U-shape notches and following the concepts of crack mouth opening displacement (CMOD) and fracture energy [6,7]. Furthermore, it is observed that the geometry and the shape of specimens proposed by standards relevant to brittle geo- materials differ, concerning both their shape and size. In any case, laboratory space and equipment restrictions together with prohibitive costs for large scale testing, make the design of large elements and structures dependent inevitably on extrapolation from test results on much smaller laboratory specimens. Design codes do not yet include explicit guidance regarding the transition from laboratory results based on smaller scale specimens to parameters suitable for the design of full size structural elements. This is attributed to the - evident in the literature - lack of unanimous scientific approach and generally accepted theory concerning the laws governing this transition, making regulatory bodies reluctant to change currently used empirical or semi-empirical formulas based on curve fitting to the experimental results [8]. Interest in the size effect goes many centuries back, with the observation that the nominal strength of structural elements changes by scaling their size been made by Leonardo Da Vinci [9]. The primal scaling idea by Galileo [10] introducing the concepts of stress and strength was much later soundly questioned by the statistical weakest-link theory by Fisher and Tippett [11], further developed by Weibull [12]. Limitations to the use of the statistical approach were posed due to discrepancies emerging from various experiments first conducted in concrete by Walsh [13]. Nowadays, two approaches are widely encountered in the literature: the deterministic energetic theory by Bazant [14], based on the observation that failure of quasi-brittle materials is characterized by both energy and stress quantities and the theory of crack fractality as described by Carpinteri [15] and Carpinteri et al. [16], associating the size effect with the fractal nature of crack surfaces. In this context and based on the results of the first stage of this investigation on both materials, a second stage of the study focuses on the influence of specimen shape and size on the mechanical and fracture behaviour of Portland limestone only. Consideration of relevant studies on marble [7, 17] and porous stone of Kefalonia, Greece [18] has paved the way for this particular investigation comprising an experimental protocol of three-point bending tests, aiming at shedding light on the dependence of flexural strength, deflection at mid-span, crack mouth opening displacement and fracture energy on specimen size and shape. This is a contribution to the wider investigation of the problem but also of applicability for the optimization of the design and rehabilitation of load-bearing structural members like lintels and sills, loaded in position in a similar way. T HE EXPERIMENTAL PROTOCOL The materials wo natural building stones widely used in Edinburgh were selected for the first stage of this experimental study: ‘Grove Whitbed’ Portland limestone and Corsehill sandstone. Portland limestone, originating from the Jurassic Period, is a grain supported biomicrite consisting of rounded micritic ooliths with concentric structures of diameters ranging from 50 μm to 300 μm, irregular quartz grains with a nominal size of 100 μm and a large quantity of bioclasts which range in size from 5 μm to 20 μm [19]. The relatively large Turreted Gastropods (fossilised shells) and clam shells found within Portland limestone are responsible for the voids (nominal size of 100 μm) that can be found throughout the stone, as the removal of these shell fragments due to percolating rain over time left behind what can be observed as holes. Portland limestone has a creamy/white hue, which can be darkened by clusters of grey shell fragments, scattered throughout the stone. It has a coarse texture and inhomo- geneous/porous properties, which contributes to the stone having a low level of durability, with a weathering rate of 3 mm to 4 mm per 100 years expected, particularly at the edges of stonework [19]. In construction projects, Portland limestone has been successfully used for load-bearing units as well as cladding. Corsehill Sandstone is a fine-grained, pale red-brown, somewhat calcareous sandstone originating from the Triassic Period. Its petrographic content comprises of detrital quartz, opaline silica, feldspars and occasional mica crystals which all have a reddish hue due to a ferruginous clay coating. There is a vast quantity of iron minerals, as both black and brown haematite are present in the sandstone. The rock fragments within the sandstone comprise mostly of polycrystalline chert and mud- stone grains. Occasionally, samples display altered quartz grains with iron oxide in the pore spaces. Samples that have a coarser texture are usually less altered, with a larger presence of silica cement [19]. Corsehill Sandstone is a durable sandstone that has been used for many applications in construction projects, such as cladding, paving and load-bearing units. It has a high resistance to air pollution and acid rain; however, it has a poor resistance to salt damage, meaning its suitability for use in harsh coastal environments could be called into question [19]. T