Discussion on the paper entitled “Mechanical and physical properties of the Kandira stone, Kandira, Turkey” by Arman et al., Bull Eng Geol Environ (DOI 10.1007/s10064-006-0082-x)

When choosing a stone for a project, little guidance is available to the architect, engineer or developer to select which acceptance criteria are appropriate for particular types of stone. Once a stone has been selected on aesthetic appearance it is important to determine whether it exhibits the necessary physical properties and durability characteristics. It is well known that slaking of rocks is an important consideration in evaluating the engineering behaviour of rock-mass and rock-materials in geotechnical practice (Gokceoglu et al. 2000; Yilmaz and Karacan 2005). The durability of a stone is a measure of its ability to resist weathering and so to retain its original size, shape, strength,and appearance over an extensive period of time (Sims 1991; Bell 1993a). As slake durability is a very important parameter in the evaluation of the usefulness of a building stone, it is regrettable the authors did not consider this.

The texture and porosity of a rock affect its ease of dressing and the amount of expansion, freezing and dissolution it may undergo. Fine grained rocks are more easily dressed than coarse varieties. Water retention in a rock with small pores is greater than in one with large pores and so the former are more prone to frost attack. However, the authors did not give any results regarding porosity, texture or void ratio.

Building stones derived from sedimentary rocks may undergo a varying amount of decay in an urban atmosphere where weathering is accelerated due to the presence of aggressive impurities such as SO₂, SO₃, NO₃, Cl₂ and CO₂ in the air, which produce corrosive acids. Bell (1993b) reported that limestones are susceptible to chemical modification. For instance, weak sulphuric acid reacts with the calcium carbonate of limestone to produce calcium sulphate. The latter often forms just below the surface of a stone and the expansion which takes place upon crystallization causes slight disruption. If this reaction continues, then the outer surface of the limestone begins to flake off. This is another important subject not considered in the paper.

There are many published papers discussing the relationships between the index and mechanical properties of the rocks such as Aufmuth (1973), Beverly et al. (1979), Kidybinsky (1980), Singh et al. (1983), Shorey et al. (1984), Haramy and DeMarco (1985), Ghose and Chakraborti (1986), O’ Rourke (1989), Sachpazis (1990), Xu et al. (1990), Gokceoglu (1996), Yilmaz (2002), Yasar and Erdogan (2004), Fener et al. (2005), Yılmaz and Yuksek (2007), Yılmaz (2007), etc. These papers all report unconfined strength increases with increasing Schmidt N, point load and Brazilian strength with strong correlation coefficients. When the authors’ test results tabulated in their Table 1 are correlated, there is no obvious relationship as seen in Figs. 1, 2 and 3 of this Discussion. The authors should have explained this in their paper, particularly as they state “As the hardness increases so does the strength”.