Schmidt hammer is an instrument that is widely used in rock mechanics field. In Schmidt hammer tests, in general terms, surface hardness of rock specimens or rocks itself is measured. By means of this measurement, uniaxial compressive strength of the intact rock which is conducted to experiment can be determined indirectly. There are two types of Schmidt hammer. These are namely, L-type and N-type hammers. Basic difference between these hammers is the impact energy. A new measurement system was developed with new digital Schmidt hammers. In this measurement system, a new value is found which is called rebound coefficient and indexed as Q. In this study, classical and digital Schmidt hammers were used on seven different rock samples. Additionally, uniaxial compressive, uniaxial tesile and point load tests were conducted on same rock samples. A comparative study has been performed between R and Q parameters' prediction ability on mechanical strength parameters.

1 Introduction

Schmidt hammer, also known as "Swiss Hammer", is a non-destructive testing method, which is firstly developed and patented by Ernst Schmidt in 1951 for determination of surface hardness of construction materials such as metal, concrete, artificial stone and, ceramic products (Schmidt 1951). Since 1960s, Schmidt hammers were used for rock mechanics field widely for various purposes (Deere & Miller 1966). General application areas of this method are geomorphological aspects such as exposure age estimation or weathering grade estimation (Awasthi et al. 2005), prediction of rock properties like uniaxial compressive strength, point load index and, elasticity modulus (Sachpazis 1990, Aggistalis et al. 1996, Kahraman 2001), prediction of cuttability, drillability, sawability, boreability and rock cutting machine performance (Tarkoy 1973, Poole & Farmer 1980, Bilgin et al. 2002 and Yasar et al. 2014). Apart from these purposes, this method can be used in miscellaneous area of usage as effect of fire on concrete's surface hardness (Annerel & Taerwe 2011). Schmidt hammer is used for these aspects due to certain assets as below:

  • Simplicity,

  • Cost and time effective,

  • Nondestructive,

  • Portable.

Schmidt hammers can be separated in two types according to their impact energy that are L and N type hammers, and impact energies of these hammers are 0.735 and 2.207 Nm respectively. After impacts, surface hardness of sample is determined by rebound value, indexed as R. Since 2010, a new measurement system was introduced with digital Schmidt hammers that is called as rebound coefficient and indexed as Q.

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