A new test for rock strength: Double-punch test

This paper proposes a new test for rock strength- the double-punch test, which was introduced by Chen (1970) for the determination of the indirect tensile strength of concrete. In this test, a circular cylindrical specimen of diameter 2 b and length 2h is subjected to two rigid circular punches of diameter 2a (< 2 b) at the top and bottom of the specimen. The testing machine for uniaxial compressive strength (UCS) can be used, and failure load is much lower than that for UCS. The normally observed fracture surfaces involve more than one fractures through the axis of symmetry. A new elastic theory for the test proposed recently by Wei and Chau (1999) will be discussed and compared to experimental results. The tensile strength interpreted from the present analysis is found comparable to the formula proposed by Bortolotti (1988) for a/b > 0.2 and agrees well with experimental data, and thus provides an improvement over Chen's (1970) original formula.