1. Gaede K. Ball impact testing for concrete (Die Kugelschlagprufung von Beton), Bull. no. 107, Deutscher Ausschuss fur Stahlbeton, Berlin; 1952. p. 15.
2. A method for the estimation of compressive strength of concrete in the field;Williams;Struct Eng (London),1936
3. The concrete test hammer (Der beton-prufhammer);Schmidt;Schweiz Bauz (Zurich),1950
4. Facaoaru I. Non-destructive testing of concrete in Romania. In: Proceedings of the symposium on non-destructive testing of concrete and timber, London; June 1969. Institute of Civil Engineers, London; 1970. p. 39.
5. An ultrasonic method of studying deterioration and cracking in concrete structures;Leslie;ACI J Proc,1949