1. Ultrasonic Methods of Testing Materials;Filipczynski,1966
2. Ultrasonic Testing of Material;Krautkramer,1983
3. Properties of Concrete;Neville,1995
4. R. Jones, E.N. Gatfield, Testing concrete by an ultrasonic pulse technique, DSIR Road Research Tech. Paper No. 34 (London, H.M.S.O., 1955).