Erjola Reufi and  Jozefita Marku and  Thomas Bier,  Ultrasonic Pulse Velocity Investigation of Polypropylene and Steel Fiber Reinforced Concrete.   journal   = {International Journal of Civil and Environmental Engineering}, [online]. World Academy of Science, Engineering and Technology.
    February 2016, vol. 111(3). 332 - 335
    [viewed 20 October 2019]. Available from: https://publications.waset.org/pdf/10003932.