Toughness properties
There is a general consensus that introducing fibre reinforcement the typical brittleness of the plain concrete can be overcame . Discrete fibres into a concrete matrix improve its mechanical characteristics and , above all , supply a good resistance to crack opening and propagation because of their crack arrestor role . In fact , steel fibres modify the brittle concrete into a ductile material , able to hold up large deformations before losing its bearing capacity . Toughness is well recognized as the characteristic that most clearly distinguishes fibre reinforced concrete from plain concrete . Basically flexural toughness can be defined as the area under the complete load-deflection curve . The standard test for the evaluation of toughness properties is usually done on a notched beam , because the presence of the notch ( as crack inducer ) helps to control the crack position and ensures the failure in a well-known section . All the flexural tests were executed on prismatic specimens ( 150 mm x 150 mm x 600 mm ) according to the Italian standard ( UNI 11039 2003 ) with the set-up showed in Fig . 7 . Three full-bridge resistive clip gauges were used for experimental measurements : one clip gauge was applied at the notch mouth and the others on the two opposite faces of the beam at the height of notch tip , ( Fig . 7 ). The former gauge monitored the CMOD ( Crack Mouth Opening Displacement ), while the others recorded the CTOD ( Crack Tip Opening Displacement ). The specimens were tested with a 300 kN servo-hydraulic machine under CMOD control at a constant speed of 50 μm / min .
Figure 7 : Flexural test set-up
As can be observed from Fig . 8 , plain concrete has no residual strength , the specimens with industrial steel fibres are characterized by an average residual load of 3 kN , while the average residual load with recycled fibres is equal to 4 kN .
Application of steel fibres derived from scrap tires as reinforcement in concrete 781