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