**4. Test Results and Their Analysis**

#### *4.1. Shear Behaviour of Bending Infibre-Reinforced Concrete Beams*

Having analysed the beam deformation maps obtained from the Aramis 4M system for various levels of loading, five stages of functioning under shear conditions could be distinguished for the SFRWSC.

When no cracks occur in a given element, longitudinal deformations of steel and fibre composite are the same. The element functions in the flexural phase until the tensile strength of the fibre composite is attained, as illustrated in Figure 9.

**Figure 9.** Stage I of shear behavior of element—no cracks.

Once the fibre composite tensile strength is exceeded, which corresponds to the occurrence of cracks, the beam starts functioning in stage II, as a cracked element. Cracks perpendicular to the element axis appear in the middle of its span. The beam functions in this phase until the diagonal tensile strength of the composite in the shear area is reached (Figure 10).

**Figure 10.** Stage II of shear behavior of element—flexural cracks have occurred.

In Figure 11, the occurrence of several diagonal cracks between the tensioned reinforcement and the compressed area of the cross-section of the analysed beam is already visible. At the same time, the direction of perpendicular cracks changes, slanting from perpendicular to diagonal position.

A subsequent increase in beam loading results in increased crack width in the existing cracks, which means that the element is now in stage IV. In the case of beams with strong shear reinforcement, subsequent diagonal cracks may occur at this stage. The diagonal cracks become longer and approach the main reinforcement area. In stage IV, a critical diagonal crack may also appear, depending on the element shear capacity. Reaching of the shear capacity is shown in Figure 12.

**Figure 11.** Stage III of shear behavior of element—occurrence of diagonal cracks in both shear areas.

**Figure 12.** Stage IV of shear behavior of element—stabilization of the diagonal crack and arriving at the shear capacity.

Stage V is the stage of destruction of the element under shear force, through arriving at the composite diagonal compression strength (Figure 13). This type of destruction has been described, in [70], as a shear compression failure. It is observed in those beams that have strong main reinforcement with the simultaneous absence of or very poor transversal reinforcement. The cause of failure is a split fracture of the composite structure in the so-called compressed area (i.e., above the diagonal crack end), where a sort of pivot appears to occur.

**Figure 13.** Stage V of shear behavior of element—shear compression failure of the beam.

Beam failure, due to loss of adhesion of the fibre composite to the reinforcement originating from elongation of the longitudinal crack at the main reinforcement height, is illustrated in Figure 14.

Beams with strong shear reinforcement (BFSb series) were destroyed due to their arriving at the yield point of the tensioned reinforcement (Figure 15). The load shear capacity of an element is decided by its most strained and/or weakest cross-section. However, cracks originating in various element cross-sections are very important for its deformation, as they have an impact on rigidity of the entire element.

**Figure 14.** Stage V of shear behavior of element—shear tension failure of the beam.

**Figure 15.** Stage V of shear behavior of element—element bending failure through yielding of the tensioned reinforcement.

The application of steel fibres led to the failure images of SFRWSC beams shown above having a more ductile character, compared to fine aggregate cement composite beams without fibre reinforcement. The shear force–deflection relation, in the first stage of the test (pt. 3), is illustrated in Figure 16.

An analysis of Figure 16 clearly indicates the influence of steel fibers on the bearing capacity of the beams made of fine-aggregate fibre-composite near the support zone, and on the nature of the work of the beams after the appearance of the first diagonal crack. In the B series beams (without stirrups and steel fibers), when the crack appeared, the loading force remained at the same level. The appearance of successive diagonal cracks resulted in temporary load drops. However, in the remaining beams with shear reinforcement (stirrups and steel fibers), after the appearance of a diagonal crack, the load continued to increase and no decrease in the loading force was observed at the time of appearance of subsequent cracks. Both the stirrups and steel fibers affect this behavior of the beams, where the steel fibers can "bridge" cracks by transferring tensile stresses. In addition, the addition of steel fibers reduced the brittle nature of the material more effectively than stirrups. Analyzing the slope of the force (V)–deflection (δ) curves shown in Figure 16, it can be concluded that the steel fibers affected the bending stiffness, but the impact was not significant; which has been confirmed, in [15], in the case of elements with a high degree of main reinforcement. Yoo [71] and Ashour [72] also came to similar conclusions. It should be emphasized that, despite the slight influence of the addition of steel fibers on the stiffness, the final values of deflections at the maximum transverse force were much greater for beams reinforced with stirrups and steel fibers (approx. 13 mm), compared to those without fibers and stirrups (approx. 7 mm). The deflection values for the BF (with fibres) and BSa (with stirrups) series elements were similar, amounting to approx. 10 mm.

**Figure 16.** Relation between mean shear force and mean deflection for tested beams.
