## 4 Conclusions  
In the current research, the structural behavior of hybrid-fiber-reinforced UHPC beams containing steel bars under four-point flexural loading was studied. The basic variable in the experimental investigations was the steel bar depths. The corresponding bar reinforcement percentages ranged between 1.2 and 2.0%. The developed UHPC contained about 2.5% hybrid microsteel straight fibers and fine desert aggregates. The following conclusions were drawn based on this investigation. The microcracks in the UHPC beams were typically initiated at the beam's softit surface and propagated vertically toward its top surface. The fiber's pullout from the concrete caused these microcracks. When the load on a single fiber exceeded the fiber-matrix bonding resistance, the pullout of the fiber ensued, thereby increasing the load, which the other fibers had to overcome. This mechanism made the fiber undergo a progressive pullout (collapse) mode. The enhanced deformability of the bar-reinforced UHPC beams was noted compared to the control beam; however, the majority of beams failed due to one major flexural crack. This cracked section developed when fiber pullout explicitly occurred. This phenomenon was typically caused when the beam approached its ultimate load, followed by bar yielding. The typical load-deflection behaviors of the bar-reinforced and -unreinforced UHPC beams started with linear relations and ended with the development of flexural cracks that decreased the flexural stiffness of the beams (decreasing the slope of the load-deflection curve). The end of this second linear stage of the load-deflection response was characterized by bar-reinforcement yielding. Afterward, the beam's deflection increased nonlinearly up to the peak load. The plain UHPC beam experienced the minimum cracking load and the lowest average load-carrying capacity of the beams. The maximum load of UHPC beams was notably increased by the introduction of the bar reinforcement. The ductility of the UHPC beam decreased as the beam's effective depth decreased. Moreover, the deflection ductility parameter based on the ultimate defection was more practical in evaluating the ductility of UHPC beams, as it provided positive expected results for all the investigated beams. The introduction of tensile reinforcement bars in the UHPC beam notably increased the beam's cracking capacity before failure. Additionally, the maximum crack width increased as the reinforcement effective depth decreased. After reaching the maximum load, as the effective depth increased, the rate of crack propagation decreased. This favorable behavior indicated that the efficient design of the bar reinforcement in an UHPC beam can help control the development of cracks, which plays an important role in the serviceability and durability of beams. All the studied UHPC beams had an uncracked phase that extended to a curvature of about \(7.5\times 10^{-6}\) rad, which occurred at about 10 kNm during the first cracking moment. Beyond this curvature, the moment-curvature behavior was shown to be linear. In this linear cracked phase, the curvature quickly increased for the unreinforced beam; however, the bar reinforcement helped control the curvature deformations of the UHPC beams. The bar-effective depth of the UHPC beam will likely enhance the beam's curvature capacity. The design of the experimental numerical modeling approach was utilized to develop a prediction model for \(M_{u}/\left(bd^{2}~{}f_{c}^{\prime}\right)\) in terms of \(f_{c}^{\prime}\), \(f_{t}^{\prime}\), \(\rho\), and \(d/h\) and prepare a wide-ranging database based on the present experiments and independent data. The suggested prediction model offered reasonable performance for the majority of the investigated data points, with a mean prediction-to-test ratio of 0.92. The conclusions drawn here are perhaps limited to lab-sized beams; however, normalized (size-free) design formulas have been proposed. Prospective research would investigate a size-based model of UHPC beams. Conceptualization, M.I.K.