The influence of utilizing waste concrete aggregates on the flexural behavior of external reinforced concrete beams has been studied. Seven mixtures were prepared for this investigation where the concrete mixtures had different waste concrete percentages and admixtures. Also, seven beams were modeled by Ansys program and the properties of the seven mixtures have been used in the models to study their effects. It was found that using waste concrete aggregates has decreased the load bearing capacity and concrete ductility. It was found that the beam bearing capacity was decreased by 10.7% when using only waste concrete. Using admixtures have enhanced the concrete properties where the load capacity of beams has been increased by 39% when using silica fume and superplasticizer and the load capacity has increased by 44.6% when multi-admixtures have been used. Besides, it was found that using additives has enhanced the beam ductility significantly.