This experimental study investigates the influence of replacing certain ratios of natural
aggregate by recycled concrete aggregate on properties of fresh and hardened concrete. Two
maximum nominal sizes of aggregate used in this study 40 mm and 20 mm, the source of used
recycled concrete aggregate comes from the demolitions of the upper skeleton of Al-AZHAR
building in Sohag. Thirty six mixes were designed and carried out to study the effect of both
aggregate size and replacement ratios on concrete properties, the replacement ratio of recycled
aggregate changed from 0 to 50% using of two sizes of aggregate 20 and 40 mm. A slump and
compacting factor were measured to express fresh concrete properties, hardened concrete
properties were expressed by compressive strength tests after 7 and 28 days, pull out bond
strength, splitting tensile strength and flexure strength. Test results shows the bad effect of
replacing natural aggregate by recycled concrete aggregate on both fresh and hardened concrete
properties.