Experiment Study On Size Effect On Shear Strength Of Reinforced Concrete Deep Beams Without Stirrups

In the reinforced concrete beams without stirrups, with concrete in the same property, the same anchoring, the same reinforced ratio and the similar dimension in the beam plane, if the average nominal ultimate shear strength decreases with the increasing effect depth of she beam,the phenomenon is called size effect of the beams without stirrups. The size effect in shear strength of slender beams without stirrups is obvious. However, the size effect in shear strength of deep beams without stirrups is confusing. The loading and supporting plate widths keep the same proportion to the beam height because of the influence of the plate widths on the shear strength; the plates are bonded with the beams by building structure adhesive for uniform contact.Eight reinforced concrete deep beams without stirrups, which were simply supported, were point loaded to shear failure. The heights of the beams are 200mm、400mm、800mm and 1600 mm. The size effect of short beams without stirrups exists when the shear span-to-depth ratio is 2 and the ratio of the lengths of supporting and bearing plates to the heights of the specimens is constant.The morphology of the shear zone before and after failure were analyzed, it was found that the height of crushing concrete is approximately proportional to the shear capacity. For micro crack distribution pattern of shear-compression zone has been carried on the comparative analysis, the micro crack distribution pattern changes with the change of beam size. For small size beams, the micro cracks distribute throughout the shear zone; micro cracks of large beams concentrate up on the edge of the loading plate and appear inclined downward. It indicates that the failure criteria of the concrete of this zone changes with the increase of test beam size. It was analyzed that the aggregate interlock force changes with the changes of test beams size. The open and sliding motion of inclined cracks were analyzed, and the mechanical model of aggregate interlock is also analyzed. It can be make sure that there is no aggregate interlock force in the largest beams, there may be aggregate interlock force in the smallest beams and the possibility that aggregate interlock force existing in the beams with 400 mm and 800 mm height is small. Almost all tearing cracks, which lift the restraint provided by the around concrete of longitudinal reinforcement to the vertical movement of the longitudinal reinforcement, are closely horizontal on the level of longitudinal reinforcement. As a result, the contribution of dowel action to shear strength is very limit.The shear strength and size effect were compared with several theoretical models and present design codes. It can be found that Mihaylov’s 2PKT model is better than the other existing theoretical models to predict the size effect of the deep beams; Chinese code GB50010-2010 is more conservative and safer than ACI 318-11, however they, also CSA23.3-04, failed to predict the size effect of the beams in this test.