Shear performance of ultra-high performance concrete deep beams

To promote the application of ultra-high performance concrete(UHPC) deep beams, shear performance tests were conducted on four UHPC deep beams by taking the concrete strength as the main parameter, and the C40 and C80 concrete deep beams were tested for comparison. The load-deflection curves, failure modes, reinforcement strains, crack patterns and ultimate loads of UHPC deep beams were analyzed. The method specified in the Code for Design of Concrete Structures(GB 50010—2010) was used to determine the shear strengths of six deep beam specimens, to discuss whether the current method for calculating the shear capacity of ordinary concrete deep beams is applicable to UHPC deep beams. Research result shows that the greater the concrete strength is, the greater the stiffness of the deep beam under the same load is. The stiffness of UHPC specimen in its elastic stage before the cracking increases slightly with the increase of steel fiber content. Similar to the C40 and C80 concrete deep beams, the UHPC deep beams exhibit bending-shear and web-shear cracks. When the load reaches 13%-22% and 18%-34% of the ultimate load, the two types of cracks appear successively. Beam and arch stress mechanisms are both present during the loading of UHPC deep beams. The beam stress mechanism dominates the early phases of loading process, whereas the arch stress mechanism dominates the latter stages of this process. The UHPC deep beams exhibit a large number of densely and concentrated cracks and undergo shear-compression failure, without showing any cracks in the inverted arch area above their supports. In contrast, the C40 and C80 concrete deep beams undergo diagonal compression failure, and cracks appear in the inverted arch area above their supports. The shear bearing capacity of test beam increases in an approximately exponential manner with the increase of concrete strength. When the concrete strength increases from C40 to C80 and C190, the shear bearing capacity increase by 30.76% and 201.92%, respectively. When using the method specified in the Code for Design of Concrete Structures(GB 50010—2010) to calculate the shear bearing capacities of UHPC deep beams, the calculated and experimental shear bearing capacities show an average ratio of 0.89 and a mean squared error of 0.15. Therefore, this method can be temporally used to calculate the shear bearing capacity of UHPC deep beams, until a more accurate calculation method is available.