| Steel Bars for Prestressed Concrete (PCB) is a new type of medium-strength reinforcing steel developed in China in recent years. For medium-span concrete slabs, cracking control would be difficult due to the large spacing between the rebar and full use of the material properties can not be achieved if over-reinforced when prestressing steel strand is used for reinforcement, besides, reinforcing with ordinary rebar would cause problems such as difficulty of construction caused by the crowding of the rebar. However, if PCB is adopted as the reinforcing steel for medium-span prestressed members, the strength of the steel would be full used besides ensuring the ductility and facilitating the cracking control. As the tensile strength and conditional yield strength of PCB are lower than that of high strength prestressing steel strand, whereas the ultimate tensile strain of these two types of reinforcing steel are almost the same, the influence of the hardening stage of the PCB on the normal cross sectional load bearing capacity of the prestressed concrete slabs reinforced with PCBs needs to be investigated. An experimental study on the flexural load bearing capacity of prestressed concrete slabs reinforced with PCBs is carried out and presented in this thesis.The tensile strength, yield strength, strength at proportional limit, elongation, modulus of elasticity as well as tensile stress-strain curves are obtained through property tests. The stress-strain curves of PCB are fitted with the quartic polynomial constitutional model.Tests on the normal cross section load bearing capacity of 28 slabs are completed, in which the influence of the tensile control stress and the relative height of the compression zone on the mechanical properties of the normal cross section is considered and the cracking load, ultimate load as well as the load-displacement curves of the tested slabs are measured. As a consequence, the characteristics of the three working phases of the concrete slabs prestressed with PCBs are summarized.For prestressed concrete slabs reinforced with PCBs, the actual stress level of the PCB under ultimate limit state is different from its conditional yield strength due to the influence of the tension control stress level as well as the relative depth of the compression zone; besides, the compressive strength of the concrete under eccentric compression is higher than that under axial compression as a result of the impact of the strain gradient of the concrete in the compression zone. It is found through analyses that the ratio of the compressive strength of the concrete determined by the equivalent rectangular stress graphics of the compression zone to its axial compressive strengthαincreases as the relativedepth of the compression zone decreases, hence a formula to calculateαis proposed withthe relative depth of the compression zoneξas the independent variable. Considering that the stress of the PCBs under limit state is different from their conditional yield strength, Formulas for the calculation of stress variation coefficient of PCBs is putforward via theoretical derivation and numerical fitting, respectively. On this basis, a formula for the calculation of the ultimate flexural load bearing capacity of prestressed concrete flexural members with PCBs is established, which provides a reference for the calculation of flexural load bearing capacity as well as design of prestressed concrete slabs with PCBs. |
