Experimental Study On Flexural Properties Of Unbonded Prestressed Concrete Beams At Cryogenic Temperatures

The “experimental study on flexural properties of unbounded prestressing concrete beams at cryogenic temperatures” is a sub-topic of the “experimental study on material properties and structural behaviour of prestressed concrete at cryogenic temperatures”(Natural Science Foundation(51478309)).To promote the safe use of unbonded prestressed concrete structures under low temperature and cryogenic environment, meanwhile, to provide reference for the safety assessment of existing structures under low temperature and cryogenic environment. Cryogenic temperature cold storage and the self-designed cold insulation-loading system were used to test flexural properties of the 19 unbonded prestressed concrete beam at different temperatures(20℃、-40℃、-70℃、-100℃) and different tension stresses(0.4fptk、0.6fptk、0.75fptk).Relevant works are as follows:1) With the help of ultra-low temperature cold storage as well as self-made thermal insulation system, a cooling-insulation device was made for cryogenic beam experiment. The existing loading device in the structure lab was modified to create the loads-measurement system for bending performance test at ultra-low temperature. Eventually a complete test standard process was formed.2) Prestress losses of steel strands at room temperature and in cooling process were tested.3) Considering the effects of temperature and tension control stress, the influence of unbonded prestressed concrete beam crack, crack load, yielding load, ultimate load and stiffness by tension control as well as temperature was studied. Considering the effect of low temperature to material, the applicability of calculation formulas for cracking load and ultimate bearing capacity in current code are verified at cryogenic temperatures.Conclusions can be summarized as follows:1) In the cooling process, the tendon stress increases linearly as temperature decreases. When the temperature falls below-20℃、-40℃、-70℃ and-100℃, the prestress increases at approximately 4%, 5.9%, 8.8% and 11.6% of tensioned control stress.2) In the cryogenic environment, plane-section assumption of concrete part is still reliable at ultimate state, temperature nearly has no impact on plane-section assumption of flexural member.3) Specimen cracking load, yielding load, ultimate load decreases as temperature rises and the increased degree of cracking load is greater than the ultimate load. At the same temperature, with the increase of tensioned control stress, cracking load linearly increases, yield load rise is not obvious, and ultimate load is essentially the same.4) As the temperature decreases, stiffness of prestressed concrete girder increases. While specimens yielding load and ultimate load ratio show little change with decreasing temperature, and cracks develop fully, which indicates low temperature does not significantly reduce ductility of prestressed concrete beams.5)Considering the effect of low temperature to material, the calculated cracking loads by code formula coincide well with test values, while the ultimate bearing capacities are quite different from experimental values, which deserves further research.