Experimental Study On Large Scale Prestressed Concrete Spiral Case

Reinforced concrete spiral case is usually used in riverbed hydropower station because of its low-head.Aiming at the problem of limiting crack and seepage control of high water head concrete spiral case,the domestic Gaobazhou and Yinpan hydropower station have achieved good reinforcement effect by using prestressed reinforcement technology.However,at present,the research results and engineering cases of prestressed concrete spiral case are insufficient.The anchorage cable design of prestressed especially for higher water head and larger size spiral case still lacks the necessary theoretical and practical basis.In this paper,the simulation model test of large prestressed concrete spiral case is carried out with engineering examples,and the reinforcement effect of prestressed anchor cable is verified by three-dimensional nonlinear finite element analysis,and the conclusions are as follows:(1)No prestressed spiral case experiment.Under the design of internal water pressure,tensile stress of circumferential and water flow direction of steel bars on each section is small and does not exceed the design strength.The tensile stress of the circumferential steel bar and the tensile strain of the concrete are much greater than the corresponding values of the water flow,and the maximum values on each test section are decreasing along the direction of the flow.There is stress concentration near the inner corner of the side wall of the spiral case,and the maximum stress value of the steel bar has reached 23.2MPa,and the inside of the spiral case concrete is close to cracking or just cracking.No visible cracks were found on the outside of the concrete,and the maximum tensile stress of the central concrete on the outer side of the left wall is close to the design tensile strength 2.01 MPa of concrete and is at the crack edge.(2)Prestressed spiral case experiment.Vertical prestressed anchor cable is arranged on the outside of the left wall within the 1#-2# section.Loading to normal operating conditions,the stress and strain distribution curve law of steel bars and concrete on each section before and after applying prestress is basically the same,except that the vertical tensile strain of concrete in the middle of the left wall of 1# and 2# sections is reduced by 5% and 7.7% respectively,which indicates that the vertical prestressed anchor cable can effectively reduce the vertical tensile stress at the position,but the decline was modest.The results of overload test show that the cracking load of concrete on the outer surface of the left wall of the spiral case is 0.62 MPa,which exceeds the design internal water pressure by 33.3%.When loaded into overloaded conditions,the structure can still meet the requirements of bearing capacity.It is indicated that the concrete spiral case has high overload ability after setting the vertical prestressed anchor cable.(3)Nonlinear analysis of concrete spiral case considering temperature action.Three-dimensional nonlinear calculation analysis of non-prestressed spiral case structure under different temperature loads is carried out.The results show that under the summer working conditions,the stress of the carrying bar is increased by 21% and 20.6% respectively,the influence of temperature load on the structure is relatively large under summer conditions,and the maximum width of the whole structure crack is less than 0.30 mm of the standard limit value.However,the main tensile stress value of concrete in the middle of the left wall is close to 2.01 MPa and at the crack edge.(4)Nonlinear analysis of prestressed concrete spiral case considering temperature action.The anchor cable force is increased to 3000 kN for nonlinear calculation and analysis.The results show that the vertical tensile stress and the principal tensile stress drop of the middle concrete in the outside of the left wall can reach 10.7% and 8.4% on summer working condition,and the maximum crack width of the whole structure can be reduced by 0.03 mm,which indicates that the reinforcement effect of the prestressed anchor cable will be more obvious after the force is increased to 3000 kN.