Study On Cracking Characteristics And Time-Dependent Reliability Of Steel Lined Reinforced Concrete Penstock

With the flourishing development of large hydropower station and water transfer project,steel lined reinforced concrete penstock(SLRCP)has been widely used as the key structure of water diversion and transmission.Although the SLRCP has brought many benefits such as reduced construction cost and good adaptability to high head due to the bearing characteristics of concrete encasement,numerous pressing problems remain unsolved and require immediate attention.It is designed to allow concrete to crack but the crack width must under limitation to ensure its performance throughout life cycle.Nevertheless,the problem of crack width calculation has not been agreeably solved.It remains unknown that whether the penstock can maintain its bearing capacity during its designed service life under cracking work condition.What’s more,both results of the numerical simulation methods and theoretical solutions have differences with the monitoring data.As a result,this paper focuses on the studying of numerical simulation methods,bond-slip mechanism,and crack width calculation methods under the coupling effect of loads and adverse external environmental factors to find better solutions of the above problems.The main investigations and accomplishments are delineated as follows:(1)The cohesive crack model was adopted to simulate concrete cracking by means of node separation to overcome the crack width calculation problem.A MATLAB script was utilized to globally embed cohesive elements in the concrete elements to achieve random cracking simulation.Moreover,the cohesive parameters,a key for simulating concrete cracking,were extensively discussed to provide theoretical guidance.It is verified that this model can be used in SLRCP cracking simulation by comparing the results of cracking propagation and steel stress dynamic evolution processes with the traditional smeared crack model and the large-scale model test.Consequently,this model can acquire high precision numerical crack width solution within the reasonable cohesive parameters.Both crack width and crack shape can be synchronously obtained through node separation,which avoids the disadvantages that the smeared crack model cannot get the crack width directly,and the discrete crack model can only model pre-existing cracks.(2)Bond-slip behavior at the steel-concrete interface plays a critical role in reinforced concrete structures since the bond action is the mechanism that ensures the two materials work in co-operation.However,there are few studies on the bond-slip mechanism of SLRCP.Given the pivotal role of the bond action,it is necessary to investigate the bonding mechanism of the SLRCP.Thin-thickness cohesive elements were inserted at the interface between the reinforcements and concrete to simulate the bond-slip behavior.Three interface models were implemented to represent the different interaction conditions at the steel-concrete interface.In addition,sensitivity analyses were performed to study the impact of interface bond condition and concrete tensile strength on the bond performance and structural behavior.The simulation results show that the bond-slip behavior can be ignored in structural safety design owing to its limited effect on steel stress while cannot be neglected in durability safety design due to its sensitivity to crack widths.Good bond between reinforcements and concrete can be observed attributing to the ring-closed geometry,circular tensile reinforcement,and radial internal water pressure loading of SLRCP.(3)There is no widely accepted crack width theoretical method which contradicts with the urgent need of crack width calculation.A steel-concrete composite ring method based on the theory of orthotropic and thick-walled cylinder was proposed combining the geometric features,loading conditions,and cracking characteristics of SLRCP.Subsequently,the amplification coefficient between the average crack width and the maximum crack width can be acquired by mathematical statistics method,whose sample data was obtained from the numerical simulation analysis.In addition,the temperature effect on crack width was defined utilizing the thermomechanical cohesive crack model,and the corresponding crack width calculation program was further developed.It is validated that the theoretical method given in this paper fits the monitoring data best and exhibits strong adaptability among SLRCP projects.As a result,this method can be widely applied to the crack width calculation with high precision.(4)In order to study the material degradation and structure resistance decline of SLRCP,the carbonization depth prediction model and the corrosion crack width prediction model after considering the cracking condition of SLRCP were proposed based on the studies of reinforced concrete structures.Accordingly,the time-dependent reliability analysis model was then established to explore the structural deterioration and crack evolution processes.Whereafter,the design method of crack limiting reinforcement was constructed and proved to be effective in reducing crack width to prolong the service life of SLRCP.It can be seen that the crack width of SLRCP has beyond the limit value under different crack width limit values of specifications before the designed service life,and corresponding maintenance measures must be taken to ensure the penstock safety.