Researches On The Reinforcement Of Prestressed Concrete Cylinder Pipes With External Prestressed Steel Strands

Prestressed Concrete Cylinder Pipe(PCCP)is a composite structure composed of a concrete pipe core,a steel cylinder,prestressing wires,and a mortar coating.It has several advantages,involving high strength,strong impermeability,good durability,and low maintenance cost and so on.PCCPs have been widely utilized in long-distance pressured water transportation systems and municipal draining projects in the United States,Mexico,Canada,China and many other countries.The PCCP suffers from wires breakage,longitudinal cracks and degrades due to various factors such as irrational design,manufacturing defects,substandard construction quality,inappropriate construction operations,changes in loads and external environmental conditions.These problems will seriously affect the safe operation of the pipelines.The significant advantage of the external reinforcement is that it is unnecessary to take the deteriorating pipes out of service,so as to restore the pipe to its original bearing capacity.Furthermore,the reinforcement of PCCP with external prestressed steel strands is particularly suitable for the single-line pipeline.The domestic research on the reinforcement of PCCP with external prestressed steel strands is still blank.Therefore,this paper focuses on the critical technical problems to be solved in the the reinforcement of PCCP with external prestressed steel strands and develops a series researches on key issues such as the stress of each components,prestress loss and so on by the mean of theoretical analysis,prototype test and finite element analysis.The following research work was mainly carried out:1.A review involving the replacement,internal reinforcement(steel cylinder relining,steel cylinder slip-lining,carbon fiber reinforced polymer(CFRP)reinforcement),external reinforcement(CFRP tendons,prestressed steel strands)and other methods were developed.The advantages and disadvantages were compared in order to provide references for the selection of reinforcement methods under different working conditions.2.In order to determine the final effective prestress of the prestressed strands,a calculation theory for the prestress loss of prestressed strands is proposed based on the PCCP structure.The prestress loss includes the losses caused by friction resistance between the surface of the pipe wall and the steel strands,the anchor deformation,the concrete elastic compression induced by stepwise tensioning operation,cracks closures,shrinkage and the creep losses of concrete,and the long-term relaxation losses of prestressed steel strands etc.Thus,the effective prestress at the final stage can be calculated.The determination of the prestress loss provides support for the calculation bearing capacity of the pipe in the external reinforcement design.The design framework of the external reinforcement technology is proposed in this paper.Taken the concrete core and the mortar coating as the research object,the cross-sectional area of the prestressed strands and the corresponding center distance are calculated according to the ultimate limit states of the bearing capacity.The checking computation of the stress of the core concrete and the mortar coating are conducted in the serviceability limit states and the quasi-permanent limit states.Only when all of the calculation results meet the judgment criteria can the determination of the reinforcement scheme be feasible.3.Based on the PCCP structure type,a prestressed anchoring device was developed for the surface characteristics of the PCCP to avoid the damage caused by the traditional anchorage to the solid addition.4 A prototype test of PCCP reinforced by the external prestressed strands was performed in an assembled apparatus.The apparatus was mainly constituted by two PCCPs whose internal diameter is 2000 mm.The resistance strain gauges were arranged at the spring-line,the crown and the inverted of thress sections along the longitudinal direction,respectively.The stress response curve of the core concrete and the prestressed strands before and after the reinforcement was measured.Based on the analysis of the mechanical properties of PCCP with broken wires,this paper evaluates the improvement effect of the external prestressed reinforcement technology on the bearing capacity of pipe,and provides support for the implementation of this technology.Cut the prestressing wires from the middle to the sieds of the pipe at the working pressure of 0.6 MPa until the cracks propagate in the concrete core.The percentage of brokenwires was up to 37.7%at that time.Tensioning operation was performed at the arterial pressure of 0.2MPa.The strengthened pipe was capable of sustaining the design internal water pressure of 0.9 MPa.The cracks exsiting before were still closed and the water tightness property was in a good state after the reinforcement.The strains of the core concrete are significantly lower than that before the reinforcement under the same internal water pressure.The reinforcement of the PCCP with external prestressed steel strands is able to meet the strengthen requirement of the test.5.Taking the same PCCP as the research object,a three-dimensional solid finite element model was established.The whole test process of loading,wires breakage,decompression,reinforcement and re-pressurization after the reinforcement was simulated to obtain the stress and structural response of the pipe under various working conditions.The comparison between the simulation results and the prototype experimental data is developed.The correctness,rationality and reliability of the theoretical analysis of the prestress loss,the design theory and the structural type were verified.A concrete damaged plasticity(CDP)was adopted to simulate the core concrete and mortar coating.The effects of non-structural cracks and the decrease in stiffness during sthe reinforcement weren all considered.A three-dimensional finite element numerical model of three consecutive PCCP prototype pipe joints was established considering the particularity of the bell and spigot.Furthermore,the influencing area of the broken wires was divided into sections to optimize the previous model considering the effect of the bond between the mortar coating and the prestressing wires.The previous calculation model is optimized on the premise of considering the actual type of wire wound and the prestress of the wires is applied by spiral winding.6.The influence factors of the external prestressed reinforcement technology are analyzed to determine the degree of influence of various related factors on the bearing capacity of the PCCP structure by means of three-dimensional solid finite element model,including the center spacing of the strands,the tensile strength of the strands,and pipe parameters etc.The index system of the reinforcement of the PCCP with external prestressed technology was proposed with the help of the Fuzzy Analytical Hierarchy Process to provide support for the optimization and implementation of the reinforcement of the PCCP with external prestressed technology in the future.