Experimental Study On Fatigue Behavior Of Retard-bonded Partially Prestressed Concrete Beams

With the development of prestressed concrete and super retarder, there is a strong needfor finding a suitable construction technique that can solve the problems of bonded andunbonded post-tensioned prestressed concrete. Retard-bonded prestressed concrete meets therequirements. Retard-bonded prestressed concrete is a new type prestressed system which hasappeared in recent twenty years. At present, the study on retard-bonded prestressed concrete isvery limit home and abroad. Compound retarder and retarding mortar which meet therequirements of bonded post-tensioned prestressed concrete are developed by myself.Retard-bonded prestressed strands are made by manual work and 21 retard-bonded partiallyprestressed concrete (P.P.C) T-beams are poured. Experimental study and theoretical analysison retard-bonded P.P.C beams are carried out under fatigue loadings. Connected with theproject of Doctoral Specialty Foundation of Ministry of Educaton—Study on the fatiguebehavior in normal cross-section of P.P.C beams with mixed reinforcement(No.20020141026), the fatigue behaviors of two kinds of construction crafts are contrastedwith each other. The contents which include eight chapters are as follows:In the first chapter, the significance of this study and the relevant literatures areexpounded. Meanwhile, the historical and present background of the research on fatigue ofretard-bonded P.P.C beams is introduced systematically. Based on the existing problems, theobjectives of this paper are put forward.In the second chapter, experimental study on compound retarder and retarding mortar iscarried out. The material selection and mixture ratio are introduced. The influence ofquantities on setting time and 28d compressive strength is studied; the relationship betweensetting time and average curing temperature is investigated; the interaction of admixtures isstudied and the optimum quantities are obtained; and retarding mechanism is discussed.In the third chapter, the experimental programs including the mechanics properties ofmaterials, details of beam specimens, apparatus of static and fatigue test are introduced. Thebasic rules and parameters of retard-bonded prestressed strands when they are tensioned areobtained from test. In addition, construction craft of retard-bonded P.P.C is introduced and thematters that need attention are pointed out. Moreover, the test procedure, the test parametersand the test data acquisition metheod are determined. In the fourth chapter, experimental study on retard-bonded P.P.C beams underconstant-amplitude fatigue loadings is carried out. Based on the test results, the strainincrement of the reinforcement and concrete, the change of deflection and ultrasonic wavevelocity versus number of cycles under constant-amplitude fatigue loadings are obtained.Fatigue failure pattern, fatigue life and the effect of fatigue loading on the postcyclic staticstrength are investigated. The test results of two kinds of construction crafts are contrasted.The formula of prestressed loss and fatigue check are recommended. The static design andfatigue check of P.P.C beams are compared. The calculating results are matched with the testresults well.In the fifth chapter, experimental study on retard-bonded P.P.C beams under two-stagevariable-amplitude fatigue loading is carried out. Based on the tests, the nonlinearcharacteristics of cumulative damage under two-stage variable-amplitude fatigue loadings arepresented, which is due to loading sequence and loading level. Meanwhile, the critical valueis obtained. The change of concrete and reinforcement strain, deflection and ultrasonic wavevelocity versus number of cycles under two-stage variable-amplitude fatigue loadings areobtained. Finally, the test results of P.P.C beams with different construction crafts arecompared.In the sixth chapter, experimental study on cracking and crack closing of retard-bondedP.P.C beams is carried out. Based on the test results, crack moment and crack closing momentunder static loading and S-N curve of flexural fatigue cracking under constant-amplitudefatigue loadings are obtained. Based on the tests, it can be found that cracking calculationformula of prestressed concrete is suitable for the retard-bonded P.P.C members. Themechanism of cracking and cracking close is discussed under static and fatigue loadings.Comprared with the P.P.C beams by traditional construction craft, the compressive stress ofretard-bonded P.P.C beams is much greater, in order to close the cracks. According to the testand analysis results, reduction coefficient of tensile fatigue strength is determined.In the seventh chapter, experimental study on maximum and average crack width ofretard-bonded P.P.C beams is carried out. The main factors that influence the crack width areinvestigated. Curves of maximum and average crack width under static loading,constant-amplitude and variable-amplitude fatigue loadings are obtained from the tests. Therelationship between maximum and average crack width is obtained. Based on the test results,calculating formula on maximum crack width of retard-bonded P.P.C beam under fatigueloadings is presented. The calculating results are matched with the test results well.In the eighth chapter, the main achievements of this paper are summarized and thefurther research of this field is pointed out.