Fatigue Behavior Of Simply Supported Beams Reinforced By ICCP-SS System

Sudden destruction of bridge structures occurs from time to time in engineering practice,and the consequences are also very serious.After eliminating the insufficiency of ultimate bearing capacity in design,the main reason is that the bridge structure belongs to a special structure,which is different from the main static load of buildings and other buildings.The bridge structure also bears a large number of reciprocating vehicle dynamic loads,namely fatigue load.On the other hand,many bridge structures are still in a bad environment,and steel corrosion will occur in service for many years,which further affects the service life.Based on the above practical problems,the organic unification of cathodic protection(ICCP)and structural reinforcement(SS)can not only effectively prevent the steel bar from continuing to rust,but also restore the mechanical properties of the structure and effectively prolong the service life of the bridge structure.In the formal test,13 simply supported beams were subjected to dry-wet cycling to simulate natural corrosion.Most of them were strengthened by ICCP-SS and compared with a small number of non-electrified or non-strengthened beams.Finally,fatigue loading was carried out to investigate the bending fatigue performance.In the experimental design and preparation stage,all simply supported beams were corroded by dry-wet cycling simulation for 9 months after pouring and curing,then 11 beams were strengthened by CFRCM consisting of two CFRP grids and three layers of cement base,and the remaining two beams were unreinforced.At the same time,0,20,60 and 100 m A/m2 current density variables were introduced into cathodic protection,respectively,for a total of 15 months.Reference electrodes and multimeters are used to monitor the electrochemical signals during the electrification period to ensure the normal operation of the system and continue to simulate the wet and dry cycle corrosion.In the stage of fatigue test,the load ratios of 0.85,0.75,0.65 and 0.55 are used as the upper limit of fatigue load,while the lower limit of fatigue is determined by the fixed stress ratio of 0.2.The loading frequency is 5 Hz and the waveform is a sinusoidal curve.After the fatigue test is completed,the longitudinal reinforcement of the beam is taken out to measure the corrosion rate and the static load and fatigue test of the reinforcement are supplemented.The test results show that the fatigue life of simply supported beams is lower than that of low loads at high load ratio,and the service life is increased by more than 100% through strengthening.However,because of the deterioration of CFRCM caused by the electrification of ICCP system,and the corrosion rate of steel bars simulated by experiments is low,it may appear that the deterioration rate of CFRCM is faster than that of steel bars.At the same time,the fatigue life of simply supported beams with high current density(100,60 m A/m2)is significantly lower than that of beams with low current density(20 m A/m2)or without electricity.Comparing the fatigue life of simple supported beams with low current density(20 m A/m2)and without electrification,the fatigue life of simple supported beams with low current density under 0.65 load is longer than that of simple supported beams without electrification under 0.75 and 0.55 load ratios.This may be due to the discreteness of fatigue test or the better effect of ICCP-SS system than that of SS under low current density.On the other hand,the fatigue life of simple-supported beams with the same size and reinforcement is different and gradient,which indicates that the assumption of flat section is not suitable for simple-supported beams strengthened by ICCP-SS system under medium-high load ratio,and CFRCM has entered its resistance decline section during fatigue loading process to provide resistance.Then,several commonly used fatigue life estimation methods are introduced and compared,and the test data are fitted with S-N curve formula.Finally,the fatigue design references provided by Chinese code GB50010-2010,Japanese code JSCE Standard Specifications for Concrete Structures-2007(Design),European code Fib model 2010 and American code ACI 215R-1992(R1997)are analyzed and compared.The Chinese and American codes adopt the design method of infinite fatigue life limiting the stress of reinforcing steel bars,without establishing a relationship with the fatigue life N;while the Japanese and European codes have given the formulas related to the stress and fatigue life of reinforcing steel bars through research.The calculation of the formulas shows that the two codes are relatively conservative,and the stress level of reinforcing steel bars is relatively low when reaching the.