| Due to durability problems reinforced concrete structures often failed prematurely in service, therefore, domestic and foreign scholars had done a great deal of research for such problems, but mostly based on single factor or neglecting load influence, which had many discrepancies with the surroundings the concrete structures subject to in practical projects. Concrete structures located in harsh environments as coastal or cold zones are often subject to freezing-thawing cycles and steel corrosion and other effects in addition to loads in service. To fully address the impact of multiple factors coupling on the concrete structure and clearly know the deterioration characteristics and the resistance performance, it is necessary to take the durability study of concrete structure under coupling factors such as salt freezing, load, steel corrosion and etc.To this end, the possible differences between damage characteristics of BC beams which were degraded accelerately in the laboratory and degraded in the natural environment were taken into consideration firstly. And the similarity between the electrical corrosion and natural corrosion of reinforced concrete beams was researched, a more reasonable method of accelerated corrosion was proposed. Based the above, a series of experiments were made by using a counter-force apparatus system developed by research group. And the deterioration characteristics and resistance performance of RC beams were studied under the coupling of multi factors such as load, salt frost and steel corrosion. Specifically, the main work carried out and the obtained results are as follows:1)The corrosion characteristics of RC beams caused by the reinforcement methods including stirrups, no stirrups and insulation treatment of the intersection of stirrups and longitudinal reinforcement and the position of negative electrode were studied. The results show that: the stirrup of normal beams would prevent the longitudinal reinforcement intersected to stirrup from corrosion, but it was not obvious for the insulation treatment beams. The first crack caused by corrosion raised on the top of the beams due to different kinds of corrosion methods. The corrosion efficiency was higher when the negative electrode was on the top of the beams, but the corrosion products were not fully oxidized, and it presenting ink green color. At last, the reasonable power-corrosion method of RC beams was put forward combining with the natural corrosion beam.2)Based on the power-corrosion method, the corrosion features of RC beams under different load levels and different corrosion time at the normal temperature were studied. Experimental results show that: in the beam reinforcement skeleton, stirrups corroded the most serious, followed by tensile longitudinal reinforcement, compressive longitudinal reinforcement the slightest, and the average corrosion rate of stirrup was 2~3.4 times than that of tensile longitudinal reinforcement. Load has no obvious effect on the mean corrosion ratio of tensile longitudinal reinforcement, but with the increase of load level, the tensile reinforcement pit corrosion was more obvious. It’s thought that the essential reason of the difference of beam corrosion feature is the resistance and the change of distribution form of the reinforcement protective layer caused by different loads effect.3)The deterioration characteristics of concrete beams was researched at the coupling action of single salt frost, load and corrosion under cold conditions by using large scale step type artificial environment test chamber to simulate the freeze-thaw alternating environment. The evaluation method for assessing the degree of freeze-thaw damage of RC beams was put forward. The influence of loading on freeze-thaw damage of concrete beam and the influence of freeze-thaw damage on characteristic of electric corrosion of RC beam were summarized and analyzed. The results show that: freeze-thaw damage of concrete beam is aggravated by the load action, and the degree of damage was significantly larger than that of single factor under the coupling action of multiple factors. Under freezing and thawing environment, the corrosion efficiency of steel bars is reduced, but after the freeze-thaw damage, at the normal temperature, the corrosion efficiency of steel bars is accelerated.4)Based on test results, the corrosion of beam without load in different degree was researched, the bearing capacity and deformation of perpendicular section under same corrosion time in different load is researched and the deformation of RC beams during the continuous load and bonding performance degradation of reinforced concrete beam in the process of the whole loading had been analyzed in detail. Experimental results show that: the ultimate bearing capacity of loaded beams is slightly higher than that of unloaded beams when the tensile longitudinal bars anchored in good condition. The load history could influence the bending stiffness, and its stiffness was increased by 2.23~6.77 times when the load was continued at the initial stage after loading. It is not necessary to consider the cooperative work coefficient of the reinforced concrete when calculate the bearing capacity of the beams, and for the low corrosion rate(<6%) of high reinforcement ratio beams could even ignore bar’s effective area reduces, according to estimation of sound beams.5)The resistance properties of unbonded beams were studied through experiments and numerical simulation. The impact of failure of binding action in tension zone for corrosion, freezing and its coupling on resistance properties of reinforced concrete beam were researched. Furthermore, the influence of the length and distribution of unbonded zone on failure characteristics, bearing capacity and deformation properties of reinforced concrete beam were analyzed. The results indicated that: while the unbonded zone located in the middle of the test beam(symmetrically along the midspan, including pure bending section), with the increase of the length of the unbounded zone, the ultimate bearing capacity of test beam increased corresponding, but bending stiffness decreased; While it located in the bending shear section, ultimate bearing capacity didn’t increase obvious, but to some extent the bending stiffness decreased, with the increase of the distribution length of unbonded zone, a reduction amplitude of the flexural rigidity increased, and the less range the unbonded area distributed in the pure bending section, the greater amplitude the flexural rigidity reduced; In the case of the tensile longitudinal reinforcement anchored intact, rigidity reduction of the unbonded beam was mainly due a larger average strain of the tensile longitudinal reinforcement.6)Based on the results of the resistance performance of RC beams influenced by the coupled action of load, salt frost and steel corrosion, the degradation of bond behavior of tensile longitudinal bars was discussed after the freeze-thaw damage, at the same time, the bearing capacity and deformation performance also being discussed. The results show that the test deflection changes quickly, and the greater the degree of the freeze-thaw damage is, the greater the deflection increment is during the period of continuous loading. In addition, the ultimate bearing capacity is not lower than that of the intact one in case of the two ends of the tensile longitudinal reinforcement were in good condition and there was no damage of concrete in compression zone. |
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