Study On The Mechanical Property Of Corroded Anchorage Structure And Its Durability Evaluation

Bolt, one kind of reinforcement measure, is well accepted in rock and soil engineering practice. The aggressive medium and stray earth in rock and soil where bolt works always cause fast aging and significant damage to bolt, which eventually results in decreased durability, especially when bolt mostly has deficiency in self-protective system. Many anchorage engineering projects have suffered from this and failed in the end. Apparently, deficiency of durability has an adverse effect on the service and safety of anchorage engineering, so durability is one of the key factors of safety.According to the statistical data, corrosion is a leading factor in decreasing durability. It has an effect on the capacity of bolts at two aspects: firstly it leads to the cracking of the mortar cover due to the expansion of corrosion products; secondly it results in the imperfect bonding between bolts and mortar. In the dissertation, some mechanical study is done on the mechanical properties of corroded anchorage system by theoretical analysis, experiments and numerical analysis; and at the same time a method of evaluation of durability of corroded anchorage system is also proposed. The principle achievements in the dissertation is as follows.Firstly, according to the actual corrosion occurred to bolts in the work environment, a model of dynamic outline of bolts non-uniformly corroded is established. Based on the position of bolts and the streaming direction of aggressive medium, the fictitious displacement due to the expansion of corrosion products on the surface of bolts can be determined by the model. Then the mortar cover is numerically modeled by element SOLID65 in ANSYS8.0 and fictitious displacements are loaded on the inwall of the hole of bolts, so process of the cracking of mortar cover is simulated by adjusting the coefficient of thickness of rust. Based on the result of numerical simulation, the predictive formula of critical corrosion products leading to the cover splitting is proposed. Then the method is verified by experiment results. By the use of an example, it is proved that when the corrosion quantity of bolts uniformly and non-uniformly corroded are equal, cracks in the mortar cover and the destructive effect on it are obviously different, so the assumption of uniform corrosion should be used with caution.Secondly, a 3-D analytic model of corroded anchorage system isestablished to study the stress field under the anchorage force, confining pressure and expansion of corrosion products, and mathematical solution of all stress components are also obtained. However, this model can only be applicable to the conditions that materials are all in the regime of elastic deformation, that bonding along the whole anchorage length is perfect and that the mortar cover has not cracked because of the corrosion of bolts yet. The conclusions which can be drawn from the analysis of examples are: 1. the axial stress in the bolts is distributed as the function of exponential decrease; 2. on the arbitrary plane along the anchorage length, the radial stress in the mortar cover transferred from tensile stress to compression stress as the distance to the edge of the bolt increases, and the radial tensile stress beside the circumference of bolts is maximum ; 3. the hoop stress in the mortar along the whole anchorage length is maximum on the anchorage end and besides the edge of bolts on the arbitrary plane; 4. the shear stress between the bolt and mortar distributes linearly form negative to positive passing a neutral point at which the value of shear stress is zero, and the shear stress at the anchorage end is maximum; 5. confining pressure and expansion of corrosion products is favorable to increase the withdrawal resistance of bolts before cracks initiated in the mortar cover because they produce radial compress stress on the interface .Thirdly, by use of data obtained in the pull-out experiment of bars with strain gauge sticking inside, the effect of corrosion and different positions of it along the anchorage length on the capacity and bond of bolts is studied. The conclusions which can be drawn from the results of experiments are: 1. the ultimate withdrawal resistance of non-corrosion bars is maximum, and that of bars corroded at different positions along the anchorage length show no evident difference,but still reveal some regularity that among the three groups of specimens the withdrawal resistance of bars corroded in the front part along the anchorage length is minimum, and that of bars corroded in the rear part is maximum; 2. under the same pull-out force, the slid of bars corroded at front part along the anchorage length is maximum at loading end and free end, so the corrosion located at the front part has a most unfavorable effect on the withdrawal resistance of bolts; 3. corrosion acts as lubricating effect between bars and mortar, so in the curve of stress in bars, the shape of it is prominent atthe position of corrosion which indicates that the decrease of stress is slow, in the curve of the distribution of bond stress, the peak value of bond stress never appears in the position of corrosion, and in the curve of bond-slid at different positions along the anchorage length, the slope of the ascent part at the position of corrosion is evidently smaller than that at the position of non-corrosion.Fourthly, a non-linear finite element analysis method of pull-out experiments is proposed based on the surface to surface contact analysis function of ANSYS8.0. In the method, the surface of bolts is used as contact surface, while the surface of mortar is used as target surface, and the different properties of the two surfaces are modeled by two parameters of friction coefficient and initial cohesive strength. The method is used to analyze the sensibility of parameters and the results indicate that: 1. thick mortar cover is favorable to the withdrawal resistance of bolts, but when it is thick enough, the increase of thickness of it does not work; 2. the friction coefficient and initial cohesive strength are correlated, and the ultimate capacity of bolts increases linearly by the increase of both of them; 3. the effect of confining pressure on the withdrawal resistance becomes evident after the initial cohesive strength fails, and if the cover is in good condition, the confining pressure is favorable to the withdrawal resistance of bolts by increasing the frictional resistance on the surface of bolts; 4. the anchorage length is favorable to the withdrawal resistance of bolts.Fifthly, based on the dynamic cluster analysis of fuzzy equivalence relation, an evaluation method of durability of anchorage systems corroded is proposed. The method measures the similarity of durability index series among anchorage systems by use of similarity coefficient, and then forms a fuzzy equivalence matrix with sort function. Then anchorage systems are dynamically clustered according to different degrees of confidence. The method proves to be correct by examples. It avoids the irrationality in the conventional method and can be easily realized by programming.