Research On Maintenance And Reinforcement Strategy Of Concrete Structures In Marine Environment

The concrete structure has been widely used in the construction of infrastructure exposed to the marine environment.Under the combined effect of erosive environment and overload work,the concrete structure has already suffered from problems of insufficient durability such as cracking and steel bar corrosion ahead of time.In order to prolong the service life of the concrete structure used in the marine environment,and reduce the huge operating costs in the later stage of the structure,an appropriate and complete maintenance and reinforcement strategy prepared for these structures need to be framed immediately.At present,investigation of the repair and reinforcement schemes of concrete structures has attracted much recent attention from domestic and foreign scholars.However,the key issues of the existing concrete structures such as the optimal repair and reinforcement time and the maintenance methods has not been accepted.Therefore,with the support of the National Natural Science Foundation of China（No.51378241）,this paper proposes a dynamic monitoring framework for durability performance based on the perspectives of safety,economy,and sustainability;discusses the combination state of degradation index,dynamic reliability index and economic benefit index of maintenance strategy;analyzes the optimal maintenance time and maintenance method.And the crack repair method and comprehensive performance evaluation of the service concrete under the cracking state have been investigated and evaluated.The specific contents are summarized as follows:1.Study on the dynamic monitoring framework for the durability of serviced concrete structures in the marine environment.Firstly,the deterioration index of the structural durability is determined according to the degradation process of the concrete in the marine environment.Secondly,the evaluation factors based on security,economy and sustainability,as well as the hierarchical evaluation values of the structural deterioration state are proposed.Finally,based on the hierarchical evaluation values,a dynamic monitoring framework for the durability of the serviced concrete in the marine environment has been formed via maintenance and reinforcement methods.2.The decision-making scheme of repair and reinforcement of the serviced concrete structure based on the durability deterioration index has been explored.First of all,the maintenance decision tree framework is proposed depending on the crack width and local corrosion depth of reinforcement as indicators of the state of durability degradation.In other words,the preventive maintenance measures should be taken when the crack width is between0.03mm and 0.12mm.When the local corrosion depth h of the reinforcement reaches the maximum corrosion depth h_maxand the protective layer is expanded and cracked,the necessary reinforcement should be carried out.Then,the time-varying failure probability of the concrete structure during service is simulated by Gamma stochastic process as the dynamic reliability index.Finally,based on the deteriorating state of durability and dynamic reliability,the calculation model of cost-benefit expectation and the optimization model of maintenance and reinforcement decision scheme are established.A concrete structure in service under marine environment is used as an example,the resultant indicates the most cost-saved project is preventive maintenance previous to necessary reinforcement under three restoration schemes.3.The crack repair methods and performance evaluation experiments of concrete structures in service under cracking state have been investigated.Firstly,two kinds of cracking specimens,the preloading cracked specimens and the bending cracked specimens,under the action of load are obtained by selecting the splitting and the four-point bending method respectively.Secondly,the surface sealing,grouting and microorganism induced calcium precipitation（MICP）methods are carried out to repair two cracked concrete specimens,and the crack repair effect has been preliminary evaluated by appearance detection and ultrasonic detection.Then,the precise evaluation is obtained by rapid chloride migration test.The results show that those three methods can both repair the concrete cracks under load,and bending cracked specimens is better than preloading cracked specimens.In the case of improvement of chloride ion penetration resistance,the effect of three methods is ranked as:MICP>surface sealing method>grouting method.Compared with untreated specimens,the chloride ion migration coefficients of preloading cracked specimens and bending cracked specimens after MICP decrease by 83.1%and 58.0%respectively.The influence of surface sealing method on the chloride ion migration coefficient of bending cracked specimens is higher than that of preloading cracked specimens,while the influence of grouting method on the chloride ion migration coefficient of preloading cracked specimens is almost zero.Finally,the mechanism of microbial repair is further analyzed by scanning electron microscope,and it is found that the calcium carbonate precipitation induced by microbial bacteria is the main mechanism and product of the fracture repair.4.Evaluation of durability analysis and comprehensive performance of marine concrete structures has been completed after maintenance and reinforcement.The durability,economic performance and environmental protection performance of the repaired concrete structures via the sealing,the pouring joint and the MICP method has been comprehensively evaluated.The analysis results indicate that the comprehensive effect of MICP method is the best,and the durability of preloading cracked and bending cracked components are extended to 3.2 times and 4.5 times respectively.In addition,compared with the sealing and the pouring joint method,the repair cost of MICP is reduced by 87.5%and 81.5%respectively.It should be pointed out that the repair cost analysis is mainly based on material cost in the laboratory,and does not involve other costs in the actual engineering construction,which should be paid more attention to in the follow-up work.