The Prediction Of Fracture Properties Of Hard Rock Or Concrete And The Repair Technique Of Their Defects

With the rapid development of transportation in China,the traffic tunnels have become indispensable parts of transport facilities,which are constructed massively in the whole area of China,especially in the western region.Rock and concrete are two main materials involved in tunnel construction,their mechanical properties are critical.Generally,the tensile strength of hard rock and concrete materials is far less than their compressive strength,these materials often fracture under a light tensile load.Therefore,the study on the fracture properties of hard rock and concrete is the key part of the researches on their mechanical properties,becomes the important basis for the application of these two materials.When the tunnel construction is completed and come into service,cracks of various sizes and forms will inevitably appear in tunnel structures due to the characteristics of concrete and the influence of the surrounding environment,which seriously threaten the life of the structures.Hence,to guarantee the safety of traffic tunnels,the study on the repair technology of concrete have to be carried out.With the help of technical survey,theoretical analysis,laboratory tests and numerical simulation,a series of studies are carried out to study the fracture of quasi-brittle materials and its repair techniques,based on the observation of fracture process of quasi-brittle materials.It covers the size effect of strength,the stress distribution in fracture process zone,the Boundary Effect Model,the discrete rules of fracture properties of concrete and hard rock,the effect of specimen size and shape on predicting fracture properties,the role of micro-structure in fracture process,the feasibility of using small specimen to predict the fracture properties of huge specimens,and the repair technique of micro-defects in quasi-brittle materials.This dissertation studies the fracture process zone of quasi-brittle materials firstly.Then,based on the Boundary Effect Model,the predicting method of fracture properties is concluded by analyzing the stress distribution around the crack tip.Next,the concrete and hard rock are used to study the affecting factors of prediction.After studying the fracture of quasi-brittle materials,the relavant repair technique is put forward in the end.The main achievements in this dissertation are listed as follows:The necessity of study on fracture prediction method and repair technique of quasi-brittle materials is analyzed from the view of traffic tunnel.The development of fracture mechanics,the main fracture models of quasi-brittle materials,the numerical simulation method of quasibrittle materials and the repair technology of quasi-brittle materials are summarized,which provides the direction for the work in this study.When applying classical strength theory to calculate the strength of specimens with different sizes,the size effect will appear.Hence,such phenomenon should be analyzed by fracture mechanics.By using the method in the Fictitious Crack Model,the stress distribution in the area near the crack tip is analyzed,and a simple and easy to use one parameter fracture model is derived from the Boundary Effect Model,which can be used to determine the fracture properties and predict the failure loads of specimens with different sizes and geometries.Three point bending tests are carried out among the granite(2.0mm average grain size)and medium sandstone(0.35 mm average grain size).Firstly,the fracture properties of granite are calculated by one parameter fracture model,the distribution of fracture properties is found to follow the normal distribution.Therefore,the one parameter fracture model and normal distribution analysis could be combined to predict the fracture properties of hard rock.The influence of the specimen sizes,shapes and crack length height ratios on the prediction of rock fracture performance is also studied.The results show that the prediction of rock fracture performance is relatively accurate,the results are constant relavant to the material,have nothing related to the specimen sizes,shapes,crack length height ratios and other factors.At the same time,it is verified that the effect of the errors in determining the rock average grain size on the accuracy of fracture performance calculation can be ignored.For medium sandstone,when the specimen size reaches 300 mm,its fracture process conforms to the fields of linear elastic fracture mechanics.The one parameter fracture model can also conclude similar results.It should be noted that the equivalent initial crack length of a small specimen without initial crack should be regarded as a positive number during analyzing its fracture process.With the one parameter fracture model,the fracture properties of concrete are calculated based on the three point bending tests results.Then,the dispersion of the fracture properties is analyzed,and it is found that the calculated results follow normal distribution.Then,the normal distribution analysis method and the one parameter fracture model are combined to study the fracture properties of concrete.The influence of the specimen sizes,geometries,crack length height ratios and other factors on the prediction of concrete fracture performance is also studied.The results tend to be similar with the rock's results,the fracture properties vary with the change of aggregate sizes.Finally,based on the safety evaluation standard of concrete structure,a new evaluation criterion is put forward with the index of crack depth.Combined with the mesoscopic parameters obtained from the concrete biaxial compression tests,the numerical model of concrete fracture prediction is established by utilizing the Particle Flow Code.According to the concrete test conditions in chapter 4,the concrete fracture process is simulated,and the fracture properties are calculated with the one parameter fracture model.The results show that the Particle Flow Code not only reflect the fracture process of concrete,but also predict the fracture failure load accurately.The simulating results appear to be similar with the experiment.If the computer hardware permitted,the Particle Flow Code has obvious advantages in the simulation of large size quasi-brittle fracture.Based on the analysis of the advantages and disadvantages among the existing quasi-brittle material repair methods,a new method for repairing the micro-defects in the substrate is proposed due to the characteristics of quasi-brittle material fracture.By comparing the secondary fracture properties,fracture paths and microscopic observation results of the specimens repaired with different methods,it could be concluded that the pre-coating method can effectively repair the micro-defects in the substrates,and the repair efficiency is greatly improved as well.In addition,the effect of microstructure on the repair efficiency is also studied.The method of predicting fracture properties and repairing cracks for quasi brittle materials are studied in this dissertation.The research findings would provide the theoretical support for tunnel and underground engineering construction from the fracture mechanics' view,provide a sample for repairing the disease in tunnel,also provide the theoretical basis and experience for other similar underground engineering at home and abroad.This study may promote the development of underground engineering construction and operation technology in China.