Study On Anti-erosion Properties Of Concrete For Bridge Substructure In Freeze-thaw Environment

With the rapid development of civil construction in China, length of railroad lines in service of nation will reach to120,000km before2020in the layout of medium-term and long-term railway network. The bridge substructure will use reinforce concrete structure as load-carrying members. In the special erosion environment, the durability of concrete which can ensure engineering long-term quality and operation safety gets great attention. Frost resisting property is an important index weighting the durability of concrete. There exists environment of freeze-thaw cycle in most regions along the line of Beijing-Shanghai high speed railway, which runs north and south, acrosses four large water systems. The groundwater of some intervals alone the line contains salt corrosion, which will cause concrete structures damage easily and threat the safety of structures. So, study on frost-resistance and salt erosion properties of concrete and additional protective measures for bridge substructure in freeze-thaw environment have important realistic meaning.The dissertation summarizes research about concrete frost resisting property and protection measures from domestic and overseas. Atmospheric temperature in the line of Beijing-Shanghai high speed railway and times of freeze-thaw cycle of most cities are investigated. The concentration of corrosion ionics around bridge substructure is also investigated. Anti-erosion properties of concrete are studied based on some bridge pile foundations and piers of northern Beijing-Shanghai high speed railway. These bridge substructure are in the waterline, where exist freeze-thaw cycle(D3or D4) and sea water salt erosin.Frost resisting properties of concrete are studied under freeze-thaw cycle and different corrosion surroundings by means of experiments, numerical simulation and micro-analysis. Two different kinds of materials are used to protect concrete, which are steel pipes and spray polyurea elastomer. Main research and mainly conclusions follows:1. Three grades of high performance concrete C25, C35and C50are designed based on freeze-thaw cycle and salt erosion environments where Beijing-Shanghai high speed railway is. The durability experiments are carried out under freeze-thaw cycle and in different solutions containing distilled water, sodium sulfate or sodium sulfate and sodium chloride. Two kinds of protection measures are adopted. They are rigid and flexible protection, respectively. Indexes of samples testing contain mass loss rate, strength, dynamic modulus of elasticity, stress-strain and surface topography. Variation tendency has been acquired after diffetent times of freeze-thaw cycle. Mass loss of non-protectied HPC becomes bigger with the increase of times of freeze-thaw cycle. But the mass of protected samples is gradually increasing primevally and becomes steady in later period. Freeze-thaw cycle in saline solution produces more serious damage to non-protected samples. The erosion of mix solution of sodium sulfate and sodium chloride is most serious.2. Concrete stresses change cyclically with freeze-thaw cycle. Strain and stress increase gradully with lowering of temperature in one freeze-thaw cycle. When temperature increases, the interior strain and stress reduce gradually. But the interior deformation doesn’t recover and there is residual deformation, which shows that damage occurs in the interior concrete after every freeze-thaw cycle. The different defeatures and damage mechanisms are compared between HPC and normal concrete after freeze-thaw cycle.3. Steel pipes produce hoop stress during the change of temperature, which reduces frost heaving stress and make surface layer concrete in tensile condition change to compressive stress state. The mechanisms of rigidity and flexibility protection are different. They can both stop or postpone harmful saline solution or liquor entering into the concrete. These measures can avoid concrete damage and increase service life.4. Deformation component, range of temperature and frost heave distortion are used to represent physical equation of stress component for concrete under freeze-thaw cycle. The displacement of elastic body caused by range of temperature and frost heave distortion under certain displacement boundary conditions equals to the displacement caused by range of temperature and frost heave distortion as additional forces when the temperature is invariant.5. The physical models of concrete subjected to freeze-thaw cycle are set up and the corresponding behaviors are analyzed by ANSYS. The models contain none protected concrete samples and samples protected by steel pipes. Temperature field and temperature stress of different places in the interior concrete are obtained. The changing regulity of temperature and stress in different depth of concrete are calculated by changing parameters which contain structure size, protection conditions, freeze-thaw rate and concrete grade.6. Microscopic tests are carried out to affirm the accelerating effect of saline solution to concrete damage. The effects of admixture are analysed for improving concrete property. A lot of research work has been done to improve HPC frost resisting property, master freeze-thaw damage mechanism and use appropriate protection measures, which is in favour of intensive study and setting up scientific and comprehensive evaluating system of concrete durability. The dissertation provides scientific basis to practical engineering of HPC.