Durability Of Concrete In Coastal Wetland Environment

In the coastal environment,environmental factors induced deterioration on the durability of reinforced concrete structures is very common.In this thesis,two kinds of salt erosion solutions are prepared to simulate the service environment of the sampling sites in an intertidal zone of a coastal wetland in Jiangsu province.According to the characteristics of the natural environment of the sampling sites,three kinds of test conditions are designed: long-term immersion,immersion and dry-wet cycle,and immersion and freeze-thaw cycle.Then,the morphology,strength,chloride ion content,quality loss rate and relative dynamic elastic modulus of concrete under the three test conditions are analyzed.Meanwhile,the erosion degree of concrete under the same test conditions but in different erosion solutions is analyzed.Based on the test data,the evaluation results are analyzed by four durability evaluation indexes of concrete salt frost resistance.The theory of Grey Relational Analysis(GRA)and the Analytic Hierarchy Process(AHP)are used to analyze and evaluate the durability of concrete.The main contents are as follows:(1)Through the test of long-term immersion in erosion solutions,the changes of microstructure and mechanical properties of concrete in tidal-flat wetland environment are studied and the change rule of concrete performance in chloride environment is obtained.The test results show that under the condition of long-term immersion,the compressive strength of concrete increases significantly,while its flexural strength only increases slightly.After 90 d of immersion,both the compressive strength and flexural strength become stable.The higher the solution concentration is,the higher the concrete strength improvement is.The content of chloride ion is the highest in the surface layer of concrete,decreases successvily towards the inner layer,and becomes constant at a depth of 30 mm from the outer surface.The content of chloride ion is also closely related to concrete strength grade,soaking age,solution type and concentration,and diffusion depth.(2)Through the test of immersion in erosion solutions and simultanously subject to dry-wet cycle,the durability of concrete in intertidal environment of tidal flat wetland is studied,and the deterioration mechanism of concrete under the action of chlorine and dry-wet cycle is revealed.The test results show that in the early stage of the dry-wet cycle,the compressive strength of concrete increases slightly.In the later stage(after mutiple dry-wet cycles),however,the compressive and flexural strength of concrete decrease gradually.The changing trend of chloride ion is similar to that under the long-term immersion test.(3)Through the test of immersion in erosion solution and simultaneously exposure to freeze-thaw cycle,the influence of concrete durability in intertidal environment of tidal flat wetland in winter is studied,and the change rule and deterioration mechanism of concrete performance are revealed..The test results show that with increasing the freeze-thaw cycles,the compressive strength and flexural strength of concrete decrease significantly.The quality loss rate of concrete shows an upward trend,with a slight rise in the initial stage and a larger increase in the later stage.Moreover,the quality loss rate is related to the concentration of the erosion solution,with the highest mass loss rate observed in the solution with a 5% concentration.The relative dynamic elastic modulus of concrete shows a decreasing trend,which decreases slightly in the initial stage and experiences an accelerated decrease in the later stage.The changes of these four indexes are affected by strength grade of concrete,freeze-thaw cycle times,solution type and concentration.(4)The test results under three test conditions are compared and analyzed,and based on the concrete mix proportion and surface protection,a method to improve the frost resistance of concrete is proposed.The comparative analysis shows that the concrete erosion is in a decreasing order of immersion and freeze-thaw cycle > immersion and dry-wet cycle > long-term immersion.In the surface layer of the concrete with the same strength grade,the chloride ion content is higher under test condition II(immersion and dry-wet cycle)than under test condition I(long-term immersion).At a depth of 30 mm,the difference of chloride ion content between the two conditions is negligible.After immersion and freeze-thaw cycle for 200 times,the compressive and flexural strength of concrete specimens with the same strength grade are in a decreasing order of concrete mixed with silica fume > concrete mixed with high-efficiency air-entraining agent > ordinary concrete.The quality loss rate of the ordinary concrete is much higher than that of those either with high-efficiency air-entraining agent or with silica fume.The relative dynamic elastic modulus of concrete is in a decreasing order of concrete mixed with high-efficiency air-entraining agent > concrete mixed with silica fume > ordinary concrete.In addition,the durability of concrete specimen is improved after surface protection treatment.(5)The durability of concrete under the environment of salt marsh wetland is analyzed by the theory of Grey Relational Analysis(GRA),and the influence of each influencing factor on the concrete durability and the ranking of relation degree are established.The results show that among all the influencing factors,the initial concrete strength is the biggest grey relational grade affecting the two evaluation indexes of the compressive strength and flexural strength,which is followed by the erosion solution concentration.On the other hand,the treatment time of immersion and freeze-thaw cycle is the biggest grey relational grade affecting the two evaluation indexes of the concrete quality loss rate and relative dynamic elastic modulus,which is followed by the content of silica fume.(6)By analyzing the correlation of the analysis results of the four evaluation indexes of concrete,the weight of four evaluation indexes of concrete is calculated by Analytic Hierarchy Process(AHP)theory,and a calculation model for the evaluation and prediction of concrete durability in salt-frost environment is constructed.The durability of concrete after immersion and freeze-thaw cycle is evaluated.The evaluation results are in good agreement with the test results.