Study On Chloride Transport Behavior Of Recycled Concrete In Alternating Dry-Wet Marine Environment

The accelerated process of urbanization consumes a large number of non-renewable natural sand and stone resources,but also brings the problem of construction waste disposal and utilization.Meanwhile,based on the marine strategy,the construction of coastal infrastructure and major marine engineering has developed rapidly.If local construction waste can be prepared into recycled aggregate and used in marine concrete structures,it can not only solve the shortage of natural sand and stone materials in coastal areas,but also effectively improve the utilization rate of construction waste resources.However,the research on the durability of recycled concrete in marine environment,especially the permeability of chloride ions,is still insufficient,which limits the practical application of recycled concrete in marine engineering.Therefore,this thesis mainly studies the chloride transport behavior and mechanism of recycled concrete in marine environment,aiming to provide a new theoretical basis for durability assessment and life prediction of recycled concrete structures,and promote the application and development of recycled aggregate in marine engineering concrete structures.In this research,the environmental object of the study is the alternating dry and wet areas of the sea,and the indoor accelerated test of recycled concrete under the coupled environment of erosive ion and dry-wet cycles as well as the sea service test in the tidal range zone are carried out.The effects of recycled aggregates and erosive ions(SO₄^2-、Mg²⁺)on the chloride ion transport behavior of recycled concrete and its mechanism were revealed through the multi-scale analysis of the macro and micro properties of recycled concrete in the indoor accelerated test.Based on the mechanism,the chloride ion transport model of recycled concrete was modified and finally the modified model is verified by the result of the sea service test.The main conclusions of this research are as follows:（1）The addition of recycled aggregate improves the capillary water absorption rate and chloride ion mobility coefficient of recycled concrete.The capillary water absorption of recycled concrete is mainly affected by the content and porosity of old mortar carried by recycled aggregate,while the chloride migration coefficient is affected by the old interfacial transition zone of recycled aggregate and the properties of old mortar.Based on the influence mechanism,the influence models of recycled aggregate on capillary water absorption and chloride migration coefficient were established.（2）Based on the chloride ion transport behavior of recycled concrete under the coupled environment of chloride ion and dry-wet cycles,the influence and mechanism of recycled aggregate on the chloride ion transport behavior of concrete were analyzed from a macro and micro perspective.（1）The total volume of pores in recycled concrete is higher than that of ordinary concrete,which provides more space for chloride ion transport and accumulation,thereby accelerating the increase rate of the maximum free chloride ion concentration.（2）In the process of chloride invasion,the chloride diffusion coefficient decreases with time.The larger the initial porosity of recycled concrete,the slower decrease rate of chloride ion diffusion coefficient.（3）The incorporation of recycled aggregate can improve the chloride ion binding ability of concrete,because the residual AFm and C-S-H in old mortar can bind a certain amount of chloride ions.Based on the above mechanism,the influence parameters of recycled aggregate are proposed,and the chloride ion transport model of recycled concrete is modified.（3）Based on the chloride ion transport behavior of recycled concrete under the coupled environment of composite ions and dry-wet cycles,the influence and mechanism of composite ions on the chloride ion transport behavior of recycled concrete were analyzed from macroscopic,mesoscopic and microscopic perspectives.（1）The growth rate of the maximum free chloride ion concentration mainly depends on the amount of pores on the concrete surface.The higher the corrosion degree of magnesium ion,the larger the surface porosity,resulting in the faster growth rate of the maximum free chloride ion concentration.（2）The decrease rate of chloride ion diffusion coefficient depends on the corrosion ion concentration and the initial porosity of recycled concrete.（3）Multi-ion erosion reduces the chloride ion binding capacity of recycled concrete,because SO₄^2-converts part of Friedel’s salt into AFm,and Mg²⁺converts part of C-S-H into M-S-H.Based on the above mechanism,the influence parameters of multi-ion are proposed for the decrease coefficient of chloride ion diffusion coefficient of recycled concrete,and the model is further modified.（4）The corrosion mechanisms of recycled concrete in the indoor accelerated test under the coupled environment of composite ion and dry-wet cycles as well as the sea service test are the same.The indoor test only accelerates the corrosion process of concrete without changing the transmission mechanism of chloride ion.The transmission parameters from the sea service test,including the chloride ion concentration distribution,the time-varying maximum free chloride ion concentration,the time-varying chloride ion diffusion coefficient,and the chloride ion binding capacity,are all in line with the model expressions obtained from the indoor tests.The modified model of chloride ion transport in recycled concrete from this paper can be used to predict and evaluate the durability of recycled concrete in the real sea environment.