Diffusion And Distribution Of Chloride Ion In Reactive Powder Concrete

Steel corrosion is still the most severe durability problem facing reinforced concrete structures.On the one hand,marine engineering and concrete buildings in coastal cities have long been eroded by environmental chloride ions.On the other hand,river sand resources are increasingly depleted,and the utilization of sea sand resources is becoming more and more widespread,while marine sands and even desalinated marine sands whose chloride ion content still exceeds the standard limits,which will lead to severe corrosion damage to the reinforced concrete structure.There is currently no effective method to prevent the occurrence of steel corrosion in normal concrete.Reactive Powder Concrete?RPC?is a new type of ultra-high strength cement-based composite material with high mechanical properties and good ductility,and good resistance to chloride ion permeability,such as its application to the ocean.In engineering or sea sand concrete,it can effectively reduce the penetration rate of chloride ions and prevent or greatly reduce the occurrence of steel corrosion.In this paper,the improved rapid electromigration test method was used to study the diffusion distribution of extravasated chloride ions and internal chloride ions in RPC.The chloride ions of silica sand RPC,river sand RPC,marine sand RPC and normal mortar specimens were compared.The diffusion rate,combined with the pore structure test results,discussed its diffusion rate.At the same time,the actual engineering environment was simulated,and the diffusion distribution of external chloride ions in the RPC beam and normal concrete beam under dry-wet cycle of chloride salt,and the distribution of chloride ions of the RPC beam and normal concrete beam under admixture of marine sand were investigated.The main research contents and results are as follows:?1?After 30V constant voltage 192h rapid electromigration test,the migration of extravasated chloride ions in RPC is much lower than that of normal mortar;the chloride ion content in RPC specimens show a significant downward trend with increasing thickness,and extravasated chlorine ions are not completely saturated in the specimens,and the RPC is in the unsteady electromigration stage;while the chloride ion content in the normal mortar specimen is basically unchanged with the thickness increase,the extravasated chloride ion has completely saturated the specimen,and the normal mortar is in steady state electromigration.Chloride ion diffusion coefficient in silica sand RPC,river sand RPC and river normal sand mortar are respectively 6.50×10^-14m²/s?7.48×10^-14m²/s with1.51×10^-12m²/s.The resistance to chloride ion diffusion of RPC is much higher than that of river normal sand mortar.The resistance to chloride ion diffusion of silica sand RPC is slightly higher than that of river sand RPC.When chlorine ions are doped,the chloride ion migration in RPC is much lower than that in normal mortar,whether it is doped with NaCl or chloride ion introduced from marine sand.The chloride ion migration in silica sand RPC was only 1/2 of that in marine sand RPC,this indicates that RPC,especially silica sand RPC,can significantly reduce internal chloride ion migration.?2?The penetration and diffusion of chloride ions in concrete mainly depend on the microstructure inside the concrete,especially the pore structure.Among them,the smaller the micro-capsules with a pore diameter of 10-100 nm,the larger the pore volume fractal dimension,the more resistant to chloride ion diffusion.The results of mercury intrusion test show that the micro-capsule content of RPC is much lower than that of normal mortar,and the fractal dimension of pore is larger than that of normal mortar.Therefore,the resistance to chloride ion diffusion of RPC is much better than normal mortar.Silica sand RPC,river sand RPC and marine sand RPC have similar micro-pores,but the pore volume fractal dimension of silica sand RPC is larger than the other two,so the anti-chloride ion diffusion performance of Silica sand RPC is optimal.?3?Taking RPC beam and normal concrete beam as test objects,the diffusion distribution of chloride ion in concrete beam under the condition of dry-wet cycle of chloride salt is studied.The results show that the diffusion depth of chloride ions in all directions in the RPC beam is less than 10mm,while the diffusion depth of chloride ions in normal concrete is less than 20mm.The distribution of chloride ion content in the cross-section of concrete beams with the distance between the two ends of the beam is greater than the diffusion depth.The chloride ion content at different depths in the cracked concrete is generally higher than the chloride ion content at the same depth in the un-cracked concrete.After the distance from the crack is more than 10 mm,the chloride ion concentration distribution in the concrete is not formed by the chloride ions formed in the crack.In the range of 0⁵mm depth,the presence of cracks significantly increases the chloride ion content in RPC,but has little effect on normal concrete.Using Fick's second law as a time prediction model,the time of steel corrosion in RPC beams and normal concrete beams is predicted to be 121.3 years and 2.2 years respectively.It is predicted that the time of steel corrosion in cracks in cracked RPC beams and cracked normal concrete beams are 2.3 years and 0.4 years.?4?Taking RPC beam and normal concrete beam as test objects,the diffusion distribution of chloride ions in concrete beams under the condition of admixture of marine sand is studied.The results show that,under the condition of admixture of marine sand for18 months,the concentration of chloride ions in the RPC beam and the normal concrete beam decreased with the increase of the distance from the steel bar.The amount is 0.0016%,which is only about 1/10 of the 0.0156%increase of the chloride ion in the steel bar in the normal concrete beam.Based on the principle that the transport flux of chloride ions under the action of electric field is constant,the time of steel corrosion in RPC beam and normal concrete beam under sea sand conditions is predicted to be 100.5 years and 8.9 years respectively.