Study On The Characteristics Of Steel-Concrete Interface Defects And Their Effect On The Corrosion Of Steel Bars

Corrosion of steel bars is the primary factor affecting the durability of reinforced concrete structures.Previous studies have shown that the presence of defects in the steel-concrete interface has a significant effect on the corrosion process of steel bars.At present,the characteristics of steel-concrete interface defects and their effect on the corrosion behavior of steel bars are not clear.This study is of great significance to the safety and durability of reinforced concrete structures.In order to reveal the formation and evolution of steel-concrete interface defects and their effect on the corrosion process of steel bars,reinforced concrete specimens with different directions of arranged steel bars,heights of arranged steel bars,precast defect sizes,and external load cracks were designed.The backscattered electron(BSE)imaging and nanoindentation technology were used to study the microstructure characteristics of steel-concrete interface.A test method of chloride penetration depth along the interface was proposed to characterize the permeability of steel-concrete interface.The specimens were exposed to wetting/drying cycles in sodium chloride solution,and open circuit potential(OCP)of steel bars was measured to monitor the corrosion state.After 80 cycles,specimens were broken to check the corrosion state of steel bars,the distribution of corrosion products and interface defects,and analyze the composition of concrete interfaces.Finally,ordinary Portland concrete and lightweight aggregate concrete of the same strength grade were designed to study the influence of fresh concrete stability on the steel-concrete interface.Through the above research and analysis,the following conclusions were obtained:1 Due to the settlement,segregation and bleeding of fresh concrete,and up-floating of air bubbles entrained during mixing of concrete,defects mainly gathered in the bottom part of steel-concrete interface.As the height of steel bars increased,the defect area under the horizontal steel bars rose up.The defect area in the bottom part of steel-concrete interface increased from 0.27mm~2to 10.83mm~2when the height of steel bars grew from 50mm to 850mm.2 The existence of defects made the larger porosity of steel-concrete interface,resulting in the chloride permeability of the bottom interface was greater than that of the top interface and the concrete matrix.As the height of casting concrete beneath the reinforcement increased,the result became more obvious.Correlation analysis of chloride penetration depth and defect area in the bottom interface of different height steel bars shows that there was a significant positive correlation between chloride penetration depth and defect area,and the correlation coefficient was 0.9712.3 After 80 wetting/drying cycles,the steel bar at the height of 850mm had begun to corrode,while the steel bars at other heights were still passivated.The specimens were broken and it was found that in the steel-concrete interface at the height of 850mm,the top interface of concrete was dense,and there were obvious pores in the bottom interface.In addition,corrosion phenomenon at the bottom of the steel bar was obvious.It could be considered that the presence of steel-concrete interface defects promoted the corrosion initiation of steel bars.4 The effect of the height of arranged reinforcement on the corrosion initiation time of steel bars could be attributed to:The uneven distribution of steel-concrete interface defects caused differences in humidity and oxygen content at the interfacial defects and dense area,resulting in a potential difference and macrocell around the steel bar;Due to the higher permeability of the steel-concrete interface,the migration and diffusion rate of harmful substances such as chloride ions,oxygen,and moisture in the interface were faster;The resistance to chloride permeability of concrete cover was non-uniformly distributed along the casting height,and it was found that the concrete with higher position had poor chloride penetration resistance.5 At the same height of steel bars,the fresh concrete with poor stability had the larger defect size and worse permeability of steel-concrete interface.The addition of silica fume could effectively increase the plastic viscosity of the mixture,reduce the settlement,segregation and bleeding of fresh concrete,and control the occurrence of steel-concrete interface defects.At the same time,the silica fume particles were very fine and would fill the micropores of concrete to improve the permeability of steel-concrete interface.