Study Of The Predicting Models For Corrosion-induced Cracking Of Concrete Cover And Rust Penetration Process

Rebar corrosion is one of the main causes affecting the durability of reinforced concrete(RC)structures,because it will result in the reduction of rebar cross-section,cracking of concrete cover and the deterioration of bond strength between concrete and rebar.Especially,the cover cracking will provide extra tunnels for aggressive agents,such as chloride ions,oxygen,moisture and carbon dioxide,thus it accelerates the structural degradation and rebar corrosion.This combined and superposition effect extremely endangers the durability of RC structures.Besides,the cracking time of concrete cover is one part of the service life of concrete structures,which is also regarded as an important index to determine the structual statein practical engineering.Therefore,in order to accurately evaluate the durability state,predict the service life and determine suitable maintenance strategy of RC structures,it is necessary to study the cracking time and cracking mechanism of concrete cover in theory and engineering.The main work on corrosion-induced cracking of concrete cover and main conclusions of the present study are as follows:(1)The development process of thick-walled cylinder models for corrosion-induced cover cracking,was sysmetrically analyzed and compared,and 57 groups of test data for cracking time of concrete cover were collected from literature to evaluate the accuracy of these cylinder models.Then,the differences of these models,such as the number of cylinders,crack morphologies,constitutive models for cracked concrete and values of model parameters,were studied.These cylinder models were classified according to their characteristics,and their shortcomings in model assumptions and parameter values in the models were pointed out.The empirical models for predicting the corrosion-induced cracking time of concrete cover were also summarized,and their accuracy was evaluated.The results found that the development of the thick-walled cylinder models experienced single-cylinder,double-cylinder and N-ring model,with various softening constitutive models for cracked concrete,such as linear,bilinear and power function expressions.There are large differences on the values of model parameters,such as porous zone thickness and rust volumetric expansion factor.The current available models exhibits the shortcomings of ignoring the contraint effect of the concrete section outside of the cylinder zone,failing to distinguish the differences between corner rebar or multi rebars with central single rebar conditions.The model errors induced by model assumptions and uncertain values of model parameters are summarized as the main reasons for the limited accuracy of these cylinder models.(2)Macro and meso scale numerical simulation methods for corrosion-induced cracking of concrete cover were proposed based on the Rigid Body Spring Model(RBSM).Through these numerical models,the mechanism and process of the corrosion-induced concrete cover cracking were analyzed,as well as the influence of cover thickness and rebar diameter on the critical corrosion pressure and critical radial displacement.The homogeneous material assumption for concrete in thick-walled cylinder models was analyzed by comparing the simulated corrosion-induced cracking process of macro and meso scale models.The circular section assumption for concrete section was analyzed by comparing the simulated cracking process through rectangular and circular section models.The results showed that the development of the corrosion-induced cracks has three stages,i.e.the micro crack initiation,macro crack formation and main crack propagation,during which the aggregate-mortar interface(ITZ)has significant impact.Besides,the critical corrosion pressure and critical radial displacement increase with the increase of cover thickness,and decrease with the increase of rebar diameter.Compared with the macro concrete model,the meso model has a shorter cracking time of concrete cover.In addition,the restraint of the concrete section outside of the cylinder zone is a nonuniform kind of restraint which improves the resistance of concrete cover on rebar corrosion expansion,but alters the cracking mode at the same time.When the restraint of the outer zone is weak,the cracking of the rectangular concrete specimen with restraint occurs earlier than that of the circular concrete specimen without restraint.(3)Through accelerated corrosion test and a coupled mechanical-penetration numerical model,the distribution of rust penetration(filling)into corrosion-induced concrete cracks was studied under electric field and wet-dry cycling conditions.Based on the numerical model,the influences of rust penetration on the radial expansion displacement at the steel-concrete interface and crack width of concrete surface were studied,along with the influences of boundary moisture content at the steel-concrete interface,initial moisture content of concrete and rust solubility on the amount of rust penetration into concrete cracks.Experimental results showed that the filling degree of rust in the cracks can be divided into three cases:large amount of filling,small amount of filling and no filling,but there is no statistical characteristic of the rust filling in concrete cracks.And the numerical results showed that after 400 days of rebar corrosion,the volume ratio of cracks occupied by rust to total cracks(rust-crack volume ratio)is 0.29,and after 1000 days of rebar corroson,the mass ratio of penetrated rust to total rust(rust mass ratio)is about 0.20.When rust penetration is considered,the radial displacement at the concrete-steel interface and crack width of concrete surface are found to be both smaller than the case without rust penetration.The rust mass ratio increases with the increase of rust dissolved degree,but the rust-crack volume ratiois not influenced.While the rust mass ratioand rust-crack volume ratioboth increase with the increase of relative water content at the concrete-steel interface,but they have no change with the initial water content of concrete.(4)The composite and structure of rust were analyzed,and the research methods on rust volumetric expansion factor,elstic modulus and Possison’s ratio were summarized.According to the nature of rust,the rationality of these methods was analyzed and reasonable ranges of the physical and mechanical properties of rust were recommanded.Based on these analyses,it was found that rust is a kind of composite made of multi kinds of single rust type,and has a multi-layer structure.The physical and mechanical properties of rust are not determined values,instead they have certain ranges.By analyzing the methods for studying rust properties,it was found that the experimental calculation method for determing rust volumetric expansion factor and the prode test method for rust elastic modulus are relatively reliable methods.On the basis of these methods,it can be inferred that the rust volumetric expansion factor ranges from 2.6 to 4.0,and the rust elastic modulus ranges from 40 to 90 GPa.In addition,when predicting the cracking time of concrete cover,the compression of rust and the influence of rust Possion’s ratio are no need to be considered.(5)The uncertain factors in relating to corrosion-induced cracking of concrete cover were summarized which include the concrete properties(creep coefficient),electrochemical properties(corrosion current density,corrosion current efficiency)and rebar corrosion morphologies(longitudinal non-uniform corrosion,circumferential non-uniform corrosion and pitting corrosion).It was found that these uncertain factors have great influence on the accuracy of the current methods for predicting the cracking time of concrete cover.The main sources of these uncertain factors were summarized as:insufficient studies on some uncertain parameters(i.e.physical and mechanical properties of rust),simplification of long-time property of materials(concrete creep coefficient)and different cracking mechanism under various corrosion environments.Based on these analyses,improving measurements and methods were proposed for each uncertain factor.Finally,considering the influence of uncertain parameters,the thick-walled cylinder models and empirical models were improved in terms of collected test data.