A Model For Prediction Of Time To Corrosion-Induced Concrete Cover Cracking And Analysis Of Meso-Crack Propagation

As one of the most notorious factors that influence the structural durability of reinforced concrete,reinforcement corrosion can result in the mechanical performance degradation and concrete cover spalling,which will reduce the effective bearing area of member sections and thus lower the service life.Significant works have been done to investigate the corrosion mechanism as well as modify the rust expansion theoretical model and the corrosion rate model.Highlighting the formation and expansion process of corrosion-induced cracks in the concrete cover,this study carries on theoretical derivation,numerical analysis,and practical survey.Then,comprehensive comparisons are implemented among theoretical,modelling and experimental results.The achievements in this study can perfect the theoretical system of the concrete structure durability,and provide certain insights into predicting the corrosion-induced cracking process in the practical concrete engineering.Main conclusions are reached as follows:The concrete cover structure can be simplified as a plane model of semi-infinite body in the condition that efforts have been paid to consider the thickness and geometrical boundary condition of concrete cover as well as the relative positions of steel bars.Thus,the corrosion-induced expansive stress of the concrete cover can be expressed through theoretical analysis.It is demonstrated that the corrosion-induced cracks normally originate on the interface between concrete and the corrosion layer.Moreover,if the internal cracks are not along the steel bar,they might be vulnerable to inward expansion due to the horizontal tensional stress induced by the uplift on the surface of the concrete cover.In this context,it is critical to increase the thickness and diameter of the concrete cover as well as raise the grade of the concrete,so as to enhance the anti-rust ability of reinforced concrete structures.In addition,the influence of the spacing of side-by-side steel bars on the corrosion-induced expansive stress is discussed.The corrosion layer can be simplified as a semi-ellipse with non-uniform distribution according to the corrosion morphology of steel bars before the corrosion-induced cracking in the natural environment.Thus,the mechanic model clarifying the non-uniform corrosion of steel bars can be established,and the theoretical solution of the corrosion-induced expansive stress can be achieved,which indicates that the maximum circumferential stress should be on the horizontal axis.Therefore,it can be inferred that the development of the corrosion layer can induce the horizontal internal cracks inside the cover.In order to effectively reduce the corrosion-induced expansive stress and thus enhance the anti-rust capacity,it is suggested to increase the steel bar diameter.Compared with the critical thickness of corrosion layer in the uniform corrosion model,that in the non-uniform model is much larger.The finite element model of non-uniform corrosion-induced cracking of concrete cover is built by introducing the extended finite element method（XFEM）and interference assemble method.It is demonstrated that a crack band consisting of several micro-cracks first forms in the fan-shaped areas 10°³0°away from the contact point with the adjacent steel bars on both sides,which further develops away from the steel bar axis symmetrically.A diagonal main crack then forms in the crack band when the corrosion reaches a certain level,resulting in wedge-shaped destruction in the concrete cover.Through the secondary development of ABAQUS that provides information about lengths of corrosion-induced cracks,an exponential relationship between the crack length and the steel bar corrosion rate is identified,which indicates that there should be three stages in terms of crack development,namely the initiation stage,the extending stage,and the stable stage.The structure of the steel bars in the stable stage should be evaluated,and the dividing criterion should be provided.Moreover,the precision and accuracy of the results by the proposed model is compared with that by the model for the analysis of highway bridge concrete cover spalling,which are proved to be consistent.Based on the XFEM calculations,the results are analysed,comprehensively considering the concrete cover thickness,the steel bar diameter and the concrete strength,which contribute to the establishment of the model of time to cover cracking and the validation of the reliability of the model.Given that the concrete cover cracking is in the mesoscopic level(10^-410^-1 m),this study proposes to regard the concrete in the model as three-phase composite material comprised of aggregate,mortar and interface layer,rather than homogeneous material.Then a mesoscopic modelling method is put forward for the concrete with two-and three-dimensional arbitrary shape aggregates based on pre-fabricated grids.Such method can effectively solve the grid quality problem in the mesoscopic three-phase medium by using pre-fabricated grids for aggregate packing.The employment of the arbitrary shape aggregates makes the model more realistic and avoids inaccurate description on aggregates.The packing contents can reach 75%and 65%,respectively,for the two-and three-dimensional arbitrary shape aggregates.It should be noted in the case of three-dimensional arbitrary shape aggregates that the tetrahedron segmentation method is adopted to identify the spatial relationship between three-dimensional aggregates and nodes.Both the two methods are demonstrated to guarantee the randomness of packing and the rationality of graded distribution.Finally,the concrete damage plastic（CDP）model is constructed to analyse the failure process of the mesoscopic fracture in the concrete cover due to inhomogeneous corrosion-induced cracking.The mesoscopic morphologies in the initiation,extending,and stable stages are numerically illustrated,which are in good agreement with the experimental results.