Investigating the spatial development of corrosion of corner-located steel bar in concrete by X-ray computed tomography

In this paper, the chloride-induced corrosion progression of a corner located steel bar in concrete is investigated by X-ray computed tomography (i.e., X-CT). Corrosion of steel bar is accelerated by placing the reinforced specimen in a wetting and drying cyclic corrosive environment, rather than by the impressed current method. 3D X-CT images are obtained and processed to characterize the different material phases, consisting of, steel bar, mortar, corrosion products and voids/cracks. The corrosion products expansion and concrete cracking are analysed and discussed. It has been found that pitting corrosion is prone to appear around the voids close to the steel bar, mainly due to the pre-existing supply of oxygen and moisture. In addition, a distinct transverse crack has been identified which is caused by non-uniform corrosion along the reinforcing steel bar. Within the cross-section, corrosion has also been found non-uniformly distributed, with the maximum rust layer pointing to the corner edge of the sample. Moreover, the corrosion rust distributions are used to parameterize a recently developed non-uniform corrosion model. This experimentally validated non-uniform corrosion model can be applied to corrosion-induced concrete cracking problems with confirmed accuracy. The combination of the use of wetting and drying cyclic corrosive environment and the X-CT scanning can provide a new method to the non-destructive investigation of corrosion process, rust distribution and corrosion-induced concrete cracking in the reinforced concrete structures

Elastic fracture toughness for ductile metal pipes with circumferential surface cracks

Surface cracks have long been recognized as a major cause for potential failures of metal pipes. In fracture analysis, the widely used method is based on linear elastic fracture mechanics. However, for ductile metal pipes, it has been known that the existence of plasticity results in easing of stress concentration at the crack front. This will ultimately increase the total fracture toughness. Therefore, when using linear elastic fracture mechanics to predict fracture failure of ductile metal pipes, the plastic portion of fracture toughness should be excluded. Otherwise, the value of fracture toughness will be overestimated, resulting in an under-estimated probability of failure. This paper intends to derive a model of elastic fracture toughness for steel pipes with a circumferential crack. The derived elastic fracture toughness is a function of crack geometry and material properties of the cracked pipe. The significance of the derived model is that the well-established linear elastic fracture mechanics can be used for ductile materials in predicting the fracture failur

Numerical modelling of non-uniform corrosion induced concrete crack width

Corrosion of reinforced concrete is one of the major deterioration mechanisms which result in premature failure of the reinforced concrete structures. Crack width is often used as an effective criterion to assess the serviceability of concrete structures. However, research on prediction of corrosion-induced concrete crack width, especially by considering the corrosion as a non-uniform process, has still been scarce. This paper attempts to develop a finite element model to predict the crack width for corrosion-affected concrete structures under realistic non-uniform corrosion of the reinforcement. A non-uniform corrosion model was first formulated as a function of time. To simulate arbitrary cracking in concrete, cohesive elements are inserted in the sufficiently fine mesh which is achieved through a script written in Python. The surface crack width is obtained as a function of service time and verification against experimental results from literature is conducted. Accurate prediction of crack width can allow timely maintenance which prolongs the service life of the reinforced concrete structures

Meso-scale Fracture Modelling of Concrete Cover Induced by Non-uniform Corrosion of Reinforcing Bar

Corrosion-induced concrete cracking is a significant durability problem for reinforced concrete structures. In practice, critical corrosion degree to surface cracking and crack width evolution are of significance in regards to the assessment of serviceability of reinforced concrete structures. Literature review suggests that, although considerable research has been undertaken on corrosion-induced concrete cracking, little has been focused on non-uniform corrosion of reinforcing bar, especially by considering concrete as a three-phase materials. In this paper, a meso-scale fracture model, consisting of aggregates, cement paste/mortar and ITZ, is established. To simulate arbitrary cracking in concrete, cohesive elements are inserted in the fine meshes and the process is achieved through a script written in Python. It has been found that some microcracks occur before they are connected to form a dominating discrete crack approaching to the surface. The surface crack width is obtained as a function of corrosion degree and verification against experimental results from literature is conducted

A New Model for Corrosion-induced Concrete Cracking

Corrosion of reinforced concrete is one of the major deterioration mechanisms which result in premature failure of the reinforced concrete structures. Due to the actual diffusion of chloride ingress, the corrosion products distribution is seldom uniform along the reinforcing bar. Recently, some non-uniform corrosion models have been proposed to investigate the corrosion-induced cracking mechanism of concrete. In this paper, a new corrosion model based on von Mises distribution is formulated and validated against experimental data. The developed model is then compared with the existing non-uniform models and the advantages are discussed. To demonstrate the application of the developed corrosion model, a concrete cover structure, containing aggregates, cement paste/mortar and ITZ, is simulated to predict the cracking phenomena of the concrete cover under different non-uniform coefficients in the developed corrosion model. It has been found that the non-uniform corrosion model can be used to simulate the realistic corrosion rust progression around the reinforcing bar, with the best accuracy. Moreover, parametric studies are conducted to investigate the effects of the basic factors formulated in the corrosion model on the surface cracking of the reinforced concrete structures

Modelling progressive failure in multi-phase materials using a phase field method

In this paper, a new phase field method is proposed for modelling of progressive failure in multi-phase materials. Material properties of the interface between inclusion and matrix are regularized by an auxiliary interface phase field. In addition, crack initiation and propagation are simulated by using another crack phase field. Different failure mechanisms such as interface debonding, matrix cracking and the interaction between these two failure mechanisms are modelled in a unified framework. For general application of the framework, an image processing method is employed to identify the individual phases for a given multi-phase material. The proposed method is implemented into the commercial software ABAQUS through a user subroutine UEL (user defined element). The derived method is validated through an example of a single fiber reinforced composite system. Moreover, a procedure for choosing parameters of the proposed phase field model is discussed. Further, the validated method is applied to fracture analysis of a multi-phase concrete structure and complex failure mechanisms within and across the phases are captured

Time to surface cracking and crack width of reinforced concrete structures under corrosion of multiple rebars

Concrete cover cracking caused by corrosion of reinforcement is one of major deterioration mechanisms for reinforced concrete structures. In practice, time to surface cracking and crack width evolution are of significance in regards to the assessment of serviceability of reinforced concrete structures. Literature review suggests that, although considerable research has been undertaken on corrosion-induced concrete cracking, little has been focused on corrosion of multiple reinforcing bars, especially by considering the non-uniform corrosion process. In this paper, a time-dependent non-uniform corrosion model is established. A cohesive crack model is then formulated to simulate arbitrary cracking in the whole cover of concrete structures. Two typical cover failure modes (i.e., “delamination” and “combined delamination and corner spalling”) have been simulated under the non-uniform corrosion of multiple reinforcing bars and found dependent on spacing of reinforcement and fracture energy of concrete. The effects of corrosion, geometric and mechanical parameters on the time to surface cracking after corrosion initiation and the crack width evolution are also investigated and discussed. The developed model is partially verified by comparing the results with those from experimental tests on uniform corrosion of multiple reinforcing bars

Mechanical properties of C-S-H globules and interfaces by molecular dynamics simulation

At meso-scale, Calcium Silicate Hydrate (C-S-H) can be considered as randomly packed globules (about 4.2 nm), which forms the basic unit cell, with water molecules and voids. In this paper, the nanostructures for the globules are developed based on some plausible atomic structures of C-S-H. The mechanical properties for the C-S-H globules are determined through molecular dynamics simulation. Interfaces between the C-S-H globules are also simulated with different amount of water molecules. Key material parameters, e.g., Young’s modulus, strength and fracture energy, are obtained. It has been found that longer mean chain length of silicate tends to increase the strength of C-S-H and change the fracture behavior from brittle to ductile failure, in the chain length direction. In the other direction, however, silicate chains do not play an important role while interlayer structure matters. Moreover, pores in the C-S-H nanostructures can considerably reduce the strength of the globule structures in the normal direction to silicate chain but the weakening effect becomes substantially less in silicate chain direction. Further, it has been found that for all types of the interfaces between C-S-H globules, the interface with no extra water molecules has the greatest tensile/shear strength. The mechanical properties obtained in this paper for C-S-H nanostructures and interfaces could be necessary inputs to the meso-scale modelling of C-S-H via either granular mechanics, i.e., DEM, or continuum mechanics, i.e., FEM

A non-linear cohesive zone model for low-cycle fatigue of quasi-brittle materials

The low-cycle fatigue behaviour of quasi-brittle materials (e.g., concrete and rock) that is characterized by fatigue induced inelastic deformation significantly affects the integrity and serviceability of engineering structures. However, the low-cycle fatigue mechanism and fatigue-controlled fracture process of quasi-brittle materials is not clear. This paper develops a new cyclic cohesive zone model (CZM) for low-cycle fatigue of quasi-brittle materials. Based on in-situ stress and damage state, a nonlinear fatigue damage model is proposed and implemented into the cyclic CZM. The fatigue parameters are determined based on S-N curve. A worked example for monotonic and cyclic loading of concrete beam under three-point bending is presented to demonstrate the application of the developed numerical model. After validation against experimental data, the fatigue crack mechanisms are discussed and a comprehensive parametric study is carried out to investigate the effects of fatigue parameters, stress levels and loading sequences on the fatigue failure. It has been found that there are three stages for the development of crack mouth of displacement, i.e., crack initiation, stable growth and rapid fracture which are caused by combined static and fatigue damage, fatigue damage, and combined static and fatigue damage dominated by static damage, respectively. The developed cyclic CZM is practically significant and its parameters are easy to be determined based on S-N curve. It provides a new and useful tool for low-cycle fatigue crack modelling of quasi-brittle materials

Corrosion Mechanisms of Reinforced Alkali-Activated Concrete

Alkali-activated concrete (AAC), which exhibits good mechanical strength and chemical resistance properties, has attracted emerging interest from the research perspective considering the sustainable development of construction materials. However, the corrosion mechanism at the steel-AAC interface is not yet well understood including the physical and chemical aspects, which leads to different accumulation and evaluation of corrosion products, compared to ordinary Portland cement (OPC) concrete. In this paper, concrete pull-out test and electrochemical techniques were used to investigate the bond-slip behaviour and the evolution of deterioration of AAC respectively. In addition, the current guidance of corrosion evaluation used for OPC concrete based on ASTM C876 is not suitable for AAC. Five mixed ratios of blended fly ash and slag AAC were investigated under two chloride environments and one non-chloride environment, i.e., (1) 3.5% NaCl salt fog spray in the environmental chamber; (2) 3.5% NaCl saltwater immersion; (3) tap water immersion. Electrochemical techniques include half-cell potential, linear polarization resistance and Tafel extrapolation method were used to determine the corrosion rate. The electrochemical results are validated through the comparison of the gravimetric loss of steel after corrosion and electrochemical loss from calculation