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Journal of Applied Mathematics
Volume 2012 (2012), Article ID 769132, 25 pages
http://dx.doi.org/10.1155/2012/769132
Research Article

Analytical Solutions for Corrosion-Induced Cohesive Concrete Cracking

1School of Engineering, University of Greenwich, Chatham Maritime, Kent ME4 4TB, UK
2College of Civil Engineering and Architecture, Zhejiang University, Hangzhou 310058, China

Received 8 July 2011; Revised 16 September 2011; Accepted 30 September 2011

Academic Editor: Wolfgang Schmidt

Copyright © 2012 Hua-Peng Chen and Nan Xiao. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The paper presents a new analytical model to study the evolution of radial cracking around a corroding steel reinforcement bar embedded in concrete. The concrete cover for the corroding rebar is modelled as a thick-walled cylinder subject to axisymmetrical displacement constraint at the internal boundary generated by expansive corrosion products. A bilinear softening curve reflecting realistic concrete property, together with the crack band theory for concrete fracture, is applied to model the residual tensile stress in the cracked concrete. A governing equation for directly solving the crack width in cover concrete is established for the proposed analytical model. Closed-form solutions for crack width are then obtained at various stages during the evolution of cracking in cover concrete. The propagation of crack front with corrosion progress is studied, and the time to cracking on concrete cover surface is predicted. Mechanical parameters of the model including residual tensile strength, reduced tensile stiffness, and radial pressure at the bond interface are investigated during the evolution of cover concrete cracking. Finally, the analytical predictions are examined by comparing with the published experimental data, and mechanical parameters are analysed with the progress of reinforcement corrosion and through the concrete cover.

1. Introduction

The serviceability and durability of concrete structures may be seriously affected by the corrosion of steel reinforcement in structures that are exposed to aggressive environments, such as motorway bridges, car parks, and marine structures. Reinforcement corrosion consumes original steel rebar, generates much lighter rust products, and creates expansive layer at the interface between the reinforcement and the surrounding concrete cover. As corrosion progresses, the expansive displacement at the interface generated by accumulating rust products causes tensile stress in the hoop direction within the concrete cover, leading to radial splitting cracks in the concrete. The cracking and eventually spalling of the concrete cover significantly affect the bond strength between the rebar and the surrounding concrete cover and consequently influence the service ability and resistance of reinforced concrete structures [15]. Therefore, correct predictions of the evolution of cracking in cover concrete and evaluations of residual strength and stiffness of the cracked concrete are of great importance to estimate the remaining life and prevent the premature failure of reinforced concrete structures.

Many investigations have been undertaken during the last two decades regarding the influence of reinforcement corrosion and concrete cracking on the performance of reinforced concrete structures. Al-Sulaimani et al. [6] investigated the influence of reinforcement corrosion on the bond behaviour and strength of reinforced concrete members based on their experimental results. Andrade et al. [7] conducted experiments to monitor the development of crack width on the concrete cover surface induced by the reinforcement corrosion with time. Liu and Weyers [8] presented a model for estimating the time to cracking of concrete cover based on the experiments on various specimen dimensions and corrosion rates. Pantazopoulou and Papoulia [9] proposed a numerical model to study the mechanical implications of cover concrete cracking due to reinforcement corrosion and provided estimates for the time to cover cracking over corroded rebar. Coronelli [10] presented a bond-strength model for predicting the bond strength affected by reinforcement corrosion with reference to rebar position and concrete cover thickness. Recently, Bhargavaa et al. [11] proposed an analytical model for predicting the time required for concrete cover cracking and the weight loss of reinforcement bars due to rebar corrosion. Although considerable research has been conducted on the predictions of the time to concrete cover cracking due to steel rebar corrosion based on the experimental results and the numerical models, limited work has been done on the theory of cracking evolution in cover concrete during the progress of reinforcement corrosion with reference to realistic concrete material properties such as tensile softening behaviour of the cracked concrete and crack band spacing in the concrete cover.

The paper presents a new approach for studying the evolution of cover concrete cracking due to reinforcement corrosion, based on the thick-walled cylinder model for the concrete cover and the cohesive crack model for the cracked concrete. A governing equation for directly solving crack width within cover concrete is established with considering the realistic bilinear softening curve for the cracked concrete and the estimated number of cracks in the concrete cover. The closed-form solutions to crack width are then obtained for various cases that may occur during the evolution of cover concrete cracking. The propagations of the cracked front and critical crack front are investigated, and the time to concrete cover cracking is predicted. Mechanical parameters, such as residual tensile strength, reduced tensile stiffness, and radial pressure at the bond interface, are also studied with the progress of rebar corrosion. Finally, the developed analytical model is examined through its ability to reproduce reported experimental measurements and theoretically provides the evolution of concrete cracking and the deterioration of tensile stiffness and strength of the cracked concrete over the time of reinforcement corrosion.

2. Modelling of Mechanical Problem

The thick-walled cylinder model for concrete cover, initially proposed by Tepfers [12] to analyse the splitting bond strength of reinforcing bars, has been often used for predicting the time for cover concrete cracking due to reinforcement corrosion [8, 9, 11]. A common limitation of most existing analytical models for cover concrete cracking is in the representation of the realistic tensile softening behaviour of the cracked concrete, the evolution of cover concrete cracking, and the evaluation of residual tensile strength and stiffness with the progress of reinforcement corrosion.

2.1. Boundary Value Problem for Corrosion-Induced Concrete Cracking

In the thick-walled cylinder model for cover concrete cracking induced by reinforcement corrosion, as shown in Figures 1(a) and 1(b), the reinforcing steel bar has an initial radius 𝑅𝑏 embedded in concrete with a clear cover thickness 𝐶. The restraint at the internal boundary of the concrete cover could be represented by a prescribed displacement caused by expansive steel rebar corrosion products. Liu and Weyers [8] reported that a steel rebar may expand by as many as six times its original volume depending on the level of oxidation and estimated the mass of rust products 𝑀𝑟 over time 𝑡 from𝑀𝑟(𝑡)=4.2×102𝜋𝑅𝑏𝑖corr𝑡1/2,(2.1) where 𝑖corr is mean annual corrosion current per unit length at the surface area of the steel rebar. The increase of volume per unit length due to the rebar corrosion can be obtained from the volume of corrosion rust minus the volume of the original steel rebar consumed, namely, 𝑀Δ𝑉=𝑟𝜌𝑟𝑀𝑠𝜌𝑠=𝛾𝑚𝑀𝑟,(2.2) where 𝜌𝑠 and 𝜌𝑟 are densities of original steel and corrosion rust, respectively, the mass of original steel consumed 𝑀𝑠 is estimated from 𝑀𝑠=𝛾𝑀𝑟 in which coefficient 𝛾 is related to the ratio of the mass of rebar consumed over the mass of corrosion rust and could be measured from experiments, and the coefficient 𝛾𝑚 is calculated from 𝛾𝑚=1/𝜌𝑟𝛾/𝜌𝑠. From the obtained increase of volume, the rust front can be calculated from𝑅𝑟=𝑅𝑏2+Δ𝑉𝜋.(2.3)

fig1
Figure 1: Thick-walled cylinder model for cover concrete cracking evolution due to reinforcement corrosion.

To accommodate the volume increase due to steel corrosion, the prescribed displacement at the interface between the steel rebar and the surrounding concrete over time 𝑡 is given by𝑢𝑏(𝑡)=𝑅𝑟𝑅𝑏=𝑅𝑏2+𝛾𝑚𝜋𝑀𝑟(𝑡)𝑅𝑏.(2.4)

The prescribed displacement 𝑢𝑏(𝑡) will be considered as the internal boundary condition of the boundary value problem for the evolution of cover concrete cracking. The method described above is based on the general assumption that reinforcement corrosion occurs uniformly and thus the expansion is uniform around the internal boundary of the concrete cover. A recent study by Jang and Oh [13] suggests that in actual aggressive environments reinforcement corrosion may start from the places close to the free surfaces of the concrete cover and thus the rebar may not corrode uniformly in a cross-section. However, the difference in crack development between uniform expansion and nonuniform expansion is small in the case when the corrosion distribution coefficient (i.e., the ratio of the depth of nonuniform corrosion to that of uniform corrosion) does not exceed 2. The uniform corrosion of reinforcement in concrete then could be utilised for the cases with relatively small corrosion distribution coefficients, as shown in many studies such as Bhargavaa et al. [11], Chernin et al. [14], Pantazopoulou and Papoulia [9].

In the case when the prescribed displacement is given, the mass of rust products can be calculated by𝑀𝑟𝜋(𝑡)=𝛼𝑚2𝑅𝑏𝑢𝑏(𝑡)+𝑢𝑏2(𝑡).(2.5)

Based on the assumption that the steel rebar has uniform corrosion at the surface, the thick-walled cylinder model for cover concrete cracking can be considered as an axis symmetrical problem. The thick-walled cylinder model could be further treated as a plane stress problem because the normal tension-softening stress in the direction of longitudinal axis could be ignored [9], although the approach discussed in this study can also be applied to a plane strain problem. Therefore, the hoop stress in the cylinder is typically a principle tensile stress whereas the radial stress is a principle compressive stress. When the hoop stress reaches the tensile strength of concrete, the radial splitting cracks propagate from the bond interface (𝑅𝑏) in axis symmetrical directions to the same radius (𝑟𝑦) until reaching the free surface of concrete cover (𝑅𝑐), as shown in Figure 1(b). As corrosion progresses the surrounding concrete becomes completely cracked through the cover.

2.2. Cohesive Crack Model for Cracked Concrete

Concrete cracking could be modelled as a process of tensile softening if the cracking is considered as cohesive and the crack width does not exceed a limited value [15, 16]. In cohesive crack model for quasibrittle materials such as concrete, the stress transferred through the cohesive cracks is assumed to be a function of the crack opening [17]. The function (softening curve) can be determined from experiments and may be utilised to replace the stress-strain relations in the theories such as plasticity. Numerous experiments showed that the shapes of various softening curves for different mixes of ordinary concrete are very close to each other. Meanwhile, the bilinear softening curve has been accepted as reasonable approximations of the softening curve for cracked concrete in tension. The bilinear softening curve adopted in the present study is shown in Figure 2 and expressed as 𝜎𝑤=𝑓𝑡(𝑎𝑏𝑊),(2.6) where 𝜎𝑤 is the tensile stress crossing cohesive cracks, 𝑓𝑡 is the tensile strength of concrete, and 𝑊 is dimensionless variable that normalises actual crack width 𝑤(𝑟) to a nondimensional form and defined as 𝑓𝑊=𝑡𝐺𝐹𝑤(𝑟),(2.7) where 𝐺𝐹 is the fracture energy of concrete. The coefficients 𝑎 and 𝑏 in (2.6) for the bilinear softening curve are given by𝑎=𝑎cr=1,𝑏=𝑏cr=(1𝛼)𝑊cr,if0𝑊𝑊cr,(2.8a)𝑎=𝑎𝑢=𝛼𝑊𝑢𝑊𝑢𝑊cr,𝑏=𝑏𝑢=𝛼𝑊𝑢𝑊cr,if𝑊cr𝑊𝑊𝑢,(2.8b)where coefficient 𝛼, normalised critical crack width 𝑊cr, and normalised ultimate cohesive crack width 𝑊𝑢 may be determined from experiments. In the CEB-FIB Model Code [18], the coefficient 𝛼 is given as 𝛼=0.15 and 𝑊cr and 𝑊𝑢 could be evaluated from the maximum aggregate size of concrete materials.

769132.fig.002
Figure 2: Bilinear softening curve for cohesive cracking in concrete.

From the crack band theory for the fracture of concrete [19], the total number of cracks 𝑛𝑐 separating cracking bands in concrete cover and appearing at cover surface (𝑅𝑐) may be estimated from 𝑛𝑐=2𝜋𝑅𝑐𝐿𝑐,(2.9) where 𝐿𝑐 is minimum admissible crack band width estimated from 𝐿𝑐3𝑑𝑎 in which 𝑑𝑎 is maximum aggregate size of concrete. The typical value of total crack number 𝑛𝑐 in the thick-walled cylinder model for cover concrete cracking is approximately three or four from the experimental data available [20].

3. Basic Equations

From the results for a thick-walled cylinder subject to internal pressure given by Timoshenko and Goodier [21], the radial stress at a radius follows an inverse square law and diminishes quickly over the radius, approaching zero when the radius is sufficiently large. The effect of the concrete locating at the outside of the thick-walled cylinder shown in Figure 1(a) could be ignored due to the sufficient concrete cover thickness in practice comparing with the diameter of the corroded rebar. The axis symmetrical thick-walled cylinder model with free external surface shown in Figure 1(b), which has been widely utilised in the studies of corrosion-induced concrete cracking such as Bhargavaa et al. [11], Chernin et al. [14], Pantazopoulou and Papoulia [9], can therefore be adopted in this study to represent the surrounding concrete of the corroded bar with a reasonable accuracy. Hence, the boundary value problem of the thick-walled cylinder model for reinforcement corrosion-induced concrete cracking could be considered as an anisotropic nonlinear elastic problem subject to axis symmetrical prescribed displacement at the internal boundary. Based on the cohesive crack model for the radial splitting cracks in the cover concrete, the governing equation associated with crack width for the cracked concrete is derived as follows.

3.1. Equations for Anisotropic Thick-Walled Cylinder

It is well known that, for an anisotropic thick-walled cylinder subject to axis symmetrical actions, radial strain 𝜀𝑟 and hoop strain 𝜀𝜃 are only related to radial displacement 𝑢 at radius 𝑟, expressed by𝜀𝑟=𝑑𝑢,𝜀𝑑𝑟(3.1a)𝜃=𝑢𝑟.(3.1b)For an anisotropic elastic material, the general constitutive relations between radial and hoop stresses (𝜎𝑟 and 𝜎𝜃) and strains are𝜎𝑟=11𝜐𝑟𝜃𝜐𝜃𝑟𝐸𝑟𝜀𝑟+𝜐𝑟𝜃𝐸𝜃𝜀𝜃,𝜎𝜃=11𝜐𝑟𝜃𝜐𝜃𝑟𝐸𝜃𝜀𝜃+𝜐𝜃𝑟𝐸𝑟𝜀𝑟,(3.2) where 𝐸𝑟 is modulus of elasticity in radial direction and 𝐸𝜃 is modulus of elasticity in hoop direction associated with the corresponding crack width of the cracked concrete, 𝜐𝑟𝜃 and 𝜐𝜃𝑟 are Poisson’s ratios satisfying the requirement of anisotropic elasticity 𝜐𝜃𝑟𝐸𝑟=𝜐𝑟𝜃𝐸𝜃.

The stress equilibrium equation for the thick-walled cylinder is𝑑𝜎𝑟+1𝑑𝑟𝑟𝜎𝑟𝜎𝜃=0.(3.3) By substituting (3.1a), (3.1b), and (3.2), (3.3) is rewritten as𝑑2𝑢𝑑𝑟2+1𝑟𝑑𝑢𝑢𝑑𝑟𝛽𝑟2=0,(3.4) where tangential stiffness reduction factor 𝛽 is introduced to reflect the reduction of the secant tensile stiffness of the cracked concrete in hoop direction during concrete cracking evolution, defined as𝐸𝛽=𝜃𝐸𝑟=𝐸𝜃𝐸.(3.5) The stiffness in radial direction 𝐸𝑟 is assumed to equal the initial stiffness 𝐸 of concrete because the radial stress is typically in compression for the boundary value problem considered. By using the approximation 𝜐=𝜐𝑟𝜃𝜐𝜃𝑟, the stress and strain relations for the boundary value problem given in (3.2) are rewritten as𝜎𝑟=𝐸1𝜐2𝜀𝑟+𝜐𝛽𝜀𝜃,𝜎(3.6a)𝜃=𝐸1𝜐2𝜐𝛽𝜀𝑟+𝛽𝜀𝜃.(3.6b)

3.2. Governing Equations for Cracked Concrete

From the cohesive crack model, the residual tensile stress in hoop direction for the cracked concrete can be obtained from 𝜎𝜃=𝜎𝑤=𝑓𝑡(𝑎𝑏𝑊).(3.7) The total hoop strain 𝜀𝜃 of the cracked concrete consists of fracture strain𝜀𝜃𝑓 and linear elastic strain between cracks 𝜀𝜃𝑒. The fracture strain is generated by a total number of 𝑛𝑐 cracks, whereas the linear elastic strain between cracks is associated with the residual tensile hoop stress 𝜎𝜃, defined as 𝜀𝜃𝑓=𝑛𝑐𝑤(𝑟)2𝜋𝑟=𝑏𝑙0𝑓𝑡𝐸𝑊𝑟,𝜀𝜃𝑒=𝜎𝜃𝐸=𝑓𝑡𝐸(𝑎𝑏𝑊),(3.8) where material coefficient 𝑙0=𝑛𝑐𝑙ch/2𝜋𝑏 in which 𝑙ch is characteristic length 𝑙ch=𝐸𝐺𝐹/𝑓𝑡2 defined in Hillerborg et al. [17]. The total hoop strain 𝜀𝜃 of the cracked concrete is then given by 𝜀𝜃=𝜀𝜃𝑓+𝜀𝜃𝑒=𝑓𝑡𝐸(𝑎𝑏𝑊)+𝑏𝑙0𝑊𝑟.(3.9) The radial displacement 𝑢 of the cracked concrete, from (3.1b), is calculated from𝑢=𝜀𝜃𝑓𝑟=𝑡𝐸(𝑎𝑏𝑊)𝑟+𝑏𝑙0𝑊.(3.10) And the radial strain, from (3.1a), is given by𝜀𝑟=𝑓𝑡𝐸(𝑙𝑎𝑏𝑊)+𝑏0𝑟𝑑𝑊𝑑𝑟.(3.11) The reduction factor of residual tensile stiffness 𝛽 defined in (3.5) can be expressed as𝜀𝛽=𝜃𝑒𝜀𝜃𝑒+𝜀𝜃𝑓=11+𝑏𝑙0𝑊/(𝑎𝑏𝑊)𝑟.(3.12) By substituting (3.9) and (3.11), the radial stress in (3.6a) is rewritten as𝜎𝑟=𝑓𝑡1𝜐21+𝜐𝛽(𝑙𝑎𝑏𝑊)+𝑏0𝑟𝑑𝑊𝑑𝑟+𝜐𝛽𝑏𝑙0𝑊𝑟.(3.13) The governing equation for directly solving normalised crack width 𝑊 now can be established by substituting (3.10) and (3.12) into (3.4), expressed here as𝑙0𝑑𝑟2𝑊𝑑𝑟2+𝑙013𝑟𝑟𝑑𝑊𝑑𝑟=0.(3.14) The general solution to the second-order linear homogeneous differential equation is𝑊=𝐶11𝑙0𝑙01𝑟𝑙02||𝑙ln0||𝑟𝑟+𝐶2,(3.15) where constant coefficients 𝐶1 and 𝐶2 in the general solution can be determined from two boundary conditions of the boundary value problem. To calculate radial strains and stresses, the first derivative of the normalised crack width 𝑊 with respect to radius 𝑟 is required and given as𝑑𝑊𝑑𝑟=𝐶11𝑟𝑙0𝑟2.(3.16) To simplify the general solution, a crack width function associated with material coefficient 𝑙0 and radius 𝑟 within the concrete cover is defined as𝛿𝑙0=1,𝑟𝑙0𝑙01𝑟𝑙02||𝑙ln0||𝑟𝑟.(3.17) The general solution of normalised crack width given in (3.15) can now be rewritten as𝑊=𝐶1𝛿𝑙0,𝑟+𝐶2.(3.18) Once the normalised crack width is obtained, mechanical parameters, such as actual crack width, hoop residual strength, and stiffness and radial stress, can be calculated from the corresponding developed equations.

4. Crack Propagation through Cover Concrete

Cracks initiate in cover concrete when the tensile hoop stress at the internal boundary reaches tensile strength and then propagate through the concrete cover until reaching the free cover surface. Depending on the crack width at the internal boundary 𝑊𝑏, three cases are considered at the stage of partially cracked concrete cover, crack initiation at the internal boundary, crack propagation when 𝑊𝑏 does not exceed the critical value (𝑊𝑏𝑊cr), and crack propagation when 𝑊𝑏 exceeds the critical value (𝑊𝑏𝑊cr).

4.1. Crack Initiation at Internal Boundary

Since the cover concrete remains intact and elastic before the tensile hoop stress reaches the tensile strength of concrete, the classical elastic solution of radial displacement 𝑢 to an axis symmetrical thick-walled cylinder [21] is expressed here as 𝑢=𝐷1𝑟+𝐷21𝑟,(4.1) where 𝐷1 and 𝐷2 are constant coefficients to be determined by boundary conditions. The radial stress 𝜎𝑟 and hoop stress 𝜎𝜃 for isotropic elastic materials are given by𝜎𝑟=𝐸𝐷1𝜐1𝐸𝐷1+𝜐21𝑟2,𝜎𝜃=𝐸𝐷1𝜐1+𝐸𝐷1+𝜐21𝑟2.(4.2) The displacement boundary condition at the internal boundary (𝑅𝑏) and the free surface condition at concrete cover surface (𝑅𝑐) are now introduced:𝑢|𝑟=𝑅𝑏=𝑢𝑏(𝑡),𝜎𝑟||𝑟=𝑅𝑐=0,(4.3) where the prescribed displacement 𝑢𝑏(𝑡) is given by (2.4). After the constant coefficients 𝐷1 and 𝐷2 are determined from the boundary conditions, the radial and hoop stresses are obtained from𝜎𝑟=𝐸𝑅𝑏(1𝜐)𝑅𝑏2+(1+𝜐)𝑅𝑐2𝑅1𝑐2𝑟2𝑢𝑏,𝜎(4.4a)𝜃=𝐸𝑅𝑏(1𝜐)𝑅𝑏2+(1+𝜐)𝑅𝑐2𝑅1+𝑐2𝑟2𝑢𝑏.(4.4b)It can be seen that the hoop stress is in tension whereas the radial stress is in compression over the concrete cover. The cover concrete initiates cracking when the hoop stress 𝜎𝜃 at the internal boundary reaches the tensile strength 𝑓𝑡. From (4.4b), the radial displacement at the internal boundary at the time to crack initiation 𝑇𝑖 can be calculated from𝑢𝑏𝑇𝑖=(1𝜐)𝑅𝑏2+(1+𝜐)𝑅𝑐2𝑅𝑏2+𝑅𝑐2𝑓𝑡𝐸𝑅𝑏.(4.5) The corresponding mass of corrosion rust at the time to crack initiation is obtained from (2.5), and then the time when cracking initiates at the internal boundary (𝑇𝑖) can be estimated from (2.1).

4.2. Crack Propagation before Crack Width at Rebar Surface Reaches Critical Value

The thick-walled cylinder is now divided into two zones, an intact outer ring (𝑟𝑦+𝑟𝑅𝑐) and a cracked inner ring (𝑅𝑏𝑟𝑟𝑦). In the intact outer ring, the tensile hoop stress reaches the concrete tensile strength 𝑓𝑡 at the crack front (𝑟𝑦+) and the external surface (𝑅𝑐) remains free, expressed as𝜎𝜃||𝑟=𝑟𝑦+=𝑓𝑡,𝜎𝑟||𝑟=𝑅𝑐=0.(4.6) From (4.2) and by using the constant coefficients 𝐷1 and 𝐷2 determined from the boundary conditions, the radial and hoop stresses are given by𝜎𝑟=𝑓𝑡𝑟𝑦2𝑟𝑦2+𝑅𝑐2𝑅1𝑐2𝑟2,𝜎(4.7a)𝜃=𝑓𝑡𝑟𝑦2𝑟𝑦2+𝑅𝑐2𝑅1+𝑐2𝑟2.(4.7b)

In the cracked inner ring, where the crack width at the internal boundary does not exceed the critical value, the displacement condition at internal boundary (𝑅𝑏) described in (4.3), by using (3.10) and considering (2.8a), is rewritten as the boundary condition for the normalised crack width𝑊𝑏cr1(𝑡)=𝑏cr𝑙0cr𝑅𝑏𝐸𝑓𝑡𝑢𝑏(𝑡)𝑎cr𝑅𝑏,(4.8) where material coefficient 𝑙0cr=𝑛𝑐𝑙ch/2𝜋𝑏cr. Meanwhile, considering zero crack width at the crack front (𝑟𝑦), the boundary conditions for the cracked zone are expressed as𝑊||𝑟=𝑅𝑏=𝑊𝑏cr||,𝑊𝑟=𝑟𝑦=0.(4.9) From (3.18) and by using the boundary conditions, the normalised crack width over the cracked inner ring is given by𝛿𝑙𝑊=0cr𝑙,𝑟𝛿0cr,𝑟𝑦𝛿𝑙0cr,𝑅𝑏𝑙𝛿0cr,𝑟𝑦𝑊𝑏cr.(4.10) The crack front (𝑟𝑦) can be determined by the continuity condition of radial stress crossing the intact and cracked zones, namely,𝜎𝑟||𝑟=𝑟𝑦+=𝜎𝑟||𝑟=𝑟𝑦.(4.11) From (4.7a), the radial stress at the internal boundary of the intact zone (𝑟𝑦+) is given by𝜎𝑟||𝑟=𝑟𝑦+=𝑟𝑦2𝑅𝑐2𝑟𝑦2+𝑅𝑐2𝑓𝑡.(4.12) By using (4.10) and considering 𝛽=1 at the crack front, the radial stress at the external boundary of the cracked zone (𝑟𝑦), from (3.13), is given by𝜎𝑟||𝑟=𝑟𝑦=𝑓𝑡1𝜐2(1+𝜐)(1𝛼)𝑟𝑦𝑙0cr𝑟y𝛿𝑙0cr,𝑟𝑦𝑙𝛿0cr,𝑅𝑏𝑊𝑏cr𝑊cr.(4.13) The boundary condition in (4.11) for determining the crack front (𝑟𝑦) gives𝑟𝑦𝑙0cr𝑟𝑦𝛿𝑙0cr,𝑟𝑦𝑙𝛿0cr,𝑅𝑏(1+𝜐)+1𝜐2𝑅𝑐2𝑟𝑦2𝑅𝑐2+𝑟𝑦2𝑊=(1𝛼)𝑏cr𝑊cr.(4.14) When the crack front reaches the concrete cover surface (𝑟𝑦=𝑅𝑐), the normalised crack width at the internal boundary at the time to cracking on cover surface (𝑇𝑐) is calculated from𝑊𝑏cr𝑇𝑐=(1+𝜐)𝑅𝑐𝑙0cr𝑅𝑐𝛿𝑙0cr,𝑅𝑐𝑙𝛿0cr,𝑅𝑏𝑊cr1𝛼.(4.15) From (4.8), the corresponding displacement at the internal boundary of the thick-walled cylinder at time 𝑇𝑐 can be determined from𝑢𝑏𝑇𝑐=𝑅1+(1+𝜐)𝑐𝑅𝑏𝑙0cr𝑅𝑏𝑙0cr𝑅𝑐𝛿𝑙0cr,𝑅𝑐𝑙𝛿0cr,𝑅𝑏𝑓𝑡𝐸𝑅𝑏.(4.16) Consequently, the time to cracking 𝑇𝑐 can be estimated from (2.5) and (2.1). It can be seen that the time to cracking is a function of concrete cover dimensions, material properties of cover concrete, and reinforcement corrosion rate.

4.3. Crack Propagation When Crack Width at Rebar Surface Exceeds Critical Value

In this case, the thick-walled cylinder is divided into three zones, as shown in Figure 1(b), an intact outer ring (𝑟𝑦+𝑟𝑅𝑐), a cracked middle ring, where crack width does not exceed critical value (𝑟cr+𝑟𝑟𝑦), and a cracked inner ring, where crack width exceeds critical value (𝑅𝑏𝑟𝑟cr). The intact outer ring of this case has the same results given in Section 4.2. For the cracked middle ring, considering the critical crack width at the internal boundary (𝑟cr+), the boundary conditions for the cracked middle ring are𝑊||𝑟=𝑟cr+=𝑊cr||,𝑊𝑟=𝑟𝑦=0.(4.17) The normalised crack width within the cracked middle ring is then given by𝛿𝑙𝑊=0cr𝑙,𝑟𝛿0cr,𝑟𝑦𝛿𝑙0cr,𝑟cr𝑙𝛿0cr,𝑟𝑦𝑊cr.(4.18) For the cracked inner ring, because the crack width exceeds the critical value, from (3.10) and (2.8b), the normalised crack width at its internal boundary is given by𝑊𝑏𝑢1(𝑡)=𝑏𝑢𝑙0𝑢𝑅𝑏𝐸𝑓𝑡𝑢𝑏(𝑡)𝑎𝑢𝑅𝑏,(4.19) where material coefficient 𝑙0𝑢=𝑛𝑐𝑙ch/2𝜋𝑏𝑢. The boundary conditions for the cracked inner ring are𝑊||𝑟=𝑅𝑏=𝑊𝑏𝑢||,𝑊𝑟=𝑟cr=𝑊cr.(4.20) Therefore, the normalised crack width within the cracked inner ring is given by𝛿𝑙𝑊=0𝑢𝑙,𝑟𝛿0𝑢,𝑟cr𝛿𝑙0𝑢,𝑅𝑏𝑙𝛿0𝑢,𝑟cr𝑊𝑏𝑢+𝛿𝑙0𝑢,𝑅𝑏𝑙𝛿0𝑢,𝑟𝛿𝑙0𝑢,𝑅𝑏𝑙𝛿0𝑢,𝑟cr𝑊cr.(4.21) Considering the condition of radial stress continuity at the crack front (𝑟𝑦) between the intact ring and the cracked middle ring described in (4.11), an equation similar to (4.14) is obtained𝑟𝑦𝑙0cr𝑟𝑦𝛿𝑙0cr,𝑟𝑦𝑙𝛿0cr,𝑟𝑐𝑟(1+𝜐)+1𝜐2𝑅𝑐2𝑟𝑦2𝑅𝑐2+𝑟𝑦2=1𝛼.(4.22) Meanwhile, the condition of radial stress continuity at the critical crack boundary (𝑟cr) between the cracked middle ring and the cracked inner ring gives𝜎𝑟||𝑟=𝑟cr+=𝜎𝑟||𝑟=𝑟cr.(4.23) The hoop stress in (3.7) and stiffness reduction factor in (3.12) at the critical crack boundary, which are utilised for calculating the radial stress, are given by𝜎𝜃||𝑟=𝑟cr+=𝜎𝜃||𝑟=𝑟cr=𝛼𝑓𝑡,𝛽||𝑟=𝑟cr+||=𝛽𝑟=𝑟cr=1𝑛1+c𝑙ch𝑊/2𝜋𝛼cr/𝑟cr.(4.24) And the radial strains in (3.11) at the critical crack boundary are given by𝜀𝑟||𝑟=𝑟cr+=𝑓𝑡𝐸𝛼+1𝛼𝑟cr𝑙0cr𝑟cr𝛿𝑙0cr,𝑟cr𝑙𝛿0cr,𝑟y,𝜀𝑟||𝑟=𝑟cr=𝑓𝑡𝐸𝛼𝛼+𝑟cr𝑙0𝑢𝑟cr𝛿𝑙0𝑢,𝑅𝑏𝑙𝛿0𝑢,𝑟cr𝑊𝑏𝑊cr𝑊𝑢𝑊cr.(4.25) Consequently, (4.23) is expressed as𝑙0𝑢𝑟cr𝛿𝑙0𝑢,𝑅𝑏𝑙𝛿0𝑢,𝑟cr𝛼𝑊1𝛼𝑏𝑢𝑊cr𝑊𝑢𝑊cr𝑙0cr𝑟cr𝛿𝑙0cr,𝑟cr𝑙𝛿0cr,𝑟𝑦=0.(4.26) The cracked front (𝑟𝑦) and the critical crack front (𝑟cr) can be determined from the set of nonlinear equations, (4.22) and (4.26).

5. Completely Cracked Concrete Cover

After the crack front reaches the external surface, the concrete cover becomes completely cracked. Depending on the crack widths at the internal and external boundaries, three cases are considered, crack width within the concrete cover does not exceed the critical value (𝑊𝑏𝑊cr and 𝑊𝑐𝑊cr), critical crack propagates through the concrete cover (𝑊𝑏𝑊cr and 𝑊𝑐𝑊cr), and crack width within the concrete cover exceeds the critical value (𝑊𝑏𝑊cr and 𝑊𝑐𝑊cr).

5.1. Crack Width within Concrete Cover Not Exceeding Critical Value

A single cracked zone within the concrete cover exists in this case, and the crack width at the internal boundary does not exceed the critical value when the cover surface is cracked. To determine the two constant coefficients 𝐶1 and 𝐶2 in the general solution in (3.18), the unknown crack width at the external boundary 𝑊𝑐, together with the prescribed displacement at the internal boundary, is now utilised𝑊||𝑟=𝑅𝑏=𝑊𝑏cr||,𝑊𝑟=𝑅𝑐=𝑊𝑐.(5.1) The normalised crack width over the concrete cover is then given by𝛿𝑙𝑊=0cr𝑙,𝑟𝛿0cr,𝑅𝑐𝛿𝑙0cr,𝑅𝑏𝑙𝛿0cr,𝑅𝑐𝑊𝑏cr+𝛿𝑙0cr,𝑅𝑏𝑙𝛿0cr,𝑟𝛿𝑙0cr,𝑅𝑏𝑙𝛿0cr,𝑅𝑐𝑊𝑐.(5.2) To determine the unknown 𝑊𝑐, the free surface condition at the external boundary (𝑅𝑐) described in (4.3) is adopted. The radial stress at the external boundary can be calculated from (3.13) by using the stiffness reduction factor in (3.12) and the first derivative of normalised crack width in (3.16), namely, 𝛽||𝑟=𝑅𝑐=1𝑙1+0cr/𝑅𝑐(1𝛼)𝑊𝑐/𝑊cr(1𝛼)𝑊𝑐,𝑑𝑊|||𝑑𝑟𝑟=𝑅𝑐=𝑊𝑏cr𝑊𝑐𝑅𝑐𝑙0cr𝑅𝑐2𝛿𝑙0cr,𝑅𝑏𝑙𝛿0cr,𝑅𝑐.(5.3) Then, the free surface condition at concrete cover surface gives𝑊cr(1𝛼)𝑊𝑐+𝑊(1𝛼)𝑏cr𝑊𝑐𝑅𝑐𝑙0cr𝑅𝑐𝛿𝑙0cr,𝑅𝑏𝑙𝛿0cr,𝑅𝑐+𝜐𝑊cr(1𝛼)𝑊𝑐𝑊cr(1𝛼)𝑊𝑐+𝑙0cr𝑅𝑐(1𝛼)𝑊𝑐=0.(5.4) Once 𝑊𝑐 is obtained, mechanical parameters of the completely cracked cover concrete, such as actual crack width, hoop residual strength and stiffness, and radial stress, can be calculated.

5.2. Critical Crack Propagation through Concrete Cover

The critical crack front divides the thick-walled cylinder into two zones: a cracked outer ring, where crack width does not exceed the critical value (𝑟cr+𝑟𝑅𝑐), and a cracked inner ring, where crack width exceeds the critical value (𝑅𝑏𝑟𝑟cr). For the cracked outer ring, two boundary conditions are considered: the critical crack width at the internal boundary (𝑟cr+) and the unknown crack width at the external boundary (𝑅𝑐):𝑊||𝑟=𝑟cr+=𝑊cr||,𝑊𝑟=𝑅𝑐=𝑊𝑐.(5.5) The normalised crack width within the cracked outer ring is then expressed as𝛿𝑙𝑊=0cr𝑙,𝑟𝛿0cr,𝑅𝑐𝛿𝑙0cr,𝑟cr𝑙𝛿0cr,𝑅𝑐𝑊cr+𝛿𝑙0cr,𝑟cr𝑙𝛿0cr,𝑟𝛿𝑙0cr,𝑟cr𝑙𝛿0cr,𝑅𝑐𝑊𝑐.(5.6) For the cracked inner ring, the boundary conditions are𝑊||𝑟=𝑅𝑏=𝑊𝑏𝑢||,𝑊𝑟=𝑟cr=𝑊cr.(5.7) The normalised crack width within the cracked inner ring is then given by𝛿𝑙𝑊=0𝑢𝑙,𝑟𝛿0𝑢,𝑟cr𝛿𝑙0𝑢,𝑅𝑏𝑙𝛿0𝑢,𝑟cr𝑊𝑏𝑢+𝛿𝑙0𝑢,𝑅𝑏𝑙𝛿0𝑢,𝑟𝛿𝑙0𝑢,𝑅𝑏𝑙𝛿0𝑢,𝑟cr𝑊cr.(5.8) The free surface condition at the external boundary (𝑅𝑐) of the cracked outer ring in this case gives an equation similar to (5.4) but involving unknown 𝑟cr, namely,𝑊cr(1𝛼)𝑊𝑐+𝑊(1𝛼)cr𝑊𝑐𝑅𝑐𝑙0cr𝑅𝑐𝛿𝑙0cr,𝑟cr𝑙𝛿0cr,𝑅𝑐+𝜐𝑊cr(1𝛼)𝑊𝑐𝑊cr(1𝛼)𝑊𝑐+𝑙0cr𝑅𝑐(1𝛼)𝑊𝑐=0.(5.9) Meanwhile, the continuity condition of radial stresses at the critical boundary (𝑟cr) between the outer ring and the inner ring, described in (4.23), gives𝑊cr𝑊𝑐𝑊cr𝑙0𝑢𝑟cr𝛿𝑙0𝑢,𝑅𝑏𝑙𝛿0𝑢,𝑟cr𝛼𝑊1𝛼𝑏𝑢𝑊cr𝑊𝑢𝑊cr𝑙0cr𝑟cr𝛿𝑙0cr,𝑟cr𝑙𝛿0cr,𝑅𝑐=0.(5.10) Consequently, the two unknowns, 𝑊𝑐 and 𝑟cr, can be determined from the set of nonlinear equations, (5.9) and (5.10).

5.3. Crack Width Exceeding Critical Value within Concrete Cover

A single cracked zone is considered for the thick-walled cylinder in this case, and the crack width over the concrete cover now exceeds the critical value. The boundary conditions for this case are given by𝑊||𝑟=𝑅𝑏=𝑊𝑏𝑢||,𝑊𝑟=𝑅𝑐=𝑊𝑐.(5.11) The normalised crack width within the cracked concrete cover is expressed as𝛿𝑙𝑊=0𝑢𝑙,𝑟𝛿0𝑢,𝑅𝑐𝛿𝑙0𝑢,𝑅𝑏𝑙𝛿0𝑢,𝑅𝑐𝑊𝑏𝑢+𝛿𝑙0𝑢,𝑅𝑏𝑙𝛿0𝑢,𝑟𝛿𝑙0𝑢,𝑅𝑏𝑙𝛿0𝑢,𝑅𝑐𝑊𝑐.(5.12) Similarly, from the free surface condition at the external surface (𝑅𝑐), the unknown 𝑊𝑐 can be determined from𝑊𝑢𝑊𝑐+1𝑅𝑐𝑙0𝑢𝑅𝑐𝛿𝑙0𝑢,𝑅𝑏𝑙𝛿0𝑢,𝑅𝑐𝑊𝑏𝑢𝑊𝑐+𝜐𝑊𝑢𝑊𝑐𝑊𝑢𝑊𝑐+𝑙0𝑢𝑅𝑐𝑊𝑐=0.(5.13) When cracks in the cover concrete reach the ultimate cohesive width, the cracks become cohesionless and no residual strength exists in the cracked cover concrete. From (5.13), it can be seen that cracks at both the internal and external boundaries reach the ultimate cohesive width at the same time (𝑇𝑢). The displacement at the internal boundary at time 𝑇𝑢 is calculated from𝑢𝑏𝑇𝑢=𝛼𝑊𝑢𝑊𝑢𝑊cr𝑓𝑡𝐸𝑙0𝑢.(5.14) The time at the end of cohesive cracking stage 𝑇𝑢 can be then estimated by using (2.5) and (2.1).

6. Validation and Parameter Studies

6.1. Comparison of Theoretical Predictions with Experimental Data

To validate the proposed approach, the published experimental data such as the time to cracking on the concrete cover surface and the concrete crack width with reinforcement corrosion progress are adopted. Liu and Weyers [8] conducted experiments to measure the time to cracking induced by steel rebar corrosion for specimens with various corrosion rates and cover dimensions, as given in Table 1. The material properties for the specimens utilised in their study are taken as compressive strength 𝑓𝑐=31.5MPa, tensile strength 𝑓𝑡=3.3MPa, elastic modulus of concrete 𝐸=27GPa, Poisson’s ratio 𝜐=0.18, concrete creep coefficient assumed here 𝜃=1, density of corrosion rust products 𝜌𝑟=3600kg/m3, density of steel 𝜌𝑠=7850kg/m3, and coefficient 𝛾=0.57. Other material properties adopted in the predictions are evaluated from the given concrete properties with assumed maximum aggregate size 𝑑𝑎=25mm, such as fracture energy 𝐺𝑓=83N/m, total crack number𝑛𝑐=4, critical crack width 𝑤cr=0.03mm, and ultimate cohesive crack width 𝑤𝑢=0.2mm. The theoretical predictions of the time to cracking from the developed approach are then compared with the experimental data observed by Liu and Weyers [8], as shown in Table 1. As it can be seen from Table 1 the predicted results in general agree with the experimental results for various cover dimensions and corrosion rates. The discrepancy of the time to cracking for specimen S3 may be related to the fact that not all corrosion products are activated in the generation of radial pressure at the internal surface of the concrete cover. It should be noted that part of these products penetrates into the porous voids between the steel rebar and the surrounding concrete and a considerable amount of the rust transports into the surrounding cracks, in particular in the case when the corrosion rate is relatively higher, as discussed in the studies by Chernin et al. [14], Pantazopoulou and Papoulia [9], Liu and Weyers [8].

tab1
Table 1: Predicted and observed times to cracking on concrete cover surface.

The predicted results for the crack width on concrete cover surface with the progress of reinforcement corrosion are now compared with the experimental measurements presented by Andrade et al. [7], Molina et al. [22]. The experiments were carried out for steel rebar of 16 mm in diameter embedded into a concrete specimen with clear cover of 30 mm. The material properties utilised in their studies are taken as tensile strength 𝑓𝑡=3.55MPa, elastic modulus of concrete 𝐸=36GPa, and Poisson’s ratio 𝜐=0.20. The decrease in diameter of the steel rebar due to corrosion over time is estimated from the corrosion rate 𝑖corr=100𝜇A/cm2. Other material properties adopted in predictions include fracture energy 𝐺𝑓=200N/m, total crack number 𝑛𝑐=4, critical crack width 𝑤cr=0.05mm, and ultimate cohesive crack width 𝑤𝑢=0.4mm. The theoretical predictions of the crack width on the concrete cover surface over time are shown in Figure 3 to compare with the experimental measurements by Andrade et al. [7]. It can be seen that the predicted results lie between the maximum and minimum measured crack widths and are in good agreement with the experimental data.

769132.fig.003
Figure 3: Comparison of predicted crack width over time with published experimental results.
6.2. Mechanical Parameter Studies

The specimen S1 shown in Table 1 and tested by Liu and Weyers [8] is now utilised to analyse the mechanical parameters with the reinforcement corrosion progress and through the concrete cover. The radial displacement 𝑢𝑏 at the internal boundary of the concrete cover due to steel rebar corrosion is plotted with time in Figure 4. The expansive displacement at the rebar surface increases sharply at the early stage of corrosion and then grows steadily with corrosion progress, reaching 127.3 μm when cracks get to ultimate cohesive width. Similar shape of curve is obtained for the crack width at the internal boundary of the concrete cover 𝑤𝑏, as shown in Figure 5. The crack width at the concrete cover surface 𝑤𝑐increases abruptly when crack front reaches the free cover surface due to sudden release of energy. After the time to cracking, the crack width at the cover surface 𝑤𝑐 is close to that at the rebar surface 𝑤𝑏 and becomes ultimate cohesive width at the time of 42.9 years.

769132.fig.004
Figure 4: Radial displacement at rebar surface with time 𝑡.
769132.fig.005
Figure 5: Crack widths at rebar surface (𝑤𝑏) and at cover surface (𝑤𝑐) with time 𝑡.

The plot in Figure 6 presents two sets of curves, the cracked front 𝑟𝑦 and critical crack front 𝑟cr propagating with time from the rebar surface to the concrete cover surface. The cracked front starts at the crack ignition time of 0.014 year when cracks appear at the internal boundary of the concrete cover and reaches the concrete cover surface at the predicted time to cracking of 1.83 years. Faster propagation of the cracked front is noted when the cracked front is near the internal boundary and the external boundary due to energy release. The critical crack front propagates gradually from the time of 0.97 year, but suddenly jumps to the concrete cover surface at the time to cracking.

769132.fig.006
Figure 6: Propagation of cracked front (𝑟𝑦) and critical crack front (𝑟cr) with time 𝑡.

Figure 7 gives the history of hoop stresses 𝜎𝜃 at the rebar surface and at the concrete cover surface with the progress of rebar corrosion. The hoop stress at the rebar surface quickly reaches tensile strength 𝑓𝑡 at the crack ignition time of 0.014 year, followed by steady decrease to the time of 0.97 year when the crack width at the rebar surface becomes critical. From this point, the residual strength in the hoop direction at the rebar surface gradually reduces to zero at the time when cracks get to ultimate cohesive width. The residual strength at the cover surface experiences similar history to that at the rebar surface, but peaks at the time to cracking followed by a sudden drop. The residual strengths at the rebar surface and at cover surface are very close to each other during the stage when the cover concrete is completely cracked. Figure 8 shows the histories of tangential stiffness reduction factor 𝛽 at the rebar surface and at the concrete cover surface in logarithmic scale. Sharp changes are noted at the time of cracking initiation and at time when cracks become critical for the stiffness reduction factor at the rebar surface, and at the time to cracking for the stiffness reduction factor at the cover surface. It can be seen that the tangential residual stiffness decays faster than the tangential residual strength during the development of cracking in the cover concrete.

769132.fig.007
Figure 7: Hoop stress (𝜎𝜃) at rebar surface and at cover surface with time 𝑡.
769132.fig.008
Figure 8: Hoop stiffness reduction factor (𝛽) at rebar surface and at cover surface with time 𝑡.

The bursting pressure 𝜎𝑟 exerted by the accumulating corrosion products at the internal boundary of the concrete cover is plotted in Figure 9. Radial pressure at the internal boundary builds up as cracks propagate from the rebar surface, reaching a peak value of 15.2 MPa (well below concrete compressive strength of 31.5 MPa) at the time of 1.21 years when crack front travelled about 2/3 of the concrete cover. Sudden release of the radial pressure occurs at the time when crack front reaches the cover surface, and residual radial pressure maintains only about 1/4 of the peak value after the time to cracking. The sudden release of radial pressure indicates significant reduction of the bond strength between the steel rebar and the surrounding concrete cover after the time to cracking.

769132.fig.009
Figure 9: Radial pressure (𝜎𝑟) at rebar surface with time 𝑡.

Figures 1013 present results of crack width 𝑤, hoop stress𝜎𝜃, hoop stiffness reduction factor 𝛽, and radial stress 𝜎𝑟 varying with the radius within the concrete cover. Six important times during concrete cracking evolution are selected and listed in Table 2. The results in Figure 10 show that the crack widths over the concrete cover indicate cracks open approximately in a wedge shape before the time to cracking. The crack width has no significant change over the concrete cover thereafter and eventually gets to the ultimate cohesive width through the concrete cover. As shown in Figure 11, the peak values of hoop stress indicate the cracked front propagation through the concrete cover before the time to cracking. The residual strength in hoop direction has little change over the concrete cover after the time to cracking, becoming zero at the time when the cracks reach the ultimate cohesive width. The results plotted in Figure 12 show that the hoop stiffness reduction factors change significantly at the cracked fronts during crack propagation through the concrete cover and reduce sharply after concrete is cracked. The radial stresses shown in Figure 13 have peak values at the internal boundary of the concrete cover, decreasing fast with the increase of radius within the concrete cover to a value of zero at the free surface boundary. The radial stresses drop to only approximately 1/5 of the peak value at the middle of concrete cover.

tab2
Table 2: Selected times during cover concrete cracking evolution.
769132.fig.0010
Figure 10: Crack width (𝑤) varying over concrete cover at various times.
769132.fig.0011
Figure 11: Hoop stress (𝜎𝜃) varying over concrete cover at various times.
769132.fig.0012
Figure 12: Stiffness reduction factor (𝛽) varying over concrete cover at various times.
769132.fig.0013
Figure 13: Radial stress (𝜎𝑟) varying over concrete cover at various times.

7. Conclusions

A new method for theoretically analysing the evolution of cracking in concrete cover subject to expansive internal displacement caused by steel rebar corrosion is presented based on the thick-walled cylinder model for the concrete cover and the tensile softening model for the cracked concrete. The governing equation for directly solving the crack width in the cracked concrete is established and a general closed-form solution is obtained for the proposed boundary value problem. The formulas for calculating actual crack width as well as other mechanical parameters of the cracked concrete, including residual strength, residual stiffness, and radial stress, are derived for various stages during the cracking evolution in the cover concrete. The predicted results for the time to cracking for various concrete cover dimensions and reinforcement corrosion rates and for the crack width over time are examined and demonstrated to be in good agreement with the published experimental measurements.

The time taken for cracked front to propagate from the internal boundary of the concrete cover to the cover surface is substantially long, and the existing models for estimating the time to cracking on the cover surface by ignoring the crack propagation through the concrete cover may be improper. The time to cracking is a function of cover dimensions, concrete material properties, and reinforcement corrosion rate. The crack width of the concrete cover depends on concrete material properties and the expansive displacement developed at the internal boundary due to reinforcement corrosion. The residual stiffness in hoop direction reduces significantly when concrete is cracked and decays faster than the hoop residual strength. The radial pressure at the interface between the steel rebar and the concrete cover reaches peak value well before the cracks occur at the cover surface, drops suddenly when concrete becomes completely cracked through the cover, and decays fast from the bond interface over the concrete cover. The time taken for cracks to reach the ultimate cohesive width and for hoop residual strength and stiffness to vanish is relatively long, comparing with the time to cracking.

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