PSI - Issue 64

Antonio Bossio et al. / Procedia Structural Integrity 64 (2024) 56–64 Author name / Structural Integrity Procedia 00 (2019) 000–000

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when cracks appear in the concrete (based on research by Bossio et al., 2011). Basically, the more oxide there is (thinner bar), the sooner cracks will form. The speed of oxidation and cracking depends on several factors. The properties of the oxide itself (elastic modulus of oxide, E o ) inducing how it expands compared to the original steel (volumetric expansion factor, n) has a big impact. The less the oxide expands, the faster the bar thins, and cracks appear. The size of the rebar and the surrounding concrete (R 4 -R 0 ) has an impact since bigger bars and thicker concrete covers take longer to crack. Interestingly, the model shows that from a structural standpoint, the thickness of the concrete cover itself (excluding the oxide layer) becomes less important once cracks appear. This is because the cover mainly slows down the initial corrosion process, not the cracking itself. A thicker cover will still take longer to let oxidation, but the impact on cracking after a certain point is less significant.

Fig. 1. Geometric scheme for analytical modeling of crack: (a) initiation, (b) propagation.

2.4. Cracking propagation Cracks appear when the pressure inside the concrete overcomes the concrete tensile strength. The study by Bossio et al. (2011, 2015) explored the connection between this mechanical process (cracking) and the reduction in the steel rebar's size due to corrosion (electrochemical process). To understand this, a mathematical model considered the stresses acting on the rebar, oxide layer, and both cracked and uncracked concrete (four layers total). This model takes into account how these layers interact and deform. Even though the model had four layers, the factors affecting crack growth are similar to those for crack initiation. One difference is that the amount of oxide produced increases slightly as the elastic modulus, E o , decreases, for a given amount of crack growth. The model can also simulate how different factors influence corrosion and cracking. These factors include the type and size of aggregate in the concrete (which affect how easily it cracks and its stiffness) and the creep of the concrete over time. (Figure 1b shows the model schematically). By setting an exposure environment and a level of corrosion, the model can be used to predict various things. For example, it can estimate the average compressive stress inside the concrete and how this stress affects the bond between the rebar and the concrete. The key findings from this analysis are summarized in Figure 2a. 3. Available bond increase models Existing analytical models were used to explore how corrosion affects the bond between the rebar and the concrete. Bond refers to how well the rebar and concrete interact each other and transfer forces. The impact of corrosion on the ratio between the ultimate bond stress ( τ b,f ) and the maximum bond stress ( τ 0 ) was analyzed ( Ω p,tr ). The symbol f l represents the internal pressure created by the oxide expanding inside the concrete.