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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4.6. Effect of volumetric expansion of oxide Oxide occupies more space than the original steel it replaces, this is the volumetric expansion factor ( n ), which is typically between 2 and 6 depending on the specific oxides formed (as reported by Pedeferri & Bertolini, 2000). Figure 5e explores how this volumetric expansion factor ( n ) affects the bond between the rebar and concrete. This analysis considers a specific scenario: 30mm concrete cover, 16mm diameter rebar, and C25/30 concrete strength. A higher volumetric expansion factor (more space occupied by oxides) leads to less reduction of the rebar before the concrete cover cracks completely. This is because the expanding oxide creates more pressure inside the concrete, causing cracks to appear sooner. However, the volumetric expansion factor has little impact on bond development. 4.7. Effect of elastic modulus of oxide The oxide elastic modulus, E o , is not well-known. Figure 5f examines how this uncertainty in stiffness affects the bond between the rebar and concrete. The analysis focuses on an environment with moderate exposure (XC2) and a 16mm diameter rebar. Three possible values for the oxide elastic modulus were used: 130 MPa (based on research by Carè et al., 2008), 1000 MPa (an intermediate value) and 210 GPa (assuming the oxide is as stiff as original steel). The results show that the stiffness of the oxide has very little impact on the improvement in bond strength ( Ω p,tr ratio). This is likely because the oxide layer is much thinner compared to the other layers involved (concrete and uncorroded steel). Even if the oxide is very stiff, it doesn't significantly affect the overall bond behavior due to its small size. 5. Conclusions This research is part of a larger project investigating how different factors affect Reinforced Concrete (RC) structures that are corroding. It proposes a novel model and is based on previously presented and validated modelling approaches. Experimental validation of the theoretical results of present proposed model for bond is foreseen as a future research need. This specific paper focuses on how seven key factors influence the bond between the rebar and the concrete. Various concrete conditions (exposure classes) were used to simulate the corrosion process using a complex mathematical model. The study involved simulating corrosion with different concrete and steel properties to see how these properties affect the bond strength in corroded RC elements. Oxide buildup initially strengthens the bond. As oxide forms around the rebar, it creates pressure pushing outwards on the concrete. This pressure initially makes the bond between the rebar and concrete stronger. Once cracks appear in the concrete, the bond weakens. As the pressure from the oxide continues to build, cracks start to develop in the concrete. These cracks relieve the pressure and allow the bond strength to drop back down to its original level. Over time, the bond weakens further. As corrosion continues, the rebar gets thinner, and the cracks get wider. This further weakens the bond between the rebar and the concrete. Concrete stiffness (creep) has a significant impact. The stiffer the concrete (lower creep factor), the less the rebar reduces for a given amount of pressure from oxide expansion. Concrete strength itself has little effect. The pressure created by oxide is similar for different concrete classes, but in a higher-strength concrete it takes more reduction of rebar to reach that pressure. The thickness of the concrete cover significantly affects how much the rebar reduces before cracking. A thicker cover provides more protection and slows down the process. It has little effect on the overall bond strength improvement though. The size of the rebar has a small effect. Rebar diameter has little impact on the bond strength itself. However, thinner rebars tend to crack the surrounding concrete sooner due to oxide pressure. Different theories about how the bond develops give similar results. There are some differences, but Eurocode 2 limitation seems to have the biggest effect on the predicted improvement in bond strength. The amount of space occupied by oxide (volumetric expansion factor) is a major factor. The less space the oxide occupies compared to the original steel (lower factor), the more the rebar reduces before the concrete cracks. However, the stiffness of the oxide itself seems to have little effect.