Numerical prediction of the structural response of corroded existing RC frames exposed to differential settlements

This work is focused on the numerical evaluation of the effects of corrosion on the structur- al response of an existing Italian RC framed building designed for gravity loads only, subjected to differential settlements. To this end, nonlinear analyses are carried out on a three- dimensional FE model of the structure, by considering different corrosion levels and by progressively scaling the amplitude of the imposed based displacements. The final aim of the analyses is to identify relevant structural response parameters that can be monitored during on-site inspections, as well as to evaluate their possible variation in relation to the corrosion level of the structure.

The vulnerability assessment of existing Reinforced Concrete (RC) buildings against different sources of hazard has been traditionally carried out with reference to the as-built condition, thus neglecting the detrimental effects of ageing and corrosion (Parisi et al., 2017; Fotopoulou et al., 2021).

However, in Italy a large part of the existing building stock is close to reaching, or has already exceeded, the expected design life. For this reason, recent years have seen a growing focus on assessing the impact of corrosion degradation on the response over time of RC structures, particularly within the context of seismic performance evaluation (e.g., Dizaj et al., 2018; Michelini et al., 2024). Conversely, this aspect has been less investigated so far in relation to other sources of hazard such as differential ground set- tlements, which may be due to various causes (e.g., landslides, subsidence, human activities, etc.). Aim of this work is to investigate the combined effect of material degradation and differential settlement on a typical existing RC framed structure through nonlinear Finite Element (FE) analyses, by applying different levels of corrosion to the base of the columns at the ground level of the building. The results obtained confirm that corrosion alters the response of the structure even under service loading and that its effect, combined with ground subsidence, can potentially induce undesirable brittle failure mechanisms (such as rebar buckling or shear failure), that are difficult to detect in advance by traditional monitoring systems.

Description of the case study and of the numerical model

Numerical analyses are carried out on a three-story pre-code RC framed building. The building is characterized by six primary frames in the transverse direction, as can be seen in Fig.1, which shows the in-plan view of the structure at the ground floor and a three- dimensional view of the Finite Element (FE) model. A_s the building was designed for gravity loads only, following the Italian practice of the 50s and 60s, the primary frames are only connected by longitudinal beams around the perimeter of the building, with no internal frames parallel to the direction of the floors. The same beam layout is also assumed at the foundation level.

More information on the case study, both in terms of the geometric configuration of the building and the mechanical properties of the materials, can be found in De Risi et al. (2023) and in Belletti et al. (2023). As mentioned above, the main focus of this work is to analyze the combined effect of differential soil settlements and material degradation due to rebar corrosion.

Material degradation induced by rebar corrosion

Chlorides are a common cause of corrosion for RC structures in marine environments, due to marine aerosol exposure and deposition, or to the direct contact with salts in tidal, splash and spray zones. Corrosion initiation occurs when the concentration of aggressive agents penetrating into the concrete cover at the level of reinforcing rebars reaches a critical con- tent Ccrit, which is assumed here to be 0.4% according to LNEC E 465 (Marques et al., 2012). Corrosion initiation time is estimated through the fib Bulletin 76 (2015) equation:

being C₀ and Cs,Dx the initial chloride content and the chloride content at a depth Dx at time t (wt.-%/c), respectively; c is the concrete cover depth (here equal to 30 mm for stirrups and 36 mm for longitudinal rebars), Dx is the depth of the convection zone, Dapp(t) is the appar- ent chloride diffusion coefficient (m²/s), e_rf is the error function. In the absence of observational data, the parameters in Eq. (1) can be estimated according to the literature (fib Bulle- tin 76, LNEC E 465). The amount of corrosion caused by chloride attack is quantified geo- metrically in terms of section loss (e.g. assuming hemispherical pitting, according to Val & Melchers, 1997) starting from the pitting depth:

where I_corr is the corrosion rate, t_p = t- t_i is the propagation period and Rp is the pitting factor, representing the ratio between the maximum pit depth and the average corrosion penetration (set equal to 10, according to the CONTEVECT manual, 2001). If on-site measurements of the corrosion rate are not available, the values suggested in the CONTEVECT manual for the different exposure classes can be used for calculations (I_corr = 2.75 mA/cm² in this work). Based on the pitting depth obtained from Eq. 2, the cross-sectional area of a pit A_p can first be calculated, and finally the rebar section loss can be obtained as the difference between the initial uncorroded area and A_p. Figure 2a shows the reduction of steel area over time with reference to the columns of the considered case study building, which are reinforced with 4f14 longitudinal rebars and f6/140 mm stirrups.

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