Seismic Vulnerability of RC Buildings under the Effect ... - ScienceDirect

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ScienceDirect Procedia Environmental Sciences 38 (2017) 461 – 468

International Conference on Sustainable Synergies from Buildings to the Urban Scale, SBE16

Seismic Vulnerability of RC Buildings under the Effect of Aging S.T. Karapetroua,*, S.D. Fotopouloua, K.D. Pitilakisa a

Aristotle University of Thessaloniki/ Department of Civil Engineering, Thessaloniki 54124, Greece

Abstract The present study aims at the assessment of the seismic vulnerability of reinforced concrete (RC) buildings considering performance degradation over time due to aging effects. Chloride induced corrosion is taken into account based on probabilistic modeling of corrosion initiation time and corrosion rate. Two-dimensional incremental dynamic analysis (IDA) is performed to assess the seismic performance of the initial uncorroded (t=0 years) and corroded (t= 25, 50, 75 years) RC fixed base frame structures designed based on different seismic code levels. The time-dependent fragility functions are derived at the various time periods in terms of spectral acceleration corresponding to the fundamental mode of the structure Sa(T 1,5%) for the immediate occupancy (IO) and collapse prevention (CP) limit states. Results show an overall increase in seismic vulnerability over time due to corrosion indicating the significant effect of deterioration due to aging effects on structural behavior. 2017 ThebyAuthors. Published B.V. © 2017©Published Elsevier B.V. This isby an Elsevier open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility ofthe organizing committee of SBE16. Peer-review under responsibility of the organizing committee of SBE16. Keywords: RC buildings; seismic vulnerability; time-dependent fragility curves; aging effects; IDA

1. Introduction In order to design efficient assessment tools that could be utilized by civil protection authorities, decision makers and end users, a reliable risk model for a region or for a specific structure under consideration needs to be compiled in order to predict future losses due to seismic events with a high accuracy level. In this context, the reliable vulnerability assessment of existing structures and infrastructures is a prerequisite for seismic loss estimation, risk mitigation and management. Vulnerability is commonly expressed through fragility functions representing the probability of exceeding a prescribed level of damage for a wide range of ground motion intensities. Traditionally, it is implicitly assumed that the structures are optimally maintained during their lifetime and the impact of the progressive deterioration due to various time-dependent mechanisms on structural performance is commonly

* Corresponding author. Tel.: +30 2310 994208; fax: +30 2310 994208. E-mail address:[email protected]

1878-0296 © 2017 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of SBE16. doi:10.1016/j.proenv.2017.03.137

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S.T. Karapetrou et al. / Procedia Environmental Sciences 38 (2017) 461 – 468

neglected. Two different sources of deterioration, which cause damage to accumulate with time are generally recognized (e.g. [1]): one has substantially slow, progressive effects and is usually linked to environmental and operating conditions (e.g. aging); the other has effects that are superposed occasionally to the first effects, and are usually related to external sudden changes in the structural capacity (e.g. due to cumulative earthquake damage, e.g. [2]).Owing to deterioration, the physical vulnerability of the system, as may be estimated at the time of construction and at different points in time, is actually time-dependent, and may increase with time, thus causing the risk of structural failure to accelerate. On this basis deterioration of the material properties caused by aggressive environmental attack is not accounted for. One of the primary sources of structural degradation is the corrosion of reinforced concrete (RC) members, generally associated to carbonation process and chloride penetration, leading to the variation of the mechanical properties of steel and concrete over time. In the case of significant loss of ductility due to high corrosion levels, a reduction in the load-carrying capacity of the structure, as well as a shift to more brittle failure mechanisms is expected. Consequently, both safety and serviceability of RC structures may be affected under the action of seismic (or even static) loading, compromising the capability of the structures to withstand the loads for which they are designed. This study aims to highlight the effects of chloride induced reinforcement corrosion on the response and vulnerability of structures subjected to seismic excitation and to derive time-dependent fragility functions for different time-scenarios and building typologies. Different corrosion effects are considered in the analysis including the loss of reinforcement cross-sectional area, the degradation of concrete cover and the reduction of steel ultimate deformation. 2. Aging effects: Corrosion of reinforcement Aging of structures can be defined as partial or total loss of their capacity via a slow, progressive and irreversible process that occurs over a period of time. The effects caused by aging processes lead to the degradation of engineering properties, affecting the static and dynamic response of the structures, their resistance/capacity and failure mode as well as the location of failure initiation. Thus, the ability of the structural system to withstand various challenges from operation, environment and natural events may be reduced. Once the structural capacity falls below a given performance threshold, the structure may be intervened, leading to a new initial capacity, diminishing progressively over time the ability to withstand future operating conditions. Aging processes decrease the reliability of the structural systems over time, accelerating the risk of structural failure. Since the time-dependent changes are random in nature, the safety evaluation of the existing structures can be conducted rationally within a probabilistic framework [3], taking into account various sources of uncertainty with respect to the deterioration process and rate. The rate of degradation of the structural components generally depends on the age of the structure as well as on the exposure conditions, and for its efficient determination stochastic approaches are possible. Overall, the identification of aging structural components and their probabilistic modeling over time may play a significant role in mitigating structural risk. Among the most common environmental deterioration factors, reinforcement corrosion, generally associated to carbonation and chloride ingress, is considered the most significant degradation mechanism, leading to the adverse variation of the mechanical properties of steel and concrete over time [4]. Corrosion is a complex process that may affect a RC structure in a variety of ways, including, among others, cover spalling, loss of steel-concrete bond strength and loss of reinforcement cross sectional area, potentially resulting to the reduction of the resistance and load bearing capacity of the structure and to the variation of the failure mechanism from ductile to fragile type (e.g. [5-7]).The corrosion mechanism is a time-dependent process leading progressively to reduction of the strength and serviceability of structures in relation to their as-built state. The deterioration related to the corrosion of reinforcement steel bars in concrete structures, called hereafter “aging effect”, is basically a two-phase process consisting of the initiation and the propagation phase. As soon as the concentration of chlorides or carbon dioxides exceeds a critical value, the so called “passive layer” protecting the outer reinforcement is destroyed signifying the initiation of corrosion. Then the corrosion is gradually propagating causing the formation of corrosion products (rust), leading progressively to concrete cracking and spalling as the volume of rust increases and finally resulting to significant structural damage. The parameters that affect the corrosion initiation and its progress in time may be categorized based on whether they are associated with the design

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and execution phase (e.g. concrete cover depth, water/cement ratio) or with the environmental exposure (humidity, temperature, carbon dioxide or chlorides concentration) (e.g. [8]). Several models have been proposed to quantify and account for corrosion in the design, construction, fragility analysis and maintenance of RC structures. A summary of these models can be found e.g. in DuraCrete [9]. 2.1. Corrosion modeling Under the aforementioned considerations, the uncertainties involved in corrosion phenomena point out the need for a probabilistic approach to predict the level and effects of degradation [10]. Recognizing the importance of this issue, several probabilistic models have recently been introduced into the time-variant vulnerability assessment of corroded RC structures (e.g.[11], [12]). In the present study the corrosion of reinforcing bars due to the ingress of chlorides is considered, as it is reportedly one of the most serious and widespread deterioration mechanisms of RC structures. The probabilistic model proposed by FIB- CEB Task Group 5.6 [13] is adopted to model corrosion initiation time due to chloride ingress that is expressed as:

Tini

§ § § C D2 ¨ erf 1 ¨1 crit n ¨ ¨ ¨ 4 ke kt DRCM ,0 t0 © Cs © ©

§ 1 · 2 ¨© 1n ¸¹

·· ¸ ¸¸ ¹¹

· ¸ ¸ ¹

(1)

where Tini=corrosion initiation time (years); α=cover depth (mm) ; Ccrit.=critical chloride content expressed as a percentage by weight of cement (wt % cement); Cs = the equilibrium chloride concentration at the concrete surface expressed as a percentage by weight of cement (wt % cement); t0= reference point of time (years); DRCM,0=Chloride migration Coefficient (m2/s); ke=environmental function; kt=transfer variable defined deterministically according to Choe et al. [14] equal to 0.832; erf=Gaussian error function and n=aging exponent. The statistical quantification of the model parameters describing the chloride induced corrosion adopted for the study is presented in detail in Karapetrou [1]. It is noted that the adopted corrosion rate implies a relatively high corrosion level [15]. Once the protective passive film around the reinforcement dissolves due to continued chloride ingress, corrosion initiates and loss of reinforcement cross-sectional area is observed with time. Moreover due to the radial pressure developed along the steel bar surfaces, caused by the increasing volume of the corrosion products, the tensile stresses in the concrete surrounding the rebars may exceed the tensile strength leading thus to the cracking of the concrete cover. Thus, the chloride induced corrosion effects that can be taken into account are the section area loss of reinforced bars (Ghosh and Padgett [11]), the concrete cover strength reduction (Coronelli and Gambarova [16]; Simioni [17]) and the loss of steel ductility (Rodriguez and Andrade [18]). 3. Application study 3.1. Prototype structural models Three moment resisting frames have been selected in the frame of this study to highlight the effects of corrosion reinforcement in the response and vulnerability of RC buildings. They have been designed according to different seismic code levels (Fig. 1) in order to capture the different periods of construction. The SYNER-G (www.synerg.eu) taxonomy for RC structures is used to describe the different building typologies (e.g. [19]). The first one is a three storey-three bay frame model originally designed for the purpose of an experimental study (Bracci et al. [20], total mass m=207t, initial fundamental period T 1=0.98sec, concrete strength fc=24MPa, steel strength fy=276MPa) that is representative of low rise buildings designed for gravity loads only with no seismic provisions (Low rise-No code MRF). The second is a nine storey-three bay frame model (Kappos et al. [21], m=334t, T1=0.89sec, fc=14MPa, fy=400MPa) that is considered typical of high rise buildings with low level of seismic design according to the 1959 Greek seismic code (‘Royal Decree’ of 1959) (High rise-Low code MRF). In the latter regulations, the ductility and the dynamic features of the constructions are completely ignored. Finally, the third one is a four storey-three bay

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frame structure (Kappos et al. [21], m=130t, T1=0.66sec, fc=20MPa, fy=400MPa) that represents mid rise buildings designed following the provisions of the Greek modern seismic code (EAK 2000) (Mid rise-High code MRF). This code is characterized by enhanced level of seismic design and ductile seismic detailing of RC members according to the new generation of seismic codes (similar to Eurocode 8). The numerical modeling of the structure is conducted using OpenSees finite element platform (Mazzoni et al. [22]). Inelastic force-based formulations are employed for the simulation of the nonlinear beam-column frame elements. Distributed material plasticity along the element length is considered based on the fiber approach to represent the cross-sectional behavior. The modified Kent and Park model [23] is used to define the behavior of the concrete fibers, yet different material parameters are adopted for the confined (core) and the unconfined (cover) concrete. The uniaxial ‘Concrete01’ material is used to construct a uniaxial Kent-Scott-Park concrete material object with degraded linear unloading/reloading stiffness according to the work of Karsan-Jirsa [24] with zero tensile strength. The steel reinforcement is modeled using the uniaxial ‘Steel01’ material to represent a uniaxial bilinear steel material with kinematic hardening described by a nonlinear evolution equation. The three models are analyzed assuming fixed base conditions for their un-corroded (t=0 years) and corroded state (t=25, 50, 75 years). The methodology presented in section 2.1 was applied to take into account the effects of corrosion. Based on the probabilistic model proposed by FIB- CEB Task Group 5.6 [13] (Eq. 1), mean values for Tini are estimated as 7.01for the structural models designed with no and low seismic provisions and 14.11 years for the one designed with modern seismic code. The effects of corrosion are assumed to be distributed uniformly around the perimeter and along the concrete members. The distribution of the loss of reinforcement area as well as the reduction in concrete cover strength due to corrosion of the RC elements for the considered corrosion scenarios (t=25, 50, 75 years) are calculated as a function of the corrosion rate and the corrosion initiation time variables.

Fig. 1. Reference MRF models used for time – dependent vulnerability assessment: (a) Low rise-No code, (b) High rise-Low code, (c) Mid rise-High code.

Table 1 summarizes the mean percentages (%) of reinforcement area loss, cover concrete strength and steel ultimate deformation reduction due to corrosion within the elapsed time (t-Tini). Overall, for a given corrosion scenario, it was observed that beams are more affected compared to columns. Furthermore, an increase of the initial fundamental period of the fixed base structures is expected since corrosion effects cause progressive stiffness

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degradation. The structural models under study present a percentage increase in the natural period that varies between 4-17 % for the transition from 0 years to 25 years, 6-9% from 25 to 50 years and 3-5% from 50 to 75 years, depending on the characteristics of the initial and corroded structures. Table 1. Loss in reinforcement area (%), cover concrete strength (%) and steel ultimate deformation (%) for the considered corrosion scenarios. Steel area loss (%) / Cover concrete strength reduction (%) / Steel ultimate deformation reduction (%) t (years)

Low rise -No code

High rise-Low code

Mid rise-High code

25

5 / 47 / 9

6 / 43 / 10

4 / 35 / 6

50

12 / 68 / 20

13 / 64 / 24

10 / 63 / 20

75

17 / 77 / 33

20 / 73 / 35

17 / 74 / 33

3.2. Seismic input motion The selected scenario earthquake consists of a set of 15 real ground motion records obtained from the European Strong-Motion Database (http://www.isesd.hi.is). They are all referring to outcrop conditions recorded at sites classified as rock according to EC8 (soil type A) with moment magnitude (M w) and epicentral distance (R) that range between 5.8