Corrosion of reinforcing steel is widely accepted as the

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1 Strength eterioration of Reinfored Conrete Columns Exposed to Chloride Attak R. Greo, G.C. Marano Abstrat This paper fouses on reinfored onrete olumns load arrying apaity degradation over time due to hloride indued steel pitting orrosion. The strutural element is exposed to marine environment and the effets of orrosion are desribed by the time degradation of the axial-bending interation diagram. Beause hlorides ingress and onsequent pitting orrosion propagation are both time-dependent mehanisms, the study adopts a time-variant preditive approah to evaluate residual strength of orroded reinfored onrete olumns at different lifetimes. Corrosion initiation and propagation proess is modelled by taking into aount all the parameters, suh as external environmental onditions, onrete mix proportion, onrete over and so on, whih influene the time evolution of the orrosion phenomenon and its effets on the residual strength of RC olumns. Keywords onrete strutures, diffusion oeffiient, marine environment, pitting orrosion, surfae hloride onentration, strength deterioration. I. INTROUCTION Corrosion of reinforing steel is widely aepted as the main reason of Reinfored Conrete (RC) strutures premature deterioration. One of the main soures of orrosion phenomenon is exposition to hloride attak in marine environments and de-iing salts. In these irumstanes, RC members may undergo strutural strength degradation due to loss of steel rebar ross setional area and loss of bond between steel and onrete. While the strength redution of onrete members suffering orrosion an be diretly related to the redution of the rebar area, the effets on stiffness and dutility of the overall struture are assoiated with omplex mehanisms. These inlude lak of onfinement due to orrosion of transversal reinforements and bond deterioration between reinforement and surrounding onrete. These ombined mehanisms an signifiantly redue in long-term loal strength and global dutility and modify sensibly the strutural performane during lifetime. Moreover at present, for design of RC elements in view of speial servieability limit states, modern Codes and Standards [1] define all possible soures of RC elements deterioration and group them in environmental lasses. Moreover, for eah lass different sub-lasses are defined in relation to the aggressive level of the degradation soure. Then, for eah lass some rules and limitations are imposed for strutural design. These regard mix proportion, materials mehanial properties, working and uring onditions, minimal strutural dimensions and so on. In other words, one the soures of the hemial and/or physial attaks have been identified, Tehnial Codes [1] impose to designers speifi requirements in projets, typially onerning the minimum onrete over, the maximum water/ement ratio, the minimum ement ontent, the rak width limitation, the air ontent, the ement type, the oating of onrete surfae and others. However, no proedure is provided to assess the time dereasing of strutural load arrying apaity with orrosion proess evolution. The present study fouses on the evaluation of residual strength of RC olumns subjet to pitting orrosion in hloride aggressive environments. Beause hlorides ingress and onsequent pitting orrosion propagation are both timedependent mehanisms, the study adopts a time-variant preditive approah to evaluate residual strength of orroded olumns at different lifetimes. Even if the approah here presented an be developed for all hloride indued orrosion senarios, the study fouses on marine environments and their orresponding lasses, as urrently defined by European Standards [1]. In more details, the residual strength of RC olumns designed aording to presriptions for XS exposure lasses is evaluated in terms of axial-bending interation diagrams at seleted lifetimes. Therefore, for eah XS exposure lass, a onsistent orrosion initiation and propagation model is developed, taking into aount all the relevant fators affeting the orrosion phenomenon. These are the minimum water/ement ratio, minimum over and minimum ement ontent presribed for the speifi lass, ritial hloride onentration, surfae hloride onentration haraterizing the exposure lass and others. Based on this model, time-variant axial-bending interation diagrams for olumns subjet to pitting attak in marine environments are obtained, showing strutural load arrying apaity redution over time. II. MOELLING OF PITTING CORROSION A typial ause of RC elements degradation in aggressive hloride environments is pitting (or loalized) steel orrosion. Pitting generates an iron oxide that is different from rust resulting from uniform orrosion; it presents a lower volume for unit mass, therefore it is not able to ause the disruption of the onrete over. In partiular, salts pray in marine environments and the use of deiing salts may generate ISBN:

2 hlorides penetrating the onrete over and initiating orrosion. Chlorides from the environment penetrate the onrete, produing a onentration profile haraterized by high hloride ontent at the exterior surfae and dereasing ontent at a growing depth. The hloride ontent needs to arrive at a ritial value to begin on the steel surfae the orrosion proess. After this starting event, pitting orrosion propagates and indues loss of steel rebar ross-setional area. ifferent experimental investigations show that the maximum pit depth due to pitting orrosion, P max, is greater than the one due to a general orrosion, P av. The pitting fator R [] is defined as: Pmax R = > 1 (1) Pav The maximum pit depth in a reinforing bar is given by: p(t) =.116i orr Rt () In equation (1) p is in mm, i orr is the orrosion rate, expressed as urrent density in µ A / m (for steel, 1 µ A / m 11.6 µa / yr ) t is the time (in year) sine orrosion initiation time. Vu and Stewart [3] give the orrosion rate: ( ) 1.54 (1) w i orr = (3) where w is the water/ement ratio and is the depth of the reinforement (m). However, the orrosion rate is not a fixed parameter as its value dereases with time. uprat [4] used a dereasing fator of t.3. In this study, the following law is adopted: (1).9 iorr( t) =.85iorrt (4) In order to define the residual area of a steel bar subjet to pitting orrosion, the hemispherial- form model represented in Fig.1 is assumed [5]. Through geometrial onsiderations, it is possible to obtain residual steel bar area for a maximum pit depth p(t) for a bar with diameter. pt () at () = pt () 1 (5) A1+ A if p() t π Apit () t = A + A if p() t 4 π if p() t 4 1 at () at () ϑ1( t) = arsin ; ϑ( t) = arsin pt () (6) (7) pt A1() t =.5 ϑ1() t at () A t ϑ t pt at () =.5 () () () () pt () π Ast () t = Apit () t (9) 4 Fig. 1 Pitting orrosion model ifferent models for hloride penetration have been proposed. A simplified proedure is based on the hypothesis that the ingress of hlorides is a diffusion proess modeled by means of Fik s seond law, as first realized by Collepardi et al. [6]: x C(x, t) = Cs 1 erf (1) t a (where C(x,t) is the hloride ontent (kg/m3) at distane x (m) from the surfae at a generi time instant t (s); Cs is the surfae hloride onentration (kg/m3) and a is the apparent diffusion oeffiient (m/. Although equation (5) adequately desribes the behavior of the onentration of hlorides in the RC strutures, the assumption of a onstant diffusion oeffiient restrits the use of this equation. The hloride diffusion oeffiient dereases with time due to several issues suh as ontinued hydration and hloride binding. Therefore, the diffusion oeffiient ould be written as a power funtion [7]: m tref m () t = ref = Kt t (11) where (t) is the diffusion oeffiient at time t; ref is the diffusion oeffiient at the referene time t ref and m is a onstant that depends on the mix proportions. K inorporates all the onstants and is defined as the effetive diffusion oeffiient at time t ref. Generally, is evaluated onsidering the effetive diffusion oeffiient at 8 days: K ( w ) ref (8) = 1 (1) The ageing oeffiient m in equation (11) is: m =.5w.6 (13) For a more realisti predition of hloride diffusion in onrete, therefore the time dependene of the diffusion oeffiient needs to be inorporated into the analysis proedure, thus properly deriving Fik s seond law. This model was developed by Mangat and Molloy [8], leading to the following equation: (8) ISBN:

3 x C(x, t) = C s 1 erf K t 1 m 1 m III. CRITICAL ION CONCENTRATION (14) The ritial hloride ion onentration (or hloride onentration threshold level) C rit is one of the main parameters affeting the servie life of RC strutures. In fat, when a ertain amount of hlorides, ommonly referred "ritial hloride ontent" C rit, penetrates into the onrete over at the level of the reinforement deep, there is a high probability of depassivation. Shiessl and Raupah [9] stated that the ritial hloride ion onentration ould be defined as the hloride ontent that was neessary to sustain loal passive film breakdown at the steel depth before the proess of orrosion initiation. JSCE [1] defined the ritial value of 1. kg/m 3 to initiate reinforement orrosion. In effet, ritial hloride value depends on the roughness of steel surfae, onrete properties, and the aggressiveness of the environment. Even if the methodology proposed in this study an be applied for all environments the study will be arried out onsidering hloride attak in marine environments and their orresponding lasses, aording to definitions urrently given by the European Standards EN [1] (Table I). TABLE I EXPOSURE CLASSES RELATE CORROSION INUCE BY CHLORIES FROM SEA WATER Class esription Informative examples XS1 Exposed to airborne salt Strutures near to or on the ost but not in diret ontat with sea water XS Permanently submerged Parts of marine strutures XS3 Tidal, splash and spray zones Parts of marine strutures In the following, the environmental exposure lass XS (orrosion indued by hlorides from sea water) will be onsidered. Three sublasses are distinguished: XS1, areas exposed to airborne salt but not in diret ontat with sea water; XS, permanently submerged areas; and XS3, tidal, splash and spray zones. With regard to C rit, the following values are adopted in this study aording to eah exposure lass and w ratio [11]. TABLE II CRITICAL CHLORIE CONTENT CCRIT FOR XS EXPOSURE CLASSES Water/ement ratio XS1; XS XS3 w.3.6%.5%.3 w.4.5%.4% w.4.4%.3% The surfae hloride ion onentration depends on many fators, for example, the distane of the struture from the sea, the region of struture (i.e., atmospheri, tidal, splash, or submerged zones), and onrete properties. Several researhers have proposed lose-formed solutions for both time independent and dependent surfae hloride ion onentration. Costa and Appleton [1] presented an experimental study where the parameters used in the penetration model were alibrated to allow the predition of long term hloride ontent in onrete. In the study, the surfae hloride models were onsidered depending on onrete mix proportion and exposure onditions. The results showed however that the onrete mix does not affet signifiantly the surfae hloride ontent. In this study the values listed in table 3 are adopted for surfae hloride onentration in marine exposure lasses XS [11]. In table III, nominal values and ovariane oeffiients of surfae hloride ion onentration for eah environmental lass are furnished. These data are used to define a nominal and a worst environment senario, respetively: nominal senario is obtained onsidering nominal value of hloride surfae onentration, while in worst senario, hloride surfae onentration is defined as µ + ov µ. TABLE III SURFACE CHLORIE ION CONCENTRATION IN XS EXPOSURE CLASSES Exposure lass Nominal value Cov XS1 XS XS3,31% (weight of 6,93% (weight of 4,5% (weight of IV. AXIAL BENING INTERACTION IAGRAM FOR ETERIORATE RC COLUMNS IN XS EXPOSURE CLASS For design of RC elements in view of speial servieability limit states suh as orrosion due to hloride penetration, urrent Tehnial Codes impose some rules and limitations. These presriptions regard materials ompositions and their mehanial properties, working and uring onditions, minimal strutural dimensions and so on. In other words, one the soures of the hemial and/or physial attaks have been identified, Tehnial Codes impose to designers speifi requirements in projets, typially onerning the minimum onrete over, the maximum water/ement ratio, the minimum ement ontent, the rak width limitation, the air ontent, the ement type, the oating of onrete surfae and others. Therefore, one the aggressive environment has been fixed, designers an define the maximum water/ement ratio, the strength lass, the minimum ement ontent and the minimum onrete over, as in Table IV. Presription in table 4 are appliable for an assigned expeted lifetime for ommon strutures (5, and for Portland ement. For strutures whose expeted servie life is 1 years, a larger minimum onrete over is speified.. ISBN:

4 Class TABLE IV INICATIVE REQUIREMENTS FOR XS CLASSES Max w/ ratio Strength lass (MPa) Minimum ement ontent (kg/m 3 ) Minimum onrete over (mm) 35 (45 mm 1 XS1.5 C3/37 3 4(5 mm 1 XS.45 C35/ (55 mm 1 XS3.45 C35/45 34 In this study a preditive model to evaluate deterioration of load arrying apaity of olumns designed in XS exposure lasses is developed. The initiation and propagation model is elaborated for XS1 exposure lass assigning the admissible minimum values of onrete over and ement ontent, and the maximum value of w ratio (Table IV). Moreover, ritial hloride onentration and hloride surfae onentration are assumed aording to literature data for the seleted lass. It is obvious that the quality of the evaluation depends strongly on input data. In ase of hloride ingress models, a wide variation in some of involved parameters exists. Partiularly, the surfae hloride onentration is one of the less ertain parameter ([14], [15]). stresses by the orresponding areas on whih they at. Finally, the alulus resistant axial fore Prd is omputed by adding fores in onrete and steel, and the alulus resistant bending moment Mrd is alulated by adding moments of these fores around setion plasti entroid. The ross setion of reinforing bars varies with the time aording to equations (9). Fig. 3 Conrete and steel alulus stress-strain diagrams Fig. Retangular RC setion whit symmetri reinforing steel area: ultimate strain distribution diagrams and orrespondent resultant fores Interation diagrams for RC setion are generally omputed by assuming a series of strain distributions, eah orresponding to a partiular point of the interation diagram, and then omputing the orresponding values P rd and M rd. One enough suh points have been omputed, the results are summarized in an interation diagram. A retangular ross setion whit symmetri reinforing steel area is onsidered in this study (figure ). For onrete, the parabola retangle stress strain diagram is adopted with alulus ompression strength fd. The maximum ompressive strain is set at.35 for neutral axis internal to the setion and varying between.35 and. for neutral axis external to the setion (valid for fk lower than 5MPa, where fk is the onrete harateristi ube strength). For steel, the stress strain diagram is desribed by an elasti perfetly plasti model in both tension and ompression, with the following alulus parameters: yielding strength fyd =391,3 MPa; elasti modulus Es=.MPa (figure 3). The loation of the neutral axis and the strain in eah level of reinforement are omputed from the strain distribution. This information is then used to ompute the size of the ompression region and the stress in eah layer of reinforement. The fores in the onrete and steel layers are omputed by multiplying the A RC olumn with a retangular setion with dimensions b=3m and h=5m and symmetri reinforing steel area is examined. The aim is to investigate the evolution of the axialbending interation diagram as onsequene of the pitting phenomenon indued by hlorides. Calulus ompression strength of the onrete is fd=14,167 MPa. It is assumed that the strutural element has been designed aording to the requirements on minimum onrete over, maximum water/ement ratio and minimum ement ontent for XS1 lass (Table 1) for 5 years expeted lifetime. Axial-bending interation diagram is evaluated at t= years, t=5 years, t=1 years and t=15 years. Figure shows time-variant interation diagrams for the RC olumn in the XS1 exposure lass assuming a nominal aggressive senario. A low pitting fator (R=4) is onsidered. Results show that the strength of the olumn designed for a servie life of 5 years dereases over the target lifetime and obvious over inreasing lifetimes (1 years and 15. This result points out that presriptive limits onerning the minimum onrete over, the maximum water/ement ratio and the minimum ement ontent for the XS1 lass don t guarantee that initial load arrying apaity of the olumn is the same over 5 years. If the ultimate strength orresponding to balaned rupture is onsidered, the redution over the time is reported in Table 5 with referene to Mrd. The ultimate bending Mrd after 5 years is redued of 3.65%. ISBN:

5 Fig. Time -variant interation diagrams for XS1 lass [9] Shiessl P, Raupah M.. Influene of onrete omposition and mirolimate on the ritial hloride ontent in onrete. In Pro. 3rd Int. Symp. "Corrosion of Reinforement in Conrete". Wishaw, UK. Elsevier Applied Siene, (199) [1] Japan Soiety of Civil Engineering: Standard speifiations for onrete strutures-, Materials and onstrution () [11] Marques PF, Costa A, Lanata F: Servie life of RC strutures: hloride indued orrosion: presriptive versus performane-based methodologies. Materials and Struture 45, (1). [1] Costa A, Appleton J: Chloride penetration into onrete in marine environment - Part I1: Predition of long term hloride penetration. Materials and Strutures 3, (1999) [13] Marano GC, Quaranta G, Sgobba S, Greo R, Mezzina M. Fuzzy reliability analysis of RC strutures by using an improved timedependent model of hloride ingress. Struture and Infrastruture Engineering 6(1-): 5-3,(1). [14] Marano GC, Quaranta G, Mezzina M: Fuzzy Time-ependent Reliability Analysis of RC Beams Subjet to Pitting Corrosion. Journal of Materials in Civil Engineering, (9): , (8). Corrosion of steel rebar is the most ommon ause of deterioration of RC members. It is the primary state that limits the servie life of onrete strutures, partiularly, in severe environments suh as marine ones. This paper presents a strength deterioration evaluation method for RC olumns exposed to hloride pitting attak in marine environments, obtaining time-variant axial-bending interation diagrams. The initiation and propagation model is elaborated for XS exposure lasses, assigning the admissible minimum values of onrete over and ement ontent, and the maximum value of w ratio presribed for eah lass. Moreover, ritial hloride onentration and hloride surfae onentration are assumed aording to literature data for the seleted lasses. The results obtained show that presriptive limits for 5 years are unable to guarantee strutural performanes over these target lifetimes. More in detail, the minimum onrete over presribed for 5 years lifetime is insuffiient and therefore it should be inreased to guarantee unhanged performane over this target lifetime. Moreover, also the maximum water/ement ratio seems to be inadequate. The proposed model furnishes a good quantitative estimate of the remaining strength of orroded onrete olumns members, whih helps in establishing design details. REFERENCES [1] EN Euroode esign of onrete strutures. Part 1-1: general rules and rules for buildings. CEN, Brussels (4) [] Gonzalez, J.A., Andrade C., Alonso C., Feliu S.: Comparison of rates of general orrosion and maximum pitting penetration on onrete embedded steel reinforement. Cement Conrete Researh 5 (), (1995) [3] Vu, K.A., Stewart, M.G.: Strutural reliability of onrete bridges inluding improved hloride indued orrosion models. Strutural Safety, (4) [4] uprat, F.: Reliability of RC beams under hloride ingress. Constrution and Building Materials 1 (8), (7) [5] Val,.V., Melhers, R.E.: Reliability of deteriorating RC slab bridges. Journal of Strutural Engineering 13 (1), () [6] Collepardi, C.M., Marialis, A., Turriziani, R.: Penetration of hloride ions into ement pastes and onrete. Journal Amerian Cerami Soiety 55 (1), (197) [7] Tapan, M., Aboutaha, R.S.: Load arrying apaity of deteriorated reinfored onrete olumns. Computers and Conrete 6(6), (9) [8] Mangat, P.S., Molloy, B.T.: Predition of longterm hloride onentration in onrete. Material Strutures 7 (6), (1994) ISBN: