FEM analysis of reinforcement corrosion effects on RC members degradation

Transcription

1 Le Nuove Frontiere del Calcestruzzo Strutturale Università degli Studi di Salerno ACI Italy Chapter, Aprile 2010 FEM analysis of reinforcement corrosion effects on RC members degradation G.P. Lignola 1, G.P. Menichino 2, M. Montuori 3, F. Bellucci 4, E. Cosenza 5 and G. Manfredi 6 ABSTRACT: Sound concrete is an ideal environment for steel but the increased use of deicing salts and the increased concentration of carbon dioxide in modern environments principally due to industrial pollution, has resulted in corrosion of the bars becoming the primary cause of failure of Reinforced Concrete (RC) structures. The scale of this problem has reached alarming proportions in various parts of the world. The present paper summarizes the progress made in a wider research program on corrosion control of reinforcing steel in RC. This paper deals with both experimental activities on single bar/cover concrete specimens and numerical Finite Elements (FE) analyses aiming at demonstrating the possibility to use the results for a single bar also for more complex RC members. Preliminary outcomes of about one hundred parametric analyses allowed providing a methodology to evaluate the parameters to be inserted in single bar simplified model and the maximum expected difference. 1 INTRODUCTION 1.1 Corrosion of steel in concrete Throughout the world Reinforced Concrete (RC) is the most widely used construction material for buildings and civil engineering structures. Most RC structures have performed satisfactory over many decades, but there is still an unacceptable large number of structures that deteriorate prematurely. Reinforcement corrosion is identified to be the foremost cause of deterioration. Many structures in adverse environments have experienced unacceptable loss in serviceability or safety far earlier than expected, due to the corrosion of reinforcing steel and thus need replacement, rehabilitation, or strengthening. Corrosion is an electrochemical reaction between a material, usually a metal and its environment that produces a deterioration of material s proprieties. However, several cases show that RC structures may not perform adequately and are not as durable as expected. The lack of durability is a result of several factors. Among them, it is worth to mention the poor design, and a more severe environment than expected, lack of maintenance, or a combination of these factors. Generally, steel is passivated by a protective oxide due to the alkaline environment provided by the pore solution in concrete and the bulk of surrounding concrete, which acts as a physical barrier towards aggressive agents. The passive state of iron is stable 1 Assistant Professor, Department of Structural Engineering, University of Naples Federico II, Italy 2 PhD Candidate, Department of Materials and Production Engineering, University of Naples Federico II, Italy 3 Post Doc, Department of Materials and Production Engineering, University of Naples Federico II, Italy 4 Full Professor, Department of Materials and Production Engineering, University of Naples Federico II, Italy 5 Full Professor, Department of Structural Engineering, University of Naples Federico II, Italy 6 Full Professor, Department of Structural Engineering, University of Naples Federico II, Italy

2 with time unless the protective environment is modified either by neutralization or by ingress of depassivating ions, such as Cl -. One of the most common destroyers of the passive layer is chlorine (Cl - ), which is introduced into the system through the use of deicing salts such as those used on bridge decks. The most dangerous aspect of chloride induced corrosion is that it is a form of pitting corrosion, meaning that it is highly localized at the point of high chloride concentration. As corrosion affects the exposed portion, it reduces the cross-sectional area of the steel, causing the formation of a weak point. Carbonation induced corrosion, the second major cause of the breakdown of the passive layer, occurs through a multiple step chemical process. First, carbon dioxide, CO2, dissolves into the water in the cement pores and bonds with the water to produce carbonic acid, H2CO3. This acid neutralizes the alkalis in the water in the pores, causing calcium carbonate, CaCO3 to precipitate out of the liquid. This solid precipitate forms a lining around the inside of the pore. In this case the ph drops significantly to a level that induces steel corrosion. When steel corrosion occurs the volume of rust products is about four to six times larger than iron volume. This volume increase induces internal tensile stresses in the cover concrete, and when the tensile stress exceed the tensile strength of the concrete, the concrete cover is damaged by cracking and spalling. In addition to loss of concrete cover, a reinforced-concrete member may suffer structural damage due to loss of bond between steel and concrete and significant reduction of rebar cross-sectional area. As previously described, steel corrosion became a critical issue in the management of RC structures. The high costs related to maintenance and repair of corrosion damaged structures have led to the awareness that steel corrosion has to be prevented since the early design stage in order to reduce the cost of structures. 2 EXPERIMENTAL PROGRAM 2.1 Materials and design of specimen In the experimental phase (Montuori et al. 2009), a large number of concrete cylindrical specimens were casted. Concrete mixtures with a maximum aggregate size of 20 mm and a slump of 70 mm were used. The diameter of the specimens was 100 mm and the height was 150 mm (Fig. 1). A 10 mm diameter rebar was embedded in the center of specimen. Figure 1. Details of tested columns. The concrete mix design is listed in table 1; cement content was 350 Kg/m³ and the water cement ratio was The compressive strength of concrete at 28 days was about 25 MPa. Reinforcing bars were FeB44K steel with nominal yielding strength of 580 MPa. Table 1. Mix design of concrete for specimens. Cement type Aggregates (Kg) Cement (Kg) W/C fcm (MPa) Portland Cement II/B.LL.32,5R

3 In order to study the effect of the corrosion products on the time of concrete cracking, several specimens were placed in chloride environment (immersion in a 3.5%wt aerated sodium chloride solution) in accelerated corrosion condition. The reinforcement bar was connected to the circuit so as to serve as anode in the corrosion cell; the circuit was closed by means of a reference electrode (Saturated Calomelan Electrode SCE) and a stainless steel bars (counter electrode). Anode and cathode were connected to a constant power supply of 5 Volt. The evaluation of the corrosion rate and the time to cracking was carried out by the measurement of the electrical current and the polarization resistance. 2.2 Measurement techniques The degradation of RC specimens was evaluated by electrochemical measurements. The electrical current passing through each specimen (Icorr) was measured by interpolating ampere meters between anode and power supply. The corrosion potentials were monitored using a high impedance voltmeter with a saturated calomel reference electrode (SCE). The corrosion rate (Vcorr) was determined by linear polarization resistance method (LPRM) using a Potentiostat/Galvonostat. The resistance to polarization (Rp) was determined by conducting a linear polarization scan in the range of ± 20 mv of the corrosion potential. The corrosion current density was determined using Stern-Geary formula. B Icorr = (1) R p where: Icorr = corrosion current density, µa/cm²; Rp = polarization resistance, Ω cm²; B = (βa βc)/2.3(βa+βc) where βa and βc are the anodic and cathodic Tafel constants, respectively. At a steel corrosion current density of 1 ma/m² corresponds to a corrosion speed of 1.17 µm/year. For the case of steel embedded in concrete, a value of 26 mv was found for the active state (corrosion), whereas B = 52 mv is more appropriate for passive steel. 2.3 Accelerated corrosion test in chloride environment The corrosion rate of low carbon steel rebar and the corrosion current density (Icorr) circulating in the circuit were plotted against time of exposure in Figure I corr [μa/cm 2 ] C 1 carbon steel rebar C 2 carbon steel rebar Vcorr [μm/year] ,1 0,1 C1 carbon steel rebar C2 carbon steel rebar 0, Time [days] 0, Time [days] Figure 2. Current density and corrosion rate of carbon steel rebar in chloride environment during accelerated corrosion test.

4 The time-current plots were analyzed in order to evaluate the time to cracking of concrete specimens due to reinforcement corrosion, which was taken as the point at which a significant increase in the current or a change in the slope of the time-current was detected. The corrosion current density results indicated that low carbon steel RC specimens showed an increase to a maximum value, approximately after 10 days, of about 1000 µa/cm². At the same time, hairline cracks, filled with corrosion products, were observed on the surface of the specimens (Fig. 3). Simultaneously the Vcorr value increased to about 100µm/year. Figure 3. Cracks on the surface of the cylindrical specimen after accelerated corrosion test. The time to cracking and corrosion rate of the concrete specimens subjected to an impressed potential are presented in table 2. All the acquired experimental data can be used in a useful manner to validate FEM numerical models, and move to virtual testing. Table 2. Time to cracking of the concrete specimens subjected to an impressed potential of + 5 Volt. Specimen Time to Cracking (days) C1 8 C2 6 3 FEM MODELING 3.1 Experimental tests modeling The multipurpose software SIMULIA ABAQUS was used to simulate the tested specimens in the elastic range, to evaluate the beginning of the cracking process in concrete due to the expansion of corroded steel reinforcing bar. Just the concrete cover was modeled because the bar was simply considered as pressure load pushing the inner boundary of the circular crown made of concrete. The mechanical properties needed for concrete were the Poisson ratio, ν, and the Young modulus, Ec. The former was always assumed equal to 0.2, while the latter was related to the concrete strength fcm, according to Italian code (NTC 2008) equation eq. (2); the same code was considered to evaluate the tensile strength of concrete, fct eq. (3). fcm Ec = f = 0.3 f 8 (2) ; (3) ; ( ) 2 3 ct cm Both 3D and 2D (plain strain) analyses (Fig. 4) were performed to evaluate the influence of the modeling type. Always fully comparable results were found so that only 2D plain strain analyses were performed thereafter. An increasing pressure was applied to let the tensile principal stress attain the tensile strength of concrete. The main outcome of the analyses is that the maximum tensile stress is attained close to the steel bar and is in the circumferential direction. This confirms experimental outcomes on single bar system.

5 a) b) Figure 4. a) 3D FEM tensile stresses (horizontal x component); b) 2D FEM tensile stresses (principal stress). It is worth noted that the FEM model is axi-symmetric so that also the stress is uniform in the circumferential direction Parametric analyses The effect of different concrete classes, namely C20/25, C25/30 and C30/37, was analyzed. Furthermore two other variables were considered: three different diameters for the bars, namely D12, D16 and D20 and two different covers of the bar, 20 mm and 30 mm, respectively. The selection of these parameters was driven by the worst (in terms of concrete cover dimensions and concrete strength) exposure classes reported in UNI EN 206-1, assuming the reinforcement corrosion due to carbonation (XC1), chlorides from sea water (XS1) or freeze/thaw attack with de-icing agents (XF2). The outcomes of these 18 analyses are summarized in the following table 3. Class Table 3. Main outcomes of the FEM simulation of the experimental single bar specimen. fctm (MPa) Ec (GPa) Bar diameter (mm) Concrete Cover (mm) Inner Displacement (μm) Inner Pressure (MPa) C20/ C25/ C20/ C25/ C20/ C25/ C20/ C25/ C20/ C25/ C20/ C25/ It is clearly shown that the influence of the tensile strength on internal displacement is weak. In fact, increasing the concrete class from C20/25 to C30/37 the tensile strength increase is about 30%, while the increase in terms of internal displacement is about 19%, almost independently on the bar diameter and concrete cover.

6 Conversely, the increase in terms of internal pressure is about 30%, almost independently on bar diameter and concrete cover, so it has the same increase as the tensile strength of concrete. The influence of the elastic modulus is even more reduced, in fact increasing the class from C20/25 to C30/37, the elastic modulus increase is about 9%. The influence of bar diameter is relevant on internal displacement (extending the bar diameter about 70% from 12 mm to 20 mm, the same increment is found on displacement), while it is almost negligible on inner pressure, and leads to a pressure reduction of about 10%. The influence of concrete cover is negligible on internal displacement (increasing the concrete cover of 50%, the increment on displacement is about 1%), while it leads to a pressure increment ranging between 5% and 10%. 3.2 Real RC members modeling Real RC members have been also numerically simulated. In Figure 5a, the analyzed RC beams are shown, where the reinforcement bars were arranged to fulfill the indications of Eurocode 2 (sec. 8.2): the clear distance, ic, between individual bars and between horizontal layers of bars should not be less than the bar diameter, the aggregate size + 5 mm, or 20 mm, whichever is the greatest. Figure 5b reports the deformed shape of a quarter of the beam after the expansion exerted by the rust formed on the corroded bar. It is worth noted that the corrosion is simulated as a pressure induced by the rust on the concrete circular hole, in place of the bar. It is clearly shown that there is a higher displacement outwards the cross section. It is caused by the constraint exerted by the concrete core to the expansion of rust and, consequently, of concrete. a) b) Figure 5. a) typical beam considered in the analyses; b) deformed shape of a quarter of beam with D12 bar, C20/25 concrete, cc=ic=20 mm. The expansion due to corrosion leads to circumferential tensile stresses around the bars, similarly to the case of single bar specimens. The main aim of this modeling is the comparison between the single bar model and the real case of an RC beam. To provide a comparison, and especially to evaluate the differences between the tensile stress predictions of the simple single bar model, and the effective stress in a real RC beam (given a bar diameter, concrete cover and clear distance), the same pressure applied to the single bar model to reach the tensile strength of concrete, was applied also in the bars slot of the RC beam. Due to modeling issues the pressure load was preferred to the displacement load to avoid forcing the shape of the expanded corroded bar; the scatter in terms of principal tensile stress prediction between the two load schemes (displacement vs. pressure) was always lower than about ±10% Parametric analyses To analyze the effect of different parameters, three clear distances, ic, were considered, namely, 20 mm, 30 mm and 40 mm, while the same concrete covers, cc, con-

7 crete properties and steel bars adopted for parametric analyses were replicated in this case. The outcomes of the subsequent 54 analyses are summarized in the following table 4 showing the main results in the case of real RC beams. Principal tensile stresses are related to the same values of inner pressure adopted in the single bar modeling. Table 4. Main outcomes of the FEM simulation of the real RC beam (in brackets is the increment of predicted principal tensile stresses in real RC beam compared to single bar model). Class fctm Ec Bar diameter Concrete Max Principal Tensile Stress (MPa) (MPa) (GPa) (mm) Cover (mm) ic=20 mm ic=30 mm ic=40 mm C20/ (+16%) (+0%) (-1%) C25/ (+16%) (+0%) (-1%) (+16%) (+0%) (-1%) 12 C20/ (+19%) (+8%) (+3%) C25/ (+19%) (+8%) (+3%) (+19%) (+8%) (+3%) C20/ (+27%) (+17%) (+15%) C25/ (+27%) (+17%) (+15%) (+27%) (+17%) (+15%) 16 C20/ (+30%) (+16%) (+8%) C25/ (+30%) (+16%) (+8%) (+30%) (+16%) (+8%) C20/ (+34%) (+12%) (+0%) C25/ (+34%) (+12%) (+0%) (+34%) (+12%) (+0%) 20 C20/ (+47%) (+23%) (+11%) C25/ (+47%) (+23%) (+11%) (+47%) (+23%) (+11%) Contour plot of principal tensile stresses is provided for 4 combinations only (Fig. 6) of the analyzed parameters, for a given concrete class (results are clearly independent on concrete classes, see Table 4). The maximum tensile stresses are always in the direction of the layer of bars, while in the direction of the concrete cover, that is toward the outside of the cross section, the stresses are usually smaller than about 20% (being the difference smaller in the case of larger clear distances, ic). a) D12 bar, cc=20 mm, ic=20 mm b) D12 bar, cc=20 mm, ic=40 mm c) D12 bar, cc=30 mm, ic=20 mm d) D16 bar, cc=20 mm, ic=40 mm Figure 6. Real RC beam: C20/25 concrete.

8 It is clearly shown that increasing the clear distance, ic, (Fig. 6a vs. Fig. 6b) the bars interaction reduces and also the tensile stress (and stress difference, table 4); increasing the concrete cover (Fig. 6a vs. Fig. 6c) the interaction and the tensile stress (and stress difference, table 4) are almost similar; while increasing the bar diameter (Fig. 6b vs. Fig. 6d), the interaction increases and also the tensile stress (and stress difference, table 4). Figure 7a shows the stress percentage increment between the tensile stresses provided by single bar model and real RC beam simulation (given identical values for the parameters, neglecting the different clear distance values, ic). To reduce the remarkable effect of the clear distance, ic, (leading to stress increments up to 47%) the analyses were repeated considering in this case a concrete cover for the single bar model equal to the minimum between cc and half the clear distance, ic. This means that two virtual circular crowns of concrete are assumed around each bar and they are not intersecting, but, at least, tangent each other. The pressure exerted by the single bar model (under these assumptions) was applied again to the previously analyzed 54 cases, and the stress increment (Fig. 7b) was found to be dramatically reduced and always smaller than about 10%. 50% D12 C=20mm D12 C=30mm 50% D12 C=20mm D12 C=30mm 40% D16 C=20mm D16 C=30mm 40% D16 C=20mm D16 C=30mm 30% D20 C=20mm D20 C=30mm 30% D20 C=20mm D20 C=30mm 20% 20% 10% 10% 0% i c (mm) -10% 0% i c (mm) -10% a) b) Figure 7. Scatter of tensile stresses: considering, in single bar model, a concrete cover: a)cc; b) min(cc;ic/2). 4 CONCLUSIONS The aim of the work is to check the possibility to compare single bar simplified models with real RC members behavior in terms of concrete cracking. An experimental methodology to assess corrosion of steel reinforcing bars in concrete was shown and FEM simulations were performed for both single bar and real beam schemes, stressing the influence of concrete classes, diameters and clear distances of bars and concrete covers on concrete cracking. The proposed methodology was verified by means of about a hundred parametric comparative FEM analyses. The average underestimation of the proposed method is almost negligible only if, in single bar models, concrete cover equal to min(cc;ic/2) is adopted. Maximum underestimation of the model was found equal to about 10% thus still comparable to usual uncertainty on concrete tensile strength. 5 REFERENCES Comite Europeen de Normalisation, EN Concrete Part 1: Specification, performance, production and conformity ; Eurocode 2. Design of concrete structures Part 1-1 General rules and rules for buildings. Bruxelles: (EN :2004: E); NTC Ministero delle Infrastrutture. DM 14 gennaio 2008, Norme tecniche per le costruzioni, Suppl. or. n.30 alla G.U. n.29 del 4/2/2008 (in Italian). M. Montuori, G. Serroni, T. Monetta and F. Bellucci, Corrosion of Carbon Steel and Galvanized Rebar on RC Columns Wrapped with FRP Sheets, EUROCORR, September 2009, Nice.