CONSIDERATIONS ON THE THRESHOLD CHLORIDE CONTENT VALUES ON THE CORROSION OF STEEL BARS IN CONCRETE

Transcription

1 CONSIDERATIONS ON THE THRESHOLD CHLORIDE CONTENT VALUES ON THE CORROSION OF STEEL BARS IN CONCRETE N.Otsuki*, Tokyo Institute of Technology, Japan T. Nishida, Tokyo Institute of Technology, Japan M. Madlangbayan, Tokyo Institute of Technology, Japan 32nd Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2007, Singapore Article Online Id: The online version of this article can be found at: This article is brought to you with the support of Singapore Concrete Institute All Rights reserved for CI Premier PTE LTD You are not Allowed to re distribute or re sale the article in any format without written approval of CI Premier PTE LTD Visit Our Website for more information

2 32 nd Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2007, Singapore CONSIDERATIONS ON THE THRESHOLD CHLORIDE CONTENT VALUES ON THE CORROSION OF STEEL BARS IN CONCRETE N.Otsuki*, Tokyo Institute of Technology, Japan T. Nishida, Tokyo Institute of Technology, Japan M. Madlangbayan, Tokyo Institute of Technology, Japan Abstract In some recommendations, including those of the JSCE and ACI standards, the threshold content values of chloride ion on the corrosion of steel bars in concrete are introduced, partly because of practical reasons. The readers might believe that this value is so accurate that values over this limit will directly lead to loss of passivity and the corrosion of steel bar can immediately start. However, this belief is not true. This paper is aimed at reconfirming the meaning of the threshold content values, including the statistical nature of the values and the influence of ITZ and kind of cement on the values. Special emphasis is given to the influence of ITZ as clarified from the intensive laboratory and exposure tests. Keywords: deterioration process, temperature, reinforced concrete, chloride induced corrosion, Cl - diffusivity, corrosion rate, Arrhenius theory, activation energy 1. Introduction It is very important to determine the threshold chloride content value on the corrosion of steel bars in concrete for many reasons, especially to predict the incubation period. In the JSCE standard, the stated value for threshold chloride content is 1.2kg/m 3 of concrete. However, this value is considered to be on the safer side and not theoretical at all. In this paper, some considerations on this value are presented for a better understanding on this matter. The considerations are as follows; - The determination methods of the threshold values are important. - The solution and ion concentration of the solution around the steel bar should be more important. - The ions other than chloride ion also have an influence on the corrosion. - The soluble ions affect the corrosion but the fixed ions do not. - The threshold chloride content values on the corrosion of steel bars in concrete show the probabilistic nature. - In existing structures, the threshold values were found to be more than 10kg/m 3. - The influence of bleeding is very large. So, the influence of interfacial transition zone is also very large. - The effect of temperature is also very large. 2. Determination methods of the threshold values 2.1 Conventional methods There have been many methods and they can be classified into 2 categories.

3 1) Visual observation The procedure of this type is shown below; - Manufacturing of concrete (or mortar) specimens with steel bar(s) included (pre-included or after included). - Splitting of the specimen and observing the steel bar(s) whether they are corroded or not. - Also, checking the chloride content. - Then, judging whether the chloride content is below threshold value or not. 2) Abrupt change of the mix potentials The procedure of this type is shown below; - Manufacturing of concrete (or mortar) specimens with steel bar(s) included (pre-included or after included). - Measuring the mix potentials of the embedded steel bar(s) continuously as shown in Fig.1 (by K.Horiguchi, T.Maruya and K.Takewaka). - Checking the time of abrupt change of the mix potential. - Then measuring the chloride concentration of the concrete near the bar, this value thus becomes the threshold value Half-cell Potential (mv CSE) Single Steel Bar-Concrete Cover 20mm -Cell 25mm-Dry Condition Multiple Steel Bars-Concrete Cover 20mm -Cell 25mm-Wet Condition Multiple Steel Bars-Concrete Cover 25mm -Cell 25mm-Wet Condition Time (h) Fig. 1 Measured mix potentials continuously over time 2.2 Evaluation of passivity The most theoretical method to determine the threshold value is to check the passivity. And the evaluation of passivity is also a very difficult matter and should be discussed. In this paper, however, the comprehensive evaluation method proposed by one of the authors is adapted [1]. This method is based on the study of the potentiostatic anodic polarization curves (with 1mV/sec of the scanning speed) of mild steel bars in different solutions. The solutions were; a) saturated calcium hydroxide solution (it is considered that the passivation film around steel bars would exist in this solution), b) extracted cement paste solution (ph=12), c) 0.1% NaCl solution, d) 3.6% NaCl solution, e) 1.8% NaCl in extracted cement paste solution, f) 3.6% NaCl in extracted cement paste solution. A passivation film is generally considered to exist in solutions a) and b), whereas for solutions c) through f), it is unlikely to exist. The anodic polarizations curves of steel bars in each solution are shown in Fig 2. Based on Fig. 2, the passivity of a steel bar in concrete could be graded as shown in Fig. 3. The details of the passivity grades are as follows; Grade 0:(no passivity exists): potentials between 0.2 and 0.6V (vs.sce), current density is over 100μA/cm 2 at least one time. Grade 1:(certain degree of passivity exists): potential between 0.2 and 0.6V, current density is

4 between 10 to 100μA/cm 2. Grade 2:(certain degree of passivity exists): potential between 0.2 and 0.6V, current density is over 10μA/cm 2 at least once but not qualified to Grades 0 and 1. Grade 3:(certain degree of passivity exists): potential between 0.2 and 0.6V, current density is between 1 to 10μA/cm 2. Grade 4:(certain degree of passivity exists): potential between 0.2 and 0.6V, current density is over 1μA/cm 2 at least once but not qualified to Grades 0-3. Grade 5:(excellent passivity exists): potential between 0.2 and 0.6V, current density is less than 1μA/cm 2. Potential vs S.C.E(V) a Saturated Ca(OH) 2 solution b Extracted cement c 0.1 % NaCl solution d 3.6% NaCl solution e 1.8% NaCl in extracted cement solution f 3.6 % NaCl in extracted cement solution a d c b e f Electrode Potential (V vs S.C.E) Grade 5 Grade 4 Grade 3 Grade 2 Grade 1 Grade Current Density(nA/cm 2 ) Current Density (μa/cm 2 ) Fig. 2 Polarization curves of steel bars in various solutions Fig. 3 Concept of passivity grade 3. Influence of Chloride concentration around the steel bar There have been many methods to measure the chloride contents, but almost all are aimed at measuring the contents of chloride in concrete, mortar or paste. The authors believe that the chloride content of the solution around the steel bar is most influential one. So, in this paper, measurement of the chloride concentration around steel bars is proposed. 3.1 Measurement of the chloride concentration around steel bar Mortar specimens, 50x100 mm cylinders, each embedded with a 50 mm long round bar, were prepared as shown in Fig. 4. The chemical composition of the steel bar and the mix proportions of mortar are shown in Tables 1 and 2 respectively. The cement type used is OPC Reinforcement Mortar (unit in mm) Fig. 4 Mortar specimen with steel bar Table 1. Chemical composition of steel bars. Chemical composition, percent C Si Mn P S Table 2. Mix proportions of mortar. W/C Water, kg Cement, kg Fine, kg

5 After 28 days curing in water, the specimens were immersed in NaCl solution (20,000 ppm), and after the specified immersion period, the chloride concentration at the surface of the round steel bar was measured as shown in Fig. 5. Pincette Sample container Electronic balance Taking out the reinforcing bar Weighing Distilled water Reweighing after drying for 24 hr in a desiccators after the weighing (measurement of adherent) Washing with distilled water Measuring the Cl - concentration in wash liquor Fig. 5 Outline of the measurement of chloride concentration around steel bar 3.2 Relation between the passivity grade and the chloride concentration around the steel bar Figure 6 shows the relationship between chloride concentration around steel bar and the depth of color change measured by the silver nitrate spray method. In this case the cover depth of the steel bar is 20.5mm. Also in our previous study [1], the region of color change shown by the silver nitrate spray method was confirmed to have a (free chloride)/(cement) ratio (weight basis) of more than 0.15%. Table 3 shows the relationship between the depth of color change region and the passivity grade. Figure 6 shows that in the region where (free chloride/ cement) is less than 0.15%, the chloride concentration around steel bar is less than 10,000ppm, and from Table 3 there exists a passivity with a possibility of 100%. Also if the chloride concentration is over 10,000ppm, the possibility of having passivity is almost 0 %. So the authors are confident that if the chloride concentration around the steel bar was measured, the possibility of having passivity can be evaluated with a good possibility in the case of using ordinary portland cement. Cl - Depth (mm) Cl - around Steel (ppm) Fig. 6 Relationship between chloride concentration around steel bar and discolored distance Table 3. Relation between the passivity grade and color changed depth (grade 0 means no passivity). Cl - depth, mm Grade, percent

6 4. Probabilistic nature of the threshold chloride content values 4.1 Outline The influence of soluble chloride ion concentration of mortar on the passivity was investigated at 20 o C temperature. A total of 226 specimens with various contents of chloride ion were fabricated and tested. 4.2 Specimens A total of 226 mortar specimens (40x40x160 mm) with 100 mm long (9mm diameter) round bar and 15mm cover depth were fabricated. The cement used was ordinary portland cement, and the sand used was from Chiba prefecture. Also for controlling the concentration of chloride, sea water was partly used as mixing water. The s/a of the mortar was fixed at 2.0 and the water cement ratios were 0.40, 0.45, 0.50, 0.55, 0.60 and Also, the chloride contents of the mixing water were changed by using tap water, sea water and condensed sea water. Mortar was mixed with 20-liter mortar mixer and placed in the mold in which the steel bar was set as a cover depth of 15mm. Then the specimens were cured in room temperature condition (20 o C and 80%RH). After the curing process, the specimens were exposed in tap water or in sea water. The properties of cement are shown in Table 4. Table 4. The properties of cement. Specific gravity Specific surface (cm 3 /g) Stability Compressive strength (N/mm 2 ) days days days GOOD Measurement of chloride contents and passivity The mortar around the steel bar was taken and the chloride ion was extracted in 20 o C distilled water for 24 hours. In this case the targeted chloride ion is water soluble ion. And the ratio of (water soluble ion/ total chloride ion) was about 36%. The passivity of the steel bar was measured using the method mentioned in Chapter 2. In this case, when the passivity grade was judged as 0, the situation was described to be without passivity or passivity does not exist, and when the passivity grade was judged to be 1 to 5, the situation was described to be with passivity or passivity exists. 4.4 Experimental results The experimental results of the relation between the existence of the passivity of the steel bar and chloride content in concrete with the assumption of the mortar content in 1m 3 concrete is 960 kg is shown in Table 5. Based on Table 5, the distribution of probability obtained by logistic regression analysis is shown in Figure 7. In this figure, line 1 is obtained with the original data and line 2 is obtained without the data ( ) between 2.10 and The probability ofspecim en with passivity(%) P1=1/(1+EXP(-z)) z= x 2P2=1/(1+EXP(-z)) z= x Chloride contentin concrete(kg/m 3 ) Fig. 7 Soluble chloride content and the possibility of passivity.

7 Table 5. Experimental results of the relation between existence of the passivity of steel bar and chloride content in concrete. Chloride content in concrete (kg/m 3 ) Average (kg/m 3 ) Steel bar in concrete with passivity (105 specimens) Steel bar in concrete without passivity (118 specimens) Ratio of steel bar in concrete without passivity Ratio of steel bar in concrete with passivity Number of specimens < 1.20 G 1340kg/m 3 (Mortar=960kg/m 3 ) 1.20< < < < < 5.70 From the experimental data, when the chloride content is less than 0.3kg/m 3, there is 100% passivity and over 9.3kg/m 3 there is no possibility of passivity at all. In the case of logistic regression analysis, even though the chloride content is less than 0.3 kg/m 3, there is still about 10% possibility of non passivity. Here, further discussions are difficult to make and the authors would like to show that the existence of passivity seems to be very probabilistic. 5. Durability of PC piles exposed in Kobe port for 37 years [2] Four PC piles (denoted as A-1, A-2,B-1 and B-2) were tested. Fig. 8 shows the cross section of the PC piles. As shown in this figure, A-type is smaller than B-type. According to the survey for the former engineers who were involved in the design and construction of these piles, the estimated unit cement content was 450 kg/m 3, water cement ratio was 0.45, and the slump was 8 to 10 cm. River sand and gravel (maximum size: about 25mm) were used as fine and coarse aggregates. High-early strength portland cement was used. The piles were exposed to marine environment such that their vertical profiles were divided into four different environmental zones, such as 1) splash zone, 2) tidal zone, 3) submerged zone, and 4) under ground zone. Core samples, whose diameter and length were 75mm and 150mm respectively, were taken form these piles. Test items for the cores were 1) compressive strength and modulus of elasticity, 2) chloride ion concentration distribution, 3) ultimate flexural moment capacity, and 4) corrosion of pre-stressing steel wires. 5.70< < < < < < < >12.0 Type-A-- Diameter 664mm Type-B-- Diameter 748mm PC wire In A-type piles where the cover depths were less than 15mm, several longitudinal cracks were observed. However, in areas where concrete cover depth was larger than 20mm, no cracks were observed. Fig. 8. Cross section of PC piles

8 In Table 6, compressive strength and Young s modulus of concrete cores after 37 years exposure are shown. Roughly speaking, the values of compressive strength and Young s modulus are satisfactory, however, in splash zone, the properties are less than those of the other zones. But the strength is still over 65 MPa. Table 6. Compressive strength and Young s modulus of concrete cores after 37 years exposure. Pile No. Zone Compressive Strength Young s Modulus (x10 4 A-1 A-2 B-1 B-2 Splash Tidal Submerged Underground Splash Tidal Submerged Underground Splash Tidal Submerged Underground Splash Tidal Submerged Underground (MPa) MPa) In Fig. 9, the acid soluble chloride concentrations (nearly equal to the total chloride concentrations) are shown against the depth of concrete cover. At the cover depth of 20mm, where no corrosion was observed, the chloride concentration was about 10 kg/m 3. So, in this case, the threshold value was over 10 kg/m Influence of ITZ around Horizontal Steel Bars on Corrosion behavior in Concrete 6.1 Purposes The influence of ITZ or gap at steel-concrete interface was reported significant [3]. Also as shown in Fig.10 [4], the void percentage in ITZ has a significant effect on the threshold value. Then, the purposes of this research were set as follows: (1) To reconfirm the influence of chloride ion concentration on the corrosion of horizontal steel bars (kind of patterns), (2) To reconfirm the influence of ITZ around the horizontal steel bars on the corrosion in concrete, and (3) To reconfirm the combined effect of chloride concentration and ITZ thickness on corrosion of horizontal steel bars in concrete. Acid-soluble chloride content (kg/m 3 ) Tidal Submerged Underground Depth from the concrete surface(cm) 18 Fig. 9 Acid soluble chloride concentrations in PC piles Fig. 10 Chloride threshold values vs. void percentage in ITZ

9 Epoxy resin Lead wire Ammeter A A Steel Bar (X Type, Bamboo Type, Round Type) Fig. 11 Outline of divided steel bar used in this study 400mm 50mm 100mm 200mm 100mm 200mm 750mm 100mm 100mm 1500mm Fig. 12 Outline of Type-A and Type-B specimens 750mm 50mm 150mm 150mm 6.2 Experimental details Generally the corrosion formation of steel bars in concrete is divided into macrocell and microcell. In the case of macrocell corrosion, the anodic region is clearly separated from the cathodic region. On the other hand, in the case of microcell corrosion, the anodic region and cathodic region are uniformly distributed on the surface of steel. In this study these corrosion formations were separately evaluated using divided steel bars as shown in Fig. 11. Each steel element was attached together by epoxy resin. Lead wires were also soldered on each of steel elements in order to measure the macrocell current density flowing in the steel bar. And in order to control the size of ITZ, three types of specimens were prepared as follows; (1) The specimen with small size of ITZ (type-a specimen): The outline of concrete specimen is shown in Fig. 12. The height of horizontal steel bar was set at 50mm and horizontal steel bar was covered by cement paste before casting in order to avoid the formation of ITZ cased by bleeding water. Prior to the casting of concrete, the horizontal steel bars were covered by cement paste (W/C=0.30 with Cl - ). (2) The specimen with normal size of ITZ (Type-B specimen): The outline of the Type-B specimen was the same with Type-A specimen. The height of horizontal steel bar was set at 50 mm without cement paste coating. (3) The specimen with large size of ITZ (Type-C specimen): The outline of the Type-C specimen is shown in Fig. 13. The height of horizontal steel bar was set at 1450 mm in order to generate the large size of ITZ due to bleeding water. Also each specimen has 3 levels of vibrating time (0, 30 and 120 seconds) and 3 kinds of steel type (X type, bamboo type and round type) in order to control the size of ITZ. The experimental cases are shown in Table 7.The mixing proportion of concrete is shown in Table 8. The water content was set at 225 kg/m 3 in order to simulate high bleeding concrete. Ordinary Portland cement was used River sand and crashed stones were used as fine and coarse aggregates respectively. Tap water was used as mixing water. In order to simulate different Cl - concentration in the specimens, Cl - content of 0.3 kg/m3, 0.6 kg/m3, 5.0 kg/m3,10.0 kg/m3 of Cl- was used in the concrete mix. Wet-dry condition was investigated as external environment for the simulation of tidal zone. Specimens were submerged in water (20 o C) for 24 hours and exposed to dried condition(20 o C, 75 % 100mm Fig. 13 Outline of Type-C specimen

10 R.H.) for 72 hours. Table 7. Experimental cases for various ITZ. Cl - concentration (kg/m 3 ) Time of vibrating (second) Bleeding ratio of concrete (%) (All the specimens, 3 kinds of steel bar (X type, bamboo type and round type) were embedded in concrete.) Table 8. Mixing proportion of concrete. W/ C W (kg/m 3 ) C S 6.3 Measured Items The measured items considered in this study are as follows; (1)Measurement of macrocell corrosion current density using a zero resistance ammeters, (2) Measurement of microcell corrosion current density using a corrosion monitor, (3) Calculation of total current density by summation of macrocell and micro cell corrosion density, and total corrosion rate,from corrosion current density considering electro-chemical process (100μA/cm2 (current density) = 1.16mm/year (corrosion rate)), (4) Measurement of the size of thickness of ITZ formed around the horizontal steel bar using a digital microscope. 6.4 Influence of chloride concentration on corrosion The relationship between the chloride ion concentration and the total corrosion rate is shown in Fig 14. From this figure, it is confirmed that the total corrosion rate of the horizontal steel bar in concrete increase as the chloride concentration increases. However, even if the Cl- content is same, it can be seen from this figure the total corrosion rate of specimen widely varies. Therefore it can be concluded that the Cl - concentration is not only factor that influences the corrosion of the horizontal steel bars G Cl - AE (g/m 3 ) AE- WRA (g/m 3 ) Total corrosion rate (mm/year) kg/m 3 5 kg/m 3 10kg/m Cl - concentration (kg/m 3 ) Fig. 14 Relationship between chloride ion concentration and total corrosion rate 6.5 Influence of ITZ around horizontal steel bars on corrosion The relationships between the total corrosion rate and thickness of ITZ in different chloride ion concentration (0.3 kg/m 3 and 5.0 kg/m 3 ), are shown in Figs 15 and 16 respectively. In these figures, the broken line shows the corrosion rate ( mm/year) which is considered to be the rate that the crack due to steel corrosion occurs within 35 years. Especially, in 35 years the total corrosion will

11 Total corrosion rate (mm/year) mm/year Thickness of ITZ (μm) Fig.15 Relationship between total corrosion rate and thickness of ITZ. (Chloride ion concentration = 0.3 kg/m 3 ) Total corrosion rate (mm/year) mm/year Thickness of ITZ (μm) Fig.16 Relationship between total corrosion rate and thickness of ITZ. (Chloride ion concentration = 5.0 kg/m 3 ) Thickness of ITZ (μm) Crack due to steel corrosion occurs more than 35 years Crack due to steel corrosion occurs less than 35 years Threshold of Cl - Concentration Proposed by JSCE (1.2kg/m 3 ) Crack due to corrosion occurs 80 after 35 years 60 Sever corrosion Slight corrosion Cl - concentration in concrete (kg/m 3 ) Fig. 17 The combined effect of chloride ion concentration and ITZ thickness on corrosion be mm ( x35 mm) and this value is equivalent to 10mg/cm 2 which is considered to be the value at which the crack in the longitudinal direction of the steel bar occurs. This value is based on the standard specification of Japan Society of Civil Engineers. From these figures, it is confirmed that the total corrosion rate in horizontal steel bar increases as the ITZ thickness becomes larger in any case of chloride ion concentration. As shown in Fig. 15, it is confirmed that the total corrosion rate of steel bar becomes mm/year in the case that the thickness of ITZ larger than 100μm with 0.3 kg/m3 of Cl - concentration. On the other hand, as shown in Fig. 16, it is confirmed that the total corrosion rate also becomes mm/year in the case that the thickness of ITZ is smaller than 20 μm with 5.0 kg/m3 of Cl - concentration. This is because the space of corrosion reaction increases as the ITZ thickness becomes larger. From above results it can be said that the properties of ITZ, especially the thickness, is one of the important factors against corrosion behavior in horizontal steel bars in concrete. The combined effect of the thickness of ITZ under the horizontal steel bar and chloride ion concentration on corrosion rate is shown in Fig 17. From this figure, it is confirmed that the steel bar in concrete with more than 60 μm of thickness of ITZ is corroded severely even if the chloride ion concentration is less than 1.2 kg/m 3. Also it is confirmed that the steel bar in concrete with less than 10 μm of thickness of ITZ is not corroded severely even if the chloride ion concentration is more than 1.2 kg/m 3. Also the dotted line shows the imaginary boundary between a longer and a shorter life. In other words, below this boundary, the longitudinal crack will not appear up to 35 years, and above the boundary the crack will appear within 35 years.

12 From these results it can be concluded that the thickness of ITZ is an important factor from the view point of durability of reinforced concrete structures. Especially when the steel bar is coated with cement paste before casting was used as an effective method to prevent the corrosion in horizontal steel bar. 7. Conclusions From the results obtained in Chapters 2 to 6, the conclusions are as follows; 1) It is impossible to determine one threshold value for corrosion of steel bars in concrete. 2) The threshold value has a very probabilistic nature. Even in cases containing more than 10 kg/m 3 of chloride ion concentration, there is a possibility to have passivity. 3) The ITZ has a great influence on corrosion and so with the threshold value. References [1] Otsuki, N., Nagataki, S. & Nakashita, K Evaluation of AgNO 3 solution spray method for measurement of chloride penetration into hardened cementitious matrix materials. ACI Materials Journal Vol.89, No.6: [2] Otsuki, N., Yokota, H., Hamada, H. & T.U.Mohammed Lifecycle design for durability of offshore concrete structures. Proceedings of the 1 st FIB Congress session 4: [3] Mohammed, T.U., Otsuki, N., Hamada, H. & Yamaji, T Chloride induced corrosion of steel bars in concrete with presence of gap at steel- concrete interface. ACI Materials Journal Vol.99, No.2: [4] Glass et al., PCT Patent App WO 01/55056 A1, 2001