STRESS CORROSION CRACKING SUSCEPTIBILITY OF STEEL STRANDS IN CONCRETE CONTAMINATING SALT

Transcription

1 STRESS CORROSION CRACKING SUSCEPTIBILITY OF STEEL STRANDS IN CONCRETE CONTAMINATING SALT Fu M. Li, Ying S. Yuan, Jian H. Jiang and Bo Wang School of Architecture and Civil Engineering, China University of Mining and Technology, China Abstract Stress corrosion cracking (SCC) of steel strands are the significant durability problem of prestressed concrete structures. The theoretical analysis shows that in concrete contaminated with salt, it is difficult to have the SCC with anodic dissolution in steel strands for the microstructure of tensile pearlites. To find the susceptibility of SCC with hydrogen induced cracking (HIC), the occluded cell corrosion (OCC) model of steel strands in concrete contaminated with salt is presented on the base of the microstructures. Hydrogen may be produced in OCC, but it is difficult to get a high concentration in hydrogen traps; or even if some hydrogen traps get a high concentration and cracking is caused, the cracking is approximately parallel to the longitudinal axis of steel wires. So, in general, the HIC is difficult to occur in steel strands. The long-term experimental investigations also show that in concrete contaminated with salt, the conditions of HIC of steel strands are not easy to be realized. 1. INTRODUCTION With the development of prestress technique, the modern prestressed concrete structures have been applied into construction fields extensively. Some of the structures are in bad environment of corrosive industry, marine, deicing salt and so on. The steel strands in these structures are corroded very easily; some structures are in normal atmospheric environments, but the carbonization can lead steel strands to corrode with service time increasing. According to a survey,there have been a series of accidents with durability failure of prestressed concrete structures in worldwide during the last few decades, and the mainly damage is caused by corrosion of prestressing steels [1-3].. There are some studies about the SCC of prestressing steels, but the conclusions are inconsistency because they were drawn from different simulation test conditions [4-9]. In this article, the SCC susceptibility of both anodic dissolution and HIC is analyzed theoretically firstly. Then, the long-term experimental study under the chloride salt 463

2 environment is made. Through these works, the sufficient conclusions about SCC susceptibility of steel strands in concrete contaminated with salt may be got. 2. THEORETICAL ANALYSIS ON SUSCEPTIBILITY OF SCC WITH ANODIC DISSOLUTION When the carbon content of steel is about 0.12 percent, the susceptibility of SCC with anodic dissolution is greatest; otherwise it will gradually reduce when the carbon content is below or over this value [10]. The explanations are not yet unified. The author suggests that the analysis can be taken from the microstructure of carbon steel. The microstructure of eutectoid steel with higher carbon content is pearlite composed of homogeneous mixture of ferrite and cementite in a certain proportion. The pearlites form different orientations in space, which makes the corrosion difficult (or impossible) to develop towards depth along one or several easy transverse channels. So, the corrosion occurred only on the surface generally (ferrite as anode and cementite as cathode), and gradually develop inward overall, thus the SCC with anodic dissolution is not easy to occur. The hypereutectoid steel with higher carbon content is similar to that. For the hypoeutectoid steel, which has a lower carbon content and consists of pro-eutectoid ferrite and eutectoid pearlite, when the carbon content is appropriate (0.12 percent), there will be a interface between pearlite and ferrite with better horizontal connectivity, the ferrite on the two sides of interface is the anode, the cementite is the cathode, and the corrosion develops in depth because of having formed one or several easy transverse corrosion channels, thus the SCC with anodic dissolution easily occurs; when the carbon content is away from the value(greater or less than), it is difficult to form two-phase interface with better horizontal connectivity, and thus the sensitivity of SCC is reduced. As shown in Figure 1. Dissolve path through interface Dissolve path through interface Dissolve path through interface F P P F P F (a)wc<0.12% (b)wc=0.12% (c)wc>0.12% F- ferrites,p- pearlites, Figure 1: Microstructures and dissoluting accesses through interfaces of carbon steel The carbon content of prestressing steel strands is close to that of eutectoid steel, and the structure is mainly flaky fine pearlite (sorbite). In addition, there is a little pro-eutectoid ferrite (when w C <0.77%) or pro-eutectoid cementite (when w C >0.77%). The orientation of flaky fine pearlite after drawing is roughly parallel to the direction of longitudinal axis. Based on the above analysis, the probability of SCC with anodic dissolution in the structure of such organizations is very small. 3. THEORETICAL ANALYSIS ON SUSCEPTIBILITY OF SCC WITH HIC The probability of SCC with horizontal anodic dissolution in steel strands is very small for the microstructure. However, if there is hydrogen diffusing into steel strands, then some of 464

3 them directly enter into the hydrogen traps (such as crystal defects, the second phase, as well as some elements of solute and the three-dimensional tensile stress region); some of them exist in the location of the lattice space. The hydrogen atoms at the location of the space lattice will diffuse under the concentration gradient and stress gradient. In the diffusion process, hydrogen atoms will trap into the traps again, which leads to the hydrogen concentration in the traps. The concentration of hydrogen to a certain extent can cause HIC. Clearly, so long as there is adequate hydrogen diffusing into the strands, then the HIC is bound to occur under the external stress. Therefore, the supply of hydrogen being sufficient or not is the key that whether HIC can occur in the strands. There are often three possible sources of hydrogen for steel strands in the prestressed concrete structures, firstly, in the conditions of full water supply, the OCC may occur when the partial breakdown of the passive film on the surface of strands caused by the corrosion medium (such as Cl - ), and this corrosion can produce hydrogen atoms; secondly, the cathodic protection measures are applied to the strands in some projects, then the hydrogen will be produced if controlled inproperly such as over-protection; thirdly, the pickling in the process of production of strands may produce hydrogen. For the second and third conditions, the production of excessive hydrogen can be prevented through the appropriate process control. The first is discussed mainly in the following. 3.1 OCC and the production of hydrogen The accumulation of Cl - in concrete or cement paste surrounding steel strands can cause pitting on the surface,or crevice corrosion in contact gaps of steel wires,,which may both develop into the so called OCC. According to the microstructure of tensile pearlites of steel strands, the author presents an OCC model shown in Figure 2. O2 H2O O2+2H2O+4e O2 H2O 4(OH) - - Concrete conteminated with salt Cl - Fe2O3 H2O. H2 FeCl2 HCl +Fe(OH)2 H2O Fe(OH)2 Passive film or cementite plate Products same as left Pearlites H Fe 2+ e - H2 Fe Fe +2e H + e H + 2H H2 Figure 2: The OCC model of steel strands in concrete contaminated with salt 465

4 Firstly, although the overall orientation of pearlite colonies is parallel to the longitudinal axis of steel wires, but there are still some few pearlite colonies with non-parallel orientation. To the pearlite colonies with a small angle between the orientation and longitudinal axis, if the outer surface is the layer of cementite, then the cementite layer becomes directly a protective film; if the outer surface is the ferrite layer, then the ferrite layer is passivated into a protective film in the alkaline environment provided by concrete, or directly corroded to expose the cementite protective film; To the pearlite colonies with a larger angle, a good protective film can be produced in the alkaline environment provided by concrete, because the interphase arrange of cementite layers and ferrite layers can form a good micro-battery corrosion. After the Cl - arrives at the surface of steel strands, the passivating film formed by pearlite colonies with the larger angle is destroyed and creating a source of pit or crack. At first, the anode reaction with dissolution of iron and the cathode reaction with oxygen ionization are proceeding simultaneously in the source, which is Anode: 2+ Fe Fe +2e (1) Cathode: O +2H O+4e 4(OH) Fe +2OH Fe(OH) Fe(OH) +O +2H O 4Fe(OH) (4) 2Fe(OH) 3 2H2O+Fe2O3 H2O (5) However, the communication between within and outside the cracks is poor due to the accumulation of cathodic corrosion products in the mouth of cracks, the oxygen in the cracks is consumed quickly and only the anodic reaction can proceed, the cathode reaction transfers to the outside of cracks. Because the anodic reaction products of Fe 2 + is unable to transport to outside the cracks, the only cathode reaction of equation (2) proceed outside the cracks. The cathodic reaction products of OH - are also unable to enter the cracks, resulting in the increasing of ph near the mouth of cracks. The quantity of Fe 2+ in the crack becomes more and more, the Cl, with negative electricity outside the crack diffuses into the crack and reacts with Fe 2+ generating FeCl 2, then the electric neutral of solution is maintained. The FeCl 2 is hydrolyzed into the indissoluble rust (deposited in the wall of crack) and soluble HCl: + - FeCl +2H O Fe(OH) +2H Cl (6) With the producing of soluble acid, the ph in the crack drops at 1.5 to 5.0, and the potential gets more negative. Until the potential below the liberating hydrogen potential, the atom H can be generated: + H +e H (7) The atom H not only can enter the steel strands by diffusion, but also can be synthesized into the molecule of H 2 with escaping from the gaps in the form of bubbles: 2H H 2 (8) (2) (3) 466

5 The self-catalyzed anode reaction of the iron in crack tip is proceeding continuously in the mentioned above process, and therefore, the atom H will continue to be generated. In addition, the propelling path of the OCC depends on the space orientation, as well as the joint relations between each other of pearlite colonies near the pits or the cracks source. Within each pearlite colony, the occlusion zone propels towards two directions along the pearlite plate plane, which will not expand in the vertical direction of pearlite plate plane due to the protection of cementite plate, but the scope in this direction narrows continuously because of the dissolution of occlusion zone tip. When the propelling along the direction of pearlite plate plane reaches to the end of the pearlite colony, there is a joint encountered between the pearlite colony and another pearlite colony. There are two types of joint, namely, the L-type and T-type, as shown in Figure 3. If the L-type joint is encountered, the OCC will entry into the next pearlite colony smoothly and continue to propel forward; otherwise, if the T-type joint is encountered, the OCC will stop progressing due to the protection of cementite plate in this joint. (a)l Joint (b)t Joint Figure 3: The effects of the joint relations between adjacent pearlite colonies on the progressing of OCC Obviously, the model emphasizes two key issues, one is the location of pits or cracks sources, and the other is the propelling path of the OCC. It is worth noting that, because the overall orientation of pearlite colonies with tensile pearlite microstructure in the strands is parallel to the longitudinal axis of the wires, the pits or cracks from OCC are often on the shallow inside surface with the development along the longitudinal direction of wires, and the channel of SCC with anodic dissolution that develops in depth along the transverse direction of wire is not easy to form. However the SCC with HIC is possible to occur due to the generation of hydrogen. 3.2 The concentration of hydrogen and HIC After the atoms H enter into the metal, they will be found in the internal metal in different ways according to the specific conditions. For the steel strands in service, the atoms H generated by OCC enter into the inner of strands and mainly exist in two kinds of form, one is the dissolved solid within the lattices of ferrite and cementite in the form of atoms, and the second is to be arrested into the various hydrogen traps of steels in the form of atoms. Comparing with the lattice, the hydrogen in the traps has a higher concentration. In addition, stress will induce hydrogen to enrich towards the high stress region, thus the hydrogen in the traps further enriches. 467

6 Only when hydrogen is enriched into the critical concentration C lim, which can cause sufficient plastic deformation and strain highly localized and fully reducing of linkage force between atoms, the cracking may occurs under low external stress. 3.3 Susceptibility of SCC with HIC For the special microstructure, steel strand has lots of hydrogen traps such as dislocation, phase-boundary, grain-boundary and micro-crack etc. It is difficult to get a high concentration of hydrogen in these traps, because the sufficient production and diffusion of hydrogen are needed for that. At the same time, lots of traps such as phase-boundary and grain-boundary are approximately parallel to the longitudinal axis of steel wires, so, even if cracking occurs on these regions, the effect on transverse failure of steel wires is little. Accordingly, the HIC (especially transverse cracking) is difficult to occur in steel strands from the qualitative point of view. 4. EXPERIMENTAL INVESTIGATION 4.1 Experimental program (1): Climate conditions Climate 1: the simulated climate of constant temperature and humidity. The temperature is 30 o C±2 o C and the relative humidity is 80%±5%. Climate 2: the natural climate under canopy in Xuzhou outdoor. (2): Concentration of chloride ions The following two concentrations of chloride ion are drafted: 0.6% (corresponding content of NaCl is 0.994% 1%) and 3% (corresponding content of NaCl is 4.971% 5%). (3): Stress level Following three stress levels are determined: 0.7 f ptk, 0.5 f ptk, 0.3 f ptk. Moreover, the comparison with 0 stress is made in each kind of stress level. (4): Structure conditions The structure conditions include pre-tensioned prestressed concrete structure system and post-tensioned prestressed concrete structure system. The latter includes the poreforming system by either steel pipe or bellows. (5): Experimental conditions combination and specimen serial numbers The details are listed in Table 1. Table 1: Experimental conditions combination and specimen serial numbers Climate Climate 1 Climate 2 Structure types Concentration Stress and specimen serial numbers of Cl - σ pe =0.7f ptk σ pe =0.5f ptk σ pe =0.3f ptk Pre-tensioned 3% CRO1 CRO2 CRO3 Pre-tensioned 0.6% CRO4 CRO5 CRO6 Pre-tensioned 0.6% CRO7 CRO8 CRO9 Post-tensioned, poreforming by steel pipe 0.6% CRO10 CRO11 CRO12 Post-tensioned, poreforming by bellows 0.6% CRO13 468

7 Post-tensioned, poreforming by bellows 3% CRO14 (6): Specimen design and testing The photograph of specimens shows in Figure 4. The tendons are steel strands of φ s 12.7(1 7) 1860; The design strength grade of concrete is C30, and the mixture ratio is: W:C:S:G=0.44:1:1.35:2.36; The mixture ratio of cement paste is: W/C=0.40/1. The cement is R325 ordinary silicate cement made in Xuzhou. The sand is fine river sand locally made in Xuzhou. The coarse aggregate is crushed stone locally made in Xuzhou and its particle size is 10mm to 20mm. The water is general tap water supplied by Xuzhou waterworks. The salt is directly doped into the concrete or cement paste. Figure 4: Stress monitoring Figure 5: Potential monitoring The specimens are corroded in the design climate conditions after fabrication. The whole experiment lasts for one year. The stress of tendons in CRO7, CRO8 and CRO9 is monitored during experiment. The monitoring device is sensor of tensile and compression (Figure 4). The corrosion potential of steel strands in all specimens are monitored weekly during experiment, the measuring instrument is Corrosion Rate Meter (GECOR 8) (Figure 5). At the same time, the temperature and relative humidity under outdoor canopy are monitored synchronously. 4.2 Experimental results and analysis (1): Results of stress monitoring The long-term monitoring results of the stress of tendons in CRO7, CRO8 and CRO9 are shown in Table 2. Table 2 Stress variation of partial samples of steel strands Time/d CRO7 CRO8 CRO9 Stress/MPa Stress level Stress/MPa Stress level Stress/MPa Stress level f ptk 1048, f ptk 668, f ptk f ptk 974, f ptk 649, f ptk 469

8 f ptk f ptk f ptk f ptk f ptk f ptk f ptk f ptk f ptk (2): Climate parameters The temperature and relative humidity under canopy are monitored weekly for the outdoor specimens, the monitoring time is at 8 to 9 o'clock, and the values are considered as represents of corresponding parameters for the day and next one week. The results are shown in Figure T / o C RH / % (a) Temperature (b) Relative humidity Figure 6: Climate parameters under canopy in Xuzhou outdoor (3): Corrosion potential The long-term monitoring results of the corrosion potential of partial specimens show in Figure 7. φ /-mv CRO1 Linear fit: R 2 =0.48 φ /-mv CRO4 Linear fit: R 2 =

9 φ /-mv CRO7 Polynomial fit: R 2 =0.41 φ /-mv CRO13 Polynomial fit: R2=0.16 Figure 7: Partial results of corrosion potential The results show that there are some general rules in either the simulative environment of constant temperature and humidity (artificial climate room) or the natural environment of outdoor canopy. To the samples placed in the simulative environment, the absolute value of the corrosion potential declines over time in linear trend overall; But, to the samples placed in the natural environment of outdoor canopy, the absolute value of the corrosion potential varies over time in quadratic trend overall, namely, the absolute value of the corrosion declines in the praphase and increases in the anaphase. There are two reasons for the linear degressive trend in the simulative environment, one is reducing of the water on the surface of steel strand gradually, and the other is the positive moving of potential caused by the corrosion products. The reasons for the overall trend of degression in the natural environment of outdoor canopy are the same as the above reasons; simultaneously, the trend of degression in praphase and increasing in anaphase is consistent with the change of temperature and humidity of the environment. It is known that -600 mv is the upper limit potential of releasing hydrogen, while when the scope of the potential less than -900 mv is very large, it ll release enough hydrogen to cause HIC of steel strand [6]. Looking at the whole corrosion potential of all the samples, it has found that there is still a significant gap between them and -900 mv. Therefore, it can be inferred that HIC will not happen in these specimens. This conclusion has been proved by the broken observation. 5. CONCLUSIONS The corrosion of steel strands is difficult (or impossible) to develop towards depth along one or several easy transverse channels for the microstructure of tensile pearlites, which determines the probability of SCC with anodic dissolution in steel strands is very small. On the base of microstructures, the OCC model of steel strands in concrete contaminated with salt is presented. Hydrogen may be produced in OCC, but the HIC is difficult to occur in steel strands. The reasons lay in 2 aspects: firstly, the steel strands has lots of hydrogen traps such as dislocation, phase-boundary, grain-boundary and micro-crack etc. it is difficult to get a high concentration of hydrogen in these traps; secondly, lots of the traps are 471

10 approximately parallel to the longitudinal axis of steel wires, so, even if cracking occurs on these regions, the effect on transverse failure of steel wires is little. The long-term experimental investigation for one year shows that in concrete contaminated with salt, the difference between corrosion potential and HIC potential(- 900mV) of steel strands is remarkable, so the conditions of HIC for steel strands are not easy to be understood; the fracture caused by HIC can not be found when the samples were broken. ACKNOWLEDGEMENTS This work was funded by the National Nature Science Foundation of China (No , No ), the Public Affairs Development Foundation of Xuzhou (No , No. XM07C082), and the Youth Research Foundation of CUMT(0B060086). REFERENCES [1] Schupack, M. A survey of the durability performance of post-tensioning tendons[j]. ACI Journal, 1978, 75(10): [2] Schupack, M, Suarez M G. Some recent corrosion embrittlement failures of prestressing systems in the United States[J]. PCI Journal, 1982, 27(2): [3] Walter, P. J. Corrosion of prestresing steels and Its mitigation[j]. PCI Journal, 1992(5): [4] Klodt, D. T. Studies of electrochemical corrosion and brittle fracture susceptibility of prestressing steel in relation to prestressed concrete bridges[c]. Proceedings of 25th Conference of National Association of Corrosion Engineers. Houston, 1969: [5] Cherry, B. W., and Price, S. M. Pitting, crevice and stress corrosion cracking studies of colddrawn eutectoid steels[j]. Corrosion Science, 1980, 20: [6] Parkins, R. N. and Elices, M. and Sanchez-Galvez, V.. Environment sensitive cracking of prestressing steels[j]. Corrosion Science, 1982, 22: [7] Proverbio, E. and Longo, P. Failure mechanisms of high strength steels in bicarbonate solutions under anodic polarization[j]. Corrosion Science, 2003, 45: [8] Toribio, J. and Ovejero, E. Microstructure-based modelling of localized anodic dissolution in pearlitic steels[j]. Materials Science and Engineering, 2001, A : [9] Toribio, J. and Ovejero, E. Microstructure-based modeling of hydrogen assisted cracking in pearlitic steels[j]. Materials Science and Engineering, 2001, A : [10] Zheng, W. L. and, Yu, Q. Fracture of steel for susceptibleness to environment [M]. Beijing: Chemical Industrial Publisher, 1988:34.(in Chinese) 472