THE IMPACT OF CHLORIDE INDUCED CORROSION AND LOW CYCLE FATIGUE ON REINFORCING STEEL

Transcription

1 МАТЕРИАЛЫ MATERIALS THE IMPACT OF CHLORIDE INDUCED CORROSION AND LOW CYCLE FATIGUE ON REINFORCING STEEL ВОЗДЕЙСТВИЕ ХЛОРИДНО ВЪЗВАННОЙ КОРРОЗИИ И НИСКОЦИКЛОВОЙ УСТАЛОСТИ У АРМИРОВОЧНОЙ СТАЛИ Ch. Alk. Apostolopoulos 1, D. Mihalopoulos* DEPARTMENT OF MECHANICAL ENGINEERING AND AERONAUTICS, UNIVERSITY OF PATRAS, PATRAS, GREECE 265. *Corresponding author. Tel./Fax , 1 CAPOSTOLO@TEE.GR, * MIXALOP@MECH.UPATRAS.GR Abstrat: Reinfored onrete is affeted by hloride orrosion and the original mass of the steel reinforement is redued thus endangering the integrity and lifetime of the struture. An experimental study was onduted on BSt5 s steel whih showed that the level of orrosion had a diret impat on the mass loss and low yle fatigue (LCF). ontrolled LCF testing under ±1 and ±2.5% onstant amplitude strain showed that the orroded bars exhibit gradual redution in available energy, remaining life and their load bearing ability. Pits and nothes were formed on the orroded steel surfae and stress onentration points were developed whih are highly loalized at imperfetions and espeially at the rib bases. The experimental investigation of the speimens subjeted to LCF showed that the life expetany, the remaining energy density and the strength properties were redued as a result of the formation of pits and nothes ombined with the mass loss and redution of the exterior hard layer of martensite. Seismi design that does not aount for redution of the load bearing ability, life expetany and umulative plasti deformation of the steel due to the loading and orrosion history that a struture will suffer under severe weather and ground motion ould lead to unpreditable performane. KEYWORDS: MASS LOSS; CHLORIDE CORROSION; LOW CYCLE FATIGUE; BSt5 s STEEL ; PLASTIC DEFORMATION. 1. Introdution Corrosion deterioration of the steel reinforement in onrete strutures is a serious problem with high eonomi impat. Strutures exposed to harsh environments suh in oastal sites or ontaining deiing salts are partiularly at risk. Sine the steel reinforement annot be inspeted visually, orrosion often remains undeteted until extensive raking or spalling has ourred. This type of damage an redue the servie life of the strutural member and an also reate a safety hazard. Reinforement embedded in onrete will not orrode under ideal onditions sine onrete having a ph of approximately provides a protetive environment and formation of a thin film of passivating iron oxide on its surfae. The two proesses leading to a breakdown of the passivating film and initiation of orrosion, whih are essentially of the same type, are: a) Development of an aidi environment when arbon dioxide from the air mixes with water in the onrete pores (known as arbonation) that removes the passivating layer and b) The passivating layer an beome permeable due to the presene of hloride ions that penetrate into the onrete from seawater, salt and saltwater, deiing hemials, brakish water or spray from these soures and marine environments [1-5]. Carbonation redues the ph to approximately 8 or 9, destabilizes the oxide film and with adequate supply of oxygen and moisture initiates orrosion. The penetration of onrete strutures by arbonation is a slow proess, the rate of whih is determined by the penetration of arbon dioxide into the onrete and also its degree of porosity and permeability [3,6]. Chloride ion orrosion is initiated by their entrane into the onrete from de-iing salts or from seawater in oastal sites. The hlorides within the salt at as a atalyst in the natural orrosion proess. At the start of orrosion the reinforement is eventually replaed by rust, a porous produt with volume of 3-8 times higher than the reinforing steel. As orrosion ontinues the rust exerts tensile fores on the surrounding onrete and auses delamination along the steel-onrete interfae. Eletrohemial orrosion starts due to the existene of an anode a athode an eletrolyte and a metalli path. The alkaline solution that protets the rebar remains stable in the alkaline onrete environment, but begins to deteriorate when the ph of the pore solution drops below 11 [7-1]. Rust ours beause of differenes in eletrial potential between small areas on the steel surfae involving anodes, athodes and an eletrolyte. This results in rak formation from the steel bar to the onrete surfae or between bars allowing oxygen and moisture to attak the bars faster and inrease the orrosion rate. The rust redues the bond strength and results in the loss of steel-onrete omposite ation, whih affets the servieability and performane of strutures [3,1-13]. In oastal loations the limati onditions onstitute one of the most aggressive environments for onrete strutures due to the severe ambient salinity, high temperature and humidity and also due to the ingress of hlorine through wind borne salt spray [7-8]. Chloride orrosion leads to onrete raking, spalling, destrution of the protetive steel barrier and formation of pits, nothes and avities on the steel surfae [2-3,9]. The urrent design odes do not aount for orrosion of steel sine they are unable to quantify it. The redution in the strutural performane of reinfored onrete members due to orroded steel is aused by the loss in the effetive ross-setional area of onrete due to raking in the over onrete, loss in the mehanial properties and performane of reinforing bars due to redution of their ross-setional area and also loss of bonding[7,1,14-18]. Seismi loads at on the load bearing elements of strutures in the form of high strain reversals, whih an be simulated as single axis LCF.The Fourier spetra of ground movement during an earthquake that ourred in Japan showed that the loading was yli and the frequeny orresponding to the maximum amplitude was approximately 2 Hz. Investigation of the atastrophi earthquake of Tang Shan in China onfirmed that the failure mode of the building strutural steel was LCF [ 4]. Earthquakes inflit umulative damage on reinfored onrete strutures but most odes do not expliitly take it into aount. In urrent design pratie a displaement dutility fator is used whih however fails to aount for the aumulated damage sine it is impliitly assumed that strutural damage ours only due to the maximum response deformation and is independent of the number of non-peak inelasti yles or strain energy dissipation. However all inelasti yles must be onsidered ontributory to damage the aumulation of whih may beome important depending on the harateristis of the ground motion. During strong earthquakes, yielding strutures undergo inreased number of yles into the inelasti range and the aumulated damage may signifiantly affets their overall performane. This type of damage may also arise from multiple seismi events, in whih ase, a series of pre or post shoks in ombination with the main shok may be treated as a single event of extended duration. Assessment of seismi damage is МАШИНЫ, ТЕХНОЛОГИИ, МАТЕРИАЛЫ, 1-27 MACHINES,, MATERIALS, No I-27

2 usually assumed to be similar to metal fatigue under variable amplitude yli loading. The level of orrosion ombined with the LCF is an issue of great onern for designers sine eah one of these fators affets the rebar durability and performane and shortens the design life of strutures [4-6,19-22]. The results of an experimental investigation are presented in this paper showing the influene of hloride indued orrosion on the mass loss and LCF testing at onstant strain amplitude of BSt5s tempore 12 mm diameter steel speimens, whih are important for the assessment of the safety and residual life of orroded strutures that are loated within seismi zones suh as Greee, Japan, Western United States, et. 2. Preonditions and means for solving the problem 2.1. Corrosion indued mass loss Ribbed steel bars of 12 mm diameter were artifiially orroded in a salt spray orrosion hamber, aording to ASTM B standard, for 1, 2, 3, 45, 6 and 9 day. The spray solution was 5% sodium hloride and 95% distilled water, with ph range of and spray hamber temperature of ± 1.7 o C for different durations so that different orrosion levels were obtained. BSt5s steel in maximum permissible values of the final produt ontains C=.24%, P=.55%, S=.55% and N=.13% and its mehanial properties are shown in Table 1. Table 1. Mehanial properties of steel BSt5s speimen Yield stress (MPa) Ribbed bar 5 Tensile strength (MPa) Elasti modulus (GPa) % Elongation after breaking The properties of the orroded material were ompared against the requirements set in the standards for involving steels in reinfored onrete strutures. It should be emphasized that aelerated salt spray orrosion tests on bare bars and steel embedded in onrete theoretially lead to different results, but they represent results represent a good first approximation of the influene of both plasti deformation and quik orrosion on steel dutility properties versus exposure of embedded steel in a natural orrosive environment whih would require years to reah similar levels of deterioration sine the onrete delays the hloride penetration depending on its physial and hemial harateristis. When the hlorides reah the reinforement and exeed a ritial onentration level then the orrosion proess takes plae almost similarly with the ase of bare bars. Pitting was observed to have started progressively on the speimens after 1, 2 and 3 days orrosion level whih beame progressively more severe. After salt spray exposure the speimens were washed with lean water aording to ASTM G 1-72 proedure in order to remove any left over salt deposits and then dried. The pitting developed on the steel surfaes even after removal of the rust is shown in Fig. 1. non orroded 3 days orrosion Fig. 1. Stereosopi images of speimens The relatively large pits at 3 days of salt spray exposure suggest that these are the ative sites at whih orrosion is primarily taking plae. Pitting appeared to be initiated at the reinforement veins of the steel bars and proeeded to the intermediate spae. The orrosion proess reated pitting and formed avities and nothes on the steel surfae and espeially in the rib bases, whih beame progressively more severe as the orrosion level inreased, reahing an average pit depth after 9 days, aording to ASTM G46-94 standard guide for examination and evaluation of pitting orrosion, of approximately.25 mm with maximum value of.5 mm. The degree of orrosion was measured as the perentage mass loss given by the ratio of the differene between the initial and final masses (M i - M f ), before and after ompletion of the orrosion proess, divided by M i, multiplied by 1. The remaining mass loss and remaining diameter versus orrosion level for the steel speimens is shown in Table 2. The area As used in the fatigue alulations was determined aording to DIN speifiation from: (1) M f A s = l where As is in mm2, Μ f is in g and l is the length in mm. Table 2. Mass loss and remaining diameter at different orrosions Exposure time[days] Mass loss [%] Diameter [mm] Exposure to salt spray orrosion environment Low Cyle Fatigue The steel speimens were subjeted to uniaxial sinusoidal loads of 1Hz frequeny and onstant strain amplitude of ±1 and 2.5% after being exposed to salt spray for different durations. A total of 42 low yle fatigue tests were onduted, 3 for eah orrosion level at, 1, 2, 3, 45, 6 and 9 days. The length of the 12 mm diameter speimens was 172 mm and the free length between grips was set at an empirial value of 6x12 = 72 mm, sine it represents typially the free bar length between stirrups in ritial regions of olumns in seismi areas. The speimens were subjeted to LCF testing without modifiations of their ross setional area sine this would alter the nature of the material thus giving misleading results. The analytial expression for total strain is given by [24]: (2) Δ ε t Δ ε e Δ ε σ P f b = + = ( 2 Ν ) + ε f ( 2 Ν ) Ε where Δε t, Δε e and Δε p are the total, elasti and plasti strain amplitudes respetively, as shown in the steady state loops of Fig. 2, ε f and are the fatigue dutility oeffiient and exponent respetively. σ f and b are the fatigue strength oeffiient and exponent respetively and Ε is the modulus of elastiity. Figure 2 shows representative typial hysteresis loops for LCF for different orrosion levels and ±1 and ±2.5% applied strain, where progressive redution of stress is observed. The total dissipated energy density was evaluated as the sum of the areas formed within the hysterisis loops. This measures the apaity of the material to absorb energy during seismi ativity. After evaluating the total energy absorbed the remaining energy density was plotted after eah yle as a funtion of the number of subjeted yles [8,21-26]. Even though random non orroded and orroded speimens are represented it is learly shown that plasti deformation (Δε p ) is predominant. Therefore sine the plasti deformation (Δε p ) oupies the 33.5% of the applied width at ±1% strain level as expeted, this amount is inreased to 66.5% when the strain level is ±2.5%. The measured plasti deformation orresponds approximately to the average life expetany of the material. Cyling with large or small strain amplitudes are additive and highly redue the useful life expetany of steel in aordane to plastiity theory. Knowing the mehanial behaviour of the gradually orroded steel and as shown in Fig. 4, the redution in МАШИНЫ, ТЕХНОЛОГИИ, МАТЕРИАЛЫ, 1-27 MACHINES,, MATERIALS, No I-27

3 dutility properties suh as elongation and energy density, is evident. It is finally apparent that the redution of the useful life of 1th - 15th yle 6 steel is losely related to the loading history, the width of the 25th - 255th yle subjeted strain and the imposed orrosion level [8,14,21,24,27-3]. 5th - 55th yle 4 Sine the LCF tests were performed at onstant strain amplitude, 2 the maximum resisting fore exerted by the speimens for eah Strai n yle was gradually redued. Furthermore, an overall redution of -,1 -,5,,5,1 the applied fore was observed for the pre and post orroded -2 speimens whih was expeted sine the ross setional area was redued with advaning orrosion and imposed yling, ontrary to -4 the expetation that the reinforing steel is expeted to arry a -6 onstant load throughout its servie life. Typial hysterisis loops of non-orroded and orroded speimens Δεe/2 Δε ρ Δεe/2 are shown in Fig. 2 where progressive redution of stress is Δεt observed. From the LCF tests, the total dissipated energy density was evaluated as the sum of the areas formed within the hysterisis loops. This is a measure of the apaity of the material to absorb energy during seismi ativity. After evaluating the total energy 1th - 15th Cyle absorbed the remaining energy density was plotted after eah yle 6 1th - 15th Cyle as a funtion of the number of subjeted yles. Corrosion has a 2th - 25th Cyle 4 diret effet on the exponential oeffiients and b of the reinforing steel bars and their average value (for eah group of 2 three measurements) was alulated from the LCF test results for eah orrosion level and applied strain as shown in Table 3 and Fig. -,1 -,5,,5, The variation is espeially more evident in oeffiient sine it determines the material behaviour in the plasti region. -4 Seismi damage assessment is usually assumed to be similar to metal fatigue under variable amplitude yli loading. Seismi -6 damage, however, is normally assessed in terms of only the plasti strain omponent sine a relatively small number of load yles is imposed by the earthquake ground motion. Assuming a orrespondene between steel bar material and strutural damage [21,23] the low-yle fatigue model under onstant displaement amplitude yles may be written as: Δm Δ y = ( Δum Δy)(2Ν f) b (3) Where Δ m = peak displaement under onstant amplitude yles Δ y = yield displaement; Δ um = ultimate displaement under monotoni loading; N f = number of yles to failure; = low-yle fatigue onstant (-.5 to -.8); Δ indiates displaement with plasti deformation. Knowing the signifiane of the plasti strain to the life expetany of steel reinforement and from Fig. 2 it an be seen for example that for 1% applied strain the plasti deformation (Δεp) in the non orroded ase oupies approximately 22% of the width of the loop while after 9 days orrosion level it inreases to approximately 29%. For 2.5% applied strain the orresponding values for plasti deformation (Δε p ) in the non orroded ase and 9 days orrosion level are approximately 63 and 67% of the width of the loop. Therefore orrosion and applied strain levels are the predominant fators to material inflited damage. Figures 5 and 6 show the orrelation of the maximum tensileompressive fore versus applied yles for ±1 and 2.5% strain level for three random speimens. Table 3 Effet of orrosion on oeffiients and b amplitude ± 1% ± 2.5% Coeffiients b b Samples days " " " " [%] 9 " Fig. 4. Redution of elongation and energy density МАШИНЫ, ТЕХНОЛОГИИ, МАТЕРИАЛЫ, 1-27 MACHINES,, MATERIALS, No I-27 Corrosion level a) non orroded 1% strain 2.5% strain b) 9 days 1% strain 2.5% strain Fig. 2. Typial LCF stress-strain diagrams, at ±1 and 2.5% applied strain, showing hysterisis loops for initial, middle and final yles for non orroded and 9 days orroded steel - b, -,1 -,2 -,3 -,4 -,5 -, Duration exposure [Days] - b, -,1 -,2 -,3 -,4 -,5 -,6 -, Duration exposure [Days] 1) 1% strain level 2) 2.5% strain level Fig. 3. Variation of oeffiients and b versus orrosion duration Days 6 Days 1st yle 1th yle 2th yle -,3 -,2 -,1,,1,2,3 5th Cyle 1th Cyle 15th Cyle 45 Days 3 Days Δε e/2 Δε ρ Δε e/2 Δε t ,3 -,2 -,1,,1,2, Days non-orroded b

4 of different orrosion levels Maximum fore [KN] Subjeted yles Unorroded 1 Days a. orr. 2 Days a. orr. 3 Days a. orr. 45 Days a. orr. 6 Days a. orr. 9 Days a. orr. Fig. 5 Correlation of maximum tensile-ompressive fore versus applied yles for ±1% strain level Maximum fore [KN] Subjeted yles Fig. 6 Correlation of maximum tensile-ompressive Fore versus applied yles for ±2.5% strain level 3. Results and disussion Unorroded 1 Days a. orr. 2 Days a. orr. 3 Days a. orr. 45 Days a. orr. 6 Days a. orr. 9 Days a. orr. The experimental results represent a good indiation of the influene of plasti deformation and orrosion on steel dutility properties. The aelerated hloride orrosion testing of bare steel bars represent a good first approximation of the influene of both plasti deformation and quik orrosion on steel dutility properties versus exposure of embedded steel in a natural orrosive environment whih would require years to reah similar levels of deterioration. Corrosion was found to be detrimental for the integrity of steel while pitting was evident after a few days of salt spray and beame progressively more severe as the orrosion level inreased, as shown in Fig. 1. As the steel surfae beame rougher avities and nothes were formed whih redued the steel diameter. Considerable redution in the fatigue limit took plae sine the mass loss led to redution of the exterior hard layer of martensite and drasti drop in the energy density of the orroded speimens thus developing stress onentration points whih are highly loalized at imperfetions and espeially in the pits and nothes of the rib bases of the orroded steel. Thus the inreased orrosion led to a derease of the useful life of steel. Table 2 shows the mass loss and remaining steel bar diameter versus orrosion level. It was observed that a small mass loss reated great redution in the strength and life expetany of steel. The rib height was also measured during the different levels of orrosion and at 9 days, orresponding to 1.4% mass loss, the average height of the ribs was redued by approximately 8%. The stress-strain behaviour shown in Fig. 2 indiates data for reinforing steel subjeted to reversed yli strain with symmetri inelasti strain intervals. In engineering pratie suh data exhibit partiular response harateristis that are not obviously realized by tensile tests. Important harateristis of response of the speimens shown in Fig. 2 and 4 inlude : a) Loss of linearity during unloading, known as Baushinger effet, prior to ahieving the yield strength in the opposite diretion. b) Exhibition of isotropi strain hardening haraterized by inreasing strength under inreasing inelasti strain demand. This is observed under yli as well as monotoni loading. ) The initial tangent to the unloading stress-strain response is slightly less than the initial elasti stiffness. d) Exhibition of yli strain softening, known also as redued tangential stiffness under multiple yles to partiular strain limits. Figure 3 shows that there is a diret effet on the exponential oeffiients and b of the reinforing steel bars and their average value was alulated from the LCF test results for eah orrosion level and applied strain. Figure 4 shows the stress-strain variation versus orrosion level and the energy density as the area under these urves. It an be observed that as the orrosion level inreases not only the strength properties, of yield and frature points, but also the dutility properties suh as energy density and elongation to frature derease. In severe limates and seismi areas steel bars must have great amounts of energy density, whih haraterizes the resistane to failure and may be used to evaluate the frature under both stati and dynami fatigue loading onditions [14,31]. Signifiant redution in energy density was observed as the orrosion level inreased. The non orroded material has greater energy density than any of the orroded bars, as shown by the area under the stressstrain urve. For smaller strains and longer lives the elasti strain omponent is more predominant while for larger strains and shorter lives the plasti omponent is predominant. At larger strains the life expetany of smooth speimens is extended due to dutility while at smaller strains stronger material is required. From strain versus life expetany tests of pre and post orroded speimens it was realized that life expetany is indiretly dependant on the strain amplitude. The experimental results of uniaxial fatigue tests have shown an overall degradation of the orroded BSt5 s steel reinforing bars. The formation of an oxide layer on the surfae of the speimens made them rougher by forming pits, nothes and avities and thus allowing formation of stress onentration points leading to umulative damage under alternating fatigue loading. Furthermore, the oxide layer redued the nominal ross setional area of the speimens. Considerable degradation of the steel speimens due to orrosion reates a drop in energy density, Fig. 5, ombined with the gradual loss of the exterior hard layer of martensite. Corrosion initiation in hloride environments as well as fatigue rak initiation and propagation are phenomena known to be of stohasti nature. Reproduibility of the results was found to be within aeptable levels. Figures 5 and 6 show the orrelation of the maximum tensileompressive fore versus applied yles for ±1 and 2.5% strain level for three random speimens. It an be seen that as the orrosion level inreases the maximum fore that the steel bar an sustain, both in tension and ompression, dereases. Also as the strain level inreases from 1 to 2.5% the maximum fore dereases by approximately 8-14 %. 4. Conlusions Corrosion of BSt5 s steel has diret effet on the mass loss and LCF: 1. Steel orroded for 1-9 days reated a mass loss from %. For LCF loading a small mass loss reated great redution in the strength and life expetany of steel. 2. The rib bases appeared to be inflited first and as the orrosion level inreased their size was redued. At 9 days or 1.4% mass МАШИНЫ, ТЕХНОЛОГИИ, МАТЕРИАЛЫ, 1-27 MACHINES,, MATERIALS, No I-27

5 loss, their average size was redued by approximately 8% whih has a diret impat on the steel-onrete bonding mehanism. 3. The fatigue limit and energy density of the orroded steel were redued onsiderably due to loss of the exterior hard martensite layer. This developed stress onentration points whih are highly loalized at the imperfetions and espeially in the pits and nothes of the rib bases and shortened the useful life of steel. 4. In engineering pratie the gradual energy density redution due to orrosion is overlooked by the odes and should be re-evaluated aordingly. 5. Considerable derease of the LCF resistane of the orroded steel has a synergisti effet on the aumulated damage due to fatigue and orrosive environment. 6. The maximum load bearing apaity was redued during LCF loading and further redution was observed for orroded speimens. Older strutures in earthquake prone areas are not expeted to display a onstant load bearing ability beyond a ertain servie life. Progression of orrosion aused signifiant redution of dutility and life expetany. 5. Referenes 1..Hausmann, D.A Eletrohemial Behaviour of Steel in Conrete, Journal, Amerian Conrete Institute, 1964, 61, No. 2, Glass, GK, Buenfeld NR., Chloride-indued orrosion of steel in onrete. Prog. Strut. Eng. Mater,. 2 (No. 4), 2, Capozua, R. Damage to reinfored onrete due to reinforement orrosion, Constrution and Building Materials, Vol 9 (No. 5), 1995, Sheng, G.M. SH.Gong, Investigation of low yle fatigue behaviour of building strutural steels under earthquake loading, Ata Metallurgia Sinia (English letters), 1 (1),1997, Tegoshi Yoshaki, Proeedings of Aademial Letures of JAS. Tokyo, 1983, Hakuto Shigeru, Researh report, Retrofitting of reinfored onrete moment Resisting Frames, supervised by Park, R and Tanaka, H. ISSN , August Maslehuddin, M, Ibrahim IM, Huseyin S., Abdulaziz l. 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Researh Counil, Charlottesville, VA, VTRC 3- R7, Ma, S.Y.M. V.V. Bertero, E.P. Popov, Experimental and Analytial Studies on the Hystereti Behaviour of Reinfored Conrete Retangular and T-Beams. Earthquake Eng. researh report 76 (No.2),1976, Berkeley: Univ. of California. 21. Chai, Y.H. Inorporating low yle fatigue model into durationdependent inelasti design spetra, Earthquake Engng Strut. Dyn., 25, 34, Krawinkler, H. Performane assessment of steel omponents, Earthquake Spetra 1987, 3 (1), Fuhs H.O and Stephens R.I. Metal fatigue in engineering. USA: John Wiley & sons In, 198, Krawinkler, H. A. Nassar, Seismi Design based on dutility and umulative damage demands and apaities, Nonlinear seismi analysis and design of reinfored onrete buildings,elsevier Applied Siene, 1992, Riva, P. A. Franhi and D. Tabeni,Weld tempore reinforement behavior for seismi appliations,materials and strutures, Mater strut, Vol 34, 21, Franhi, A. P. Riva, P. Rona, R. Roberti, M. La Vehia, Failure Modalities of Reinforement Bars in reinfored Conrete Elements Under Cyli Loading. Studi e roerhe, Vol 17, 1996, Almusallam, A. A. A. S. Al-Gahtani, A. R. Aziz, F. H. Dakhil, Rasheeduzzafar, Effets of reinforement orrosion on flexural behaviour of onrete slabs, J. of mat. in ivil engineering, 8(3), August 1996, I. Kasiraj, I. JTP. Yao, Fatigue damage in seismi strutures, Journal of the Strut. Division (ASCE) 1969, 95(8), Morinaga, S. Predition of Servie Lives of Reinf. Conrete Build. based on Corros. Rate of Reinforing Steel, Pro. 5th Intern. Conf. on Durability of Build. Materials and Components, Brighton, U. K., Nov. 199; Edited by J.M. Baker, P.J. Nixon, A.J.Majumdar, and H. Davies, 5-16, E. & F.N. SPON, London Gaidis, J.M. A.M. Rosenberg, Avoiding Corrosion Damage in Reinfored Conrete, Conrete International, 21, 23(11), Sih, G. C. C. K.Chao, Failure initiation in unnothed speimens subjeted to monotoni and loading, Theor. Appl. Frat. 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