Analytical modeling of bond stress at steel-concrete interface due to corrosion

Transcription

1 yerson University Digital yerson Theses and dissertations Analytial modeling of bond stress at steel-onrete interfae due to orrosion Luaay Hussein yerson University Follow this and additional works at: Part of the Civil Engineering Commons eommended Citation Hussein, Luaay, "Analytial modeling of bond stress at steel-onrete interfae due to orrosion" (011). Theses and dissertations. Paper 698. This Thesis is brought to you for free and open aess by Digital yerson. It has been aepted for inlusion in Theses and dissertations by an authorized administrator of Digital yerson. For more information, please ontat bameron@ryerson.a.

2 ANALYTICAL MODELING OF BOND STESS AT STEEL-CONCETE INTEFACE DUE TO COOSION By: Luaay Hussein B. S. in Civil Engineering, University of Baghdad, 1989 MAS. in Civil Engineering, Saddam University, 1993 A thesis presented to yerson University in partial fulfillment of the requirements for the degree of Master of Applied Siene in the program of Civil Engineering Toronto, Canada, 011 Luaay Hussein, 011

3 Author s Delaration I hereby delare that I am the sole author of this thesis or dissertation. I authorize yerson University to lend this thesis or dissertation to other institutions or individuals for the purpose of sholarly researh. Luaay Hussein I further authorize yerson University to reprodue this thesis or dissertation by photoopying or by other means, in total or in part, at the request of other institutions or individuals for the purpose of sholarly researh. Luaay Hussein ii

4 Borrower's Page No. Borrower's Name Address Phone No. Date Signature iii

5 ANALYTICAL MODELING OF BOND STESS AT STEEL- CONCETE INTEFACE DUE TO COOSION Luaay Hussein MAS., Department of Civil Engineering, yerson University, 011 Abstrat An analytial model that desribes the deterioration of bond strength, due to orrosion of steel reinforement, at the steel-onrete interfae in a reinfored onrete is developed. Conrete is assumed as a thik-walled ylinder subjeted to internal pressure exerted from the growth of orrosion produts on the onrete at the steel-onrete interfae. The onrete in the inner ylinder is onsidered as an anisotropi material with stiffness degradation fator as an exponential funtion, while at the outer ylinder, the onrete is treated as isotropi material. A fritional model is used to ombine the ation of onfining pressure resulted from radial pressure produed by prinipal bar ribs on surrounding onrete, and orrosion pressure resulted from the expansion of orrosion produts. The results of the proposed model are validated with experimental results by several researhers and a good agreement was noted; this shows that the derived analytial model was able to satisfatory predit the redution of bond strength between steel and onrete. iv

6 Aknowledgment The author would like to express his deepest appreiation to his supervisor Dr. Lamya Amleh, for her invaluable advie, guidane, suggestion, patient and enouragement throughout the exeution of this researh program. Her unfailing optimism and onstant enouragement always prompted the author to overome the diffiulties in ompleting this researh. The author would like to express his sinerely thanks to the Civil Engineering Department in yerson University. Finally, the author is grateful to his family, olleagues, and all friends for their support, and enouragement throughout this work. v

7 To My Family with all espet and Appreiation vi

8 Table of Content Author s Delaration... ii Borrower's Page... iii Abstrat... iv Aknowledgment... v Table of Content... vii List of Figures... ix List of Tables... xi List of Symbols... xii Chapter INTODUCTION Introdution Sope and objetive of the study Thesis layout... 5 Chapter... 6 FUNDAMENTALS OF BOND AND COOSION BETWEEN STEEL AND CONCETE Fundamentals of Bond Introdution Bond Stress Bond Mehanisms Bond Failure Modes Fators Affeting the Bond Strength Measurement of Bond Fundamental of Corrosion Mehanism of einforement Corrosion Effets of Corrosion on einfored Conrete Summary of esearh at yerson Chapter EVIEW OF BOND MODELS Lundgren s model (00) Coronelli s model (00) Wang-Liu model (004) Ghosh-Amleh model (006) Bhargava-Ghosh-Mori-amanujam model (007) Chapter ANALYTICAL MODELING OF BOND STESS AT STEEL-CONCETE INTEFACE DUE TO COOSION Introdution Modeling bond-stress at steel-onrete interfae Proposed analytial model for ontat pressure Assumption for the study vii

9 4.3. The expression for ultimate bond strength Derivation of the expression for Adhesion Derivation of the expression for onfining pressure Derivation of the expression for orrosion pressure Stiffness redution fator Solution proedure Chapter NUMEICAL EXAMPLE AND VEIFICATION OF POPOSED MODEL Numerial example for the proposed model Comparison of esults Using Different Analytial Bond Strength Models Comparison with Wang and Liu model (004) Comparison with Bhargava et al. (007) Comparison with Chernin et al. (009) Validation of the Model with Experimental esults by Other esearhers Validation of the model with the results of Almusallam et al. (1996) Validation of the model with the results of Al-Sulaimani et al. (1990) Validation of the model with the results of Cabrera and Ghiddoussi (199) Validation of the model with the results of Lee et al. (00) Chapter CONCLUSIONS AND FUTUE ECMMENDATIONS Summary and Conlusions Future reommendations EFEENCES viii

10 List of Figures Figure.1 Idealized fore transfer mehanism (ACI Committee 408, 199)... 7 Figure. Typial bond stress-slip relationship (Girard and Bastien, 00)... 9 Figure.3 Separation between the reinforing bar and onrete near primary rak (Lutz and Gergely, 1967) Figure.4 Formation of internal raks (Goto, 1971) Figure.5: Modes of bond failure... 1 Figure.6 Failure mehanisms at the ribs of deformed bars (ehm, 1968) Figure.7 Bond strength as a funtion of bar loation within a wall speimen Figure.8 Shemati diagram of a pullout test (Amleh, 000)... 1 Figure.9 Effet of embedment length on the distribution of bond (Leonhardt, 1964)... Figure.10 Variation of steel, bond and onrete stresses in a tension speimen (Amleh, 000)... 4 Figure.11 Free body diagrams showing fores on the steel bar in a tension speimen with no, one and three raks (Amleh, 000)... 5 Figure.1 Bond beam test National Bureau of Standard bond test beam (Ferguson, 1988)... 5 Figure.13 Bond beam test- University of Texas (Ferguson, 1988)... 6 Figure.14 Shemati bond test (Kemp et al., 1968)... 7 Figure.15 Miro-orrosion ell formations in reinfored onrete (Azher, 005) Figure.16 The relative volumes of iron and its orrosion reation produts (Nielsen, 1985) Figure.17 Stages in Corrosion-indued damage (ihardson, 00) Figure.18 Corrosion indued damage of C strutures (Zhou et al., 005) Figure.19 elationship between the ultimate bond strength and different degrees of orrosion for antilever beam test (Almusallam et al., 1996) Figure.0 Different types of failure modes (odrigues et al., 1997) Figure.1 Different Failure forms of beam speimens (a) BD1 (b) BD10 (Jin and Zhao, 001) Figure 3.1 Physial interpretation of variables t n, t t, u n and u t (Lundgren and Gylltoft, 000) Figure 3. The stress in the inlined ompressive struts determines the upper limit (Lundgren and Gylltoft, 000) Figure 3.3 The yield lines (Lundgren and Gylltoft, 000) Figure 3.4 Physial interpretation of the variables in the orrosion model Figure 3.5 The funtion k(x/r) vs x/r (Lundgren s model, 005) Figure 3.6 Corrosion depth (x) and bar expansion (t) (Coronelli s model, 00) Figure 3.7 Corrosion rak patterns: (a) orrosion raks smaller over; and (b) orrosion raks both sides (Coronelli s model, 00) Figure 3.8 Wang and Liu Model Figure 3.9 Average stress strain relationship of onrete in tension (Wang and Liu, 004)... 5 ix

11 Figure 3.10 (a) Geometry of a ribbed bar and the mehanial interation between bar and onrete; (b) Point A at the end of onrete key; () Stresses of Point A; (d) Prinipal stresses of Point A (Wang and Liu 004) Figure 3.11 Fores ating at the steel onrete interfae (Amleh-Ghosh model, 006) Figure 3.1 Two zones for over onrete due to orrosion raking proess Figure 4.1 Tensile stress in onrete ring due to the fore transfer between steel and onrete (Tepfers, 1979) Figure 4. Fores exerted by the onrete on a ribbed bar in a reinfored onrete Figure 4.3 Fritional model for bond (Pantazopoulou and Tastani, 00) Figure 4.4 Interation between onrete and steel at the interfae: (a) thik-walled ylinder approah of the prism ross-setion, and (b) stresses in onrete Figure 4.5 Shemati of proposed orrosion model Figure 4.6 Models for rak initiation and propagation through the onrete over Figure 4.7 esidual stiffness of partially raked thik-walled onrete ylinder (FE analysis) (Chernin et al. 009) Figure 4.8 Typial stress-strain softening urve for onrete subjet to tensile stress (Zhong et al., 010) Figure 4.9 Stiffness redution fator of partially raked thik-walled ylinder Figure 5.1 Variation of onfining, orrosion, and total pressure with the orrosion depth x Figure 5. Variation of onfining, orrosion and ultimate bond stresses with orrosion depth x Figure 5.3 Variation of orrosion pressure with radius of inner ylinder Figure 5.4 Variation of orrosion depth x with front rak Figure 5.5 Variation of orrosion pressure with orrosion depth x Figure 5.6 Variation of maximum orrosion pressure with /d Figure 5.7 Variation of orrosion pressure with front rak Figure 5.8 Comparison of Predited Bond stress versus Corrosion Depth with Al- Musallam et al. (1996) Experimental esults Figure 5.9 Comparison of Predited Bond stress versus Corrosion Depth with Al- Sulaimani et al. (1990) Experimental esults Figure 5.10 Comparison of Predited Bond stress versus Corrosion Depth with Cabrera and Ghoddoussi (199) Experimental esults Figure 5.11 Comparison of Predited Bond stress versus Corrosion Depth with Lee et al. (00) Experimental esults x

12 List of Tables Table 5.1 Variation of orrosion pressure with rak front for partially raked ylinder Table 5. Variation of bond stress with orrosion depth for partially raked ylinder.. 98 Table 5.3 Variation of orrosion pressure with orrosion depth for fully raked ylinder Table 5.4 Variation of bond stress with orrosion depth for fully raked ylinder xi

13 List of Symbols υ υ a = Stiffness redution fator = Frition angle between steel and onrete = Maximum size of aggregate µ = Coeffiient of frition µ k = Coeffiient of kineti frition µ s = Coeffiient of stati frition υ rs δ γ eq σ θ n = atio between the volumes of the orroded and virgin steel = Angle between fae of the rib and the bar axis = Slip = Tangential stress = adial pressure produed by prinipal bar ribs on surrounding onrete = esidual tensile stress in raked onrete r σ σ s τ t b o (x) = adial bond stress = Stress in steel = Bond stress = Bond strength when the onrete raks = Cohesive bond strength ontribution = Splitting bond anhorage strength rx = Ultimate bond strength for orroded reinforing bar; bu = Bond strength ontribution of maximum onfining pressure at CP anhorage = Bond strength ontribution due to adhesion between orroded steel and AD onrete = Bond strength ontribution due to expansion of orrosion produts CO between orroded steel and onrete =Tangential strain xii

14 = Tensile strain apaity of onrete t rs A r A s A t d d o D r D i d b E 0 = atio between the volumes of the orroded and virgin steel = ib area in the plane at right angles to the bar axis = Area of steel = Conrete area subjeted to tension = Conrete over thikness = Effetive rust layer = Thikness of the porous zone = edued diameter of the orroded reinforing bar = Initial diameter of the reinforing bar = Diameter of the steel reinforing bar = Young modulus of onrete E = Effetive modulus of elastiity of onrete ef E s E st f oh f t = Elasti modulus of steel = Modulus of elastiity of stirrups = Adhesion strength = Conrete tensile strength f = Maximum residual strength of raked onrete at onset of raking t 0 f h r k L l b = Compressive strength of onrete = ib height = Experimentally determined oeffiient related to frature energy = Perentage loss of ontat pressure = Bar embedded length l r M n n s = ib spaing = Perentage mass loss of steel bar = Number of transverse ribs at a setion = Number of legs of the stirrups in the ross setion xiii

15 n b = Number of reinforing bars P = Contat pressure P = The average radial fore rx P orr (x) P max (x) = Pressure developed by orrosion produt expansion = Maximum pressure at anhorage bond failure P = Applied fore at failure max P = Corrosion pressure or P = Confining pressure onf r b o i p s r r S r S v t T C T u V a V b V f w W r = adius of the bar = Outer radius of thik wall ylinder = Inner radius of thik wall ylinder = Crak front = adius of porous zone = adius of initial bar after orrosion = adius of initial bar inluded the expansion of orrosion produts = adius at any point of thik wall ylinder = ib spaing = Spaing of the stirrups = Thikness of orrosion produts = Tensile apaity at a setion = Steel fore = Displaement = Adhesion resistane = Bearing of the lug = Fritional resistane = Crak width = Crak width at over raking xiv

16 w iorr x x r xp = opening of eah single radial rak = depth of the orrosion attak = Corrosion depth at the onset of primary raking = Corrosion level xv

17 Chapter 1 INTODUCTION 1.1. Introdution One of the major degradation proesses of reinfored onrete strutures is the orrosion of the steel reinforement. The orrosion problem of the build infrastruture has a signifiant impat on the eonomy, aording to a study done by the Federal Highway Administration (FHWA) by Koh et al. (001) on the total diret ost of orrosion in the U.S., whih is estimated at $67 billion per year, whih is equivalent to 3.1% of the U.S. gross national produt (GNP). For instane, approximately 15 perent of the 586,000 bridges in the U.S. are reorded as struturally defiient, primarily due to orrosion of steel and steel reinforement. The annual diret ost of orrosion for highway bridges is estimated to be $8.3 billion. In addition, the onsequenes of this problem are numerous, inluding redued safety, servieability and servie life, whih lead to inreased risk of injuries and fatalities; and inreased maintenane osts and user osts. In summary, orrosion of steel reinforements is the foremost ause of damage and early failure of reinfored onrete strutures, leading to huge osts for inspetion, maintenane, rehabilitation and replaement of the infrastruture worldwide. The repair and maintenane of reinfored onrete strutures is beoming inreasingly important and extensive. In order to inrease the reliability of the struture and to redue maintenane osts, eliminating or at worst impeding the orrosion problem is very important. Also, to design new onrete strutures and to repair existing deteriorated onrete strutures requires an understanding of the various auses and mehanisms of orrosion in reinforing and prestressing steel along with their performane in the varying aggressive environments. The main reason assoiated with the deterioration of reinfored onrete due to steel bar orrosion is not the redution in mehanial strength of the reinforing bar itself, but rather than that the pressure exerted from the expansion of the orrosion produts whih annot be supported by the limited tensile strength of onrete. Therefore, this weakens 1

18 the bond between steel and onrete whih immediately affets the servieability and ultimate strength of reinfored onrete strutures (Cabrera 1996). The onrete over ats as a physial barrier to the aess of aggressive agents beause of its strength and resistane to wear and tear, and to permeation of fluids ontaining harmful ompounds. Normally the steel in reinfored onrete is proteted from orrosion beause of the high alkalinity of the onrete; when the ph of the pore water is greater than 1.5, a passive layer forms on the steel surfae whih naturally protets it from orroding. However when the ph of the onrete redues from 1.5 to 9.5 due to arbonation or inrease of the hloride ions onentration near the steel, the passive layer gets destroyed and an no longer protet the embedded steel from orrosion attak. Furthermore, the onrete made through using low water-ement ratios and good uring praties have a low permeability that minimizes the penetration of the orrosion induing ingredients. In addition, low permeability inreases the eletrial resistivity of the onrete to some degree, thus helping in reduing the rate of orrosion by retarding the flow of the eletrial urrent within the onrete that aompanies the eletrohemial orrosion proess. Consequently, orrosion of the embedded steel requires the breakdown of its passivity. Conrete is relatively weak in tension, and it raks when the tensile strength is exeeded in a reinfored member. Craking is an important phenomenon speifi to reinfored onrete, and it an have a signifiant influene on the durability of a onrete struture. The influene of new materials and new tehnologies being used presently, on the onrete tensile strength and the bond harateristis is not well established; the examples are the use of high-strength onretes, the use of fiber-reinfored plasti rebars, and the use of epoxy-oated rebars. Conrete tensile strength and toughness are fundamental properties that ensure bond effiieny at the steel-onrete interfae, as relatively low values of the bond stress-tensile strength ratio ( loal stress and strain state (Gambarova and osati, 1996). f t = 0.5 to 0.8) an exhibit a omplex

19 When the embedded reinforement orrodes, the strength of a reinfored onrete member is damaged in a variety of ways. The expanded volume of orrosion produts on the steel bar surfae develops internal pressure at the steel-onrete interfae whih auses high tensile stresses in the onrete member. When the tensile stress in the onrete exeeds its tensile strength, raks will form in the surrounding onrete. Also, with the inrease of orrosion, existing fine and miro raks in the surrounding onrete tend to enlarge and form a network of interonneted raks, providing inreased ioni transport between the surfae of the onrete and the surfae of the reinforing steel, effetively promoting the orrosion proess. Crak growth dereases onrete stiffness and tensile strength, while the formation of a network of raks inreases onrete permeability. Thus, the holding apaity and onfinement of the onrete member is dereasingly ompromised as raking progresses. As orrosion inreases, the normal ontat pressure at steel-onrete interfae is redued, ausing a onsiderable amount of deterioration of bond between the reinforing steel and onrete. In addition to the bond deterioration and with the inrease of orrosion, the ross setional area of the steel reinforement redues signifiantly and hene, it an no longer withstand the load and leads to the ollapse of the struture. Hene, it is essential to prevent the premature failure of reinfored onrete strutures by appropriate ontrolling and monitoring of reinforement orrosion. In spite of the well understanding of the eletrohemial proesses indued by orrosion, the effets of orrosion on bond apaity, and the determination of residual bond strength, whih is an important fator in prediting the servie life of strutures, are not well established. Bond between the reinforing steel and the onrete is dependent on ohesion and adhesion at the steel-onrete interfae and the mehanial interloking between the lugs or deformations of the reinforing bar and the surrounding onrete. Corrosion results in an early loss of both ohesion and adhesion. As steel orrodes, the orrosion produts at first improve bond by a slight amount, however, the inreasing levels of orrosion an result in longitudinal and transverse raking whih auses a release in the hold of the onrete on the bar and dereases the bond apaity at the steel-onrete interfae. 3

20 In summary, the reinforing steel is provided in reinfored onrete strutures to resist the tensile fores, and to produe ontrolled raking within that zone. In reinfored onrete members, onrete forms the body of the member and provides stiffness and resistane to ompression loads. While the steel reinforing bars (rebar) are plaed where tensile loads are expeted, so that one the onrete raks, the steel is present to resist the tension. However, orrosion not only deteriorates the steel bar and its funtion of transferring the tensile fores, but also it deteriorates the onrete through spalling of the over. Therefore, orrosion of the reinforement has a strong influene on the bond behaviour at the interfae between the steel reinforement and the onrete. As orrosion of the reinforing steel progresses, the bond strength between the reinforing steel and the onrete diminishes progressively, and major repairs or replaement are needed. While onsiderable researh has been undertaken about the problem, and numerous reports have disussed how this orrosion an be ontrolled, only limited data are available about its influene on the bond behaviour at the steel-onrete interfae. Some researhers have proposed analytial models to study the bond behavior of orroded reinforement (Coronelli (00), Wang and Liu (004), Bharagava et al. (007)); However, onsiderable variations in the predition of bond loss have been reported. Hene, a better understanding of the mehanism through whih orrosion affets bond is neessary to enable the ontrolling fators to be better understood, to resolve the apparent inonsistenies between different studies, and to enable effetive models to be developed. 1. Sope and objetive of the study The purpose of this researh is to model the ontat pressure at steel- onrete interfae by ombining the ation of onfining and orrosion pressure using a fritional model, and assuming the onrete as anisotropi material in the rak zone. The main objetives of the researh are: 1. to develop an analytial model for orrosion pressure at steel-onrete interfae, assuming the onrete in the raking zone as anisotropi material,. to develop an analytial model for onfining pressure at steel-onrete interfae, 4

21 3. to add the affet of adhesion to the developed model, and finally, verifiation, and alibration of the model for reinfored onrete systems of inreasing omplexity Thesis layout This thesis is omposed of six hapters. Chapter one addresses the sope and objetive of the present study. Chapter two presents fundamentals of bond and orrosion at steelonrete interfae. Chapter three reviews some of the latest models of bond behavior for orroded reinforing steel. Chapter four presents the analytial modeling of bond stress at the interfae between onrete and reinforing steel due to orrosion. Chapter five illustrates an analytial example, with results, inluding a disussion of the analysis. Finally, Chapter six presents a brief summary of the analytial observations, as well as the onlusions and reommendations for further researh and development on the influene of orrosion on bond behavior. 5

22 Chapter FUNDAMENTALS OF BOND AND COOSION BETWEEN STEEL AND CONCETE The behaviour of a reinfored onrete struture is influened by the bond at steel-onrete interfae. This hapter presents some basi information on bond behaviour between the onrete and the reinforing steel suh as bond mehanisms, failure modes of bond, raking behaviour and fators affeting the bond strength. It also presents some basi information on orrosion and the effets of orrosion on bond behaviour at steel-onrete interfae..1 Fundamentals of Bond.1.1 Introdution Bond between reinforing steel bar and surrounding onrete is neessary to ensure omposite ation of the two materials, and the load transfers between steel and onrete is required to maintain this omposite ation. This load transfer is named bond whih is idealized as a ontinuous stress that develops in the viinity of steel onrete interfae. aymond and Henry (1965) defined bond as that property whih auses hardened onrete to grip an embedded steel bar in suh a manner as to resist fores tending to slide the bar longitudinally through the onrete. At the steel-onrete interfae, bond failure will prevent the tensile fore to be developed in the steel bar, thus influening the resistane of the strutural element. In reinfored onrete strutures, an interation between the steel bar and the surrounding onrete is essential to transfer a fore between the two materials. Therefore, bond is fundamental beause it influenes many aspets of the behaviour of reinfored onrete suh as raking, deformability, and instability. 6

23 .1. Bond Stress Bond stress is defined as the shear stress at the steel-onrete interfae whih modifies the steel stress by transferring the load between the steel and the surrounding onrete (ACI Committee 408, 1966). Bond stress an be alulated as the stress per nominal unit area of the bar surfae. Also, bond stress an be measured by the rate of hange of steel stress in the bar. Thus, there will not be any hange in bar stress without bond stress or vie versa..1. Bond Mehanisms Aording to ACI Committee 408 (199), an effiient and reliable fore transfer from the reinforement to the surrounding onrete depends on three mehanisms; namely, adhesion, frition and mehanial interloking as shown in Fig..1 where V a is the adhesion, V b is the mehanial anhorage due to bearing of the lug and V f is the fritional resistane. Figure.1 Idealized fore transfer mehanism (ACI Committee 408, 199) 7

24 Adhesion: Adhesion is the hemial bond between the bar and the onrete whih is related to the shear strength at the steel-onrete interfae. For a small load, the basi resisting mehanism is the hemial adhesion; however when a deformed bar moves with respet to the surrounding onrete due to inrease in the loads, the hemial adhesion along the bar surfae is lost. Treee and Jirsa (1989) studied the adhesion mehanism for both unoated and epoxy oated steel reinforing bars. They found that there was no evidene for hemial adhesion between the epoxy oated and the onrete while the unoated bar was adhered to the onrete. Similarly, Cairns and Abdullah (1994) studied the bond harateristis at the steel-onrete interfae for unoated and epoxy oating steel plates. They noted that the unoated steel plates were overed with a layer of rushed mortar after failure, while the oated plates were observed to be lean after failure. Frition: Frition is the fore resisting the parallel displaement between two surfaes sliding against eah other. Frition plays a signifiant role in fore transfer between the onrete and the steel bar. Based on the work of Treee and Jirsa (1989), the ACI Committee 408 (199) suggested that frition an ontribute up to 35% of the ultimate strength governed by the splitting of the onrete over. Mehanial interloking: For deformed steel bars, bond depends primarily on mehanial interloking between the ribs and the onrete keys. In addition, the mehanial interloking of the deformed steel bar depends on the geometry of the ribs along the steel bar. As the ultimate bond strength is reahed, shear raks begin to form in the onrete between the ribs as the interloking fores indue large bearing stresses around the ribs, and slip ours. Therefore, the bar ribs restrain the slip movement by bearing against the onrete keys. The slip of a deformed bar may our in two ways, either through pushing the onrete away from the bar by the ribs, i.e. wedging ation, or through rushing of the onrete by the ribs. 8

25 Perfet bond for reinfored onrete members provides omplete ompatibility of strains between onrete and steel. However, in reality, perfet bond ours only in the regions where negligible stress transfers between onrete and steel. Whereas, in the regions where high stress transfers along the steel onrete interfae, suh as in the viinity of raks, the bond stress is related to the relative displaement between reinforing steel and the surrounding onrete. Therefore, strain ompatibility does not exist between reinforing steel and surrounding onrete near raks. The relation between bond stress and the relative displaement between reinforing bar and onrete is due to strain inompatibility and the rak propagation is known as bond-slip as shown in Fig.. Initially, with unraked onrete, bond stress is assured by the hemial adhesion between the steel and the onrete up to the point A as shown in Fig.. where the slip is relatively negligible. As mentioned earlier, one a deformed bar moves with respet to the surrounding onrete, surfae adhesion is destroyed as a onsequene of the wedging ation of the ribs whih pushes the onrete away from the steel. Figure. Typial bond stress-slip relationship (Girard and Bastien, 00) With the onset of slippage between the reinforing steel and the onrete, bond resistane will be developed by frition and mehanial interloking between the bar and the surrounding onrete. However, the bearing of the lugs beome signifiant for the bond 9

26 between steel and onrete. The onentrated bearing fores in front of the lugs will split into two diretions: Parallel omponents to the bar axis represents the bond stresses and the radial (perpendiular) omponents to the bar axis represents the irumferential tensile stresses. When these tensile stresses exeed the tensile strength of the onrete r, internal raks develop around the bar, and the deformation of onrete resulting from generated stresses tend to pull the onrete away from the reinforing bar in the viinity of a major rak as shown in Fig..3. Therefore, at point B in Fig.., the stiffness of the onrete is redued and longitudinal splitting raks are initiated by the inlined ompressive fores spreading from the lugs into onrete. The internal raks reah the onrete surfae at point C, and the bond resistane will drop to zero if suffiient onfinement is not provided. Thus bond failure due to splitting ours (Lundgren, 005). However, with the presene of suffiient onfinement, the load an be inreased further and pull out failure will our instead of splitting failure. At point D, shear raks will initiate in the onrete keys between ribs whih orrespond to the point of maximum bond resistane. The bond resistane is dereased with the inreasing slip due to spreading of shear raks through the onrete. Hene the fritional resistane of onrete along the failure surfae remains the only mehanism that exists at point E. Figure.3 Separation between the reinforing bar and onrete near primary rak (Lutz and Gergely, 1967) 10

27 When the tensile stress at a given loation exeeds the tensile strength of onrete, rak develops around the bar, and it is manifested by a separation of the onrete at this loation. Further loading will lead to loss of adhesion near the rak, and different seondary internal raks will form lose to the main rak whih may not propagate to the external surfae of the onrete. Steel stresses at the rak will reah a loal peak while between the raks; the steel stress is lower than that due to the onrete ontribution. The separation between steel and onrete in plain bars leads to omplete loss of bond stresses in the viinity of the rak. However, in the deformed bars, separation does not lead to omplete loss of bond, and bond fores are transmitted by the rib bearing in the viinity of main raks, as shown in Fig..4. Figure.4 Formation of internal raks (Goto, 1971).1.3 Bond Failure Modes Prinipally, the bond failure between steel and onrete an be desribed by two modes: pull-out and splitting failures. If the onrete is well onfined or the ratio of onrete over to bar diameter is more than three (Cairns and Abdullah, 1996), splitting does not 11

28 our and bond failure is aused by bar pullout due to the shearing off of the onrete keys between the bar ribs. The mehanism of fore transfer hanges from rib bearing to frition along the vertial line between the tops of the ribs as shown in Fig..5a. In the ase of medium onfinement where a suffiient amount of transverse reinforement is provided, rushing or shearing-off of the onrete below the ribs aompanied by longitudinal raks will our through the entire over thikness as shown in Fig..5b. If the onrete over to bar diameter ratio is less than three (Cairns and Abdullah, 1996) or the steel bars are losely spaed, the longitudinal raks aompanied by slip on the rib fae break out through the entire over thikness as shown in Fig..5. Figure.5: Modes of bond failure (a) heavy onfinement pull-out; (b) medium onfinement, splitting indued pull-out aompanied by rushing and/or shearing-off in the onrete below the ribs; and () light onfinement splitting aompanied by slip on the rib fae (Task group bond model, 000) 1

29 Slipping of the deformed bars an our due to rushing of the onrete in front of the ribs, and splitting of the onrete by wedging ation (ehm (1968) and Lutz and Gregely (1967)). ehm (1968) related the bond failure modes to the ratio of rib height to rib spaing. When the ratio is greater than 0.15, the bond failure ours due to the shearing off of the onrete keys between the bar ribs and the bar will pull out as shown in Fig..6a. When the ratio is less than 0.15, the bond failure ours due to rushing of the onrete in front of the ribs, and the deformed bar will split from the surrounding onrete as shown in Fig..6b Figure.6 Failure mehanisms at the ribs of deformed bars (ehm, 1968) Longitudinal splitting raks develop when the onrete separates from the reinforing bar at a primary rak due to an inrease in the irumferential tensile stresses. Tepfers (1973) studied the irumferential stress distribution over a thik walled ylinder onfining the reinforing bar as desribed later on in setion 4.1. Tepfers assumed three stages of bond response of the onrete ylinder: the unraked stage, partially raked stage and the plasti stage. Tepfers (1979) derived equations for the three stages by assuming short anhorage lengths, and found good agreement between the measured values of the short anhorage tests with the partially raked theory. Aording to the 13

30 raked elasti behaviour, the bond strength, at the raking of the onrete over, is given by equation (0.5 / d ) f (.1) b t For larger onrete over thikness, the assumption of a plasti behaviour at the steelonrete interfae, gives: ( / d ) f (.) b t Where, Bond strength when onrete raks = minimum onrete over thikness d b = diameter of the steel reinforing bar f t = onrete tensile strength.1.4 Fators Affeting the Bond Strength Bond strength between the steel and onrete depends on several fators suh as onrete and steel strengths; bar size and profile; onrete over thikness; embedment length of steel; spaing of bars; stirrups; temperature; orrosion et. A brief desription of some of these fators that influene the bond strength at steel-onrete interfae is presented in the following setions Conrete Strength Compressive strength is onsidered to be a signifiant parameter in bond behaviour beause the fore between steel and onrete is transferred mainly by bearing and bond (Orangun et al. 1977). Tepfers (1973) showed that the slope of the bond stress distribution varies onsiderably over the splie length with a higher onrete strength 14

31 when ompared to that with lower onrete strengths. Sine bond failure an our by tensile splitting and shearing off of the onrete, the ompressive strength is onsidered to be a signifiant key in bond behavior (ACI Committee 408, 199). It has been found that the bond of high strength onrete is proportional to the ompressive strength of onrete (Alavi-Fard and Marzouk, 00). However, test result indiates that the square root has proven to be adequate as long as onrete strengths remain below about 55 MPa (ACI Committee 408, 199), while for high strength onrete, it is observed that 1/ f 4 provides the best fit for the effet of ompressive strength on the onrete ontribution to bond strength for bars not onfined by transverse reinforement (Zuo and Darwin 1998, 000). Zuo and Darwin (1998, 000) found that 3 / f 4 provides a good fit for the effet of ompressive strength on the onrete ontribution to bond strength for bars onfined by transverse reinforement. The tensile and ompressive stresses of onrete ontribute to the development of bond stresses. For example, miro raks are ontrolled by the tensile stresses of the onrete, while bearing stresses indue high ompressive stresses in front of the ribs. Martin (198) observed that for a slip range of 0.01 to 1 mm, the bond stress is proportional to the onrete ompressive strength, based on the pullout test results, with onrete strengths varying from 16 to 50 MPa; However, for very small slip less than 0.01 mm, and for high slip larger than 1 mm, the effet of the onrete ompressive strength is less important and proportional to / 3 f Conrete Cover Thikness and Bar Spaing Bond strength inreases with inreasing over thikness and bar spaing (ACI Committee 408, 003). Tepfers (1973), Orangun el al., (1977), and Eligehausen (1979) observed that the onrete over and the bar spaing signifiantly influene the type of bond failure. Splitting tensile failure ours with small onrete over and bar spaing, while pullout 15

32 failure ours with large onrete over and bar spaing. For most strutural members, splitting failure is expeted and an ours between the bars, between the bars and the free surfae, or both, while pullout failure an ours with some splitting if the member has signifiant transverse reinforement to onfine the anhored steel (ACI Committee 408, 003) Transverse einforement The amount and distribution of transverse reinforement influenes the type of bond failure (Tepfers, 1973; Orangun et al., 1977; Eligehausen, 1979). The inrease in the transverse reinforement inreases the onrete onfinement whih results in an inrease in bond fore, and onverts the splitting failure to a pullout failure. Additional transverse reinforement, above that needed to onvert the splitting failure to a pullout failure beomes less effetive, eventually providing no inrease in bond strength (Orangun et al., 1977) Bar Size The relationship between bar size and bond strength is not always appreiated due to the following reasons (ACI Committee 408, 003): 1. The inrease in the bar size inreases the length of development.. For a ertain development length, larger bars ahieve higher bond fores than smaller bars for the same degree of onfinement. Therefore, it is desirable to use several of the small bars instead of using a few large bars and maintain a reasonable lear distanes between the bars (ACI Committee 408, 003). The bar size also plays an important role in the ontribution of onfining transverse reinforement to bond strength. When the larger bars slip, higher stresses are mobilized in the transverse reinforement, thus better onfinement for onrete is provided. 16

33 Therefore, the effet of transverse reinforement on the bond strength is the same as the bar size effet (ACI Committee 408, 003) Bar Profile The stress transfer between the reinforing bar and the surrounding onrete depends on the resistane to relative motion or slippage between the onrete and the surfae of the embedded steel bar due to the bond at steel-onrete interfae. It is well known that this mehanism of stress transfer is the base of the theory of reinfored onrete. The geometry of the bar rib has great influene on the bond strength due to the importane of the mehanial interloking to the bond strength. Previous studies (ehm, 1961; Lutz et al., 1966; Darwin and Eheneze, 1993) indiate that the geometry of the lugs affet the bond strength of anhored bars. It was onluded from their studies that bond strength of deformed bars would improve with an inrease in the rib bearing area (projeted rib area normal to the bar axis) to the rib shearing area (bar perimeter times enter-to-enter distane between ribs) ratio. This ratio is known today as the relative rib area. r Lutz et al. (1966) showed that slip ours due to the rushing of the onrete in front of the ribs when the rib fae angle (the angle between the fae of the rib and the longitudinal axis of the bar) is greater than 40 degrees produing a fae angle between 30 to 40 degrees from rushed onrete, and when rib fae angle is less than 30, no rushing of the onrete ours in front of the rib. In addition, if the fae of the rib formed an angle of 90 degree with the axis of the bar, all of the bond strength will be arried out by the diret bearing of the rib against the onrete key. In this ase, frition between the onrete and steel will not ontribute to the bond strength, but this ase an not be ahieved due to insuffiient ompation of the onrete in front of the rib whih oppositely affets the bond strength. However, if the rib fae angle is zero degree as in a plain bar, the frition aused by adhesion between the onrete and steel will be the only bond omponent, and loss of this adhesion will destroy the bond. 17

34 As a result from the rushing of the onrete in front of the rib, Choi and Lee (00) found that the range of the effetive rib fae angle was between 5 and 35 degrees, whih is lower than the atual rib fae angle, and when the bars are not onfined by transverse reinforement, the relative rib area has a little effet on the bond strength of deformed bars Steel Yield Strength The bond stress is related to the fore in the steel. When the strain in the bar exeeds the yield strain, the interloking effet between the bar ribs, and the onrete is dereased due to the influene of the effet of lateral bar ontration on the frition mehanism; Therefore, the bond stress dereases signifiantly after steel yielding (Task Group Bond Models, 000). Aording to the ACI Committee 408 (003) report, the average bond stress for bars that yielded before bond failure is signifiantly lower than that of the bars with high strength steel. Studies show that when the onrete is not onfined by transverse steel reinforement, % of the bars yielded before bond failure produe average bond stresses, and 10% yielded after bond failure produe average bond stresses when onfined by transverse reinforement, ompared to similar bars with the same bonded lengths made of higher strength steel that does not yield (Darwin et al. 1996a; Zuo and Darwin 1998, 000) Bar Casting Position It was observed that bar asting position plays an important role in the bond strength between onrete and reinforing steel. Top-ast bars have lower bond strengths than bottom ast bars (Jeanty, Mithell, and Mirza 1988). Luke et al. (1981) studied the affet of asting position on the bond strength; they found that the bond strength dereases with inreasing the depth of onrete below the bar as shown in Fig..7. It an be noted from Fig..7 that bond strength dereases with 18

35 inreasing slump. However; this derease is mostly for top-ast bars while for bottom ast bars, slump appears to have little effet. The reason for that is the water and air trapped will be greater under top bars. In addition, the relative downward movement of the surrounding onrete aused by settlement of the fresh mixture inreases with the inrease of the depth of onrete below the bar. Figure.7 Bond strength as a funtion of bar loation within a wall speimen High slump = 8-1/ in. (15 mm). Low slump = 3 in. (75 mm) (Luke et al., 1981) Effets of Corrosion For very low levels of orrosion, when there is no longitudinal raking; the orrosion produts have a benefiial effet of improving the bond harateristis at the steelonrete interfae beause it inreases the surfae roughness and hene the fritional fore. While at high levels of orrosion, the steel bars display loalized pitting and loss of some of the ribs over the bar length, result in the weakening of mehanial interloking mehanism at the steel-onrete interfae. When reinforement orrodes, the strength of a reinfored onrete member is undermined in a variety of ways. The expanded volume of orrosion produts on the steel bar surfae develops internal pressure at the steel-onrete interfae whih auses high tensile stresses in the onrete speimen. When the tensile stresses in the onrete exeed its tensile strength, raks will form in the onrete. With the inrease of orrosion, 19

36 existing fine and miro raks in the surrounding onrete tend to enlarge and form a network of interonneted raks, providing inreased ioni transport between the surfae of the onrete and the surfae of the reinforing steel, effetively promoting the orrosion proess. Crak growth dereases onrete stiffness and tensile strength, while the formation of a network of raks inreases onrete permeability. Thus, the holding apaity and onfinement of the onrete member is dereasingly ompromised as raking progresses. As orrosion inreases, the rak width inreases, and this results in the breakdown of ohesion, adhesion and frition at the steel-onrete interfae. Amleh and Mirza, (1999) examined the affet of the orrosion on the number and spaing of the transverse raks. They found that as the level of orrosion inreases, the transverse rak spaing inreases, refleting the deterioration of bond harateristis at the steel-onrete interfae..1.5 Measurement of Bond Many different methods have been used to investigate the bond harateristis of the steel reinforement in the onrete. Aording to Nawy (1996), bond tests an be lassified into three groups: pull-out tests, embedded bar tests and beam tests. This lassifiation inludes the pullout tests (both the onentri and the eentri), variety of bond beam tests (the National Bureau of Standards beam, the University of Texas beam), semi beam speimen test, and the standard tension speimen. A good overview of the urrent tests used to determine the bond harateristis of reinforing bar an be found in ACI Committee 408 (199), Park and Paulay (1975), and MaGregor (1997), Ferguson (1988),. The main aim of the bond test is to determine the stresses transferred from steel to onrete and vie versa under servie onditions Pullout Tests Pullout test is the most widely used by researhers beause of its simpliity. In this test, a bar is embedded in the entre of a onrete ylinder or prism, and the fore required to 0

37 pull out the bar or make it slip exessively is measured, as illustrated in Fig..8. In this type of test, a small load auses a slip and develops a high bond stress near the loaded end, leaving the upper part of the bar totally unstressed as shown in Fig..9. However, this test appears useful where relative bond resistane is ompared rather than real bond resistane is obtained (Ferguson 1988). The slip at the loaded end inreases when the applied load is inreased whih leads to the high bond stress and the slip extends deeper into the onrete speimen. If the embedment is long enough, the bond strength is higher than tensile strength of bar, and failure ours due to bar rupture, while if the bar is very short, or light weight aggregates were used, the bond strength is less than the tensile strength of the bar, and failure ours due to bar pullout. In ase longitudinal splitting of the onrete ours, failure is initiated due to onrete raking. Figure.8 Shemati diagram of a pullout test (Amleh, 000) 1

38 Figure.9 Effet of embedment length on the distribution of bond (Leonhardt, 1964) This method is not intended for establishing bond strength values for strutural design purposes beause in reinfored onrete beams or slabs, the onrete surrounding the tensile reinforement is in tension, whereas the onrete in this test is in ompression, whih not only inreases the bond strength but also eliminates tension raks in the speimen. Furthermore, this type of test is not subjeted to external shear or bending moments, whih are present in the atual strutures. Therefore, the failure patterns in the pullout test are not realisti (Almusallam et al. 1996) Tension Tests Modifiation to onentri pull out test to eliminate ompression on the onrete speimen is alled the tension pullout test (Ferguson 1988). However, the interation between spaed splies and rak pattern introdues problems in this test (Ferguson 1988). Goto (1971) performed this type of test to larify the pattern of raks around the tensile reinforing bars. In this test, a steel bar is embedded in the entre of a onrete ylinder, and subjeted to applied loads at its ends as shown in Fig..10. When the applied fores at the speimen

39 ends on the steel bar are inreased, the bond stresses at the steel-onrete interfae inrease gradually; this leads to an inrease in the fore transmitted to the onrete until it reahes the tensile apaity onrete at a setion an be obtained from equation (.4). TC at whih the setion raks. The tensile apaity of T f. A (.4) C t t Where f t is the tensile strength of the onrete, and A t is the onrete area subjeted to tension. Note that just before the rak forms, the fore transferred from the steel to the onrete is dependent on the bearing of the lugs beause the adhesion between the steel bar and the onrete is exhausted. The onrete setion with the tensile fore TC leads to redistribution of the stresses in the steel and the onrete, and the bond stresses. At the setion where the rak formed, the steel fore is equal tot, while the applied fore at the end of the speimen, and the resultant onrete fore is zero. The redistribution of the various stresses is shown in Fig..10. If the raks are widely spaed, this redistribution an lead to inrease the tensile fore in the onrete somewhere between the rak and the free end tot C, whih in turn leads to form a rak at this setion, and the steel, onrete and the bond stresses will be redistributed as shown in Fig..10. Fig..11 is showing the free body diagram of the steel bar for different onditions of raks. This proess will be repeated as long as stress in the onrete between the raks will reaht C, otherwise, the raking proess will stabilize, and no further raking will our in the speimen. One the raks have stabilized, any further inrease in the load applied to the speimen will ause only an inrease in the steel fore at the rak setion, until finally the bar yields, and will not result in any additional raks. Aording to that, the bond stress between the raks remains almost onstant. Therefore, the spaing between the raks will be inreased as a result of orrosion at the steel-onrete interfae. 3

40 Figure.10 Variation of steel, bond and onrete stresses in a tension speimen (Amleh, 000) 4

41 Figure.11 Free body diagrams showing fores on the steel bar in a tension speimen with no, one and three raks (Amleh, 000) Beam Tests The influene of flexural tension raks is inluded in beam tests, therefore they are onsidered more reliable than other bond tests (Ferguson 1988). Beam tests an be divided into two types: National Bureau of Standards beam test as shown in Fig,.1, and the University of Texas beam test as illustrated in Fig..13. Figure.1 Bond beam test National Bureau of Standard bond test beam (Ferguson, 1988) 5

42 Figure.13 Bond beam test- University of Texas (Ferguson, 1988) The results of this type of test are onsidered more reliable beause the tests truly represent the atual bond stress onditions enountered in the flexural members. However, the major onern in the bond beam test is the reation restraint that might inrease the onfining of the onrete over the bar at the supports by inreasing the splitting resistane (Ferguson 1988). Semi-beam speimen or antilever beam tests have been used to redue the speimen size and its ost. This test was developed by Kemp et al. (1968) to overome some of the disadvantages in the pullout test as shown in Fig..14. Some of the advantages of this test are (Kemp et al., 1968): 1. The bond stress obtained from this test is similar to that in the atual flexural members beause of the presene of both external shear and bending moments in the test speimen.. The tensile strains in the steel bar and onrete are similar to those ourring in atual strutures 3. Different types of failure an be produed 6

43 The disadvantages of this test are the onfining pressure on the steel bar, whih inreases the beam length to overome the splitting resistane, and the low ratio of shear to bond stress (Ferguson 1988). Figure.14 Shemati bond test (Kemp et al., 1968). Fundamental of Corrosion..1 Mehanism of einforement Corrosion Corrosion of steel embedded in onrete is an eletrohemial proess of the transformation of a metal towards its "natural" form whih is its ore state. This transformation ours beause the metal in its ore state suh as the oxides ontain less energy than pure metals; therefore, they are more thermodynamially stable. The orrosion proess takes plae as a series of eletrohemial reations with the passage of an eletri urrent only when both anodi and athodi reations are possible. Corrosion depends on the type and nature of the metal, the immediate environment, temperature and other related fators. 7

44 The steel bar embedded in onrete is normally proteted from orrosion as a result of the high alkalinity of the onrete; the ph of the pore water an be greater than 1.5, whih protets the embedded steel against orrosion. At this high ph level, a mirosopi oxide layer known as the `passive' film, forms on the steel surfae during the early stages of ement hydration whih naturally protets the embedded steel from orroding. However, when the ph of the onrete hanges from 1.5 to 9.5 due to arbonation or inrease of the hloride ions onentration near the steel, this layer is destroyed and an no longer protet the embedded steel from orrosion attak. Carbonation refers to the reation between the arbon dioxide, present in the air, penetrates into the onrete and the alium hydroxide whih is a primary hydration produt that provides the pore solution with its alkalinity. As a result of these reations, ph in the pores of the ement paste dereases to about 9.5 (Kyle et al, 1999). The penetration of the arbon dioxide depends on the quality of the onrete suh as waterement ratio, and hydration and the degree of saturation of the pores in the ement paste. The presene of the hloride ions, either in the onrete mix or due to the ingress from the immediate environment, also breaks down the passive layer, when the hloride ions reah a threshold value. Chloride ions reat with the passive film to form a soluble iron hloride omplex, [FeCl] + (Mindess, 003). Subsequently this hloride omplex reats further with the hydroxyl groups in the solution resulting in the subsequent release of hloride ions. This release of hloride ions allows for the proess to propagate itself, as well as simultaneously bonding free alium hydroxide. As a result, the orrosion proess fouses at loal area instead of spreading along the bar, and this result in the formation of deep pits and loal loss of bar ross setional area. Therefore, the damage due to hloride ingress is so dangerous. In order for orrosion to take plae there are four riteria that must be met (Corrosion in reinfored onrete strutures 005): 8

45 1. An anodi reation must be possible by the breakdown of the passive layer that protets the steel at high alkalinities beause of lowered ph in pore water due to arbonation or ingress of hloride into onrete reahing a ritial level. The anodi reation is haraterized by: Fe Fe e (.5). A athodi reation must be possible due to the presene of oxygen at the steel interfae. The athodi reation is written as: O 4e H O 4 OH (.6) 3. A flux of ions is possible. Within onrete the eletrolyte pore solution serves as a bridge for the transport of ions from athode to anode 4. A flux of eletrons is possible. The reinforement itself serves as the medium for the transport of eletrons between the sites on anodi and athodi reations. Anodi and athodi reations take plae at the surfae of the orroding steel whih funtions as a mixed eletrode that is eletrially onneted through the body of steel itself. eations at anodes and athodes are referred to as half-ell reations. The anodi reation is the oxidation proess, whih results in dissolution or loss of metal (loss of eletrons) while the athodi reation is the redution proess whih results in the redution of dissolved oxygen forming hydroxyl ions. At anodi area, the arrived hydroxyl ions OH eletrially neutralize the form a solution of ferrous hydroxide at the anode: Fe ions dissolved in pore water and Fe OH Fe( OH) (.7) 9

46 This ompound Fe(OH) reats further with additional hydroxide and available oxygen, to form the water insoluble red rust (ferri hydoxide): 4Fe( OH) OH O H O 4Fe( ) 3 (.8) Anodi and athodi sites are eletronially onneted as they exist on the same rod and they are ionially onneted by onrete pore water funtioning as a omplex eletrolyte as shown in Fig..15. Figure.15 Miro-orrosion ell formations in reinfored onrete (Azher, 005) ed rust is not the only orrosion produt of steel in onrete. Other ompounds suh as blak rust, Fe O, green rust, 3 4 FeCl, and other ferri and ferrous oxides, hydroxides, hlorides, and hydrates are also formed. Their omposition depends on the availability of the pore water, its ph and omposition, and oxygen supply. Fig..16 shows the relative 30

47 inrease in the volumes of the various oxides and hydroxides of iron, whih inreases onsiderably when water moleules ombine with them. Figure.16 The relative volumes of iron and its orrosion reation produts (Nielsen, 1985) This rust an have a volume two to six times that of the parent iron from whih it is formed. The rust produt an exert large pressures (similar to bursting pressures in pipes) and ause raking of the onrete over leading to its eventual spalling. In addition to loss of over onrete, a reinfored onrete member may be damaged due to the loss of bond between steel and onrete and loss of rebar ross setion. Therefore, it an be noted that oxygen and moisture are the most important elements for reinforement orrosion to our and the ingress of these omponents through the onrete must be ontrolled to avoid orrosion. Aording to the different spatial loations of anode and athode, orrosion of steel in onrete an our in two forms: 1. As miroells, where anodi and athodi reations are adjaent to eah other, and the distane between them may be a miron. Miroell orrosion leads to a 31

48 uniform iron dissolution over the whole surfae whih is generally aused by arbonation of onrete or by very high hloride ontent at the steel surfae.. As maroells, where anodi and athodi reations are separated by a finite distane, whih may be entimeters or meters. The anode and athode may our at the same bar or on different bars with eletrial ontinuity. Maroell orrosion is more important beause the redution in ross-setional area of the rebar may be extremely aelerated due to the large athode to anode area ratio whih may lead to strutural safety problems... Effets of Corrosion on einfored Conrete When the reinforing bar orrodes, the properties of onrete an be affeted in a variety of ways depending on the environment, length of exposure, and onrete type. Corrosion results in the loss of strength, loss of stiffness, loss of bond strength, loss of servieability, and raking and spalling of reinfored onrete whih an perform individually in ombination with eah other in order to provide a large variety of potential effets. Spalling of onrete leads to a redution in the ultimate apaity, and more signifiantly, a redution in the stiffness and dutility of the reinfored onrete setion due to the loss or breakdown of the bond at steel-onrete interfae. Craking of onrete results in redution in stiffness of the material, and inreases the permeability of the onrete that leads to more ritial environmental effets. When reinforement orrodes, the formation of ferri hydroxide Fe(OH) 3 is aompanied by a large expansion of volume. The expanded volume of orrosion produts on the steel bar surfae exerts an outward pressure on the onrete and as the pressure builds, the tensile stresses of the onrete may be exeeded the tensile strength of onrete aused through build-up of even a mirosopially thin layer of orroding reinforement (ihardson, 00). The ultimate result is raking of the onrete, whih in turns results in delamination and spalling stages as illustrated in Fig..17. The first sign of distress ould be pop outs or long thin raks along the line of the reinforement. 3

49 Figure.17 Stages in Corrosion-indued damage (ihardson, 00) At initial stage, the formation of small amounts of orrosion produts inreases the bond strength by reduing the onrete porosity (Kyle et al, 1999). However, further inrease in the orrosion level develops internal pressure at the steel-onrete interfae whih auses high tensile stresses in the onrete speimen that leads to onrete raking. Craks an redue the overall strength and stiffness of the onrete struture. In addition, these raks an inrease the ingress of aggressive ions whih result in onrete deterioration. The formation of large quantities of orrosion produts may result in loal expansions. Thus, raking, spalling and delamination of the onrete take plae, resulting in failure of the struture as shown in Fig..18 (Zhou et al, 005). 33

50 Figure.18 Corrosion indued damage of C strutures (Zhou et al., 005) The bond strength inreases in the beginning up to a ertain level of orrosion then dereases when orrosion is very high. The reason for that is the inreases in the roughness of the reinforing bar surfae with the growth of a firm layer of orrosion, whereas the loss in bond with further orrosion is due to the severe degradation of bar ribs, the lubriating effet of the flaky orroded metal on the bar surfae, and the redued onrete onfinement of the bar due to the widening of the longitudinal orrosion rak (Al-Sulaimani et al. 1990). Similarly, Almusallam et al. (1996) showed that the ultimate bond strength inreases with orrosion level from 0 to 4% of mass loss. The reason for that is the inrease in the onfinement pressure due to the pressure exerted from the expansive orrosion produts on the surrounding onrete as well as an inrease in the bar roughness in the initial stage. The ultimate bond strength initially inreased with an inrease in the degree of orrosion until it attained a maximum value of 4% rebar orrosion after whih there was sharp derease in the ultimate bond strength up to 6% rebar orrosion. Beyond the 6% rebar orrosion level the ultimate bond strength did not vary muh even up to 80% orrosion as shown in Fig..19. Almusallam et al. onluded that the signifiant redution in the bond strength due to signifiant degradation whih redued the mehanial interloking of the ribs of the lugs ausing the deformed bar to at as a plain bar. In addition, a redution in the frition 34

51 Ultimate bond strength KN between the bar and the onrete due to aumulated rust layer around the bar, and the redution of the onfinement of the onrete around the steel bar due to the formation of the rak was observed Degree of Corrosion, perent loss in weight Figure.19 elationship between the ultimate bond strength and different degrees of orrosion for antilever beam test (Almusallam et al., 1996) Another effet is the loss of steel area; orrosion is one of the important auses of steel area loss whih appears uniformly along the length of the reinforement. In general, orrosion has two effets: firstly, it will redue the ross-setional area of the steel and seondly, it will reate loal disontinuities in the steel surfae. These effets redue the tensile apaity of the steel due to the loss of its ross-setional area. Thus, the rosssetional area of steel dereases as long as the orrosion produts inrease; therefore, the ultimate moment apaity of struture also dereases, in addition to the bond deterioration, till the area of the steel beomes so small that it an no longer withstand the load and leads to the ollapse of the struture. Yoon et al. (000) stated that the orrosion redues the ross setional area of the reinforing steel that may ause some stress onentrations in the reinforing steel, whih results in dereasing the dutility of the struture espeially when pitting orrosion ours. 35

52 It was determined that orrosion not only affets the bond strength, but it an hange the mode of failure as well. odrigues et al. (1997) notied that with orrosion, the failure mode was shifted from bending to shear failure. This hange in mode of failure was attributed to the redution of onrete setion due to spalling of top onrete over and redution of stirrup setion due to pitting. The failure mode in beams with low tensile reinforement was in bending while the beams with high ratio of shear reinforement and low unorroded tensile reinforement failed by bending in onrete. Fig..0 shows the different types of failure modes that were observed by odriguez et al. as detailed below: Type 1 - ourred in both orroded and un-orroded beams with a low tensile reinforement ratio. Type - ourred in beams with high ratio of un-orroded tensile reinforement and most orroded beams with a high ratio of shear reinforement. Type 3 - ourred in almost all the beams with high ratio of orroded tensile bars and large stirrup spaing. Type 4 - ourred in orroded and un-orroded beams with urtailed tensile reinforement. 1) Failure by bending (yielding of tensile reinforement). ) Failure by bending (rushing of onrete). 3) Failure by shear. 4) Failure by both shear and bond splitting. Figure.0 Different types of failure modes (odrigues et al., 1997) 36

53 It was also observed that the orrosion inreases the rak width and defletions at servie load, leading to a derease in the bond strength, and an inrease in both spaing and raking width at ultimate load (odrigues et al. 1997). Jin and Zhao (001) observed that the failure mode of orroded reinfored onrete beams hanged from dutile mode to brittle mode and was similar to that of plain onrete with the inrease of the bar orrosion as shown in Fig..1. Both beams BD1, and BD10 failed in flexure; however, with the slightly orroded beam BD1, there are several main raks that appeared at the bottom of the beam while in highly orroded beam BD10; the raks appeared only in one plae. They found that the distribution of raks of orroded reinfored onrete beams beame onentrated instead of sattered. Figure.1 Different Failure forms of beam speimens (a) BD1 (b) BD10 (Jin and Zhao, 001) 37