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Thesis_2015

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Thesis_2015

  1. 1. 1 Faculty of Engineering and Physical Sciences Civil Engineering Effect of Reinforcement Corrosion on Flexural Strength of Reinforced Concrete Beams Varun Poddar A dissertation submitted in partial fulfilment of the requirements for the Degree of Master of Science in MSc Bridge Engineering 2014-2015
  2. 2. 2 Declaration of Originality I confirm that the dissertation entitled “Effect of Reinforcement Corrosion on Flexural Strength of Reinforced Concrete Beams” for the partial fulfilment of the degree of MSc in MSc Bridge Engineering, has been composed by myself and has not been presented or accepted in any previous application for a degree. The work, of which this is a record, has been carried out by myself unless otherwise stated and where the work is mine, it reflects personal views and values. All quotations have been distinguished by quotation marks and all sources of information have been acknowledged by means of references including those of the internet.’
  3. 3. 3 Summary This report deals with the effects the corrosion on flexural strength of reinforced concrete beam. The report follows the logical order to first describe the corrosion process i.e initiation and propagation of corrosion. The report also discusses different mechanisms of corrosion possible in reinforced concrete beam. Different techniques available for preventing corrosion in RC structures have also been discussed in detail. Once the basics of corrosion are dealt with, different deterioration models for flexural beams have been discussed. There are three ways to estimate the residual strength of corroded beams namely, empirical, experimental and analytical by use of modelling softwares. Experimental evaluation gives the most accurate results. However, because it is destructive in nature, it cannot be used for structures that are already in use. Therefore, empirical models and analytical models are important tools for carrying out the analysis of damaged structure which are already in use. This report deals only with empirical and analytical models. As a part of dissertation project, corroded reinforced concrete beam has been modelled and analysed in Ansys 14.5. Bar diameter has been varied from 32 mm to 27 mm. Flexural strengths of the beam is analysed by first considering that there is no bond degradation at concrete steel interface and then considering that there is bond degradation, which is calculated using empirical models. The result of this analysis are then compared with those obtained from empirical models. It is observed that results from two models are similar when bond degradation is not considered. However, there is significant divergence in results when bond degradation is considered. This may be attributed to the absence of consideration of spalling effects in Ansys model. However, to be certain, experimental studies will need to be conducted to verify the results. Based on the experimental observations, the analytical model can be further improved to better represent the real world scenario.
  4. 4. 4 Acknowledgements First of all, I am particularly grateful for the assistance given by Dr Stergios Mitoulis for his valuable advice and supervision of this dissertation. I am extremely grateful to my family and friends for their unconditional support and understanding at all times. Thanks for always being there when I needed it. Last, but not least, I would like to say thanks to all those living with me in Hamilton Drive 13, for the improvised Zumba lessons and the laughs (and sometimes tears) shared.
  5. 5. 5 Contents Summary.................................................................................................................... 3 Acknowledgements .................................................................................................... 4 Abstract...................................................................................................................... 7 Introduction ................................................................................................................ 8 Research Gap and Objectives ................................................................................. 10 Research Gap....................................................................................................... 10 Objectives ............................................................................................................. 10 Literature Review ..................................................................................................... 11 Basics of Corrosion............................................................................................... 11 Corrosion of Steel in Concrete.............................................................................. 13 Approaches available for prevention of corrosion ................................................. 15 Fusion bonded epoxy coated reinforcement...................................................... 15 Waterproofing membranes ................................................................................ 16 Corrosion Inhibitors............................................................................................ 16 Cathodic Protection ........................................................................................... 17 Inspection.............................................................................................................. 21 Need for detection ............................................................................................. 21 Half Cell Potential Measurements...................................................................... 21 Chloride Content determination ......................................................................... 21 Polarization decay testing.................................................................................. 21 Case Study I ......................................................................................................... 22 Case Study II ........................................................................................................ 23 Abilities for maintaining RC structures .................................................................. 24 Effects of Corrosion .............................................................................................. 24 Loss of Bond Strength ....................................................................................... 25 Loss of Area....................................................................................................... 27 Effects of Reinforcement Corrosion on reinforced concrete.................................. 28 Cracking ............................................................................................................ 28 Spalling.............................................................................................................. 29 Mode of Failure of beam with Corroded Steel....................................................... 30 Empirical Models................................................................................................... 31 Sample Calculation............................................................................................ 33 Methodology............................................................................................................. 35
  6. 6. 6 Introduction ........................................................................................................... 35 Problem description .............................................................................................. 36 Preprocessing....................................................................................................... 36 Element type......................................................................................................... 36 Real constants ...................................................................................................... 39 Material Properties................................................................................................ 40 Modeling ............................................................................................................... 41 Meshing ................................................................................................................ 44 Solution................................................................................................................. 45 Analysis type...................................................................................................... 45 Boundary conditions .......................................................................................... 45 Post processing ................................................................................................. 46 Results and Discussions .......................................................................................... 47 First crack induced Load....................................................................................... 47 Results Contours .................................................................................................. 48 Conclusion ............................................................................................................... 56 Future Work ............................................................................................................. 57 Experimental Studies ............................................................................................ 57 Analysis of Prestressed Beams ............................................................................ 58 Determination of shear strength............................................................................ 58 References............................................................................................................... 59
  7. 7. 7 Abstract Majority of infrastructure in developed countries like UK and USA was built before 1970. This has led to large number of structures needing maintenance to be able to carry on with their intended purpose. Among many forms of deterioration, corrosion is the largest contributor to the degradation of structures. According to the NACE International which is a world’s leading professional organisation for the corrosion control industry, corrosion amounts to 3.1% loss of GDP of developed countries every year. There are different techniques available to counter corrosion in built structures like use of waterproof membrane, corrosion inhibitors and electrochemical repairs among many others. This report briefly describes the process of corrosion, different techniques available to quantify the extent of deterioration and ways to protect structures against corrosion. It further delves into mechanisms which leads to loss in strength. As a part of dissertation project, a beam was modelled in Finite Element Software with varying cross sections of reinforcement. Subsequently, flexural strength variation with the varying cross section has been discussed. The results from the software analysis have been compared to the ones derived from empirical analysis.
  8. 8. 8 Introduction At the time of construction of reinforced and prestressed structures, it was thought that they would be maintenance free because the reinforcement and tendons were embedded inside the concrete and therefore there was no risk of corrosion. However, problems related to corrosion damage to structural components have come to the fore in last few decades. One such example is the sudden collapse of Ynys-y Gwas Bridge in 1985 and Malle Bridge in 1992. These collapses were attributed to the corrosion of tendons inside the beams. In case of Ynys-y-Gwas Bridge, ingress of chloride ions through segmental joints was believed to lead to pitting corrosion of tendons which finally led to the collapse of bridge (Woodward et al., 1988) Due to no certain way to know the condition of tendons and further estimate the remaining strength of the beams, bridge owners are left with very little choice other than to replace the whole bridge, which is generally very expensive and budget is not always available for the same. One of the major reasons for degradation of reinforced concrete beams is due to the corrosion of bars in the concrete. It is important to consider that however, loss in strength and failure of the beam does not occur solely due to loss in strength of steel, but there are many more contributing factors. Whenever, the reinforcing bars corrode in the structure, this is accompanied by expansion of corroded products, leading to cracking and eventual spalling of the concrete cover. Also, there is loss of bond between concrete and bars and loss of cross sectional area of reinforcement which contribute to the failure of the structure. Initial bar rusting is not necessarily disadvantageous to the bond. Various studies have shown that corrosion up to the extent of 1% loss in cross sectional area of bars actually make the bond stronger. The estimation of residual capacity of reinforced and prestressed structures can aid in taking the right decisions regarding the repair/ rehabilitation of the structure. Residual structural capacity estimation also helps in planning for the maintenance of
  9. 9. 9 structures and assigning priority to the structures which are in the urgent need of repair or replacement. The calculation of remaining flexural strength of such corroded beams can be either carried out using finite element modelling software tools or using various empirical models suggested by different researchers based on experimental studies. One such empirical model proposed by Bhargava et al., 2008 has been used in this study to model corroded beams. Alternatively, finite element software tool Ansys has been used to model the corroded beams. Results obtained from both empirical and software model have been discussed and compared in this report.
  10. 10. 10 Research Gap and Objectives Research Gap Corrosion is one of the major drags on developed economies in the modern world. An estimated 3.1% of GDP is lost to the corrosion related issues every year. Corrosion is a field of active research around the world. Most of the research is directed towards understanding how corrosion works, on different ways to protect structures against corrosion and quantifying the damage caused by the process. Corrosion of steel structures is easy to understand and there are plenty of options available for protecting such structures and quantifying the damage. However, reinforced concrete structures are more complex to understand. As the steel bars are embedded inside the concrete, there is no definitive way to ascertain the extent of corrosion. It is even more difficult to protect such structures which were built without any provision for protection against corrosion effects. Other major issue with such structures is determination of residual strength available in the structure. Many empirical models have been proposed to this affect, however the results from such models vary to great extent depending on quality of concrete, reinforcement position, type of corrosion etc. This report uses one of such proposed empirical model to compare the results with finite element software analysis. Objectives 1. Discussion on corrosion initiation and propagation in reinforced concrete structures. 2. Brief description about different techniques for quantifying the extent of corrosion in RC structures and different ways for mitigating corrosion. 3. Review of failure mechanism in corrosion damaged RC beams and different deterioration models. 4. Modelling a corroded RC beam in finite element software package and comparing the results with empirical models.
  11. 11. 11 Literature Review Basics of Corrosion Corrosion can be defined as a process by which any material reacts with its environment and in the process deteriorates leading to changed physical and chemical properties. It is an exothermic process and therefore most metals have a tendency to return to their natural state and in the process release energy in the environment (Broomfield, 2002) Corrosion process can be effectively explained using the concept of electrochemical cell. Therefore, proper understanding of this phenomenon can enable us to design structures, which are more resistant to the effects of corrosion and thus increase the life of the structure. Figure 1 is a representation of typical electrochemical cell. Figure 1. Electrochemical cell (Holze, 2008)
  12. 12. 12 Corrosion is a chemical reaction and therefore, there has to be an exchange of ions and electrons in the process. The corrosion mechanism can be described as an electrochemical process. Just as in electrochemical process, there is an anode, cathode, electron path and electrolyte in the corrosion (Jones, 1996). Oxidation takes place at the anodic site, in the process releasing electrons which then travel to cathodic site where reduction takes place. These components of the corrosion cell are briefly described here. Anode Anode is the site where metal cations are formed and electrons are released. The anode is the material which lies in the lower echelon of the galvanic series meaning that it is less noble then the cathode. The mass loss occurs at the surface of the metal in to the environment whereas electrons in the metal are free to move based on the voltage gradients. In Figure 1 Zn acts as an anode. Cathode Reduction takes place at the cathodic site. Cathode is the material which is nobler than the anode of corrosion cell. The electrons produced at the anode migrate to the cathodic site where they are consumed to form by products. Therefore, no loss of mass occurs here. Electron Path Electron path is required for the electrons to travel from the anode to cathode to maintain the reaction equilibrium. This movement of electrons is governed by the voltage difference between the cathodic and anodic site. This path has to be metallic in nature for the electrons to travel as electrons cannot travel through the solution medium. In case of corrosion of in steel bar, this path is generally the corroding material itself. Electrolyte Electrolyte is a solution through which the movement of anions and cations takes place. This is necessary to maintain the charge equilibrium and counter the movement of electrons through the metallic path. As the anions and
  13. 13. 13 cations are unable to move through the metallic path, electrolyte solution is required. Therefore, in an electrochemical cell, cations tend to move towards the cathodic site whereas anions move towards the anodic site. In the case of corrosion of steel bar in concrete, role of electrolyte is played by the pore solution in concrete (Broomfield, 2002). Corrosion of Steel in Concrete Corrosion of reinforcement in concrete structures is one of the most common reasons for cause of deterioration of the structures in coastal areas. India has extensive coastal region, which suffers from severe corrosion (Lahiri, 1970). Because, the volume of corrosion byproduct is much higher in volume then the steel bar itself, this may lead to cracks and eventual spalling of concrete cover (Alonso et al., 1998). Corrosion is a reason for large amount of economic cost associated with failure of old structures. When the new structure is built, the alkaline nature of pore solution keeps the steel reinforcement passivated. This prevents the corrosion of bars. Initiation of the corrosion can occur either by the reduction in alkalinity from carbonation or breakdown of passive layer by chloride attack (Hausmann, 1967). Time taken to initiate corrosion in the structure is largely determined by the cover provided to the reinforcement as well as the permeability of the concrete (Rasheeduzzafar et al., 1992). Once the corrosion process has started, the rate of propagation is largely decided by the anodic and cathodic reactions and electrolytic properties of the concrete (Broomfield, 1997). Corrosion process in the concrete can be described in terms of corrosion cell. Just like the electrochemical cell, it contains anode, cathode, electrolyte and the electron path. Both anodes and cathodes are formed on the steel bar whereby electrochemical reactions take place. The steel bar itself acts as an electron path and the pore solution in the concrete acts as an electrolyte for transporting different anions and cations (Broomfield, 1997). In this way a complete corrosion cell is formed. The mechanism of corrosion in concrete reinforcement is demonstrated in Figure 2.
  14. 14. 14 Figure 2. Mechanism of corrosion (Ahmad, 2003) Anodic and Cathodic Reactions Separate reactions at anode and cathode are known as half-cell reactions. Oxidation reactions take place at the anodic site whereas reduction takes place at the cathode. These reactions are governed by the pH of pore solution in concrete, presence of aggressive ions and prevailing electrochemical potential in different portions of steel bar surface (Moreno et al., 2004). Different reactions taking place at anodic and cathodic sites are listed below in Table 1 and 2 respectively. Table 1 Anodic Reactions Table 2 Cathodic Reactions
  15. 15. 15 Approaches available for prevention of corrosion There are several measures available for prevention and protection of rebars against corrosion such as use of epoxy coating, corrosion inhibitors, cathodic protection, etc. Corrosion initiation can also be postponed by increasing cover depth over the rebar, using high cement content, and use of admixtures for decreasing the permeability of concrete. Few of the widely used measures are discussed briefly here. Fusion bonded epoxy coated reinforcement Epoxy coating can be applied to the rebars in a factory process. The bars are properly cleaned and then heated in induction furnace and then coated with fine epoxy powder by spraying. The powder fuses with the hot bar, after which the bar is quenched. The epoxy thus obtained has a dielectric nature which is very important for preventing charged ions like chloride from penetrating to the bar (Broomfield, 1997). Further, the epoxy coating also has a good adhesion to the steel. Disadvantages The epoxy coat insulates the steel bar from electrical current. This poses a major hindrance during the maintenance work of the structure. Because of the electrical insulation provided by the epoxy coating, electrical and electrochemical techniques for determining the state of the bar as well as carrying out the repair work is difficult (Broomfield, 1997). It is difficult to interpret the results of half-cell potential and linear polarization technique. In case of extensive damage due to corrosion, it is more difficult to carry out the repair works such as realkalization, chloride removal and cathodic protection. Epoxy coating also presents increased risk of pitting corrosion if the coating breaks down over the small portion of rebar (Vaca-Cortés et al., 1998). Moreover, there are high chances of damage to coating during the transportation and handling.
  16. 16. 16 Waterproofing membranes Waterproof membranes are used to waterproof the concrete structure. In this way, conditions necessary for the initiation of corrosion are prevented. Membrane comes in two forms, in a liquid form that solidifies in its place and a sheet which is glued to the concrete surface (Broomfield, 1997). These waterproof membranes have a lifetime of around 15 years, after which they need to be replaced. Disadvantages For the membrane to be effective, it should be applied with minimum number of defects. These defects can occur as blow holes, penetrations or mechanical damage to the liquid applied types, or cuts, tears, bad joints or perforations in the sheets. Further, the membrane may be damaged at the time of overlay of asphalt, either by its heat or mechanical damage from aggregate particles. Other important disadvantage of using the membrane is the inability to use the cathodic protection system at later stage (Vassie, 1994). If the anode is used outside of the membrane, it blocks the electrical path, thereby leaving the protection ineffective. Application of anodes below the membrane creates the problem of accumulation of gases released from the anode (Broomfield, 1997). Corrosion Inhibitors Corrosion inhibitors are chemical substances, which are capable of retarding or preventing corrosion of steel in concrete. They are added in small quantities to the concrete mix (Elsener et al., 1997). They should not have adverse effects on the concrete properties like compressive strength. One of the major advantage of using corrosion inhibitor is that unlike epoxy coating and waterproof membranes, they do not hinder the application of electrical and electrochemical repair and measurement techniques (Broomfield, 1997).
  17. 17. 17 Mechanism There are various ways in which different corrosion inhibitors work to prevent and retard the action of corrosion. Based on the mechanism of action, corrosion inhibitors can be divided into following types (Trabanelli, 1987).  Adsorption inhibitors: These inhibitors prevents or retards either the anodic or cathodic partial reaction or both reactions.  Film forming inhibitors: These inhibitors prevent corrosion by forming a film on the surface of steel bar.  Passivators: They help in corrosion prevention by favouring the formation of passivation layer on the steel bar.  Buffering: They act as a buffer to maintain high pH in the local pit environment. Disadvantages As the inhibitor is itself consumed during the mechanism of corrosion prevention, it is very important to predict the right amount required based on the design life of structure and environmental conditions around the structure (Broomfield, 1997). Cathodic Protection Cathodic protection derives its name from the fact that it protects the metal from corrosion by changing the role of corroding metal from anode to cathode. Current flows between anode and cathode because of potential difference between the two. In the corrosion cell, anode potential is more negative than the cathode, which acts as a driving force for current flow and hence corrosion (Broomfield, 1997). Cathodic protection helps in preventing corrosion by minimizing this potential difference between the anode and cathode. This is done by supplying current from outside source and when enough current is provided, the complete steel bar comes at the same potential and starts acting as an anode. This source of current can either be some sacrificial metal/alloy or battery supplying direct current. Based on the
  18. 18. 18 source of current, cathodic protection can be divided into two sub categories namely Galvanic and Impressed Current Cathodic Protection (Broomfield, 1997). Cathodic protection is an effective for both prevention as well as protection of concrete from cathodic protection (Bertolini et al., 2009). It is effective against both carbonate and chloride affected concrete. Further, it can be used for realkalization and repassivation (Bertolini et al., 2008) of corrosion infected concrete. Galvanic System Galvanic method of cathodic protection uses galvanic series to decide on metal, which has more negative potential than the metal which has to be protected. For example, zinc can be used to protect the steel bar because zinc is more corrosive than the steel bar and therefore act as an anode and in the process transforms the steel bar in to cathode. This is done by either embedding the anode in the concrete or superimposing it on the surface of concrete. Concrete acts as an electrolyte and an outside wire connection is provided between the anode and the steel bar. In the process of protection of steel bar, these galvanic anodes corrode themselves and are therefore called sacrificial anodes. Galvanic anodes are usually made of magnesium or zinc because of their higher potential compared to steel structures (Broomfield, 1997). Impressed Current Cathodic Protection Systems Impressed Current Cathodic Protection (ICCP) systems are extremely accurate and can be controlled to deliver optimal protective current to the structure. Anodes are connected to a DC power supply to deliver low-voltage electric currents and are controlled by strategically placed reference electrodes. These reference electrodes are connected to a monitoring system that observes and verifies the system performance. The rectifier is used to transform alternating current (AC) to direct current (DC), which is then passed through the structure using non sacrificial anode to provide protection to the steel. The anode can be embedded in the concrete in the form of a rod or can be superimposed on the concrete in the form of fabric. Common choices for the anodes include mixed metal oxide, high silicon iron, platinized titanium/ niobium/ zirconium.
  19. 19. 19 Criteria for successful implementation of ICCP system include following characteristics (Vukcevic, 2010).  Operating at all times (i.e. being continuously powered-on).  Functioning at all times (i.e. delivering CP by continuously operating within a regulating loop).  Providing optimal protection (i.e. not over-protecting or under protecting after initial steel polarization period).  Working autonomously (i.e. not needing operator attendance, also being self- adjusting and self-regulating).  Reliable (i.e. being fault-tolerant, redundant, self-protecting, self-calibrating).  Incorporating alarm functions (i.e. reporting malfunctions, operation out of range, independent watch-dog status).  Everlasting (i.e. having average operating lifetime equal to or longer than projected infrastructure lifetime). It is very important to determine the right amount of current needed to achieve the desired prevention/protection. The current should be such that the structure is neither under protected nor over protected. Over protection of the structure may lead to hydrogen embrittlement on prestressed structures (Bertolini et al., 2002). Table 3 mentions the current requirement for various real world conditions (Chess et al, 1998) Table 3 Current requirement for real word conditions (Chess et al., 1998)
  20. 20. 20 Figure 3 is a demonstrative figure of ICCP setup. Primary anodes here is an inert material so that it does not decompose in a very short time. This primary anode can be made of CFRP or titanium. As is clear from the figure, for protection to be effective, the current has to pass through the concrete medium. However, due to high resistivity values of concrete, there is non uniform distribution of current on the steel bar. This necessitates the research into making concrete less resistive for efficient use of cathodic protection system. Figure 3 Impressed cathodic current protection setup (Bertolini et al., 98) Rectifier and system component evaluation It is important to check components of the system. The components include rectifier, anode, and wiring. Rectifiers should be housed in a box and should not be accessible to the public to avoid vandalism. Periodic inspection are necessary to inspect the conditions of anodic systems. If sacrificial anode have been used, they need to be replaced and replenished after certain period. Non sacrificial anodes should also be checked for delamination.
  21. 21. 21 Inspection Need for detection Majority of the structures in second half of the century have been built with the use of RC. At the time of their construction, it was thought that these structures are going to be durable. It was only much later, that corrosion related problems became widespread. However, unlike steel structures, where it is easier to ascertain the health of the structure and maintain the structure, it is very difficult to do the same in case of RC members. Therefore, innovative ways are needed for ascertaining the level of corrosion in the reinforcement members as well as for maintenance of the structure if so required. Half Cell Potential Measurements Half-cell potential measurements are very handy and fast way to ascertain the rate of corrosion in the rebar. It measures the potential difference between the concrete and rebar. However, for taking half-cell potential measurements, clear connection to rebar on which corrosion has to be measured needs to be established. Chloride Content determination Chloride content can be found in the concrete to determine the risk of corrosion to rebars. As chloride penetrates concrete and reaches rebars, passivation layer is broken down which makes rebars susceptible to corrosion. Effective cathodic protection is able to drive out chloride ions away from the rebar. Polarization decay testing Polarization decay testing takes longer time to do, however it is much more reliable test for effectiveness of cathodic protection. Cathodic protection system is disconnected and potential measurements between concrete and rebar are taken at
  22. 22. 22 the instant off and then again after 4 hours and 24 hours. As the time passes by, if there is a potential decay, it establishes that cathodic protection was effective before switching off and corrosion potential has again started to build up after the system stopped working. Usually, potential decay of more than 100 mV implies that cathodic protection is effective. Case Study I Strategic Highway Research Program (USA) had supported a study on cathodic protection of bridge elements. This case study describes briefly the methodology used and the performance of cathodic protection. Location: Yaquina Bay, Oregon, USA Anode Type and Location: Porter Paint and Coating on beams and soffit Conductive coating was applied to the underside of the bridge deck. Structural cracks has appeared on the underside of deck owing to the corrosion of reinforcement in rebars due to highly corrosive marine environment. Methodology: Cathodic protection system (Impressed Current Cathodic Protection) was installed on the structure. Rectifier with specification of 8V, 2A was installed. Porter Paint was applied on beams and soffit which acted both as the anode as well as the conductive layer for uniform distribution of current through the concrete cover to the rebars (cathode). Thus, forming the electrochemical cell and reversing the function of rebar from anode to the cathode. The current density for the CP was set to 0.5 to 0.6 mA/sqft. Results: The best way to check the efficiency of the Cathodic Protection system is to measure the potential decay after 4 Hrs and 24 Hrs of switching off of the CP system. It was observed that after 24 Hrs, potential has decayed from -387mV at the switch off instant to -186mV after 4 Hrs and to -100mV after 24 Hrs of switching off. This establishes that installed cathodic protection system is effective in controlling corrosion of rebars in reinforced concrete. Further, concrete cores were taken from the areas with active cathodic protection and control areas. On measuring the chloride content in the samples, it was found that chloride content in the CP core
  23. 23. 23 was less than the control sample. This establishes that cathodic protection is also effective in repelling of chloride ions from the rebar area. This further helps in repassivation of rebars and thus preventing corrosion. Case Study II Location: East Camino under crossing, California State Highway System, USA Anode type and location: Metallized Zn Anode on deck and soffit surface. Protection type: Impressed current cathodic protection Impressed current cathodic protection (ICCP) was applied to the bridge which is a 3 span (27 m long) reinforced concrete structure and was built in 1964. There were large scale corrosion in reinforcements owing to chloride ingress due to deicing operations during winter. Methodology: Deck and soffit surface were coated with thermal sprayed Zn which acts as an anode in the ICCP scheme. In addition to acting as anode, Zn layer also helps in uniform distribution of current through the concrete. Sprayed layer of Zn was 0.6 mm thick. The system was energized in 1985 and current density was set to 0.43 mA/ft2. Results: The ICCP system was analysed after 10 years of service. It was observed that driving voltage varied from 26 to 40 V. Further, it was observed that CP currents were higher in winter when compared to summer months. This can be attributed to change in conductivity of concrete. Due to higher moisture content in winter, concrete becomes more conductive resulting in higher CP currents thereby making the system more efficient. On core examination, it was found that there were significant amount of corrosion product of Zn deposited under the Zn coating. This lead to debonding of the anode layer from the concrete surface and thus affecting the performance of the ICCP system. This is one of the major drawback with using Zn as anode material. High amount of chloride ions were also observed in the core owing to the extensive de- icing operations during the winter months.
  24. 24. 24 Abilities for maintaining RC structures For maintaining RC structure (mainly from the view point of corrosion), following abilities are required. 1. To determine the level of corrosion in the structure. Which can be done using many techniques such as half-cell, AC impedance, LPR etc. However, it should be kept in mind that none of these techniques are definitive and that there are high chances of error in the results from such techniques. Therefore, it is required that more than one method is used to determine the level of corrosion in the structure. 2. Once the level of corrosion in the reinforcement is determined, it is important to find the residual strength of the member. However, it is necessary to remember that strength of the member is not dependent solely on the cross section area of the member. It also depends on the bond strength between concrete and steel. Effects of Corrosion As the reinforcement corrodes, it grows in volume and thus resulting in formation of crack which eventually lead to spalling. However, even before the spalling happens, the structure can fail due to reduction in the load carrying capacity of the structure or beam. Corrosion also reduces the cross sectional area of the reinforcement and reduces the bond strength between the concrete and steel, leading to reduction in flexural strength of the beam (Mehta and Monterio 1997). As the reinforcement corrodes, volume of the byproducts formed from corrosion is substantially higher than the original bars. This results in exertion of expansive radial pressure at the steel-concrete interface leading to hoop tensile stresses in the surrounding concrete. Once, the maximum hoop tensile stress exceeds the tensile strength of the concrete, cracking follows (Bhargava et al., 2008)
  25. 25. 25 Loss of Bond Strength Many researchers have conducted research in the bond behavior of corroded steel. A lot of research has been done to predict the loss in bond strength due to the loss resulting from corrosion. While, some research papers have proposed empirical formulas derived from experimental results (Cabrera 1996; Stanish et al., 1999; Lee et al., 2002; Chung et al., 2004), others have proposed analytical models (Lundgren 2002; Coronelli 2002; Wang and Liu 2004). However, despite a lot of research in this field, there exists considerable discrepancies between the predicted and experimentally observed values. This may be the result of difference in testing procedures, varying material strengths of concrete and steel and variation in corrosion inducing conditions. There are a number of tests available to estimate the bond strength in RC structures. Some of the most commonly used ones are concentric pullout test, tension pullout test, bond beam test, Bureau of Standards test, cantilever bond test and University of Texas beam test. The results may vary depending on the type of test used for calculating the bond strength value. Bond between concrete and reinforcement bars play an important role in the composite action of reinforced structure. With the reduction in the bond strength between the bar and concrete, it is imperative that there will be reduction in flexural strength of the concrete. The bond strength increases slightly with the initial corrosion which can be attributed to the expansive nature of iron oxides due to which roughness in the concrete reinforcement interface increases leading to increased bond strength. However, with increased corrosion, there is build-up of loose corrosion products at the interface leading to loss in bond strength. The bond stress declines consistently until about 7.5% loss in the initial area of the rebars, after which it becomes negligible. This loss in bond strength can be further attributed to loss of mechanical interlocking rebars and concrete and lubrication effects produced by the flaky corroded materials (Naus et al). Experimental bond values tend to increase up to 1-4% corrosion level (Al- Sulaimani et al., 1990) after which it starts to decline again. As the corrosion increases, accumulation of corrosion products exerts hoop tensile stress in the
  26. 26. 26 surrounding concrete leading to development of cracks and loss of adhesion and friction between the concrete and reinforcement. Figure 4. Variation in bond strength with corrosion (Bilcik et al., 2012) Figure 5. Expansive pressure on steel concrete interface due to corrosion products (Zhao et al., 2011) Studies have shown that loss of bond strength for unconfined reinforcement is more critical than the loss of reinforcement area. Therefore, it is recommended that the
  27. 27. 27 longitudinal and transverse reinforcement be provided with the confining bars to counteract the bond less. Bhargava et al 2008, has proposed empirical models to evaluate bond degradation for concrete and steel reinforcement without stirrups. The empirical formula has been proposed on two different experimental data of Pullout testing and flexural testing. Based on pullout testing experimental data R = 1.0 for X ≤ 1.5 %; R = 1.192e-0.117X for X > 1.5% Based on flexural testing experimental data R = 1.0 for X ≤ 1.5 %; R = 1.346e-0.198X for X > 1.5% Where R is nominal bond strength and X is the weight loss in percentage. As the bond strength in the above formula has been normalized, this negates to great extent the difference in material strengths of concrete and steel, testing procedures and corrosion process used in different experiments. Loss of Area The loss in the area of the bars depends on the corrosion mechanism namely chloride or carbonation induced corrosion. Carbonation induced corrosion leads to a uniform corrosion throughout the bar length and therefore, reduction in cross section area is limited. Whereas chloride induced mechanism leads to severe pitting of bars and localized loss in steel cross section area. Cairns and Millar, 1999 have shown that there is balance between the strength loss from the loss in cross section and strength gained as a result of hardening of undamaged material in the bar.
  28. 28. 28 Effects of Reinforcement Corrosion on reinforced concrete Deterioration model Deterioration of reinforced concrete can be subdivided into three stages, namely initiation, propagation and finally serviceability/ ultimate failures. Initiation and propagation of corrosion have already been explained in previous sections. Different modes of failures have been described here briefly. Figure 6. Deterioration model of service life due to reinforcement corrosion (Bilcik et al., 2012) Cracking As the corrosion occurs, the resulting oxidation of iron leads to products formation which has volume of up to 5 times or more the volume of the original iron. Therefore, the reinforcement tends to expand and the resulting pressure on surrounding concrete leads to cracking, spalling and delamination of the concrete. In addition to the reduction in strength of the member, cracking also makes the structure vulnerable to further ingress of chloride ions and therefore, accelerating the pace of corrosion.
  29. 29. 29 Figure 7. Oxidation states of iron and representation of different forms of corrosion (Naus et al., 2007) Morinaga, using the laboratory data has suggested an empirical equation for determining the time taken for cracking of concrete. Where Qcr = critical mass of corrosion products (10-4g/cm2) c = concrete cover to the reinforcement d = diameter of reinforcing bars icorr = corrosion rate (g/day) tcr = time to cracking Spalling (Du et al., 2013) has shown that bar clear distance is the determining factor when the spalling of the concrete cover arising from corrosion is considered. Bar clear distance can be described as ratio of distance between consecutive bars and concrete cover (s/c). This terminology further becomes clear from the subsequent Figure 8.
  30. 30. 30 Therefore, when value of s/c decreases, less amount of radial expansion from corrosion is required to cause internal penetration and therefore, spalling occurs more easily. Therefore, it is important to take care of both concrete cover and internal spacing of bars when designing RC structures. Figure 8. Representation of cracking mechanism in corroded beam It is preferable that bar clear distance is higher than 2.2 times the concrete cover. This will help in identifying that corrosion is occurring in the early stage as the corrosion cracks will appear on external surface before spalling begins. It is recommended that for maintenenace of existing corroded structures, one should be aware of internal penetration happening where ever the clear distance is less than 2.2 times the concrete cover. In such cases, exterior concrete surface might not give any hints to the corrosion occurring inside the concrete. Mode of Failure of beam with Corroded Steel Mode of failure of RC beams with corroded is depended on the extent of corrosion that has already occurred in the beam. However, the failure pattern in both corroded and non-corroded beams before the cracking stage is quite similar (Almusallam et al., 1996).
  31. 31. 31 The deflection increases gradually with the increasing load, after which there are transverse flexural cracks in middle of the beam. This is accompanied by sudden increase in deflection. With further increase in the loading, flexural cracks propagate across the width and depth of the beam. The crack width increases with increasing loads. With further increase in the load, there is yielding of bars leading to huge deflection. Once the bars have yielded, flexural cracks propagate upwards reducing the compression zone, leading to secondary compression failure. With increasing load, flexural cracks propagate and merge with the longitudinal cracks which originate from the corrosion in the reinforcing steel leading to sudden longitudinal splitting of the beam along with the reinforcing bar. This failure is a result of the bond failure between the rebars and the concrete. (Uomoto et al., 1984) observed that shear bond failure was the predominant mode of deterioration in beams with corroded reinforcements. Empirical Models Bhargava et al 2008, has proposed empirical models to evaluate bond degradation for concrete and steel reinforcement without stirrups. The empirical formula has been proposed on two different experimental data of Pullout testing and flexural testing. Based on pullout testing experimental data R = 1.0 for X ≤ 1.5 %; R = 1.192e-0.117X for X > 1.5% Based on flexural testing experimental data R = 1.0 for X ≤ 1.5 %; R = 1.346e-0.198X for X > 1.5% Where R is nominal bond strength and X is the weight loss in percentage. Flexural strengths of corroded beams can be estimated using empirical models and the procedure for the same is shown as below: Tension in the bottom reinforcement can be given by: Ftx = nst x π x Dstx x ld x τbux(Bhargava et al., 2008)
  32. 32. 32 Where Ftx = Tensile force generated in the corroded tensile reinforcement at any stage of corrosion nst = number of bottom tensile steel rebars Dstx = Diameter of bottom tensile steel bar ld = Development length in tension τbux = Bond strength at the corrosion level X Development length in this expression is calculated as follows: Ld = 0.025fy α β λ (db)/fc 0.5 Where fy = specified yield strength of reinforcement bars in psi α = 1.3 for top bars and 1.0 for all other bars β = 1.0 for uncoated bars and 1.5 for epoxy coated bars λ = 1.0 for nominal weight concrete db = nominal diameter of the bars in inches fc = specified compressive strength of concrete in psi Calculation of bond strength at corrosion level X Bond strength when there is no corrosion: Ft = 0.3 (fc)0.6 Bond strength at corrosion level X can be calculated using nominal bond strength Therefore, τbux = 1.346e-0.198X x 0.3 (fc)0.6 Calculation of Design Flexural Strength for a beam Depth of equivalent compressive block, a a = Asfy/0.85fc.b Where Asfy = Ftx (Tensile force generated in the corroded tensile reinforcement at any stage of corrosion)
  33. 33. 33 fc = compressive strength of concrete b = width of the beam Nominal moment is then given by the expression: Mn = Ftx (d-a/2) Where d = distance between the centroid of tensile steel and the edge of the compression zone Design Moment Strength Md = 0.9 Mn Critical point load when applied at the centre of the beam w = 4Md/L Where L is the length of the beam Sample Calculation Sample calculation of uncorroded beam using the above described empirical method: For uncorroded beam Ld = 0.025fy α β λ (db)/fc 0.5 α = β = λ = 1 fy = 415 MPa db = 32 mm fc = 25 MPa Therefore, Development length, Ld = 797 mm Bond strength at the corrosion level X, τbux = 0.3 (fc)0.6 = 2 MPa Tension in the bottom reinforcement, Ftx = nst x π x Dstx x ld x τbux = 3 x 3.14 x 32 x 797 x 2 Ftx = 480 kN
  34. 34. 34 Depth of equivalent compressive block, a = Asfy/0.85fc.b a = 482/(0.85 x 25 x 300) = 75 mm Nominal Moment, Mn = 482 (250 – 37.5) = 102.425 kNm Design Moment, Md = 92.205 kNm Critical point load when applied at the centre of the beam w = 73.6 kN
  35. 35. 35 Methodology Reinforced concrete beam was modelled using Finite Element software Ansys. The beam was taken to be of the dimensions 5m x 0.3m x 0.3m. In this report, nonlinear finite element analysis for the reinforced beam has been carried out. The beam is simply supported with pinned support at one end and roller supports at the other end. The beam is placed on the pair of bearings. Point load has been applied at the centre of the beam, the load was applied increased until the beam failed in flexure. Ansys Workbench 14.5 has been used to carry out the modelling and analysis of the beam. Introduction Structural concrete beams require the understanding of responses of flexural strength and crack initiation and propagation subjected to a variety of loadings. There are various available procedures for modelling concrete beams using both empirical and numerical/analytical methods. Finite elemen analysis (FEA) is a numerical method which is widely used to model concrete beams using non linear elements available in different software tools. Ansys 14.5 is capable of simulating and predicting the responses of reinforced and prestressed beam members. In addition to Ansys, there are many other commercial tools for analysis reinforced concrete members. I recent years, use of finite element analysis has grown because of increase in processing power of computers and increased availability of software option. Since, each material has its own complicated stress-strain behaviour, nonlinear modelling of structural members play an important role in carrying out a nonlinear analysis. ANSYS incorporates a 3D element SOLID65 with the nonlinear properties similar to that of concrete. SOLID65 element hasa smeared crack analogy for cracking in tension zones. It also has a plasticity algorithm to simulate crushing behaviour of
  36. 36. 36 concrete in compression zones. SOLID65 is a solid isoperimetric element with the integration points for crushing as well cracking checks. Problem description Flexural strength analysis of reinforcement steel concrete beam study has been carried out in this report. A rectangular cross section beam was used with three reinforcement steel bars at equally spacing. Bearing surfaces were applied at both ends as well as at centre. Roller support was applied at one end while pinned support was applied at other end. A point load was applied at the centre bearing surface. Beam dimension was 5m X 0.3m X 0.3m. Rebars radius was varied ranging from 16 mm to 13.5 mm with step size of 0.25 mm. Main concern of this study was to determine the load at which a first crack generates and corresponding stresses at that point. Second, objective was to optimize the diameters of steel reinforcements. Lastly, flexural strength of beam under analysis was evaluated and the stresses at which beams fails to bear more load. ANSYS APDL module was used to obtain all described goals. Deformations, von mises stresses as well as cracks propagations has been evaluated here using FEA (finite element analysis) techniques. Preprocessing For the Finite element model to run in Ansys, there are multiple tasks that need to be taken care of. Finite element model can be either created using command prompt line or with the use of Graphical use interface. For this report, graphical user interface method has been used. The subsequent section describes different parameters and entries used to model the reinforced concrete beam. Element type The element types for this model are shown in Table 4. The Solid65 element was used to model the concrete and concrete bearing support. The element has eight different
  37. 37. 37 nodes in a shape of a cube. Each node has three degrees of freedom, i.e translation in x, y and z directions. The element is able to deform plastically, crack in all the three orthogonal directions, and crush as well.. A schematic of the element is shown in Figure 9. Material Type ANSYS Element Concrete SOLID65 Reinforcement Steel LINK180 Concrete bearing SOLID65 Table 4. Element Type Figure 9. Ansys SOLID65 Element A Link180 element was used to model steel reinforcement. This element is a 3D spar element and it has two nodes with three degrees of freedom – translations in the
  38. 38. 38 nodal x, y, and z directions. This element is also capable of plastic deformation. This element is shown in Figure 10. Figure 10. Ansys LINK180 Element Additionally, COMBIN39 element was used to model the interface between the steel and concrete. COMBIN39 is a nonlinear unidirectional element. The element can be either used for longitudinal or torsional capability. In this study, longitudinal option was used zwhich has uniaxial tension – compression ability with three degrees of freedon at each node. Each node is allowed to translate in nodal x, y and z directions. The element is not allowed to bend. Figure 11. COMBIN39 Element
  39. 39. 39 Real constants The real constants for this model are shown in Table 5. Table 5. Real Constants for calibration model Real Constant Set 1 is used for the Solid65 element. Custom values can be input for volume ratio. The material number in the table refers to the material type of reinforcement bars and volume ratio refers to the volumetric ratio between concrete and steel in the element. Real Constant Sets 2, 3, 4, 5, 6, 7, 8,9,10 are defined for the Link180 element. Values for cross-sectional area and initial strain have also been entered. Initial strain value is set to zero as there is no initial or residual stress in the reinforcement bars. Real constant set no Element Type Parameters Value 1 SOLID65 Material number 0 Volume ratio 0 2 LINK180 Initial strain 0 Cross sectional area(m2) 0.000803 3 LINK180 Initial strain 0 Cross sectional area(m2) 0.000754 4 LINK180 Initial strain 0 Cross sectional area(m2) 0.000706 5 LINK180 Initial strain 0 Cross sectional area(m2) 0.000660 6 LINK180 Initial strain 0 Cross sectional area(m2) 0.000615 7 LINK180 Initial strain 0 Cross sectional area(m2) 0.000572 8 LINK180 Initial strain 0 Cross sectional area(m2) 0.000530 9 LINK180 Initial strain 0 Cross sectional area(m2) 0.000490 10 LINK180 Initial strain 0 Cross sectional area(m2) 0.000452 11 LINK180 Initial strain 0 Cross sectional area(m2) 0.000415
  40. 40. 40 Material Properties Parameters required to define concrete and rebars are shown in Table 6. There are multiple parts of the material model for each element. Material Number Element Type Material properties 1 SOLID Density(Kg/m3) Density 2400 Linear Isotropic EX 2.5e10 N/m2 PRXY 0.2 Concrete Open shear transfer coef 0.3 Closed shear transfer coef 1 uniaxial cracking stress 3.58MPa Uniaxial crushing stress -1MPa Multilinear elastic Point Strain Stress(N/m2) 1 0.0003 7.5E+006 2 0.00054 1.268E+007 3 0.00124 2.239E+007 4 0.00184 2.491E+007 5 0.00237 2.5E+007 2 LINK180 Density Density 7850(Kg/m3) Linear Isotropic EX 2e11(N/m2) PRXY 0.3 Bilinear Isotropic Yield Strength 4.15e8 Tang Mod 2e7 Table 6. Material Properties
  41. 41. 41 Material Model Number 1 in the above table refers to Solid65 element. The Solid65 element makes use of linear isotropic material properties to adequately model concrete properties. The Multilinear isotropic material uses the von Mises failure criterion to define the failure of the concrete. EX is the modulus of elasticity of the concrete, and PRXY is the Poisson’s ratio (ν). The Multilinear isotropic stress-strain implementation requires the first point of the curve to be defined by the user and it also must satisfy the Hooke’s law. Implementation of the material model in ANSYS requires that different constants be defined. These constants are 1. Shear transfer coefficients (open crack) 2. Shear transfer coefficients (closed crack) 3. Uniaxial tensile cracking stress 4. Uniaxial crushing stress 5. Biaxial crushing stress Material Model Number 2 in Table 6 refers to the Link180 element. The Link180 element has been assigned to all the steel reinforcement in the RC beam model. Bilinear isotropic material is also based on the von Mises failure criteria. The hardening modulus have been defined as 2e7 N/m2 and the yield stress for steel has been used as 415 MPa. Modeling Beam and bearing supports were modelled as volume. Beam dimensions were 0.3m x 0.3m x 5m and bearing supports size was 0.3m x 0.2m x 0.3m. Two bearing supports were applied at beam ends while one was at the centre of top. Steel reinforcement link was created from element attributes by just selecting the nodes created from solid65 volumes so they do not need to be meshed again. The steel bars were at same plane 50mm above the beam bottom surface and distance of 1st,
  42. 42. 42 2nd and 3rd bar from side surface of beam was 50mm, 150mm and 250mm. Geometry of problem description has been shown in Figure 12. Figure 12. Beam Geometry Beam dimensions and bearing dimensions are shown in the Figure 13. Length of the beam is 5m and the bearing dimensions are 0.3 m x 0.3 m x 0.2 m. Figure 13. Beam dimensions
  43. 43. 43 The contact force between the steel and concrete was varied to stimulate reduction in bond strength as a result of corrosion. This bond strength was calculated using empirical models discussed in methodology section of the report. Diameter Bond Strength (Mpa) 32 2.36 31.5 1.72 31 0.94 30.5 0.52 30 0.29 29.5 0.16 29 0.09 28.5 0.05 28 0.03 27.5 0.02 27 0.01 Table 7. Bond strength vs with diameter as a result of corrosion Figure 14. Bond Strength vs Diameter Strength of bond reduces exponentially as the diameter of the bar is decreased as a result of the corrosion proces. 0 0.5 1 1.5 2 2.5 26 27 28 29 30 31 32 33 BondStrength Diameter
  44. 44. 44 Meshing The element type number, material number, and real constant set number for the calibration model were set for each mesh as shown in Table 8. Model Parts Element type Material number Real constant set Concrete SOLID65 1 1 Steel bars LINK180 2 2 Bearings SOLID65 1 1 Table 8. Meshing Attributes of the Model Beam Rectangular mesh has been used for element SOLID65 to obtain simulate the behaviour of concrete in real world. Meshing for bearing supports have been done using volume sweep command in the Ansys toolbox. Use of volume sweep command sets the width and length of elements in the bearing plates to be consistent with the elements and nodes of the main beam. This is an important stp avoid the problems related with convergence.The overall mesh of the concrete and bearing support volumes is shown in Figure 15.
  45. 45. 45 Figure 15. Mapped meshing of model beam Solution Analysis type Static analysis type was selected in new analysis tab of solution analysis type and in the solution control automatic time stepping was kept on. The number of sub steps was kept to 5 and maximum sub steps were selected as 10 while keeping the minimum number of sub steps as 5. Boundary conditions Displacement boundary conditions are needed to constrain the model to get a unique solution. Roller boundary conditions were applied at left end by keeping displacement values in Y and Z directions zero and pinned support was applied right end by keeping displacement in Y direction as zero. The load was applied at top surface nodes of centre bearing as shown in Figure 16..
  46. 46. 46 Figure 16. Boundary conditions Post processing Convergence history of problem has been shown in Figure 17. Figure 17. Convergence History
  47. 47. 47 Results and Discussions First crack induced Load Initially, beam with 16mm radius bar were used by selecting the real constant set number 2 for that and a load of 75KN were applied at centre and it was observed many number of cracks, then load was reduced until it was examined where the first crack propagates and it was just above the 71.5 KN. Similarly, first crack induced load were manipulated for different sizes of steel bar, the safe load for this beam has been tabulated in given table. S.No Bar diamter(mm) Bar cross sectional area(mm2) Load(KN) 1 32 803 88.4 2 31.5 778 64 3 31 754 44 4 30.5 730 36 5 30 706 25.6 6 29.5 683 20
  48. 48. 48 7 29 660 16 8 28.5 637 8 9 28 615 4 10 27.5 593 3.6 11 27 572 2.4 Table 9. First crack induced load Results Contours Stresses corresponding to first crack were also examined and it was noted that these stresses range from 10 Mpa to 12 Mpa where concrete strength was considered as 35 Mpa. Contours of different parameters like stresses, deformation, steel bar stress as well as deformation for a specific case has been shown in onward pictures.
  49. 49. 49 Figure 18. Stresses at safe load Figure 19. Stresses at load beyond first crack
  50. 50. 50 Figure 20. Total deformation beyond the first crack Figure 21. Total deformation at steel reinforcement at safe load
  51. 51. 51 Figure 22. No crack just before first crack load Figure 23. Cracks beyond the first crack load
  52. 52. 52 As is evident from the figure 24, loss in bond strength resulting from corrosion plays an important role in the determination of the flexural strength. Flexural strength varies linearly with the diameter of the reinforcement when the consideration is made that there is no reduction in bond strength. However, the flexural strength starts to decline exponentially when the bond degradation is considered along with the reduction in the area of the reinforcement. The same phenomenon is demonstrated in the Figure 25. Figure 24 Variation of Flexural Strength vs Reinforcement Diameter (Empirical Model) The same relation between the flexural strength and reinforcement diameter is observed in the Ansys results. However, the graph are not as streamlined as the one derived from the empirical model. This may be the result of the method Ansys uses to analyse the model and the elements used for modelling of different structural elements. Figure 25 Variation of Flexural Strength vs Diameter of Reinforcement (Ansys Model) When comparing the flexural strength from empirical and ansys model (no bond degradation), the results are similar. This shows that empirical models are efficient in predicting the flexural strengths. 0 20 40 60 80 100 120 140 26 27 28 29 30 31 32 33 FlexuralStrength(kNm) Reinforcement Diameter (mm) No Bond Degradation Bond Degradation
  53. 53. 53 Figure 25. Flexural strength vs diameter for Ansys model Figure 26. Flexural Strength vs reinforcement diameter without bond degradation However, the flexural strength reduction in empirical model is more radical when bond degradation is considered. One reason for such discrepancy is the effect of spalling. The empirical model takes into account the spalling of concrete as the reinforcement corrodes and exerts outward pressure on the surrounding concrete. However, this effect is not taken into account in the Ansys modelling. 0 20 40 60 80 100 120 26 27 28 29 30 31 32 33 FlexuralStrength(kNm) Reinforcement Diameter (mm) No Bond Degradation Bond Degradation 0 20 40 60 80 100 120 26 27 28 29 30 31 32 33 FlexuralStrength(kNm) Reinforcement Diameter (mm) Ansys Model Empirical Model
  54. 54. 54 Figure 27. Flexural Strength vs reinforcement diameter with bond degradation The analysis carried out in the finite element software assumed that the corrosion is uniform through the bar length which may not be the case in the reality. If the corrosion is chloride induced, there might be pitting corrosion which will lead to flexural strengths much less than predicted in this report. Results for flexural strength from empirical and ansys model are tabulated here for reference. Diameter(mm) Flexural Strength (kNm) -No bond degradation Flexural Strength kNm - Bond degradation 32 106.5541924 132.4162753 31.5 103.9603911 80.40663157 31 101.3630674 45.68772355 30.5 98.76433412 25.33560096 30 96.16627037 13.95032112 29.5 93.57092134 7.688105981 29 90.98029848 4.25728995 28.5 88.39637938 2.373477507 28 85.82110787 1.333576273 27.5 83.25639395 0.755546011 27 80.70411382 0.431749855 Table 10. Flexural strength results for empirical model 0 20 40 60 80 100 120 140 26 27 28 29 30 31 32 33 FlexuralStrength(kNm) Reinforcement Diameter (mm) Ansys Model Empirical Model
  55. 55. 55 Diameter(mm) Flexural Strength (kNm) -No bond degradation Flexural Strength kNm - Bond degradation 32 110.5 110.5 31.5 95.6 80.3 31 88 55.2 30.5 91.2 45.6 30 85.3 32.6 29.5 80.6 32.3 29 70.5 19.8 28.5 65.5 11.3 28 62.2 4.5 27.5 60.5 3.2 27 58 0.9 Table 11. Flexural strength results for Ansys model
  56. 56. 56 Conclusion Bending moment capacity of corroded reinforced beams has been calculated using empirical model as well as Ansys model. The flexural capacity of beam is affected by two major factors resulting from corrosion namely mass loss of steel and loss in bond strength between concrete and reinforcement. Other factors responsible for strength reduction are spalling and cracking. Both empirical and FE software model considers the effects of loss in bond strength and mass loss, however they do no account for the effects of spalling. It is observed that there is similarity in the strengths of both the models when no bond degradation is considered. However, there is marked difference between the flexural strengths of both models when bond degradation is taken into the account. It is observed that loss in bond strength is the most critical factor effecting the flexural strength of corroded beams and loss in steel area contributes negligibly to the strength loss.
  57. 57. 57 Future Work As is evident from the results, there is a considerable difference in flexural strength of RC beams when the results from the finite element software and empirical methods are compared. Few of the steps that can be taken to further the scope of the study are described briefly here. Experimental Studies The beams can be cast in the lab and then allowed to corrode for some time. The corrosion in such beams could be expedited by the use of salt contaminated water. This is done by creating a small dam on top of the specimen beam. Figure 28. Setup to expedite corrosion process The above figure illustrates a mechanism to expedite the corrosion mechanism. The dam is filled with varying level of chloride contaminated solution to effect the rate of corrosion. Level of corrosion can be monitored by measuring Icorr and thus deducing the extent of mass loss. Once the desired corrosion has happened, the beam can be tested for flexural strength.
  58. 58. 58 These results can then be corroborated with the observations obtained from analysis in finite element software and empirical models. With the use of these results, finite element model can be further optimized and the formulas used in empirical models can also be tweaked to get more consistent results with the experimental value. Analysis of Prestressed Beams Similar studies can be conducted for studying the effect of corroded prestressed tendons on the flexural strength of prestressed beams. However, the major factor affecting the corrosion of tendons is the process used for grouting and the quality of the overall process. However, for modelling the beam with tendons without the surrounding duct can be carried out on the similar lines as used in this report. Another important consideration will be design and modelling of anchorage for such beams. Determination of shear strength This report doesn’t cover the effects of corrosion on the shear strength of the beam. This can be another area for further research in to the effects of corrosion.
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