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A structure refers to a system of connected parts used to support loads. In buildings, structure consists of the walls, floors, roofs and foundation. In bridges it may be deck, supporting systems and foundation. In towers, the structure consists of vertical, horizontal and diagonal members. Concrete is a composite building material made from combination of aggregates (coarse and fine) and binder as cement. The most common form of concrete consists of mineral aggregate (gravel and sand), Portland cement and water. After mixing, the cement hydrates and eventually hardens into a stone like material. Hardened concrete has high compressive strength and low tensile strength. Concrete is generally strengthened using steel bars or rods known as "rebars" in tension zone. Such elements reinforce the concrete, so that it may be mould into any complex shape using suitable form work and it has high ductility, better appearance, fire resistance and it is economical. So use of plain concrete as a structural member is limited to the situations where significant tensile stresses and strains do not develop as solid or hollow concrete blocks, pedestal and in mass concrete dams. The steel bars are used to resist tensile, while concrete provides compressive strength.
Cracking of concrete section is nearly impossible to prevent. However, the size and location of cracks can be limited and controlled by the control joints, appropriate reinforcements, curing methods and concrete mix design. The various phenomena or agents, which may lead to the corrosion of rebar are Carbonation, Chlorides, Alkali- silica reaction, sulphates, high alumina content etc. Corrosion of steel reinforcements leads to the swelling and cracking of the concrete structure, which may lead to the reduction of the serviceability of the structural member. Corrosion of rebars may cause the reduction of the yield strength of steel and affect the bond strength due to delamination of rust formed on the rebar surface.
Technological advances during the last few years in the field of deformed bar production steel manufacturing industry has successfully developed a corrosion-resistant variety of rebar for the construction industry. This type of steel, called thermo-mechanically treated (TMT), which imparts the better corrosion resistance against severe environment conditions. In the TMT process the hot rebars emerging out of the final rolling stand are subjected to rapid on-line cooling through a series of water jackets. Direct water quenching results in the formation of martensite at the surface layers of the rebars, while the core remains austenitic. The thermal gradient existing across the rebar section emerges from the quenching zone causes heat to flow from the hot austenitic core toward the rebar surface. This results in the tempering of the surface martensite, and an equalization of the surface and core temperatures takes place. Lower equalization temperatures result in higher yield strengths. During subsequent atmospheric cooling of the rebar on the cooling bed, the hot austenitic core gradually transforms to a ferrite-pearlite microstructure. This composite structure, in which the rim of the tempered martensite acts as the load-bearing constituent and the relatively soft ferrite-pearlite core provides the rebars with ductility and cold formability, helps in imparting superior mechanical proper-ties to TMT rebars compared to the conventionally used carbon steel rebars [Ray, 1997]. TMT rebars have recently been used as advancement over the conventional mild steel rebars in reinforced concrete structure in order to enhance the durability in corrosive environments.
Reinforced concrete structures exposed to marine environment are subjected to simultaneous action of number of physical chemical and electro chemical deterioration process. In the view of immense cost involved in initial construction and in repair and rehabilitation, it is quite important that these structures should able to resist the ravages of time and deleterious effects of harsh environmental condition with minimum maintenance cost and hence reinforcement corrosion has been identified as the predominant deterioration mechanism for reinforced concrete structures, which seriously affects the serviceability and the safety of the structures. It has long been recognized that carbon steel reinforcing bars have a low resistance to corrosion in chloride-bearing environments, resulting in many and marine structures having been severely damaged by corrosion of the reinforcement. Although concrete provides protection for embedded steel, the penetration of oxygen, water and chloride to the carbon steel allows rapid deterioration of the entire structure [Castro, 1997]. An easy way to overcome the problem of corroding carbon steel, which leads to reinforced concrete failure, structural problems and costly repairs, is to replace the reinforcing carbon steel with reinforcement produced with highly corrosive resistant materials or metal alloys.
In view of the above, it is important to study the corrosion behavior of TMT bar in cement environment, as the durability problems encountered in the reinforced concrete structures across the world. The durability of the reinforced concrete structures is dependent upon the conditions of the exposure and surrounding environment [Sakr, 2005]. One of the major factors responsible for deterioration of concrete structure is chloride induced reinforcement corrosion. Reinforcement corrosion is generally accompanied by loss of cross section and build-up of the corrosion products. The corrosion products occupy a larger volume than the original volume of the steel from which they were produced. This in turn generates tensile stresses in concrete causing spalling and cracking of the cover [Poupard, 2006]. Though, rebar is surrounded by the concrete and protected with the corrosion by the alkanity of the pore solution [Ahmad, 2003], leading to the formation of the passive film on the rebar surface. But, passive surface of the rebar can be destroyed by the presence of the chloride ions or presence of acidic atmosphere around the rebar formed due to the carbonation process of cement concrete. In addition to this, reinforcement corrosion may lead to the loss of the interfacial bonding between rebar and concrete. The source of chloride ion may be internal or external. The chloride ions introduced into concrete at time of the preparation i.e., from mixing water, chloride contaminated aggregates, chloride containing admixtures etc. on the other hand, chloride entering the hardened concrete by the application of deicing salts in bridge decks and parking structure, from sea water in marine structure and from the ground water containing chloride salts is known as external chloride.
The corrosion of the rebars is the electrochemical process [Elsener, 2002], which may be defined as the rust formation in the presence of the oxygen and water. In this electrochemical process, at anode Fe is oxidized to Fe ions that pass into solutions and at cathode oxygen is reduced into hydroxyl ions. This electrochemical cell forms a short circuit corrosion cell, with the flow of electrons in the steel of rebar and the ions into the pore solution of the concrete.
The corrosion behavior of the steel reinforcement can be described by means of polarization curves that represent the relationship between the potential and the anodic or cathodic current density. Such polarization curves can be obtained by conducting potentiostatic studies, as shown in Fig. 1. The structure of concrete is extremely complicated and the characteristics of concrete and its electrolyte dominantly affect the corrosion process. In addition to the moisture content that affects conductivity and, imperviousness that affects polarization; electrolytic properties of concrete also vary from place to place. Due to this variability in the electrolytic environment and high resistivity of concrete, it is difficult and complicated to conduct potentiostatic study directly on steel embedded in concrete. However, polarization curves can be obtained indirectly by conducting the potentiostatic study in solution that simulates concrete pore solution. Thus, potentiostatic study on steel reinforcement in concrete powder solution extracts can be carried out to obtain the anodic polarization curves as the concrete powder solution extract represents nearly the electrolytic condition of the concrete pore solution and in addition, the concrete powder solution extract is low resistive unlike concrete [Katat, 2006]. In an attempt [Katat, 2006], an experimental study was conducted to identify different zones of corrosion for three types of steel in concrete powder solution extracts made from one type of cement, three w/c ratios and without deaeration of the solution extract. In that study chloride was admixed to concrete as calcium chloride at different levels.
Figure 1: Polarization graph obtained for CuTMT bare steel specimen (Mix-1, 0% admixed CaCl2). [Katat, 2006]
The various non-destructive techniques used for the determination of corrosion rate are linear polarization resistance (LPR) method, AC impedance spectroscopy and Tafel plot technique. The gravimetric (mass loss) measurement is a destructive technique for obtaining the corrosion rate. LPR technique provides a simple method for the determination of corrosion rate both in the laboratory and field studies. In LPR measurements, the steel reinforcement is polarized by the application of a small perturbation from the equilibrium potential through an auxiliary electrode. The polarized surface area of the reinforcing steel is assumed to be that area which lies directly below the auxiliary electrode. However there is considerable evidence that current flowing from the auxiliary electrode is not confined and can spread laterally over an unknown large area of the reinforcing steel, which may lead to the inaccurate estimation of the corrosion rate [Law, 2000]. Therefore in order to avoid the problem of the confinement of the current to predetermined area of the reinforcing steel, the use of a second auxiliary guard ring electrode surrounding the inner auxiliary electrode has been developed. The principle is to maintain the confinement current by the outer guard ring electrode during LPR measurement. This confinement current prevents the perturbation current from central auxiliary electrode spreading beyond a known area. Due to the sophistication of the measurement, AC impedance spectroscopy technique is more frequently used in laboratory studies rather than in field surveys. Further, often it is difficult to interpret and is a time-consuming technique. Nevertheless, this technique is used as a powerful tool to understand the behavior of the steel/concrete interface and provides information about corrosion rate of the steel reinforcement [Ismail, 2006] [Montemor, 2003]. Visual observation and gravimetric (mass loss) measurement are also used as performance evaluation techniques. Visual observation gives qualitative results about corrosion performance of different types of steel embedded in concrete made with different types of cement. On the other hand, gravimetric (mass loss) measurement is used as a destructive test, generally conducted in laboratory. However it serves as the most reliable reference method. It is simple, although a time-consuming technique for determination of corrosion rate. The corrosion rate obtained by different non-destructive electrochemical techniques must be compared with those obtained from the gravimetric measurement. From the literature, it is observed that the work on the corrosion performance of different types of cement and rebar individually in chloride contaminated concrete using various corrosion rate techniques is scanty. In addition the work on the performance evaluation of various combinations of cement and rebar by different corrosion rate techniques against chloride induced corrosion is also very little. Therefore a study involving the performance evaluation of different type of cement (typical cements used in various study is given in table-1), steel and their combinations against chloride environment using different corrosion rate techniques can furnish useful information regarding the selection of suitable cement-steel combinations and also provide the correlation between different corrosion rate techniques.
Table 1: Typical composition of cements frequently used for corrosion degradation studies. OPC; Ordinary Portland Cement, PPC; Portland Pozzolonna Cement and PSC; Portland Slag Cement. [Pradhan, 2009]