Thermo-mechanically Treated Rebar Research Articles

An overview of corrosion behavior and contemporary management techniques of thermomechanically treated rebars in concrete structures

ABSTRACT The corrosion of reinforcement possesses a huge problem for our present infrastructure both in terms of human lives and monetary ground. Understanding the corrosion of thermomechanically treated (TMT) rebar in concrete structures is essential as it represents a large segment of reinforcement materials. In this review, we have tried to scrutinize this issue from different directions. The established corrosion model for rebar, especially for TMT rebar, has been examined. The main contributing factor for rebar corrosion is how passivation occurs and its disintegration in contact with aggressive ions. The effects of composition, microstructure, concrete-rebar interface, concrete type, and corrosion media on this phenomenon have been analyzed. We have realized that the exact time of chloride ion attack determines the effectiveness of the passive layer in inhabiting the corrosion initiation. A combination of suitable alloying and controlled thermomechanical treatment ensures corrosion resistant rebar. Concrete also plays an important role in corrosion prevention as it helps passivation step and stops aggressive ions from reaching the rebars. Finally, we have discussed some recent trends in corrosion management technologies and their effectiveness.

Evaluation of the fracture behaviour of concrete prisms reinforced with regular and high-strength steel rebars using acoustic emission technique

This study examines the potential of high-strength steel strands of smaller diameter as a substitute for regular steel in normal concrete, aiming to reduce steel consumption and increase sustainability. The fracture behaviour of concrete prisms reinforced with high-strength steel strands is examined using acoustic emission (AE) and digital image correlation (DIC) techniques and compared with those reinforced with thermo-mechanically treated (TMT) bars. The fracture behaviour is examined for the following parameters: the type of reinforcement (TMT and HSS), the number of rebars (1 and 2 rebars), and the percentage of reinforcement (0.22 % of TMT vs. (0.18 % & 0.27%) of HSS, 0.45% of TMT vs. 0.47% of HSS). Test results showed that replacing a similar percentage of TMT rebars with HSS rebars increased load-carrying capacity in serviceability and ultimate load regimes. The enhancement in performance can be attributed to the increased surface area of HSS due to the smaller diameter of bars, which provides better bonding with concrete when compared to TMT rebars. Micro and macro cracking characteristics and damage progression are studied by analysing the peak amplitude from the AE waveform. AE results show that tensile mode cracks dominated initially, and shear mode cracks formed after the specimens reached their first cracking load due to internal reinforcement.

Fire residual stress–strain curves of corroded thermo-mechanically treated (TMT) reinforcing steel bars

Thermo-mechanically treated (TMT) reinforcing steel bars are extensively employed in RC buildings these days because of their superior thermal- and seismic-resisting characteristics, which enable them to sustain most of their strengths during earthquakes. The distinctive cross-sectional phase distribution (CSPD) of martensite, bainite, and pearlite in TMT rebars makes them outstanding. The current study examines the extent to which TMT reinforcing steel bars' mechanical properties deteriorate as a result of exposure to a combination of corrosion-induced CSPD damage and high temperatures. The research attempted to model an accidental fire in an aging structure with corroded members as a result of exposure to extreme environmental conditions. Heated residual tensile tests on rebar specimens subjected to various combinations of corrosion-induced damage and high temperatures were used in the experiment. The results show a significant loss in the key mechanical characteristics of reinforcing steel due to the superimposition of elevated temperature exposure on corrosion-induced damage to CSPD.

Evaluation of Greener Corrosion-Inhibiting Admixtures for Steel Reinforcements in Concrete

In this study, corrosion of thermo-mechanically treated (TMT) rebars with and without 3.5% NaCl for various concentrations (0.1%, 0.2%, 0.3% and 0.4%) of green corrosion inhibitors, namely glucosamine and stevioside, was systematically studied in simulated concrete pore solution (SCPS). Open-circuit potential measurements revealed that inhibited system showed passive behaviour compared with control system in SCPS. Potentiodynamic polarization studies revealed that inhibitor stevioside offers more than 88% inhibition efficiency at 0.3% concentration in SCPS containing 3.5% NaCl. Similarly, linear polarization resistance and impedance measurements showed higher polarization resistance and charge transfer resistance values, respectively, at 0.3% concentration for inhibited systems when compared with control (without inhibitor).In the second part, the optimized concentration of glucosamine and stevioside showed better performance against corrosion of rebar in the solution study admixed into concrete. The corrosion of TMT rebar embedded in concrete with and without inhibitors is subjected to simulated marine exposure for a period of 180 days and evaluated the corrosion inhibition. Gravimetric weight loss and electrochemical techniques were employed to study the efficacy of inhibitors in SCPS and concrete medium. It is found that the time taken for corrosion-induced cracking in concrete admixed with stevioside inhibitor is approximately 20 times that of control concrete and that stevioside inhibitor is relatively better corrosion inhibitor than other inhibitor reported.

Behavior of thermo-mechanically treated rebar exposed to elevated temperatures

PurposeThe purpose of the paper is to mathematically model and predict the characteristics of thermo-mechanically treated (TMT) rebar when subjected to elevated temperatures.Design/methodology/approachData were collected from a few selected studies for developing the constitutive relations. Using the exposed temperature and the duration of heating as independent variables, the empirical relations were developed for determining the changes in mechanical properties of TMT rebars at elevated temperatures.FindingsRecrystallization of TMT rebar crystals took place around 500 °C, which led to a decrease in the dislocation density along with the increase of large-sized grains, resulting in the degradation of strength. Up to a temperature range of 500 °C, the normalized fracture strength was higher, while the normalized fracture strain is not so high. This indicated a failure of brittle nature.Originality/valueThis is an original work done by others as a study to theoretically predict the mechanical behavior of TMT rebars when exposed to elevated temperature.HighlightsThe TMT bars showed brittleness characteristics up to 500 °C and showed ductility characteristics after that on account of its recrystallization and extensive tempering of the outer martensitic rim around that temperature.The comparison between the super ductile (SD) TMT and the regular TMT exhibit shows that the SD-TMT bars were about 1.5 times more ductile than the normal ones.

Corrosion Resistance Behavior of Cr–Cu Alloyed Thermo-Mechanically Treated Reinforced Bars in 3.5% NaCl Solution

The corrosion resistance of reinforcing steels are of interest to the building and construction industry as it decides the durability of structures. These steels passivates under very high alkaline pore solution (leached solution from concrete). However, they corrode once they come in direct contact with chlorides ions (near sea coast and/or de-icing material) causing premature deterioration of the entire concrete structure. To protect steel bar from corrosion, they are alloyed with chromium (up to 0.50%) in steps to steel and their corrosion resistance behavior are compared with unalloyed rebar in chloride environment. In general, chromium addition in steel provides better corrosion protection through surface passivation than unalloyed one. However, in the present study, increase in chromium content in steel did not show a direct relationship to the corrosion protection of thermo mechanically treated (TMT) rebars. To ensure the corrosion protection of TMT rebars, the critical limit of chromium addition should be 0.3%. The corrosion resistance of the TMT with varying chromium content followed the following trends: TMT-0.5Cr > TMT-0.3Cr > TMT > TMT-0.1Cr.

Tensile and impact behaviour of thermo mechanically treated and micro-alloyed medium carbon steel bar

In the present study, a comparison between thermo mechanically treated (TMT) and direct air cooled micro-alloyed hot rolled (MAHR) steel bar has been made in terms of microstructure, texture and mechanical properties. Tensile and Charpy impact tests were performed as a part of the mechanical investigation. Tensile strength and impact toughness values of TMT bar were higher than those of MAHR bar. It was also noticed that TMT bar exhibited better resistance to both ductile and brittle fracture at high and low temperatures compared to MAHR steel. This can be corroborated owing to the presence of fine tempered martensite at the periphery, a mixed microstructure of granular bainite, ferrite, and degenerated pearlite as an intermediate layer and a mixture of ferrite and pearlite at the core of the TMT bar. On the other hand, a coarse ferrite-pearlite phase mixture was present throughout the cross section for MAHR steel and sounded responsible for lower strength and impact toughness properties. Additionally, fractions of higher angle grain boundaries (HAGBs) in the core and smaller martensite packet around the rim section were also contributed beneficially to resist the cleavage crack propagation in TMT rebar. The higher intensity of gamma fiber of MAHR rebar has contributed lowest ductile-brittle transition temperature. Numerous cleavage cracks which were initiated from inclusions or carbide particles were also found responsible for crack propagation during both ductile and brittle types of fracture.

Standard requirements of hot rolled thermo-mechanically treated reinforcement bars

Reinforcement bars (rebars) are thermo-mechanically treated (TMT) products with a microstructure comprising a tempered martensite at the periphery and a ferrite-pearlite structure at the core. Billets made from scrap and direct reduced iron (DRI) were hot rolled (HR) to produce TMT rebars.It has been observed that, there is correlation between hardness values, ultimate tensile strength (UTS) and microstructure of the rebars.Other findings indicate that,appropriate quenching and tempering techniques have an influence on the outcome of the mechanical properties of TMT rebars. Theresults obtained are in agreement with standard requirements for production and quality of a rebar.

Low Cycle Fatigue Performance Evaluation of TMT rebar

In this study, the strain controlled low cycle fatigue performance of thermo-mechanically treated (TMT) rebar is experimentally evaluated and compared with results of finite element simulation considering Chaboche and Ohno-Wang phenomenological cyclic plasticity models. The experimental results reveal that TMT rebar undergoes cyclic softening in all strain ranges and the fatigue crack is observed at the transverse rib region. The finite element simulation results demonstrate that localized plastic deformation and positive strain accumulation occurs at the transverse rib root from which fatigue damage initiates. Evaluation of ductile damage criteria and stress triaxiality shows that fatigue crack initiate and propagate along the transverse rib.

Effect of Thermal Exposure on the Charpy Impact Properties of Thermo-Mechanically Treated Reinforcement Steel Bar

The reinforcing bars provide necessary structural support for the buildings made with concrete under normal and demanding conditions. The two most structurally demanding situations in building can arise under seismic activities and fire. This paper deals with the behavior of normal Fe 500D (ASTM 1786) rebars under fire situations. The thermo-mechanically treated (TMT) 12 mm rebar samples are heat-treated at different temperatures (150–650 °C for 3 h) and are subjected to microstructural, textural characterization, and tensile and Charpy impact testing. Experimental results show that reheating the rebars below 550 °C does not cause much deterioration in its properties. However, when the same is heated above 550 °C, the temper martensite structure at the rim and mixed ferrite-pearlite-bainite structure at the core region of the rebar is converted into coarse polygonal ferrite grains and carbide particles. These not only reduce the strength, but also significantly increase the impact transition temperature of the TMT rebar. The intensification of detrimental cube and Goss texture with the expense of beneficial gamma-fiber texture and the release of beneficial compressive residual stress upon reheating treatment can also hamper the impact transition behavior of the heat-treated rebar.

Microstructure, mechanical properties and corrosion performance of a few TMT rebars

The present work aims to study the evaluation of the microstructure, tensile properties, hardness, corrosion behaviour of the various grades of rebars. The microstructures of all rebar samples comprise an outer tempered martensite ring with an inner core of ferrite-pearlite in between a narrow bainitic transition zone. Maximum hardness is achieved at the periphery which gradually decreases towards the centre. Chinese grade has a similarity with the Fe 600 rebar in terms of strength and % elongation, whereas Fe 500D and Fe 500 have lower strength but higher ductility. The EDS analyses of corrosion products obtained after immersion test in 5% NaCl solution for a period of 7 days apparently indicate the occurrences of iron oxy-hydroxides and iron oxides. X-ray diffraction and FTIR studies of the corrosion products formed on all thermo-mechanically treated (TMT) rebar surfaces after the aforesaid immersion test primarily indicate the presence of Lepidocrocite (γ-FeOOH), Goethite (α-FeOOH), Magnetite (Fe3O4) and Maghemite (γ-Fe2O3). Fe 500 (12 mm) and Chinese rebars have lower corrosion resistance as compared to the other rebars in 5% NaCl solution. Nevertheless, all the TMT rebars are corrosion resistant and can be satisfactorily used for construction purposes.

Effect of microstructure and texture on the impact transition behaviour of thermo-mechanically treated reinforcement steel bars

In order to develop earthquake resistant or blast resistant structures for freezing environmental condition, it is necessary to evaluate the impact toughness and impact transition behaviour of thermo-mechanically treated (TMT) reinforcement steel bars. Considering the difficulty in preparing standard geometry Charpy V-notch specimens from the TMT rebar, a new specimen design, on semi-circular cross-section, is proposed in the present study for the impact testing of rebars. Tensile testing and impact toughness testing have been carried out on the TMT rebar samples of different diameters (10–16mm) to study the processing-microstructure-mechanical property relationship. Among the investigated rebars, higher section rebar (16mm) showed the best combination of strength, ductility and impact toughness possibly due to the higher tempered martensite rim fraction and the presence of degenerated pearlite and lower bainite at the core region of that rebar. Large fraction of high-angle boundaries and the presence of gamma-fibre texture could have contributed to the lowest ductile–brittle transition temperature (DBTT) in 12mm rebar. Impact toughness can also be influenced by the residual stress distribution inside the rebars.

Time-dependent variation of the electrochemical impedance for thermo-mechanically treated versus plain low alloy steel rebars in contact with simulated concrete pore solution

Electrochemical studies (AC impedance and DC polarization) have been used to study the protective properties of passive films that form on reinforced steel embedded in concrete as a function of time and space. Experiments have been performed on passive/oxide films grown on the surface of steel reinforcement bars exposed to chloride-contaminated simulated concrete pore solution (SPS) prepared by dissolving a representative amount of sodium hydroxide and potassium hydroxide in distilled water and saturated with lime. A common type of locally available concrete reinforcement TMT (thermo-mechanically treated) mild steel with TM (tempered martensitic) and PF (pearlite ferrite) rings (source “A”) was investigated in comparison to a type of hot rolled low-alloy steel without thermo-mechanical treatment (source “B”). The results of the elemental analysis and metallographic microstructure coupled with electrochemical studies indicate that the coupled surface and sub-surface composition of TMT rebar has a time-dependent reversing effect on the protective properties of the passive oxide films that provide durability and protection to the steel rebars against corrosion in concrete. Steel with TM–PF rings has a better initial corrosion resistance in comparison to low alloyed steel. However, with the passage of time, its corrosion resistance decreases, and alloyed steel finally dominates over the TMT rebar.

Performance Enhancement of TMT Rebar in Accelerated Corrosion

Thermo-Mechanically-Treated (TMT) bars have recently been used as advancement over the conventional mild steel bars in reinforced concrete structures in order to enhance the durability in corrosive environment. Corrosion of steel reinforcement leads to cracking of reinforced concrete sections and thus may further reduce the load carrying capacity and serviceability of the structural members. Corrosion of rebars may cause reduction in yield strength of steel, affect the bond strength due to delamination of rust formed on the rebar surface. In this paper, the results of an experimental investi- gation on effects of corrosion on the mechanical strength of TMT steel rebars are presented. Corrosion is induced on the bars by impressed current method. The rebars are subjected to varying levels of corrosion and their stress strain characteris- tics are studied. The stress strain characteristics indicate a decrease in the area under stress strain curve with increasing level of corrosion. Further, it is observed that the yield strength of steel bars is not affected at a lower percentage of corro- sion, but for higher percentage of corrosion there is a considerable reduction in yield strength, percentage of elongation. It was also observed that at a higher percentage of corrosion the failure pattern of bars had changed from ductile mode to brit- tle mode.

Study of Inclusion Bands in Continuously Cast Steel Billets for Rolling Thermomechanically Treated Rebars

The article presents the investigation results of inclusion bands generally formed toward the loose end/inner radius of continuously cast (CC) strands of thermomechanically treated (TMT) rebar grade cast through curved molds. The main analytical tool used for this purpose was an ultrasonic C-scan image analysis system to reveal this band, and the defects detected in this band using the preceding technique were further measured and analyzed in an optical microscope and a scanning electron microscope (SEM) using energy-dispersive system (EDS). The investigation results revealed the presence of macrolevel globular macroinclusions in the size range 50 to 711 μm. The main constituents of the globular inclusions were found to be SiO2 and MnO. Impact test results revealed reduction in ductility in these bands when compared with the billet material without inclusion band toward the opposite face from the inner radius face. In Charpy testing, reduction in the impact energy in these bands was found to be 0.2 kg m. It is important to monitor the severity of macroinclusions present in these bands to avoid the cracking of TMT rebars during hot rolling.

Phosphate treatment of TMT rebar bundle to avoid early rusting: An option for single step process

Thermo-mechanically treated (TMT) rebars are extensively used for reinforcement of cement concrete. In a large tropical country like India, where transit times are long and moisture levels are high, the rebars get easily rusted leading to a poor appearance. The present paper documents the work done by the authors to alleviate this problem. The work involved phosphate treatment on the surface of TMT rebars in bundle without additional process steps such as degreasing, rinsing and pickling. Rebars were treated in two alternate phosphate solutions one containing nitric acid and another nitric acid free. The phosphate layer obtained on the rebar surface was dense and black in appearances. When treatment was done in phosphate solution containing nitric acid, Zinc phosphate (hopeite) and zinc–iron phosphate (phosphophyllite) compounds formed. On the other hand, zinc phosphate (hopeite) in combination with iron phosphate (ludllamite) compounds formed when surface treatment was done in nitric acid free phosphate solution. The treated rebars were tested for extended exposure in normal atmosphere as well as under high humidity conditions. Also, Salt spray and Tafel study were conducted. In all cases the results were compared with those for untreated bars as control. Surface treatment carried out in nitric acid free phosphate solution showed 3–4 times improved resistance to red rust formation compared to bars treated in phosphate solution containing nitric acid. The bond strength of the rebar with concrete also increased marginally when treated in nitric acid free phosphate solution, whereas it reduced 10–15% for treatment in phosphate solution containing nitric acid.

Modelling of Microstructure and Heat Transfer during Controlled Cooling of Low Carbon Wire Rod

The microstructure of TMT (Thermo-Mechanically-Treated) rebars consists of four phases, tempered martensite, bainite, ferrite and pearlite. These four phases are non-homogeneously distributed across the cross section of the wire rod which gives rise to a complex hardness distribution. A coupled heat transfer and phase transformation model has been developed at the R & D Division of Tata Steel that can predict the temperature profile and the fraction of various phases in this type of wire rods efficiently. A fully implicit finite difference scheme has been used for solving the heat transfer equation in cylindrical coordinates; the recalescence due to phase transformation has been taken into consideration that makes the model suitable for use in any industrial application. A model has also been developed based on the volume fraction and strength of the individual phases to predict the hardness at different sections of the wire rod. The predicted hardness distribution has been found to agree well with the micro hardness measurements at various points across the section of the wire rod.