Production and Characteristics of High Strength Reinforcement Bars
Production and Characteristics of High Strength Reinforcement Bars
During the last few decades, reinforced concrete construction has generally utilized reinforcement bars with yield strength of 415 MPa (415 N/sq mm) for most beams, girders, and columns, and less frequently reinforcement bars with yield strength of 500 MPa for columns which are not a part of a special moment resisting frame. However presently, high strength reinforcement bars are replacing earlier used reinforcement bars with yield strength of 415 MPa for the construction of high rise concrete buildings and structures especially in the areas with high seismic risk. This is being done for resisting earthquake forces. High-strength reinforcement bar is normally defined as that reinforcement bar which has a YS of 500 MPa or more. In Japan, reinforcement bars with yield strength as high as 690 MPa is presently being used in building members designed to resist earthquake forces.
A number of studies have been done for evaluating the use of reinforcement bars with higher strengths for beams, girders, and columns supporting live and dead loads. These studies have indicated that there is an increasing need for higher strength reinforcement bars in seismic and non-seismic applications. There are many potential benefits to the use of high-strength reinforcement bars in concrete construction. These include cost savings, reduced construction time, and reduction in reinforcement congestion.
In USA presently steel producers are developing reinforcement bars with YS strengths reaching 830 MPa and with varying mechanical and chemical properties. The new high-strength reinforcement bars are being produced using different methods of production. However, none of the high-strength reinforcement bars are able to match the benchmark mechanical properties of 415 MPa grade reinforcement bars. Each of the high-strength variant differs from benchmark behaviour in different ways. There is concern that the less ductile high-strength reinforcement bars can fracture at the bends and can need larger bend diameters.
In Japan, a 5 year project under the code name ‘New RC’ project was taken up which ended in 1993. One of the tasks of this project was to establish standard for high strength reinforcement bars for use in buildings in seismic regions. Comparison of this standard with some of the other standards for high strength reinforcement bars is given in Tab 1.
|Tab 1 Comparison for some of the standards for high-strength reinforcement bars|
|Sl. No.||Country||Standard||Yield strength in Mpa||Remarks|
|1||Japan||New RC project 1993||980||Also includes 1275 MPa grade but only for transverse reinforcement applications|
|2||USA||ASTM 1035-14||830||High yield strength by controlling microstructure|
|3||India||IS 1786 – 2008||650||Microalloyed steel with maximum CE of 0.53|
|4||Russia||GOST 10884-94||1200||High yield strength with addition of silicon upto 2.3 %|
|5||Korea||KS D3504-11||700||CE increase allowed upto 0.63|
|7||UK||BS 6744-01 + A2:09||650||Stainless steel rebar|
|8||China||GB 1499.2 -07||500||CE 0.55 maximum|
|Note: Carbon equivalent CE = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/15|
Production of high-strength reinforcement bars
There are three methods normally used for the production of high-strength reinforcement bars. These are (i) cold working, (ii) addition of alloying elements in the steel composition, and (iii) quenching and tempering of steel during its rolling. High-strength reinforcement bars produced through quenching and tempering typically show relatively low tensile strength to yield strength ratios and relatively high strains at fracture. High-strength reinforcement bars produced by micro-alloying have a relatively high tensile strength to yield strength ratio and relatively high strains at fracture. These methods are shown in Fig1 and described below.
Fig 1 Production of high strength reinforcement bars
Cold working – Cold working is a long-standing method of producing high-strength reinforcement bars. In cold working of the steel, the steel deformation is carried out by any of the cold working processes such as cold rolling, cold twisting, or cold drawing etc. This method enables the production of high strength reinforcement bars from low carbon and manganese steels which are weldable. In this method, the reinforcement bars are submitted to a strain hardening after hot rolling. For such reinforcement bars, the yield strength can be increased by increasing the extent of straining. Cold working is carried out below the recrystallization temperature of steel. The process causes dislocation generation and movements within the crystal structure of the steel material. A dislocation is a crystallographic defect or irregularity within a crystal structure. The presence of these dislocations strongly affects the yield strength and ductility of the steel material. Cold working eliminates a yield plateau and hardens the steel. While cold working improves yield strength, it reduces both ductility and the ratio of tensile strength to yield stress. Hence, it is normally not an appropriate method of producing high-strength reinforcement bars for members resisting earthquake effects.
Addition of alloying elements – In this method, the yield stress of the steel material is increased by modifying the chemical composition by the addition of alloying elements but the carbon and manganese contents are kept low in order to avoid a significant decrease in the weldability of the steel. The high strength reinforcement bars produced by the addition of the alloying elements are used in as-rolled condition after slow cooling in air. Normally, the high strength of the steel material is achieved by adding small amounts of titanium, niobium, or vanadium, which is referred to as micro-alloying. Micro-alloying is a process which involves introducing small amounts of alloying elements in order to achieve the desired properties in the reinforcement bars. Micro-alloying can produce a marked yield point and a tensile strength/yield stress ratio larger than that from quenched and tempered steel reinforcement bars (of the order of 1.25 for 690 MPa grade reinforcement bars).
Micro-alloying forms inter-metallic carbides which produce fine-grain strengthening and precipitation hardening. Fine-grain strengthening occurs by the pinning of planar defects (grain boundaries) during thermo-mechanical processing (rolling), which produces a very fine grain size in the steel reinforcement bars. In general, the finer the grain size, the higher is the yield stress. This relationship is known as the Hall- Petch effect (Hall–Petch relationship tells the strength in materials that is as high as their own theoretical strength can be achieved by reducing grain size. Indeed, the material strength continues to increase with decreasing grain size to around 20 nano-meters to 30 nano-meters where the strength peaks.). When these inter-metallic carbides are dispersed through the ferrite grains, pinning line defects (dislocations) occur, which further raise the yield stress of the material. This mechanism is known as precipitation hardening.
Titanium micro-alloying contributes to precipitation hardening, but the strong tendency of titanium to combine with oxygen, sulphur, and nitrogen makes it difficult to control the strengthening effects. Niobium micro-alloying is widely used in steel sheet and strip production, in which the temperature at the end of production is relatively low and the deformation is high. Reinforcement bar production requires high rolling temperatures and less deformation, making niobium micro-alloying ineffective for the high-strength reinforcement bar production.
Vanadium is one of the most commonly used alloying elements to increase the strength of the reinforcement bars. Vanadium or vanadium-nitrogen micro-alloying is normally used to produce high-strength reinforcement bars which are weldable. Vanadium addition increases yield stress and fracture toughness primarily due to inhibition of grain growth during heat-treatment and the precipitation of carbides and nitrides. Vanadium-only micro-alloying results in 35.5 % of the vanadium forming carbide and nitride precipitates, while 56.3 % of the vanadium ends up as solid solution dissolved in the matrix, which does not improve the yield stress of the reinforcement bar. The amount of vanadium forming precipitates can be increased upto 70 % with the addition of nitrogen. Another advantage of vanadium-nitrogen micro-alloyed reinforcement bars is that it eliminates the adverse effects of strain aging on properties of steel because it pins the soluble nitrogen. The use of vanadium can reduce the amount of carbon needed to achieve higher strengths and is therefore useful for achieving weldable high-strength reinforcement bars.
Quenching and tempering – Quenching is the rapid cooling of steel which has been heated to the austenitic phase (at which solid steel material recrystallizes). The process of quenching and tempering consists of quenching the steel immediately after rolling and then allowing the reinforcement bar to be tempered by the heat remaining in the core while it gradually cooling on the cooling bed. As a result, this process produces steel with mechanical properties which vary significantly between its inner core layer and its outer skin layer, with the inner core having lower yield strength and more ductility than the outer layer. Quenching and tempering treated reinforcement bars retain their yield plateau since they have not been strain hardened and since the overall chemical composition has not been altered. These reinforcement bars can be weldable if their chemistry satisfies the requirements. These reinforcement bars typically show a low tensile strength to yield stress ratio (of the order of 1.15 for 690 MPa grade reinforcement bars).
The steel is quenched normally in water, which results in a hard and brittle material structure. Tempering is the heating of the quenched steel, which modifies the microstructure to decrease the hardness and increase the ductility of the material.
The production process of high-strength reinforcement bars is based on the thermo-mechanical processing. Thermo-mechanical process is a metallurgical process which combines plastic deformation process with the thermal processes such as the heat-treatment, water quenching, heating, and cooling at various rates into a single process. The process imparts high strength to the reinforcement bars by the technique of thermo mechanical treatment as against mechanical working by cold working. The strength of the reinforcement bars is due to the tempered martensite outer layer while the ductility of the reinforcement bars is due to the ferrite- pearlite structure in the core of the reinforcement bars.
The thermo mechanical treatment converts the surface of the reinforcement bars to a hardened structure (martensite) and subsequently the phase evolves by cooling at ambient temperature to allow the hot core to temper the surface through thermal exchange. This results in a unique composite microstructure comprised of tempered martensite in the peripheral zone/case, transition zone of pearlite and bainite just after the martensite periphery and a fine grain ferrite-pearlite at the central zone/core (Fig 2). Due to the quenching and self-tempering production process, high-strength reinforcement bars produced are also called ‘quenched and self-tempered (QST) rebars’.
Fig 2 Microstructure of quenched and tempered reinforcement bars
There is one more production method for the production of high-strength reinforcement bars. This method is covered in ASTM specification number ASTM A1035; 2011. These reinforcement bars typically have large tensile strength to yield strength ratios but relatively low strains at fracture. This production process for high-strength reinforcement bars is a patented process and is known as ‘Microstructure Manipulation (MMFX)’ process. The patented MMFX process involves manipulating the microstructure of steel to obtain the desired mechanical properties and strength. The process generates reinforcement bars with stress-stain relations that do not have a well-defined yield point, show a relatively high tensile strength to yield stress ratio, but have relatively low fracture elongations. The MMFX of high-strength reinforcement bars satisfy the ASTM A1035 specifications.
Other production aspects – High strength reinforcement bars are normally produced in straight lengths at the rolling mill. However, coiling smaller sized reinforcement bars is becoming a practice. Bars are coiled soon after rolling, which traps heat in the coil. Thus, the cooling rate of coiled reinforcement bars is somewhat slower than for straight bars. Since the testing samples are required to be straightened before testing, coiled bars tend to have a lower yield stress, and the shape of the stress-strain curve can be somewhat rounded. To counteract this effect, high-strength coiled reinforcement bars needs higher quantities of micro-alloying elements as compared to corresponding sizes of straight reinforcement bars.
In the production process for the reinforcement bars, bar identification marks are added during rolling, and mechanical properties are tested after the bars are rolled (and marked). If the mechanical properties of high-strength reinforcement are not achieved, then the outcome is that the reinforcement bars do not meet the specifications associated with the mark.
Fabrication issues – Issues with fabrication of high-strength reinforcement bars can be grouped into the two categories namely (i) the introduction of multiple grades of reinforcement bars which need to be scheduled, received, and stored at a fabrication facility prior to use, and (ii) changes in the fabrication process needed as a result of the properties of high-strength reinforcing bars.
Fabrication processes of shearing and bending are impacted by the properties of high-strength reinforcement bars. High-strength reinforcement bars result in higher shearing and bending forces for the same size bar, and experience more elastic rebound after bending, leading to fabrication concerns regarding (i) wear and tear on existing equipment and the possible need for new, higher capacity equipment, (ii) safety of workmen in the event of bar or equipment failure during bending operations, and (iii) compliance with bar fabrication tolerances. There is more frequent equipment failures associated with the fabrication of high-strength reinforcement bars. Concerns regarding safety are heightened in cases where bar defects have caused fracture during bending operations at higher force levels. Extra precautions are necessary to maintain a safe work environment, which can impact the efficiency of fabrication operations.
The strength and ductility of the high-strength reinforcement bars are defined in different ways. The tensile properties and other requirements defining strength and ductility which are specified include (i) minimum or lower bound yield stress, as specified in the relevant specification, (ii) maximum or upper bound yield stress, as specified in the relevant specification, (iii) length of the yield plateau or strain at the end of the yield plateau, (iv) tensile strength, (v) uniform elongation and total elongation, (vi) ratio of tensile strength to yield stress or its inverse (referred to as the yield ratio), and (viii) results of a bend test or a bend-rebend test. Some, but not all, of these properties and tests are specified for each high-strength reinforcement bars. Several of the tensile properties are indicated on the idealized stress-strain curve shown in Fig 3.
Fig 3 Idealized stress-strain curve showing the properties of strength and ductility
Yield stress – For all the reinforcement bars, either the yield point or yield stress are normally specified. However, in some standards (e.g. ASTM), the strain at the end of the yield plateau is specified for some of the high-strength reinforcement bars. These standards specify the strain at the end of the yield plateau. ASTM standards do not allow the measured value of the yield stress to fall below the grade of steel, whereas other standards (such as the Australian/New Zealand standards) use the 5 % fractile concept, in which a small percentage of tests are allowed to be below the minimum strength. ASTM A370, ‘Standard Test Methods and Definitions for Mechanical Testing of Steel Products’ defines yield point as ‘the first stress in a material less than the maximum obtainable stress, at which an increase in strain occurs without an increase in stress’. Yield point is applicable to reinforcement which shows an increase in strain without an increase in stress, which generally only occurs in reinforcement bars with lower strengths.
High-strength reinforcement bars typically do not have a definitive yield point, so another means of defining the yield stress is necessary. ASTM A370 defines yield stress as ‘the stress at which a material exhibits a specified limiting deviation from the proportionality of stress to strain’. The yield stress can be determined by the 0.2 % offset method or the ‘extension under load’ (EUL) method of ASTM A370. The 0.2 % offset method is used for computation of the yield stress for the reinforcement bars, but an additional check using the EUL method for a strain of 0.0035 is also required to define the minimum yield stress ofr the reinforcement bars. The EUL method with a strain of 0.0035 produce minimum stresses of 550 MPa and 620 MPa for 690 and 830 grades of high-strength reinforcement bars respectively. The 0.2 % offset method is also used to define the yield stress in specifications in many countries.
Tensile strength – In the specifications, tensile strength is consistently defined as the peak stress on the stress-strain curve. Tensile strength is calculated by dividing the maximum load which the specimen sustains by the nominal bar area.
Elongation – Elongation is normally reported as the total elongation over a prescribed gauge length which extends across the fracture of a bar. ASTM A370 provides two methods for the determination of the ‘total elongation’. In one method, a bar is marked with an initial gauge length of 200 mm and pulled to fracture. This method does not account for elastic elongation. For the first method, the ends of the fractured bar are fit together and the gauge length is re-measured. The elongation is then reported as the percentage increase in length relative to the original gauge length. In the second method, the elongation at fracture can be measured using an extensometer, in which case elastic elongation is included. Both these methods include the additional localized elongation at the necked-down region plus the elongation along the non-necked-down portions of the bar within the gauge length.
Uniform elongation is the strain which occurs as the bar reaches its peak stress (tensile strength), expressed as a percentage. Its name originates from the fact that this is the largest deformation in the test bar while the tensile strains are uniform throughout the length between the test grips. It occurs right before the onset of necking in a bar. The uniform elongation is typically measured with an extensometer while a bar specimen is being tested. It includes both plastic strain and the strain which is recovered upon unloading the bar. It can also be determined by measuring the plastic elongation upon removal of the bar specimen from the test machine and then adding the recovered strain. In this case, plastic strain is measured away from the necked-down region, and the recovered strain is added to it to obtain the uniform elongation.
The uniform elongation calculated using the ‘Canadian Associations Standard CSA G30.18 (CSA, 2009)’ assumes linear unloading with a modulus equal to the initial modulus of steel, Es, of 200,000 MPa. However, test data on 415 MPa grade reinforcement bars indicate (i) the unloading modulus decreases with an increase in the tensile strain, (ii) the unloading curve is linear only during the initial phase of unloading, and (iii) the response becomes progressively nonlinear as the bars are fully unloaded. A linearization of the unloading response can result in an unloading modulus around two-thirds of the initial loading modulus. For high-strength reinforcement bars, the recovered strain can be as high as 1 %. Some standards (e.g. Australian/New Zealand Standard 4671, 2001) require reporting the uniform elongation.
Uniform elongation is a useful property for seismic design since it is more closely related to the maximum elongation (the useable elongation) which is relied upon in a location of yielding, i.e., a plastic hinge region. Useable elongation is to be taken as 75 % or less of the uniform elongation, because under cyclic loading conditions, reinforcement bars can achieve the equivalent damage state associated with uniform elongation at a smaller elongation. Reinforcement bars typically have a characteristic ratio of uniform elongation to fracture elongation, which varies by reinforcement type.
Ductility – The bend and bend-rebend tests are two ways of evaluating ductility of reinforcement bars. Normally, the reinforcement bar specifications include a bend test in which bars are bent around a pin or mandrel of a specified diameter and to a specified degree of bending. The bend diameter varies with the bar diameter. The test specimen passes if no cracks appear on the outside of the bent portion of the bar.
Three main categories of experimental tests are useful for investigating the behaviour of bends in reinforcement bars, with each category of tests geared to answer a particular set of questions. These categories are (i) visual inspections of bends (ASTM bend tests), (ii) bend/re-bend tests, and (iii) bend tests in concrete.
ASTM specifications for reinforcement bars specify the bending requirement as ‘The bend test specimen shall withstand being bent around a pin without cracking on the outside of the bend portion’. The required bend test hence involves bending bars to 180 degrees (or 90 degrees for 43 mm diameter and larger bars) at a specified pin bend diameter. A visual inspection is then performed to identify cracking at the bend. If no cracking is visually observed, a specimen is deemed to pass the bend test. The test though simple to perform, does not provide a measure of the reserve strength and ductility of bar bends, as a load-test can. It is possible that micro-cracking not visible to the eye can compromise the performance of the reinforcement bars in-situ.
In the bend and re-bend tests, bar samples are bent to the required angle and bend diameter, and then straightened at either quasi-static or dynamic loading rates. For 415 MPa grade bars, work hardening increases steel strength at the bends and typically causes the samples to fracture away from the bends in a ductile manner. However, if the reinforcement bars have limited ductility such as high-strength reinforcement bars, strain demands at the bends can cause cracks, which can make bends weaker than the unbent portions of the reinforcement bars and more susceptible to brittle fracture. If a reinforcement bar fails in a brittle manner at a bend, it is considered to have failed the bend/re-bend test. If, however, a reinforcement bar fails in a ductile manner, then it is deemed to have passed the test. This type of test has the advantage of putting bar-bends under load and therefore provides a direct measure of the strength and ductility performance of bar bends.
Some standards need a bend-rebend test for smaller bar diameters and a bend test for larger bar diameters. For the bend-rebend test, the bar sample is to be bent around a mandrel of a specified diameter to an angle of 90 degrees at the mid-length of the sample. Two additional 45 degrees bends are made so that the sample is straight for a portion, v-shaped near its middle length, and straight at the other end, as shown in Fig 4. The sample is then aged in oil for an hour at 100 deg C, cooled, and rebent in the reverse direction by applying a tension force to the ends of the sample. The aging and cooling steps are necessary to simulate the detrimental effects of strain aging. When reinforcement bars are bent, nitrogen can be released from the steel, which can lead to embrittlement. Upon rebending, the embrittled steel is more likely to crack. No cracks are to be evident in the rebent bar to pass the test.
Fig 4 Test samples of reinforcement bars showing bend-rebend
It is to be noted that the bend/re-bend tests apply larger demands on the bar bends than they normally see in a concrete structure. For this reason, it is best to compare the bend/re-bend performance of the high-strength reinforcement bars to that of 415 MPa grade bars, which have been used for decades and have shown adequate performance in concrete members. Bends in reinforcing bars can also be tested in concrete. In such tests, the interaction between the concrete and bar-bends can be investigated. Simplified versions of the test include embedding a hooked bar into a concrete block and pulling on it until failure. Possible failure modes which can be expected in block tests include (i) bar fracture outside the block where demands on the bar are highest, (ii) bar failure inside the block closer to or at the bend, or (iii) splitting of the concrete block. Such tests, however, may not expose bends to the worst loading which can be experienced in a structure, as the surrounding concrete can relieve the bends of some load. In contrast, some of the worst loading on bar-bends can arise in confinement applications, where an expanding concrete core partially straightens hoop bends while applying high tensile loads to them. Another critical application for bar-bends is in damaged regions, where bond to concrete and its beneficial effects on bends are reduced (e.g., joints under severe seismic loading, or severely cracked regions). However, tests of bar bends in concrete members are essential for validating the adequate performance of bar bends in high-strength reinforcement bars. Though, such tests are expensive to conduct and do not easily lend themselves to the task of determining minimum bend diameters while exploring the numerous variables that affect the performance of bar bends.
Strain aging – Strain aging is defined as the process by which steel strained beyond its elastic limit undergoes time dependent changes in it mechanical properties. Typically, reinforcement bars strained beyond their elastic limit will, over time, see an increase in their tensile strength and a decrease in their ductility (Fig 5). Strain aging is also proven to affect the brittle transition temperature in steel. Factors affecting strain aging include the steel composition, temperature, and the time elapsed since large strains were incurred. Strain aging is mostly attributed to nitrogen reallocation within the steel matrix. Higher temperatures accelerate this process. Hence, strain aging occurs much faster in warmer regions.
Typically, most of the effects of stain aging in steel reinforcement bars occur within a few months after inelastic strains are incurred. As reinforcement bars are bent, they experience large inelastic strains. Bar bends are therefore prone to strain aging embrittlement, which can cause them to fracture prematurely and limit their ability to sustain inelastic deformations during structural loading.
Fig 5 Typical stress strain curves showing the effects of strain aging
A study carried out on the strain aging of reinforcement bars suggests that micro-alloyed steel including titanium and vanadium can lower the effects of strain aging on reinforcement bars. Such alloying elements have properties which allow them to bond with the nitrogen in the composition to form nitrides. These reactions limit the amount of free nitrogen throughout the steel that is attributed to strain aging effects.