Awarded as the best online publication by CIDC
1) Prof.M.V.Kuriakose B.Tech(Ed), M.I.E. (Retd.Principal, Govt.Polytechnic, Perinthalmanna)
2) Mr.Jismon Issac B.E (Mech) A.I.E, MBA
[Prof.M.V .Kuriakose is having around 35 years of expertise in Structural Engineering and Designing. He is an active member of Lensfed.]
[Mr.Jismon Issac has 13 years of experience in the manufacturing & Quality testing of Construction Steel bars especially High grade TMT steel bars]
Now days some contractors report a tendency among few design Engineers that they specify grade Fe-500 or higher steel in residential buildings, citing its high strength. If they design buildings considering the use of Fe 500 steel in construction, it could help reduce the volume of steel used and reduce the column size.
The rosy part aside, Fe 500 grade steel could pose quite a few site specific issues during construction, especially for small builders. Considering the reported failures and problems with grade Fe-500 or higher, it is advisable to use Fe 415 in residential and commercial buildings and Fe500 could be used only when the entire design is made according to that grade. This is explained below;
Usually, Concrete, a mixture of cement, sand and aggregate, is considered by far as the most stable of building compounds. But it has negatives also: low tensile strength and ductility. This means that concrete’s ability to stretch and to withstand pressure at an angle without breaking is very less.
Here Steel has its role. Steel in the form of bars has great tensile strength and ductility. It can reinforce concrete. Thus the quality of steel has an important role in deciding the quality of concrete. Quest for better material has produced various kinds of steel bars with qualities that make concrete more stable. It started with mild steel Plain bars, evolved through deformed plain bars and then came Cold Twisted Steel Bars (TOR steel) .Now the Market is dominated by various grades of TMT.
Fe 415 TMT bars with much higher strength than Cold Twisted Deformed bars (TOR steel) provide an extra cushion to the factor of safety of the design. And so, Fe 415 TMT bars became the most commonly used steel grade in construction of houses in the State.
High rise buildings call for steel of higher strength, and hence higher grades of TMT steel- Fe 500 and above – are used in such buildings. This is because using Fe 415 steel will require a large number of rebars to provide the required strength. That would increase the size of the columns. This is a serious concern at a time when space is at a premium else why should one go for high rises.
Fe 500 is produced using the same TMT process that is used to produce Fe 415. The beauty of the TMT process is that it can produce different grades of steel by making slight changes in the process. If the steel is quenched a little more, the outer martensite layer that provides the high strength to the steel rebar becomes thicker at the expense of the soft inner core that endows it with ductility. This is exactly what happens in Fe 500 grade TMT steel.
Not only using Fe500 allows us to saves cost
Usually steel producers suggest their clients that use of Fe500 grade steel instead of Fe 415 saves cost. But it is true only, when the entire design is made according to that grade. Indian standards specify that the MS bars have a tensile strength of 250 N/mm2.
When steel is detailed it is designed to resist a higher load than what it really has to struggle with. Similarly while designing RCC for buildings factor of safety usually‘3’ is considered for concrete with respect to the cube strength. In the case of steel “1.8” is considered as factor of safety. In short permissible strength is equal to yield strength /1.8.Thus while calculating load of a structural member and on designing its steel, we provide sufficient factors of safety.
Thus an engineer who makes his design with Fe 415 usually considers the steel to deform at 230 N/ mm2. But as you know even if it is considered to deform at lower loads, Fe-415 resists changes only if it crosses its yield strength. As an example for a residential building or a 3 storied commercial complex, the engineer designs at 230N/mm2 and the use of Fe-415 already gives sufficient protection. Here using Fe-500 is not economical as it costs more. It’s true only if the design is made accordingly.
Practically the civil designs are made by considering the factor of safety aspect. Now days Engineers make use of modern limit state method for detailing. Normal loads taken into consideration are dead load, live load, earthquake load, wind load etc., and their combinations. While designing special attention is taken to consider both acting load & material strength. If the engineer underestimate the load, it is unsafe. If load is overestimated, it is safe but turns uneconomical
(Design load = Characteristic load x practical safety factor for load)
where characteristic load is the maximum working load that the structure could with stand.
Design Engineer take partial safety factor. Factor 1.5 is taken when loads like dead load, live load, wind load are considered separate .When combinations of loads are taken into consideration, they consider safety factor 1.2. It means that the maximum working load is considered 1.2 times. If designs are altered by rendering quantity of steel, Fe- 500 grade is ok. Otherwise it costs more and is a waste.
Balancing of Tensile strength Vs Ductility
Dual core in Thermo mechanical Steel bars contributes to two distinct characteristics of steel bars. The outer tempered Martensite layer gives required tensile strength to the TMT while the inner ferrite –pearlite core give ductility property. In any TMT grade, these should be in equilibrium. If one core exceeds the other, TMT will not have sufficient characteristics. Suppose outer core is more than inner core, TMT bars will have more strength; but compromising on its ductility .If the inner core is more, TMT will be called more ductile but with less strength.
It is clearly observed by Bureau of Indian Standards that in Fe-415 grade ductility is standardized as minimum 14.5 % elongation. Yield strength of Fe-415 is standardized to be minimum 415 N/mm2. Now consider the case of Fe-500 grade. Here yield strength should be minimum 500 N/ mm2. It’s appreciable; but compromising on its ductile property. Ductility of fe-500 grade is standardized to be 12% minimum. For 550 grade ductility (ability to deformation without breaking) again reduce to 8% min.
Steel Bars used in civil constructions must have sufficient yield strength. More over it should be ductile. Then only steel can elongate or deform on heavy loads and safeguard the buildings without breaking up. So the point is that Fe-500 or high grade must be used only when design usage requires it. Otherwise use of Fe-415 will be safer.
Bending problems associated with Fe- 500 grade
A higher strength and lower ductility means that Fe 500 bars do not bend easily. Fe 500 grade shown itself sensitive to high strains induced in the bending process. It is not tolerant of bending to diameter higher than the minimum bend diameters specified. Reports indicate that if bend diameters are frequently less than the minimum specified that lead to problems failure. In certain cases, the steel won’t break. But compressed side of bend shows ribs spitted up.
For example, a 12 mm rebar of Fe 500 breaks when bent into a perfect ‘U’, unlike a similar rebar of Fe-415 grade. It might cause it to crack, too. Manual bending takes its toll on the masons. Hence hydraulic bending machines have to be used to bend the bar.
Bending a Fe-500 grade bar should be carried out very slowly, not with a jerk. Bending done on a bar bending table/ block is always very sharp. It weakens the TMT at tension side of bend portion as tempered martensite layer there gets softened and it breaks. So on practical use, Fe-500 fails at construction sites.
The builders of most high rises that use Fe 500 grade rebars procure them in factory cut sizes, avoiding the need to work on them further. Their designers provide them with the steel detail that helps them procure items of the shelf. Small builders may not have such luxuries. In the first place, their design might not be for Fe 500 steel, negating the savings in quantity of steel.
Even if their design is for Fe 500 steel, they might not be able to take the due advantage due to a variety of reasons. They might not have access to a steel detail that gives the precise number of various types of structural steel elements needed for the building. Even if they have the steel detail and can buy factory-cut steel bars based on it, transporting them to work sites in the interiors through roads that hardly allow truck to pass is a difficult task. That is the main reason why factory cut steel has not picked up in Kerala.
In such a situation they will have to resort to fashioning the required steel elements at the work site itself. And then, an absence of the machinery required to bend the Fe 500 rebars would lead to an increase in labour costs and a decline in quality. This is especially true if the rebar has to be shaped into tight curves. In, short they would have to incur the extra cost without getting the perceived benefits.
Welding problems associated with Fe -500 grade
Weldability too is an issue with Fe 500 grade steels. The amount of Carbon content in steel has been a major deciding factor for engineers as a minimum level of carbon content is essential in steel to achieve the required strength. At the same time, excess carbon content threatens its property of weldability. Even though the carbon content in Fe -500 is advised to keep max 0.30%; practically steel with C ? 0.25 is better weldable. It is observed that in the case of welding a Fe -500 or Fe- 550 steel bar, the bar is raised to a temperature above its tempered temperature. Then without controlled quenching and tempering process, it is cooled to the ambient temperature. Through this cycle, steel bar lose its strength of its external case and revert back to steel with lower yield strength. In short designers should not rely on welding Fe -500 or 550 grade steel bars.
ReBending problems associated with Fe- 500 grade
Even though Re bending or reverse bending is not advisable for TMT grades, occasions do arise on construction sites where they are unavoidable. It is reported that significant softening of tempered martensite layer can happen at relatively low temperature in TMT high grades while rebend / reverse bending. This results in reduction of steel strength. The notching strain develops in rebend of Fe 500 grade is considerably high which leads the bar to get snapped off. In certain cases snapping won’t be there. But steel got surface cracks and strain leading to excessive corrosion. It is recommended to preheat Fe 500 grade to a temperature 100 Degree Centigrade.ie, above softening temperature, before rebending. This minimizes work hardening& loss of ductility. But practically it is difficult in sites.
Performance of Fe 500 & Fe 550 at elevated fire temperatures.
Reinforced concrete buildings are exposed to the elevated temperatures during a fire event. Most often the elevated temperatures exceed 500 degree C. Unfortunately this is also about the tempering temperatures of TMT bars. Thus prolong exposures to elevated temperatures would result in retempering of the outer skin resulting in reversion to the strength of the core steel which is vastly reduced.
Thus accelerated failure of the RCC building frame during a fire is more likely for a building designed with Fe 500 or 550 grades.
Seismic Performance Considerations in using Fe-500 grade
The main assignment against the use of higher grade TMT is its behavior under cyclic loading. Studies in several parts of the world notably New Zealand, Australia, Italy etc.have pointed to the difficulties associated with the use of Fe-500 and Fe 550 grades under cyclic loading particularly in Seismic zone 3 and 4. Kerala State is in Seismic Zone 3. A maximum limit for yield strength is desirable to be specified in standards used for earthquake design. The absence of such a maximum limit may lead to brittle shear failure of the structure. Requirements specified is IS: 1786 for Fe- 415 grade TMT bars are in line with the requirements of other countries for ductile design. However this doesn’t hold well for rebars of grade Fe 550 as per IS 1786. Cautious approach should be adopted in using rebar grades higher than Fe 415 especially Fe 550 grade where ductility of rebars is necessary for inelastic deformation of structural members as demanded by design philosophies. Such design cases are earthquake designing, designing for impact load, designing of beams/ slabs with adjustment of support moments load, against gravity load etc.
In short, engineers must be cautious enough to use steel of higher grades where yield strength in maximum is not limited and where ductility is lower, while doing building designs for seismic zone areas.
Importance of Static Stress Strain Diagram
TMT bars presently are used in India for construction of concrete structure. IS 456 provides design stress strain curve of TMT bars. Uses of design curve of CTD bars while doing design for TMT grades are not correct. If BIS comes out with design stress strain curve and design valve of the yield strength of TMT bars, then only design turns to be economical. Using Fe 500 or Fe 550 grades using design of CTD bars doesn’t yield profit.
Avoid possible mix-ups of different grades
Some engineers show a tendency to specify Fe-500 or Fe-550 grade steel when it is not required. It may be used in one part of the building. But the pragmatic decision is frequently taken to make all steel the same grade to avoid possible mix ups. However what happens in practice is that suppliers offer alternatives in order to reduce costs. Sometimes clients also look for other grades. Decision must be so cautious in recognizing this possibility of mix-ups in sites.
Problems associated with stacking and storing high grade of TMT at sites & at shop godown.
Stacking higher grade TMT bars should be considered from the quality point of view. The stacking height must be optimized for different dia bars. The more the stacking height, the more load on the bars at the lower layers. Excessive load may damage the surface characteristic of TMT as a result of which tensile strength & bond strength is reduced. Rough handling shock loading and dropping of Fe 500 & Fe 550 grade steel from a height is also to be avoided.
Importance of testing of Fe 500 & 550 grades.
Designers/ Engineers should accept TMT Bars only after proper testing & verification of the same irrespective of the name of the brand/ manufacturer. Nowadays the market is flooded with so much inferior products which fail in actual mechanical testing in labs. They are marketed as Fe 500 or Fe 550 grade for cheating customers having half knowledge. Asking for a test certificate & a computerized plotted stress – strain graph will solve the issue. Also be vigilant if some suppliers give you Fe 500 or 550 grades at the cost of Fe 415. It is impossible as much care, systems & cost is involved in production of 500 or 550 grade.
One should also be on the lookout for fraudsters who sell other grades of steel under the Fe 500 label. The increasing demand for Fe 500 grade steel in the market and the inability to solve the -problems associated with it provides ample room for such fraudsters.
If the Fe 500 steel you bought bends easily and offers no issues with workability, you might have been taken for a ride. So, before you set out to purchase steel for your dream home, assure yourself that you go for the right grade.
An Fe 415 bar would be an optimal choice for nor mal buildings due to its right combination of strength and ductility. Adding more strength at the cost of ductility might not be the best solution for your dream home. In fact, IS: 13920, the code of practice for ductile detailing for structures for seismic forces, recommend steel reinforcements of grade Fe 415 or less. Only select grades of Fe 500 rebars having an elongation more than 14.5 per cent, against the normal 12per cent can be used for the purpose.
If you have a premium for space, a design that has factored in the use of Fe 500 steel, and have access to factory-cut steel, it is all for you. For the rest, Fe 415 will be the choice. And when in doubt, ask your engineer.
1) “Critical Properties of Seismic Resistant Rebars” – Dr.A.M.Elmaghrabi P.hD (Inorganic and analytical Chemistry)
2) “New Zealand Standard 3101:2006”: Concrete Structures
3) “Indian Standard Specification for High Strength deformed bars and wires for concrete reinforcement (third revision),IS 1786:1985”; Bureau of Indian Standards,New Delhi
4) “The impact of 500 Mpa reinforcement on the ductility of concrete structures”-Proceedings of the concrete Institute of Australia 2001.
5) “A clear and present danger in use of High Grade TMT in Seismic Zones”-Emilio M.Morales(Fellow in Civil Engineering, Carnegie Mellon University)
We at engineeringcivil.com are thankful to Sir Jismon Issac for submitting this very important paper on “TECHNICAL PAPER PRESENTATION ON THE PROBLEMS OF USING FE- 500 GRADE AND HIGHER GRADE TMT STEEL BARS IN RESIDENTIAL BUILDINGS IN INDIA” to us. We are hopeful that this will be of great use to all civil engineers working on construction related projects in India.