SOME ISSUES IN THE USAGE OF FE- 500 OR HIGHER GRADE TMT STEEL BARS IN RESIDENTIAL BUILDINGS
Some contractors report a tendency among few design Engineers to specify Fe-500 or higher grade steel in residential buildings citing its high strength. Customers are advised that if they use Fe 500 or higher grade steel in building construction, that would help in decreasing the quantity of steel used and reduce the column sizes.
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 became the most commonly used steel grade in construction of houses in the state as it is easier to produce the desired strength equivalence of CTD bars through the TMT process than through the conventional CTD bar production process apart from the other advantages of TMT over CTD.
Higher grades of TMT steel- Fe 500 and above –may be used in high rise buildings. This is because using Fe 415 steel will require a larger 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.
FE500 ALLOWS US TO SAVE 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 ‘3’ is considered for cement concrete for 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 divided by 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. Savings materialize only if the design is made for Fe-500. Practically the civil designs are made considering the factor of safety aspect. Engineers make use of 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 underestimates the load, it is unsafe. If load is overestimated, it is safe but turns uneconomical and against the basics of engineering
(Design load = Characteristic load x practical safety factor for load)
Where characteristic load is the maximum working load that the structure could withstand
Design Engineer take partial safety factor. A factor of safety of 1.5 is taken when loads like dead load, live load, wind load are considered separately. But when combinations of loads are taken into consideration, they consider a factor of safety of 1.2. It means that the maximum working load is considered 1.2 times. If all the designs are altered by considering the parameters of Fe- 500 grade, then it’s usage is OK. Otherwise it costs more and is a waste and is against engineering.
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 gives it ductility. 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 reduces to 8% min.
Steel Bars used in civil constructions must have sufficient yield strength. More over it should be ductile. Then only can steel 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 is sensitive to higher strains induced in the bending process. It is not tolerant of bending to diameters higher than the minimum bend diameters specified. Reports indicate that if bend diameters are frequently less than the minimum specified then that can lead to problems like failures. In certain cases, the steel won’t break. But compressed side of bend shows ribs splitted 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 and at the same time paying a higher purchase price.
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 steel. 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 be kept at max 0.30%; practically steel with C ≥ 0.25will be 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 loses the strength of its external case and reverts 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 Rebending or reverse bending is not advisable for TMT grades, occasions do arise in construction sites where it is unavoidable. It is reported that significant softening of the tempered martensite layer, can happen in higher grade TMT bars at relatively lower temperatures while doing rebend / reverse bending. This results in reduction of steel strength. The notching strain developed during 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 will devolope surface cracks and strain, leading to excessive corrosion. It is recommended to pre-heat Fe 500 grade to a temperature of 100 degree centigrade.{ie, above the softening temperature} before rebending. This minimizes work hardening& loss of ductility. But practically it is difficult at sites.
PERFORMANCE OF FE 500 & FE 550 AT ELEVATED TEMPERATURES
Reinforced concrete buildings are exposed to elevated temperatures during a fire event. Most often the elevated temperatures exceed 500 degree C. Unfortunately this level of heating is also above the tempering temperature of Fe-500 TMT bars. Thus prolonged exposure to elevated temperatures would result in retempering of the outer skin resulting in reversion to the strength of the core steel, which is vastlylesser.
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 argument against the use of higher grade TMT is its behavior under cyclic loading. Studies in several seismic prone parts of the world notably New Zealand, 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 in 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 in the 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 for construction of concrete structures. IS 456 provides design stress strain curves of TMT bars. Usage of design curves of CTD bars, while doing design for TMT grades is not correct. If BIS comes out with design stress strain curve and design value of the yield strength of TMT bars, then only the design turns out to be economical. Using Fe 500 or Fe 550 grades using the design curves of CTD bars doesn’t yieldany economic benefit.
AVOID POSSIBLE MIX-UPS OF DIFFERENT GRADES
Some engineers show a tendency to specify Fe-500 or Fe-550 grade steel where 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 IN THE STACKING AND STORAGE OF HIGHER GRADES OF TMT BARS AT SITES & AT SHOPS/ GODOWNS.
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 steels 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 many inferior products which fail in 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 its production process 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 in IS: 13920, the code of practice for ductile detailing for structures for seismic forces, recommends 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.
AUTHOR
Jismon Issac, Mechanical Engineering graduate with nearly 14 years experience in steel manufacturing and quality control.
Contact no: 09447065360
REFERENCES:
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)
No comments:
Post a Comment