History of Rebar

Year Event
1848Jean-Louis Lambot
Used iron bars and wire mesh to reinforce several concrete rowboats…
1854William B. Wilkinson
Used iron bars and wire rope to reinforce the concrete floors of a two-story cottage.
1850-1880 François Coignet
A pioneer of reinforced concrete, Coignet was the first to use iron reinforced concrete in construction on a widespread scale. France
1889Gyozo Mihailich
Mihailich is credited with designing the first arch bridge to use reinforced concrete. Hungary
1911ASTM A15 published with grades 33 and 50
till 1967Plain MS bar of grade FE-250 in India
1968ASTM A615 published (replaced ASTM A15, ASTM A408, ASTM A431, ASTM A432, and portions of ASTM A305) with grades 40, 60, and 75
1973Epoxy-coated rebar first used in a U.S. bridge
1979ASTM A767 published for zinc-coated (galvanized) rebars
1981ASTM A775 for epoxy-coated rebars and ASTM D3963 for handling of epoxy coated bars published
1983Stainless steel rebar first used in U.S. bridges.
upto 1990CTD bars of grade Fe-405-415 used in India
1980-85TMT bars introduced in India
1985IS-1786 first published in India
1993IS-13620 for epoxy coated bars published in India
1996ASTM A955/A955M published for stainless steel rebars
2008ASTM A1055 published for zinc and epoxy dual coated rebars
2017IS-16651 for stainless rebar published in India

Rebar fabrication process

The fabrication of reinforcing steels into shapes suitable for fixing into concrete formwork is normally performed in advanced countries by specialist reinforcement fabrication centres. In these advanced countries very little reinforcement is cut and bent on-site. In India, the reinforcement bar fabrication is predominantly done at site by employing huge labour force and semi-automatic machines. Although the cutting and bending of steel reinforcement appears relatively straightforward, the steel reinforcement fabrication service centres are well equipped to do so in a consistently accurate manner. The service centres works under defined set of quality management criteria to ensure the accuracy of cutting and bending operations in order to achieve proper fit at site, and to maintain the required technical parameters defined by the clients.

Rebar Processing
The basic fabrication processes consist of cutting and bending the reinforcement steel. The actual process employed depends on the material being processed, whether bar or coil. At the Rebar processing centre coated or bare reinforcement are fabricated by cutting on shear lines, and thereafter bending on power bending machines. The processing ranges of these machines are from 16 mm diameter to 50 mm diameter. The reinforcement bar, in coil form, is obtained from the mills in Coil form in the size range of 6mm to 12mm and is bent to stirrups and most complicated shapes through the Automatic Link Bending machine to promise the delivery of dimensionally accurate stirrups and complex shapes just in time (JIT delivery).

Benefits to Construction Industry:

Image courtesy : unionrebar

• Accurate planning of the site programme
• An answer to severe skill and labour shortage
• Weather-independent construction
• Reduced construction time, thereby avoiding time and cost overrun
• Efficient use of limited space at congested sites
• Improved safety
• High accuracy on specified tolerances thereby avoiding rework at site
• Minimum wastage
• Dependency avoidance on power availability and power saving at site
• No investment in machinery for fabricating the reinforcement
• On time delivery every time

Benefits to Coated bars:

Image courtesy : ISPC

* No touch up at the site
* No damage to coatings
* Reduced transportation cost for the contractor
* Full range of rebar processing service-improved image
* Value-added service + saving generated due to a reduction of patch up compound

Myths about TMT bar

1. TMT has higher bond
Process of making TMT as such has no role in bond strength between rebar and concrete. Factors responsible for higher or lower bond are Rib geometry and concrete properties.
Rib geometry
-transverse rib height
-transverse rib length
-transverse rib angle of inclination
-transverse rib spacing
Concrete properties
-strength
-vibration/compactness
-use of chemical ingredient
Bond strength of any type of rebar in concrete can be tested with pull out test

2. TMT is corrosion resistant
in fact reverse is the truth. TMT bars are highly prone to corrosion due to its dual microstructure ie. Martensite and Perlite, Which provides galvanic coupling for corrosion. TMT bars at site shows corroded layer on outer surface.

3. TMT bar once in concrete never corrode
This is equally false belief that TMT bars gets corroded in outer atmosphere only and not inside concrete. As concrete is permeable to corrodents like moisture, oxygen, chloride etc Rate of corrosion can be different based on severity of atmosphere

4. TMT never fails in bend/strength
Every material has a defined range of properties which is prescribed in various nation/international standards beyond which it fails.

5. Any water-cooled bar are TMT
Water cooling hot bars are critical part of TMT bar processing but the process is called Quenching and is carried out under certain range of temperature to get desired microstructure which is responsible for higher strength of TMT bar. Water quenching if not done properly creates two process issues
Improper or peripheral quenching- this creates bar with variable mechanical properties in the bar, which is an safety hazard.
Tempering-this process imparts ductility and stress relieving effects to bar which harmonizes properties of rebar

6. TMT bars are earth quake resistant
In case of earthquake combined effects of concrete and TMT bar plays important role, TMT bar alone can’t resist forces of earthquake. Hence judicious selection of properties and integration of both is required

7. Scrap heated and rolled into bars are also TMT
Any scrap without known material history does not provide physical and mechanical properties defined in rebar standard, it may provide such wide variation that structural design requirement becomes first casualty and hence a safety concern.

8. TMT is the only rebar used in RCC

Now a days lot of innovations are happening in rebar field. Variety of Bars depending on site and environmental concerns are deployed. Some of the widely used type are Stainless steel, FRP, basalt, galvanized bars, epoxy coated bars

What is basalt rebar?

Rebars are backbone of construction industry. Rebars are made of steel and are in use ever since its innovated. Need is the mother of innovation. Problems like rebar corrosion led to various new type of rebar like epoxy coated bars, galvanized bars, stainless steel and frp rebars. Places where constructions are exposed to high levels of radio frequency radiations alongwith corrosion FRP and new entrant –rebar made of basalt are highly suitable.

Basalt rebars are made of volcanic rock called basalt- which is present abundantly in earh’s layer below crust

Basalt filaments are made by melting crushed volcanic basalt rock of specific mineral mixture at 1700 C, hot material is drawn through platinum  bushings and cooled to form filaments. This filaments are stronger then steel or FRP. Basalt rebar are made by pultrusion process using 80% basalt filaments and 20% epoxy as binder.

No chemicals are added to basalt during its manufacturing unlike FRP – so its environment friendly

Physical properties of basalt filaments like tensile strength, modulus of elasticity, temperature tolerance and resistance to acid & alkali damage are better than steel & glass fibers..

Basalt rebars have following advantages:

  1.    It does not corrode
  2.    Its stronger than steel and FRP rebars (tensile strength)
  3.    Higher chemical stability
  4.    Thermal expansion coefficient is similar to concrete.
  5.     Lighter than steel
  6.     Non conductor to heat & electricity.

                                                                                                                        Image courtesy: technobasalt

 

Basalt rebar clearly is ready to be used as a substitute for both steel and fiberglass rebar.  It is clear that steel in a concrete construction is a rust-spalling failure waiting to happen. Eventually moisture will get to steel wherever it is and no matter how well it is protected. It will then rust, swell, and cause the concrete to fail. With basalt rebar this issue is avoided forever.

The lack of spalling leads to one more advantage for basalt rebar. Construction codes call for concrete cover- spacing steel reinforcement at least 3 inches from the surface of the concrete. This is not necessary when using basalt rebar. A slab or panel can be made as thick or thin as is needed for structural integrity. If one inch of concrete is sufficient, a panel can be one inch thick with no risk of failure from spalling.

Comparison of physical properties- Steel, FRP and Basalt rebars.

ItemSteelGFRPBasalt rebar
Tensile strength(MPa)350-650500-900700-1300
Yield strength(MPa)280-420--
Compressive strength(MPa)350-650400-550450-550
Tensile elastic modulus(GPa)20035-4555-75
Coefficient of Thermal
expansion(*10-6 /C)-transverse
11.721-2321-22

courtesey: pulwell composites co.Ltd

 

FRP rebar-will it replace steel bars?

FRP rebars have generated lot of hope in construction industry largely as a result of high decibel marketing and aspirations generated due to its earlier applications in aircraft industry.

The use of high performance FRP in primary structural applications, however, has been slower to gain acceptance although there is much development activity.

Their benefits of corrosion resistance and light weight have proven attractive in many low stress applications. Composites present immense opportunities to play increasing role as an alternate material to replace timber, steel, aluminium and concrete in buildings.

What is FRP

Demand for steel rebar in India in 2016-17 was 30.39 million ton, it is projected to grow in 2020-21 to 41.82 million ton. Due to increased competition,  producers have focused on branding and value adding to their products.

FRP materials were developed primarily for aerospace and defense industries in the 1940s and are widely used in many industries today, including aeronautic, marine, automotive and electrical engineering. With the continuing cost reduction in high-performance FRP materials and the growing need for new materials to renovate civil infrastructures, FRP materials are now finding some acceptance among civil engineers

A Fiber Reinforced Polymer (FRP) composite is defined as a polymer (plastic) matrix, either thermo set or thermoplastic, that is reinforced (combined) with a fibre or other

A Fiber Reinforced Polymer (FRP) composite is defined as a polymer (plastic) matrix, either thermo set or thermoplastic, that is reinforced (combined) with a fibre or other reinforcing material with a sufficient aspect ratio (length to thickness) to provide a discernable reinforcing function in one or more directions. FRP composites are different from traditional construction materials such as steel or aluminium. FRP composites are anisotropic (properties apparent in the direction of the applied load) whereas steel or aluminium is isotropic (uniform properties in all directions, independent of applied load). Therefore, FRP composite properties are directional, meaning that the best mechanical properties are in the direction of the fiber placement.

FRP composites are composed of:

Epoxy-transfers load to reinforcing fibres and acts as glue

Reinforcenments-  provides mechanical properties aramid, carbon and glass. In construction mainly glass fibers are used due to cost

Filler-improve performance and reduce the cost of resit & fibres

Additives-expand the usefulness of polymers, enhance process ability

 

Why it looks so attractive

. corrosion proof

.  high tensile strength

.  light weight

.  electrical and magnetic properties

Some time facts are more fictitious than reality.  let us examine the claim of advantages of FRP bars in construction.

.  Corrosion proof

steel gets corroded and its value gets deteriorate, it’s a so deep rooted fact that when we consider alternative we conveniently degradation mechanism of the material say FRP.    FRP too deteriorates but it follows different mechanism.  So let’s consider environmental conditions of concrete constructions and its impact on durability on FRP:

              -moisture  : 

water molecules act as plasticizer disrupting van der waals bond in polymerchains. This will  lead to                                  changes in modulus, strength, strain to failure and toughness. Swelling stresses may cause                                                 permanent polymer  matrix cracking, hydrolysis and  a degree of fibre-matrix debonding Effects                                       will aggravates further with rise in temperature. FRP have 0.5 % water retention.

               -chlorides :

In broad terms, CFRP bars show very little degradation with time, exposure or temperature. AFRP                                   and GFRP elements may show reductions in strength and stiffness of up to 50%, pre-stress                                                 relaxation of up to 30% and moment reduction in reinforced beams of up to 20%.

               -alkali     :

Performance of FRP reinforcement and pre-stressed tendons in alkali varies with the materials                                        (fibres and resins) used and manufacturing processes. Literature suggests that bars or tendons                                          deteriorate much faster in alkaline solution than in concrete, probably due to the relative mobility                                    of  OH+ ions.  Specific observations for generic material types are given below:

 

􀁸 With GFRP extensive degradation can be caused by exposure to high

Temperature alkaline solutions. Bars embedded in concrete at various

temperatures and with good fibre–resin combinations show only limited

degradation, but this increases with temperature and stress level.

 

􀁸 Alkali affects AFRP bars and tendons less than GFRP, but a combination of

frp

Alkali solution and high tensile stress (in the order of 0.75 UTS) may damage AFRP bars significantly.

 

                 -temperatue :

Deterioration by thermal action may occur in composite materials when constituents have                                                  different coefficients of thermal expansion. With FRP-reinforced concrete members, transverse                                        thermal expansion is particularly important for a good bond.

Resins will soften due to excessive heat. The tensile, compressive, and shear properties of the                                             resin diminish when temperatures approach the Glass Transition Temperature, Tg.  Tg values are                                    approximately 250oF (120oC) for vinylestes resins which are typically used with GFRP rebars.  Tg                                     lowers as a  result of moisture absorption.

Thermal expansion in longitudinal and radial direction different generally 2 or more times than                                       concrete

            – UV radiation :

                                   Ulta Violet  rays may cause degradation during storage or if the FRP is used in external                                                   environment. Degradation mechanism is benign to the surrounding concrete unlike steel that                                           expands and causes failure of the member. So its silent killer.

 High tensile strength

strength depends on type and composition of fibre.

 Following Factors Affects  Material Characteristics:

 Type of fiber

 Fiber volume

 Type of resin

 Fiber orientation

 Dimensional effects

 Quality control procedures during manufacturing

 Rate of curing

 Void content

 Service temperature

Tensile property of some FRP is comparable to steel but GFRP- which is mainly used in construction , there is not much gain.

Steel                                                           GFRP

Tensile strength. in mpa                             483 to 1600                                               483 to 690

 

. Light weight

This benefit is of not much functional value except very long bridge(with low stress) other than transportation and installation cost.

Limitations:

 Other than different degradation mechanism FRP has following inherent limitations or disadvantages

  • Low ductility and No yield point-fickly plastic behavior- structure will fail suddenly, heavy safety risk
  • Susceptible to local unevenness
  • High cost
  • Low shear strength
  • Tensile strength of bent bar is significantly lower
  • Brittle – so risky in earthquake prone zone, area with high wind velocity and presence of machinery vibrations
  • Different design criteria so cannot replace steel directly  in use.
  • Anisotropic so bend, shear strength, dowel action will be affected
  • Creep rupture endurance time can decrease in high temperature, UV radiation, high alkalinity, wet & dry cycle, freezing & thaw cycle.
  • Resin will control much of the property as dia of bar increase, strength decreases.
  • Recycling is either not easy or not possible
  • Fatigue- temperature and moisture decrease the fatigue endurance
  • Anchorage- gripping the end of FRP bar is difficult and costly
  • Lower modulus of FRP bars, design for serviceability controls
  • Endurance in fire and elevated temperature is less than steel

As the material is in its infancy as far as rebar application is concerned, only guidelines for use are formed, extensive data on durability is not available, FRP lacks in value proposition compared to epoxy coated bars, not only that from review of limitations we can say that FRP will never replace even steel bars except in some low stress applications.