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.

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