Strengthening of old structure by Carbon Fiber Reinforced Polymer Wrapping (CFRP)

What is CFRP?

CFRP is carbon fiber reinforced polymers which are composite materials consist of two main parts namely carbon fiber (Reinforcement) and matrix to bind it (Epoxy Resin).

Uses of it?

It is very light in weight comparison to its strength so that whenever it is  required to high strength to weight ratio or increment of large amount of stiffness without increasing the existing structure weight it is highly recommended. Followings are the main performances of using it

  1. ·   Increases load carrying capacity.
  2. ·       Increases seismic performance of structure.
  3. ·       Increases strength and durability.
  4. ·       Substitute missing reinforcement.
  5. ·       Increases load bearing capacity
  6. ·       Flexural strengthening
  7. ·       Shear strengthening

Limitation of uses?

But it has also some limitations like different strength characteristic in different direction unlike isotropic materials and also have some fatigue loss of strength with repetitive action.

When it to be installed?

Generally in old structure specially in bridges girder is most important component to carry out the load. With time concrete of that part gets distressed day by day with fatigue mechanism. It may be due to disintegration of micro elemental parts from each other in very microscopic level which may cause due to vibration of structure during application of  load or due to repetitive impacts or due to microscopic cracks inside it.

The above factors generally affects the structural integrity between each and every elements and finally retards it rigidity. 

There are various health test are in market to detect the need

Rebound Hammer Test or SCHMIDT Hammer Test-

Working principle is  the rebound of an elastic mass depends on the hardness of the surface against which mass strikes. From relationship between rebound number and compressive strength (The compressive strength can be read directly from the graph provided on the body of the hammer.) we can determine the approximate compressive strength of structure. If the value is very low than what it was previously design for , CFRP can be used in that case. 

UPV Test (Ultrasonic Pulse Velocity Test )- 

The fundamental rule of the test to asses quality of concrete  in such a way that  for ‘Good’ concrete in terms of  density, homogeneity  speeds are more than that of ‘weak’ concrete containing  defect/voids/porosity and In order to examine and evaluate the conditions of in-situ concrete this test will fundamentally require evaluation of homogeneity of concrete in terms of  compactness, nonattendance of  voids or intermittence utilizing instrument capable of transmitting ultra-sonic pulse of 50-60 Hz and assessing the quality of concrete integrity and homogeneity by interpreting the pulse velocity.

If found velocity through concrete is medium to poor there may be any discontinuity which may ends at lower the strength of that concrete structure and in that case CFRP comes in to action.


How to install ?

It can be applied on beam/girder , column or may be in slab structure but each and every application process are different as each member has its own individual working principle.

From my experience I have experience of using CRFP of Sika brand.

CRFP of Sika made may be of various types but we use Sikawrap-230C

Which was of mid strength range unidirectional fiber wrap. It has mainly two parts-

Sikadur 330– acts as priming coat resin and also as impregnating resin

Sikadur-330 Material Test Report


Sikawrap 230C- Woven unidirectional carbon fiber fabric

Sikawrap 230 c Test Report

Application process may be of two types namely Dry application or Wet application.

We here use dry application method as wet application method is suitable only for non-woven carbon fabric.

Sikadur 300 is generally used for wet impregnation of carbon fabric before apply it on primer surface of Sikadur 330.

Sikadur 300 can also be used as primer when concrete surface is very smooth as it is very liquid and for rougher surface Sikadur 330 is good as it is denser and fill the voids of rougher surface suitably.

 Substrate cleaning- 

Preparation of substrate with dust free clean surface is very much needed. Grinding , sand blasting , brushing , air blowing are the process you can adopt to get cleared surfaces.

If there is any surface defects or undulation can be repaired with compatible epoxy mortar or other compatible materials. 

Application of Priming Resin over clear substrate


Primer application-

Primer mainly containing two parts which are epoxy resin and hardener. Mix it in applicable ratio using blender for a while until it getting uniform/homogeneous colour.

Primer can be applied on surface with help brush and application may be continued until application surface area getting fully wet.

 CFRP application-

Now apply fabric over priming surface within its remaining pot life and apply it by using hand with gloves over hand or press it by roller to get perfect flat surface with no air bubble or entraps inside the FRP.

For wet application fabric before application need to impregnated  with resin by saturation in suitable impregnation resin.

Resin Impregnation of carbon fabric-

Suitable resin mixed with hardener in proper proportion shall be applied over fabric with the help of roller or brush.

Impregnation should be done until  the  woven fabric surface gets fully wet condition and no void left behind the fabric sheet.

In case of multilayer next fabric should be applied as early as possible before end of impregnation resin pot life and if not possible then next layer should be applied after 12 hrs with another application of  primer resin over dry fabric layer.


Overlaying Protection-

This may be of cementitious overlay or it may be overlaying paint coat.

CFRP (Carbon Fiber)


  • What is CFRP?
CFRP is carbon fiber reinforced polymers
  • Uses of CFRP ?
Increases load carrying capacity, Increases seismic performance of structure, Increases strength and durability, Substitute missing reinforcement, Increases load bearing capacity, Flexural strengthening, Shear strengthening.
  • What are the limitation of uses?
Different strength characteristic in different direction and also have some fatigue loss of strength with repetitive action.
  • How come to know when structure need that?
Using NDT test eg Rebound Hammer Test or SCHMIDT Hammer Test and UPV Test (Ultrasonic Pulse Velocity Test ).

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