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European Network for Durable Reinforcement and Rehabilitation Solutions

Contract Number: MC-ITN-2013-607851

Use of NSM FRP for the strengthening of thin wall tubular RC structures

Chandan Gowda

The University of Minho



The main idea of the present research activity is to strengthen thin walled tubular reinforced concrete structures subjected to torsion, using fibre reinforced polymers (FRP’s) with near surface mounted (NSM) technique.

NSM technique is chosen over externally bonded reinforcement (EBR) since the FRP material is more effectively utilised in the former and is also practically easier to apply on the structure.

This type of strengthening is mainly useful for box structures like bridges which are subjected to wind loads and solid sections in circular staircase, spandrel beams etc. all producing torsional effect. As a result new innovative strengthening types are proposed for the reason. They not only increase the torsional capacity of the beams, but also the flexural and shear capacity.

In order to validate the proposed strengthening types, primarily numerical analysis is performed using finite element method, which includes the study of different parameters; variation of longitudinal reinforcement, transverse reinforcement and the concrete strength class. Based on the obtained results, the experimental campaign is under preparation and will be tested as shown in Fig.1.

Fig.1 - Typical testing configuration


The research consists of four types of strengthening with different combination of longitudinal and transversal fibre reinforced polymers. All proposed techniques increase the elasto-plastic stiffness and the torsional capacity of the beams, thereby increasing the load carrying capacity by arresting the crack propagation.

The preliminary results of the finite element analysis of the whole experimental setup is shown in Fig.2.

Fig.2 - FEM results of the experimental setup


Aims and Objectives

The objective of the research is to develop innovative strengthening techniques to strengthen the thin walled tubular reinforced concrete structures subjected to torsion. The influence of other parameters is also studied, namely:

  • Ratio of existing internal longitudinal steel reinforcement
  • Interaction between the longitudinal steel bars and FRP laminates
  • Spacing between external NSM laminates
  • Effect of longitudinal and transverse reinforcement
  • Thickness of adhesive layer and its properties
  • Concrete strength class
  • Different combination of strengthening techniques


Methodology and Results

The first part of the research involves numerical analysis of beams with the finite element method and the second part deals with the experimental investigation. The beam has a geometry of 400 mm x 400 mm with a length of 1900 mm, the reinforcement details are as shown in Fig.3 (a) & (c) with 8 mm transverse bars and 10 mm longitudinal bars. The transverse reinforcements are placed at a distance of 200 mm center to center in the central study area of 1000 mm and at a distance of 100 mm at the ends.

Advanced 3D smeared crack model is used for the numerical simulations in GiD using FEMIX computer code as the processor. The beam model consists of 50 mm mesh size for the concrete (hexahedron), reinforcements (linear: 3D embedded cable) and steel loading section (quadrilateral: mendlin shell elements). The analysis is performed with displacement control using arc-length method. A tri-linear tensile softening diagram is used for concrete to simulate crack opening propagation.

Fig.3 - Cross section (a), FE mesh with details (b) and longitudinal section of the beam (c)


Four types of strengthening using FRP material is proposed with a combination of transverse and longitudinal arrangement of the fibre reinforced polymer. The strengthening typologies are as described below

  • Type 1: 4 transverse FRP laminates (2 vertical and 2 horizontal) of 400 mm length, between 2 steel stirrups and 2 longitudinal FRP laminates on each side of the beam running throughout the beam
  • Type 2: 8 transverse FRP laminates (2 vertical and 2 horizontal) of 400 mm length, between 2 steel stirrups and 2 longitudinal FRP laminates on each side of the beam
  • Type 3: 4 transverse FRP laminates (2 vertical and 2 horizontal) of 400 mm length, between 4 steel stirrups and 2 longitudinal FRP laminates on each side of the beam
  • Type 4: 8 transverse FRP laminates (2 vertical and 2 horizontal) of 400 mm length, between 4 steel stirrups and 2 longitudinal FRP laminates on each side of the beam

The results of the torque-angle of rotation curves for the strengthened beam with the unstrengthened (reference) beam is shown in Fig.4. As seen the stiffness of the elasto-plastic section of the strengthened beam increases with the increase in the percentage of FRP laminates. The torsional capacity of the beam is increased upto 20.56% in type 4 strengthening. The results of all beams with maximum torque and maximum angle of rotation are presented in Table 1.

The proposed strengthening methods are effective in increasing the torsional capacity of the beams as well as arresting the crack propagation.

Fig.4 - Torque – Angle of rotation


In order to analyse the effects of testing on the experimental frame in the laboratory, a linear analysis of the whole test setup is performed as shown in Fig.1.

Table 1 - Results of the numerical analysis


March 2016

As, the results of the torsional strengthened beams do not show more than 20% increase in the torsional moment carrying capacity of the beams, pre-stress is applied to NSM FRP laminates (both longitudinal and transverse direction) for type 1 strengthening technique. The result of the torsional moment and the torsional angle of rotation for the beam with and with out prestress is presented in Fig.5(a). The torsional cracking moment of the beam is increased by 45%, providing more FRP contribution in serviceability limit state. The evolution of crack width with respect to torsional moment angle is also shown in Fig.5(b), showing the effectiveness of crack arrest mechanism by the applied prestress.

Fig.5 - Torsional moment and torsional angle of rotation for type 1 strengthening technique with and without prestress


Dissemination Activities

Conferences and meetings 

  • 4th endure meeting Ghent, Belgium 26th Jan 2015
  • COST TU1207 meeting Lecce, Italy 19-21 May 2015
  • 5th endure meeting Dubendorf, Switzerland 29-03 July 2015
  • COST TU1207 meeting Barcelona, Spain 15-16 Oct 2015
  • Conference paper submitted to 11th fib PhD symposium, titled “Exploring NSM technique for torsional strengthening of tubular type RC structures”.
  • Participation in FRP course: Reinforcing and strengthening of structures with FRP reinforcement, University of Ghent, Belgium.
  • COST TU1207 meeting Lodz, Poland 4-6 April 2016

Membership in international organizations

  • Fib TG5.1
  • COST TU1207
  • American Concrete Institute