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European Network for Durable Reinforcement and Rehabilitation Solutions

Contract Number: MC-ITN-2013-607851

Bond Behaviour of NSM FRP

Mohamed Reda Emara

Universitat de Girona



Use of FRPs in the strengthening of reinforced concrete (RC) structures has been proven to be an efficient technique due to their unique properties. The near-surface mounted (NSM) FRP technique, in which FRP reinforcement is bonded into grooves cut in the concrete cover, has more recently attracted researchers’ attention due to several advantages over the Externally Bonded (EB) FRP systems. The success of a strengthening system is highly dependent on the interfacial bond properties between reinforcement and concrete which depends upon a number of parameters. Although many studies were carried out on the short-term bond behaviour of NSM and few others on long-term, still there is a lack and uncertainty about the long-term bond behaviour.

An experimental programme is being carried out to investigate the mechanics of bond behaviour between NSM FRP reinforcement and concrete when subjected to various environmental conditioning regimes under the effect of immediate and sustained loading. The effect of adhesive type, bond length, temperature, humidity and sustained load level, which are considered to be critical parameters that may affect the bond performance, is being studied. A new test set-up is designed for carrying out the experimental program. Deformations will be measured by means of electrical strain gauges and transducers, and at the same time suitability of innovative measuring systems will be studied (fibre optic sensors, digital image correlation). The experimental results will be used to validate the existing bond models

The results of the project will allow expanding the database on bond behaviour of NSM FRP strengthening systems, as well as providing guidance for the pull-out testing of NSM FRP systems under sustained loading and combination of temperature and humidity. Furthermore, the results will be used to check validity of existing bond models and propose modifications when needed with the objective of improving the design of NSM FRP strengthened RC elements


Aims and Objectives

The main objectives of this research project is to investigate the mechanics of bond between NSM FRP and concrete and analysing the effect of the most critical parameters in order to validate bond models and to improve existing design guidelines. These global objectives can be developed in the following specific objectives:

  • Characterization of instantaneous and time-dependent mechanical properties of FRP reinforcement, adhesives and concrete.
  • Design and execution of an experimental programme on long-term pull-out bond tests of NSM FRPs in concrete to analyse the influence of different parameters that may affect the bond behaviour.
  • Comparison of results of bond behaviour from long-term and short-term tests and identification of the most critical factors.
  • Checking the validity of different available bond models by using the obtained experimental results.
  • Development of bonds-slip models for time-dependent bond behaviour of NSM FRP reinforcement bonded to concrete under different environmental conditions.


Methodology and Results

An experimental program including 72 test specimens (200 mm x 200 mm x 300 mm) forming six test series is being carried out to study the long-term bond behaviour of NSM FRP concrete through single shear pull-out test (Fig.1). Different combinations of type of groove filling material (adhesive), bonded length, temperature and humidity will be tested under different levels of sustained loading as shown in Table 1.

The main studied parameters are:

  • Service stress level (25 % and 50 % of the maximum load).
  • Temperature (20 and 40 °C).
  • Humidity (65%, 75% and 90 %).
  • Adhesive type ( two adhesives)
  • Bond length (short, medium and long)

Fig.1 - Specimens preparation


Table 1 - Test series


Two displacement transducers (LVDTs) are being used to measure the loaded end and the free end slips. The strain distribution along the FRP bonded length is being monitored by means of Electrical Resistance strain gauges. The suitability of innovative measuring systems as fibre optic sensors or digital image correlation will be also studied.

The specimens will be subjected to sustained load, by a new manufactured test set-up, in the humidity chamber till slip stabilization, and then will be loaded till failure.

Instantaneous and time-dependent mechanical properties of FRP reinforcement, adhesives and concrete will be characterized through execution of short and long-term tests under different conditions of temperature and humidity; and sustained load levels. Evaluation of glass transition temperature of the resin and determination of concrete immediate and time-dependent properties like creep and shrinkage will be also carried out.

Based on the experimental results, existing bond models will be validated and a new bond test will be designed, the effect of each studied parameter on the bond performance will be analysed and finally conclusions will be driven aiming to help improving existing design guidelines.

At present the main part of an extensive literature review on experimental tests and numerical modelling of NSM strengthening systems has been done. This has allowed designing the test set-up and the test matrix (specimens and variables) including the design of the devices for the applications of sustained loading after trying a number of possible solutions. Furthermore, selection of materials (FRP and resins), selection and acquisition of a specific climate chamber for these tests as well as contacts with suppliers have been successfully carried out. After finishing the checking of all the aspects related with the specific designed test set-up it is expected to start shortly with the first series of tests.

Test for the characterization of the used material is being carried out and the first series of tests was cast and been prepared for testing (Fig.2 and Fig.3).

Fig.2 - Adhesive (A), CFRP laminates (B) and concrete (C) specimens for material characterization


Fig.3 - Instrumentation (A) and specimens (B) for short and long term tests


June 2016

The first two series of the experimental programme started and the specimens will remain loaded till the end of June, 2016. Each series contains 12 concrete specimens(200mm * 200 mm * 250 mm) to study the effect of the bonded length(60, 90 and 120 mm) and stress level (25% and 50% of the pull-out failure load) on the bond behaviour between FRP NSM and concrete. The first series is subjected to a sustained load within the normal lab conditions while the second one is subjected to a sustained load inside a climate chamber with 20°C and 95 % of relative humidity (Fig.4). Loaded and free end slips for each specimens are being registered. Initial results were obtained and being analyzed.

On the other hand, short term pull-out test was carried out to obtain the ultimate strength of the strengthened system in order to apply a percentage for long term test.

Fig.3 - Long term test (a) Climate chamber (b) Lab conditions


Dissemination Activities

Some activities have been made during endure official meetings and other at the host research group AMADE at the University of Girona:

  • Participation in the poster competition celebrated in the endure official meeting in Kaiserslautern, Germany, during 22-24 October 2014. Title and authors: “ Bond Between NSM FRP and Concrete”, M. Emara, L. Torres, C. Barris
  • A presentation during endure official meeting in Ghent, Belgium, 26 January 2015. Title and authors: “ Bond Between NSM FRP and Concrete”, M. Emara and L. Torres
  • Participation and attendance at the endure official meeting in Ghent, Belgium, 26 January 2015.
  • A presentation during the research group AMADE special meeting with visiting students from U. Lyon 1 (France), Vilnius Gediminas Technical University (Lithuania), Chulalongkorn University in Bangkok (Thailand), Girona, Spain, 18 February 2015. Title and authors: “A study of the bond and flexural behaviour of reinforced concrete elements strengthened with near surface mounted (NSM) FRP reinforcement”, M. Emara and L. Torres
  • A presentation during the research group AMADE weekly meeting, Girona, Spain, 26 February 2015. Title and authors: “Bond behaviour of reinforced concrete elements strengthened with near surface mounted (NSM) FRP reinforcement”, M. Emara and L. Torres
  • FRP Lab Award for best predictions/report during the laboratory competition, EMPA, Dübendorf, Switzerland, 2 July 2015.