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European Network for Durable Reinforcement and Rehabilitation Solutions

Contract Number: MC-ITN-2013-607851

Fatigue Behaviour of FRP Reinforced Concrete Structural Elements

Ana Veljkovic

Politecnico di Milano

 

Overview

The occurrence of new fiber reinforced plastic materials of high strength and low weight is related to developing of aerospace industry in the period of 50s and 60s years of last century. Meanwhile, civil engineering branch met new problems with maintenance and durability of their constructions. Steel corrosion caused severe damage in reinforced concrete structures in last decades. Chloride was recognized as important cause of steel corrosion in classically reinforced concrete structures. Especially bridges experienced many problems because of great exposure to environmental effects, especially those ones positioned near sea coasts. Beside natural conditions, all bridges were exposed to additional and very aggressive impact of chlorides from salt for de-icing process during cold periods.

Development of FRP (fibre reinforced polymer) bars technology for use in civil engineering concrete structures took part mainly for its favourable effect on durability, cost and safety of such structures. But, there are not enough research data and design regulations considering FRP reinforced concrete structures, so the usage and application of this methodology is still very limited.

One of the main durability aspects under investigation is the fatigue behaviour. Fatigue properties of reinforced or prestressed concrete structures are important factors to be considered in design. Fatigue behaviour is especially important limit state for concrete bridge decks, parking garages and other structures subjected to cyclic loading. Since bridge deck slabs directly sustain traffic (repeated moving) loads, they are one of the most vulnerable bridge elements regarding fatigue. 

Currently, fatigue of FRP reinforced structures is one of the areas that is still of limited knowledge. Accordingly, additional research in this field is needed, in order to standardize its use, to make it more accurate and safe, but still fast, clear and straightforward. Following this way, it becomes possible to make many successful infrastructure projects such as bridges, with the possibility to build them easily and quickly, but at the same time with improved safety and prolonged service life, which represents the aspiration of modern ways of constructing

 

Aims and Objectives

Fatigue is one of the most important issues in FRP concrete design to be clarified and understood. It becomes even more important having in mind the fact that FRP reinforcement has one of its biggest implementation in bridges which are constantly exposed to fatigue influence. It is important to estimate and anticipate structure behaviour during the period of whole designed life, so to be able to take preventive measures already in design and constructing period and to plan maintenance and servicing actions and costs in future.

Many researches regarding the quasi-static behaviour of FRP reinforced concrete have been done in past two decades. Still few experimental data exist on the long-term fatigue performance of FRP reinforced concrete structural elements. Additional investigations are also needed for determining better process of FRP bars – concrete debonding and to understand completely the failure mechanism of this composite system. So far, recommendations and standards don’t consider enough the design of concrete elements reinforced with FRP bars under fatigue loadings and there is also a lack of appropriate standard test methods for fatigue behaviour of these bars.

My research will be focused in obtaining the in-depth knowledge of the FRP rebar and concrete adhesion behaviour under cyclic loading, by conducting experimental tests and implementation of numerical modelling. I will try to establish more accurate numerical modelling, to make prediction of expected service life, residual stiffness and remaining life of structures internally reinforced with FRP subjected to cyclically repeated loads. As use of FRP material tends to become a part of standard engineering practice, the final product of my research should be the possibility to use it in developing and upgrading existing regulations and testing procedures in order to make them easy understandable, clear, straightforward and suitable for common use.

 

Methodology and Results

My research project should consist of several parts. First it is essential to gain appropriate knowledge about this topic, considering all available literature, papers and standards published so far. Then it is relevant to obtain the in-depth knowledge of the FRP rebar and concrete adhesion behaviour under cyclic loading, by implementation and interpretation of experimental tests and predictions by analytical / numerical modelling. The next step is the application of gained knowledge and experience to understand the behaviour of full size structural elements during long term cyclic loading. As the final product, the obtained knowledge have to be adopted for assessing and improving the current recommendations and rules (e.g. in Eurocodes and fib guidelines for steel RC structures), extending them to account for the unique properties of FRP reinforcement.

As scheduled research duration is three years, here is the plan of activities proposed for this period:

In the Year 1 this research project should consist of theoretical and experimental part.

First period was dedicated to writing State-of-the-Art and obtaining appropriate theoretical knowledge in the area of internally reinforced FRP concrete structures, its mechanical characteristics and fatigue behaviour. It was and will be investigated fatigue itself as a phenomenon and the ways of structure responses.

Year 1 also already included experimental investigation performed in the Laboratory of the Technical University of Kaiserslautern, Germany. This experimental investigation studied static and fatigue behaviour of the bond between GFRP (glass fibre reinforced polymer) rebar and concrete, including several parameters: rebar diameter, rebar external surface, concrete quality and concrete cover.

Upcoming new experimental research will take place in Year 2 and it will also consider the real response of appropriate specimens under fatigue loading and take part in Laboratory in Kaiserslautern, Germany. These results will be used later as benchmark while proceeding with numerical modelling and eventually statistical predictions. The other part of secondment considering the numerical modelling should be done in cooperation with University of Girona, Spain.

Year 3 will comprise of finalizing of activities that started in Year 2, writing, reviewing and submission the PhD thesis.

So far, it is done State-of-the-Art, by studying the available literature, papers, recommendations and standards related to my topic, and it is still updating. Work in progress is obtaining appropriate knowledge by attending and passing the exams and courses held on Politecnico di Milano or other European institution partners of the “Marie Curie ITN Endure”. Till now, two courses are completed and another two are attended but the exams are not yet taken. One course is currently ongoing. Training activities overview is given in section 7 of this document.

During last 2 months it was conducted experimental investigation of influence of many factors on static and fatigue behaviour of the bond between GFRP rebar and concrete. This research is done in collaboration with colleagues from Technical University of Kaiserslautern, Germany. It is analysing static and fatigue behaviour of the bond between GFRP rebars and low concrete cover. More than 100 pull-out set-ups with eccentrically GFRP and steel bars are casted to study the influence of following parameters: thickness of the concrete cover, diameter, surface and material of the bar and concrete mechanical properties. This study consists of two parts. The first part is dedicated to the measurement of the static bond strength and the observation of the failure mechanisms and it is already finished. Second part considers the tension-tension cyclic loading of the bar with several minimum to maximum load ratio values, it requires more time than static part and it is still on-going process. As soon as it is ended-up, fatigue life and the evolution of the damage are going to be analysed.

 

March 2016

As it was stated in the last report, during first year, the first part of experimental investigation was performed and it took place in the Laboratory of the Technical University of Kaiserslautern, Germany. This experimental investigation studied mainly static behaviour of the bond between GFRP (glass fibre reinforced polymer) rebar and concrete, including several parameters: rebar diameter, rebar external surface, concrete quality and concrete cover.

Another part of this experimental research continued in collaboration with colleagues from Technical University of Kaiserslautern, during October and November 2015, and it considered the real response of appropriate specimens under tension-tension cyclic loading. This time experiment comprised of three different classes of concrete (C20/25, C30/37 and C45/55), one GFRP bar diameter (Schöck bar ∅8, as it showed better bonding characteristics during part I of testing), two concrete covers (10 and 20 mm) and centric specimens beside, for comparison. As new concrete specimens were cast for this part of research, static tests were done again for correctly estimating the average static bond strength for each type of specimens. Then, fatigue experiments were performed with two fatigue load levels - 60% and 70% of the average static bond strength. One hundred specimens were cast in total for this phase of experiments, but as fatigue testing requires more time than static one, approx. 70% of specimens were tested so far. Therefore, this is still on-going process and 30% of this should be done in near future (March 2016), in order to complete all required data planned for this investigation.

Briefly, quasi-static results showed similar shear strength for the different concrete covers and the shear strength increased with the concrete mechanical properties. Cyclic tests provided a fatigue life exceeding one million cycles for the minimum load level considered, while for the maximum, the number of cycles to failure extremely increased considering the higher concrete quality.

Collected results are used for writing a Journal paper ‘Experimental investigation of the static and fatigue bond behaviour of GFRP and concrete’, which should comprise of parametric and analytic evaluation of performed tests. These results will be used later as benchmark while proceeding with numerical modelling and eventually statistical predictions

June 2016

On-going process of closing fatigue testing series, planned to be done until the end of the month.

On-going process of writing a Journal paper ‘Experimental investigation of the static and fatigue bond behaviour of GFRP and concrete’.

 

September 2016

The last part of experimental campaign in collaboration with Technical University of Kaiserslautern is done during spring months of 2016. The overall number of tested specimens during this large campaign is more than 200. This last part considered testing the remained specimens under tension-tension cyclic loading. Eccentric specimens were recorded with high speed camera in order to assess the surface deformations during fatigue loading. These recordings should help in understanding the development of damage along the fatigue life of each type of specimen.

All results are systematically analysed and some of these were presented in two conference papers. One conference already took place in August and useful comments are collected in order to analyse data more systematically. Final paper that is supposed to be submitted for journal is planned to be finished till the end of September.

So far, the main conclusions of the campaign are following:

Static tests

  • GFRP rebars give comparable bond strength with steel rebars, for all concrete covers,
  • Ribbed GFRP rebars show better softening behaviour, very high bond fracture energy,
  • Reducing the thickness of the cover from 20 mm to 10 mm, for this type of the bar of diameter 8 mm, affects bonding conditions in different manner: it lowers bond fracture energy, but it does not lower the value of the bond strength,
  • Concrete compressive strength influence on bond properties is much more pronounced than that of the concrete cover,
  • Increasing the mechanical properties of the concrete and the concrete cover has the consequence to modify the failure mode transferring the damage from the concrete to the interface to the external surface of the bar.

Fatigue tests

  • Specimens of all combinations did not fail after one million cycles of cyclic loading at R = 60%. Moreover, they showed an unchanged post-fatigue bond strength compared to the pre-fatigue quasi-static one,
  • For cyclic loading with R = 70%, the higher concrete quality demonstrated a longer fatigue life with the lower concrete cover, that is consistent with the results of the quasi static pull out tests,
  • The fatigue failure mode for concrete cover 10 mm is separation of the bar and concrete for lower concrete class, opposite to failure mode due to static loading; while splitting of the concrete is observed for higher concrete class. For the cover of 20 mm, failure mode was mainly pull out, as observed for static loading. Increasing the mechanical properties of the concrete and the concrete cover, the failure mechanism involves more damage on the bar surface then in concrete.

 

Dissemination Activities

Conferences and meetings

  • endure meeting 03, COST meeting, fib TG9.3 Meeting, workshop and Industry seminar in Kaiserslautern, Germany, 20 – 24 October, 2014 
  • Milestone 01, ABC Department, Politecnico di Milano, Italy, 12 December 2014
  • endure meeting 04 and Training School in Ghent, Belgium, 27 – 29 January 2015
  • Milestone 02, ABC Department, Politecnico di Milano, Italy, 28 April 2015
  • COST Action TU1207 general meeting in Lecce, Italy, 19-21 May 2015
  • endure meeting 05 (Mid-term review meeting), fib task group 5.1. meeting, Industry seminar and Training School in EMPA, Zurich, Switzerland 29 June – 3 July 2015
  • COST Action TU1207 general meeting in Lecce, Italy, 19-21 May 2015
  • ENDURE meeting 05 (Mid-term review meeting), fib task group 5.1. meeting, Industry seminar and Training School in EMPA, Zurich, Switzerland, 29 June – 3 July 2015
  • SMAR 2015 Conference – The Third Conference on Smart Monitoring, Assessment and Rehabilitation of Structures, 7-9 September 2015, Antalya, Turkey
  • COST Action TU1207 general meeting, fib task group 5.1. meeting and ESR meeting in Barcelona, Spain, 8-10 October 2015
  • Milestone 03, ABC Department, Politecnico di Milano, Italy, 19 October 2015
  • Training School in Ghent, Belgium, 25 – 29 January 2016
  • MCAA (Marie Curie Alumni Association) Annual Conference, 4 March 2016 at  Ca' Foscari University, Venice, Italy (Presenting a poster)
  • COST Action TU1207 general meeting, Industry Seminar, fib task group 5.1. meeting, ESR meeting and Poster competition for Early Stage Researchers in Łódź, Poland, 4 – 6 April 2016 (Presenting a poster and participating all meetings and seminars) 
  • Milestone 04 – preliminary (Deep review) meeting, ABC Department, Politecnico di Milano, Italy, 10 May 2016 (Discussing with the Board the document about development of my research project)

Posters

  • Kaiserslautern Endure/Cost Meeting

Papers

  • Submitted the paper ‘Experimental investigation of the static bond of GFRP rebar and concrete’ for the proceeding of the SMAR 2015 Conference – The Third Conference on Smart Monitoring, Assessment and Rehabilitation of Structures, 7-9 September 2015, Antalya, Turkey.
  • Accepted paper ‘Experimental investigation of the static and fatigue bond behaviour of GFRP bars and concrete’ for presenting at The 11th fib International PhD Symposium in Civil Engineering, The University of Tokyo, Tokyo, Japan, 29 – 31 August 2016,
  • Submitted paper ‘Static and fatigue bond behaviour of GFRP bars and concrete’ for the proceeding of The 8th International Conference on Fibre-Reinforced Polymer (FRP) Composites in Civil Engineering (CICE 2016), The Hong Kong Polytechnic University, Hong Kong, China, 14 – 16 December 2016,
  • Milestone 04, ABC Department, Politecnico di Milano, Italy, September 2016 (Presenting the document about development of my research project)
  • endure meeting 07, Padua, Italy, 11 – 13 July 2016 (Presenting the document about development of my research project)