My research topic is focused on shear strengthening of reinforced concrete beams using fibre reinforced composites.
Traditional retrofitting techniques come with some drawbacks such as working with heavy steel parts, corrosion related risks, increase and change in weight and section of the elements and possible modifications in stiffness and global behaviour of the structure.
In order to solve these issues, a technique using fibre reinforced polymers (FRP), comprised of a polymer matrix reinforced with fibres in the form of sheets, meshes, or any other fibre form, has been developed during the past years.
However, shear strengthening of RC members with FRP is actually a research argument far to be completely solved and it is still under investigation. The actual most diffused codes/recommendations quantify the nominal shear strength by means of a simple sum of the contributions of concrete, steel, and FRP. This kind of approach has been discussed by various authors observing that the presence of the FRP could influence the effective stress in the internal steel, sometimes leading to non-conservative results. Furthermore, common analytical models adopted in some of the most spread recommendations are generally referred to a fixed slope of the shear cracks equal to 45°, although many experimental works have shown that such inclination may vary. Last, but not least, equations for the evaluation of the effective bond length, show contradictory results. The effective bond length, which plays a key role on the load-carrying capacity of the system, can be defined as the length beyond which any increase in bonded length does not provide an increase in the bond strength.
The first part of my project will, consequently, be focused on the evaluation and assessment of existing current guidelines for FRP shear strengthening, aiming to develop or update formulations for the design of concrete structures strengthened in this fashion.
Additionally, the project will concentrate as well in the study of a new strengthening technique known as fibre reinforced cementations matrix (FRCM), in which inorganic mortars replace resins. This technique allows overcoming some limitations linked to the use of the resins in FRP. The literature in the topic is still scarce and at the moment there is not a European guideline devoted to its design. Therefore, my research project will include the study of reinforced concrete beams strengthened in shear with FRCM, planning to contribute to a better understanding of its behaviour and its comparison with FRP strengthened elements.
This work includes the following objectives:
The methodology proposed for the research project is divided in an analytical approach and laboratory related activities. The analytical approach comprises:
The laboratory campaign is divided in two main parts.
FRCM bond behaviour
It has been observed that shear failure of strengthened beams with FRCM is related to debonding on to the fabric-matrix interface or delamination between the composite and the concrete substrate but research regarding this topic is still scarce. Therefore, in order to obtain insight in this phenomenon prior to perform tests on strengthened concrete beams, the first part of the laboratory campaign includes the investigation of the bond behaviour of FRCM strips when applied on concrete substrates. This includes the testing of FRCM-concrete joints of different length and width made of carbon, basalt and glass fibres and cement matrix applied on concrete substrates, in order to assess the existence of an effective bond length and compare its behaviour to FRP elements. Being aware of the need of research on the subject, the campaign includes FRCM-masonry joints too. In Figure 2, photos of FRCM-concrete and masonry joints as well as the test set-up are presented.
Fig.1 - FRCM-concrete and masonry joints. Test set-up
Shear behaviour of FRCM and FRP strengthened RC beams
This part of the laboratory campaign will comprehend the test of RC beams, strengthened in shear with FRP and FRCM. The variables to study will be: Influence of internal reinforcement steel and spacing, type of fibre/mortar, wrapping configuration and comparison between FRP and FRCM performance.
Regarding instrumentation, linear Variable Differential Transducers (LVDTs) will be use to monitor beams deflections. Electrical strain gauges will be implemented to monitor strains in the web reinforcement, concrete and fabrics.
Fig.2 - Proposed geometry for shear strengthened beams
The results obtained so far are related with a better insight of the bond behaviour of FRCM strips when applied on concrete and masonry substrates. The laboratory campaign carried out has allowed getting a better understanding of the performance of FRCM-concrete and masonry joints and highlights the main differences with the better-investigated behaviour of the FRP technique. The results obtained had been reported in three articles that were be presented in international conferences this year.
At the moment, a second experimental campaign regarding the bond behaviour of FRCM strips applied on concrete substrates is being carried out in the Laboratory. This work is being developed with the cooperation of the ESR from the Lulea University of Technology as part of his secondment activities in the University of Padua. The objectives of this work are related with the better understanding of the influence of the type of fibre on the bond behaviour of FRCM composites. The suitability of the use of DIC systems to measure slip/strain in single shear lap tests is also being studied.
In addition, all of the planned RC beams that will be tested to evaluate the shear behaviour of elements strengthened with FRP and FRCM composites have been casted. The testing campaign is planned to begin in the month of March 2016.
Conferences and meetings