Shear carrying mechanism in RC structures is a complex phenomenon which combines many internal mechanisms acting at the same time. The development of this phenomenon, apart from the contribution of the web reinforcement, rests on the shear carrying mechanisms offered by compressed concrete, friction of the concrete surfaces along the shear crack, and shear resistance of the longitudinal reinforcement.
Although the same mechanisms occur in RC structures with FRP reinforcement, their development and magnitude are different. In fact, due to the different properties of FRP reinforcements, a more brittle behaviour of the RC member is expected and higher deflections and strains are observed. In addition, the depth of the RC member influences the shear resisting mechanisms indicating a decrease in shear strength with an increase in the cross‑section depth.
The complex nature of the shear in FRP RC structures is the reason why the performance of this force carrying mechanism is still not fully clear and not predictable at the full range of load. Hence, a better understanding of the shear behaviour of the FRP RC structures is needed.
The research program of this project consists of four rectangular FRP RC beams casted without vertical shear reinforcement (Fig.1 Left). Beams differ in height of the cross‑section to study the size effect in the specimens.
In order to track the development of strains on the concrete surface and deflection through the load history of the beams a three dimensional digital image correlation (3D‑DIC) system is used in this study (Fig.1 Right).
The high accuracy and small resolution offered by this contactless measuring technique are expect to yield more detailed outcomes and help to investigate deeper the resisting shear carrying mechanism.
Fig.1 - Beams casting (Left) and DIC configuration (Right)
The project aims to investigate the shear behavior of RC beams with internal FRP reinforcement using 3D DIC system as a measuring technique and focusing on size effect in FRP RC members. The main goals of the project can be drawn as:
Experimental program consists of four FRP RC beams with clear span of 2300mm and rectangular cross section 150mm wide and the different height varied from 250mm to 450mm (see Fig.2). To preserve similar longitudinal reinforcement ratio beams were reinforced with proportional amount of GFRP bars. On the other hand, to keep the same shear span to beam depth ratio beams are going to be tested in three point bending putting the load point in appropriate distance from the support of each beam.
A total of 6 bending tests will be carried out on the specimens; two tests on each shear span of the beams GB53 and GB54 and (because of the long shear span) only one test per beam on the specimens GB54 and GB55. Strains in the GFRP reinforcement and deflections along the span of the beam will be monitored using strain gauges and LVDTs respectively.
Fig.2 - Reinforcement scheme of the beam GB54 and sections of all specimens
In this study, DIC measuring system is also used to investigate displacement and strain in a predefined region of full scale concrete beam reinforced with GFRP bars and tested in four point bending. The DIC setup is designed to minimize the uncertainty in the measured in-plane displacement, which is achieved by using three-dimensional DIC and a fully controlled surface texturization. Results will be used to assess the compatibility of DIC displacement and strain with those obtained with well-established measurements techniques (i.e., strain gauges and potentiometers).
Failure mode, as expected, was a diagonal shear failure which can be described as brittle failure caused by the propagation of a diagonal crack toward the point of load, initiated from the tip of the flexural crack, close to the support (Fig.3 Left)
Preliminary results from the first phase of the test of the beam GB54 (Fig.3 Right) shows the load-deflection relationship compared with predictions offered by Eurocode 2 and ACI 440 guidelines. It can be noticed that the behaviour of the specimen at the beginning follows the calculations from the codes, however, at higher level of load a significant underestimation in deflections can be observed. The reason of that is the present do not include shear inducted deflections after serviceability limit state especially after the appearance of the diagonal cracking.
Fig.3 - Load–deflection response of beam GB54 (Left) and failure mode (Right)
Video 1 - DIC strain measurments during the test of beam GB54
In the next step, the results from DIC measurements will be analysed and compared with those obtained from the strain gauges spaced on reinforcement. Sequentially, residual beams with the different depth will be tested and analysed for better comprehend the shear transfer in FRP RC members.
The first part of the experimental programme was designed primarily to investigate the size effect in shear in FRP RC beams without shear reinforcement. As part of this programme, tests were carried out on 3 geometrically similar specimens as shown in Fig. 4. The longitudinal reinforcement consisted of GFRP bars in tension and additionally BFRP bars in compression to allow the installation of strain gauges at various locations of interest within the shear spans. Concrete strength, longitudinal reinforcement ratio and the shear span to depth ratio were kept constant for all specimens, while the overall depth of the beams varied from 260 mm to 460 mm.
Tests were performed on each of the specimen in two different phases keeping the same shear span length. During the first phase of testing (GB54 and GB58), damage was induced primarily along the shorter of the shear spans (A). This procedure ensured that the opposite shear span (B+C) would remain relatively undamaged. Before the retests (GB55 and GB59), the damaged portion of the beams (shear span A) were removed and post-tensioned using metal straps (PTMS). The tests GB56 and GB57 were performed with the same load configuration using PTMS on the longer shear span of the beam.
Fig.4 - Reinforcement scheme of the FRP RC beams without shear reinforcement
Failure modes in all of the beams, as expected, were identified as a diagonal shear failure which can be described as brittle failure caused by the propagation of a diagonal crack toward the point of load, initiated from the tip of the flexural crack, close to the support (see Fig. 5A-D). The shear behaviour of the beams was monitored on the full load path of the beam up to shear failure performing two initial load cycles corresponding to the service strain and maximum allowed strain level in the longitudinal reinforcement, respectively.
Fig.5 - Load–deflection response of the beam GB54 (a), GB55 (b), GB56 (c), GB58 (d)
A comprehensive database on FRP RC beams without shear reinforcement was collected to investigate the shear strength performance in these structures. The strength of the beams at the failure is greatly affected by their size, which is known as size effect in shear. The recent studies on shear in FRP RC beams shows similar results, indicating the decrease in shear strength with an increase of the overall member depth of the (see Fig. 6A). The same trend was observed from the outcome of the conducted experimental programme showing a significant loss in shear strength up to 130%, while the geometrical and mechanical parameters of the beams were kept the same (Fig. 6B).
Fig.6 - Size effect in the beams without FRP reinforcement - data collected from the literature (A) and size effect in the beams from the present study (B)
In the next step, the results from DIC measurements will be analysed and compared with those obtained from the strain gauges spaced on reinforcement. Sequentially, residual beams with the different depth will be tested and analysed for better comprehend the shear transfer in FRP RC members. Furthermore, a second part of the experimental tests with the same number of geometrically similar beams is planned. In this part, the study will be focused on the behaviour of the FRP RC beams with FRP shear reinforcement, examining in particular the shear strength development in terms of the size effect in these members.
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