Data Pubblicazione:

Reinforced concrete t-beams strengthened in shear with steel fiber reinforced polymers – CICE 2012

Reinforced concrete t-beams strengthened in shear with steel fiber reinforced polymers – CICE 2012

Proceedings of CICE 2012 6th International Conference on FRP Composites in Civil Engineering - Rome, Italy, 13-15 June 2012 

This paper presents results of an experimental study on the seismic response of reinforced concrete (RC) T-beams with shear deficiencies strengthened with externally bonded steel fiber reinforced polymer (SFRP) tapes. Four cantilever RC beams were strengthened with externally bonded uni-axial SFRP tapes in a U-shape configuration and were tested under cycling loading conditions. The main variable was the use or not of an anchoring system for the SFRP tapes. The results from the presented study were compared with results reported in the literature from similar T-beams strengthened with carbon fibers with and without anchoring. The examination of the results leads to the conclusion that that steel fibers have a great potential to be used for shear strengthening, especially since the use of mechanical anchoring systems does not seem to negatively affect their performance. As, expected, the lack of mechanical anchoring results in premature delamination of the strengthening system, and thus an undesirable SFRP material performance.

Keywords: Anchorage, Cycle loading conditions, Reinforced concrete, Steel fiber reinforced polymers (SFRP), T-Beam section.

The use of fiber reinforced polymers (FRP) for strengthening of reinforced concrete structures has shown great potential as it provides a relatively easy and inexpensive way to provide additional strength and prolong the life of structures. FRP overlays have been investigated and used for flexural and shear strengthening of reinforced concrete (RC) members. Most of the research work related to FRP strengthening systems focused on flexural strengthening and design and analysis models have been adopted. On the contrary, there is relatively less available experimental data on shear strengthening. This is especially true for the best performing strengthening schemes that include anchoring mechanisms or full wrap of the RC member.
It should be noted that there are several parameters that play important role in the shear strengthening of RC concrete beams. Some of these parameters are the bond strength, alignment of the fibers, resin type, anchoring system, brittleness of the fibers. The existing experimental results are limited in terms of variability and although very useful they only
shed limited light to the problem.
It has been proven that externally bonded glass and carbon FRPs can provide a significant increase in the maximum shear capacity of RC structural members. Strengthening systems could either consist of continuous fabrics covering the whole length of the specimen or uniaxial tapes placed at some intervals. However, it has been identified that the continuous fabrics do not result in better performance and therefore they do not provide an economical solution. In most cases the fiber direction is typically vertical to the longitudinal axis of the structural element. Due to the nature of the FRP tapes, positioning of the fibers at 45 degrees presents difficulties especially in the case of a U-shape configuration.
Most of the studies in the literature deal with the use of carbon fibers and few with the use of glass fibers. Furthermore, there is a lack of experimental data on the use of other type emerging fibers, such as the high strength steel fibers.
High strength steel fibers have been used recently by several researchers. Tapes made with high strength steel fibers (known as SRG or SFRP) have been used either with cementitious grouts (SRG) or with organic resins (SFRP) by several researchers [1-4]. It was shown that the addition of these strengthening systems could be an effective alternative for repairs and retrofit of reinforced concrete structural elements. The SFRP is relatively lightweight in comparison to steel plates and is ductile unlike the carbon, glass or aramid fibers. In all published studies the mode of failure was based on the delamination of the SFRP [1-4], which resembles the most common mode of failure of typical FRP systems based on organic resins [5, 6]. To the best of our knowledge there are no experimental data on the use of SFRP tapes in shear strengthening applications and also in combination with mechanical anchors.
In terms of shear strengthening reinforced concrete beams subjected to cyclic load the number of experimental results is quite limited. More specifically, Anil and Tanarslan [7-10] reported on RC shear deficient T-beams strengthened with CFRP systems with and without mechanical anchoring in several different configurations. Their findings suggest that the shear strengthening effectiveness of the CFRP strips varies with CFRP width, strip orientation, and anchorage usage. The latter was found to be the dominant parameter in order to prevent premature failure (debonding) and to maximize shear strength. Another significant finding was that the use of CFRP tapes significantly improved the cumulative energy dissipation
capacity of the strengthened specimens. However, it was reported that side bonding of the tapes without mechanical anchoring system did not produce significant increase of shear strength.
The present study deals with the use of SFRP shear strengthening systems on reinforced concrete T-beams subjected to seismic loading conditions. More specifically, a series of cantilever reinforced concrete T-beams were reinforced with tapes consisting of steel fiber reinforced polymers (SFRP). The SFRPs were attached using a U-shape configuration and in some cases additional mechanical anchoring devices were used as well.