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Inácio, M, Isufi B, Lapi M, Ramos AP.  2020.  Rational Use of High-Strength Concrete in Flat Slab-Column Connections under Seismic Loading. ACI Structural Journal. 117(6) Abstractmanuscript_aci_accepted.pdfWebsite

High Strength Concrete (HSC) slab–column connections with relatively low concrete strengths compared to today’s capabilities have been tested under seismic-type loading in the past. Herein, the hybrid use of HSC with compressive strength around 120 MPa and Normal Strength Concrete (NSC) is investigated through three reversed horizontal cyclic loading tests with different geometries of the HSC region and a reference NSC specimen. The results show that HSC applied in the vicinity of the column can significantly enhance the seismic performance of slab–column connections. The best result in terms of drift capacity and economic use of HSC was achieved in the case of full-depth HSC extended from the column’s face up to 2.5 times the effective depth. Drift ratios up to 3.0% were achieved. A comparison with previous tests showed that the hybrid use of HSC and NSC can achieve similar results to the provision of punching shear reinforcement.

Peixoto, J.  2020.  Tensile behaviour characterization of a high performance fiber reinforced concrete. FEUP - Faculdade de Engenharia da Universidade do Porto. (Sandra Nunes, Amin Abrishambaf, Eds.)., Porto, Portugal: FEUP
Blazy, J, Nunes S, Sousa C, Pimentel M.  2021.  Development of an HPFRC for Use in Flat Slabs. Fibre Reinforced Concrete: Improvements and Innovations. BEFIB 2020. RILEM Bookseries, vol 30.. :209-220.: Springer
Rossi, M.  2021.  Flat Slabs with Different Longitudinal Reinforcement Ratios Under Horizontal Cyclic Loading. NOVA School of Science and Technology. (António Pinho Ramos, Brisid Isufi, Eds.)., Caparica: NOVA School of Science and Technology Abstract

The following dissertation studies the behavior of flat slabs when subjected to constant vertical loads and cyclic horizontal displacements, as a continuation of previous studies developed at FCT/UNL. The main focus of this research is to study the influence of flexural reinforcement on the seismic response of flat slabs. Therefore, three reinforced concrete flat slabs with varying flexural reinforcement ratio were tested, two having the same top reinforcement ratio of !=0,64% and one with !=1,34%. One of the specimens with lower longitudinal ratio was reinforced with studs as specific punching shear reinforcement. All slabs had overall dimensions of 4,15 × 1,85 × 0,15 m3 and a gravity shear ratio, ratio between the gravity load and the punching shear resistance, approximately equal to 55%. For a more complete analysis the results obtained were compared to two other specimens from previous experimental campaigns also conducted at FCT/UNL. These two slabs were designed with top flexural reinforcement ratio (!=0,96%) that lies between the two tested in this dissertation, one with no shear-reinforcement and the other with headed studs. Results showed that the reduction of flexural reinforcement resulted in a more ductile behavior of the specimens and in a higher drift capacity. The high flexural ratio added to one specimen improved the maximum unbalanced moment capacity but also made the slab fail in a more brittle mode. As expected, the specimen with shear headed studs supported the highest drifts and ended up not failing during this experimental campaign, reaching the test setup upper limit.

Isufi, B, Rossi M, Ramos AP.  2021.  Influence of flexural reinforcement on the seismic performance of flat slab – column connections. Engineering Structures. 242(September 2021):112583. AbstractWebsite

The behavior of flat slab – column connections under seismic-type loading is complex and not exhaustively studied. Among the many variables involved, this paper focuses on the influence of flexural reinforcement on the seismic performance of such connections. Three specimens were tested and analyzed in conjunction with two previously published specimens tested under similar conditions, under constant vertical loading and cyclic horizontal displacements, resulting in a total of five specimens. Among these specimens, the top flexural reinforcement varied from 0.64% to 1.34% and the approximate value of applied gravity shear ratio (GSR, equal to the ratio between the applied gravity load and the punching shear resistance) was around 55%. Two of the specimens (low and median reinforcement ratio) were also reinforced with headed studs against punching shear to study the unbalanced moment transfer capacity of the slab – column connections. The specimens are described and analyzed in detail. The results show that the performance under cyclic loading is affected by the amount of flexural reinforcement, even though GSR was almost the same for all specimens. It is shown that current code-based approaches for the estimation of unbalanced moment capacity, as well as drift capacity, are generally safe sided for the specimens under investigation but do not fully capture the trends observed in the experimental campaign.

Isufi, B, Ramos AP.  2021.  A review of tests on slab-column connections with advanced concrete materials. Structures. 32(August 2021):849-860. AbstractWebsite

Advances in concrete technology during the last decades have resulted in the development of materials with enhanced mechanical properties, such as High Strength Concrete (HSC), Fibre Reinforced Concrete (FRC) and Ultra-High Performance Fibre Reinforced Concrete (UHPFRC). The application of these materials in flat slabs, which are a popular structural solution in Reinforced Concrete (RC) buildings worldwide, has the potential of significantly reducing raw material consumption by enabling the design of slenderer and therefore lighter structures. However, flat slabs are susceptible to punching shear failure, which is a complex phenomenon that remains challenging, even though significant efforts have been made to experimentally study it. For advanced concrete materials (HSC, FRC and UHPFRC), the challenge is further accentuated by the continuous and rapid development of these materials. With the purpose of identifying and highlighting gaps in the published literature, a review of tests with HSC, FRC and UHPFRC slab-column connections in non-seismic and seismic loading applications is presented in this paper. It is shown that future research directions in this field include, among others, testing thicker slabs, HSC slabs with higher concrete compressive strength, HSC combined with FRC and several more cases related to seismic loading conditions.

Rossi, M, Isufi B, Ramos AP.  2021.  Seismic behavior of slab-column connections with varying flexural reinforcement ratio, 14-16 June 2021. fib Symposium 2021. , Lisbon
Isufi, B, Marreiros R, Ramos AP, Lúcio V.  2021.  Seismic behaviour of slab – column connections with various punching shear enhancement methods, 14-16 June 2021. fib Symposium 2021. , Lisbon
Isufi, B, Marreiros R, Ramos AP, Lúcio V.  2021.  Comportamento sísmico da ligação laje-pilar considerando diferentes soluções de reforço, 3-5 November. Reabilitar & Betão Estrutural 2020. , Lisbonfullpaperbe2020_pt_final.pdf
Rossi, M, Isufi B, Ramos AP.  2021.  Comportamento sísmico de ligações laje-pilar com variação da taxa de armadura de flexão, 3-5 November. Reabilitar & Betão Estrutural 2020. , Lisbon