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Isufi, B, Marchão C, Marreiros R, Ramos AP.  2022.  Experimental Investigation on the Behaviour of Hybrid HPFRC Flat Slabs, June 2022. fib Congress 2022, Oslo. , Oslomonotonicoslo.pdf
Ramos, A, Isufi B, Marreiros R, Marchão C.  2022.  Utilização de BEDRF em Ligações Laje-Pilar sujeitas a Ações Horizontais Cíclicas, 2022. 6ªs Jornadas Portuguesas de Engenharia de Estruturas. , Lisbonartigo_bedrf_ciclico_jpee2022_final.pdf
Ramos, A, Isufi B, Marchão C, Marreiros R.  2022.  Utilização Racional de BEDRF em Ligações Laje-Pilar, 2022. 6ªs Jornadas Portuguesas de Engenharia de Estruturas. , Lisbonartigo_hpfrc_monotonico_jpee2022_final.pdf
Isufi, B, Relvas JP, Marchão C, Ramos A.  2022.  Behavior of flat slabs with partial use of high-performance fiber reinforced concrete under monotonic vertical loading. Engineering Structures. 264(August 2022):114471. AbstractWebsite

Reinforced concrete flat slabs are used worldwide in multi-story buildings. In these slabs, the design is often governed by punching shear and serviceability. The mitigation of these issues during design usually leads to increased raw material consumption and costs. Previous studies have shown that using Fiber Reinforced Concrete (FRC) or High-Strength Concrete (HSC) only at the vicinity of the column, while casting the rest of the slab with Normal Strength Concrete (NSC), can lead to an improved behavior under gravity loads in terms of both serviceability and ultimate capacity. Motivated by these results and the scarcity of previous tests, the present paper experimentally investigates the applicability of High-Performance Fiber Reinforced Concrete (HPFRC) as an alternative material that can be seen as an improvement over FRC and HSC, allowing a combination of ductility and strength. In addition, the HPFRC used in this paper is self-compacting, thus reducing the labor costs associated with concrete vibration. Five 150 mm thick flat slabs were tested under monotonically increasing punching load. The experimental variables were the flexural reinforcement ratio and the extent of the HPFRC zone. One of the specimens was cast only with NSC and served as a reference slab. Results show that the solution was effective for both flexural reinforcement ratios considered. Cracking load, maximum load, as well as the displacement capacity were increased significantly, even for a small extent of HPFRC (1.5 times the effective depth from the face of the column). Regarding the ultimate load capacity, it was observed an increase of 44% to 58% for the specimens with lower reinforcement ratio (0.64%) and between 15%–21% for the specimens with higher reinforcement ratio (0.96%). The results indicate that the use of HPFRC is a promising solution regarding both serviceability and ultimate limit state design of reinforced concrete flat slabs under gravity loading, with obvious advantages in material savings and labor costs.

Ramos, AP, Isufi B, Marreiros R, Marchão C.  2022.  Hybrid use of HPFRC in Slab-Column Connections under Cyclic Laterial Loading. fib Congress 2022, Oslo. , Oslocyclicoslo.pdf
Ramos, A, Isufi B, Marreiros R, Bolesova M, Gajdsova K.  2022.  Rational Use of FPFRC in Slab-Connections Under Reversed Horizontal Cyclic Loading. Engineering Structures. Accepted for publication Abstract

Slab – column connections that are subjected to combined gravity and horizontal loading during an earthquake are prone to premature failure due to punching shear. Traditional solutions to avoid punching failure and to increase the displacement capacity of this type of connection include using stirrups and double-headed studs as shear reinforcement. The use of High-Performance Fiber Reinforced Concrete (HPFRC) in a small region of the slab around the column as a substitute for traditional solutions is investigated in this paper, because this material has the potential to reduce labor and material costs. To fulfill this objective, four slab specimens with a thickness of 150 mm were tested under combined gravity and reversed horizontal drifts. The results are discussed in detail. The experimental variables considered were the top flexural reinforcement ratio, the size of the HPFRC zone and the intensity of the gravity load. Previously published tests that serve as reference specimens are used to compare the results. The behavior of the specimens with HPFRC was substantially improved compared to the reference specimens in terms of drift capacity: from only 1.0% drift to above 5.5%, even though a very small quantity of HPFRC was used, extended up to only 1.5 times the effective depth of the slab from the face of the column. Specimens with HPFRC also behaved better when compared to specimens with High-Strength Concrete (HSC). Side effects of using HPFRC in the slab in the vicinity of the column include an increase of the unbalanced moment transfer capacity and lateral stiffness, as well as a reduction of the deflections of the slab.

Isufi, B, Almeida A, Marreiros R, Ramos A, Lúcio V.  2022.  Slab – column connection punching and ductility improvement methods for seismic response of buildings with flat slabs. Structural Concrete. 23:1385–1398.Website
Relvas, JP.  2022.  Utilização Racional de Betões de Alta Resistência Reforçados com Fibras de Aço em Lajes Fungiformes. NOVA School of Science and Technology. (António Pinho Ramos, Brisid Isufi, Eds.)., Caparica
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
Sine, A, Pimentel M, Nunes S.  2021.  Punching Shear Tests on RC Flat Slabs Strengthened with an UHPFRC Layer, 2021. fib Symposium 2021. , Lisbonsine-fib2021_169_punching.pdf
Pimentel, M, Sine A, Nunes S.  2021.  Resistência ao punçoamento de lajes fungiformes reforçadas com UHPFRC, 2021. Reabilitar & Betão Estrutural 2020. , Lisbonrbe2020_artigo_puncoamento_1.pdf
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
PEREIRA, JOÃOFILIPESOUSA.  2021.  CARATERIZAÇÃO DE UMA MISTURA COMERCIAL DE UM MATERIAL CIMENTÍCIO DE ULTRAELEVADO DESEMPENHO. Faculty of Engineering of the University of Porto. (Sandra Nunes, Ed.)., Portotese_final_final_signed.pdf
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.

Nunes, S, Pimentel M, Sousa C.  2021.  Mechanical and Fracture Behaviour of an HPFRC. Proceedings of RILEM-fib International Symposium on Fibre Reinforced Concrete, BEFIB2021. , Valencia Abstract

The current paper analyses the mechanical and fracture behaviour of a High-Performance Fibre Reinforced Concrete (HPFRC). An HPFRC was developed in a previous stage aiming to simultaneously, maximise aggregates content, achieve a compressive strength of 90–120 MPa and maintaining self-compactability (SF1+VS2). The benefits of fibres hybridisation (using fibres with lengths of 13, 35 and 60 mm) on flexural strength are investigated using the wedge-splitting test, in order to achieve the highest performance while keeping a relatively low fibre content. The final selected mixture was characterised in terms of workability, compressive strength and modulus of elasticity. Six notched prismatic specimens were subjected to three-point bending tests, according to EN 14651, for classification according to the MC2010. Based on the bending tests data, the simplified linear characteristic tensile stress vs. crack opening displacement relationship of the HPFRC was evaluated according to MC2010 and two other analytical approaches available in the literature.

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.

Sine, AG.  2021.  STRENGTHENING OF REINFORCED CONCRETE ELEMENTS WITH UHPFRC. Faculty of Engineering of the University of Porto. (Mário Pimentel, Sandra Nunes, Américo Dimande, Eds.)., Portostrengthening_of_rc_elements_with_uhpfrc_agsine.pdf
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