H. Marzouk

Fiber-Reinforced Polymer Strengthening of Two-Way Slabs

This paper experimentally evaluates the strengthening of 2-way slabs using fiber-reinforced polymers (FRPs). Two different types of FRP materials were evaluated: carbon FRP strips and glass FRP laminates. The dominating failure mode for 2-way slab, flexural, or punching shear is based on the slab steel reinforcement ratio. The reinforcement ratios were chosen to serve the purpose of demarcating the 2 possible modes of failure. The tested specimens were classified according to the purpose of strengthening into specimens strengthened in flexure and specimens strengthened in punching shear. Specimens strengthened in flexure had 2 steel reinforcement ratios: 0.35 and 0.5%. Results show that the flexural capacity of 2-way slabs can increase to an average of 35.5% over that of the reference (unstrengthened) specimen. An increase of the initial stiffness was achieved for flexural specimens; however, an apparent decrease in the overall ductility was evident. FRP materials can be used to increase the flexural capacity of 2-way slabs. However, an average decrease in the values of the energy absorption of approximately 30% for flexural strengthening specimens was observed. Specimens strengthened for punching shear have an original slab reinforcement ratio of 1.0%. A strengthening technique that combines the use of carbon FRP strips and steel bolts increases the strength of the slab by 9.0%. An analytical model for the analysis of FRP strengthening of 2-way slabs under flexure or punching shear is introduced.

STRENGTHENING OF TWO-WAY SLABS USING STEEL PLATES

This paper introduces a strengthening technique of two-way slabs using steel plates and steel bolts. The effectiveness of two configurations of steel plates and four different arrangements of steel bolts were evaluated. The strengthening steel plates were extended to twice the slab depth around the column and acted as a drop panel of an equivalent concrete depth. Steel bolts were used as vertical shear reinforcement. Eight bolts were sufficient to transfer the horizontal forces from the steel plates to the concrete and the confining concrete sandwiched between the steel plates. The strengthened slabs showed an increase in stiffness and energy absorption. In addition, the ductility was slightly improved. The load-carrying capacity of the strengthened slabs was increased by 56.55, 57.76, and 64.56% over that of the control specimen with slabs that had eight, 12, and 16 bolts, respectively. The research presents a strengthening concept that can be used to strengthen two-way slabs in multistory structures. A simple approach that was based on the yield line theory showed good agreement with test results.

Tension-stiffening model for FRP-strenghened RC concrete two-way slabs

The main focus of this paper is to present a tension-stiffening model that is suitable for finite element analysis (FEA) aimed at investigating the effect of FRP strengthening on the tensile behaviour of concrete slabs. Available experimental results of the FRP-strengthened reinforced concrete slabs are used to calibrate the finite element model based on the ultimate load carrying capacity of the two-way slabs. The proposed tension-stiffening model is implemented into the constitutive concrete model defined in a general-purpose finite element code. Reinforced concrete behaviour in tension can signifcantly be changed due to strengthening. An overall increase in the post-peak stiffness based on the tensile stress-strain relationship is observed. A simplified bilinear model is introduced to define the behaviour of the FRP-strengthened concrete in tension. An expression of the fracture energy density is introduced to define the area under the concrete tensile stress-strain relationship. The tensile stress-strain relationship of concrete is referred to as the tension-stiffening model. It is shown numerically that the ultimate load capacity of two-way slab specimens is sensitive to the fracture energy density. Hence, a distinction has to be made between the definitions of the tension-stiffening model of FRP-strengthened and un-strengthened concrete. This distinction is the focus of this paper.RésuméLobjectif principal de cet article est de présenter un modèle approprié de raidissement en traction pour lanalyse déléments finis (AEF). Lanalyse est destinée pour létude de leffet du renforcement en polymère renforcé de fibres (PRF) sur le comportement en traction des dalles en béton armé. Les résultats expérimentaux des dalles renforcées sont employés pour calibrer le modèle déléments finis basés sur la capacité ultime des dalles bidirectionnelle. Le modèle de raidissement en traction proposé, est appliqué dans un code général délément finis. Le comportement du béton armé renforcé en traction peut être changé dune manière significative due au renforcement. On observe une augmentation en tension de la rigidité du poteau crête basée sur la relation contrainte-déformation. Un modèle bilinéaire simplifié est présenté pour définir le comportement du béton renforcé de PRF en traction. Une expression de la densité dénergie de rupture est présentée pour définir laire sous la courbe contrainte-déformation. La relation contrainte-déformation du béton en tension est appelée modèle raidissement en traction. On montre numériquement que la capacité de la charge ultime de spécimens bi-directionnels de dalle est affectée par densité dénergie de rupture. Par conséquent, une distinction doit être faite entre les définitions du modèle raidissement en traction du béton renforcé de PRF est béton non renforcé. Cette distinction est lobjet de cet article.

CYCLIC LOADING OF HIGH-STRENGTH LIGHTWEIGHT CONCRETE SLABS

Ductility often governs the design of structures in seismic zones, while monotonic punching shear strength governs the design of flat-plate structures in nonseismic zones. Since the ductility of concrete increases as strength increases, high-strength lightweight concrete (HSLW) located in low to moderate seismic zones can be used to enhance ductility, energy dissipation, and seismic energy. This paper describes the testing of 6 interior slab-column connections under simulated seismic conditions. Loading was investigated to study the effect of using HSLW concrete. Two slabs were made of 70 MPa concrete, 2 slabs were made of 35 MPa normal-strength lightweight (NSLW) concrete, and 2 were made of 35 MPa normal-strength normal aggregates (NSNW) concrete. The steel reinforcement ratios were 0.5 and 1.0% for all 3 types. Under seismic loading, HSLW concrete slabs showed a better performance in general than the other 2 types of concrete. Results show that HSLW concrete had a higher ductility compared with NSLW and NSNW concrete. Additional results are discussed.

Finite element evaluation of the boundary conditions for biaxial testing of high strength concrete

This paper represents a numerical investigation, using a non linear finite element approach, to evaluate the different load application platens used in the biaxial testing of concrete. Three methods are evaluated numerically: the dry ordinary solid steel testing platens, the brush platens, and the friction reducing Teflon sheets. The effect of confinement on the displacement field in addition to the stress distribution in the loading direction are presented and discussed. The shear stresses induced in the specimen are evaluated for both uniaxial and biaxial loading conditions. Finally, the buckling capacity of the brush platens is examined. The finite element results indicate that the brush platens provide the most homogeneous displacement fields. The displacement fields are close to those obtained without lateral confinement. In addition, the shear stresses induced in the specimen are the lowest for the brush platens. The current study was used to design brush bearing platens for biaxial testing of high strength concrete. A brush platen with rod dimensions of 5×5 mm cross-section and 75 mm height can be safely used in testing high strength concrete with compressive strength up to 100 MPa with a reasonable factor of safety against buckling of the brush rods.RésuméCet article présente une analyse numérique, se servant dune approche non-linéaire aux éléments finis, pour évaluer les platines à application de charge variée utilisées pour éprouver de façon biaxiale le béton. Trois méthodes sont évaluées de façon numérique: les platines dépreuve ordinaires sèches en acier solide, les platines à brosses, et les feuilles en Téflon qui réduisent la friction. Le présent article aborde également leffet de limitation sur le champ de déplacement, aussi bien que la distribution des contraintes dans le sens de la charge. Les contraintes de cisaillement provoquées dans le spécimen sont évaluées pour les conditions de charge uniaxiales et biaxiales. Les résultats à éléments finis indiquent que les platines à brosse donnent les champs de déplacement les plus homogènes. Les champs de déplacement sont proches de ceux obtenus sans limitation latérale. En plus les contraintes de cisaillement apportées dans le spécimen sont des plus basses pour les platines à brosse. Létude actuelle a été conçue pour créer des platines à brosse dans les épreuves biaxiales de béton à haute résistance. Une platine à brosse avec des barres dun profil de 5×5 mm et dune longueur de 75 mm peut être utilisée en toute sécurité pour éprouver le béton à haute résistance ayant une résistance en compression pouvant aller jusquà 100 MPa avec une marge de sécurité raisonnable contre le flambage des barres de brosse.