Giuseppe Campione

Strength and strain capacities of concrete compression members reinforced with FRP

The analytical compressive behavior of concrete members reinforced with fiber-reinforced polymer (FRP) was examined. The variation in the shape of the transverse cross-section was analyzed. The bearing capacity and the increase in the maximum strain for members having a cross-section which was circular, square or square with round corners reinforced with FRP were determined. The proposed analytical model allows one to evaluate the confining pressure in ultimate conditions considering the effective confined cross-section and also allows one to determine the ultimate strain corresponding to FRP failure through a simplified energetic approach. Analytical results are then compared to experimental values available in the literature, showing good agreement.

Mechanical properties of steel fibre reinforced lightweight concrete with pumice stone or expanded clay aggregates

This paper presents basic information on the mechanical properties of steel fibre-reinforced light-weight concrete, manufactured using pumice stone or expanded clay aggregates. Results are presented for standard compressive tests and indirect tensile tests (splitting tests on cylinder specimens and flexure tests on prismatic beams using a three-point loading arrangement) under monotonically increasing or cyclically varying loads. The influence of steel fibres and aggregate types on modulus of elasticity, compressive and tensile strength and post-peak behaviour is evaluated. Test results show that compressive strength does not change for pumice stone aggregates, while an increase is observed for expanded clay; tensile strength and fracture toughness are significantly improved for both pumice stone and expanded clay. The results also show that with both expanded clay and pumice stone lightweight aggregates a suitable content of fibres allows one to obtain performances comparable with those expected from normal weight concrete, the important advantage of lower structural weight being maintained.RésuméCet article présente des informations de base sur le comportement mécanique des bétons légers fabriqués avec des granulats d’argile expansé et de ponce renforcés de fibres d’acier. En particulier, on présente les résultats d’essais de compression et de traction indirecte (essais par fendage et essais de flexion sur de petites poutres prismatiques appuyées aux extrémités et chargées dans la section médiane). Les essais ont été réalisés en agissant sous contrôle des déformations et en imposant des histoires de déformations monotoniques et cycliques. L’étude a montré l’influence des divers pourcentages de fibres et du type de granulat léger sur le comportement mécanique du béton, en particulier sur le module d’élasticité en compression et sur la résistance maximum en compression et en traction. Les résultats des essais ont montré que l’ajout des fibres au béton comportant des granulats de ponce ne produit pas de variation de la résistance maximum à la compression. En revanche, l’introduction des fibres dans le béton avec granulats d’argile expansé entraîne une augmentation significative de la résistance maximale à la compression. Pour les deux bétons, on constate que l’ajout des fibres augmente la résistance à la traction et la ténacité en flexion. L’introduction des fibres d’acier dans tous les bétons légers testés donne au matériau des prestations élevées certainement comparables à celles des bétons normaux, mais avec les avantages évidents liés à un poids inférieur.

Behavior in compression of lightweight fiber reinforced concrete confined with transverse steel reinforcement

Abstract The compressive behavior of lightweight fiber reinforced concrete confined with transverse reinforcement consisting of steel stirrups or spirals was analyzed. Pumice stone and expanded clay aggregates were utilized to decrease the weight of the composite; hooked steel fibers were also added. The investigation was carried out by testing cylindrical and prismatic specimens of different sizes in compression using an open-loop displacement control machine, recording the full load–deformation curves. The influence of the dimensions and shape on the bearing capacity and on the ductility of the specimens confined with transverse steel reinforcements was analyzed. The results show the possibility of obtaining high confinement level through the coupled effect of fibers and steel transverse reinforcement.

Steel Fiber-Reinforced Concrete Corbels: Experimental Behavior and Shear Strength Prediction

Corbels are structural members often used in reinforced concrete structures to transfer vertical and horizontal forces to principal members. This paper presents experimental research regarding the flexural behavior of corbels in plain and fibrous concrete and in the presence of steel bars. The study considers the influence of the type of concrete grade (normal- and high-strength concretes), of the fiber percentage and of the arrangement and percentage of the steel bars on the flexural behavior of the corbels. The results in terms of load-deflection curves and crack patterns show the effectiveness in using fibrous reinforced concrete corbels as well as in the presence of stirrups ensuring adequate strength and deformation capacity in reinforced concrete corbels. A simplified analytical model is proposed to calculate the shear strength of fibrous reinforced concrete corbels as well as in the presence of stirrups. The expression proposed for the calculation of the shear strength is then compared with other expression from the literature. The experimental results generated in terms of bearing capacity and those available in the literature are compared with the results obtained through the proposed model and with existing analytical expression. They show a good level of approximation, highlighting the role of the individual contributions from bars, concrete and fibers in the resistant mechanisms.

Simplified Model for Compressive Behavior of Concrete Columns Strengthened by Steel Angles and Strips

This paper presents an analytical model to determine the compressive response of concrete members, having a square transverse cross section and reinforced with steel angles at the corners and steel strips connecting the steel angles. The model evaluates the effects of transverse strips and steel angles on confinement pressures, including strength and buckling phenomena. Compressive response was evaluated in cases of directly and indirectly loaded elements. The data produced correlate well with available experimental data.

Evaluation of infilled frames: an updated in-plane-stiffness macro-model considering the effects of vertical loads

The influence of masonry infills on the in-plane behaviour of RC framed structures is a central topic in the seismic evaluation and retrofitting of existing buildings. Many models in the literature use an equivalent strut member in order to represent the infill but, among the parameters influencing the equivalent strut behaviour, the effect of vertical loads acting on the frames is recognized but not quantified. Nevertheless a vertical load causes a non-negligible variation in the in-plane behaviour of infilled frames by influencing the effective volume of the infill. This results in a change in the stiffness and strength of the system. This paper presents an equivalent diagonal pin-jointed strut model taking into account the stiffening effect of vertical loads on the infill in the initial state. The in-plane stiffness of a range of infilled frames was evaluated using a finite element model of the frame-infill system and the cross-section of the strut equivalent to the infill was obtained for different levels of vertical loading by imposing the equivalence between the frame containing the infill and the frame containing the diagonal strut. In this way a law for identifying the equivalent strut width depending on the geometrical and mechanical characteristics of the infilled frame was generalized to consider the influence of vertical loads for use in the practical applications. The strategy presented, limited to the initial stiffness of infilled frames, is preparatory to the definition of complete non-linear cyclic laws for the equivalent strut.

Behavior of fiber-reinforced concrete columns under axially and eccentrically compressive loads

Fiber-reinforced concrete increasingly is being used for structural members. This study investigates the compressive behavior of reinforced concrete columns with and without steel fibers under axial and eccentric loads. The 16 confined columns had a concrete core 165 x 165 mm (6.49 x 6.49 in.) at the midsection and were hunched at the ends to apply eccentric loading and prevent boundary effects. The specimens were tested to failure at different strain rates under two loading schemes: concentric compression and eccentric compression with a constant eccentricity. The axial load and axial strains were obtained to evaluate the effects of the presence of steel fibers, the thickness of the cover concrete, and the eccentricity of the applied axial load. A comparative analysis of the experimental results showed that the presence of steel fibers delayed the spalling of concrete cover and increased the strain capacity and ductility. The eccentricity of the applied axial load caused substantial variation in the peak load, ultimate strength, and failure modes. The structural response of cross sections of normal concrete and steel fiber-reinforced concrete columns subjected to compressive concentric and eccentric loading was numerically modeled to compare the experimental results. Adequate reproduction of experimental results was obtained using a suitable choice of constitutive laws for concrete and reinforcing steel bars and a reasonable calibration criterion of the model. Directions for future research are discussed.

RC Columns Strengthened with Steel Angles and Battens: Experimental Results and Design Procedure

AbstractIn this paper, an analytical model for the design of axially loaded strengthened RC columns with steel angles and battens is presented. Cases of directly loaded and not directly loaded steel angles are considered. The model considers the contribution in confinement pressures caused by transverse battens and steel angles and the contribution in terms of the load-carrying capacity of steel angles subjected to axial force and the bending moment. An experimental investigation on the compressive behavior of eight short, confined, RC columns externally strengthened with steel angles and steel battens with various pitches was carried out, and has been utilized as support for the analytical model. The analytical results generated are compared with the experimental data available in the literature and with those obtained by using existing analytical models. The comparison shows the acceptable prediction of the experimental results in both cases of directly loaded and not directly loaded steel angles. Final...

Flexural and Shear Resistance of Steel Fiber–Reinforced Lightweight Concrete Beams

In the present paper an analytical model is proposed that is able to determine the shear resistance of lightweight reinforced concrete beams with longitudinal bars, in the presence of reinforcing steel fibers. The model is based on the evaluation of the resistance contribution resulting from beam and arch actions. For the resistance contribution of main bars in tension, the residual bond adherence of steel bars and the crack spacing of reinforced concrete (RC) beams in the presence of fibers are considered. The contribution in terms of postcracking resistance in the tension zone of the beams is also included. The model was verified on the basis of experimental data available in the literature and is able to include the following variables in the resistance provision: diameter and number of steel bars, depth to shear span ratio, resistance of materials and fiber characteristics, crack spacing, tensile stress in main bars, residual bond resistance, postcracking tensile resistance, and size effects. Finally, some of the more recent analytical expressions able to predict the shear resistance of fibrous concrete beams are mentioned and a comparison is made with experimental data.

Flexural Behavior of Steel Fibrous Reinforced Concrete Deep Beams

Experimental research was carried out regarding the flexural behavior of deep beams cast with plain and fibrous concrete with hooked steel fibers, and subjected to monotonic vertical loads. Four deep fiber-reinforced concrete beams were cast. Two of them were made of plain concrete with main and web steel reinforcements (RC), and two were made of hooked steel fiber-reinforced concrete (SFRC) with main steel reinforcements. The experimental results show the brittle behavior of reinforced deep RC members characterized by crushing of concrete struts and fracture of web steel bars. SFRC deep beams exhibit higher strength and, above all, ductility with respect to RC members due to the bridging actions of fibers across main and secondary cracks. From the analytical point of view, a softened strut-and-tie macromodel able to reproduce the flexural behavior (in terms of multilinear load-deflection curves) of deep beams in plain and fibrous concrete and with the presence of horizontal steel bars, including softening of compressed struts and yielding of steel bars, is presented. The model is able to take into account the tensile behavior of main bars embedded in the surrounding concrete and the softening of the compressed strut, the arrangement and percentage of the steel bars, and the percentage and aspect ratio of steel fibers. The experimental results obtained here and those available in the literature are compared with the results obtained through the proposed model and with the other models given in the literature, showing good agreement. Finally, a validation of the proposed model is made numerically by using a nonlinear finite-element program (ATENA-2D) able to analyze the flexural behavior of deep members.