Maurizio Papia

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.

PUMICE CONCRETE FOR STRUCTURAL WALL PANELS

Some properties of lightweight pumice stone concrete (LWPSC) are discussed, on account of a possible structural use of this material. Then the results of an experimental investigation are described, in order to show that pumice can really be considered an alternative to common artificial lightweight aggregates, taking into account the performance pointed out by loading tests carried out on structural systems made of LWPSC. Three different kinds of reinforced wall panels were made using LWPSC, lightweight expanded clay concrete and normal weight concrete; then their structural responses under horizontal cyclic and constant vertical forces were compared, above all with reference to lateral stiffness, cracking pattern, ultimate strength and associated plastic deformations. This comparison shows the effectiveness of pumice as an aggregate in manufacturing concrete, at least for this type of structural element.

A new dynamic identification technique: application to the evaluation of the equivalent strut for infilled frames

Abstract A new time domain identification technique for systems under Gaussian white noise input is presented, requiring for its application the measurement of the system response but no information about input intensity. The technique proposed is based on the statistic moment equations derived by using a special class of mathematical models named “potential models”. These models allow one to determine fundamental properties of the response statistics, making it possible to identify stiffness and dissipation features of a structural system, and also to determine the excitation input. The technique proposed is here applied to the identification of the strut equivalent to the infill of a single story-single bay frame subjected to lateral loads, showing a reduced effort compared to any procedure based on static experimental tests and a higher reliability of the results compared to identification procedures for the strut based only on mathematical assumptions.

Masonry infills and RC frames interaction: literature overview and state of the art of macromodeling approach

The issue of the influence of masonry infills within RC frames structures has been widely investigated in the last decades by several researchers. The large interest addressed to this topic depends on the actual observation that when in presence of seismic events, the response of framed structures is strongly conditioned by the interaction with the infill walls, which however are considered as non-structural elements and not included in the models. The influence of masonry infills role in structural response is so much relevant to affect not only the overall strength and the stiffness but it may also radically change the possible collapse mechanisms of the overall structural complex under the effect of strong ground motions. Infill panels may have a beneficial effect on the structural response, being able in some cases to supply the lack of resistance of structures to lateral actions, or an adverse contribution inducing unexpected and dangerous non-ductile collapse mechanisms. However, the studies carried out on this topic have demonstrated that, independently from the beneficial or adverse contribution of masonry infills on structural response, their presence cannot be neglected in structural modelling both in design and verification phases. The paper provides a large literature review regarding the modelling techniques developed in the last decades, going from refined nonlinear FE micromodel approaches to simplified equivalent single or multiple strut macromodels including also different technical code statements. The reliability of these approaches is discussed highlighting advantages and weakness points. Macromodelling approach is particularly pointed out since it constitutes the most attractive technique to perform complex nonlinear analyses (static and dynamic). A state of the art of the main issues regarding equivalent strut identification (stiffness, constitutive law and cyclic behaviour) across scientific literature is provided describing in detail noteworthy aspects of some approaches.

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.

Seismic response of braced frames with and without friction dampers

Abstract The dynamic response of friction damped bracing systems and ordinary cross-bracing systems inserted into surrounding frames without lateral stiffness is analysed in order to compare the typical behaviour of the two different bracing systems. The cyclic force-displacement relationship in the two cases is accurately modelled on the basis of experimental and related numerical models which are available in the literature, and the structural dimensionless parameters defining the shapes of the cycles are stressed. Systems having the same average period of vibration in the elastic phase of the response are assumed to be comparable with one another, which leads to a relationship linking the structural parameters of the two kinds of system. Extending the ductility concept to the friction devices, by a step-by-step integration of the equation of motion for the two systems the dimensionless structural responses are derived with variations in the strength level factor. Computing the average values of the maximum responses to five accelerograms artificially generated to be compatible with the normalized elastic spectrum proposed by Eurocode 8, the behaviour factor versus period curves are also deduced with variations in the available ductility. These curves show the different reliability levels provided by the two kinds of bracing systems considered and they are useful for practical applications. Two meaningful numerical examples, carried out by assuming an input accelerogram corresponding to an actual seismic record (EI Centro earthquake, 1940) confirm these design suggestions.

Stress-Strain Law for Confined Concrete with Hardening or Softening Behavior

This paper provides a new general stress-strain law for concrete confined by steel, fiber reinforced polymer (FRP), or fiber reinforced cementitious matrix (FRCM), obtained by a suitable modification of the well-known Sargin’s curve for steel confined concrete. The proposed law is able to reproduce stress-strain curve of any shape, having both hardening or softening behavior, by using a single closed-form simple algebraic expression with constant coefficients. The coefficients are defined on the basis of the stress and the tangent modulus of the confined concrete in three characteristic points of the curve, thus being related to physical meaningful parameters. It will be shown that if the values of the parameters of the law are deduced from experimental tests, the model is able to accurately reproduce the experimental curve. If they are evaluated on the basis of an analysis-oriented model, the proposed model provides a handy equivalent design model.

Analytical prediction of ultimate moment and curvature of RC rectangular sections in compression

This paper presents closed form expressions linking the ultimate bearing capacity to the ultimate curvature of rectangular RC sections subjected to axial load and bending moment acting in one of the two symmetry planes of the section. With respect to possible simplified formulations the following effects are also considered: confinement of the concrete, hardening of the longitudinal reinforcement, and presence of reinforcing bars distributed orthogonally to the neutral axis. The formulation is proposed in dimensional terms after a preliminary definition of the geometrical and mechanical parameters governing the structural response of the class of sections considered. The analytical expressions derived using the proposed approach also allow one to determine the compression level that makes the ultimate bending moment maximum as well as to evaluate the curvature corresponding to the first yielding of the principal reinforcement in tension. Comparing this value of curvature with the ultimate one, an approximate estimation of the available ductility of curvature of the section can be made. The analytical procedure is validated by comparing the results with those obtained using a typical numerical approach. Some experimental results are also considered.

Boundary element model for bond problems in reinforced concrete members

Abstract A numerical procedure for the analysis of the bond stress distribution along the steel-concrete interface of reinforced concrete members is presented. Considering a reinforcing steel bar embedded in a surrounding cylinder of concrete, the analysis is based on a boundary element formulation in axisymmetric elasticity and on a local bond stress-slip nonlinear relationship which is able to model the contact interface behaviour observed by experimental tests. The results of practical application of the procedure to the case of uniaxial tension are in very good agreement with those presented by other authors using a finite element model and they are in sufficient agreement with those which for the particular scheme examined can be obtained by an analytical approach. The proposed model is advantageous with respect to a finite element model because it does not need special bond elements, allows a reduced number of unknowns and transfers the nonlinearity of the problem only to a number of equations equal to the number of nodal points at the contact interface.

Flexural bearing capacity and related ductility demand for masonry sections under nonlinear constitutive law

This paper aims at examining the maximum bearing capacity of rectangular masonry cross-sections subjected to eccentric compression, by assuming a nonlinear constitutive law characterized by two parameters defining stressstrain curve of the material. The variation of these parameters permits us to represent a wide variety of materials; therefore the ductility required of these materials in order for a section to achieve the maximum bending moment compatible with an average normal stress is also determined. The proposed approach can be utilized to evaluate the safety condition of masonry bearing walls, like those characterizing buildings of historical andfor monumental interest, for which the stress state can become dangerous because of eccentricity in the loads or due to horizontal seismic forces. Some experimental tests confirm the reliability of the proposed procedure.