Thanasis C. Triantafillou

Failure surfaces for cellular materials under multiaxial loads—I.Modelling

Abstract Materials with a cellular structure are increasingly used in engineering. Proper design requires an understanding of the response of the materials to stress; and, in real engineering design, the stress state is often a complex one. In this paper we model the elastic buckling, plastic yield and brittle fracture of cellular solids under multiaxial stresses to develop equations describing their failure surfaces. The models are compared to data in the following, companion, paper.

Failure surfaces for cellular materials under multiaxial loads—II. Comparison of models with experiment

Abstract The behaviour of cellular materials under multiaxial loads was modelled in the previous companion paper. Here we present data for the failure of foams by elastic buckling and plastic yielding and compare them to the results of the models. The models describe the main features of the multiaxial behaviour of foams well.

Concrete Confinement with Textile-Reinforced Mortar Jackets

The application of textile-reinforced mortars (TRMs) as a means of increasing the axial capacity of concrete through confinement is investigated experimentally in this study. TRM may be thought of as an alternative to fiber-reinforced polymers (FRPs), addressing many of the problems associated with application of the latter without compromising performance by a significant degree. Based on the response of confined cylinders and short rectangular columns, it is concluded that textile-mortar jacketing provides a substantial gain in compressive strength and deformability; this gain is higher as the number of confining layers increases and depends on the tensile strength of the mortar. Compared with their resin-impregnated counterparts, mortar-impregnated textiles may result in reduced effectiveness. This reduction was more pronounced in cylindrical specimens but rather insignificant in rectangular ones. Favorable confinement characteristics on rectangular columns were also obtained by using helically applied unbonded strips with end anchorages-an interesting concept that deserves further investigation. Modeling of concrete confined with jackets other than resin-impregnated ones is presented by the authors as a rather straightforward procedure through the proper introduction of experimentally derived jacket effectiveness coefficients. From the results obtained in this study, it is believed that TRM jacketing is an extremely promising solution for the confinement of reinforced concrete.

Textile-Reinforced Mortar versus Fiber-Reinforced Polymer Confinement in Reinforced Concrete Columns

Poorly detailed reinforced concrete (RC) columns have limited deformation capacity under seismic loads due to buckling of the longitudinal bars. This study experimentally investigates the effectiveness of textile-reinforced mortar (TRM) jackets as a means of confining these columns. The effectiveness of TRM is evaluated by comparing TRM jackets with fiber-reinforced polymer (FRP) jackets of equal stiffness and strength. Tests were carried out both on short prisms under concentric compression and on nearly full-scale, nonseismically detailed, RC columns subjected to cyclic uniaxial flexure under constant axial load. The compression tests on 15 RC prisms show that TRM jackets provide a substantial gain in compressive strength and deformation capacity by delaying buckling of the longitudinal bars. This gain increases with the volumetric ratio of the jacket. Compared with their FRP counterparts, TRM jackets used in this study are slightly less effective in terms of increasing strength and deformation capacity by approximately 10%. Tests on nearly full-scale columns under cyclic uniaxial flexure show that TRM jacketing is very effective (and equal to the FRP jacketing) as a means of increasing the cyclic deformation capacity and the energy dissipation of RC columns with poor detailing by delaying bar buckling.

Composites: a new possibility for the shear strengthening of concrete, masonry and wood

Abstract The paper presents a compilation of some of the authors research results related to the use of composites as shear strengthening materials for concrete, masonry and wood members. The main emphasis is on the presentation of analytical models for the contribution of composites to the shear capacity of strengthened elements, within the framework of ultimate limit states. It is demonstrated that in the case of concrete and masonry, the design of FRP-strengthened members can be treated on the basis of the classical truss analogy and by accounting for an effective FRP strain, which depends on the product of the FRP elastic modulus and the area fraction, while the analysis of shear strength in the case of wood members is quite straightforward, following basic principles of mechanics.

Strengthening of historic masonry structures with composite materials

This paper deals with the applications of unidirectional fibre-reinforced polymer tendons for the reversible strengthening of masonry monuments. The tendons, anchored to the masonry only at the ends, are circumferentially applied on the external face of the structure and posttensioned to provide horizontal confinement. The relevant properties of fibre-reinforced polymer materials and prestressing systems are summarised; in addition, the concepts for their application, including anchorage, to masonry structures are developed, and a general design procedure is presented. The effectiveness of the strengthening technique is established both analytically, for structures with simple geometries, and numerically, for a real three-dimensional structure with openings, based on the finite element method. The effects of temperature changes on the tendons and the masonry are shown to be negligible. It is concluded that the effectiveness of the proposed method in the consolidation of historic masonry structures is quite satisfactory, especially when the strengthening elements are made of carbon fibre-reinforced polymer.RésuméCet article présente l’utilisation de câbles unidirectionnels en polymère renforcé de fibres pour le renforcement réversible des monuments en maçonnerie. Les câbles, ancrés dans la maçonnerie uniquement aux extrémités, sont appliqués de manière circonférentielle sur la face externe de la structure et précontraints pour fournir un confinement horizontal. On résume les propriété des matériaux polymères renforcés de fibres et les systèmes de précontrainte. Les concepts pour leur application, y compris l’ancrage, aux constructions en maçonnerie sont discutés, et une procédure générale de conception est présentée. L’efficacité de la technique de renforcement est établie à la fois analytiquement, pour des constructions à gémétrie simple, et numériquement, pour de véritables constructions à trois dimensions ayant des ouvertures, sur la base de la méthode des éléments finis. Il est montré que les effets des variations de température sur les câbles et sur la maçonnerie sont négligeables. On conclut que l’efficacité de la méthode proposée pour la consolidation des constructions historiques en maçonnerie est tout à fait satisfaisante, surtout lorsque les éléments de renforcement sont constitués de polymère renforcé de fibres en carbone.

Textile-Reinforced Mortar versus FRP Jacketing in Seismic Retrofitting of RC Columns with Continuous or Lap-Spliced Deformed Bars

The effectiveness of a new structural material, namely, textile-reinforced mortar (TRM), was investigated experimentally in this study as a means of confining oldtype reinforced concrete (RC) columns with limited capacity due to bar buckling or due to bond failure at lap splice regions. Comparisons with equal stiffness and strength fiber-reinforced polymer (FRP) jackets allow for the evaluation of the effectiveness of TRM versus FRP. Tests were carried out on nearly full scale nonseismically detailed RC columns subjected to cyclic uniaxial flexure under constant axial load. Ten cantilevertype specimens with either continuous or lap-spliced deformed longitudinal reinforcement at the floor level were constructed and tested. Experimental results indicated that TRM jacketing is quite effective as a means of increasing the cyclic deformation capacity of oldtype RC columns with poor detailing, by delaying bar buckling and by preventing splitting bond failures in columns with lap-spliced bars. Compared with thei...

Fiber-Reinforced Polymer Retrofitting of Rectangular Reinforced Concrete Columns with or without Corrosion

20 concrete columns, with a 250 x 500 mm section and materials and detailing emulating older construction, were tested to study, in a systematic way, the effect of important parameters of seismic retrofit with fiber-reinforced polymer (FRP) wraps, as well as effects of reinforcing bar corrosion on the effectiveness of the retrofitting. As far as the number of FRP layers and fiber material is concerned, it is concluded that replacing carbon fibers by glass fibers, while maintaining the same extensional stiffness of the FRP jacket in the circumferential direction, leads to about the same performance. Nonetheless, FRP extensional stiffness seems to be the controlling factor up to a certain limit, as increasing the number of carbon FRP (CFRP) layers from 2 to 5 does not materially improve performance. Prior damage left unrepaired reduces effectiveness of rehabilitation with FRP wrap. Confinement by the FRP is very effective in increasing concrete strain capacity to levels of 5-6% even in the middle of a wide side of the column. Even so, rectangular columns tested in the strong direction (with a 250 mm-wide compression zone) are found to benefit more from FRP wrapping than when tested in their weak direction (w/ a 500 mm-wide compression zone). Although wrapping with FRP is found to greatly improve seismic performance of columns that suffer from both lack of seismic detailing and reinforcement corrosion, such corrosion materially reduces the effectiveness of FRP wraps as a strengthening measure, as the corroded bars become the weak link of the column, instead of the confined compression zone.

Flexural Strengthening of Reinforced Concrete Columns with Near-Surface-Mounted FRP or Stainless Steel

Near-surface mounted (NSM) reinforcement involves cutting grooves into a concrete cover and bonding reinforcing bars inside the grooves using an appropriate filler. A large-scale experimental program was conducted to study the behavior of reinforced concrete (RC) columns under simulated seismic loading, strengthened in flexure with different types and configurations of NSM reinforcing materials. Lateral load-versus-displacement response characteristics (peak force, drift ratios, energy dissipation, and stiffness) were compared in order to examine the role of different parameters. The parameters include carbon or glass fiber-reinforced polymers (FRP) versus stainless steel, configuration and amount of NSM reinforcement, confinement via local jacketing, and type of bonding agent (epoxy resin or mortar). The results show that NSM FRP or stainless steel reinforcement is a viable solution toward enhancing the flexural resistance of RC columns subjected to seismic loads. All types of NSM reinforcing elements reached large axial strains. Local confinement with textile-reinforced mortar jackets was effective in controlling buckling of the NSM reinforcement, thus enabling this reinforcement to reach higher strains at failure. Epoxy-based bonding agencies inside grooves were more effective than cement-based mortar bonding agents.

Use of Anchors in Shear Strengthening of Reinforced Concrete T-Beams with FRP

This paper presents an experimental investigation on the effectiveness of various types of spike anchors in combination with U-shaped fiber-reinforced polymer (FRP) jackets for shear strengthening of reinforced concrete T-beams. The parameters examined include the orientation, the number and spacing of anchors, and the role of carbon versus glass fibers in the anchors. It is concluded that anchors placed inside the slab are many times more effective than those placed horizontally inside the web, and anchors of similar geometrical characteristics (e.g., embedment length) display similar effectiveness despite the difference in fiber type. DOI: 10.1061/(ASCE)CC .1943-5614.0000316.