Michael N. Fardis

DEFORMATIONS OF REINFORCED CONCRETE MEMBERS AT YIELDING AND ULTIMATE

The inelastic deformation capacity of reinforced concrete (RC) members is important for the resistance of RC structures to imposed deformations, and especially so for seismic loads as earthquake-resistant design relies on the ability of RC members to develop (cyclic) deformations well beyond elastic limits without significant loss of load-carrying capacity. This study develops expressions for the ultimate deformation capacity and for the deformation at yielding of RC members in terms of their geometric and mechanical characteristics. Such expressions are essential for the application of displacement-based procedures for earthquake-resistant design of new RC structures and for seismic evaluation of old ones. They are also essential for a realistic estimation of the effective elastic stiffness of cracked RC members and structures, which is important for the calculation of seismic force and deformation demands.

Fundamental Modeling and Experimental Investigation of Concrete Carbonation

The physicochemical processes in concrete carbonation are presented and modeled mathematically. These processes include the diffusion of CO2, in the gas phase of concrete pores, its dissolution in the aqueous film of these pores, the dissolution of solid Ca(OH)2 in the water of the the pores, the diffusion of dissolved Ca(OH)2 in pore water, its ultimate reaction with the dissolved CO2, and the reaction of CO2 with CSH and with the yet unhydrated C3S and C2S. In addition, the parallel processes of production of materials susceptible to carbonation during the hydration and carbonation, are included in the model.

Physical and Chemical Characteristics Affecting the Durability of Concrete

The durability of reinforced concrete is influenced by those physical characteristics of concrete that control the diffusion of gases, such as CO2 and O2 or of liquids (mainly water) through its pores, and the diffusion of ions, such as Cl-, dissolved in the pore water. These physical characteristics depend on the composition of concrete, the chemical composition and type of cement, and the relative humidity and temperature of the environment. In the present paper, these characteristics of concrete are determined analytically and/or experimentally in terms of the composition parameters and environmental condtions; the molar concentration of those constituents that are susceptible to carbonation; the porosity and pre-size distribution; the degree of saturation of the pores; and effective diffusivity of gases through the concrete.

Experimental investigation and mathematical modeling of the concrete carbonation problem

Abstract Carbonation of concrete is the major time-limiting factor for the durability of reinforced concrete structures. The carbonation reaction between atmospheric CO 2 and Ca(OH) 2 of the concrete mass destroys the high pH environment of surrounding concrete which protects the steel bars of reinforced concrete from corrosion. In this paper we present experimental results obtained in an accelerated carbonation apparatus using a variety of techniques, including TGA, and we extend the mathematical model developed recently to include the entire range of ambient relative humidities.

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.

Concrete Encased in Fiberglass-Reinforced Plastic

Fiberglass-reinforced plastics (FRP) have very high tensile strength but relatively low modulus and poor stability in compression. 1t is proposed co construct FRP plain concrete composite members, in which an FRP casing is used as a form that stays permanently with the member, confining the concrete and acting as tensile reinforcement. Such a combination can result in significant savings in material and construction costs. Circular FRP-encased concrete cylinders tested in concentric compression exhibit very high strength and ductility. Rectangular FRP-encased concrete beams were constructed also with varying amounts of unidirectional FRP reinforcement added at the bottom. Such beams have very good strength and ductility, and their deflections are almost completely reversible, even after loading co their peak capacity, provided that enough FRP reinforcement has been added at the bottom to prevent brittle failure by fracture of the FRP in tension.

Seismic Retrofitting of Columns with Lap Spliced Smooth Bars Through FRP or Concrete Jackets

The effectiveness of RC jacketing or FRP wrapping for seismic retrofitting of rectangular columns having smooth (plain) bars with 180° hooks lap-spliced at floor level is experimentally investigated. The relatively low deformation capacity and energy dissipation of five unretrofitted columns is found not to depend on lap length, if lapping is not less than 15 bar-diameters. Six columns cyclically tested up to ultimate deformation after RC concrete jacketing demonstrate force and deformation capacity and energy dissipation sufficient for earthquake resistance, regardless of the presence or length of lap splicing in the original column. Another ten columns cyclically tested to ultimate deformation after wrapping of the plastic hinge region with CFRP show that FRP wrapping of the splice region is more effective than concrete jackets for enhancement of the deformation and energy dissipation capacity of old-type columns with smooth bars lap-spliced at floor level, provided that wrapping extends over the member length sufficiently to preclude plastic hinging and early member failure outside the FRP-wrapped length of the column.

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.

Seismic response and design of RC structures with plan-eccentric masonry infills

The bidirectional response of a two-storey RC frame structure with two adjacent sides infilled is studied through shaking table tests and non-linear dynamic analyses. The pre-cracking stiffness of the infills is large enough to impose twisting of the infilled structure about the common corner of the two infilled sides, with predominant period close to that of translation of the symmetric bare structure in the two horizontal directions. Parametric analyses and test results show that the peak displacement components of the corner column of the two open sides are about the same as (or slightly less than) those of the bare structure under the same bidirectional excitation, but take place simultaneously. This simultaneity of peak local demands from the two components of the motion seems to be the only effect of plan-eccentric infilling that needs to be taken into account in the design of the RC structure. Despite their very high slenderness (height-to-thickness ratio of about 30), infill panels survive out-of-plane peak accelerations of 0.6g at the base of the structure or 1.3-1.75g at their centre.

Analysis of Coherent Multistate Systems

The structure and reliability of homogeneous s-coherent multistate systems is studied. Homogeneous systems have two properties: 1) their components can be rearranged internally in any arbitrary way without affecting the mapping of the component states onto the system state; 2) all components, all subsystems composed of them, and the system have the same set of states. Because of the s-coherence and homogeneity properties, mapping of the component states onto the system state, as well as calculating probabilities of system states from those of the component states, become very simple. In homogeneous systems, states can be ranked according to their relative dominance, and the state ranking completely determines the structure of such a system. Any homogeneous s-coherent system belongs to one of four categories which differ in whether the identity property is satisfied or not, or on details of the state ranking. As an application, the high pressure injection system of a pressurized water reactor is modeled as a multistate system composed of homogeneous s-coherent multistate subsystems.