Mechanical behaviour of corroded reinforced concrete beams—Part 1: Experimental study of corroded beams

Steel corrosion in reinforced concrete leads to crack occurrence along the reinforcement (secondary cracks), to a reduction in bond strength and a reduction in steel cross section. The purpose of this study is to determine the effect of these deteriorations on the global behaviour of reinforced concrete structural elements in their service and ultimate states. Mechanical experimentation was carried out on fourteen-year-old reinforced concrete beams, on two control elements and two corroded beams. A comparative analysis of the results obtained on the beams showed that concrete cracking in the compressive area had no significant influence on the behaviour in service of the corroded elements. However, significant modifications of service behaviour were observed, due to the degradations in the tensile zone, namely: loss of bending stiffness, dissymmetrical behaviour. Finally, the measure of the residual steel cross-section of the corroded re-bars showed that the loss of bending stiffnes due to steel corrosion cannot be merely explained in terms of steel cross-section reduction. Concerning the ultimate behaviour, the loss of steel cross-section is the main parameter which leads to a reduction of bearing capacity and ductility. Another part will explain the separate and coupling effects of bond strength and steel cross-section loss on the mechanical behaviour of corroded beams.     

Nonlinear three–dimensional finite–element modelling of reinforced–concrete beams: Computational challenges and experimental validation

Three–dimensional nonlinear finite–element models have been developed to investigate the loading–unloading–reloading behaviour of two reinforced–concrete beams under four–point bending using explicit dynamics in ABAQUS. The damaged–plasticity model proposed by Lubliner and collaborators was employed for the plain concrete, and elastic–perfectly plastic models were employed for the steel reinforcement. A perfect bond was assumed between the steel rebars and concrete, whereby the bond–slip behaviour, as well as damage along crack patterns, were modelled through concrete damage. The influence of the shape of the tension–softening law on the numerical load–deflection response was studied by considering bi–linear, exponential and linear post–failure stress–displacement and stress–strain relationships. The effect of modelling steel rebars with truss or beam elements was also investigated. Structured meshes of linear hexahedral elements either with incompatible modes or with reduced integration, and unstructured meshes of either linear or ‘modified’ quadratic tetrahedral elements were considered. In terms of load–deflection curves, both the structured and the unstructured meshes gave results in very good agreement with test results. In terms of crack patterns, results predicted by the structured meshes exhibited some mesh bias, which was less pronounced with the unstructured meshes. In the post–yield phase, if a geometrically nonlinear model is used, discrepancies were found when truss elements were used for steel rebars, whereas good agreement was found if the bending stiffness of the rebars is included using beam elements. This is a non–obvious result that may be important to consider when studying the progressive collapse of RC structures.     

Preparation of Ti–Ni–Fe phase by levitation and its structural characterisation

Abstract

This paper presents results concerning the electromagnetic levitation of solid metal samples. It describes the levitation system and some coils designed to melt 10 g metal samples under controlled atmosphere, as well as melting the process. The main advantage of the method is the absence of any type of crucible and owing to this, the effect of side reaction can be avoided, resulting from the influence of the active metal on the walls of the ceramic crucible in the physicochemical investigations. In many cases, the side reaction makes the correct interpretation of the experimental results sometimes difficult and/or impossible. In order to describe the crystallisation process and the change in the microstructure of the samples obtained during electromagnetic melting at the intermetallic Fe–Ni–Ti phase, a scanning electron microscopy analysis and an X-ray diffraction analysis were used.     

Mechanical behaviour of corroded reinforced concrete beams—Part 2: Bond and notch effects

This paper deals with the effect of steel cross-section and bond strength reduction on the mechanical behaviour of corroded RC-beams. In the case of corroded reinforced concrete members, those effects are always coupled and a previous study (part one) has shown that it is not realistic to forecast the behaviour of corroded beams merely in terms of steel cross-section reduction. The object of the study is thus to understand the separated and coupled effects of the reduction in bond strength and steel cross-section. These investigations are carried out in order to be able to model the behaviour of corroded structural members and to predict how and when repairing is necessary. Different experimental simulations of corrosion were made. The results show the significant impact of coupling between reduction of bond strength and steel cross-section.

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Résumé Cet article traite de l’influence de la réduction de section d’acier et de l’adhérence acier béton en partie tendue sur le comportement mécanique des poutres corrodées. L’étude précédente (partie 1) semble montrer qu’il n’est pas réaliste de vouloir prédire le comportement mécanique des éléments de structures corrodées en ne tenant compte que de la réduction de section des aciers tendus. Par conséquent, l’objectif de ce travail est de mieux comprendre et quantifier les effets couplés et découplés de ces deux paramètres. Pour cela, plusieurs simulations expérimentales des effets de la corrosion ont été réalisées sur poutres ou sur échantillons d’armature non corrodés. Les expérimentations mécaniques sont réalisées en service et à rupture. Les résultats obtenus confirment largement qu’une prédiction réaliste du comportement mécanique résiduel en service des poutres corrodées, ne sera obtenue qu’en prenant en compte de l’effet couplé de la réduction de section d’acier et de l’adhérence acier béton en partie tendue.

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Editorial Note Prof. Ginette Arliguie is a RILEM Senior Member. She works at LMDC (Laboratoire Matériaux et Durabilité des Constructions), a RILEM Titular Member.     

Fracture properties of concrete reinforced with steel–polypropylene hybrid fibres

This research discusses polypropylene fibres and three sizes of steel fibres reinforced concrete. The total fibre content ranges from 0% to 0.95% by volume of concrete. A four-point bending test is adopted on the notched prisms with the size of 100×100×500 mm³ to investigate the effect of hybrid fibres on crack arresting. The research results show that there is a positive synergy effect between large steel fibres and polypropylene fibres on the load-bearing capacity and fracture toughness in the small displacement range. But this synergy effect disappears in the large displacement range. The large and strong steel fibre is better than soft polypropylene fibre and small steel fibre in the aspect of energy absorption capacity in the large displacement range. The static service limitation for the hybrid fibres concrete, with “a wide peak” or “multi-peaks” load–CMOD patterns, should be carefully selected. The ultimate load bearing capacity and the crack width or CMOD at this load level should be jointly considered in this case. The K_IC and fracture toughness of proper hybrid fibre system can be higher than that of mono-fibre system.     

Strengthening of non-seismically detailed reinforced concrete beam–column joints using SIFCON blocks

This article aims to propose a novel seismic strengthening technique for non-seismically detailed beam–column joints of existing reinforced concrete buildings, typical of the pre-1975 construction practice in Turkey. The technique is based on mounting pre-fabricated SIFCON composite corner and plate blocks on joints with anchorage rods. For the experimental part three 2/3 scale exterior beam–column joint specimens were tested under quasi-static cyclic loading. One of them was a control specimen with non-seismic details, and the remaining two with the same design properties were strengthened with composite blocks with different thickness and anchorage details. Results showed that the control specimen showed brittle shear failure at low drift levels, whereas in the strengthened specimens, plastic hinge formation moved away from column face allowing specimens to fail in flexure. The proposed technique greatly improved lateral strength, stiffness, energy dissipation, and ductility.     

The use of wire mesh–epoxy composite for enhancing the flexural performance of concrete beams

This paper presents a study of an ongoing research project on the use of new composites for enhancement of the performance of concrete beams. A plain concrete beam was externally bonded with wire mesh–epoxy composite using one to five wire mesh layers. The flexural performance of the beam specimens bonded with wire mesh layers was compared with the beam specimens bonded with carbon fibre as well as a hybrid of wire mesh–epoxy–carbon fibre composite. The test results show that the use of wire mesh with epoxy is an efficient way to improve the flexural performance of concrete beam specimens. The increase in wire mesh layers significantly enhances the flexural strength, cracking behaviour and energy absorption capability. In comparison with carbon fibre, wire mesh–epoxy composite is more efficient in flexural strength and ductility. In addition, it was found that a concrete beam bonded with a hybrid wire mesh–epoxy–carbon fibre composite has significantly more energy absorption capability compared to specimens bonded with only carbon fibre.     

Behaviour of hybrid fibre reinforced concrete beam–column joints under reverse cyclic loads

An experimental investigation was carried out to study the effect of hybrid fibres on the strength and behaviour of High performance concrete beam column joints subjected to reverse cyclic loads. A total of 12 reinforced concrete beams column joints were cast and tested in the present investigation. High performance concrete of M60 grade was designed using the modified ACI method suggested by Aïtcin. Crimped steel fibres and polypropylene fibres were used in hybrid form. The main variables considered were the volume fraction of (i) crimped steel fibres viz. 0.5% (39.25 kg/m³) and 1.0% (78.5 kg/m³) and (ii) polypropylene fibres viz. 0.1% (0.9 kg/m³), 0.15% (1.35 kg/m³), and 0.2% (1.8 kg/m³). Addition of fibres in hybrid form improved many of the engineering properties such as the first crack load, ultimate load and ductility factor of the composite. The combination of 1% (78.5 kg/m³) volume fraction of steel fibres and 0.15% (1.35 kg/m³) volume fraction of polypropylene fibres gave better performance with respect to energy dissipation capacity and stiffness degradation than the other combinations.     

Synthesis of β-MoO3 by vacuum drying and its structural and electrochemical characterisation

The β-MoO₃ was obtained successfully free of α-MoO₃ through soft chemistry methods. The formation of β-MoO₃ with high purity was determined by the formation of the precursor MoO₃·2H₂O when a solution of Na₂MoO₄·2H₂O was passed through a cation-exchange resin. A structural, spectroscopic and thermal study of the polymorph synthesised was made by XRD, electron dispersion spectroscopy (EDS), FTIR and TGA/DTA techniques, respectively, in order to make a study about the possibilities of β-MoO₃ as active material in a lithium battery. Electrochemical experiments showed a high ability of the β-MoO₃ to form lithium molybdenum bronzes via a lithium insertion reaction.     

Shear deformation effect in flexural–torsional vibrations of beams by BEM

In this paper, a boundary element method is developed for the general flexural–torsional vibration problem of Timoshenko beams of arbitrarily shaped cross section taking into account the effects of warping stiffness, warping and rotary inertia and shear deformation. The beam is subjected to arbitrarily transverse and/or torsional distributed or concentrated loading, while its edges are restrained by the most general linear boundary conditions. The resulting initial boundary value problem, described by three coupled partial differential equations, is solved employing a boundary integral equation approach. Besides the effectiveness and accuracy of the developed method, a significant advantage is that the displacements as well as the stress resultants are computed at any cross-section of the beam using the respective integral representations as mathematical formulae. All basic equations are formulated with respect to the principal shear axes coordinate system, which does not coincide with the principal bending one in a nonsymmetric cross section. To account for shear deformations, the concept of shear deformation coefficients is used. Six boundary value problems are formulated with respect to the transverse displacements, to the angle of twist, to the primary warping function and to two stress functions and solved using the Analog Equation Method, a BEM based method. Both free and forced vibrations are examined. Several beams are analysed to illustrate the method and demonstrate its efficiency and wherever possible its accuracy.     

Comparative performance of sub-standard interior reinforced concrete beam–column connection with various joint reinforcing details

This paper discusses the effect of some important parameters on cyclic behavior of sub-substandard interior beam–column connection. The objective is to investigate the effect of joint shear stress, anchorage bond of longitudinal beam bar within the joint and horizontal joint reinforcements on the joint performance. The experiment consisted of five half-scale beam–column specimens. The control specimen (J1) represented a typical non-ductile beam–column joint in mid-rise RC buildings constructed in low seismic zone. In specimen J2, the bond between concrete and longitudinal bars was completely removed initially. In specimen J3A and J3B, a substantial amount of horizontal joint reinforcement was provided in joint core. In specimen J4, the column size was enlarged to reduce shear stress in joint. The experimental result demonstrated brittle joint shear failure in control specimen (J1), specimens J3A and J3B, beam splitting failure in specimen J2 and ductile flexural failure in specimen J4. Based on experimental results, it was found that the initial lost of bond did not cause a substantial reduction in joint capacity. Moreover, provision of substantial horizontal joint reinforcements in specimen J3A and J3B did not produce a comparable improvement in the seismic performance. With increased column size in specimen J4, the energy dissipation characteristics were greatly improved as indicated by large spindle-shaped cyclic loops.     

Electrical characterisation of stearic acid/calix[4]amine Langmuir–Blodgett thin film

Within this work we deposited 16 monolayers of stearic acid alternated with 15 monolayers of calix[4]amine to form a non-centrosymmetric Langmuir–Blodgett (LB) thin film onto an aluminized (50 nm coated) glass microscopic slide. Dielectric constant and dielectric loss for the film were determined using C-f and tan (δ-f) measurements. The value of the pyroelectric figure of merit was determined as 1.73 μC m⁻² K⁻¹. To elucidate the conduction mechanism of stearic acid/calix[4]amine LB film, DC current–voltage measurements between −4 and +4 V were carried out. The I(V) behaviour shows a symmetrical and highly non-linear behaviour. Analysis of this behaviour of the stearic acid/calix[4]amine LB film showed a conductivity value of 1.12 × 10⁻¹³ S m⁻¹ for ohmic region. The exponential part of I(V) dependence obeyed the Schottky conduction mechanism with a barrier height of 1.67 eV. This LB film structure shows a typical insulating behaviour for low voltage values and the Schottky effect becomes dominant when the voltage increases. The frequency dependence of conductivity shows a power law relationship between conductance and frequency.