S. J. Pantazopoulou

MICROSTRUCTURAL ASPECTS OF THE MECHANICAL RESPONSE OF PLAIN CONCRETE

A simplified constitutive model is presented that recognizes and incorporates the properties of microstructure and its influence on mechanical response of plain concrete. The model evaluates uniaxial compressive stress at any level of axial strain, using a strain-dependent estimate of material stiffness. The initial elastic modulus of uncracked concrete is assessed, based on water-cement ratio, age, volume fraction of aggregates, paste porosity, degree of hydration, and paste-aggregate interface properties. Reduction of the initial modulus with increasing load is modeled by the application of a factor that depends on the natural porosity of the material and mechanically induced porosity as it is assessed by the area strain that develops in the cross section supporting the load. Using this approach, it is possible to model the change effected on the initial resistance of the material from progressive microcrack growth and internal damage occurring in the concrete. The sensitivity of the proposed model to several variables was evaluated from parametric studies and by comparisons with available experimental data.

Numerical solution of mass transport equations in concrete structures

Abstract To calculate the service life of reinforced concrete (RC) structures, the process of reinforcement corrosion was modeled using a numerical formulation of the associated mass transport partial differential equations. Migration of chlorides, moisture and heat transfer within concrete, resulting from seasonal variations in the surface conditions of the RC member, form a coupled boundary-value problem, which was solved in space using a finite element formulation and in time using a finite difference marching scheme. The stage of active corrosion was modeled by including in the numerical algorithm of the FE formulation the mass conservation equation that describes diffusion of oxygen in the concrete cover. The paper presents details of the FE formulation and computed results from selected case studies.

CORROSION EFFECTS ON BOND STRENGTH IN REINFORCED CONCRETE

Corrosion damage of reinforced concrete (RC) structures is a serious problem responsible for billions of dollars in repairs of highway structures every year. Corrosion of reinforcing steel in RC affects structural performance in two different ways: by loss of steel section and through deterioration of steel-concrete bond. This experiment investigated the effects of corrosion products on bond strength. To test the strength of corroded reinforcing bar anchorages and to quantify the effect of deterioration of the bar surface on development length, RC slabs were cast with the ends of the reinforcing bars anchored in the concrete for a known length. Bond breakers were used over the center portion of the bars to control the force demand input to the test anchorage. This measure also served to protect those regions from corrosion. The anchorage zones were corroded to various degrees by applying electrical voltage to the bar ends. Specimens were tested in flexure to assess strength and mode of failure. Results were summarized by means of simple parametric design expressions that relate bond strength to steel area loss.

Consideration of Questions about Beam-Column Joints

The mechanics of beam-column joints in laterally loaded frame structures are investigated in this paper. The formulation presented establishes compatibility of strains and stress equilibrium. Factors known to affect the behavior of connections, including the effect of lateral restraint, as well as the reduction of concrete strength associated with diagonal tensile strains, are considered. The role of stirrups and axial load on the behavior of the joint are illustrated clearly. In addition to providing an improved understanding of joint behavior, the derivation reflects the relationships between the design limit states and corresponding deformations, giving an opportunity to link joint design to the overall lateral drift of the structure.

Behavior of Corroded Bar Anchorages

This paper uses analytical modeling and correlation with experimental testing to evaluate the influence of rust accumulation on bond strength and the implications for development capacity of bar anchorages. The model is a frictional construct in which bond strength is estimated from the coefficient of friction and the normal confining pressure along the anchorage. Both variables are evaluated considering the effects of iron depletion and relevant design parameters such as cover, shrinkage, and transverse reinforcement. The model is used to interpret the behavior of corroded anchorages as documented in previous research. To supplement model calibration with data representative of long anchorages, two series of flexural specimens designed to fail in anchorage after yielding are tested after being conditioned in accelerated corrosion to a predefined damage level in the anchorage zones. The effects of corrosion on bond strength are also considered in carbon fiber-reinforced polymer-patch repaired anchorages. Findings show that the model consistently reproduces the magnitude and parametric sensitivity of corrosion-induced bond degradation demonstrated in previous experiments.

Structural health monitoring of smart structures

An intelligent monitoring procedure has been applied to static and dynamic field data collected from two innovative structural systems that have recently been constructed in the provinces of Manitoba and Nova Scotia, Canada. These structures incorporate innovative materials and systems and have been extensively instrumented with both fiber optic and electrical strain gauge sensors. Sensor data are remotely accessed and analysed, and performance indices appraising the structural health and performance of the structures are computed.

Global Interventions for Seismic Upgrading of Substandard RC Buildings

A methodology is developed in this paper for the design and proportioning of interventions for seismic upgrading of substandard reinforced-concrete (RC) buildings. The retrofit approach is presented in the form of a simple design tool that aims toward both demand re- duction and enhancement of force and deformation supply through controlled modification of stiffness along the height of the building. This objective is achieved by engineering the translational mode-shape of the structure, so as to optimize the distribution of interstory drift. Results from the proposed approach are summarized in a spectrum format in which demand, expressed in terms of interstory drift, is related to stiffness. Design charts, which relate the characteristics of commonly used global intervention procedures to influence drift demands, are developed to facilitate the retrofit design. The intervention procedures considered in this paper are reinforced-concrete jacketing, the addition of reinforced- concrete walls, and the addition of masonry infills. The proposed methodology is also amenable to adaptation to other strengthening methods, such as the addition of cross-bracing. DOI: 10.1061/(ASCE)ST.1943-541X.0000474.

Design Methodology for Seismic Upgrading of Substandard Reinforced Concrete Structures

This article presents a design methodology for seismic upgrading of existing reinforced concrete (RC) buildings. The methodology is based on the modification of the deflected shape of the structure so as to achieve a near-uniform distribution of interstorey drift along the building height, thereby eliminating damage localization. Yield Point Spectra are utilized for the definition of demand and a direct displacement-based design approach is implemented. The fundamental steps of the method are described in detail, including a systematic evaluation of assumptions and limitations. A full-scale tested structure is used as a case study for assessment and verification of the proposed methodology. Alternative retrofit scenarios are set according to target response and performance levels. The role of the target deflected response shape and its influence on the outcome of the retrofit strategy is investigated. The viability of the alternative retrofit scenarios is studied for different ground motions including near-fault earthquake records.

Fiber-Reinforced Polymer Retrofitting of Predamaged Substandard RC Prismatic Members

An experimental study was conducted to investigate the efficiency of FRP jackets in upgrading the seismic behavior of lightly reinforced concrete prismatic members previously damaged under a combination of axial compression and a reversed cyclic lateral displacement history simulating earthquake effects. The test program comprises 13 cantilever prismatic specimens, which, owing to substandard reinforcing details representative of older construction practices in southern Europe, were susceptible to various undesirable modes of damage such as web-shear cracking, longitudinal bar buckling, or lap-splice failure. After repair, the specimens were retested using the same load combination. The efficiency of the repair options considered in the study, which refer to alternative strengthening systems (with glass or carbon wraps), was investigated with reference to the design parameters of the intervention, the type of the applied lateral displacement history, and the mode of failure that had occurred previously in the initial phase of the tests. The results provided valuable insight regarding participation of the FRP jackets in the various mechanisms of resistance, their ability to reverse the effect of initial damage, and to impart deformation capacity to the structural member.

Time-frequency analysis of sensor data for detection of structural damage in instrumented structures

An experimental time-frequency analysis framework is developed for monitoring the occurrence and location of structural damage in instrumented structures such as bridges. We make available a software package written in Matlab that provides assistance in assessing the condition of the structural system with excellent visualization tools and interaction with the user. Data were obtained from an actual experimental bridge, instrumented with strain gauges which is located in Nova Scotia, Canada. Experimental results are in close agreement with analytical calculations of natural frequencies and damping ratios and provide remotely, useful information on the location and the possibility of damage.