Kent A. Harries

Behavior and Modeling of Concrete Subject to Variable Confining Pressure

This paper presents a study of the behavior of concrete subject to variable levels of confining pressure under concentric axial loading. A detailed experimental investigation of this behavior, using fiber-reinforced polymer-confined concrete cylinders, is used to develop an understanding of the relationships required to accurately predict the behavior of concrete subject to passively induced, varying levels of confinement. In particular, the relationship between transverse and longitudinal strains-the dilation relationship--is investigated, and a model for this behavior, based on the stiffness of the confining materials, is proposed. The established relationships are combined into an iterative model for determining the complete stress-strain relationship of concrete subject to variable confining pressure.

Shape and 'gap' effects on the behavior of variably confined concrete

Abstract Factors affecting the behavior of variably confined concrete are presented. The effect of debonding the fiber-reinforced polymer (FRP) jacket to the concrete substrate and providing a gap between the concrete and confining jacket is investigated. A second parameter—the shape of the cross section—is also investigated. An experimental program involving the compression testing of standard cylinders and similarly sized square specimens having external FRP jackets providing passive confinement is presented. Factors affecting jacket efficiency and the appropriateness of factors accounting for specimen shape are determined experimentally and discussed. The provision of a gap affected the axial stress at which the confining jacket was engaged, resulting in a reduced maximum attainable concrete strength. The jacket efficiency was not affected by the provision of the gap. The shape of the specimens was observed to affect the level of confinement generated. Square specimens exhibit lower confinement levels than circular specimens having the same jacket.

Axial behavior and modeling of confined small-, medium-, and large-scale circular sections with carbon fiber-reinforced polymer jackets

This article reports on a study of the axial behavior and modeling of confined circular sections of concrete with carbon fiber-reinforced polymer (FRP) jackets. The authors note that the axial stress-strain behaviors of unconfined and confined concrete differ significantly. Confined concrete shows improved compressive strength and axial strain capacity over unconfined concrete. The study involved axial load tests of small-scale (152 mm diameter by 305 mm tall cylinders), medium-scale (254 mm diameter by 762 mm tall), and large-scale (610 mm in diameter by 1.83 m tall) circular concrete specimens, all of which had a height-to-diameter ratio less than or equal to 3.0. At the relatively high level of confinement provided, a scale effect was not observed; similar results were observed regardless of column size. The authors conclude by presenting appropriate parameters for modeling confined concrete and by making recommendations for the modeling and design of axially-loaded confined concrete.

The Effect of the Presence of Water on the Durability of Bond between CFRP and Concrete

This research assesses the effects of the presence of water during the carbon fiber reinforced polymer (CFRP) application and after the CFRP cure on the bond between the CFRP and the concrete. Modified double cantilever beam (MDCB) specimens are used to determine the interfacial energy release rate, G, of the CFRP—concrete bond. A CFRP fabric is applied to the specimens with three different initial levels of water/moisture presence. Other specimens, with a CFRP fabric applied in dry conditions and allowed to cure, are conditioned in a saturated environment for different lengths of time. The test results indicate that the presence of water during the CFRP application decreases the bond quality significantly and most of the resulting failures are adhesive failures along the primer/concrete interface. The use of a specially formulated primer results in a slightly higher bond capacity but the same undesirable failure. High quality CFRP applications, conditioned in a saturated environment for a relatively short period of time, from 3 to 8 weeks, demonstrate that exposure to water degrades the bond between CFRP and concrete after the epoxy has cured.

Behavior and Design of Reinforced Concrete, Steel, and Steel‐Concrete Coupling Beams

The efficiency of coupled wall systems to resist lateral loads is well known. In order for the desired behavior of the coupled wall system to be attained, the coupling beams must be sufficiently strong and stiff. The coupling beams, however, must also yield before the wall piers, behave in a ductile manner, and exhibit significant energy-absorbing characteristics. This paper reviews the current state of the art for the design of conventional reinforced concrete, diagonally reinforced concrete, steel, and composite steel-concrete coupling beams. Although not exhaustive, critical aspects of the design of these systems are presented.

A Nonlinear Acoustic Technique for Crack Detection in Metallic Structures

A crack detection technique based on nonlinear acoustics is investigated in this study. Acoustic waves at a chosen frequency are generated using an actuating lead zirconate titanate (PZT) transducer, and they travel through the target structure before being received by a sensing PZT wafer. Unlike an undamaged medium, a cracked medium exhibits high acoustic nonlinearity which is manifested as harmonics in the power spectrum of the received signal. Experimental results also indicate that the harmonic components increase nonlinearly in magnitude with increasing amplitude of the input signal. The proposed technique identifies the presence of cracks by looking at the two aforementioned features: harmonics and their nonlinear relationship to the input amplitude. The effectiveness of the technique has been tested on aluminum and steel specimens. The behavior of these nonlinear features as crack propagates in the steel beam has also been studied.

Experimental investigation of the behavior of variably confined concrete

The behavior of concrete subject to variable levels of confining pressure under concentric axial loading is presented. An extensive experimental investigation of this behavior, using FRP-confined concrete cylinders, is used to develop an understanding of the relationships required to accurately model the behavior of concrete subject to passively induced varying levels of confinement. In particular, the relationship between transverse and longitudinal strains—the dilation relationship—is investigated and a model for this behavior, based on the stiffness of the confining materials, is proposed. Concrete compressive strength is observed to increase with increasing confinement. Axial strain capacity is observed to increase to a greater degree than the compressive strength resulting in a more ductile axial stress–strain behavior for confined concrete as compared to unconfined concrete. The axial stress–strain behavior is also observed to change from parabolic to bilinear as the level of confinement is increased.

Ductility and Deformability of Coupling Beams in Reinforced Concrete Coupled Walls

The seismic response of coupled wall structures is discussed. It is demonstrated through a review of large-scale experimental investigations of coupling beam behavior and analytic studies of coupled wall behavior that often the coupling beam ductility demand exceeds the expected available ductility. As a result, it is possible that coupled wall structures will not behave as desired in the course of a significant seismic event. Limits to the allowable degree of coupling are proposed as a remedy to this apparent deficiency. Additional design and analysis issues are discussed including reduced section properties and wall overstrength requirements.

PREDICTIVE RESPONSE OF NOTCHED STEEL BEAMS REPAIRED WITH CFRP STRIPS INCLUDING BOND-SLIP BEHAVIOR

This paper presents the flexural behavior of notched steel beams repaired with carbon fiber-reinforced polymer (CFRP) strips. A combined experimental and computational approaches are used to examine local plasticity near the damage and the effects of CFRP-repair. A modeling approach is proposed to take into account the bond-slip behavior of CFRP-steel interface. The experimentally validated models are further used to conduct a parametric study addressing various engineering properties of CFRP composites and adhesives. The CFRP-repair is shown to restore the strength of the damaged beam. The CFRP strip relieves the stress concentration resulting from the presence of the notch, reducing the high local plasticity. The parametric study confirms the improved effectiveness of high modulus CFRP (i.e., exceeding 150 GPa) in affecting repairs of steel members. Under static loading conditions, the stiffness of the adhesive bond line influences the local behavior of the CFRP-steel interface but has little effect on the overall member behavior.

FLEXURAL BEHAVIOR OF CONCRETE COLUMNS RETROFITTED WITH CARBON FIBER-REINFORCED POLYMER JACKETS

A major deficiency in many existing nonductile reinforced concrete frames is the inability of the columns to undergo significant deformations while maintaining their load-carrying capacity. As a result, relatively brittle modes of column failure, accompanied by soft story structural failure mechanisms, are possible. Providing additional confinement to the columns allows them to behave in a more ductile manner. This paper investigates the use of carbon fiber-reinforced polymer composite jackets as a method of retrofitting existing nonductile reinforced concrete structural columns. A method of designing fiber-reinforced polymer composite jackets for columns to achieve the desired flexural behavior is presented and verified through a full-scale experimental study.