External confinement of concrete by means of high-strength fiber composites can significantly enhance its strength and ductility as well as result in large energy absorption capacity. The confinement mechanism may include fiber-wrapping of existing columns as a retrofitting measure or encasement of concrete in a fiber reinforced plastic (FRP) tube for new construction. Proper design of such hybrid columns, however, requires an accurate estimate of the performance enhancement. Current design methods use simple extension of the models developed for conventional reinforced concrete columns. Results from a series of uniaxial compression tests on concrete-filled FRP tubes are compared with the available confinement models in the literature. The present study indicates that these models generally result in overestimating the strength and unsafe design. The study also shows a unique characteristic of confinement with fiber composites in that, unlike steel, FRP curtails the dilation tendency of concrete, as it re...

Behavior of Concrete Columns Confined by Fiber Composites

FRP-confined concrete in circular sections: Review and assessment of stress-strain models

Finite element modeling of confined concrete-I: Drucker–Prager type plasticity model

Triaxial compression tests were carried out to evaluate the effect of randomly distributed fiber reinforcement and cement inclusion on the response of a sandy soil to load. Cemented specimens were prepared with cement contents of 0% and 1% by weight of dry soil and cured for seven days. Fiber length was of 12.8 mm, in the contents of 0% and 3% by weight of dry soil-cement mixture. Test results indicated that the addition of cement to soil increases stiffness, brittleness, and peak strength. The fiber reinforcement increases both he peak and residual triaxial strength, decreases stiffness, and changes the cemented soils’ brittle behavior to a more ductile one. The triaxial peak strength increase due to fiber inclusion is more effective for uncemented soil. However, the increase in residual strength is more efficacious when fiber is added to cemented soil. Peak strength envelopes indicate that the friction angle is increased from 35° to 46° as a result of fiber inclusion. The cohesion intercept is affected slightly by fiber addition, being basically a function of cementation.

Influence of fiber and cement addition on behavior of sandy soil

Finite element modeling of confined concrete-II : plastic-damage model

FRP-confined concrete members: Axial compression experiments and plasticity modelling

A method is presented for the assessment and calibration of the elastoplastic behaviour of FRP confined concrete. The method is based on the evaluation of permanent deformations from observed experimental deformations and theoretical elastic response of confined concrete. The inelastic response of concrete and the parameters of its mathematical modelling are investigated. Closed form expressions are produced to relate the model parameters to the mechanical properties of the material. A strain-hardening Drucker–Prager model is developed which simulates both the hardening and softening material response with reasonable agreement to the experimental observations. The predictive ability of the model is verified through comparisons to numerous published experimental data and analytical models.

Analytical modelling of plastic behaviour of uniformly FRP confined concrete members

the authors note, this is a very difficult problem to solve, even numerically, since it involves large deformations, moving boundaries and non-linear material behav- iour. The authors have used an axi-symmetric finite element analysis and presented numerical results for the displacement, stress and pore water press- ure fields around the cone tip at failure for penetra- tion under undrained conditions. Failure is defined as the condition where penetration proceeds in- definitely at a constant cone pressure. Results are also given for the relationships between penetra- tion resistance and penetration depth. In the analysis reported in the paper it was assumed that the initial effective stress state in the soil was hydrostatic with a magnitude of

A large strain theory and its application in the analysis of the cone penetration mechanism

This paper is accompanied by a study on constitutive modelling issues of cemented sands. The concentration here is on experimental issues related to the triaxial testing of cemented sands. A preliminary investigation is performed aiming to identify potential effects of specimen size and slenderness on the stress–strain–strength characteristics of cemented sands. A comprehensive experimental study follows where clean sand specimens, as well as specimens with 2, 4 and 6 per cent cement content, are tested. The aim of the study is to examine the effects of cement content and confinement on the shear strength, stiffness, softening and dilation characteristics of cemented sand. © 1997 John Wiley & Sons, Ltd.

Behavior of cemented sands—I. Testing

A novel formulation aiming to achieve optimal design of reinforced concrete (RC) structures is presented here. Optimal sizing and reinforcing for beam and column members in multi-bay and multi- story RC structures incorporates optimal stiffness correlation among all structural members and results in cost savings over typical-practice design solutions. A Nonlinear Programming algorithm searches for a minimum cost solution that satisfies ACI 2005 code requirements for axial and flexural loads. Material and labor costs for forming and placing concrete and steel are incorporated as a function of member size using RS Means 2005 cost data. Successful implementation demonstrates the abilities and performance of MATLAB's (The Mathworks, Inc.) Sequential Quadratic Programming algorithm for the design optimization of RC structures. A number of examples are presented that demonstrate the ability of this formulation to achieve optimal designs. This paper presents a novel optimization approach for the design of reinforced concrete (RC) structures. Optimal sizing and reinforcing for beam and column members in multi-bay and multi- story RC structures incorporates optimal stiffness correlation among structural members and results in cost savings over typical state-of-the-practice design solutions. The design procedures for RC structures that are typically adapted in practice begin by assuming initial stiffness for the structural skeleton elements. This is necessary to calculate the internal forces of a statically indeterminate structure. The final member dimensions are then designed to resist the internal forces that are the result of the assumed stiffness distribution. This creates a situation where the internal forces used for design may be inconsistent with the internal forces that correspond to the final design dimensions. The redistribution of forces in statically indeterminate structures at incipient failure, however, results in the structural performance that is consistent with the design strength of each member. Although this common practice typically produces safe structural designs, it includes an inconsistency between the elastic tendencies and the ultimate strength of the structure. In some cases this can cause unsafe structural performance under overloads (e.g. earthquakes) as well as unwanted cracking under normal building operations when factored design loads are close to service loads, (e.g. dead load dominated structures). This inconsistency also implies that such designs are unnecessarily expensive as they do not optimize the structural resistance and often result in

Panos D. Kiousis

Papers

FRP-confined concrete members: Axial compression experiments and plasticity modelling

Analytical modelling of plastic behaviour of uniformly FRP confined concrete members

A large strain theory and its application in the analysis of the cone penetration mechanism

Behavior of cemented sands—I. Testing

Design optimization of reinforced concrete structures