Amir Mirmiran

Tests and modeling of carbon-wrapped concrete columns

Abstract There is an urgent need for models that can accurately predict performance of fiber-wrapped concrete columns. Axial compression tests on a total of 45 carbon-wrapped concrete stubs of two batches of normal and high-strength concrete and five different number of wraps were used to verify a confinement model, which was originally developed for concrete-filled glass FRP tubes. Also, a nonlinear finite element model with a non-associative Drucker–Prager plasticity was developed. Both models compared favorably with test results. It was concluded that the adhesive bond between concrete and the wrap would not significantly affect the confinement behavior. Moreover, the same confinement model can be applied to carbon and glass fibers, as long as the model has incorporated the dilation tendency of concrete as a function of the stiffness of the jacket. However, it is of utmost importance to establish the effective hoop rupture strain of the wrap through a reliability analysis by setting proper confidence level for design purposes.

A new concrete-filled hollow FRP composite column

An effective use of fiber reinforced plastic (FRP) shapes in infrastructure is in the form of composite construction with reinforced concrete. A novel composite column is proposed that is similar to the classic concrete-filled steel tubes, except that steel has been replaced with a hollow FRP shell. The FRP shell, while an integral part of the structure, is also the pour form for concrete. The shell may be a filament-wound or a multi-layer FRP pipe with a layer of longitudinal fibers sandwiched between two piles of circumferential fibers. The proposed column offers high strength and ductility in addition to excellent durability. Behavior of the proposed column is studied by developing two analytical tools; a new passive confinement model for externally reinforced concrete columns, and a composite action model that evaluates the lateral stiffening effect of the jacket. Results are compared with recent studies of fiber-wrapped columns.

Nonlinear finite element modeling of concrete confined by fiber composites

Abstract Since use of fiber composites for confining concrete columns is relatively new, analytical models are limited to those of steel ties and hoops, which tend to overestimate the strength of cross section. A nonlinear finite element model with a non-associative Drucker–Prager plasticity is developed to account for restraint pressure sensitivity of concrete. The predicted stress–strain curves compare favorably with test results. Volumetric expansion tendency of confined concrete, however, is not preserved. The model shows stress concentrations around corners of square sections, as previously observed by the experiments. The stress concentration depends on corner radius. The model, however, does not allow for stiffness degradation under cyclic loading.

LARGE BEAM-COLUMN TESTS ON CONCRETE-FILLED COMPOSITE TUBES

The concept of concrete-filled composite tubes was developed to address the corrosion problems associated with reinforced and prestressed concrete piles. Beam-column tests on a total of 16 2.75-m specimens demonstrated the feasibility of off-the-shelf fiber-reinforced polymer products. Two types of tubes were used: spin-cast (I) and filament-wound (II). Based on their respective brittle-brittle reinforcement ratios, Type I and II specimens were considered as over-reinforced and under-reinforced concrete sections, respectively. The tests showed that over-reinforced specimens performed superior as beam-columns. They deflected to a lesser extent and failed at much higher axial and lateral loads, while their failure was still gradual and ductile. They were also more efficient, as a smaller portion of their sectional capacity was consumed by secondary moment effects. Bond failure was not an issue in beam-columns. Therefore, off-the-shelf tubes can be used as long as end conditions and connections are properly designed. It is necessary, however, to provide a shear transfer mechanism for beams. In comparison with prestressed concrete columns, the 348 mm diameter Type I specimens were found comparable to 584 mm diameter circular sections prestressed with 20 strands of Grade 1862 MPa, whereas the 369 mm Type II specimens were found comparable to 460 mm square sections prestressed with eight strands.

Behavior of Ultrahigh-Performance Concrete Confined by Fiber-Reinforced Polymers

Over a decade of studies have demonstrated the benefits of ultra high performance concrete (UHPC) in terms of damage tolerance, energy absorption, crack distribution, and deformation capacity. However, little information is available on the confinement behavior of UHPC, especially when confined with fiber-reinforced polymers (FRP). Sixteen UHPC-filled FRP tubes with different fiber type and tube thickness were tested under monotonic uniaxial compression. All specimens failed by rupture of the tube at or near the midheight. Similar to conventional concrete, test results showed significant enhancements in the ultimate strength and strain of UHPC—up to 98% and 195%, respectively, compared with its unconfined counterpart. The experimental results were compared with a number of available confinement models. Although one of the models provided a reasonable fit for the stress-strain response in most cases, all models generally underestimated the effectiveness of FRP confinement at higher confinement ratios. The study demonstrated the need for confinement models that could accurately predict the behavior of FRP-confined UHPC in terms of the stress-strain relationship and the respective ultimate strengths and strains.

Analysis and field tests on the performance of composite tubes under pile driving impact

Abstract Composite tubes provide a feasible alternative to concrete piles by eliminating formwork, reinforcing cage, and additional corrosion-deterrent cover. Field driving of concrete-filled composite tubes showed no damage to the tube or concrete. Driving stresses in filled tubes were found comparable to those for prestressed concrete piles. Empty tubes may buckle or rupture under driving impact, unless driven at shallow depths and in soft soils, or with a steel mandrel. A detailed parametric study using the wave equation further confirmed that there is no difference in the drivability of filled FRP tubes and prestressed concrete piles of the same cross-sectional area and concrete strength. The typical refusal rate for conventional concrete piles can be safely adopted for filled tubes. However, empty tubes are susceptible to compression failure, and can only endure diving stresses up to 40–50% of the refusal rate of concrete piles.

Design for Slenderness in Concrete Columns Internally Reinforced with Fiber-Reinforced Polymer Bars

In spite of their successful introduction into the construction market, widespread acceptance of fiber-reinforced polymer (FRP) reinforcement by the engineering community depends on timely development of design guidelines and specifications. Many properties can influence the slenderness of FRP reinforced concrete (RC) columns. In this paper, a rational method is developed for the analysis of slender FRP-RC columns. A detailed parametric study is also carried out. This study recommends reducing the current slenderness limit of 22 to 17 for FRP-RC columns bent in single curvature. It is also shown that the moment magnification method can be extended to FRP-RC columns. The current stiffness formulas, and the stiffness reduction factor, however, need to be adjusted either as sole functions of modular ratio or as functions of both modular ratio and eccentricity ratio.

Creep modeling for concrete-filled steel tubes

Using the rate of flow method and the double power law function for basic creep of concrete, an algorithm is developed for the time-dependent behavior of concrete-filled steel tube (CFT), with or without the interface bond. The model adheres to geometric compatibility and static equilibrium, and considers the effects of sealed concrete, multi-axial state of stresses, creep Poisson’s ratio, stress redistribution, variable creep stress history, and creep failure of the column. The model is verified against previous creep tests for bonded and unbonded specimens. A study is then carried out on the practical design parameters that may affect creep of CFT columns under service loads, or lead to their creep rupture at high levels of sustained load. The study indicates that creep of CFT columns should be considered in the design, however, with creep coefficients much lower than those prescribed in the current ACI. Creep of CFT is shown to be a function of concrete mix, column geometry, and interface bond. Therefore, a single ultimate creep coefficient cannot be used for all concrete mixes, column geometries, and construction types. Bonded tubes curtail creep of concrete much more than the equivalent unbonded ones, mainly because of the stress relaxation phenomenon, which is more pronounced for smaller diameter-to-thickness ratios. For diameter-to-thickness ratios of 40 or less, bonded tubes are more durable in creep rupture than the equivalent unbonded ones. Creep rupture life of 75 years is quite feasible in bonded CFT, with diameter-to-thickness ratio of 40 or less, for sustained loads as high as 65% of the static capacity of the column.  2003 Elsevier Ltd. All rights reserved.

Ultra-High-Performance Concrete Bridge Deck Reinforced with High-Strength Steel

Open-grid steel bridge decks are lightweight, but present noise, fatigue damage, ride comfort and maintenance challenges. A lightweight bridge deck system with a solid riding surface could eliminate these problems. Ultra-high-performance concrete (UHPC) shows promise as a material for a new lightweight bridge deck system. This paper describes the development of a low-profile UHPC deck system that addresses the concerns with open-grid steel decks, satisfies strength and serviceability requirements, and meets the strict self-weight requirements for movable bridges. The experimental study shows that the proposed system has great potential to serve as an alternative to open-grid steel decks. The ultimate load capacity of all specimens exceeded the target load for the bridge deck. Although shear was the governing mode of failure in most of the specimens, it was not as abrupt and catastrophic as the commonly seen shear failure mode. The use of standard 180-degree hooks on both ends of the flexural reinforcement helped avoid bond failure. Further research is needed before field implementation of the proposed deck system.

TIME-DEPENDENT BEHAVIOR OF FIBER-REINFORCED POLYMER-CONFINED CONCRETE COLUMNS UNDER AXIAL LOADS

This paper presents an investigation into the long-term behavior of fiber-reinforced polymer (FRP)-confined concrete columns. Shrinkage, interface bond, creep, creep recovery, static, and reserved strength of several concrete-filled FRP tubes (CFFTs) and fiber-wrapped concrete columns (FWCCs) were measured. The study showed shrinkage of the concrete core in CFFT columns to be quite negligible. Bond strength at the interface of the concrete core and FRP tube was found to be lower than that in steel tubes, but still large enough to counteract the axial shrinkage of the concrete core. The effect of lateral confinement on the creep of the concrete core was found to be not as significant as sealing of the concrete core and the stress distribution that takes place between the concrete and the FRP tube in the axial direction. As the stiffness of the tube increases relative to that of the concrete core, a larger stress redistribution can occur over time, yielding a lower creep coefficient. The ACI 209 model was shown to overestimate creep of FRP-confined concrete. The creep coefficients for CFFTs may be as low as 22% of those recommended by ACI 209 for an equivalent sealed concrete. The difference between ACI 209 and test results of the FWCC specimens, however, is not as significant. While no strength degradation was observed for FWCC specimens, FRP tubes showed ~30% strength degradation.