International Concrete Abstracts Portal

Editor: Riadh Al-Mahaidi

This CD contains 8 papers that were presented at a session sponsored by Joint ACI-ASCE technical commttee 447 at the ACI Fall Convention, October 2011 in Cincinnati, Ohio. The papers cover the modeling for strengthening for flexure, shear, torsion, and confinement of concrete. Where applicable, the papers cover comparisons of modeling results with experimental tests performed around the world.

Note: The individual papers are also available. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP-301

Fiber reinforced polymers (FRP) is an attractive material to the field of strengthening and confining new and existing structures. FRP is usually used to wrap columns to increase the ultimate strength and strain of the concrete through confinement. Existing columns typically have spiral steel reinforcement (SS) in the section when wrapped with FRP. The nature of the problem becomes totally different since there are two systems with different behavior engaged in confinement. Several models were proposed to depict the behavior of the FRP alone in confining concrete. On the other hand, the literature has limited studies assessing the behavior of FRP and SS working on confining concrete simultaneously. This paper proposes a model addressing the two materials engagement in circular columns. The development of the effective lateral confinement pressure is based on Lam and Teng model for FRP action and Mander Model for SS action. It also introduces the force eccentricity as a new parameter that plays an important role in estimating the amount of confinement involved. Hence, the level of strength and ductility vary based on the eccentricity. The proposed model considers the fully confined curve as an upper bound curve with zero eccentricity and the unconfined curve as a lower bound curve with infinite eccentricity. In between these two curves, infinite numbers of stress-strain curves can be generated based on the eccentricity. Generalization of the moment of area approach is utilized based on proportional loading, finite layer procedure and the secant stiffness approach, to achieve equilibrium points of P-e and M-f diagrams up to failure. Finally the model is validated by showing good conservative correlation to experimental data.

The increasing demand to strengthen existing infrastructure has resulted in growing popularity of advanced fiber composite materials (FRPs) applied to reinforced concrete (RC) members as externally bonded reinforcement. Although FRPs contain very high tensile strengths, premature debonding usually prevents the material from reaching its full potential. Research is currently underway to address this shortcoming by the provision of anchorages to the ends of FRP reinforcement. Bi-directional fiber patch anchors have been found to be one of the most effective anchorages available, which are particularly suitable in shear strengthening applications. The ongoing need for verification of the various influencing parameters such as anchor size, spacing and fiber thickness have inspired further numerical and experimental studies resulting in the present work. The paper will investigate the effect of such parameters highlighting key relationships that may be applied for future use in anchorage strength models.

This paper presents the development of a finite element (FE) model of a rectangular reinforced (RC) beam externally strengthened with a carbon fibre reinforced polymer (CFRP) plate to capture the response of firetested beams conducted by other researchers. The developed model considers the variations in the thermal and mechanical properties of the beam’s constituent materials with temperature. In addition, cohesion elements are placed at the interface between the CFRP and concrete materials to simulate debonding. Transient time domain thermal-stress analysis is performed to obtain the heat transfer distribution and deformation within the beam. The model is validated by comparing the predicted progression of temperature at certain specified locations across the beam’s cross-section to that of the measured fire test data. In addition, the predicted beam’s mid-span deflection during fire exposure is compared to the measured experimental data. In general, good correlation was observed between the measured and predicted results. Furthermore, the developed FE models were able to capture the debonding failure mode that was observed in the experimental tests. It is concluded that the developed model could be used as a valid tool to investigate the fire performance of RC beams externally strengthened with CFRP laminates.

Fiber-reinforced Polymer (FRP) started to find its way as an economical alternative material in civil engineering from the early 1970s. The behavior and failure modes for FRP composite structures were studied through extensive experimental and analytical investigations. While research related to the flexural behavior of FRPstrengthened elements has reached a mature phase, studies related to FRP shear strengthening is still in a less advanced stage. In all proposed models to predict the shear capacity, the constitutive behavior of concrete and FRP was described independently. The true behavior, however, should account for the high level of interaction between the two materials. In this research, new constitutive relations for FRP-strengthened reinforced concrete elements subjected to pure shear are developed. In order to generate these relations, large-scale tests of a series of FRPstrengthened reinforced concrete panel elements subjected to pure shear are conducted. The University of Houston is equipped with a unique universal panel testing machine that was used for this purpose. These constitutive laws are implemented into fiber-based finite element models to predict the behavior of externally bonded FRP strengthened beams. The newly developed model proved to provide a good level of accuracy when compared to experimental results.